Pyrolysis

Pyrolysis

The thermal decomposition of organic compounds is known as pyrolysis, when applied to alkanes, is known as cracking.

When heated to about 500-600^oC, alkanes are decomposed into smaller molecules, and the products obtained from a given alkane depend on (i) the structure of the alkane; (ii) the pressure under which cracking is carried out; and (iii) the presence or absence of catalysts such as silica-alumina, silica-alumina-thoria, silica-alumina-zirconia.

The mechanism of carking is still obscure. Many theories have been suggested, and one that is highly favoured is a free-radical mechanism, evidence for which has been obtained from the observation that at cracking temperatures many hydrocarbons produce alkyl free radicals.

When petroleum is cracked, of all the compounds produced, the most important are those containing up to four carbons atoms : methane, ethane, ethylene, propane, propene, butane, butane and isobutene. All of these have found wide application as the materials for the preparation of a large number of chemicals.

By using suitable catalysis, alkanes containing six or more carbon atoms may be catalytically cyclised, e.g.,  n-hexane, under pressure, passed over chromic oxide carried on an alumina support and heated at 480-550^oC.

C₆H₁₄  -  C₆H₆ + 4H₂