Page 188 - Adsorbents fundamentals and applications

UNIQUE ADSORPTION PROPERTIES: ANIONIC OXYGENS AND ISOLATED CATIONS 173

ammonium, tetrapropyl ammonium, and tetrabutyl ammonium ions. A modified
templating theory is the “lock-and-key” version, which envisions that the zeolite
structure grows around the template, thus stabilizing certain pore/cavity struc-
tures or cages. In some cases, the additive acts not only as a template for a
given structure to crystallize, but also prohibits another structure from forming
during nucleation.
Organic molecules react with silica in aqueous solution-forming complexes
and thus increase the solubility of silica in the solution. Hence another role of
the organic additive is to act as a gel modifier that influences both the gelling
and crystallization processes. An extensive critical review and discussion on the
subject of templating has been given by Szostak (1998).

7.3. UNIQUE ADSORPTION PROPERTIES: ANIONIC OXYGENS
ANDISOLATEDCATIONS

Zeolites exhibit many unique adsorption properties, mainly because of their
unique surface chemistry. The surface of the framework is essentially oxygen
atoms, whereas Si and Al are buried or recessed in the tetrahedra of oxygen
atoms. They therefore are not fully exposed and cannot be readily accessed by
adsorbate molecules. Also, the anionic oxygen atoms are more abundant and
are much more polarizable than the Al and Si cations. Therefore, the numerous
anionic oxygen atoms dominate the van der Waals interactions with the sorbate
molecules, that is, the φ D + φ R (dispersion + repulsion) terms (see Chapter 2).
Besides the anionic oxygen, cations are located at certain sites. Some of these
sites are hidden or inaccessible to the adsorbate molecules. However, some other
cations are located above the oxide surfaces and are fully accessible. For adsor-
bate molecules with permanent dipoles and quadrupoles, the interactions with
these exposed cations often dominate the total interaction potential (see, for
example, Table 2.1 and discussion in 7.4).
The anionic surface oxygens carry negative charges. The charge depends on
the location of the oxygen relative to the cation sites, and also on the cation.
In Monte Carlo simulations, a constant charge is usually assigned to all surface
oxygen atoms and the value is usually determined by fitting the experimental data
(of isosteric heat of adsorption or the isotherm). For example, for each oxygen,
acharge of −1/3 was used by Razmus and Hall (1991) and −1.2 was used by
Mellot and Lignieres (1997).
It is instructive to compare the relative anion electronegativities of the zeolite
framework with simple anions such as halides. Such a comparison can be made
by calculating the net charges of the anions (or cations) by using molecular orbital
2
theories. The Gaussian 94 Program in Cerius molecular modeling software from
Molecular Simulations, Inc. was used for the calculation (Takahashi et al., 2000;
Yang and Yang, 2002). The calculations were performed at the Hartree–Fock
(HF) and density functional theory (DFT) levels by using effective core potentials
(ECPs). The LanL2DZ basis set was used for both geometry optimization and
natural bond orbital (NBO) analysis. The net charges were calculated by using