Hydrogenation of carbon monoxide over rhodium/silica catalysts promoted with molybdenum oxide and thorium oxide

Abstract

The promotion of silica-supported rhodium catalysts in the hydrogenation of carbon monoxide by molybdenum oxide and thorium oxide has been examined. Temperature programmed reduction studies indicated the formation of rhodium molybdates, while no evidence was found for the formation of such mixed oxides in the thorium oxide-promoted catalysts. Hydrogen and carbon monoxide chemisorption were suppressed by the presence of molybdenum oxide, pointing to a coverage of the rhodium particles by this promoter oxide. The catalysts with Mo:Rh ratios exceeding one even exhibited an almost complete suppression of the rhodium chemisorption capacity. In the thorium oxide-promoted catalysts the chemisorption of hydrogen and carbon monoxide were not suppressed. Infrared spectroscopy of adsorbed carbon monoxide showed that molybdenum oxide completely suppressed bridge-bonded and linearly bonded carbon monoxide, as well as the gemdicarbonyl species. Thorium oxide addition resulted in a minor decrease of the linearly bonded carbon monoxide, while the bridge-bonded carbon monoxide was suppressed to a greater extent. The IR spectra of the thorium oxide-promoted catalysts also exhibited a broad absorption band between 1300 and 1750 cm−1, which is thought to be due to carbon monoxide bonded with the carbon atom to the metal and with the oxygen atom to the promoter ion. Carbon monoxide hydrogenation was greatly enhanced by the presence of both molybdenum oxide and thorium oxide. Thorium oxide-promoted catalysts had a high selectivity to C2-oxygenates, while the molybdenum oxide-promoted catalysts exhibited a high methanol selectivity. Ethylene addition to a working catalyst showed that the carbon monoxide insertion reaction, which is thought to be responsible for the formation of oxygenates, was not enhanced by molybdenum oxide, nor by thorium oxide. The ethylene addition experiments indicated that the role of the promoter is to enhance carbon monoxide dissociation. The results can be understood by assuming that side-bonded carbon monoxide, with its weakened CO bond, is responsible for the higher carbon monoxide dissociation activity.