Vanillin production from lignin depolymerization by acidic deep eutectic solvent and its possible mechanism

Abstract As one of the most abundant renewable resources and complex aromatic polymers, lignin holds great promise for producing monophenolic compounds. However, its depolymerization into valuable monomers is hindered by intricate side reactions. In this study, alkaline lignin was converted into vanillin using nine choline chloride‐based deep eutectic solvents (DES) under an eco‐friendly and mild process. Among them, choline chloride /lactic acid (molar ratio 10:1) containing 30% water at 80°C for 120 min achieved the highest vanillin yield (1.92 wt %), outperforming oxalic‐, levulinic‐, and glycol‐based DES by 1.5 to 5‐fold. Single‐factor assays showed that acidity (lactic acid fraction), water content (20%–40%), and reaction time (≤120 min) were decisive. GPC revealed that the weight‐average molar mass dropped from 3000 to 420 g mol −1 and PDI narrowed to 1.25, confirming extensive yet selective cleavage of β‐O‐4 linkages. 2D‐HSQC indicated simultaneous demethoxylation and minor condensation of side chains. The depolymerized products exhibited strong antibacterial activity toward Staphylococcus aureus (79.6% inhibition) and Escherichia coli (65.5% inhibition). A mild acid–water synergistic oxidation pathway is proposed, in which protons generated from DES dissociation attack ether bonds and H 2 O activates oxygen transfer, enabling greener lignin without noble metals or high pressure.