Zeolites are naturally occurring minerals with the formula [M2/a(SiO4)x(AlO4)y]- formed from the reaction of aluminosilicate, water, and other metallic and non-metallic elements such as iron, titanium dioxide, lithium, sodium, antimony, aluminum, and vanadium. A zeolite, also called a microporous or micaceous zeolite, is a hydrated aluminum silicate containing both silica and alumina. Zeolites may also include metals such as iron, chromium, gallium, manganese, zinc, magnesium, calcium, and molybdenum. Zeolite minerals are formed when aluminum atoms occupy a portion of the coordinated tetrahedral positions in a silica-alumina framework. The necessary aluminum is introduced into the zeolite by various means, such as by the addition of aluminum salts to a solution of the silicon-containing preparation of the zeolite or by thermal decomposition of aluminum alkoxides or hydroxides. The incorporated aluminum may be substituted for silicon, as would normally occur by replacement of an equimolar amount of silica. Consequently, a very large variety of compositional variations can be effected by appropriate selection of the various reagents.
Degradation of the cation-exchange capacity of a zeolite is relatively easy to a greater or lesser extent depending upon the preparation and on the ultimate uses of the zeolite. As mentioned before, the contact of acid with a zeolite can result in the removal of protons and a total or partial conversion of the silicon to the tetrahedral sites of the zeolite, a process referred to as ion-exchange. However, as discussed above, the removal of the protons can be obtained not only by NaOH or other hydroxides but also by other bases, such as NH4OH or Na2CO3. Zeolites have been described as selective adsorbent materials based on their ability to affect the sorption of molecules with different charges. By altering the composition of the zeolite, the type or amount of charge on the zeolite, its pore size and its hydration, one can alter the molecular sorption capacities of zeolites. d2c66b5586