Kinetics of inhibited oxidation

This greatly alters the kinetics of inhibited oxidation by making it dependent on p02, since chain termination by reaction (16) competes with reaction (1) of chain propagation. The rate of oxidation becomes dependent on the dioxygen partial pressure. If the radical formed in the reaction of an acceptor with the alkyl radical does not participate in chain propagation, the following equations are valid for description of t, v, and A[02] ... 

Quasistationary Kinetics of Inhibited Autoxidation Topological Kinetics of Inhibited Oxidation of Hydrocarbons... 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1], The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16], Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction [6], However, the addition of an aqueous solution of Na2C03 or NaOH in combination with vigorous agitation of this system accelerates the oxidation process [1-17]. The addition of an aqueous phase accelerates the oxidation and withdrawal of water retards it [6]. The addition of surfactants such as salts of fatty acids accelerates the oxidation of cumene in emulsion [3], The higher the surfactant concentration the faster the cumene autoxidation in emulsion [17]. The rates of cumene emulsion oxidation after an induction period are given below (T = 353 K, [RH] [H20] = 2 3 (v/v), p02 = 98 kPa [17]). 

Depending on the oxidation conditions and its reactivity, the inhibitor InH and the formed radical In can participate in various reactions determining particular mechanisms of inhibited oxidation. Of the various mechanisms, one can distinguish 13 basic mechanisms, each of which is characterized by a minimal set of elementary steps and kinetic parameters [38,43 15], These mechanisms are described for the case of initiated chain oxidation when the initiation rate v = const, autoinitiation rate fc3[ROOH] -C vy and the concentration of dissolved dioxygen is sufficiently high for the efficient conversion of alkyl radicals into peroxyl radicals. The initiated oxidation of organic compounds includes the following steps (see Chapter 2). 

The mechanisms by which an inhibitor adds to an oxidized hydrocarbon exerts its influence may differ depending on the reaction conditions. If the rate constants of the elementary reactions of RH, InH, R02 , In, ROOH, and 02 are known, the kinetics of the inhibited oxidation of RH can mathematically be described for any conditions. However, such an approach fails to answer questions how the mechanism of inhibited oxidation is related to the structure and reactivity of InH, RH, and R02 or what inhibitor appears the most efficient under the given conditions, and so on. At the same time, these questions can easily be clarified in terms of a topological approach whose basic ideas are the following [43-45,70-72] ... 

Kinetics of the dimethyidioxirane oxidation of adamantane in an oxygen atmosphere support a radicai mechanism. The kinetics of the oxidation of 2-methyibutane by DMDO in acetone soiution have been studied and the mechanisms of the reaction and of inhibition of the reaction by O2 were discussed. 

Although Equation C often adequately describes the kinetics of hydrocarbon oxidation inhibition by phenols, more complicated kinetics are common (12), particularly for hydrocarbons which yield hydroperoxide as the primary oxidation product. The complications arise mainly from reversibility of Reaction 7 but can also result from a chain-transfer reaction ... 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1]. The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16]. Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction... 

With respect to the kinetics of aromatic oxidations, (extended) redox models are suitable, and often provide an adequate fit of the data. Not all authors agree on this point, and Langmuir—Hinshelwood models are proposed as well, particularly to describe inhibition effects. It may be noted once more that extended redox models also account for certain inhibition effects, for mixtures of components that are oxidized with different velocities. The steady state degree of reduction (surface coverage with oxygen) is mainly determined by the component that reacts the fastest. This component therefore inhibits the reaction of a slower one, which, on its own, would be in contact with surface richer in oxygen (see also the introduction to Sect. 2). 

The kinetics of the oxidation of l-phenyl-2-thiourea by chlorite, in aqueous acidic media, are strongly influenced by the pH and show a complex acid dependence. The proposed mechanism involves HOC1 as a major intermediate whose autocatalytic production determines the observed kinetics of the reaction. The oxidation involved the formation of two stable intermediates, the sulfinic acid and the sulfonic acid, on the pathway towards total desulfurization to form phenylurea. A comprehensive 29-reaction scheme has been proposed to describe the observed complex kinetics.96 The oxidation of trimethylthiourea (TMTU) by chlorite in slightly acidic media is very fast. The oxidation of TMTU proceeds through the formation of sulfinic acid then to the sulfoxylate anion. The direct reaction of chlorine dioxide and TMTU is autocatalytic and is also inhibited by acid. A series of 28 reactions have been proposed to describe the mechanism.97... 

Dunfield P. and Knowles R. (1995) Kinetics of inhibition of methane oxidation by nitrate, nitrite, and ammonium in a Humisol. Appl. Environ. Microbiol. 61, 3129-3135. 

It was found that the simple Vw complex with A,A -ethylenediaminediacetate (EDDA) had insulin-enhancing properties. 05 The corresponding complex with A,A -ethylene bis(5)-methionine was found to have a lesser effect with regard to inhibition of the free fatty acid release from diabetic rat adipocytes.605 The kinetics of the oxidation of YIV complexes in solution with... 

An investigation of the kinetics of hydrazine oxidation [149] yielded the expression r = fe [N2H4][O2] for the rate of reaction, i.e. the reaction is inhibited by the educt oxygen. The activation energy Er of the reaction rate constant of Er =... 

This result does not agree with the actual value of 0.29, because the first-order assumption does not fit the kinetics of SO2 oxidation. Sulfur trioxide inhibits the rate of this reversible reaction. The effect of diffusion... 

The kinetics of CO oxidation on LaCoOj has been studied by Tascdn et al. (186). In the temperature range 120-155°C these authors found a strong C02 inhibition and proposed a kinetic equation by assuming the controlling step to be the surface reaction between adsorbed CO and... 

Ioffe and Sherman (149) studied the kinetics of naphthalene oxidation to phthalic anhydride on a more complex vanadium-potassium-sulfate catalyst over a wide range of conversions and temperatures. The naphthalene oxidation was found to be independent of naphthalene concentration. This reaction is first order with respect to oxygen concentration and is inhibited with reaction products. 

Temkin et al. (159) studied the kinetics of ethylene oxidation over a stationary silver surface. It was shown by means of the flow-circulating method that the rate of ethylene oxide and carbon dioxide formation was proportional to ethylene concentration in the gas phase, and that there was inhibition with reaction products. 

However, in the years after Ostwald, his former students and collaborators dominated catalysis in Germany. We can mention Bodenstein who put the study of the kinetics of heterogeneous catalysis essentially in its modern state. The Ostwald school made little contribution to mechanism, but it adhered to a view surprisingly close to that of Faraday. The famous paper of Bodenstein and Fink of 1907 interpreted the kinetics of the oxidation of sulfur dioxide on platinum in terms of the diffusion of sulfur dioxide or oxygen through a polymolecular layer of adsorbed material. In extreme cases, accord with the observed inhibition by sulfur trioxide would have required adsorbed layers so thick that they could have been pared with a razor. 

The kinetics of the oxidations of Chromatium vinosum HIPIP by several ferrocenium derivatives show no inhibitions by charged redox-inactive metal complexes, and display a pH dependence (pK = 6.90) in which protonation reduces the HIPIP reactivity by a factor of two. Electron transfer at an uncharged hydrophobic patch near Cys-46 (4 A from the Fe4S4 core to the surface), enhanced by deprotonation of His-42, is inferred from the data. The self-exchange rate constant for the HIPIPo/HIPIPr couple is estimated to be 5 x 10 s from... 

To facilitate the data presentation let us briefly describe the general kinetic laws of the inhibited chain reaction. Specific features of inhibited oxidation in the condensed phase will be discussed in detail. This is mainly conditioned by the availability of detailed data on the inhibition mechanism for the chemical reactions imder consideration. In the next paragraphs they will be examined by the value method. 

The kinetics of the oxidation of benzaldehyde in the presence of [Pd(PPh3)2(02)] was studied [265]. The initial rate of oxygen consumption was described by equation (180), where Ci and are constants. The reaction was inhibited by free phosphine and markedly retarded by perbenzoic acid while added benzoic acid had no effect. 

It can be assumed that the accumulation of an inhibiting product leads to a decrease in the autocatalysis factor , the kinetics of the oxidation being described by the equation... 

Certain important principles have been detected in the behavior of inhibitors in the oxidation of rubbers. It has been established earlier that the oxidation of rubbers, as well as that of low-molecular hydrocarbons, is accompanied by consumption of the inhibitor, which is added to the molecules of the substance to be oxidized. In a number of cases it has been shown that the consumption of the inhibitor occurs at a constant rate. This phenomenon finds a kinetic explanation if we assume that the inhibitor is consumed only as a result of its reaction with the hydrocarbon being oxidized, leading to chain termination. In this case, if termination proceeds primarily on the inhibitor, in a steady-state process the rate of inhibitor consumption is equal to the rate of initiation. It has been established, however, that such a principle is manifested only at small inhibitor concentrations (up to 1%) at concentrations exceeding 1%, the consumption or addition of the inhibitor to the polymer already does not occur linearly, but the initial rate of consumption increases with increasing inhibitor concentration. Thus, the rate of inhibitor consumption does not always reflect the process of inhibited oxidation of polymers. In the case of inhibitors of the class of secondary amines, not only addition of the amine radical to the hydrocarbon being oxidized, but also partial recombination of its radicals occurs, forming dimers of the type... 

Kinetics of the oxidation of diallyl sulphide by singlet oxygen, and its inhibition by rubrene, have been reported. Isomerization studies in which the base-catalysed prop-2-eny 1-prop-1-enyl equilibrium for allyl sulphides, sulphoxides, and sulphones were compared have been undertaken, and isolated reports on this topic have been rationalized in terms of electronic distribution in the functional groups. 

As can be seen, the kinetics of the oxidation of the PP/FGG nanocomposites has features characteristic of the inhibited oxidation of PP the parabolic law of oxidation holds, with the parameter b being approximately the same for all the samples. A significant increase in the induction period is observed at the content of FGG 1.6-3.6 wt % x=140 min for pure PP and x = 152, 310, 390 min at 0.8, 1.6, 3.6 wt% of FGG content respectively. This means that FGG is a very effective inhibitor of the oxidation of PP/FGG nanocomposites. However, at low FGG content (0.8%), the induction period is almost the same as that for pure PP. It makes sense that, under these conditions, carbon particles manifest their ability to initiate the oxidation of PP, as was observed previously in the oxidation of graphite-PP composites [12] and in the oxidation fullerene-PP nanocomposites [16]. 

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