Autoxidation of cyclohexane

Zaidi (ref. 28) has reported the autoxidation of cyclohexane in acetic acid, at 60-80 °C and 1 bar, in the presence of a Co(OAc)2/NaBr catalyst (4). Adipic acid was obtained in 31% yield. Based on the results obtained in alkylaromatic oxidations it would be interesting to try the Co/Mn/Br /HOAc system in cyclohexane oxidation. It is, however, difficult to believe that this has not already been done. 

The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

The autoxidation of cyclohexane initiated by dicyclohexyl peroxy-dicarbonate (DCPD) takes this pathway 38> ... 

A simple but effective means of preparing supported metal ion catalysts is to employ ion exchange resins. For example, a cobalt-exchanged H-type resin (Dowex 50) was shown43 to be an effective solid catalyst for the autoxidation of acetaldehyde to acetic acid at 20°C. No leaching of cobalt ions from the resin was observed and the catalyst was used repeatedly (5x) without any significant loss of activity. More recently the use of weak acid resins exchanged with cobalt ions as catalysts for the autoxidation of cyclohexane... 

Polymers with chelated Co have also been used as catalysts for alkane autoxidation. Kulkarni et al. (166) employed a tyrosine-based polymer for autoxidation in pure cyclohexane, but very different conditions were used by Shen and Weng (167,168) in the autoxidation of cyclohexane or cyclohexanone. The latter authors, used glacial acetic acid as a solvent and a Co-exchanged weak acid resin as the catalyst. At high conversions, adipic acid is formed ... 

In recent years increasing use has been made of an alternative procedure involving the oxidation of hydrocarbon substrates in polar solvents, usually acetic acid, in the presence of relatively large amounts of metal catalysts, usually the metal acetate. These reactions are characterized by high rates of oxidation, high conversions, and more complete oxidation of the substrate. For example, the classic autoxidation of cyclohexane is carried out to rather low conversions and affords mainly cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone. Autoxidation of cyclohexane in acetic acid, in the presence of substantial amounts of cobalt acetate catalyst, results in the selective formation of adipic acid at high conversions (see Section II.B.3.c). 

The solvent may also influence the rates of the various steps in the autoxida-tion to differing degrees. For example, in the autoxidation of cyclohexane in a variety of solvents,373" 6 the dielectric constant of the medium had no effect on the rate constant for propagation. The medium, however, strongly influences the rate constant for termination (R02 + R02 ), which involves an interaction of two dipoles. 

Several processes are used for the industrial production of caprolactam. Generally cyclohexanone is the key intermediate and it is produced by catalytic hydrogenation of phenol (ex benzene or toluene) or the catalytic autoxidation of cyclohexane (from benzene hydrogenation) as shown in Fig. 2.27. 

Air oxidation of /i-butane to maleic anhydride is possible over vanadium phos(4tate and, remaiicably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in conffast to the situation found in the liquid, n-allcanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

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. 

Cobalt(II) has a special place in the open framework family because it is one of the few transition metal cations that easily exhibits tetrahedral coordination, in addition to five and sixfold ones. Moreover, magnetic couplings can give rise to interesting magnetic properties, similar to those described above for iron. Furthermore, it is well known that Co doping enhances the catalytic performance of certain zeolites and related compounds. For instance, CoAPO-5 and CoAPO-11 have been used for the autoxidation of cyclohexane [77] and p-cresol [78]. 

Jacobs and coworkers [35] described the use of CoAPO-5 and CoAPO-11 as heterogeneous catalysts for the autoxidation of cyclohexane. The cobalt did not seem to be leached at low conversions. At higher conversions secondary products, such as adipic acid, leach the cobalt by forming chelate complexes. It would be interesting to investigate the activity and stability towards teaching of these and other cobalt-substituted molecular sieves [36,37] in benzylic oxidations with O. ... 

Figure 1.1 Oxygen consumption during the autoxidation of cyclohexane at 418 K with and without the addition of cyclohexanone. The red slopes signify the oxidation rate at various stages of the reaction. The initial pressure increase is due to heating, initiated at time 0.

Swaminathan R, Kuriacose JC (1970) Studies on the ketonization of acetic acid on chromia. II. The surface reaction. J Catal 16 357-362 Tinker HB (1970) The decarboxylation of carboxylic acids during the autoxidation of cyclohexane. J Catal 19 237-244... 

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