Zeolites

Zeolite is a class of hydrated aluminosilicates found in certain volcanic rocks. Geologists thought these minerals were interesting because they consist of large cage-like structures with open channel ways. Today, these materials are highly valued for their applications.

Jaujasite is a representative zeolite. This link represented its structure using large polyhedra. All lines in the structure shown are oxygen bridges -O-, the oxygen atom is located not on the line but somewhere next to it. The angle -O- of the two bonds is about 110ø. Points where lines meet are the locations of Si⁴⁺ that are surrounded tetrahedrally by four oxygen atoms. They can be substituted by Al³⁺. As a result, the frame work is negatively charged. Thus, zeolites can trap positive ions: H⁺, Na⁺, K⁺, Ca²⁺, Cu²⁺ or Mg²⁺.

Figure 1: Crystal structure of Faujasite. Image used with permission of Hermann Luyken via wikipedia.

The name 'zeolite' is said to have its origin in the two Greek words zeo and lithos which mean 'to boil' and 'a stone'. The phenomena of melting and boiling at the same time is a novel property. The name 'zeolite' was first used by the Swedish mineralogist Cronstedt to describe stilbite, the first recognized mineral zeolite, which was discovered in 1756. Over 100 years later, the reversible desorption/adsorption of water in this mineral was recognized.

The faujasite framework consists of "sodalite" cages which are connected through hexagonal prisms (Figure 1). The pores are arranged perpendicular to each other. The pore, which is formed by a 12-membered ring, has a relatively large diameter of 7.4 Å. The inner cavity has a diameter of 12 Å and is surrounded by 10 sodalite cages (Figure 2).

Figure 2: The structure of an individual cage of Faujasite. Image used with permission of wikipedia.

While you are admiring this beautiful picture of faujasite, remember that the oxygen atoms have two unshared electron pairs in addition to the (Al,Si)-O-Si(or Al) bonds. Thus the oxygen atoms are sites to interact with positive site of molecules that passes by these structures. At present over 150 synthetic zeolites & zeotypes and 40 natural zeolites are known. Synthesis of zeolite is a very active field of study.

Aluminosilicates have three major minerals: Andalusite, sillimanite, and kyanite. Zeochem has been developing and manufacturing molecular sieve adsorbents since 1977. Simply put, their adsorbents are used to "screen" out impurities from a variety of applications by attracting and trapping the targeted contaminants. For example, in natural gas processing, molecular sieves are used to remove specific molecules from the gas stream to allow for more efficient downstream processing. Faujasite is a typical zeolite.

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