Catalytic oxidation of dichloromethane on gamma-Al2O3: A combined flow andinfrared spectroscopic study

When gamma-Al2O3 is applied as a support for noble metals in the catalytic oxidation of dichloromethane, the reaction primarily takes place on the alu mina. In our flow study, dichloromethane was completely converted at 600 K on gamma-Al2O3, CO being a main product, especially at higher temperatures. Methyl chloride was observed in substantial amounts as an intermediate pro duct. The formation of CH3Cl, a reduction product formed under oxidative co nditions, has been described before, but no satisfactory explanation has be en given. To this end, reaction between alumina and CH2Cl2 has been studied by FTIR spectroscopy as well. The spectrum of the adsorbed species at 523 K was clearly different from that at 298 K. Apparently, dichloromethane rea cted with surface hydroxyl groups, which disappeared. Experiments with CD2C l2 enabled the identification of the surface species. The bands found could be attributed to surface methoxy groups and to adsorbed formate. With thes e species the products found in the flow experiments could be explained: th e formates are converted into CO and the methoxy groups can react with HCl to form CH3Cl. The first step, reaction of the adsorbed dichloromethane with a surface hyd roxyl group, yields a chloromethoxy species. This leads to a chemisorbed fo rmaldehyde analogue, which appears to disproportionate due to some hydride shift, to form the observed methoxy and formate groups. The Cl from the CH2 Cl2 is released as HCl and/or reacts with the alumina to form aluminum chlo rides. The observed rate enhancing effect of added water in the gas stream can be explained by reaction with chlorinated alumina and restoration of su rface hydroxyl groups, the active sites in the reaction with dichloromethan e. (C) 1998 Academic Press.