Role of Lewis and brønsted sites in selective oxidation of propene on MoO3–Sb2O4 catalyst

Abstract

A relationship between activity and acidity of MoO₃–α-Sb₂O₄ catalysts for selective propene oxidation was established. It was found that two types of acid sites existed on the mixed oxides containing MoO₃, i.e. the Lewis sites which are probably situated on the (010) face, and the Brønsted sites on the (001), (101) and (100) faces of MoO₃ crystallites. No acidity could be detected on pure α-Sb₂O₄, but its presence in the mixture could increase the Brønsted acidity and decrease the Lewis acidity. A linear correlation between the propene conversion and the number of Lewis sites was obtained. On the other hand, the yield of acrolein from propene depended on the number of Brønsted sites. Test results suggest that the chemisorption of propene takes place on the Lewis sites, and the subsequent abstraction of α-hydrogen, which is the rate-determining step for the selective oxidation of propene, is accomplished on the Brønsted sites.