A Comparative Kinetic Study of the Photo-oxidation of Phenol by Tetra-cationic Porphyrins and Zinc Compounds

Abstract

Kinetic of the photo-oxidation of phenol in aqueous alkaline and neutral buffer solutions was studied and compared by irradiation of visible light in the presence of tetra-cationic porphyrins and their zinc metal ion photo-sensitizers. The porphyrins 5,10,15,20-tetrakis(3-methylpyridinium)porphyrin (TMPyP(3)); 5,10,15,20-tetrakis(4-methylpyridinium)porphyrin (TMPyP(4)); 5,10,15,20-tetrakis(4-N,N,N-trimethylanilinium)porphyrin (TAPP) and their zinc ion complexes were used in this study. In the dark conditions, none of the porphyrin compounds showed any effect on the reducing of phenol. The results showed that the rate of the photo-oxidation of phenol in alkaline medium has been increased in the presence of these photo-sensitizers. Among them, the rate of photo-oxidation by TAPP was more than the other ones. In neutral medium, the rate of the photo-oxidation of all of the compounds have been shown approximately similar behavior. It was found that the dominant pathway of phenol degradation occurred in the reaction with molecular singlet oxygen (1O2). Kinetic parameters of the phenol reaction with 1O2 were estimated. According to the equation (ln A/A_0 =k_obs t) and by plotting ln A/A_0 against time, the rate of the reaction was obtained first order.