Preparation of Ti/SiO2 catalysts by chemical vapor deposition method for olefin epoxidation with cumene hydroperoxide

Ti/SiO2 epoxidation catalysts with titanium concentration ≤1.12 μmol/m2 were prepared by chemical deposition of TiCl4 vapor on silica gel at 900 °C for 0.5–3 h. A zero-order rate law was able to satisfactorily describe the deposition process in the range of Ti ≤1 μmol/m2. The CVD-prepared Ti/SiO2 catalysts were used to catalyze the epoxidation of propylene and 1-octene to the corresponding epoxides (propylene oxide and 1,2-epoxyoctane) with cumene hydroperoxide (CHP) and tert-butyl hydroperoxide (TBHP) as the oxidants. The epoxide selectivity increased rapidly with Ti concentration. For the catalyst containing the maximum Ti concentration, the epoxide selectivities were insensitive to the oxidant type, and had 97% 1,2-epoxyoctane yield and 93% propylene oxide yield. For the catalysts containing the lower Ti concentration (i.e., with residual SiOH groups), CHP exhibited significant higher epoxide selectivity than TBHP. The results suggest that the oxidant type had a stronger effect on the hydroperoxide decomposition rate (that occurred on the Si–O–H Bronsted acid site) than on the epoxidation rate (that occurred on the Si–O–Ti site), and that CHP had a significantly lower decomposition rate than TBHP. This is opposite to the results reported before for the hydroperoxide stability in the presence of liquid acid. When CHP was used as the oxidant, the rate of propylene epoxidation was found to be first-order with respect to the CHP concentration; the kinetic parameters were determined.