Preparation And Catalytic Performance Study Of Cu-based Catalysts For Slurry Methanol Synthesis From CO₂ Hydrogenation

It is well known that CO₂ is not only one of the main ―greenhouse gases‖, but also a cheap, nontoxic and abundant C1 feedstock. Therefore, chemical utilization of CO₂ has attracted considerable attention. The majority of the present research focuses on catalytic conversion of CO₂ to methanol because methanol is an important raw material in the chemical industry and can be utilized directly as a fuel additive or a clean fuel. Slurry phase methanol synthesis has been known as a potential process for large-scale methanol production because it has the advantages of high heat transfer efficiency, high conversion per pass, convenient on-line catalyst replacing and low operation cost. The stability of Cu/ZnO/Al₂O₃/Zr O₂ catalysts via hydrotalcite-like precursors, which possessed high catalytic activity for methanol synthesis from CO₂ hydrogenation in the fixed-bed reactor, was much low in slurry phase reactor due to the low attrition resistance.In the present dissertation, the influences of types and content of structural modifiers on the mechanical strength, physiochemical properties and the catalytic performance of Cu/ZnO/Al₂O₃/Zr O₂ catalysts for slurry methanol synthesis from CO₂ hydrogenation were systematically studied. In addition, micro-spherical SiO₂ support was obtained by spray-dried method and SiO₂ supported Cu/ZnO-based catalysts were successfully prepared by ammonia-evaporation methods which exhibited better resistance to attrition and higher activity. According to the above research, the main conclusions could be gained as follows:（1） The low stability of Cu/ZnO/Al₂O₃/Zr O₂ catalysts pepared by direct spray-dried process for slurry methanol synthesis from CO₂ hydrogenation was mainly attributed to the weak attrition resistance. The attrition resistance of spray-dried Cu/ZnO/Al₂O₃/Zr O₂ catalysts via hydrotalcite-like precursors was enhanced significantly by the introduction of the precipitated SiO₂, binder SiO₂ and binder alumina sol during the preparation process. With the incorporation of these structural additives, the hydrotalcite-like structure was still maintained and the Cu dispersion of Cu/ZnO/Al₂O₃/Zr O₂ catalysts increased. However, the activity for methanol synthesis from CO₂ hydrogenation decreased markedly with the introduction of precipitated SiO₂ and binder SiO₂, which was probably attributed to the formation of large amounts of copper phyllosilicate. The effect of binder alumina sol on the activity was small.（2） The content of alumina sol in Cu/ZnO/Al₂O₃/Zr O₂ catalysts would play a vital import on the physicochemical properties and catalytic activity. A series of Cu/ZnO/Al₂O₃/Zr O₂ micro-spherical catalysts with different weight percentages of alumina sol from 0 to 20 wt% were prepared by spray drying method. The as-obtained catalysts were first tested in the fixed-bed reactor for CO₂ hydrogenation to methanol. When the added alumina sol was below 10 wt%, the CO₂ conversion was close to the equilibrium conversion of CO₂（26.5%） at 523 K and 5 MPa. Further increase of alumina sol content decreased the activity due to the decreased Cu surface area and weakened interaction among Cu and ZnO. When used in slurry bed, attrition resistance of catalysts played an important role to determine the final catalytic performance. The catalyst without addition of alumina sol possessed low attrition resistance, which led to catalyst loss and deactivation during the reaction process. The introduction of suitable amount of alumina sol could enhance the attrition resistance of the catalysts markedly, and the catalytic activity could be still maintained at a high value. The spray-dried Cu/ZnO/Al₂O₃/Zr O₂ catalyst with addition of 10 wt% alumina sol exhibited the best performance with high activity and high stability.（3） In order to enhance the specific surface area and the availability of functional components of catalysts, a series of SiO₂ microsphere supported Cu/ZnO/SiO₂ catalysts with different loadings of Cu and ZnO were prepared.Typically, micro-spherical support SiO₂ with good attrition resistance was firstly synthesized by the spray-dried method, and then the Cu and ZnO components were loaded on SiO₂ support by ammonia-evaporation methods. As a result, the Cu and ZnO loading played an important role on the physicochemical properties and catalytic performance of catalysts. The outside surface of catalysts was smooth when the loadings of Cu and ZnO were below 39.36 wt%, whereas some active species were existed on outside surface of catalysts when further increase of the Cu and ZnO content. In addition, the specific surface area of catalysts increased with increasing Cu and ZnO loadings due to the formation of porous structure inside support SiO₂ when the loaded Cu and ZnO was below 39.36 wt%, while it decreased with further increase of Cu and ZnO content. Moreover, the reducibility was weakened with the weight percentages of Cu and ZnO increasing from 10.03 to 47.91 wt%, and the reduction peaks due to different Cu species appeared when the loading of Cu and ZnO was 58.23 wt%. Different from hydrotalcite-like compounds derived catalysts, both Cu+ and Cu0 existed on the reduced surface of Cu/ZnO/SiO₂ catalysts. The catalytic performance enhanced gradually with increasing Cu and ZnO loadings up to 28.23 wt%, whereas it changed slightly when further increase of the Cu and ZnO content. These results also provided evidence for the fact that both Cu+ and Cu0 species were the active sites over Cu/ZnO/SiO₂ catalysts for CO₂ hydrogenation to methanol.