Promotional Effect of Water on Direct Dimethyl Ether Synthesis from Carbon Monoxide and Hydrogen Catalyzed by Cu–Zn/Al2O3

Cu–Zn/Al 2 O 3 was prepared by using the sol–gel method and employed for the direct synthesis of dimethyl ether (DME) from synthesis gas (syngas) in a continuous-flow reactor. We studied the effect of water concentration in the feed on the formation rates of various products. With an increase in the amount of water, the formation rate of DME initially increased and then decreased. Co-feeding of 0.09% water was co-fed resulting in the highest formation rate of DME, 486 μmol g cat –1 h –1 (8.0% yield of DME), at 220 °C under 0.9 MPaG. Structural studies by XANES, EXAFS, XPS, and infrared spectroscopies showed that the treatment of the catalyst with 0.66% water at 240 °C under atmospheric pressure resulted in oxidation of the metallic copper species and increased the number of surface Cu + sites. Combined with the catalytic results, we conclude that the increase in the surface Cu + sites is responsible for the promotion effect of steam on the DME production.