Study of metal oxide catalysts in the olefin oxidation from their reduction behavior: I. Reduction of various metal oxides with propylene

Abstract

The reduction behavior of 14 metal oxides with propylene was investigated to characterize their catalytic properties in the olefin oxidation. Metal oxides were classified depending on whether the reduction was limited within a few surface layers (group (I) oxides: TiO 2, Cr 2O 3, ZnO, In 2O 3, SnO 2, and WO 3) or it proceeded into the bulk (group (II) oxides: V 2O 5, MnO 2, Fe 2O 3, Co 3O 4, NiO, Bi 2O 3, CuO, and MoO 3). The distinction was shown to result from the thermodynamic stability of metal oxides. It was found that the rate and selectivity at the initial stage of reduction were well correlated with those attained in the catalytic oxidation. The fact indicates not only that lattice oxygen participates in the catalytic oxidation but also that the reduction steps of catalysts virtually determine the catalytic rates. With the progress of reduction, the activity and selectivity changed in complex manners which were widely different for group (I) and group (II) oxides. Discussion was made on such reduction behavior from various viewpoints. An interesting case was provided by ZnO in which the reduction rate as well as the rate of 1,5-hexadiene formation went through maxima. Analyses showed that, in this case, active sites (exposed metal ions) for the propylene dimerization reaction were created by the removal of surface oxygen, and that the increase of such sites coupled by the decrease of surface active oxygen gave rise to the observed rate maxima.