Zeolites are shape selective catalyst which have honey-comb like structure. Zeolites have a pore size between 260 pm to 740 pm which makes them possess, the unique property of shape-selectivity of Zeolites. Chemically, Zeolites are micro porous alumino silicates with three dimensional networks of silicates in which some silicon atoms are replaced by aluminium atom giving Al-O-Si framework.

The general chemical formula of Zeolites is M_x/n [(AlO₂)_x (SiO₃)_y].mH₂O and may be considered as open structure of silica in which aluminium has been substituted in a fraction x/(x+y) of its tetrahedral sites.