The direct source of carbon in ethanol formation over promoted copper catalysts

Abstract

Abstract The synthesis of methanol and higher alcohols has been studied for seven decades; over the mixed oxide catalysts used prior to the 1960's and over the modern Cu/ZnO catalysts. Despite the vast effort devoted to the study of the methanol synthesis and the smaller but significant effort devoted to higher alcohol formation, there remains a lack of consensus about the mechanisms and active sites involved in these closely related reactions. Results obtained by the authors, combined with the information in some of the recent literature provides a variety of evidence (including in situ spectroscopic, adsorption, kinetic, and isotope labelling information) that allows the proposal of a surprisingly complex scheme of reactions for alcohol formation on alkali-promoted Cu/ZnO catalysts. Methanol is formed from CO at copper/alkali interfaces, while methanol is also produced from CO 2 , but on copper sites which are not associated with the alkali component of the catalyst. Carbon-13 labelling studies have recently provided evidence that both methanol and CO are direct precursors to the C 1 species that lead to ethanol production over these catalysts. However, CO 2 produces kinetic effects on the higher alcohol synthesis (HAS) which are almost identical to those it produces on methanol synthesis, suggesting that CO 2 may also participate in the HAS. Spectroscopic evidence has demonstrated that CO and CO 2 both produce formate (possibly formaldehyde) and methoxy species, and participation of some of these species in both the methanol and higher alcohol syntheses is consistent with the observations of recent kinetic and 13 C labelling studies.