Table of Contents

Cover

Wiley Series in Renewable Resources

Title Page

Dedication

Series Preface

Preface

List of Acronyms

List of Symbols

Part I: Introduction

Chapter 1: Background and Overview
1. 1.1 Introduction
2. 1.2 Lignin: Economical Aspects and Sustainability
3. 1.3 Structure of the Book
4. References

Part II: What is Lignin?

Chapter 2: Structure and Physicochemical Properties
1. 2.1 Introduction
2. 2.2 Monolignols, The Basis of a Complex Architecture
3. 2.3 Chemical Classification of Lignins
4. 2.4 Lignin Linkages
5. 2.5 Structural Models of Native Lignin
6. 2.6 Lignin–Carbohydrate Complex
7. 2.7 Physical and Chemical Properties of Lignins
8. References
Chapter 3: Detection and Determination
1. 3.1 Introduction
2. 3.2 The Detection of Lignin (Color-Forming Reactions)
3. 3.3 Determination of Lignin
4. 3.4 Direct Methods for the Determination of Lignin
5. 3.5 Indirect Methods for the Determination of Lignin
6. 3.6 Comparison of the Different Determination Methods
7. References
Chapter 4: Biosynthesis of Lignin
1. 4.1 Introduction
2. 4.2 The Biological Function of Lignins
3. 4.3 The Shikimic Acid Pathway
4. 4.4 The Common Phenylpropanoid Pathway
5. 4.5 The Biosynthesis of Lignin Precursors (the Monolignol-Specific Pathway)
6. 4.6 The Dehydrogenation of the Precursors
7. 4.7 Peroxidases and Laccases
8. 4.8 The Radical Polymerization
9. 4.9 The Lignin–Carbohydrate Connectivity
10. 4.10 Location of Lignins (Cell Wall Lignification)
11. 4.11 Differences Between Angiosperm and Gymnosperm Lignins
12. References

Part III: Sources and Characterization of Lignin

Chapter 5: Isolation of Lignins
1. 5.1 Introduction
2. 5.2 Methods for Lignin Isolation from Wood and Grass for Laboratory Purposes
3. 5.3 Commercial Lignins
4. References
Chapter 6: Functional and Spectroscopic Characterization of Lignins
1. 6.1 Introduction
2. 6.2 Elemental Analysis and Empirical Formula
3. 6.3 Determination of Molecular Weight
4. 6.4 Functional Group Analyses
5. 6.5 Frequencies of Functional Groups and Linkage Types in Lignins
6. 6.6 Characterization by Spectroscopic Methods
7. 6.7 Raman Spectroscopy
8. References
Chapter 7: Chemical Characterization and Modification of Lignins
1. 7.1 Introduction
2. 7.2 Characterization by Chemical Degradation Methods
3. 7.3 Other Chemical Modifications of Lignins
4. 7.4 Thermolysis (Pyrolysis) of Lignins
5. 7.5 Biochemical Transformations of Lignins
6. References

Part IV: Lignins Applications

Chapter 8: Applications of Modified and Unmodified Lignins
1. 8.1 Introduction
2. 8.2 Lignin as Fuel
3. 8.3 Lignin as a Binder
4. 8.4 Lignin as Chelating Agent
5. 8.5 Lignin in Biosciences and Medicine
6. 8.6 Lignin in Agriculture
7. 8.7 Polymers with Unmodified Lignin
8. 8.8 Other Applications of Unmodified Lignins
9. 8.9 New Polymeric Materials Derived from Modified Lignins and Related Biomass Derivatives
10. 8.10 Polymers Derived from Chemicals Obtainable from Lignin Decomposition
11. 8.11 Other Applications of Modified Lignins
12. References
Chapter 9: High-Value Chemical Products
1. 9.1 Introduction
2. 9.2 Gasification: Syngas from Lignin
3. 9.3 Thermolysis of Lignin
4. 9.4 Hydrodeoxygenation (Hydrogenolysis)
5. 9.5 Hydrothermal Hydrolysis
6. 9.6 Chemical Depolymerization
7. 9.7 Oxidative Transformation of Lignin
8. 9.8 High-Value Chemicals from Lignin
9. References

Part V: Lignans

Chapter 10: Structure and Chemical Properties of Lignans
1. 10.1 Introduction
2. 10.2 Structure and Classification of Lignans
3. 10.3 Nomenclature of Lignans
4. 10.4 Lignan Occurrence in Plants
5. 10.5 Methods of Determination and Isolation of Lignans from Plants
6. 10.6 Structure Determination of Lignans
7. 10.7 The Chemical Synthesis of Lignans
8. References
Chapter 11: Biological Properties of Lignans
1. 11.1 Introduction
2. 11.2 Biosynthesis of Lignans
3. 11.3 Metabolism of Lignans
4. 11.4 Plant Physiology and Plant Defense
5. 11.5 Podophyllotoxin
6. 11.6 Biological Activity of Different Lignan Structures
7. References

Part VI: Outcome and Challenges

Chapter 12: Summary, Conclusions, and Perspectives on Lignin Chemistry
1. 12.1 Sources of Lignin
2. 12.2 Structure of Lignin
3. 12.3 Biosynthesis and Biological Function
4. 12.4 Applications of Lignin
5. 12.5 Lignans
6. 12.6 Perspectives
7. References

Glossary

Index

End User License Agreement

List of Illustrations

Chapter 1: Background and Overview

Figure 1.1 Number of entries retrieved using “lignin” as topic in Scifinder® in the 1920–2014 period

Chapter 2: Structure and Physicochemical Properties

Figure 2.1 Chemical formula, and atom numbering for the hydroxycinnamyl alcohols (M1) monomers and residues of lignin.
Figure 2.2 Chemical formula, and atom numbering for the M2–M12 monomers of lignin
Figure 2.3 Scheme for the formation of lignoglucoside derivatives by means of the enzyme coniferyl-alcohol glucosyltransferase
Figure 2.4 Chemical formula for nonconventional monolignols [12, 13]
Figure 2.5 Scheme of the microfibril and structure of the plant cell wall [14]
Figure 2.6 Structural features of wood. (a) General structural features;(b) micrograph of birch surface structures. Copyright from ref. [15]. (See insert for color representation of this figure.)
Figure 2.7 Common phenylpropane linkages in lignin (carbon–oxygen bond)
Figure 2.9 Common phenylpropane linkages in lignin (carbon–oxygen and carbon–carbon bonds)
Figure 2.10 Scheme of a hardwood lignin fragment, with the notation of linkages and functional groups of Table 2.3 [23–27]
Figure 2.11 Lignin according to Neish [28]
Figure 2.12 Glasser and Glasser lignin model [29]
Figure 2.13 Lignin according to Adler [27]
Figure 2.14 Hydrolysis products (1–26) of protolignins according to Sakakibara [30]. See Figure 2.15 for the meaning of R1-R3.
Figure 2.17 Other fragments in the Sakakibara structural model for softwood lignin [30]
Figure 2.18 Spruce lignin model proposed by Brunow [31]. (See insert for color representation of this figure.)
Figure 2.19 Milled softwood-lignin model proposed by Crestini et al. [33]. (See insert for color representation of this figure.)
Figure 2.20 Structure of beech wood lignin proposed by Nimz [35]
Figure 2.21 Hardwood lignin model proposed by Boerjan et al. [36]. (See insert for color representation of this figure.)
Figure 2.22 Structure of a fragment macromolecule of reed stem lignin [38]
Figure 2.23 Main structures present in the highly acylated lignins [40]. (A) $c02-math-0061$ linked substructures; (A') $c02-math-0062$ linked substructures with acetylated $c02-math-0063$ -carbon; (A') $c02-math-0064$ linked substructures; with p-coumaroylated $c02-math-0065$ -carbon; (B) phenylcoumaran structures formed by $c02-math-0066$ and $c02-math-0067$ linkages; (C) resinol structures formed by $c02-math-0068$ , and $c02-math-0069$ linkages; and (D) spirodienone structures formed by $c02-math-0070$ , and $c02-math-0071$ linkages
Figure 2.24 Chemical structures of the various wheat straw lignin polymeric fragments [41]
Figure 2.25 Main structures of lignin fractions of Arundo donax, involving different side-chain linkages, and aromatic units identified by 2D HSQC NMR [42]. (A) $c02-math-0075$ linkages; (A') $c02-math-0076$ linkages with acetylated $c02-math-0077$ -carbon; (A') $c02-math-0078$ linkages with p-coumaroylated $c02-math-0079$ -carbon; (B) resinol structures formed by $c02-math-0080$ , and $c02-math-0081$ linkages; (C) phenylcoumaran structures formed by $c02-math-0082$ and $c02-math-0083$ linkages; (D) spirodienone structures formed by $c02-math-0084$ and $c02-math-0085$ linkages; (E) $c02-math-0086$ -diaryl ether substructures; (H) p-hydroxyphenyl unit; (G) guaiacyl unit; (S) syringyl unit; (I) cinnamyl alcohol end-groups; (J) cinnamyl aldehyde end-groups; (FA) ferulate; (PCA) p-coumarate; (T) tricin
Figure 2.26 Representation of lignin polymers from the C4H:F5H-up-regular transgenic trees, as predicted from NMR-based lignin analysis [43]. (See insert for color representation of this figure.)
Figure 2.27 Four main types of LCC linkages in wood
Figure 2.28 Sketch of a structural spiral motif of lignin molecule with $c02-math-0153$ , and $c02-math-0154$ [90]

Chapter 3: Detection and Determination

Figure 3.1 Lignification pattern in Populus tissues. (a) Scanning electron micrograph of xylem elements in a Zinnia stem. Courtesy of Kim Findlay and Copyright from ref. [7]. (b) Transverse section of stem segment. Lignin deposition, visualized under the light microscope after phloroglucinol–HCl staining (red color) (x-xylem, ph-phloem, s-sclerenchyma. $c03-math-0001$ ). Copyright from ref. [8]. (c) Secondary xylem from stem. Lignin distribution by fluorescent microscopy (autofluorescence). Copyright from ref. [8]. (See insert for color representation of this figure.)
Figure 3.2 UV/vis detection of cinnamaldehyde units based on the Wiesner reaction with phloroglucinol in acid media
Figure 3.3 Structures of acridine and acriflavine heterocycles
Figure 3.4 Cross–Bevan color reaction of syringyl lignin [26]
Figure 3.5 Mäule test reaction steps [[29], p. 26]
Figure 3.6 Adler and Stenemur color reaction of guaiacyl lignin [30]
Figure 3.7 Adler and Lundquist color reaction of guaiacyl lignin [31]
Figure 3.8 Acetyl derivatization of lignin by acetyl bromide reagent to yield lignin soluble under acidic conditions [46]
Figure 3.9 Thio derivatization of lignin by thioglycolate reagent to yield lignin soluble under alkaline conditions [46]

Chapter 4: Biosynthesis of Lignin

Figure 4.1 Structure of monolignols
Figure 4.2 Primary and secondary metabolic pathways leading to the biosynthesis of lignin and other wood components [13]
Figure 4.3 The shikimate–chorismate pathway [17]
Figure 4.4 Biosynthetic pathway from chorismate to L-tyrosine and L-phenylalanine via arogenate [17]
Figure 4.5 Common phenylpropanoid pathway. PAL, Phenylalanine ammonia-lyase; TAL, tyrosine ammonia-lyase; COMT, CoA O-methyltransferase; C4H, cinnamic acid 4-hydroxylase; C3H, emphp-coumarate 3-hydroxylase; NADPH, nicotinamide adenine dinucleotide phosphate; CCoAOTM, caffeoyl-CoA O-methyltransferase; F6H, flavonol 6-hydroxylase; 4CL, emphp-coumaroyl-CoA ligase; HSCoA, coenzyme A (CoA, CoASH, or HSCoA); CCoA3H, 4-hydroxycinnamoyl-CoA 3-hydroxylase [42]
Figure 4.6 Schematic view of the central role of cinnamoyl-coenzyme A in the phenylpropanoid metabolism [48]
Figure 4.7 Monolignol-specific pathway to lignins. CCoAOMT, caffeoyl-CoA O-methyltransferase; CCR, cinnamoyl-CoA reductase; NADP oxidoreductase; CAD, cinnamyl alcohol dehydrogenase; F5H, ferulate 5-hydroxylase [50]
Figure 4.8 Chemical formula and atom numbering for the M2–M12 monomers of lignin
Figure 4.9 Monolignol biosynthesis and transport in the plant cell. Three main lignin building blocks (p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol) are synthesized in the cytosol. Monolignols are transported from the cytosol to different locations: to the cell wall for oxidative cross-linking by apoplastic peroxidases (PER) and laccases (LAC) into lignins; into the vacuole for storage as glucoconjugates and, for ferulic acid, into the Golgi apparatus for incorporation into polysaccharides (pectins or arabinoxylans). Most of these transport routes still remain to be discovered. In principle, these hydrophobic molecules may passively diffuse through membranes, undergo active transport through membrane transporters or through Golgi-derived vesicles or simply be released from dying cells. p-Coumaryl alcohol is exported across the plasma membrane by the AtABCG29 transporter, whereas free coniferyl alcohol and sinapyl alcohol are exported by other, so far unknown ABC transporters. Monolignols are selectively imported into the vacuole as glucoconjugates by unknown ABC transporters. Monolignol glucoconjugates are generated by cytosolic glucosyl transferases (GT) and need to be deglucosylated by the cell wall-associated glucosylhydrolases (GH) before incorporation into lignin polymers. A hypothetical transport route of ferulic acid into Golgi vesicles is also represented. Copyright from ref. [67]. (See insert for color representation of this figure.)
Figure 4.10 Resonance Lewis structures for the coniferyl alcohol radical [42]
Figure 4.11 Proposed scheme of reactions generating $c04-math-0008$ by wall-bound/associated malate dehydrogenase and peroxidase in the plant cell walls [77]. In summary, malate is oxidized by $c04-math-0009$ to yield oxaloacetate and $c04-math-0010$ that can be utilized for the formation of monolignol radicals to initiate lignin formation. (1) Well-documented malate–oxaloacetate shuttles transport reducing equivalents across the plasma membrane; (2) bound malate dehydrogenase (MDH) in lignifying the cell walls of Forsythia differs from other MDHs; (3) most efficient monophenol (ROH) stimulation of $c04-math-0011$ formation in the cell walls was affected by coniferyl alcohol; (4) high concentrations of Mn in wood have been reported; (5) $c04-math-0012$ production via the superoxide radical $c04-math-0013$ cycle is driven by $c04-math-0014$ , ROH, and wall-bound peroxidase; (6) $c04-math-0015$ formation largely depends on the availability of monolignols in the cell walls [78]
Figure 4.12 Major linkages present in lignins [43]
Figure 4.13 Major linkages present in lignans [43]
Figure 4.14 Dimerization of monolignols: coniferyl alcohol dehydrodimerization [100]. (See insert for color representation of this figure.)
Figure 4.15 Dimerization of monolignols: sinapyl alcohol dehydrodimerization [100]. (See insert for color representation of this figure.)
Figure 4.16 Formation of different interunit linkages in lignin via addition reactions to a quinone methide [12]
Figure 4.17 Lignification of monolignols: monolignol cross-coupling with a G-end unit [100]. (See insert for color representation of this figure.)
Figure 4.18 Lignification of monolignols: monolignol cross-coupling with an S-end unit [100]. (See insert for color representation of this figure.)
Figure 4.19 Coupling reactions (endwise polymerization or dimerization) for the major structural units of lignin polymer ( $c04-math-0045$ -O-4 and $c04-math-0046$ -5 linkages) [116]
Figure 4.20 Coupling reactions (oligolignol cross-coupling) for the major structural units of lignin polymer (4-O-5 and 5-5 linkages) [116]
Figure 4.21 The diaryl propane ( $c04-math-0056$ -1) interunit linkages of lignin [12]
Figure 4.22 Coupling reactions (5-hydroxyguaiacyl and coniferaldehyde ones) for the major structural units of lignin polymer ( $c04-math-0061$ -O-4 linkage) [116]
Figure 4.23 Coupling reaction (dimerization) for the major structural units of lignin polymer ( $c04-math-0062$ linkage) [12]
Figure 4.24 Dibenzodioxocin present in softwood lignin
Figure 4.25 Bimolecular phenoxy radical coupling products from E-coniferyl alcohol. (a) Dimeric lignans formed via “random” coupling. (c) Stereoselective coupling to give (+)-pinoresinol. (b and d) HPLC profiles show chirality of pinoresinol obtained for each case, respectively [5]
Figure 4.26 The scheme for monolignol deposition and the subsequent initiation of lignin polymerization within the cell wall. Symplastic transport of monolignols may export them to the cell wall through active transport or by passive diffusion. Alternatively, they may be sequestrated and stored as glucoconjugates into the vacuoles in gymnosperms, before their subsequent transport to the cell wall and hydrolysis to free monolignols for polymerization. The deposited monolignols in the cell wall diffuse to initiation sites where the polymerization process begins. The polymerization to form different bond-linkages of lignin is known to be a random chemical process. However, the nature of initiating sites and the way in which the amount and type of lignin formation is controlled across the cell wall are poorly understood. Copyright from ref. [2]. (See insert for color representation of this figure.)
Figure 4.27 Monolignols and differential cell wall targeting. Telescopic representation of a conifer tracheid. p-Coumaryl alcohol is preferentially deposited in the middle lamella whereas secondary alcohols in the secondary wall [6]
Figure 4.28 Plant phylogenetic tree marked with the major milestones of evolution of lignin biosynthesis. Strikethrough text means extinct lineage [43]

Chapter 5: Isolation of Lignins

Figure 5.1 Degradation of monosaccharide units to dialdehydes
Figure 5.2 Oxidative cleaving of the aromatic ring, via o-quinone, to muconic acid
Figure 5.3 Lignin fragmentation reaction suggested to take place during the milling of wood [11]
Figure Table 5.4 Kraft and sulfite processes of pulp formation [82]
Figure 5.5 Main reactions involved in the formation of Kraft lignin during pulping [128]
Figure 5.6 Model addressing the structural characteristics of pine Kraft lignin [13]
Figure 5.7 Model depicting the structural characteristics of lignosulfonate lignin [13]
Figure 5.8 Main reactions involved in the formation of soda lignin [128]

Chapter 6: Functional and Spectroscopic Characterization of Lignins

Figure 6.1 Main reactions of HI with functional groups present in lignin [91]
Figure 6.2 The aminolysis method [112]
Figure 6.3 Oxidation of guaiacyl moiety with sodium periodate [23]
Figure 6.4 Reaction of quinone monochloroimide (4) with p-hydroxybenzyl alcohol groups (3) [23]
Figure 6.5 Reaction of DDQ with p-hydroxybenzyl alcohol groups (6) [23]
Figure 6.6 Color reaction of coniferyl alcohol groups [23]
Figure 6.7 Scheme of a hardwood lignin fragment, with the frequency of linkages [164]
Figure 6.8 Typical linkage units in lignin [23]
Figure 6.9 Determination of condensed units by oxidation of guaiacyl nucleus with Fremy's salt [23]
Figure 6.10 The hemipinic and metahemipinic acids
Figure 6.11 Dibenzodioxocin (DBDO)
Figure 6.12 FTIR spectra of MWLs from oak, birch, and spruce [23]
Figure 6.13 Example of a UV/vis spectra of lignin [197]
Figure 6.14 2D NMR spectra revealing lignin unit compositions. Partial short-range $c06-math-0441$ (HSQC) correlation spectra (aromatic regions only) of cell wall gels in $c06-math-0442$ (4:1, v/v) from (a) two-year-old greenhouse-grown poplar wood, (b) mature pine wood, (c) senesced corn stalks and (d) senesced Arabidopsis inflorescence stems. Contours in this region are used to measure S/G/H ratios, as well as relative p-hydroxybenzoate (PB, in poplar), p-coumarate (pCA in corn) and ferulate (FA in corn) levels [238]. (See insert for color representation of this figure.)
Figure 6.15 Derivatization of hydroxylic structures with TMPD [254, 255]

Chapter 7: Chemical Characterization and Modification of Lignins

Figure 7.1 Mechanism of lignin oxidation meditated by nitrobenzene [[27], p. 434]
Figure 7.2 Products identified in nitrobenzene and cupric oxide oxidations of lignins [28]
Figure 7.3 Reaction sequence for the oxidative degradation of lignin with $c07-math-0017$ [13]
Figure 7.4 Main carboxylic acid methyl esters formed in the oxidation of lignin with $c07-math-0018$ [13]
Figure 7.5 Products obtained by mild hydrolysis of protolignins with aqueous dioxane at $c07-math-0023$ [84]
Figure 7.6 Solvolytic reactions of benzyl alcohol and benzyl ether groups [95]
Figure 7.7 Dimeric lignin acidolysis products [100]
Figure 7.8 Degradation of lignin by thioacetolysis with thioacetic acid [122]
Figure 7.9 Dilignols obtained from beech protolignin by thioacetolysis (% of lignin) [84, 122]
Figure 7.10 Reaction mechanism of $c07-math-0057$ -O-4 substructure units by thioacidolysis and subsequently desulfurization with Raney nickel [17, 84]
Figure 7.11 Thioethylated monomers and Raney nickel-desulfurated dimers produced by thioacidolysis of spruce MWL [84, 126]
Figure 7.12 Cleavage patterns in the hydrogenolysis of interunitary lignin linkages [6]
Figure 7.13 Dilignols isolated from the hydrogenolysis of protolignins [6]
Figure 7.14 Trilignols isolated from the hydrogenolysis of protolignins [6]
Figure 7.15 Reactions of the DFRC degradation method on lignin structures. Degradation of H ( $c07-math-0077$ ), G ( $c07-math-0078$ , $c07-math-0079$ ), and S ( $c07-math-0080$ ) lignin units involved only in arylglycerol- $c07-math-0081$ -ether structures [19]
Figure 7.16 Main dimer recovered from G units involved in phenylcoumaran structures. The chiral center at $c07-math-0082$ -carbon is denoted with an asterisk (*) [19]
Figure 7.17 Formation of catechol (126) from softwood lignin in the NE method [178]
Figure 7.18 Mechanism of Criegee ozonolysis [188, 189]
Figure 7.19 Formation of two tetronic acids from side chains of arylglycerol-β-aryl ether structures by ozonolysis [203]
Figure 7.20 Initial radical reactions of conjugated structures with chlorine dioxide (formation of chlorous ester intermediates) [263]
Figure 7.21 Mechanism of the lignin oxidation meditate by $c07-math-0171$ [[27], p. 474]
Figure 7.22 Oxidation meditate by $c07-math-0172$ of several lignin model compounds [27]
Figure 7.23 Oxidative rupture of the aromatic ring of lignin with peracids [27]
Figure 7.24 Lignin oxidation mediated by $c07-math-0195$ [[27], p. 472]
Figure 7.25 Lignin oxidation mediated by Fremy's salt [[27], p. 473]
Figure 7.26 Lignin oxidation mediated by DDQ [[27], p. 473]
Figure 7.27 Proposed mechanism of the role of activated oxygen in degradation of a monomeric lignin model compound, veratryl alcohol (V), by lignin peroxidase. In the absence of oxygen, veratraldehyde (VI) is the only product formed from the cation radical. In the presence of oxygen, seven other products resulting from radical chemical reactions were detected [341]

Chapter 8: Applications of Modified and Unmodified Lignins

Figure 8.1 Percentages of different plant components
Figure 8.2 Sources of lignin and main types produced in industry
Figure 8.3 Syntheses of urea–formaldehyde and phenol–formaldehyde polymers
Figure 8.4 General scheme for the polymerization of alkene into polyethylene
Figure 8.5 General scheme for the formation of polyester from dicarboxylic acid and dialcohol
Figure 8.6 Lignin polymerization with acrylamide [54]
Figure 8.7 Synthesis of polyurethane from 1,6-diisocyanatohexane and butane-1,4-diol
Figure 8.8 Schematic representation of the network structure of the polyurethane–lignin polymer
Figure 8.9 Template-guided polymerization of aniline with lignosulfonic acid as the dopant [65]
Figure 8.10 Schematic representation of the oxidative reactions of quinone groups in a lignosulfonate biopolymer with a polypyrrole matrix [66]
Figure 8.11 Schematic representation of the lignin resin composed by a polyphenol fragment [86, p. 84]
Figure 8.12 Steric hindrance effects that hamper cross-inking in lignin polymers. Steric hindrance sites are marked with solid spheres
Figure 8.13 Phenolization reaction in lignin. Active sites are indicated with an asterisk (∗) [88]
Figure 8.14 Demethylation reaction in lignin
Figure 8.15 Lederer–Manasse reaction of lignin with formaldehyde in dilute basic medium [89]
Figure 8.16 Fragments of modified lignin with carbon atoms (in bold marked with arrows) identified by ¹³C NMR spectroscopy [90]
Figure 8.17 Synthesis of epichlorohydrin from glycerol and propene [94]
Figure 8.18 Synthesis of epoxy resin (bisphenol A diglycidyl ether) from epichlorohydrin and bisphenol A
Figure 8.19 Polymerization reactions mediated by epichlorohydrin [88]
Figure 8.20 Lignin modification with polyurethane [100]
Figure 8.21 Scheme of the lignin–PU preparation [100]
Figure 8.22 Scheme for the lignin–polybutadiene linkage: ester linkage (a), ionic linkage (b), and interpenetrating network (c) [103]
Figure 8.23 Lignin modification by means of the Mannich reaction
Figure 8.24 Synthetic protocol for the generation of hollow lignin nanocontainers by inverse miniemulsion [112]

Chapter 9: High-Value Chemical Products

Figure 9.1 Phenolic parent compounds obtained from depolymerization of lignin
Figure 9.2 Thermochemical and chemical lignin transformation processes [8]
Figure 9.3 Directional synthesis of ethylbenzene through catalytic transformation of lignin [10]
Figure 9.4 Valorization of lignin via reduction or liquid-phase reforming [33]
Figure 9.5 Proposed lignin chlorination mechanism [[52], pp. 125–160]
Figure 9.6 Proposed mechanism for chlorinated reagents at the (a) phenolic units of lignin and (b) the corresponding conjugate double bonds [[52], pp. 125–160]
Figure 9.7 Proposed mechanism for the oxidation of lignin by hydrogen peroxide [55]
Figure 9.8 Syntheses of vanillin
Figure 9.9 Four classical syntheses of vanillin from guaiacol, eugenol, coniferyl alcohol, and curcumin [59]
Figure 9.10 Synthesis of vanillin from curcumin induced by bismuth salts under MW irradiation [60]
Figure 9.11 Rhoda's synthesis of vanillin
Figure 9.12 Borregaard's synthesis of vanillin [59]
Figure 9.13 Production of DMSO from lignin [59]
Figure 9.14 Schematic production of carbon fiber from industrial lignin [77]

Chapter 10: Structure and Chemical Properties of Lignans

Figure 10.1 Atom numbering in monolignols and lignans
Figure 10.2 The eight subgroups of lignans [32]
Figure 10.3 Main aromatic substituents of lignans
Figure 10.4 Neolignan structures resulted by the different bonding of two cinnamyl units
Figure 10.5 Sesquilignans and dineolignans
Figure 10.6 Chemical structures of norlignans
Figure 10.7 Hybrid lignans (Flavono-, coumarino-, and stilbenolignans)
Figure 10.8 Hybrid lignans (xantholignan, terpene lignan, and macrocyclic lignan)
Figure 10.9 Nomenclature of lignans. Step 1: Fundamental parent structure according to Moss [43]
Figure 10.10 Nomenclature of lignans. Step 2: Modifications of the fundamental parent structures
Figure 10.11 Nomenclature of lignans. Step 3: Changes in the hydrogenation level
Figure 10.12 Nomenclature of lignans. Step 4: Derivatives
Figure 10.13 Nomenclature of lignans. Step 5: Stereochemistry
Figure 10.14 Lignans most commonly distributed in plants [9]. See Figure 10.3 for the meaning of R groups
Figure 10.15 Trigonotins A–C [143]
Figure 10.16 Monobenzylbutyrolactone transformations [145]
Figure 10.17 Asymmetric synthesis of lignans using the Koga approach [145]. (a) i. LDA, HMPA, $c10-math-0096$ ii. PhSeBr at $c10-math-0097$ 19 h rt; (b) $c10-math-0098$ , MeOH/ $c10-math-0099$ , 30 min, rt; (c) i. (48), THF ii. substituted benzyl bromide; (d) Ra-Ni, EtOH, reflux; (e) THF, $c10-math-0100$ , rt; (f) $c10-math-0101$ , t-BuOH, rt; (g) $c10-math-0102$ , pyridine, $c10-math-0103$ , rt
Figure 10.18 Asymmetric synthesis of dibenzylbutyrolactones and dibenzylbutanediols [186, 187]. (a) LDA, THF, $c10-math-0104$ , $c10-math-0105$ , 24 h; (b) LDA, hexane/THF, $c10-math-0106$ , DMPU, $c10-math-0107$ , 12 h; (c) THF, LiOH, $c10-math-0108$ , rt 12–24 h; (d) LiOH, THF/ $c10-math-0109$ , reflux 24–48 h; (e) i. $c10-math-0110$ , MeOH $c10-math-0111$ ii. $c10-math-0112$ , 1 h; (f) $c10-math-0113$
Figure 10.19 Asymmetric synthesis of ( $c10-math-0121$ )-deoxypodophyllotoxin (45) of Bogucki and Charlton [191]. (a) n-BuLi, THF, $c10-math-0122$ ; (b) toluene, reflux, 44 h; (c) $c10-math-0123$ , $c10-math-0124$ , $c10-math-0125$ then $c10-math-0126$ , $c10-math-0127$ ; (d) Pd/C, $c10-math-0128$ , MeOH/AcOH, rt, 89 h; (e) $c10-math-0129$ , reflux, 2 h; (f) $c10-math-0130$ , i-PrOH, 15 h, rt; (g) benzene, p-TsOH, reflux, 17.5 h
Figure 10.20 Dihydroguaiaretic acid structure
Figure 10.21 Synthesis of NDGA from 1,4-di-(substituted-phenyl)-2,3-dimethyl-butan-2-ol [197]
Figure 10.22 Synthesis of secoisolariciresinol of Wang et al. [44, 196]
Figure 10.23 Direct oxidative coupling of ethyl ferulate [196]
Figure 10.24 Synthesis of secoisolariciresinol of Morreel et al. from ferulic acid [208]
Figure 10.25 Synthesis of matairesinol (21) and secoisolariciresinol (19) [215]
Figure 10.26 Synthesis of hydroxymatairesinol (57) [216]
Figure 10.27 Synthesis of secoisolariciresinol from pinoresinol mediated by PLR enzyme. See Figure 10.3 for the meaning of R groups
Figure 10.28 Butyrolactones as key intermediates in the synthesis of lignans [214]
Figure 10.29 Isomerization reactions for podophyllotoxin [227]
Figure 10.30 Four main pathways to the synthesis of podophyllotoxin. (a) The oxo-ester pathway; (b) the dihydroxy acid pathway; (c) Diels–Alder pathway, and (d) the tandem conjugate addition pathway [49]
Figure 10.31 Synthesis of aryltetralin lactone
Figure 10.32 Synthesis of podophyllotoxin via Diels–Alder reaction
Figure 10.33 Synthesis of podophyllotoxin by using a Michael-induced ring closure [228, 239]
Figure 10.34 Synthesis of podophyllotoxin using an intramolecular Heck reaction [240]
Figure 10.35 Synthesis of podophyllotoxin in nine steps [252]. a: (1) $c10-math-0139$ , $c10-math-0140$ , DMAP cat., $c10-math-0141$ ; (2) dimethylmalonate, $c10-math-0142$ , toluene; b: $c10-math-0143$ , dppe, NaH, allyl acetate, DMF; c: same as b and emphn-Bu4NOAc; d: same as c and NaH; e: (1) NMO, $c10-math-0144$ , THF/ $c10-math-0145$ ; (2) $c10-math-0146$ , acetone/ $c10-math-0147$ ; f: (1) NaOH, MeOH; (2) descarboxylation; g: NaOH; h: Zinc borohydride, then Gensler enolate quenching procedure or Jones reagent, then acid workup
Figure 10.36 The two major strategies for the synthesis of dibenzocyclooctadiene lignan derivatives
Figure 10.37 Ullmann reaction of biphenyl coupling
Figure 10.38 Four different synthetic pathways to the synthesis of ( $c10-math-0157$ )-steganone (42) by pathway A strategies
Figure 10.39 Intramolecular coupling of eight-membered ring
Figure 10.40 Synthesis of dibenzylbutane lignans [168, 282, 283]
Figure 10.41 McMurry reaction for the coupling of two carbonyl groups mediated by Ti [285, 286]
Figure 10.42 Synthesis of nonsymmetrical lignans using the strategy of Minato et al. [207]
Figure 10.43 Synthesis of nonsymmetrical diaryl lignans using the strategy of Krauss et al. (acyl anion equivalent) [287–291]
Figure 10.44 Synthesis of nonsymmetrical diaryl lignans using the strategy of Takeya et al. [292] ( $c10-math-0160$ )
Figure 10.45 Synthesis of dibenzylbutyrolactone lignans
Figure 10.46 Synthesis of dibenzylbutyrolactone lignans using the strategy of Belletire et al. [293]
Figure 10.47 Total synthesis of steganone by Magnus et al. [295]
Figure 10.48 Strategy for the synthesis of a series of lignans type wuweizi by Chang et al. [298, 299].
Figure 10.49 Synthesis of ( $c10-math-0161$ )-steganone and ( $c10-math-0162$ )-isosteganone by Robin et al. [267]
Figure 10.50 Synthesis of steganone precursor by Faruque [29]
Figure 10.51 Asymmetric synthesis of ( $c10-math-0163$ )-steganone by Meyers et al. [311]
Figure 10.52 Synthesis of steganone by Monovich et al. [265]
Figure 10.53 Synthesis of steganone precursor by Hughes and Raphael [309]
Figure 10.54 Synthesis of steganone precursor by Kende and Liebeskind [312]
Figure 10.55 Total synthesis of steganone by Meyers et al. [311]

Chapter 11: Biological Properties of Lignans

Figure 11.1 Biosynthesis of the precursors of lignans (hydroxycinnamyl alcohol monomers) [23]
Figure 11.2 Enantioselective formation of (+)-pinoresinol with the aid of dirigent protein (DIR) [23]
Figure 11.3 Conversion of pinoresinol into matairesinol [16]
Figure 11.4 Biosynthetic pathways for cyclolignan and cyclooctadiene lignans starting from pinoresinol [16]. See Figure 11.7 for the meaning of R groups
Figure 11.7 Common lignan substituents $c11-math-0005$ used in Figure for clarity
Figure 11.5 Biosynthetic pathway for (+)-sesaminol from (+)-pinoresinol. Undetermined pathways are indicated with a dashed arrow and “?” symbol [32]
Figure 11.6 Biosynthetic pathway for yatein and bursehemin in Anthriscus sylvestris [17]. See Figure 11.7 for the meaning of R groups
Figure 11.8 Biosynthetic pathway for podophyllotoxin (PPT) (14) and 6-methoxy-PPT [36]. See Figure 11.7 for the meaning of R groups
Figure 11.9 Isoeugenol formation from coniferyl alcohol by isoeugenol synthase and its conversion into verrucosin lignan in Virola surinamensis [16]
Figure 11.10 Biosynthetic pathways for cyclooctadiene lignans, starting from coniferyl alcohols [16]. See Figure 11.7 for the meaning of R groups
Figure 11.11 Proposed biosynthetic pathway to neolignans containing 8–5′ linkage present in Cryptomeria japonica [24]
Figure 11.12 The cinnamate/monolignol biosynthetic pathway [48]
Figure 11.13 Proposed biosynthetic pathway of $c11-math-0019$ -linked norlignan hinokiresinol (32) [48]
Figure 11.14 Proposed biosynthetic pathway of $c11-math-0020$ -linked norlignan Sequirin D (33 [49]
Figure 11.15 Bioconversion of plant lignans to enterolignans in the human gut, mediated by facultative aerobes [60–65]
Figure 11.16 Matairesinol (MAT) metabolism by intestinal bacteria [68]. See Figure 11.7 for the meaning of R groups
Figure 11.17 Arctiin (ART) metabolism by intestinal bacteria [68]. See Figure 11.7 for the meaning of R groups
Figure 11.18 Sesaminol triglucoside (STG) metabolism by intestinal bacteria [68, 69]. See Figure 11.7 for the meaning of R groups
Figure 11.19 Metabolism of lignans: tracheloside and olivil glucoside. See Figure 11.7 for the meaning of R groups
Figure 11.20 Lignans and neolignans with fungal activity. See Figure 11.7 for the meaning of R groups
Figure 11.21 Antitumor PPT-glycosyl derivatives. See Figure 11.7 for the meaning of R groups
Figure 11.22 Illustration of Podophyllum peltatum (American Podophyllum).
Figure 11.23 Plant sources of podophyllotoxin [132, 150]
Figure 11.24 Podophyllotoxin and its glycosidic derivative mode of action [133]
Figure 11.25 Main aryltetralin lactone lignans with biological activity. I: Podophyllotoxin (PPT) (14) $c11-math-0055$ ; deoxy-PPT (13) $c11-math-0056$ ; 6-methoxy-PPT (26) $c11-math-0057$ ; PPT glucoside (71) $c11-math-0058$ ; $c11-math-0059$ -demethyl-PPT (72) $c11-math-0060$ ; acetyl-PPT (78) $c11-math-0061$ ; $c11-math-0062$ -demethyl-PPT (81) $c11-math-0063$ ; $c11-math-0064$ -desmethoxy-PPT (82) (morelsin) $c11-math-0065$ ; $c11-math-0066$ -demethyldeoxy-PPT (87) $c11-math-0067$ . II: $c11-math-0068$ -Peltatin (PT) (75) $c11-math-0069$ ; $c11-math-0070$ -PT (24) $c11-math-0071$ ; $c11-math-0072$ -PT A methyl ether (25) $c11-math-0073$ ; $c11-math-0074$ -PT-glucoside (73) $c11-math-0075$ ; $c11-math-0076$ -PT-glucoside (74) $c11-math-0077$ ; $c11-math-0078$ -desmethoxy- $c11-math-0079$ -PT-glucoside (76) $c11-math-0080$ . See Figure 11.7 for the meaning of R groups
Figure 11.26 SAR keypoints for podophyllotoxin derivatives [133]
Figure 11.27 Main dibenzylbutane lignans with biological activity. See Figure 11.7 for the meaning of R groups
Figure 11.28 Main dibenzylbutyrolactone lignans with biological activity. See Figure 11.7 for the meaning of R groups
Figure 11.29 Main dibenzylbutyrolactol lignans with biological activity [373]. See Figure 11.7 for the meaning of R groups
Figure 11.30 Main furanoid lignans with biological activity. See Figure 11.7 for the meaning of R groups
Figure 11.31 Main furofuranoid lignans with biological activity. See Figure 11.7 for the meaning of R groups
Figure 11.32 Main $c11-math-0230$ -dihydroxyaryltetralin lignans with biological activity. See Figure 11.7 for the meaning of R groups
Figure 11.33 The main arylnaphthalene lignans with biological activity. See Figure 11.7 for the meaning of R groups
Figure 11.34 Main dibenzocyclooctadiene lignans (lactones) with biological activity. See Figure 11.35 for the meaning of fused rings X, Y, and R
Figure 11.35 Fused rings used in Figure for clarity
Figure 11.36 The main dibenzocyclooctadiene lignans with biological activity. See Figure 11.35 for the meaning of fused rings X, Y, and R
Figure 11.37 New dibenzocyclooctadiene lignans (marlignans A–L) with biological activity. See Figure 11.37 for the meaning of substituent [557]
Figure 11.38 Neolignans with biological activity

Chapter 12: Summary, Conclusions, and Perspectives on Lignin Chemistry

Figure 12.1 Sugar–lignin platform potential for value-added chemical production.

List of Tables

Chapter 1: Background and Overview

Table 1.1 The most common plant phenolic compounds listed according to the count (content) of carbon atoms^a

Chapter 2: Structure and Physicochemical Properties

Table 2.1 Amount of the different monolignols in lignin from various plant types^a
Table 2.2 Percentage of total linkages present in softwood and hardwood lignins
Table 2.3 Types and frequencies of linkages and main functional groups in softwood and hardwood lignins (dilignol/functional groups per 100 ppu) [23]
Table 2.4 Semiempirical formulas of the lignins of some herbaceous plants^a
Table 2.5 Guaiacyl/syringyl ratio (G/S) in the lignins of birch cells and their structural and rigidity level, respectively^a

Chapter 3: Detection and Determination

Table 3.1 Color reactions of lignin with phenol derivatives^a
Table 3.2 Color reactions of lignin with amines^a
Table 3.3 Color reactions of lignin with heterocycles^a
Table 3.4 Color reactions of lignin with inorganic compounds
Table 3.5 Lignin content in various types of plants^a
Table 3.6 Chemical lignin content of common natural and wood fibers^a
Table 3.7 Lignin determination with 72% and 80% sulfuric acid methods^a
Table 3.8 Relationships for the interconversion of lignin content and chlorine (hypo), permanganate, and Kappa numbers^a
Table 3.9 Typical Kappa numbers range for representative pulps^a
Table 3.10 Lignin contents of woods and pulps given by different methods^a
Table 3.11 Lignin concentrations given by four analytical procedures^a,b

Chapter 4: Biosynthesis of Lignin

Table 4.1 The eight core monolignol biosynthetic enzymes^a
Table 4.2 Types and frequencies of linkages in softwood and hardwood lignins[128]^a

Chapter 5: Isolation of Lignins

Table 5.1 Main methods used for the isolation of lignin
Table 5.2 Acid-insoluble (Klason) lignin contents of lignified materials [[32], p. 37]^a
Table 5.3 Representative data on MWL isolated from different wood species^a
Table 5.4 Yield of organosolv lignin under various extraction conditions^a,b
Table 5.5 Yield of organosolv lignin in EtOH/water at $c05-math-0051$ under various extraction conditions^a,b
Table 5.6 Lignin yields after microwave extraction^a of various biomass^b
Table 5.7 Yield and analytical data of Kraft lignins obtained after solvent fractionation of isolated black liquor lignin^a
Table 5.8 Yield of Kraft lignin isolated from pine Kamyr black liquor at different pH^a
Table 5.9 Sulfite-pulping procedures used to extract lignin from wood^a
Table 5.10 Amines and solvents used in the extraction of lignosulfonates^a
Table 5.11 Lignosulfonate fractions isolated from spruce spent sulfite liquor by addition of octylamine^a
Table 5.12 Chemical properties of lignin sulfonate^a
Table 5.13 Organosolv lignins [137]

Chapter 6: Functional and Spectroscopic Characterization of Lignins

Table 6.1 Elementary composition of various lignin preparations^a
Table 6.2 Example of calculation of empirical $c06-math-0007$ formula^a
Table 6.3 Formulas for milled wood lignins (MWLs)^a
Table 6.4 Elemental analysis, methoxyl content, and calculated $c06-math-0038$ formula of various microwave extracted grass lignins^a
Table 6.5 Average molar mass data $c06-math-0063$ and dispersity index $c06-math-0064$ of organosolv lignins extracted from various types of herbaceous biomass^a
Table 6.6 Molecular weight $c06-math-0075$ , dispersity index $c06-math-0076$ , and glass-transition temperatures $c06-math-0077$ of selected lignins^a
Table 6.7 $c06-math-0084$ of different lignins determined by light scattering^a
Table 6.8 Molecular weight $c06-math-0087$ of spruce dioxane lignin^a
Table 6.9 $c06-math-0088$ of spruce dioxane lignin fractionated by preparative SEC^a
Table 6.10 $c06-math-0094$ of black cottonwood lignins determined by VPO measured at various temperatures in 2-methoxyethanol^a
Table 6.11 $c06-math-0097$ of different Kraft lignins determined by VPO^a
Table 6.12 Comparison of $c06-math-0102$ from different methods^a,b
Table 6.13 Functional group of spruce MWL^a
Table 6.14 Percentage of methoxyl in various lignin preparations^a
Table 6.15 Phenolic hydroxyl groups of spruce MWLs^a
Table 6.16 Phenolic hydroxyl contents of some lignins^a
Table 6.17 Total, phenolic, and aliphatic hydroxyl contents of some representative milled wood and bamboo lignins (MWL and MBL)^a
Table 6.18 Hydroxyl group content (determined by $c06-math-0154$ analysis) of organosolv lignins isolated from various types of plant biomass^a
Table 6.19 Ethylene group contents of some lignins^a
Table 6.20 Carboxyl contents of some lignins^a
Table 6.21 Sulfonic acid contents in lignin determined by different methods^a
Table 6.22 Structural units of spruce MWL^a
Table 6.23 Structural units (per 100 $c06-math-0177$ units) of birch MWL and beech lignin^a
Table 6.24 IR-absorption bands of lignins^a
Table 6.25 Assignments of FTIR peaks in lignins^a
Table 6.26 Assignments of FTIR peaks $c06-math-0245$ of lignins extracted from various herbaceous biomass^a,b
Table 6.27 Main $c06-math-0269$ chemical shifts $c06-math-0270$ assignment from TMS of acetylated spruce MWL and beech MWL^a
Table 6.28 Main $c06-math-0310$ chemical shifts ( $c06-math-0311$ , ppm) assignment from TMS of acetylated spruce MWL and beech MWL^a
Table 6.29 Main $c06-math-0354$ chemical shifts $c06-math-0355$ assignment from TMS of nonacetylated lignin^a

Chapter 7: Chemical Characterization and Modification of Lignins

Table 7.1 Chemistry methods for the analysis of lignin^a
Table 7.2 Yields and oxidation products from Picea abies wood on nitrobenzene oxidation^a
Table 7.3 Yields of phenolic aldehydes from several plant species on nitrobenzene oxidation^a,b
Table 7.4 Alkaline nitrobenzene oxidation products of the fiber fraction from birch wood soft xylem^a
Table 7.5 Nitrobenzene oxidation of birch wood decayed by four ascomycetes^a,b
Table 7.6 Yields on nitrobenzene and cupric oxide oxidation products from Picea abies and Populus tremuloides woods^a,b
Table 7.7 Relative frequency of aromatic carboxylic acids formed in the permanganate oxidation of selected woods and pulps
Table 7.8 Monomeric phenols detected in lignin acidolysis reaction mixtures^a
Table 7.9 Yields^a of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) monomeric thioacidolysis products^b
Table 7.10 Yields^a of monomeric guaiacyl (G) and syringyl (S) products recovered from poplar lignin fractions after degradation by various procedures^b
Table 7.11 Monomeric aromatic products formed in the hydrogenolysis of lignin^a,^b
Table 7.12 Fractions of condensed and noncondensed guaiacyl nuclei in softwood protolignins Mol (% of original lignin)^a
Table 7.13 Characteristics of Douglas fir lignin^a
Table 7.14 Characterization of hardwood protolignin by the NE method^a
Table 7.15 Media used in ozonolysis of lignins^a
Table 7.16 Erythro/threo ratios of isolated and in situ lignins^a
Table 7.17 Composition of halolignins prepared under different conditions^a
Table 7.18 Composition of nitrolignins prepared under different conditions^a

Chapter 8: Applications of Modified and Unmodified Lignins

Table 8.1 Physical and chemical properties of the two main types of lignin^a
Table 8.2 Comparison of physical properties of Arboform® with wood and plastics [58]

Chapter 9: High-Value Chemical Products

Table 9.1 Metallic catalyzed lignin depolymerization [12]
Table 9.2 Base-calalyzed lignin depolymerization [12]
Table 9.3 Ionic liquid-assisted lignin depolymerization [12]
Table 9.4 Supercritical fluid-assisted depolymerization [12]
Table 9.5 Potential lignin-derived products [56]
Table 9.6 Lignin producers of vanillin [62]
Table 9.7 Physical and chemical activation of lignins [65]^a

Chapter 10: Structure and Chemical Properties of Lignans

Table 10.1 Synonyms of most common lignan names used in this textbook^a
Table 10.2 Resin and heartwood sources^a,b,c
Table 10.3 Total (fresh weight) content of lignans (as aglycones) in common foods and their botanical origin^a

Chapter 11: Biological Properties of Lignans

Table 11.1 Main reported biological activities of lignans [2]
Table 11.2 Bacteria implicated in the metabolism of lignans^a,b
Table 11.3 Physicochemical properties of podophyllotoxin
Table 11.4 Short chronology of discovery and development of podophyllum drugs [153]
Table 11.5 Lignan isolates from the species of Justicia [485]

Wiley Series in Renewable Resources

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Callum A. S. Hill

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Industrial Application of Natural Fibres–Structure, Properties and Technical Applications
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Thermochemical Processing of Biomass–Conversion into Fuels, Chemicals and Power
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Biorefinery Co-Products: Phytochemicals, Primary Metabolites and Value-Added Biomass Processing
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Aqueous Pretreatment of Plant Biomass for Biological and Chemical Conversion to Fuels and Chemicals
Charles E. Wyman

Bio-Based Plastics: Materials and Applications
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Introduction to Wood and Natural Fiber Composites
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Introduction to Chemicals from Biomass, Second Edition
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Erik Meers and Gerard Velthof

Bio-Based Solvents
François Jerome and Rafael Luque

Lignin and Lignans as Renewable Raw Materials

Chemistry, Technology and Applications

FRANCISCO G. CALVO-FLORES, JOSÉ A. DOBADO, JOAQUÍN ISAC-GARCÍA

Department of Organic Chemistry, University of Granada, Spain

and

FRANCISCO J. MARTÍN-MARTÍNEZ

Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, USA

John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom

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