Zeolite ZSM-5 (Molecule of the Month for April 1997)
Zeolite-based heterogeneous catalysts are used by industrial chemical companies in the interconversion of hydrocarbons and the alkylation of aromatic compounds.
A very good example is the zeolite ZSM-5. This zeolite, developed by Mobil Oil, is an aluminosilicate zeolite with a high silica and low alumininum content. Its structure is based on channels with insecting tunnels. The aluminium sites are very acidic. The substitution of Al3+in place of the tetrahedral Si4+ silca requires the presence of an added postive charge. When this is H+, the acidity of the zeolite is very high. The reaction and catalysis chemistry of the ZSM-5 is due to this acidity.
The ZSM-5 zeolite catalyst is used in the petroleum industry for hydrocarbon interconversion.
An example use is in the isomerizations of xylene- from meta to para-xylene. The acidic zeolite promotes carbocation isomerizations. There are two suggested mechanisms for this type of isomerizations.
Firstly shape may play a role. Perhaps para-xylene has a shape which allows it to diffuse rapidly through the zeolite structure, whereas as meta-xylene takes longer to pass through the zeolite and thus has more opportunity to be converted into the para-xylene.
Secondly, is that the orientation of reactive intermediates within the zeolite channels favors specifically para-xylene.
Formal Chemical Name (IUPAC)
Update by Karl Harrison
(Molecule of the Month for April 1997 )
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