| The greenhouse effect is worsening because of a large number of cabon dioxide emissions. Make "the twenty-first century clean fule" dimetyel ether by greenhouse gas caobon dioxide hydrogenation, can not only control the emissions of carbon dioxide but also promote economic development.The reaction of carbon dioxide hydrogenation to dimethyl ether directly system is rather complicated, which mainly includes three reactions, i.e., methanol synthesis, methanol dehydration reaction and inverse water-gas shift reaction. In this paper, the thermodynamics of the reaction system were firstly analyzed to give the theoretical basis for catalytic activity evaluation. Synthesis of dimethyl ether by hydrogenation of carbon dioxide includes methanol synthesis catalyst and methanol dehydration catalyst, methanol synthesis catalyst is studied in this paper, which includes three parts, Cu, Zn-based catalysts is the main catalyst, ceria-zirconia solid solution is additives, HZSM-5 zeolite as a carrier. Methanol synthesis catalysts were mixed with HZSM-5 zeolite to synthesize dimethyl ether synthesis bifunctional catalysts by physically mixed. Based on the technologies of nitrogen adsorption-desorption(BET), X-ray diffraction(XRD), hydrogen temperature programmed reduction(H2-TPR), ammonia temperature programmed desorption(NH3-TPD), activity evaluation on a high-temperature and high-pressure fixed-bed reactor, catalytic properties of the bifunctional catalysts was systemically investigated.In thermodynamic analysis of direct synthesis dimethyl ether reaction system, effects of temperature, pressure and hydrogen-carbon ratio on the reaction system were studied. The analysis calculated results show that with the temperature rising, the conversion of CO2 decrease at first, and then increase; CO selectivity increase gradually, DME selectivity decrease gradually, equilibrium concentration of water increase with temperature decreasing, the equilibrium concentration of other products affect little by temperature; increasing pressure can improve the CO2 conversion and DME selectivity, with the pressure increasing, CO selectivity decrease rapidly, water equilibrium concentration increases, CO equilibrium concentration decrease, the equilibrium concentration of methanol and DME are subject to pressure; increasing the hydrogen-carbon ratio in suitable range could improve the CO2 conversion and DME selectivity of the reaction system, the theory of hydrogen-carbon ratio is 3:1, the equilibrium concentration are affected little by hydrogen-carbon ratio.Carbon dioxide hydrogenation to methanol catalysts was prepared by co-precipitation deposition method, In the catalyst preparation process, the precipitation temperature, Ce-Zr atomic ratio, ceria-zirconia content and calcination temperature have a great influence on the performance of the catalyst.In this paer, six different carriers(TiO2, SiO2,γ-Al2O3, and Si-Al ratio of HZSM-5 zeolite 25,38,50) on the catalytic properties were studied, the results show that the Si-Al ratio of 38's zeolite as carrier have the best catalytic effect, and the further investigation of the carrier zeolite content shows that 30% content is optimal.Based on the influence of carriers, a series of different Ce/Zr ratio and ceria-zirconia content catalysts were prepared, and focusing on the Ce-Zr atomic ratio and the ceria-zirconia content on the catalytic properties. Five different Ce-Zr atomic ratio catalysts (Ce/Zr=1:0, Ce/Zr=3:1, Ce/Zr=1:1, Ce/Zr=1:3 and Ce/Zr=0:1) were studied firest. Different Ce-Zr atomic ratio will form different solid solutions, the study found that there is a optimum value of Ce-Zr atomic ratio, when Ce/Zr=1/1, the catalyst has best catalytic performance, and the further investigation on the effect of ceria-zirconia content (0%-49%) on catalytic performance when Ce/Zr=1/1, the results show that appropriate solid solution can prevent grain growth and promot dispersion of CuO and ZnO. But a higher content of solid solution will lead to CuO form pluralism, which is bad for the reaction. Activity evaluation results showed that 21% of the ceria-zirconia content is appropriate.In this paer, we also focuses on investigation of preparation conditions on the catalytic properties. First of all, influence of precipitation temperature in the range of 30℃-80℃was studied. Precipitation temperature directly affects the generation rate of nucleation and growth rate, low temperature is conducive to the formation of small particles, but it is not conducive to the growth of nuclei, high temperature is exactly the opposite. It was found that the surface area of the catalyst prepared under the conditions of 70℃precipitation was up to 135.69m2/g, characterization results also proved the results that CuO was most evenly distributed, and easy to be restored with best catalytic performance. And then calcined the catalyst precursor in the range of 300℃-700℃, we found that the catalyst precursor calcined at 500℃has the best catalytic performance. Based on the above study, it concludes that after the prepared condition of carrier HZSM-5 (Si/Al=38) is 30%, Ce/Zr=1/1, ceria-zirconia content is 21%, precipitation temperature is 70℃, and calcination temperature is 500℃, the catalyst has the best activity, and carbon dioxide conversion rate is 22.5%, dimethyl ether selectivity is 46%. |
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