Zeolites and Related Molecular Sieves
Zeolites comprise a class of crystalline, microporous solids. They are mainly based on aluminosilicates, the structure of which are formed by corner-sharing SiO4 and AlO4 tetrahedra. Presence of three-valent aluminium in tetrahedral oxygen coordination results in negative framework charges that are compensated by extra-framework cations. The latter are generally exchangeable alkali or alkaline earth cations that are located in micropores (< 2 nm) that are an inherent feature of zeolite structures. Replacement of the cations by protons results in acidic zeolites that are excellent catalysts for petrochemical processes and the production of fine chemicals. The pore size and geometry of a given zeolite limit the sizes of molecules that can enter the pore systems, making zeolites highly selective catalysts and efficient adsorbents.