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

Abstract

The present work examines the direct synthesis of dimethyl ether (DME) from CO2 hydrogenation over novel hybrid catalysts containing a CuZnOZrO2 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 WOx 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 WOx species over the surface of Al2O3 at W surface density ∼5.1 W nm−2 and a co-existence of polytungstate species and WO3 nanoparticles at W surface density >5.1 W 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 (∼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 CuZnOZrO2 and WOx/Al2O3 weight ratio of 1:5 achieves the highest STY of DME of 165.6 gDME kgcat−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.