Hybrid Polyoxotungstate/MIL-101 Materials: Synthesis, Characterization, and Catalysis of H2O2-Based Alkene Epoxidation

Polyoxotungstates [PW4O24](3-) (PW4) and [PW12O40](3-) (PW12) have been inserted into nanocages of the metal organic framework MIL-101. The hybrid materials PWx/MIL-101 (x = 4 or 12) containing 5-14 wt % of polyoxotungstate have been obtained and characterized by elemental analysis, N-2 adsorption, FT-IR, Raman, and P-31 NMR MAS spectroscopic techniques. Their catalytic performance was assessed in the selective oxidation of alkenes with aqueous hydrogen peroxide under mild reaction conditions ([H2O2] = 0.1-0.2 M, 50 degrees C, MeCN). PWx/MIL-101 enclosing 5 wt % of polyoxotungstate demonstrated fairly good catalytic activities in the epoxidation of various alkenes (3-carene, limonene, alpha-pinene, cyclohexene, cyclooctene, 1-octene), the turnover frequencies (TOF) and alkene conversions were close to the corresponding parameters achieved with homogeneous PWx. For the oxidation of substrates with aromatic groups (styrene, cis- and trans-stilbenes), a higher level of olefin conversion was attained using PW12/MIL-101. Moreover, confinement of PW12 within MIL-101 nanocages allowed us to reach higher epoxide selectivities at higher alkene conversions. The hybrid PWx/MIL-101 materials were stable to leaching, behaved as true heterogeneous catalysts, were easily recovered by filtration, and reused several times with the maintenance of the catalytic performance.