Low-Temperature Aldol Condensation of Aldehydes on R-TiO2(100)-(1 × 1): Exceptional Selectivity for α,β-Unsaturated Enal Production.

Selective C-C coupling of oxygenates via aldol condensation has the potential to produce useful chemicals from aldehydes and ketones. Here we report a combined experimental and theoretical study on the aldol condensation of unbranched aldehydes (CnH2n+1-CHO, n = 1-4) on rutile (R)-TiO2(100)-(1 × 1). Experimental results show that the R-TiO2(100)-(1 × 1) surface has a very high reactivity and selectivity for aldol product formation from tested aldehydes at room temperature. Theoretical calculations indicate that the CH3CHO enolization and the aldol dehydration occur with low energy barriers, and the 3-butanolal intermediate adsorbs on R-TiO2(100)-(1 × 1) stably, suggesting that the surface has a "modest" acid-base strength for efficient crotonaldehyde formation. The adsorption configuration of CH3CHO and surface structure of R-TiO2(100)-(1 × 1) may contribute to the exclusive selectivity of (E)-crotonaldehyde formation, which provides us a deep insight into the high selectivity of aldol condensation of aldehydes on the TiO2 catalyst.