Autoxidation catalyst deactivation

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... 

In the autoxidation of neat hydrocarbons, catalyst deactivation is often due to the formation of insoluble salts of the catalyst with certain carboxylic acids that are formed as secondary products. For example, in the cobalt stearate-catalyzed oxidation of cyclohexane, an insoluble precipitate of cobalt adipate is formed. 18fl c Separation of the rates of oxidation into macroscopic stages is not usually observed in acetic acid, which is a better solvent for metal complexes. Furthermore, carboxylate ligands may be destroyed by oxidative decarboxylation or by reaction with alkyl hydroperoxides. The result is often a precipitation of the catalyst as insoluble hydroxides or oxides. The latter are neutralized by acetic acid and the reactions remain homogeneous. 

Zeolites have been used as (acid) catalysts in hydration/dehydration reactions. A pertinent example is the Asahi process for the hydration of cyclohexene to cyclo-hexanol over a high silica (Si/Al>20), H-ZSM-5 type catalyst [57]. This process has been operated successfully on a 60000 tpa scale since 1990, although many problems still remain [57] mainly due to catalyst deactivation. The hydration of cyclohexanene is a key step in an alternative route to cyclohexanone (and phenol) from benzene (see Fig. 2.19). The conventional route involves hydrogenation to cyclohexane followed by autoxidation to a mixture of cyclohexanol and... 

In the absence of bromide ion the p-xylene undergoes rapid autoxidation to p-toluic acid but oxidation of the second methyl group is difficult, due to deactivation by the electron-withdrawing carboxyl group, and proceeds only in low yield at elevated temperatures. Although bromide-free processes were subsequently developed (ref. 5) they require the use of much higher amounts of cobalt catalyst and have not achieved the same importance as the Amoco-MC process. Indeed, the... 

In some cases (e.g., gasoline), autoxidation of hydrocarbons is undesirable, and trace amounts of metal catalysts may often be deactivated by the addition of suitable chelating agents. The latter affect the catalytic activity of metal complexes by hindering or preventing the formation of catalyst-hydroperoxide or catalyst-substrate complexes by blocking sites of attack or by altering the redox potential of the metal ion. 

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