Zeolites

Class: Tectosilicate

Sodalite - SOD
Sodalite – SOD

Description:

Zeolites are a subclass of tectosilicates, the largest class of minerals, comprising 75% of the Earth’s crust. Zeolites are microporous aluminosilicates though a number of other microporous silicates (borosilicates, germanosilicates) may also be included given their structural similarity and applications in materials science. There are approximately 200 known zeolite phases and the synthesis of new zeolites is an ongoing challenge. However the Al:Si ratio, as well as the possibility of aliovalent doping, dramatically increases the number of unique materials. The first zeolite was realized by Axel Cronstedt in 1756 when he observed steam evolve from the mineral stilbite upon heating.

Linde Type A - LTA
Linde Type A – LTA

Chemically the base molecular formula for zeolites is SiO2, however, aluminum substitution is ubiquitous resulting in a formula, MxAlxSi(1-x)O2•nH2O. Both the aluminum and silicon sites are considered tetrahedral to a first order approximation and the charge balancing metal sites are typically Na+, Ca2+, Mg2+, Ti4+, and or course simply H+.

Generally six membered silicated rings (6r) and smaller are considered inaccessible to all but the smallest gasses. Eight-membered rings (8r) and larger are needed to catalyze reactions of organic building blocks like benzene. Common and industrial relevant structure types are listed. A formal and comprehensive account of zeolite structures can be found in the Database of Zeolite Structures and the Atlas of Zeolite Framework Types.

Essential and Industrially Relevant Zeolites:

Sodalite, Linde Type A (first synthetic zeolite), Faujasite, ZSM-5, Chabazite, Mordenite, Stilbite (first discovered zeolite), Tschernichite (BEA), MFI

Faujasite - FAU
Faujasite – FAU

As a general rule the thermodynamic stability of zeolites can be expressed in terms of its relative stability to quartz. Quartz has a framework density of 30.7 T-sites per 1000 Å3. Likewise, porosity is best described by pore volume of the largest guest that can be accommodated by the lattice. Values calculated for a variety of Zeolite frameworks are given.

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