Oxidation of hydrocarbons by molecular oxygen

Liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes,3 "16 including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p-xylene, acetaldehyde and vinyl acetate from ethylene, propylene oxide from propylene, and many others. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. 

HOMOGENEOUS CATALYTIC OXIDATION OF HYDROCARBONS BY MOLECULAR OXYGEN... 

The oxidation of hydrocarbons by molecular oxygen in the absence of metal complexes has been discussed in Chapter II. The oxidation of hydrocarbons with molecular oxygen, as well as with donors of an oxygen atom (hydrogen peroxide, alkyl hydroperoxides and some other compounds), is a very important field since many industrial processes are based on these reactions [1], In many cases, chain radical non-catalyzed autoxidation of samrated hydrocarbons is not very selective and the yields of valuable products are often low. The use of salts and complexes of transition metals creates great possibilities for solving problems of selective oxidation, as has been demonstrated for a number of important processes. 

Partial Oxidation Reactions. Oxidation is used in industry for producing aliphatic and aromatic alcohols, aldehydes, ketones and acids. Generally, oxidation involves splitting of C-C or C-H bonds and formation of C-0 bonds. For example, the partial oxidation of hydrocarbons by molecular oxygen, to form oxygenates that are used as building blocks in the manufacturing of plastics and synthetic fibers, is an important process in the chemical industry. 

Gaillon, L., F. Bedioui, P. Battioni, and J. Devynck (1993). Electroassisted biomimetic oxidation of hydrocarbons by molecular-oxygen catalyzed by manganese porphyrin complexes intercalated into montmorillonite. J. Mol. Catal. 78(2), L23-L26. 

The method is direct and based on the use of the chain reaction of liquid-phase oxidation of hydrocarbon by molecular oxygen. 

SELECTIVE OXIDATION OF HYDROCARBONS WITH MOLECULAR OXYGEN CATALYZED BY TRANSmON-METAL-SUBSTITUTED SILICOTUNGSTATES... 

The high conversion to CO, C02, and H20 hinders the formation of oxygen-containing products in direct oxidation of hydrocarbons with molecular oxygen. It is considered as firmly established that the reason for low selectivity of catalytic hydrocarbon oxidations lies in the ready subsequent oxidation and decomposition of oxygen-containing products formed by synthesis. The accepted scheme for olefine oxidation is... 

Both metals and their oxides catalyze oxidation of hydrocarbons with molecular oxygen (see books [28], reviews [29], certain original papers [30] and recent patents [31]). Possible reactions of n-pentane in oxidative transformation catalyzed by vanadyl pyrophosphate are illustrated in Scheme 111.8 [29d]. 

Scheme 9.12 Selective oxidations of hydrocarbons with molecular oxygen catalysed by transition metal-containing aluminophosphates.

In recent years, the studies in the field of homogeneous catalytic oxidation of hydrocarbons with molecular oxygen were developed in two directions, namely, the free-radical chain oxidation catalyzed by transition metal complexes and the catalysis by metal complexes that mimic enzymes. Low yields of oxidation products in relation to the consumed hydrocarbon (RH) caused by the fast catalyst deactivation are the main obstacle to the use of the majority of biomimetic systems on the industrial scale. 

However, the findings on the mechanism of action of enzymes, and, in particular, dioxygenases and their models, are very useful in the treatment of the mechanism of catalysis by metal complexes in the proeesses of oxidation of hydrocarbons with molecular oxygen. Moreover, as one will see below, the investigation of the mechanism of catalysis by metal complexes can give the necessary material for the study of the mechanism of action of enzymes. 

The heterobimetallic complexes [N(n-Bu)4] [Os(N)R2(/u.-0)2Cr02] catalyze the selective oxidation of alcohols with molecular oxygen. A mechanism in which alcohol coordinates to the osmium center and is oxidized by B-hydrogen elimination (see -Hydride Elimination) is consistent with the data. The hydroxide adduct of OSO4, [0s(0H)204], with ferric cyanide and other co-oxidants catalyzes the oxidative dehydrogenation of primary aromatic and aliphatic amines to nitriles, the oxidation of primary alcohols to carboxylic acids, and of secondary alcohols to ketones. Osmium derivatives such as OsCb catalyze the effective oxidation of saturated hydrocarbons in acetonitrile through a radical mechanism. ... 

J. E. Backvall, Stud. Surf. Sci. Catal, 1988, 41, 105-114. Metal-Catalyzed Oxidations of Unsaturated Hydrocarbons by Molecular Oxygen The Use of Multi-step Catalysis. 

Ni-complexes has been proved to be efficient in the electrocatalysis (Fig. 17.9) Electrocatalysis turned out to be efficient in a series of other reactions, such as water oxidation to molecular oxygen on iron(III) and cobalt(III) hydroxides, oxidation of hydrocarbons by hydrogen peroxide and molecular oxygen in the presence of iron ions and hemin derivatives. 

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