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 C-beta-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 O-2 capture and activation to generate O-1(2) that acts as a dominant role in promoting C-beta-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 O-2 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/g(lignin) could be obtained with C-beta-O bond cleavage. This work could offer an in-depth insight into catalyst design as well as photocatalytic valorization of biomass.