Comparison of structure, thermal stability, and pyrolysis products of lignin extracted with ChCl-formic acid/lactic acid systems

The selection of pretreatment conditions has important meaning for the high-value utilization of lignocellulose. This study explored the influence of different acid-deep eutectic solvent (DES) systems on the extraction of lignin. It was found that the lignin samples extracted with ChCl (Choline chloride)-formic acid (FDES) had a greater degree of damage, and they had a lower molecular weight (Mw < 2000) than milled wood lignin (MWL). The side-chain structure of lignin was also damaged, but the thermal stability of the lignin sample was better than that extracted with ChCl-lactic acid (ADES), and the pyrolysis products mostly came from the functional groups directly connected to the benzene ring. After ADES treatment, the carbohydrate content of lignin was higher (>10%) and the structure was less damaged, but with increasing reaction time, the polymerization effect of lignin became more obvious. This research is meaningful for exploring the process of acid DES separation of lignin and the green transformation of biomass.