From CO2 to Methanol on Cu/ZnO/Al2O3 Industrial Catalyst. What Do We Know about the Active Phase and the Reaction Mechanism?

For more than half a century, the industrial production of methanol in the world has been based on a heterogeneous Cu/ZnO/Al2O3 catalyst where Cu nanoparticles are dispersed on an alumina support with ZnO particles as promoters. The reaction starts with syngas, a mixture of CO, CO2, and hydrogen, and the hydrogenation of carbon dioxide is an essential part of the process. Despite the technological importance and the high number of studies dedicated to identifying the structure of the functioning catalyst, there are still several open questions on the nature of the active sites and phases. Over the years, various hypotheses have been formulated. The most accredited versions concern the formation of CuZn alloys, the migration of ZnO over the Cu nanoparticles which become covered by an ultrathin oxide layer, and the role of the Cu-ZnO interface and, in particular, of the presence of Cu+ ions. The role of alumina as a promoter has also been widely discussed, suggesting that the oxide provides acidic sites where intermediates are stabilized or that Al dopants enter the ZnO phase, altering its properties. The reality, as discussed in this brief overview, is that all these mechanisms are probably contributing but under different conditions of temperature, pressure, and reagent composition. This results in a dynamic catalyst that makes unambiguous identification of active sites difficult, if not impossible. Less controversial is the reaction mechanism, although even in this case the path followed by the reaction is a delicate function of various conditions, including the composition of the reaction mixture and the presence of water or other contaminants.