INVESTIGATION OF THE REACTIONS OF ACETALDEHYDE ON PROMOTED RHODIUM CATALYSTS

The reactions of acetaldehyde and ethanol on a series of supported rhodium catalysts have been investigated under different experimental conditions. The results show that pure rhodium is a poor catalyst for the hydrogenation of acetaldehyde especially in CO/H2 mixtures. In contrast, iron oxide promoted rhodium is an extremely efficient catalyst for the hydrogenation of acetaldehyde even in the presence of carbon monoxide at low temperatures. The results are consistent with the suggestion that in the synthesis of C2-oxygenates from CO/H2 mixtures on rhodium catalysts, acetaldehyde is a primary product from which ethanol is easily produced in a secondary hydrogenation reaction. Evidence from experiments using physical mixtures of Rh/SiO2 and Fe/SiO2 show a clear synergy in the hydrogenation of acetaldehyde. It is concluded that in CO/H2 reactions ethanol may be produced either by the hydrogenation of a surface species related to adsorbed acetaldehyde or by hydrogenation of acetaldehyde at isolated iron oxide sites. In the former case, sites at the Rh/oxide promoter interface are likely to be of importance, while in the latter case hydrogen spillover from rhodium to iron oxide sites is thought to be necessary for rapid hydrogenation of acetaldehyde to occur. Infrared measurements using an in situ cell show that the nature of the adsorbed species present on the rhodium surface under reaction conditions is affected by the promoter. When acetaldehyde is injected over the pure Rh/SiO2 catalyst, no adsorbed ethanol is detected, whereas with the iron oxide promoted catalyst strong bands due to adsorbed ethanol are observed. It is concluded that the hydrogenation of acetaldehyde, or equivalent adsorbed species, is controlled by the availability of surface hydrogen. One function of the oxide promoter may be to increase the amount of available hydrogen by acting as a reservoir for spillover hydrogen.