Donor–Acceptor β-Ketoenamine-Based Photocatalysts with a Tuning Band Structure for Selective Oxidation of a Biomass Derivative

Photocatalytic biomass-derivative conversion exerts infinite potential in dealing with resource recycling, but its low selectivity and slow dynamics process still hinder its practical application. The energy band structure plays a pivotal role in initiating the photocatalytic redox process. Apart from reducing the band gap, tuning the band-edge position of the semiconductor also preserves its non-negligible importance for driving photocatalytic reactions selectively. Herein, we synthesized five metal-free β-ketoenamine-based small organic molecules with different electron-withdrawing acceptor (A) units as photocatalysts for selective oxidation of 5-hydroxymethylfurfural (HMF). The influence of the A unit on the optical properties and band structure of these organic semiconductors was investigated. The band gap reduced from 2.6 to 1.5 eV and the corresponding conduction band position was more positive with the electron-withdrawing ability of the A unit increased. Different radical species were generated by moderately adjusting the energy band position, leading to selective photocatalytic conversion of HMF to 2,5-diformylfuran (DFF), with an optimum DFF generation rate of 2.91 mmol/g/h. Loading IrO2 on the organic photocatalyst could transfer the photogenerated holes and moderate the generation of •OH to inhibit its self-oxidation. Besides, photocatalytic conversion of HMF to DFF accompanied by H2 evolution was achieved. This research developed the feasibility to control radical generation and facilitate photocatalytic biomass conversion by organic semiconductors with a β-ketoenamine structure.