Inhibited liquid-phase oxidation

The reasons of critical phenomena in the inhibited liquid-phase oxidation of organic compounds are thoroughly studied. On the basis of theoretical fundamentals of degenerate branching-chain reactions developed by N.N. Semenov [1], N.M. Emanuel and A.B. Gagarina [33] explained the features of the manifestation of critical phenomena in such systems. 

EA Blumberg, YuD Norikov. Heterogeneous Catalysis and Inhibition of Reactions of Liquid-Phase Oxidation of Organic Compounds [Itogi Nauki i Tekhniki, Kinetika i Kataliz], vol 12. Moscow VINITI, 1984, pp 1-143 [in Russian],... 

Metals and metal oxides, as a rule, accelerate the liquid-phase oxidation of hydrocarbons. This acceleration is produced by the initiation of free radicals via catalytic decomposition of hydroperoxides or catalysis of the reaction of RH with dioxygen (see Chapter 10). In addition to the catalytic action, a solid powder of different compounds gives evidence of the inhibiting action [1-3]. Here are a few examples. The following metals in the form of a powder retard the autoxidation of a hydrocarbon mixture (fuel T-6, at T= 398 K) Mg, Mo, Ni, Nb V, W, and Zn [4,5]. The retarding action of the following compounds was described in the literature. 

It is considerably more difficult to inhibit oxidation in the gas phase than in the liquid phase. At the high temperatures of gas-phase oxidations the rates of the chain-propagating and branching reactions are increased to a greater extent than the rates of the chain-terminating reactions. Initiation by surfaces can also constitute a serious problem. The majority of liquid-phase antioxidants which are effective at high temperatures are too involatile to be useful in the gas phase. However, inhibition can be achieved with aliphatic amines, which are generally rather ineffective inhibitors of low temperature liquid-phase oxidations. The mechanisms by which the different types of antioxidants inhibit oxidation are briefly described below. 

The four remaining papers all deal with the catalysis of liquid-phase oxidation processes by transition metal ions (6). A. T. Betts and N. Uri show in particular how metal complexes can either catalyze or inhibit oxidation according to their concentration. In this investigation, various hydrocarbons (especially 2,6,10,14-tetramethylpentadecane) were used as substrates, and metal ions were present either as salicylaldimine or di-isopropylsalicylate chelates. These compounds are considerably soluble in non-polar media, and this makes it possible to examine their effect over a much wider range of concentration than is usually accessible in this type of work. These studies show that catalyst-inhibitor conversion is always... 

No account of liquid phase oxidations would be complete without some discussion of the inhibition of oxidation by chain-breaking antioxidants. For well over one hundred years, antioxidants have been used in a variety of commercial products to slow deterioration in air, rubber being among the first to receive attention [148]. Excellent reviews of the practical aspects of antioxidant use and development are given by Lundberg [149] and Scott [150]. 

The kinetic examination of the liquid phase oxidation of aldehydes is usually done using initial rates for small conversion ratios. This enables fortuitous auto inhibition and catalysis phenomena to be eliminated. Experimentation must be carried out on a single aldehyde batch, since considerable differences may be observed when going from one bath to another. This is a serious handicap with regard to the absolute values of the rate coefficients determined. 

The phenomenon of selective inhibition of chain reactions was for the first time explained by N.M. Emanuel, E.A. Bliunberg, L.A. Tavadyan and S.A. Maslov [11]. It was experimentally observed in a mnnber of liquid-phase oxidation reactions. Thus, introducing small amoimts of an additive stable nitroxyl radical (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, TEMPOL) and a heterogeneous inhibitor (WSc2) makes it possible to increase the selectivity of formation of imsaturated acids and epoxides in chain oxidation reactions of a-methylacrolein, 2-ethylhexenal, as well as co-oxidation of aldehydes and olefins (Table 5.1). 

Numerical identification and analysis of critical conditions for liquid-phase oxidation of ethylbenzene inhibited by butylated hydroxytoluene. The critical phenomena are studied and analyzed in detail for the liquid-phase autooxidation of organic substances, the carbochain polymers in the presence of inhibitors [32-34,52], Investigations in this direction cmrently are also urgent for predicting the antioxidant activity of compoimds, including the bioantioxidants [53-67],... 

Table 5.4. Kinetic model for liquid-phase oxidation of ethylbenzene inhibited by...

Figure 5.6. Reduced value contributions of individual steps for liquid-phase oxidation of ethylbenzene inhibited by butylated hydroxytoluene at conversions of the inhibitor 0.1% and 0.37% and temperatures 60 (a) and 120 (b). The initial concentrations of the inhibitor correspond to the critical...

From a methodical viewpoint it seems practical to address first the possibilities of the selective inhibition of the undesirable direction of the reaction for liquid-phase oxidation of a-methylaerolein, by using the formal kinetic tool for the multicentered chain reactions presented in the Section 5.1. Now, by analogy with the scheme (5.1), we present the above-listed set of steps (1-18) as the flow graph (Figmc 6.3) for the multicentered chain process [10]. 

For eoneentrations [ln]>2-10 M the condition (6.13) and consequently (6.12) are met, indicating the possibility to perform the selective inhibition of the chain liquid-phase oxidation of a-methylacrolein. 

The possibilities of the value approach are considered to solve the problem for the non-empirical selection of an effective inhibitor, based on a determined kinetic model of the inhibited reaction. The solving of such problems is demonstrated by the liquid phase oxidation of ethylbenzene with its inhibition by phenols. The transcript of molecular design of effective antioxidant from the series of similar conqxtunds is carried out by calculating the optimal value of the dissociation energy of the phenolic OH group (BDEoh ). The magnitude of BDEoh in the optimization process acts as a control parameter, which is expressed by the rate constants of reactions involving the initial antioxidant and its intermediates. 

In this part the numerical value analysis is carried out [31-33] for the kinetic model on liquid-phase oxidation of ethylbenzene inhibited by />ara-substituted phenols. 

We considered the kinetic model for the reaction under study to a certain extent as illustrative with an objective to demonstrate the capabilities of numerical analysis for kinetic models of the inhibited oxidation by the value method. It must be stated that the value ranking of individual steps by the degree of their kinetic participation in the liquid-phase oxidation of... 

Kinetic Analysis of the Mechanism of Liquid-Phase Oxidation of Ethylbenzene Inhibited by fVim-Substituted Phenols... 

The kinetic model of reactions. The inhibition mechanism of liquid-phase oxidation of hydrocarbons (RH) by phenolic compounds (InH) has been repeatedly discussed [1-16], The kinetic scheme of inhibited ethylbenzene oxidation in the presence of ora-substituted phenols, in general, includes a set of reaction steps as shown in Table 7.1. The scheme is... 

Numerical description of experiments. As we have already mentioned, the selected kinetic model for liquid-phase oxidation of ethylbenzene inhibited by />ara-substituted phenols was studied in detail and describes the available experimental data with good accuracy. Nevertheless, we have conducted special experiments [31] (see the conditions in the captions to Figures 7.4 and 7.5) to confirm additionally the adequacy of the selected kinetic model. 

Figure 7.6. Kinetic trajectories of reduced value contributions of steps over the induction period for liquid-phase oxidation of ethylbenzene inhibited by ora-methylphenol at 60 °C (a) and 120 °C (b). Initial concentrations of /lora-methylphenol and ethylbenzene hydroperoxide were lO M and 10 M, respectively (curves are numbered in accordance with step numbers in Table 7.1, the arrow points out the end of the induction period).

Figure 7.9. Reduced value contributions depending on Dqk in liquid-phase oxidation of ethylbenzene inhibited by efficient antioxidants /lura-iV-diniethylaininophenol (R =N(CH3)2, 79oh=355 kJ/mol) and / ara-methylphenol (R Hs, Z)oh=365 kJ/mol) at r= 60 C. Initial concentrations of the antioxidant and the hydroperoxide of ethylbenzene were 10 and 10 M, respectively. Conversion of ethylbenzene ...

Figure 7.16. Reduced value contributions of individual steps over the induction period for liquid-phase oxidation of ethylbenzene inhibited by BHT at different initial concentrations lO M, 10 M and 10 M, T=60 °C. The initial concentration of ethylbenzene hydroperoxide was 10 M. Step contributions are numbered according to the steps. Conversion of BHT makes up 7-10%.

Table 7.6. Base mechanisms for liquid-phase oxidation of ethylbenzene inhibited by BHT at different initial conditions including those of numerical (see captions of Figures 7.13-7.16) and experimental (see captions to Figures 7.11 and 7.12) investigations... 

As an example a model of die liquid-phase oxidation of the ethylbenzene in the presence of inhibitors, the iora-substituted phenols and the butylated hydroxytoluene, was selected. The identified dynamics of die value contribution of steps in the reaction mechanism is complicated. The dominant steps for die different time intervals of the inhibited reaction were determined. The inhibition mechanism of die ethylbenzene oxidation by sterically unhindered phenols is conditioned by establishing equilibrium (7.24) in the reaction of the chain carrier, the peroxyl radical, with the inhibitor s molecule (within sufficiently wide interval of the inhibitor s initial concentration), followed by the reaction radical s quadratic termination with the participation of the phenoxyl radical. The value analysis has established that the efficient inhibitor with low dissociation energy of the phenolic 0-H bond promotes shifting the mentioned equilibrium from the chain carrier to the direction of the phenoxyl radical formation. 

Characterizing the mechanism of the action of amines, the overwhelming majority of tiie authors believe that their basic role reduces to the termination of kinetic oxidation chains on account of reaction (3). Actually, stable radicals (CjH5)2NO have been detected by the method of electron paramagnetic resonance as a result of the interaction with peroxide radicals, formed in the liquid-phase oxidation of a hydrocarbon inhibited with diphenylamine [30]. The interaction of certain secondary amines (Ar2NH) with peroxide radicals, prepared by oxidative radiolysis. 

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