Cyclohexane autoxidation

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

Recently, ho vever, Hermans et al. [2h-m] have revisited the mechanism of cyclohexane autoxidation, and found that indeed the most efficient mechanism for chain propagation is not ... 

In one approach cyclohexane is autoxidized to a mixture of cyclohexanol and cyclohexanone in the presence of a Co or Mn naphthenate catalyst. This mixture is subsequently oxidized to adipic acid using nitric acid as the oxidant in the presence of a Cu Vv catalyst. An alternative method using dioxygen in combination with Co or Mn in HOAc gives lower selectivities to adipic acid (70% vs 95%). Alternatively, autoxidation in the presence of stoichiometric amounts of boric acid produces cyclohexanol as the major product, which is subsequently oxidized to adipic acid using HNO3 in the presence of Cu Vv. The latter step produces substantial amounts of N2O as a waste product. 

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... 

Autoxidation of the hydrazone (O2/C6H6/UV) gives the explosive isomeric 1,2-dihydroperoxy-l,2-bis(benzeneazo)cyclohexane [1], and the same is true for COT derivatives [2],... 

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

The presence of ascorbic acid as a co-substrate enhanced the rate of the Ru(EDTA)-catalyzed autoxidation in the order cyclohexane < cyclohexanol < cyclohexene (148). The reactions were always first-order in [H2A]. It was concluded that these reactions occur via a Ru(EDTA)(H2A)(S)(02) adduct, in which ascorbic acid promotes the cleavage of the 02 unit and, as a consequence, O-transfer to the substrate. While the model seems to be consistent with the experimental observations, it leaves open some very intriguing questions. According to earlier results from the same laboratory (24,25), the Ru(EDTA) catalyzed autoxidation of ascorbic acid occurs at a comparable or even a faster rate than the reactions listed in Table III. It follows, that the interference from this side reaction should not be neglected in the detailed kinetic model, in particular because ascorbic acid may be completely consumed before the oxidation of the other substrate takes place. 

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... 

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. 

The autoxidation of 4-undecanone in ah at 130 °C leads to the formation of hydroperoxides, which decompose at 120-160 °C via different radical pathways to give CO, CO2, and H2 by parallel pseudomonomolecular processes.304 An extremely sterically crowded heptatriene (141) is reported to undergo autoxidation at 25 °C in cyclohexane. The isolated products were rationalized by the dissociation of (141) to the tropyl radical (142) or fluorenyl radical (143) and subsequent attack by molecular oxygen (Scheme 22).305... 

Generally, the issue of whether a truly solid Cr catalyst has been created for the aforementioned reactions is unresolved. This point is illustrated most clearly by all the work that has been devoted, in vain, to Cr molecular sieves (55-57). Particularly the silicates Cr-silicalite-1 and Cr-sihcahte-2 and the aluminophosphate Cr-AlPO-5 have been investigated. These materials have been employed, among others, for alcohol oxidation with t-BuOOH, for allylic (aut)oxidation of olefins, for the autoxidation of ethylbenzene and cyclohexane, and even for the catalytic decomposition of cyclohexyl hydroperoxide to give mainly cyclohexanone ... 

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). 

Surprisingly, alkanes containing tertiary C—H bonds showed poor reactivity in these reactions.2943 b 29Sa d Thus, isobutane was less reactive than n-butane, and methylcyclohexane less reactive than cyclohexane (cf., lower reactivity of cumene to toluene). In the series of normal alkanes, n-butane reacted faster than n-pentane. n-Undecane was unreactive. These results are inconsistent with a normal free radical autoxidation. The authors used the analogy with arene oxidations to postulate that formation of radical cations by electron transfer from the alkane to Co(III) was a critical factor ... 

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. 

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. 

We have demonstrated recently that epoxidation and hydroxyl-ation can be achieved with simple iron-porphine catalysts with iodosylbenzene as the oxidant (24). Cyclohexene can be oxidized with iodosylbenzene in the presence of catalytic amounts of Fe(III)TPP-Cl to give cyclohexene oxide and cyclohexenol in 55% and 15% yields, respectively. Likewise, cyclohexane is converted to cyclohexanol under these conditions. Significantly, the alcohols were not oxidized rapidly to ketones under these conditions, a selectivity shared with the enzymic hydroxylations. The distribution of products observed here, particularly the preponderance of epoxide and the lack of ketones, is distinctly different from that observed in an autoxidation reaction or in typical reactions of reagents such as chromates or permanganates (15). 

What are the main products of metal-catalyzed autoxidation of methyl cyclohex-2-ene Why is cyclohexene more susceptible to autoxidation than cyclohexane ... 

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. 

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