Acidic Deep Eutectic Solvents for Lignocellulose Pretreatment: Insights into Lignin Extraction Efficiency and Structural Transformation

Lignocellulosic biomass is a vital renewable resource, but its complex structure and the recalcitrance of lignin present significant challenges for efficient fractionation and conversion. Acidic deep eutectic solvents (ADES), a subclass of green, tunable solvents, have emerged as promising media for biomass pretreatment and lignin extraction due to their low toxicity, high efficiency, and strong bond-cleaving capacity. ADES systems effectively disrupt phenyl ether (e.g., β–O–4) and ester linkages in lignin, facilitating its separation from hemicellulose and enhancing the extraction efficiency. Lignin removal or extraction rates have been reported to be high for various biomasses, e.g., 96.3% for poplar sawdust using lactic acid: ZnCl2, 99.6% for acacia wood using lactic acid: citric acid: choline chloride, and more than 97% for systems such as ethylene glycol: 4-chlorobenzenesulfonate: choline chloride and ethylene glycol: p-toluenesulfonic acid: choline chloride. These results highlight the potential of ADES for selective delignification while retaining the lignin structure for valorization. This review summarizes recent advances in ADES-mediated lignocellulosic pretreatment, focusing on solvent–lignin interactions, key process parameters, and catalytic roles in biomass conversion. Despite their advantages, further work is needed in reaction mechanism elucidation, process intensification, and industrial scalability. Finally, the paper explores the effect of solvent properties on the lignin structure as well as prospects for the application of ADES in future lignocellulosic resourcing processes and life cycle analysis.