Cobalt ions, autoxidation

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... 

However, the rate of oxidation in the presence of bromide ion (Figure 2) is exactly first order with respect to cobalt. The autoxidation of hydrocarbons catalyzed by cobalt and bromide ion is characterized by the fact that the rate increases with increasing cobalt concentration, while the rate at high cobalt concentrations reaches a limiting value in the absence of bromide ion. 

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... 

In the presence of bromide ion there is apparently no direct reaction of Co(III) with the hydrocarbon substrate, in contrast to cobalt-catalyzed autoxidations carried out in the absence of bromide. That different mechanisms are operating is illustrated by the relative rates of oxidation of alkylbenzenes catalyzed by cobalt acetate alone compared to those obtained in the presence of added bromide ion (Table VIII). In the presence of bromide ion, the relative reactivities are consistent with a mechanism involving attack by bromine atoms but not one involving electron transfer. Individual discrepancies in selectivities between bromine atom and the species active in the Co(0 Ac)2-NaBr system (Table VIII) were attributed to a bromine complex,... 

A complete mechanism for the autoxidation of alkylaromatic hydrocarbons by cobalt(n) in acetic acid has not been established,25 6 although a complex rate law has been determined for tetralin. 22 The reaction most likely proceeds by a fiiee radical chain mechanism in which the purpose of the cobalt ions is to provide a hi h steady state concentration of free radicals by catalysis of the decomposition of THP. The free radical nature of the autoxidation of tettalin with the colloidal CoPy catalysts is supported by experiments which showed inhibition of the reaction by 2,6-di-rerr-butylphenol and 2,6-di-rm-butyl-4-methylphenol, and by a shortening of the induction period and increase of the reaction rate when azobis(isobut nitrile) was added to the reaction mixture as a free radical initiator. 

It has been reported that 11,12-epoxyretinaldehyde is obtained as a product when retinol (1) is treated with peracetic acid in tetrahydrofuran (Ogata et al., 1973 Davalian and Heathcock, 1979b) 11,12-epoxyretinol has been postulated as an intermediate in the autoxidation of retinol (1) catalyzed by cobalt ions (Ogata eta/., 1971). 

In the absence of bromide ion the p-xylene undergoes rapid autoxidation to p-toluic acid but oxidation of the second methyl group is difficult, due to deactivation by the electron-withdrawing carboxyl group, and proceeds only in low yield at elevated temperatures. Although bromide-free processes were subsequently developed (ref. 5) they require the use of much higher amounts of cobalt catalyst and have not achieved the same importance as the Amoco-MC process. Indeed, the... 

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... 

The actual schemes of these reactions are very complicated the radicals involved may also react with the metal ions in the system, the hydroperoxide decomposition may also be catalysed by the metal complexes, which adds to the complexity of the autoxidation reactions. Some reactions, such as the cobalt catalysed oxidation of benzaldehyde have been found to be oscillating reactions under certain conditions [48],... 

Benzoic Acid. The industrial-scale oxidation of toluene to benzoic acid is carried out with cobalt catalysts.973,978,982 983 The process, a free-radical autoxidation, is significantly promoted by bromide ions.984 Under these conditions bromine atoms rather than alkylperoxy radicals serve as a regenerable source of chain-initiating free radicals. Substantial rate increase can be achieved by the addition of manga-nese(II) ions.984... 

Autoxidation of Hydrocarbons Catalyzed by Cobalt and Bromide Ions... 

Oxidation Products. Although the ratio of hydroxyl to carbonyl products is 1/1 or nearly so in the ordinary metal salt-catalyzed autoxi-dation of hydrocarbons, higher proportions of carbonyl compounds are obtained in autoxidations catalyzed by cobalt and bromide ion—e.g.,... 

The effect of bromide ion was more pronounced in polystyrene oxidation. Although polystyrene in a 1/1 mixture by volume of chlorobenzene and acetic acid is barely autoxidized at 100°C. in the presence of cobalt salt or initiators, the oxidation catalyzed by cobalt is so strongly accelerated by bromide ion that it proceeds rapidly even at temperatures as low as 45°C. (Figure 10). 

Allan S. Hay During the autoxidation of p-xylene catalyzed by cobalt acetate bromide, a potentiometric titration for bromide ion of an aliquot of the reaction mixture at 0°C. shows that only a fraction of the bromide is present in ionic form. If the titration is performed at room temperature, there is a gradual drift of the end point until it finally corresponds to the calculated total amount of bromide. The implication thus is that benzylic bromides are present during the reaction, and at room temperature during the titration they are slowly solvolyzed. 

The resultant hydroxyl radicals are effective in initiating many chain reactions. The number of metal ions and complexes which are capable of activating hydrogen peroxide in this manner is quite large and is determined in part by the redox potentials of the activator. Related systems in which free radicals are generated by the intervention of suitable metallic catalysts include many in which oxygen is consumed in autoxidations. Cobalt(H) compounds which act as oxygen carriers can often activate radicals in such systems by reactions of the type ... 

X lO M, respectively. These observations apply to ferrous, cobaltous, and manganous phthalocyanines, and it is noteworthy that the cupric, magnesimn, and nickel phthalocyanines had no significant catalytic activity. It appears, therefore, that oxidation of the central metal ion by one-electron transfer plays an important role in initiating autoxidation. 

Bromide, as hydrogen bromide, alkali bromide, NH4Br, or CoBr2, or organically bound bromide as in bromoform, tetrabromoethane, or monobromoacetic acid, has an expressed effect on the cobalt- and manganese-catalyzed autoxidations of al-kylaromatic hydrocarbons. The catalytic activity of the metal ions is drastically increased by an addition of bromide ions in the right molar ratio, mostly n(metal)/n(Br) = 1 1. 

One aspect which sets oxidation apart from other reactions, e.g. hydrogenation and carbonylation is the fact that there is almost always a reaction (free radical chain autoxidation) in the absence of the catalyst (Reactions 1-3). Moreover, (transition) metal ions which readily imdergo a reversible one-electron valence change, e.g. manganese, cobalt, iron, chromium, and copper, catalyze this process by generating alkoxy and alkylperoxy radicals from RO2H (Reactions 4-6). 

For example, the cobalt(II) complex for phthalocyanine tetrasodium sulfonate (PcTs) catalyzes the autoxidation of thiols, such as 2-mercaptoethanol (Eq. 1) [4] and 2,6-di(t-butyl)phenol (Eq. 2) [5]. In the first example the substrate and product were water-soluble whereas the second reaction involved an aqueous suspension. In both cases the activity of the Co(PcTs) was enhanced by binding it to an insoluble polymer, e.g., polyvinylamine [4] or a styrene - divinylbenzene copolymer substituted with quaternary ammonium ions [5]. This enhancement of activity was attributed to inhibition of aggregation of the Co(PcTs) which is known to occur in water, by the polymer network. Hence, in the polymeric form more of the Co(PcTs) will exist in an active monomeric form. In Eq. (2) the polymer-bound Co(PcTs) gave the diphenoquinone (1) with 100% selectivity whereas with soluble Co(PcTs) small amounts of the benzoquinone (2) were also formed. Both reactions involve one-electron oxidations by Co(III) followed by dimerization of the intermediate radical (RS or ArO ). 

The reactive species that initiate free-radical oxidation are present in trace amounts. Extensive studies (11) of the autoxidation mechanism have clearly established that the most reactive materials are thiols and disulfides, heterocyclic nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further complicates the control of oxidation. It is difficult to avoid some metals, particularly iron, in fuel systems. 

Ions of transition metals (homogeneously or in some cases supported on polymers [5]) also effectively catalyze the autoxidation. Salts of cobalt, manganese, iron, copper, chromium, lead, and nickel are used as catalysts that allow the reactions to be carried out at lower temperatures, therefore increasing the selectivity of the oxidation (see, for example, [6]). However, it is more important that the catalyst itself may regulate the selectivity of the process, leading to the formation of a particular product. The studies of the mechanism of the transition metal salt involvement have shown their role to consist, in most cases, of enhancing the formation of free radicals in the interaction with the initial and intermediate species. 

The RO-OH cleavage can also be catalyzed by transition metal ions that are able to undergo one-electron redox switches such as cobalt and manganese, among others. The catalysis of autoxidations is discussed in more detail in chapters dealing with specific industrial processes. 

Autoxidation reactions of organic compounds are quite widespread and Co(lll) is among the most generally active metal ions serving as a catalyst in such reactions [36]. Cobalt—phthalocyanine tetrasulfonate bound cationic latexes have been... 

A drop of the test solution is placed on filter paper and spotted with a (hrop of a saturated water solution of sodium azide. The fleck is exposed to the vapors of a saturated aqueous solution of sulfurous acid. A yellow color appears which changes to blue on treatment with a drop of a 2 % acetic acid solution of a-tolidine Idn, Limit 0.5 y Co). The test is based on the fact that the oxidation of complex Co azide to complex cobalt azide is catalyzed by the autoxidation of sulfurous acid. The color reaction with o-tolidine is due to the action of the tervalent cobalt formed. Copper and iron ions interfere and should be previously removed or masked. The test can be carried out in the presence of as much as 200 times the amount of nickel. 

On adding an excess of sodium azide to neutral or slightly acid solutions of cobalt salts, a violet color appears which is due to complex cobalt -azide anions. In contact with air an oxidation to anionic azide-complexes of tervalent cobalt takes place, shown by a change of color from violet to yellow. This very slow autoxidation is enormously accelerated by sulfurous acid or sulfite ions. Probably the autoxidation of sulfite to sulfate induces the oxidation of the complex bounded cobalt. However, the color change violet yellow is not sufficiently sensitive to serve as a test for this induction effect and therefore, for sulfite. It is better to identify the tervalent cobalt through a color reaction with an acetic acid solution of o-tolidine (formation of a blue quinoidal oxidation product of the base). Compare page 209. 

Lipid oxidation is catalysed by transition metals that form compounds in many oxidation states, due to the relatively low reactivity of unpaired d electrons. These compounds, which include mainly iron, copper, manganese, nickel, cobalt and chromium, are reduced by adopting one electron. The last three elements are indeed fairly active, but the level of their active forms is so low that these metals are almost of no significance. Other metals, as free ions or some undissociated salts or complexes, act as catalysts directly or indirectly in the initiation, propagation and termination phase of autoxidation reaction. 

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