The oxidation of propene over bismuth oxide, molybdenum oxide, and bismuth molybdate catalysts: IV. The selective oxidation of propene

Abstract

A low-pressure static system, with mass spectrometric analysis of the gas phase, has been used to study the reactions of propene with bismuth molybdate, molybdenum oxide, and bismuth oxide in the absence and in the presence of gaseous oxygen at temperatures between 425 and 500 °C. Propene reduces bismuth molybdate and the oxygen appearing in the gaseous products can be quantitatively replaced in the lattice; many layers of ions in the catalyst appear to be reduced. Acrolein forms nearly 80% of the products to nearly 50% conversion; the selectivity is higher with oxygen present, and the amount of carbon dioxide found is then greatly reduced relative to acrolein and carbon monoxide. The activation energy of acrolein formation is 31 ± 4 kcal in the absence of gaseous oxygen, and 17 ± 4 in its presence. Over molybdenum oxide the initial selectivity is much lower and more carbon monoxide is formed. The rate of oxidation of gaseous acrolein is rather slower on the molybdenum oxide than on the bismuth molybdate, and the selectivity of the bismuth molybdate is ascribed to alternative reactions of the adsorbed allylic hydrocarbon radicals. In particular, attention is drawn to the possible effects of oxygen gas and bismuth ions on the bonding of these radicals to the catalysts and the subsequent ease of desorption of acrolein.