(544fu) Dimethyl Ether Synthesis from CO2 Hydrogenation on a CuO?ZnO?Al2O3?CeO2/HZSM-5 Bifunctional Catalyst

The Cu-rich composition manifests itself in a peculiar microstructure of the Cu/ZnO/Al2O3 catalyst, which is composed of spherical copper nanoparticles of a size of 5–15 nm and often even smaller ZnO nanoparticles arranged in an alternating fashion. One important role of ZnO is to act as spacer and stabilizer avoiding direct contact of the copper particles and preventing them from sintering. Thus, porous aggregates are formed, in which the oxide particles act as spacers between Cu particles. Three requirements for high catalytic activity – large surface area of Cu, defective Cu nanoparticles and many reactive interfaces to ZnO. Preparation of this microstructure requires a homogeneous and maximized intermixing of the Cu and Zn species in order to stabilize the alternating arrangement of small Cu and ZnO nanoparticles. The main goal of catalyst synthesis is to carry over and maintain the perfectly homogeneous cation distribution in the starting mixed solutions to a maximum extent to the final catalyst. The best catalysts can be obtained by constant pH co-precipitation with Na2CO3 solution at pH 6 or 7 and at elevated temperatures around 333–343 K. Ageing of the initial precipitate is crucial and takes from around 30 min to several hours. Calcination is typically performed at relatively mild temperatures around 600–700 K. The oxide catalysts are activated with dilute hydrogen at 190–230 °C, by which copper oxide is completely reduced to metallic crystallites interspersed by ZnO/Al2O3. In this study, CuO−ZnO−Al₂O₃−CeO₂ with high Cu content (70%) was synthesized by coprecipitation at pH 7 and 338 K using Na2CO3 as the precipitating agent. Under this condition, all metals could be precipitated simultaneously. The sample was dried at 373 K, and then calcined at 573 K, then reduced by diluted hydrogen gas at 573 K. the sample was then mixed with H-ZSM-5 (Si/Al2 ratio= 35). The bifunctional catalyst had a high activity for dimethy ether (DME) synthesis in CO₂ hydrogenation with a feed of H2/CO2 ratio of 3, 523 K and 5 MPa. CO₂ conversion reached 18%. Adding CeO2 in the catalyst could decrease the Cu particle size.