Investigation of the synthesis of oxygenates from carbon monoxide/hydrogen mixtures on supported rhodium catalysts

The activity and selectivity of a range of promoted Rh/SiO2 catalysts for the reaction of CO/H2 mixtures have been investigated at high pressures. The activity and selectivity is found to depend on the choice of promoter. Pure rhodium is observed to give a high selectivity to methane but a low selectivity to methanol; a high selectivity to propane and acetaldehyde but a low selectivity to ethanol. Reducible metal oxide promoters result in a low selectivity to methane but a high selectivity to methanol; a low selectivity to propane and acetaldehyde but a high selectivity to ethanol. Alkali oxide promoters give a low selectivity to methane and methanol; a low selectivity to propane but a high selectivity to acetaldehyde; a low selectivity to ethanol but a high selectivity to acetic acid. For most catalysts there is a good correlation between the selectivities to propane and to acetaldehyde, and the implications of this observation for the mechanism of CO/H2 reactions are discussed. The results are consistent with a model in which oxygenated products are formed from a common surface acetyl intermediate. It is concluded that pure rhodium tends to produce hydrocarbons or acetaldehyde because of the relatively high activity for carbon monoxide dissociation combined with the relatively low concentration of surface hydrogen on the catalyst under reaction conditions. When promoted by reducible oxides, rhodium tends to produce ethanol because of the enhanced availability of hydrogen and the increased stability of the surface intermediates. Although it is believed that the rhodium/promoter interface is important in creating new types of active site, the hydrogenation of acetaldehyde at isolated promoter oxide sites by spillover hydrogen may also be important in certain circumstances.