Visible-light-responsive sulfated vanadium-doped TS-1 with hollow structure: Enhanced photocatalytic activity in selective oxidation of cyclohexane

Abstract

A series of visible-light-responsive hollow sulfated V-doped TS-1 photocatalysts with different dopant content has been prepared by impregnation and evaluated in the selective photooxidation of cyclohexane by molecular oxygen. The activity results have been correlated with structural, electronic, and surface examinations of the photocatalysts with the help of X-ray diffraction, N2 physisorption, transmission electron microscopy, and Raman, XPS, infrared, and UV–visible spectroscopy. Irrespective of the reaction, a photocatalytic efficiency enhancement of valuable products (alcohols and ketones) with respect to V-doped titania (P25) references and V-doped and sulfated TS-1 catalysts was observed for hollow sulfated V-doped samples with the TiO4 species (tetrahedral Ti4+). This is likely linked with an effective narrowing of the band gap and results from a cooperative V, S effect on the structural properties of the TiO4 network inducing isolated energy levels near the conduction and valence bands. The simultaneous presence of V, S in the TS-1 also promotes the process with higher efficiency (TOF=2.37h−1, based on the molar amount of V sites) and chemoselectivity (cyclohexanone/cyclohexanol molar ratio=3.46) than in the homogeneous phase. Extensive computational modeling, together with the OVO4Si and OVO4Ti bridging sites on TS-1, was performed to show that OV and O4Ti (oxygen vacancy) in OVO4Ti bridging sites are the main active sites for the selective photooxidation reactions, and this is compatible with experimental observations for varying reaction conditions.