Abstract
Understanding lignin structural transformations during DES pretreatment would facilitate to produce lignin with tailor-made properties based on intended applications. To unravel the structural variant of lignin in the DES, the alkali lignin (AL) was pretreated in choline chloride/formic acid (ChCl/FA) (the ratio of 1:2) at 80-120 oC. Characterization of the AL and regenerated lignin samples were determined to understand lignin structure, such as linkage in lignin, phenolic OH content, molecular weight, and aromatic products. ChCl/FA DES pretreatment resulted in large amounts of the dissociation of β-O-4' bonds, partial cleavage of carbon-carbon bonds (i.e., β-β′, β-5′), dehydration of hydroxyl radicals in lignin’s side-chain, and demethoxylation reaction. Furthermore, various monomeric phenols derived from AL were also determined in spite of low combined yields, which facilitate the production of fine chemicals in the future. Detailed unmasking of lignin structural transformation during the DES process is conducive to the optimizing preparation of homogeneous lignin with low molecular weight.
Highlights
INTRODUCTIONLignocellulose, mainly consisting of carbohydrates (cellulose and hemicellulose) and arylbiopolymers (lignin), is the richest, green and renewable bioresource, which can be used to produce various fuels, materials, and fine chemicals (Zhang et al, 2017; Liu et al, 2019b; Dong et al, 2020)
Lignocellulose, mainly consisting of carbohydrates and arylbiopolymers, is the richest, green and renewable bioresource, which can be used to produce various fuels, materials, and fine chemicals (Zhang et al, 2017; Liu et al, 2019b; Dong et al, 2020)
The present study deeply investigated the impact of the temperatures of choline chloride/ formic acid (ChCl/formic acid (FA)) DES pretreatment on the structure and depolymerized products of lignin
Summary
Lignocellulose, mainly consisting of carbohydrates (cellulose and hemicellulose) and arylbiopolymers (lignin), is the richest, green and renewable bioresource, which can be used to produce various fuels, materials, and fine chemicals (Zhang et al, 2017; Liu et al, 2019b; Dong et al, 2020). Lignin fractionation from lignocellulose remains the key step. Deep eutectic solvents (DES) have drawn increasing interest, which can be used as promising green solvents for lignocellulosic biomass process and delignification Wan and co-workers further revealed that ChCl/lactic acid pretreatment can extract 65–72% lignin from lignocellulose for only 45 s with microwave irradiation (Chen and Wan, 2018). Numerous DESs, especially carboxylic acid DES, have achieved the extraction of high-yield lignin among mild conditions. Zhang and co-workers reported that ChCl/ lactic acid pretreatment fractionated a large amount of lignin fragments with high purity from woody biomass, due to the breakage of β-O-4′ bonds in lignin and the suppression of lignin recondensation (Alvarez-Vasco et al, 2016). The depolymerized products of lignin were systematically determined via gas chromatography-mass spectrometry (GCMS)
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