Highly selective oxidation of cyclohexene to 2-cyclohexene-1-one over polyoxometalate/metal–organic framework hybrids with greatly improved performances

Abstract

H_3+xPMo_12−xV_xO₄₀@MIL-100 (Fe) (x = 0, 1, 2) hybrids were prepared by encapsulation of polyoxometalates (POMs) within a metal–organic framework using a direct hydrothermal method. The as-prepared samples were well characterized by X-ray diffraction (XRD), Fourier transform infrared spectrophotometry (FT-IR), transmission electron microscopy (TEM), N₂ adsorption–desorption, UV-vis diffused reflectance spectra and inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis. The catalytic performances of the samples were tested in the oxidation of cyclohexene using H₂O₂ as green oxidant. The results have shown that both H₄PMo₁₁VO₄₀@MIL-100 (Fe) and H₅PMo₁₀V₂O₄₀@MIL-100 (Fe) can effectively catalyze the allylic oxidation of cyclohexene to give 2-cyclohexen-1-one as the main product. In particular, when H₄PMo₁₁VO₄₀@MIL-100 (Fe) was employed, 85% cyclohexene conversion, 91% selectivity for 2-cyclohexene-1-one and 715 h⁻¹ of turnover frequency were obtained under optimized conditions. The catalyst can be reused at least five times without obvious loss of activity.