Synthesis of Stilbene from Benzaldehyde by Reductive Coupling on TIO2(001) Surfaces

Hicham Idriss, K G PIERCE, M A BARTEAU

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Abstract

Reductive coupling to form stilbene is the dominant reaction of benzaldehyde on TiO2(001) single-crystal surfaces reduced by argon ion bombardment. Stilbene yields of 56% of the adsorbed benzaldehyde layer and selectivities up to 72% of the benzaldehyde reacted were obtained in temperature programmed desorption experiments under ultrahigh vacuum conditions. Reductive coupling requires a reduced surface. The stilbene yield;decreased by an order of magnitude when the TiO2(001) surface was oxidized by annealing at 850 K prior to benzaldehyde adsorption. The extent of surface reduction was determined from the populations of titanium cations in different oxidation states quantified by X-ray photoelectron spectroscopy. The activity of the surface for reductive coupling was found to track the extent of surface reduction as the latter was 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 cations in the +1, +2, and +3 oxidation states which collectively effect this four-electron reduction. These results demonstrate novel carbon-carbon bond formation on oxide surfaces in UHV and illustrate the application of surface analytical techniques to probe the sites of organic synthesis at the fluid-solid interface.

Original languageEnglish
Pages (from-to)3063-3074
Number of pages12
JournalJournal of the American Chemical Society
Volume116
Issue number7
DOIs
Publication statusPublished - 6 Apr 1994

Keywords

  • single-crystal surfaces
  • low-valent titanium
  • heterogenous catalysis
  • molecular pathways
  • carbonyl-compounds
  • ion surfaces
  • zinc-oxide
  • oxidation
  • acetylene
  • propylene

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