This report deals with the results of a study on the oxidation of cyclohexanone to adipic acid with air,\ud
catalysed by Keggin-type polyoxometalates of composition H3+xPMo12−xVxO40 (x = 1 and 2), which was\ud
carried out in a semi-continuous stirred-tank reactor. It was found that when conducted in the presence\ud
of a water-only solvent, the reaction proceeds with a redox mechanism, in which the step of\ud
reoxidation of the reduced POM by oxygen is rate limiting. When, however, the reaction was carried\ud
out with an acetic acid co-solvent, a radical-chain autoxidation mechanism prevailed, especially when\ud
very low amounts of catalyst were used. Autoxidation overlapped with the redox mechanism when the\ud
catalyst-to-cyclohexanone ratio was increased. Moreover, the composition of the polyoxometalate, that\ud
is, the number of V atoms per Keggin unit, affected the relative importance of the two mechanisms. The\ud
selectivity to adipic acid achieved was a function of the reaction mechanism, but also was affected by\ud
cyclohexanone conversion, due to the presence of a complex reaction network
The aerobic oxidation of trans‐1,2‐cyclohexanediol in the synthesis of adipic acid was studied. Two classes of catalysts are compared, 1) alumina‐supported Ru(OH)3, and 2) Keggin type P/Mo/V polyoxometalates. These two classes are representative examples because they are active in alcohol oxidation under quite different reaction conditions. In the former case, basic conditions are needed in order to activate the substrate, whereas with polyoxometalates, acidic conditions are used. Their catalytic behavior showed remarkable differences; in basic conditions, the reaction network was very complex, and several side reactions led to a number of by‐products, with a low selectivity to adipic acid in the end. The supported Ru(OH)3 catalyst was very efficient in 1,2‐cyclohexanediol oxidative dehydrogenation to 1,2‐cyclohexanedione, but several undesired reactions occurred starting from this key intermediate under basic conditions: rearrangement into 6‐hydroxycaprolactone and 1‐hydroxycyclopentanecarboxylic acid, and formation of the product of aldol condensation. The former compound was also an intermediate for adipic acid formation, but this reaction gave only a minor contribution to the reactant conversion. Polyoxometalates were extremely selective in 1,2‐cyclohexanediol conversion into adipic acid, but under acidic conditions the product reacted with the unconverted reactant to yield the corresponding ester.
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