Mechanism of [3-13C]propene ammoxidation on bismuth molybdate catalyst

Abstract

To study the mechanism of acrylonitrile formation, the ammoxidation of [3-¹³C]propene was studied in the presence of a bismuth molybdate catalyst on a silica support. The reaction was carried out at 723 K with a conversion of about 40 vol % of the propene to acrylonitrile and a small amount of acetonitrile. The ¹³C distribution among the reaction products was examined quantitatively with a high-resolution mass spectrometer. Half of the ¹³C in the acrylonitrile was found to be in the CN-group and the other half in the C₂H₃-group. This result points to a symmetrical intermediate, such as an allylic species, in the ammoxidation reaction. A similar intermediate was observed in the catalytic oxidation of propene to acrolein. The total ¹³C content in the acetonitrile amounted only half that of the acrylonitrile, which indicates carbon-carbon bond fissure of the allylic intermediate in the mechanism of acetonitrile formation. Ammoxidation of propene does not seem to involve acrolein as an intermediate. As a matter of fact, the addition of ammonia to a mixture of propene + oxygen + helium completely suppresses acrolein formation at 623 K.