D. W. Breck
Union Carbide Corporation Linde Division Research Laboratory Tonawanda, New York
1
Crystalline Molecular Sieves
I
During the last fifteen years the unique propert.ies of a group of crystalline aluminosilicateshave attracted increasing attention in scientific circles and in the chemical process industry. The synthesis and coinmercial manufacture of these crystallme aluminosilicates, called zeolites, constitutes one of the more sophisticated utilizations of crystallography in molecular science and chemical engineering. Zeolite minerals were discovered and named in 1756 by Baron Cronstedt, the Swedish mineralogist. It was early recognized that these minerals are able to exchange their metal cations by treatment with aqueous solutions of various salts, and many naturally occurring or synthetic cation exchangers were called "zeolites." Amorphous, gel-type, aluminosilicates have been used for yeara for water treatment. An extensive literature on the use of natural materials, such as green sand or glauconite, and synthetic "zeolite" water softeners, has accumulated (1). The molecular sieve zeolites are the crystalline types as originally discovered. Smith has recently defined a zeolite as "an aluminosilicate with a frame work structure enclosing cavities occupied by large ions and water molecules, both of which have considerable freedom of movement, permitting ion exchange and reversible dehydration" (8). Zeolite crystals were reversibly dehydrated without any change in transparency or external crystal form by Damour in 1840. Grandjean later found that dehydrated zeolite crystals would reversibly adsorb inorganic vapors such as iodine, mercury, and ammonia. Iu 1925, Weigel and Steinhoff reported that the zeolite chabazite adsorbed water vapor, methyl, and ethyl alcohol; but acetone and benzene were largely excluded. This is perhaps the first report of the molecular sieve behavior of anhydrous crystalline zeolites. For a review of the early literature on the properties of zeolite crystals, the reader is referred to McBain who, in discussing the significance of these results, proposed the term "molecular sieve" (3). Although extensive study yielded much information about the nature of zeolite minerals, the properties of ion exchange, the reversible gain and loss of water,
and the adsorption of other gases and vapors were rather mysterious. However, a concept of a spongelike structure had arisen. The application of X-ray diffraction techniques by Pauling (4) and Taylor (5) in the early 1930's led to the determination of the crystal structures of the zeolites analcite and natrolite. A selected list of synthetic zeolites and zeolite minerals is given in Table 1. The porous nature of zeolite crystals attracted the attention of a few physical chemists, in particular R. AI. Barrer, who reported in 1938 the results of his stndies of the sorption of gases on ciystals of the zeolites chabazite and analcite (6). Many scientists attempted to prepare synthetic forms of zeolite crystals with partial success. Although the synthesis of several zeolite minerals is reported in the literature of the last 50 years, the majority of these accounts must be discredited on the basis of improper identitication (7, 8). The advent of X-ray diffraction has enabled a more positive identification of con~plex compositions and structures than the methods possessed by the early investigators. Attracted by the potential application of crystalline zeolites to the separation of gases, R. 32. Milton and associates a t Union Carbide Corporation's Linde Division in 1948 initiated a study of zeolite mineral synthesis and chracterization. Before this, Barrer had synthesized the zeolite mordenite and several other synthetic varieties (7, 8, 9). By 1952, many different species of synthetic zeolites had been prepared in the Linde research laboratory (10,11). Although some of these are analogs of zeolite minerals, many were new varieties not found in nature. Two of these, ~ ~ h i will ch be discussed in some detail, are designated as zeolite type A and type X. Since there is no systematic method for the chemical naming of complex aluminosilicates, we have used a system of capital letters to designate new zeolite structures. Proper names should not be used in the naming of synthetic "mineral" species. In general, this policy has been utilized by all workers in the zeolite area, although it has not been universally followed in the naming of other "synthetic" minerals. Zeolite Cryrfallography
'Present address: Union Carhide Research Institute, P. 0. Box 278, Tanytown, New York. 678 / Journal of Chemical Education
The first structural analysis of a synthetic zeolite, type A, was reported in 1956 (18) and was followed by
the solutions of the structures of faujasite (IS), zeolite type X (14, 15), and chabazite (16). The structures of zeolites consist of a three dimensional framework of SiOr and A104 tetrahedra (Fig. 1). The framework
Figure 1. The representolion of tetrahedral coordination of oxygen with aluminum and 4licon bv skeldol tetrahedron. ~ o c k e d rphere~orrolid tetrahedron.
silicates include the feldspars, feldspathoids, and zeolites (17). The aluminum ion is small enough to occupy the position in the center of the tetrahedron of four oxygen atoms, and the substitution of AIS+for Si4+in framework silicates is common. However each substitution requires the presence of an alkali metal or alkaline earth ion, such as Na+, K+, Ca2+, Sr2+, in order to maintain electrical neutrality. I n zeolites the maximum substitution of A13+ for Si4+is the ratio of 1:1 and leads to a complete ordering of the Al and Si ions (12). The minimum substitution occurs in the zeolite mordenite and gives an Al/Si ratio of 1:5 (18). Unlike the feldspars, zeolite structures contain large cavities filled with water molecules. The cavities are interconnected in one, two, or three dimensions. The alkali metal ions needed for charge compensation occupy sites adjacent to the cavities and are generally available for exchange with other ions. There is no known occurrence of a zeolite mineral containing a fillimg material other than water. Inert gases, for example, are found in some minerals. I n order to illustrate the relation between chemical composition and structure, we use a structural formula of the type Me.~dAlO&(SiOd,I .M H.0
where Me = metal cation, and x, y, and n are integers. This is applied to one unit cell of the zeolite structure. The portion in brackets discloses the framework composition. The ratio y/x varies between 1 and 5. The zeolite compositions given in Table 1 are shown by this type of formula. For illustration, the composition of
the zeolite chabazite is: Cad(A101)8(Si0dd.26 EL0
In chabazite the Si/Al ratio actually varies between 2 and 2.6. Although other ions on the basis of size and charge can fit the tetrahedral sites (for example P6+, Gas+, Ge4+) they are not found except in rare cases in natural zeolites. Synthetic varieties containing Ge4+and Ga3+ substituted for Si4+ and AIS+ have been -~ r e- ~ a r c d (19, 20). The structures of many zeolites consist of simple arrangements of polyhedra; each polyhedron itself is a three-dimensional array of (Si,Al04) tetrahedra in a definite geometric form. The sodalite group of zeolites (Table 1) are all based on frameworks which are simple arrangements of truncated octahedra. The tetrahedra are arranged a t corners of a truncated octahedron which, in keeping with Euler's theorem, contains eight hexagonal faces, six square faces, 24 vertices, and 36 edges (21). In the sodalite structure, the truncated octahedra share square and hexagonal faces (Fig. 2). Although sodalite is not strictly a zeolite, the framework structure is based on similar units. I n the structure of zeolite type A (IS), the octahedra are linked in a cubic array by joining them with cubes on the square faces (Figs. 3 and 4). This produces a central truncated cube octahedron (another Archimedian polyhedron) with an internal cavity of 11 A in diameter as shown in Figure 5. Each central cavity, termed the a cage, is entered through six circular apertures formed by a nearly regular ring of eight oxygen atoms with a free diameter of 4.2 A. The cavities thus are arranged in a continuous three-dimensional pattern forming a system of unduloid-like channels with*a maximum diameter of 11 A and a minimum of 4.2 A. The truncated octahedra themselves enclose a second set of smaller cavities 6.6 A an internal diameter (6 cages) and connected to the larger cavities by means of a distorted ring of six oxygen atoms of 2.2 A free diameter. In each crystallographic unit cell of zeolite type A there are 12 AlO, and 12 SiOI tetrahedra (Table 1) and therefore, 12 monovalent cations. Eight of these sodium ions lie in the center of the six-rings in the hexagonal faces and four occupy positions adjacent to the
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