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Keywords:

  • adipic acid;
  • polyoxometalates;
  • reaction mechanisms;
  • ruthenium;
  • selective oxidation

Abstract

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.