Efficient Extraction and Structural Characterization of Eucalyptus Lignin with Deep Eutectic Solvents

Abstract

Effects of deep eutectic solvents (DES) type and pretreatment time were evaluated relative to lignin extraction efficiency. Three major indicators, including lignin yield, solid residue proportion, and lignin purity, were investigated in detail at fixed molar ratios (1:8 and 1:8:1) and temperature (120 °C). The lignin yield gradually increased over 2 to 10 h, reaching 78.9% at 6 h in the choline chloride (ChCl)-formic acid (Fa)-oxalic acid (OA) system. The solid residue proportion continuously decreased to 36.4%, while maintaining lignin purity above 90% without significant variation. According to the lignin structure analysis, the basic aromatic structure of lignin, comprising syringyl (S) and guaiacyl (G) units, remained unchanged after treatment with the ternary DES system. The C/O ratio of carbon-rich macromolecular structures gradually increased with reaction time, and the carbon content of the samples exceeded 60%. This study provides a new approach to optimizing lignin extraction.