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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
- Veröffentlichungsjahr: 2023
Enables readers to convert lignin using a variety of depolymerization methods and develop it into industrially relevant and functional compounds Depolymerization of Lignin to Produce Value Added Chemicals summarizes the depolymerization and utilization of lignin from different sources and covers the emerging field of biological depolymerization, enabling the reader to identify the high added value of a biomass residue and support him/her in its possible use for mass and niche high impact application sectors. Lignin has great potential to significantly improve the economics of a biorefinery due to its conversion into value-added products. To illustrate, this book includes information on: * Feasibility of large-scale implementation of covered technologies, including thermal, biological, and chemical depolymerization, especially in relation to potential industrial applications * "Lignin-first" biorefining approach, and potential applications of lignin-derived monomers and their derivatives as bioactives in food, natural health products, and pharmaceuticals * Business and market scenarios and challenges that intersect with lignin, along with perspectives on lignin valorization * Benefits and drawbacks of a lignin-first approach to biorefining, and techno-economic considerations of lignin and its applications Depolymerization of Lignin to Produce Value Added Chemicals is an essential resource for researchers, chemists, engineers, analysts, and consultants within universities, independent research organizations, and government.
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Depolymerization of Lignin to Produce Value Added Chemicals
Pratima Bajpai
Pulp and Paper Consultant, Kanpur, India
First edition
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Set in 9.5/12.5pt STIXTwoText by Integra Software Services Pvt. Ltd., Pondicherry, India
Contents
Cover
Title Page
Copyright Page
List of Figures
List of Tables
Preface
Acknowledgements
1 General Background and Introduction
1.1 Structural and Chemical Composition of Lignin
1.2 Major Backbone Units and Representative Linkages in Lignin Molecules
1.3 Types of Lignin
2 Isolation of Lignin
2.1 Lignosulfonates
2.2 Kraft Lignin
2.3 Soda Lignin
2.4 Steam Explosion Lignin
2.5 Organosolv Lignins
3 Lignin Depolymerization Technologies
3.1 Thermal Depolymerization
3.1.1 Pyrolysis
3.1.2 Hydrothermal Liquefaction
3.2 Biological Depolymerization
3.2.1 Lignin Depolymerization by Fungi
3.2.2 Lignin Depolymerization by Bacteria
3.2.3 Lignin Depolymerization by Enzymes
3.2.3.1 Laccase
3.2.3.2 Lignin Peroxidase
3.2.3.3 Manganese Peroxidase
3.2.3.4 Versatile Peroxidase
3.2.3.5 β-etherase
3.2.3.6 Biphenyl Bond Cleavage Enzyme
3.3 Chemical Depolymerization
3.3.1 Acid-catalyzed Depolymerization
3.3.2 Base-catalyzed Depolymerization
3.3.3 Ionic Liquid-assisted Depolymerization
3.3.4 Supercritical Fluids-assisted Lignin Depolymerization
3.3.5 Metallic Catalysis
3.4 Oxidative Depolymerization of Lignin
3.5 Microwave-aided Depolymerization
3.6 Electrochemical Lignin Depolymerization
3.7 Reductive De-polymerization of Lignin
4 Lignin-first Biorefining Process
4.1 Introduction
4.2 The Revolutionary “Lignin-first” Method for Lignocellulosic Catalytic Valorization
4.2.1 Reductive Catalytic Fractionation
4.2.2 From Phenolic Units to Value-added Products
4.3 Future Challenges
5 Lignin Production
5.1 Introduction
5.2 Pilot-scale
5.2.1 Ammonia Fiber Explosion Lignin
5.2.2 Steam Explosion Process
5.2.3 BioFlex Process
5.2.4 German Lignocellulose Feedstock Biorefinery Project
5.2.5 Proesa
®
Lignin
5.2.6 FABIOLA
TM
Lignin
5.2.7 Fast Pyrolysis Lignin
5.2.8 Sequential Liquid-lignin Recovery and Purification Technology
5.3 Commercial scale
5.3.1 LignoForceTM Technology
5.3.2 LignoBoostTM Technology
5.3.3 SunCarbon Lignin
5.3.4 Production of Lignosulfonates
5.3.5 Kraft Lignin Production
5.3.6 Organosolv and Soda Lignin
5.3.7 Thermo-mechanical Pulp-bio Lignin
5.4 Future Perspectives
6 Applications of Lignin
6.1 Introduction
6.2 Applications
6.2.1 Aromatics, Phenolics and Flavoring Compounds
6.2.2 Carbon Materials
6.2.3 Lignin-based Nanomaterials
6.2.4 Biomedical Application
6.2.5 Lignin-based Nanocomposites
6.2.6 Urethanes and Epoxy Resins
6.2.7 Controlled Release Fertilizer
6.2.8 Biosensor and Bioimaging
6.2.9 Hydrogen Production
6.2.10 Battery Material for Energy Storage
6.2.11 Dust Control Agent
6.2.12 Bitumen Modifier in Road Industry
6.2.13 Cement Additives and Building Material
6.2.14 Bioplastics
6.2.15 Use of Lignin as a Binder
6.2.16 Lignin as Dispersant
6.2.17 Lignin as Food Additives
6.2.18 Lignin as Sequestering Agent
6.2.19 Lignin Bio-oil
7 Lignin – Business and Market Scenario
7.1 Introduction
7.2 Lignin Market
8 Challenges and Perspectives on Lignin Valorization
8.1 Introduction
8.2 Challenges and Perspectives on Lignin Utilization
Index
End User License Agreement
List of Tables
CHAPTER 01
Table 1.1 Value-added chemicals formed from lignin through various treatments.
CHAPTER 02
Table 2.1 Different types of lignin, their monomer’s molecular weight, and lign...
Table 2.2 Characterization of technical lignins.
Table 2.3 Characteristics of the technical lignin.
Table 2.4 Some of the major manufacturers of lignins.
Table 2.5 Sulfur content and purity of different types of lignins.
CHAPTER 03
Table 3.1 Hydrothermal liquefaction of lignin into different products.
Table 3.2 Effect of ionic liquids on lignin degradation for different feedstock...
Table 3.3 Lignin depolymerization through fungi.
Table 3.4 Fungi degradation of lignin in various biomass sources.
Table 3.5 Lignin depolymerization through bacteria.
Table 3.6 Bacterial degradation of lignin in various biomass sources.
Table 3.7 Major ligninolytic enzymes.
Table 3.8 Acid-catalyzed depolymerization of lignin.
Table 3.9 Reaction conditions and the products of base-catalyzed depolymerizati...
Table 3.10 Various phenolic products of sodium hydroxide catalyzed hydrolysis of...
Table 3.11 Ionic liquids used for depolymerization of lignin.
Table 3.12 Reaction conditions and the products of ionic liquid and deep eutecti...
Table 3.13 Reaction conditions and the products of sub- and supercritical depoly...
Table 3.14 Metallic catalyzed lignin depolymerization.
Table 3.15 Different lignin sources depolymerized with noble metal catalyst.
Table 3.16 Nickel catalyst in different support materials to depolymerize lignin...
Table 3.17 Reaction conditions and the products of oxidative depolymerization of...
Table 3.18 Reaction conditions and the products of microwave-aided depolymerizat...
Table 3.19 Depolymerization of lignin under microwave irradiation.
CHAPTER 04
Table 4.1 Reductive catalytic fractionation of biomass feedstock.
CHAPTER 05
Table 5.1 Different sources of lignin and their current volume.
Table 5.2 Lignin production at pilot scale.
Table 5.3 Lignin production on commercial scale.
CHAPTER 06
Table 6.1 Important results from lignin-based biosensors and use in bioimaging....
CHAPTER 07
Table 7.1 Lignin market key players.
Table 7.2 Current industrial applications of lignin.
Table 7.3 Lignin market value.
Table 7.4 Lignin market share (by product) 2022.
List of Illustrations
CHAPTER 01
Figure 1.1 Lignocellulose in biomass...
Figure 1.2 Major backbone units and...
Figure 1.3 The three building blocks...
Figure 1.4 Typical linkages present...
Figure 1.5 Model lignin structures...
Figure 1.6 Dehydrogenation of...
Figure 1.7 Structure of lignin...
CHAPTER 02
Figure 2.1 Typical structural...
Figure 2.2 Monolignol monomer...
Figure 2.3 Common linkages found...
Figure 2.4 Schematic representation...
Figure 2.5 Schematic representation...
Figure 2.6 Simplified and representative...
Figure 2.7 Photographs showing the...
CHAPTER 03
Figure 3.1 Different technologies...
Figure 3.2 Ranges of reaction...
Figure 3.5 Characteristic pyrolytic...
Figure 3.6 Proposed reaction pathways...
Figure 3.3 Effect of pyrolysis temperature...
Figure 3.4a Proposed pyrolytic mechanism...
Figure 3.4b Proposed reaction mechanism...
Figure 3.4c The purpose mechanism of...
Figure 3.7 Process flow configuration...
Figure 3.8 Some phenolic products...
Figure 3.9 Some guaiacol derivatives...
Figure 3.10 Structures of some...
Figure 3.11 Aromatic catabolism...
Figure 3.12 Ligninolytic enzymes...
Figure 3.13 Catalytic mechanism...
Figure 3.17 Three-dimensional...
Figure 3.14 Catalytic mechanism...
Figure 3.15 Catalytic mechanism...
Figure 3.16 Catalytic mechanism...
Figure 3.18 Mechanisms of β-O-4...
Figure 3.22 The three ionic liquid...
Figure 3.19 Cleavage of β-O-4...
Figure 3.20 Mechanism for base...
Figure 3.21 Low-molecular- weight...
Figure 3.23 Cations and anions...
Figure 3.24 Plausible reaction pathway...
Figure 3.25 The purpose mechanism...
Figure 3.26 The mechanism of cleaving...
Figure 3.27 Oxidative depolymerization...
Figure 3.28 Microwave-assisted...
Figure 3.29 Understanding lignin...
Figure 3.30 Microwave processing...
Figure 3.31 Ultrasonic and microwave...
Figure 3.32 Abbreviated reaction...
Figure 3.33 A representation of the...
Figure 3.34 Main levulinic acid-derived...
Figure 3.35 Reductive depolymerization...
CHAPTER 04
Figure 4.1 The revolution of the...
Figure 4.2 The chemical-reaction...
Figure 4.3 Three lignin-first strategies...
Figure 4.4 The evolution of reactor...
Figure 4.5 A schematic overview of potential...
CHAPTER 06
Figure 6.1 Current and potential applications...
Guide
Cover
Title Page
Copyright Page
Table of Contents
List of Figures
List of Tables
Preface
Acknowledgements
Begin Reading
Index
End User License Agreement
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List of Figures
1.1 Lignocellulose in biomass and its composition.
1.2 Major backbone units and representative linkages in lignin molecules.
1.3 The three building blocks of lignin.
1.4 Typical linkages present in lignin.
1.5 Model lignin structures: (a) softwood, (b) hardwood, and (c) grass.
1.6 Dehydrogenation of coniferyl alcohol a aterial.
1.7 Structure of lignin in lignocellulosic material.
2.1 Typical structural model of lignin.
2.2 Monolignol monomer species.
2.3 Common linkages found in lignin.
2.4 Schematic representation of hardwood lignin.
2.5 Schematic representation of softwood lignin.
2.6 Simplified and representative structures of common technical lignins.
2.7 Photographs showing the differences in physical appearance of a number of typical technical lignins.
3.1 Different technologies of lignin depolymerization.
3.2 Ranges of reaction temperatures used by different lignin thermal depolymerization methods.
3.3 Effect of pyrolysis temperature on the aromatic substitution products of G-type monomers.
3.4a Proposed pyrolytic mechanism of lignin.
3.4b Proposed reaction mechanism pathway of lignin non-catalytic/catalytic fast pyrolysis.
3.4c The purpose mechanism of cleaving the β-O-4 linkages by pyrolysis.
3.5 Characteristic pyrolytic behaviors of initial, primary, and charring stages.
3.6 Proposed reaction pathways for primary stage of lignin pyrolysis.
3.7 Process flow configuration for hydrothermal liquefaction.
3.8 Some phenolic products from HTL of lignin.
3.9 Some guaiacol derivatives from HTL of lignin.
3.10 Structures of some catechols.
3.11 Aromatics catabolism from the coniferyl, sinapyl and p-coumaryl branch.
3.12 Ligninolytic enzymes and their selective action on lignin components.
3.13 Catalytic mechanism of laccase mediated lignin degradation.
3.14 Catalytic mechanism of lignin peroxidase (LiP) mediated lignin degradation.
3.15 Catalytic mechanism of manganese peroxidase (MnP) mediated lignin degradation.
3.16 Catalytic mechanism of versatile peroxidase (VP) mediated lignin degradation.
3.17 Three-dimensional structures of ligninolytic enzymes.
3.18 Mechanisms of β-O-4 ether and biphenyl linkage degradation.
3.19 Cleavage of β-O-4 linkages by acid catalysts.
3.20 Mechanism for base catalyzed depolymerization of lignin.
3.21 Low-molecular- weight products resulting from the depolymerization of lignin with the use of sodium hydroxide.
3.22 The three ionic liquid generations.
3.23 Cations and anions of ionic liquids in lignin chemistry.
3.24 Plausible reaction pathway of lignin in supercritical water.
3.25 The purpose mechanism of cleaving the β-O-4 linkages with the metallic catalyst.
3.26 The mechanism of cleaving the β-O-4 linkages with the oxidant.
3.27 Oxidative depolymerization mechanism of lignin.
3.28 Microwave-assisted catalytic depolymerization of lignin from birch sawdust to produce phenolic monomers utilizing a hydrogen-free strategy.
3.29 Understanding lignin depolymerization to phenols via microwave- assisted solvolysis process.
3.30 Microwave processing of lignin in green solvents: a high-yield process to narrow-dispersity oligomers.
3.31 Ultrasonic and microwave assisted organosolv pretreatment of pine wood for producing pyrolytic sugars and phenols.
3.32 Abbreviated reaction scheme showing proposed electrochemical/radical mechanisms during lignin degradation.
3.33 A representation of the structure of levulinic acid.
3.34 Main levulinic acid-derived products of kraft lignin depolymerization identified by direct injection high-resolution MS.
3.35 Reductive depolymerization of kraft lignin to produce aromatics.
4.1 The revolution of “lignin-first” approach: from lignocellulosic biomasses to added value.
4.2 The chemical-reaction mechanism of lignin-first biorefinery using solvolysis and the catalytic stabilization of reactive intermediates to stable products or protection-group chemistry and subsequent upgrading.
4.3 Three lignin-first strategies.
4.4 The evolution of reactor configurations for reductive catalytic fractionation.
4.5 A schematic overview of potential added value products of lignin biorefinery.
6.1 Current and potential applications of technical lignin.
List of Tables
1.1 Value-added chemicals formed from lignin through various treatments.
2.1 Different types of lignin, their monomer’s molecular weight, and lignin content.
2.2 Characterization of technical lignins.
2.3 Characteristics of the technical lignin.
2.4 Some of the major manufacturers of lignins.
2.5 Sulfur content and purity of different types of lignins.
3.1 Hydrothermal liquefaction of lignin into different products.
3.2 Effect of ionic liquids on lignin degradation for different feedstock.
3.3 Lignin depolymerization through fungi.
3.4 Fungi degradation of lignin in various biomass sources.
3.5 Lignin depolymerization through bacteria.
3.6 Bacterial degradation of lignin in various biomass sources.
3.7 Major ligninolytic enzymes.
3.8 Acid-catalyzed depolymerization of lignin.
3.9 Reaction conditions and the products of base-catalyzed depolymerization of lignin.
3.10 Various phenolic products of sodium hydroxide catalyzed hydrolysis of lignin at 300 °C and 250 bar.
3.11 Depolymerization of lignin by catalytic oxidation in ionic liquids.
3.12 Reaction conditions and the products of ionic liquid and deep eutectic solvent-based depolymerization.
3.13 Reaction conditions and the products of sub- and supercritical depolymerization of lignin.
3.14 Metallic catalyzed lignin depolymerization.
3.15 Different lignin sources depolymerized with noble metal catalyst.
3.16 Nickel catalyst in different support materials to depolymerize lignin.
3.17 Reaction conditions and the products of oxidative depolymerization of lignin.
3.18 Reaction conditions and the products of microwave-aided depolymerization of lignin.
3.19 Depolymerization of lignin under microwave irradiation.
4.1 Reductive catalytic fractionation of biomass feedstock.
5.1 Different sources of lignin and their current volume.
5.2 Lignin production at pilot scale.
5.3 Lignin production on commercial scale.
6.1 Important results from lignin-related biosensors and applications in bioimaging.
7.1 Lignin market key players.
7.2 Current industrial applications of lignin.
7.3 Lignin market value.
7.4 Lignin market share (by product) 2022.
Preface
Lignin is a potential feedstock due to its energy content and its abundant availability from pulp and paper mills and biomass based biorefinery. Lignin has great potential for its conversion into value-added products, which could significantly improve the economics of a biorefinery. Emerging opportunities exist in generating high-value small molecules from lignin through depolymerization. This book discusses technologies of lignin depolymerization. Compared with thermal and chemical depolymerization, bioprocessing with microbial and enzymatic catalysis is a clean and efficient method for lignin depolymerization and conversion. Biological methods of lignin depolymerization are gaining significance due to their economic and environmentally benign nature. The feasibility of large-scale implementation of these technologies, including thermal, biological, and chemical depolymerizations is discussed in relation to potential industrial applications. The “lignin-first” biorefining approach and potential applications of lignin-derived monomers and their derivatives as bioactives in food, natural health products, and pharmaceutical sectors; lignin – business and market scenario and challenges and perspectives on lignin valorization are also covered.
This book will help readers to identify the high added value of a biomass residue and support them in its possible use for mass and niche high impact application sectors.
Acknowledgments
I am grateful for the help received from many people and companies/organizations who provided information. I am also thankful to various publishers for allowing me to use their material. My deepest appreciation is extended to Elsevier, Springer, RSC, ACS Publications, John Wiley & Sons, Frontiers Media SA, Hindawi, MDPI, IntechOpen, SpringerOpen, and other open-access journals and publications. My special thanks to Dr. KK Pant IIT Delhi, India and Dr. Weckhuysen, Distinguished Professor, Utrecht University, Netherlands who allowed me to use their material.
1 General Background and Introduction
Abstract
Lignin, a complicated organic polymer, plays a significant structural function in the support tissues of vascular plants. It is particularly prevalent in woody plants and is highly polymerized. Lignin is one of the three crucial elements of wood, along with extractives and carbohydrates. Lignin, a three-dimensional amorphous polymer made of methoxylated phenylpropane structures, is important for the survival of vascular plants. In nature, lignin polymer generally forms ether or ester linkages with hemicellulose which is also connected with cellulose. The structural and chemical composition of lignin; representative linkages in lignin molecules and types of lignin are presented in this chapter.
Keywords Lignin; Phenylpropane units; Monolignols; Sinapyl alcohol; Coniferyl alcohol; p-coumaryl alcohol; Hardwood lignin; Softwood lignin;
With the rapid growth of populations and rising living standards in developing nations, global energy demand is rapidly rising. To meet this rising energy demand, fossil resources alone will not be sufficient. At the same time, there are significant concerns regarding the impact of climate change, which may be linked to the combustion of fossil fuels that are not renewable. As a result, it is crucial to develop technologies that can use new energy solutions on a large scale and provide more environmentally friendly alternatives to the current economy based on fossil fuels. Because this renewable feedstock can theoretically be incorporated into a carbon dioxide-neutral energy cycle, biomass is an option for the production of sustainable fuels and chemicals. Cellulose, hemicellulose, and lignin are the three main components of biomass.
Aromatic compounds, which can be used as fuel or as intermediate chemicals in the industry, can be obtained from lignin, the organic biopolymer that is found in the second highest concentration anywhere on the planet. Biomass conversion technology’s viability can be improved by incorporating lignin into biorefineries. The recalcitrant and complicated nature of the lignin feedstock presents the primary obstacle in this situation. It is a huge challenge to properly convert lignin into functional polymers, but this is a fascinating area of research in both industry and academia (Guvenatam, 2015).
1.1 Structural and Chemical Composition of Lignin
The Swiss botanist Augustin Pyramus de Candolle was the first person to use the term lignin, which comes from the Latin word lignum, which means wood (Candolle et al., 1821). Lignin, a complicated organic polymer, plays a significant structural role in the support tissues of vascular plants. It is particularly prevalent in woody plants and is highly polymerized. Lignin is one of the three crucial elements of wood, along with extractives and carbohydrates (Sarkanen and Ludwig, 1971; Sjöström, 1982). Protolignin is the name given to lignin when it is in its natural state, as it is in plants. Lignin, a three-dimensional amorphous polymer made of methoxylated phenylpropane structures, is essential for the survival of vascular plants. In nature, lignin polymer usually forms ether or ester linkages with hemicellulose which is also associated with cellulose. Therefore, these natural polymers construct a complicated and valuable lignocellulose polymer (Figure 1.1).
Figure 1.1 Lignocellulose in biomass and its composition. Chonlong Chio et al. 2019/ Reproduced with permission from Elsevier.
1.2 Major Backbone Units and Representative Linkages in Lignin Molecules
It is generally acknowledged that the polymerization of three types of phenylpropane units, also known as monolignols, initiates the biosynthesis of lignin (Freudenberg and Neish, 1968; Lewis, 1999; Ralph, 1999; Sarkanen and Ludwig, 1971). These units, sinapyl, coniferyl, and p-coumaryl alcohol, are linked by the chemical bonds of aryl ether (β-O-4), phenylcoumaran (β-5), resinol (β-β), biphenyl ether (5-O-4), and dibenzodioxocin (5–5) (Figure 1.2). In Figure 1.3, the three structures are shown. The most typical linkage among the various typical linkages (β-O-4, β-5, β-1, 5–5, α-O-4, 4-O-5, β-β) (Figure 1.4) is the β-aryl ether (β-O-4), which accounts for more than half of the structure of lignin (Dutta et al., 2014; Rinaldi et al., 2016). Figure 1.5 shows model lignin structures: A softwood, B hardwood, and C grass (Lu and Gu, 2022).
Figure 1.2 Major backbone units and representative linkages in lignin molecules. (a) The building blocks of lignin consist of three primary types of monolignols, namely p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The alcohols form the corresponding phenylpropanoid units like p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) in lignin polymer, respectively. (b) Backbone units are conjugated via different chemical bonds (e.g., β-O-4, β-β, 5–5, and β-5) resulting in high resistance to lignin depolymerisation Weng et al. (2021) / Springer Nature / Public Domain CC BY 4.0.
Figure 1.3 The three building blocks of lignin. Chakar and Ragauskas, 2004 / with permission of ELSEVIER.
Figure 1.4 Typical linkages present in lignin. Agarwal et al. (2018) / with permission of ELSEVIER.
Figure 1.5 Model lignin structures: (A) softwood, (B) hardwood, and (C) grass. Lu and Gu (2022) / Springer Nature / Public Domain CC BY 4.0.
Figure 1.6 demonstrates the phenoxy radicals that are resonance-stabilized and the dehydrogenation of coniferyl alcohol (Chakar and Ragauskas, 2004). The polymerization interaction is set up by the oxidation of the monolignol phenolic hydroxyl groups. It has been demonstrated that an enzymatic pathway catalyzes the oxidation itself. An electron transfer initiates the enzymatic dehydrogenation, resulting in reactive monolignol species and free radicals, which are able to pair with one another. The aromaticity of the benzene ring will be restored by a subsequent nucleophilic attack by water, alcohols, or phenolic hydroxyl groups on the benzyl carbon of the quinone methide intermediate. Polymerization will continue on the produced dilignols.
Figure 1.6 Dehydrogenation of coniferyl alcohol and the mesomeric radicals. Chakar and Ragauskas, 2004 / with permission of ELSEVIER.
Architecturally, lignin forms a complex, three-dimensional heterogeneous network thanks to the chemical bonds it forms with cellulose and hemicellulose via covalent and non-covalent bonds (Figure 1.7) (Agarwal et al., 2018). Due to lignin’s irregular, heterogeneous structure, it remains difficult to produce commodity chemicals with added value.
Figure 1.7 Structure of lignin in lignocellulosic material. Agarwal et al. (2018) / with permission of ELSEVIER.
The primary sources of lignin that can be utilized on a larger scale are spent cooking liquor and the chemical extraction of wood fibers from the pulp and paper industry. Over 50 million tons of lignin-based materials and chemicals are produced annually worldwide. Despite the fact that most lignin in the world is still used as boiler fuel in facilities that process carbohydrates, its low value and abundance indicate that it might be used to create high-value new products.
If converted into chemical compounds, bioproducts based on lignin could lead to a multibillion-dollar industry. Several million tonnes of lignin are produced as a low-value byproduct of industrial cellulosic bioethanol production. It is anticipated that the US bioethanol industry alone will produce up to 60 Mt./year of lignin by the end of 2022 (Holladay et al., 2007a; Joffres et al., 2014).
1.3 Types of Lignin
Sinapyl alcohol, coniferyl alcohol and p-coumaryl alcohol, are frequently joined by non-hydrolysable linkages during the dehydrogenation of phenylpropanoid precursors that is carried out by free radicals with the assistance of peroxidase and results in the formation of lignin. The aromatic amorphous heteropolymer lignin lacks any optical activity. These three monoolignols are present in varying amounts in various plant species. For instance, softwood lignin contains a lot of coniferyl alcohol, whereas hardwood lignin contains both sinapyl and coniferyl alcohols and grass lignin contains all the three monolignols (Duval and Lawoko, 2014). The extracted lignin has been divided into four major categories—lignosulfonates, kraft lignin, soda lignin, and organosolv lignin based on the chemical pre-treatment method used: sulfur-free soda lignin is produced when biomass is treated with sodium hydroxide, whereas kraft lignin is produced when biomass is treated with sodium sulfide and sodium hydroxide. The ethanol-water extraction method and the pre-treatment of biomass with aqueous sulfur dioxide produce lignosulfonates and organosolv lignin, respectively. In addition to the four primary types of lignin, ionic liquid lignin, which is produced by treating biomass with ionic liquid is attracting a lot of interest because of its condensed structure and a low β-O-4 content (Wen et al., 2013). Due to the structural changes that take place when lignin is separated from lignocellulose biomass, chemical properties vary between lignin types. Contrary to kraft lignin and lignosulfonates, organosolv lignin, which is practically insoluble in water and natural solvents, contains a higher level of β-O-4 linkages (Bauer et al., 2012).
Even though lignin accounts for 15–40% of a plant’s dry weight, it is still not considered a high-value-added product in biorefinery processes (Cao et al., 2017). Indeed, the utilization of lignin has the potential to significantly boost the cost-effectiveness of biorefinery processes based on biomass (Ragauskas et al., 2014). Only 5% of the lignin produced by the paper and pulp industry is used to produce low-quality fuel for use in heat and electricity applications through combustion (Cao et al., 2018). The well-organized valorization of lignin produced by various industrial processes may result in the proliferation of economic and environmental sustainability (Wu et al., 2018).
The highly asymmetrical polymeric structure is the most significant impediment to the lignin conversion process. The fractionation process’s effect on product recovery was discussed by a number of researchers (Anderson et al., 2019). Bio-oil yield and quality could be impacted by minor structural changes. By selectively fractionating lignin from other biomass components with fewer structural changes for efficient lignin application, attempts have been made to valorize the lignin conversion process. Compared to extracted lignin whose structure has been altered, the direct hydrogenolysis process yielded aromatic monomers from native lignin at rates of 40–50%, which is five to ten times higher (Shuai et al., 2016).
There have been a number of studies on lignin valorization, in which the monomers and oligomers produced by depolymerizing lignin through thermal, chemical, and biological, pre-treatments can be turned into fuels and other chemicals (Beckham et al., 2016; Ragauskas et al., 2014).
Typically, heterogeneous aromatic compounds are produced following lignin depolymerization based on the feedstock and pre-treatment used (Schutyser et al., 2018). Fine chemicals can only be made with aromatic compounds of high purity. As a result, lignin upgrade is hindered by the heterogeneity of aromatics produced by depolymerization (Liu et al., 2017; Schutyser et al., 2018).
According to Abdelaziz et al. (2016), large quantities of lignin have been produced, estimated at 5–36 × 108 tons annually. The pulp and paper and biomass refinery industries each contribute approximately 6.2 × 107 and 5 × 107 tons of lignin annually, respectively, which includes soda lignin, kraft lignin, and lignosulfonate (Zakzeski, 2010).
Most of the time, lignin is used for energy or thrown away as waste. Due to its rich aromatic skeleton and high carbon-to-oxygen ratio, lignin is a promising feedstock for the production of biofuels and biochemicals (Vishtal and Kraslawski, 2011). In order to take advantage of lignin valorization, it is urgently necessary to acquire an understanding of the degradation procedure and create an efficient metabolic pathway for conversion. Lignin’s recalcitrance and complicated structure make it difficult to depolymerize and use effectively. Currently, the most common approaches for lignin depolymerization are thermochemical and biological ones. Pyrolysis (thermolysis), gasification, hydrogenolysis, and chemical oxidation are thermochemical processes that call for extreme conditions, a lot of energy, and costly facilities (Bandounas, 2011). On the other hand, bioprocessing lignin has the advantages of higher specificity, reduced energy consumption, and affordability (Chen and Wan, 2017). In the specific cleavage of lignin linkages, biological depolymerization has demonstrated a number of benefits, and its nature makes this process environmentally friendly (Xu et al., 2019). But there are some disadvantages, such as the difficulty of genetically altering the microbes and their high sensitivity to changes in pH, temperature, and oxygen levels in the reaction system (Chauhan, 2020). Thermochemical methods have been used frequently for a long time, and these can be categorized into various groups based on the catalyst, heating technology, solvents, temperature ranges, and other factors. The liquid that comes out of lignin depolymerization, bio-oil, will have different percentage yields and different kinds of monomers and oligomers because of these factors (Agarwal et al., 2018; Lopez-Camas et al., 2020).
Holladay et al. (2007b) provide an economical evaluation of lignin feedstock-based chemical conversion technologies. Despite its potential, lignin is underutilized by industry as a chemical conversion raw material (Doherty et al., 2011; El Mansouri and Salvadó, 2006).
Biorefining natural feedstocks looks like a good way to use more lignin. The intricate utilization of biomass-derived feedstocks like lignocelluloses, oil and sugar crops, and algae is the foundation of the biorefinery concept (Cherubini, 2010; Demirbas, 2009). The three main components of lignocellulosic materials are: lignin, cellulose, and hemicelluloses (Sjöström, 1982). The majority of biorefineries are presently concentrating on the sugar-based platform for the valorization of hemicelluloses and cellulose (FitzPatrick et al., 2010), whereas lignin is typically regarded as a low-value product (Cherubini et al., 2010; Doherty et al., 2011). In contrast to sugars, which are released as uniformly monomeric carbohydrates, lignin is released as a complex and polydisperse compound. Limited use of lignin in biorefineries is primarily due to its complex structure and uncertain reactivity.
One strategy for realizing the full potential of lignin is through its transformation into useful products. Table 1.1 shows value-added chemicals formed from lignin through various treatments.
Table 1.1 Value-added chemicals formed from lignin through various treatments.
Lignin
Hydrogenation
Pyrolysis
Oxidative hydrolysis
Fast thermolysis
Alkali fusion
Enzymatic oxidation
Microbial conversion
Phenol, cresols, substituted phenols
Phenol, acetic acid, carbon monoxide, methane
Vanillin, dimethyl sulfide
dimethyl sulfooxide
Ethylene, acetylene
Catechol and phenolic acid
Oxidized lignin
Lignin with high level of polymerization
ferulic, coumaric, vanollic, and other acid
Ullah et al. (2022) / MDPI / Public Domain CC BY 4.0.
Technical lignins, such as kraft lignin, soda lignin, and lignosulphonates, are obtained in processes that deal with treating lignocelluloses, making them an intriguing raw material. Additionally, many technical lignins can be obtained in large quantities and are readily available. However, hydrolysis, organic solvents, and ionic liquids only yield a small fraction of the potentially valuable lignins. These are produced in relatively smaller quantities but may eventually develop into products on a commercial scale. Removing lignin from the product streams of small non-wood mills permits the elimination of recovery boiler bottlenecks and offers a solution to some environmental issues but may eventually develop into products on an industrial scale (Gosselink et al., 2004).
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2 Isolation of Lignin
Abstract
Lignin is produced by the cell walls of plants, agricultural crops, and wood. Between 15 and 40% of the dry matter in woody plants is made up of lignin, which is mainly a structural material that gives cell walls more strength and rigidity. Compared to cellulose and other structural polysaccharides, lignin is more resistant to the majority of biological attacks. Currently, chemical pulping of wood yields lignin. However, a number of biomass refineries are now operational. P-coumaryl, coniferyl, and sinapyl alcohols are the three basic phenylpropane units that make up lignin’s chemical structure. The pulping process produces the technical lignins as a byproduct. The most commonly used commercial lignins are kraft lignin and lignosulfonates. Organosolv lignins are another type of lignin. Isolation and characterization of different types of lignin are presented in this chapter.
Keywords Lignin; Technical lignin; Kraft lignin; Lignosulfonates; Organosolv lignins; Soda lignin; Steam explosion lignin; Phenylpropane units; Monolignols; Sinapyl alcohol; Coniferyl alcohol; p-coumaryl alcohol
Currently, lignin is produced through the chemical pulping of wood. Due to the fact that numerous biomass refineries are currently in operation, the lignin that is produced as a byproduct of the production of cellulosic ethanol would be an excellent feedstock for the production of products with added value. Melt-spinnable lignins made by oganosolv pulping are simple to make. Compared to lignins obtained through chemical pulping, these lignins are purer. The biosphere contains more than 300 billion tonnes of lignin, which grows by approximately 20 billion tonnes per year.
Lignin is mostly found in the cell walls of club mosses, ferns, and vascular plants (Akin and Benner, 1988; Baurhoo et al., 2008; Gregorováa et al., 2006; Kirk, 1971; Matsushita, 2015; McCrady, 1991; Miidla, 1980; Piló-Veloso et al., 1993; Rosas et al., 2014; Souto et al., 2018). In contrast to polysaccharides, which have a clear structure, lignin has unique properties like being aromatic and having less oxygen. Because of these properties, lignin is a desirable feedstock for the transformation of it into useful chemicals or materials, as well as renewable chemical building blocks. However, the potential applications of lignin are highly dependent on its availability, a thorough understanding of the source of lignin, method of isolation, and the anticipated applications’ technical requirements.
Lignin is an organic polymer with three dimensions. It creates crucial structural components for vascular plants’ support tissues. It is primarily found in woody plants, where it is more complex and heavily polymerized. Lignin is absorbed into the wood’s cellulose walls. The term for this process is lignification. It gives trees more rigidity and significantly increases the cell’s strength and hardness. According to Rouhi and Washington (2001), this is crucial to enable woody plants to stand straight and vertical. According to Nordström (2012), the complex structure of natural lignin, which is found in a variety of plants, includes both aromatic and aliphatic components. Even though information about lignin has been available for more than a century, its significance has generally been recognized since the early 1900s (Glasser et al., 2000).
The complex structure of lignin limits our understanding of it. The field of lignin has seen significant growth in recent years as new chemical analysis techniques have been implemented. Because of this, we now know about the structure of lignin and how it can be used. Lignin is a random, three-dimensional network polymer made of phenylpropane units that are linked in different ways.
Plant fibers are held together by mechanical supports, which is made possible by lignin. Lignin is also important in the transportation of water and nutrients because it reduces the amount of water that penetrates the xylem’s cell walls. Lastly, because it prevents destructive enzymes from passing through the cell wall, lignin is a key component of a plant’s natural defense mechanism against deterioration (Bajpai, 2017; Sarkanen and Ludwig, 1971; Sjöström, 1993).
Figure 2.1 shows a typical structural model of lignin (Lu et al., 2017). Sinapyl, p-coumaryl, and coniferyl alcohols give lignin its three fundamental phenylpropane units (Figure 2.2). Figure 2.3 shows common linkages found in lignin.
Figure 2.1 Typical structural model of lignin. Lu et al., 2017 / John Wiley & Sons / Public Domain CC BY 4.0.
Figure 2.2 Monolignol monomer species. (a) p-coumaryl alcohol (4-hydroxyl phenyl, H), (b) coniferyl alcohol (guaiacyl, G), (c) sinapyl alcohol (syringyl, S). Based on Bajpai (2017); Ekielski and Mishra (2020).
Figure 2.3 Common linkages found in lignin. Ekielski and Mishra (2020) / MDPI / Public Domain CC BY 4.0.
During biological lignification, radical coupling reactions join these to form a complex three-dimensional macromolecule. The important linkages include β-O-4, β-5, β-b, and 5–5 linkages among others. The structure and quantity of lignin found in different species vary. Coniferyl and sinapyl alcohols, for instance, can be found in small amounts in hardwood lignin; coniferyl alcohol is the primary component of softwood lignin. Sinapyl, p-coumaryl, and coniferyl can be found in grass lignin (Matsushita, 2015). Hardwood and softwood lignin’s structures are depicted in Figures 2.4 and 2.5 (Gargulak and Lebo, 1999; Nimz, 1974; Zakzeski et al., 2010).
Figure 2.4 Schematic representation of hardwood lignin. Reproduced with permission Zakzeski et al., 2010 / American Chemical Society.
Figure 2.5 Schematic representation of softwood lignin. Reproduced with permission Zakzeski et al., 2010 / American Chemical Society.
Lignin is produced by the cell walls of plants, agricultural crops, and wood. In plants, cellulose and lignin work together to serve a structural purpose. Additionally, it serves as a strong defence against insect and fungal assaults. The structure and composition of lignin is found to vary. It depends upon the species of tree or plant, the time of year, the climate, and the age of the plant. In terms of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) composition, as well as the relative abundance of chemical linkages in the polymer, the composition of lignin is highly dependent on the two plant species. Table 2.1 shows different types of lignin, their monomer’s molecular weight, and lignin content.
Table 2.1 Different types of lignin, their monomer’s molecular weight, and lignin content.
Types of Lignin
Source
Monomer Molecular Weight (g mol
−1
)
(mmol g
−1
) Lignin Content
Chemicals/catalysts
Sulfur process
Kraft lignin
Wood chips, softwoods, hardwoods
2000–3000
1.25
NaOH, Na
2
S
Lignosulfonates
Softwoods, hardwoods, annual plants
20 000–50 000
1.25–2.5
Ca(HSO
3
)
2
or Mg(HSO
3
)
2
Sulfur free process
Organosolv lignin
Hardwoodm, Softwood, and wheat straw
2000–5000
0
Methanol, ethanol, various bronsted acid catalysts (H
2
SO
4
)
Alkali/soda lignin
Hardwood, bagasse, wheat straw, and flax
5000–6000
0
NaOH, NH
4
OH, Ca(OH)
2
Based on Aro and Fatehi, 2017; Basakçılardan Kabakcı and Tanis, 2021; Kim et al., 2016; Ullah et al., 2022; Xu et al., 2020; Yoo et al., 2020.
Almost always, the process of separating biomass into its component parts will have a significant impact upon the molecular structure of lignin, resulting in the release of lignin (technical lignins). Consequently, the technical lignins have distinct characteristics (Table 2.2). Figure 2.6 shows simplified and representative structures of common technical lignins.
Figure 2.6 Simplified and representative structures of common technical lignins. Ekielski and Mishra (2020) / MDPI / Public Domain CC BY 4.0.
Table 2.2 Characterization of technical lignins.
Characterization of kraft lignin
Species
Hydroxyl group
Molecular weight
References
Total
(mmol g−1)
Phenolic (mmol g
−1
)
Mw (×103)
Mn (×103)
Mw Mn
−1
Softwood
6.5–8.6
2.7–3.5
1.1–45.7
0.5–7.7
2.2–13.4
(EI Mansouri and Salvadó, 2006, 2007; Ekeberg et al., 2006; Mansson, 1983; Ponomarenko et al., 2014)
Hardwood
6.5–8.4
4.3–4.7
2.4–4.8
0.4–1.3
1.8–12.0
(Mansson, 1983; Pan and Saddler, 2013; Ponomarenko et al., 2014)
Characterization of Lignosulfonate
Species
Hydroxyl group
Molecular weight
References
Total
(mmol g−1)
Phenolic
(mmol g−1)
Mw (×103)
Mn (×103)
Mw Mn
−1
Softwood
Not available
1.2–1.9
10.5–60.2
2.7–6.5
6.7–22.3
(Alonso et al., 2001; EI Mansouri and Salvadó, 2007; Ekeberg et al., 2006)
Hardwood
Not available
1.4–1.5
6.9–7.8
2.4–4.6
1.7–3.0
(Alonso et al., 2001; Ekeberg et al., 2006; Ye et al., 2013; Zhou et al., 2013)
Characterization of ogranosolv lignin
Species
Hydroxyl group
Molecular weight
References
Total
(mmol g−1)
Phenolic
(mmol g−1)
Mw (×103)
Mn (×103)
Mw Mn
−1
(ethanol)
Softwood
6.3–10
2.7–3.1
2.9–5.4
1.8–3.1
1.6–1.8
(Pan et al., 2005; Sannigrahi et al., 2010)
Hardwood
5.7
2.8
2.0–2.6
1.3–1.6
1.5–1.6
(Pan et al., 2005; Pan and Saddler, 2013)
(Formic acid)
Miscanthus
3.7–4.9
1.6–2.4
2.8
1.1
2.5
(EI Mansouri and Salvadó, 2006, 2007; EI Mansouri et al., 2012)
(Acetic acid)
Hardwood
5.5–5.8
3.5–4.0
0.9
Not available
Not available
(Benar et al., 1999; Kin, 1990)
(Acetic acid/formic acid)
Wheat straw
3.4
1.0
2.2
1.6
1.3
(Delmas et al., 2011)
Bajpai (2021) / with permission of ELSEVIER.
The various pre-treatment-derived lignins differ in terms of physicochemical properties and their chemical structure.
Presently, lignin is available in large quantities and can be transformed into a variety of other raw materials through fractionation, purification, and chemical alterations (functionalized).
Table 2.3 presents the characteristics of the technical lignin. Table 2.4 lists some of the biggest companies that make these products.
Table 2.3 Characteristics of the technical lignin.
Molecular weight
Water solubility
Degree of contamination (example remaining covalently bound sugar residues or incorporation of non-native elements, such as sulfur)
Extent of condensation
Functional group decoration of the macromolecule.
Bajpai (2021) / with permission of ELSEVIER.
Table 2.4 Some of the major manufacturers of lignins.
Alberta Pacific
Borregaard LignoTech,
CIMV
Domtar
Domsjö
Tembec
UPM
Weyerhaeuser
Bajpai (2021) / with permission of ELSEVIER.
The pulping process produces the technical lignins as a byproduct. The most commonly used commercial lignins are kraft lignin and lignosulfonates. Organosolv lignins are another type of lignin. These lignins are made by pulping materials with organic solvents like formic acid, acetic acid, and ethanol among others. The pulping method determines how the lignin is structured. Additionally, different lignins have distinct functional groups and molecular weights. Industrial uses of lignin are constrained by type of phenylpropane units, its functional groups, molecular weight distributions, and linkage between structural units (Matsushita, 2015). The technical lignins differ greatly in physical characteristics such as solubility, hydrophilicity, and hydrophobicity, as well as in molecular structure, weight, and chemical composition (including impurities). The strategies that are viable for further valorization will largely be determined by these characteristics (Bruijnincx et al., 2016).
2.1 Lignosulfonates
Lignosulfonates are produced by sulfite pulping processes. The liquor used to cook the wood is made by combining sulfur dioxide and an aqueous base. Sulfuric acid is produced when sulfur dioxide reacts with water to form sulfur dioxide, which then breaks down and sulfonates the lignin by substituting a hydroxyl group for a sulfonate one. Because of this, the lignin can be solubilized and removed from the cellulose without getting precipitated. The sugars in the spent sulfite liquor—mostly monosaccharides—must be destroyed before the lignosulfonate can be used as a concrete additive to reduce water content (Niaounakis, 2015).
In the lignosulfonate process, sulfite with either magnesium or calcium as the counterion is used. The process takes place over a broad pH range, from 2 to 12. Water and some organics and amines with high polarities dissolve the product. Due to the inclusion of sulfonate groups on the arenes, the lignosulfonate produced by the sulfite process has a higher average molecular weight and monomer molecular weight than kraft lignin. Lignosulfonates are distinct due to the sulfonate groups that are primarily introduced in the α-position of the propyl side chain. Kraft lignins, on the other hand, have a sulfonate on the aromatic ring and are sulfonated.
Consequently, the resulting lignin is water-soluble, distinguishing it from other technical lignins. Lignosulfonates are used as adhesives, stabilizers, dispersants, and surfactants due to the unique colloidal properties they possess thanks to their high density of functional groups. The lignosulfonates have a generally higher sub-atomic weight and higher ash content and still contain a significant amount of carbohydrates. Hardwoods and softwoods both contain lignosulfonates, which can be purchased commercially (www.dutchbiorefinerycluster.nl).
2.2 Kraft Lignin
Sodium sulfide and sodium hydroxide are used in the kraft pulping method of alkaline pulping. It has control of 96% of the market. Phenols and lignins with high and low molecular weights that are bound to carbohydrate residues give soda and kraft process lignin its highly polydisperse nature. The native lignin structure is substantially destroyed during kraft pulping. In contrast to soda lignin, kraft lignin contains sulfur. Since the black liquor is burned to produce energy and recover chemicals, it is not free.
The lignin that is produced in the soda and kraft processes is characterized by greater lignin fragmentation. The production of carbon fiber from kraft lignin in the past was unsuccessful; particularly the softwood kraft lignin, which only produces char on heating (Kubo et al., 1977, 1996, 1997; Kubo and Kadla, 1987). The lack of a primary lignin fraction with plasticizing and softening properties could be the cause of this behaviour. Utilizing highly purified hardwood lignin, carbon fiber was successfully produced from industrial kraft lignin (Kadla et al., 2002). The lignin was first treated with heat for 60 minutes at 145 °C in a vacuum to make fibers. The molecular weight was increased and the volatile components of the lignin were removed.
The lignin’s spinnability increased when a small amount of poly(ethylene oxide) was added as a plasticizer. In addition, spinning ought to be feasible at a lower temperature than when using only lignin. When poly(ethylene oxide) was added at a concentration greater than 10%, the lignin fibers self-fused. Due to carefully monitored thermostabilization conditions, such as a very slow increase (12 °C h⁻1) in temperature of the lignin fiber to 250 °C, the ensuing carbonization enhanced the strength characteristics as shown in Table 2.2. Lignin was thermostabilized at 250 °C for 60 minutes in air before being carbonized at 1000 °C to produce carbon fiber. Before thermo-stabilization and carbonization, 5% poly(ethyleneterephtalate) could be added to the lignin to further enhance its strength properties (Kubo and Kadla, 2005; www.benthamscience.com).
Technical kraft lignin was first extracted from black liquor in 1942 by the MeadWestvaco Corporation, the largest kraft lignin producer in the world. It was the only factory in the world by 2011 that offered technical lignin for sale to businesses. Later, Innventia and Chalmers University Technology created the LignoBoost® process, which reduces the amount of ash and carbohydrate in the recovered lignin to produce technical kraft lignin with fewer impurities. Metso Co. bought the technology. In Canada FP Innovation also developed a low-ash, low-carbohydrate, and low-sulfur recovery process called Lignoforce®. GreenValue in Switzerland also offers commercially available, low impurity, non-wood, technical soda lignin (Gosselink, 2011; Smolarski, 2012; Zhu, 2013; iopscience.iop.org).
Extraction of highly pure lignin from a kraft pulp mill forms the basis of the LignoBoost technology, which was developed by Innventia and Chalmers University of Technology in Sweden. Carbon dioxide is used for lowering the pH of the black liquor in order to precipitate the lignin. A filter press is used to extract water from the precipitate.
Problems with sodium separation and conventional filtration are addressed by redissolving the lignin in spent wash water and acid. In order to produce pure lignin, water is removed from the resulting slurry once more and washed with acidified water. When acidified, carboxylic acids and all phenols undergo protonation. The result is pure lignin containing between 2 and 3 weight % sulfur and little to no contamination from carbohydrates or ash. The lignin is chemically linked to about half of the sulfur.
Metso expanded the technology after acquiring it in its entirety in 2008. At the end of 2011, Metso sold Domtar its first commercial LignoBoost technology plant. The plant is located in a pulp mill in Plymouth, North Carolina, USA. It produced 25 000 tons of kraft lignin in 2013. A second plant in Sunila, Finland, which was acquired by Stora Enso was established and started producing 50 000 tonnes of dried lignin annually in the third quarter of 2015. Mead-Westvaco produces a similar lignin for commercial use in Charleston, USA. Approximately 30 000 tonnes are produced annually (www.dutchbiorefinerycluster.nl).
There is a growing number of commercial lignin suppliers. However, some purity is required for high-value applications, so numerous technologies are being developed. Commercially available lignosulphonates are also available. However, due to the presence of impurities, they are utilized for low-value tasks (Chen, 2014; Gosselink, 2011). Kraft lignin has a very condensed structure, many C–C bond links that will not break (like biphenyl and methylene-bridged ones), and a few easy-to-break ether bonds.
Covalently, sulfur species, especially thiols, are also incorporated into the structure making them significant impurities that may prevent subsequent valorization (for sulfur-based catalytic depolymerization, a known poison for many metal catalysts or in material applications). Softwoods and hardwoods are used in the commercial production of kraft lignins. These lignins possess a high hydroxyl content.
2.3 Soda Lignin
Soda lignins are distinct from lignosulfonates and kraft lignins in that they do not contain any sulfur-containing reagents. Lignin becomes resistant and condensed as a result of the pulping process’s relatively harsh conditions. Soda lignins, like kraft lignins, have a low to moderate purity and only a small amount of ash and carbohydrates. Vinyl ethers are present in soda lignins, in contrast to kraft lignins and lignins produced under acidic conditions. Both hardwoods and annual crops are used for the commercial production of soda lignins.
2.4 Steam Explosion Lignin
Steam explosion involves pre-treatment of woody biomass for a brief period of time with steam at high pressure and a temperature of 200 °C or higher, followed by rapid decompression (Krutov et al., 2017; Palmqvist et al., 1996; Soederstroem et al., 2004; Stenberg et al., 1998). The material obtained after explosion is extracted using either an organic solvent or an aqueous alkali, and the lignin, a byproduct with few impurities (carbohydrates and wood extractives), is obtained. Steam explosion lignin is more like native lignin in terms of the quantity and composition of functional groups than any other produced technical lignin. However, the molecular weight of the lignin decreases significantly.
The biomass is rapidly decompressed after treatment with steam to temperatures of 200–220 o
