Pyrolysis free radical production

Basic Catalysis. The catalytic properties of alkali zeolites free of acidic sites have been investigated for the cracking of hexanes (25, 26). At 500 C K-Y zeolite cracks easily n-hexane and its isomers resulting in product distributions markedly different from those obtained over acidic zeolites or even by thermal cracking (pyrolysis). Free radical-type mechanism predominates on the zeolite surface. The relative rates of H atom abstraction (bimolecular) and B-scission (unimolecular) are greatly affected by the zeolite matrix. Zeolites also concentrate hydrocarbon reactants within the crystal, which enhances the rate of bimolecular reaction step. Comparison with silicalite (Al-free ZSM-5 zeolite) and quartz chips has been done in order to characterize the zeolitic effect. Silicalite behaves as inert quartz chips with no effect on the rate of H-abstraction step,... 

It contains free radical species which react with the gaseous free radical products formed during the degradation of PP. These species may also play a part in termination steps in the free radical reaction scheme of the pyrolysis of the PP and of the degradation of the protective material in the condensed phase,... 

In contrast, the ultrasonic irradiation of organic Hquids has been less studied. SusHck and co-workers estabHshed that virtually all organic Hquids wiU generate free radicals upon ultrasonic irradiation, as long as the total vapor pressure is low enough to allow effective bubble coUapse (49). The sonolysis of simple hydrocarbons (for example, alkanes) creates the same kinds of products associated with very high temperature pyrolysis (50). Most of these products (H2, CH4, and the smaller 1-alkenes) derive from a weU-understood radical chain mechanism. 

Pyrolysis of more complex molecules proceeds via production of free radicals. Then formula (4.5) fails, because reactions of creation and recombination of radicals in these systems are irreversible. Therefore, the steady-state concentration of active particles in these systems depends on conditions of pyrolysis, determining the first or the second order of recombination of active particles, and is governed by the following equations [8]... 

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

Three-membered rings can also be cleaved to unsaturated products in at least two other ways. (1) On pyrolysis, cyclopropanes can undergo contraction to propenes.121 In the simplest case, cyclopropane gives propene when heated to 400 to 500°C. The mechanism is generally regarded122 as involving a diradical intermediate123 (recall that free-radical 1,2... 

Chlorination of Alkanes. The most direct and economical method for the manufacture of chloromethanes is the thermal free-radical chlorination of methane.176 177 Whereas in the 1940s and 1950s photochlorination was practiced in some plants, thermal chlorination is the principal industrial process today. The product chloromethanes are important solvents and intermediates. Commercial operations perform thermal chlorination at about 400-450°C. Vapor-phase photochemical chlorination of methane may be accomplished at 50-60°C. Fast and effective removal of heat associated with thermally induced free-radical substitution is a crucial point. Inadequate heat control may lead to explosion attributed to the uncontrollable pyrolysis liberating free carbon and much heat ... 

The mechanisms for ambient temperature oxidations become more complex as the alkyl radical R becomes more complex and as the reactions proceed. Thus, with azo-2-methylpropane (52) the pyrolysis of the tert-butoxy radical must be included, and in all but the initial stages, reactions of alkoxy radicals with products containing weak C—H bonds must be included. Numerous tertiary reactions can then occur. As with most free radical systems, useful information can be obtained only if the degree of conversion of the starting material is kept low. 

The diazonium salts (C6H.)RN2)+ (1,2-C2B9Hii)2Co undergo an interesting free-radical reaction upon pyrolysis. Nitrogen gas is liberated and the product is found to consist of a symmetrical sandwich cewmo-mctallo-carborane with an ortho-disubstituted phenyl ring bonded to polyhedral boron atoms and bridging between the two icosahedra (35). 

Termination reactions also occur when a free radical in the gas phase reacts or couples with a free radical on a solid surface. The solid coke formed as a by-product during pyrolysis is essentially pure carbon, which has numerous free radicals on its surface. The exothermic heats of such... 

While there is controversy as to whether or not this Cellulose species exists, experimental evidence for the Cellulose species was obtained by Price et al.,60 who suggested that it could be a free radical in nature. At lower temperatures, oxygen plays a dominant role in cellulose degradation, and pyrolysis is faster in an oxidative atmosphere than in an inert one.61 Oxygen catalyzes the formation of both volatiles and char-promoting reactions.62 At higher temperatures, the degradation products are little affected.61... 

A previous investigation indicated that the pyrolysis of methyl 3-bromopropionate alone was autocatalyzed by the HBr gas product. However, under maximum free-radical inhibition the reaction was homogeneous, unimolecular and of first order111. Moreover, the acidity of the hydrogen adjacent to the COOCH3 was held responsible for the faster... 

Qualitative work on the pyrolysis of bromochlorodifluoromethane, either pure or mixed with 1 1 air, at 400-800 °C showed it to proceed, as expected, through a complex free radical type mechanism140. The overall reaction yielded the products described in equation 57 ... 

The mechanism of thermal decomposition (pyrolysis) of methane has been extensively studied.11 Since C- H bonds in methane molecule are significantly stronger than C-H and C-C bonds of the products, secondary and tertiary reactions contribute at the very early stages of the reaction, obscuring the initial processes. It has been shown11 that the homogeneous dissociation of methane is the only primary source of free radicals and controls the rate of the overall process ... 

This mechanism has been found to exactly reproduce evolution of the major products at low extents of reaction between 350°-425°C. Bibenzyl pyrolysis proceeds by conventional well-understood, free-radical reaction steps. 

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