The polymerization of ethylene on chromium oxide catalysts II. Infrared studies

Abstract

Ethylene reduces silica supported chromium(VI) oxide catalyst at 300 °C and adsorbs as ethyl groups by a self-hydrogenation mechanism. No exchange of hydrogen atoms between ethylene and the hydroxyl groups in the oxide surface occurs. Ethyl groups are adsorbed on chromium atoms in both high (probably + 5) and low (probably + 3) oxidation states and are partially desorbed, particularly from the former, by evacuation at 300 °C. The adsorption of ethylene confers catalytic activity for the polymerization of ethylene at 50 °C on chromium(v) atoms but chromium(III) atoms on which there are adsorbed ethyl groups are inactive. The catalytic activity of the high oxidation state sites from which ethyl groups desorbed during evacuation at 300 °C depends upon the form of the adsorption isotherm of ethylene on the sites at 50 °C. The adsorption of ethylene as ethyl groups constitutes the initiation step in the polymerization reaction. The products from the oxidation of ethylene caused by contact with catalyst appear in part as adsorbed water, carboxylate, carbonate, and carbonyl species. The catalytic activity of the oxide is poisoned by the presence of the adsorbed oxidation products but is enhanced by evacuation at 300 °C which causes their desorption.