Green Sustainable Process for Chemical and Environmental Engineering and Science: Green Solvents and Extraction Technology 0323951562, 9780323951562

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Green Sustainable Process for

CHEMICAL AND ENVIRONMENTAL ENGINEERING AND SCIENCE

Green Solvents and Extraction Technology Edited by

INAMUDDIN Department of Applied Chemistry, Zakir Husain College of Engineering and Technology Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India

TARIQ ALTALHI Department of Chemistry, College of Science, Taif University, Taif, Saudi Arabia

Green Sustainable Process for

CHEMICAL AND ENVIRONMENTAL ENGINEERING AND SCIENCE

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-95156-2 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Joseph P. Hayton Editorial Project Manager: Kathrine Esten Production Project Manager: Kamesh R Cover Designer: Mark Rogers Typeset by Aptara, New Delhi, India

Contributors

Muhammad Sajid Hamid Akash Department of Pharmaceutical Chemistry, Government College University, Faisalabad, Pakistan Mohankumar Anandraj Department of Mechanical Engineering, Kongu Engineering College, Perundurai, Erode, Tamil Nadu, India Mukesh Kumar Awasthi College of Natural Resources and Environment, Northwest A & F University,Yangling, Shaanxi, China Mostapha Bachir-bey Department of Food Science, Laboratory of Applied Biochemistry, Faculty of Nature and Life Science, University of Bejaia, Bejaia, Algeria Jhonatas Rodrigues Barbosa Institute of Technology (ITEC), Faculty of Food Engineering (FEA), Federal University of Para (UFPA), Belém, PA, Brazil Yassine Benchikh Laboratory of Biotechnology and Food Quality, Institute of Nutrition, Food and Agro-Food Technologies, University of Constantine 1, Constantine, Algeria; Laboratory of Applied Biochemistry, Faculty of Nature and Life Science, University of Bejaia, Bejaia, Algeria Achinta Bera Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Priscila do N. Bezerra LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Parameswaran Binod Microbial Processes and Technology Division, CSIR-National Institute of Interdisciplinary Science and Technology (CSIR-NIIST), Trivandrum, Kerala, India Makhlouf Chaalal Laboratory of Biotechnology and Food Quality, Institute of Nutrition, Food and Agro-Food Technologies, University of Constantine 1, Constantine, Algeria; Laboratory of Applied Biochemistry, Faculty of Nature and Life Science, University of Bejaia, Bejaia, Algeria Moganapriya Chinnasamy Department of Mining Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India

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Contributors

Vânia M.B. Cunha LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Marcilene P. da Silva LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Raul N. de Carvalho, Jr LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Sérgio H.B. de Sousa LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Maria C.R. Ferreira LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Anirban Garg Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India; Department of Chemistry, J.B. College (Autonomous), Jorhat, Assam, India Durga Rao Gijjapu Applied Research Center for Environment and Marine Studies at Research Institute, King Fahd University for Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia Kamran Haider Department of Pharmacy, University of Agriculture, Faisalabad, Pakistan Himshweta Biosensor Technology Laboratory, Department of Biotechnology, Punjabi University, Patiala, Punjab, India Muhammad Ibrahim Department of Applied Chemistry, Government College University, Faisalabad, Pakistan Saravana Kumar Jaganathan Biomedical Engineering, School of Engineering, College of Science, University of Lincoln, UK Shreya Juneja Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi, India Shagufta Kamal Department of Biochemistry, Government College University, Faisalabad, Pakistan Djamel Edine Kati Department of Food Science, Laboratory of Applied Biochemistry, Faculty of Nature and Life Science, University of Bejaia, Bejaia, Algeria Poonam Khandelwal Department of Chemistry, Mohanlal Sukhadia University, Udaipur, Rajasthan, India Vaishali Khokhar Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi, India

Contributors

Aravind Madhavan Rajiv Gandhi Centre for Biotechnology, Thiruvananthapuram, Kerala, India Ritu Mathur Department of Chemistry, Zakir Husian Delhi College, University of Delhi, Delhi, India Nomvano Mketo Department of Chemistry, College of Science and Engineering and Technology, Florida Science Campus, University of South Africa, Johannesburg, South Africa Rashmy Nair Department of Chemistry, S. S. Jain Subodh P.G. College, Jaipur, Rajasthan, India Mazen Khaled Nazal Applied Research Center for Environment and Marine Studies at Research Institute, King Fahd University for Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia Marioara Nechifor “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania Kamaraj Nithya Department of Chemical Engineering & Materials Science, Amrita School of Engineering, Coimbatore Amrita Vishwa Vidyapeetham, India Philiswa N. Nomngongo Department of Chemical Sciences, University of Johannesburg, Johannesburg, South Africa Samir Kumar Pal Department of Mining Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Sathish Kumar Palaniappan Department of Mining Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Siddharth Pandey Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi, India Ashok Pandey Centre for Innovation and Translational Research, CSIR- Indian Institute for Toxicology Research (CSIR-IITR), Lucknow, Uttar Pradesh, India; Centre for Energy and Environmental Sustainability, Lucknow, Uttar Pradesh, India; Sustainability Cluster, School of Engineering, University of Petroleum and Energy Studies, Dehradun, Uttarakhand, India Shelly Pathania Department of Pharmaceutical Chemistry, ISF College of Pharmacy, Moga, Punjab, India; Maharaja Ranjit Singh Punjab Technical University, Bathinda, Punjab, India Sherely Annie Paul Post Graduate and Research Department of Chemistry, Bishop Moore College, Mavelikara, Kerala, India

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Contributors

Flávia C.S. Pires LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Reshmy Rajasekharan Department of Science and Humanities, Providence College of Engineering, Chengannur, Kerala, India; Post Graduate and Research Department of Chemistry, Bishop Moore College, Mavelikara, Kerala, India Rajasekar Rathanasamy Department of Mechanical Engineering, Kongu Engineering College, Perundurai, Erode, Tamil Nadu, India Ravindra K. Rawal Natural Product Chemistry Group, Chemical Sciences and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat, Assam, India Kanwal Rehman Department of Pharmacy, The Women University, Multan, Pakistan Marielba de los Ángeles Rodríguez Salazar LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Diganta Sarma Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India Asha Sathish Department of Sciences, Amrita School of Engineering, Coimbatore Amrita Vishwa Vidyapeetham, India Anam Shabbir LIAS College of Pharmacy, Faisalabad, Pakistan Rahul Shrivastava Department of Chemistry, Manipal University Jaipur, Jaipur, Rajasthan, India Ana P. de S. e Silva LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Raveendran Sindhu Department of Food Technology, T K M Institute of Technology, Kollam, Kerala, India Swati Department of Chemistry, Maharishi Markandeswar (Deemed to be University), Mullana, Ambala, Haryana, India Fulga Tanasă “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania Carmen-Alice Teacă “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania Mohanraj Thangamuthu Department of Mechanical Engineering, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India

Contributors

Shruti Trivedi Department of Chemistry, Institute of Science, Banaras Hindu University,Varanasi, India Glides Rafael O. Urbina LABEX (Extraction Laboratory), UFPA (Federal University of Para), Para, Brazil Neelam Verma Chemistry and Division of Research and Development, Lovely Professional University, Phagwara, Punjab, India; Biosensor Technology Laboratory, Department of Biotechnology, Punjabi University, Patiala, Punjab, India Siham Ydjedd Laboratory of Applied Biochemistry, Faculty of Nature and Life Science, University of Bejaia, Bejaia, Algeria; Department of Nature and Life Science, Faculty of Nature and Life Science and Earth and Universe Science, University 8 May 1945 Guelma, Algeria

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CONTENTS

Contributors 

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1.  Utilization of green solvents for synthesis of biodiesel 

1

Sathish Kumar Palaniappan, Rajasekar Rathanasamy, Moganapriya Chinnasamy, Samir Kumar Pal, Saravana Kumar Jaganathan 1.1  Introduction  1.2  Feedstocks  1.3  Biodiesel production technologies  1.4  Biodiesel reaction medium  1.5  Conclusions  References 

2.  Chemistry of ionic liquids in multicomponent reactions 

1 2 4 6 10 11

17

Rashmy Nair, Rahul Shrivastava, Ritu Mathur, Poonam Khandelwal 2.1  Introduction  2.2  Three-component reactions using ionic liquids as solvents  2.3  Three-component reactions using ionic liquids as catalysts  2.4  Four-component reactions in ionic liquids as solvents  2.5  Four-component reactions in ionic liquids as catalysts  2.6  Solid support ionic liquids  2.7  Biodegradable ionic liquids  2.8  Ionic liquids in nanoform  2.9  Conclusion  Abbreviations  References 

3.  Green solvents in polymer synthesis 

17 19 29 34 37 39 42 43 44 44 46

51

Marioara Nechifor, Fulga Tanasă, Carmen-Alice Teacă 3.1  Introduction  3.2  Ionic liquids  3.3  Supercritical carbon dioxide  3.4  Polymerization reactions in water  3.5  Conclusions  References 

51 52 67 71 73 74

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4.  Click reaction in micellar media: A green and sustainable approach toward 1,2,3-triazoles synthesis 

85

Anirban Garg, Diganta Sarma 4.1  Introduction  4.2  Amphiphiles—a brief idea  4.3  Click reaction  4.4  Conclusions  References 

5.  Industrial application of green solvent for energy conversion and storage 

85 87 92 107 107

113

Mohanraj Thangamuthu, Rajasekar Rathanasamy, Mohankumar Anandraj, Moganapriya Chinnasamy 5.1  Introduction  5.2  Green solvents  5.3  Applications  5.4  Conclusion  References 

6.  Applications of ionic liquids as green solvents in enhanced oil recovery 

113 113 116 121 121

125

Achinta Bera 6.1  Introduction  6.2  Properties of ionic liquids  6.3  Ionic liquids in enhanced oil recovery  6.4  Advantages and disadvantages of ionic liquids  6.5  Future prospects and challenges  6.6  Summary and conclusions  Acknowledgments  References 

7.  Solvation within deep eutectic solvent-based systems: A review 

125 126 128 137 138 139 139 140

145

Shruti Trivedi, Shreya Juneja, Vaishali Khokhar, Siddharth Pandey 7.1  7.2  7.3  7.4  7.5  7.6 

Introduction  Spectroscopy within DESs  Polarity of and solvation within DES-based systems  Aggregation within DESs-based systems  Thermosolvatochromism within DES-based systems  Conclusion 

145 146 151 161 177 183

Contents

Acknowledgments  References 

8.  Introductory chapter: Understanding green chemistry principles for extraction of green solvents 

183 183

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Kamaraj Nithya, Asha Sathish 8.1  Introduction  8.2  Basic green chemistry principles  8.3  Conclusions  Abbreviations  References 

9.  Ionic liquids for phenolic compounds removal and extraction 

193 194 213 214 215

217

Durga Rao Gijjapu, Mazen Khaled Nazal 9.1  Introduction  9.2  Physicochemical properties of phenols  9.3  Faith and degradation of phenols  9.4  Reactivity of phenolic compounds in aquatic system  9.5  Toxicity of phenolic compounds  9.6  Methods for the phenolic compounds removal  9.7  Conclusions  References 

10.  Recovery of natural polysaccharides and advances in the hydrolysis of subcritical, supercritical water and eutectic solvents 

217 218 218 221 221 223 234 234

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Jhonatas Rodrigues Barbosa 10.1  Introduction  10.2  Importance and applications of natural polysaccharides  10.3  Main techniques for polysaccharides extraction  10.4  Extraction of polysaccharides with subcritical and supercritical fluid  10.5  Polysaccharides extraction, pretreatment, and modifications with eutectic solvents  10.6  Hydrolysis of polysaccharides with subcritical, supercritical water, and eutectic solvents  10.7  Conclusive observations  Additional reading  Author contributions  Ethical approval  Declaration of competing interest  Acknowledgment  References 

239 240 242 244 247 250 254 255 255 255 255 255 256

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11.  Green strategies for extraction of nanocellulose from agricultural wastes—Current trends and future perspectives 

269

Reshmy Rajasekharan, Sherely Annie Paul, Aravind Madhavan, Raveendran Sindhu, Parameswaran Binod, Mukesh Kumar Awasthi, Ashok Pandey 11.1  Introduction  11.2  Agricultural waste—a major source of cellulose  11.3  Green approach for extraction of nanocellulose  11.4  Application of nanocellulose  11.5  Conclusions and future scope  Acknowledgments  References 

12.  Antioxidants extraction from vegetable matrices with green solvents 

269 269 271 282 282 283 283

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Marielba de los Ángeles Rodríguez Salazar, Glides Rafael O. Urbina, Priscila do N. Bezerra, Vânia M.B. Cunha, Marcilene P. da Silva, Flávia C.S. Pires, Ana P. de S. e Silva, Maria C.R. Ferreira, Jhonatas Rodrigues Barbosa, Sérgio H.B. de Sousa, Raul N. de Carvalho, Jr 12.1  Introduction  12.2  Antioxidants  12.3  Antioxidant extraction techniques with green solvents  12.4  Main methods for in vitro antioxidant activity quantification  12.5  Considerations  Acknowledgments  References 

13.  Green methods for extraction of biomolecules 

289 290 290 300 301 302 302

309

Muhammad Sajid Hamid Akash, Kanwal Rehman, Kamran Haider, Anam Shabbir, Shagufta Kamal 13.1  Introduction  13.2  Carbohydrates extraction  13.3  Protein extraction  13.4  Lipid extraction  13.5  Nucleic acid extraction  13.6  Anions-exchange materials  13.7  Conclusion  Summary  Conflict of interest  References 

309 311 314 318 320 323 324 325 325 325

Contents

14.  Extraction of phenolic compounds 

329

Yassine Benchikh, Mostapha Bachir-bey, Makhlouf Chaalal, Siham Ydjedd, Djamel Edine Kati 14.1  Introduction  14.2  Chemistry of phenolic compounds  14.3  Factors affecting extraction of phenolic compounds  14.4  Extraction techniques of phenolic compounds  14.5  Conclusion  References 

15.  Extraction of phenolic compounds by conventional and green innovative techniques 

329 330 332 336 348 349

355

Neelam Verma, Himshweta 15.1  Introduction  15.2  Classification and properties of phenolic compounds  15.3  Conventional extraction methods  15.4  Concept of green technologies  15.5  Conclusion and future perspectives  References 

16.  Application of ionic liquids for extraction of phenolic compounds and dyes: A critical review 

355 357 359 363 382 383

395

Nomvano Mketo, Philiswa N. Nomngongo 16.1  Introduction  16.2  Determination of dyes and phenolic compounds in various matrices  16.3  Summary  16.4  Conclusion  Acknowledgments  References 

17.  Green methods for extraction of phenolic compounds 

395 397 404 405 405 406

409

Muhammad Sajid Hamid Akash, Kanwal Rehman, Anam Shabbir, Shagufta Kamal, Muhammad Ibrahim 17.1  Introduction  17.2  Methods of extractions of phenolic compounds  17.3  Conclusion  17.4  Summary  Conflict of interest  References 

409 410 416 416 416 416

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18.  Current prospective of green chemistry in the pharmaceutical industry 

419

Swati, Shelly Pathania, Ravindra K. Rawal 18.1  Introduction  18.2  Design of green chemistry  18.3  Applications of green chemistry in pharmaceuticals  18.4  Conclusion  Acknowledgments  Abbreviations  References  Index 

419 422 426 444 444 444 445 451

CHAPTER 1

Utilization of green solvents for synthesis of biodiesel Sathish Kumar Palaniappana, Rajasekar Rathanasamyb, Moganapriya Chinnasamya, Samir Kumar Pala, Saravana Kumar Jaganathanc

Department of Mining Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Department of Mechanical Engineering, Kongu Engineering College, Perundurai, Erode, Tamil Nadu, India c Biomedical Engineering, School of Engineering, College of Science, University of Lincoln, UK a

b

1.1 Introduction Due to the decreasing oil reserve and related economic impacts, the use of fossil fuel has turn out to be a significant problem in the last century. In the tenure of next 10 centuries, the world’s oil capability will decrease. In order to mitigate this issue with a focus on its renewability, financial, and social effects, alternative energy has been extensively studied by several researchers. The progressive decrease in availability of fossil fuels and the emergent emission of carbon dioxide from the environment finds its need for the development of sustainable unconventional fuels. The recommended alternative which can substitute standard oil petrol is biodiesel, a combination of fatty proteins alkyl esters. Biodiesel has drawn countless attention to scientists around the globe due to their prominent performances. Biodiesel’s physical characteristics are comparable with petroleum diesel which can be employed exclusive of change in current engine specifications (Demirbas, 2009; Fjerbaek, 2009). It is also a sustainable, nontoxic, and biodegradable gas, which reduces natural energy dependency and pollution of hazardous gasses to the environment (Bajpai, 2006; Fellows, 2000; Al-Zuhair, 2007; Demirbas, 2007; Fukuda, 2001; Ranganathan, 2008). The biological propensities of petrodiesel gas are very much comparable. However, biodiesel engines release less particulate matter, carbon monoxide, and hydrocarbons than petrodiesel motors which reduce the greenhouse gas effect. Biodiesel with a splash point of more than 266°F is much easier than petrodiesel as opposed with approximately 126°F for periodic diesel No. 2 (American Society for Testing and Materials Standard D6751). In addition, the sulfur and aromatic content of biodiesel are lesser. Based on the report from United States Department of Energy Statistics and Analysis, production of biodiesel has achieved around 1.2 × 109 gallons in the year 2015. Similarly, in the first 6 months of 2016, the information from concern department indicates that the output of biodiesel is about 21% greater than that of 2015, over the same time period. Biodiesel’s commercial value is determined mainly by the structure of its fatty acids and thus differs between the various feedstocks of biodiesel. Biodiesel prices are linked Green Sustainable Process for Chemical and Environmental Engineering and Science DOI: https://doi.org/10.1016/B978-0-323-95156-2.00011-8

© 2023 Elsevier Inc. All rights reserved.

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straight away to biodiesel feedstock costs. The waste substance alone is expected to account for about 75–90% of the complete production cost of biodiesel. Since the price of production is immediately related to raw material costs, the exploration to choose the finest biodiesel feedstock becomes more and more a subject of concern when developing an economical basis for the production of biological diesel. Enzyme lipase transesterification is regarded to be very efficient because of readily available glycerol and economic in nature. However, the foremost hinder for its marketable usage in production of biodiesel is excess-cost of lipase. Lipase can be immobilized in a strong base to enable frequent usage in batch or ongoing response procedures to resolve this issue. The selection of an oxygen immobilization aid matrix is focused on a number of factors: charging ability, recovery convenience, movable stabilization, available functional organizations, microbial resistance, mechanical strength, and costs. Different products such as porous crystals and monoliths are also accessible for assistance. Chemical catalysts are commonly used for the transformation of oil into biodiesel, which are both homogenous and heterogeneous. However, the corrosion effect and formation of soap in the high free fatty acids or water reaction media, which lead to noncatalytic supercritical transesterification, are often associating with both types of chemical catalysts. Due to the lack of poisonous chemicals, this catalyst-free reaction is much easier and environmentally friendly. The production of supercritical biodiesel, which exceeds critical liquid values at altitudes and stresses, offers a better volume transportation speed while supporting elevated esterification and transesterification (Jayanth et al., 2018). The supercritical gas production of biodiesel thus offers an option to a costly supercritical methanol-based technique. At gentle heat and stresses, supercritical gas can be run. Integration with supercritical carbon dioxide of the enzyme-monolithic reactor is now becoming a leading study project in the biofuel sector. There is also less publication about a comprehensive evaluation of this technique in production of biodiesel.This chapter seeks to explore the possibilities of integrating green feedstock of enzymes for biodiesel production. The first discussion will be about the conventional feedstock and some green feedstocks for production of biodiesel. The green catalysts used for the fatty acid ester transesterification of petroleum are also outlined in the following chapter.

1.2 Feedstocks 1.2.1  Conventional feedstocks for production of biodiesel Extensive usage of conventional feedstocks, triglycerides, prepared from oil-rich feedstocks like soybean, rapeseed, canola, sunflower, and palm due to their rich in abundance (Antunes, 2008; Liu, 2008; Dube, 2007; Al-Zuhair, 2007; Kalam, 2002; Saka, 2001; Mekhilef, 2011). The first initiation of this project was to use peanut oil for testing the diesel engine which was performed by foremost inventor, Rudolf Diesel. Natural oils also have an inadequate number of cetane present on it. The concept was therefore not

Utilization of green solvents for synthesis of biodiesel

embraced and plant products have been substituting for petroleum oils (Fukuda, 2001; Akoh, 2007; Basha, 2009; Sharma, 2008). Recent concerns over incomplete oil basin and increased oil outburst have reminded to utilize oil-rich feedstocks. The overcoming of this limitation of viscosity was suggested by dilution of oils by solvent, thermal cracking, pyrolysis, microemulsion, and transesterification reactions (Fan, 2009; Helwani, 2009). In the existence of appropriate catalyst, transesterification processes of brief chain alcohols like methanol and ethanol is the most favored and widely used strategy. Although immense capacity of vegetable oils can be found, biodiesel manufacturing from these vegetable oils is very competitive with their food application. Moreover, budding oil enriched crops need more soil and fresh water. Vegetable oils represent more than 60% of the overall production cost of biodiesel (Al-Zuhair, 2007; Lai, 2005) have been reported. So, nonedible oils were suggested from this perspective, such as those of nonedible crops not in nutrition and which may cultivate in unfertile territories. But there is still a need for fresh water. It was suggested to utilize the disposal oils and fats, where waste management is used (Phan, 2008), but a big numbers of free fatty acids and water were exist to increase the cost of manufacturing. In addition, everincreasing global demands for diesel cannot be met (Predojevi, 2008; Taufiqurrahmi, 2011; Zhang, 2003). 1.2.2  Green feedstocks for production of biodiesel The critical move to get cost-efficient biodiesel is the choice of selecting a cheaper and more sustainable oil feed. The microalgae currently acquire successful interest in practical use, and they are microorganisms in nature. Due to their elevated oil substance and development speed, the prospective feedstock tends to substitute the traditional oil has been considered (Adamczak, 2009; Chisti, 2007; Spolaore, 2006; Vijayaraghavan, 2009; Sheehan, 1998). Moreover, development of microalgae cells requires no soil or fresh water production. In sea water and waste water, several types of algae have been identified. The petroleum components of such feedstocks generally range from 20% to 50% and dry in some types to 80%. This can also alter its structure through changes in circumstances of development, such as sunshine, nutrients, and temperature.The stressful atmosphere generally increases the production efficiency of oil. Generally, microalgae bodies comprise proteins, carbohydrates, and lipids that stretch the implementation areas of manufactured materials from food to biofuels. In addition, microalgae cells were also utilize to mitigate carbon dioxide. However, several phases like choice of algae type, biomass manufacturing and processing, as well as oil extraction and transformation, are very much needed to exploit microalgae biomass for biodiesel production. 1.2.2.1  Algae: feedstock for biodiesel production During previous years, growths of algae have been developed as a capable feedstock for manufacture of biodiesel.Various examinations have shown algae growth as a better

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feedstock for biodiesel creation as first and second era biodiesel feedstocks because of its high ecological index and high eco-friendliness. The development of microalgae can be performed on nonarable grounds accordingly which evading the expansion of farmlands required for oleaginous plants. Microalgae additionally have high photon transformation proficiency and the capacity to make use of salt and waste water streams, therefore altogether lessening utilization of freshwater. Under reasonable circumstances, this can twofold its biomass and turn out up to half lipid to dried up biomass proportion. This doesn’t contend with different feedstocks for characteristic assets and further; they don’t affect nourishment costs. Oil extricated from vegetable plants or microalgae’s presents a novel issue which is straightforwardly utilized in accessible motors. A few strategies have been acquainted to take care of these issues including microemulsions, pyrolysis reaction, and transesterification process with the exclusive incentive behind lessening the oil thickness extricated from corresponding plants. Among every one of these choices, transesterification process is found to be the best as the procedure is basic, and their physical attributes of biodiesel are near to that of diesel fuel. It is the reactant procedure of substituting ester alkoxy group by alcohol which changes to biodiesel and glycerol. However, some biological-based enzymes or acids/bases can be utilized adequately as possible catalysts.

1.3  Biodiesel production technologies Extraction of oils through transesterification processes was the mainly used system. It is typically a response between petroleum and short-chain liquor that forms a combination of esters and glycerol as a secondary item. Commonly, the existence of catalysts can accelerate the response, which requires three moles of alcohol for reaction with oil. However, additional alcohol is generally used to generate more biodiesel. 1.3.1  Utilization of conventional catalysts Transesterification responses, as stated previously, are chemically catalyzed and may be basal or acidic catalysts based on free fatty acids and humidity substance of oil. Because of their little cost and attainable rates over 98% at sensible temperature of 60°C in an hour, alkaline catalysts like sodium hydroxide and potassium hydroxide are majorly employed (Fukuda, 2001; Atadashi, 2012). The response begins with preparation the alcohol alternative and loading it into the furnace with the presence of oils. Therefore, the heating reaction carried out for a few hours to that of reaction temperature. So, products are divided by its severity to attain coarse biodiesel, it also requires additional cleaning in order to retrieve unreacted oils and alcohols. Although it is a straight forward and commercially operated method, it is not practical for feedstocks that contain free fatty acids and water content such as soap-based nonalimentable oil and waste oil, which decreases output generally and needs immense

Utilization of green solvents for synthesis of biodiesel

amounts of catalysts (Ma, 1998; Sivasamy, 2009). This procedure is easy and widely applicable. Prior to transesterification, pretreatment of oil through acid esterification was recommended, where sulfuric acid was usually implemented. The quality of the oil has been improved; also the process is very time-consuming and requires a bunch of alcohol. Furthermore, acids are also corrosive in nature (Al-Zuhair, 2007; Akoh, 2007; Marchetti et al., 2007). 1.3.2  Utilization of green catalysts Generally, it is recommended that enzymes can be used that are green catalysts. Lipases, hydrolytic enzymes have received better attention in the production of biodiesel among the numerous existing enzymes. Because of their capacity to deal with ester bonds at moderate pressures and less power consumption, lipase-catalyzed biodiesels were produced (Marchetti et  al., 2007). Without certain pretreatment requirements, lipases, that is, nonspecific oils, can transform oil from various ingredients, even in hose cooking, with simple materials segregation technique and no formation of soap. Among the numerous researched lipases, Pseudomonas fluorescens (Guldhe, 2015; Devanesan, 2007), Pseudomonas cepacia (Noureddini, 2005), Candida rugosa (Moreno-Pirajan, 2011; Tan, 2014; Lee, 2011), Candida antartica (Nelson, 1996; Shimada, 1999; Watanabe, 2000; Samukawa, 2000; Fedosov, 2013; Watanabe, 2002; Taher et al., 2014a; Taher et al., 2011; Al-Zuhair, 2012), and Rizhomucor miehei (Huang, 2012; Huang, 2014) are frequently used in this field. Even if lipases are inferior to chemistry catalysts, glycerol concentration that is a byproduct which badly affects the activity of enzyme and production yield. Accumulation of glycerol raises the viscosity of the blend and creates hydrophilic foam around the enzyme, stopping the response substrate to reach the effective location (Dossat, 1999; Xu, 2011). Silica, the highest microporous design, is used in the lipase immobilization protocol to obtain maximum glycerol inhibition effect. It was recommended that generated glycerol ought to be continuously detached from reaction blend and utilized catalyst of tert-butanol (Chen, 2011; Azocar, 2014). Also in such cases, it is advantageous to employ the silica gel onto it, which can absorb glycerol efficiently (Modi, 2007). The enzyme’s output was decreased with the usage of more than 1.5 molar alcohol equivalents. At a certain level, hydrophilic alcohols become insoluble in oil and tend to remove the water hydration layer from lipase, which results in activity loss (Fjerbaek, 2009; Du, 2004; Li, 2006; Zheng, 2009). Many alternatives were suggested to resolve the restriction on the inhibition of short-chain alcohol. These include step-by-step alcohol supplement (Shimada, 2002), lipase pretreatment and exploitation of acetate as acceptors (Modi, 2006, 2007; Du, 2004). This latter technique is usually used. On the other side, because of the elevated cost of enzymes, the manufacturing of enzymatic biodiesel is not yet applicable for practical use. In several periods, lipase immobilization is normally observed to reuse the protein. This can also improve the

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stabilization as well. For example, Novozym®435 was used again with methanol transesterified nonedible oil of jatropha for 12 ongoing periods without a detectable reduction in operation (Modi, 2007). In disparity, action of Novozym®435 is retained over 200 times with the usage of tert-butanol as a response medium (Lu, 2009).

1.4  Biodiesel reaction medium In enzyme catalytic processes, solvent-free reaction systems are constantly preferable, but the application of solvents is very much necessary for preventing inhibition during lipase catalyst reaction. The solubility of response components improves by incorporating hydrophobenic solvents onto it, which results in reduced hydrophilic substrate/ product binding impact. In fact, the response mixture’s viscosity and transportation constraints decreased on effective locations, resulting in enhanced response output (Klibanov, 2001; Zaks, 1984). 1.4.1  Possible conventional organic solvents During production of biodiesel, numerous organic solvents were employed. Hydrophobicity, which was the primary variable, considered for selection of proper solvent (Adamczak, 2004).The production speed of biodiesel increased due to the liquid hydrophobicity and, as a result, hydrophilic solvents lean for removal of water from the enzyme surfaces (Samukawa, 2000; Klibanov, 1997; Doukyu, 2010;Yang, 2004; Gorman, 1992).The commonly utilized n-hexane has capability to improve the output was noted in several research studies, as opposed to solvent-free methods. This comprises the study on evaluating the impact of Mucor miehei lipase catalyst with the application of n-hexane in tallow fats transesterification process with proportion of methanol and oil molar as 3:1 (Nelson, 1996). A high output of 95% was achieved relative to 19% without liquid. Another useful solvent employ in lipase catalyzed procedures for biodiesel production is tert-butanol. A competent solution for n-hexane, tert-butanol, cannot soften the glycerol and the inhibitor impact is reduced to a minimal level (Al-Zuhair, 2007; Demirbas, 2009;Yang, 2010; Peng, 2001; Lai, 2012; Royon, 2007). In the transesterification process of soybean oils with Novozym®435, the molar proportion of methanol to oil as 6:1 was used toward obtain 60% production, contrast to 10% in the solvent-free scheme. 1.4.2  Green solvents for production of biodiesel Although an organic solvent increases the manufacturing output, the solvent gets separated, which entails extra manufacturing costs. In addition, organic solvents are highly toxic and volatile which can be utilized to minimize a variety of economic related problems. Unconventional nontoxic and environment-friendly solvents have been identified after several attempts by many researchers. Supercritical carbon dioxide and ionic liquids were suggested in this respect.

Utilization of green solvents for synthesis of biodiesel

1.4.2.1  Supercritical carbon dioxide Supercritical fluids are liquids with heat and stresses above its critical points. They were used in a number of applications. Supercritical carbon dioxide, the most extensively utilized distinct liquids, is nontoxic and inexpensive, with a large number of mild critical parameters (Rathore, 2007). Supercritical carbon dioxide has fluid solutions, air diffusiveness (and viscosity), compared with natural solvents, where tiny modifications in circumstances can boost their characteristics considerably. It can be used for several applications with these distinctive physiochemical characteristics, including segregation and reaction (Del Valle, 2004; Reverchon, 2001; Sovova, 2001). In addition, supercritical carbon dioxide can be used for simple product segregation purposes. Although supercritical carbon dioxide is frequently used in the existence of lipase in ester transesterification, it remains fresh in biodiesel production (Romero, 2005). It is well known that supercritical carbon dioxide is compatible with lipases and would improve the mass transition of response materials to enzyme-active locations through its usage in biodiesel production. Despite the use of elevated stress, it was obviously confirmed that the impact on inhibition of the enzyme at pressures below 200 bar (Celia, 2005; Novak, 2003) was minimal. When the supercritical carbon dioxide (Rathore, 2007; Oliveira, 2001; Madras, 2004; Varma, 2007) was transesterified in the presence of Novozym®435, natural drug returns were comparable with those obtained from palm fruit and jatropha seeds. During microalgae lipid transesterification in the presences of the same lipase, supercritical carbon dioxide achieved a higher output of 80 times (Taher et al., 2014b). The extraction of oils for biodiesel products by employing supercritical carbon dioxide occurs on fruit crop oil (Del Valle, 2004; Reverchon, 2001), livestock flesh fats, and microalgae cells (Taher et  al., 2014a; Andrich, 2006; Mendes, 2003; Mendes, 1995; Cheng, 2011; Halim, 2011). Supercritical carbon dioxide, an alternative to toxic n-hexane and its use as a power efficient method, is used to diminish the utilization of poisonous solvents and utilization of residue removal in further applications, for example in cooking related and pharmaceuticals. The specific extraction parameters like temperature, flow rate, and pressure influences their effectiveness, whereas rise in pressure enhances density of supercritical carbon dioxide and extraction output. Similarly, temperature replies reverse impact which turns out to be constant at certain pressure values. Decrease in the concentration of supercritical carbon dioxide reduces its capability to dissolve with elevated extraction temperature. However, the solvent removal improves by raising the solute vapor stress. The efficacy of supercritical carbon dioxide had been examined in several research studies. For instance, utilization of Spirulina platensis (Andrich, 2006), Spirulina maxima (Mendes, 2003), and Pavlova sp. (Cheng, 2011) microalgae cells for oil extraction by employing n-hexane results in similar output performances. During oil extraction from Chlorococum sp. fluids and Nannochloropsis sp., greater effectiveness was recorded (Halim, 2011; Andrich, 2005).

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As stated previously in the above sections, supercritical carbon dioxide has many advantageous benefits. However, the boiling and the supercritical condition of carbon dioxide require a strong stress, rendering the method very expensive. Depressurizing process to take apart biodiesel from enhanced supercritical carbon dioxide could have a negative effect on lipases and on their stabilization. Uninterrupted action was also taken into account to minimize the impact. The soybean (Dalla Rosa, 2009), maize oil (Ciftci, 2011; Ciftci, 2013), microalgae, and sunflower oil (Rodrigues, 2011) have been effectively used for the same. It is unclear if a combined oil conversion to biodiesel in supercritical carbon dioxide could be achieved on one embedded scheme. However, the extra heating costs could be tolerated for power manufacturing and allow the general method to be more viable (Taher et al., 2014a; Al-Zuhair, 2012, 2016). The existence of water may lead, on the other side, to the creation of carbonic acid that changes the pH reaction and denature of lipase. Carbon dioxide could also be used to blend with amine clusters on the lipase surface to obtain carbamates (Wright, 1948). 1.4.2.2  Ionic liquids Ionic liquids are fluids with a small crystallize affinity.They consist of cations and anions, and their nonvapor stress function distinguishes themselves from standard solvents. Thus, they have been created to substitute the traditional, unstable solvents in numerous processes, together with biodiesel production, as a green alternative solvent. The choice of selecting the correct ionic liquids depends on its impact to improve the substrate/ product solubility of the protein interaction reaction (Lozano, 2001, 2003; Dang, 2007; Kaar, 2003; Klahn, 2011). The physiochemical characteristics of developed ionic liquids can usually be adjusted by careful choice of the alkaline chains on the cation and anion groups. For instance, symmetrical and smaller alkyl bonds in the ionic liquids lead to a greater boiling temperature (Ohno, 2002; Endres, 2006), and improves chain branching outcome in greater boiling point (Xue, 2016). Still, it also reduces with rise in anion volume. Instead, ionic liquids with symmetrical and fluorinated anions are highly viscos in protein catalyzed responses, which is generally not preferable. Ionic liquids related to cations along with an aromatic phenyl ring resulted in high viscosity (Ochedzan-Siodlak, 2013). The miscibility of response substrate with ionic liquids hydrophobicity is the major variable influencing global reaction outcomes, where elevated fluid solubility substrates are the required characteristics in biodiesel production to improve reaction speed and poor biodiesel solubility in ionic liquids.The anions used primarily depend on the hydrophobicity of the ionic liquids. The extent of alkyl string on cation can affect hydrophobicity of ionic liquids, which leads to more hydrophobic ionic liquids of large alkyl chains (Aki, 2001; Huddleston, 2001). In the catalytic enzyme biodiesel system, hydrophilic ionic liquids could remove basic hydration coating and deactivate the lipase, whereas hydrophobe ionic liquids are preferably same as like organic solvents. In addition, the

Utilization of green solvents for synthesis of biodiesel

nucleophilicity of anion used in this ionic liquids mixture has an impact on lipase development and stabilization, where the elevated nucleophilicity of ionic liquids can influence the development of the lipase composition associated with positive locations of lipase (Lozano, 2001). In relation to application of ionic liquids in lipase-catalytic reactions, they are also used as green catalyst to solve problems of response complications and material purification problems in chemical catalytic reactions. Oils were extracted by ionic fluids, often from microalgae structures be used in the midst of moist soils devoid of the requirement for disturbances in plant ceilings. Hydrophilic ionic liquids are used for such procedures where algal cell components can be dissolved and oils can be left insoluble and floatable. Testing ionic liquids with removal from moist cells of Chlorella vulgaris was done, which is 40 times greater than n-hexane-methanol (7:3 volume ratio) combination (Choi, 2014). Also, impact of the addition of a polar liquid with ionic liquids was assessed (Young, 2010). For instance, Chlorella sp. cells (70% water) were screened with a combination of methanol to obtain 75% yield at the solvent proportion of 1:1.2 (weight ratio) accordingly. Ionic liquids at manufacturing scale has major problem as heavy expensive in nature. The recycling process is therefore very essential. Moreover, the removal phase of biodiesel from reactive blend is not simple, when the lengthy alkyl bonds on cation-based ionic liquids are employed. Recently, it was proposed by combining the ionic liquids and supercritical carbon dioxide, where biodiesel can be retrieved efficiently using supercritical carbon dioxide. This has also been tested with Novozym®435 with several ionic liquids for biodiesel production and 98% higher yields were attained after 6 hours (Young, 2010). 1.4.2.3  Deep eutectic solvents The green chemical revolution has been recognized as deep eutetic solvents prepared in different quantities which can also be called as green solvents for production of biodiesel. Choline chloride–urine, glycerol-based acetate acid was one of the first deep eutetic solvents literature reports for biodiesel processing (Palaniappan et al., 2020). In many applications, this mixture was subsequently used because of ease in preparing procedures. The above deep eutetic solvent mixture was used in electrodeposition of zinc–tin alloys and polyoxometalate-based variants. During production of biodiesel, the utilization of deep eutetic solvent as a suitable cosolvent was noticed. Deep eutetic solvent decreases end reactions like saponification and readily help for purification. In some instances, glycerol and alkaline catalysts are efficiently removed from natural biodiesel. Prepared deep eutetic solvent decreases the use of organic solvents that are normally contaminated. These are cheaper and made from organic products, were suggested to decrease the price of ionic liquids used in the production of biodiesel. Through a mild temperature, it was blended in an adequate way until a consistent liquid has been created. The purity of the following blend relies on the purity of each of the preparing components. The H-bond donor ability interacts with the anion component of water and improves

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efficacy. Choline chloride, a cost less toxic compared with each other’s individual component of the mixture. Thus, the initial manufacture of deep eutetic solvents has been made by combination of choline chloride onto it. It is now used as a complement for animal feed, called vitamin B4. In several implementations, deep eutetic solvents were used in electrodeposition of zinc–tin metals, electropolishing of stainless steel, and several other chemical processes. Deep eutetic solvents have many comparable physical features, like conductivity, polarity, thickness, and viscosity, alongside ionic liquids. Generally, deep eutetic solvents are nonvolatile in nature and noninflammable too, similar to other ionic liquids. They are also much more biodegradable, less toxic and less inert to air responses than standard ionic liquids. Furthermore, their groundwork in elevated purity is extremely easy with no waste production and without additional cation requirements, leading them cheaper than ionic liquids. Deep eutetic solvents were employed as a catalyst and as a reaction solvent during separation processes of biodiesel production. The deep eutetic solvents based on choline chloride were used effectively as catalysts in transesterification process of oils comprising significant free fatty acids (~10 weight percentages). For example, to reduce the quantity of free fatty acids contained in acidic crude palm oil from 9% to below 1%, choline chloride: p-toluenesulfonic acid mixed at 60°C was used as pretreatment catalyst. The output of biodiesel generated in this method after the alkaline transesterification was around 92%. Deep eutetic solvents is also interested in the fact that side-derivative glycerol is employed as an H-bond donor ability, an extra benefit in the manufacture of biodiesel. In the activation of calcium oxide strong catalyst by separating dormant parts of calcium carbonate and calcium hydroxide, the usage of choline chloride:glycerol was discovered to be more effective. At 65°C and 14:1 molar ratios, the output of biodiesel manufacture in the occurrence of these deep eutetic solvents has reached 92% as explained earlier. In the existence of choline chloride:glycerol deep eutetic solvents, calcinated calcium oxide yields of 95% were also attained, compared to deep eutetic solvents of 87%. Deep eutetic solvents have also been used in alkaline processes to purify crude biodiesel. Deep eutetic solvents were also used efficiently in catalyzed lipase processes, demonstrating elevated efficiency and stabilization of the enzymes. Miglyol®812 oil, a blend of caprylic and capric proteins with fatty acids in short chains was used as reference oil. In another study, the choline chloride:glycerol ratio of Novozym®435 was used, which reduced soybean oils to biodiesel by achieving 88% in 24 hours. Due to the existence of longchains fatty acids on soya oils, this extended reaction time was necessary.

1.5 Conclusions The manufacturing of biological diesel using chemical and solvent catalysts has been focused on replacing standard diesel fuel. Nevertheless, the method is not commercialized because of numerous inadequacies. So, utilization of green-based catalysts

Utilization of green solvents for synthesis of biodiesel

and solvents has been focused with numerous technological boundaries for biodiesel production, which can replace conventional biodiesel effectively. The use of embedded procedures combining the utilization of several green-based catalysts and solvents in single phase is discussed for improving material segregation and recovery of solvent. However, the potential of utilizing ionic liquids in production of biodiesel has achieved ever-increasing attention, because of distinctive and tunable characteristics. As an end, the investigation of green feedstocks on behalf of biodiesel production is as yet progressing and promising. Microalgal biomass was projected as cutting edge feedstocks for production of biodiesel predominantly, because of its higher lipid substance and elevated output. Obviously, the consolidation of solid enzymatic reactors, especially in a microalgal-based biodiesel creation stage, demonstrates an exceptionally encouraging monetary potential. A coordinated framework that consolidates the extraction of oil from green feedstock and the immediate bioconversion of lipid into biodiesel utilizing an enzymatic reactor in supercritical carbon dioxide has been examined.

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Lozano, P., De Diego, T., Carrie, D., Vaultier, M., Iborra, J.L., 2001. Over-stabilization of Candida antarctica lipase B by ionic liquids in ester synthesis. Biotechnol. Lett 23 (18), 1529–1533. https://doi.org/10.10 23/A:1011697609756. Lozano, P., de Diego, T., Carrié, D., Vaultier, M., Iborra, J.L., 2003. Lipase catalysis in ionic liquids and supercritical carbon dioxide at 150°C. Biotechnol. Progr. 19 (2), 380–382. https://doi.org/10.1021/ bp025759o. Lozano, P., de Diego,T., Guegan, J.-P.,Vaultier, M., Iborra, J.L., 2001. Stabilization of achymotrypsin by ionic liquids in transesterification reactions. Biotechnol. Bioeng. 75 (5), 563–569. https://doi.org/10.1002/ bit.10089. Lu, J., Chen, Y., Wang, F., Tan, T., 2009. Effect of water on methanolysis of glycerol trioleate catalyzed by immobilized lipase Candida sp. 99-125 in organic solvent system. J. Mol. Catal. B Enzym. 56 (2-3), 122–125. https://doi.org/10.1016/j.molcatb.2008.05.004. Ma, F., Clements, L.D., Ha, M.A., 1998. Biodiesel fuel from animal fat. Ind. Eng. Chem. Res. 37 (9), 3768–3771. https://doi.org/10.1021/ie980162s. Madras, G., Kolluru, C., Kumar, R., 2004. Synthesis of biodiesel in supercritical fluids. Fuel 83 (14-15), 2029–2033. https://doi.org/10.1016/j.fuel.2004.03.014. Marchetti, J.M., Miguel,V.U., Errazu, A.F., 2007. Possible methods for biodiesel production. Renew. Sustain. Energy Rev. 11, 1300–1311. https://doi.org/10.1016/j.rser.2005.08.006. Mekhilef, S., Siga, S., Saidur, R., 2011. A review on palm oil biodiesel as a source of renewable fuel. Renew. Sustain. Energy Rev. 15 (4), 1937–1949. https://doi.org/10.1016/j.rser.2010.12.012. Mendes, R.L., Fernandes, H.L., Coelho, J.P., Reis, E.C., Cabral, J.M.S., Novais, J.L.M., Palavra, A.N.F., 1995. Supercritical CO2 extraction of carotenoids and other lipids from Chlorella vulgaris. Food Chem. 53 (1), 99–103. https://doi.org/10.1016/0308-8146(95)95794-7. Mendes, R.L., Nobre, B.P., Cardoso, M.T., Pereira, A.P., Palavra, A.N.F., 2003. Supercritical carbon dioxide extraction of compounds with pharmaceutical importance from microalgae. Inorg. Chim. Acta 356, 328–334. https://doi.org/10.1016/S0020-1693(03)00363-3. Modi, M.K., Reddy, J.R.C., Rao, B.V.S.K., Prasad, R.B.N., 2006. Lipase-mediated transformation of vegetable oils into biodiesel using propan-2-ol as acyl acceptor. Biotechnol. Lett 28 (9), 637–640. https://doi. org/10.1007/s10529-006-0027-2. Modi, M.K., Reddy, J.R.C., Rao, B.V.S.K., Prasad, R.B.N., 2007. Lipase-mediated conversion of vegetable oils into biodiesel using ethyl acetate as acyl acceptor. Bioresour. Technol. 98 (6), 1260–1264. https:// doi.org/10.1016/j.biortech.2006.05.006. Moreno-Pirajàn, J.C., Giraldo, L., 2011. Study of immobilized Candida rugosa lipase for biodiesel fuel production from palm oil by flow microcalorimetry.Arab. J. Chem. 4 (1), 55–62. https://doi.org/10.1016/j. arabjc.2010.06.019. Nelson, L.A., Foglia,T.A., Marmer,W.N., 1996. Lipase-catalyzed production of biodiesel. J.Am. Oil Chemists’ Soc. 73 (9), 1191–1195. https://doi.org/10.1007/BF02523383. Noureddini, H., Gao, X., Philkana, R.S., 2005. Immobilized Pseudomonas cepacia lipase for biodiesel fuel production from soybean oil. Bioresour. Technol. 96 (7), 769–777. https://doi.org/10.1016/j. biortech.2004.05.029. Novak, Z., Habulin, M., Krmelj,V., Knez,Å.E., 2003. Silica aerogels as supports for lipase catalyzed esterifications at sub- and supercritical conditions. J. Supercrit. Fluids 27 (2), 169–178. https://doi.org/10.1016/S08968446(02)00233-4. Ochędzan-Siodłak,W., Dziubek, K., Siod?ak, D., 2013. Densities and viscosities of imidazolium and pyridinium chloroaluminate ionic liquids. J. Mol. Liq. 177, 85–93. https://doi.org/10.1016/j.molliq.2012.10.001. Ohno, H.,Yoshizawa, M., 2002. Ion conductive characteristics of ionic liquids prepared by neutralization of alkylimidazoles. Solid State Ionics 154, 303–309. https://doi.org/10.1016/S0167-2738(02)00526-X. Oliveira, D., Oliveira, J.V., 2001. Enzymatic alcoholysis of palm kernel oil in n-hexane and SCCO2. J. Supercrit. Fluids 19 (2), 141–148. https://doi.org/10.1016/S0896-8446(00)00068-1. Palaniappan, S.K., et al., 2020. Synthetic Binders for Polymer Division. Green Adhesives: Preparation, Properties and Applications, pp. 227–272. Peng,W.,Wu, Q.,Tu, P., 2001. Pyrolytic characteristics of heterotrophic chlorella protothecoides for renewable bio-fuel production. J. Appl. Phycol. 13 (1), 5–12. https://doi.org/10.1023/A:1008153831875. Phan, A.N., Phan, T.M., 2008. Biodiesel production from waste cooking oils. Fuel 87 (17-18), 3490–3496. https://doi.org/10.1016/j.fuel.2008.07.008.

Utilization of green solvents for synthesis of biodiesel

Predojevi, Z.J., 2008.The production of biodiesel from waste frying oils: a comparison of different purification steps. Fuel 87 (17-18), 3522–3528. https://doi.org/10.1016/j.fuel.2008.07.003. Ranganathan, S.V., Narasimhan, S.L., Muthukumar, K., 2008. An overview of enzymatic production of biodiesel. Bioresour. Technol. 99 (10), 3975–3981. https://doi.org/10.1016/j.biortech.2007.04.060. Rathore,V., Madras, G., 2007. Synthesis of biodiesel from edible and non-edible oils in supercritical alcohols and enzymatic synthesis in supercritical carbon dioxide. Fuel 86 (17-18), 2650–2659. https://doi. org/10.1016/j.fuel.2007.03.014. Reverchon, E., Marrone, C., 2001. Modeling and simulation of the supercritical CO2 extraction of vegetable oils. J. Supercrit. Fluids 19 (2), 161–175. https://doi.org/10.1016/S0896-8446(00)00093-0. Rodrigues, A.R., Paiva, A., da Silva, M.G., Sim??es, P., Barreiros, S., 2011. Continuous enzymatic production of biodiesel from virgin and waste sunflower oil in supercritical carbon dioxide. J. Supercrit. Fluids 56 (3), 259–264. https://doi.org/10.1016/j.supflu.2010.10.031. Romero, M.D., Calvo, L., Alba, C., Habulin, M., Primoži? M., Knez, Ž., 2005. Enzymatic synthesis of isoamyl acetate with immobilized Candida antarctica lipase in supercritical carbon dioxide. J. Supercrit. Fluids 33 (1), 77–84. https://doi.org/10.1016/S0896-8446(04)00114-7. Royon, D., Daz, M., Ellenrieder, G., Locatelli, S., 2007. Enzymatic production of biodiesel from cotton seed oil using t-butanol as a solvent. Bioresour. Technol. 98 (3), 648–653. https://doi.org/10.1016/j. biortech.2006.02.021. Saka, S., Kusdiana, D., 2001. Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel 80 (2), 225–231. https://doi.org/10.1016/S0016-2361(00)00083-1. Samukawa, T., Kaieda, M., Matsumoto, T., Ban, K., Kondo, A., Shimada, Y., Noda, H., Fukuda, H., 2000. Pretreatment of immobilized Candida antarctica lipase for biodiesel fuel production from plant oil. J. Biosci. Bioeng. 90 (2), 180–183. https://doi.org/10.1016/S1389-1723(00)80107-3. Sharma,Y.C., Singh, B., Upadhyay, S.N., 2008.Advancements in development and characterization of biodiesel: a review. Fuel 87 (12), 2355–2373. https://doi.org/10.1016/j.fuel.2008.01.014. Sheehan, J., Dunahay,T., Benemann, R., Roessler, G.,Weissman, C., 1998.A Look Back At the U.S. Department of Energy- Aquatic Species Program: Biodiesel From Algae. National Renewable Energy Laboratory, USA. https://doi.org/10.2172/15003040. Shimada,Y., Watanabe,Y., Samukawa, T., Sugihara, A., Noda, H., Fukuda, H., Tominaga,Y., 1999. Conversion of vegetable oil to biodiesel using immobilized Candida antarctica lipase. J. Am. Oil Chem. Soc. 76 (7), 789–793. https://doi.org/10.1007/s11746-999-0067-6. Shimada,Y.,Watanabe,Y., Sugihara,A.,Tominaga,Y., 2002. Enzymatic alcoholysis for biodiesel fuel production and application of the reaction to oil processing. J. Mol. Catal. B Enzym. 17 (3-5), 133–142. https://doi. org/10.1016/S1381-1177(02)00020-6. Sivasamy,A., Cheah, K.Y., Fornasiero, P., Kemausuor, F., Zinoviev, S., Miertus, S., 2009. Catalytic applications in the production of biodiesel from vegetable oils. ChemSusChem 2 (4), 278–300. https://doi.org/10.1002/ cssc.200800253. Sovova, H., Zarevucka, M.,Vacek, M., Stransky, K., 2001. Solubility of two vegetable oils in supercritical CO2. J. Supercrit. Fluids 20 (1), 15–28. https://doi.org/10.1016/S0896-8446(01)00057-2. Spolaore, P., Joannis-Cassan, C., Duran, E., Isambert, A., 2006. Commercial applications of microalgae. J. Biosci. Bioeng. 101 (2), 87–96. https://doi.org/10.1263/jbb.101.87. Taher, H., Al-Zuhair, S., Al-Marzouqi, A.H., Haik, Y., Farid, M., 2014a. Enzymatic biodiesel production of microalgae lipids under supercritical carbon dioxide: process optimization and integration. Biochem. Eng. J. 90, 103–113. https://doi.org/10.1016/j.bej.2014.05.019. Taher, H., Al-Zuhair, S., Al-Marzouqi, A.H., Haik,Y., Farid, M.,Tariq, S., 2014b. Supercritical carbon dioxide extraction of microalgae lipid: process optimization and laboratory scale-up. J. Supercrit. Fluids 86, 57–66. https://doi.org/10.1016/j.supflu.2013.11.020. Taher, H., Al-Zuhair, S., AlMarzouqui, A., Hashim, I., 2011. Extracted fat from lamb meat by supercritical CO2 as feedstock for biodiesel production. Biochem. Eng. J. 55 (1), 23–31. https://doi.org/10.1016/j. bej.2011.03.003. Tan, Z., Fang, M., Du, H., Song, L., Ren, D., Tang, X., Han, X., Liang, X., 2014. Production of biodiesel catalyzed by Candida rugosa lipase at interface of W/O microemulsion system. J. Braz. Chem. Soc. 25, 1704–1711. https://doi.org/10.5935/0103-5053.20140165.

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Taufiqurrahmi, N., Mohamed,A.R., Bhatia, S., 2011. Production of biofuel from waste cooking palm oil using nanocrystalline zeolite as catalyst: process optimization studies. Bioresour.Technol. 102 (22), 10686–10694. https://doi.org/10.1016/j.biortech.2011.08.068. Varma, M.N., Madras, G., 2007. Synthesis of biodiesel from castor oil and linseed oil in supercritical fluids. Ind. Eng. Chem. Res. 46 (1), 1–6. https://doi.org/10.1021/ie0607043. Vijayaraghavan, K., Hemanathan, K., 2009. Biodiesel production from freshwater algae. Energy Fuels 23 (11), 5448–5453. https://doi.org/10.1021/ef9006033. Watanabe,Y., Shimada,Y., Sugihara, A., Noda, H., Fukuda, H.,Tominaga,Y., 2000. Continuous production of biodiesel fuel from vegetable oil using immobilized Candida antarctica lipase. J. Am. Oil Chem. Soc. 77 (4), 355–360. https://doi.org/10.1007/s11746-000-0058-9. Watanabe,Y., Shimada,Y., Sugihara, A.,Tominaga,Y., 2002. Conversion of degummed soybean oil to biodiesel fuel with immobilized Candida antarctica lipase. J. Mol. Catal. B Enzym. 17 (3-5), 151–155. https://doi. org/10.1016/S1381-1177(02)00022-X. Wright, H.B., Moore, M.B., 1948. Reactions of aralkyl amines with carbon dioxide. J. Am. Chem. Soc. 70 (11), 3865–3866. https://doi.org/10.1021/ja01191a097. Xu, Y., Nordblad, M., Nielsen, P.M., Brask, J., Woodley, J.M., 2011. Situ visualization and effect of glycerol in lipase-catalyzed ethanolysis of rapeseed oil. J. Mol. Catal. B Enzym. 72 (3-4), 213–219. https://doi. org/10.1016/j.molcatb.2011.06.008. Xue, L., Gurung, E., Tamas, G., Koh, Y.P., Shadeck, M., Simon, S.L., Maroncelli, M., Quitevis, E.L., 2016. Effect of alkyl chain branching on physicochemical properties of imidazolium-based ionic liquids. J. Chem. Eng. Data 61 (3), 1078–1091. https://doi.org/10.1021/acs.jced.5b00658. Yang, L., Dordick, J.S., Garde, S., 2004. Hydration of enzyme in nonaqueous media is consistent with solvent dependence of its activity. Biophys. J. 87 (2), 812–821. https://doi.org/10.1529/biophysj.104.041269. Yang, Z., Zhang, K.-P., Huang,Y.,Wang, Z., 2010. Both hydrolytic and transesterification activities of Penicillium expansum lipase are significantly enhanced in ionic liquid [Bmim][Pf6]. J. Mol. Catal. B Enzym. 63 (1-2), 23–30. https://doi.org/10.1016/j.molcatb.2009.11.014. Young, G., Nippgen, F., Titterbrandt, S., Cooney, M.J., 2010. Lipid extraction from biomass using co-solvent mixtures of ionic liquids and polar covalent molecules. Sep. Purif.Technol. 72 (1), 118–121. https://doi. org/10.1016/j.seppur.2010.01.009. Zaks, A., Klibanov, A., 1984. Enzymatic catalysis in organic media at 100 degrees C. Science 224, 1249–1251. https://doi.org/10.1126/science.6729453. Zhang, Y., Dube, M.A., McLean, D.D., Kates, M., 2003. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresour.Technol. 89 (1), 1–16. https://doi.org/10.1016/ S0960-8524(03)00040-3. Zheng,Y., Quan, J., Ning, X., Zhu, L.-M., Jiang, B., He, Z.-Y., 2009. Lipase-catalyzed transesterification of soybean oil for biodiesel production in tert-amyl alcohol.World J. Microbiol. Biotechnol. 25 (1), 41–46. https://doi.org/10.1007/s11274-008-9858-4.

CHAPTER 2

Chemistry of ionic liquids in multicomponent reactions Rashmy Naira, Rahul Shrivastavab, Ritu Mathurc, Poonam Khandelwald Department of Chemistry, S. S. Jain Subodh P.G. College, Jaipur, Rajasthan, India Department of Chemistry, Manipal University Jaipur, Jaipur, Rajasthan, India Department of Chemistry, Zakir Husian Delhi College, University of Delhi, Delhi, India d Department of Chemistry, Mohanlal Sukhadia University, Udaipur, Rajasthan, India a

b c

2.1 Introduction Ionic liquids, also known as room temperature ionic liquids, are basically salts made of short-lived ion-pairs with melting point below 100°C. These substances are also known as liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses (Welton, 1999; Endres and Zein El Abedin, 2006; Freemantle, 2009). The definition of ionic liquid is inclusive of such a wide scope and flexibility that they are said to be designer solvents (Freemantle, 1998). Ethanol ammonium nitrate, the first ionic liquid, was reported by S. Gabriel and J. Weiner in 1888 and then by Paul Walden in 1914 (Gabriel and Weiner, 1888; Walden, 1914). Later interest was generated with the discovery of chloroaluminates synthesized by combination of alkyl-substituted imidazolium and pyridinium cations with aluminum chlorides (Chum et  al., 1975; Wilkes et  al., 1982). Although these ionic liquids had several potential uses but also suffered from the major drawback of high moisture sensitivity. In 1990–1995, Wilkes and Zaworotko contributed majorly by synthesizing moisture stable ionic liquids by combining imidazolium and pyridinium cations with anions like hexafluorophosphate and tetrafluoroborate (Wilkes and Zaworotko, 1992). A number of different ionic liquids are available with unique features and applications. In general, cationic component in ionic liquids are thiazolium, pyridinium, ammonium, phosphonium, 1-alkyl pyridinium, 1-alkyl-3-methylimidazolium, triazolium, or 1-methyl-1-alkyl pyrrolidinium species (Handy, 2005; Kim and Varma, 2005; Hajipour et al., 2008; Cui et al., 2006; Hajipour et al., 2007; Li et al., 2006; Zhang et al., 2006;Yavari and Kowsari, 2007) whereas the anionic components are triflate, tetrafluoroborate, hexafluorophosphate, triflimide, bis(trifluorosulfonyl)amide, tosylate, and dicyanamide (Gong et al., 2008, 2009; Hajipour et al., 2008). Ionic liquids can be divided mainly into three generations as shown in Table 2.1 (Hough et al., 2007). First generation of ionic liquids is designed and synthesized by the careful variation of both cation and anions. These ionic liquids have unique physical properties such as thermal stability, Green Sustainable Process for Chemical and Environmental Engineering and Science DOI: https://doi.org/10.1016/B978-0-323-95156-2.00003-9

© 2023 Elsevier Inc. All rights reserved.

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non or low volatility, lower viscosity, hydrophobicity or hydrophilicity, large liquid range, etc. Second generation of ionic liquids has tunable physical properties combined with targeted chemical properties. These ionic liquids have useful applications in materials like energetic materials, lubricants, metal ions complexation, and many more because properties of ionic liquids can be modified by altering properties of anions and cations independently as per design and requirement of targeted materials. The ionic liquids of the third generation have used biologically active ions with low toxicity. These ionic liquids exhibited biological activities like antibiotic, antibacterial, anti-inflammatory, or local anesthetic effect (Hough et al., 2007). The distinctive properties of ionic liquids such as thermal stability, low combustibility, favorable solvating properties for a variety of nonpolar and polar compounds, high viscosity, low vapor pressure, poor electrical conductivity, and nonionizing properties make them attractive for wide range of applications. They serve as solvents and catalyst for a variety of chemical and biological reactions (Walker and Bruce, 2004). The different types of anions and side chain length on cations variate the miscibility of ionic liquids with water or other organic solvents. Ionic liquids are synthetic materials that can be tailor-made in the laboratory as per requirements of targeted chemical transformations with additional features of high efficiency, scalability, and being an inexpensive one.The general synthetic route for the synthesis of room temperature ionic liquids includes alkylation of heteroatom containing compound followed by metathesis.These steps of the reaction have some disadvantages like slow alkylation reaction, high temperatures, and slow anion metathesis. Therefore, nonconventional methods of synthesis involving microwave and sonication are used recently (Namboodiri and Varma, 2002; Varma and Namboodiri, 2001; Frejaville et  al., 1994). Several other methods have been reported, which include one-pot reactions having both alkylation and metathesis (Estager et al., 2007). These methods utilize alkyl sulfonates for alkylation reaction thereby eliminating the anion metathesis step (Narodai et al., 1998). Ionic liquids gained immense interest in the earlier years as green and recyclable solvents for many organic transformations (Hallett and Welton, 2011). Almost all organic reactions have been reported in ionic liquids. Multicomponent reactions are one of the most versatile and eco-friendly synthetic methods for organic transformations and it was observed that lot of multicomponent reactions are carried out in ionic liquids. Before we get into the details of some of the multicomponent reactions, let us have a brief idea of the multicomponent reactions. Multicomponent reactions may be termed as convergent reactions as the chemical reaction involves combination of three or more starting materials leading to the formation of a product in single step, in such a way that the majority of atoms of starting materials are found in the product. The quick and simple implementation, saving time and energy, environment friendliness and high atom efficiency are some of the key features that contribute to an ideal, target and diversity-oriented synthesis (Wender, 2014).

Chemistry of ionic liquids in multicomponent reactions

Multicomponent reaction (MCR)

Multicomponent reactions are of paramount importance in the field of synthetic organic chemistry. These reactions have been classified in a number of ways based on name reactions, nature of substrates, starting materials, number of reactants (components) involved in the reactions etc. The simplest and easiest classification of multicomponent reactions is the one based on the number of reactants (components) involved in the reactions. Accordingly, they are termed as three-component reactions and four-component reactions. Some common examples of multicomponent reactions are Strecker, Biginelli, Passerini, Ugi, Bienayme-Blackburn–Groebke, Mannich, Prins, Hantzsch reactions. Sustainable chemistry has been the driving force that has led to the design and development of eco-compatible and highly selective reaction sequences leading to structurally diverse molecular scaffolds. The synthesis of important and useful structurally diverse heterocyclic moieties through multicomponent reactions using ionic liquids either as solvents or as catalyst are best alternative methods due to advantages of multicomponent reactions as well as ionic liquids such as environmentally benign methodology, high atom economy, easy process, and cost-effective methods. In next section, we will discuss some important three- and four-component reactions using ionic liquids as solvents and as catalysts.

2.2  Three-component reactions using ionic liquids as solvents The most studied and well-known three-component reaction involving carbonyl compounds is Biginelli reaction in which 3,4-dihydropyrimidin-2-one derivatives are

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Table 2.1  Different generations of ionic liquids (ILs). Cations

Anions

First generation of ionic liquids R1 N

R1

N

R4

N R3

N N

O

Cl

PF6

FeCl4

O

R3 R2

S

HO

PH R2

R1 R4

O

R1

R1

N

R

F3C

N OH

O

O

S N

S

O

O

O CF3

R O

S

O

O O

BF4 O

Second generation of ionic liquids

Third generation of ionic liquids

obtained by reaction of aldehyde with urea and β-ketoester under acidic conditions in refluxing ethanol (Biginelli, 1893). First generation ionic liquids are used as alternative solvents to carry out Biginelli reaction and synthesize medicinally useful heterocyclic compounds. For instance, an efficient Biginelli reaction in ionic liquids was established for synthesis of pharmaceutically

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.1  Biginelli synthesis of quinazolin-2,5-dione derivatives using 1-n-butyl-3-methyl imidazolium bromide ionic liquid and solid supported acid catalyst.

valuable quinazolin-2,5-dione derivatives through one-pot reaction by using dimedone, aldehydes, and urea or thiourea in 1-n-butyl-3-methyl imidazolium bromide ionic liquid and silica sulfuric acid as a solid acid catalyst as shown in Scheme 2.1 (Shaabani et al., 2007). The targeted products were obtained in excellent yield with short reaction times. Moreover, ionic liquids were recyclable up to four times keeping yields close to 80%. The additional acid catalyst in Biginelli reaction was avoided when reaction was conducted in a Bronsted acidic ionic liquid. For example, dihydropyrimidine derivatives were synthesized by using acidic ionic liquids 1-n-butyl imidazolium tetrafluoroborate under ultrasound irradiation (Scheme 2.2) (Gholap et  al., 2004). The short reaction time, excellent yields and ambient temperature are some additional advantages of this ionic liquid. The detailed nuclear magnetic resonance and infrared spectroscopic studies suggested that acidic ionic liquid acted as both promotor and favorable reaction medium in Biginelli reaction. The combined synergistic effects of ultrasound activation and acidic ionic liquid are responsible for completion of reaction at room temperature in short times.

Scheme 2.2  Synthesis of dihydropyrimidine (DHPM) derivatives using 1-n-butyl imidazolium tetrafluoroborate [Hbim]Br under ultrasonic irradiation at room temperature.

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Scheme 2.3  One-pot synthesis of 2,3-dihydroquinazolin-4(1H)-one derivatives using [bmim]Br ionic liquid.

Another pharmaceutically valuable 2,3-dihydroquinazolin-4(1H)-ones were synthesized through one-pot reaction using 2-aminobenzothiazole, isatoic anhydride and different aldehydes in 1-n-butyl-3-methyl imidazolium bromide ionic liquid (Scheme 2.3). The target pharmacophore was obtained in 0.5 hours in excellent yields between 73% and 93%. Interestingly, the ionic liquid could be recycled up to five times with comparable yields of the product. It was worthy to note that trace amounts of desired products were obtained without using 1-n-butyl-3-methyl imidazolium bromide ionic liquid even at high temperatures (Shaabani et al., 2008). A three-component reaction was carried out in acidic ionic liquid which allowed the formation of 2-styryl quinazolinones under environmentally benign conditions through one-pot condensation reaction. In this synthesis, again isatoic anhydride along with different primary amines were reacted with triethyl orthoacetate in 1-methyl imidazolium trifluoroacetate resulting in corresponding 2-methyl quinazolinones and then followed by addition of aromatic aldehyde leading to targeted 2-styryl substituted products in good to excellent yields (Scheme 2.4).The successful formation of expected product revealed that acidic 1-methyl imidazolium trifluoroacetate ionic liquid could act both as an efficient solvent system as well as reaction promoter for heterocyclic synthesis (Zhao et al., 2004; Darvatkar et al., 2006).

Scheme 2.4  One-pot three-component reaction for synthesis of 2-styryl quinazolinones under environmentally benign conditions using 1-methyl imidazolium trifluoroacetate [Hmim]TFA ionic liquid.

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.5  Synthesis of 1-pyridylimidazo[1,5-a] pyridines using 1-n-butyl imidazolium tetrafluoroborate [Hbim]BF4 at 100°C.

Another valuable medicinally relevant heterocycle 1-pyridylimidazo[1,5-a] pyridines were efficiently synthesized in Brønsted acidic ionic liquid 1-n-butyl imidazolium tetrafluoroborate, through one-pot condensation of aromatic aldehyde, ammonium acetate and 1,2-dipyridylketone (Scheme 2.5). Moreover, the yield of reaction was intensely increased in comparison to similar reactions carried out in traditional solvents (Siddiqui et al., 2006). A three-component reaction led to the synthesis of pyrimido-[4,5-b] quinolines in different ionic liquids in high yield, which overwhelmed the problems faced in previous classical volatile organic solvents. Therefore, reaction between dimedone, 6-aminopyrimidine-2,4-dione, and aromatic aldehydes in 1-n-butyl-3-methyl imidazolium bromide resulted in corresponding pyrimido-[4,5-b] quinolines in short reaction time and easily isolated in good yields (Scheme 2.6). In another reaction,

Scheme 2.6  A three-component synthesis of pyrimido-[4,5-b] quinolines in high yields using 1-nbutyl-3-methyl imidazolium bromide [bmim]Br at 95°C.

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Scheme 2.7  Synthesis of pyrimido-[4,5-b] quinoline derivatives using 1-n-butyl-3-methyl imidazolium bromide [bmim]Br.

corresponding oxidized products indeno [20,10: 5,6] pyrido[2,3-d] pyrimidine was isolated by using phenylacetaldehyde instead of aromatic aldehyde in similar sequences of reagents (Scheme 2.7) (Shi et al., 2008, 2009). The method was an environment friendly procedure with merits like high yields and easy work-up in comparison to classical methods. 6-Methyl-4-hydroxypyran-2-one establishes an exciting raw material having unusual reactivity in the presence of ionic liquids. For instance, this substrate was condensed with primary amine or ammonium acetate and aldehyde leading to bislactam systems (Scheme 2.8) (Shi et al., 2008). The highest yield was obtained in 1-n-butyl3-methyl imidazolium bromide and reaction came to completion in less than 2 hours. Additionally, ionic liquid was recycled five times and desired product was obtained with similar yields up to 95%. In another set of reactions, the amine partner was changed with β-naphthol along with the use of p-toluene sulfonic acid as catalyst under similar reaction conditions giving tetrahydrobenzoxanthenone derivatives in less than 4 hours with high yields (Scheme 2.9) (Khurana and Magoo, 2009). Ionic liquid was proven to be an excellent solvent for the three-component reaction between malononitrile, cyanothioacetamide, and aldehyde leading to catalyst-free formation of thiopyran derivatives (Zhang et  al., 2009). In this reaction, 1-n-butyl3-methyl imidazolium tetrafluoroborate ionic liquid was found to be best solvent along with better yield and shorter reaction times. Moreover, high selectivity, simple work-up, reusability of catalyst and easy recovery are some additional advantages (Scheme 2.10).

Scheme 2.8  One-pot condensation to bislactam derivatives at 100°C using 1-n-butyl-3-methyl imidazolium bromide [bmim]Br.

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.9  Synthesis of tetrahydrobenzoxanthenone derivatives using p-toluene sulfonic acid p-TSA and [bmim]BF4.

Scheme 2.10  Synthesis of thiopyran derivatives in [bmim]BF4 solvent leading to better yield and high selectivity.

Isocyanide-based multicomponent reactions are most studied reactions due to its structurally diverse heterocyclic scaffolds having wider applications in biological and medicinal fields. These reactions are widely developed in conventional solvents especially three-component Passerini, conventional four-component Ugi, and threecomponent Bienayme–Blackburn–Groebke reactions and recently much attention have been focused on the uses of ionic liquids in these reactions. The Passerini reaction was discovered in 1921 by Passerini (Passerini and Simone, 1921) in which reaction between carboxylic acid, carbonyl compound, and an isocyanide offered direct access to α-hydroxy carboxamides. In this context, Passerini reaction was successfully carried out using ionic liquids 1-n-butyl-3-methyl imidazolium hexafluorophosphate as greener solvent system to get access of α-acyloxy carboxamides under mild reaction conditions as shown in Scheme 2.11. These environmentally benign solvents are efficient alternatives to volatile organic solvents. Furthermore, easy work-up procedures combined with possibility of easy recyclability and reusability of ionic liquids make them excellent solvents for further use in similar multicomponent reactions (Andrade et al., 2006). The imidazo[1,2-a] pyridine derivatives have attracted massive interest in medicinal and biomedical areas of relevance, in view of their utility as anti-inflammatory,

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Scheme 2.11  Three-component Passerini reaction for synthesis of α-acyloxy carboxamides in the presence of greener solvents.

antibacterial, anticancer, and antituberculosis agents. The imidazo[1,2-a] pyridine scaffolds are prevalent in several drugs such as Zolpidem (hypnotic), Olprinone (cardiotonic agent), Alpidem (a nonsedative anxiolytic), Levamisole (anticancer), Zolimidine (antiinflammatory). The imidazo[1,2-a] pyridine derivative was synthesized by three-component Bienayme–Blackburn–Groebke reaction by the condensation of an aldehyde, 2-aminoazine and an isocyanide using strong acid as catalyst.This reaction required long reaction time to obtain desired product as well as needed tedious isolation procedure for separation of product from impurities.The use of ionic liquids as solvent helped to overcome these problems and provide environmental benign methodology for this reaction. For example, the reaction of an isocyanide, an aldehyde, and 2-amino-5-methylpyridine was conducted in 1-n-butyl-3-methyl imidazolium bromide ionic liquid proceeded with high yields within a period of only 3 hours (Scheme 2.12) (Shaabani et al. 2006). Another one-pot three-component reaction between cyclohexyl isocyanide, dimethyl acetylenedicarboxylate, and different aldehydes in hydrophilic 1-n-butyl3-methyl imidazolium tetrafluoroborate ionic liquid afforded poly substituted 2-aminofuran derivatives in high yield in less than 2 hours. The enhanced reactivity of this reaction in 1-n-butyl-3-methyl imidazolium tetrafluoroborate was probably owing to

Scheme 2.12  Three-component Bienayme–Blackburn–Groebke reaction in presence of 1-n-butyl3-methyl imidazolium bromide [bmim]Br ionic liquid leading to high yield of imidazo[1,2-a] pyridine derivatives.

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.13  1-n-Butyl-3-methyl imidazolium tetrafluoroborate ionic liquids promoted synthesis of 2-aminofuran derivatives in high yield and low reaction times at room temperature.

the zwitter ionic intermediate formation from the addition of isocyanide on dimethyl acetylene dicarboxylate (Scheme 2.13) (Yadav et al., 2004). Liu et al. reported the asymmetric Mannich reaction of a series of methyl ketones, aromatic amines, and isovaleraldehyde mediated by highly efficient task-specific ionic liquids and L-proline leading to the corresponding β-aminoketones (Scheme 2.14). The adducts were obtained in high yields (up to 96%) and high enantiomeric excess values (up to >99%). The ionic liquids used in reaction were 1-diethylcarbamoylmethyl3-methyl imidazolium tetrafluoroborate, 1-diethylcarbamoylmethyl-3-ethyl imidazolium tetrafluoroborate, 1-n-butylcarbamoylmethyl-3-ethyl imidazolium tetrafluoroborate,1ethyl-3-(2-morpholin-4-yl-2-oxomethyl) imidazolium tetrafluoroborate, and 1-ethyl3-(2-piperidin-1-yl-2-oxomethyl) imidazolium tetrafluoroborate. All these ionic liquids along with L-proline showed good catalytic activity and selectivity. Among these ionic liquids, 1-diethylcarbamoylmethyl-3-methyl imidazolium tetrafluoroborate was found to act as the best reaction medium and could be recycled at least for three times without much loss of the catalytic activity (Liu et al., 2007). Zheng and Li prepared a series of task-specific ionic liquids which have Lewis basic nature. These ionic liquids included ethylenediammonium acetate, triethylenetetraammonium acetate, triethylenetetraammonium p-toluenesulfonate, triethylenetetraammonium tetrafluoroborate, and triethylenetetraammonium trifluoroacetate. The study of the catalytic activities of the five ionic liquids in the model reaction of dimedone, malononitrile, and 4-chlorobenzaldehyde proved that triethylenetetraammonium trifluoroacetate

Scheme 2.14  Synthesis of β-aminoketones through asymmetric Mannich reaction in the presence of task-specific ionic liquid 1-diethylcarbamoylmethyl-3-methyl imidazolium tetrafluoroborate [DEMim] [BF4]/L-proline as the best reaction medium.

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Scheme 2.15  Synthesis of tetrahydrobenzo [b] pyran through three-component reaction in presence of 5% triethylenetetraammonium trifluoroacetate [TETA][TFA].

Scheme 2.16  Synthesis of pyrano[c] chromene derivatives in the presence of 5% triethylenetetraammonium trifluoroacetate [TETA][TFA], the optimum catalyst.

was the optimum catalyst for such multicomponent reactions providing high yields up to 95% and in a low reaction time. Thus, a series of tetrahydrobenzo[b]pyran (Scheme 2.15) and pyrano[c]chromene (Scheme 2.16) derivatives were obtained by the reaction of different aromatic aldehydes, malononitrile, and cyclohexane-1,3-dione/4-hydroxy coumarin in the presence of triethylenetetraammonium trifluoroacetate as catalyst. The ionic liquid was recyclable up to eight uses thus making it better than any other catalyst, had short reaction times and good yields (Zheng and Li, 2011). The task-specific ionic liquid 2-hydroxyethanaminium acetate acted as a reusable and highly efficient catalyst for the environmentally benign synthesis of chromene derivatives by room temperature pulverization of the three-components aliphatic/ aromatic aldehydes, 4-hydroxycoumarin, and malononitrile. The 2-hydroxyethanaminium acetate exhibited environment friendly catalytic behavior with reusability up to four runs, excellent yields and shorter reaction times. (Scheme 2.17) (Shaterian and Honarmand, 2011).

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.17  Synthesis of chromene derivatives in the presence of 2-hydroxyethanaminium acetate HEAA.

Scheme 2.18  Synthesis of chromene derivatives in the presence of 3-hydroxypropanaminium acetate HPAA.

After the synthesis of 2-hydroxyethanaminium acetate, Shaterian et al. synthesized another novel ionic liquid 3-hydroxypropanaminium acetate which played the role of a solvent as well as a weak basic catalyst. It was again used in the synthesis of another chromene derivatives from the same set of reactants as used in Scheme 2.17 at room temperature by “Grindstone Chemistry.” 3-Hydroxypropanaminium acetate worked as a recoverable catalyst with good to excellent yield of products with much less reaction time and could be reused at least four times without considerable loss in catalytic activity (Scheme 2.18) (Shaterian and Oveisi, 2011).

2.3  Three-component reactions using ionic liquids as catalysts This section of the chapter deals with some selected examples in which ionic liquids are used either as catalyst or sub stoichiometric in multicomponent reactions. Most of the ionic liquids described in this section are task-specific ionic liquids that are designed for their acidic/basic characteristics. Some multicomponent reactions require harsh reaction conditions such as high temperature, strong acids/bases and toxic organic solvents. Task-specific ionic liquids as catalysts have potentials to promote these multicomponent reactions in milder reaction conditions combined with excellent yields and shorter reaction times. In this context, ionic liquid catalyzed Biginelli reaction was carried out under solvent-free conditions. The corresponding product of Biginelli reaction dihydropyrimidines was obtained in excellent yield within 30 minutes at 100°C in presence of 0.4 mol% of 1-n-butyl-3-methyl imidazolium hexafluorophosphate or

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Scheme 2.19  Ionic liquids N,N,N’,N’-tetramethyl-N,N’-bis(sulfo)ethane1,2-diaminium mesylate [TMBSED][OMs]2) promoted synthesis of 3-methyl-4-arylmethylene-isoxazole-5(4H)-ones under solventfree conditions.

1-n-butyl-3-methyl imidazolium tetrafluoroborate ionic liquid (Peng and Deng, 2001). Both cationic and anionic parts of ionic liquids play a critical role in completion of reaction as no product was obtained while using either n-Bu4Cl or 1-n-butyl-3-methyl imidazolium chloride ionic liquid as catalyst. Halogen-free and nontoxic ionic liquid like 1-n-butyl-3-methylimidazolium saccharinate (Ming et al., 2006) and tetraalkylammonium sulfonic acid/HSO4− (Dong et al., 2007) derivatives also catalyze the Biginelli reaction with even more eco-friendly pathway. A dicationic ionic liquid, N,N,N’,N’-tetramethyl-N,N’-bis(sulfo)ethane1,2-diaminium mesylate was used to synthesize 3-methyl-4-arylmethylene-isoxazole-5(4H)ones from reaction between arylaldehydes, hydroxylamine hydrochloride and ethyl acetoacetate under solvent-free conditions as shown in Scheme 2.19. This protocol had several advantages like good yields, easy purifying of products by crystallization and relatively short reaction times, simple experimental procedure, broad substrate range, clean reaction profile, and low cost (Irannejad-Gheshlaghchaei et al., 2018). Similarly, N-methylimidazolium perchlorate was proven to be excellent catalyst for the formation of 4,4′-[(4-chlorophenyl)-methylene] bis(3-methyl-1-phenyl1H-pyrazol-5-ol) under solvent-free conditions from the reaction of ethyl acetoacetate, 4-chloro-benzaldehyde and phenylhydrazine (Scheme 2.20). The detailed experimental studies revealed that superior catalytic performance was obtained because of synergistic effect of perchlorate and imidazolium cation. This methodology has several advantages like simple operation procedure, high yields, short reaction times, and clean transformations. Additionally, N-methylimidazolium perchlorate was recycled and then reused without much loss of activity (Khaligh et al., 2016). The magnetic ionic liquids 1-n-butyl-3-methylimidazolium tetrachloroferrate has gained considerable attention due to its remarkable characteristics which includes high catalytic activity, ease of synthesis and high stability. Moreover, due to the paramagnetic

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.20  N-methylimidazolium perchlorate catalyzed solvent-free 4,4′-[(4-chlorophenyl)-methylene] bis(3-methyl-1-phenyl-1H-pyrazol-5-ol).

synthesis

of

Scheme 2.21  Magnetic ionic liquids 1-n-butyl-3-methylimidazolium tetrachloroferrate [bmim]FeCl4 promoted synthesis of quinazoline derivatives under solvent-free conditions.

FeCl4− anion in ionic liquid, it responds to presence of a magnet. The 1-n-butyl-3-methylimidazolium tetrachloroferrate ionic liquid was used as catalyst to produce quinazoline derivatives from substituted benzaldehydes, 2-aminoarylketones and ammonium acetate at moderate temperature under solvent-free conditions in excellent yields as shown in Scheme 2.21 (Panja and Saha, 2013). Another more work on the similar lines, elaborated a green and convenient procedure for the synthesis of 4H-pyran derivatives through three-component reaction of diversely substituted aldehydes, 1,3-diketones and malononitrile using 20 mol% 1-n-butyl-3-methyl imidazolium hydroxide as a basic task-specific ionic liquid at 50–60°C (Scheme 2.22). They also used the optimized 20 mol% of [bmim]OH in the cyclocondensation of various aldehydes, malononitrile, and pyrazolone, leading to formation of pyrazole derivatives (Scheme 2.23). The basic ionic liquid led to an environmentally benign procedure with ease of recovery, easily reusable up to three times, good yields and shorter reaction times (Khurana and Chaudhary, 2012). A novel environmental friendly methodology was undertaken for the synthesis of spiro[dibenzo[a,i]-xanthene-14,3′-indoline]-2′,8,13-triones via cyclo-condensation of isatin, 2-hydroxynaphthalene-1,4-dione and β-naphthol using catalytic amount of 1-hexyl-3-methylimidazolium hydrogen sulfate (Scheme 2.24). The one-pot

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Scheme 2.22  One-pot environmentally benign synthesis of 4H-pyran derivatives in the presence of basic task-specific ionic liquid, 1-n-butyl-3-methyl imidazolium hydroxide [bmim]OH.

Scheme 2.23  One-pot synthesis of 4H-pyrano[2,3-c] pyrazole derivatives via three-component reaction in the presence of 1-n-butyl-3-methyl imidazolium hydroxide ionic liquid.

Scheme 2.24  Synthesis of spiro[dibenzo[a,i]-xanthene-14,3′-indoline]-2′,8,13-triones in the presence of 1-hexyl-3-methyl imidazolium hydrogen sulfate under solvent-free conditions.

three-component condensation of isatin derivatives, β-naphthol and barbituric acids with 1-hexyl-3-methylimidazolium hydrogen sulfate under solvent-free conditions at 100°C affording spironaphthopyrano[2,3-d] pyrimidine-5,3′-indolines was also carried out (Scheme 2.25). On systematically evaluating the effect of a variety of catalysts such as sulfuric acid, hydrochloric acid, p-TsOH, NH2SO3H, ferric chloride, zinc chloride, 1-n-butyl-3-methyl imidazolium bromide, 1-hexyl-3-methyl imidazolium hydrogen sulfate, 1-ethyl-3-methyl imidazolium hydrogen sulfate, and 1-n-butyl-3-methyl imidazolium hydrogen sulfate on the yield of spiro[dibenzo[a,i]-xanthene14,3′-indoline]2′,8,13-triones, the results showed that the reaction proceeded with good yields and low reaction times in 1-ethyl-3-methyl imidazolium hydrogen sulfate, 1-n-butyl3-methyl imidazolium hydrogen sulfate, and 1-hexyl-3-methyl imidazolium hydrogen sulfate ionic liquids. The reaction did not occur efficiently in 1-n-butyl-3-methyl imidazolium bromide. This confirmed to the fact that catalytic activity was independent of the cation and dependent on the Bronsted acidity of the hydrogen sulfate counteranion.

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.25  One-pot solvent-free synthesis of spironaphthopyrano[2,3-d] pyrimidine-5,3′-indolines in the presence of 1-hexyl-3-methyl imidazolium hydrogen sulfate.

After completion of reaction, the reaction mixture was worked up with water and the insoluble crude products were separated by filtration and pure products were attained by recrystallization. The catalyst was recovered and reused for three cycles of reactions (Yang et al., 2012). The merits of the ionic base, tetrabutylammonium hydroxide over the classical organic amine base piperidine in terms of catalytic activity, were enumerated in the synthesis of pyridine-3,5-dicarbonitriles scaffolds through the three-component reaction of malononitrile, thiophenols, and aldehydes with diverse substituents. The ionic (Ranu et al., 2007) and nitrogenated (Evdokimov et al., 2006) bases lead to similar yields, but using 50 mol% tetrabutylammonium hydroxide in acetonitrile resulted in lesser reaction times (1 hour) whereas 30 mol% piperidine resulted in longer reaction times (24 hours) (Scheme 2.26) (Guo et al., 2009). Task-specific basic ionic liquid, 1-n-butyl-3-methyl imidazolium hydroxide in aqueous system was found to be a good catalytic medium for the one-pot condensation of substituted acid chlorides, different amino acids, and dialkylacetylenedicarboxylates affording highly functionalized pyrroles (Scheme 2.27) (Yavari and Kowsari, 2008). The 1-n-butyl-3-methyl imidazolium tetrafluoroborate catalyst was less effective catalyst in comparison to 1-n-butyl-3-methyl imidazolium hydroxide thus proving the vital role of the hydroxy counterion of the catalyst in the reaction. Thus, it was an eco-friendly protocol offering significant advantages such as low reaction times, excellent yields, green and benign solvent usage, easy procedure and reusability over the classical Hantzsch method for the synthesis of pyrroles.

Scheme 2.26  Ionic liquid base, tetrabutylammonium hydroxide and nitrogen base, piperidine catalyzed multicomponent synthesis of pyridine-3,5-dicarbonitriles.

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Scheme 2.27  Synthesis of polysubstituted pyrroles in the presence of 1-n-butyl-3-methyl imidazolium hydroxide–aqueous [bmim]OH–H2O system.

Scheme 2.28  Bronsted acidic ionic liquid 3-methyl-1-(4-sulfonylbutyl) imidazolium hydrogen sulfate catalyzed synthesis of 2,4,6-triarylpyridines under solvent-free conditions.

Pharmacological importance of Kröhnke pyridines has led to the development of multicomponent reaction of acetophenones, aryl halides and ammonium acetate using Bronsted-acidic ionic liquids 3-methyl-1-(4-sulfonylbutyl) imidazolium hydrogen sulfate [HO3S(CH2)4MIM] [HSO4] under solvent-free conditions (Scheme 2.28). The introduction of sulfonic acid groups into the cationic or anionic counterpart of the ionic liquids, increased their water solubilities and acidic character. This modified form of ionic liquid, that is, Bronsted acidic ionic liquid can be utilized as highly efficient, reusable, and green acid catalyst and can be used as a substitute for conventional catalysts like sulfuric acid, hydrofluoric acid, and aluminum chloride in different chemical processes. The efficiency of the reaction was examined for different parameters like amount of catalyst, temperature, and different solvents. It was found that the reaction led to better yields with 20 mol% of Bronsted acidic ionic liquid, optimum temperature of 120°C and under solvent-free conditions. Thus, a very simple method for synthesis of 2,4,6-triarylpyridines was developed in presence of an eco-friendly homogeneous catalyst under solvent-free conditions, with the merits like easy work up procedure, good to excellent yields with low reaction times, reusable with a little reduction in catalytic activity, absence of hazardous and volatile organic solvents (Davoodnia et al., 2010).

2.4  Four-component reactions in ionic liquids as solvents In this section of the chapter, selected examples of four-component reactions using ionic liquids as solvent and their advantage over conventional methodology are discussed in details. Ionic liquids-mediated four-component reactions offer improved ecofriendly procedure, high atom and step economy and high structural diversity which

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.29  Four-component reaction for facile synthesis of 1,4-dihydropyridine derivatives in the presence of [bmim]BF4.

Scheme 2.30  Synthesis of tetra-substituted imidazoles derivatives in N-methyl-2-pyrrolidonium hydrogen sulfate at 100°C.

results in facile access of highly decorated heterocyclic scaffolds. For instance, facile synthesis of substituted 1,4-dihydropyridine derivatives was accomplished by reaction between Meldrum's acid, aromatic or aliphatic aldehydes, 1,3-dicarbonyl compounds, and ammonium acetate as source of nitrogen in 1-n-butyl-3-methyl imidazolium tetrafluoroborate ionic liquid (Scheme 2.29) (Zhang et al., 2006). Imidazole derivatives are biologically and pharmaceutically active analogues and core moiety of several drugs molecules (Breslow, 1995). The substituted imidazole derivatives were synthesized through one-pot four-component reaction in ionic liquid. For example, 1,2,4,5-tetrasubstituted imidazoles derivative was synthesized from the reaction between benzyl, ammonium acetate, substituted benzaldehydes and phenyl hydrazine in N-methyl-2-pyrrolidonium hydrogen sulfate at 100°C (Scheme 2.30) (Shaterian and Ranjbar, 2011). The use of ionic liquids was found to much better than all previously reported solvents. A rapid and efficient access of 4H-pyrano[2,3-c] pyrazoles were achieved by cyclocondensation reaction of ethyl acetoacetate, aldehydes, phenyl hydrazine, and malononitrile and using L-proline as catalyst at room-temperature in 1-n-butyl-3-methyl imidazolium tetrafluoroborate ionic liquids. Moreover, recyclability of 1-n-butyl-3-methyl imidazolium tetrafluoroborate ionic liquid without remarkable loss of activity was an additional advantage (Scheme 2.31) (Khurana et al., 2011). Another similar four-component condensation reaction between ethyl acetoacetate, substituted aldehydes, hydrazine monohydrate, and malononitrile in the presence

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Scheme 2.31  Synthesis of 4H-pyrano[2,3-c]pyrazoles through condensation reaction using L-proline as catalyst in [bmim]BF4 ionic liquid.

Scheme 2.32  Preparation of 4H-pyrano[2,3-c] pyrazoles in 20 mol%1-n-butyl-3-methyl imidazolium hydroxide [bmim]OH, a task-specific ionic liquid.

of 20 mol% task-specific ionic liquid 1-n-butyl-3-methyl imidazolium hydroxide at an optimum temperature of 50–60°C achieved the formation of 4H-pyrano[2,3-c] pyrazoles (Scheme 2.32). These pyrazole derivatives could also be obtained by a threecomponent reaction which has been mentioned earlier in this chapter. It was found that both the three- and four-component reactions led to comparative yields, but threecomponent condensation required shorter time for completion of reaction (Khurana and Chaudhary, 2012). In another report, ionic liquid 1-n-butyl-3-methyl imidazolium tetrafluoroborate was used as solvent for the synthesis of chromeno[2,3-d] pyrimidin-8-amines in reasonable yields from the simple starting materials of malononitrile, α-naphthol, aryl aldehyde, and ammonium chloride in the presence of trace amounts of triethyl amine as depicted in Scheme 2.33. The simple and easy isolation of target analogue along with better yield are merits of this methodology (Kanakaraju et al., 2012). The[bmim]BF4 ionic liquid was also used as solvent for synthesis of different 2H-indazolo[2,1-b]phthalazinetriones through one-pot four-component reaction of phthalic anhydride, aldehyde, hydrazine and dimedone under ultrasonic irradiation in excellent yield without using any catalyst (Scheme 2.34). The present methodology was superior to the other reported procedures (Shekouhy and Hasaninejad, 2012).

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.33  Synthesis of chromeno[2,3-d]pyrimidin-8-amines derivatives in the presence of [bmim] BF4 as solvent.

Scheme 2.34  One-pot four-component reaction for synthesis of 2H-indazolo[2,1-b]phthalazinetriones using 1-n-butyl-3-methyl imidazolium tetrafluoroborate ionic liquid under ultrasonic irradiation.

2.5  Four-component reactions in ionic liquids as catalysts There are few examples available in the literature in which ionic liquids are successfully utilized as catalyst in solvent-free conditions to conduct one-pot four-component reactions. In this context, the Hantzsch reaction for successful preparation of polyhydroquinoline derivatives was carried out using 1-hexyl-3-methyl imidazolium tetrafluoroborate as catalysts (Scheme 2.35). The use of 1-hexyl-3-methyl imidazolium tetrafluoroborate shortened reaction times hours to a few minutes along with a drastic increase in the product yields (Ji et al., 2004). The pyrroles derivatives were synthesized from one-pot four-component reaction using a reusable ionic liquid as reaction medium. For example, coupling of different amines, 1,3-pentanedione, nitromethane and aromatic aldehydes in the presence of n-butylimidazolium tetrafluoroborate afforded tetra-substituted pyrroles in good yields as shown in Scheme 2.36 (Meshram et al., 2013). Some biologically important 1,2,4,5-tetrasubstituted imidazoles were synthesized through one-pot four-component cyclo-condensation of benzil, diversely substituted aldehydes, aliphatic/aromatic amines, and ammonium acetate in the presence of 1,3-disulfonic acid imidazolium hydrogen sulfate dual catalyst at 90°C under

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Scheme 2.35  Ionic liquid [hmim]BF4 catalyzed Hantzsch synthesis of polyhydroquinoline derivatives with shorter reaction times.

Scheme 2.36  One-pot four-component synthesis of tetra-substituted pyrroles in the presence of 1-nbutyl imidazolium tetrafluoroborate, a reusable ionic liquid.

Scheme 2.37  1,3-Disulfonic acid imidazolium hydrogen sulfate [Dsim]HSO4 catalyzed preparation of 1,2,4,5-tetrasubstituted imidazoles under solvent-free conditions.

solvent-free conditions (Scheme 2.37). Molecular self-assembly is generated through hydrogen bond donors which was the characteristic feature of the Bronsted acidic ionic liquid 1,3-disulfonic acid imidazolium hydrogen sulfate. Benzil and aldehydes were activated by such assemblies by the ionic liquid and simultaneously activated the nucleophile, that is, amine and ammonium acetate to attack the aldehydes and benzil at the carbonyl carbon. This twofold catalytic ability of 1,3-disulfonic acid imidazolium hydrogen sulfate had improved its efficiency. The pivotal points of this methodology were excellent yields, easy workup, high efficiency and reusable catalyst for four successive runs (Zolfigol et al., 2013).

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.38  Synthesis of 1,2,4,5-tetrasubstituted imidazoles in 15 mol% of ionic liquid 3-methyl-1-(4sulfonic acid) butyl imidazolium hydrogen sulfate under solvent-free conditions.

Tetrasubstituted imidazoles were also synthesized using the same four components as utilized by Zolfigol et al. but in the presence of 15 mol% of 3-methyl-1-(4-sulfonic acid) butyl imidazolium hydrogen sulfate [(CH2)4SO3HMIM][HSO4], a novel Bronsted acidic ionic liquid which behaved as a catalyst under solvent-free conditions (Scheme 2.38). An eco-friendly protocol was developed with no use of hazardous organic solvents, completion of reaction in lesser time, easy workup procedure, reuse of catalyst with little decline in its activity and high yield of products (Davoodnia et al., 2010).

2.6  Solid support ionic liquids Ionic liquids although have a large number of unique properties as mentioned in the earlier sections of the chapter, but their widespread applications in various organic transformations was also hampered by the huge number of disadvantages like homogeneous reaction and high viscosity. Due to high viscosity only a small part of the ionic liquid took part in the reaction and being a homogeneous reaction the separation and reuse procedures were tedious. Thus in order to overcome these issues, a solid support ionic liquid has been developed as a novel solid catalyst having all the advantages of ionic liquid incorporated in it. One such novel solid support recyclable catalyst was 1-methyl-3-(3trimethoxysilylpropyl)-1H-imidazol-3-ium chloride supported nano-Fe3O4@SiO2 used in the synthesis of a series of 1,3-thiazolidin-4-one compounds through an easy, environment friendly and efficient one-pot three-component cyclo-condensation of substituted anilines, diversely functionalized arylaldehydes and thioglycolic acid under solvent-free conditions at an optimum temperature of 70°C (Scheme 2.39). The nanoFe3O4@SiO2 supported ionic liquid was supermagnetic and could be separated from the reaction mixture by a permanent magnet. The recovered catalyst could be reused up to 10 successive cycles without major loss in their catalytic activity was the advantage offered by this procedure, along with the usual advantages offered by an ionic liquid catalyzed reaction (Azgomi and Mokhtary, 2015).

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Scheme 2.39 Synthesis of 1,3-thiazolidin-4-one compounds in the presence of 1-methyl-3-(3trimethoxysilylpropyl)-1H-imidazol-3-ium chloride supported nano-Fe3O4@SiO2 (MNP@SiO2-IL) under solvent-free conditions.

The ferrocene labeled supported ionic liquid phase containing L-prolinate anion catalyst, grafted on Merrifield resin was utilized as an efficient catalyst for the three-component green synthesis of 1-amidoalkyl-2-naphthols from aryl aldehydes, acetamide/urea/benzamide and 2-naphthol at 100°C under solvent-free conditions (Scheme 2.40). The solid support ionic liquids could be readily recovered by simple filtration and could be reused up to five runs without significant loss in the yield of the products. The supported ionic liquid phase catalyst was prepared in three steps which included the synthesis of ferrocene labeled supported ionic liquid phase containing chloride anion followed by the synthesis of ferrocene labeled supported ionic liquid

Scheme 2.40  Synthesis of 1-amidoalkyl-2-naphthols promoted by ferrocene labeled supported ionic liquid phase containing L-prolinate anion catalyst [FemSILP]L-prolinate.

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.41 Polystyrene-supported 1-methylimidazolium tetrachloroferrate PS[mim][FeCl4] promoted synthesis of 2-amino-4H-chromene-3-carbonitrile in water.

phase containing hydroxide anion and this finally led to ferrocene labeled supported ionic liquid phase containing L-prolinate anion catalyst (Rashinkar and Salunkhe, 2010). A heterogeneous catalyst, polystyrene-supported 1-methylimidazolium tetrachloroferrate was utilized in the synthesis of 2-amino-4H-chromene-3-carbonitriles having potential medicinal applications, through the cyclo-condensation reaction of malononitrile, various aromatic aldehydes and dimedone (Scheme 2.41) or 2-naphthol (Scheme 2.42) in water at 80°C. The reaction with this heterogeneous catalyst was an environmentally benign procedure with several advantages like better catalytic efficiency, easy recovery of the catalyst with reuse up to five times with almost no loss in the yields of the product, good yield in short reaction times, etc. (Taheri et al., 2018). The novel bifunctional periodic mesoporous organosilica-based ionic liquid containing sulfonic acid group was prepared and investigated for its catalytic activity on one-pot Biginelli condensation of different aldehydes with urea and alkylacetoacetates under solvent-free conditions at 75°C (Scheme 2.43). The bifunctional periodic mesoporous

Scheme 2.42 Polystyrene-supported 1-methylimidazolium tetrachloroferrate PS[mim][FeCl4] promoted synthesis of 2-amino-4H-chromene-3-carbonitrile derivatives in water.

Scheme 2.43  Bifunctional periodic mesoporous organosilica-based ionic liquid containing sulfonic acid group (BPMO–IL–SO3H) nanocatalyst catalyzed one-pot synthesis of dihydropyrimidone derivatives under solvent-free conditions.

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organosilica-based ionic liquid containing sulfonic acid group catalyst gave good yields of the corresponding dihydropyrimidone products with excellent selectivities within 50–110 minutes. The large surface area of bifunctional periodic mesoporous organosilicabased ionic liquid material, ionic liquid nature of mesopores with sulfonic acid groups are probably responsible for the high reactivity and stability of the catalyst. Under applied reaction conditions, the catalyst could be recovered and reused up to 10 successive runs without significant loss of yield and selectivity of product (Elhamifar et al., 2014).

2.7  Biodegradable ionic liquids Of the various merits of the ionic liquids, another more feature biodegradability was added to the same thus making ionic liquids a green reaction media for the multicomponent reactions. Thus, a simply efficient and environmentally friendly protocol involving novel basic, readily accessible, biodegradable, and cost-effective ionic liquid choline hydroxide as catalyst was used for the synthesis of tetrahydrobenzo[b]pyrans, owing to the broad spectrum biological and pharmacological properties of the oxygen containing heterocycles, in aqueous solution (Scheme 2.44). It was a one-pot reaction between aromatic aldehydes, active methylene compounds, and dimedone and choline hydroxide as catalyst. The desired yield of products was excellent. The biodegradability, easy recovery, and reusability up to six times make it a practical alternative to the previously known conventional methods for the synthesis of 4H-pyran derivatives (Hu et al., 2014). Another biodegradable and recyclable organocatalyst, tetrabutylammonium glycinate ionic liquid was used for efficient and selective synthesis of 3-substituted indoles and indolyl-4H-chromenes. Ten mol% tetrabutylammonium glycinate catalyzed onepot three-component condensation of substituted aldehydes (aliphatic/aromatic) with malononitrile and indoles led to excellent yields of 3-substituted indole derivatives under solvent-free conditions at 60°C (Scheme 2.45). Under above-mentioned reaction

Scheme 2.44  Synthesis of 4H-pyran derivatives using choline hydroxide as catalyst in aqueous solution.

Scheme 2.45  Tetrabutylammonium glycinate [TBA][Gly] promoted synthesis of 3-substituted indoles under solvent-free conditions.

Chemistry of ionic liquids in multicomponent reactions

Scheme 2.46  Synthesis of indolyl-4H-chromenes derivatives using tetrabutylammonium glycinate [TBA][Gly].

Scheme 2.47  Synthesis of indolyl-4H-chromenes derivatives using tetrabutylammonium glycinate [TBA][Gly].

conditions and catalyst, indolyl-4H-chromenes derivatives were attained in excellent yields by reaction between active methylene compounds, salicylaldehyde, and substituted indoles (Schemes 2.46 and 2.47). The catalyst could be recycled till six successive runs without significant loss in catalytic activity. It was found to be a greener protocol with high atom economy and metal-free reaction (Chinna Rajesh et al., 2015).

2.8  Ionic liquids in nanoform A novel green, recoverable and recyclable heterogeneous catalyst, Zr metal–organic framework functionalized with Bronsted acidic ionic liquid like triethylenediamine or imidazole is used for the synthesis of pharmacologically active dihydropyrido[2,3-d] pyrimidine derivatives via three-component reaction of different aromatic aldehydes, 6-amino-1,3-dimethyl uracil, and acetyl acetone under solvent-free conditions at a temperature of 100°C. The catalyst could be reused up to six times without considerable loss in its catalytic activity. The Bronsted acidic ionic liquid @ UiO-66 through a firm co-ordinate bond enhanced the electrophilicity of the carbonyl group present in aldehyde and acetyl acetone. It is possible to separate and purify the respective products easily using crystallization. We can recycle the catalysts six times without losing any major activity. Also, the characterization of the catalyst was done by energy-dispersive X-ray, field emission scanning electron microscopy, Fourier transform infrared, Brunauer−Emmett−Teller, X-ray diffraction, and thermogravimetric analysis analyses (Mirhosseini-Eshkevari et al., 2019) (Scheme 2.48).

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Scheme 2.48  Synthesis of dihydropyrido[2,3-d] pyrimidine derivatives using Zr metal–organic framework functionalized with Bronsted acidic ionic liquid like triethylenediamine or imidazole (TEDA/IMIZBAIL@UiO-66) under solvent-free conditions.

2.9 Conclusion In conclusion, we have demonstrated different generations of ionic liquids and their specific physical and targeted chemical properties like low volatility, significant thermal stability, low viscosity, and large liquid range. These ionic liquids have substantial utility in three- and four-component reactions. These multicomponent reactions result synthesis of important and useful structurally diverse heterocyclic scaffolds and use of ionic liquid as promotor or as solvent provide best alternative methods with advantage of environmentally benign methodology, high atom economy, easy process, and cost-effective methods. Overall, the ionic liquids-mediated multicomponent reaction approach is an exceptionally useful, especially for easy and rapid synthesis of medicinally useful structurally divergent heterocyclic molecules in excellent yields.

Abbreviations RTIL room temperature ionic liquids ILs ionic liquids TSIL task-specific ionic liquids MCR multicomponent reactions 3CR three-component reaction 4CR four-component reaction SSA silica sulfuric acid DHPM dihydropyrimidine p-TSA p-toulene sulfonic acid DMAD dimethyl acetylenedicarboxylate NMR nuclear magnetic resonance IR infrared spectroscopy [bmim]Br 1-n-butyl-3-methyl imidazolium bromide [bmim]OH 1-n-butyl-3-methyl imidazolium hydroxide [bmim]BF4 1-n-butyl-3-methyl imidazolium tetrafluoroborate

Chemistry of ionic liquids in multicomponent reactions

[bmim]PF6 [bmim]Cl [bmim]Sac [bmim]FeCl4 [Hbim]BF4 [Hmim]TFA [Hbim]BF4 [hmim][HSO4] [hmim]BF4 [emim][HSO4] [bmim][HSO4] [DEMIm][BF4] tetrafluoroborate [DEEIm][BF4]

1-n-butyl-3-methyl imidazolium hexafluorophosphate 1-n-butyl-3-methyl imidazolium chloride 1-n-butyl-3-methyl imidazolium saccharinate, 1-n-butyl-3-methylimidazolium tetrachloroferrate 1-n-butyl imidazolium tetrafluoroborate 1-methyl imidazolium trifluoroacetate 1-n-butyl imidazolium tetrafluoroborate 1-hexyl-3-methyl imidazolium hydrogen sulfate 1-hexyl-3-methyl imidazolium tetrafluoroborate 1-ethyl-3-methyl imidazolium hydrogen sulfate 1-n-butyl-3-methyl imidazolium hydrogen sulfate 1-diethylcarbamoylmethyl-3-methyl imidazolium

1-diethylcarbamoylmethyl-3-ethyl imidazolium tetrafluoroborate [BEIm][BF4] 1-n-butylcarbamoylmethyl-3-ethyl imidazolium tetrafluoroborate [MEIm][BF4] 1-ethyl-3-(2-mor pholin-4-yl-2-oxomethyl) imidazolium tetrafluoroborate [PEIm][BF4] 1-ethyl-3-(2-piper idin-1-yl-2-oxomethyl) imidazolium tetrafluoroborate [EDA][OAc] ethylenediammonium acetate [TETA][OAc] triethylenetetraammonium acetate [TETA][PTS] triethylenetetraammonium p-toluenesulfonate [TETA][BF4] triethylenetetraammonium tetrafluoroborate [TETA][TFA] triethylenetetraammonium trifluoroacetate HEAA 2-hydroxyethanaminium acetate HPAA 3-hydroxypropanaminium acetate [Ch][OH] choline hydroxide [TBA][Gly] tetrabutylammonium glycinate [TMBSED][OMs]2 N, N, N, N’-tetramethyl-N, N’-bis(sulfo)ethane1,2diaminium mesylate [MIm]ClO4 N-methylimidazolium perchlorate TBAH tetrabutylammonium hydroxide [NMP]HSO4 N-methyl-2-pyrrolidonium hydrogen sulfate [Dsim]HSO4 1,3-disulfonic acid imidazolium hydrogen sulfate [(CH2)4SO3HMIM][HSO4] 3-methyl-1-(4-sulfonic acid) butyl imidazolium hydrogen sulfate MNP@SiO2-IL 1-methyl-3-(3-trimethoxysilylpropyl)-1H-imidazol-3-ium chloride supported nano-Fe3O4@SiO2 SILP supported ionic liquid phase

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[FemSILP]L-prolinate

supported ionic liquid phase (SILP) containing L-prolinate anion catalyst (PS[mim][FeCl4]) polystyrene-supported 1-methylimidazolium tetrachloroferrate BPMO–IL–SO3H bifunctional periodic mesoporous organosilicabased ionic liquid containing sulfonic acid group TEDA/IMIZ-BAIL@UiO-66  Zr metal–organic framework functionalized with Bronsted acidic ionic liquid like triethylenediamine or imidazole

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Shi, D.-.Q., Ni, S.-.N., Yang, F., Ji, S.-.J., 2008. An efficient and green synthesis of 3,3′-benzylidenebis (4-hydroxy-6-methylpyridin-2(1H)-one) derivatives through multi-component reaction in ionic liquid. J. Heterocycl. Chem. 45, 1275–1280. doi:10.1002/jhet.5570450503. Shi, D.-.Q., Ni, S.-.N.,Yang, F., Shi, J.-.W., Dou, G.-.L., Li, X.-.Y., Wang, X.-.S., Ji, S.-.J., 2008. An efficient synthesis of pyrimido[4,5-b]quinoline and indeno[2′,1′:5,6]pyrido[2,3-d]pyrimidine derivatives via multicomponent reactions in ionic liquid. J. Heterocycl. Chem. 45, 693–702. doi:10.1002/jhet.5570450310. Shi, D.-.Q., Yang, F., Ni, S.-.N., 2009. A facile synthesis of furo[3,4-e]pyrazolo[3,4-b]pyridine-5(7H)-one derivatives via three-component reaction in ionic liquid without any catalyst. J. Heterocycl. Chem. 46, 469–476. doi:10.1002/jhet.103. Siddiqui, S.A., Potewar, T.M., Lahoti, R.J., Srinivasan, K.V., 2006. Ionic liquid promoted facile one-pot synthesis of 1-pyridylimidazo[1,5-a]pyridines from dipyridylketone and aryl aldehydes. Synthesis 17, 2849–2854. doi:10.1055/s-2006-942522. Taheri, N., Fallah-Mehrjardi, M., Sayyahi, S., 2018. Polystyrene-supported 1-methylimidazolium tetrachloro ferrate: synthesis, characterization, and application as an efficient and reusable heterogeneous catalyst for one-pot synthesis of 4h-chromene derivatives in aqueous media. Bull. Chem. Soc. Ethiop. 32 (3), 531–540. doi:10.4314/bcse. v32i3.12. Varma, R.S., Namboodiri, V.V., 2001. An expeditious solvent-free route to ionic liquids using microwaves. Chem. Commun. 7, 643–644. doi:10.1039/B101375K. Walden, P., 1914. Molecular weights and electrical conductivity of several fused salts. Bull. Acad. Imper. Sci. St Petersburg, 8, 405–422. Walker,A.J., Bruce, N.C., 2004. Cofactor-dependent enzyme catalysis in functionalized ionic solvents. Chem. Commun. 22, 2570–2571. doi:10.1039/b410467f. Welton, T., 1999. Room-temperature ionic liquids. Chem. Rev. 99 (8), 2071–2084. doi:10.1021/cr980032t. Wender, P.A., 2014. Toward the ideal synthesis and molecular function through synthesis-informed design. Nat. Prod. Rep. 31, 433–440. doi:10.1039/c4np00013g. Wilkes, J.S., Levisky, J.A.,Wilson, R.A., Hussey, C.L., 1982. Dialkylimidazolium chloroaluminate melts: a new class of room-temperature ionic liquids for electrochemistry, spectroscopy and synthesis. Inorg. Chem. 21 (3), 1263–1264. doi:10.1021/ic00133a078. Wilkes, J.S., Zaworotko, M.J., 1992. Air and water stable 1-ethyl-3-methylimidazolium based ionic liquids. Chem. Commun. 13, 965–967. doi:10.1039/c39920000 965. Yadav, J.S., Subba Reddy, B.V., Shubashree, S., Sadashiv, K., Naidu, J.J., 2004. Ionic liquids-promoted multicomponent reaction: green approach for highly substituted 2-aminofuran derivatives. Synthesis, 2376–2380. doi:10.1055/s-2004-831181. Yang, X., Yang, L., Wu, L., 2012. [hmim][HSO4]: an efficient and reusable catalyst for the synthesis of spiro[dibenzo[a,i]-xanthene-14,30-indoline]-20,8,13-triones and spironaphthopyran[2,3-d]pyrimidine5,30-indolines. Bull. Korean Chem. Soc. 33, 714–716. doi:10.5012/bkcs.2012.33.2.714. Yavari, I., Kowsari, E., 2007. Ionic liquids as novel and recyclable reaction media for N-alkylation of amino-9,10-anthraquinones by trialkyl phosphites. Tetrahedron Lett. 48, 3753–3756. doi:10.1016/j. tetlet.2007.03.077. Yavari, I., Kowsari, E., 2008.Task-specific basic ionic liquid: a reusable and green catalyst for one-pot synthesis of highly functionalized pyrroles in aqueous media. Synlett 6, 897–899. doi:10.1055/s-2008-1042912. Zhang, J., Jiang, T., Han, B., Zhu, A., Ma, X., 2006. Knoevenagel condensation catalyzed by1,1,3,3tetramethylguanidium lactate. Synth. Commun. 36, 3305–3317. doi:10.1080/00397 910600941190. Zhang, X., Li, X., Fan, X., Wang, X., Li, D., Qu, G., Wang, J., 2009. Ionic liquid promoted preparation of 4H-thiopyran and pyrimidine nucleoside-thiopyran hybrids through one-pot multi-component reaction of thioamide. Mol. Diversity 13, 57. doi:10.1007/s11030-008-9098-4. Zhang, X.Y., Li, Y.Z., Fan, X.S., Qu, G.R., Hu, X.Y., Wang, J., 2006. Multicomponent reaction in ionic liquid: a novel and green synthesis of 1, 4-dihydropyridine derivatives. Chin. Chem. Lett. 17, 150–152. Zhao, G., Jiang, T., Gao, H., Han, B., Huang, J., Sun, D., 2004. Mannich reaction using acidic ionic liquids as catalysts and solvents. Green Chem. 6, 75–77. doi:10.1039/B309700P. Zheng, J., Li, Y., 2011. Basic ionic liquid-catalyzed multicomponent synthesis of tetrahydrobenzo[b]pyrans and pyrano[c]chromenes. Mendeleev Commun. 21, 280–281. doi:10.1016/j.mencom.2011.09.017. Zolfigol, M.A., Khazaei, A., Moosavi-Zare, A.R., Zare, A., Asgari, Z., Khakyzadeh,V., Hasaninejad, A., 2013. Design of ionic liquid 1,3-disulfonic acid imidazolium hydrogen sulfate as a dual-catalyst for the onepot multi-component synthesis of 1,2,4,5-tetrasubstituted imidazoles. J. Ind. Eng. Chem. 19, 721–726. doi:10.1016/j.jiec.2012.10.014.

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CHAPTER 3

Green solvents in polymer synthesis Marioara Nechifor, Fulga Tanasă, Carmen-Alice Teacă “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania

3.1 Introduction The polymer industry plays a significant role in our society as polymers have become ubiquitous and an inseparable part of our daily life. The impact of synthetic polymers and functional materials on human life is profound. Processed polymers are used in various technical applications and different forms as fundamental components of practically any kind of consumer product that satisfies the highly broaden demands of contemporary society, because they can be designed to be conducting or insulating, optically transparent or opaque, rubbery, permeable or impermeable, soft, stiff, stable, or (bio) degradable. Their applications include food and medical packaging materials, medical devices that improve quality of life, and new materials for biomedical applications such as air and water purification, artificial hearts, and implants, dental fillings, drug delivery systems, wound dressing, membranes for artificial kidneys, building materials for transportation and infrastructure, high-strength fibers for functional textiles and composite materials, high-tech devices for communication and information processing, printed circuit boards and photoresists for microelectronics, sustainable power generation and energy storage, solvent-free coatings for corrosion protection, adhesives, lightweight engineering plastics in the automotive and aerospace industries, and a host of other farreaching capabilities (Mülhaupt, 2013; Worthington et al., 2017). Taking into consideration their importance in various applications, increasing attention is being given not only to synthesis, but also to polymer processing. Various traditional synthesis methods and processing techniques of polymers have been performed using organic solvents. In complete agreement, our world can be inconceivable without solvents, and water is probably the most important solvent of all. Solvents are an important type of chemical product, with a multimillion tons annual market. Usually, they are low- or high-boiling organic liquids that are capable to dissolve, dilute and diffuse gases, liquids or solids, but without altering them chemically. Typical solution phase processes require large amounts of solvents with significant impact on solvents emissions and generation of aqueous waste streams. At the same time, intensive distillations before their use or for their recovery are expensive consuming energy processes. The use of solvents is always critical in terms of environmental protection and occupational health and safety. The reputation of solvents is strongly linked to environmentally hazardous volatile organic compound emissions with huge impact on air quality. Environmental Green Sustainable Process for Chemical and Environmental Engineering and Science DOI: https://doi.org/10.1016/B978-0-323-95156-2.00002-7

© 2023 Elsevier Inc. All rights reserved.

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issues have come to the attention of academic and industrial R&D specialists who seek to find new and cleaner methods in synthesizing and processing polymers by reducing the volume of waste scattered over the water, air, and soil. Currently, the idea of sustainable chemistry has received increasing interest in polymer research because almost all aspects of polymerization processes, such as the synthetic method, reagents, reaction medium, and the characteristics of the resulted polymer are revealed in its nonbiodegradability or inherent toxicity. Polymeric materials should not have any negative effect on the environment during their production or service and at the end of their life. The durability of polymers can present problems for the wildlife and long-term disposal. Green or sustainable chemistry has emerged as a result of the need to obtain chemicals through processes that reduce or eliminate the use and production of harmful substances (Anastas and Warner, 1998; Anastas, 2011; Elhenshir and Subkha, 2013). Green chemistry has found a wide range of applications in new areas of research such as atom-economical synthesis, molecular self-assembly, alternative energy science and development of new ways to product solar cells, fuel cells, and batteries for storing energy, bio-based transformations and materials, molecular design for reduced hazard, next-generation catalyst design, and green solvents (Erdmenger et al., 2010; Dube and Salehpour, 2014). As a new field of chemistry with ecological approaches, green of sustainable chemistry is envisaged as the future of all chemistry. “Solvents should be made unnecessary wherever possible and innocuous when used” claims one of the 12 principles of green chemistry (Anastas and Warner, 1998). Auxiliary substances are used to perform a reaction, but are not incorporated into the final product. As such, they become part of the waste stream and most of them are problematic due to environmental pollution. Much progress has been made in the last years in the continuing quest for greener, environmentally acceptable solvents and more sustainable solvent use in polymer synthesis. The most effective approaches to influence the safety and efficiency of a process or product consist in finding and applying more safer and efficient alternatives or even removing solvents from a reaction system (Welton, 2015; Sheldon, 2017). There has been extensive research into developing and applications of so-called green solvents, and neoteric solvents are slowly being integrated into industrial processes. The most promising types of solvents are ionic liquids, supercritical carbon dioxide, liquid polymers, deep eutectic solvents, gas expanded solvents, and switchable solvents (Clarke et al., 2018).The purpose of this review is to provide appropriate details concerning the applications of more sustainable green solvents in polymer synthesis.

3.2  Ionic liquids Ionic liquids are generally defined as liquid electrolytes composed of organic cations and (in)organic anions (molecular ionic liquids), which are per definition in liquid state below 100°C and display ionic-covalent crystalline structure (Rogers and Voth, 2007).

Green solvents in polymer synthesis

Most of ionic liquids keep their liquid state at temperatures between 0°C and up to 400°C.The cationic parts of most ionic liquids are organic-based moieties, such as imidazolium, pyridinium, ammonium, pyrrolidinium, sulfonium, and phosphonium ions. The anionic parts can be organic or inorganic entities, for example, halide (Cl-, Br-, I-), nitrate (NO3)-, acetate ((CH3COO)-, hexafluorophosphate [PF6]-, tetrafluoroborate [BF4]-, sulfate (SO4)-, thiocyanate (SCN)-, trifluoracetate ((CF3CO2)-, p-­toluensulfonat (CH3C6H5SO3)-, dicyanamide ((N(CN)2-, tris(pentafluoroethyl)trifluorophosphate ((C2F5)3PF3-, bis(trifluoromethylsulfonyl)imide ((CF3SO2)2N-, and trifluoromethanesulfonate ((CF3SO3)- (Xie et al., 2009; Olivier-Bourbigou et al., 2010). The interchangeability among thousands of possible cations and anions allow these salts to be used in a tunable fashion. When the nature of the cation or anion changes subtly, some of the physical properties of these salts, such as melting point, hydrophobicity, solvation strength, density, and viscosity, change significantly (El Seoud et  al., 2007; Wilpiszewska and Spychaj, 2011). As designer solvents, ionic liquids have been named “task specific ionic liquids” because they can be modulated to suit the reaction conditions. Some specific physicochemical properties, such as negligible vapor pressure, nonflammability, high thermal stability, high ionic conductivity, a broad liquid range, high polarity, (electro)chemical stability, catalytic activity, facile preparation, and recyclability have led to the intensification of the study of ionic liquids as green solvents (Parvulescu and Hardacre, 2007; Martins et al., 2008; Greaves and Drummond, 2008; Riccardo et al., 2008; Ranieri et  al., 2008; Zicmanis et  al., 2009; Hallett et  al., 2009; Lu et  al., 2009; Mallakpour and Dinari, 2010; Mallakpour and Dinari, 2011; Figoli et al., 2014; Cevasco and Chiappe, 2014; Kowsari and Fakhraee, 2015). During the last 20 years, ionic liquids have been considered as “green” media of the future especially because they do not produce volatile organic compounds, and “designer solvents” to demonstrate their potential as an valuable approach to replace conventional and harmful organic solvents in chemical reactions (Rantwijk and Sheldon, 2007; Wang et al., 2009; Freemantle, 2010). Ionic liquids are good solvents for a large number of monomers in polymerization processes, because they are polar and noncoordinating compounds. Their highly ionic character allows them to interact through specific interactions, such as coulombic, hydrogen bonding, and van der Waals interactions (Martins et  al., 2008; Greaves and Drummond, 2008). Liquid state is a result of specific chemical structure, large organic asymmetric cations and anions with low coordination properties (El Seoud et al., 2007; Dommert et  al., 2014; Kudlak et  al., 2015). Ionic liquids exert a screening effect on solute ions, especially on reactions entailing charged species during the course of the reaction. Anions and solute cations from ionic liquids are totally dissociated and occur as free solvated ions due to the screening effect, because the solute ions behave as an ion-pair in polar molecular solvents, as evidenced by spectroscopic (Hallett et al., 2009; Giernoth et  al., 2014) and computational (Lynden-Bell 2010; Kirchner 2010; Zhang et al., 2017) studies.

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Thanks to their unique and attractive properties, ionic liquids are used in many fields of life sciences and industry, for example, in chemistry, biotechnology, materials, physics, medicine, nanotechnology, electrochemistry, and power engineering (Kudlak et  al., 2015). The physical and chemical properties of ionic liquids, mixtures of ionic liquids, and liquid ionic solutions must be completely investigated to maximize their potential as new reaction media in chemical processes (Petkovic et  al., 2011; Spange et al., 2014). One of the goals of green chemistry is to promote ionic liquids because they introduce a clean and sustainable chemistry and work as environmentally friendly solvents for a large number of synthetic (Mallakpour and Rafiee, 2011a, 2011b; Li et al., 2015; Lafuente et al., 2016) and catalytic (Chakraborti and Roy, 2009; OlivierBourbigou et al., 2010; de Melo, 2015; Vekariya, 2017) and biocatalytic processes (van Rantwijk and Sheldon, 2007). The use of ionic liquids as solvents for polymerization reactions (Kubisa, 2009) has been reported for free radical polymerization (Jelicic et al., 2009), controlled/living polymerization (Brusseau et  al., 2011), atom transfer radical polymerization (Zhang et  al., 2008; Hou et  al., 2008), reversible addition–fragmentation transfer (Puttick et al., 2009), ionic and coordination polymerization (Kokubo and Watanabe, 2008; Aoshima and Kanaoka, 2009), polycondensation (Zhang et al., 2012), electrochemical polymerization (Singh et al., 2012; De Bon et al., 2018; Guo and Zhou, 2019), photoinitiated polymerization (Andrzejewska, 2017), and biochemical synthesis (Muginova et al., 2010). Despite their attractive properties, the applications of ionic liquids in the chemical industry are still limited due to their rather expensive price (complex synthesis and purification, i.e., high production costs) and their relatively reduced commercial availability. Actually, few works report on the replacement of a conventional solvent by a green one in an existing commercial process just for the purpose of creating a greener process (Welton, 2015). Moreover, the green character of ionic liquid is still debatable in terms of toxic influence on the environment and poor biodegradability (Clark and Taverner, 2007; Jessop, 2011; Kudlak et al., 2015). Despite of their appealing properties, some properties of the ionic liquids such as high viscosity and sensitivity to moisture are not beneficial in certain chemical syntheses. 3.2.1  Radical polymerization in ionic liquids 3.2.1.1  Free radical polymerization Ionic liquids, as alternative solvents over conventional organic solvents used in polymerization processes, offer general advantages such as low volatility and nonflammability, but the rate constant and conversion of free radical polymerization are also influenced by the structure of the ionic liquid. Radical polymerization in ionic liquids produces higher molecular weight polymers and/or higher polymerization rates due to decreasing of the activation energy of propagation as a consequence of high polarity and interactions of ionic liquids and monomers (Strehmel et al., 2014a; Low et al., 2018).

Green solvents in polymer synthesis

Study on different 1,3-dialkylimidazolium and tetraalkylphosphonium ionic liquids used in free radical polymerization of methyl methacrylate and acrylonitrile revealed that 1-ethyl-3-methylimidazolium trifluoromethane sulfonate was the best solvent for the two monomers, resulting in high conversions and high molar weights (Vygodskii et al., 2007). The yield of the free radical polymerizations in ionic liquids is controlled by the nature and concentration of the ionic liquid used as reaction medium (GuerreroSanchez et al., 2007).The rate constants in free radical polymerizations are determined by the polarity and viscosity of the ionic liquids (Winterton, 2006; Barth et al., 2009). Ionic liquids decrease the activation energy of the propagation due to the increased polarity of the reaction media, which results in an enhanced contribution of charge-transfer structures to the transition state. On the opposite, the termination rate is decreasing with viscosity, which might be ascribed to translational diffusion of the radicals.This behavior is attributed to the “diffusion control termination” because of high viscosity of ionic liquids and solubility problems of monomers and resulting polymers in ionic liquids. the influence of the structure of the ionic liquid on the free radical polymerization process of methyl methacrylate and acrylonitrile was found to be relevant, depending on the four anions namely, tris(pentafluoroethyl)trifluorophosphate, trifluoromethanesulfonate, trifluoroacetate, and tetracyanoborate. Trifluoromethanesulfonate- and trifluoroacetatebased ionic solvents yielded the formation of polymers with the highest molecular weight (Mw ≈ 8.8 × 105 g/mol for polyacrylonitrile and Mw ≈ 18.9 × 105 g/mol for polymethylmethacrylate in quantitative yield (96% and 92%, respectively) (Vygodskii et al., 2011). The influence of the ionic liquid 1-ethyl-3-methylimidazoliumethylsulfate on the polymerization kinetics of methyl methacrylate was studied. The polarity and viscosity of ionic liquids were found to be in correlation with the rate coefficients of propagation kp and termination kt of polymerizations performed in ionic liquids. When the concentration of ionic liquid in the system increased, an increase in kp and decrease in kt occurred (Schmidt-Naake et al., 2008). During the course of reaction, hydrogen bonds between both the polar monomer molecules and the growing radical chains and the cations and anions of the ionic liquids were formed. These conditions were beneficial to a very high propagation rate coefficient kp and the reduction of the activation energy of propagation. The high molar mass and high polydispersity of the final polymers were ascribed to the high viscosity of the ionic liquids themselves and reaction medium during the polymerization process (Schmidt-Naake et al., 2009; Strehmel et al., 2014b; Fumino and Ludwig, 2014). The termination rate coefficients, kt, for free radical polymerization of methyl methacrylate dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide and in 1-butyl-3-methylimidazolium tetrafluoroborate were estimated using the single pulse-pulsed laser polymerization-electron paramagnetic resonance technique. The absolute kt in ionic liquid solution was by about one order of magnitude lower than that

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found using bulk polymerization of methyl methacrylate. Therefore, the chain-length dependence of kt was similar in both liquid environments (Barth et al., 2009). Free radical homopolymerization of methyl methacrylate and glycidyl methacrylate were performed in ionic liquids based on imidazolim salts. The experimental results evidenced that propagation rate coefficients (kp) were increased by four times for methyl methacrylate and twice for glycidyl methacrylate by comparison with the values found for bulk polymerization. The explanation resides in a lower activation energy resulted upon partial replacement of monomer by ionic liquid species (bulk: AE = 22.4 kJ/mol, 50 v% ionic liquid: AE = 20.4 kJ/mol) due to the high polarity of the ionic liquid (Woecht et al., 2008). The degree of polymerization was proved to be dependent on the liquid state range of the ionic liquid used in the synthesis of polymer. A high viscosity of the ionic liquid resulted in a high degree of polymerization. Moreover, the micropolarity effects of ionic liquids influenced the composition of the polymer (Strehmel, 2007). Imidazolium type ionic liquids proved to induce a “protected” radical mechanism in polymerization of methyl methacrylate at temperatures up to 120°C when almost quantitative yields were obtained. However, almost no polymer was formed in common organic solvents (i.e., xylene) due to rapid initiator burnout (rapid initiator consumption in xylene). Preparation of the block copolymer poly(styrene-block-methyl methacrylate) in ionic liquids was not accompanied by producing poly(methyl methacrylate), because methyl methacrylate was initiated only by “protected” polystyrene macroradicals. This reaction mechanism may explain the high values of polymerization rate, high molecular weight and high yield obtained in the presence of ionic liquids. In addition, these results may suggest that ionic liquids could be used to prepare “catalyst-free” high molecular weight copolymers (Thurecht et al., 2008). As compared with conventional polymerization, microwave-assisted radical polymerization showed a more efficient heating profile in the presence of water-soluble ionic liquids as reaction media. Ionic liquids can be used as heat transfer fluids due to their microwave absorption capability (GuerreroSanchez et  al., 2007). Ionic liquids accelerated the living radical polymerization of methyl methacrylate by an efficient initiation of 100%, removing the induction time of catalyst, achieving a polydispersity index of 1.1 and perfect bifunctional chain-ends (MacFarlane et al., 2007). A higher percentage of grafting was obtained when styrene and methyl methacrylate were subjected to radical graft polymerization using silica particles and carbon black surfaces functionalized with azo groups as initiators and ionic liquid (1-butyl3-methylimidazolium hexafluorophosphate) as solvent, as compared with the same process developed in dioxane. (Ueda et  al., 2007). Though carbon black retards the polymerization of monomers in organic solvents to an extent, this is not the case in the presence of ionic liquids. A possible reason consists in the prolonged lifetime of the surface radicals formed by the azo groups ascribed to the high viscosity of the ionic liquid in comparison with dioxane. Grafting styrene, methyl methacrylate, and vinyl acetate

Green solvents in polymer synthesis

onto carbon black using as initiators 2,2′-azobisisobutyronitrile and benzoyl peroxide, respectively, led to similar results (Ueda et al., 2008; Wu et al., 2013). 3.2.1.2  Controlled radical polymerizations in ionic liquids 3.2.1.2.1  Atom transfer radical polymerization The studies regarding the atom transfer radical polymerization in the presence of ionic liquids monitored especially their influence on different ligands, catalysts, initiators, and temperature reaction (Maria et al., 2007; Li et al., 2007; Zhang et al., 2008; Xiao et al., 2008). Ionic liquids allow the polymer to be separated from the residual catalyst and reduce the magnitude of the side reactions when used in atom transfer radical polymerization. In addition, an amine ligand for the catalyst is not necessary (Kubisa, 2004). 1-Methyl-imidazolium acetate, 1-methylimidazolium propionate, and 1-methylimidazolium butyrate were found to be good solvents for the atom transfer radical polymerization of methyl methacrylate in the presence of ethyl 2-bromoisobutyrate/ CuBr as the initiating system. All the polymerizations proceeded in a well-controlled manner. There is a strong dependence of the reaction rate on the length of the substituted groups of anions in the ionic liquids as revealed by the values of the apparent constants of the polymerization rate (Lai et al., 2007). The living radical polymerization of methyl methacrylate, methyl acrylate, and styrene was performed in ionic liquids in the presence of organotellurium derivatives (Feng et al., 2012). The polymerization rate of methyl methacrylate and methyl acrylate was significantly increased as compared with organic solvents media, and the control of the polydispersity index was also improved. For example, the resulted poly(methyl methacrylate)s had polydispersity indexes lower than 1.1 and nearly full conversion in half an hour in the absence of dimethyl ditelluride. The key role in the control of polydispersity index for this reaction was a faster degenerative chain transfer reaction. The Lewis acid–base interaction between ionic liquids and the tellurium atom and the polar effect of ionic liquids lowered the activation energy of the degenerative chain transfer process. 1-Butyl-3-methylimidazolium hexafluorophospate was used as solvent in atom transfer radical polymerization of methyl acrylate in the presence of a bromine-containing initiator and a chloride-containing catalyst (Maria et al., 2007). The polymerization proceeded in a controlled manner and the reversibly deactivated macromolecules had both bromine and chloride end groups. Three ionic liquids, 1-methyl-imidazolium acetate, 1-methylimidazolium valerate, and 1-methylimidazolium caproate were used in atom transfer radical polymerization of acrylonitrile with FeBr2 as the catalyst system, and ethyl 2-bromoisobutyrate as the initiator, respectively, which led to a well-controlled polymerization (Hou et al., 2008). The chain extension polymerization in ionic liquids was facilitated by the resulting polyacrylonitrile that acted as macroinitiator. All the ionic liquids and catalyst were

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easily recovered and recycled after usual purification. The length of the substituted groups of the anions in the ionic liquids influenced the reaction rate that was evidenced by the sequence of the rate coefficients of polymerization. In the above-studied systems, both the monomer and resulted polymer are soluble in the ionic liquid phase and polymerization proceeded almost under homogeneous conditions. N-butyl-N-methyl morpholinium tetrafluoroborate was synthesized and used as reaction media for heterogeneous atom transfer radical polymerization of methyl methacrylate in the presence of CuBr/2,2′-bipyridine as catalyst and methyl 2-bromopropionate as initiator (Xiao et al., 2008). The resulted poly(methyl methacrylate) having a well-defined molecular weight and narrow polydispersity acted as a macroinitiator in the block polymerization with t-butyl methacrylate as comonomer. The ionic liquid was recovered and recycled, and its catalytic activity was similar to that of the fresh catalyst. Atom transfer radical polymerization using activators regenerated by electron transfer of acrylonitrile was carried out in three ionic liquids, namely 1-dodecyl-3methylimidazolium tetrafluoroborate, 1-octyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methylimidazolium tetrafluoroborate (Chen et  al., 2011a). Under the same experimental conditions, the rate of polymerization in 1-octyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate was considerably lower than in 1-dodecyl-3-methylimidazolium tetrafluoroborate. Gel permeation chromatography method proved that the molecular weights of the resulting polymers increased with conversion, and the concordance between the found values of molecular weights and those calculated was better in 1-dodecyl-3-methylimidazolium tetrafluoroborate than in 1-octyl-3-methylimidazolium tetrafluoroborate and 1-butyl3-methylimidazolium tetrafluoroborate. The polydispersity was narrower in 1-dodecyl3-methylimidazolium tetrafluoroborate than that found in 1-octyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate. Atom transfer radical polymerization of acrylonitrile was carried out in three ionic liquids, namely 1-methylimidazolium acetate, 1-methylimidazolium propionate, and 1-methylimidazolium butyrate, and in the presence of activators generated by electron transfer (Chen et  al., 2010). The reaction proceeded in the presence of ethyl 2-bromoisobutyrate as initiator, ascorbic acid as a reducing agent, FeBr3 as catalyst and no additional ligand. Kinetic studies showed that activators generated by electron transfer in atom transfer radical polymerization of acrylonitrile proceeded in a well-controlled manner both in the absence of oxygen and in the presence of air. The sequence of the apparent polymerization rate constants was kapp(1-methylimidazolium butyrate]) < kapp(1-methylimidazolium propionate])< kapp(1-methylimidazolium acetate). The chain end analysis and block copolymerization of methyl methacrylate with resulted polyacrylonitrile as macroinitiator confirmed the living nature of the polymerization. The catalyst and ionic liquids were recovered and reused and the living nature of subsequent polymerization was not altered.

Green solvents in polymer synthesis

Four ionic liquids, namely 1-methylimidazolium acetate, 1-methylimidazolium butyrate, 1-methylimidazolium caproate, and 1-methylimidazolium heptylate, were used as reaction media in reverse atom transfer radical polymerization of methacrylonitrile initiated by 2,2′-azobisisobutyronitrile. The reaction system was completed by FeCl3 as catalyst and no additional ligand was used. The best control of molecular weight and its distribution were found in 1-methylimidazolium acetate when a more rapid reaction rate was observed (Chen et al., 2011b). A conventional atom transfer radical polymerization process in 1-methylimidazolium acetate was chosen to prepare the block copolymer poly(methacrilonitrile)-b-polystyrene.The resulted poly(methacrylonitrile) acted as a macroinitiator. The catalyst and 1-methylimidazolium acetate were easily recycled and reused and the living nature of the subsequent reverse atom transfer radical polymerization of methacrylonitrile was not influenced. Besides atom transfer radical polymerization, some other controlled polymerization techniques were carried out in ionic liquids. Nitroxide-mediated polymerization of methyl methacrylate with a low percentage of styrene was carried out in 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide as reaction medium (Brusseau et al., 2011). The control over the polymer molar mass and polydispersity index was significantly better in comparison with previously results reported in the literature on nitroxide-mediated polymerizations in ionic liquids, and slightly better than that obtained in bulk or conventional solvent solutions for the same system. The polymerization rate and the final conversion were higher, and according to increasing of kp and decreasing of kt, which are key features of free radical polymerization in ionic liquids. 3.2.1.2.2  Reversible addition–fragmentation chain transfer polymerization The reversible addition–fragmentation chain transfer polymerization-mediated polymerization of styrene using cumyl phenyldithioacetate as reversible addition–fragmentation chain transfer polymerization agent and biodegradable ionic liquids having alkoxycarbonyl side chains and a trifluoromethanesulfonate counterion resulted in high conversions and slightly higher polymerization rates than those observed for toluene (Johnston-Hall et al., 2009). The polymerization exhibited a high “controlled” character and the polydispersity index was lower than 1.2.The lack of “trapped” or “protected” radicals was revealed by the high value of the termination rate. Based on the evidence suggesting that an ionic liquid creates a dynamic environment which involves polar and nonpolar domains controlled by all the weak interactions and self-assembly, Puttick et al. (2013) studied the kinetics in the reversible addition–fragmentation transfer controlled free radical polymerization of methyl methacrylate in several ionic liquids at room temperature. The authors found that in almost all cases dithiobenzoate-moiety of the reversible addition–fragmentation chain transfer polymerization agent was preferential fragmented into the ionic domains of the ionic liquid.

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3.2.2  Metathesis polymerizations in ionic liquids 3.2.2.1  Ring-opening polymerizations The ring-opening polymerization of lactones in the presence of rare-earth metal triflates as catalyst evidenced that ionic liquids used as solvents had a strong effect on the molecular weights of the resulted polymers (Nomura et  al., 2007). No polymers (in the best case oligomers) were obtained when tetrafluoroborate-containing ionic liquids were used, while polymers with relatively high molar masses were prepared in ionic liquids with hexafluorophosphate or hexafluoroantimonate counterions. This might be attributed to the complexes formed between the counterion of the ionic liquid and the rare-earth metals from the catalyst that facilitated the polymerization. ε-Caprolactone was subjected to ring-opening polymerization in ionic liquids instead of toluene, and a polymer-supported scandium (III) trifluoromethanesulfonate as catalyst. The experimental results proved that 1-butyl-3-methylimidazolium hexafluorophosphate and 1-ethyl-3-methylimidazolium hexafluorophosphate accelerated the ring-opening polymerization of the monomer (Oshimura et al., 2009). Cyclohexene oxide underwent ring-opening polymerization in 1‐n‐butyl‐3‐methylimidazolium tetrafluoroborate using scandium (III) trifluoromethanesulfonate as catalyst. The reaction took place at room temperature involving a two‐phase polymerization system and immiscible monomer in ionic liquid (Ling et  al., 2012). The catalyst was immobilized in the ionic liquid phase where polymerization occurred. The yield of poly(cyclohexene oxide) in ionic liquid was higher than that in bulk. The resulted polymer was extracted by the monomer, and the catalytic activity of the recovered ionic liquid phase decreased concurrently with the recycle batches of polymerization due to the loss of scandium (III) trifluoromethanesulfonate during the extraction process of polymers. The polymerization of lactones by enzymatic ring-opening polymerization in various ionic liquids evidenced that hydrophobic ionic liquids were superior to hydrophilic ones, and an improved degree of polymerization took place by decreasing the water content in the enzymatic reaction (Gorke et  al., 2007). Polymerization of N-carboxy-α-amino acid anhydride using amine-initiated ring-opening polymerization in ionic liquid yielded in polymers with narrow polydispersity index values (lower than 1.3) (Mori et al., 2007). By comparison with the reaction performed in organic solvents, the polymerization rate was lower while the induction period was shorter in ionic liquids. The spectroscopic investigations indicated the existence of the interactions between the ionic liquid and N-carboxy-α-amino acid anhydride monomer, as well as ionic liquid and the initiator, which should be related to the polymerization behavior. Microwave-assisted ring-opening polymerization of trimethylene carbonate in 1-butyl-3-methylimidazolium hexafluorophospate resulted in a polymer with a molecular weight much higher than that produced in bulk at the same reaction time (Liao et al., 2007).

Green solvents in polymer synthesis

Study on the ring-opening polymerization of cyclooctene in ionic liquids came to the conclusion that the propagation rate was improved by increasing the polarity of the reaction medium. In addition, the contribution of charge-transfer structures to the transition state was enhanced because high molar masses and narrow polydispersity indices were obtained (Han et al., 2007). The lower conversions obtained in this reaction system are explained by the interaction between the charge of the monomer and the ionic liquid. Ring‐opening metathesis polymerization using a Grubbs catalyst was applied to a series of nornenyl derivatives with pendant oligoethyleneoxy spacer and imidazolium salt, and 5-substituted cyclooctene with a pendant imidazole-based ionic salt. Polymerization behavior of these monomers was studied at various temperatures in different ionic liquids including hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate and hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate that proved to be suitable media for ring‐opening metathesis polymerization (Han et  al., 2007, 2008; Dang et  al., 2008; Xie et al., 2008). 3.2.2.2  Cationic ring-opening polymerizations The cationic ring-opening polymerization of 3,3-bis(chloromethyl)oxacyclobutane carried out in 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate led to high conversions but low molecular weights, similar to the bulk polymerization or polymerization in organic solvents (Wang et al., 2007). The recycling of 1-butyl-3-methylimidazolium hexafluorophosphate was accomplished by adding water or organic solvents to the reaction solution. In the case of cationic ring-opening polymerization, the polymerization rates were increased due to the presence of additional ionic species belonging to the ionic liquid that changed the association between the living polymer chain ends and their respective counterions. Consequently, the reaction time was shortened. Water-soluble ionic liquids, for example, 1-butyl-3-methylimidazolium trifluoromethane sulfonate and 1-butyl-3-methylimidazolium tetrafluoroborate, were used in cationic ring-opening polymerization of 2-(m-difluorophenyl)-2-oxazoline and 2-phenyl-2-oxazoline (Guerrero-Sanchez et  al., 2007). The polymers obtained when 2-(m-difluorophenyl)-2-oxazoline was used had low polydispersity indexes values, but much higher molecular weights than expected, and a broad polydispersity in the case of 2-phenyl-2-oxazoline. The ionic liquids were recovered by precipitating the polymer in water, its filtering and removing the water by microwave-assisted distillation. 3.2.3  Anionic/cationic polymerizations in ionic liquids Ionic liquids have been attractive solvents for ionic polymerization processes due to their high polarity and ability to dissolve a wide range of organic and ionic compounds to an appreciable extent. It is expected that the high polarity and possible interactions of

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counteranions of ionic liquids with growing carbocations to greatly affect polymerization behavior. The huge amount of ionic species generates a highly polar environment that facilitates the dissociation of the counteranion of the growing species, allows counteranions of ionic liquids to interact weakly with Lewis acid catalysts, and surrounds the growing carbocations with counteranions of ionic liquids (Yoshimitsu et al., 2016). Ionic liquids are regarded as polar but noncoordinating solvents with a high charge density due to their ionic nature; consequently, they act differently from conventional solvents used in ionic polymerization.The detailed mechanism of the cationic polymerization was not elucidated (Vijayaraghavan and MacFarlane, 2007, 2012; Wu et al., 2015) even though air- and water-stable ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate (Bueno et  al., 2009; Basko et  al., 2009), trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)amide (Vijayaraghavan and MacFarlane, 2012), 1-octyl-3-methylimidazolium tetrafluoroborate (Wu et  al., 2015), and N-butylN-methyl pyrrolidinium bis(trifluoromethanesulfonyl)amide (Vijayaraghavan and MacFarlane, 2007) have been successfully applied in cationic polymerization. Moreover, ionic liquids seem unsuitable solvents for anionic polymerization. The acidic proton at the 2-position of the imidazolium ring makes imidazole-based ionic liquids unsuitable for anionic polymerization because they are not entirely stable under basic conditions (Biedron and Kubisa, 2007; Kokubo and Watanabe, 2008). Cationic polymerizations in ionic liquid media are more feasible than anionic polymerizations. Imidazolium-based ionic liquids are less stable and deactivation of the initiator and/or chain transfer to ionic liquid occur under basic conditions. Some ionic liquid-mediated cationic polymerizations enable an increased activity of the catalyst. It seems that the ionic liquids can stabilize the charged intermediates, though the mechanism is not completely elucidated. This is due to the moderate polar and noncoordinating nature of ionic liquids. The influence of imidazolium ionic liquids with hexafluorophosphate and tetrafluoroborate on anionic polymerization of methyl methacrylate resulted in high yields and low molecular weight polymers ended in imidazolium moiety due to significant chain transfer to these ionic liquids (Biedron and Kubisa, 2007).The same phenomenon seems plausible to occur in cationic polymerization. Cationic polymerization of isobutyl vinyl ether was carried out in 1-butyl-3-octylimidazolium bis(trifluoromethanesulfonyl)imide. The authors used 1-(isobutoxy)ethyl acetate/TiCl4 initiating system, ethyl acetate as base and 2,6-di-tert-butylpyridine as a proton trap reagent (Yoshimitsu et  al., 2016). The control of polymerization is based on a proper choice of metal halide catalysts, counteranions of ionic liquids, and additives. The high polarity generated by the increased population of ionic active species in ionic liquid promotes a polymerization much faster unlike the reaction performed in CH2Cl2. Polymers with low polydispersity index were prepared in the ionic liquid with a bis(trifluoromethanesulfonyl)imide anion and no added base. The explanation lies in

Green solvents in polymer synthesis

possible interactions between the counteranion of the ionic liquid and the growing carbocations. The cationic polymerization of 1-(2-vinyloxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide took place in a homogeneous phase in 1-methyl3-octylimidazolium bis(trifluoromethanesulfonyl)imide. The solubility of the resulted polymers was controlled by counteranion exchange. The living cationic polymerization of styrene in N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide as solvent led to polystyrenes with low molecular weights and low polydispersity (Vijayaraghavan and MacFarlane, 2007). Solubility and viscosity of p-methylstyrene during its cationic polymerization in imidazolium-based ionic liquids were investigated depending on the nature of anions, cations, and alkyl chain length of ionic liquids (Zhang et  al., 2016). The experimental results revealed that polymerization occurred in a milder exothermic system in ionic liquids compared to a traditional organic solvent. Controlled polymerizations were achieved in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with a 2-phenyl-2-propanol/BF3OEt2/2,6-di-tertbutylpyridine initiating system at −25°C. The results of density functional theory and the terminal structures of polymers were the basis of the cationic polymerization mechanism of p-methylstyrene in ionic liquids. Similar results were obtained by cationic polymerization of styrene using 1-butyl-3-methylimidazolium hexafluorophosphate as solvent and α,α-dimethylbenzyl chloride as an initiator at −15°C (Han et al., 2016). The authors proposed the following reaction mechanism: (a) in the initiation step, Lewis acid metal halide extracted a Cl atom from α,α-dimethylbenzyl chloride and generated a carbocation and a metal halide-based counterion which attacked the styrene to initiate the polymerization; (b) chain termination reactions occurred by means of halide-based counterion and not via hexafluorophosphate anion, because hexafluorophosphate anion was a very weakly nucleophilic species. The anionic polymerization of methyl methacrylate in 1-n-butyl-3-methylimidazolium was investigated (Biedron and Kubisa, 2007). Resulted polymers with low molecular weights ended in an imidazolium moiety indicated that chain transfer reactions to the ionic liquid took place. A possible mechanism involved an attack of the anionic growing species to the nitrogen atom of the ionic liquid displacing the larger alkyl side chain of the imidazolium ring. The anionic polymerization of styrene in the presence of s-butyl lithium as initiator and trihexyl-(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)imide as reaction medium resulted in high molecular weights polymers, but low yields (Vijayaraghavan et al., 2008). A zwitterion (butyl imidazolium sulfonate) added as coinitiator improved the yields. The zwitterion was thought to bind Li + ions, which allowed an increased amount of initiator to be involved in polymerization. Generally, anionic polymerizations performed in ionic liquids are influenced by much milder Lewis base initiators. The polymerization is controlled by the Brønsted acidity of the initiator and the very weak basicity of the ionic liquid (Vijayaraghavan and MacFarlane, 2007).

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The cationic polymerization of styrene in 1-n-butyl-3-methylimidazolium heptachlorodiferrate was performed in miniemulsion with water as continuous phase in the temperature range from 70°C to 90°C (Alves et  al., 2018). Polystyrenes with much higher molecular weight than those usually obtained in cationic polymerizations were obtained. Compared to miniemulsion polymerization, bulk polymerization in the same ionic liquid resulted in higher reaction rates but much lower molecular weights. The miniemulsion polymerization was easier to control because it took place slower than the conventional one. The cationic polymerization of isobutylene was investigated in 1-butyl-3-methylimidazolium hexafluorophosphate at −10°C (Li et al., 2019). A highly reactive polyisobutylene with a high exo-olefin end group content (>80%) was synthesized in the presence of H2O/TiCl4 initiating system and 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid. The polymerization occurred at the interface of ionic liquid particles and hexafluorophosphate anions facilitated the ionization of the initiating system and stabilized the carbocation active center. The cationic copolymerization of isobutylene with p-methylstyrene was investigated using different initiating systems in 1-hexyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide at −30°C (Zhang et al., 2018). The chain transfer mechanism of copolymerization was influenced by high viscosity, high polarity, and ionic environment of ionic liquids. 3.2.4  Polycondensation in ionic liquids Step-growth polymerization consists of the stepwise condensation of functional groups of difunctional monomers and requires severe conditions such as high temperatures and vacuum conditions for prolonged reaction time in the presence of protic acid or organometallic catalysts. The reaction conditions favor side reactions resulting in polymers with low molecular weights with varying microstructures, besides environmental problems attributed to toxic reagents and volatile solvents. Due to their tunable solvent properties, ionic liquids are considered as efficient substitutes for organic solvents. Research in this area has been preponderantly focused on polyamides, polyimides, and polyesters synthesis. Some studies evidenced the catalytic effect of ionic liquids during the polycondesation reaction. Solubility of 4,4′-oxydianiline and pyromellitic dianhydride in 1-benzyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide was significantly increased in the presence of imidazolium type zwitterions without heating. The subsequent imidation of the resulted poly(amide acid) afforded high molecular weight polyimides (Tamada et al., 2007). Similarly, 4,4′-oxydianiline, pyromellitic dianhydride, and 1,3,5-tri(4-aminophenyl)benzene in ionic liquid/zwitterion mixture resulted in a branched polyimide by in situ polycondensation. The gel shape and ionic conductivity was controlled by the compatibility between network and ionic liquids (Tamada et al., 2008).

Green solvents in polymer synthesis

Synthesis of aliphatic polyesters from diols and diacids in ionic liquid 1-butyl3-methylimidazolium hexafluorophosphate catalyzed by SnCl2 yielded in polymers with lower molar masses (Mw lower than 1.8 × 104 g/mol) than those obtained in catalyst-free bulk polymerization (Fu and Liu, 2008). Changing the counteranion from hexafluorophosphate to bis(trifluoromethanesulfonyl)imide in the ionic liquid structure resulted in increased molecular weights of polyesters. This kind of catalyst deactivation was explained based on hard and soft acid–base principle. Accordingly, Sn2+, which is a hard acid, reacts with hexafluorophosphate counternion to give metal fluoride complex in ionic liquid and becomes inactive for the polymerization. The strong covalent bond C–F makes improbable such complexation in bis(trifluoromethanesulfonyl)imide. Sn2+ is catalytically active and relative high molecular weight polyesters were obtained in bis(trifluoromethanesulfonyl)imide-based ionic liquids. Sebacic, adipic, and succinic acid and aliphatic diols underwent polycondensation by a two-step procedure and post polycondensation in ionic liquid (Kubisa, 2009). Aliphatic polyesters with relative low molecular weights were obtained (Mw lower than 6 × 104 g/mol) which indicated that solubility of polyesters in ionic liquid was a restricting factor. The solubility of the resulted polymers was significantly influenced by the nature of cation and anion from the structure of ionic liquids. It was found a correlation between the miscibility of aliphatic polyester/ionic liquid system and the extent to which their solubility parameters matched. Organosoluble poly(amide-ether-imidazole)s having different functional groups were synthesized by direct polycondensation in the presence of ionic liquids and triphenyl phosphite without pyridine, lithium chloride, or N-methyl-pyrrolidone (Ghaemy et al., 2013). Room temperature ionic liquids with anions such as Br-, tetrafluoroborate and hexafluorophosphate, and 1,3-dialkylimidazolium cations were used as solvents. The reaction time was shorter than that required in N-methyl-2-pyrrolidone (2.5 vs. 12 hours). Ionic liquid 1,3-dipropylimidazolium bromide was tested as a substitute for volatile toxic organic solvents in direct polycondensation of a chiral dicarboxylic acid monomer with various diamines to prepare aromatic polyamides (Mallakpour and Rafiee, 2011c). The optically active polyamides were obtained in 80–98% yields and their inherent viscosity varied between 0.39 and 0.69 dL/g. The inherent viscosity, yields, and thermal stability were comparable to analogous products obtained in organic solvents under milder reaction conditions. 1-Methyl-3-alkyl imidazolium bromides (alkyl=C3–C8) served as solvents to prepare high molecular weight polyamides (Mansoori et al., 2012). The highest molecular weight was obtained in 1-butyl-3-methyl imidazolium bromide. No relationship could be established between the inherent viscosity and the molecular weight of the polymer obtained and the length of the alkyl chain in the ionic liquids.

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Synthesis of poly(ethylene terephthalate) was carried out in a series of phenylalkyl pyrrolidinium ionic liquids with specific functional groups ([YBPy][X], Y = NO2, CH3, F, H; B = benzyl, phenethyl; X = bis(trifluoromethanesulfonyl)imide) (Dou and Liu, 2012). Ionic liquids exhibited high thermostability and attractive properties in preparing poly(ethylene terephthalate) at lower temperature (190–240°C) and pressure (500 Pa). The resulted poly(ethylene terephthalate) had Mw up to 1.9 × 104 g/mol. Except for the hexafluorophosphate-based ionic liquids, the others ionic liquids were recovered and reused after usual purification. This process promotes an important and environmentally friendly choice to the usual method used for the industrial synthesis of poly(ethylene terephthalate). Another series of ionic liquids based on benzyl imidazolium ([YBMIM][X],Y = NO2, CH3, F; B = benzyl; X = bis(trifluoromethanesulfon yl)imide) was used to study the preparation of poly(ethylene terephthalate) at low temperature and pressure (Dou et al., 2012). A two‐step polycondensation in the abovementioned ionic liquids yielded in high molecular weight poly(ethylene terephthalate) (Mw up to 2.6 × 104 g/mol). The temperatures used in solution reaction (230–240°C) were lower than those used in traditional melt polycondensation (270–290°C). The catalysts (Sb2(OCH2CH2O)3, Sb(OAc)3, and Sb2O3) used in the formation of poly(ethylene terephthalate) had insignificant effect on the thermal stability of the ionic liquids. The ionic liquids decreased the viscosity of the reaction mixture, and small molecules were easily removed from the system. High performance poly(phenylene sulfide sulfone)s based on 4,4-difluorodiphenylsulfone and sodium sulfide (Na2S) were prepared under mild conditions in imidazolium-based ionic liquid and zwitterionic type ionic liquid/1-methylimidazolium3-butylsulfonate zwitterion as solvents (He et  al., 2017). The resulted polymers had high molecular weights (2.5 × 104 g/mol) and yields (higher than 99%). Imidazoliumbased ionic liquids with short alkyl chain were favorable to synthesize high molecular weight polymers. In addition, ionic liquids were recycled and reused and had excellent recyclability in this polymerization system. A successful synthesis of polysulfones with high molecular weight and high yield occurred when a similar ionic liquid/zwitterion system was used as a reaction medium (Wang and Liu, 2012). As expected, the solubility of 2,2-bis(4-hydroxyphenyl) propane (bisphenol A) was significantly improved due to the zwitterion. Sulfonic-type ionic liquid generated intermediary between zwitterion and bisphenol A has a high acidity which has led to an 80% reduction in dehydration time compared to conventional methods. Polyetheretherketones were successfully synthesized in 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide by polycondensation reactions of 4,4′-dihalobenzophenones with hydroquinone using K2CO3 at high temperatures (up to 320°C) (Gunaratne et al., 2013). The properties of the resulted polymers were similar to analogous polymers prepared in diphenyl sulfone, the solvent currently used in industrial processes. The molecular weights were lower but the separation efficiency was significantly

Green solvents in polymer synthesis

improved in ionic liquid. An efficient synthesis of poly(ether sulfone)s (ηinh = 0.10−0.31 dL/g) was carried out using ionic liquid ionic liquid/zwitterion as reaction medium (Wang et al., 2016). At elevated temperatures, the interaction between 4,4′-dihydroxydiphenylsulfone (bisphenol-S) and zwitterion resulted in reduction of dehydration time, and no agglomeration was observed. The polymerization temperature (150°C) was significantly lower than that used in conventional methods (220−300°C) and because of the high solubility of bisphenol-S dipotassium salt in ionic liquid/zwitterion. The efficiency of ionic liquid/zwitterion as reaction medium was also proved using sulfolane as solvent and the same reaction conditions used in ionic liquid/zwitterion polymerization. Various dicarboxylic acids and a new diamine-bisphenol were subjected to direct polycondensation in the presence of imidazolium-based ionic liquids that proved to be very efficient solvents and catalysts for the selective synthesis of aromatic polyamides (Taghavi et al., 2013). High performance polyamides with molecular weights (Mw) up to 61,000 g/mol were obtained at 110°C for 2.5 hours, which is significantly shorter than that required in conventional direct polycondensation. Recently, synthetic methods based on microwaves and ionic liquids have become frequently used in polymer synthesis due to their high efficiency and low energy consumption. Having high flammability and volatility, conventional organic solvents are not safe at high-temperatures developed in closed-vessel using microwaves. On the contrary, ionic liquids have high-boiling point, low vapor pressure, high thermal stability, moderate dielectric constant (between 10 and 15), and low heat capacity (1–2 J/g °K). Owing to this combination of specific properties, ionic liquids are able to absorb microwaves efficiently. Under microwave irradiation, ionic liquids with high polarity are heated within a short time, volumetrically and simultaneously. Microwave irradiation as a heating source and ionic liquids as solvents has developed as a completely new technique in the recent years in the field of polymer synthesis (Mallakpour and Rafiee, 2011d; Tarasova et al., 2015; Wang et al., 2019).

3.3  Supercritical carbon dioxide For over five decades, supercritical fluids have been studied in both industry and academia, intensively and extensively, as viable alternatives to classic solvents, which led to a broad range of applications. A supercritical fluid is defined as a substance for which both pressure and temperature are above the critical values. Similar to the gaseous state, supercritical fluids possess high diffusivity. Just like liquids, they exhibit relatively high density and low viscosity.Their high diffusivity creates favorable conditions for increased reaction rates. They are appropriate media for reactions of gaseous substrates because they allow a high solubility of gases. Operating small changes in pressure within the critical interval limits, it is possible to finely tune the density of supercritical fluids. The supercritical fluids have been successfully employed as solvents, antisolvents, or

67

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plasticizers in polymer synthesis and in polymer processing, such as polymer blending, polymer composites, polymer modification, particle production, and microcellular foaming (Eckert et al., 1996; Alsoy and Duda, 1999; Yoganathan et al., 2010; Yoon and Byun, 2012; Knez et al., 2014; Campardelli et al., 2015). Supercritical carbon dioxide has proved to be environmentally benign, which corroborated with its attractive physical properties made it extensively studied as medium for organic transformations and polymerization reactions. Furthermore, it is a valuable option by choice from practical environmental and economic considerations too. It proved to be a clean and versatile solvent, a promising alternative to toxic organic solvents and chlorofluorocarbons. Supercritical CO2 is inexpensive and relatively inert, nontoxic, nonflammable, odorless. It is easily removed after the completion of the reaction and it can be recycled by ecologically friendly processes. Additionally, its ubiquity makes it inexpensive and readily available. Supercritical CO2 is considered a green solvent that can be conveniently used in both synthesis and processing of polymers (Kendall et al., 1999; Cooper, 2000; Tomasko et al., 2003; Nalawade et al., 2006; Villarroya et al., 2007; Davies et al., 2008; Ramsey et al., 2009; Naylor et al., 2011; Boyère et al., 2014). The conditions for its supercritical state (Tc = 304°K, Pc = 7.38 MPa) are easily attainable compared to other substances and it can be removed by simple depressurization. Above the critical values of pressure and temperature, CO2 reaches the supercritical state and displays properties characteristic to both gas and liquid states. Therefore, it exhibits a viscosity comparable to that of a gas and a liquid-like density, but a very high diffusion coefficient compared to the liquid. Low molecular weight nonpolar molecules are soluble in supercritical CO2. Despite that, numerous small polar molecules, such as acetone, methanol, tetrahydrofuran, and vinyl monomers, are also soluble. On the opposite, water and ionic compounds are insoluble. Being relatively inert and nonpolar, the number of polymeric systems completely soluble in supercritical carbon dioxide is very limited (Licence et al., 2003; Beckman, 2004; Erdmenger et al., 2010; Cummings et al., 2011; Picchioni, 2014; Clarke et al., 2018). Since common hydrocarbon polymers are insoluble in CO2, the application of homogeneous polymerization in supercritical CO2 is limited and preponderantly replaced by processes in heterogeneous systems, that is, precipitation, dispersion, suspension, and emulsion polymerization. However, supercritical carbon dioxide exhibits relevant solubility values (up to a few weight percent) in different polymeric systems. Several polymers swell extensively in CO2 at moderate pressure, even though they are poorly soluble under mild conditions (97.5

86–99 96.7 – 99.1

87.72, 64.77, and 63.47

97.95

82–100

ND ND

(Li et al., 2019) (Hou et al., 2013)

(Kermanioryani et al., 2016) (Sidek et al., 2017)

(Chen et al., 2013) (Sadeghi and Nasehi, 2018)

(Mahajan et al., 2019)

References

ND l

(Continue)

(Deng et al., 2011)

(Brinda Lakshmi et al., 2013) l ND (Fan et al., 2014) 1.00 μg/L (Yu et al., 2016)

ND

l

l

l

ND l

ND l

2.7  and 1.4 μg/L

89.09 and 64.14 lND

89–98

Detection Extraction effitechnique ciency (%)

Table 16.1  Ionic liquid-assisted liquid–liquid extraction sample preparation methods for preconcentration of dyes or phenols in various matrices.

Water Water Waste water Water Bio-oil Water Model oil

Water Water

4 Phenols

4 Phenols Phenol, o-cresol, and resorcinol 4 Phenols

Selected phenols

4 Phenols

Phenol

Selected bis phenols

Phenols

c

b

a

      d   e   f   g   h   i   j   k   l  

SADBME b LLE c

DLLME

a

LLE

b

LLE

b

LLE

b

a

DLLME

b

b

LLE LLE

12

Dicationic liquid

[N8 8,8,1[FeCl4]

UV–Vis UV–Vis

HPLC i

HPLC– UV–Vis e

60–99.7

80.1

85.8–117.0

>90.0 ND

81.6 - 119.4

98.2–100.8

>99

90.2

GC-FID >96

h

HPLCUV–Vis g GCFID/MS d UV–Vis e

d

d

CE– UV–Vis f

Detection Extraction effitechnique ciency (%)

3 and 5 jUV–Vis

1.2

[PMIM][NTf2], ND [BMIM][NTf2] and [HMIM][NTf2] [Et2NEMIM][Cl]2, ND [Et2NEMMOR] [Cl]2, [Et2NEMPYR][Cl]2 and  [Et2NEMPIC] [Cl]2 [BMIM-Cl] and] ND BMIM-NTf2]

[Choline][NTf2]

3

7

5.07.0

[C4MIM][PF6], [C6MIM][PF6] and [C8MIM][PF6] [C2MIM]FSI [HMPYR][NTf2] and [HMIM][NTf2] [C8MIM][PF6]

DLLME

a

pH

Extraction Ionic liquid

DLLME, dispersive liquid–liquid microextraction. LLE, liquid–liquid extraction. SADBME, stirring-assisted drop-breakup microextraction. UV–Vis, ultraviolet–visible. HPLC–UV–Vis, high performance liquid chromatography–ultraviolet–visible. CE–UV–Vis, capillary electrophoresis–ultraviolet–visible. GC-FID/MS, gas chromatography-flame ionization detection/mass spectrometry. GC-FID, gas chromatography-flame ionization detection. HPLC, high performance liquid chromatography–ultraviolet–visible. UV–Vis, ultraviolet–visible. LOD, limit of detection. ND, not determine.

Malachite green oxalate Water dye

Matrix

Pollutant

(Zhou et al., 2012)

References

ND

(Cacho et al., 2016)

(Yao et al., 2018)

1.05–33.0 (Chatzimitakos µL−1 et al., 2016) l ND (Lv et al., 2018)

0.8–4.8 ng/mL

l

(Sas et al., 2018) (González et al., 2018) 0.27–0.68 (Zhou et al., μg/L 2011) l ND (Cesari et al., 2019) l ND (Sas et al., 2019)

5, 5, 8, and 100 ng/mL l ND l ND

LOD

k

Application of ionic liquids for extraction of phenolic compounds and dyes: A critical review

time, another report was published, which described the selected bis-phenol extraction from water by using dispersive LLME followed by HPLC–UV–Vis detection. In this reported study, two imidazolium-based ionic liquids with different anions were investigated and resulted into more than 90% recoveries, with good detection limits of 0.8–4.8 µg/L (Cacho et  al., 2016). In the same year, Yu and coworkers published dispersive liquid–liquid microextraction of 2-nitrophenol in water using [P6,6,6,14+]2[MnCl42−] ionic liquid, followed by high performance liquid chromatography–ultraviolet–visible analysis. However, poor extraction recoveries of 59.4–78.2% were obtained with good detection limits of 1.00 μg/L (Yu et al., 2016). It has to be noted that, Sadeghi and Nasehi were the first researchers to report the use of ionic liquid [OMIM][PF6]-assisted dispersive liquid–liquid microextraction of dyes (brilliant green and crystal violet) in water. The removal of dyes was monitored by the use of ultraviolet–visible analysis and extraction efficiencies ranging from 82% to 100% (Sadeghi and Nasehi, 2018). Crystal violet and Eriochrome black T dyes were also extracted from water by dispersive liquid–liquid microextraction, using three imidazolium-based ionic liquids such as [C6MIM][BF4], [C4MIM][PF6], and [C6MIM] [PF6], resulted in good recoveries (89–98%) (Mahajan et al., 2019). It has to be noted that, the use of dicationic liquid is also possible in LLE and the first findings on this concept were recently reported, whereby dyes from aqueous solution were extracted (98.2–100.8%) by using thermoregulated dicationic ionic liquids (Lv et al., 2018). Lastly, magnetic ionic liquid ([N8,8,8,1[FeCl4]) was use in stirring-assisted drop-breakup microextraction of phenolic endocrine disrupters in aqueous solutions. This new developed method showed excellent recoveries (more than 99%) and analysis was conducted by using HPLC–UV–Vis detector (Chatzimitakos et al., 2016). 16.2.1.2  Ionic liquid-assisted solid phase extraction of dyes and phenolic compounds Adsorption is one of the most powerful technologies used in analytical chemistry for removal of notorious pollutants from the environment and the tunable features of ionic liquids makes the adsorption concept to develop even more. These ionic liquids are normally used as modifiers to enhance the removal of target pollutant. Table 16.2 shows the ability of ionic liquids to act as modifiers in various adsorbents for removal of dyes or phenolic compounds in mainly water matrices. The first developments of the use of ionic liquids as adsorbent modifiers in solid phase extraction were reported in 2011. In this work, performance of supported ionic liquid membrane (SLM[CnMIM]+[X]−) was monitored (ultraviolet–visible) for removal of phenol in aqueous solution (Nosrati et al., 2011). In 2013, Gao and coworkers developed a similar concept whereby ionic liquid functionalized cross-linked polymers were used for removal of anionic azo dyes from aqueous solution (Gao et al., 2013a, 2013b). In both studies, polymeric imidazolium-based ionic liquids were used and adsorption models were pseudosecond order and Langmuir with adsorption capacities of 547.17–2100.98 mg/g. Then the report of Zarezadeh and

401

Polymeric imidazolium chloride Polymeric imidazolium chloride Polymeric vinylimidazole

IM-NPS

d

hb-PIm+PF6− PDVB-IL-OH

methyl orange

Anionic dyes

Orange II and sunset yellow FCF dyes Methyl blue dye e

r-GO-PIL

Imidazolium chloride

PIL-POM

a

DMDVAC

c

Acid red 87 dye j

5–10

[AMIM]Cl

ND

4

ND

ND

ND

5.6 3–7

2.0

Polymeric pyrro- No effect lidinium

Imidazolium

pH

Polymeric imidazolium chloride j DMDVAC

Montmorillonite

Amaranth dye

Ionic liquid

Orange G, orange Poly ionic liquid II and sunset yel- Pyr+AA-TFSI− low dyes Orange II, sunset aPDVB-IL-OH yellow FCF, and amaranth dyes 4 ionic dyes Fe3O4@SiO2@ poly ionic liquid b Congo red dye IL-CCS

Adsorbent

Analyte

Pseudosecond order and Langmuir Langmuir

Pseudosecond order and Langmuir Freundlich and Langmuir Pseudosecond order and Freundlich Pseudosecond order Pseudosecond order and Langmuir Langmuir

Pseudosecond order and Langmuir Freundlich and Langmuir

(Lawal and Moodley, 2015)

1910

2095.80 and 2100.98

2050

135

1108.9

288.63 

510

925.09, 734.62, and 547.17

(Zhao et al., 2015)

(Gao et al., 2013b)

(ZarezadehMehrizi et al., 2013) (Song et al., 2016)

(Yang et al., 2019)

(Lyu et al., 2019)

(Yang et al., 2019)

(Gao et al., 2013a)

198.4, 279.3, and (Makrygianni 316.5 et al., 2019)

263.2

Adsorption capacAdsorption model ity (mg/g) References

Table 16.2  Ionic liquid-assisted solid phase extraction followed by ultraviolet–visible or high performance liquid chromatographic analysis for determination of dyes or phenols in various matrices.

402 Green sustainable process for chemical and environmental engineering and science

Chitosan ionic liquid beads IL-P PILs@GO@Sil PS-CH2

f

g

SLM

Malachite green dye

3 Phenols

Phenolic acid p-Nitrophenol

Phenol

Si-DHIM-NH2 4.0

2.0

2.0–10.0

[CnMIM]+ X−

5.5

Polymeric 7.0 imidazolium bromide VHIM+PF6− 8.0–10.0 [C2NH2MIM] Br 1.0–4.0

[BMIM] Ac and [BMIM] Cl

k

[AMIM]Cl

b

a

  PDVB-IL-OH, hydroxyl-functionalized ionic liquid-based cross-linked polymer.   IL-CCS, ionic liquid functionalized cross-linked chitosan. c   PIL-POM, poly-ionic liquid-based polyoxometalate hybrid. d   IM-NPS, imidazolium nanoporous silica. e   r-GO-PIL, reduced graphene oxide-poly-ionic liquid. f   IL-P, ionic liquid functionalized polymer. g   PILs@GO@Sil, poly-ionic liquid-based graphene oxide and silica. h  PS-CH2, chloromethyl polystyrene resin. i   SLM, supported liquid membrane. j   DMDVAC, dimethyl-dodecyl-4-vinylbenzyl ammonium chloride. k  Si-DHIM-NH2, amino functionalized ionic liquid. l   ND, not determined.

i

h

Fe3O4@nSiO2@mSiO2

Orange II and amaranth dyes

IL-CCS

b

Sunset yellow FCF dye

Langmuir and pseudosecond order Langmuir and pseudosecond order Langmuir and pseudosecond order Langmuir and pseudosecond order ND Pseudosecond order and Freundlich l ND

(Lyu et al., 2019)

ND l

ND 1269.8

239.7, 68.39, 56.86 and 64.28

8.07 and 0.24

(Nosrati et al., 2011)

(Hou et al., 2018) (Cheng et al., 2019)

(Zhu et al., 2019)

(Naseeruteen et al., 2018)

153.06 and 84.40 (Cheng et al., 2016)

300.28 

Application of ionic liquids for extraction of phenolic compounds and dyes: A critical review

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Green sustainable process for chemical and environmental engineering and science

coworkers followed, which described the use of ionic liquid functionalized nonporous silica for removal of anionic dye from water with 135 mg/g maximum adsorption capacity (Zarezadeh-Mehrizi et al., 2013). Song et al. reported another imidazolium-based polymeric ionic liquid for removal of anionic dyes with 2050 mg/g adsorption capacity with Langmuir better fit model (Song et al., 2016). The removal of anionic dyes from aqueous solution by novel pyrrolidinium-based polymeric ionic liquid adsorbent also showed good adsorption capacities of 198.4–316.5 mg/g (Makrygianni et al., 2019). Recently, interesting work was published by Zhu et al. on the same concept, where an ionic liquid functionalized polymer was used to facilitate simultaneous removal of four phenolic pollutants in real environmental samples with adsorption capacities of 64.28–239.7 mg/g, depending on the phenolic compound investigated (Zhu et  al., 2019). Nanocomposites were also reported to be good adsorbents when modified with ionic liquid (Lawal and Moodley, 2015; Yang et al., 2019; Cheng et al., 2016). Additionally, carbon-based adsorbents were also reported to be good when combined with ionic liquids (Naseeruteen et al., 2018; Hou et al., 2018). It has to be noted that imidazolium-based ionic liquids were the most reported solvents for the extraction of dyes and phenolic compounds. 16.2.1.3  Ionic liquid in biphasic extraction methods Aqueous  two-phase systems composed of nonionic surfactants and an ionic liquid are termed as biphasic. These systems are sometimes called aqueous biphasic systems and are promising tools for extraction of industrial pollutants in various matrices (Poole and Poole, 2010). Aqueous biphasic systems or aqueous two-phase systems based on ionic liquids are environmentally safer option to replace conventional LLE. These systems take advantage of water-miscible ionic liquids, and are composed of two macroscopic liquid phases, typically formed by different pairs of aqueous solutions (polymer–polymer, polymer–salt, or salt–salt) at suitable concentration levels. Current research activities have reported the exceptional performance of these systems in different separation and extraction procedures (de Souza et al., 2014; Ferreira et al., 2014; Escudero et al., 2019). However, there are few literature reports that have investigated the use of these systems on the extraction of phenols and dyes.

16.3 Summary Fig. 16.1 shows number of publications that have been reported for extraction of dyes and phenols prior to their determination. From this figure it can be observed that, phenols were mostly determined after LLE sample preparation method (11 papers), while dyes were extracted by using SPE methods (13 papers). However, DLLME was only reported in eight papers (six: phenols and two: dyes). It has to be noted that aqueous biphasic extraction systems are only reported for the removal of dyes with three papers, while stirring-assisted drop-breakup microextraction was only reported for phenols

Application of ionic liquids for extraction of phenolic compounds and dyes: A critical review

Fig. 16.1  Published sample preparation methods (ABE, aqueous biphasic extraction and other: stirring-assisted drop-breakup microextraction; DLLME, dispersive liquid–liquid-microextraction; LLE, liquid–liquid extraction; SPE, solid phase extraction; ) for extraction of phenols and dyes in various matrices prior to their chromatographic or spectrophotometric determination.

with one paper. The trend illustrated in Fig. 16.1 shows that there is still more work that still need to be done on the removal so phenols from environmental matrices.

16.4 Conclusion Ionic liquids are indeed the future of sample preparation technology in analytical chemistry at large. The tunable features of these room temperature solvents make it easy to develop more concepts that are fast, environmentally friendly and reliable. From the published work it can be observed that, imidazolium-based ionic liquids are the most investigated solvents in all the above discussed extraction procedures. Additionally, phenolic compounds were more dominated on the liquid–liquid extraction methods, while dyes were mostly removed by using various adsorbents. Both dyes and phenolic compounds are environmental pollutants, but the most investigated matrix was water. However, the developed methods can either remove or detect dye or phenolic compounds, therefore, more work need to be done on the design of extraction systems that can remove both dyes and phenols simultaneously, since these two normally occur concurrently in the environment. In addition, there is still more work that can be performed on the use of aqueous biphasic system and the combination of liquid–liquid and solid phase extraction methods.

Acknowledgments The South African National Research Foundation-THUTHUKA (TTK170418227444), University of South Africa, and University of Johannesburg are acknowledged for their financial assistant.

405

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Application of ionic liquids for extraction of phenolic compounds and dyes: A critical review

Han, D., et al., 2012. Application of ionic liquid in liquid phase microextraction technology. J. Sep. Sci. 35 (21), 2949–2961. doi:10.1002/jssc.201200486. Han, D., Row, K.H., 2010. Recent Applications of Ionic Liquids in Separation Technology. 15, 2405–2426. doi:10.3390/molecules15042405. Holkar, C.R., et al., 2016. A critical review on textile wastewater treatments: possible approaches. J. Environ. Manage. 182, 351–366. doi:10.1016/j.jenvman.2016.07.090. Hou, X., et  al., 2018. Graphene oxide reinforced ionic liquid-functionalized adsorbent for solid-phase extraction of phenolic acids. J. Chromatogr. B 1072 (November 2017), 123–129. doi:10.1016/j. jchromb.2017.11.013. Hou,Y., et al., 2013. Separation of phenols from oil using imidazolium-based ionic liquids. Ind. Eng. Chem. Res. 52 (50), 18071–18075. doi:10.1021/ie403849g. Isosaari, P., Srivastava, V., Sillanpää, M., 2019. Ionic liquid-based water treatment technologies for organic pollutants: current status and future prospects of ionic liquid mediated technologies. Sci. Total Environ. 690, 604–619. doi:10.1016/j.scitotenv.2019.06.421. Kaur, P., et al., 2018. Performance and mechanism of dye extraction from aqueous solution using synthesized deep eutectic solvents. Colloids Surfaces A: Physicochem. Eng. Aspects 539 (October 2017), 85–91. doi:10.1016/j.colsurfa.2017.12.013. Kermanioryani, M., et al., 2016. Using tunability of ionic liquids to remove methylene blue from aqueous solution. J. Environ. Chem. Eng. 4 (2), 2327–2332. doi:10.1016/j.jece.2016.04.008. Lawal, I.A., et al., 2019a. Brief bibliometric analysis of “ionic liquid” applications and its review as a substitute for common adsorbent modifier for the adsorption of organic pollutants. Environ. Res. 175 (May), 34–51. doi:10.1016/j.envres.2019.05.005. Lawal, I.A., et al., 2019b. Theoretical and experimental adsorption studies of phenol and crystal violet dye on carbon nanotube functionalized with deep eutectic solvent. J. Mol. Liq. 288, 110895. doi:10.1016/j. molliq.2019.110895. Lawal, I.A., Moodley, B., 2015. Synthesis, characterisation and application of imidazolium based ionic liquid modified montmorillonite sorbents for the removal of amaranth dye. RSC Adv. 5 (76), 61913–61924. doi:10.1039/c5ra09483f. Li, Z., et al., 2019. Anionic structural effect in liquid–liquid separation of phenol from model oil by choline carboxylate ionic liquids. Green Energy Environ. 4 (2), 131–138. doi:10.1016/j.gee.2019.01.011. Lv, R., et al., 2018. Solid-liquid separation properties of thermoregulated dicationic ionic liquid as extractant of dyes from aqueous solution. J. Anal. Methods Chem. 2018. doi:10.1155/2018/3980364. Lyu, H., Fan, J., et  al., 2019. Functionalized cross-linked chitosan with ionic liquid and highly efficient removal of azo dyes from aqueous solution. Int. J. Biol. Macromol. 126, 1023–1029. doi:10.1016/j. ijbiomac.2018.12.117. Lyu, H., Ling,Y., et al., 2019. Preparation of ionic liquid-functionalized layered double hydroxide via thiolene click chemistry for highly efficient removal of azo dyes during broad pH range. J. Cleaner Prod. 211, 1026–1033. doi:10.1016/j.jclepro.2018.11.260. Ma, J., Hong, X., 2012. Application of ionic liquids in organic pollutants control. J. Environ. Manage. 99, 104–109. doi:10.1016/j.jenvman.2012.01.013. Mahajan, S., et al., 2019. Evaluation and mechanism of cationic/anionic dyes extraction from water by ionic liquids. Chem. Eng. Commun. 206 (6), 697–707. doi:10.1080/00986445.2018.1520706. Makrygianni, M., et  al., 2019. Removal of anionic dyes from aqueous solution by novel pyrrolidiniumbased polymeric ionic liquid (PIL) as adsorbent: investigation of the adsorption kinetics, equilibrium isotherms and the adsorption mechanisms involved. J. Environ. Chem. Eng. 7 (3), 103163. doi:10.1016/j. jece.2019.103163. Moradi, M., et  al., 2010. Application of surfactant assisted dispersive liquid-liquid microextraction for sample preparation of chlorophenols in water samples. Talanta 82 (5), 1864–1869. doi:10.1016/j. talanta.2010.08.002. Naseeruteen, F., et al., 2018. Adsorption of malachite green from aqueous solution by using novel chitosan ionic liquid beads. Int. J. Biol. Macromol. 107 (PartA), 1270–1277. doi:10.1016/j.ijbiomac.2017.09.111. Nosrati, S., Jayakumar, N.S., Hashim, M.A., 2011. Performance evaluation of supported ionic liquid membrane for removal of phenol. J. Hazard. Mater. 192 (3), 1283–1290. doi:10.1016/j.jhazmat.2011.06.037. Pinheiro, H.M., Touraud, E., Thomas, O., 2004. Aromatic amines from azo dye reduction: status review with emphasis on direct UV spectrophotometric detection in textile industry wastewaters. Dyes Pigments 61 (2), 121–139. doi:10.1016/j.dyepig.2003.10.009.

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Poole, C.F., Lenca, N., 2015. Green sample-preparation methods using room-temperature ionic liquids for the chromatographic analysis of organic compounds.TrAC - Trends Anal. Chem. 71, 144–156. doi:10.1016/j. trac.2014.08.018. Poole, C.F., Poole, S.K., 2010. Extraction of organic compounds with room temperature ionic liquids. J. Chromatogr. A 1217 (16), 2268–2286. doi:10.1016/j.chroma.2009.09.011. Rodríguez, I., Llompart, M.P., Cela, R., 2000. Solid-phase extraction of phenols. J. Chromatogr. A 885 (1–2), 291–304. doi:10.1016/S0021-9673(00)00116-3. Rovira, J., Domingo, J.L., 2019. Human health risks due to exposure to inorganic and organic chemicals from textiles: a review. Environ. Res. 168 (August 2018), 62–69. doi:10.1016/j.envres.2018.09.027. Sadeghi, S., Nasehi, Z., 2018. Simultaneous determination of Brilliant Green and Crystal Violet dyes in fish and water samples with dispersive liquid-liquid micro-extraction using ionic liquid followed by zero crossing first derivative spectrophotometric analysis method. Spectrochim. Acta - Part A: Mol. Biomol. Spectrosc. 201, 134–142. doi:10.1016/j.saa.2018.04.061. Sajid, M., 2019. Magnetic ionic liquids in analytical sample preparation: a literature review. TrAC - Trends Anal. Chem. 113, 210–223. doi:10.1016/j.trac.2019.02.007. Sas, O.G., et  al., 2018. Liquid-liquid extraction of phenolic compounds from water using ionic liquids: literature review and new experimental data using [C 2 mim]FSI. J. Environ. Manage. 228 (September), 475–482. doi:10.1016/j.jenvman.2018.09.042. Sas, O.G., et  al., 2019. Using bis(trifluoromethylsulfonyl)imide based ionic liquids to extract phenolic compounds. J. Chem. Thermodyn. 131, 159–167. doi:10.1016/j.jct.2018.11.002. Sekar, S., et al., 2012. Choline-based ionic liquids-enhanced biodegradation of azo dyes. Environ. Sci.Technol. 46 (9), 4902–4908. doi:10.1021/es204489h. Sidek, N., Ninie, N.S., Mohamad, S., 2017. Efficient removal of phenolic compounds from model oil using benzyl imidazolium-based ionic liquids. J. Mol. Liq. 240, 794–802. doi:10.1016/j.molliq.2017.05.111. Song,W., et al., 2016. Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes. Chem. Eng. J. 287, 482–491. doi:10.1016/j.cej.2015.11.039. Yagub, M.T., et al., 2014. Dye and its removal from aqueous solution by adsorption: a review. Adv. Colloid Interface Sci. 209, 172–184. doi:10.1016/j.cis.2014.04.002. Yang, H., Bai, L., et al., 2019. Ionic self-assembly of poly(ionic liquid)-polyoxometalate hybrids for selective adsorption of anionic dyes. Chem. Eng. J. 358 (October 2018), 850–859. doi:10.1016/j.cej.2018.10.100. Yang, H., Zhang, J., et al., 2019. Rapid removal of anionic dye from water by poly(ionic liquid)-modified magnetic nanoparticles. J. Mol. Liq. 284, 383–392. doi:10.1016/j.molliq.2019.04.029. Yao, C., et al., 2018. Efficient separation of phenolic compounds from model oils by dual-functionalized ionic liquids. Chem. Eng. Process. - Process Intensification 128 (December 2017), 216–222. doi:10.1016/j. cep.2018.04.026. Yu, H., Merib, J.,Anderson, J.L., 2016. Faster dispersive liquid-liquid microextraction methods using magnetic ionic liquids as solvents. J. Chromatogr. A 1463, 11–19. doi:10.1016/j.chroma.2016.08.007. Zarezadeh-Mehrizi, M., Badiei, A., Mehrabadi, A.R., 2013. Ionic liquid functionalized nanoporous silica for removal of anionic dye. J. Mol. Liq. 180, 95–100. doi:10.1016/j.molliq.2013.01.007. Zhao, L., Lee, H.K., 2001. Determination of phenols in water using liquid phase microextraction with back extraction combined with high-performance liquid chromatography. J. Chromatogr.A 931 (1–2), 95–105. doi:10.1016/S0021-9673(01)01199-2. Zhao, W., et al., 2015. Functionalized graphene sheets with poly(ionic liquid)s and high adsorption capacity of anionic dyes. Appl. Surface Sci. 326, 276–284. doi:10.1016/j.apsusc.2014.11.069. Zhou, C., et al., 2012. Ionic liquid-based dispersive liquid-liquid microextraction with back-extraction coupled with capillary electrophoresis to determine phenolic compounds. Electrophoresis 33 (8), 1331–1338. doi:10.1002/elps.201100469. Zhou, F., Li, X., Zeng, Z., 2005. Determination of phenolic compounds in wastewater samples using a novel fiber by solid-phase microextraction coupled to gas chromatography. Anal. Chim. Acta 538 (1–2), 63–70. doi:10.1016/j.aca.2005.02.009. Zhou, Q., et al., 2011. Sensitive determination of phenols from water samples by temperature-controlled ionic liquid dispersive liquid-phase microextraction.Anal. Meth. 3 (3), 653–658. doi:10.1039/c0ay00619j. Zhu, G., et al., 2019. An ionic liquid functionalized polymer for simultaneous removal of four phenolic pollutants in real environmental samples. J. Hazard. Mater. 373 (January), 347–358. doi:10.1016/j.jhaz mat.2019.03.101.

CHAPTER 17

Green methods for extraction of phenolic compounds Muhammad Sajid Hamid Akasha, Kanwal Rehmanb, Anam Shabbirc, Shagufta Kamald, Muhammad Ibrahime Department of Pharmaceutical Chemistry, Government College University, Faisalabad, Pakistan Department of Pharmacy, The Women University, Multan, Pakistan c LIAS College of Pharmacy, Faisalabad, Pakistan d Department of Biochemistry, Government College University, Faisalabad, Pakistan e Department of Applied Chemistry, Government College University, Faisalabad, Pakistan a

b

17.1 Introduction Phenolic compounds contain aromatic ring conferring hydroxyl group. The compounds having phenolic groups more than one are termed as polyphenols. Phenolic compounds being originated from natural sources are also called as phytochemicals. They have variable distribution is in all parts of the plant, including edible and nonedible parts like fruits, seeds, leaves, stems, roots. There are more than 8000 different phenolic compounds found in plants, with distinct variety of structures. Phenolic compounds can be classified majorly into phenolic acids, flavonoids, and tannins. The largest group of the plant phenolic compounds is flavonoids that can be further classified into six classes like flavonones, flavones, flavanols, isoflavones, flavanols, and anthocyanadins. The other important class of phenolic compounds is phenolic acids which can also be further classified into two subclasses on the basis of chemistry: hydroxybenzoic acid and hydroxycinnamic acids (Martins et al., 2011). Tannins basically are natural polyphenolics. Tannins are classified into condensed and hydrolysable tannins (Soto-Vaca et al., 2012). In the past years, phenolic compounds have been gained a serious intention for their beneficial role in human health. These phytochemicals have been investigated for their specific features and health benefits. Flavonoids and phenolic acids are biologically active compounds and synthesized as secondary metabolites. Some of their valuable roles is to improve the human health by acting as an antioxidant and having anticancer potential (Gawlik-Dziki et al., 2013), possessing antidiabetic activity (You et  al., 2012), reducing risk factors of cardiovascular diseases (Jiménez et  al., 2008), by acting as anti-inflammatory (Fang et al., 2008; Hsu et al., 2013), and having antimicrobial effects (Cicerale et al., 2012). Because of such great valuable features for human health, extensive researches are being conducted to explore the more sources

Green Sustainable Process for Chemical and Environmental Engineering and Science DOI: https://doi.org/10.1016/B978-0-323-95156-2.00015-5

© 2023 Elsevier Inc. All rights reserved.

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of phenolic compounds for their beneficial effects and to increase the quality of health of human beings. The extraction of phenolic compounds is carried out through various processes by the application of different techniques which can be classified into conventional techniques and modern techniques (Azmir et al., 2013). The extensively used extraction technique is liquid–liquid extraction. It is an economical method that involves the utilization of organic solvents, but with limitations of extract degradation and more time consumption. Similarly, solid-phase extraction may be applied in liquid samples. There are some modern and nonconventional techniques are: pressurizedliquid extraction, supercritical fluid extraction, ultrasound-assisted extraction, and microwave-assisted extraction. These nonconventional methods considerably minimize the utilization of solvent as well as speed up the process of extraction (GarciaSalas et al., 2010).

17.2  Methods of extractions of phenolic compounds Extraction is an isolation process in which analyte of phenolic compounds is distributed between two immiscible liquid phases in such a way that to attain a suitable distribution coefficient (Dobiáš et  al., 2010). For extraction of phenolic compounds, the process comprises sequenced and systematic based, and with the utilization of organic aqueous solvent from different sources and parts of plants. This conventional technique is known as liquid–liquid extraction. For this method of extraction, different solvents such as acetone, ethanol or methanol, or their combination with water are used (Ross et al., 2009). The conventional Soxhlet system is still a reference method for the extraction and used for comparison of newly developing techniques. This system has extensively been applied to extract the important bioactive natural compounds from different sources (Azmir et al., 2013). Extraction process should be executed with preset analytical conditions like pH, pressure, and temperature with suitable solvent.Variety of techniques must be utilized to optimize the extraction of phenolic compounds from different plant sources. Basically, all these different techniques have some common goals, (1) extraction of required bioactive compounds from multipart plant samples, (2) enhancement of the selectivity of analytical methods, (3) enhancement of sensitivity of bioassay by gradually increasing the bioactive compounds concentration, (4) conversion of targeted compounds into appropriate formation for separation as well as detection, and (5) establishment of most suitable technique that has no dependency of changes in sample medium (Smith, 2003). Conventional methodologies have certain disadvantages, particularly usage of bulk quantities of solvents and long extraction time. However, these conventional methods provide base to utilize variable experimental conditions for optimization for large-scale applications.

Green methods for extraction of phenolic compounds

17.2.1  Liquid–liquid extraction Chemical nature in plant origin is very important factor that governs the solubility of phenolic compounds. Different parts of the plants contain variable quantities of phenylpropanoids, phenolic acids, tannins, and anthocyanins. Phenolic compounds may also form conjugated compounds by the interaction of other organic molecules like protein and carbohydrates which are insoluble. Similarly, the degree of polarization of solvent influences the solubility of phenolic compounds. Consequently, it is not possible to fix the constant extraction technique suitable for different phenolic compounds extraction. Plant-source extracts of phenolic compounds are usually diversified mixture which is soluble in the system of solvent used and may be required further processes to isolate the unwanted phenolic compounds and also to remove the nonphenolic constituents such as chlorophylls, terpenes, fats, and waxes (Naczk and Shahidi, 2006). Liquid–liquid extraction technique requires the hazardous and expensive organic solvents that are not suitable for health, and more time consuming that may possibly cause the degradations. Other factors of degradation are light, air, and temperature. The temperature for extraction is usually high in order to accelerate the process. Due to said reasons, these methodologies are being substituted by modern techniques (Fig. 17.1) because they are more fast, sensitive selective, and friendly to environment (Mahugo Santana et al., 2009).

Fig. 17.1  Schematic representation of conventional and green methods for the extraction of phenolic compounds.

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17.2.2  Solid-phase extraction It is a very suitable technique for the preparation of sample. The cost of equipment used in solid-phase extraction is higher than that of the liquid–liquid extraction but some problems are related with later, like usage of expensive organic solvents in bulk quantities, incomplete phase partitioning. Solid-phase extraction technique is often used for the preparation of liquid extract and samples of none or semivolatile analytes. It is also used for pre-extracted solids into solvents. In solid-phase extraction, varieties of solid phases have been experienced for the determination of phenolic compounds in grapes or beverages. Styrene-divinylbenzene polymers exhibited good results, but less satisfactory results showed by bases comprise carbon-18 based for polar phenolic compounds (Gómez Caravaca et al., 2005; Palma et al., 2002). 17.2.3  Supercritical fluid extraction It is a very useful technique for the extraction of phenolic compounds. Optimal utilization of temperature and pressure in supercritical fluid extraction increases the rate of extraction. Supercritical fluid extraction is most often utilizing carbon dioxide. Carbon dioxide has the ability to preserve the bioactive compounds in extract from atmospheric oxidation and low critical temperature (Da Silva et al., 2016). Other advantage is the less energy consumption as well as obtaining high quality product because solute phase contains no solvent. Though, this methodology exhibits certain limitations for low and medium polarity compounds. Basically, supercritical fluid system comprises the following parts: a mobile phase (usually carbon dioxide) tank, a pressure making pump, a pump for cosolvent and vessel, a vessel for extraction, a trapping vessel, and a controller. According to the requirement, other meters like wet gas meter, dry gas meter, and flow meter can be installed in the system. Carbon dioxide has low critical temperature (31 °C) and pressure (74 bars) that makes the possibility to work at moderate pressures 100–450 bar (Temelli and Güçlü‐Üstündağ, 2005). Low polarity of carbon dioxide makes it best choice for nonpolar substances like lipid, fat, etc. but meanwhile it is not applicable for most drug samples and pharmaceuticals which is the only limitation of this technique. This limitation can be overcome by using different chemical modifiers. Extraction techniques for phenolic compounds by supercritical fluid extraction with the utilization of variable percentages of organic modifier requirements are well described in literature (Garcia-Salas et al., 2010). Usually, during the extraction procedure, various steps are involved: loading of samples sorbent placed in a cartridge. This cartridge is introduced into the extraction cell. Then supercritical liquid like carbon dioxide passes through the hydrolyzed sample-filled supercritical fluid extraction cartridge. A trapping solvent is used to trap the analyte containing phenolic compounds. When the carbon dioxide expands it decreases the temperature of trapped solvent naturally and trapped solvent cools down and the extracts are dried by evaporation. Then for analysis in high-performance liquid

Green methods for extraction of phenolic compounds

chromatography/electrospray ionization-mass spectrometry system, the extracts are dissolved in mobile phase (Mahugo Santana et al., 2009). The applications of supercritical fluid extraction in different areas such as food science, pharmaceutical, and environmental science have opened new era of advancement and utilization of modern techniques to optimize the extraction methodology (Herrero et al., 2010). 17.2.4  Pressurized liquid extraction It is also known as high-pressure solvent extraction (Nieto et al., 2010) that utilizes the high temperature above than the boiling point of solvents with high pressure (Fig. 17.2). The main features of pressurized-liquid extraction technique are automation process, less solvent requirement, and decrease extraction time. High pressure and temperature both techniques favor the less utilization of solvent as well as extraction time. The application of higher temperature improves extraction rate by promoting increase analyte solubility by minimizing the surface tension as well as viscosity of the solvents and also by increasing the rate of mass transfer (Ibañez et al., 2012). Phenolic compounds like isoflavones have been successfully extracted with degradation by the application of pressurized-liquid extraction with optimized condition from natural source. Generally, a solid sample is loaded into extraction cell made up stainless steel and a suitable solvent used for extraction under high pressure (500–3000 lbf/in2), high temperature (40–200 °C), and for less time period (5–15 minutes). Extract sample is collected in collection vial by using compressed gas (Garcia-Salas et al., 2010). 17.2.5  Microwave-assisted extraction It is a modern technology which is useful for the extraction of phenolic compounds from different plant materials. The microwave-assisted extraction, operates with the effect of microwave energy to expedite the partition analyte into solvent. In microwaveassisted extraction main physical factors are extraction time, microwave power, solvent property, solubility, and dielectric constant. Among these, solvent property is a critical factor as more microwave energy can be absorbed by solvents possessing high dielectric constants like water, methanol, ethanol, etc. are generally used for the extraction of phenolic compounds (Fang et al., 2015). In microwave-assisted extraction, bioactive compounds can be extracted more rapidly with better recovery than that of the conventional extraction techniques. The frequency range of electromagnetic fields is 300 MHz–300 GHz. Electromagnetic field comprises on two fields, an electric field and a magnetic field, both are perpendicular to each other. Dipole rotation and ionic conduction, these mechanisms are followed by the conversion of electromagnetic energy into heat. Polar materials are greatly affected by heat produced from microwaves. Microwaveassisted extraction is also known as environment friendly because it facilitates the less utilization of solvent (Alupului et al., 2012). It also prevents the phenolic compounds from the exposure of light and prevents them to undergo the degradation as they are

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Fig. 17.2  Operational diagrammatic mechanism and principle of pressurized liquid extraction method.

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Table 17.1  Applications of microwave-assisted extraction to phenolic compound extraction.

Plant material

Targeted phenolic compound

Citrus mandarin

Phenolic antioxidants

Citrus mandarin (Peels)

Phenolic acids

Silybum marianum

Silybinin

Eucommia ulmodies

Phenolic acids

Physalis angulata

Flavonoids and Phenolic acids

Radix astragali

Flavonoids

Hippophae rhamnoides

Flavonoids

Cortex fraxini

Flavone and coumarin

Extraction conditions

Microwave power (400 W); extraction time (3 min); solid to solvent ratio (1:2); & extraction temperature (383°C) Microwave power (400 W); extraction time (15 min); & no addition of solvent Microwave power (1000 W); & 1.0 g sample in 40 mL polyethylene glycol solution

Microwave system

References

Microwave oven

(Ahmad and Langrish, 2012)

Microwave oven

(Dhobi et al., 2009) (Hayat et al., 2009)

Domestic microwave Open vessel microwaveassisted extraction Microwave reactor model Open vessel microwaveassisted extraction Multimode microwave reactor MAS-II microwave oven

(Li et al., 2004) (Carniel et al., 2017) (Xiao et al., 2008) (PérinoIssartier et al., 2011) (Zhou et al., 2011)

very sensitive to light and may undergo the degradation as in pressurized-liquid extraction and supercritical fluid extraction. Table 17.1 represents the various applications of microwave-assisted extraction to phenolic compound extraction. 17.2.6  Ultrasound-assisted extraction It is an economical and more convenient technique as low-cost instruments are utilized. Sonication is produced by sound waves that causes the cavitation bubbles, which destroy

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cell walls of plant cell causing release of cellular content (Cheng et  al., 2015). This radiation technique is a powerful accelerator to speed up the process of extraction. In ultrasound-assisted extraction process, the energy is utilized for pretreatment like homogenization of sample to speed up and accelerate the processes of extraction particularly organic and inorganic. Ultrasound-assisted extraction utilizes less time duration with more efficient technology for the extraction of different compounds (Dobiáš et al., 2010).

17.3 Conclusion In this chapter, different techniques for the extraction of phenolic compounds have been discussed. Conventional extraction techniques are still being applied for both the extraction as well as reference procedures. Some modern and advanced techniques of extraction like ultrasound-assisted extraction, solid–liquid extraction, microwave-assisted extraction, supercritical fluid extraction, and pressurized-liquid extraction have improved the yield of extraction.These techniques not only environment friendly but also maintain the quality of extract and are very helpful to prevent the phenolic compounds from degradation. Among these techniques, solid–liquid extraction is the most convenient and simple technique that is applicable in wide extractions procedures of phenolic compounds, but with limitation of low efficiency of the extraction. Ultrasound-assisted extraction is an economical, simple, and efficient technique which is generally applied in laboratory. For thermostable phenolic compounds, microwave-assisted extraction is more efficient technique with less utilization of time as well as solvents.

17.4 Summary The importance of phenolic compounds has been accepted due to their various beneficial effects on health. The modern techniques have efficiently improved the extraction processes of phenolic compounds by less utilization of expensive solvents with less consumption of time and are gradually replacing the outdated techniques.

Conflict of interest Nothing to declare.

References Ahmad, J., Langrish, T., 2012. Optimisation of total phenolic acids extraction from mandarin peels using microwave energy: the importance of the Maillard reaction. J. Food Eng. 109, 162–174. Alupului, A., Calinescu, I., Lavric,V., 2012. Microwave extraction of active principles from medicinal plants. UPB Sci. Bull. Ser. B 74, 1454–2331. Azmir, J., Zaidul, I., Rahman, M., Sharif, K., Mohamed, A., Sahena, F., Jahurul, M., Ghafoor, K., Norulaini, N., Omar, A., 2013. Techniques for extraction of bioactive compounds from plant materials: a review. J. Food Eng. 117, 426–436. doi:10.1016/j.jfoodeng.2013.1001.1014.

Green methods for extraction of phenolic compounds

Carniel, N., Dallago, R.M., Dariva, C., Bender, J.P., Nunes, A.L., Zanella, O., Bilibio, D., Luiz Priamo, W., 2017. Microwave-assisted extraction of phenolic acids and flavonoids from Physalis angulata. J. Food Process Eng. 40, e12433. Cheng, X., Zhang, M., Xu, B., Adhikari, B., Sun, J., 2015. The principles of ultrasound and its application in freezing related processes of food materials: a review. Ultrason. Sonochem. 27, 576–585. doi:10.1016/j. ultsonch.2015.1004.1015. Cicerale, S., Lucas, L., Keast, R., 2012. Antimicrobial, antioxidant and anti-inflammatory phenolic activities in extra virgin olive oil. Curr. Opin. Biotechnol. 23, 129–135. doi:10.1016/j.copbio.2011.1009.1006. Da Silva, R.P., Rocha-Santos,T.A., Duarte,A.C., 2016. Supercritical fluid extraction of bioactive compounds. Trends Anal. Chem. 76, 40–51. https://doi.org/10.1016/j.trac.2015.1011.1013. Dhobi, M., Mandal, V., Hemalatha, S., 2009. Optimization of microwave assisted extraction of bioactive flavonolignan-silybinin. J. Chem. Metrol. 3 (1), 13–23. Dobiáš, P., Pavlíková, P., Adam, M., Eisner, A., Beňová, B.,Ventura, K., 2010. Comparison of pressurised fluid and ultrasonic extraction methods for analysis of plant antioxidants and their antioxidant capacity. Open Chem. 8, 87–95. https://doi.org/10.2478/s11532-11009-10125-11539. Fang, S.-C., Hsu, C.-L., Yen, G.-C., 2008. Anti-inflammatory effects of phenolic compounds isolated from the fruits of Artocarpus heterophyllus. J. Agric. Food Chem. 56, 4463–4468. doi:10.1021/jf800444g. Fang, X., Wang, J., Hao, J., Li, X., Guo, N., 2015. Simultaneous extraction, identification and quantification of phenolic compounds in Eclipta prostrata using microwave-assisted extraction combined with HPLC– DAD–ESI–MS/MS. Food Chem. 188, 527–536. doi:10.1016/j.foodchem.2015.1005.1037. Garcia-Salas, P., Morales-Soto, A., Segura-Carretero, A., Fernández-Gutiérrez, A., 2010. Phenoliccompound-extraction systems for fruit and vegetable samples. Molecules 15, 8813–8826. doi:10.3390/ molecules15128813. Gawlik-Dziki, U., Świeca, M., Sułkowski, M., Dziki, D., Baraniak, B., Czyż, J., 2013. Antioxidant and anticancer activities of Chenopodium quinoa leaves extracts–in vitro study. Food Chem.Toxicol. 57, 154–160. doi:10.1016/j.fct.2013.1003.1023. Gómez Caravaca,A.M., Carrasco Pancorbo,A., Cañabate Díaz, B., Segura Carretero,A., Fernández Gutiérrez, A., 2005. Electrophoretic identification and quantitation of compounds in the polyphenolic fraction of extra-virgin olive oil. Electrophoresis 26, 3538–3551. doi:10.1002/elps.200500202. Hayat, K., Hussain, S., Abbas, S., Farooq, U., Ding, B., Xia, S., Jia, C., Zhang, X., Xia, W., 2009. Optimized microwave-assisted extraction of phenolic acids from citrus mandarin peels and evaluation of antioxidant activity in vitro. Sep. Purif. Technol. 70, 63–70. Herrero, M., Mendiola, J.A., Cifuentes, A., Ibáñez, E., 2010. Supercritical fluid extraction: recent advances and applications. J. Chromatogr. A 1217, 2495–2511. doi:10.1016/j.chroma.2009.2412.2019. Hsu, C.-L., Fang, S.-C., Yen, G.-C., 2013. Anti-inflammatory effects of phenolic compounds isolated from the flowers of Nymphaea mexicana Zucc. Food Funct. 4, 1216–1222. doi:10.1039/c1213fo60041f. Ibañez, E., Herrero, M., Mendiola, J.A., Castro-Puyana, M., 2012. Extraction and characterization of bioactive compounds with health benefits from marine resources: macro and micro algae, cyanobacteria, and invertebrates. In: Hayes, M. (Ed.). Marine Bioactive Compounds. Springer, Boston, MA, pp. 55–98. https://doi.org/10.1007/1978-1001-4614-1247-1002_1002. Jiménez, J.P., Serrano, J.,Tabernero, M., Arranz, S., Díaz-Rubio, M.E., García-Diz, L., Goñi, I., Saura-Calixto, F., 2008. Effects of grape antioxidant dietary fiber in cardiovascular disease risk factors. Nutrition 24, 646–653. doi:10.1016/j.nut.2008.1003.1012. Li, H., Chen, B., Nie, L.,Yao, S., 2004. Solvent effects on focused microwave assisted extraction of polyphenolic acids from Eucommia ulmodies. Phytochem. Anal. 15, 306–312. Mahugo Santana, C., Sosa Ferrera, Z., Esther Torres Padrón, M., Juan Santana Rodríguez, J., 2009. Methodologies for the extraction of phenolic compounds from environmental samples: new approaches. Molecules 14, 298–320. doi:10.3390/molecules14010298. Martins, S., Mussatto, S.I., Martínez-Avila, G., Montañez-Saenz, J.,Aguilar, C.N.,Teixeira, J.A., 2011. Bioactive phenolic compounds: production and extraction by solid-state fermentation. A review. Biotechnol. Adv. 29, 365–373. doi:10.1016/j.biotechadv.2011.1001.1008. Naczk, M., Shahidi, F., 2006. Phenolics in cereals, fruits and vegetables: occurrence, extraction and analysis. J. Pharm. Biomed. Anal. 41, 1523–1542. doi:10.1016/j.jpba.2006.1504.1002.

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Nieto, A., Borrull, F., Pocurull, E., Marcé, R.M., 2010. Pressurized liquid extraction: a useful technique to extract pharmaceuticals and personal-care products from sewage sludge.Trends Anal. Chem. 29, 752–764. doi:10.1016/j.trac.2010.1003.1014. Palma, M., Piñeiro, Z., Barroso, C.G., 2002. In-line pressurized-fluid extraction–solid-phase extraction for determining phenolic compounds in grapes. J. Chromatogr.A 968, 1–6. https://doi.org/10.1016/S00219673(1002)00823-00823. Périno-Issartier, S.,Abert-Vian, M., Chemat, F., 2011. Solvent free microwave-assisted extraction of antioxidants from sea buckthorn (Hippophae rhamnoides) food by-products. Food Bioprocess Technol. 4, 1020–1028. Ross, K., Beta, T., Arntfield, S., 2009. A comparative study on the phenolic acids identified and quantified in dry beans using HPLC as affected by different extraction and hydrolysis methods. Food Chem. 113, 336–344. doi:10.1016/j.foodchem.2008.1007.1064. Smith, R.M., 2003. Before the injection—modern methods of sample preparation for separation techniques. J. Chromatogr. A 1000, 3–27. https://doi.org/10.1016/S0021-9673(1003)00511-00519. Soto-Vaca, A., Gutierrez, A., Losso, J.N., Xu, Z., Finley, J.W., 2012. Evolution of phenolic compounds from color and flavor problems to health benefits. J.Agric. Food Chem. 60, 6658–6677. doi:10.1021/jf300861c. Temelli, F., Güçlü-Üstündağ, Ö., 2005. Supercritical technologies for further processing of edible oils. In: Shahidi, F. (Ed.), Bailey’s Industrial Oil and Fat Products. Wiley Online Library, Texas, United States. https://doi.org/10.1002/047167849X.bio047167057. Xiao, W., Han, L., Shi, B., 2008. Optimization of microwave-assisted extraction of flavonoid from Radix Astragali using response surface methodology. Sep. Sci. Technol. 43, 671–681. You, Q., Chen, F.,Wang, X., Jiang,Y., Lin, S., 2012.Anti-diabetic activities of phenolic compounds in muscadine against alpha-glucosidase and pancreatic lipase. LWT-Food Sci. Technol. 46, 164–168. doi:10.1016/j. lwt.2011.1010.1011. Zhou,T., Xiao, X., Li, G., Cai, Z.-w., 2011. Study of polyethylene glycol as a green solvent in the microwaveassisted extraction of flavone and coumarin compounds from medicinal plants. J. Chromatogr. A 1218, 3608–3615.

CHAPTER 18

Current prospective of green chemistry in the pharmaceutical industry Swatia, Shelly Pathaniab,c, Ravindra K. Rawald

Department of Chemistry, Maharishi Markandeswar (Deemed to be University), Mullana, Ambala, Haryana, India Department of Pharmaceutical Chemistry, ISF College of Pharmacy, Moga, Punjab, India Maharaja Ranjit Singh Punjab Technical University, Bathinda, Punjab, India d Natural Product Chemistry Group, Chemical Sciences and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat, Assam, India a

b c

18.1 Introduction The safety of modern society is unimaginable without the industrial organic products. The human life is strongly dependent on chemical products like medicines, soaps, detergents, polymers, plastics, flavoring agents in foods, dyes, etc. The various global challenges, such as environmental pollution, poverty, urbanization, biodiversity loss, climatic change, energy, and basic human needs are interdependent. Nowdays, green chemistry plays a vital role in sustainability for energy and the environment. Green chemistry also called as sustainable chemistry, used to design chemical products with less hazardous impact on environment and humans. It helps us to develop new safer drug synthesis and drug action mechanism which are more useful and less toxic in nature. In chemical industries, there is emission of carbon dioxide by the use of various synthetic processes this supercritical carbon dioxide can be used as a green solvent with less harmful impact on the environment (Talaviya and Majmudar, 2012). The uses of green catalysts and alternative heating sources have ensured the sustainable development in chemical industries (Mooney et al., 2015). With advancement in the green chemistry, the pharmaceutical industries turn toward the alternate sources of energy like wind, solar, and other renewable sources of energy, so that burden on the environment can be reduced. The basic idea of green chemistry has practiced over nearly two decades around worldwide, which aimed at completion of “triple bottom line”-sustainability in society, economically, and environmentally (Clark, 2006). The basic idea of green chemistry was introduced as a response to pollution prevention act of 1990, which was in favor of reduction of pollution by reducing the waste production rather than waste disposal and treatment (Sheldon, 2017). In 1991, the US Environmental Protection Agency (EPA) had launched a research grant program to reconstruct the chemical products with new greener route to reduce the adverse effect on human health and environment (Paul and Joseph, 1997). The EPA’s first major decision was to ban the use of DDT and some other pesticides. In support of pollution prevention act (PPA), Paul Anastas and John C. Green Sustainable Process for Chemical and Environmental Engineering and Science DOI: https://doi.org/10.1016/B978-0-323-95156-2.00001-5

© 2023 Elsevier Inc. All rights reserved.

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12 Minimize potential for accidents

1. Prevention waste

11 Real-time analysis

2 Maximize atom economy 3 Reduce toxicity

10 Design chemical and products degrade after use

4 Safer chemicals and products

Twelve principles of green chemistry

5 Green solvents

9 Use catalyst 8 Avoid dcerivatizati on

7 Use renewable feedstock

6 Increase energy efficiency

Fig. 18.1  Twelve principles of green chemistry.

designed the 12 principles of the green chemistry as a set of guidelines for further motivation in academic and industrial science and research (Paul and Warner, 1998). Poliakoff and coworkers put forward a mnemonic, PRODUCTIVELY, which traps the 12 principles of green chemistry (Fig. 18.1) in one pot (Tang et al., 2005). In the early 1990s, the European Community’s Chemistry Councils published papers on “chemistry for a clean world.” The members of Environmental Chemistry of the American Chemical society have organized first conference in 1994 at Chicago with ideas, “Benign by design: Alternative synthetic design for pollution prevention.” In 1995, the US EPA with the US president of Bill Clinton established the annual “Presidential Green Chemistry Challenge Awards” to highlight the scientific contribution in academia and industry (Hanno et al., 2018). In 2001, Knowles, Noyori, Sharpless and in 2005, Chauvin, Grubbs, Schrock won the Noble Prize for their great contribution in research areas of green chemistry (Rouhi, 2005). In 2005, American Chemical Society (ACS) and Green Chemistry Institute (GCI) organized an industrial roundtable for green chemistry and engineering that promote the chemical manufacturing with a safer route (Constable et al., 2007). These various achievements inspired many researchers to further carry out research in the field of

Current prospective of green chemistry in the pharmaceutical industry

green chemistry. The introduction of greener synthetic strategies for the development of new pharmaceutical products has made a revolutionary change in the medication, human health, and drug delivery systems (Paul and Tracy, 1998). Various sustainable strategies like reduction of waste, less use of raw material, use of green solvent, biocatalyst, and more energetics efficient processes were introduced. The efficiency of sustainability depends upon the E-factor introduced by Roger Sheldon (Sheldon, 1992, 2007) and atom economy introduced by Trost (Trost, 1991). The E-factor is defined as the ratio of total mass of waste produced to the mass of product formed in kg.

E-factor =

Total mass of waste produced Total mass of product produced

The large value of E-factor corresponding to higher amount of waste production, thus indicates less efficient process with negative impact on the environment. The high E-factor value is due to the complexity of reaction substrate and complex mechanism followed by the reaction for the synthesis of active pharmaceutical ingredients (Sheldon, 2017). Similarly, atom economy represented as the ratio of molecular weight of desired product to the molecular weight of all obtained chemicals as a product.

Atom economy =

Molecular weight of thedesired product × 100 Molecular weight of allobtained product

The Baylis–Hillman reaction is an excellent example of atom economy in which all the atoms of reactant gets incorporated into the products (Mansilla and Saa, 2010). This is a condensation reaction of an α,β-unsaturated systems with the C=O functionality present in aldehydes, ketones, or α-keto esters catalyzed by Lewis base like tertiary amine or phosphine with certain additive like Bronsted acids (water, methanol, thioureas, and ureas) (Scheme 18.1). Another metrics that can be used is process mass intensity (PMI), it is the ratio of total mass of all materials used in a process (substrate, solvents used in the reaction, reagent used, and catalyst) to convert into products to the mass of product. Other commonly used matrix for sustainability and efficiency of a reaction include chemical yield, environment impact factor, mass intensity, product mass efficiency, mass productivity, CO2R

+

α,β unsaturated ester

O

LBBA

R R' ketone

R

OH

R'

substituted hydroxy keto ester

LBBA= mixture of lewis base and bronsted acid

Scheme 18.1  Baylis Hillman reaction catalyzed by LBBA.

CO2R

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efficiency mass yield, carbon efficiency, reaction mass efficiency, reaction mass intensity, solvent intensity, waste water intensity (Roschangar et al., 2015). The waste produced by the chemical industries cause destruction to the environment, thus, there is a dire need to design new manufacturing process with less waste production. For this purpose, the selection of starting material, catalyst, solvent, and reagents must be chosen carefully and must avoid the use of carcinogenic substances. Keeping the principles of green chemistry in our mind, we looking forward to the day when all the pharmaceutical processes become totally greener and the use of hazardous substances completely disappear from the modern strategies. The future perspective of green chemistry will be emphasizing to adopt new and safe greener strategies in the field of synthesis, research, and education. The application and various approaches of green chemistry have summarized in Table 18.1.

18.2  Design of green chemistry In nature, green chemistry may be defined as the philosophy of chemistry as well as engineering that emphasized on the construction of products and synthetic processes that reduce the production of harmful substances and wastes. In past decades, the chemists used the natural sources and focused only on the synthetic processes and its utilization. But with increase in the population, the natural resources get depleted at much higher rate and thus provide loss to the environment. Thus, green chemistry becomes a fundamental guide-map to prevent these problems. It encourages the innovations and promotes environment benign and economical sustainable products. The various green solvents, catalysts, and stoichiometric reagents are the most important features of the green chemistry (Fig. 18.2) and are applied in various greener strategies. 18.2.1  Choice of starting material In traditional method of synthesis, the focus was laid mainly on the development of large quantity of products, but not on the choice of starting material and the wastes generated. In green approaches, first the reactant substrates were chosen carefully so that it can incorporate all the atoms into the products, that is, maximize the atom economy. Examples of atom economy included addition reactions like halogenation of alkene, cycloaddition reactions, and some rearrangement reactions. Elimination reactions, Wittig and Grignard reagent’s reaction are not considered to be atom economic as the eliminated group is totally converted into waste. The alternative starting material for coal and petrochemical products for energy production are biological sources includes cultivated crops, organic waste, some oils like sunflower oil, rape seed oil, corn, wood etc. (Ahluwalia and Kidwai, 2004).

Current prospective of green chemistry in the pharmaceutical industry

Table 18.1  Overview of the various aspects of green chemistry. 1. Green solvents

Applications

a) Water

Used as solvent and reaction media in C–C bond formation organic synthesis, Hetro Diels–Alder reaction, Heck reaction, cycloaddition reactions, etc. Used as a reaction media and solvent in hydroformylation, homogenous and hetrogenous hydrogenation, polymer improvement, organometallic catalysis, etc. Used as solvents, catalysts, lubricants, electrolytes, in purification, in substitution reaction of aromatic compound with improved rate and percentage yield in ionic liquids, used to produce biodiesel.

b) Supercritical carbon dioxide c) Ionic Liquids

2. Green catalysts

a) Solid acid catalystfluorided silica–alumina catalyst b) Metal catalysts in oxidation reaction c) Biocatalyst d) Phase transfer catalyst e) Photo catalyst f) Polymer supported catalysts

Michael reaction, Friedel Craft reactions, Mannich reaction, etc. Hydrogenation, oxidation, hydroformylation, etc. Fermentation, hydrolysis of biomolecules. Preparation of diazomethane, nitriles from alkyl or acyl halides. Purification of water. Preparation of ethers from alcohols, cracking, and isomerization of alkanes.

3. Safer chemicals and starting materials

a) γ-Valerolactones (GVL)

b) Dimethyl carbonate, dibenzyl carbonate c) Biomass

Used as reaction media for many reactions include Ullamann-type cross coupling, hydrogenation reactions, hydroxyamides and for production of 1,4-pentanediols, 2-MeTHF, valerate-based ionic liquids, alkanes, butenes, alkylvalerates, 4-hydroxy pentanamides. Green methylating agent. Used for the production of energy, decomposed into sugars and their products. Organic reaction carried out at room temperature in less time include substitution, addition, saponification, alkylation esterification, etc.

4. Ultrasonic-assisted reaction

18.2.1.1  Choice of reagent The choice of reagent is another important feature for the development of product with safety and minimum waste loss. In nineteenth century, trinitrotoluene (TNT) was oxidized in the presence of acidic dichromate and then reduced with iron and HCl (hydrochloric acid) and in situ decarboxylation, afforded 1,3,5-triaminobenzene.

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Fig. 18.2  Design of green chemistry.

Further, heating of 1,3,5-triaminobenzene in acidic solution gave phloroglucinol. In this method, per kilogram of phloroglucinol produced ca. 40 kg of solid waste containing Cr2(SO4)3, NH4Cl, FeCl3, and KHSO4 and was less atom efficient. The reaction of syn gas for the production of methanol is 100% atom economic as given in Scheme 18.2 (Sheldon, 2007). Some examples of green reagents include DMC, supercritical carbon dioxide and polymers supported reagents. The recycled metals aluminum, copper, and iron can also be used a better substitute for the heavy metals in many reactions. 18.2.2  Choice of solvent Nowadays, solvents are also responsible for the generation of undesired product and wastes in various synthetic processes. The commonly used solvents like trichloroethylene, benzene, toluene, esters, ethers, amines, and nitrated and halogenated hydrocarbons are toxic, harmful, and volatile in nature and hence contribute to air pollution and health issues to workers. Solvents are also used for reaction work-up and washing the products and intermediates so most of the solvents also get wasted. Thus, there is a need of finding safer and more efficient approaches of conventional solvents including use of greener solvents and strategies that use carbon dioxide, water, or avoid the use of solvents completely. In supercritical carbon dioxide (scCO2), palladium-catalyzed C–C coupling processes occur at high rates and with excellent selectivity.

Current prospective of green chemistry in the pharmaceutical industry

Scheme 18.2  Syn gas utilization.

One of the good examples includes use of cellulosic ethanol from corncobs and stalks instead of corn kernels as solvent in Tide Coldwater Clean, a laundry detergent. This process repurposed more than 7000 tons of agricultural waste yearly and saved 1  month energy needed for washing clothes in homes across the California (Mccoy, 2015). Then cellulosic ethanol was prepared as a green solvent for detergents and other cleaning household and industrial products. Some other green solvents include propanediol, ethyl levulinate glycerol ketals, glycerin and methyl esters, butyl 3-hydroxybutyrate, etc. Propanediol on esterification with terephthalic acid produced an important polymer polytrimethylene terephthalate. 18.2.3  Choice of catalyst The traditional organic reaction features with stoichiometric quantities of catalysts like HF, AlCl3, and H2SO4 result into large quantities of byproducts, which contribute to the lot of wastage. So, it is necessary to find out the appropriate catalyst to enhance the product value with minimum waste. Catalysts also have remarkable impact in the synthesis of active pharmaceutical ingredients (API’s). Mainly two types of catalysts are used in chemical reactions including chemical catalysts and biocatalysts (Fig. 18.3). Enzymes are the highly selective biocatalyst used to catalyze two out of three of chiral products on large scale by reducing the amount of byproducts, like inorganic salts, crystalline salts, and avoid recycling and breaking of chiral auxiliary. Biocatalyst minimized the production of waste, number of synthetic steps, with excellent yield and high degree of chemo-, regio-, and stereo selectivity. Chemo catalyst includes different types of catalyst such as homogenous, heterogeneous, and organocatalyst. Homogenous Catalyst

Chemical catalyst

Biocatalyst homogenous

heterogenous organocatalyst

Fig. 18.3  Various types of catalyst used in green synthesis.

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R1

O +

alkyl benzene

R2

Solid acid catalyst

R1

OH

carboxylic acid

R2 O

+

H2O

Aromatic ketone

Scheme 18.3  Reaction catalyzed by solid acids, a heterogeneous catalyst.

catalyst gives rise to highly reactive and selective reaction products, but the limitation associated is the recovery and recyclability as it is present in same phase of reactant. This limitation can be overcome by using the heterogeneous catalyst. For example, acid catalyzed Friedel Craft reactions for the synthesis of alkylbenzene and aromatic ketones in the presence of soluble aluminum chloride or hydrofluoric acid results into large amount of hazardous waste production. This synthetic route was improved by using solid acids, such as MCM-41, Cs2.5H0.5PW12O40, H+-ZSM-5 Mn+-mont, and SO42−/ ZrO2 as shown in Scheme 18.3 (Kaneda et al., 2010). Metal nanoparticles are also an important class of heterogeneous catalyst, used to catalyze the synthetic reactions of fine chemicals and pharmaceutical industry.

18.3  Applications of green chemistry in pharmaceuticals Currently, most of the pharmaceutical and chemical industries have shifted their interest toward the use of greener approaches improve the performance and quality of products. Many applications of green chemistry have been applied for the drug discovery and manufacturing processes. The risk to human health and environment also get reduced by following the principles of green chemistry. Several important parameter used for green chemistry includes green solvents, catalyst, biocatalyst, waste minimization, etc. 18.3.1  Green solvents Green solvents, which produced less waste in chemical reactions, are tremendously used by the researchers. The most commonly used green solvent is water. It is a naturally occurring, cheap, environment friendly, and nonhazardous substance that is used in many organic syntheses. The separation of product can be easily done as it is polar in nature. Some limitations are also associated with water that it cannot be recycled after the extraction of product (Narayan et al., 2005). Another approach used to improve synthetic efficiency is to reduce the unnecessary steps occurring in the reaction mechanism.Water helps to reduce these numbers of steps in synthetic route, if the synthetic strategies are properly designed (Li, 2005). Rideout et al. reported that Hetro Diels–Alder reaction (Scheme 18.4) containing nitrogen and oxygen as dienophile in alcoholic medium is 12-fold faster than in isooctane while in aqueous media the rate becomes 730-fold faster than isooctane. The polarity difference of the solvent is responsible for increased rate (Breslow, 1992).

Current prospective of green chemistry in the pharmaceutical industry

+ cyclopentadienyl

H2O

O

+

COCH3

COCH 3 Endo

Butenone

Exo

Scheme 18.4  Hetro Diels–Alder reaction in the presence of water. H2N

CO2H H

H

Br Pd(OAc)2, Base, +

H2O, TPPTS, 120 o C

N H (S)-4bromotryptophan 1,1-dimethylallyl alcohol

HN

C

CO2H H

N H clavicipitic acid

Scheme 18.5  Heck reaction.

The optically active clavicipitic acid was obtained by the Heck reaction of (S)4bromotryptophan with 1,1-dimethylallyl alcohol in aqueous media, catalyzed by lead acetate and water-soluble ligand TPPTS (triphenylphosphine trisulfonate sodium). The product was obtained in alkaline aqueous media with a high yield of 91%, but in the presence of organic solvents like dioxane or DMF, the reaction gave a complex mixture Scheme 18.5 (Yokoyama et al., 1999). Lubineau et al. reported [4+3] cycloaddition reaction of α,α-dibromo ketones with furan or cyclopentadiene mediated by iron or copper, and water with a high percentage yield of 76–88% as shown in Scheme 18.6. Water used as solvent to prevent the polymerization of substrate (Lubineau and Bouchain, 1997). Another most used solvent is supercritical carbon dioxide with minimum hazards (Leitner, 2002). The supercritical carbon dioxide as solvent possesses several applications in pharmaceuticals, polymer improvement (Wells and DeSimone. 2001), organometallic catalysis (Jessop, 1999) radical polymerization (McCoy, 1999), and heterogeneous and homogeneous hydrogenation (Freemantle, 2001; Jessop, 1996). Daniel Koch et al. explained the rhodium catalyzed (unmodified and modified with phosphorus donor ligand) hydroformylation of 1-octene mediated by supercritical carbon dioxide. This reaction was studied with various unsaturated substrate with high reaction rate and efficiency. The greater solubility of catalyst and low solubility of Br Ph

O

Z +

Fe(2 eq.), 20 o C, H 2O

Br Ph α,α-dibromoketone Z=O, CH 2

Scheme 18.6  [4+3] Cycloaddition reaction.

Z

Ph Ph O endo cis

+

Z Ph

Ph O

exo cis

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Scheme 18.7  Rhodium catalyzed hydroformylation.

Scheme 18.8  Homopolymerization of 1,1-dihydroperfluoro octyl acrylate.

unsaturated substrate in supercritical carbon dioxide than conventional solvents ensured the rapid precipitation of obtained product (Scheme 18.7) (Koch and Leitner, 1998). J.M. DeSimone et al. reported the homopolymerization of 1,l-dihydroperfluorooctyl acrylate (FOA) with azobisisobutyronitrile (AIBN) as shown in Scheme 18.8 and the copolymers of FOA with ethylene, styrene, butyl acrylate, and methyl methacrylate in supercritical CO2. The high molecular mass fluoropolymers are soluble in supercritical carbon dioxide, but insoluble in organic solvents. The rate of decomposition of azobisisobutyronitrile is much slower in supercritical carbon dioxide than in benzene (DeSimone et al., 1992). Ionic liquids are another interesting solvent system used, which comprised of ions (Fig. 18.4) having melting point less than 100°C. These liquids are thermally stable and nonvolatile in nature. The polarity, hydrophobicity, and solvent miscibility completely depend upon the nature of ions present in it. The reactions occurring in ionic media are highly reactive and selective (Stepnowski, 2007). J. Earle et al. had explained the nitration of toluene in three different ionic liquids with 67% nitric acid to obtained three different products in quantitative yield of

Fig. 18.4  Common ions involved in ionic liquids.

Current prospective of green chemistry in the pharmaceutical industry

Scheme 18.9  Reactions of toluene in different ionic liquids.

(85–99)%. Ionic liquids used in this synthetic route also act as catalyst and water was only byproduct formed (Scheme 18.9) (Earle et al., 2004). 18.3.2  Green catalyst Catalysis is an important part in the chemical reactions, taken into consideration by the pharmaceutical industry. The selection of an efficient green catalyst gives more atom economic reactions, thus reducing the cost, energy and waste production in many synthetic processes. Catalyst also enhances the selectivity of product, and allows the reactions to occur at comparatively mild conditions. Some reactions with green catalyst include reduction with metal and metallic hydride, oxidation with permanganate, and dichromate {or Cr (VI)} reagents and electrophilic substitution reaction of aromatic compounds catalyzed by Lewis acids and mineral acids. Yan Shi et al. reported an alternate pathway for the preparation of monoethers from linear glycerol and diglycerol in the presence of 0.5 mol% of Pd/C under 10 bars of hydrogen using a Bronsted acid as cocatalyst instead of using Williamson synthesis (Scheme 18.10).This method was highly selective, having high atom economy, and with no inorganic byproduct (Shi et al., 2010). Sheldon et al. reported aqueous biphasic aerobic solvent-free oxidation of alcohols catalyzed by palladium complexes but the limitation of palladium is that it is difficult to reoxidize to Pd (II). This limitation was overcome by use of water-soluble bathophenanthroline disulfonate (PhenS) ligand. The catalyst can easily recycle in aqueous media and product can be collected by decantation (Scheme 18.11) (Sheldon et al., 2002). The Hoechst–Celanese reaction is an excellent example for the preparation of the ibuprofen involving two steps and a metallic catalyst. The first step includes the hydrogenation of substituted ketones whereas in second step carbonylation occurs to get the

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Scheme 18.10  Catalytic pathway for the preparation of monoethers.

Scheme 18.11  Solvent-free catalytic oxidation of alcohols.

final product as shown in Scheme 18.12.This process with higher E-factor is better than the classical route, which is more tedious and lengthy (Elango et al., 1991). The other simple example of C–C bond formation is Heck reaction which provides an alternate route to Friedel Craft reactions to introduce carbon fragments to an aromatic ring system. Due to the high activity of catalyst the product obtained in less time at 95–105°C (Heck, 1985) shown in Scheme 18.13.

Scheme 18.12  Hoechst–Celanese process.

Current prospective of green chemistry in the pharmaceutical industry

Scheme 18.13  Palladium catalyzed Heck reaction.

A highly enantioselective synthetic strategies developed by Novartis for the preparation of (S)-metolachlor catalyzed by an iridium complex of a chiral ferrocenyldiphosphine (Blaser, 2002) (Scheme 18.14). The activity of catalyst increased by a factor of 10 and enantioselectivity by 5–6% with the use of acetic acid as solvent instead of toluene, benzene, and alcohol. There are other numerous reactions reported in the literature like reduction, oxidation, isomerization, and hydrolytic reactions catalyzed by biocatalyst (Sun et al., 2018). 18.3.3  Waste water treatment The main sources of waste production in pharmaceutical industries are manufacturing processes, synthesis of drugs excipients, and modification of raw material. The amount of waste produced is more than the active pharmaceutical ingredients (API’S) production. Therefore, waste management becomes a major challenge for researchers as well as for pharmacists. There are different possible ways for the management and treatment of waste, depending upon the type of waste produced like chemical waste (by-products of a chemical reaction), drugs, cytotoxic waste, radioactive waste, hazardous waste, and clinical waste. The liquid waste containing high concentration of organic matter cannot be disposed directly to municipal sewer system. The waste that cannot be recycled, reused

Scheme 18.14  Iridium catalyzed preparation of metolachlor.

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should not dump directly to landfill and should treat by incineration. Incineration is an oxidation of combustible and organic matter at high temperature that results in the reduction of volume and weight of wastes. The liquid wastes generated by a pharmaceutical industry are carbon-rich and can be treated by incineration. The carbon-rich waste can also be used for the treatment of domestic waste water and for anaerobic biogas production. The volatile component of liquid waste can be easily separated and solvent can be recycled, while the nonvolatile residual component can be removed by wet oxidation and further can be biologically treated (Hosseini et al., 2011). Earlier, conventional methods used for water purification are biological treatment, sedimentation, electrochemical methods, membrane procedures, and adsorption, etc., but due to their awful effects on the environment, they have been replaced by new, efficient, and environmentally benign methods. Radiation technology is an advance oxidation process which is considered as an effective method against waste water treatment. This radiation technology commonly used in textile industry to decolorize the red acid azo dye using chemical oxygen demand (COD) and UV–visible spectroscopy techniques.The radical OH• and hydrated electrons in equal amount with small amount of H• radicals are used as reactive intermediate in water radiolysis. The hydrated electrons, H• attacks the azo bond and destroyed the conjugation, causing decoloration of dye, and OH• radicals react with aromatic ring to destroy it completely. Irradiation doses of 10 kGy are sufficient for decoloration of a solution of 10−3 to 10−4 mol/dm3 (Scheme 18.15) (Foldvary and Wojnarovits, 2007). Victor Sarria et al. developed a two-step photochemical and biological system for the biorecalcitrant waste water treatment by taking into consideration four different nonbiodegradable pollutants, that is, p-nitrotoluene-ortho-sulfonic acid (p-NTS), 5-amino-6-methyl-2-benzimidazolone (AMBI), metobromuron (MB), and isoproturon (IP). They used three types of photochemical and biological system photo-Fenton, or Fe3+/UV, or TiO2, supported on glass rings for the photo catalytic pretreatment (Sarria et al., 2002). The biodegradability of these four pollutants was tested by Zahn–Wellens test and the toxicity of photo treated solution was found to be reduced and further subjected to biological treatment includes decolorization, ozonation, and oxidation of organic matter with H2O2 (OECD, 1996; Oloman, 1981).

Scheme 18.15  Structure of acid red dye (Azophloxine).

Current prospective of green chemistry in the pharmaceutical industry

Scheme 18.16  Reactions occurring during the electrochemical oxidation of phenol using platinum electrode.

Comninellis et al. reported electrochemical anodic oxidation of phenols (waste water pollutants) using platinum electrodes. Phenol, as a pollutant, is generated by many chemical industries, including chemical and plastic plants, coke plants, and oil refineries. In the first step, hydroxyl ion is generated from the electrochemical oxidation of water, and the hydroxyl ion thus formed on the electrodes surface and react with phenol to give out hydroquinone, benzoquinone, maleic acid, oxalic acid, and carbon dioxide. Phenol can be removed from waste water using chemical oxidants include ozone, hydrogen peroxide, and chlorine but the total organic matter of waste water cannot be destroyed using these oxidants. Thus, the electrochemical oxidation process was more efficient than chemical oxidants as it completely remove the carbon or organic matter as shown in Scheme 18.16 (Comninellis and Pulgarin, 1991). Further, an improved strategy for oxidation of phenol was proposed by Comninellis et al., waste water using doped SnO2 anode. First, hydroxyl radicals are generated by the electrochemical oxidation of water. Then the hydroxyl ion formed reacts with phenol to give carbon dioxide and oxygen (Scheme 18.17). It was found that electrochemical oxidation using doped SnO2 anodes was much more effective than platinum electrode as very small amount of intermediate is formed on doped SnO2 anodes than platinum electrodes and aliphatic acid formed as intermediate during the oxidation of phenols which readily oxidized to carbon dioxide (Comninellis and Pulgarin, 1993). Waste water generated from textile industries contains high color (dyes), metal ions, and dissolved salt. So, before discharging waste water it should be first treated by

Scheme 18.17  Electrochemical oxidation of phenol using doped SnO2 electrode.

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Scheme 18.18  Preparation of alkene from GLV.

physical, chemical, and biological methods. The physical treatment involves coagulation, adsorption, and filtration technique like ultrafiltration, nanofiltration, and reverse osmosis which are useful for discoloration of waste water. The chemical treatments involve the advanced oxidation with hydroxyl radicals and chemical oxidation with oxidizing agents like ozone and H2O2 for complete degradation of toxic substances and their byproducts. The biological treatment like aerobic and anaerobic method is the final step for waste treatment that removes the dissolved organic matter (Holkar et al., 2016). 18.3.4  Safer chemical Millions of chemical products such as drugs, medicines, additives in food, soap and detergents, agrochemicals, cosmetics, fuel, and fertilizers, etc. have made human life very comfortable but also harm the organisms and environment due to their some deleterious effects. So, green chemistry provides an alternate solution to these existing challenges. Environmentally friendly methodologies were designed for the synthesis of safer chemicals. Horvath et al. demonstrated an excellent example of first sustainable liquid γ-valerolactones (GVL) as safe chemical. The safety is due to its physical and chemical properties including low melting point (231°C), high boiling point (207°C) open cup flash point (96°C), easily biodegradable, characteristics smell for leakage identification and miscibility with water. It does not form azeotropes with water and can be easily separated and its low vapor pressure resists its emission (Horvath, 2008). J.Q. Bond et al. had reported the preparation of liquid alkene used as transportation fuel from decarboxylation of γ-valerolactones catalyzed by SiO2/Al2O3 and further oligomerized to form higher olefins as shown in (Scheme 18.18) (Bond et al., 2010). GLV acts as reaction media for many reactions like Ullamann-type cross coupling, hydrogenation reactions, hydroxyamides and for production of 1,4-pentanediols, 2-MeTHF, valerate-based ionic liquids, alkanes, butenes, alkylvalerates, 4-hydroxy pentanamides, etc. (Havasi et al., 2016; Orha et al., 2018; Chalid et al., 2011). Dimethyl carbonate (DMC) is another organic carbonate with low toxicity, nonhazardous, and safer to organisms and environment. DMC provides an alternate synthetic agent for numerous alkylating reagent including carboxy methylating, dimethyl sulfate, methyl halides, and phosgene, etc. (Arico and Tundo, 2010; Tundo and Selva, 2002). DMC is considered as most suitable green methylating reagent due to its simple synthesis from the catalytic oxidative carbonylation of methanol and oxygen as shown

Current prospective of green chemistry in the pharmaceutical industry

Scheme 18.19  Oxidative carbonylation of methanol to DMC.

Scheme 18.20  C-methylation of phenyl acetonitrile.

in Scheme 18.19. It is also a superior alternate of phosgene as it does not produce any inorganic salt as byproduct on methylation (Rivetti et al., 1996). Selva et al. examined the methylating efficiency of dimethyl carbonate (DMC), dimethyl sulfate (DMS), methyl iodide (MeI), and methanol (MeOH). DMC was found to be most suitable green compound to most of the methylation reaction such as O-methylation of phenol, mono-C-methylation of phenyl acetonitrile (Scheme 18.20) (Selva and Perosa, 2008), and mono-N-methylation of aniline due to its nontoxic and biodegradable nature (Arico and Tundo, 2010). Marques et al. prepared N-methyl oxazolinones by the reaction of ketoxime with DMC in the in the existence of potassium carbonate at 180–190°C. This method was applicable to both aliphatic and aromatic ketone oximes, containing a methylene near the C–N bond (Scheme 18.21) (Marques et al., 1993). Higher homologues of DMC also show remarkable alkylating and carboxy alkylating reactivity and selectivity as that of dibenzyl carbonate and asymmetrical alkyl methyl carbonates, etc. The rate of reaction is found to be slow with use of higher carbonates (Selva and Perosa, 1995). 18.3.5  Renewable feedstock Today’s the largest part of energy requirement are accompanied by carbon-based fossils resources, coal, and petroleum. The main fossils products are petrol, diesel, gasoline, kerosene which are used for transportation of fuels.The energy produce from fossil fuels is convenient as it readily compete with energy demands of population. But now these days, the depletion of fossil is at much faster rate than its generation. So, our researcher

Scheme 18.21  Reaction of ketoxime to yield N-methyl oxazolinones.

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Fig. 18.5  Source and products of biomass.

faced a major challenge to find the alternate feedstock and energy sources of fossils (Deffeyes, 2005). A renewable feedstock must be replenished at the same rate at which it gets consumed, so that it can meet the demands of all organisms. For the sustainable achievement in chemical and allied industry, there should be transition between traditional feedstock sources (fossils) and alternate bio-based sources. An example of a renewable feedstock is biomass, any material derive from living organism (plants) is a renewable source of energy and fuel after the coal, petroleum, and natural gas to compete with energy demand of population (Palkovits and Delidovich, 2017). The breakdown of biomass also releases long chain polysaccharides, starch, cellulose, and free sugars which on further hydrolysis gives 5 or 6 carbon containing sugars (Fig. 18.5). The carbohydrates including fructose, glucose, and sucrose derived from feedstock can be fermented by microbes or biocatalyst to form different chemical products (Rubin, 2008). Some of the fermented and chemical products of glucose are listed in Fig. 18.6 (Corma et al., 2007). Cellulose, a constituent of plant cell wall, also called as lignocellulose biomass, can be easily transformed into biofuel (Himmel et al., 2007). Cellulose-based biomass is an excellent alternate energy source to fossils. The enzymatic or biocatalytic breakdown of cellulose and its constituent types release the polysaccharide in the presence of acids at high temperature. The most common enzymes are involved in microbial degradation of lignocellulose are Trichoderma reese, a fungi, and the bacterium Clostridium thermocellum (Gilbert, 2007). Mascal et al. reported the extraction of various intermediates from carbohydrates by using aqueous media as solvent (Mascal and Nikitin, 2008) the major product obtained was 5-(chloromethyl) furfural with yield 71–76% which on further treatment with

Current prospective of green chemistry in the pharmaceutical industry

Fig. 18.6  Fermented and chemical products of glucose.

ethanol converted into ethoxymethylfurfural, and on hydrogenation with PdCl2 gives 5-methylfurfural (Scheme 18.22) an alternate fuel to oxygenated fossil fuels (Gruter and Dautzenberg, 2007). The advantage of using renewable feedstock over fossils is lowering the demand of crude oil and its products, recycling of carbon dioxide, avoid the dependence on politically unstable nation for the supply of renewable feedstock, reduction in the challenges for achieving sustainable development. 18.3.6  Synthesis of carbon dots Carbon dots (C-dots), a new member of the carbon nanomaterial family, are spherical, nontoxic, biocompatible, and discrete particles less than 10 nm in diameter, are

Scheme 18.22  Conversion of 5-(chloromethyl) furfural to alternate oxygenated fuel.

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produced from any carbon source (Wang and Hu, 2014). Carbon dot have sp2 hybridized structure with different oxygen containing functional group like hydroxyl (–OH) carboxyl (–COOH), and aldehyde (–CHO). C-dots have well-defined arrangement of carbon atom with large surface area, excellent thermal and chemical stabilities, and an efficient drug-loading capacity comparable to that of larger particles, such as quantum dots (Q-dots) (Zuo et al., 2015). Different natural and synthetic precursor for the carbon dots synthesis are natural precursors like grass (Liu et al., 2012), sugars (Palashuddin et  al., 2012), eggs in vitro bioimaging (Wang et  al., 2012), glycerol (Lai et  al., 2012), banana and orange juice for bioimaging (Sahu et al., 2012; De and Kark, 2013), soya milk for electrocuted of metals (Zhu et al., 2012), amino acids (Jahan et al., 2013), solid biomass (Zhang and Yu, 2016), potato (Xu et al., 2015), whereas synthetic precursor are electrochemical synthesis (Li et al., 2011), ultrasonic/microwave (Hui et al., 2009), and hydrothermal treatment (Dong et al., 2013). Yoon et al. had first time reported the recycling methods of cellulose waste paper to form carbon dots by approaching green synthetic protocol, based upon ionic liquids. The waste paper was dissolved in an ionic liquid 1-allyl-3methylimidazolium chloride ([Amim][Cl]) to destroy the chemical structure of cellulose. The ionic liquid can be recovered easily at the end of the process using an additional antisolvent, that is, absolute ethanol. The waste paper can be dissolved into ionic liquids by microwave irradiation. The carbon dot thus prepared and characterized by using transmission electron microscopy (TEM) and X-ray diffraction (XRD) (Jeong et al., 2018). Fadllan et al. had also reported a versatile method for the preparation of carbon dot from waste paper using hydrothermal method. The synthesis of C-dots was done by heating mixture of 5 g waste paper in 40 mL water, 30 mL H2SO4, and 50 mL NaOH and urea under hydrothermal conditions using a furnace at 150–300°C for 50 minutes (Fadllan et al., 2017). Liu et al. also reported the synthetic route of carbon dot as fluorescence nanomaterial for targeting tumor cells imaging with high quantum yield.This was a one-step preparation from folic acid precursor by hydrothermal method. The carbon dots produced by this method exhibited an elegant photo luminescent activity, high photo stability, chemo stability, and biocompatibility. The folic acid residues present on the surface of carbon dots are responsible for the excellent activity toward tumor cells (Liu et al., 2018). Aji et al. reported the preparation of carbon nanodots from waste of frying oil that can be used as a photo catalyst in the treatment of water on exposure to solar light. The purification test was conducted with a sample of sewage methylene blue and prepared C-dots were compared waste frying oil as a photo catalyst and methylene blue sample without photo catalyst C-dots. The results were analyzed by absorbance spectrum and it was found that C-dots showed excellent performance at 664 nm (Aji et al., 2016). Ramanan et al. developed a green synthetic methodology to convert a dangerous nonbiodegradable environmental pollutants expanded polystyrene (EPS) into highly

Current prospective of green chemistry in the pharmaceutical industry

luminescent carbon dots. The EPS-derived carbon dots are photo stable, freely soluble in water, show high photoluminescence and quantum yield and remain unaffected with a wide range of pH and ionic strength. This ecofriendly approach found significance in the selective and sensitive fluorimetric detection of Au3+ ions (a toxic heavy metal ion). The chlorine-rich nature of carbon dots makes it as an elegant candidate for photo catalysis (Ramanan et al., 2018). Another remarkable preparation of water-soluble, fluorescent carbon dots were reported by Jumeng Wei et al. from waste paper. These carbon dots exhibited an amorphous structure, small particle size, excitation wavelength-dependent photoluminescence behavior, and high quantum yield. In addition, it also exhibited high photoluminescence, excellent water-solubility, and fairly low toxicity. These are found to be potential against human L02 hepatic cells and S180 sarcoma cells and thus exhibited a great application in the area of biolabeling (Wei et al., 2014). Meiqin He et al. proposed the synthesis of water-soluble fluorescent carbon quantum dots using lemon juice as carbon resource by a simplistic hydrothermal reaction. The carbon quantum dots thus prepared and showed remarkable optical properties and can be used for bioimaging of plant cell (He et al., 2018). 18.3.6.1  Organic solvent recovery Solvent recovery is a process of waste reduction or extraction of useful material from waste generated in various synthetic approaches. The recovery and subsequent reuse of organic solvents are very important factors to be considered during the production processes by chemical and pharmaceutical industries, as large volume of solvents was used in heat transfer reactions and for cleaning of apparatus. Most of the API’s synthesis occurred through multistep processes and needs solvent elimination at each step. The major part of solvent waste is disposed by incineration at every step of production (Gonzalez et al., 2004). Cui et al. proposed an organic solvent forward osmosis (OSFO) in which at the membrane compartment, the solvent from the feed solution transports through the membrane to the draw solution while the solutes are rejected for the concentration of API’s and organic solvent recovery. Organic solvent was recovered from pharmaceutical products from different kind of organic solvent systems including pure ethanol and hexane as organic feed solution and LiCl, citric acid, and methyl palmitate as draw solute (Cui et al., 2018). The solvent selection guide is a program preceded by GlaxoSmith Kline (GSK), which provides guidelines manufacturer about the safety issues of environment and health concerned with chemicals and solvents. This program includes the life cycle inventory and assessment information of solvent using some metrics, which explain the potential issues associated with a solvent and the reduction of complex solvent use in chemical reactions according to their ranking. The overall amounts of materials

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and energy used during the synthesis of solvents, waste production, and their systematic disposal and their use in pharmaceutical reactions was identified by A Life Cycle Inventory/Assessment optimization. The solvent ranked poor by life cycle is not easy to recycle, important step should be followed for the alternatives of solvents. According to GSK guideline less than 50% solvent can be recovered and reused in the API’s synthesis (Gonzalez et al., 2004). Because of poor quality of organic solvents used in synthesis, these cannot be reused or recovered and hence discharged. Thus, organic solvents used for the synthesis of API’s should be of high purity and chromatographically pure liquid so that they can be recovered easily. The nanofiltration is widely used process for solvent separation from API’s like methanol, toluene, and methylene chloride, as they contribute majority of mass waste production (Geens et al., 2007). The nanofiltration process is energetically more efficient than distillation with 200 times reduction of energy consumption ca. Rundquista et al. demonstrated another solvent separation technique from mother liquors for separation of organic solvents, known as organic solvent nanofiltration. Crystallization is an important purification technique which results in the generation of mother liquors containing amount of API’s. The recovery of solvents and API’s from mother liquor increased atom economy and mass efficiency of reaction. However, the nanofiltration-based solvent recovery depends on the solubility of API’s in waste generated and a better alternate to distillation used for solvent recovery (Rundquista et al., 2012). 18.3.7  Separation of natural products from agrochemical Any substances that are produced from life, that is, living organism are considered as natural product. The semisynthetic and artificially synthesized natural product have also a vital role in organic chemistry. These provide useful substrate and products on modification like drugs and medicines, cosmetics, artificial food, fibers, etc. to fulfill the human needs. The greatest challenge nowadays is the separation, purification, and modification of biomass into useful environment benign products (Long et al., 2017). Green solvents exhibit an important advantage for the separation of natural products like near-supercritical or supercritical fluids, which have an elegant mass transport properties, polarity, and easy handling of solvent removal techniques after extracting the compound of our demand. The green solvents are the best alternatives for the distillation separation of natural products as it costs very high and constitutes more than half of the total cost.Water can also be an alternate option, but sometimes there is less solubility of the compounds.The supercritical water solvent is also better option for many organic reactions like depolymerization, hydrolysis, and carbonization of biomass into important bioactive substances (Shehada et al., 2016). Faulds et al. had reported the extraction of bioactive phenolic compound, that is, ferulic acid, from cereal cell walls (brans from wheat and maize, sugar beet pulp, and

Current prospective of green chemistry in the pharmaceutical industry

spent grain from barley) and agroindustrial waste by microbial esterase, an enzyme. The phenolic residue hindered the ease of hydrolysis of polysaccharides in the presence of other enzyme endo-b-1,4-xylanase. The residue obtained from enzymatic removal of ferulic acid is eco-friendly and can be further treated. The ferulic acid can be further utilized in the synthesis of natural products by biotransformation (Faulds et al., 1997). Sagar et al. described the various conventional and nonconventional extraction technique of bioactive compound from edible and waste part of fruits and vegetables. The extracted bioactive compound can further used in pharmaceuticals and chemical industries, cosmetic, food, and the development of functional foods (Sagar et al., 2018). 18.3.8 Sonochemistry Sonochemistry is considered an effective and greener method in organic synthesis. It involves the utilization of sonic and ultrasonic waves to chemical processing. Ultrasound waves work at a frequency of 16 kHz, above than the hearing range of humans. It also works on the phenomena of cavitation. The propagation of ultrasonic waves takes place through a medium by alternate compression and rarefaction. When the intermolecular forces of molecules were lower than rarefaction, rarefaction bubbles are formed, which grows in size by absorbing air pressure from medium and collapse. This cavitation is responsible for the higher chemical reaction rate due to the formation of highly reactive radicals during cavitation (Serpone and Colarusso, 1994). Visscher et al. studied the sonochemical degradation of benzene, styrene, ethyl benzene, and o-chlorotoluene in aqueous solution at a frequency of 520 kHz by taking different initial millimolar concentrations of substrates. The breakdown follows first-order kinetics and reaction rate depends on the initial concentration and sonication time. The pyrolysis in the cavitations occurs by lowering the maximum cavitation temperature and affords both reactive/volatile and inert/nonvolatile products (Visscher et al., 1996). The first-order breakdown of ethyl benzene was also carried out in aqueous solution at 520 kHz ultrasound frequency. Several monosubstituted, monocyclic, bicyclic aromatic hydrocarbons, and some oxygenated products were extracted during the pyrolysis of ethyl benzene. The product formed was analyzed by a sampling technique, solid-phase microextraction that allows convenient GC–MS and GC–FID analysis in micromolar range (Visscher et al., 1997). Rao et al. have reported the easy and simple green synthetic process for the synthesis of β-unsaturated nitriles from 2-cyanothiomethylbenzimidazole in aqueous medium under ultrasonic irradiation for 10–13 minutes at a frequency 36 ± 3 kHz (Scheme 18.23). The process was highly atom economical, efficient, and environmentally benign and free from tedious work-up methods (Rao et al., 2014). Li et al. reported the Knoevenagel condensation reaction of ethyl cyanoacetate with aromatic aldehydes catalyzed by pyridine under ultrasound irradiation with a frequency

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Scheme 18.23  Ultrasonic-assisted synthesis of α, β-unsaturated nitrile.

Scheme 18.24  Ultrasonic-assisted Knoevenagel condensation.

of 25 kHz and power of 500 W for 2–3 hours and obtained product with a high yield of 80–96% as shown in Scheme 18.24 (Li et al., 1999). Ultrasonic-assisted Michael addition of chalcone with active methylene compound catalyzed by potassium hydroxide in anhydrous ethanol was studied by Li and his coworker. The product formed with a high yield of 75–98% in 25–90 minutes as shown in Scheme 18.25 (Li et al., 2003). 18.3.9  Green chemistry considerations in APIs Green chemistry has a safe impact on every pharmaceutical API synthesis and design. Many active pharmaceutical ingredients (APIs) and intermediates are synthesized using green approaches like biocatalysts, green catalysts, green solvents, and solvent reduction by researchers and chemists. Some of FDA-approved drugs involving green synthesis include Atorvastatin and Montelukast. 18.3.9.1 Atorvastatin Atorvastatin with trade name Lipitor having chemical formula C33H35FN2O5. The drug is orally administered to treat cardiovascular disease of high risk and abnormal lipid

Scheme 18.25  Ultrasonic-assisted Michael addition of chalcone.

Current prospective of green chemistry in the pharmaceutical industry

Scheme 18.26  Synthesis of Atorvastatin.

levels. S.K. Ma et al. developed two-step green synthesis involving three enzymes as biocatalyst for the preparation of an intermediate of Atorvastatin. In the first step, there is biocatalytic reduction of ethyl-4-chloroacetoacetate using a ketoreductase (KRED) in combination with glucose and a NADP-dependent glucose dehydrogenase for cofactor regeneration. In second step, the chloro substituent is replaced with cyano group in the presence of hydrogen cyanide at an ambient temperature and neutral pH value catalyzed by halohydrin dehalogenase (HHDH) (Scheme 18.26). Formally, on industrial scale it was prepared by the reaction of an ethyl 3-hydroxy-4-halobutyrate with a cyanide ion in alkaline medium at high temperature. However, the final product obtained was purified by high-vacuum fractional distillation (Steven et al., 2010). 18.3.9.2 Montelukast Montelukast is a leukotriene receptor antagonist, works by inhibiting the action of leukotriene D4 in the lungs, thus reduces the inflammation and relaxation of smooth muscles. It was first developed by Merck and sold under the trade name Singulair and used for the treatment of asthma, allergic rhinitis, and hives of long duration. The synthetic method used by Merck required large volume of solvent and used toxic, corrosive, and moisture-sensitive chiral reagent (−)-β-chlorodiisopinocampheylborane [(−)-DIPchloride] to introduce a chiral center in the bulky and highly functionalized ketone. Thus, an alternate green enzymatic synthetic route was developed at Codexis and Arch Pharm Labs Limited for reducing the starting ketone to alcohol. In this reaction KRED-NAD(P) complex transfers a hydride from isopropanol (IPA) to the ketone to generate the (S)-alcohol and the acetone is produced as coproduct to transfer hydrogenolysis/ionic hydrogenation path (Scheme 18.27) (Liang et al., 2010).

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Scheme 18.27  Synthesis of Montelukast.

18.4 Conclusion Green chemistry is a new philosophical approach that can help the pharmaceutical industry to achieve its goal of sustainable development. Synthetic routes involving the use of green reagents, solvents, and catalysts, if utilized properly, can reduce the environmental burden. In short, it is concluded that green chemistry is leading to fundamental, green innovations in organic synthesis that benefits pharmaceutical and chemical industries. The parameters like economic and societal benefits, and reduction of environmental destruction, etc. also get improve by following the principles of green chemistry. This chapter explained the importance of different green metrics with their diversified applications.

Acknowledgments We are sincerely thankful to the CSIR-NEIST, Jorhat and Maharishi Markandeshwar (Deemed to be University), Mullana, India for their guidance, support, and advice at all times.

Abbreviations EPA Environmental Protection Agency PPA Pollution Prevention Act PMI process mass intensity API’s active pharmaceutical ingredients DMC dimethyl carbonate GLV gamma-valerolactones C-dot carbon dots

Current prospective of green chemistry in the pharmaceutical industry

p-NTS p-nitro toluene-ortho-sulfonic acid AMBI 5-amino-6-methyl-2-benzimidazolone MB metobromuron IP isoproturon

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Index Page numbers followed by “f ” and “t” indicate, figures and tables respectively.

A Accelerated solvent extraction (ASE), 345, 372–373 Accelerated solvent extraction apparatus, 346f Aerobic method, 434 Affinity chromatography technique, 315–316 Agricultural waste, 269 cellulose, 270 nanocellulose, 270 Alkaline extraction, 320 American Chemical Society (ACS), 200, 420 Ammonium persulfate oxidation, 280 Amphiphiles, 87 different classes, 88 micellar system, 91 micelles, 89 surfactants, 88 surfactants in catalysis, 91 vesicles and Langmuir monolayers, 90 Anaerobic method, 434 Anion-exchange resin, 323 Anionic surfactant, 163 Antioxidants, 290, 291, 295, 329 compounds, 296–297 extraction, 295 extraction techniques, 290–291 techniques for extracting, 292t–293t Aqueous biphasic systems, 404 Aqueous counter collision, 275 Atom economy, 421 Atom transfer radical polymerization (ATRP), 57, 69 Atorvastatin, 442–443 synthesis, 443f

B Ball milling, 273 Baylis Hillman reaction, 421f Bienayme-Blackburn-Groebke reactions, 25 three-component, 26f Biginelli reaction, 19–20 Bioactive compounds, 290

Biodegradable ionic liquids, 42 Biodiesel algae, 3 conventional catalysts, 4 deep eutectic solvents, 9 green catalysts, 5 green feedstocks, 3 green solvents, 6 ionic liquids, 8 possible conventional organic solvents, 6 production, 2 production technologies, 4 reaction medium, 6 supercritical carbon dioxide, 7 value determination, 1–2 Biomolecules, 309 Bligh and Dyer method, 310, 318 Bume method, 318 Butanol-methanol extraction, 310

C Carbohydrates extraction, 309, 310, 311 Carbon dots, 437–438 CATB extraction method, 321–322 Cationic ring-opening polymerizations, 61 Cationic surfactant, 162 Cellulose, 270, 436 Cellulose-based biomass, 436 Cesium chloride gradient centrifugation, 321f Cesium chloride gradient centrifugation method, 321 Cetyltrimethylammonium bromide (CTAB), 164, 322 Chaotropic agent, 322–323 Chelex extraction, 322 Chemical oxygen demand (COD), 432 Click reaction azide-alkyne cycloaddition, 104 carbon-carbon multiple bonds, 93 classification, 92 copper-free micelle promoted, 101

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cycloadditions, 93 in-water and on-water reactions, 86 multicomponent, 99 nonaldol type, 93 nucleophilic ring-openings, 93 solvent and its impact, 85 Compact fluorescent light (CFL), 203 Conventional extraction methods, 359 Conventional extraction techniques, 336 Copper-free micelle promoted click reaction, 101 Critical aggregation concentration, 164–165 Critical micellar concentration (CMC), 88, 91 Cryocrushing, 273 Cu catalyzed azide-alkyne cycloaddition reaction, 94 Cycloadditions, 93 Cyclopentyl methyl ether (CPME), 116

D Decoction extraction technique, 338, 370–371 Decyltrimethylammonium bromide (DeTAB), 164 Deep eutectic solvent (DES), 9, 10, 146, 280 aggregation, 161 carbon dioxide capture, 159 cosolvent-modified, 155 dye aggregation, 169 lithium chloride, 180 neat, 151 polarity and solvation within, 151 spectroscopy, 146 surfactant aggregation, 161 thermosolvatochromism, 177 type I, 148 type II, 148 type III, 148 type IV, 148 Deoxyribonucleic acid units, 320 Dimethyl carbonate, 434–435 Dipole rotation, 413–415 Dipole rotation and ionic conduction, 314, 413–415 DNA mutations, 329 DPPH method, 300–301 Dyes, 395–396 ionic liquid-assisted solid phase extraction, 401–404 liquid-liquid extraction, 398 Dynamic light scattering (DLS) method, 91

E E-factor, 198–199, 421 Electromagnetic field, 413–415 Elimination reactions, 422 Endocrine disrupting compounds (EDC), 217 Enhanced solvent extraction (ESE), 345, 372–373 Environmental pollution, 395 Environmental Protection Agency (EPA), 193, 200 Enzymes, 425–426 assisted extraction technique, 347 associated extraction, 313 EPS-derived carbon dots, 439 Extraction process, 309, 410 schematic representation of method, 312f Extraction techniques, 365

F First generation ionic liquids, 20–21 Flavonoids, 332f, 333f, 356, 409–410 Folch’s method, 318 schematic representation, 319f Food and pharmaceutical industry, 294 FRAP antioxidant assay, 300 FRAP method, 300 Free radical polymerization, 54 Friedel Craft reactions, 430

G Gas chromatography, 397 Gel electrophoresis, 314–315, 315f Gel filtration chromatography technique, 316–317 schematic representation, 316f Glass particles, 324 Glucose fermented and chemical products, 437f G protein-coupled receptors (GPCR), 106 Green catalyst, 429 Green Chemistry Institute (GCI), 420–421 Green solvents, 9, 113, 426, 440 applications, 116 and biodiesel production, 6 energy conversion, 116 energy storage, 118 ionic liquids, 114 solvent-free conditions, 114 supercritical carbon dioxide, 115 supercritical water, 115 water, 114 Green synthesis catalyst, 425f Green technology foundation, 363–364

Index

H Halogen compounds, 296–297 Hantzsch reaction, 37 Heck reaction, 427f, 430 Hetro Diels-Alder reaction, 426, 427f High-density supercoiled molecules, 321–322 Higher frequency ultrasound waves, 366–367 High hydrostatic pressure extraction (HHPE), 380–381 High-intensity ultrasonication, 272 See also Nanocellulose High performance liquid chromatography (HPLC), 397 High pressure solvent extraction, 345, 372–373 Hoechst-Celanese reaction, 429–430, 430f Hot water extraction, 242 Hydrocinnamic acids, 331–332 Hydrodistillation process, 362 Hydrogen bond donor, 146–147 Hydrophilic compounds, 300

I In vitro antioxidant activity quantification TEAC test, 300 Ionic conduction, 413–415 Ionic liquid in biphasic extraction methods, 404 Ionic liquids, 8, 17, 52, 401–404, 428f, 428, 429 adsorption, 133 advantages, 137 alteration of wettability, 131 anion and cation, 127t atom transfer radical polymerization, 57 biodegradable, 42 cationic ring-opening polymerizations, 61 controlled radical polymerizations, 57 defined, 17, 52–53 different generations, 20t disadvantages, 137 enhanced oil recovery, 128 first generation, 17–18 four-component reactions, 34, 37 free radical polymerization, 54 future prospects and challenges, 138 interfacial tension, 128 metathesis polymerizations, 60 microemulsions, 134 multicomponent reactions, 18, 19 in nanoform, 43 oil recovery, 136 polycondensation, 64

polymerizations, 61 properties, 18, 126, 127t radical polymerization, 54 reversible addition-fragmentation chain transfer polymerization, 59 ring-opening polymerizations, 60 second generation, 17–18 solid support, 39 three-component reactions, 19, 29 Ionic liquids in extraction methods, 397–398 Iridium catalyzed preparation of metolachlor, 431f Isoelectric focusing technique, 317 schematic representation, 317f

K Knoevenagel condensation reaction, 441–442

L Lignocellulose biomass, 436 Lipase transesterification, 1–2 Lipids, 310 extraction, 318 Lipophilic compounds, 300 Liposomes, 90 Liquid-liquid extraction technique, 397, 411 Liquid-liquid microextraction (LLME), 397

M Maceration method, 336–337 Magnetic bead-based method, 323 schematic representation, 324 Methyl-tert-butyl ether, 310 Micelles, 89, 163 head group, 89 tail group, 89 Microwave-assisted approach, 370–371 Microwave-assisted extraction, 292t–293t, 297, 298, 310, 330, 341, 342, 368, 369, 413–415 apparatus, 341f process, 298 techniques, 313–314 Microwave-assisted extraction to phenolic compound extraction, 415t Microwave-assisted hydrodistillation, 298 Microwave extraction method, 298 hydrodiffusion, 298 microwave-assisted hydrodistillation, 298 solvent, 298 solvent-free extraction, 298

453

454

Index

Microwave hydrodiffusion gravity extraction, 369 Modern extraction techniques, 348t Modern extraction technologies, 290–291 Monomeric, 330 Monomers, 329 Montelukast, 443 synthesis, 443 Multicomponent reactions (MCR), 18, 19

N Nanocellulose, 270 ammonium persulfate oxidation, 280 application, 282 aqueous counter collision, 275 ball milling, 273 cryocrushing, 273 deep eutectic solvent method, 280 enzyme-assisted process, 276 extraction, 271 future scope, 282 green catalyst strategies, 278 high-intensity ultrasonication, 272 high pressure homogenization, 274 ionic liquids, 278 mechanical methods, 271 microfluidization, 275 one pot green synthesis, 279 organoclick strategy, 279 phosphotungstic acid, 278 pressure-induced methods, 274 static culture method, 276 steam explosion, 274 stirred culture method, 277 subcritical water method, 276 twin screw extrusion, 273 ultrafine friction grinding colloider, 271 Natural antioxidants, 289, 290 Natural phenolic compounds, 357 Natural polysaccharides chemical extraction, 244 extraction techniques, 242 hot water extraction, 242 importance and applications, 240 microwave extraction methods, 244 Nernst’s distribution law, 320 Nonconventional extraction techniques, 340–341 Nonpolar solvents, 298 Nucleic acid extraction, 311, 320 Nucleophilic ring-openings, 93

O One pot green synthesis, 279 Organic pollutants, 395 Organic solvent forward osmosis (OSFO), 439 Organic solvent recovery, 439 Organic solvents-free extracts, 291 Organoclick strategy, 279 Oxidative stress, 289 Oxidative stress-related diseases, 357 Oxygen reduction reaction (ORR), 203

P Palladium catalyzed Heck reaction, 431f Passerini reaction, 25 three-component, 26f Percolation method, 340, 361 Phenolic acids, 329, 409–410 Phenolic compounds, 329, 330, 334f, 336, 338, 347, 355, 357, 359, 370, 371–372, 382–383, 395, 398, 401–404, 409, 413 classification and properties, 357 classification of, 331t extraction method, 337f, 355, 410, 412 physicochemical and environmental factors, 332 plant-source extracts, 411 Phenols, 397 adsorption, 223 in aquatic system, 221 biological treatment technique, 227 catalytic wet air oxidation process, 225 chemical oxidation process, 224 extraction method, 227 faith and degradation, 218 fenton and electro-fenton method, 225 liquid-liquid extraction, 228 membrane separation technique, 226 physicochemical properties, 218 removal, 223 solid-phase extraction, 227 toxicity, 221 Photodiode array ultraviolet detection, 397 Physical parameters equipment power and frequency, 299 intensity and length, 299 pulsating bubble acoustic pressure, 299 waves amplitude, 299 Plant-based phenolics, 355 Polar molecules, 298 Polyacrylamide gel electrophoresis, 314–315

Index

Polychlorinated biphenyls, 395 Polymeric molecules, 330 Polymerization reactions, in water, 71 heterogeneous radical polymerization systems, 72 homogenous radical polymerization reactions, 72 Polymers, 329 Polyphenolic compound, 321–322 Polysaccharides biomass hydrolysis, 251 diffusivity affect solubility, 246 extraction, 244 extraction mechanisms, 247 hydrolysis kinetics, 253 lignocellulosic biomass hydrolysis, 253 medium acidification, 246 pretreatment and modifications, 247 processing by green solvents, 250 subcritical and supercritical water, 244 temperature increases extraction yield, 246 Power conversion efficiency (PCE), 116 Pressurized fluid extraction (PFE), 345 Pressurized hot water extraction, 378 Pressurized liquid extraction (PLE), 292t–293t, 296, 297, 297f, 311, 376, 379t, 413 operational diagrammatic mechanism, 414f principle, 414f schematic representation, 313f Preyssler heteropolyacids, 278 Processed polymers, 51 Process mass intensity (PMI), 421 Protein extraction, 314 Proton exchange membrane (PEM), 204 Pulsed-electric field, 330, 346 Pulse electric field, 380

R Reactive compounds, 290 Reflux extraction, 363 Renewable feedstock, 435–436 Reversible addition-fragmentation chain transfer polymerization (RAFT), 69 Rhodium catalyzed hydroformylation, 428f Ring-opening polymerizations, 60

S Salting out technique, 316 Selective extraction methods, 289

Silica materials, 322–323 Simvastatin, 198 Size exclusion chromatography technique, 316–317 Small angle neutron scattering (SANS), 91 Small angle X-ray scattering (SAXS), 91 Sodium dodecyl benzenesulfonate (SDBS), 164 Sodium dodecyl sulfate (SDS), 164, 314–315 Solid phase extraction (SPE), 298, 397, 410, 412 Solid to solvent ratio, 336 Solvents, 291 feed system, 294 free catalytic oxidation, 430f free extraction, 298 recovery process, 439 Sonication method, 367 Sonochemistry, 441 Soxhlet apparatus, 338, 339–340 Soxhlet extractor, 339–340 Soxhlet method, 359, 360 Soxhlet system, 410 Spectroscopy and deep eutectic solvents, 146 Subcritical water extraction, 292t–293t, 295, 296f Subcritical water extraction method, 276, 372–373, 376 Sugar compound, 321–322 Supercritical carbon dioxide, 7, 67, 115, 291, 424 polycondensation reactions, 71 polymerization reactions, 69 Supercritical extraction system, 347f Supercritical fluid extraction, 291, 292t–293t, 314f, 345–346, 371, 412 process, 291 schematic representation, 314f system, 294f technique, 311 Supercritical fluid state, 311 Supercritical technology, 289, 291 Supercritical water, 115 Surfactant, 88 See also Amphiphiles aggregation, 161 anionic, 163 cationic, 162 classification, 162 nonionic, 163 Zwitterionic, 163 Syn gas utilization, 425 Synthetic antioxidants, 290 Synthetic polymers, 51

455

456

Index

T Tannic acid, 334f Tannins, 332, 409 Thermal quenching, 180–182 Three-component reactions, 19, 29 Toxic solvents, 296–297 Twin screw extrusion, 273

U Ultrasonic-assisted Knoevenagel condensation, 442f Ultrasonic bath devices, 299 Ultrasound apparatus, 343f Ultrasound-assisted extraction, 292t–293t, 299, 342, 343, 344, 356–357, 365–366, 415–416 natural products, 299 process, 299–300

Ultrasound waves, 365–366 UV-visible spectroscopy techniques, 432

W Waste pine, 367 Waste water treatment, 431–432 Water dielectric constant, 295 Wittig and Grignard reagent’s reaction, 422

X X-ray diffraction (XRD), 438

Z Zahn-Wellens test, 432 Zwitterionic surfactant, 163