10.5.3 SBU Terminated by Cations
Infrared spectra are affected not only by the substitutions in tetrahedral positions but
also by the exchange of the so-called extra-framework cations. In a mid-IR region,
this influence is quite well known, thanks—among others—to the support of
experimental techniques of vibrational spectroscopy with computational methods
[36, 38] as well as other measurement methods [53, 54]. For example, Ma et al.
found [53] that the Na
+
! Ca
2+ ion exchange in stilbite structure results in shifting
the bands associated with 4- and 5-membered rings’ characteristic vibrations by
about 20 cm
−1 toward higher wave numbers. These changes are explained by the
position of individual cations relative to the STI framework (changes in unit cell
parameters) and the degree of hydration of these cations, which was confirmed on
the basis of XRD studies. Similar observations were made by Mozgawa et al. [48,
55, 56], who analyzed the process of ion exchange on natural clinoptilolite. In this
case, changes in the intensity of the band at about 675 cm
−1 , attributable to
4-membered ring vibrations in the 4–4=1 unit, were considered as an indicator of
the occurrence of ion exchange process.
In order to check how the presence of non-tetrahedral cations affects the spectra
envelope, including the change in the position of the bands associated with SBUs
characteristic vibrations, the theoretical computational methods have proved again
to be useful. As a model for the calculation, pseudomolecules being equivalents of
the SBUs were once again used, this time, however, terminated with metal cations
(K
+ , Na
+ , Ca
2+ , and Zn
2+ ). Some controversy in this case may cause the level of
interaction of these cations with isolated molecules, especially in the case of
non-alkaline cations, since there is a possibility that small d block cations will
interact stronger with the isolated SBUs than with the same units embedded in
stiffening crystal lattice framework, which should translate into their significant
deformation, and thus change in spectrum envelope.
In contrast to earlier described SBUs terminated with protons, metal cations
change the distribution of electron density and thus change the nature of chemical
bonds within a given unit. This applies primarily to Si–O
− terminal, but also to
bridge bonds. In contrast, the bonds between cations and terminal oxygen ions in
tetrahedra have already ionic character (much stronger than in the case of protons),
so the bonds become undirected; i.e., the individual cations do not form bonds with
a particular tetrahedron, but interact electrostatically with several surrounding
tetrahedra at the same time. This means that the proposed models will be the most
suitable for interpreting the vibrational spectra of cyclosilicates [57]; however,
attempts are also made to transfer the interpretation to tectosilicates.
For example, Fig. 10.7 shows the calculated spectra of D4R unit terminated with
sodium and potassium cations. Król et al. made an attempt [36] to use the obtained
results to interpret experimental spectra. It was found that vibrational spectroscopy
can be used in the study of sorption properties of zeolites and based on the model
spectra it is possible to confirm and analyze the changes in the experimental spectra
of zeolites due to the immobilization process of metal cations within their structure.
312
M. Król et al.
Infrared spectra are affected not only by the substitutions in tetrahedral positions but
also by the exchange of the so-called extra-framework cations. In a mid-IR region,
this influence is quite well known, thanks—among others—to the support of
experimental techniques of vibrational spectroscopy with computational methods
[36, 38] as well as other measurement methods [53, 54]. For example, Ma et al.
found [53] that the Na
+
! Ca
2+ ion exchange in stilbite structure results in shifting
the bands associated with 4- and 5-membered rings’ characteristic vibrations by
about 20 cm
−1 toward higher wave numbers. These changes are explained by the
position of individual cations relative to the STI framework (changes in unit cell
parameters) and the degree of hydration of these cations, which was confirmed on
the basis of XRD studies. Similar observations were made by Mozgawa et al. [48,
55, 56], who analyzed the process of ion exchange on natural clinoptilolite. In this
case, changes in the intensity of the band at about 675 cm
−1 , attributable to
4-membered ring vibrations in the 4–4=1 unit, were considered as an indicator of
the occurrence of ion exchange process.
In order to check how the presence of non-tetrahedral cations affects the spectra
envelope, including the change in the position of the bands associated with SBUs
characteristic vibrations, the theoretical computational methods have proved again
to be useful. As a model for the calculation, pseudomolecules being equivalents of
the SBUs were once again used, this time, however, terminated with metal cations
(K
+ , Na
+ , Ca
2+ , and Zn
2+ ). Some controversy in this case may cause the level of
interaction of these cations with isolated molecules, especially in the case of
non-alkaline cations, since there is a possibility that small d block cations will
interact stronger with the isolated SBUs than with the same units embedded in
stiffening crystal lattice framework, which should translate into their significant
deformation, and thus change in spectrum envelope.
In contrast to earlier described SBUs terminated with protons, metal cations
change the distribution of electron density and thus change the nature of chemical
bonds within a given unit. This applies primarily to Si–O
− terminal, but also to
bridge bonds. In contrast, the bonds between cations and terminal oxygen ions in
tetrahedra have already ionic character (much stronger than in the case of protons),
so the bonds become undirected; i.e., the individual cations do not form bonds with
a particular tetrahedron, but interact electrostatically with several surrounding
tetrahedra at the same time. This means that the proposed models will be the most
suitable for interpreting the vibrational spectra of cyclosilicates [57]; however,
attempts are also made to transfer the interpretation to tectosilicates.
For example, Fig. 10.7 shows the calculated spectra of D4R unit terminated with
sodium and potassium cations. Król et al. made an attempt [36] to use the obtained
results to interpret experimental spectra. It was found that vibrational spectroscopy
can be used in the study of sorption properties of zeolites and based on the model
spectra it is possible to confirm and analyze the changes in the experimental spectra
of zeolites due to the immobilization process of metal cations within their structure.
312
M. Król et al.