Improving the Selectivity of Aldehyde–Ketone Cross-Aldol Condensation Reactions with Phosphate-Modified TiO₂ Catalysts

Aldol condensation of mixtures of acetaldehyde and acetone was investigated over TiO2 catalysts modified with phosphate groups of varying coverages to probe effects of surface composition on reaction selectivity. Whereas the self-aldol condensation of acetaldehyde to crotonaldehyde was observed to dominate on unmodified TiO2, we found that modifying the TiO2 catalyst with surface phosphate groups resulted in improved selectivity toward cross-aldol condensation. Moreover, whereas high phosphate coverages led to lower overall condensation rates, decreasing the density of the phosphate layer resulted in activities on par with the unmodified catalyst, while maintaining higher selectivity. Characterization of reactant binding affinities and the electronic nature of the catalyst using temperature-programmed desorption, diffuse reflectance Fourier transform spectroscopy, and X-ray photoelectron spectroscopy suggested that the changes in selectivity stem from changes in reactant binding driven by the phosphates modifying the electronic nature of the catalyst. This approach represents a promising path toward tuning selectivity for aldol condensation reactions in heterogeneous systems with a relatively facile catalyst modification technique.