Defect Engineering Promoted Photocatalysis for Lignin Depolymerization: Performance and Mechanism Insight

Defect engineering has shown a pronounced promise in photocatalysis. In this study, ZIF-8-NH2@Bi/Bi2MoO6 with oxygen vacancies was elaborately developed for efficient Cβ–O bond cleavage of a typical lignin model, 2-phenoxy-1-phenylethanol (PP-ol) and alkali lignin. The surface plasmon resonance effect of Bi nanoparticles, the heterojunction between ZIF-8-NH2 and Bi2MoO6, and the Schottky junction between Bi and Bi2MoO6 together facilitate charge separation and transfer. More significantly, the oxygen vacancy could efficiently facilitate O2 capture and activation to generate 1O2 that acts as a dominant role in promoting Cβ–O bond cleavage while holes could only convert PP-ol into 2-phenoxy-1-acetophenone. As a result, 93% of PP-ol was converted into phenol and acetophenone (AP) with yields of 57% and 48%, respectively, over ZIF-8-NH2@Bi/Bi2MoO6 in air, which is obviously higher than those over ZIF-8-NH2@Bi2MoO6 (46% of PP-ol conversion, 8.7% of phenol yield and 6.2% of AP yield). Moreover, in a pure O2 environment, 100% of PP-ol conversion, 69% of phenol yield, and 68% of AP yield could be achieved. As for alkali lignin extracted from Pinus massoniana, a high vanillin yield of 25 mg/glignin could be obtained with Cβ–O bond cleavage. This work could offer an in-depth insight into catalyst design as well as photocatalytic valorization of biomass.