The competition between propylene epoxidation and hydrogen oxidation and the effect of water on Au/TS-1/MOx catalysts

Abstract

Water is the only by-product of propylene epoxidation theoretically and there is not much research on the effect of water at present. In the paper, through encapsulating metal oxides with the difference on the ability to adsorb O2 into titanium silicalite-1 (TS-1) shell, the TS-1/MOx carriers (MOx = CeO2, MnO2, Fe2O3 and Co2O3) with core-shell structure were prepared. After loading Au, they were used for propylene epoxidation. With the reactant mixture of C3H6: H2: O2: N2 = 1:1:1:7 Au/TS-1/Fe2O3 shows better performance than Au/TS-1 and Au/TS-1/CeO2 performs badly because of the strong ability of CeO2 to adsorb and activate O2. A large amount of O2 is adsorbed on Au/TS-1/CeO2 but only a part can be used for propylene epoxidation, and the excess part of O2 will react with H2 to form water. Thus the hydrogen oxidation is dominant. Interestingly, reducing the oxygen concentration in the reactant mixture can weaken the hydrogen oxidation, and the amount of water generated can be reduced. Besides, the PO formation rate and H2 efficiency of Au/TS-1/CeO2 can be greatly increased. XPS and in-situ DRIFTS are employed to study the mechanism of the formation of water and its effect on the reaction. The results show that the generation of excessive water results in the competitive adsorption with propylene, affecting the catalytic performance. Additionally, some of the adsorbed water increases the amount of hydroxyl structure in the catalyst, leading to the ring-opening of the PO and impacting the PO selectivity of the catalyst.