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Zeolites structure and types

Chemical structure

Zeolites are three-dimensional, microporous, crystalline solids with well-defined structures that contain aluminium, silicon, and oxygen in their regular framework; cations and water are located in the pores.
The silicon and aluminium atoms are tetrahedrally coordinated with each other through shared oxygen atoms. Compositionally, zeolites are similar to clay minerals. More specifically, both are alumino-silicates. They differ, however, in their crystalline structure.

Many types of clay have a layered crystalline structure (similar to a deck of cards) and are subject to shrinking and swelling as water is absorbed and removed between the layers. In contrast, zeolites have a rigid, 3-dimensional crystalline structure (similar to a honeycomb) consisting of a network of interconnected tunnels and cages.
Water moves freely in and out of these pores but the zeolites framework remains rigid. Another special aspect of this structure is that the pore and channel sizes are nearly uniform, allowing the crystal to act as a molecular sieve.

The porous zeolite is host to water molecules and ions of potassium and calcium, as well as a variety of other positively charged ions, but only those of appropriate molecular size to fit into the pores are admitted creating the "sieving" property. Because of their regular and reproducible structure, they behave in a predictable fashion.

Types of zeolite

Zeolites are natural minerals that are mined in many parts of the world; most zeolites used commercially are produced synthetically. When developing applications for zeolites, it is important to remember that not all of these minerals are the same.

There are nearly 50 different types of zeolites (clinoptilolite, chabazite, phillipsite, mordenite, etc.) with varying physical and chemical properties. Crystal structure and chemical composition account for the primary differences. Particle density, cation selectivity, molecular pore size, and strength are only some of the properties that can differ depending on the zeolite in question. It is important to know the specific type of zeolite one is using in order to assure that it is appropriate for one's needs.

There are numerous naturally occurring and synthetic zeolites, each with a unique structure. The pore sizes commercially available range from approximately 3 Å to approximately 8 Å. Some of the commercial materials are: A, beta, mordenite, Y, ZSM-5.

The biggest differences between natural and synthetic zeolites are:

  • Synthetics are manufactured from energy consuming chemicals and naturals are processed from natural ore bodies.
  • Synthetic zeolites have a silica to alumina ratio of 1 to 1 and clinoptotilite (clino) zeolites have a 5 to 1 ratio.
  • Clino natural zeolites do not break down in a mildly acid environment, where synthetic zeolites do. The natural zeolite structure has more acid resistant resistant silica to hold it’s structure together. The clino natural zeolite is broadly accepted for use in the agriculutral industry as a soil amendment and as a feed additive.

In 1948, Richard Barrer first produced a synthetic zeolite that did not have a natural counterpart. At approximately the same time, Milton made the first materials that had no natural counterpart such as zeolite A. New natural zeolites are still being discovered, and new synthetic zeolites are being invented in many laboratories around the world.

Click here for an introduction about zeolite or find more information about zeolite applications.

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