To put it simply, shape-selective catalysis combines catalysis with the molecular sieve effect. Because of the reactant's or substrate's form or size, the catalyst in this case exhibits preference or selectivity towards it. These catalytic processes are influenced by the substrates' and products' sizes or shapes as well as the catalyst's pore structure. Zeolites are an excellent example of this kind of catalyst. In its purest form, a zeolite is a "boiling stone." This is due to the fact that they are stones with extremely long-lasting heat retention. Zeolites have unique properties when heated because they are highly porous and can trap water in their pores. They are extremely stable and tolerant of challenging environmental factors like high temperature and high pressure.