Catalytic conversion of CO2 to methanol via CO2 hydrogenation : An investigation of metal oxide promoted catalysts

Abstract

CO2 hydrogenation to methanol is a promising process for converting renewable energy into valuable fuels and chemicals, which can combat the emissions of greenhouse gases associated with the use of fossil resources. Developing novel reactors to overcome the severe thermodynamic restrictions and designing high-performance catalysts are of vital importance for the industrial implementation of the CO2-to-methanol process. In this work, a thorough investigation of the thermodynamics and the role of metal oxide on the activity of Co- and Cu-based catalysts for CO2-to-methanol is carried out. First, the most promising catalyst systems for the industrial implementation of the CO2-to-methanol process are reviewed. For the conventional Cu-based catalysts, the interaction between the active metal and the metal oxide promoters is a determining factor for the methanol synthesis activity of the catalyst. Alloying and metal-oxide interaction also play a determining role in the performance of the catalysts. The interaction between the main active component and the promoter(s) is also a determining factor for the performance of other methanol synthesis catalysts based on In2O3 or other transition metals. Thereafter, a comprehensive thermodynamic analysis of CO2 hydrogenation to methanol/CO and methanol/dimethyl ether/CO is performed. It is demonstrated that product condensation occurs at relevant reaction conditions for the CO2-to-methanol process, which could be utilized to bypass the thermodynamic restrictions on the methanol yield. The condensation of products allows almost complete conversion of CO2 into methanol and increases the methanol selectivity. Another option to increase the CO2 conversion is to produce methanol and dimethyl ether in a single-step process. Product condensation also improves the yield of methanol and dimethyl ether. The new catalyst system comprising of Co-Mn oxides is investigated. Mesoporous Co, Mn, and Co-Mn spinel oxide catalysts is prepared by a modified sol-gel inverse micelle method. The activity tests reveal that the Co-Mn oxide catalysts contain highly active sites for methanol synthesis that are not present on the monometallic Co and Mn oxide catalysts. Furthermore, the Co-Mn oxide catalyst exhibits very high methanol formation rates at low pressure compared to conventional Cu-based catalysts. Thus, the Co-Mn oxide system is a promising candidate for the low-pressure methanol synthesis process. However, further effort is needed to limit the formation of hydrocarbons to reduce the high methane selectivity. For Cu based catalysts, the role of the Cu-ZnO interaction is studied by comparing the performance of Cu/ZnO/Al2O3 obtained from a hydrotalcite-like (HT) precursor to that of a malachite-derived Cu/ZnO catalysts. The HT-derived catalysts contain Cu particles partially embedded within a Zn-Al oxide matrix. The results show that the stronger Cu-ZnO interaction of the HT-derived catalyst increases the intrinsic activity and methanol selectivity. Sintering of the Zn-Al oxide phase during long-term tests is observed to decrease the methanol formation rate and selectivity. The influence of In promotion is also investigated. The addition of In can stabilize the Zn-Al oxide phase but the presence of In on the Cu surface seems to inhibit the active sites. The results demonstrate that optimizing and stabilizing the Cu-ZnO interaction is crucial to enhance the performance of Cu/ZnO-based catalysts. The Cu-oxide interaction is further investigated to elucidate the role of Zn, Zr, and In oxide as promoters for CO2 hydrogenation to methanol. The activity of Cu/ZnO and Cu/ZrO2 is strongly linked to the Cu-oxide interaction. A facile approach to increase the activity of Cu/ZnO is presented in which impregnating a small amount of ZrO2 onto the catalyst can increase the activity and methanol selectivity, which is attributed to the formation of Cu-ZrO2 interfacial sites. It is found that In inhibits the active sites but increases the methanol selectivity of Cu/ZnO. However, new active sites for methanol synthesis are present for the In-doped Cu/ZrO2 catalyst. This is attributed to the formation of In-Zr oxide sites that enhance the methanol formation rate, methanol selectivity, and stability of the catalyst above 250 °C. This study highlights that tuning both the Cu-oxide and oxide-oxide interaction is key to develop more active and stable Cu-based catalysts.