Deep Eutectic Solvents for Pretreatment of Lignocellulosic Biomass (SpringerBriefs in Applied Sciences and Technology) 9811640122, 9789811640124

Table of contents :
Preface
Contents
List of Figures
List of Tables
1 Background and General Information
Bibliography
2 Deep Eutectic Solvents and Their Physicochemical Properties
Bibliography
3 Cellulose, Hemicelluloses and Lignin Solubilization in DESs
Bibliography
4 Processing of Biomass by DESs
Bibliography
5 Compatibility of DES with Enzymes and Microorganisms
Bibliography
6 Recycling of DESs
Bibliography
7 Comparison of Deep Eutectic Solvents and Ionic Liquids
Bibliography
8 Challenges and Opportunities
Bibliography

Citation preview

SPRINGER BRIEFS IN APPLIED SCIENCES AND TECHNOLOGY

Pratima Bajpai

Deep Eutectic Solvents for Pretreatment of Lignocellulosic Biomass 123

SpringerBriefs in Applied Sciences and Technology

SpringerBriefs present concise summaries of cutting-edge research and practical applications across a wide spectrum of fields. Featuring compact volumes of 50 to 125 pages, the series covers a range of content from professional to academic. Typical publications can be: • A timely report of state-of-the art methods • An introduction to or a manual for the application of mathematical or computer techniques • A bridge between new research results, as published in journal articles • A snapshot of a hot or emerging topic • An in-depth case study • A presentation of core concepts that students must understand in order to make independent contributions SpringerBriefs are characterized by fast, global electronic dissemination, standard publishing contracts, standardized manuscript preparation and formatting guidelines, and expedited production schedules. On the one hand, SpringerBriefs in Applied Sciences and Technology are devoted to the publication of fundamentals and applications within the different classical engineering disciplines as well as in interdisciplinary fields that recently emerged between these areas. On the other hand, as the boundary separating fundamental research and applied technology is more and more dissolving, this series is particularly open to trans-disciplinary topics between fundamental science and engineering. Indexed by EI-Compendex, SCOPUS and Springerlink.

More information about this series at http://www.springer.com/series/8884

Pratima Bajpai

Deep Eutectic Solvents for Pretreatment of Lignocellulosic Biomass

Pratima Bajpai Pulp and Paper Consultant Kanpur, Uttar Pradesh, India

ISSN 2191-530X ISSN 2191-5318 (electronic) SpringerBriefs in Applied Sciences and Technology ISBN 978-981-16-4012-4 ISBN 978-981-16-4013-1 (eBook) https://doi.org/10.1007/978-981-16-4013-1 © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 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

Preface

Lignocellulosic biomass conversion to biofuels, biochemicals and other value-added products has attracted global attention because it is a readily available, inexpensive and renewable resource. Lowering recalcitrance of biomass in a cost-effective manner is a challenge to commercialize biomass-based technologies. Deep eutectic solvents (DESs) are new “green” solvents that have a high potential for biomass processing because of their low cost, low toxicity, biodegradability, easy recycling and reuse. This SpringerBrief focuses on properties of DESs and recent advances in their application for lignocellulosic biomass processing. It begins with the current status of lignocellulosic biomass pretreatment followed by the discussion on physiochemical properties of DESs, key findings on the effects of DES on cellulose, hemicellulose and lignin solubilization and then biomass pretreatment and changes in biomass crystallinity. Then, it progresses to enzymatic hydrolysis performance of DES-pretreated solids, compatibility of DESs with enzymes and microorganisms and recycling potential of DESs. Finally, it covers the comparison of DESs with ILs, and challenges and opportunities for furthering DESs’ use in processing of lignocellulosic biomass. Kanpur, India

Pratima Bajpai

Acknowledgements I am grateful for the help of many people, companies and publishers for providing information and granting permission to use their material. Deepest appreciation is extended to Elsevier, Springer, John Wiley and Sons, Royal Society of Chemistry, De Gruyter, IntechOpen and other open-access journals and publications.

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Contents

1 Background and General Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Deep Eutectic Solvents and Their Physicochemical Properties . . . . . . . 9 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3 Cellulose, Hemicelluloses and Lignin Solubilization in DESs . . . . . . . . . 21 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4 Processing of Biomass by DESs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 5 Compatibility of DES with Enzymes and Microorganisms . . . . . . . . . . . 55 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6 Recycling of DESs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 7 Comparison of Deep Eutectic Solvents and Ionic Liquids . . . . . . . . . . . . 81 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 8 Challenges and Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

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List of Figures

Fig. 1.1 Fig. 2.1

Fig. 2.2 Fig. 4.1

Fig. 4.2

Fig. 4.3

Fig. 4.4

Fig. 5.1

Fig. 6.1 Fig. 6.2

Schematic of pretreatment effect on lignocellulosic biomass. Based on Mosier et al. (2005) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of recent literature available on DESs via SciFinder. a Number of publications per year; b distribution of number of publications under different sections. Satlewal et al. (2018). Reproduced with permission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical structures of hydrogen bond donors (HBDs) and bond acceptors (HBAs) for DES synthesis. Xu et al. (2017) . . . . . . . . . . . Mechanism involved in the β-O-4 linkage cleavage of lignin during DES pretreatment based on a study of the model compound guaiacylglycerol-β-guaiacyl ether Alvarez-Vasco et al. (2016). Reproduced with permission . . . . . . . . . . . . . . . . . . . . Visual observation of the process steps involved during the DES pretreatment, a DES components before reagent preparation, b DES reagent after preparation, c DES-pretreated rice straw, d lignin precipitate, e recovered DES reagent. Kumar et al. (2016). Reproduced with permission . . . . . . . . . . . . . . . . . . . . . . . . . Schematic illustration of the effect of DES on the lignocellulosic structure to yield sustainable processes, Ünlü and Takaç (2020). Reproduced with permission . . . . . . . . . . . . . . . . . . . . . . . . . Schematic representation of reaction between DES (ChCl:Urea) and lignin–carbohydrate complexes Yongzhuang et al. (2017). Reproduced with permission . . . . . . . . . . . . . . . . . . . . Cellulase activity in the presence of different DESs (HBA as choline chloride and HBD as ethylene glycol, glycerol or malonic acid at various concentrations after a 24 h, and b 48 h incubation times. Gunny et al (2015). Reproduced with permission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reuse of ChCl/2PTSA for Biginelli reaction Cui et al. (2019). Reproduced with permission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saccharification yield from the recycled ChCl–PCA pretreated Switchgrass Kim et al. (2018). Reproduced with permission . . . . .

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Fig. 6.3

Fig. 7.1 Fig. 7.2

List of Figures

The original AhG DES (a), and recycled AhG DES (b–f) after one to five times of reuse at room temperature. Li et al. (2018). Reproduced with permission . . . . . . . . . . . . . . . . . . . . . . . . . Similarities and differences of ILs and DESs. Reproduced with permission Płotka-Wasylka et al. 2020 . . . . . . . . . . . . . . . . . . . A comparison of the main synthesis processes of ILs and DESs. Reproduced with permission Płotka-Wasylka et al. 2020 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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84

List of Tables

Table 2.1 Table 2.2 Table 3.1 Table 3.2 Table 3.3 Table 3.4 Table 4.1 Table 4.2 Table 4.3 Table 5.1 Table 6.1

General formula for the classification of DESs . . . . . . . . . . . . . . . Properties of some common DES combinations . . . . . . . . . . . . . . Reported DESs for cellulose dissolution . . . . . . . . . . . . . . . . . . . . . Solubility of the isolated biomass components in various DESs at 60 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solubility of lignin and cellulose in various DESs . . . . . . . . . . . . . Extractability of lignin of various DESs . . . . . . . . . . . . . . . . . . . . . Performance of DESs in assisting delignification and/or sugar recovery from lignocellulosic biomass . . . . . . . . . . . . . . . . . . . . . . Overview of previous works on biomass delignification using DES, sorted by biomass type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystallinity index (CrI) of lignocellulosic biomass after DES pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enzymatic hydrolysis efficiency after DES pretreatment . . . . . . . DESs application and strategies for their recycling and reuse . . . .

13 15 23 24 25 26 42 44 48 56 71

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Chapter 1

Background and General Information

Pretreatment of lignocellulosic biomass with deep eutectic solvents (DESs) is a promising and challenging process for the production of biofuels and value-added chemicals. The environment is suffering from climate change, aggravated by overutilization of resources thereby escalating greenhouse gas emission (Anderson et al. 2019; Hassan et al. 2019). Sustainable and environment-friendly energy based on renewable resources are needed to meet the future energy requirements of the world. Lignocellulose is abundantly available renewable biomass on earth and is recognized as an alternative source for producing renewable fuels and chemicals. The production rate is 170–200 billion tons annually (Luo et al. 2014). Landfilling or burning is the common practices of biomass disposal. These practices do not provide any economic value to the industry. Lignocellulosic biomass can be converted into biofuels, energy sources or chemicals instead of disposing as waste (Iqbal et al. 2013; Loow et al. 2017). The term “waste valorization” is the process of reusing, recycling or composting the waste materials and converting them into useful products including chemicals, materials, fuels or other sources of energy (Kabongo, 2013). Therefore, the valorization of biomass for producing value-added products could be a sustainable solution to the abovementioned global issue. Moreover, lignocellulosic biomass is available in huge quantities at a lower cost, thus providing a more viable option than the use of food crops as a sustainable resource for producing chemicals (Thompson and Meyer 2013). The production of ethanol from lignocellulosic biomass is gaining increasing attention (Yusuf et al. 2011). The annual production capacities of bioethanol obtained from banana residual, corn and sugarcane are 0.019, 39.5 and 30 billion liters per year, respectively (Guerrero et al. 2016; Liguori et al. 2013). The recent focus on biorefinery concepts, especially in developed countries, is expected to contribute to the growth of biochemical pathways for biomass exploitation (Achinas and Euverink 2016).

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 P. Bajpai, Deep Eutectic Solvents for Pretreatment of Lignocellulosic Biomass, SpringerBriefs in Applied Sciences and Technology, https://doi.org/10.1007/978-981-16-4013-1_1

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1 Background and General Information Lignocellulosic biomass includes the following: • Forestry wastes (e.g., wood chips and sawdust) • Agricultural residues (e.g., corn stover (cob and stalk), rice straw, bagasse, cotton gin trash, etc.) • Bioenergy crops (sweet sorghum, switchgrass and common reeds) • Industrial wastes (e.g., paper sludge, recycled newspaper) • Municipal solid wastes. Unlike food-based (starch-derived) biomass, it shows a series of advantages such as low cost, abundant supplies, non-competition with grain as food (Bajpai 2016; Sathisuksanoh et al. 2009).

Lignocellulose is a natural complicated composite. It basically consists of cellulose, hemicellulose and lignin. “Cellulose and hemicellulose are tangled together and wrapped by lignin outside (de Vries and Visser 2001). Depending on sources and cell types, the dry weight typically makes up of around 35–50% cellulose, 20–35% hemicellulose and 10–25% lignin (Demirbas 2005). Cellulose, the most abundant natural carbon bioresource on the earth, is a homopolysaccharide of anhydroglucopyranose linked by ß-1, 4-glycosidic linkages (McMillan 1997). Adjacent cellulose chains are coupled via orderly hydrogen bonds and Van der Waal’s forces, resulting in a parallel alignment and a crystalline structure (Zhang et al. 2007). Several elementary fibrils gather, forming much larger microfibrils, which are further bundled into larger macrofibrils, leading to the rigidity and strength of cell walls. Efficient conversion of cellulose into glucose has been a central topic for long. Hemicellulose, the second main polysaccharide, is a polymer containing primarily pentoses (xylose and arabinose) with hexoses (glucose and mannose), which are dispersed throughout and form a short-chain polymer that intertwines with cellulose and lignin like a glue (Wilkie 1979). Lignin is a polymer consisting of various aromatic groups. It can be converted into numerous chemical products that are made from fossil-based chemical industry, including coal, oils and natural gas. The production of cellulosic ethanol holds promise for an improved strategic national security, job creation, strengthened rural economies, improved environmental quality, nearly zero net greenhouse gas emissions and sustainable local resource supplies (Demain et al. 2005; Lynd et al. 1991, 1999, 2002; Zhang 2008). So far, more companies are working on reducing costs of cellulosic ethanol production, such as Iogen Corporation, Abengoa Bioenergy, Dupont, British Petroleum, Mascoma (Biotechnology Industry Organization 2008). The price of E85 is still as high as $1.81 per gallon” (Bajpai, 2016; DOE 2009). Soluble sugars can be produced from lignocellulosic biomass using a large number of approaches. Conversion of lignocelluloses is much more complicated and difficult, taking into consideration their complex and recalcitrant structures (Jorgensen et al. 2007; Yang and Wyman 2008). “The biological conversion of cellulosic biomass into bioethanol is based on the breakdown of biomass into aqueous sugars using chemical and biological means, including the use of hydrolytic enzymes. From that point, the fermentable sugars can be further processed into ethanol or other advanced biofuels.

1 Background and General Information

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Fig. 1.1 Schematic of pretreatment effect on lignocellulosic biomass. Based on Mosier et al. (2005)

Therefore, pretreatment is required. The main objectives of the pretreatment process are to remove lignin and hemicelluloses, increase the porosity of the lignocellulosic materials and reduce the crystallinity of cellulose (Fig. 1.1). Pretreatment should meet the following requirements: • Low capital and operational cost • Effective on a wide range and loading of lignocellulosic materials • Should result in the recovery of most of the lignocellulosic components in a useable form in separate fractions • Need for preparation/handling or preconditioning steps prior to pretreatment such as size reduction should be minimized • Avoid the formation of by-products that are inhibitory to the subsequent hydrolysis and fermentation processes. Pretreatment can be the most expensive process in biomass-to-fuels conversion but it has great potential for improving its efficiency and lowering of costs through further research and development. Pretreatment is an important tool for biomass-to-biofuels conversion processes” (Bajpai 2016). A wide array of pretreatment technologies has been studied for lignocellulosic biomass valorization for producing biofuels and chemicals with high cost efficiency (Abo-Hamad et al. 2015; Agrawal et al. 2015; Chandra et al. 2007; Dale 1985; Eggeman and Elander 2005; Himmel et al. 2007; Ladisch et al. 1992; Lin et al. 1981; Lynd 1996; Lynd et al. 2003, 2008; McMillan 1994; Mosier et al. 2005; Office of

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1 Background and General Information

Energy Efficiency and Renewable Energy and Office of Science 2006; Ragauskas et al. 2006; Sun and Cheng 2002; Vertès et al. 2006; Wyman 2007; Wyman et al. 2005a, b; Bensah and Mensah 2009; Zheng et al. 2009; Chaturvedi and Verma 2013; Agbor et al. 2011; Kumar et al. 2009; Avgerinos and Wang 1983). “Most notably, a collaborative team called consortium for applied fundamentals and innovation (CAFI) funded by the Department of Energy and Department of Agriculture has formed and focused on several leading pretreatment technologies including dilute (sulfuric) acid pretreatment, flow-through pretreatment, ammonia fiber expansion (AFEX), ammonia recycle percolation (ARP) and lime pretreatment for the past several years” (Bajpai 2016; Moxley 2007). Among the several pretreatment methods, the preference of the convenient one depends upon the type of lignocellulosic biomass used as there is variation in the composition of cellulose, hemicellulose and lignin (Dahadha et al. 2017). The major types of pretreatment methods include the thermochemical methods, for example steam explosion, chemical treatment methods with acid or alkali, and biological treatment methods with whole microbial cells or enzymes. Physical treatments like drying, grinding and granulometric classification are preliminary steps common to most processes which involve the use of lignocelluloses. These are important steps for the standardization of the material, making sure that the particle size, flow and reaction properties are the same for the same type of raw material. Thermochemical methods disrupt the structure of the material, degrades hemicellulose and cellulose and transforms lignin, thereby facilitating the subsequent cellulose hydrolysis. Steam explosion, washing with alkali and use of dilute acid hydrolysis are some thermochemical methods for pretreatment and hydrolysis of the lignocellulosic biomass. Steam explosion causes the material to explode because of the high temperature and pressure. Acid hydrolysis is the most frequently used pretreatment method, and sulfuric acid is the most commonly used acid, but other acids, such as hydrochloric acid, phosphoric acid, and nitric acid have also been used. Organic or aqueous-organic solvents and also catalysts, such as oxalic acid, salicylic acid and acetylsalicylic acid, can be used in the organosolv pretreatment of lignocellulosic biomass at temperatures of 150–200 °C. Different types of organic solvents such as organic acids, ethers, esters, alcohols, ketones, glycols and phenols have been used. Biological treatment with whole microbial cells can also remove lignin. However, it needs long retention times in comparison to thermochemical methods. Treatment with enzymes is very selective and requires only a few hours. So, more suitable as compared to the treatment with microbial cells. But, the high capital investment associated with biomass pretreatments often makes these technologies cost prohibited (Himmel et al. 2007; Michelin et al. 2013). Over the recent decades, ionic liquids (ILs) have been in the limelight as an alternative solvent method for the pretreatment of lignocelluloses for improving the economics of downstream processes (Haregewine and Rafael 2011). This is because of its distinctive properties, like non-volatility and the ability to be varied according to the type of desired extractions (Kumar et al., 2016). Under gentle operating conditions, ILs show higher efficiency for lignin extraction, reduce cellulose crystallinity

1 Background and General Information

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and improve enzymatic digestibility. But, their application on an industrial scale has been limited by high costs, incompatibility with microbes and enzymes and recycling challenges (Yoo et al. 2017). Therefore, several factors play a crucial role in the selection of the right pretreatment method for lignocelluloses based upon the nature of the feedstock (i.e., hardwoods, softwoods, agricultural residues, grasses etc.), capital and operational expenses, energy investment, yields, efficiency and environmental sustainability (Satlewal et al., 2018). So, there is still a large scope to innovate and develop novel and disruptive biomass pretreatment technologies. In the recent years, the emergence of DESs in processing of lignocellulosic biomass is becoming more important because they are greener solvents as compared with ILs (Yoo et al. 2017; Satlewal et al. 2018). Although DESs offer more advantages as compared to ILs, they are still not extensively implemented in the field of biomass processing because DES-related studies are still in an early phase. DESs have been reported to dissolve and extract lignin of high quality with more than 90% purity, and almost 60% of the total lignin present in rice straw (Kumar et al. 2016; van Osch et al. 2017). Cellulose solubility was found to be very little (Oliveira et al. 2015). Selected DESs worked very well during biomass pretreatments with ethyl ammonium chloride:ethylene glycol (EAC:EG) for oil palm trunk fiber pretreatment with 74% glucose production (Zulkefli et al. 2017). Pretreatment of corncob with ChCl: glycerol gave glucan conversion of 92%. With ChCl:imidazole, glucan conversion of 95% was obtained (Procentese et al. 2015). Pretreatment of rice straw with choline chloride:oxalic acid and choline chloride:urea showed a glucose yield of 90.2% (Hou et al. 2017). Pretreatment of choline chloride: formic acid with corn stover showed a hydrolysis yield of 99% (Xu et al. 2016).

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A.A. Vertès, M. Inui, H. Yukawa, Implementing biofuels on a global scale. Nat. Biotechnol. 24, 761–764 (2006) K. Wilkie, The hemicelluloses of grasses and cereals. Adv. Carbohydr Chem. Biochem. 36(1), 215–264 (1979) C.E. Wyman, What is (and is not) vital to advancing cellulosic ethanol. Trends Biotechnol. 25(4), 153–157 (2007) C.E. Wyman, B.E. Dale, R.T. Elander, M. Holtzapple, M.R. Ladisch, Y.Y. Lee, Comparative sugar recovery data from laboratory scale application of leading pretreatment technologies to corn stover. Biores. Technol. 96, 2026–2032 (2005) C.E. Wyman, B.E. Dale, R.T. Elander, M. Holtzapple, M.R. Ladisch, Y.Y. Lee, Coordinated development of leading biomass pretreatment technologies. Biores Technol. 96, 1959–1966 (2005) G.-C. Xu, J.-C. Ding, R.-Z. Han, J.-J. Dong, Y. Ni, Enhancing cellulose accessibility of corn stover by deep eutectic solvent pretreatment for butanol fermentation. Biores. Technol. 203, 364–369 (2016) B. Yang, C.E. Wyman, Pretreatment: the key to unlocking low-cost cellulosic ethanol. Biofuels Bioprod Biorefin 2(1), 26–40 (2008) C.G. Yoo, Y. Pu, A.J. Ragauskas, Ionic liquids: Promising green solvents for lignocellulosic biomass utilization. Curr. Opin. Green. Sustain. Chem. 5, 5–11 (2017) N.N.A.N. Yusuf, S.K. Kamarudin, Z. Yaakub, Overview on the current trends in biodiesel production. Energy Convers Manag. 52, 2741–2751 (2011) Y.-H.P. Zhang, S.-Y. Ding, J.R. Mielenz, R. Elander, M. Laser, M. Himmel, J.D. McMillan, L.R. Lynd, Fractionating recalcitrant lignocellulose at modest reaction conditions. Biotechnol. Bioeng. 97(2), 214–223 (2007) S. Zulkefli, E. Abdulmalek, M.B. Abdul Rahman, Pretreatment of oil palm trunk in deep eutectic solvent and optimization of enzymatic hydrolysis of pretreated oil palm trunk. Renew. Energ. 107, 36–41 (2017) J.D. McMillan, Bioethanol production: status and prospects. Renew Energy 10(2–3), 295– 302 (1997) Y. Zheng, Z. Pan, R. Zhang, Overview of biomass pretreatment for cellulosic ethanol production. Int J Agric Biol Eng 2, 51–68 (2009)

Chapter 2

Deep Eutectic Solvents and Their Physicochemical Properties

Deep eutectic solvents (DESs) are one of the most popular green solvents that are mostly non-toxic, recyclable and non-flammable and have low vapor pressures (Abbott et al. 2003; Zhang et al. 2012; Garcia et al. 2015). DESs have attracted much attention in many fields (Smith et al. 2014; Carriazo et al. 2012; Zhang et al. 2012; Wang et al. 2018; Smith et al. 2014; Abbott et al. 2004; Xu et al. 2017; Xing et al. 2017; Tang and Row, 2013; Tang et al. 2017; Ren et al. 2016; Pandey et al. 2017; Oliveira et al. 2015; Mbous et al. 2017; Abbott et al. 2004; Lynam et al. 2017; Chen and Mu, 2019). DESs share the promising solvent properties of ILs, typically including low volatility, wide liquid range and biocompatibility (Francisco et al. 2012). DESs are low cost, show lower toxicity, higher biodegradability and can be prepared easily. Furthermore, DESs show several benefits over traditional ILs like their ease of synthesis and wide availability from relatively low-cost components, allowing large-scale production (Kim et al. 2018). “They can be easily prepared in the laboratory using numerous substances in different molar ratios that result in diverse properties of DES such as polar, nonpolar, acidic and basic. The most common method to prepare a DES is to mix the constituents in a certain molar ratio at a certain temperature until a homogeneous liquid form is obtained. In another method, the constituents (mostly solid) are mixed together with water, and subsequent evaporation of the excess water under vacuum is performed, which is called evaporation method. Similar steps are followed for the freeze-drying method; indeed, water is removed by freeze-drying. In grinding method, a glove box in nitrogen atmosphere is used to grind the solid in a mortar till clear liquid is obtained” (Ünlü and Takaç 2020; Abbott et al. 2003; Dai et al. 2013a, b; Gutierrez et al. 2009; Florindo et al. 2014). If a novel DES to be synthesized for the first time, several tests should be conducted for proving the “deep eutectic” properties of the solvent. Otherwise, published articles’ protocols should be followed precisely for synthesizing DES in the right form.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 P. Bajpai, Deep Eutectic Solvents for Pretreatment of Lignocellulosic Biomass, SpringerBriefs in Applied Sciences and Technology, https://doi.org/10.1007/978-981-16-4013-1_2

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2 Deep Eutectic Solvents and Their Physicochemical Properties

The concept of DES was first described by Abbot et al. (2003) as the liquid formed between a variety of quaternary ammonium salts and carboxylic acid. Afterward, DESs were introduced as low-cost eutectic mixtures, with chemical and physical properties analogous to ILs (Abbott et al. 2004). They are synthesized by combining hydrogen bonding donors (HBDs) and hydrogen bonding acceptors (HBAs) to form eutectic mixtures (Fig. 2.2) ( Xu et al. 2017). DESs are preferred over conventional ILs as they can be synthesized easily at a competitive cost and usually most of them are environmentally friendly (Satlewal et al. 2018; Mbous et al. 2017). According to Gorke et al. (2010) the components for DESs were ten times less costly than the components used for preparation of the ionic liquids and according to Xu et al. (2016), the cost for synthesizing DES was only 20% of that of an IL. But, the relationship between molecular composition and the solvent properties of the resulting eutectic mixtures is not completely understood. Nonetheless, a number of promising DESs systems have been reported. The numbers of publications on DESs are growing exponentially during the last few years (Fig. 2.1a, b) (Satlewal et al. 2018). DESs show potential applications mainly in the areas of lignocellulosic biomass processing, fermentation and bioindustrial chemistry, electrochemistry, fossil fuels, pharmaceuticals, food and feed industry, nanomaterials, separation and metal processing (Smith et al. 2014; Zainal-Abidin et al. 2017; Shishov et al. 2017; Hadj-Kali, 2015; Lee, 2017; Isaifan and Amhamed, 2018). The use of DESs in different fields has been increasing since 2003, when it was first described. In the recent years, they were shown to be used as solvents in several types of reactions such as polymerization, transesterification, esterification and hydrolysis (Wang et al. 2013; Tran et al. 2016; De Santi et al. 2012; Unlu et al. 2017; Arıkaya et al. 2019). Their catalytic effects in many diverse types of reactions have also been reported (Wang et al. 2013; Tran et al. 2016; De Santi et al. 2012; Musale and Shukla, 2016; Patil et al. 2014; Mondal et al. 2016; Singh et al. 2011; Keshavarzipour and Tavakol, 2015). DES-related publications were more than 300 between 2009 and 2013, whereas in 2008, it was only 29 (Tang and Row, 2013). In 2017, the number of publications on DESs reached up to almost 750 (Satlewal et al. 2018). Biocompatibility of the DESs with biomolecules, i.e., nucleic acids, proteins, enzymes and microorganisms is one the most noteworthy properties of DESs which is attracting recent interest for their applications in biopharma industries for molecular extractions, bioorganic catalysis and biotransformation (Mbous et al. 2017). The application of DESs as an alternative to ILs in dissolving the lignin and polysaccharides present in biomass is attracting a vast interest worldwide for producing biofuels, value-added products and commodity chemicals (Oliveira et al. 2015). The knowledge of DESs’ physicochemical properties has been critically reviewed because of the need to precisely understand the properties of DESs for application in different areas (Garcia et al. 2015).

2 Deep Eutectic Solvents and Their Physicochemical Properties

11

Fig. 2.1 Analysis of recent literature available on DESs via SciFinder. a Number of publications per year; b distribution of number of publications under different sections. Satlewal et al. (2018). Reproduced with permission

“Deep eutectic solvents can be described by the general formula + − Cat X zY (1) where Cat+ is in principle any ammonium, phosphonium or sulfonium cation, and X is a Lewis base, generally a halide anion. The complex anionic species are formed between X− and either a Lewis or Brønsted acid Y (z refers to the number of Y molecules that interact with the anion). The majority of studies have focused on quaternary ammonium and imidazolium cations with particular emphasis

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2 Deep Eutectic Solvents and Their Physicochemical Properties

Fig. 2.2 Typical structures of hydrogen bond donors (HBDs) and bond acceptors (HBAs) for DES synthesis. Xu et al. (2017)

being placed on more practical systems using choline chloride, [ChCl, HOC2H4N+ (CH3)3Cl−]” (Smith et al. 2014). DESs are broadly classified into four categories depending on the nature of the complexing agent used. Type I DESs are composed of quaternary ammonium salt (QAS) and metal chloride, Type II are composed of QAS and metal chloride hydrate, Type III are composed of QAS and hydrogen bond donor (HBD) and Type IV are composed of metal chloride and HBD, as shown in Table 2.1. “In essence, the potential suitability of Type I DESs for application in biomass processing or other applications has been limited due to the high melting point of the non-hydrated metal halides used for Type I DES synthesis (Smith et al. 2014). Conversely, the inherent air and moisture insensitivity of most hydrated metal salts employed in the synthesis of Type II DESs allow for a much wider scope of application for this type of DES. Nevertheless, Type III DESs have been receiving the spotlight among all DESs due to their favorable advantages. Specifically, there is a huge array of hydrogen bonds selectable for Type III DES synthesis, resulting in the high adaptability of the DES as a form of extraction solvent. To date, various Type III

2 Deep Eutectic Solvents and Their Physicochemical Properties

13

Table 2.1 General formula for the classification of DESs Types

General formula

Terms

Example

Type I

Cat + X- + zMClx

M = Zn, In, Sn, Al, Fe

ChCl + ZnCl2

Type II

Cat + X- + zMClx.yH2O

M = Cr, Ni, Cu, Fe,

Co ChCl + CoCl2.6H2O

Type III

Cat + X- + zRZ

Z = OH, COOH, CONH2

ChCl + Urea

Type IV

MClx + zRZ

M = Zn, Al and Z = OH, CONH2

ZnCl2 + Urea

Cat + , any ammonium, phosphonium or sulfonium cation. X, a Lewis base,generally a halide anion. Y, a Lewis or Bronsted acid. z, number of y molecules that interact with the anion. Based on Degam, (2017); Satlewal et al. (2018); Loow et al. (2017); Smith et al. (2014)

DESs have been developed using various biodegradable and low-cost components consisting of amides, carboxylic acids and alcohols (Smith et al. 2014). Finally, Type IV DESs incorporate the use of inorganic transition metals with urea to form eutectic mixtures, even though metal salts would not normally ionize in non-aqueous media” (Loow et al. 2017). Another type of DESs, “natural deep eutectic solvent (NADES),” are developed by using several primary metabolites, like amino acids, organic acid, sugars and choline (Choi et al. 2011). “NMR-based metabolomics analysis have shown that NADES indeed existed as the third liquid other than water and lipid in an organism. For instance, a mixture of sucrose, fructose and glucose in a molar ratio of 1:1:1 forms uniform and clear liquid at room temperature. They can help plants survive anhydrobiosis and play an important role in the synthesis of intracellular macromolecules. These NADES may preserve the linkage between understanding of cellular metabolism and physiology” (Xu et al, 2017). Several researchers have presented many different types of DESs from several different types of molecules that the definition of DES converged to a simple form: “DESs are composed of two or more components, which in minimum two of them have a hydrogen bonding interaction ability: one as a HBD and one as a hydrogen bond acceptor (HBA) (Durand et al. 2013). On the other hand, DESs formed by natural compounds called NADESs may be classified as sugar based (glucosefructose-water, glucose-fructose-sucrose water), polyol based, acid based and so on (Dai et al. 2013a). Since DESs can be formed by a number of components, physicochemical properties vary from type to type. Therefore, one can tune the physicochemical property by changing the type and the molar ratio of the constituents. Depending on the type of the constituents, viscosity of DESs may be low or high. High viscosity DESs are hard to be handled, but in some cases, they are preferred to be used as a mixture of alcohol and water to decrease the viscosity” (Ünlü and Takaç 2020).

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2 Deep Eutectic Solvents and Their Physicochemical Properties

DESs usually have low-melting points. This is related to the hydrogen bond interaction. Between the constituents, some DESs were reported to have a glass transition temperature (Dai et al. 2013b; Florindo et al. 2014; Zubeir et al. 2014). Density was found to range between 800–1600 kg/m3 but generally, they have higher density than water (Dean et al. 2010; Fredlake et al. 2004; Almeida et al. 2012). In contrast, hydrophobic DESs are reported to have lower density in comparison to hydrophilic DESs (Van Osch et al. 2015, 2016; Ribeiro et al. 2015). The understanding of physiochemical characteristics of DESs is important for its industrial applications (Satlewal et al. 2018). The key properties of DESs are freezing point, density, viscosity, surface tension and conductivity. Table 2.2 presents the summary of the properties of some common DES combinations.

HBA

ChCI

ChCI

ChCI

ChCI

ChCI

ZnCl2

Bu4NBr

ZnCl2

ChCI

ChCI

HBD

Urea

Ethylene glycol

Glycerol

CFsCONH2

ZnCl2

Urea

Imidazole

Ethylene glycol

2,2.2Triflutroacetamide

Acrylic acid

1.6:1

1.6:1

4:1

7:3

3.5:1

2:1

2:1

2:1

2:1

2:1

Molar ratio (HBD:HBA)

Table 2.2 Properties of some common DES combinations

Liquid at 25 °C

Liquid at 25 °C

–

–

9

–

51

–

1.342

1.45

–

1.63

–

1.342

1.18

1.12

−12.9 17.8

1.25

Density (g cm−3 )

12

Freezing part (o C) 52 (25 °C)

Surface tension (mN m−1 )

55.8 (25 °C)

–

–

–

–

115 (22 °C)

–

77 (40 °C) 35.9 (25 °C)

–

810 (20 °C)

11,340 (25 °C)

85,000 (25 °C)

77 (40 °C) –

259 (25 °C)

37 (25 °C) 49 (25 °C)

750 (25 °C)

Viscosity (cP)

–

–

–

0.24 (20 °C)

0.18 (42 °C)

0.06(42 oC)

–

1.05 (25 °C)

7.61 (25 °C)

0.75 (25 °C)

Conductivity (mS−1 )

(continued)

Ato-Hamad et al. (2015)

Smith et al. (2014)

Smith et al. (2014) and Zhang et al. (2012)

References

2 Deep Eutectic Solvents and Their Physicochemical Properties 15

Methyltriphenylphosphonium bromide

Methyltriphenylphosphonium bromide

Methyltriphenylphosphonium bromide

ChCI

ChCI

ChCI

EtNH3 CI

EtNH3 Cl

AcChCl

Glycerol

Ethylene glycol

Triethylene glycol

Malonic acid

1.4-Butanediol

Imidazole

Acetamide

Urea

Urea

Loow et al. (2017). Reproduced with permission

HBA

HBD

Table 2.2 (continued)

2:1

1.5:1

1.5:1

7:3

3:1

1:1

5:1

4:I

3:1

Molar ratio (HBD:HBA)

–

–

–

56

1.206

1.140

1.041

–

1.06

1.19

−21

−32

1.23

−49.34

–

1.3

−5.55

10

Density (g cm−3 )

Freezing part (o C)

47.17 (25 °C)

65.7 (25 °C)

49.58 (25 °C)

51.29 (25 °C)

58.94 (3 °C)

Surface tension (mN m−1 )

2214 (40 °C)

128 (40 °C)

64

–

15 (70 °C) −

140 (20 °C)

721 (25 °C)

–

–

–

Viscosity (cP)

0.017 (40 °C)

0.348 (40 °C)

0.688 (40 °C)

12 (60 °C)

1.64 (25 °C)

0.55 (25 °C)

–

1.092 (25 °C)

0.062 (25 °C)

Conductivity (mS−1 )

Tang and Row (2013) and Zhang et al. (2012)

References

16 2 Deep Eutectic Solvents and Their Physicochemical Properties

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Chapter 3

Cellulose, Hemicelluloses and Lignin Solubilization in DESs

“At present, the main hurdle for the commercial feasibility of biobased refineries is the separation of lignin from polysaccharides at low costs for the production of fermentable sugars and other high-value products from both sugars and lignin” (Saltewal 2018). Deep eutectic solvents (DESs) are able to donate and accept protons and this characteristic enables the formation of hydrogen bonds with other compounds which results in an increases in its solvation properties (Pandey et al. 2017). Choline chloride is mostly used for producing DESs as a hydrogen bond acceptor, and therefore, the studies on DESs for cellulose dissolution started with choline chloride-derived DESs. Miller (2011) studied the dissolution of several cellulosic polymers like sodium carboxymethyl cellulose, xanthan gum, modified guar gum, carboxymethyl tamarind in choline chloride urea eutectics. The cellulosic polymers were found to be soluble at a temperature of 65 °C. Pan et al. (2017) reported pretreatment with DES of choline chloride (ChCl)/urea mixtures with rice straw and its chemical fractions of holocellulose, α-cellulose, and acid-insoluble-lignin (AIL). The pretreatment with ChCl/urea was considerably affected by the treatment temperature prior to the treated time. Optimum condition for ChCl/urea pretreatment was a temperature of 130 °C and time of 4 h. “The separation capacity of ChCl/urea on the chemical fractions was in an order of AIL (22.87%) > hemicellulose and amorphous cellulose (16.71%) > α-cellulose (9.60%). ChCl/urea had a higher selective solubility on lignin. The solubility of the whole fractionation of rice straw affected by ChCl/urea was a combination of solubilization on cellulose, hemicellulose and lignin. ChCl/urea pretreatment increased crystallinity index (CrI) of rice straw residue and α-cellulose, while had no obvious influence on CrI of holocellulose” (Pan et al. (2017). Even, microcrystalline cellulose (MCC) was only sparingly soluble and almost no solubilization was found in choline chloride/urea (molar ratio 1:2) and choline chloride/ZnCl2 (molar ratio 1:2) (Tenhunen et al. 2017; Zhang et al. 2012b). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 P. Bajpai, Deep Eutectic Solvents for Pretreatment of Lignocellulosic Biomass, SpringerBriefs in Applied Sciences and Technology, https://doi.org/10.1007/978-981-16-4013-1_3

21

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3 Cellulose, Hemicelluloses and Lignin Solubilization in DESs

Hertel et al. (2012) did not observe any dissolution of MCC in the DESs of choline chloride and other chemicals including malonic acid, oxalic acid, glycerine, phenylacetic acid, glycerine, formamide and acetamide showing that the choline chloride-derived DESs might not be effective for dissolution of the original cellulose. Ren et al. (2016a) activated the cellulose by the use of ultrasound-assisted saturated calcium chloride solution for improving the solubility of cellulose in choline chloride-derived DESs. The solubility of cellulose was found to be 1.43 wt% in choline chloride and urea. Higher solubility of cellulose (2.48 wt%) was observed when urea was replaced with imidazole. The solubility further increased to 4.57 wt% in choline chloride and imidazole by the addition of 5 wt% polyethylene glycol. Malaeke et al. (2018) produced several types of DESs including choline chloride/maleic acid, choline chloride/resorcinol, choline chloride/phenol and choline chloride/alpha-naphthol. Cellulose was found to be dissolved in these DESs by the use of ultrasound irradiation. The maximum solubility of cellulose was observed in choline chloride and resorcinol with 6.10 wt%. Francisco et al. (2012) synthesized 21 kinds of DESs with different molar ratios for finding DESs for cellulose dissolution. The maximum solubility of cellulose was found to be 0.78 wt% in a DES consisting of malic acid and proline at 100 °C. Zhou and Liu (2014) synthesized DES by using urea and caprolactam at a molar ratio of 1:3. The maximum solubility of cotton-ramie pulp was 2.83 wt% at 50 °C. Zhang et al. (2012a) found that MCC (i.e., Avicel PH-105) was not soluble in ChCl:ZnCl2 (molar ratio1:2) and ChCl:urea (molar ratio 1:2) even after treatment at high temperature of 110 °C for a period of 12 h. Ren et al. (2016b) synthesized a new allyl-functionalized choline DES for promoting dissolution of cellulose. The allyl-functionalized choline ([ATEAm]Cl) was synthesized by using triethylamine and allyl chloride. Then the [ATEAm]Cl (hydrogen bond acceptor) and oxalic acid (hydrogen bond donor) were mixed and heated to obtain [ATEAm]Cl-Oxa. The maximum solubility of cellulose in this DES was found to be 6.48 wt%. Ren et al. (2016a, b) found that amorphous cellulose (cotton linter pulp) was solubilized by 1.43 wt% in ChCl:urea and 2.48 wt% in ChCl:imidazole. Pulp solubility was further increased to 4.57 wt% in ChCl:imidazole by the addition of 5 wt% polyethylene glycol as cosolvents, which was used for reducing the hydrophobicity of cellulose (Tang et al. 2017). Cellulose solubility was found to be inversely proportional to the crystallinity of the substrate. Hemicellulose was also found to be sparingly soluble in DESs (Satlewal et al. 2018). In contrast to both cellulose and hemicellulose, DESs particularly acidic DESs, i.e., lactic, malic and oxalic acid-based DESs were found highly effective for lignin dissolution (Satlewal et al. 2018). “One of the reasons for selective solubilization of lignin over cellulose is that, both cellulose and DESs possess strong hydrogen bonding networks, and dissolving cellulose in a DES needs the two hydrogen bond networks to be dissociated and reorganized to form a thermodynamically more stable system” (Vigier et al. 2015). But the cohesive energy of cellulose is so strong that it may hinder its dissolution in any DES.

3 Cellulose, Hemicelluloses and Lignin Solubilization in DESs

23

Kumar et al. (2016) observed that lignin isolated from rice straw was solubilized to a larger extent as compared to lignin embedded in rice straw structure (in its native state). The most probable reason for this could be the disintegration of highly cross-linked architecture of biomass and strong bonding between lignin carbohydrate complexes. Synthesis of a novel DESs having a strong ability for solubilizing hemicelluloses and cellulose remains a gray area. Other main issues for the industrial application of DESs-based biomass processing are their recyclability and thermal stability (Yoo et al. 2017). The recovery and reuse of DESs after biomass processing is a cost and energy intensive process. The release of trimethylamine from ChCl-based solvents at high temperatures (i.e., Hoffman elimination reaction) is a harmful component for the economic viability of this technology. These limitations must be overcome before DESs could be largely implemented on a large scale for biomass processing (Vigier et al. 2015; Satlewal et al. 2018). In Table 3.1, DESs for cellulose dissolution are presented (Chen et al. 2018). Table 3.2 shows the solubility of lignin, hemicelluloses and cellulose in various DESs (Chen and Mu, 2019; Lynam et al. 2017). Table 3.3 shows solubility of lignin and cellulose Table 3.1 Reported DESs for cellulose dissolution Raw material

DESs

Molar ratio

Method

Cellulose solubility (wt%)

References

Microcrystalline cellulose Avicel PH 105

ChCl/urea

1:2

Processing at 110 °C

98% ChCl/maleic acid mass fraction purity) purchased from Merck

1:1

Processing with ultrasonic irradiation (20 Hz) at 90 °C

3

Malaeke et al. (2018)

Same as above

ChCl/α-naphthol

1:1

Same as above

3

Same as above

Same as above

ChCl/phenol

2:1

Same as above

4.70

Same as above

Same as above

ChCl/resorcino

1:1

Same as above

6.10

Same as above

1:1

Chen et al. (2018). Reproduced with permission Table 3.2 Solubility of the isolated biomass components in various DESs at 60 °C Hydrogen bond donor

Hydrogen bond Mole ratio acceptor

Cellulose solubility (%)

Xylan solubility (%)

Lignin solubility (%)

Lactic acid

Betaine

2:01