Page 190 - Adsorbents fundamentals and applications

INTERACTIONS OF ADSORBATE WITH CATIONS 175

+
should have an empty 5s orbital. Again, it is seen that Ag in AgZ has the lowest
−
occupancy in its 5s orbital, indicating that the Z (i.e., zeolite framework anion)
is the most electronegative anion.
The strong anionic nature of the zeolite framework and the correspondingly
strong cations that are bonded to the framework zeolite are unique with zeolites.
Furthermore, the cations and anions are not located closely to each other. Thus,
these cations and anions exert strong electric fields and field gradients over the
surface. The surfaces of zeolites are different from ionic crystals such as NaCl or
cement (that has the same chemical composition as zeolites). On the surfaces of
ionic crystals, the cations and anions are closely and periodically spaced. Thus,
for an adsorbate molecule several angstroms in size, no net strong electric field
and field gradient is exerted by the surface for interaction.

7.4. INTERACTIONS OF ADSORBATE WITH CATIONS: EFFECTS
OF CATION SITE, CHARGE, AND IONIC RADIUS
In earlier discussions, the strong or dominating contributions of cation-dipole
and cation-quadrupole interactions to the total bonding energy for adsorption
on zeolites are already seen (see Table 2.1). Unfortunately, except ionic radii,
information on the cation sites and the charges has not been well determined.
Hence they are often treated as fitting parameters in molecular simulations of
adsorption. However, the strong effects of these parameters on adsorption are
well established.

7.4.1. Cation Sites
Zeolite frameworks usually have more sites for the number of charge-balancing
cations that occupy them. The cations distribute themselves in a manner to min-
imize the free energy of the system. The distribution of the cations on the sites
depends on (1) the temperature of heat-treatment, (2) the cationic species, and
(3) the degree of hydration. X-ray diffraction has been the main tool for cation
siting (Barrer, 1978). Neutron diffraction has advantages over X-ray diffraction
because the X-ray scattering is nearly indistinguishable between Si and Al. Also,
the X-ray scattering by small cations, such as Li, is too weak to detect (Hutson
et al., 2000). Rietveld refinement, a trial-and-error procedure, is now the standard
technique for determining the cation sites as well as the structure (Rietveld, 1967;
Hutson, et al., 2000). It is difficult to determine all the cation sites in zeolites
because of the relatively small number of cations compared with the large number
of other atoms in the structure (Al, Si, and O). Other techniques, such as solid-
state NMR (Engelhardt et al., 1994), infrared spectroscopy (Ozin et al., 1983),
and diffusion studies (of probe molecules) (Ackley and Yang, 1991) have also
assisted in cation siting.
Due to their importance, the most studied cation sites are for zeolites A,
X, and Y. Results on cation siting in these zeolites will be summarized first.
Cation sites in another main type of structure with cage-topology (i.e., chabazite)