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

Ordered mesoporous chromium(III) oxide-phosphotungstic acid (PWA) nanocomposite structures with controllable composition (similar to 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 similar to 67 to 80 m(2) g(-1), and uniform pores of similar to 3 to 4 nm size according to small-angle X-ray scattering, high resolution transmission electron microscopy and N-2 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.