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 (MOx, including SiO2, gamma-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 (Delta H-epox(double dagger)) and entropies (Delta S-epox(double dagger)). 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 Delta H-epox(double dagger) 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. (c) 2022 Elsevier Inc. All rights reserved.

Automated summary

This study found that atomically dispersed Ti sites on metal oxides can activate H2O2 to create intermediates active for alkene epoxidation reactions. By comparing reaction rates, electronic properties, and non-covalent interactions on different metal oxide catalysts, the researchers discovered a correlation between water adsorption at the solid-liquid interface and the stability of epoxidation transition states.