Direct synthesis of dimethyl ether from CO2 and H2 over novel bifunctional catalysts containing CuO-ZnO-ZrO2 catalyst admixed with WOx/ZrO2 catalysts

The present work examined the performance of WOx/ZrO2 catalysts calcined at different temperatures admixed with a Cu-ZnO-ZrO2 catalyst for CO2-to-dimethyl ether (DME) hydrogenation. The effect of operating conditions including reaction temperature, pressure and space velocity was also investigated. The calcination temperature was found to play a crucial role on the physicochemical properties of the WOx/ZrO2 catalysts as well as their catalytic performance. Increasing calcination temperature from 600 to 800 degrees C led to a gradual increase of W surface density (2.82-4.82 W nm(-2)) which was accompanied by an increase of both quantity and strength of acid sites. The WOx/ZrO2 catalyst calcined at 900 degrees C showed W surface density of 9.63 W nm(-2) which was higher than theoretical monolayer surface coverage of W (similar to 8 W nm(-2)), indicating the saturation coverage of WOx species coexisted with a small portion of nanocrystalline WO3. At calcination temperature of 1000 degrees C, the monoclinic ZrO2, bulk WO3 particles and ZrW2O8 phase were formed because of a drastic reduction in BET surface area (similar to 5 m(2)g(-1)). The CuO-ZnO-ZrO2 catalyst admixed with WOx/ZrO2 catalyst calcined at 900 degrees C exhibited the highest space-time yield of DME of 271.2 g(DME) kg(cat)(-1) h(-1) during a 48-h test at 260 degrees C, 30 bars and a space velocity of 4333.33 mL g(cat)(-1) h(-1).