Mechanism of catalytic decomposition of pentachlorophenol by a highly recyclable heterogeneous SiO 2–[Fe-porphyrin] catalyst

Abstract

A novel heterogenized FeR 4P–SiO 2 shows enhanced catalytic efficiency for PCP conversion vs. the homogeneous FeR 4P catalyst. The heterogenized FeR 4P–SiO 2 catalyst is highly recyclable in the presence of imidazole in solution. EPR and DR-UV–Vis data provide direct evidence that high-valent iron species [R 4P + Fe IV O] are formed in the heterogenized FeR 4P–SiO 2 system. The electron spin density of the a 1u cation radical (Por + ) is mainly localised on the tetrapyrole frame, and this results in the observed weak magnetic coupling between the S = 1 oxo-ferryl moiety (Fe IV O) and the S ′ = 1/2 porphyrin cation radical (Por + ). A catalytic cycle mechanism is suggested. Accordingly, the reduction in [R 4P + Fe IV O] can proceed via a substrate molecule, in a one electron-transfer, thus producing [R 4PFe IV] plus a radical entity derived from the substrate. Then, a second electron-transfer to [R 4PFe IV] leads to the regeneration of the initial R 4PFe III state. This second electron can originate from either a substrate molecule or a radical substrate species.