Sol-Gel Synthesis Strategies for Tailored Catalytic Materials 303120722X, 9783031207228

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SpringerBriefs in Materials Serena Esposito

Sol–Gel Synthesis Strategies for Tailored Catalytic Materials

SpringerBriefs in Materials Series Editors Sujata K. Bhatia, University of Delaware, Newark, DE, USA Alain Diebold, Schenectady, NY, USA Juejun Hu, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA Kannan M. Krishnan, University of Washington, Seattle, WA, USA Dario Narducci, Department of Materials Science, University of Milano Bicocca, Milano, Italy Suprakas Sinha Ray , Centre for Nanostructures Materials, Council for Scientific and Industrial Research, Brummeria, Pretoria, South Africa Gerhard Wilde, Altenberge, Nordrhein-Westfalen, Germany

The SpringerBriefs Series in Materials presents highly relevant, concise monographs on a wide range of topics covering fundamental advances and new applications in the field. Areas of interest include topical information on innovative, structural and functional materials and composites as well as fundamental principles, physical properties, materials theory and design. Indexed in Scopus (2022). SpringerBriefs present succinct summaries of cutting-edge research and practical applications across a wide spectrum of fields. Featuring compact volumes of 50 to 125 pages, the series covers a range of content from professional to academic. Typical topics might include • A timely report of state-of-the art analytical techniques • A bridge between new research results, as published in journal articles, and a contextual literature review • A snapshot of a hot or emerging topic • An in-depth case study or clinical example • A presentation of core concepts that students must understand in order to make independent contributions Briefs are characterized by fast, global electronic dissemination, standard publishing contracts, standardized manuscript preparation and formatting guidelines, and expedited production schedules.

Serena Esposito

Sol–Gel Synthesis Strategies for Tailored Catalytic Materials

Serena Esposito Department of Applied Science and Technology and INSTM Unit Politecnico di Torino Turin, Italy

ISSN 2192-1091 ISSN 2192-1105 (electronic) SpringerBriefs in Materials ISBN 978-3-031-20722-8 ISBN 978-3-031-20723-5 (eBook) https://doi.org/10.1007/978-3-031-20723-5 © 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

To my mum, who taught me to never give up and to always believe that my future could be whatever I dreamt it to be. To my husband, my son and my daughter, who have always supported me

Preface

Provides an overview of the sol–gel synthesis of supported metal and metal oxide catalysts from the perspective of reliable, sustainable and cost-effective processes. Introduces sol–gel chemistry from its origins, and then gradually moves on to the most current fascinating strategies for the preparation of tailor-made catalysts. Organises and develops the different subjects in such a way that goes beyond theoretical considerations and rather provides strategies for the rational design of highperformance catalysts, abandoning the time-consuming trial-and-error approach. Torino, Italy

Serena Esposito

vii

About This Book

This book provides the reader with an understanding of the basics of sol–gel chemistry as a fundamental tool for the rational design of heterogeneous catalysts based on supported metals and metal oxides. One of the main objectives is to illustrate the versatility of sol–gel preparation, highlighting its advantages over other preparation methods. The reader is accompanied through each step of the sol–gel synthesis in order to show the impact that the process parameters have on the final product. In this regard, the reader is given some hints for the manipulation of structural, textural and morphological properties within the different types of chemistry that can be considered under the heading ‘sol–gel’. The target audience of this book is researchers working on the catalytic process who can benefit from knowing how to master the sol–gel synthesis method to prepare tailored and multifunctional inorganic catalysts.

ix

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 The Multifaceted Role of Oxides in Heterogeneous Catalysis . . . . . . 1.2 Synthesis Approaches to Nanocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Impregnation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 (Co)precipitation Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Hydrothermal/Solvothermal Process . . . . . . . . . . . . . . . . . . . . . 1.3 Sol–Gel Chemistry for Oxides Preparation . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 3 5 6 6 7 9

2 “Traditional” Sol–Gel: The Chemistry of Alkoxides . . . . . . . . . . . . . . . . 2.1 Hydrolysis and Condensation Reactions: An Overview . . . . . . . . . . . . 2.2 Silicon Alkoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Acid and Base Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Transition Metal Alkoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 13 14 16 18 19

3 From (Sub)colloidal Growth to the Gel Structure . . . . . . . . . . . . . . . . . . . 3.1 Polymeric Gel Versus Colloidal Gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Water Can Be a Strategic Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

21 21 24 25 29

4 From Wet Gel to the Final Product: Draw Your Way . . . . . . . . . . . . . . . 4.1 Drying Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Thin Film Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33 33 36 40

5 Evolution of Sol–Gel Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Organic–Inorganic Hybrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Non-hydrolitic Routes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 The Pechini Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

43 43 46 48 50

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6 Synthetic Strategies for (Supported) Metal and Metal Oxide Catalysts: Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Acid Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Highly Dispersed Supported Metal Catalysts . . . . . . . . . . . . . . . . . . . . 6.3 Reverse Micelle Approach for Photocatalysts . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53 53 57 65 67

Chapter 1

Introduction

Abstract Chemistry of materials gathers a wide range of chemical routes that allow producing materials for different processes. Researchers working on chemical synthesis constantly explore new material formulations with tailored properties and functionalities to run chemical processes at relatively low environmental costs and/or at high efficiency. Many impressive advances have been made in the chemistry of materials, from each of the major material classes, with a remarkable impact on novel and emerging technologies in the chemical industry. In this scenario, the synergy between chemists and chemical engineers can boost the modernization of chemical industry although many challenges still need to be addressed. Materials such as catalysts are regarded as the key parameter of a chemical process and their efficiencies do not only depend on the nature of the active phase but also on numerous features such as porosity (size, homogeneity, interconnectivity) and morphology, which are strongly dependent to the designed synthesis strategy. Moreover, most of the materials produced at lab scale in the form of micron-sized powders need to be extruded or wash-coated to be used industrially. These processes require binders and additives, whose interaction with the active phase can modify material properties. A comprehensive characterization of the prepared materials for a greater understanding of the complex relationships between specific modifications and the surface and bulk properties cannot be ignored. For this reason, development of modern tools to evaluate the materials structure and assembly processes are of utmost importance. Besides the conventional methods (ion-exchange, impregnation, coprecipitation and hydrothermal method), advanced preparation techniques such as those relying upon the sol–gel chemistry can lead to smart materials with control of particle and/or pore size and/or shape at molecular scale. Keywords Oxides · Nanocatalysts · Impregnation, (co)precipitation · Sol–gel method

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Esposito, Sol–Gel Synthesis Strategies for Tailored Catalytic Materials, SpringerBriefs in Materials, https://doi.org/10.1007/978-3-031-20723-5_1

1

2

1 Introduction

1.1 The Multifaceted Role of Oxides in Heterogeneous Catalysis Metal oxides and mixed metal oxides are of tremendous current interest to scientists and engineers because of their potential applications in most industrial chemical processes [1]. These materials are known to possess some unique characteristics among inorganic materials such as high stability, in some cases, biocompatibility, cost-effectiveness, distinctive acid–base and redox properties. In addition, some metal oxides show exceptional ability to generate charge carriers under light irradiation resulting in improved electrical and optical characteristics. In the field of heterogeneous catalysis, they are used either as catalysts or support for active species characterized by decisive physico-chemical properties and covering the largest family of catalysts. The main catalytic domains cover oxidation (selective or total), acid and base catalyses, hydrogenation, hydrotreating, photocatalysis, depollution and biomass conversion [2, 3]. Research in the field of metal oxides is always abuzz, both in academia and in industry, with the definition of novel catalytic formulations for targeted applications. Moreover, the ever-growing need for a green chemistry approach calls for new sustainable and reliable catalysts and processes. Silica, Al2 O3 , TiO2 , ZnO, CeO2 , ZrO2 , porous and mesoporous metal oxides, multicomponent mixed oxides, polyoxometallates (POMs), the perovskites, and more recently the high entropy oxides (HEO) are some examples of heterogeneous catalysts, whether bulk or support, which play a decisive role in industrial chemistry [4–10]. The entry of nanotechnology into materials synthesis has further boosted the development of catalysts with enhanced properties with respect to the bulk counterparts. In this scenario, the bottom up synthetic strategies can help in the design of engineered materials replacing the trial and error approach. Considering structure sensitive reactions, besides the definition of the most suitable chemical composition, the research has devoted much effort in tailoring the size and shape of solid particles, favouring morphology with specific crystal facet. The intent is always to increase the number of active sites and to control the nature of the exposed sites since the conversion of the reactants into the product occurs on sets of surface atoms. A “creative disturbance” is a fascinating way to improve the activity of a metal oxide surface. This concept refers to the possibility of improving the catalytic activity of an oxide by replacing a small fraction of the cations of a "host oxide" with a different cation. An alternative way comes through the deposition of small MeOx clusters on the surface of another oxide (or any other insulator) [10]. Viewed from the perspectives of achieving designed properties and functionalities, the sol–gel method can be considered a powerful synthesis route possessing many advantages with respect to the conventional synthesis methods, such as the preparation of multicomponent materials with a one-pot process, the low temperature process, and high homogeneity. For this reason, by sol–gel synthesis a large

1.2 Synthesis Approaches to Nanocatalysts

3

Fig. 1.1 Nano-engineered oxides and application by sol–gel approach

number of sophisticated materials have been prepared whose properties are a function of the synthesis condition, Fig. 1.1.

1.2 Synthesis Approaches to Nanocatalysts The progressive increase of the global population, the depletion of natural resources and the need to find alternative energy sources to fossil fuels related to the rising concerns over the emission of CO2 have placed catalysis scientists towards new challenges in the context of climate change, pollution and sustainable energy [11]. In this scenario, many innovative strategies and approaches have been developed for the generation of renewable fuels, use of solar and wind energy, sustainable electricity generation and long-term safe and efficient energy storage [12–14]. In the materials science, more focus is currently being placed on manufacturing advanced catalytic formulations. However, to support industrial processes in the direction of an effective ecological transition, helping to ensure the resilience of our society, the improvement of the catalyst performance is not enough. The design of reliable, sustainable and costeffective synthesis protocols is also highly demanded. A proper and systematic understanding of how effectively catalysts can be manipulated avoiding sophisticated synthesis route and harsh conditions can boost much more progress to scale up and meet practical production. Nowadays, researchers focus most of the effort toward the synthesis of nano-scale catalysts with targeted properties for specific applications. Nonetheless, the effective utilization of nanocatalysts still involves many challenges, but the growing potential of nanotechnology sees more and more goals being achieved with the support of increasingly powerful materials characterization techniques [15].

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1 Introduction

Nanotechnology is concerned with the synthesis, characterization and application of materials and devices, changing their size and shape at the nanoscale. The concept of nanotechnology was first introduced by Nobel prize laureate Richard Feynman in 1954, whose famous statement is “There’s a Plenty of Room at the Bottom”. Since then, nanotechnology has quickly captured the attention of researchers who immediately realized the potential offered by this emerging technology. In fact, this field is constantly evolving to prepare materials with enhanced properties compared to their bulk counterpart. While nanotechnology is a powerful tool for governing and manipulating matter at the atomic level, nanoscience provides knowledge about the arrangement of atoms, molecules, clusters at the nanoscale [16]. The building blocks of nanotechnology are the “nanomaterials” whose properties depend on their size and shape. The definition of nanomaterial is still controversial but we can refer to nanomaterials when at least one of the dimensions is in the range 1–100 nm [17]. Different morphologies can be obtained depending on the number of dimensions on the nanoscale: nanoparticles, nanorods, nanosheets. Although nanomaterials are penetrating almost every field of science and technology, people are unknowingly using them in their daily lives. Compared to their large-scale counterpart, nanomaterials show distinct and sometimes unexpected properties mainly due to increased relative surface area and quantum effects. These factors can change or enhance properties such as the chemical reactivity, mechanical strength, thermal and electrical conductivity, antimicrobial activity, optical absorption. Since catalytic chemical reactions take place at the surface, the nano-structuring of catalysts ensures superior reactivity than bulk material. As a particle decreases in size, it exposes a greater proportion of atoms on the surface than on the inside. Therefore, nanoparticles provide more active sites and favourable surface features than larger particles [18] The synthesis of nanomaterials can be viewed from two perspectives: on the one hand, the top-down approach in which typically mechanical methods (cutting, etching, grinding or ball milling techniques), electron beam lithography, atomic force manipulation, gas-phase condensation, aerosol spray, are used to cut and shape bulk materials. They are inherently simple with the possibility of large-scale production and no need for chemical purification. In addition to the limited versatility, the downsides are the imperfection of surface structure and the wide distribution of particle size and shape. Alternative to the top-down method is the bottom-up approach, in which the nanomaterial is build up through assembling atoms, molecules and clusters. This approach has many merits that make it attractive: fewer defects, a more homogenous chemical composition and better ordering. Furthermore, size, shape and surface properties can be engineered by selecting the building block and manipulating the process parameters. Nano-architectures based on metal or metal oxide particles are the most widely used formulations in industrial processes. The common criteria for a highperformance catalyst are narrow size distribution and high dispersion on a selected support. In recent decades, the literature can count a proliferation of scientific works on the preparation of catalysts at nano-scale using a variety of synthesis

1.2 Synthesis Approaches to Nanocatalysts

5

methods. However, coprecipitation and impregnation are still the most used methods and probably the simplest approaches for the synthesis of different types of nanocatalysts. They often prove ineffective since they possess poor reproducibility and a scarce control of the particle size and shape, dispersion, crystallinity, and magnetic properties.

1.2.1 Impregnation Method The impregnation method, traditionally used for the synthesis of supported catalysts for a variety of heterogeneous reactions, involves loading a porous substrate with a metal precursor solution. The porous substrate can be prepared in a previous step if specific textural properties are desired, or it could be a commercial product. The amount of precursor solution in relation to the pore volume of the substrate reveals whether it is wet impregnation (WI) or incipient wet impregnation (IWI). In both methods, the metal precursors are typically inorganic metal salts such as chlorides, nitrates and acetates. Although the type of metal precursor used in these syntheses is often the result of laboratory availability, the correct choice could have a crucial effect on the final dispersion as well as the drying and calcination temperatures [19]. The impregnation method implies the suspension of the catalytic support in an excess amount of a diluted aqueous solution of the metallic precursor. The most commonly used solvent for inorganic salts is water because of the high solubility of many precursors, whereas organic solvents are mainly used for organometallic precursors [20]. Uptake of the liquid into the pores of the support mainly involves diffusion phenomena. After an appropriate contact time, the mixture is filtered to recover the powder and leaving the liquid phase containing any precursor not retained by the support. This results in the need to recycle excess liquid to minimise precursor waste. Impregnation is called incipient, or dry impregnation (DI), when the quantity of precursor solution is just enough to fill the pore volume of the substrate. In this case, the solution is drawn into the pores by capillary pressure. The lack of filtration step during DI synthesis of catalyst means that any counterions from the metal precursor salt, will be retained in the dried catalyst. When the solvent is water and the substrate an oxide, the liquid is considered wetting and will spontaneously penetrate the substrate. If the liquid is not wetting, external pressure is required to force the liquid into the pores [20]. To obtain the final catalysts with the zero-valent metal particles anchored onto the support, the impregnated and dried powder is subjected to thermal treatment in a reducing environment. The catalysts obtained by WI method usually present higher metal dispersion and small clusters, which depending on the reaction, usually improves the catalytic activity.

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1 Introduction

1.2.2 (Co)precipitation Method The simplicity of the experimental set-up, the low cost, the absence of organic solvents and the high yield are some of the features that account for the widespread use of the precipitation method. It can be used to prepare either single component catalysts or supported and mixed catalysts [21, 22]. In a typical co-precipitation reaction, hydroxides and/or insoluble carbonates are obtained by precipitation from an aqueous solution of metallic salts, usually nitrates, a chlorides or oxychlorides. The precipitation process can be induced by a change in conditions, such as temperature, pH value, solvent evaporation [23]. The filtered and washed co-precipitate is heat-treated at a specific temperature chosen according to the application. The most critical aspect concerns the degree of homogeneity of the system when precipitation involves more than one precursor salt. The weak point arises from the difficulty in managing the different precipitation rates and their pH dependence. From this point of view, the method shows low versatility. The precipitation rate is also crucial for controlling the particle size and distribution [24]. Co-precipitation is spotlighted as a simple, cost-effective and fast process for the synthesis of magnetic nanoparticles, such as pure iron oxides and ferrites in which iron oxides are chemically combined with one or more additional metals. Ferrites that can be represented by the formula MFe2 O4 consist of a cubic crystalline structure of oxygen atoms with M (II) and Fe (III) in two different crystallographic positions. Different ferrites can be obtained with high purity under oxygen-free atmosphere without requiring hazardous organic solvents, nor treatments under high pressure or temperature. Nevertheless, a few challenges still limit this method such as the control of the particle size and shape, crystallinity, and magnetic properties. Many authors have emphasised the importance of process parameters, in particular the type of alkaline agent, in controlling the chemical composition of nanoparticles and improving the magnetic response [25, 26].

1.2.3 Hydrothermal/Solvothermal Process The term hydrothermal is used when the synthesis is carried out in an aqueous solution in a sealed reactor, known as an autoclave, under autogenous pressure. The strengths of hydrothermal synthesis are the relatively mild operating conditions (reaction temperatures Si(OEt)4 > Si(OnPr)4 > Si(OiPr)4 > Si(OnBu)4 . The possible substitution of –OR groups for –R' groups (R' x Si(OR)4-x ) changes the electronic density around the silicon and brings us back to consider the impact of inductive effects on precursor reactivity. Effects that, as previously discussed, proceed in opposite directions for acidic and basic catalysis. The seemingly simple fact of being able to replace OR groups with

(a)

(b)

Fig. 2.3 Mechanism of a acid catalysed hydrolysis of silicon alkoxides and b base catalysed hydrolysis of silicon alkoxides

2.2 Silicon Alkoxides

17

(a)

(b)

Fig. 2.4 Mechanism of a Acid catalysed condensation of silicon alkoxides and b Base catalysed condensation of silicon alkoxides

nonhydrolyzable groups has several far-reaching chemical consequences in terms of catalyst engineering. In principle, it is possible to govern the textural properties and introduce specific functionalities on the surface [14, 15]. The electron density at the silicon atom decreases in the following order: alkyl group, alkoxy group, hydroxyl group and Si–O–Si group. Similar consideration can be done for the condensation reactions whose extent depend on the hydrolysis degree, Fig. 2.4. Unlike hydrolysis, where both steric and inductive factors are relevant, the condensation reaction is mainly influenced by steric effects. Although the hydrolysis and condensation reactions have been described as consecutive, they actually compete with each other during all steps of the sol–gel process. Furthermore, the rates of these reactions have an inverse dependence on pH, which is reflected in the kinetics and thus in the final gel structure [16]. Considering that the PZC of silica is around pH 4.5, condensation is the rate determining step at pH 5 which leads to the prompt consumption of the hydrolysed species. Connected to this, there is the profound and fascinating difference in the structure of the silica gel as a result of the type of catalysis adopted, Fig. 2.5. Under acidic conditions, driven by inductive effects, the reaction proceeds mainly at the extremities of the chains where unreacted –OR groups are able to stabilise the positively charged transition state to a greater extent. Conversely, the reactions proceed preferentially on the silicon atoms at the centre of the growing chain when a base is adopted. More branched (i.e. more highly condensed) networks are thus attained with the basic conditions whereas chain-like networks result under acidic conditions. The type of catalyst results to be one of the key factors to tailor the porosity and the structure-adsorption characteristics in final materials: use of a basic catalyst results in a larger amount of mesopores in the final material while an acidic catalyst shifts porosity towards microporosity [17, 18].

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2 “Traditional” Sol–Gel: The Chemistry of Alkoxides

Fig. 2.5 Silica gel structure under basic (left) and acidic (right) conditions

2.3 Transition Metal Alkoxides The greater electropositive character of transition metals compared to silicon, together with the ability to expand their coordination sphere beyond 4, makes transition metal alkoxides more prone to nucleophilic attack than silicon alkoxides. Thus, the big issue when the preparation involves the transition metal alkoxides is their very fast reaction with water that leads to precipitation instead of a homogeneous sol, with poor control on composition and homogeneity. For example, the hydrolysis rate of Ti(OR)4 is about 105 higher than the corresponding Si(OR)4 under the same conditions [19]. A further consequence of the pronounced Lewis acidity of metals in alkoxides is the inclination of metal alkoxides to associate via OR bridges or to coordinate alcohol molecules [20]. On the other side, silicon alkoxides are present in solution as monomeric forms. It is clear that the reactivity of transition metal alkoxides, besides the inductive and steric size factors already discussed for silicon precursors, can also be traced back to the possible degree of oligomerisation. Ti(OEt)4 , for example, is more reactive than Ti(OiPr)4 because the former in alcohol is present as a trimeric structure while the latter as a monomer in isopropanolic solution. Metal size can also increase the tendency to form complex structures. If the goal of the research is to take a step forward in the rational design of catalysts based on transition metal oxides, an understanding of the unexpected chemistry of such precursors is required. Just a short while ago, in March 2022, Ulrich Schubert reported interesting findings from his own research focused on the in-depth inspection of clusters formation in “sol” prepared starting from transition metal alkoxides. The study sheds light on the oligomeric structures, [M(OR)m ]n , present in the first step of gel formation [21]. Another important aspect that should not be surprising considering the different chemistry of the two families of alkoxides considered so far is the final gel structure. Silicon is in fact tetrahedrally coordinated to oxygen and the interconnected Si tetrahedra only share corners rather than edges or faces while metals have usually an octahedral coordination and less degree of freedom of the polyhedra to reach the final stoichiometry MO2 [21]. Semi or microcrystalline oxides are obtained by sol– gel processing of transition metal precursors whereas silica gel retains its amorphous nature.

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The reactivity of the transition metal alkoxides is to some extent moderated with the use of complexing ligands which are commonly—diketones, polyhydroxy acids, or carboxylic acids. The chelating action, by blocking an additional coordination site, increases the metal coordination number and decreasing the number of easily hydrolysable groups. The bi- or (tri-)dentate ligands can also bridge two metal atoms in dimeric structures [20]. These modified precursors have consequence on reaction pathway and thus on the microstructure and texture of the obtained gels. In recent times, the study of the formation of metal oxo clusters has gained even more prominence not only in the specific field of sol–gel but also because they are considered to be the core of MOFs, heterogeneous catalysts with extremely high potential [22]. From the above, it is evident that the gel obtained is the result of a delicate balance among the many factors involved since many different pathways from the molecular precursor to the final gel are possible. This should by no means be underestimated if rational catalyst design is to be done. Other parameters that influence the rate of the hydrolysis and condensation reaction include pH, temperature, and reaction time, the concentration of reagents, the type and concentration of surfactant or structure directing agents and H2 O/M(OR)4 molar ratio. A detailed understanding of the parameters influencing the reaction rates and thus the structure evolution is necessary in order to tailor the texture and properties of sol–gel materials.

References 1. J.J. Ebelmen, Untersuchungen über die verbindung der barsaure und kieselsaure mit aether. Ann. Chim. Phys. Ser. 57, 319–355 (1846) 2. T. Graham, On the properties of silicic acid and other analogous substances. J. Chem. Soc. 17, 318–327 (1864) 3. C.B. Hurd, Theories for the mechanism of the setting of silicic acid gels. Chem. Rev. 22, 403–422 (1938) 4. S. Sakka, History of the sol-gel chemistry and technology, in Handbook of Sol-Gel Science and Technology Processing, Characterization and Applications, ed. by L. Klein, M. Aparicio, A. Jitianu. https://doi.org/10.1007/978-3-319-32101-1 5. L.L. Hench, J.K. West, The sol-gel process. Chem. Rev. 90, 33–72 (1990) 6. R.J. Hook, A 29 Si NMR study of the sol-gel polymerisation rates of substituted ethoxysilanes. J. Non-Cryst. Solids 195, 1–15 (1996) 7. R.J.P. Corriu, D. Leclercq, Recent developments of molecular chemistry for sol-gel processes. Angew. Chem. Int. Ed. Engl. 35, 1420–1436 (1996). https://doi.org/10.1002/anie.199614201 8. J. Livage, C. Sanchez, Sol-gel chemistry. J. Non-Cryst. Solids 145, 11–19 (1992) 9. C.J. Brinker, G. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, 1st edn. (Academic Press Inc., New York, NY, USA, 1990) 10. J. Livage, Sol–Gel synthesis of inorganic materials, in Encyclopedia of Materials: Science and Technology, 2nd edn., pp. 4105–4107 (2001) 11. E.J.A. Pope, J.D. Mackenzie, Sol-gel processing of silica: II. The role of the catalyst. J. Non Cryst. Solids 87, 185–198 (1986). https://doi.org/10.1016/S0022-3093(86)80078-3

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12. A.E. Danks, S.R. Hall, Z. Schnepp, The evolution of ‘sol–gel’ chemistry as a technique for materials synthesis. Mater. Horiz. 3, 91 (2016) 13. U. Schubert, New materials by sol–gel processing: design at the molecular level. J. Chem. Soc., Dalton Trans. 3343–3348 (1996) 14. R. Ciriminna, A. Fidalgo, V. Pandarus, F. Béland, L.M. Ilharco, M. Pagliaro, The sol−gel route to advanced silica-based materials and recent applications. Chem. Rev. 113, 6592−6620 (2013). https://doi.org/10.1021/cr300399c 15. M. Pagliaro, Sol–gel catalysts for synthetic organic chemistry: milestones in 30 years of successful innovation. J. Sol-Gel Sci. Technol. 95, 551–561 (2020). https://doi.org/10.1007/ s10971-020-05266-3 16. H. Li, X. Chen, D. Shen, F. Wu, R. Pleixats, J. Pan, Functionalized silica nanoparticles: classification, synthetic approaches and recent advances in adsorption applications. Nanoscale 13, 15998–16016 (2021) 17. S. Esposito, “Traditional” sol-gel chemistry as a powerful tool for the preparation of supported metal and metal oxide catalysts. Materials 12, 668 (2019). https://doi.org/10.3390/ma12040668 18. A.F. Sierra-Salazar, A. Ayral, T. Chave, V. Hulea, S.I. Nikitenko, S. Abate, S. Perathoner, P. Lacroix-Desmazes, Chapter 18—Unconventional pathways for designing silica-supported Pt and Pd catalysts with hierarchical porosity, in Studies in Surface Science and Catalysis, vol. 178, pp. 377–397 (2019). https://doi.org/10.1016/B978-0-444-64127-4.00018-5 19. X. Lu, K. Kanamori, K. Nakanishi, Hierarchically porous monoliths based on low-valence transition metal (Cu Co, Mn) oxides: gelation and phase separation. Natl. Sci. Rev. 7, 1656–1666 (2020). https://doi.org/10.1093/nsr/nwaa103 20. U. Schubert, Organically modified transition metal alkoxides: chemical problems and structural issues on the way to materials syntheses. Acc. Chem. Res. 40, 730–737 (2007) 21. U. Schubert, En route from metal alkoxides to metal oxides: metal oxo/alkoxo clusters. J. Sol-Gel Sci. Technol. (2022). https://doi.org/10.1007/s10971-022-05774-4 22. V. Pascanu, G.G. Miera, A.K. Inge, B. Martín-Matute, Metal-organic frameworks as catalysts for organic synthesis: a critical perspective. J. Am. Chem. Soc. 141, 7223–7234 (2019). https:// doi.org/10.1021/jacs.9b00733

Chapter 3

From (Sub)colloidal Growth to the Gel Structure

Abstract An important consideration in the study of the sol–gel method is that there is no universal recipe for the preparation of a selected catalytic formulation. This means that it is not possible to provide a scheme for the preparation of a catalyst using the sol–gel process, as the synthesis route depends on the type of the selected approach. Furthermore, since its discovery, the sol–gel method has undergone continuous development and new synthesis routes have been designed. Generally speaking, we can consider the following as starting materials: alkoxides and organic molecules (chelating agents, surfactants, esters, organic acids), inorganic salts (nitrates, chlorides), solvent—usually alcohol (methanol, ethanol)-water. Catalysts when it is necessary to increase the speed of hydrolysis or simply to change the particle growth mechanism. The structure of the gel, responsible for the properties of the final material, is closely related to the hydrolysis and condensation pathway and the cluster growth mechanism. In ‘colloidal’ gels, the network consists of the agglomeration of dense colloidal particles, whereas in ‘polymeric’ gels, the particles have a polymeric substructure resulting from the aggregation of sub-colloidal chemical units. The latter are an entanglement of randomly branched polymer chains. Actually, it is not possible to predict which type of cluster will form in a specific case, due to the complexity of the system. However, if a particular synthesis protocol is followed exactly, the same cluster can be reliably synthesised in most cases, very often with high yields and in large quantities. This is one of the great advantages of this methodology. Among the factors influencing hydrolysis and condensation reactions, water is a key parameter regulating the sol-to-gel transition and gelation time. In this chapter, we will see how it also affects the textural and structural properties of the synthesised material. Keywords Polymeric gel · Colloidal gel · Gelation time · Aging · Synthesis parameters

3.1 Polymeric Gel Versus Colloidal Gel The transition from sol to gel and the pathway followed by the growing particles enclose most of the factors that define the final characteristics of the gel. For example, © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Esposito, Sol–Gel Synthesis Strategies for Tailored Catalytic Materials, SpringerBriefs in Materials, https://doi.org/10.1007/978-3-031-20723-5_3

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3 From (Sub)colloidal Growth to the Gel Structure

the textural properties in terms of surface area and average pore diameter are strongly dependent on the size and structure of the primary particles or polymers formed by condensation reactions, as well as the organisation of these structures. In order to tailor the catalyst using the sol–gel procedure, it is useful to delve into the mechanisms of particle growth revealing the key factors ruling the process. As was mentioned in the previous chapter, small linear and cyclic oligomers are the first ‘sol’ forming products and serve as the building blocks for gel formation. This results in a broad spectrum of structures at different degrees of crosslinking, as evidenced by 29 Si NMR spectroscopy. Also contributing to the formation of these particles is the possible depolymerisation with the opening of ring structures and the resulting renewed availability of monomers [1]. The fate of the primary (nano)particles depends on the reaction conditions: clusters can condense with other clusters or clusters can grow by adding monomer [2]. Considering for silicon alkoxides the use of a catalyst to increase the reaction rates, it must be borne in mind which is the rate determining step in order to work out the path followed by the clusters. At acid pH, since hydrolysis is faster than condensation, small clusters are formed which condense together as the reaction proceeds. Reactions occurring on the terminal silicon atoms are favoured under these conditions, while the formation of Si–O–Si bonds can be considered irreversible. The result is the formation of small clusters that condense together, reminiscent of a classic organic polymerisation reaction. Gels formed under these conditions are typically microporous. Unlike what happen in polymeric gels, the particles in colloidal gels interact via short-range forces with depolymerisation and re-polymerisation phenomena that allow the clusters to be reorganised towards more stable configurations. Under basic conditions, hydrolysis is the rate-determining step, and the clusters grow by adding monomer whose supply is ensured by the hydrolysis of siloxane bonds, Si–O–Si, more favoured at high pH. Highly branched clusters are so formed in this condition. At high pH, an ammonia-catalysed hydrolysis of silicon alkoxides produces a uniform distribution of particles, known as Stöber silicas. In this case, the delicate balance between hydrolysis and condensation kinetics influences the amount of non-hydrolysed groups, which can alter the external morphology and affect the homogeneity in the internal structure of silica particles [3]. The small surface area of Stöber’s silica limits its applications. The need to overcome this disadvantage has promoted intensive research to establish how to fine-tune the size, size distribution, internal structure and chemical characteristics of silica particles [3, 4]. The use of secondary alkanolamines (diethanolamine) and tertiary alkanolamines (triethanolamine) as catalysts instead of ammonia has proven to be a powerful means of altering the particle size and thus changing the textural properties [5]. In addition to pH, which has a strong impact on the mechanism of particle growth, cluster-monomer or cluster–cluster, and gel structure, there are other factors to consider, including solvent and temperature. As mentioned in the previous chapter, a solvent is generally used to mix alkoxide and water because water and alkoxide are immiscible in all proportions and avoid

3.1 Polymeric Gel Versus Colloidal Gel

23

nanoparticles precipitation. As the hydrolysis reaction proceeds, partially hydrolysed species are formed that are soluble in water. Alcohols are generally used as solvents, ethanol typically in the presence of TEOS. The choice of solvent is more critical in the case of metal alkoxides where alcohol-alkoxide exchange and the formation of oligomeric structures can affect the cluster structure [6]. Generally speaking, speed can be an insidious enemy in sol–gel preparations. Forcing the gelation could lead to a cloudy sol and a non-transparent gel. Excessive acceleration could then lead to the formation of precipitates. For these reasons, an ageing step is often added to the preparation. The reaction temperature is crucial in regulating the chemical kinetics of the different reactions involved in nanoparticle formation and the assembly of nanoparticles into a gel network. It has been said that in the case of silicon alkoxides, gelation can take weeks or months; in this case, an increase in temperature can be beneficial, but if the temperature is too high, the reactions that assemble the nanoparticles into a gel network occur so quickly that clumps form (instead) and the solid precipitates from the liquid. It is therefore of utmost importance to be able to recognise what the product of a controlled gelation should be. How can we recognise the gelation point? To answer this question, we must continue to distinguish between polymeric and colloidal routes. In the former, there is an unperceivable gradual increase in viscosity of the sol with the time. At the gelation point, the viscosity increases abruptly and a three-dimensionally continuous solid network is obtained. The gelation time (tg) is usually determined by the intercept between the two linear phases in Fig. 3.1 [7, 8]. At the gelation point, no liquid escapes by turning the container upside down, because all the residual liquid is retained in the entire volume of the gel and cannot flow out of the becker, Fig. 3.2. In the case of colloidal gels, for example those obtained by the Stöber method, there is first an increasing opalescence of the mixture in a time dependent on the concentration of the initial reagents. Finally, the solution takes on the appearance of a cloudy white suspension, indicating the formation of colloidal silica nanoparticles [9]. For stable colloidal particles, aggregation can be induced by removing the solvent. Alternatively, altering the pH, salinity or temperature can induce destabilisation of the particles. Fig. 3.1 Evolution of viscosity versus time as reported in Ref. [7]

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3 From (Sub)colloidal Growth to the Gel Structure

Fig. 3.2 Wet gel by polymeric route

Although several valid attempts to define a gel can be found in the literature, there is still the difficulty of giving a definition that can encompass the different types of structures that a gel can assume. For this reason, it may be worth relying on Nijenhuis’s more philosophical definition: ‘A gel is a gel, as long as one cannot prove that it is not a gel’ [10].

3.2 Aging The gelation point could be read as a rapid solidification, but it is actually more properly described as a freezing of a particular polymeric structure. This threedimensional structure, which occupies the entire volume previously occupied by the liquid, consists of the polymeric lattice that incorporates within its condensable particles, by-products and possibly monomers. Freezing is due to the high viscosity value, but the flexibility of the system allows reactions to continue over time. This time-consuming step is called aging and although it could be seen as a disadvantage of the sol–gel method, it is a very important phase in the preparation of the catalyst because it allows the porosity and structure of the gel to be altered. Ageing encompasses a wide range of processes, including the further formation of a crosslinked structure, associated shrinkage and hardening of the gel. Syneresis is one of the processes observed during ageing and consists of the expulsion of liquid from the pores, causing further shrinkage. Coarsening is another age-related effect that strengthens the gel by changing the size and shape of the pores. It consists of the dissolution of the mass from thermodynamically unstable regions and re-condensation into thermodynamically

3.3 Water Can Be a Strategic Parameter

25

more stable regions. Like the previous steps in the sol–gel process, it depends on temperature and pH, pressure and solvent type. Many oxides used as catalysts or supports for catalysts exist in two or more polymorphic forms. Although they are isochemical, these crystal forms have distinct crystal structures and thus different physical properties. Anatase, rutile and brookite are the three main crystalline polymorphs of titania, an oxide with a great number of important technological applications and probably the most studied photocatalyst. An extensive literature shows that titania activity is closely related to the presence of a crystalline phase or mixed crystalline phases and the degree of crystallinity. Beyond the type of sol–gel route followed to prepare titanium oxide, the aging phase can decisively affect the crystallinity of the sample [11–13].

3.3 Water Can Be a Strategic Parameter Tuning the porosity features of the substrate is an essential task in the design of functional catalysts. The effect of water on the porous structure of the xerogel has long been debated, as it depends on the structure of the particles growing out of the sol. Although the simplest and most immediate conclusion that can be reached is the formation of more hydrolysed species as the water content increases, the effect of dilution must also be considered. Early studies conducted by Brinker, considered the pioneer of sol–gel science and the synthesis of materials from soluble molecular precursors, reported that the dilution effect for water to alkoxide ratio (Rw) > 5 changes the rate of hydrolysis and condensation in the direction that leads to longer gelation times [14–16]. Back to considering the hydrolysis and condensation reactions. According to reaction 1 of Fig. 3.3, the stoichiometric ratio of water to alkoxide (Rw) is 4, which implies that the hydrolysis of a tetravalent alkoxide M(OR)4 , such as the tetraethylorthosilicate (TEOS), Si(OC2 H5 )4 , requires four water molecules to be completed. However, if one considers the condensation reaction that occurs between two hydroxyl groups, reaction 2 of Fig. 3.3, water is produced as a result of the formation of a Si–O–Si bond. This suggests that it is possible to work with a under-stoichiometric addition of water as indicated by reaction 3 of Fig. 3.3. The effect of the ratio of water to alkoxide in the acid catalysed hydrolysis is generally discussed by considering three ranges. For an under-stoichiometric addition of water, the reaction product consists of linear chains with residual alkoxide groups. The explanation can be sought in the steric factors as the monomers are more easily hydrolysed than the dimers or end groups of the chains, which in turn are more easily hydrolysed than the intermediate groups of the chains. Moreover, the presence of alkoxy groups on the polymer chain could cause a low packing density [16]. At an intermediate water content, Rw > 4, hydrolysis of the alkoxide is more pronounced given the excess water in the solutions. The growth of silicon-containing species again leads to the formation of linear chains, but the higher concentration of

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3 From (Sub)colloidal Growth to the Gel Structure

Fig. 3.3 Hydrolysis of tetraalkoxysilane with under-stoichiometric addition of water

the latter favours intramolecular reactions. Strawbridge et al. [16], in agreement with the results of Brinker et al. [14] report that gelling occurs through the entanglement of linear species to give rise to very small overlapping clusters. These clusters then aggregate to give rise to a dense gel. In this respect, Yoldas reports that a certain residual of alkoxide groups is always possible even in cases of high-water content [16, 17]. With a Rw above 25, the extent of hydrolysis could strongly influence subsequent condensation reactions. Strawbridge et al. [16] hypothesise that after an initial development of linear chains, high dilution in water favours intramolecular reactions, leading to the formation of cycles. Gelation then occurs from the nuclei formed as a result of the rearrangement of these cyclic species. The different growth mechanism at high water contents may introduce a different type of porosity from previous situations. To better understand the relevance of water content and bring this topic into an application scenario, the reader is presented with some important results obtained in the field of adsorption and drug delivery, where organic substances are trapped in sol–gel matrices and subsequently released. Although these examples deviate slightly

3.3 Water Can Be a Strategic Parameter

27

from the catalysis, it is important to emphasise that the reported findings could be fruitful for the preparation of hybrid systems for biocatalysts, where tailored sol– gel materials are successfully used for trapping a wide variety of biomolecules. An important example is the trapping of enzymes in inorganic matrices. The firs example concerns with the design of materials for the removal of pesticides from water [18–25]. For this specific application the textural properties of the material are particularly relevant. An ideal sorbent should have high surface area (i.e., a large number of sorption sites), accessible pores and physical and chemical stability. The sorbents are often modified to enhance their adsorption efficiency to improve the affinity to a specific organic compound. Viewed from the perspective of matching all the requirements, the sol–gel method is a powerful platform. The achievements of my research group concerning the preparation of porous silicas with acid catalysis will be discussed below. Figure 3.4a shows a ‘standard’ procedure in which TEOS is added to ethyl alcohol in a molar ratio 1: 4 and water, in a ratio lower than stoichiometric, is added for hydrolysis. The catalysis is acidic due to the addition of hydrochloric acid. This procedure is time-consuming as evidenced by the gelation time, tgel, of about 2 months. In Fig. 3.4b, a solvent-free route was explored in which TEOS is hydrolysed in water at 50 °C for 1 h. The system is initially biphasic because TEOS and water are immiscible, but when the reactions start, alcohol is produced by condensation reactions with the partially hydrolysed species soluble in the hydroalcoholic medium. The Rw values explored are 5, 10 and 20 corresponding to SG-5, SG-10 and SG-20, respectively. This procedure in the first place allows a drastic reduction in the time required for gel formation toward tgel more useful for practical applications. The wet gels are dried and then heat-treated to remove all organic residues. The temperature of heat treatment is stabilized based on the results of the thermal analysis [26–28]. Based on the results obtained by the authors [26–28], the xerogels were heat treated at 400 °C in order to burn all organic residues and give the material adequate mechanical properties.

(a)

(b)

Fig. 3.4 Flow chart of a silica gel preparation in acidic condition, Rw = 2 and b silica gel preparation at Rw = 5, 10 and 20

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3 From (Sub)colloidal Growth to the Gel Structure

The gelation time is not the only feature that differentiates the two sol–gel approaches. Textural properties undergo considerable change, which testifies the relevance of synthesis parameters in the rational design of a functional material. In particular, as evident from the adsorption/desorption isotherms shown in Fig. 3.5, the porous texture changes strongly by increasing the water content in the series of samples prepared without alcohol. The isotherm related to this silica prepared with a conventional method, SG-2, is of type I, according to the IUPAC classification, and is characteristic of a microporous material with high quantities of N2 adsorbed at low pressures and a large “plateau” at P/P0 > 0.1. The knee of the isotherms progressively shifts towards higher partial pressure values by increasing Rw, suggesting the presence of mesopores and thus a gradual deviation from an exclusively microporous structure. The surface area increases up to a remarkable value of 705 m2 g−1 with the highest water content, the average pore diameter increases and enters the mesopore range and the volume of the micropores becomes negligible compared to the total pore volume [26]. These results suggest that in order to obtain a product with the desired characteristics, it is not always necessary to turn to complex synthesis procedures. We have shown how important it is to know the relationship between the synthesis parameters and the final properties of the final material. The H2 O/TEOS ratio is a simple strategy to change the textural properties of silica. By simply changing the value of Rw, a silica gel with a surface area comparable to that of some zeolites can be obtained, albeit with a disordered porous structure. Silica samples were used as adsorbents for the removal of simazine (2-chloro-4,6bis(ethylamino)-s-triazine), a common herbicide belonging to the s-triazine family. Striazines are selective persistent herbicides that are widely studied for their still wide

240 2

-1

2

-1

2

-1

SG-20

SBET 705 m g

SG-10

SBET 588 m g

SG-5

SBET 528 m g

200

3

Vads, cm g

-1

220

180 160 140 120

SG-2 100 0.0

0.2

0.4

0.6

2

-1

SBET 410 m g

0.8

1.0

Relative Pressure (P/P0)

Fig. 3.5 N2 adsorption–desorption isotherms of SiO2 prepared at different Rw values

References

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application in forestry and agricultural soils. Porous silica sample SG-20 showed a high affinity for simazine, making it potentially usable for practical applications in iterative removal experiments [18–25]. The water-alkoxide ratio is not only a critical parameter in the hydrolysis of silicon precursors but also in the case of metal alkoxides. Lópex et al. synthesised a sol–gel titania containing encapsulated the sodium salt of 5,5-diphenylhydantoin, an anti-epileptic drug commonly used to suppress abnormal brain activity during a seizure. The release profile of this drug delivery system is influenced by the physicochemical and surface properties of the matrix. The authors demonstrated how the water/alkoxide ratio impacts on the type and strength of drug-matrix interactions of the phenytoin-titania complex, resulting in changes in short- and long-term release kinetics [29]. Titania is a material with great potential and is therefore widely used in numerous applications. In catalysis, titania is often used as a support for the catalytic active phase, often a transition metal. One of the key parameters for increasing catalytic performance is the degree of metal dispersion, which according to the authors T. Umegaki et al. can be improved by increasing the Rw ratio in the preparation of titania nickel nanocomposites. The authors report that textural properties are also changed, with an increase in specific surface area and an increase in pore diameter. All these changes result in a higher rate of hydrogen evolution from aqueous borane ammonia [30]. The role played by the sol–gel reaction conditions in the design of bulk and surface physico-chemical properties has also been reported in the case of sulphated zirconia obtained by a one-pot sol–gel approach. The water/alkoxide ratio plays no role in obtaining a specific crystalline phase, but is important for the development of a high surface area and in the retention of sulphur after the calcination step [31].

References 1. C.J. Brinker, R. Sehgal, S.L. Hietala, R. Deshpande, D.M. Smith, D. Lop, C.S. Ashley, Sol-gel strategies for controlled porosity inorganic materials. J. Membr. Sci. 94, 85–102 (1994) 2. U. Schubert, Chemistry and fundamentals of the Sol–Gel process, in The Sol-Gel Handbook: Synthesis, Characterization, and Applications, ed. D. Levy, M. Zayat (Wiley-VCH Verlag GmbH & Co. KGaA, 2015) 3. Y. Han, Z. Lu, Z. Teng, J. Liang Z. Guo, D. Wang, M.Y Han, W. Yang, Unraveling the growth mechanism of silica particles in the Stöber method: in situ seeded growth model. Langmuir 33, 5879−5890 (2017) 4. S.-H. Wua, C.-Y. Moua, H.-P. Lin, Synthesis of mesoporous silica nanoparticles. Chem. Soc. Rev. 42, 3862–3875 (2013) 5. S. Nandy, D. Kundu, M. Kanti Naskar, Synthesis of mesoporous Stöber silica nanoparticles: the effect of secondary and tertiary alkanolamines. J. Sol-Gel Sci. Technol. 72, 49–55 (2014) 6. J. Caruso, T.M. Alam, M.J. Hampden-Smith, A.L. Rheingoldh, G.A. P. Yap, Alcohol-alkoxide exchange between Sn(OBut )4 , and HOBut in co-ordinating and non-co-ordinating solvent. J. Chem. SOC. Dalton Trans. 2659–2664 (1996) 7. M. Borlaf, R. Moreno, Colloidal sol-gel: a powerful low-temperature aqueous synthesis route of nanosized powders and suspensions. Open Ceramics 8, 100200 (2001)

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8. S. Sakka, Viscosity and Spinnability of Gelling Solutions. Handbook of Sol-Gel Science and Technology, pp. 1–33. https://doi.org/10.1007/978-3-319-19454-7_41-1 9. W. Stöber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range. J. Colloid Interface Sci. 26, 62–69 (1968) 10. K.T. Nijenhuis, Thermoreversible Networks: Viscoelastic Properties and Structure of Gels; Advances in Polymer Science, vol. 130 (Springer, Berlin, 1997) 11. V.A. Lebedev, D.A. Kozlov, I.V. Kolesnik, A.S. Poluboyarinov, A.E. Becerikli, W. Grünert, A.V. Garshev, The amorphous phase in Titania and its influence on photocatalytic properties. Appl. Catal. B: Environ. 195, 39–47 (2016) 12. S. Paul, A. Choudhury, Investigation of the optical property and photocatalytic activity of mixed phase nanocrystalline Titania. Appl. Nanosci. 4, 839–847 (2014) 13. A. Wong, W.A. Daoud, H. Liang, Y.S. Szeto, The effect of aging and precursor concentration on room-temperature synthesis of nanocrystalline anatase TiO2 . Mater. Lett. 117, 82–85 (2014) 14. C.J. Brinker, G. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, 1st edn. (Academic Press Inc., New York, NY, USA, 1990) 15. S. Sakka, K. Kamiya, The sol-gel transition in the hydrolysis of metal alkoxides in relation to the formation of glass fibers and films. J. Non-Cryst. Solids 48, 31–46 (1982) 16. I. Strawbridge, A.F. Craievich, P.F. James, The effect of the of the H2 O/TEOS ratio on the structure of gels derived by the acid derived by the acid catalysed hydrolysis of tetraethoxysilane. J. Non-Cryst. Solids 72, 139–157 (1985) 17. C.J. Brinker, K.D. Keefer, R.A. Assink, B.D. Kay, C.S. Ashley, J. Non-Cryst, Solids 63, 45 (1984) 18. S. Esposito, F. Sannino, M. Pansini, B. Bonelli, E. Garrone, Modes of interaction of simazine with the surface of model amorphous silicas in water. J. Phys. Chem. C 117, 11203–11210 (2013) 19. F. Sannino, S. Ruocco, A. Marocco, S. Esposito, M. Pansini, Simazine removal from waters by adsorption on porous silicas tailored by sol-gel technique. Microporous Mesoporous Mater. 180, 178–186 (2013) 20. S. Esposito, F. Sannino, M. Armandi, B. Bonelli, E. Garrone, Modes of interaction of simazine with the surface of amorphous silica in water. Part II: Adsorption at temperatures higher than ambient. J. Phys. Chem. C 117, 27047–27051 (2013) 21. F. Sannino, M. Pansini, A. Marocco, B. Bonelli, E. Garrone, S. Esposito, The role of outer surface/inner bulk Brønsted acidic sites in the adsorption of a large basic molecule (simazine) on H-Y zeolite. Phys. Chem. Chem. Phys. 17, 28950–28957 (2015) 22. S. Esposito, E. Garrone, A. Marocco, M. Pansini, P. Martinelli, F. Sannino, Application of highly porous materials for simazine removal from aqueous solutions. Environ. Technol. 37, 2428–2434 (2016) 23. V. Addorisio, S. Esposito, F. Sannino, Sorption capacity of mesoporous metal oxides for the removal of MCPA from polluted waters. J. Agric. Food Chem. 58, 5011–5016 (2010) 24. V. Addorisio, D. Pirozzi, S. Esposito, F. Sannino, Decontamination of waters polluted with simazine by sorption on mesoporous metal oxides. J. Hazard. Mater. 196, 242–247 (2011) 25. F. Sannino, S. Ruocco, A. Marocco, S. Esposito, M. Pansini, Cyclic process of simazine removal from waters by adsorption on zeolite H-Y and its regeneration by thermal treatment. J. Hazard. Mater. 229–230, 354–360 (2012) 26. S. Esposito, “Traditional” sol-gel chemistry as a powerful tool for the preparation of supported metal and metal oxide catalysts. Materials 12, 668 (2019). https://doi.org/10.3390/ma12040668 27. S. Esposito, M. Turco, G. Ramis, G. Bagnasco, P. Pernice, C. Pagliuca, M. Bevilacqua, A. Aronne, Cobalt–silicon mixed oxide nanocomposites by modified sol-gel method. J. Solid State Chem. 180, 3341–3350 (2007). https://doi.org/10.1016/j.jssc.2007.09.032 28. S. Esposito, M. Turco, G. Bagnasco, C. Cammarano, P. Pernice, New insight into the preparation of copper/zirconia catalysts by sol-gel method. Appl. Catal. A Gen. 403, 128–135 (2011) 29. T. López, K.A. Espinoza, A. Kozina, P. Castillo, A. Silvestre-Albero, F. Rodriguez-Reinoso, R. Alexander-Katz, Influence of water/alkoxide ratio in the synthesis of nanosized sol-gel titania on the release of phenytoin. Langmuir 27, 4004–4009 (2011)

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30. T. Umegaki, Y. Yamamoto, Q. Xu, Y. Kojima, Influence of the water/titanium alkoxide ratio on the morphology and catalytic activity of titania-nickel composite particles for the hydrolysis of ammonia borane. Chem. Open 7, 611–616 (2018) 31. S. Melada, S.A. Ardizzone, C.L. Bianchi, Sulphated zirconia by sol–gel route. The effects of the preparative variables. Microporous Mesoporous Mater. 73, 203–209 (2004)

Chapter 4

From Wet Gel to the Final Product: Draw Your Way

Abstract Once the gel is obtained, the water and alcohol that fill the porosity of the solid network must be removed through drying treatments to readily convert the wet gel in a dry solid. The methods of liquid extraction, as well as the other steps in the sol–gel process, can play an important role in determining the surface properties of the final material. Moreover, the combination of the drying procedure and the protocol to be adopted can be a decisive factor in yielding a highly porous material. In most cases, wet gels are dried in an oven at a temperature of around 100 °C to produce socalled xerogels. The difference in the contraction speed between inside and outside the gel is responsible for the formation of cracks. Outstanding and somewhat unusual properties can be achieved if the gels are dried by supercritical drying, resulting in what are called aerogels. In particular, aerogels are characterised by very low apparent densities, large specific surface areas and, in most cases, have amorphous structures. In more recent times, other drying methods have become popular that allow for high pore interconnection and large surface to volume ratio, usually between xerogel and aerogel. These include the freeze-drying process from which cryogels are obtained. Keywords Wet gel · Drying · Xerogel · Aerogel · Cryogel · Film formation

4.1 Drying Methods During classic stove drying, under constant temperature, pressure and humidity conditions, the water and alcohol trapped in the gel matrix are removed in a process driven by capillary tension, which, if not properly controlled, leads to a simultaneous collapse of the gel structure, Fig. 4.1. In the first step of this process, the gel structure is compliant and responds to the increase in capillary tension with a gradual shrinkage, leaving liquid–vapor interface at the exterior gel surface [1, 2]. The reduction in volume is simultaneously accompanied by further cross-linking of the lattice due to the approaching of condensable groups and the formation of new siloxane bonds. This phenomenon leads to a stiffening of the gel, which begins to resist further contraction until it reaches a critical point where stress is at its highest. It is at this point that the textural properties of the dry product, called xerogel, are defined. In order to prevent fracture formation, the © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Esposito, Sol–Gel Synthesis Strategies for Tailored Catalytic Materials, SpringerBriefs in Materials, https://doi.org/10.1007/978-3-031-20723-5_4

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4 From Wet Gel to the Final Product: Draw Your Way

Fig. 4.1 Schematic representation of wet gel formation and possible drying procedures. Adapted with permission from [2] Copyright 2017 Springer

rigidity of the gel must be increased, providing resistance to collapse, and the capillary pressure that causes the gel to collapse must be limited. Slow drying certainly causes a more uniform compression of the lattice, decreasing the tendency to fracture, but it would increase the xerogel formation time, negatively affecting the entire process. The average pore size formed during the drying process contributes largely to the effectiveness of solvent removal while the strength of the gel helps to contrast capillary tension. Careful control of the molar ratios of alkoxide/water/alcohol and the type of catalysis can be a means of achieving pore enlargement and network strengthening [1, 3]. A more innovative strategy for crack-free gels is the adoption of a surfactant as a model for the uniform pore network. This decreases the capillary tension gradient in the gel, which is the real factor causing the formation of cracks [4]. Another strategy to circumvent gel collapse involves drying under supercritical conditions, a more sophisticated procedure that removes the obstacle of the capillary pressure gradient by working at a pressure and temperature higher than those corresponding to the critical point of a liquid trapped in the gel pores, Fig. 4.1. After replacing the water with a specific solvent, the system is brought to supercritical fluid conditions, at this point the pressure and temperature are gradually released, allowing the gas to escape and leaving a dried product. Among possible supercritical solvents, carbon dioxide has the advantage of less drastic conditions, in particular the critical temperature is close to the room temperature: 31 °C and 7.4 MPa (for ethanol it is 243 °C and 6.4 MPa). Aerogels possess outstanding physical properties, such as very low thermal conductivity, high temperature resistance, and good mechanical properties combined

4.1 Drying Methods

35

with very low density, the pore volume is above 90% of the sample volume. The name aerogel does not refer to a specific chemical composition but, rather, to this unique structure composed essentially of air. The most prominent features that have aroused the interest of researchers working in the field of catalysis are the very high porosity, high specific surface areas and the possibility of modelling the material [5, 6]. Although the industry is dominated by silica aerogels and their use as composite blankets in energy and industrial insulation, the strong impetus from scientific research is broadening the fields of application of aerogel. Indeed, a wide range of substances prepared in the form of aerogels are used in optical, electrical, environmental, biomedical and catalytic applications [7–12]. The lack of an effective boom in the aerogel industry is largely linked to the fragility of these materials, which limits their commercial use as part of a composite design, and the need to reduce the cost of the supercritical drying process. For this

(a)

(b)

Fig. 4.2 Textural properties of a Ni/Al2 O3 gel and b TG profiles of carbon formation on the catalysts under CH4 –CO2 atmosphere at 800 °C. Adapted with permission from [15] Copyright 2021 Springer.

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4 From Wet Gel to the Final Product: Draw Your Way

reason, improvements to supercritical drying continue to be sought along with the development of alternative processes, including ambient pressure drying and freezedrying. Freeze-drying, also known as Lyophilisation or cryo-drying, is a slow process that removes water or other solvents from frozen samples through sublimation and desorption under vacuum, Fig. 4.1. The final porous architecture, benefiting from the ’ice crystal model effect’, is represented by highly interconnected macroporous structures [13, 14]. Toshihiko Osaki [15] showed how the freeze-drying process can be a strategy to impact the structural and textural properties of catalysts. Ni/Al2 O3 cryogel was synthesised from aluminium sec-butoxide and nickel acetate by a one-pot sol–gel process followed by lyophilisation. Comparison with an analogous Ni/Al2 O3 xerogel gel revealed the characteristics of the cryogel that led to higher performance in reforming CH4 from CO2 . In particular, an increase in surface area and pore volume was observed, Fig. 4.2. It was also found that the average nickel particle size for cryogel was about 87% of that of xerogel, thus limiting carbon formation, Fig. 4.2 [15].

4.2 Thin Film Formation The drying of the gel can be forced if the surface area in contact with the atmosphere is increased, which is what happens in the production of thin films. The preparation of thin films, using dip-coating or spin-coating techniques, is indeed one of the most important and intriguing applications of sol–gel processing. While from the point of view of instrumentation, film deposition can be considered a rather simple procedure, from the point of view of the chemistry of the solution, adhesion to the substrate and the formation of the film itself, the process can be challenging. In the preparation of thin films, the drying stage overlaps with the aggregation/gelation and ageing steps. Consequently, the formation of a crack-free film, as well as its final structure, are the consequence of competitive phenomena such as evaporation (which compacts the film), condensation reactions (which strengthen the film, increasing its resistance to compaction) and shear-induced ordering [16]. The ’sol’ that meets the requirements for film deposition must be stable and of adequate viscosity. As mentioned in previous chapters, throughout the gel formation, viscosity increases very slowly, then undergoes a sudden exponential increase followed by instantaneous gelling. It is therefore crucial to monitor the parameters affecting the polymerisation kinetics of metal alkoxide, in order to have a sol with a high degree of hydrolysis and adequate viscosity. But, at the same time, the factors that rule the film deposition process on the substrate can be decisive in obtaining a uniform crack-free film [17–19]. A successful attempt to prepare humidity-sensitive CoOx-SiO2 thin films in a reasonable time scale was realised by Esposito et al. [18]. The strategy adopted was

4.2 Thin Film Formation

(a)

37

(b)

(c)

Fig. 4.3 CoOx-SiO2 thin films prepared by dip coating and heat treated at a 1000 °C; b 300 °C and c 400 °C

to hydrolyse TEOS under acidic conditions in the absence of alcohol, resulting in a sol with a high degree of cross-linking after 2 days. Moreover, the absence of alcohol makes easier to solubilise an high amount of inorganic salt precursors, up to 30 mol%. To achieve the appropriate viscosity for the deposition of a uniform crackfree film, the solution was subsequently diluted with anhydrous ethanol. Silica glass slides were dipped into the solution and withdrawn at a speed of 100 mm min−1 . Transparent pinkish films were obtained that were fully dried and then annealed at different temperatures in the range 400–1000 °C, Fig. 4.3. Film thickness was estimated to be approximately 0.8 ± 0.03 μm [18]. One of the advantages of the sol–gel method over conventional coating methods such as sputtering/evaporation and CVD (chemical vapor deposition) is the possibility to control the microstructure of the film (pore volume, pore size and surface area) [20]. The microstructures of thin films obtained by dipping and spinning are largely dependent on process parameters such as withdrawal rate, spinning speed, surface tension, viscosity and evaporation rate. The dip coating process can be seen as the sum of several steps, Fig. 4.4. First, the substrate is immersed in the sol at a constant rate and for a period of time sufficient to ensure the interaction of the substrate with the film precursors. In a next step, the substrate is pulled upwards at a constant speed. If the sol has the right viscosity characteristics and the right degree of cross-linking, the substrate drags the solution towards the deposition region. The adhesion to the substrate that Fig. 4.4 A schematic view of the dip-coating method

38

4 From Wet Gel to the Final Product: Draw Your Way

ensures subsequent film formation is a competition between the viscous drag that holds the fluid on the substrate and the gravity force that moves the fluid away from the substrate [16]. The extraction of the substrate from solution causes a rapid increase in the liquid– vapour separation surface, triggering rapid evaporation of the volatile components. The resulting increase in the concentration of the reagents in the forming film compared to the concentration in solution strongly fosters polycondensation reactions leading to almost instantaneous gelation. The obtained coating undergoes further heat treatment, which burns off residual organics and induces crystallisation of the functional oxides. The heating stage can be remarkably critical in the formation of cracks and unavoidable peeling off of local areas. Indeed, fractures are assumed to be caused by stress resulting from the deformation introduced by the condensation of the silanol groups and the evaporation of the residual organic substances [21, 22]. The thickness of the film is related to the processing parameters and can reasonably calculated from Eq. 4.1 derived from Landau and Levich model [23]. According to this model, the thickness of the film is proportional to the viscosity of the colloidal solution and the extraction rate, and inversely proportional to the liquid vapor surface tension according to the following equation h = 0.94

ην 2/3 γ 1/6 (ρg)1/2

(4.1)

In this equation, η is the viscosity of the solution, ν is the withdrawal speed from the solution, ρ is the density of the solution, g is the gravity acceleration and γ is the liquid–vapour surface tension. In addition to all the parameters that have been mentioned, the roughness of the support and its chemical affinity with the film can affect the formation of a uniform coating and its final thickness [23]. The strategy, used by Emiel J. Kappert et al. [24], to prevent crack tendency in silica films was to replace part of the TEOS with an alkyl-substituted silane. Although the synthesis was designed to retain the same microstructure for silica and organosilica, the results obtained show that organosilica materials have much larger critical thicknesses than silica [24]. The spin coating method overcomes some of the limitations of dip coating, such as double side coating and non-uniformity at the wafer edge. In the spin-coating method, the sol is dripped onto the substrate in an adequate quantity to subsequently cover the entire surface. When the substrate is spun at a predetermined speed, the liquid flows radially outwards due to the centrifugal force. As the film thins, the speed at which the excess liquid is removed slows down. The deposited film assumes uniform thickness as a result of balancing action between two types of forces, i.e. centrifugal force and viscous (friction) force [25]. Promising new fields for nanostructured oxides, which include photocatalysis and electrocatalysis, require their processing as mesostructured thin films [26, 27].

4.2 Thin Film Formation

39

Fig. 4.5 Schematic representation of EISA process for the synthesis of highly organized mesoporous TiO2 films. Adapted with permission from [32]; Copyright 2011 Elsevier

The Evaporation Induced Self-Assembly Method, EISA, was first reported by Osawa and Brinker [28–30] to prepare mesoporous silica thin film. The technique is based on the same principle as mesostructured silicas, that is the ability of surfactants to self-assemble into various structures. However, in the deposition of mesoporous thin film, the self-assembly is driven by the evaporation of the solvent during the withdrawal of the support from the solution, Fig. 4.5 [31, 32]. The process of forming mesoporous thin films using EISA begins with the preparation of a dilute solution containing mainly inorganic (or hybrid) precursors, solvents, catalysts and a surfactant, Fig. 4.5. When the substrate is extracted from the solution, rapid evaporation of the solvent occurs and the concentrations of the metal oxide oligomers and the non-volatile surfactant increase. When the surfactant concentration is equal to the critical micelle concentration, cmc, micelles form and an organised Liquid Crystal (LC) mesophase is obtained while the inorganic network is not completely condensed. In this stage of film formation, the relative humidity (RH) conditions play a key role. In the final stage, the template is removed to impart porosity and completely condense the inorganic network [33]. A big advantage of the EISA process when combined with dip-coating is the flexibility afforded in controlling the final mesostructure of the thin film by careful control of processing parameters such as temperature, time, RH, vapor pressure and chemical parameters such as composition and pH. The preparation of reproducible mesoporous thin films requires understanding and control at three main levels: The chemistry associated with the initial solution, the processes linked to the layer deposition technique and the treatment aimed at eliminating the organic phase and stiffening the network without pore collapse [34].

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4 From Wet Gel to the Final Product: Draw Your Way

References 1. F. Kirkbir, H. Murata, D. Meyers, S.R.A.Y. Chaudhuri, A. Sarkar, Drying and Sintering of Sol-Gel Derived Large SiO2 Monoliths. J. Sol-Gel Sci. Technol. 6, 203–217 (1996) 2. S. Dervin, S.C. Pillai. An introduction to sol-gel processing for aerogels. In: S. Pillai, S Hehir (eds.) Sol-Gel materials for energy, environment and electronic applications. Advances in SolGel derived materials and technologies. Springer, Cham. (2017) https://doi.org/10.1007/9783-319-50144-4_1 3. A. Fidalgo, M.E. Rosa, L.M. Ilharco, Chemical control of highly porous silica xerogels: Physical properties and morphology. Chem. Mater. 15(11), 2186–2192 (2003) 4. M.J. Mosquera, D.M. de los Santos, L. Valdez-Castro, L. Esquivias, New route for producing crack-free xerogels: Obtaining uniform pore size. J. Non-Cryst. Solids, 354, (2008) 645–650 5. T. Wu, M. Chen, L. Zhang, X. Xu, Y. Liu, J. Yan et al., Three-dimensional graphene-based aerogels prepared by a self-assembly process and its excellent catalytic and absorbing performance. J. Mater. Chem. A. 1, 7612–7621 (2013) 6. Z. Zhou, X. Zhang, C. Lu, L. Lan, G. Yuan Polyaniline-decorated cellulose aerogel nanocomposite with strong interfacial adhesion and enhanced photocatalytic activity. RSC Adv. 4, 8966–72 (2014) 7. A.C. Pierre, G.M. Pajonk, Chemistry of aerogels and their applications. Chem. Rev. 102, 4243–4265 (2002) 8. J. Choi, D.J. Suh, Catalytic applications of aerogels. Catal. Surv. Asia 11, 123–133 (2007) 9. Y. Guo, C. Zhao, J. Sun, W. Li, P. Lu Facile synthesis of silica aerogel supported K2CO3 sorbents with enhanced CO2 capture capacity for ultra-dilute flue gas treatment. Fuel. 215, 735–43 (2018) 10. Z. Ali, A. Khan, R. Ahmad, The use of functionalized aerogels as a low-level chromium scavenger. Microporous Mesoporous Mater. 203, 8–16 (2015) 11. S. Zhao, B. Jiang, T. Maeder, P. Muralt, N. Kim, S.K. Matam et al., Dimensional and structural control of silica aerogel membranes for miniaturized motionless gas pumps. ACS Appl. Mater. Interfaces. 7, 18803–18814 (2015) 12. T.W. Hamann, A.B. Martinson, J.W. Elam, M.J. Pellin, J.T. Hupp, Atomic layer deposition of TiO2 on aerogel templates: new photoanodes for dye sensitized solar cells. J. Phys. Chem. C. 112, 10303–10307 (2008) 13. X. Wang, A. Sumboja, E. Khoo, C. Yan, P.S. Lee, Cryogel synthesis of hierarchical interconnected Macro-/Mesoporous Co3 O4 with superb electrochemical energy storage. J. Phys. Chem. C. 116, 4930–4935 (2012) 14. L.B. Chiriac, M. Todea, A. Vulpoi, M. Muresan-Pop, R.V.F. Turcu, S. Simon, Freeze-drying assisted sol–gel-derived silica-based particles embedding iron: synthesis and characterization. J. Sol-Gel. Sci. Technol. 87, 195–203 (2018) 15. T. Osaki, Synthesis of porous and homogeneous Ni/Al2O3 cryogel for CO2 reforming of CH4. J. Sol-Gel Sci. Technol. 97, 291–301 (2021) 16. C.J. Brinker, A.J. Hurd, G.C. Frye, K.J. Ward, C.S. Ashley, Sol-Gel thin film formation. J. Non-Cryst. Solids 121, 294–302 (1990) 17. M. D’Apuzzo, A. Aronne, S. Esposito, P. Pernice, Sol-Gel synthesis of humidity-sensitive P2O5-SiO2 amorphous films. J. Sol-Gel Sci. Technol. 17, 247–254 (2000) 18. S. Esposito, A. Setaro, P. Maddalena, A. Aronne, P. Pernice, M. Laracca, Synthesis of cobalt doped silica thin film for low temperature optical gas sensor. J. Sol-Gel Sci. Technol. 60, 388–394 (2011) 19. G. Dell’Agli, S. Esposito, G. Mascolo, M.C. Mascolo, C. Pagliuca, Films by slurry coating of nanometric YSZ (8 mol% Y2 O3 ) powders synthesized by low-temperature hydrothermal treatment. J. Eur. Ceram. Soc. 25, 2017–2021 (2005) 20. K. Haas-Santo, M. Fichtner, K. Schubert, Preparation of microstructure compatible porous supports by sol–gel synthesis for catalyst coatings. Appl. Catal. A. 220(1–2), 79–92 (2001)

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21. Sumio Sakka in handbook of advanced ceramics: Chapter 11.1.2. Sol–Gel process and applications. materials, applications, processing, and properties, book, second edition 2013 academic Press, Editor in chief Shigeyuki Somiya. https://doi.org/10.1016/B978-0-12-385469-8.010 01-7 22. T.J. Garino, The cracking of Sol-Gel films during drying. in MRS Online Proceedings Library, 180, 497 (1990) 23. J.P. Fernández-Hernán, A.J. López, B. Torres, J. Rams, Influence of roughness and grinding direction on the thickness and adhesion of sol-gel coatings deposited by dip-coating on AZ31 magnesium substrates. A Landau–Levich equation revision. Surface & Coatings Technology, 408, 126798 (2021) 24. E.J. Kappert, D. Pavlenko, J. Malzbender, A. Nijmeijer, N.E. Benes, P.A. Tsaj, Formation and prevention of fractures in sol–gelderived thin films. Soft Matter 11, 882–888 (2015) 25. K. Vorotilov, V. Petrovsky, V. Vasiljev, Spin coating process of Sol-Gel silicate films deposition: Effect of spin speed and processing temperature. J. Sol-Gel Sci. Technol. 5, 173–183 (1995) 26. M. Einert, M. Mellin, N. Bahadorani, C. Dietz, S. Lauterbach, Jan P. Hofmann, Mesoporous High-entropy oxide thin Films: Electrocatalytic water oxidation on high-surface-area spinel (Cr0.2 Mn0.2 Fe0.2 Co0.2 Ni0.2 )3 O4 Electrodes. ACS Appl. Energy Mater, 5, 1 717–730 (2022) 27. W. Zhou, W. Li, Q.J. Wang, Y. Qu, Y. Yang, Y. Xie, K. Zhang, L. Wang, H. Fu, D. Zhao, Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst. J. Am. Chem. Soc. 136, 9280–9283 (2014) 28. M. Ogawa, Preparation of layered silica−dialkyldimethylammonium bromide nanocomposites. Langmuir 13, 1853–1855 (1997) 29. C.J. Brinker, Y.F. Lu, A. Sellinger, H.Y. Fan, Evaporation-Induced Self-Assembly: Nanostructures made easy. Adv. Mater. 11, 579–585 (1999) 30. C.J. Brinker, Evaporation-Induced Self-Assembly: Functional nanostructures made easy. MRS Bull. 29, 631–640 (2004) 31. T. Brezesinski, M. Groenewolt, A. Gibaud, N. Pinna, M. Antonietti, B.M. Smarsly, EvaporationInduced Self-Assembly (EISA) at Its Limit: ultrathin, crystalline patterns by templating of micellar monolayers. Adv. Mater. 18, 2260–2263 (2006) 32. J.H. Pan, X.S. Zhao, W.I. Lee, Block copolymer-templated synthesis of highly organized mesoporous TiO2 -based films and their photoelectrochemical applications. Chem. Eng. J. 170, 363–380 (2011) 33. L. Mahoney, R.T. Koodali, Versatility of Evaporation-Induced Self-Assembly (EISA) method for preparation of mesoporous TiO2 for energy and environmental applications. Materials 7, 2697–2746 (2014). https://doi.org/10.3390/ma7042697 34. C.J. Brinker, R. Sehgal, S.L. Hietala, R. Deshpande, D.M. Smith, D. Loy, C.S. Ashley, Sol-gel strategies for controlled porosity inorganic materials. J. Membr. Sci. 94, 85–102 (1994)

Chapter 5

Evolution of Sol–Gel Chemistry

Abstract Since the early studies on hydrolysis and condensation reactions of silicon alkoxides, progress in the knowledge of sol–gel chemistry and its potential has never stopped. The possibility of varying both parameters and precursors is boundless, resulting in ever more finely tailor-made materials. Furthermore, the continuous development of this technique has produced an impressive number of new syntheses and procedures that exploit different chemistries. A typical example of this is the development of organic–inorganic hybrid materials that combine the benefits of the organic matrix with the properties of the inorganic part, leading to new fields of application. The non-hydrolytic sol–gel is, on the other hand, an elegant and versatile method that allows remarkable levels of homogeneity to be achieved in the preparation of mixed oxides, as well as favouring the formation of defined polymorphs. Another particularly valuable route for the preparation of mixed oxides is the Pechini method, which overcomes most of the complexities and inconveniences that often occur in the alkoxide-based sol–gel process. In this chapter we retrace the main developments in the sol–gel method, reporting on the most interesting advances in the framework of catalytic application. Keywords Organic–inorganic hybrids · Pechini method · Non-hydrolitytic sol–gel

5.1 Organic–Inorganic Hybrids Hybrid materials can be defined as the nano-scale combination of two components, one (bio)organic and one inorganic. This class of material is not an ingenious human invention but rather one of the countless expressions of nature’s magnificence. The physico-chemical properties are enhanced by the synergistic effect of the two components, giving rise to interesting and intriguing properties that have opened up promising horizons for a new generation of multifunctional materials [1, 2]. A productive and multi-disciplinary field of research arose with a view to imitating nature in order to design and produce sophisticated and inexpensive materials that would respond to the industrial needs triggered by technological progress. The sol– gel method, offering extreme versatility in underlying chemistry and mild operation

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Esposito, Sol–Gel Synthesis Strategies for Tailored Catalytic Materials, SpringerBriefs in Materials, https://doi.org/10.1007/978-3-031-20723-5_5

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conditions, immediately emerged as a leading player in the development of tailormade hybrid materials with well-defined architecture [3]. Hybrid materials, often biologically inspired, resulting from sol–gel synthesis strategies are countless. Their progressive and ever-increasing penetration in various industrial fields can also be attributed to the easiness of processing them into different shapes [1]. The distinctive properties of this class of materials are closely dependent on the nature of the interaction between the inorganic matrix and its organic counterpart. A rational design of advanced hybrid materials necessitates an understanding of the type of interface that can be generated in the coupling of the different components. This enables the best synthetic route to be outlined to enhance the right properties for applications in the fields of catalysis, sensor technology, optics and nanomedicine. A distinction made on the type of interaction that can occur between the organic and inorganic fractions allows hybrids to be divided into two classes. Class I contains hybrid materials characterized by the absence of strong bonds between the organic and inorganic components. They rather interact through weak interactions: Van der Waals forces, π– π interaction, hydrogen bonds or electrostatic interactions. When, on the other hand, the two components are connected through covalent bonds, we obtain what are classified as class II hybrids [4]. The organic moiety can consist of monomers, oligomers or polymers and may contain any functionality. The organic block can also be represented by a biologically active molecule, such as an enzyme. The inorganic species are molecular systems able to form inorganic networks and can also contain organic functionalities. Actually, a large number of hybrid architecture can be designed and Fig. 5.1 reports two examples of class I hybrid materials [5]. TEOS can be mixed with an organic phase (e.g. poly(vinylpyrrolidone)) allowing SiO2 network to be formed within the organic phase. In this case, the organic polymer and the silica network are held by hydrogen bonds formed between hydroxyl groups of silica and the carbonyl groups in polymer chains, Fig. 5.1. Another approach that leads to a type I hybrid can be the simultaneous gelation of the inorganic and organic components that form an entangled independent network of an organic polymeric gel and a silica gel like, Fig. 5.1b. Although these hybrid materials featured appealing properties for heterogeneous catalysis, some downsides limited their use. Easy separation of products and the possibility of catalyst reuse are some of the main advantages of heterogeneous catalysis. As fully explained by Diaz et al. [6], the use of class I hybrid catalysts, in which the organic phase is generally trapped in the pores of the inorganic network, exhibit leaching, deactivation and desorption problems of the active organic moiety. The need for a more robust hybrid matrix that preserves the integration of the two fragments has driven catalytic research toward synthetic strategies for the preparation of class II hybrid catalysts. The challenge is to have the organic fraction uniformly distributed over the entire catalyst without being confined within the walls of the solid by limiting its porosity [6]. Some type of fascinating architecture within the class II hybrid materials are mentioned in the book of Alain C. Pierre [7].

5.1 Organic–Inorganic Hybrids

(a)

45

(b)

Fig. 5.1 Schematic representation of a molecular structure of PVP-silica hybrid and b hybrid with interpenetrating SiO2 and organic gel networks structure [5]

A possible arrangement arises from the presence of chains with alternating organic and inorganic moieties; in this subclass, neither of the two components can participate in an independent 3D network and the precursors must contain groups that allow copolymerisation. The two components do not necessarily have to be linear, but can also consist of rather dense nanoparticles or clusters. Additionally, it is possible to have core–shell structures in which the organic phase covers the colloidal silica particles. The use of alkoxides modified with organic groups can be extremely effective in the preparation of Class II hybrid catalysts. The hydrolysis and condensation of an organically modified precursors leads to the formation organically modified sol–gel materials, known as Ormosils (organically modified silicates), whereas Ormocers (organically modified ceramics) are obtained when an alkoxide of a metal other than silicon is used [8]. The physical and chemical properties of ormosils can be easily tailored by modulating the amount of modified precursor or changing the type of functional group. The introduction of various organic functional groups can either be an attachment site for subsequent covalent anchoring of active organic groups or serve for copolymerisation with organic monomers. The utilisation of an organically modified precursor was also proposed to tune the gelification kinetics and better control the shaping process. A further advantageous feature of hybrids is the possibility of juggling with the hydrophobic character through the introduction of specific organic groups. Organic–inorganic precursors can be monomeric species, R’Si(OR)3 , bridged ((R’O)3 SiRSi(OR’)3 ) or polyhedral oligomeric (POSS) silsesquioxanes [6]. The flexibility of chemistry relating to the preparation of silicon alkoxides provides a very large number of organically modified alkoxides where the functionalities introduced can be of any nature (alkyl, aryl, acrylic, epoxy...). The 3(methacryloyloxy)propyl trimethoxysilane, Fig. 5.2a contains both hydrolysable

46

(a)

5 Evolution of Sol–Gel Chemistry

(b)

+

Fig. 5.2 a Precursor of an hybrid material of class II: 3-(methacryloyloxy)propyl trimethoxysilane; b Si alkoxide and 2-hydroxyethyl methacrylate

alkoxide groups and organic groups susceptible to polymerization so it can lead itself to a hybrid material of class II. Hybrids can also be obtained by the simultaneous polymerization of alkoxide or alkyl alkoxides and polymer monomers like 2hydroxyethyl methacrylate, Fig. 5.2b. Regardless, synthesis strategies should always be designed for obtaining hybrid networks at molecular level, bridging the inorganic and organic structures. Indeed, the introduction of one or more organic groups leads to different reactivity with respect to TEOS because the steric and electronic properties of the R’ group on the silicon atom strongly impact on the nucleophilic substitution. A valuable and significant contribution to the development of hybrid catalytic formulations with advanced properties over single-functional solids or homogeneous catalysts was conducted by Katz et al., Davis et al. and Corma et al. [9–12]. Working mainly on amorphous silica, they evaluated the cooperative effect of several different active groups obtaining multifunctional and versatile heterogeneous catalysts. We conclude this section with a quick mention of the possibility of obtaining biocatalysts by using biological molecules such as enzymes as organic fraction. Enzymes are well known as highly effective and efficient catalysts of a wide variety of processes characterised by high selectivity and activity. To improve the stability of the biomolecules under various reaction conditions and to enhance the reusability of biomolecules over successive catalytic cycles, enzyme immobilisation is the widely used technique. The material chosen to support the enzyme must meet very strict criteria and from this point of view the flexibility of the sol–gel method makes it a powerful platform through which it is possible to tailor the porous structure, surface area, hydrophobicity, biocompatibility, surface properties, surface functionalization [13, 14].

5.2 Non-hydrolitic Routes The Non-Hydrolytic Sol–Gel, NHSG, is a fascinating and alternative bottom-up route to overcome certain limitations of the classical alkoxide-based sol–gel method. The name ”non-hydrolytic” refers to the drastic change of the reaction medium as the processes take place in a water-free environment.

5.2 Non-hydrolitic Routes

47

This method can be particularly beneficial for the preparation of mixed oxide catalysts or hybrid systems with strictly controlled morphologies, crystallinity and porosity [15, 16]. In the preparation of mixed oxides, where alkoxide precursors are typically used, a first obstacle encountered is the difference in reaction rates. The production of a homogeneous gel from hetero-condensation reactions is pursued using a some “gimmicks”. A relatively straightforward approach involves acid prehydrolysis of low reactive alkoxides, such as silicon precursors, and chemical modification of the metal alkoxides. However, controlling the reaction pathway remains challenging making the material hardly tailorable. Moreover, the addition of certain additives can make starting materials more costly and less environmentally friendly. The presence of water can also be a drawback in view of the miscibility of precursors and hydration phenomena affecting the reactivity of species and the surface energy of clusters. Non-Hydrolytic Sol–Gel can be seen as a way to ensure a better homogeneity in the case of mixed oxides. To properly define a synthesis pathway as non-hydrolytic, it is necessary to completely rule out the presence of water, including as a product of polycondensation reactions [16, 17]. As represented in Scheme 5.1, the chemistry of NHSG relies on a sort of condensation where the formation of the M–O-M bond is ensured by an oxygen donor other than water. Typically, organic substances (e.g. alcohols, carboxylates or the alkoxides themselves) are used as oxygen donors. The reactions are generally much slower than those in an aqueous environment providing improved control over the composition, structure, and texture of the resulting materials. Indeed, the performance of heterogeneous catalysts is often strictly correlated to the presence of specific polymorphs as well as a well-defined morphology. A noteworthy example is titania, a material featuring unique properties that trigger its cross-cutting presence in catalytic processes of industrial interest. The non-hydrolytic sol–gel enables some of titania’s main characteristics to be modulated by playing with synthesis parameters, such as the nature and type of oxygen donor, solvent or temperature, avoiding elaborate synthetic procedures [18, 19]. Another important feature that should not be ignored in catalyst design is the type of porosity and relative pore size distribution. Conventional methods, with particular reference to polymer networks, generally produce poorly cross-linked monolithic gels that under unfavourable capillary forces collapse into microporous structures. For this reason, templates or drying systems under supercritical conditions are often used. Although NHSG also requires a solvent removal step, a greater extent of condensation capable of withstanding capillary stresses and lower surface tension of organic solvents compared to water yield mesoporous xerogels sometimes featuring distinctive textural properties. Further tailoring of the porosity toward an ordered mesoporosity can be achieved with the addition of a structure-directing agent [15]. The water-free reaction medium has also a remarkable impact on the surface chemical properties. Unlike oxides obtained by the conventional method that are rich in hydroxyl groups, the surfaces of nanoparticles obtained by the nonhydrolytic

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5 Evolution of Sol–Gel Chemistry

Scheme 5.1 M–O-M bonds formation by nonhydrolitic sol–gel approaches

method expose organic groups that may belong to the precursors, the oxygen donor, or the solvent. In addition to the hydrophobic character they impart to the particles, these organic groups can promote the formation of nanoscale crystallites and possibly be used for subsequent surface functionalization. In conclusion, non-hydrolytic sol–gel can be seen as a versatile one-pot procedure that relies on organic reactions while providing ample leeway to address specific requirements concerning the composition, texture, structure, and morphology of the final material. NHSG is a particularly well-established and powerful methodology for the synthesis of mixed oxide catalysts, where adjustable porosity, homogeneous dispersion of metal cations and active surfaces are particularly sought-after features [16].

5.3 The Pechini Method When Maggio Paul Pechini worked for the Sprague Electric Company in Massachusetts in the 1960s, ceramic capacitors were prepared by a conventional solid-state reaction method. The limitations associated to the latter, such as the nonhomogeneity of the final product, long milling times, high temperature, and the presence of contaminants, prompted Pechini to investigate an alternative preparation process [20]. The method named after him was developed for the preparation of lead and alkaline-earth titanates and niobates and patented in 1967 [21]. The Pechini method is a wet chemical method that can be considered a modification of the sol–gel method. This procedure is particularly helpful when alkoxides cannot be used because they are expensive, too reactive or simply unavailable. Since its original definition, the ’Pechini process’ has continuously evolved towards the synthesis of many metal oxides in bulk shape, as nanocrystalline powders and in thin films, and it is currently applied all over the world at the industrial level.

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Fig. 5.3 Pivotal reactions of the Pechini process. Adapted with permission from [22]; Copyright 2004 Springer

The chemical reactions behind the Pechini approach, and which replace those of hydrolysis and condensation of alkoxides, are mainly complexation and esterification [22]. As illustrated in Fig. 5.3, a complex is generated between the metal cation and a hydroxycarboxylic acid. The polybasic carboxylic acid chelates are then cross-linked with the addition of polyalcohols that bind to the carboxyl groups via esterification reactions. The demand for less organic matter, more nature-friendly compounds for a more sustainable process, and an extension of the method to a large number of metals drove research towards a modification of the method by introducing water as the main solvent medium and replacing citric acid and ethylene glycol with other carboxylic acids and polyols, respectively. In a typical preparation, metal salts (typically nitrates, chlorides, acetates) are dissolved in water in the presence of a tricarboxylic acid and a polyalcohol. The mixture is then heated to remove the water and obtain the polymer -like resin. Finally, the elimination of organic compounds is promoted by the subsequent heat treatment at around 400 °C [23]. The feasibility, versatility, and relatively low cost together with the high degree of purity and homogeneity achievable in the preparation of mixed oxide systems makes the method very attractive. Nevertheless, some weaknesses of the method in synthesis design must be considered. Careful pH control is imperative to avoid precipitation of individual hydroxides that would deviate the composition of the final product from its nominal composition. It is generally optimized using ammonia, ammonium hydroxide or other bases. pH is also a pivotal factor in determining the stability of the complex. From this point of view, for some metals, such as alkaline earth metals, the stability of the chelated complex may require the use of additional complexing agents, like EDTA (Ethylenediaminetetraacetic acid).

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References 1. L. Nicole, C. Laberty-Robert, L. Rozes, C. Sanchez, Hybrid materials science: a promised land for the integrative design of multifunctional materials. Nanoscale 6, 6267 (2014) 2. M. Faustini, L. Nicole, E. Ruiz-Hitzky, C. Sanchez, History of Organic-inorganic hybrid materials: Prehistory, art, science, and advanced applications. Adv. Funct. Mater. 28, 1704158 (2018) 3. J.D. Mackenzie, Sol-Gel Research—Achievements since 1981 and prospects for the future. J. Sol-Gel Sci. Technol. 26, 23–27 (2003) 4. C. Sanchez, F. Ribot, Design of hybrid organic-inorganic materials synthesized via the sol-gel process. J. Chem. 18(10), 1007–1047 (1994) 5. G.J. Owens, R.K. Singh, F. Foroutan, M. Alqaysi, C.M. Han, C. Mahapatra, H.W. Kim, J.C. Knowles, Sol-Gel based materials for biomedical applications. Prog. Mater Sci. 77, 1–79 (2016) 6. U. Díaz, D. Brunel, A. Corma, Catalysis using multifunctional organosiliceous hybrid materials. Chem. Soc. Rev. 42, 4083 (2013) 7. Alain C. Pierre, Introduction to Sol-Gel processing, cap 10 Hybrid Organic–Inorganic and Composite Materials https://doi.org/10.1007/978-3-030-38144-8 8. M.M. Collinson, Recent trends in analytical applications of organically modified silicate materials. TrAC, Trends Anal. Chem. 21(1), 31–39 (2002) 9. J.M. Notestein, A. Katz, Enhancing heterogeneous catalysis through cooperative hybrid organic–inorganic interfaces. Chem.–Eur. J. 12, 3954–396 (2006) 10. E.L. Margelefsky, R.K. Zeidan, M.E. Davis, Cooperative catalysis by silica-supported organic functional groups. Chem. Soc. Rev. 37, 1118–2112 (2008) 11. A. Corma, U. Díaz, T. García, G. Sastre, A. Velty, Multifunctional hybrid organic−inorganic catalytic materials with a hierarchical system of well-defined micro- and mesopores. J. Am. Chem. Soc. 132(42), 15011–15021 (2010) 12. V. Ayala, A. Corma, M. Iglesias, J.A. Rincón, F. Sánchez, Hybrid organic—inorganic catalysts: a cooperative effect between support, and palladium and nickel salen complexes on catalytic hydrogenation of imines. J. Catal. 224, 170–177 (2004) 13. C. Ortiz, E. Jackson, L. Betancor, Immobilization and stabilization of enzymes using biomimetic silicification reactions. J. Sol-Gel Sci. Technol. 102, 86–95 (2022) 14. E. Parandi, M. Safaripour, M.H. Abdellattif, M. Saidi, A. Bozorgian, H. Rashidi Nodeh, S. Rezania. Biodiesel production from waste cooking oil using a novel biocatalyst of lipase enzyme immobilized magnetic nanocomposite. Fuel Volume, 313, 123057 (2022) 15. A. Styskalik, D. Skoda, C.E. Barnes, J. Pinkas, The power of non-hydrolytic sol-gel chemistry: A review. Catalysts 7, 168 (2017). https://doi.org/10.3390/catal7060168 16. D.P. Debecker, P.H. Mutin, Non-hydrolytic sol–gel routes to heterogeneous catalysts. Chem. Soc. Rev. 41, 3624–3650 (2012) 17. Y. Wang, M. Bouchneb, R. Mighri, J.G. Alauzun, P.H. Mutin, Water formation in non-hydrolytic sol-gel routes: selective synthesis of tetragonal and monoclinic mesoporous zirconia as a case study. Chem. Eur. J. 27, 2670–2682 (2021) 18. P. Arnal, R.J.P. Corriu, D. Leclercq, P.H. Mutin, A. Vioux, Preparation of anatase, brookite and rutile at low temperature by non-hydrolytic sol–gel methods. J. Mater. Chem. 6, 1925–1932 (1996) 19. Y. Wang, M. Bouchneb, J. G. Alauzun, P. Hubert Mutin, Tuning texture and morphology of mesoporous TiO2 by non-hydrolytic sol-gel syntheses. Molecules, 23, 3006; (2018) https:// doi.org/10.3390/molecules23113006 20. L. Dimesso, Pechini processes: an alternate approach of the sol–gel method, preparation, properties, and applications in handbook of sol-gel science and technology, L. Klein et al. (eds.), Springer Int. Publ. Switz. (2016). https://doi.org/10.1007/978-3-319-19454-7_123-1 21. M.P. Pechini, Method of preparing lead and alkaline earth titanates and niobates and coating method using the same to form a capacitor. US Patent 3330697A, 7 November (1967)

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22. D.S. Gouveia, R. Rosenhaim, M.A.M.A. de Maurera, S.J.G. Lima, C.A. Paskocimas, E. Longo, A.G. Souza, I.M.G. Santos, Thermal study of CoxZn7-xSb2O12 spinel obtained by pechini method using different alcohols. J. Therm. Anal. Calorim. 75, 453–460 (2004) 23. T.O.L. Sunde, T. Grande, M.A. Einarsrud, Modified pechini synthesis of oxide powders and thin films in handbook of sol-gel science and technology, L. Klein et al. (eds.), Springer Int. Publ. Switz. (2016). https://doi.org/10.1007/978-3-319-19454-7_130-1

Chapter 6

Synthetic Strategies for (Supported) Metal and Metal Oxide Catalysts: Case Studies

Abstract As outlined in previous chapters, the sol–gel method, which originated around the preparation of silica, has developed massively over time, generating a plethora of different methodologies and chemistries. This inexhaustible source of synthetic pathways has been a powerful tool at the research community’s disposal to prepare increasingly advanced catalytic formulations. Nevertheless, by using more sophisticated chemistry compared to conventional synthesis methods, challenges can be encountered in obtaining the desired products. These can be overcome by adopting the appropriate strategies and selecting the most suitable route bearing in mind the final composition and required properties. The optimal way to understand how to approach this subject in order to design reliable and sustainable syntheses that allow the control of composition, structure and surface properties, is to exploit some case studies. The selected examples cover some of the main fields of application of metal and oxide supported catalysts in heterogeneous catalysis. Keywords Acid catalysts · Supported metal catalysts · Reverse micelles approach · Photocatalysts

6.1 Acid Catalysts • P2 O5 –SiO2 Oxide-based systems containing P2 O5 have been the subject of considerable interest in the field of sensor chemistry due to their high proton conductivity [1–4]. In particular, phosphosilicate glasses have proven to be excellent candidates as sensing elements in integrated humidity sensors. The need to understand the conduction mechanism and to identify the role and contribution of surface hydroxyl groups and phosphorus distribution within the silica matrix has promoted extensive research on these materials using complementary characterization techniques, among which nuclear magnetic resonance has played a primary role. The authors [5, 6] performed 1 H MAS-NMR spectroscopy with 1 H T1 and T1ρ relaxation times as well as low-temperature 1 H solid-echo NMR on gels of composition 10P2 O5 -90SiO2 and 30P2 O5 -70SiO2 to correlate differences in proton conductivity with the size of phosphorus domains. A uniform distribution of small POH © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Esposito, Sol–Gel Synthesis Strategies for Tailored Catalytic Materials, SpringerBriefs in Materials, https://doi.org/10.1007/978-3-031-20723-5_6

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domains that give rise to dynamic processes was showed only by the sample with a high phosphorus content. As evidence of this, a single narrow 1 H resonance connected to the POH groups was shown. The sample with the lowest phosphorus content heattreated at 400 °C present an inhomogeneous distribution of POH groups, not allowing a proton hopping pathway to be formed. More recently, Esposito et al. [7] used an extrinsic probe, the muon, to further investigate the mechanism of proton conductivity in phosphosilicate gels. The work explored both the nature of the sites involved and the proton dynamics using muon spin relaxation. Another peculiar feature of silicophosphates is the formation of hexacoordinate silicon in certain crystalline phases, including Si5 O(PO4 )6 , Si3 (PO4 )4 and SiP2 O7 . A reproducible synthesis of silicophosphates could also be complicated by the existence of numerous polymorphs of crystalline silicon diphosphate with octahedrally coordinated silicon [8–10]. With regard to catalysis, the presence of Brønsted acid sites makes silicophosphates valuable materials for acid catalysis [8, 11]. A significant number of processes in the petrochemical industry requires the use of acid catalysts, which, depending on the type of reaction, must feature Brønsted or Lewis sites with a certain strength [12, 13]. Although they are widely used in hydrocarbon processing reactions, there is much interest in the development of advanced active and low-cost formulations for the transformation of biomass into value-added products. A great deal of effort is devoted to the production of green products through the use of renewable resources to improve environmental impact. Efficient conversion pathways, such as hydrolysis of cellulose, dehydration of saccharides, acetylation, alkylation, and acylation, are catalysed by acids. Environmentally friendly chemical processes [14]. In this scenario, design of solid acid catalysts has a central role to establish environmentally friendly catalytic processes. A basic task of the present review is the pivotal role of the synthesis route to consciously tailor the features of the final product to comply with the requirements of the catalytic reaction. The first bottleneck encountered in the synthesis of silico-phosphates by the conventional hydrolytic method is the choice of silicon and phosphorus precursors. While the choice of silicon precursor seems obvious, considering the well-known sol–gel chemistry of TEOS, the exact choice of phosphorus precursor can be critical and dramatically affect the extent of copolymerization between silicate and phosphate units, the type and the relative amount of crystalline phases, the final composition and the textural and surface properties [15]. Phosphates and silicates have similar crystal structures, consisting of tetrahedral [PO4 ]3 and [SiO4 ]4 units, respectively. Despite this, the reactivity of TEOS can be increased with acidic or basic catalysis whereas drastic conditions, such as heating under reflux for prolonged times, result in only partial hydrolysis of the triethylphosphate PO(OEt)3 . The homo-condensation of the silanol groups is thus favoured and the silica fraction grows around the phosphorus precursor mostly present in monomeric form [15–17]. Phosphorus-containing molecules trapped and weakly interacting with the silica matrix easily evaporate during heat treatment and the composition of the final product deviates from the nominal one. Furthermore, it is worth bearing in mind that the Si–O–P bond is sensitive to the presence of water, which easily causes its hydrolysis [18, 19].

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The effectiveness of the phosphorus precursor was revealed by in-depth studies of the structure of the silicophosphate gels, with particular reference to the extent of Si–O–P bond formation. Nuclear magnetic resonance in the liquid and solid state on the 1 H, 31 P and 29 Si nuclei was decisive in establishing that, after 10 months, phosphorus was still only present in the pristine alkoxide TEP (triethylphosphate PO(OEt)3 ) [17]. In the same study, Schrotter et al. revealed how the use of alternative precursors to TEP, namely P(OEt)3 and (OEt)2 P–O–P(OEt)2 , led to complete hydrolysis to phosphoric acid via intermediate species. However, rapid and uncontrolled hydrolysis could cause precipitation or the formation of inhomogeneous gels. Furthermore, different crystallisation behaviour was observed by varying the phosphorus precursor, with quite controversial results, probably due to the Si/P ratio and the adopted synthesis route. Szu et al. [20] obtain amorphous gels even after prolonged treatment at 800 °C using TEP. Conversely, Matsuda et al. [21] observe crystallisation of tetragonal and monoclinic SiP2 O7 in gels calcined at 450 and 600 °C. There is a quite large numbers of paper reporting attempts to prepare crosslinked Si–O–P inorganic network using phosphoric acid as precursor. As mentioned above, the major concern arises from its very rapid hydrolysis compared to the silica precursor. However, the presence of monomeric or dimeric units containing POH groups leads to greater interaction with the silica matrix and, therefore, less P losses during heat treatment compared to preparations with TEP [19–21]. Still within the hydrolytic route, Esposito et al. reported an intriguing synthetic strategy that overcomes the obstacles encountered with the phosphorus precursors mentioned above. The authors succeed in obtaining gels with 10 and 30 mol % of P2 O5 without loss of phosphorus, avoiding time-consuming steps and drastic operating conditions [9, 18]. The adopted approach entails the use of phosphorus oxychloride, POCl3 , whose reactivity is controlled by replacing part of the chlorine atoms with OR groups. POCl3 is mixed with anhydrous ethyl alcohol in a 1:6 molar ratio obtaining POCl3 1x(OEt)x species bearing a reduced partial positive charge on phosphorus compared with the initial precursor. MAS-NMR characterization provides evidence that the degree of Si–O–P cross-linking is closely dependent on phosphorus content, while the temperature of heat treatment may contribute to the self-condensation of POH groups. Evidence of Si–O–P bond formation in the gel containing 30 mol% P2 O5 is provided by the 29 Si resonances at −211 and −215 ppm, which are characteristic of six-coordinate silicon in phosphosilicates. An equally fascinating approach is that proposed by Styskalik et al. [8] They obtain an inorganic Si–O-P cross-linked network featuring surface residual organic groups by selecting a nonhydrolytic ester elimination pathway. The degree of Si–O– P condensation can be handled through the synthesis parameters, such as solvent, temperature and reaction time. Conversely, formation of trapped oligomeric phosphorus species is not observed. This procedure, in its optimized condition, produces xerogels with remarkable surface area. • Ru-Nb2 O5 –SiO2 A worldwide matter of concern that has a dramatic environmental and social impact is the growing demand for energy and the urgent need for renewable resources.

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Fig. 6.1 Hydrogenation of levulinic acid

Currently, biomass-based energy system is the most reliable option to diminish reliance on fossil fuels through the development of green and sustainable chemistry. One of the main benefits of biomass-based fuel (sometimes called as biofuel) is zero carbon dioxide release as compared to fossil fuel, not contributing to the global warming [22]. In addition, biomass has a high potential for the production of value-added products that drive the research community to constantly search for valuable and sustainable reaction pathways [23]. From this point of view, a notable example of biomass valorisation is represented by the hydrogenation of levulinic acid (LA) into γ-valerolactone (GVL), Fig. 6.1. The levulinic acid can be considered a valuable and versatile molecule for the production of renewable fuels, fuel additives, green solvents, polymers, value-added fine chemicals, flavouring agents and resins. Actually, LA grabs the industrial interest because it possesses both keto and carboxylic acid groups and it can undergo a series of chemical reactions such as esterification, hydrogenation, halogenation, and oxidation. However, to highlight the value of LA, the most promising reaction is the hydrogenation to γ-valerolactone (GVL), a green platform molecule that has gained considerable attention in the last decade [24–26]. The reaction pathway of levulinic acid to GVL at low temperature can be described by a first step of hydrogenation to γ-hydroxypentanoic acid (HPA) followed by a second step in which intramolecular lactonisation to GVL occurs [27, 28]. A literature survey of the aqueous-phase hydrogenation of biosourced molecules shows that ruthenium metal particles supported on various carbons and oxides were the most efficient catalysts to achieve a rapid and selective conversion of carbonyl functionalities into the corresponding alcohols. This type of mechanism highlights the need for catalysts characterised by a dual redox-acid functionality. Ru/C, being characterised by the presence of both metal and acid sites, is the commercial catalyst commonly used for this reaction, but it has the non-negligible inconvenience of a slow but irreversible deactivation, probably due to the sintering of the ruthenium particles and the formation of carbon deposits on the surface of the catalyst [29]. Furthermore, the authors [28] point out that the yield at GVL can be significantly increased by using an acidic co-catalyst, such as commercial niobium phosphate (NbOPO4 , NBP) or niobium oxide. This implies that the process can be further optimised with the use of a ternary system containing both functionalities. The preparation of a ternary system by the wet route could be rather complicated, as the design of the synthesis route should consider the different chemistry of the precursors.

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To prepare a bifunctional catalyst, Sun et al. [30] followed a wet chemistry route using H3 PO4 and NbCl5 as precursors for the acid fraction. They succeeded in obtaining a mesoporous niobium phosphate using a structure directing agent P123 through a somewhat time-consuming multistep procedure with a double autoclave treatment initially at 313 K and subsequently at 348 K. The fraction with redox properties consisting of Ru particles is embedded by adsorption of colloidal Ru nanoparticles on solid acids. The colloidal Ru nanoparticles were prepared by the reduction of RuCl3 with ascorbic acid in a suitable solvent. The autoclave time, the type of solvent and the temperature proved to be useful parameters for tailoring the size of Ru NPs. Rather than anchoring or trapping ruthenium in the acid matrix, the authors [27, 29] developed a one-step procedure by operating with a reaction environment containing both niobium and silicon precursors. The purpose was to guarantee significant dispersion of the ruthenium nanoparticles for an active and stable catalyst. Niobium chloride was mixed with ethyl alcohol to control the rate of hydrolysis, obtaining less reactive species, Nb(OEt)5-x (Cl)x . This idea echoes the one devised for the precursor POCl3 in the preparation of silico-phosphates, vide supra. The solution containing niobium was mixed with an alcoholic solution of TEOS containing a certain amount of RuCl3 ·3H2 O. A dark gel was obtained by dropwise addition of water to a final H2 O/TEOS ratio equal to 4/1. The procedure is efficient, requiring few and simple steps, without the use of elaborate equipment and conducted at room temperature. The obtained catalysts can be described as mixed oxo network of niobium and silicon in which ruthenium was uniformly dispersed, avoiding segregation. Notable surface area (between 400 and 500 m2 g−1 ) were obtained without any surfactant. Catalysts containing Ru NPs are obtained through a reduction pre-treatment in H2 . While ruthenium effectively hydrogenated LA to HPA, the subsequent lactonisation reaction was promoted by the presence of niobium oxide, which was responsible for the increased acidity of the catalyst. Finally, cyclic tests have proven the stability of the catalysts [27, 29].

6.2 Highly Dispersed Supported Metal Catalysts • Co-SiO2 Cobalt-based catalysts play a starring role in the field of heterogeneous catalysis [31–37]. They can offer comparable or even better performance than noble metals, with the considerable advantage of being more affordable. In oxidised form, they are successfully used in the oxidation reactions of organics [31]. A reaction with a strong environmental impact is the oxidation of volatile organic compounds, VOCs, whose presence is harmful to air quality and human health. The origin of VOCs emissions is diverse, including mobile sources in urban settings and manufacturing

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and processing industries [32, 33]. Mixed valence cobalt oxide, Co3 O4 , is extensively reported as the most active metal oxide for total oxidation [34, 35]. The redox chemistry of cobalt, Co3+ ↔ Co2+ ↔ Co, hence the cobalt species that are formed and their interaction with the support represent a crucial point in the design of a material with high performance. For this reason, a big effort has been spent to tailor the materials features through the variation of the synthesis parameters [36]. Cobalt oxide chemistry is characterized by two stable oxides, CoO and Co3 O4 , being the latter the most stable at room temperature. Co3 O4 can be described as a mixed valence oxide, (AB2 O4 ), where the Co2+ ions occupy A sites with tetrahedral (Td) symmetry, while those in the B sites with octahedral (Oh) symmetry are trivalent (Co3+ ) [38]. In wet chemical methods, Co2+ salts are generally used because [Co(H2 O)6 ]2+ is stable in aqueous solution. Conversely, [Co(H2 O)6 ]3+ is a strong oxidising agent and in aqueous solution, unless acidic, it rapidly decomposes by oxidising water with oxygen evolution. Consequently, unlike Co (II), Co (III) provides few simple salts and those obtained are unstable. B. Puértolas et al. [32] used a modified Pechini method to prepare unsupported Co3 O4 as a catalyst for the total oxidation of propane. Cobalt II nitrate and organic acids (glyoxylic acid or ketoglutaric acid) were employed in the synthesis. Despite the simplicity of the procedure, the method proved ineffective in achieving an improvement in catalytic activity compared to the reference catalysts. The low surface area was considered the main factor negatively affecting catalytic performance. To improve textural and structural properties without involving too complex and difficult operations, cobalt oxide is generally dispersed on a porous support, with an increase in the number of active sites. SiO2 is one of the most attractive media on account of its versatility, tailorable porosity, excellent chemical and thermal stability and, not least, because it can be easily surface-modified with organic groups allowing the anchoring of specific catalytic centres. In addition, the type of silica morphology and texture can drive the formation of specific Co phase(s) and also affect the cobalt oxide particles size and their distribution in silica structure [39, 40]. An intriguing and fascinating approach to shed light on the relationship between structure and activity of silica-supported cobalt oxide in the catalytic oxidation of carbon monoxide was reported by Eurov et al. [40]. They prepared a series of silicas with different pore structures using a sol–gel approach, following the protocol described in Fig. 6.2. Mesoporous silica particles were obtained by hydrolysing TEOS in a hydroalcoholic solution in the presence of cetyltrimethylammonium bromide (CTAB) while micro-mesoporous SiO2 particles with a pore size of 0.9–3.5 nm were synthesised using a modified silica precursor, 3-(methacryloxy)propyl]trimethoxysilane (MPTMOS), along with TEOS. The particles were decorated with cobalt oxides using the capillary impregnation method. Cobalt is essentially found as Co3 O4 located on the outer surface of the silica particles or as Co2+ interacting with the substrate inside the pores. Cobalt, in its oxidised form, has gained a distinctive position in one of the most significant processes in the field of renewable and clean energy resources, the oxygen

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Fig. 6.2 Schematic illustration of the Co-SiO2 composites preparation. Adapted with permission from [40]; Copyright 2022 Elsevier

evolution reaction (OER) [41]. In the work of Tüysüz et al. [42], porous silicas were used as hard templates, taking the advantage of the nanocasting method, for the preparation of designed Co3 O4 catalysts as electrocatalyst for water oxidation. In particular, cubic ordered mesoporous silicas, KIT-6, with different textural parameters, were used as a hard template to fabricate mesoporous ordered cubic Co3 O4 . The adopted strategy provided a catalyst with an open structure and a high surface area, which proved to be the pivotal factors in achieving remarkable activity. Mesoporous cobalt oxide also showed excellent structural stability during the electrolysis of water [42]. Cobalt is an efficient catalyst not only as a stable oxide but also in its metallic state. Indeed, there are processes, such as the Fischer–Tropsch synthesis for the production of liquid hydrocarbons from syngas, which require the catalyst to be activated by reduction treatments at a temperature above 350 °C in order to obtain the active phase Co0 . In this case, mixed cobalt oxide is considered the precursor to metallic cobalt owing to its reducibility at low temperature, but, above a certain particle size, its formation is symptomatic of a low dispersion of cobalt species, which is detrimental to catalytic activity. On the other hand, increased interaction between the metal and the matrix can lead to a cobalt silicate phase that is hardly reducible [43, 44]. Therefore, an effective synthesis design should allow the definition of new pathways driving the formation of specific cobalt phases by achieving the delicate balance between reducibility, dispersion and reactivity. The use of surfactants in the supramolecular templating approach has been massively explored. For this reason, some strategies that make unconventional use of surfactants will be reported.

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Recently, Olguin et al. [45] employed a short cationic surfactant, 3-hexyl triethyl ammonium bromide, at a concentration below the critical micelle concentration point (CMC), to prepare microporous cobalt oxide silica. They observed how the amount of the Co3 O4 is tunable through the concentration of the quaternary ammonium surfactant. Indeed, at low surfactant amounts, the formation of CoBrx 2−x species in the xerogel facilitates the formation of Co3 O4 during heat treatment. The reverse effect is observed with increasing surfactant due to interactions between the CoBrx 2−x species, the surfactant head groups and the silica surface. Keeping the cobalt precursor content unchanged, the authors also explored the effect of the carbon number of quaternary ammonium surfactants. The point at which the decrease in cobalt oxide occurs was found to be a function of the length of the alkyl chain of the surfactant. Quaternary ammonium salts with a longer chain cause an earlier transition to the formation of cobalt species more strongly interacting with the silica matrix. The pore size was also tailored by the increase in surfactant, with higher loading promoting mesoporosity. Still working under conditions that circumvent amphiphile aggregation or micelle formation, the authors [39] explored a challenging route for the immobilization of highly dispersed cobalt species in the silica framework in high metal loading using a non-ionic surfactant. The foremost task of this approach was to identify a single molecule acting both as a porogenic template to modify textural properties and as an oxygen-rich complexing agent for metal species. In this way, the dual result of modifying textural properties while simultaneously acting on cobalt dispersion, even at high cobalt contents, can be attained. Following the procedure, the polyoxyethylene (10) cetyl ether, Brij C10, was added to a solution containing partially hydrolyzed Si(OR)x(OH)y and [Co(H2 O)6 ]2+ aquo-ion, Fig. 6.3. TEOS was hydrolysed at 50 °C without any alcoholic solvent, by using concentrate HNO3 as catalyst, in the following molar ratio TEOS: H2 O: HNO3 = 1: 4: 0.01. The absence of an alcoholic solvent sets the system in the “immiscibility” state. However, as TEOS hydrolysis takes place with both water consumption and alcohol production, the system moves towards the miscibility region. This approach allowed high concentrations of hydro-soluble inorganic salts to be introduced into the reaction system. For comparison, the authors prepared the same compositions, CoOxSiO2 featuring 10 and 30 mol % Co, without using the surfactant. The pre-reduced catalysts were tested in the ethanol steam reforming reaction [39]. The steam reforming of biofuels, such as ethanol, is an excellent opportunity to produce hydrogen from renewable sources [46]. The main challenge, as we have seen for the processes mentioned aforementioned, is the development of cost-effective catalysts with a high conversion efficiency, while limiting the use of noble metals. Several studies have recognised nickel and cobalt as very valuable alternatives due to their high C–C bond cleavage activity [47]. The overall ethanol steam reforming (ESR) reaction can be represented by Eq. (6.1), but this process involves several steps and reactions leading to the formation of by-products, such as carbon monoxide, methane, acetaldehyde or ethylene, which are also coke precursors.

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Fig. 6.3 Non-ionic surfactant assisted Sol–Gel route

CH3 CH2 OH + 3H2 O → 2CO2 + 6H2

(6.1)

A suitable ESR catalyst must then meet stringent requirements, not only promote an increased conversion of ethanol into hydrogen, but also prevent possible undesirable reactions and limit cook formation. Surfactant-assisted sol–gel synthesis has emerged as a viable strategy to deal with the redox chemistry of cobalt while simultaneously achieving outstanding surface area. Notably, the chelating action of the surfactant substantially increased the fraction of reducible dispersed cobalt species in gels containing 10 mol % Co. At higher cobalt loadings, the surfactant pathway prevents Co agglomeration leading to finely dispersed species. Although reducible at higher temperatures compared to the segregated phase of Co3 O4 obtained in the reference sample, they potentially produce the same amount of metallic cobalt [15]. • Cu-ZrO2 Compared to natural gases, alcohols derived from biomass are undoubtedly fascinating sources for hydrogen production, as they are carbon–neutral. In particular, methanol features extremely attractive characteristics compared to alcohols with a higher carbon number. The absence of carbon–carbon bonds makes hydrogen extraction more feasible, with considerably lower operating temperatures than those required for ethanol steam reforming [48, 49]. Although the selection of the best cost-effective raw material should be based on the overall economics of the process and the local government policies, methanol is highly recommended for its safe handling, low cost, high energy density and flexibility, being generated by from multiple sources [48]. Extensive research has been conducted over the years to find valuable and efficient alternatives to noble metals-based catalysts, with the aim of ensuring maximum activity and selectivity for higher hydrogen yield and lower CO production.

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Copper-based catalysts have been identified as particularly promising in the steam reforming reaction though the literature clearly reveals that catalyst performance can be dependent on many cross-cutting factors such as composition, the presence of promoters, the nature and acidity of the support, and the physico-chemical properties often closely related to the adopted synthetic route. The selection of the carrier is then of utmost importance because it can contribute decisively to the dispersion of the active phase and its availability. It can also improve selectivity towards the desired product and improve the stability of the catalyst itself. Compared to the more common Al2 O3 and SiO2 supports, ZrO2 is of particular interest due to its weak acidity and basicity, its mechanical and thermal stability, and its redox behaviour [50–52]. Actually, Cu-ZrO2 is not only a suitable candidate for steam reforming, it is a high-potential catalyst in a number of reactions concerning the production of liquid fuels and chemicals from renewable raw materials such as biomass [50, 52, 53]. Polymorphic transitions in oxides, like those occurring in zirconium oxide, can have a strong impact on surface properties and, consequently, on the amount and distribution of active sites. From this point of view, the most sought-after synthesis routes are those allowing the tailoring of the crystalline phase. The preparation have to address some important key factors: metal dispersion, overall surface area and particles size. Well, despite the many advantages of the sol–gel procedure, the big issue when the preparation involves the transition metal alkoxides is their very fast reaction with water that lead to a precipitation instead of a homogeneous sol. As a consequence, there is poor control on composition and homogeneity. Although the hydrolysis may be controlled by coordination with suitable ligands, the addition of a metal salt can further complicate the system because it might hinder, in some extent, the formation of Zr complexes. The authors [50, 51] designed two different sol–gel routes that successfully led to homogeneous polymer gels consisting of CuOx copper oxide dispersed in a zirconia matrix, Fig. 6.4. The procedures were conceived based on two different copper precursors and two different approaches to control the hydrolysis of the zirconium precursor. The high polarity of the Zr-O bond makes Zr alkoxides highly susceptible to nucleophilic attack. They can hydrolyse upon simple exposure to ambient moisture. In addition, coordination expansion is a general tendency of zirconium alkoxides. It occurs spontaneously through solvation or oligomerisation and some zirconium alkoxides would present oligomeric molecular structures [54]. For these reasons, zirconium alkoxides must be stored in humidity-controlled environments and preparations of catalysts using zirconium alkoxides must be carried out in dry boxes. In a first approach, the hydrolysis of zirconium(IV) propoxide, used as a precursor in all preparations, was thoroughly controlled with the use of acetylacetone. Acetylacetone, however, can also coordinate Cu2+ in solution thus reducing its efficacy as a chelating agent for Zr. This assumption is supported by the increased content of

6.2 Highly Dispersed Supported Metal Catalysts

(a)

63

(b)

Fig. 6.4 Flow chart of the hydrolytic sol–gel synthesis of CuOx-ZrO2 by using a acetylacetone and copper nitrate or b acetic acid and copper acetate

acetylacetone required for a stable sol, compared to the preparation of pure zirconia [51]. Copper nitrate hydrate, Cu(NO3 )2 ·2.5H2 O, used as a precursor, was added dissolved in 1-propanol to counteract the formation of a precipitate caused by the water included in the salt structure. Finally, water for hydrolysis was added in mixture with 1-propanol, and the best ratio to avoid the formation of insoluble oligomeric species was (1/1.9), Fig. 6.4a. The bluish transparent gel obtained in 30 min can be described as a zirconia matrix obtained by polymerization of Zr(OPr)x(OH)y oligomers trapping copper species. The use of acetic acid and a different copper precursor, in a second alternative route, has required the development of a new procedure, Fig. 6.4b. Acetic acid needs to be handled carefully as it possesses contrasting functionalities. On the one hand it can behave as an acid catalyst in the hydrolysis reaction promoting further precipitation, on the other hand its function as a bidentate ligand agent of Zr alkoxide is reported [55]. The role of acetic acid as a modifier of transition metal alkoxide through ligand exchange can further complicate the control of hydrolysis kinetics. All the above may interfere with the condensation pathway and with the whole process of sol formation [56–60]. The authors studied the effect of the HAc/Zr molar ratio on gel formation observing that a rapid reaction with water, with massive precipitation, occurred for an acid/alkoxide ratio of less than 1.6. The formation of a stable sol without limiting gel formation is ensured by a HAc/Zr ratio of two, in agreement with the results of other authors [15, 61]. The in-situ water source from the esterification reaction between acetic acid and ethanol explained the lower amount of added water compared with the acetylacetone preparation.

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The preparation of copper zirconia catalysts is an example of the dramatic impact that the synthesis route can have on the performance of a catalytic material. The first noteworthy result was the modification of the thermal behaviour induced by the synthesis procedure with a shift to higher temperature of the crystallization peak for the sample prepared by acetylacetone and copper nitrate [50, 51]. This important finding is related to the possible modification of the crystal lattice of zirconium oxide by copper impurities. This exclusively involves Cu2+ ions since Cu+ ions have an ionic radius much larger than the one of Zr4+ . In this phenomenon, the copper precursor played a critical role since nitrate promotes an oxidizing environment favouring the formation of CuO whereas acetate decomposes initially giving cuprous oxide, Cu2 O, as the main product. The textural properties of the catalysts, obtained by reduction at 300 and 450 °C, were severely affected by the crystallization behaviour of the xerogels. Indeed, the materials prepared by acetylacetone and copper nitrate retained remarkable values of surface areas and pore volumes after the reduction treatment. Finally, outstanding values of metal dispersion, function of the temperature of the reduction, were obtained. On account of the aforementioned remarkable properties, the catalysts obtained by the route a, Fig. 6.4 offered the best catalytic results in terms of methanol conversion, H2 yield, and lower amount of secondary products. These results were also very satisfying compared with the one reported in literature considering the lower amount of copper and considering the absence of a promoter. A hydrolytic sol–gel procedure was also proposed by Algorabi et al. [52] for the preparation of Cu-ZrO2 catalysts for the hydrogenation of furfural, Fig. 6.5. The synthesis strategy adopted to obtain homogeneous gels called for the use of zirconium propoxide diluted in isopropyl alcohol and ethylacetoacetate as a complexing. Copper (II) nitrate trihydrate is added to the reaction mixture maintained at 60 °C until gelation. Cu/ZrO2 sol–gel catalysts loading 4, 12, and 16% were prepared using this protocol. Crystallisation of tetragonal zirconia was observed in all samples but only for low copper contents a shift towards higher theta degrees was detected, indicating a certain isomorphic substitution of Zr4+ by Cu2+ . Evidence of the homogeneous nature of catalysts obtained from alkoxide-based sol–gel chemistry is provided by X-ray Photoelectron Spectroscopy (XPS) investigations. The authors speculate that the peak at 935.1 eV can be attributed to strongly interacting Cu2+ –O–Zr species.

Fig. 6.5 Schematic representation of hydrogenation of furfural

6.3 Reverse Micelle Approach for Photocatalysts

65

The results of the catalytic activity were very promising, although they revealed that the stability of the catalyst might be a matter of concern that deserves in-depth investigation. Indeed, the characterisation of the post-reaction materials showed a significant change in textural properties that could be the cause of the decrease in activity with reuse. Despite being less versatile than the alkoxide route, Pechini method may be the right strategy to make preparation less expensive, using non-toxic products, and more environmentally friendly. In the work of Majedi et al. [62], zirconium acetate was used as a zirconium precursor, while lemon juice extracted from lemon (Citrus aurantifolia) fruits was used as a source of organic α-hydroxycarboxylic acid with the function of a metal chelating agent. The polymer resin was obtained by polyesterification with ethylene glycol (EG) after reaction at 90 °C for 3 h. The authors also reported a successful attempt to limit particle aggregation phenomena with the use of a natural additive, sucrose. Structural and morphological characterizations of the samples revealed that zirconia crystallizes in its cubic phase with unit cell distortion from doping with Mg2+ and Ca2+ ions contained in lemon juice. The green method presented by the authors is certainly appealing and could be explored to prepare transition metal supported catalysts on zirconia.

6.3 Reverse Micelle Approach for Photocatalysts A major global concern affecting the environment and human health is climate change, mainly caused by the excessive use of fossil fuels and the release of CO2 . Simultaneously, human activities, agricultural production and growing population pressure all contribute to increasing levels of air and water pollution. On account of its low-cost, versatility and environmental friendliness, photocatalysis is considered one of the most promising green chemistry technologies to address environmental concerns [63, 64]. However, in order to look optimistically at photocatalysis and envisage it as a leading technology in the chemical industry, further investigations are needed to resolve some crucial aspects that limit the scale-up of the laboratory experiment. One of the main shortcoming lies in the low quantum efficiency of existing photocatalysts. Among semiconductors, TiO2 undoubtedly presents numerous advantages, as widely reported in the literature, in terms of low cost, durability and long-term photostability [65]. However, only about 4 per cent of the solar spectrum can be exploited with TiO2 catalysts, due to a relatively high band gap (about 3.2 per anatase) which precludes the adsorption of visible light. Furthermore, the high recombination rate of photogenerated electron–hole pairs on the semiconductor surface may further limit its photocatalytic activity. A possible strategy for increasing the efficiency of photocatalytic activity is the doping of the TiO2 lattice with heteroatoms [66]. The nature of the dopant, their concentration and the different positions in the TiO2 lattice, whether interstitial or substitutional, can have a different impact on

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semiconductor properties. Transition metals are often used as dopants in semiconductors to improve the optical response and harvesting efficiency of visible light, inducing the formation of band gap states and promoting the formation of oxygen vacancies [67]. In this scenario, it is extremely relevant to have a synthesis method enabling the chemical-physical properties of the catalyst to be shaped through surface engineering strategies. Among the different types of wet chemical processes, the reverse micelle approach is particularly effective in controlling the nucleation and growth of nanoparticles (NPs), while improving dopant dispersion. This is achieved by channelling reactions that usually take place in aqueous media into the small domains of reverse micelles, Fig. 6.6. The aqueous core of these micelles can be regarded as an ideal reactor boosting intimate contact between the precursors, achieving homogeneity and mixing on an atomic scale and ultimately promoting the inclusion of the dopant in the titania lattice [68, 69]. In addition, the use of the reverse micelle approach can be seen as a valuable alternative to traditional procedures where the use of complexing agents is mandatory to avoid uncontrolled precipitation. Outstanding values of surface area can be also achieved by water-in-oil microemulsion compared to conventional methods. The textural properties of the photocatalyst play a decisive role in improving the adsorption/desorption mechanism with the selected pollutant molecule by contributing to improved photocatalytic performance. Varma et al. [70] prepared a series of Cu–TiO2 samples with varying copper content (0.25, 0.5, 0.75, 1 wt%) using inverse micelle nanodomains where the oil phase consists of triton X-114 in hexane-toluene mixture (7:3 volume ratio). They found improved properties compared to Cu–TiO2 nanomaterials obtained by conventional sol–gel methods. In particular, the crystallite size of the Cu–TiO2

Fig. 6.6 Synthesis of doped titania by reverse micelles approach

References

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samples was much smaller, the surface area increased consistently with 0.5 wt. % of copper, and the absorption wavelength was shifted towards visible light with the increased doping of Cu on TiO2 . The sample with 0.5 wt% copper proved to be the most efficient in degrading levofloxacin (LFX) under a 40 W visible LED light source. Higher copper contents favour the formation of Cu-LFX complexes, worsening the performance of the photocatalysts [70]. A cage-like environment for the preparation of manganese-doped titania was obtained employing polyoxyethylene (20) oleylether (Brij O20) as surfactant and cyclohexane as the oil phase [68]. 97% titanium(IV)butoxide (Ti-(BuO)4 ) and manganese(II) nitrate tetrahydrate were used as precursors. The use of the reverse micelle method resulted to be extremely effective in increasing the dispersion of manganese both in bulk and on the titania surface. Furthermore, considering the ionic radii of Mn4+ and Ti4+ (octahedral Ti4+ r = 0.605 Å, octahedral Mn4+ = 0.53 Å), the progressive shrinkage of the unit cell as the manganese content increases evidenced the success of the preparation method in promoting substitutional doping [68]. Of all possible alternative to titania, cerium oxide has gained increasing attention as a photocatalyst due to its photochemical stability, ‘cost-effectiveness’ and ‘ecofriendliness’, and above all its peculiar redox chemistry. Indeed, cerium’s unique ability to switch from the Ce(III) to the Ce(IV) state generates a strong catalytic potential due to high oxygen mobility in the CeO2 lattice, without any structural modification of the fluorite structure. Moreover, titanium has been added to the list of critical raw materials since 2020 in UE justifying the urgent need for implementation of strategies toward new catalytic formulation [71]. Modification in size, shape and defects can be controlled by varying the synthesis conditions, in particular the type and nature of surfactant and the water/surfactant ratio. Reaction time and aging can be also investigated as impact factors on crystal growth and crystallinity [72]. The authors of Ref. [19] observed that textural and optical properties can be shaped by using non-ionic surfactants (Triton-x type) of different polar tail lengths. Indeed, the surface area increases significantly as the length of the polar tail increases, the particle size varies in relation to the radius of the aqueous centre, and the bad-gap decreases.

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