Electrostatic immobilization of phosphomolybdic acid on imidazolium-based mesoporous organosilicas for catalytic olefin epoxidation

Abstract

Polyoxometalate-based heterogeneous materials were prepared by the immobilization of 12-phosphomolybdic acid (PMA) on periodic mesoporous organosilicas containing embedded imidazolium cations (PMO-ILs). The resulting hybrid materials (PMA@PMO-ILs) were characterized by N 2 adsorption-desorption, powder X-ray diffraction, atomic adsorption spectroscopy, thermogravimetric and differential thermal analyses, Fourier transform infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, and solid-state cross-polarization magic angle spinning nuclear magnetic resonance. PMA was electrostatically immobilized on the surface and in the channels of PMO-ILs. The PMO-IL support and PMA structures were retained during the preparation processes. The catalytic properties of the PMA@PMO-ILs were evaluated for the liquid-phase epoxidation of cyclooctene. PMA@PMO-ILs were catalytically active, with nearly 100% selectivity to cyclooctene epoxide using tert -butyl hydroperoxide as the oxidant. The catalysts could be reused four times without obvious loss of activity or selectivity under identical reaction conditions. Imidazolium cations in the PMO-IL framework improved the stability and recyclability of the PMA immobilized catalysts. Polyoxometalate-based heterogeneous materials were prepared by immobilizing 12-phosphomolybdic acid on imidazolium-based periodic mesoporous organosilicas. These hybrid materials were active and stable heterogeneous catalysts for olefin epoxidation.