Comparison of the reactions of branched alcohols and aldehydes on Rh(111)

Alcohols adsorbed on the Rh(lll) surface have been suggested to decompose. via unstable surface oxametallacycle intermediates rather than via aldehydes. The chemistry of alcohols and aldehydes containing multiple methyl groups at the beta-position was examined in this study to determine whether metallacycle formation could be blocked. Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) studies demonstrated that complete substitution of beta-hydrogens with methyl groups did lead to common alcohol and aldehyde decomposition pathways. 2,2-Dimethyl-1-propanol and 2,2-dimethyl-1-propanal decarbonylated to deposit isobutene on the surface; the sequence of subsequent dehydrogenation steps was the same, whether adsorbed isobutene was generated from these oxygenates, from t-butanol, or by isobutene exposure. In contrast, partial substitution at the beta-position did not produce a common path for alcohol and aldehyde decarbonylation. 2-Methyl-1-propanol decomposition resulted in fragmentation of the hydrocarbon backbone of the molecule, generating C-1 and C-2 fragments from the reaction of the oxametallacycle intermediate. 2-Methyl-1-propanal, however, decarbonylated cleanly to form surface propylidyne intermediates, analogous to the chemistry observed for other aldehydes on Rh(lll). These results illustrate the importance of beta-CH activation in producing the oxametallacycle-mediated reaction pathways characteristic of alcohols on the Rb{111} surface.