Production of Materials from Sustainable Biomass Resources [1st ed.] 978-981-13-3767-3;978-981-13-3768-0

Table of contents :
Front Matter ....Pages i-xviii
Front Matter ....Pages 1-1
Isolation, Purification, and Potential Applications of Xylan (Gen-Que Fu, Ya-Jie Hu, Jing Bian, Ming-Fei Li, Feng Peng, Run-Cang Sun)....Pages 3-35
Front Matter ....Pages 37-37
Development of Lignin-Based Antioxidants for Polymers (Afsana S. Kabir, Zhong-Shun Yuan, Takashi Kuboki, Chunbao (Charles) Xu)....Pages 39-59
Nanocellulose Applications in Papermaking (Carlos Salas, Martin Hubbe, Orlando J. Rojas)....Pages 61-96
Front Matter ....Pages 97-97
Recent Advances in Cellulose Chemistry and Potential Applications (Poonam Trivedi, Pedro Fardim)....Pages 99-115
Production, Characterization and Alternative Applications of Biochar (Aldrich Ngan, Charles Q. Jia, Shi-Tang Tong)....Pages 117-151
Carbons from Biomass for Electrochemical Capacitors (Lu Wei, Wen Zhao, Gleb Yushin)....Pages 153-184
Carbonaceous Catalysts from Biomass (Melanie J. Hazlett, Ross A. Arnold, Vicente Montes, Ye Xiao, Josephine M. Hill)....Pages 185-231
Synthesis and Design of Engineered Biochars as Electrode Materials in Energy Storage Systems (Omid Norouzi, Pejman Salimi, Francesco Di Maria, S. E. M. Pourhosseini, Farid Safari)....Pages 233-265
Front Matter ....Pages 267-267
Biomass Pelletization: Contribution to Renewable Power Generation Scenarios (Roberto García, María V. Gil, María P. González-Vázquez, Fernando Rubiera, Covadonga Pevida)....Pages 269-294
Biocarbon Production and Use as a Fuel (Pietro Bartocci, Liang Wang, Øyvind Skreiberg, Federica Liberti, Gianni Bidini, Francesco Fantozzi)....Pages 295-324
Mechanical Aspects and Applications of Pellets Prepared from Biomass Resources (Pietro Bartocci, Øyvind Skreiberg, Liang Wang, Hu Song, Hai-Ping Yang, Mauro Zampilli et al.)....Pages 325-358
Front Matter ....Pages 359-359
Microbial Production and Properties of LA-based Polymers and Oligomers from Renewable Feedstock (John Masani Nduko, Seiichi Taguchi)....Pages 361-390
Back Matter ....Pages 391-401

Citation preview

Biofuels and Biorefineries 9

Zhen Fang Richard L. Smith, Jr. Xiao-Fei Tian Editors

Production of Materials from Sustainable Biomass Resources

Biofuels and Biorefineries Volume 9

Editors-in-Chief: Professor Zhen Fang, Nanjing Agricultural University, Nanjing, China Editorial Board Members: Professor Jamal Chaouki, Polytechnique Montréal, Canada Professor Liang-shih Fan, Ohio State University, USA Professor John R. Grace, University of British Columbia, Canada Professor Vijaya Raghavan, McGill University, Canada Professor Yonghao Ni, University of New Brunswick, Canada Professor Norman R. Scott, Cornell University, USA Professor Richard L. Smith, Jr., Tohoku University, Japan Professor Ying Zheng, University of Edinburgh, UK

Aims and Scope of the Series The Biofuels and Biorefineries Series aims at being a comprehensive and integrated reference for biomass, bioenergy, biofuels, and bioproducts. The Series provides leading global research advances and critical evaluations of methods for converting biomass to biofuels and chemicals. Scientific and engineering challenges in biomass production and conversion are covered that show technological advances and approaches for creating new bio-economies in a format that is suitable for both industrialists and environmental policy decision makers. The Biofuels and Biorefineries Series provides readers with clear and concisely written chapters that are peer reviewed on significant topics in biomass production, biofuels, bioproducts, chemicals, catalysts, energy policy, economics, and processing technologies. The text covers major fields of plant science, green chemistry, economics and economy, biotechnology, microbiology, chemical engineering, mechanical engineering, and energy. Series Description Annual global biomass production is about 220 billion dry tons or 4,500 EJ, equivalent to 8.3 times the world’s energy consumption in 2014 (543 EJ). On the other hand, world-proven oil reserves at the end of 2011 reached 1652.6 billion barrels, which can only meet just over 50 years of global production. Therefore, alternative resources are needed to both supplement and replace fossil oils as the raw material for transportation fuels, chemicals, and materials in petroleum-based industries. Renewable biomass is a likely candidate because it is prevalent over the Earth and is readily converted to other products. Compared with coal, some of the advantages of biomass are (i) its carbon-neutral and sustainable nature when properly managed, (ii) its reactivity in biological conversion processes, (iii) its potential to produce bio-oil (ca. yields of 75%) by fast pyrolysis because of its high oxygen content, (iv) its low sulfur and lack of undesirable contaminants (e.g., metals, nitrogen content), (v) its wide geographical distribution, and (vi) its potential for creating jobs and industries in energy crop productions and conversion plants. Many researchers, governments, research institutions, and industries are developing projects for converting biomass, including forest woody and herbaceous biomass, into chemicals, biofuels, and materials, and the race is on for creating new “biorefinery” processes needed for future economies. The development of biorefineries will create remarkable opportunities for the forestry sector, biotechnology, materials, and the chemical processing industry and stimulate advances in agriculture. It will help to create a sustainable society and industries that use renewable and carbon-neutral resources. More information about this series at http://www.springer.com/series/11687

Zhen Fang  •  Richard L. Smith, Jr.  Xiao-Fei Tian Editors

Production of Materials from Sustainable Biomass Resources

Editors Zhen Fang College of Engineering Nanjing Agricultural University Nanjing, China Xiao-Fei Tian School of Biology and Biological Engineering South China University of Technology Guangzhou, China

Richard L. Smith, Jr. Graduate School of Environmental Studies, Department of Chemical Engineering, Research Center of Supercritical Fluid Technology Tohoku University Sendai, Japan

ISSN 2214-1537     ISSN 2214-1545 (electronic) Biofuels and Biorefineries ISBN 978-981-13-3767-3    ISBN 978-981-13-3768-0 (eBook) https://doi.org/10.1007/978-981-13-3768-0 Library of Congress Control Number: 2019931852 © Springer Nature Singapore Pte Ltd. 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Lignocellulosic biomass consists of the biopolymers (cellulose, hemicellulose, and lignin) that form a natural structural matrix with structural similarities but uniqueness among its many forms. As one of the most abundant renewable resources, lignocellulosic biomass can be transformed into materials, chemicals, and energy with sustainable chemistry and engineering.  The substitution of traditional fossil resources by the three major biopolymers as sustainable feedstocks has been extensively investigated for the manufacture of high value-added products, including biofuels, commodity chemicals, bio-based functional materials, and heterogeneous catalysts that could be directly applied for promoting the manufacturing processes. Aimed at developing innovative materials and fuels for practical application, this book was conceived for the collection of studies on state-of-the-art techniques developed specifically for lignocellulose component derivation and producing functional materials, composite polymers, carbonaceous biocatalysts, and pellets from lignocellulosic biomass with emphasis on sustainable chemistry and engineering. Technological strategies in terms of physical processing or biological conversion of biomass for material production are also included. Each individual chapter was contributed by experts or professionals in the field and globally selected to provide a broad perspective of applications on the frontier. This book is the ninth book of the series entitled “Biofuels and Biorefineries,” and it contains 12 chapters contributed by leading experts in the field. The text is arranged into five key areas: Part I: Isolation and Purification of Lignocellulose Components (Chap. 1) Part II: Composite Polymers Derived from Lignin and Cellulose (Chaps. 2 and 3) Part III: Functional Materials Derived from Cellulose and Lignocelluloses (Chaps. 4, 5, 6, 7, and 8) Part IV: Biomass Pellets as Fuels (Chaps. 9, 10, and 11) Part V: Biosynthesis of Polymers from Renewable Biomass (Chap. 12)

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Chapter 1 summarizes current reports on extraction, purification, chemical components, structural features, and functional properties of xylan. The preparation of xylan derivatives and xylan-based materials, as well as their potential applications, is discussed. Chemical modifications applied to functionalize xylan, especially to modify its thermo-plasticity, hydrophobicity, conductivity, and stimuli responsiveness, are highlighted. Chapter 2 introduces a detailed literature review on how lignin fits into the growing market for antioxidants, especially as replacements for polyolefins, and discusses hydrolytic depolymerization processes showing how depolymerization can improve the antioxidant activity of commercial lignin and how the mechanical properties are affected after incorporating lignin into polymer matrixes. Chapter 3 focuses on the properties and use of nanocellulose to achieve favorable strength and barrier properties, as well as in coating and paper sheet forming. Chapter 4 describes cellulose derivatization approaches and advanced material designs that have been realized and materials that have potential application in biomedical areas. Chapter 5 gives a state-of-the-art overview of biochar production, characterization, and applications in nontraditional areas. Potential use of biochar for environmental remediation and for water desalination is demonstrated, as well as sustainable energy applications related to supercapacitors and electrochemical sensors. Chapter 6 covers progress on fabrication of biomass-derived nanostructured carbon materials for use as carbon electrodes. Correlations between carbon structures and electrochemical properties are summarized along with performance aspects. Chapter 7 summarizes the characteristics and properties of biomass-derived catalysts and metal-free functionalized carbocatalysts and shows comparisons to catalysts from other carbon sources and materials. The biomass-derived carbonaceous catalysts used in biodiesel production, gasification, and electro-Fenton oxidation reaction are reviewed. Chapter 8 introduces designs, structural features, and chemical and physical activation of engineered biocarbon-based materials with a focus on the application and performance differences of novel engineered biochars in lithium-ion batteries. Chapter 9 provides a state-of-the-art review of biomass pelletization on the laboratory, pilot, and industrial scales with particular emphasis on its implementation in power generation. The chapter is rich with examples on the status of large-scale biomass pellets firing and cofiring of worldwide operations. Chapter 10 presents an overview of property differences among biocarbons produced from different thermal processes, including pyrolysis, gasification, and hydrothermal treatment. Of particular importance is the use of biocarbon in the steel industry to reduce carbon dioxide emissions. Techno-economic and environmental aspects of biocarbon pellet combustion are analyzed. Chapter 11 highlights the effects of the raw materials, binders, pretreatment, and process parameters on pellet design and the pelletization process and includes practical models for development. Mechanical aspects of charcoal and wood pelletization are covered with actual examples that have resulted in commercial materials. Chapter 12 provides opportunities and challenges regarding the production of lactic-acid-based polymers and related oligomer precursors using genetically modified organisms and engineered enzymes. Future developments show the advantages of using biological techniques to replace fossil fuel feedstocks.

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The text should be of interest to professionals in academia and industry who are working in the fields of natural renewable materials, biorefinery of lignocellulose, biofuels, and environmental engineering. It can also be used as comprehensive references for university students with background in chemical engineering, material science, and environmental engineering. Nanjing, China Sendai, Japan Guangzhou, China

Zhen Fang Richard L. Smith, Jr. Xiao-Fei Tian

Acknowledgments

First and foremost, we would like to cordially thank all the contributing authors for their great efforts in writing and revising the chapters and insuring the reliability of information given in their chapters. Their contributions have really made this project realizable. Apart from the efforts of the authors, we would also like to acknowledge the individuals listed below for carefully reading the book chapters and giving many constructive comments that significantly improved the quality of the chapters: Prof. E.J. “Ben” Anthony, Cranfield University, UK Dr. Ing. Marco Barbanera, Università degli Studi di Perugia, Italy Dr. Marc Borrega, VTT Technical Research Centre of Finland Ltd, Finland Dr. Pedro Castaño, University of the Basque Country (UPV/EHU), Spain Dr. Greta Faccio, Empa Swiss Federal Laboratories for Materials Science & Technology, Switzerland Prof. Dr. Anton Friedl, Technische Universität Wien (TU Wien), Austria Mr. Stefan Frodeson, Karlstad University, Sweden Prof. Thomas Heinze, Friedrich-Schiller-University Jena, Germany Prof. Fang Huang, Fujian Agriculture and Forestry University, China Mr. Fohr Jarno, Lappeenranta University of Technology, Finland Prof. Charles Q. Jia, University of Toronto, Canada Dr. Junhua Jiang, Idaho National Laboratory, USA Dr. Andreas Koschella, Universität Jena, Germany Dr. Konstantina Kourmentza, University of Reading, UK Prof. Richard A Larson, University of Illinois at Urbana-Champaign, USA Dr. Seung Woo Lee, Georgia Institute of Technology, USA Dr. James McGregor, The University of Sheffield, UK Prof. Emilia Morallon, Universidad de Alicante, Spain Dr. Oleksandr Nechyporchuk, Swerea IVF; Chalmers University of Technology, Sweden Dr. Ioanna G. Ntaikou, Foundation for Research and Technology, Hellas (FORTH/ ICE-HT), Greece ix

x

Acknowledgments

Dr. Tobias Placke, University of Münster, Germany Dr. Hema Ramsurn, University of Tulsa, USA Prof. Dr. Junli Ren, South China University of Technology, China Dr. MA Sanchez-Monedero, CEBAS-CSIC, Campus Universitario de Espinardo, Spain Dr. Takafumi Sato, Utsunomiya University, Japan Dr. Olena Sevastyanova, Fibre and Polymer Technology/Wood Chemistry and Pulp Technology, Sweden Dr. Arjan Smit, Energy research Centre of the Netherlands (ECN), the Netherlands Dr. Juan José Villaverde, INIA  - National Institute for Agricultural and Food Research and Technology, Spain Prof. Shubin Wu, South China University of Technology, China Dr. Seunghyun Yoo, North Carolina State University, USA We are also grateful to Ms. Becky Zhao (senior editor) and Ms. Abbey Huang (editorial assistant) for their encouragement, assistance, and guidance during the preparation of the book. Finally, we would like to express our deepest gratitude toward our families for their love, understanding, and encouragement, which help us in the completion of this project.

Zhen Fang, October 18, 2018 in Nanjing

Richard L. Smith, Jr., October 18, 2018 in Sendai

Xiao-Fei Tian, October 18, 2018 in Guangzhou

(Zhen Fang)

(Richard L. Smith, Jr.)

(Xiao-Fei Tian)

Contents

Part I Isolation and Purification of Lignocellulose Components 1 Isolation, Purification, and Potential Applications of Xylan����������������    3 Gen-Que Fu, Ya-Jie Hu, Jing Bian, Ming-Fei Li, Feng Peng, and Run-Cang Sun Part II Composite Polymers Derived from Lignin and Cellulose 2 Development of Lignin-Based Antioxidants for Polymers ������������������   39 Afsana S. Kabir, Zhong-Shun Yuan, Takashi Kuboki, and Chunbao (Charles) Xu 3 Nanocellulose Applications in Papermaking ����������������������������������������   61 Carlos Salas, Martin Hubbe, and Orlando J. Rojas Part III Functional Materials Derived from Cellulose and Lignocelluloses 4 Recent Advances in Cellulose Chemistry and Potential Applications����������������������������������������������������������������������   99 Poonam Trivedi and Pedro Fardim 5 Production, Characterization and Alternative Applications of Biochar ��������������������������������������������������������������������������  117 Aldrich Ngan, Charles Q. Jia, and Shi-Tang Tong 6 Carbons from Biomass for Electrochemical Capacitors����������������������  153 Lu Wei, Wen Zhao, and Gleb Yushin 7 Carbonaceous Catalysts from Biomass��������������������������������������������������  185 Melanie J. Hazlett, Ross A. Arnold, Vicente Montes, Ye Xiao, and Josephine M. Hill

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8 Synthesis and Design of Engineered Biochars as Electrode Materials in Energy Storage Systems������������������������������  233 Omid Norouzi, Pejman Salimi, Francesco Di Maria, S. E. M. Pourhosseini, and Farid Safari Part IV Biomass Pellets as Fuels 9 Biomass Pelletization: Contribution to Renewable Power Generation Scenarios ������������������������������������������������������������������  269 Roberto García, María V. Gil, María P. González-Vázquez, Fernando Rubiera, and Covadonga Pevida 10 Biocarbon Production and Use as a Fuel����������������������������������������������  295 Pietro Bartocci, Liang Wang, Øyvind Skreiberg, Federica Liberti, Gianni Bidini, and Francesco Fantozzi 11 Mechanical Aspects and Applications of Pellets Prepared from Biomass Resources��������������������������������������������������������������������������  325 Pietro Bartocci, Øyvind Skreiberg, Liang Wang, Hu Song, Hai-Ping Yang, Mauro Zampilli, Gianni Bidini, and Francesco Fantozzi Part V Biosynthesis of Polymers from Renewable Biomass 12 Microbial Production and Properties of LA-based Polymers and Oligomers from Renewable Feedstock ������������������������������������������  361 John Masani Nduko and Seiichi Taguchi Index������������������������������������������������������������������������������������������������������������������  391

Contributors

Ross A. Arnold  Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada Pietro Bartocci  Department of Engineering, University of Perugia, Perugia, Italy Jing Bian  Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China Gianni Bidini  Department of Engineering, University of Perugia, Perugia, Italy Francesco Di Maria  Department of Engineering, University of Perugia, Perugia, Italy LAR5 Laboratory, Dipertimento di Ingegneria, University of Perugia, Perugia, Italy Francesco Fantozzi  Department of Engineering, University of Perugia, Perugia, Italy Pedro  Fardim  Laboratory of Fibre and Cellulose Technology, Åbo Akademi University, Turku, Finland Department of Chemical Engineering, KU Leuven, Leuven, Belgium Gen-Que  Fu  Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China Roberto García  Instituto Nacional del Carbón, INCAR-CSIC, Oviedo, Spain María V. Gil  Instituto Nacional del Carbón, INCAR-CSIC, Oviedo, Spain María  P.  González-Vázquez  Instituto Nacional del Carbón, INCAR-CSIC, Oviedo, Spain Melanie  J.  Hazlett  Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada Josephine M. Hill  Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada xiii

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Contributors

Martin  Hubbe  Department of Forest Biomaterials, North Carolina State University, Raleigh, NC, USA Ya-Jie Hu  Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China Charles  Q.  Jia  Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON, Canada Afsana S. Kabir  Institute for Chemicals and Fuels from Alternative Resources, Western University, London, ON, Canada Takashi Kuboki  Department of Mechanical and Materials Engineering, Western University, London, ON, Canada Federica Liberti  CRB (Biomass Research Centre), University of Perugia, Perugia, Italy Ming-Fei  Li  Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China Vicente  Montes  Department of Organic Chemistry, University of Cordoba, Cordoba, Andalucia, Spain John  Masani  Nduko  Department of Dairy and Food Science and Technology, Faculty of Agriculture, Egerton University, Egerton, Kenya Aldrich  Ngan  Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON, Canada Feng Peng  Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China Covadonga Pevida  Instituto Nacional del Carbón, INCAR-CSIC, Oviedo, Spain S.  E.  M.  Pourhosseini  School of Chemistry, College of Science, University of Tehran, Tehran, Iran Orlando J. Rojas  Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Espoo, Finland Fernando Rubiera  Instituto Nacional del Carbón, INCAR-CSIC, Oviedo, Spain Farid Safari  Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, ON, Canada Omid Norouzi  Department of Engineering, University of Perugia, Perugia, Italy LAR5 Laboratory, Dipertimento di Ingegneria, University of Perugia, Perugia, Italy Carlos Salas  Department of Forest Biomaterials, North Carolina State University, Raleigh, NC, USA

Contributors

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Pejman  Salimi  Department of Physical Chemistry, Faculty of Science, Tarbiat Modares University, Tehran, Iran Øyvind Skreiberg  SINTEF Energy Research, Trondheim, Norway Hu  Song  China-EU Institute for Clean and Renewable Energy, Huazhong University of Science and Technology, Wuhan, China State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, China Run-Cang  Sun  Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China Seiichi  Taguchi  Department of Chemistry for Life Sciences and Agriculture, Faculty of Life Sciences, Tokyo University of Agriculture, Tokyo, Japan CREST, JST, Tokyo, Japan Shi-Tang  Tong  Department of Chemical Engineering, Wuhan University of Science and Technology, Wuhan, China Poonam  Trivedi  Laboratory of Fibre and Cellulose Technology, Åbo Akademi University, Turku, Finland Liang Wang  SINTEF Energy Research, Trondheim, Norway Lu  Wei  School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, China Ye  Xiao  Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada Chunbao  (Charles)  Xu  Institute for Chemicals and Fuels from Alternative Resources, Western University, London, ON, Canada Hai-Ping Yang  State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, China Zhong-Shun Yuan  Institute for Chemicals and Fuels from Alternative Resources, Western University, London, ON, Canada Gleb  Yushin  School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, USA Mauro Zampilli  Department of Engineering, University of Perugia, Perugia, Italy Wen Zhao  School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, China

About the Editors

Zhen Fang is professor and leader of biomass group, Nanjing Agricultural University. He is the inventor of the “fast hydrolysis” process. He is listed in the “Most Cited Chinese Researchers” in energy for 2014–2017 (Elsevier-Scopus). Professor Fang specializes in thermal/biochemical conversion of biomass, nanocatalysts synthesis and their applications, pretreatment of biomass for biorefineries, and supercritical fluid processes. He obtained his PhDs from China Agricultural University and McGill University. Professor Fang is associate editor of Biotechnology for Biofuels and the Journal of Supercritical Fluids. He has more than 20-year international research experiences at top universities and institutes around the world, including 1 year in Spain (University of Zaragoza), 3 years in Japan (Biomass Technology Research Center, AIST; Tohoku University), and more than 8 years in Canada (McGill) in renewable energy and green technologies. He has worked for 7 years as engineer in energy, bioresource utilization, and engine design in industry before moving to academia.  

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About the Editors

Richard L.  Smith, Jr. is professor of Chemical Engineering, Graduate School of Environmental Studies, Research Center of Supercritical Fluid Technology, Tohoku University, Japan. Professor Smith has a strong background in physical properties and separations and obtained his PhD in chemical engineering from the Georgia Institute of Technology (USA). His research focuses on developing green chemical processes especially those that use water and carbon dioxide as the solvents in their supercritical state. He has expertise in physical property measurements and in separation techniques with ionic liquids and has published more than 200 scientific papers, patents, and reports in the field of chemical engineering. Professor Smith is the Asia regional editor for the Journal of Supercritical Fluids and has served on editorial boards of major international journals associated with properties and energy.  

Xiao-Fei Tian is associate professor of School of Biology and Biological Engineering, South China University of Technology. Dr. Tian obtained his PhD from the University of Chinese Academy of Sciences. His research focuses on solvent deconstruction and enzymatic saccharification of lignocelluloses for renewable cellulosic materials and biofuels. He also has experience in the development of fermentation techniques for functional fungal pigments. Dr. Tian has coauthored more than 30 research papers, reviews, and patents in his research specialty and has served as peer reviewer for major scientific journals.  

Part I

Isolation and Purification of Lignocellulose Components

Chapter 1

Isolation, Purification, and Potential Applications of Xylan Gen-Que Fu, Ya-Jie Hu, Jing Bian, Ming-Fei Li, Feng Peng, and Run-Cang Sun

Abstract  There is great interest in replacing fossil fuel resources with renewable raw materials. Widely distributed lignocellulosic biomass is viewed as a potential candidate to address energy and environmental demands. In this chapter, the use of the hemicellulose component of lignocellulosic biomass, xylan, is considered. Due to the presence of ester and ether lignin-carbohydrate linkages, extraction of xylan is generally restricted to cell wall matrices of wood and lignified grass. Many extraction methods of xylan have been proposed, however, the subsequent purification and analyses are needed on its fine structure. An effective way to modify xylan with advanced properties is through etherification. This chapter points out new functionalities, for example, thermoplasticity, hydrophobicity, conductivity, and stimuli-­ responsiveness of xylan by chemical modification and it summarize recent reports on xylan, including extraction, purification, chemical components, structural features, and functional properties. Xylan derivatives, xylan-based materials, and their potential applications are discussed and future research areas are highlighted.

1.1  Introduction Demand for energy is high worldwide and with the instability of oil resources and concerns about global climate change, initiatives have been issued for alternative energy development that can replace fossil fuels. Consequently, much interest exists in developing alternative fuels and sources of chemicals. For creation of a sustainable society, renewable feed stocks must be used in the production of life necessities. Presently, lignocellulosic biomass provides numerous industrial and non-food consumer products for society, including materials, chemicals, and fuels. However, every year, there are many biomass residues produced in agriculture that are treated G.-Q. Fu · Y.-J. Hu · J. Bian · M.-F. Li · F. Peng (*) · R.-C. Sun Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 Z. Fang et al. (eds.), Production of Materials from Sustainable Biomass Resources, Biofuels and Biorefineries 9, https://doi.org/10.1007/978-981-13-3768-0_1

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Fig. 1.1  Main sugar units of hemicelluloses

as wastes and used inefficiently. Wastes cause many problems related to space, release of greenhouse gases such as methane and contamination with chemicals such as dioxins [1]. Renewable and abundant lignocellulosic biomass is regarded as a sustainable resource and since it is widely distributed in nature, it can be used to produce fuels and necessary chemicals [2]. Cellulose, lignin, and hemicelluloses are well known as the major constituents of lignocellulosic biomass [3]. Up to now, cellulose and lignin are being used for commercial applications, while the hemicellulose constituent of biomass has practically no commercial application. As an enormous biopolymer resource, hemicelluloses have a wide variety of structural types and consist of main sugar units including xylose, mannose, D-glucose, galactose, arabinose, and glucuronic acid as shown in Fig.  1.1. The β-(1→4)-D-­ xylopyranose is the basic structure unit of xylan, and the 2- or 3- position is randomly substituted with side groups. Gradually, researchers have focused more on xylan to make progress on its application to the food and non-food fields. Xylan has been studied in many fields and some of its promising properties are documented in reviews [4–10].

1.2  Structure of Xylan The structure diversity of xylan affects its functionality [11]. Generally, xylan can be divided into several parts according to its primary structure in plant tissues: homoxylans and glucuronoxylan (GX), arabinoglucuronoxylan (AGX), glucuronoarabinoxylan (GAX), arabinoxylan (AX) and heteroxylan [12].

1  Isolation, Purification, and Potential Applications of Xylan 3

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(b) Fig. 1.2 Structural conformations of homoxylan, (a) β-(1→3)-D-xylan; (b) β-(1→3 and 1→4)-D-xylan

1.2.1  Homoxylan Xylan polysaccharides can be found in the oldest plant species botanically. Homoxylan is the form of xylan that consists of xylose residues only. It has been reported that homoxylan with β-(1→3) glycosidic linkages (Fig. 1.2a) is from green algae (Caulerpa sp.) and homoxylan, which has a structure of β-(1→3 and 1→4) (Fig. 1.2b), can be found in Palmariales and Nemaliales [11, 13]. Analyses indicate 1, 3-linkages are regularly distributed in the form of pentamers in the cell wall [14].

1.2.2  Arabinoxylan (AX) The primary structure of arabinoxylan is shown in Fig. 1.3. AX readily occurs in a variety of tissues in many kinds of plants [15–18]. AX is primarily substituted by arabinose at carbon position 2 or 3 (may be di-substituted or mono-substituted, Fig. 1.3a, b) of the xylose units along the backbone of xylan [13, 19]. The degree of substitution affects the solubility of AX in water or other solvents. Water-insoluble AX is mono-substituted by α-L-arabinose at position 3 of the xylan backbone and the ratio of arabinose to xylose is in the range of 0.3–1.2 [20].

1.2.3  Glucuronoxylan (GX) In hardwood, glucuronoxylan is the largest hemicellulose constituent and it makes up 15–30% proportion of the dry mass weight of hemicellulose [21, 22]. The backbone of GX is (1→4)-linked β-D-xylopyranosyl (β-D-Xylp), and it is always substituted by 4-O-methyl-α-D-glucopyranosyl uronic acid residues (4-O-MeGlcA) at

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G.-Q. Fu et al. O OH HO O

OH O

O

HOH2C

5 4

O

O OH HOH2C

O 3

1

2 O OH

HOH2C

O

O

OH O

OH O

O

OH

OH

(a) HO O

OH O

O

O 4

O OH HOH2C

3

O

OH

HO

1

5

O OH

2

OH O

O

(b)

Fig. 1.3  Primary structure of arabinoxylan (a) 2, 3-arabino-D-xylan; (b) 3-arabino-D-xylan

HO O O

OH O

5 4

O

1

O 3

HO O

2

HO O OH

O

OH O

OH MeO COOH

Fig. 1.4  Structure of glucuronoxylan, 4-O-methyl-D-glucurono-D-xylan

position 2 (Fig. 1.4). The polysaccharide framework consists of around 200 β-D-­ Xylp residues. Some xylose units only carry a single and terminal side chain of glucuronic acid residue at position 2 [23]. Teleman et al. [24] extracted hemicelluloses from birch and beech by dimethyl sulphoxide (DMSO). Through NMR analysis, they found that the main constituent is O-acetyl-(4-O-methylglucurono)-xylan containing one 4-O-MeGlcA substituent for approximately every 15 D-xylose residues. Mais et  al. [25] found that the main hemicellulose in hardwood pulp is 4-O-methylglucurono-xylan with a 4-O-MeGlcA: Xyl ratio of 5:100 at maximum.

1  Isolation, Purification, and Potential Applications of Xylan

7 COOH O

HO O O

OH O O OH HOH2C

1

5 4O

O 3

HOH2C OH

HO O

2

O

O OH

O

1

O

MeO OH 3

2 5

4

OH O

OH

Fig. 1.5 Structure of glucuroarabinoxylan (GAX), 2, 3-arabino-2-(4-O-methylglucurono acid)-xylan

1.2.4  Arabinoglucuronoxylan (AGX) and Glucuroarabinoxylan (GAX) The predominant fraction of non-woody materials is GAX, which has a backbone of β-(1→4)-D- Xylp substituted at C-2 or 3 with MeGlcA and α-L-arabinofuranosyl (α-L-Araf) units (Fig. 1.5), and it might also be slightly acetylated [3]. GAX has an AX backbone, and its uronic acid side chain is ten times less in number than α-L-­ Araf, and some Xylp residues are double in number of those sugars [14]. Different extraction methods and sources finally lead to different degrees and substitution pattern of GAX [26, 27]. Shi et al. [28] examined alkali-extracted dendrocalamus hemicelluloses. Neutral sugar analysis suggested that the soluble hemicelluloses were mainly GAX with structure of linear β-(1→4)-Xylp backbone substituted by α-L-Araf and/or 4-O-MeGlcA units at position 2 or 3. Arabinoglucuronoxylans were isolated from the holocellulose of sugi (Cryptomeria japonica) and Hinoki (Chamaecyparis obtusa) by Yamasaki et al. [29]. The obtained AGX was mainly contained one 4-O-MeGlcA per 6.2 D-Xylp and one 4-O-Me-D-GlcA residue per 3.8 D-Xylp. By means of size exclusion chromatography and electrospray-­ionization mass spectroscopy, the presence of xylooligosaccharides was found and was determined to have a degree of polymerization of 2–8 in addition to D-Xyl, suggesting that the AGXs from Sugi and Hinoki contained unsubstituted chains consisting of at least eight D-Xyl residues.

1.3  Isolation of Xylan In daily life, xylan is used in cosmetics and shaving creams, while it also found in wood meal, forest chips, and plant crops. Xylan can be extracted directly from plants or it can be obtained as a by-product in wood and agriculture processing. Xylan occurs in sources of oligomers or mono sugars from degradation of

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hemicelluloses mixed in black liquor in conventional Kraft pulping industries. For the case of Kraft pulping however, xylan is rarely recovered as the black liquor is used to produce steam and electricity. For the case of bioethanol production, it is necessary to remove hemicelluloses for high accessibility of cellulosic materials and to improve the yields of ethanol. Much attention has been paid to the isolation of xylan that has high purity and high yield. Typical fractionation methods of xylan from biomass materials will be introduced in the next sections.

1.3.1  Alkali Isolation Hemicelluloses are interconnected with other cell wall components via covalent linkages and hydrogen bonds. Alkaline treatment is a commonly used method to destroy the covalent link or H-bond between hemicelluloses and some other components such as cellulose, lignin, and silica. On the other hand, cellulose swells with alkali treatment and its crystallinity decreases [30]. Yields of hemicellulose are influenced by the type of alkali used in the isolation process. NaOH and KOH are both recommended because they are effective to obtain a high yields of xylan, and NaOH is better than KOH for the same concentration, but KOH-extracted fractions tend to have higher purity [31–34]. On the other hand, Ca(OH)2 can be used to extract xylan, but the obtained xylan fraction has low solubility in water. It has been reported that Ba(OH)2 is effective in the fractional extraction of AX from some crops [18, 35–37]. Hutterer et al. [38] found that xylan is effectively fractionated by alkaline treatment in the study of hardwood kraft pulps. In their work, uronic acid was marked with fluorescencing compounds, and this indicated that a large amount of uronic acid in the cellulosic fraction was separated as the alkalinity increased during pulp treatment. In the work of Singh et al., an alkali extraction method with two-stages was evaluated [39]. In the first stage, the xylan obtained was from canut husk, which was treated with 10% NaOH for 8 h at 65 °C, and then with hydrothermal treatment for the second-step. About 40–52% of the available xylan was obtained in the first step, and the overall xylan recovery was more than 90%. Arumugam et al. [40] conducted one and two-step alkaline-extractions of peanut shell. On the basis of characterization by NMR and MALDI ToF MS analysis, the extracted hemicelluloses were mainly GX, which was similar to a hardwood GX, with a high degree of substitution of the main chain with 4-O-methyl-Dglucuronicxylan acid (Xyl: MeGlcA ratio ~6–7).

1.3.2  Organic Solvent Isolation Organic solvent extraction of biomass preserve the structure of xylan, such as its acetyl groups. Some organic solvents including alcohols or organic acids are considered to be neutral solvents for xylan due to the process of extracting xylan

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without major alteration or degradation of its native structure [41, 42]. Researchers found that the proportion of DMSO-extracted xylan husk of sorghum grain was about 24% of the total lignocellulosic content, and the extracts were highly branched AX, and had negative specific optical rotations [43]. DMSO/LiCl system has also been used to extract xylan from vegetables or fruits, and relatively fine structures were obtained [44]. Fu et al. [45] compared properties of DMSO and NaOH extractable hemicelluloses from Neosinocalamus affinis and found that ethanol precipitation was effective after extraction with NaOH, but that ethanol precipitation could not separate the fraction with different degrees of branching for DMSO-extracted hemicellulose (main component, xylan). However, DMSO-extracted fractions had fine structures of 4-O-methyl-glucuronoarabinoxylans with acetyl substitution at C-2, C-3.

1.3.3  Steam Explosion For lignocellulosic biomass, high-pressure steam treatment is popular due to its high-efficiency [46, 47]. With high-pressure steam, raw materials are saturated and the pressure is released abruptly so that the materials are rapidly degraded. Steam explosion destroys the basic structure of plant materials and hydrolyzes the xylan [48]. Steam explosion works well for a variety of biomass feedstock such as wood and agricultural residues [49–53]. The production of xylooligosaccharides from corncobs was studied by Teng et al. with a two-stage process involved steam explosion pretreatment and alkali treatment [54] in which conditions for steam explosion were 188 °C to 204 °C for 2.5–2.7 min; optimal conditions were determined to be at 196  °C for 5  min with a yield of 22.8%. Although steam explosion is an environmentally-­friendly method, problems still remain including degradation and solubilization of xylan yielding oligomeric products [55–57].

1.3.4  Hydrothermal (Autohydrolysis) The hydrothermal method or autohydrolysis method is highly consistent with the concept of “biomass refining” and it is very successful for separating lignocellulosic materials. The application of hydrothermal technology to biomass has application for reactions and separations. At high temperatures and pressures, hydronium ions from both water and in situ-generated compounds catalyze xylan depolymerization [58]. Kim et al. [59] proposed a low acid hydrothermal fraction method. The yield of xylose was up to 74.8% when the Miscanthus sacchariflorus were treated with 0.3 wt% of sulfuric acid for 10 min at 180 °C. In this condition, the decomposed xylose was transformed into furfural, and the maximum concentration of furfural was up to 1.87 g/L (the loss of xylan was 25.9%). Chen et al. [60] developed a flow-­ through hydrothermal system for increasing xylan recovery. It was found that the

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xylose recovery, lignin removal and total sugar yield were improved by this flow-­ through hydrothermal system when compared with the batch hydrothermal process. The maximum sugar yield was 84% with the optimum conditions being 200 °C for 10 min at a flow rate of 160 mL/min.

1.3.5  Microwave Irradiation Microwave-assisted extraction has been deemed as an alternative method to traditional extraction methods, because of its unique mechanism, low-cost, high extraction rates, and selective product isolation that proceeds, without changing the molecular structure of the target compounds [61]. Panthapulakkal et al. [62] have found that the result of microwave treatment is significantly better than the traditional methods, and that the recovery of xylan from birch wood is up to 60%, while the xylan is degraded to some degree during microwave treatment. Panthapulakkal et al. [63] hypothesized that the selective heating ability of microwaves leads to the generation of hot spots through interaction with the alkali present in the fibers, and the resulting “explosive effect” relaxes the stubborn fibrous structural network. Under optimized extraction conditions of a 8 wt % NaOH solution with a 1:8 (g:mL) solid-to-liquid ratio for 25 min, low power input microwave heating can be used to extract about 75% of the xylan present in birch wood. However, obtaining good yields without degradation of xylan and contamination is still an issue in microwave-­ assisted extractions.

1.3.6  Ultrasonic Treatment Ultrasonic irradiation during the separation of plant material with a solvent can significantly improve the extraction of polysaccharides, in particular, the extraction of low molecular weight substances [64–66]. With ultrasonic treatment, the swelling and hydration of cell walls in biomass promote diffusion and mass transfer [67–69]. It has been shown that the ultrasonic treatment is applicable to xylan [70]. Under ultrasonic conditions, no substantial changes in the structural and molecular properties of the corn cob and corn shell xylan are detected [71].

1.3.7  Subcritical or Supercritical Fluids Researchers are using subcritical or supercritical fluids, with or without the use of catalysts, to remove hemicellulose from biomass. Microwave hot water extraction of arabinoxylans, galactomannans, arabinogalactans and mannans from hops grains (BSG) and waste coffee grounds (SCG) has been proposed [72–74]. The results

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confirm that hot water extraction is an effective technique for the quantitative recovery of xylan from BSG and SCG. Subcritical CO2 is also popular for the pretreatment for extraction of xylan. Researchers [75] studied CO2-assisted autohydrolysis of wheat straw and found that the initial formation of carbonic acid resulted in high dissolution of xylose and xylooligosaccharides. In addition, CO2-assisted autohydrolysis towards elevated xylooligosaccharides at elevated levels under very severe conditions has been proposed.

1.3.8  Ionic Liquid Extraction Ionic liquids (ILs) are non-volatile and green solvents that dissolve lignocellulosic materials under mild conditions and they also have applications as pretreatment solvents [12, 76, 77]. Due to the dual ionic and organic properties of ILs, the intractability of biomass is effectively reduced, so that they can be better processed in other solvent systems [78, 79]. Imidazolium-based ILs such as 1-ethyl-3-­ methylimidazolium acetate [C2C1Im][OAc] is well known to be effective at reducing biomass recalcitrance by disrupting the lignin-carbohydrate complex and increasing the accessibility of the cellulose to saccharification. New types of ILs, bio-derived cholinium ILs, are regarded as promising solvents to improve the biocompatibility [80–83]. Wang et al. [84] compared imidazolium-based ILs with different anionic constituents ([HSO4]−, [Cl]−, [MeCO2]−) in pretreatment of hardwood. The acidity of [HSO4]− caused extensive hydrolysis of xylan, which facilitated pretreatment of xylan-rich hardwood. Half or more of the xylan in materials can be dissolved in [C4C1im][HSO4], but many of the products were recovered as xylose or xylooligosaccharides. Payal et al. [85] studied the dissolving mechanism of xylan in ILs with molecular simulation. They found that inter- and intramolecular hydrogen bonding are important in the dissolution processes and that complexes of xylan with the ions of the ionic liquid were stable with large negative binding energies ranging between −21 and −55 kcal/mol−1. Fewer number of hydroxyl groups in xylan would not affect the number of cations present in its first solvation shell while the number of anions would be reduced.

1.3.9  Deep Eutectic Solvents Extraction As a potential extraction method with ideal physical and chemical properties, deep eutectic solvents (DESs) were introduced for fractionation of lignocellulosic fractions [86]. DESs have attracted interest in dissolving polysaccharides such as cellulose, xylose, arabinose, starch and chitin, and they are regarded as alternatives to ILs [87]. In comparison with conventional solvents, DESs are easy to synthesize, renewable in nature, cost-effective, biocompatible and biodegradable [88]. Usually, there are two components in a DES solvent system: one component is a hydrogen

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Table 1.1  General formulas for classification of DESs Type Components I Metal salt + organic salt II Metal salt hydrate + organic salt III HBD + organic salt IV Zinc/aluminium chloride + HBD

General formula Cat+ X− zMClx; M = Zn, Sn, Fe, Al, Ga, In Cat+ X− zMClx.yH2O; M = Cr, Co, Cu, Ni, Fe Cat+ X− zRZ; Z = CONH2, COOH, OH MClx + RZ = MClx-1+ RZ + MCl-x+1-;M =Al, Zn & Z = CONH2,OH

Example ZnCl2+ ChCl CoCl2.6H2O + ChCl Urea + ChCl ZnCl2 + urea

Reprinted with permission from Ref. [89]. Copyright © 2018, Elsevier

bond acceptor and the other component is a hydrogen bond donor. Table 1.1 shows a classification of DESs [89]. Morais et al. [90] used DESs (choline chloride and urea or acetic acid) as solvents to extract hardwood xylan. The yield of solubilization of xylan was 328.23 g/L under conditions of using 66.7 wt% DES in water at 80  °C.  They also found that 4-O-methyl groups were eliminated from the 4-O-methylglucuronic acids moieties and uronic acids (15%) were cleaved from the xylan backbone during this process. In the works of Agrawal et al. [91], about 80% hemicelluloses were hydrolyzed into monosaccharides including xylose, arabinose, mannose, and galactose above 120 °C and acidic pH (~1.5 to 2) in less than 30 min. A big disadvantage of DESs is the high viscosity, thus, further research needs to focus in developing DESs with favorable transport properties so that applications on an industrial scale can be realized [92].

1.3.10  Twin-Screw Extruder Harper suggested starch conversion in twin-screw extruders for applications in the agricultural industry [93]. The high shear forces generated by the twin-screws on the material disrupt the cell structure and provide an easier way for the hemicellulose to be removed with an extractant. In the screw extrusion process, xylan will not degrade if a lower barrel temperature is used, but higher temperatures used in hydrothermal pretreatment causes structural changes [94]. Marechal et al. [95] developed a method for directly atomizing extracts by using twin-screw extrusion and ultrafiltration. Twin-screw extrusion is effective in alkali impregnation of wheat bran, although the shortcomings in this method should not be ignored, namely, that the xylan extraction needs large liquid/solid ratios.

1.4  Purification of Xylan Extracted xylan cannot be used directly because the product normally contains contaminants. Thus, fractions need further purification to meet the specifications required for a chemical product. Some hemicelluloses combine with lignin through

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covalent bonds and this kind of structure is called lignin-carbohydrate complexes, which are obstacles to the total separation of the hemicelluloses from other components in the plant cell wall. The next section provides some methods for improving the purity of xylan.

1.4.1  Ethanol Precipitation In continuous and exploratory experiments, it has been found that ethanol is a good anti-solvent for some hemicelluloses, so that it can be applied to the separation and purification of hemicellulose polysaccharides. Similarly, acetone, methanol, and other organic solvents also have similar properties to ethanol for this purpose [96]. Graded ethanol precipitation has been successfully used as an evaluation tool for xylan purification. It has been reported that, the less branched hemicelluloses with large molecules can be precipitated by ethanol with low concentration, while with an increase in ethanol concentration, more branched hemicelluloses with low molecular weights are obtained [3, 97–101]. Compared with other purification methods, fractionated ethanol precipitation has several advantages such as high efficiency, low-cost, simple operation, and ethanol recovery.

1.4.2  Ammonium Sulfate Precipitation Selective precipitation by (NH4)2SO4 is generally used in the fractionation of proteins, although this precipitation method has received little attention in the area of fractionation of polysaccharides from aqueous solution [102]. Peng et  al. [103] reported that alkali-soluble bamboo (Sinocalamus affinis) xylans could be fractionated by gradient ammonium sulphate precipitation. It was found that more linear xylan was obtained at relatively low concentrations of saturated ammonium sulfate. Using precipitation by stepwise addition of 50% and 80% saturated (NH4)2SO4 solutions, DMSO, Ba(OH)2, and NaOH-extracted Arundo donax hemicelluloses could be fractionated into two products [97]. Sugar composition and NMR analysis indicated that the highly acetylated and linear xylans were precipitated at the relatively lower concentrations of 50% saturated (NH4)2SO4, however, the DMSO-­ extracted hemicelluloses obtained by saturated (NH4)2SO4 (80%) mainly consisted of highly branched xylans and β-D-glucan. Therefore, this method can separate acetyl and non-acetyl xylan. Peng et al. also found that stepwise precipitation with (NH4)2SO4 was effective for separating β-glucans from arabinoxylan.

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1.4.3  Iodide Complex Precipitation Iodine complex precipitation is obtained when iodide and KI are added to a solution of hemicelluloses with CaCl2 (KCl may also be used) [99, 104, 105]. This process of fractionation of hemicellulose involves a linear portion of the hemicellulose forming a precipitate upon addition of iodine and the branched portion being insoluble [105]. Peng et  al. [99, 104] reported that the alkali-soluble xylan from the delignified Peashrub (Caragana korshinskii) and Poplar (Populus gansuensis) could be fractionated by the iodine-complex precipitation technique in the presence of aqueous KCl. The hemicellulosic fractions precipitated by iodine-KI solution were linear and they contained more xylose (81.7–87.6%) and lower uronic acids/xylose ratios (0.10–0.14) than the xylan sub-fractions remaining in the solution, which were more branched and contained a higher content of uronic acids (15.7–19.7%). Thus, this is a useful method for obtaining low-branched xylans for further production of xylose, which an intermediate for the production of xylitol, and a variety of xylo-oligosaccharides.

1.4.4  Column Chromatography Although standards have been added for the quality control of xylooligosaccharides and its products, the preparation of pure xylooligosaccharides standards is still not satisfactory. For now, chromatography is one way to separate and purify xylan fractions with high purity. Multiple columns are needed to obtain highly pure hemicellulose [106–108]. Alkali-extracted wheat arabinoxylan from was fractionated using an anion-exchange chromatography in the work of DuPont and Selvendran [109] and Gruppen et  al. [110]. The diethylaminoethyl-bound arabinoxylan was acidic and had high overall Ara/Xyl ratios compared with the corresponding unbound fractions. Diethylaminoethyl-52 chromatography was used by Peng et al. [111] to fractionate alkali-soluble xylan from sugarcane bagasse, with eluting of water, 0.1 M and 0.3 M NaCl solution, respectively. As the NaCl concentration increases, xylan with a higher Ara/Xyl ratio and a higher molecular weight is eluted. Column chromatography provides reliable xylan purification. In the research of Yang et al. [112], the crude xylooligosaccharides from acid hydrolysis was isolated and separated by fast protein liquid chromatography-refractive index detection on the basis of anion exchange chromatography and size exclusion chromatography, respectively. Five final fractions had a purity of 95% and the degree of the polymerization of the xylooligosaccharides was from 2 to 6.

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1.4.5  Membrane Separation Membrane separation technology is a new method to separate and purify hemicellulose. The working principle is to achieve molecular weight exclusion by controlling the size of the internal pore structure of the membrane. Only molecules that are equal to or less than a certain molecular weight can pass through the membrane [113–115]. The factors affecting the filtration of xylan are the solubility, degree of substitution, linkage, degree of polymerization degree and branching [116]. Viscosity of the solution is another property that affects fractionation efficiency [114]. Typical membrane filtration techniques such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis, have been implemented in a continuous process to provide a good fractionation range of xylan and other wood constituents. Ko et al. [117] used UF with molecular weight cut-offs (MWCOs) of 5000 and 10,000 made of polyethersulfone polyethersulfone and UF with MWCO 30,000 made of polysulphone to separate xylan for production of xylooligosaccharides. Concentrations of total xylose were raised from 11 mg/mL to about 52 mg/ mL. Baktash et al. [118] separated xylan from pre-hydrolysis liquor before production of furfural by the means of membrane filtration (MWCO, 150–300). High temperatures (>170  °C) and low pH were required for furfural production, thus, the operation with membrane effectively decreased the water quality and increased the concentration of xylan. After membrane treatment, the sugar content reached 104 g/L from 37 g/L. Furfural and acetic acid concentrations in the retentate were 0.014 wt% and 0.56 wt%, respectively.

1.4.6  Supercritical Anti-solvent Precipitation Carbon dioxide is in its supercritical condition has special physicochemical properties, and it is non-toxic, stable, low cost, and easy to remove [119]. Due to the low polarizability and low cohesion energy density per unit volume of supercritical carbon dioxide (scCO2), scientists have applied the solvent to the separation and purification of biodegradable polymers such as hemicellulose [85, 120–122]. Haimer et al. [85] applied scCO2 to obtain purified xylan and mannan precipitates, which had a wide particle size distribution ranging from less than 0.1 μm to greater than 5  μm. The xylan and mannan with nano- and micro-structures obtained by the method can be used for preparing active alcohols, and the technology can be further developed.

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1.5  Chemical Modification of Xylan Although natural xylan polymers have many excellent properties, they still have many shortcomings in product applications. Therefore, through chemical modification, its structure is modified to give more functional properties of xylan molecules to meet product specifications.

1.5.1  Esterification Researchers have studied the esterification of xylan. Since xylan has a large number of hydroxyl groups in its sugar unit structure, esterification reactions are attractive since xylan has specific active reaction sites (Fig. 1.6) [123]. Acid halides, anhydrides and carboxylic acids have been successfully esterified with xylan. The common esterification systems for xylan are as follows: N, N′-dimethylformamide (DMF), N, N′-dimethylacetamide (DMAc)/lithium chloride (LiCl), DMSO/tetrahydrofuran (THF), or various ionic liquids by acyl chlorides. By using trimethylamine (TEA)/dimethylaminopyridine (DMAP) as a base and a catalyst under mild reaction conditions, conversion with fatty acid chloride, hydrophobic xylan can be uniformly obtained in DMF/LiCl [124]. After reaction of xylan with acetic anhydride in a formamide/pyridine solvent system for 4 h, complete acetylation of Aspen MeGlp-­ xylan (DS = 1.9) was achieved. MGX acetate exhibits good solubility in DMSO and chloroform, as well as thermoplastic properties, which allows film production using autoclave processing techniques [21]. Succinylation is a commonly used esterification method. Succinic anhydride esterifies xylan and introduces carboxyl functional groups on its structure [125, 126]. The hydrophilicity of xylan is increased due to

Fig. 1.6  Reaction of the esterification of the xylan. (Reprinted with permission from Ref. [123]. Copyright © 2018, Springer Nature)

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the introduction of carboxyl groups, and the derivatives have pH response characteristics under specific conditions.

1.5.2  Etherification Etherification is another commonly used hemicellulose chemical modification. The ether bond formed after the reaction is more stable than the chemical bond and is not easily hydrolyzed. Nonetheless, a major disadvantage of the etherification reaction is that the formation of the ether bond requires a higher pH to deprotonate the hydroxyl group and react with an etherifying agent (e.g. an epoxide or halide compound), which may result in almost half of the cellulose being degraded. To reduce the degree of degradation, a homogeneous reaction can be performed in a mixed system of water and an organic solvent. In an alkali solution system, beech wood MGX undergoes etherification reaction with p-carboxybenzyl bromide. The resulting etherified xylan derivative has good water solubility, good emulsifying and protein foam-stabilizing activity, and the degree of substitution can reach 0.25 without severe depolymerization. Sedimentation rate tests show that the molecular weight of ether xylan-derivatives is about 27,000  g/mol [127, 128]. Cationic groups are usually introduced into cellulose or chitosan to fabricate various reagents (Fig. 1.7) [14]. Some xylan-rich waste materials have been modified with 3-chloro-2-­ hydroxypropyltrimethylammonium chloride (CHTMAC) to be cationic xylan derivatives. Result showed that, the monosubstitution tends to occur at C-2 with a

Fig. 1.7  Reaction pathways for the introduction of cationic moieties into xylan. (Reprinted with permission from Ref. [14]. Copyright © 2005, Elsevier)

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Fig. 1.8  Oxidation of xylan by sodium periodate

low degree of substitution [129]. Sulfoethyl beech GX derivatives can be obtained by reacting GX with 1-bromododecane in dimethyl sulphoxide. This modified derivative has amphiphilicity and achieves the purpose of reducing GX surface tension [130, 131]. Introduction of other groups such as amines, azides, thiols, and double bonds to xylan can also be achieved by etherification.

1.5.3  Oxidization Dialdehyde functional groups can be introduced into xylan by oxidation with periodic acid or sodium periodate (Fig. 1.8). Water-soluble dialdehyde xylan was synthesized by Hassan et al. [132]. In their experiments, periodate-oxidized was used as oxidizer, and the final oxidation degree of xylan was 91.5%. After oxidation, the xylan derivative has lower thermal stability and a greater amount of residual char compare to the original xylan. Chemin et  al. [133] designed a functionalized 4-O-methylglucuronoxylans (MGX) derivative by oxidation. MGX from beech wood was oxidized by sodium periodate. The NaIO4/xylose ratio was 0.2 and the obtained xylan derivative had a degree of oxidation of 0.15 with 75% yield. They also further studied the oxidation of MGX by sodium periodate. [134]. The NaIO4/xylose molar ratios were set as 0.05, 0.20 and 1.00. In those experiments, no depolymerization was observed at a NaIO4/xylose ratio of 0.05, while it was found when the ratio was 0.20, and was even complete at 1.00. Additionally, increasing the temperature (up to 80 °C) led to an increase of the oxidation rate with no effect on the depolymerization. Oxidization of xylan is a popular modification method, because it provides many possibilities to change the property of xylan and to make new applications.

1.5.4  Other Chemical Modifications Many chemical modifications have been exploited for xylan-type polysaccharides. Apart from those introduced in the previous section, there are some other interesting modifications. The product from condensation of the xylan with the organostannyl halide has been characterized by Naoshima et al. [135] Elemental analysis shows that the presence of tin was consistent with the presence of the tin portion. The result of the control reaction was consistent with the product containing the portion from both reactants. The presence of the Sn-O-R ether linkage was found by infrared

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spectroscopy. Homogeneous ring-open graft polymerization of ε-caprolactone (ε-CL) onto xylan in ionic liquid (IL) 1-allyl-3-methylimidazolium chloride ([Amim]Cl) was reported by Zhang et al. as shown in Fig. 1.9 [136]. Around 38.8% and 61.2% of PCL were attached to C2 and C3 positions of xylan, respectively. After oxidization, the thermal stability of xylan was increased and the morphologies changed significantly with increased degree of substitution. Fully sulfated xylan films were fabricated by Simkovic et al. [137]. The quaternized xylan was prepared firstly, then, the sulfation of quaternized xylan was conducted in the second step with a degree of substitution of 1.67. The degree of substitution decreased when sulfated xylan was used in the first step. After the modifications, the xylan derivatives could form mechanically more stable films than xylan specimens. Now, more attention is being focused on modification of xylan, and the methods highlighted in this section broaden the applications of xylan and its derivatives.

Fig. 1.9  Grafting copolymerization of PCL onto xylan backbone in [Amim]Cl with DMAP as catalyst (a) and the possible mechanism (b). (Reprinted with permission from Ref. [135]. Copyright © 2015, Elsevier)

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1.6  P  otential Applications of Xylan-Based Materials and Chemicals 1.6.1  Polymeric Films In a contemporary society in which energy conservation and environmental protection concepts are being discussed, the consumer is aware of products that use renewable resources to produce degradable food packaging [137–140]. Xylan, itself, has a certain degree of hydrophilicity. Membrane materials prepared from xylan have good properties such as oil resistance, low oxygen permeability, and high light transmittance. Therefore, they are suitable for application as biodegradable package materials [141–143]. However, they also have some inherent disadvantages. The film materials prepared from high-purity raw materials are high in brittleness, are hygroscopic to some degree, and have high production costs. To improve the performance of xylan-based membrane materials, many proposals are being studied [144]. In the research of the authors, hemicelluloses composite films have been prepared by combination of hemicelluloses and montmorillonite (MMT) [145–148]. By adding different proportions of polyvinyl alcohols (PVA), chitin nanowhiskers (NCH), and cationic biopolymer chitosans (CS), the performance of the composite membrane can be improved, providing a compact and powerful nano-composite membrane. The tensile strength and oxygen transmission rate of quaternized hemicelluloses-­MMT-PVA films are better than those of quaternized hemicellulose/ MTT films. The tensile strengths of films quaternized hemicelluloses-MMT-CS were 57.8 MPa. Due to the small amount of CS, the thermal stability of the nanocomposite film, oxygen permeability and water vapor permeability are improved [145]. In addition, biobased nanocomposite films have been developed using a wood hydrolysate (WH) and the effects of MMT and carboxymethyl cellulose (CMC) on the physical properties of the WH-MMT films have been determined (Fig. 1.10) [147]. The WH contained lots of hemicelluloses and small amounts of lignin. The results showed that the mechanical properties of the film designated FCMC0.05 (91.5  MPa) were dramatically enhanced. Optimized films (WH-MMT-­ CMC) exhibit an oxygen permeability below 2.0  cm3 μm/(d·m2·kPa). Further, a novel ternary bio-inductive hydrophobic nanocomposite membrane with high strength and flame retardant properties can be obtained from the combination of wood hydrolysate with montmorillonite (MMT) and a small amount of graphene oxide (GO). The preparation of the composite material creates an important foundation for further research and development of high-performance biomass-based nanocomposites. Xylan is also a good candidate in fabrication of films for food packaging. Plasticized arabinoxylan films were made by Zhang et al. [149] that had thicknesses of 22–32 μm. A homogeneous morphology could be seen in the scanning electron micrographs. When grapes were packaged in the plasticized films, weight loss rates of the fruit decreased 41%, after 7 days. Xylan, from cotton stalks containing residual lignin, was used by Goksu et al. [150] to form films intended for food packaging

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Fig. 1.10  Fabrication of functional wood hydrolysate- montmorillonite (WH-MMT) films. (Reprinted with permission from Ref. [147]. Copyright © 2018, Elsevier)

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applications. The films were fabricated using 8–14% (w/w) xylan without complete removal of lignin during xylan isolation. The water vapor transfer rates (WVTR) decreased with increasing xylan concentration, and the mechanical properties of the films obtained by using 8% xylan were lower in comparison with the ones containing 14% xylan. In general, films having properties similar to those of many other biopolymer-based films were obtained from xylan with the introduction of lignin. Under various conditions, with these particular modifications, the properties of xylan are improved, thus, the modified xylan-based films are promising materials in the field of food packaging, wound dressing, and other applications.

1.6.2  Hydrogels Hydrogels are water-dispersible polymers. A water-soluble polymer having a net-­ like crosslinked structure incorporates part of a hydrophobic group and a hydrophilic residue, then a hydrophilic residue binds to a water molecule, and a water molecule connects to a net-like interior, and the hydrophobic residue swells in water. The hydrogel polymer soft network system is able to maintain a certain shape and absorb large amounts of water ranging from 10% to thousands of times their dry mass [151, 152]. Xylan-based hydrogels have been developed and applied to a wide range of products. Acetylated xylan hydrogels doped with Fe3O4 for the detection of hydrogen peroxide were made by Dai et al. [153]. The sensitivity of limited concentration towards hydrogen peroxide was 5 μM. Wallenius et al. [154] studied propionic acid produced from lipase immobilized on a xylan hydrogel. Compared with results reported elsewhere, the productivity of propionic acid in their works was up to 1.3 g L−1 h−1. Wheat straw xylan hydrogels have been prepared and theophylline was loaded as model drug by Sun et al. [155, 156]. In vitro tests showed that the loaded hydrogels had pH sensitivity and were biodegradable, thus, this is a potential material for oral drug delivery.

1.6.3  Nanoparticles Nanoparticles (NPs) are being used widely in our daily life and they have a huge potential for use in medicines and drug delivery. Due to some advantages over synthetic polymers, lignocellulosic polysaccharides are ideal raw materials for NP fabrication, because they are biocompatibile, abundant, cost-efficient, easy-to-use, and they can be modified due to several nucleophilic groups in their structures [157]. Xylan as a typical hemicellulosic polysaccharides, that has great potential to be modified and applied to NP fabrication. Xylan phenyl carbonate with a maximum DS value of 2 has been prepared and characterized [158]. Ionic liquids seem to be good reaction medium, and provide good reaction efficiency, however, the conversions are influenced by the type of xylan. The pH-responsive xylan-curcumin

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nanoparticles were studied as a therapeutic prodrug in cancer therapy [159]. The functional NPs automatically released drug loaded into its surrounding at low pH, and they exhibited greater cytotoxic effect than curcumin. NPs with high drug loadings were synthesized by modification of xylan with ibuprofen and carboxylic acid with N, N- carbonyldiimidazole [160]. Subsequent sulfation of xylan ibuprofen esters was conducted to improve the hydrophobicity of the xylan derivatives. The resulting products self-assemble, and spherical nanoparticles are formed with mean diameters in the range of 162–472 nm. Preliminary stability measurements indicate that hydrolytic stability decreases with an increase in the degree of substitution of sulfate groups. Thus, a new concept towards improved drug delivery from polysaccharide-­based nanoparticles has been established.

1.6.4  Bioconversion and Chemicals Enzymatic degradation of xylan is an environmentally-friendly, mild and specific transformation process with low by-product formation, and it seems to be an alternative to the traditional chemical process. There are many kinds of enzymes that can degrade xylan, and the largest group of enzymes responsible for the hydrolysis of xylan is xylanases (Table  1.2) [3]. Endo-1, 4-β-xylanase (EC 3.2.1.8) and β-xylosidase (EC 3.2.1.37) are two important enzymes in xylan degradation. Endo-­ 1, 4-β-xylanase cleaves the linkages of the main chains randomly and produces mixtures of xylooligosaccharides. The β-xylosidase cuts the terminal monosaccharides from the non-reducing end of the short oligosaccharides. Additionally, other accessory enzymes such as α-L-arabinofuranosidase, α-D-glucuronidase, and acetylxylan esterase are also helpful in debranching xylans [161, 162]. After fermentation, lignocellulosic biomass transforms into ethanol with the help of microorganisms [176]. In the conversion, temperature, pH, osmotic pressure, inhibiters, and purity have a large influence on the performance of those microorganisms [177]. Figure  1.11 shows the use of bacteria or fungi (yeast) to produce ethanol from xylose. Lactic acid can also be obtained from fermented xylan. Hydrogenation of xylose is a good way to prepare xylitol [178]. Xylitol is quite an important in food and medical fields. As an alternative sweetener, it is highly recommended for diabetics, and it also plays an important role in protection of dental pastes [179]. Xylitol can be absorbed slowly and involved in metabolic activation independently of insulin without rapid increase of blood glucose level [180]. Now, xylitol has been studied and applied in daily life widely based on its intrinsic properties [181–183]. Yeast and bacteria are used to produce other chemicals from xylose. Bacteria can convert xylose to xylitol, but yeast converts xylose to xylitol more efficiently. Detoxified hydrolase solution is required to reduce inhibitors and improve the fermentation yield of xylose [184, 185]. Therefore, methods including sulphite treatment, vacuum evaporation and ion exchange resins have been chosen to detoxify the hydrolase solutions [178, 184, 185]. Furfural is derived only from lignocellulosic biomass through dehydration of pentose (or xylose). To increase the effective

Enzyme Endo-­1,4-βxylanase (3.2.1.8)

Linkage hydrolyzed Internal β-1, 4

7.0

5.0

7.0

6.5

190

21.2

31

Paenibacillus barcinonensis

Streptomyces mayensis

30

6.0

170– 700 43

Ammonium sulfate precipitation, ion exchange and gel filtration chromatography Precipitation with 20% ammonium sulfate and cation exchange chromatography

Precipitation with 60% saturation of ammonium sulfate and His-Tag Ni-affinity column Sephacryl S-100 HR chromatography

55–65 50 mM sodium citrate

7.0

57

Ultrafiltration and AKTA Fast Protein LC system Filtration and overnight dialysis

6.0

60

65

50

90

37

50 mM phosphate

20 mM citrate (pH 3.0–6.0), sodium phosphate (pH 6.0–8.0), Tris-HCl (pH 8.0–9.0), sodium carbonate (pH 8.0–11.0) 50 mM MOPS

50 mM citrate

50 mM Tris-HCl

50 mM phosphate

50 mM Acetate

40

55

Metal affinity resin and batch/ gravity-­flow column

7.0

22.1

Gel filtration and IEC

Buffer solution Citrate

Acrophialophora nainiana Bacteroides xylanisolvens XB1A Sulfolobus solfataricus Streptomyces sp. strain AMT-3 Glaciecola mesophila KMM 241 Bacillus sp. YJ6

Conditions Temp pH (°C) 6.0 60

Purification Ultrafiltration, anion exchange and chromatography

Mw (kDa) 33

Origin Aspergillus niger B03

Table 1.2  Endo-1, 4-β-xylanase (EC 3.2.1.8) and β-xylosidase (EC 3.2.1.37)

[171]

[170]

[169]

[168]

[167]

[166]

[165]

[164]

Refs. [163]

24 G.-Q. Fu et al.

β-xylosidase (3.2.1.37)

Terminal β-1,4

DEAE-­Sephadex and HPLC GF-510 gel filtration

Humicola grisea var. Ultrafiltration, ion-exchange, thermoidea DEAE-Sepharose and PhenylSepharose resins Penicillium Ultrafiltration and DEAEcapsulatum cellulose anion-exchange column Cell wall of maize Precipitated with ammonium sulfate and CM-sephadex Cation-­exchange chromatography

Aspergillus versicolor

4.5 4.2

68.5 60/66

60

37

48

50 mM sodium acetate

50 mM sodium acetate

100 mM citrate-­phosphate Ryan et al

3.8

22

29

McIlvaine (pH 4.0–8.0), Tris-HCl (pH 8.0–9.0), glycine-NaOH (9.0–9.5) 4.5– 55–60 100 mM sodium acetate 6.5

6.0– 55 7.0

32

[175]

[174]

[173]

[172]

1  Isolation, Purification, and Potential Applications of Xylan 25

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G.-Q. Fu et al. XI

D-xylose (Bacteria)

D-xylulose D-xylose (Fungi)

XR

xylitol

XK

D-xylulose-5-P

XDH

Pentose Phosphate Pathway

Fig. 1.11  Use of bacteria or fungi (yeast) to produce ethanol from xylose

recovery of furfural, the solvent in liquid-liquid extraction is a widely promoted method. The mixture of solvent and furfural can be separated by steam distillation [186]. Supercritical CO2 and polymer sorbent have also been considered to recover the furfural from aqueous mixture [185, 187]. However, the mechanism by which furfural is formed from xylose, or any other pentose sugar, is still unknown [188].

1.7  Conclusions and Future Outlook Hemicelluloses are types of plant cell wall hetero-polysaccharides, and the world’s second most abundant renewable polymers after cellulose in lignocellulosic biomass. To efficiently utilize the biomass component and create new value and offset biorefinery processing cost, it is necessary and important to use the hemicelluloses. As an important part of hemicelluloses, xylan has been studied for a long time due to its favorable characteristics. There is a long history of research on the structural features of various xylan-type polysaccharides. Methods of isolation, fractionation, and purification have been used for obtaining ideal xylan fractions. Alkali extraction is widely used to obtain xylan fractions, and the auto-hydrolysis method is conducted mainly in pulping factories. Innovative isolation methods such as supercritical fluids, ionic liquids, and deep eutectic solvent extraction have been studied extensively. Ethanol precipitation is the most popular way of xylan fractionation in the laboratory. Membrane filtration has been used in practice for obtaining xylan with high purity in the viscose fiber factories. Considerable research on the chemical modification of xylan has been made, and the obtained xylan derivatives usually have improved characteristics, such as anti-bacterial, higher thermal stability and higher hydrophobicity than unmodified xylans. The usefulness of xylan-based products, for example, xylooligosaccharide, xylose, arabinose, xylitol, and furfural, on an industrial and biomedical scale is now beyond dispute. Xylan derivatives-based functional materials with favorable properties in the forms of films, hydrogels and nanoparticles are a reality and can be used in many practical applications. The physico-chemical properties of xylan polysaccharides are strongly related to the structural and molecular properties, which depend on the botanic origins, and the methods of isolation, fractionation and purification. Therefore, structure-­ property relationships are needed to understand the design of xylan-based products with stable properties. For example, the solubility of xylan is affected by the specific patterns of intra- and inter-molecular hydrogen bonds which are natural or formed during the drying process. Thus, the dissolution of low-branched hetero-xylans is a

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big challenge. Although some dissolution methods have been proposed, homogeneous xylan solutions prepared in short periods of time at mild conditions remain to be found and require further research. Xylan-based films are promising oxygen, grease, and aroma barrier materials. Indeed, due to their short molecular chain and high hydrophilicity, the major drawback of xylan based films is the sensitivity of moisture and unsatisfactory mechanical properties. Chemical modification or lamination technology is an effective method to improve the film performance of xylan-­ based films for maintaining their sustainability and safety for food-packaging use. For future studies, further attention should be paid to the “Green synthesis” of xylan-based materials. It is necessary to make isolation, fractionation, purification, and modification of xylan green and environmentally-friendly. Nontoxic solvents, low energy, nontoxic additives or chemicals are strongly recommended. The hope of the authors is that the account given in this chapter will serve as a “road map” for further research in the field of applications of xylan-type polysaccharides. Urgent research effort is needed to understand and develop new applications with promising and renewable xylan-type polysaccharides. Acknowledgments  This work was supported by the Fundamental Research Funds for Central Universities (JC2015-03), Beijing Municipal Natural Science Foundation (6182031), Author of National Excellent Doctoral Dissertations of China (201458), and the National Program for Support of Top-notch Young Professionals.

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Part II

Composite Polymers Derived from Lignin and Cellulose

Chapter 2

Development of Lignin-Based Antioxidants for Polymers Afsana S. Kabir, Zhong-Shun Yuan, Takashi Kuboki, and Chunbao (Charles) Xu

Abstract  The growing interest in lignin as a potential source for biofuels and biochemicals is driven by multiple factors: (1) relative abundance, (2) absence of competition between food and fuel, and (3) recent legislation and mandates promoting a green economy. This book chapter presents a detailed literature review on how lignin fits into the growing market for antioxidants especially fpr polyolefins, and discusses previous studies on lignin as a bio-based chemical. There is a scarcity of the literature addressing the effects of adding technical lignin and its de-polymerized products in polyolefins and their antioxidant properties. In this context, the authors explored lignin de-polymerization as a promising approach to improve the reactivity of the lignin-based antioxidants for polymers (polyethylene, PE and polypropylene, PP). A proprietary hydrolytic de-polymerization process was utilized to increase the antioxidant activity of two types of technical lignin: Kraft lignin, KL (a by-product from the pulp and paper industry) and hydrolysis lignin, HL (a by-product from the pre-treatment processes in cellulosic ethanol plants). This book chapter discusses some of the results showing how de-polymerization can improve the antioxidant activity of commercial lignins, and how the mechanical properties are affected after incorporating lignin into polymer matrixes.

A. S. Kabir · Z.-S. Yuan · C. Xu (*) Institute for Chemicals and Fuels from Alternative Resources, Western University, London, ON, Canada e-mail: [email protected] T. Kuboki Department of Mechanical and Materials Engineering, Western University, London, ON, Canada © Springer Nature Singapore Pte Ltd. 2019 Z. Fang et al. (eds.), Production of Materials from Sustainable Biomass Resources, Biofuels and Biorefineries 9, https://doi.org/10.1007/978-981-13-3768-0_2

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2.1  Introduction Lignin, which is one of the primary constituents of plant cell walls, constitutes about 10–40  wt% of plant biomass [1, 2]. Pulp and paper mills are the largest commercial sources of lignin in the form of Kraft lignin as “black liquor”. As of 2013, the global production of Kraft lignin was approximately 50 million tonnes per annum, most of which is used as a low-value fuel, with less than 5% of it being used as chemicals or other products [3]. According to an estimate by Ragauskas et al. [4], second generation of biorefineries will produce an additional 62 million tonnes of lignin annually, which is a 60% surplus after meeting the refineries’ internal energy requirement. The depletion of fossil fuels and increasing concern about climate changes have motivated governments to initiate mandates towards having a carbon neutral green economy. The U.S.  Department of Agriculture and U.S.  Department of Energy have aimed to derive 20% of transportation fuels and 25% of U.S. chemical commodities from biomass by 2030 [5]. In Europe, the Dutch Ministry of Economic Affairs have set ambitious goals to substitute 30% of transportation fuels by biofuels and 20–45% of fossil-based raw materials by biomass-based chemicals by 2040 [5]. As a result, there is an up-and-coming need to develop value-added bio-products from lignin. The objectives of this chapter are to present a detailed overview of how lignin fits into the green economy as a bio-based chemical, especially as an antioxidant for polymers. Along with discussion of previous studies on the effect of lignin as a stabilizer for polymers, this chapter also presents some of the authors approaches on improving antioxidant activity of technical lignins. The antioxidant market is expected to grow with the expanding of the plastic industry owing to the growing demand to replace metal parts in the automotive and aerospace industries with lightweight engineering plastics [6], and the growing demand for plastic in packaging, durable goods, automotive and other industrial applications [7]. Polyolefins (e.g., PE and PP) represent around 60% of the thermoplastics market, and the global demand for PE and PP reached 150 million tonnes in 2015. Asia is the largest consumer of polyolefins, seconded by North America and Europe (with around 34% of the market share) [7]. As a result, there would be a steep demand for antioxidants to ensure processing stability and protect the finished products. The segment revenue from antioxidants in European and North American markets was $1.02B in 2014, which is projected to be $1.62B in the next few years [6]. As per a study by Oak Ridge National Laboratory, the available biomass resources in the forms of forest and agricultural residues will amount to 1.5 billion tonnes in 2030 in the United States alone [8], and the amount of lignin produced far exceeds the amount needed to fulfill the internal energy necessity of the pulp and paper mills and the biorefineries. Considering the growing market for polyolefins and thus antioxidants, and the surplus amount of lignin being produced, lignin has a great potential of occupying a significant portion of the antioxidants market.

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2.1.1  Polymers and Stabilization of Polymers Polymers are a broad class of materials that are composed of repeating units of smaller molecules called monomers. Polymers are useful in many applications because of their strength and durability. Polymers can be natural in origin, such as cellulose or lignin, while others derived from petroleum-based monomers (e.g. polyethylene and polypropylene) [9, 10]. One of the challenges of working with polymers is their degradability when used in high-temperature conditions or in outdoor applications, which can result in the breaking of polymer chains, the production of free radicals and the subsequent reduction in molecular weight, thereby deteriorating their chemical and mechanical properties. Therefore, almost all synthetic polymers require stabilization against adverse environmental effects. It is necessary to find methods to reduce or prevent damage induced by environmental components such as heat, light, and oxygen. The stabilization of polymers may be achieved in many ways, including addition of some additives such as hindered phenolic antioxidants, phosphite antioxidants, thiosynergists, hindered amine stabilizers, and UV light absorbers. The additive used depends on the extent of the stabilization needed, and the adverse environmental conditions for the end use product of the polymer [9, 11]. Polymer degradation is the change in molecular weight/chain length of a polymeric material leading to loss of desired properties in the end-use product. The weak sites inherent in the polymer are affected by various thermal and chemical factors, such as, heat, light (UV), and mechanical stress, all of which initiate the degradation of a polymer, leading to eventual major mass loss through chain scission, crosslinking, and branching. Thermal degradation is usually accelerated by the presence of oxygen. For example, in polyethylene, thermo-oxidative degradation starts at 423 K in air, whereas in the inert atmosphere it is delayed until the temperature reaches 565  K [12]. Antioxidants have been exploited to retard thermo-oxidative degradation of many polymers. The mechanism of thermo-oxidative degradation in polymers involves a series of chain reactions. The first step in the degradation process is usually the loss of a hydrogen atom from the polymer chain. The degradation process can be initiated by multi-initiators: high temperature (energy) input, O2, O3 and transition metal ions. For example, a high energy input can abstract an H from the polymer chain (RH) and create alkyl radicals (R•) and hydrogen radicals (H•). O2 can react with the molecule (RH) to create two free radicals (R• and •OOH). In propagation, the free radicals then can react with O2 and extract a hydrogen from another polymer molecule (RH), giving rise to peroxy radicals (ROO•), or more free alkyl radicals (R•). The peroxy radicals can extract another hydrogen atom from the polymer chain to form hydroperoxide species (ROOH) that could undergo homolytic cleavage to create secondary initiators RO• and OH•, which continue to propagate the reaction

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to other chains. Some of the free radicals created can self-terminate themselves, so that the overall process can be described as below [13]: Initiation:

RH - energy ® R • + H •

(2.1)



RH + O2 ® R • + • OOH

(2.2)



R • + O2 ® ROO•

(2.3)



ROO• . + RH ® ROOH + R •

(2.4)



RH + H • ® H 2 + R •

(2.5)

ROOH ® RO• + OH •

(2.6)



RO• + H • ® ROH

(2.7)



ROO• + H • ® ROOH

(2.8)



RO• + R • ® ROR

(2.9)

Propagation:

Chain transfer or branching: Self-termination:

The amount of propagation eventually exhausts the amount of self-termination, leading to degradation of the polymer through chain scission or crosslinking [13]. For certain polymers, such as polypropylene (PP), the degradation happens through beta scission of the polymer chain containing a free radical, whereas, for polyethylene (PE), the free radical often causes one chain to graft onto another chain, leading to crosslinking and thus furthering the degradation phenomenon [13, 14]. Peterson, et al. [15] proposed the following bimolecular decomposition mechanism for hydroperoxide decomposition:

ROOH + RH ® RO. + R. + HOH,

(2.10)

and suggested that it is the rate limiting step of thermo-oxidative degradation in a polymer. In thermal degradation under inert conditions, however, random scission is the rate-limiting step, and it has a higher activation energy than thermo-oxidative degradation. The mechanism of UV degradation is similar to thermo-oxidative degradation. UV radiation causes photooxidative degradation, which results in breaking of the polymer chains, produces free radicals and reduces molecular weight, causing

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deterioration of mechanical properties [9, 11, 16]. Similar to the thermo-oxidative degradation of polymers, photodegradation also goes through the steps of initiation, propagation, chain branching and termination [9, 16]. Even though UV radiation represent only 4% of total radiation reaching the earth, the energy of the radiation in the UV spectrum if absorbed by a chromophore is enough to break down the C-H and C-C bonds in polyolefins. Although polyolefins do not have functional groups that can absorb the UV rays, during the processing stage, the polyolefins become oxidized to create various compounds containing C=O groups, which can absorb the UV rays and create an unstable, excited state. The excited molecules can transfer their energy through a variety of mechanisms, including decomposition of hydroperoxides, formed in the early stages of oxidation, that are unstable and provide a source of free radicals for further initiation; chain scission; and crosslinking. Decomposition of hydroperoxides produces carbonyl-­containing structures such as aldehydes, ketones, acids, and similar compounds during thermos-oxidative degradation of polyolefins. With increasing degradation, the carbonyl content increases and causes a decrease in tensile strength and elongation. There are various factors that affect this photodegradation, including UV energy, temperature, the orientation and the thickness of the sample [16]. Degradation reduces a polymer’s lifetime and deteriorates the characteristics necessary for its end-use purposes, such as tensile strength, aesthetic appeal, electrical conductivity and melt flow instability, and thus necessitates stabilization additives. There are many kinds of additives used in the polymer blend depending on the stage and extent of stabilization needed. Antioxidants are additives used in polymer processing to prevent oxidative degradation during the lifetime of the polymer produced [11, 16, 17]. Primary antioxidants, such as hindered phenol compounds, are free radical scavengers designed to react with the initial free radicals that are formed by donating a hydrogen atom [18, 19]. Hindered phenols have a reactive hydroxyl group in the benzene ring that is sterically shielded by hydrocarbon units connected to each neighboring atom. The electron donating large groups weaken the OH bond by pushing it to donate the hydrogen atom. This hydrogen atom can then react with the free radicals in the polymer and scavenge the free radicals and stabilize the polymer. The antioxidant when sterically hindered turns itself into a stable, inactive phenoxy radical that will prevent the initiation of new radicals in the polymer. Some of the popular commercially-used hindered phenolic antioxidants are BHT (2,6-di-tert-­ butyl-4-methylphenol), Irganox 1076, and Irganox 1010. BHT is the first generation antioxidant, and the latter ones are advanced versions of BHT that improve the secondary structure of the BHT molecule. Incorporation of the secondary structure into BHT’s 2,6-di-tert-butyl phenol moiety helps to increase the molecular weight of the antioxidant. Low molecular weight antioxidants pose the problem being volatille, whereas whereas high molecular weight antioxidants pose the problem of diffusing into the polymer matrix. New generations of antioxidants aim at optimizing the molecular weight without sacrificing phenol concentration [20, 21]. The secondary antioxidants, such as phosphite compounds, interrupt the degradation cycle by taking

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the oxygen from the hydroperoxides and transforming them into more stable alcohol (ROH) forms, and make the processes cost-effective by lowering the amount of primary antioxidants needed [17]. Photo stabilization of polymers may be achieved in many ways. The following stabilizing systems have been developed based on the action of a stabilizer: (1) light screeners, (2) UV absorbers, (3) excited-state quenchers, (4) peroxide decomposers, and (5) free radical scavengers. Usually, a combination of antioxidant and UV absorbers are used to extend the lifetime of the polymer [9]. Many natural compounds with polyphenolic structure (such as flavonoids, vitamin E) could also have antioxidant properties and have been studied as stabilizers, especially in the food industry [22, 23]. In this context, the hindered phenolic structure inherent in lignin has been studied in the literature as a free radical scavenging antioxidant for polymers [24–29].

2.1.2  Lignin from Natural Sources Lignocellulosic biomass is composed of cellulose, hemicellulose, lignin and some minor components [2]. Cellulose and hemicellulose are polymers of sugar and can be converted to produce biofuels (e.g., bio-ethanol, bio-butanol, etc.) and platform chemicals such 5-hydromethyl furfural (5-HMF) [30]. Lignin is the second most abundant natural polymer, just after cellulose. Depending on the type of the wood, the lignin content varies from 10 to 40 wt% (dry basis) [1, 2]. It is an amorphous polymer of phenylpropanoid monomer structures, and acts as a thermoplastic material [2]. It affects the transportation of water and different nutrients in a plant and creates a binder between the plant cells to provide resistance to sunlight, frost, fungi and other such biological attacks [2, 3]. The structure of lignin is extremely complex. It is a three-dimensional polymer with three major phenyl propane derived alcohols: p-coumaryl alcohol (4-hydroxyl phenyl, H), coniferyl alcohol (guaiacyl, G) and sinapyl alcohol (syringyl, S) as the primary building blocks. The structure includes a variety of functional groups, namely hydroxyl, methoxyl, carbonyl and carboxyl moieties [2, 3]. Hydroxyl groups and the aromatic/phenolic rings are the characteristic functional groups in lignin, and determine its reactivity and constitute the reactive sites to be exploited in macromolecular chemistry [31]. Lignin accouts for 20–30 wt% of mass in a woody biomass. Woody plants can be divided into two categories: hardwood (angiosperm) and softwood (gymnosperm) [2]. It has been identified that more than 90% of the lignin contained in softwood is made up of coniferyl alcohol (G), with the remaining being mainly p-coumaryl alcohol units (H). In contrast, hardwood lignin is formed of varying ratios of G and sinapyl (S) alcohol types of units and grass lignin is made up of mostly p-coumaryl alcohol units (H) [2, 3].

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2.1.3  Lignin from Industrial Sources The main source of technical lignin is the pulp and paper industry with a lignin production capacity of about 50 million tonnes (mainly Kraft lignin) per year [6]. There are two main pulping processes: mechanical and chemical pulping. The mechanical process, which is mainly for the production of newsprint and paperboards, keeps both cellulose and lignin intact in the fibers, resulting in paper that is weak in strength. However, mechanical pulping results in larger pulp yields than chemical pulping. In chemical pulping, wood chips are treated with chemicals to remove lignin and hemicellulose, thus yielding purer and cleaner fibers. Delignification gives the pulp and papers greater flexibility and strength at the expense of fiber yield [32]. Chemical pulping processes treat lignocellulosic material with chemicals so that the lignin can be dissolved and separated from the fibers. There are two major commercial routes for producing chemical pulps: Kraft (sulfate) pulping and sulfite pulping. Kraft pulping involves treating the wood chips and sawdust with a sodium sulfide and sodium hydroxide solution. Most of the lignin and hemicellulose are dissolved and separated in the black liquor stream, which is routed to a chemical recovery plant [32]. Most of the waste liquor is burned in a recovery boiler to produce energy for the plant, while a not very significant amount of commodity chemicals (e.g., turpentine, tall oil, and resin) is extracted [32, 33]. In North America 60–70% of Kraft pulping mills have a production bottleneck due to the thermal capacity of their recovery boilers. Thus, there is an opportunity to isolate lignin from the black liquor using acid precipitation and then to use it as a valuable chemical resource, which would also diversify the product portfolios of the pulp and paper mills [32, 33]. The second route of chemical pulping is sulfite pulping, where almost pure cellulose fibers are produced by using various salts of sulfurous acids to extract the lignin from wood chips. The major by-product from sulfite pulping is lignosulfonates. There are also some newer pulping methods in the market, including organosolv, alkaline and soda pulping processes. All of these processes produce lignin as a by-product [32]. Several processes exist for the recovery of lignin from black liquor, including Westvaco, LignoBoost, and LignoForce System. The uniqueness of LignoForce System, developed by FPInnovations, is that the black liquor is oxidized under controlled conditions before the acidification step in the conventional lignin recovery process takes place. This improves the filterability of lignin, along with reducing the TRS (totally reduced sulfur compounds) and SO2 emissions [33]. Some of the experiments reported in this chapter use KL that is supplied by FPInnovations. Another prospective commercial source of lignin could be the many modern third generation biorefineries, producing cellulosic sugar-based ethanol/butanol or chemicals and a large amount of hydrolysis lignin (HL). FPInnovation has patented a biomass conversion process, producing sugars from the cellulose and hemicellulose components of a hardwood, while generating a significant amount

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of solid residue, that contains HL (56–57 wt%), cellulose, and mono and oligosaccharides. HL can also be used for production of valuable products after proper modification [31].

2.1.4  Applications of Lignins in Polymer Blends Traditionally, lignin is used as a low-cost fuel and in leather tanning [31]. Nevertheless, a wide variety of bulk and fine chemicals, particularly aromatic compounds, and bio-based materials can be obtained from lignin [31, 34, 35]. Table 2.1 summarizes the applications of lignins in polymer-lignin blends. Besides the bio-based materials shown in Table 2.1, Isikgor and Remzi Becer present a collection of 200 lignocellulose derived value-added compounds and suggest how the combination of new and current technologies can lead to the realization of commodity polymers from lignin [36]. A few examples of the compounds mentioned in this refernce incldue polyethylene terephthalate (PET), polystyrene, Kevlar, unsaturated polyesters, polyaniline, benzene, toluene, xylene, phenols, hydroxybenzoic acids as well as coniferyl, sinapyl, and p-coumaryl

Table 2.1  Applications of lignins in polymer-lignin blends Polymer matrix Protein-lignin blends

Main function of lignin Reduced water absorption and improved mechanical properties Starch-lignin blends Reduced water absorption and improved mechanical properties Polyhydroxyalkanoates Improved recyclability, Tg, melting point, and Young’s modulus Polylactides and Reduced flammability, improved thermal degradation, polyglycolides improved processing performance Epoxy-lignin composites Substitution of bis phenol-A as the raw material in preparing epoxy based adhesives Phenol-formaldehyde Substitute of phenol for preparing phenol-formaldehyde resin resin for adhesive and foam Polyolefin-lignin blend UV and thermal stabilizer, plastisizers, filler, fire retardant Vinyl polymer-lignin UV and thermal stabilizer blend Lignin-polyester blend Improved mechanical properties and processability Lignin in polyurethanes Replacement for polyols. Increased crosslinking of the polyurethane networks, (b) increased Tg, (c) increased tensile strength, (d) increased curing rates and, (e) increased thermal stability Synthetic rubber–lignin Filler blends

References [3] [3] [3] [3] [3, 34, 35] [3, 34, 35] [6, 34] [3] [3] [3, 31, 35]

[3]

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compounds [36]. Ragauskas et al. suggested the potential use of lignin-derived carbon fiber for light weight vehicles [4]. Lignins can act as an effective radical scavenger due to its naturally occurring hindered phenolic structure and can be added to polyolefins for improving their thermo-oxidative resistance. Radical scavenging activity of lignin and the addition of lignin in polyolefin blends are discussed in detail in Sects. 2.2.1 and 2.2.2, respectively.

2.2  Lignin as an Antioxidant 2.2.1  Lignin as a Radical Scavenger As discussed previously, hindered phenols are primary antioxidants that function by scavenging the peroxy radicals created in the oxidation process. The growing interest in the substitution of synthetic antioxidants by natural ones has fostered research in exploiting forestry and agricultural residues as stabilizers in the food, cosmetics, pharmaceuticals and plastics industries [22, 23, 37–39]. Similar to the commercial hindered phenol-based antioxidants, the hindered phenolic structure inherent in lignin enables it to work as a free radical scavenging antioxidant (see Fig. 2.1). Technical lignins consist of complex polyphenolic polymers that contain numerous chemical functional groups, such as phenolic hydroxyl, carboxylic, carbonyl, and methoxyl groups. The phenolic hydroxyl and methoxyl groups

Fig. 2.1  Trapping of peroxy radicals by lignin and eventual delocalized stabilization of the phenoxy radical created from the lignin. (Modified from the MSc thesis [43] of co-author Afsana Kabir who owns the copyright)

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present in lignin possess antioxidant, antimicrobial, antimutagenic, etc. [40]. It should however be noted that the solubility of high MW lignin in polyolefins is very low that the quenching of radicals generated in the polymer chain can only occur at the boundaries of the incorporated mixtures. The antioxidant activity of lignin for scavenging free radicals in polymer materials are summarized below. Dizhbite et al. [41] reported on the antioxidant activity of lignins extracted from coniferous and deciduous wood species and concluded that non-etherified phenolic hydroxyl groups, aliphatic hydroxyl groups in the side chain, high molecular weight, enhanced heterogeneity, and polydispersity are the main factors that decrease the radical scavenging activity of lignin. The radical scavenging ability of phenolic compounds depend not only on the capacity to form a phenoxyl radical, but also on the stability of the phenoxyl radical. Dizhbite et al. [41] postulated that phenolic structures with substituents that can stabilize the phenoxyl radicals have higher antioxidant activity than those that do not. Methoxyl groups at the ortho position stabilized phenoxyl radicals by resonance as well as by hindering their propagation, thereby increasing the antioxidant activity. Also, the purity (presence of hemicellulose or other non-lignin compounds) and heterogeneity of lignin also diminished the antioxidant capacity since carbohydrates can generate hydrogen bonding with lignin phenolic groups, thereby interfering with the antioxidant properties of the lignin. The group of Telysheva [42] performed systematic and thorough investigation of the structure and antioxidant properties of different technical lignins employing Py-GC/ MS as an effective method to quantitatively determine the structural descriptors needed for rationalization of lignin antioxidant activity. Addler [26] analyzed 21 organosolv ethanol lignin samples from hybrid poplar trees and concluded that lignins with more phenolic hydroxyl groups, fewer aliphatic hydroxyl groups, lower molecular weight, and narrow polydispersity showed high antioxidant activity. It was explained that the low molecular weight fraction of the lignin possesses more aromatic hydroxyls than the high molecular weight fraction and thus had higher antioxidant activity. García et al. [24] investigated the effect of different fractionation processes on the antioxidant activity of Miscanthus sinensis. They established that the organosolv fractionation process of Miscanthus sinensis had the highest radical scavenging activity, followed by autohydrolysis and alkaline samples. These results were in agreement with Dizhbite et al. [38], where alkaline processes produced lignins with higher hemicellulose contamination than organosolv treatments, generating hydrogen bonding between carbohydrates and lignin phenolic groups and resulting in lower antioxidant activity. García et al. [24] also inferred that lignin with a lower hydroxyl content would lead to higher compatibility with the thermoplastic matrix and thus act as a better thermal stabilizer for polymers in practice. García et al. [44] studied the effect of processing parameters on the lignins’ antioxidant activity by analyzing the capacity of various lignins from apple tree pruning to reduce the ABTS radical. In the studies, it was concluded that source and purification affect antioxidant activity and that radical scavenging is directly related to total phenolic content. It was also shown that lignin, though at higher amounts,

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could attain the same level of antiradical activity as those of some powerful and well-known commercial antioxidants [44]. Kaur and Uppal [45] investigated the capacity of lignin derived from sugarcane bagasse in reduction of DPPH radicals. The lignin was found to exhibit greater antioxidant activity than its oxidized derivative. They attributed this finding to the higher content of phenolic hydroxyl groups in lignin than oxidized lignin. By comparing the antioxidant activity of the lignin with some commercial antioxidants, they found that the antioxidant activity of both lignin and oxidized lignin was higher than that of BHT (3,5-di-tert-butyl-4-hydroxytoluene) whereas lower than that of BHA (3-tert-butyl-4- hydroxyanisole), and concluded that the sugarcane bagasse lignin has the potential to be used as an antioxidant for food oils and fats [45].

2.2.2  Application of Lignins as Antioxidants in Polyolefins The addition of lignin to polyolefins, such as low-density polyethylene (LDPE) and high-density polyethylene (HDPE), has attracted growing attention since the late 1970s [3]. The presence of lignin in the lignin-polyolefin blends provides the polymer resistance against UV radiation and improves its thermal stability. Levon et al. monitored the stabilization effects of three kinds of lignin: lignosulfonate, KL, and desulphonated lignin in LDPE and HDPE, and reported that among these three kinds of lignin, only lignosulfonate worked as a thermal stabilizer, but for LDPE, the two other lignins did not perform well due to their poor compatibility with the polymer matrixes [46]. Alexy et al. [29] blended lignin from prehydrolysis of beech wood with PE and PP as a stabilizer at 10–30 wt% addition levels. They measured the tensile strength of the composite samples after 113 h exposure to UV with a QUV tester, and 500 h exposure in an oven at 130 °C. With PE, lignin addition of up to 10 wt% retained the mechanical properties during UV exposure, while the tensile strength of the PE-lignin composites (after 20 wt% lignin addition) increased after heat-exposure. With PP, however, the additon of lignin did not affect the resistance of the composites to UV radiation, but deterioriated the mechanical properties of the composites after exposition to heat [29]. Pouteau et al. [27] used industrial KL from wood and lignin samples from various botanical sources in PP and reported that lignin with a low molecular weight and low phenolic content had better compatibility and antioxidant activity, which was however in contradiction to much of the literature that discuss radical scavenging activity of lignin. Pucciariello et al. [47] investigated blends of a straw-lignin with LDPE, linear low-density polyethylene (LLDPE), HDPE and polystyrene (PS). Although the modulus of most lignin-polymer blends slightly increase, both the tensile stress and elongation properties are reduced, which is likely due to the poor compatibility between lignin and the synthetic polymers leading to non-uniform distribution of

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the lignin in the matrix. This problem may be resolved by employing efficient mixing techniques and compatibilizing agents. On the other hand, the research [47] demonstrated that lignin could be an effective antioxidant to increase the resistance of PS, LLDPE and LDPE to UV radiation, although adverse effects were observed when blending lignin with HDPE due to the poor compatibility of lignin in the HDPE matrix. Gregorová et  al. reported on the antioxidant properties of a sulfur free lignin from beech wood prehydrolysis when it was blended with both neat and recycled PP [48]. Similarly, Canetti et al. confirmed the enhancement of the thermal stability of an isotactic PP by addition of lignin under oxidative conditions [28]. Piña et al. [49] investigated the antioxidant effect of Kraft lignin in HDPE, compared with a commercial antioxidant. They concluded that although Kraft lignin could act as an antioxidant, it had much less effectiveness than commercial antioxidants probably due to its larger molecular weight and poorer compatibility with the polymer. Chemical modification of lignin to either reduce its molecular weight or enhance its compatibility with other polymers is thus beneficial for improving the effectiveness of lignin as an antioxidant. For instance, Sailaja [50] blended lignin grafted poly (methyl methacrylate) (LPMMA) with LDPE in the presence of a small amount of compatibilizer. The grafting modification of lignin contributed to increasing the hydrophobicity and thermal stability of the lignin compared with the untreated lignin, resulting in improvement in mechanical and thermal stability of the LPMMA-LDPE composite compared with the untreated lignin-LDPE blend. Sailaja and Deepthi [51] blended esterified lignin with LDPE with the addition of maleic anhydride grafted LDPE as a compatibilizer. The results revealed that the esterification modification of lignin and the usage of the compatibilizer improved the dispersion of the lignin particles in the polymer matrix, resulting in favorable mechanical properties and thermo-oxidative stability of the blend. Ye et al. [52] investigated the thermo-oxidative performance of blends of esterified lignin with PP in terms of oxidation induction time and induction aging time. Despite the decrease in phenolic content due to the esterification of the lignin, it improved the compatibility of the lignin with the polymer and increased the thermooxidative stability of the composites. Dehne et al. [53] evaluated the effects of lignin types (Kraft lignin, soda lignin, hydrolysis lignin and organosolv lignin) and esterification on the properties of the PE-lignin blends, and concluded that the type of lignin did not noticeably affect the mechanical properties blend, while esterification of the lignins greatly improved the mechanical strength of the blends. Lignins after modifications can be effective antioxidants to substitute the petroleum-based antioxidants in polyolefins (e.g., PE and PP). Compared with the commercial petroleum-based antioxidants, lignin-based antioxidants are not only renewable, but are also inexpensive and widely available.

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2.3  L  ignin Modification and Applications of Modified Lignins as Antioxidant in Polyolefins 2.3.1  Lignin Modification Techniques Chemical modification of lignin to either reduce its molecular weight or enhance its compatibility with other polymers is beneficial for improving the effectiveness of lignin as an antioxidant. Typical lignin modification techniques involving functionalization of hydroxyl groups in lignin by esterification, etherification, phenolation, and urethanization [3]. Compatibility of lignin in the polymer matrix can be improved by modifying lignin using esterification or grafting techniques. However, esterification decreases the phenolic content of lignin and grafting adds additional cost to the process. There are various approaches for lignin modification, mainly including thermochemical and catalytic degradation/depolymerization approaches. Pandey and Kim [54] published a comprehensive review on different thermochemical methods for lignin modification and conversion, such as pyrolytic and oxidative de-polymerization of lignin. Pyrolysis refers to the process of heating an organic substance in the absence of air so that the large molecular structure is thermally cracked into smaller units, while the limited oxygen available for the reaction ensures that there is no further combustion to carbon dioxide. When pyrolysis is performed in the presence of hydrogen, the process is also called hydrogenation or hydrogenolysis. In oxidative depolymerization processes, however, lignin rings, aryl ether bonds, or other linkages within the lignin are cleaved via oxidative cracking. Ragauskas et al. [4] published a review concerning oxidative processes for lignin de-polymerization, involving catalytic side-chain oxidation and fragmentation reactions. The main products from lignin oxidation include aromatic acids and aldehydes. Catalytic lignin depolymerization has been considered as an important approach for lignin modification. Mahmood et al. [29] reviewed different catalytic strategies for lignin depolymerization, based on solvent and catalyst selection: (1) acid catalysis, (2) metallic catalysis, (3) base catalysis, (4) ionic liquid assisted, (5) sub or supercritical fluids assisted, (6) oxidative route, and (7) de-polymerization under low pressure. Catalytic lignin depolymerization is more often achieved at 300– 500 °C in a suitable solvent (preferable hydrogen donor solvent) in the presence of a high-pressure reductive atmosphere (such as H2) and a catalyst such as Ni-Mo or Co-Mo/Al2O3 in order to achieve high lignin conversion and suppress char formation, while keeping the reaction severity under a permissible limit [55]. A novel low-T/ low-P lignin de-polymerization process was developed by the authors’ group, achieving cost-effective and efficient conversion of Kraft lignin (Mw ~10,000  g/ mol) and hydrolysis lignin (Mw >20,0000 g/mol) into de-polymerized lignin (DL) of a lower molecular weight (Mw ~1000 g/mol to 2000 g/mol) at a high yield (>70– 90%) [55]. Details of this technology are provided in Sect. 2.3.2.1.

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2.3.2  A  pplication of De-polymerized Lignin as an Antioxidant in Polyolefins As discussed previously, there has been much research on increasing the compatibility of lignin in the polymer matrix by esterifying or grafting of the lignin. However, there does not seem to be published works for using catalytic depolymerized lignin (with reduced molecular weight and improved phenolic content) as an antioxidant in polyolefins. This Section discusses preliminary results from our trials with de-polymerized Kraft and hydrolysis lignins (produced by our low-T/low-P lignin de-polymerization process) as an antioxidant in PE and PP [56]. 2.3.2.1  L  ow-T/Low-P Lignin De-polymerization Process and Materials Characterization Softwood Kraft lignin (KL) used in this study was provided by FPInnovations produced in its pilot plant in Thunder Bay, Ontario. It is a yellow-brown powder with weak odor and specific gravity of 0.80. The relative weight-average molecular weight (Mw) of KL is ≈10,000 g/mol (PDI ≈2.0) based on our GPC-UV analysis. The original KL has an ash content of 0.5 wt.% and >99% purity. Hydrolysis lignin (HL) used in this study was also supplied from FPInnovations, obtained from its TMP-Bio™ hydrolysis process comprising of mechanical refining to disintegrate a hardwood biomass feedstock, hemicellulose extraction, enzymatic hydrolysis, sugar/lignin separation, and fermentation. After hemicellulose extraction and a subsequent hydrolysis, the remaining substrate (solid) residue recovered is the HL used for this study. Mw of HL was not possible to determine by using GPC-UV as it is insoluble in THF or in any organic solvents like ethanol, methanol or acetone etc. The HL used contains 45.7 wt% lignin, approx. 30 wt% carbohydrates and about 1.0 wt% ash. The pH value of original HL was neutral. The other chemicals used in the study were NaOH, ethylene glycol (EG), acetone, sulfuric acid (H2SO4), pyridine, acetic anhydride, d-chloroform, tetrahydrofuran (THF, HPLC grade), etc. were all reagent grade purchased from Sigma-Aldrich, used without further purification or any treatment. KL and HL was depolymerized via a proprietary process using water- EG mixture as a solvent under alkaline or acid conditions using NaOH, or H2SO4, respectively, as a catalyst under low pressure DHL > KL > HL, although the phenolic content of HL was not measurable due to its insolubility in solvents. Figure 2.3 presents the relationship between the content of DKL in PE and the thermo-oxidative stability of the lignin-polymer composites. As demonstrated in Fig. 2.3, the addition of 2.5 wt% DKL can reach the same level of OIT achieved by the addition of 0.5 wt% Irganox 1010 (~35.9 min). DKL has a weight-average molecular weight (Mw) of 1517 g/mol which is close to the molecular weight of Irganox 1010 (1178 g/mol). Hence, DKL was expected to provide similar level of antioxidant activity of Irganox 1010. However, as demonstrated in Fig.  2.3, it takes five times more DKL to provide the same level of efficacy of Irganox 1010.

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Fig. 2.2  Thermos-oxidative stability of polyethylene after incorporating crude lignins and depolymerized lignins Fig. 2.3 Oxidation induction time of PE vs. the content of DKL in PE

This might be attributed to the lower phenolic content of DKL and DHL as compared to Irganox 1010. According to the literature, the structure of Irganox 1010 is composed of multiple hindered phenols tethered together, which have improved the phenolic concentration without sacrificing the molecular weight [17]. The phenolic content of Irganox 1010 is 3.6 mol phenol/kg [17]; whereas DKL has a phenolic content of 2.7 mol phenol/kg. Moreover, DKL has polar polyol components in their side chains whereas Irganox 1010 has nonpolar components in its secondary structure. Hence, the hydrophilicity of DKL could also contribute to the reduced compatibility with hydrophobic PE, as compared to commercial antioxidant. Figure 2.4 shows tensile strength of selected PE blends. In many literature studies with polyolefin blends containing a crude (or unmodified) lignin at a

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18000

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16000 14000

12000 10000 8000 6000 4000 2000

0

PE

PE-2.5DKL

PE-0.5irg

Fig. 2.4  Tensile strength of selected PE blends

high level of addition, decrease in tensile strength was observed due to the poor compatibility between polar lignin and the non-polar polymer matrix [26, 46, 57]. In this study, the addition of 2.5 wt% DKL did not alter the tensile strength of the PE may be owing to the improved compatibility between PE and DKL that has a low molecular weight and decreased aliphatic hydroxyl content. After 200 h thermo-oxidative exposure, the tensile strength of all the samples including the PE and the PE-DKL and PE-Irganox 1010 blends decreased slightly. After 200 h UV exposure, surprisingly, PE-DKL and PE-Irganox 1010 blends have better tensile strength than the neat PE. The PE-2.5DKL blend has better tensile strength than that of either the neat PE or the PE-0.5irg after 200  h thermo-oxidative exposure.

2.4  Conclusions Lignin as a radical scavenger has attracted lots of attention for application to polyolefins as bio-antioxidant to provide resistance against thermo-oxidative degradation and photodegradation. It has been demonstrated in literature work that lignins can act as effective radical scavenger and can be added to polyolefins for improving their thermo-oxidative resistance. There had also work conducted on improving the compatibility of lignin in polymer matrix by modifying lignin using esterification or grafting techniques. There is not much literature work addressing the performance of using technical lignin (i.e., Kraft lignin or hydrolysis lignin) and their de-polymerized products as an antioxidant in polyolefins. Effects of depolymerized technical lignins on antioxidant properties of polyolefins are yet to be explored. Preliminary trials have been performed by the authors with de-polymerized Kraft or hydrolysis lignins (produced by our low-T/low-P lignin de-polymerization process), compared with the raw Kraft and hydrolysis lignins and a commercial antioxidant – Irganox

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1010 as an antioxidant in polyethylene (PE) and polypropylene (PP). The results revealed that The effects of the lignin-based antioxidants follow the order of: depolymerized KL (DKL) > de-polymerized HL (DHL) > KL > HL, which is in the same order as their phenolic content and in the reverse order of their molecular weights. Thus, it was concluded that de-polymerization of lignin, which decreases its molecular weight, increases its phenolic content and improves its compatibility to the polymer matrix, led to an improved its antioxidant activity. The addition of 2.5 wt% DKL can reach the same level of antioxidant effect as that of the addition of 0.5 wt% Irganox 1010. The addition of 2.5 wt% DKL did not alter the tensile strength of the PE, owing to the improved compatibility between PE and DKL that has a low molecular weight and decreased aliphatic hydroxyl content. Interestingly, the PE-DKL and PE-Irganox 1010 blends have better tensile strength than the neat PE after 200 h UV exposure for all samples. The PE-2.5DKL blend has better tensile strength than that of either the neat PE or the PE-0.5irg after 200  h thermooxidative exposure. Acknowledgements  The authors gratefully acknowledge the funding support of Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation, and BioFuelNet Canada. We also thank Mrs. Fang Cao for her assistance in analysis of some samples.

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Chapter 3

Nanocellulose Applications in Papermaking Carlos Salas, Martin Hubbe, and Orlando J. Rojas

Abstract  Research on the utilization of biomass feedstocks has evolved rapidly in the past decades. Key developments include the production of materials with a more sustainable footprint than those derived from petrochemicals. Among associated materials, nanocelluloses have been produced from different sources and routes, such as high shear fibrillation and hydrolysis (chemical or enzymatic) or their combinations. The unique properties of nanocelluloses have sparked a myriad of uses including those related to the fields of oil and gas, adhesion, film formation, coating, packaging, food and composite processing. High end uses include the development of advanced lightweight materials, biosensors and energy harvesting systems; however, central to this review are uses closer to the source itself, namely fiber processing and, in particular, papermaking. In this chapter, the literature in these latter applications is discussed with emphasis on the use of nanocellulose to achieve favorable strength and barrier properties as well as in coating and paper sheet-forming.

3.1  Introduction There is plenty already written about nanocelluloses and their applications. However, when it comes to aspects that are closer to the “parent” fibers and the industry that makes the heaviest use of such materials, namely, the pulp and paper industry, much less is known about their practical use. This is somewhat surprising since there are already major investments in pilot, semi-commercial and commercial C. Salas (*) · M. Hubbe Department of Forest Biomaterials, North Carolina State University, Raleigh, NC, USA e-mail: [email protected]; [email protected] O. J. Rojas Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Espoo, Finland e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 Z. Fang et al. (eds.), Production of Materials from Sustainable Biomass Resources, Biofuels and Biorefineries 9, https://doi.org/10.1007/978-981-13-3768-0_3

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facilities that use nano/micro-celluloses (MNFC) in one form or another. A field that is reminiscent of the use of “nanocelluloses” in papermaking is that of fines engineering. Keeping in mind the differences in morphology and size, several aspects can be translated from one to the other. The role of fines in paper products have been always recognized as critically important, and nowadays research efforts are devoted to fines fractionation (from primary and secondary, for example, fibrillar or flake types). These are being analyzed in terms of their composition and surface energy, depending on the type and origin from the cell wall and, ultimately, methods to isolate and re-suspend them in strategic sections of the paper production process have been proposed. More closely related to the actual size domain of MNFC, one finds structures derived from the cell walls of fibrillated fibers, for example, after extensive beating or refining, which are known to aid in the development of bond strength and bond density. These fibrils are quite challenging to observe or measure, not only because of their dimensions but their close physical proximity to the large features typical of fibers. Also, their highly hydrated state is a factor. Already in 1985 Clark [1] indicated that cellulose chains on the surface of fibers exist like “eel grass on the bottom of a pond”. In fact, it was relatively recently, in the 1990s, when Neuman et al. [2] provided experimental evidence of “water-swollen cellulose surface comprised of dangling tails” or molecular fibrils. This was done by making use not of microscopy or imaging techniques but also the less common interferometric surface force apparatus, which enables the quantification of surface interaction forces and the absolute inter-surface distance. It was then hypothesized that such fibrils gave rise to very strong adhesive forces between cellulosic surfaces. Similar points of discussion can be made regarding free nanocelluloses added to the furnish. To what extent do cellulose nanofibrils or cellulose nanocrystals produce related effects when added ex-situ to fiber dispersions, for example, to develop paper strength? The evidence points to this possibility, which will be discussed in more detail in this chapter. One can reasonably ask whether other effects observed and widely reported for fines translate to MNFC, for instance, reduced rates of dewatering. Additional questions that emerge in relation to MNFC addition include the mechanism of paper strengthening, pore filling, changes in smoothness, water absorption, flexibility, light scattering and bulk/density. Is this the same story as for fines? Recent commercial efforts involve the use of co-grinding fibrous precursors with minerals to, in effect, produce hybrid materials for addition in the wet end. Here some of the benefits come from the possibility to produce paper grades with higher filler loading, without compromising strength and other properties. The possibilities go further, for instance the development of cellulosic “filaments” that exhibit larger axial aspect and produce large gains in strength properties upon addition in paper products. Nanocelluloses carrying residual lignin are also interesting options in efforts to control wettability, transport and other properties of paper products. Another option is highly branched or “hairy” nanocelluloses that expose large surface areas to enable additional features. The possibilities are countless, and this discussion entails the very basics so that the reader can be equipped with fundamen-

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tal information to develop new efforts and material combinations that can open new property spaces in the paper products of the future. When it comes to the nanocelluloses themselves, apart from the types, one wonders about the possibilities of extensive refining in producing products similar to “nanocelluloses” and, as a question of discussion, what difference it makes to have sizes in the nano- or micro-scales. And what about the charges, for instance in TEMPO-oxidized nanocelluloses or carboxymethylated grades? Then, one can ask what form of application is most useful: on the surface, as a coating material or in the bulk? And when it comes to the form of delivery: does it makes a difference to use nanocelluloses in integrated processes or as a product in the dry form? What about the re-dispersability of MNFC? Answers to some of these questions are found in this chapter, which by no means is exhaustive but can be taken as the starting point for further inquiries.

3.2  Nanocellulose Nanocelluloses refer to nano-size particles obtained after subjecting cellulose fibers to either high shear mechanical disintegration or hydrolysis. In some cases, special pretreatments such as enzymatic pretreatment or chemical oxidation are applied before the final processing of cellulose into nanocellulose. Different acronyms are used in the literature for nano- and microcelluloses. The common ones are cellulose nanocrystals (CNC), cellulose nanofibrils (CNF or NFC), and microfibrillated cellulose (MFC). Sometimes the terminology is used interchangeably in the case of nanofibrillar and microfibrillar cellulose, which can be confusing. For simplicity, as was noted in the previous section, we adopt here the MNFC as a generic term to loosely refer to both MFC and NFC. Numerous review articles have discussed the properties and applications of nanocellulose [3–8], including papermaking applications [9–11]. The properties of nanocellulose affect its interactions with the different components of the pulp slurry, including mineral fillers and pigments. Among these properties, the surface charge plays an important role, as it affects the electrostatic interactions with these components as well as polymers and agents added as retention, drainage, sizing and formation aids, among others. Especially, both surface chemistry and particle morphology, which largely depends on the raw material, play a role in the interactions of nanocellulose with mineral fillers. In addition, different chemistries can be used for chemical modification of cellulose, which offer additional functionalities. Cellulose nanocrystals (CNC) are produced by strong acid hydrolysis under controlled temperature conditions [4]. Different acids have been used for this purpose, most commonly sulfuric and hydrochloric acids. The surface chemistry of the obtained cellulose nanocrystals varies depending on the type of acid. For instance, if sulfuric acid is used, then the surface of cellulose nanocrystals will carry negatively charged sulfate half-esters, which endow them with electrostatic stability in water and redispersion ability. If hydrochloric acid is used, then the lack of negative charges makes the particles more prone to aggregate in aqueous dispersions [4].

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Unlike cellulose nanocrystals, in which disordered cellulose chains have been removed by the hydrolysis, cellulose nanofibrils (CNF) and microfibrils (MFC) are produced by high shear defibrillation of cellulose fibers, the nano and microfibrils contain both the disordered and ordered cellulose domains. TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation of cellulose fibers introduces negative charges due to the formation of carboxylate (and a small amount of aldehyde) groups by oxidation of C6 primary OH [6]. Further conversion of TEMPO-oxidized cellulose fibers to nanofibers can be achieved by mechanical disintegration, usually at milder shear conditions than those used for non-oxidized cellulose fibers. The final conversion of the oxidized fibers to nanofibers depends on the carboxylate content. Individual nanofibers of paper with 20% chalk>paper with 20% nanocellulose. Reportedly, these effects were due to the ability of the cellulose nanocrystals to engage in hydrogen bonding with the cellulose fibers and surface fibrils, and its ability to work as a filler to close the gaps between fibers [76]. The paper formed when using CNF-cationic starch-PCC composite filler exhibited higher burst and tensile index compared to paper formed using only PCC [48]. This has been attributed to the ability of CNF of engaging in hydrogen bonding with paper fibers and to act like a bridge to increase bonding area in paper. The optical properties of paper were also improved, the opacity of paper using the system PCCCNF was 1–2% higher than that of paper produced using only PCC [48].

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When evaluating the effect of order of addition of CNF on the properties of 60 g/ m2 TMP handsheets using GCC as filler and a two-component retention system consisting of bentonite and a commercial cationic polyacrylamide, the results indicate small gains in tensile strength with addition of up to 5  wt% CNF and a reduction in the permeability of paper with the amount of CNF in all addition strategies [23].

3.5  Applications of Nanocellulose on Paper Coatings Coated paper is utilized in a variety of applications that require special texture such as smooth finish and gloss. For example, printing paper requires a smooth surface to avoid ink smearing. Coated paper is used in the manufacture of catalogues, brochures, magazines, and advertisement paper in general. Coatings are also used in industrial packaging paper products and to produce fine paper which is used in art, crafts, business, labels. Of these applications, coating applications in packaging have seen a continuous increase, a trend apparently driven by consumer preferences for environmentally friendly packaging and online trading, which has boosted the shipping of packed goods (www.technavio.com). If the trend continues, some market analysts predict that the global market will grow to $7221 million by 2021 (www.technavio.com). These trends have opened new R&D opportunities on the use of sustainable materials that are environmentally friendly, readily available and for which the price is less affected to the fluctuation in crude oil price. Among these opportunities, key aspects include the utilization of sustainable lightweight materials for packaging, including paper coating applications. In that regard, carbohydrate polymers have been the subject of research interest not only because of their renewable source, but also because of their interesting properties. Starch and modified starch are already used in paper coatings and wet-end chemistry applications in the paper industry. Similarly, interest in the utilization of nanocellulose obtained from abundant biomass has steadily increased in the last decade, with a myriad of possible applications, including paper coatings. This section presents a review of recent literature on the utilization of nanocellulose in paper coating applications.

3.5.1  Paper Coatings Coatings are applied to either impart a particular characteristic on the paper surface according to end uses or to hide undesired variations in the surface due to the nonuniformity of paper formation. Specifically, coatings are applied for the enhancement of paper surface properties (smoothness, surface energy, printability and barrier properties). The coating formulations used in papermaking, which are referred to as coating color, are composed of mineral pigments, a polymeric latex binder, a thickening agent and dispersing/wetting agents. The mineral pigments in

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coating color include titanium dioxide, kaolin, zinc oxide and calcium carbonate. The surface charge of pigment particles depends on their isoelectric pH. Below the isoelectric pH the particles become positively charged, whereas above the isoelectric pH they become negatively charged. In practice, organic (anionic polyelectrolytes) or inorganic (polyphosphates) dispersants are used, which impart negative charges to the particles keeping them well dispersed. Polymer lattices used in barrier dispersion coatings include different type of copolymers of acrylate and styrene-­ butadiene, plasticizers are also used to decrease the glass transition temperature of the polymers. Among the different thickeners used carboxymethyl cellulose and other hydrocolloids are common [77].

3.5.2  E  ffect of Nanocellulose on Rheology and Application of Coating Colors The rheology of coating color plays an important role in the uniformity of an applied coating layer, pigment binders’ distribution, process runnability, and final properties of paper. The rheology is, in turn, affected by the properties of each of its different components including solid content, binder, pigment particle size, shape and particle size distribution, the chemistry and type of thickener, its molecular weight and dosage [77]. Furthermore, good runnability of the process requires a high solid content, which demands the use of dispersants to avoid particle agglomeration. Depending on the coating process, during application the coating formulation undergoes high shear forces, typically of the order of 104–107 s−1. Nanocellulose suspensions exhibit non-Newtonian shear thinning behavior and gel-like behavior at low solids content, which makes it an interesting material to tune the rheological properties of coating formulations [78]. Because MFC/CNF already exhibit high viscosity at very low solids content, one of the key aspects for their utilization at industrial scale is how to increase the solids content while at the same time keep viscosity within the target range. Some alternatives have been proposed, such as use of plasticizers, lubricants, surfactants and the chemical pretreatment of fibers. Nanocellulose exhibits shear thinning behavior (pseudo plastic) with a yield stress; flow curves of MFC exhibit a hysteresis loop attributed to the high network forming capacity of MFC (see Fig. 3.7) [79]. A slot die roll to roll coating process was proposed to measure rheology of nanocellulose suspensions at high shear rates; the same device was used on a roll to roll coating application [80]. It was found that at high shear rates (1000 s−1) the slot to die device produces reproducible data that extend beyond that from low shear rate rheology. In addition, the data measured using different slot gaps overlapped, regardless of the slot size. This was explained by the dynamic yield behavior, where a flow of CNF occurs at the center along two slip layers at the edges [81].

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Viscosity (Pa.s)

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Shear rate (1/s) Fig. 3.7  Flow curves of 1% nanocellulose suspensions nanocellulose indicating hysteresis loop between 0 and 50 s−1, shear rate ramp from 0 to 1000 s−1 and 1000 to 0 s−1 at 25 °C. (Reprinted from Iotti et al. [79] by permission from Springer Nature. Copyright © 2010)

Different processes are used industrially for paper coating. The application method varies, depending on the end use of desired product. For instance, dispersion and extrusion coatings are used to impart barrier properties in grades such as food packaging. Regardless of the process, after the coating is applied, the solvent is evaporated, leaving behind the dry coating layer. Different aspects have been identified to influence such film formation, namely particle size and particle size distribution of components, rheological properties of the polymer particles, water evaporation rate, drying temperature, and chemical composition of the latex [77]. Other aspects to consider are the effect of the coating on the properties of the finished products. These include the barrier properties and the mechanical properties of the paper.

3.5.3  A  pplication of MNFC Using Different Coating Processes and its Effect on Paper Properties Nanocellulose is being scrutinized by the scientific community to enhance the barrier properties of paper, including barrier to vapor transmission, oxygen transmission, air permeability and grease barrier. Though the hydrophilic nature of nanocellulose adds a challenge in these applications; some tools explored to overcome this challenge are the chemical modification of nanocellulose and the utilization of nanocellulose as a minor component in the formulation as a replacement of one of the components in the coating formulation. It is expected that the different properties of nanocellulose (aspect ratio, crystallinity, surface charge, chemical modification) will have a different effect on enhancing paper barrier properties. The

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coating application process also influences the final barrier properties. Different studies have been reported in the literature regarding the application of coatings with nanocellulose, including bar coating, size press, dip coating, blade coating and spray coating. In most cases, the efforts have been directed towards proof of concept, mostly at laboratory scale, with only a few reports addressing conditions relevant to the industrial application of such coatings. For example, MNFC of different surface charge was produced by enzymatic fiber pretreatment and carboxymethylation of cellulose fibers and used in coating formulations with kaolin [83]. The coating with carboxymethylated MNFC produced a significant reduction in air permeability, reportedly due to the formation of denser films. The coat weight required to cover the fiber network was less for the carboxylmethylated MNFC (4.7 g/m2) than for the low charge MNFC (6 g/m2). However, the hydrophilicity of the coatings increased with the amount of MNFC [82]. Some studies highlight the use of crosslinkers and plasticizers to enhance barrier properties. Paper coatings were produced by dip coating using nanocellulose with sorbitol as a plasticizer and citric acid as crosslinker. The coated paper exhibited lower vapor transmission, Samples coated with formulations containing plasticizer showed enhanced oxygen barrier properties, in particular the film with three coat layers exhibit an oxygen permeability of 0.7 mL µm/day, whereas those with crosslinking reduced the oxygen barrier ability [83]. The coat weight and coat thickness were found to vary with the wire diameter of rods when applying nanocellulose coating using a laboratory bar coater. The increase in wire diameter did not have a significant effect on air permeability of the coated paper at different coating speeds. However, reducing the wire diameter showed a steady decrease of air permeability with rod speed [84]. The mechanical and barrier properties of papers coated with MNFC by bar coating and size press were compared against a benchmark paper. The application of coating with MFC using the bar coating process was found to significantly improve the bending stiffness and reduced the air permeability at coat weights of 7  g/m2, whereas applying a coat weight of 4 g/m2 by size press process did not improve the paper properties, even after ten coating layers [85]. An auto bar coater was used to coat handsheets with one or two layers of either 1.5 wt% or 3 wt% CNF using at a speed of 50 mm/s. A coating weight of up to 5 g/ m2 was achieved with two coating layers of 3  wt% suspensions of CNF.  The permeability, water absorption and surface roughness of the coated paper was reduced after coating with CNF. The reduction of permeability was also doubled with each additional layer at each CNF concentration [86]. Carboxymethylated microfibrillar cellulose (0.85  wt% dispersions) was evaluated for the coating of kraft paper and grease-proof paper using a rod coater. For different coat weights, the coating with nanocellulose produced a uniform film on the paper surface, the coverage improved with the cot weight (see Fig. 3.8). The air permeability of coated paper decreased and the oil resistance increased with the coat weight. The oxygen transmission rate of the MFC coated papers increased with the relative humidity, attributed to the mobility of polymer chains and the plasticization of the amorphous regions of MFC by water [26].

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Fig. 3.8  E-SEM micrographs of uncoated (a) and MFC-coated unbleached papers with coat weights of ca. 0.9 (b), 1.3 (c) and 1.8 g/m2 (d), respectively. The scale bar is 100 μm. (Reprinted from Aulin et al. [26] by permission from Springer Nature. Copyright © 2010)

A different approach aimed to improve the holdout of the NFC on the paper surface. Wood-free copy paper of basis weight 80 g/m2 was precoated with a layer of modified calcium carbonate prior to coating it with MNFC.  The coat weight of MNFC was reduced when the paper was precoated with modified calcium carbonate compared with the uncoated paper. The results showed full coverage of the surface after applying the MNFC coating on the precoat layer. The permeability and roughness of both calendered and uncalendered samples was reduced by this approach, which was enhanced with the increasing layers of MNFC and MCC coat layer. Uncalendered paper coated using the modified calcium carbonate precoat before the MNFC exhibited higher bending stiffness than the uncalendared paper with only the calcium carbonate precoat [87]. Nanocellulose coating has also been used as the precoat layer before applying alkyd resins on paper substrates with different porosity. The paper substrates were precoated with suspensions of 0.85% nanocellulose using a rod coater. One precoat layer of 3 g/ m2 of nanocellulose was enough to form a smooth and uniform film and served as a primer for the further coating of alkyd resin. Paper coated with a coat weight of 3 g/m2 nanocellulose and a 20 g/m2 alkyd resin coat weight exhibited a significant reduction in water vapor transmission rate at different relative humidity compared to the starting paper substrate and to the paper coated with only nanocellulose [88].

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Fig. 3.9  Low shear rate viscosity with different CNF levels for calcium carbonate (left) and kaolin (right) colors. The sum of latex and CNF was 10 pph. (Reproduced from Rautkoski et al. [89], Copyright © 2015, with permission from DeGruyter)

CNF produced by enzymatic pretreatment or TEMPO-mediated oxidation of wood fibers were evaluated as substitute for synthetic latex binders and thickeners in board colors. CNF produced from the enzymatically treated fibers, which had lower surface charge, exhibited a lower water retention than the reference at constant solids, whereas the TEMPO-oxidized CNF of higher surface charge exhibited the lowest water retention. Rheology measurements indicated a significant increase in viscosity of the coating color (at 24.55 s−1) when latex binder was replaced by CNF (see Fig. 3.9). Coating color containing CNF exhibited greater shear thinning than those containing latex only. The higher viscosity was observed for the CNF produced from the enzymatically treated fibers versus TEMPO-oxidized CNF. These results indicate that CNF can be used as a thickener but not as a binder [89].

3.5.4  A  pplication of Coatings with Nanocellulose in Semi-­ industrial Operations Different research efforts have been directed towards the scale-up and application of coatings with MNFC at pilot scales that mimic the real process conditions found in industry. It is important to evaluate the performance of these formulations at the speeds typically found in commercial processes and using the equipment that is typically used in these applications. Some of the challenges for industrial application of nanocellulose on paper coatings include the high viscosity of MNFCs suspensions at low concentrations, and

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the low solid concentration of MNFC suspensions. The amount of water can also weaken the paper’s web structure and create runnability issues [80]. MNFC was used in the coating of paperboard (250 g/m2 basis weight and 340 μm thickness) on a semi industrial process. The paperboard was coated with MNFC on the side that has not been coated with minerals. The addition of rheology modifiers allowed the reduction of hysteresis behavior in MNFC samples and formation of a uniform film on the paper surface. The samples coated with MFC exhibited reduced water vapor transmission rates [90]. CNF suspensions of 1 and 2 wt% were used alone or in combination with carboxymethyl cellulose for the coating of packaging paperboard (178 g/m2) on a continuous process operation using a roll-to-roll method [80]. The effect of different variables on runnability and coating quality were studied; namely slot gap, thickness of wet coating layer, coating speed, concentration of CNF suspension and addition of carboxymethyl cellulose. Several parameters were optimized including: drying capability, machine speed, slot die gap, and the gap between the paperboard surface and the slot die, referred as slot web gap (SWG). The system is illustrated in Fig. 3.10. The optimum formulation was achieved when using 2% CNF suspensions with 5 pph of CMC. The coating weight showed strong dependency on the slot web gap. Coating weights of up to 16 g/m2 were achieved at slot web gaps of 700 μm. In addition, coating uniformity also improved with slot web gap. Figure 3.11 shows results of ink penetration test for coating at different slot web gaps. As the slot web gap increases, so does the amount of coating applied, and the surface becomes more closed [80]. The water vapor transmission rate (WVTR) was reduced with increasing SWG. For instance, a paper coated with 2% CNF containing 5 pph of CMC and 700 SWG exhibited a WVTR 85% less than uncoated paper. This was explained by the ability of CNF to form a tight coating film on the paper surface. Similar results were observed for heptane vapor transmission rate and air permeability. The strength of coated paper also was increased with increasing thickness of CNF coating. For instance, when using 3% CNF coating, the tensile strength index and strain at break were increased by 10% and 20%, respectively. This effect was attributed to the formation of a strong CNF network and the filling of void spaces on the paper surface by CNF [80]. In a recent study, a high speed (10 m/s) blade coater was used to coat unmodified CNF on paperboard (200 g/m2 bleached, wood-free), achieving coat weights up to 13 g/m2 at CNF concentrations of 4 wt% when carboxymethyl cellulose was used in the coating formulation [91]. The addition of carboxymethyl cellulose as a rheology modifier enhances the uniformity of the CNF coatings and facilitates the coating from high solids content suspensions. The barrier properties of paperboard were significantly improved when coated with CNF.  For example, by using the high-­ speed blade coater, the paperboard coated with 3 wt% CNF exhibit enhanced an air resistance (13,000 Gurley seconds vs 57 Gurley seconds for uncoated paper), enhanced grease resistance (kit number of 12 with a coat weight of 9 g/m2), and enhanced resistance to water penetration (400 s for CNF coated paperboard versus 90 s for uncoated paperboard as determined by Hercules sizing test) [91].

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Fig. 3.10  Slot die unit used for application of nanocellulose coatings. (Reprinted (Adapted) with permission from Kumar et al. [80]. Copyright © 2016. American Chemical Society)

Fig. 3.11  Results from print penetration test show a correlation between surface porosity with stain length, i.e. the stain length is higher on a more closed surface. Different SWGs correlate to different coat weights as follows. SWG-50: 1 g/m2, SWG-100: 4 g/m2, SWG-200: 5 g/m2, SWG-­ 300: 6  g/m2. (Reprinted (adapted) with permission from [80]. Copyright © 2016, American Chemical Society)

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3.6  Conclusions and Future Outlook In summary, recent publications show the benefits of MNFC application in papermaking, under the right conditions. Nanocelluloses have various effects on paper properties, depending on the type and properties of nanocellulose, the dosage, the retention system and the presence of other components in the pulp slurry. The addition of MNFC can help increase paper strength with less refining of recycled fiber furnish. Because of its eco-friendly nature, MNFC offers a unique advantage compared to typical synthetic additives. However, more research is needed to achieve a wider adoption and to overcome certain challenges associated with MNFC utilization in papermaking, including: • Slow drainage, which leads to lower production rates. • High energy costs to make nanocellulose, leading to relatively high cost of use of nanocellulose, • Batch to batch differences in MNFC properties, • Difficulties in retaining MNFC during papermaking and determination of retained amount. • Paper machine process instabilities due to build-up of unretained MNFC • Two-sided nature of the paper due to uneven distribution of MNFC in the z-direction of the sheet • Curl and other problems resulting from uneven distribution and the high shrinkage tendency of nanocellulose. The surface charge of MNFC plays an important role in the flocculation and retention of fillers. TEMPO-oxidized, carboxymethylated and cationic nanocelluloses are useful to tune the balance of charges in the system and, as a result, the colloidal interactions. MNFC suspensions exhibit a shear-thinning behavior, making them good candidates for coating formulations or to reduce the use of other additives in the mixture. The utilization of MNFC and their synergies with other components in the formulation offer an opportunity to tune the properties of the coating, even if added at small dosages. MNFC can improve coating coverage, which in turn has potential to improve gas barrier properties. In these applications, the hydrophilic nature of nanocellulose undermines the moisture barrier properties; however, chemical modifications are possible to address this challenge. The demonstration of semi-­industrial coating units with nanocellulose opens the opportunity for scale-up into commercial applications. Overall, the properties of MNFC offer potential advantages in papermaking. Challenges remain with respect to the adoption of nanocelluloses and their potential value creation in commercial papermaking operations.

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59. Arnold E (1985) New possibilities of paper coloring with cationic direct colors. Wochenbl Pap 113(8):267–270 60. Daniels CR, Reznik C, Landes CF (2010) Dye diffusion at surfaces: charge matters. Langmuir 26(7):4807–4812. https://doi.org/10.1021/la904749z 61. Hubbe MA, Nanko H, McNeal MR (2009) Retention aid polymer interactions with cellulosic surfaces and suspensions: a review. Bioresources 4(2):850–906 62. Salmi J, Nypelö T, Österberg M, Laine J (2009) Layer structures formed by silica nanoparticles and cellulose nanofibrils with cationic polyacrylamide (C-Pam) on cellulose surface and their influence on interactions. Bioresources 4(2):602–625 63. Meng Q, Li H, Fu S, Lucia LA (2014) The non-trivial role of native xylans on the preparation of TEMPO-oxidized cellulose nanofibrils. React Funct Polym 85:142–150. https://doi. org/10.1016/j.reactfunctpolym.2014.07.021 64. Campbell WB (1959) The mechanism of bonding. TAPPI J 42:3 65. Page DH (1993) Quantitative theory of the strength of wet webs. J  Pulp Paper Sci 19(4):J175–J176 66. Herbert H (2006) Handbook of paper and board. Wiley-VCH Verlag GmbH &Co. KGaA 67. Hubbe MA, Gill RA (2016) Fillers for papermaking: a review of their properties, usage practices, and their mechanistic role. BioRes 11(1):2886–2963 68. Lagerwall JPF, Schütz C, Salajkova M, Noh J, Hyun Park J, Scalia G, Bergström L (2014) Cellulose nanocrystal-based materials: from liquid crystal self-assembly and glass formation to multifunctional thin films. Npg Asia Mater 6:e80. https://doi.org/10.1038/am.2013.69 69. Revol JF, Bradford H, Giasson J, Marchessault RH, Gray DG (1992) Helicoidal self-ordering of cellulose microfibrils in aqueous suspension. Int J Biol Macromol 14(3):170–172. https:// doi.org/10.1016/S0141-8130(05)80008-X 70. Korhonen MHJ, Laine J (2014) Flocculation and retention of fillers with nanocelluloses. Nord Pulp Paper Res J 29(1):119–128. https://doi.org/10.3183/NPPRJ-2014-29-01-p119-128 71. Ämmälä A, Liimatainen H, Burmeister C, Niinimäki J (2013) Effect of tempo and periodate-­ chlorite oxidized nanofibrils on ground calcium carbonate flocculation and retention in sheet forming and on the physical properties of sheets. Cellulose 20(5):2451–2460. https://doi. org/10.1007/s10570-013-0012-6 72. Lourenço AF, Gamelas JAF, Nunes T, Amaral J, Mutjé P, Ferreira PJ (2017) Influence of TEMPO-oxidised cellulose nanofibrils on the properties of filler-containing papers. Cellulose 24(1):349–362. https://doi.org/10.1007/s10570-016-1121-9 73. Chen D, van de Ven TGM (2016) Flocculation kinetics of precipitated calcium carbonate induced by electrosterically stabilized nanocrystalline cellulose. Colloids Surf A Physicochem Eng Asp 504:11–17. https://doi.org/10.1016/j.colsurfa.2016.05.023 74. He M, Cho B-U, Lee YK, Won JM (2016) Utilizing cellulose nanofibril as an eco-friendly flocculant for filler flocculation in papermaking. BioResources 11(4):10296–10313 75. Rantanen J, Dimic-Misic K, Kuusisto J, Maloney TC (2015) The effect of micro and nanofibrillated cellulose water uptake on high filler content composite paper properties and furnish dewatering. Cellulose 22(6):4003–4015. https://doi.org/10.1007/s10570-015-0777-x 76. Ioelovich M, Figovsky O (2010) Structure and properties of nanoparticles used in paper compositions. Mech Compos Mater 46(4):435–442 77. Brander J, Thorn I (eds) (1997) Surface applications of paper chemicals. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-1457-5 78. Hubbe MA, Tayeb P, Joyce M, Tyagi P, Kehoe M, Dimic-Misic K, Pal L (2017) Rheology of nanocellulose-rich aqueous suspensions: a review. Bioresources 12(4):9556–9661 79. Iotti M, Gregersen ØW, Moe S, Lenes M (2011) Rheological studies of microfibril lar cellulose water dispersions. J  Polym Environ 19(1):137–145. https://doi.org/10.1007/ s10924-010-0248-2 80. Kumar V, Elfving A, Koivula H, Bousfield D, Toivakka M (2016) Roll-to-roll processed cellulose nanofiber coatings. Ind Eng Chem Res 55(12):3603–3613. https://doi.org/10.1021/acs. iecr.6b00417

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81. Kumar V, Bousfield D, Toivakka M (2018) Slot die coating of nanocellulose on paperboard. TAPPI J 17:11–19 82. Nygårds S (2011) Nanocellulose in pigment coatings: aspects of barrier properties and printability in offset. Dissertation, Linköping University 83. Herrera MA, Mathew AP, Oksman K (2017) Barrier and mechanical properties of plasticized and cross-linked nanocellulose coatings for paper packaging applications. Cellulose 24(9):3969–3980. https://doi.org/10.1007/s10570-017-1405-8 84. Matikainen L (2017) Nanocellulose as barrier coating deposited using a laboratory rod coater. Master’s Thesis 85. Lavoine N, Desloges I, Khelifi B, Bras J (2014) Impact of different coating processes of microfibrillated cellulose on the mechanical and barrier properties of paper. J Mater Sci 49(7):2879– 2893. https://doi.org/10.1007/s10853-013-7995-0 86. Afra E, Mohammadnejad S, Saraeyan A (2016) Cellulose nanofibrils as coating material and its effects on paper properties. Prog Org Coat 101:455–460. https://doi.org/10.1016/j. porgcoat.2016.09.018 87. Ridgway CJ, Gane PAC (2012) Constructing NFC-pigment composite surface treatment for enhanced paper stiffness and surface properties. Cellulose 19(2):547–560. https://doi. org/10.1007/s10570-011-9634-8 88. Aulin C, Ström G (2013) Multilayered alkyd resin/nanocellulose coatings for use in renewable packaging solutions with a high level of moisture resistance. Ind Eng Chem Res 52(7):2582– 2589. https://doi.org/10.1021/ie301785a 89. Rautkoski H, Pajari H, Koskela H, Sneck A, Moilanen P (2015) Use of cellulose nanofibrils (CNF) in coating colors. Nord Pulp Paper Res J  30(3):511–518. https://doi.org/10.3183/ NPPRJ-2015-30-03-p511-518 90. Iotti M (2010) Semi industrial application of MFC barrier coating. A complete rhological and tehcnological study. Paper presented at the 2010 TAPPI International Conference on Nanotechnology for the Forest Product Industry, Otaniemi, Espoo, Finland 91. Mazhari Mousavi SM, Afra E, Tajvidi M, Bousfield DW, Dehghani-Firouzabadi M (2018) Application of cellulose nanofibril (CNF) as coating on paperboard at moderate solids content and high coating speed using blade coater. Prog Org Coat 122:207–218. https://doi. org/10.1016/j.porgcoat.2018.05.024

Part III

Functional Materials Derived from Cellulose and Lignocelluloses

Chapter 4

Recent Advances in Cellulose Chemistry and Potential Applications Poonam Trivedi and Pedro Fardim

Abstract  Cellulose, which is the most abundant organic compound of natural origin, has wide application in technical and biomedical fields. Cellulose can be chemically derivatized in to cellulose intermediates, such as cellulose tosylate or carbonates. The synthesised intermediates can be further transformed into cellulose derivatives of biological interest, for instance, amino cellulose. The reaction parameters such as homogeneous/heterogeneous mode, molar ratio of reagent, temperature, and solvent affects the efficiency of derivatization, substitution pattern and the physicochemical properties of the final product obtained. Derivatized cellulose has been applied to advanced materials for diagnostics and biomedical areas in the form of fibres, nanoparticles microbeads. This chapter provides an integrated overview on cellulose derivatization approaches and advanced material design that can be obtained from cellulose derivatives and which have potential application in biomedical areas.

4.1  Introduction The historical relevance of cellulose and cellulose based products for their use on a bulk scale has been known for centuries [1]. Cotton, which is native cellulose, has been the prime component in textile industries, but due to its high production cost and limited geographical availability, there are strong reasons to develop wood-­ derived cellulose for textiles. The dissolving pulp (>90% cellulose content) obtained from wood is processed via different solvents and techniques and is available in the P. Trivedi Laboratory of Fibre and Cellulose Technology, Åbo Akademi University, Turku, Finland e-mail: [email protected] P. Fardim (*) Laboratory of Fibre and Cellulose Technology, Åbo Akademi University, Turku, Finland Department of Chemical Engineering, KU Leuven, Leuven, Belgium e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 Z. Fang et al. (eds.), Production of Materials from Sustainable Biomass Resources, Biofuels and Biorefineries 9, https://doi.org/10.1007/978-981-13-3768-0_4

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market by brand names such as Viscose, Rayon, Lyocell [2]. The possibility of tuning the properties of cellulose by chemical derivatization has provided a vision to develop highly-engineered cellulose products. In the modern era, the largest cellulose derivative used in bulk is cellulose acetate with production being approximately 900,000 tonnes per year [3]. The organic esters are used in pharmaceutical industry and have many applications in controlled release preparations, enteric and osmotic drug delivery systems, bio and mucoadhesives, thickeners, stabilisers, free flowing agents. The main cellulose ester derivatives are cellulose acetate (CA), cellulose acetate phthalate (CAP), cellulose acetate butyrate (CAB), hydroxypropyl methyl cellulose phthalate (HPMCP). The Eastman Chemical and Dow Chemical companies are some of the major producers of cellulose esters and ethers. A wide range of products is available as binders, additives, film formers, modifiers for paints and coating materials. They are also used as enamels for automotive, wood, plastic, paper, leather applications, in graphic arts, inks and overprint varnishes. The stereoregularity of the cellulose polymer has also lead the researchers to envision the use of it for separation and purification of chiral moieties. The stereoregularity of the polymers is not enough for the desired separation of enantiomers, so derivatization into esters or carbamates is required. In 1973 Hesse and Hagel successfully used microcrystalline triacetate cellulose as chiral stationary phase (CSPs) [4]. In 1984, Okamoto and Diacel company reported that cellulose esters and carbamates coated on silica gel are very attractive CSPs [5]. In 2004, chiral stationary phases based on cellulose tri benzoate and triphenyl carbamate was commercialised by the brand name chiralcel OD [6]. The use of functional cellulose particles also finds application in chromatography due to hydrophilicity and the potential for derivatization. Functionalized cellulose beads, for example, can be used in purification of biomolecules [7]. The growing research in the field of application based products and the establishment of biocompatibility and abundant availability of cellulose has attracted chemists, biochemists, material scientists and medical doctors to focus on new and advanced materials from cellulose. Chemical modification of cellulose is a prerequisite in developing engineered application based products such as nanoscale structures in the form of functional beads, crystals, nanoparticles, microparticles, biosensors and textile fibres. The progress in green cellulose chemistry is encouraging young researchers to foresee, a better world using products manufactured from sustainable, renewable bioresources. Historical and current research have shown that the cellulose biopolymer is a worthy candidate for advanced biocompatible products for emerging bioeconomies.

4.2  Cellulose Structure and Modes of Derivatization Cellulose biopolymer has a complex structure in which various levels of organisation exists. The highest level or organisation is its morphology that consists of micro and nanofibrils. The supramolecular level is comprised of crystalline and

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amorphous regions. The crystalline (highly ordered) and amorphous (disordered) regions in cellulose fibrils are dependent upon the species and biosynthetic procedure. The linear chains in the form of flat ribbons interact with each other through hydrogen bonding and have a strong influence on its chemical behavior [8–10]. The molecular level that in which the cellulose consists of linearly arranged D-anhydroglucopyranose units (AGU) linked via β-(1→4) glycosidic bonds [11]; each repeating AGU possesses three hydroxyl groups with one being primary and two being secondary. The primary hydroxyl group is present at C-6, and the secondary groups are at C2 and C3 positions. There are two types of terminal ends in a cellulose chain. The reducing end, in which the C1 hydroxyl is present in hemiacetal form and the non-reducing end in which C4 hydroxyl group is free and not involved in any linkage [12]. The presence of two-fold helical conformations in the crystal structure results in 180° orientation in adjacent molecules and gives stereoregularity to the polymer. There also exists extensive inter- and intra- molecular hydrogen bonding which provides mechanical strength to the fibres. The intramolecular hydrogen bonding is considered to be present between hydroxyl groups of C2, C6 and C3 and the endocyclic oxygen atom [13]. The intermolecular hydrogen bonding occurs between C3 and C6 OH of successive AGUs. The structure of cellulose at the molecular level is fascinating and decides various properties of cellulose such as its solubilization and functionalization.

4.2.1  Homogeneous and Heterogeneous Derivatization To introduce functional groups into the cellulose backbone, the mechanism of reaction is highly crucial. The presence of three hydroxyl groups in an AGU and the accessibility of these groups is the deciding factor of cellulose dissolution in any solvent and chemical modification. The chemical derivatization of cellulose can be classified into two modes: (i) homogeneous and (ii) heterogeneous. Homogeneous chemical derivatization can be defined as the method in which all the reactants, comprising cellulose and the reagents are in the same phase as that of the solvent employed or transformation of cellulose from heterogeneous state to homogeneous by reactive dissolution in a solvent system. Direct dissolution of cellulose has been achieved in ILs 1-butyl-3-methylimidazolium chloride, [BMIM]Cl, 1-allyl-3-methylimidazolium chloride, [AMIM]Cl and organic solvent metal salts dimethylacetamide/lithium chloride (DMA/LiCl) at certain conditions and some desirable derivatives have been synthesised [14, 15]. In the esterification of cellulose, homogeneity is obtained after the intermediate formation during the reaction course, which in fact is dependent upon the molar ratio of reagent used per AGU. The homogeneous mode of derivatization is comparatively efficient and results in products with higher functionalization [16]. In heterogeneous derivatization, the cellulose is generally solvated by the solvent molecules and the reagents used are soluble. This method of derivatization is comparatively less efficient than the homogeneous, but the recovery of solvent is easier. Heterogeneous derivatization has been proven

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to be very beneficial in the case of preparation of biochromatography media for the separation of biomolecules such as proteins and enzymes [7, 17–19].

4.2.2  Degree of Substitution (DS) The DS of cellulose can be defined as the number of substituent groups attached to the three hydroxyl groups per AGU after functionalization. It is a crucial factor, which dictates various properties of cellulose derivatives, for example, the solubility of cellulose in organic or aqueous solvents, its structure formation and biological properties. The historical example is the synthesis of cellulose esters and ethers. CA with DS 0.49 is water-soluble, while a sample with DS >1.0 is organosoluble. The basic concept here is that the substitution of hydroxyl groups of cellulose with hydrophobic moieties reduces the hydrophilicity of the cellulose. While substitution with ionic groups such as protonated amines, quaternary ammonium groups enhances the hydrophilicity of the biopolymer. The type of chemistry performed and the functional group introduced significantly influences the performance for certain applications [20].

4.3  Cellulose Intermediates To perform nucleophilic displacement reactions on cellulose, activation of the hydroxyl groups is necessary. Therefore, cellulose intermediates can be synthesised to improve the leaving ability of the hydroxyl groups present in the AGU. Up to now, examples of the activating agents that have been applied are trimethylsilyl chloride (TMSCl), cyanogen bromide (CNBr), phosgene (COCl2), p-toluene sulphonyl chloride (p-TsCl) (Fig. 4.1). The cyanate ester intermediates are not considered useful due to the formation of inert carbamates in case of dextran. Therefore, further discussion is focussed on the advancement in cellulose intermediates, i.e., cellulose tosylate and carbonate, for the synthesis of novel cellulose derivatives [21].

4.3.1  Cellulose Tosylate Cellulose tosylates are one of the prominent class of sulphonic acid ester intermediates synthesised by the reaction of cellulose with p-toluene sulphonyl chloride in organic and aqueous solvent systems [15, 22]. The degree of tosyl substitution depends on upon the molar ratio of reagent and the reaction conditions. The reaction performed in organic solvents at (8 to 10) °C for (5 to 24) h resulted in products with DS (0.4 to 2.3). The complete substitution of C6 position can be achieved at DS 1.89. The cellulose tosylates with more than a DS value of 0.4 are soluble in dipolar

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Fig. 4.1  Cellulose intermediates (a) chloro formate, (b) tosylate (c) trimethylsilyl ether (d) phenyl carbonate, (e) propyl phosphonate (f) cyanate

aprotic solvents such as dimethyl sulphoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA). A stronger organic base trimethylamine (TEA) is considered as best-suited due to its formation of less reactive species with the solvent DMA/LiCl in comparison to weak organic bases that ultimately give tosylate cellulose. The major side products formed during the reaction are (i) chloride ions from TsCl or LiCl which subsequently lead to the formation of chlorodeoxy cellulose and (ii) formation of crosslinking between hydroxyl groups thus resulting in the gelation and insoluble product [15, 21]. The tosylation of cellulose in a mixture of ionic liquids and base (BMIMCl and pyridine) have also been investigated [23]. That research reveals that the amount of tosyl and subsequent nucleophilic displacement with amines can be controlled by the reaction parameters. The reaction is energy efficient and can be performed at ambient temperature in at reaction times of (8 to 16) h. An increase in DSTos results in phase separation of the product. The researchers claim that, to optimise ILs as a solvent for tosylation, the miscibility of the solvent system towards products should be increased to avoid phase separation and ILs with low viscosity enhance the mixing of reagents [23]. The toxicity of the organic amines is undesirable and their energy consumption is high for bulk scale synthesis. Therefore, to establish a sustainable, eco-friendly solution to synthesise tosyl cellulose, the investigation of water-based solvent ­systems have been studied. The sodium hydroxide (7%) – urea (12%) – water (81%)

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solvent system was used to conduct tosylation of cellulose at ambient temperature 25  °C.  The obtained product was insoluble in organic solvents namely DMSO, DMF. To obtain a soluble product, ionic (sodium dodecyl sulphate) and non-ionic surfactant (polyethylene glycol alkyl C11- C15- imbentine) were added during the reaction. The nonionic surfactant resulted in cellulose tosylate which was soluble in DMSO with DS (0.43–0.95) and in DMF with DS (0.95). The PEG product had comparatively higher DSTos. The cellulose tosylates obtained are intermediates for further substitution with nucleophiles for instance amines, heterocycles, azides leading to the formation of derivatives 6-deoxy-6-substituted cellulose [22].

4.3.2  Cellulose Carbonate Cellulose carbonates are intermediates synthesised by the reaction of cellulose hydroxyl groups with alkyl or aryl chloro/fluoro formates. In 1968, the first cellulose carbonate was synthesised by the treatment of cellulose with ethyl chloroformate in DMSO/TEA solvent system under heterogeneous conditions, which resulted in the formation of a mixture of trans-2,3- cyclic carbonate and acyclic O-ethoxy carbonyl moieties [24, 25]. The cyclic moieties were extensively studied for the immobilisation of enzymes, antibiotics, isolation and purification of antibodies [26– 29]. In recent years, this intermediate has regained interest due to potential to design moieties for immunoassay applications. Cellulose phenyl carbonates with different degrees of substitution are being synthesised in BMIMCl and pyridine ionic liquid mixtures. In DMA/LiCl solvent systems, it is difficult to synthesise cellulose phenyl carbonates with high DS due to side reactions of chlorophenylcarbonate with the solvent that result in the formation of unstable intermediates [30, 31]. The phenyl carbonates with DS >0.73 are soluble in aprotic organic solvents (DMF, DMSO), while the intermediates synthesised in DMA/LiCl are insoluble. The effect of reaction temperature can be considered as the limiting factor, as the reactivity of phenyl chloroformate would be higher at room temperature than at 8 °C. The products with DS 2.83 and 2.93 are soluble in acetone, chloroform and toluene respectively. The solubility of high DS (ca. 2.8) cellulose in nonpolar and polar aprotic solvents can be explained due to the non-availability of hydroxyl groups in cellulose for hydrogen bonding [31]. Reaction of cellulose carbonate with amines lead to the formation of carbamates and is known as aminolysis. The DS of carbonate and the type of amine used for aminolysis decides the final properties of the product. The possibility of cross-­ linking cannot be overruled and depends on the kind of amine used for the reaction. In the case of cellulose with high DS phenyl carbonate, primary amines are more reactive than secondary and tertiary amines. It is claimed that primary amines result in 90 % carbamate conversion while secondary amines lead to major crosslinking and only (13 to 25) % aminolysis. The low DS cellulose carbonate 97.5 97.9–99.5

[36] [36] [36] [36] [44]

Rapeseed oil

2

3.5

89.91–91.44

[45]

Binder type Lignosulphonate Maize starch Lignosulphonate Maize starch Lignosulphonate Maize starch Lignosulphonate + Maize starch Lignosulphonate + Maize starch Lignosulphonate + Maize starch Lignosulphonate + Maize starch Lignosulphonate Lignosulphonate Wheat starch

Potato starch

Bark-free Scotch pine

Potato flour Potato peel residue

Sawmill residues Untreated wood

Inclusion level (% by weight) 2.5 2.5 5 5 7 7 1.05 (LS) + 0.95 (MS) 1.06 (LS) + 1.94 (MS) 1.06 (LS) + 2.94 (MS) 1.07 (LS) + 3.93 (MS) 1 2 0.7

[41]

Besides solid bridges, the following forces act to bond the particles: molecular forces, electrostatic and magnetic forces. Molecular forces can be of three types: valance forces (i.e. free chemical bonds), hydrogen bridges and van der Waals’ forces. Valance forces act only if the distance between the particles is about 10 Å. Van der Waals’ forces are responsible for adhesion between particles with distance less than 1 μm (see Kaliyan and Morey [48]). The effectiveness of binding forces

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decrease with the increase of particle size and the increase of inter-particle distance (see Pietsch [49]). The establishment of solid bridges can be explained with several mechanisms. Due to high pressure and high temperature conditions, the diffusion of molecules from one particle to another can form solid bridges at the point of contact. Solid bridges can be formed also from crystallization of some ingredients, chemical reactions, hardening of binders and solidification of melted components. Solid bridges are formed during cooling/drying process after pelletization, while during the compression phase fibers and particles can do interlocking bonds, which are mainly mechanical bonds. An action comparable to that of solid bridges can be exerted by viscous binders (e.g. tar) which adhere to the surface of solid particles to form strong bonds. Adhesion forces at the interface between solid particles and the binder and cohesion forces within the viscous binder can bond the solid particles until the weaker of the two fails. Viscous binders can also harden after cooling and form solid bridges. Also the presence of liquids can be of fundamental importance to build bridges among biomass particles. Free moisture between particles creates cohesive forces. If the biomass powder is soluble in the liquid binder (i.e. water), it can diffuse in the interstices between primary particles forming liquid bridges that hold particles together by capillary and viscous forces. When drying happens the liquid evaporates from the bridges and leaves “necks” between the particles. These solid bridges are formed through recrystallization (or precipitation) of the biomass powder. From the review of Back [50] it results that plasticization of wood polymers like lignin above their glass transition temperature is necessary to produce effective bonds. These are produced by the inter-diffusion of amorphous polymer molecules from one biomass particle to an adjacent other. The key parameter which regulates the inter-diffusion process is the glass transition temperature. When passing the glass transition temperature the polymer switches from a glassy into a rubbery state and its viscosity decreases resulting in improved flow properties. This promotes the inter-diffusion phenomenon. Among the molecular forces (valence forces, hydrogen bridges and van der Waals’ forces), the main bonding mechanism is represented by the establishment of hydrogen bonds at lignin and cellulose surface areas. The formation of covalent bonds is possible at a minor extent during hot pressing reactions. Covalent bonds are stronger than hydrogen bonds which are stronger than non-polar van der Waals’ forces. When switching from biomass to torrefied material or pyrolysed material, it has to be taken into account that torrefaction and pyrolysis degrade hemicellulose, cellulose and lignin and remove moisture from the material. The breakdown of carbohydrates implies a reduced ability to establish hydrogen bonds between polymer chains of adjacent particles (see Stelte [51]). The lack of moisture increases the glass transition temperature and this reduces the possibility to form solid bridges. A way to solve these problems is to find proper binders and additives that would compensate for the loss of hydrogen bonding sites.

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11.4  Pre-treatment of Raw Materials 11.4.1  P  yrolysis, Torrefaction and Hydrothermal Carbonization Lignocellulosic biomass is often difficult to transport and handle due to its low bulk density, that is in the range of (200 to 600)  kg/m3 for woody biomass and (60 to 80) kg/m3 for agricultural material [52, 53]. Thermal pretreatment can facilitate the transportation, handling and storage of biomass [54], but it cannot totally solve the problem because the solid product, biochar or charcoal, is dusty and friable. Pelletization increases mass and energy density of char and produces a uniform shape and size product and reduces dust formation. These processes have gained great interest in the field of bioenergy as an important preprocessing phase, to achieve improved physicochemical properties of biomass for combustion [55]. Thermal pretreatment combined with densification improves storage and logistics for seasonal crops, which are harvested only a few months a year [56]. Thermal pretreatment achieves improved storability and reduces the hydrophobicity of biomass [57]. There are three different thermal treatment processes that can be applied to biomass: pyrolysis, torrefaction and hydrothermal carbonization, also called wet torrefaction at moderate process temperature. Pyrolysis is a thermochemical decomposition process of organic material at elevated temperatures (typically 300 °C to 600 °C) in the absence of oxygen. The process results in the production of three different substances: char (solid product), “non-condensable” gases, such as CO, CO2, CH4 and H2, and condensables, i.e. a liquid oily phase [58]. Depending on the temperature and time of the process, the percentages of the three products change and four different types of processes can be identified: slow, fast, flash and intermediate pyrolysis [59]. Compared to other pyrolysis processes, slow pyrolysis yields as a main product biochar. Low temperature and low heating rate give typically a solid yield of about 25–30% [60]. During torrefaction, or mild pyrolysis, biomass is heated in absence of oxygen to a temperature of (200 to 300) °C, for typically 30  min to 2  h [61]. The torrefaction reactions include: devolatilization and carbonization of hemicelluloses, and depolymerization and devolatilization of lignin and cellulose. Torrefaction of biomass improves pelletability index ratings, decomposing the hemicelluloses and softening the lignin, which results in better binding during pelletization and increases the carbon content and decreases the moisture and oxygen content. Therefore, torrefaction of biomass develops a uniform feedstock with minimum variability in moisture content [62]. Compared to biochar from the higher temperature pyrolysis process, the solid by-product of the torrefaction process contains more volatile organic compounds. Hydrothermal carbonization (HTC) is performed at a temperature of (180 to 260) °C, during which biomass is heated in the presence of water, under pressure of (2 to 6) MPa for typically (5 to 240) min [63]. Hydrochar, the solid by-product, is the main product of the process, with a mass yield of about 40–70%, but the percentage distribution of the final products depends on the process conditions, mainly temperature [64]. In some

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studies, it has been reported that the energy densification ratio, calculated as the ratio of the higher heating value of biochar to the HHV of raw biomass, obtained via torrefaction is lower than that obtained via hydrothermal carbonization [65]. The high energy density of hydrochar, compared to biochar, is due to the rapid decomposition of hemicellulose during the HTC process [65]. The HHV of hemicellulose is considerably lower than the HHV of lignin [66] and the more selective removal of this component leads to a product with higher lignin content and increased energy density, with an increased C:O ratio. The removal of hemicellulose from biomass also improves its hydrophobicity, because hemicellulose has the capacity of adsorbing water [67]. This allows for better and extended self-­storage, with lower risk of biodegradation [68]. The densification of torrefied biomass, biochar and hydrochar have been studied extensively [15]. Hu et al. [15] investigated the influence of pyrolysis processing temperature on the densification of biochar and found that for production of renewable biofuels, the optimum pelletization conditions are 128 MPa compressive pressure, (550 to 650) °C pyrolysis temperature, 35% of water content and 10% of lignin content. The results show how strongly the water content influences the densification process. Mixtures with less than 15%w water content cannot be used and the optimal range is from 20%w to 40%w. Another study [69] indicates that acceptable biochar pellets can be made by adding 20% water and 10% of binders (lignin or calcium hydroxide Ca(OH)2). In this case, the pellet production process had the following benefits: low compressive energy consumption, acceptable moisture uptake and improved mechanical strength. Other binders have been studied by many researchers. Peng et al. [70] report that the use of starch and lignin can improve the durability of biochar pellets, because the starch improves the adhesion forces between biochar particles, while lignin is responsible for the formation of the solid bridges between particles. For torrefied biomass, the densification process under the same operating conditions was more difficult than for raw biomass [71]. To produce torrefied pellets of a similar quality as for wood pellets, it is necessary to process the torrefied sawdust with high pressure (125 MPa to 280 MPa) [72] and high die temperature (170 to 230)  °C [73]. Compared to raw and torrefied pellets, the hydrochar pellets have improved hydrophobicity, improved grindability and reduced inorganic elements content in the ashes, which means better storage and combustion properties [74]. Increasing the HTC process temperature, the mass and energy density of the pellets increases, while the durability decreases [74].

11.4.2  Steam Pre-treatment Steam explosion, which is a technique typically applied to second generation bioethanol production, can be used also as a pre-treatment to produce pellet. The process separates biomass main components (cellulose, hemicellulose and lignin) through both chemical and mechanical separation. Main phenomena are: adiabatic

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expansion of water inside the wood pores and hydrolysis of cell components. Biswas et  al. [75] conducted batch steam explosion tests on Salix. They discovered that steam explosion can increase the heating value of the pellet, due to carbonization and removal of oxygenated compounds. The total amount of ash of the pellet can be decreased due to water leaching of the ashes once the cell structure has been destroyed. A disadvantage could be the decrease of ash fusion temperature. Generally steam treated pellet has also higher density, impact resistance and abrasive resistance. Lam et al. [76] performed steam explosion of Canadian Douglas fir and then, after drying the output material (until 10% moisture was reached), they used it to produce pellet. Steam explosion was performed with saturated steam at the temperatures of 200 °C or 220 °C and a pressure of 1.6 MPa or 2.4 MPa. treatment time was 6 min or 10 min. To identify clearly different combinations of reaction parameters the severity index (R0) has been defined:

0 æ ( T - 100 ) ö R0 = ò exp çç ÷÷ dt t è 14.75 ø 

(11.1)

where t is the residence time (expressed in minutes) and T is the reaction temperature (°C). The experiments of Lam et al. [76] confirmed the results found by Biswas et al. [75] and showed that steam exploded pellet has higher hydrophobicity. Lam [77] also discovered that steam treated biomass requires 12–81% more energy to form durable pellet. The improved durability of steam exploded pellet can be explained with the formation of pseudolignin during steam explosion process.

11.5  Main Parameters Affecting the Pelletization Process For the mechanical aspects of the pelletization process, the most important variable to take into consideration is the pelletizing pressure. This parameter is the one which determines the mechanical quality of the pellet, which is given by: its density and its durability. It is clearly demonstrable that with the increase of the pelletizing pressure, the density of the pellet increases. This is verifiable until a pressure of about 250 MPa is reached. After passing this threshold no sensible increase in the density can be measured, due to approaching the basic density. Besides the pressure, which is an external parameter, other parameters have to be considered: the die temperature (which is an external parameter) and sample moisture content and compression ratio (which are internal parameters and intrinsic to the raw material composition). Water content in particular, according to Tumuluru [78], affects densification in three ways: 1 . it lowers the glass transition temperature; 2. it promotes a solid bridge formation; 3. it increases the contact area of particles through van der Waals forces.

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Mani et  al. [79] observed that moisture acts as a binder during pelletization and increases the bonding via van der Waals forces. The optimal concentration of moisture in the pelletizing material depends on the nature of the material and it is not always the same. Another important parameter affecting biomass pelletization is the temperature of the die. This affects indirectly the water content of the raw material, because it can reduce it through evaporation during pelletization. The temperature of the die affects the state of lignin, which is important for its glass transition. In Fig. 11.1 the influence of pelletizing temperature on pelletizing pressure is shown. The pressure decreases with the increase of die temperature (Fig. 11.1) for the given cases. It is reported that a decrease in friction results from an increase in die temperature and this is greater for hardwood (beech) than for softwood (pine), according to Nielsen et  al. [81]. Besides this, Finell et  al. [82] report that wood extractives (i.e. resins, fatty acids, waxes and sterols) might act as lubricants in the press channel. This effect can be increased for spruce (which contains about 1.8% extractives) compared with beech (which contains about 0.6% extractives), as reported by Stelte et al. [26]. Another process to understand the decrease of pressure is the glass transition of hemicellulose and lignin. In fact, both wood materials show a sudden change in the slope of the curve at 100 °C for beech and 70 °C for spruce. This indicates thermal softening of the polymers. It is possible that pelletization of such softened polymers (i.e. hemicellulose and lignin) results in significantly lower pressures. The transition temperature at which polymers pass from a glassy to a plastic state (Tg) has been determined by Chow [83] and Goring [84] and ranges from (77 to 128) °C for lignin and (54 to 142) °C for hemicellulose, depending on the wood moisture content.

Fig. 11.1  Pressure variation depending on die temperature for different woody biomasses. (Adapted with permission from Stelte et al. [80]. Copyright © 2011 Elsevier)

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The “compression ratio” affects biomass pelletization and is defined as the ratio between the pressures which involve the pellet particle, that are mainly: the radial pressure and the longitudinal pressure. This parameter represents the ratio between the longitudinal and the radial forces. These forces can be expressed using the elastic modulus of the specific material in the directions parallel and perpendicular to the fiber orientation or in terms of Poisson’s ratio (νRL) and the size of the volume element. Starting from the ratio of the forces, shown in Eq. (11.2), the compression ratio is derived, as shown in Eq. (11.3):

(11.2)



AL EL Dr FL A R E R r 2dx E L = = v RL du FR r ER dx

(11.3)



FL PL A L FL r E 2dx E L r = = = = L v RL v RL FR FR 2dx r ER PR 2dx E R AR

where E is a constant of proportionality known as the modulus of elasticity of Young’s modulus and it is typical of the substrate. Both Eqs. (11.2) and (11.3) represent an application of Hooke’s law to an infinitesimal volume element of the pellet cylinder and are taken from the work of Holm et al. [85, 86], which has provided a pioneer model for biomass pelletization. Once the way to define the pressure acting on the pelletizing channel has been decided, this model can be used to describe the function of the pelletizer, as reported in Xia et  al. [87] and Chłopec et  al. [88]. The approach to model the pelletizer depends on the type of operation (flat die or ring die) or configuration (fixed die or moving die) or equipment (moving rollers).

11.6  Modeling Flat Die and Ring Die Pelletizers 11.6.1  Modeling of Flat Die Pelletizer A scheme of the forces acting on a flat die pelletizer is shown in Fig. 11.2. The pellet dies are usually made of alloy steel grades, such as X46Cr13, 20 MnCr5 and 18NiCrMo5. The last two steel grades offer better performances when dealing with materials with hardness above 65 HB.  The selection of the die is based on the knowledge of the properties of the material to be compacted as well of the properties of the final product. The rolls are typically made of alloy steel grades NCI ILV. They are quenched until the hardness of 58–60 HRC is reached. The average lifetime of the rolls is about 500 working hours.

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Fig. 11.2  Force balance in a flat die pelletizer. (Redrawn from [88], with permission of dr. Tomasz Dzik and prof. Marek Hryniewicz)

Industrial pellet presses use dies with a diameter comprised between 175 mm and 1250  mm. The perforated surface areas of the meshes are between 0.06 and 0.59 m2 and the mesh ratio is between 0.38 and 0.60. The factor to calculate the installed power based on the perforated surface of the die falls within the limits of (0.002 to 0.0067)  W/m2. The diameters of the rolls are between 175 mm and 400 mm, while their width ranges from 70 mm to 190 mm. A significant mathematical problem is to find the friction force momentum that acts on the contact surface between the roller and the material to be compacted. In Fig. 11.2, the symbol M stands for the momentum, while Q and F are the forces that act on the center of the roller, T is the point of contact between the material and the roller, N is the force exercised by the material onto the roller and is expressed by Eq. (11.4):

N = pr ´ a ´ B

(11.4)

pr is the average force acting on the contact surface between the roller and the material, a is the length of the chord that corresponds to the arc of nip, b is the roller width. The friction coefficient, μ, between the roller and the material can be expressed based on the value of the friction force:

m = Ff / ( pr ´ a ´ B )

(11.5)



Equation (11.5) is correct when the roller surface is plain. The coefficient of external friction can be increased by the application of special profiles of corrugation to the roller surfaces. The momentum for the force of external friction can be determined with the following equation:

M Ff = pr ´ a ´ B ´ mz ´ R



(11.6)

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The presented model assumes that pr determination is a separate problem, in fact pr can be determined with a force balance on the pelletizing channel, as reported in Sect. 11.6.3.

11.6.2  Modeling of Ring Die Pelletizer The active component of a flat die pelletizing machine is the die, while the active component in a ring die pelletizing machine is the roller. From Fig. 11.3 it can be seen that the forces acting on the roller include F and M which are the forces that the shaft acts on the center of the roller; N1 and N2 represent the pressure generated on the surface of the roller and on the two opposed friction forces F1 and F2, and are attributed to the material compaction during compression and extrusion. Once the material is completely filled into the die holes, the forming process can be divided in two phases: the compression stage and the extrusion stage. The compression stage is the phase in which the material fills the cavity and gradually becomes compacted. The pressure of the roller increases quickly and the biomass material is pushed into a smaller region by the frictional force of the roller surface. Material density increases in this phase until reaching a constant value; then the extrusion phase begins. The material will not be further compacted and the applied pressure is maintained constant and only the shape changes. The partition of the areas is shown in Figure 11.3, as an extrusion zone from 0 to α1 and as a compression zone from α1 to αmax. Where α1 is the wrap angle in correspondence of K1 and αmax is the wrap angle in correspondence of K.

Fig. 11.3  Force balance in a ring die pelletizer. (Redrawn from [87], with permission of dr. Yu Sun)

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Assuming that the material starts compressing at point K and reaches required density ρ1 at point K1, the material height is h1 and the wrap angle is α1. Given the previously cited data, the value of α1 can be obtained using Eq. (11.7):



é r0 h max æ r0 h max öù - 2l ÷ ú ê ç r rr r rr øú a1 = arccos ê1 - 1 è 1 ú ê 2 - 2l ú ê úû êë

(11.7)

As shown in Fig. 11.3, the directions of friction forces are contrary in the two areas for the following reason: the roller spins around point O driven by the spindle, with the biomass material generating the friction force F2 to prevent the roller from turning. Under the action of F2 the roller rotates, grabbing the biomass material and generating a frictional force F1 opposite to the rotational direction. The force balance is shown in Fig. 11.3. According to the momentum equilibrium condition, the momentum of each force to point O1 is zero. Then, it can be concluded that F1=F2. This means that the frictional force value of the compression and extrusion forming zones are equal. To calculate the driving torque it has to be taken into account that in the compression zone, the stress σαx increases from zero to the maximum value gradually, while in the extrusion zone σmax remains constant. To determine the value of the N1 force and the N2 force, analysis of a small segment of the roller has to be performed. Assuming that the stress is σαx, then the pressure force dN acting on the roller can be expressed by Eq. (11.8).

dN = s a x ´ B ´ rr ´ da

(11.8)

where σαx is the stress in the compression zone, B is the roller effective working width, rr is the radius of the roller and α is the central angle of the resultant force. The quantities αN1 and αN2 can be determined by Eqs. (11.9) and (11.10) as presented below.

a N2 = a1 / 2



(11.9)

In Eq. (11.10) αN1 can be calculated as follows:

a N1

a1

a1

a a1

a a1

a

a

ò a dN = ò = ò dN ò

as a x da ds a x da

(11.10)

Due to the curvature of the working surface of the roller, dN should be decomposed along the αN1 and αN2 directions, for solving the resultant forces N1 and N2.

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N1 and N2 can be then calculated through the following equations taken from [89]. N1 = N2 =

a N1

a1

a max

a N1

ò s a x cos (a - a N1) Brda +

aN 2

ò s ma x cos (a - a N 2 ) Brda +

a1



òs

ax

cos (a N1 - a ) Brda

0

òs

(11.11)

ma x

cos (a N 2 - a ) Brda

aN 2

(11.12)

The momentum balance is presented in Eq. (11.13):

F ( R - rr ) + F1 éë r + ( R - rr ) cos a N1 ùû - F2 éë rr + ( R - rr ) cos a N 2 ùû -



N1 sin a N1 ( R - rr ) - N 2 sin a N 2 ( R - rr ) = 0

(11.13)



Since F1=F2 and F2=N2*f, F can be determined by Eq. (11.14): F = N 2 ´ f ´ ( cos a N 2 - cos a N1 ) + N1 sin a N1 + N1 sin a N 2





(11.14)

The driving torque is calculated from Eq. (11.15): T = 2 F ( R - rr )



(11.15)



To determine the value of torque T, the solution of σαx is needed. This can be determined in two ways: 1 . a mechanical model based on material compaction; 2. a calculation model based on die-hole pressure. The mechanical models based on material compaction are listed in Table 11.4. Using the mechanical model 4 (Table 11.4), σax can be calculated by:

s a x = Ae



B( h max - ha )

(11.16)



Table 11.4  Mechanical models [87] Number 1

Scholar name Skalweit

Calculation model

2

Faborode

3

Panelli-Filho

æ 1 ö ln ç ÷ = b+A P è 1- g ø

[92]

4

Jianjun Hu

P = Aebx

[93]

P P = 0b g b g0 P=

Ag 0 é b(g -1) ù e -1 û b ë

Refs. [90] [91]

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The mechanical model 2 (Table 11.4) provides an effective equation to calculate the die-hole pressure. The pressure σmax at the inlet of the die hole can therefore be calculated by Eq. (11.17).



s max = s L =

s 0 2´f ´n ´L / rh s 0 e n n

(11.17)

where ν is Poisson’s ratio; σ0 is pre-stressing pressure; f is friction coefficient; rh is the die hole diameter and L is the die hole length.

11.6.3  Modeling a Single Pelletization Channel The state of the art model on pelletization has been developed by Aalborg University and Andritz Feed & Biofuel and it is adapted to the pelletization of charcoal and wood pellets [94]. The model is based on a single pelletization channel. The densification process is simulated only in one direction, which corresponds to the axial direction of the pelletization channel. The analyzed forces are: –– –– –– ––

Frad: force due to radial expansion of the material; Fμ: friction force acting on the surface of the channel; Ftop: force of the roller on the material to be pelletized; Fout: force at the exit of the material.

To solve the force balance, illustrated in Fig. 11.4, the model starts from the exit condition: Fout=Fres and calculates Ftop based on Eq. (11.18). Ftop =

Fout ö æ E L 2L 1 - ç · ·n RL ·m ÷ çE r ÷ è R p ø

(11.18)

Fig. 11.4  Force balance in the pellet channel. (Redrawn from [94] with permission of dr. Simon Klinge Nielsen)

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For the next element, the value of Fres is the last value calculated for Ftop. The iterative calculation process re-starts for each element of the pelletizing channel moving to the top, until the final value of Ftop is found. This represents the maximum and is due to the action of the rollers which push the biomass inside the pelletizing channel. Once the force acting on the top of the material has been calculated, the pressure is derived from Eq. (11.19):



ö 1 æ Ftop P= ç + 2·Fm ÷ 3 è AL ø

(11.19)

Integrating the pressure on the infinitesimal volumes of the pellet material in the pellet channel, the compression work can be found. Pressure is also correlated with the density of the material through an empirical equation. To calculate the velocity of the material in the pellet channel, the law of mass conservation has been applied. vP =

m r P · AL

(11.20)

The velocity of the material can be used to calculate the specific work of the friction force ws _ u. 



ws _ m =

Wm  m

=

Fm .vP 3600.m



(11.21)

The specific work of compression ws _ c is derived using the pressure calculated for the volumes Vs.1 and Vs.2, which are the minimum and maximum compression volumes: ws _ c =

Vs .1

ò PdV

s

Vs .2



(11.22)

11.7  A  pplying Single Pellet Channel Model to Charcoal and Wood Pelletization The aim of the above described model, in this case, is to examine pelletizing performance of softwood and biochar, through the comparison of the following parameters: –– –– –– –– –– ––

pelletizing force; pressure on biomass; biomass velocity; biomass density; compression work and friction work; specific compression and friction works.

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Channel geometry parameters are: –– –– –– –– ––

inlet height z = 1 mm; inlet angle θ = 70°; channel length L = 50 mm; channel diameter D = 6 mm; biomass flow ṁ = 0.000211 kg/s

Material parameters have been taken from [95, 96] and are: –– ratio between the longitudinal and radial elastic modules of spruce fir wood: Gs =

EL = 20.7 ER

(11.23)

–– ratio between the longitudinal and radial elastic modules of biochar Gb =

EL = 4.06 ER

(11.24)

–– friction coefficient μ = 0.4 between spruce fir wood and the steel of the die (AISI stainless steel 420) –– friction coefficient μ = 0.6 between biochar and the steel of the die (AISI stainless steel 420). Assuming a moisture content of the biochar equal to 30%w/w; –– Poisson coefficient for spruce fir wood: νs = 0.025 –– Poisson coefficient for charcoal: νc = 0.08 In Fig. 11.5a the compression force is shown with a continuous line, the geometry of the channel is shown with a dotted line. The value of the exerted force decreases while the material flows through the channel. The maximum value of the exerted force is at the inlet and it is equal to Fmax = 9600 N. The force is reduced to about half of the initial force after the first 5 mm of channel length. Figure 11.5b shows the trend of pressure and density for the spruce fir wood pellet. In particular the pressure starts from 30 MPa, increases in the first part of the pellet channel, due to the compression action. Once the maximum peak of 102.5 MPa has been reached, the pressure decreases along the pellet channel, due to the decrease of the compression work. The density is indicated with a dotted line and it has an immediate increase in the first part of the compressing channel, in correspondence of the initial chamfer (that is made with a conical shape to facilitate biomass entering the channel). The density of the pellet presents a different trend, compared to pressure, it increases until it reaches the maximum value of 1512 kg/m3. Then it remains constant. In Fig. 11.6, the trends of the compression force, of the pressure and of the density of a biochar pellet are shown. In this case, the compression force is about 6413  N with a decrease of 33.1%, compared to the case of spruce fir wood. Consequently, the pressure has a maximum peak which is about 68.94 MPa, with a decrease compared to the value of the spruce fir wood of about 32.7%.

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Fig. 11.5  Spruce fir wood: (a) Compression force and die geometry section; (b) Pressure and density

Fig. 11.6  Biochar: compression force and die geometry section (a); Pressure and density (b)

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The density of the biochar pellet has a similar trend as the spruce fir wood pellet, but has a lower maximum value of =1340 kg/m3. In this case the reduction of the final value, compared to the spruce fir wood pellet, is about 11.38%. In Fig.  11.7, the trends of pellet velocity inside the compression channel are shown.

Fig. 11.7  Velocity of spruce fir wood pellet (a); velocity of biochar pellet (b)

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In agreement with Eq. (11.5) the velocity of the material is mainly influenced by its density and by the section crossed by the material. In particular, at the inlet, the density increases while the section area decreases. That implies an increase also in the velocity, which reaches its maximum value. For the spruce fir wood pellet the maximum velocity is about 4.8 × 10−3  m/s, while for biochar pellet it is slightly higher and equal to 5.6 × 10−3 m/s. In both cases the pellet velocity is stabilized at the maximum value along the length of the channel, which is due to the density being constant and to the area of the section being constant. In Fig. 11.8, the consumption of energy inside the pelletizing channel is shown. In particular, Fig. 11.8a shows how much energy that is consumed for compression and how much for friction, while Fig. 11.8b shows the cumulated consumption of energy along the pelletizing channel. At the entrance of the channel the two peaks of energy consumption are attributed to: one for compression and one for friction. They are equal respectively to 2.31·10−3  J and 1.87·10−3  J.  The compression energy then decreases until a null value is achieved, and the friction energy decreases less rapidly and reaches the null value only after 70% of the total length of the channel is reached. Cumulated energy reaches a maximum of 0.15 J. 60% of the total value is reached after the first 5 mm of the pellet channel being passed. The energy required to pelletize biochar is shown in Fig. 11.9. The trends are similar to those of the energy required to pelletize spruce fir wood. The total cumulate energy is equal to 0.1 J, it is 67% of the energy required for the spruce fir wood pellet.

Fig. 11.8  Spruce fir wood: Compression energy and friction energy (a); cumulate pelletization energy required (b)

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Fig. 11.9  Biochar: Compression energy and friction energy (a); cumulate pelletization energy required (b)

Fig. 11.10  Density gradient for spruce fir wood pellet (a); density gradient for biochar pellet (b)

In Fig. 11.10, the gradients of the density of the pellet produced from spruce fir wood and biochar are shown. For both materials, two characteristic areas are clearly recognizable: (i) the inlet: it is the zone where the material undergoes compression and increases its density value, until the maximum density is reached;

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Fig. 11.11  Samples of spruce fir wood pellet (a); Samples of biochar pellets (b)

(ii) pelletizing channel: it is the zone with constant pressure, in which the density does not change and remains equal to the constant maximum. The density of the two materials is respectively equal to: • spruce fir wood pellet: ρmax = 1512 kg/m3 • biochar pellet: ρmax = 1340 kg/m3 To verify the simulation data, experimental pelletization tests were performed on a laboratory scale in the Biomass Research Centre labs of the University of Perugia, using a SMARTEC PLT 100 pelletizing machine, with a flat die. The density of the pellets obtained from spruce fir wood and biochar (Fig. 11.11) has been measured, and also their durability, using a Holmen Tester TekPro. Both pellet samples have a durability higher than 99%, so that both can be commercialized according the standard ENPLUS A1. Pellet density was calculated as follows:

rP =

mP mP = VP p ·rP2 ·l P

(

)

(11.25)

where ρP is the pellet density [kg/m3], mP is the pellet mass; VP is the pellet volume, calculated as the volume of the cylinder of measured radius and length; rP is the measured pellet radius and lP si the meaured pellet length. To describe the difference between the measured density and the simulated value the following error was considered: %err =

rav, P - rsim ravm

•100

(11.26)

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Table 11.5  Calculation of the average density of spruce fir wood pellet and estimate of the error between measured value and simulated value Average values

mP [g] 1.03

rP [mm] 3

lP [mm] 26.5

ρmed, P [kg/m3] 1376

ρsim [kg/m3] 1512

%err +9.86%

Table 11.6  Calculation of the average density of biochar pellet and estimate of the error between measured value and simulated value Average values

mP [g] 0.79

rP [mm] 3

lP [mm] 23.09

ρmed, P [kg/m3] 1213

ρsim [kg/m3] 1340

%err +10.44%

where ρavm is the average measured pellet density [kg/m3] and ρsim is the calculated pellet density [kg/m3]. Measurements were performed on 30 samples of spruce fir wood and biochar. An average of all the measurements has been calculated and these values have been used as an input for Eq. (11.25). The results of the comparison between the two densities (Tables 11.5 and 11.6) show that the difference between the measured values and the simulated values is not very big. Nevertheless, the precision of the model should be increased taking into account for example the temperature of the die. In general, the model is too “ideal” and overestimates the density in both cases: for the spruce fir wood and the biochar. The obtained error is acceptable given that the machine is not a commercial pelletizer.

11.8  Conclusions This chapter has described parameters that influence biomass pelletization, with emphasis on the mechanism of the pelletization process, and its performance. Raw materials’ and binders’ characteristics have been evaluated to better understand the importance of each parameter and their role in the densification process. The effect of biomass moisture content, the temperature of the die and the use of binders has been discussed in depth. While for sawdust, the initial moisture can be around 10–15%, for mixtures of biochar and various binders the moisture should be around 30–40% in weight. The mechanical forces acting during pelletization are described for two cases: flat die and ring die. In particular, for the ring die, it has been shown in detail how the solution of the force balance passes through the identification of a law which relates the mechanical stress to the pressure exerted by the rolls on the die. Modelling of this aspect with empirical equations allows solution of the entire balance of forces. This is also the approach of the model presented in this chapter and used to calculate the energy consumption for pelletizing biochar. The results show a general reduction of the energy requirement for biochar compared to spruce fir wood. The

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reduction in the consumption of energy is directly linked to the reduction of the process pressure, which in the end implies also a reduced density of the material at the exit of the machine. The reduced density of biochar (about 1340  kg/m3), compared to spruce fir wood (1512 kg/m3) has been confirmed by experimental tests performed to verify the model. Pelletization of biomass is a key technology when this fuel is used to produce heat and power, given the diffusion of CHP plants fed with pellets. In fact biomass pellets can give more stable conditions also during the thermal degradation process (i.e. combustion, gasification). For this reason the technology appears to have a promising market and can be used with different materials as feedstock (e.g. biochar, agricultural waste and herbaceous crops) and as additives (see for example pyrolysis oils used as a binder). Acknowledgments  The authors would like to thank eng. Michele Oligarchi and eng. Micro Cesca for the help during pelletizing tests and simulation running. The authors acknowledge the financial support by the Research Council of Norway and a number of industrial partners through the project BioCarb+ (“Enabling the biocarbon value chain for energy”). The authors would like to acknowledge the help of dr. Tomasz Dzik and prof. Marek Hryniewicz from AGH University of Science and Technology, Cracow, Poland. The authors would like to acknowledge the help of dr. Yu Sun from Nanjing University of Science and Technology, China. The authors would like to acknowledge the help of professor Mathias Mandø from Department of energy Aalborg University, Denmark and dr. Andreas Brinch Rosenørn and dr. Simon Klinge Nielsen from Andritz Feed and Biofuel. The authors would like to acknowledge the help of eng. Lorenzo Riva from University of Agder, Norway. The authors want also to acknowledge the help of reviewers.

References 1. Saidur R, Abdelaziz EA, Demirbas A, Hossain MS, Mekhilef S (2011) A review on biomass as a fuel for boilers. Renew Sust Energ Rev 15(5):2262–2289 2. Kambo HS, Dutta A (2015) Comparative evaluation of torrefaction and hydrothermal carbonization of lignocellulosic biomass for the production of solid biofuel. Energy Convers Manag 105:746–755 3. Nunes LJR, Matias JCO, Catalão JPS (2014) A review on torrefied biomass pellets as a sustainable alternative to coal in power generation. Renew Sust Energ Rev 40:153–160 4. Tarasov D, Shahi C, Leitch M (2013) Effect of additives on wood pellet physical and thermal characteristics: a review. ISRN For 2013:1–6 5. Tumuluru J, Wright C, Kenny K, Hess R, (2010) A review on biomass densification technologies for energy application. 1–59 6. Kaliyan N, Morey RV (2009) Factors affecting strength and durability of densified biomass products. Biomass Bioenergy 33(3):337–359 7. Whittaker C, Shield I (2017) Factors affecting wood, energy grass and straw pellet durability – a review. Renew Sust Energ Rev 71:1–11 8. Mola-Yudego B, Selkimäki M, González-Olabarria JR (2014) Spatial analysis of the wood pellet production for energy in Europe. Renew Energy 63:76–83 9. IEA BIoenergy Global wood pellet industry and trade study 2017. http://task40.ieabioenergy. com/wp-content/uploads/2013/09/IEA-Wood-Pellet-Study_final-july-2017.pdf

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Part V

Biosynthesis of Polymers from Renewable Biomass

Chapter 12

Microbial Production and Properties of LA-based Polymers and Oligomers from Renewable Feedstock John Masani Nduko and Seiichi Taguchi

Abstract  Most plastics, materials, fuels and other organic chemicals are presently derived from fossil fuel feedstocks. Due to the finite nature and foreseeable depletion potential of these raw materials, concerted efforts are being explored to find sustainable alternatives to the fossil fuel feedstock-derived products. Among these, bioplastics and oligomers derived from fermentation of the renewable plant biomass are promising candidates to replace fossil-fuel-derived plastics. Bioplastics are a class of storage polymers synthesized by microorganisms. Natural plastics can also be produced via a bio-chemo process that combines fermentative production of monomers or oligomers, followed by a chemical synthesis process to produce a variety of polymers. These polymers, particularly polyhydroxyalkanoates (PHAs) represent futuristic biomaterials owing to their biodegradability and biocompatibility. Furthermore, PHAs have physicochemical properties that are similar to petrochemical-­based plastics hence their potential replacement. Designing efficient processes holds the key towards their adoption. This chapter discusses opportunities and challenges regarding the production of lactic acid (LA)-based polymers and related oligomers that can act as precursors for catalytic synthesis of polylactic acid (PLA). It covers crucial steps of their production using genetically modified organisms and engineered enzymes as well as providing future developments.

J. M. Nduko Department of Dairy and Food Science and Technology, Faculty of Agriculture, Egerton University, Egerton, Kenya e-mail: [email protected] S. Taguchi (*) Department of Chemistry for Life Sciences and Agriculture, Faculty of Life Sciences, Tokyo University of Agriculture, Tokyo, Japan CREST, JST, Tokyo, Japan e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2019 Z. Fang et al. (eds.), Production of Materials from Sustainable Biomass Resources, Biofuels and Biorefineries 9, https://doi.org/10.1007/978-981-13-3768-0_12

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12.1  Introduction Plastics are ubiquitous synthetic polymeric molecules that have become an indispensable part of our daily life [1]. The majority of monomers such as ethylene and propylene used to produce plastics are derived from petrochemical feedstocks [2]. The polymers that are used the most are polyethylene (PE), polypropylene (PP), nylon, and polyvinyl chloride (PVC) with different properties. The main reasons for the broader use of plastics are; they are inexpensive, corrosion-resistant, are strong and durable, are light-weight, and due to the diversity of polymers and the versatility of their properties, they can be molded into a vast array of products for broad applications [3, 4]. As a consequence, the production of plastics worldwide has increased exponentially since 1950 and surpassed 300 million tonnes in the 2010– 2015 time period, and in 2015, over 320 million tonnes were produced [5, 6]. However, plastics are resistant against physical, chemical and microbial attack such that they tend to accumulate rather than decompose in landfills or in the natural environment where they are often improperly discarded [7, 8]. The contamination of the natural environment with plastics is a major global concern; plastics have been found in all ocean basins, fresh water ecosystems and in terrestrial habitats [6]. Landfills for the materials are becoming rare and disposal by incineration may result in the formation of carcinogenic polychlorinated dibenzo-p-dioxins/furans (PCDD/ Fs) and additional toxic, persistent organohalogens thus, escalating problems regarding plastic waste treatment [7, 9]. Due to the destructive effects of petroleum-­ based plastic products on the environment such as the production of greenhouse gases in the industrial production of petroleum-based plastics coupled with the finite nature of the raw materials and predicted increase in prices of plastics [10, 11], public apprehension has increased over a few decades leading to an upsurge in the demand for greener materials with a lower carbon footprint, hence giving impetus to the development of the ecofriendly biodegradable biopolyesters derived from renewable resources as alternatives. Among the renewable polymers, there are polyesters; polyhydroxyalkanoates (PHAs) and polylactic acid (PLA) [12–14]. These materials are considered green polymers with similar physicochemical, thermal, and mechanical properties to polypropylene (PP) and low-density polyethylene (LDPE) [5, 15, 16]. PHAs are a class of natural biodegradable thermoplastics intracellularly accumulated by various microbes as carbon and energy reserves [17]. PHA production occurs when there is deficiency in an essential growth nutrient in the medium, but there is excess carbon. However, there are several bacteria that accumulate PHAs without essential nutrient deficiency [16]. The stored carbon is later utilized by the bacteria as a source of fatty acids and is crucial for the survival of the organisms [18]. More than 150 PHA structures have been identified and all of them are linear polyesters containing 3-hydroxy fatty acid monomers as shown in Fig. 12.1 [19, 20]. Depending on the chain length of the monomeric unit (R), PHAs have been categorized into three classes:

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12  Microbial Production and Properties of LA-based Polymers and Oligomers… Fig. 12.1  Chemical structure of PHAs; R=Methyl (C1) to tridecyl (C13)

O

R CH

(CH2)

C

n

(i) Short-chain-length PHAs (scl-PHAs) that consist of three to five carbon atoms such as polyhydroxybutyrate [P(3HB)] and P(3HB-co-3-hydroxyvalerate (3HV) and are obtained from several bacteria such as Cupriavidus necator and Alcaligenes latus [8, 17, 21, 22]. The SCL-PHAs exhibit thermoplastic material properties similar to polypropylene [23]. (ii) Medium-chain-length PHAs (mcl-PHAs) that consist of 6–14 carbon atoms in the chain. The mcl-PHAs are obtained from several bacteria such as Pseudomonas putida and Pseudomonas mendocina and possess elastic material properties similar to rubber [12, 24, 25]. (iii) Long chain PHAs having carbon atoms greater than 14, which have been reported in bacteria including Shewanella oneidensis and Aureispira marina [21]. Interestingly, some microorganisms have been reported to accumulate PHAs that have both SCL and MCL-monomers that display material properties analogous to low density polyethylene [20, 24, 26]. Among the PHAs, poly-3-hydroxybutyrate P(3HB) is the most characterized scl-PHA that is brittle and has difficult processability because of its crystalline nature [16]. To improve its properties, formation of copolymers with PHB has been attempted. In effect, incorporation of 3-­hydroxyvalerate (HV) into PHB to form a copolymer; poly(3-­hydroxybutyrate-­ co-3-hydroxyvalerate) [P(3-HB-co-3HV)] makes the material tough, more flexible and gives it broader thermal processing properties [27]. Polylactic acid (PLA) is an aliphatic polyester derived from 100% renewable carbohydrate-rich resources, such as corn, cane sugar, and sugar beets that is highly versatile and biodegradable. PLA is a thermoplastic, high-strength, and high-­ modulus polymer that yields materials with potential use in industrial packaging or as biocompatible medical devices [28, 29]. PLA has good processability and can use the conventional plastic equipment for moulded parts, films or fibers. PLA has a stereochemical structure that is amenable to modification by polymerizing both the L-or D-isomers to produce high-molecular-weight crystalline or amorphous polymers that can be used in food contact surfaces due to their ‘generally recognized as safe (GRAS)’ status and excellent organoleptic characteristics [30, 31]. PLA is mostly prepared by the ring-opening polymerization (ROP) of the cyclic lactide dimer although it can also be synthesized by the direct condensation of lactic acid. Most of the PLA is produced by ROP of lactides to obtain high molecular weight PLA using catalysts such as tin [28]. In ROP, lactic acid is first produced by fermentation of carbohydrates by microorganisms, purified from the fermentation medium by calcium salt precipitation, then followed by acidification that generates huge amounts of calcium salts as a byproduct [32]. The purified lactic acid is then condensed at high temperatures to produce lactate oligomers, that are d­ epolymerized

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to lactides, which are then polymerized to generate high molocelar weight PLA [33]. Although, this process has gained prominence for PLA synthesis, the multistep chemo-bio process is complex, rendering the ultimate products more expensive with regard to their petroleum-derived counterparts [33]. The production of lactide from lactic acid alone contributes up to 30% of the PLA cost and ring-opening polymerization reaction requires harmful chemical catalysts, which hampers the economic and environmental impact of the material [34, 35]. The establishment of a single-pot bioprocess for PLA synthesis analogous to PHAs or the microbial synthesis of lactate oligomers, followed by lactide synthesis and subsequent ring-opening polymerization could overcome the challenges with the use of metal catalysts and purification and oligomerization of lactic acid; hence the cost of production can be reduced [36, 37]. In the year 2008, a whole cell biosynthesis system for P(LA-co-3HB) polyester production without the use of heavy metal catalysts was constructed in engineered bacteria [36]. In addition, a system for the synthesis and secretion of lactate oligomers by microorganisms has been established [37], thereby shortening the process for PLA synthesis. This chapter reviews the literature on the production of polymeric materials including P(LA-co-3HB) copolymers and lactate oligomers using microbes. The chapter has discussions on the properties and degradation of the LA-based polymers, the use of lignocellulosic biomass as inexpensive substrates for polymer and oligomer production and the enabling technologies such as metabolic engineering, protein engineering, and genetic engineering and fermentation media manipulation strategies for the production of the renewable polymers.

12.2  B  iomass-Derived Biopolyesters: Polyhydroxyalkanoates (PHAs) and Polylactic Acid (PLA) The ecofriendly biopolyesters expected to replace conventional petrochemical-­ derived plastics are polyhydroxyalkanoates (PHAs) and polylactic acid (PLA) that have similar physicochemical, thermal, and mechanical properties as polypropylene (PP) and low-density polyethylene (LDPE) [5, 15, 19, 38].

12.2.1  Polyhydroxyalkanoates (PHAs) Polyhydroxyalkanoates (PHAs) are the largest group of biopolyesters with over 150 different types of hydroxyalkanoic acids reported as monomers [39]. The PHAs are produced by both native and recombinant microorganisms via fermentation and are biodegradable. They comprise of a big family ranging from brittle plastics to elastomers to rubbers depending on monomer composition. They are mainly composed of 3-, 4-, 5-, and 6- (R)-hydroxyalkanoic acids. PHAs are synthesized intracellularly and accumulated as hydrophobic inclusions and then they are recovered from cell

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biomass by extraction using organic solvents such as chloroform. However, PHAs can be purified efficiently by mild treatments with alkaline solutions from organisms such as recombinant E. coli that accumulates over 85% of the cell biomass [22].

12.2.2  Polylactic Acid (PLA) PLA is produced by chemical polymerization of fermentative lactic acid (chemo-­bio synthesis), while PHAs are polymerized by enzymes and accumulated as intracellular granules by microbes as carbon and energy storage compounds [19, 38]. PLA is a benign linear aliphatic polyester entirely produced from renewable resources such as corn starch and is readily biodegradable [28, 40]. The initial applications of PLA were limited to biomedical uses such as sutures and drug delivery systems due to high production costs [35, 41]. However, cost-effective large scale operations for the production of PLA have been established hence availing PLA for packaging and fiber applications [42]. PLA can be formed by either direct condensation of lactic acid or by ring opening polymerization (ROP) via the cyclic intermediate dimer (lactide) (Fig. 12.2). Production of PLA by polycondensation approach was used by Carothers who discovered PLA in 1932. The process involves the removal of water by condensation and use of solvent under high vacuum and temperature. However, only low to intermediate molecular weight PLA polymers could be obtained with Sugar Microbial fermentation D-Lactic acid O

Lactic acid purification

HO

OH

Polycondensation O

O

O ROP

Depolymerization O

O

O

x

O

n

O D-LA oligomer

D-Lactide

Poly(D-Lactate)

Fig. 12.2  Scheme of conventional route for PLA production via the ring opening polymerization (ROP) chemistry

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this method due to difficulties in removing water and other impurities. The process also requires a large reactor, there is need for evaporation, solvent recovery and increased color and racemisation, hence there has been focus on ROP. Mitsui Toatsu chemicals developed and patented an azeotropic distillation process using a high boiling solvent to drive water removal in the direct esterification process to obtain high molecular weight PLA [43]. This process has been adapted commercially and NatureWorks LLC, the major global producer of PLA [43], synthesizes high-molecular weight PLA via ring-opening polymerization (ROP). Using ROP coupled with advances in lactic acid production through cost-­effective fermentation, the cost of producing PLA has gone down [42]. Although ROP chemistry has improved industrially and life-cycle assessments and comparisons with equivalent fossil-sourced plastics being positive, the multistep chemo-bio process is still considered to be complex and expensive relative to petroleum-based plastics production process [35, 44–46]. The production of lactide from lactic acid involves synthesis of oligomers, lactides that comprise many purification steps including esterification–distillation– hydrolysis procedure [44]. This latter procedure alone contributes up to 30% of the cost of PLA, which hampers its applications [34, 47].

12.2.3  The First Microbial Factory of LA-Based Polymers Unlike PLA, PHAs are wholly synthesized in vivo via a two-step biosynthetic pathway of monomer supply and polymerization [17, 48]. A typical PHA is polyhydroxybutyrate [P(3HB)], whose biosynthetic pathway is shown in Fig. 12.3, and is extensively investigated in Ralstonia eutropha. In R. eutropha, two molecules of acetyl-CoA are coupled to each other by a 3-ketothiolase (PhaA) to generate acetoacetyl-­CoA.  Acetoacetyl-CoA is then reduced to 3-hydroxybutyryl-CoA by NADPH-dependent acetoacetyl-CoA reductase(PhaB), which can then be polymerized by a PHA synthase (PhaC) to form P(3HB) [17, 49–51]. Renewable Carbohydrates Glycolysis Acetyl-CoA PhaA

HS-CoA

Acetoacetyl-CoA

PhaB

3-hydroxybutyryl-CoA

NADPH NADP+

PhaC

P(3-HB)

HS-CoA

Fig. 12.3  Biosynthetic pathway for polyhydroxybutyrate [P(3HB)] in Ralstonia eutropha. PhaA β-ketothiolase, PhaB NADPH-dependent acetoacetyl-CoA reductase, PhaC polyhydroxyalkanoate (PHA) syynthase

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The establishment of a single-pot bioprocess for PLA synthesis analogous to PHAs will overcome the need for the use of metal catalysts and purification of lactic acid; hence the cost of production will possibly be reduced. In 2008, an ‘LA-polymerizing enzyme (LPE)’, functioning as an alternative to a metal catalyst was discovered and enabled the establishment of a whole cell bioprocess for the production of the LA-based polyester, poly(lactate-co-3-hydroxybutyrate) [P(LA-­ co-­3HB)] in engineered bacteria (Fig. 12.4) [36, 46]. Lactic acid is an uncommon monomer unit to PHA synthases, and microbially synthesized LA-based polymers had not been reported before. Essentially, polymerization of LA unit was achieved after the discovery of a Ser325Thr/Glu481Lys mutant of PHA synthase from Pseudomonas sp. 61-3 [PhaC1PS(ST/QK)] that had acquired LA-polymerizing activity. Due to molecule similarity, LPE could incorporate LA unit together with 3 HB to form P(LA-co-3HB) [12, 13]. This discovery laid the foundation for the advancement of the system to polymerize varied monomers including LA, glycolate and 2-hydroxybutyrate (2HB) [52].

12.2.4  Production of LA-Based Polymers The first LA-based polymer [P(LA-co-3HB)] synthesized by microorganisms using glucose had 6 mol% of LA incorporated into the polymer [36]. The composition, ratio and distribution of the monomeric constituents inside the molecule chain are Fig. 12.4  The biosynthetic pathway for the LA-based polyester in E. coli. PCT propionyl-CoA transferase, PhaA β-ketothiolase, PhaB NADPH-dependent acetoacetyl-CoA reductase, LPE Lactate (LA)polymerizing enzyme

Glucose

Pyruvate Lactate

Acetyl-CoA PhaA

PCT

Acetoacetyl-CoA

PhaB 3-Hydroxybutyryl-CoA

Lactyl-CoA LPE

O

O O O

n P(LA-co-3HB)

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known to affect chemical, thermal, optical or mechanical properties of the copolymer due to the altered crystallinity and melting temperatures (Tm) [53, 54]. The P(6 mol% LA-co-94 mol% 3HB) was found to have reduced melting temperature (Tm) compared to P(3HB) and PLA homopolymers [36]. The P(LA-co-3HB) properties can be altered by varying the monomeric units to obtain new materials [52, 55–57]. To develop a system for the varied LA-based polymer production, the first experiments were conducted in vitro using a chemo-enzymatic arrangement [58]. In this system, P(LA-co-3HB) containing 36 mol% of LA was achieved. Under this arrangement, the monomer units; LA-CoA and 3HB-CoA were supplied in equimolar quantities [58]. The supply of lactic acid or LA-CoA was limiting LA incorporation into P(LA-co-3HB) in the in vivo system using E. coli [36], suggesting that native bacteria could be rewired to synthesize LA-enriched polymers/PLA in a similar way as the PHAs. Yamada et  al. [56] employed a ∆pflA mutant (E. coli JW0885) that carries a mutation for the pyruvate formate lyase activator protein (pflA−) in an attempt to improve lactic acid in E. coli so as to reinforce LA incorporation into the copolymer. Employing this mutant expressing the same set of enzymes responsible for P(LA-­ co-­3HB) synthesis under similar culture conditions as the parent strain resulted in an increase of LA into P(LA-co-3HB) from the initial 6 to 26 mol% [56]. These results confirmed that metabolic engineering strategies could be effectively employed to channel the carbon flux towards lactic acid overproduction thus enriching the LA fraction in the P(LA-co-3HB) copolymer. Although this strategy was effective, in E. coli, pyruvate is converted into acetyl-CoA under aerobic conditions but into lactic acid under anaerobic conditions. It was then evident that for improved LA incorporation into P(LA-co-3HB), recombinant E. coli JW0885 cells had to be cultivated under anaerobic conditions. When this growing condition was attempted using glucose, the LA fraction in P(LA-co-3HB) improved to 47 mol% [56], suggesting that high lactic acid production under anaerobic conditions was effective in raising the LA ratio in the copolymers. Although there were improvements in the LA fractions in P(LA-co-3HB) under anaerobic conditions, the polymer content reduced significantly giving less than 2% of the total weight of the dry cells, implying that further advances were necessary to improve LA fractions without necessarily affecting polymer content in cells. In subsequent attempts, enzyme engineering of LPE was employed as a strategy to improve the LA fraction and polymer productivity. This was demonstrated whereby E. coli JW0885 expressing an evolved LPE (eLPE) and other requisite enzymes were expressed to produce P(LA-co-3HB). Under aerobic conditions, these cells produced P(LA-co-3HB) with 47 mol% LA and the polymer content was 62% from glucose as the carbon source [54]. Cultivating the same under anaerobic conditions produced P(LA-co-3HB) copolymer with 62 mol% LA however, with less than 1% polymer content. These results demonstrated the effect of enzyme evolutionary engineering in finding suitable polymerase mutant with enhanced LA-polymerization capability and high polymer productivity.

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12.2.4.1  I mprovement of LA Fraction in P(LA-co-3HB) by Elimination of 3HB Supply In the pioneering study of Taguchi et  al. [36] when P(6  mol% LA-co-3HB) was produced, it was found that LA-based copolymer is only synthesized in the presence of the co-monomer; 3HB-CoA, which demonstrated that LA polymerization was dependent on 3HB. The NMR analysis demonstrated the existence of LA-LA-LA triad sequence in the copolymer, which suggested LA linking to another LA molecule and thus the potential of synthesizing P(LA-co-3HB) with high LA ratio/PLA-­ like polymers [57]. Since the necessity of 3HB-CoA was demonstrated in the polymerization of LA, Shozui et al. [59] eliminated the 3HB-CoA supplying pathway by removing the PhaA and PhaB enzymes and replacing them with PhaJ that could supply 3-HV-CoA from the extraneously supplemented valerate via the β-oxidation pathway. When recombinant E. coli cells were cultivated and supplied with glucose and 0.4–0.7 g/L of sodium valerate, terpolymers, P(LA-co-3HB-co3HV) comprising very high LA fractions (90–96  mol%) could be obtained [59]. Although 3HB was eliminated, small fractions were detected in the polymer and this could have been due to a small basal amount of (R)-3HB-CoA that intrinsically exists in E. coli in the presence of a fatty acid, probably associated with the enzymes involved in fatty acid metabolism including 3-ketoacyl-ACP reductase (FabG) [60, 61] and 3-ketoacyl-ACP synthase III (FabH) [61, 62]. Thus, the elimination of 3HB is an effective strategy to produce PLA-like polymers although production of 100% PLA was not possible without further evolution of LPE [59]. 12.2.4.2  Production of P(LA-co-3HB) in Corynebacterium glutamicum In early experiments, LA-based polymers incorporating 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), and 3-hydroxyhexanoate (3HHx) had only been established in engineered E. coli [36, 54, 57, 59, 63]. The use of E. coli, a Gram-negative bacterium could present a number of challenges especially if the resultant polymers are targeted for the food-grade and biomedical applications. This is because Gram-­ negative bacteria produce endotoxins and lipopolysaccharides that are potentially harmful [64–66]. To upgrade this prototype to practical scale, an endotoxin-free Gram positive bacterium with GRAS status was sought. Effectively, Corynebacterium glutamicum that is widely used as chassis for industrial production of a number of food-grade bioproducts such amino acids including L-glutamate and L-lysine, feeds, and pharmaceutical products for several decades [67], was selected. The organism had also been used as a host for PHA production [68, 69] hence; its potential for LA-based polymer production was highly likely. Song et al. (55) assessed the potential of this approach and assembled the requisite P(LA-co-3HB) synthesis pathways in C. glutamicum then cultivated the cells on glucose for polymer production [55]. Analyzing the recombinant cells after 72 h of cultivation indicated that P(LA-co-3HB) copolymers having 97 mol% LA were synthesized, which was significantly different from those synthesized by E. coli (47 mol% of LA) expressing

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the same set of genes [53, 55]. The high LA ratios in P(LA-co-3HB) synthesized by C. glutamicum were attributed to low 3HB supply in E. coli, which was supported by the low P(3HB) yield compared to P(LA-co-3HB) yield when the D-LA-CoA supplying enzyme, PCT was omitted from the system. This prompted the researchers to investigate the necessity of 3HB-CoA in P(LA-co-3HB) synthesis in C. glutamicum. By eliminating PhaA and PhaB from the system, unlike E. coli, C. glutamicum could produce P(LA-co-3HB)s with almost 100% LA fraction [P(99.3 mol% LA-co-3HB)]. The appearance of 3HB monomer in the copolymer could be attributed to the possible presence of an endogenous 3HB-CoA supplying pathway in C. glutamicum. Analyzing the PLA-like polymers produced by C. glutamicum by 1H-NMR showed strong resonances of LA, which were characteristically similar to those of chemically synthesized PLA. Matsumoto et al. [70] assessed the differences in the synthesis of PLA-like polymers in C. glutamicum and P(LA-co-3HB) in E. coli in Vitro. It was found that LPE could actually polymerize LA-CoA as a sole substrate however, achieving low molecular weights, which was attributed to low mobility of the synthesized polymer [70]. It was also observed that copolymerization of LA-CoA and 3-HB-CoA only proceeds at a low concentration of LA-CoA and this was demonstrated in P(LA-co-­ 3HB)-producing E. coli where intracellular LA-CoA concentration was below the detection limit, while that in C. glutamicum was detectable at levels similar to other metabolites such as acetyl-CoA [70]. This study revealed that the mobility of the synthesized polymer and LA-CoA concentration are important factors in microbial PLA and P(LA-co-3HB) biosynthesis. This also supports an earlier prediction that a weak 3HB-CoA supplying pathway is the key to the biosynthesis PLA and that the bacterium milieu and genetic diversity of hosts are important in modulating the monomer composition of P(LA-co-3HB) [71].

12.2.5  Latest Approaches to P(LA-co-3HB) Production Metabolic and fermentation engineering of PHA producers had been exploited to modulate monomer composition or polymer yields of P(LA-co-3HB). Kadoya et al. [72] explored a new approach based on the disruption of σ factors that globally govern the transcription of genes to indirectly achieve positive effects on beneficial performance, increased production, and altered monomer composition of P(LA-co-­ 3HB). First, the researchers examined the effect of disrupting four non-essential σ factors RpoS, RpoN, FliA and FecI, that are known to be found in E. coli on the polymer biosynthesis and LA fraction in P(LA-co-3HB). The researchers thus recruited the σ factors disruptant strains and the parent strain E. coli BW25113 expressing LPE and respective LA and 3HB monomer supplying enzymes for P(LA-co-3HB) production from glucose. From the analysis of the polymers produced, ΔrpoN mutant exhibited higher polymer yield and LA fraction compared to the parent strain while the ΔrpoS strain only had increased polymer yield without

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affecting the LA ratio [72]. The ΔrpoN was the only mutant that was beneficial in terms of polymer yield and LA fractions. The higher polymer yields and LA fractions in P(LA-co-3HB) produced by ΔrpoN strain was attributed to its higher production of lactic acid. Nduko et al. [73, 74] demonstrated the contribution of xylose towards increased LA fraction compared to glucose. Effectively, Kadoya et al. [75] sought to exploit the synergistic benefits of xylose utilization and ΔrpoN disruption to increase LA ratios in P(LA-co-3HB). When the ΔrpoN strains were cultivated on xylose, the LA fraction in P(LA-co-3HB) was higher (33.9 mol%) than the 26.2 mol% that was produced by the same cells but cultivated on glucose [75]. However, the LA fraction in P(LA-co-3HB) produced by the ΔrpoN disruptant cultivated on xylose was similar to that produced by the BW25113 parent strain also cultivated on xylose, indicating that a synergistic effect of xylose utilization and ΔrpoN disruption did not exist. The improvement of polymer production by rpoN deletion motivated the same researchers to explore genome-wide transposon mutagenesis of P(LA-co-3HB)producing E. coli to obtain beneficial mutants [76]. The mutagenesis was accompanied by a high-throughput screening strategy to identify high P(LA-co-3HB)-accumulating mutants. The mutagenesis generated approximately 10,000 colonies from which 100 colonies were chosen as the first stage candidates. The candidates were then subjected to polymer production and the polymer contents were estimated by HPLC. The analysis identified one mutant, mtgA disruptant which exhibited enhanced P(LA-co-3HB) accumulation (5.1 g/l polymer compared to the recombinant parent, 2.9 g/l). The MtgA is involved in peptidoglycan strand formation in E. coli and the disruptant strain had an enlarged cell size. This knowledge was applied in E. coli BW25113 strain that is superior in P(LA-co-3HB)accumulation. Therefore, an mtgA-deleted derivative of BW25113, E. coli JW3175 harboring LPE and 3HB and LA monomer supplying genes was used for P(LA-co-­ 3HB) production. The E. coli JW3175 had higher amounts of P(LA-co-3HB) (7.0 g/l) compared to the parent recombinant strain (5.2 g/l), implying that mtgA gene deletion increases P(LA-co-3HB) production in E. coli with little effect on the LA/3HB ratio. The mtgA gene encodes a monofunctional peptidoglycan transglycosylase that is involved in the polymerization of lipid II molecules into glycan strands of peptidoglycans [77]. The researchers then evaluated the morphology of the mtgA-­ deleted strain under P(LA-co-3HB) accumulation and non-accumulation conditions and found that the mutant strain had similar cell size as the parent strain BW25113 under polymer non-producing conditions. Under P(LA-co-3HB)-producing conditions, the mtgA-deleted strain had a 1.4-fold increase in size compared to the P(LA-­ co-­3HB)-producing parent strain (BW25113). The effect of mtgA-mutation was however found to increase the cell diameter and not along the polar axis [76]. The mtgA deletion therefore increased the flexibility of the cell wall that could allow the recombinant cells to expand in width to accommodate more polymer accumulation. Along with the ΔrpoN, mtgA deletion is an additional strategy that could be used for P(LA-co-3HB) production.

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12.2.6  A  pplication of Lignocellulosic Biomass for the Production of P(LA-co-3HB) 12.2.6.1  Use of Xylose for the Production of P(LA-co-3HB) Despite PLA and PHAs having huge potential for replacing petrochemical-based plastics, their impact has been insignificant owing to their high cost of production, resulting in higher prices compared to their conventional polymer counterparts [16, 19, 38]. The major reason for the high cost of PLA and PHAs synthesized by biosynthetic pathways is due to high purity substrates, such as glucose mainly sourced from edible plants such as sugarcane, sugar beet and starch feedstocks. To lower the cost of production, several groups have demonstrated the use of lignocellulosic biomass as a substrate [12, 77–79]. Lignocellulosic biomass is inedible, carbon rich, renewable, abundant, and inexpensive hence could be used as a carbon source for P(LA-co-3HB) production. Lignocellulosic biomass consists of 30–55% cellulose, 25–50% hemicellulose and 10–40% lignin [80]. Whereas cellulose has been hydrolyzed into glucose and used for the production of bioethanol and a number of bioproducts, the hemicellulose portion that is mainly composed of xylose is underutilized [81–83]. For example, in Kraft pulping processes, hemicellulose is dissolved in black liquor along with lignin and combusted for power generation [84]. Hemicellulose has a lower heating value as compared to lignin, therefore its utilization for the production of polymers, fuels and other chemicals including lactic acid could lead to an efficient use of resources [85, 86]. On this basis, Nduko et al. [73] explored the potential of xylose in contributing to the efficient production of P(LA-co-3HB) for the first time. In that study, E. coli JW0885 (pflA−) strain expressing LPE or eLPE along with the 3HB-CoA and LA-CoA monomer supplying enzymes were cultivated on media containing xylose as the carbon source for P(LA-co-3HB) production. In the analysis, the cells expressing LPE [phaC1 Ps(ST/ QK)] and grown on xylose produced P(LA-co-3HB) having 34 mol% LA with a polymer content of 61%, while the same cells expressing the same set of enzymes but cultivated on glucose produced P(LA-co-3HB) having 26 mol% LA with a polymer content of 62%. On the other hand, when the same cells expressing eLPE [phaC1 Ps(ST/FS/QK)] were cultivated, those growing on xylose produced P(LA-­ co-­3HB) with 60 mol% LA and a polymer content of 70%. In contrast, those grown on glucose produced P(LA-co-3HB) with 47 mol% LA and a polymer content of 81% [73]. From these studies, the advantages of utilizing xylose over glucose substrate for the production of P(LA-co-3HB) became apparent owing to the higher cellular polymer content and LA fraction of the copolymer. In previous studies using glucose, P(LA-co-3HB) production with LA fractions exceeding 50  mol% were characterized with low polymer yields (less than 1 g/L) from 20 g/L glucose, which limited the investigation of polymer properties [54]. To exploit the benefits of using xylose for P(LA-co-3HB) production, Nduko et al. [74] employed two strategies to enrich LA fractions and polymer yields. The first strategy capitalized on the knowledge of the supply of lactic acid based on prior

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studies that had demonstrated that a ΔpflA mutant of E. coli strengthened the supply of lactic acid that could ultimately be incorporated into P(LA-co-3HB) [56, 87]. Effectively, Nduko et al. [74] applied other gene disruptants (Δpta, ΔackA, ΔpoxB, and Δdld) that are known to increase lactic acid production for P(LA-co-3HB) production from xylose. These mutants reroute the carbon flux at the pyruvate node into lactic acid [88]. When these mutants were transformed with the set of genes for P(LA-co-3HB) production and cultivated on xylose, it was found that Δpta, ΔackA, and Δdld mutants exhibited higher polymer productivities (6.5 to 7.4 g/L) than the parent strain BW25113, that had a polymer productivity of 6.3 g/L [72]. Similarly, the Δpta and Δdld mutants produced P(LA-co-3HB) polymers having higher LA fractions (58 and 66 mol%, respectively) than the parent strain, BW25113 that had 56 mol% LA in the copolymer, demonstrating the effectiveness of metabolic engineering in modulating the LA fractions in the polymers and polymer yields. Two single mutations (ΔpflA and Δdld) that had proved effective in improving LA fractions were combined to create a dual mutant, E. coli JWMB1 and evaluated for P(LA-co-3HB) production. The dual mutant strain exhibited an additive effect whereby the LA fraction in P(LA-co-3HB) increased to 73 mol% with high polymer content (96%), which was the highest recorded LA fraction among the P(LA-­ co-­3HB) production conditions with polymer contents being greater than 1  g/L [74]. The second strategy in the exploitation of xylose substrate was pegged on the enhancement of xylose transport by E. coli. The E. coli microbe has two different D-xylose-specific transport systems: the XylE, which is a member of the major facilitator superfamily of transporters and the ATP-binding cassette (ABC) transporter encoded by the xylFGH operon [89]. The latter system is the one mostly employed by E. coli to transport xylose, which could reduce the amount of ATP available for other cellular activities. To eliminate this possibility, Nduko et al. [74] overexpressed a non-ATP consuming galactitol permease, GatC that has been demonstrated to transport xylose efficiently for lactic acid production [88] in E. coli harboring P(LA-co-3HB) production genes. When the cells were cultivated on xylose, the GatC overexpression in E. coli parent strain, BW25113 increased the polymer productivity up to 7.7 g/L without significant change in the LA/3HB ratio. On the other hand, the GatC overexpression increased the LA fraction in some mutants. To fully exploit this system, xylose concentrations were increased up to 40 g/L and this increased polymer yields with a P(66 mol% LA-co-3HB) copolymer to 14.4 g/L yields obtained in BW25113 overexpressing GatC. This is the highest LA-based polymer yield obtained from sugars under the same culture conditions. Carbon flux was investigated and results showed that strains producing P(LA-co-­ 3HB) with high LA ratios had high lactic acid in the medium. Examination of the carbon balance revealed that compared to the polymerization of 3HB, the polymerization of LA is more carbon efficient because in generating a 3HB molecule (C4) from pyruvate, two pyruvate molecules (C3 × 2) are utilized with concomitant loss of two carbons in terms of CO2, whereas a single LA unit (C3) is generated from one pyruvate molecule (C3). This stoichiometry could result in high copolymer yields

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when an LA-rich polymer is produced for example, as in the case of xylose compared with glucose [74]. These studies demonstrate a direct relationship between lactic acid production in some strains with the ratio of LA in P(LA-co-3HB), which gave an indication of carbon redistribution in the mutations. However, some mutants have incomplete utilization of xylose which suggests that the polymerization is the limiting step, possibly due to the LA-incorporation capacity of LPE. In addition to the mutations, overexpression of GatC was found to beneficially improve polymer productivity. The hemicellulose sugar, xylose could be efficiently utilized for production of P(LA-co-3HB). Since xylose is a major component of lignocellulosic biomass, its utilization for the production of P(LA-co-3HB) could improve the economics of biomass conversion pathways where currently only glucose is used efficiently. 12.2.6.2  U  se of Lignocellulosic Biomass Hydrolysates for the Production of P(LA-co-3HB) The production of P(LA-co-3HB) from the major constituent sugars (pure glucose and xylose) of lignocellulosic biomass has been demonstrated [56, 74] so that it is possible that lignocellulosic biomass could be a feedstock for P(LA-co-3HB) production. Sun et al. [90] examined the production of P(LA-co-3HB) from lignocellulose biomass-derived sugars. The researchers used Miscanthus × giganteus (hybrid Miscanthus), a large perennial grass and rice straw. The two sets of biomass were treated with acidified sodium chlorite (NaClO2) and sodium hydroxide (NaOH) then hydrolyzed using the cellulase enzyme complex. The hydrolysate was mainly converted into xylose and glucose that were used as carbon substrates for P(LA-co-­ 3HB) production in E. coli BW25113 expressing LPE and monomer supplying enzymes. The cells grew on the hydrolysates and produced P(LA-co-3HB) polymers. First, the weight of cells cultivated on both hydrolysates had higher cell weight compared with cells growing on a mixture of pure glucose and xylose, which could be due to higher carbon sources in the hydrolysates such as arabinose that were detected in small quantities. The polymer content in the cells was similar for both the hydrolysates and pure sugars. The polymers produced had LA fractions ranging between 8.7 and 16.9 mol% [90]. Takisawa et al. [91] applied eucalyptus hemicellulose hydrolysate for P(LA-co-­ 3HB) production in recombinant E. coli. The hydrolysate was mainly composed of xylose and galactose and traces of acetate as impurities that were removed by activated carbon and ion-exchange columns. When the cells were grown on purified hydrolysate, they attained a dry cell weight of 8.6 g/L and produced P(LA-co3HB) having an LA fraction of 5.5 mol% and polymer content of 62.4%, which was ­comparable to a pure mixture of xylose and galactose [91]. These results demonstrated that lignocellulosic biomass hydrolysates could give similar amounts of polymer as pure sugars, hence signifying their potential application. However, the LA fractions in the copolymers were lower than when pure sugars are used

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separately, which could be attributed to catabolite repression in E. coli when different sugars are availed simultaneously in the medium. During the pretreatment of lignocellulosic biomass, toxic byproducts that could negatively affect yields through slowing microbial growth or inhibiting fermentative processes can be produced. Examples include acetate, furfural, and hydroxymethylfurfural that result from the thermochemical treatment of hemicellulose. Many efforts of detoxifying the hemicellulose hydrolysates have been demonstrated, however, these additional steps increase the cost of production. A more practical strategy therefore is identifying microbial strains that are resistant to the byproducts. In a study designed to assess P(LA-co-3HB) copolymer-producing strains for acetate tolerance, Salamanca-Cardona et al. [92] investigated four E. coli strains using xylose and acetate feedstocks. From the investigation, some strains were shown to be tolerant to acetate hence they could be useful in the production of P(LA-co-3HB) from lignocellulosic biomass hydrolysates. Plant biomass-derived xylose for P(LA-co-3HB) production is a promising strategy to achieve lower production cost. However, the system requires hydrolysis of the lignocellulosic biomass prior to fermentation. However, hydrolysis using enzymes or at high temperatures and low pH using acids and alkali compounds could increase the production cost. Therefore, a consolidated process that couples hydrolysis and fermentation for polymer production could be an attractive strategy for reducing production cost. Coupling these steps in E. coli, is however, not possible because E. coli is deficient of genes that express the hydrolytic enzymes for xylan hydrolysis, endoxylanases and β-xylosidases [93]. To obtain a xylanolytic E. coli strain, Salamanca-Cardona et al. [94] expressed an endoxylanase (XylB) from Streptomyces coelicolor and a β-xylosidase (XynB) from Bacillus subtilis in a P(LA-co-3HB) producing E. coli strain. When the E. coli LS5218 co-expressing the P(LA-co-3HB) and the xylanase system was cultivated on a media containing xylan, P(LA-co-3HB) was produced, demonstrating for the first time a consolidated bioprocess (CBP) for the production of P(LA-co-3HB) from xylan as a carbon source. In the study of Salamanca-Cardona et al. [94], the P(LA-co-3HB) produced had low LA ratio (100 kDa) are stronger [102–106]. However, these increases in strength are accompanied by a decrease in the elongation-­at-break from 6.0% to 3.3% [107]. On the other hand, PHB homopolymer is very brittle at low molecular weights namely, the material elongates 2% at break with a strength of 8.5  MPa at 22  kDa [108]. At 370  kDa, PHB strength improves to 36 MPa with little change in the strain behavior [107, 109]. At 11 MDa, PHB strength and ductility improves to 62 MPa and 58% elongation-at-break [110]. In a study of the mechanical properties, the Young’s modulus of P(LA-co-3HB) s were in a range of (148 to 905) MP compared with that of PLA (1020 MPa) and P(3HB) (1079  MPa) homopolymers. The Young’s modulus of P(LA-co-3HB)s decreased along with the increase of the LA fraction. P(LA-co-3HB)s exhibited advanced flexibility compared to PLA and P(3HB) homopolymers [54]. For instance, the elongation at break of the P(LA-co-3HB) with 29 mol% LA reached 156% [54], which proved that P(LA-co-3HB)s are a more elastic material than the respective homopolymers, hence variation in LA fractions could generate materials with target mechanical properties.

12.4  Microbial Degradation of P(LA-co-3HB) Due to the high potential of P(LA-co-3HB) as a practical polymer for society, it is important to study its biodegradability. The biodegradability of P(3HB) by microorganisms possessing P(3HB) depolymerases has been reported, while PDLA is not readily biodegraded [111]. As a hybrid of PDLA and P(3HB), the biodegradation of P(LA-co-3HB) should be dependent on the degradability of both homopolymers. To demonstrate the biodegradability of P(LA-co-3HB), degrading microorganisms were screened using soil samples collected around Sapporo, Japan and some isolates degraded an LA-enriched P(LA-co-3HB) copolymer containing 67 mol% of LA, indicating the existence of P(LA-co-3HB)-degrading microorganisms [112]. One particular microorganism identified as Variovorax sp. C34, had high P(LA-co-­ 3HB) and P(3HB) degradation ability but not PDLA. When a phylogenic tree was constructed based on the 16S rDNA sequence, Variovorax sp. C34 was found close to extracellular P(3HB) depolymerase-secreting bacteria, such as Acidovorax sp. TP4 [113], Comamonas sp. [114] Comamonas testosteroni [110] and Delftia acidovorans [115], which are the members of Comamonadaceae family. The enzymatic activity towards P(LA-co-3HB) degradation was detected in the culture supernatant of Variovorax sp. C34, suggesting that the P(LA-co-3HB) degrading enzyme was an extracellular depolymerase. For further characterization, the P(LA-co-3HB) depolymerase was purified and tested for P(LA-co-3HB), P(3HB), PLLA and PDLA degradation. The enzyme exhibited degradation activity towards both P(LA-co-3HB) and P(3HB), but not PLLA or PDLA. During the deg-

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radation of the polymers that were treated by P(LA-co-3HB) depolymerase were found to degrade P(LA-co-3HB) faster than that of P(3HB), attributable to the lower crystallinity of P(LA-co-3HB) compared with that of P(3HB) [116–118].

12.5  Production of LA Oligomers Since the establishment of the first microbial cell factory in 2008 for the production of D-LA-based polyesters using an engineered PHA synthase (PhaC) (D-specific lactate-polymerizing enzyme (LPE)) [36], a repertoire of enantiopure LA-based copolymers have been produced in recombinant microorganisms [54, 55, 57, 59, 73, 74]. During the course of these studies, it was found that when poly (D-lactide) (PDLA)-like polymers were synthesized, the resulting polymers tended to have low molecular weight [55, 59]. These results hinted that the recombinant organisms used for P(LA-co-3HB) production could be synthesizing oligomers and thus, could be used for PDLA oligomer production that could create a short-cut route for PLA production as shown in Fig. 12.5. Utsunomia et al. [37] explored the presence of oligomers in LA-based polymer production system using recombinant E. coli expressing LPE. In the experimental set up, the cells expressing P(LA-co-3HB) production requisite enzymes as shown in Fig. 12.5, were cultivated on glucose and then the compounds in the supernatant extracted and analyzed by NMR. The chemical shifts (δ in ppm) in 1H NMR of the Glucose

Pyruvate

D-Lactate 3-Hydroxybutyryl-CoA D-Lactyl-CoA Convent ional route for PLA synthesis

D-LA-based polymer

D-LA-based Oligomer Secretion

D-Lactate

Polycondensaon

D-LAOs

Depolymerization

Fig. 12.5  Adapted P(LA-co-3HB) scheme for D-LA oligomer synthesis

Lactide

ROP

PLA

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extracted fraction at 4.9–5.2 ppm [1H, m, LA (1)] and 1.4–1.6 ppm [3H, m, LA(2)] were almost identical to those of P(LA-co-3HB), with the slight differences in chemical shifts being attributed to the molecular weight difference between the polymers and oligomers [12, 37, 57]. Therefore, based on the 1H NMR results, the extracted compounds were determined to be oligomers consisting of 63 mol% LA [37]. The ESI-TOF-MS analysis of the extracted samples identified a bimodal distribution of the oligomers with periodic m/z values in the range of approximately 400–1400 that corresponds to ∼4- to 19-mer oligomers consisting of LA and 3HB units. The peak tops of the bimodal distribution were 7- and 12- mer, respectively. These results motivated the researchers to enhance the secretion of D-LAOs in P(LA-co-3HB) polymer-producing E. coli by the addition of chain transfer (CT) agents, based on the reports that a PHA synthase loses its polymerization activity/ transfers the polymer chain to a hydroxyl group-containing CT agent [119]. This would expedite the production of oligomers since at the termination step during polymer synthesis, increasing the frequency of the CT reaction results in the production of polymers with lower molecular weights [120]. Studies have shown that low molecular weight PEGs and short chain alcohols are effective as CT agents that significantly reduce molecular weight of the PHAs when added to the medium [120–124]. For this purpose, Utsunomia et al. [37] evaluated the ability of four CT agents; PEG200, DEG, EG, and ethanol to enhance oligomer synthesis based on the amount of oligomeric LA secreted into the medium. DEG was found to be the most effective CT agent for D-LAO production, whereby it also effected drastic reduction in the molecular weight of the intracellularly accumulated polymers [37]. The enhanced production of extracellular D-LAOs was not as a result of cell lysis because microscopic observations of the cells revealed that they remained intact upon DEG addition at various concentrations [37]. Moreover, viable cell counts based on CFU/mL did not exhibit significant differences between the conditions of 5% DEG addition and control (no CT supplement), which indicated that D-LAOs are secreted by E. coli into the medium and are not in the medium as a result of cell lysis. The highest production of (8.3 ± 1.5)  g L−1 of D-LAOs containing 86.0 ± 4.5 mol% LA was achieved at 5% DEG supplementation level. A two phase extraction of 5% DEG culture supernatant protocol recovered 40% D-LAOs (3.2 g L−1) and the extracted D-LAOs were not harboring free DEG.  However, the D-LAOs were likely to be capped with DEG at their carboxyl terminal. The extracted D-LAOs were subjected to 1H−1H COSY-NMR, which indicated that DEG was covalently bound to the carboxyl terminal of D-LAOs [37]. The intracellularly accumulated high molecular weight polymer was also demonstrated to be terminated with DEG at the carboxyl terminal as revealed by 1H−1H COSY-NMR and 1H−1H DOSY NMR. The ESI-TOF-MS analysis of the extracted D-LAOs-DEG detected periodic m/z values in the range of approximately 400–800, which corresponded to ∼4- to 10-mers, that suggested that shorter oligomers were synthesized relative to those obtained without DEG [37]. The efficiency of D-LAO secretion into the medium was evaluated. It was found that the concentration of intracellular D-LAO was higher (1.8 g/L) compared to the extracellular concentration (0.4 g/L), which implied the presence of a secretion bar-

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rier [37]. To identify the D-LAO secretion route in E. coli, a loss-of-function screening strategy was devised using 209 single-gene membrane protein deletants, which are involved in the transport of organic compounds [124]. The mutants expressing P(LA-co-3HB) and D-LAO enzymes were cultivated on glucose and evaluated for D-LAO secretion. Among the mutants, 55 of them had decreased amount of extracellular D-LAOs in comparison to the control (parental strain) expressing the same set of enzymes. Furthermore, there was focus on mutants having elevated intracellular D-LAOs as this could indicate disruption of the secretory mechanism. As a result, seven mutants (∆mpF, ∆argT, ∆mngA, ∆macA, ∆ompG, ∆citA and ∆cpxA) that exhibited both diminished D-LAO secretion and enhanced intracellular accumulation were selected. These deletion mutants when grown without plasmids grew normally as the parental strain, indicating that the membrane proteins that were deleted did not hinder cell growth. Based on the results, the passage of D-LAOs through the outer membrane was proposed to be mediated by passive diffusion through porins such as OmpF and OmpG. To cross the inner membrane, the inner membrane-associated proteins including CitA, ArgT, MacA and CpxA could be involved in the transport of D-LAOs [125]. The same researchers sought to convert D-LAOs into lactides that could be used for PLA production thus eliminating the costly chemo-oligomerization step in the PLA production process [126]. Lactide was synthesized from extracted D-LAOs-­ DEG having 68 mol% LA and ranging from 3 to 7 mers produced by recombinant E. coli growing on glucose supplemented with 5% DEG.  As controls, extracted D-LAOs having 61 mol% LA and ranging from 3 to 16 mers produced by recombinant E. coli growing on glucose and L-LA homo-oligomers having 100% mol% LA and ranging from 3 to 14 mers, were used. All the oligomers were heated with zinc oxide as a catalyst to synthesize lactides, whose synthesis was confirmed by 1H NMR analysis [126]. This showed that D-LAO-DEG can be converted into lactides similar to D-LAOs and L-LAOS oligomers. Although the microbial D-LAOs were converted into lactides, the conversion rates (25% for D-LAOs and 13% for D-LAO-­ DEG) were lower compared to that of L-LAOs (77%), which was attributed to the presence of 3HB units in D-LAOs that could inhibit conversion rates. Thus increasing the LA fraction in D-LAOs could increase the conversion rate. Thus, xylose that had been demonstrated to be superior in increasing LA fractions in P(LA-co-3HB) [72] was used in the synthesis of D-LAOs. With the use of xylose, D-LAOs-DEG containing 97 mol% LA were produced and with the use of microaerobic culture conditions, extracted D-LAOs-DEG containing 89% LA fraction were obtained. Subsequent synthesis experiments indicated that lactide yield increased with increase in LA fraction in the oligomers hence further increase in LA fractions in the oligomers is essential for increased lactide yield. Another factor that could affect lactide yield is the molecular weight of the oligomers. Short oligomers vaporize during heating and are lost [126], hence increasing the oligomer length should be a focus area to improve lactide synthesis.

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12.6  Conclusion and Future Outlook The establishment of a microbial cell factory for the synthesis of LA-based polyesters was hinged on the engineering of a PHA synthase (LPE) to acquire LA-polymerizing activity and integrating it into a PHA synthetic pathway. The pioneering experiment yielded an LA-based polymer with low LA fractions. This groundbreaking study however laid the foundation for consequent studies that recruited enzyme evolutionary engineering, metabolic engineering, and fermentation techniques to improve the LA fractions in P(LA-co-3HB) as summarized in Table 12.1. With an evolved LPE, the LA fraction in P(LA-co-3HB) was significantly improved with relatively high yields. To further increase the LA fraction, elimination of the 3HB-CoA supply pathway was done by exclusion of the monomer supplying pathway and replaced with extraneous valerate as a 3HV monomer precursor. This led to the synthesis a PLA-like terpolymer (96 mol% of LA), which was similar to chemically synthesized PLA in terms of properties. Transfer of the P(LA-co-­ 3HB) biosynthetic system into C. glutamicum resulted also into the synthesis of PLA-like polymers, suggesting that change of a host organism is a useful strategy to explore in the synthesis of PLA-like polymers with improved yields. Other new approaches to attain positive effects on P(LA-co-3HB)’s increased production and altered monomer composition and beneficial performance of production strains includes the disruption of σ factors that globally govern the transcription of genes. These strategies combined together with exploration of substrates could achieve modulated polymer yields and varied monomer compositions with varied polymer properties and thus extensive studies are necessary to find optimum conditions that can be used for industrial processes. Investigation on depolymerases demonstrated that P(LA-co-3HB) could be degraded hence it is a biodegradable polymer and thus an ecofriendly alternative to the petrochemical-based plastics. The production cost of P(LA-co-3HB) and other PHAs is the limiting factor towards their widespread industrial production and application owing to the overreliance on purified glucose. Lignocellulosic biomass is one of the alternatives that could inexpensively be used for LA-based polymer production. As has been demonstrated, xylose, a major component of lignocellulosic biomass is a good substrate for the production of LA-based polymer with high LA fractions. Moreover, the applicability of lignocellulosic biomass hydrolysate and xylan has been demonstrated and all these strategies could be critical in reducing the cost of producing LA-based polymers. On the other hand, the P(LA-co-3HB) production system was found to produce LA oligomers that were transformed into lactides, precursors for chemical synthesis of PLA. Efforts have been made to understand the secretion mechanism of the LA-oligomers, however investigations to pinpoint the exact mechanism/better understand candidate transporters are necessary as this will guide designs to improve the secretion of the oligomers. Although the secreted oligomers were found to form lactides under reactions using a catalyst, the conversion rates were low possibly due to the presence of 3HB components in the chain. Thus,

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Table 12.1  History of microbial LA-based polymer production LA fraction S.No. (Mol%) 1 6

Host E. coli JM109

Culture conditions Aerobic

2

26

E. coli JW0885 (∆pflA)

Aerobic

3

47

E. coli JW0885 (∆pflA)

Anaerobic

4

47

E. coli JW0885 (∆pflA)

Aerobic

5

62

E. coli JW0885 (∆pflA)

Anaerobic

6

90–96

E. coli LS5218

Aerobic

7

97–99.3

Corynebacterium glutamicum

Aerobic

8

34

E. coli JW0885 (∆pflA)

Aerobic

9

60

E. coli JW0885 (∆pflA)

Aerobic

10

66

E. coli JW2121 (∆dld)

Aerobic

11

73

E. coli JWMB1 (∆dld, ∆pflA)

Aerobic

Specific features of host Expresses LPE and PHB biosynthetic pathway Expresses LPE and PHB biosynthetic pathway Expresses LPE and PHB biosynthetic pathway Expresses evolved LPE (eLPE) and PHB biosynthetic pathway Expresses evolved LPE (eLPE) and PHB biosynthetic pathway Expresses evolved LPE (eLPE) and PHB biosynthetic pathway omitted Expresses LPE only/ and PHB biosynthetic pathway Expresses LPE and PHB biosynthetic pathway Expresses evolved LPE (eLPE) and PHB biosynthetic pathway omitted Expresses evolved LPE (eLPE) and PHB biosynthetic pathway omitted Expresses evolved LPE (eLPE) and PHB biosynthetic pathway omitted

Substrate Glucose

Reference [36]

Glucose

[56]

Glucose

[56]

Glucose

[54]

Glucose

[54]

Glucose and valerate

[59]

Glucose

[55]

Xylose

[73]

Xylose

[73]

Xylose

[73]

Xylose

[73]

efforts to increase LA fraction in the oligomers and fine tuning the molecular weight of the oligomers similar to those directed towards P(LA-co-3HB) should be explored. Moreover, improving the yields of oligomers should be another frontier of research. The approaches for high yield production of P(LA-co-3HB) and

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LA-oligomers together with the application of lignocellulosic biomass have potential for the creation of a robust biorefinery for the production of the biodegradable LA-based materials. The systems discussed are therefore potentially capable of being scaled up hence improving the competitiveness of LA-based polymers against the petrochemical-derived plastic counterparts.

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Index

A Acidity, 11, 175, 202, 205, 210 Acid treatments, 198, 238, 250 Activated carbons (ACs), 136, 138, 154–157, 161, 163, 168, 176, 185, 187, 188, 190–197, 200–206, 208, 210, 212, 215, 216, 242, 252, 260, 374 Activating agents, 102, 124, 127, 157, 210 Activation, vi, 23, 102, 105, 111, 118, 124–125, 137, 155, 157–159, 161, 163, 166, 167, 170, 172, 176, 186, 194–198, 202, 210, 216, 237, 239, 241–243, 245, 246, 248, 249, 252, 254 Activation energy, 42, 314, 315, 319 Active sites, 142, 165, 168, 186, 191, 197, 198, 201, 202, 204–206, 208, 210, 214–216, 246, 247, 252, 314 Additives, 27, 41, 43, 72, 73, 80, 91, 100, 135, 140, 203, 241, 273–277, 312, 321, 328, 332, 333, 335, 354, 373 Adsorption, 64, 73, 80, 129, 130, 134–137, 141, 154, 161, 165, 173, 203, 205, 210, 212, 215, 239, 242, 246, 247, 297 Agricultural residues, 9, 40, 47, 170, 328, 329, 331 Agricultural wastes, 176, 236, 239, 240, 286, 354 Agriculture, 3, 7, 40, 118, 128, 252 Air permeability, 70, 71, 85, 86, 89 Algae, 5, 110, 162, 163, 248, 259, 270, 274 Algal biomass, 162, 248, 260 Algal cell, 259 Alginate, 158, 166, 167, 193, 204, 259 Alkali-extracted, 7, 8, 14, 26

Alkali metals, 211, 212, 216, 283, 286 Alternative applications, 118–143 Amino celluloses, 105–108, 111 Aminolysis, 104, 105 Ammonium acetate, 128 Ammonium sulfate precipitation, 13, 24 Amorphous, 44, 86, 101, 120, 121, 132, 133, 135, 188, 194, 202, 205, 207, 209, 235, 236, 247, 335, 363, 376, 377 Amount of exchangeable cations, 128 Amphiphilicity, 18 Anaerobic, 271, 368 Anaerobic conditions, 368 Anionic, 11, 73, 77, 80, 84, 105, 108, 111, 259 Anodes, 136, 138, 214, 236, 252, 259 Antimicrobial activity, 111 Antioxidants, vi, 40 Apparent density, 65–68, 71 Arabinofuranosyl, 7 Arabinoglucuronoxylan (AGX), 4, 7 Arabinose, 4, 5, 11, 12, 26, 374 Arabinoxylan (AX), 4–10, 13, 14, 20, 120 Aromatic, 44, 46, 48, 51, 121, 131, 193, 204, 207, 246, 296 Aromaticity, 125, 126, 128, 132–133, 194 Aromatic rings, 194, 204 Ash contents, 52, 122, 125, 126, 139, 157, 186–189, 191, 193, 197, 277, 284, 285, 297, 298, 302, 317, 320, 328, 331–333 Ashes, 283–287, 297, 328, 329, 337, 338 Atomic absorption spectroscopy (AAS), 126 Autohydrolysis, 9–11, 48

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392 B Bacteria, 23, 26, 108, 111, 362–364, 367–369, 378 Bacterial cellulose, 64, 166, 167 Bamboo chopsticks, 257 Banana peels, 258 Bar coater, 86 Barium chloride, 125, 128 Barrett-Joyner-Halenda (BJH), 129 Barrier properties, vi, 83, 85, 86, 89, 91 Basic treatment, 238, 250 Batteries, 138, 153, 154, 213, 234–236, 255 Benzene ring, 43 Bifunctional, 210 Bio-based chemicals, 40 Biocarbons, vi, 177, 260 Biochar, vi, 118–143, 170, 187–189, 191–195, 197, 201, 202, 204, 206, 208, 210, 212, 215, 216, 234–260, 271, 274, 296, 321, 329, 336, 337, 346, 347, 349–353 Biochemical, 271 Biocompatibility, 11, 100, 108 Bioconversion, 23–25 Biodegradability, 282, 378 Biodegradable, 11, 15, 20, 22, 176, 362–365, 382, 384 Biodegradation, 337, 378 Biodiesel, vi, 141, 186, 204, 206–208, 271 Bioenergy, 270, 271, 296, 336 Bioethanol, 8, 271, 337, 372 Biofilm formation, 108 Biofuels, v, vii, 40, 44, 186, 204, 239, 260, 270, 329, 337, 345 Biogas, 271 Biological, v, vi, 44, 102, 105, 127, 140, 333 Biomass, v, 3, 40, 83, 157–162, 185, 236, 270, 327, 364 Biomedical applications, 108, 111–112, 369 Biopolyesters, 362, 364 Bioproducts, 40, 369, 372 Biorefineries, v, vii, 26, 40, 45, 384 Biosynthetic pathway, 366, 367, 370, 372 Black liquor, 8, 40, 45, 372 Black pellets, 281, 282, 288, 289 Blade coater, 89 Bleached, 68, 70, 78, 80, 82, 89 Boilers, 45, 272, 276, 277, 280, 282–288, 298, 308–310, 312, 313, 320, 329 Bonding structures, 118, 125, 132–133 Boron (B), 165, 214 Breaking, 41, 42, 68 Briquettes, 270, 302, 305 Brunauer-Emmett-Teller (BET), 122, 128, 129, 159, 161, 163, 166, 168, 173, 175, 237–239

Index Bulk, 46, 62, 63, 65, 76, 99, 100, 103, 118, 125, 127, 129, 142, 176, 272, 273, 277, 279, 320, 328, 333, 336 By-products, 7, 23, 45, 176, 238, 329, 336 C Calcination, 204, 242 Caliper, 66, 76 Capacitances, 137, 155, 156, 158, 159, 161–163, 165–168, 170, 172–177, 252 Capacitive deionization (CDI), 136–137 Carbocatalysts, vi, 186, 187, 197, 198, 200, 201, 205–210, 216 Carbohydrates, 48, 52, 75, 83, 121, 163, 239, 335, 363 Carbon, 5, 40, 102, 118, 154, 185, 234, 271, 296, 327, 362 Carbonaceous, v, vi, 137, 138, 185–216, 236, 240, 241, 243, 260, 296, 297 Carbonate-based electrolytes, 235 Carbon blacks, 188, 189, 191, 201, 202, 213, 214 Carbon debt, 271 Carbon dioxide, vi, 15, 51, 118, 124, 195, 212, 213, 299, 314, 320 Carbon fibers (CFs), 47, 136, 165–167, 257 Carbon flux, 368, 373 Carbonization, 119, 123–124, 126, 127, 132, 137, 142, 143, 157, 163, 167, 172, 176, 191, 193, 207, 216, 238–241, 243–246, 250–252, 258, 296, 297, 299, 311, 312, 329, 336–338 Carbon microspheres, 163, 165, 241 Carbon nanofibers, 167, 258 Carbon nanosheets (CNs), 142, 169–171, 251, 257 Carbon nanotubes (CNTs), 138, 140, 142, 155, 168–169, 186, 213, 250, 255 Carboxylated cellulose, 108–109 Carboxymethylated, 63, 72, 77, 86, 91 Catabolite repression, 375, 376 Catalysis, 51, 128, 185, 186, 207–210, 216 Catalysts, v, vi, 10, 16, 19, 51, 52, 108, 129, 141–142, 185–216, 252, 286, 309, 363, 364, 367, 381, 382 Catalytic depolymerization, 9, 51, 52 Cathodes, 136, 137, 139, 213–215, 236 Cation, 24, 128, 135 Cation exchange capacity (CEC), 125, 128 Cationic, 17, 20, 66, 67, 72–74, 77, 78, 80–83, 91, 105–107, 111 Cattail, 166, 167 Cell disruption, 371, 381, 382 Cellulose acetate (CA), 100, 108

Index Cellulose beads, 100, 110, 112 Cellulose carbonates, 104–105, 107, 111 Cellulose nanocrystals (CNC), 62–64, 73, 76, 82 Cellulose nanofibrils (CNF/NFC), 62–64, 74, 77–84, 86, 88, 89, 186, 203, 205 Cellulose tosylates, 102–104 Cellulosic materials, 8 Chain reactions, 41 Chain scission, 41–43 Chain transfer (CT) agents, 380 Char, 18, 51, 120, 123, 124, 139, 188, 191–196, 211, 239, 317, 319, 336 Characterization, vi, 8, 52–53, 118–143, 187, 205, 245, 260, 297, 299, 378 Charcoals, vi, 126, 188–192, 205, 270, 271, 336, 345–353 Charge-discharge, 161, 167, 174, 175, 243, 246, 259 Chemical activation, 124, 157, 159, 167, 170, 174, 190, 195, 237, 242–243, 245 Chemical modifications, 16–19, 26, 27, 50, 51, 63, 77, 85, 91, 100, 101, 201, 237, 238, 250 Chemical property, 125 Chemicals, vi, 3, 40, 63, 100, 118, 154, 186, 235, 271, 296, 328, 362 Chips, 7, 45, 270, 276, 277, 279, 285, 286 Chiral nematic, 76 Chitin, 11, 20, 162, 163 Choline acetate, 12 Chromatography, 7, 14, 24, 25, 100, 110 Cladophora glomerata, 244, 249–252 Climate changes, 3, 40 Clover stems, 169, 170 Coal, 157, 186, 188, 189, 192, 194, 204, 210, 211, 215, 240, 269, 272–274, 277, 280–289, 298, 303, 314, 317–320 Coatings, vi, 63, 65, 71, 72, 76, 83–89, 91, 100, 108, 111, 168, 252 Co-combustion, 284, 318, 319 Coconut, 142, 157, 158, 162, 163, 169, 170, 176, 186, 192, 203, 239 Coconut oil, 250, 257 CO2 emissions, 269, 271, 284, 285, 287, 300, 301 Co-feeding, 194 Co-firing, vi, 272, 277, 280, 282–284, 288, 289 Co-generation, 286, 289 Co-generation/combined heat and power (CHP), 277, 278, 280, 285, 286, 354 Coke, 157, 189–191, 194, 197, 210, 212, 298–305, 320 Collapse, 243

393 Combustion, vi, 51, 157, 195, 269, 271, 276, 282, 283, 288, 298, 301, 306–309, 311–314, 316–320, 331, 336, 337, 354 Compatibility, 48–52, 54–56, 140 Composite, v, 20, 49, 50, 80, 82, 136–138, 141, 142, 155, 167, 235, 257, 259 Composite biochars, 246–248 Connectivity, 167 Consolidated bioprocess (CBP), 375, 376 Contaminants, 12, 128, 130, 134–136, 213 Corncobs, 9, 10, 121, 137, 138, 157, 158, 169, 170, 211, 212, 250, 257, 274 Corn stalks, 188, 250, 256, 274 Corynebacterium glutamicum, 369–370 Cotton, 20, 64, 99, 118, 138, 158, 166, 169, 170, 251 Columbic efficiency, 256 Crops, 7, 8, 127, 189, 251, 270, 328, 329, 336, 354 Crosslinking, 22, 41–43, 46, 86, 103, 104, 110, 296 Crystalline, 100, 120, 132, 133, 235, 363, 376 Crystalline cellulose, 101, 120 Crystallinity, 8, 85, 118, 125, 132, 368, 377, 379 Current density, 156, 161, 163, 167, 253, 254, 259 Cyclability, 163 D Dandelion fluff, 168 Deactivation, 204, 206, 208, 210, 211, 283 Decomposition, 42, 43, 118–120, 122, 124, 156, 186, 189, 200–202, 204, 205, 212–214, 336, 337 Deep eutectic solvents (DESs), 11, 12, 26 Degradation, 7, 9, 10, 17, 23, 41–43, 119, 142, 175, 188, 215, 272, 354, 378, 379 Degree of substitution (DS), 5, 8, 15, 17–19, 23, 102 Dehydration, 23, 120, 121, 124, 190, 194, 205, 216, 241, 243, 246, 297 Dehydrogenation, 190, 200, 201, 203, 205, 206, 216 Delignification, 45 Densification, 66, 68, 74, 272, 273, 281, 304, 320, 328, 329, 336–338, 345, 353 Density, 15, 62, 67, 68, 109, 129, 155, 186, 234, 272, 304, 326, 363 Density functional theory, 129 De-polymerization, vi, 51–53, 56 Depolymerized Kraft lignin (DKL), 52–56 Destruction, 127, 362 Dewatering, 62, 65, 68, 71–74, 82

394 Diagnostics, 105, 108, 112, 140 Dialdehyde, 18, 110 Dialdehyde cellulose (DAC), 110–111 Die, 84, 89, 273, 328, 331, 337–348, 352, 353 Digestion, 271 Dimensional stability, 66, 75 Dimethyl sulphoxide (DMSO), 6, 18, 103 Dispersions, 50, 62, 63, 72, 78, 84–86, 195, 197, 198, 201, 212 Disruption, 370, 371, 381, 382 Dissolving pulp, 99 District heating (DH), 277, 278, 285, 286, 312 Drainage, 63, 66, 67, 70–73, 75, 76, 82, 91 Dual component systems, 80 Dual mutant, 373 Durability, 41, 140, 153, 246, 273, 277, 278, 309, 312, 313, 320, 328–330, 332, 333, 337, 338, 352 Dye, 74, 108, 203, 205, 215 E Efficiency, 9, 13, 15, 22, 73, 77, 124, 130, 134, 135, 137, 214, 234, 235, 254, 285, 287, 289, 301, 308, 312, 320, 328, 330, 380 Eggplant, 168–170 Egg yolk, 246, 251, 256 Electrical conductivity, 43, 118, 124, 125, 127–128, 132, 142, 155, 162, 165, 203 Electrocatalyst, 212 Electrochemical capacitors, 153–177 Electrochemical sensor, vi, 140–141 Electrodes, 118, 154, 188, 234, 255 Electro-Fenton oxidation (EF oxidation), vi, 187–189, 200, 202, 207, 212–216 Electrolyte, 125, 134, 154–157, 161–163, 165, 166, 168, 170, 172, 174–176, 213–215, 235, 236, 243, 246, 247, 254, 255, 260 Electrolyte engineered biochar, 236, 260 Electron transport, 168, 169 Electrospinning, 167 Electrosterically stabilized, 78 Elemental composition, 125, 126 Emissions, vi, 45, 137, 141, 203, 234, 269, 271, 277, 283, 285, 287, 299–301, 306–310, 313, 320, 327 Enantiopurity, 376 Energy, 3, 40, 62, 103, 118, 154, 185, 234, 269, 298, 327, 362 Energy consumption, 103, 140, 270–273, 281, 331, 337, 350, 353 Energy density, 15, 139, 155, 156, 161, 163, 165, 175–177, 234, 272, 279, 282, 302, 304, 312, 320, 329, 336, 337

Index Energy dispersive X-ray spectroscopy (EDX), 125, 130, 131 Energy storage, 118, 124, 125, 127, 128, 137, 139, 153, 167, 172, 212, 213, 234–260, 365 Engineered bacteria, 364, 367 EN-Plus, 277, 278 Enteromorpha, 249, 251 Environmental, vi, vii, 20, 23, 27, 41, 83, 137, 140, 157, 162, 165, 176, 236, 243, 249, 255, 260, 275, 283, 327, 328, 364 Environmental remediation, vi, 124, 131, 133–137, 142 Enzymatic, 86, 109, 207, 378 Enzymatic degradation, 23 Enzymatic hydrolysis, 52 Enzymatic pretreatment, 63, 88 Enzyme, vi, 23–25, 70, 102, 105, 109, 110, 207, 365, 367–370, 372, 374–376, 378, 379, 381, 382 Escherichia coli, 111, 365, 367–375, 379–381, 383 Esterification, 16, 50, 51, 55, 64, 101, 142, 186, 189, 200, 201, 205–210, 216, 366 Ethanol, 8, 13, 23, 26, 45, 48, 52, 78, 328, 380 Ethanol precipitation, 9, 13, 26 Etherification, 17, 51, 200, 201, 206 Ethylene glycol (EG), 52 EU28, 280 Eucalyptus, 78, 82, 136, 158, 191, 192, 202, 240, 274, 277, 330, 332, 374 European Union (EU), 271, 276, 279–282, 288 Evolved-Lactate polymerizing enzyme, 368, 382 Extraction, 7, 8, 10–12, 26, 52, 76, 254, 258, 259, 328, 365, 380 F Feedstock, v, vi, 9, 52, 123, 128, 132, 134, 139, 186, 188, 192, 194, 202, 206, 210, 248, 269, 271–275, 277, 279, 281, 282, 311, 328, 330–333, 336, 354 Fermentation, 23, 52, 271, 363, 364, 366, 370, 375, 382 FESEM, 244, 247, 248, 253 Fibers, 10, 45, 61, 109, 136, 154, 250, 274, 317, 335, 363 Fibrillation, 68, 70, 71, 80, 110 Filler materials, 76 Films, 16, 19, 20, 26, 27, 76, 85–87, 89, 100, 138, 328, 363 Filtration, 15, 24–26, 53, 187, 201

Index Firing, vi, 278, 282, 284, 285, 287, 288 Fischer-Tropsch synthesis, 186, 203, 204, 212 Fixed carbon, 188, 194, 297, 302, 304, 316, 317, 320 Flocculation, 77, 78, 80, 82, 91 Fluidized bed, 284–286 Food packaging, 20, 22, 27, 85 Forces, 12, 62, 64, 74, 75, 84, 134, 329, 330, 332, 333, 335, 337, 338, 340–343, 345, 347, 348, 353 Forest, 7, 40, 118, 270, 273, 278, 289, 311 Forest residues, 304 Formulation, 72, 83–85, 88, 89, 91, 108, 192 Fossil fuel, vi, 3, 40, 137, 157, 236, 271, 275, 283, 286, 299, 327 Fractionation, 8, 11, 13–15, 26, 27, 48, 62, 79 Free radicals, 41–44, 47, 48, 140, 296 Fresh juice, 162 Friction, 273, 286, 330, 339, 341–343, 345–347, 350, 351 Fuel, 3, 40, 139, 203, 234, 270, 296, 328, 372 Fuel cells, 138, 139, 188, 189, 206 Functional groups, 16, 18, 43, 44, 47, 101, 102, 110, 111, 118, 122, 125, 131, 134, 136, 140, 142, 155, 161, 172, 174, 175, 177, 187, 194, 197, 198, 200, 205, 206, 214, 236, 238, 242, 243, 252 Functionalization, 51, 101, 107, 108, 111, 136, 240, 260 Functionalized biochar, 134, 140, 247 Furfural, 9, 15, 23, 26, 44, 193, 194, 201, 204, 238, 246, 297, 375 Furnish, 62, 64, 65, 68, 70–73, 79, 82, 91 G Galactitol permease, 373 Galactose, 4, 12, 374 Gas diffusion electrodes (GDE), 213–215 Gasification, vi, 119, 186, 189, 195, 197, 200–202, 207, 210–212, 216, 243, 271, 296, 297, 299, 303, 314, 316, 317, 354 Genetic engineering, 364 Geo-thermal, 234 Glass transition temperature, 84, 274, 332, 335, 338, 377 Glucopyranosyl uronic acid, 5 Glucose, 4, 23, 120, 168, 193, 194, 201, 207, 241, 243, 245, 249, 251, 252, 255, 274, 275, 367–372, 374, 379, 381, 382 Glucuronic acid, 4, 6 Glucuronoarabinoxylan (GAX), 4, 9 Glucuronoxylan (GX), 4–6, 120

395 Graphene, 20, 127, 138, 142, 154, 155, 162, 176, 186–188, 201, 202, 205, 209, 213, 235, 249, 255 Graphite, 127, 132, 138, 142, 188–191, 202, 213, 214, 235–236, 252, 255, 256, 296 Graphitization, 132, 161, 167, 173, 187, 189, 194, 236, 243, 299 Grate, 284 Green economy, 40 Greenhouse gases, 4, 234, 271, 277, 282, 362 Green polymers, 362 Grindability, 272, 282, 288, 329, 337 Grinder, 80 Ground calcium carbonate (GCC), 75, 78, 79, 82 H Handling, 109, 272, 276, 279, 286, 289, 328, 336 Hard carbon electrodes, 235–236 Hard templates, 162 Hardwood, 5, 6, 8, 11, 12, 44, 45, 52, 64, 67, 80, 119, 129, 273, 277, 298, 302, 331, 332, 339 H/C ratio, 126, 191, 192, 296 Heat, 41, 49, 127, 157, 270, 271, 275, 277, 280, 282, 283, 286, 288, 302, 311–313, 318, 328, 354, 377 Heating rate, 119, 132, 188, 193, 239, 316, 318, 336 Heating value, 191, 272, 277, 288, 297, 312, 332, 333, 337, 338, 372 Heat treatment, 119, 123, 127, 138, 172, 174, 236 Heavy metal, 131, 135–136, 364 Hemicelluloses, v, 4, 6–8, 10, 12–15, 17, 20, 26, 44, 45, 48, 52, 120, 121, 192, 194, 239, 243, 252, 272, 330, 335–337, 339, 372, 374, 375 Hemocompatibility, 108 Heteroatom-doping, 161, 165, 169, 212, 245–247, 254, 255, 260 Heteroatoms, 165, 170, 186, 198, 201, 212, 214, 216, 238, 239, 243, 245, 246, 251, 254 Heteroxylan, 4 Hierarchically porous carbon, 136 Hierarchical structure, 167, 168 Hindered phenolic, 41, 43, 44, 47 Holdout, 65, 87 Holocellulose, 7 Homogeneous chemical derivatization, 101 Homoxylans, 4, 5

396 Honeysuckle, 246, 251, 258 Husks, 8, 9, 197, 270, 274 Hybrid electric vehicles, 234 Hydrogels, 22, 26, 161 Hydrogenation, 23, 51, 186, 189, 200, 201, 203, 204, 206, 271 Hydrolysate, 20, 374, 375, 382 Hydrolysis, 11, 14, 15, 23, 26, 52, 63, 64, 76, 141, 194, 201, 206, 209, 241, 271, 297, 338, 366, 375 Hydrolysis lignin (HL), 45, 50–56 Hydrophilic, 16, 20, 22, 27, 54, 85, 91, 100, 102, 105, 108, 109, 111, 134, 174, 187, 204, 208, 216 Hydrophobicity, vi, 16, 20, 22, 23, 26, 50, 53, 65, 70, 102, 105, 106, 108, 134, 174, 187, 203, 245, 272, 288, 336–338, 364 Hydrothermal, vi, 8–10, 12, 167, 169, 204, 240, 241, 246, 252, 255, 298 Hydrothermal carbonization (HTC), 119, 123–124, 137, 172, 193, 238, 240, 241, 245, 246, 250, 260, 296, 297, 313, 329, 336, 337 Hydroxyl groups, 11, 16, 17, 43, 44, 48, 49, 51, 79, 101–104, 108, 111, 175, 205, 243, 380 Hysteresis, 84, 89 I Immobilisation of enzymes, 104, 110 Immunoassay applications, 104 Impregnation, 12, 135, 137, 157, 161, 195, 210, 238, 250 Inductively coupled plasma mass spectroscopy (ICP-MS), 125, 126 Industrial pellets, 275, 277, 279, 285, 341 Industrial wastes, 186, 236, 270, 273 Initiation, 42, 43 Initiators, 41 In situ, 82, 211, 213 Interface, 154, 162, 168, 335 Intermolecular hydrogen bonding, 101 Ion, 23, 24, 134, 136, 137, 160, 168, 169, 172, 173, 177, 203, 213, 234, 243–247, 249, 254, 258, 260 Ion diffusion, 160, 168, 169, 172, 248 Ionic liquids (ILs), 11, 16, 19, 22, 26, 103, 104, 156, 161, 162, 174, 243, 245 Ionotropic gelation, 109 Irganox 1010, 43, 53, 55–56 Iron, 129, 205, 206, 211, 212, 214, 299–303 Iron composite, 246, 247 Irradiation, 10, 187

Index Isolation, 3–27, 104 Isothermal carbonization, 303 J Juncus roemerianus, 168 K Kaolin, 75–77, 80, 84, 86, 88 Kapok fiber, 169, 170 Kinetics, 122, 138, 204, 215, 248, 254, 313, 314, 316–320 Kraft lignin (KL), 40, 45, 49–56 Kraft pulps, 8, 45, 68, 70, 78, 80, 82, 372 L LA-based polymer, vi, 362–384 Lactic acid (LA), 23, 363–376, 380, 382 Lactides, 363–366, 376, 379, 381, 382 LA-fractions, 368–378, 381–383 LA-oligomers, 379–382 LA-polymerizing enzyme (LPE), 367–372, 374, 376, 377, 379, 382, 383 LA unit, 367, 373, 377 Lightweight, 40, 83, 307 Lignin, v, vi, 4, 8, 10, 12, 20, 22, 40–56, 62, 120–122, 165, 167, 188, 192–194, 201, 204, 239, 243, 252, 272, 274, 305, 328, 329, 332, 333, 335–337, 339, 372 LignoBoost, 45 Lignocellulose, vii, 46, 374 Lignocellulosic, v Lignocellulosic biomass, v, 3, 4, 9, 23, 26, 44, 118, 121, 192, 210, 243, 318, 336, 364, 372–376, 382, 384 Lignocellulosic materials, 9, 11, 45, 192, 194, 332 LignoForce, 45 Lignosulfonates, 45, 49, 127, 162, 165, 332, 333 Lipids, 239, 371 Lithium-ion batteries (LIBs), vi, 138, 234–236, 240, 241, 243, 246, 248, 249, 252, 255–260 Loss, 9, 20, 41, 127, 132, 161, 170, 208, 255, 299, 303, 304, 312, 320, 335, 373 Lysis, 380 M Macropores, 129, 157, 191, 197, 247, 249, 252 Maize, 332 Mannose, 4, 12

Index Maple, 119, 127, 130 Marine biomasses, 236, 239 Matrixes, v, vi, 43, 48–52, 55, 56, 105, 142, 167, 168, 243, 296 Mechanical, vi, 19, 45, 50, 52, 63–65, 75, 86, 101, 142, 187, 188, 202, 273, 304, 306, 327–354 Mechanical property, vi, 20, 22, 27, 41, 43, 49, 50, 85, 140, 167, 362, 364, 368, 377, 378 Mechanical stability, 50, 162, 194, 235 Mechanical stress, 41, 353 Melting temperatures, 368, 377 Membrane separation, 15 Mental-organic frameworks, 162 Mercury porosimetry, 129 Mesopores, 129, 157, 160, 163, 167, 172, 176, 191, 195, 197, 212, 242, 245, 247 Metabolic engineering, 364, 368, 373, 382 Methanation, 186, 204 Methane, 4, 204, 210, 377 Methylglucuronoxylans (MGX), 16–18 Methyl orange, 215 Microalgae, 163 Microbes, 362, 364, 365, 373, 377 Microbial factory, 366–367 Microfibrillated cellulose (MFC), 63, 64, 66, 72, 80, 84, 86, 89 Microorganisms, 23, 363, 364, 367, 378, 379 Micropores, 122, 125, 129, 157, 160, 162, 169, 172, 176, 191, 195, 203, 212, 215, 242, 247, 248, 252 Microwaves, 10, 274 Microwave treatment, 10 Milling, 188, 272, 286 Mills, 40, 45, 76, 283, 285–289 Mineral fillers, 63, 75–83 Miscanthus giganteus, 374 MNFC, 62, 63, 65–75, 82, 85–88, 91 Modifications, vi, 16–19, 22, 23, 26, 27, 46, 50–55, 63, 64, 77, 85, 100, 101, 135, 201, 205, 237, 240, 242, 260, 283, 287, 289, 363 Modified biochar, 135, 260 Moisture, 27, 91, 188, 239, 272, 275, 277, 300–302, 328–333, 335–339, 347, 353 Molecular weights, 10, 13–15, 17, 41–43, 48–53, 55, 56, 77, 81, 84, 194, 363, 365, 366, 370, 376–381, 383 Momentum, 341, 343 Mono-heteroatom, 165 Monomers, 41, 120, 121, 193, 194, 297, 362–364, 367–370, 376, 377, 382 Monomer sequence, 376

397 Monomer supply, 366, 370–372, 374, 382 Monosaccharides, 12, 23 Montmorillonitem (MMT), 20, 73 Morphology, 19, 20, 62, 63, 76, 77, 80, 100, 110, 129–130, 141, 162, 167, 241, 243, 245, 248, 249, 371 Municipal solid waste, 273 N Nanocellulose, vi, 61–91 Nano-fibrillar cellulose, 109 Nanoparticles, 22, 23, 26, 64, 73, 74, 82, 100, 108, 129, 141, 176, 203, 206, 252, 297 Nanoprecipitation, 107 Negative electrodes, 154, 234, 235, 246, 249, 252, 254–260 Neutralization, 78 Nitrogen, 52, 122, 126, 131, 132, 134–136, 139, 142, 161, 165, 193, 198, 199, 205, 206, 212, 214, 216, 243, 246, 252, 254, 271, 302 Nitrogen-doped carbon (NDC), 167, 214, 241 Nitrogen oxides (NOx), 200, 203–205, 271, 283, 285, 287, 306, 307, 320 Nuclear magnetic resonance (NMR), 6, 8, 13, 133, 177, 296, 369, 370, 376, 377, 379–381 Nutrients, 44, 128, 134–135, 163, 362 O O/C ratios, 126, 192 Oil palm, 168 Oils, 3, 20, 45, 49, 65, 83, 86, 141, 193, 194, 204, 206–210, 269–271, 285, 312, 328, 354 Oligoamine, 105 Optical properties, 75, 76, 82, 368 Optimization, 176, 279, 281, 312, 313 Organic contaminants, 134–135, 213 Organic solid waste, 273 Organosolv, 45, 48, 50 Oxidation induction time (OIT), 50, 53, 54 Oxidative cleavage, 110 Oxidative de-polymerization, 51 Oxidization, 18, 19 Oxyanions, 134–136 Oxygen, 20, 27, 41, 44, 51, 65, 86, 101, 118, 119, 122, 126, 140, 161, 165, 174, 175, 186, 187, 191, 194, 195, 198, 205, 206, 210, 212–214, 216, 239, 298, 310, 314, 336

398 Oxygen content, 187, 192, 194, 209, 243, 272, 336 Oxygen reduction reaction, 205, 206, 213 Oxygen transmission rate, 20, 86 Ozonation, 124 P Packaging, 20, 22, 27, 40, 83, 85, 89, 363, 365 Paper machines, 65, 66, 74, 91 Paper products, 62, 63, 70, 83 Paper properties, 64, 66, 68, 71, 82, 85–88, 91 Paper strength, 62, 65, 69, 73–75, 82, 91 Particulate matter (PM), 275, 307–310, 320 Peanut shell, 8 Pelletization, vi, 269–289, 308, 309, 312, 328–330, 332, 333, 335–340, 345, 346, 350–354 Pellets, 211, 269–289, 297, 301, 308–313, 317, 327–354 Permeability, 20, 67, 70–71, 75, 83, 85–87, 89, 300, 301 Petrochemical feedstocks, 362 pH, 12, 15, 17, 22, 23, 52, 53, 84, 111, 122, 125, 127, 128, 131, 134, 175, 214, 240, 297, 375 Phenolic rings, 44 Phenoxyl, 48 Phenyl propane, 44 Phosphite antioxidants, 41 Phosphorus, 126, 139, 163, 165, 198, 200, 212, 284 Photocatalytic, 142, 186 Photodegradation, 43, 55 Physical, v, vi, 11, 20, 62, 76, 118, 124, 129, 134, 140, 142, 154, 157, 167, 174, 177, 186, 195, 237, 239, 240, 271, 277, 303, 320, 332, 362 Physical activation, 124, 157, 170, 195, 237, 239 Physical modification, 237, 250 Physicochemical, 15, 239, 329, 336, 362, 364 Physisorption, 125, 128, 129 Pine, 130, 133, 141, 212, 240, 275, 277, 331, 333, 339 Pineapple leaf, 169, 170 Plasma treatment, 170, 174 Plastics, 40, 84, 100, 188, 273, 339, 362–364, 366, 372, 382, 384 Platanus fruit, 166, 167 Pollens, 162–164 Polyacrylamide (PAM), 72, 73, 78 Polycationic cellulose derivatives, 105

Index Polyesters, 46, 362–365, 367, 379, 382 Polyethylene (PE), 40–42, 49, 50, 52–56, 77, 362, 363 Polyhydroxyalkanoates (PHAs), 46, 362–377, 379, 380, 382 Polyhydroxyalkanoate synthase (PhaC), 366, 367, 376, 379, 380, 382 Polyhydroxybutyrate, 363, 366 Polylactic acid (PLA), 362–379, 381, 382 Polymerization, 7, 14, 15, 19, 241, 246, 297, 365–367, 369, 371, 373, 374, 380 Polymer-lignin blends, 46 Polymers, v, vi, 15, 16, 22, 26, 40–56, 63, 64, 72–75, 80, 83–86, 100, 101, 108, 109, 121, 122, 154, 162, 194, 213, 259, 332, 335, 339, 363, 364, 367–380, 382, 383 contents, 368, 371–374 degradation, 41 Polyolefins, vi, 40, 43, 47–55 Polypropylene (PP), 40–42, 49, 50, 52, 56, 362–364 Polysaccharides, 5, 6, 10, 11, 13, 18, 22, 26, 27, 105, 120, 246, 259 Polytetrafluoroethylene (PTFE), 213–215 Pond water, 62 Pores, 124, 129, 142, 155, 161, 162, 172, 173, 176, 193, 197, 201, 203, 210, 212, 216, 238, 241, 245–247, 250, 260, 304, 315, 338 Pore size distribution (PSD), 123, 125, 128–129, 136, 155, 162, 171, 172, 197, 202, 203, 242, 247, 249 Porosity, 82, 87, 90, 111, 118, 124, 125, 135, 138, 140, 157, 162, 165, 167, 188, 191, 192, 194, 195, 200, 201, 213, 238, 241, 243, 246, 249, 297, 303, 304, 317, 320, 321 Porous, 72, 110, 118, 124, 129, 134–137, 139, 140, 155, 160, 163, 167–169, 195, 202, 212, 237, 243, 245–247, 254, 301 Porous carbons, 124, 135–137, 154, 155, 157, 161, 165, 170, 172, 173, 175, 199, 248, 251, 252, 255, 257 Positive electrodes, 154, 156, 234, 255, 258, 260 Potassium hydroxide (KOH), 8, 124, 157, 195, 242 Potassium ion batteries, 236 Power, 10, 138, 153, 155, 163, 167, 168, 173, 176, 177, 214, 234, 235, 259, 269–289, 328, 329, 332, 341, 354, 372 densities, 138, 139, 155, 163, 176, 234 generation, vi, 269–289, 328, 372

Index Precipitated calcium carbonate (PCC), 72, 75–78, 80–82 Precursors, vi, 62, 118, 119, 127, 129, 137, 138, 142, 157, 159, 161–163, 167–170, 187, 192, 194, 196–198, 202, 203, 205, 206, 210, 236, 237, 239–241, 243, 246, 252, 260, 382 Pressures, 9, 23, 51, 52, 66, 78, 123, 129, 141, 161, 203, 272–274, 301, 304, 315, 320, 321, 327–329, 333, 335–340, 342–348, 352–354 Pretreatment, 9, 11, 12, 63, 84, 86, 88, 163, 203, 247, 254, 308, 336, 375 Probe, 125 Processability, 46, 110, 363 Propagation, 41–43, 48, 140 Properties, vi, 4, 41, 62, 100, 155, 186, 235, 271, 296, 329, 362 Proteins, 13, 14, 24, 102, 163, 239, 246, 275, 328, 364, 368, 381 Pseudocapacitance, 161, 165, 174 Pseudocapacitors, 154 Pulp and papers, 40, 45, 61 Pulps, 6, 8, 45, 63, 64, 68–71, 78, 80, 82, 91, 99 Pulverized fuel, 284, 289 Purification, v, 3, 48, 52, 100, 104, 110, 125, 155, 157, 364, 366, 367 Purpureus seeds, 162, 163 Pyrolysis, 118, 160, 188, 239, 271, 296, 329 Pyrolytic de-polymerization, 51 Pyruvate formate lyase, 368 R Raman spectroscopy, 132, 187, 194 Ramie fibers, 250, 257 Ramp rates, 119, 120, 123 Rate capacity, 259 Reactor conditions, 163 Recalcitrance, 11 Rechargeable batteries, 234 Recovery, 8–11, 13, 26, 45, 101, 135, 141, 196, 366 Reducing agents, 187, 320 Refining, 9, 52, 62–66, 69, 70, 91 Refractive index, 76 Regioselective, 105, 111 Regioselective synthesis, 105 Renewable energies, 137, 271, 275, 276, 287, 327 Renewable polymers, 26, 362, 364

399 Residence times, 119, 123, 191, 194, 239, 273, 297, 338 Retention, 63, 67, 72, 73, 75–78, 80, 82, 83, 88, 91, 109, 128, 129, 161, 165, 167, 234, 252, 256 Retention aids, 72–75, 78–80, 82 Rheology, 84–85, 88, 89 Rice hulls, 189 Rice-husk, 159, 202, 208, 209, 212, 239, 240, 250, 252, 256, 274, 276 Ring-opening polymerization (ROP), 363–366 Rollers, 273, 326, 327, 340–343, 345, 346 Runnability, 84, 89 S Saccharides, 206, 207, 210 Sawdust, 45, 159, 211, 240, 274–277, 313, 317, 328, 329, 331, 337, 353 Scanning electron microscopy (SEM), 79, 81, 125, 129–131, 160, 164, 166, 212, 241, 242, 248, 305 Scavengers, 43, 44, 47–49, 55 Schiff-base formation, 110 Seaweed, 176 Selective catalytic reduction (SCR), 203, 284, 286 Selectivities, 130, 140, 141, 185, 197, 201–206, 208, 212, 214 Self-templates, 163 Semi-industrial, 88–89, 91 Shear thinning, 84, 88, 91 Shrimp shells, 246, 258 Sigma (σ) factors, 370, 382 Single-pot bioprocess, 364, 367 Slot die, 84, 89, 90 Sludge, 118, 123, 134, 211, 270, 273–275, 286 SO2, 45, 271, 283, 285, 287 Sodium hydroxide (NAOH), 8–10, 45, 52, 103, 106, 124, 131, 157, 163, 195, 196, 206, 250, 276, 374 Sodium metaperiodate reagent (NaIO4), 18, 110 Sodium periodate, 18 Soil amendment, 118, 127, 128, 131, 133, 142, 191 Soil fertility, 127, 128 Solar, 137, 234 Solid-acid catalysts, 206–208 Solubilization, 9, 12, 101 Solvation, 11, 172, 173

400 Solvents, 5, 8–13, 15–17, 26, 27, 51–53, 85, 99, 101–104, 107, 108, 161, 172, 240, 243, 245, 365 Sonication, 78 Sorption, 135, 136 Soybean milk, 249, 251, 257 Specific surface area (SSA), 76, 79, 124, 137, 154, 155, 159, 163, 166, 168–170, 237, 238, 241–243, 260, 304 Spores, 162, 163 Spruce bark, 275, 277 Stability, 23, 40, 48, 50, 53, 54, 63, 75, 120, 122, 131, 132, 140, 155, 157, 161, 165, 167, 168, 170, 175, 177, 194, 203, 205, 210, 216, 235, 236, 245, 269, 301 Stabilization, 41–44, 47, 49 Stabilizers, 40, 41, 44, 46–49 Starch, 11, 12, 65–67, 70, 72, 73, 78, 80, 81, 83, 157, 159, 162, 163, 193, 204, 207, 274–276, 305, 328, 332–334, 337, 365, 372 Steam, 8, 9, 26, 124, 157, 158, 195, 197, 239, 283, 286, 314, 337, 338 Steam explosion, 9, 327, 337, 338 Steel, vi, 282, 289, 298–304, 340, 347 Stereoregularity, 100, 101 Stoker, 284 Straws, 49, 127, 157, 166, 167, 214, 270, 273, 274, 284–286, 374 Strength properties, 62, 64, 68–70 Stress, 49, 69, 84, 326, 327, 343, 353 Subcritical fluids, 10 Sucrose, 157, 159, 241, 274, 275 Sugarcane bagasse, 14, 49, 245, 256, 274 Sugars, 4, 7, 10, 13, 15, 16, 26, 44, 45, 52, 119, 120, 157, 159, 189, 241, 251, 363, 372–375 Sulfur, 45, 50, 122, 126, 165, 300, 302, 333 Sulfuric acid (H2SO4), 9, 52, 53, 63, 76, 157, 158, 161, 166, 168, 170, 174, 175, 195, 198, 206, 207, 209, 214, 246, 250, 251 Sulphonation, 206, 208–210, 216 Sulphonic acids, 102, 201, 205–208 Sulphur, 196, 198, 199, 205, 212, 271, 302, 331 Supercapacitors, vi, 119, 153, 198, 235 Supercritical anti-solvent, 15 Supercritical carbon dioxide (scCO2), 15 Supercritical fluids, 26, 51 Support matrix, 105 Supports, 105, 111, 142, 168, 169, 186, 187, 189–191, 193, 194, 200–205, 210, 212, 213, 216, 370

Index Surface, 18, 62, 108, 118, 154, 187, 235, 273, 296, 335, 363 areas, 62, 65, 71, 73, 122–125, 128–129, 134, 136–138, 140, 141, 155, 157, 160, 162, 172, 174, 175, 188, 189, 194, 195, 197, 200–202, 210, 212, 215, 235–239, 245–249, 252, 254, 297, 303, 335, 341 charges, 63, 73, 77–81, 84–86, 88, 91, 136 chemical compounds, 125, 131 Sustainable, v, vi, 3, 4, 83, 100, 103, 137–138, 142, 157, 162, 163, 167, 168, 170, 185, 188, 202, 216, 240, 260, 289 Switchgrass, 186, 189, 191, 211, 275, 276, 298 Synergistic effects, 141, 169, 371 Synthesis strategies, 170, 199 T Talc, 75, 76 Tars, 141, 189, 204, 210, 211, 276, 296, 335 Tea-seed shells, 257 Technical lignins, 40, 45, 47, 48, 55 Temperatures, 9, 41, 63, 103, 119, 157, 186, 239, 272, 328, 363 TEMPO-oxidation, 77 TEMPO-oxidized nanofibers, 109 Tensile strength, 20, 43, 46, 49, 54–56, 68–70, 83, 89, 187, 188, 377 Termination, 43, 380 Thermal, vi, 18, 41, 119, 161, 194, 234, 272, 336, 362 decomposition, 120 stability, 18–20, 26, 46, 49, 50, 125, 131, 245 Thermochemical, 51, 118, 239, 260, 271, 336, 375 Thermogravimetric analysis (TGA), 125, 132, 161, 316, 318 Thermo-oxidative degradation, 41–43, 55 Thermoplastic, 16, 40, 44, 48, 362, 363 Torrefaction, 272–275, 288, 289, 313, 318, 329, 335–337 Tosyl cellulose, 103, 106, 107, 111 Total phenolic content, 48 Toughness, 140 Transesterification, 142, 186, 189, 201, 204, 206–210, 271 Transportation fuels, 40 Treated biochar, 242, 244, 252 Turbostratic, 188, 194 Twin-screw extruder, 12

Index U Ultrasonic, 10 irradiation, 10 Uncalendered, 87 Uncoated, 87, 89 UV, 41–44, 46, 49, 50, 55, 56, 187 V Van Krevelen plot, 126, 191 Viability, 109, 118, 192 Vilsmeier-Haack-type reaction, 106 Volatiles, 120, 122, 140, 188, 195, 204, 239, 272, 297, 302, 303, 308, 318, 336 Voltages, 136, 156, 161, 234 W Walnut shells, 258 Water treatment, 133 Water vapor transmission rate (WVTR), 22, 87, 89 Waxberry, 162, 163 Waxy rice, 330 Wettability, 62, 125, 155, 161, 165, 174, 213, 243, 247 Wheat chaff, 11, 22, 159, 161, 246, 247, 249, 256, 275 Wheat straw, 11, 22, 161, 246, 247, 249, 250, 256, 274–276, 331 Willow catkins, 159, 166–170 Wind, 137, 234, 271, 285

401 Wood, vi, 7, 44, 87, 99, 118, 157, 186, 239, 270, 328 pellets, 211, 275–277, 279–289, 297, 308–310, 312, 328, 332, 337, 345–353 sawdust, 165, 166, 275, 276 X X-ray diffraction (XRD), 125, 127, 132, 211, 239, 240 X-ray photoelectron spectroscopy (XPS), 131, 174, 205, 247, 248 Xylan, vi, 4, 120, 375 Xylanases, 23, 375 Xylitol, 14, 23, 26 Xylooligosaccharides, 7, 9, 11, 14, 15, 23, 26 Xylopyranose, 4 Xylose, 4–6, 9–12, 14, 15, 18, 23, 26, 193, 371–376, 381, 382 Xylosidase, 23–25, 375, 376 Y Yeast, 23, 26 Yields, 8–10, 12, 18, 23, 45, 51, 68, 72, 84, 107, 111, 118, 120–124, 157, 193, 194, 207–210, 242, 243, 245, 259, 308, 312, 336, 363, 370–373, 375, 381–383 Young’s modulus, 46, 327, 340, 378 Z Zwitterionic cellulose beads, 111, 112