Catalysis of metalloporphyrins for selective hydroxylation of phenol by H2O2

Abstract

Liquid phase catalytic selective hydroxylation of phenol to catechol and hydroquinone was carried out in the presence of metalloporphyrins using hydrogen peroxide as oxidant and water as solvent. Five kinds of metal tetra(p-chlorophenyl)porphrin ( T (p- Cl ) PPMCl , M = Fe , Co , Mn , Cu , Zn ) were studied. It was found that T (p- Cl ) PPFeCl had high catalytic activity and diphenol selectivity for the hydroxylation of phenol to catechol and hydroquinone. The influence of various reaction parameters, namely, reaction temperature, solvent, ratio of substrate and oxidant, substrate concentration, the amount of catalyst, reaction time and pH value were investigated systematically. When water was used as solvent (10 mL), the optimum conditions were following: pH = 7, the concentration of phenol was 0.3 g/mL, the molar ratio of phenol and H 2 O 2 was 1/2, the molar ratio of catalyst and phenol was 7/100000, the reaction temperature was 65°C and the reaction time was 1.5 h. Under above optimum conditions, the phenol conversion was up to 55.1%, and the selectivity of diphenol was almost up to 99.9%, the molar turnover numbers of the catalyst was about 7500. A possible mechanism was also proposed.