geometry of the entire zeolite framework. These subtle changes have a relatively
large impact on the position and shape of individual bands, both in the mid- and in
the far-infrared regions.
Accordingly, an attempt was made to use periodic models with a different
content of selected extra-framework ions (both cations and anions), to analyze and
interpret changes in the structure of zeolites due to the ion exchange process. This
approach also required appropriate modification of silicate framework through
substitution of part of the silicon atoms with aluminum in order to maintain charge
neutrality of model structure. The calculations were based on the SOD [69, 70] and
the LTA model structure [63, 64]. The initial positions of non-tetrahedral cations
were selected based on the literature data. The optimized structures were analyzed
in terms of structure (electron density topology, changes in lattice parameters,
changes of positions of extra-framework ions relative to the aluminosilicate
framework and framework deformation), and theoretical vibrational spectra were
simulated. Obtained results and calculated theoretical spectra were used to interpret
structural changes undergoing during sorption processes.
Figure 10.11 presents theoretical spectra of the optimized LTA structure with
various heavy metal ions [63]. Selected structural parameters of the single cationic
forms of LTA are collected in Table 10.2. The obtained results clearly indicate that
the tendency of cations to be coordinated by particular oxygen atoms and thus the
deformation of individual SBUs is strictly dependent on the type of position and
size of cations. The topological analysis of total electron density indicates that
cations can be coordinated by either all 6 or only by 3 bridge oxygen atoms
belonging to a 6-membered ring, as indicated by the number and position of
oxygen–cation bond critical points. The values of electron density q BCP and electron density Laplacian r
2 qðrÞ (Table 10.2) for these bond critical points
unequivocally suggest that they are relatively weak chemical bonds with predominantly ionic, closed-shell character. For comparison, all Si–O bonds are characterized by significantly higher values of q BCP ¼ 0:14À0:15 and r
2 q ¼ 1:04À1:09,
as well as a strongly negative value of the total electronic energy density H e qðrÞ
½
,
which confirms that they are strong, covalent–ionic bonds with predominantly ionic
character. This type of Si–O bond character agrees with the concept of Schwartz
and Berry [71]. The results of electron density topology analysis also confirm that
Al–O bonds are even more ionic in nature than Si–O (Table 10.2).
When assessing the effect of the cation type on the structure of zeolites, it should
be noted that cations with a small ionic radius such as Li
+ occupy positions close to
the plane of the ring, whereby the oxygen atoms coordinating them move toward
the center of the ring. Thus, both 6- and 4-membered rings undergo the greatest
deformation, which translates into an increase in the number of bands in the IR
spectra (Fig. 10.11), best seen in m as Si–O–(Si,Al) vibration range. The calculated
spectra of the LTA structure with large ionic cations, in the range related to m as Si–O
(Si,Al) vibrations, are characterized by much smaller band splitting. Greater distance of cations with a large ionic radius from framework oxide atoms results in
much weaker interaction with the framework (Table 10.2), and thus the degree of
320
M. Król et al.
large impact on the position and shape of individual bands, both in the mid- and in
the far-infrared regions.
Accordingly, an attempt was made to use periodic models with a different
content of selected extra-framework ions (both cations and anions), to analyze and
interpret changes in the structure of zeolites due to the ion exchange process. This
approach also required appropriate modification of silicate framework through
substitution of part of the silicon atoms with aluminum in order to maintain charge
neutrality of model structure. The calculations were based on the SOD [69, 70] and
the LTA model structure [63, 64]. The initial positions of non-tetrahedral cations
were selected based on the literature data. The optimized structures were analyzed
in terms of structure (electron density topology, changes in lattice parameters,
changes of positions of extra-framework ions relative to the aluminosilicate
framework and framework deformation), and theoretical vibrational spectra were
simulated. Obtained results and calculated theoretical spectra were used to interpret
structural changes undergoing during sorption processes.
Figure 10.11 presents theoretical spectra of the optimized LTA structure with
various heavy metal ions [63]. Selected structural parameters of the single cationic
forms of LTA are collected in Table 10.2. The obtained results clearly indicate that
the tendency of cations to be coordinated by particular oxygen atoms and thus the
deformation of individual SBUs is strictly dependent on the type of position and
size of cations. The topological analysis of total electron density indicates that
cations can be coordinated by either all 6 or only by 3 bridge oxygen atoms
belonging to a 6-membered ring, as indicated by the number and position of
oxygen–cation bond critical points. The values of electron density q BCP and electron density Laplacian r
2 qðrÞ (Table 10.2) for these bond critical points
unequivocally suggest that they are relatively weak chemical bonds with predominantly ionic, closed-shell character. For comparison, all Si–O bonds are characterized by significantly higher values of q BCP ¼ 0:14À0:15 and r
2 q ¼ 1:04À1:09,
as well as a strongly negative value of the total electronic energy density H e qðrÞ
½
,
which confirms that they are strong, covalent–ionic bonds with predominantly ionic
character. This type of Si–O bond character agrees with the concept of Schwartz
and Berry [71]. The results of electron density topology analysis also confirm that
Al–O bonds are even more ionic in nature than Si–O (Table 10.2).
When assessing the effect of the cation type on the structure of zeolites, it should
be noted that cations with a small ionic radius such as Li
+ occupy positions close to
the plane of the ring, whereby the oxygen atoms coordinating them move toward
the center of the ring. Thus, both 6- and 4-membered rings undergo the greatest
deformation, which translates into an increase in the number of bands in the IR
spectra (Fig. 10.11), best seen in m as Si–O–(Si,Al) vibration range. The calculated
spectra of the LTA structure with large ionic cations, in the range related to m as Si–O
(Si,Al) vibrations, are characterized by much smaller band splitting. Greater distance of cations with a large ionic radius from framework oxide atoms results in
much weaker interaction with the framework (Table 10.2), and thus the degree of
320
M. Król et al.