The Mineral ZeoliteThe term "zeolite" was coined in the year 1756 by the Swedish amateur mineralogist A. F. VON CRONSTEDT. He had observed that certain minerals release a large quantity of water when they are heated and seem to boil. He therefore named them "boiling stones", in Greek, zeolites. Nowadays, "zeolite" is a collective term for crystalline metal aluminosilicates with the general formula:
|x [(M I, M II ) AlO2 ] y SiO2 z H2O with:||M I||Alkaline metal cation, e.g.:
Li+, Na+, K+, Rb+, Cs+, Fr+
|M II||Alkaline earth metal cation, e.g.:
Mg2+, Ca2+, Sr2+, Ba2+
Zeolites are characterised by a large inner surface of over 1000 m²/g, strong electrostatic fields in their crystal lattice and by a specific bulk density of around 750 kg/m³. Their most important property is that they can be reversibly dehydrated, whereby their lattice structure remains unchanged even after several thousand cycles as long as pressure and temperature do not exceed certain limits.
Zeolites are non-toxic, incombustible, occur in large quantities in nature and are therefore environmentally-compatible. There are around 40 known natural zeolites and over 140 synthetic versions. Since the beginning of the fifties, many types of zeolite have been created synthetically by the chemical industry on a large industrial scale and put to a wide variety of uses such as:
- low-cost water softeners in detergents (ion exchangers)
- catalysts in cracking processes
- fillers for paper
- dehumidifiers for refrigerators and freezers, cupboards, cars, double-glazed windows, etc.
|Photo: Zeolite moulded bodies
The structure of zeolite
Photo: Structure of A-zeolite
|(AlO4) tetrahedrons each have one negative charge in the lattice, which needs to be balanced out by metal cations, e.g. sodium. Linking of the primary SBUs results in secondary SBUs. A number of secondary SBUs combine to form tertiary SBUs. The graphic shows a cuboctahedron. Then, a number of tertiary SBUs, linked by oxygen bridges (shown here as cuboids), form a crystal body in whose interior a pore structure is created with a defined pore diameter and large volume.|
The inner surface of the zeolite crystal shown in the graphic is approx. 1000 m²/g, the pore diameter 0.4 nm and the specific pore volume about 0.47 cm³/cm³