Sulfur vacancy induced radical generation in ZnIn2S4 for lignin photocatalytic Cα–Cβ cleavage†

The low efficiency of photogenerated charge transfer to generate reactive oxygen species is the main factor that limits the photocatalytic activity of ZnIn₂S₄ (ZIS)-based materials in lignin depolymerization. In this work, we prepared an efficient ZIS catalyst with a hexagonal crystal structure and three-dimensional flower-like micromorphology by a solvothermal method. The ZIS catalyst possessed abundant active sites and efficient light-harvesting ability. Sulfur vacancies (Sv) were introduced into the ZIS photocatalyst (ZIS-Sv) to enhance its electron-withdrawing ability and promote the separation and transfer of photogenerated charges. Using the ZIS-Sv photocatalyst, the conversion rate of C_α–C_β bonds in the β-O-4 dimer reached 96.31% and the molar yields (selectivity) of phenol and acetophenone were 695.17 μmol g⁻¹ h⁻¹ (85.14%) and 610.66 μmol g⁻¹ h⁻¹ (74.73%), respectively (with a ZIS-Sv_0.6 concentration of 1 mg ml⁻¹, irradiation time of 6 h, ethanol content of 90%, pH of 5, and 2-phenoxyacetophenone concentration of 1 mg ml⁻¹). The photocatalytic depolymerization of dioxane lignin by the ZIS-Sv photocatalyst reached an efficiency of 80.42%. Catalyst characterization results and mechanism studies indicated that Sv promoted effective electron adsorption to generate oxygen radicals ˙OH and ˙O₂⁻ through O₂ single-electron reduction and H₂O decomposition, respectively. Then, the oxygen radicals combined with C-centered radicals to form unstable intermediates, further breaking lignin linkages, which finally formed an efficient indirect oxidation process. It is expected that introducing Sv into ZIS materials is a useful strategy to obtain photocatalysts for C_α–C_β cleavage to realize efficient photocatalytic lignin depolymerization.