Role of cobalt molecular sieves in the liquid-phase oxidation of cyclohexane to adipic acid

The catalytic activity of CoAPO-5 in the oxidation of cyclohexane to adipic acid has been re-examined in order to specify the actual role of the solid. It was shown that the catalytic activity in acetic acid was ascribable to the leaching of cobalt. The activity of CoAPO-5 was comparable to that of cobalt(II) acetate at very low concentration. Surprisingly, a maximum of activity was found at 0.17 mmol dm(-3), which corresponds to the concentration of cobalt leached from the solid in the reaction. Such a concentration is far below the concentration usually used for homogeneous catalysis. Variations in adipic acid selectivity, which followed the variations in activity, were interpreted by changes in mechanism.