Study On The Preparation And Properties Of Direct Synthesis Of Dimethyl Ether By Hydrogenation Of CO_2

DME can be synthesized from nature gas, coal, crude oil, residual oil, waste products and biomass. DME is nontoxic, non corrosive and no carcinogenic, which has been widely used. DME is considered not only as an important organic chemical material but a promising new clean alternative fuel. Therefore, CO2 converted into DME is recognized as significant challenges for the chemicals industry, environmental protection and other fields.We used one-step method for CO2 hydrogenation to DME. A series of HTlc-x/HZ bifunctional catalyst were prepared by co-precipitation method and physically mixing method. The influences of the Zr content on catalytic activity are explored, and the optimum Zr content is found. Second, in order to further improve the performance of bifunctional catalyst, we also attempts to adding the Mg, Ce, Y additives. We also studied the effects of two active phase ratio and HZSM-5 molecular sieve silicon aluminum ratio on the catalytic activity.The physico-chemical characteristics of the catalysts were characterized by X-ray diffraction, Nitrogen physisorption analysis and also hydrogen temperature programmed reduction techniques. Their catalytic activities were carried out in a continuous flow, fixed bed reacter-gas chromatography. Based on the results of this study, we conclude the following.(1) All the Zr-modified precursors were prepared by co-precipitation method exhibited hydrotalcite-like structure of typical hydroxy carbonate Cu3Zn3Al2(OH)16CO3-4H2O phase. The bifunctional catalysts HTlc-x/HZ showed good performance for CO2 hydrogenation to DME.(2) CO2 conversion was mainly caused by the methanol synthesis rate on the HTlc-x/HZ methanol synthesis catalyst. The experimental results demonstrated that the performance over the catalyst with atomic ratio of n(Zr4+):n(Al3++Zr4+)=0.1 was superior to the other catalysts. Proper addition of ZrO2 increased the bifunctional catalysts specific surface area, enhanced the synergy of CuO and ZnO, improved the dispersion of the oxide particles and reduced the reduction temperature of CuO. At the same time, the proper addition of ZrO2 could state low active sites to prolonging the service life of the catalysts. The HT1c-0.1/HZ catalyst was stable after 30 h of running.(3) Different Zr content on the CO2 conversion and DME selectivity possessed the same change trend. With the increase of reaction temperature, CO2 conversion first increased obviously, and then increased gradually. DME selectivity increased gradually with the temperature increased from 493 to 533 K, then an obviously decline. When n(Zr4+):n(Al3++Zr4+)=0.1, The catalytic activity is superior to the other catalysts and obtains the highest CO2 conversion at 25.87% and the selectivity of DME at 45.15% under the optimal reaction conditions of 533K,3.0MPa, H2/CO2=3/1 and GHSV=2400 mL/(h·gcat). We selected the suitable pressure was 3 MPa, airspeed was 1800 mL/(h-gcat).(4) The catalysts specific surface area, pore volume and average pore diameter increased with the introduction of Mg, Y and Zr additives. The introduction of additives, no other crystalline phase formated in the bifunctional catalysts, possessed higher dispersion and lower reduction temperature of CuO and corresponding to the better catalytic performance. Especially the introduction of Y addictive, the catalytic activity was higer than others under the conditions of 3.0 MPa, H2/CO2=3/1 and GHSV=1800mL/(h-gcat).(5) When the ratio of methanol synthesis catalyst and HZSM 5 molecular sieve was 2:1, the ratio of silicon to aluminum was lower, the bifunctional catalysts showed better DME selectivity and yield.