Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(Ⅳ) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation

摘要

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]arene-Ti~(IV) single-sites grafted on amorphous vs crystalline delaminated zeotype (UCB-4) silicates as supports. A chelating calix[4]arene macrocyclic ligand helps enforce a constant Ti~(IV) inner-sphere, as characterized by UV–visible and X-ray absorption spectroscopies, thus enabling the rigorous comparison of outer-sphere environments across different siliceous supports. These outer-sphere environments are characterized by solid-state ~(1)H NMR spectroscopy to comprise proximally organized silanols confined within 12 membered-ring cups in crystalline UCB-4, and are responsible for up to 5-fold enhancements in rates of epoxidation by Ti~(IV) centers.