Synthesis and characterization of a highly active alumina catalyst for methanol dehydration to dimethyl ether

A simple sol–gel method was adopted to synthesize boehmites with high surface area using aluminum iso-propoxide (AIP), acetic acid (AA) and 2-propanol, and the effects of surface area and methanol dehydration on activity were investigated. The hydrolysis conditions of AIP in the presence of AA in 2-propanol solvent were systematically varied to observe their effect on phase formation, crystallinity, surface area and pore size distribution of the alumina. The surface area and the number of acidic sites varied considerably with the variation in the molar ratio of AA/AIP. This study revealed that a high surface area boehmite (in the range of 628–717 m2/g) could be obtained by keeping the molar ratio of AA/AIP as 0.5 and that of H2O/AIP at ∼3. Rod shaped, porous γ-Al2O3 powder with a high surface area of 438 m2/g was obtained after calcination of the boehmite at 500 °C for 5 h in air. The temperature programmed desorption of ammonia (NH3-TPD) of the γ-Al2O3 samples demonstrated higher concentration of acidic sites when acetic acid was used during preparation than when it was not used. The vapor phase dehydration of methanol (containing 20 mol% H2O) to dimethyl ether (DME) was conducted on the prepared aluminas. With increasing surface area of γ-Al2O3, the temperature required to reach 50% conversion of methanol decreased due to the increased number of acidic sites which are favorable for methanol dehydration with low byproduct formation. The catalytic activity for methanol dehydration to DME correlated well with the total number of acidic sites of γ-Al2O3, which was controlled by changing the AA/AIP and H2O/AIP molar ratios.

The laboratory-made γ-Al2O3 showed an enhanced catalytic activity due to the enhanced number of available acidic sites on γ-Al2O3 which is one of the crucial factors for methanol dehydration to dimethyl ether. A good correlation is obtained between the number of acid sites and the temperature required to reach 50% conversion of methanol. Figure optionsDownload as PowerPoint slide