Homogeneous oxidation of cyclohexane by a Ru(III) analogue of the model peroxidase system Ru(III)-EDTA-ascorbate-H₂O₂

Abstract

A ruthenium(III) analogue of the model peroxidase system Ru(III)-EDTA-ascorbic acid-H₂O₂ is an efficient catalyst for the oxidation of cyclohexane. The kinetics of the oxidation of cyclohexane to cyclohexanol by hydrogen peroxide catalyzed by the Ru(III)-EDTA-ascorbate system was investigated by spectrophotometric techniques in the pH range 1.50 to 2.50 at 30°C, μ=0.1 M KNO₃ in a 1:1 (v/v) mixture of 1,4-dioxane and water. The rate of oxidation of cyclohexane is first order with respect to the concentrations of ascorbate, Ru(III)-EDTA and H₂O₂, inverse first order with respect to hydrogen ion concentration, and fractional order with respect to cyclohexane. The kinetic parameters of the oxidation of cyclohexane to cyclohexanol were determined by potentiometric, spectrophotometric and gas chromatographic methods. On the basis of the kinetic and experimental results, a rate law for the reaction was derived and the mechanism discussed. In the proposed mechanism, the function of the two-electron donor, ascorbic acid, is to reduce the metal chelate Ru(III)-EDTA to Ru(II)-EDTA. The reduced metal chelate Ru(II)-EDTA catalyzes the oxidation of cyclohexane to cyclohexanol.