Improvement in the catalyst activity for direct synthesis of dimethyl ether from synthesis gas through enhancing the dispersion of CuO/ZnO/gamma-Al2O3 in hybrid catalysts

Based on the knowledge of the mechanisms of methanol synthesis and methanol dehydration, it was predicted that if the dispersion of CuO/ZnO/gamma-Al2O3 in hybrid catalysts for direct synthesis of dimethyl ether from synthesis gas was enhanced, the formation rate of dimethyl ether would be increased. In order to justify the prediction, five preparation methods, including two mechanical mixing ones and three co-precipitation ones were used to prepare the catalysts. It was found that co-precipitation impregnation and co-precipitation of Cu/Zn by NaAlO2 provided the catalyst with the highest activity and co-precipitation of Cu/Zn/Al by Na2CO3 resulted in the worst catalyst. Combined with the XRD results it is deduced that the active phases for direct synthesis of dimethyl ether from synthesis gas are highly-dispersed fine crystallites of CuO/ZnO/gamma-Al2O3. Calcination temperature of the precursor of gamma-alumina used in the methods of mechanical mixing was also studied. It was found that a calcination temperature of 550 degrees C brought about the most active dehydration catalyst with the largest surface area and nearly pure gamma-alumina of poor crystallinity. So it might be concluded that the active phase for methanol dehydration is fine crystallites of gamma-alumina.