Cyclic oxidation process

Scheme 2 Schematic presentation of the cyclical oxidation process and some of the main reactions/products formed from the propagating radicals. The antioxidant mechanisms interrupting the oxidative cycles are also shown. AO antioxidant, CB-A chain breaking acceptor, CB-D chain breaking donor, PD peroxide decomposer, UVA UV-absorber, MD metal deactivator...

SCHEME 22.1 Schematic presentation of cyclical oxidation process according to the mechanism presented by Bolland (1946). 

One possible approach to avoid cryogeiuc air separation is the cyclic oxidation process, in which a reducible metal oxide is used as an oxygen source (Eq. 8.12)... 

In addition. Figure 14.1 illustrates the cyclic nature of the initiation reactions. The carbon radicals (C ) enter into a cyclic oxidation process. In the presence of oxygen, peroxy radicals (COO ) are formed. The peroxy radicals can abstract a hydrogen atom from the backbone of the polymer chain to generate another carbon radical and a hydroperoxide group (COOH). The hydroperoxide group will decompose under heat to form an alkoxy radical (CO ) and a hydroxyl radical (HO ). Each of these radicals can abstract hydrogen from the polymer backbone to form yet more carbon radicals [12]. 

FIGURE 14.1 Cyclic oxidation process. (From Ohm, R.F., Improving the Heat Resistance of HNBR, ACM, EAM by the Use of Antioxidants, Crompton, presented at Connecticut Rubber Group Meeting, April 6, 2004. With permission.)... 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

Figure 5.2 Cyclic deracemization process involving sequential enzyme-catalyzed oxidation and nonenzymatic reduction.

The coordination of redox-active ligands such as 1,2-bis-dithiolates, to the M03Q7 cluster unit, results in oxidation-active complexes in sharp contrast with the electrochemical behavior found for the [Mo3S7Br6] di-anion for which no oxidation process is observed by cyclic voltammetry in acetonitrile within the allowed solvent window [38]. The oxidation potentials are easily accessible and this property can be used to obtain a new family of single-component molecular conductors as will be presented in the next section. Upon reduction, [M03S7 (dithiolate)3] type-11 complexes transform into [Mo3S4(dithiolate)3] type-I dianions, as represented in Eq. (7). 

Mixed-metal dendrimers containing up to 6 Pt(IV)-based organometallic species in the branches and 12 peripheral ferrocene units (8) have recently been synthesized and their electrochemical behavior investigated [13]. As in the previously discussed examples, multi-electron reversible oxidation processes, assigned to the equivalent, non-interacting ferrocene units, have been observed. The authors point out that cyclic voltammetry is a powerful tool to support the structure of the dendrimers containing ferrocene units. 

SCHEME 1 Schematic illustration of the biological process of 02 dismutation into 02 and H202 catalyzed by Cu, Zn-SOD via a cyclic oxidation-reduction electron transfer mechanism. (Reprinted from [98], with permission from Elsevier.)... 

The superoxide ion is a very weak hydrogen atom abstractor, which cannot continue the chain, and is destroyed via disproportionation with any peroxyl radical. So, the studies of the mechanisms of cyclic chain termination in oxidation processes demonstrate that they, on the one hand, are extremely diverse and, on the other, that they are highly structurally selective. The 20 currently known mechanisms are presented in Table 16.6. 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

The oxidation processes shown in reactions 4 and 5 are the basis for differential pulse polarography/cyclic voltametry determination of dialkyllead and trialkyllead species135. 

LO-CAT A process for removing hydrogen sulfide and organic sulfur compounds from petroleum fractions by air oxidation in a cyclic catalytic process similar to the Stretford process. The aqueous solution contains iron, two proprietary chelating agents, a biocide, and a surfactant the formulation is known as ARI-310. The sulfur product is removed as a slurry. Developed in 1972 by Air Resources (now ARI Technologies) and first commercialized in 1976. Over 125 units were operating in 1996. An improved version, LO-CAT II, was announced in 1991. 

Persson A process for making chlorine dioxide by reducing sodium chlorate with chromium (III) in the presence of sulfuric acid. The chromium (III) becomes oxidized to chromium (VI) and is then reduced back to chromium (III) with sulfur dioxide. This cyclic redox process with chromium avoids complications that would occur if sulfur dioxide itself were used as the reductant. Installed at the Stora Kopparbergs paper mill, Sweden, in 1946. Sheltmire, W. H., in Chlorine, its Manufacture, Properties and Uses, Sconce, J. S., Ed., Reinhold Publishing, New York, 1962,275,538. 

A related method of preparation involves the oxidative addition of a tin(II) salt to propargylic iodides, which yield mixtures of allenyl- and propargyltin halides on treatment with SnCl2 in DMF-DMI (l,3-dimethylimidazol-2-one) (Eq. 9.75) [68], These intermediates react in situ with aldehydes to afford mixtures of propargylic and allenic carbinols via a cyclic SE2 process (Eqs. 9.76 and 9.77). As explained in the Introduction, the ratio of these two products reflects the relative transition-state energies of the addition reactions. 

To obviate possible erroneous interpretation of the cyclic voltammetric profiles (particularly in those cases in which an oxidation process occurs at negative potential values as well as a reduction process occuring at positive potential values), it is always wise to perform preliminarily hydrodynamic tests. 

In confirmation, Figure 44 compares the cyclic voltammogram illustrated in Figure 43b with the voltammogram obtained through the use of a platinum electrode with periodical renewal of the diffusion layer. As seen, it confirms that the species undergoes consecutive oxidation processes. 

As seen, it affords a single oxidation process the cyclic voltammetric profile of which is very close to that expected for the mechanism ... 

It exhibits a single oxidation process (Ea = + 0.59 V, vs. SCE) affected by some adsorption problems. These adsorption phenomena, which typically affect the electrochemical response of these derivatives, sometimes make it difficult to ascertain by controlled potential coulometry the effective number of electrons involved in the oxidation step. In this case, the (approximate) number of electrons involved per molecule of dendrimer, nd, can be roughly calculated by comparing the cyclic voltammetric responses of the dendrimer with that of the ferrocene monomer using the following empirical equation.27,40... 

As illustrated in Figure 79, just like triferrocene, it displays three well separated (one-electron) oxidation processes, all possessing features of chemical reversibility on the cyclic voltammetric time scale. They are clearly attributed to the step-by-step electron removals Fen3/ Feni3(Eo/+ — +0.56 V vs. SCE EV,2 + = +0.93 V E2 + 13 + = + 1.25 V).122... 

Cyclic voltammetry revealed four reversible one-electron oxidation processes. The ground state structures for HOC and HOC " are aromatic . The structures for HOC" and HOC " are doublets and show Jahn-Teller distortions. The structures for HOC " in the solid is a Jahn-Teller distorted, closed-shell singlet with a canonical structure [37 ], as represented by cyanine/p-phenylenediammonium fragments. The one-to-one complex of HOC" TCNE" was indeed synthesized but found to be antiferromagnetic. 

That specific hydride transfer from carbon to carbon does occur, was established by showing that use of labelled (Me2CDO)3Al led to the formation of RjCDOH. The reaction probably proceeds via a cyclic T.S. such as (47), though some cases have been observed in which two moles of alkoxide are involved—one to transfer hydride ion, while the other complexes with the carbonyl oxygen atom. The reaction has now been essentially superseded by MH reductions, but can sometimes be made to operate in the reverse direction (oxidation) by use of Al(OCMc3)3 catalyst, and with a large excess of propanone to drive the equilibrium over to the left. This reverse (oxidation) process is generally referred to as the Oppenauer reaction. 

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