Single-step synthesis of dimethyl ether from biomass-derived syngas over CuO-ZnO-MOx (M = Zr, Al, Cr, Ti)/HZSM-5 hybrid catalyst: Effects of MOx

A series of CuO-ZnO-MOx (M = Zr, Al, Cr, Ti) catalysts were prepared by co-precipitation method and characterized by ICP-OES, XRD, N-2 adsorption, N2O titration, H-2-TPR, and XPS. The CuO-ZnO-ZrO2 catalyst exhibits the highest BET surface area and Cu surface area. For all the CuO-ZnO-MOx catalysts, Cu-0 was the predominant copper species detectable on the surface of both reduced and spent samples. As-prepared CuO-ZnOMOx catalysts were mixed physically with HZSM-5 zeolite to synthesize dimethyl ether (DME) via biomass derived syngas. The highest CO conversion and DME yield were obtained over a CuO-ZnO-ZrO2/HZSM-5 hybrid catalyst. The CO conversion increases with the increase in the Cu surface area, but the relationship between them is not linear. Due to the H-2-deficient characteristic of biomass-derived syngas, the water-gas shift reaction, by which H-2 can be produced in-situ for the hydrogenation of CO, plays an important role in the direct DME synthesis.