Study Of Cu-based Catalysts For Methanol Synthesis By CO₂ Hydrogenation

With an increase in carbon dioxide (CO2) concentration in the atmosphere, global warming problems are becoming more and more serious in recent years. Conversion of CO2 to useful chemicals is one of the most promising ways to utilize CO2. Methanol synthesis by CO2 hydrogenation is of great significance from the viewpoint of environmental protection, energy sources and chemical industry, because methanol is a common chemical feedstock for several important chemicals and a potential alternative energy to fossil fuels.In this work, the research progress of Cu-based catalysts for methanol synthesis from CO2 has been reviewed, and then the preparation method and composition of Cu-based catalyst and the mechanism of catalytic reaction were investigated. The results obtained are mainly as follows.1. Studies on the preparation methods for Cu-based catalystsCuO-ZnO-ZrO2 catalysts were prepared by the combustion method. The effects of fuel amount, fuel type and ignition manner on the properties of catalysts have been investigated. Many efforts have been focused on elucidating the relationship of the composition structure—property of the catalyst. The results show that the physicochemical and catalytic properties of CuO-ZnO-ZrO2 are affected strongly by fuel content and fuel type. Changing the fuel amount would lead to the variation of the combustion enthalpy, the duration of combustion and the amount of the gases evolved in the combustion process, finally resulting in a change of the combustion temperature, which is related closely to the properties of catalysts. The variation regularities of the properties of CuO-ZnO-ZrO2 with the change of fuel content are different for different types of fuel. The combustion intensity of citric acid-nitrate is markedly weak in comparison with glycine-nitrate and urea-nitrate. The reason can be ascribed to the difference in composition and structure of fuel.The maximum methanol yields of 9.6%,9.9% and 8.1% under the reaction conditions of T=220℃, P=3.0 MPa and GHSV=3600 h-1 were obtained over the CuO-ZnO-ZrO2 catalysts prepared by combustion method using urea, glycine and citric acid as fuel, respectively. In comparison with the catalysts prepared by co-precipitation method, the catalyst prepared by combustion method exhibit a higher activity for methanol synthesis. This is due to the high temperature produced during combustion process favoring an interaction between the components of catalyst in a short time. The results show that the catalytic properties of catalysts are related to the dispersion of Cu, the phase state of ZrO2 and an interaction between the compositions in catalysts. The combustion method is a simple, fast and effective method for the preparation of CuO-ZnO-ZrO2 catalysts, and it can be used to prepare other complex oxide powder.The CuO-ZnO-ZrO2 catalysts were synthesized by a route of solid-state reaction, and the effects of calcination temperature and the complexant amount on the properties of catalysts have been investigated. Furthermore, a mechanism of solid-state reaction was proposed. The results show that, the complexes of metal-citrate at ambient temperature can be formed by the solid-state reaction, because the crystallization water in hydrated metal salts decreases the lattice energy. The CuO-ZnO-ZrO2 catalyst can be obtained by the decomposition of the Cu-Zn-Zr citrate precursors. With an increase in calcination temperature, the dispersion of copper species decreases, resulting in a low catalytic activity for methanol synthesis from CO2 hydrogenation. When the transformation of t- ZrO2 to m- ZrO2 occurred in this catalyst, the CO2 conversion is reduced and the methanol selectivity is increased. The route of solid-state reaction is a solvent-free method that meets the requirement of green chemistry, and can be used to prepare the complex oxide catalysts.2. Studies on the composition of catalyst and the promoterThe effects of La2O3 and alkaline-earth oxide doping on the properties of CuO-ZrO2 catalysts were investigated and the role of ZnO in CuO-ZnO-ZrO2 was studied. The results show that, the presence of La2O3 affects the dispersion of Cu and the property of basic sites in the catalysts. With an increase in La loading, the Cu surface area takes on a volcano variation trend, and the amount and density of basic sites over CuO-ZrO2 catalysts increase continually. The conversion of CO2 depends on the surface area of metallic Cu, and there is a linear relationship between them. The methanol selectivity is related to the distribution of basic sites on the surface of catalyst. A suitable amount of La doing is beneficial for the catalytic activity of CuO-ZrO2 and a maximum methanol yield is obtained as the La loading is 5% of the total amount of Cu2+ and Zr4+. With the doping of alkaline-earth metal oxide, the surface area of Cu in catalysts is increased obviously, but the interaction between CuO and ZxO2 became weaker, leading to the rise in the reduction temperature of CuO, in which the effect extent is in the sequence of Mg<Ca<Sr<Ba. The doping of a suitable amount of Mg results in an increase in CH3OH selectivity and further an increase in CH3OH yield. The presence of ZnO in CuO-ZnO-ZrO2 catalysts can promote the crystallization of CuO, and depress the growth of CuO crystal and its conglomeration. While the amount of basic sites on the catalyst surface increases markedly by an introduction of ZnO, resulting in a remarkable increase of the catalytic activity. The highest activity can be obtained when the ratio of Zn/Zr is 2/3 in the CuO-ZnO-ZrO2 catalyst.3. Roles of carrier in the Cu-based catalyst and the reaction mechanisms for methanol synthesis from CO2 hydrogenationThe roles of the carrier in Cu-based catalysts were studied, and the valence of Cu and the intermediate species in the catalytic reaction were investigated by in-situ XRD and in-situ DRIFT techniques. The results show that there are three roles of the carrier in the Cu-based catalysts for methanol synthesis from CO2 hydrogenation:(ⅰ) the carrier can disperse the CuO component; (ⅱ) there is an electronic effect between the carrier and the CuO, which affects the reduction performance of CuO; (ⅲ) it is very important that the carrier participates the catalytic reaction directly as an active sites of adsorbing CO2 and reaction intermediate species. The results of in-situ XRD show that, the Cu species exists in the form of Cu0 during the catalytic process, and no Cu+ can be detected. With the in-situ DRIFT technique, the reaction intermediate species including bicarbonate, formate, surface-bound formaldehyde and methoxide can be detected, which were hydrogenated step by step to form methanol. The dual-site mechanism is reasonable for the methanol synthesis by CO2 hydrogenation over Cu-based catalysts.