JEE Main & Advanced Chemistry Surface & Nuclear Chemistry / भूतल और नाभिकीय रसायन Zeolite (Shape selective catalysis)

(1) Zeolite are alumino–silicates of the general formula, \[{{M}_{x/n}}{{[Al{{O}_{2}}]}_{x}}.{{(Si{{O}_{2}})}_{y}}.m{{H}_{2}}O\], where, M may be simple cation like \[N{{a}^{+}}\], \[{{K}^{+}}\]or \[C{{a}^{2+}}\], n is the charge on the simple cation, m is the number of molecules of water of crystallization.

(2) Some well known zeolites are as follows,

Erionite \[\to N{{a}_{2}}{{K}_{2}}CaMg{{(Al{{O}_{2}})}_{2}}{{(Si{{O}_{2}})}_{2}}.6{{H}_{2}}O\]

Gemelinite \[\to N{{a}_{2}}Ca{{(Al{{O}_{2}})}_{2}}{{(Si{{O}_{2}})}_{4}}.6{{H}_{2}}O\]

Faujasite (natural) \[\to N{{a}_{56}}{{(Al{{O}_{2}})}_{56}}{{(Si{{O}_{2}})}_{136}}.250{{H}_{2}}O\]

ZSM-5 \[\to {{H}_{x}}[{{(Al{{O}_{2}})}_{x}}{{(Si{{O}_{2}})}_{96-x}}].16{{H}_{2}}O\]

Linde-A (synthetic) \[\to {{[N{{a}_{12}}{{(Al{{O}_{2}})}_{12}}{{(Si{{O}_{2}})}_{12}}.27{{H}_{2}}O]}_{8}}\]

(3) The characteristic feature of zeolites is the openness of the structure, which permits cavities of different sizes.

(4) The open structure is provided by silica in which aluminium occupies x/(x+y) fraction of the telrahedral sites.

(5) The negative charge of the aluminosilicate framework is neutralized by the replaceable cations.

(6) The void space forms more than 50% of the total volume, which is occupied by water molecules.

(7) The reaction- selectivity of zeolites depends upon the size of cavities (cages), pores (apertures) and the distribution of pores in the structure. The pore size in zeolites generally varies from 260 pm to 740 pm. 

(8) Zeolite have high porosity due to the presence of one, two, or three dimensional networks of interconnected channels and cavities of molecular dimensions.

(9) There is a new class of highly siliceous zeolites with an optimal pore diameter of 550pm. ZSM-5 is one such zeolite having the formula. \[[{{H}_{x}}{{(Al{{O}_{2}})}_{x}}.{{(Si{{O}_{2}})}_{96-x}}].16{{H}_{2}}O\]

(10) The zeolite catalyst ZSM-5 converts alcohols to gasoline (petrol) by dehydrating the alcohol and producing a mixture of wide variety of hydrocarbons.