Metal Phosphates and Phosphonates: Fundamental to Advanced Emerging Applications 3031270614, 9783031270611

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Engineering Materials

Ram K. Gupta   Editor

Metal Phosphates and Phosphonates Fundamental to Advanced Emerging Applications

Engineering Materials

This series provides topical information on innovative, structural and functional materials and composites with applications in optical, electrical, mechanical, civil, aeronautical, medical, bio- and nano-engineering. The individual volumes are complete, comprehensive monographs covering the structure, properties, manufacturing process and applications of these materials. This multidisciplinary series is devoted to professionals, students and all those interested in the latest developments in the Materials Science field, that look for a carefully selected collection of high quality review articles on their respective field of expertise. Indexed at Compendex (2021) and Scopus (2022)

Ram K. Gupta Editor

Metal Phosphates and Phosphonates Fundamental to Advanced Emerging Applications

Editor Ram K. Gupta Department of Chemistry Pittsburg State University Pittsburg, PA, USA

ISSN 1612-1317 ISSN 1868-1212 (electronic) Engineering Materials ISBN 978-3-031-27061-1 ISBN 978-3-031-27062-8 (eBook) https://doi.org/10.1007/978-3-031-27062-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Contents

An Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sunil Kumar Baburao Mane, Naghma Shaishta, G. Manjunatha, and Asif Hayat Hierarchically Porous Metal Phosphates and Phosphonates: Emerging Materials Toward Advance Applications . . . . . . . . . . . . . . . . . . . Umair Azhar, Muhammad S. Bashir, Muhammad Arif, and Muhammad Sagir Rich Structural Chemistry of Metal Phosphates/Phosphonates for Emerging Applications: V, Ti-containing Materials . . . . . . . . . . . . . . . . Wei Ni and Ling-Ying Shi

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Electrochemistry of Metal Phosphates and Phosphonates . . . . . . . . . . . . . . Hülya Silah, Cem Erkmen, and Bengi Uslu

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Fundamentals of Electrochemical Energy Devices . . . . . . . . . . . . . . . . . . . . Abhinay Thakur and Ashish Kumar

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Principles of Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ruchi Jha, Ranita Pal, Debdutta Chakraborty, and Pratim K. Chattaraj

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Metal Phosphates/Phosphonates as Catalysts for HER . . . . . . . . . . . . . . . . 115 Changrui Feng, Meng Chen, Abuliti Abudula, and Guoqing Guan Metal Phosphate/Phosphonates for Hydrogen Production and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Rabia Sultana, Yinghui Han, Xin Zhang, and Lijing Wang Polyphosphate-Based Electrocatalysts for Oxygen Evolution . . . . . . . . . . . 151 Md. Yeasin Pabel, Akash Pandit, Tabassum Taspya, and Md. Mominul Islam Metal Phosphates/Phosphonates for Membranes . . . . . . . . . . . . . . . . . . . . . 171 Peng Sun, Lei Zhang, Hongsen Hui, and Zhongfang Li

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Metal Phosphates/Phosphonates for Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . 193 Runwei Mo Metal Phosphates/Phosphonates for Biofuels . . . . . . . . . . . . . . . . . . . . . . . . . 209 Shan E. Zahra Jawad, Batool Fatima, and Muhammad Najam-ul-Haq Phosphates and Phosphonates as Photocatalysts for Environmental and Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Demet Ozer Metal Phosphates/Phosphonates for Supercapacitor Applications . . . . . . 245 Nawishta Jabeen, Ahmad Hussain, and Jazib Ali Polyoxometalate Archetypes as Supercapacitor Materials . . . . . . . . . . . . . 267 Md. Akib Hasan, Ahammad Musa, Mohy Menul Islam, and Md. Mominul Islam Transition Metal Phosphates/Phosphonates for Lithium-Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 C. Nithya Metal Phosphates for Environmental Remediation: Adsorptive Removal of Dyes from Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Turkan Kopac Recent Insights in the Utilization of Metal Phosphonates for Remediation of Dye-Polluted Wastewaters . . . . . . . . . . . . . . . . . . . . . . . . 323 Turkan Kopac Metal Phosphates: Their Role as Ion Exchangers in Water Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 Amita Somya Metal Phosphates/Phosphonates for Biomedical Applications . . . . . . . . . . 357 Aditya Dev Rajora, Trishna Bal, Snigdha Singh, Shreya Sharma, Itishree Jogamaya Das, and Fahad Uddin Metal Phosphate and Phosphonate Application for Imaging and Diagnosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 Hamide Ehtesabi and Seyed-Omid Kalji Advances and Challenges in the Fabrication of Porous Metal Phosphate and Phosphonate for Emerging Applications . . . . . . . . . . . . . . . 393 Ababay Ketema Worku and Delele Worku Ayele

An Introduction Sunil Kumar Baburao Mane, Naghma Shaishta, G. Manjunatha, and Asif Hayat

Abstract The type of inorganic-organic hybrid polymeric material formed by the coordination of phosphonate ligands to metal ions, resulting in multi-dimensional extended assemblies is metal phosphonates (MPs). The discipline of MPs chemistry has developed progressively over the last few decades, fueled by interest in applications in a wide range of fields. Synthetic technologies of MPs are lacking on the way to domestic, more efficient alternatives. For the characterization, the advancement of electron diffraction as an instrument for crystal structure determination and the use of in situ characterization techniques have allowed for a better understanding of reaction pathways. Metal phosphonates have been discovered to be appropriate materials for a wide range of applications. This chapter continues to concentrate on advanced emerging applications of MPs in bio-ceramics, electrochemical energy devices, fuel cells, state-of-the-art hydrogen evolution rate (HER), oxygen evolution rate (OER), and water splitting catalysts. The remaining eighteen chapters in the book demonstrate the vast expansion and diversity of metal phosphonate chemistry research briefly. Keywords Metal phosphate and phosphonates · Water splitting catalysts · Electrochemical energy devices · Fuel cells

S. K. B. Mane · N. Shaishta (B) Department of Chemistry, Khaja Bandanawaz University, Kalaburagi, Karnataka 585104, India e-mail: [email protected] G. Manjunatha Department of Chemistry, Shri Siddhartha Institute of Technology, Tumkur, Karnataka 572101, India A. Hayat (B) State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, P. R. China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_1

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1 Introduction The goal of this chapter is to compile important and contemporary research publications on the intriguing chemistry of metal phosphate and phosphonates. Metal phosphates (MPs) are a large group of structurally adaptable acidic solids that perform exceptionally well in a variety of applications such as catalysts, fuel cells, batteries, biomedical devices, and so on. These P-containing synchronization primitives can be synthesized using a variety of methods which are frequently simple to access, providing a relatively vast array of key components. Reliant on metal phosphate mixtures and synthetic techniques, MP solids can be generated in a wide range of crystalline shapes, from 1D polymeric structures to layered networks to 3D open frameworks [1]. These solids’ benefits include being inexpensive and simple to prepare, being hydrophilic and thermally stable, and having some structural design ability. Their structures can also be altered after syntheses, such as by adding ionic or neutral species that have a big impact on their functionality and other crucial characteristics, like the development of hydrogen bonding networks [2]. The main group, alkaline, transition, and rare-earth metals can be combined with these ligands to create strong, nanocrystals that can be used in a wide range of processes, including electrostatic interaction, gas adsorption, molecular recognition, catalysis, and reinforcement for therapeutic systems [3]. Such materials have an ancient legacy that started in the 1970s with the autonomous groundbreaking studies done by Giulio Alberti and Abraham Clearfield. Relying on phenyl phosphonic acid, Alberti published the discovery of the inaugural multilayer Zr phosphonate in 1978 [4]. Clearfield subsequently discovered the crystal structure of this compound in 1993 [5]. This Zr-modified version revealed a brand-new chemistry premised upon the efficient artificial material scheme owning custom-made characteristics as a result of the synergistic participation of both the metal type and organic component of the linkers. It is regarded as the quintessential framework of all metal phosphonates. Past developments have seen the publication of many thorough assessments of this subject [6]. But, the in-depth study of novel ligands with varying degrees of complexity and utility propelled this chemistry in unanticipated directions and to thrilling breakthroughs in the realm of the development of novel nanomaterials. The First European Workshop on Metal Phosphonate, which took place in Swansea (UK) in September 2018, gathered the most recent aids from many subject-matter specialists, and this chapter compiles them. The workshop was a one-day gathering that was planned to provide the top researchers in the area of MPs chemistry with a place for debate. The seminar’s invited speakers addressed a wide range of subjects, including new synthetic techniques, porous materials, catalysis, batteries, and tissue engineering. The collaborative viewpoint paper titled “New Directions in Metal Phosphonate and Phosphinate Chemistry” [7] compiles a thorough summary of the workshop. This viewpoint enumerated potential fresh ideas for research study and summarized all of the writers’ workshop presentations. Due to the First European Workshop on Metal Phosphonate Chemistry’s success, a second one has been

An Introduction

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Fig. 1 The number of publications with entitled metal phosphate and phosphonates from Science Direct (2002–2022)

planned for Berlin (Germany Federal)’s Institute for Materials Research and Testing (BAM) on September 24, 2019. Figure 1 shows the two-decade (2002–2022) data of many publications with entitled metal phosphate and phosphonates from Science Direct, which shows the advancement of research work going on in this field. This chapter begins by going back in time and recounting a couple of the most significant phases of historical research with a particular emphasis on architectural chemistry because the field of MPs has advanced significantly. The methodologies for fabrication and characterization are covered in the following section, with a focus on cutting-edge techniques like rising production for the creation of novel substances, mechano-chemical synthesis, techniques for creating porous architectures, framework remedy from the diffraction pattern, and in situ analytical techniques. The next segment concentrates on innovative MP applications, such as medication administration, electrochemical devices, catalysis, and gas sorption/separation. Lastly, we consider what the field’s prospective new may include while attempting to pinpoint the much more potential fresh lines of inquiry.

2 Ancient Point of View The research from Clearfield et al. in the area of tetravalent metal phosphates was recognized for strong ion exchange capabilities since the 1950s and sparked initial interest in the field. In 1968, Clearfield used single-crystal X-ray diffraction (SCXRD) data to determine the crystal structure of -zirconium bis(monohydrogen orthophosphate) monohydrate [Zr(HPO4 )2 ·H2 O, referred to as -ZrP] [8]. The layered assembly of α-ZrP is made up of Zr atoms that are tridentate monohydrogen phosphate subunits that are octahedrally linked, allowing free hydroxy groups that face toward the interlayer space and form hydrogen bonds with the molecules of water fitted between the sheets. An extensive study was conducted to take benefit of the acidic hydrogen on

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the exterior of the layers, particularly for ion exchange and complexation reasons, as a result of the atomic level comprehension of the construction of α-ZrP. An arena of MPs was introduced in 1978 [4] by Alberti and Costantino et al. Three zirconium phosphonates were created: zirconium ethylphosphate (Zr(C2 H5 OPO3 )2 ), zirconium phenyl phosphonate (Zr(C6 H5 PO3 )2 , and zirconium hydroxymethyl phosphonate (Zr(HOCH2 PO3 )2 ). They were unable to find acceptable single crystals for use in the SCXRD to solve the structures because of the extremely high poor solubility of these substances. The researchers speculated that these substances might well produce a similar network structure to α-ZrP because of their comparable synthesis procedure and focused primarily on powder X-ray diffraction (PXRD) patterns ruled by intense basal peaks at low angles. In α-ZrP, the hydroxyl groups in the interlayer are replaced by the organic substituent, which decides the interlayer distance. In the years that followed, several comparable substances were created using both mono and di phosphonates to produce comparable stacked or walled layer structures. The first MP derivatives have been based on tetravalent metals, particularly zirconium, but in the late 1970s and 1980s, numerous frameworks premised on divalent metal ions have been developed. Cunningham et al. [9] conducted preliminary research and reported an easy synthetic procedure on divalent metal phenyl phosphonate and phenylarsonates, M(C6 H5 PO3 ) and M(C6 H5 AsO3 ) [M2+ = Mg, Mn Fe, Co, Ni, Cu, Zn, and Cd] by a simple reaction with the chloride or sulfate metal salts. Appreciation to the inferior solubility of these materials, associated with tetravalent MPs, single crystals might be developed. A variety of important articles documenting the crystal assemblies of numerous divalent MPs appeared toward the conclusion of the 1980s. Each of these substances had layers formed by connecting metal atoms and phosphonate groups, with the organic substituent residing in the interlayer space. One of the instances given by Cao et al., where a variety of structures based on divalent metals remained published, relying on SCXRD measurements, encompassed: Mn, Mg, Ca, Cd, and Zn and explained the layered crystal structure of Mn(C6 H5 PO3 )·H2 O [10]. Beginning in the 1990s, MPs structures spread out across the entire periodic table, including all of the lanthanide series, a variety of transition metals, and more than half of the alkali and alkaline earth elements. Major advances had also been achieved in resolving the crystal structures using PXRD observations during this development over the periodic table. This one was crucial for the area MPs because it has frequently been challenging to create an appropriate single crystal for SCXRD, particularly when using metals having high oxidation levels. A case in point is the groundbreaking research of Alberti and Costantino et al. who’ve been unsuccessful in obtaining massive crystals of Zr(C6 H5 PO3 )2 . Poojary et al. finally resolved the framework from PXRD statistics in 1993 [5] using a mixture of optimization techniques, Patterson strategies, and Rietveld processes, proving that Alberti and Costantino et al. 15-yearold’s expectation was accurate and that the structure of Zr phenyl phosphonate was predicated on the identical layered setup of -ZrP. The advent of more potent crystallographic programs, better accessible radiation accelerators, and the production of more advanced laboratory powder X-ray

An Introduction

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diffractometers have all contributed to the design approaches from PXRD becoming a powerhouse for scientists dealing with MPs. Although the vast number of MPs identified in the first 15 decades of studies had layered architectures, some open framework MPs were discovered in the early to mid-1990s. The earliest instances were all built on the tiny ligand methyl phosphonic acid, which, when mixed with Cu, Zn, and Al, produced frameworks with a broadcast organization that was evocative of several zeolite structures. Further research in this area was encouraged by the structural resemblance to zeolites which were at the time the most significant class of crystalline and microporous materials. As a result, further principles set substances were discovered, the majority of which were built on mono phosphonates or di phosphonates having short alkyl chains. In virtually all cases, lengthening the alkyl chains led to the formation of multilayered architectures, prohibiting the enlargement of the channel width and the production of more porous materials [7]. Three methods were developed to stop the enlargement of the structure from growing to combat the high tendency of MPs to polymerize: Incorporating sterically demanding moieties onto the framework of phosphonic acids, utilizing terminal auxiliary ligands that really can fill coordination sites on the metal ions, employing premade clusters, and conducting regulated ligand interchange are all examples of this [11]. The researcher can consult the book “Metal Phosphonate Chemistry: from Synthesis to Applications” which also has a chapter on the early history of MPs chemistry, written by Abraham Clearfield [12] if they are eager for a more thorough overview of the development of the field up to 2011.

3 Synthesis of Metal Phosphate and Phosphonates The various synthetic route for the production of metal phosphate and phosphonates were represented in Fig. 2.

Fig. 2 Various synthetic routes for metal phosphate and phosphonates

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3.1 Micro Porous Metal Phosphate Numerous research activities were undertaken for the designing of metal phosphates via hydrothermal technique after the finding of microporous alumino-phosphonates using amino-centered organic molecules which possess 2D and 3D construction assembly. As a result, over time, a diverse set of outline configurations, as well as isomorphous replacement by responsive catalytic metal atoms, have emerged. With the help of various organic pattern fragments, microporous aluminophosphates made of substitute alumina and phosphate tetrahedrons with impartial outline structures are disclosed with a wide variety of pore sizes. As models, a variety of primary, secondary, and tertiary amines, quaternary ammonium cations, alcohols, and so on are used to create these microporous materials. Later on, a hypothetical example constructed on host-guest communications to elucidate the outline nanostructures of microporous materials using solvo or hydrothermal processes was proposed. In addition to the standard pathways, the ionothermal process, in which an ionic liquid can be used as both a solvent and a model, was shown to be valuable for its structuredirecting character in constructing these metal phosphate-based nano-architectures. These porous nanostructures can be created using the microwave-assisted heating technique, which significantly reduces the time needed for crystallization and the entire synthetic process to just an insufficient minute. Microwave production frequently results in minor constituent parts using greater morphological regularity while maintaining crystallinity in aluminum phosphates. The resultant materials have important adsorption and catalytic capabilities and can comprise a huge amount of 3d transition elements and also non-transition elements [13]. Owing to the mixture of the Lewis acidity and redox characteristics of these transition metals in heterogeneous catalysis, their isomorphous substitution in AlPO4 networks is frequently highly advantageous. Due to their great chemical constancy and extensive potential for adjusting the superficial acidity to meet the requirements for an anticipated catalytic response, these silico alumino phosphates have drawn specific consideration to isomorphous replacement of Al or P by Si in the corresponding AlPO4 outlines [14]. The embedding of organic assemblies at the pore superficial can add elasticity, performance, and hydrophobicity to the subsequent organic-inorganic hybrid resources in addition to the integration of the inorganic substituent in these phosphate-built constituents. In this regard, a phenylfunctionalized large pore alumino phosphate resource using phenyl phosphoric acid as the phosphate source has been reported [15]. Owing to the large toughness, phenyl assemblies can be additionally functionalized with responsive organic clusters to complete the anticipated request potential. Constructing metallo phosphate molecular filters requires a careful fundamental route, which is facilitated by models. The pH of the production units for these phosphate-based microporous resources is reserved slightly acidic, in contrast to the manifestation of high silica zeolites, which frequently includes tetra alkyl ammonium cations. Maximum amines protonate in slightly acidic circumstances and can

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cooperate electrostatically with phosphate anions to steady the formation of the operational distinct components, which serve as nucleating types in guiding the crystal structures. In this area, work has been effective in creating microporous phosphatebased resources with an AFI outline and globular constituent part morphologies using tiny biomolecules as structure guiding representatives, such as tetramethylguanidine. It has also been reported that crystalline microporous aluminophosphate and metallo alumino phosphate resources have been developed using the Ni(II) complex as a prototype [16]. Here, the size of the relevant multifaceted and the kind of nonbonding purposeful assemblies are important structural factors that influence the development of porous structures. When crystallization is in process, in situ XRD and spectroscopic analyses of the manufactured gel frequently shed light on the crystallization process and the function of substrates in these preparations. To determine the crystallization methodology of Co-APO-5, Weckhuysen and colleagues examined the time-resolved SAXS/WAXS and spectroscopic investigations [17]. Their study revealed the organization of predominant amorphous metal and phosphate concrete members, followed by accelerated evaporation to linear Al-O-P groups in the proximity of template molecules, and eventually quick restructuring to establish the porous crystalline structure. Some of the most common synthetic approaches for manufacturing microporous and mesoporous metal phosphates and phosphonates were sol-gel and ligand-assisted solvothermal synthetic processes, as well as hydrothermal methods, are among them.

3.2 Mesoporous Metal Phosphates The complex molecular surfactant arrangement aided material production by the morality of charge transfer among both zirconium sulfate polyanions and the cationic surfactant in the synthesis of ordered mesoporous zirconium oxophosphate material has been well published. A large well-organized surface area of mesoporous titanium phosphates with cationic and anionic surfactant molecules self-assembling as structure guiding mediators was nicely synthesized. Ti(IV) positions may perhaps engage with anionic surfactants, however anionic phosphate possibly will connect with a cationic surfactant to maintain the well-arranged mesophases in organic molecules. In the corresponding synthetic routes, charged Ti(II) sites and negatively charged phosphate sites could interact with each other to form new molecules. By using self-assembly with cetyltrimethylammonium bromide (CTAB) as a template underneath hydrothermal circumstances, scientists were able to create mesoporous cobalt phosphate with episodic nanostructures and crystalline pore walls [18]. This mesoporous material’s highly precise superficial area, crystalline pore wall, and plenty of catalytic spots significantly aid in its electrocatalytic movement in the oxygen evolution reaction. The wormhole-based designed mesoporous phosphate titania in the company of Pluronic P123, where almost even pore channels of dimensions are arbitrarily organized to form a 3D structure has been testified [19].

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Furthermore, the robust P-O-Ti attachment in this mesoporous phosphate titania encourages supplementary cross-linking of the surface imperfection spots to steady the mesostructure. With the help of UV/visible light irradiation, the vast surface region structured with tetrahedral Ti(IV) centers maintains its catalytic function in the degradation of n-pentane. Zhao et al. have tuned the ordered mesophase of titanium phosphate from 2D-hexagonal to cubic, and then into the lamellar nanostructure in this context by varying the surfactant (P123)/(Ti+P) molar ratio [20]. Here, the creation of stably ordered mesophases was substantially guided by the links among the acid-base pairs of the metals and phosphate. Triblock copolymer supramolecular construction is repeatedly detected to be an extremely helpful framework for the production of spherical nanoparticles of materials depending on aluminum organo phosphonate (AOP) and aluminum phosphate. In contrast hand, Liu et al. created an AlPO polymer using a hydrothermal synthesis gel that contained a tiny organic molecule called citric acid that might engage with the aluminophosphate matrix during the nucleation and polycondensation stages and produce consistent mesopores [21].

3.3 Mechanochemical Synthesis Mechanochemistry has a lengthy history dating back more than a thousand years but has only subsequently gained popularity as a practical technique in chemical processing. Since the past 20 years, there has been a massive campaign for environmentally friendly and renewable chemistry, which may be one of the chief factors why mechanochemistry has since attracted much increasing emphasis in a variety of useful sectors. The general idea is evident, although there is still a variety of specifics to be worked up regarding the precise mechanisms that underlie mechanochemical production. The interaction among two or more solids to create the quality output is commonly driven by the supply of mechanical power, i.e., through crushing or milling, and frequently occurs with minimal or no solvent. Mechanochemistry is more interesting in part because processes can go forward through routes that aren’t available by traditional techniques [7]. The cadmium-based Mps Cd(O3 PPh)·H2 O and Cd(HO3 PPh)2 were obtained via a vibration ball milling process when cadmium acetate dehydrate and phenyl phosphonic acid was shared inside a reaction container in several ratios (1:1, 1:2, and 1:4) and then milled for 15 min along with stainless steel balls [22]. Because water and acetic acid were released during the reaction, causing all of the compounds acquired through milling to be moist, this indicates that every step of the production was actually helped by liquids. Further investigation towards replacing cadmium with Mn, Co, and Ni was done M(HO3 PPh)2 (H2 O3 PPh)2 (H2 O)2 MPs in the same year [23]. Following 15 min of smooth crushing, the scientists succeeded in synthesizing three pure chemicals using the outlined method. For each of the components, researchers also conducted a liquid-assisted grinding (LAG) run, and they discovered that it had no impact on the results acquired relative to the dry

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course. The researchers realized that such a method may be used to quickly and simply synthesize different molecular MPs. Because of the flexibility, minimal influence on the environment, rapidity of production, and additionally, the potential for commercial uses exists.

4 Applications With particular emphasis on the recent 20 years (from 2002–2022) of advances, this chapter covers the current state of the art in drug delivery, proton conductive or fuel cell, electrochemical water splitting, and bio-ceramaic applications of metal phosphates. Figures 3 and 4 shows a summary of publishing patterns which includes book chapter, review, and research articles taken from the science direct collection, which attests to the rising popularity of this subject over time.

4.1 Medication Transport Implementation For the creation of ionic-strength pharmaceutical delivery systems, the bidirectional protonation-deprotonation of the oxidative stress activity in permeable nanoarchitectures is very essential. By jointly employing mono- and bis-phosphonic acid functionalities like organophosphorus substrates, together with built-in L-proline and piperazine functions for such colon-focused ionic strength discharge of DNA

Fig. 3 Numerous applications of metal phosphate and phosphonates. Where, I: metal phosphonated MOF (fuel cells), II: biomaterial (drug transport and bioceramics), III: membrane (ion exchange and gas sorption), IV: energy storage (battery and supercapacitors), V: photocatalysis and electrochemical cells (water splitting, HER, OER and CO2 reduction and VI: catalysts of synthesis (biofuel and petrochemical)

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Fig. 4 Statistical data from Science Direct (2002–2022) on Metal phosphate and phosphonates in various fields

molecules, a report disclosed highly porous zirconium phosphonates. The main benefits of using phosphonate functionalized porous polymers as pharmaceutical logistics companies also include their minimal cytotoxicity rate and good biocompatibility. To create covalent imine group-based organic nanostructured resources, research has been undertaken to functionalize cellulose nanocrystals with (bis) phosphonate, which encompasses alendronate and 3-aminopropyl phosphoric acid. Phosphonates substituents in such components seem to be especially beneficial in the prevention of bone density, and they can also be connected to fluorophores to deliver effective therapeutic agents for bone biosensing. Supramolecular nano valves, for instance, predicated on the synthesis process of phosphonated column arteries over mesoporous silica nanoparticles (MSNs), could indeed perform extremely proficiently as just a vehicle for drug delivery [24]. The phosphonate groups in the MSNs regulate the discharge of prescription medications for tumor photothermal chemotherapy by ion-pairing with quaternary ammonium ions in the nanostructure. Guest molecules frequently prevent the mesopores of these MSNs, preventing the stimuli-responsive intercellular delivery of medications on supply. Rim et al. demonstrated that coating the pore exterior of MSNs with the environmentally benign biological material calcium phosphate could perhaps perform extremely proficiently as just an ionic strength pore inhibition for the continuous flow of apprehended therapeutic agents [25]. Underneath slightly acid pH circumstances, intracellular partitions which include lysosomes and endosomes might discharge the encapsulated active ingredients kept inside the porous structure. Covalent bonding using phosphonate assemblies allows biopolymer-like chitosan to somehow be successfully deposited over functionalized MSNs, which in turn causes the painkiller ibuprofen to discharge in a pH-responsive manner. Via scientific investigations, it had been discovered that fructose 1,6-bisphosphate can be employed as a phosphonate linkage representative for the production of a blended highly porous

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strontium-loaded calcium phosphate nanomaterial. This material serves as an effective drug-delivery motor for antibiotic drugs such as vancomycin and significantly improves the skeletal capacity when combined with collagen. For the manufacture of highly porous hydroxyapatite, a sodium hexa metaphosphate as a phosphate precursor had been employed to transport the pH-responsive anticancer medication doxorubicin (Dox). Zhang et al. have created a mixed nanomaterial for the loading of the anticancer medication Dox that has covalently linked highly porous calcium phosphate nanoparticles with a cylindrical enclosure nanostructure and contains photoresponsive components encapsulating polydopamine [26]. In medical research, chemo-photo thermal treatment and improved cinematography are both simultaneously provided by this hybrid nano-architectured polymer.

4.2 Electrode Materials/Proton Conduction for Fuel Cells For the production of numerous divalent and trivalent metal phosphates, the existence of organic molecules as charge-compensating ions was required, whereas tetravalent and monovalent metal phosphates are typically prepared by orthodox synthetic methods [27]. For a high intrinsic proton conduction process, the metal phosphate 2− must contain an acidic group namely H2 PO− 4 and H2 PO4 . Furthermore, a particular chemical alteration or influenced morphological alterations, such as the creation of nanoplatelets or nanorods particles, might result in extrinsic proton conduction, related to associated epidermal proton transportation. As both proton transporters as well as in the creation of H-bonding connections within the structure, water molecules play a role. Additionally, several techniques, like intercalation and post-synthesis adsorption, may be used to incorporate additional foreign entities, like organic components, into their architecture. Consequently, the hydrophilic system boundaries and water/guest molecule interactions play a unique role in MP hydrogen transportation [28]. As a result, the proton-transfer process in metal phosphates like CsH2 PO4 or CaHPO4 ·2H2 O [29] includes both the free rotation of the phosphate groups and the H+ jump. Other times, the proton-transfer process behaves incorrectly, as it does in the instance of KH2 PO4 [30]. A “proton jump” or the Grotthuss mechanism, both of which have a lower ionization potential, are frequently used to describe proton transportation within a quasi-liquid state [29]. Identifying proton propagation pathways is crucial to creating proton conductor materials. Therefore, proton-containing opposing charges such as hydronium ions, NH+ 4 , protonated amine groups, as well as protonated organic compounds in particular ought to be included in the framework when an automotive process is required. It can also be necessary to immobilize particular functional groups and their accompanying counter ions onto the architecture. However, materials that show proton transportation via a Grotthuss-type mechanism primarily have persistent H-bond systems which encourage H+ conduction

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having low activation energy (Ea). The latter substances’ fundamental flaw is that, at a specific temperature, the H-bond systems may rupture, releasing water molecules in the process, necessitating the development of persistent H-bonding systems containing hydrophilic pathways or the investigation of another suitable conductive medium [31]. The advancement of fuel cells in relation to energy production, environmental concerns, as well as the steady degradation of traditional energy reserves is a research subject of utmost importance. Perfluorinated sulfonic acid (PFSA) ionomers, including Nafion® as well as Aquivion® , are frequently utilized in PEMFCs and DMFCs as polymer electrolyte vesicles (PEM) for working temperatures below 120 °C from the numerous sort of solid electrolytes that are created in the last numerous centuries [32]. Improved conductivity characteristics, varying between 9 × 10−3 and 1.2 × 10−1 S/cm, as well as mechanical and chemical durability, are to blame for this [33]. Nafion® does, though, have a few significant disadvantages, including methanol absorption, increased price, complicated processing method, operating under 100 °C to retain the hydration of the membrane, as well as additional problems with catalyst performance. Because of everything said above, advanced membrane substances must be developed [34]. The utilization of low-loading expensive metal catalysts is made possible by intermediate temperature fuel cells (ITFCs) that run from 120 to 300 °C and avoid certain of these issues. They also accomplish lower catalyst toxicity and improved management of the hydration parameters. However, potential electrolyte breakdown must be researched mostly in terms of methanol HT-PEMFCs, which must run over 250 °C and under elevated pressure [35]. Throughout this book chapter, we update significant innovations in super protonic and tetravalent metal phosphate/pyrophosphate proton conductors. The creation of new proton carrier systems that depend on divalent and trivalent metal phosphates is also updated, as well as their development and design. To build an effective power source that uses H2 (fuel) and O2 molecules to produce electrical energy, it’s indeed extremely desirable to develop a porous membrane that exchanges protons with substantial proton conductivity [36]. In this regard, a ternary nanocomposite membrane composed of branched-chain triphosphonic acid NMPA, sulfonated polysulfone, and titania has been described [37]. This nanocomposite membrane had a proton conductivity of 0.002 S/cm at 150 °C, making it perfect for use in fuel cells. Since the labile protons of the free phosphate groups can move between different acid sites via their pore channels, porous nanoarchitecture built on phosphates and phosphonates are considered necessary in this aspect. A phosphonic acid-functionalized mesoporous silica was produced using CTABassisted hydrothermal co-condensation of different percentages of diethyl phosphate, ethyltriethoxy silane, and tetraethoxysilane [38]. For the fabrication of a multilayer Co–Ca bimetallic MOF, Zheng et al. used a bridging tetraphosphonic acid–based ligand, and it demonstrated a significant shift in the proton conductivity across various humidity levels [39]. Investigating their viability as a sensing element requires a significant shift in the proton conductivity with humidity. 5-(phosphonomethyl)isophthalic acid was employed by Wei et al. [40] as a ligand in the production of a Eu-based MOF with a multilayer anionic framework

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structure. Proton transportation is made easier by the hydrogen-bonded links that are formed along the pore axis by this Eu-MOF carrying surface phosphonate groups. The zirconium phosphonate-based material Zr(O3 P-OH)(O3 PC6 H4 SO3 H) was created by Alberti et al. using phosphoric acid and meta sulfophenylene phosphonic acid as the phosphate origin, and it displayed an incredibly high proton conductivity of 0.1 S/cm at a reasonably low temperature (100 °C) in 90% RH [41]. To comprehend the unrestricted passage of protons within the nanospheres in the phosphonate-based NiMOF, Schroder et al. have used quasielastic neutron scattering investigations [42]. Contrarily, Krautscheid et al. used solid-state MAS NMR spectroscopic investigations on a phosphonate-based La-MOF to determine the movement of molecules of water within the pore axis of the MOF with a pore size of 1.9 nm [43].

4.3 Electrochemistry of Water Splitting Another active field of study from an energy and sustainability standpoint is the creation of noble-metal-free electrochemical accelerators for such process of hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) through the electrochemical separation of water. The metal complexes are frequently unsustainable in water electrolysis separation, although certain heterocyclic compounds of transition elements have strong initial reactivity. As a result, metal phosphonate-based hybrid nanostructures, which are significantly stable over a broader pH scale, may be extremely beneficial again for the redox process. IrO2 -based nanostructures perform well for OER, however, their expensive price is a key limitation for these catalysts. Pt NP-based nanomaterials are indeed the reference HER electrocatalysts. As a result, nonprecious metal-based catalytic systems, particularly those that use metal phosphates and phosphonates, can provide a significant benefit in this situation [28]. A study on a nanocomposite consisting of nano carbon and N-doped cobalt phosphate for well-organized ORR at alkaline pH is given in this background [44]. Ndoping in graphitic carbon and Co–N interaction with cobalt phosphate were the two factors that contributed to the substantial ORR performance. Utilizing NMPA as little more than a phosphate precursor, F127 and polyvinyl alcohol as templates, work has been reported on a porous cobalt phosphonate exhibiting multilayered porosity with sizes of 1.5–5.0 micropore/mesopore and nanocages (20–60 nm). This cobalt phosphonate outline’s large area of surface and nanoscale permeability was discovered toward being extremely advantageous because of its excellent electrochemical performance within the alkaline water oxidation process. The production of cobalt phosphonates ornamented N-doped carbon using a phosphonate-bridged MOF has additionally been done. Such material has been used as an accelerator for electrochemical water-splitting reactions [45]. It has been hypothesized that greater OER action is caused by deformed geometry at the metal centers and extended Co–O and Co–Co bond lengths based on XAS and XPS research. Pinna and colleagues have compiled many coordination polymers

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and MOFs based on metal phosphonates for creating metal phosphates, phosphides, and oxyhydroxides, and the end products demonstrated significant electrocatalytic activity in HER or OER [46]. Furthermore, a nanocomposite material that is made of cobalt phosphate NPs supported over nitrogen and phosphorus co-doped mesoporous carbon has been reported and a high HER (acidic pH) and OER (alkaline pH) are produced by such multi-component catalytic system, which effectively supports the overall water separation response [47]. Transition metal phosphate electrocatalysts are emerging as a potential for OER or ORR in water splitting or metal-air batteries since they are inexpensive with good electrochemical behavior. Nevertheless, it remains difficult to find polyfunctional electrocatalysts with good electrochemical results for OER and ORR. In this perspective, for zinc-air batteries, Pan et al. have reported a simple self-template fabrication method for 2D amorphous N-doped CoFe-mesoporous phosphate micro sheets [48]. The ideal 2D amorphous N-doped CoFe-phosphate shows greater electrocatalytic activity for both OER and ORR, which displays an inferior overpotential of 313 mV at 10 mA/cm2 for OER besides a high half-wave potential of 0.74 V for ORR. The Zn-air battery that this multifunctional electrocatalyst was used in also had a high power density (74.6 m W/cm2 ), precise capacity (750 mAhgZn−1 ), and long-lasting consistency (over 30 h at 10 mA/cm2 ). Hence such research offered a fresh approach to synthesizing new 2D heteroatom-doped electrocatalytic components with mesoporous structures for power conversion purposes, particularly in rechargeable Zn-air batteries.

4.4 Bioceramics In bioceramics, where inorganic materials are primarily employed to fix and replace damaged human skeletal system components, metal phosphates have a considerable opportunity. Highly permeable metallo phosphates are bioactive glasses that can be used to engraft both hard and soft tissues and to keep giving bones the necessary tenacity and stretchability. Calcium orthophosphate, which is frequently used as a biocompatible substance for bone, achilles tendon, and tooth replacement, in addition to skeletal tissue creation, is one of the crucial elements in this frame of reference [28]. The effective synthesis of calcium orthophosphate, commonly known as hydroxyapatite (HA), can indeed be accomplished by condensing calcium(II) salts with triethyl phosphite in a sol-gel process. As bone fillers, hybrid fiber-reinforced composites made of biodegradable polymers and HA-type bioceramics are frequently used because they may quickly create pore structures in bone formation and speed up the healing course. In this regard, Bernstein and colleagues explored bone tissue development by implanting micro porous beta-tricalcium phosphate [49] and it was found that by their physical adsorption, genuine tissue and good skeletons could be formed in 52 weeks. Microspores were discovered to be extremely helpful for the regeneration of tissues including bone, muscle, and perhaps other vital organs in the 3D calcium

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phosphate nanostructure. When designing frameworks for bone and tissue regeneration, natural polymers are frequently combined alongside bioactive ceramics [50]. For the creation of Nano synthetic structures in biomedical applications, hierarchical permeability ranging from the nanoscale to macrospores is typically very advantageous. Such hybrids’ polymers afford the required rigidity and flexibility, and their metallo phosphates offer them the strength properties that may be appropriate for human bone. Such artificial biomaterials’ permeability may significantly support the development of connective tissues and keep the right amount of fluid inside the bones. Restorative dentistry procedures frequently employ metallic titanium discs. It is easier for the skeletal components and the phosphonates mounted Ti prosthesis to communicate chemically when the implants are coated with phosphonic acids at their edges, which results in a much-enhanced interaction with bones. As a result, the crosslinking of phosphate can provide an ecologically friendly method for placing dental implants. For instance, poly(beta-amino ester)-based gels and cryogels with phosphonate functionalization are redox-responsive, exhibit low cytotoxicity, and are biodegradable when used in in vitro cell culture investigations. Consequently, there is a significant chance that these phosphonated materials will be investigated as tissue manufacturing platforms.

5 Standpoints and Obstacles for the Forthcoming The successful synthesis of mesoporous silico alumino phosphate is one of the most difficult assignments to investigate in the coming years. The addition of regular mesopores in conjunction with a zeolite-like polycrystalline pore wall should bypass the transport restriction of porous structure materials and so make it an ideal contender in heterogeneous catalysis, despite studies on compounds with progressive meso-micro highly skewed permeability. Relative to microporous H-ZSM-5 and H-SAPO-5, hierarchically porous H-SAPO-5 did in fact exhibit improved catalytic performance for the benzene alkylation utilizing benzyl ethanol as an efficient catalyst. A strong Bronsted acidity can be achieved through pyridine-IR in mesoporous silicoalumino phosphate materials such as MESO-SAPO-5. The enhanced catalytic efficiency again for the alkylation of phenol using propylene oxide is mostly attributable to the highly skewed permeability in this catalyst’s solid acid structure. Because the production gel for each of these metallo phosphates- and phosphonate-based compounds contain metal ion progenitors, the utilization of tiny chelating agents like salicylic acid may act as a blueprint for creating the corresponding highly porous nanostructural design. By utilizing unusual framework compounds generated through biomass resources, like glucose, citric acid, or biopolymers including sodium alginate, inter crystallite meso porosity in the microporous materials can be established. Sustainable development is among the key challenges in chemical studies. The production of such porosity nano-architectured compounds will undoubtedly be very efficient thanks to certain framing techniques utilizing biomaterials. The variety of

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the scaffolding formulations is one of the special benefits of metal phosphate- and phosphonate-based porous nanostructural design. Based on the choice of SDAs, a wide range of polyvalent metals can produce matching metal phosphates and phosphonates. By employing water-in-oil surfactant micelles with surfactant patterning for just a slow crystallization approach, work has been reported in which CePO4 nanowires are made of linear or twisted strands with a consistent thickness of 3.7 nm. Extended production times could be controlled using preservatives or even microwave heating conditions to create nanomaterials with precisely defined particle shapes and porosities. Another significant area is chiral catalysis, where phosphonate-based homochiral porous materials have a lot of unrealized potential. Hexa-connected Ni(II) containing bridging bi naphthyl bisphosphonate binders could be synthesized by a hydro or solvo thermal synthesis method to create 1D zigzag chain-like phosphonate-based MOFs. These phosphonate-based MOFs might function as an efficient catalyst for the enantioselective hydrogenation process due to the structural characterization of these kinds of low dimensional conjugated polymers that include pretty distorted octahedral Ni(II) centers. In light of their content, structure architecture, pore diameters, and simplicity of postsynthetic surface modification with reactive organic groups, one can examine the applicability of these porous nanomaterials. The viability of such components in different energy, environmental, and biological requests has now been expanded by the use of heterogeneous ligands in addition to phosphates to create chemically stable and mechanically resilient phosphonate-based MOFs. By having interstitial organic functional groups, highly porous phosphonates have the potential to create multi-constituent catalytic converters through post-synthetic functionalization and carbonization. Future research would only further enhance the chemical properties of such porous materials by identifying innovative nano infrastructures of metal phosphate- and phosphonate-based microporous and mesoporous components for catalysis, adsorption, photonics, electrochemical cells, battery storage, and biomedical applications

6 Conclusion Nano structural design with very large areas of surface and nanoscale pores that vary in length from a few nanometers to 50 nm. The porous metal phosphate and phosphonates reported above-mentioned are composed of appropriate metals and phosphate/phosphonate-based ligands. Porous nanomaterials have found many practical applications in catalysis, gas storage/adsorption, optoelectronics, electrochemical cells, fuel cells, and medicinal fields thanks to their structural stability and diversity in compositional changes. Because diverse metal ions have substantial binding affinities for phosphate-based ligands, these nanostructures are extremely resistant to harsh chemical conditions. Additionally, by combining these metal phosphates and phosphonates with 2D nanostructures like reduced graphene oxide, graphitic carbon nitride, etc., they will significantly increase their conductivity and mobility

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of ions and electrons, increasing their potential for use in electrochemical and optoelectronic applications. Furthermore, the development of novel phosphonate-based fusions with photo-, pH-, or thermo-responsive functionalities may enhance their possibilities for use in many cutting-edge fields including energy, the environment, biomedical sciences, etc.

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Hierarchically Porous Metal Phosphates and Phosphonates: Emerging Materials Toward Advance Applications Umair Azhar, Muhammad S. Bashir, Muhammad Arif, and Muhammad Sagir

Abstract The ability to create hierarchically porous nanostructures using materials based on metal phosphate and phosphonates is very astonishing. The major drivers of the scientific community are focused on the need to rationalize novel synthetic ways to synthesize these materials under controlled settings especially related to morphology. In this chapter, we have highlighted different synthetic techniques that have been employed in the synthesis of metal phosphonates and how the properties of porous metal phosphonates and phosphates are being impacted. Nanoporous metal phosphonates are proliferating rapidly owing to their versatile applications in different areas, including energy storage, catalysis, environmental intervention, and biology, among others, which are also discussed in this chaptesr. It is expected that the chemistry of porous metal phosphonates and phosphates would advance as a result of their utilization in domains like biology and fuel cells. Keywords Hierarchical · Porous nanostructures · Phosphates · Phosphonates · Fuel cells

1 Introduction The research in the field of inorganic–organic hybrid materials has expanded over the last two decades, thanks to the advent of hierarchically porous materials including metal–organic frameworks (MOFs) [1–3]. Hybrid materials have a lot of potential U. Azhar · M. Arif · M. Sagir Institute of Chemical and Environmental Engineering, Khwaja Fareed University of Engineering and Information Technology, Rahim Yar Khan 64200, Punjab, Pakistan M. S. Bashir (B) Institutes of Physical Science and Information Technology, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui University, Hefei 230601, Anhui, China e-mail: [email protected]; [email protected] CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, Anhui, China © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_2

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in both material chemistry and daily applications. Fine-tuning the matrix and interfacial characteristics of both organic and inorganic processes have been made easy. Modification of chemical or physical properties or the ability to manipulate specific chemical reactions or interactions is possible. As a result, hybrid porous materials like MOFs have been employed in many research reports. The class of chemicals synthesized via the reaction of metals and organophosphonic units is called porous metal phosphonates and is noteworthy since it is more specific and has been less researched. With their malleable surfaces, tunable pore sizes, readily accessible channels, and controllable pore environments, porous materials are used in a wide range of proven applications and cutting-edge technologies to address complex economic and energy challenges [4–9]. Structures of hybrid porous metal phosphonates are shown in Fig. 1, and these structures are compared in great detail to those of many other common materials. The unique inorganic–organic hybrid structure of porous metal phosphonates makes them stand out from others of the same rank [11, 12]. There are various types of porous materials, but porous metal phosphonates stand out by their unique properties [13]. Beyond the straightforward physical pairings of organic in organic– inorganic metal with holes, especially phosphonates, the molecular interweaving of organic and inorganic components in nanocomposites goes a step further. This is because nanocomposites include both organic and inorganic materials in their molecular interweaving. To sum up, hybrid porous metal phosphonates have the best features of both organic and inorganic frameworks. All other properties of MOFs are equivalent to those of porous organic–inorganic metal phosphonates by taking advantage of the malleability of organic chemistry to get the superior chemical and physical stability it provides [14–16]. The benefits of porous metal phosphonates over other forms of porous materials are their chemical versatility and their low processing complexity. In addition, the development of metal phosphonates over the last two decades, beginning with layered metal phosphonates and progressing via phosphonates-based metal–organic frameworks and onto templated (supramolecular) mesoporous metal phosphonates is of worthy discussion. From layered metal phosphonates to phosphonate-containing metal–organic frameworks, this transformation is rather extensive. The degree of long-range organization and the presence or absence of pores in a polymeric network is two additional criteria that may be used to classify such a network. Metal-layered phosphonates, metal–organic frameworks with phosphonates, and templated (supramolecular) metallic phosphonates are all included in this evolution.

2 Synthesis of Porous Metal Phosphonates Prior to the metal phosphonates, we will discuss some of the metal phosphonate networks produced in recent years and the challenges faced in obtaining these networks. In this section, we address the three distinct structural morphology classes

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Fig. 1 Characteristics of porous metal phosphonates also comparison to other common materials. Adapted with permission [10]. Copyright (2021), Wiley–VCH

(Fig. 2) that are dictated by the conditions under which the porous metal phosphonates are synthesized, as well as the precursors and kind of organophosphonic acid utilized.

Fig. 2 Classification of porous metal phosphonate based on structural morphological properties. Adapted with permission [17]. Copyright (2021), MDPI

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Fig. 3 Divalent metal phosphonates: Recent advances in synthesis, on-site characterization, and structural determination. Adapted with permission [19], Copyright (2016), De Gruyter

2.1 Layered Metal Phosphonates Similar to the process used to produce metal phosphates, layered metal phosphonates were created by directly precipitating metal precursors and organophosphonic acids [18]. Similar to layered metal phosphates, but without the interlayer of stacked metal phosphates, the morphologies of resulting compounds were studied. The preparation of divalent and trivalent metal phosphonates will be covered first, followed by tetravalent phosphonates. One key difference is how metal ions react in certain solvents. It is to be noted that there is a robust interplay between phosphoric acid and transition metals. However, when tetravalent metal precursors speed up the formation of a metal phosphonate coordination network, uncontrolled precipitation of the combinations occurs, even under highly acidic circumstances, destroying the regulated structural features. To crystallize, however, phosphonic precursors must first have their metal moieties sufficiently solubilized to allow for uniform interactions with the other molecules during the controlled assembly of these molecules into a hybrid network. When this happens, only the phosphonic precursors will be able to solidify. Multiple methods for the modern synthesis, characterization, and solution of divalent metal structures are shown in Fig. 3. A decrease in the pH of the reaction fluid may be employed to reduce the concentration of divalent and trivalent metals during synthesis.

2.2 MOFs–Phosphonates In structures consisting of metal nodes and organo-functional units, metal–organic frameworks are a prominent type of coordination polymeric networks known for their high surface areas and permeability. Specific classes of MOFs, those with metallic units joined by phosphonates are promising materials. When compared to carboxylate MOFs, which have been available since the late 1990s, metal phosphonate-based

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frameworks are in more recent developments. Frameworks based on rare metal phosphonates have just recently been synthesized successfully. When compared to its carboxylate counterpart, the phosphonate moiety interacts with metals more favorably [20]. Multilayer phosphonate frameworks that resemble metal phosphonates, such as the divalent/trivalent metal phosphonate frameworks that gave rise to the more complicated tetravalent metal phosphonate frameworks are also reported. The solubility issue prevents the creation of porous structures, just as it prevents the creation of layered ones. The literature reveals that most “ultra-stable” MOFs feature nodes made of Ti (IV) or Zr (IV) [21]. To this end, it is crucial to learn how phosphonic linkers can be included in the design of such structures. This chapter will explain layered metal phosphonates by looking at how their lamellar structure may be altered to create open porous structures. Three-dimensional open frameworks with micropores were also created by mixing ethylenediaminetetrakis (methylenephosphonic acid) with lead (II) and zinc (III), as seen in Fig. 4. The eight phosphonic acid sites in this phosphonic ligand made possible a wide range of coordination modes, with the mode ultimately decided by the metal of choice and the pH of the reaction medium. To create the lead-based structure, square-pyramidal lead and RPO3 phosphonate units were arranged using helical chains. However, the zinc atom’s amino group was not involved in the metal-to-metal contact; therefore, the phosphonate layer and the zinc were clustered together in a square wave shape.

Fig. 4 Metal–organic framework phosphonates. Adapted with permission [22]. Copyright (2020), Wiley–VCH

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The 1D inorganic building units (IBU) are shown in section B from the side; it is composed of a zigzag chain of copper dimers that share a corner and have Cu-Cu distances of less than 3 Å [22]. Different coordination modes between metal units and phosphonic acid units may be created depending on reaction circumstances, metal precursors, and phosphonic acid precursors. The addition of extra molecules may lead to different kinds of coordination interactions. As a consequence of introducing a new chemical, it is feasible that several coordination modes may emerge. To create a structure with 1D pores, many materials were employed, including copper (II), benzene diphosphonic acid, and amino triazole. The pillared form and minute openings make this pattern easy to recognize (only 0.45 nm in diameter). The presence of an amino group in the triazole molecule led to the formation of 6-coordinated copper organized in 1D columns. Due to the absence of an amino group in triazole, the conventional pillared copper benzene was created along with the perfectly monoclinic crystals of diphosphonate, with no air gaps between the layers [23]. Separate research found that a diphosphonic ligand, organo-piperazine, reacted with lead (II) to form an early 2D layered structure. Due to the low strength of Pb–O(N) interactions, supramolecular assembly was used to transform this 2D structure into a 3D framework throughout the synthesis. Hydrogen bonding, van der Waals forces, and other factors in the creation of supramolecular assemblies were assumed to be responsible for the electric framework interactions seen [24].

2.3 Templated Porous Metal Phosphonates The last step in making a metal phosphonate with a porous textured surface is by using supramolecular templates. Inspired by the PMO hybrid system, these materials use a heterocondensation phase to link the metal oxide and organic phosphonate precursors into a hybrid network rather than relying on a simple chemical reaction. Porous metal oxide systems are required for the synthesis of these mesoporous metal phosphonates [25]. For the most part, sol–gel procedures, which are typically employed to create metal oxides, are adapted for this class of materials synthesis. A final calcination process is often needed to crystallize the amorphous metal oxide network created by sol–gel-based materials [25, 26]. When a calcination process is needed, and the required temperature is greater than the thermal stability linked with the organic functional group, the organic molecules included in the inorganic–organic network or hybrid material often fail to operate. In these instances, extraction is necessary, sometimes followed by a brief heating step. Templated metal phosphonate networks are often not thought out well enough when it comes to texturing or creating amorphous structures. The primary intermediates in the synthesis of inorganic phosphorus are titanium isopropoxide and phosphoric acid whereas an example of a weak acidic SDA is octadecyl trimethyl ammonium chloride/bromide. Figure 5 shows the distinctions between the three architectural strategies.

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Fig. 5 Crystalline mesoporous material productions using a template: an example of the synthetic techniques/procedures and their benefits/drawbacks (hard, soft, and colloidal templating). Adapted with permission [27]. Copyright (2019), Frontiers Media SA

3 Applications of Porous Metal Phosphonates These hybrid porous metal phosphonates combine the benefits of adsorbents with those of periodic organic–inorganic skeleton topologies. Because of its wellstructured mesoporosity, sensitive regulated functional group, and flexible inorganic units, mesoporous materials find utility in a broad range of applications. Adsorption, electrochemical energy conversion and storage, and catalytic support are only a few examples of their uses. It is because they can store energy and maintain their electrochemical stability. Here novel applications for porous metal phosphonates will be discussed, along with the current benefits and their potential in the future.

3.1 Adsorption The removal of greenhouse gases, the storage of clean fuel gases, and the capture of poisonous gases are just a few examples of how fundamentally important gas adsorption is to modern society. With huge surface areas, high porosities, and adaptable porous architectures, crystalline porous materials are making tremendous strides in the field of gas adsorption. It works very well with porous metal phosphonates [28]. A few distinguishing characteristics of porous metal phosphonates for gas absorption include the following. (i)

Many different synthetic methods may be utilized to create porous metal phosphonates with a wide range of pore sizes and a wide range of structural topologies.

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(ii) A robust organic–inorganic framework with outstanding stability allows several usage cycles, and the rigorous basic components produce a long-lasting porous structure. (iii) Rapid reaction from tunable substituents and scalable inorganic modules makes the adsorption of a large variety of compounds manageable.

3.2 Heterogeneous catalysis The fields of electrochemistry and pharmaceutical development all rely heavily on heterogeneous catalysis. Some of the distinctive properties of porous metal phosphonates and their use in catalysis have been explored. Pores of a tunable size that aid in the selective transport of molecules of varying sizes for size-selective catalysis; (i) predefined catalytic moieties that can be freely incorporated on the pore surfaces or confined within the pore space; (ii) abundant active sites that are both exposed and well-distributed; (iii) surfaces that are both highly accessible and facilitate functionalization. Due to their exceptional properties, porous metal phosphonates may be the best carriers for heterogeneous catalysts [29].

3.2.1

Metal Complexes as Catalytic Sites

A possible method for producing porous materials like zeolite, metal–organic frameworks, and porous polymers is to include predefined metal sites in building blocks [30]. Producing effective porous metal phosphonates may also be accomplished using this technique, which involves integrating metal active sites into a hybrid skeleton. Because of their great capacity for earth-abundance, proper electron structure, and favorable redox capabilities, most terminal metal elements (such as Ti, Co, Cu, and Mn) have long been regarded as excellent metal centers to form metal phosphonate skeletons. It has been shown that reactive transition metal elements may be successfully incorporated into the skeletons of the porous phosphonates, hence providing essential reactive catalytic sites [31].

3.2.2

Organic Moieties as Catalytic Sites

An improvement in catalytic efficiency may be possible with heterogeneous catalysis, but this would require packing a lot of different functional groups onto a single host. The catalytic performance of porous materials may be improved by organophosphonic functionalization. Most of the credit for this goes to the enhanced coordination chemistry of phosphonate ligands. When phosphonate ligands are attached to hybrid skeletons, the resulting structure becomes an ideal foundation for designing heterogeneous catalysts. Since this is the case, changing a material’s pH, alkalinity, hydrophobicity, or hydrophilicity is a straightforward process. A newly designed

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structure with functional compositions built with great care may speed up synergistic catalysis by positioning different active parts close to one another. In the hydrothermal synthesis of acid-based bifunctional mesoporous titanium phosphonates, for instance, alendronate sodium trihydrate, which includes amino acids, was utilized as a coupling molecule. Bifunctional phosphonic linkages with both acidic P-OH and basic -NH2 groups as activators for aziridine and CO2 , respectively, were shown to be active and persistent in the diffusely catalyzed cycloaddition of aziridine with CO2 to create oxazolidinone. Both aziridine and carbon dioxide were activated by these bonds [32].

3.2.3

Catalysis Based on Metal Phosphonate Derivatives

Metal phosphonates are widely prepared because they are efficient precursors, and this has led to the discovery of several metal phosphates and phosphides that might be valuable compounds. Materials like carbon with a heteroatom-doped organophosphonic moiety show promise as next-generation catalytic sites for a wide range of reactions. This is because metal phosphides, phosphates, and phosphonates all have similar chemical properties. It can be concluded that calcining phosphonate precursors are a productive and promising method for producing metal phosphate, after reviewing the extensive work done by the Pinna group on the utilization of metal phosphonate coordination frameworks as precursors for transition-metal phosphates or phosphides and their electrocatalytic activities in water oxidation and reduction reactions also a new class of porous materials with useful functional properties was developed. The data analysis proved this to be true. Exploration of metal phosphonate coordination frameworks led to this breakthrough. Here is only a small sampling of the numerous positive outcomes that may result from using this tactic: Increases in both conductivity and electro-catalytic specific surface area which may result from the carbon matrix produced from the organic material in the precursor. Coordination sphere flexibility is greatly improved for organophosphonic ligands with many heteroatoms. Due to the malleability of the precursors’ chemical compositions and nanostructures, phosphonate derivatives can be synthesized with tunable properties and bespoke structures. When working with organophosphonic acids, clever modifications to the geometric and electrical structures of the materials are possible due to the presence of heteroatoms. Introducing these alterations has the potential to enhance host–guest interactions and create new active sites at interfaces. Since heteroatoms may be found in organophosphonic acids, this is not wholly implausible. Heating metal phosphonates in the presence of hydrogen can result in the formation of metal phosphides and heteroatom-doped carbon composites. Heating the phosphonates triggers this reaction. Thanks to its N-doping, unique core–shell design, and easily accessible active sites, as well as the synergistic interactions between its components. The modified CoP/Co2 P@NC-2 catalyst for the hydrogen evolution process (HER) exhibits enhanced electrochemical performance across a wide pH range. This progress was measured over a longer time frame.

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3.3 Electrochemical Energy Storage Electrode materials with high efficiency are a primary focus of energy storage devices, and this is of tremendous significance to fast social growth and a sustainable economy. Many energy-storage technologies such as Li-ion batteries, fuel cells, and supercapacitors make use of porous metal phosphonate hybrids because they are such promising materials [33]. These characteristics are unique to electrodes fabricated from porous metal phosphonates. (I)

By introducing certain molecular redox functional groups into pore walls or onto the surface, the redox potential may be altered. (II) The nanoporous structure also allows electrolyte ions to intercalate with relative ease. (III) The rapid kinetics of a reaction may be explained by the skeleton’s open and connected channels, which provide an unimpeded pathway for ion diffusion. (IV) Strong construction for extensive battery cycling. 3.3.1

Li-Ion Batteries

For a very long time, due to their very high energy density, lithium-ion batteries have been used almost exclusively to power portable consumer gadgets [34]. The use of electrodes in nanoporous materials has become more common in lithiumion battery production. Metal oxides, alloys, chalcogenides, and phosphates are all examples of such substances. This is because the porous nature of the materials allows for a high concentration of active sites, good contact between the electrolyte and the electrodes, rapid proton and electron transport, and a reduction in volume expansion. As a result of its adaptable hybrid skeleton and many potential uses, porous metal phosphonate materials are rapidly growing in popularity. Thanks to their high surface-to-volume ratio of phosphate groups and their precisely adjusted porosity, metal phosphonate hybrids are efficient in storing vast quantities of Li ions. Metal phosphonates are promising candidates for use in lithium-ion batteries due to their porous structure, which facilitates the transport of ions. Mesoporous iron phosphonates were synthesized by Yamauchi and coworkers and found to be beneficial in lithium-ion batteries via a cooperative assembly method using positively charged CTAB and negatively charged precursors [35].

3.3.2

Supercapacitors

Supercapacitor devices are becoming more popular in applications where a quick reaction time is essential because of their high power density, rapid charge/discharge rate, and exceptional cycling characteristics. Energy storage capacitors may benefit from using porous metal phosphonates as hybrids. Because of their open porosity structure, ions can travel quickly through them, and the double-layer capacitance is effectively boosted by their varied skeleton structure with a large surface

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area [36]. Additionally, they have an abundance of surface phosphate groups that form controlled redox-active moieties, making them excellent candidates for faradaic (pseudo-capacitive) energy storage. Mesostructured manganese phosphonate hybrids, with their high pseudo-capacitance and remarkable rate capability, are prime examples of supercapacitor materials. Thanks to their unique mesostructured design, MnPs have many active sites strategically placed throughout the molecule, allowing them to efficiently transport ions and electrons. Nickel phenyl phosphonate microspheres in floral shape self-assembly from stacked nanosheets are promising materials, which might be used as electrodes in supercapacitors.

3.3.3

Fuel Cells

Materials that conduct protons are crucial for the proper functioning of proton exchange membrane fuel cells. These parts enable the proton to go through while electrically separating the other side of the process. The exceptional physicochemical features of porous materials, such as oxide ceramics and porous polymers, have contributed significantly to recent developments in PEMFC research. The acidic salts of Oxo-acids have the adverse properties of being very soluble in water and having weak conductivity at ambient temperature, making it challenging to uncover the definitive proton routes at the atomic level in these compounds. In this way, pinpointing the specific proton paths on a molecular level is challenging. The already complex proton pathways are further complicated by these two additional factors. The potential application of hybrids of porous organic and inorganic metal phosphonates in porous electrochemical metal hybrid fuel cells is intriguing. This is because porous organic–inorganic metal phosphonate hybrids may be tailored to specific applications by adjusting the proportions of metal ions and organic ligands. Because of their sturdy hybrid skeletons, metal phosphonates can withstand higher temperatures than metal–organic frameworks (MOFs) and porous polymers. These materials are in great demand because of their utility as proton conductors in PEMFCs and their prospective usage as intrinsic proton sources. A single or double proton is released from each molecule of phosphonic acid (RPO3 H2 ) when it is diluted with water. The partial protonation of phosphonate groups at low pH or the incorporation of acidic functional groups are two possible mechanisms for their generation. They are similar because they both need the presence of fluids. Tetravalent Zr and Ti phosphonates were the first MOFs to be investigated for their potential as proton conductors due to their well-designed structures and remarkable chemical and thermal stability. Exciting compounds with the potential to behave as proton conductors span a wide variety of chemical and physical categories. Compounds with both carboxylic and phosphonic groups, such as multilayer mixed zirconium phosphate/phosphonate, are an example of this kind.

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3.4 Solar Cells and Photoelectrochemical Reactions Since Ru(II) bipyridyl compounds include phosphonic acid groups, they attach strongly to metal oxide surfaces. These parts are often implemented as a dye in solar cells that follow the dye-sensitized method of operation. Meyer and coworkers have synthesized an extensive library of Ru(II)-bipyridyl moieties that are all connected to the phosphonic acid group, which may be represented by either CH2 PO3 H2 or PO3 H2 . Many different compounds have been identified that contain these moieties. Fifteen hundred phosphonate binding sites have been ligated to more than a dozen different Ru(II) complexes. The use of metal phosphonates synthesized on the semiconductor metal oxide surface has the potential to enhance charge transfer in DSSCs. To improve the efficiency of photovoltaics, Mutin and coworkers noticed that the porous nanomaterials had naturally occurring phosphonate linkers, which might alter the features of the surface and interface [37].

3.5 Electrochemical Water Splitting Electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) are currently being developed by researchers [38–40]. These electrocatalysts do not need the use of noble metals to function properly. This is being done so that the full potential of electrochemical water splitting may be realized. Further research into this area is being conducted to reduce wasteful energy use and maximize positive environmental outcomes. Several coordination compounds of transition elements show high initial activity in the electrochemical process of water splitting [41–43]. Nevertheless, metal complexes are notoriously unsteady in such environments. Electrochemistry is employed in the process of water molecule separation. It is possible that the electrochemical process would benefit greatly from the use of hybrid nanostructures based on metal phosphonates since these chemicals are stable across a wider pH range. These results add to the growing body of data supporting the use of metal phosphonate-based hybrid nanostructures. Despite the high manufacturing cost, IrO2 nanoparticles have proven effective as electrocatalysts in oxygen evolution processes (OER) [44, 45]. These methods employ a Pt NP-based electrocatalyst system for HER, rather than more conventional Pt-based systems. One alternative is to employ metal phosphates and phosphonates, which are based on common metals and function as catalytic systems. When exposed to an alkaline environment, researchers prepared nanocomposite which efficiently removed harmful organic radicals. The nanocomposite material is made up of nanocarbon and N-doped cobalt phosphate. The existence of N-doping in the graphitic carbon nanostructure and the presence of Co–N bonding in the cobalt phosphate may be responsible for the very high ORR activity. To synthesize porous cobalt phosphonate with hierarchical porosity of dimensions 1.5–5.0 micropore/mesopore and nanocages, scientists used NMPA as a phosphate source, F127 as a precursor, and poly (vinyl alcohol) as a template. A mold

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made of poly (vinyl alcohol) was used to achieve this. Micropores and mesopores were found at varied densities across the pore distribution. Researchers have been working hard to construct nanocages to contain these subatomic particles in a safe environment. The nanoscale porosity and huge surface area of cobalt phosphonate make it a promising option for use in an alkaline water oxidation process.

3.6 Membrane Materials The engineering field of membrane reactors (MRs) has demonstrated some promising results in ion exchange and gas separation. Conversion and isolation are included in a single MR for convenience. The malleability of porous metal phosphonates in magnetic resonance is primarily attributable to the exceptional coordination chemistry that has risen as a result of the plentiful availability of phosphonate legends. To be more specific, (i) the sturdy hybrid skeleton of metal phosphonate materials can survive severe reaction conditions, and (ii) the pore size, pore shape, and pore periodicity may be carefully regulated to meet a broad range of separation aims and needs.

3.6.1

Ion Exchange

Many porous organic–inorganic metal phosphonate hybrids have been investigated for possible application in ion exchange due to their flexible skeletons and enhanced resilience to chemicals and heat. This is because of the hybrid nature of these substances. A metal phosphonate is potentially useful as a cation exchanger due to the presence of a phosphonic acid group (RPO3 H2 ) that allows protons to be exchanged for cations [46]. In particular, Zr phosphonates and Sn phosphonates are two examples of ion-exchange compounds that are radiologically stable. The lack of phosphate in the synthesis, stability, and reproducibility of these Zr/Sn layered phosphonates is a major plus for the materials. Scientists found that hybrid materials made of Zr phosphonate and phosphate could perform ion exchange. The selectivity of the separation was discovered to be due, for instance, to ion-exchange interactions or chelation involving connected carboxylic or phosphonic acid groups. Zirconiumhydroxylethylene diphosphonate, produced using sol–gel processing, shows promise as an effective ion exchanger due to its high ion exchange capacity, good chemical resistance, and high thermal stability. Because of this, it is a useful material for a variety of applications. Increased ion-exchange capacity in porous metal phosphonate hybrids may be achieved by (a) tuning the P/Me ratio, (b) protecting organic amino acids, and (c) sulfonating the hybrids. One such method for doing this is shown in Fig. 6. Forming metal phosphonate hybrids always occurs in the more stable and H+ exchangeable POMe bonding state as opposed to the POH bonding state. The element Me might stand in for Ti, Zr, V, or even Al. In light of these findings, it is clear that metal phosphonates represent a potential class of ion exchange.

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Fig. 6 The ion exchange capacity of porous metal phosphonate hybrid materials may be increased by (a) adjusting the P/Me ratio, (b) covering the organic amino groups, and (c) sulfonate treating the material. These three steps can be taken in that sequence. Adapted with permission [10]. Copyright (2021), Wiley–VCH

3.6.2

Gas Separation

Owing to their capacity to selectively segregate ions and gas molecules (such as CO2 and CH4 ) that are transferred across the membrane, porous metal phosphonates are being researched for use as membrane materials. There has been a lot of interest in the potential of porous metal phosphonates for application in gas separation. The higher charge and greater availability of donor atoms in the –PO3 group may make metal phosphonate hybrids more stable than those with –CO2 groups, according to some studies. This takes place because the –PO3 group has access to a larger pool of donor atoms. Moreover, the increased coordination flexibility of the phosphonate groups has led to a drop in the efficiency of metal phosphonates in gas separation. For that purpose, Wright and his colleagues collaborated to synthesize N, N' -piperazine bis metal and analyze its crystal chemistry. This substance belongs to the STA-12 (methylenephosphonic) family. The excellent permanent porosity and homogenous microscopic particle size of the synthetic STA-12(Ni) material led to not only extraordinary thermal and hydrothermal endurance but also remarkable separation performance. The increased permanent porosity of this material was the determining factor.

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3.7 Biomaterials Biomaterials serve an essential role in helping people recover from disease or damage. Biomaterials are materials used in medicine that mimic the properties or perform the same activities as natural materials. Metals, ceramics, polymers, glasses, and even living cells and tissues are all possible components of a biomaterial. These may also be used in drug delivery systems, which provide healing substances to afflicted areas. Bioengineers determine whether a biomaterial is effective for a certain application. Calcium phosphates, a key component of bone, are used not just by the body but also in the manufacture of biomaterials for use in fracture repair. It’s well-accepted that calcium phosphate biomaterials stimulate bone formation and closely integrate with the bone that forms around them.

3.8 Proton Conduction Porous metal phosphonates can be very advantageous in the field of proton conduction. Proton conduction was tested on a framework made of zinc precursor and BTP as the phosphonate ligand (referred to as PCMOF-3). The primary source of protons to be delivered was thought to be the aqua ligands on zinc. Under conditions of 44% RH and 25 °C, the proton conductivity was 4.5–108 S/cm. At 0% RH and constant temperature, this value plummeted to 1–105 S/cm. Using bulk conductivity; researchers determined that the activation energy (Ea) for proton transport inside the PCMOF-3 structure is 0.17 eV. However, activation energy measurements suggest that the reaction may begin with as little as 0.22 eV of Nafion and 1 mol/L of HCl. This material relies on the proton-conducting metal phosphonate PCMOF3 as its base. The framework’s polar interlayer highly organized water molecules provide an outline for the Grotthuss proton transfer mechanism suggested by a low Ea. According to the study’s findings, both the “O” and “N” forms of phosphonate were effective in this mechanism [47].

3.9 Other Applications Including Challenges and Future Outlook The applications for porous metal phosphonates are already listed in previous sections. Moreover, Langmuir–Blodgett, molecular sensors, and film-based material production are all exciting areas of study related to these metal phosphonates. Scientists discovered the anti-corrosion properties of mild steel by using a metal phosphonate structure such as Co(O3 PCCH2 ) H2 O (or CoVP). At even the lowest concentration, it was found that the components of CoVP significantly influenced the formation of a protective coating over the iron’s surface. Using this method

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has a 96.6% probability of avoiding adverse effects [48]. Hydrolysis of the film’s protective layer led to a steady release of phosphonic acid, which formed stronger connections with the metal’s surface over time [48]. The same research also reported that 1,4-((H2 O3 PCH2 ) (HOOCCH2 ) (NCH2 )2 C6 H4 may be utilized to detect UV light. We may also recognize this chemical as Zn2 3D metal phosphonate (H2 L) H6 L [49]. Zn2 (H2 L) can withstand temperatures up to 300 °C in the air without degrading, and emitting bright light whose intensity can be adjusted, and can be used to detect nitrobenzene in both directions [49]. These findings have undoubtedly led to enormous advancements in research across a wide range of applications, from cutting-edge new technologies to established fields, thanks to their intriguing pore characteristics, regular structure, and flexible porous metal phosphonate skeletons [50]. Polymers materials, for example, are benefiting greatly from the novel development platforms made possible by hybrid nanoporous metal phosphonate materials and an increasing number of disciplines are exploring the possibilities of porous materials. Ion exchange, catalysis, adsorption, and separation are only a few of the many industrial uses for microporous materials. With the use of alum inosilicates or metal phosphates, two common “porous materials”, new porous metal phosphonate compounds are anticipated to emerge from the proposed effort. These materials differ from more common zeotypes due to the inclusion of organic functional groups on their inner surface. It is possible to build a superior chemical with enhanced characteristics in both categories by fusing the structural advantages of inorganic frameworks with the functional diversity of organic chemistry. The introduction of additives can change the synthesis time, similarly, the use of promoters or microwave heating conditions is desired to prepare well-defined particle shape nanomaterials and porosity. It is also possible to create phosphonate-based MOFs using a HydroHex coordinated synthetic approach for (solvo)thermal Binaphthylbisphosphonate ligand-bridged Ni(II).

4 Conclusion The study of porous metal phosphonates is in its infancy compared to the decades of research devoted to carboxylate metal–organic frameworks, layered hybrid materials, and organic hybrid solids. First, we looked at the different coordination network structures present in porous metal phosphonates. As an example, it was shown that layered-based porous metal phosphonates were the ancestors of porous hybrid inorganic–organic systems and microporous phosphonate metal–organic frameworks. The development of mesoporous metal phosphonates still needs kinetic control to create intriguing porous structures. It is feared that heterocondensation or phase separation between metal precursors and phosphonic moieties may speed up the precipitation process. Limiting contacts and avoiding HF during the production of porous materials is challenging and time-consuming because of the risk of damage and the complexity of the manufacturing process. Although the phosphate group’s

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interaction with the chelated metal (IV) precursors is well-documented, its precise nature remains poorly understood. To create novel coordination networks of metal phosphonates, it is crucial to regulate the reactivity of the interaction between the metal and the phosphoric precursors. This is essential for creating novel coordinating networks. As a result of this link, a network with hybrid porosity requires careful construction to avoid premature precipitation. Metal phosphonates with porous skeletons tend to be both malleable and periodic. To utilize in electrochemical cells, optoelectronics, adsorption, and catalysis, it is necessary to develop novel microporous and mesoporous materials based on nanoarchitecture and composed of metal phosphates and phosphonates. Increased interest in phosphate-based biomaterials is likely to materialize as chemical engineering, materials science, and biology continue to develop.

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Rich Structural Chemistry of Metal Phosphates/Phosphonates for Emerging Applications: V, Ti-containing Materials Wei Ni and Ling-Ying Shi

Abstract Transition-metal phosphates/phosphonates have demonstrated remarkable performances for their unique physicochemical properties. Compared with transition-metal oxides, phosphate/phosphonate groups in transition-metal phosphates/phosphonates show flexible coordination with diverse orientations, endowing them with an ideal platform for many promising applications. In this chapter, we focus on the rich structural chemistry and design strategies of efficient, high-valent V/Ticontaining transition-metal phosphate/phosphonate materials for emerging applications, with special emphasis on the tuning of transition-metal-center coordination environment, optimization of electronic structures, increase of catalytically active site densities, and design of heterostructures, to address the urgent issues confronted by transition-metal phosphates/phosphonates such as low intrinsic catalytic efficiency and low electronic conductivity. The major challenges, opportunities, and prospective solutions are also discussed for further development of V/Ti-containing transition-metal phosphates/phosphonates-based materials with ultimate practical applications. Keywords Phosphates · Phosphonates · Ti-containing · V-containing · Rich structural chemistry · Structure–property relationships

1 Introduction Transition-metal phosphates/phosphonates have attracted increasing interest because of their diversity of phosphorous chemistry, enriched redox behavior, high chemical/thermal stability, tunable porous structures, and widespread applications including selective adsorption of gases/ions/dyes, ionic conductor, heterogeneous W. Ni (B) State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization, ANSTEEL Research Institute of Vanadium & Titanium (Iron & Steel), Chengdu 610031, China e-mail: [email protected]; [email protected] L.-Y. Shi College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_3

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catalysis, semiconductor, optoelectronics, energy storage and conversion, and biomedicine [1–4]. Metal phosphates (commonly orthophosphates) are usually purely inorganic materials consisting of metal cations bound with phosphoric anions, while metal phosphonates are somewhat like organic–inorganic hybrids (or metal– organic frameworks, MOFs) due to the common use of organophosphonate ligands [2, 4, 5]. As layered or porous frameworks, phosphates/phosphonates are more robust thanks to strong affinity between corresponding ligands and metal cations, and are showing higher thermal/chemical stability but with less predictable coordination chemistry due to the more ligating modes, three protonation states, compared to the normal one (e.g., COO− ligand); also the high oxidation states of 3d transition metals such as Ti4+ , V4+ will make the phosphates/phosphonates more complicated in structural characterization and properties [1, 6, 7]. Clearfield et al. edited a book titled “Metal Phosphonate Chemistry,” in which the specific structural chemistry of Mo, Co, Zr, Al, Zn, U (and transuranium, lanthanide)-based phosphonates are emphasized, and the impact of structural diversity of phosphonate frameworks on applications are evaluated; however, the content on V and Ti-based phosphonates with metal ions in high valence and various oxidation states along with diverse phosphonate/phosphate ligand selection is missing [8, 9]. As we know, high-valent Ti/V-containing metal(IV or V) phosphates, phosphonates, or their hybrids (e.g., phosphate–phosphonates) have attracted increasing interest owing to their practical/emerging application in the fields of catalysis, intercalation chemistry, photochemistry, ion exchange, sorption, sensors, proton conduction, clean energy, etc. [10, 11]. These phosphonate/phosphate materials are usually present in the form of 0D clusters, 2D films/layers, or 3D porous structures including coating, ceramics, MOFs, etc. Here, in this chapter, we present a selective overview of the interesting chemical and structural features of high-valent V/Ti-based metal phosphates/phosphonates, as well as the emerging applications and perspectives. Also, the structure–performance relationship will be explored and discussed.

2 Structures and Merits The phosphates generally have a central phosphorus atom surrounded by four oxygen atoms in a tetrahedral (i.e., orthophosphoric acid, or PO4 3− ) arrangement, while the phosphonates consist of a central phosphorus atom surrounded by three oxygen atoms and one other atom in a similar phosphonic acid (PO3 3− ) arrangement [12]. Depending on the metal ions and phosphoric/phosphonic moieties, the phosphates/phosphonates vary widely in solubility in water, alcohols, or organic solvents. Generally, the transition-metal (i.e., divalent, trivalent, and tetravalent) phosphates/phosphonates exhibited much lower solubility than alkali-metal ones (i.e., monovalent metal phosphates/phosphonates), viz., slightly soluble or insoluble in water. The insolubility, as well as stability against air and thermal treatment for phosphates/phosphonates, increases with the rising valence of the central metal ions, partly because of the enhanced activity/coordination ability of the high-valent metal

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ions. These trivalent/tetravalent metal phosphates/phosphonates are highly insoluble, not only in water but even in highly acidic solutions; thus, compared to monovalent/divalent ones they have attracted greater attention for many specific fields including stable and efficient energy storage and conversion, liquid-phase catalysis, sensors, ion exchange and adsorption [7]. Different metallic central ions coordinated with phosphonic acids via various metallic precursors can endow the phosphonatebased MOFs with distinct physicochemical properties, e.g., photoluminescence, magnetism, and catalysis [3]. Metal phosphonates usually exist in the form of unconventional MOFs; although relatively rarer compared to conventional carboxylate-based MOFs, these phosphonate-based MOFs are showing novel and specific advantages owing to the rich coordination chemistry and higher diversity of ligating modes [3, 5, 7, 13]. The phosphonate-based MOFs tend to form densely layered structures or microporous structures due to the strong binding ability of corresponding phosphonic ligands (or linkers). In recent years, more and more works are focused on mesoporous and/or macroporous phosphonate-based MOFs with open-framework structures, and some works on tuning the interlayer distance of layered phosphonate-based MOFs by incorporating/changing pillar groups/spacer groups (sometimes pendant functional groups or clusters) or by exfoliating the layers [3, 7]. The choice of phosphonate bridging moieties with varying chain lengths or anchored functional groups, e.g., alkyl phosphonates or heteroatomic groups, can remarkably alter the corresponding MOF structures, especially for metal phosphonate-based MOFs of 3D open frameworks [3]. And the large and multidimensional (poly)phosphonic bridging moieties would prefer an open framework structure [3]. For example, the complicated organophosphonic ligand 1,3,5,7-tetrakis(4-phenylphosphonic acid)adamantane (TPPhA) containing four radical phosphonic acid moieties can be used in the synthesis of lots of metal phosphonate-based MOFs including vanadium phosphonates and titanium phosphonates with improved open frameworks, i.e., enlarged pore size (~4 nm), higher specific surface area (e.g., up to ~550 m2 g−1 for TPPhA-Ti) or enhanced mass transfer kinetics (Fig. 1) [14, 15]. To further enhance the mass transport performance, hierarchically porous phosphates/phosphonates are designed, where multiple templates or structure-directing agents (including self-templating precursors) are usually needed [6]. It should be mentioned that not only the microscopic intramolecular structures can be tailored, but the mesoscopic and/or macroscopic morphologies can also be controlled via soft/hard-templating method(s). Most of the mesoporous phosphonates, e.g., mesoporous titanium phosphonates (MTP) are prepared by surfactant-assisted hydrothermal synthesis [3, 16] and evaporation-induced self-assembly (EISA), some by microwave-assisted hydrothermal method or sol–gel processing [6, 17]. And hierarchically porous phosphonates/phosphates could be prepared with ionic/oligomeric surfactants or block copolymers for mesoporosity (usually 2–10 nm) and with colloidal templates for larger mesopores or macropores [7]. In addition, it is of great interest that these phosphonates, phosphates, and other kinds of metal-like phosphides could even undergo mutual transformation under appropriate conditions.

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Fig. 1 a Molecular formula of the dendritic tetraphosphonate ligand (tetrahedral TPPhA) for constructing nanoporous vanadium phosphonate and titanium phosphonate (TPPhA-V, TPPhA-Ti). b and c Typical SEM and TEM images of as-prepared sponge-like amorphous mesoporous TPPhAV. Adapted with permission [14]. Copyright (2006) American Chemical Society. d Computergenerated model of the as-synthesized porous TPPhA-Ti. e and f Typical TEM images of the lamellalike folded structure and hollow nanosphere structure of TPPhA-Ti. Adapted with permission [15]. Copyright (2006) Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim

3 Typical Applications Owing to their rich chemistry and extensively tailorable composition/structures, V- and Ti-containing phosphonates/phosphates have been exploited as multifunctional materials for emerging applications, e.g., energy storage and conversion, sensors, biomedical engineering, (surface) hardness enhancement and environmental protection, etc., beyond conventional catalysts and adsorbents.

3.1 Heterogenous Catalysis Transition-metal ions, especially V4+ and Ti4+ , isomorphously substituted/incorporated molecular sieves are promising catalysts for selective oxidation [18]. The oxidation state, as well as coordination (sphere) and distribution (topology) of V or Ti in the inorganic framework is a principal determinant of the catalysis and is quite sensitive to the synthesis method [18–20], which should be controlled or tailored for optimized performance. Some preconditioning or in-situ/ex-situ characterization methods such as UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) or electron

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paramagnetic resonance (EPR) spectroscopy, and hyperfine sublevel correlation (HYSCORE) spectroscopy are powerful ones [18, 19, 21, 22]. Vanadium or titanium phosphonates with mesoporous structures are one of the most prominent candidates for heterogeneous catalysis, e.g., aerobic oxidation [14], photocatalysis for H2 production, or CO2 reduction/conversion. Via alteration of the functional groups of phosphoric/phosphonic acids, the porous structure and surface area could be tuned, thus enhancing catalysis due to the easier accessibility of adsorbed molecules. A mesoporous vanadium phosphonate (denoted as TPPhA-V, with a V/P ratio of 1:1, a surface area of 118 m2 /g, and a pore size of 3.8–3.9 nm) synthesized through the non-hydrolytic condensation of an arylphosphonic acid (TPPhA) and vanadium alkoxide for oxidation of benzylic alcohols to benzylic aldehydes with high conversion and selectivity. The TPPhA-V catalysts are also endowed with shape selective for some specific benzylic alcohols such as 2,4,6-trimethylbenzyl alcohol with high steric hindrance or inductive effect [14]. Furthermore, these metal phosphonates could be further functionalized (e.g., by – SO3 H) for monolithic acid catalysts with the advantage of separation and recovery compared to liquid acid catalysts [23]. Titanium phosphonates can be used as support of specific heterogeneous catalysts, for example, titanium phosphonate supported Pt for photocatalytic water splitting (or hydrogen evolution reaction); titanium phosphonate supported Pd for various reactions including hydrogenation (of acetophenone); titanium phosphonate supported CuO nanoparticles for CO oxidation; titanium phosphonate supported molybdenum oxide for oxidative desulfurization; titanium phosphonate supported peptide for heterogeneous enantioselective hydration (of epoxides, styrene to diols); titanium phosphonate supported Ru/homochiral peptide for enantioselective hydrogenation (of ketones to secondary alcohols). Sunlight-driven water splitting for hydrogen production is an increasingly active field due to the green and sustainable aspects of renewable energy sources to address climate change and environmental pollution [24]. MOFs with specific organic linkers (e.g., organophosphonic acids) demonstrate a higher level of structural/functional tunability; by incorporation of Ti-oxo clusters, it further endows the MOFs with semiconductivity for enhanced solar energy conversion efficiency via the facilitated photoexcited linker-to-cluster charge transfer (LCCT mechanism, i.e., forming Ti3+ and holes at linkers) [25, 26]. Alshareef group designed a highly stable titanium phosphonate-based MOF (TiPNW) with unique one-dimensional (1D) nanowire topology and tunable bandgaps via a stirring hydrothermal self-assembly method (otherwise, in static conditions, it will lead to the formation of nanoparticles), which may enhance the photoinduced charge carriers separation/transport, thus for a remarkable improvement in photocatalytic hydrogen evolution reaction (HER) under irradiation of either visible light or sunlight simulator (Fig. 2) [25]. The MOF with – OH group (original) shows superior photocatalytic activity to that with –Br group via post-modification (tuning the electronic structure through functionalizing linkers), due to the weaker electron-donating ability and the larger bandgap for the latter one. This new approach allows a facile and scalable production of 1D photoactive MOFs via the stirring hydrothermal method. However, the as-synthesized TiPNW is

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Fig. 2 a and b SEM image and TEM-EDS elemental mapping of the as-synthesized titanium phosphonate nanowires (TiPNW). c Schematic illustration of original TiPNW and the derived TiPNW-Br. d Proposed photocatalytic HER mechanism over TiPNW catalyst, where TEOA is used as the electron donor. e UV–vis diffuse reflectance spectra (DRS) of TiPNW compared to anatase TiO2 . f Time course of photocatalytic HER activity delivered by the as-synthesized photocatalysts under visible light irradiation (TiPNP: titanium phosphonate nanoparticles). Adapted with permission [25]. Copyright (2020), WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim

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amorphous, high HER efficiency may be achieved by engineering the 1D MOF with higher crystallinity and/or porosity. Li et al. elaborately fabricated a kind of hierarchically mesoporous titanium phosphonate (HM-TiPPh) nanospheres (sphere diameter 400–600 nm, pore size ~3 nm) via a facile dual-surfactant-directed hydrothermal method and studied the photocatalysis performance for HER. The well-structured MOF-like material with uniformly incorporated phosphonate groups greatly enhanced the mass transfer, optical absorption, and photocatalytic reaction thereof (i.e., HER) under both visible light and simulated sunlight conditions. Compared to the pristine titania (TiO2 ), the homogeneous incorporation of P and C species (i.e., the phosphonic functional linkers) into the framework shifts the absorption edge to the visible region (Fig. 3) [27]. Although the titanium phosphonates show enhanced HER performance, the photocatalytic system generally includes a precious/noble-metal cocatalyst (e.g., Pt) and a sacrificial agent (e.g., triethanolamine, TEOA), thus a high cost may limit its practical application and new strategies are needed for further improvement. Qiao group designed an ordered mesoporous titanium phosphonate with acid–base bifunctional group via a facile surfactant-assisted one-pot hydrothermal approach; the new hybrid material (organic–inorganic framework) integrates dual functions (uncoordinated acidic P–OH and basic –NH2 ) into one host, which is usually challenging but quite promising to improve catalytic performance. When evaluated for CO2 conversion, e.g., cycloaddition reaction with aziridine, it showed a high conversion of >99%, yield of 98%, and regioselectivity of 98:2 (Fig. 4) [28]. This synergistic and heterogeneous catalysis exhibited superior performance to previous heterogeneous ones and was comparable to that of some homogeneous systems. Also worth mentioning is the easy separation and repeated reuse without activity loss, and not involving the use of organic solvents, hazardous halogen ions, or cocatalysts. Overall, the facile one-pot method for bifunctional mesoporous catalysts could be an environment-friendly strategy for practical high-performance heterogeneous catalysis including carbon capture, utilization, and storage (CCUS). The titanium-substituted lithium carbanions, i.e., lithium titanium phosphonate, with high configurational restrictions (featured by planar four-membered Ti–C–P–O chelate ring) and enhanced stability (owing to the introduction of Ti atom for hyperconjugation), may facilitate highly stereoselective asymmetric transformations in C–C bond-forming reactions [29]. Ti/V-containing phosphates are usually inorganic materials that generally need annealing or heat treatment during the synthesis. For example, mesoporous titanium phosphate (or named phosphate mesoporous titanium dioxide, PMT) synthesized by triblock copolymer-assisted sol–gel process shows a high surface area of over 300 m2 /g after calcination (400 °C), and significant activity on the photocatalytic oxidation of n-pentane due the extended band gap energy, Ti ion centers in tetrahedral coordination and the large surface area [30]. The PMTs possess higher photocatalytic activity than mesoporous titanium dioxide (MT) and commercial P25, however, the appropriate calcination temperature is a necessity for optimized structure and performance. Furthermore, these transition-metal phosphonate frameworks may also serve as precursors for electrocatalysts (e.g., phosphides).

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Fig. 3 a–d SEM and TEM images of hierarchically mesoporous titanium phosphonate (HM-TiPPh) nanospheres. e and f XPS spectra of HM-TiPPh (Ti 2p and P 2p). g N2 adsorption–desorption isotherm (inset: corresponding pore size distribution curve). h Typical time course of HER catalyzed by the as-synthesized HM-TiPPh photocatalyst under visible light irradiation compared to its counterparts. i Long-term durability test of HM-TiPPh under visible light illumination. Adapted with permission [27]. Copyright (2018), Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim

Aluminophosphates and silicoaluminophosphates have rich structure chemistry [31], and so do V and Ti-containing phosphates or phosphonates with rich stoichiometries and enhanced mixed bonding, compared to di-/trivalent transition-metal cations (M2+ , M3+ ). The vanadium incorporation or substitution in molecular seize or mesoporous frameworks such as V-AlPO can effectively enhance the catalytic activity/dynamics, resulting in higher substrate conversion or product selectivity in oxidation (dehydrogenation) of organic alcohols and other hydrocarbon-based small molecules, compared to pristine aluminophosphates [32]. And by the introduction of titanium or vanadium centers, Ti-AlPO or V-AlPO catalysts have also been synthesized for allylic oxidation/epoxidation (e.g., cyclohexene) or for cycloalkane oxidation (e.g., cyclohexane) with high activity/selectivity, representing a new kind of (partial) oxidation catalyst.

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Fig. 4 a–e TEM images and EDS elemental mapping images of the as-synthesized periodic mesoporous titanium phosphonate by using amino-containing alendronate sodium trihydrate ligand (Ti-AST). f CO2 adsorption isotherms and g CO2 -TPD curves of Ti-AST and Ti-MDA (synthesized from methylene diphosphonic acid ligand), showing typical chemical adsorption for Ti-AST with lower activation energy than physical adsorption for Ti-MDA. TPD: temperature-programmed desorption. h Ti-AST catalyzed cycloaddition reaction of aziridine and CO2 . i Proposed reaction mechanism triggered on acid–base bifunctional Ti-AST catalyst. Adapted with permission [28]. Copyright (2014), American Chemical Society

3.2 Ion Exchange and Adsorption/Separation The tailorability of such mesoporous titanium phosphates/phosphonates not only showed great promise for advanced photocatalysis but also other environmental applications such as efficient dye removal, toxic/heavy metal ions liquid-phase adsorption, and CO2 gas-phase adsorption [33–35]. For example, titanium phosphonates or their coating layer supported on other frameworks can also work as absorber for rare-earth, heavy/toxic, intra-lanthanide metal ions (e.g., Sc3+ , Cs+ , Sr+ , Cu2+ , Cd2+ , Pb2+ , U4+ , Lu3+ , Dy3+ ), dyes, and ammonia owing to the excellent ion exchange and/or photocatalytic characteristics of these insoluble acid salts of tetravalent metals (or named tetravalent metal acid (TMA) salts), which are beneficial to the metal ion recovery and/or environmental remediation. Mesoporous titanium phosphates with cationic framework topologies can serve as a new open-framework molecular sieve, and the

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most stable octahedral state of Ti in the framework is achieved by the stoichiometric incorporation of phosphorus (at Ti/P = 1:1), which shows high anion/cation exchange capacity due to phosphonium cation/defective P–OH groups [35]. For example, Yuan and coworkers prepared a periodic hexagonal mesoporous titanium phosphonate (PMTP) via a surfactant-assisted hydrothermal and self-assembly process [36]. The organophosphate groups and titanium units are homogeneously located in the network of mesoporous solids. After high-temperature calcination (up to 450 °C), the PMTP hybrid material can transform into conformal inorganic titanium phosphonates or partly/completely collapsed titanium phosphates at even higher temperatures (over 550 °C). The PMTP hybrid shows a promising application as an efficient adsorbent (e.g., for Cu2+ , Cd2+ , and Pb2+ owing to the ethylenediamine ligands, corresponding the adsorption capacity of 3.21, 2.10, and 2.05 µmol/g, respectively) and photocatalyst for degradation (e.g., of organic cationic dye rhodamine B and methylene blue, with photodegradation degree higher than that of commercial P25). It can serve as a multifunctional and low-cost photocatalyst for integrated organic/inorganic pollutants-containing wastewater treatment. Particularly noteworthy is that the degradation degree of PhB under simulated solar light irradiation is enhanced in the integrated wastewater treatment due to the newborn broad adsorption peak (600–900 nm) induced by the coordination of these metal ions with chelating ethylenediamine groups in the framework (Fig. 5) [36]. The ion adsorption could be even higher up to 28.5 µmol/g or 139 mg/g for Cu2+ with good reusability via tailoring the bridging of ligands (e.g., hydroxyethylidene) [33, 37]. Titanium phosphate can also serve as an ion exchanger for rare earth elements (REEs) recovery, e.g., trace scandium (Sc3+ ) from complex bauxite residue acid leachate. The amorphous titanium phosphate demonstrated the highest Sc3+ exchange capacity of up to 1.74 mequiv/g, with a separation factor in the magnitude of 10– 1000 (for Sc/Fe2+ , Sc/Al, Sc/Ca), and could increase by a factor of 8.8 and 265 for the concentration ratio of Sc/Fe and Sc/Al, respectively, with a Sc recovery ratio of 91.1% in a single cycle of chromatographic separation. The high selectivity for Sc3+ by amorphous titanium phosphate is probably due to the matched Sc3+ size with Ti4+ lattice (both 1.49 Å) (Fig. 6) [38]. These amine-containing titanium phosphonates not only show great potential in heavy metal ion adsorption but also have promising applications in CCUS (CO2 adsorption). For example, a hierarchical titanium phosphonate with wormlike mesostructure and parallel macroporous channels exhibits a CO2 uptake of up to 0.89 mmol/g or even higher [34, 37]. Ti(IV) phosphate–phosphonate can also perform as a proton conductor, dependent on both temperature and relative humidity (RH) and comparable to the conductivity of Nafion [10]. In addition, the highly crystalline hexagonal mesoporous metal (Ti,V)phosphonates synthesized via a microwave-assisted templating (triblock copolymer F127) strategy, may also be applied in open-tubular capillary electrochromatography (OT-CEC), an advanced hybrid chromatographic separation technique combining liquid chromatography and capillary electrophoresis. The hierarchical micro/mesoporous (wall/channel) structure with functional groups and specific framework

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Fig. 5 a and b TEM images of the as-synthesized periodic mesoporous titanium phosphonate (PMTP-1) hybrid material. c UV–vis diffuse reflectance spectra of PMTP-1 before and after adsorption of Cu2+ ion (Cu-PMTP-1). d One-pot wastewater treatment for heavy metal ion (Pb2+ , Cu2+ ) adsorption and dye RhB photodegradation by the as-synthesized PMTP-1 under simulated solar light irradiation. Adapted with permission [36]. Copyright (2010), The Royal Society of Chemistry

and structural stability endows the as-synthesized metal (Ti,V)-phosphonate materials with improved selectivity, thus showing promising potential as stationary phase for OT-CEC separation of neutral, acidic, and basic compounds [39]. Also, hybrid porous titania phosphonates with varied functional groups could be used as efficient solvent separation materials, e.g., basic heterocyclic compounds containing nitrogen such as pyridine can be facilely and completely removed from methanol.

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Fig. 6 a Schematic illustration of selective recovery of trace scandium (Sc3+ ion) by inorganic ion exchanger titanium phosphate from leachates of waste bauxite residue (BR, red mud). b Chromatographic elution of simulated BR leachate-loaded amorphous titanium phosphate (am-TiP) column using three different acid solutions. Adapted with permission [38]. Copyright (2017), American Chemical Society

3.3 Surface Modification (Organo)phosphorus acids and their derivatives (salts, esters) are very promising coupling molecules for surface modification of oxides and alloys to enhance their

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hydrolytic stability and/or interfacial compatibility [40]. For example, phosphonate/phosphate groups can be grafted to solid surfaces of mesoporous transitionmetal oxide films or xerogels via post-functionalization (forming P–O–M, M = Si, Ti, Zr, or V, etc.) for well-defined surface chemistry and the further advanced applications thereof [7, 40]. Phosphonates/phosphates anchored titanium and its alloy implants, by the formation of Ti–O–P bond, can also be used for further biomolecular coupling, thus for appropriate orthopedic, dental, or antimicrobial applications [41, 42]. In addition, phosphonate or phosphinate coupling molecules were also utilized for the modification of titania (TiO2 ) particles.

3.4 Selective Sensing Nanoporous Ti-phosphonate MOFs can also be utilized as a selective sensor for explosives, e.g., 2,4,6-trinitrophenol (TNP) [3]. The specific MOF with tetradentate phosphonate ligand (H8 L–Ti–MOF) may reach a detection limit of 3.6 µM (or 0.82 ppm) in an aqueous phase via the mechanism of (complete) fluorescence quenching. The Ti/P ratio in the framework could also be tuned to form a core–shell structure, e.g., mesoporous titanium phosphonate–TiO2 nanospherical material as a nanocarrier to load fluorescent agents (labeled oligonucleotide, chemically grafted) and drugs (ibuprofen, physically adsorbed) for bioresponsive sensing (e.g., detection of DNA or protein) and controlled drug release [43]. The simultaneous and highefficiency biomolecule sensing along with controlled drug release opens a new way for advanced biomedical diagnosis and therapy.

3.5 Energy Storage and Conversion Phosphorus-based materials with intrinsic electrochemical activity and structural superiority have attracted huge attention as potential catalysts/electrode materials for clean/renewable energy storage and conversion [4, 44]. For example, mesoporous vanadium phosphate nanosheet (2D VOPO4 with hexagonal mesostructure) was first synthesized via a lyotropic liquid crystal (LLC, i.e., a triblock copolymer P123) templating method by Mei et al. [45]; the as-prepared 2D mesoporous VOPO4 after calcination showed a crystalline framework and less-ordered mesostructure. When used as electrode material for supercapacitors, it can deliver a superior pseudocapacitance of 767 F/g (at 0.5 A/g) and high rate performance (434 and 283 F/g at 5 and 10 A/g, respectively, much higher than that of the bulk one), due to the abundant and facilely accessible redox active sites (based on reversible two-stage valence switching of V5+ ↔ V4+ ↔ V3+ ) along with the enhanced ion diffusion and electron transfer. The LLC or block copolymer templating opens up a novel route for designing mesoporous vanadium phosphate nanosheets for advanced energy storage devices. Some similar systems using self-supported vanadium phosphate nanostructures have also

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been investigated for high-voltage (up to 2.8 V) aqueous supercapacitors [46]. H8 L– Ti–MOF (containing tetradentate phosphonate ligand) based asymmetric supercapacitor shows an areal capacitance of 20.0 F/cm2 (at 0.1 mA/cm2 ) and a relatively high cycling stability (82.7% capacitance retention after 1000 cycles) [3]. Rechargeable aqueous zinc-ion batteries (ZIBs) are promising green, safe, and low-cost energy storage devices as an alternative to alkali-metal-ion batteries (AMIBs). Vanadium/titanium phosphates are also exploited as anodes of AMIBs, and some are emerging as cathodes for aqueous ZIBs. For example, layered vanadium phosphate (VOPO4 ·2H2 O) with unique layered structures and high discharge plateau are superior candidates for ZIBs. Via tuning the interlayer spacing (from original 0.74 nm to 1.65 nm) with phenylamine (PA) intercalation, the hydrated VOPO4 (VOP) as ZIB cathode showed enhanced Zn2+ diffusion kinetics (diffusion coefficient of 5.7 × 10−8 cm2 /s compared to pristine VOP of 6.2 × 10−13 cm2 /s), high specific capacity (268 mAh/g−1 at 0.1 A/g−1 ), and excellent electrochemical stability (201 mAh/g−1 at 5 A/g−1 after 2000 cycles, capacity retention of 92.3%) (Fig. 7) [47]. The intercalation of hydrophobic/conjugated molecules into layered metal phosphates (partly inhibiting decomposition/dissolution) is of great potential for practical high-performance aqueous ZIBs. Lithium and sodium (transition) metal phosphates are among the most promising cathode (or anode) materials for high-rate, high-capacity, and high-security metal-ion batteries [48–52]. Owing to the ‘inductive effect’ of PO4 polyanion, the transitionmetal ion redox couple in the cathode materials could be elevated by ca. 1.5–2 V, thus endowing them with significant advantages as attractive/practical cathode materials. Lithium/sodium vanadium phosphates especially their mesoporous nanocomposites (e.g., with highly conductive graphene, carbon nanotubes, or carbon cloth) are attracting increasing attention due to the high capacity (approaching theoretical capacities), superior charge/discharge rates (up to 20C) and good cycling stability (e.g., over 96% after 1000 cycles) as cathodes, which are of great potential for highpower Li/Na-ion batteries [53]. Sodium titanium phosphate (NaTi2 (PO4 )3 , NTP), lithium titanium phosphate (LiTi2 (PO4 )3 , LTP), sodium-substituted lithium titanium phosphate (Li1−x Nax Ti2 (PO4 )3 ) or sodium vanadium titanium phosphate, etc. can be used as electrodes for alkali-metal-ion batteries, either with organic, aqueous, or solid electrolytes [54, 55]. These phosphate-based NASICON materials may also be a class of promising solid electrolytes for all-solid-state alkali-metal-ion batteries. For the NASICON-structured phosphates with promising applications in energy storage, one may refer to some recent reviews [48, 51, 54].

4 Challenges and Perspectives Metal phosphates/phosphonates comprised of suitable metal centers and phosphate/phosphonate ligands provide a wide range of nanoarchitecture, structural

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Fig. 7 a Schematic illustration of capacity regulation for zinc-ion storage by phenylamine (PA)intercalation engineering. b and c GCD curves and rate performance of layered vanadium phosphate (VOP) based zinc-ion battery (ZIB) and PA–VOP based ZIBs with tunable interlayer spacing (14.8, 15.6, and 16.5 Å, compared to pristine VOP of 7.4 Å). GCD: galvanostatic charge–discharge. d Long-term cycling performance of Zn//VOP and Zn//PA–VOP (16.5 Å phase) batteries using 2 M Zn(CF3 SO3 )2 aqueous electrolyte. Adapted with permission [47]. Copyright (2021), The Royal Society of Chemistry

robustness, and compositional diversity as well as rich chemistry, e.g., the phosphates/phosphonates with 3d transition metal elements (Ti and V) possess superior redox properties and rich structural chemistry. These low-cost Ti/V-containing phosphate/phosphonate-based nanomaterials are showing great potential for various promising applications, including heterogeneous catalysis, photocatalytic pollutant degradation, water splitting, ion exchange, adsorption/separation, surface modification for biomedical applications, sensors, and energy storage and conversion. Some

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major challenges and perspectives are listed for further development and the final practical applications. Structural engineering especially tuning the layer spacing or porosity is of great importance to make phosphates/phosphonates meet the diverse and specific requirements for practical applications. The 1D/2D nanostructures and mesoporous, hierarchical structures are promising candidates, some 0D nanostructures such as clusters are also of interest in specific fields including biomedicine. More attention should be paid to the control of the hydrolysis rate of metal precursors and the removal of templates to avoid structural collapse or low yield. Another key consideration is to take full advantage of the physicochemical features of the precursors via selection, modification, and optimization for a well-controlled self-assembly process. For example, the precursors such as phosphonate ligands bearing pH-, photo-, or thermoresponsive moieties may extend the potential application in environmental, energy, and biomedical fields. The V/Ti-containing phosphates/phosphonates are relatively posing a greater challenge to achieve sufficient lattice structures owing to the limited reaction reversibility and rapid precipitation, thus for a less ordered material with lower crystallization. Fortunately, some elaborate templating/self-assembly techniques are adopted for great improvement; more structural characterization and the illustration of structure–performance relationship need further investigation. High-efficiency synthesis techniques under mild conditions should be developed for facile, fast, and scalable production and practical applications with desired properties. Hybridization or incorporation of these phosphates/phosphonates with other functional materials such as 2D nanostructures or 0D clusters may greatly enhance their conductivity and ion/electron transfer and their applications thereof, especially in photocatalysis and electrochemical fields. Post-treatment is also a facile way to enhance their performance or extend their application.

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Electrochemistry of Metal Phosphates and Phosphonates Hülya Silah, Cem Erkmen, and Bengi Uslu

Abstract Worldwide, various factors have caused an increase in energy consumption. So, the requirement for renewable and green energy sources becomes a gradually important problem. Nowadays, improvements in energy-related conversion methods such as metal-air batteries, electrochemical water splitting, supercapacitors, etc., are very significant fields of research. Nanostructures of metal phosphate and phosphonate compounds are of tremendous attention in today’s technological applications, because of their presence of hollow spaces, high surface area, and straightforward adjusting composition and dimensions. Phosphorus compounds like metal phosphates and metal phosphonates have demonstrated perfect performances and excellent potential in electrochemical applications and devices. This chapter reviews some electrochemical applications such as water splitting and energy storage devices using metal phosphate and phosphonates were examined in the last two decades of improvement. Keywords Batteries · Energy · Metal phosphonates · Metal phosphates · Storage · Supercapacitor

1 Introduction The usage of hybrid substances containing nearly crosslinked inorganic and organic structures at the molecular scale intends to combine the supremacy of both these substances within the backbones, ensuring the generation of materials with the structural and mechanical stability of inorganic substances, and with the elasticity and functionality properties of organic substances [1]. Metal phosphates and phosphonates represent a significant class of materials with settled industrial implementations H. Silah Faculty of Sciences, Department of Chemistry, Bilecik Seyh Edebali University, 11210 Bilecik, Turkey C. Erkmen · B. Uslu (B) Faculty of Pharmacy, Department of Analytical Chemistry, Ankara University, 06560 Ankara, Turkey e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_4

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that have been attracting private scientific attention, on account of their distinguished chemical and physical features [2]. The design of chemically synthesized inorganic metal phosphates, metal phosphonates, and metal hybrid phosphates implicate an important investigation area that has been broadly examined both in academia and industrial applications [3]. Poriferous metal phosphate and phosphonate compounds symbolize the good composition of changeable pore structures and controlled organophosphonic chemistry, which has noteworthy features and has taken extensive notice for various implementations. Metal phosphate and phosphonate compounds act as characteristic samples of the developing applications because these materials have the superiority such as inexpensive, environmental friendliness, safety, more stability, high natural abundance, mechanical flexibility, low toxicity, easy preparation, unique physicochemical properties, hydrophilic and thermally stability, charge transport, tunable multifunctionality, superior conductivity, and structural designability to a certain extent [4–6]. Metal phosphates and metal phosphonates-based materials have taken remarkable concern because of their great potential in various application areas such as catalysis, heterogeneous catalysis, redox catalysis, proton conductors, drug delivery, ion exchange, clean energy technologies, environmental restoration, photoluminescence, biomedical devices, diffusion applications, and so on [3, 7]. In this chapter, a concise overview of current developments about phosphorusbased compounds for electrochemical devices and applications, comprising metal phosphates and phosphonates has been provided.

2 Properties and Applications of Metal Phosphates and Phosphonates Phosphorus element is a multivalent nonmetal, which is further one of the most convenient and well-known donor atoms in coordination compounds [8]. Metal phosphate compounds are described as merely inorganic substances constituted from the binding of metal units with phosphoric acid, however metal phosphonate compounds are inorganic–organic hybrid poriferous nanostructures, where organophosphate ligands are utilized as phosphorus welding [9]. The covalent bond between phosphorus oxygen in phosphate groups is comparatively robust, making them chemically steady and balancing the lattice oxygen even at a high state of charge [6, 8]. Metal phosphate compounds have increased interest as active catalyst materials due to their bifunctional characteristics such as redox ability and solid acidity. Due to their properties as intercalation hosts and conductors, many transition metal phosphates, including those of vanadium, manganese, zirconium, nickel, etc., are suitable for forming layered structures with open frameworks where the protons of acid groups can distribute throughout the interregional [10]. Also, metal phosphonate compounds are usually maintained by powerful bonds that render them very stable materials [11]. Because of these properties, metal phosphates are used

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as catalysts in the catalysis process, fuel cells, batteries, and biomedical applications [4]. Metal phosphonate compounds are significant groups of metal–organic frameworks or coordination polymers. The phosphonic acids have two pKa values 2.0 for the first proton and 6.59 for the second. These compounds (RPO3 H2 ) can ionize in solution, releasing one or two protons. Also, since each phosphonate group formed as a result of the dissociation reaction has three oxygen atoms, the phosphonate group can bind up to nine metal atoms [11]. Depending on the experimental conditions applied, di-, tri-, tetra-, penta-, and hexavalent metal compounds of metal phosphates and phosphonates can be formed [9]. The phosphonate group has a vigorously anionic moiety. Therefore, this group is disposed to create powerful bonds with metal ions Mn+ (n = 1–4) [1]. Compared to other classical ligands such as alkoxy groups, sulfonic acids, carboxylic acids, and polyazaheteroaromatic ligands, the role of organophosphonic acids in creating hybrid matter is significantly interesting because the organophosphonic acids can generate robust bonds and more coordination modes with metal-based clusters and metal nodes [7]. The different kinds of bonding formats between the organophosphorus ligand and transition metal may form microporosity through the framework of metal phosphonates. Additionally, the coordination chemistry of the phosphonate group can assist in form various coordination environments with the metal centers with superior catalytic efficiency and affirmative kinetics [12]. So, metal phosphonate compounds are a group of hybrid organic–inorganic polymeric substances formed via the binding of phosphonate ligands to metal ions, creating extended structures of diverse dimensions [13]. Compared with most of the other porous coordination polymers and metal–organic frameworks, metal phosphonate compounds usually demonstrate higher thermal and water stability based on the existence of inorganic components in the structure such as chains, clusters, or layers [11, 14]. Because of their superior properties such as thermal and chemical stability, also high solubility in different solvents, these materials have been utilized in various applications such as intercalation, ion exchange, fuel cells, magnetism, adsorption, catalysis, proton conduction, nanotechnology, gas sorption, electronics, medicine, and so on [13]. Poriferous inorganic–organic metal phosphonate hybrid compounds have taken an increasing grade in investigation concerns because of their capability to combine the advances of both poriferous materials and inorganic–organic skeleton structures [15]. The improvement of poriferous proton change membrane materials having great proton conductance is extremely charming for the development of a yielding fuel cell, that employs O2 and H2 (as fuel) gases to form electrical power [9]. In new-generation fuel cell systems, different metal phosphates (MP2 O7 , M = Zr, Ti, etc.) are used as proton conductive membranes, because of many structure-based proton bonding units and transportation ways [16]. Metal phosphonates have been used as metal–organic frameworks. Recently, these compounds are a novel kind of poriferous materials with fascinating structures and geometry that have acquired remarkable care. So, many studies have been carried out based on the application of metal–organic frameworks for the adsorption of different pollutants from various kinds of wastewater [17].

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The existence of powerful affinity between phosphate/phosphonate ligands and metal centers causes the design wide kind of metal phosphate and phosphonate compounds [5]. The nanoporous metal phosphate/phosphonate compounds with different morphologies via using various synthesis methods are one of the popular topics because of their incomparable features containing a lot of active sites for reactions with large surface areas and quick interfacial transportation of protons/electrons by diminishing the diffusion way lengthiness across the porous structure [18]. These nanostructured metal phosphate materials propose an unmatched occasion to design hierarchically poriferous nanoarchitectured structures are formed interconnected micro-, meso-, and macropores [9]. Metal phosphate compounds, particularly aluminophosphates with wealthy two/three-dimensional structures, compose materials having different porosities altering from micropores to mesopores and macropores. Also, trivalent metal cations like gallium(III), chrome(III), manganese(III), cerium(III), etc. can readily replace the framework aluminum(III) in the aluminophosphate structure without changing the charge neutrality of the structure [9]. Different synthesis methods are used for the synthesis of metal phosphate compounds such as hydrothermal/solvothermal method, chemical precipitation method, aqueous-based reflux method, ball milling method, calcination process, sol–gel method, one pot oil in water emulsion process, direct growth strategies like atomic layer deposition and so on [6, 8]. Hollow nanostructures are very important for many applications and devices because of their low mass density, large pore volume, and high surface/volume ratio. So these features make the metal phosphate/phosphonates advantageous for implementations involving catalysis and energy storage [3].

3 Electrochemistry of Metal Phosphates and Phosphonates and Their Applications Worldwide, various factors such as thriving industrialization, rapid world population growth, urbanization, civilization, diverse agricultural activities, therewithal, global, environmental, and geological alterations have caused an increase in energy consumption. Today, carbon-based conventional fuels comprising coal, biofuels, oil, and natural gas are used to meet around 80% of the world’s energy needs. The usage of these fuels causes the formation of CO2 , toxic gases, and some particles. So, energy storage and new renewable energy sources have become two of the most critical issues that humankind has to face [19, 20].

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3.1 Energy Storage Devices Supercapacitors, fuel cells, also batteries are among the most efficient and many trustworthy applications for electrochemical energy storage and conversion [8], and proton-conducting materials are of excellent attention because they can be nominees for electrolytes in these applications [11]. Electrochemical energy storage is used as a strengthened interface between energy conversion and generation in all power storage technologies. Ion and electron conduction, which depends on the electrode material’s surface area, electrochemical active units, and conductivity, creates capacitive energy storage [21]. Metal hydroxides/oxides and sulfides are a type of electrode that shows strong characteristic capacitance, although these materials have certain limitations because of their poor electrical stability and conductivity. The open framework of many metal phosphate compounds with substantial voids and solid channels, however, has lately demonstrated perfect ion conductivity and has been investigated as a potential pseudocapacitive electrode material [21]. These properties of metal phosphates and phosphonates, linked with the natural structural variability real to any metal–organic framework compound, are the basis of their concern as proton conducting materials [11]. Supercapacitors are energy repository apparatuses that fill the space among batteries and dielectric capacitors in terms of energy density and power. One of the most important features of supercapacitors, as designed and manufactured, is that they are energy storage apparatuses alike rechargeable cells. These electrochemical devices contain two extremely conductive electrodes divided by the electrolyte including mobile ionic kinds. The energy is physically stored at the electrolyte/electrode interface by the genesis of an electric double layer or by rapid redox reactions on the surface of the electrode. As a result, the discharge and charge can occur very swiftly, providing high power intensities and long cycling decisiveness [8, 22]. One of the parameters that largely determines the efficiency of the supercapacitor is the electrode material. It is crucial to conduct an additional study on cathode materials because there are far fewer studies on cathode materials in the literature of supercapacitor studies than there are on anode materials. The development and design of new electrode materials with high efficiency and inexpensive are the main focuses of cathode material research nowadays [23]. Lu and coworkers synthesized a new bouquet-like Co3 (HPO4 )2 (OH)2 by hydrothermal method using Co(NO3 )2 6H2 O, red phosphorus, and ammonia. This mixture remained at 180 °C for 12 h in the hydrothermal autoclave. Physical characterization of Co3 (HPO4 )2 (OH)2 was carried out X-ray diffractometry, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (SEM), and field emission scanning electron microscopy (FESEM). Using a conventional threeelectrode system and hybrid supercapacitor, the electrochemical characteristics of the Co(NO3 )2 6H2 O compound were studied (Fig. 1). The prepared Co(NO3 )2 6H2 Obased electrode showed an ultrahigh rate efficiency of 83.6 mA h/g at 100 A/g and a high perfect capacity of 119.2 mAh/g at 1 A/g. Additionally, the combined

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Fig. 1 a Schematic diagram of the Co3 (HPO4 )2 (OH)2 //porous carbon hybrid supercapacitors. b Cyclic voltammogram profiles of the porous carbon and Co3 (HPO4 )2 (OH)2 electrodes. c Cyclic voltammogram profiles of the Co3 (HPO4 )2 (OH)2 //porous carbon hybrid supercapacitors d Galvanostatic charge–discharge profiles of the Co3 (HPO4 )2 (OH)2 //porous carbon hybrid supercapacitors. e The corresponding specific capacitance. f Ragone plots of the Co3 (HPO4 )2 (OH)2 //porous carbon hybrid supercapacitors. g Cycle feature and coulombic yield of the Co3 (HPO4 )2 (OH)2 //porous carbon hybrid supercapacitors after 10,000 cycles at 3 A/g. h An LED screen is lighted up, and i a fan is driven by two-hybrid supercapacitor devices. Adapted with permission [23]. Copyright (2022), Springer

Co3 (HPO4 )2 (OH)2 /porous carbon hybrid supercapacitor instrument has maximum energy and power densities of 44.6 W h/kg and 33.75 kW/kg, respectively [23]. Li et al., synthesized nanoporous aluminum, nickel, and zirconium phosphates by a thermal conversion method and studied their electrochemical properties and performances. Thermogravimetric, SEM, FESEM, and X-ray diffractometry (XRD) analyses of the resulting phosphate material were performed to determine its chemical composition. According to these findings, the structure of the synthetic nickel phosphate compound has a globular morphology and a wide surface area, which allows it to be extremely useful as an electrode material for supercapacitors [8]. Metal phosphates and phosphonate compounds are usually synthesized in diverse forms to improve intended implementations. Pujari and coworkers applied the hydrothermal method to synthesize a copper phosphate hydroxide compound that can be used as an electrode in supercapacitor applications (Fig. 2). For the synthesis of this compound, a bath was prepared using cupric sulfate, ammonium phosphate

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Fig. 2 Preparation of copper phosphate thin film electrode via hydrothermal method. Adapted with permission [21]. Copyright (2020), Springer

monobasic, and urea. The prepared mixture was autoclaved at 17 psi and 393 K. A stainless steel substrate was dipped vertically into this prepared mixture and left for a certain period. XRD, Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) analysis showed that Cu2 (PO4 )(OH) materials have a macroporous and microstrip-like structure with a large surface area of 5.26 m2 /g. For the optimum thickness of 12.85 µm, obtained phosphate film electrode displayed a maximum capacitance of 280 F/g. The Cu2 (PO4 )(OH) electrode showed a power intensity of 264.70 W/kg and an energy intensity of 3.85 Wh/kg after 2000 galvanostatic charge/discharge measurements [21]. Transition metal phosphates stand out as excellent nominees for supercapacitors on account of their improved conductivity and longer period stability. In their work, Patil et al. proposed cobalt cyclotetraphosphate (Co2 P4 O12 ) as a new supercapacitor electrode substance. This material provides a capacitance of 437 F/g, maintaining 90% of its stability even more than 3000 cycles. To demonstrate the practical suitability of the developed material, an asymmetrical device capable of operating up to 1.4 V was produced using Co2 P4 O12 and activated carbon as positive and negative electrodes, respectively. In this study, with the addition of redox-active molecules such as potassium iodide (KI) to the main KOH solution medium, the capacitance value for the Co2 P4 O12 //activated carbon cell increased from 120 to 156 F/g. Thanks to the extra voltage and advancement in capacitance the developed cell provided energy with a power intensity of 2.3 kW/kg. Even at a high power of 7.6 kW/kg, the cell’s 48 Wh/kg energy was perfectly preserved. This showed that the developed cell performed good performance. In addition, there was a negligible loss in energy with the developed asymmetric device. The high Coulomb efficiency even over 5000 cycles showed that the developed metal cyclotetraphosphate is an up-and-coming electrode material with a view to energy storage [24].

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Due to their numerous benefits, including safety, excellent thermal stability, longtime of life, low cost, and environmental friendliness, lithium-ion batteries are now the most popular rechargeable batteries. Lithium-ion batteries are progressively utilized in electric appliances, drones, hybrid electric devices, hybrid vehicles, and electric vehicles. And these batteries are necessary energy sources for power instruments and contemporary electronics, involving computers and cell phones. The two most considerable features of lithium batteries are their characteristic energy and power levels, which are largely limited by the voltage and capacity of cathodes [25, 26]. To acquire high-performance lithium-ion batteries, electrode materials with appropriate chemical features (for example composition, crystallinity, and stability) and physical features of structures (for example porosity, surface area, and pore size) are consequently necessary that can allow effective mass transfer and reversible faradic reactions throughout charging/discharging [6]. Because of the plenty of surface phosphate sites, these materials can be stored in large quantities of lithium ions, which is very important for their implementation potential in lithium-ion batteries [9]. Additionally, the lithiation/delithiation process throughout the pore canals of metal phosphonate and phosphate compounds supports the diffusion of lithium ions [9]. In particular, lithium iron phosphate compounds are widely used commercially due to their high energy capability and low-cost availability [16]. Han and coworkers manufactured three-dimensional macroporous graphene aerogelsupported iron(III) hydroxide phosphate dihydrate [Fe5 (PO4 )4 (OH)3 2H2 O] microspheres via hydrothermal mineralization of Fe3+ and PO4 3− ions in the existence of graphene oxide. The structure and morphology of the synthesized phosphate materials were investigated by SEM, XRD, and FTIR. A thermogravimetric analysis was used to describe the materials’ thermal properties as well. The results of the structural characterization studies show that the synthesized phosphate materials have a three-dimensional macroporous structure encapsulated in flexible graphene sheets. The iron(III) phosphate compound’s electrochemical characteristics, which make it a potential cathode material for lithium-ion batteries were investigated, and a good reversible specific capacity value of 155 mA h/g at a current density of 50 mA/g after 300 cycles was measured [27]. For the development of other ion batteries as rechargeable zinc-air batteries, it is important to develop a highly efficient and durable bifunctional electrocatalyst in ORR and OER. In their work, Wang et al. developed a manganese phosphate-based electrocatalyst (Mnx (PO4 )y /NPC). This catalyst had a very high specific surface area (1106.9 m2 /g). The developed catalyst exhibited a more positive half-wave potential of 0.87 V for ORR at 0.1 M KOH. Also, this catalyst exhibited an overpotential of 0.37 V for OER to obtain a current density of 10 mA/cm2 at 1 M KOH. A rechargeable zinc-air battery combined with Mnx (PO4 )y /NPC provides a new type of electrocatalyst to the non-noble metal-based bifunctional electrocatalyst family thanks to its superior performance [28].

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3.2 Water Splitting Applications Hydrogen makes a promise to replace fossil fuels on account of its renewable nature, clean oxidation product, and superior energy density. Hydrogen has been used in a variety of industrial processes, including the manufacturing of ammonia, as well as in cars, fuel cells, and rocket fuels [20]. Thus electrolysis, or the process of splitting water into hydrogen, is an important alternative method for creating renewable energy sources [20]. The improvement of renewable energy can be significantly attributed to efficient hydrogen generation via water splitting [29]. The water-splitting processing has been composed of the basis for many various energy conversion or storage apparatuses, of which the most effective and powerful are fuel cells, electrolyzers, and metal-air batteries. Hybrid inorganic systems demonstrate a fast-thriving field of exploration and are exciting instances of integrating water splitting [30]. But the electrocatalytic yield of water splitting is hindered by the oxygen evolution reaction (OER) in the anode and hydrogen evolution reaction (HER) in the cathode. Additionally, this reaction is more complicated and the slow kinetics of the OER reaction, which produces molecular oxygen via a complicated pathway including the extraction of four protons and four electrons as well as two water molecules generating one oxygen molecule, is a typical constraint of water splitting systems. Lately, a spawning number of implementations have been needed that require water splitting at neutral solution pH and under natural conditions. This necessity brings about further disadvantages as the electrolysis of water is appropriate in alkaline and acidic solutions. For these reasons, electrocatalytic materials encouraging water electrolysis have been studied to advance electrolytic yield [29, 30]. To decrease the energy decrement and overcome the overpotential, scientific studies about to design of low-cost electrocatalysts with extremely yielding for HER and OER have been performed [31]. A very important factor in evaluating the activity of electrocatalysts is catalytic efficiency. The number of active units and the actual activity of each unit are the two criteria that characterize efficiency [32]. These active units can be experimentally developed by designing pore structure, crystallinity, and morphology [31]. As it is known, noble metal electrocatalysts like platinum on carbon (often referred to as Pt/C) and ruthenium(IV) oxide, iridium(IV) oxide are one of the newest catalysts used in HER and OER processes. But these electrocatalysts have some disadvantages such as the rareness and high cost of the overall process in terms of being produced on a large scale. For these reasons, there is a high requirement in the water-splitting process of research to produce new more effective, cheap, and readily reachable electrocatalysts compared to traditional electrocatalysts [29, 33]. In water splitting, some transition metal compounds like metal borides, hydroxides, oxides, carbides, sulfides, nitrides, selenides, phosphides, and phosphates have been investigated due to their remarkable stability and small overpotential [29]. A phosphate group is a fascinating and growing attention due to its high catalytic action and matchless structure of atomic geometry. Especially, transition metal phosphonate compounds are a substantial group of materials of their bonding style

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between organophosphorus ligands and cations [33]. The phosphate group not only makes easy the oxidation of metal atoms throughout proton-coupled electron transfer processing but also distorts the natural atomic geometry and in this way promotes the oxidation of water molecules [29]. Nowadays, metal phosphonates, produced by the introduction of organic moieties into the inorganic framework, have latterly been taken into account as a new kind of electrocatalyst in addition to inorganic metal phosphate complexes [34]. Some metal phosphate and metal phosphonate-based electrocatalysts for OER are given in Table 1. Compared with some other cobalt-based catalysts, cobalt phosphate compounds have usually been studied as a low-cost, multifunctional material, earth-abundant, and environmentally friendly. Yuan et al. [40] researched the first sample of cobalt phosphate nanoparticles embellished with nitrogen-doped carbon layers. These particles were made utilizing a straightforward hydrothermal process using o-phospho-DL serine as both the carbon and phosphate source. The produced Co3 (PO4 )2 @N–C material demonstrated effective and consistent OER performance in highly alkaline conditions, according to experimental data. Due to their high number of active units, good conductivity of the carbon structure, and the synergistic interaction between N-doped carbon layers and cobalt phosphate nanoparticles, the prepared cobalt phosphate catalysts have excellent electrocatalytic OER activity with a low overpotential of 317 mV and a small Tafel slope of 62 mV per decade in 1 M KOH electrolyte. Yuan and colleagues demonstrated that cobalt phosphate carbon material is a great and enticing candidate as the catalyst for the OER process to replace the pricey and uncommon trade noble metal catalysts such as IrO2 for practical electrochemical water electrolysis and generating oxygen for potential implementations in metal-air batteries [40]. As mentioned before, due to the slow kinetics of HER and OER reactions, there is a need for new catalysts, and one of the catalysts that can be used for eliminating the need is bifunctional catalysts [35]. For extremely effective and environmentally friendly energy conversion processes, the invention of bifunctional electrocatalysts made using 3D transition metal-based materials for both OER and HER may be a solution [32]. Bifunctional electrocatalysts free of noble metals hold promise as cost-effective, energy-efficient, and rechargeable metal-air battery materials. In addition, by exploiting quick reactions of metal ions in the catalyst layer, facilitated by OER/ORR reactions, the attributes of these metal-air batteries, such as power density, energy density, and cycle life, can be further enhanced. Senthilkumar et al. suggested mixed metal phosphates of cobalt and nickel as multifunctional air cathodes with bifunctional and electrocatalytic activity in their research. In this study, Nix Co3−x (PO4 )2 particles of submicron size were produced utilizing the combustion synthesis method. By altering the Ni-to-Co ratio, the electrocatalytic activity of the material produced for OER/ORR was carefully studied. This material offered a specific capacity of 110 mAh/g when used as a catalyst material under optimum conditions. The air cell developed using this catalyst exhibited more than 78% energy efficiency with stable cycle performance [47]. Bhanja and coworkers synthesized three novel transition metal-based phosphonate compounds such as cobalt phosphonate, nickel–cobalt phosphonate, and nickel

1.0 M KOH 1.0 M PBS 0.1 M PBS 1.0 M KOH 0.5 M NaOH 1.0 M KOH

Solution combustion synthesis process

Hydrothermal process

Precipitation method

Precipitation method

In situ vertical growth

Calcination

Hydrothermal process

Electrodeposition method

Electrodeposition method

Precipitation method

Hydrothermal process

Electrodeposition method

Hydrothermal process

Hydrothermal reaction

NaCoP2 O7

Cobalt phosphonate Co3 (O3 PCH2 –NC4 H7 –CO2 )2 4H2 O

4NiH–PO4 ·Ni3 (PO4 )2 ·22H2 O

Nickel (II) phosphate

FePO4 /NF

Co3 (PO4 )2 –Co3 O4 MNSN

Co3 (PO4 )2 @N–C

CoPi NA/Ti

Co-Pi/TM

FeCo phosphate

Iron selenophosphate

Ni–Fe phosphate film/Ni foam

3D flower like nickel phosphate

CoEDA

70.1

91

88

117

35.9

183

187

62

39

42.72

49

48

83

52

51

Tafel slope (mV dec−1 )

318

442

376

430

273

430

450

317

270

218

70

360

484

386

365

OER overpotential

[12]

[46]

[45]

[44]

[43]

[42]

[41]

[40]

[39]

[32]

[38]

[37]

[36]

[35]

[35]

References

CoEDA: Cobalt phosphonate, CoPi NA: Cobalt phosphate nanoarray, Co-Pi/TM: Cobalt phosphate nanowires films on Ti mesh, Co3 (PO4 )2 @N–C: Co3 (PO4 )2 decorated with nitrogen-doped carbon; dec: decade, MNSN: mesoporous nanosheet networks, NF: Nickel foam, PBS: Phosphate buffer solution

1.0 M KOH

1.0 M KOH

1.0 M KOH

1.0 M KOH

0.1 M KOH

0.1 M KOH

0.1 M PBS

1.0 M NaOH

1.0 M NaOH

Solution combustion synthesis process

Na2 CoP2 O7

Electrolyte

Synthesized method

Electrocatalysts material

Table 1 Metal phosphate and metal phosphonate-based electrocatalysts for OER

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phosphonate via a hydrothermal process. In their synthesis reactions, etidronic acid (1-Hydroxyethane 1,1-phosphonic acid) was used as the organophosphorus resource at pH 6.5. Amongst these phosphonate compounds, the structural analysis showed that cobalt phosphonate has a superior surface area with convenient microporous canals. In the electrochemical investigations, cobalt phosphonate ensured perfect electrocatalytic efficiency toward OER with a Tafel slope of 70.1 mV/dec and a lower overpotential of 318 mV. The excellent catalytic efficiency towards the OER process can be correlated with the existence of the inorganic–organic hybrid materials involving the Co–O–P bond and the genesis of extremely electroactive cobalt oxyhydroxide compounds on the pore barrier of the microporous canal. Also, obtained cobalt phosphonate catalysts represent distinguished electrochemical stability for 25 h with a very little alteration in current density of only 6% [12]. According to Xie et al. research’s, the cobalt phosphite nanoarray on the Ti network substrate was topotactically transformed into a cobalt phosphate nanoarray (Co-Pi NA) in the phosphate buffer by oxidative polarization. Co-Pi NA/Ti used as the catalyst electrode for a 3D OER at neutral pH exhibited an exceptionally high catalytic activity. The designed electrode only requires a 450 mV overpotential to operate and also offers a geometric catalytic current density of 10 mA/cm2 . Moreover, as can be seen in Fig. 3, SEM, TEM, and XRD analysis provide another piece of evidence to support the chemical production of cobalt phosphate nanoarrays. The good catalytic activity provided by Co-Pi NA/Ti is based on the presence of more active sites in this type of 3D nanoarray configuration, allowing electrolytes and oxygen to diffuse more easily [41].

Fig. 3 a XRD patterns for blank Ti mesh, CoP NA/Ti, and Co-Pi NA/Ti. b, c SEM images of b CoP NA/Ti and c Co-Pi NA/Ti. d, f TEM images of d CoP and f Co-Pi nanowire. e, g SAED patterns for e CoP and g Co-Pi nanowire. Adapted with permission [41]. Copyright (2017), Wiley

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In their research, Yang et al. developed a novel method for improved water splitting employing 2D amorphous FePO4 nanolayers on Ni foam (Am FePO4 /NF). Due to the synergistic effects of the 2D morphology, amorphous structure, conductive substrate, and Ni–Fe mixed phosphate, the produced structure exhibited higher electrocatalytic activity against OER and HER, according to data from experimental studies and density functional theory calculations. Additionally, the Am FePO4 /NFs electrocatalyst used in this work at both electrodes performed better than the bifunctional electrocatalyst reported in the literature at current densities of 10 and 100 mA/cm2 . The data gathered in the current work could be useful for developing 3D electrocatalysts based on transition metals [32]. For the OER half-reaction of water electrolysis, iron-containing nickel-based compounds are promising catalysts. Furthermore, nickel has increased catalytic activity for methanol oxidation when it is present as a bimetallic catalyst. In their study, Candelaria et al. synthesized bimetallic iron-nickel nanoparticles in water under ambient conditions. The synthesized bimetallic material shown significantly greater catalytic activity for both OER and methanol oxidation than the monometallic structures of iron and nickel. At 1 mA/cm2 , the overpotential for a monometallic iron particle is 421 mV, while the overpotential for a monometallic nickel particle is 476 mV. Bimetallic Fe–Ni particles synthesized in the study have an overpotential of 311 mV at 10 mA/cm2 . The obtained results showed that bimetallic particles can be useful for various alkaline electrochemical applications [48]. Liu and coworkers designed new mesoporous nanosheet materials involving cobalt phosphate (Co3 (PO4 )2 ) and cobalt oxide (Co3 O4 ) utilizing a simple hydrothermal process via calcinating the cobalt hydroxy carbonate, carbon, and cobalt phosphate composite precursor. Transfer of electron from Co3 O4 to Co3 (PO4 )2 based on lewis acid–base interaction, which made Co3 O4 molecule more acidic as Lewis acid and simplified the activation of H2 O molecules as Lewis base cause to an advanced high OER activity of the hybrid material with an overpotential of 270 mV at a current density of 10 mA/cm2 , and a Tafel slope of 39 mV/dec in alkaline mediums [39]. In recent years, active electrocatalysts with structural properties similar to photosynthesis II systems (PS-II) have attracted attention due to their ability to efficiently catalyze OER, to increase energy conversion efficiency. In their study, Shao et al. designed Crystalline cobalt phosphate nanolayers that act as an efficient OER catalyst in a neutral environment. According to experimental and computational findings, the active sites derived from molecular PS-II’s shape promote OER reaction intermediate adsorption, lower the energy barrier, and enhance OER kinetics [49]. Bifunctional catalyst materials based on noble metal-free are very important to developing low-cost and high-performance water-electrolysis systems. Gond and coworkers investigated the two sodium cobalt phosphate compounds (NaCoPO4 and Na2 CoP2 O7 ) as a bifunctional electrocatalyst. The electrocatalytic efficiencies of Na2 CoP2 O7 and NaCoPO4 were studied utilizing linear sweep voltammetry. The obtained results showed that these phosphate compounds can be used as electrocatalysts for effective ORR and OER activity in alkaline mediums because of their good stability and bifunctional activity [35].

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Depending upon the different metal precursors as well as experimental conditions, the synthesized catalysts can be acquired with the discriminating nano-structural motifs which will have tunable microporosity with a high surface area and can be influenced for the catalytic activity towards OER [50]. Zhu et al. [50] synthesized novel iron-anchored microporous cobalt-based phosphonate materials under the hydrothermal reaction condition using nitrilotri(methylphosphonic acid) in absence of any structure-directing agent. Moreover, obtained cobalt-based phosphonate material displayed perfect catalytic performance towards electrochemical OER with the low overpotential value of 289 mV at the current density of 10 mA/cm2 in 1.0 M basic solutions. In their studies, the Tafel slope was 59.3 mV/dec [50].

4 Conclusion Nowadays, attention to metal phosphate/phosphanate based materials has increased remarkably because of their original chemical and physical features. These materials have excellent potential in different energy storage and conversion apparatus such as supercapacitors, ion batteries, rechargeable metal-air batteries, and watersplitting applications. The surface area of the electrode material and diffusion of ions in electrodes is related to the structure of the phosphate/phosphonate material. Functionalization and modification of metal phosphate/phosphonates can be carried out to improve their activity and efficacy. Despite all of these efforts, there is still much that can be learned about transition-metal phosphate/phosphonatebased materials, particularly those that are used in electrocatalysis applications. To develop extensive and useful applications in sustainable energy storage and conversion devices, great efforts are required to create simple and scalable methodologies for designing transition-metal phosphates/phosphonate-based materials. Designing new metal phosphate and phosphonate-based micro-and mesoporous nanoarchitectures for adsorption, catalysis, biomedical applications, electrochemical cells, optoelectronics, and fuel cells will anymore prosper the chemistry of these porous materials in the future. Acknowledgements The Council of Higher Education (YOK) was greatly appreciated for supporting scholarships under the special 100/2000 scholarship program to Cem Erkmen. Cem Erkmen also thanks to the financial support from TÜB˙ITAK under the BIDEB/2211-A doctoral scholarship program.

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Fundamentals of Electrochemical Energy Devices Abhinay Thakur and Ashish Kumar

Abstract Due to their enormous theoretical capability and potential application in electrochemical energy systems, curiosity about metal phosphates and phosphonates has considerably expanded over the past couple of years in light of the significance of renewable power, economic, and ecological challenges. Several of these metal phosphates, phosphides, and hybrids outperform transition metal hydroxides/oxides, a family of electrode materials that have received substantial study for supercapacitor applications. We thoroughly cover the properties and fundamentals of metal phosphates with regard to electrochemical energy devices in this chapter. The synthetic procedures used to manufacture these metal phosphides and phosphates, as well as the underlying scientific principles, are explored since they have a significant impact on the effectiveness of electrochemical energy retention. Several aspects of metal phosphates-based electrochemical devices along with their basic operating principles and material compositions have been examined. Keywords Electrochemical · Energy · Metal phosphates · Efficacy · Power density · CV

1 Introduction The significant rise in electrical energy utilization over the past 50 years is attributed mostly to the tremendous expansion in the world’s demographic, especially in emerging countries, and technical advancements. As a consequence of continued population expansion (the world’s demographic is forecast to hit 9 billion by 2048) and rising electrical energy requirements in the commercial and industrial segment, the world’s energy usage is anticipated to rise [1–3]. By the midst of the century, it A. Thakur Department of Chemistry, School of Chemical Engineering and Physical Sciences, Lovely Professional University, Phagwara, Punjab 144411, India A. Kumar (B) NCE, Department of Science and Technology, Government of Bihar, Chandi, Bihar 803108, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_5

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is predicted that energy consumption will have doubled, and by the turn of the era, it will have tripled. At the moment, fossil fuels including gas, oil, and coal serve as the world’s main sources of energy, accounting for more than 85% of all energy production [4–6]. The excessive dependency on fossil fuels for energy generation has resulted in a variety of ecological issues, such as poor air quality, unanticipated climate changes, water or soil contaminants, and a massive rise in greenhouse gas releases. The use of more environmentally friendly, low-carbon, or even decarbonized renewable sources like solar, wind, and tidal is crucial to halting or reversing these patterns. Moreover, because these renewable energy sources are site-specific and occasionally erratic, operation agility is essential for integrating them into energy systems. This adaptability is possible by utilizing the right energy retention systems, which may manage variations and imbalances in energy production and consumption [7, 8]. By closing the deficit between generation and usage, these technologies assist to increase the efficiency of the generation and energy distribution systems. Additionally, when the amount of energy supplied to the electric grid increases due to the penetration of solar resources like solar and wind power, some abridgments could be avoided by adopting the right storage solutions. Energy retention and transition strategies come in a wide variety, comprising mechanical, chemical, thermochemical, electrical, and electrochemical storage arrays among them. Owing to its remarkable columbic efficiency, electrochemical energy retention and transmission technologies, including electrochemical batteries/capacitors and fuel cells, have attracted the greatest interest and have been used in a variety of everyday purposes [9]. Due to the wide range of uses for electrochemical energy devices, such as sodium or Li-ion batteries and supercapacitors, comprising electrical gadgets, electric cars, and energy storage systems (ESSs). Due to their enormous theoretical capacities (3862 mAh/g for Li-ion batteries and 1165 mAh/g for Na-ion batteries), extensive research has been done on the creation of enormous sodium/lithium ionbased electrochemical energy systems. Although electrochemical energy gadgets have advanced significantly over the past 20 years, they are still not suited for use in elevated ESSs or electric vehicles that need a prolonged consistency or a huge battery volume [10]. Further precisely, graphite, that is presently accessible commercially, serves as the anode material in Li-ion batteries. Although they possess a considerably reduced theoretical potential of 372 mAh/g, Li ions are deintercalated and intercalated throughout the charge and discharge cycle to generate a steady electrochemical response. Moreover, due to the enormous mass of Na ions, it is challenging to employ the identical graphite (d002 = 0.334 nm) utilized in Li-ion batteries as a negative electrode element in Na-ion batteries [9, 11]. Additionally, the insufficient energy density of commercialized supercapacitors made of carbon-based substances is problematic. It makes sense to look for innovative electrochemical devices that can be used in electrochemical systems to have better efficiency. Metal phosphates, one of the several capacitive alternative substances, have drawn a lot of interest because of their superior capacity to store electrons and emit light when subjected to a chemical redox process in an energy storage system. Furthermore, because of the conversion events that take place with the guest ions,

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a metal phosphates-based electrode substance experiences significant volume variations throughout the discharge/charge operation (for example, delithiation/lithiation and desodiation/sodiation). Research on the production of a composite containing metallic phosphates has been conducted in order to address the issues of poor electric conductivity and substantial volume shift of the active materials throughout the discharging/charging operation of the energy device, that are drawbacks of metal oxides. Numerous metal phosphides and phosphates exhibit considerably improved electrical conductivity or greater flexibility in optimizing the meso and nanostructures when contrasted to transition metal hydroxides/oxides and conducting polymers, which could provide greater electrochemical energy storage capabilities resulting from the Faradaic redox reactivity than either carbon-based material. In comparison to popular conducting polymers like polypyrrole (PPy), polyaniline (PANI), polythiophene, and its variants, they are also far more resilient. In fact, throughout the deintercalating and intercalating processes, conducting polymers may expand and contract, which causes mechanical deterioration and hence poor cycling resilience [3, 12–15]. Metal phosphides and phosphates are significant factors as a class of electrodes wherein both decent inter- and intra-particle conductivity are needed because they share similarities with carbon-based components in that their nanomaterials and mesostructure are strongly controllable for enhanced availability by electrolytes. Transition metal hydroxides/oxides and conducting polymers, most of which have been intensively investigated for the hybrid-type electrode substances angling for enhanced electrochemical effectiveness. Many of the metal phosphides and phosphates will be preferable replacements when they are fully developed, even if significant advancements have been made with transition metal hydroxides/oxides, carbon-based materials, conducting polymers, and their composites. Using a metal–organic framework (MOF)-based nanosheet method, Li et al. [16] presented a one-step production of nanosheets combining cobalt phosphides and amorphous carbon (indicated as CoP-NS/C). Figure 1 depicts the method for the synthesis conceptually. ZIF-67 was created in a conventional procedure by reacting with a combination of 2-methyl imidazole and Co2+ . Following solvothermal processing, ZIF-67 nanosheets (referred to as ZIF-NS) were created. Utilizing NaH2 PO2 as the phosphorus resource, ZIF-NS were subsequently converted into CoP-NS/C via moderate temperature phosphodation. This method of creating a novel type of metal phosphide ultrathin nanosheets proved simple and controlled. The extensively accessible adsorption sites in the MOF-derived porous crystalline CoP nanosheets are guaranteed, and the presence of carbon makes the composites conductive, which is essential for electron transmission. With Tafel inclines of 64 and 59 mV/dec and a current concentration of 10 mA/cm2 at overpotentials of 292 and 140 mV, in both, it exhibits outstanding electrocatalytic outcomes for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline and acidic medium. These productions are noticeably better than those of CoP/C, CoP particulate, and similar to those of conventional noble-metal catalysts. Following a lengthy evaluation, the CoP-NS/C also displays good resilience. Because of this, the enhanced CoP-NS/C demonstrated good electrocatalytic capabilities and significant

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Fig. 1 Depiction of manufacture process for CoP-NS/C utilized as the OER and HER catalyst. Figure adapted with permission [16]. Copyright (2018), MDPI. Distributed under a Creative Commons Attribution License 4.0 (CC BY)

durability for both OER and HER in alkaline and acidic conditions, accordingly. Its electrocatalytic efficiency in OER was noticeably better than that of commercialized IrO2 . Utilizing phytic acid, melamine, and ferric trichloride as precursors, Rong et al. [17] created nitrogen/phosphorus/iron co-doped carbon electrocatalysts (NPFe-C) having multilevel porous morphology utilizing the self-template approach. Therefore, self-template facilitated fabrication was used to create a 3D N/P/Fe co-doped substance with appropriate pore diameter and heteroatom dispersion. The surface region and porous construction were designed to enhance the active areas accessible to the interface of the catalytic process. To create a phytic acid (PA), hydrogel and melamine combine with Fe. In the pyrolysis procedure, the breakdown of melamine using PA might produce a lot of small-sized mesopores and micropores, exposing the doped atoms at the 3-phase border and producing a lot of active areas that are accessible (Fig. 2). It enhanced surface area, discloses more active areas, and encourages the creation of carbon porous frameworks. The ORR half-wave potential in an alkaline solution is 0.867 V (vs. RHE), which is comparable to that of platinum-based catalysts. Interestingly, NPFe-C operates superior in respect of specific capacity and power density than the conventional Pt/C catalyst. The enhanced effectiveness could be ascribed to the preceding factors: (1) P-, Fe- and N-triple doping formed abundant active surface area, devoting to the catalyst’s greater intrinsic action; and (2) The heteroatom doping function endows the catalyst to higher mass transition capacity and plenteous available reaction mechanism. (3) Melamine and power density were crosslinked to create hydrogel, and the carbonized by-products of this process had large specific surface areas, which is advantageous for exposing a lot of active spots at the reaction junction. (4) The porous 3D carbon matrix makes it easier to transmit electric charge as well as reactants, intermediaries, and products. As a

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Fig. 2 Illustration of the preparation of the NPFe-C. Figure adapted with permission [17]. Copyright (2022), MDPI. Distributed under a Creative Commons Attribution License 4.0 (CC BY)

result, Fe/N/P Co-doped 3D porous carbon materials generated by a simple and reproducible pyrolysis process show promise for use in the transmission and retention of energy. The ORR performance of NPFe-C was initially evaluated by cyclic voltammetry (CV) in 0.1 M KOH media saturated using N2 /O2 (Fig. 3a). In the O2 -saturated electrolyte, a cathode spike was found at 0.81 V. In comparison, these cathode spikes were not present in the electrolytes that were N2 -saturated. This demonstrates the electrocatalytic properties of NPFe-C for ORR. Spinning disc electrode experiments were used to study the ORR efficiency of NPFe-C electrocatalysts (Fig. 3b). The polarization graphs reveal that the electrocatalytic performance of the NPFe-C catalysts was even similar to that of the commercialized catalyst at a half-wave potential of 0.867 V when contrasted to NP-C (E1/2 = 0.712 V). They noticed that the ORR performance of N/P/Fe–C was considerably impacted by the annealed temperature. The catalyst having the highest efficiency was achieved at 800 C as per the E1/2 frequency of ORR and the minimum current density. Additionally, the efficiency of the ORR and the N/P/Fe–C synthesis were both strongly impacted by the reagent ratios. In comparison to the specimens having the ratios of 0.5 g:0.06 mL, 0.5 g:0.04 mL, 0.5 g:0.15 mL, and 0.5 g:0.2 mL, the ORR activities of the specimens with the proportion of 0.5 g:0.1 mL of melamine to phytic acid was substantially superior. The ORR function appears to rise with the addition of more FeCl3 , however, adding more FeCl3 than 0.12 g prevents the formation of the hydrogel. In saturated 0.1 M KOH media, the ORR polarisation curves at various rotational velocities were obtained to better examine the electron transmission of NPFe-C catalysts. The electron transport ratio was near 4, suggesting strong specificity for the entire four-electron oxygen reduction reaction when the kinetic variables were examined utilizing the Koutecky Levich (K-L) equation (Fig. 3c, d). The rotating ring disc electrode (RRDE) was then used to evaluate the selectivity of ORR (Fig. 3e, f). Over 0.8 V, the disc current density (ID) ascended significantly. The disc current (IR), which corresponds to a decreased H2 O2 output of 7–12%, was insignificant in the region of 0.2–0.8 V, much like RDE had reported.

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Fig. 3 a CV curves of NPFe-C in an O2 - and N2 -saturated 0.1 M KOH media. b LSV of NPFe-C, NP-C and conventional Pt/C in an O2 -saturated 0.1 M KOH media. c LSV of NPFe-C at various rotation rates. d Koutecky- Levich of NPFe-C. e, f Rotating ring-disk electrode voltammograms attained at 1600 rpm in O2 -saturated 0.1 MKOH media. Figure adapted with permission [17]. Copyright (2022), MDPI. Distributed under a Creative Commons Attribution License 4.0 (CC BY)

As a consequence, the value of n in this possible region was approximately 3.88, which is comparable with the RDE findings. The cobalt-doped nickel phosphite was created by Li et al. [18] for superior electrochemical energy retention. At a power density of 0.60 mW/cm2 , the aqueous apparatus displays an energy density of 15.48 μWh/cm2 . At a power density of 0.60 mW/cm2 , the solid-state device exhibits an energy density of 14.72 μWh/cm2 . At a power density of 0.60 mW/cm2 , the aqueous device displayed an energy density of 15.48 μWh/cm2 (55.72 mWs/cm2 ). Additionally, the solid-state device showed a 14.72 μWh/cm2 (52.99 mWs/cm2 ) energy density at a 0.60 W/cm2 power density. Additionally, For purposes involving hybridized electrochemical energy storage devices, Mirghni et al. [19] reported the impact of various graphene foam (GF) concentrations on the electrochemical impedance of nickel phosphate Ni3 (PO4 )2 nano-rods as an electrode component. Hydrothermal synthesis was used to create pure Ni3 (PO4 )2 nano-rods and Ni3 (PO4 )2 /GF composites having various GF mass loadings of 30, 60, 90, and 120 mg. 3-electrode cells with 6 M KOH electrolyte were used to investigate the electrochemical behavior of pure Ni3 (PO4 )2 and Ni3 (PO4 )2 /GF composites. At a current density of 0.5 A/g, the Ni3 (PO4 )2 /90 mg GF specimen had the maximum specific capacity, 48 mAh/g. In a 2-electrode hybridized asymmetric apparatus, the electrochemical behavior of the Ni3 (PO4 )2 /90 mg GF composite was subsequently examined. With carbonized iron cations (Fe3+ ) immobilized onto polyaniline (PANI) (C-FP) as the anode substance and Ni3 (PO4 )2 /90 mg GF as the cathode, a composite asymmetric system was created and evaluated in a broad potential window region of 0.0–1.6 V utilizing 6 M KOH. This composite device demonstrated long-term cycling reliability and reached maximal energy and power densities of 49 Wh/kg and 499 W/kg, correspondingly, at 0.5 A/g.

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2 Synthesis Methods 2.1 Hydrothermal/Solvothermal Method The typical “hydrothermal/solvothermal” method is to carry out the specified chemical reactions in chosen solvents in sealed containers for transition metal phosphates. There have been reports of numerous forms of cobalt phosphates, such as Co11 (HPO3 )8 (OH)6 , CoHPO4 3H2 O, and Co3 (PO4 )2 , nickel phosphates, such as VSB-5, Ni11(HPO3 )8 (OH), and Ni2 P2 O7 , manganese phosphates, such as MnPO4 H2 O and Mn3 (PO4 )2 , vanadyl phosphates Bimetallic phosphates, such as Co–Ni pyrophosphates, have demonstrated enhanced capabilities when contrasted to mono-metallic phosphates, and these phosphates have been prepared via hydrothermal/solvothermal techniques. Transition metal phosphates could have a variety of stages and topologies depending on the interaction of various mineralizers, precursors, additives, and even solvents [20–22]. Consequently, many formations, such as 1D nanorods/wires, two-dimensional nanosheets/flakes, and even certain micro-sized constructions, could be created using the hydrothermal/solvothermal method. In an experiment, Rahman et al. [23] concentrated on the creation and DFT computations of a multilayer MnO2 redox-active Na3 -δ-MnO2 nanocomposite based on carbon fabric (preparation method indicated in Fig. 4). MnO2 is regarded as the ideal substrate for intercalation because it contains 2D diffusion pathways and is resistant to compositional alterations throughout intercalation. To increase inter-layered separation, optimize the crystalline architecture, and enhance the essential electrochemical behavior of asymmetric aqueous supercapacitors, cation pre-intercalation has been successfully used. Through one-pot hydrothermal production, they were able to effectively create Na+ pre-intercalated δMnO2 nanosheets upon carbon fabric. Their produced substance had a significant specific capacitance of 546 F/g (300 F/g at 50 A/g) as a cathode having an expanded potential window of 0–1.4 V. Additionally, it demonstrated excellent effectiveness (64 Wh/kg at a power density of 1225 W/kg), an unmatched potential window of 2.4 V, and 83% capacitance preservation over 10,000 cycles to a high Columbic efficiency when this cathode was combined with an N-AC anode in an asymmetric aqueous supercapacitor.

2.2 Chemical Precipitation Method The required element is precipitation in a liquid during the chemical precipitation of metal phosphates, which includes liquid blending, polymerization, crystal development, and agglomeration of the basic particulates [24–29]. The chemical precipitation method has the benefits of (i) minimal energy consumption, where the reaction could be done even at ambient temperature; (ii) non-toxic reagents; and (iii) labor savings as a moderate, typically straightforward, and environmentally friendly

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Fig. 4 Production of Na-MnO2 @CC. Figure adapted with permission [23]. Copyright (2022), MDPI. Distributed under a Creative Commons Attribution License 4.0 (CC BY)

method. For instance, the precipitating methods were used to create cobalt phosphate Co3 (PO4 )28H2 O micro-flowers, Co3 (PO4 )2 nanoflakes, and Co3 P2 O88 H2 O particles. Manganese phosphates and nickel phosphates were two additional metal phosphates. The integration of ultrasound into chemical processes and activities is known as sonochemistry. The ultrasound is used to fine-tune nanoparticles having customizable topologies. Colloidal precipitates were produced after sonicating a solution of Na2 HPO4 and Ni(CH3 COO)2 (950 W) for 30 min. The result of the ensuing calcination process was Ni3 (PO2 )4 . By using a simple one-pot hydrothermal method, Padmanathan et al. [30] showed how to create a layer of Ni3 (PO4 )28H2 O nano/micro flakes on nickel foam (NF) and studied this electrode for a variety of uses, such as sweat-based glucose and pH sensors and hybrid energy storage devices, such as supercapacitors. At an induced current of 5 mA/cm2 , the electrode has a specific capacitance of 301.8 mAh/g (1552 F/g) and can maintain 84% of its baseline capacity following 10,000 cycles. Additionally, supercapacitors with activated carbon as the negative electrode and Ni3 (PO4 )28H2 O/NF as the positive electrode may provide significant specific energy of 33.4 Wh/kg having a power of 165.5 W/kg. The Ni3 (PO4 )28H2 O/NF exhibits remarkable sensitivity (24.39 mA/mM cm2 ) and a LOD of 97 nM (S/N = 3) as an electro-catalyst for non-enzymatic glucose sensors. Additionally, the electrode’s sweat-based pH sensor can monitor pH levels in the range of 4–7 in human perspiration. Owing to its superior electrochemical efficiency, this 3D nanoporous Ni3 (PO4 )0.28H2 O/NF electrode could be effectively used for biosensor purposes and electrochemical energy storage. Cobalt phosphate served as the positive electrode in a model supercapattery that Shao et al. [31] created, and AC served as the negative electrode. This positive electrode has the highest specific capacity for a metal phosphate-based electrode, 215.6 mAh/g (1990 F/g). The supercapattery produces significant energy densities of 3.53 mWh/cm3 (43.2 Wh/kg) and 425 mW/cm3 (5.8 kW/kg), respectively. Additionally, the gadget could maintain 79% voltage even following a 4-min self-discharge, which is sufficient to supply the electricity in cardiac emergencies. With 68% of its specific

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capacity remaining upon 100,000 cycles at ambient temperature (25 °C) and up to 81.5% upon 20,000 cycles at 38 °C, this hybrid device exhibits outstanding stability and performance within the physiological temperature range (35–41 °C), proving its efficacy as a prospective source of power for the next era of implanted medical gadgets.

2.3 Other Methods The aqueous-based reflux approach is an easy, inexpensive procedure that yields the required product with fine precision over metal phosphate reaction conditions. VOPO4 2H2 O has been utilized to produce many nanostructured substances. It was possible to create VOPO4 ultrathin nanosheets having a greater surface area and improved electrochemical capabilities by exfoliating bulk VOPO4 2H2 O generated by the reflux procedure. Nix Co3−x (PO4 )2 hollow frameworks that resemble mesoporous shells have been created using a one-pot oil-in-water emulsion method. Additionally, metal phosphates made from other sources are also available. Using cobalt and nickel phosphate as an instance, Co2 P2 O7 could be produced by calcining NH4 CoPO4 H2 O. Additionally, Nix Py Oz , a nickel phosphate generated from Ni-MOF, has been successfully prepared. In one-step electrodeposition at ambient temperature, Huang et al. [15] demonstrated the synthesis of transition-metal phosphate electrodes having an ultrathin sheetlike grid topology. A transition-metal phosphate component of NiCo(HPO4 )23H2 O having an extremely thin nanosheet structure (thickness 2.3 nm) was created and studied as a proof-of-concept. The NiCo(HPO4 )23H2 O electrode exhibits a remarkable rate capability of 1144.8 C g1 at 100 A/g, outstanding electrochemical durability, and an extremely high specific capacity of 1768.5 C/g at 2 A/g, the maximum priority for transition-metal phosphates/phosphides documented to date. Additionally, the homogeneous deposition of the transition-metal phosphate nanosheet arrangement on a variety of conductive substrates proves the applicability of our method. Hence, using this straightforward electrodeposition technique, it is possible to create ultrathin transition-metal phosphate nanostructured nanomaterials for catalytic performance, power storage and transmission, and other electrochemical energy-related devices. As part of their research, Guo et al. [32] investigated the electrochemical efficiency of many nickel phosphide heterostructure composites made utilizing a temperature-programmed phosphating technique in 2 mol/L KOH media. The produced Ni2 P/Ni3 P/Ni (Ni/P = 7:3) exhibited a specific capacitance of 321 mAh g1 over 1 A g1 , and the produced Ni2 P/Ni5 P4 (Ni/P = 5:4) obtained a specific capacitance of 218 mAh/g over 1 A/g. This was due to the interfacial impact that may enhance the catalytic active areas and enhance ion propagation. Figure 5 displays the SEM scans of the single-phase Ni2 P and each Ni/P proportion. Each image shows two unique configurations in contrast to the single-phase nickel phosphide, demonstrating that the composite phase was successfully prepared. The SEM picture with a

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Ni/P proportion of 7:3 is shown in Fig. 5d. The linear component is seen to be encircling the particulate framework in this view, expanding the area of contact across the two. The SEM picture of a Ni/P proportion of 5:4 is shown in Fig. 5f. This picture shows that little particles are distributed amongst the spherical granules and that the hyphal morphology forms on top of them. Such a mixture of several stages will result in lattice strain at the interfaces, increasing the catalytic performance and charge transfer effectiveness. The SEM picture of a 3:1 Ni/P proportion is shown in Fig. 5a. This graphic demonstrates how such conditions result in the generation of braided linear patterns and granular formations that overlap. The SEM picture of a 5:2 Ni/P proportion is shown in Fig. 5b. From this picture, it could be seen that agglomerated and granular patterns develop in this situation, with the majority of the acicular structure being embedded among big and tiny particles. The SEM picture with a Ni/P proportion of 12:5 is shown in Fig. 5c. This image shows the formation of prismatic and granule formations in this context, including granular formations encircling the prism. Additionally, a variety of tiny acicular components work together to generate the prism. The SEM picture of a 2:1 Ni/P proportion is shown in Fig. 5e. The equiaxed and granular formations are connected in this micrograph, as could be seen. The SEM picture of a 1:1 Ni/P proportion is shown in Fig. 5g. This image shows that the lamellar component and the bean-sprout-shaped architecture are merged. The single-phase Ni2 P SEM picture is shown in Fig. 5h. This graphic shows that the distribution of massive and tiny particles is not uniform. 76% of the specific capacity was still present following increasing the current density from 0.5 to 5 A/g. The capacity retaining performance was higher than 82% following 7000 cycles. The electrochemical behavior of Ni2 P/Ni5 P4 and Ni2 P/Ni3 P/Ni was significantly superior to that of single-phase N2 P caused by the temperature-splitting phenomenon. The Ni2 P/Ni5 P4 /AC produced an energy density of 27 Wh/kg and a power density of 800 W/kg and Ni2 P/Ni3 P/Ni/AC exhibited an energy density of 22 Wh/kg and a power density of 800 W/kg. These results were obtained following integrating the produced composite and AC into a supercapacitor.

3 Fundamental Mechanisms 3.1 Intercalation Mechanism Metal phosphides (MPn) undergo topotactic Li intercalation throughout the reduction reaction, which results in the disruption of P–P bonds in the metal phosphides and the production of LixMPn, which is then reoxidized to MPn throughout the charging process avoiding rupturing the M–P bonds. As a consequence, the insertion reaction produces Lix MPn , which has good electrochemical characteristics and a robust crystalline structure. The response might be expressed as: MPn + xLi+ + xe− ↔ Lix MPn

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Fig. 5 SEM scans of several Ni/P proportions and single-phase Ni2 P. a–g SEM scans of Ni/P proportions 3:1, 5:2, 12:5, 7:3, 2:1, 5:4, and 1:1. h SEM scans of single-phase Ni2P. Figure adapted with permission [32]. Copyright (2022), MDPI. Distributed under a Creative Commons Attribution License 4.0 (CC BY)

In particular, the simple Li-ion intercalation process results from the presence of a di-phosphorous pair, specifically a remnant P atom that is physically bound to a metal ion. Investigators discovered that an electrochemical redox mechanism led to the topotactic crystalline transition of MnP4 and Li7 MnP4 at room temperature after studying the low-potential Li+ intercalation in a solid-state MnP4 (0.57–1.70 V). When Li was inserted into the MnP4 framework, the P–P bonds were broken, forming the crystallized Li7 MnP4 , and they were then reversed following reoxidation, which was connected to the electrochemical recrystallization of MnP4 . MnP4 served as an electron storage buffer as a result. By using X-ray diffraction (XRD) analysis, Kim et al. looked into the Li storage process of teardrop-shaped SnP0.94 particles. The XRD revealed that even after repeated cycling, the SnP0.94 ’s initial composition persisted. Additionally, rather than through bulk lattice architectural change of the layered SnP0.94 , the charge equalization by Li-ion insertion took place in the short-range-ordered architecture surrounding the Sn. The polymeric –[Sn–P–P–Sn]– slabs were not disrupted by the Li-ion activity in the SnP0.94 system, according to XRD spectra, which also revealed that the Sn oxidation status has not changed. It was also discovered that SnP0.94 was a promising intercalation substance for LIBs because it demonstrated excellent electrochemical cycling capabilities as a consequence of the architectural tunability of the lithium deintercalation/intercalation process across molecular pathways without a phase transformation. In a moderate oxalic acid environment, Wu et al. [14] created a novel route for the layered acid niobium phosphate (2NbOPO4 H3 PO4 H2 O) production. By utilizing this technique, self-standing electrodes for solid-state supercapacitors can be created by in situ growing sub-5 nm 2NbOPO4 H3 PO4 H2 O nanosheet (NPene) arrays on conductive carbon fiber cloth (CFC) substrates. It’s worth noting that in aqueous media, the

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NPene@CFC electrode displays a standard cation (H+ or Li+ )-intercalation dynamics having a broad potential window of 0–1.0 V. The solid-state asymmetric supercapacitors made from such an NPenes@CFC electrode exhibit exceptional wearable properties, an excessive working potential of 2.0 V, an energy density of 122.2 Wh/kg at a power density of 589.7 W/kg, cycle stability with a capacitance retention of 94.2% after 10,000 cycles, and cycle stability with a capacitance retention of 94.2% upon 10,000 cycles.

3.2 Conversion Reaction Mechanism As a consequence of the electrochemical process, which results in the creation of Liphosphides and nanoscale metal particulates throughout the discharge/charge cycles. The following format could be used to express the reaction: MPn + 3nLi+ + 3ne− ↔ nLi3 P + M The redox character of the P dominates the interaction among Li and metal phosphides, and the capacities rely on how many electrons the anion has. Certain metal phosphides experience the direct creation of metallic particles and Li3 P throughout the conversion reaction conditions, whilst others begin with deposition and conclude with conversions throughout a continual discharge/charge phase with regard to potential versus Li/Li+ . Contrary to monoclinic NiP2 , which underwent an intercalation phase accompanied by a conversion phase in the first discharge, cubic NiP2 underwent a straight conversion process, according to Gillot et al. Electronic configuration estimates were used to explore the Li-uptake method, which was based on subtle structural reforms. The results demonstrate that the cubic NiP2 was more likely to undergo a direct conversion response into Ni and Li3 P because of its densely packed configuration, which prevented any Li+ insertion. However, several of the monoclinic NiP2’s diffusion layer was still exposed to Li+ , which encouraged the creation of the monoclinic Li2 ·5NiP2 stage. CoP, CoP3 , Co2 P, and Cu3 P electrodes undergo a direct conversion procedure to achieve their potential Li uptake. Moreover, throughout the discharging and charging operations, sequential insertion and conversion occur after the reactivity of CoP, Cu3 P, NiP3 , FeP, and Sn4 P3 . Using (NH4 )(Ni,Co)PO4 ·H2 O nanosheets @ single crystal microplatelets (NH4 )(Ni,Co)PO4 H2 O (NCoNiP@NCoNiP), Wang et al. [33] created a unique interface-rich core configuration using a simple two-step hydrothermal technique, making use of the Kirkendall phenomenon and Ostwald ripening caused by etching. By adding additional charges (such as pits, electrons, or gaps) to the interface, this particular arrangement could achieve synergy and quick charge retention. In particular, a 3-electrode experiment showed a maximal specific capacity of 190.3 mAh/g and ultrahigh conversion efficiency exhibiting capacity retention of 96.1% over 1– 10 A/g. According to the kinetics model, the electrochemical reaction of the hybrid battery-supercapacitor storage systems exhibits clear supercapacitor characteristics,

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particularly at higher scanning speeds. While concurrently displaying a remarkable energy density of 44.5 Wh/kg, hybrid battery-supercapacitor devices constructed on NCo-NiP@NCoNiP at 150 W/kg of power density, which keeps a 30Wh/kg at a high 7.4 kW/kg power density with a 77.5% retention of capacitance following 7000 cycles. By facilitating the capacitive charge retention process of rechargeable batteries by nanostructure engineering, this research offers a unique approach to the usage of battery materials in elevated power applications. For the production of nickel–cobalt phosphate 2D nanosheets, Li et al. [6] developed a straightforward one-step hydrothermal method. Using electrochemical evaluation, the morphological impact on the pseudocapacitive behavior of the resulting nickel–cobalt phosphate was examined. Having a maximal specific capacitance of 1132.5 F/g, it is discovered that ultrathin nickel–cobalt phosphate 2D nanosheets having a Ni/Co proportion of 4:5 exhibit the greatest electrochemical efficiency for energy storage. The assembly of an aqueous and solid-state flexible electrochemical energy storage system is more significant. A significant energy density of 32.5 Wh/kg at a power density of 0.6 kW/kg was displayed by the aqueous device, while a great energy density of 35.8 Wh kg−1 at a power density of 0.7 kW/kg was displayed by the solid-state device. These outstanding results demonstrate the potential of the nickel–cobalt phosphate 2D nanosheets as materials for electrochemical energy storage technologies.

4 Conclusion For use in electrochemical energy preservation, phosphates, phosphatides, and their hybrids combined with other functional regimes have received extensive research. These phosphide and phosphate equivalents demonstrate significantly improved technical efficiency in various particular features and important metrics as compared to the transition metal hydroxides and oxides, which have been quite well documented up to this point. First off, the wide range of compounds and structural variations among metal phosphides and phosphates allows for the customization of a variety of technical factors, paving the path for their continued advancement and advancement in supercapacitor uses. The open framework of metal phosphates allows for outstanding ion/charge conductivity and plenteous active places, which, along with other special characteristics including the chemically steady configuration resulting from the comparatively potent P–O covalent bonds in phosphates, create phosphates/phosphides encouraging as a subject of electrochemical devices for electrochemical energy retentions. Numerous metal phosphides are recognized to acquire outstanding electric conductivity. A large-scale study on many novel metal phosphates, phosphides, and their composites has only lately begun. However, a significant advance has been accomplished in comprehending their inherent geometries and characteristics, particularly at the nanoscale and micrometer scales. With the refinement of various metal phosphate and phosphide compositions as well as

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the technological advancements concerned, particularly those aiming for greater efficiency as electrodes, fantastic improvement has proceeded unquestionably swiftly. Since a powder having a nanostructure has a lower density than the identical substance with micrometer-sized components, reduced volume energy density is the additional difficulty for nanostructured metal phosphides. It could be lessened by creating hierarchical micro-/nanoarchitecture containing fundamental construction elements that are nanometer in size, as well as with a well-delineated interior void region. Furthermore, the investigation of binders related to the metaphase of electrode substances will be crucial. Additionally, more research is needed to create metal with optimized nanoarchitectures and great efficiency for the industrialization of metal phosphides. This requires developing cost-efficient and large-scale synthesis methodologies.

References 1. Raju, K.: Ammonium metal phosphates: emerging materials for energy storage. Curr. Opin. Electrochem. 21, 351–357 (2020) 2. Baumann, A.E., Burns, D.A., Liu, B., Thoi, V.S.: Metal-organic framework functionalization and design strategies for advanced electrochemical energy storage devices. Commun. Chem. 2, 1–14 (2019) 3. Sumboja, A., Liu, J., Zheng, W.G., Zong, Y., Zhang, H., Liu, Z.: Electrochemical energy storage devices for wearable technology: a rationale for materials selection and cell design. Chem. Soc. Rev. 47, 5919–5945 (2018) 4. Kuznetsov, V., Fesenko, G., Schwenk-Ferrero, A., Andrianov, A., Kuptsov, I.: Innovative nuclear energy systems: state-of-the art survey on evaluation and aggregation judgment measures applied to performance comparison. Energies 8, 3679–3719 (2015) 5. Ambrozkiewicz, B., Litak, G., Wolszczak, P.: Modelling of electromagnetic energy harvester with rotational pendulum using mechanical vibrations to scavenge electrical energy. Appl. Sci. 10, 1–14 (2020) 6. Li, B., Gu, P., Feng, Y., Zhang, G., Huang, K., Xue, H., Pang, H.: Ultrathin nickel–cobalt phosphate 2D nanosheets for electrochemical energy storage under aqueous/solid-state electrolyte. Adv. Funct. Mater. 27 (2017) 7. Sui, Y., Zhou, J., Wang, X., Wu, L., Zhong, S., Li, Y.: Recent advances in black-phosphorusbased materials for electrochemical energy storage. Mater. Today 42, 117–136 (2021) 8. Wang, F., Zhao, H., Liang, J., Li, T., Luo, Y., Lu, S., Shi, X., Zheng, B., Du, J., Sun, X.: Magnetron sputtering enabled synthesis of nanostructured materials for electrochemical energy storage. J. Mater. Chem. A 8, 20260–20285 (2020) 9. Kyne, D., Bolin, B.: Emerging environmental justice issues in nuclear power and radioactive contamination. Int. J. Environ. Res. Public Health 13 (2016) 10. Bao, X., Bin Zhang, W., Zhang, Q., Zhang, L., Ma, X.J., Long, J.: Interlayer material technology of manganese phosphate toward and beyond electrochemical pseudocapacitance over energy storage application. J. Mater. Sci. Technol. 71, 109–128 (2021) 11. Agrawal, R., Wang, C.: On-chip asymmetric microsupercapacitors combining reduced graphene oxide and manganese oxide for high energy-power tradeoff. Micromachines 9 (2018) 12. Tiwari, A.P., Novak, T.G., Bu, X., Ho, J.C., Jeon, S.: Layered ternary and quaternary transition metal chalcogenide based catalysts for water splitting. Catalysts 8 (2018) 13. Naveed, A., Yang, H., Yang, J., Nuli, Y., Wang, J.: Highly reversible and rechargeable safe Zn batteries based on a triethyl phosphate electrolyte. Angew. Chem. Int. Ed. 58, 2760–2764 (2019)

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14. Wu, Z., Jiang, L., Tian, W., Wang, Y., Jiang, Y., Gu, Q., Hu, L.: Novel sub-5 nm layered niobium phosphate nanosheets for high-voltage, cation-intercalation typed electrochemical energy storage in wearable pseudocapacitors. Adv. Energy Mater. 9 (2019) 15. Huang, J., Xiong, Y., Peng, Z., Chen, L., Wang, L., Xu, Y., Tan, L., Yuan, K., Chen, Y.: A general electrodeposition strategy for fabricating ultrathin nickel cobalt phosphate nanosheets with ultrahigh capacity and rate performance. ACS Nano 14, 14201–14211 (2020) 16. Li, H., Ke, F., Zhu, J.: MOF-derived ultrathin cobalt phosphide nanosheets as efficient bifunctional hydrogen evolution reaction and oxygen evolution reaction electrocatalysts. Nanomaterials 8 (2018) 17. Rong, Y., Huang, S.: Self-templating synthesis of N/P/Fe Co-doped 3D porous carbon for oxygen reduction reaction electrocatalysts in alkaline media. Nanomaterials 12 (2022) 18. Li, B., Shi, Y., Huang, K., Zhao, M., Qiu, J., Xue, H., Pang, H.: Correction to: cobaltdoped nickel phosphite for high performance of electrochemical energy storage. Small 14, 13, (1703811) (2018); Small 17, 59–60 (2021) 19. Mirghni, A.A., Madito, M.J., Oyedotun, K.O., Masikhwa, T.M., Ndiaye, N.M., Ray, S.J., Manyala, N.: A high energy density asymmetric supercapacitor utilizing a nickel phosphate/graphene foam composite as the cathode and carbonized iron cations adsorbed onto polyaniline as the anode. RSC Adv. 8, 11608–11621 (2018) 20. Thakur, A., Kumar, A., Kaya, S., Vo, D.V.N., Sharma, A.: Suppressing inhibitory compounds by nanomaterials for highly efficient biofuel production: a review. Fuel 312, 122934 (2022) 21. Kumar, A., Thakur, A.: Encapsulated Nanoparticles in Organic Polymers for Corrosion Inhibition. Elsevier Inc. (2020) 22. Thakur, A., Kaya, S., Kumar, A.: Recent innovations in nano container-based self-healing coatings in the construction industry. Curr. Nanosci. 18, 203–216 (2021) 23. Rahman, A.U., Zarshad, N., Jianghua, W., Shah, M., Ullah, S., Li, G., Tariq, M., Ali, A.: Sodium pre-intercalation-based Na3-δ-MnO2@CC for high-performance aqueous asymmetric supercapacitor: joint experimental and DFT study. Nanomaterials 12 (2022) 24. Thakur, A., Kumar, A., Kaya, S., Marzouki, R., Zhang, F., Guo, L.: Recent Advancements in surface modification, characterization and functionalization for enhancing the biocompatibility and corrosion resistance of biomedical implants. Coatings 12, 1459 (2022) 25. Thakur, A., Kumar, A.: Sustainable inhibitors for corrosion mitigation in aggressive corrosive media: a comprehensive study. J. Bio- Tribo-Corros. 7, 1–48 (2021) 26. Thakur, A., Kaya, S., Abousalem, A.S., Sharma, S., Ganjoo, R., Assad, H., Kumar, A.: Computational and experimental studies on the corrosion inhibition performance of an aerial extract of Cnicus Benedictus weed on the acidic corrosion of mild steel. Process Saf. Environ. Prot. 161, 801–818 (2022) 27. Thakur, A., Kaya, S., Abousalem, A.S., Kumar, A.: Experimental, DFT and MC simulation analysis of Vicia Sativa weed aerial extract as sustainable and eco-benign corrosion inhibitor for mild steel in acidic environment. Sustain. Chem. Pharm. 29, 100785 (2022) 28. Thakur, A., Kumar, A., Sharma, S., Ganjoo, R., Assad, H.: Computational and experimental studies on the efficiency of Sonchus arvensis as green corrosion inhibitor for mild steel in 0.5 M HCl solution. Mater. Today Proc. (2022) 29. Thakur, A., Kumar, A.: Recent advances on rapid detection and remediation of environmental pollutants utilizing nanomaterials-based (bio)sensors. Sci. Total Environ. 834, 155219 (2022) 30. Padmanathan, N., Shao, H., Razeeb, K.M.: Multifunctional nickel phosphate nano/microflakes 3D electrode for electrochemical energy storage, nonenzymatic glucose, and sweat pH sensors. ACS Appl. Mater. Interfaces 10, 8599–8610 (2018) 31. Shao, H., Padmanathan, N., McNulty, D., O’Dwyer, C., Razeeb, K.M.: Cobalt phosphate-based supercapattery as alternative power source for implantable medical devices. ACS Appl. Energy Mater. 2, 569–578 (2019) 32. Guo, S., Zhang, W., Yang, Z., Bao, X., Zhang, L., Guo, Y., Han, X.: The preparation and electrochemical pseudocapacitive performance of mutual nickel phosphide heterostructures. Cyrstals 12, 469 (2022)

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33. Wang, M., Zhao, Y., Zhang, X., Qi, R., Shi, S., Li, Z., Wang, Q., Zhao, Y.: Interface-rich coreshell ammonium nickel cobalt phosphate for high-performance aqueous hybrid energy storage device without a depressed power density. Electrochim. Acta. 272, 184–191 (2018)

Principles of Catalysis Ruchi Jha, Ranita Pal, Debdutta Chakraborty, and Pratim K. Chattaraj

Abstract Catalysts alter the rate of reactions by providing an alternative route to the reactants for product formation. Physically feasible reactions when uncatalyzed in the real world may take hundreds of years to complete but with the help of catalysts, they take a reasonably small amount of time. There are many ways in which a particular reaction can be catalyzed. To this end, geometrical confinement provides an attractive route through which a given chemical reaction could be catalyzed. It has been shown that confining the reactants inside suitable cavitands such as cucurbit[n]urils can reduce the activation barrier associated with the reactions. Geometrical confinement impacts the nature of bonding, and energy gaps among the frontier orbitals within the concerned reactants thereby affecting their reactivity. The cavitands’ size and shape (and thereby the nature of the confining regime), however, plays a significant role in determining the rate of the reaction as shown by Diels–Alder reactions occurring inside organic hosts like ExBox4+ and cucurbit[n]uril. The cavitands (e.g. cucurbit[n]uril) which promote suitable alignment of the reactants can facilitate the reaction effectively. Furthermore, the cavitands can exhibit fluxional behavior (as in B40 molecule) and thus the nature of the confining regime can change as a function of time. In such systems, the reaction dynamics get affected by this fluxional behavior of the host. Keywords Catalyst · Confinement · Fluxionality · Cavitand · Reactivity

R. Jha · R. Pal Advanced Technology Development Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India D. Chakraborty Department of Chemistry, Birla Institute of Technology, Mesra, Ranchi 835215, Jharkhand, India P. K. Chattaraj (B) Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_6

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1 Introduction Chemical reactions are ubiquitous in every aspect of human life as well as far away galaxies where “life” may or may not exist. Not only chemical reactions are responsible for the proper functioning of our bodies, even the formation of stars is due to them. Many of the reactions are spontaneous in the given environmental conditions. To characterize a spontaneous reaction, the change in free energy ΔG plays a major role. A negative value of ΔG signifies a spontaneous reaction and vice-versa. It is well known that the spontaneity of a reaction is not the only factor that should determine the fate of a reaction. Many reactions that are characterized by negative ΔG are at the same time tremendously slow to be observed. For example, the reaction of the breakdown of glucose into carbon-di-oxide water and energy, even being spontaneous at room temperature is not observed outside of our bodies. The reason for this is that the rate at which this reaction processes is too slow to be observed. For such slow reactions, catalysts come to the rescue. The same reaction mentioned above is happening all the time inside our bodies due to the presence of the enzyme catalyst hexokinase. By definition, a catalyst is a species that accelerates the reaction rate without getting consumed during the reaction. Although a catalyst always participates in the reaction mechanism and appears in the rate law, it regenerates itself at a later step. Thermodynamically, the free energy barrier, (ΔG‡ ) of the catalyzed reaction is lower than the uncatalyzed reaction but ΔG is unaltered. There are many different ways to catalyze reactions, for example, there are acid– base catalysis, enzyme catalysis, transition metal catalysis, etc. A catalytic reaction can go through multiple steps but the rate-accelerating effect of the catalyst can be associated with the stabilization of its rate-determining transition state. One of the ways to catalyze a reaction is by confining them inside suitable cavitands. Confinement leads to quantization and can bring about many fascinating outcomes in terms of change in chemical reactivity as compared to their unconfined counterparts [1–6]. Shaowei Li et al. have shown experimentally that the molecular dihydrogen can dissociate into hydrogen atoms by confining the molecule between a copper substrate and Scanning Tunneling Microscope (STM) [7]. Geometrical confinement plays a major role in affecting the physicochemical properties of the chemical compounds confined inside and a new concept fairly known as supramolecule catalysis is widely explored [8–11]. This provides a new avenue towards catalysis sans a catalyst. To date, many organic supramolecular frameworks have been used to examine the optical response properties as well as the bonding of many encapsulated chemical entities and it has produced favorable outcomes as well [12–15]. Although different confinement conditions are used for different chemical entities to facilitate different outcomes, it is still difficult to guess a priori if a confinement condition will work for a given reaction. Cucurbit[n] uril (CB[n]) [16, 17] is an organic host which is hydrophobic and can facilitate many reactions, especially thermal and photochemical ones. In many cases, the same CB[n] vessel can decelerate the reaction as well [18]. Therefore, it is challenging to get a conclusive argument about any such host due to many factors such as

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the proper orientation of the reacting species inside the host, concentration of the guest molecules, the stabilization of the rate-determining transition state involved in the reaction, etc. [19, 20]. Apart from CB[n] uril cavities many different confining hosts can be suggested. The well-known fullerene cages of varying sizes have been studied extensively as well. The molecular surgery methods have even made it possible to confine a few molecules inside the fullerene cages experimentally as well [21]. Stoddart et al. synthesized ExBox4+ complex that has a box-like geometry. This complex is fairly known as a high affinity “scavenger” due to its ability to strongly bond with electron-rich polycyclic aromatic hydrocarbons (PAHs) [22, 23]. Mostly the host–guest interactions are non-covalent. The size of the guest molecules confined and the size of the cavity also play a major role in deciding the fate of the reaction. A very important and unexplored aspect of catalysis inside confinement can be the fluxionality exhibited by the host systems [24]. The dynamic interchange of atoms of a molecule between symmetry equivalent positions is termed fluxionality. There is a possibility that fluxionality, due to its random change in orientations between energetically very close structures can have a positive impact on reaction rate acceleration. Confinement inside such cavities is less explored in terms of this dynamic feature.

2 Theoretical Background It is well known that for a reaction to proceed in the forward direction and form a product, the reactants are required to be activated and form an activated complex which is known as the transition state (TS) of the reaction. For different reactions, the energy required to get activated is different and is known as the activation energy for the reaction (Ea ). This energy is gained by the reactants by collision. At a given temperature T only a fraction of the reactants can collide to get enough energy to be activated. −E a According to Arrhenius, this fraction is given by e RT where E a is the activation energy. Now the rate of reaction is quantified in terms of k and is given by, k = Ae

−E a RT

(1)

This is the well-known Arrhenius equation where A is a parameter that depends on the frequency of collision and is known as the frequency factor. When a catalyst is added, the activation energy, E a gets lowered as a catalyst can absorb the reactants and activate them and allow them to form products easily by making them follow a different reaction path. As E a gets lowered, the value of k (Eq. 1) gets significantly increased and the reaction becomes faster.

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When we consider confinement-induced catalysis, there can be many factors that can result in accelerated reaction rates. Mostly confinement alters the reaction environment and the interaction between the host and the guest is the major reason responsible for these reactions. To understand these interactions many theoretical tools have been developed. The working theory of a few of these is described here. The nature of bonding and interactions can be analyzed with the help of the following tools: (a) (b) (c) (d)

Atoms-in-molecules (AIM) Non-covalent interactions (NCI) Natural bond orbitals (NBO) Energy decomposition analysis (EDA)

(a) Atoms-in-molecules (AIM) analysis The Quantum Theory of Atoms-in-molecules (QTAIM) given by Bader is a very useful theoretical tool to get insight into the bonding nature of the systems considered [5]. This theory utilizes the concept of electron density (ED), that in quantum mechanical terms is the probability of finding an electron around an atom or molecule. It is more likely to find an electron in areas having high ED (and vice-versa) and is calculated by taking the square of the wave function. As the system consists of electrons and nuclei, the attractive forces due to nuclei are responsible for the topography of ED. The local maximum of ED is positioned at the nuclear coordinates only. Figure 1 shows the ED distribution, ED relief map, and molecular graph of benzene [25]. The critical points (CPs) of ED are the points where ED is a maximum, a minimum, or a saddle point, which simply means that the first derivative of ED vanishes at CPs. The second derivative of ED gives useful information regarding the characteristics of the chemical bond and its sign tells us whether the extremum is maxima or minima. The nine second-order derivatives constitute the Hessian of ρ(rc ) (ED at critical point rc ). This matrix can be diagonalized to obtain the eigenvalues λ1 , λ2, and λ3 . The critical points are characterized by (ω, σ) where ω is the number of nonzero eigenvalues and is known as the rank of CP and σ is the sum of the signs of

Fig. 1 ED distribution, ED relief map, and molecular graph of benzene. Reprinted with permission [25]. Copyright (2016), Springer Nature

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eigenvalues. Therefore, for a CP of rank 3, four values of σ can be there and these are (3, −3), (3, −1), (3, +1), and (3, +3). The CP (3, +3) represents the local minimum at rc, and (3, −3) represents the local maximum at rc . It can be observed from Fig. 1, the positioning of (3, −3) CPs at the nuclear coordinates shows the local maxima of ED. The CP (3, −1) is located between the nuclei having bonds and is therefore called Bond critical points (BCPs). The CP (3, +1) is positioned at the center of the benzene ring and is known as the ring critical point (RCP). If the Laplacian of the ED (∇ 2 ρ(rc )) at the BCP is less than zero, this implies that there is an accumulation of charge towards the interaction line which results in contraction of ED perpendicular to the interaction line, and potential energy is decreased. This creates an attractive shared interaction. In contrast, if the Laplacian of the ED is greater than zero at BCP, net repulsive interaction is observed. Along with the Laplacian of ED, other important quantities have been defined and their conditions (stated below) give useful insights into the bonding. Condition If

−G(rc ) V (rc )

>1

If |V (rc )| > 2G(rc )

Inference Interaction is purely non-covalent Interaction is covalent in nature

Here G(r c ) and V(r c ) are the kinetic and potential energy densities at the BCP respectively [26, 27]. (b) Non-covalent interaction (NCI) Non-covalent interactions (NCI) are everywhere and are of many types arising from many different origins like hydrogen bonds, van der Waals interactions, halogen bonds, etc. [28]. Similar to AIM theory, electron density ρ(r) plays a major role in describing its approach. The deformation in ρ(r) corresponds to all the contributors towards the interaction energy except for the Zeroth-order electrostatic energy term. The NCI index corresponds to the study of reduced density gradient s(r), which is defined in terms of electron density as follows, s(r) =

1 |∇ρ(r)| 4 Cs ρ(r) / 3

(2)

Here, C s is a constant of value 2(3π2 )1/3 . To qualitatively classify interactions, s(r) is plotted against sign(λ2 ) ρ(r). This quantity is just the product of the sign of the second eigenvalue of Hessian matrix and the density. The change accumulation perpendicular to the interaction plane is given by the sign of λ2 and its product with electron density can classify the interaction types as follows: sign(λ2 ) ρ(r) < 0

Strongly attractive

sign(λ2 ) ρ(r) ≈ 0

Van-der-Waals

sign(λ2 ) ρ(r) > 0

Steric clashes

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Fig. 2 NCI surfaces of SCF (left) and promolecular densities (right), corresponding to s = 0.6 au and with a color scale of −0.03 < ρ < 0.03 au for the former, and s = 0.5 au (water and methane dimers) or s = 0.35 au (bicyclo[2,2,2]octene) with a colour scale of −0.04 < ρ < 0.04 au for the latter. Reprinted with permission [29]. Copyright (2011), American Chemical Society

These values are color coded for visualization. Figure 2 shows an example taking water and methane dimers and bicycle [2] octane [29]. Favorable interactions appear on the left side, van der Waals appear near the zero and unfavorable interactions appear on the right side. (c) Natural bond orbitals (NBO) NBO stands for Natural Bond Orbital, and is obtained by transforming a wave function into a localized form [30]. This is an important computational tool to get useful insight into the physical and chemical properties of the system. It is done by studying the charge transfer of the hyper-conjugative interaction in the system. In NBO analysis the transformation of the atomic orbital basis set is done to get the Natural Bond orbitals following a hierarchy as shown below: (Atomic Orbitals) → (Natural Atomic Orbitals) → (Natural Hybrid Orbitals) → (Natural Bond Orbitals). (d) Energy Decomposition Analysis (EDA) Developed by Morokuma [31], and Ziegler and Rank, Energy Decomposition analysis is another useful theoretical tool to get insights into the interacting energies between the fragments of the system considered. Whenever a molecule is formed

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(say A-B) a corresponding wavefunction (denoted by ψAB ) having an energy eigenvalue EAB is associated with it. The fragments A0 and B0 have their electronic and geometrical ground states represented by the wavefunction ψA 0 and ψB 0 with energy values EA 0 and EB 0 , respectively [32]. These geometries are distorted to get the geometry of the product. The distorted wavefunctions of the fragments can be represented by ψA and ψB and with energies EA and EB , respectively. The preparation energy can be defined as, ΔE pr eparation = E A − E 0A + E B − E 0B

(3)

The interaction energy can be calculated as, ΔE interaction = E AB − E A − E B

(4)

Now this interaction energy can be decomposed into four major contributing energy terms as follows, ΔE interaction = ΔE elstat + ΔE Pauli + ΔE or b + ΔE disp

(5)

Out of these four energy terms, only the Pauli energy is repulsive. Electrostatic energy (ΔE elstat ), orbital energy (ΔE or b ) and dispersion energy (ΔE disp ) terms are all attractive.

3 Confinement-Induced Catalysis When a reaction is confined inside a suitable cavitand, the chemical environment can act as a catalyst and the reaction can be facilitated kinetically and thermodynamically with ease. Chakraborty et al. have explored this method of catalyzing some very important Diels–Alder reactions [33]. The cavitand used by them is cucurbit [7] uril, which in itself is very useful in facilitating various photochemical as well as thermal reactions. Four reactions were considered which are as follows: Reaction 1

Benzene + ethylene

Reaction 2

Furan + ethylene

Reaction 3

Cyclopentadiene + ethylene

Reaction 4

Thiophene + ethylene

The reactants, transition states, and the products of all the reactions mentioned above were optimized first in the free state (without confinement) and then inside the confinement using DFT theory employing wb97xd functional and 6-311G(d,p) basis set. Frequency analysis has also been carried out to check whether the optimized structures are at their minimum. The reactants and products were found to have no

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imaginary frequency and the transition states had only one imaginary frequency. All the calculations were done using the Gaussian software. It is observed from the optimized structures that the host system got tad distortions along vertical as well as horizontal directions upon confinement. The minimum energy structures of the reactants, TS, and products of all the reactions have been provided in Figs. 3 and 4. For reaction 1, there is no significant change in the orientation of the reactants due to confinement. The Transition state (inside cavitand) for reaction1 undergoes significant distortion with respect to the unconfined state which is inferred by the corresponding change in bond lengths and angles. This shows that the TS inside the cavity is different due to the effects of confinement geometrically. For the products of reaction 1, the structure is more or less similar with or without confinement. For all the other cases as well, the TS geometry of guests inside confinement is quite different from their respective unconfined geometry. Thus we can say that the change in the structure of TS inside confinement results in a different bonding scenario due to interactions with the host. For a reaction to being thermodynamically viable and to be able to. proceed in the forward direction (sponteneity), change in Gibbs free energy (ΔG) and change in enthalpy (ΔH) should be taken into consideration. The ΔG and ΔH values of all the reactions in free as well as confined states have been provided in

Fig. 3 Optimized geometries of the a reactants b transition state c products for reaction 1 and d reactants e transition state f products for reaction 3. Reprinted with permission [33]. Copyright (2017), John Wiley and sons

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Fig. 4 Optimized geometries of the a reactants b transition state c products for reaction 2 and d reactants e transition state f products for reaction 4. Reprinted with permission [33]. Copyright (2017), John Wiley and Sons

Table 1. All the calculations have been simulated at 298.15 K temperature and 1 atm pressure. It can be observed from the table that in the free state, only reaction 3 is favorable as the ΔG value is negative. All the other reactions are non-spontaneous in a free state. Reaction 1 is the most unfavorable as the value of ΔG is positive and the highest. The enthalpy change is favorable for reaction 2 to reaction 4. It can be said that the entropy factor associated with reaction 2 and reaction 4 makes them unfavorable. For reaction 3 the highly negative ΔH factor dominates and makes the reaction favorable at room temperature and pressure in the free state. Inside CB[7] confinement all the reactions become comparatively more favorable thermodynamically. Reaction 2 and reaction 4 which were non-spontaneous in the free state become spontaneous inside confinement (negative ΔG values). Although the enthalpy change inside the confinement is unfavorable for the reactions as ΔH values get increased, it means that the entropic factor plays a major role in making the reactions spontaneous inside CB[7]. Now considering the kinetic aspect of the reactions, the free energy of activation (ΔG‡ ) and the enthalpy barrier ( ΔH‡ ) for all the reactions in the free state as well as in the confined, along with the rate constant k has been provided in Table 1. The rate constant k at 298.15 K and 1 atm pressure, gives us an idea about the kinetics

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Table 1 Change in Gibbs free energy and enthalpy for the reactions in free and confined states. Reprinted with permission [33]. Copyright (2017), John Wiley and Sons ΔG‡ (kcal mol−1 )

ΔH‡ (kcal mol−1 )

K (s−1 )

3.63

49.95

36.93

1.50 * 10–24

2.54

−11.28

37.07

24.19

4.14 * 10–15

(Reaction 3)free

−13.52

−27.70

32.52

19.46

9.05 * 10–12

(Reaction 4)free

3.56

−10.35

45.66

32.71

2.10 * 10–21

(Reaction 1)CB[7]

13.21

9.51

43.93

41.15

3.90 * 10–20

(Reaction 2)CB[7]

−5.39

−9.26

30.34

27.70

3.59 * 10–10

(Reaction 3)CB[7]

−17.67

−21.28

25.92

23.03

6.15 * 10–07

(Reaction 4)CB[7]

−1.90

−5.07

39.60

36.69

5.83 * 10–17

ΔG (kcal mol−1 )

ΔH (kcal mol−1 )

(Reaction 1)free

17.95

(Reaction 2)free

System

of these reactions. The lower the value of k smaller the time taken to complete and the faster the reaction. It is calculated using the Eyring formula given by, k=

k B T −ΔG ‡√ RT e h

(6)

From the table, it is evident that the ΔG‡ value for all the reactions gets significantly reduced but the ΔH‡ becomes more unfavorable upon confinement. This means that the entropic factors dominate in facilitating the reaction inside the cavity. It is interesting to observe that the value of rate constant k gets reduced many folds inside the cavitand. So we can say that the confined environment facilitates the reactions as well as catalyze them and the reaction rates get improved. To check if the reacting moieties become more reactive upon confinement the energy gap between the HOMO of dienophile and LUMO of diene as well as HOMO of diene and LUMO of dienophile for the reactions in free as well as confined states have been calculated and provided in Table 2. From the table, it is visible that the energy gap HOMOdiene and LUMOdienophile is less than the energy gap between HOMOdienophile and LUMOdiene . Upon confinement, all the energy gaps get reduced, which makes these reactions quite reactive. After establishing the fact that the reaction rates have been improved upon confinement, few analyses on bonding and reactivity have been carried out to get an idea about the reasons responsible for such increased reactivity inside the cavity. The NBO analysis tells us that upon confinement the reactants acquire more negative

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Table 2 The energy gap (in eV) for the HOMO and LUMO for the reacting species in free and confined states. Reprinted with permission [33]. Copyright (2017), John Wiley and Sons System

DienophileLUMO -DieneHOMO (eV)

DieneLUMO -DienophileHOMO (eV)

(Reaction 1)free

11.24

11.43

(Reaction 1)CB[7]

9.65

10.55

(Reaction 2)free

10.68

12.04

(Reaction 2)CB[7]

9.14

10.54

(Reaction 3)free

10.36

11.19

(Reaction 3)CB[7]

8.78

10.53

(Reaction 4)free

10.90

11.22

(Reaction 4)CB[7]

9.34

10.57

charges. Due to this the TS inside the cavitand becomes more polar and facilitates the reaction at a faster rate. As compared to the free state, the extent of charge variation on various atoms is quite minimal inside the confinement which suggests that the interaction between the host and the guest is not quite high. To get a better understanding of the bonding situation AIM analysis has been done. All the newly formed C–C bonds from the TS structures have been considered. As described in the theoretical section, relevant information can be obtained based on electron density descriptors like electron density (ρ(rc )) at the bond critical point (BCP), laplacian of electron density (∇ 2 ρ(rc )), local electron energy density (H(rc )) and Electron localization function (ELF). ELF is used to get an idea about the probability of an electron localizing around a bond. All these descriptors have been provided in Table 3. To categorize a bond as partially covalent, its H(rc ) value should be less than zero and ∇ 2 ρ(rc ) value should be greater than zero. Therefore all the C–C bonds (Table 3) formed at TS structure in free and confined state can be said to have partially covalent character. The ELF values of reactions 1, 3 and 4 increase upon confinement whereas for reaction 3 it gets slightly reduced upon confinement. This observation can be related to the corresponding NBO analysis. Few regions inside the host contribute towards the facilitation of bond formation. To identify those regions, NCI analysis is carried out. From the generated NCI isosurface plots, it can be observed that the guest systems are bound to the host (CB[7]) system by non-covalent interactions. These interactions can be visualized by the green color in the NCI plot provided in Fig. 5. The host–guest interactions can also be characterized by the plot of reduced density gradient (RDG) versus sign(λ2 )ρ as explained in the theoretical background section. From those plots(SI of ref V) it is inferred that the reactant and product states inside confinement are stabilized by weak attractive forces whereas the TS inside the cavitand is stabilized by strong attractive forces. This might be due to increased orbital overlap in the TS structure. The host–guest interaction and binding can be analyzed in more detail with the help of Energy decomposition analysis (EDA) which has been explained in the theoretical background section. The interacting energies are calculated by considering fragments and the total interaction energy is decomposed into contributing energy

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Table 3 AIM descriptors at BCP of C–C bond formed at TS structures of all the reactions in free and confined states. Reprinted with permission [33]. Copyright (2017), John Wiley and Sons System

BCP

ρ(rc )

∇ 2 ρ(rc )

H(rc )

ELF

TS1(free state)

C-Cforming

0.07

0.03

−0.02

0.63

C-Cforming

0.07

0.03

−0.02

0.63

C-Cforming

0.07

0.03

−0.02

0.63

C-Cforming

0.07

0.02

−0.02

0.65

C-Cforming

0.07

0.03

−0.02

0.62

C-Cforming

0.07

0.03

−0.02

0.62

C-Cforming

0.07

0.03

−0.02

0.62

C-Cforming

0.07

0.03

−0.02

0.61

C-Cforming

0.06

0.04

−0.01

0.52

C-Cforming

0.06

0.04

−0.01

0.52

C-Cforming

0.06

0.04

−0.01

0.54

C-Cforming

0.06

0.04

−0.01

0.54

C-Cforming

0.06

0.03

−0.02

0.66

C-Cforming

0.06

0.03

−0.02

0.66

C-Cforming

0.07

0.03

−0.02

0.61

C-Cforming

0.06

0.03

−0.02

0.61

TS1@CB[7] TS2(free state) TS2@CB[7] TS3(free state) TS3@CB[7] TS4(free state) TS4@CB[7]

factors that are stabilizing and destabilizing energies. The electrostatic interaction, orbital interaction, and dispersion interaction come under the stabilizing energies and are attractive whereas the Pauli interaction is the only destabilizing energy and is repulsive. The fragmentation of the systems has been done in two ways. Firstly, ethylene is considered as fragment 1 whereas the dienophiles have been considered as the second fragment. In the second set ethylene@CB[7] has been considered as fragment 1 and the dienophiles have been considered as the other fragment. The results of EDA have been provided in Table 4. From the table, it is evident that the TS@CB[7] structure is more stable as the total interacting energy is more negative as compared to the reactant @CB[7]. The orbital and the electrostatic contributions in the case of TS@CB[7] are more which stabilizes the system. Product systems inside confinement interact favorably with the host and the major contributor to this stabilization is the dispersion interaction. It can be noted that all the reactants, TS, and products interact with the cavitand attractively as is evident from the negative total energy values.

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Fig. 5 NCI isosurface of a reactant 1@CB[7], b TS1@CB[7], c product1@CB[7], d reactant 2@CB[7], e TS2@CB[7], f product2@CB[7] respectively. Reprinted with permission [33]. Copyright (2017), John Wiley and Sons

4 Fluxionality Designing clusters for catalysis has been explored far and wide due to its most obvious application of drastically cutting down reaction times. Although clusters with smaller sizes showing catalytic activities are preferred over the larger-sized ones, hence making the designing process easier, there are still many complexities in the whole process. As with any computational chemical study, the most fundamental step is to identify the minimum energy structure (also known as the global minimum structure, GM) of the cluster. Several global optimization algorithms, most commonly, Particle Swarm Optimization, Basin Hopping, Generic Algorithm, Simulated Annealing, when applied along with quantum chemical electronic structure methods and potential energy surface (PES) fitting techniques, have proven to be very efficient in detecting the global optimum structure. In the case of larger clusters, the use of empirical potentials can accelerate the process. The PES of these systems contains several local minima, some of which may become accessible in the higher temperature range used during the catalytic process, causing significant interactions between the isomers. Now, these isomers may have high kinetic barriers, but some of them can be suspected to be to be present in the catalytic system. Also, it is noteworthy to consider that the most stable isomer may not have catalytic properties.

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Table 4 EDA results of Guest@CB[7] moieties. Reprinted with permission [33]. Copyright (2017), John Wiley and Sons Systems

Fragments

DE el

DE Pauli

DE orb

DE disp

DE tot

TS1 (Free state)

[Bz] + [ET]

263.51

138.55

271.91

28.41

25.28

TS2 (Free state)

[Fur] + [ET]

261.70

127.35

266.84

26.99

28.18

TS3 (Free state)

[Cb] + [ET]

249.43

104.86

256.25

28.07

28.89

TS4 (Free state)

[Thio] + [ET]

261.68

130.26

265.00

28.01

24.43

Reactant 1@CB[7]

[ET] + [Bz@CB[7]]

28.13

18.99

22.75

218.77

210.66

Reactant 1@CB[7]

[Bz] + [ET@CB[7]]

214.41

30.85

24.68

232.52

220.77

TS1@CB[7]

[ET] + [Bz@CB[7]]

267.78

146.50

275.01

216.94

213.23

TS1@CB[7]

[Bz] + [ET@CB[7]]

276.95

163.43

277.48

234.46

225.45

Pdt1@CB[7]

[Pdt + ] + [CB[7]]

213.19

27.51

24.45

234.77

224.90

Reactant 2@CB[7]

[ET] + [Fur@CB[7]]

27.00

15.72

22.46

216.85

210.59

Reactant 2@CB[7]

[Fur] + [ET@CB[7]]

214.24

25.68

24.32

225.33

218.21

TS2@CB[7]

[ET] + [Fur@CB[7]]

265.80

135.93

267.54

217.62

215.04

TS2@CB[7]

[Fur] + [ET@CB[7]]

273.74

147.62

269.30

227.86

223.28

Pdt2@CB[7]

[Pdt2] + [CB[7]]

218.00

31.69

25.46

232.33

224.09

Reactant 3@CB[7]

[ET] + [Cb@CB[7]]

27.97

17.35

22.75

217.94

211.31

Reactant 3@CB[7]

[Cb] + [ET@CB[7]]

212.57

27.92

24.60

228.87

218.12

TS3@CB[7]

[ET] + [Cb@CB[7]]

255.21

116.85

259.99

218.59

216.94

TS3@CB[7]

[Cb] + [ET@CB[7]]

261.15

128.22

261.74

231.23

225.89

Pdt3@CB[7]

[Pdt3] + [CB[7]]

210.96

25.88

24.13

232.10

221.32

Reactant 4@CB[7]

ET] + [Thio@CB[7]]

27.75

18.51

22.97

218.39

210.60

Reactant 4@CB[7]

[Thio] + [ET@CB[7]]

214.91

29.13

24.92

230.18

220.88

TS4@CB[7]

[ET] + [Thio@CB[7]]

265.85

138.15

267.90

217.24

212.84

TS4@CB[7]

[Thio] + [ET@CB[7]]

272.73

150.54

269.99

230.89

223.08 225.71

Pdt4@CB[7]

[Pdt4] + [CB[7]]

214.45

24.02

24.78

230.50

TS1@CB[7]

[Bz] + [ET]

265.25

141.98

275.30

28.37

26.94

TS2@CB[7]

[Fur] + [ET]

261.64

127.70

266.51

27.00

27.46

TS3@CB[7]

[Cb] + [ET]

251.67

109.42

259.54

28.05

29.83

TS4@CB[7]

[Thio] + [ET]

263.29

133.60

267.14

28.00

24.83

Here, ET, Bz, Cb, Fur, and Thio stand for ethylene, benzene, cyclopentadiene, furan, and thiophene, respectively

It is logical to assume that the higher energy isomers, being more reactive, might be more likely to show catalytic activity. Thus, the detection and further study of the GM alone are not enough to describe the cluster’s catalytic ability. The concept of fluxionality comes into play when these aforementioned isomers undergo interconversion within themselves. In such cases, the relation between this interconversion and the reaction being catalyzed is very important. A controversial

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Fig. 6 Probabilities of five lowest-energy isomers of Pt13 cluster at varying temperatures. Reprinted with permission [34]. Copyright (2016), American Chemical Society

point rises whether the reorganization of the cluster shape occurs during the reaction, i.e., it interconverts within the given free energy well of the reactants till a catalytically active isomer is formed which then reduces the activation barrier of the reaction. If this is assumed, however, the cluster may interconvert back to its previous isomer in the next free-energy well (following the intermediate of the reaction). Zhai et al. [34] have proposed a way to study the fluxionality in catalysis by creating a statistical ensemble (Boltzmann statistics) of possible isomers including global as well as local minimum structures populated in order of their free energies. They have calculated probabilities of the five lowest-energy isomers of Pt13 cluster as a function of temperature (Fig. 6). These isomers have different binding sites and thus varied affinity towards small molecules [35], where the latter can be effectively described by a ‘nearest-neighbor count’ descriptor (CN) [36, 37]. Since the catalytic activity of these isomers depends upon the site-reactivity as well as its probability to exist in the population of isomers at the given temperature, an ensemble average of the activity over the thermally accessible isomers needs to be considered. The aforementioned cases are possible when all the isomers are low-lying minima on the PES. In cases where they are protected by high kinetic barriers, however, their accessibility reduces and they might not undergo interconversion among themselves. Some pure platinum clusters are reported to exhibit this behavior (barrier ∼1 eV) and are stuck in rough isomeric forms. Whereas, when deposited on carbon nanotubes, they can easily undergo interconversion [38]. For gold clusters (except Au20 ), exhibit rapid interconversion at 300 K [39, 40]. Again, this barrier can decrease or disappear completely upon co-adsorption of H2 O and CO, [41] suggesting that ligand binding can transform the free energy landscape of the isomerization process. Another case of fluxionality is reported [42] in the borospherene cage (B40 ) as a result of interconversion between two types of ring structures of the cage (B6 and B7 ) [43]. This happens when one of the boron atoms moves from the B7 to the neighboring B6 ring, with an associated energy barrier of 14.3 kcal mol−1 . This fluxional behavior is also observed after confining a noble gas atom within its cavity with a kinetic barrier for

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Fig. 7 Activation-free energy barriers associated with the fluxionality of Ng@B40 during the interconversion of B6 and B7 rings. Reproduced with permission [42]. PCCP Owner Societies

the interconversion in the range 16.0 (Xe)–18.9 (He) kcal mol−1 , where that of the empty cage is 16.4 kcal mol−1 (Fig. 7). A few points need to be addressed while studying catalysis of fluxional clusters, viz., accessible local minimum structures are also to be considered along with the global minimum in an ensemble, the extent by which these local isomers are kinetically accessible, and analyzing whether there exists a dynamic coupling between the fluxional conversion and the reaction (if so, whether that happens simultaneously or sequentially). The role of this fluxionality as an additional degree of freedom along with the confinement effects on certain chemical reactions can lead to some interesting and unusual results.

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5 Conclusion It becomes quite evident from the aforementioned discussion that geometrical confinement can help to bring about changes in chemical reactivity. This fact could be exploited in designing suitable ‘reaction vessels’ which can facilitate many chemical reactions [44]. Given the availability of wide arrays of potential organic (such as CB[n], Octa acid cavitand as well as many other cavitands) as well as inorganic (such as metal–organic frameworks (MOF)) host systems for confining a given set of reactants, this route offers an attractive option for catalyzing chemical reactions. However, if the host binds the guest in a ‘non-productive’ alignment, then the concerned reaction would be difficult to catalyze. Therefore, it needs to be seen how to bring the given reactants within a ‘productive’ alignment within the host via suitable ‘molecular tailoring’. In summary, more research efforts are needed to fully understand confinement-induced catalysis, and suitable experimental, as well as computational developments in this direction, can turn out to be quite beneficial for several practical applications. Acknowledgements PKC would like to thank DST, New Delhi, for the J. C. Bose National Fellowship. DC thanks SERB, New Delhi (File Number: SRG/2022/001280) for financial assistance.

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Metal Phosphates/Phosphonates as Catalysts for HER Changrui Feng, Meng Chen, Abuliti Abudula, and Guoqing Guan

Abstract As the cleanest renewable resource with the highest gravimetric energy density, hydrogen fuel is considered the preferred alternative to fossil fuels for the supply of future energy. Water electrolysis provides an attractive strategy to achieve efficient and sustainable hydrogen generation for the future hydrogen economy. As the vital half-reaction of water electrolysis for hydrogen production, cathodic hydrogen evolution reaction (HER) is always intensely researched over the past decades. The employment of highly active HER catalysts can make water electrolysis more economical and energy-efficient. Metal phosphates/phosphonates (MPi/MPPi) are promising and burgeoning HER catalysts owing to their special electronic structure, tunable composition, and multi-functionality. In this chapter, states of the art of main metal-based phosphates/phosphonates catalysts including cobalt, nickel, iron, and other metal phosphates/phosphonates for HER are summarized. While those effective strategies for the design and construction of MPi/MPPi-based catalysts with high HER activities and the catalysis mechanisms are also introduced and discussed. In addition, the application of MPi/MPPi-based catalysts in the future for HER has been prospected. Keywords Hydrogen evolution reaction · Metal phosphates · Metal phosphonates · Electrolysis · Water electrolysis

1 Introduction Energy is not only a foundation of human survival and development but also a guarantee of economic and socially sustainable development. For nearly a century, the energy demand has been greatly increasing with the development of the global C. Feng · M. Chen · A. Abudula · G. Guan (B) Graduate School of Science and Technology, Hirosaki University, 1-Bunkyocho, Hirosaki 036-8560, Japan e-mail: [email protected] G. Guan Energy Conversion Engineering Laboratory, Institute of Regional Innovation (IRI), Hirosaki University, 3-Bunkyocho, Hirosaki 036-8561, Japan © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_7

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economy. Nevertheless, the world’s energy structure still mainly relies on fossil energy including coal, oil, and natural gas [1]. With continuous consumption of fossil energy, it brings about a gradually aggravated energy crisis and environmental pollution. In addition, the proposal of carbon neutrality and peak carbon dioxide emissions have also demonstrated the significant urgency of the development of renewable clean energy. As the cleanest renewable resource with the highest gravimetric energy density (120 MJ/kg at 25 °C), hydrogen is deserved as the primary replacement for fossil fuel for energy supply in the future [2]. Water electrolysis is a highly efficient and pollution-free hydrogen production technology that can convert renewable electricity into high-purity (99.99%) hydrogen resources [3]. As a vital part of water electrolysis for hydrogen production, cathodic hydrogen evolution reaction (HER) has been extensively researched over the past decades. However, the existence of overpotential (η) always needs additional energy input to overcome the barrier in the electrolysis process. Thus, the fabrication of highly active HER catalysts to minimize overpotential is essential to ensure efficient and sustainable hydrogen production from water electrolysis. In recent years, phosphorous-contained materials have emerged with remarkable catalytic performance and thus have attracted numerous attention and been intensively studied [4–8]. It has been reported that the phosphorus atom can regulate charge density and tune the charge state of the catalyst’s local surface to accelerate catalysis performance [9–11]. Owing to their special electronic structure, tunable composition, and multifunctionality, metal phosphates and phosphonates (MPi and MPPi) are regarded as very representative and burgeoning catalysts in recent years. In either MPi or MPPi, the P is in a chemically-stable P5+ state. MPi is a kind of inorganic salt that is formed through a strong ionic bond between metal ions and phosphate. Generally, MPis have a layer structure with an open framework. The development of MPi as electrocatalysts for water electrolysis can be tracked back to 2008 when cobalt phosphate was first confirmed to have good OER activity in an electrolyte with a neutral pH value [12]. Herein, the superior performance of MPis electrocatalysts can be attributed to the phosphate as the proton acceptor that endow MPi with excellent protonic conductivity and stabilize the active center [13, 14]. At the same time, it may also induce a local metal geometry deformation, which is a benefit for water adsorption [15, 16]. While MPPi is a typical organic–inorganic hybrid material that is closely related to metal–organic frameworks (MOFs) with inorganic metal ions coordinated with organophosphonic acid ligands. As such, a controllable porous structure is always formed in MPPi, which is conducive to mass transfer and gaseous product release. In addition, the surface morphology and functionality of MPPi could be tuned by altering metal cations or organophosphonic ligands, which is why the MPPis are structurally superior to other phosphorous-contained catalysts. Besides, the strong R–P–O–metal bonds in MPPi always has excellent thermal, mechanical and acidic stability, ensuring the applications in those strict conditions [17]. Based on the above unique characteristics, MPi and MPPi have received numerous attention as electrocatalysts for HER. In recent years, cobalt, nickel, iron, and some other metal-based phosphates and phosphonates have been developed with satisfactory catalytic performance. In this chapter, the fundamental mechanism

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of HER over such heterogeneous catalysts and some essential parameters for catalytic performance evaluation are introduced first, and then, the recently developed MPi/MPPi-based catalysts for HER are reviewed and the related mechanisms for the enhancement of performance are analyzed. In addition, the futural development of MPi/MPPi-based catalysts for HER has been prospected.

2 Fundamental Mechanisms of HER HER is a classic two-electron transfer reaction and its reaction rate is dependent on the amounts of protons in the electrolyte. Therefore, there is a distinction in the mechanism of HER over heterogeneous catalysts in different pH media [18, 19]. Generally, HER in acidic media consists of the following three steps. H3 O+ + e− → Hads + H2 O Hads + H+ + e− → H2

(Volmer step)

(1)

(Heyrovsky step)

(2)

Tafel step Hads + Hads → H2

(Tafel step)

(3)

For the Volmer step (Eq. (1)), the hydronium cation (H3 O+ ) acts as a proton source reacting with an electron to yield an adsorbed hydrogen (Hads ) on the electrocatalyst surface. After that, the Hads acts as the precursor to generate hydrogen molecule from the Heyrovsky step (Eq. (2)) or Tafel step (Eq. (3)). In the Heyrovsky step, the Hads combines with another proton and an electron to generate hydrogen molecule. In addition, in the Tafel step, the two Hads species interact with each other to generate hydrogen directly. In neutral or alkaline media, HER also proceeds through three steps (i.e., Eqs. (4)– (6)). The overall difference is: instead of protons, the water molecule is reduced to hydrogen on the surface of the electrocatalyst. In the Volmer step, the water molecule on the electrocatalyst is coupled with one electron to be reduced into Hads and adsorbed OH− . The formed Hads can result in the generation of hydrogen molecules through either Heyrovsky or Tafel steps. The first is to combine with another water molecule and an electron to form a hydrogen molecule, the other is the same as the acidic HER process. The broken H–O–H bond can promote the formation of protons, which is a vital part of HER in neutral/alkaline media, also known as the rate-determine step. Therefore, both the hydrogen adsorption-free energy and the ability to decompose water should be considered when evaluating the performance of electrocatalysts for neutral or alkaline HER. H2 O + e− → Hads + OH−

(Volmer step)

(4)

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Hads + H2 O + e− → H2 + OH− Hads + Hads → H2

(Heyrovsky step) (Tafel step)

(5) (6)

3 Parameters for HER Activity Evaluation 3.1 Overpotential Based on the Nernst equation, the standard potential of HER is zero referenced to a normal hydrogen electrode (NHE). Nonetheless, a higher potential is always needed due to the sluggish reaction kinetics for a practical electrode [20–22]. Thus, ion and gas diffusions in the electrode, the impedance of the electrode, the formation of bubbles, and heat release should be taken into consideration in the evaluation of HER performance since all these parameters will lead to additional potential beyond the standard potential, which is called as overpotential (η). Therefore, the applied potential in fact (without iR correction) should be expressed as: E = E 0H E R + η

(7)

The overpotential value at a current density (j, a catalytic current normalized to geometric area or electrochemical active surface area of working electrode) of 10 mA/cm2 is usually used for the comparison of HER activities of different electrocatalysts. This criterion is derived from the solar-to-energy efficiency for photoexcitation in a one-step, in which ca. 10% of solar-to-energy efficiency should be achieved to enable the solar hydrogen generation to be cost-competitive. However, for a practical application, it is more significant to reach a higher current density. For example, in the industrial proton-exchange membrane (PEM) electrolyzer and alkaline water electrolyzer (AWE), the current densities always ranged from 200 to 2000 mA/cm2 . Therefore, the overpotential at a current density above 200 mA/cm2 or more should be also taken into consideration for the evaluation of catalysis activity promising for practical water electrolysis.

3.2 Tafel Slope Tafel slope describes the current density slewing rate with the changing of overpotential, which represents the intrinsic properties of an electrocatalyst. The mathematical relationship between η and j is described by the following equation. η = a + b log j

(8)

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where, b is the Tafel slope, which is a very important parameter in the deducing of possible reaction mechanisms and kinetics of electrocatalysts [23, 24]. Under the alkaline condition, the HER rate over the Pt/C electrode is determined based on the Volmer step, whose Tafel slope is ~120 mV/dec. In the acid electrolyte, when the reaction rate on the electrode is controlled by the Volmer step or Volmer-Heyrovsky step, the value of the Tafel slope can also reach up to 120 mV/dec. While, if the rate-determine step is Heyrovsky or Tafel reaction, a smaller Tafel slope (40 mV/dec or 30 mV/dec) is shown for the electrode [25, 26]. Nevertheless, the Tafel slope is interfered with by various other parameters like the applied voltage, the existence of adsorbents in porous structure, and mass transportation. Therefore, the mechanism of the reaction process inferred based on the Tafel slope can only be used as a reference, more experimental characterizations and theoretical calculations are required to truly determine the reaction mechanism. Generally, a smaller Tafel slope means a high HER performance since a smaller overpotential is required to reach the same current density in that case, which also demonstrates faster electron-transfer kinetics. When η is assumed as zero, the current density calculated through the above formula is called exchange current density (j0 ). Herein, the magnitude of j0 reflects the intrinsic bonding/charge-transferring interactions between the electrocatalyst and reactants. A higher j0 usually indicates better performance of electrocatalyst for HER.

3.3 Electrochemical Impedance Electrochemical impedance spectroscopy (EIS) can be used for the exploration of kinetics on the electrode. In a Nyquist plot, the semicircle part in high-frequency area and the linear part in the low-frequency area represent charge transfer and diffusion processes, respectively. Besides, the diameter of the semicircle indicates the charge transfer resistance (Rct ), which can be determined by equivalent circuit fitting. With regard to HER, a smaller Rct value demonstrates a faster reaction rate on the electrode, resulting in a smaller overpotential.

3.4 Stability Stability testing is important for the evaluation of electrocatalyst performance in practical applications. Two ways are often utilized to carry out stability tests. The first one is galvanostatic or potentiostatic electrochemical testing, in which the change of potential or current density over time at a constant current density or overpotential will be measured. In the potentiostatic electrochemical testing, the current density should be at least 10 mA/cm2 , and the testing time should not be less than 10 h. The second method is cyclic voltammetry (CV) or linear sweep voltammetry (LSV) testing. The specific implementation is to compare the overpotential variation after certain cycles (e.g., 10,000) in the range including the onset potential. As such,

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a smaller chance of overpotential after multiple cycles manifested a more stable performance of electrocatalysts.

3.5 Turnover Frequency Turnover frequency (TOF) is defined as the number of reactants that can be converted to the desired product per unit of time on each catalytic site, which indicates the inherent activity of the catalysts. For electrocatalysts, it can be calculated by using the following formula, TOF =

jA αFn

(9)

where, j (mA/cm2 ) is current density; “A” is electrode surface area (cm2 ); α is transferred electrons number relating to the reactions over the electrode with the generation of per mole of the product (mol−1 ); F is Faraday constant (C/mol), and n is active sites amount (mol), which can be calculated from an electrochemical active surface area (ECSA). However, the TOF calculated by this method is not so precise since for most heterogeneous catalysts, particularly those with complicated structures, not all of the surface sites are active or equally accessible. Even so, it offers an effective way the comparison the activities of different catalysts, especially those within a similar system.

3.6 Faradic Efficiency Faradic efficiency (FE) is always used to describe the efficiency of electrons that participated in the target electrochemical reaction. FE of HER is the ratio of the actually produced amount of hydrogen gas (na ) to the theoretically generated hydrogen gas amount (nt ). Herein, nt is determined by the integration from galvanostatic or potentiostatic electrolysis at a given current density while the generated gases are quantified by the drainage method or gas chromatography. Then, the FE of HER over the heterogeneous catalysts can be calculated by the following equation. FE =

bFn a na × 100% × 100% = nt It

in which, b refers to the number of transferred electrons in the generation of H2 , F is Faradic constant.

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4 MPis/MPPis as Catalysts for HER 4.1 Cobalt Phosphates/Phosphonates Cobalt phosphate (CoPi) theoretically has a low energy barrier for hydrogen atom adsorption as a potential HER catalyst. In recent years, some strategies have been employed for further modification of electronic structure and morphology to improve HER performance of CoPi. For instance, Liu et al. [27] developed plasma-activated Co3+ -rich Co3 (PO4 )2 (PA-CoPO) nanosheet arrays (Fig. 1a, b) as the bifunctional catalyst and found that the plasma treatment resulted in the oxidization of Co2+ to more catalytical active Co3+ species, which can not only act as active sites but also improve the electrical conductivity, thereby enhancing the catalytical performance. In addition, it is observed that the original nanosheet array morphology of Co3 (PO4 )2 can be preserved. As shown in Fig. 1c, many holes are created during the plasma treatment, which is important in maximizing the surface area and geometrically exposing of active sites. As a result, the as-prepared PA-CoPO exhibited excellent catalytic performance and satisfying stability for HER in an alkaline solution with an ultra-low overpotential of 50 mV at 10 mA/cm2 (Fig. 1d, e).

Fig. 1 a, b The FESEM and c TEM of PA-CoPO nanosheet. The red arrows in (c) showing those nanoholes. d Polarization curves for IrO2 /C, CoPO nanosheet and the PA-CoPO catalysts for HER. e I-t curve for PA-CoPO nanosheet and HER curve before and after 10,000 cycles. Adapted with permission [27]. Copyright (2018), Elsevier

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The performances of CoPis with different crystalline states (amorphous or crystalline) have been also investigated for HER. Menezes’s group [28] reported a crystalline lazulite-like Co3 (OH)2 (HPO4 )2 as a precursor for HER in an alkaline solution and the pre-catalyst displayed an overpotential of 130 mV at 10 mA/cm2 . Interestingly, it is validated that crystalline Co3 (OH)2 (HPO4 )2 can be in situ converted into a highly active amorphous CoOx (OH) phase. The TEM and HR-TEM morphologies (Fig. 2a–c) after HER CA (chronoamperometric) displayed a complete morphological change of Co3 (OH)2 (HPO4 )2 phase, in which the original rod-type structure is converted to a porous nanostructure. While the SAED patterns indicated the in situ new generations of an amorphous phase in potential-inducing condition during the HER process (insets of Fig. 2c). Combing with the EDX, ICP-AES, and XPS analyses, one can see that the P species have been moved out from crystalline structure so that the restructuring occurred and led to a highly active amorphous CoOx (OH) phase (Fig. 2d–f). Remarkably, the fully converted catalytically active amorphous CoOx (OH) gave an extremely low overpotential of 87 mV for HER at 10 mA/cm2 and exhibited excellent stability under alkaline media. Similarly, Nie’s group [29] directly electrodeposited amorphous CoPi on tris-(4-fluorophenyl) phosphane (PF) functionalized carbon nanotubes (PF-CNTs). The obtained catalyst exhibited a large ECSA, which showed a satisfactory electrocatalytic HER performance with a low overpotential of 105 mV at 10 mA/cm2 as well as good stability in 0.5 mol/L H2 SO4 solution.

Fig. 2 a, b TEM images of Co3 (OH)2 (HPO4 )2 (Inset (b): SAED pattern). c TEM images of Co3 (OH)2 (HPO4 )2 pre-catalyst after the CA HER stability tests. d–f High-resolution XPS spectra of Co 2p, P 2p and O 1s. Adapted with permission [28]. Copyright (2019), John Wiley and Sons

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For an HER catalyst, the hydrogen adsorption energy is very critical, which must be not so high (adverse to the adsorption process) or so low (adverse to the desorption process). Noble metals are always considered to be promising catalysts for HER due to their appropriate hydrogen adsorption ΔGH* in a volcano plot. However, since their atoms/clusters are prone to aggregate, the fabrication of ideal noble metalbased electrocatalysts for water electrolysis is still challenging. Zheng et al. [30] combined iridium oxide (IrO2 ) and CoPi with carbon nanotubes (CNTs) to achieve suitable adsorption/desorption properties (|ΔGH* | → 0) for facilitating the kinetics of HER. The XPS characterization and DFT calculations in Fig. 3 demonstrated that the phosphate species (P/O bridges) can donate electrons to IrO2 , thereby modulating their electronic structure of them and resulting in a more suitable ΔGH* value (ca. −0.13 eV) for HER. Herein, the electron-donating phosphate bonds can also hinder IrO2 particles from further aggregation. In addition, the CoPi nanoparticles can promote water dissociation and deliver H* intermediate to nearby active sites of IrO2 , promoting hydrogen molecule generation and release. Strong synergistic effects of IrO2 and CoPi-CNTs make the resultant compound perform a Pt-like HER performance with an ultra-small overpotential of 29 mV at 10 mA/cm2 and a low Tafel slope of 27 mV/dec. As mentioned earlier, MPPi is a kind of coordination polymer, in which the metal ion center and phosphonate ligand are two major impact factors of their electrocatalytic activity. Cai et al. [31] synthesized a bio-inspired cobalt phosphonate (CoPPi) coordination polymer nanosheets by directly self-assembling phosphonate ligand, (3-methoxyphenyl) phosphonic acid, with cobalt acetate in an aqueous solution with sonication at room temperature. Herein, the introduction of the phosphonate ligand can help to tune the electronic activity of the cobalt ion center and as a result, the as-proposed catalyst showed superior activity for HER with an onset potential of 84 mV even in a neutral condition. Similarly, Chakraborty et al. [32] constructed bioinspired Co-MOFs by using a novel tetradentate phosphonate as a ligand and explored the HER activity of it in different media. As a result, a low overpotential of 243 mV at 10 mA/cm2 with a Tafel slope of 102 mV/dec was achieved in the acidic medium. In addition, CoPPi as the pre-catalyst for HER was also investigated by Indra et al. [33]. It is proved that the metallic Co species can be generated as active sites for hydrogen production, and a combination of it with the spinel Co3 O4 and Co(OH)2 can further promote the catalytic performance by employing quasi-in-situ and ex-situ ways. Only 144 mV of overpotential was needed to achieve the current density of 10 mA/cm2 .

4.2 Nickel Phosphates/Phosphonates From theoretical calculations and experiments, it is found that the Ni species has a low ΔGH* with a high j0 . Thus, nickel-based HER electrocatalysts including nickel phosphates/phosphonates (NiPis/NiPPis) have been frequently utilized in recent years.

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Fig. 3 High-resolution XPS spectra of CoPi-CNTs, IrO2 -CNTs, and IrO2 -CoPi-CNT hybrid catalyst for a Co 2p, b P 2p, and c Ir 4f. d Top and front views of IrO2 -CoPi complex and e partial density of states (PDOS) in DFT simulations. f Diagram of Gibbs free energies (DG) for HER at CoPi-IrO2 complex’s active sites. g Schematic illustration of a catalytic pathway of CoPi-IrO2 hybrid catalyst towards HER in acid electrolyte. Adapted with permission [30]. Copyright (2020), Royal Society of Chemistry

For example, Theerthagiri et al. [34] reported a platinum-decorated Ni pyrophosphate (β-Ni2 P2 O7 /Pt) HER catalyst, in which Pt nanoparticles are well distributed inside the ‘cages’ of β-Ni2 P2 O7 with a crystalline pattern of 0.67 nm in distance. By using this electrocatalyst, the Volmer-Tafel mechanism with the Tafel reaction was found to be the major rate-limiting step. In addition, it is found that the β-Ni2 P2 O7 cage can help the inside Pt nanoparticles to endure a harsh acidic environment, which is conducive to promoting stability. As a result, the synergic effect between Pt and β-Ni2 P2 O7 prompted resulted in superior HER activity in acidic media with an ultra-low overpotential of 28 mV at 10 mA/cm2 and excellent stability for even 12 days. Generally, the synthesis of MPis utilized NaH2 PO2 as the phosphorous source. Ying et al. [35] innovatively employed phosphonium-based ionic liquid (IL) and n-octylammonium hypophosphite as precursors for the construction of nickel metaphosphate (Ni2 P4 O12 ), in which n-octylammonium hypophosphite served as

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both reactant and solvent. The as-synthesized Ni2 P4 O12 presented a porous structure, which guaranteed a high surface reaction area. As a result, the obtained electrocatalysts exhibited impressive electrocatalytic HER performance and good durability in alkaline media with an overpotential as low as 116 mV at 10 mA/cm2 and a small Tafel slope of 97 mV/dec. Fabrication of nanostructure to increase the active sites is a general method for HER activity enhancement. However, owing to the easy aggregation of nanosized catalysts, it is usually difficult to maintain the trade-off between active sites and stability. It is found that carbon material can help metal active species well dispersed, which is beneficial to guarantee the trade-off. As such, Li’s group [36] designed a hybrid nanostructure, in which nanosized Ni(PO3 )2 particles were landed on carbon nanotubes (Ni(PO3 )2 /CNTs) through a self-assembly process followed by a sequential pyrolysis and phosphidation process (Fig. 4a). As shown in Fig. 4b, c, Ni(PO3 )2 nanoparticles are homogeneously loaded on the surface of the CNTs. Herein, the nickel phthalocyanine (NiPc) contacts with CNTs through a π-π interaction, which is conducive to tightly attaching active species on the substrate while the inlaid structure hinders the Ni(PO3 )2 nanoparticles corrosion/aggregation effectively, thereby enhancing the catalyst stability. Accordingly, the as-fabricated hybrid catalyst performed excellent HER activity (Fig. 4d, e) in either alkaline (η10 = 71 mV, Tafel slope: 65 mV/dec) and acidic (η10 = 87 mV, Tafel slope: 46 mV/dec) electrolytes with excellent stability. Due to the unique and easily tailorable properties of MPPi, NiPPi has been also used as the electrocatalyst precursor. For example, Lv et al. [37] utilized NiPPi as a precursor to synthesize a core–shell N-doped carbon-coated nickel phosphide nanoparticles, which were in situ grown on Ni foam (Ni2 P@NC/NF). The inherent catalytic active Ni2 P and conducive 3D core–shell framework endowed Ni2 P@NC/NF with excellent HER activity and stability in all-pH electrolytes. To reach 10 mA/cm2 , the required overpotentials were as low as 68, 84, and 155 mV in 0.5 M H2 SO4 , 1.0 M KOH, and 1.0 M PBS solutions, respectively.

4.3 Iron Phosphates/Phosphonates As one of the most abundant and cheapest transition metal, iron and iron derived materials have been extensively researched and applied in the field of catalysis. Among them, iron phosphates (FePis) have been widely investigated as HER or OER electrocatalyst. For instance, Ronge et al. [38] employed Atomic Layer Deposition (ALD) to obtain FePi film without the need for phosphidation step. However, the overpotential was as high as 416 mV at 10 mA/cm2 for HER in alkaline media because of the low mass loading of the catalyst by using this method. Zhang et al. [39] utilized Fe2 P2 O7 as dopants to synthesize phosphate (i.e., Fe2 P2 O7 ) doped FeP nanosheet (FeP-I NS) for HER in acidic media. The atomic force microscopy (AFM) characterization demonstrated that the thickness of FeP-I NS is only ~0.7 nm (Fig. 5a). As such, ultra-thin and mesoporous structural properties endowed this catalyst with

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Fig. 4 a Synthesis process of Ni(PO3 )2 /CNTs. b SEM and c TEM images of Ni(PO3 )2 /CNTs. Polarization curves for Ni(PO3 )2 /CNTs, Ni(PO3 )2 , CNTs and GCE in 1.0 M KOH (d) and 0.5 M H2 SO4 (e). Adapted with permission [36]. Copyright (2022), Elsevier

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Fig. 5 a AFM analysis results for the FeP-I NS with the corresponding height profiles. b Total and partial electronic density of states for FeP and Fe2 P2 O7 /FeP (dashed green line: Fermi level at 0 eV). Inset: Optimized surface structures of H-Fe2 P2 O7 /FeP. Fe: blue, P: purple, O: red, H: white. Adapted with permission [39]. Copyright (2019), John Wiley and Sons

higher conductivity as well as more exposed active sites. Furthermore, the introduction of the acid-resisting Fe2 P2 O7 doping not only resulted in the reduction of ΔGH* , but also enhanced surface electronic properties of the catalyst as shown in Fig. 5b. These contributions endowed the catalyst with superior HER catalytic performance in 0.5 M H2 SO4 solution with overpotentials of 96 and 160 mV at 10 and 100 mA/cm2 , respectively. In addition, Chen et al. [40] synthesized iron phosphate (Fe7 (PO4 )6 ) through a simple and rapid microwave treatment method using ionic liquids (ILs) as both iron and phosphorus sources. A series of characterizations demonstrated that the IL had an impact on the morphology and electrocatalytic performance of catalysts and the introduction of CNTs facilitated the formation and dispersion of Fe7 (PO4 )6 nanoparticles. Ultimately, the obtained catalyst showed an onset overpotential of 120 mV and an overpotential of 185 mV at 10 mA/cm2 for HER.

4.4 Other Metal Phosphates/Phosphonates Except for familiar Co, Ni, and Fe-based phosphates/phosphonates, some other metal-based phosphates/phosphonates have been also developed for HER. For instance, Xia et al. [41] embedded crystalline P-doped WTe2 core in an amorphous phosphate shell (am-WPO@WTe2 ) as the HER electrocatalyst. Since plentiful

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unsaturated defect sites, as well as active sites, can be provided from the relatively disordered structure and atomic arrangement as well as the amorphous surface, and the crystalline P-doped WTe2 core can accelerate electron transfer to active sites, the obtained HER electrocatalyst exhibited low overpotential in both acidic (η10 = 220 mV) and alkaline (η10 = 280 mV) electrolytes. Zhang et al. [42] fabricated a metal phosphate-based highly active electrocatalyst for HER by combining cerium phosphate with molybdenum phosphide nanobelts on carbon cloth (CePO4 /MoP/CC, Fig. 6a–d). During the HER process, it is found that the CeO2 can be in situ generated on the CePO4 surface, which can generate abundant oxygen vacancies (Ov) from the XPS and EPR results (Fig. 6e–g), thereby significantly modulating d-band electronic density-of-states of MoP, further increasing the number of active sites and enhancing vectorial electronic transfer. Besides, by comparing the calculated ΔGH* values on different catalyst models, it is verified that CePO4 and CeO2 (Ov) co-modified MoP (100) surface could have a ΔGH* of only −0.02 eV with further creating of oxygen vacancy by the generation of CeO2 (Fig. 6h), which can explain why HER activity of CePO4 /MoP/CC electrocatalyst was greatly enhanced. As a result, the synthesized catalyst exhibited an excellent HER performance with an overpotential of only 48 mV at 10 mA/cm2 and a satisfactory Tafel slope as low as 38 mV/dec (Fig. 6i, j), which is comparable with benchmark Pt/C catalyst. Kumar et al. [43] synthesized tin polyphosphate with both amorphous and crystalline phases on NF (SnPx Oy /NF) as the HER electrocatalyst. Herein, the wellconductive and 3D-opened NF provided a large surface area for the attachment of SnPx Oy . By synergistically combining catalytic active Sn2+ /Sn4+ species and polyphosphate ions, the resulting SnPx Oy /NF presented impressive catalytic performance toward HER with only 100 mV overpotential at 10 mA/cm2 in 1 M KOH solution, superior to crystalline counterpart. It is considered that the amorphous phase is more likely to provide enhanced exposed active sites, and the excessed partial negatively charged (δ−) polyphosphate ions (P2 O7 4− ) could also preferentially adsorb more H+ , resulting in more efficient HER at lower overpotential. Yadav’s group [44] successfully prepared rare earth metal-based nanomaterials, i.e., Gd2 O3 :GdPO4 and Nd2 O3 :NdPO4 via a simple one-step hydrothermal method as the HER electrocatalysts in acidic medium (0.5 M H2 SO4 ). Due to its unique surface morphology and strong interaction between the present elements, the asprepared electrocatalyst showed efficient activity. In particular, the Nd2 O3 :NdPO4 nanocomposite exhibited enhanced electrical conductivity and a larger surface area with more active edges, which are beneficial for the HER process. As a result, the overpotential of as developed catalyst for HER was only 134 mV to reach 10 mA/cm2 with good stability.

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Fig. 6 a–c SEM images and d TEM images of CePO4 /MoP/CC. XPS of e Ce 3d and f O 1 s for MoP/CC, CePO4/MoP/CC. g EPR spectra of MoP/CC and CePO4/MoP/CC. h Free-energy diagram of HER. i HER performance and j Tafel plots of bare CC, MoP/CC, CePO4 /MoP/CC, and Pt/C electrocatalysts. Adapted with permission [42]. Copyright (2021), John Wiley and Sons

4.5 Polymetallic Metal Phosphates/Phosphonates To further enhance the activities of those MPis/MPPis for HER, an extra metallic element was intentionally introduced to fabricate binary or trinary, and even highentropy MPis/MPPis. Compared with those monometallic phosphates/phosphonates, multi-metallic elements in polymetallic phosphates/phosphonates tend to exhibit a synergistic effect with a more optimized electronic structure, thereby resulting in better electrocatalytic performance. For instance, Li et al. [45] reported a selfsupported amorphous iron and cobalt-based phosphate catalyst on NF through a colloidal chemical method for HER. It is found that the existence of Fe ions can achieve a partial-charge-transfer activation influence and as a result, the assynthesized binary MPi manifested the minimal charge transfer resistance. Besides, the morphology of the catalyst exhibited a nanosheet structure, which is favorable for reactant up-taking and gas release. By altering the Fe/Co ratio, the finally

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obtained optimum Fe0.72 Co0.42 PO4 /NF achieved an ultralow overpotential of 77 mV at 10 mA/cm2 , which is much better than that of the corresponding monometallic phosphates (FePO4 and Co3 (PO4 )2 ). For any water electrolysis catalysts, the fabrication of appropriate nanostructure is conducive to exposing more active sites and enhancing mass transfer and gas release. Sial et al. [46] fabricated microporous 2D NiCoFe phosphate nanosheets on NF as HER electrocatalysts. A series of characterizations demonstrated that the assynthesized nanosheet presented porous, amorphous, oxygen and defect-enriched characteristics. Herein, the microporous structure could reduce the ion transport resistance and induce the additional edge active site exposure. Besides, the abundant defects that existed in the nanosheets could be significantly helpful in promoting electrocatalytic activity. By further bonus, the synergistic effect of multiple metallic elements, the 2D NiCoFe phosphate nanosheets formed on NF manifested high catalytic activity for HER with a low overpotential of 231 mV at 10 mA/cm2 . Kong et al. [47] synthesized amorphous NiMoPOx supported by NF as the precatalyst for water electrolysis. During the electrocatalytic process, it is found that the NiMoPOx can achieve an in situ electrochemical self-tuning process to generate metallic Ni on the catalyst surface, resulting in a hybrid material with a special heterostructure. Herein, the in-situ generated nanosized Ni particles are beneficial for the generation of adsorbed H atoms to promote the HER kinetics and activity. Ultimately, the overpotential to reach 10 mA/cm2 was only 30 mV and even at 600 mA/cm2 , the overpotential was just 280 mV, and high stability was still maintained after over 40 h electrochemical test. Similarly, Zhang et al. [48] fabricated a hybridized Ni(PO3 )2 -MnPO4 nanosheet array on NF (Ni(PO3 )2 -MnPO4 /NF) as high-performance electrode material, which exhibited good catalytic activity for HER (η10 = 125 mV). He et al. [49] constructed a Ni-Co phosphide@phosphate nanocage-type (NCPP NCs) electrocatalyst by phosphorization of Ni-Co layered double hydroxide (Ni-Co LDH). It is worth noting that the as-prepared NCPP NCs displayed outstanding wettability with a contact angle of approximately 0, which is conducive to the contact between the catalyst and the electrolyte and thereby further facilitating gas evolution. As a result, the required overpotential was only 140 mV at 10 mA/cm2 for HER. Yang et al. [50] fabricated a Fe-doped cobalt-phosphate nanosheets-packing micro-sized 3D sphere on a Cu support (Fex Co3−x (PO4 )2 /Cu) through a facile onestep electrodeposition process. Herein, the Fe doping optimized the adsorption of hydrogen species on those active sites by modifying the local electronic structure of O-sites in phosphate. In the case of Fe doping, the theoretical calculations demonstrated that the density of state (DOS) of Fe0.43 Co2.57 (PO4 )2 displayed an enhanced charge density around Fermi level compared with that of pristine Co3 (PO4 )2 , resulting in an enhanced conductivity. Besides, Fe doping also optimized ΔGH* on the O site to close thermoneutral (ΔGH* = −0.06 eV). Ultimately, the as-prepared catalyst showed good performance over a wide range of pH for HER. Especially, the Fe0.43 Co2.57 (PO4 )2 /Cu required only overpotentials of 108.1, 128.8, and 291.5 mV at 100 mA/cm2 in 1.0 M KOH, 0.5 M H2 SO4 , and 1.0 M phosphate-buffered solutions, respectively, with excellent durability.

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Furthermore, Gao et al. [51] fabricated Nix Co1−x (PO3 )2 nanoparticles embedded in N-doped carbon (NC) on partially phosphatized NF (P-NF). With regard to selfsupported transition metal phosphate, the inner nano-sized Nix Co1−x (PO3 )2 particles provided active sites. While, the wrapped-around NC can not only improve the electrical conductivity but also hinder corrosion from the electrolyte. The excellent synergistic effect between them endowed the electrocatalyst with outstanding HER catalysis property. The required overpotential at 10 mA cm−2 in acidic and alkaline solutions were 138.2 and 73.1 mV, respectively. Liu et al. [52] utilized the surface reconstruction method to successfully construct Co2+ or Co3+ -rich (oxy) hydroxide surface on ZnCo phosphate through anions and cations etching treatment for HER at high current density in alkaline electrolytes. It is found that the obtained reconstructed CoOOH covered defective ZnCoPi (VZn -ZnCoPi-OH) with excellent interfacial interaction and charge transfer ability. As a result, the electrocatalyst exhibited an excellent HER performance with low overpotentials of 63, 280, and 338 mV at 10, 500, and 1000 mA/cm2 , respectively. Furthermore, it maintained stability even at 1200 mA/cm2 over 50 h. In addition, except for hydrogen evolution from freshwater, such MPis have been also applied for HER from seawater electrolysis. For instance, Kim et al. [53] prepared Co-Fe-phosphate ((Co,Fe)PO4 ) as the electrocatalysts in alkaline seawater electrolysis, which displayed an overpotential of only 137 mV at 10 mA/cm2 . As a kind of newly emerged material in recent years, high-entropy materials have attracted numerous attention for energy conversion and storage. The strong synergistic effect among multiple elements makes high-entropy materials promising as electrocatalysts for highly efficient water electrolysis. In 2021, Qiao et al. [54] for the first time synthesized high-entropy phosphates (HEPi) in the form of highly homogeneous spherical particles via a high-temperature fly-through method for OER. Thereafter, Wang et al. [55] synthesized high-entropy phosphate/carbon (HEPi/C) hybrid nanosheet, (W28 Ni24 Co24 Mo17 Ru7 )POx /C, for the first time and applied for acidic HER by using a mechanochemical method to synthesize high-entropy MOF as the precursor followed by a high-temperature phosphating process (Fig. 7a–d). The hybrid nanosheet with a thickness of only 22 nm provided a large surface area and abundant active sites for HER. With the attribution of strong synergistic effect and high entropy properties of HEPi/C, the required overpotential was only 40 mV at 10 mA/cm2 in 0.5 M H2 SO4 solution with satisfying stability (Fig. 7e, f). As for polymetallic phosphonate, Bhanja et al. [56] synthesized Ni-Co phosphonate (NiCoDPA) with a microporous morphology using diphenylphosphinic acid as an organophosphorus source via a simple hydrothermal process. The microporous channel existed in NiCoDPA provided a high specific area with accelerated reaction kinetics since a larger amount of electrons/ions can transfer in the porous channels. Besides, the synergistic effect between the two metals endowed the electrocatalyst with higher intrinsic HER activity. As a result, the as-proposed NiCoDPA presented a low overpotential of 112 mV at 10 mA/cm2 with a low Tafel slope of 78 mV/dec toward HER in 1 M KOH electrolyte and outstanding 100 h stability.

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Fig. 7 a SEM, b TEM, c HRTEM characterizations of HEPi/C hybrid nanosheet. d EDS mappings for Ru, Ni, Co, Mo, W, P, and O. e HER polarization curves of HEPi/C hybrid nanosheets, HEMOF nanosheets, and commercial Pt/C. f The chronopotentiometry measurement of HEPi/C hybrid nanosheets at 10 mA/cm2 . Adapted with permission [55]. Copyright (2022), Elsevier

5 Conclusions and Prospects As is generally acknowledged as the cleanest energy in the world, hydrogen is emerging beyond doubt as the most promising renewable energy in the twenty-first century. Hydrogen production from water splitting is a convenient and efficient way to convert unstable renewable electricity into high-purity stable hydrogen energy. In recent decades, numerous catalysts have been developed to achieve sustainable

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hydrogen production with high efficiency. This chapter mainly summarized the stateof-the-art metal phosphates/phosphonates-based HER electrocatalysts. Different strategies like changing the phase structure, combining with carbon-based materials (carbon nanotubes, graphenes), heteroatomic doping, polymetallic compound fabrication based on synergistic effect, etc. have been employed to improve the catalytic performance. By taking advantage of the above-mentioned methods, it can achieve the purposes of improving the surface area, modulating the electronic structure, and enhancing the electrical conductivity. Therefore, the as-synthesized metal phosphates/phosphonates usually exhibit very impressive catalytic performance and some of them are even comparable with commercial Pt/C. Even so, more efforts are still needed to fabricate metal phosphates/phosphonates-based catalysts with high performance for HER. In addition, the electrocatalytic performance of metal phosphates/phosphonates for HER still needs further enhancement when compared with those of noble metalbased catalysts, especially, since more active sites are needed to be generated and/or exposed to improve the activity. Generally, NaH2 PO2 , phosphorus, and thrioctylphosphide are the typical P sources in the synthesis of metal phosphates/phosphonates. However, the employment of these raw materials always needs extra pretreatment and high-temperature phosphatization steps, which makes the fabrication process tedious and energy-consuming so it is not conducive to practical application for industrialization. Furthermore, the catalytic activity and stability under a high current density should be also taken into consideration when it comes to industrialization. The fabrication of metal phosphates/phosphonates with a porous nanostructure and a hydrophilic surface is conducive to promoting mass transfer and suppressing the formation of bubbles in the practical electrolysis process, thereby avoiding the falling out of catalysts. Moreover, theoretical calculations and in-situ characterization techniques should be introduced to combine with experimental investigations to get more insights into the reaction mechanism. Acknowledgements This work is supported by ZiQoo Chemical Co. Ltd., Japan and Hydrogen Energy Systems Society of Japan. Feng and Chen gratefully acknowledge the State Scholarship Fund of China Scholarship Council, China.

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43. Kumar, A., Kim, I.H., Mathur, L., Kim, H.S., Song, S.J.: Design of tin polyphosphate for hydrogen evolution reaction and supercapacitor applications. J. Korean Ceram. Soc. 58, 688– 699 (2021) 44. Majhi, K.C., Yadav, M.: Neodymium oxide doped neodymium phosphate as efficient electrocatalyst towards hydrogen evolution reaction in acidic medium. J. Environ. Chem. Eng. 10, 107416 (2022) 45. Li, C., Mei, X., Lam, F.L.Y., Hu, X.: Amorphous iron and cobalt based phosphate nanosheets supported on nickel foam as superior catalysts for hydrogen evolution reaction. ACS Appl. Energy Mater. 1, 6764–6768 (2018) 46. Sial, M.A.Z.G., Lin, H., Wang, X.: Microporous 2D NiCoFe phosphate nanosheets supported on Ni foam for efficient overall water splitting in alkaline media. Nanoscale 10, 12975–12980 (2018) 47. Kong, F., Sun, L., Huo, L., Zhao, H.: In-situ electrochemical self-tuning of amorphous nickel molybdenum phosphate to crystal Ni-rich compound for enhanced overall water splitting. J. Power Sources 430, 218–227 (2019) 48. Zhang, X., Li, J., Sun, Y., Liu, Q., Guo, J.: Hybridized Ni(PO3 )2 -MnPO4 nanosheets array with excellent electrochemical performances for overall water splitting and supercapacitor. Electrochim. Acta 299, 835–843 (2019) 49. He, L., Gong, L., Gao, M., Yang, C.W., Sheng, G.P.: In situ formation of NiCoP@phosphate nanocages as an efficient bifunctional electrocatalyst for overall water splitting. Electrochim. Acta 337, 135799 (2020) 50. Yang, C., He, T., Zhou, W., Deng, R., Zhang, Q.: Iron-tuned 3D cobalt-phosphate catalysts for efficient hydrogen and oxygen evolution reactions over a wide pH range. ACS Sustain. Chem. Eng. 8, 13793–13804 (2020) 51. Gao, H., Wang, Y., Zhou, S., Song, S., Tian, X., Li, W., Yuan, Y., Zang, J.: Nickel-cobalt phosphate nanoparticles wrapped in nitrogen-doped carbon loading on partially phosphatized foamed nickel as efficient electrocatalyst for water splitting. Chem. Eng. J. 426, 130854 (2021) 52. Liu, H., Cao, S., Zhang, J., Liu, S., Chen, C., Zhang, Y., Wei, S., Wang, Z., Lu, X.: Facile control of surface reconstruction with Co2+ or Co3+ -rich (oxy)hydroxide surface on ZnCo phosphate for large-current-density hydrogen evolution in alkali. Mater. Today Phys. 20, 100448 (2021) 53. Kim, C., Lee, S., Kim, S.H., Park, J., Kim, S., Kwon, S.H., Bae, J.S., Park, Y.S., Kim, Y.: Cobalt–iron–phosphate hydrogen evolution reaction electrocatalyst for solar-driven alkaline seawater electrolyzer. Nanomaterials 11, 2989 (2021) 54. Qiao, H., Wang, X., Dong, Q., Zheng, H., Chen, G., Hong, M., Yang, C.P., Wu, M., He, K., Hu, L.: A high-entropy phosphate catalyst for oxygen evolution reaction. Nano Energy 86, 106029 (2021) 55. Wang, Z., Zhang, X., Wu, X., Pan, Y., Li, H., Han, Y., Xu, G., Chi, J., Lai, J., Wang, L.: High-entropy phosphate/C hybrid nanosheets for efficient acidic hydrogen evolution reaction. Chem. Eng. J. 437, 135375 (2022) 56. Bhanja, P., Mohanty, B., Chongdar, S., Bhaumik, A., Jena, B.K., Basu, S.: Novel microporous metal phosphonates as electrocatalyst for the electrochemical hydrogen evolution reaction. ACS Appl. Energy Mater. 4, 12827–12835 (2021)

Metal Phosphate/Phosphonates for Hydrogen Production and Storage Rabia Sultana, Yinghui Han, Xin Zhang, and Lijing Wang

Abstract As a kind of green energy, hydrogen is an effective way to solve energy shortages, environmental pollution, and climate change. At present, the cost of hydrogen energy production and storage is high. Metal phosphide is expected to be the upstream main raw material to support the hydrogen economy instead of precious metals due to its low hydrogen overpotential, adjustable electronic structure, high conductivity, and low price. This chapter reviews the R&D status of metal phosphates/phosphonates as electrocatalysts, photocatalysts, biological starters, and thermochemical stabilizers in hydrogen energy preparation and storage technology, respectively. The progress of phosphorous-based fuel cells is also summarized. Finally, the application of metal phosphate/phosphonates in the related fields of hydrogen energy development has prospected. This chapter aims to provide technical support and theoretical reference for researchers and beginners in related industries. Keywords Metal phosphate · Metal phosphonates · Hydrogen production · Hydrogen storage

1 Introduction Hydrogen, the most ubiquitous element in the universe, is seen as an effective path to a low-carbon future. Hydrogen is a kind of energy carrier with low density, cleanness, great power-to-weight ratio (approximately 143 MJ/kg), and high thermal efficiency. Its calorific value is high, non-toxic, tasteless, colorless, odorless, and can be converted into other kinds of energy. Instead of combustion, it may store and distribute electrical energy through chemical reactions, so hydrogen energy can be a compelling alternative to the fossil fuel economy for climate change. Hydrogen is more efficient than diesel and gasoline. The wastewater from the regeneration process can be used to make hydrogen. Although the initial installation cost of hydrogen production is high, the cost during maintenance is low. Different source materials can be converted into R. Sultana · Y. Han (B) · X. Zhang · L. Wang College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_8

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Fig. 1 Application of metal phosphate and phosphonates for hydrogen production and storage

hydrogen using different techniques. Hydrogen produced from fossil fuels is known as “grey hydrogen” and is not a completely clean energy source, while hydrogen produced from photobiological, electrochemical, photochemical, and thermochemical processes is known as “green hydrogen” and is considered the cleanest energy source. Hydrogen is often expensive to produce, store and transport. This is one reason why hydrogen has not yet completely replaced fossil fuels. Hydrogen must be produced in an economically viable manner before it can be safely stored and distributed for use in a practical form. Transition metal phosphide, phosphates, and phosphonates because of their richness in active elements in the earth’s crust and natural organisms, as well as their unique lattice geometrical structure and price advantage, have become a series of replacing precious metal catalyst, photocatalyst, the thermal stability of substrate materials, etc., used in the preparation, storage, and transportation of hydrogen [1]. Figure 1 provides an overview of the main applications and roles of metal phosphate/phosphonates in the hydrogen energy supply chain. In this chapter, we will introduce these aspects in detail.

2 Roles of Metal Phosphate/Phosphonates for Hydrogen Production and Storage Metal phosphate/ phosphonates materials have been widely concerned in hydrogen energy production and storage because of their cost-effective, high-efficient, and stable characteristics. In particular, the nanoscale metal phosphate/ phosphonates are more prominent in the optics, electricity, heat, magnetic, and acoustic performances,

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so it is often used for hydrogen production and storage by photolytic or electrolysis of water.

2.1 Metal Phosphate/Phosphonates as Electrocatalysts Hydrogen production by water electrolysis has attracted wide attention because of its mild and simple method, high purity of hydrogen production products, and no greenhouse gas emissions. Electrocatalysts are required to fully utilize water splitting and produce hydrogen. Due to the kinetic constraints of the two watersplitting half-reactions, HER, and in particular, OER, electrocatalysts are needed to both accelerate and reduce the energy requirements of these reactions [2]. A metal phosphating agent is a compound with both metal and semiconductor properties. However, due to the complex, diverse chemical compositions and variable crystal structures of metal phosphate, it is easy to crystallize out in solution, which makes the controlled synthesis of metal phosphate catalysts are faced great difficulties. At present, there are various methods to prepare metal phosphonate catalysts, such as temperature-programmed reduction, phosphite pyrolysis, and solvothermal synthesis. The problem to be solved in the synthesis of metal phosphate catalyst is how to control the particle size by structure design. Some studies have investigated the hydrolysis activities of different transition metal phosphates at the same active sites [3]. Figure 2 shows the hydrolysis activities comparison of several typical metal phosphates. Ni2 P has the highest activity, while Fe2 P is the weakest. CoP, MoP, and WP showed moderate activity and increased in turn. To improve the operation economy of the producing hydrogen process by water splitting while ensuring environmental safety, it is crucial that the right nanoelectrocatalysts be multifunctional, affordable, and environmentally benign. An electrochemical deposition-phosphatization strategy is considered to be an effective approach for hydrogen evolution reactions. For example, phosphate and phosphide electrocatalysts have been synthesized for hydrogen production via 1, 1' -bis (diphenylphosphino) ferrocene as a precursor to supply a base source of phosphorus, iron, and carbon. Profiting from the synergistic action of FePO4 /PdC, Ni2 P, NC

Fig. 2 Activity comparison of transition metal-rich phosphides in hydroprocessing. Reprinted with permission [3], Copyright 2014, Elsevier

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matrix, and organophosphine ligand, the FPPC/Ni2 P@NC/NF catalyst can produce hydrogen efficiently at low pressure during urea oxidation, meanwhile, it can exhibit excellent OER activity [4]. However, metal phosphate catalysts still face technical bottlenecks such as weak conductivity and a lack of active site, which are the main factors restricting the improvements of catalytic performance of UOR. Co/NiCoP nano-heterojunctions have good HER stability in an alkaline medium, and only 54 mV can achieve a current density of 10 mA/cm2 [5]. By in-situ electrochemical reduction, the bimetallic cobalt-iron phosphate electrode can be reconstructed, and the oxygen-nucleophilic groups can enhance HER activity at the nucleophilic bimetallic phosphate sites [6].

2.2 Metal Phosphate/Phosphonates as Photocatalysts In general, water can be photo split or organic species can be photo reformed to produce H2 through photocatalysis. These procedures are usually aided by catalysts that can convert the absorbed light energy into the energy needed to excite electrons to produce H2 . The current H2 production efficiencies (solar-to-fuel) that may be achieved with the photocatalysts are insufficient and fall far short of the necessary operational standards for industrial applications [7]. Based on the mechanism of ligand-to-metal transfer photocatalysis, some transition metal ion phosphonates act as isomorphic organic–inorganic hybrids and are frequently utilized as photocatalysts for hydrogen production from water splitting [8]. In the renewable hydrogen economy, these non-noble metal catalysts (Co, Ni, W, Fe, and so on) have economic benefits over precious metals (Pt and Pd). For example, an active and steady photocatalyst of tungsten phosphide (WP) auxiliary cocatalyst loaded on CdS can be used for efficient hydrogen evolution [9]. In addition, phosphate-based metal–organic frameworks (MOFs) have been paid attention to as potential materials for hydrogen production and storage. However, compared with other traditional metal–organic frameworks, phosphonate-based organic skeleton materials are still underdeveloped to a large extent. Zhu et al. [10] designed a one-dimensional titanium phosphate MOF photocatalyst. The nanowire architecture can help improve photocarrier transit and separation. The activity of photocatalytic hydrogen evolution of titanium phosphate nanowires is improved under visible light.

2.3 Metal Phosphate/Phosphonates for Hydrogen Production from Biomass Biological hydrogen production is the process of producing molecular hydrogen in the physiological metabolism of biomass through gasification and microbial catalytic

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dehydrogenation. Pentose phosphate can be used to increase the hydrogen production rate of recombinant Escherichia coli, which is a dark fermentation process. To improve hydrogen production in Escherichia coli, it is necessary to activate the PP pathway by increasing gluconeogenic flux through glpX overexpression [11]. To overcome the slow yield deficiency of NADPH-dependent directly producing H2 from thermodynamically unfriendly soluble [NiFe] hydrogenase (SH1), a solution strategy to construct the bionic electron transport chain from NADPH to H2 by using in vitro enzymes consisting of ultra-high temperature glucose 6-phosphate dehydrogenase, 6-phosphate glucose colactolase, 6-phosphate gluconate dehydrogenase, NROR and SH1 [12].

2.4 Metal Phosphate/Phosphonates for Thermochemical Hydrogen Production Several different procedures can be used to generate hydrogen. Thermochemical processes use heat and chemical reactions to extract hydrogen from organic sources as well as from liquids. Solar hydrogen production could be carried out by a thermochemical reaction cycle. As early as 1984, a preliminary study evaluated the feasibility of a demonstration project to produce two tons of hydrogen per day based on a ceria-sodium phosphate/carbonate thermochemical cycle [13].

3 Hydrogen Production Technologies by Metal Phosphate/Phosphonates 3.1 Biohydrogen Technology Biological hydrogen production technology is generally considered to be one of the most economical hydrogen production technologies. The following three biological hydrogen production technologies use metal phosphates [14].

3.1.1

Direct Biophotolysis

Due to the photosynthetic ability of microalgae, oxygen and hydrogen are produced through water splitting in photosystems located on the thylakoid membrane [15]. Oxygen and plastoquinone-bound hydrogen (pqH2 ) are generated by splitting water, which is initiated by the absorption of optimal sunrays. However, the oxygen produced as a byproduct of PS11 suppresses the hydrogen-related activities, limiting the capability of direct photolysis. Hydrogen can be preserved in microalgae by

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removing inert gases or creating a scavenger to remove the oxygen produced by photosynthesis [16].

3.1.2

Hydrogen Production by Algae

Algae harvested in large quantities from nature can produce fuel H2 through efficient photosynthesis. The process of producing hydrogen energy from algal biomass usually includes two types: the thermochemical process and the biological process. There are generally four types of thermochemical processes: combustion, pyrolysis, liquefaction, and gasification. There are usually five kinds of biological processes: direct biophotolysis, indirect biophotolysis, biological water–gas conversion reaction, light fermentation, and dark fermentation. Hydrogen production by algae mainly consists of direct and indirect biological photolysis [17]. The inclusion of sulfur dramatically increased hydrogen productivity as well as its performance in numerous biological processes.

3.1.3

Indirect Biological Photolysis

Typically, the algae will be in sulfur-free reaction conditions, resulting in anaerobic conditions that stimulate continued H2 production. The four steps of indirect biological photolysis are usually as follows: (1) photosynthesis to produce biomass; (2) biomass concentration; (3) algal cells were fermented in aerobic darkness, producing 4 mol H2 per mole of glucose, accompanied by 2 mol of the acetate; (4) Convert 2 mol of acetate into H2 alone. However, this process is not as efficient as direct photolysis.

3.2 Fermentative Hydrogen Production The waste-activated sludge anaerobic fermentation broth is high in organic matter and phosphate ions, making it a suitable supply of carbon and electrolytes for MEC. Under the condition of equal conductivity, electrolytes phosphate buffer can significantly promote the hydrogen production of sludge fermentation solution [18, 19], and phosphate can be added to the co-substrate of glucose and landfill leachate for hydrogen production. The introduction of OP significantly promoted hydrogen production by providing nutrients. The hydrogen yield gradually slows down with the increase of pH value, indicating that an acidic environment is more conducive to the development of the hydrogen production capacity of photosynthetic bacteria than an alkaline environment [20].

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Fig. 3 Schematic diagram of hydrogen production from water by photocatalytic electrolysis

3.3 Photoelectrolytic and Electrochemical Method The typical photoelectrolytic hydrogen production method is solar or photovoltaic hydrogen production. Photovoltaics for power generation can achieve 10–20% efficiency, which is likely to rise when new solar light-absorbing materials are introduced [14]. The nanostructured cobalt phosphides have demonstrated good hydrogen generation activity, as well as strong catalytic current and low onset potential (Co2 P). In the basic medium, nanostructured Co2 P particles with tiny holes exhibit substantial hydrogen generation activity. By substituting earth-abundant metal-based catalytic systems for precious metal-based systems, highly efficient and cost-effective hydrogen-generating systems can be achieved [21]. Metal phosphate electrocatalysts are facing many technical bottlenecks to overcome, such as high temperature and pressure and high purity requirements for raw materials, and the decomposition of phosphorus compounds to produce toxic gases that are easy to threaten human health and the environment. Whether photocatalytic or electrochemical catalysis, hydrogen is produced by the reduction of hydrogen ions with electrons in the cathode [22]. However, the simultaneous production of hydrogen and oxygen also limits the efficiency of the battery, because the reaction of oxygen and hydrogen could result in an explosion, and the semiconductor electrode will corrode [23]. Some studies have used photocatalysts to produce heterostructured photocathodes to produce hydrogen in neutral pH phosphate buffer solutions by photochemistry [24]. Phosphate groups, as the proton supply medium, can enhance the electrostatic interaction between photocatalytic materials and ions in solution, effectively sending protons to the active site, to improve the efficiency of photocatalytic hydrogen production [25]. The process of hydrogen production from water by photocatalytic electrolysis is shown in Fig. 3.

3.4 Photocatalytic Hydrogen Production The photocatalytic system can reduce greenhouse gas emissions into the environment by producing alternative types of energy such as hydrogen [26, 27]. Photocatalysts

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containing phosphorus are significant members of the H2 -producing photocatalyst family. Yu et al. theoretically studied the process of hydrogen evolution of red phosphorus catalyzed by visible light in 2012 [28]. Following that, a series of red phosphorus complexes were prepared to improve the charge separation efficiency and improve hydrogen yield [29–31]. Furthermore, phosphorus can persist in a very stable state as phosphate (PO4 3− , 5+ P ). Because of the highest valence of P5+ , phosphate is chemically stable, but it can also rapidly combine with metal cations to form metallic phosphates with strong ionic bonds that act as photocatalysts for hydrogen production [32]. Additionally, iron hydroxyl phosphate [33], bismuth phosphate (BiPO4 ) [34], and silver phosphate (Ag3 PO4 ) [35, 36] are considered as promising candidates for H2 production. Furthermore, metal phosphate is a molecule consisting of a P3− anion and its equivalent metal cation, which can be applied for H2 photocatalytic production, such as cobalt phosphide nanowires [37], nanocrystalline nickel phosphide [38], WP2 particles [39], and so on.

3.5 Thermo-chemical Water Splitting Recycling solar energy through thermocouples to split water is seen as a promising technology because it could eliminate dependence on fossil fuels [40]. Hydrogen evolution, oxygen production, and material regeneration are the necessary steps in the production of hydrogen by thermochemical splitting water. The thermochemical water decomposition cycle of cerium and alkaline earth phosphate was studied at the Oak Ridge Laboratory as early as 1979 [41]. At 600–1000 °C, cerium oxide (IV), CeO2 , interacts with phosphate derivatives, metaphosphate, and pyrophosphate to create oxygen and cerium phosphate (III). Steam oxidizes cerium (III) phosphate to cerium oxide at 700–1200 °C in the presence of alkaline earth oxides, carbonates, or halides. These two sets of reactions provide the foundation of the thermochemical cycle family that underpins the REDOX pair Ce(IV)/(III). Hydrogen can be thermally generated from water by injecting water vapor into the tungsten phosphate glass thick membrane with Pd composite surface at temperatures below 500 °C [42]. Its advantages are low reaction temperature and fewer processes, but the separation and purification of the produced hydrogen is still a big problem.

4 Phosphorous-Based Hydrogen Fuel Cells A phosphoric acid fuel cell, or PAFC for short, is a fuel cell with liquid phosphoric acid as an electrolyte, which is the earliest commercial fuel cell. PAFC can be divided into hydrogen fuel type, alcohol fuel type, and other types. The electrolyte is a pure liquid phosphoric acid (H3 PO4 ) at high concentrations saturated in a silicon carbide (SiC) matrix. It uses cheap carbon material as the skeleton. Carbon paper electrodes

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were covered with finely scattered platinum catalysts. PAFC is an excellent contender for early fixed applications because of these characteristics. In addition to hydrogen, methanol, natural gas, municipal gas, and other low-cost fuels can be used directly. The main advantage of PAFC over alkaline hydrogen and oxygen fuel cells is that it does not require CO2 treatment equipment. Moreover, PAFC can tolerate about 1.5% CO concentration, which broads the range of options for fuel they can use. When using gasoline, sulfur must be eliminated. Furthermore, the operating temperature range is around 150–210 °C, making them more resistant to contaminants. Even if the reactant contains 1–2% carbon monoxide and a few parts per million sulfur, phosphoric acid fuel cells can function. Unlike existing power generation methods such as internal combustion engines, fuel cells generate electricity directly without converting the energy of fuel into thermal and mechanical energy. Therefore, although the fuel cell has a small capacity, it can obtain high power generation efficiency, and it is a non-combustion power generation mode, and the amount of NOx in the PAFC exhaust is below 5 ppm, which is very clean. In addition, there is no movable part, so the vibration and noise are very low. Figure 4 shows the power generation principle of the phosphoric acid fuel cell. Battery internal reaction is the reverse reaction water electrolysis, in particular, is not the direct reaction of fuel gas and oxygen in the air but relies on an electrolyte (phosphoric acid) to make the positive (air) and negative (fuel) REDOX reaction respectively, thus electron flow connect to the positive and negative external circuit for electric power. Although PAFCs are more mature than other fuel cells, they also face some urgent subject to be solved, such as further improving the battery power density, prolonging service life and enhancing its operation consistency, and lessening the manufacturing costs. Preparation of catalysts with high activity and excellent stability, optimization of the porous gas electrode structure, and preparation of ultra-thin electrode matrix materials with good thermal and electrical conductivity will improve the output performance. Phosphonated polysulfones can be used to prepare durability proton exchange membranes in fuel cells [43]. The challenge of evolving phosphor-based Fig. 4 Schematic diagram of the power generation principle of phosphorous-based fuel cells

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fuel cells is to advance durable membranes that permit fuel cells to run at high temperatures without humidifying requirements [44]. The usage of the membranes provides significant advantages in terms of fuel cell system complexity, cost, and performance. Such membranes with phosphonic groups perhaps have several major advantages over routinely utilized sulfonated membranes in this regard. At low humidity, the front membrane can transport protons via structural diffusion due to the hydrogen bonding and amphoteric nature of phosphonic acid. Protons can be transported by the kinetics of water in high water-content membranes in much the same way as in typical sulfonated membranes. Because of the strength of carbon-phosphorus linkages, phosphate polymers have strong hydrolytic and thermal stability, which is particularly important in high temperatures. Whereas, it is evident that to attain these improved qualities, the molecular structure of phosphine polymers must be carefully designed. The proton conductivity, ion exchange capacity, and stability of the membrane can be further enhanced by adding boric acid (H3 BO3 ) and boron phosphate (BPO4 ) to the membrane matrix in different proportions [45]. The results show that the water holding capacity is 56% for the membrane containing 15% boron phosphate, the swelling rate is 8%, the ion exchange rate is 1.36 meq/g, and the proton conductivity is 0.37 S/cm at 0.6 V and 750 mA/cm2 . These values are close to those of the PFOS film. The loss of water from the electrolyte membrane was the main problem of the operation of the fuel cell at high temperatures. The 2D zirconium phosphate display layered structures[46]. The proton conductivity of the compound varies with temperature and relative humidity.

5 Hydrogen Storage by Metal Phosphate/Phosphonates The main links of the hydrogen energy industry chain include the preparation, storage, transportation, and utilization of hydrogen. The link between hydrogen storage, hydrogen generation, and hydrogen production application in the middle of the industrial chain is the critical technology and prerequisite for accomplishing large-scale hydrogen application. Hydrogen storage is critical for advancing hydrogen applications forward in stationary power, transit, and portable power systems. To develop a clean hydrogen economy, effective and sustainable hydrogen storage technologies are required. But hydrogen is difficult to store because it is a very low-density gas, with a density of 40.8 g/m3 . Hydrogen storage methods can usually be classified into three strategies, i.e. high-pressure gaseous hydrogen storage, low-temperature liquid hydrogen storage, and solid hydrogen storage. Finding and rationally designing affordable, effective materials for diverse energy-related chemical reactions is the main problem in the development of hydrogen storage technologies. Mesoporous materials’ high surface area and changeable porosity have been demonstrated to be useful in energy-related applications. Their interactions with incoming species result in a surplus of readily available active sites, which facilitates simple mass/charge transfer. Because of their structural benefits and inherent electrochemical activity, mesoporous materials based on phosphorus are advantageous for energy storage.

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Fig. 5 Schematic illustration of metal phosphates/phosphonates for hydrogen storage. Reprinted with permission [50, 51]

Due to their versatile electrocatalytic activity, metal phosphates have distinct benefits in hydrogen storage [47]. Metal phosphates exhibit remarkable performance in these reactions due to their exceptional physicochemical properties. Transition metal phosphates and phosphate groups in phosphates have flexible coordination and various orientations in comparison to transition metal oxides, making them excellent building blocks for creating active electrocatalysts. But unfortunately, there is not much research on the use of metal phosphate/ phosphonates for hydrogen storage. This may be because although considerable effort has been given to the development of transition metal phosphates and phosphates, the technical bottlenecks that limit their application in hydrogen storage, such as minimal intrinsic catalytic activity, low electronic conductivity, poor selectivity, and instability, need to be addressed urgently. Some studies try to solve these problems by optimizing the coordination environment and electronic structure of transition metal centers, improving the density of active sites, and constructing heterogeneous structures [48, 49]. Figure 5 gives a schematic of the use of metal phosphates/phosphonates for hydrogen storage.

6 Conclusion The most appealing clean energy is hydrogen energy, which is the most advanced in terms of production, storage, transportation, and consumption. The primary obstacle in the development of hydrogen energy is the high cost. Because of the advantages of hydrogen production potential, adjustable electronic structure, high conductivity and low price, metal phosphates and phosphonates, acting as electrocatalysts, photocatalysts, biological starter cultures and thermochemical stabilizers, are expected to replace precious metals as the main upstream raw materials to support hydrogen energy economy. In phosphoric acid solvent, inorganic doping can enhance the mechanical properties and thermal stability of polymer membranes, reduce electroosmotic resistance (by increasing membrane swelling), and enhance the ability to trap acid. Metal phosphates provide additional proton transport channels than inorganic oxide silicon dioxide and in turn have higher proton conductivity. Phosphate ions have specific selectivity and effectively provide targeted diffusion and transport

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of hydrogen ions. Therefore, metal phosphide may shine in the widely concerned application of proton exchange membrane. Acknowledgements This work was financially supported by the National Key R&D Program of China (2018YFA0209302), the National Natural Science Foundation of China (21976177 & 22276191), the Industry-Academy cooperation project (E2021000435), and the Innovative practice training program for college students of Chinese Academy of Sciences (117900M002).

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34. Pan, B., Wang, Y., Liang, Y., Luo, S., Su, W., Wang, X.: Nanocomposite of BiPO4 and reduced graphene oxide as an efficient photocatalyst for hydrogen evolution. Int. J. Hydrogen Energy 39, 13527–13533 (2014) 35. Martin, D.J., Liu, G.G., Moniz, S.J.A., Bi, Y.P., Beale, A.M., Ye, J.H., Tang, J.W.: Efficient visible driven photocatalyst, silver phosphate: performance, understanding and perspective. Chem. Soc. Rev. 44, 7808–7828 (2015) 36. Yi, Z.G., Ye, J.H., Kikugawa, N., Kako, T., Ouyang, S.X., Stuart-Williams, H., Yang, H., Cao, J.Y., Luo, W.J., Li, Z.S., Liu, Y., Withers, R.L.: Nanocomposite of BiPO4 and reduced graphene oxide as an efficient photocatalyst for hydrogen evolution. Nat. Mater. 9, 559–564 (2010) 37. Tian, J., Cheng, N., Liu, Q., Xing, W., Sun, X.: Cobalt phosphide nanowires: efficient nanostructures for fluorescence sensing of biomolecules and photocatalytic evolution of dihydrogen from water under visible light. Angew. Chem Int. Ed. 54, 5493–5497 (2015) 38. Wu, W., Yue, X., Wu, X.-Y., Lu, C.-Z.: Efficient visible-light-induced hydrogen evolution from water splitting using a nanocrystalline nickel phosphide catalyst. RSC Adv. 6, 24361–24365 (2016) 39. Pi, M., Wu, T., Zhang, D., Chen, S., Wang, S.: Facile preparation of semimetallic WP2 as a novel photocatalyst with high photoactivity. RSC Adv. 6, 15724–15730 (2016) 40. Wajda, T., Gabriel, K.: Thermolysis reactor scale-up for pilot scale Cu–Cl hybrid hydrogen production. Int. J. Hydrogen Energy 44(20), 9779–9790 (2019) 41. Pobinson, P.R., Bamberger, C.E.: Thermochemical water-splitting cycles based upon reactions of cerium- and alkaline earth phosphates. In: Conference: 2. Miami International Conference on Alternative Energy Sources, Miami Beach, FL, USA, 10 Dec 1979 (1979) 42. Tawarayama, H., Utsuno, F., Inoue, H., Fujitsu, S., Kawazoe, H.: Low temperature thermochemical water splitting using tungsten phosphate glass/Pd laminated membrane. J. Power Sources 161(1), 129–132 (2006) 43. Tang, H., Geng, K., Hu, Y., Li, N.: Synthesis and properties of phosphonated polysulfones for durable high-temperature proton exchange membranes fuel cell. J. Membrance Sci. 605(15), 118107 (2020) 44. Lafitte, B., Jannasch, P.: Chapter Three—On the prospects for phosphonated polymers as proton-exchange fuel cell membranes. Advanced in Fuel Cell. 1, 119–185 (2007) 45. Sahin, A., Ar, I.: Synthesis, characterization and fuel cell performance tests of boric acid and boron phosphate doped, sulphonated and phosphonated poly(vinyl alcohol) based composite membranes. J. Power Sources 288(15), 426–433 (2015) 46. Segawa, K., Funamoto, T., Ando, J., Yamaguchi, C., Kaneko, K., Takeoka, Y., Rikukawa, M.: Molecular design of layered zirconium phosphonates for fuel cell applications. Stud. Surface Sci. Catal. 154(Part A), 1096–1102 (2004) 47. Mei, P., Kim, J., Kumar, N.A., Pramanik, M., Kobayashi, N., Sugahara, Y., Yamauchi, Y.: Phosphorus-based mesoporous materials for energy storage and conversion. Joule 2, 2289–2306 (2018) 48. Zhao, H., Yuan, Z.: Design strategies of transition-metal phosphate and phosphonate electrocatalysts for energy-related reactions. Chemsuschem 14(1), 130–149 (2021) 49. Dong, J., Ban, G., Zhao, Q., Liu, L., Liu, J.: Hydrogen storage in several metal-phosphate molecular sieves. Environ. Energy Eng. 54(1), 3017–3025 (2008) 50. Rivard, E., Trudeau, M., Zaghib K.: Hydrogen storage for mobility: a review. Materials 12(12), 1973 (2019) (Open Access) 51. Chen, X., Peng, Y., Han X., Liu, Y., Lin, X., Cui, Y.: Sixteen isostructural phosphonate metal-organic frameworks with controlled Lewis acidity and chemical stability for asymmetric catalysis. Nat. Commun. 8, 2171 (2017) (Open Access)

Polyphosphate-Based Electrocatalysts for Oxygen Evolution Md. Yeasin Pabel, Akash Pandit, Tabassum Taspya, and Md. Mominul Islam

Abstract The efficient and scalable production of H2 from the water splitting is fundamentally reliant on the development of active, robust, and economical electrocatalysts for oxygen evolution reaction (OER). Transition metal phosphates and phosphonates are promising materials due to their distinct layered architectures, high catalytic activity, low-cost, facile synthesis, and environmental friendliness. Metal phosphates with open, gigantic, framework structures, e.g., polyoxometalates (POMs), provide ion transport channels and cavities along with abundant redox active sites. POMs containing primarily oxygen atoms and transition metals like M = Mo, W, V, etc., and a central addenda heteroatom like P, Si, etc., have shown great promise toward the electrocatalytic OER. In this chapter, the overviews of the fundamentals of OER and the catalytic characteristics of POMs, and the insights into OER electrocatalysis are discussed. Keywords Electrocatalysts · Water splitting electrolysis · Oxygen evolution reaction · Polyoxometalates · Phosphate

1 Introduction Energy resources have emerged as a major area of contention between economic development and environmental preservation in the modern period. The usage of fossil fuels causes many environmental issues. Renewable energy sources such as hydrogen fuel, solar electricity, and wind energy are promising solutions to rising energy demand. Among these, H2 fuel may be the greatest alternative because it can be generated efficiently at any time and from anywhere using simple processes such as H2 O splitting as represented by Eq. 1 [1]. 2H2 O → 2H2 + O2

(1)

Md. Y. Pabel · A. Pandit · T. Taspya · Md. M. Islam (B) Department of Chemistry, University of Dhaka, Dhaka 1000, Bangladesh e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_9

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However, the sluggish and intricate oxygen evolution reaction (OER) limits the efficiency of H2 generation from the H2 O electrolysis. An efficient OER electrocatalyst is desired to address this issue. Generally, a state-of-art OER catalyst must not only be quick, capable of splitting water at a potential just above the thermodynamic value but also be cheap and highly stable in a reaction medium. The precious metals, e.g., Pt-, Ir- and Ru-based catalysts fulfill the criteria of OER catalysts. Polyoxometalates (POMs), polynuclear metal–oxygen clusters comprised of a metal, oxide, and the main group oxyanion (phosphate, silicate, etc.), could be a promising choice as an OER catalyst due to their high redox ability and oxidative, hydrolytic, and thermal stability [2]. Transition-metal-substituted POMs containing phosphate group possess outstanding OER performances [3] with some shortcomings related to conductivity [4]. Developing nanocomposite of POMs with organic/inorganic structures is an attractive way to modify the physical and chemical properties and to broaden their application in OER [5]. The paramagnetic characteristic of POMs opens a new avenue to enhance the OER catalysis with an external magnetic field. This chapter focuses on the development of OER catalysts based on POMs and their composites. For the development of sophisticated OER electrocatalysts, it is essential to have a clear understanding of the OER mechanism. Thus, the fundamentals of the OER and POM family and the effect of a magnetic field on the OER are discussed. The examples of the application of POMs-based electrocatalysts for enhancing the OER are summarized. The challenges and prospects of using polyphosphate-based OER catalysts are highlighted.

2 Fundamentals 2.1 OER H2 O decomposes into H2 and O2 gases (Eq. 1). This reaction is endothermic with a thermodynamic energy of 1.23 V (25 °C and 1 atm). In practice, to perform this reaction electrochemically at a practical rate, one must apply extra potential on the cell (Fig. 1). The overall operational potential (ηop ) is the combination of the potentials required to overcome the internal resistance (ηΩ ) and the overpotentials required to surmount the intrinsic activation barriers of the OER and hydrogen evolution reaction (HER). However, the OER is very complicated involving a 4e− /4H+ removal and the formation of a new oxygen–oxygen bond that implies severe kinetic hurdles and high activation energy, which can be overcome by using suitable catalysts. The most common pathway of the OER is based on the conventional adsorbate evolution mechanism (AEM), where O2 is primarily derived from adsorbed H2 O molecules (see Fig. 1). For each O2 molecule to be generated, four electrons must be transferred over the electrode as expressed by Eqs. (2–9). In general, the O2 evolution occurs at the

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Fig. 1 Electrocatalytic OER pathways in alkaline media a AEM of the Eley–Rideal (ER) type, b AEM of the Langmuir–Hinshelwood (LH) type. “Adapted with permission [9], Copyright (2021), Springer”. c A diagram showing how the lattice oxygen participation mechanism and the AEM compete in alkaline media. “Adapted with permission [10], Copyright (2018), American Chemical Society”. d LaCoO3 and SrCoO3 rigid band diagrams in a schematic. “Adapted with permission [11], Copyright (2017), Springer Nature”. e Magnet-assisted mechanism of OER. Upon application of the magnetic field, the spin anti-parallel pathway (Left) and spin parallel pathway emerge (Right), f Electrochemical set-up for oxygen evolution reaction (a: Ni–foam/NiZnFeOx working electrode; b: Pt mesh counter electrode; c: Ag/AgCl 3.5 M KCl reference electrode; d: FOXY probe). “Adapted with permission [12], Copyright (2019), Springer Nature.”

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metal oxide surfaces rather than clean/bare metal surfaces, i.e., the formation of O2 by cleavage of O–H is followed by the development of metal oxide under applied potential [6, 7]. As a result, the mechanism of OER differs depending on the surface structure of metal oxides as can be seen below [7]: In acidic media ∗

Step 1:

+ H2 O → ∗ OH + H+ + e− ∗

OH → ∗ O + H+ + e−

(3)

O + H2 O → ∗ OOH + H+ + e−

(4)

Step 2: Step 3:

∗

(2)

∗

Step 4:

OOH → O2 + H+ + e−

(5)

In alkaline or neutral media ∗

+ OH− → ∗ OH + e−

(6)

OH + OH− → ∗ O + H2 O + e−

(7)

Step 1: Step 2:

∗

∗

Step 3: Step 4:

∗

O + OH− → ∗ OOH + e−

(8)

OOH + OH− → O2 + H2 O + e−

(9)

where the symbol ‘*’ suggests the active sites of the electrocatalyst. For the fourstep process, there are three reaction intermediates namely *OH, *O, and *OOH representing the adsorbed oxygen-related species on the surface of catalysts. The oxidation of water/hydroxyl ions is often initiated under electrochemical oxidation conditions to make *OH (Eqs. 2 or 6), followed by deprotonation (Eqs. 3 or 7) and further oxidation to generate the intermediates of *O and *OOH (Eqs. 3, 4 or 7, 8), and finally O2 product (Eqs. 5 or 9). In the acidic solution, H2 O molecules serve as the oxygen source, while in neutral or alkaline solutions, oxygen comes from the hydroxyl ions. The most difficult step in OER is the formation of *OOH species on the surface of electrode material by breaking H2 O (Eq. 4) [6]. There are two different modes of AEM-based reaction pathway: the Eley–Rideal (ER) type mechanism and the Langmuir–Hinshelwood (LH) type mechanism [8]. In the case of ER type mechanism one metal center (active site) plays a role to bind one reactant species (Fig. 1a) but in the case of LH type mechanism two adjacent metal cation active sites bind the reactant species (Fig. 1b). The formation of O2 in the electrocatalytic OER on perovskite oxides involves the participation of lattice oxygen. A kinetic lattice oxygen-mediated mechanism (LOM) is more favorable for OER catalyzed by perovskite oxides over the general AEM. The

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18

O isotope identification of the product and density functional theory (DFT)-based calculations support the involvement of the lattice oxygen anion from the perovskite lattice in the production of O2 [11]. A thorough description of the occurrence of lattice oxygen evolution reaction (LOER) concurrent with the standard OER mechanism based on thermodynamic considerations has been proposed elsewhere [13]. In the LOER, the reaction is initiated by the oxidation of the lattice oxygen ions rather than the oxidation of the cations to a higher valence state (Fig. 1c). The OH− ions from alkaline solution can replace the lattice oxygen that the LOER is used up. As a result, a stable dynamic cycle is created, enabling the coexistence of the original perovskite structure with a self-assembled active surface layer. The LOM-type mechanism involving a strong OER activity of strontium cobaltite, or SrCoO3–δ (SCO) has been evidenced with both theoretical and experimental initiatives [10]. The necessary covalency of metal–oxygen in the case of perovskite oxide catalysts is enhanced lattice-oxygen oxidation and enables non-concerted protonelectron transfers during OER. The antibonding states below the Fermi level reveal higher oxygen character as the Fermi level drops down in energy and closer to the O 2p states from LaCoO3 to SrCoO3 , indicating greater covalency of the metal– oxygen bond (Fig. 1d) [14]. Thus, in the case of SrCoO3 , the oxidation of lattice oxygen is more favorable than that of LaCoO3 . The oxidations of lattice oxygen of La0.5 Sr0.5 CoO3−δ , Pr0.5 Ba 0.5 CoO3−δ, and SrCoO3−δ have been evidenced to take place during OER. These oxides show strong pH-dependent OER kinetics [11]. The most stable and common allotrope of molecular oxygen (O2 ) is its triplet state. The generation of the triplet state of O2 through OER is energetically advantageous. In forming the paramagnetic, triplet state of O2 after the rupturing of the two H2 O molecules, spin conservation must be followed [15]. The electrochemical OER is thus spin-controlled and the spin state of the anode determines the OER mechanism. During electrochemical OER, the singlet O2 is first formed on the catalyst surface, and the thus-formed singlet state is converted to a triplet state by spending time and energy in the solution state. However, the magnetic electrode accepts all of the electrons from the reactants with the same (parallel) spin shown in Fig. 1e, which is essential for quickly achieving the triplet form of oxygen [12]. Thus, theoretical studies imply that spin polarization of the active catalyst surface may favor parallel spin alignment of oxygen atoms during the reaction to increase the effectiveness of the process. An increment of the current density by 100% (over 100 mA cm−2 ) has been observed when a magnetic field of ≤450 mT has been applied [12] during the OER on mixed oxide NiZnFe4 Ox catalyst. The magnetic catalysts with unpaired spin facilitate the reaction rate of electrochemical OER at lower overpotentials under an applied external uniform magnetic field.

2.2 POMs POMs have drawn a lot of interest as the materials used in multipurpose applications including catalysis because of their distinctive qualities, including strong protonic

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conductivity, acid tolerance, multi-electronic redox capabilities, etc. [16]. POMs are clusters of transition metal oxyanions that are connected by oxygen atoms to form a three-dimensional metal–oxygen cage, with MOX (X = 5, 6) oxometalpolyhedra constituting the basic building blocks [2]. Here, M typically refers to early transition metals, such as W(VI), Mo(VI), V(V/VI), Nb(V), and Ta(V). The most prevalent POMs are based on W(VI) and Mo(VI) as represented in Scheme 1. In contrast to isopolyanions, which exclusively contain the atoms M and O, most POMs have one or more heteroatoms (usually from the p or d block) [2]. The metal of POMs has vacant d orbitals that are ready to receive electrons from an electron donor. Thus, POMs can act as Lewis acids. These metal ions are considered to be active sites for oxidation catalysis. POM surfaces, on the other hand, have an abundance of O atoms that can donate electrons. POMs can thus be considered soft bases. As a result, POMs can act as both Lewis acid and Lewis base under certain conditions. Due to the available multiple active sites including metal, oxygen atoms, and even protons, POMs can be thought of as versatile catalysts [17]. However, the metals in POM catalysts should receive the most attention because they are active sites in all oxidative reactions, including OER. The fascinating properties of POMs, such as tunable acidity and redox properties, impressive sensitivity to electricity, and appealing stability encourage their use in catalysis. These extraordinary properties are closely related to their structures and compositions [18]. Another major issue with catalysts especially in OER is their stability. Due to the absence of organic ligands, POM catalysts have appealing hydrolytic and oxidative stability that is essential for OER catalysis since the catalyst must react with H2 O producing O2 . Polyanions of POM have a high resistance to oxidants. As a result, POMs-based catalysts are suitable catalysts for use in rigorous conditions including in both acidic and alkaline solutions. The POMs such as polytungstates, polymolybdates, and polyvanadates shown in Scheme 1 have fundamental hydrolytic properties that are compatible with lower

Scheme 1 Common POMs used as OER catalysts [3, 4, 18, 41]

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pH values, whereas polyniobates and polytantalates are compatible with higher pH values [19]. When these elements are combined in an appropriate synthesis, such as to prepare polytungstotantalates, the resulting POM is hydrolytically stable in intermediate pH values [19]. Most of the POMs are diamagnetic except for polyoxovanadates(IV) due to the presence of transition metal ions having a d 0 electrical configuration. However, these diamagnetic POMs can be converted into paramagnetic in two ways [20]: Either POMs can serve as ligands that coordinate with paramagnetic ions, such as Ni(II), Fe(II), Fe(III), Cu(II), Co(II), and Mn(II) resulting in paramagnetic POMs; or they can be reversibly reduced to mixed-valence species by injection of an electron.

3 Electrocatalytic OER The catalytic performance of electrochemical OER is examined in terms of onset overpotential (ηonset ), characteristic overpotential at the current density of 10 mA/cm2 (η10 ), Tafel slope, turn-over frequency (TOF), turn-over number (TON), etc. In addition, these parameters are generally compared with those of noble catalysts of OER such as IrO2 and RuO2 . It is mentioned that the transition-metal oxides show remarkable activity in OER in alkaline media, but their stability decays in acidic media. POMs-based metal-phosphates that are highly stable in acidic and basic conditions are the promising choice as OER catalysts. However, the redox potential of the high-valent core metals of POMs is insufficient for the abstraction of an electron from H2 O. The necessary activation of POMs for OER is generally accompanied by incorporating active transition metals. The catalytic performance of different forms of POMs towards OER in different media is described below.

3.1 Metal Phosphates and Phosphonates As efficient OER catalysts, transition metal (especially 3d transition metal) phosphate (PO4 3− ) and phosphonate (R3 O2 P = O) have attracted a lot of attention. Due to high structural and chemical stability, they can withstand drastic application conditions such as they are quite stable both in acidic and basic media. 3d transition metals can provide a wide range of frameworks that frequently undergo redox reactions involving mixed-valence oxidation states of the respective metals during traditional catalytic reactions [21]. Co-based phosphates and phosphonates are the most studied electrocatalysts used for OER. The catalytic performances of different catalysts are represented in Fig. 2 and Table 1. Typically, Co-based phosphate exhibits better OER performance depicted in Fig. 2a [22]. The surface area and porosity of these catalysts may influence the OER catalysis as evidenced by Brunauer–Emmett–Teller (BET) analysis [22]. For example, the Co-phosphate, Ni-phosphate, and Ni-Co-phosphate have surface

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areas of 154, 121, and 117 m2 /g with corresponding pore volumes of 0.0929, 0.0758, and 0.0523 cc/g, respectively (Fig. 2b). Heteroatom-doped metal phosphates that can easily be synthesized have been used as efficient OER catalysts. Fe-doped three-dimensional Ni-phosphate/Ni-foam exhibits better OER performance compared to Ni-phosphate/Ni-foam [24] due to the prominent synergistic effect between Ni-phosphate and Fe. The consensus is that iron doping can enhance the electrical conductivity of the catalysts and act as a charge regulator for Ni–Fe dual sites, which would increase OER performance compared to single metal sites [24]. More than one metal heteroatom can also be doped and the

Fig. 2 a LSV curves of CoPO, NiPO, and NiCoPO in 1 M KOH measured at a scan rate of 10 mV/s, b Curves showing the distribution of pore size of (a) CoPO, (b) NiPO, and (c) NiCoPO. “Adapted with permission [22], Copyright (2020), Chemistry Europe”. c LSV curves of CoPIm, NiCoPIm and NiPIm in 1 M KOH at a scan rate of 10 mV/s. “Adapted with permission [23], Copyright (2020), Elsevier”

Table 1 Metal phosphates and phosphonates-based OER electrocatalysis in 1 M KOH solution Catalyst

Synthetic Route Overpotential (mV) ηonset

η10

Tafel Slope (mV/dec)

References

CoPO

Hydrothermal

–

350

60.7

[22]

NiPO

Hydrothermal

–

378

85.4

[22]

A-CoPOx porous nanosheet

Chemical precipitation

–

278

80

[28]

Co3 (PO4 )2 @N–C

Hydrothermal

–

317

62

[29]

FeNiPi/NF

Hydrothermal

–

220

37

[30]

FePO4/NF

–

–

218

42.72

[31]

Fe0.43 Co2.57 (PO4 )2 /Cu

–

290

310

40.2

[32]

Co-phosphonate

Hydrothermal

–

334

58.6

[23]

Ni-Co-phosphonate

Hydrothermal

–

351

66.5

[23]

CoNiPP-600

–

–

264

60

[27]

CoPi-HSNPC

–

270

320

85

[26]

◦ C,

CoNiPP-600: Cobalt nickel phenylphosphonate calcined at 600 CoPi-HSNPC: Cobalt phosphate and sufficient heteroatoms doped carbon; CoPO: Cobalt phosphate; FeNiPi: Iron-nickel phosphate; N–C: Nitrogen doped carbon; NF: Nickel foam; NiPO: Nickel phosphate

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resulting modified catalytic system can catalyze the OER more efficiently. Integrating crystalline Ni1.5 Sn nanoparticles into an amorphous matrix of triMPO4 (M = Sn, Ni, Fe) produced an efficient glass ceramic (Ni1.5 Sn@triMPO4 ) which has a special dualphase structure that can promote the surface rebuilding to an OER electrocatalyst with a high activity [25]. Organic–inorganic phosphonates can be utilized to produce metal phosphates incorporating N, P-doped carbon substrate with intrinsically active metal phosphates and hollow spherical structure [26] for OER in an alkaline solution. Cophosphonate (CoPIm) exhibited great electrocatalytic activity towards OER (Fig. 2c) because of the large catalytic surface area and the development of active CoOOH species during the OER process [23]. The value of η10 for the OER studied with cobalt–nickel-phenyl phosphonate calcined at 600 °C (CoNiPP-600) is 264 mV [27].

3.2 Transition Metal-Based Heteropolyanions: POMs The most popular POMs are those containing Ru, Co, Ni, or Mn since they are active in photochemical, and electrochemical OER. But Co-containing POM is extensively used as a promising OER catalyst [33–41] (Table 2). The first POM-based OER electrocatalyst was Ru containing POM, Na14 [RuIII 2 Zn2 (H2 O)2 - (ZnW9 O34 )2 ] [33]. The electrochemical generation of O2 for this di-Ru-substituted POM molecule has been investigated in a phosphate buffer solution. It has been revealed that Ru-containing POM exhibits an enhanced performance compared to its oxide, RuO2 . This improved activity is thought to be a result due to the proximity of two Ru atoms in POM [33]. However, the noble-metal Ru-containing POM is not feasible for a large-scale application. The abundant metal Co or other metal is more feasible [3, 35–41]. Extraordinary catalysis of OER has been achieved with tetra-Co-sandwiched POM Na10 [Co4 (H2 O)2 (α-PW9 O34 )2 ] [34]. Since then, most of the POM-based metalphosphates containing Co have been studied for electrochemically driven OER such as [Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]16– [3]. Co-POMs maintain their remarkable catalytic activity in OER in acidic media as insoluble cesium/barium salts of [Co9 (H2 O)6 (OH)3 (HPO4 )2 (PW9 O34 )3 ]16– cluster [3]. This improvement suggests a strong influence of the counter-cation in electrocatalysis OER kinetics [3, 37] since both salts contain the same active Co-POM. More surprisingly, barium salts of CoPOMs exhibit superior performance to the state-of-the-art IrO2 in sulfuric acid solution [3]. Lower ηonset , higher current densities at lower overpotentials, and faradaic oxygen evolution demonstrate that barium Co-POM salts are a real alternative to the corresponding noble-metal champions. Mn- and Ni-containing POMs have also been shown to be promising catalysts for OER [35, 36]. There has an effect of POM structure and composition, for example, different nuclearity, charge, or heteroatom on OER activity [3, 35, 37, 38]. In polyoxotungstate based compounds, W may exist as W5+ and W6+ states and the relative percentage of the two oxidation states is an important factor to consider in studying

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Table 2 Electrocatalytic performance of POM-based catalysts Electrolyte ηonset (mV) η10 (mV) Tafel References slope (mV/dec)

Catalyst

Synthetic route

Ba[Co-POM]/CP

Metathesis 1.0 M H2 SO4

88

361

97

[3]

Cs[Co-POM]/CP

Metathesis 1.0 M H2 SO4

196

466

98

[3]

AlO7

SCS

1.0 M H2 SO4

4

620

370

[37]

CoPOM-APTS-rGO

–

Aq. (Neutral)

–

128

74

[50]

Ru4 Si2 -APS/NF

–

Na3 PO4

–

550

172

[51]

Co4 (PW9 )2 @Co/Ni@C

Bulk 0.1 M deposition KOH

–

400

67

[52]

PW12 /Ag/G

–

Aq. (Neutral)

–

540

190

[53]

NiCoFe/TX/PW-CNTPE –

1.0 M KOH

–

210

83

[54]

Co4 POM@CNTF

–

0.1 M KOH

–

323

69

[55]

POM@ZnCoS/ NF

–

1.0 M KOH

–

CV > R6G > PFG

Characteristics of adsorption

• Efficient MG removal

• Potential for reactive textile dyes removal

• Greater potential for cationic dyes removal

• Higher potential for MB sorption

Comments

(continued)

[60]

[59]

[58]

[15]

Reference

Metal Phosphates for Environmental Remediation: Adsorptive Removal … 307

Basic/cationic dye • CV

• γ-ZrP Zr(PO4 )(H2 PO4 )·nH2 O • γ-ZrP/SUR+ (Intermediation of cationic surfactant Praepagen HY) • Intercalation of HY surfactant in γ-ZP (γ-ZrPHY)

• [λ-carrageenan-CaP] • [Sodium alginate-CaP] modified with dimethyl diallyl ammonium chloride and diallylamin co-polymer

• Cellulose and silk fibroin Textile/basic/cationic dye • MB dissolved in 1-Butyl-3methylimidazolium chloride [Bmim][Cl] • And regenerated with ethanol to form homogenous blend of regenerated cellulose/silk fibroin

Layered zirconium phosphates (ZrP) • γ-ZrP • γ-ZrP/SUR+

Alginate and carrageenan bio-polymers • [λ-carrageenan-CaP] • [sodium alginate-CaP]

Calcium phosphate biocomposites • Cellulose/silk fibroin/CaP biocomposite • Regenerated cellulose/silk/CaP biocomposite

Acid/anionic dye • Acid Blue 25

Dye

Synthesis method

Metal Phosphate

Table 1 (continued)

• Pseudo-2nd order • Langmuir

• Pseudo-2nd order (cationized hybrid materials) • Chemisorption (cationized hybrid materials) • 1st order [unmodified sodium alginate-CaP] • Freundlich

• Unmodified [sodium alginate-CaP]: ≤ 3.4 • Cationized [sodium alginate-CaP]:446 • [λ-carrageenan-CaP]: 195

• Biocomposite: 172.4 • Regenerated cellulose/silk fibrin blend: 120.4

–

Characteristics of adsorption

• γ-ZrP: 320.20 • γ-ZrPHY: 322.48 (RT) dye uptake (%) • γ-ZrP: 18 • γ-ZrPHY: 19

Adsorption capacity (mg/g)

• Efficient for organic dyes

• Effective for cationic/ anionic dyes • Effective for acid dyes

• Efficient for triphenylmethane dyes (MG, CV)

Comments

(continued)

[3]

[9]

[4]

Reference

308 T. Kopac

Synthesis method

• Cobalt ferrite NPs capped with ultrathin phosphate layer synthesized by co-precipitation followed by hetero condensation of free surface OH- groups of H2 PO4 on CoFe2 O4 particles

Modification of barium phosphate composites by changing reactants/ Surfactants to form • Barium sodium phosphate (NaBaPO4 ) (NBP) • Barium phosphate (Ba3 (PO4 )2 ) (BP) • Barium hydroxyl phosphate (Ba5 (PO4 )3 OH) (BPO) • Barium hydrogen phosphate (BaHPO4 ) (BHP) using a one-step hydrothermal method Positively charged cetyl trimethyl ammonium bromide (CTAB) on phosphate composites

• Synthesis of HAP via the green chemistry approach using leaf extract of copper pod tree

Metal Phosphate

Phosphate capped CoFe2 O4 NPs (CFP)

Barium phosphate composites • Barium sodium phosphate (NaBaPO4 ) • Barium phosphate (Ba3 (PO4 )2 ) • Barium hydroxyl phosphate (Ba5 (PO4 )3 OH) • Barium hydrogen phosphate (BaHPO4 )

Synthetic hydroxyapatite (HAP)

Table 1 (continued)

Textile/acid/anionic/diazo dye • Acid Blue 113 (AB113)

Textile/basic/cationic dye • MB

Cationic/anionic dyes • MB (textile, basic/ cationic) • Brilliant Blue R (textile/ acid/anionic/ anthraquinone) • Bromo Phenol Blue (slightly anionic) • Rhodamine B (basic/ cationic)

Dye –

Characteristics of adsorption

• 120.48 (RT) Dye removal (%) 92.72

• Pseudo-2nd order (chemisorption) • Freundlich dominates Langmuir • Spontaneous, exothermic process

• NBP(249.6) > BP(226.0) > BPO(162.5) > BHP(85.84) • Pseudo-2nd order • Langmuir (NBP, BPO) • Freundlich (BP, BHP)

• MB: 4.5 • BBR: 3.6 • BPB: 1.9 dye removal (%) • MB: 89.5 • BBR: 72 • BPB: 74

Adsorption capacity (mg/g)

• Economical ecofriendly promising alternative for AB113

• NBP suitable for MB removal (high alkalinity)

• Phosphate moieties enhance MB removal • Dye removal with no labor intensive steps

Comments

(continued)

[2]

[17]

[61]

Reference

Metal Phosphates for Environmental Remediation: Adsorptive Removal … 309

• Fe3 O4 NPs synthesized Textile/acid/anionic/diazo using Thunbergia dye • AB113 grandiflora leaf extract doped with HAP from waste bivalve clamshells to produce HA/Fe3 O4 NPs

Hydroxyapatite/magnetite nanocomposite (HA/Fe3 O4 NPs)

Dye

Synthesis method

Metal Phosphate

Table 1 (continued)

• 109.98 Dye removal (%) 94.38

Adsorption capacity (mg/g) • Pseudo-2nd order • Langmuir • Exothermic process

Characteristics of adsorption • Novel nanosorbent for anionic dyes

Comments [6]

Reference

310 T. Kopac

Metal Phosphates for Environmental Remediation: Adsorptive Removal …

311

Alginate and carrageenan bio-polymers are known to be suitable sorbents for cationic dyes. Sebeia et al. [9] studied the synthesis of [λ-carrageenan-CaP] and [sodium alginate-CaP] modified with diallylamin co-polymer and dimethyl diallyl ammonium chloride and reported the sorption of an anionic dye Acid Blue 25. The potential of cellulose/silk fibroin/CaP biocomposite for the adsorptive removal of organic dyes (MB) was investigated by Salama [3], in which the biocomposite was prepared by dissolving the silk fibroin and cellulose in 1-Butyl-3-methylimidazolium chloride [Bmim] [Cl], then regenerating with ethyl alcohol to obtain a homogenous mixture of regenerated cellulose/silk fibroin. The study was further extended for the investigation of the bioactivity of the materials. In a study, cobalt ferrite (CoFe2 O4 ) NPs (CFP) capped with a layer of ultrathin phosphate were synthesized via co-precipitation followed by hetero condensation of free surface OH- groups of phosphoric acid on CFP [61]. The formation of CFP and the identification of surface-bound phosphate groups were confirmed by the XRD and the FTIR results, respectively. The sorption of water molecules via hydrogen bonding with PO4 3− moieties capped on CFP caused the hydrophilic nature of particles. The effect of various functional groups of dyes with PO4 3− groups on CFP, along with the modes of interactions were studied for the adsorption of different dyes [61]. Zhaozhao et al. [17] studied the modification of barium phosphate composites by the variation of the reactants/surfactants to obtain barium sodium phosphate (NBP), barium phosphate (BP), barium hydroxyl phosphate (BPO), and barium hydrogen phosphate (BHP) employing a one-step hydrothermal method, and investigated the MB dye sorption in a batch system. The measurements were conducted for the optimization of the experimental variables such as the temperature, pH, initial dye concentration, contact time, and the adsorbent quantity. The effect of the cetyl trimethyl ammonium bromide (CTAB), which is positively charged, on the dye removal capacities of phosphate composites was also studied. HAP synthesis utilizing leaf extracts of copper pod tree and the adsorption capability in the removal of AB113 dye was reported by Vinayagam et al. [2] (Fig. 4). The HAP formation in the form of rod shapes with prominent elements P and Ca was confirmed by the FESEM-EDS, the TEM, and the XPS characterizations. The crystal structure, the thermal and the colloidal suspension stabilities, and the porous structure were analyzed by XRD, TGA, Zeta potential, and the BET measurements, respectively. The sorption of AB113 was investigated by the variation of the process parameters. The magnetite NPs (Fe3 O4 NPs) sourced from Thunbergia Grandiflora leaf extract synthesized by Pai et al. [6] were doped with HAP using bivalve clamshells waste to obtain hydroxyapatite/magnetite (HA/Fe3 O4 NPs) nanocomposite (Fig. 5). The crystal structure and the irregular spherical particle shape of the HA/Fe3 O4 NPs were confirmed through the XRD and FESEM analyses. The identification of specific elements along with functional groups of both iron oxide and hydroxyapatite NPs were analyzed by the EDAX and FTIR techniques, respectively. The superparamagnetic property was examined by VSM analysis. The adsorption process for AB113 dye removal from aqueous solutions was optimized by employing the central composite design.

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Fig. 4 The synthesis process of CP-HAp nanoparticles. Adapted with permission [2]. Copyright (2022), Elsevier

Based on the literature findings on the use of metal phosphates for the adsorptive removal of dyes from aqueous media, the major results reached can be given as follows: For cation-exchange materials used as sorbent, the interactions taking place between the dye functional groups and the material matrix could be in the form of either Coulombic, covalent, weak van der Waals forces, or hydrogen bonding [15]. The natural phosphate NPs could remove the cationic dyes (BY 28, MB), whereas showed a weak adsorption affinity for the anionic dye (RY 125). The sorption process was pH and temperature dependent with high sorption ability for cationic dyes (BY 28, MB) in the basic medium, and high sorption of RY 125 in the acidic medium. The temperature rise increased the MB, BY 28 adsorbed amounts, while decreasing the RY 125. The NPs showed a higher sorption capability for the cationic dyes with no additional costly equipment requirement [58]. The use of synthetic calcium phosphates (PTCa, OCPa) reduced significantly the pollution discharge from textile wastewater. The adsorption kinetic study for the textile dye RY4 from aqueous solutions showed that the equilibrium was established fastly for both phosphates. PTCa and OCPa showed a higher potential for the sorption of reactive textile dyes from industrial wastewater, offering the advantages of easy and low-cost preparation in a large scale [59]. For MG intercalation in γ-ZrP and γ-ZrP/SUR+ , about 12–16% maximum ionexchange capacity was reported by molecular modeling calculations based on the structure of the material and the dye dimensions, reflecting a high capacity in the uptake of dye. Impure phases with γ-ZrP and γ-ZrP/SUR+ (14, 18% dye loading)

Metal Phosphates for Environmental Remediation: Adsorptive Removal …

313

Fig. 5 Synthesis of HA/Fe3 O4 NPs. Adapted with permission [6]. Copyright (2022), Elsevier

were observed via XRD measurements. The observation of the difference in the dye loadings was due to the interlayer distance and the variable cross-section of dye relying on its orientation and/or conformation within the interlayer region. Both layered materials were efficient in the removal of MG [60]. In the intercalation study using γ-ZrP and γ-ZrP/SUR+ for the removal of CV from an aqueous solution, the pristine γ-ZrP and γ-ZrPHY showed higher uptake behavior (ion exchange) for CV (18, 19%), that were close to the molecular modeling (20%) calculation results. The dye uptake was also dependent on the interlayer distances of the layered composites and the mode in which dye molecules were arranged inside the interlayer matrix. The maximum values of dye uptakes were obtained for the case where the dye molecules were arranged perpendicular to the inorganic layers. Pre-intercalating γ-ZrP with the surfactant molecules facilitated the diffusion of the dye molecules for a smooth exchange within the interlayer gallery [4]. The γ-ZrP and

314

T. Kopac

γ-ZrPHY composite were extremely effective for the removal of triphenylmethane dyes MG and CV [4, 60]. The evidence of interactions between the apatite structure exposed to modification with dimethyl diallyl ammonium chloride and diallylamin co-polymer (alginate and carrageenan bio-polymers) based hybrid materials and the cationized agent was examined through various analysis methods [9]. The materials were confirmed to be effective sorbents for Acid Blue 25, while the sorption process displayed dependence on the experimental variables. It was favorable at low temperatures with decreasing randomness. The [λ-carrageenan-CaP] and [sodium alginate-CaP] exposed to dimethyl diallyl ammonium chloride and diallylamin co-polymer were found to be effective for anionic dyes, while alginate and carrageenan bio-polymers were reported to be suitable for cationic dyes in the previous works. High sorption affinities of cationized hybrid compounds suggested their use as efficient sorbents for acid dyes from wastewater [9]. The cellulose/silk fibroin/CaP biocomposite indicated high MB removal efficiency compared to the regenerated cellulose/silk fibrin blend. The adsorption was favorable at a slightly alkaline medium. The biocomposite exhibited high homogeneity between the inorganic and organic phases, and the role of CaP NPs as crosslinking points between the polymer chains was supported by swelling tests. The regenerated cellulose/silk/CaP composite provided promise for further research for the remediation of dye-containing wastewater. The cellulose/silk fibroin/CaP material was presented as a cost-effective and sustainable platform for effective dye removal from aqueous media [3]. Related to the phosphate-capped CoFe2 O4 NPs (CFP), phosphate ions were adsorbed over CFP at pH ≥ 9 by the hetero condensation of free surface OH- groups on the particles, where surface OH- groups were exchanged by phosphate ions. The phosphate ions adsorbed caused the particles to be water-dispersible and hydrophilic greatly enhancing MB adsorption through electrostatic interaction. The smaller and linear MB dye molecular structure facilitated its adsorption on the CFP, however, the anionic BBR indicated weak electrostatic adsorption owing to the protonation of some OH- groups over phosphate moieties of the CFP, while BPB had even less adsorption on CFP. The phosphate ions on CFP induced aggregation of RB molecules, causing the broadening and distortion of absorption peaks at higher RB concentrations, overweighing the inherent electrostatic attractions between the phosphates on CFP and the cationic RB. Dye molecule adsorption over the particles was dependent on the type of interactions between the available functional moieties. Adsorption over CFP did not require any further intensive treatment steps like centrifugation, precipitation, or filtration [61]. Regarding the BP, BHP, BPO, and NBP composites synthesized via a simple onestep hydrothermal treatment by varying the reactants and surfactants, while cetyl trimethyl ammonium bromide (CTAB) and SDBS indicated no significant effect on the materials crystal type and composition, they caused small differences in the MB removal efficiencies. Positively charged CTAB on phosphate composites led to a slight increase in the MB removal capacity though it did not have any appreciable effect on the material composition and crystal type. NBP was most effective in MB

Metal Phosphates for Environmental Remediation: Adsorptive Removal …

315

sorption owing to its high alkalinity. Increasing solution pH values upon the addition of the materials led to improved MB color removal and thus enhanced material removal capacities (Fig. 6). Hydrogen bonds formation between the PO4 3− content of composites and N–H or C–H of MB, along with strong Lewis acid–base interaction between cations (Ba2+ /Na+ ) in NBP and –SO3 − in MB were effective for the removal of dyes [17]. The hydroxyapatite/magnetite nanocomposite (HA/Fe3 O4 NPs) prepared from bivalve clamshells as a calcium precursor for HAP and Thunbergia Grandiflora leaf’s extract (TGLE) as a reducing agent for Fe3 O4 NPs, showed the presence of crystalline phase, irregular spherical particles, specific elements along with functional groups of both iron oxide and HAP NPs. HA/Fe3 O4 NPs were effective for the removal of AB113 dye and were proposed as an effective novel nanosorbent for the removal of anionic dyes from wastewater [6]. The leaf extract obtained from the copper pod tree was powerful for HAP NPs (CP-HAp NPs) synthesis with suitable physicochemical and morphological properties [2]. A mesoporous structure with an appreciable high specific surface area was obtained. The NPs dose, pH, initial AB113 concentration, agitation speed, and contact time were the affecting factors

Fig. 6 Removal capacity of MB by NBP, BP, BPO, and BHP from water (C 0 (MB) = 250 mg/L, 50 mL, 25 °C, 60 min): (a) at a different temperature, (b) at different pH, and (c) with different dosages. Adapted with permission [17]. Copyright (2021), Elsevier

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on the adsorption. CP-HAP NPs were proposed as an eco-friendly, cost-effective promising sorbent to remove AB113 from waste streams [2]. It is desirable that the optimal sorbents should be chemically stable, cheap, possess a high surface area with the availability of a large number of active sites, easily regenerable and recoverable. Additionally, when the dyes are adsorbed onto the solid material, they should not be degraded to toxic components, while photocatalytic degradation is preferred usually [57]. Layered zirconium phosphate is a promising material in the removal of dyes because of its convenient thermal and chemical stability, and excellent intercalation and cation exchange properties. The uptake of organic dyes by ZrP can occur via both acid–base interactions and ion exchange processes. ZrP upon combination with photosensitive materials, such as graphene oxide, titania, silver NPs, carbon nitrides, and silver/silver halide heterojunctions displays improved photocatalytic efficiency because of a more effective electron–hole separation and light absorption [57]. HAP-based sorbents are promising for the adsorptive removal of heavy metals and dyes, as well as some other pollutants, owing to their unique properties conferring on their thermal stability, low water solubility, ionic exchange capability, and adsorption affinities towards many pollutant species. The process costs can be lowered via the utilization of naturally available materials for the calcium and phosphate precursors and the choice of green synthesis methods using minimized chemicals amount and energy consumption requirements. The complication in the removal of the sorbent and its inconsistency at severe conditions related to HAPs can be prevented by the formation and utilization of HAP composites. Their specific properties can be improved by the addition of materials such as cobalt, silver, zirconium, iron, chitosan, and other metals/non-metals resulting in higher adsorptive capacity, and increased stability sorbents. The mechanical and physicochemical properties can be improved by employing HAP composites formed by HAP doping, for the efficient removal of dyes from aqueous media. Dye removal from aqueous media by HAP is a multiparametric process depending mostly on parameters such as temperature, dye concentration, pH, and contact time. HAP-based sorbents are recommended as a cost-effective alternative for wastewater treatment [7].

4 Conclusions Based on the literature findings on the use of metal phosphates for the adsorptive removal of dyes from wastewater, the following conclusions were drawn: The optimal sorbents should be mechanically and chemically stable, low cost, possessing a high specific surface area with a large number of available active sites, and easily regenerable and recoverable. It is desirable that when the dye adsorption process on the porous solid material takes place, they should not be degraded to toxic products, while photocatalytic degradation is most commonly employed [57]. The costs related to the adsorptive treatment process can be minimized by lowering the

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sorbent costs, through the employment of naturally occurring materials as alternatives for the calcium and phosphate precursors, and through selecting a synthesis methodology involving less chemical and energy requirements [7]. Natural phosphate NPs have a high potential for the effective removal of cationic dyes without the need for any further costly equipment. The use of synthetic CaPs (PTCa, OCPa) showed potential for the removal of reactive textile dyes from industrial wastewater, offering advantages like low-cost and easy preparation on large scale. The layered materials γ-zirconium phosphate (γ-ZrP) and surfactant-treated γZrP (γ-ZrP/SUR+ ) revealed efficient removal of MG dye. The differences in the dye loadings between the pristine γ-ZrP and γ-ZrP/SUR+ were due to the differences in the interlayer distance and the variable cross-sections of the dyes that depended on its orientation and/or conformation within the interlayer space. γ-ZrP and zirconium phosphate/surfactant composite (γ-ZrPHY) were extremely efficient in the adsorptive removal of triphenylmethane dyes MG and CV. The [λ-carrageenan-CaP] and [sodium alginate-CaP] modified with dimethyl diallyl ammonium chloride and diallylamin co-polymer were found to be effective for the removal of anionic dyes, while in previous works alginate and carrageenan bio-polymers were reported to be suitable for cationic dyes. The high sorption capabilities obtained for the cationized hybrid compounds suggested their application as effective sorbents for acid dyes. The cellulose/silk fibroin/CaP biocomposite was a cost-effective sustainable platform for the effective removal of organic dyes from aqueous media. The regenerated cellulose/silk/CaP biocomposite provided prospects for further applications in the remediation of dye-containing wastewater. The adsorbed phosphate moieties over CoFe2 O4 particles (phosphate-capped CoFe2 O4 NPs (CFP)) caused the particles to be water dispersible and hydrophilic and greatly enhanced the MB dye adsorption by electrostatic interactions. The smaller linear MB molecular structure facilitated its adsorption on CFP, whereas the anionic BBR exhibited weak electrostatic interactions owing to the protonation of some OHgroups on phosphate ions of CFP. BPB exhibited much lower adsorption levels on CFP. The phosphate groups on CFP induced aggregation of RB dye molecules, which overweighed the inherent electrostatic attractions between cationic RB and phosphate ions on CFP. Dye adsorption on the particles depended on the type of interactions between the functional groups. The dye removal process with the phosphate-capped CoFe2 O4 nanoparticles (CFP) did not require any further intensive steps such as precipitation, centrifugation, or filtration [61]. Regarding the phosphate composites (NBP, BP, BPO, BHP), positively charged cetyl trimethyl ammonium bromide (CTAB) had a slight effect to increase their MB sorption capacities. NBP was the most suitable material for MB removal owing to its high alkalinity. The increased pH solution values due to the addition of the materials enhanced MB decoloration and hence improved material removal capabilities. Hydrogen bonds formed between PO4 3− in materials and N–H or C–H in MB, along with the stronger Lewis acid–base interactions between cations (Ba2+ /Na+ ) in NBP and –SO3 − in MB were effective in the sorption process [17].

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The hydroxyapatite/magnetite nanocomposite (HA/Fe3 O4 NPs) prepared from bivalve clamshells (calcium precursor) and Thunbergia Grandiflora leaf’s extract (TGLE) as a reducing agent for Fe3 O4 NPs was suitable for the adsorption of AB113 dye from aqueous media. It was proposed as an effective nano sorbent for the adsorptive removal of anionic dyes from aqueous waste streams [6]. The leaf extracts obtained from the copper pod tree were effective for the synthesis of HAP NPs (CP-HAp NPs) with convenient physicochemical and morphological properties, and a mesoporous structure with a significant specific surface area. CP-HAp NPs dosage, solution pH, AB113 concentration, agitation speed, and contact time were the affecting parameters on the adsorption. The CP-HAp NPs were proposed as an ecofriendly, low-cost promising sorbent to remove AB113 from waste streams [2]. Layered zirconium phosphate was offered as an effective material for the removal of dyes owing to its good thermal and chemical stability, and its excellent intercalation and cation exchange properties. The uptake of organic dyes by ZrP occurs through both ion exchange processes and acid–base interactions. Upon combination with photosensitive materials, such as graphene oxide, titania, silver NPs, carbon nitrides, and silver/silver halide heterojunctions, ZrP composites show improved photocatalytic activity because of a more effective electron–hole separation and light absorption [57]. HAP-based sorbents are promising for the removal of dyes, heavy metals, and various other emerging pollutants, due to their unique structures and properties conferring their thermal and chemical stability, low water solubility, ionic exchange capability, and high adsorption affinity towards many pollutants [7]. The difficulty in the removal of the sorbents and their instabilities at severe conditions related to HAPs can be inhibited by forming HAP composites. The specific properties can be improved by using cobalt, iron, chitosan, silver, zirconium, and other metal/non-metal compounds, resulting in higher adsorption affinity, improved thermal and chemical stability, improved physicochemical properties and easy removal of the sorbent for the efficient removal of dyes from aqueous media. HAp-based sorbents were revealed as a cost-effective option for adsorptive wastewater treatment [7]. Adsorptive removal of dyes is a multi-parametric process mostly depending on factors like solution pH, temperature, contact time, and dye concentration, optimization of these factors should be considered for the maximal effectiveness of the process. Henceforward, metal phosphates and their hybrid composites could be among the promising sorbents for wastewater treatment-based technologies. The subject of metal phosphates and their utilization as sorbents for environmental remediation will continue to attract attention in near future.

5 Prospects The following future perspectives could be recommended on the use of metal phosphates for the adsorptive removal of dyes from wastewater:

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Further investigations are necessary for further developments of the topic. Several challenges related to the utilization and application of metal phosphates, such as the synthesis techniques, their stability, production and regeneration costs, and real-time applications should be overcome. The works could further be extended for their investigation into other environmental remediation applications, such as the removal of pesticides and heavy metal ions. The viability of natural metal phosphates as a safe, cost-effective sustainable sorbent derives from its easy synthesis and the precursors sourced from biogenic wastes. However, the use of HAP is usually limited by issues related to its stability, porosity, and surface functionality which negatively affects its adsorption performance. Thus, a study on the enhancement of its functionality by the variation of the preparation conditions could be helpful. Attention should be given to the development of HAP composites either via doping or functional modification. Future work should be extended focusing on the assessment of the performance of HAP-based composites for the remediation of multi-component systems. Further studies would be helpful in the development of sorbents with high stability under acidic and basic conditions and in improving their stability. Most of the published studies refer to laboratory scale experiments, so the studies need to be extended to prove the effectiveness of the applications of these materials on a larger scale, where other aspects, such as the economic advantages or disadvantages, should also be assessed. It is important to improve the reusability of sorbents. Some attention needs to be focused on the reusability and regeneration potential of sorbents for establishing their sustainability. There are only a few studies reported on the regeneration of sorbents. Operational parameters (temperature, pH, dye concentration, sorbent dosage, time) affecting regeneration should be investigated. Only a few studies have been made with real effluents using most of the sorbents. The issues related to the disposal of used sorbents and left dyes following the treatment processes should seriously be considered. Future advances in wastewater treatment should cover investigations with a special focus on the synthesis of sorbents with an eco-friendly approach, large-scale production, high surface area with maximum available active sites, higher stability, high selectivity, fast removal rate, high sorption capability, and repeated reusability.

References 1. Sriram, G., Bendre, A., Mariappan, E., Altalhi, T., Kigga, M., Ching, Y.C., Jung, H.-Y., Bhaduri, B., Kurkuri, M.: Recent trends in the application of metal-organic frameworks (MOFs) for the removal of toxic dyes and their removal mechanism-a review. Sustain. Mater. Technol. 31, e00378 (2022) 2. Vinayagam, R., Pai, S., Murugesan, G., Varadavenkatesan, T., Kaviyarasu, K., Selvaraj, R.: Green synthesized hydroxyapatite nanoadsorbent for the adsorptive removal of AB113 dye for environmental applications. Environ. Res. 212, 113274 (2022)

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Recent Insights in the Utilization of Metal Phosphonates for Remediation of Dye-Polluted Wastewaters Turkan Kopac

Abstract The field of metal phosphonates (MPs) has gained importance for sustainable energy and environmental applications over recent years. They are a prominent kind of metal–organic hybrid structures, revealing potential appliances in materials sciences, catalysis, ion- exchange, separation, and sorption, owing to their superior stability and insolubility in most solvents, which could be attributed to the hard nature of the phosphonate oxygen atoms and higher coordination affinity for metallic atoms. In the remediation of wastewater treatment, the utilization of novel materials with high adsorption capability and removal efficiency is a matter of major significance. The principal aim of this Chapter is to present a survey on the MP-based systems in the elimination of dye-polluted wastewater. The adsorptive removal of dyes from aqueous solution has been presented in the framework of the recently published works that demonstrated the feasibility and efficiency of the materials. Keywords Adsorption · Dye removal · Dye degradation · Metal phosphonates · Photocatalysis · Wastewater

1 Introduction 1.1 Remediation of Dye-Polluted Wastewater by Adsorption The remediation of dye-polluted wastewater comprises significant environmental importance and thus has highly attracted the scientific community. Developing effective wastewater treatment systems is necessary for the remediation of dye-containing effluents discharged into the environment through robust removal technologies [1–5]. Numerous approaches have been described for the recovery of dye-contained effluents. Among the options, adsorption via the use of effective sorbents has emerged as one of the competitive methods, and hence called considerable attention owing to T. Kopac (B) Department of Chemistry, Zonguldak Bülent Ecevit University, 67100 Zonguldak, Türkiye e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_18

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the high efficiency, versatility, simplicity, and low energy requirement of the method along with the possibility of reusability of the sorbing materials [6, 7].

1.2 Adsorbents for Environmental Remediation To date, many research efforts have been realized on the development and analysis of various sorbents for the adsorptive removal of dyestuffs from wastewater, such as clays, ion exchange resins, zeolites, polymeric materials, activated carbons, biochar, carbon nanomaterials, and metal–organic frameworks (MOFs) [2, 8–15]. Although reasonable efficiencies have been reported with most of them, one significant constraint of their implementation is the higher costs very much related to their synthesis and usage. Waste-derived sorbents have low costs, and however, are usually characterized by low sorption capacities. To overcome the challenges related to the application of sorbents, scientific studies have been forwarded recently towards developing eco-friendly, cost-effective, renewable, biodegradable, effective novel materials [7, 16]. In relation to the remediation of dye-polluted wastewater by adsorption techniques and the commonly used adsorbents for environmental remediation, a survey on the MP-based systems in the domain of the elimination of dyepolluted wastewater will be presented in this Chapter, in the framework of the recently published works that demonstrated the feasibility and efficiency of the materials.

2 Metal–Organic Frameworks Among the sorbents, MOFs and their hybrid structures have appeared to be promising, owing to their high effectiveness in environmental remediation [2]. They are a novel type of considerably nanoporous, crystalline compounds that are constructed from metal clusters, or metallic ions attached via organic ligands, exhibiting numerous attractive characteristics, such as great specific surface area, controllable surface properties, well-managed pore structure, and pore size distribution [17, 18]. These aspects make MOFs promising for a large number of areas, including separation, catalysis, gas storage, drug delivery, ion exchange, adsorption, and environmental applications [19, 20]. The diversity of the structural configurations enables MOFs distinctive characteristics of higher chemical, thermal, and mechanical stabilities with application possibilities in a broad range of fields [21]. Their adsorption capabilities are in pretty good correlation with the pore sizes and electrostatic attractions of targeted compounds [20, 22]. MOFs consisting of differing central metal ions connected to organic linkers were examined in many studies as sorbent materials from wastewater [23].

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3 Metal Phosphonates MPs are a type of organic/inorganic hybrid polymeric components constructed by the coordination of phosphonate ligands to metal ions, resulting in numerous structures of many different dimensions [24]. The compounds have been recognized for their complex diversified structural units and topologies. Thus, the field of MPs has been undergoing notable developments in recent years [25, 26]. Mesoporous MP materials as a significant class of non-silica-based inorganic/organic hybrid materials were described to have potential utility in versatile applications such as adsorption, separation, solar light utilization, gas storage, ion-exchange, catalysis, proton conduction, corrosion inhibition, materials chemistry [24, 25, 27–31]. They can be employed as effective sorbents for the adsorption of heavy metal ions from solution, gas-phase adsorption of CO2 , and separation of hydrocarbons. They can also be utilized as photocatalysts for the degradation of organic dyes revealing their potentiality in environmental applications. When further functionalized, the synthesized structures can be employed as oxidation and acid catalysts, with striking efficiencies for sustainable environmental and energy applications. Their high specific surface areas and narrow mesopore size distributions enable them to serve as perfect sorption materials [28–30]. As regards to most of the other metal–organic structures, MPs usually display greater thermal and water stability, based on the existence of inorganic components involved in the structure, the hardy nature of phosphonate oxygen atoms, and stronger coordination affinity towards metallic atoms [24, 31]. The phosphonate groups (– PO3 H− /–PO3 2− ) demonstrate a high affinity for metal ions [25, 32]. The modification of the organic moieties of phosphonate ligands (RPO3 2− ) could be possible through the employment of further functional groups giving rise to the formation of MPs with various versatile structures [31]. Inorganic/organic hybrid mesoporous MP structures can be synthesized using a set of polyphosphonic acids. The network can then be tuned to cellular foam, wormhole-like, cubic and hexagonal morphologies. By the use of organically linked polyphosphonic acids as coupling molecules, the incorporation of a significant amount of organic functional groups into the MP hybrid structures could be obtained. The pH of reacting solution and the availability of organic additives have a significant influence on the textural properties and the morphology of the final MP structures. Nonionic or cationic surfactants could be employed as templates for the preparation of mesoporous MPs [29]. Transition-MPs are MP coordination polymers produced by introducing organic moieties into the inorganic structure. The organic groups are homogeneously distributed within the network, regulating their physicochemical characteristics. They display remarkable stability based on the powerful R–P–O-metallic bondings, where R designates the organic functional groups [33–35]. The attractiveness of MPs is based on the controllable and functional characteristics of the modulation of both phosphonic and metallic components, which considerably extend their feasible relevancy. Relating to transition-metallic oxides, the ligand groups contained in transition-MPs exhibit versatile coordination and orientations, leading

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to achieving superior performance in catalytic applications [36]. MPs have been extensively examined as selective catalysts in many oxidative reactions. Sodium exchanged zirconium phosphate phenylphosphonate amorphous [Zr(O3 PC6 H5 )2 ], zirconium [Zr(O3 PCH2 CH2 COOH)2 ], and aluminum carboxymethylphosphonate [Al3 (OH)3 (O3 PCH2 CH2 CO2 )2 ·3H2 O] have been employed as selective catalysts in the Baeyer–Villiger oxidation reactions [37]. The hexagonal and cubic periodic mesoporous titanium phosphonates (PMTP) linked with distinct organic groups were obtained via the surfactant-templating method [30, 38–40]. The organic functional groups involved mainly assist in the formation of complexes with metal ions via the acid–base reactions. The solid support enables the straightforward recovery of the loaded sorbent from the polluted aqueous phase [28, 30, 41]. The synthesis along with the physicochemical characterization of MPs and phosphonate MOFs and their application as sorbent materials for the remediation of colored wastewaters have been described in the literature [20, 21, 27, 30, 42–48].

4 Use of MPs in the Adsorptive Removal of Dyes from Aqueous Media Table 1 shows some examples of MPs examined for the remediation of dye-polluted wastewaters reported in the literature, involving the type of MP, the followed synthesis method, the type of dye, the notes related to the characteristics and the mechanism of adsorption, and the corresponding references. Among the MPs examined in the reported studies were titanium, calcium, cadmium, manganese phosphonates, phosphonate-based MOFs, and some novel MP-based nanomaterials. Titanium-hydroxyethylidene diphosphonate (TiHEDP), which is an ion exchanger hybrid material of a tetravalent metal acid salt was prepared using a sol–gel technique. Its chemical stability in organic solvents, acids, and bases was examined [27]. Sorption characteristics of TiHEDP towards dyes pink FG (PFG), methylene blue (MB), crystal violet (CV), and rhodamine 6G (R6G) were investigated using adsorption isotherms and thermodynamic analysis [27]. Nanoscale calcium aminodiphosphonates (CadP) having different morphologies, such as nano-networks, nanorods, and nanobelts were synthesized using surfactant-assisted solvo/hydrothermal methods, and the effect of experimental factors including solvent, base, and the surfactant types, along with concentration was examined [42]. Sorption of the cationic basic dye MB from aqueous media on the CadP-N and the effects of adsorbent morphology, dose, contact time, pH, and initial dye concentration on sorption were evaluated [42]. Periodic mesoporous titanium phosphonates (PMTP-1) were prepared and examined for the elimination of MB from aqueous media using batch-wise adsorption analysis under varying experimental conditions, such as adsorbent dosage, initial dye concentration, contact time, temperature, and, solution pH with the optimization of the impact factors. Adsorption isotherms were evaluated for the analysis of the adsorption process [30]. A MnII phosphonate [Mn(H2 L)2 (H2 O)2 (H2 bibp)]

Adsorption capacity (mg/g) • 617.28 (MB)

Adsorption capacity (mg/g) • 79.37 (313 K) (CV) 15.6 (323 K) (CV) • 56.49 (313 K) (R6G) 416.66 (323 K) (R6G) • 500.0 (313 K) (MB) 454.54 (323 K) (MB) • 27.17 (313 K) (PFG) 25.64 (323 K) (PFG)

Periodic mesoporous titanium • Synthesis using EDTMPS (coupling Cationic/basic dye phosphonate (PMTP-1) molecule), TiCl4 (inorganic • MB precursor), cationic surfactant CTAB (structure-directing agent) • Stirring, autoclaved, crystallization, evaporation, filtration, extraction

• Surfactant-assistant solvo/hydrothermal methods • CadP-N with different morphologies (nanobelts, nanorods, nano-networks)

Nanoscale calcium aminodiphosphonates (CadP-N)

Basic/cationic dyes • Crystal violet (CV) • Rhodamine 6G (R6G) • Methylene blue (MB) • Pink FG (PFG)

Characteristics of adsorption

• Langmuir isotherms (monolayer) • Pseudo-second-order kinetics • Chemisorption

• Langmuir, Freundlich isotherms

• Langmuir isotherms (CV, PFG) • Freundlich isotherms (R6G, MB) • Exothermic (CV, MB, PFG) • Endothermic (R6G)

• Useful photocatalysts for organic dye degradation

Adsorption capacity (mg/g) • 138.89 (MB)

• Sol–gel method • Calcination

Titanium-hydroxyethylidene diphosphonate (TiHEDP)

Type of dye • Organic dyes

Basic/cationic dye • MB

Synthesis method

• Organic/inorganic hybrid mesoporous MPs from polyphosphonic acids • Tuning structure to wormhole-like, cellular foam, hexagonal, cubic morphologies • Using cationic/nonionic surfactants as templates

Type of MP

MP hybrid mesostructures

Table 1 Metal Phosphonates for dye removal

[27]

• Efficient for the removal of textile dyes (MB) from effluents

(continued)

[30]

• Efficient MB removal [42] from wastewater • Ability for recycling use

• Sorption affinity MB > CV > R6G > PFG • Higher potential for MB sorption

References [29]

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• Synthesis of nanomaterial containing • MB a tri-cationic phosphonate ionic • Allura red (AR) liquid (TCPIL), copper ferrite magnetic nanoparticles (CuFe2 O4 NP), partially oxidized modified boron nitride nanosheet (BNONS) • Synthesis of 3 ionic liquids (ILs) separately [DABCO]+[PDOL]−, [APIC]+[PDOL]−, [PYR]+[PDOL]−, then using for preparing TCPIL {[DABCO, PYR, APIC-PDOL]+[ATMP]−}ionic liquid. Then using CuFe2 O4 NPs, BNONS to synthesize (TCPIL/CuFe2 O4 /BNONS)

Triphosphonated ionic liquid-CuFe2 O4 -modified boron nitride (TCPIL/CuFe2 O4 /BNONS)

Type of dye • Methyl orange (MO) • Safranine O (SO) • CV • Rhodamine B (RB) • MB • R6G

Synthesis method

• In the form of a layered motif with protonated H2 bibp2+ cations embedded in the channels • 1 Y (H4 L = thiophene-2-phosphonic acid, bibp = 4,4' -bis(1-imidazolyl)biphenyl) • Light-yellow block-shaped crystals of 1 Y from MnCl2 ·4 H2 O, H4 L, bibp under solvothermal conditions • Elemental analysis 1 Y C22 H26 N4 O8 P2 SMn (M r = 623.41): C 42.38, H 4.20, N 8.99

Type of MP

Mn-based photochromic MOF MnII phosphonate MOF [Mn(H2 L)2 (H2 O)2 (H2 bibp)]

Table 1 (continued)

• Making different • Zero order (AR) nanomaterials by • Pseudo first order (MB) combining different d-block metals (Zn,Pd,Pt,Mn) with BNONSs

• Efficient for the reduction of dyes in aqueous solution at ambient temperature • Easily recovered nanomaterial • Reusable (≥7) with negligible loss of catalytic activity • Potential biomedical, sensors, catalytic applications

• Rapid, significant • Excellent for rapid decrease from yellow to separation of MO from colorless for MO aqueous solution • Fast adsorption rate, solution high uptake capacity, • Rapid photochromism selective MO removal, upon irradiation with reusability visible light at RT • The first Mn-based photochromic MOF, showing LLCT mechanism with non-photochromic components

Characteristics of adsorption Adsorption capacity (mg/g) • 1337 (MO)

References

(continued)

[44]

[43]

328 T. Kopac

Type of dye • MB • MO

Anionic azo dyes • Congo red (CR) • Reactive red 2 (RR2)

Synthesis method

• Synthesis of organic ligand (H4 L) • Preparing 1,4-piperazinediylbis (methylene) phosphonic acid (H2 O3 P-CH2 -NC4 H8 N-CH2 PO3 H2 ) • Synthesis of STA-12(M), (M = Mn, Fe, Co, Ni) • Straight hydrothermal method

• Preparation of P,P-bis (2-oxooxazolidin-3-yl)-N-(3(triethoxysilyl)propyl)phosphinic amide (APTES-BOP)-modified SBA-15 (SBA-15-BOP) by a post-synthesis grafting method

Type of MP

Phosphonate-based MOFs STA-12(M) (M = Mn, Fe, Co, Ni)

Organophosphorus ligand-modified SBA-15 (SBA-15-BOP)

Table 1 (continued)

Adsorption capacity (mg/g) • 518.1 (CR) • 253.8 (RR2)

• Langmuir isotherms • Pseudo-second-order kinetics • Spontaneous • Exothermal process

• Pseudo first-order kinetics • Photo-degradation rates of dyes over STA-12(Mn), STA-12(Fe), STA-12(Co) and STA-12(Ni) increased with H2 O2 • Synergistic index in the STA12(Fe)/sunlight/H2 O2 system ≈ 431% • Degradation of dyes faster with STA-12(Fe) than other MOFs

Characteristics of adsorption • Photocatalytic activity for degradation of dyes from aqueous solution under natural sunlight irradiation • Capability of STA-12 series MOFs for H2 O2 excitation to high competence under natural sunlight photocatalysis • Efficiency of STA-12(Fe) in photo-Fenton process

References

(continued)

• Promising for the [47] removal of anionic dyes from aqueous solutions

• The first example of [45] facilitating photo-Fenton excitation of H2 O2 via phosphonate based MOFs as photocatalysts • Opportunity for sunlight-induced photocatalytic environmental remediation and protection

Recent Insights in the Utilization of Metal Phosphonates … 329

Type of dye

• CR

Cadmium-phosphonate network Cd4 (H4 L)2 (phen)2 (H2 O)4 (1)

• Prepared based on a tetrahedral shaped tetraphosphoric acid linker resulting in 3D -compound Cd4 (H4 L)2 (phen)2 (H2 O)4 (1)

• MO • RB

Anionic/cationic dyes • Acid Orange 7 (anionic) • Basic Fuchsine (cationic)

phosphonate-based MOFs • Synthesis of organic ligand (H4 L) STA-12(M)(M = Mn, Fe, Co, • preparing 1,4-bis (phosphomethyl) Ni) piperazine, H2 O3 P–CH2 – NC4 H8 N–CH2 –PO3 H2 • synthesis of STA-12(M), (M = Mn, Fe, Co, Ni) • Straight hydrothermal method

Synthesis method

• Prepared by reaction of divalent inorganic salts (CoSO4 ·7H2 O, NiSO4 ·6H2 O, CuSO4 ·5H2 O) with vinyl phosphonic acid in hydrothermal conditions

Type of MP

Phosphonate MOFs cobalt, nickel, copper vinylphosphonate (CoVP, NiVP, CuVP)

Table 1 (continued)

Adsorption capacity (mg/g) • 684 (CR)

• Highest photocatalytic activity of dyes degradation and Cr (VI) reduction at dye/Cr (VI) weight ratio 3:1

• Pseudo-second-order kinetics • Chemisorption

• Cr (VI) reduction ratio increases by addition of dyes

• Langmuir isotherms • Pseudo second-order kinetics • Chemisorption • Spontaneous • Endothermic process

Characteristics of adsorption Adsorption capacity (mg/g) • 35.83 (Acid Orange 7) (CoVP) • 32.63 (Basic Fuchsine) (CoVP) • highest sorption capacities at optimum conditions (pH 4.2 Acid Orange 7, pH 10 Basic Fuchsine, 25 °C) • Adsorption capacities increase order CuVP < NiVP < CoVP in correlation with metallic Radius increase

• Rapid, efficient dye adsorption • Excellent structural stability/ recyclability for 3 adsorption/desorption cycles

• Considerable synergistic photocatalytic activity for dyes degradation and Cr (VI) reduction

• Efficiency for diverse initial dye concentrations, pH, temperatures • Efficiency in elimination of anionic/cationic dyes from aqueous solutions

References

[48]

[46]

[20]

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Fig. 1 Preparation of TCPIL/CuFe2 O4 /BNONS Nanomaterial. Adapted with permission [44], Copyright (2018), Elsevier

was adopted in the form of a layered pattern having protonated H2 bibp2+ cations incorporated into the channels (H4 L:thiophene-2-phosphonic acid; bibp:4,4' -bis(1imidazolyl)biphenyl) [43]. The removal of methyl orange (MO) from aqueous media and the photochromism behavior under irradiation with visible light at ambient temperature were studied [43]. A nanomaterial involving a new tri-cationic phosphonate ionic liquid (TCPIL), copper ferrite magnetic nanoparticles (CuFe2 O4 NPs), and partially oxidized modified boron nitride nanosheet (BNONS) was prepared and evaluated as a sorbent for the removal of dyes [44]. Three different ionic liquids (ILs) were prepared, then used in the preparation of the new TCPIL {[DABCO, PYR, APICPDOL]+[ATMP]−}ionic liquid. Subsequently, CuFe2 O4 NPs and BNONS were utilized for the synthesis of the (TCPIL/CuFe2 O4 /BNONS) nanomaterial (Fig. 1). The catalytic reduction of dyes Allura red (AR) and MB in aqueous media with the synthesized materials at ambient temperature was investigated [44]. The photocatalytic activity of MB and MO dyes degradation from aqueous solution over phosphonate-based MOFs, in the form of STA-12(M)(M = Mn, Fe, Co, Ni) was studied under natural sunlight irradiation [45]. The photo-Fenton oxidative decoloration of dyes was examined using H2 O2 catalyzed with STA-12(M), and dyes mineralization was evaluated through TOC and spectroscopic measurements. The effectiveness of STA-12(Fe) in photo-Fenton oxidation was searched via the identification of reactive radicals, stability, and reusability of the material, along with the impact of experimental variables (solution pH, H2 O2 dosage, initial dye concentration) [45]. In another study [46], the photocatalytic reduction of MO, RB dyes, and Cr (VI) over STA-12(M) was investigated, and the important species affecting the photocatalytic reduction was determined with the optimization of the affecting variables via the central composite design [46]. P,P-bis (2-oxooxazolidin3-yl)-N-(3-(triethoxysilyl)propyl)phosphinic amide (APTES-BOP)-modified SBA15 (SBA-15-BOP) was synthesized through a post-synthesis grafting strategy for the sorption of anionic azo dyes Congo Red (CR) and Reactive Red 2 (RR2) from aqueous media. Equilibrium and kinetics of adsorption were investigated [47]. Nistor et al. [20] prepared phosphonate MOFs, that were formed from different central

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metallic ions linked with organic connectors, through the reaction of divalent inorganic salts (CoSO4 ·7H2 O/NiSO4 ·6H2 O/CuSO4 ·5H2 O,) and vinyl phosphonic acid under hydrothermal conditions, resulting in cobalt/nickel/copper vinylphosphonate (CoVP/NiVP/CuVP). The experiments were conducted under various conditions of pH, time, and temperature. The effectiveness of synthesized materials was examined for the removal of cationic and anionic dyes from aqueous media at different experimental conditions [20]. In another study, a cadmium-phosphonate structure depending on a tetrahedral-shaped tetraphosphoric acid binder was prepared [48] to obtain a 3D- material Cd4 (H4 L)2 (phen)2 (H2 O)4 (1). The adsorption capacity, structural stability and recyclability for adsorption and desorption of CR dye, the kinetics and the mechanism of dye adsorption were investigated [48].

4.1 Results and Discussion of Findings Sorption affinity of dyes towards TiHEDP materials synthesized by sol–gel method followed the order MB > CV > R6G > PFG [27]. In the synthesis of the calcium aminodiphosphonate (CadP) nanostructures, the synthesis parameters such as the selection of the type of solvent, base, and surfactant along with their concentration have a powerful influence on the resulting nanostructures generated by the surfactant-assistant solve-hydrothermal method. For the specific sodium dodecyl sulfate (SDS) concentration (0.02 M) and trimethylammonium hydroxide (TMAOH) base, obtained samples consisted of highly-developed 3D-nano networks. The adsorption was found to be greatly directed by the factors such as the material morphology and dosage, initial dye concentration, and contact time. It was confirmed that CadP-N was an efficient sorbent in the elimination of MB from wastewater, and had the ability for recycling use [42]. The mesoporous titanium phosphonate (PMTP-1) powder was found to have a high surface area (606 m2 /g) with a uniform mesopore size (2.9 nm). It exhibited a rapid adsorption rate reaching equilibrium within 30 min. The influencing factors like temperature, contact time, solution pH, and initial dye concentration needed to be regulated for the optimization of the process. The results indicated that the PMTP-1 could be utilized efficiently for the elimination of dyes (cationic dye MB) from wastewater [30]. The constructed MnII phosphonate MOF was proposed to be an efficient sorbent for the speedy elimination of MO dye from wastewater, indicating advantages such as rapid adsorption rate, high uptake capacity, selectivity, and reusability. The compound also displayed a fast visible-light-induced photocatalytic activity based on a ligand-to-ligand electron-transfer mechanism without photochromic components. The sample subjected to irradiation possessed an ultra-long-lived charge-separated state, appearing to be promising for the solar energy conversion application. The compound was proposed to be the primary Mnbased photochromic MOF, and the model exhibited LLCT with non-photochromic constituents [43].

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The synthesis of a tri-cationic phosphonate IL involving differing heterocyclic cations was reported. TCPIL/CuFe2 O4 /BNONS nanomaterial (Fig. 1) was prepared using the separately prepared lBNONS (partially oxidized) and CuFe2 O4 NP. The nanomaterial was successfully applied for the effective removal of MB and AR dyes from aqueous media at ambient temperature. Kinetic studies indicated that increasing temperature and the catalyst amount led to the reaction occurring faster. The recovery and reusability of the material were simple and performed properly in aqueous media, with no requirement for additional heating, reducing agent, or specific devices. Constructing diverse nano-structures via the combination of different dblock elements (Mn, Zn, Pt, Pd) together with BNONSs for biomedical, sensor, and catalytic applications was achievable. The material was easily recoverable and reusable for (≥7) with minor losses of catalytic activity [44]. Phosphonate MOFs structured as STA-12(M) (M = Mn, Co, Fe, Ni) showed photocatalytic behavior in the degradation of MO and MB dyes from aqueous media under natural sunlight irradiation. STA-12(Fe) performed faster dye elimination in comparison to the other MOF structures. The study was the demonstration of the typical example of the promoting photo-Fenton excitation of H2 O2 employing phosphonate MOFs as photocatalysts, explaining possibilities for sunlight-induced photocatalytic protection and remediation of the environment [45]. In some other research, Farrokhi et al. [46] examined the photocatalytic reduction of Cr (VI) over the similar structure phosphonate-based MOFs. The Cr (VI) elimination was quickly under natural sunlight when STA-12(Fe) was used. STA-12(Fe) indicated significant synergic photocatalytic activity for MO and RB dyes degradation along with the reduction of Cr (VI), with an extremely increasing Cr (VI) reduction rate by MO/RB addition. The maximum photocatalytic activity for the degradation of dyes and the reduction of Cr (VI) was reported for the weight ratio of dye/Cr (VI): 3:1 (Fig. 2) [46]. The prepared organophosphorus group-modified SBA-15 (SBA-15-BOP) was applied for the elimination of the anionic azo dyes RR2 and CR from aqueous media. SBA-15-BOP revealed appreciable adsorption capability for the investigated dyes in the closely-neutral pH and acidic media. The prepared material was reported as an appropriate sorbent for the elimination of anionic dyes from aqueous media

Fig. 2 Simultaneous photocatalytic reduction of Cr (VI) and degradation of organic dyes MO (a) and RB (b) over STA-12(Fe)/sunlight system. Conditions: 40 ml mixture solution containing Cr (VI)/dye, pH = 3, Catalyst = 10 mg, illumination time = 20 min, temperature = 30 °C. Adapted with permission [46]. Copyright (2020), Wiley

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[47]. Divalent phosphonate MOF materials synthesized through the reaction of divalent inorganic salts and vinyl phosphonic acid in hydrothermal conditions yielded cobalt/nickel/copper vinyl phosphonates (CoVP/NiVP/ CuVP), and revealed appreciable effectiveness for the removal of anionic/cationic dyes (Acid Orange 7, Basic Fuchsine) from aqueous media (Fig. 3). The dye removal efficiency depended on medium pH and increased with an increase in initial dye concentration and temperature. The highest adsorption efficiencies were obtained at optimized conditions. The adsorption capacities were according to the order: CuVP < NiVP < CoVP, associated with increase in metallic radius. Also adsorption capacities were dependent on the distance between the layers of materials that showed a similar decrease: CoVP(10.948 Å) > NiVP(10.127 Å) > CuVP(9.911 Å) (Fig. 4). The materials had promising applications for the remediation of colored wastewater [20]. The 3D-structured cadmium-phosphonate compound Cd4 (H4 L)2 (phen)2 (H2 O)4 (1) that was obtained using a tetrahedral-shaped tetraphosphoric acid linker indicated a fast and effective sorption for the CR dye. In the process of chemisorption, hydrogen bonds between the amide groups of the dye and the uncoordinated phosphonate oxygen atoms of the compound played a crucial role. The material indicated superior structural stability and recyclability for three adsorption–desorption cycles [48]. MOFs prepared through the solvothermal and hydrothermal techniques indicated higher surface areas than the ones prepared by other methods [2]. MOFsbased porous structures revealed potential in the elimination of anionic and cationic dyes from water due to their higher surface areas, pore geometries, easily functionalization. Hybrid MOFs showed higher efficiency as compared to bare MOFs, and surface charges of MOFs played a crucial role in the elimination of dyes [2]. Layered zirconium phosphonate materials involving amino and carboxyl groups, covalently bonded to the inorganic layers are promising for the elimination of dyes. The amino and carboxyl groups improved the affinity of the material towards transition metals owing to their coordinative capabilities and concurrently, the presence of acidic groups could facilitate the sorption of basic dyes [49]. Over the last few years, MPs-based materials have been considered in detail in the domain of dye removal, and revealed superior functioning. They are promising

Fig. 3 Samples of dye solutions a before the adsorption process and after using, b CuVP, c NiVP, and d CoVP as adsorbents. Adapted with permission [20]. Copyright (2020), Wiley

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Fig. 4 Interlayer metal–metal distance for a CoVP, b NiVP, c CuVP. Adapted with permission [20]. Copyright (2020), Wiley

candidates in the arena of water treatment. The subject of MPs and their utilization as adsorbents for environmental remediation will continue to receive attention for the years to come.

5 Conclusions Experimental factors such as type of base, solvent, and surfactant along with their concentrations have considerable influence on the resulting CadP nano networks generated by the surfactant-assistant solvo/hydrothermal approach. The adsorption was found to be closely dependent on variables such as the adsorbent’s morphology

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and quantity, initial dye concentration, contact time, and pH. CadP-N was an effective sorbent having the ability to remove MB from wastewater for recycling use [42]. The influencing factors such as initial dye concentration, contact time, temperature, and pH needed to be optimized. PMTP-1 could be employed as an effective sorbent for the elimination of textile dyes (cationic dye MB) from solution [30]. Novel nanomaterial TCPIL/CuFe2 O4 /BNONS could be efficiently utilized as a heterocatalytic reduction of MB and AR dyes in an aqueous solution at ambient temperature. Kinetics investigation revealed that increasing temperature and the catalyst amount led to the reaction occurring faster. The recovery and reusability of the nanomaterial were easier and performed properly in an aqueous solution, with no requirement for additional heating, reducing agent, or special equipment [44]. The constructed MnII phosphonate MOF was proposed to be an attractive sorbent for the quick elimination of MO dye from aqueous media, indicating advantages such as rapid adsorption rate, high uptake capacity, selectivity, and reusability. The compound also displayed fast visible-light-induced photocatalytic activity depending on a ligand-to-ligand electron-transfer process without photochromic components. Phosphonate MOFs structured as STA-12(M) (M = Mn, Fe, Co, Ni) showed effective photocatalytic characteristics for MB, and MO dyes degradation from aqueous media under natural sunlight irradiation. STA-12(Fe) performed a faster degradation in comparison to the other MOF structures. STA-12(Fe) displayed powerful synergic photocatalytic activity for MO, RB dyes degradation, and Cr (VI) reduction, with an extremely increasing Cr (VI) reduction ratio by the addition of MO/RB. Organophosphorus group-modified SBA-15 (SBA-15-BOP) was proposed as a promising sorbent for the elimination of anionic azo dyes CR and RR2 from aqueous media [47]. The divalent phosphonate MOF materials cobalt/nickel/copper vinyl phosphonates (CoVP/NiVP/CuVP) revealed appreciable efficiency for the adsorption of anionic/cationic dyes (Acid Orange 7, Basic Fuchsine) from aqueous media. The dye removal efficiency depended on medium pH and showed an increase with increasing temperature and initial dye concentration. The sorption efficiencies increased according to the order CuVP < NiVP < CoVP, in good agreement with increasing metallic radius. Also, adsorption capacities were correlated to the distance between the layers of materials that showed a similar decrease. The materials had potential utilization in the purification of colored wastewater [20]. The 3D-cadmium-phosphonate Cd4 (H4 L)2 (phen)2 (H2 O)4 (1) network prepared using a tetrahedral-shaped tetraphosphoric acid linker revealed efficient and fast adsorption of CR dye, along with superior structural stability and recyclability for three adsorption/desorption cycles [48]. MOFs act as excellent sorbents in the elimination of anionic and cationic dyes from wastewater owing to their higher surface area, pore structures, and simplicity of functionalization. Hybrid ones show higher removal efficiency and surface charges play a major role in the removal of dyes [2]. Layered zirconium phosphonate materials possessing amino and carboxyl groups, covalently bonded to the inorganic layer are promising sorbents for the elimination of dyes. The presence of the amino and the carboxylic groups improves the material affinity towards transition metals owing to

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their coordinative capacities, simultaneously, the presence of acidic groups enhances the adsorption of the basic dye [49]. Over the last few years, MPs and their hybrid structures have been examined for dye removal applications and revealed excellent performances showing their potential in the domain of water treatment. The subject of MPs and their utilization for environmental sequestration will continue to draw attention for the years to come.

6 Prospects Further works could be considered to extend the studies to cover: • trial of MPs in further environmental relevancy, such as the remediation of heavy metals, pesticides • developing higher and improved stability materials in acidic/basic media • the impact of parameters (temperature, pH, material dosage, dye concentration, time) on the regeneration and the selectivity of materials • further development of cost-effective synthesis with minimum time limits set out • sorption studies with real effluents • the aspects considering the disposal of waste dyes and sorbents • challenges considering the synthesis and regeneration costs • possibility of synthesis through an easy, large-scale, green approach • safe, reusable, regenerable, sustainable, cost-effective sorbent development • improving stability, surface functionality impacting adsorption performance • enhancing the functionality of sorbents by manipulating the preparation conditions. • development of sorbents through functional modification or doping • assessing the performance of sorbents for multi-component media

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Metal Phosphates: Their Role as Ion Exchangers in Water Purification Amita Somya

Abstract Ion exchange is entrenched as an efficacious contrivance in enormous water purification processes where it depends chiefly on the performance of the exchangers, utilized. Metal phosphates have played a crucial role as ion exchangers for the same processes. This chapter emphasizes the history, recognition of copious metal phosphates as ion exchangers, their classification, key characteristics of metal phosphates being used as ion exchangers, profuse stages of development with novel modifications, and a focus on current advancements in the field of analytical chemistry, especially surfactant based hybrid metal phosphates, both in fibrous and nonfibrous forms. The applications of these hybrid metal phosphates in the separation and removal of different toxic metals, in particular from the point of view of water purification are also explained. Keywords Metal phosphates · Ion exchangers · Applications Hybrid metal phosphates · Metal ion separations

Abbreviations AAmCeP AAmSnP AAmThP AAmZrP ANCeP ANZrP AOT Ce Cr DPC

Acrylamide cerium (IV) phosphate Acrylamide tin (IV) phosphate Acrylamide thorium (IV) phosphate Acrylamide zirconium (IV) phosphate Acrylonitrile cerium (IV) phosphate Acrylonitrile zirconium (IV) phosphate Sodium bis(2-ethylhexyl)sulfosuccinate Cerium Chromium N-dodecyl pyridinium chloride

A. Somya (B) Department of Chemistry, School of Engineering, Presidency University, Itgalpura, Rajanukunte, Bengaluru 560064, Karnataka, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_19

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DPCCeP Fe IEC PANThP POPE POPEThP PSCeP PSThP SDBS SDS SDSCeP SLS Sn SZrP Ta Th TX-100 TXCeP TXThP U Zr

A. Somya

N-dodecyl pyridinium chloride cerium (IV) phosphate Iron Ion exchange capacity Polyacrylonitrile thorium (IV) phosphate Polyoxyethylene octyl phenyl ether Polyoxyethylene octyl phenyl ether thorium (IV) phosphate Polystyrene cerium (IV) phosphate Polystyrene thorium (IV) phosphate Sodium dodecyl benzene sulphonate Sodium dodecyl sulphate Sodium dodecyl sulphate cerium (IV) phosphate Sodium lauryl sulphate Tin Styrene zirconium (IV) phosphate Tantalum Thorium Triton X-10 Triton X-100 cerium (IV) phosphate Triton X-100 thorium (IV) phosphate Uranyl Zirconium

1 Introduction One of the chromatographic methods that have garnered the most interest from analysts in real use is ion exchange. Ion exchange is not a recently discovered phenomenon. Its historical context is fascinating. The Holy Bible has the earliest references, establishing Moses as the first person to successfully use an ion-exchange process to make brackish water into drinkable water [1]. Later, Aristotle discovered that some types of sand cause seawater to lose some of its salt content. Base exchange (also known as cation exchange) in soils was discovered in 1850 by two English chemists, Thompson [2] and Way [3]. Since their discovery, these substances have been widely used in various plants for water softening and further other uses, as well as to create products with comparable qualities. Throughout the nineteenth century, several scientific investigations into natural, refined, and synthetic inorganic materials were conducted to learn more about their ion exchange characteristics. E. A. Adams and E. L. Holmes [4] described the first synthetic granular ion-exchange resin in 1935, however naturally occurring zeolites had been used to purify water since the nineteenth century. However, this discovery brought to light, investigated, and resolved issues with adding more effective ionic groups to a polymer backbone structure. According to the demands of the period, these new resins were created, enhanced, and are currently being enhanced. The manufacture of and research into

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synthetic organic and fibrous ion exchangers has increased dramatically since 1935. The development of ion exchange technology may have occurred throughout a variety of periods.

1.1 Ion Exchanger Ion exchangers are immiscible liquids or insoluble solid materials that contain exchangeable ions (with regards to liquid ion exchangers). On coming into contact with an electrolyte solution, those ions can be converted into stoichiometrically correspondent ions of the identical sign. Ion exchangers can be classified as anion, cation and, amphoteric ion exchangers, depending on whether they can exchange anions, cations, or both. A structure with a negative charge makes up a cation exchanger, whereas a structure with a positive charge makes up an anion exchanger. The oppositely charged counter-ions, being mobile, balance out the structure’s positive or negative charge. An illustration of an identical ion exchange reaction is as follows: R − A + B(aq) ⇌ R − B + A(aq) where R denotes the ion structural exchanger’s unit (matrix), A and B stand for the interchangeable counter ions, and aq refers to the aqueous phase. This process can be stopped by appropriately altering the amount of these ions in the solution, making it reversible.

1.2 Classification Any ion exchanger can be distinguished as organic and inorganic, contingent upon the type of matrices it has been made of. Organic ion exchangers are extremely reproducible in their ion exchange conducts and depict chemical stability. However, they mislay their ion exchange characteristics with respect to radiations and elevated temperatures. Inorganic ion exchangers have entrenched their position in ion exchange chemistry because of their tenaciousness on the appearance of strong radiations and high temperatures and their specificity for several metal ions. As a result, they have numerous uses in nuclear research, including radioisotope separations and the remediation of radioactive waste. They are employed as catalysts, packing matter in ion exchange chromatography, ion selective electrodes, and in the identification of metals in medication and biological products, alloy investigations, and rock analysis. They are used in environmental analysis as well. Hetropolyacid salts, insoluble metal ferrocyanides, polyvalent metal hydroxides, oxides, and insoluble acid salts are the accustomed types of inorganic ion exchangers.

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1.3 Metal Phosphates as Ion Exchangers The aforesaid materials are typically created by mixing oxides of elements from the periodic table’s 3rd, 4th, 5th, and 6th groups. Phosphoric, antimony, arsenic, molybdic, and vanadic acids have been combined with the metals such as antimony, titanium, tantalum, zirconium, thorium, cerium, tin, iron, chromium, nickel, niobium, bismuth, cobalt, and other elements to create a variety of such materials. Owing to their special selectivity concerning particular metal ions, among them, metal phosphates switch better than others. Many metal phosphates are reported in Table 1, as per literature, those have exhibited excellent ion exchange properties. Nonetheless, the primary disadvantage of these inorganic exchangers or metal phosphates was precariousness towards some chemicals and mechanical stress. Farther, they do not reveal substantial reproducibility in ion-exchange conducts. The pitfalls and benefits of both, organic and inorganic ion exchangers have facilitated the investigations on properties of organic-based inorganic or hybrid ion exchangers where metal phosphates have been the backbone of such materials. Therefore, hybrid ion exchange materials [19] gained the contemplation of the researchers owing to the harmonious enthralling characteristics of hybrid ion exchangers which are absent in both pure organic and inorganic ion exchangers. The search for increased chemical, thermal, and mechanical strength, reproducibility in ion exchange behavior, and specific adsorption of metal ions, has been sparked by these hybrid ion exchangers. Figure 1 exhibits the phases of the evolution of ion exchangers/sorbents and the recognition of metal phosphates as ion exchangers. In a Table 1 Metal phosphates used as inorganic ion exchangers and their specific adsorption for metals S.N

Name

1

Ce phosphate

Nature

Selective adsorption for

References

Amorphous

Cs(I), Rb(I),

[5]

Crystalline

Ba(II), Pb(II), Ag(I)

[6]

2

Ce phosphate sulphate

Crystalline

Ag(I), Ca(II), Na(I), Sr(II), Cs(I) [7]

3

Cr phosphate

Amorphous

Ba(II), Ca(II), Sr(II)

[8]

4

Fe (III) phosphate

Amorphous

Eu(III), Pb(II), Ga(III)

[9]

7

Sn (IV) pyrophosphate

Amorphous

Bi(III), Th(IV), Y(III), Zr(IV)

[10]

10

Th (IV) phosphate

Amorphous

Bi(III), Pb(II), Fe(III),

[11]

12

Zr (IV) phosphate

Amorphous Crystalline

Na(I), Cs(I), Rb(I), K(I), Co(II), Eu(III), Sr(II), Zn(II), Ni(II) UO2 (II),Ca(II), Na(I), Ag(I), NH4 (I),Ce(III), Sr(II),UO2 (II)

[12–16]

15

Zr (IV) hypophosphate

Amorphous

For multivalent metal ions

[17]

17

S-supported-Zr tungstophosphate

Amorphous

Hg(II)

[18]

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Till 1850

345

• The very first experimental observation on ion exchange materials though principle was not discovered.

1850 to 1905

• Principle of ion exchange was discovered and very first technical utilization was explored.

1905 to 1935

• Applications of inorganic ion exchangers and modified natural organic ion exchangers were explored.

1935 to 1940

• Swift developemnt in organic exchangers with complete elimination of inroganic exchangers.

1940 till date

• Introdcution of metal phosphates as inorganic ion exchangers. Researches on numerous metal phosphates. • Latest developements in hybrid metal phosphates and profuse applications in analytical chemistry.

Fig. 1 Phases of the evolution of ion exchangers/sorbents and recognition of metal phosphates as ion exchangers

variety of ways, the hybrid ion exchangers or hybrid metal phosphates (HMPs) are ameliorated. Either of them is its dimensioned features, which makes it ideal for use in column processes. Better mechanical qualities are also added to the final product, which is hybrid ion exchange materials, by binding with an organic species. One of the largest risks to civilization today is environmental pollution, which is incredibly complicated and huge due to unchecked technological advancements brought about by humans. Owing to enhancement in urbanization and industrialization water is getting polluted. The discharge of industrial effluents in the surface water has been noticed as one of the reasons. Consequently, a lot of toxic metals, organic pollutants, and dyes are released into the water systems which are the cause of decay of water animals and bad impact on other living organisms depending on water.

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Hence, before the wastewater is discharged into freshwater bodies, the most essential objective is to eliminate harmful heavy metal pollutant ions since these ions can directly or indirectly cause a multitude of health issues in humans and aquatic animals. Mercury, Cadmium, Lead, and Nickel [20, 21] are the most toxic metals among other heavy metals, found owing to their implications on living organisms. Mercury is the most noxious metal which plunges into surroundings via many industries, such as paints and pharmaceuticals, the paper and pulp industry, molding processes, chloralkali plants, etc. Its contamination is much more dangerous since it can affect the lungs, digestive system, or bloodstreams. Even Hg(II) fumes are very harmful, and few mercury combinations are extremely volatile or insoluble, posing possible risks to human beings. The other poisonous heavy metal is Cd which builds up and is retained by the body, leading to kidney damage, lung cancer, and demineralization of the bones [20]. Many products, including electroplating, storage batteries, vapor lamps, and some soldiers, employ cadmium. Two to four hours after exposure, symptoms may not start to appear. Fatigue, headaches, nausea, vomiting, cramps in the stomach, diarrhea, and fever can all result from excessive exposure to cadmium. Both man-made and natural sources introduce lead (Pb) into the environment. The soil contains lead that is naturally occurring. The enormous growth in lead use over the last couple of decades has raised environmental challenges due to its toxicity. It occupies a significant position among the harmful elements found whose traces have been detected in human cells. The primary origin of its exposure is Pb paints used to paint steel, washbasins, kitchen sinks, water storage tanks, Pb pipes utilized to link plumbing fixtures, and some plastic pipes containing stabilizers made of lead compounds. Ni is an immunotoxic and carcinogenic substance that can have a range of negative health consequences on the body, including contact dermatitis, cardiovascular illness, respiratory tract cancer, asthma, and lung fibrosis, based on the dosage and time of exposure. Comparatively, excessive Ni exposure has more detrimental consequences on humans, including birth abnormalities, heart disease, respiratory failure, and pulmonary embolism. Therefore, to eliminate these heavy metal pollutant ions coming from drain water, an appropriate, simple, and advanced method must be used. Flotation, electrochemical deposition, adsorption, and ion exchange have been just a few of the established methods used to eliminate heavy metal pollutants from industrial effluents.

2 Hybrid Metal Phosphates (HMPs) The pervasively used method for the elimination of heavy metal pollutants from industrial wastewater has been the chemical precipitation method. However, it fails in case of disruptive ions are present, like complex formers in the wastewater. Among all methods, adsorption has been observed as a convenient and simple technique to remove toxic metal pollutant species from the effluents. In recent years, several materials and procedures, such as adsorbents were developed for the elimination of the hazardous heavy metal pollutant ions coming from wastewater which is based

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on adsorption processes. But the sky-high costs and stagnant nature of the materials restrain their employment in large-scale water treatment processes. However, the shortcomings of conventional adsorbents necessitated the development of innovative materials that were inexpensive, biocompatible, biodegradable, simple to synthesize, easy to regenerate, and recyclable with high efficacy. The aforementioned limitations can thus, be overcome by research on the development of hybrid metal phosphates (HMPs). The HMPs have demonstrated advancements in many areas. One among them is its granulometric characteristics, which make it better suited for use in column operations. Better mechanical qualities are also added to the final product, or hybrid ion exchange materials, through the binding with an organic species. HMPs can be made as layered compounds containing phosphate groups as 3-dimensional absorbent materials with cross-linked films. Both organic and inorganic precursors typically exhibit relatively varied levels of reactivity, and phase separation frequently takes place. In addition to organic and inorganic moieties, the interface between the two phases also affects the properties of hybrid materials. Therefore, the tendency is to strengthen interfacial contacts by intimately mixing or penetrating organic and inorganic networks. Additionally, phase separation would be prevented by the development of chemical bonding between an organic and inorganic species, enabling the creation of composite materials or organic–inorganic copolymers. Hence, two groups of hybrid materials can be distinguished as follows: • Class 1 relates to a hybrid structure where weak connections between organic and inorganic phases are produced, such as H- bonds, van der-Waal forces, and/or electrostatic attractions. Small organic species contained within an oxide matrix make up the majority of this class. • Class 2 refers to hybrid structures that have both the organic and an inorganic constituent connected by potent covalent chemical interactions.

2.1 Synthesis of HMPs Many HMPs have been created by the combination of organic species into inorganic metal phosphates. Varshney and his team have synthesized many hybrid metal phosphates [22–37] by introducing an organic monomer or polymer species into the inorganic ion exchange matrices such as acrylamide, acrylonitrile, styrene, pyridine, and based metal phosphates where the majority of the phosphates were of cerium, thorium, tin, and zirconium. In general, hybrid phosphates of cerium and thorium were prepared by intermixing the solutions of corresponding salt, organic species, and orthophosphoric acid in optimum molar ratio and heated at 45–55 °C with perpetual stirring using a magnetic stirrer followed by filtering the gel after holding the mixture overnight. The dried sheet was then, cut up into tiny pieces and transformed into H+ ionic structure by the treatment with acid whereas hybrid phosphates of tin and zirconium were made by sol–gel procedure but excluding the

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supply of heat. The filtered and dried gel was cracked with the application of acid and then, excess acid was eliminated with proper washing by demineralized water.

2.2 Types of HMPs K. G. Varshney and co-workers [33, 35–37] found that few fibrous inorganic metal phosphates, such as Ce (IV) phosphate and Th (IV) phosphate, also possess fibrous nature when mixed with the organic species. It was decided to call this latest kind of metal phosphates “Hybrid Fibrous Metal Phosphates”. Because hybrid fibrous metal phosphates have exceptional thermal, chemical and, mechanical durability in addition to selective adsorption of metal ions, there has been a recent increase in interest in their manufacture. The benefits of both of their counterparts are also present in these HMPs. In contrast to organic resins, they have demonstrated high-rise chemical and mechanical firmness together with superior thermal stability, an attribute of inorganic ion exchangers. By incorporating organic monomers like acrylamide, nbutyl acetate, cellulose acetate, etc. and polymers such as PAN, PS, etc. into inorganic metal phosphates i.e., Cerium (IV) and Thorium (IV) phosphates, Varshney and coworkers have created the significant extent of hybrid fibrous metal phosphates (listed in Table 2) which have revealed outstanding ion exchange properties. Broadly the phosphates of cerium, thorium, etc. revealed fibrous nature and tin, zirconium, etc. as nonfibrous characteristics among all. Thus, the aforesaid HMPs, based on their appearance during synthetic procedures, are of two types named- hybrid fibrous and non-fibrous metal phosphates. Further, based on the introduction of organic moiety into inorganic phosphates, HMPs are of three categories, as depicted in Fig. 2. The worldwide utility of surfactants or, surface active agents in realistic applications, and scientific compassion with regard to their specific nature and unique properties have expedited opulent literature on this topic. The extraordinary capacity of surfactants to affect the properties of surfaces and interfaces has been one of the main drivers behind their widespread use. In many industrial applications, surfactants are used such as adhesives, agrochemicals, coatings, lubricants, petroleum, pharmaceuticals, paints, processing of foods, photographic films, in many laundries and personal care products. K. G. Varshney [48, 49] well-employed surfactants as the media in their studies on the adsorption property of various alkaline piles of earth and heavy metal ions on the surface of inorganic and HMPs and discovered that their contiguity boosted the adsorption of metal ions on the surface of ion exchange materials. The extraordinary elevation in adsorption of aforementioned metal ions has, therefore, widened up opportunities in the realm of material chemistry. Thus, it was deemed useful to include different surfactants in a matrix of inorganic metal phosphates to examine how the surfactant molecules would alter the ion exchangers’ properties. Surfactants and inorganic metal phosphates have lately been intermixed by researchers to create a new category of HMPs both, fibrous and nonfibrous forms

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Table 2 Different types of hybrid metal phosphates Monomer-based hybrid metal phosphates S. N

Name

IEC

Selectivity for

Nature

References

1

Acrylamide Ce (IV) phosphate

2.6

Mercury

Fibrous, crystalline

[22]

2

Acrylamide Sn (IV) phosphate

2.1

Mercury

Non-fibrous, poorly crystalline

[23]

3

Acrylamide Th (IV) phosphate

2

Lead

Fibrous, poorly crystalline

[24]

4

Acrylamide Zr (IV) phosphate

2.26

Mercury

Non-fibrous, crystalline

[25]

5

Acrylonitrile Ce (IV) 2.86 phosphate

Mercury

Fibrous, poorly crystalline

[26]

6

Acrylonitrile Zr (IV) phosphate

2.08

Strontium

Non-fibrous, semicrystalline

[27]

7

Cellulose acetate Th (IV) Phosphate

1.7

Lead

Fibrous, amorphous

[28]

8

n-Butyl acetate Ce (IV) phosphate

2.25

Mercury

Fibrous, amorphous

[29]

9

Pectin Ce (IV) phosphate

1.78

Mercury

Fibrous, amorphous

[30]

10

Pectin Th (IV) phosphate

2.15

Lead

Fibrous, amorphous

[30]

11

Pyridine Ce (IV) phosphate

2

Mercury

Fibrous, amorphous

[31]

12

Pyridine Sn (IV) phosphate

2.1

Lead

Non-fibrous, poorly crystalline

[32]

14

Pyridine Zr (IV) phosphate

2

Mercury

Non-fibrous, poorly crystalline

[32]

13

Pyridine Th (IV) phosphate

2.1

Lead

Fibrous, amorphous

[33]

15

Styrene Zr (IV) phosphate

2.18

Lead

Non-fibrous, semi-crystalline

[34]

Polymer-based hybrid metal phosphates 1

Polyacrylonitrile Th (IV) Phosphate

3.9

Lead

Fibrous, microcrystalline

[35]

2

Polystyrene Th (IV) phosphate

4.52

Cadmium

Fibrous, crystalline

[36]

3

Polystyrene Ce (IV) phosphate

2.95

Mercury

Fibrous, microcrystalline

[37]

Fibrous, amorphous

[38]

Surfactant-based hybrid metal phosphates 1

TX-100 based Ce (IV) phosphate

3

Mercury

(continued)

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Table 2 (continued) 2

AOT Ce (IV) phosphate

3.02

Copper, cadmium, lead, zinc, mercury

3

AOT Sn (IV) phosphate

2.4

Cadmium, mercury, Fibrous, zinc semicrystalline

[40]

4

DPC Ce (IV) phosphate

3.15

Mercury

Fibrous, amorphous

[41]

5

DPC Sn (IV) phosphate

2.39

Cadmium

Fibrous, amorphous

[42]

6

POPE Th (IV) phosphate

3.25

Mercury

Fibrous, amorphous

[43]

7

SDBS Ce (IV) phosphate

2.17

Lead

Fibrous, semicrystalline

[44]

8

SDBS Sn (IV) phosphate

2.2

Mercury and calcium

Fibrous

[45]

9

SDS Ce (IV) phosphate

2.92

Lead

Fibrous, amorphous

[46]

10

SLS Th (IV) phosphate

3.1

Mercury and calcium

Fibrous, poorly crystalline

[47]

Based on appearance

Hybrid Metal Phosphates

Fibrous, amorphous

[39]

Hybrid fibrous metal phosphates Hybrid non fibrous metal phosphates

Monomer based hybrid metal phosphates Based on organic moiety in the structure

Polymer based hybrid metal phosphates Surfactant based hybrid metal phosphates

Fig. 2 Types of hybrid metal phosphates

[38–47]. Surfactant-based metal phosphates are analogous to metal–organic frameworks because surfactants are an organic equivalent placed in the inorganic metal phosphates by connecting in between the layers of metal phosphates. Due to the amorphous and imperfectly crystalline nature of materials, the structure could not be explained at this level either. By including anionic, cationic, and, nonionic surfactants

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in the matrices of inorganic metal phosphates, A. Somya [38, 41–43, 46, 47] have likely made the initial application of surfactants in the synthesis of unique HMPs (fibrous and non-fibrous both). Later, N. Iqbal [39, 40, 44, 45] combined sodium bis (2–ethylhexyl) sulfosuccinate and sodium dodecyl benzene sulphonate in a matrix of cerium and tin in their + 4 oxidation states to create the same class of HMPs.

2.3 Applications of HMPs in Water Purification These synthesized HMPs were well characterized by several characterization techniques like, Fourier transform infrared (FTIR) spectroscopy, X-Ray diffraction (XRD) study, Thermal Gravimetric Analysis (TGA)/Differential Thermal Analysis (DTA)/Differential Thermal Gravimetry (DTG), Scanning Electron Microscopy (SEM), elemental analysis, etc. to explain their structure and composition. And, their applications were explored by studying their ion exchange properties under variable conditions of medium, pH, temperature, chemicals, acids, etc. Most importantly, their adsorption properties were explored by K. G. Varshney and his team [25–38, 48, 49] for numerous metals including toxic metals which are often found in industrial effluents, discharged into different sources of surface water and reported remarkable selectivity of HMPs for specific metal. And, based on the output of HMPs selectivity, they conducted several separations on their columns in presence of other metals which is useful in water pollution control to eliminate toxic metals from industrial effluents. Applications of HMPs in the separation of mercury (II), cadmium and lead ions from other metals are depicted in Figs. 3, 4 and 5. Though applications of these phosphates with real industrial effluents have not been explored much, however, these separation studies prove their properties about the same. Due to their selectivity for various metal ions, these HMPs have demonstrated remarkable ion exchange properties and are being used in the elimination of the same metal ions from others. Metal phosphates, including those of Sn (IV), Ce (IV), Zr (IV), and others, were discovered to be excellent ion exchangers and also, intercalating agents. The entire concept behind turning them into HMPs was to increase interspaces between all the layers of metal phosphates by adding organic species to these metal phosphates’ measurements, leading to better ion exchange characteristics. These HMPs, also known as hybrid ion exchangers, are analogous to metal–organic-frameworks (MOFs) which exhibit characteristics of organic and inorganic, both of the compeers in terms of ion exchange capacity, thermal stability, chemical stability, and mechanical resistance, not only-but also selective adsorption of metal ions. However, because these materials were discovered to be amorphous or imperfectly crystalline, hence, their structures could not be explained. Table 2 reveals the IEC, selectivity, and nature of different monomer, polymer, and surfactant-based HMPs, synthesized so far. The real utility of metal phosphates for being an ion exchanger is mostly determined by their ion exchange properties, including ion exchange capacity, elution, concentration, pH titration, and distribution characteristics. Ionic radii that are hydrated and selectivity affect ion exchange capacity. Any ion-functional exchanger’s

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From Ni(II),Pb(II), Cd(II) by AAmCP

From Ni(II),Pb(II), Cd(II), Mg(II) by AAmZP

From Cd(II), Pb(II), Sr(II) by AAmSP From Pb(II), Fe(III), Ba(II) by TXTP From Pb(II),Ca(II) Ni(II) by DPCCP

From Mg(II), Cd(II), Pb(II) by PSCP

From Mg(II), Pb(II),Cd(II) Zn(II), by ANCP

From Pb(II), Ni(II), Ca(II) by TXCP

Separation of Hg (II)

Fig. 3 Separation studies of mercury (II) ions by different HMPs

From Mg(II) by ANZP

From Cd(II), Mg(II),Cu(II), Hg(II) by AAmTP

From Cd(II), Zn(II) by PANTP

From Mg(II), Ba(II) by SZP

Separation of Pb (II)

From Cd(II),Mg(II) Hg(II), Ni(II) by SDSCP

Fig. 4 Separation studies of lead (II) ions by different HMPs

group’s characteristics and degree of cross-linking, in turn, have an impact on how selective it is. Stronger adsorption of certain ions occurs in ion exchangers that have groups capable of forming complexes with those specific ions. The exchanger turns even additional selective toward ions of various magnitudes as the magnitude of cross-linking rises. The concentration of eluant determines how much H+ ions will

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353

From Pb(II) by PSCP

From Pb(II) by AAmCP

From Zn(II), Pb(II) by ANCP

From Hg(II), Zn(II), Ba(II) by PSTP

Separation of Cd (II)

From Ca(II) by ANZP

Fig. 5 Separation studies of cadmium (II) ions by different HMPs

be eluted from an ion exchanger column, but the nature of the ionogenic moieties in the exchanger matrices which, mostly depends on values of pKa for the acids utilized in the application, determines an optimum concentration of the eluant required for highest elution of H+ ions. The following basic exchange reactions determine any ion-exchangers effectiveness: • Exchange equivalence. • The degree to which one ion is preferred over another, taking into account instances in which the employment of chelating and complexing agents have altered the various affinities of the ions. • The capacity to reject ions but, often, not undissociated compounds, is known as Donnan exclusion. • The incapacity of particularly big ions or polymer molecules to be adsorbed to a significant level is known as the “screening effect.” • Variations in the rates at which adsorbed compounds migrate down a column, which is principally a result of affinities. • Ion mobility is limited to counterions and exchangeable ions alone. • Other mechanical qualities, including swelling, surface area, and others. First and foremost ion exchangers were most often applied for water softening, but in due course, they found extensive applications in many different sectors, including syntheses and certain preparative tasks. In addition to meeting the needs of contemporary laboratories, the use of ion exchangers gave analysts new techniques that helped them solve issues that had previously been intractable. As a result, laboratories and businesses have come to rely on the ion exchange technique as an analytical

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tool. As the range of their applications widens, ion exchange processes are becoming more and more popular in industries. Today, they are a highly beneficial addition to other processes including filtration, distillation, and adsorption. Different plants are in operation all over the world, carrying out duties ranging from the catalysis of organic reactions to the purification of aqua in cooling systems of nuclear reactors from the recapture of metals from industrial discharges to the separation of rare earth metals.

3 Conclusion According to the investigations carried out thus far, it is evident that HMPs are previously owned as ion exchange materials. These HMPs have depicted improved characteristics of both the adsorption properties of different metal ions as well as ion exchange capabilities with the introduction of organic moieties i.e., monomers, polymers, and surfactants to their matrices. These HMPs have demonstrated discernible adsorption characteristic features with particular metal selectivity. As a result, these materials provide analysts in the fields of analytical chemistry and environmental science with a wealth of options where they might be employed to reduce water contamination. Acknowledgements The author is grateful and thankful to the Chancellor, Presidency University, Bengaluru for the seed grant RIC/funded projects/IR 1, dated 11.07.2018. Thanks are also due to Mr. Amit Prakash Varshney, Mr. Anirudh, and Miss. Anushi Varshney for supporting to compile this work.

References 1. 2. 3. 4. 5.

6.

7.

8.

The Second Book of Moses. Exodus. Chapter 15, Verse 25 Thompson, H.S.: On the absorbent power of soils. J. Roy Agr. Soc. Engl. 11, 68 (1850) Way, J.T.: On the power of soils to absorb manure. J. Roy Agr. Soc. Engl. 11, 313 (1850) Adams, B.A., Holmes, E.L.: Adsorptive properties of synthetic resins. J. Soc. Chem. Ind. 54, 11 (1935) Alberti, G., Costantino, U., Gregorio, F.D., Galli, P.C., Torracca, E.: Crystalline insoluble salts of polybasic metals—III: preparation and ion exchange properties of cerium(IV) phosphate of various crystallinities. J. Inorg. Nucl. Chem. 30, 295 (1968) Alberti, G., Casciola, M., Costantino, U., Luciani, M.L.: Crystalline insoluble acid salts of tetravalent metals: XXIV. Ion-exchange behaviour of fibrous cerium(IV) phosphate. J. Chromatogr. 128, 289 (1976) Konig, K.H., Eckstein, G.: Amorphe und kristallinecer(IV)-phosphate alsionenaustauscherIII Herstellung, chemische und kationenaustauschereigenschaftenkristalliner cerium (IV)phosphat-sulfate. J. Inorg. Nucl. Chem. 31, 1179 (1969) Akiyama, T., Tomita, I.: Preparation and some properties of chromium phosphate ion exchanger. J. Inorg. Nucl. Chem. 35, 2971 (1973)

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Metal Phosphates/Phosphonates for Biomedical Applications Aditya Dev Rajora, Trishna Bal, Snigdha Singh, Shreya Sharma, Itishree Jogamaya Das, and Fahad Uddin

Abstract The human body is composed of many vital elements and phosphorus is one such element that forms the basic ingredient of our biological constitution. Recently, researchers are more lenient on fabricating and applying materials for biomedical applications which pose the least threat to the human body with high mechanical strength and degradability opportunities. This provides an interesting platform and opens many avenues for the utilization of versatile metal conjugates of phosphorus in the form of phosphates/phosphonates as nanodevices or nanostructured moieties for varied biomedical applications. In this regard, metal phosphates/phosphonates have come out with such ideal properties making our biological functions better and comfortable with the least adverse reactions. Ideal metal phosphates/phosphonates are inert and degradable materials without any probability of releasing toxic contents and are generally employed for different applications namely in the field of dentistry as composite material for dental caries, cardiovascular stent fabrication, orthopedic applications, etc. As such metal phosphates are purely inorganic materials attached to phosphoric acid units and metal phosphonates are nanostructures comprising organophosphonate ligands attached to the organic– inorganic hybrid structures. These hybrid materials have a well-defined structural build triggering their usage as body implants and other in vivo applications. Thus, the current chapter highlights all the major strategies involving the fabrication of these hybrid structures with their latest biomedical applications involving biocompatibility studies. Keywords Magnesium Phosphate · Zirconium Phosphate · Cytotoxicity · Tissue engineering · Drug Delivery · Gene Delivery

A. D. Rajora · T. Bal (B) · S. Singh · S. Sharma · I. J. Das · F. Uddin Department of Pharmaceutical Sciences & Technology, Birla Institute of Technology, Mesra, Ranchi, Jharkhand 835215, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_20

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1 Introduction Metals are an important component in the design of biomedical devices for ages, but they suffer from versatile problems viz corrosion leading to their deformation when they are used for different applications from cardiovascular to otorhinlogy. Metal phosphonates are fabricated from inorganic and organic scaffolds built over a polymeric structure by coordination with phosphonate ligands of different dimensions, whereas the metal phosphates are acidic moieties that are fabricated in the bound form of metal with phosphoric acid groups and they are hydrophilic. Both metal phosphonates and phosphates are exceptionally chemically and thermally stable with large surface areas which makes their application more feasible in the field of ion exchange, proton conduction, catalysis, etc. The organophosphonate ligands are used as the phosphate source for the fabrication of phosphates/phosphonates. These metal-phosphorus-polymer hybrid systems represent a metal–organic framework (MOF) exhibiting crystalline characteristics as 1Dimensional, 2Dimensional, or 3Dimensional porous nano-assemblies. Phosphorus is one of the chief components present in biomolecules and biological metabolites like phospholipids, nucleic acids, proteins, polysaccharides, or nucleotide cofactors and thus phosphorus enrolls itself in maintaining important functions as metabolic intermediates, common regulatory switches for proteins, and as a backbone for the genetic information. Thus, these metal phosphates/phosphonates nanoarchitectures can very well help in sustaining the basic metabolic functions in the body when they are implemented in the form of biomedical devices without any toxicity issues [1, 2]. These meal-organic frameworks (MOFs) can range in a variety of porosity from being mesoporous MOF where there is a combination of metal–ligand with a single molecule template in presence of surfactants which are responsible for their porosity and macroporous MOF structures where the crystallization process impacts their structure during formation [1]. These pores in the MOFs represent high-definition geometry which can determine the degree of entrapment of biomolecules and pharmaceuticals within them making the MOFs appropriate candidates for theranostic applications [3]. The particle size of the metal phosphates/phosphonates should be optimized for their penetration within cells when used in the biomedical field and thus in this aspect, nanoscale MOFs are in research for such applications. Moreover, the type of synthesis method applied, temperature provided during their synthesis, as well heating rates, impact a lot on the nucleation process of crystal formation and the size of the crystal formed during the MOFs preparation. Metal phosphonates can be easily fabricated into different shapes with versatile morphologies. Conjugation of different organophosphonic acids like methylene diphosphonic acid,1hydroxydiethylidene 1,1-diphosphonic acid, amino trimethylene phosphonic acid, etc. with inorganic moieties can have a dual impact on the hybrid structures by improving the hydrophilic and hydrophobic properties of the pore wall surface [4]. The metal phosphates/phosphonates have varied applications as a drug delivery vehicle for pH-responsive anticancer drugs like sodium hexametaphosphate which is used as a precursor for the synthesis of mesoporous hydroxy apatite for the

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delivery of doxorubicin [5]. Metal Phosphonates are used as bioceramics for the replacement of different parts in the skeletal system and their porous forms are used as bioactive glass for the growth of soft tissues as well as bone tissue regeneration thereby providing toughness and elasticity to the bones [6]. The most naturally available metal phosphates are calcium Phosphate present abundantly in the human body and thus they are highly biocompatible and can be undoubtedly applied as bioceramics in different maxilla facial, dental, orthopedic, otolaryngology, total joint replacement (bone augmentation), craniomaxillofacial reconstruction applications, etc. [7]. Besides, calcium phosphates, other metal phosphates have wide biomedical applications like the synthesized mesoporous zirconium phosphonate prepared with the aid of mono- and biphosphonic acid functionalized with L-proline and piperazine moieties for colon-targeted delivery of nucleic acid as reported by Tang et al. 2013 [8]. In another study done by Gorgieva et al. functionalized cellulose nanocrystals with bis(phosphonates) embedding alendronate and 3aminoropylphosphoric acid comprising of a covalent Schiff-base nanostructured material can be applied for different theranostic applications in bones disorders [9]. Also, mesoporous silica nanoparticles (MSN) functionalized with phosphonated pillars can be efficiently utilized for controlled photothermal-chemotherapy for tumor ailments [10]. Also, it was reported that when these MSNs are coated with calcium phosphates(hydroxyapatite) can efficiently control the release of drugs, as these nano-systems upon entering the cells confront the weak acidic conditions of lysozyme and thus get triggered at that pH, controlling the release of drugs from these mesopores [11]. Also, polymers like chitosan can be used for coating these functionalized MSN by the formation of bonds with the phosphonate group for stimulating the pH-sensitive release of some analgesic drugs like Ibuprofen [12]. Also, research has been carried out on the synthesis of functionalized mesoporous calcium phosphate nanoparticles for the delivery of cytotoxic agents like Doxorubicin [13]. Thus, these metal phosphates/phosphonates have been efficiently explored for the successful delivery of biologicals and pharmaceuticals. With this background, the current chapter is designed to provide insight into the recent applications of metal phosphates/phosphonates in the biomedical field with their synthesis protocols and highlights of cytotoxicity tests conducted for such materials.

2 Preparative Techniques In this section, the general methods of synthesis for metal phosphates/phosphonates are outlined below:

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2.1 Soft Templating Route This method is used for metal–organic framework moieties by sol–gel and hydrothermal method by use of varied surfactants which help in directing the hydrophilic or the hydrophobic structures on the hybrid skeleton [14].

2.2 Hard Templating Route This method is also known as nano-casting technology, where highly ordered mesoporous silicates are applied as hard templates. The precursors which are embedded into pores of silicates moieties by either sorption, ion exchange, phase transition, complex, or by covalent grafting, are excluded by heat treatment which results in the unification of individual nanoforms of mesoporous silica during the crystallization process, leaving behind porous byproducts. This technique is used for the synthesis of non-silica mesoporous materials like metals, metal oxides, carbon, carbides, and nitrides [14]. Microwave-assisted heating is more beneficial when used for heat treatment in this technique, as synthesis is hastened with smaller and more uniform crystalline nanostructures [15].

2.3 Template-Free Synthesis Route The microemulsion technique is used for the preparation of mesoporous materials with different shapes and the pH of the reaction mixture along with the type of organic additive used impacts the formed porous hybrid systems as reported by Ma et al. [16]. Another low-cost method involving the electrochemical method was developed by Quian et al. for the synthesis of LiFePO4 -Carbon hybrid nanocrystal system and it was observed that on utilization of layered materials, there was a structural change in the hybrid system leading to the formation of a mesoporous system [15].

3 Cytotoxicity of Metal Phosphate and Phosphonates Cytotoxicity is an essential in vitro test tool carried out to investigate the toxicity of material on living cells and tissues. It is necessary to determine whether the material causes any cell damage or cell death in presence of the material. Metalbased components show surface and quantum properties that leads to a potential risk of abnormal chemical reaction, tissue penetration, and adsorption. So, cytotoxicity studies are important to assess the risk for the development of safe material loaded

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with metal for biomedical applications [17]. Various in-vitro assays performed to determine the cell viability are outlined below:

3.1 Tetrazolium Reduction Assay XTT (soidum3' -[1-phenylaminocarbonyl)-3,4-tetrazolim]-bis (4-methoxy 6- nitro) benzene sulfonic acid hydrate) assay is performed to determine the cell viability. The viable cells reduce the XTT into orange-colored formazan, a non-toxic, aqueous soluble colored liquid after incubating at 37 ºC for 4 h and the absorbance is to be measured at 450 nm [18]. Tavare et al. prepared magnesium-loaded βtricalcium phosphate granules (β-TCP) and evaluated its toxicity using XTT assay on MC3T3 pre-osteoblasts cells. Their result showed that the cells when treated with β-TCP showed similar mitochondrial dehydrogenase activity with the control group (untreated cells), thus the Mg loaded β-TCP granules were found to be biocompatible [19].

3.2 MTT Assay MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) is a yellowcolored tetrazolium salt. The metabolically active cells, when treated with the MTT, converted MTT into aqueous insoluble purple formazan crystals due to the presence of NAD(P)H- dependent oxidoreductase enzyme present in the cells [18, 20]. Among all the samples, the darkest sample possesses the greater number of viable cells. Su et al. prepared a zinc phosphate-coated Zn disc and investigated the cytotoxicity on pre-osteoblast and endothelial cells. The zinc phosphate-coated material showed maximum cell viability of pre-osteoblast and endothelial cells containing 25% extract media. In another study, the cytotoxicity of pH-sensitive Iron phosphate (FePOs) nanozymes was evaluated against noncancerous L929 fibroblast cells and it was found that the material was safe for biomedical applications [21]. Beigoli et al. prepared calcium phosphate nanopowder from aloe vera plant extract and evaluated its cytotoxicity on oral squamous cell (KB cell line) and found that 0.1–0.75 mg/ml concentration of calcium phosphate does not shows any cytotoxicity against these particular cell lines but the concentration of 1 mg/ml showed significant difference [22].

3.3 Neutral Red (NR) and Coomassie Blue (CB) Assay This test helps to determine the survival and protein content within the cells after getting exposed to the test sample. In this test, the NR dye is taken up by the lysozymes

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of the living cells and the quantity of proteins are being quantified as per the method provided by Liebsch Spielmann in 1995 using Coomassie brilliant blue dye in place of Kenacid Blue R dye. Here in this test, the test sample has to be incubated for 24 h with preferred cells in presence of fetal calf serum(FCS) in a 96-well plate and after 24 h, the media has to be replaced with fresh 5% FCS containing the NR and again the cells have to be incubated for another 3 h duration to let the NR dye being taken up by the cells and later the quantity of dye absorbed by the viable cells has to be extracted using acetic acid–ethanol mixture and measured the absorbance at 540 nm. The NR dye cannot be retained by the dead cells. The increased quantity of NR dye within the cells is a direct estimation of viability of cells. CB dye assays are to be performed for the quantification of proteins in viable cells under acidic conditions, as the presence of large amounts of proteins helps in cell proliferation which is a direct assessment of live cells. The test is performed by application of the CB dye to the 96 well plate containing cell lines and shaking for 10 min, later the dye is to be washed with an acetic-ethanol solution and the absorbed dye which has formed a complex with the viable cell proteins are to be extracted using 1(M) Potassium acetate solution and the extracted dye is to be measured at 570 nm with the help of a microplate reader [23].

3.4 Lactate Dehydrogenase (LDH) Leakage Assay This test is based on the amount of LDH released from the destroyed cells in presence of a test substance and the content is to be analyzed using a commercially available kit.

3.5 Highest Tolerance Doses This study is done for assessing any morphological changes in the cells when in contact with the test. The highest tolerance dose is the maximum dose that can result in the least morphological changes in the cells in presence of the test material [23].

4 Applications Metal Phosphates/Phosphonates have been explored for a variety of applications in terms of the biomedical field and drug delivery platforms along with other units in areas of membranes, separators, energy storage devices, photocatalysts, etc. The applications are highlighted in a nutshell in Fig. 1.

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Fig. 1 Applications of metal phosphates and phosphonates. Adapted with permission [24]. Copyright (2019), American Chemical Society

4.1 Biomedical Applications A small amount of metals are already present in the body which contribute to the proper functioning of biological processes including nerve cells, muscle cells, bones, the brain, and the heart but most of the pure metals cannot be used in biomedical applications because of their toxicity, corrosiveness, and non-biocompatibility. However, their alloys contain non-metallic elements predominantly stainless steel, titanium, magnesium, and zirconium with controllable corrosion [25].

4.1.1

Bone Tissue Regeneration

Phosphates regulate the growth and the mineralization of bones, insufficiency of phosphates can lead to bone weakness, fractures, rickets, and osteomalacia [26]. Being an essential element of bone, phosphates enhance the regeneration process. Metal phosphates accelerate the regeneration process and also enhance the mechanical and biological properties of implants. Magnesium phosphate, calcium phosphate, and titanium phosphate is majorly used metal phosphates for bone tissue engineering.

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4.1.1.1 Magnesium Phosphate Being an essential element in the human body magnesium helps in DNA stabilization, skeletal development, and bone metabolism. Magnesium also enhances the activities of osteoblasts and osteoclasts. The amorphous form of magnesium phosphate (AMP) promotes the rapid differentiation of and mineralization of preosteoblasts as compared to crystalline magnesium phosphate (CMP). Debey et al. prepared an extracellular matrix/amorphous magnesium phosphate bio-ink for 3D bioprinting and used that ink for craniomaxillofacial bone tissue regeneration. The scaffold increased bone volume and bone density [27]. Zhao et al. prepared a magnesium phosphatebased composite for bone tissue regeneration and increased its biocompatibility and biodegradability using gelatin crosslinking. The composite demonstrated excellent cell viability on MC3T3 osteoblast precursor cells which assist angiogenesis activity [28]. Golafshan et al. investigated the effectiveness of Magnesium phosphate (MPs) modified with strontium (Sr2+ ) and polycaprolactone (PCL) composite systems on tuber coxae of ponies. The bone growth was observed not only in the periphery but also in its center. The scaffold degraded itself after 6 months of implantation [29]. Cao et al., fabricated MgP/SrHPO4 3D porous composite scaffold for bone tissue regeneration because the porous scaffold exhibits good biocompatibility and cell affinity, sustained in vitro degradation, and suitable pore structure. Thus, the material shows a potential application in the field of bone repair [30]. Amorphous magnesium phosphate/graphene oxide (AMP/GO) bioceramic was used for bone tissue regeneration. AMP along with the GO sheet not only, decreased the degradation rate of bio-ceramic as well as improved the antibacterial activity of the formulation. GO nanosheets in the AMP structure increased the cell viability against MG63 cells. The prepared AMP/GO ceramic exhibits promising potential for bone tissue engineering along with antimicrobial activity [31].

4.1.1.2 Calcium Phosphate Xu et al. developed injectable calcium phosphate cement (CPCs) for the regeneration of bone tissues. In-vivo CPCs were implanted on 8-mm critical bone defect in rats and new bone formation was observed with microcapillary-like blood vessels. The implantation of CPCs enhanced osteogenesis as well as angiogenesis of cranial bones [32]. Gellynck et al. reported the effects of adding tricalcium phosphate to a degradable bone adhesive on cell proliferation or attachment and the activation of the hedgehog pathway. The addition of a high level of tricalcium phosphate decreased the photocured rate of the bone adhesive leading to the prolonged development of in vivo bone repair [33]. The calcium phosphate glass was doped with copper (Cu2+ ) and regenerative property, as well as cytocompatibility, was measured on HTB85 osteosarcoma cells. A significant increase in cell proliferation was observed. The incorporation of Cu2+ shows an antimicrobial effect against gram-positive bacteria S. aureus. A positive correlation between Cu2+ and the bactericidal property was observed. Calcium-strontium-zinc-phosphate coated titanium implants (CSZP +

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IL 4) were used for the femoral bone tissue regeneration in rats assisted with Interleukin-4. CSZP coating on Ti enhanced osteogenic activity along with IL-4 [34].

4.1.1.3 Zirconium Phosphate Kalita et al. 2017 prepared metal (Ca, Mg, Ti) doped zirconium phosphate/polyvinyl alcohol nanocomposite calcium/zirconium phosphate (CaZrP), magnesium zirconium phosphate (MgZrP) and titanium zirconium phosphate (TiZrP) was used as filler in the nanocomposite. Among all the nanocomposite films TiZrP nanocomposite possesses the highest mechanical property but CaZrP doped composite showed enhanced bioactivity [35].

4.1.1.4 Titanium Phosphate (TiP) Titanium phosphate could be an interesting alternative for bone tissue regeneration. The incorporation of bio-functional nanoparticles could improve the properties of implants and help in the repair of bone. TiP scaffold loaded with silver nanoparticles enhanced the antimicrobial activity of the implant with outstanding cytocompatibility. The tiP is implanted as bone cement as a surface layer covering. Titanium, with high osseointegration capacity and ease of physical attachment to bone, owing to the spontaneously formed oxide layer on its surface, does not cause denaturation of proteins in the proximity of the implant [36].

4.1.2

Dental Applications

Dental implant therapy has become a successful replacement for missing teeth. The load-bearing capacity of metals makes them useful for dental implants. Mestres et al. prepared metal phosphate cement (MPCs) for endodontic application and observed that these MPCs possess a low degradation rate as well as adequate mechanical properties and stable sealing ability [37]. A calcium phosphate-coated zirconia dental implant was prepared using an injection molding technique. The prepared implants were inserted and screwed into the dental block, an increase in strength was observed due to the implants [38].

4.1.3

Drug Delivery Applications

Due to their distinctive properties, viz resemblance to the inorganic component of bone, favorable adsorption capability to different proteins and biomolecules as well as biodegradability in moderately acidic medium, calcium phosphate-based nanoparticles have been investigated for targeted drug and gene delivery applications. Calcium

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phosphate nanoparticles smaller than 200 nm can enter cells through endocytosis by ending up in lysosomes [39]. Self-setting calcium phosphate has been researched as a promising option for a drug carrier due to its characteristics like biodegradability, injectability, and consists of large surface area which can help in the incorporation of drugs and other molecules. These formulations are potential materials employed as drug delivery systems for antibiotics, anticancer, anti-inflammatory medicines, or bone morphogenetic proteins [40]. pH-sensitive carriers were prepared using mesoporous zirconium diphosphonates and a controlled oral colon-targeted drug delivery system was developed. High drug loading was observed with the mesoporous zirconium diphosphonates due to its huge pores size and large surface area [41]. In 2012, Diaz and co-workers intercalated doxorubicin successfully in the novel zirconium phosphate nano-platelets for breast cancer therapy. These intercalated nano-platelets showed 34.9% (w/w) drug loading. The cytotoxicity observed was higher than the free doxorubicin [42]. Hollow nanostructures of transition metal phosphate due to their high surface area, presence of hollow void, and easy tuning of compositions and dimensions applications, including drug delivery. The hollow hydroxy zinc phosphate nanospheres (HZnPNSs) with the size of 30–50 nm and wall thickness of about 7 nm anticancer doxorubicin as a model drug was used to evaluate the entrapment efficiency and drug loading capacity of HZnPNSs, which showed high loading capacity (>16 wt%) for doxorubicin. The confocal laser scanning microscope (CLSM) observations showed that the drug could be efficiently delivered into cells [43]. Tang et al. prepared colon-targeted bifunctional mesoporous zirconium phosphonates for oral delivery of nucleic acids. Two phosphonic acids in the framework are incorporated simultaneously. L-proline group and the piperazine group from both the phosphonic acids provides pH-controlled release of nucleic acids. The main benefits of using phosphonate-functionalized porous polymers as drug delivery vehicles are their biodegradability and low cytotoxicity level [8]. Wang et al. designed a novel iron phosphate nanozyme displaying tri-enzyme-like activities, exhibiting strong anti-tumor activities. Intertumoral injection of FePOs + H2 O2 may decrease tumor growth by as much as 84.4% without causing physical toxicity. The FePOs nanozyme maintains the redox balance in non-cancerous cells leading to minimum side effects. This research introduces a novel Fe-based nanozyme that activates the Fenton reaction by controlling the tumor microenvironment, holding promise for improved antitumor effectiveness with the benefits of selective tumor killing with minimal side effects [44]. In another study, europium-doped calcium phosphate nanospheres were used for drug release as it was found that the combination converted to hydroxyapatite during drug release, and europium was used for providing photoluminescence [45]. Sriram and Lee utilized the fact that MicroRNA-21(miR-21) can be overexpressed in some cases of cancer like breast cancer and small cell lung cancer. So, they designed a calcium phosphate-coated nanoparticle system to deliver mirR-21 inhibitor combined with doxorubicin for the synergistic action, the former being hydrophilic and the latter being hydrophobic which showed to be a promising candidate for targeted combination therapy overcoming drug resistance [46]. To overcome various demerits of oral delivery of vaccines, calcium phosphate nanoparticles were designed with the coating of chitosan and alginate polysaccharides. Encapsulation

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of protein antigens into the core of calcium phosphate was done efficiently. The coating of polysaccharide helped in protecting the protein antigens from the acidic environment of the gastro-intestine, while sustained release at pH 6.8 and 7.4 were observed. These Calcium phosphate nanoparticles showed potential for vaccines oral route administration prompting immune response [47].

4.1.4

Gene Delivery

Razieh Khalifehzadeh et al. developed strontium-doped calcium phosphate nanoparticles using DNA as a template for its synthesis for targeting the specific genes present in the bone cells and regulating their function. This study showed that the prepared material helped in triggering bone tissue regeneration as well as treating different bone diseases [48]. Calcium phosphates being positive can efficiently bind with the negatively charged deoxyribose nucleic acid (DNA) and this complex in the form of positively charged nanoparticles can effectively penetrate the cell membranes by endocytosis mechanism and deliver the genes at the targeted area.

4.1.5

Biosensors

Metal phosphates can serve as biosensors for early diagnosis of disease and for monitoring the response of the treatment or the efficacy of the treatment. Cheng et al. prepared streptavidin-Titanium phosphate-metal ion functionalized nanosphere and used it as an electrochemical biosensor for the detection of miRNAs. Different types of miRNAs are responsible for different types of human cancers, neurological disorders, and viral infections. Therefore, metal phosphates can be used for diagnosing and preventing various types of diseases [45]. The radiopacity of magnesium phosphate scaffold is insufficient like other natural materials but the incorporation of strontium hydrogen phosphate (SrHPO4 ) increases the radiopacity of the material so, the material effectively absorbed the X-rays and provide improved radiographic images [32].

4.1.6

Other Applications

Heavy Metal Removal (Ion Exchange Materials) Zirconium phosphate is a very promising material for water treatment due to its water insolubility. Zirconium phosphate nanoparticles exhibit maximum absorption of water polluted with lead. Silbernagel and co-workers explored zirconium hybrid materials by adding a surplus amount of phosphates. The addition led to the removal of all the ions from the solution without any discrimination based on charge. War et al. prepared potato starch -sodium alginate-Zr phosphate bio-nanocomposite for the removal of impurities present in water. The prepared nanocomposite is used for the

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removal of cationic dye (methylene blue) from the water based on photodegradation [49]. Ma et al. designed meso-/macroporous titanium tetraphosphonate materials with intraframework ethylenediamine groups using a surfactant-free process. The porous structure led to better photolytic activity and also adsorbent for heavy metal ions like Cu2+ , Cd2+ , and Pb+2 [18]. Catalyst Metal phosphates can be used as a catalyst. Nickel phosphate was used as a catalyst for the oxygen evolution reaction. Manjhi et al. prepared neodymium oxide doped neodymium phosphate and used it as a catalyst for the evolution of hydrogen in an acidic medium [50].

5 Conclusion Metal phosphates/phosphonates serve as a versatile platform for diverse applications possessing high surface area and porosity. Metals like calcium, magnesium, strontium, etc. have a strong affinity towards the phosphate/phosphonates making their metal–organic framework exceptionally strong, and such mechanical durability makes them appropriate in orthopedic applications whereupon invading the targeted area, triggering bone cells for their regeneration. Metal phosphates are also good candidates for delivering genetic material at the cellular level in form of nanosystems as metal-phosphate/phosphonate configurations are positively charged by which they can easily permeate negatively charged cells by endocytosis and deliver the desired moieties. Metal phosphates/phosphonates are also used for the fabrication of biosensors as a part of diagnostic applications. Thus, unexplored applications can be devised in the future by doping these metal phosphates/phosphonates with different pH, photo, or thermosensitive entities to bring a revolution in the biomedical field.

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Metal Phosphate and Phosphonate Application for Imaging and Diagnosis Hamide Ehtesabi and Seyed-Omid Kalji

Abstract Metal phosphate and phosphonate are hybrid materials prepared from the chemical interaction between organic and inorganic material that owns the favorable properties of both metal ions and phosphate-containing organic agents. The synthesis of metal phosphate hybrid material in mild conditions from the wide variety of existing reactants is a promising methodology to obtain new advanced materials with various building parts and chemical groups. Owing to their homogenous porosity, adjustable composition, low toxicity, and controllable structures, metal phosphates are used in lots of applications from catalysts, energy storage, and energy conversion to biotechnology, medical diagnosis, and therapy. The resolution and body retention time of metal phosphate as a medical imaging agent is significantly enhanced by using metal ions in the structure of these hybrid materials. The recent progress in cancer imaging and disease detection by metal phosphates and phosphonates are discussed in this chapter and exemplified according to the used metal types. Keywords Inorganic–organic hybrid · Nanoparticle · Nuclear imaging · Diagnosis

1 Introduction The combination of desirable properties of organic and inorganic material to obtain advanced hybrid materials in the mild condition has been an old wish of scientists from the stone age until the past century when chemists introduces wet chemistry as a new approach to synthesizing composites of two materials such as titanium dioxide (TiO2 ) without needs to high temperature [1]. Organic–inorganic materials are nanocomposites made from organic and inorganic components that chemically interact at molecular scales. This definition owns a different concept in comparison with the simple and physical mixing of inorganic and organic parts [2]. These H. Ehtesabi (B) Faculty of Life Sciences and Biotechnology, Shahid Beheshti University, Tehran, Iran e-mail: [email protected] S.-O. Kalji Faculty of Biological Sciences, Tarbiat Modares University, Tehran, Iran © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_21

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types of hybrid materials are categorized into two classes: in the first class, the organic and inorganic components are gathered via weak and noncovalent chemical bonding including electrostatic forces, van der Waals, and hydrogen bonding. In contrast, in the second class, the organic and inorganic materials are in strong covalent chemical interactions [3]. The strong chemical affinity of organophosphonic linkers to metal ions creates one of the most attractive organic–inorganic hybrids known as metal phosphates and metal phosphonates material. Metal phosphates and phosphonates have remarkable thermal and chemical stability and gain a wide application in energy conversion, catalysis, and biotechnology [4]. Metal phosphate is commonly categorized into three groups: (1) phosphonate-based metal–organic frameworks (MOFs) or crystalline metal phosphonates (from layered structures to microporous or open-framework structures), (2) mesoporous metal phosphonate, and (3) surface modification of oxides with phosphonate layers [1]. Nuclear imaging is a new diagnostic procedure based on radionuclides metal ions that collects information from the physiological status of the body to detection of diseases. The metallic radionuclides emit photons including gamma rays or the annihilation photons produced by positron β+ decay. The irradiated photons from the metal ions have limited interaction with the body tissue which creates spatial imaging information [5]. Accordingly, nuclear imaging can be categorized into positron emission tomography (PET) and single-photon imaging based on their different nuclear decay mechanism and consequently different detector optics to create images. In PET imaging, the emitted positron from the nucleus like cobalt (55 Co) passed a small distinctive route before the collision with an electron. This collision causes the annihilation of positron and electron, which resulted in the production of two photons (511 keV). These two irradiated photons create two locations spot on the circular ring of the detector and finally revealed the image of the location. On the other side, in single-photon imaging, gamma rays are emitted from radioactive atoms such as technetium 99 m (99m Tc). This kind of irradiation is a single event. To visualize this kind of single-event emission, a lead collimator containing lots of holes is placed between the source and the detector that are perpendicular to the face of the detector [6, 7]. Nanoparticles exhibit their peculiar behavior nature and biological environment due to their specific shape, size, high surface-to-volume ratio, and inner structure. Nanoparticles have a remarkable ability for cellular endocytosis and tissue penetration. They also show prolonged circulation time in the body and partial accumulation in tumor tissue [8]. Therefore, nanoparticles are widely used in the preparation of contrast agents (CA) to enhance the resolution and efficiency of nuclear imaging such as magnetic resonance imaging (MRI) and optical imaging (OI) [9]. In this chapter, the common metal phosphate and phosphonate utilized in medical nuclear imaging are separately discussed and exemplified. In addition, the newest advanced progress in the field of medical imaging by metal phosphates-phosphonates and their novel derivations such as nano-based metal agents are presented.

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2 Technetium The only transition metal without stable isotopes is 99m Tc. As a naturally occurring 238 uranium (238 U) fission product, 99 Tc can be found in pitchblende ore that contains U. Other long-lived Tc isotopes have included reactor-produced 97 Tc and 98 Tc [10]. More than 85% of all nuclear medicine experiments use 99m Tc radiopharmaceuticals, which are widely used for single-photon imaging in nuclear medicine applications. In addition to having appealing emission characteristics, 99m Tc can be easily produced from the molibden 99 molybdenum (99 Mo) [11]. A recent global 99 Mo scarcity caused by issues with aging reactor facilities in Canada and other countries sparked interest in alternate 99 Mo and 99m Tc manufacturing techniques as well as other Tc radionuclides. For instance, 94m Tc is seen as a potential PET substitute for 99m Tc, even though both its positron energy and half-life are substandard [12]. It was looked into using 99m Tc-imidodiphosphonate (99m Tc-IDP) as a nuclear medicine imaging agent for acute myocardial infarctions. An appropriate animal model (rat) was selected for this purpose. With 80% of the animals that survived coronary artery closure, repeatable myocardial infarcts were found. A high-resolution gamma camera was used to collect scans of the myocardial infarcts, and high-quality photos were recorded. Calculated 99m Tc-IDP ratios for healthy and infarcted tissue were compared to information from other 99m Tc-labelled phosphates.99m Tc-IDP is the most effective radiopharmaceutical to date for nuclear medicine imaging of necrosed cardiac muscle has the infarct/normal ratio of 21:1. As soon as six hours after infarction, pictures of the myocardial infarcts have been captured. One millicurie of 99m Tc pertechnetate is applied to the vial at room temperature; the average labeling efficiency ranges from 95 to 97%. One millicurie of 99m Tc-IDP was injected into the rat tail vein, and imaging of these animals was done an hour later. On a Nuclear Enterprises Mk, animal imaging was performed [13]. A popular physiologic isotonic medium used in biotechnological research is phosphate-buffered saline (PBS). PBS is made up of phosphate salts and sodium chloride in an aqueous solution with a defined pH, typically 7.4. PBS was mentioned as a potential factor that could affect the labeling stability of molecules with 99m Tc and is used in the research area as a buffer system for radiolabeling of molecules to establish a suitable environment for intravenous delivery (Fig. 1a) [14, 15]. In a study, the phosphate ions in the PBS solution were radiolabeled with 99m Tc to assess the possible applications of the system as a bone tracker with an efficient cost ratio. This was done based on the radiolabeling attributes of phosphate-based substances and the affinity of these complexes with the bone tissue. Thus, the objective of this study was to 99m Tc radiolabel PBS and show in vivo bone uptake of [99m Tc]Tc-PBS system in healthy mice. To achieve this, 800 mL of double-distilled water was used to dissolve certain values of NaCl, KCl, Na2 HPO4 , and KH2 PO4 to create PBS solution. Following complete solubilization, pH was raised to 7.4 using HCl solution, and the volume was then finished to a volume of 1000 mL using water. 300 μL of the freshly prepared PBS solution and 50 μL of a SnCl2 ·2H2 O in HCl solution were added to a vial of 2 mL. The pH was then brought back to 7 using

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Fig. 1 a Biodistribution of bevacizumab-loaded HSA nanoparticles (NP-Ab) crosslinked with PEG35000 by SPECT/CT in vivo imaging. NP-Abs were prepared by a desolvation process, coated with PEG35000 and radiolabeled with 99m Tc using a pre-tinning method ([99m Tc]Tc-NP-Ab). Adapted with permission [14], Copyright (2021), Elsevier. b, c Immobilized modified probes on slides using Zr organophosphonate. If the substrate is a hydrophobic surface, such as OTS-coated glass, Langmuir–Blodgett method (X is an aliphatic chain, ODPA) can be used to deposit the phosphonate layer. Alternately, it can be made by making changes to a layer that is covalently connected, like an aminopropylsilane film (X is aminopropylsilane). Adapted with permission [19], Copyright (2004), American Chemical Society

a NaOH solution. The mixture was then mixed with 100 μL of a Na[99m Tc]TcO4 (37 megabecquerel) solution and was left to react at room temperature for 15 min. 100 μL of 18 megabecquerel of [99m Tc]Tc-PBS were administered intravenously into healthy Swiss mice to perform scintigraphic imaging. Anesthetized mice were horizontally placed beneath the collimator of a gamma camera combined with a low-energy high-resolution collimator. The system was successfully radiolabeled, producing radiochemical purity of about 90%. Mouse plasma and saline medium

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were used to assess the system’s stability, and the results showed that [99m Tc]TcPBS had excellent long-term stability in both media. A biphasic clearance profile was seen, with renal filtration providing quick removal from the circulation. The radiolabeled method demonstrated high specific bone accumulation, showing an uptake pattern that was 20–50 times greater than muscle, the reference tissue. From all of the data, the [99m Tc]Tc-PBS system can be viewed as a possible selective agent for bone imaging [16].

3 Zirconium Zirconium (Zr) is an early transition metal with three known isotopes including 86 Zr, 88 Zr, and 89 Zr has gained lots of application in nuclear imaging, organometallic polymerization, and ceramics. The various isotopes of Zr are synthesized via the cyclotron technique. Because of the low energy of emitted positrons from 89 Zr, its related half-life is limited and comparable with the circulation times of monoclonal antibodies (about 78 h) in the blood. Therefore, this isotope of Zr can be conjugated with antibodies for targeted clinical applications such as imaging and therapy [17]. The use of protein immobilization on solid supports for analytical purposes such as functional proteomics, diagnostics, and drug screening is fast growing. The main difficulties in creating protein microarrays are maintaining the protein functions, immobilizing the protein in a special and oriented manner on the surface, creating a surface with a strong affinity capability, and developing alternatives to antibodies to serve as recognition agents in protein microarrays. However, the construction of readable and rewritable surfaces using such metal chelates surfaces has generated a lot of interest in the nanotechnology field. The nature of the immobilized metal ion can be altered to change the chelate’s affinity for particular chemical groups. It has been noted that Zr4+ ions adsorbed on an alkyl phosphonate monolayer show higher phosphopeptide selectivity than normal ions. For the creation of protein microarrays, a unique method of protein binding to glass slides covered with Zr phosphonate is available. A novel peptide tag, which could be genetically attached to either the C- or the N-terminal end of recombinant proteins and effectively phosphorylated in vitro by CKII, is described in the presented research. The resultant proteins have a cluster of four serine molecules that have been phosphorylated, which enables direct and selective immobilization of the proteins on the Zr phosphonate surface as well as orientation control. Incredibly high signal-to-noise ratios are achieved and protein concentrations in the picomolar level can be identified when this technique is utilized to create protein microarrays. Lastly, proteins with a multi-phosphate tag may also present advantageous chances for a straightforward coating of metal phosphate [18]. In molecular biological studies, DNA arrays have become a useful tool for quick and reliable gene mapping, DNA sequencing, mRNA expression study, and the identification of genetic illnesses. Most sensors are made up of single-stranded oligonucleotides with diverse sequences, known as probes that are attached to a surface and ready to be hybridized with targets. There are two standard methods for

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creating arrays. In-situ fabrication, such as the Affymetrix photolithographic on-chip production. In this method, the oligonucleotides are obtained via enzymatic chemical synthesis approaches. Then, the arrays of prepared oligonucleotides are created via robotically controlled “spotting” methods, which may harm the effectiveness of hybridization. Another strategy is to develop reactive functional groups that can make covalent connections on the surface of the oligonucleotide probe, often at its 5' end, to offer particular nucleobase-level attachment. However, our knowledge about the immobilizing of oligonucleotide probes into arrays on a surface via “organic–inorganic” interactions is still not sufficient. Zr phosphonate is a new advanced approach to achieving microarrays through the covalent immobilization of oligonucleotides on glass slides. Toward this aim, a monolayer of Zr phosphonate was coated on the glass surface. Then, Oligonucleotide probes are covalently attached to this substrate via their free phosphate ends. This reported method has enough potential to create effective oligonucleotide arrays because benefits from the appropriate selectivity of attachment through the terminal phosphate. The method depends on adding a free phosphate group to the oligonucleotides’ 5' -position, a straightforward change that can be accomplished by enzymatic means (Fig. 1b, c). Following hybridization with the fluorescent target, signal-to-noise ratios up to 1000 have been seen [19].

4 Iron Oxide Magnetic iron oxide nanoparticles (IONPs) are the nano-size form of iron metal with remarkable physical and chemical properties that recently emerges as an MRI agent (T2 contrast improvement), drug delivery, cell separator, and therapy. The most common form of IONPs is core–shell nanoparticles containing magnetite (Fe3 O4 ) and its oxidized version (Fe2 O3 ). These types of nanoparticles are controllable because their magnetic features vanished after removing the external magnetic field. The other amazing property of IONPs that proved them for biological and theranostics applications is their feasibility of surface modification to improve their biocompatibility and bioactivity. Due to the simplicity of surface functionalization, IONPs have a large loading capacity through functional groups and colloidal stability via steric and electrostatic interactions. This allows them to bind with numerous targeting compounds such as antibodies, aptamers, and peptides [20]. To develop a powerful imaging agent that can travel through the bloodstream in the time it takes to concentrate in the target organ, the colloidal stability of IONPs must be considered. To create a powerful imaging agent that can travel in the bloodstream in however long it takes the target organ to concentrate, the colloidal stability of IONPs must be taken into consideration. Uncoated nanoparticles, on the other hand, have the propensity to aggregate in living tissue, resulting in the creation of micro-sized particles that mostly collect in the organs of the reticuloendothelial system (RES) and quickly sequester from blood circulation. On the other hand, IONPs are given stealth qualities, greater stability, and decreased cytotoxicity when their surfaces are coated with the correct organic

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and inorganic compounds, such as phosphate. This increases their possibility for usage in vivo studies. Researchers have worked extremely hard to create non-invasive, multimodal imaging agents that can more accurately identify anomalies in vivo in response to the demand for early cancer diagnosis. Because of their ability to get beyond the limits of a single imaging modality, dual-modality imaging agents, including radiolabeled IONPs, have become more necessary in recent years in the field of biomedicine. PET, single-photon emission computed tomography (SPECT), X-ray computed tomography (CT), OI, and MRI is frequently utilized in nuclear medicine imaging with their all benefits and drawbacks. For example, PET and SPECT imaging creates limited information from the imaging area and MRI provides proportional spatial resolution and contrast but shows low sensitivity. Accordingly, the combination of such imaging techniques can cover their defects and improve their potential for biomedical imaging [21]. The imaging agent presented in a study is made up of Fe3 O4 nanoparticles that have been directly radiolabeled with 68 Ga for PET imaging and functionalized with hydrophilic stabilizer 2,3-dicarboxypropane-1,1-diphosphonic acid (DPD). Due to its hydrophilicity and biocompatibility, DPD was chosen since it offers the necessary dispersion stability and decreases the potential toxicity of the bare Fe3 O4 nanoparticles. DPD also works well as a chelating agent because it contains two phosphonates and two carboxylate groups in its structure. By using of a particularly designed apparatus for small animal PET/X-ray imaging, dynamic and cumulative images of healthy Swiss mice were obtained (Fig. 2a). Owing to the liver’s large concentration and the kidneys’ overlapping with the liver and intestines on PET imaging, kidney absorption cannot be seen. 68 Ga-Fe3 O4 -DPD nanoparticle investigations on ex vivo biodistribution and in vivo PET imaging demonstrated significant accumulation in the RES organs, but they also showed adequate blood retention at 30, 60, and 120 min (Fig. 2b). The potential of 68 Ga-Fe3 O4 -DPD nanoparticles as a PET/MRI agent is therefore very high [20]. The development of MNPs as an excellent MRI contrast agent has opened up new avenues for scientific investigation and clinical diagnostics. MNPs need to meet a few more requirements for this approach, namely having strong colloidal stability at physiological salt concentrations and at various pH levels to provide intravenous administration. As a result, selecting the best functionalizing agent also influences how well MNPs perform as MRI contrast agents. The most common capping materials include polyethylene glycol, polyacrylic acid, carboxymethyl cellulose, chitosan, and polyethylenimine. Due to the significant attraction of phosphate/phosphoric acid groups for the Fe atoms on the surface of MNPs, another kind of functionalizing agent is based on phosphonate molecules. Phosphonate derivatives can enhance chemical stability versus hydrolysis, pH changes, dilutions, and oxidation. Interesting characteristics offered by external phosphonate surface groups include interaction with rare earth elements, which may result in a more adaptable usage profile for MNPs, the ability to diagnose bone-related illnesses through its attraction for bone, phosphonate groups induce an improvement in relaxivity via second sphere effect in the context

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Fig. 2 a 68 Ga-Fe3 O4 -DPD nanoparticles were injected into a healthy Swiss mouse at 20, 30, and 60 min after injection. The steady change in color denotes a progression from fewer to more recorded counts. b At 30, 60, and 120 min after injection, 68 Ga-Fe3 O4 -DPD nanoparticles were studied for ex vivo biodistribution in healthy Swiss mice. a, b Adapted with permission [20] Copyright The Authors, some rights reserved; exclusive licensee [Wiley]. Distributed under a Creative Commons Attribution License 4.0 (CC BY). c DTPMP’s chemical structure and methods for making DTPMPcoated MNPs. Adapted with permission [22], Copyright (2021), Elsevier

of their use as MRI contrast agents because of their potent hydrogen bonding interactions with molecules of water. Diethylenetriaminepenta (DTPMP) was later used to create an amino phosphonate Fe3 O4 nanoparticle (Fig. 2c). Sonochemistry and hydrothermal techniques were used to create the DTPMP-coated Fe3 O4 material. It is also crucial to emphasize that, with a 17× quicker reaction time, sonochemistry has proven to be a more efficient synthesis approach. The MRI contrast agent capability of the DTPMP-coated Fe3 O4 nanoparticles was further supported by relaxivity and cytotoxicity tests, which revealed high transverse relaxivity rates and a non-cytotoxic nature. Thus, this work introduces a brand-new and superb alternative magnetic material for both biological and technological usage, with an MRI contrast agent as the primary focus [22].

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5 Gadolinium The contrast of tissues is frequently improved by the introduction of small-molecule paramagnetic agents, particularly Gd3+ compounds, which boost the T1-weighted MRI contrast by raising the longitudinal relaxation rate (R1 = 1/T1) of protons in water. However, due to their poor concentration and quick elimination from tumors, Gd3+ is frequently restricted in the imaging of tumors. Due to the enhanced permeability and retention (EPR) effect, the incorporation of Gd3+ complexes into nanoparticles may boost the concentration of contrast agents and, as a result, increase the signal intensities for tumor imaging [23]. Gadolinium-DTPA (pentetic acid)-HPDP (1-hydroxo-3-aminopropane-1, Idiphosphonate) are recently being investigated as sophisticated MRI agents for their biodistribution and imaging capabilities. The free diphosphonate acts as the targeting component while the DTPA chelates the paramagnetic metal. Testing was done on two classifications of phosphonated agents’ sample substances. The free diphosphonates were represented by Gd-DTPA-HPDP and HEDP ((hydroxyethyl-1, diphosphonate), while the bound phosphonates were represented by Gd-EDTMP (ethylenediamine-N,N,V' ,N' -t etrarnethylrnephosphonate). The current investigation shows that when the sodium salt of these compounds is rapidly delivered, the free diphosphonate ends of phosphonate drugs can have negative effects at the levels necessary for MR image enhancement. Therefore, suitable procedures should be taken to prevent adverse inotropic effects if one intends to utilize these drugs in MR imaging of acute myocardial infarcts [24]. Clinical research is very interested in tumor identification. It is well known that solid tumors have certain vascular pathologic characteristics known as the EPR effect, which causes blood macromolecules to accumulate and be retained for an extended time inside the tumor. As a new MRI contrast agent, new dextran-coated paramagnetic Gd phosphate nanoparticles (PGP/dextran) were created. The main features of this new material are a positive contrast material for higher image resolution, a size of a few tens of nanometers for accumulation and retention in tumors, and a highly environmentally friendly dextran coating to prevent rapid blood flow elimination. The findings of pharmacokinetic investigations, toxicity tests, and MRI employing PGP/dextran to visualize tumors are discussed and compared with the clinically utilized contrast agent Magnevist@. Using the particulate and positive PGP/dextran and the EPR effect, tumors in a rabbit were successfully identified on traditional T1-weighted MR imaging with only a 1/10 reduction in applied dose as compared to Magnevist@. As a result, PGP/dextran could be employed as a medication delivery system for tumors as well as a tumor-specific contrast agent [25].

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6 Copper For eukaryotes, copper (Cu) is a necessary metal ion, and it has a significant function in the active sites of enzymes that are critical for the redox chemistry, O2 binding, and electron transfer of diverse organic material. Because Cu is seen as a potential theranostics element, research interest in Cu radioisotopes is rising [5]. Particularly, the nuclides 61 Cu and 64 Cu are potential PET imaging radioisotopes. Numerous cancers have been successfully imaged by PET using 64 Cu-labelled cyclam derivatives and their conjugates (Fig. 3) [26, 27]. A novel cyclam derivative with a phosphinate-bis(phosphonate) pendant (H5 te1PBP ) was described to show the ability of bis(phosphonate) carrying macrocyclic ligands as a Cu radioisotope carrier. The bis(phosphonate) group was not correlated in the Cu2+ complex and interacted aggressively with other metal ions in the solution. The ligand demonstrated a high selectivity to Cu2+ over Zn2+ and Ni2+ ions. At pH 5, at a millimolar scale, and in 1 s, the Cu2+ compound formed. Due to the generation of chemical intermediates with different metal-to-ligand ratios and protonation states, respectively, the complexation rates under a ligand or metal ion excess greatly varied. Additionally, the [64 Cu]-PBP complex displayed strong acidassisted hydrolysis tolerance and was successfully adsorbed on the hydroxyapatite surface. The outcomes of this study showed that [64 Cu]-PBP was very well suited

Fig. 3 a Synthetic protocol for the preparation of the ligands H2 te1pyp and H2 cb-te1pypa with phosphonate appended pyridine side arms for the coordination of Cu2+ ions in the context of 64 Cu PET imaging. b PET-CT imaging of Balb-C mice model injected with [64 Cu(te1pyp)] (left) and [64 Cu(cb-te1pyp)] (right) at t = 2 h post-injection. Adapted with permission [27], Copyright (2021), American Chemical Society

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for imaging active bone compartments by focused small animal PET/CT in healthy mice, followed by in a rat femoral defect model [28]. A study’s objective was to compare phosphonate-based cross-bridged chelators to 64 Cu-CB-TE2A, which had previously demonstrated preferential binding to integrin αvβ3 on osteoclasts, in the 4T1 mouse mammary tumor bone metastases model. 4T1/Luc cells were injected into the left ventricle of BALB/c mice for in vivo testing. Bioluminescence imaging (BLI) was used to monitor the development of metastases, and then small-animal PET/CT was used two hours after radiotracer administration. The chelator-peptide conjugates have low nanomolar levels of integrin αvβ3 affinity. All 64 Cu-labeled phosphate analogs showed a greater absorption in bones with metastases during PET imaging compared to bones from animals without tumors. These results indicate the potential of c(RDGyK) peptide phosphonate chelator conjugates as PET tracers for imaging tumor-associated osteoclasts in bone metastases [29].

7 Titanium 44

Ti and 45 Ti are the main titanium (Ti) radionuclides of relevance for nuclear medical applications [30]. The 45 Sc(p,n)45 Ti reaction can be used to create the positronemitting radionuclide 45 Ti [31]. The two most frequent oxidation states of Ti are Ti3+ and Ti4+ , with Ti4+ being preferred in physiologically relevant aqueous solutions. Ti aqua complexes are vulnerable to the production of Ti(OH)3 + and Ti(OH)4 at low pH and cluster formation at high pH due to the strong Lewis acidity of Ti4+ . While MRI offers a unique spatial resolution, its sensitivity and specificity are very low. Conversely, OI has very strong resolution and outstanding sensitivity, but its depth of detection limits it. The application potential for combined MRI-OI drugs in targeted tumor labeling for oncological assessment and surgery is quite high [32]. The MRI sensitivity issue can be solved by gathering several CA molecules into one entity. Additionally, the tumbling of the chelate in solution slows down following attachment, increasing the R1 relaxivity, which expresses the effectiveness of MRI CA [33]. Due to its inertness and lack of toxicity, TiO2 was used to make MRI and luminescence imaging nanoprobes. However, electron excitation takes place when UV light is applied to TiO2 , and the electron and its energy can be transported to another molecule and be in charge of chemical alterations. TiO2 that has been exposed to radiation creates OH extremely hazardous radicals. This characteristic may be exploited to kill cancer cells. A recent study reports the creation of TiO2 nanoparticles wrapped in phosphonated Gd chelates and their potential MRI characteristics. This investigation found that the surface of TiO2 is highly interacted with by bisphosphonate. A coordination polymer made of P–O–Ti–O–P chains is created as a consequence of the interaction and coats the surface of the nanoparticle (Fig. 4a). By co-adsorbing both sorbates in one step in an aqueous medium, the bimodal MRI OI probe is made in one pot. With the help of phosphonates and geminal bisphosphonates, the established technology makes it possible to create monodisperse nano colloids of stable anatase nanoparticles (12 nm). The outcomes demonstrate the procedure’s ease of use

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and applicability, which opens the door to the large manufacture of TiO2 nanoprobes coated with a variety of contrast agent molecules. On the stem, HeLa, and T cells that were tagged and observed by MRI and fluorescence microscopy, the probe’s potential for use in cell labeling is assessed (Fig. 4b, c). TiO2 @RhdGd multimodal imaging therapeutic nanoprobes were created and successfully employed for in vitro cancer cell killing as well as in vivo cell tracking. It was demonstrated that UV light irradiation could transform the probe TiO2 @RhdGd into a cancer cell killer. The data mentioned above serve as a significant demonstration of a theory demonstrating the suitability of the probe design for both diagnosis and treatment [9]. When miRNAs attach to their related messenger RNAs (mRNAs), they cause the mRNAs to degrade or stop translation, which controls many genes linked to human malignancies, neurological disorders, and viral diseases. miRNAs are a type of nonprotein-coding short RNA molecules of 17–25 nucleotides. Different tumor forms are linked to distinct miRNA expression profiles. The investigation of miRNAs using traditional methods such as northern blotting, microarrays, and quantitative reversetranscriptase polymerase chain reaction (qRT-PCR) is extremely difficult due to their small size, high sequence homology, and low expression levels. Using cadmium ions (Cd2+ ) modified Ti phosphate nanoparticles as the signal unit, two DNA serve as

Fig. 4 TiO2 phosphonate assembly (a), Fluorescent microphotography of cryocut lymph nodes from control mouse using phosphonate TiO2 assemblies (b), and from the mouse injected with lymph node-derived cells labeled with TiO2 @RhdGd (c). Red signal: fluorescence of TiO2 @RhdGd in the lysosome, Blue signal: nuclei stained with DAPI. Adapted with permission [9], Copyright (2011), American Chemical Society

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capture probes, and Ru(NH3 )3+ 6 acts as an electron transfer mediator, a unique and straightforward electrochemical miRNA biosensor has been constructed. To produce the electrochemical signal, significant amounts of Cd2+ were placed in Ti phosphate spheres. The addition of Ru(NH3 )3+ 6 molecules, which operated as an electron wire with DNA base pairs, caused a considerable rise in the electrochemical signal of more than five times. With an ultra-low limit detection and a broad dynamic linear range, this method significantly increased sensitivity. Additionally, the suggested biosensor could be employed for a quick and direct study of miRNAs in human serum and was adequately selective to distinguish the target miRNAs from homologous miRNAs (Fig. 5a). For medical research and clinical diagnostics, this technique thus offers a novel and highly sensitive framework for miRNA expression analysis [34].

Fig. 5 a Schematic for the step-by-step sensor construction process. Reprinted permission from ref [34]. b DPA detection by porous TbP-CPs as fluorescent probes, c, d SEM images of TbP-CPs. Adapted with permission [35], Copyright (2019), American Chemical Society

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8 Terbium Bacillus anthracis is the cause of the acute contagious disease known as anthrax, which primarily manifests as a spore. Due to its longevity in soil, the spore can be found anywhere in the world. Without prompt access to adequate medical care, an inhalation of more than 104 spores can result in death in 24–28 h. Dipicolinate acid (DPA), a key chemical component of Bacillus anthracis, was once thought to be a special biomarker of either the anthracis infection or Bacillus anthracis spores. Thus, numerous efforts have been made to investigate quick and effective detection techniques. Conventional organic dyes can occasionally be challenging to dissolve in water and are quickly photobleached. Background noise can readily disrupt several fluorescent probes, which has a significant impact on the detection outcomes. The efficient synthesis of a novel probe based on multiporous terbium phosphonate coordination polymers (TbP-CPs) for anthrax was reported. This structure might have a large surface area to improve interactions between DPA and Tb3+ (Fig. 5b-d). For more than 15 days, TbP-CPs’ fluorescence intensity in water remained nearly unchanged. TbP-CP sensors demonstrated a low detection limit of 5 nM, high selectivity, and a rapid response time of about 30 s when compared to other fluorescence-based DPA detection methods. It was also precise and reliable for real sample detection, including those of urine and bovine serum. In addition, the TbP-CPs are convenient and easy to make. These findings illustrated an important method for broadening the scope of applications for TbP-CPs. This study predicts that TbP-CPs with good biocompatibility could be used for additional applications in chemical sensors, biomedicine, and cell microscopic imaging [35].

9 Lutetium As a bone-seeking radiopharmaceutical, sodium pyrophosphate (Na-PYP) tagged with a radionuclide is used. For more than 30 years, radionuclide therapy has been used successfully to relieve bone pain. It is easy to deliver radiotherapy for bone pain, and research has shown that it increases life quality, decreases dependency on narcotic and non-narcotic analgesics, and improves mobility in many patients. With beta particle emissions with an Emax of 497 keV for therapeutic action and gamma emissions at 113 keV (6.4%) for imaging, 177 lutetium (177 Lu) is a suitable radionuclide for therapy. Since 177 Lu decays to stable 177 Hf and has a long half-life, it may be supplied to locations far from the reactor more easily. Because 176 Lu has a high thermal neutron capture cross-section and can be produced on a wide scale with great radionuclide purity and sufficient specific activity, 177 Lu has many advantages. The researcher has effectively tagged MDP (methylene diphosphonate) with 177 Lu. MDP is a bisphosphonate that is commonly employed as a radiopharmaceutical for bone scintigraphy in situations of metastatic bone disease. This article reports on the labeling of 177 Lu with PYP and its gamma camera images in rabbit models after

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intravenous administration of 177 Lu-PYP complex to show its viability for the relief of bone pain. To confirm the skeletal uptake, bioevaluation procedures with rabbits in front of cameras were also carried out. The research shows that Na-PYP may be radiochemically tagged with 177 Lu with high yields. The negatively charged 177 LuPYP combination maintained its stability throughout the day at high temperatures. High skeletal uptake was seen in gamma-camera images of 177 Lu-PYP in normal rabbits 24 h after injection, indicating that it might be effective as a bone-pain reliever for the management of bone metastases [36].

10 Indium Short-lived radionuclide indium-113 m (113m In) has exceptional physical properties, including a monoenergetic gamma emission of 393 keV and a half-life of 100 min. The daughter nuclide of this isotope’s long-lived parent, 113 Sn, is commercially available. This nuclide’s producers have been created and are readily available in the marketplace. Before the skeleton, this radionuclide was utilized in nuclear medicine to image the majority of man’s principal organs. Experimental animals exhibited preferential skeletal localization for 113m In complexed with the polyfunctional phosphonates EDTMP (an analog of ethylene-diamine-tetra-acid with carboxylic groups substituted by phosphate groups), and DTPMP (an analog of diethylene-triaminePenta-acid). Using the scintillation camera as well as the 113m In and 111 In compounds, excellent photos of the rabbit skeleton were captured. 113m In-EDTMP compound demonstrated higher concentration in the skeleton than the DTPMP complex in tissue radio assay using 85 Sr as a contemporaneous biologic reference, and its bone absorption was comparable to that of 85 Sr (Fig. 6). The DTPMP complex had a higher rate of renal excretion and cleared blood more quickly than EDTMP. Where 99m Tc bone-imaging agents are not accessible, 113m ln-EDTMP may be used for bone scanning in humans due to its good skeletal localization and low levels of soft tissue. These substances may also help demonstrate acute myocardial infarcts, especially in follow-up studies after the administration of 99m Tc bone agents [37].

11 Conclusion The synthesis of metal phosphate and phosphonate materials in the mild reaction condition through a wet chemistry procedure is one of the most attractive inventions of recent decades because provides the possibility of gathering the favorable properties of both organic and inorganic material into a hybrid product. The vast variety of designed properties with high structural and functional diversity is available in metal phosphates, which candidates them for biochemistry and biotechnology applications such as cancer imaging and infection diagnosis. The new participant of meal

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Fig. 6 Following injection of each chelate containing 5–10 mg of phosphonates, composite posterior pictures of rabbits using both the EDTMP and DTPMP chelates, 113m ln and 111 ln, were obtained. For 113m ln and 111 ln, respectively, parallel-hole collimators of 410 and 250 keV were employed. Three different photos that each collected 300,000 counts were combined to create a whole-body image of each rabbit. Figure 6 adapted with permission [37] Copyright The Authors, some rights reserved; exclusive licensee [Society of Nuclear Medicine and Molecular Imaging]. Distributed under a Creative Commons Attribution License 4.0 (CC BY)

phosphates is nanoparticle and nanostructure materials that enhanced their detection and therapy abilities. Metal phosphate and phosphonate field of science are in progress toward their other biological application such as DNA/RNA microarrays and electrochemical biosensors. Therefore, the new hybrid material such as nanometal phosphates assemblies with new properties for this application will be the future requirement of this part of technology.

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Advances and Challenges in the Fabrication of Porous Metal Phosphate and Phosphonate for Emerging Applications Ababay Ketema Worku and Delele Worku Ayele

Abstract Recently, nanomaterials made of phosphates and phosphonates have opened up a rare occasion to create hierarchically porous nanomaterials with interconnected macropores, mesopores, and micropores. They provide appealing qualities for new applications that call for control over the interface, which is essential for functional technology, catalysis, and adsorption. For material scientists and engineers, the necessity to develop a justification for new preparation techniques that will allow for the regulated production of these materials is a crucial driving force. In this article, we will provide a quick summary of recent advancements in many development tactics, emphasizing many metal phosphonate technology and how they affect the characteristics of porous metal phosphates/phosphonates. Manufacturing of innovative metal phosphate/phosphonate-based materials with nanostructures for adsorption, catalysis, optoelectronics, fuel cells, and electrochemical cells will also be covered. The final section will give challenges and prospects for future development in the areas of mesoporous metal phosphates, increasing crystallinity, and low-cost and scalable mesoporous metal phosphide synthesis. Keywords Nanoarchitectures · Porous materials · Phosphates · Hybrid porous materials

Nomenclature NPs MOFs LMPs

Nanoparticles Metal–organic frameworks Layered metal phosphonates

A. K. Worku (B) · D. W. Ayele Bahir Dar Energy Center, Bahir Dar Institute of Technology, Bahir Dar University, P.O. BOX 26, Bahir Dar, Ethiopia e-mail: [email protected] D. W. Ayele Department of Chemistry, College of Science, Bahir Dar University, P.O. Box 79, Bahir Dar, Ethiopia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. K. Gupta (ed.), Metal Phosphates and Phosphonates, Engineering Materials, https://doi.org/10.1007/978-3-031-27062-8_22

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TMPs LIBs MR MSNs HER OER ORR MSNs ATMP

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Transition Metal Phosphates Li-ion batteries Membrane reactor Mesoporous silica nanoparticles Hydrogen evolution reaction Oxygen evolution reaction Oxygen reduction reaction Mesoporous silica nanoparticles Nitrilotrimethylene triphosphonic acid

1 Introduction Phosphonate-based and metal phosphate porous nanoparticles (NPs) have enormous potential for use in biological, environmental, and energy research [1]. Due to the high affinity among metal cations and phosphonate/phosphate-based ligands, a wide variety of porous NPs can be produced. Contrary to metal phosphates, which are fully inorganic materials formed by the binding of metal sites with phosphoric acid, metal phosphonates are organic–inorganic hybrid porosity NPs that use organophosphonate ligands as phosphorus sources [2]. Hybrid materials are useful for different applications because they allow for the modification of the matrix and interface characteristics of materials. The specialized qualities and control with regard to the physical properties, particular chemical reactivity, structure, and interactions are created by a special blend of inorganic and organic traits [3]. The resultant materials, where phosphonate-based ligands are employed to create the crystalline porous nanoarchitectures, are organic–inorganic hybrids that are very similar to MOFs [4]. In contrast, the source of mesoporosity in these porous nanomaterials may be the interparticle void space or supramolecular assembly of cationic/anionic or nonionic surfactants. These syntheses commonly employed single molecule templates and extended metal–ligand coordination, which resulted in gaps on the micropore length scale [5]. On the other hand, the macroscopic phase separations that took place during the crystallization process were primarily responsible for the creation of macropores in these materials. Due to this, hybrid porous materials like MOFs and PMOs have been actively and widely created. Porous metal phosphonates are a fascinating, more specialized, and less well-researched family of materials [6]. They are produced through the interaction of metals and organophosphonic units. For applications requiring stable performance under occasionally difficult conditions and specific surface interactions, such as the sorption of metal ions, heterogeneous catalysis, thin-film-based devices, chromatography, or membrane technology, these hybrid porous metal phosphonate materials are particularly intriguing [7]. In this chapter, we have highlighted the development of metal phosphonates over the last few years, from layered metal phosphonates through MOFs based on phosphonates to templated mesoporous metal phosphonates. According to their long-range order

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and/or pore form, all of these substances, as seen in Fig. 1, fall under the category of coordination polymeric networks and can be further divided into LMPs, supramolecular metal phosphonates, and phosphonate MOFs [8]. Since the interaction between the metal precursors and phosphonic moieties in solution during preparation produces structures with a diversity of coordination modes, this categorization is therefore exclusively based on the final structure/texture of the porous coordination networks [9]. We would want to underline that this is not a comprehensive examination of each of these types of materials, but rather a comparison and contrast of some of the alternatives for metal, metal valency, and phosphonic linkers, as well as some of the intriguing topologies of porous coordination networks. For a more thorough study on the synthesis and/or applications for each family of metal phosphonates, we direct readers to some of the intriguing reviews [10]. This review is divided into two sections where we first present a summary of the synthetic procedures used to produce porous metal phosphonates and then give an overview of the possible advantages of porous metal phosphonates in certain important fields of application.

Fig. 1 Based on structural and morphological characteristics, a classification scheme for porous metal phosphonates. Adapted with permission [11]. Copyright The Authors, some rights reserved; exclusive licensee [MDPI]. Distributed under a Creative Commons Attribution License 4.0 (CC BY)

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2 Synthesis Methods of TMPs TMPs can be produced as 3D porous structures, 2D nanosheets/nanoplates, or 1D nanowires/nanotubes. The superior charge transport, chemical stability, and mechanical flexibility of transition metal phosphate 1D nanostructures make them intriguing for electrochemical applications. Due to their huge surface area, higher conductivity, and mechanical flexibility, ultrathin 2D structures are also appealing. A 3D structure of materials can expose more active sites than 1D or 2D materials and significantly speed up the diffusion of gases and electrolytes [12]. This section discusses the numerous synthesis methods for transition metal phosphates (Fig. 2).

Fig. 2 Key synthetic pathways for creating porous NPs based on phosphate and phosphonates. Organic molecules are represented by blue spheres, oxygen atoms by red spheres, metal ions by gray spheres, and phosphorus atoms by green spheres. Adapted with permission [13]. Copyright (2019), American Chemical Society

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2.1 Co-precipitation Method Another straightforward approach for obtaining transition metal phosphates in ambient conditions is precipitation. It has been stated that FeCo phosphate can be produced via deionized water as the solvent, (Fe(NO3 )3 ·9H2 O Co(NO3 )2 ) and Na2 HPO4 as the metal, and phosphate precursors, respectively. In alkaline conditions, the resultant FeCo phosphate nanoparticles had better electrocatalytic activity toward OER. During the precipitation process, selecting the appropriate solvent and precipitation agent is essential. By employing different phosphate precursors, such as Na3 PO4 , Na2 HPO4 , and NaH2 PO4 , one can alter the morphologies and architectures of cobalt phosphate. Co(CH3 COO)2 with Na2 HPO4 and NaH2 PO4 produces cobalt phosphates that have a 3D architecture and flower-like morphologies; in contrast, the sample made with Na3 PO4 is made up of a lot of erratic and agglomeration particles. Additionally, the pH levels of the reaction process are crucial in regulating the morphology and structure of the end products [11].

2.2 Structure-Directing Method Both the soft and hard templating methods based on the sol–gel route and using soft templates as structure-directing agents can be used to create porous materials. As is well knowledge, porous materials including metal oxides, carbons, and carbides may typically be synthesized using SiO2 as a hard template. The traditional method of synthesis for transition metal phosphates relies on a precipitation reaction between phosphate and a soluble metal salt. The precipitation of metal phosphates is a very quick process, though, and it can be completed before the entry of metal and phosphorous sources into the pores of the template, or if it occurs inside the pores too quickly, can plug the pores. As a result, the hard-templating method cannot achieve the precipitation required to create porous transition metal phosphates [14].

2.3 Hydrated Metal Phosphate-Derived Method Nanowires of Ni2 P2 O7 can be produced via calcination and chemical precipitation exhausting hydrated metal phosphates as a precursor, such as NiNH4 PO4 ·H2 O. Mesoporous crystal structures could be created by liberating water in the metal phosphate hydrates and dehydrating the crystal. Magnesium phosphate with a constant pore structure and substantial surface area was also created using the thermaldecomposition method. It used MgNH4 PO4 ·6H2 O as a precursor. Through varying the preparation conditions, such as concentration and pH, it is possible to change the shape and structure of the precursor and the finished product [15].

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2.4 Organophosphonate-Derived Method Despite being widely utilized to create mesostructured materials, the structuredirecting technique frequently necessitates risky and complicated synthesis procedures. Therefore, it is vital to investigate some unique, inexpensive, simple, and cost-effective ways to produce porous metal phosphates without the use of surfactants or templates. A hybrid substance called metal organophosphonate is made up of metal cations linked to organophosphonate ligands. The uniform distribution of organic groups makes it easier to adjust their stability, physicochemical characteristics, and density. ATMP, ethylenediamine tetramethylene phosphonic acid, 1hydroxyethylidene-1, 1-diphosphonic acid, and Phytic-acid, are only a few examples of the many different types of organophosphonates. The creation of graphitic carbons in-situ, which are then coated on the metal phosphate particles, is made possible by the simple calcination of metal phosphonates to produce metal phosphates. The catalyst may have improved conductivity and stability thanks to the covering of graphitic carbon that is created. In particular, the N-containing phosphonate can be converted into a metal phosphate layer coated in N-incorporated carbon, which could provide more active sites for electrocatalytic reactions [16].

2.5 Other Methods Additionally, some innovative methods for making porous transition metal phosphates have been devised. For instance, substituting the ligand with phosphoric ions can yield transition metal phosphate. By replacing benzene-1,3,5-tricarboxylic acid ligands with phosphoric ions, hollow porous nickel phosphate (Nix Py Oz ), produced from Ni-MOFs, was created [17]. Except the evident volume expansion and looser surface brought on by the substitution effect, the Nix Py Oz retains the exterior morphology from the Ni-MOF. Additionally, one frequent technique for creating transition metal phosphate coatings is electrodeposition. Aqueous solutions of metal salts can be electrolyzed with phosphate to produce transition metal phosphate in situ as films on conductive substrates. The electrodeposition method is simple since it uses common water sources and operates in comfortable settings. Furthermore, research into new tactics has not stopped. Transition metal phosphates can be created as 1D, 2D, and 3D materials, as was already mentioned. Because of their large specific surface area, low-dimensional transition metal phosphates such as 1D nanotubes/nanowires and 2D nanosheets/nanoplates have received a lot of attention in the field of electrocatalysis. Due to dimensional confinement, these lowdimensional metal phosphates can expose more surface-facet atoms, creating more active sites to increase catalytic activity. Through careful management of the essential synthesis parameters, many forms of metal phosphates can be produced. It has been observed that the concentration of surfactant is essential to the structure-directing method to influence the synthesis of mesoporous iron phosphate nanotubes. When the

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concentration of sodium dodecyl sulfate is fixed in the transition zones between the hexagonal and lamellar mesophases, it is possible to produce the 2D flake-like mesoporous iron phosphate. Mesoporous iron phosphate nanotubes are created through subsequent solvothermal processes. The choice of organophosphonate and calcination temperature is crucial for the organophosphonate-derived strategy. For instance, regulated calcination of the 2D cobalt phosphonate has allowed for the production of cobalt phosphate in a variety of morphologies. During calcination at 600 °C, porous structures resembling 2D sheets can emerge. Additionally, altering the electrostatic interactions in the development process between the associated intermediates and alkali metal ions can lead to the controlled synthesis of nanomaterials. The strength of the electrostatic interaction between different alkali metal ions, such as Li+ , Na+ , K+ , and Cs+ , and early-stage produced intermediates can be adjusted to produce nickel phosphate single-walled nanotubes with tunable diameters and lengths. By utilizing particular sacrificial templates, transition metal phosphate nanotubes can also self-assemble into 2D sheet-like structures. It has been extensively investigated how a material’s porous structure affects the catalytic activity. A high surface area microporous structure favors good exposure to active areas. Nevertheless, because of the limited reactant accessibility, it frequently results in inferior mass transfer. The macroporous structure, on the other hand, can provide efficient mass transport, but the low surface area restricts the exposure of active sites, which is bad for catalytic performance. The benefits of both microporous and macroporous structures are carried over into mesoporous structures. Up until recently, it has been very difficult to obtain porous transition metal phosphate with predictable structure and shape. On the one hand, the intrinsic growth habit is related to the high hydrolysis and condensation reactivity of phosphate and soluble metal salts, making the synthesis of ordered mesostructured metal phosphates with regulated morphologies and dimensions challenging. This calls for further development of the mesoporous metal phosphate construction strategy, which has a specified porous structure and certain morphologies [18]. Additionally, Fig. 3 illustrates the principle of a brand-new technique for recovering phosphorus from aqueous solutions.

3 Applications Porous hybrid metal phosphonates have a lot of potential in a variety of applications. However, we will focus on those important application areas in this part that, in our opinion, profit the most from the synergistic interaction between the metal units and the organophosphonate functionality. The materials that have high specific surface areas brought on by a porous network will be the focus of our attention since they offer the potential for an adjustable interface between the substrate and the interactive/reactive species. Applications such as electrochemical energy storage, separation/extraction processes, drug delivery catalysis, Fuel Cells electrode materials, and proton conduction require materials that may be customized and have

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Fig. 3 Detailed diagram of the proposed technique for recovering phosphorus from aqueous solutions as a concentrate and a dry residue from the concentrate. Adapted with permission [19]. Copyright The Authors, some rights reserved; exclusive licensee [Nature]. Distributed under a Creative Commons Attribution License 4.0 (CC BY)

surface-specific capabilities (Fig. 4). We concentrate on these fields since these are the target domains where porous metal phosphonates would make a smart choice.

3.1 Energy Storage For a sustainable economy and rapid social growth, it is increasingly important to create superior energy-storage systems, and energy-storage technologies place a lot of emphasis on highly efficient electrode materials. In several energy-storage reactions, porous metal phosphonate hybrids have been employed as highly promising materials (e.g., fuel cells, supercapacitors, and LIBs) [20]. The following exceptional characteristics are found in porous metal phosphonate electrodes: The well-structured nanopores facilitate the deintercalation/intercalation of electrolyte ions; the open and interconnected channels within the skeleton provide an unhindered pathway for ion diffusion, leading to the fast reaction kinetics; and the predefined molecular redox functional groups can be flexibly integrated into pore walls or on the surface to

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Fig. 4 Phosphate- and phosphonate-based porous nanoarchitectures possible applications. Adapted with permission [13]. Copyright (2019), American Chemical Society

control the redox potential; Materials with the strong skeletal structure have good battery cycling characteristics [21].

3.2 Separation and Extraction Zirconium phosphonate-based hybrid material was used to separate three types of metals: f-block metals (Th 4+ and Ce3+ ), heavy metals (Cd2+ , Hg2+ , and Pb2+ ), and transition metals (Zn2+ , Cu2+ , Co2+ and Ni2+ ). The phosphonic ligands 1,4bis(phosphomethyl)piperazine (BPMP) and 1-phosphomethylproline were used in zirconium hybrid phosphonate frameworks to produce a pH-sensitive porous structure. Using this framework, nucleic acids were supplied and adsorbed without any extra alterations or post-treatment. The Langmuir method of adsorption was used to adsorb the DNA from salmon sperm onto the framework. Among the several hybrids created using this method, the ZrBF-2 hybrid had the highest equilibrium monolayer capacity, measuring 238.6 mg/g. The porous structure of the hybrid material was found to contain uncondensed P-OH groups and pyridinic N-atoms, which was assumed to be the reason for the improved absorption. This hybrid framework absorption is comparable to the magnetic mesoporous silica adsorbents’ uptake of 375 and 110.7 mg/g salmon sperm DNA. The adsorption method was based on pseudosecond-order kinetics. The framework could also perform adsorption–desorption cycles in response to pH changes because it was pH responsive. The structure

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revealed a cationic surface that could electrostatically adsorb negatively charged DNA molecules under acidic and neutral pH conditions. The nucleic acid was then released by increasing pH, changing surface charge, and inducing desorption. In a different application, immobilized metal affinity chromatography was employed to selectively enrich phosphopeptides using zirconium phosphonate frameworks. These frameworks were made by combining phenyl phosphonic acid and zirconium(IV) with an SDS template [22].

3.3 Fuel Cells Electrode Materials To build an effective fuel cell that uses H2 (fuel) and O2 gases to generate electrical energy, it is very desirable to design a porous proton exchange membrane material with strong proton conductivity [20, 23]. Phosphate and phosphonate-based porous nanoarchitectures are especially important in this situation because the labile protons of the free phosphate groups can move between various acid sites through their pore channels [24, 25]. By using varying amounts of diethylphosphatoethyltriethoxysilane and tetraethoxysilane in a hydrothermal co-condensation with the aid of CTAB, Jin et al. have created phosphonic acid-functionalized mesoporous silica.

3.4 Catalysis The solid-acid catalyst used in the production of methyl-2,3-o-isopropylidene-dribofuranoside was zirconium phosphonates. Along with NKC-9, commercial acidic resin, and concentrated hydrogen chloride, the hybrid substance was contrasted. Another instance was the evaluation of porous zirconium phosphonates as a solidacid catalyst for the hydrolysis of ethyl acetate in aqueous media as well as the esterification of acetic acid with cyclohexanol. In another instance, hybrid iron (III) phosphonate was used as a solid-acid catalyst for transesterification reactions. It was produced via the reaction of iron (III) chloride with BTP precursors. Additionally, both the metal center and the organic group have crucial functional roles to perform in catalysis. The hybrid zirconium phosphonate was used in the reaction, which was carried out at 70 °C and resulted in 28% after 2 h before leveling off at 35.6% after 3 h. In another instance, transesterification reactions were aided by the employment of hybrid iron (III) phosphonate as a solid-acid catalyst. It was created by combining BTP precursors and iron (III) chloride [26]. The electrophilicity of the carbonyl carbon in the ester reactant was believed to be the main catalyst for the reaction. Pelectron cloud-rich molecules might not be able to enter the porous channels because of the hybrid materials’ negatively charged free phosphonic acids and their structural design. To oxidize cyclohexanone without the use of peroxides as an oxidant, porous tin phosphonate, synthesized from methylphosphonic acid, tin(IV) chloride and hexadecenoic sodium, was used. Additionally, under solvent-free conditions,

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1,4-dihydropyridines were made from 1,3-dicarbonyls and ammonium acetate using hybrid tin(IV) phosphonates made from tin (IV) chloride and BTP [27].

3.5 Biomaterials Due to the enormous potential of these materials in the growing bio-applications such as drug delivery and bioceramics, the fabrication of innovative porous materials with well-designed network topologies is one of the most difficult directions in numerous study domains. Extraordinarily, porous metal phosphonate hybrid materials stand out from other porous materials due to their outstanding capacity to have well-defined porosity valuable biocompatibility, and predetermined functionality. They have a great deal of potential to become a new class of biomaterials. Applications of porous metal phosphonates in biomaterials include biofuels, drug delivery, and bioceramics [28].

3.5.1

Drug Delivery

The main advantages of employing porous polymers with phosphonate functionalization as drug delivery systems also include their low cytotoxicity and biodegradability. Kokol et. al., developed cellulose nanocrystals with (bis) phosphonate, which comprises alendronate and 3-aminopropyl phosphoric acid, to produce covalent Schiff-base organic NPs. To successfully deliver this therapeutic chemical for bone theranostics, these materials’ phosphonate moieties can be attached to fluorescent molecules in addition to being particularly useful for treating osteoporosis. On the other hand, a very efficient drug delivery technique can be achieved by functionalizing the phosphonated pillar arteries over MSNs in supramolecular nanovalves. The phosphonate groups in the MSNs enable the controlled release of drugs for tumor photothermal-chemotherapy by ion partnering with quaternary ammonium ions in the nanostructure [29]. The guest molecules frequently choke the mesopores of these MSNs, preventing the stimuli-responsive, on-demand intercellular dispersion of medications. As established by Rim et al., calcium phosphate (hydroxyapatite), a benign biomaterial, may function extremely well as a pH-responsive pore blocker for the controlled release of drug molecules when coated at the pore surface of MSNs. The drug molecules that were captured and kept in the mesopores might be released at mildly acidic pH conditions in lysosomes and endosomes, two intracellular compartments [30].

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3.6 Proton Conduction A framework consisting of zinc precursor and BTP as the phosphonate ligand was used to test proton conduction (referred to as PCMOF-3). The aquo ligands on zinc were assumed to be the main source of protons to be given. The proton conductivity of the framework was 4.5 × 10–8 S/cm and 1 × 10–5 S/cm, respectively, at 44% relative humidity (RH) and 25 °C. The activation energy (Ea) for the proton transfer was calculated using the bulk conductivity of the PCMOF-3 framework and was found to be 0.17 eV. In contrast, it has been noted that 1 mol/L HCl and Nafion both have activation energies of 0.11 eV and 0.22 eV, respectively. A framework consisting of zinc precursor and BTP as the phosphonate ligand was used to test proton conduction (referred to as PCMOF-3). The aquo ligands on zinc were assumed to be the main source of protons to be given. The proton conductivity of the framework was 4.5 × 10–8 S/cm and 1 × 10–5 S/cm, respectively, at 44% relative humidity (RH) and 25 °C. The activation energy (Ea) for the proton transfer was calculated using the bulk conductivity of the PCMOF-3 framework and was found to be 0.17 eV. In contrast, it has been noted that 1 mol/L HCl and Nafion both have activation energies of 0.11 eV and 0.22 eV, respectively [31].

3.7 Membrane Materials MR engineering has demonstrated considerable potential benefits for a variety of applications, including ion exchange and gas separation. MR engineering combines the conversion and separation effect within a single unit. Porous metal phosphonates are the perfect materials for MR because of the numerous kinds and amazing coordination chemistry of phosphonate ligands, which have given them tunable functions [32]. The particular characteristics are as follows: To meet various separation requirements and objectives, metal phosphonate materials’ porosity (periodicity, shape, pore size) well-designed functional groups inside the phosphonate framework, improved selectivity, and robust hybrid skeleton can all be taken into consideration. Metal phosphonate materials with variable permeability and high specific surface area show promise for low-cost membrane engineering. These benefits have led to the widespread usage of porous metal phosphonate hybrids as MR materials for gas separation and ion exchange [33].

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4 Summary and Future Outlook Comparatively speaking to layered hybrid materials, organic–inorganic hybrid silicas, and carboxylate MOFs, porous metal phosphonates are still a relatively young field of study. We concentrated on the various types of porous metal phosphonates coordination networks in our initial investigation. We observed the evolution of porous metal phosphonates into microporous phosphonate MOFs and porous hybrid inorganic–organic systems based on multilayer metal phosphonate systems (with or without supramolecular templating). Particularly when aiming for mesoporous metal phosphonates, kinetic control continues to be a major obstacle in the development of intriguing porous topologies. One of the issues that need to be overcome is the rapid precipitation caused by the heterocondensation/phase separation between metal precursors and phosphonic moieties. When building porous structures without the use of toxic HF, there are still difficulties in controlling the interactions, especially when group IV metals are taken into account. It is still completely unknown how various kinds of (chelated) metal (IV) precursors interact with phosphonate moieties. It is thus possible to imagine that the difficulty in developing new coordination networks for metal phosphonates lies in controlling the interaction between the metal and phosphonic precursors’ reactivity, to prevent rapid precipitation and achieve the controlled formation of a hybrid porous network. This issue is made even more difficult when porosity must be produced by the use of SDAs, particularly in the case of ionic templates because of their interaction with the metal ion or the phosphonic anion, which affects reactivity and kinetics. Therefore, in contrast to porous metal oxide systems, the role of templating in these metal phosphonate systems is frequently more complex and difficult. The interaction of ionic and/or polymeric SDAs with the phosphonic moieties during the creation of porous metal phosphonate networks has received little attention up to this point. The areas in which porous metal phosphonates are used are extremely diverse. However, we focused on the leading candidates where porous metal phosphonates are the best option. This is frequently the case in situations where multifunctionality is advantageous (for instance, when structural characteristics are combined with the specific chemistry of the metal and phosphonate unit). To improve the interface for catalyzing processes, heterogeneous catalysis by hybrid metal phosphonates may profit from the synergy of combining metal centers with tunable organo-functionalities. Metal phosphonates are an excellent choice for applications including proton conduction, solid-acid catalysts, and metal sorption due to the existence of hydrophilic/acidic porous architecture. In separation/extraction applications, the porous amorphous coordination networks may be employed as solid-phase adsorbents. They make excellent solid-phase extraction candidates, particularly when it comes to removing actinides and lanthanides from highly acidic solutions, where the material’s stability in highly acidic conditions and radiolytic stability are crucial considerations (SPE). It would be advantageous to use crystalline metal phosphonate MOFs as a thin film for fuel-cell applications, sensors, or other purposes where ordered channels with certain functional groups

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are essential, such as in manufactured devices. As a result, porous metal phosphonates play a significant role in the organic–inorganic coordination networks, which offer material chemists considerable potential for developing novel porous structures with multifunctional properties that can be advantageous to a variety of application domains. Moreover, long-term stability, bifunctional activity, and selectivity need to be given more consideration in the creation of catalysts that are specifically aimed at a given application. In neutral or alkaline conditions, transition-metal phosphates and phosphonates have demonstrated effective catalytic performance toward OER. Long-term stability is still unsatisfactory, though. Additionally, these metal phosphates/phosphonates can be hybridized with other substances like conductive carbons, nickel foam, and transition-metal phosphide to give them the dual functionality of ORR/OER/ or HER/OER, which makes them suitable for use in water splitting and rechargeable metal-air batteries.

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