Catalytic conversion of CO2 to methanol over Cu/Zn/Al catalysts promoted with cerium and lanthanum
Abstract
Carbon dioxide is one of the most prominent greenhouse gases, and the rise in its emissions iscausing an increase in the world’s temperature, which can have catastrophic effects if it is notcontrolled. Governments and organizations are working hard to implement sustainablesolutions to address the issue of climate change. Among proposed solutions, the capture of CO2and its recycling to valuable fuels and chemicals has grabbed the attention of scientists. CO2hydrogenation to methanol is a promising technological solution as methanol is an importantcomponent in the fuel sector and carbon utilization is an environment friendly way to reducethe emissions.Heterogenous catalytic systems are applied to convert CO2 to methanol, and the commercialcatalyst, Cu/ZnO/Al2O3, is one of the most used catalysts because of its relatively highperformance and relatively low cost compared to other heterogenous catalyst. However,promoters were added to this catalyst to improve its performance and make it effective forindustrial utilization of CO2 hydrogenation to methanol. Cerium and lanthanum precursorsshowed good performance when added to the Cu/ZnO/Al2O3 catalyst when the catalyst isprepared with the co-precipitation method. However, it was demonstrated that more promisingperformance could be obtained when lanthanum was added to the conventional catalyst througha sequential preparation method. In this study, two series of catalysts were prepared by thecoprecipitation method using different amounts of cerium and lanthanum as promoters. Theperformance of the promoted catalysts was compared to a conventional Cu/ZnO/Al2O3 catalyst.Furthermore, a La promoted catalyst was also prepared using a sequential preparation methodby impregnating the Cu/ZnO/Al2O3 catalyst with lanthanum. The catalysts were characterizedby XRD, TPR, TEM, N2 adsorption-desorption, and N2O chemisorption.The catalytic performance of the catalysts was tested in a fixed bed reactor at 230°C, 40 bar,and H2/CO2/N2 ratio of 3/1/1. The lanthanum promoted Cu/ZnO/Al2O3 catalyst prepared by thesequential preparation method showed the highest methanol selectivity of 49.9% and space timeyield of 1357 mmol·gcat-1·h-1. For the catalyst prepared by co-precipitation, the catalystscontaining 2% cerium and 4% lanthanum showed the best catalytic performance. It was alsoobserved that the promoted catalysts showed superior performance at different reactionconditions (temperature, pressures, and gas-hour space velocity) compared to the unpromotedcatalyst. Furthermore, long-term tests over 72 h indicate that the stability of the catalyst isenhanced for the La and Ce promoted catalysts.