On the mechanism of catalytic oxidation of CH2Cl2 on gamma-Al2O3 and its oscillatory behaviour

Adsorption of CH2Cl2 and its oxidation with dioxygen have been investigated on pure gamma-Al2O3 by means of infrared spectroscopy and gas chromatographic analysis. Infrared spectroscopy showed that CH2Cl2 may be adsorbed on the alumina surface through chlorine ions either on two adjacent exposed Al ions at the (111) plane of Al2O3, or on the isoloated Lewis acid sites. The oxidation of CH2Cl2 at temperatures below 450 degrees C yields an intermediate product CH3Cl with 50% of selectivity whereas the other half of the CH2Cl2 amount is totally oxidized to CO2 and HCl. At temperatures higher than 450 degrees C practically total conversion of CH2Cl2 to CO2 and HCl takes place. Selfsustained oscillations of the CH2Cl2 conversion in a broad range are observed when water vapour is added to the feed. The mechanism explaining the nature of the activity of alumina in the CH2Cl2 oxidation and selectivity to CH3Cl as well as a possible reason of oscillations of the conversion are proposed.