Page 191 - Adsorbents fundamentals and applications

176 ZEOLITES AND MOLECULAR SIEVES

and that in a typical type with tubular topology (i.e., heulandite) will also be
included. Adsorption of molecules with strong dipoles and/or quadrupoles (e.g.,
NH 3 and SO 2 ) is known to cause redistribution of some cations (Barrer, 1978). As
mentioned, the remaining water molecules have a strong influence on cation sites.
Because water is removed for adsorption applications, only highly dehydrated
zeolites will be discussed.
Cation sites for zeolite A, with the common alkali and alkaline earth cations,
are listed in Table 7.3. The cation sites in A and X zeolites from Figure 7.1 are
shown again in Figure 7.11. The three sites in zeolite A, sites I, II and III, are
only approximate indications of the actual sites. For example, some of the cations
associated with 6-oxygen rings (i.e., site I) are extended into the large cavity;
while some others, also referred to as being at site I, are recessed into the sodalite
cage or are nearly in the plane of the 6-ring (e.g., Firor and Seff, 1979; McCusker
and Seff, 1981). Although they are all referred to as being at site I, they are clearly
different in terms of their abilities to interact with adsorbate molecules. The
cation sites can be predicted by calculating the cation-lattice interaction energies
(consisting of electrostatic, polarization, dispersion/repulsion, and charge-transfer
energies), and the sites with the highest energies will be preferred (Ogawa et al.,
1978). This technique can also be used to predict the sites for zeolites with mixed
cations that are obtained by partial ion exchange. As a result, the site selectivities
2+
+
+
determined for cations are as follows: Li ,Na ,Ca ,and Sr 2+ prefer 6-ring
sites (site I); whereas K ,Cs ,and Ba 2+ prefer the 8-ring sites (site II) (Ogawa
+
+
et al., 1979). The predictions are in agreement with experimental data shown in
Table 7.3.
Table 7.3. Cation site occupancies in dehydrated
zeolite A
Sites
Zeolite I II III Others

Li-A a 8 3 1 —
Na-A b,c 8 3 1 —
NaCa-A (4Na + 4Ca) d 8 0 0 —
K-A e 6 3 3 e
Ca-A f 5 1 0 —
Sr-A f 5 1 0 —
Ba-A g 4 2 — —
a Vance and Seff, 1975.
b Reed and Breck, 1956; Smith and Dowell, 1968.
c Yanagida et al., 1973.
d Breck, 1974.
e Barrer, 1978.
f Firor and Seff, 1979.
g Dyer et al., 1971; Ogawa et al., 1978.
12 Monovalent cations per unit cell.