Integrated photocatalysts for hydrocarbon oxidation: polyoxotungstates iron porphyrins systems in the reductive activation of molecular oxygen

摘要

The photochemical and photocatalytic properties of (nBu(4)N)(3)PW12O40(PW12O403-), (nBu(4)N)(4)W10O32 and (Et4N)(4)W10O32 (W10O324-) were investigated in the presence of several Fe(III) meso-tetraarylporphyrins (Fe(III)Por). Laser flash photolysis experiments show that in the (nBu(4)N)(3)PW12O40/Fe(III)Por integrated systems the electron transfer from the photoreduced polyoxotungstate to the Fe(III)Por is the first step for the subsequent reductive activation of O-2 by the ferrous complex. In contrast, the direct reduction of O-2 by the photoreduced polyoxotungstate is the kinetically favored process in the (nBu(4)N)(4)W10O32/Fe(III)Por systems. A comparison between the results obtained using (nBu(4)N)(4)W10O32 and (Et4N)(4)W10O32 shows that the decrease in the steric hindrance of the counterion, surrounding the photochemically active polyoxoanion, increases the rate of electron transfer between W10O325- and Fe(III)Por, thus enabling the decatungstate to contribute to the activation of dioxygen through the reduction of the Fe(III)Por. Continuous irradiation of the POTn-/Fe(III)Por integrated systems under controlled aerobic conditions leads to the oxidation of cyclohexane to cyclohexanol and cyclohexanone. In comparison with POT(n-)s alone a higher cyclohexanol/cyclohexanone concentration ratio is obtained, while in comparison with Fe(III)Por alone, higher yields of oxidation products and a higher stability of the photocatalyst are observed. The effect of the iron porphyrin on the products distribution can be ascribed to the ability of the complex in both the ferrous and the ferric forms to generate monooxygenating intermediates as a consequence of its reactions with O-2, alkylhydroperoxide and radical species. (C) 1998 Elsevier Science S.A. All rights reserved. [References: 37]