One often confuse with the combustion as well as pyrolysis however pyrolysis is completely a different process than combustion.

In pyrolysis, a bigger hydrocarbon in the absence of air breaks into smaller fragments by the application of heat. Since the pyrolysis happens in the absence of oxygen (air) so oxidation of hydrocarbon does not take place. Pyrolysis is more of a decomposition process carried out by thermal energy.

Generally, pyrolysis of alkanes is termed as cracking also.

Pyrolysis Process

In the absence of oxygen, the alkane in the vapour forms when passed over the red hot metal, it breaks down into the simpler hydrocarbons.

Very high temperature and pressure is required for the process to get executed in the absence of a catalyst. The catalyst such as Palladium and Platinum can help to carry out the process at low temperature and pressure.

This process is generally helpful in the fractional distillation of the petroleum. The large hydrocarbons obtained during the fractional distillation of crude oil can be broken down into smaller hydrocarbon using pyrolysis. The hydrocarbon molecule breaks down into the random manner and even some compound obtained after pyrolysis carries double bond too.

Pyrolysis of Alkanes

As the molecular weight and branching in alkane increases, the rate of pyrolysis is increases. This is opposite to what happens during combustion. The fission of C-C bond leads to the formation of alkanes and alkenes whereas the fission of C-H bond leads to the formation of methane (CH4) and Hydrogen (H2).

The fission of C-H bond takes place due to the catalytic action of Cr₂O₃, V₂O₂, MoO₃ and C-C bond fission occur under the presence of SiO₂, Al₂O₃, and ZnO.

The cracking of alkane is followed by the free radical mechanism.

The pyrolysis process is of prime importance for the petroleum industry. For e.g. Dodecane ( a component of kerosene oil) gives a mixture of heptane and pentane as pyrolysis products, on heating it to a temperature of 973K under the catalytic action of platinum, palladium or nickel.

C₁₂H₂₆ → C₇H₁₆   +  C₅H₁₀ +  other products

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