SYNTHESIS OF STILBENE FROM BENZALDEHYDE BY REDUCTIVE COUPLING ON TIO2(001) SURFACES

Reductive coupling to form stilbene is the dominant reaction of benzal dehyde on TiO2(001) single-crystal surfaces reduced by argon ion bomba rdment. Stilbene yields of 56% of the adsorbed benzaldehyde layer and selectivities up to 72% of the benzaldehyde reacted were obtained in t emperature programmed desorption experiments under ultrahigh vacuum co nditions. Reductive coupling requires a reduced surface. The stilbene yield;decreased by an order of magnitude when the TiO2(001) surface wa s oxidized by annealing at 850 K prior to benzaldehyde adsorption. The extent of surface reduction was determined from the populations of ti tanium cations in different oxidation states quantified by X-ray photo electron spectroscopy. The activity of the surface for reductive coupl ing was found to track the extent of surface reduction as the latter w as varied either by annealing or ion-bombardment procedures. However, in contrast to the mechanism commonly proposed for reductive coupling of aldehydes with titanium-based reagents in liquid-solid slurries, no Ti-0 was observed by XPS even on the most active surfaces. The active site required for gas-solid reductive coupling is an ensemble of Ti c ations in the +1, +2, and +3 oxidation states which collectively effec t this four-electron reduction. These results demonstrate novel carbon -carbon bond formation on oxide surfaces in UHV and illustrate the app lication of surface analytical techniques to probe the sites of organi c synthesis at the fluid-solid interface.