Green Solvent Pretreatment of Biomass: Insights into Lignin Structural Evolution in Organosolv and Deep Eutectic Solvent Systems.

Lignocellulosic biomass is a key renewable resource for producing biofuels, biomaterials, and high-value chemicals, but its efficient utilization is hindered by the recalcitrant lignin-carbohydrate complex (LCC) network. Organosolv (OS) and deep eutectic solvent (DES) pretreatments have emerged as promising green strategies for biomass fractionation and lignin extraction. The present work summarizes recent advances in OS and DES pretreatment, with a particular focus on the mechanisms of lignin depolymerization and structural evolution, including changes in major bonds, functional groups, S/G ratio, molecular weight, and its distribution. The selection of solvents, recovery strategies, and the influence of solvent composition and process conditions on lignin recovery and biomass fractionation are critically discussed. The challenges and prospects for improving solvent recyclability and lignin valorization in industrial-scale applications are also highlighted. This review provides insights into green solvent-mediated lignin transformation and supports the development of more sustainable biorefinery processes.