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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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Adipic acid (AA) is an important chemical intermediate [1]. The production of Nylon-6,6 (for fibers and resins) accounts for 63% of AA consumption; the fibers are used in applications such as fishing lines, tires, carpets, home furnishings and in tough fabrics for parachutes, backpacks, luggage and business cases. Nylon engineering resins are chiefly used in electrical connectors, auto parts and items such as self-lubricating bearings, gears, and cams.The major non-nylon uses of AA are in polyester polyols (for polyurethane resins, 25% of AA production), in plasticizers (7%: dioctyl adipate, diisodecyl adipate, etc. for vinyl chloride, nitrocellulose and cellulose acetate polymers), resins (2%: unsaturated polyesters) and 3% for miscellaneous applications, such as a food ingredient in gelatins, and as a component in cosmetics, pharmaceuticals, fertilizers, paper, cements, waxes, and so on.Currently, the overall growth for AA is close to 3% per year, while demand is growing faster and is forecast to be around 5-6% per year by 2010. The expected growth is 12% in Asia-Pacific, around 8-9% in Asia/Middle East, while remaining stable in Western Europe and in the USA. The most rapidly growing sector is that of nylon resins, which has grown by 8-10% per year during the past decade. However, the market anticipates that the substitution of nylon engineering plastics into automobile parts is becoming saturated. Moreover, with 62% of the AA take, a slowdown in nylon fiber production is retarding the AA market. Nylon fibers are highly sensitive to the performance of the housing sector, which has been growing slowly for some years now, by about 1.7% on an annual basis. Nevertheless, despite the new plants being installed in China, capacity will fall short by 2010, because of the poor profitability that has hindered investments.In 2006, the global AA capacity was around 2.8 million metric tons per year; nearly 1 M-ton yr À1 was produced in Western Europe, 1 M-ton yr À1 in the USA (where 62% of the total global production is consumed), 0.5 M-ton yr À1 in Asia (excluding Japan), with the remainder being produced in Japan, Canada and Latin