Biomass-Derived Humins: Formation, Chemistry and Structure 9819919908, 9789819919901

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Li Liu

Biomass-Derived Humins Formation, Chemistry and Structure

Biomass-Derived Humins

Li Liu

Biomass-Derived Humins Formation, Chemistry and Structure

Li Liu College of Chemistry and Chemical Engineering Inner Mongolia University Hohhot, Nei Mongol, China

ISBN 978-981-99-1990-1 ISBN 978-981-99-1991-8 (eBook) https://doi.org/10.1007/978-981-99-1991-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Contents

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

Structure and Formation Mechanism of 5-Hydroxymethylfurfural (HMF)-Derived Humins . . . . . . . . . . . . . 2.1 Polymerization of HMF and Aromatic Decomposition Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Humins Formation via HMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Humins Formation via DHH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Aldol Addition/Condensation of DHH with HMF . . . . . . . . . . . . . 2.5 Aldol Condensation and/or Etherification of HMF with DHH or HMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Polymerization of HMF Through Electrophilic Substitution . . . . 2.7 Humins Formation via α-Carbonyl Aldehyde . . . . . . . . . . . . . . . . . 2.8 Etherification, Esterification, Aldol Condensation, and Acetalization of HMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

4

1 3 7 8 9 10 11 13 14 15 16 18 19

Structure and Formation Mechanism of Furfural-Derived Humins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Humins Formation via Furfural . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Electrophilic Substitution of Furfural . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Condensation of Furfural/ring-Cleavage Compounds . . . . . . . . . . 3.4 Humins Formation via α-Carbonyl Aldehyde . . . . . . . . . . . . . . . . . 3.5 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23 24 25 26 28 30 30

Structure and Formation Mechanism of Glucose-Derived Humins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Intermolecular Condensation of Glucose . . . . . . . . . . . . . . . . . . . . .

33 34

v

vi

Contents

4.2

Polymerization/Condensation Between Glucose and Decomposition Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Humins Formation via HMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Polycondensation of HMF Through Electrophilic Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Aldol Addition/condensation of DHH with Other Aldehydes and Ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Aldol Condensation of DHH and Limited LA with Aldehyde Groups of HMF/DHH . . . . . . . . . . . . . . . . . . . . . . . 4.7 Aldol Condensation Between HMF, DHH and WSO . . . . . . . . . . 4.8 Humins Formation via α-Carbonyl Aldehyde . . . . . . . . . . . . . . . . . 4.9 Etherification of Dehydrated Glucose and Electrophilic Substitution of Furfural Alcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

6

Structure and Formation Mechanism of Xylose-Derived Humins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Humins Formation via Furfural . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Electrophilic Substitution of Furfural . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Polymerization of Furfural and Xylose Oligomers . . . . . . . . . . . . . 5.4 Humins Formation via Furfural . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Humins Formation via α-Carbonyl Aldehyde . . . . . . . . . . . . . . . . . 5.6 Humins Formation via Xylose Oligomers . . . . . . . . . . . . . . . . . . . . 5.7 Copolymerization Between Furfural and Ethylene Glycol (EG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Condensation Between Isomerized Xylose and Furfural/ring-Cleavage Compounds . . . . . . . . . . . . . . . . . . . . . 5.9 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure and Formation Mechanism of Cellulose-Derived Humins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Polymerization/condensation Between Decomposition Products of Cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Humins Formation via HMF or Intramolecular Condensation, Dehydration and Decarboxylation . . . . . . . . . . . . . 6.3 Aldol Addition/condensation of DHH with Other Aldehydes and Ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Polymerization of HMF and Glucose by Acetalization and Etherification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35 37 39 40 43 45 46 49 50 53 57 58 60 61 63 63 66 66 68 69 70 73 75 76 78 79 81 82

Contents

7

Structure and Formation Mechanism of Pseudo-Lignin Derived from Lignocellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Lignin Maintaining the Natural Macrostructure . . . . . . . . . . . . . . . 7.2 Polycondensation and/or Polymerization Between DMC/BTO and Furfural/HMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Pseudo-Lignin Formation via Xylose Mostly . . . . . . . . . . . . . . . . . 7.4 Linkage of Lignin with DMC/BTO and Furfural/HMF . . . . . . . . . 7.5 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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85 87 92 93 96 96 98

8

Analytical Methods for Humins Characterization . . . . . . . . . . . . . . . . 8.1 FT-IR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 13 C Solid-State NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 1D 13 C Solid-State NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 2D 13 C Solid-State NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Elemental Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 X-Ray Photoelectron Spectroscopy (XPS) . . . . . . . . . . . . . . . . . . . 8.5 Dynamic Lights Scattering (DLS) . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . . . . . . . . 8.7 Pyrolysis–GC–MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8 HPLC–MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9 Gel Permeation Chromatography (GPC) . . . . . . . . . . . . . . . . . . . . . 8.10 Theoretical Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

101 101 103 103 105 108 108 112 113 114 117 120 121 128

9

Humins Valorization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Depolymerization of Humins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.1 Gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.2 Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.3 Hydrotreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Adsorbents for CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Heavy Metal Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Thermoset Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 Approach of Auto-Crosslinking . . . . . . . . . . . . . . . . . . . . . . 9.4.2 Approach of Chemical Modifications . . . . . . . . . . . . . . . . . 9.5 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

131 131 132 132 133 135 138 140 141 142 144 145

10 Outlook and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Chapter 1

Introduction

The demand of global energy is elevating at a high rate. Nevertheless, over 80% of the global energy is mostly from nonrenewable fossil fuels, which can lead to the highest growth rate of greenhouse gas emissions, thereby causing the climate change (2019). Extensive explorations on developing cost-effective systems for harnessing energy from a carbon–neutral renewable resource and carbon-free renewable sources including solar, wind, geothermal, tidal, nuclear, and hydroelectric energy are being undertaken with the aim of seeking alternatives to fossil fuels (Klass, 1998; KoohiFayegh & Rosen, 2020). Biomass is the sole renewable source of organic carbon in nature (Huber et al., 2006; Klass, 1998; Lin & Huber, 2009), which arouses wide concern as a reproducible carbon source for producing various fuels, chemicals and materials (Alonso et al., 2012; Chatterjee et al., 2015; Gallezot, 2012; Tuck et al., 2012; Wang et al., 2015), apart from its part as the world’s fourth greatest source of energy to generate power and heat after oil, coal, and natural gas (Sawin & Sverrisson, 2014). Converting biomass to fuels and chemicals efficiently assists in alleviating the great burdens on fossil resources, which is identified to be the practicable approach for reducing CO2 net emissions through combining chemical approaches with photosynthesis (He et al., 2013). Additionally, using renewable biomass with great abundance in the nature to be the feedstock for value-added products can incorporate the green chemistry principles in sustainability. Consequently, as more and more global concerns have been paid to nonrenewable fossil carbon reserve depletion as well as the associated climate change, biomass transformation arouses increasing attention from industry and academia in the last few decades. Certainly, producing fuel products, functional materials and value-added chemicals based on biomass is becoming a more and more attractive research field (De Lasa et al., 2011; Ennaert et al., 2016; Luterbacher et al., 2014; Rinaldi et al., 2016; Straathof, 2014). Lignocellulosic biomass, such as softwoods, hardwoods, urban wastes, grasses, and agricultural residues, has been the renewable carbon–neutral resource representing the most abundant biomass in the world. Under such circumstances, using lignocellulosic biomass seems to be one of the most promising alternatives to produce © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 L. Liu, Biomass-Derived Humins, https://doi.org/10.1007/978-981-99-1991-8_1

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1 Introduction

chemicals and fuel in a sustainable and environmentally-friendly manner (Alonso et al., 2010, 2012; Besson et al., 2014; Bozell & Petersen, 2010; Centi et al., 2011; Corma et al., 2007; Gallezot, 2012; Isikgor & Becer, 2015; Jin et al., 2018; Kamm et al., 2006; Lange et al., 2012; Pfaltzgraff et al., 2013; Sheldon, 2014; Sudarsanam et al., 2018; Tuck et al., 2012; Zakzeski et al., 2010; Zhou et al., 2011). Lignocellulosic biomass mainly includes cellulose (30–50 wt%), hemicellulose (20–40 wt%) and lignin (10–20 wt%), and their weight percentages greatly change from plants to wastes and residues (Chen, 2014; Xu et al., 2020). Due to the complex and recalcitrant structure, pretreatment of lignocellulose by catalytic, mechanical, physico-chemical, and biological means has drawn a lot of attention (Paone et al., 2020; Renders et al., 2019; Schutyser et al., 2018). With the deconstruction of lignocellulose, cellulose, hemicellulose and lignin fractions can be transformed into platform chemicals, which will be further converted into target molecules, added-value chemicals, and materials (De et al., 2015; Deuss & Barta, 2016; Gu & Jérôme, 2013; Liu et al., 2014; Ruppert et al., 2012; Serrano-Ruiz et al., 2011; Sudarsanam et al., 2019; Xu et al., 2014; Zhang & Huber, 2018; Zhang et al., 2016). In 2004, the U.S. Department of Energy published a list of the most potential biomass-derived platform molecules, which was updated by Bozell et al. in 2010 (Bozell & Petersen, 2010; Werpy & Petersen, 2004). Hereinto, 5-hydroxymethylfurfural (HMF) and furfural, as versatile biomass-based C6 and C5 platform molecules, are attracting increasing interests. Nonetheless, during the acid-catalyzed biomass conversion procedure, the darkbrown insoluble byproducts (also called humins) are formed inevitably. In general, the yield of humins is 20–50% with 55–65% carbon content, suggesting that ~30– 80% of carbon (40%) is not adopted for producing targeted chemicals, reducing the target products’ yield (Corma et al., 2007). It is of note that humins’ relative high yield is inevitable in pilot or plants of industrialized scale. For instance, about 25–30 wt% of carbohydrate materials finally become humins byproducts in the Biofine procedure (Kamm et al., 2008). Besides, deposition of humins on the continuous tube reactor’s inner wall is possible, so that the reactor is clogged, the pressure is elevated and the thermal transfer is less efficient. To evade the safety issue resulting from pressure elevation, the requirements placed on manufacturing of reactors should be more stringent. Additionally, apart from absorbing carbohydrates, HMF and LA (Hoàng, 2014; Tarabanko et al., 2015), humins are also capable of absorbing homogeneous catalysts like H2 SO4 . According to a report, following the filtration elimination of humins, the amount of H2 SO4 retained in the aqueous solution was merely ~90.5% (Kang & Yu, 2016). To recover the targeted chemicals thoroughly, washing by water is thus necessary. Otherwise, humins would partially adsorb the catalysts and final products to cause their partial loss. Hence, humins become the bottle neck problem in biomass, due to their competing with desired products, restraining the activity of catalyst, as well as hindering the recycling of catalyst and separation of products. With the purpose of further improving the efficiency of acid-catalyzed biomass conversion, unambiguous elucidation of the chemical structure and formation mechanism of humins are prerequisite.

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Over the previous few decades, many research groups have made tremendous attempts to speculate the chemical structure and formation mechanism of humins from a variety of feedstocks. A lot of novel technologies, methods and characterizations have been developed to unveil the evolution process of humins, which offer novel insights to enhance the efficiency of biomass conversion. On the other hand, the elucidation of the chemical structure and formation mechanism of biomass-derived humins can be also beneficial for the valorization of humins. This book focuses on the chemical structure analysis and formation mechanism of various biomass-derived humins, including 5-hydroxymethylfurfural (HMF), furfural, glucose, xylose, cellulose and lignocellulose, from simple molecular models to monosaccharides, until raw biomass resources. The chemical similarities and differences of various biomass-derived humins have been systematically summarized according to advanced analytical interpretation. Furthermore, the progresses that have been achieved on humins valorization and future perspectives are discussed.

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Serrano-Ruiz, J. C., Luque, R., & Sepulveda-Escribano, A. (2011). Transformations of biomassderived platform molecules: From high added-value chemicals to fuels via aqueous-phase processing. Chemical Society Reviews, 40, 5266–5281. Sheldon, R. A. (2014). Green and sustainable manufacture of chemicals from biomass: State of the art. Green Chemistry, 16, 950–963. Straathof, A. J. J. (2014). Transformation of biomass into commodity chemicals using enzymes or cells. Chemical Reviews, 114, 1871–1908. Sudarsanam, P., Peeters, E., Makshina, E. V., Parvulescu, V. I., & Sels, B. F. (2019). Advances in porous and nanoscale catalysts for viable biomass conversion. Chemical Society Reviews, 48, 2366–2421. Sudarsanam, P., Zhong, R., Van Den Bosch, S., Coman, S. M., Parvulescu, V. I., & Sels, B. F. (2018). Functionalised heterogeneous catalysts for sustainable biomass valorisation. Chemical Society Reviews, 47, 8349–8402. Tarabanko, V. E., Smirnova, M. A., Chernyak, M. Y., Kondrasenko, A. A., & Tarabanko, N. V. (2015). The nature and mechanism of selectivity decrease of the acid-catalyzed fructose conversion with increasing the carbohydrate concentration. Journal of Siberian Federal University Chemistry, 8, 6–18. Tuck, C. O., Perez, E., Horvath, I. T., Sheldon, R. A., & Poliakoff, M. (2012). Valorization of biomass: Deriving more value from waste. Science, 337, 695–699. Wang, H. L., Ruan, H., Pei, H. S., Wang, H. M., Chen, X. W., Tucker, M. P., Cort, J. R., & Yang, B. (2015). Biomass-derived lignin to jet fuel range hydrocarbons via aqueous phase hydrodeoxygenation. Green Chemistry, 17, 5131–5135. Werpy, T., & Petersen, G. (2004). Top value added chemicals from biomass volume I-Results of screening for potential candidates from sugars and synthesis gas. National Renewable Energy Laboratory (NREL). Xu, C., Arancon, R. A. D., Labidi, J., & Luque, R. (2014). Lignin depolymerisation strategies: Towards valuable chemicals and fuels. Chemical Society Reviews, 43, 7485–7500. Xu, C., Paone, E., Rodriguez-Padron, D., Luque, R., & Mauriello, F. (2020). Recent catalytic routes for the preparation and the upgrading of biomass derived furfural and 5-hydroxymethylfurfural. Chemical Society Reviews, 49, 4273–4306. Zakzeski, J., Bruijnincx, P. C. A., Jongerius, A. L., & Weckhuysen, B. M. (2010). The catalytic valorization of lignin for the production of renewable chemicals. Chemical Reviews, 110, 3552– 3599. Zhang, X. G., Wilson, K., & Lee, A. F. (2016). Heterogeneously catalyzed hydrothermal processing of C-5-C-6 sugars. Chemical Reviews, 116, 12328–12368. Zhang, Z., & Huber, G. W. (2018). Catalytic oxidation of carbohydrates into organic acids and furan chemicals. Chemical Society Reviews, 47, 1351–1390. Zhou, C. H., Xia, X., Lin, C. X., Tong, D. S., & Beltramini, J. (2011). Catalytic conversion of lignocellulosic biomass to fine chemicals and fuels. Chemical Society Reviews, 40, 5588–5617.

Chapter 2

Structure and Formation Mechanism of 5-Hydroxymethylfurfural (HMF)-Derived Humins

5-Hydroxymethylfurfural (HMF) was initially reported by Dull et al. (Dull, 1895) in the late nineteenth century, where they synthesized HMF through pressurized thermal processing of inulin in a solution of oxalic acid. On account of its bifunctional hydroxyl and aldehyde groups, the U.S. Department of Energy has enlisted HMF, a representative product from cellulose and glucose dehydration, as one of the major biomass-derived platform chemicals for yielding various precious chemicals (Fig. 2.1), such as 2,5-furandicarboxylic acid (monomer substitute for terephthalate) (Davis et al., 2012; Hayashi et al., 2019), 2,5-dimethylfuran (biofuel) (Guo et al., 2016; Saha et al., 2014), furan-2,5-dicarbaldehyde (chemical intermediate) (Fang et al., 2016; Zhang & Huber, 2018), as well as levulinic acid (platform chemical) (Hou et al., 2018, 2020; Liu, 2021; Liu et al., 2018; Ren et al., 2013, 2015; Zhao & Liu, 2021). The process of biomass transformation inevitably generates humins. Over the last several decades, a few researchers have focused on exploring the formation mechanism and structure of humins from diverse feedstocks varying from HMF, glucose to cellulose (Chuntanapum & Matsumura, 2009; Horvat et al., 1985, 1986; Patil & Lund, 2011; Patil et al., 2012; Sevilla & Fuertes, 2009a, 2009b; Sumerskii et al., 2010; Titirici et al., 2008; Tsilomelekis et al., 2016; Van Zandvoort et al., 2013). A substantial portion of references (Chuntanapum & Matsumura, 2009; Patil & Lund, 2011; Patil et al., 2012; Sumerskii et al., 2010; Titirici et al., 2008; Tsilomelekis et al., 2016; Van Zandvoort et al., 2013) support the idea that HMF contributes to the formation of humins. In terms of structure, the humins acquired from HMF are less sophisticated than those acquired from glucose or cellulose. Therefore, the research of humins was simplified by selecting HMF as the model molecule. With this measure, tremendous reduction was achievable concerning the complexity of reaction resulting from the extra system constituents, thereby increasing the opportunity to recognize critical intermediates in the course of humins formation.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 L. Liu, Biomass-Derived Humins, https://doi.org/10.1007/978-981-99-1991-8_2

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2 Structure and Formation Mechanism of 5-Hydroxymethylfurfural …

Fig. 2.1 HMF as platform chemical (Reproduced from Van Putten et al. [2013] with permission. Copyright 2013 American Chemical Society)

2.1 Polymerization of HMF and Aromatic Decomposition Compounds Humins obtained by hydrothermally processing biomass without catalyst are often referred to as hydrochar or hydrothermal carbon (HTC), which have been characterized more comprehensively as compared to those acquired by acid-catalyzed conversion (Sevilla & Fuertes, 2009a, 2009b). No unified consensus has been reached, despite the close emphasis on the HTC structure and formation. Hydrothermal processing of HMF was explored by Matsumura et al. in both supercritical (25 MPa, 450 °C) and subcritical (25 MPa, 350 °C) settings, whereby HTC was merely produced in the subcritical setting (Chuntanapum & Matsumura, 2009). Under the supercritical scenario, fast HMF degradation was noted, without generating HTC. Contrastively, in the subcritical setting, HMF underwent degradation (gasification) in synchronization with polymerization (generation of HTC). In the first of these two reaction pathways, degradation of HMF into a liquid product was noted, which consisted of aromatic and acidic compounds like 1,2,4-benzenetriol (BTO), 5-methyl-2-furaldehyde (MF), 1,4-benzenediol (BDO), furfural and levulinic acid. Pyrolysis and hydrolysis of furan rings were encompassed in the reactions. Subsequently, the aromatics were partially degraded and also contributed a bit of gaseous product, while the majority of acidic compounds were gasified to H2 , CO and CO2 . Regarding the second reaction pathway, the Raman spectrometry and FT-IR spectroscopy revealed that the HTC particles displayed features of cyclic

2.2 Humins Formation via HMF

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Fig. 2.2 Reaction pathway of HMF (Reproduced from Chuntanapum and Matsumura [2009] with permission. Copyright 2009 American Chemical Society)

and aromatic structures for both furan and benzene rings, despite the deficiency of aliphatic fragments. Accordingly, they suggested that through polymerization, the HMF and degraded liquid aromatics (e.g. BTO, BDO) formed HTC that was insoluble in water (Fig. 2.2). This was achieved principally by the ring substitution reactions and the substituent functional groups.

2.2 Humins Formation via HMF In their exploration of glucose-derived HTC, Titirici et al. carbonized HMF, the primary dehydration intermediate of glucose, under identical conditions of carbonization (Titirici et al., 2008). As displayed by SEM, the microstructure of C-HMF comprises small (160) were needed for aqueous glucose solutions for the transformation into porous carbon sphere dispersion, whereas fructose dehydrated within water at 3–4 atm under the deceased temperature (120 °C) owing to the more reactive furanose form of fructose, in comparison to glucose present in the form of pyranose. Consequently, the authors suggested that, due to the stable pyranose structure, glucose lost water by the intermolecular condensation reaction during heating under pressure. But, fructose first formed HMF via the intramolecular dehydration and then formed carbon through water loss. The continuous intermolecular dehydration later led to rough surfaces (raspberry structure) in the generation of carbon spheres.

4.2 Polymerization/Condensation Between Glucose and Decomposition Products As reported by Sevilla et al., during hydrothermal treatment process, poly-saccharides (starch) and disaccharides (sucrose) were hydrolyzed to produce relevant monosaccharides (namely glucose for starch, whereas glucose/fructose for sucrose) (Sevilla & Fuertes, 2009a). Additionally, monosaccharides were decomposed, as a result, organic acids (such as lactic, acetic, levulinic, propenoic, and formic acids) were generated (Antal et al., 1990, Bobleter, 1994; Schuhmacher et al., 1960), thus

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4 Structure and Formation Mechanism of Glucose-Derived Humins

decreasing pH to pH ~3 in a short time. The hydronium ions from the acids then act as catalyst to achieve oligosaccharides decomposition (for starch) (Garrote et al., 1999). For instance, glucose can be dehydrated to 1,6-anhydroglucose apart from being fragmented to erythrose, glyceraldehyde and dihydroxyacetone (Kabyemela et al., 1999). Furfural-like compounds can be further degraded to produce phenols, as well as acids/aldehydes (Aida et al., 2007; Asghari & Yoshida, 2006; Chheda et al., 2007; Kabyemela et al., 1999; Luijkx et al., 1995; Sinag et al., 2003). Next step is condensation or polymerization of glucose and/or its degradation products, so that soluble polymers can be generated (Asghari & Yoshida, 2006). The intermolecular dehydration [Eq. (4.1)] or the aldol condensation [Eq. (4.2)] is the probable inducer of the foregoing two reactions.

(4.1)

(4.2)

After hydrothermal processing of glucose above 170 °C, a carbonaceous solid residue (hydrochar) is yielded, which comprises uniform microspheres at a micrometer-scale (0.4–6.0 mm). Through synthesis condition alteration, the diameter adjustment is achievable for these microspheres. As the reaction temperature rises, reaction duration prolongs or glucose content increases, the diameter of microspheres enlarges. The XPS and FT-IR results suggest that substantial oxygenfunctional groups are contained in the glucose-derived hydrochar microspheres. Despite the homogeneous distribution of oxygen along the microspheres, the oxygenfunctional groups vary in nature between the particle shell and core. Accordingly, as depicted in Fig. 4.3, the microspheres possess a core–shell structure, where the hydrophobic core is highly aromatic and contains stable oxygenic groups (e.g. pyrone, ether, quinone) and the hydrophilic shell encompasses hydrophilic/reactive oxygen-containing groups (e.g. carboxylic, carbonyl, ester, hydroxyl, phenolic) that are highly dense. Owing to the following two features, the hydrochar microspheres are applicable in the areas like drug delivery, catalysis and enzyme fixation. Firstly, they are perhaps

4.3 Humins Formation via HMF

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Fig. 4.3 (a) TEM image and (b) schematic illustration of the core–shell chemical structure (Reproduced from Sevilla and Fuertes [2009a] with permission from Wiley–VCH)

easily linked to other functionally complementary substances that are conducive to the functional nanocomposite preparation, since they contain high content of surface oxygen groups. Secondly, their synthesis into uniform size (220 °C. The cellulose hydrolysis at this phase is catalysed by the hydronium ions resulting from the autoionization of water (Tanger & Pitzer, 1989), thereby yielding various oligomers (cellohexaose, cellopentaose, cellotetraose, cellotriose and cellobiose), as well as glucose (Garrote et al., 1999; Ogihara et al., 2005; Sasaki et al., 2000), which then undergoes isomerization to generate fructose (Bobleter, 1994, Nagamori & Funazukuri, 2004). The monomer degradation brings about organic acids (acetic, levulinic lactic, propenoic and formic) (Bobleter, 1994; Antal et al., 1990). In succeeding reaction phases, the degradation is catalysed by the hydronium ions generated from the foregoing acids (Sinag et al., 2003). The hydrolysis of oligomers also leads to their monomer form. These monomers are dehydrated and fragmented (i.e. ring opening and C–C linkage breaking) to yield diverse solubles, including erythrose, furfural-like compounds (i.e. 5-methylfurfural, furfural, HMF), HMF-associated 1,2,4-benzenetriol, 1,6anhydroglucose, acids, as well as aldehydes (acetonylacetone, acetaldehyde, pyruvaldehyde, glycolaldehyde, glyceraldehyde) (Aida et al., 2007; Asghari & Yoshida, 2006; Chheda et al., 2007; Kabyemela et al., 1999; Luijkx et al., 1995; Sasaki et al., 1998; Sinag et al., 2003). The furfural-like compounds can also be degraded to produce phenols and acids/aldehydes (Ogihara et al., 2005). In the next reaction phase, condensation or polymerization occurs to result in the soluble polymer generation (Asghari & Yoshida, 2006). Aldol condensation or intermolecular dehydration are the probable inducers of the above two reactions. There is a concurrent event of polymer aromatization. Since water is dehydrated from the monomers’ equatorial hydroxyl groups, C=O groups emerge (Tang & Bacon, 1964). Alternatively, intramolecular dehydration or keto–enol tautomerism of dehydrated species

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6 Structure and Formation Mechanism …

probably leads to the C=C linkage emergence (Tang & Bacon, 1964). The aromatized molecules produced during the degradation/dehydration of monosaccharides or oligosaccharides can be condensed due to the intermolecular dehydration, which is the possible cause of aromatic cluster formation. A burst nucleation happens upon reaching of the aromatic cluster content in aqueous solution to a critical point of supersaturation. Through diffusion, the nuclei created therefrom show outward growth towards the chemical species surfaces existing in the solution. There is linkage of such chemical species to the microsphere surfaces via the reactive oxygen functionalities (e.g. carbonyl, carboxylic, hydroxyl) existing in both the reactive species and the external particle surfaces, giving rise to the formation of stable oxygen groups like quinone or ether. In these contexts, upon termination of the growth process, substantial reactive oxygen groups will be encompassed in the external surfaces of hydrochar microspheres, while the reactive groups produced from the oxygen in the core will be less in quantity. Accordingly, Fig. 6.2 displays a chemical model of hydrochar microspheres, which describes the chemical differences of the microsphere core from the corresponding shell.

6.2 Humins Formation via HMF or Intramolecular Condensation, Dehydration and Decarboxylation Titirici and co-workers investigated the HTC derived from cellulose (Falco et al., 2011). As shown by SEM images, a significant difference in the mechanism of particle formation can be found between glucose and cellulose. Besides, as shown in the former case, following the HMF generation and polymerization, particles form via a nucleation process from a homogeneous solution (Falco et al., 2011). With the extension of residence duration, they grow persistently until final size is reached, depending on the temperature of HTC treatment. Contrastively, for cellulose, it is unsusceptible to the hydrothermal treatment when the temperature of HTC process is low. Its fibers remain intactness, which are organized in the distinctive cellulose network (O’Sullivan, 1997). Upon elevation of the HTC temperature, disruption of the fibrous network commences at a few points, so that nano/micro-sized cellulose fragments are created, which are insoluble in water. By adopting a spherical morphology, the contacting interface between them and surroundings could be minimized. Furthermore, as demonstrated by the solid-state 13 C NMR patterns, the hydrothermal treatment remains uninfluential to the crystalline cellulose at 180 °C. The characteristic peaks (65, 72, 75, 84, 89 as well as 105 ppm) (Link et al., 2008) exist and well resolved in the NMR spectra whereas no peaks can be found within the 110–150 ppm region, indicating that HTC has not been formed thus far. Within the range between 180 and 200 °C, significant changes can be found on the spectrum. All the characteristic peaks of pure cellulose fully disappear, whereas a new peak occurs at 129 ppm, suggesting that the aromatization of the structure, begins

6.2 Humins Formation via HMF or Intramolecular Condensation, …

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Fig. 6.2 Mechanism of cellulose-derived HTC (Reproduced from Sevilla and Fuertes [2009] with permission. Copyright 2009 Elsevier)

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6 Structure and Formation Mechanism …

noticeable. In addition, the sharp transition is completely consistent with the SEM results. Moreover, similar to glucose, the hydrothermal carbon acquired from cellulose can experience a functional group loss and an elevated aromatic character when the processing temperature is increased. Apparently, the HTC carbons acquired from the two different feedstocks remain basically the same from a chemical perspective over 200 °C. Nevertheless, a significant difference should be emphasized between the HTC mechanism of glucose and cellulose. In accordance with the 13 C NMR spectra acquired from hydrothermally treated cellulose at 240 °C at varying residence times, all the obtained carbon samples from cellulose can be featured by the existence of the central aromatic peak at 125–129 ppm, having the tendency of becoming the most abundant chemical species. Unlike the time-dependent observation result for glucose treatment at an identical temperature, the existence of this feature is noted since the early reaction phases. Moreover, this important finding implies that unlike the case of glucose, the cellulose HTC fails to proceed via a mere polyfuranic intermediate. Instead, direct conversion of cellulosic substrate into a carbonaceous material, which comprises vast aromatic networks, is supposedly the primary mechanism route. As summarized in Fig. 6.3, hydrolysis into glucose is regarded as the postulated HTC mechanism for cellulose when strong acidic catalysts are adopted or under harsher processing scenarios (Sevilla & Fuertes, 2009). Subsequently, in the course of hydrothermal treatment, identical reaction pathway to glucose should be followed. When hydrothermal processing is accomplished under mild conditions and no extra catalysts are utilized, the structure of cellulose is kept almost unimpacted below 180 °C. When this temperature is exceeded, an HTC material is produced from the cellulosic substrate, whose aromaticity is highly developed (peak locations 125– 129 ppm) following the early reaction phases. Based on the previous explanation, the existing evidence emphasizes that the HTC mechanism for cellulose does not completely experience a polyfuranic intermediate, as in the case of glucose, while it could mainly contain reactions associated to the classical pyrolysis process, whereas the cellulosic substrate experiences intramolecular condensation, dehydration as well as decarboxylation reactions, contributing to producing HTC comprised of extensive aromatic networks.

6.3 Aldol Addition/condensation of DHH with Other Aldehydes and Ketones As discussed in Chap. 4, According to Lund et al., glucose should be transformed into HMF, before its transformation into DHH, and it experienced aldol addition/condensation with additional ketones and aldehydes for forming glucose-derived humins (Patil et al., 2012). Furthermore, they started to think about cellulose-derived humins. Therefore, it may be possible that cellulose is directly transformed into humins. Consequently, this work analyzed cellobiose (an effective cellulose polymer

6.4 Polymerization of HMF and Glucose by Acetalization and Etherification

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Fig. 6.3 Proposed model for cellulose HTC (Reproduced from Falco et al. [2011] with permission. Copyright 2011 Royal Society of Chemistry)

that contains only two glucose units) conversion under identical conditions to those for glucose. As observed from IR spectrum, glucose-derived humins were not differentiated from cellobiose-derived ones. Therefore, plausible reaction pathways for humins formation from cellobiose, and by extension cellulose, were proposed in Fig. 6.4, which entail sequential hydrolysis to glucose, dehydration of the glucose to HMF, formation of DHH from HMF, as well as aldol addition/condensation polymerization of DHH.

6.4 Polymerization of HMF and Glucose by Acetalization and Etherification Bell and co-workers studied the acid-catalyzed hydrolysis of cellulose dissolved in 1-ethyl-3-methylimidazolium chloride ([Emim][Cl]) and 1-butyl-3methylimidazolium chloride ([Bmim][Cl]) (Dee & Bell, 2011). The main reaction products were glucose, cellobiose and HMF. Figure 6.5 displays one potential mechanism underlying the process. The mechanism put forward was based on the fact that cis-diol protected the aldehyde functionality during organic synthesis (Greene & Wuts, 1999). As for this investigated system, aldehyde group in HMF was protonated and later reacted glucose. The resultant product was further protonated for the formation of one oxocarbonium ion, which reacted with the second cis-hydroxyl group in glucose for forming the cyclic compound. Hydroxyl group protonation on HMF or the residual hydroxyl group protonation on glucose took place to form the novel oxocarbonium ion after polymerization. The resultant second oxocarbonium

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6 Structure and Formation Mechanism …

Fig. 6.4 Possible pathways for cellulose conversion to humins (Reproduced from Patil et al. [2012] with permission. Copyright 2012 American Chemical Society)

ion later reacted with hydroxyl group in glucose or HMF for generating the novel aldehyde group on the other side of this compound for propagating reaction to products with high molecular weights. The mechanism accounted for the recent results that humin generation was of first order in HMF and glucose concentrations, although it remains hypothetical (Sievers et al., 2009a, 2009b; Weingarten et al., 2010). Ma and co-workers studied the conversion of cellulose into platform chemical HMF in water-tetrahydrofuran (THF) co-solvents under acidic condition (Shi et al., 2014). 38.6% HMF could be acquired under a low cellulose concentration (2.4 wt%), however, solid humins and levulinic acid (LA) were the major products under a high cellulose level. HPLC/multiple stage tandem mass spectrometry were conducted to analyze soluble byproducts, then chemicals (formulas, C9 H16 O4 , C10 H14 O4 , C11 H12 O4 , C12 H10 O5 and C12 H16 O8 ) were analyzed. Figure 6.6 displays reactions taking place in this process. THF was related to the reaction by ringopening in 1,4-butanediol and then esterification with HMF in C10 H14 O4 or with

6.5 Chapter Summary

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Fig. 6.5 Proposed reaction mechanism for humins formation (Reproduced from ref Dee and Bell [2011] with permission from Wiley–VCH)

LA in C9 H16 O4 . HMF esterification with LA yield C11 H12 O4 , whereas HMF selfetherification produced C12 H10 O5 , and HMF acetalization with glucose generated C12 H16 O8 . HMF etherification with 1,4-butanediol or its self-etherification represented two major side reactions. Regardless of the low C12 H16 O8 yield (