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Mechanisms of catalytic cracking
Strongly acidic zeolites, including dealuminated Y zeolites (faujasite), are used in catalytic cracking (Fig. 1). The stability of these zeolites can be increased by incorporating cerium or rare earth ions.
The following figure shows the cleavage mechanism using the example of n-hexane:
This takes a central position here -Cbenium ion, which can form
- by adding the proton of the surface hydroxide group to a trace amount of hexene in the educt,
- by the direct cleavage of a C-C or C-H bond according to the non-classical Haag-Dessau mechanism.
That -Carbenium ion splits into in the β-position on the catalyst surface -Olefin and a-Carbenium ion or in a -Olefin and a -Carbenium ion. The react with hydride transfer/-Cbenium ions with another molecule of hexane, with a new -Carbenium ion and the corresponding /-Carbons are formed. Another possible reaction of the -Carbenium ionis that it is formed by the addition of/-Olephins to the/-Carbenium ions react. Via cleavage in the β position and subsequent hydride transfer- and -Hydrocarbons are built up.
High molecular weight coke precursors are built up through the consecutive addition of further olefins.