Direct synthesis of dimethyl ether from CO2 hydrogenation over novel hybrid catalysts containing a Cu-ZnO-ZrO2 catalyst admixed with WOx/Al2O3 catalysts: Effects of pore size of Al2O3 support and W loading content

The present work examines the direct synthesis of dimethyl ether (DME) from CO2 hydrogenation over novel hybrid catalysts containing a Cu-ZnO-ZrO2 catalyst admixed with WOx/Al2O3 catalysts. The effect of pore sizes (6.6, 33 and 51 nm) of Al2O3 support and W loading contents (5, 10, 15 and 20 wt%) on the physicochemical properties of WOx/Al2O3 catalysts as well as their catalytic performance is also investigated. Characterization results from XRD, XPS and UV-vis reveal that the structure of WO species on the surface of Al2O3 is mainly related to W surface density, i.e. W loading content, which can be classified into three regions: a mixture of monotungstate species and polytungstate species at W surface density < 5.1 W nm(-2), a monolayer coverage of WO species over the surface of Al2O3 at W surface density similar to 5.1 W nm(-2) and a co-existence of polytungstate species and WO3 nanoparticles at W surface density > 5.1W nm(-2). The space-time yield (STY) of DME exhibits a volcanic trend as a function of W surface density with the maximum values at nearly monolayer coverage (similar to 4.7 W nm(-2)) for large pores (33 and 51 nm), and above monolayer coverage (8.0 W nm(-2)) for small pore (6.6 nm). The hybrid catalyst with optimum Cu-ZnO-ZrO2 and WOx/Al2O3 weight ratio of 1:5 achieves the highest STY of DME of 165.6 g(DME)kg(cat)(-1) h(-1). The long-term stability test shows a gradual decrease in activity of the hybrid catalyst which is attributed a combination of coke deposition, sintering of Cu-based catalyst and WOx/Al2O3 catalyst and strongly adsorbed water molecules.