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+center dot FeIV)=O] are formed in the heterogenized FeR4P-SiO2 system. The electron spin density of the al, cation radical (Por(+center dot)) 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(+center dot)). A catalytic cycle mechanism is suggested. Accordingly, the reduction in [(R4P+center dot FeIV)=O] 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. (C) 2009 Elsevier Inc. All rights reserved.