Catalytic conversion of chloromethane to methanol and dimethyl ether over mesoporous γ-alumina

Mesoporous gamma-alumina, mp-gamma-Al2O3, possessing surface area around 385 m(2)/g and total pore volume of 2.0 cm(3)/g was prepared via template-free sol-gel synthesis. The catalytic activity of the prepared alumina for the conversion of chloromethane to dimethyl ether, DME, in the presence of water or methanol was studied in the temperature range of 170-450 degrees C employing FTIR spectroscopy. In the absence of water and methanol, the fresh surface of mp-gamma-Al2O3 showed 100% conversion of chloromethane to DME at temperatures between 250 and 350 degrees C. However, rapid deactivation of the catalyst resulted in a sharp decrease in the conversion to <5% within a few minutes of reaction. The catalytic activity was noticeably enhanced by adding water vapor to the gas feed resulting in higher conversions to DME and methanol. The catalytic activity and DME selectivity were further enhanced in the presence of methanol instead of water. In the temperature range of 200-300 degrees C, complete conversions were obtained at the beginning of reactions before they declined to values between 31 and 45% depending on the reaction temperature. It was proposed that the surface hydroxyl groups are the active sites where chloromethane molecules dissociatively adsorb forming adsorbed methoxy ion intermediates. The adsorption of molecular methanol regenerates the surface hydroxyl groups and enhances the formation of adsorbed methoxy ions on the surface which react further with chloromethane or methanol molecules to produce more DME. (C) 2011 Elsevier B.V. All rights reserved.