What is shape selective catalysis?

Shape selective catalysis refers to a catalytic reaction that depends on the pore structure of the catalyst and the size and shape of the reactant and product molecules. In this type of catalysis, the catalyst shows preference or selectivity towards a reactant based on its size or shape. If the size of the reactant molecule is larger than the pores of the catalyst, it cannot diffuse into the catalyst. 

Zeolites are excellent shape-selective catalysts because of their honeycomb-like structures and uniform size and shape of the micropores. When zeolite is used in a reaction, the reactant molecules are diffusers in the zeolite pores and later adsorbed in the active sites of the chemical reaction.