Immobilization of Phosphotungstic Acid (PTA) on Imidazole Functionalized Silica: Evidence for the Nature of PTA Binding by Solid State NMR and Reaction Studies

Abstract

Phosphotungstic acid (PTA) immobilized onto imidazole functionalized fumed silica and was used as an efficient catalyst for epoxidation of a variety of olefins using aqueous H2O2 as an oxidant. Negligible leaching of PTA under the reaction conditions employed indicates a strong interaction between PTA and imidazole. The immobilized catalysts could be separated and reused after the catalytic cycle. Evidence for the heterogenization of PTA on the imidazole functionalized fumed silica has been inferred from different spectroscopic techniques like IR, UV−vis, and NMR. Importantly, the nature of binding of PTA on the support has been studied in detail by solid state NMR spectroscopy using 15N labeled imidazole support. It is clear from the NMR studies that the effective heterogenization of PTA is mainly due to imidazolium ion formation on the support by the acidic protons of PTA and the resultant ion pair.