REINVESTIGATION OF EPOXIDATION USING TUNGSTEN-BASED PRECURSORS AND HYDROGEN-PEROXIDE IN A BIPHASE MEDIUM

The reaction of hydrogen peroxide with tungstic acid, "H2WO4", and various types of polyoxometalates (isopolyanions, [MnOy]x-, and heteropolyanions, [XM12O40]x'- and [X2M18O62]x"-, where M = Mo and W and X = P, B, and Si) has been studied by several physicochemical techniques (UV, IR, Raman, and P-31 and W-183 NMR spectroscopy). On the basis of these results, an interpretation of the differences in the catalytic activity of these precursors in the epoxidation of 1-octene by H2O2 in a biphase medium is proposed. Heteropolyacids (HPA) with the Keggin structure, H3[PM12O40] (M = Mo, W), are degraded in the presence of excess H2O2 to form peroxo species {PO4[M(O)(O2)2]4}3- and [M2O3(O2)4(H2O)2]2-. These anions were isolated in the form of salts and identified by elemental and spectroscopic analyses; it appears that the former is responsible for the catalytic activity of phase-transfer systems involving H3[PM12O40] as a precursor.