SBUs are the most characteristic part of the zeolite structure, and it is most
convenient to describe their framework in terms of such units [10]. SBUs are also
most commonly used as a criterion for designing zeolitic structure systematic and
assignment of particular zeolites into various structural groups. In commonly used
and recommended by IUPAC systematics based precisely on the SBU, seven basic
structural groups are distinguished: S4R, S6R, D4R, D6R, 5–1, 4–1, and 4=1–1. Of
course, such a division is a conventional one (and obviously not perfect), due to the
fact that in the vast majority of zeolite structures more than one SBU can be
distinguished (Fig. 10.1e). Nevertheless, the classification of zeolites based on
SBUs seems to be the most appropriate and is widely used in the literature devoted
to this subject [10, 11].
It is also worth noting that within each SBU group there may be different types
of frameworks, differing only in chemical composition, i.e., Si/Al ratio in the
framework or the content of extra-framework ions. Each such framework type,
included in given SBU group, is labeled with a three-letter code, e.g., LTA (zeolite
A, zeolite ZK-4), or FAU (zeolite X, zeolite Y).
10.3 Application of QM Methods in Interpretation
of Zeolite Spectra
As already mentioned, vibrational spectroscopy occupies an important place among
many research methods used to describe the zeolite frameworks [11–13]. However,
obtaining structural information based on such spectra is usually associated with a
rather cumbersome process of their interpretation. As indicated by Ermoshin et al.
[14] and Pechar and Rykl [15], difficulties arise, inter alia, when considering the
whole unit cell in vibrational analysis. The analysis of normal modes of vibration
for such large systems predicts several dozens or even several hundreds of IR or
Raman active modes in the vibrational spectra [16], while in the experimental IR
and Raman spectra the number of bands is much smaller. Among other things, for
this particular reason, the use of computational methods is now a very valuable tool
supplementing the interpretation process in vibrational spectroscopy.
There have been, and are, attempts to describe the vibrational spectra of zeolites
based on the assignment of bands to appropriate normal modes of vibration that use
interpretation methods developed for solids [17–19]. In such attempts [20], using
the normal mode analysis, vibration classification is made for the known structure
symmetry. For this purpose, for example, analytical or correlational methods are
used [21–23]. However, due to the usually very few information provided by the
spectrum, these methods are quite difficult to use experimental spectra interpretation. There is, therefore, a need to use more straightforward methods that would
allow the band assignments to be as precise as possible and bypass the presented
difficulties. Such methods include theoretical ones based on the formalism of
quantum mechanics, providing detailed information on the frequency and intensity
10 Vibrational Spectroscopy of Zeolites …
303
convenient to describe their framework in terms of such units [10]. SBUs are also
most commonly used as a criterion for designing zeolitic structure systematic and
assignment of particular zeolites into various structural groups. In commonly used
and recommended by IUPAC systematics based precisely on the SBU, seven basic
structural groups are distinguished: S4R, S6R, D4R, D6R, 5–1, 4–1, and 4=1–1. Of
course, such a division is a conventional one (and obviously not perfect), due to the
fact that in the vast majority of zeolite structures more than one SBU can be
distinguished (Fig. 10.1e). Nevertheless, the classification of zeolites based on
SBUs seems to be the most appropriate and is widely used in the literature devoted
to this subject [10, 11].
It is also worth noting that within each SBU group there may be different types
of frameworks, differing only in chemical composition, i.e., Si/Al ratio in the
framework or the content of extra-framework ions. Each such framework type,
included in given SBU group, is labeled with a three-letter code, e.g., LTA (zeolite
A, zeolite ZK-4), or FAU (zeolite X, zeolite Y).
10.3 Application of QM Methods in Interpretation
of Zeolite Spectra
As already mentioned, vibrational spectroscopy occupies an important place among
many research methods used to describe the zeolite frameworks [11–13]. However,
obtaining structural information based on such spectra is usually associated with a
rather cumbersome process of their interpretation. As indicated by Ermoshin et al.
[14] and Pechar and Rykl [15], difficulties arise, inter alia, when considering the
whole unit cell in vibrational analysis. The analysis of normal modes of vibration
for such large systems predicts several dozens or even several hundreds of IR or
Raman active modes in the vibrational spectra [16], while in the experimental IR
and Raman spectra the number of bands is much smaller. Among other things, for
this particular reason, the use of computational methods is now a very valuable tool
supplementing the interpretation process in vibrational spectroscopy.
There have been, and are, attempts to describe the vibrational spectra of zeolites
based on the assignment of bands to appropriate normal modes of vibration that use
interpretation methods developed for solids [17–19]. In such attempts [20], using
the normal mode analysis, vibration classification is made for the known structure
symmetry. For this purpose, for example, analytical or correlational methods are
used [21–23]. However, due to the usually very few information provided by the
spectrum, these methods are quite difficult to use experimental spectra interpretation. There is, therefore, a need to use more straightforward methods that would
allow the band assignments to be as precise as possible and bypass the presented
difficulties. Such methods include theoretical ones based on the formalism of
quantum mechanics, providing detailed information on the frequency and intensity
10 Vibrational Spectroscopy of Zeolites …
303