Pyrolysis, of alkanes

Pyrolysis of alkanes is referred to as eraeking. Alkanes from the paraffins (kerosene) fraetion in the vapor state are passed through a metal ehamher heated to 400-700°C. Metallie oxides are used as a eatalyst. The starting alkanes are broken down into a mixture of smaller alkanes, alkenes, and some hydrogen. 

In the 1930s it was discovered that the pyrolysis of alkanes produced large quantities of olefins. This pyrolysis process is not very selective, but the costs of separation were cheaper, and scaleup was simpler and safer in making ethylene rather than acetylene so during the 1940s ethylene and other small olefins replaced acetylene as the major building block in chemical synthesis. We will consider the reactions and reactors used in olefin synthesis from alkanes in the next chapter. 

Pyridinium chlorochromate, 305 Pyrimidine, 458 Pyrolysis of alkanes, 58 Pyrrole,... 

Ten types of elementary processes were defined, which included all the reaction types characteristic for the pyrolysis of alkanes. To speed up the process of reaction generation, five basic structural attributes were attached to each species. These attributes described the type of the species (molecule, radical), the length, the saturation, etc. The application of attributes enabled the sensible application of reaction types, since, for example, molecules do not take part in radical recombination reactions, and short radicals cannot take part in 1,4-isomerization. Where possible, rate parameters were taken from databases and others were estimated on the basis of ten rules which were applied according to the type of reaction and reaction partners. 

The addition reactions take place at a carbon-carbon multiple bond, or carbon-hetero atom multiple bond. Because of this peculiarity, the addition reactions are not common as the first step in pyrolysis. The generation of double bonds during pyrolysis can, however, continue with addition reactions. The additions can be electrophilic, nucleophilic, involving free radicals, with a cyclic mechanism, or additions to conjugated systems such as Diels-Alder reaction. This type of reaction may explain, for example, the formation of benzene (or other aromatic hydrocarbons) following the radicalic elimination during the pyrolysis of alkanes. In these reactions, after the first step with the formation of unsaturated hydrocarbons, a Diels-Alder reaction may occur, followed by further hydrogen elimination ... 

A similar computational modelling approach has been shown to be useful, for example, in studying the mechanism of low-temperature oxidation of alkanes (4), pyrolysis of alkanes (5-7), other gas-phase reactions (8), the formation of photochemical smog (9,10), and peroxide decomposition (11), among others. It is not uncommon to begin with all possible species and by permutation and combination derive a complete set of reactions, and then eliminate a subset by chemical... 

More generally, a ratio of alkene to alkane is determined by the relative production rate of each. During free-radical cracking, pyrolysis of alkanes yields either alkenes by unimolecularly decomposing free radicals, alkanes by free radicals abstracting a hydrogen from another source, or both. For example, ethene can be formed by decomposition of primary radicals (including ethyl),... 

Numerical methods for the solution of the resulting large set of coupled differential equations have been developed. The method originally due to Gear( ) has been applied by Allara and Edelson for the pyrolysis of alkanes( 7) and later by others(8,9) for similar processes. Smog formation and detailed small-molecule photochemistry have also been studied by these numerical simulations(lO). Semi-quantitative... 

All the studies mentioned determined the position of the label as CO2 after proper oxidation processes. Another possible method can be pyrolysis of alkanes.Pulses of various [ C] labelled hydrocarbons (butadiene, pentene, cyclopentadiene, alkanes, alkylbenzenes) were introduced into an inert gas stream, then passed through a heated reactor, and the pyrolysis products (methane, >Ci fragments and benzene) were analysed by a gas chromatograph equipped by a mass and a radioactivity detector. The temperature was kept between 873 and 1050 K, while the decomposition of aromatics required higher temperatures. With conversions between 5 and 90%, an accuracy of 5% could be reached. 

The pyrolysis of alkanes can be effective in decreasing the overall chain length of the molecule and in introducing imsaturation. Let s consider the pyrolysis of butane. First, there are two possible homolytic cleavages of carbon-carbon bonds... 

However, it was suggested that if the residual anions could be completely removed from the organoclays, the primary degradation pathway would switch to an elimination-type mechanism [16]. The Hofmann elimination of ammonium compounds was most probably the source of additional amounts of vinyl-type unsaturation found in melt-processed OMMT-PE relative to both the polymer control and the Na+MMT-PE samples [28]. On the other hand, the presence of alkenes was also explained by three possible routes of decomposition (i) pyrolysis of alkanes derived from the major component of the organic part, i.e., hydrogenated tallow (HT) (ii) pyrolysis of the tallow (unsaturated fatty acids used for the preparation of the quaternary ammonium salt) and (iii) decarboxylation of RCOO and RCO radicals [17, 29]. 

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