S-vacancy regulation over ultra-thin ZnIn₂S₄ for enhanced photocatalytic valorization of biomass-derived 5-hydroxymethylfurfural to 2,5-diformylfuran

Employing photocatalysis for the conversion of biomass into high-value chemicals represents a green and sustainable development model. Herein, ZnIn₂S₄ (ZIS) nano flower with different S vacancy concentration were synthesized to catalyze the conversion of biomass-based 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF). The result shows that S vacancy can enhance the optical and electrical properties, as well as alter surface characteristics to speed the surface reaction. The optimal ZIS-(1:6) performed well with 96.5 % HMF conversion and 77.1 % DFF yield as well as the negligible amount of over-oxidized compound FFCA (<5%) for only 3 h reaction. It is noteworthy that the conversion rate of HMF reaches 2030 μmol·g⁻¹·h⁻¹ in the first hour of the reaction with DFF formation rate of 1610 μmol·g⁻¹·h⁻¹, which is higher than most reported photocatalytic HMF aerobic oxidation system. The ZIS-(1:6) effectively overcomes the low charge utilization rate and low surface reaction efficiency of the common ZIS. By free radical capture experiment and EPR characterization, the roles of active species (·O₂⁻, ¹O₂ and R-·CHOH radical) and photogenerated h⁺ and e⁻ in the reaction were investigated, leading to a plausible reaction mechanism. This work contributes to the development of green and stable catalysts for visible light driven HMF and similar biomass molecular transformation.