Unusual kinetic role of a water-soluble iron(III) porphyrin catalyst in the oxidation of 2,4,6-trichlorophenol by hydrogen peroxide

The oxidation of 2,4,6-trichlorophenol (TCP) to 2,6-dichloro- 1,4-benzoquinone (DCQ) by hydrogen peroxide using iron(Ill) meso-tetra(4-sutfonatophenyl) porphine chloride, Fe(TPPS)Cl, as a catalyst was studied with stopped-flow UV-vis spectrophotometry and potentiometry using a chloride ion selective electrode. The observations are interpreted by a three-step kinetic model: the initial reaction of the catalyst with the oxidant (Fe(TPPS)(+) + H2O2--> Cat') produces an active intermediate, which oxidizes the substrate (Cat' + TCP --> Fe(TPPS)+ + DCQ + Cl-) in the second step. The third step is the transformation of the catalyst into a much less active form (Cat' --> Cat") and is responsible for the unusual kinetic phenomena observed in the system. (C) 2004 Wiley Periodicals, Inc.