Activation of C-H, C-C, and C-O Bonds of Oxygenates on Rh(111)

Decarbonylation of oxygenates to liberate CO, H₂, and volatile or adsorbed hydrocarbons has been observed on single crystal surfaces of a number of Group VIII metals. The details of this chemistry hold the promise of providing new insights into catalytic processes which are essentially its reverse, e.g., higher oxygenate synthesis from CO and H₂, and heterogeneous hydroformylation of olefins. The results of our studies of ten C₂ and C₃ oxygenates on Rh(111) demonstrate, however, that the decarbonylation network for these probe molecules is far more complicated than previously recognized. Higher aliphatic aldehydes release CO plus alkyl groups via decarbonylation on Rh(111), these alkyls are hydrogenated to volatile alkanes. Higher alcohols do not dehydrogenate to aldehydes, and do not release volatile hydrocarbon products. Parallels between the reactions of alcohols and epoxides suggest that both form oxametallacycle intermediates on the surface. The divergence of alcohol and aldehyde decarbonylation pathways persists for the unsaturated oxygenates acrolein and allyl alcohol; although both ultimately deposited CO, H, and CH₃C≡ species on the surface, only acrolein liberated volatile C₂ hydrocarbons. Of the ten oxygenates examined, decarbonylation reactions must release at least five different hydrocarbon ligands to the surface in order to account for the range of behavior observed in TPD and HREELS experiments. This suggests that the identities of the primary products of CO insertion on metal catalysts may depend on the identities of the surface hydrocarbon ligands undergoing insertion.