[γ-1,2-H₂SiV₂W₁₀O₄₀] immobilized on surface-modified SiO₂ as a heterogeneous catalyst for liquid-phase oxidation with H₂O₂

An organic-inorganic hybrid support has been synthesized by covalently anchoring an N-octyldihydroimidazolium cation fragment onto SiO₂ (denoted as 1-SiO₂). This modified support was characterized by solid-state ¹³C, ²⁹Si, and ³¹P NMR spectroscopy, IR spectroscopy, and elemental analysis. The results showed that the structure of the dihydroimidazolium skeleton is preserved on the surface of SiO₂. The modified support can act as a good anion exchanger, which allows the catalytically active polyoxometalate anion [γ-1,2-H ₂SiV₂W₁₀O₄₀]^4- (I) to be immobilized onto the support by a stoichiometric anion exchange (denoted as I/1-SiO₂). The structure of anion I is preserved after the anion exchange, as confirmed by IR and ⁵¹V NMR spectroscopy. The catalytic performance for the oxidation of olefins and sulfides, with hydrogen peroxide (only one equivalent with respect to substrate) as the sole oxidant, was investigated with I/1-SiO₂. This supported catalyst shows a high stereospecificity, diastereoselectivity, regioselectivity, and a high efficiency of hydrogen peroxide utilization for the oxidation of various olefins and sulfides without any loss of the intrinsic catalytic nature of the corresponding homogeneous analogue of I (i.e., the tetra-n-butylammonium salt of I, TBA-I), although the rates decreased to about half that with TBA-I. The oxidation can be stopped immediately by removal of the solid catalyst, and vanadium and tungsten species' can hardly be found in the filtrate after removal of the catalyst. These results rule out any contribution to the observed catalysis from vanadium and tungsten species that leach into the reaction solution, which means that the observed catalysis is truly heterogeneous in nature. In addition, the catalyst is reusable for both epoxidation and sulfoxidation without any loss of catalytic performance.