Hybrid monometallic and bimetallic copper–palladium zeolite catalysts for direct synthesis of dimethyl ether from CO2†
Currently, carbon dioxide in the atmosphere is the major contributor toward global climate change. Direct CO2 hydrogenation to dimethyl ether produces an important platform molecule for the synthesis of fuels and chemicals and at the same time, utilizes large amounts of this greenhouse gas. In this paper, we prepared a series of hybrid catalysts, which are composed of alumina supported copper–palladium nanoparticles and HZSM-5 zeolite for the direct synthesis of dimethyl ether from CO2. Copper active sites showed a higher intrinsic activity for CO2 hydrogenation compared to palladium. The low palladium content in the copper–palladium bimetallic catalysts was particularly beneficial for the dimethyl ether production. Undesirable methane and ethane production was completely suppressed, while the dimethyl ether selectivity was considerably increased. Extensive catalyst characterization combined with catalytic measurements was indicative of the presence of copper and palladium monometallic and bimetallic particles with different sizes and reducibility in the hybrid catalysts. The presence of even small amounts of palladium significantly improved copper reducibility and copper dispersion. Some decrease in the Brønsted acidity in the copper containing catalysts was due to the migration of unreduced copper ions in the zeolite channels during the reduction. The methanol dehydration to dimethyl ether was only slightly affected by the amounts of Brønsted acid sites in the hybrid catalysts.