Free radicals, liquid-phase chain oxidation

Hock and Kropf [253] studied cumene oxidation catalyzed by Pb02. They proposed that Pb02 decomposed cumyl hydroperoxide (ROOH) into free radicals (R0 , R02 ). The free radicals started the chain oxidation of cumene in the liquid phase. Lead dioxide introduced into cumene was found to be reduced to lead oxide. The reduction product lead oxide was found to possess catalytic activity. The following tentative mechanism was proposed. 

Catalytic surface is active toward hydroperoxide and decomposes it to free radicals. The free radicals initiate the chain oxidation of RH in the liquid phase. 

Oxidation of organic compounds by dioxygen is a phenomenon of exceptional importance in nature, technology, and life. The liquid-phase oxidation of hydrocarbons forms the basis of several efficient technological synthetic processes such as the production of phenol via cumene oxidation, cyclohexanone from cyclohexane, styrene oxide from ethylbenzene, etc. The intensive development of oxidative petrochemical processes was observed in 1950-1970. Free radicals participate in the oxidation of organic compounds. Oxidation occurs very often as a chain reaction. Hydroperoxides are formed as intermediates and accelerate oxidation. The chemistry of the liquid-phase oxidation of organic compounds is closely interwoven with free radical chemistry, chemistry of peroxides, kinetics of chain reactions, and polymer chemistry. 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

The chain mechanism is complicated when two hydrocarbons are oxidized simultaneously. Russell and Williamson [1,2] performed the first experiments on the co-oxidation of hydrocarbons with ethers. The theory of these reactions is close to that for the reaction of free radical copolymerization [3] and was developed by several researchers [4-9], When one hydrocarbon R H is oxidized in the liquid phase at a sufficiently high dioxygen pressure chain propagation is limited only by one reaction, namely, R OO + R H. For the co-oxidation of two hydrocarbons R1 and R2H, four propagation reactions are important, viz,... 

Combustion processes are fast and exothermic reactions that proceed by free-radical chain reactions. Combustion processes release large amounts of energy, and they have many applications in the production of power and heat and in incineration. These processes combine many of the complexities of the previous chapters complex kinetics, mass transfer control, and large temperature variations. They also frequently involve multiple phases because the oxidant is usually air while fuels are frequently liquids or solids such as coal, wood, and oil drops. 

Oxidative chain reactions of organic compounds are current targets of theoretical and experimental study. The kinetic theory of collisions has influenced research on liquid-phase oxidation. This has led to determining rate constants for chain initiation, branching, extension, and rupture and to establishing the influence of solvent, vessel wall, and other factors in the mechanism of individual reactions. Research on liquid-phase oxidation has led to studies on free radical mechanisms and the role of peroxides in their formation. 

Production of phenol and acetone is based on liquid-phase oxidation of isopropylbenzene. Synthetic fatty acids and fatty alcohols for producing surfactants, terephthalic, adipic, and acetic acids used in producing synthetic and artificial fibers, a variety of solvents for the petroleum and coatings industries—these and other important products are obtained by liquid-phase oxidation of organic compounds. Oxidation processes comprise many parallel and sequential macroscopic and unit (or very simple) stages. The active centers in oxidative chain reactions are various free radicals, differing in structure and in reactivity, so that the nomenclature of these labile particles is constantly changing as oxidation processes are clarified by the appearance in the reaction zone of products which are also involved in the complex mechanism of these chemical conversions. 

The effect of the medium on the rates and routes of liquid-phase oxidation reactions was investigated. The rate constants for chain propagation and termination upon dilution of methyl ethyl ketone with a nonpolar solvent—benzene— were shown to be consistent with the Kirkwood equation relating the constants for bimolecular reactions with the dielectric constant of the medium. The effect of solvents capable of forming hydrogen bonds with peroxy radicals appears to be more complicated. The rate constants for chain propagation and termination in aqueous methyl ethyl ketone solutions appear to be lower because of the lower reactivity of solvated R02. .. HOH radicals than of free RO radicals. The routes of oxidation reactions are a function of the competition between two R02 reaction routes. In the presence of water the reaction selectivity markedly increases, and acetic acid becomes the only oxidation product. 

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... 

The liquid-phase oxidation of toluene with molecular oxygen is another example of a well established process (Table 4, entry 40). A cobalt catalyst is used in the process and the reaction proceeds via a free-radical chain mechanism. Heat of reaction is removed by external circulation of the reactor content and both bubble columns or stirred tanks are employed. It is important to note that air distribution is critical to prevent the danger of a runaway. Another example of direct oxidation is the commercial production of nitrobenzoic acid by oxidation of 4-nitrotoluene with oxygen (Table 4, entry 41). 

The holy grail of oxidation chemistry is the design of catalytic systems capable of mediating oxygen transfer from dioxygen, without the need for a sacrificial reductant, i.e. a Mars-van Krevelen mechanism [18] in the liquid phase. Indeed, the confinement of substrate molecules in the micropores of molecular sieves might be expected to create quasi gas phase conditions conductive to such a mechanism at the expense of free radical chain autoxidation (we note, however, that a mechanism involving two metal centres as shown in eq. 12 would be difficult to achieve in a molecular sieve). 

In the gas phase a Mars-van-Krevelen type mechanism can compete successfully with free radical autoxidation because if free radicals are formed they are not surrounded by substrate molecules (RH) as in the liquid phase, i.e. free radical chains are very short. Conversely, a Mars-van-Krevelen type mechanism is difficult to achieve with 02 in the liquid phase (but see later) due to competing facile autoxidation. The key to designing selective catalysts for liquid phase oxidations is to create a gas phase environment in the liquid phase. 

In very general terms, the Co-Br-catalyzed oxidation is a particular case of the free radical chain oxidation, common for all liquid phase oxidations of hydrocarbons [8-10]. The free radical chain oxidation occurs with four types of free radicals alkyl, alkoxy, alkylperoxy, and acylperoxy radicals [11, 12]. Other key active intermediates are hydroperoxides and peracids [11,12]. The nomenclature and structures are displayed in Figure 4.2. 

Parallel to these developments, observations made in the 19th century linked the deterioration of many organic materials, such as natural oils and fats, to the absorption of dioxygen. Around the turn of the century it was recognized that these processes involved organic peroxide intermediates. Subsequently, detailed mechanistic studies with simple hydrocarbons led to the free radical chain theory of autoxidation [7]. Following close on the heels of these mechanistic developments several important catalytic oxidation processes, in both the gas and liquid phase, were developed in the period 1945-1960. Some examples are shown in Table 1. 

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