The Zeolite Faujasite (also known as Zeolite X or Y) framework showing the large pores in which catalytic cracking occurs.
The naturally occuring zeolite Faujasite consists of linked AlO4 and SiO4 tetrahedra. The diagram above demonstrates the relationship between the size of the pore diameter and the size of atom that can fit through these pores. A larger pore can accomodate a larger molecule.