Shape-Selective Catalysis by Zeolites: Concepts, Applications, Examples

As you have already learnt, one of the important features of a catalytic reaction is the selectivity of the catalyst. Did you know that there are some catalytic reactions that selectively proceed if the size and shape of the reactants or products fit the pore size of the catalyst! We call such a reaction, a shape-selective catalysis and the catalysts are called as zeolites. Let us learn about this interesting concept in further detail.

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What is Shape-Selective Catalysis?

In simple words, shape-selective catalysis is the combination of catalysis and the molecular sieve effect. Here, the catalyst shows preference or selectivity towards a reactant or substrate because of its shape or size. These catalytic reactions depend on the pore structure of the catalyst and the size or shape of the substrates and products. A good example of these type of catalysts is zeolites.

What are Zeolites?

A Zeolite literally means a ‘boiling stone’. This is because they are stones that can retain heat for a very large period of time. They are extremely porous and can trap water in their pores such that when heated, a large amount of steam is generated off their surface.

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Chemically speaking, they are microporous aluminosilicates. They have a 3D network of silicates where aluminium atoms replace some of the silicon atoms giving an Al-O-Si framework. Within the gaps of this network, alkaline earth metals (sodium, potassium, magnesium) and water are trapped.

They form many different crystalline structures with very large open cavities in a very regular arrangement. There exist about 40 naturally occurring zeolites and a number of artificial ones or synthetic zeolites have also been designed.

Zeolites have special properties. They are very stable and resistant to harsh environmental conditions such as high temperature and high pressure. But the most interesting property is their framework and how they can trap other molecules in it. They have honeycomb-like structures which make them good shape-selective catalysts.

Zeolites as Catalysts

We use Zeolites as catalysts for many important reactions with organic molecules. They act as shape-selective catalysts either by transition state selectivity or excluding competing reactants based on their molecular size. When some of the reactant molecules are too large to diffuse into the zeolite pores, it is reactant shape selectivity.

Contrarily, when only those products with the proper dimensions can diffuse out of the zeolite pores, it is product shape selectivity. Certain reactions are also prevented because the transition state reactants don’t have enough space within the pores.

Applications of Zeolites as Catalysts

Zeolites have wide applications as catalysts in the pharmaceutical and also, petrochemical industry. In the petrochemical industry, they act as ‘catalytic crackers’ to break large hydrocarbon molecules into diesel, kerosene, gasoline, waxes and other petroleum by-products.

One of the most important zeolites used in the petroleum industry is ZSM-5. It dehydrates alcohols into a mixture of hydrocarbons and directly gives gasoline (petrol).