On the Oxidative Valorization of Lignin to High-Value Chemicals: A Critical Review of Opportunities and Challenges

Abstract

The efficient valorization of lignin is crucial if we are to replace current petroleum-based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second-generation biorefineries produce large quantities of low-value technical lignin as a by-product, which is often combusted on-site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

Keywords: analytical methods; biomass; heterogeneous catalysis; lignin valorization; oxidative depolymerization.

Figure 1

Model of the polymer structure of lignin, showing its general features and the main types of linkage. Adapted from Ref. [25], under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).

Figure 2

Chemical structures of the monolignols p‐coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, and the corresponding aromatic residues they form in lignin p‐hydroxyphenyl (H), guaiacyl (G), and syringyl (S).

Figure 3

Main interunit linkages formed in the lignin biopolymer. Adapted from Ref. [30].

Figure 4

Schematic depiction of lignin degradation, where ether bonds are represented by O‐linkages. Adapted with permission from Ref. [4]; Copyright 2018, the Royal Society of Chemistry.

Figure 5

Theoretical monomer yield as a function of cleavable linkages in different lignins. Reproduced from Ref. [33], under the terms of the Creative Commons Attribution‐NonCommercial 4.0 International License (https://creativecommons.org/licenses/by‐nc/4.0/).

Figure 6

Elemental mapping of (a) fresh and (b) spent Cu−Mn catalyst by scanning electron microscopy–energy dispersive X‐ray spectroscopy (SEM–EDX). A significant increase can be seen in the amount of sulfur (yellow) in the spent catalyst, compared to the fresh catalyst. The signal intensities of Cu (aqua) and Mn (pink) also appeared to be lower in the spent catalyst. Scale bars: 100 μm. Adapted from Ref. [73], under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).

Figure 7

Proposed reaction pathway for oxidative lignin conversion into vanillin and vanillic acid over Cu‐based catalysts. Assuming that the starting compound for the production of vanillin from lignin is coniferyl alcohol, the oxidizing agent H₂O₂ is dissociatively adsorbed onto active Cu−Mn sites on the catalyst surface and decomposed to O₂, which is then transformed into surface oxygen species (O*) at the oxygen vacancy (Mn⁴⁺−Ο*−Cu⁺), leading to the formation of Mn³⁺−O*−Cu²⁺. The lignin‐derived coniferyl alcohol is converted to vanillin by means of the surface oxygen species, where the vanillin produced can be oxidized to vanillic acid, depending on the Cu content in the catalyst. Reproduced with permission from Ref. [75]; Copyright 2020, Elsevier.

Figure 8

(A) Overview of commonly addressed research questions, methods, and detectors employed in studies of oxidative lignin conversion. The lignin structure is adapted from Ref. [25], under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). (B) Analysis of bio‐oil from oxidative lignin degradation with ¹H–¹³C HSQC spectroscopy. 2D NMR methods can benefit from the appropriate choice of spectral width in the second dimension and are highly reproducible. The measurement time in the right‐most spectrum (acquired in DMSO‐d ₆) was less than 15 min.

Figure 9

Illustration of centrifugal partition chromatography as an effective means of obtaining pure fractions of aromatic chemicals from oxidative lignin depolymerization. Reproduced from Ref. [103], under the terms of the Attribution‐NonCommercial‐NoDerivatives 4.0 International License (https://creativecommons.org/licenses/by‐nc‐nd/4.0/).