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Lignin Conversion Catalysis E-Book

Chaofeng Zhang

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Herausgeber: Wiley-VCH GmbH
Kategorie: Wissenschaft und neue Technologien
Sprache: Englisch

Beschreibung

This book focuses on the research development of lignin conversion to aromatic compounds. It provides systematic results for the study of lignin conversion by giving details of lignin depolymerization and mechanisms.

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Seitenzahl: 820

Veröffentlichungsjahr: 2022

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Leseprobe

Cover

Title Page

Dedication

Preface

List of Abbreviations

List of Symbols

Part I: Book Introduction

1 Background and Overview

1.1 Introduction

1.2 Lignin: A Natural and Sustainable Aromatic Bank

1.3 Structure of This Book

References

Part II: Lignin Introduction

2 Lignin Biosynthesis and Structure

2.1 Lignin Biosynthesis

2.2 Lignin Structure

2.3 Chapter Summary

References

3 Lignin Isolation, Physicochemistry Properties, and Chemical Properties

3.1 Lignin Polymer Physical Properties

3.2 Lignin Isolation from Lignocellulose and Technical Lignins

3.3 Lignin Spectroscopy Properties

3.4 Lignin Chemical Properties

3.5 Chapter Summary

References

Part III: Lignin Depolymerization: Scientific Questions, Challenges, and Current Progress

4 Scientific Questions for Lignin Conversion and a Brief Summary of Methods for Lignin Depolymerization

4.1 Opportunity and Challenges of New Biorefinery Approaches for Lignin Valorization

4.2 Scientific Questions Involved in Lignin Depolymerization and the Foundation of Strategies

4.3 Two Different Approaches for the Foundation of Lignin Depolymerization Strategies

4.4 Classification of Lignin Conversion Methods by Reaction Types

4.5 Brief Index of Progress of Native/Technical Lignin Conversion

4.6 Chapter Summary

References

Part IV: Review on Lignin Linkages Cleavage Strategies and Mechanisms via an IDA Method

5 The Inverse Disassembly Analysis Method for Classifying Lignin Conversion Strategies

5.1 Introduction of Inverse Disassembly Analysis for Lignin Conversion

5.2 Different Analysis Modes for Lignin Depolymerization

5.3 IDA Catalogue of Lignin Conversion Methods Discussed in the Following Chapters

5.4 Chapter Summary

References

6 Direct Lignin C–OAr, ArO–Ar or C–Ar Bonds Cleavage without First Activation of the Adjacent Chemical Bonds

6.1 Brönsted/Lewis Acid + Metal Systems for the Direct Hydrogenative Cleavage of Ether Bonds and C–Ar Bonds

6.2 Base/Organometallic Systems for the Direct Hydrogenative Cleavage of Ether Bonds

6.3 Other Heterogeneous Catalytic Systems for the Direct Hydrogenative Cleavage of C–OAr Ether Bonds

6.4 Direct Reductive Cleavage of Ether Bond with Hydride Reagents

6.5 Direct Reductive Cleavage of Lignin Ether Bond with e

–

6.6 Chapter Summary

References

7 Lignin C–C/C–O Bonds Cleavage via First Phenolic Hydroxyl Group Dehydrogenation or First Aromatic Rings Activation

7.1 Lignin C

–C

Bonds Cleavage after the First Phenolic Hydroxyl Group Dehydrogenation to the Phenolic Radical

7.2 Lignin C

–C

bonds Cleavage via the First Single‐Electron Transfer (SET) of the Aromatic Ring

7.3 Lignin C

–OC/C

–C Bonds Cleavage via First Partly‐Hydrogenation or Partly‐Addition of the Neighbouring Aromatic Ring

7.4 Lignin C(sp

)–C(sp

) σ Bond and C(sp

)–OAr Bonds Cleavage via Adjacent Aromatic Groups Activation or Extra Radicals Attack

7.5 Chapter Summary

References

8 Lignin Linkages Cleavage Beginning with C

O–H/ArO–H or C

–OH Bond Heterolysis

8.1 Base‐catalyzed C

–OAr Bond Cleavage Beginning with C

O–H or ArO–H Heterolysis

8.2 Acid‐catalyzed C

–OAr Bonds Cleavage Beginning with C

–OH Heterolysis

8.3 Chapter Summary

References

9 Lignin Linkages Cleavage Beginning with C

–H, C

–OH, or C

O–H Bond Non‐ionized Activation

9.1 Lignin C

–OAr Bond Cleavage via a Transfer Hydrogenation or Dehydrogenation‐hydrogenation Process Beginning with the First Activation of C

–H(O–H) to C

==O

9.2 Lignin C

–OAr Bond Cleavage in the Dehydrogenation/Oxidation‐Hydrogenation (Reduction) Process Beginning with the First Activation of C

–H(OH) to C

9.3 Lignin C

–C

–OAr Bonds Cleavage via Multiple Oxidation Process Beginning with the First Activation of C

–OH to C

9.4 Lignin C

–C

, C

–C

, or C

O–C

Bonds Cleavage by Inserting an O‐ or N‐containing Fragment after the First Oxidation of C

–OH to C

9.5 Embellishing Lignin β‐O‐4 Linkages Hydrolysis Involving the C

–OH First Oxidation and C

–OH Transformation

9.6 Lignin C

–OAr Bond Cleavage after the First Activation of C

–H, C

–OH, or C

O–H to C

•

Radical

9.7 Lignin C

–C

Bond Cleavage after the First Activation of C

O–H Bond to C

•

Radical via PCET Strategies and LMCT Mechanisms

9.8 Chapter Summary

References

10 Lignin Linkages Cleavage Beginning with C

–H, C

–H, or C

O–H Direct Activation

10.1 Lignin C–C/C

–OAr Bond Cleavage Beginning with C

–H Bond Direct Activation

10.2 First —C

OH Activation to —C

HO/—C

OOR Inducing Lignin C–C Bonds Selective Cleavage and Its Derivative Methods

10.3 Lignin C

–OAr Bond Cleavage Beginning with C

First Sulphonation

10.4 Chapter Summary

References

11 Lignin Linkages Cleavage Considering Fragments Condensation

11.1 Different Mechanisms of Lignin Fragments Condensation

11.2 Methods for Restraining Lignin Fragments Condensation

11.3 Chapter Summary

References

Part V: Outcome and Outlook

12 Summary on Lignin Utilization and Perspectives on Preparation of Aromatic Chemicals

12.1 Brief Summary on Lignin Utilization as Materials

12.2 Outlets of Lignin Resources Beyond Aromatic Chemicals

12.3 Standardized Lignin Substrate and Standardized Products

12.4 Concluding Remarks

References

Index

End User License Agreement

List of Tables

Chapter 3

Table 3.1 Typical Method Used for the Lignin Isolation.

Table 3.2 Different Commercial Organosolv Lignins.

Table 3.3 Sulfite‐pulping Procedures Used to Extract Lignin from Wood.

Table 3.4 IR‐absorption Bands of Lignins.

Table 3.5 Main

H Chemical Shifts (δ) of Acetylated Spruce MWL and Beech MW...

Table 3.6 Main

C Chemical Shifts (δ, ppm) Assignment of Lignin.

Chapter 4

Table 4.1 A Brief Summary of Main Methods for Lignin Conversion by Reaction...

Table 4.2 A Brief List of Some Representative Systems for Native/Technical ...

Chapter 5

Table 5.1 Bond Dissociation Energies of the C

–O and C

–C

B...

List of Illustrations

Chapter 1

Figure 1.1 The Carbon‐Increasing Process Based on the Non‐renewable Fossil R...

Figure 1.2 Lignocellulose (LC) from Biomass and its Structural Composition....

Figure 1.3 Number of Entries Retrieved Using “Lignin” as the Research Topic...

Chapter 2

Figure 2.1 The Three‐step for Lignin Biosynthesis.

Figure 2.2 Primary and Secondary Metabolic Pathways Leading to the Biosynthe...

Figure 2.3 The Shikimate–Chorismate Pathway (from PEP/E‐4‐P to Chorismate vi...

Figure 2.4 Common Phenylpropanoid Pathway (a) and the Monolignol‐Specific Pa...

Figure 2.5 Chemical Formulas of the Other Monomers of Lignin.

Note:

M1monoli...

Figure 2.6 Resonance Structures of Monolignols Radicals.

Note:

‐II ≈ R

‐II...

Figure 2.7 Major Linkages Present in Lignins and Frequencies of Linkages and...

Figure 2.8 Dehydrogenation of Coniferyl Alcohol and Its Mesomeric Radicals C...

Figure 2.9 Dehydrogenation of Coniferyl Alcohol and Its Mesomeric Radicals C...

Figure 2.10 Dehydrogenation of Coniferyl Alcohol and Its Mesomeric Radicals ...

Figure 2.11 Dehydrogenation of Coniferyl Alcohol and Its Mesomeric Radicals ...

Figure 2.12 Dehydrogenation of Coniferyl Alcohol and Its Mesomeric Radicals ...

Figure 2.13 The Formation of Dibenzodioxocin Unit from the 5‐5 Linkage.

Figure 2.14 The β‐1 Cross‐coupling Mechanisms and the Corresponding Lignin U...

Figure 2.15 Mechanism of 5‐Hydroxyconiferyl Alcohol Incorporation into a Gua...

Figure 2.16 The Coniferaldehyde Coupling with Lignin Fragment to the β‐O‐4 L...

Figure 2.17 The Generation of α,β Ester Structure from the Quinone Methide I...

Figure 2.18 Structure Model of Beech Wood Lignin.

Figure 2.19 2D NMR Spectra Revealing Lignin Unit Compositions. Partial Short...

Figure 2.20 The Spruce Lignin Model.

Figure 2.21 Structural Model for Softwood Lignin.

Figure 2.22 The Spruce Lignin Model.

Figure 2.23 The Milled Softwood‐lignin Model.

Figure 2.24 Structure Model Reed Stem Lignin.

Figure 2.25 Five Main Types of LCC Linkages in the Wood.

Chapter 3

Figure 3.1 Lignin Fragmentation Reaction Taking Place during the Milling of ...

Figure 3.2 Degradation of Monosaccharide to Dialdehydes in the Periodate Lig...

Figure 3.3 Main Reactions Involved in the Formation of Kraft Lignin during P...

Figure 3.4 Structure Model of Lignosulfonate.

Figure 3.5 Main Reactions Involved in the Formation of Alkali Lignin.

Figure 3.6 Derivatization of Lignin Hydroxylic Structures with TMPD.

Figure 3.7 Derivatization of Lignin Hydroxylic Structures with HMDS and Me3S...

Figure 3.8 Derivatization of Lignin Hydroxylic Structures with Fluorine Reage...

Figure 3.9 Lignin Oxidation with Nitrobenzene.

Figure 3.10 Formation of Low‐molecular Aliphatic Acids from Ozonolysis of β‐...

Figure 3.11 Lignin H2O2 Oxidation with Broken Down of the Aromatic Ring.

Figure 3.12 Methylation Reaction in Lignin.

Figure 3.13 Phenolization Reaction in Lignin.

Figure 3.14 Demethylation Reaction in Lignin.

Figure 3.15 Hydroxymethylation Reaction in Lignin.

Figure 3.16 The lignin Mannich Reaction.

Figure 3.17 Nucleus‐Exchange Reaction in Lignin.

Chapter 4

Figure 4.1 Lignin Depolymerization to Aromatic Chemicals.

Figure 4.2 Frequencies of MajorLignin Linkages.

Figure 4.3 The Solubility of Birch Lignin in Various Solvents Characterized ...

Figure 4.4 MALDI‐TOF Profiles of Dissolved Birch Lignin in 1,4‐Dioxane (A), ...

Figure 4.5 Two Different Approaches for the Foundation of Lignin Depolymeriz...

Figure 4.6 Definitions of Conversion and Product Yield for Lignin (A) and Li...

Chapter 5

Figure 5.1 The Retrosynthetic Analysis of the Natural Product to Confirm the...

Figure 5.2 Bond Dissociation Enthalpy of Typical Bonds in Lignin Linkages....

Figure 5.3 Adjacent Functional Group Modification (AFGM) Strategy for the Cl...

Figure 5.4 The β–O–4 Linkage Model with Atoms Denomination.

Note:

The phenol...

Figure 5.5 Direct Lignin C–OAr, ArO–Ar or C–Ar Bonds Cleavage without First ...

Figure 5.6 Lignin C–C/C–O Bonds Cleavage via First Phenolic Hydroxyl Group D...

Figure 5.7 Lignin Linkages Cleavage Beginning with C

O–H/ArO–H Heterol...

Figure 5.8 Lignin Linkages Cleavage Beginning with the Activation of C

...

Figure 5.9 Lignin Linkages Cleavage Beginning with Cβ–H, Cγ–H, or CγO–H Dire...

Chapter 6

Figure 6.1 Stoichiometric Reactions for C–O Ether Bonds Cleavage (A and B), ...

Figure 6.2 Acidolysis‐hydrogenation Tandem Pathway for Lanthanide Triflate/P...

Figure 6.3 Step‐wise Reaction Profile for Lignin C

−OAr Bond Hydrogenol...

Figure 6.4 Tandem Catalytic System for the Reductive Fractionation of Woody ...

Figure 6.5 Hydrogenolysis of Lignin‐derived Aromatic Ethers Catalyzed by a S...

Figure 6.6 Noble‐metal Catalyzed Hydrodeoxygenation of Biomass‐derived Ligni...

Figure 6.7 Catalytic Fast Pyrolysis of Lignin over ZSM‐5 Zeolite for the Pro...

Figure 6.8 Direct Hydrodeoxygenation of Woody Biomass into Liquid Alkanes an...

Figure 6.9 Size‐dependent Catalytic Performance of Ru Nanoparticles of Ru/Nb...

Figure 6.10 Lignin Fragmentation over Magnetically Recyclable Composite Co@N...

Figure 6.11 MOF‐based Catalysts for Selective Hydrogenolysis of Carbon−oxyge...

Figure 6.12 Hydrogenolysis of Aryl Ether Bonds over a Nickel Ni Carbene Cata...

Figure 6.13 Mechanisms for the Conversion of Aryl Alkyl Ethers to Arenes....

Figure 6.14 Reaction Mechanism for C–O Bond Hydrogenolysis of DiphenylEther ...

Figure 6.15 Catalytic C−OAr Bond Hydrogenolysis of Methyl Phenyl Ether with ...

Figure 6.16 The Role of the Excess Base on the Mechanism of Ni‐NHC Catalyzed...

Figure 6.17 Nickel‐catalyzed Reductive Cleavage of Aryl Alkyl Ethers to Aren...

Figure 6.18 Metal‐catalyzed Transformations of Aryl Ethers with Various Nucl...

Figure 6.19 Hydrogenolysis of Diaryl Ethers over a Heterogeneous Nickel Cata...

Figure 6.20 Ni‐catalyzed Cleavage of Aryl Ethers in the Aqueous Phase.

Figure 6.21 One‐pot Catalytic Hydrocracking of Raw Woody Biomass into Chemic...

Figure 6.22 Lignin Conversion in Methanol with Ni/AC Catalyst.

Figure 6.23 Representation of the Ni/AC Catalyst Reduced by H

(a) and Activ...

Figure 6.24 Depolymerization of Various Lignin over the Ni/C Catalyst in Met...

Figure 6.25 Hydrogenolysis of Diaryl Ether C–O Bonds by a Heterogeneous Ni/C...

Figure 6.26 Hydrogenation of Lignin to Aromatic Chemicals over a Ni‐Mo

C/C C...

Figure 6.27 Catalytic Lignin Ether Bond Cleavage over the Supported Ru‐WO

C...

Figure 6.28 Reaction Pathways of β‐O‐4 Model Transformation into Monomeric a...

Figure 6.29 Low‐temperature Cleavage of Ethers with Boranes/Metal Borohydrid...

Figure 6.30 NiCl

/NaBH

in Methanol Achieves the Dealkylation or Debenzylati...

Figure 6.31 Palladium‐Catalyzed Formal Cross‐Coupling of Diaryl Ethers with ...

Figure 6.32 Selective Reductive Cleavage of Inert aryl C–O Bonds by an Iron ...

Figure 6.33 Rh‐catalyzed Reductive Cleavage of the C–O Bond of Acetals and w...

Figure 6.34 Ni‐catalyzed Reductive Cleavage of Aryl ether with Hydrosilane....

Figure 6.35 Ni‐catalyzed Protocol for the Reductive Cleavage of Inert Ar–OR ...

Figure 6.36 Reductive Cleavage of Aryl ethers with Silane and Base..

Figure 6.37 Reductive Cleavage of Lignin Model Compounds to Phenols and Prim...

Figure 6.38 Reductive Degradation of Lignin and Model Compounds by Hydrosila...

Figure 6.39 Pd‐catalyzed Hydrogenolysis of Dibenzodioxocin Lignin Model Comp...

Figure 6.40 Electrochemical Cleavage of Aryl Ethers Promoted by Sodium Boroh...

Chapter 7

Figure 7.1 Phenoxyl Radical Induce the Cleavage of Lignin C–C Bonds.

Figure 7.2 Catalytic Oxidation of Syringyl Alcohols to the Corresponding Ben...

Figure 7.3 Oxidative Degradation of Monomeric and Dimeric Phenylpropanoids w...

Figure 7.4 The Oxidation Mechanism of Veratryl Alcohol with Co(salen) Media....

Figure 7.5 Catalytic Oxidation of Veratryl Alcohol via a Co/ [EMIM][DEP]/OH ...

Figure 7.6 The Catalytic Oxidation Mechanism of Guaiacyl Models with a Hinde...

Figure 7.7 Reaction Mechanism Proposed for Alkaline Oxidation of Lignin to V...

Figure 7.8 Lignin Oxidation Mechanism with Cu

and Fe

in NaOH.

Figure 7.9 Proposed Mechanism of Lignin Oxidation in the Presence of Cu‐Subs...

Figure 7.10 Mechanism of Electrooxidation Cleavage of Lignin Model Dimers....

Figure 7.11 Cleavage of C

–C and C

alkyl

O–Ar Bonds by the In‐situ Generated ...

Figure 7.12 High‐temperature Electrolysis of Kraft Lignin for Selective Vani...

Figure 7.13 Degradation of Lignin to BHT by Electrochemical Catalysis on Pb/...

Figure 7.14 Photocatalytic Chemoselective C−C Bond Cleavage at Room Temperat...

Figure 7.15 Mechanism of C–C Bonds Cleavage in the Catalytic Oxidation the β...

Figure 7.16 Proposed Mechanism for Side‐chain Cleavage of a Phenolic β‐O‐4 L...

Figure 7.17 Proposed Mechanisms for C

Oxidation (A) and Alkyl‐phenyl B...

Figure 7.18 Proposed Mechanism for C

–C

Cleavage of Phenolic β‐1 ...

Figure 7.19 Lignin C

–C

Bonds Cleavage via the First Generation o...

Figure 7.20 Proposed Radical Depolymerization Route of a Model Compound of β...

Figure 7.21 Possible Reactions of the Veratryl Alcohol Cation Radical and a ...

Figure 7.22 Structures of Porphyrin and Phthalocyanine Catalysts Used for Li...

Figure 7.23 Single‐electron Transfer Mechanism for Oxidation of the β‐1 Lign...

Figure 7.24 Oxidative Cleavage of Lignin to Aromatic Aldehydes by Metal Salt...

Figure 7.25 HPA‐5 Catalyzed Oxidative Cleavage of Non‐phenolic β‐O‐4 Bonds (...

Figure 7.26 Oxidation of Lignosulfonate with Persulphate and Various Metal S...

Figure 7.27 Oxidation of Lignosulfonate with Nitrobenzene and Metal Salts.

Figure 7.28 Triarylamine Mediated the Electrochemical Oxidation of Nonphenol...

Figure 7.29 Regioselectivity of Enzymatic and Photochemical Single Electron ...

Figure 7.30 Depolymerization Route of β–1 Linkage via the First One‐electron...

Figure 7.31 Light/Copper Relay for Aerobic Degradation of Lignin Model Compo...

Figure 7.32 The mechanism for Selective C

–C

Bond Cleavage by Ir ...

Figure 7.33 Uranyl‐Photocatalyzed Hydrolysis of Diaryl Ethers.

Figure 7.34 Synthesis of Ketones from Lignin‐derived Methyl Aromatic Ether....

Figure 7.35 Palladium‐Catalyzed Hydrolytic Cleavage of Aromatic C–O Bonds....

Figure 7.36 Palladium‐Catalyzed Reductive Insertion of Alcohols into Aryl Et...

Figure 7.37 Cleaving Lignin Ether Bond via First Partly‐addition of the Neig...

Figure 7.38 Ni‐catalyzed Reductive Cleavage of Inert C

–O Bonds with the Ex...

Figure 7.39 Pd‐catalyzed Ether Bond Cleavage and Rearrangement of 4‐O‐5 Lign...

Figure 7.40 Methoxybenzenes Oxidation to Benzoquinones Catalyzed by MTO/H

Figure 7.41 Cleavage of a C(sp

)–C(sp

) σ Bond between Two Phenyl Groups und...

Figure 7.42 Functionalized Spirolactones by Photoinduced Dearomatization of ...

Figure 7.43 Visible‐Light Photoredox‐Catalyzed C

–O Bond Cleava...

Figure 7.44 Strategy for the Catalytic Activation of C(aryl)−C(aryl) Bonds o...

Chapter 8

Figure 8.1 Alkaline Cleavage of α‐aryl and β‐aryl Ether Bonds in Phenolic Ar...

Figure 8.2 Competitive Addition of Nucleophiles Hydrosulfide Ion (a) or Phen...

Figure 8.3 Alkali‐promoted Condensation Reactions in Phenolic Units with For...

Figure 8.4 Decomposition of Phenolic Lignin Models over Organic N‐bases in a...

Figure 8.5 Degradation of β‐O‐4 Lignin Model Compounds by Solvent‐free Grind...

Figure 8.6 Lignin Depolymerization by Nickel‐supported Layered Double Hydrox...

Figure 8.7 Lignin Depolymerization with Nitrate‐Intercalated Hydrotalcite Ca...

Figure 8.8 Vinylation of Lignin β‐O‐4 Linkage with Calcium Carbide through C...

Figure 8.9 Pathways for the Acid‐mediated Cleavage of the Lignin β‐O‐4 Linka...

Figure 8.10 Depolymerization of Lignin Linkage with Hydrogen Iodide.

Figure 8.11 Selective Ether Bonds Cleavage in Lignins by the DFRC Method.

Figure 8.12 Degradation of Lignin by Thioacetolysis with Thioacetic Acid.

Figure 8.13 Acid‐catalyzed Mechanism for Hydrolysis of the β‐O‐4 Bonds in Io...

Figure 8.14 Cleavage of Lignin β‐O‐4 C–O Bonds Catalyzed by Methyldioxorheni...

Figure 8.15 Microwave‐assisted Fast Conversion of Lignin β‐O‐4 Linkage over ...

Figure 8.16 Proposed Reaction Pathway for the W2C/AC‐catalyzed Deconstructio...

Chapter 9

Figure 9.1 The Mechanism for Ru Catalyzed C–O Bond Cleavage in 2‐Aryloxy‐1‐a...

Figure 9.2 Mechanism of 2‐Aryloxy‐1‐arylethanol Catalytic Deconstruction by ...

Figure 9.3 Dehydrogenation and C–O Cleavage by Intermolecular Hydrogen Trans...

Figure 9.4 Cleavage of the β‐O‐4 Linkage with Pincer Complexes.

Figure 9.5 Binuclear Rh‐catalyzed Redox‐neutral System for Lignin Depolymeri...

Figure 9.6 Ru‐terpyridine Catalyzed Redox‐neutral Depolymerization of Lignin...

Figure 9.7 Proposed Mechanism of β‐O‐4 Ether Bond Cleavage via the C

–M...

Figure 9.8 Proposed Mechanism for the Transformation of β‐O‐4 Ether Bond Cle...

Figure 9.9 Proposed Mechanism of Aryl Propene Formation from Wood over the P...

Figure 9.10 Proposed Reaction Mechanisms for the Redox Neutral Transfer Hydr...

Figure 9.11 Pd/C‐Catalyzed Redox Neutral Cleavage of the Model I to Products...

Figure 9.12 Reaction Network for the β‐O‐4 Bond Cleavage of 2‐Phenoxy‐1‐phen...

Figure 9.13 The Optimized β‐O‐4 Molecules/Pd(111) Structures. In Each Row, t...

Figure 9.14 ZnIn

‐catalyzed Photocatalytic Self‐transfer Hydrogenolysis of...

Figure 9.15 The Transformation of Lignin β‐O‐4 Models to Benzylamines.

Figure 9.16 Dehydrogenation/Oxidation‐hydrogenation (Reduction) Strategies f...

Figure 9.17 Isolation of Functionalized Phenolic Monomers through Selective ...

Figure 9.18 Chemoselective Oxidant‐free Dehydrogenation of C

H–OH in Li...

Figure 9.19 Nucleophilic ThiolsReductively Cleave Ether Linkages in Lignin M...

Figure 9.20 The reaction of the LigDFG Enzyme System with AVR.

Figure 9.21 DFRC Method for Lignin Analysis with C

–OAr Cleavage.

Figure 9.22 The Conversion of Birch Sawdust to Aromatic monomers via an Oxid...

Figure 9.23 Carbon Modification of Nickel Catalyst Ni/MgAlO‐C for Depolymeri...

Figure 9.24 Catalytic Depolymerisation of Oxidized Lignin over TiN‐Cu Nanoca...

Figure 9.25 Two‐step Visible‐light Mediated Depolymerization Strategies for ...

Figure 9.26 Selective C−O Bond Cleavage of Lignin Systems and Polymers Enabl...

Figure 9.27 The One‐pot Electrocatalytic Oxidation−reductive Cleavage of β‐O...

Figure 9.28 Proposed Reaction Mechanism for Photocatalytic C−O Bond Cleavage...

Figure 9.29 Further Deoxygenation of Aromatic Ketones to Alkyl Arenes over t...

Figure 9.30 Solar‐driven Lignin Oxidation via Hydrogen Atom Transfer with a ...

Figure 9.31 Fine Tuning the Redox Potentials of Carbazolic Porous Organic Fr...

Figure 9.32 Visible Light‐enabled Selective Depolymerization of Oxidized Lig...

Figure 9.33 Two‐step Oxidation Strategy for β‐O‐4 Cleavage.

Figure 9.34 Possible Reaction Mechanism of the Copper ComplexCatalyzed Oxida...

Figure 9.35 Oxidative C(OH)–C Bond Cleavage of Secondary Alcohols to Acids o...

Figure 9.36 Cobalt Nanoparticles‐Catalyzed Successive C–C Bond Cleavage in A...

Figure 9.37 Possible Mechanism of Oxidation of Lignin Model Compound in Ioni...

Figure 9.38 Amine‐Mediated Bond Cleavage in Oxidized Lignin Models.

Figure 9.39 Dual ILs‐promoted Photochemical Degradation of Pre‐oxidized Lign...

Figure 9.40 Selective Electrochemical C–O Bond Oxidative Cleavage of β‐O‐4 L...

Figure 9.41 The β‐O‐4 Models Transformation via a ‘Wedging’ Mode.

Figure 9.42 Chemoselective Aerobic Alcohol Oxidation in Lignin and C

–C

Figure 9.43 Transformation of Lignin Model Compounds to N‐substituted Aromat...

Figure 9.44 NH

OH−mediated Lignin Conversion to Isoxazole and Nitrile.

Figure 9.45 Introducing N‐radical at C

Mediated Lignin Aryl Ether Clea...

Figure 9.46 Nucleophilic Aromatic Substitution for the Cleavage of Aryl C–O ...

Figure 9.47 From Alkylarenes to Anilines via Site‐directed Carbon‐carbon Ami...

Figure 9.48 Selective Utilization of the Methoxy Group in Lignin to Produce ...

Figure 9.49 C

–OAr Cleavage of Oxidized Lignin Model Compounds with For...

Figure 9.50 C

–OAr Bond Cleavage after the First Activation of C

–...

Figure 9.51 Possible Radical Intermediates for the Degradation Reactions of

Figure 9.52 Boosting Laccase/HBT‐Catalyzed the Cleavage of β‐O‐4' Linkage in...

Figure 9.53 Proposed Pathways for HDO of β‐O‐4 Lignin Substrate over Zn(II)‐...

Figure 9.54 Zn(II)/Pd/C Catalyzed Cleavage and Hydrodeoxygenation of Lignin ...

Figure 9.55 Catalytic Cleavage of Aryl–ether Bonds in Lignin Model Compounds...

Figure 9.56 The Potential Dehydroxylation‐hydrogenation Strategy for β‐O‐4‐A...

Figure 9.57 Solar Energy‐driven Lignin‐first Approach to Full Utilization of...

Figure 9.58 Ligand‐controlled Photocatalysis of CdS Quantum Dots for Lignin ...

Figure 9.59 Enhancing Photocatalytic β‐O‐4 Bond Cleavage in Lignin Model Com...

Figure 9.60 Visible‐Light‐Driven Cleavage of C−O Linkage for Lignin Valoriza...

Figure 9.61 Catalytic Cleavage of Ether C–O Bonds by Pincer Iridium Complexe...

Figure 9.62 Multiple Mechanisms Mapped in Aryl Alkyl Ether Cleavage via Aque...

Figure 9.63 Vanadium‐catalyzed Non‐oxidative Cleavage of β‐O‐4 Linkage.

Figure 9.64 C–C or C–O Bond Cleavage in a Phenolic Lignin Model Compound: Se...

Figure 9.65 Overall Mechanistic Proposals for C(sp

)−OAr and Ar−C(sp

) Bond ...

Figure 9.66 Catalytic Cleavage of the C–C Bond with an Adjacent Alcohol Grou...

Figure 9.67 Proposed Mechanistic Cleavage Pathways Schematic BQ−BQH

Redox C...

Figure 9.68 Catalytic Ring‐opening of Cyclic Alcohols Enabled by PCET Activa...

Figure 9.69 Light‐driven Depolymerization of Lignin Enabled by PCET.

Figure 9.70 Photocatalytic C

–C

Bond Cleavage in Lignin

‐O‐...

Figure 9.71 Selective Lignin β‐O‐4 C–C Bond Cleavage with a Vanadium Photoca...

Figure 9.72 Visible‐Light‐Induced Oxidative Lignin C−C Bond Cleavage to Alde...

Figure 9.73 Photocatalytic C–C Bond Cleavage and Amination of Cycloalkanols ...

Figure 9.74 CeCl

‐promoted Photocatalytic Cleavage of C

‐C

Bond i...

Chapter 10

Figure 10.1 Lignin Linkages Cleavage Beginning with C

O–H, or C

Figure 10.2 Photocatalytic C−C Bond Cleavage of β‐1 Lignin Models to Aromati...

Figure 10.3 Photocatalytic Cleavage of C–C Bond in Lignin Models under Visib...

Figure 10.4 VO(acac)2‐catalyzed Oxidative Cleavage of 2‐Phenoxy‐1‐phenyletha...

Figure 10.5 Pt1/N‐CNTs Catalyze the Electrooxidation of β‐O‐4 Linkage with C

Figure 10.6 Retro‐aldol Strategies for Biomass C–C Bond Cleavage.

Figure 10.7 The Transfer Hydrogen‐based Retro‐aldol Mechanism for the C–C Bo...

Figure 10,8. Organocatalytic Chemoselective Primary Alcohol Oxidation and Su...

Figure 10.9 Iridium‐catalyzed Primary Alcohol Oxidation and Hydrogen Shuttli...

Figure 10.10 Electrochemical Aminoxyl‐mediated Oxidation of Primary Alcohols...

Figure 10.11 Selective Copper−N‐heterocyclic Carbene‐catalyzed Aerobic Cleav...

Figure 10.12 The Transformation of Alcohol(butanol)‐pretreated Lignin Linkag...

Figure 10.13 Sequential Catalytic Modification of the Lignin α‐Ethoxylated β...

Figure 10.14 The Degradation of β‐O‐4 Model Compounds by a TIZ Method.

Figure 10.15 Multi‐steps γ‐TTSA Method for β‐O‐4 Linkage Cleavage.

Chapter 11

Figure 11.1 Lignin Fragments Condensations or New Stable C−C Bond Generation...

Figure 11.2 Alkaline Cleavage of C

–OR Bonds in Phenolic Arylpropane U...

Figure 11.3 Some Examples of β–O‐4 Lignin Fragments Recondensation Involving...

Figure 11.4 The Transformation of β‐1 Linkages to Dihydrobenzofuran Structur...

Figure 11.5 Depolymerization of Lignin by Microwave‐assisted Methylation of ...

Figure 11.6 Production of Phenolic Alcohols from Woody Biomass via an In‐sit...

Figure 11.7 Formaldehyde Stabilization Facilitates Lignin Monomers Productio...

Figure 11.8 α,γ‐Diol Lignin Stabilization Strategies besides the Formaldehyd...

Figure 11.9 Mechanistic Insights into Formaldehyde‐Blocked Lignin Condensati...

Figure 11.10 Pre‐protection of Lignin: Diol Etherization, Acetylation, and S...

Figure 11.11 First Transformation of the Active Groups to Promote the Depoly...

Figure 11.12 Selective Ether Bonds Cleavage in Lignins by the DFRC Method.

Figure 11.13 The Lignin Depolymerization Strategies with β‐O‐4 Fragments Gen...

Figure 11.14 In Situ Conversion of the Reactive Aldehyde Intermediate in the...

Figure 11.15 Lewis Acid‐catalyzed β‐O‐4 Linkage Cleavage with Rh‐catalyzed D...

Figure 11.16 Preventing Lignin Fragments Condensation by Diffusion Control B...

Figure 11.17 The Reactor Installation with Membrane Separation.

Figure 11.18 The Normal Lignin Structure and Benzodioxane Structure of C‐lig...

Chapter 12

Figure 12.1 The Lignin Polymers Model from the C4H:F5H‐up‐regular Transgenic...

Figure 12.2 The Typical Structure Model of the Catechyl lignin.

Figure 12.3 Schematic Flow of Maleic Acid Promote Sustainable Fractionation ...

Figure 12.4 Catalytic Hydrogenation Depolymerization of Organosolv (Methanol...

Figure 12.5 Hydrogenolysis Monomer Yields of C‐lignin over the Pd/C in Metha...

Figure 12.6 Defunctionalization of Lignin Depolymerization Monomers to Stand...

Figure 12.7 Catalytic Upgrading of Alkylphenols Derived to Phenol and Olefin...

Figure 12.8 Proposed Reaction Pathway of Dealkylation, Isomerization, Dispro...

Figure 12.9 Proposed Mechanism for the O‐ and C‐dealkylation of Ferulic Acid...

Figure 12.10 Lignin Valorization to Phenol by Direct Transformation of Csp2–...

Figure 12.11 La(OTf)3 Catalyzed the Selective Transformation of Lignin to Gu...

Figure 12.12 Lignin Dealkylation to Phenol via Hydrogenation‐oxidation‐decar...

Figure 12.13 Reaction Pathways of the Self‐Reforming‐Driven Depolymerization...

Figure 12.14 Catalytic Lignin Decomposition to Benzene over the RuW/Zeolite ...

Figure 12.15 Production of Terephthalic Acid (TPA) from Lignin‐Based Phenoli...

Figure 12.16 A Three‐step Strategy for the TPA Production from Corn Stover L...

Guide

Cover Page

Table of Contents

Title Page

Dedication

Preface

List of Abbreviations

List of Symbols

Begin Reading

Index

End User License Agreement

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Lignin Conversion Catalysis

Transformation to Aromatic Chemicals

Chaofeng ZhangFeng Wang

Authors

Dr. Chaofeng Zhang

College of Light Industry and Food Engineering

Nanjing Forestry University

159 Longpan Road

210037 Nanjing

China

Prof. Feng Wang

Dalian Institute of Chemical Physics

Dalian Nat. Lab. for Clean Energy

Chinese Academy of Science

457 Zhongshan Road

116023 Dalian

China

Cover Image: Courtesy of Chaofeng Zhang and Feng Wang

All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing‐in‐Publication Data: A catalogue record for this book is available from the British Library.

Bibliographic information published bythe Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de.

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Print ISBN978‐3‐527‐34973‐9

ePDF ISBN978‐3‐527‐83501‐0

ePub ISBN978‐3‐527‐83502‐7

oBook ISBN978‐3‐527‐83503‐4

To Yiyao Zhang

Preface

The development of civilization and human society can be regarded as the changing history of utilization of various resources and materials. In history, drilling wood for fire lightened the glimmer of the development of human civilization, heating water for steam power by burning coal allowed development to catch the train of the steam age, and petroleum utilization built the cornerstone of modern civilization. Each technical revolution based on the utilization of fossil resources promoted the qualitative development of human civilization. However, just as coins have two sides, a large number of fossil resources have been quickly consumed in a short time and the carbon element that has been stored in Earth's crust is concurrently converted into CO2 gas that enters the atmosphere of the ecosphere where its presence causes corresponding environmental and climate problems. When human society is at a loss about this dilemma, nature has already prepared a solution for us with biomass as the renewable carbon resource. Photosynthesis on Earth can convert CO2 and water into organic carbon resources and O2 with renewable solar energy, which produces about 170 billion tonnes of lignocellulose every year. If the renewable biomass resource can be efficiently transformed to the desired chemical stocks with sustainable energy input, the increasing CO2 concentration will be not a hot potato, primarily because of the potential CO2‐neutral cycle that transforms solar energy, H2O, and CO2 into chemical stocks and fuels via biomass. Different from the utilization of the fossil resource that can be regarded as the conversion and stitching of small molecules into macromolecules, biomass utilization is more like a process of tailoring and converting natural polymers into corresponding small‐molecule chemicals, all of which requires the development of more sophisticated strategies and efficient implementation methods.

Lignin is the third abundant organic carbon resource on the earth. As a precious resource given to mankind by nature, especially for its potential as a treasure house of aromatic chemicals, research on lignin depolymerization and the transformation of its downstream products have drawn much attention from scientists and companies worldwide in the recent decade aimed at harvesting aromatic compounds and hydrocarbon stocks/fuels from this abundant and renewable natural polymer. In November 2020, we received an invitation from Wiley to prepare a book on the topic of lignin conversion/transformation to chemicals. Perhaps it is a coincidence, but our literature retrieval process revealed that the electronic literature on the topic of lignin can be traced back to the 1920s with the SciFinder database, 100 years from 2020. Thus, it is meaningful to receive the invitation to record the research process of lignin conversion. Before setting a topic line of this book from the complex content of lignin catalytic conversion, we first reviewed previous records to determine the likely readership of the book. We hope that this book is useful for readers in academia, industry, and education at the introductory, advanced, and specialist levels who are interested in wood chemistry or fields of lignin utilization research.

For this reason, Chapter I of the book briefly introduces the structure of the book. In Part II, the basic knowledge of lignin, including lignin biosynthesis, lignin structure, and lignin isolation, physicochemistry properties, and chemical properties are introduced. Then, lignin depolymerization issues, involving scientific questions, challenges, classification of lignin conversion methods by reaction type, and current progress, are discussed in Part III. For the main topic, the critical book aims to provide an overview of key advances in the field of lignin depolymerization to aromatic monomers and more attention is paid to the generalization and summarization of the mechanism of lignin linkage cleavage. It is challenging to systemically summarize all of the strategies via the traditional classification method by reaction types. In Chapter 5 of Part IV, inspired by the retrosynthetic analysis of the natural product to confirm the synthons proposed by Professor Cory, we herein put forward an inverse disassembly analysis (IDA) method to explore the efficient strategies for lignin depolymerization, which can be also used in the classification and discussion of the various methods for lignin conversion viewed from the cleavage order and type of corresponding chemical bonds over various catalytic systems. With β‐O‐4 linkage as the main lignin mode, we further introduce the lignin depolymerization strategies with the IDA thoughts from Chapter 6 to Chapter 11. Finally, in Part V, exploring lignin depolymerization to aromatic chemicals and how to make lignin depolymerization profitable, we provide some viewpoints for outlets of the lignin resource that highlights the importance of high‐value medicine and synthetic block preparation or the utilization of lignin from natural polymer to artificial polymer. To accelerate the lignin utilization from lab to industry application, we discussed the critical roles of the standardized lignin substrate and standardized products, during which we support the idea that funneling and functionalization of a mixture of lignin‐derived monomers into a single high‐value chemical is fascinating and promising.

Finally, given the complexity of the catalytic conversion of lignin and the multiformity of the catalytic methods, which involves the research fields of different disciplines, the discussion of some relevant catalytic methods for lignin conversion may be omitted because of the limited knowledge reserve of the authors. In addition, if there are any improper points in this book, we encourage readers and experts to constructively criticize and correct them with us.

November 2021, in Dalian

Chaofeng Zhang

Feng Wang

List of Abbreviations

Activated carbon

ACT

4‐Acetamido‐TEMPO

AFEX

Ammonia fiber explosion

AFGM

Adjacent functional group modification

BDE

Bond dissociation enthalpy

BNL

Brauns native lignin

BTX

Benzene–toluene–xylenes

C3H

Coumarate 3‐hydroxylase

C4H

Cinnamic acid 4‐hydroxylase

CAD

Cinnamyl alcohol dehydrogenase.

Conduction band

CCoAOTM

Caffeoyl‐CoA O‐methyltransferase

CCR

Cinnamoyl‐CoA reductase

CCTC

Contact charge transfer complex

CDE

explosion

CEL

Cellulolytic Enzyme Lignins

4CL

4‐coumarate CoA ligase

CoA

Coenzyme A

COMT

CoA O‐methyltransferase

CP/MAS

It is a combination of cross‐polarization sequence (CP) and magic angle transformation technology (MAS).

DAIB

Diacetoxyiodobenzene

DBAD

Diisopropyl azodiformate

DDQ

2,3‐Dichloro‐5,6‐dicyano‐1,4‐benzoquinone

DEPT

Distortionless enhancement by polarization transfer

DES

Deep eutectic solvent

DFRC

Derivatization followed by reductive cleavage

DFT

Density functional theory

DIPEA

N,N‐diisopropylethylamine

Dioxane (acidolysis) lignins

DMSO

Dimethyl sulfoxide

DSPEC

Dye‐sensitized photoelectrochemical cell

E‐4‐P

Erythrose‐4‐phosphate

EMAL

Enzymatic mild acidolysis lignin

ESR

Electron spin resonance absorption

EXAFS

Extended X‐ray absorption fine structure

F5H

Ferulate 5‐hydroxylase

Formic acid

FTIR

Fourier transform infrared spectroscopy infrared absorption spectroscopy

GFC

Gel‐filtration chromatography

Glucosylhydrolases

GPC

Gel‐permeation chromatography

Glutathione

Glucosyl transferases

G‐unit

Coniferyl alcohol, or lignin phenylpropanoid unit with one –OMe connecting with the aromatic ring

HAADF‐STEM

High angle annular dark‐field scanning transmission electron microscopy

HAT

Hydrogen atom transfer

HBS

High‐boiling‐point solvent

HCT

Hydroxycinnamoyltransferase

HMBC

Heteronuclear multiple bond correlation

HMQC

Heteronuclear multiple‐quantum correlation

HPAs

Heteropoly acids

HPSEC

High‐performance size‐exclusion chromatography

HTC

Hydrotalcite

H‐unit

‐Coumaric alcohol, or lignin phenylpropanoid unit without –OMe connecting with the aromatic ring

IDA

Inverse disassembly analysis

ILs

Ionic liquids

INADEQUATE

Incredible natural abundance double quantum transfer experiment

Lewis acid

LAR

Lignin isolation from lignocellulose with lignin as the residue

LBD

Lignin isolation from lignocellulose by first lignin dissolution

Lignocellulose

LCC

Lignin‐carbohydrate complex

LDH

Layered‐double hydroxide catalyst

LDP

Lignin depolymerization

LMCT

Ligand‐to‐metal charge‐transfer

Lignocellulose

MALDI‐TOF

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry

mCPBA

‐Chloroperbenzoic acid

MDO

Methyldioxorheniu

MOF

Metal‐organic framework

MTO

Methyltrioxorhenium

MWD

Molecular weight distribution

MWL

Milled‐wood lignin

NCS

N‐chlorosuccinimide

Nucleus exchange

NHC

N‐heterocyclic carbene

NHPI

N‐hydroxyphthalimide

NMR

Nuclear magnetic resonance spectroscopy

NPs

Nanoparticles (noble metal)

OAT

Oxygen atom transfer

OMS

Open metal sites

OPCs

Organic (visible light) photocatalysts

OSL

Organosolv lignin

PAL

Phenylalanine ammonia‐lyase;

PAR

Phenylseleninic acid resin

‐Hydroxybenzoate

PCET

Proton‐coupled electron transfer

PDI

Perylene diimide

Polyethylene

PEDOS

Partial electronic density of states

PEP

Phosphoenol pyruvate

POFs

Porous organic frameworks

POM

Polyoxometalate

ppu

Phenylpropanoid units

PTH

Phenothiazine

RCF

Reductive catalytic fractionation

RSA

Retrosynthetic analysis

scMeOH

Supercritical methanol

SEC

Size‐exclusion chromatography

SET

Single‐electron transfer

S‐unit

Sinapyl alcohol, or lignin phenylpropanoid unit with two ‐OMe connecting with the aromatic ring

TAL

Tyrosine ammonia‐lyase

TBD

1,5,7‐Triazabicyclo[4.4.0]dec‐5‐ene

BuONO

tert‐Butyl nitrite

TDTS

TOF‐determining transition state

TEMPO

2,2,6,6‐tetramethyl‐1‐piperidinyloxy

THF

Tetrahydrofuran

TMSI

1‐(Trimethylsilyl)imidazole

TPA

Terephthalic acid

Ultraviolet absorption spectroscopy

UV−vis DRS

The ultraviolet‐visible diffuse reflectance spectrum

Valence band

XPS

X‐ray photoelectron spectroscopy

XRD

X‐ray diffraction

List of Symbols

NMR chemical shift

number‐average molar mass

weight‐average molar mass

dispersity index, Đ=

max

The wavelength at which the largest amount of absorption occurs

acidity or basicity of an aqueous solution

ppm

parts per million

glass transition temperature

thermal softening temperature

room temperature

molar absorption coefficient (UV‐vis)

Part IBook Introduction

1Background and Overview

1.1 Introduction

The world currently is relying more and more on non‐renewable fossil resources. In 2019, the global primary energy consumption reached 583.9 EJ (1 EJ=1018 J) [1] and the 84.3% of this energy consumption was from traditional fossil fueld resources: coal, natural gas, and crude oil. Besides fuel production and energy output from these resources, billions of tons of additional fossil resources are transformed into chemicals and materials to support the explosive development of human civilization. However, this represents the negative side of a double‐edged sword because the immoderate utilization of fossil resources has released billions of tons of CO2 into the atmosphere within the last century. This CO2 release has considerably exceeded the capacity of the earth's ecological cycle and therefore caused global climate problems such as global warming and problems deriving from global warming. Against this background of ecological problems, carbon peaking and carbon neutrality have become topics of concern to all those concerned with the future of humanity. Meanwhile, increasing awareness of dwindling fossil resources and the ever‐increasing need for more chemical feedstock and energy supply has led to extensive research into more efficient and sustainable methods to meet these demands. To make the carbon cycle more efficient and controllable, optimizing existing technologies and reducing dependence on fossil resources will be insufficient and therefore utilizing sustainable carbon resources is an important and promising option to help address the problem.

As one kind of carbon and solar energy storage form, biomass is any organic matter that is renewable over time and it is only renewable organic carbon resource in nature. Biomass is generally plant‐based and includes agricultural residues, forestry wastes, and energy crops [2]. Every year, global photosynthesis can produce about 170 billion tons of lignocellulose (LC) by capturing the low concentration CO2 from the atmosphere, immobilizing the carbon, and releasing O2[3]. However, besides the timber industry and papermaking, only a small proportion of biomass carbon resources is utilized to prepare chemicals. The rest of biomass carbon is primarily used to generate heat and power or abandoned. However, biomass is still the fourth largest source of energy in the world after oil, coal, and methane, supplying 10% of the world's primary energy. Considering another perspective, we can reduce millions of years of fossil resources generation if we can efficiently transform the renewable biomass resource to the desired chemical stocks with an acceptable CO2 release during their preparation [4–12]. This would reduce the concern regarding increasing CO2 concentration, primarily because CO2 released during further utilization of chemicals prepared in this way originates from CO2 in the modern‐day atmosphere. It can be a perfect CO2‐neutral cycle that transforms solar energy, H2O, and CO2 into chemical stocks and fuels via biomass (Figure 1.1).

Figure 1.1The Carbon‐Increasing Process Based on the Non‐renewable Fossil Resources Compared with the Carbon‐neutral Cycle Connecting Solar Energy, H2O, and CO2with Chemicals/Fuels via Biomass. Source: Zhang and Wang [38].

1.2 Lignin: A Natural and Sustainable Aromatic Bank

Aromatic chemicals are important bulk chemicals widely used in the chemical industry for the production of polymers, solvents, medicines, pesticides, and many other materials. With the rapid development of economic and industrial globalization, the demand for aromatic chemicals is increasing sharply every year. At present, aromatic chemicals are primarily obtained through petroleum catalytic reforming or steam cracking followed by a complex functionalization process.

These chemical feedstocks are obtained primarily from fossil resources (coal and oil) formed by the photosynthesis of ancient plants. Ancient plants used solar energy to convert CO2 into biomass, then the residue of animals and plants in food webs were transformed to fossil resources after millions of years underground.

The technical revolution based on the utilization of these fossil resources to produce power and chemicals promoted the development of human society. However, it is now clear that the inherent non‐renewability of fossil resources and tough environmental protection requirements have begun to affect the sustainable production of aromatic chemicals [13]. Therefore, the development of new technologies to replace or supplement fossil resources to prepare aromatic chemicals has become increasingly important [214–16]. In the past decades of research, people have successively developed methane aromatization [17], methanol aromatization [18], CO2 conversion [19], C2 compound synthesis [20], and other routes. However, using these low‐carbon substrates as a raw material to obtain aromatic hydrocarbon monomers with high selectivity is still a serious challenge [21].

Lignocellulose (LC), with an annual production of around 170 billion tons, is the most abundant biomass form [3]. The potential use of non‐edible lignocellulosic biomass for the production of value‐added chemicals could provide an attractive alternative for fossil‐based processes is therefore of great significance. As shown in Figure 1.2, LC comprises three main components: hemicellulose, cellulose, and lignin. Among them, hemicellulose (20−30 w% of LC) and cellulose (40−50 w% of LC) are the polymers of C5 and C6 sugars (Figure 1.2) [22] that can be further converted to the low‐carbon alcohols, aldehydes, ketones, or acid. These compounds can be used in some biomass‐based reforming processes to obtain aromatic hydrocarbon compounds, as exemplified by the BioFormPX project from the Virient Company. Additionally, the C5 and C6 polymers or their monomers can be transformed to furan‐based chemicals or other low‐carbon chemicals, which can also be used in C6 aromatization via a typical 4+2 Diels‐Alder reaction [23–25]. At the same time, lignin (10‐35 w% of LC) is a complex three‐dimensional amorphous polymer, consisting of various methoxylated phenylpropanoid units, which can be regarded as a potential resource to replace fossil resources to directly prepare functionalized aromatic chemicals and non‐aromatic hydrocarbon stocks/fuels [1426–28]. Therefore, the direct conversion of lignin in LC is a more attractive process. However, the current paper industry and, more recently, biorefineries produce large quantities of lignin that is currently considered almost a by‐product, not to mention the transformation of the more complex native lignin. Not only from an economic standpoint but also from a sustainability perspective, the misuse or even nonuse of lignin appears to be a colossal mistake. Lignin, whether as native lignin from plants or partially transformed by industrial separation procedures, is a complex material with great potential by itself or as a source of chemicals. In a world where raw materials are in constant demand, having a renewable source such as lignin should be considered a gift from nature to technology [29].

Figure 1.2 Lignocellulose (LC) from Biomass and its Structural Composition.

Unlike other bio‐platform molecules with a distinct structure, lignin structures are much more complex. The key issue for the lignin transformation/depolymerization lies in the development of strategies with highly selective and active catalysts to effectively cleave the ubiquitous C–O/C–C bonds [2, 14, 16, 30] while leaving the aromatic benzene rings unconverted. Although lignin research can be traced back to 1819, when the Swiss botanist A. P. Candolle (1778–1841) first used the term lignin [31] from the Latin word lignum meaning wood, and various non‐catalytic and catalytic methods have been explored to cleave the various lignin C–C or C–O linkages bonds from the 1920s [29], it remains a big challenge to selectively convert lignin extracts, or even original lignin, to aromatics. For detailed reasons, this can be attributed to the much more complex mechanism caused by disturbing the lignin structure with various targeted linkages, combined with other factors such as transformations of lignin structure and different condensation modes of lignin fragments. With the development of lignin depolymerization in the last decade, many efficient methods and strategies have been reported for lignin utilization to prepare chemicals and materials [2, 14, 16,32–35]. To better make use of lignin to prepare aromatic chemicals, it is necessary to summarize the existing catalysts and processes. Based on the obtained knowledge about lignin conversion regimens, we can update the catalysts and develop new routes for lignin efficient conversion.

The critical target of this book is to provide an overview of key advances in the field of lignin depolymerization to aromatics. Unlike previous review work, which classified the research by the lignin substrate (native lignin, kraft lignin, organosolv lignin, etc.), cleavage methods (oxidation, hydrogenation, etc.), or main products (arenes, phenol, acid, etc.), we provide a brief introduction to lignin, past technologies, and new catalytic methods or strategies for lignin depolymerization to monomers and then pay more attention to the relationships among the lignin depolymerization strategies/methods, catalysts, and mechanisms viewed from cleavage order and type of corresponding chemical bonds over various catalytic systems [36, 37]. Based on this summary of reported processes and their results, we hope to partly pull off the lid of the black box of lignin depolymerization.

1.3 Structure of This Book

The structure, reactions, and utilizations of the lignins have been studied for about two centuries from 1819 [31]. Lignin has been widely used in the preparation of various functional materials, chemicals, fuels, medicines, polymers, and others. As shown in Figure 1.3, entering the term “lignin into the SciFinder database by the end of December 2020 generated more than 140,000 entries over the last 100 years. Furthermore, the ever‐increasing growth curve displays that about half of them (76554) have been published in the last 10 years and over 8000 references were reported in each of the last four years, reflecting the fact that lignin is a popular and evolving topic. As a star molecule, lignin and its utilization have drawn much attention from academia to industry worldwide.

Figure 1.3 Number of Entries Retrieved Using “Lignin” as the Research Topic in SciFinder from 1920 to 2020.

One important utilization approach is to efficiently depolymerize the natural lignin polymer to easily‐handled monomers as this is a precondition of lignin utilization to prepare chemicals that may be potential alternatives to fossil resources. In recent decades, many attempts have been carried out to achieve this target [2, 12, 14, 16, 37]. Given the complexity of lignin structure, numerous strategies and methods taking varying approaches from different research fields have been developed to effectively degrade lignin into monomer chemicals by targeting linkages and chemical bonds cleavages. These include the traditional methods of pyrolysis, gasification, liquid‐phase reforming, supercritical solvolysis, chemical oxidation, hydrogenation, reduction, acidolysis, alkaline hydrolysis, and alcoholysis, as well as the newly developed redox‐neutral process, biocatalysis, and combinatorial strategies. Therefore, there is a strong demand to summarize this research, providing a research history to show the connections among different research published in different periods and also providing an overview of current research into lignin depolymerization, which may provide useful suggestions for this vigorous research field.

Although it is quite a challenging project to give a systematic summary of so much research from different research fields with different backgrounds, it is possible using a viewpoint that is not just based on the reaction types or what kinds of catalyst are needed to achieve lignin depolymerization. Classifying the related strategies/methods and establishing their connections needs to come from a deeper perspective. Taking the perspective of a detailed discussion of mechanisms, this book examines the science and technology of lignin depolymerization conversion by using a multidisciplinary approach. About 2000 bibliographical references have been compiled to provide the reader with a complete and systematic overview of research into lignin depolymerization to produce chemicals, mainly aromatic chemicals. To handle such a vast amount of information about different lignin conversion strategies/methods, this book is divided into several parts that give a wide vision of the science and technology of lignin.

Considering that not all readers are professionals in the field of lignin research, we added some brief introductions about lignin near the beginning of the book (Part II, Chapter 2, and Chapter 3). In Chapter 2, we briefly introduce the biosynthesis of lignin from monolignols generation to their transport and then polymerization to the lignin molecule. Then, we provide an introduction of the lignin structure, viewing from the lignin linkages, linkages generation mechanism, structure models of different kinds of protolignin, and lignin‐carbohydrate complex. In Chapter 3, after a brief introduction about the lignin polymer physical properties and methods to isolate lignin from lignocellulose, we present a basic introduction about the spectroscopy properties for the lignin structure characterization. At the end of Chapter 3, lignin chemical properties related to pre‐chemical modification are discussed.

Next, focusing on lignin depolymerization, Part III, Chapter 4 first provides a discussion about scientific questions regarding lignin conversion and new biorefinery approaches. Next, we summarize scientific questions focusing on the topic of lignin depolymerization to aromatic monomers. Then, two different approaches for the foundation of lignin depolymerization strategies are introduced: direct lignin conversion and the bottom‐up approach. To provide a macro‐level understanding of the research field of lignin conversion, the classification of lignin conversion methods by reaction types is summarized. Then, as an important complementary issue, a brief list of systems for lignin/models conversion based on the catalysts is provided and targeted to provide knowledge about the catalyst/reagent that can be used in the lignin conversion via corresponding reaction types. The chapter concludes with summary of systems recently reported for native/technical lignin conversion.

Part IV, Chapters 5 to 11, comprises the main part of this book. As discussed in Chapter 4, multitudinous methods have been developed to depolymerize lignin to provide aromatic monomers. It is hard work to systemically summarize all of these strategies. Facing this question in Chapter 5, inspired by retrosynthetic analysis of the natural product to confirm the synthons and synthetic equivalents, we put forward an inverse disassembly analysis (IDA) method to explore the efficient strategies for lignin depolymerization. The IDA method, which can be used in the classification and discussion of various methods of lignin conversion, considers the cleavage order and type of corresponding chemical bonds over various catalytic systems. Then, to generalize as many lignin conversion methods with various routes and intermediates as possible, four thinking modes based on the IDA for lignin conversion are emphasized: (1) tailoring mode; (2) wedging mode; (3) protection mode; and (4) cascaded mode. With the β‐O‐4 linkage as the main lignin model, we further introduce the lignin depolymerization strategies with the IDA method from Chapters 6 to 11.

In Chapter 6, we firstly review the stoichiometric reactions and catalytic systems for the direct cleavage of ether bonds with a strong nucleophile or a strong electrophilic reagent. Then, modifying and updating the above methods, Brönsted/Lewis acid and metal systems, base/organometallic systems, and other heterogeneous catalysts used in the direct hydrogenative cleavage of C–OAr ether bonds are systemically introduced. In addition, for the direct reductive cleavage of ether bond, a brief review of the lignin conversion methods with hydride reagents, and the e– species from chemical conversion or surface of the cathode or semiconductors is provided.

In Chapter 7, lignin depolymerization strategies beginning with the first activation/transformation of the neighboring lignin phenolic hydroxyl group or aromatic rings to the active radicals are summarized, which includes the following strategies: (1) lignin CAr–C bond cleavage of after the first phenolic hydroxyl group dehydrogenation to the phenolic radical; (2) lignin C–C bond cleavage via the first single‐electron transfer from the aromatic ring; (3) lignin C–O bond cleavage of via the first semi‐hydrogenation/addition of one aromatic ring to a cyclohexenyl ether; and (4) lignin C–O or C–C bond cleavage via the first inserting a cleavage reagent (metal catalyst center or extra radical) between the target bond with the assistance of the aromatic rings conjoint groups.

In Chapter 8, strategies for lignin linkages cleavage beginning with CαO–H/ArO–H heterolysis or Cα–OH bond heterolysis are summarized, which is mainly focusing on the lignin alkaline hydrolysis, acidolysis, and their corresponding tandem processes.

In Chapter 9, strategies for lignin catalytic depolymerization which begin with the activation of Cα–H, Cα–OH, or CαO–H bonds via a non‐ionized route are mainly summarized. The first cleavage of CαO–H and Cα–H bonds to a β‐O‐4 ketone intermediate can weaken the Cβ–OAr bond and make a slightly stronger Cα–Cβ bond, but with an active Cβ–H. Therefore, focusing on this molecule intermediate, the lignin depolymerization strategies of catalytic transfer hydrogenation, dehydrogenation/oxidation‐hydrogenation (reduction), and the multiple oxidation process are first summarized. Given that the Baeyer‐Villiger oxidation and Beckmann rearrangement can be used in the cleavage of C–C bonds in lignin linkages that contain a ketone substrate site, the corresponding strategies and their derivative methods are then summarized. In addition, the first oxidation of Cα–OH can promote lignin hydrolysis, so an embellished β‐O‐4 linkage hydrolysis method involving the Cα–OH first oxidation and Cγ–OH transformation is discussed. In addition to the first cleavage of CαO–H and Cα–H bonds to Cα=O at the beginning of the β‐O‐4 model transformation, some catalytic strategies beginning with the Cα–H or CαO–H activation to the corresponding active radical intermediates can also lead to the cleavage of the lignin fragments. The first activation of Cα–OH(–H) to a Cα• radical intermediate can reduce the bond dissociation enthalpy (BDE) of the Cβ–OAr bond for the following self‐cleavage of β‐O‐4 linkage. Additionally, the first activation of CαO–H bond to the CαO• via a hydrogen atom transfer (HAT), proton‐coupled electron transfer (PCET) oxidation, or redox‐neutral ligand‐to‐metal charge transfer (LMCT) process can lead to the selective cleavage of Cα–Cβ bonds.

In Chapter 10, lignin cleavage strategies beginning with Cβ–H, Cγ–H, or Cγ

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