Zeolites is the name given to some aluminesllicates of metals with the genral formula `[M_(x//n )(AlO_(2))_(x)(SiO_(2))_(3)] zH_(2)O`, where n is the charge on the metal cation while z is the number of molecules of water hydration present. The metal cations participating in the zeolite formation are usually `Na^(+),K^(+)` or `Ca^(2+)`. the zeolites in the hydrated form are used as ion-exchanges in removing hardness of water. However. if these are heated in vacuum for sometime, they low the molecules of water and get dehydrated . pores or cavities are formed at the places wehre `H_(2)O` molecules were present. Thus, they become highly porous and can act as catalysis in certain reactions.
(i) Why are zeolites called shape-selective catalysis?
(ii) What is the value associated with zeolites?

Explanation

(i) Zeolites are known as shape selective catalysts because their activity is linked with pore size. In other words, they act as molecular sleves. For example, zeolite sodium alumino silicate can adsorb straight cahin hydrocarbons since their molecules can fit into the pores. the isomeric branched hydrocarbons donot fit into the pores becaue of smaller size.
(ii) Zeolites act as useful chain catalysts for petro-chemical industry for carrying out cracking of certain hydrocarbons and isomerisation. For example, in the presence of zeolite `ZSM-5` (Zeolite sleve of porosity5) methanol is converted into gasoline of high octane number.