only predict a single stable structure instead of the two phases reported experimentally. Subsequently, Balestra et al. [19] reported that the extra framework cations
play an important role in phase transition in this type of zeolite, finding that the force
exerted by the cations has a similar effect to applying an external pressure to the
zeolite. Besides, they found a direct relation between the composition, concentration,
and distribution of the cations and the distortion of the framework rings. For these
systems, the polarizability of the oxygen atoms needs to be explicitly modeled to get
a realistic description of the phenomena. Several such studies show that a careful
selection of models for the elements of the system is required, and further research
on force field development is needed to accurately describe structural changes in
zeolites.
Pure silica zeolites only have two types of atoms: oxygen and silicon. Usually, a
given set of point charges (for the electrostatic interactions) and Lennard-Jones or
Buckingham parameters (for the van der Waals interactions) is used to describe the
interactions of atoms in a given system. However, one can use other approximations
for modeling the zeolite, such as having gases and silicon atoms interact only via
long-range forces (i.e., electrostatic), resulting in a model in which the silicon atoms
are described only with point charges and the oxygen atoms with both point charges
and Lennard-Jones parameters. These parameters are calculated to include the van
der Waals interaction corresponding to the silicon atoms. Since the primary building
units of zeolites are oxygen tetrahedra with a silicon atom in the middle, it is
reasonable to assume that the van der Waals interaction between the silicon atom
and the guest gases is screened by the surrounding oxygen atoms.
2.1.1 Zeolites with Aluminum and Germanium
Most zeolites have some silicon atoms replaced by aluminum atoms. Early models
described for these zeolites assign the same partial charge to both silicon and
aluminum atoms [37, 38]. However, this approach fails to place the non-cations in
their proper positions. To model a zeolite specifically distinguishing the two types of
atoms, Lowenstein’s rule is usually applied. It states that zeolites are more stable
when no two atoms of alumina are connected to the same oxygen atom [39, 40], and
therefore, there should be at least one Si-tetrahedra between any two Al-tetrahedra.
The atoms of silicon can also be substituted by other types of atoms such as
germanium, modifying the properties of the framework. Gutierrez-Sevillano et al.
[13] combined an experimental and theoretical study to show that the presence of
atoms of germanium in zeolites confers dynamic flexibility to the framework. This
results in extensive, breathing-like pore behavior, and, in the zeolite ITQ-29 (pure
silica LTA), the diffusion coefficients of alkanes increase at least by a factor of
3. The large pore window deformations in the framework shown in Fig. 1 are linked
to the breathing dynamics and to the faster diffusion of the gas despite the fact that
the molecules are attracted more strongly to the zeolite framework in the presence of
Ge. Later, Rigo et al. [41] studied in detail the systematic substitution of Si-atoms by
Ge-atoms in the zeolite STW. They generated more than 4,000 configurations
60
J. J. Gutiérrez-Sevillano and S. Calero
play an important role in phase transition in this type of zeolite, finding that the force
exerted by the cations has a similar effect to applying an external pressure to the
zeolite. Besides, they found a direct relation between the composition, concentration,
and distribution of the cations and the distortion of the framework rings. For these
systems, the polarizability of the oxygen atoms needs to be explicitly modeled to get
a realistic description of the phenomena. Several such studies show that a careful
selection of models for the elements of the system is required, and further research
on force field development is needed to accurately describe structural changes in
zeolites.
Pure silica zeolites only have two types of atoms: oxygen and silicon. Usually, a
given set of point charges (for the electrostatic interactions) and Lennard-Jones or
Buckingham parameters (for the van der Waals interactions) is used to describe the
interactions of atoms in a given system. However, one can use other approximations
for modeling the zeolite, such as having gases and silicon atoms interact only via
long-range forces (i.e., electrostatic), resulting in a model in which the silicon atoms
are described only with point charges and the oxygen atoms with both point charges
and Lennard-Jones parameters. These parameters are calculated to include the van
der Waals interaction corresponding to the silicon atoms. Since the primary building
units of zeolites are oxygen tetrahedra with a silicon atom in the middle, it is
reasonable to assume that the van der Waals interaction between the silicon atom
and the guest gases is screened by the surrounding oxygen atoms.
2.1.1 Zeolites with Aluminum and Germanium
Most zeolites have some silicon atoms replaced by aluminum atoms. Early models
described for these zeolites assign the same partial charge to both silicon and
aluminum atoms [37, 38]. However, this approach fails to place the non-cations in
their proper positions. To model a zeolite specifically distinguishing the two types of
atoms, Lowenstein’s rule is usually applied. It states that zeolites are more stable
when no two atoms of alumina are connected to the same oxygen atom [39, 40], and
therefore, there should be at least one Si-tetrahedra between any two Al-tetrahedra.
The atoms of silicon can also be substituted by other types of atoms such as
germanium, modifying the properties of the framework. Gutierrez-Sevillano et al.
[13] combined an experimental and theoretical study to show that the presence of
atoms of germanium in zeolites confers dynamic flexibility to the framework. This
results in extensive, breathing-like pore behavior, and, in the zeolite ITQ-29 (pure
silica LTA), the diffusion coefficients of alkanes increase at least by a factor of
3. The large pore window deformations in the framework shown in Fig. 1 are linked
to the breathing dynamics and to the faster diffusion of the gas despite the fact that
the molecules are attracted more strongly to the zeolite framework in the presence of
Ge. Later, Rigo et al. [41] studied in detail the systematic substitution of Si-atoms by
Ge-atoms in the zeolite STW. They generated more than 4,000 configurations
60
J. J. Gutiérrez-Sevillano and S. Calero