Highly dispersed zinc oxide species on silica as active sites for photoepoxidation of propene by molecular oxygen

The activity of silica-supported zinc oxide containing from 0.01 to 5 mol% of Zn was investigated in the photoepoxidation of propene by molecular oxygen at room temperature. With an increase in the Zn loading amount, the conversion of propene increased monotonically, while the yield of propene oxide (PO) increased up to 1 mol% of Zn and then decreased. The selectivity to PO was almost constant up to 0.1 mol% of Zn and then decreased. By means of XRD, UV-vis, and XANES/EXAFS spectroscopy, it was found that the highly dispersed (isolated) tetrahedral zinc oxide species were predominant in the ZnO/SiO2 samples of low Zn loading amounts, while the aggregated species such as [ZnO]n clusters were dominant in the ZnO/SiO2 samples of higher Zn loading. The former species were found to be responsible for the photoepoxidation of propene.