Thermal cracking of alkanes

Acceleration, Inhibition, and Stoichiometric Orientation, by Hydrogenated Additives, of the Thermal Cracking of Alkanes at ca. 500 C... 

NiCLAUSE ET AL. Thermal Cracking of Alkanes with a bimolecular process of isomerization (26)... 

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

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 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. 

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. 

Table II gives the product distribution for thermal cracking of shale oil. We defined oil as the sum of condensed oil and C5-C9 hydrocarbons in the gas. The amount of each gaseous product was determined from the slope of the curve plotting gas production versus cracking loss (conversion) (14). The amount of coke produced was determined by difference, but it agreed well with the measured value for the few experiments in which carbon was analyzed in the shale from the bottom reactor. The alkene/alkane ratios in the gas depended more strongly on the cracking temperature than on the extent of conversion. This topic is discussed in greater detail in another paper published in these proceedings (20).

The thermal cracking of higher alkanes becomes significant above 650°C [374] [532] with the formation of alkenes, aromatics and coke. This is applied in steam crackers in ethylene plants, where steam is added as a diluent and for minimising coke formation. 

So far, we have seen a variety of reactions of radicals with alkenes. Alkanes are much less reactive than alkenes, and we have not seen ways to induce these notoriously unreactive species into reaction in a specific way. We have seen pyrolysis, the thermal cracking of saturated hydrocarbons (p. 470), but this method is anything but useful in making specific molecules. Here we find a way to use alkanes in synthesis. There still will be little specificity here, and these reactions will not find a prominent place in your catalog of synthetically important processes. Yet they are occasionally useful, and provide a nice framework on which to base a discussion of selectivity. 

Today, the majority of ethylene is produced by thermal cracking of hydrocarbon feedstocks ranging fi-om ethane to heavy vacuum gas oils. Over 60% of the world s propylene is produced as a by-product of thermal cracking, with the balance being supplied from refinery sources and others. Raw materials are mosdy natural gas condensate components (principally ethane and propane) in the US and Mideast and naphtha in Europe and Asia. Alkanes/olefins are broken apart at high temperatures, often in the presence of a zeolite catalyst, to produce a mixture of primarily aliphatic alkenes and lower molecular weight alkanes. The mixture is feedstock and temperature dependent and separated by fractional distillation. 

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. 

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