Catalytic behavior of molybdenum suboxides in the selective oxidation of isobutene to methacrolein

The catalytic behavior of "Mo8O23", a sample composed mainly of Mo8O23, was investigated in the oxidation of isobutene to methacrolein and compared to that of fully oxidized MoO3. "Mo8O23" was initially more active and selective than MoO3 but exhibited a deactivation with time-on-stream. Conversely, when used in a physical mixture with alpha-Sb2O4, the catalytic performance of "Mo8O23" progressively increased. In both cases, the bulk of "Mo8O23" was reoxidized to MoO3 after the catalytic reaction. However, after being catalytically tested alone, the surface of "Mo8O23" was more reduced compared to that of the fresh material, whereas it was more oxidized, with a stoichiometry likely corresponding to Mo8O52, when reacted in the presence of alpha-Sb2O4. The results indicate that the shear structures in the bulk of the molybdenum suboxides are not necessarily the origin of the high levels of catalytic performance observed for molybdenum oxide-containing oxidation catalysts. The results also show that the key to maintaining high catalytic performance is to stabilize the surface of the molybdenum oxide in a stoichiometry close to Mo18O52 Under our reaction conditions, this happened only when alpha-Sb2O4 was present. The role of alpha-Sb2O4 is to irrigate the catalyst with spillover oxygen, which facilitates the reoxidation of the surface of "Mo8O23" after incorporation of its oxygen atoms in oxidation products. Because of the spillover oxygen, the structure of the "Mo8O23" stabilized during the reaction in the presence of alpha-Sb2O4 is identical to that obtained when using MoO3 under the same conditions, namely in mixture with alpha-Sb2O4. In the presence of spillover oxygen, the steady-state molybdenum oxide catalyst is composed of a core of MoO3 and a superficial layer possessing the stoichiometry of Mo18O52 These findings confirm predictions made by the remote control theory about the dynamics of oxides "at work" in the presence of spillover oxygen.