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Mechanism of Catalysis

 

 

 

 

 

Hydrogen transfer reactions

One reaction occurs mainly between cycloalkanes and alkenes over zeolites to give aromatics.

 

 

 

 

Catalytic cracking involves ions not molecules or radicals. That is why acids catalyse the reactions. To give an idea of the nature of the reactions involved we will look at the example of an alkane.
An H+ ion can be returned to the zeolite catalyst to terminate the process
The other, which occurs more readily over zeolites than over silica/alumina catalysts, means that the cracking of the hydrocarbon chain is intercepted in the pores of the zeolite by transfer of a hydride ion from an alkane to the carbocation. Because of this reaction zeolites give fewer very small molecules of product, and more of the useful C5 to C10 products
Hydrogen transfer reactions

One reaction occurs mainly between cycloalkanes and alkenes over zeolites to give aromatics.

The secondary carbocation, with the positive charge on a carbon connected to two other carbons, is free to isomerise to a more stable tertiary carbocation where the positive charge is on a carbon connected to three other carbons.

This favoured rearrangement is mainly responsible for the large number of branched products from catalytic cracking over zeolites.

There are then different ways in which bonds can break to give products and a new carbocation.

Or there could be a trace of alkene from some thermal cracking which takes an H+ from a Bronsted acid site on the catalyst
Initiation
A carbocation can form by direct attack on an alkane by a Lewis acid site of the zeolite

change to hydrocarbon                                                              change on zeolite

This reaction gives an alkene and a new smaller carbocation.
Isomerisation
Breaking of a carbon-carbon bond