Carbon-carbon bond formation on metal oxides: From single crystals toward catalysis

Abstract

Previous work in our laboratory indicated that carbonyl metathesis of aldehydes can be accomplished as a gas-solid reaction on a reduced TiO₂(001) single crystal under ultrahigh vacuum (UHV). This reaction requires the presence of cations in lower oxidation states on the surface, but does not require Ti⁰ as suggested by previous studies in the liquid phase. In the course of attempts to carry out this reaction catalytically, TPD experiments on TiO₂ single crystals and powders were conducted. In addition to the aldolization of acetaldehyde to crotonaldehyde, carbonyl metathesis of acetaldehyde to butene was observed. Benzaldehyde TPD on reduced TiO₂ powder gave substantial amounts of styrene (PhCH=CH₂) which is most likely formed by disproportionation of stilbene (PhCH=CHPh), the metathesis product observed in single crystal studies. Addition of Ti metal to titania resulted in the formation of large amounts of toluene (by unimolecular reduction) rather than stilbene or styrene (by bimolecular reductive coupling). More importantly, it was possible to form 1-phenylpropene (PhCH=CH-CH₃), a cross-coupling product, by the co-metathesis of benzaldehyde and acetaldehyde on reduced titania, ceria and iron oxide powders during TPD at atmospheric pressure. These studies represent an extension of stoichiometric reactions observed on single crystals in UHV toward novel catalysis. While carbonyl metathesis has still not been accomplished catalytically, its observation during TPD of aldehydes on dihydrogen-reduced oxides has demonstrated the feasibility of each of the steps required to close a catalytic cycle.