Catalytic Behavior of Molybdenum Suboxides in the Selective Oxidation of Isobutene to Methacrolein

The catalytic behavior of “Mo 8 O 23 ”, a sample composed mainly of Mo 8 O 23, was investigated in the oxidation of isobutene to methacrolein and compared to that of fully oxidized MoO 3 . “Mo 8 O 23 ” was initially more active and selective than MoO 3 but exhibited a deactivation with time-on-stream. Conversely, when used in a physical mixture with α-Sb 2 O 4, the catalytic performance of “Mo 8 O 23 ” progressively increased. In both cases, the bulk of “Mo 8 O 23 ” was reoxidized to MoO 3 after the catalytic reaction. However, after being catalytically tested alone, the surface of “Mo 8 O 23 ” was more reduced compared to that of the fresh material, whereas it was more oxidized, with a stoichiometry likely corresponding to Mo 18 O 52, when reacted in the presence of α-Sb 2 O 4 . 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 Mo 18 O 52 . Under our reaction conditions, this happened only when α-Sb 2 O 4 was present. The role of α-Sb 2 O 4 is to irrigate the catalyst with spillover oxygen, which facilitates the reoxidation of the surface of “Mo 8 O 23 ” after incorporation of its oxygen atoms in oxidation products. Because of the spillover oxygen, the structure of the “Mo 8 O 23 ” stabilized during the reaction in the presence of α-Sb 2 O 4 is identical to that obtained when using MoO 3 under the same conditions, namely in mixture with α-Sb 2 O 4 . In the presence of spillover oxygen, the steady-state molybdenum oxide catalyst is composed of a core of MoO 3 and a superficial layer possessing the stoichiometry of Mo 18 O 52 . These findings confirm predictions made by the remote control theory about the dynamics of oxides “at work” in the presence of spillover oxygen.