Study Of Rh-based Catalysts For C₂Oxygenates Synthesis By CO Hydrogenation

With the escalating oil crisis, how to develop new technology route to synthetize fuels and chemical raw materials has become a focus of scientific research. Ethanol and other C2oxygenates synthesis by CO hydrogenation is of great significance on the basis of the viewpoint of environmental protection, energy sources and chemical industry, because they are not only the best alternative fuel of the future, but also an important chemical raw material and clean source of hydrogen.In this dissertation, the research progress of Rh-based catalysts for C2oxygenates synthesis by CO hydrogenation has been reviewed, and then the effects of promoters, preparation methods of catalysts, and properties of SiO2on the catalytic performance of Rh-based catalyst were investigated. The results obtained are mainly as follows.1. The effects of methods for introducing Mn and La on the catalytic performance of Rh/SiO2catalystsThe effects of impregnation sequences for Rh and Mn upon SiO2on the catalytic performance of Rh-Mn/SiO2were investigated. When Mn was impregnated second onto a calcined Rh/SiO2catalyst, the Rh-Mn interaction enhances, which can increase the high activated Rh-Mn interface and promote the formation of tilted CO species, resulting in a high CO hydrogenation activity and selectivity of C2+oxygenates.The effects of La doping on the catalytic performance of Rh/SiO2during CO hydrogenation were investigated. When the La doped into the support of SiO2, the Rh dispersion is improved, thus the selectivity of oxygenates can be increased, such as the methanol selectivity and C2+oxygenates selectivity reach40.5%and37.6%, respectively. When La is co-impregnated with Rh, the effective Rh-La interface increases because of the strong Rh-La interaction, which promotes the selectivity of C2+oxygenates.2. The effects of promoters of Mn and Li on the catalytic performance of Rh/SiO2catalystsThe effects of Mn and Li on the catalytic performance of Rh/SiO2catalysts as well as their role in the catalyst were studied. The results show that, the presence of Mn and Li can improve the CO conversion and selectivity of C2oxygenates. Since the catalyst containing1.5wt.%of Rh,1.5wt.%of Mn, and0.075wt.%of Li has a strong ability of CO adsorption, slow desorption rate of adsorbed CO and high activity of hydrogenation, the highest selectivity of C2oxygenates can be obtained, that is, the yield of C2oxygenates reaches309.1 g/(kg-h).It has been found that the doping of Mn and Li inhibits the reduction of Rh. When the ratio of Rh versus Mn is1:1, the (RhxRhy)-Mn active structures can be formed to promote the CO dissociation. The presence of Mn enhances the ability of CO adsorption, and the doping of Li weakens the CO adsorption capacity. The doping of Mn and Li both can suppress the desorption of adsorbed CO and promote the H2dissociation and reaction. When the contents of Mn and Li are too high, the promotional effect of Mn and Li on the H2dissociation can be weakened. According to the mechanism of C2oxygenates synthesis by CO hydrogenation, the increase in the ability of CO adsorption and hydrogenation is beneficial to CO conversion, and the decreased desorption rate of adsorbed CO is conducive to CO insertion, which promotes the formation of C2oxygenates.3. The effects of Fe doping on the catalytic performance of Rh-Mn-Li/SiO2catalystsThe effects of Fe doping on the catalytic performance of Rh-Mn-Li/SiO2were investigated. The results show that, the transformation of adsorbed CO and formation of C2+oxygenates are promoted at the lower Fe loading (Fe≤0.1wt.%), and at the higher Fe loading (Fe>0.1wt.%), the CO conversion decreases due to the decreasing its ability of CO adsorption. The facile transformation of dicarbonyl Rh+(CO)2into H-Rh-CO and Rh-CO-Fe is responsible for the higher selectivity of C2+oxygenates over the catalyst, in which Fe was impregnated first followed by Rh-Mn-Li impregnation. Moreover, the presence of Fe can increase the hydrogenation of acetaldehyde to ethanol, because of its high hydrogenation ability.4. The effects of properties of SiO2on the catalytic performance of Rh-based catalystsThe effect of nature of SiO2on the catalytic performance of the Rh-Mn-Li/SiO2catalyst for C2oxygenates synthesis by CO hydrogenation was investigated. The results show that the types and amount of surface hydroxyl groups on the SiO2can affect the properties of Rh particles and the interaction among Rh, Mn and Li, which further affects the catalytic performance of Rh-Mn-Li/SiO2catalysts.Compared with the SiO2synthesized by the sol-gel and industrial methods, in the reaction condition of300℃,3.0MPa and GHSV of10000h-1, the Rh-Mn-Li catalyst supported on the SiO2synthesized by the Stober method exhibits an excellent CO conversion (8.2%) and higher selectivity to C2+oxygenates (59.1%). The results show that, the weakly H-bonded hydroxyls on the SiO2prepared by the Stober method is favorable for the moderate Rh-Mn interaction and weakens the Rh-CO bond strength, which facilitates the transformation from Rh+(CO)2into Rh-CO and desorption/reactivity of adsorbed CO, which accelerates the CO conversion and formation of C2+oxygenates.The effects of the calcination temperature and ammonia concentration in the preparation of SiO2by the Stober method on the surface properties of SiO2and catalysts were investigated. The results show that, an appropriate amount of surface Si-OH groups on the SiO2calcined at350℃can gain a moderate Rh-Mn interaction, which can achieve the right ability of CO adsorption and dissociation, is beneficial to the reaction of CO insertion, resulting in a higher C2+oxygenates selectivity. On the other hand, if the ammonia concentration in the preparation process is too low, the surface of prepared SiO2is uneven, the supported Rh particles will grow up during the reaction, and the Rh-Mn interaction is weakened, which promotes hydrogenation rather than the formation of C2+oxygenates.