Catalytic oxidation. IV. Ethylene and propylene oxidation over gold

The heterogeneous oxidation of C ₂H ₄ and C ₃H ₆ was studied over several gold catalysts. The principal products from C ₂H ₄ were CO ₂ and H ₂O, but small amounts of acetic acid and acetaldehyde were also found. In addition to these products, up to 3% acetone and 50% acrolein were obtained from C ₃H ₆. Gold sponge and Au/SiO ₂ were much more selective for acrolein formation than Au/α-Al ₂O ₃. For both olefins, the rate of total oxidation was first order in oxygen pressure and zero order in olefin pressure at low pressures. The apparent activation energies (except for C ₂H ₄ over Au sponge) ranged from 17 to 22 kcal mol ^-1. Acrolein formation had a much lower apparent activation energy (9-12 kcal mol ^-1), was also first order in O ₂, but showed a maximum rate for C ₃H ₆ pressures near 30 Torr. Oxidation of propylene-1- ¹³C produced acrolein molecules in which the ¹³C was equally distributed between the end positions. Similarly, CD ₃CHCH ₂ and CH ₃CHCD ₂ both yielded CH ₂CHCDO and CD ₂CHCHO in about equal amounts, although the rate of the reaction with the former olefin was only about 40% that of the latter or that of CH ₃CHCH ₂. These results indicated that the rate-determining step in acrolein formation was abstraction of a methyl hydrogen (or deuterium) to yield a symmetrical allylic intermediate. The modes of fragmentation of acrolein during mass spectral analysis are discussed on the basis of high-resolution measurements of the labeled acroleins formed in the present work.