Identity of the metal oxide support controls outer sphere interactions that change rates and barriers for alkene epoxidations at isolated Ti atoms

Atomically disperse Ti sites on metal oxides (MO_x, including SiO₂, γ-Al₂O₃, ZnO, GeO₂) activate H₂O₂ to create intermediates active for alkene epoxidations. Turnover rates for 1-hexene epoxidation in acetonitrile vary 1000-fold at identical conditions due to differences in apparent activation enthalpies (ΔH^‡_epox) and entropies (ΔS^‡_epox). Ligand-to-metal charge transfer energies and vibrational frequencies of reactive species assessed by in situ UV–Vis and Raman spectroscopy, respectively, indicate supports do not detectably change electronic properties of H₂O₂-derived intermediates. However, isoelectric points and solution-phase water uptakes for these metal oxides correlate with ΔH^‡_epox and suggest that non-covalent interactions at the solid-liquid interface influence the stability of epoxidation transition states. Supports with lower pK_a values concentrate water near the solid-liquid interface and enthalpically stabilize the transition state. These findings illustrate that outer sphere interactions impact epoxidation reactions upon metal oxide catalysts including titanium silicates.