Catalyst-inhibitor conversion

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... 

Metal catalysis, which is claimed to have an important role in initiating autoxidation, appears to be so complex that in some systems catalysts are converted to inhibitors when their concentrations are increased. The additives examined include the N-butylsalicylaldimino and N-phenylsalicylaldi-mino chelates of cobalt(ll), copper(11), nickeVJl), and zinc as well as a number of 3,5-diisopropylsalicylato metal chelates. Some were autoxidation catalysts, some were inhibitors, and some exhibited catalyst-inhibitor conversion. Reaction mechanisms which account for most of the observed phenomena are proposed. The scope for developing metal chelates as antioxidants and the implications concerning the critical antioxidant concentration are outlined. 

Tphe major objective of this work was to understand better the effect - of heavy metals in autoxidation reactions in view of the importance of trace metals in oils, fats, rubber, plastics, and other materials. Because of our interest in the stability of polyolefins such as polyethylene and polypropylene the major model substance used was 2,6,10,14-tetramethyl-pentadecane. With its four tertiary C—H bonds it is a suitable model for either polypropylene or branched polyethylene. Hexadec-l-ene was also used since its mono-olefinic character could be typical of some residual unsaturation in polyethylene. N-alkylamides served as model substances for polyamides, and a few experiments were also carried out with methyl linoleate. While studying the causes of initiation of the autoxidation of these substances we observed that certain compounds were catalysts at low concentrations but became inhibitors at higher concentrations. The phenomenon was called catalyst-inhibitor conversion. ... 

Catalyst-Inhibitor Conversion. The system 2,6,10,14-tetramethyl-pentadecane-bis(N-butylsalicylaldimino)cobalt(II) at 50°C. illustrates well the observed catalyst-inhibitor conversion (Figure 2). At low concentrations up to M/20,000 the metal chelate is a conventional catalyst no induction period is observed, and the reproducible initial autoxidation rates are proportional to the square root of catalyst concentration. From the curves shown in Figure 2 catalyst deactivation becomes apparent at... 

Effects of Different Metal Salicylaldimine Chelates. Varying the central metal profoundly affected catalytic and inhibitory properties. There were only small quantitative variations, however, between N-phenyl- and V-butylsalicylaldimines having the same central metal atom. The only other salicylaldimines where catalyst-inhibitor conversion could be demonstrated were those of copper (II). With copper (II) both the catalytic and the inhibitory effects are much less pronounced than for cobalt (II). Surprisingly nickel (II) complexes behaved like conventional catalysts for hydrocarbon autoxidation—i.e., the rate is proportional to... 

Catalysis and Inhibition. The varied behavior of most of the metal chelates we studied is summarized in Tables II and III. Table II indicates which chelates exhibited catalyst-inhibitor conversion in TMPD at 50°C., and generally, analogous observations were made with hexadec-l-ene. N-butyl- and N-phenylsalicylaldimino ligands are presented in Tables II and III as BuSal and PhSal, respectively in addition the metal and its... 

Table II. Catalyst-Inhibitor Conversion in the Autoxidation of 2,6,10,14-Tetramethylpentadecane at 50°C.a...

One of the most surprising findings is the observed catalyst-inhibitor conversion with copper complexes. Since a copper (III) chelate is not regarded as feasible, a possible explanation might be that copper(I) formed by reducing impurities is the active species at least as far as catalysis is concerned. 

In recent years Emanuel, Neiman, and their respective schools have greatly contributed to the theory of antioxidant action by studying the phenomenon of the critical antioxidant concentration in terms of a degenerate branched chain reaction. The critical antioxidant concentration, a well-established feature of phenolic antioxidants, is one below which autoxidation is autocatalytic and above which it proceeds at a slow and steady rate. Since the theory allowed not only a satisfactory explanation of the critical antioxidant concentration itself but elucidation of many refinements, such as the greater than expected activity of multifunctional phenolic antioxidants (21), we wondered whether catalyst-inhibitor conversion could be fitted into its framework. If degenerate chain branching is assumed to be the result of... 

Reaction of Metal Catalysts with Free Radicals-Catalyst-Inhibitor Conversion... 

The phenomenon of catalyst-inhibitor conversion1 2,143,356 may be understood and critical concentration of metal can be deduced by reference to Eq. (280). If decomposition of the hydroperoxide is the source of initiation, it must be formed as rapidly as it is consumed to maintain a steady rate. If termination by metal complex predominates, a steady state occurs when the right-hand side of Eq. (280) equals unity. No oxidation will occur when this quantity is less than unity. Hence, catalyst-inhibitor conversion is observed as the metal concentration is increased to the point that the chain length becomes less than unity. If termination occurs by the bimolecular reaction of peroxy radicals, a chain length of less than unity will result in the depletion of the hydroperoxide until the rate of initiation has decreased to the point where the chain length is unity again. No inhibition is expected or observed. 

Note that even critical phenomena for the catalyst-inhibitor conversion were observed in alkene epoxidation by O2/IBA system in the presence of PWnTi and PWnV catalysts [19]. All these data provide evidence in favor of the radical chain mechanism of the epoxidation in most 02/IBA/Co(II) systems studied where acylperoxyradicals are most probably the main epoxidizing species. 

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