Shape-Selective Catalysis by Zeolites

The catalytic reaction that depends upon the structure of pores 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 honeycomb-like structures. Zeolites are aluminosilicates i.e., three dimensional network silicates in which some silicon atoms are replaced by aluminium atoms. They are found in nature as well as synthesized for catalytic selectivity. Zeolites, before using 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. 

The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of the zeolites. That is why these types of reactions are called ‘shape-selective catalysis’ reactions. 

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. It converts alcohols directly into gasoline (petrol) by dehydrating them so that a mixture of hydrocarbons in formed.

 

Name
Email Id
Mobile

Exam
Target Year

Related Resources
Characteristics of Catalytic Reactions

Characteristics of Catalytic Reactions (i) The...

Enzyme Catalysis

Enzyme Catalysis Living organisms carry out...

Catalysts in Industries

Catalysts in industries 1. Haber’s process...

Nature of Solid Catalysts

Nature of Solid Catalysts Solid catalysts may be...

Summary of Adsorption and Catalysis

Summary of Adsorption & Catalysis This section...

Types of Catalysts

Types of Catalysts (i) Positive catalysis: The...