Effect of Mg-Modified HZSM-5 on Catalytic Performance of Cu-ZnO-Al_2O_3/HZSM-5 for Direct Synthesis of Dimethyl Ether from Syngas

A series of HZSM-5 zeolites modified by MgO were prepared by impregnation and characterized by N_2 adsorption, XRD, NH_3-TPD, CO_2-TPD and Py-IR methods. The direct synthesis of dimethyl ether (DME) from syngas was carried out over Cu-ZnO-Al_2O_3∥MgO/HZSM-5 under pressurized fixed-bed continuous flow conditions. The results indicated that the HZSM-5 modified with suitable amount of MgO could significantly enhance the selectivity for DME from 49 1% to 64 8%,and decreased the selectivity for the undesired hydrocarbons and CO_2 from 9 3% and 37 1% to 0 4% and 30 2%, respectively. However, when the MgO contents were higher than 5%, both the CO conversion and DME selectivity were decreased remarkably. A mechanism for methanol dehydration to DME involving both acidic and basic sites on MgO/HZSM-5 was proposed based on the experimental results.