11.0 Zeolites as shape-selective catalysts

  Surface Chemistry

• Zeolites are aluminosilicates with the general formula, ${M_{x/n}}\left[ {{{\left( {Al{O_2}} \right)}_x}{{\left( {Si{O_2}} \right)}_y}} \right].z{H_2}O$, where $n$ is the charge on the metal cation ${M^{n + }}$, which is usually $N{a^ + }$, ${K^ + }$ or $C{a^{2 + }}$ and $z$ is the number of water molecules of hydration which is highly variable.

• They are microporous three dimensional net work silicates in which some silicon atoms are replaced by aluminium atoms giving $Al - O - Si$ frame work.

• They have honey comb like structure. It consists of truncated octahedra as the building blocks. This structure is called $\beta - cage$ or sodalite cage.

• They form an important class of oxide catalysis.

• Zeolites to be used as catalysts are heated in vacuum so that the water of hydration is lost. As a result, zeolite becomes porous , i.e., the cavities in the cage-like structure which were occupied by the water molecules become vacant.

• The size of the pores generally varies between 260 pm and 740 pm. Thus, only those molecules can be adsorbed in these pores whose size is small enough to enter these cavities and also leave easily.

• Thus, zeolites act as selective adsorbents and hence as 'molecular sieves'. For example, the zeolites, sodium aluminosilicate, can adsorb straight chain hydrocarbons and not branched chain or aromatic ones.

• Consequently, the reaction taking place in zeolites depends upon the size and shape of reactants and product molecules as compared to those of the pores and cavities of the zeolites. That is why these types of reactions are called 'shape-selective catalysis'.

• Zeolites are being very widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerisation. An important zeolite catalyst used in the petroleum industry is $ZSM-5$ (Zeolite Sieve of Molecular Porosity $5$).

• It converts alcohols directly into gasoline (petrol) by first dehydrating then so that a mixture of hydrocarbons is formed. \[\begin{array}{c}{Alcohols}&{\xrightarrow{{ZSM - 5}}}&{Hydrocarbons} \end{array}\]

• It may be mentioned that hydrated zeolites are used as 'ion-exchangers' in softening of hard water.