Abstract
Atomically disperse Ti sites on metal oxides (MOx, 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 pKa 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.
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