Zeolites, or molecular sieves, occur naturally and can also be synthesized in the laboratory. The unusual properties of these aluminosilicates and aluminophosphates are a result of their cage structure, the size of which varies for different zeolites. In synthetic zeolites, the size of the cage can be tailored to fit molecules of different sizes, thus providing selective reaction or adsorption sites. Zeolites have many uses–in the refining of gasoline and diesel fuel, as water softeners in combination with laundry detergents, to separate air and other gaseous mixtures into valuable components, and to act as ion-exchangers that can selectively trap radioactive waste elements from military production facilities and commercial nuclear reactors (1). In addition, the specificity of cavity sizes in the new transition metal zeolites has made it possible to selectively oxidize simple organic molecules into useful chemicals.

Sodalite, a transparent or translucent mineral frequently found in igneous rocks, is a typical zeolite. It has the formula Na₁₂(AlO₂)₁₂(SiO₂)₁₂ ?xH₂O. The negatively charged framework is formed of aluminum, silicon, and oxygen atoms, while positive counter ions in the cavities balance the negative charges of the framework. One way to show the structure of zeolites is the so-called framework representation, in which only the structural units forming the wall of the cavity are shown (each vertex of the polygon represents an atom).

Framework representation of a zeolite

The representation of sodalite shows a clearer illustration of the cage structure.

Structure of sodalite

How are zeolites formed? The primary building unit is commonly a tetrahedron of AlO₄^5-, SiO₄^4-, or PO₄^3-.

Tetrahedral building blocks of zeolite

These tetrahedra may then link together by sharing vertices to form secondary building units in the form of rings and cages. In these extended structures, atoms of aluminum, silicon, or phosphorus are linked by oxygen atoms, so that the rings and cages of the zeolite framework are formed by interlocking chains with the structure –O–X–O–X–O–, where X represents the atoms of Al, Si, or P.

Secondary building units of zeolite formed by linking of tetrahedra

The ability to prepare a zeolite-type structure containing large amounts of cobalt (up to 90% of the aluminum sites contain cobalt) by a fairly general synthetic method (2,3) opens the way to new materials that are expected to have a broad range of catalytic properties. G. Stucky and his group prepared several dozen of these zeolites and characterized their structures by x-ray crystallography. They were successful because cobalt is a transition metal having a preference for a tetrahedral environment and therefore can substitute for Al, Si , or P atoms. A typical structure is shown below.

Typical transition metal zeolite structure

The syntheses of these new zeolites are based on the clever technique of matching the charge and shape of the framework cavity with simple organic molecules used as templates (4). A zeolite is neutral, so the species occupying the cage must be a cation capable of canceling the negative charge of the framework. In these syntheses, a quaternary ammonium salt is used to counter the charge of the desired cage structure. The size and shape of the organic cation are chosen so that it can act as a template around which the desired cage structure assembles. The pores of the crystalline structure are filled with organic ammonium cations and water, which stabilize the structure as it crystallizes. These “guest” filler materials determine the backbone structure, and especially cavity size, of the particular zeolite desired. An example of such an organic cation is the anilinium ion, which is seen below in the structure of anilinium hydrochloride.

Anilinium hydrochloride

The organic material is then usually removed by heating the zeolite. Heating the ammonium or organic quaternary ammonium salt will decompose it into volatile products, which then escape, freeing the pores of the framework (hydrogen ions, H⁺, are retained to maintain charge neutrality). The challenge is to find materials that retain the framework structure when this final heating step is carried out.

Transition metals such as cobalt give these structures interesting properties. Ordinary aluminophosphates act only as ion exchangers and are otherwise chemically inert. However, zeolites incorporating transition metals are able to exhibit a wider range of reactions. The wide variety of oxidation states available to transition metals points to activity as oxidation-reduction catalysts, and the magnetic behavior of transition metals suggests that the new zeolites could have some interesting magnetic properties (5). Zeolites are already in use as absorbants to separate radioactive wastes, but being able to design unique cage sizes and other properties may improve their usefulness in this regard.

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