Bright Side of Lignin Depolymerization: Toward New Platform Chemicals

Abstract

Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the future considered as potential lignin-derived platform chemicals.

Figure 1

(Left) A representative lignin structure displaying typical lignin subunits and linkages encountered. (Right) General strategies for depolymerization of lignin and application of lignin-derived platform chemicals.

Figure 2

A summary of procedures for isolation of lignin from lignocellulose.

Figure 3

Types of starting materials used for the development of novel catalytic methods targeting high yield production of aromatics from lignin. (a) Isolation of lignin by lignocellulose fractionation prior to catalytic processing. (b) Reductive catalytic fractionation (RCF) using lignocellulose in the presence of a catalyst. (c) Complete conversion of all lignocellulose components by one-pot catalytic processing.

Figure 4

Strategies and yields of main products established for the depolymerization of lignins isolated from lignocellulose prior to catalytic treatment.

Figure 5

Proposed reaction mechanism for vanillin formation during alkaline oxidation of lignin. Reproduced with permission from ref (111). Copyright 2000, Springer Nature. Reproduced with permission from ref (112). Copyright 2004, Springer Nature.

Figure 6

First example of transition metal free, room temperature reductive depolymerization of formacell lignin using B(C₆F₅)₃/Et₃SiH.

Figure 7

Highly efficient catalytic conversion of lignin through formaldehyde stabilization (top) and product distribution for beech wood and F5H poplar lignin with or without formaldehyde stabilization (bottom). Reprinted with permission from ref (158). Copyright 2017 Wiley-VCH.

Figure 8

Acid catalyzed depolymerization of lignin in conjunction with stabilization of reactive C2-aldehydes. Comparison of the yields of aromatic C2-acetals obtained by the addition of ethylene glycol obtained from various lignin sources.

Figure 9

Selective oxidation and cleavage of isolated lignin into monomeric aromatic products.

Figure 10

Biochemical transformation of lignin to polyhydroxyalkanoates and muconic acid.

Figure 11

Summary of processes including the isolation and depolymerization of lignin.

Figure 12

Summary of reductive catalytic fractionation processes developed to obtain aromatic monomers at high yield and selectivity. (The ball size represents the total monomer yield).

Figure 13

Structures of phenolic monomers derived from native lignin of different resources. On the basis of results of ref (188). Reaction conditions: 2 g of substrate, 0.3 g of 5% Ru/C, 40 mL of methanol, 250 °C, 3 h, 30 bar H₂.

Figure 14

Correlation between the yield of phenolic monomers and the frequency of the β-O-4 moiety in native lignin for different lignocellulose substrates. Reprinted with permission from ref (193). Copyright 2016 Wiley-VCH.

Figure 15

Structures of main products depending on type of catalyst after reductive catalytic fractionation under H₂ pressure.

Figure 16

Proposed mechanism of cleavage and hydrodeoxygenation of β-O-4 ether linkage by Pd/C catalyst and Pd/C and Zn^II catalysts. Adapted with permission from ref (210). Copyright 2016 Royal Society of Chemistry.

Figure 17

Reductive catalytic fractionation using Pd/C only or Pd/C in combination with different additives.

Figure 18

Reductive depolymerization of wood lignin into phenolic monomers over a tandem Pd/C and Al(III)-triflate catalyst system, at different Pd/Al ratios. Adapted with permission from ref (200). Copyright 2017 Royal Society of Chemistry.

Figure 19

Birch delignification vs solvent polarity as described by the reichardt parameter (E_T^N). Reproduced with permission from ref (198). Copyright 2015 Royal Society of Chemistry.

Figure 20

Proposed mechanism of aryl propene formation during Pd-catalyzed hydrogenolysis, established by model compound studies. Adapted with permission from ref (187). Copyright 2014 Wiley VCH.

Figure 21

Solutions developed for the separation of catalysts from solid residue after reductive catalytic fractionation process. (a) Separation of a magnetic catalyst by application of magnetic field, (b) Using a microporous catalyst cage, and (c) Liquid–liquid extraction.

Figure 22

Possible applications of the solid residue after reductive catalytic fractionation.

Figure 23

A summary of lignin-based monomers obtained via the different catalytic processes in section 2.

Figure 24

Summary of strategies for the conversion of lignin-derived monomers to emerging structures and bulk chemicals.

Figure 25

General strategies for functionalization of aromatic monomers obtained upon lignin depolymerization.

Figure 26

Direct functionalization of the phenol moiety in guaiacol.

Figure 27

Functionalization of the phenol moiety in guaiacol through protective groups, applying homogeneous catalysts.

Figure 28

Catalytic strategies for the functionalization of the side chain of lignin-derived monomers.

Figure 29

Strategies for defunctionalization of lignin-derived monomers.

Figure 30

General strategies for production of polymers from lignin.

Figure 31

Polymers produced from lignin-derived monomers by functionalization of only the phenol moiety or both the phenol moiety and the side chain.

Figure 32

Polymers produced from lignin-derived monomers by functionalization of the aromatic ring or only the side chain.

Figure 33

Synthesis of cross-linked thermosets by epoxy/amine reaction and epoxy polymers from model mixtures of G- and GS-type monomers. Adapted with permission from ref (348). Copyright 2016 Royal Society of Chemistry.

Figure 34

Approaches to prepare cured epoxy resins from lignin hydrogenolysis products and the cured epoxy resin specimens. Adapted from ref (394). Copyright 2017 American Chemical Society.

Figure 35

Synthesis route of fully renewable triphenylmethane-type polyphenols and corresponding polymers from lignin-derived aldehydes and para-substituted guaiacols. Adapted from ref (396). Copyright 2017 American Chemical Society.

Figure 36

Synthesis of lignin-based epoxy resins by using lignin-derived monomer (M31G) and acetal-modified lignin.

Figure 37

Strategies for transformation of muconic acid to biobased polymers or polymer building blocks.

Figure 38

Polymer materials obtained in feasible starting from lignin.

Figure 39

Monomers frequently originating from lignin depolymerization methods (left) and monomers very frequently used for polymer production (right).

Figure 40

Summary of structures of natural products obtained from lignin-derived monomers. (a) Number of steps required to synthesize the natural products versus overall yield of the process. (b) Monomers used as starting material. (c) Chemical structures of natural products separated by dotted line based on different starting material.

Figure 41

Summary of structures of pharmaceutical products obtained from lignin-derived monomers.

Figure 42

Summary of structures of drug lead compounds obtained from lignin-derived monomers. (a) Number of steps required to synthesize the drug lead compounds versus overall yield of the process. (b) Monomers used as starting material. (c) Chemical structures of drug lead compounds separated by dotted line based on starting material.