Carbon Dioxide Hydrogenation Direct Synthesis Of Dimethyl Ether Bifunctional Catalyst Activity

Dimethyl Ether(DME)is not only an important chemicals,but also an ideal substitute for diesel or liquefied petroleum gas. Furthermore, DME is also an important chemical feedstock for the preparation of light alkenes.So,the application prospect of DME becomes broader and broader. In recent years, DME synthesis from synthesis gas has attracted much attention in research fields and industrial fields for its low cost and low energy consumption.Carbon dioxide, the richest carbon resource in the world,is a greenhouse gas which influences the global environmental in a significant manner.On the other hand, the yield of crude oil will decrease due to the growth in demand being faster than the newly detected storage. New methods of sources utilization must be exploited to balance the great demand of energy. Consequently, the development of clean fuels as a substitute for the fossil fuels is suggested in order to reduce the discharged amount of carbon dioxide. The technological development for direct synthesis of DME via CO2 hydrogenation is of great importance to utilization of CO2 resources, to dispel the greenhouse effect of CO2 to improve the current structure of power source, and to promote the utilization of reproducible energy.The synthesis of DME by CO2 hydrogenation is a complicated process:it consisted of the CH3OH synthesis step by CO? hydrogenation and the dehydration step of formed CH3OH to DME. In this thesis, the Cu-based CO2 hydrogenation component and the HZSM-5 zeolite dehydration component were selected as main research object to constitute the hybrid catalyst for direct DME synthesis, the effects of preparation conditions of catalyst, including precipitation temperatures, promoters, combination modes of two different functional components, calcinations temperatures and MgO-modified HZSM-5 zeolite on the performance of hybrid catalyst were investigated systematically. Using the techniques of BET,XRD,TG-DSC,H2-TPR,NH3-TPD,some physic-chemical properties of the hybrid catalyst samples were characterized more roundly.The catalytic performances of hybrid catalyst samples were evaluated in a continuous flow pressurized slurry reactor-gas chromatographic apparatus combined equipment under various temperature, various pressure, various H2/CO2 and various gas hourly space velocity. At the beginning of the dissertation, effects of methanol synthesis components on the synthesis of DME from CO2 hydrogenation were further investigated in this dissertation. Among the catalyst samples prepared with different methods, such as dry-mixing, wet-mixing, coprecipitation-sedimentation and coprecipitation-impregnation, the catalyst prepared with coprecipitation-sedimentation method showed better catalytic performance for DME synthesis. Among the composite catalyst samples prepared by adding different promoters to the methanol synthesis component, the catalyst sample promoted with ZrO2 showed more ideal catalytic activity. Because the presence of ZrO2 promoted the dispersion of CuO-ZnO-Al2O3/HZSM-5 catalyst and decreased the reduction temperature of CuO, especially a small amount of ZrO2 added to the catalyst greatly improved the conversion of CO2 and the selectivity of DME. When the suitable precipitation temperature is70℃, the obtained catalyst samples showed higher specific surface area and better dispersion degree, which could contribute to its better catalytic performance for target reaction.The calcination temperature also affected some physic-chemical performances,such as the composition and structure of crystal phases,the reduction property, the specific surface area and the catalytic activity of the catalyst samples.The catalyst sample calcined at 400℃showed larger specific surface area,and was reduced more easily,which could contribute to its better catalytic performance for target reaction.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,can the bifunctional catalysts exhibited excellent catalytic performance. Aiming at this catalytic character, effects of methanol dehydration components on the synthesis of DME from CO2 hydrogenation were investigated. The bifunctional catalysts were prepared via coprecipitation-sedimentation method,using theCuO-ZnO-Al2O3 catalyst as methanol synthesis component, using various solid acid catalysts HZSM-5 and MgO modification of HZSM-5 as the methanol dehydration component. It was found that compared with the composite catalyst sample prepared with unmodified HZSM-5, a series of HZSM-5zeolites modified with various contents(0～10.0%wt)of magnesium oxide(MgO)were prepared by impregnating with aqueous solutions of magnesium acetate showed better catalytic activity, the modification of HZSM-5 with a suitable amount of MgO can significantly enhance the selectivity for DME from 33.2% to more than 45.1%,while the selectivity for the undesired byproducts like CO and hydrocarbons from 50.8% and 6.5% to less than40.5% and 0.1%, respectively. However, when the MgO contents were equal to or higher than 5.0wt%, the selectivity for DME decreased markedly because of the decreased activity for methanol dehydration, the decrease in the formation of hydrocarbons and CO can be attributed to the significant decline in the amount of strong acid sites. In addition, increasing reaction temperature was beneficial to improve the conversion of CO2, while decreased the selectivity of DME, increasing pressure and H2/CO2 molar ratio were contributable to improve the conversion of CO2 and the selectivity of DME, and increasing the space velocity would decrease both the conversion of CO2 and the selectivity of DME.