Efficient demethylation of lignin for polyphenol production enabled by low-cost bifunctional protic ionic liquid under mild and halogen-free conditions

The utilization of lignin is of high interest since it represents the most abundant polyphenol that has found multiple applications especially in functional material preparation. Demethylation is a promising way to produce polyphenol from lignin but is still challenging owing to the inactive/poorly soluble nature of lignin, and the majority of processing solvents remaining halogenated and high-cost, which stand as critical issues for commercialization. Herein, low-cost bifunctional protic ionic liquids (PILs) were successfully applied for dissolving and demethylating real representative lignin (e.g. enzymatic lignin, alkali lignin, and sodium ligninsulfonate) under mild and halogen-free conditions for the first time. As a probe, acetate anion countered with ethanolamine cation exhibited effectively synergistic activity for enzymatic lignin demethylation with a rate as high as 73%, which is better than most recently reported methods. Systematical characterizations including 2D HSQC NMR provided a comprehensive insight into the structural changes of lignin after treatment in PILs. In the case of ethanolamine acetate, both a significant increase in the Ph-OH group content and significant reductions in the β-O-4 bond and S/G values in the treated lignin were achieved, leading to an increase in the number of activation sites in the lignin macromolecule. In addition, ethanolamine acetate could be easily recovered at least three times with an average recovery rate around 97%, and maintain high lignin recovery and lignin conversion in all the cycles. Overall, this study presents a new, simple, and halogen-free method for lignin demethylation, which opens up a strategy for improving the valorization of lignin.