Polyoxometalate-based ionic complexes immobilized in mesoporous silicas prepared via a one-pot procedure: Efficient and reusable catalysts for H2O2-mediated alcohol oxidations in aqueous media

Abstract

Fabrication of 12-tungstophosphoric acid-(dihydro)imidazolium cation ionic complexes incorporated into SBA-15 mesoporous silicas was achieved via a one-pot procedure including prehydrolyzation of tetraethoxysilane in the presence of surfactant (EO20PO70EO20), addition of 1-trimethoxysilylpropyl-3-methylimidazolium chloride or N-(3-triethoxysilylpropyl), N(3)-(3-trimethoxysilylpropyl-4,5-dihydroimidazolium chloride, and successive addition of 12-tungstophosphoric acid. The obtained composite materials were well characterized. XRD, TEM, and nitrogen adsorption–desorption results indicated that the composite materials possessed good mesoporous character. FT-IR, ICP, and solid-state NMR analyses showed 12-tungstophosphoric acid-(dihydro)imidazolium cation ionic complexes had been incorporated into SBA-15 mesoporous silicas with intact structure of organic moieties and Keggin units. Using H2O2-mediated selective alcohol oxidations in aqueous media as test reactions, it was found that the occasion of adding HPW, together with the location of the (dihydro)imidazolium cations in the mesostructures, played a crucial role in 12-tungstophosphoric acid loading and ultimate catalytic performance. Under optimum conditions, our catalysts showed better catalytic efficiency and reusability than their analog prepared by a multistep process due to better dispersion of the Keggin units, higher structural stability, and lower leaching level of active Keggin units.