More about Catalytic Cracking
The key reaction in catalytic cracking is the addition of H+ to the hydrocarbon to form a positively charged ion - under the conditions of catalysis, the catalysts used are very strong acids. The multiple paths by which the positively charged hydrocarbon (carbocation) can lose its charge often lead to its decomposition.
Any acid would do, but in a conventional chemical reaction of hydrocarbons with a strong acid (e.g., H2SO4), it would be kind of difficult both to separate out what we wanted afterward and avoid corroding our reactors. The catalysts used are solids with acidic surfaces, so they stay where they're put.
Surface Reactions with Inorganic Catalysts
As you know, a catalyst is a species that accelerates the rate of a chemical reaction, but is not consumed in the reaction itself. The catalysts used in catalytic cracking are usually zeolites, compounds of aluminium, silicon, and oxygen containing strongly acid sites. In addition, zeolites have an extremely high surface area, which provides many sites where reactions can occur.
Find out more at the World of Zeolites.
X and Y zeolites are the major catalysts used in catlytic cracking. They have chemical formulae that may appear unusual, since they are minerals, rather than the nicely defined chemical compounds chemists are used to dealing with: [NapAlpSi192-pO384.gH2O, where p = 96-74 (x) or 74-48 (y) and g is 270-250, lower with higher aluminium. The sodium ions are typically exchanged with other cations to give zeolites with different catalytic properties]
Zeolites have a "sieving effect" - molecules that are too big will not be able to enter.
Zeolites have a "caging effect" - once inside, a molecule will find it difficult to get out and will undergo many successive reactions.
Rearrangement of Charged Hydrocarbons
There are a large number of ways a molecule might rearrange itself when heated up to ridiculous temperatures and given a charge, as happens in a catalytic cracker.
Examples of these reactions are inter- and intra-molecular hydrogen transfer reactions. These reactions are involved in the decomposition pathways that the catalytic crackers use to make more useful products.
Another class of reactions that occurs change the actual skeletal structure of the molecules being used.
As we will see, a highly branched hydrocarbon will have a lower
melting point and density than an unbranched one, making it more
suitable for use as a fuel - hence, these reactions might actually