Study On Catalysts For Direct Synthesis Of Dimethyl Ether From Carbon Dioxide Hydrogenation

Carbon dioxide is one of the main greenhouse gases. Dimethyl ether is a important organic chemical product. Carbon dioxide in the atmosphere is more and more increasing with the industrial development, so the environment we live in are aggravated. Synthesis of dimethyl ether from carbon dioxide hydrogenation is an important way of using carbon dioxide efficiently. The reactions studied is a composite system, which involves three reactions i.e. synthesis of methanol from carbon dioxide hydrogenation, dehydration of methanol to dimethyl ether and the reversed water-gas shift reaction. Experiments had been carried out over the hybrid catalysts, which were prepared by the mechanical mixing of C207 commercial catalyst or CuO-ZnO-Al2O3 catalyst prepared with co-precipitation method (as methanol synthesis catalyst) and HZSM-5 (as dehydration catalyst). Using the technologies of BET, XRD, H2-TPR, NH3-TPD, IR and activity-evaluation in a laboratory fixed-bed reactor, the effects of Si/Al ratio in HZSM-5, the mass ratio of C207 to HZSM-5, precipitation agents, precipitation orders, precipitation methods and Cu/Zn ratio in CuO-ZnO-Al2O3 catalyst on the physico-chemical properties and reaction performance of catalysts were systemically investigated. And the effects of reaction conditions including temperature, pressure and space velocity on the reactivity over the optimized catalyst were also tested. Through the experimental results, the feasible reaction conditions were suggested. According to the results of activity-evaluation and NH3-TPD, it was concluded that the stronger acidic sites could be active for methanol dehydration. The hybrid catalyst with 25 of Si/Al ratio in HZSM-5 was of larger amount of stronger acidic sites, and showed lower reduction temperature, thus demonstrated the best dehydration performance for synthesis of dimethyl ether. Among the CuO-ZnO-Al2O3 catalysts prepared by co-precipitation method, the one by two-step co-precipitation in the solution of sodium carbonate showed ideal catalytic activity.Good catalytic activity and dimethyl ether selectivity had been obtained on the hybrid catalysts under the reaction conditions of H2/ CO2=3, 518K, 3Mpa and GHSV＝1600h-1. With the hybrid catalyst of C207 as methanol synthesis catalyst, the conversion of CO2 is 19.82％, the overall yield of methanol and dimethyl ether is 11.79％. With the hybrid catalyst of CuO-ZnO-Al2O3 prepared by co-precipitation method as methanol synthesis catalyst, the conversion of CO2 is 26.48％, the overall yield of methanol and dimethyl ether is 17.95％.