Photocatalytic depolymerization of lignin to produce high value-added chemicals by carbon quantum dots modified WO3 nanosheets

Photocatalysis, a sustainable development strategy, employed to produce value-added chemicals from lignin, has attracted much attention due to its low energy consumption and mild reaction conditions. Carbon quantum dots (CQDs) and tungsten oxide (WO3) composite photocatalysts (CQDs/WO3) were prepared by ultrasonic assisted wet impregnation method in this study. The CQDs/WO3 nanosheets exhibit good optical properties and satisfactory performance of lignin depolymerization. Characterization results embody that the addition of CQDs significantly changes the morphology of WO3. Compared with bulk pure WO3, CQDs/WO3 becomes a nanosheet structure in the size of ~ 200 nm. Furthermore, CQDs can absorb photogenerated electrons as well as reduce the recombination of photogenerated electrons and holes. The strong oxidizing ability of n-type semiconductor WO3 enables it to effectively break the typical bonds like C-O or C-C near polar functional groups in lignin. Due to this ingenious design, dealkaline lignin conversion was up to 34.21% as well as the highest yields of vanillin and vanillic acid in the form of lignin were 24.74 mg/g and 8.02 mg/g in 2 h at room temperature, visible light illumination and air atmosphere, respectively. And optimized CQDs/WO3 also had satisfactory effects on three natural lignin (grass lignin, pine lignin and birch lignin), which proved its universality. Electron spin resonance experiments and mechanism investigation demonstrated that ·O2- and ·OH play a weighty role in photocatalytic depolymerization of lignin. This study provides a new gambit for converting lignin to high value-added chemicals (vanillin and vanillic acid) over modified WO3.