Protic solvent-boosted lignin demethylation in protic ionic liquids

Ethanolamine-based protic ionic liquids (PILs) have shown promising efficiency for lignin demethylation, offering a greener alternative to traditional halogenated Brønsted acids for polyphenol production. For precipitation of demethylated lignin, protic anti-solvents (e.g., water, methanol) are typically added and then completely removed by distillation to recover PILs. Considering that protic solvents could affect the ionicity and viscosity of the PILs, their addition was hypothesized to promote S_N2-based demethylation by further increasing the extent of ion dissociation of the PIL. Herein, typical protic co-solvents were evaluated for enhancing lignin demethylation in ethanolamine-based PILs. Commercial alkali lignin (AL) and corncob alkali lignin (CAL) were studied for comparison. Co-solvents improved demethylation efficiency in the order of water > methanol > ethanol > isopropanol. A 50.0 wt% water-[EOA][OAc] system achieved a threefold increase in AL demethylation efficiency (25.3 %, PhOH content = 5.05 %) compared to [EOA][OAc], due to enhanced ionicity (confirmed by the Walden plot slope) and lower viscosity. Notably, water exhibited a bifunctional role, including co-solvent for lignin demethylation and anti-solvent for lignin precipitation. Besides, due to the decreased solubility of lignin in [EOA][OAc]-water system, an efficient heterogeneous demethylation of lignin was achieved. Structural analysis of lignin further revealed that water addition promoted lignin linkage cleavage and condensation. For CAL, water increased demethylation efficiency from 45.4 % to 67.2 %, indicating the good substrate versatility of [EOA][OAc]-water system. Overall, this study improves the environmental and operational viability of lignin demethylation with [EOA][OAc], and highlights the critical role of tuning PIL ionicity and solvent environment in biomass processing.