Microporous Solids
Zeolites are aluminosilicate solids that are based on a framework of corner-sharing SiO4 and AlO4 tetrahedra. These frameworks consists pores and channels of molecular dimensions, that in natural minerals (or after laboratory synthesis, generally by hydrothermal methods,) consist species like water and hydrated ions. Removal of these species (example by careful heating under vacuum) leads to microporous materials with empty pores and channels. It is feasible to make synthetic zeolites of composition SiO2 with no aluminum but when AlIII is provide the framework formula is [AlxSi1-xO2]x- and the charge must be compensated through extra-framework cations. In as-prepared zeolites these might be alkali cations NH4+, or organic amines but while the pore materials are removed they are replaced through H+, which creates strong Brønsted acid sites in the pores.
The structure of the zeolite faujasite is displayed. In this conventional representation the framework structure is displayed without depicting atoms directly. Each line shows an Si-O-Si or Si-O-Al connection. Four lines get together at tetrahedral vertices showing the positions of the four-coordinate Si or Al atoms. Space-filling models of this zeolite depict that the pores can accommodate molecules up to about 750 pm in diameter.
In their hydrated forms zeolites are employed for ion exchange purposes, for instance, water softening through replacement of Ca2+ with Na+ or another ion. When dehydrated they have significant catalytic applications, promoted through the Bronsted acid sites and by the large area of 'internal surface'. They are employed for the cracking of petroleum and for the isomerization of hydrocarbons, in which limited pore size exerts a 'shape selectivity', which permits one desirable result to be formed in high yield.