Ordered mesoporous Cr2O3 frameworks incorporating Keggin-type 12-phosphotungstic acids as efficient catalysts for oxidation of benzyl alcohols

Abstract

Ordered mesoporous chromium(III) oxide–phosphotungstic acid (PWA) nanocomposite structures with controllable composition (∼17 to 49 wt% in PWA) have been successfully prepared via an ultrasound-assisted nanocasting route, using mesoporous SBA-15 silica as a rigid mold. These materials possess 3D hexagonal mesostructure, large internal BET surface areas of ∼67 to 80 m² g⁻¹, and uniform pores of ∼3 to 4 nm size according to small-angle X-ray scattering, high resolution transmission electron microscopy and N₂ physisorption. The Keggin-type structure of [PW₁₂O₄₀]³⁻ anions is preserved intact into the Cr₂O₃ framework, as confirmed by total X-ray diffuse scattering and pair distribution function analysis and infrared and diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy. The integration of regular porosity, large internal surface area, and Cr₂O₃–PWA composition makes these materials highly promising for applications in oxidation catalysis. Although pure mesoporous Cr₂O₃ and PWA compounds exhibit low catalytic activity, the mesoporous Cr₂O₃–PWA composites showed superior activity and selectivity for the oxidation of selected secondary benzyl alcohols, giving good-to-high yields within a short reaction time. Furthermore, the Cr₂O₃–PWA composite frameworks demonstrated remarkable durability and reusability upon multiple usages without leaching or decomposition of the incorporated PWA clusters. This enhancement is attributed to the synergistic interactions between the PWA and Cr₂O₃ components as well as the well-ordered open-pore structure and large catalytically active surface area.