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.

Recent advances in the valorization of plant biomass

“…a The partial chemical structural of cellulose; b The partial chemical structural of hemicellulose; c The structural characteristics of three typical units in lignin; d the common linkages between different lignin units ( Adopted from Ref. [ 34 , 253 ]) …”

Exploiting Nature's Most Abundant Polymers: Developing New Pathways for the Conversion of Cellulose, Hemicellulose, Lignin and Chitin into Platform Molecules (and Beyond)

“…In terms of the manipulation (valorisation) of lignin, which can, as highlighted above, take many structural forms, a wide range techniques has been investigated. Since conventional deligninification processes that deliver cellulose significantly degrade the lignin and so potentially devaluing it, a so‐called “lignin‐first” approach to deriving useful chemicals from it has attracted attention [48m] . This can take various forms including “organosolv” fractionation and wherein organic solvents such as methanol, ethanol, butanol, acetone, formic acid, acetic acid or even ionic liquids are used [48d,h,k,l,v] .…”

Recent Catalytic Approaches for the Production of Cycloalkane Intermediates from Lignin‐Based Aromatic Compounds: A Review

“…phenols, guaiacols, syringols, alkylated guaiacols/syringols, benzenes, alkylated/hydroxyl benzenes, cyclohexanols, and cyclohexanes are mainly obtained during reductive disconnection (Figure 2). [13,29] The technical lignins (co‐product of sugar/paper‐pulp/bioethanol industries) are consisting of C−C linkages and these are difficult to cleave. Reductive depolymerisation is an efficient tool for the conversion of technical lignins.…”

“…Reductive depolymerisation is an efficient tool for the conversion of technical lignins. This process decreases the complexity in the product mixtures and avoids repolymerization of formed phenolic monomers [29] …”