Tandem strategy of photocatalytic preoxidation-ultrasonic cavitation depolymerization for lignin valorization

Tandem strategy for lignin utilization with photocatalytic preoxidation and ultrasonic cavitation depolymerization was proposed. Cornstalk residual lignin from industrial bioethanol process was first photocatalytically preoxidized under visible light by g-C₃N₄ and WO₃/g-C₃N₄/h-BN (WCB) photocatalysts respectively, then obtained lignin samples were characterized to confirm the preoxidation with raw lignin as a blank. During photocatalytic preoxidation, benzyl hydroxyls in lignin was transformed to carbonyls, but a certain degree of lignin degradation and condensation was observed. In comparison, WCB-catalyzed photopreoxidation was more effective. Thereafter, lignin depolymerization was achieved by ultrasonic cavitation-assisted ethanololysis under optimal conditions. Compared with the mere ultrasonic cavitation depolymerization of pristine lignin, WCB-induced photocatalytic preoxidation improved the conversion rate by 14%, the light-oil yield by 26%, and the phenolic monomer yield by 35%. In general, the reported tandem method worked very well for the enhancement of lignin depolymerization and provided a new idea for the development of lignin valorization.