Metal chloride-based acidic deep eutectic solvent for efficient lignin separation and furfural co-production from solid residue

A “lignin-first” strategy was proposed employing an acidic DES for the preliminary separation of lignin. The DES, formulated with ZnCl₂, levulinic acid, and ethanol used as an additive (molar ratio of 1:1:2), was used to investigate the effects of pretreatment conditions on the efficiency of lignin separation from pine wood. The results demonstrated that both temperature and pretreatment time were critical factors determining lignin recovery efficiency, with 120 °C and 6 h identified as the optimal reaction conditions. The lignin recovery yield initially increased with higher proportion of DES, followed by a decrease upon further addition of DES. Excessive DES addition resulted in excessive fragmentation of lignin, thereby reducing recovery efficiency. A maximum lignin recovery ratio of 94.12% was achieved under the conditions of 120 °C, 6 h, and a solid-to-liquid ratio of 1:15. Characterization analysis revealed that the extracted lignin exhibited structural features similar to those of milled wood lignin (MWL), along with a relatively low molecular weight (1500–2500 g/mol) and a narrow dispersion coefficient (PDI < 2.0). Hydrolysis of the solid residue confirmed that DES pretreatment effectively enhanced cellulose conversion (increasing from 41.11% to 93.26%) and yielded 36.74% furfural. This study provides valuable insights for future biomass refining processes.