Evidence for the presence of alternative mechanisms in the oxidation of cyclohexanone to adipic acid with oxygen, catalysed by Keggin polyoxometalates

We report here about the results of a study on the oxidation of cyclohexanone to adipic acid with air, catalysed by Keggin-type polyoxometalates of composition H3+xPMo12-xVxO40 (x=1 and 2), which was carried out in a semi-continuous stirred-tank reactor. It was found that when conducted in the presence of a water-only solvent, the reaction proceeds with a redox mechanism, in which the step of reoxidation of the reduced POM by oxygen is rate-limiting. When, however, the reaction was carried out with an acetic acid co-solvent, a radical-chain autoxidation mechanism prevailed, especially when very low amounts of catalyst were used. Autoxidation overlapped with the redox mechanism when the catalyst-to-cyclohexanone ratio was increased. Moreover, the composition of the polyoxometalate, that is, the number of V atoms per Keggin unit, affected the relative importance of the two mechanisms. The selectivity to adipic acid achieved was a function of the reaction mechanism, but also was affected by cyclohexanone conversion, due to the presence of a complex reaction network.