Framework vibrations

Infrared spectra of zeolites in the 1300-200 cm region are widely used for the investigation of their framework properties. According to Flanigen et al. [46] the observed bands can be classified into two types  

Some of the structure-sensitive bands are shifted towards lower frequencies with increasing A1 content, which can be used to determine the framework aluminium atoms in dealumination studies after calibration. It should be emphasized that a recent study has shown that the Flanigen classification needs to be modified due to the strong coupling between different framework vibrations [47]. 

In the framework region bands arising from other than TOx species can also be observed. This is important for monitoring the replacement of framework constituents by other elements or modification of the external surface by deposition of heteropolyoxometalates [48]. 

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In addition, there are some weaker broad bands that are typically observed between 1900 and 1500 cm" that have been assigned to overtones of framework vibrations. The IR spectra in Figure 4.22 are truncated at 1300 cm" because the absorbance of the sample is too high to measure at lower frequencies (<1200 cm" ). This is due to the very strong T-O-T stretching vibrations of the zeolite as mentioned in the previous section on framework IR measurements. 

The Influence of Exchangeable Cations on Zeolite Framework Vibrations... 

The influence of exchangeable monovalent cations on the framework vibrations for the hydrated zeolites Linde A and X has been investigated. An approximately linear relationship is found between the frequency of some absorption bands and the inverse of the sum of the cation and framework oxygen ionic radii. It is proposed that the shift in framework vibrations is largely caused by those cations which are strongly interacting with the zeolite framework. Thus the linear relationship indicates that these monovalent cations are all similarly sited in the zeolite lattice. This is consistent with the presently available x-ray analyses on some of these zeolites. Since Rb + and Cs + are only partially exchangeable in both Linde A and Linde X, these cations deviate from this linear relationship. 

However, the influence of the exchangeable cation on the framework vibrations has not been systematically investigated. From x-ray diffraction studies (2) on zeolites it is known that most of the exchangeable cations are firmly bound onto the negatively charged framework. Therefore these cations might have some influence on the lattice vibrational modes. 

The exchangeable monovalent cations have a marked influence on the framework vibrations of hydrated Linde A and X. For some vibrational modes the frequency shifts appear to give a quantitative measure of the interaction between cations and lattice. A regularity is found for Li+, Na+, Ag+, K+, and T1+ exchanged forms which implies a similar distribution of cation sites for both zeolites. It is further deduced that in the Cs+ and Rb+ exchanged forms there is only a relatively weak interaction between the cations and the zeolite framework. This technique can be readily extended to study cation siting in other zeolites in both hydrated and dehydrated forms. 

Figure 2. IR spectra of framework vibrations of MCM-41 and amorphous Si02-...

In the electron-diffraction jargon it is often referred to the framework vibration in contrast to the large amplitude motion. The idea is to try to separate the large amplitude motion, as for example a torsional motion, from the small amplitude vibration also taking place in rigid molecules. This practical approach does not lead to semantic difficulties, but the approach, of course, meets with the well known difficulty in any theoretical treatment of this kind, namely the problem of separability of the energy and consequently of the Hamiltonian operator. 

Pfr(r,q) is related to the framework vibration and represents the probability distribution of the individual distance of a hypothetical molecule with a fixed value for q. P(q) is the large amplitude probability distribution, which in a classical approximation may be expressed as... 

Fig. 3. Radial distribution curves for hexachloroethane. The vertical lines give the Cl Cl positions in gauche ( ) and anti (a). Curve A is experimental, the dashed line combined with the other part, indicates the torsional dependent contribution, obtained by subtracting the theoretical torsional insensitive part from the experimental curve. Curves B-E are theoretical torsional dependent distribution curves. (B) based on a rigid, staggered model with ug = 14.3, ua = 6.7 (pm). (C-E) calculated for large amplitude models, using framework vibrations and a torsional potential 5-V3 (1 +cos 30) with V3 equal to 12.5,4.2, andO(kJ /mol), respectively. The scaling between A and the other curves is somewhat arbitrary, and the damping factors and modification functions slightly different...

Ug, Ugfr The barrier has been estimated from the torsional contribution to Ug, obtained by correcting ug for framework vibration (ugfr) calculated from spectroscopic data. 

Also in the staggered model approach the u-values for the halogen-halogen distances are composed of contribution both from framework vibration and torsional motion. The torsional motion part may be expressed by athe root-mean-square deviation from the minimum position. For the molecules so far described, the value of 00 is to a good approximation equal for the gauche and tram peaks. (This is of course not the case for molecules like 1,2-dihaloethanes). 

Since the a //-distance varies so little with high barrier cases 09). The anti-peak is accordingly not suited for direct determination of 00. On the other hand, the torsional motion leads to an asymmetry in the anti-peak due to the functional relation between r and . For a low barrier case this asymmetry may be appreciable, while in a high barrier case it may be observed only as a shrinkage effect for the anti-distance. The asymmetry or the shrinkage may be used to derive a value for 00. 

A similar analysis of data obtained from molecules with asymmetric end groups is more complicated. Apart from the problems connected with the separability of the torsional motion from the framework vibration, experience shows that several more terms have to be included in the Fourier series to describe the torsional potentials properly. On the other hand, the electron-diffraction data from asymmetric molecules usually contain more information about the potential function than data from the higher symmetric cases. In conformity with the results obtained for symmetric ethanes the asymmetric substituted ethanes, as a rule, exist as mixtures of two or more conformers in the gas phase. Some physical data for asymmetric molecules are given in Table 4. The electron-diffraction conformational analysis gives rather accurate information about the positions of the minima in the potential curve. Moreover, the relative abundance of the coexisting conformers may also be derived. If the ratio between the concentrations of two conformers is equal to K, one may write... 

An approximation approach to study the torsonial amplitudes in biphenyls without ortho substituents using electron-diffraction data leads to rather large amplitudes216. The two molecules chosen were 33 -dibromobiphenyl and 3,5,3 5 -tetra-bromobiphenyl. The u-value for the Br3. ..Br3.-distance was calculated from the electron-diffraction data. Only the larger of the two Br3. .. Br3.-distances appeared suited for the study. Since the total u-value is composed of contributions both from the framework vibration and from the torsional motion, an estimate of the framework vibration amplitude is needed in order to obtain the rotational amplitude, a In order to estimate the framework vibration, 3,5,4 -tribromobiphenyl was studied. 

The Br3. .. Br4-distance in 3,5,4 -tribromobiphenyl is nearly of the same length as the larger Br3. .. Br3--distance of the two other molecules. But as the Br4-atom lies on the axis of rotation, the Br3. .. Br4>-distance is independent of the angle of torsion. Consequently the corresponding u-value is due to framework vibration only. As a rough approximation the latter u-value was used as an estimate for the u-frame-work of the longer Br3. .. Br3-distance both in 3,3 -dibromobiphenyl and in 3,5,3 ,5 -tetrabromobiphenyl. This led to a value of the of 19 ° and 17 ° for the... 

Raman spectra of hydrazine (a) and of the kaolinite-hydrazine (KH) intercalate (b) suspended in liquid hydrazine are shown in Fig. 1. In contrast to the strong IR-active absorption bands characteristic of clay minerals below 1200 cm-1, the corresponding Raman bands of kaolinite are relatively weak. Nonetheless, both the kaolinite and the hydrazine bands can clearly be resolved (Fig. lb). Hydrazine bands occur at 903,1111,1680, 3200,3280, and 3340 cm-1, whereas the kaolinite bands are found at 140 (not shown), 336, 400, 436, 467, 514, 636, 739, 794, and 3620 cm-1. Observation of lower-frequency adsorbate modes below 1200 cm-1 are often obfuscated in IR absorption spectra because of the strong lattice- framework vibrational modes. As the Raman spectrum of the KH complex shown in Fig. la indicate, the lower-frequency modes of hydrazine below 1200 cm-1 can readily be resolved. The positions of die hydrazine bands in the KH spectrum (Fig. lb) are similar to those of liquid hydrazine (Fig. la) and agree well with published vibrational data for hydrazine (22.23.29-31). The observed band positions for the KH complex, for hydrazine, and for kaolinite are listed in Table 1. 

Infrared spectra. IR absorption spectra in the framework vibration region (400-1400 cm-1, resolution 1 cm 1) were obtained with a Nicolet MX-1 Fourier transform spectrometer using the KBr pellet technique. 

The position of the asymmetric stretch T-O-T vibration in infrared spectra of the framework vibration region (at about 1100 cm 1) is a sensitive probe of Si/Al and Si/B ratios in aluminosilicates (27) or borosilicates (31). Infrared spectra of the boronated samples revealed a slight shift of the asymmetric stretch silicalite band at 1100 cm 1 to about 1098 cm 1, consistent with the boron content calculated from NMR line intensities (Jansen et al. (31) reported a 10 cm 1 shift upon substitution of 4.1 boron atoms per unit cell of ZSM-5). 

Further support for the conclusion that extensive realumination has taken place comes from the increase in unit cell parameter and from IR spectra (4). The spectra of treated samples show shifts to lower frequencies in the framework vibration region with respect to the untreated samples, except for the T-O bending at ca. 450 cm 1 which is known (17) to be insensitive to framework composition. These changes are considerable and consistent with the increase in the framework aluminium content. The band at ca. 730 cm 1, related to the symmetric Si-O-Al or to "isolated" AIO4 tetrahedra, increases in intensity following the treatment. Furthermore, the band at ca. 812 cm-1 which is known to shift to lower frequencies and decrease in intensity with an increase in framework aluminium is clearly observed in the realuminated product. 

FIGURE 3 IR spectra for ECR-1, mordenite and Linde Omega (mazzite), showing common framework vibrations, and a distinctive 5-ring doublet at 1210 and 1240cm.-1 for ECR-1. 

Thus 28 IR active modes are expected to fall in the regions of the vibrations of the orthosilicate anions. Of these, we can expect five modes associated with V3 (asymmetric stretching) and two modes associated with Vi (symmetric stretching), three modes associated with the symmetric deformation (V2) and five with the asymmetric deformation V4, four hindered rotations, four hindered translations, and, finally, five modes associated with Al—O tetrahedra. We actually observe at least 10 components for framework vibrations. Additionally, the low-frequency modes of Na ions are expected to fall in the FIR region [68], where several bands are indeed observed. 

ZSM-5 zeolites modified by conventional and solid-state ion-exchange were characterized by X-ray diffraction, BET measurements, derivatography, IR spectroscopy in the framework vibration range and acidity measurements with pyridine as probe. NO adsorption and transformation on Cu-, Co-, Ni- and FeZSM-5 zeolites were followed by IR spectroscopy. Mono- and dinitrosyl surface species, adsorbed NjO and NO were detected in different concentrations on the tested catalysts. Differences in adsorption behaviour were observed for samples exchanged by the conventional and solid-state procedures. 

The KBr matrix technique of IR spectroscopy was utilized for characterization of the materials in the framework vibration range 400-1600 cm. ... 

The dependence of the framework vibrational frequencies on the Si/Al ratio and aluminosilicate ring size were also examined with Raman spectroscopy, shown in Figure 22. The Si/AI ratio was varied from 1.0 to 2.7 in a series of zeolite A materials. The low-frequency bands at 337 and 410 cm were not found to change with the Si/Al ratio. The strong band at 489 cm exhibits a weak dependence on Si/Al ratio. The 700 cm band, however, shows the most rapid and almost linear increase in frequency with Si/Al ratios. The bands in the 900 to 1100 cm region exhibit a complicated dependence on the Si/Al ratio. The strong Raman band at about SOO cm, which possesses a weak dependence on the Si/Al ratio, however, is very sensitive to the... 

Key-words Raman spectroscopy, TEOS, synthesis, zeolite, templates, framework vibrations... 

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