Special Features of Cyclohexane Oxidation

The first step, oxidation of cyclohexane to cyclohexanol and cyclohexanone, follows the general mechanism outlined by reactions 8.13 to 8.17. Trace quantities of cyclohexyl hydroperoxide 8.9 can initiate the radical chain, where the radicals 8.10 and 8.11 take part in the propagation steps. 

One of the termination steps that lead to the simultaneous formation of both cyclohexanol and cyclohexanone deserves attention. The peroxy radical dimerizes to give the intermediate 8.12. The dimer as shown by reaction 8.20, undergoes intramolecular rearrangement to give the products. 

The oxidation of cyclohexane is accompanied by the oxidative degradation of the products. In other words, the products themselves undergo metal-initiated radical chain oxidation. For this reason oxidation is normally carried out at low conversions when the selectivity to the desired products is high. The catalysts used are a mixture of hydrocarbon soluble carboxylate salts of Co2+ and Mn2+ or Cr3+. Due to the better solubility properties, salts of long-chain carboxylic acids such as 2-ethyl hexanoic or naphthenic acids are favored. As already mentioned, the primary role of the metal ions is to act as catalysts for the initiation steps. In other words, the metal ions undergo redox reactions with 8.9 to give 8.10 and 8.11. 

The mixture of cyclohexanone and cyclohexanol is oxidized by nitric acid in the presence of V5+ and Cu2+ ions. This is shown in Fig. 8.4. Since some of the nitric acid is lost as N2 and N20, the oxidation is semistoichiometric in nitric acid. However, the NO and N02 produced in this oxidation are recycled back. 

The roles of vanadium and copper are complex and not fully understood. A large number of intermediates, including the nitroso and the diketo compound as shown by reaction 8.21 are formed. The ion V02 stoichiometrically oxidizes the diketo compound to adipic acid. This is shown by reaction 8.22. 

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