Styrene epoxidation over V-SBA-15 with alkaline-earth metal ion promotion under photo-assisted conditions

The photo-assisted epoxidation of styrene over V/SBA-15 modified by alkaline-earth metal ions was investigated with hydrogen peroxide at room temperature. The results indicated that the isolated VO4 tetrahedron dispersed on the SiO2 was the photo-assisted catalytic active site for the formation of styrene oxide, while V5+ polymer species in V/SBA-15 with highly loaded V species caused deep oxidation of styrene oxide. Among the catalysts modified by alkaline-earth metal ions, Mg–V/SBA-15 displayed the best catalytic performance. It was probably due to the fact that the metallic character of Mg was weaker in alkaline-earth metals, so it was easier to disperse V5+ polymer species and the dispersion effect was especially obvious in the comparatively high loading weight of vanadium oxide. Moreover, addition of alkaline-earth metals decreased the acidic sites of V/SBA-15, inhibiting the ring-opening reaction on which the deep oxidation occurred. The structures of all catalysts were thoroughly investigated using different characterization techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), NH3-temperature programmed desorption (TPD), H2 temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS), etc. A possible catalytic mechanism was proposed.

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► V-SBA-15 with alkaline-earth metal ion was synthesized.
► Catalysts with low acidic sites avoided the ring opening reaction.
► Addition of alkaline-earth metals favor of dispersion of V5+ species.
► Possible mechanism for styrene epoxidation was discussed.