I think zeolites possess acid sites that are catalytically active in many hydrocarbon reactions. The pore system only allows molecules that are small enough to enter, hence it affects the selectivity of reactions by excluding both the participation and formation of molecules that are too large for the pores. Is that right or not ?

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    – bon
    Jun 15 '16 at 11:01

Alumina containing zeolites have composition that can be represented as

enter image description here

where M is metal ion with m charge, and so on, respectively. Meaning you have $[SiO_4]^{4-}$ and $[AlO_4]^{5-}$ tetrahedra which make up negatively charged framework. Positive ions compensate for this negative charge. There is a rule from 1954. called Löwenstein rule, which states that x/y has to be less than 1, which in turn implies there are no Al-O-Al bonds, as two tetrahedra have Al atoms in their tetrahedral positions. This gives really great internal specific area and resistance to high temperature, which makes this type of zeolite a great candidate for catalysis (heterogenous). The following picture nicely represents the active sites in zeolites - they act as molecular sieves, as only the specific molecules fit in the pores:enter image description here


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