Methanol Synthesis

This paper reviews the results of many years work in ICI into the kinetics and mechanism of methanol synthesis over copper/zinc oxide/alumina and other oxide supported catalysts. It was carded out under the precept that an understanding of the mode of action of the catalyst was a pre-requisite to the systematic improvement of the catalyst. The kinetics and mechanism of methanol synthesis over the multicomponent catalyst have been elucidated by the following process. The adsorptive and reactive interactions of each of the gas phase species (CO, CO2, H2) with each of the catalytic components (Cu, ZnO, Al2O3) separately have been studied. The interactions of combinations of the gas phase species with the individual catalytic components and then with pairs of the catalytic components etc were then studied, ultimately building up to a study of the full system. What has emerged is that it is the CO2 component of the CO, CO2 mixture which is the immediate precursor to methanol, being adsorbed on the partially oxidised copper as a symmetric carbonate. This carbonate is hydrogenated on the copper component of the catalyst to a formate species, the most stable and longest lived intermediate to methanol, hydrogenation of the formate being the rate determining step of the reaction. The specific activity of copper in synthesising methanol is unaffected by the nature of the oxide support, indicating that no unique synergy attaches to the copper/zinc oxide combination. The role of the CO is to keep the copper in a more reduced (more active) state than could be achieved with hydrogen alone.