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

This report deals with 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.