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Question
What do you understand by shape selective catalysis? Why are zeolites good shape selective catalysts?
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Solution
The catalytic reaction which depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape selective catalysis. Zeolites are good shape selective catalysts because of their honey comb like structures.
(i) Zeolites are microporous aluminosilicates of the general formula \[\ce{M_{x/n}[(AlO2)_{x}(SiO2)_{y}]mH2O}\]
(ii) Zeolites have an enormous surface area which is largely on the inside of the solid. The zeolites can permit the entry and exit of molecules of a certain size into the active regions within the holes.
(iii) They are used in petrochemical industries for cracking of hydrocarbons and isomerization.
(iv) The reactions in zeolites depend upon the size of the cavities (cages) and pores (tunnels) present in them. The pore size in zeolites generally varies between 260 pm and 740 pm.
(v) Zeolite catalyst known as ZSM-5 converts alcohols to gasoline. The alcohol is dehydrated in the cavities and the hydrocarbons are formed.
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