Highly efficient synthesis of dimethyl ether from syngas over the admixed catalyst of CuO-ZnO-Al2O3 and antimony oxide modified HZSM-5 zeolite

A series of HZSM-5 zeolites modified with various contents of antimony oxide (0-30 wt.%) were prepared by solid state ion reaction at 500 degrees C, and the acidities of the resulted materials were characterized by temperature-programmed desorption of NH3. The direct synthesis of dimethyl ether (DME) from syngas was carried out over the admixed catalysts of an industrial CuO-ZnO-Al2O3 methanol synthesis catalyst and the parent and antimony oxide modified HZSM-5 zeolites under pressurized fixed-bed continuous flow conditions. The results indicated that modification of HZSM-5 with suitable amount of antimony oxide significantly decreased the selectivity for undesired byproducts like hydrocarbons and carbon dioxide from 9.3% and 32.4% to less than 1% and 28%, respectively, so the selectivity for DME was enhanced greatly from 55% to 69% under temperature of 260 degrees C, pressure of 4 MPa and gas hourly space velocity of 1500 mL h(-1) g(cat)(-1). The decrease in the formation of hydrocarbons and carbon dioxide can be attributed to the significant decline in the amount of strong acid sites of the HZSM-5 zeolite induced by antimony oxide modification. Additionally, the influences of the operating parameters on the performance of the most efficient catalyst were also investigated. The results showed that high reaction temperature and high gas hourly space velocity resulted in both lower carbon monoxide conversion and lower dimethyl ether selectivity, so they should be no higher than 280 degrees C and 3000 mL h(-1) g(cat)(-1), respectively. (C) 2009 Elsevier Ltd. All rights reserved.