Homogenization autoxidation

Examples include high-pressure hydrolysis of a nitrile paraffin autoxidation homogeneous aldehyde hydrogenation olefin hydroformylation to alcohol with paraffin by-product formation, aldehyde condensation to heavy ends, and olefin isomerization cyclo-addition reactions and hydrogen-halide reactions. 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

There is specificity of the antioxidant action in the presence of heterogeneous catalyst. The kinetics of ionol retarding action on the oxidation of fuel T-6 catalyzed by the copper powder and homogeneous catalyst copper oleate was studied in Ref. [12]. Copper oleate appeared to be very active homogeneous catalyst it was found to catalyze the autoxidation of T-6 in such small concentration as 10 6 mol L-1 (T = 398 K). The kinetics of autoxidation catalyzed by copper salt obeys the parabolic law (see Chapter 4) ... 

A number of autoxidation reactions exhibit exotic kinetic phenomena under specific experimental conditions. One of the most widely studied systems is the peroxidase-oxidase (PO) oscillator which is the only enzyme reaction showing oscillation in vitro in homogeneous stirred solution. The net reaction is the oxidation of nicotinamide adenine dinucleotide (NADH), a biologically vital coenzyme, by dioxygen in a horseradish peroxidase enzyme (HRP) catalyzed process ... 

Present developments. One might think that an established reaction such as aerobic oxidation (or autoxidation) is not the subject of further research and improvement, but this is definitely not the case and both new homogeneous and heterogeneous catalysts are in development. In the introduction we already mentioned the drawbacks of oxidation of cyclohexene to adipic acid and several researchers address this challenge. Also a highly developed reaction such as the oxidation of paraxylene is subject to further improvements. 

This indicated unambiguously that Co-POM leaching from the solid catalyst into the solution occurred in the co-oxidation process and catalysis, at least, partially is homogeneous in nature. Indeed, the elemental analysis data confirmed substantial leaching of Co-POM during a-pinene/IBA co-oxidation (see Table 1). The difference observed for the autoxidation... 

Both homogeneous and heterogeneous catalytic oxidations can be divided into the same three categories based on the type of mechanism involved (a) autoxidation, (b) direct oxidation of (coordinated) substrates and (c) catalytic oxygen transfer. 

Similarly, cobalt(ll)-pyridine (CoPy) complexes bound to copolymers of styrene and acrylic or methacrylic acid, cross-linked with divinylbenzene, catalyze the autoxidation of tetralin dispersed in water at 50°C and 1 bar.45 The rate of oxidation with the colloidal CoPy catalyst was twice as fast as with homogeneous CoPy and nine times as fast as with cobalt(II) acetate in acetic acid. 

In a variation on this theme cobaltphthalocyaninetetrasulfonate (CoPcTs) was bound via the anionic sulfonate groups to styrene-divinylbenzene copolymer latexes containing quaternary ammonium ions.46 The resulting colloidal catalyst was used to effect the autoxidation of 2,6-di-tert-butylphenol in aqueous solution, to the corresponding diphenoquinone (reaction 21). The rate of oxidation was ten times faster than with homogeneous CoPcTs in water. 

Metal oxides have often been used as catalysts for the autoxidation of hydrocarbons.1 In many cases the metal probably dissolves in the reaction medium and catalysis involves homogeneous metal complexes. However, according to a recent report56 cerium oxide catalyzes the liquid phase oxidation of cyclohexanone in acetic acid (5-15 bar and 98-118°C) without dissolving in the reaction medium. 

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 this article I have calculated concentrations of linoleic acid, vitamin E, the initiator, and Op in the micellef since this is where the autoxidation occurs. Thus, the concentrations quoted in the text are 100 times larger than the values that would be obtained if the solution were assumed to be a single homogeneous phase and average concentrations were used. Similarly, therefore, the value of R. I have used is 100 times larger than the value calculated fronrthe rate of... 

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 mentioned in Section II.B.2 studies of the oxidation of olefins by molecular oxygen in the presence of low-valent Group VIII metal complexes, with the expectation of effecting homogeneous, nonradical oxidation processes. However, these reactions were shown to involve the usual free radical chain autoxidation, and no direct transfer of oxygen from a metal-dioxygen complex to an olefin was demonstrated. 

Thus, mechanistic studies of autoxidations in heterogeneous systems suffer from ambiguities similar to their homogeneous counterparts. There is no unequivocal evidence for the direct reaction of chemisorbed oxygen with hydrocarbon substrates under mild conditions. Catalysis of autoxidations proceeding by reaction with intermediate hydroperoxides is a more likely explanation. 

Figure 9.11. Dioxygen uptake plots for the autoxidation of 1-decanethiol in the presence of [CoPcTs]4- as catalyst (a) unsupported and (b) supported on a MgAl-LDH. A shorter induction time and a larger turnover frequency were obtained for the LDFI assembly compared with the homogeneous case. After Perez Bernal et al. [124],...

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