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The Catalyst
The regular micro pore structure of zeolites is probably responsible for the selectivity which makes them so useful.

Before being used for cat cracking zeolite Y is converted to its acid form by ion exchange. The cations are replaced by hydrogen ions.

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The Bronsted acid sites catalyse the cracking reactions, with the reactants held in the large network of pores in the zeolite.










Zeolites can be 10000 times more active as cracking catalysts than silica/alumina. But commercially the cracking rate is matched to the catalyst regeneration rate . The zeolite is dispersed on SiO2-Al2O3, rather than being used pure.
Zeolite Y is particularly effective at giving good yields of products with high octane numbers. The diagrams show the structure of Zeolite Y, and an artist's impression of cracking reactions on a zeolite