Electron-transfer and the catalytic activity of oxide catalysts. Part 1.—Electron-transfer activity of alumina catalysts and their activity for the catalytic exchange of the hydrogen in propane with deuterium

Abstract

The exchange reaction of propane with deuterium has been studied over a number of oxide surfaces. Exchange activity in aluminas is virtually confined to catalysts activated in the temperature range 400–800°C, with a sharp maximum in activity around 530–560°C. Since these temperatures are well in excess of those leading to maximum surface area and to maximum propane adsorption (∼350°C), some specific property of the surface must be involved in the exchange reaction. The property of the pure alumina catalysts which correlates best with the hydrogen-deuterium exchange activity is the Lewis acidity. Blocking experiments with electron-acceptor molecules show, however, that the reducing centres on the surface play an essential role. In each case the weaker sites are important. The hydroxyl groups in alumina are not apparently involved as vehicles for the deuterium, and exchange of hydrogen between the alumina hydroxyl groups and the hydrocarbon is not an intermediate step in the exchange reaction. Silica-alumina catalysts containing less than ∼80% Al2O3 show no detectable exchange activity for the propane-deuterium reaction at room temperature under any conditions of activation and only moderate rates of exchange at higher temperatures. On all the oxides studied the exchange of propane with deuterium is a stepwise process involving replacement of one hydrogen atom at a time.