Sustainable Utilization of Carbon Dioxide: From Waste to Product 9789819928897

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Sustainable Materials and Technology

Mohammad Jawaid  Anish Khan Editors

Sustainable Utilization of Carbon Dioxide From Waste to Product

Sustainable Materials and Technology Series Editors Mohammad Jawaid,Laboratory of Biocomposite Technology, Universiti Putra Malaysia, INTROP, Serdang, Selangor, Malaysia Anish Khan,Centre of Excellence for Advanced Materials, King Abdulaziz University, Jeddah, Saudi Arabia

Sustainable Materials and Technology (SMT) book series publishes research monographs (both edited and authored volumes) showcasing the latest developments in the field and comprehensively covering topics such as: • • • • • • • • • • • • • • • • • • • • • • • • •

Recycling of waste into useful material and their energy applications Catalytic action of Nano oxides for efficient carbon reforming process Sustainable technologies for plastic transformation Bifunctional nanoparticles for sustainable water splitting applications Sustainable dying and printing New materials from waste Sustainable Manure Management and Technology: Potentials, Uses and limitations Sustainable Mechanical Engineering Approach Sustainable biochemistry for the improvement of health Sustainable development of Mechanical recycling of automotive components Sustainable -waste recycling and conversion in useful materials for different applications Sustainable development of inexpensive Nano-photo catalysts Sustainable development of recycling of discarded lithium ion batteries Modern sustainable cement and concrete Sustainable adsorbent for hazardous removal Sustainable superior electromagnetic shielding materials Excellent sustainable nanostructured materials for energy storage device Sustainable development of heavy metal detoxification from water Carbon dioxide utilization for sustainable energy Sustainable development in green syntheses of materials Environment friendly and sustainable cloth for garments application Sustainable design and application of eco-materials Nanoparticles for sustainable environment applications Sustainable remediation of industrial contaminated water towards potential industrial applications Biomaterials for sustainable bioremediations

Mohammad Jawaid · Anish Khan Editors

Sustainable Utilization of Carbon Dioxide From Waste to Product

Editors Mohammad Jawaid Laboratory of Biocomposite Technology Institute of Tropical Forestry and Forest Products Universiti Putra Malaysia Serdang, Malaysia

Anish Khan Center of Excellence for Advanced Materials Research King Abdulaziz University Jeddah, Saudi Arabia

ISSN 2731-0426 ISSN 2731-0434 (electronic) Sustainable Materials and Technology ISBN 978-981-99-2889-7 ISBN 978-981-99-2890-3 (eBook) https://doi.org/10.1007/978-981-99-2890-3 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 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 Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Contents

Organocatalytic Reductive Functionalization of Carbon Dioxide .......... 1 Vitthal B. Saptal, Swapna M. Gade, Gianvito Vilé, J˛edrzej Walkowiak, and Bhalchandra M. Bhanage CO2 Conversion via Catalytic Hydrogenation to Methanol, DME and Syngas .................................................................................................... 37 Muhammad Usman, Mustapha D. Garba, Zonish Zeb, Muhammad Israr, Safia Safia, Fatima Javed, Munzir S. Suliman, Bandar Alfaify, Mohammed A. Sanhoob, Naseem Iqbal, Muhammad Humayun, and Aasif Helal Similar Life Cycle Evaluation of Microalgae Development ...... for Non-energy Purposes Utilizing Diverse Carbon Dioxide Sources R. Gayathri, J. Ranjitha, and Vijayalakshmi Shankar

61

Microalgae Biotechnology and Chemical Absorption as Merged Techniques to Decrease Carbon Dioxide in the Atmosphere ..................... 91 Michele Greque de Morais, Gabriel Martins da Rosa, Luiza Moraes, Thaisa Duarte Santos, and Jorge Alberto Vieira Costa Hydrogen Creation and Carbon Sequestration by Fracking Carbon Dioxide ............................................................................................. 111 Zohal Safaei Mahmoudabadi and Alimorad Rashidi Carbon Dioxide Utilization and Biogas Upgradation Via Hydrogenotrophic Methanogenesis: Theory, Applications, and Opportunities ......................................................................................... 137 Thiyagarajan Divya, Kalyanasundaram Geetha Thanuja, Desikan Ramesh, and Subburamu Karthikeyan

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Contents

Carbon Dioxide Capture and Bioenergy Production by Utilizing the Biological System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 R. Gayathri, J. Ranjitha, and Shankar Vijayalakshmi A Review on Water–Gas Shift Reactions Energy Production by Carbon Dioxide Capture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Sanjeev Kumar Gupta

About the Editors

Dr. Mohammad Jawaid is currently working as Senior Fellow (Professor) at Biocomposite Technology Laboratory, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Serdang, Selangor, Malaysia, and also Distinguished Visiting Professor at Malaysian Japan International Institute of Technology (MJIIT), Malaysia. Dr. Mohammad Jawaid received his Ph.D. from Universiti Sains Malaysia, Malaysia. He has more than 20 years of experience in teaching, research, and industries. His area of research interests includes hybrid reinforced/ filled polymer composites and advanced materials. So far, he has published 75 books, 85 book chapters, more than 450 peer-reviewed international journal papers and several published review papers under top 25 hot articles in science direct during 2013–2018. He also obtained 6 Patents and 6 Copyrights. H-index and citation in Scopus are 79 and 28008, and in Google scholar, H-index and citation are 92 and 37479. He is founding Series Editor of Composite Science and Technology Book, Sustainable Materials and Technology and Smart Nanotechnology of Springer Nature. Beside that he is also Editor of Springer Proceedings in Materials. He is also an International Advisory Board member of Springer Series on Polymer and Composite Materials. He also in Editorial Board Member of Journal of Polymers and The Environment, Journal of Natural fibres, Journal of Plastics Technology, Applied Science and Engineering Progress Journal, Journal of Asian Science, Technology and Innovation and the Recent Innovations in Chemical Engineering. Besides that, he is also reviewer of several high-impact international peer-reviewed journals of Elsevier, Springer, Wiley, Saga, ACS, RSC, Frontiers, etc. Presently, he is supervising 8 Ph.D. students (5 Ph.D. as Chairman, and 3 Ph.D. as Member) and 5 Master’s students as Member in the fields of hybrid composites, green composites, nanocomposites, natural fiber-reinforced composites, nanocellulose, etc. 32 Ph.D. and 14 Master’s students graduated under his supervision in 2014–2022. Dr. Mohammad Jawaid received Excellent Academic Award in Category of International Grant-Universiti Putra Malaysia-2018 and also Excellent Academic Staff Award in industry High Impact Network (ICAN 2019) Award. Beside that Gold Medal-Community and Industry Network (JINM Showcase) at Universiti Putra Malaysia. He also Received Publons Peer Review Awards 2017, and 2018 (Materials Science), Certified Sentinel vii

viii

About the Editors

of science Award Receipient-2016 (Materials Science) and 2019 (Materials Science and Cross field). He is also Winner of Newton-Ungku Omar Coordination Fund: UKMalaysia Research and Innovation Bridges Competition 2015. Recently he recognized with Fellow and Charted Scientist Award from Institute of Materials, Minerals and Mining (IOM), UK. He is also life member of Asian Polymer Association, and Malaysian Society for Engineering and Technology. He has professional membership of American Chemical Society (ACS), and Society for polymers Engineers (SPE), USA. Dr. Anish Khanis currently working as an Associate Professor in the Centre of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia. Completed Ph.D. from Aligarh Muslim University, India in 2010. Completed Postdoctoral from the School of Chemical Sciences, University Sains Malaysia (USM) on electroanalytical chemistry in 2010). Working in the field of biosensors, polymers composite, organic-inorganic electrically conducting nanocomposites. More than 300 research articles, 103 book chapters, 61 books, two US Patent published in referred international Publishers also editorials also more than 30 Research projects completed, and more than 20 international conferences/workshops. Member of American Nano Society, Field of specialization is polymer nanocomposite/cation-exchanger/chemical sensor/micro biosensor/ nanotechnology, application of nanomaterials in electroanalytical chemistry, material chemistry, ion-exchange chromatography, and electro-analytical chemistry, dealing with the synthesis, characterization (using different analytical techniques) and derivatization of inorganic ion-exchanger by the incorporation of electrically conducting polymers. Preparation and characterization of hybrid nano composite materials and their applications, Polymeric inorganic cation–exchange materials, Electrically conducting polymeric, materials, Composite material used as Sensors, Green chemistry by remediation of remediation of pollution, Heavy metal ion selective membrane electrode, and Biosensor on neurotransmitter.

Organocatalytic Reductive Functionalization of Carbon Dioxide Vitthal B. Saptal , Swapna M. Gade, Gianvito Vilé, J˛edrzej Walkowiak, and Bhalchandra M. Bhanage

Abstract The catalytic transformation of carbon dioxide (CO2 ) into value-added chemicals and high-value fuels is the most promising strategy to mitigate the concentration of CO2 in the atmosphere. Among the various catalytic approaches used in the conversion of CO2 , its reductive functionalization is a very attractive way to access the array of interesting scaffolds such as formic acid, formates, formamides, methanol, and methane as key C1 building blocks for energy and synthetic chemistry. Several reports are available for the reductive functionalization of CO2 with homogeneous and heterogeneous catalysts using molecular hydrogen and other soft reducing agents like hydrosilanes and hydroboranes. Although these catalysts demonstrated excellent activity and selectivity, they suffer from several drawbacks like use of precious, toxic metals, and air- and moisture-sensitive complexes and generally follow the lengthy synthetic process. However, the use of transition metal-free catalysts or organocatalysts has attracted much attention in recent years because it follows the green chemistry rules, is cost-effective, and avoids contamination of products that are common by using transition metal catalysts. In recent years, numerous reports are raised in the utilization of well-defined organocatalysts for the reductive functionalization of CO2 due to its activity, selectivity, and sustainability. Here, we overview the use of organocatalysts for the reductive functionalization of CO2 using hydrosilanes, hydroboranes, and hydrogen as a reducing agent for the synthesis of value-added chemicals and fuels in recent days. Homogeneous transition metal and heterogeneous and heterogenized organocatalysts are out of the scope of this chapter. Keywords Catalytic reduction · Carbon dioxide · Organocatalysis · Hydrosilanes · Hydroboranes · CO2 reduction V. B. Saptal · J. Walkowiak Center for Advanced Technology, Adam Mickiewicz University, Uniwersytetu Pozna´nskiego 10, 61-614 Pozna´n, Poland S. M. Gade · B. M. Bhanage (B) Department of Chemistry, Institute of Chemical Technology (ICT), Mumbai 400019, India e-mail: [email protected] G. Vilé Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Politecnico Di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jawaid and A. Khan (eds.), Sustainable Utilization of Carbon Dioxide, Sustainable Materials and Technology, https://doi.org/10.1007/978-981-99-2890-3_1

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V. B. Saptal et al.

Abbreviations aNHC 9-BBN Ar atm Bu Cat Cat. DABCO DBU DMAP Et FLPs h Me Mes MIC NHC NHO Ph Pin Pr rt TBD THF

Abnormal N-heterocyclic carbene 9-Borabicyclo[3.3.1]nonane Aryl Atmosphere Butyl Catechol Catalyst 1,4-Diazabicyclo[2.2.2]octane 1,8-Diazabicyclo[5.4.0]undec-7-ene (4-Dimethylaminopyridine) Ethyl Frustrated Lewis pairs Hour(s) Methyl Mesityl; 2,4,6-trimethylphenyl Meso-ionic carbene N-heterocyclic carbene N-heterocyclic olefin Phenyl Pinacol Propyl Room temperature 1,5,7-Triazabicyclo [4.4.0] dec-5-ene Tetrahydrofuran

1 Introduction Anthropogenic activity generated excess amounts of greenhouse gases, creating significant environmental threats to humankind and the ecosystem in terms of global warming, ocean acidification, rising sea level, etc. Among them, the generation of excess amount of CO2 as a greenhouse gas by the burning of fossil fuel is highly responsible for global warming. Thus, the mitigation of CO2 is the highest interest of research from academic, industry, and environmental points of view [1]. Various strategies have been developed to capture conversion and use of anthropogenic CO2 to value-added chemicals and fuels [2]. Among them, the transformation of CO2 to value-added chemicals and fuel would be a great alternative to the conversion of CO2 [3]. However, the transformation of CO2 is a challenging task due to the highest oxidized form of carbon and its inert nature [4]. In addition to this, its thermodynamic stability and kinetic inertness hamper its applications in the synthesis of chemicals and fuel. Due to this CO2 is sluggish to react, it generally needs strong reagents, harsh

Organocatalytic Reductive Functionalization of Carbon Dioxide

3

conditions, and potential catalysts [5]. In this regard, catalytic conversion of CO2 is taking intense attention, and a range of well-defined homogeneous and heterogeneous catalysts has been developed. Although homogeneous organometallic catalysts are widely used, they utilize precious metals, are highly air- and moisture-sensitive, and require a lengthy ligand synthesis method [6]. In heterogeneous catalysis, there is a lack of well-defined active centers, and low yields and poor selectivity are the major issues. Recently, the use of well-defined organocatalysts for various transformations has been of high interest because they feature well-defined active centers, avoid the use of precious transition metals, are easy to synthesize, and can act as a replacement for transition metals [7]. Recently, a range of organocatalysts such as frustrated Lewis’s acid–base pairs (FLP), NHC’s, Lewis’ acids such as B(C5 F5 )3 , bases, and functional ionic liquids (ILs) demonstrated potential applications in catalytic activation of CO2 . Especially, reductive functionalization of CO2 with reducing agents like hydroboranes, hydrosilanes, and hydrogen is widely explored for various organic transformations (Scheme 1a). These reducing agents offer very exciting products from CO2 (Scheme 1b) such as formic acid (2e− ), formaldehyde (4e− ), methanol (6e− ), and methane (8e− ) [8]. Along with these reactions and using suitable coupling partners like amines, various reactions like N-formylation, N-formylation/cyclization, and Nmethylation were also achieved to synthesize the very important functional groups under mild reaction conditions. We sensed that no one has summarized this topic systematically, with mechanistic aspects toward the active centers of organocatalysts and its diverse substrate scope. Hence, in the present book chapter, we will review the recent progress by organocatalytic reductive functionalization of CO2 with hydroboranes, hydrosilanes, and hydrogen concisely and mechanistically. Reductive f unctionalization of CO 2 (a)

(b)

O H

H

OE

O C

OH

H3C-OH Organocatalyst

O C O

+ [E]H

R

E = Si, B, H R

H N H N

2eCHO

H H

CH3

C

H

8e-

4e-

CO2

H

O C

H

H 6e-

N H X

H

X = N, S

H C

OH

H

Scheme 1 a Reduction of CO2 to different chemicals and b reduction level of CO2 to various products

4

V. B. Saptal et al.

Organocatalytic hydroboration of CO2 Hydroboranes are very important reducing agents and are considered as soft reducing agents for a broad spectrum of reductive transformations. In recent years, various boranes have been used to reduce the CO2 . In 2013, Fontaine et al. used 1-Bcat-2PPh2 -C6 H4 (cat = catechol) as an ambiphilic organocatalyst for the reductive functionalization of CO2 using catechol boranes to synthesize CH3 OBR2 or (CH3 OBO)3 . Their easy hydrolysis generates methanol (Scheme 2a) [9]. High TON and TOF up to > 2950 and 853 h−1 have been noted. A range of boranes like catecholborane (HBcat), pinacolborane (HBpin), 9-borabicyclo[3.3.1]nonane (9-BBN), BH3 ·SMe2 , and BH3 ·THF has been screened. A reaction mechanism has been anticipated, as the presence of active centers of Lewis acid and base on the catalyst assists the catalysis by liberating the reduction products. In an isotope experiment study, it was found that when the adduct 1 formed, it remains as it is and acts as an active catalyst (Scheme 2b) [10]. Cascade activation of borane and CO2 using an ambiphilic catalyst is the key for these transformations. This concept of CO2 activation was inspired by the work of G. Erker, where they have used the base tBu3 P and B(C6 F5 )3 as an acid pair and FLP-CO2 as a solid product that was isolated [11]. Stephan and coworkers in 2014 applied stoichiometric tri-tert-butylphosphine (tBu3 P) for the reduction of CO2 using 9-BBN as a reducing agent which yielded (R3 PCH2 O)(HC-(O)O)B(C8 H14 ) [12]. The catalyst works like FLP pathway, and at the 60 ˚C, around 98% yields were observed with TON up to 5500 and a TOF of 170 h−1 (Scheme 3). Experimental evidence suggested that the reaction mechanism follows FLP to activate the CO2 molecule and the subsequent transfer of hydride to the electrophilic carbon center.

a)

B OB

1 mol% O

O

O

PPh2

O O

O

Bcat

B Ph

HB O

CO2 (1 atm), 23oC, 24h

Ph

O H3COB

R2P

O B O H H

CO2 + H[B]

cat.

CH3O[B]

C H2 1

O

b)

O

P

R2P

[B] = Bcat, BH2 R = Ph or iPr

Scheme 2 Reducing CO2 to methanol using frustrated Lewis pairs

O O B O

O

Organocatalytic Reductive Functionalization of Carbon Dioxide

CO2 R3P

R3P O [B]

H[B]

HH

H[B] = 9-BBN R3P

5

O

O

R = tBu

H R3P-[B]

H[B]

H

O R3P

O

[B]H

H[B]

O

-R3P H

O

[B]

+R3P

H

O [B]

H

O [B]

R3P H[B]

- O[B]2

H[B]

O H

PR3

H

O [B]OCOH

H

O

[B]

H

PR3

H

O [B]H

H[B] - O[B]2

H

H

O[B]

H

Scheme 3 FLP-catalyzed reduction of CO2 to methanol

As established by Stephan, FLP for CO2 activation, phosphine-bearing NHC (XX) adduct was synthesized and used as an intramolecular FLP by NHC ring expansion and 9-BBN (Scheme 4) [13]. This FLP comprises a strong basic site (P) and weak Lewis acidic site (B), which act as suitable FLP reagent for the hydroboration of CO2 with boron hydride and resulted in methoxyborane as a product with a TON of 240. In 2014, Fountain screened various base catalysts for the hydroboration of CO2 using the borane-dimethylsulfide (BH3 .SMe2 ) as hydroborating agent, among the various bases like MeONa, TBD, terpyridine, etc., and non-nucleophilic proton sponge demonstrated the highest activity with TOF up to 64 h−1 at 80 °C (Scheme 5) [14]. Hydroboration of CO2 was slow at room temperature, while at 80 °C, proton sponge and BH3 .SMe2 react rapidly and exothermically and accelerate the rate of reaction very efficiently. This observation also suggests that the generation of 2b from 2a is the rate-limiting state of this catalytic cycle. Then Cantat and coworkers used 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a base for stabilizing the silylium species as FLP organocatalysts in the hydroboration of CO2 to afford methoxide using 9-BBN, catBH, and pinBH as reducing agents (Scheme 6) [15]. The reaction of CO2 and catalyst afforded the N/Si+ FLP-CO2 as an active adduct which is responsible for the catalytic cycle. In the reaction mechanism, initially, FLP activates the CO2 molecule in the electrophilic carbon by the nitrogen Lewis base, while silicon acts as Lewis’s acid. Again, they have synthesized an ambiphilic phosphinoborane complex supported by the ferrocene backbone for the hydoboration of CO2 [16]. With phosphine

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V. B. Saptal et al.

R2P N B

H B R2P N

N PHR 2

R = tBu, N(iPr) 2

H

FLP FLP C6D5Br

CO2 + BH3.SMe2

(CH3OBO)3

R2P N B

(CH3OBO)3 HCOO[B]

R2P N [B]O B H O

N PR 2

N PR 2 H

CO2

N PR 2 H

R2P N B O O

N PR 2 H

H[B]

Scheme 4 FLP-catalyzed reduction of CO2 to methanol

(PPh2 ), boranes such as 3-Bpin and 3-BMes2 were installed on the ferrocene scaffold (Scheme 7). The 3-BMes2 as a Lewis acid demonstrated excellent catalytic activity for this reaction, and TON up to 1980 and TOF up to 250 h−1 were noted. The presence of phosphines and boranes functional groups on ferrocene presented synergistic catalysis for the hydroboration of CO2 . However, when individually tested, no catalytic reduction in CO2 was observed. In the reaction mechanism, both active centers participate in the activation of borane and CO2 to furnish CH3 O–BBN. Song et al. used organic ligands based on N-methyl-4,5-diazafluorenide, a molecule containing three as well as two rings for the reductive hydroboration of CO2 to methyl boryl ether, which can be further hydrolyzed to methanol (Scheme 8) [17]. Carbon-centered reversible binding of CO2 from these organocatalysts demonstrated excellent catalytic activity with wide boranes. In 2018, Garca-Vivo used carbodiimides as organocatalyst for CO2 reduction with hydroboranes such as 9-BBN or BH3 SMe2 at 1 atm CO2 to lead CH2 (OBBN)2 and CH3 OBBN or (MeOBO)3 and B(OMe)3 at 25–60 °C (Scheme 9) [18]. Experimental and computational studies demonstrated that the formation of formaldehyde and

Organocatalytic Reductive Functionalization of Carbon Dioxide

7

H H B

N

N

[BH4]2b (H3COBO)n

BH3.SM2

CO2

N

(H3COBO)3

N 2 BH3.SM2

HCOOBH2

2a H H N

B

H H

[HCOO]N

N

3c

B

N

[BH4]-

2b

CO2

BH3

Scheme 5 Non-nucleophilic proton sponge-catalyzed reduction of CO2 to methanol Cat. R R Si Cl OBR2

Cat. CO2 + R2BH

H

R2BOBR2

H H

N

N

O

O N

N

R Si N

R

N

R = Me, iPr, Ph H H O BR2

H

CO2

N N

O

Cl

H

N

N N

Si R R

Cl

O BR2

N N N R Si H O R Cl O BR2

N N N Si O O R Cl

R

HBR2

Scheme 6 N/Si+ FLP for reductive activation of CO2

N

C O Si R RO

8

V. B. Saptal et al. cat. O[B]

cat. CO2 + H[B]

[B]O[B]

H

BR2

H H

BR2 = BPin, BMes2

H[B] = 9-BBN

Ph P

Ph

PPh2

PPh2

Fe

Ph [B] H

H[B]

Ph P

Ph [B] H

CO2

Ph P

H

O O

C

O

O BR2

BR2

BR2

[B]

B R2

+ HCO2[B]

Scheme 7 Phosphine-borane FLP for reductive activation of CO2 with boranes Cat. CO2 + HBR2

H2O

CH3OH

H3COBR2

Cat.

H

H

N

N

N

H

O

OH

N R

N

CO2 N R

N

-CO2

Scheme 8 Organocatalyzed activation of CO2 and reduction with boranes

triazaborinaneimine-BH3 adducts (4a and 4b) was responsible for CO2 activation and subsequent reduction. Recently, Ramos and coworkers synthesized phosphinoborane Ph2 P(CH2 )2 BBN by reacting diphenyl(vinyl)phosphine and 9-BBN and used for the hydroboration of CO2 to CH3 OBBN (MeOH), and TON up to 764 is noted (Scheme 10) [19]. It was found that this catalyst reacts with CO2 and boranes to generate the formaldehyde adduct (5) Ph2 P(CH2 )2 BBN(CH2 O) as in previous reports and is responsible for hydroboration reactions. Cantat et al. used nitrogen bases like TBD, Me-TBD (MTBD), and DBU for the hydroboration of CO2 with 9-BBN or CatBH and achieved a TON of up to 648 at room temperature (Scheme 11) [20]. The basic sites of TBD activate CO2 and generate a TBD-CO2 adduct which is the key step in the reduction of CO2 . Based on

Organocatalytic Reductive Functionalization of Carbon Dioxide BH3.SMe2 OCH3 B O O B B H3CO OCH3 O + B(OCH3)3

iPrNCNiPr

BH3

9

9-BBN

CO2

O 9-BBN

H H

O 9-BBN + H

H

O 9-BBN

iPrNCNiPr

4b

4a

H

NiPr iPr N

iPr N

B O

NiPr iPr N

iPr N

BH

CH2

Scheme 9 Carbodiimides as catalysts for the reduction of CO2 with boranes Cat. Cat.

PPh2

O

B CO2 + H[B] H[B] = 9-BBN

O[B] H

H H

B

Ph2P

5 f ormaldehyde-adduct

Scheme 10 Phosphinoborane as catalysts for the reduction of CO2 with boranes

the experimental study, a reaction mechanism has been proposed, and TBD catalyst acts as Lewis’s base to activate CO2 and coordinates with hydroborane to generate an adduct 6b, which facilitates the hydride transfer from borane to carbon, to give adduct 6c with generation of boron formate intermediate which will further go reduction reaction to methoxyborane. Mandal and coworkers used an abnormal N-heterocyclic carbene (aNHC) borane adduct formed by the reaction of aNHC with 9-BBN to capture CO2 from the atmosphere and its integrated reduction into formic acid and methanol under mild conditions (Scheme 12) [21]. Reaction of 9-BBN and CO2 in presence of aNHC afforded a TON of 6000 to methanol in C6 D6 at the room temperature for 12 h. Initially, it is assumed that a reaction of CO2 and aNHC afforded aNHC-CO2 which is the key catalyst for the reduction of CO2 . This adduct activates the B–H bond of borane which further undergoes an insertion with another CO2 molecule to lead to fourcoordinated boron species. Then, the insertion of two molecules of 9-BBN to adduct gives a zwitterionic borondiformate with the releasing of a dimer B–O–B. Afterward, reaction of other 9-BBN results in the regeneration of catalyst with methoxide derivative. Reactions with amines The first metal-free catalytic methylation of amines using CO2 as a C1 source with boranes was reported by Cantat et al. in 2014 using proazaphosphatrane superbases as catalysts (Scheme 13) [22]. Boranes such as 9-BBN, HBcat, and HBpin were

10

V. B. Saptal et al.

CO2

2 R2BH

H[B] = 9-BBN, HBCat

24h, 25oC, THF

N Me Me-TBD

TBD

O H3C

O

BR2

O

CH3OH

DBU

NMe2NMe2

N

N

N

N

DABCO

DMAP

PS

N

N N

H2O BR2

N

N

N

N H

N

O

R2OB-BOR2

N

N

H3C

O

9-BBN

9-BBN2O

O B 6a

HCOO[B]

N H N [B]O

O

N

N

N H

H3C

N N

O B 6c

N O

N O B

[B]H 6b

Scheme 11 TBD base-catalyzed reduction of CO2 with boranes

screened, but the use of 9-BBN gave excellent yields. For monomethylation 4 equivalent and for dimethylation of amines 8 equivalent of the reducing agents were required. Various other organocatalysts were also screened such as TBD, NHCs, and other phosphine ligands, but the only substituted proazaphosphatrane afforded excellent yields and selectivity. Using this protocol, a large number of substrates tolerated excellent yields for the mono- as well as dimethylation of amines. Basic phosphine acts as a key catalyst for this transformation and tolerates the reaction effectively. Ong and coworkers in 2015 synthesized novel carbodicarbenes (CDCs) as strong σ -donating ligands analogues to the NHC and used for the reductive N-methylation of amines using 9-BBN as reducing agent (Scheme 14) [23]. Wide range of functional amines that undergoes N-methylation efficiently afforded good to excellent yields of the products. Upon reaction, the CDC reagent ligand with borane adduct is generated which was assumed to be key catalytic active species for the reductive functionalization of CO2 . A reaction mechanism is also proposed similar to the previous scheme promoted by CDC catalyst. The CDC catalyst activates both the borane and CO2 by a nucleophilic attack to generate formate intermediate, which subsequently reacts with amine molecule. Bhanage and coworkers in 2016 synthesized a series of N-heterocyclic olefin (NHO) ligand systems and used them as catalysts for the reductive N-formylation of amines with CO2 (Scheme 15) [24]. The presence of an ylidic exocyclic carbon atom of NHO ligand is a very powerful nucleophile and acts as a scavenger for CO2 by

Organocatalytic Reductive Functionalization of Carbon Dioxide

11

Ph Cat. 9-BBN + CO2

CH3OBBN

C6D6, rt, 12h Ph O O

Cat.

TON: >6000

Dipp N Ph N Dipp

Dipp N Ph N Dipp 9-BBN Ph H B O O

NBBOBBN

Dipp N Ph N Dipp

CO2

CH3OBBN O H 9-BBN

Ph O B O O

H O

O

O B

O Ph

H H

Dipp N Ph N Dipp

-CO

Dipp N Ph N Dipp

29-BBN + CO2

NBBOBBN

Scheme 12 aNHC-catalyzed reduction of CO2 with boranes

forming an NHO-CO2 adduct. This adduct is the key active catalyst for the reductive functionalization of CO2 at mild conditions. 9-BBN and polymethylahydrosiloxane (PMHS) from silicon industry waste as a reducing agent were used for the various amines. Bhanage et al. in 2018 used B(C6 F5 )3 as a Lewis acid catalyst for CO2 reduction with amines and dimethylamine borane as a reducing agent for N-formylation and Ncyclization of diamines to benzimidazoles [25]. High TONs up to 1237 and TOFs up to 309 h−1 were observed (Scheme 16). The catalyst B(C6 F5 )3 is highly responsible for the activation of CO2 with the amine molecule by acting as a FLP pair. Also, the activation of B–H bond is processed by the catalyst. Various amines including aliphatic and aromatic and diamines were tolerated effectively. Recently, Gao and coworkers used a borane-trimethylamine complex as a reducing agent for the methylation and formylation of amines in presence of 6-amino-2picoline as an organocatalyst with 1 atm of CO2 (Scheme 17) [26]. A wide range of primary, secondary, and aliphatic amines tolerated under the optimized reaction condition for the methylation as well as formylation reaction. Synthetically important

12

V. B. Saptal et al. CH3 Me Cat. H Me + CO2 + 4 H[B] N N N P N Me o R R THF, 1h, 90 C R R N R = alkyl, aryl H[B] = 9-BBN, up to 99% HBpin, N HBcat CH3 N Ph Ph

CH3 N Me Ph

CH3 N Et Ph

91%

99%

99%

CH3 N

CH3 N Et Et 87%

99%

CH3 N O 99%

Di-methylation H3C CH3 H3C CH3 N N iPr iPr

85%

Ph

R = 4-OMe, 99% H3C 4-Cl, 99% N Me 4-F, 99% 3-Me, 99% 3-Cl, 99%

99%

52%

99%

N

CH3

OH 59%

CO2 + H[B]

R 97%

CH3 N

H3C

H3C

Cat. HCOO[B] H N

N

CH3 H3C

BuO O 69% H

O

R

CH3 N Me

CH3 Ph N

R

N

R

Cat.

N

CH3

33% CH3 N R R

R

Scheme 13 Proazaphosphatrane superbases-catalyzed hydroboration of CO2

molecules such as chlorcyclizine derivatives have also been formylated and methylated using this method. Organocatalyst 6-amino-2-picoline acts as intramolecular FLP to activate the CO2 molecule. In the absence of the catalyst, the only formylated product was observed. An experimental and mechanistic study suggested that borane plays a dual role, acting as Lewis’ acid for the generation of intramolecular FLP and reducing agent for CO2 . Very recently, So et al. synthesized N-phosphinoamidinato NHC-diborene as an active catalyst for the hydroboration of CO2 with HBpin to synthesize the MeOBpin and for the N-formylation of amines (Scheme 18) [27]. In the mechanistic study, it was found that the presence of double-bonded boron centers is the key catalytic center for the activation of CO2 and further hydroboration reaction. Very recently, Mandal et al. used mesoionic N-heterocyclic olefin (NHO) as an active catalyst for the N-methylation of amines with CO2 using 9-BBN as reducing agent using THF as solvent at 40 ˚C (Scheme 19) [28]. As a NHC ligand, the NHO ligands have more electron density at the olefinic carbon, which unveiled the activation of borane and CO2 . A wide range of secondary and primary amines undergoes mono- and dimethylation reactions under mild reaction conditions. Additionally, the

Organocatalytic Reductive Functionalization of Carbon Dioxide

13

Cat. Dipp CH 3 H C N N N N R + 9-BBN Toluene, 1.5 h, R R R N N up to 97% 100 oC Dipp R = alkyl, aryl CO2 Cat.

CH3 N Ph Ph

CH3 N Me Ph

CH3 N iPr Ph

90%

75%

52%

CH3 N

CH3 N

Ph

R

86% CH3 N CH3

R = 4-OMe, 74% 4-Me, 35% 2,5-iPr, 80% CH3 N Ph

Ph

O 53% Dipp N

97%

C

80%

53%

N

THF, RT

N Dipp

+ 9-BBN CO2

O [B] H

R

H[B]

N

N

CDC H[B]

O N R

H

H[B]

H[B] = 9-BBN

R

B

Dipp N

N

N Dipp

CH3 N Me

CH3 N

-H2

N R

CDC

O

H

[B]

R

N R

R

H

N R

Me

[B]O[B]

Scheme 14 CDC-catalyzed hydroboration of CO2

R

H N

Cat.

Cat. PMHS/9-BBN R

+ CO2 2 Mpa

THF, 80oC, 6h

Ph

N O

CHO N R R

N

H

71/80% PMHS/9-BBN

N

O

H

92/97%

H

O

89/93%

O

N

H

R N

R = Me, iPr, Et R N

H

52/72%

N R

N R O

O NHO-CO2 adduct

Scheme 15 NHO-catalyzed hydroboration and hydrosilyation of CO2

14

V. B. Saptal et al. B(C6F5)3

NH2 or R

NH2

H N

N

CHO + N R R

BH3.NHMe2

+ CO2 CH3CN, 80oC, 4h R

N H Ph

N

N O

H

O

H

N

N

N H

N H

N H

94% TOF: 294h-1

RH N R

80% TOF: 250h-1

N H

O

O

H

87% TOF: 272h-1

99% 99% -1 TOF: 309h-1 TOF: 309 h

N

99% TOF: 309h-1

H

O

94% -1 TOF: 294 h

72% -1 TOF: 225 h

N

N

O O B(C6F5)3

Scheme 16 Lewis’s acid-catalyzed hydroboration of CO2

H N

+ CO2 R R R = H, alkyl, aryl

Ph

6 h, 100 oC

CH3 N R R up to 93%

R = 4-CN, 63% 4-NO2, 64% 4-CF3, 85% 4-Et, 80% 4-Ph, 84% 4-Ms, 83% CH3 CH3 N N 4-Me-Ph OH 4-Me-Ph

CH3 N Me

R

Cat. DMF BH3.NMe3 (4 equiv.)

CH3 N

R = H, 84% 4-Me, 88% 4-OMe, 66% 4-F, 67% 4-Cl, 85% 4-I, 84%

Me

NH2

N H3C

N

Me

73% CH3 N n-Bu

93%

52% Ph N

45%

N CH3

71%

Cl

S

84%

N

N CH3

56% CH3 N Me

67%

69%

N CH3

CO2

Mechanism

Me

Cat.

NH2 N + B.NMe3

Me

NH BH2

N

Me

N O

O

R R

R

N CHO

N CH3 R

R

Scheme 17 6-amino-2-picoline-catalyzed hydroboration of CO2

N H R + B.NMe3

NH BH2

Organocatalytic Reductive Functionalization of Carbon Dioxide Cat. CH3OBpin C6D6, 110oC + pinBOBpin >99% H N CHO R Cat. R N R R C6D6, 90 oC

CO2 + 3HBpin 97.8% -1 TOF: 33.3 h

HBpin + CO2

15 Ph

Cat. tBu P tBu

N N Ar B B

Br

N N

Ph N O

N H

O

99% -1 TOF: 13.3 h

H

99% -1 TOF: 50 h

tBu P tBu

N N Ar B B

Br

N N

H

O

H N

R

N H

91% -1 TOF: 3.1 h

82% -1 TOF: 1.4 h

+ HBpin

Ph N tBu N Ar P tBu B B HBpin Br N O O H N B O O

O

O

H

98% -1 TOF: 33.3 h

R Ph

N

N

-H2 R

Bpin N R

O

CO2

H

O

Bpin

CHO N R R + pinBOBpin

Scheme 18 N-phosphinoamidinato NHC-diborene-catalyzed hydroboration of CO2

protocol demonstrated the synthesis of pharmaceutically important drugs such as angustureine as an antimalarial drug and pempidine as a ganglion-blocking drug. Mechanistic assumptions were made based on the various computational and experimental studies, and it is assumed that the nucleophilic mesoionic NHO attack on the boron center of the 9-BBN makes hydrogen more hydridic to transfer to the CO2 effectively. Organocatalytic hydrosilylation of CO2 Among all hydroelements, silanes are important and versatile reducing reagents. Reduction of CO2 with silane soft reducing agent with homogeneous catalysts is well established. Cantat and coworker demonstrated the reactivity of amines and alcohols with CO2 in the presence of a base to yield carbamate and carbonate salts, respectively (Scheme 20) [29]. The reaction of the CO2 -amine/alcohol-silane tricomponent systems was elucidated for the reductive functionalization of CO2 . With various bases as catalysts such as TBD, Methyl-TBD, DBU, DABCO, and DMAP, the results obtained showed that stronger bases such as Me-TBD, DBU, and DMAP proved to be efficient catalysts for the formylation product. Organosilanes can also be used as a reducing agent without additional energy expenditure. This is particularly attractive to be used as a reducing agent with added benefits of low to non-toxicity

16

V. B. Saptal et al. Cat. Ph

R' NH R or R

NH2 + CO2

CH3 CH3 or N N R' R CH3 THF, 40 C, 24h R 9-BBN Cat.

CH3 R = H, 95% 4-Me, 79% N Ph 4-OMe, 83% 4-iPr, 80% 4-Br, 73%

R

H2C

o

CH3 N R

R = H, 80% 4-Me, 79% 4-OMe, 74% 4-Br, 79% 4-Cl, 72%

R = 4-I, 88% R = 4-Br, 78% CH3 4-Me, 84% 2-Me, 77% N CH3 4-OMe, 81% 2-Br, 73% 4-iPr, 66% 2-I, 68% H, 82% 2-Ph, 59%

R

CH3 N CH3 CH3 N 61% Ph Ph H2C

N CH3 73% H3C CH3 N

N N 3 CH3 CH3 Angustureine 84% Pempidine 73%

Dipp 9-BBN N Ph N Dipp

Ph H B

Dipp N N Dipp

Ph

69% O

CO2 H

Dipp N Ph N Dipp

O B

Ph

Dipp N Ph N Dipp

Scheme 19 mNHO-catalyzed hydroboration of CO2

and cost-effectiveness. Furthermore, dialkylamines such as piperidine, morpholine, Me2 NH, and Et2 NH were used as carbonate substitutes that were converted to their corresponding formamide derivatives with good yield. The yield ranged between 47 and 65% with minimum catalyst loading, that is, 5 mol%. Following this work, Cantat et al. have further demonstrated the use of a TBD catalyst for the selective reduction of CO2 with silane and amines in aminals (Scheme 21) [30]. In this case, interesting results were found for the ‘four-electron reduction of CO2 ’ in the presence of secondary aromatic amines. In the reaction describing the metal-free hydrosilylation conditions, the CO2 bonds were cleaved, and the organocatalysts attack on CO2 to balance with the reactivity and to endorse the selective development of two C–OH and two C-N bonds. The methodology allows the formation of various unsymmetrical as well as symmetrical aminals efficiently. Potential reaction paths are suggested in the scheme, and the reductive functionalization of CO2 to formamides is demonstrated, specifically TBD’s role in catalysis of this process. Interestingly though, reactivity is absent when N-methylformanilide is combined with PhSiH3 in the presence of 5 mol% TBD at 80 °C, even when N-methylaniline is present (or absent; under Ar or CO2 ). It therefore appears that

Organocatalytic Reductive Functionalization of Carbon Dioxide

H N

+ CO2 + PhSiH3

R R R = alkyl, aryl CHO N Et Et

CHO N Me Me

47%

64%

solvent, 25h, 100 oC

N

O

O

N

N H

N H

O 65% R'OH

N N

N

CHO N

49%

N H

Cat.

up to 99%

CHO N

N

CO2 +

CHO N R R

Cat.

17

N H R' = Me iPr, etc HO Ph3SiH

N N H OH

H isolated Ph3Si-OR'

Scheme 20 TBD-catalyzed reduction of CO2 to formamides using amines and PhSiH3

formamides manifest as competition products in the conversion of CO2 to aminals. The activation of CO2 to aminals proceeds via reduction of CO2 to a silyl acetal species which undergoes two successive nucleophilic attacks by the amine group. The N-methylation and N-formylation strategies of amines were further studied by Li et al. by using the DBU as a base catalyst [31]. In DBU catalysis, it was found that a temperature switch (30–100 °C) with a corresponding change in reaction time from 72–48 h controls the reaction mechanism from N-formylation to N-methylation. Ying and coworker in 2009 for the first time used NHC as an organocatalyst for the CO2 reduction with hydrosilanes [32]. The reaction conditions used were 1,3-bis(2,4,6trimethylphenyl)imidazolium carboxylate (IMes-CO2 ) in N,N-dimethylformamide (DMF) through H2 SiPh2 at 1 bar of CO2 pressure carried out at room temperature. After 24 h, the yield observed was the methoxide species (CH3 O)2 SiPh2 and [(CH3 O)SiPh2 O]. The TON and the TOF using the NHC catalyst at the mentioned reaction conditions reached 1840 and 25.5 h−1 , respectively. From the obtained results, nucleophilic NHC turns as an excellent CO2 activator. In the reaction mechanism, they proposed that imidazolium carboxylate was formed which reacts with silanes even if a free carbene might also trigger the Si–H bond. Followed by this work, Cantat et al. used very active organocatalysts based on NHCs for the N-formylation of NH bonds [33]. Various forms of nitrogen molecules and heterocyclic groups were applied using CO2 and polymethylhydrosiloxane (PMHS), which are the two chemical wastes from the industries. IPr NHC was unveiled high activity in the formylation of morpholine with CO2 and PhSiH3 , which showed in that TOF was 160 h−1 and also on further investigation of the scope of reaction for formylation with various types of N–H bonds, like PhSiH3 as reductant and IPr as organocatalyst. Aliphatic secondary amines, such as piperidine, morpholine,

18

V. B. Saptal et al. R3

N H +

R4 CO2

R2 R1 R = alkyl, aryl

N

N

R Ph

Ph N

Ph

N

H

N

H

Ph N H

94%

88%

N

N

R

88%

H H R = Me, 88% = F, 62%

CO2 + [Si]-H

TBD

Ph

H H 76%

N

H H 87%

N

N N

H H

H H [Si]O

N R1

N R1

O[Si]

+ R1R2NH -[Si]-OH H H

H H R2

N

N

O

H

[Si]O

Ph

94%

O

TBD

N H

N

N

N

H H

N

N

N H Ph 95%

H H

Cat.

up to 99%

R = 3-Me, 79% 3-OMe, 76% 3-Cl, 91% N N Ph Ph R 4-F, 98% H H 4-CN, 95% 73% 4-COCH3, 98%

H H

N

R N 4 CH2 N R2 R1

CD3CN, 2.5h, 80 oC - siloxanes

H N

Ph

R3

Cat. + PhSiH3

R2

+ R1R2NH -[Si]-OH

R2

N R1

O[Si]

Scheme 21 TBD-catalyzed reduction of CO2 to formamides using amines and PhSiH3

and diethylamine, were transformed to corresponding formamides quantitatively after 24 h at r.t., under 1 bar of CO2 pressure (Scheme 22). Dimethylformamide (DMF) was obtained as the only product when dimethylammonium dimethylcarbamate was employed as a substrate. Primary aliphatic amines showed particular reluctant substrates using nitrogen bases as catalysts (e.g., TBD). Various simple aliphatic, bulky, and aromatic substrates were also tolerated under the optimized reaction condition. Electron-donating groups have a constructive impact on the translations observed for aniline derivatives. The scope of catalyst extended by using less expensive and active organosilanes like polymethylhydrosiloxane (PMHS) is one of the by-products as a waste produced from silicon industries. PMHS is more advantageous over other silanes because of its non-toxicity, moisture stability, and cost effectivity. PMHS was an active reducing reagent for the formylation of N−H bonds in primary and secondary amines, hydrazines, imines, anilines, and N-heterocycles as showed with the decent to outstanding yields found in the synthesis of DMF.

Organocatalytic Reductive Functionalization of Carbon Dioxide Cat.

iPr

iPr

N

N iPr H N

+ CO2

R R R = alkyl, aryl

CHO N H Ph 35%

iPr

PhSiH3,

Cat.

iPr CHO CHO or N N R R R CHO up to 99% R = H

99%

99%

iPr

THF, 1h, rt

CHO CHO N N Et Me Me

Et

CHO N

99%

CHO N

CHO N Me Ph

CHO N H tBu

99%

99%

CHO N H

CHO N H Mes

O

N Ph

Ph 60%

H

O

HN H PhN H N N Ph Ph Ph Ph 66% 35%

99% CHO N H Mes 8%

71%

86% O

CHO N Me Ph

O 99%

CHO N Ph CHO 31%

O

19

N N

N

H

O N H

O 99%

99%

Scheme 22 NHC-catalyzed reduction of CO2 to formamides using silanes

Dyson et al. describe the green and practically scalable route for the hydrosilylative N-formylation and N-methylation of various primary and secondary amines using carbon dioxide CO2 , by applying relatively cheap thiazolium carbene-type NHC ligand as a catalyst [34]. The catalyst is air-sensitive and must be generated fresh just before the use; consequently, the techniques used to prepare and manipulate the catalyst are described. The synthetic approach described in this protocol does not use any toxic reagents. The appropriate catalyst furnished N-formylated or N-methylated products with high selectivity. Cantat’s group presents advancement in metal-free organocatalysis of CO2 utilization, as discussed earlier, and subsequent in-depth mechanistic analysis of the same is reported by several groups working on efficacy of carbenes and formates as catalysts. For instance, phenyl substitution on amine-N atom providing high reaction, the efficacy with good conversion and yields, while a biphenyl substitution preventing activity, has been discussed by Cao et al [35]. A theoretical approach was involved to demonstrate the reaction mechanism of amide formation from amine substrates using CO2 (Scheme 23). The reaction mechanism was categorized in four key steps, (a) silane activation, (b) CO2 incorporation in an intermediate, (c) NHC driven amineintermediate conjugation, and (d) C–O bond cleavage to get amide product. Two mechanisms are revealed through DFT analysis, sequential covalent bonding and general base catalysis, with different Gibbs free energy profiles. DFT reveals the

20

V. B. Saptal et al.

most favorable mechanism being general base catalysis with a lower overall Gibbs barrier driven by NHC. However, the duality of the reaction mechanism will generate by-products such as carbamate anion. The reaction mechanism and predicted product formation are closely aligned with Cantat’s findings. This DFT study also provides an explanation for the differential reactivity of substrates conjugated on the amine nitrogen. While methyl and phenyl conjugates are highly active, biphenyl substitution results in complete loss of activity, shown earlier by Cantat. Wang and Cao show how biphenyl substitution presents a greater thermodynamic barrier resulting in selective reaction preference. The phenomenon is explained by the nucleophilic enhancement of the substitution atom due to the aromatic substitution. Dual-phenyl substitution renders the substitution atom (N) highly nucleophilic leading to reaction arrest. Further, based on the conversion of CO2 to methanol [32], Huang et al. demonstrated detailed DFT calculations to study the reaction mechanism catalyzed by NHC [36]. They reveal the presence of formaldehyde as a productive intermediate in a reaction scheme examined earlier by other groups. While previously unreported, the formaldehyde along with other intermediates such as formoxysilane (FOS) and bis(silyl)acetal forms a compelling reaction complex to lead to the formation of methanol which is a desirable output for CO2 utilization. NHC plays a crucial role here by activating the Si–H bond in silanes and promoting the formation of electrondense hydridic H atoms which can couple with the electrophilic C center of the appropriate substrate, in this case, CO2 , CH2 O, and FOS (Scheme 24). Interestingly, Huang et al. also highlighted the phenomenon first described by Cantat wherein biphenyl substitution is counterproductive compared to phenyl or methyl substitution in performing the conversion, as discussed earlier. Further, Cantat and coworker studied complete catalytic deoxygenation of CO2 into formamidine derivatives [37]. In this study, they found application for the synthesis of derivatives of benzimidazole by the reductive functionalization of CO2 in Scheme 23 NHC promoted activation of CO2 and silanes in various pathways

N

N

O

O

N

N H N

C O

H N Covalent bonding mechanism

O

General base mechanism N

N N + N H N Ph Si O O H H H Covalent bonding mechanism

H N

O C H O SiH2Ph

General base mechanism

Organocatalytic Reductive Functionalization of Carbon Dioxide NHC

3R3SiH + CO2

Favourable

N

N

H2O

R3SiOCH3 + R3SiOSiR3

21 MeOH + R 3SiOH

O

N C

Ph

H

Si H

H

C O

O

O or

H

N

O or O[Si] H

H

H O

H Si H Ph

Not f avourable

Scheme 24 Possible pathways for NHC-catalyzed activation of CO2 and silanes

the existence of hydrosilanes such as H3 SiPh or poly(methylhydrosiloxane) (PMHS) replacing TBD with IPr (NHC) as catalysts and ortho-diamines as coupling partners. Very recently, Yang and coworkers synthesized NHC-CO2 adduct as a catalyst for the N-formylation and cyclization reactions of CO2 (1 atm.) with amines using silane as a reducing agent [38]. At the specific temperature, this NHC adduct decarboxylates and removes the CO2 , and hence, it acts as not only the nucleophilic center but also the C1 source. As stated by Huang et al., aldehydes may be underreported in their role and presence in the system of formylation reactions. Murata et al. report a study detailing utilization of CO2 in the downstream synthesis of important aldehydes [39]. As a twopart process, solvent-free preparation of silyl formats is elucidated driven by tetrabutylammonium acetate (TBAA) as a catalyst (Scheme 25). The subsequent process while driven by Grignard’s reagent is easily substituted for metal-free catalysts. Various aldehydes have been tolerated efficiently using the developed system. Based on the DFT calculations, a reaction mechanism has been described, where an OAc anion of catalyst attacks on electrophilic Si center of hydrosilane which led a pentacoordinate silicate intermediate. Further transfer of hydride ion to CO2 from intermediate affords silyl acetate and Bu4 N+ HCO2 – . Generated HCO2 – acts as a nucleophile to form another pentacoordinate silicate intermediate. Finally, hydride transfer on carbon dioxide generated HCO2 SiR3 which subsequently undergoes nucleophilic attach by Grignard’s reagent to form the corresponding aldehyde. In a parallel study with a counter-point focus, Motokura et al. investigated the effect of various formate salts on the hydrosilylation of CO2 , and it was found that the tetrabutylammonium (TBA)formate demonstrated excellent catalytic activity [40]. Experimental study like in-situ FT-IR and 1 H NMR spectrometry suggested that along with the formate anion, Lewis basic solvents like NMP and DMSO enhance the rate of reaction by donating electrons to the Si center. Motokura et al. demonstrate a mechanism aligned instead, with that elaborated by Murata et al., which emphasizes the role of formates in aiding hydrosilylation of CO2 (Scheme 26). Based on reaction acceleration by in-situ-formed formate in a typical organometal/transition metalcatalyzed silylation reaction, the application of formates, using the class-typical

22

V. B. Saptal et al. Bu4+NOAcH N

O

PhSiH3

+ CO2

O N

CHO

O

H3O+

+

Bu4N

H

R

CHO

59% R AcO Si H R R

-

Ph

CHO

S 62% CHO

76% R3SiH

O

N CHO

Ph

71%

TBAA

RMgBr THF

CHO S 57%

Bu4N+OAc-

H

CO2

56% 4-tBuPh 64%

R AcO Si H R R

iR 3 OS -Ac

O C O

Bu4N+

Bu4N+HCO2-

CHO

HCO2SiR3

R3SiH

R HCO2 Si H R R Bu4N+

R HCO2 Si H R R

-

Bu4N+

CO2

O C O

Scheme 25 ILs-catalyzed activation of CO2 and silanes for the formation of various bonds

example of TBA formate, derived from TBAA is shown in the hydrosilylation of CO2 [41]. Importantly, a CO2 can be used as the very efficient C1 source for the synthesis of heterocycles like benzimidazoles. In 2016, Sun used B(C6 F5 )3 as Lewis’s acid catalyst for the synthesis of benzimidazoles from o-phenylenediamine as an amine source and PhSiH3 as a reducing agent at 120 °C for 24 h in THF as a solvent (Scheme 27) [42]. A wide range of derivatives of o-phenylenediamine was tolerated using this catalytic system and afforded moderate to good yields. To elucidate the reaction mechanism, experimental studies such as 13 C NMR were carried out, which Scheme 26 Reaction mechanism for the formate-catalyzed activation silane

R Si H + CO2 R R

R Si F R R R Si H + CO2 + F R R

O H

O R H O Si R R O

Organocatalytic Reductive Functionalization of Carbon Dioxide

+ CO2 + PhSiH3

-H2O

NH2 N

N H

83%

N

N H

O

F 76%

61%

85% N

N

N

N H

N H N

N N H

Br

94%

88%

N H

R N

N

N H

N

N

B(C6F5)3

NH2 R

23

91%

N H

O2N 25%

NH2

N B(C6F5)3

+ CO2 NH2

N H O

NH

B(C6F5)3

NH3

CHO NH OSiH2Ph

PhSiH3 PhSi-H H H

-H2O

O B(C6F5)3

NH2

NH B(C6F5)3

N H3

O B(C6F5)3

PhSiOH

Scheme 27 Lewis’s acid-catalyzed synthesis of benzimidazoles using amine, CO2 , and silane

suggested that the reaction mechanism follows the FLP pathway for the activation of amine, silane, and CO2 . In an exploration of additional catalysts used in the formylation of amines applying the metal-free green route, studies published by Liu and coworkers demonstrated the use of ionic liquids (ILs) as potential catalysts (Scheme 28) [43]. 1-Alkyl-3methylimidazolium [BMim]Cl was used as a catalyst for the N-formylation of amines with CO2 and PhSiH3 as reducing agent. 1-Butyl-3-methylimidazolium acetate, [Bmim][OAc] catalyzes the N-formylative cyclization of CO2 using EtO3 SiH as a reducing agent. The derivatives of benzothiazoles and benzimidazoles were afforded [44], while, equally functional as NHCs, ILs are the advantages in terms of physical state, reactivity, and freedom from the use of precious metals. ILs have been studied extensively for multiple catalytical roles such as those in the formation of carbamates or carbonates. ILs demonstrate a fundamental difference in mechanism of CO2 uptake, compared to NHC. IL, embodied here as imidazolium ion and counter-ion system, directly activate silane Si–H bond to accept CO2 , leading to direct formation of formoxysilanes, whereas NHC itself connects with CO2 to form an adduct which is passed onto the silane. Further, they also evaluated the role of various cations and anions in ILs. Ions comprising Cl− , NO3 − , and Br− exerted a positive effect on the activation of Si–H, whereas the cation with complex anions such as BF4 − and PF6 − reacted poorly. The reaction is effective at room temperature which makes it

24

V. B. Saptal et al.

valuable for commercialization, relying on relatively low CO2 pressure. Employed ILs shown reusability of the catalyst which can be employed in cost-effectiveness for large-scale conversions. Further, in 2017 Liu and his group investigated metal-free and versatile route for synthesis of unsymmetrically N,N-disubstituted formamides (NNFAs) using CO2 [45]. For the reaction, primary amines and aldehyde were used with ionic liquid (1butyl-3-methylimidazolium chloride) at room temperature. IL was recycled five times without loss in its catalytic activity. This was the novel approach, based on metal-free system, which was applied in simple and versatile route for the synthesis of unsymmetrical N,N-disubstituted formamides (NNFAs) by using three-component reductive coupling of primary aldehyde, amine, and CO2 with phenylsilane in the presence of [BMIm]Cl at room temperature. The substrate scope was screened providing various unsymmetrical NNFAs in good to excellent yields. Following the classic route of CO2 hydrosilylation, the efficiency of which rides on the choice of catalyst, and reaction conditions, most groups working on the area attempt to optimize high-functioning catalysts with the highest potential for conversion and yield. Fluorides have been demonstrated earlier, in the form of CsF in the

R1

H N

30oC, 5h

R1

NH2

[BMim]OAc

N

-H2O

S

+ CO2 + (EtO)3SiH SH N

S

99%

S 48%

N

N Br 91%

O

O

[BMim]OAc

S 54%

N O2N

O2N

N H

90%

N

(EtO)3Si N

+

N

H N CHO

N S

+

-H2O

N H

O

H Si(OEt)3

CO2 N

R2

N

73%

N S

O

S

O

84%

86%

N

H

N

N

S

Br

H

[BMim]Cl

O

+ CO2 + PhSiH3 R2 1 MPa

XH

H

O O H2 N O X O H X = S or N

Scheme 28 ILs-catalyzed synthesis formylation and cyclization reactions using amine, CO2 , and silane

Organocatalytic Reductive Functionalization of Carbon Dioxide

25

effective hydrosilylation of CO2 . The chemistry of fluorine allows potent interaction with Si reaction center of silanes due to high electronegativity to form penta- or hexa-coordinated silicon intermediates, thereby enhancing the reduction potential of silanes. He et al. used tetrabutylammonium fluoride (TBAF) as a potent metal-free organocatalyst with a wide selectivity spectrum for amines [46]. The TBAF and acetonitrile offered high conversion and yields (92–99% and 90–99%, respectively), compared to other catalysts. It was observed that polar solvents are more suited for catalyst coordination chemistry. The silane choice itself allows tunability for reaction selectivity. For instance, triethoxysilane allows N-formylation more readily, while phenylsilane promotes N-methylation. Hu et al. reported the use of lecithin, a commonly occurring natural product in eggs, soy beans, etc., in the hydrosilylation of CO2 [47]. The common trait that enables the two unusual catalysts is their zwitterionic nature. They report very high conversion rates and yields (~99 and 99%, respectively) with lecithin using aniline, p-Br-aniline, and morpholino substrates and further describe lecithin as being selective toward the formation of mono-formylated products due to steric restriction of the catalyst. N-methylation as catalyzed by lecithin is also highly efficient with conversion rate and yield reported up to 99% each, using substrates such as aniline, p-methyl, and p-chloroaniline. The reaction conditions and molar ratios of components can be modulated to switch between formylation and methylation; e.g., formylation is carried out at room temperature, 0.5 MPa CO2 pressure, 1 mmol phenylsilane, and 5 mol% of lecithin. Methylation is proceeded under lower CO2 pressure (0.2 MPa), 2 mmol phenylsilane, and at 100 °C. Further, So and coworkers have investigated the use of silicon (II) complex of the NHC-silyliumylidene cation [(IMe)2 SiH]I (1, IMe = :C{N(Me)C(Me)}2 ) as a catalyst for the N-formylation using CO2 and PhSiH3 under mild reaction conditions furnishing the relevant formamides (Scheme 29) [48]. For primary amines, the reaction time was 4.5 h; a typical yield obtained was up to 95%, and the average TOF was 8 h−1 . While in the case of secondary amines, average yield obtained was 98%, and the typical TOF was 17 h−1 . The catalytic activity of silicon(II)complex and product yields were found to be more superior to the currently available non-transition metal catalysts used for this reaction. Mechanistic studies suggested that the silicon(II) center in complex catalyzed the C−N bond formation through a different pathway in comparison with non-transition metal catalysts. The trail of activation was CO2 , PhSiH3 , and amines, which proceeded through a dihydrogen elimination mechanism, to synthesize formamides, dihydrogen gas, and siloxanes. They have explained the mechanistic pathway for the formylation of amine. Silyene compound has the presence of lone pair of electrons and a P vacant orbital on the silicon(II). Due to the unique properties of the silyene complex, it comprises both nucleophilic and electrophilic characters, leading to Lewis ambiphilicity. It was shown for the first time that catalytically or stoichiometrically activating both N−H bond and CO2 bond simultaneously, resulting in the formation of the C−N bond. Catalyst activates both amines and CO2 simultaneously for formation of C−N bond. It has been proven that NHC-silyliumylidene cation complex selectively catalyzed N-formylation of CO2

26

V. B. Saptal et al. via

Cat. N

R R R = alkyl, aryl

R

o

O C

85%

H

I H H

N

Si

H R

N

H

R

N

up to 99% CHO N

O HN

CHO N R R

Cat. PhSiH3 C6D6, 60 C

CHO N Et Et 99%

O

N

H + CO2

Si

N

N Si

H N

H

Ph

N

N

CHO N

99%

CHO N

CHO N O 99%

CHO N Si Si

99%

R = H, 99% 3-Me, 99% CHO 2,4,6-Me, 75% Ph NH 2-OMe, 99% 99% 2-NO2, 99% 4-Cl, 99%

99%

CHO NH 99%

CHO N 99%

Scheme 29 NHC-silyliumylidene cation catalyzed formylation of amines

and amines to formamides. The activity of catalyst was because of transition metallike catalysis, where the silicon(II) center sequentially activates CO2 , amines, and PhSiH3 and further proceeds via a dihydrogen elimination mechanism, leading to form siloxanes, formamides, and dihydrogen gas. Lu and coworkers in 2015 synthesized a series of phosphorus ylide (P-ylide) CO2 adducts and used them as catalysts for the various organic transformations, including reductive formylation and methylation of amines with CO2 and amines (Scheme 30) [49]. Phosphorus ylide reacts with the CO2 molecule and generates the P-ylide-CO2 adduct, which is a key catalytic center for the activation of silane for the reductive functionalization. Mandal et al., in 2018, used abnormal N-heterocyclic carbene (aNHC) as a catalyst for the N-formylation of challenging substrate like amides with CO2 using hydrosilane at mild reaction conditions (Scheme 31) [50]. Various NHC-based ligands were tested for this transformation which represented a good to moderate activity at the atmospheric pressure of CO2 and room temperature. A range of amides

H N

+ CO2

R R R = alkyl, aryl

PhSiH3, Cat.

CHO CH3 or N N R R R R up to 91%

Scheme 30 Phosphine ylide-catalyzed activation of CO2 and silane

Cat.

O O

Ph3P R

Organocatalytic Reductive Functionalization of Carbon Dioxide

27

undergoes formylation reaction at room temperature and represented good yields. A reaction mechanism is proposed, where the aNHC ligand initially reacts with CO2 molecule and forms the aNHC-CO2 adduct which subsequently attacks the silane molecule to afford the formoxysilanes intermediate. This intermediate then undergoes nucleophilic attack by amide to furnish final product. Following this work, again very recently, they have used another organocatalytic approach of NHC stabilized phosphinidene 1,3-dimethyl-2(phenylphosphanylidene)-2,3-dihydro1H-imidazole for the N-formylation of amides using silane as reducing agent at atmospheric pressure of CO2 (Scheme 32) [51]. Based on the experimental findings and DFT studies, a reaction mechanism has been developed. Phosphinidene catalyst assists the dehydrogenation reaction of phenylsilane and amide to form N-silylated amide with the liberation of H2 (identified by 1 H NMR and GCMS spectrometry). After this, N-silylated amide reacts with the phenylsilyl formate generated by another catalytic cycle of catalyst and formyl transfer occurred to furnish the final product with siloxanes as a by-product. Complex chemistry to expand the depth of CO2 application is a major challenge. In 2017, Zhu et al. took an innovative green route to access the high-value products using stepwise C–C and C-N bond formation using CO2 and silylation chemistry for the synthesis of spiro-indolepyrolidine (SIP), catalyzed by TBD (Scheme 33)

Ph Cat. PhSiH3

O NH2 R R = R, Ar O

R

O

CH3CN, rt, 24h

O R

O N H

H

Cat.

R = 2-Br, 64% 77% O R = H, 74% 2-Me,4-Br, 68% 4-Me, 70% Ph N H 3,4-Me, 69% 2-OMe, 78% H 3-NO2, 53% 4-NO2, 51% 3-CF3, 49% 2-Cl, 63% 74%

O N H

R

+ CO2

H

Ph

+ R3SiOSiR3 O

R3Si

Dipp N Ph N Dipp

O H

N H O

O N H

H

H

O

Ph

NH2 Ph O R

O

CO2

O R

Dipp N Ph N Dipp

Si O R R H

Dipp N Ph N Dipp

O O

Dipp N Ph N Dipp

R3SiH

Scheme 31 aNHC-catalyzed activation of CO2 and silane for the formylation of amides

28

V. B. Saptal et al. Cat. PhSiH3

O

O

Cat.

O

P N

O

SiH3 NH2

R

N

PhSiH3 -H2

P N

O R O R

O

N H H + Siloxanes

Ph

N

NH2 + CO2 R N H CH3CN, rt, 24h R H R = R, Ar up to 82% 25 entries

N H

SiH2Ph O PhSiH2

O

H

N P CO2

N

H3Si DFT

Scheme 32 Phosphine-catalyzed activation of CO2 and silane for the formylation of amides

[52]. Here, CO2 as a green and cost-effective C1 feedstock was used to reform tryptophan into the SIP products. The innovative approach emphasizes the versatility of silane reagents in hydrosilylation permitting effective utilization of CO2 into valueadded processes. The general reaction scheme is illustrated showing the primary that gives the integrated product with CO2 -derived C–C and C-N bonds. Subsequent reductive reaction with sodium borohydride gives a saturated product with a chiral arm which is instrumental in the formation of SIP-based chiral compounds. The reported yields are typically very good, depending on the solvent system used and the catalyst loading (0.2–0.5 mmol). In acetonitrile yields, 93–95% were obtained. Catalyst substitution with pyridine or solvent switching with THF resulted in very poor reactivity (~15% yield). Although Zhu et al. present a viable route of valueadded utilization of CO2 , process requirements of high temperature over a significant period of time along with relatively high catalyst loading in some cases may require optimization before commercial impact. The proposed mechanism based around the use of TBD as the best-suited catalyst implies the formation of formate silane and subsequently bis(silyl) acetate. This generated acetate that reacts with the tryptophan in acetonitrile at relatively high temperature to form reactive intermediates which are further converted to the final product by nucleophilic addition at the carbon center. Organocatalyzed CO2 reactions are mainly divided into three main paths: (a) direct functionalization (i.e., carbonates, carbamates); (b) reduction into formaldehyde, formic acid, and methanol; and (c) reductive functionalization. The reductive functionalization of CO2 with an amine gives formamides, N-methylamines, or aminals, which are very important value-added products. This class of processes has been investigated exhaustively with myriad catalysts, including carbenes, Lewis’s acids, formates, ionic liquids, ylides, hydroxyl compounds, and to some extent guanidines.

Organocatalytic Reductive Functionalization of Carbon Dioxide NR1

NHR1

N H

a

R2

Cat. N

CO2 + PhSiH3 1 atm. CH3CN o

100 C, 30h

R = H, 90% 4-OMe, 80% 3-Me, 92% R 4-CF3, 55% 4-CN, 73% 4-Br, 81%

Me

N

N

N H

MeO

Me

N

TBD

O Si

O

PhSiH3

H

N H

N H

N

R2

Me

NR1 R = 4-OMe-Ph, 75% 4-Me-Ph, 65% 4-F-Ph, 71% 4-I-Ph, 77% R2 4-Br-Ph, 81%

N H

Et 70% R1N

R1N N H

R2

N H

N

86%

CO2 PhSiH3 a

Cat. N

NPh

N

70%

R2 NR1

Cat. NaBH4 0 oC, 2h

N

29

OSi

R2

N H

H H SiO

OSi NR1 a

N

R2

Scheme 33 TBD-catalyzed activation of CO2 and silane to the synthesis of spiroindolepyrrolidines

Given the exhaustive work done to unravel the mechanisms of typical reductive processes described above, Nicholls et al. explored the various steps of the reductive amination of CO2 using a morpholine base [53]. At the same time, He et al. [54] and Han and coworkers [55] have reported naturally occurring glycine betaine as an efficient, active, and renewable catalyst for C-N bond formation using CO2 and amines with PhSiH3 as the reducing agent. A broad range of amines was screened for this reaction, which gave a significant yield. Also, they studied, by controlling reaction parameters, that selectivity of products can be tuned. Glycine betaine is a quaternary ammonium alkaloid having a zwitterionic structure. It is basic in nature, so it can be used in various chemical syntheses. The Han group proposed a plausible reaction mechanism on the basis of NMR results and literature. First, glycine betaine interacts with amines and PhSiH3 to create Intermediates I and II (Scheme 34). Further, the Si–H bond in PhSiH3 gets activated by Intermediate II, which further on insertion of CO2 forms silyl active species. This was dependent on

30

V. B. Saptal et al.

the molar ratio of the reactants and the reaction temperature. Also, simultaneously N–H bond in glycine betaine gets activated by the formation of Intermediate I. At the end, nucleophilic amine attacks the silyl active species for the formation of C-N bonds to make products. Similar to silanes, phosphines have been used to attempt hydrophosphination of CO2 . Due to the nucleophilic nature of phosphines, successful hydrophosphination of CO2 has been rarely reported. To overcome this barrier, Chong and Kinjo adapted the principle of hydrophosphination to diazophospholene resulting in a successful reaction and formation of phospholene formate, which upon further CO2 exposure transfers the formate to silyl group (Scheme 35) [56]. They proposed the role of formate species in catalytic activity driving the formylation of amines. The use of diazophospholene enables the route of one-pot synthesis of formamides in milder conditions.

H N

+ CO2

Cat. PhSiH3

R R R = alkyl, aryl

O N

H Si H H

Ph

I

CO2

II

H Ph

Si

O

O

O O

N

R H N R

O

R H N R

O

Cat.

H H CH3 CHO R R or or N N N N R R R R R R

O N II

O OSi PhSiH3 PhSi(OCH3)3 or H H H H Methoxysilane Bis(silyl)acetal

PhSiH3 SiO

H H Silyl formate

I

I

I

H H

CHO N R R

R

N R

N R

CH3 N R R

R

Scheme 34 Betaine-catalyzed activation of CO2 and silane for formylation and methylation reaction Scheme 35 Phosphine-catalyzed activation of CO2 and silane for formylation and methylation of amines

N

O

N

PH + CO2 N 1 atm

P

CH3CN, rt

N

O

H

O N + P N O Cat.

R1

H N

Cat. R2

H

O

or R1

N

R2

CH3 N R1 R2

N PH N

H

Organocatalytic Reductive Functionalization of Carbon Dioxide

31

Organocatalytic hydrogenation of CO2 Among other reducing agents, hydrogen is always considered superior in terms of atom economy, cost, and so on. However, the activation of dihydrogen is quite challenging through organocatalysts, and very few reports are available. In 2009 Stephen, Erker and coworkers pioneered the activation of CO2 using a phosphino-borane FLP system [57]. From this discovery, reductive functionalization of CO2 has taken intense attention over the last decade. In 2009, Ashley and O’Hare developed the first FLP-mediated activation of CO2 and subsequent hydrogenation into methanol using hydrogen gas (Scheme 36) [58]. This work was inspired by the concept of FLP which split the H2 heterolytically and then consecutive CO2 activation to achieve the thermodynamically difficult task by organocatalysts. Under thermal conditions, B(C6 F5 )3 and TMP (TMP = 2,2,6,6-tetramethylpiperidine) reacted with CO2 and H2 afforded formatoborate complex, [TMPH]-[HCO2 B(C6 F5 )3 ]. Stepwise activation of H2 and CO2 occurred, where heterolytical cleavage of H2 was promoted by an equimolar amount of B(C6 F5 )3 and TMP to give [TMPH][HB(C6 F5 )3 ]. Afterward, CO2 (1 atm) in toluene at higher temperature (>110 ˚C) directed to the generation of a formatoborate complex [TMPH] [HCO2 B(C6 F5 )3 ]. Finally, a quantitative amount of [CH3 OB(C6 F5 )3 ][TMPH] was obtained by purging of CO2 in a mixture of TMP/ B(C6 F5 )3 under an H2 atmosphere. Then, in 2015 Fountain used an intramolecular FLP system of N/B for the hydrogenation of CO2 with H2 to obtain the reduced derivatives of formate, acetal, and methoxy (Scheme 37) [59]. In FLP, weakly Lewis acidic boron activates the H2 H2, CO2 NH + B(C6F5)3

NH2 + HCO2B(C6F5)3 B

TMP A HB(C6F5)3

TMPH

CO2, 110oC H

TMPH

TMPH

B(C6F5)3

B(C6F5)3 O

B(C6F5)3 O

H

O B(C6F5)3

O

HO-B(C6F5)3

A

TMPH TMP H H H

TMPH

H O B(C6F5)3

TMPH

H

H H

O B(C6F5)3 [TMPH]2

Scheme 36 FLP-catalyzed activation of CO2 and H2 to methanol

B(C6F5)3 O

O B(C6F5)3

32

V. B. Saptal et al.

molecule and generates the NH and BH species, which are responsible for the concurrent reduction of CO2 . The NMR investigation study demonstrated that upon exposure of H2 gas with ambiphilic FLP catalyst resulted in the synthesis of 7 at 80 ˚C by removing of mesitylene on boron by photodeborylation reaction. Stephan and coworkers recently used FLP obtained from the 2,6-lutidine and B(C6F5)3 for the hydrogenation of CO2 with H2 and silylhalide used as an oxophile to remove the oxygen atom (Scheme 38) [60]. Due to well-established steric characteristics and suitable nature of silylhalide and solvent selection like C6 D6 and CDCl3 , different types of hydrogenated products such as methoxysilane, Me3 SiOCH3 , the acetal (Et3 SiO)2 CH2 , CH4 , and CH3 I were obtained at 100˚C using CO2 (2 atm) and H2 (4 atm). The reaction mechanism was also determined on the basis of the experimental and DFT calculations. This begins with the known activation of H2 by FLP, followed by the reaction of CO2 to the formyl borate anion.

N

BR2

N

CO2 + H2

BR'2

N +

R = 2,4,6-Me 3C6H2 R = 2,4,5-Me 3C6H2

N

N BR2

B R

R' = -OCOH = -OCH2O= -OCH3

H2

N O

H

O

B R

N

H BR2

H B

R

-RH

N

H

N

B

-RH H

7

Scheme 37 Intramolecular FLP system of N/B for the hydrogenation of CO2

CO2 + H2 + R3Si-X + 2,6-lutidine

B(C6F5)3

H H R

O

100 oC, 20-60h

R = Me or Et X = Halides

Scheme 38 FLP-catalyzed activation of CO2 and H2

O

CH3O-R

R or

CH3I

CH4

H H B R

Organocatalytic Reductive Functionalization of Carbon Dioxide

33

2 Summary and Outlook Capture and conversion of CO2 to value-added chemicals and fuels is taking enormous attention not only due to its environmental impacts but also to maintain the carbon neutral cycle by reducing CO2 concentration. Furthermore, due to current concerns about environmental degradation, urgency is required in the development of efficient capture processes and the design of highly active catalysts for CO2 reduction reactions. Here, we have summarized recent advancements in the utilization of organocatalysts for the reductive functionalization of CO2 to possible structures using borane, silane, and dihydrogen molecules. This transformation results in fundamental C1 building blocks such as HCOOH, HCOH, CH3 OH, CH4 , and C2 H5 OH. Along with these and by depending on coupling partners, other bonds such as C–N, C–C, C–O, and C–S are also formed in reductive functionalization process. Generally, CO2 transformation reactions are mainly catalyzed by transition metal catalysts, which are expensive, toxic, and sensitive to handle under normal conditions. Furthermore, the hydrogenation process demands harsh reaction conditions such as high pressure and temperature, and catalyst deactivation by leaching is always associated with this process. On the other hand, organocatalysts perform the CO2 reduction chemistry at the very mild reaction condition with high efficiency. Soft reducing agents like boranes and silanes with organocatalysts ultimately demonstrate the sustainability of processes. It is noteworthy that these reductive transformations of CO2 can generate both fuel-type structures and also pharmaceutically necessary scaffolds. Active centers of organocatalysts such as Lewis’ acids, basic amines, FLP, carbenes, ylides, cations, and anions demonstrated excellent efficiency in activating CO2 , reducing agents (boranes, silanes, and H2 ), and coupling reagents. Various active intermediates such as formates, acetals, and activated hydride species are generated with various hydroelements (B, Si), which subsequently endow the generation of exciting functional groups such as N-formyls, N-methyls, methanol, methane, aminals, and heterocycles. In organocatalysis, silyl formate is generated as an initial reaction product or as an intermediate, which has its own importance in the formation of CH3 OH, CH4 , formic acid, formamides, etc. Various active organocatalysts with well-defined active centers such as FLP, NHC, IL, amine superbases, carbene, and phosphine ylides demonstrated excellent catalytic activity for the reductive transformation of CO2 . Along with this, we have tried to give mechanistic insights on organocatalytic active centers for the activation of CO2 and reducing agents. Notes The author declares no competing financial interest. Acknowledgements The authors thank Springer Nature publisher for their kind invitation.

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CO2 Conversion via Catalytic Hydrogenation to Methanol, DME and Syngas Muhammad Usman, Mustapha D. Garba, Zonish Zeb, Muhammad Israr, Safia Safia, Fatima Javed, Munzir S. Suliman, Bandar Alfaify, Mohammed A. Sanhoob, Naseem Iqbal, Muhammad Humayun, and Aasif Helal

Abstract This chapter introduces carbon dioxide (CO2 ) hydrogenation to fuel and valuable chemicals as a best approach for CO2 mitigation. Among the useful products, methanol (MeOH), dimethyl ether (DME) and syngas are highly attractive industrial feedstocks. This chapter provides an overview of the mechanism of catalytic hydrogenation of CO2 to methanol, dimethyl ether and syngas which mainly undergoes via a methanol-mediated route or modified-CO2 Fischer–Tropsch synthesis (FTS) via two-step RWGS reaction and FT synthesis. The advancement in hydrogenation catalysts for CO2 conversion to MeOH, syngas and DME is highlighted. This chapter outlines the progress toward promoters, catalyst supports, structure–performance associations and reaction conditions’ optimization for each process. In addition, this

M. Usman (B) · M. S. Suliman · B. Alfaify · M. A. Sanhoob · A. Helal Interdisciplinary Research Center for Hydrogen and Energy Storage (IRC-HES), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia e-mail: [email protected] M. D. Garba Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK Z. Zeb · M. Israr Department of Chemistry, Tsinghua University, Beijing 100084, China S. Safia · F. Javed Department of Chemistry, Shaheed Benazir Bhutto Women University Peshawar, Peshawar, Pakistan B. Alfaify Faculty of Engineering and Information Technology, Onaizah Colleges, Unaizah, Saudi Arabia N. Iqbal U. S. Pakistan Center for Advanced Studies in Energy (USPCAS−E), National University of Sciences and Technology (NUST), Islamabad, Pakistan M. Humayun Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jawaid and A. Khan (eds.), Sustainable Utilization of Carbon Dioxide, Sustainable Materials and Technology, https://doi.org/10.1007/978-981-99-2890-3_2

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chapter describes the importance of heterogeneous catalysis and future perspectives for the design of efficient catalysts for CO2 conversion to valuable products. Graphical Abstract

Keywords CO2 reduction · Hydrogenation · Methanol · DME · Syngas · Catalysis

1 Introduction The burning of fossil fuels is the main source of energy and a big source of anthropogenic CO2 emissions into the atmosphere. Moreover, CO2 is a greenhouse gas and its excess amount in the atmosphere leads to dire consequences such as global warming and abrupt climate changes, including melting glaciers, extinction of species and rising sea levels. The increasing global energy demands have led to more CO2 in the atmosphere, 414 ppm CO2 reported in 2020, compared to 410 ppm in 2013 [1]. In 2017, energy consumption emitted 33 gigatons of global CO2 , almost two times higher than the amount accommodated by land- and ocean-based CO2 sinks

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[2]. It is expected that the energy consumption will be almost double by 2050 as compared to the current energy consumption [3]. This would drastically increase the CO2 concentration in the atmosphere. Therefore, it is of immense interest to reduce atmospheric CO2 concentration. To achieve CO2 reduction goals, many strategies, including Carbon Capture and Storage (CCS), CO2 capture and utilization (CCU) and carbon capture and sequestration (CCS), have been developed [4–9]. These strategies depict CO2 as a key component to drive chemical and fuel industries rather than an undesirable waste, thus offering a sustainable future through chemical recycling of CO2 [10]. CO2 conversion to fuels and valuable chemicals will aid in fulfilling the growing energy demand and facilitate inexpensive fuels and fine products. CO2 can be hydrogenated into liquid fuels through direct or indirect routes [11]. CO2 hydrogenation is one of the promising green strategies for reducing atmospheric CO2 by the formation of important chemical products and fuels such as syngas, methanol and DME [12–16], as schematically presented in Fig. 1. The CO2 hydrogenation can be divided into two main branches: (i) methanation reaction and (ii) synthesis of hydrocarbons through Fischer–Tropsch synthesis (FTS). In later method, first step involves reverse water gas shift (RWGS) reaction, while

Fig. 1 Schematic representation of CO2 hydrogenation to methanol, DME and syngas

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second step is conventional FTS with CO [17]. Different possible CO2 hydrogenation reactions are given in Eqs. (1–5). CO2 + H2 ↔ CO + H2 O

(1)

CO + 3H2 ↔ CH4 + H2 O

(2)

CO2 + 4H2 ↔ CH4 + 2H2 O

(3)

(2n + 1)H2 + nCO → Cn H2n+2 + nH2 O

(4)

2nH2 + nCO → Cn H2n + nH2 O

(5)

In the first step (Eq. 1), CO2 will go through hydrogenation for producing CO through the RWGS reaction. Then, further hydrogenation of CO can lead to the production of CH4 (Eq. 2). Hydrogenation can take place through direct methanation of CO2 via the Sabatier reaction (Eq. 3). Therefore, the methanation reaction is an exothermic reaction taking place at moderately high temperatures (200–500 °C), while other than methanation reactions, FTS can be used to synthesize hydrocarbons such as alkanes and alkenes as important chemicals, for example, dyes, coatings, plastics and transportation fuels. (Eqs. 4 and 5) [17, 18]. Therefore, CO as a versatile chemical compound produced through CO2 hydrogenation can serve as a direct synthesis route to methanol, hydrocarbons and dimethyl DME. CO2 hydrogenation to methanol is effective mean for the latest proposed “methanol economy” (proposed by Nobel Laureate George Olah) and “liquid sunshine”. In addition, methanol serves as an effective raw material for preparing dimethyl ether (DME) and hydrocarbons [19, 20]. Therefore, CO2 hydrogenation reactions lead to the synthesis of clean, multipurpose and green fuels that can be alternatives to the fossil fuels [21, 22]. Given the catalytic hydrogenation of CO2 through electrocatalysis, thermal catalysis, photocatalysis and photothermal catalysis into the liquid fuels such as methanol, syngas etc. is an effective method to facilitate environment-friendly fuels and reduce the need of fossil fuels [23–26]. Like water, CO2 is known to be highly stable (ΔGf ° = −396 kJ mol−1 ) and C–C coupling is challenging. Although the design of an efficient CO2 hydrogenation catalyst is critical, hydrogenation involves complex reactions that are influenced by both kinetics and thermodynamics. It is found that catalytic performances for hydrogenation are influenced by the supports, particle sizes and promoters [27, 28]. Yet, for effective CO2 hydrogenation, heterogeneous/ homogeneous catalysts are being used and have proved to be efficient materials for boosting hydrogenation into liquid fuels. A review of the supported catalyst preparation method has been reported recently [29]. In case of CO synthesis through RWGS reaction, the noble metal (Pt, Pd, Rh and Ru) catalysts have been reported as effective materials. It was found that K-Rh/Al2 O3 (with weight ratios 16.5−0.8–82.7) had 100% selectivity for CO [30], while among noble metal free catalysts, copper

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(Cu) metal-based catalysts have been recognized as best option due to their costeffectiveness and higher activities. For instance, Cu-based ternary catalysts such as Cu/ZnO supported on Al2 O3 are the most used catalysts for CO synthesis through hydrogenation of CO2, particularly at lower temperatures [19, 31, 32]. Iron (Fe)based catalysts widely support CO2 conversion to C2−C4 hydrocarbons through FTS [33]. Co and Fe have been reported to produce chemicals and fuels, while Co has been reported to produce methane selectively during CO2 hydrogenation [34]. For CO2 hydrogenation to methanol reaction, among many metal-based catalysts, the most efficient and extensively studied catalysts were modified Cu catalysts. As, the main challenge faced by the CO2 hydrogenation to methanol is the low product selectivity because of competing reverse water–gas shift reaction (RWGS; CO2 + H2 → CO + H2 O). Hence, Cu catalysts are found to be effective in overcoming this challenge. CuO/ZnO/Al2 O3 (CZA) is a highly active conventional catalyst for methanol synthesis from syngas [21, 35]. The thermodynamic behavior of CO2 hydrogenation to DME is similar to methanol synthesis. Therefore, Pd and Cu-based catalysts are prerequisites of DME synthesis through CO2 hydrogenation. Similar to syngas conversion to DME, direct CO2 hydrogenation to DME involves bifunctional catalysts consisting of CO2 hydrogenation catalysts and solid acid catalysts (methanol hydrogenation) for effective methanol synthesis and methanol dehydration [19, 35, 36]. Moreover, it is evaluated that CO2 hydrogenation to methanol and DME is favored by high pressure, low temperature and higher H2 /CO2 ratio [37]. However, the gaps in the fabrication of perfect hydrogenation catalysts exist which can be fulfilled through the optimization of active sites, interaction among different components responsible for catalytic activity and the insight into reaction mechanisms (kinetic and thermodynamic analysis) [37, 38]. Methanol (CH3 OH) is the simplest form of alcohol with a unique volatility and inflammable characteristics under normal conditions. It is used as a feedstock in many chemical industrial processes such as paints, explosives, fuel and spirit. Methanol has substantial applications in both chemical industries and the research field. CO2 conversion into CH3 OH is one of the multi-beneficial options for utilizing CO2 [39] with environmental sustainability. This is because CO2 gas contributes negatively in our environment with atmospheric destruction. Recent records show that methanol can be utilized for the purposes as follows: (a) Energy > 40% of global usage due to output energy of ~20 MJ/Kg. (b) Solvents (in the manufacture of printing ink and pharmaceuticals). The CO2 hydrogenation reactions to MeOH proceed via stoichiometric and rational interaction of the CO2 (obtained from fossils and renewable sources) with hydrogen. This produces methanol under variable reaction conditions (i.e., high pressure, 200–300 °C temperature and supported metal oxide catalyst) as shown below: CO2 + 3H® 2 CH3 OH + H2 O

ΔH298K = 49.5 kJ/mol

(6)

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The mechanistic insight of the CO2 → MeOH via the formate HCOO* is COOH* pathway widely reported in literatures, but may also include CH3 O2 *, CH2 O* and CH3 O*. The HCOO* hydrogenation leads to the production of HCOOH*, where the hydrogenation surface intermediates species assisted by CO using HCO* as the hydrogen donor are the mechanism contributing intermediates for methanol formation. The CO2 hydrogenation to methanol is limited by the CH3 O* hydrogenation which is the common slow step. However, the individual contribution of each steps is determined by their respective slow step illustrated below: HCO ∗ + H ∗® CH2 O ∗ + ∗

(7)

HCOOH ∗ + H ∗® CH3 O2 ∗ + ∗

(8)

As such, the H2 is attributed with reductions with low intermediate generation [40, 41].

2 Catalytic CO2 Conversion to Methanol Several reports discussed the progress of catalytic methanol production via CO2 hydrogenation [42, 43]. Emphasis was made on catalysts based on supported oxides of noble and transition metals, which could lead to the production of other hydrocarbons with little desired methanol. Consequently, research focuses on Cu particles and ZnO have been extensively exploited with promising high methanol yield and CO2 conversion. Other reported catalysts include Cu/ZrO2 and Zn/ZrO2 . Cu-based catalysts have been widely used for CO2 to methanol conversion. Liu et al. [44] synthesized Cu/TiO2 -MgO catalyst with variable MgO content. Their finding shows that the MeOH production appreciably depends on the MgO loading. As shown in Fig. 2, about 56% methanol was produced with 5%wt. MgO which is the optimal content. The ~56% production of MeOH indicates improved selectivity (>85%) compared to the Cu/TiO2 system without MgO at content reaction conditions. The addition of the MgO improved the catalyst activity by enhancing the Cu-support interaction [44]. The structure–activity relationship of Cu and ZrO2 in the synthesis of methanol via CO2 hydrogenation is studied by Witoon et al. [45]. Different phases of the ZrO2 (amorphous (a), monoclinic (m) and tetragonal (t)) were dispersed onto the surface of copper. The Cu/a-ZrO2 exhibited the highest surface area and CO2 conversion to methanol; however, Cu/t-ZrO2 revealed higher TOF(methanol) compared to Cu/a-ZrO2 and Cu/m-ZrO2 . Figure 1(a) compares the CO2 conversion rates at different temperatures and Cu/a-ZrO2 showed the highest CO2 conversion rate due to higher surface area of copper. However, the Cu/t-ZrO2 had the highest methanol selectivity at 240 °C (Fig. 2b). The selectivity dropped for all the catalysts with increasing the temperature. Figures 2c, d show the space time yield of methanol and CO which is increased with rise in temperature.

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Fig. 2 CO2 conversion (a), selectivity to methanol (b), space time yield of methanol (c) and space time yield of CO (d). Reproduce with permission from Ref. [45]

Dang and coworkers [46] reported > 66% MeOH selectivity using Cu/Zn/ZrO2 treated with hydrocarbon. However, work by Lei et al. [47] shows that replacing the ZrO2 with Al2 O3 improved the CO2 activation to ~16% from ~14% with merely about the same CH3 OH (i.e., 66%) selectivity and good stability of the catalyst. A study by Huang et al. [48] shows that the incorporation of ZrO2 into CuO/ZnO catalyst increases the Cu-particle dispersion and specific surface area. Both the CO2 conversion and CH3 OH selectivity are greatly enhanced by the addition of ZrO2 . Nano-structured Cu-particle with ZrO2 [49], porous and structured Cu–Zn/A-forms [50] all provide enhanced CO2 conversion and CH3 OH selectivity. The reaction temperature of 260 °C provides an optimum CO2 conversion and ~16 and 51% CH3 OH selectivity and yield, respectively, whereas lower reaction temperatures produced poor results. Similarly, pressure variation (i.e., 2–5 MPa) enhanced the CO2 conversion from ~12 to 20% and CH3 OH yield. Meanwhile, increasing the WHSV from 5,000 to 20,000/gh affects and reduces the CO2 conversion from ~15 to 9%, increasing the CH3 OH yield similar with other findings [51–54]. Deerattrakaul et al. [55] investigated the extent of catalyst reduction time using Cu–Zn/NMGO (where NMGO = N-modified graphene oxide) catalyst. The catalyst was reduced in situ with variable time using hydrazine prior to the carbon dioxide hydrogenation (i.e., 30–180 min) at 250 °C and 15 bar pressure. Their finding shows

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that at 90 min, the highest CH3 OH conversion (~30%) was attained, using 591 mgMeOH g−1 cat h−1 . However, the catalyst activity drastically reduced at high temperature due to catalyst sintering. Garba et al. [1] reported that the catalyst performance depends on the catalyst synthesis procedure, thus having an important effect for obtaining good results. For example, Tursonov et al. [56] reported that Cu–Zn supported on alumina or silica catalyst prepared by impregnation methods produced better results compared to the ones prepared by precipitation method. This is because high dispersion of Cu species with high surface area was obtained which are more active for the H2 and CO2 activation. Wisaijorn [57] modified Cu/ZrO2 and Cu/ ZrO2 /TiO2 catalysts by CaO incorporation and utilized for CO2 hydrogenation. The addition of CaO to the catalysts significantly increased the CO2 conversion as well as the CH3 OH selectivity (>25%) at 250 °C, 3 MPa and 79,900 h−1 reaction conditions. Their finding reveals that the addition of CaO improves the basicity of the catalyst active site. Metal–organic frameworks (MOFs) are highly crystalline porous materials and have been extensively used in CO2 capture, separation and conversion [58–70]. A detailed review on the applications of MOFs for methanol synthesis has been reported previously [71]. Rungtaweevoranit et al. [58] showed that the performance of Cu can be further improved by encapsulating Cu nanoparticles in zirconium-based MOF. The Cu ⊂ UiO-66 was synthesized by adding the as-prepared copper nanocrystals (Cu-NCs) in solution containing the MOF precursors (Fig. 3a), which lead to the formation of UiO-66 wrapped Cu-NCs. The CO2 conversion performance of the Cu ⊂ UiO-66 was compared with Cu on UiO-66 by depositing Cu-NCs on a surface of the pre-synthesized UiO-66. Figure 3c shows the enhancement of the TOF of methanol formation for Cu ⊂ UiO-66 compared to the Cu-NCs on UiO-66. The TOF of methanol formation for the Cu ⊂ UiO-66 was also compared with the benchmark (Cu/Zn/Al2 O3 ) for methanol and CO formation at different temperatures (Fig. 3d).

3 CO2 Hydrogenation to Dimethyl Ether (DME) DME is an important chemical and can be produced via CO2 hydrogenation [72, 73], and about 5 metric tons is produced every year globally and is projected to grow up steadily in the future [74]. There are numerous uses of DME such as in refrigerant, in the production of alkyl-aromatics, propellant, light olefins, fuels and lots of other important chemicals [75–78]. DME is produced industrially by methanol dehydration obtained from syngas, as illustrated below. The methanol in the second equation is converted into the DME in a subsequent reactor. ◦ CO2 + 2H® 2 CH3 OH ΔH = −90.6 kJ/mol

2CH3 OH® CH3 OCH3 + H2 O ΔH◦ = −23.5 kJ/mol

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Fig. 3 a Crystal structure of UiO-66 where Zr oxide SBUs are linked with BDC to form an ordered array of the SBUs. b Illustration of active site of Cu NC-UiO-66 catalyst. One Zr oxide SBU [Zr6 O4 (OH)4(−CO2 )12 ] is used as a representative of ordered array of SBUs. Atom labeling scheme: Cu, brown; C, black; O, red; Zr, blue polyhedra. c Initial TOFs of methanol formation over Cu ⊂ UiO-66 and Cu on UiO-66. The reaction rates were measured after 1 h. Reaction conditions: 7 SCCM (standard cubic centimeters per minute) of CO2 , 21 SCCM of H2 , 10 bar and 175 °C. d TOFs of product formation over Cu ⊂ UiO-66 catalyst and Cu/ZnO/Al2 O3 catalyst as various reaction temperatures. No CO is produced in the case of Cu ⊂ UiO-66 under all reaction temperatures. Reproduce with permission from Ref. [58]

However, there is also a one-step process, where both equations above are combined in the same reactor using a hybrid catalyst [79, 80], but could be a bit challenging due to correct ratio of the hybrid metals with controlled interaction as illustrated in the equation below. CO + H2 O® CO2 + H2 ΔH◦ = −41.2 kJ/mol ◦ 3CO + 3H® 2 CH3 OCH3 + CO2 ΔH = −245.8 kJ/mol

Research has also indicated that DME can be produced directly from CO2 hydrogenation, instead of the syngas way. However, this process requires extra H to remove the oxygen from the CO2 , plus the thermodynamics is not as favorable as syngas to DME. As such, the process gives very low yield of the DME [81].

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Fig. 4 Effects of temperature and pressure on the yield of methanol and dimethyl ether (DME) in the thermodynamic equilibrium of the direct synthesis of methanol and DME from syngas (a) and CO2 (b). Reproduce with permission from Ref. [82]

Ateka et al. [82] compared the production of DME and CH3 OH from both syngas and CO2 hydrogenation which shows that the obtained DME low yield forms CO2 and H2 feed and is associated with low equilibrium constant of the CH3 OH formation than for the syngas. The DME formation was shown to be controlled by RWGS reaction participation. However, the reaction temperature (200–400 °C) and pressure (10–100 bar) play an important role in the yield of DME as illustrated in Fig. 4. The catalytic CO2 conversion to DME requires the combination of efficient metal and acid site for CH3 OH synthesis and subsequent dehydration to DME. Both acid function and metal side of the catalyst can be adjusted necessary for hydrogenation to methanol (i.e., high activity at low temperature), and the acid site stabilizes and strengthens to obtain high dehydration at identical low temperature [83].

3.1

Methanol Dehydration to DME

Alumina-based materials (such as γ-Al2 O3 and η-Al2 O3 ) are considered as the substrate materials for the dehydration conversion of CH3 OH to DME [84, 85]. This is because the reaction occurs on the Bronsted or Lewis acid site. Both γ-Al2 O3 and η-Al2 O3 are characterized with medium and weak-strength acidity; alumina is also cost effective with high surface area, mechanical strength and excellent thermal stability. This is the reason why alumina is suitable for this operation. Alumina is also modified to avoid adsorption of H2 O and subsequent loss of activity. Yaripour et al. [86] studied the activity and physicochemical properties of γ-Al2 O3 by varying the synthesis conditions. The study shows that the CH3 OH dehydration to DME on the γ-Al2 O3 is influenced by pH and precipitation method of the alumina synthesis. Crystalline γ-alumina synthesized with ~ 252 m2 /g surface areas yields methanol conversion of ~78%. Another Cu–ZnO/Al2 O3 catalyst prepared by

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Carvalho et al. [87] derived by Si insertion to the alumina exhibited direct CO2 hydrogenation to DME at 270–290 °C temperature and 30–50 bar pressure. Osman et al. [88] studied the modification of alumina weak Lewis acid site with metals such as Au, Ag and Cu as electron acceptors. Variable Ag loading (i.e., 1, 10 and 15% w/w Ag) was used to modify η-Al2 O3 and the catalyst performance was evaluated for DME production using fixed-bed reactor at 180–300 °C, 48.4 h−1 WHSV. The catalyst activity improved, and the 10% w/w Ag/ η-Al2 O3 was more pronounced. This improvement was linked to improved Lewis acidity and surface hydrophilicity [85]. Armenta et al. [89] reported the formation of DME through CH3 OH dehydration using hematite (Fe2 O3 )-promoted Cu/γ-Al2 O3 catalyst (i.e., Cu-Fe2 O3 / γ-Al2 O3 ). The CH3 OH conversion attained 70% using 290 °C and atmospheric pressure. The activity is similar to those obtained at harsh conditions. About 100% DME selectivity was reported with the catalyst and CH3 OH conversion was maintained after the regeneration at 600 °C for 2 h in air environment. Zeolitic-based catalysts have also been reported with excellent CH3 OH dehydration to DME, but their activity depends on the zeolite physicochemical properties. However, in comparison with Al2 O3 , zeolite shows more tolerant water adsorption and is a sign of better activity [90]. Meanwhile, zeolites are characterized with strong acidity which might cause some site products and coke formation. The CH3 OH dehydration to DME can also be affected by the zeolite morphology and acidity [90] and the hydrophilicity was reported to be affected by the Si/Al ratio. Catizzone et al. [91] reported the activity of BEA, MFI and FER zeolites with same Si/Al ratio and compared their activity. The best result was achieved over BEA with largest pore-3D framework, but the catalyst fastly deactivated. Low carbon formation was observed with MFI, while FER showed best stability and DME selectivity at high temperatures. Other forms of zeolite such as H-ZSM-5, H-Y and H-mordenite have also been used for the operation. Furthermore, smaller crystalline H-ZSM-5 with low Bronsted acid site concentration could be more beneficial in activity compared to alumina. Magzoub et al. [92] reported that monolith form of H-ZSM-5 catalyst give enhanced selectivity to DME (96%) and ~70% methanol conversion at 180 °C. This is because monolith zeolite exhibits low Bronsted acid site concentration and high microporosity that facilitates the process. Other acid function catalyst such as SAPO, natural clays, WOx /Al2 O3 , WOx /TiO2 and Nb-doped TiO2 has also been reported in literatures [93–95].

3.2

Direct DME Synthesis from CO 2

The catalytic function for direct CO2 conversion to DME requires combine methanol synthesis and subsequent dehydration to DME. The catalyst should have adequate properties to be operated under reaction conditions for the direct DME production from the CO2 (i.e., 250–280 °C and 20–50 MPa). Hybrid catalyst is usually employed for this operation that contains both metallic and acidic components. The catalyst’s precise metal/acid site control is a key factor for efficient direct DME production

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[96]. The use of hybrid and bifunctional catalysts has been reported to show high DME activity in a fixed-bed reactor [36]. Hybrid catalyst prepared by Bonura et al. [36] (i.e., CuZnAlZr, CuZnZr and CuZnAl with H-ZSM-5 and or γ-Al2 O3 ) showed high activity toward DME production in a single fixed-bed reactor. Their findings suggest that H-ZSM-5 is more effective acid component compared to the γ-Al2 O3 . It was also shown that CuZnAlZr catalyst works better than the CuZnZr and CuZnAl with ~ 400 g DME/Kgcat /h STY. Studies by Jeong et al. [97] reveal the use of Al2 O3 / Cu/ZnO hybrid catalyst with variable Al loading, for direct CO2 conversion to DME. The studies show that the hybrid catalyst with high Al loading and acidity exhibits higher CO2 conversion (25%) and ~ 75% DME selectivity at 300 °C and 50 bar. The La doping in Cu/ZrO2 catalyst as investigated by Guo et al. [98] showed that 1% La2 O3 doping exhibits best CO2 conversion and DME selectivity (i.e., ~34 and 57%), respectively. The catalyst performance was linked to the optimal adsorption of the CO2 and smaller Cu active species formation. In another report, Jiang et al. [99] studied the nature and strength of metal–acid interaction in hybrid catalyst (Cu/ ZnO-ZrO2 ) with amorphous mesoporous alumina silicate acid substrate. The catalyst prepared by physical mixing led to the higher DME production of ~ 41 gKgcat −1 h−1 at 260 °C and 20 bar. This is better than same catalyst prepared by coprecipitation, where the CH3 OH was actively produced. The lower activity observed with coprecipitation was attributed to the close contact between Cu and Bronsted acid sites. Po et al. reported the direct CO2 conversion to DME using a membrane reactor [100]. The authors optimized the parameter of the sweep gas to feed flow (SW) and the pressure across the membrane and their influence on the CO2 conversion, CO yield, DME yield and water removal as shown in Fig. 5.

4 CO2 Hydrogenation to Syngas The production of syngas via the CO2 hydrogenation is a major step in the fight for sustainable environment and reduction of the carbon footprint in the future, since CO is the intermediate form during the CO2 hydrogenation to valuable chemicals. The CO2 conversion to CO requires high temperature and pressure, because CO formation is not favored thermodynamically. Therefore, the CO2 and CO in the process are referred to as reverse water–gas shift (RWGS) as shown in equation below: CO2 + H® 2 CO + H2 O ΔH = −141.2 kJ/mol The above reaction is endothermic noticeable from the Gibb’s free energy and as such favored at high temperature. High temperature is therefore required for this reaction [101]. The stability of those catalysts which exhibit high activity and selectivity is also very important. Monometallic, bimetallic and nanoparticles’ catalyst systems have previously been exploited, playing an important role in CO selectivity. Some transition metals (i.e., Ru, Pd, Ni, Rh and Pt) supported on metal oxide are

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Fig. 5 Effect of the SW ratio and the ΔP on the reactor performances in terms of (a) CO2 conversion (XCO2 ), (b) DME yield (YDME), (c) CO yield (YCO) and (d) water removal (WR), (H2 :CO2 = 3; T0P = 473 K. Reproduce with permission from Ref. [100]

known to be very active for the production of CO with Ag, Au, Cu and Mo which provide more selectivity toward CO. The CO2 hydrogenation to CO using transition metal such as Au, Ag and Cu produces CO via non-dissociative C-O bond mechanism, with other transition metals such as Ni, Rh, Pt, Pd, Fe, Ru and Co, produces CH4 instead. Usually, the CO formed is adsorbed as a final product via the RWGS reaction [104, 105]. Meanwhile, a CO and or CH4 production depends on CO and the catalyst surface bond. As such, strong interaction of the produced CO and the catalyst surface results in the C-O bond dissociation and helps in the production of CH4 in contrast to Au, Ag and Cu, and transition metals such as Pt, Rh, Ni, Pd, Co, Fe and Ru promote the production of CH4 due to the strong CO adsorption with subsequent C-O dissociation and formation of C-H bond [102]. Usually, CO is formed on the catalyst surface that is either desorbed as a final product (weak adsorption) or as an intermediate (strong adsorption), further hydrogenated to form the CH4 . Strong interaction with metal-Np can results in C-O bond dissociation leading to the CH4 formation [106, 107]. Kwak

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et al. [108] reported that CO2 activation on Pd clusters led to the formation of strong CO bond and subsequent methanation reaction. Studies by Chen et al. [109] show that both CO and CH4 are produced on supported Pt catalyst via the CO2 hydrogenation. It was highlighted that noble metal plays an important role in the determination of product selectivity for CO2 hydrogenation. Relatively large Pt-Nps are responsible for the CH4 production, but when combined with adjacent oxygen vacancy [Pt-Ov] on the catalyst support, CO is suggested to form [110]. Similarly, Ni is also known for CH4 formation via the CO2 hydrogenation [111–113]. However, various metal oxides’ supports such as Al2 O3 , TiO2 , CeO2 -ZrO2 are used as the active site for CH4 production [113–115]. Furthermore, dispersed Ni site is shown to selectively produce CO in contrast to CH4 formation [116] and it is related to the influence of metal particle size. Work by Bahmanpour et al. [101] made a computational calculation of the adsorption energy of CO on various transition metals (i.e., Ag, Au, Rh, Pd, Cu, Pt and Ni). The studies did not include Ru–CO interaction due to the already known strong interaction in the literature [117] and it is not FCC metal. As shown in Fig. 6, metal with high CO desorption energy (i.e., Pt, Pd and Rh) tend to produce both CO and CH4 , while Cu, Au and Ag will selectively produce CO. Recent studies show that monometallic catalysts can selectively produce CO [107, 118]. Cu, Au and Ag are known transition metal with CO weak desorption energy, and this property is well known for high CO selectivity via the CO2 hydrogenation. Au nanoparticles have been reported to form CO from CO2 hydrogenation [119, 120]. Cu-based catalysts have been reported to catalyze CO2 hydrogenation to CO [121, 122]. Although, Cu faces major drawback due to its low thermal stability which leads to the fast deactivation of the catalyst at high temperatures. However, recent studies show that modified Cu is relatively stable. Fig. 6 Calculated CO desorption energies for the (1 1 1) surface of different metals studied for the RWGS and methanation reaction. Reproduce with permission from Ref. [101]

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Zhang et al. [123] reported that Cu-Np dispersed on β-Mo2 C significantly enhanced the catalyst stability compared to the commercial Cu/ZnO/Al2 O3 catalyst for WGS reactions. Cu/β-Mo2 C loses only ~ 15% of its initial activity compared to > 60% obtained using Cu/ZnO/Al2 O3 after 40 h on stream reaction. In another attempt to get a highly stable catalyst, Bahmanpour et al. [124] prepared in situ Cu–Al spinel. The spinel 4Cu-Al2 O3 was compared with reference catalysts Cu/Al2 O3 and known catalyst Cu/ZnO/Al2 O3 . The Cu/Al2 O3 showed around 20% conversion with about 50% loss in the activity after 5 h and Cu/Al2 O3 loss more than 70% of its initial activity, while the spinel 4Cu-Al2 O3 reveals a CO2 conversion with 47% and 100% CO selectivity with an excellent stability for 40 h. The performance was normalized to the copper amount, and the spinel 4Cu-Al2 O3 still showed higher CO2 conversion ratio. A Cr2 O3 /Cu catalyst designed by Shen and coworkers showed high activity for RWGS reactions, better than those of noble metal catalysts [125]. The catalyst was synthesized using dealloying of CrCuAl and formation of the Cr2 O3 layer over the Cu surface. Yu et al. [126] reported that Cu+ and Cu2+ formation both favored the RWGS reactions due to their ability to form oxygen vacancies. Results by Konsolakis et al. [127] demonstrated that Cu/CeO2 nanorods are more active than the nanocubes counterpart in CO2 conversion, due to their higher potential to form oxygen vacancies under reduction. The proposed Cu/CeO2-δ catalyst by Zhon et al. [128] shows that Cu° and oxygen vacancies are actively participating in the RWGS reaction, and its role was proven by Bahmanpour et al. [129] while comparing the Cu-Al and Co-Al spinel oxides for RWGS reactions. Dispersed monometallic Pd/Al2 O3 reported by Kwak et al. [108] selectively produces CO via the CO2 hydrogenation. Their finding reveals that the CO2 activation on the Al2 O3 surface contributes to the formation of weak CO bond and subsequent desorption as a product. Xu et al. [130] reported that monometallic Au-Np encapsulated in Zr-based MOFs (i.e., Au@UiO-67) selectively produces CO. Study by Beh’ms group [131] shows that both CO and CH3 OH are produced initially on the AuCeO2 surface. However, inhibition of CO is faster than CH3 OH, leading to the high CH3 OH selectivity. Bimetallic catalysts with high activity and stability have also been used in CO production. Studies show that Pb-based metallic catalysts shift CO production selectivity from CO and CH4 formation. Ye et al. reported the fabrication of Pd-In/SiO2 catalyst that selectively produced CO as the CO2 hydrogenation product [107]. Ni-Pd bimetallic catalyst was used by Braga et al. [132] to weaken the CO binding energy. The catalyst showed high CO selectivity compared to monometallic Pd. Au–Pd-Np encapsulated in Zn and Zr-based MOFs were fabricated to improve CO production during CO2 hydrogenation [133, 134]. This was attributed to the increase in Au electron density caused by the presence of Pd. Qian et al. [118] revealed that CO is selectivity produced via the active participation of SiH using Pd nanoparticle catalyst supported on the SiH. ZrO2 –Pd_ZrO2 synthesized by Du et al. [135] was used for RWGS and methanation reactions. Subsequent coke formation selectively blocked the CH4 production active sites and led to the 100% CO production. A series of ZrO2 (ZrO2 @Pd/SiO2 , Pd/ZrO2 , and

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Fig. 7 CO2 hydrogenation catalyzed by a ZrO2 @Pd/SiO2 , b Pd/ZrO2 and c ZrO2 @Pd/ZrO2 . To be able to compare the conversions directly, the mass of the catalyst in the reactor was adjusted so as to obtain the same WHSV (1060 LCO2 ·g–1 ·Pd·h–1 for solid lines and 3000 LCO2 ·g–1 ·Pd·h–1 for dash lines) d CO chemisorption measurements of the fresh and spent catalysts (targeting total and reversible deactivations by first and second titrations, respectively). Reproduce with permission from Ref. [135]

ZrO2 @Pd/ZrO2 ) were tested for the CO2 conversion (Fig. 7). The Pd/ZrO2 exhibited the highest CO2 conversion activity with high CO selectivity (i.e., 92%) (Fig. 7b). However, as it can be seen in Fig. 7d, the Pd/ZrO2 catalyst has deactivated quickly.

5 Summary and Outcomes It is important to take immediate action to alleviate CO2 mainly formed by fossil fuels’ combustion. CO2 capture and hydrogenation into fuels and chemicals are essential methods to reduce our too much dependency on fossil fuels. Recent decades have perceived a rush in academic concern in catalytic CO2 hydrogenation into important products such as methanol, syngas and DME. In the hydrogenation of CO2 to methanol, catalytic CO2 conversion to methanol is thermodynamically favored by high pressure and low temperature. Among the applicable catalysts for hydrogenation of CO2 to methanol, copper-based catalytic materials have gained considerable attention, owing to the low cost, high catalytic activity

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and extensive abundance nature in comparison to the Pd-based catalytic materials. ZnO acts as an effective support/promoter to increase copper dispersion and create active sites. ZrO2 represents other dominant support/promoter that has been extensively considered due to its weak hydrophilicity, great redox ability, oxygen mobility and basicity. The preparation method of the catalyst and support also greatly influence the catalyst performance such as Cu–Zn supported on alumina or silica catalyst obtained through impregnation methods to show better results than that obtained by precipitation method. An important effort has been dedicated to the catalysts by specific prominence on the supports, active phases and roles of CO2 activation. Researchers are trying to increase the catalytic performance by enhancing the dispersion and active sites’ reducibility, accompanied by a synergy between supports and active phases modified by a proper preparation method and supports’ selection. Two possible paths are recognized in CO2 hydrogenation to DME: (i) a two-step route synthesis of methanol followed by dehydration to respective DME and (ii) one-step route direct DME synthesis. The thermodynamic issue for DME production is closely linked to methanol preparation. In addition, low temperature and high pressure are thermodynamically suitable for the DME production. For this combination process, the bifunctional catalysts are required to combine catalysts for methanol production (such as Cu-based material catalysts and Pd-based material catalysts) and solid acid catalysts for CH3 OH dehydration to DME (such as γ-Al2 O3 -based catalyst and zeolites-based catalyst). γ-Al2 O3 -based catalysts have been widely used for CH3 OH dehydration due to their suitable acidity, outstanding lifetime, low cost and extraordinary mechanical resistance. However, the key drawback of γ-Al2 O3 -based catalysts is the water affinity which results in the active site blockage by adsorption on the surface of the catalysts. Consequently, solid acid catalyst materials with great water resistance are good replacements for γ-Al2 O3 . On the other hand, zeolitesbased catalysts with strong acidity could further improve the DME transformation to respective hydrocarbons, which results in the catalyst deactivation. As the byproducts obtain by DME preparation such as hydrocarbons deposited on the surface of the catalyst due to the existence of strong acidic active sites, additional work is necessary to modify the acidic properties of acidic solid material catalysts and achieve high selectivity for the production of DME by applying additional basic elements. In hydrogenation of CO2 to syngas, CO2 conversion to CO via the reverse water gas shift (RWGS) is endothermic reactions and thermodynamically recommended at high temperatures. Non-noble/noble metal-containing catalysts and carbides of transitional metal-based catalysts with different preparation methods, supports and promoters are being established to achieve high catalytic CO2 conversion and selectivity of CO. Noble metal-containing catalysts such as Pt, Pd, Rh and Ru have shown high CO2 conversion activity through methanation; however, their use is limited due to high cost. Non-noble metal-containing catalysts, such as a supported Cu catalyst, are more desirable because of their high activity and selectivity toward CO during CO2 hydrogenation. However, it is quickly deactivated at high temperatures due to its little thermal stability. However, recently, numerous alterations have been developed to often relatively stable and highly active supported copper-based catalysts such as highly dispersed Cu nanoparticle on the supported β-Mo2 C show higher stability compared to the Cu/ZnO/Al2 O3 catalysts.

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Acknowledgements We are thankful for the support from Saudi Aramco Chair Programme (ORCP2390). Conflicts of Interest The authors declare no conflict of interest.

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Similar Life Cycle Evaluation of Microalgae Development for Non-energy Purposes Utilizing Diverse Carbon Dioxide Sources R. Gayathri, J. Ranjitha, and Vijayalakshmi Shankar

Abstract The potential of microalgal species to biofix carbon and synthesize profitable chemical substances has scope in various fields like nutrition, pharmaceutics and cosmetology resulting in commercial exploiting, specifically in the perspective of blue bioeconomy. Among the microalgal species, the strain Phaeodactylum tricornutum serves as a key source for omega-3 polyunsaturated fatty acids like eicosapentaenoic acid, a vital PUFA which cures inflammation and microbial infections. This chapter focuses on the (LCA analysis) “similar life cycle assessment for the development of microalgal strain for non-energy purposes”, by cultivating the strain P.tricornutum. The comparative study on the development of P.tricornutum is to produce biologically fictional compounds with improved quality by supplying two different CO2 sources (synthetic and waste carbon dioxide) and its life cycle assessment. The symbiotic association of waste CO2 with algal cultivation and also the usage of photovoltaics with normal electricity mix are to reduce the environmental impacts. The average impact score was reported as suggested by ILCD Handbook. Sensitivity analysis was performed for cultivating, recirculating culture media and cleansing agents, solvents, freeze-drying, etc., to improve the ecological scores of the impact categories. Demonstration of LCA comparison for both CO2 supplied and evaluating its feasibility for further usage. Ultimately, the result of sensitivity analysis states that improvements on the ecological functions can be done if the essential factors such as power resource, nutritional supply, growth media and cleansing agents were alternated or used in limited amounts. Keywords LCA · Microalgae · Non-energy · Cultivation · Carbon dioxide

R. Gayathri · J. Ranjitha · V. Shankar (B) CO2 Research and Green Technologies Centre, VIT University, Vellore-14, Tamilnadu, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jawaid and A. Khan (eds.), Sustainable Utilization of Carbon Dioxide, Sustainable Materials and Technology, https://doi.org/10.1007/978-981-99-2890-3_3

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General concepts of LCA Direct applications of LCA •

Goal and scope

• Inventory analysis

Interpretation

Impact assessment

• • • •

Development of products Improvement of products Strategic planning Public policymaking Marketing Other uses

Fig. 1 Basic framework of LCA and its applications

1 Introduction Life cycle analysis/assessment (LCA) is a method used to assess the ecological impacts of a product in its entire life cycle accompanying all the stages of a commercial product during the manufacturing process beginning from initial raw materials to final stage resulting in the end product. To examine the environmental impacts caused by a product during its production process at various steps like harvesting, extracting compounds, separation, purification, distribution, usage, repair, maintenance, waste disposal and it recycles, based the results obtained the technique with least environmental impact is considered to proceed with entire product system thereby eliminating the necessity to optimize the system [1]. The objective of LCA is associated with the documentation and improvement of the entire environmental profile of a product, thereby analysing the aggregate potent impact caused by the product on the environment [2]. The LCA encloses various stages (Fig. 1) starting with the goal and scope, definition, inventory analysis and impact analysis along interpreting the entire process [3].

2 Steps of Life Cycle Analysis 2.1 Goal and Scope It explains the purpose and objectives of the study.

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Goal

To define the objective of the study with proper rationale and result comparison, and it is open to public access.

2.1.2

Scope

It should outline the qualitative and quantitative factual data already been obtained describing the in-depth details of the study and demonstrating that the study could fulfil its goal within the specified limits. According to ISO LCA Standard guidelines, the scope must explain the factors that follow.

2.1.3

Product System

It is the collective process in which the process involved in the transformation of input into output required for performing a specific function within the system limits, representing the entire process involved in the life cycle of a commercial product.

2.1.4

Functional Unit

Describes the answers to queries such as what, how, time duration, number of studies, where, when and how effective. Quantitative result of the system’s output enables the comparison of relation between the effort and performance to a reference and presents the necessary alternate material/facility. This stage is the most significant part of the LCA, and it requires a clear definition. It is essential to select the product system (single/multiple) delivering the activity. Thus, it allows and highlights the equal performance exhibited by different systems as a whole functional unit. This must include items in quantifying means including their units, considering temporal coverage and eliminating the input and output of the product system.

2.1.5

Reference Flow

The quantity of material or power required to carry out the functional unit. In general, the quantitative and qualitative reference flow varies for diverse materials or systems among the same reference flow and also be similar in certain cases.

2.1.6

System Boundary

It selectively allows processing techniques involved in the assessment of a product system thereby stating the production of any co-products accounting for expanding

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or allocating the system. The system limitation must reside within the given study goal.

2.1.7

Assumptions and Limitations

This stage enables to incorporate any postulations or finalizing any decisions which would be an impact on the output. Ancillary assumptions and boundaries essentially required for accomplishing the project were necessarily done and documented at this stage to avoid misinterpretation of the output.

2.1.8

Data Quality Requirements

This specifies the type of data to be added with specific limitations. Based on the ISO 14044, documentation of data quality contemplation should be included under its scope, which involves the coverages such as temporal, geographical, technological coverages, complete precise representation of data, uniformity and duplicability of the methodology adopted for this study, data sources, the ambiguity of data obtained or recognition of data breakage.

2.1.9

Allocation Procedure

This aids in separating the products input from the output and essential for the manufacturing processes that yield various secondary products and is commonly termed as multifunctionality of a product system. ISO 14044 has provided a hierarchy to resolve such issues regarding the impact caused by choices for expansion/allocation for secondary products on the output of LCA. The solutions listed by ISO were: (i) Eliminate the allocation by sub-division: This results in disaggregating the unit process into small scale sub-processes for the separation of the product and secondary product at the level of fabrication itself. (ii) Allocation via system expansion must be avoided: This approach helps in expanding the processes of the secondary product production being provided with an alternate method of function for the independent production of the secondary product. The influence made by the production of a secondary product using an alternate method is being eliminated from the determined product to isolate the impacts on the system. (iii) Allocation based on physical relationship: This approach results in dividing the input and output. According to the physical relationship shared among the products, separation will be done. Example: The weight or amount of power required by the system. (iv) Separation based on other relationships: This approach leads to the separation of input and output according to the non-physical relationship. Example: Allocation of products based on their economical values.

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Impact Analysis

This encloses the framework of a diverse class of effects recognized underneath the area of study and utilizing the methods adapted for calculating the significant effects associated with it. Precisely, the life cycle inventory data had been converted into environmental impact scores grouped into human toxicity, smog, global warming and eutrophication. Only an outline of this must be included in the scope, and detailed impact assessment will be provided under the section of life cycle impact assessment (LCIA).

2.1.11

Documentation of Data

In this section, the input and output utilized in the study will be precisely documented with the specific individual flow. This part is essential since the majority of the analysis fail to contemplate the entire input and output of a product system; hence, this gives out the spectators a clear and transparent view of the data being nominated. This clarifies the rationale behind the selection of system limitation, product system and functional unit [3].

2.2 Life Cycle Inventory Life cycle inventory (LCI) assessment encloses the generation of an inventory flow ranging from initial to waste disposal and recycle for a product system. This includes the quantification of resources, basic materials, the power required, various types of operations ranging from atmosphere, land, H2 O, utility of the source material and further emission of substances across the entire life cycle of the product/process. In general, it is a cumulative basic flow associated with all the individual unit process comprised in a single product system. For developing an inventory, construction of a flow model of the technical system is advised utilizing the data input and output required by the product system, and this is represented by illustrating the functions coming under the analysis associated with the appropriate supply chain into a flow diagram that clarifies the limitations of the technical system. In general, the accuracy of the study and results will be reflected in the complexity of the flow diagram with elaborate details. The input and output data required to construct the model are usually obtained for every individual function under the system limitations, which includes the energy consumed by the supply chain, mentioning the technosphere input. Based on the ISO 14044, the documents required for framing an LCI must include the steps as follows: (1). To prepare the data, the goal and scope must be highly focused while collecting it. Collecting, validating and allocating the data if in case it is essential data,

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(3).

(4).

(5).

(6).

(7).

(8).

(9).

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then the data have to be related with the unit process as well as a functional unit and aggregating the data were mains steps involved. Since it is mandatory to align the data with a functional unit, goal and scope based on the ISO 14044 standard, collecting data might result in deviation of data from its goal and scope which would result in extra activities such as incorporation/omission of data (before and after deviation) that had been already obtained in the LCI. The outcome of the LCI includes the compilation of inventory enclosing the basic flow for all individual processes examined under the product system. Generally, there exists data complexity in the form of charts; hence, structural representation with hierarchy is required to simplify the data for clarity. According to the ISO LCA standard, to collect data for every individual process enclosed in a system boundary, a quantitative representation of all individual functions within the product system is required for measurement. Data collection should be focused on the primary sources, and generally, questionnaire is used for onsite data collection and sometimes sent to the production company to obtain the data completely. The questionnaire should contain the following: Product for collecting the data, investigator and date for data collection, time duration, details explaining the process, inputs describing the raw materials, additional materials, power required, mode of transport, outcome describing the waste emitted into the atmospheric including land, H2 O and air and qualitative and quantitative data for all input and outputs. Sometimes collecting the primary data is impossible if the data are highly confidential, and in such cases, secondary data obtained from the LCA databases, literature sources and recorded in previous studies were widely used. The use of secondary data requires proper documentation regarding the variations in the data along with its source, consistency, temporal, geographical and technological representativeness. During the identification of input and output for all individual processes enclosed in the product system of an LCI, the investigator must allocate the multiple input and output streams according to the “allocation procedure” stated with the section of “goal and scope”. The difficulty in assessing the data begins with the technosphere. It is the secondary resource to geologists as it is the human-created world, which is capable of recycling theoretically 100%, and practically, the main goal is recovery. In LCI, the technosphere supply chain product was manmade, and finishing the questionnaire regarding the process in which manmade product as a means to the end would result in the inability for specifying the amount of power required by the process. Data associated with the input and output used by the product previously produced become inaccessible. Hence, the use of secondary data must be appropriately reproduced in regional/national conditions. The LCI also contains the “process-based LCAs”, economic input–output (EIOLCA) and hybrid approach. The physical flows were mixed up with the

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information of a process within the product’s life cycle, and it utilizes data on basic flow accompanying 1 unit of economic function among various sectors. In general, the data are obtained from government agency national statistics tracking trade and services between sectors, and it is a combined form of process-based LCA and EIOLCA. In general, a pedigree matrix is used for evaluating the data quality. Varieties of pedigree matrices exist with numerous data quality indicators with a set of qualitative standards per indicator. Semiquantitative pedigree matrix is also used for evaluating the data quality of LCI using the pedigree matrix in the case of non-technical audiences in specific policymakers.

2.3 Life Cycle Impact Assessment (LCIA) The life cycle impact analysis is followed by the life cycle inventory analysis, and it is an LCA stage generally aiming to evaluate the potent ecological and human health impacts caused by the basic flow fixed in the LCA. According to the ISO 14040 and 14,044 standards, the steps recommended for completion of LCIA were listed below: (a) Selection: Selecting the impaction groups and their indicators, along with characterization models. According to ISO standards, the study must enclose multiple impacts encompassing a complete set of ecological issues. The impacts must be related to a geographical area of that study and must be justified with discussion for all selected individual impacts. In a few circumstances, completion of such is accomplished by the use of pre-existing LCIA method. (b) Classification: Classification starts with raw data obtained in the inventory analysis consisting of the flow of materials and energy required, and it encloses identification of ecological alarms/impacts proposed in the inventory analysis flow. Classification chiefly attributes to global warming, ozone depletion, acidification, formation of smog and toxicity to humans. Example: During an industrial process, the use of petroleum feedstock contains methane, butene and formaldehyde. (c) Characterization: This stage provides information regarding the amount or percentage of results contributing to the impact category commonly referred to as the equivalency factors for creating the impact category indicator. It is intended for converting all the categorized individual flows for the impact into a general unit of evaluation. Example: The unit for measuring the global warming potential is CO2-equiv or CO2-e (CO2 equivalents) in which the value assigned to CO2 is 1, and further conversion of each unit is relevant with their impacts, respectively.

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(d) Normalizing the results: The objective of this is to express temporal and spatial viewpoints for validating the results obtained in LCIA. Here, every individual impact category is assigned with a discrete reference value. The standard reference is the characteristic impact of each impact category for every individual factor such as area representing particular geography, inhabitation, in particular, are of geography (representing individual), industries and other product systems or baseline references. (e) Sorting the LCIA results: The LCIA results were grouped into solitary or multiple clusters outlined in the goal and scope. In general, the sorting is independent and might vary within the studies. (f) Weightage of Impact category: The purpose of this stage is to govern the importance of all individual categories and the level of relative importance with one another. It permits the aggregation of impact score into a solitary value to compare, and it might vary due to decisions made by the corporate associations with an ethical point of view. Commonly, three methods were used to weigh the impact category—they are the panel method, monetization method and the target method. ISO 14044 strongly advises representing the weighed result along with non-weighted results for clarity. Generally, a grouping of life cycle impacts can be done within multiple stages while developing, producing, disposing of and using the product. The impacts were widely categorized as first impacts, use impacts and end of life impacts. The first impact is associated with extracting the basic material from the resource, manufacturing, transport of products into the marketplace, construct/install, etc. Use impact deals with physical impacts during the operation of the products or facilities like power, H2 O and renovating, repairing and maintaining them for continuous usage of the facilities/products. End of life impacts are involved with demolishing and recycling process of waste materials. (g) Interpretation: Life cycle interpretation is used for identifying, quantifying, checking and evaluating the data obtained by the results of LCI or LCIA, and the data were interpreted and concluded into a final summary with a suggestion. It identifies the important problems using the results of LCI or LCIA, evaluating the research in consideration with comprehensive, sensitivity, reliability check, conclusion, limits and suggestions. The interpretation is intended for determining the confidence level of the result and to communicate it in an unbiased, comprehensive and precise way [3]. (j) Applications of LCA: To search the widely accessible life cycles. (Life cycle with least ecological effects.) To predict the determining path and significant tactic plan, product design or design for processing, etc., decided by the industries, public organization and NGO’s (Fig. 2). Choosing significant standard for ecological functions of a company which includes the quantity and evaluating practices, chiefly connecting the ecological evaluation, marketed in association with its formula for eco-labelling [3].

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Extract and process raw materials

Recycle and dispose of waste at end of its useful life

Recycle and dispose of waste at end of its useful life

Product packaging

Use, reuse and maintaining the product Marketing

Fig. 2 Process cycle as described by LCA

2.4 Life Cycle Assessment 2.4.1

Cradle-to-Grave

It is an entire analysis beginning with the production, usage and stage of waste removal or management. Each input and output for all individual stages in the entire cycle is taken into account.

2.4.2

Cradle-to-Gate

The evaluation of a partial products life cycle from the stage of production to the workshop, a preliminary stage where it is not shipped to the customer. Here, the two stages such as the products used and waste disposal were generally eliminated. This stage is significant for determining the ecological product declaration. For example, utilization of biofuel as an alternative for fossil fuel while being transported might have influenced the impact value of the results of LCA.

2.4.3

Cradle-to-Cradle

This stage specifically focuses on the reuse and recycle of the product’s end life during the phase of disposal resulting in the production of an original, new or similar product. For example, production of glass wool as a recycled product derived from the waste glass bottle.

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2.5 Life Cycle Energy Analysis (LCEA) The evaluation on complete energy requirement (direct and indirect) for the production of a product which includes energy supply for producing materials and its maintenance. The entire life cycle energy requirement is ascertained, and it is essential to identify the energy source regarding renewable or non-renewable form. For example, transport division is stated below as a typical example of LCEA. Based on the analysis of social requirements in various spatial measurements, the improvement in transport division was made. Beginning with the local stage, the government decided to construct various modes of transport facilities that allow the manufacturer to reach the market place focusing mainly on transporting the goods representing less transport of individuals. The main plan for developing such transport facilities was due to multiple markets opening and to establish shipping in large-scale goods from the producer to reach national and international levels (Fig. 3). This flow starts with the initial point requirements but gets altered due to different social factors, economical limitations, hazardous alarms, negative ecological effects and modern technology. The outcome will be a specific material of demand. The idea behind LCA leads to streamlined LCA (SLCA) for retaining the primary idea to implement it directly and most effectively. The significant impact of a particular ecological problem will be recognized using the provided guidelines associated with different relative characters (Table 1). The effects are based on a special level, how severely the product could cause damages, the quantity of materials contained, exposure to the population (materials with high toxicity are concerned comparatively with less toxic compounds) and the degree to which it is exposed. The bioeconomic significance of the algae was well identified by the European Commission due to its dual role in the environment and its commercialized products reducing the burden on products from the land. Several

Needs and wants

policy instruments Society

Modification factors

Economy

Reconstruction of the plan

Technology

Pollution Demands

Corporate plans

Product Design

Fig. 3 Interactions between industrial activities and societal systems

Products

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Table 1 Transportation evaluated at a different spatial scale S. no.

Constituency

Local

National

International

1.

Primary product

Regional development

National security

Trade competition

Dedicated system

Dedicated system

Market diversity, stability of demand

2.

Secondary product

Labour supply

Product distribution, market access

Export, market presence

3.

Consumer

Commuting, shopping

Recreation, business

Vacation, business

value-added products can be manufactured using microalgae due to their potential in converting carbon dioxide, H2 O and solar energy into biological molecules such as polypeptides, amino acids and lipids that can be transformed into value-added commercial products [4]. Since 2001, various microalgae were intensively studied and recognized as an important resource of bioenergy to produce and extract lipid/ algal oil when compared to other bioenergy raw materials. The advantage includes maximum yield of biomolecules, and the growth rate is fast when compared other to land-based products and crop sources [5]. Hence, several research works were done chiefly concentrating on the growth of microalgae to produce energy as a substitute material for hydrocarbon-based non-renewable sources. There is a huge drawback in the microalgal growth intended for producing algal fuels due to some limitations. Currently, the technologies and techniques used were insufficient for reaching the economic feasibility and inability to sustain the target [6, 7]. The chief issue concerned here is with the energy being intensively required to grow and harvest the algae and the downstream process involved in obtaining the algal fuels [8, 9]. Hence from a commercial viewpoint, cultivating microalgae to produce value-added compounds became the centre of attraction for exploiting its capabilities [10]. A variety of commercially significant biomolecules can be synthesized using algal biomass such as omega-3 fatty acids, phenol substance, carbohydrate, pigments and protein molecules. Biologically active substances necessary to maintain good health in humans can be obtained from microalgae [11] which describes their use in multidisciplinary areas involving the field of nutrition, pharma industry, cosmetic, biopesticide and plant protectant/insect repellent [12]. Especially considering their market price, the algal product is cheap, and the resultant products will counterbalance the initial, operational charges and considered to be much better an economical viewpoint [13]. Hence, there exists a significant place for the microalgae in the economy with more development and commercialisation in future [14]. Currently, the energy flow, techniques and processes for developing such value-added products have been chiefly focused, and there is a need to determine a fixed price for biomass [15, 16]. The residual material obtained from the biorefineries was originally valorized and enumerated by Greggio et al. [17] in reference with Italy.

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The European Commission 2003 has stated the significance of life cycle assessment as the finest basic plan to assess the potential ecological effects caused by a product during its entire life cycle. Currently, the number of scientific recordings on LCA addressing the ecological impacts for producing microalgae, especially for non-energy purposes and value-added products, was insufficient. Few scientists had reported LCA analysis for non-energy purposes with the systematic comparison of the same products with the use of conventional sources and microalgae. The comparative study conducted by Taelman et al. [18] focusing LCA for the production of a protein meal using soybean versus microalgae, and the results reveal that the soybean was more efficient since the low-scale underdeveloped microalgal system required higher energy; hence, it was considered to be a non-competitive system, but with slight modification with renewable power and with scale-up efficiency suggested in sensitivity analysis, this flaw can be resolved. Smetana et al. [19] compared the LCA to study various methods used for cultivating microalgae to extract protein concentrates, in which she compared the role of ecological functions with conventional protein resources and highlighted that the use of substitute appeared as an advantage in reference with the usage of meat resource. Kyriakopoulou et al. [20] compared the conventional and alternate resources to produce the same compound and cultivated Dunaliella species versus carrot to obtain beta-carotene; though the algae showed huge ecological impact with biomass, the significantly highest yield of beta-carotene content was extracted from D. salina, with low impacts. Gong and You [21] performed a study by optimizing the co-products synthesis using multiple objectives along with biofuel as the main focus in which economical and ecological suitability was studied revealing that global warming alone was a recorded impact in that analysis. Pacheco et al. [22] conducted a study in which biohydrogen along with pigment was co-produced and concluded that it is impossible to contempt the fact that producing pigments were beneficial from an economical viewpoint, but intense energy is required to extract it which exhibits a negative impact on the ability to sustain the entire process. Few researchers comparatively studied the alternate sources and suggested identifying the suitable hotspot for alternate raw materials and to potentially improve the process to implement it at a huge level. The author PerezLopez et al. [23–25] analysed and reported that five biologically active substances were synthesized from Tetraselmica suecica and astaxanthin from Haematococcus pluvialis and the eicosapentaenoic acid produced using P. tricornutum. A similar examination was conducted by combining various methods and techniques to pretreat, extract and recover biomolecule phycocyanin using Spirulina platensis reported by Papadaki et al. [26]. Espada et al. [27] did a comparative analysis on two procedures for extracting beta-carotene using D. salina mainly focusing on the ecological impacts. Bussa et al. [28] evaluated the potencies of microalgae to produce polylactic acid and highlighted two important factors such as optimal situations to grow the microalgae and the impact properties of the targeted product [18–28]. Generally, data obtained from the previously recorded studies emphasize the following facts: high energy is required to produce value-added products using algal biomass, which exhibits negative ecological impacts. Utilization of the entire

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microalgae products leads to high commercial viability, combining the bioremediation, CO2 mitigation process to produce biomass whenever it is feasible would lead to ecological sustainability and economic viability. [4, 29–31]. Currently, global warming has become a major issue, and anthropological CO2 acts as the key source; hence focusing on the sequestration of carbon dioxide using microalgae would reduce the carbon emission as well as act as an alternate carbon source to cultivate microalgae. [32–34]. Since algae are highly potential in capturing the CO2 , the quantity required for cultivating the microalgae would be determined based on physicochemical parameters and operational conditions, selected algal strain which includes temperature, light, oxygen level, temperature, the existence of growth inhibitors, etc. [35]. Based on the rate of photosynthesis and carbon dioxide assimilation, algal biomass can be directly supplied with gaseous CO2 for the improved transference of CO2 in bulk [36]. The extra CO2 is subjected to recycling with the help of various processes used in industries. For example, when energy is produced, the additional CO2 supplied along with raw materials to combust [37] was used to manufacture the cement [38] and utilized to upgrade the biogas (typically with 50–71% of methane with up to 45% of CO2 , along with H2 O and O2 ) [39]. Removal of impurities and CO2 reduction lead to improved biogas quality with the required standard. Various techniques were used to reduce the CO2 which involves physical, chemical and biological processes [40, 41]. Microalgae was recognized as a resource to upgrade biogas thereby sequestrating the CO2 with the help of the biotechnology method when compared to traditional methods. Though the biotechnology method has the potential to remove CO2 during biogas synthesis, it was conducted only at a pilot scale [42]. Recently, the potential of microalgae to biofix CO2 encompassed in postcombustive exhaust gas has been intensively investigated [43]. Most of the research work was focused on the assessment of algal productivity and response to the exhaust gas and significant impacts of the contaminants released within the exhaust gas such as carbon monoxide, nitrous oxide, sulphur dioxide, hydrocarbons, halogens, acids and heavy metals [44]. Some research work suggests that there was zero negative impact caused by CH4 on the cultivation of microalgae for producing gas anaerobically in a digester intended to upgrade biogas. It has been recorded that the quality and productivity of the microalgae were influenced by the presence of pollutant released in the wastewater from the industries; based on this fact, the parameters for biomass quality can be revealed, particularly few were noticed with alterations in lipid synthesis stimulated by the pollutants as a result of CO2 sequestration and not focusing on the value-added products obtained by converting the algal biomass [45– 48]. Since there is a necessity for ensuring the quality standard of biomass cultivated using the exhaust gas, hence a comprehensive assessment of ecological impacts is required. Considering this issue, a study was conducted by Roberto Porcelli et al. [1] mainly focusing on the utilization of carbon dioxide derived from the process to upgrade biogas in comparison with commercially available synthetic CO2 , especially to examine the ecological impacts along with LCA of microalgae for non-energy purpose. They have chosen the algal strain Phaeodactylum tricornutum to produce

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biologically active substances like polyunsaturated fatty acids. Here, the only difference is that the type of CO2 varies, and supplementary resources were comparatively examined using the LCA method. The impact of energy sources on the ecological function of the system was also analysed. Due to less research were recorded on the LCA for ecological activity relevant in cultivating P. tricornutum and the framework to produce microalgae for obtaining value-added substances, this research was the first to be recorded with the usage of analytical data reporting the benefits of waste CO2 utilization for the process of algal cultivation stated by Roberto Porcelli et al. [1].

2.6 Material and Methods The algae chosen by the author for the experiment was a marine isolate of P. tricornutum (strain PTN0301). Sample collection was done at the North Sea in 2003. The author subjected the diatom for monoclonal culture with the use of F/2 media adjusting the salinity to 20. The cultures were kept at 20 ± 1°C for a photoperiod of 16 h light: 8 h dark and about 100–110 μmol photon/m2 /s irradiation. Based on the algal potentiality to synthesize polyunsaturated fatty acid, up to 60% of protein content, eicosapentaenoic acid, concentrates on fatty acids up to 30%, it grows rapidly, and the algal strain was chosen. Life cycle assessment methodology used: This experiment uses LCA to evaluate the ecological functions comparatively under different circumstances. It is a standard method in which the ecological impacts were quantified in association with a product’s life cycle, ranging from initial to final disposal stages (enclosing the stages of feedstock harvest, transport, usage, and recycling). According to ISO 14040 series (ISO [49]), the four following stages were included in this experiment. (1) Goal and scope definition, (2) life cycle inventory (LCI) analysis, (3) life cycle impact assessment (LCIA) and (4) interpretation. This research intends to examine the ecological functions for producing the production of P. tricornutum used for the non-energy purpose at a semi-industrial scale, to evaluate the utilization of two various sources of carbon. The initial batch was supplied with synthetic CO2 and the second batch with exhaust CO2 derived from the process of biogas upgrade. These two trails act as the main focus of the study with an additional trial involving the comparative developments achieved with the replacement of CO2 source which might be derived by alteration in energy source applied to the plant. The product system was investigated at the stage of “cradle-togate”, in which the usage and disposal stage was omitted. Based on the assumptions that the structure of a chemical production plant would be lower or trivial [50, 51], the current experiment neglects the impact of constructing, equipping and maintaining the production plant. The transport phase was analysed for each product brought into the plant, as recommended by Frischknecht et al. [52]. The end product was the dry form of microalgae biomass containing 5% w/w of H2 O, with much more similar biochemical substances [53]. Subsequently, the extraction stage was also omitted in this study, and 1 kg dried weight (DW) of algal biomass was the selected

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functional unit. This analysis was dependent on the original data obtained from a semi-industrial plant (250 l column photobioreactor (PBR)), for the entire apparatus, its energy requirements, and the models developed based on the system. The rate at the microalgae was grown using the 70 l column PBR and at the laboratory was detailed under the segment (description of laboratory experimental tests). The usage of waste CO2 to cultivate the algae is still at a laboratory scale [1].

2.7 Overview of the Production System The outline of the production system enclosing the processing unit and system limitations was detailed in the figure; only four important steps were considered for the analysis purpose; they are (1). cleansing and sterilizing the algal isolates, (2). cultivating the microalgae, (3). the process to harvest and (4). the process that involved free-dry of the algal biomass. Since the microalgae are capable of growing under any environmental conditions, commonly the system used to cultivate it comprises the open and closed system [54]. Apparatus or machines involved in the open system were cost-efficient and simple when compared to the closed photobioreactor device, but it enables regulation of the entire process and factors involved, prevents the entry of contaminants and has high potential to fix the CO2 effectively [35, 48]. Hence, the closed system is considered to be a highly effective technique for growing algal biomass to synthesize biologically active substances with good quality along with the maximum level of CO2 input [55]. The photobioreactor was used to cultivate the algal biomass in this current experiment (Fig. 4).

2.7.1

The Process to Clean and Sterilize the Isolate of Microalgae

To clean and sterilize the PBR, the salinity retained in the apparatus was removed by cleansing it with the HCl solution in the initial stage, followed by sterilizing the apparatus using NaClO2 , twofold washing with H2 O and finally with demineralized H2 O. This activity occurs in-between two consecutive cycles if the PBR becomes empty when the biomass is been collected. Hypothetically, the cleansing and sterilization occur approximately at once in 20 days, based on the biomass growth and if harvested with a static concentration of the biomass. Once the cleansing process is over, the remnant HCl is subjected to wastewater recycling and recovered in a solid form due to evaporation of the solution and directed to the hazardous waste disposal process.

2.7.2

Conditions Required to Cultivate the Algal Biomass

The apparatus used to inoculate the algal isolate was a photobioreactor of 250 l capacity with a column height of 220 cm and circumference about 55 cm, containing

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The system boundary System background

System foreground Cleansing and sterilizing

Supply of H2O

Power generation

Nutrient synthesis

Cultivating algae

Synthesis of CO2

Producing cleansing agents

Harvest

Waste treatment

Freeze-dry the algal biomass

Fig. 4 Outline of the process chains involved and overview of the system limitations

the altered F/2 culture medium [56]; the original concertation of the inoculant was 0.3 g/l (biomass DW), kept under 22°C for a photoperiod of 16:8 (light/dark) reaction, with a light intensity of 150 μmol photon/m2 /s. The culture was constantly aerated to mix (≈0.5 sized bubbles) and supported with the supply of CO2 at a rate of 6.25 l/ min per every 24 min daily. The only difference to be noted was that the first culture batch was supplied with synthetic CO2 which is 99.5% pure (V/V) and the second batch with waste CO2 about 75%, containing nearly 15% of methane residue and other impurities such as hydrogen sulphide about 24 ppm released in the process of biogas upgrade. During this stage, it is taken into account that PBR required a power supply to provide artificial light and power requirement for the process to blow the air and for the maintenance of electrical apparatus.

2.7.3

The Process of the Algal Biomass Harvest

The algal culture was under continuous observation; once the concentration reaches 1 g/l, the culture was harvested, about 70% of the entire biomass yield was achieved for every cycle, and specific quantity of biomass is used as an inoculant for the succeeding cycles and centrifuged to obtain wet algal biomass containing up to 85%W of H2 O content. The remnant media used for algal cultivation were redirected to waste treatment. The energy consumed by this stage was only due to the centrifugation process used for separating biomass from the culture media.

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The Methods Used to Freeze-dry the Algal Biomass/ Cryodesiccation

The biomass after being centrifuged was freeze-dried and kept under − 20°C in a cooler followed by the process of lyophilization, finally with the water content less than 5% w/w, and the energy consumed by the equipment at this stage includes only the cooler and freeze-dryer.

2.7.5

Life Cycle Inventory

According to the author, the original experimental data include only the rate at the algal culture was grown, and rest of the data recorded were derived as primary data from a semi-industrial plant located in Italy to produce a distant algal strain the Arthrospira platensis, and hypothetically, it was altered to suit the cultivation of P. tricornutum strain. The author obtained these background data from the following databases as Gabi Professional Database and Ecoinvent v.2 Database (Fig. 5).

2.8 Energy Required The energy required for the total input and output flows for the foreground system under analysis for both the experimental conditions was mentioned as follows (the author included the database from which the original data were obtained) for the processes, respectively.

2.8.1

Batch Supplied with Synthetic CO2

The input flow was supplied according to the data obtained from the GaBi databases for the following factors such as 2475 l of EU-28 tap H2 O and 1134 l of demineralized H2 O, with a supply of 26,747 g of DE CO2 , and power supply includes 611 kWh of EU-28 electricity grid mix, 526 g of DE hydrochloric acid (32%), production mix at plant, and based on Ecoinvent database, the following chemicals were supplied which include 99.8 g of RER potassium nitrate, as N in the regional scale, RER sodium phosphate, in plant about 37.3 g, RER sodium silicate, furnace process pieces around 13.0 g, 3.59 g of RER, EDTA, ethylenediaminetetraacetic acid at the plant, about 4.73 g of CH iron (III) chloride, with 40% H2 O in the plant, RER zinc monosulphate, ZnSO4 .H2 O at the plant around 0.0137 g, around 1384 g of RER sodium hypochlorite, around 15% in water, in the plant, etc., and the output flows were recorded with following: about 1000 g of dried weight algal biomass was derived as the final product using the supply of EU-28 municipal waste water treatment mix about 2468 l, 24,897 g of CO2 as elementary flow, and based on GaBi database, around 1384 g of EU-28 glass/inert waste on landfill as a source of NaCIO was used.

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Outputs

Inputs Material acquisition

Principal products Formulating, processing and Manufacture Materials

Co-products Distributing the products H2O effluents

Energy Utilization of the product Airborne emission

H2O supply Recycling the product, its components and the materials

Solid waste

Waste management Other ecological interactions

Fig. 5 Elementary stages of inventory analysis of LCA

2.8.2

Batch Supplied with Synthetic CO2

The input flows supply for the batch cultivated using the waste CO2 as per the GaBi database includes the following factors such as 2412 l of EU-28 tap H2 O, 1087 l of EU-28 deionised H2 O, supplied with 25,617 g of waste CO2 , 597 kWh of power supplied using the EU-28 electricity grid mix, 504 g of DE hydrochloric acid (32%), production mix at plant and supplies used as per Ecoinvent database include the usage of 99.8 g of RER potassium nitrate, as N in the regional scale, 37.3 g of RER sodium phosphate, at the plant, RER sodium silicate, furnace process pieces around 13.0 g, RER, EDTA, ethylenediaminetetraacetic acid around 3.9 g, at the plant, 4.73 g of CH iron (III) chloride, with 40% H2O in the plant, RER zinc monosulphate, ZnSO4.H2O at the plant around 0.0137 g, exactly 1326 g of RER sodium hypochlorite, 15% in water, in the plant, etc., and the output flows were reported as follows: nearly 1000 g

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of dried weight algal biomass was obtained as an end product with the supply of 23767 g of CO2 , as an elementary flow, and according to GaBi database, 2406 l of EU-municipal waste water treatment mix and 1326 g of EU-28 glass/inert waste on landfill as a source of NaCIO were used. The author describes in detail about energy requirement during the operational period of the equipment, and it was constant per unit. It has been stated by the author that the CO2 type does not have any influence on the power consumed during the process of harvest, cryodesiccation, production of biomass, etc. To produce 1 kg DW of P. tricornutum, the following conditions were required for both batches. (1) Batch with synthetic CO2 : 191.7 kWh was used by the cultivation lamps, compressor required 95.9 kWh, nearly 42.7 kWh for the control unit, 0 kWh to harvest the biomass, 8.9 kWh to centrifuge it, 32.0 kWh consumed by the chiller for the process of cryodesiccation, and 240.0 kWh was used by the freeze-dryer. (2) Batch with waste CO2 : 183.6 kWh was required to operate the cultivation lamps, 91.8 kWh power consumed by the compressor, 40.9 kWh for the control unit, and no power is required to harvest the biomass; for centrifuging, the biomass 8.9 kWh was supplied, the chiller was provided with 32.0 kWh for lyophilization, and the freeze-dryer utilized 240.0 kWh of energy. The simultaneous production of P.tricornutum with the usage of 2 photobioreactors only differs in the source of CO2 supplied [53]. A fixed quantity of CO2 and nutritional supply was provided for both batches. The waste carbon dioxide was an emission from the biogas upgrade plant fitted with membrane filters executed by the GoBioM project. The algal growth was continuously observed till the stationary growth phase (1–12 days), to retrieve the data regarding the growth rate. Similarly, both the batches were reported with an equivalent amount of productivity which was 0.046 g/l/d; finally, the components of biomass were examined for key compounds such as carbohydrate, lipids and amino acids and basic cellular components showing that both the cultures were not significantly distant. The possibility for incorporating the supplementary off-gas substances was eliminated while evaluating since it was considered to be insignificant for ecological assessment. The carbon dioxide output was calculated using the difference among the input CO2 and amount of fixed CO2 rate. The rate of CO2 biofixation was calculated by estimating the productivity of biomass with the use of stoichiometric CO2 requirement factor necessary for growing the microalgae resulting in 1.85 g CO2 /g biomass [57]. As per the author’s statement, for the inventory data for cultivation, supply of H2 O, production of synthetic CO2 , power and water treatment and management, he used data from the GaBi database. Due to the unavailability of data from the GaBi database regarding the inventory data for producing the nutrients and cleansing agents, information was retrieved from the Ecoinvent database.

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2.9 Transportation The mode of transportation involved a small diesel truck with a loading capacity of 9.3 tonnes, based on assumptions that the distance travelled was noted to be 600 km for chemicals supplied to prepare the growth media, received in the form of salt, 100 km noted for receiving washing agents (sodium hypochlorite 15% and hydrochloric acid 32%), 500 km to receive synthetic carbon dioxide and 100 km to obtain waste carbon dioxide near the biogas upgrade plant. The waste carbon dioxide entered the system conveniently stating that the aftereffects associated with the biogas upgrade were completely designed to produce the main target of the biologically synthesized CH4 gas; in contrast, the waste carbon dioxide obtained was emitted as a waste flue gas; therefore, it was considered to be unaccountable for the aftereffects associated to its production, yet negative effects in association with recovering and delivering it to the microalgae cultivation plant were considered to be accountable. Subsequently, the CO2 recaptured from the biogas upgrade plant could be a direct emission into the environment, (in case of the first batch), the CO2 directly released from the production process was made within the system limitation, accurately mentioned in Haber–Bosch production process, via steam reforming of natural gas, hence involved in inventory, following the greenhouse gas accountable approach projected in Supekar and Skerlos [58], differentiating the greenhouse gas production and its release into the environment, with a suggestion to account the release of CO2 solely if it was produced, thereby avoiding the dual entry or seepage of recaptured carbon dioxide. Simultaneously, the CO2 stored in the bio-product plant was also taken into account, and (in the case of the second batch) the gas is released straightway from the production process relevant to the carbon dioxide; as a feedstock, it was also taken into consideration. The inventory data were calculated using the computer program GaBi 8.0 According to the average impact groups endorsed by the ILCD Handbook (ILCD/ PEF recommendations v1.09) (JRC European Commission 2011), the LCIA was performed. Thus, 16 average impact groups were taken into account, which include the following: the Acidification Potential (ACP), Human Toxicity Potential with cancer effects (HTPC), excluding biogenic carbon (GWPebc), biogenic carbon (GWPibc), GlobalWarming Potential, Terrestrial Eutrophication Potential (ETP), Photochemical Ozone Formation Potential (POFP), Ionizing Radiation Potential with human health impacts (IRPhh), Freshwater Aquatic Ecotoxicity Potential (FAETP), Ozone Layer Depletion Potential (ODP), Marine Eutrophication Potential (METP), Human Toxicity Potential with non-cancer effects (HTPnc), Land Use Change Potential (LUCP), Respiratory Inorganics Impact Potential with particulate matter (RIPpm), Abiotic Resources Depletion Potential (ADP), Water Resource Depletion Potential (WRDP), etc. Normalizing and weightage of the impact category were omitted since it was considered to be elective assessment insignificant for realizing the objectives of this analysis.

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Sensitivity Analysis

This analysis was done for evaluating the effects caused by the input parameters on the model outcome. [49, 59]. The three major parameters include: (1) algal productivity, (2) the culture media recirculation factor and (3) the number of solvents required per cleaning cycle. The readings gathered from a solitary test experiment were taken for use as mentioned in the part of the description of experimental analysis. Nevertheless, this factor may differ substantially due to slight variations in the ecological circumstances with significant impacts on microalgal growth [60]. Considering the possibility for alterations, distant productivity values were accounted for, derived from additional examinations performed using the same variety of the system with modification in the culture conditions such as pH, temperature and photoperiod, thereby varying from − 5 to 63% with respect to synthetic carbon dioxide batch [61]. Relevant to the culture media recirculation factor, the media were defined for single use only and directly cleared out after the process of centrifugation and harvest stage. The recycling of the culture media released in the form of discharge was anticipated only for the following productive cycles; hence, newly prepared media will not be required, yet solely a proper restoration of macronutrients (N, P); due to this fact, this factor associated to recirculation undergoes variation for both the batches and a new condition enclosing the highest supply when recirculated. To achieve this, a model for recirculation was set up for quantitative estimation of macronutrients required for restoration derived from the analytical data regarding the intake of nutrients. Finally, the third parameter accountable was the number of cleansing agents required per every cycle. Even though this procedure was omitted in the plant, there exists a circumstance for the reuse of the same cleansing agents with high possibilities for multiple photobioreactors. Hence, according to the hypothesis of reusing the same cleansing agent for nearly five photobioreactors, the number of cleansing agents around 80% was cut out.

3 Discussion The number of impact categories (scores) improved in the case of the batch supplied with waste CO2 , but the only mild difference in the percentile was recorded (Table 2). The fact behind the improved cultivation with the waste CO2 shows that the generation of synthetic CO2 was absent, and a slight increase in the yield during the phase of production yet considered to be insignificant in the statistic viewpoint. The improved yield in the case of the waste CO2 might be the resultant of the existence of impurities within the off-gas and got incorporated by the algal cell as micronutrients. This data needs to be confirmed with advanced analysis at a huge scale. It has been noticed that the impact score for the climatic change impact category (GWPebc and GWPibc) was reduced up to 14%, even though the amount of CO2 directly released while cultivating the microalgae supplied with waste CO2 was taken into account; meanwhile, the algal production system was said to be irrelevant for its release. There

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might be variations in the productivity of algae, based on the components present in the off-gas depending upon the process involved in the production plant. Though there is an improvement in the algae production using the waste CO2 , the biomass quality has to be analysed to meet the quality standard. Exploiting the off-gas CO2 was relevant to the techniques for realizing the profitable system thereby reusing the waste CO2 to cultivate biomass in the, and during the developmental phase of the CO2 utilizing technologies, its economical profitability remains unknown [62]. The author explained the relative difference between the two experimental conditions relating the four major processes such as cleaning, sterilizing, cultivating, harvesting and cryodesiccation of the microalgae according to its impact category. The impact scores for ACP, GWPebc, GWPibc, METP, ETP, IRPhh, LUCP, RIPpm, POFP, WRDP and ADP were mainly derived while cultivating and freeze-dry process with 50 and 45%, respectively. Around 5% was noted with cleansing, sterilizing and harvest process and only related to five impact groups including the FAETP, FETP and ODP which are equivalent to one-half of the entire impact so far detected, and the HTPc and HTPnc contributed up to 30 and 50%, respectively. The same type of Table 2 Ecological impact scores improved by using the waste CO2 S. no. Impact scores

Improvement in the impact score of waste CO2 recorded in the Unit study

1

ACP

0.37E − 01

2

GWPebc 0.41E + 02

kg CO2 eq

3

GWPibc

0.43E + 02

kg CO2 eq

4

FAETP

0.13E + 01

CTUe

5

FETP

0.1E − 03

kg P eq

6

METP

0.12E − 01

kg N eq

7

ETP

0.13E + 00

Mole of N eq

8

HTPc

0.3E − 07

CTUh

9

HTPnc

0.17E − 06

CTUh

10

IRPhh

0.02E + 02

kBq U235 eq

11

LUCP

0.06E + 02

kg C deficit eq

12

ODP

0.04E − 07

kg CFC-11 eq

13

RIPpm

0.13E − 02

kg PM2.5 eq

14

POFP

0.29E − 01

kg NMVOC

15

WRDP

0.06E + 01

m3 eq

16

ADP

0.03E − 03

kg Sb eq

Mole of H+ eq

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end values might be recorded for both cases, but the process to dry the biomass in case of the waste CO2 batch was somewhat reduced. For proper interpretation of the resultant values, a comparative analysis was conducted to determine the efficiency of different CO2 sources. Each process is added into any of the listed categories: (1). power requirements, (2). synthesis of nutritional supplements, (3). producing the required solvents, (4). H2 O consumed, (5). waste/water treatment, (6). the synthesis and utilization of CO2 and (7). transportation. It has been found that the power supplied to serve as a chief component and contributed remarkably for most of the impact groups especially played a crucial role at those stages where the microalgae have been cultivated and dried. Producing the solvents occurred under the impact groups of cleansing and sterilizing stages, it plays an important role including the impact score for FAETP, FETP, ODP, HTPc and HTPnc. For the same group, the synthesis of the nutrient supplements required to grow the microalgae was recorded with 50% in the ODP impact score out of the entire impact caused. For the impact groups such as FETP, HTPnc and HTPc relevant to produce H2 O, wastewater treatment was slightly influenced. The transportation stage contributed to the impact group HTPnc with more than 10% in the case of the synthetic CO2 batch, and the rest of it were recorded as insignificant. Based on the comparison, it has been observed that the impact profile derived with the utilization of commercial CO2 could be entirely eradicated by alternating it with waste CO2 . The third circumstance under analysis was meant to examine the functionality of power resource—the EU-28 electricity grid mix was partly alternated with a portion of photovoltaics which was considered to be nearly 75% of the power supply required by the production plant, accounting for the accessibility of the self-reliant photovoltaic system evaluated according to the decision of professionals. Comparatively, the ecological profile for the three analytical conditions demonstrates that modifications in the power resource could alter the impact profile at a significant level. Most of the impact group under analysis shows the advantages of replacing the power source with photovoltaics reducing the impact scores up to 50% for ACP, GWPebc, GWPibc, METP ETP, POFP and > 50% for IRPhh, LUCP, WRDP. In contrast, the FAETP, HTPc, HTPnc and ADP were recorded with an increase in the negative impact. Comparatively, the results recorded by the author with the same type of study might be promising and have their limits. In general, most of the LCA analyses for cultivating the microalgae were relevant to producing biomass for energy purposes. This helps to govern the variation between the two systems, and the preference for the open pond system comparatively to the highly equipped closed photobioreactor sets out the economical price of the product to be less [63, 64]. Ultimately, the power consumed by the open system was recorded to be low when compared [65, 66]. Comparing the energy requirement, it could be deceptive or even impacts score relevant for cultivating the algal strains exhibited with distant productivity rate. The yield obtained from various scientific reports mentioned by Ho et al. [48] shows variations, and it could be taken into account, ranging from 0.040 to 1.250 g/l/d. The value of 1.250 g/l/d was recorded with the same kind of algal culture mentioned in this current analysis—the bubble column photobioreactor, but it has been noticed that there is a difference in the biomass yield with two different scales in this current analysis (0.044 divided by 0.046 g/l/d). There

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is a significant difference shown when the power required was comparative per unit of volume/time; in the case of light and air supplied, there exists a distant power requirement for producing the biomass. Taking this factor into account, the analysis is most suitable for comparing and found to be analogous with the research performed by Pérez-López et al. [23] and LCA for cultivating the strain P. tricornutum to produce the biologically functional substances using an indoor bubble column photobioreactor. Here, there is a huge variation reporting that the productivity was eightfold high, and the stage for extracting materials was also encompassed within the system limitations, and due to the usage of the distant functional unit—kg PUFAs and distant impact methodology—CML2001, it cannot be compared directly. With respect to GWP impact, assessments used kgCO2 eq for both of them together, results reported with biomass of about 1 KgDW, the current analytical readings were noticed ranging from 257 in case of the batch with waste CO2 and 298 for synthetic CO2 , but 47.3 was the reading shown by Pérez-López et al. [23]. There exists a huge space and could be acceptable due to disproportions in the biomass yield. In contrast, the impact score of ODP recorded in the current analysis ranges from 1.73 × 10−7 to 1.69 × 10−7 kg CFC-11 eq . Here, it contributes significantly to this impact factor recorded by PérezLópez et al. [23] originated in the transportation stage, currently resulting with mild significance. The reliability of the previously recorded data was high when observed for the significance of the relative contribution of the input and another process; in those experimental work mainly focusing to produce the microalgae or biologically functional substances (Pérez-López et al., [23–25]), the culture stage shows significantly high impacts when compared to the harvesting stage which is usually minimal. The same work was reported for impacts in association with the energy required remarkably, highlighted and previously confirmed LCA analysis concerned with the production of biomass plantation to produce energy [67, 68].

3.1 Sensitivity Analysis The sensitivity analysis shows the impact caused by producing the microalgae. It has been observed that the same degree of impact was resulted for all the impacts groups and determines the alterations up to 4% in case of the negative score and 20– 26% with positive impact effects. The parameters associated with recirculating and reducing the solvents help to lower the impact scores for the following categories such as FAETP, FETP, HTPc, HTPnc and ODP but notably with mild effect on other impact groups. Based on the present analysis, it has been observed that a large number of the impact groups could be counteracted with a mild fall in the biomass yield, and the positive effects caused a result of alteration in the typology of CO2 source supplied. This analysis also recommends the use of a mixed strategic plan with the combination of photovoltaic energy in higher proportion with electric mix along with reduced nutrient supply and solvents used, for minimizing the impacts scores for all the groups. Exactly the impacts scores for FAETP, HTPc and HTPnc become worse when supplied with high photovoltaic energy, but also the nutritional

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supply and solvent use were lowered in that case. Nearly 50% of the impact groups such as ACP, GWPebc, GWPibc, FETP, METP, ETP, IRPhh, LUCP, ODP, POFP and WRDP were reduced. Only the ACP remained with a crucial point because of the non-availability of alternate abiotic sources used for manufacturing the panels of photovoltaics for counterbalancing, and the strategies were not yet improved. The absence of an alternate source for developing photovoltaic panels was responsible for ADP impacts. The key objective stated by the author was to execute an analysis for comparing the life cycle assessment to produce P.tricornutum via the supply of synthetic CO2 and waste flue gas CO2 emitted from a biogas upgrading plant. Based on the study report, it has been observed that the impacts groups were reduced when the synthetic CO2 was replaced with waste CO2 gas particularly utilizing the GHG emissions, due to the non-existence of CO2 generation and a slight increase in the yield at the algal growth phase. For both the analytical conditions, energy requirement played an important role and responsible for the ecological impacts related to most of the impact groups, especially only for the two stages which include cultivating the microalgae and cryodesiccation. In the third circumstance, the utilization of photovoltaic energy resulted in reducing the ecological impacts for most of the impact groups. Additional ecological benefits can be achieved by combining the waste CO2 with a non-conventional energy source. Ultimately, the factors influencing the final results were the data associated with the production of microalgae and its yield, a highly specific profound parameter. The sensitivity analysis assures that mild alteration could negatively affect the biomass yield supplied with waste CO2 . In contrast, the yield of biomass improved the ecological functions ranging from 20 to 25% under laboratory conditions. Simultaneously, improvement was observed for certain impact groups associated with recirculating the growth media and the recycling of cleaning solvents.

4 Conclusion The ecological impacts were scientifically proven to be improved when waste CO2 gas emitted from the biogas upgrade plant was supplied as an alternate source resulting in increased yield when compared to synthetic CO2 . The prospect of utilizing waste CO2 for producing value-added substances by cultivating microalgal species was highly sustainable, sufficient and cost-effective. The compilation of circular economy objectives with a symbiotic function of industries could enable and help to achieve in the direction of blue bioeconomy. This research work also highlights the routes to improve the ecological functions of algal biomass cultivation to synthesize value-added and high-priced commercial products, thereby altering the important factors such as the source of power supply (renewable form) and nutritional supplements. However, to cultivate the microalgae using waste CO2 , the economic and technological difficulties associated with waste CO2 emitted from the biogas upgrade plant should be rectified before its supply. The LCA analysis enables a

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calculation of the ecological impacts caused by a product at all the stages from initial to disposal stage and for microalgal cultivation, and the LCA result has conclusively shown that the impacts scores were reduced for certain impact groups when renewable sources were used. This leads to a clean environment and enables to achieve and manufacture eco-friendly products. For some of the factors associated with the worst-case scenario, it requires a lot of research work to rectify the issues. So, it could be possible to produce value-added products with the use of microalgae eliminating most of the negative effects.

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Microalgae Biotechnology and Chemical Absorption as Merged Techniques to Decrease Carbon Dioxide in the Atmosphere Michele Greque de Morais, Gabriel Martins da Rosa, Luiza Moraes, Thaisa Duarte Santos, and Jorge Alberto Vieira Costa

Abstract The increase in carbon dioxide (CO2 ) concentration in the atmosphere has been widely associated with global warming and climate change. Thus, chemical, physical and biological methods have been studied as strategies to capture CO2 . The biological method of CO2 fixation by microalgae (or CO2 biofixation) stands out because, in addition to capturing this greenhouse gas (GHG), microalgae convert it into biomass that can be applied as a feedstock for biofuels and other bioproducts. Furthermore, microalgae can be cultivated using waste resources such as flue gas and wastewater as nutrients for their growth. However, the efficiency of CO2 biofixation by microalgae may a limiting factor in this process. The integration with another method, such as chemical absorption has shown an alternative not only to improve the CO2 fixation, but also to produce biomass with higher applicability, due to the increase in the biomolecule concentration such as proteins, carbohydrates, and lipids, exopolysaccharides, and pigments. In this context, this chapter aimed to report the impact and highlight the main challenges of microalgal biotechnology and chemical absorption technologies as a combined technique in the CO2 fixation and bioproducts obtained. Keywords Biomas · Chemical solvents · Climate change · CO2 biofixation · Greenhouse gas

M. G. de Morais (B) Laboratory of Microbiology and Biochemistry, College of Chemistry and Food Engineering, Federal University of Rio Grande, Rio Grande-RS, Brazil e-mail: [email protected] G. M. da Rosa · L. Moraes · T. D. Santos · J. A. V. Costa Laboratory of Biochemical Engineering, College of Chemistry and Food Engineering, Federal University of Rio Grande, Rio Grande-RS, Brazil © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jawaid and A. Khan (eds.), Sustainable Utilization of Carbon Dioxide, Sustainable Materials and Technology, https://doi.org/10.1007/978-981-99-2890-3_4

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1 Introduction Climate change is a globally critical issue. Among the main causes, the growing population growth and the consequent effect of urbanization and industrialization to meet high energy demands stand out [1]. Anthropogenic activity associated with energy generation is the main and constant promoter of the increase in the concentration of GHG in the atmosphere, mainly CO2 [2]. CO2 emissions represent a relevant threat to the environment. In the last 20 years, the concentration of this GHG in the atmosphere has increased by about ten billion tons [2]. In the last decade, the CO2 concentration has grown by an average of 2.4 ppm per year, reaching an average concentration of 419 ppm in 2022 [3]. The predicted increase in CO2 in the atmosphere by 2050 (685 ppm) may imply an increase of up to 6°C in the global average temperature [1]. Projections for a more optimistic scenario (average increase of 1.5°C) are strongly linked to CO2 reductions of almost 50% by 2030 [4]. CO2 capture can be performed by different processes, which involve different technologies, such as: adsorption [5], membranas [6], chemical absorption [7], and microalgae cultivation [8]. Absorption is highlighted as one of the main nonbiological processes used due to CO2 capture reaching ~ 100% w w−1 [9]. This process commonly employs amine solvents to remove CO2 and other wastes present in industrial gas streams. However, chemical absorption has disadvantages such as high energy consumption to regenerate solvents and toxic liabilities [10]. Microalgae cultivation is a technology that can be less costly because productive land is not required and industrial wastes can be used to produce biomass, concomitantly with CO2 biofixation [11, 12]. In addition, value-added compounds such as pigments and fatty acids can reduce of investment time of this technology [9]. However, the optimization of microalgae cultivation, mainly related to the maximization of CO2 fixation, is still necessary. The synergy between CO2 fixation technologies through of merging of microalgae biotechnology and the chemical absorption of carbon is efficient to reduce the concentration of this important GHG [13–15]. Furthermore, the increase in the biocompounds concentrations, such as exopolysaccharides [16], pigments [17], lipids [18], and proteins [19], characterize this synergistic process as a potential source of inputs for biofuels, feed, food, pharmaceutical compounds, among others [20]. Therefore, the aim of this chapter was to present the microalgae cultivation and chemical absorption technologies as a combined technique to contribute to the reduction of CO2 concentration, demonstrate aggregated bioproducts obtained, as well as highlight the main challenges and future trends in relation to the topic.

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2 Methods and Technologies for CO2 Emissions Reductions CCS (carbon capture and storage) and CCU (carbon capture and utilization) are similar strategies with different end destinations of the GHG [21]. For CO2 , CCS aims at long-term storage of this GEE in geological formations (eg saline aquifers, depleted natural gas and oil reservoirs) and oceans (deep injection) [22]. CCU strategy has sought to capture CO2 associating processes to generate new products, reducing consumption of fossil matrix and carbon emissions in the atmosphere. Among the CCU technologies, the conversion of CO2 into chemicals and fuels, and the biological conversion of CO2 by microalgae stand out [23]. Three CO2 capture strategies are available: pre-combustion, oxy-combustion, and post-combustion [24, 25]. Depending on the strategy employed, different CO2 capture technologies are used, such as absorption (chemical and physical solvents), adsorption, membrane separation, among others [26, 27]. In this context, microalgae have potential, as they are responsible not only for CO2 capturing through its biofixation but also their biomass can be a source of bioproducts [23, 24]. Hybrid CO2 capture through the integration of two or more technologies has also been investigated for efficiency increase and cost reduction of the process. Among those, are highlighted: absorption-membrane, absorption-adsorption, adsorptionmembrane, adsorption-cryogenic, and membrane-cryogenic [27]. Another integration of technologies that have also been investigated is the absorption-microalgae system [28]. This chapter aimed to present the CO2 capture technologies separately, with the main focus on an integration CO2 fixation system composed of microalgae cultivation and chemical absorption by alkanolamines.

2.1 Adsorption Adsorption is based on the separation of CO2 from the gas stream using solid sorbent [25]. In this process, the gaseous stream containing CO2 permeates through the column packed with the adsorbent, and the gas is adsorbed on the surface of the solid until equilibrium is reached [29]. In CO2 adsorption by solid sorbent, the dominant mechanisms are classified as physisorption (weak physical interactions) and chemisorption (strong chemical interactions) [30]. Activated carbon, calcium oxides, zeolites, hydrotalcite, and metal–organic structures are examples of adsorbents that can be used to capture CO2 [30, 31]. Currently, polymeric nanofibers can be used as CO2 adsorbents [32, 33]. After gas adsorption by the solid surface, it obtains pure CO2 and the adsorbent regeneration through the desorption process [29]. For this, different strategies can be used: pressure reduction (pressure swing adsorption—PSA), temperature increase (temperature swing adsorption—TSA), low voltage electric current (electric swing adsorption—ESA), and also the use of hybrid processes, such as a combination of

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temperature and pressure (PTSA), vacuum and pressure (VPSA), or washing process [25, 26].

2.2 Membrane Separation The use of membranes for CO2 separation has been investigated over the last few years. This technology is considered a recent in comparison to CO2 absorption and adsorption methods [6, 24]. Membranes act as a semipermeable filter in order to promote the separation of a specific gas, such as CO2 , from a gas mixture [26]. Thus, permeability and selectivity are parameters that determine separation efficiency [34]. Polymeric and inorganic membranes can be used to separate CO2 from gas mixtures. The former have featured ease of fabrication, lower production cost, scalability, and good mechanical stability. As for disadvantages, there are “trade-off” limitations between permeability and selectivity parameters, and low thermal and chemical stability. Inorganic membranes have greater robustness, moderate “tradeoff” (between permeability and selectivity), and superior thermal, chemical, and mechanical stability. However, inorganic membranes have brittle, high production costs, and more difficult scale-up [35]. In a view to overcoming the disadvantages, synergistically incorporating the advantages of polymeric and inorganic membranes, the development of composite or mixed membranes has been proposed [35, 36]. These membranes are composed of a polymer matrix and micro or nanoparticles of inorganic material are incorporated into them [37]. Studies have shown that the mixed matrix membranes have favoring the process of CO2 separation due to increased permeability and CO2 selectivity, in comparison to membranes produced with pure polymers [38–42].

2.3 Absorption Absorption is one of the most consolidated and applied technologies on an industrial scale for the separation of CO2 from combustion gases of power generation. It has a high CO2 recovery rate (>90%), moderate operating cost, and is considered a flexible technology [25]. This technology is based on the CO2 separation through liquid solvents, it is classified into chemical or physical absorption [43]. In the former, solvents that chemically react with CO2 are used, while in the physical process, the solvents are inert, and mass transfer occurs at the gas–liquid interface [44]. Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), methyl diethanolamine (MDEA) [45], triethanolamine (TEA) [46], carbonates and ammonia [26, 29] are examples of solvents used in chemical absorption. MEA is the

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most mature chemical absorbent for post-combustion CO2 capture used commercially [47]. Physical absorbents such as methanol (Rectisol process) and polyethylene glycol dimethyl ethers (Selexol process) are used in the natural gas industry for CO2 separation [25, 26, 43]. Chemical absorption has been most effective for low partial pressure CO2 separation and employed for post-combustion CO2 capture. However, physical absorption has better CO2 separation performance at high gas partial pressure and has been more suitable for the pre-combustion strategy [43]. The desorption of physical solvents can be carried out by heating or by reducing pressure, while in chemical absorption, regeneration occurs by heating. Compared to chemical absorbents, physical solvents show weak interactions between CO2 and the absorbent, reducing the energy demand for the regeneration process [26]. On the other hand, physical solvents have lower capture efficiencies [23, 26]. The process of CO2 chemical fixation with MEA, for example, is corrosive to equipment and the absorbent can be degraded by irreversible reactions with oxygen (O2 ) and oxides (NOx and SOx ) present in combustion gases of thermoelectric origin. Therefore, carcinogenic products such as nitrosamines are formed in the degradation of MEA. Furthermore, for the mass ratio between MEA and captured CO2 (1.6:1,000) to be maintained, constant additions of evaporated solvent are necessary [48, 49]. The reaction between MEA and CO2 in an aqueous medium can form different molecules such as CO2 , H2 CO3 , HCO3 − , CO3 −2 and carbamate intermediate. The concentrations, as well as the chemical balance of these species, depend on the total concentration of inorganic carbon and pH [50].

2.4 Biological CO2 Fixation by Microalgae The ecosystem plays an important role in the mitigation of atmospheric CO2 since plants and photosynthetic microorganisms assimilate CO2 through photosynthesis. Among the biological methods of CO2 fixation, microalgae have received attention compared to others such as oceanic fertilization and forestation. The ocean fertilization with nutrients stimulates the growth of phytoplankton in the ocean and consequently the assimilation of CO2 . However, the decomposition of organic matter resultant reduces the concentration of oxygen available to other aquatic life. Another consequence of this method is the acidification of the ocean. Such changes may affect the marine ecosystem [51, 52]. The methods of afforestation, reforestation, and microalgae cultivation convert the captured CO2 into biomass that can be used in several applications. Among them, microalgae cultivation is not seasonal and does not require arable land. These microorganisms have higher biomass productivity and photosynthetic efficiency than higher plants [53, 54]. Furthermore, microalgae can assimilate CO2 from the atmosphere or from flue gases, which expands the possibility of the application of this biological method. The biofixation by microalgae has been widely investigated and reported as one of the main alternatives for CO2 mitigation. The average carbon in microalgae biomass

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is 50% w w−1 [55], thus ~ 1.8 kg of CO2 can be assimilated in the production of 1 kg of microalgal biomass. From this data and considering that the global production of microalgae is ~ 20 kt year−1 , ~ 36 kt of CO2 can be captured. The biomass produced is mainly used for food and feed applications [56], but it can also be used in the production of biofuels and other bioproducts, in the context of biorefinery. This is another advantage of using microalgae for CO2 mitigation. Microalgae are able to adapt to several environmental conditions, as well as tolerate different concentrations of CO2 and flow rates [23]. The suitable combination of CO2 concentration and gas flow rate can maintain the pH of the medium at an optimal value and minimize CO2 losses to the atmosphere [57]. Studies on microalgae fixation have been carried out using pure CO2 or flue gas (real or simulated) [58, 59]. Other factors investigated, regarding the effect on biomass production and biofixation efficiency, are temperature, pH, luminosity, and nutrient concentration. In this context, some conditions studied were temperature from 21 to 40°C, CO2 concentration from 0.04 to 50% v v−1 , pH from 2.0 to 9.0 [54]. The efficiency of CO2 biofixation can also be influenced by the species of microalgae. The most promising microalgae for CO2 capture belong to the genera Chlorella, Scenedesmus, and Spirulina [60]. The cultivation of microalgae for CO2 capture can be carried out in open reactors or photobioreactors [61]. Generally, in the open system, parameters such as luminosity, agitation, temperature, and pH are not controlled. On the other hand, there is no cost with artificial lighting. Cultivation of microalgae in an open reactor, with the only purpose of CO2 biofixation, may not be the most suitable process due to the short residence time of the gas [62]. As a solution to increase the gas/liquid contact time, some raceway reactors are constructed with a sump (~1.0 m deep), located after the paddlewheels, in which the CO2 is added or air is injected for O2 desorption [63]. Photobioreactors allow better control of the fundamental parameters that influence the culture than the open systems [64]. Furthermore, photobioreactors usually have larger cultivation columns to transfer CO2 from the gas to the liquid phase. These systems promote higher biomass productivity, however, microalgal production is more expensive than in open bioreactors [65]. Therefore, the biofixation of CO2 by microalgae in photobioreactors is carried out on demand for biomass production, by the maintenance of the ideal pH of the strain [66]. A reduction of costs for CO2 biofixation by microalgae in photobioreactors can be achieved by reducing nutrient costs, using wastewater [67], and waste from the power generation process, such as minerals [58, 68, 69]. CO2 biofixation is an interesting option for companies that need to adequate the level of exhaust gas from power generation. In this way, Costa et al. [58] reported that the flue gas from the coal-fired power plant, when added to the cultivation of Spirulina sp. LEB 18, increased the production of biomass (by 35%) and reduced the CO2 in the flue gas by 24%. However, a disadvantage associated with the biofixation of CO2 from flue gases by microalgae is the high temperatures of the outlet gas, and the presence of NOX , SOX , and other impurities, according to the fossil fuel used [62].

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Therefore, the selection of species tolerant to flue gas conditions, such as temperature and the presence of toxic compounds, is important to improve the efficiency of CO2 fixation. As an alternative, a screening can be carried out to select species of microalgae tolerant to higher concentrations of CO2 and other components present in the flue gas [70, 71] In addition, studies have shown that native microalgae are more tolerant to adverse conditions of the locals from which they were isolated. For example, Chlorella fusca LEB 111, isolated from an ash settling pond located near the flue gas emitters of a thermoelectric plant, showed CO2 fixation 2.6 times higher than Spirulina sp. LEB 18, when cultivated with gas from coal combustion [72]. Adaptive evolution, random mutagenesis, and genetic engineering are strategies that have been employed to improve the CO2 fixation by microalgae [54, 73]. Adaptive evolution was used to obtain a new strain of Chlorella (Chlorella sp. Cv), after 46 cycles of adaptation to simulated flue gas, with a gradual increase in CO2 , NOX , and SOX concentrations. The evolved strain showed a maximum CO2 fixation rate of 1.2 g L−1 d−1 when cultivated with the simulated flue gas (10% CO2 , 200 ppm NOX , and 100 ppm SOX ) at 0.3 vvm. The tolerance to NOx and SOX was related to the upregulations of genes [73]. As mentioned before, microalgae can be cultivated using wastes with different physicochemical characteristics. Flue gas and wastewater from industries have been investigated as alternative sources of nutrients and water for the cultivation of microalgae [53, 54]. Yadav et al. [74] conducted assays utilizing flue gas from coal-based thermal power plants and industrial wastewater (from textile and food processing industries) in the cultivation of Chlorella sp. and Chlorococcum sp. The highest nutrient removal (91.9% dissolved oxygen, 95.9% PO4 , 100% NH4 , and 98.8% NO3 ), biomass productivity (208.93 mg L−1 d−1 ), and CO2 fixation rate (187.65 mg L−1 d−1 ) was verified for Chlorella sp. cultivated with the diluted flue gas (5% CO2 ), demonstrating the possibility of integrated use of gaseous and liquid wastes in microalgae cultivation [74]. All advances to achieve the optimal conditions for CO2 biofixation by microalgae have contributed to the definition of process parameters, selection of the most promising bioreactor, and microalgae strain. However, the implementation of this biological method for CO2 capture still faces challenges such as low CO2 solubility and fixation efficiency and CO2 loss during microalgae cultivation.

3 Merging of Chemical and Biological CO2 Fixation The techniques developed to reduce atmospheric CO2 from industrial processes can be highly efficient [75]. The threshold of each system is defined by a few factors. Processes based on photosynthesis can be influenced by factors such as temperature, CO2 concentration, and light intensity [76]. On the other hand, the limit of CO2 uptake by chemical processes can be considerably predicted [23] compared to biological systems.

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The use of chemical absorption technology combined with the CO2 biofixation by microalgae, concomitantly or sequentially, can be found in an approach with cultures of different strains, for example: Anabaena sp. [16], Scenedesmus acuminatus [13], Scenedesmus sp. [14], Scenedesmus dimorphus [77], Spirulina sp. LEB 18 [17, 19, 55], Chlamydomonas sp., Chlorella sp. e Pseudochlorococcum sp. [78], Chlorella fusca LEB 111 [15, 18, 79] (Table 1). Combined systems between technologies can use open or closed systems for CO2 fixation. However, vertical or horizontal tubular photobioreactors, ordinally built by polymeric materials or glass, are preferentially used due to the longer residence time of the gas in the liquid medium [9]. The subsequent and independent combination of chemical (500 mmol L−1 of DEA) and biological (Anabaena sp. cultivation) technologies was used successfully for CO2 fixation. In this proposal, the chemical fixation occurred before the microalgal culture, which was carried out later with the culture medium adding DEA + fixed CO2 . The proposed process promoted high CO2 removal (~100%), moderate concentrations of total inorganic carbon (~2 g L−1 ) [16]. The cultivation of Scenedesmus sp. has shown an increase in DIC with the addition of chemical absorbents (MEA, 2-amino-2-methyl-1-propanol, DEA or TEA) in relation to the assay with no the compounds. CO2 fixation by the strain increased by 31% with the addition of TEA (5 mmol L−1 ) compared to the control assay. Furthermore, the repeated addition of TEA increased this combined CO2 fixation process by 39 and 19% compared to the control batch (without addition) and in batch mode with a single addition of the chemical absorbent, respectively [14]. The increase in DIC in the abiotic culture of microalgae (assay without cells) was proportional to the concentration of MEA added [18, 77]. The CO2 absorptivity remained greater than 60% for a pH range between 6.5 and 10.0. The biomass and lipid productivity of S. dimorphus were increased with MEA, as with the addition of 1.64 mmol L−1 of the absorbent, the CO2 utilization efficiency reached 76.1%. However, the microalgae had its growth inhibited with MEA concentration above 2.46 mmol L−1 [77]. Comparison with control assay of Spirulina sp. LEB 18 (free from CO2 absorption chemical technology) demonstrated that the semicontinuous cultivation of this strain can be increased by combining both processes. This was demonstrated by combining the addition of MEA (0.20 mmol L−1 per growth cycle), blend concentration (0.5 g L−1 ), and medium reuse rate (50% v v−1 ). Thus, answers such as growth cycles (60%) and CO2 fixation (40%) were increased [55]. From the combination of these technologies, but in batch and addition of 0.20 mmol L−1 of MEA, the same strain demonstrated tolerance 0.13 gMEA gbiomass −1 and protein increment [19]. Chemical absorption with DEA (10% v v−1 ) combined with microalgal biotechnology increased the growth of Chlamydomonas sp. (0.37 d−1 ), Chlorella sp. (0.35 d−1 ), and Pseudochlorococcum sp. (0.67 d−1 ). This combined system presented the highest response of capacity for CO2 reduction (135 mmol mL−1 d−1 ) which was similar in all strains. In these systems, after regeneration, the DEA solution recovered 85% w w−1 of its initial capacity [78]. The biological method (batch cultivation of Spirulina sp. LEB 18), combined chemical technology (a mixture of 1.64 mmol L−1

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Table 1 Answers to the merging of chemical absorption and microalgae cultivation technologies for CO2 fixation Strains

Absorbent

mmol L−1

Highlight

References

Anabaena sp.

Diethanolamine

500

Increased of CO2 removed (~100%), biomass productivity (400 mg L−1 d−1 ), exopolysaccharides (200 mg L−1 d−1 )

[16]

Scenedesmus acuminatus Monoethanolamine AG10316

4.91

DIC and chlorophyll concentration increased

[13]

Scenedesmus sp.

Triethanolamine

5.00

*Increased of CO2 fixation (~40%)

[14]

Scenedesmus dimorphus

Monoethanolamine

1.64

CO2 efficiency use = 76.1%

[77]

Spirulina sp. LEB 18

Monoethanolamine

0.20

*Increased of CO2 fixation (40%) and carbohydrates (50%)

[55]

Spirulina sp. LEB 18

Monoethanolamine

0.20

*Increased of proteins in 17.0%

[19]

Chlamydomonas sp., Chlorella sp. and Pseudochlorococcum sp.

Diethanolamine

0.95

Increased of [78] biomass concentration and skill to remove CO2

Spirulina sp. LEB 18

Diethanolamine + Potassium carbonate

1.64 + 0.41

*Increased of CO2 fixation rate (61.5%) and biomass productivity (46%)

[17]

Chlorella sp. and Monoethanolamine Chlorella fusca LEB 111

0.82 and Increased DIC 1.64 accumulation, CO2 use efficiency, and carbohydrate and lipid productivity

[79]

Chlorella fusca LEB 111 Monoethanolamine

0.82 *Increased of DIC every 3 d (500%) and lipids productivity (51%)

[18]

Chlorella fusca LEB 111 Monoethanolamine

0.82 per growth cycle

[15]

*

Increased of lipid concentration (~41.0% w w−1 )

It compared to a control cultivation; TIC: Total inorganic carbon; DIC: dissolved inorganic carbon

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DEA and 0.41 mmol L−1 K2 CO3 ), increased the CO2 biofixation (by 61.5%) and biomass productivity (by 46%) this strain [17]. C. fusca LEB 111 strain was used in 3 different cultivation modes, combined with the chemical absorption technique with MEA. In batch, the assayed concentrations of MEA (0.82, 1.64 and 2.46 mmol L−1 ) were added together with the inoculum and culture medium. The strain showed the highest DIC accumulation and CO2 utilization efficiency (~36.0% w w−1 ) with the two highest concentrations of the chemical absorbent assayed [79]. In the fed-batch cultivation, the selected concentration of MEA (0.82 mmol L−1 each 3 d) promoted an accumulation of 22% in the DIC response by the system, in relation to the control assay without the chemical technology [18]. In semicontinuous cultivation, with the addition of MEA (0.82 mmol L−1 per growth cycle), medium reuse rate of 70% v v−1 , and blend concentration of 1.0 g L−1 , the CO2 biofixation rate by the set (microalgal cultivation + chemical absorption) was maximized by ~ 75% [15] in relation to control cultivations without chemical fixation technology [18, 80, 81].

4 Enrichment and Applications of the Microalgae Biomass Obtained in CO2 Fixation The bioconversion of CO2 into microalgal biomass, which in turn can be applied to obtain bioproducts, in a context of circular bioeconomy, has received the attention of researchers and has been considered an alternative to make the process more viable. The biomass produced through the integrated chemical and biological fixation has the potential to be used as a feedstock for several applications. Microalgae can be considered a sustainable raw material for the production of biofuels, with the potential to be a carbon–neutral process. However, the production of biofuels from microalgae is still not economically viable. Alternatives such as the use of low-cost CO2 and nutrients and the production in an integrated biorefinery system can reduce production costs [56]. Under specific growth conditions, some microalgae species can accumulate high concentrations of lipids and carbohydrates in their biomass, which can be converted into biofuels such as biodiesel and bioethanol, respectively. Besides these biofuels, biogas can be obtained from the anaerobic digestion of biomass. However, due to the low yield and high costs of the process, its production is recommended in association with other biofuels such as bioethanol and biodiesel [82]. Biodiesel produced from microalgae has advantages when compared to fossil fuel, such as lower emissions of hydrocarbons, carbon monoxide, sulfur oxide, and particulates [83]. According to the techno-economic analysis, the estimated lowest selling price (LSP) for biodiesel derived from microalgal biomass ranged from 0.96 to 3.69 $ L−1 . The LSP was estimated for integrated production of biodiesel and other products, indicating that coproduction can have an economic impact on biodiesel production from microalgal biomass [84]. Studies reported concentrations of lipids

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from 24 to 38% w w−1 , in microalgal biomass obtained from cultivation with the addition of flue gas (real or simulated). Furthermore, the lipid profile was suitable for the production of biodiesel [85–87]. Microalgal carbohydrates can be converted into bioethanol and biobutanol via microbial fermentation by Saccharomyces cerevisiae and Clostridium acetobutylicum, respectively. Microalgae are capable of synthesizing and accumulating up to 60% w w−1 of carbohydrates in the biomass under specific cultivation conditions. The utilization of microalgal biomass in the production of bioalcohol has the advantage of the absence of lignin and low hemicellulose content, which facilitates the pre-treatment process [88]. Another promising application of microalgal biomass is the production of bio-jet fuel. The method that has been used is the hydroprocessing of oil extracted from the biomass. However, other production routes have been suggested, such as the conversion of carbohydrates into ethanol and then into jet fuel through a series of chemical reactions. Bio-jet fuel produced from microalgae may have proprieties consistent with American Society for Testing and Materials (ASTM) standards such as viscosity (2.8 mm2 s−1 ), flash point (68°C), sulfur concentration (0.27% w w−1 ), and net heating value (44 MJ kg−1 ). However, the freezing point and density of biojet fuel are the main parameters that must be improved. Another challenge of this process is to minimize production costs [89]. Biochar is another bioproduct that can be obtained from microalgal biomass. Microalgal biochar is generally produced by a slow pyrolysis process and has a high content of nutrients, mainly nitrogen, and high pH. These characteristics make it suitable for soil improvement through nutrient enrichment and acidification balance [74, 90]. Furthermore, biochar derived from microalgal biomass can be applied to remove organic contaminants in water and wastewater treatment, due to the presence of functional groups [49, 90]. Some microalgae species can naturally produce biopolymers as an energy reserve. Biopolymers have the potential to replace the use of polymers from fossil origin in certain applications. The main biopolymer synthesized by microalgae is polyhydroxybutyrate (PHB), which is a biodegradable thermoplastic polymer with physical and chemical properties similar to those of polypropylene. PHB has the potential to application in the food, pharmaceutical, and cosmetics industries. However, the production cost is a challenge to be overcome and can be minimized through biorefinery [91]. Microalgal biomass contains components of interest for application as a plant biostimulant, contributing to the development of sustainable agriculture. Microalgae can produce phytohormones such as auxin and cytokinin, in addition to containing other compounds with biostimulant activity such as amino acids, fatty acids, polysaccharides, terpenoids, polyamines, phenolic compounds, vitamins, and allelochemicals. The use of microalgal biomass as a biostimulant can accelerate plant development, increase crop yield, stimulate root development, reduce saline stress effects, and improve germination, flowering, and fruit production [92]. Besides, the use of microalgae biomass produced in wastewater treatment for the production of biofertilizers is an interesting alternative to make the process viable. This valorization of

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biomass can reduce costs from 0.17 to 0.15 $ m−3 , without considering the financial return of the product commercialization [65]. The integration of chemical and biological fixation of CO2 , through the cultivation of microalgae, not only contributes to the mitigation of the gas but can also provide an increase in the production of molecules of interest for the applications previously mentioned. The cultivation of C. fusca LEB 111 conducted in fed-batch with periodic addition of MEA (0.82 mmol L−1 ) and CO2 increased the concentration and productivity of protein (44% w w−1 and 67.1 mg L−1 d−1 ) and lipids (30.8% w w−1 and 46.9 mg L−1 d−1 ) [18]. An increase in lipid productivity was also observed in the cultivation of S. dimorphus with 0.82 and 100 mmol L−1 of MEA, in semicontinuous mode. The addition of MEA also promotes the accumulation of oleic acid, a component that can improve the properties of biodiesel [77]. Rosa et al. [15] evaluated the production of biomolecules by C. fusca LEB 111 cultivated in a semicontinuous mode with the addition of MEA and reuse of nutrients. The highest concentration of carbohydrates (35.7% w w−1 ) was verified in the assay with a medium reuse rate of 70% (v v−1 ) and three additions of monoethanolamine (0.82 mmol L−1 per growth cycle). In addition, a high concentration of lipids (41.0% w w−1 ) was obtained in the cultivation with a medium reuse rate of 50% and also with three additions of MEA (0.82 mmol L−1 ) [15]. The addition of MEA (0.20 mmol L−1 ) in the Spirulina sp. LEB 18 cultivation resulted in an increase of approximately 96.0% in carbohydrate concentration when compared to the control [55]. With this same strain, the addition of absorbents to CO2 fixation also resulted in variations in the biomass composition. A mixture of DEA (1.64 mmol L−1 ) and K2 CO3 (0.41 mmol L−1 ) increased the carbohydrate concentration in the biomass by 43.0% in relation to the control assay. Besides, the main pigment of this cyanobacterium (phycocyanin) was significantly increased (64%) when the microalgal culture was combined with the addition of K2 CO3 (1.64 mmol L−1 ) [17]. The still using DEA, but with previous use of this technology (500 mmol L−1 ) and afterward microalgal cultivation with fixed carbon with the chemical absorbent, it promoted higher biomass productivity and increased exopolysaccharide productivity (~200 mg L−1 d−1 ) of Anabaena sp. [16]. Although it has potential as a feedstock for a range of bioproducts, microalgae biomass has been commercially applied for limited purposes. Biorefinery strategies can be an alternative to value biomass and increase the possibility of final products [65], also contributing to the viability of the process. In this way, De Bhowmick et al. [93] evaluated the use of wastewater (kitchen waste and poultry litter waste) and flue gas in the cultivation of Chlorella minutissima, as well as the concomitant production of lutein and lipids by the microalga. The concentration and productivity of lipids and the productivity of lutein were higher in the assay with liquid and gaseous wastes, compared to the control. The CO2 fixation rate was 80.7 mg L−1 d−1 and the remediation of nitrate, nitrite, and ammonium was complete.

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5 Final Considerations and Perspectives In view of the above, it was confirmed that the chemical fixation of CO2 via the absorptive process with MEA is the most mature and used technology, especially in the energy sector. However, the regeneration of this absorbent contributes to the increase in atmospheric concentrations of CO2 . Microalgae biotechnology has appeared to be an environmentally friendly alternative, but with moderate CO2 capture potential. In isolation, both technologies present environmental and technological challenges, which could be solved, for example, with the development of environmentally friendly absorbents and the concomitant profitable production of value-added bioproducts. This chapter has highlighted the potential offered by the integration of microalgal chemical and biological technologies. In this combination, the environmental liabilities of the former are drastically reduced or do not exist, while microalgae biotechnology in a biorefinery approach can maximize responses such as CO2 fixation and production of bioproducts. The match of these technologies has presented promising results demonstrated for the main microalgae genera (Spirulina, Chlorella, and Scenedesmus) and chemical absorbents (MEA and DEA). In this context, the improvements were demonstrated with the substantial increase of dissolved carbon in the medium (up to 500%), CO2 utilization efficiency (~80%), microalgal biomass productivity (400 mg L−1 d−1 ), and obtaining of bioproducts such as carbohydrates (50% increase), exopolysaccharides (200 mg L−1 d−1 ) and lipid productivity (51% increase). The optimized development of a system containing chemical and biological technology, as presented here, must be efficient in the cultivation, production, and refining of microalgal biomass, compared to existing chemical and biological systems. Furthermore, all stages of microalgal biomass beneficiation must be developed in accordance with long-term environmental sustainability. For future implementation, the simplicity of processes must be concentrated on sectors more development, such as in the pharmaceutical, cosmetics, or biofuels industries. The combined CO2 fixation strategy presented has potential application in different countries, especially those with high population density, as the concepts developed in these applications meet not only the generation of financial resources and environmental protection but also food and energy security. However, further research is still needed to ratify, on a large scale, the fittest strain and cultivation method, as well as the most appropriate value-added process. Acknowledgements The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES)—Finance Code 001, National Council for Scientific and Technological Development (CNPq), and the Ministry of Science, Technology, and Innovation (MCTI) for their financial support. This research was developed within the scope of the CapesPrInt Program (Process # 88887.310848/2018-00). In addition, G.M. Rosa would like to thank the National Council for Scientific and Technological Development (CNPq) for granting a postdoctoral fellowship (process number 161006/2019-1).

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Hydrogen Creation and Carbon Sequestration by Fracking Carbon Dioxide Zohal Safaei Mahmoudabadi and Alimorad Rashidi

Abstract Carbon dioxide (CO2 ) gas is one of the most significant greenhouse gases which is the main cause of climate change. The different strategies to convert CO2 to beneficial materials could aid to the development of the implementation of novel technologies to diminish carbon emissions, products, and industries, and help to decrease climate-altering emissions. In addition, technical and economic feasibility is required to use and execute the technologies of using CO2 gas at an industrial scale. Thus, efforts should be made in order to reduce costs, fines, and energy consumption and provide incentive regulations to attract carbon capture, storage of carbon, and carbon utilization. In addition, there are ways of commercial methods for hydrogen production today, in which hydrogen production is “designed” to capture CO2 emissions. Given that the largest amount of hydrogen production is related to the use of fossil fuels today. Steam methane reforming (SMR) plays a role of about 50% in the production of hydrogen in the world. Also, SMR processes correspond to a considerable amount of carbon emission at different stages of the process. In this chapter, the latest developments in CO2 capture, utilization, conversion, and carbon capture and sequestration (CCS) are investigated through a multi-scale perspective. Also, the significant benefits and shortcomings of different carbon capture methods used in CCS technology and various new technologies to increase the feasibility of CCS technology are examined. Finally, we study the key challenges and issues that must be faced for industrial applications related to CCS technology in the future. Furthermore, the different ways of hydrogen generation with CO2 capture on sorption-increased SMR are described. Keywords CO2 utilization · Carbon capture · Carbon storage · Hydrogen creation · Climate change

Z. S. Mahmoudabadi · A. Rashidi (B) Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jawaid and A. Khan (eds.), Sustainable Utilization of Carbon Dioxide, Sustainable Materials and Technology, https://doi.org/10.1007/978-981-99-2890-3_5

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1 Introduction The enormous greenhouse gases emission is the chief cause of climate change. Industrial flue gas emissions contain nitrogen oxides (Nox ), hydrocarbons, carbon dioxide (CO2 ), carbon monoxide (CO), sulfur dioxide (SO2 ), and particulate matter which almost all these emissions are greenhouse gases. Carbon dioxide (CO2 ), as one of the greenhouse gases emissions released by human activities, has carried out with increase worldwide due to increasing requirements for energy. Also, greenhouse gases emissions possess about 77% CO2 . Figure 1 demonstrates the relation between the earth’s surface temperature and CO2 concentrations in the atmosphere. It can be observed that CO2 emissions have been increased since 1850. It is expected that carbon dioxide emissions will rise in the future relate to commercial growth. In accordance with recent the Intergovernmental Panel on Climate Change (IPCC), it announces that the worldwide mean concentration of CO2 in the environmental is now near to 400 ppm; however, the most extensive research states that the safe level of CO2 concentration is below 350 ppm [2, 3]. Therefore, the different strategies to convert CO2 to beneficial commodities could spur the development of the implementation of novel technologies to diminish carbon emissions, products, and industries, and help to decrease climate-altering emissions [4, 5]. The possible replacement for CCS would be carbon dioxide capture and utilization (CCU) which is dominant consideration related to sustainable expansion and is expected to be a constant solution for CO2 emissions. Also, tremendous advances have been made in the utilization of CO2 gas as refrigerant [6], cleaning liquid [7, 8], solvent medium [9], chemical feedstock [10], inserting agent [11], packaging gas [12, 13] (Fig. 2), etc. Thus, Sect. 2 explores carbon capture and sequestration (CCS) and presents their various uses. Section 3 illustrates hydrogen creation using steam methane reforming (SMR). Section 4 brings the conclusion.

Fig. 1 Relationship between the temperature of earth’s surface and CO2 concentrations in the atmosphere [1]

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Fig. 2 Summary of CO2 gas applications [3]

2 Carbon Capture and Sequestration CCS usually relates to the group of technologies that are employed to capture carbon emissions for electric generation from equipment using fuels, and next, transport these emissions for underground storage with pipeline to the proper locations. CCS is a proportionally modern technology which research and development is essential for the future commercialization [14]. Also, CCS consists of three stages: captures, transports, and stores carbon emissions [15]. Oxy-fuel combustion, precombustion capture, and post-combustion capture are three fundamental methods for improving to capture of carbon. These procedures each have various advantages, certain drawbacks, evaluations, and costs [16]. In addition, the captured CO2 can be employed in oil drilling operations for extraction activities [17] or to aid mine coal from beds and seams [18]. Also, anime scrubbing as a well-established technology has been widely used in industries for large-scale carbon capture [14]. Notwithstanding, the process existed shortcomings such as high regeneration energy demand, high cost, and corrosion of equipment. The main idea behind CCS is to permit the manufacturing of biofuels at the same time as lowering greenhouse gases emissions into the ecosystem and as a result of decreased global climate exchange. Thus, CCS is often investigated to be a crucial technology for the decarbonization of the worldwide energy system and can be used to produce power [19]. In recent years, CCS coupled with fossil-fueled power plants

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is a climate change mitigation option that potentially allows the continued use of fossil fuels, while decreasing greenhouse gas emissions [20, 21].

2.1 Concept of CO2 Capture and Sequestration The procedures to reduce CO2 emissions can be classified into carbon-based completely, carbon emission minimization-based and carbon sink primarily based procedure [22, 23]. Carbon capture and sequestration techniques fall into the 1/3 class, as CO2 is placed in a storage medium in preference to being emitted into the environment. Generally, the view of CCS technology is to capture CO2 using a carbon source, deliver it to an injection site, and then sequester it in the lengthy time period of storage in any of an expansion of storage [24]. In fact, CCS is affordable, and low-contaminate approach that can be applied for greenhouse gases emission, because it (i) captures CO2 at focused storage, especially commercial sites; (ii) captures CO2 from the air; and (iii) completely and safely stores CO2 away from the environmental [25]. The basic criteria that must be used in CCS technologies are involved a long storage period, low cost, the elimination of the risk of accidents, and the consideration of national and international regulations [26–28]. It should be noted that CO2 emissions released fossil fuels applied in large sources and must be carried to safe and suitable resources instead of emitted into the environment in CCS technology [26].

2.2 CCS Techniques The different techniques in CCS technology may be applied to capture CO2 , involving gas-phase separation, adsorption onto a solid, absorption into a liquid, as well as hybrid processes [29–31]. Based on the view of Herzog et al. [26], the three principle separation processes can be illustrated as: (a) Absorption process: In this process, liquid solvents can absorb CO2 gas by creating a chemical bond. Next, the solvent is transferred to the another section for heating, which leads to the separation of CO2 gas, and also the solvent is recycled and reused. (b) Adsorption process onto a surface of solid: the selective adsorption of CO2 gas, which is regenerated by enhancing the temperature or lowering the pressure to release the CO2 from solid. (c) Membrane processes: CO2 gas is selectively divorced from flue gas by a membrane systems. Figure 3 shows the overview of carbon capture technology. CO2 capture overviews are divided into three chief classes: (a) CO2 gas is released from industrial processes and stored with high purity; (b) a pure stream of CO2 gas is generated from improved industrial processes such as oxy-fueling and pre-combustion fuel gasification; and (c) direct air capture at a large scale into a pure CO2 gas or chemically constant terminal product [32].

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Fig. 3 Overviews of CO2 capture

Also, the procedures applied for CO2 capture and separation are demonstrated in Fig. 4. Briefly, the five various techniques for CO2 capture and separation have been considered such as membrane system, cryogenic distillation, absorption, adsorption, and biological fixation using bacteria or algae. Based on the regeneration procedure, various adsorption processes can be related to achieving CO2 separation, consisting of pressure swing adsorption (PSA), vacuum swing adsorption (VSA), temperature swing adsorption (TSA), electric swing adsorption (ESA), and simulated moving bed (SMB) [33].

Fig. 4 Procedures for CO2 capture and separation

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Fig. 5 CO2 capture and utilization (CCU) process [39]

One of the most challenging stages of the CCU is related to carbon dioxide utilization, which is classified into two categories: conversion processes and nonconversion processes [34–36] (Fig. 5). In the CCU process, CO2 is captured and separated at the production source. Then it is utilized in two various methods either by a non-conversion process using it as a solvent, and heat transfer agent or as feedstock for the generation of chemicals, fuels, and polymers in the CO2 conversion process [37, 38].

2.3 Capture from Major Sources The competitiveness of different CCU technologies would be explored frequently resulting in a variety of economical efficiency of the performance of CO2 emission management. The main approaches for CO2 capture can be illustrated via post-, pre-, and oxy-fuel combustion, as explained in the next sections (Fig. 6) [40–42]. Also, Table 1 exhibits the advantages and drawbacks of different carbon capture methods used in CCS technology.

2.3.1

Post-combustion Capture

The capture of CO2 from flue gases generated by the perfect combustion of fossil fuels and biomass in the air is assigned to post-combustion capture [49]. The flue gas emitted from the combustion processes contains mostly CO2 , NOx, SOx, and other

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Fig. 6 Options for CO2 capture a post-combustion, b pre-combustion, c oxy-fuels

potentially perilous compounds [50, 51]. The post-combustion capture is related to the final processing stage which is used to separation of the CO2 after the combustion process and immediately before release into the atmosphere. Wet scrubbing with aqueous amine solutions is the most generally applied chemical for post-combustion CO2 capture at low temperature because the reversible reactions between amines and CO2 through their mild interaction give effective “catch and release” [52]. The proportionately low concentration of CO2 gas needs separation prior to usage or transportation, leading to the use of the major items of equipment to establish the operation system [45]. Post-combustion capture system is located after traditional purification systems to diminish pollutants [53]. Also, Fig. 7 displays a process for post-combustion CO2 capture in a power plant, involving the elimination of particulate matter, SOx, and NOx. Post-combustion capture encounters several challenges including design, high-energy penalty, bad condition of flue gas such as low CO2 partial pressure, high flow rates [54, 55]. The low partial pressure of CO2 in the flue gas results in a low driving force for CO2 capture [56, 57]. In addition, the low concentration of CO2 in the flue gas needs strong separation processes, which is usually done by amine-based aqueous absorbents [59]. The convenient operating conditions of power plants with CO2 capture have been studied as a way to increase plant economics [59]. Performance of post-combustion capture suggests a promising view to decreasing carbon releases related to its ability to be used in available power plants with little correction [52]. Furthermore, the various projects under advancement, various pilot facilities have been carried out. It should be noted that in 2009, first pilot-scale CO2 -scrubbing plant was established in Niederaussem[60], while it has been built by a capacity of 1 ton CO2 per hour post-combustion capture pilot power plant in Denmark since 2010 [16, 61].

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Table 1 Main advantages and drawbacks of different carbon capture methods used in CCS technology [43] Post-combustion

Advantages

Aqueous ammonia (wet scrubbing) [24, 45, 46]

1. Lower heat reaction, high CO2 capacity, lower cost and regenerates at high pressure [24] 2. Saves energy up to 60% compared to monoethanolamine (MEA) [44] 3. Has high loading capacity, does not pose a corrosion risk, tolerant to oxygen in the flue gas, is low cost and there is no absorbent degradation problem, thus reducing the absorbent makeup rate, and the energy requirement for absorbent regeneration is predicted to be much lower than in the MEA process [45]

Polymer membrane (flue gas is passed through a bundle of membrane tubes, while an amine solution flows through the shell side of the bundle) [24, 31, 44]

Drawbacks

1. The concentration of CO2 in flue gases is low, which means that large quantities of gases need to be processed [44] 2. High temperature of flue gases rapidly destroys a membrane, so the gases need to be cooled to below 100 z C prior to membrane separation [24] 3. Membranes need to be chemically resistant to the harsh chemicals contained 4. Within flue gases, or these chemicals need to be removed prior to membrane separation [31]

CO2 capture sorbents (Na2 CO3 ) [45]

1. Energy-efficient [45]

Metal organic framework [24]

1. Low energy requirement for regeneration, good thermal stability, tolerance to contaminants, attrition resistance and low cost

• Solids are more difficult to work with than liquids when capturing CO2 [45]

(continued)

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Table 1 (continued) Post-combustion

Advantages

Enzyme-based system (biologically based) [24]

1. Very low heat for absorption, which reduces the energy penalty of absorption

Ionic liquid [24]

1. Recovers CO2 from flue gas without having to cool it first 2. Little heat required for regeneration 3. The viscosity of many ionic liquids is relatively high compared to conventional solvents

Monoethanol-amine 1. Produces a relatively pure CO2 stream (MEA) absorption [24]

Cryogenic distillation (air separation process; gaseous components of a mixture are separated by condensation) [44, 45]

Drawbacks

1. This scheme would increase the cost of electricity production by 70% 2. Non-economic and requires large-size equipment and intensive energy input 3. Low CO2 loading capacity and high equipment corrosion rate

1. Removes CO2 from natural gas, while avoiding the shortcomings of the conventional acid gas treatment process 2.Simultaneous separation of CO2 and H2 O from flue gas, avoiding the use of solvents and high pressures

Pre-combustion Capture technique

Advantages

Integrated gasification 1. Less expensive combine cycle [24, 44] 1. Requires less energy for regeneration Physical solvent processes (methyldiethanolamine (MDEA)) [24, 47]

Membranes [24]

Disadvantage 1. Some technical and economic uncertainties 1. The capacity is only best at low temperatures, so it is necessary to cool the syngas before carbon capture

1. Less energy-intensive process does not require a phase change and typically provides low maintenance operation 2. Polymeric and metallic membranes can produce clean fuel from the syngas obtained [44] 3. Increases the CO2 concentration in the output stream and, consequently, increases the performance of the CO2 capture system [31] (continued)

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Table 1 (continued) Pre-combustion Capture technique

Advantages

Reaction with pre-combustion sorbents: lithium silicate-based (Li4 SiO4 ) [24, 44, 46]

1. Sorbent shows excellent regenerability and attrition resistance in thermal cycling tests [48] 2. Lithium silicate-based sorbent has the capability to not only separate CO2 from syngas, but also promote the water gas shift reaction 3. Large capacity, rapid absorption and high stability[44]

Disadvantage

Oxy-fuel combustion Capture technique/ process

Advantages

Disadvantage

Fuel is burned with nearly pure oxygen (greater than 95%) mixed with recycled flue gas [24, 44]

1. Cost is lower for a conventional pulverized coal plant as a result of the decreased flue gas volume and increased concentration of CO2 [24] 1. Elimination of NOX control equipment and the CO2 separation step 2. The boiler size can be reduced because only oxygen is supplied for combustion [24]

1. The cost of air separation and flue gas recirculation significantly reduces the economic benefit [24] 1. Corrosion of equipment occurs by increased SO2 concentration in the exhaust gas stream

Capture technique

Advantages

Disadvantage

Membrane [31]

1. Produces oxygen-enriched streams from air 2. Increases the CO2 concentration in the output stream and, consequently, 3. Increases the performance of the CO2 capture system

Post-combustion

Recently, SaskPower’s power plant and NGR’s Petra Nova plant with CCS have started operating [59, 60].

2.3.2

Pre-combustion Capture

The gasification power plants carried out pre-combustion capture which involved units using integrated gasification combined cycle (IGCC). Also, this capture procedure is related to technology used in the industry to generate materials for decades [62]. Moreover, the CO2 capturing process can happen before the combustion process if fuels are converted with some steam at high pressures and substoichiometric amounts of oxygen to generate a “synthetic gas”. In addition, as seen in Eq. (1)

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Fig. 7 Post-combustion capture system for power plant [58]

when steam enters the reaction, to enable the water–gas shift (WGS) reaction takes place in the presence of catalyst beds. As displayed by the equation, adding steam and decreasing temperature assisted the production of CO2 from CO [63, 64]: CO + H2 O ↔ 3CO2 + H2

(1)

Also, the CO2 is detached to generate a hydrogen-rich fuel gas. Then, the CO2 is dissolved in the solvent at a higher pressure and released as the pressure is decreased. As the CO2 can be released at above-atmospheric pressure, not much heat is needed to regenerate the solvent. Therefore, the energy needs in pre-combustion capture processes maybe about half that of post-combustion capture for CO2 capture [65].

2.3.3

Oxy-Fuel Combustion Capture

In oxy-fuel combustion, only oxygen (O2 ) gas is provided concentrations of CO2 and H2 O [66]. The recent studies in oxy-fuel power cycles are considered focusing on the main concepts of manipulating exergy flows within each cycle and the presented thermal efficiencies. Therefore, fossil fuels use with a mixture of oxygen gas and recycled flue gases. When complete combustion takes place in the presence of rich oxygen gas, a mixture of mainly CO2 and condensable water vapor is produced which is separated and cleaned in the compression process. In the use of coal, NOx, SOx, and other contaminants must be eliminated from the CO2 compression process [52]. Many industrial pilot plants have effectively carried out oxy-fuel combustion in the past decade [67].

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Direct Air Capture

Specifically, CO2 has detached absolutely from the atmosphere and was originally presented as an idea by Lackner in 1999 in direct air capture (DAC) [45]. Also, DAC is an emerging technology that can help humankind to decrease greenhouse gases in recent years. It is impossible to neutralize the greenhouse needs by conventional CO2 capture methods [68]. The maintenance, chemicals, storage cost, and energy requirement to operate a DAC plant are lower than the cost of other CCS options [69]. The utilization of adsorbents in DAC needs minimum energy in both adsorption and regeneration without including the formation of a chemical bond resulting from the interaction between the adsorbate and adsorbent which is related to kinds of pollutants (SOx, NOx, mercury, etc.). Due to the fact that the concentration of CO2 in the atmosphere is very low, it is necessary to use adsorbents with high absorption capacity. Recent studies have been carried out on various types of materials to direct carbon capture. Some of these adsorbents include different solid materials containing alkali carbonates, metal–organic frameworks (MOFs), anionic-exchange resins; also the other group includes aqueous hydroxides [70]. DAC processes require an effective, energy-intensive process in order to decrease atmospheric CO2 [71]. Sorbents must incorporate higher capture capacity, selectivity, recyclability, low cost, and high durability in order to CO2 capture over competing gases [69–71].

2.4 Modern Technologies of CCS to Increase the CO2 Capture Several reduction emissions from fuel combustion strategies have been grown including CCS which involves membranes, liquid solvent absorption, solid sorbent materials, and metal–organic frameworks (MOFs) [72]. Table 2 demonstrates the modern technologies of large-scale CCS. It should be noted post-combustion capture based on solvent is investigated as the foremost available technology related to different flue gas streams [73]. Membranes are widely used for post-combustion CO2 capture which gas mixtures including many materials are detached by a semipermeable barrier in the physical separation process. The advantages of using membrane technology are ease of equipment by skid-mounting, ease of operation and preservation, lower energy consumption, compactness, and the need for few chemical materials as a result of lower cost rather than other separation processes [74]. Thus, the use of membrane is an economically viable and environmentally friendly technology for CO2 capture in the future.

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Table 2 Several emerging technologies available from 2015 onward to enhance the feasibility of CCS technology [43] No. References

Technologies

Research finding

Advantages

1

Francisco-Marques Single-walled and Galano [75] silicon-carbon nanotubes (SWSiCNTs)

1. The main finding supporting this proposal is SWSiCNTs with intermediate (2–33%) Si amount which are very promising agents for efficient sequestering of CO2 at room temperature

1. The rate constants for the SWSiCNT reactions with CO2 are around 105 M−1 S−1 , which suggests that they are fast enough to ensure efficient CO2 capture at room temperature 2. Promising good CO2 sequestration because (i) they present high CO2 / N2 selectivity; (ii) their reactions with CO2 are spontaneous and fast at room temperature; and (iii) they are able to capture multiple CO2 molecules

2

Basnayake et al. [76]

1. The ZIF-CO3 -1 synthesis reported in this work captures CO2 at temperatures ranging from room temperature to 85 z C and close to atmospheric pressure (1.01325 bar) which is near to post-combustion gas stream condition 2. The post-combustion gas stream is normally at a pressure of ~ 1 bar and a temperature of ~ 40 zC

1. This reaction selectively consumes CO2 because CO2 is incorporated in the ZIF as carbonate 2. CO2 can be quantitatively released (~ 12% by weight) by acidifying the ZIF, and the CO2 capture process can be repeated without significant energy

Carbonate-based zeolitic imidazolate framework (ZIF-CO3 -1)

(continued)

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Table 2 (continued) No. References

Technologies

Research finding

Advantages

3

Dave et al. [77]

Physical solvent DMEPEG I

1. In 90.7% CO2 absorption and 89% saturation of CO2 dissolved in DMEPEG solvent

1. Suitable for both sulfur compounds (such as H2 S3 ) AND CO2 (hence suitable for syngas treatment) 2. Lower heat duty for gas desorption (as compared to chemical solvent), and 3. Lower power consumption (for solvent circulation and CO2 compression) 4. Less corrosive compared to chemical solvent 5. Minimization of equipment size and utility consumption

4

Shen et al. [78]

Graphene oxide (GO) nanosheet membranes Graphene oxide (GO) nanosheets were engineered to be assembled into laminar structures having fast and selective transport channels for gas separation

1. Excellent preferential CO2 which is permeability: 100 Barrer, and ideal selectivity: 91 CO2 /N2 2. Extraordinary operational stability (> 6000 min) which are attractive for implementation of practical CO2 capture 3. Allowed fastest transport for CO2 molecules and very low permeation of other gases, with a gas permeability order of CO2 > H2 > CH4 > N2

1. Fast and selective transport channels for gas separation 2. Selective gas (CO2 or H2 ) diffusion through few-layered GO membranes can be achieved by controlling gas flow pathways and pores

(continued)

2.5 Challenge of CCS Technology Disadvantages and gaps include costs, the effects of life cycle, storage capacity and stability in the research, development, and use of CCS technology [84]. In addition,

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Table 2 (continued) No. References

Technologies

Research finding

Advantages

5

Zhao et al. [79]

Membrane evaporation system

1. It is found that both evaporation temperature and gas flow rates have significant influences on vapor and heat transfer, while liquid flow rates have limited effect on mass and heat transfer in membrane evaporation 2. The system estimated to recovered heat flux up to 32 MJm−2 h−1 and heat recovery can be over 40% when the gas/ liquid flow rate ratio is 150

1. Membrane evaporation system has great potential to save substantial energy in large-scale operation 2. Mass and heat transfer rates decrease as the concentration of the solvent increases because of the reduced vapor pressure of the liquid at higher concentration 3. The increase in evaporation temperature and gas flow rates significantly improves the evaporation efficiency and heat recovery

6

Yu et al. [80]

Superhydrophobic ceramic (SC) membrane The wetting and fouling of a membrane contactor deteriorated performance of the membrane gas absorption system for CO2 post-combustion capture

1. High CO2 removal efficiency (> 90%) and the harmful effect of wetting can be reduced by periodic drying the SC 2. Drying did not work for the polypropylene (PP) membrane because the swelling of PP fibers is irreversible

1. The SC hollow fiber membrane contactor shows good anti-wetting and anti-fouling features because the superhydrophobic surface featured of SC had a self-cleaning function 2. The fabrication cost per absorbed CO2 flux of the SC membrane was 35.4% lower than that of the PP membrane (continued)

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Table 2 (continued) No. References

Technologies

Research finding

Advantages

7

Li et al. [81]

Matrimid/NHS -s20 composite membrane

1. The CO2 permeability of Matrimid/NHS -20 composite membrane is 315% higher compared to pure Matrimid membrane (which is 278 Barrer) 2. The performance of the as-prepared composite membranes surpassed or was close to the 2008 Robeson upper bound line

1. Improve the separation performance for CO2 /CH4 and CO2 /N2 mixtures 2. The nanohydrogels homogeneously embedded in the Matrimid matrix acted as water reservoirs to not only provide more water for dissolving CO2 , but also construct interconnected CO2 transport passageways 3. The incorporation of nanohydrogels increased the fractional free volume of the composite membranes, water uptake, and water retention capacity

8

Wang et al. [82]

PI-PEGSS(20) membrane 20% loading of polyethylene glycol-containing polymeric submicrospheres (PEGSS) incorporated into polyimide (PI) matrix to prepare hybrid membranes

1. PI-PEGSS(20) membrane gives 35% of increase in CO2 permeability and 104% of increase in CO2 /N2 selectivity compared with those of original polyimide (PI) membrane

1. The hybrid membranes showed enhanced CO2 permeability and CO2 /N2 , CO2 /CH4 selectivity at low PEGSS loadings 2. The favorable attraction between PEGSS and CO2 greatly enhanced CO2 solubility and thus CO2 3. Whereas N2 and CH4 permeabilities both decreased for the tortuous gas transport pathways by PEGSS incorporation (continued)

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Table 2 (continued) No. References

Technologies

Research finding

Advantages

9

Thermally rearranged (TR) polymeric hollow fiber membranes

1. The results revealed that the CO2 permeance measured under the mixed-gas condition remained the same as the single gas CO2 permeance, while the permeances of other gases decreased, thus leading to a favored increase in selectivity

1. Displayed superior gas separation properties over conventional polymeric membranes 2. Offering great advantages in gas separation applications, due to presence of micro-pores appropriately tuned in cavities size and distribution thus offering great advantages in gas separation applications, especially in CO2 capture from post-combustion flue gas streams

Cersosimo et al. [83]

the shortage of a long-time policy framework, high-performance costs, power penalties can be troubled challenges when using CCS technology [85]. Despite these issues, CCS technology provides potential opportunities for the advancement of CO2 gas storage, commercial application of CO2 , and carbon market advancement. Despite these issues, CCS technology provides potential opportunities for the advancement of high value-added CO2 gas storage, commercial and industrial application of CO2 , and carbon market progress [86, 87]. The most controversial issue in the use of CCS technology carbon capture is expensive and high energy consumption. Also, the high cost of the carbon capture process relates to the gas separation facilities during power production, energy utilization, maintenance, and operating costs. As a result, the mean cost of CO2 capture directly relates to the real position of facilities and also the network expansion. In addition, an optimal CO2 pipeline can assist reduce the cost of operational use of CCS process [88]. One of the major challenges to deploying CCS technology is that the cost of using the CCS technology is more than the CO2 emitting, and there can never be commercial progress of CCS technology. In addition, laws in different countries restrict the volume of CO2 emissions from power resources, actually limiting some industries to use of CCS system [89]. Thus, a suitable strategy for the long-time utilization of CCS process must be considered in order to perform CCS technology [90].

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3 Hydrogen Creation Hydrogen production with steam methane reforming (SMR) is the commercial and leading method for industrial hydrogen gas production. Recent studies have focused on hydrogen generation with carbon capture. The work of Voldsund. et al. [91] studied the different ways to hydrogen generation with CO2 capture on sorption-increased steam methane reforming (SE-SMR). The work of Giuliano and Gallucci [92] who demonstrated an comprehensive investigation on the SE-SMR is related to Ni and Ca looping.

3.1 Overview and Current Status of SE-SMR Figure 8 shows the flowchart of the visualized process as four-independent variable which contains the SMR facility, carbon capture system, carbon storage, and carbon utilization systems. Also, variable 1 encompasses the SMR unit with the chief H2 generation plant and the balance of the plant (BoP) or utilities that consist of a boiler to produce high-pressure (HP) steam for the reforming process, which is improved as low pressure (LP steam) to produce electricity using a turbine, while variable 2 with 90% capture efficiency consists of an amine-based CO2 capture unit, where the CO2 is detached from flue gases generated. In addition, this captured CO2 can then be either; (i) transported and injected into store underground in variable 3 or (ii) reformed into formic acid by applying a CO2 electrolyzer in variable 4 depending on the operating scenario. Also, a combination of these blocks can be carried out depending on the operating scenarios.

Fig. 8 Flow diagram demonstrated various blocks in the SMR process and summarizes the mass/ energy balance [93]

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In fact, four SMR operational solutions are suggested for enhanced environmental performance: 1. Base case: Business as normal SMR operation with everything that is dispersed into the environment. 2. Operational solution 1: The CO2 emissions generated in the process are dispersed to the environment but taxed under a governance’s carbon tax. 3. Operational solution 2: A hub overview in which SMR is considered with CCS process with the captured emissions stored in natural reservoirs. 4. Operational solution: A utilization and storage system with both CCS and CCU is located at each hub. There is an urgent need to produce and consume hydrogen gas in the world. Therefore, there is an urgent need to not only enhance the number of low-carbon generation sites but also to raise the current SMR plants [94]. Figure 9 shows a schematic of the SMR process with carbon capture. The feedstock that enters into the reformer is compressed and heated as shown in Eq. 2, reforming of methane occurs. Because the reforming process is an endothermic process, heat has to be provided to pass through the thermodynamic energy barrier. Also, the generated syngas is then transferred to the shift converter; then the water–gas shift (WGS) reaction occurs in the presence of nickel-based catalysts (Eq. 3) [95] (Fig. 10). CH4 + H2 O ↔ 3H2 + CO H298 = 206.4 kJ mol−1

(2)

CO + H2 O ↔ H2 + CO2 H298 = −41.1 kJ mol−1

(3)

Fig. 9 CO2 capture for SMR process [95]

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Fig. 10 1.5 MWth pilot plant [94]

The purification process continues until the fuel is completely converted into H2 and CO2 . In addition, the SMR process usually applies amine scrubbing. This technology has established its worth and also has been used for decades, and despite restricted such as high temperature and pressure and low product efficiency, it can be used for rapid growth in the application of hydrogen gas as an energy carrier. One way to increase the efficiency of the SMR process is a change in the reaction equilibrium by extracting CO2 gas from the “equation” [95]. Thus, improving the reforming reactor in carbon capture unit assigned to decrease of capital cost (CAPEX) of the operation and also enhanced the product efficiency. In addition, in mild operating conditions (1.53 MPa, and 800–1000 °C) leading to produce about 70% of H2 [96] and also in other operating conditions (450–490 °C and 180–890 kPa) leading to 90–98% of H2 in the product for SE-SMR process [97]. As a result, operating costs are potentially reduced. Recently, Cranfield University is currently building a 1.5 MWth pilot plant to advance the process, in order to lift the TRL up, to 6 [94] which in this system entrained flow reactor for the calcination and a bubbling fluidized bed reactor for the reformer is used (Fig. 8) [95].

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4 Final Remarks and Conclusions Carbon dioxide, as one of the greenhouse gases emissions released by human activities, is the chief cause for climate change. Carbon capture, storage, and utilization technologies are essential to the success in order to mitigation of climate change. To reach the aim of restricting the anthropogenic enhance in the mean global surface temperature by 2100 to 2 °C, total CO2 emissions must be decreased by 50% by 2050 rather than levels in 1990. Thus, CCS technologies are in various stages of growth, and the incessant effort for making better some of them is considerable for CCS process. The main advanced capture technologies in commercial for CO2 capture at industrial point sources are pre-combustion, oxy-fuel, and postcombustion, that is economically desirable since it has decreased retrofitting costs for the plant. Also, several reduction emissions from fuel combustion strategies have been grown which consist of CCS that includes membrane system, cryogenic distillation, absorption, adsorption, and mineralization. Further, there are different ways to generation of H2 gas with CO2 capture system.

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Carbon Dioxide Utilization and Biogas Upgradation Via Hydrogenotrophic Methanogenesis: Theory, Applications, and Opportunities Thiyagarajan Divya, Kalyanasundaram Geetha Thanuja, Desikan Ramesh, and Subburamu Karthikeyan

Abstract Increasing industrialization and its impact on the environment have led us to develop an alternative renewable energy source to compensate for the growing demand. Methane, one of the predominant greenhouse gases, has a high potential to act as an energy source in the future. Biogas is an excellent resource for biomethane production, which is produced from the anaerobic digestion of organic wastes. Generally, the biogas mainly contains methane (55–65%), carbon dioxide (35–45%), and other trace gases such as hydrogen sulfide, ammonia, siloxanes, oxygen, carbon monoxide, and nitrogen. Biogas has limited applications (heating, lighting, and engine fuel) and is a poor competitor to petroleum fuels. The main reasons are (i) the low calorific value of biogas (ii) the presence of a higher percentage of non-combustible gases. High energy and chemical consumption hinder conventional biogas upgrading technologies. Alternatively, hydrogenotrophic methanogens which utilize carbon dioxide and hydrogen to produce methane can be employed to increase the calorific value of biogas generated in the anaerobic digesters. Reducing equivalents to biotransform the carbon dioxide in the biogas to biomethane can be supplied by hydrogen or electrons. Water electrolysis employing renewable energy sources is the only environmentally sustainable source to deliver hydrogen for carbon dioxide to methane bioconversion. In this context, it is noted that hydrogen’s low density usually requires large storage volumes, and their transportation and direct utilization technology is still in progress. Therefore, the possibility of converting hydrogen to biomethane is better to approach. Further, the biomethane can be injected into natural gas grids or used as vehicle fuel, an appealing choice for chemically storing energy that would otherwise be lost. This book chapter is brief about the anaerobic digestion process, biogas upgradation via either in-situ or ex-situ approaches,

T. Divya · K. Geetha Thanuja · S. Karthikeyan (B) Department of Agricultural Microbiology, Tamil Nadu Agricultural University, Coimbatore, India e-mail: [email protected] D. Ramesh · S. Karthikeyan Department of Renewable Energy Engineering, Tamil Nadu Agricultural University, Coimbatore, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jawaid and A. Khan (eds.), Sustainable Utilization of Carbon Dioxide, Sustainable Materials and Technology, https://doi.org/10.1007/978-981-99-2890-3_6

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recent technological interventions in biogas upgradation, and challenges in their implementations.

Abbreviations AD CBG CCS CNG CSTR GHG HRT TWh UASB VFAs MUR WWTP

Anaerobic digestion Compressed biogas Carbon capture and storage Compressed natural gas Continuous stirred tank reactor Greenhouse gas Hydraulic retention time Tera watt hour Upflow anaerobic sludge blanket reactor Volatile fatty acids Methane uptake rate Waste water treatment plant

1 Introduction The amplifying amount of carbon dioxide (CO2 ) in the atmosphere has driven intense interest toward renewable energy sources and technologies to reduce/observe it from the air. The excess CO2 in the atmosphere results an imbalance in the environment, causing perturbation in the carbon cycle. Several technologies are being exploited toward mitigation of CO2 by utilization via photosynthesis [62], generation of a wide variety of biochemicals by catalytic oxidation [41], employed as supercritical fluid [23]. The sustainable and economical use of CO2 is to convert it into methane, thereby trapping CO2 and exploiting it for further applications to mitigate/reduce the global warming issues [37, 40]. Besides developing biogas technology in India, it steers opportunities for CO2 utilization [31]. Biological way of methane production from CO2 gas is the stirring strategy for CO2 capture and storage (CCS) at power plants. This concept would be augmented with the use of suitable microorganisms as the inoculum for sequestering CO2 . The microbe-mediated conversion of methane to CO2 is one of the active processes in the earth’s crust and has been well documented in soil, lake, and ocean sediments. Mostly, the bacterial pathway CO2 conversion is used in the world’s largest natural gas deposits. For instance, the gas fields of Santa Barabara in the Po Valley in northern Italy, Urengoy in West Siberia, produce methane likely of bacterial origin [19, 57]. Several studies have witnessed the sequestration of CO2 by methanogens for methane/fuel production [49].

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Further, the methane (CH4 ) obtained from CO2 using hydrogen (H2 ) combines gives a dual solution against environmental concerns and fits into the currently developed area of CCS, ultimately assisting in meeting the country’s growing demand for clean fuels. The use of H2 by hydrogenotrophic methanogens assists in CO2 reduction [42].

2 Anaerobic Digestion and Merits The organic wastes originated from plant or animal sources, including animal manures, vegetable waste, kitchen waste, food waste, organic fraction of municipal solid wastes, and agro-processing industrial liquid and solid wastes. Because of higher moisture, waste management strategies should be adopted for proper and safe disposal. In other words, severe public health issues and environmental problems will emerge if they are dumped into the environment. Over 105 billion tons of organic waste are produced every year, but only 2% of them are recycled [68]. Therefore, the effective management of these wastes is vital for restoring the good quality of air, water, and the environment. There are copious methods available for converting wastes into a variety of bioproducts. Among them, anaerobic digestion (AD) serves as an excellent and economical way to unleash the great values from these wastes. It is well acknowledged, long-established technology for valorizing diverse organic wastes with concurrent by-product generation. It is evaluated that 50 million microanaerobic digesters are currently operated at globe level for biogas generation and utilized for cooking, heating, and lighting purposes [67]. In the context of organic waste management, AD curtails the emission, and the organic wastes are turned into valuable green resources like biogas, biomethane, bio-CO2 , and nutrient-rich fertilizers. AD uses technology capable of fulfilling defossilization and decarbonization, thereby reducing to nearly 10% global GHG emissions by 2030 [52]. Presently, the AD technology can generate ca. 12,000 TWh of energy annually, which is equivalent to 28% of global coal consumed during 2017. In accordance with the Paris agreement, all sectors need to provide deeper decarbonization to curb global warming to 2 °C, AD already serves as carbon-negative technology in supporting the reversal of climate change. AD units can be installed on a microlevel to recycle the household wastes, which could handle either wet or dry or a mixture of them. The AD-based energy production potential could replace 33% fossil fuel demand, whose proportion could be increased to 53% upon the integration of power-to-gas technology. Overall, it possesses advantages like diverting organics from landfills, renewable energy generation, soil health benefits, manure management, and GHG emission reduction. Furthermore, the AD acts as a multifaceted platform for biogas generation from which potential applications can be made.

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3 Biogas Upgradation The CH4 content in biogas is ranged from 55 to 65%, while CO2 accounts for 35– 45%, along with H2 S, water vapor, nitrogen, hydrogen, and oxygen. The occurrence of CO2 hinders the calorific value and limits the economic feasibility of biogas for power generation. Likewise, the other contaminant in the biogas causes detrimental effects on downstream processing. Recently, compressed biomethane (CBG) has been considered as substitute for compressed natural gas (CNG) in the vehicles. The calorific value of the biogas is directly proportional to its CH4 content. In other words, a higher calorific value of biogas is observed for biogas with higher CH4 content and vice versa. The simplest way to improve the calorific value of biogas, increase the percentage of CH4 content in that gas by removing other gasses (impurities) present in the biogas. Furthermore, the main objective of upgradation is to increase the energy density of the biogas [34]. For this work, biogas upgradation technology is employed to attain the higher energy density fuel. The final product obtained after biogas upgradation contains higher CH4 content (≥ 90%) is called biomethane. The biomethane finds application as vehicular fuel or injection into the gas grid and is promoted in various countries. The biomethane assists in GHG reduction and holds environmental benefits like less emission of hydrocarbons, nitrogen oxide, and carbon monoxide. There are several biogas upgradation technologies, and its summary is furnished in Table 1.

4 Biological Methods Compared to other biogas upgradation technologies, the biological method employs microorganisms for converting CO2 gas into methane, which results in increased CH4 content in the biogas. Chemoautotrophic, photoautotrophic, and fermentation fall under this category. In this method, the microorganisms may utilize the impurities (CO2 or H2 ) to produce valuable products. One of the examples for this case is CO2 gas into succinic acid. The microorganisms’ actions and mechanisms used for hydrogen (H2 )/CO2 utilization in the biological upgradation method are briefed.

5 Nature and Physiology of Hydrogenotrophs Hydrogenotrophic methanogens have the greatest phylogenetic diversity among the three metabolic versatile methanogens. Most methanogens in the orders Methanobacteriales, Methanococcales, Methanomicrobiales, Methanocellales, and Methanopyrales are hydrogenotrophs, and they can use H2 as an electron donor to convert CO2 to CH4 [43]. The most common hydrogenotrophic methanogens in AD are Methanobacterium, Methanothermobacter, Methanobrevibacter, Methanospirillum,

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Table 1 A summary of biogas upgrading technologies Technology

Principle

Basis of operation

CH4 (%) in biomethane

CH4 loss (%)

References

A. Absorption 1. Physical absorption (a) High-pressure water scrubbing (b) Organic physical scrubbing

Based on the solubility of various components of the biogas in a liquid solvent

Physical absorption

96–98

99

42 days under 70 °C when kept in amine solvent; meanwhile, unbound CA became inactive within 60 min E. coli MO1 carbonic anhydrase is utilized to immobilize and found to be 100% active for 28 days

Carbon nanotube (SWNTs) loaded with peptides

Electropolymerized polypyrrole films

Chitosan–alginate polyelectrolyte complex (C-APEC)

References

Even after immobilization the CA was 71% active for about 6 runs

This technique enables to develop a biologically functional hybrid substance devoid of interrupting the natural shape

[68]

[66]

Possess the ability of [65] overcoming the formation of aggregates and compression due to the process of centrifuge and filtrating it

Involves the application of bovine carbonic anhydrase (BCA)

Cross-linked amino-activated paramagnetic nanoparticles

Remarks

Mechanism

Supporting matrices

Table 1 (continued)

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by inducing site-directed mutagenesis [44]. Several solvent systems were utilized for facilitating the CO2 absorption including the carbonate, amine solvent, amino acid salt, ionic liquid, and immiscible solvents [43, 45]. In additional to the usage of solvent system, alongside the contact of gas and liquid for adsorption and desorption could be improvised by utilizing several contact devices. Diverse forms of contact devices were developed for capturing the CO2 like packed bed (PB), rotating packed bed (RPB), and membrane contractor (MC).

2.2 Packed Bed (PB) and Rotating Packed Bed (RPB) In PB, the entry of exhausts gas was setup in its bottom and exit at its top, and the entry of absorbent usually occurs at the top and exits at the bottom. Immobilization of carbonic anhydrase facilitates the conversion of CO2 into HCO3 , followed by its precipitation and collection once after the completion of its interaction with CA. Leimbrink et al. conducted an experiment to increase the adsorption in PB by immobilizing CA onto SiO2 beads loaded within a Katapak SP bags and used it for packing the PB, he compared the adsorption efficiency between the PB operated with and without the enzyme, and the result stated that PB loaded with CA enzyme showed 7.5X augmented adsorption [46]. The RPB instruments are usually enhanced for mass transfer rate among the gas-liquid by the application of the centrifugal force [47]. This mechanization is also termed as higee (high gravity), and it is capable of enhancing the process of distillation and adsorption to a substantial rate [48]. Applying centrifugal force elevates the shear force, mass transfer, and micromixing which facilitates reduction in the equipment volume [49]. For operating the RPB, the flue gas with CO2 has to be injected with intense compression from the external edge and the liquid absorbent is injected at the internal edge through the dispenser. The centrifugal force acts upon the absorbents and stimulates its movement in the direction of the edge thereby facilitating its interaction with CO2 . The gas exits the PRB at the top and absorbent containing the dissolved CO2 exists at the sides [48]. Even though this technique removes CO2 up to 90%, yet it has to be performed at industrial scale.

2.3 Membrane Contractor (MC) The membrane contractor is a combined technique of adsorption and membrane technology for avoiding the connection among gas and solvent system (Fig. 2) by the membrane barrier [47]. The entry of gas occurs form one side of the MC unit and permeation of CO2 occurs via the CA-bounded selective membrane in which hydration of CO2 into HCO3 takes place. The CO2 permeation rate is generally high when compared to nitrogen, carbon monoxide, and other contents present in the exhaust gas. In order to recover the CO2 heavy pressure or sweep gas is applied from

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CO2

CO2 Flue gas

Flue gas

Flue gas

Membrane Liquid absorbent

Membrane contractor Fig. 2 Membrane contractor designed to capture CO2

the other side [50]. Usage of solvents like ionic liquids on the opposite would result in the increased absorption and permeation due to mass transfer acceleration. The preparation of membrane includes the following substance like SiO2 , ceramic, and zeolite. Hou et al. prepared the membrane by immobilizing the CA using titanium dioxide nanoparticles with virgin polypropylene and superhydrophobic functionalized polypropylene (pp) hollow fiber membranes and observed that it could be reused for at least ten times [51]. Yong et al. engineered polypropylene membranes with mesoporous silica nanoparticles immobilized to CA using layer-to-layer technology and recorded with increase in the efficiency for CO2 hydration nearly 19 ± 4 μmol cm−2 min−1 per layer [52]. To operate the MC unit, the exhaust gas has to be compressed using intense pressure in prior to subjecting it to the process of filtration resulting in excess consumption of power. Moreover, majority of the membranes utilized were inefficient and selectively permeable for CO2 hence non-applicable for long-term actions [53]. The MC technologies were capable for reducing the volume of operational unit to 70% in comparison with the PB reactor. This mechanization remains at lab-scale due to its modular state and the mass transfer is lowered due to the membrane barrier [47].

2.4 Hydrogenation of CO2 into Formate Several chemical and biological techniques were recorded for the transformation of formate from CO2 . In chemical process, metallic catalyst was required along with increased temperature and pressure [54]. It is possible to conduct biological process precisely within mild condition. CO2 has been reduced into formate with the usage of enzyme immobilization and whole cell biocatalyst. Diverse microbial strains were scientifically recorded for its capabilities of converting CO2 into formate

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including E. coli, Methylobacterium extorquens, Acetobacterium woodii, etc. [55– 57]. For improving the steadiness and separating the enzymes, immobilization of formate dehydrogenase (FDH) onto alginate silica hybrid gel and along with usage of the electron donor-β nicotinamide adenine dinucleotide (NADH) was done by Lu et al. [58]. The enzymatic efficiency for converting CO2 was reported to be 95.6% and retains its performance up to 69% even after ten cycles. Electroenzymatic process was applied for regenerating the NADH. Candida boidinii FDH and a Cu electrode were utilized for converting CO2 into formate by Kim et al. [59]. Yadav et al. stated that regeneration of NADH could be possible with use of a non-natural photosynthesis process for the formation of formate from the CO2 when a graphene-associated visible light active photocatalyst-enzyme coupled system was used [60]. Thin films of polydopamine copolymerization of FDH and NADH via electrochemical synthesis were utilized for converting CO2 into formate, where the PDA facilitates the transportation of electron and augments the system steadiness for a prolonged period [61].

2.5 Formation of CH4 from CO2 Biogenic methane is produced by a process in which the carbon dioxide is reduced into CH4 via biological reaction. ATP-dependent nitrogenase is capable of reducing multiple electrons in an inert molecule [62]. The nitrogenase which exists in nature lacks the ability of reducing the CO2 , since numerous amino acids were approaching the FeMo-cofactor that is significant for substrate binding and reduction. Remodeled nitrogenase with the substitution of α-195 for Gln and α-70 for Ala showed the ability of reducing the CO2 into methane [63]. Fixen et al. performed an experiment which involved the development of an anoxygenic phototrophic microbe the Rhodopseudomonas palustris to express the re-designed nitrogenase with the potential of reducing CO2 –CH4 when kept in the presence of light at in vivo conditions [62]. The Fe protein present in the nitrogenase provides the electrons necessary for reducing the CO2 , and the ATP molecules supply the energy vital for performing this process. Light serves as a significant factor for regenerating the ATP molecules via the cyclic phosphorylation [62]. The alternate technique so far recorded for reducing CO2 involves the microbial electrolysis cell (MEC) [64], where CO2 sequestration and energy generation were performed simultaneously [65]. The MEC system is an altered microbial fuel cell (MFC) capable of reducing CO2 into CH4 when a potential difference is a supplied into the biocathode (a cathode with a thin layer of microorganisms) [66]. Even though the microorganism was capable of producing 0.3 V potential utilizing the C2 H3 O−2 present in the anionic chamber, it is considered to be low than the potential needed for producing hydrogen in the cathode (0.414 V); hence, extra potential is needed for generating the hydrogen [67]. Usage of a biocathode could assist in reducing the CO2 into CH4 . Biocathode was cheap substitutes for expensive cathodes like platinum. The microbial cells coupled with the biocathode could receive electron straight from

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the electrode/involves the usage of electron shuttles and cathode H2 for the purpose of reducing CO2 –CH4 . [68]. Considering the fact that heterogenous cultures were highly appropriate for producing the biofilm in biocathodes when subjected to ecological stress. Villano et al. have given a suggestion that combining the MECs with anaerobic digestor that already exists would facilitate the qualitative improvement of biogas by reduction of CO2 into CH4 since it is one of the biogas elements [69]. Recently, Dou et al. conducted a study in which he investigated the process of hybrid anaerobic digestion bioelectrochemical cell (AD-BEC) system with the supply of various potentials and observed that the methane production was high in case of the hybrid system in comparison with the traditional system. It has been found that the potential supplied to the system enriched the exoelectrogens present in the biofilm of anode and the hydrogenotrophic methanogens present in the cathodic biofilm [70].

2.6 Enzyme Cascade-Mediated Production of Methane from CO2 Reduction of CO2 into methanol can be performed with usage of several catalytic materials under increased temperature around 150–300 °C and high pressure up to 3–14 Mpa. These enzymatic processes were performed under room temperature with the use of the enzyme cascade. CO2 is reduced into methanol using 3enzymes in sequential stages. For this, NADH is required at its reduced state as a cofactor. Initially, the processes begin with the formation of formate (HCOOH) from CO2 driven by dehydrogenase (FDH), followed by its transformation into formaldehyde (HCHO) mediated by formaldehyde dehydrogenase (FaldDH) and ultimately converted into methanol by the activity of alcohol dehydrogenase (ADH). Regardless of the improvement in the technologies, still there exist some limitation for converting CO2 into methanol due to loss of enzymatic activity as CH3 OH is formed. Luo et al. performed an investigation, by co-immobilizing and sequentially immobilizing the FDH, FaldDH, and ADH into a thin sheet of polymer membranes for converting CO2 into CH3 OH [71]. The co-immobilizing tactic led to low conversion efficiency and acted as a second reaction. The formation was formaldehyde from formic acid acted as a limiting factor. Sequential immobilizing strategy was a widely chosen methodology for enzyme cascade as every single step could be under control [71]. For producing 1 mol of CH3 OH from the CO2 , it requires 3 mol of NADH, that impacts the entire process in economical viewpoint [72]. Aresta et al. explained a hybrid enzyme/photocatalyst method for converting methanol from CO2 and regenerating NADH simultaneously from NAD + with the usage of TiO2 -based photocatalyst [73]. With the application of cofactor regenerating method, it is highly possible to generate 100–1000 mol of methanol using 1 mol of NADH. To regenerate the coenzyme NAD + /NADH, the glutamate dehydrogenase (GDH) was utilized by Marques et al. Immobilization of GDH into functionalized magnetite nanoparticles laterally

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with three enzymatic cascades is vital for converting methanol and the reaction driven these four enzymes showed 64X conversion efficiency [74]. Ji et al. described the non-natural photosynthetic system to biologically convert CO2 into methanol [75]. The coupling of enzymatic cascade system for reducing reaction with the GDH coenzyme regeneration system was enclosed and hooked to a polyurethane (PU) nanofiber lumen doped with polyelectrolyte with 20 kDa of molecular weight for preventing the leakage of capsulated enzymes, and this facilitated the movement of polyelectrolyte poly(allylamine hydrochloride) (PAH) [75]. In comparison to the solution-based system, the non-natural photosynthetic system showed up to 35.6– 90.65% of methanol formation. Schlager et al. improved a system to be independent from co-enzymes to directly convert CO2 into methanol via the immobilization of CH2 O2 , CH2 O, and alcohol dehydrogenases onto an electrode with the use of alginate–silicate hybrid supporting gel and provided with direct supply of electrons for immobilizing the enzymes via the electrode [76].

2.7 Role of RuBisCo Enzyme The Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) is specific toward CO2 and O2 . Being a carboxylase, carboxylation of ribulose-1,5-biphosphate is carried out by its catalytic activity and results in the production of extremely unbalanced 6-C phosphorylated transitional product termed as 3-keto-2-carboxyarabinitol1,5-bisphosphate that forms the glucose molecule. Initiation of C2 photorespiration occurs since this enzyme also acts as an oxygenase, thereby producing the 2-phasphoglyco late with the liberation of CO2 [77]. The RuBisCo exhibits low selectivity to CO2 than the O2 thus responsible for reduced rate of CO2 incorporation. Research works were still carried out for engineering the RuBisCo enzyme but faced lot of challenges and boundaries, due to the increased activity resulting in reduced selectivity to CO2 and vice versa an improved selectivity resulting in diminished functionality. [78]. The S rel value represents the ration of selectivity to CO2 and O2 . There are two types in RuBisCo enzyme; the red and green forms were scientifically recorded. Ninomiya et al. performed a structural analysis on the protein framework present in both the RuBisCo types and the results revealed that the S rel (238) was found to be high in red type due to the presence of a handle-shaped structure sandwiched among the O2 val 332 and the side chain amide nitrogen of Gln 386. [79] Liang and Lindblad performed an experiment on Synechocystis PCC 6803 to study the overexpression of RuBisCo enzyme and improvised the strain resulting in the augmented growth and the photosynthetic function [80].

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3 Production of Bioenergy via the Biological Conversion of CO2 The CO2 sequestered from various fixed resources including the coal-fired plants, cement, and steel industries could be either stored for utilization in future or direct usage. The direct use of CO2 involves bioenergy production where the CO2 serves as a raw material for fermentation reaction using microorganism, and the resultant products include the biodiesel, bioalcohol, and innovative fuels listed in Table 2.

3.1 Biodiesel Direct usage of CO2 and its derived forms such as CH4 and formate acts as a raw material for producing biodiesel. Serval strains of bacteria and algae were scientifically reported for carbon sequestration, and it is precipitates into CaCO3. It includes the bacteria—Myxococcus xanthus, Bacillus cereus, Pseudomonas aeruginosa, Bacillus subtilis and algae—Chlamydomonas Botryococcus braunii, Chlorella vulgaris, Chlorella kessleri, Chlorocuccum littorale, etc. [77–83]. Recently, microalgae have been widely focused due to their potential for direct fixation of CO2 present in the air or capturing soluble carbonates, resulting in increased yield of lipids when cultured at industrial scale using wastewater or waste land incapable for farming converted to cultivate microalgae [84–86]. Waste H2 O containing high N2 and P content was utilized by various strains of microalgae and facilitate the removal of NH3 , NO3– , and PO4 3− present in the waste H2 O. The microalgae were cultivated using two types of cultivation system: the open and closed culture systems. In open culture system, the algae were cultivated using the non-artificial water bodies such as pond, lagoons, and circular water channels. The closed culture system aka photobioreactor is comprised a vessel made up of translucent walls capable of maintaining within controlled culture conditions [85]. Microalgae were capable of fixing carbon up to 15-fold quicker when compared to plants and no issues as a food and fuel resources. In algae, the CO2 is fixed by oxygenic photosynthetic process or involves the enzymatic action of Zn-dependent Carbonic anhydrase. The CO2 is converted into HCO3 by the enzymatic activity of carbonic anhydrase, and a particular carrier molecule aids its transportation/entry into the cell [87]. A wide range algal species possesses the ability of growth even if the concentration of CO2 is less than 5%. To culture the microorganism at industrial scale, the species must be tolerant to high concentration of carbon dioxide. For improving CO2 sequestration rate, genetical modification of microbes is necessary and it also results in increased oil deposition potential. Li et al. improved the microbial strain, it was Synechococcus obliquus WUST4 highly tolerant to CO2 concentration by chemically induced mutations, and a direct pilot study was conducted by him to evaluate the CO2 fixation rate via developing the CO2 fixation system using the discharge of a combustion chamber [20]. Lee et al. genetically modified Synechococcus elongatus PCC 7942 to produce ethanol with

Showed ability of growth with pH of wide range – An algal isolate from the sea capable of growing – in saline water with 3.5% Showed tolerance and growth under elevated 0.260 CO2 concentration ranging from 30 to 50% and noticed for rise in the polyunsaturated fatty (PUFA) production under elevated CO2 strength Optimal bio-oil production reported under a CO2 concentration of 4% Grows at maximum rate under 5% of CO2 Involves the application of Bubbling-type photosynthetic algae microbial fuel cell Algal strains were cultivated using vertical tubular photobioreactor To produce bioenergy and biofix CO2 , airlift microbial fuel cells were utilized Wastewater from seafood processing industry was supplied continuously with a CO2 concentration of 10% to cultivate the algae Airlift photobioreactor is taken into application for cultivating the microalgae with 12% CO2 concentration

Asterarcys sp.

Chlamydomonas sp. JSC4

Chlorella pyrenoidosa SJTU-2

Chlorella sp. BTA 9031

Chlorella vulgaris

Chlorella vulgaris

Chlorella vulgaris

Chlorella vulgaris

Chlorella vulgaris NIOCCV

Coelastrum sp. SM

0.302

0.43

1.29

0.43

0.605

–

0.235

0.287

Isolation of bacteria from free air CO2 enrichment (FACE) soil area

Bacillus sp. SS105

The degree of CO2 incorporation (g/(L · D))

Biodiesel

Remarks

Microbe

Class

Table 2 Various microbes capable of capturing and transforming CO2 into bioenergy

101

92

234.3

430

105.9

157

21.3

29.57

169

30

1.7

Yield (mg/ (L · D))

(continued)

[21]

[140]

[139]

[138]

References

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Ralstonia eutropha H16

Isobutanol

Elevated CO2 concentration favored the synthesis of polyunsaturated fatty acid and its deposition

Scenedesmus obliquus SJTU-3

Synechococcus elongatus PCC 7942

Involves two-stage cultivation strategy

Scenedesmus obliquus CNW-N

n-butanol

Cultivated within 1 L scale, and maximum rate for biofixation of CO2 occurs at the concentration of 15%

Scenedesmus bajacalifornicus BBKLP-07

Utilizes integrated system for the conversion of CO2 into formate electrochemically followed conversion into isobutanol by genetically manipulated R. eutropha H16

Microalgae genetically manipulated with O2 -tolerant CoA-acylating aldehyde dehydrogenases (PduP)

Capable of producing ethanol with the utilization of H2 and CO2 under 55 °C

Capable of growth even under 15% of CO2 concentration

Nannochloropsis sp.

Moorella sp. HUC22-1

N/P ratio impacts the growth of biomass and the – synthesis of lipids

Neochloris oleoabundans

–

–

–

0.288

0.549

0.12

–

–

First strain recorded for the conversion of CO2 and CO or H2 into CH3 COOH, and the second species for converting CH3 COOH into lipids

Moorella thermoacetica and Yarrowia lipolytica

The degree of CO2 incorporation (g/(L · D))

Remarks

Microbe

Ethanol

Class

Table 2 (continued)

206

51

50

32

78.73

15.7

18.93

45

4560

Yield (mg/ (L · D))

(continued)

[29]

References

Carbon Dioxide Capture and Bioenergy Production by Utilizing … 175

Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803

Ethylene

Isoprene

Limonene

–

Manipulated by the incorporation of plant limonene synthase

–

Incorporated with C5 H8 synthesizing gene from – Pueraria montana

Incorporation of an efe gene responsible for the formation of ethylene-producing enzyme derived from Pseudomonas syringae pv

Modification of glycolysis metabolic route and – the C3 cycle for increasing the carbon flux and it is redirected for carbon fixation

Synechococcus elongatus

–

Manipulated with 2,3-butanediol synthesizing metabolic route, and additionally with the cofactor biosynthetic metabolic route

Synechococcus elongatus PCC7942

–

–

2,3-butanediol

Capable of performing conversion reaction independent of B12 coenzyme

This species was manipulated in which the oxygen-sensitive CoA-acylating butyraldehyde dehydrogenase (Bldh) is replaced with oxygen-tolerant CoA-acylating aldehyde dehydrogenases (PduP)

Synechococcus elongatus PCC 7942

The degree of CO2 incorporation (g/(L · D))

Synechococcus elongatus PCC 7942

Remarks

Microbe

1,3-propanediol

Class

Table 2 (continued)

0.056

0.050

171

1100

793

20

51

Yield (mg/ (L · D))

[133]

References

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a metabolic route and ester synthase (AftA); this microbe was found to be capable of direct synthesis of fatty acid ethyl ethers (FAEEs) using CO2 as a substrate [88]. Integrating the technologies for sequestration of CO2 together with waste H2 O treatment might favor in reducing the ecological pollution caused by greenhouse gases emitted from the anaerobic digestor [89]. Rosli et al. studied the development of integrating a method to simultaneously treat the waste H2 O and CO2 sequestration. The microalgae Chlorella vulgaris was cultivated using a PU supporting material loaded within a liquid bed reactor for facilitating its harvest [81]. Biodiesel is produced by direct utilization the CO2 as a raw material by culturing microorganism resulting in the collection of biomasses when harvested possessing bio-oil/lipid content. This biooil or lipid content after its extraction will be subjected for conversion into biodiesel by the process of transesterification along with the presence of alcohol, enzymes, and chemical catalyst [90].

3.2 Ethanol (C2 H5 OH) Alcohol is widely produced by fermenting food crops such as Saccharum officinarum, Zea mays, and sorghum. Due to limitations in the agricultural land and food competition, improvement of this process becomes difficult. Cyanobacteria biologically produces alcohol by photosynthesis, and it possesses the ability for capturing the CO2 and converting into organic substance by utilizing the sun light via the Calvin cycle [91]. Genetic modification of metabolic pathways into these strains will improve the synthesis of bioalcohol. Based on a theoretical evaluation, it has been estimated that nearly 5280 gallons per acre/year of C2 H5 OH production could be achieved if microorganism capable of photosynthesis was used, and simultaneously, the yield achieved by cultivating food crops only results with 321 gallons per acre/year for corns and sugarcane with 727 gallons per acre/year [92, 93]. Synthesis of ethanol using autotrophs was initially reported by Deng and Coleman, and he produced ethanol with Synechococcus sp. strain PCC 7942 via heterologously expressing the pyruvate decarboxylase (PDC) and ADH in the species Zymomonas mobilis [94]. Gao et al. genetically manipulated Synechocystis sp. PCC6803 in a comparable way to heterologously express PDC and ADH, and he disrupted numerous genes responsible for synthesizing PHA and resulted with an ethanol yield of about 5.50 g/L. The Calvin–Benson–Bas sham (CBB) cycle is the principal route to biologically fix CO2 in cyanobacteria [91]. Liang et al. manipulated the strain Synechocystis PCC 6803 for increased ethanol production via the overexpression of four genes in the CBB pathway along with genes responsible for the production of C2 H5 OH [95]. Silva et al. done a cytometric evaluation on ethanol-tolerant level for the strain Synechocystis PCC 6803 and reported that extraction of bioethanol from the culture would prevent the unfavorable impacts caused to the biomass [96]. The key factor responsible for tolerating the ethanol is the enzyme alcohol dehydrogenase (AdhA). The removal of this enzyme would result in the lack of ability to survive when ethanol is present with

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the concentration of 4% v/v for the algal species [97]. NADPH performs a significant activity in converting the ethanol from acetaldehyde. The NADPH regenerating system driven by oxides of metal was improved by Velmurugan et al. with the ability to increase the ethanol production up to twofold than the control culture [98].

3.3 Supplementary Biofuels 1-butanol is a highly potent biochemical substance and fuel widely synthesized form the species Clostridium and some genetically modified microbes. Generally, 1-butanol cannot be produced by utilizing the cyanobacterial strains. Lan and Liao (2011) incorporated the co-enzyme-A-independent route in the strain S. elongatus PCC 7942 to biologically produce 1-butanol, and he was scientifically reported to be first person for demonstrating the production of 1-butanol from autotrophs [99]. The same scientific set were recorded for producing high energy biofuel the 2-methyl-1butanol (2 MB) utilizing the species S. elongatus PCC 7942 [100]. The complete and detailed study on the synthesis of 2-methyl-1-butanol was recorded by utilizing the recombinant E. coli via the isoleucine metabolic route by decarboxylation of 2-keto3-methylvalerate which is further reduced. Miao et al. manipulated the species Synechocystis sp. PCC 6803 intended for overexpression of α-ketoisovalerate decarboxylase derived from Lactococcus lactis and various ADHs to produce isobutanol [100]. A citramalate metabolic route was incorporated by Shen et al. for the production of isoleucine precursor 2-ketobutyrate and genes ketoacid decarboxylase (Kivd) along with alcohol dehydrogenase (YqhD) resulting with the yield of 2 MB about 20 mg/L [100]. Kusakabe et al. modified the metabolic route synthesizing isopropanol in the strain S. elongatus PCC 7942 with the incorporation of acetyl-CoA acetyl transferase [102], acetoacetate decarboxylase (adc), acetoacetyl-CoA transferase (atoAD), and secondary alcohol dehydrogenase (adh), and the isopropanol yielded within 9d was recorded to be 26.5 g/L [101]. Cyclic hydrocarbons were one of the constituents of petroleum-based fuel and their functions could be improved [103]. Limonene is transparent liquid grouped as cyclic monoterpene a major constituent of oil produced from citrus plants and shows similarity to biofuel and jet fuel. The cyanobacterial strain Anabaena sp. PCC 7120 was incorporated with genes producing limonene for its secretion taken from Sitka spruce. Wang et al. manipulated the species Synechocystis sp PCC 6803 to overexpress the innate genes acyl-acyl carrier protein reductase and aldehyde-deformylating oxygenase to produce an advance biofuel called alka(e)nes [103]. The strain Synechocystis sp. PCC 6803 was manipulated by Yao et al. to produce fatty alcohol via the heterologous expression of the fatty acylcoenzyme A (acyl-CoA) reductase gene obtained from the bacteria Marinobacter aquaeolei VT8 along with the deletion of aldehyde-deformylating oxygenase gene producing the alkenes [104]. Levulinate is a fuel additive substance produced as a secondary product when the microalgae were subjected to the process of in situ transesterification. The microalgae with high carbohydrate content will increase the yield of levulinate [105].

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4 Alternative Techniques to Sequester the CO2 In addition to the biological techniques used for CO2 , there exist many techniques which involves the (i) absorption, (ii) adsorption, (iii) cryogenic techniques, and (iv) membrane techniques. Adsorption is the single method commercially in practice for capturing the CO2 , and the remaining methods were still under development and yet to reach the industrial scale.

4.1 Absorption The process of absorption involves two kinds. They are (i) chemical absorption and (ii) physical absorption. (i) Chemical adsorption: The chemical absorption method involves minimal pressure and temperature ranging from 40 to 60 °C to capture CO2 with the usage of aqueous alkanolamine including the MDEA—methyl diethanolamine, DEA— diethanolamine, MEA—monoethanolamine, tetramethylene sulfone, diethylenediamine (PZ), aminoisobutanol (AMP), and Dipotassium carbonate (K2 CO3 ) [106, 107]. The reaction among (1°) and (2°) amine with CO2 results in the formation of CO3 and CO2 reacting with (3°) amine leads to the formation of HCO3 [108]. The amines act a buffer for the reacting solution and chelates the metallic ions and function as an activator substance for the CO2 [109]. For capturing the CO2 , these technologies have been utilized in case of exhaust gas with minimal concentration. A slightly bounded transitional material is formed when there is a reaction between the solution and the CO2 . When an aqueous solution of aminoisobutanol is added to Diethylenediamine will endorses the absorption rate once if the partial pressure of CO2 is maximum and shows negative effects when at minimal pressure [110]. Further reaction includes the recovery of CO2 along with the regeneration of the solvents with the application of heat around 100–140 °C and the resolvents were reused. (ii) Physical absorption: In physical absorption, the CO2 is sequestered at highest pressure and lowest temperature. Heat is supplied for recovering the CO2 and solvent regeneration. This technology is applicable for the exhaust gases with up to 40% of CO2 and requires a physical solvent like Selexol comprising polyethylene glycol [111]. Integration of gasification combine cycle (IGCC) was improvised by Belaissaoui et al. for capturing the CO2 obtained by the gasification of carbon containing fuels, with an attachment of a void fiber gas–liquid membrane contactor loaded with a physical solvent under applied pressure. As per the authors’ statement, it only requires 50% of power to absorb and desorb when compared to other methods so far recorded [112]. This technology involves few drawbacks like heavy power consumption for recovering CO2 and regenerating the solvents, controlling corrosion is necessary,

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and the efficient was low as result of impurities present including nitrogen oxide and sulfur oxide [111].

4.2 Adsorption The adsorption process involves the sequestration of carbon via interacting to a solid surface like active charcoal and an adsorption bed made up of aluminum oxide and zeolite. Adsorption involves three kinds, and they are (i) thermal swing adsorption (TSA), (ii) pressure swing adsorption (PSA), and (iii) electric swing adsorption (ESA) [167]. They are grouped based upon the techniques involved in recovering the CO2 and regenerating the adsorbent compound. The thermal swing process involves the application of temperature, PSA encompasses pressure application, and ESA includes the passage of electricity with low voltage via the adsorption material. An innovative hybrid model was developed for capturing the CO2 , comprising the combination of the zeolite/activated carbon honeycomb which is capable of performing ESA with vacuum swing adsorption (VSA), and it rapidly recovered up to 72% of CO2 which is 33% pure in 30 s when operated with a slight pressure of about 10 kPa [113]. Hong et al. established a carbon chamber with micropores by carbonizing wheat flour and recorded with carbon sequestration about 5.70 mol/kg under 0 °C and 3.48 mol/kg at 25.1 °C [114]. Adsorption materials made by impregnating amines were generally utilized for capturing CO2 present in the exhaust gas that have been diluted. Peng et al. organized a mesoporous carbon substance derived from deacetylchitin infused in the pentaethylenehexamine, and the material thus formed exhibited a tendency to adsorb CO2 selectively [115]. In recent studies, a novel adsorbent material called covalent organic polymers (COPs) was scientifically recorded. It has good chemical tuning flexibility, highest hydrothermal stability, and inexpensive. It has a high potential for removal of CO2 present in the non-artificial gas [116]. For the direct sequestration of CO2 present in the atmosphere, Sujan et al. operationalized a polymer/silica fiber with (1˚) amine—poly(ethylenimine) sorbent, resulting in the formation of highly pure CO2 obtained from dry and as well as moisture air mixture present in the surrounding [117]. Ramadass et al. discovered the innately accessible halloysite nanotubes (HNTs) and HNT/Kaolin combinations to adsorb the CO2 resulted with 120 μmol/m2 of carbon sequestration rate, which the maximum level ever recorded when compared to other commercially used adsorbent material like mesoporous silica, activated carbon, and carbon nitride [118]. The porous tubular structure and its exceptional surface facilitated the augmentation of CO2 adsorption and due to the existence of unbound hydroxy groups in the inner and outer surface of HNTs.

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4.3 Cryogenic Method This method performs on the principle of cooling and condensation. Here, the CO2 gets cooled at the temperature ranging from −100 °C to −135 °C for its solidification and separation from the gaseous mixture [119]. Once the CO2 is solidified, it is pressurized, subjected for melting and transportation at its liquid state. This method is applicable only for recovering CO2 from the flue gas with the concentration more than 50%, intense energy requirement in order to refrigerate impacts the economical perspective, incase used for diluting the CO2 streams [120]. The key benefit of this technique is, it is free from chemical absorbents and capable of function under extremely low pressure. This technique involves the direct conversion of CO2 into liquid, facilitating its transportation via pipeline with ease. The H2 O component present in the gas blend utilized for recovering the CO2 must be controlled due to the fact that sudden fall in temperature would block the system as a result of H2 O solidification, thereby making the process less efficient. Stirling cooler technology operates on the principle that variation among dehydration synthesis and deposition, used by Song et al. (2012). The condensation and desublimation of water and carbon dioxide occur at various segments in the capture system, which makes the process free from the application of solvents and pressure [121]. The Stirling methods was developed by integrating thermal exchanging networks, where the heat in association with the disconnected stream (water after condensation, gas residue, and the sequestered CO2 ) for its recovery and utilized to operate the unit [122]. By applying this method, the authors could potentially lower the power consumed for capturing the CO2 in that unit around 195.29–88.0 MW [122]. Knapik et al. improved a combined method to remove CO2 present in the exhaust gas emitted in the process of oxycombustion, where the N2 obtained from the nitrogen elimination unit subjected for reuse intended for cooling the condenser to liquefy the CO2 [123].

4.4 Membrane Technology The membrane-dependent technique involves the separation of CO2 present in the gaseous mixture through a semi-permeable membrane. It is suitable for low carbonemission plants, which allows continuous activity. The key factor which mediates this process to separate gas involves the variation in partial pressure among the feedstock and the permeate sides [179]. There are three kinds of material used for membrane separation, and it includes the (i) ceramic, (ii) polymeric, and (iii) hybrid membranes [124]. Different ceramics were used so far to sequester CO2 including the usage of an inorganic material like graphene oxides, silica, and alumina. The polymeric membrane contains nylon, polypropylene thermoplastic polymer, polyvinylidene difluoride (PVDF), and polytetrafluoroethylene (PTFE). Polymer-based membrane was generally operated under low temperature, with better mechanical characteristics, produced in huge scale with ease [124]. Ceramic membranes exhibit high

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chemical and thermal stability with surface of hydrophilic nature and could get wet easily which is a major issue associated with it. In addition to that, producing the ceramic membrane containing huge surface areas is also a challenge [125]. Qu et al. established an innovative polyelectrolyte membrane quaternary ammonium polysulfonate to intensify the CO2 permeation rate. When the condition is humid, the hydrophilic quaternary ammonium groups lead to the formation of cluster of ions thereby promoting the specificity for the transportation of CO2 than methane [126]. Xu et al. organized a combined matrix membrane via the formation of crosslinks among the UIO-66-NH2 nanoparticles with poly(vinylamine) (PVAm) [127]. After its fabrication, the MMMs have exhibited an outstanding rate of separating the flue gas and were highly stable for extended period [127]. Lim et al. conducted an experiment to study the utilization of membranes formed by hybrid composite for elevated CO2 sequestration. The hybrid composite membrane was designed by combining the poly (vinyl benzene chloride) (PVBC) beads operationalized with an ionic liquid (bis (trifluoromethane)sulfonimide (BITFSI)). This hybrid PVBC– BITFSI membrane was found be highly specific for CO2 , based on its potential for controlling the solubility and diffusivity [128]. The usage of nanocellulosic material as an additive to prepare the poly (vinyl alcohol) (PVA) nanocomposite to separate CO2 /N2 was scientifically proven. When the nanocellulosic material is added, improvement in the permeability of CO2 at a rate of 127.8 ± 5.5 GPU and augmented the separation factor of CO2 /N2 around 39 ± 0.4 of the PVA composite membranes [129].

5 Difficulties in Capturing the CO2 and Transforming It into Energy 5.1 Difficulties Faced While Capturing the CO2 1. The most significant factor limiting the process is that this method is extremely expensive, and government policies and subsidies were required for accelerating its execution. 2. The relationship among the CO2 sequestration and transforming it as a resource remains at the beginning phase and lot of research works were required for its improvement and making them real. 3. A huge challenge is that absence of co-operation between professional for capturing the CO2 and converting it as a resource which is two different separate independent processes [132]. 4. There is a necessity for integrating the two factors such as reducing the expense of storing the CO2 and transporting it.

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5. In addition to the above-mentioned challenges, there exist another couple of difficulties such as availability of technologies for commercial scale, involving the process for regenerating the solvents and to compressing the CO2 resulting in the excessive energy consumption. 6. Hence, constructing the plants incorporated with technologies for capturing CO2 is expensive. So, it is essential to discover innovative technologies for capturing the CO2 as other alternate methods so far recorded such as biological CO2 sequestration and membrane separation still remaining in the beginning phase. 7. CO2 captured in the low concentrated exhaust gas is expensive, and application of these methods was highly suitable for coal-fired plants releasing exhaust gas with more than 15% of CO2 [133]. 8. Presence of impurities in the exhaust gas such as oxides of nitrogen and sulfur oxides would result in the reduction of CO2 sequestration and its separation rate making this process less efficient. 9. Developing a process capable of pre-treating the exhaust gas with a simultaneous function of capturing the CO2 would assist in reducing the expenses. Despite widespread research, still additional works are required under majority of the research area to convert the CO2 capturing technology to be highly effective (Table 3).

5.2 Difficulties in Converting the CO2 into Bioenergy In general, the CO2 appears to be a waste product; nevertheless, its proper management would transform it into a useful resource. Transforming CO2 into energy by utilizing the biological system is highly achievable and a suitable solution for reducing its atmospheric discharge and might result in the reduction of greenhouse gases. The principal issue associated with the conversion of CO2 into bioenergy is due to the physiochemical conditions such as the pH, concentrations of CO2 , and the temperature. The concentration of CO2 exhibits adverse impacts on microorganism making it less effective for reducing the CO2, and a wide range of microalgal species were found to be highly efficient under 2–5% of CO2 concentration [132]. It is mandatory to improvise the microbes to become CO2 tolerant thereby increasing the rate of reduction. In general, majority of the biological process were operated under room temperature ranging between 15 and 26 °C, simultaneously the exhaust gas released would be hot, and hence, it has to be cooled in order to supply it to the biological system [133]. It is essential to find microbial strains that are capable of performing under elevated temperature, since the temperature of exhaust gas would rise up to 120 °C. The pH plays a vital role in controlling the growth rate of microbes, and majority of the microbial species show optimal growth around the neutral pH. To achieve increased biomass yield, the CO2 must be fed at high concentration, but it must be taken into consideration that elevated CO2 level would lead to pH reduction around 5—a slightly acidic scale with adverse impact on the growth of microbes.

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Table 3 Technologies’ difficulties and boundaries for several CO2 capturing methods Technologies

Limits and difficulties

References

Carbonic anhydrase (CA)

1. The huge difficulty faced includes extremely low mass transfer and the production of bicarbonates results in the inhibition of enzyme function 2. Regressive dehydration was also challenging. Requirement of novel compound to immobilize for improving and making the enzyme highly stable and capable of reuse 3. Identification of alkaliphilic CA becomes essential due to the requirement of an alkaline pH to form carbonates 4. It requires heat treatment for recovering CO2 and regenerating the solvents resulting the inhibition of CA activation

[36]

Hydrogenation and reduction of CO2

1. Requirement of cofactors making this process expensive 2. It requires development of a high-efficiency cofactor regenerating system or a system free from cofactor which involves electrons directly transferred into the electrode 3. Identification of a highly appropriate supporting matrix capable of immobilizing and encapsulating many enzymes, along with the capacity of retaining their individual enzymatic function

[81, 104]

Microbial electrolysis cell Electrodes should be improved to achieve maximum (CO2 is reduced into CH4 ) efficiency due to the limitation in the 2D assembly of the electrode with lack of surface area availability thereby lowering the electrocatalytic action Unavailability of data relevant to hybrid AD-MEC systems operating procedures and scale-up processes Requires lot of scientific findings in this area Regulation on cathode potential is necessary due to the formation of several products supplied with variations in the potential

[88]

RuBisCo enzyme

Improvement for affinity toward CO2 than O2 is required for its selective function

[108]

Chemical absorption

Identification of absorbing materials with minimal energy consumption while regenerating and possessing maximum absorbance efficiency, with less heat reaction, is necessary for making process cheap and feasible. Amine-derived absorbent material shows less CO2 load caused by the production of carbamates with high stability. Requirement of amine enhancers such as piperazine and corrosion inhibiting agents were required by K2 CO3 absorbent during its utilization in carbon steel (continued)

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Table 3 (continued) Technologies

Limits and difficulties

Adsorption

Material shows less specificity and the adsorption rate is also reduced. Working with solid material becomes more challenging due to the solvent decay that occurs when the solvents were recycled

Cryogenic method

Significant amount of energy is required in this method, nearly 80% of energy consumption for capturing the CO2 , 10% for storing it, and 10% for its transport This process is based on the accessibility of natural gases It is appropriate for exhaust gas having a CO2 concentration more than 20% and gas with less moisture content

Membrane separation

This process is expensive due to overconsumption of energy for compressing the gas and requires additional process for transportation and cleansing activities Membrane usually becomes unstable after few cycles and lack of stability when fuel gas is present, this remains as a huge challenge. Existence of impurities and increased temperature also contribute for lack of stability. Requires improvement of mechanically tough, ionic liquid stable membranes

References

The above-mentioned factor was the difficulties faced and requires research focus for making CO2 to bioenergy concept into real.

6 Process Design for Improving the Transformation of CO2 into Bioenergy Based on the review of various accessible technologies for capturing the CO2 and bioenergy producing methods, the two technologies listed below would improvise the CO2 to bioenergy and make it more economically efficient.

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6.1 Developing a System by Integrating the CO2 Sequestration and Producing Bioenergy 6.1.1

The Electrical–Biological Hybrid System

The availability of CO2 is abundant as a feedstock and capable of conversion into distinct types of economically profitable substances like CH3 COOH, PHA, and biofuels [134, 135]. This substance shows higher stability, and in order to reduce this molecule, intense amount of energy is required to perform a multiple electron reducing stages. Application of biological mechanism for reducing reactions is striking method due to the fact that it could be carried out under room temperature and pressure or under natural environmental conditions. The biocatalyst is capable of regenerating itself on its own and highly appropriate for prolonged activity [23]. Equations state the workable pathways to reduce CO2 resulting in the form of different fuels and its respective theoretical potential necessary for carrying that reaction [76]. In order to rectify the energy blockage and for the conversion of biofuel form CO2 , it requires the catalytic activity of enzymes. Different types of dehydrogenases enzymes along the cofactor NADH were used to perform such reactions and reported scientifically, in which the NADH serves as an energy carrier for reduction of CO2 . In all individual steps, two electron particles were formed the CO2 along with the oxidation of one molecule of NADH to produce NAD+ . Even though it is capable of performing an enzyme-catalyzed reaction at room/environmental temperature and pressure, there is an impact in the economical perspective, as it requires additionally the NADH. Developing a reaction capable of performing independently without requiring a co-enzyme to reduce CO2 for the production of biofuel still remains challenging, and several scientific groups were still trying to solve this problem. For overcoming the problems such as the expense and requirement of cofactor, an electrochemical approach is a suitable alternate which includes the electrons that are injected directly into the enzymes to replace the usage of NADH donating the electron donor. Immobilization of enzymes into an appropriate matrix would help and enable us to check whether the enzyme is stable and capable of reuse. Co-encapsulation of three dehydrogenase enzymes on a supporting matrix such as alginate–silica hybrid gel was done by Xu et al. for converting methanol from CO2 and resulted with a conversion rate of about 98.1% [136]. Regeneration of NADH was recorded by Addo et al. via the coupling of dehydrogenase enzyme cascade to a poly (neutral red) altered electrode [137]. Schlager et al. [76] improvised an electrochemical system independent from coenzyme by immobilizing the dehydrogenases on to a carbon felt electrodes with direct injection of electron on to the biocatalyst. It is possible to utilize an identical electrochemical system—a photoelectric system to reduce CO2 into a biofuel with the help of sunlight to split water by replacing process of electrolysis. In general, the photosynthetic reaction facilitates the conversion of CO2 and H2 O into biomolecules using solar radiation. The combination of an inorganic and organic process together with H2 O, CO2, and solar radiation to mimic this process results in the producing of economically profitable substances. In this reaction, the

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sun light stimulates the release of H2 from the water via the catalytic activity of an inorganic enzyme which splits the water. The H2 thus liberated is utilized by the microbial mechanism serving as a reducing equivalent to reduce the CO2 into CH4 . Nichols et al. improved a bioinorganic hybrid system with a Pt catalyst to split H2 O and Methanosarcina barkeri for capturing the CO2 as well as a biocatalyst to convert CO2 into CH4 which was found to be 86% efficient [138]. Moreover, this process exists at the beginning phase only and never applied to large scale industries.

6.1.2

A Combined Process for Capturing and Producing Bioenergy

In general, various tactics were used to convert bioenergy from the CO2 source which includes (i) capturing the CO2 emitted in the coal-fire plant with the use of biological and other technologies to perform microbial fermenting with the captured CO2 as a feed, (ii) sequestration of CO2 is used as a feed to produce organic acids, followed by its subsequent conversion into bioenergy via two-step fermentation process utilizing microbes, and (iii) simultaneously capturing the CO2 and producing bioenergy with the use of a single-plot system. Lehtinen et al. established a two-step bioprocess to convert alkane from CO2 [139]. In this process, the initial phase includes sequestration of CO2 by Acetobacterium woodii followed by the production of CH3 COOH and the second phase involves the utilization of CH3 COOH by genetically modified strain Acinetobacter baylyi ADP1 for the production of alkane. Same type of approach was recorded for the production of biologically synthesized chemical compounds than CH3 COOH with the utilization of the CO2 feedstock. Sciarria et al. worked for the development of an electrosynthesis system coupled with microbe to produce volatile fatty acids exploiting the CO2 followed by its extraction and utilization as a feed, supplied to a mixed culture and fermented to synthesize polyhydroxyalkanoates [140]. This approach becomes expensive due to the usage of extra CO2 capturing technologies. There is a necessity for developing a single-plot system via integration with a potential for capturing the CO2 and transforming into bioenergy with the simultaneous elimination of extra unit required for storing and transporting the CO2 . Such kind system might be improved with the use of genetically manipulated microbial strains capable of exploiting CO2 via metabolic route for producing bioenergy or microbial cell incorporated with metabolic route of CO2 sequestration.

7 Conclusion Sequestration of CO2 and converting it into bioenergy technologies were appealing. But these technologies were underdeveloped remaining at its beginning phase, and recently, numerous disadvantages turn the processes into costly and hinder its practical applications. The usage of biological systems was found to be less productive due to the prerequisites of cofactors and the microbial cell that are highly sensitive toward the higher concentration of CO2 . Hence, highly dynamic enzymes that are

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capable of withstanding critical conditions such as intense temperature, presence of impurities in the exhaust gas and ionic liquids, regeneration of several cofactor necessary for perfuming the reactions were found be essential. To achieve this scientist industries and international agencies must be highly co-operative. The Government must offer subsides and frame policies to the industries as an encouragement for adapting CCS technologies. Collaborations must be established among the industries and researchers for redesigning and integrating the plants at industries for simultaneously capturing the CO2 and producing bioenergy. Integrating the processes and utilizing the hybrid systems for CO2 sequestration and generating bioenergy might assist in overcoming the difficulties of this particular area (to store and transport CO2 ) thereby making this approach economically feasible.

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A Review on Water–Gas Shift Reactions Energy Production by Carbon Dioxide Capture Sanjeev Kumar Gupta

Abstract One important method for the industrial hydrogen production for renewable energy is the conventional water–gas shift reaction (WGSR). It is necessary to separate H2 from products including carbon dioxide, methane, and remnant CO in this process since it runs at high pressures and temperatures. The development of H2 as clean energy and capturing and storing of carbon for the creation of alternative source of energy and the reduction of the climatic greenhouse effect has made the WGSR an essential method for meeting the demands of H2 production and CO2 enrichment at the same time, improving CO2 capture. The study on the water–gas shift reaction is thoroughly reviewed in this article. The thermodynamics and kinetics involved in WGSR is also discussed and reviewed in this article. Keywords WGSR · Greenhouse gas · Thermodynamics · Kinetics · Catalyst

Abbreviation HTSR HTWGSR LTSR SEM WGSR XRD

High-temperature shift reaction High-temperature water–gas shift reaction Low-temperature shift reaction Scanning electron microscopy Water–gas shift reaction X-ray diffraction

S. K. Gupta (B) Department of Mechanical Engineering, IET, GLA University Mathura, Mathura, U.P 281406, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Jawaid and A. Khan (eds.), Sustainable Utilization of Carbon Dioxide, Sustainable Materials and Technology, https://doi.org/10.1007/978-981-99-2890-3_8

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1 Introduction The worldwide energy crisis was a result of the rapidly increasing global population and industrialisation. Additionally, the global energy crisis is caused by the quickening development of socioeconomic necessities. Traditional petroleum fuels are being used more and more, yet they are becoming less and less readily available. Fossil fuels still account for roughly 81% of the world’s primary energy output. Fossil fuels have historically been the most significant sources of fuel for energy production [1, 2]. At the moment, the two biggest problems afflicting the entire world are the energy crisis and emissions of greenhouse gases. The majority of fossil fuels are burned during their use, which releases a lot of air pollutants into the atmosphere, including carbon monoxide (CO), sulphur oxides, nitrogen oxides and many more. A substantial quantity of CO2 , specific consequence from the combustion of fuel, is also discharged into the environment along with air pollution. Despite not being an air pollutant, CO2 increases the greenhouse effect in the atmosphere, which in turn contributes to climate change and global temperature rise. From 2011 to 2021, CO2 emissions increased from 390 parts per million to 414.47 parts per million [3]. Reducing global CO2 emissions has emerged as the world community’s top concern and the greatest environmental sustainability problem as a result of the rise in CO2 . Developments in renewable energy sources including solar, geothermal, hydropower, and wind energy as well as carbon–neutral fuels like biofuels have drawn a lot of attention in an effort to reduce the atmospheric greenhouse effect [4]. However, as previously mentioned, fossil fuels continue to be the primary source of energy. Consequently, creating strategies for lowering CO2 emissions is a matter of great importance. Using carbon capture, storage, and use is thought to be a successful strategy for reducing manmade CO2 emissions. Three alternative capture methods have been explored to date: precombustion, postcombustion, and oxyfuel combustion [5]. Synthesis gas, also known as syngas, is the primary gaseous species produced during precombustion, which occurs when fuels combine with insufficient oxidants to create product gases. Synthesis gas is a mixture of carbon monoxide and hydrogen. The WGSR can be used to transform carbon mono oxide into CO2 as a result of the water gas. In the meantime, the reaction’s steam can be used to enhance hydrogen levels even more. The extracting and storing of CO2 from water vapour is much simpler. Le Chatelier’s theory also suggests that using a membrane or a sorbent to increase H2 production and separate CO2 could improve the WGSR. After CO2 and H2 have been separated, the refined H2 can be used as a fuel. Hydrogen has been viewed as a potential replacement for non-renewable fuels used to produce heat and electricity throughout the past few decades. This results from the fact that hydrogen is the most prevalent element in the universe and a non-carbon fuel. Hydrogen is a promising energy conveyor that may be used in the shipping and transport sector, unlike electricity produced by solar and wind energy, which is hard to store [6, 7]. The industrial applications of hydrogen are

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Fig. 1 Industrial applications of hydrogen

shown in Fig. 1. Despite being the most prevalent element in nature, hydrogen is mostly bound to other elements like carbon and oxygen and found in molecules like water, fossil fuels, biomass, and hydrocarbons. In order to extract hydrogen from these substances, many methods including thermochemical, electrochemical, photobiological, and photochemical ones have been devised [8]. When compared to the other ways, the thermochemical process has a significant impact on the production of commercial hydrogen. The WGSR is predicted to be crucial in these thermochemical processes for the future hydrogen financial system and CO2 depletion. This is due to the ability of this chemical process to produce hydrogen while simultaneously enriching CO2 . Recent advancements and advances in hydrogen manufacturing and CO2 depletion from the WGSR will be examined in order to give a thorough overview of the WGSR. There will also be discussion of the elemental properties of the WGSR, particularly from a thermodynamic perspective.

2 Methodology of Water–Gas Shift Reaction (WGSR) Felice Fontana, an Italian physicist, made the discovery of the WGSR in 1780. The transformation of CO and water vapour into CO2 and H2 is known as the water–gas shift reaction (WGSR) [9]. The following is supposed to be how WGSR works: CO + H2 O → CO2 + H2 H0 = −41 kJ/mol

(1)

According to the equation above, the chemical reaction is reversible and somewhat exothermic. When carbon monoxide and steam are present in the same atmosphere

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and the chemical reaction’s energy barrier is broken, the WGSR is activated. Water serves as the source of H2 synthesis because the oxygen in H2 O is moved from H2 to CO in the WGSR, which is then converted to CO2 . WGSR is the most widely used reaction in the industry for hydrogen production [10–13]. Reducing the CO agglomeration in a vapour stream can be accomplished using the WGSR. After fuel processing is put into place, the CO content can be brought down to a maximum of 0.1–0.3% using the WGSR [14]. Depending on whether catalysts are utilised to perform the transformation of carbon mono oxide and water to carbon dioxide and hydrogen, respectively, the WGSR could be divided into catalytic and non-catalytic WGSR. The catalytic WGSR, as its name suggests, describes a chemical reaction that is accelerated by the presence of catalysts. There are many different catalysts that can start WGSR. In specific settings, like rarefied water and plasma environments, the non-catalytic WGSR may be chosen.

3 Thermodynamics of Water–Gas Shift Reaction (WGSR) The WGSR is a model reaction with a strong relationship to temperature in its reaction behaviour. Therefore, the designing of the reactor and its operation benefit from knowledge of the chemical reaction’s thermodynamic properties. Today’s industrial applications need the use of catalysts in the WGSR. This suggests that the chemical kinetics is a significant matter when it comes to the WGSR. As a result, the WGSR’s kinetics and thermodynamics will be demonstrated in the discussion that follows. The equilibrium constant, in terms of thermodynamics, controls the concentrations of the gas species at the equilibrium state. In several studies, the WGSR’s chemical kinetics has been described in terms of the equilibrium constant [15, 16]. This demonstrates that when the WGSR is explored, the equilibrium constant plays a crucial role. The following is the definition of the gas phase reaction’s equilibrium constant (K p ): Kp =

PCO2 PH2 X CO2 X H2 1+1−1−1 X CO2 X H2 = Ptotal = PCO PH2 O X CO X H2 O X CO X H2 O

(2)

where P is the partial pressure, X is the mole fraction, and Ptotal is the total pressure. The WGSR’s equilibrium constant, which depends on temperature, can be deliberated thermodynamically using the given equations.  Kp =

4577.8 − 4.33 T

 (3)

where T is absolute temperature in K. In general, somewhere around 400 K and 900 K, Eq. (3) provides good correlation between the equilibrium constants.

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Fig. 2 a Variation of equilibrium constant b variation of Gibbs free energy

Demirel et al. [17] developed thermodynamic model of WGSR in supercritical water to estimate the equilibrium composition with reaction temperature. Figure 2a [18] and Fig. 2b [18] show, respectively, how the WGS equilibrium constant and the Gibbs free energy change with temperature. When the temperature hits about 1100 K, the reaction’s Gibbs free energy turns positive. Gibbs free energy increases with temperature. This finding demonstrates that at high temperatures, reaction becomes thermodynamically unfavourable. When the temperature drops from 1400 to 600 K, the WGSR equilibrium constant increases by roughly 60 times. Larger CO transformation and higher hydrogen emission are preferred at colder temperatures because of the WGSR’s exothermic nature. As temperature rises, the equilibrium concentrations of CO and H2 O rise while those of H2 and CO2 fall. The conversion of CO and subsequent creation of H2 are significantly influenced by the water concentration. Chein et al. [17] used Gibbs free energy method to analyse the thermodynamic equilibrium of WGSR. They have used different conditions such as steam to CO ration, temperature, pressure etc. Based on the findings, it was discovered that as syngas CO2 level increased, CH4 and carbon formations were boosted. High S/C ratios can provide carbon-free WGSR. It was discovered that the removal of CO2 and H2 S by CaO improved the performance of the WGSR.

4 Kinetics of Water–Gas Shift Reaction (WGSR) Conceptually, the WGSR ought to be activated whenever CO and H2 O interact in a medium. In reality, though, the reaction’s energy barrier prevents it from happening if the temperature is too low. As a result, catalysts are needed to lower the activation energy and break through the WGSR’s energy hurdle [19]. The WGSR is commonly divided into two separate reactions, namely HTSR and LTSR [20]. The temperature

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range for the HTSR is approximately 350 to 500 °C, and for LTSR is 150 to 250 °C. Fe/Cr- and Cu/Zn-based catalysts are most popular [21]. As per thermodynamics, lower-level reaction temperatures are advantageous for better CO transformation & H2 production as a result of the heat-releasing reaction involved. On the other hand, the Arrhenius Law demonstrates that speeding up the reaction temperature could speed up the process. Chemical kinetics frequently governs the HTSR, while thermodynamic equilibrium mostly governs the LTSR [22]. Because of this, the WGSR is typically carried out in two stages to improve CO conversion, production of hydrogen, and carbon dioxide enrichment in the effluents [23]. The HTSR is turned on in the first stage so that the chemical reaction as well as CO conversion can happen quickly and at greater temperatures. The LTSR is then activated in the second stage at lower temperatures to speed up the WGSR reaction and enhance CO2 and H2 in the effluent.

4.1 High-Temperature Shift Reaction (HTSR) Catalyst Iron oxide is the primary component of the high-temperature shift (HTS) catalyst, and chromium oxide acts as a stabilising agent to slow the pace at which the active iron crystallites sinter at high temperatures [24]. Recent formulations have used copper to boost interaction/volume and protect catalyst over-reduction at reduced steam/gas ratios [25]. At these temperatures, a new catalyst charge must be able to lower the CO proportion at the reactor exit to levels that are adjacent to the equilibrium proportion of the operation situations, typically in the span of 2–3 mol%. These temperatures are typical working ranges for a HTSR catalyst. Iron is sufficiently active at these temperatures to provide the necessary performance. Park et al. [26] examined two mercantile Fe–Cr catalysts, namely HTC1 and HTC2 under varied inlet gas combinations that represented actual operating conditions. The amount of Fe–Cr in the two catalysts is similar; hence, the same patterns were observed under various conditions. The maximal response rate was attained when the intake CO level was at its highest. After then, the reaction rate slowed down as CO2 and H2 concentrations increased and CO concentration declined. According to the rate expression, the kinetics was highly correlated with the CO concentration. H2O, on the other hand, had a favourable influence on the reaction rate of HTC2, while having no effect on the total reaction rate in the HTC1 case. Jeong et al. [27] performed a comparative study on WGSR using two different catalysts namely Fe-Al-Cu and Fe-AL-Ni. They also used synthesis gas which is derived from waste materials. The Fe-Al-Cu catalyst demonstrated the maximum CO conversion at a rate of about 84% which is shown in Fig. 3 and 100% specificity to CO2 at an extremely elevated gas hour space velocity of 40,057 per hour, even with the elevated CO agglomeration in the synthesis gas. The superior catalytic activity is mostly attributable to the magnetite’s stability, easier reducibility, and the synergistic effects of Cu and Al.

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Fig. 3 Variation of CO conversions with reaction temperature for various catalysts [27]

Liang et al. [28] used Pd-doped catalyst in HTSR. They used Pd in active phase in in-situ formation. Pd addition led to the separation of PdOx compounds. A thorough examination of in-situ CO adsorption revealed that the water–gas shift reactions (WGSR), which generate metallic Pd in-situ, were the primary cause of this reaction. Bahmani et al. [29] utilised Fe2 O3 –Cr2 O3 –CuO catalyst in HTWGSR to check its mechanical strength. The oxidation/precipitation technique was used to create a number of Fe2 O3 –Cr2 O3 –CuO catalysts. The binder utilised was MgCO3 and many more. The produced samples were characterised using XRD and SEM examinations. To assess the squeezing power data of the new and utilised catalysts, the Weibull model was applied. The findings indicate that altering the mechanical strength of the catalyst during its manufacture by oxidising Fe2+ ions in low pH environments. The sample made with MgCO3 displayed the best mechanical behaviour among the new catalysts. Popa et al. [30] investigated the viability of substituting Al for Cr in addition to Ni. In contrast to the Fe–Cr–Cu reference catalyst, the CO transformation rate of the Fe–Al–Cu catalyst was subpar under all test circumstances. They also tested Fe–Al–Cr–Cu catalysts, which are produced by combining various components. The outcomes showed that efficiency with regard to reaction rate and thermal reliability had significantly increased. Under various temperatures and retention times over the optimised catalyst, the reaction rate remained at its highest.

4.2 Low-Temperature Shift Reaction (LTSR) Catalyst Copper and zinc are frequently found in low-temperature catalysts, which work between 350° and 650 °F. They are utilised in situations where very low carbon monoxide levels in the produced gas are necessary [31]. With low-temperature catalysts, the reducing gas generator is not necessary; an indirect heating system could

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be employed in its place. The feed gas must be completely desulfurised before coming into contact with the catalyst since these catalysts are particularly sensitive to sulphur compound poisoning. The LTSC can also become contaminated by chlorides and silica from entrained boiler feed water in the steam. LTSC can convert carbon monoxide almost completely, on the order of 99%, because the shift reaction’s equilibrium is more favourable at lower temperatures [32]. Some recent development in LTSC is discussed below: Temperature-programmed reduction (TPR) was used by Hossain et al. [33] to assess the reducibility of commercial CuO-ZnO/Al2 O3 catalysts and the depletion kinetics of Cu–Fe–Mn and Cu–Fe–Cr catalysts in a fixed-bed reactor. The activity of the metal oxide catalysts showed a substantial association with the reducibility. According to their findings, the Mn addition considerably improved the active Cu species. The second-order power law provided good fitting to the TPR data among a number of kinetic models. Sekine et al. [34] used LTSC in electric field. The effects of applying an electric field to the catalyst bed on catalytic water–gas shift enabling hydrogen formation in the 423–873 K temperature range were investigated. Based on chemical kinetic law and thermal equilibrium, reaction trends were examined. Through our screening experiments, Pt-La-ZrO2 was selected as an active catalyst, and the impact of the electric field on the catalytic activity was examined by varying reaction temperatures and applying electric currents. By applying an electric field to the catalyst bed, a significant drop in the apparent activation energy for WGS was seen which is shown in Fig. 4. Queiroz et al. [35] used ruthenium as LTSC for kinetics of WGSR. To predict the kinetic behaviour of the operation, the process of the WGS reaction over Ru–TiO2 Fig. 4 Apparent activation energy with and without electric field [34]

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and RuAl2 O3 catalysts was studied. For the Ru–TiO2 and Ru–Al2 O3 catalysts, the LH model yielded activation energy values of 12 kJ/mol and 12.9 kJ/mol, respectively, whereas the Redox model yielded values of 63.63 kJ/mol and 69.24 kJ/mol. The power-law and LH-type models for the Ru–TiO2 catalyst were validated by simulating the operational behaviour in a fixed-bed reactor.

5 Conclusion and Challenges Despite more than a century of development, there are still significant obstacles and issues to be resolved with the WGSR. For instance, the majority of the WGSR publications shared two primary processes, namely redox and association. A few research groups have also postulated a number of additional mechanisms. On the other hand, it is still unclear which mechanism is the most significant and dominant. Additionally, thorough examinations of the mechanisms should be carried out because it is still uncertain whether they are suitable for each catalyst, and after grasping the fundamental idea, numerous improvements can be made. The use of hydrogen as a sustainable and renewable fuel has currently drawn a lot of interest due to growing environmental concerns in many nations. Compared to fossil fuels, hydrogen has an elevated intrinsic energy density, and its only byproduct following burning is water. In many industrial processes for the synthesis and purification of hydrogen, the water–gas shift reaction is a crucial step. This research studied the WGSR from both the thermodynamics as well as kinetics perspectives for numerous requisitions in order to better understand the reaction. Due to its exothermal nature, the water–gas shift reaction generally prefers low temperatures, although its kinetics leans towards high-temperature operation. Even though the low temperature increases CO conversion, the cost of the reactor must also rise because a larger reactor is required. Fast temperatures, on the other hand, result in a high response rate but poor conversion efficiency.

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