Autooxidation catalysts

Polyoxometalates with the required redox properties can also be used in a straightforward manner as autooxidation catalysts. In this way the trisubstituted Keggin compound, [M3(H20)3PW9037] (M = Fe" and Cr ") and [Fe2M(H20)3PW9037 (M = Ni", Co , Mn and Zn ) were used in the autooxidation of alkanes such as propane and isobutane to acetone and tert-butyl alcohol [61]. Later [Fe2. Ni(0Ac)3PW9037] was prepared and used to oxidize alkanes such as adamantane. 

Because the chemiluminescence intensity can be used to monitor the concentration of peroxyl radicals, factors that influence the rate of autooxidation can easily be measured. Included are the rate and activation energy of initiation, rates of chain transfer in cooxidations, the activities of catalysts such as cobalt salts, and the activities of inhibitors (128). 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

In another reactor with a different catalyst this molecule decomposes with water to form acetone and phenol (Figure 10-5). Most industrial acetone and phenol are now made by this process, in which the key step is autooxidation. 

Hydrogen peroxide is commercially produced by autooxidation of ethyl anthraquinol in a solvent such as toluene or ethylbenzene. The product ethyl anthraquinone is reduced by hydrogen over supported nickel or platinum catalyst to regenerate hack the starting material, ethyl anthraquinol for a continuous production of H2O2. The reaction steps are ... 

Phenanthroline in the presence of heavy metals acts as an activator of the polymerization of vinyl compounds558,559 and other olefins.560-564 It also assists the dimerization of olefins in the presence of titanium catalysts.565,566 It enhances the metal catalyzed oxidation of ascorbic acid567 and dimethyl sulfoxide.568 On the other hand, on its own it can inhibit several polymerization processes.545,569 It also stabilizes butadiene and isoprene and prevents their dimerization.570 It prevents peroxide formation in ether,571 inhibits the vinylation of alcohol572 and stabilizes cumyl chloride.573 It accelerates the vulcanization of diene rubbers574 and copolymers.575 1,10-Phenanthroline catalyzes the autooxidation of linoleic and ascorbic acids in the absence of metals.567... 

In a brief survey of other simple binary carbonyls we find that the compounds M(CO)6(M = Cr, Mo, W) and Ru3(CO)12 have only minimal catalytic activity for autooxidizing alcohols or ketones. The compounds Fe(CO)5 and Fe3(CO)12 are decomposed completely when we try to use them as catalysts. When the compound Mn2(CO)i0 is used, there is a considerable enhancement in acid formation. During this reaction there is extensive decomposition to manganese dioxide, and we believe that this compound is the one primarily involved in the catalytic oxidation. 

In its ground state, the oxygen molecule exists as a diradical it is a stable molecule in isolation, and its reactivity is greatly enhanced in the presence of catalysts. In alkali-catalyzed autooxidation, fragmentation of carbon-hydrogen bonds is either caused or greatly facilitated by alkali subsequent transfers of electrons are relatively rapid and facile processes. 

In the autooxidation system, the initial stage only (dissociation) is catalyzed. Other stages are usually insensitive or less sensitive to the catalyst action. This situation strictly limits the abilities of catalyst selection for selective homolytical oxidation. 

From a practical point of view the study of the chemistry of dioxygen complexes has considerable interest. Complexation of molecular oxygen by a transition metal has been widely adopted by biological systems as a means of reducing the considerable kinetic barrier to the reduction of O2. Quite apart from the inherent interest of the biological systems, the transition metal complexes offer the possibility of efficient catalysis of autooxidation reactions, and have recently attracted interest as possible catalysts for the reduction of O2 in fuel cells ... 

Molecular oxygen can also oxidize a variety of organic compounds, including hydrocarbons, aldehydes, amines, ethers and ketones. These autooxidation reactions can be used to make a variety of small molecules and a number of industrial processes rely on the controlled oxidation of organics using molecular oxygen (often with a metal catalyst). Examples include the formation of phenol and acetone from cumene (isopropylbenzene) and cyclohexanone from cyclohexane. Phenol is a popular starting material for a number... 

The addition of trace levels (>1M) of bis(bipyridine)cobalt(II) to O2-saturated solutions of aldehydes in acetonitrile initiates their rapid autooxidation to carboxylic acids. 0 Figure 6-1 illustrates the CoIKbpy)2 -induced autooxidation of hexanal [CH3(CH2)4CH(O)] for 02-saturated (8.1 mM) and air-saturated (1.6 mM) acetonitrile. The apparent reaction dynamics for the catalyzed auto-oxidation of PhCH(O) and of CH3(CH2)4CH(O) during the first hour of their auto-oxidation is summarized in Table 6-1. The initial reaction rates appear to be first order in catalyst concentration, first order in substrate concentration, and first order in O2 concentration (Fig. 6-1). However, within one hour the autooxidation process is almost independent of catalyst concentration. Although the Fellfbpy) and Mnii(bpy)complexes also induce the auto-oxidation of aldehydes, they are much less effective initiators, and the propagation dynamics are much slower. 

It is evident from the experiments in this study that both the CrY catalyst and NMP must be present to achieve 20 % yield of oxygenated products and close to 90 % selectivity towards the 1-one product. However, it is also clear that the bulk autooxidation of cyclohexene gives a considerable contribution to the total conversion of cyclohexene. The experiments also suggest that this represents two different mechanisms, where the bulk autooxidation of cyclohexene proceeds as discussed above. This is in accordance with the fact that both CrY and NMP separately increase the conversion of cyclohexene, but without any increase in selectivity. 

Catalytic hydrogenation with cobalt/aluminum catalyst gave polyethylene copolymers (from the hydrogenation of butadiene copolymers) or ethylene/propylene copolymers (from isoprene copolymers) containing 2,6-ditertiarybutyl-il--vinylphenol or 2,6-ditertiarybutyl-4-iso-propenylphenol in the polymer. These polymers have been used as polymeric antioxidants and axe effective in retarding autooxidation of polyolefins ( 0). 

It is known that the redox potential of the Fe+3/Fe+2 pair can vary by complexing ligands (27). EDTA reduces the redox potential of Fe+2 (28) and this increases the rate constant transfer of the electron from Fe+2 to H202, which is formed during autooxidation of ascorbic acid (29), and decomposition of the latter to H0-. However, at low pH 3-4, EDTA was found to inhibit ascorbic acid oxidation by ferric ions (29). Thus, the form the metal chelate takes, as a function of pH, plays a key role in its effectiveness as a catalyst. Cupric ions are known to accelerate ascorbic acid oxidation however, EDTA inhibits its catalytic effect at both neutral and low pH (24). 

The hydroxylation of DPQ (steps F-H in Figure 7) is mimicked by the copper(II)-catalyzed aqueous autooxidation of 4-alkylcatechols to 2-alkyl-5-hydroxybenzoquinones (Figure 10). The role of the catalyst here is to promote oxidation of the catechol precursor the subsequent hydrolysis step is promoted by base, but not by copper. Interestingly, the hydrolysis is in fact a result of 1,4-addition of H2O2 to C5 of the catechol, rather than H2O (Figure 10) " H2O2 is produced in the reaction mixture as a by-product of catechol oxidation. Hence, this apparently simple synthetic reaction does not accurately model the biogenesis of TPQ, in which the C2 O atom in the final product is derived from water. 

The effect of surfactants on the autooxidation of different thiols such as 2-mercaptoethanol (ME), 3-mercapto-1,2-propanediol (MPD), mercaptoacetic acid (MAAc) and dodecylmercaptan (DDM) in the presence of a polymeric catalyst, has been studied [155,156]. Fig. 20 illustrates the change in the initial rate of ME oxidation and the change in solution turbidity as a function of the concentration of the added anionic surfactant, sodium dodecylsulfate (DDS). The strong drop in the reaction rate and the increase in the turbidity of the system are apparently due to the formation of a complex between the polyion and the surfactant. However, the nonionic surfactant, oligoethylene oxide, does not influence ME oxidation. 

The applications reported for polymer-supported, soluble oxidation catalysts are the use of poly(vinylbenzyl)trimethylammonium chloride for the autooxidation of 2,6-di-tert-butylphenol [8], of copper polyaniline nanocomposites for the Wacker oxidation reaction [9], of cationic polymers containing cobalt(II) phthalocyanate for the autooxidation of 2-mercaptoethanol [10] and oxidation of olefins [11], of polymer-bound phthalocyanines for oxidative decomposition of polychlorophenols [12], and of a norbornene-based polymer with polymer-fixed manganese(IV) complexes for the catalytic oxidation of alkanes [13], Noncatalytic processes can also be found, such as the use of soluble polystyrene-based sulfoxide reagents for Swern oxidation [14], The reactions listed above will be described in more detail in the following paragraphs. 

Isolation of a cobalt phthalocyanine catalyst known to be active in autooxidation and to be deactivated by dimerization has been reported by Schutten (36). In this case, a polyvinylamine poly-dentate ligand was added to a dilute aqueous solution of the cobalt(II) phthalocyanine tetra(sodium sulfonate) in order to prepare a thiol oxidation catalyst. By employing dilute solutions, the polydentate polyamine polymer in effect isolated the cobalt(II) catalyst within an individual polyamine coil minimizing dimerization and significantly increasing catalyst activity. 

Figure 1.16. Cyclic voltammograms under N2 (A,C) and rotating ring-disk current-potential curves in aqueous air-saturated pH 7 buffers (B,D) of 2FeCu and 2Fe-only directly adsorbed on a graphite electrode (A,B) and as a 0.7% (mol) suspension in a 1-/rm-thick phosphadytilcholine film on the electrode surface (C.D). The rapid charge transfer within the films of adsorbed catalysts is supported by the linear dependence of the peak currents on the scan rate. The non-ideal shape of the peaks is due to cooperative behavior of the catalytic films as a whole. The Fe / and Cu / potentials are the same in the adsorbed catalysts (A) but separate when the catalysts are in the lipid film (C). Autooxidation of the catalyst-02 complex is the major source of ring-detectable byproducts (see below) and accounts for the potential-dependent selectivity of electrode-adsorbed catalysts (B). The measured collection efficiency of the ring electrode toward H2O2 in these experiments was 15%.

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