Mechanism of catalytic decomposition of pentachlorophenol by a highly recyclable heterogeneous SiO2–[Fe-porphyrin] catalyst

A novel heterogenized FeR4P–SiO2 shows enhanced catalytic efficiency for PCP conversion vs. the homogeneous FeR4P catalyst. The heterogenized FeR4P–SiO2 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 [R4P+radical dotFeIVdouble bond; length as m-dashO] are formed in the heterogenized FeR4P–SiO2 system. The electron spin density of the a1u cation radical (Por+radical dot) is mainly localised on the tetrapyrole frame, and this results in the observed weak magnetic coupling between the S = 1 oxo-ferryl moiety (FeIVdouble bond; length as m-dashO) and the S′ = 1/2 porphyrin cation radical (Por+radical dot). A catalytic cycle mechanism is suggested. Accordingly, the reduction in [R4P+radical dotFeIVdouble bond; length as m-dashO] can proceed via a substrate molecule, in a one electron-transfer, thus producing [R4PFeIV] plus a radical entity derived from the substrate. Then, a second electron-transfer to [R4PFeIV] leads to the regeneration of the initial R4PFeIII state. This second electron can originate from either a substrate molecule or a radical substrate species.