A different structural group consists of zeolites built mainly from the D6R unit.
This group includes FAU structures (X and Y zeolites used in the catalysis and
natural faujasite) and CHA (one of the more common zeolites—chabazite). It is
worth noting that while the structure of the CHA type is built only from D6R units,
the S4R units can also be distinguished in the structure of the FAU type. The
visualization of D6R unit vibrations [37] allowed to distinguish in theoretical
spectrum the bands related to vibrations of the whole unit, i.e., PO D6R at
584 cm
−1 , as well as RO vibrations of 4- and 6-membered rings. Of particular
importance should be the band at 584 cm
−1 , which is reported [65] as a band related
to PO D6R vibrations. Meanwhile, in the calculated IR spectra of FAU- and CHAtype periodic structures (Fig. 10.10), only the bands related to the vibration of
single 4- and 6-membered rings are observed. A small amount of infrared active
vibrations can be explained by the absence of aluminum and/or extra-framework
ions in the proposed periodic model.
However, when comparing the experimental and theoretical spectra, it can be
concluded that the spectra of the proposed models correspond very well to the
experimental spectra (although these models do not contain aluminum and
extra-framework ions). When analyzing the pseudolattice region, it should be noted
that in the range of 800–700 cm
−1 there are bands, which until now were attributed
to m s Si–O–Si vibrations within 4-membered rings (RO S4R) [37]. Visualizations of
respective vibrations of FAU framework show that the bands present in this range
should be assigned to bridge vibrations (but these are not RO-type). Next, in both
experimental spectra (zeolite Y and chabazite) a band with a complex envelope at
about 630 cm
−1 appears, which is assigned to RO vibrations of double 6-membered
rings. At lower wavelengths, bands appear at about 580 and 510 cm
−1 in faujasite
spectrum and about 510 and 470 cm
−1 in the chabazite spectrum, which by many
authors are attributed to PO vibrations of the D6R unit [52, 66, 67]. However, as
already mentioned, no PO-type D6R vibrations were identified on the basis of
aluminum-free periodic models, although the bands in the presented ranges should
actually be associated with vibrations of this structural element and more specifically with vibrations of 4- and 6-membered rings in D6R (RO S4R vibration).
The zeolite structures built of 5–1 units (i.e. the so-called pentasils) belong to the
most important ones due to their application in catalytic processes. In the literature,
however, only a few band assignments can be found for 5-membered ring vibrations, e.g., at about 450 cm
−1 in the experimental spectrum of stilbite [53] or about
675 cm
−1 in the experimental spectrum of clinoptilolite (belonging to another group
4–4=1) [48]. However, these are not the results based on theoretical calculations.
The exception is the work of Król et al. [58], in which the authors identified bands
associated with vibrations of 5-membered rings at 560 and 524 cm
−1 ; however, the
authors of the work generated the theoretical IR spectrum of the 5–1 unit which was
simply cut from the ferrierite structure and terminated with sodium and potassium
cations (but the geometry of the unit was not optimized so reported results should
be taken with a pinch of salt). Figure 10.10 shows spectra of pure silica structures
of MOR and FER types juxtaposed with experimental spectra of mordenite and
ferrierite, respectively. These theoretical spectra are characterized by a very good
10 Vibrational Spectroscopy of Zeolites …
317
This group includes FAU structures (X and Y zeolites used in the catalysis and
natural faujasite) and CHA (one of the more common zeolites—chabazite). It is
worth noting that while the structure of the CHA type is built only from D6R units,
the S4R units can also be distinguished in the structure of the FAU type. The
visualization of D6R unit vibrations [37] allowed to distinguish in theoretical
spectrum the bands related to vibrations of the whole unit, i.e., PO D6R at
584 cm
−1 , as well as RO vibrations of 4- and 6-membered rings. Of particular
importance should be the band at 584 cm
−1 , which is reported [65] as a band related
to PO D6R vibrations. Meanwhile, in the calculated IR spectra of FAU- and CHAtype periodic structures (Fig. 10.10), only the bands related to the vibration of
single 4- and 6-membered rings are observed. A small amount of infrared active
vibrations can be explained by the absence of aluminum and/or extra-framework
ions in the proposed periodic model.
However, when comparing the experimental and theoretical spectra, it can be
concluded that the spectra of the proposed models correspond very well to the
experimental spectra (although these models do not contain aluminum and
extra-framework ions). When analyzing the pseudolattice region, it should be noted
that in the range of 800–700 cm
−1 there are bands, which until now were attributed
to m s Si–O–Si vibrations within 4-membered rings (RO S4R) [37]. Visualizations of
respective vibrations of FAU framework show that the bands present in this range
should be assigned to bridge vibrations (but these are not RO-type). Next, in both
experimental spectra (zeolite Y and chabazite) a band with a complex envelope at
about 630 cm
−1 appears, which is assigned to RO vibrations of double 6-membered
rings. At lower wavelengths, bands appear at about 580 and 510 cm
−1 in faujasite
spectrum and about 510 and 470 cm
−1 in the chabazite spectrum, which by many
authors are attributed to PO vibrations of the D6R unit [52, 66, 67]. However, as
already mentioned, no PO-type D6R vibrations were identified on the basis of
aluminum-free periodic models, although the bands in the presented ranges should
actually be associated with vibrations of this structural element and more specifically with vibrations of 4- and 6-membered rings in D6R (RO S4R vibration).
The zeolite structures built of 5–1 units (i.e. the so-called pentasils) belong to the
most important ones due to their application in catalytic processes. In the literature,
however, only a few band assignments can be found for 5-membered ring vibrations, e.g., at about 450 cm
−1 in the experimental spectrum of stilbite [53] or about
675 cm
−1 in the experimental spectrum of clinoptilolite (belonging to another group
4–4=1) [48]. However, these are not the results based on theoretical calculations.
The exception is the work of Król et al. [58], in which the authors identified bands
associated with vibrations of 5-membered rings at 560 and 524 cm
−1 ; however, the
authors of the work generated the theoretical IR spectrum of the 5–1 unit which was
simply cut from the ferrierite structure and terminated with sodium and potassium
cations (but the geometry of the unit was not optimized so reported results should
be taken with a pinch of salt). Figure 10.10 shows spectra of pure silica structures
of MOR and FER types juxtaposed with experimental spectra of mordenite and
ferrierite, respectively. These theoretical spectra are characterized by a very good
10 Vibrational Spectroscopy of Zeolites …
317