Study On Cu-Mn-Based Catalysts For Synthesis Of Dimethyl Ether From Carbon Dioxide Hydrogenation

Direct synthesis of dimethyl ether (DME) from carbon dioxide by catalytic hydrogenation is of great economical and environmental importance, not only to synthesize the useful chemical products, which will utilize carbon dioxide more efficiently, but also to reduce the greenhouse effect resulting from carbon dioxide.The bifunctional catalysts used for this reaction were composed of the methanol synthesis component and the methanol dehydration component, only when the two components catalyzed synergistically, the bifunctional catalysts could exhibit excellent catalytic performance. In this paper, Cu-Mn-based catalyst prepared by co-precipitation method was selected as methanol synthesis catalyst, and HZSM-5 zeolite was selected as methanol dehydration catalyst. The effects of precipitation temperatures, pH values, Cu/Mn ratios, prepared methods, calcination temperatures and modifications of promoter on the physico-chemical and catalytic properties of the composite catalysts were systemically investigated, using the technologies of X-ray diffraction (XRD), nitrogen adsorption-desorption (BET), temperature programmed reduction (H₂-TPR), Scanning Electron Microscope (SEM), hydrogen temperature programmed desorption (H₂-TPD), ammonia temperature programmed desorption (NH₃-TPD), X-ray photoelectron spectra (XPS),and activity evaluation on a laboratory fixed-bed reactor. The results were obtained as follows:The influences of precipitation temperatures, pH values, and Cu/Mn ratios on the properties of catalysts prepared by co-precipitation method were investigated. The results indicated that the Cu-Mn/HZSM-5 catalysts showed the best catalytic activity when the Cu/Mn ratio was 4/3, the precipitation temperature was 70℃Calcination temperature was 350℃and pH values were 7-8. The conversion of CO₂ reached 21.83% and the yield of DME reached 6.68% under the conditions of 2.0MPa, 250℃, GHSV 2100h^-1, and H₂/ CO₂ volume ratio of 3.2.The addition of SiO₂ could obviously improve the performance of the Cu-Mn/HZSM-5 catalysts. When the amount of SiO₂ was 3.49% of the catalyst mass(calculated as oxides), the conversion of CO₂ reached 23.86% and the yield of DME reached 9.33% under the conditions of 2.0MPa, 250℃, GHSV 2100h^-1, and H₂/CO₂ volume ratio of 3.2. The XRD and H₂-TPR results suggested that the suitable introduction of SiO₂ can weak the peak intensity of crystal phase CuO, and promote disperse of Cu. The copper-manganese compound oxide translates into amorphous phase from crystal phase, which prevents deep reduce of Cu to promote catalytic activity. The XPS results proved that the active sites of copper species of Cu-Mn/HZSM-5 catalysts might be Cu⁺, which might support the mechanism that Cu⁺ species might compose the active sites for methanol synthesis.The effect of operating conditions (temperature, pressure, space velocity, and mole ratio of H₂/CO₂) on the conversion of CO₂, selectivity to DME, yield of DME and product distribution on the Cu-Mn-Si/HZSM-5 catalysts prepared by co-precipitation method was investigated. CO₂ conversion is up to 26.03% at 260℃and 2.0 MPa, with a selectivity to DME of 42.42% and a yield of DME of 11.04%, for space velocity of 2100h^-1 and a feed made up of H₂/CO₂=3.2. It was also found that increasing reaction temperature could improve the conversion of CO₂, while it might decrease the selectivity of DME. Increasing pressure and H₂/CO₂ molar ratio were contributable to improve the conversion of CO₂ and the selectivity of DME. Furthermore, increasing the space velocity would decrease both the conversion of CO₂ and the selectivity of DME.The effect of calcination temperature on the performance of the Cu-Mn-Si/HZSM-5 prepared by co-precipitation method was also studied. The results showed that moderate temperature could be beneficial to catalytic hydrogenation. When calcination temperature was 400℃, more Cu⁺ existed in the reaction process. When calcination temperature was higher than 400℃, the particles of CuO became bigger resulting in the decrease of catalytic activity. The results of CO₂-TPD showed that the adsorption center to CO₂ was related to the synthesis of dimethyl ether. The moderate temperature was 400℃ in the researched temperature range.To further study the active sites of the catalysts, ultrasonic treatment was carried out to investigate its effect on the catalytic activity of Cu-Mn-Si/HZSM-5. The results showed that ultrasonic treatment could make more Cu⁺ and Mn³⁺ formed, promote the interaction of Cu⁺ and Mn³⁺ which increase the adsorption center to CO₂, and increase surface acidity and the amount of strong acid sites, which is beneficial to the reaction of dimethyl ether, improving the catalytic activity. In this paper, it was thought that the activity sites were both Cu⁺ and Mn³⁺.