Kinetic study of the partial oxidation of propene and 2-methylpropene on different phases of bismuth molybdate and on a bismuth iron molybdate phase

Abstract

Kinetic studies of the partial oxidation of propene to acrylaldehyde and 2-methylpropene to methacrylaldehyde have been carried out on various bismuth molybdates [Bi₂Mo₃O₁₂(α), Bi₂Mo₂O₉(α+γ or β) and Bi₂MoO₆(γ)] and Bi₃FeMo₂O₁₂(phase X). The partial pressures of O₂ and alkene have been varied and a maximum in the yield of aldehyde has been observed for a value of the alkene pressure which depends on the sample. In contrast, no maximum has been observed for carbon dioxide. A simple mechanistic model has been proposed involving as a first step the rapid adsorption of the alkene. This is followed by the rate-determining step, the formation of a π-allyl intermediate with a surface site S; the methacrylaldehyde is then formed relatively fast. Next 2-methylpropene reacts with site S to give an oxygenated hydrocarbon species which is strongly adsorbed and which blocks the site active for aldehyde formation. Finally the blocking hydrocarbon is oxidized into CO₂, liberating the active surface sites. The model fits the experimental data well and allows one to differentiate different catalysts by the rate constant of step (1): the rate constants of the other steps are approximately equal. The study clearly shows that a comparison between catalyst performances is misleading, particularly for 2-methylpropene oxidation, under the same experimental conditions, since the relative ordering depends on the pressure of alkene with respect to that of oxygen. Carbon dioxide does not result from a total oxidation of the aldehyde under our conditions of low conversion levels. The initial activity for aldehyde formation is much higher in the case of 2-methylpropene than for propene and follows the order X α+γ > α > γ > β. The alkane adsorption equilibrium may play the major role in catalytic activity.