Heteronuclear iron (III)-chromium(III) hydroxo complexes and hydroxides, and their catalytic activity towards peroxidative oxidation of alkanes

The equilibrium compositions of the products of hydrolysis of Fe(III) and Cr(III) in their single or binary nitrate solutions, as a function of pH and neutralisation degree have been established and Fe(III)-Cr(III) hydroxides and products of their thermolysis have been obtained and characterized by potentiometric titration, thermal analysis, IR spectroscopy and X-ray powder diffraction, and an Fe(III)-Cr(III) interaction recognised in the binary systems. The obtained aquahydroxo complexes and metal hydroxides have been applied as catalysts for the homogeneous or heterogeneous, respectively, peroxidative oxidation of cyclohexane and cyclopentane to the corresponding ketones and alcohols, at room temperature. The effects of various parameters, such as pH, degree of neutralisation and reagents molar ratios, have been investigated and correlations between the catalytic activity and the form of the hydroxo complex were recognised for the homogeneous systems, whereas for the heterogeneous mixed (heteronuclear) hydroxides a synergic effect has been detected for the oxidation of cyclohexane, the catalytic activity (which can reach 30% yields and 30 turnover numbers (TONs)) being above those of the individual hydroxides. (C) 2003 Elsevier B.V. All rights reserved.