Unraveling the Structural Transformation of Wood Lignin During Deep Eutectic Solvent Treatment

Shuizhong Wang,Helong Li,Guoyong Song,Ling-Ping Xiao

doi:10.3389/fenrg.2020.00048

Abstract

Deep eutectic solvents (DESs) can efficiently promote the efficiency of cellulose enzymatic hydrolysis through the removal of lignin component in lignocellulosic biomass pretreatment. Unraveling the fundamental structural variant of lignin during DES treatment would facilitate to understand the DES-based biomass pretreatment in a clear perspective. Herein, an enzymatic mild acidolysis lignin (EMAL) and a series of β-O-4 lignin model compounds were employed to be treated with ChCl/LA (1:2) DES, from which the structural variant of all lignin fractions can be realized in a detailed version. The β-O-4 linkages, existed in either realistic lignin or model compounds, could be cleaved by ChCl/LA, thus leading to the decrease of molecular weight and the rise of hydroxyl groups. The influence of reaction temperature and time was also examined in view of some key structural parameters. Experimental evidences from model compounds confirmed that the repolymerization occurs with the depolymerization of lignin, which may account for the low production of monomeric products during DES treatment.

Highlights

Lignocellulosic biomass, which possesses cellulose, hemicellulose, and lignin, is recognized as a complementary and alternative hydrocarbon resource to fossil feedstocks (Tuck et al, 2012; Galkin and Samec, 2016)
After the treatment of enzymatic mild acidolysis lignin (EMAL) with choline chloride (ChCl)/lactic acid (LA) Deep eutectic solvents (DESs) (1:2 of molar ratio), the reaction mixture was diluted with EtOH and HCl solution, leading to obtain regenerated lignin (RL, DES-1 to DES-5) as a precipitate
In the case of condensed OH, a similar variation trend was observed, and a maximum value of condensed OH was obtained in DES-4 (140◦C, 0.64 mmol/g). These results suggested that the lignin experiences depolymerization and repolymerization upon the cleavage of βO-4 linkages in a synchronous manner, and both of them would be enhanced under harsh conditions, being in line with a previous report (Chen et al, 2019)

Summary

Introduction

Lignocellulosic biomass, which possesses cellulose (ca. 30–50 wt%), hemicellulose (ca. 20–30 wt%), and lignin (ca. 15–30 wt%), is recognized as a complementary and alternative hydrocarbon resource to fossil feedstocks (Tuck et al, 2012; Galkin and Samec, 2016). The pretreatment of biomass, which aims to remove the lignin and promote the efficiency of cellulose enzymatic hydrolysis, remains an essential step (Behera et al, 2014; Rastogi and Shrivastava, 2017). The efforts at developing novel and efficient pretreatment technologies still stay an important one. Deep eutectic solvents (DESs) have gained considerable attention as a new medium for biomass pretreatment due to their significant merits, including ease to prepare, stable chemical property, low cost, recyclability, and environment amity (Mbous et al, 2017; Satlewal et al, 2018). In DESs, the lignin and hemicellulose components can be deconstructed and removed

Methods

Results

Conclusion