No abstract

Kinetic studies of the partial oxidation of propene to acrylaldehyde and 2-methylpropene to methacrylaldehyde have been carried out on various bismuth molybdates [Bi,Mo,O,, (a), Bi,Mo,O, (a + y or B) and Bi,MoO, (y)] and Bi,FeMo,O,, (phase X). The partial pressures of 0, 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 n-ally1 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 CO,, 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 % a+y > a > y > p. The alkane adsorption equilibrium may play the major role in catalytic activity.

Die α‐, β‐ und γ‐Phasen von Bi‐molybdat mit den Zusammensetzungen Bi2Mo3O", Bi2MO2 O9 bzw. Bi2MoO6 sowie innige äquimolare Gemenge der a‐ und γ‐Phasen werden auf ihre katalytische Aktivität für die milde Oxidation von Propen zu Acrolein hin untersucht.