A solid acid-catalyzed depolymerization of pine lignin to obtain guaiacol using a hydrogen-free strategy

Guaiacol is an important raw material to produce many value-added chemicals. Here, we report a strategy for directly cleaving the Car-Cα and Cβ-O bonds in pine lignin over a solid acid to generate guaiacol. The guaiacol yield in 90% methanol aqueous solution is up to 18.2 wt%, which is 5.6 wt% higher than that in methanol. The reaction pathway is proposed by combining controlled experiments with density functional theory. Different from previous views, the Car-Cα bond cleavage was found to be related to deacetylation, while the formation of Cα=O bond was found to change the intramolecular charge distribution and promote the Car-Cα bond cleavage through validation experiments. Adding water promotes methanol decomposition to produce more active hydrogen species and thereby promote the Car-Cα bond cleavage. This work provided a feasible pathway for obtaining guaiacol by directly cleaving the Car-Cα and Cβ-O bonds in lignin.