Dimethyl Ether Synthesis from Syngas over the Admixed Cu/ZnO/Al₂O₃ Catalyst and Alkaline Earth Oxide-Modified HZSM-5 Zeolite

HZSM-5 zeolites modified with various alkaline earth oxides (MgO, CaO, and BaO) were prepared by using impregnation and mixed physically with Cu/ZnO/Al₂O₃ to perform the direct synthesis of dimethyl ether (DME) from syngas in a pressurized fixed-bed under continuous flow conditions. The results indicated that the modification effectively enhanced the DME selectivity; MgO and CaO are more effective modifiers than BaO and they improved the DME selectivity up to 67 % as compared to only 54 % over the parent HZSM-5. Moreover, the catalyst prepared from magnesium nitrate exhibits higher DME selectivity than those prepared from magnesium acetate and chloride. The enhancement in the DME selectivity resulted from the effective suppression of hydrocarbon and CO₂ formation, which can be attributed to the significant decline in the amount of strong acid sites of the HZSM-5 zeolite resulting from the modification.