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More about Catalytic Cracking

 
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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.


The attack on an alkene by the catalyst


The attack on an alkane by the catalyst


The decomposition of the charged species

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]


Pore structure of a ZK-5 Zeolite, Na30 [Al30 Si66O192] . 98 H2O. Each bend or junction in the lines is a fully fledged atom.

As well as having high surface acidity, zeolites are fantastically porous, which makes them selective catalysts. Because they are so porous, and because their pores are so regular in size and shape:

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.


An intermolecular hydrogen transfer reaction

An intramolecular hydrogen transfer reaction

Another class of reactions that occurs change the actual skeletal structure of the molecules being used.


A reaction that rearranges the carbon backbone of the molecule inside a cat-cracker.

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 be useful.