Surfactant-assisted sol-gel synthesis of zirconia supported phosphotungstates or Ti-substituted phosphotungstates for catalytic oxidation of cyclohexene

The Keggin-type polyoxometalates (POMs) including phosphotungstate and Ti-substituted phosphotungstates were supported on zirconia (ZrO₂) matrix by a surfactant-assisted sol-gel copolymerization technique. The resultant ZrO₂ supported POMs composite materials (ZrO ₂-POMs) were characterized by X-ray power diffraction, transmission electron microscopy, Fourier transform infrared spectra, diffuse reflectance ultraviolet-visible spectra, thermogravimetry-differential scanning calorimetry analysis, and ³¹P magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis. The results showed that the ZrO₂-POMs materials have worm-like disordered mesoporous, and the primary structure of POMs is preserved after the formation of the composite materials. All the ZrO₂-POMs materials are catalytically active for the H ₂O₂-mediated oxidation of cyclohexene with relatively high selectivity to allylic oxidation products. Moreover, these materials exhibit high stability against leaching of active species, and can be easily recycled without obvious loss of activity and selectivity. The existence of relatively strong interaction between the POM units and the ZrO₂ framework should play important role in fabricating the stable heterogeneous ZrO ₂-POMs catalysts.