Cyclohexane homogeneous autoxidation

The synthesis of cyclohexanone, which is an intermediate in the manufacture of nylon 6 and nylon 6,6 is an important industrial process [1], One of the major current routes for the synthesis of cyclohexanone is the liquid-phase autoxidation of cyclohexane at 125-160 °C and 10 bar followed by the selective decomposition of the intermediate cyclohexyl hydroperoxide, using a soluble cobalt catalyst, to a mixture of cyclohexanol and cyclohexanone [2]. These severe conditions are necessary due to the low reactivity of cyclohexane towards autoxidation. Due to the high reactivity of the products in the autoxidation step conversions must be kept low (<10%) [3,4]. Heterogeneous catalysts potentially offer several advantages over their homogeneous counterparts, for example, ease of recovery and recycling and enhanced stability. Recently we found that chromium substituted aluminophosphate-5 and chromium substituted silicalite-1 (CrS-1) are active, selective and recyclable catalysts for the decomposition of cyclohexyl hydroperoxide to cyclohexanone [5j. 

The compounds Rh6(CO)16 and Re2(CO)10 are also effective homogeneous catalysts for autoxidating cyclic alcohols to dicarboxylic acids. Solvent effect data for cyclohexanol are shown in Table IV. Again low yields are found in benzene solvent, and considerably higher conversions in cyclohexane. The yields of carboxylic acids obtained from both cyclic and acyclic alcohols are shown in Table V. It is apparent that the acid yields are small for acyclic alcohols. There is no difference in catalytic activity whether the compound Rh6(CO)16 or Re2(CO)10 is used and low yields are obtained from both primary and secondary alcohols. 

In the autoxidation of neat hydrocarbons, catalyst deactivation is often due to the formation of insoluble salts of the catalyst with certain carboxylic acids that are formed as secondary products. For example, in the cobalt stearate-catalyzed oxidation of cyclohexane, an insoluble precipitate of cobalt adipate is formed. 18fl c Separation of the rates of oxidation into macroscopic stages is not usually observed in acetic acid, which is a better solvent for metal complexes. Furthermore, carboxylate ligands may be destroyed by oxidative decarboxylation or by reaction with alkyl hydroperoxides. The result is often a precipitation of the catalyst as insoluble hydroxides or oxides. The latter are neutralized by acetic acid and the reactions remain homogeneous. 

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