Page 195 - Adsorbents fundamentals and applications

180 ZEOLITES AND MOLECULAR SIEVES
◦
intersects them at an angle of 55 . Thus, the channel system is two-dimensional.
Each unit cell of clinoptilolite contains 6 Al atoms, hence there are 6 monovalent
cations or 3 bivalent cations. These cations are located at sites M(1), M(2), M(3),
and M(4), as indicated in Figure 7.1. Each unit cell contains only 4 combined
M(1)/M(2) sites, 4 M(3) sites, and 2 M(4) sites, or a total of 10 sites. Different
cations have their own preferred sites as indicated in the figure. M(1)/M(2) sites
are at the intersections of channels A/B with channel C. Na + and Ca 2+ may
occupy both M(1) and M(2) sites. The natural clinoptilolite has mixed cations.
+
+
In fully K exchanged form, 4 K ions occupy the M(3) sites and the other two
occupy M(2) sites. Thus, diffusion in the K- clinoptilolite is one-dimensional
(since channel C is closed), whereas diffusion in all other pure cation forms is
two-dimensional (since channel C is open) (Ackley and Yang, 1991).
7.4.2. Effects of Cation Sites on Adsorption

The effects of cation sites can be best illustrated by the important system of N 2 /O 2
on type X zeolites. NaX (or 13X) has been used commercially for air separation
since the 1970s. Li-LSX is the best sorbent that is commercially available today
(Chao, 1989). Mixed-cation AgLi-LSX (with 1–3% Ag cations) has been shown
to be even better than Li-LSX for air separation (Yang and Hutson, 1998; Hutson
et al., 1999; Hutson et al., 2000).
As shown in Figure 7.1, there are 192 possible cation sites in a unit cell of
faujasite (or X zeolite) and only a maximum of 96 cations to occupy them, i.e.,
LSX has 96 cations (when monovalent cations are used). Upon activation of the
◦
zeolite, i.e., heating at 350 C, the cations migrate to the sites with the lowest
energies. Migration is an activated process, which depends on the temperature,
time, as well as the size of the cation. Unfortunately, the most stable sites (those
at the lowest energies) are hidden and are not exposed to the supercage cavity.
These are the sites with the maximum coordination. From Table 7.4, only about
1/3 to 1/2 of the cations are located at exposed sites.
By ion exchange of Na + with Li + in the LSX, Chao obtained significantly
improved N 2 /O 2 selectivity (Chao, 1989). This improvement is the result of the
smaller ionic radius of Li + (0.68 ˚ A) compared to that of Na (0.97 ˚ A). Since
+
+
Li and Na have the same charge, N 2 interacts much more strongly with Li +
+
(electric field gradient - quadrupole) potential.
due to a significantly higher φ ˙ FQ
However, no improvement is seen until over approximately 70% ion exchange is
made. N 2 adsorption increases linearly with ion exchange beyond this threshold
value (see Figure 7.14). (Figure 7.14 actually shows LiX with different Si/Al
ratios, or different number of Li cations/unit cell. However, it illustrates the
same phenomenon.) This point is discussed further in Chapter 10 (Figure 10.7).
The reason for this significant phenomenon is that Sites I, I’ and II’ are lower-
energy sites and are preferred by Li + (Chao et al., 1992; Coe, 1995). Sites II
and III are exposed but have lower coordination and are less preferred. These
exposed sites are most important for adsorption.
+
Although Ag has a larger ionic radius (1.26 ˚ A) than Li ,a weak π-complexa-
+
tion bond can be formed between Ag (in AgZ) and N 2 (Chen and Yang, 1996).
+