Dimethyl ether synthesis via methanol dehydration over Ta-supported catalysts

Dehydration of methanol to dimethyl ether (DME) and the plausible side reactions have been investigated over Ta oxide supported on alumina (Ta/Al2O3) and titania (Ta/TiO2) catalysts with a Ta-load of 1 at-Ta. nm(-2). The surface atomic ratio of Ta/Al and Ta/Ti was 0.17 and 0.92, respectively; being isolated Ta species and Ta-O-Ta spreaded polymeric species the main species detected for Ta/Al2O3 and Ta/TiO2, respectively. The acidic character depends on the support, detecting a higher ammonia adsorption capacity for titania with respect to alumina. The incorporation of tantalum increases the number of acid sites, with a more remarkable effect for Ta/Al2O3. Lewis acid sites were the predominant centers and the distribution and the strength were modified by the presence of Ta. Reactivity studies of methanol adsorption and evolution in temperature were performed by in situ-FT-IR and non -isothermal TG-MS runs. Methanol dehydration was the main reaction and the methanol decomposition by two-routes and DME decomposition occur in the temperature range between 200-400 degrees C. Over Ta/Al2O3, methanol reacts more selectively to form DME at temperature close to 200 degrees C, a hundred degrees lower than for Ta/TiO2. The high acidity of Ta/TiO2 and the TaOx polymeric surface species favour these decomposition processes.