Deep eutectic solvent-facilitated demethylation and depolymerization for producing highly reactive and antioxidative lignin.

Demethylation is a promising way for lignin valorization but is still challenging due to its reliance on hazardous solvents, super-stoichiometric halogenated reagents, and harsh reaction conditions. This study developed a facile and efficient strategy for lignin demethylation and depolymerization using a sustainable acidic deep eutectic solvent (DES) coupled with a remarkably reduced hydrobromic acid (HBr) loading (4.7 wt%). Structural characterizations revealed that the DES composed of choline chloride (ChCl) and lactic acid (LA) significantly promoted β-O-4 ether bond cleavage and demethylation process, achieving a dramatic efficiency enhancement from 1 % to 45 % compared to the conventional HBr/N,N-dimethylformamide system. The resulting lignin exhibited substantially reduced molecular weight and syringyl/guaiacyl (S/G) ratio, as well as a 76 % increase in phenolic hydroxyl content (from 2.76 mmol g to 4.87 mmol g). Notably, the demethylated lignin demonstrated superior antioxidant activity compared to the original lignin and the commercial antioxidant 3,5-di-tert-butyl-4-hydroxytoluene, BHT. Preliminary application studies confirmed its effectiveness as a multifunctional additive in protein-based composites, simultaneously enhancing the tensile strength, toughness, and water resistance of the composite films. This work highlights the synergistic combination of green DES mediation and low-concentration catalytic systems for lignin demethylation, establishing an efficient strategy for sustainable lignin valorization.