Catalytic autoxidation

The various methods that are used for the production of aromatic acids from the corresponding substituted toluenes are outlined in Figure 1. The first two methods -chlorination/hydrolysis and nitric acid oxidation - have the disadvantage of relatively low atom utilization (ref. 13) with the concomitant inorganic salt production. Catalytic autoxidation, in contrast, has an atom utilization of 87% (for Ar=Ph) and produces no inorganic salts and no chlorinated or nitrated byproducts. It consumes only the cheap raw material, oxygen, and produces water as the only byproduct. 

Consequently, as a result of increasing environmental pressure many chlorine and nitric acid based processes for the manufacture of substituted aromatic acids are currently being replaced by cleaner, catalytic autoxidation processes. Benzoic acid is traditionally manufactured (ref. 14) via cobalt-catalyzed autoxidation of toluene in the absence of solvent (Fig. 2). The selectivity is ca. 90% at 30% toluene conversion. As noted earlier, oxidation of p-xylene under these conditions gives p-toluic acid in high yield. For further oxidation to terephthalic acid the stronger bromide/cobalt/manganese cocktail is needed. 

Moreover, the efficiency of catalytic autoxidation in Scheme 26 is also attributed to the short lifetime of the hydroquinone cation radical (t < 10 10 s-1)254, which renders the sequential electron-transfer/proton-trans-fer cycles extremely efficient. 

The Fe(III)/S(IV) reaction has long been of interest because of its importance in the catalytic autoxidation of S(IV). The latter reaction is known to have a complex chain mechanism, and the production of SOr radicals has been considered to be the essential chain-initiating step. It is also widely believed that the direct oxidation of S(IV) by Fe(III) is the source of SO -. There is little agreement among the various papers published on the direct reaction of Fe(III) with S(IV) with regard to its mechanism, and much of this disagreement can be traced to the potential for Fe(III) to bind several S(IV) ligands under the typical conditions of excess S(IV). 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Zwart and co-workers confirmed that the catalytic autoxidation produces cystine (RSSR) with 100% selectivity and the actual stoichiometry can be given by the following equation in alkaline solution ([NaOH] = 0.25 M) (64) ... 

Co(HI) and Mn(ffl) salts are used as initiators for the autoxidation of methylaromatics to carboxylic acids. The metal "radicals" assist in making the first organic radicals which will subsequently enter into the common catalytic autoxidation cycle for RH molecules ... 

It should be borne in mind, however, that the foregoing reaction is a stoichiometric one. In a catalytic autoxidation such a process would be readily masked by one involving metal-catalyzed decomposition of the hydroperoxide. 

Perez Bernal, M. E., Ruano Casero, R. and Pinnavaia, T. J. (1991). Catalytic autoxidation of 1-decanethiol by cobalt(II) phthalocyaninetetrasulfonate intercalated in a layered double hydroxide. Catal. Lett. 11, 55. 

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. 

T. Iwahama, G. Hatta, S. Sakaguchi, Y. Ishii, Epoxidation of alkenes using alkyl hydroperoxides generated in situ by catalytic autoxidation of hydrocarbons with dioxygen, Chem. Commun. (2000) 163. 

Calvert (2 ) has pointed out that gas-phase reactions of SO2 with ozone (O3), hydroxyl radical (OH ), and hydroperoxyl radical (HOp ) are too slow to account for the aforementioned rates of sulfate production. Consequently, the catalytic autoxidation of SO2 in deliquescent haze aerosol and hydrometeors has been proposed as a viable non-photolytic pathway for the rapid formation of sulfuric acid in humid atmospheres (30-35). In addition, hydrogen peroxide and ozone have been given serious consideration as important aqueous-phase oxidants of dissolved SO2 as discussed by Martin (35). Oxidation by H2O2 seems to be most favorable under low pH conditions (pH < 4) because of a rapid rate of reaction anc[ a negative pH-dependence that favors the facile conversion of HSO3 to sulfate. 

Transition-metal ions such as Fe(III), Cu(II), Co(II), Co(III), and Mn(II) have been shown to be effective homogeneous catalysts for the autoxidation of sulfur dioxide in aqueous solution. Hoffmann and coworkers have shown that Fe(III) and Mn(II) are the most effective catalysts at ambient concentrations for the catalytic autoxidation of S(IV) to S(VI) in cloudwater and fogwatet (Jacob and Hoffmann, 1983 Hoffmann and Jacob, 1984 Hoffmann and Calvert, 1985). Mechanisms for the homogeneous catalysis by Fe(lII) and Mn(II) that have been proposed include a free-radical chain mechanism, a polar mechanism involving inner-sphere complexation followed by a two-electron transfer from S(IV) to bound dioxygen, and photoassisted electron transfer. 

Hoffmann, M. R., and S. D. Boyce (1983), Theoretical and Experimental Considerations of the Catalytic Autoxidation of Aqueous Sulfur Dioxide in Relationship to Atmospheric Systems, Adv. Environ. Sci. Technol. 12, 149-148. (Wiley-Interscience, New York). 

Hong, A. P., S. D. Boyce, and M. R. Hoffmann (in press), Catalytic Autoxidation of Chemical Contaminants by Hybrid Complexes of Co(II) Phthalocyanine, Environ. Sci. Technol. 23. 

Japanese chemists3 have used the related cobalt catalyst (di-(3-salicylidene-nminopropyOamine cobalt(ll) (1) for catalytic autoxidation of 4-alkyl-2,6-di-/-buty lphenols (2) to the corresponding p-quinols (4) in good yield. Hydroperoxides (3) are formed as intermediates but during workup are reduced to quinols (4). 

The primary product of the photochemical or catalytic autoxidation of an aldehyde (RCHO) in the liquid phase by dissolved molecular oxygen is the corresponding peracid, RC03H, viz. 

Early work on the catalytic autoxidation of carboxythiols confirmed the effectiveness of manganese, iron, cobalt, copper, and arsenic, but the first major assault on the mechanism of the reaction was due to Michaelis and Barron [123,124]. The oxidation of cysteine at pH 7—8 was found to be zero order in cysteine and to involve metal—cysteine complexes as active intermediates. Several studies of metal—thiol complexes have been... 

Other zeolitic materials with adsorbed porphyrins on external surface include the immobilization of Sn(TMPyP)Cl2 andSn(TPyP)Cl2onzeoliteYforuseasa solid photosensitizer material in the photodegradation of organic substrates. Zeolite supported Fe[T(n-MPy)P] (n =2, 3 or 4) was explored in heterogeneous catalytic autoxidations of sulfite. 

However, all types of alkenes are not always used in the peroxidation. The most efficient alkene is the 1,1-disubstituted alkene due to the stability of the intermediate tertiary radicals B in Scheme 31. Aliphatic terminal alkenes such as n-hexene are a poor substrate. In addition, ambient air is the best atmosphere for the catalytic autoxidation since overoxidation occurs under an oxygen atmosphere. 

The reaction (Equation 7.10), together with subsequent reoxidation of nitric oxide with oxygen, constitutes the reduction-oxidation cycle for the catalytic autoxidation with NO. On the basis of the understanding of the interaction of nitrosonium with aromatic donors [15], hydroquinones are expected to strongly shift the equilibrium (Equation 7.3) to favour the nitrosonium complex in conformity with enhanced donor properties of the aromatic substrates. Accordingly, the oxidation of hydroquinone with NO proceeds via the nitrosonium nitrate ion pair. The overall two-electron oxidation of hydroquinone to quinone by 2 equivalent NO probably proceeds via successive one-electron steps. The labelling studies in the process... 

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