Mechanism of formation of C 2-oxygenated compounds from CO + H 2 reaction over SiO 2-supported Rh catalysts

Abstract

The mechanism of acetaldehyde and ethanol formation from the CO + H 2 reaction below atmospheric pressure has been investigated by combining infrared spectroscopic measurement and 13CO and C 18O isotopic tracer studies with reaction kinetics. The rates of acetaldehyde and ethanol formation are markedly dependent on the nature of metal precursors employed. The addition of sodium cations depresses the total catalytic activity, while the selectivity for ethanol is increased by the addition of manganese cations. From the behavior of surface species under reaction conditions, it is concluded that acetaldehyde is formed through the following two steps: (i) CO insertion into C 1 species which are reaction intermediates for not only hydrocarbons but also for the methyl group in acetaldehyde, and (ii) subsequent formation of acetate ions whose one oxygen atom is supplied from the support, finally producing acetaldehyde. Differences in 18O distribution in acetaldehyde and ethanol during the C 18O + H 2 reaction indicate that ethanol is not produced via direct hydrogenation of acetaldehyde.