Catalytic oxidation. V. Mechanisms of olefin oxidation over supported iridium

The reactions of ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene, iso-butene and the two 2-pentenes were compared over supported Ir catalysts. The rates of total oxidation decreased in the order given, while the corresponding apparent activation energies increased. With ethylene, rate maxima appeared on varying the pressure of either reactant; with propylene and 1-butene, maxima were found with oxygen only; with the other olefins, the rates increased with olefin pressure and were inhibited by oxygen over the entire range studied. About 40% of all the olefin oxidized became partial oxidation products. The most important of these were acetic acid from ethylene, propylene, the 2-butenes and the 2-pentenes, and propionic acid and acetone from 1-butene and isobutene, respectively. Smaller amounts of unsaturated aldehydes, diones and other ketones and acids were also detected. The acetic acid formed from propylene-1-¹⁴C was not radioactive. A large kinetic isotope effect was found for the total oxidation of propylene labeled with deuterium in the methyl group. Possible mechanisms are discussed.