Metal-Catalyzed Liquid-Phase Autoxidation

As mentioned earlier, soluble salts of cobalt and manganese catalyze oxidation of cyclohexane by oxygen to cyclohexanol and cyclohexanone. Cyclohexanol and cyclohexanone are oxidized by nitric acid to give adipic acid. The oxidation by nitric acid is carried out in the presence of V5+ and Cu2+ ions. These reactions are shown by Eq. 8.8. Adipic acid is used in the manufacture of nylon 6,6. 

The oxidation of cyclohexanone by nitric acid leads to the generation of nitrogen dioxide, nitric oxide, and nitrous oxide. The first two gases can be recycled for the synthesis of nitric acid. Nitrous oxide, however, is an ozone depleter and cannot be recycled. Indiscriminate nitrous oxide emission from this process is therefore the cause of considerable concern. As shown by 8.9, part of the cyclohexanone can also be converted to the corresponding oxime and then to caprolactam—the monomer for nylon 6. Phthalic acids are one of the monomers for the manufacture of polyesters. As shown by Eq. 8.10, it is made by the oxidation of p-xylene. A general description of polyamides (nylons) and polyesters are given in Section 8.4. 

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Metal oxides have often been used as catalysts for the autoxidation of hydrocarbons.1 In many cases the metal probably dissolves in the reaction medium and catalysis involves homogeneous metal complexes. However, according to a recent report56 cerium oxide catalyzes the liquid phase oxidation of cyclohexanone in acetic acid (5-15 bar and 98-118°C) without dissolving in the reaction medium. 

Although liquid-phase oxidations of alkanes can be carried out even in the absence of any metal derivative (the role of an inihator of chain radical process can be played by a non-metal compound), derivahves of transition metals are often used in these reactions. Metal-catalyzed autoxidation will be considered in Chapter IX. 

The vast majority of liquid phase transition metal catalyzed oxidations of organic compounds fall into these three broad categories (a) free radical autoxidation reactions, (b) reactions involving nucleophilic attack on coordinated substrate such as the Wacker process, or (c) metal catalyzed reactions of organic substrates with hydroperoxides. Of these three classes of oxidations only the first represents the actual interaction of dioxygen with an organic substrate. The function of oxygen in the Wacker process is simply to re-oxidize the catalyst after each cycle [2]. 

It has long been known that metal salts and complexes promote the reaction of olefins with oxygen in the liquid phase. Early work ([429] and references cited therein) established that during olefin oxidation in the presence of various copper, cobalt and manganese salts, free radicals arise via decomposition of a catalyst-hydroperoxide complex formed from allylic hydroperoxide generated in situ. Although the metal modifies the nature of the observed products in many cases, most homogeneous metal-catalyzed oxidations exhibit characteristics of free radical initiated autoxidations. 

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