Alkanes thermal cracking

Ethylene, propylene, and butene are synthesized industrially by thermal cracking of light (C2-Cg) alkanes. 

Cracking (Chapter 3 Focus On) A process used in petroleum refining in which large alkanes are thermally cracked into smaller fragments. 

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

Thermal cracking of higher components of crude oils can produce low molecular weight alkanes having isomeric compositions similar to the alkanes found in crudes. 

Thermal cracking The decomposition of higher alkanes to alkenes of lower relative molecular mass at high temperatures, 800-850 °C. 

Other pure mechanisms of cracking are thermal cracking and hydrogenolysis over metals. Very detailed investigations on platinum catalyzed hydrogenolysis of various alkanes, e. g., the isomeric hexanes, have been published recently (6-8). 

Thermal cracking of organic substances is an important reaction in the petroleum industry and has been extensively studied for over seventy years. At least for simple alkanes, the decay is first order in good approximation and therefore was long believed to occur in a single, unimolecular step [21]. However, in the 1930s, Rice and coworkers [22-24] established the presence of free radicals under the conditions of the reaction by means of the Paneth mirror technique [25,26], This observation led Rice and Herzfeld to propose a chain mechanism [22,27,28], Extensive later studies proved the essential features of their mechanism to be correct not only for hydrocarbons, but also for many other types of organic substances. 

The continuous increase in world consumption of MTBE has created a strong incentive to increase the production of isobutylene. Isobutylene can be produced by catalytic dehydrogenation of isobutane. However, the largest production of C4 olefins comes from the thermal cracking processes for the manufacture of ethylene which generate as by-products C4 mixtures containing C4 olefins and C4 alkanes plus butadiene. Isobutylene is also a product of fluid bed catalytic cracking units. 

Higher alkenes can be obtained from thermal cracking of wax, and although a thermodynamic mixture of internal alkenes might have been expected, the wax-cracker product contains a high proportion of 1-aIkenes, the kinetically controlled product. For the cobalt-catalyzed hydroformylation the nature of the alkene mixture is not relevant, but for other derivatizations the isomer composition is pivotal to the quality of the product. Another process involves the catalytic dehydrogenation of alkanes over a platinum catalyst. 

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