Page 213 - Adsorbents fundamentals and applications

198 π-COMPLEXATION SORBENTS AND APPLICATIONS

energy change of the reaction. Because ion exchange is usually performed in
aqueous solutions, steric hindrance can be a factor when the size of the cation, or
the hydrated cation, is larger than the aperture. This is particularly the case with
type A and type X zeolites, where only small cations can penetrate the 6-oxygen
rings into the sodalite cage. For zeolite A, large cations may also be excluded
from the supercage by the 8-ring windows.
Cation exchange in zeolites has been discussed in detail by Breck (1974). The
cation exchange behavior depends on (1) the nature of the cation, its size and
charge; (2) the temperature; (3) the concentration of cations in solution; (4) the
anions and the solvent; and (5) the structure of the zeolite. As a general rule,
the equilibrium selectivity favors cations of a higher valence. The selectivity
favors cations with a higher atomic weight for those with the same valence. The
selectivity follows the relative order of free energies of reaction for different
cations, favoring the reaction with the most negative free energy of reaction.
For type X and Y zeolites, below a level of 34 cations/unit cell (or, 40%
ion exchange of a typical X zeolite with 86 cations per unit cell), the order of
selectivity for univalent ions follow (Sherry, 1966; Breck, 1974):

Ag Tl > Cs > Rb > K > Na > Li (8.2)

This series corresponds to occupancy of the most accessible cation sites (sites III
and IV) within the supercage. At 50% exchange of X zeolite, which includes site II
in the 6-ring adjacent to the supercage, the selectivity series is (Breck, 1974)

Ag Tl > Na > K > Rb > Cs > Li (8.3)

These sites (II, III, and IV) are exposed to the supercage and hence important
for adsorption.
2+
The selectivity for Ca ,Sr ,and Ba 2+ is similar to that of Rb and K ,
2+
+
+
whereas the selectivity of Ce 2+ and La 2+ is similar to that of Ag + (Sherry,
1967; Sherry, 1968; Breck, 1974). The selectivity for the important ion used in
preparing π-complexation sorbents, Cu , is not available. However, Cu 2+ can
2+
be exchanged with ease for type X, Y, and ZSM zeolites (e.g., Huang, 1973;
Rabo et al., 1977; Takahashi et al., 2001a).
In tailoring sorbents for π-complexation, both the cation–sorbate bond strength
and the total number of cations are important. The density of cations depends on
the cation exchange capacity of the zeolite. Table 8.3 provides useful information
on the total cation capacities for a number of zeolites.
Ag Exchange. Ag-Y has been shown to be an excellent sorbent for a number of
+
purification processes, including the removal of dienes from olefins. This process
has been used commercially. As discussed above, Ag + has a high selectivity
for zeolites; hence it can be exchanged readily at the ambient temperature and
low concentrations. A typical sample preparation procedure is described here for
+
Ag exchange in Y zeolites with different Si/Al ratios, and also for partial ion
exchange for a given Y zeolite. (Takahashi et al., 2001b).