Abstract: A series of γ-Al2O3 samples modified with borate, phosphate, and sulfate were prepared by the impregnation technique and physically mixed with a methanol synthesis component (CuO-ZnO-Al2O3) at a mass ratio of 1∶2 to form hybrid catalysts. The direct synthesis of dimethyl ether (DME) from syngas was carried out over the prepared hybrid catalysts under pressurized fixed-bed continuous flow conditions. The results indicated that the modification of γ-Al2O3 with SO2-4 could significantly enhance the activity for methanol dehydration and hence remarkably increase the selectivity for DME and the conversion of CO. The effects of SO2-4 content, calcination temperature, and type of SO2-4 precursor on the performance of the SO2-4/γ-Al2O3 hybrid catalyst were further investigated. The results revealed that the selectivity for methanol decreased when the content of SO2-4 increased from 5% to 10%, resulting in the increase in both the selectivity for DME and the conversion of CO. However, when the SO2-4 content was further increased to 15%, although the activity for methanol dehydration was increased continuously, the selectivity for DME decreased slightly due to the increased formation of byproducts hydrocarbons and CO2. Moreover, when the calcination temperature of SO2-4/γ-Al2O3 increased from 350 to 550 ℃, both the CO conversion and the DME selectivity increased gradually, resulting from the decreased formation of CO2. Nevertheless, a further increase in calcination temperature of SO2-4/γ-Al2O3 to 750 ℃ resulted in a significant decline in both DME selectivity and CO conversion because of the decreased activity for methanol dehydration. On the other hand, the type of SO2-4 precursors exerted no noticeable influence on the performance of the prepared hybrid catalysts.

Key words: syngas, dimethyl ether, hybrid catalyst, methanol, dehydration, alumina, modification