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Ion-Exchange 515

Types= FAU= Related Materials. The IZA web-site has links 16.1.4.3.4 Charge
to numerous animated 3-D images, which give a perspective The proportion of silicon to aluminum has little effect on
of any of the respective Framework Types in their database. the overall structure of a zeolite. But whether the zeolite
The publisher’s abstract of the book by Baerlocher et al. may function as an adsorbent or as an ion-exchanger is
(2007) describes further the idea of a Zeolite Framework Type: affected. The two extremes are: (1) a zeolite with the same
Each time a new zeolite framework structure is reported, it is number of silicon and aluminum atoms (always, it must be
examined by the Structure Commission of the International
true that: # aluminum atoms # silicon atoms in a crystal),
Zeolite Association (IZA-SC), and if it is found to be unique and (2) a zeolite with nearly all silicon atoms. The first case
and to conform to the IZA-SC’sdefinition of a zeolite, it is results in a net negative charge in the pores; free ions are
assigned a three-letter framework type code. This code is part needed to balance the charge and most often these are Na ,
þ
of the official IUPAC (International Union of Pure and Applied K ,Ca ,Mg . In the second case, there is no net charge
2þ
2þ
þ
Chemistry) nomenclature for microporous materials. The Atlas and the structure is hydrophobic and will attract neutral mol-
of Zeolite Framework Types is essentially a compilation of data ecules based on van der Waal’s forces (Kato, 1995, p. 8).
for each of these confirmed framework types. These data
include a stereo drawing showing the framework connectivity, 16.1.4.3.5 Clinoptilolite
features that characterize the idealized framework structure, a Clinoptilolite is the most abundant naturally occurring zeolite
list of materials with this framework type, information on the with cation-exchange capacity, CEC ¼ 150 meq=100 g
type material that was used to establish the framework type, (Marshall, 1964, p. 120). The channel structure has a large
and stereo drawings of the pore openings of the type material. cavity size measuring 4.4 7.2 Å (Haggerty and Bowman,
As may be evident, the field of zeolite structures and 1994, p. 452). The chemical composition was determined as
chemical compositions is specialized. For water treatment
[Na 2 O] 0.92 [K 2 O] 0.78 [Fe 2 O 3 ] 0.27 [MgO] 0.21 [Al 2 O 3 ] 1.93 [SiO 2 ] 9.70
applications, usually related to removals of specific ions, (Kesraoul-Ouki, 1993, p. 1115). Figure 16.2 is a scan-
particular zeolites may be recommended based on past use. ning electron microscope (SEM) photomicrograph of a
The IZA web-site gives access to in-depth structural know- Clinoptilolite surface, showing its irregularity.
ledge of any zeolite.
16.1.4.3.6 Greensands
16.1.4.3.1 Characteristics
One of the best known of the natural ion-exchangers is glauco-
The zeolites are soft minerals and therefore have low resist- nite, which is a sodium aluminosilicate (Powell, 1954), com-
ance to abrasion. They have internal cage-like structures with monly called ‘‘greensand (which is classed by mineralogists as a
surface areas up to several hundred square meters per gram mica clay, not a zeolite).’’ Zeo-Durt and Inversandt have been
and cation-exchange capacities up to several equivalents per trade names for this material. The particles are greenish-black
kilogram (Haggerty and Bowman, 1994, p. 452). with exchange capacities of 6.18–8.70 kg=m (2700–3800
3
3
g=ft ). Shallow deposits in New Jersey were reported as being
16.1.4.3.2 Pores
almost inexhaustible (Babbitt and Doland, 1949, p. 513).
A zeolite has a unique crystal structure with a center channel
of uniform size. Zeolite pores range in size, 2.5–8Å
(2.5 10 10 8.0 10 10 m) diameter, depending on the struc-
ture (Kerr, 1989, p. 100; Kato, 1995, p. 7). The pores may be:
(1) three-dimensional, i.e., with channels that intersect from
three directions such as Type A synthetic; (2) two-dimen-
sional, i.e., with two intersecting channels such as ZSM-5
synthetic; and (3) one-dimensional, i.e., resembling a pack
of straws (Vaughan, 1988, p. 27).

16.1.4.3.3 Structure
The basic building block in all zeolites is a tetrahedral struc-
ture of an aluminum or silicon atom surrounded by four
oxygen atoms; each tetrahedron is connected with others
through shared oxygen atoms to form a framework. The
tetrahedral building blocks can arrange themselves in varied
combinations resulting in different framework geometries, 20 KV 1.10KX 10 V 0001
i.e., different crystal structures (Vaughan, 1988, p. 25; Kerr,
1989, p. 100). Different combinations of the same secondary FIGURE 16.2 SEM photomicrograph of clinoptilolite surface.
building unit may give many kinds of distinctive zeolite (Reprinted from INZA, International Natural Zeolite Association
structures; theoretically, thousands are possible (Vaughan, web-site, http:==inza.edu=pics_crystals.php, 2010. With permission
1988, p. 25). from Steve Chipera.)