Investigation of the mechanism of catalytic oxidation of propylene to acrolein and acrylonitrile

Deuterated propylene (96% 1-propene-3d with 4% ordinary propylene) was selectively oxidized with molecular oxygen in the vapor phase over two catalysts, cuprous oxide and bismuth molybdate, to acrolein, and in the presence of ammonia, to acrylonitrile. There was considerable retention of deuterium in both products. With the cuprous oxide catalyst, 61% of the acrolein formed at 350 °C was monodeuterated and 1.3% dideuterated, while 41% of the acrylonitrile formed at 410 °C was monodeuterated and 2.9% dideuterated. With the bismuth molybdate catalyst at 450 °C, 62% of the acrolein was monodeuterated and 1.6% dideuterated, while 44% of the acrylonitrile was monodeuterated and 0.3% dideuterated. These results suggest that the mechanism of propylene oxidation in these two catalyst systems is the same. A mechanism consistent with these data involves the formation of an allyl intermediate by abstraction of a hydrogen or deuterium from the methyl group, followed by further abstraction at either end of this symmetric intermediate. The isotope discrimination effect kDkH, assumed to be the same for all the abstractions, was calculated to be 0.58 (ex acrolein results) and 0.49 (acrylonitrile) for the cuprous oxide, and 0.50 (acrolein) and 0.49 (acrylonitrile) for the bismuth molybdate. The constancy of these results indicates that the same mechanism is inherent in the formation of both products.