changes in the location of the zeolite atoms. This is related to the coupling of the
dipole of the water molecules with the electric field induced by the zeolite. Therefore, one has to be cautious when computing the properties of water and highly polar
molecules in zeolites [55].
2.3 Models for Extra Framework Cations
Non pure silica zeolites generally contain extra framework cations to compensate the
net negative charge of the structure. From the point of view of modeling, it is
necessary to model these cations as well. Calcium and sodium cations are modeled
as point charges that can interact with the zeolite and gases; they can move freely in
the available space inside the zeolite and can block the access of guest molecules to
certain parts of the material. Available experimental works reporting the position of
the cations can be used as initial position of the cations during simulation [56, 57]
Otherwise, ab initio calculations or energy minimization can be performed to predict
the most stable positions [58, 59].
The presence of cations in the zeolite usually modifies the adsorption of water in
the material, turning hydrophobic pure silica zeolites into hydrophilic zeolites. At the
same time, water adsorption has an influence on the cation distribution in the
framework [60, 61], making the simulation of water in zeolites more complex.
There is evidence that water confined in the zeolites is hydrogen bonded
[62, 63]. While in pure silica zeolites water rapidly nucleates, in zeolites with
cations, the degree of association gradually increases with coverage of cations, but
the number of waterÀwater hydrogen bonds decreases. The average number of
hydrogen bonds in confined water with bulk-like densities was found to be about
2.6 and 2 in all silica and cationic zeolites, respectively, far from bulk liquid (3–3.5)
[62]. Gómez-Álvarez et al. [63] have shown that cations play a key role in the
adsorption and clustering of water molecules inside the pores, due to the very nature
of cations rather than the concentration of cations itself.
Besides changing the hydrophobicity/hydrophilicity of the zeolites, cations can
produce other unexpected effects on the frameworks such as the induction of local
chirality in non-chiral zeolites [64]. Although most aluminosilicates are non-chiral,
some of them can discriminate chiral molecules, as long as the adsorbing gas
consists of a scalemic mixture. Initially, one of the enantiomers is present in higher
concentrations than the other in the mixture, and once adsorbed, this concentration
can either increase or decrease. These two types of enantiospecific adsorption are
called homoselective and heteroselective. According to the work of van Erp et al.
[65], this adsorption behavior is enforced by a particular positioning of the cations in
the framework. For example, the inclusion of mono or divalent cations in the
aluminum-substituted MFI zeolites results in heteroselective or homoselective
adsorption of the 4-ethyl-4- methyloctane. This enantioselectivity can be explained
by introducing the concept of chiral cells: when one enantiomer is adsorbed, the
cations are relocated. This relocation can originate a configuration where the same
Computational Approaches to Zeolite-Based Adsorption Processes
63
dipole of the water molecules with the electric field induced by the zeolite. Therefore, one has to be cautious when computing the properties of water and highly polar
molecules in zeolites [55].
2.3 Models for Extra Framework Cations
Non pure silica zeolites generally contain extra framework cations to compensate the
net negative charge of the structure. From the point of view of modeling, it is
necessary to model these cations as well. Calcium and sodium cations are modeled
as point charges that can interact with the zeolite and gases; they can move freely in
the available space inside the zeolite and can block the access of guest molecules to
certain parts of the material. Available experimental works reporting the position of
the cations can be used as initial position of the cations during simulation [56, 57]
Otherwise, ab initio calculations or energy minimization can be performed to predict
the most stable positions [58, 59].
The presence of cations in the zeolite usually modifies the adsorption of water in
the material, turning hydrophobic pure silica zeolites into hydrophilic zeolites. At the
same time, water adsorption has an influence on the cation distribution in the
framework [60, 61], making the simulation of water in zeolites more complex.
There is evidence that water confined in the zeolites is hydrogen bonded
[62, 63]. While in pure silica zeolites water rapidly nucleates, in zeolites with
cations, the degree of association gradually increases with coverage of cations, but
the number of waterÀwater hydrogen bonds decreases. The average number of
hydrogen bonds in confined water with bulk-like densities was found to be about
2.6 and 2 in all silica and cationic zeolites, respectively, far from bulk liquid (3–3.5)
[62]. Gómez-Álvarez et al. [63] have shown that cations play a key role in the
adsorption and clustering of water molecules inside the pores, due to the very nature
of cations rather than the concentration of cations itself.
Besides changing the hydrophobicity/hydrophilicity of the zeolites, cations can
produce other unexpected effects on the frameworks such as the induction of local
chirality in non-chiral zeolites [64]. Although most aluminosilicates are non-chiral,
some of them can discriminate chiral molecules, as long as the adsorbing gas
consists of a scalemic mixture. Initially, one of the enantiomers is present in higher
concentrations than the other in the mixture, and once adsorbed, this concentration
can either increase or decrease. These two types of enantiospecific adsorption are
called homoselective and heteroselective. According to the work of van Erp et al.
[65], this adsorption behavior is enforced by a particular positioning of the cations in
the framework. For example, the inclusion of mono or divalent cations in the
aluminum-substituted MFI zeolites results in heteroselective or homoselective
adsorption of the 4-ethyl-4- methyloctane. This enantioselectivity can be explained
by introducing the concept of chiral cells: when one enantiomer is adsorbed, the
cations are relocated. This relocation can originate a configuration where the same
Computational Approaches to Zeolite-Based Adsorption Processes
63