Development of 'Lignin-First' Approaches for the Valorization of Lignocellulosic Biomass

Abstract

Currently, valorization of lignocellulosic biomass almost exclusively focuses on the production of pulp, paper, and bioethanol from its holocellulose constituent, while the remaining lignin part that comprises the highest carbon content, is burned and treated as waste. Lignin has a complex structure built up from propylphenolic subunits; therefore, its valorization to value-added products (aromatics, phenolics, biogasoline, etc.) is highly desirable. However, during the pulping processes, the original structure of native lignin changes to technical lignin. Due to this extensive structural modification, involving the cleavage of the β-O-4 moieties and the formation of recalcitrant C-C bonds, its catalytic depolymerization requires harsh reaction conditions. In order to apply mild conditions and to gain fewer and uniform products, a new strategy has emerged in the past few years, named 'lignin-first' or 'reductive catalytic fractionation' (RCF). This signifies lignin disassembly prior to carbohydrate valorization. The aim of the present work is to follow historically, year-by-year, the development of 'lignin-first' approach. A compact summary of reached achievements, future perspectives and remaining challenges is also given at the end of the review.

Keywords: lignocellulose valorization; reductive catalytic fractionation; ‘lignin-first’.

Figure 1

General strategies for lignocellulose valorization and application of lignin-derived platform chemicals with representative lignin structure displaying typical lignin subunits and linkages.

Figure 2

Types of lignocellulose valorization processes. (A) Harsh (pulping) fractionation. (B) Mild (enzymatic) fractionation. (C) One-step reductive catalytic processing. (D) Reductive catalytic fractionation (RCF).

Figure 3

Schematic representation of chemical processes involved in the ‘lignin-first’ biorefining. For clarity, hemicellulose sugars and their degradation products were omitted.

Figure 4

Main lignin monomeric products with functionalized sidechains, the date of their first isolation from lignocellulose as main product, typical catalysts, and additives with added hydrogen or hydrogen donors applied in the process, and literature references. For simplicity, guaiacols are not shown if the same syringol derivatives exist as main product. The detailed process characteristics and monomer yields of each individual study can be found in Table 1.

Figure 5

Schematic representations of semi-continuous flow-through reactor systems. (Left) Combination of a percolation reactor filled with woody biomass and a fixed catalytic bed reactor filled with Pd/C catalyst (Adapted from [61]). (Right) Schematic of the semi-continuous dual-bed flow reactor. Biomass in lower beds, catalyst in upper bed. (Adapted from [62,63]).