Zeolite framework vibrations

In the development of zeolite science, infrared spectroscopy has been one of the major tools for structure and reactivity characterization. However, the field of zeolite Raman spectroscopy is gaining importance. The Raman effect is an intrinsically weak phenomenon, and Raman spectra of zeolites are often obscured by a broad fluorescence. Just like IR spectroscopy, Raman can detect small. X-ray amorphous zeolite particles. Therefore, Raman spectroscopy has been used to examine zeolite synthesis mixtures with ex-situ methods (with separation of solid and liquid) and in-situ methods. In this work we give an overview of the zeolite framework vibrations, zeolite synthesis, adsorption on zeolites and metal substitution and ion exchange in zeolites. 

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. 

Infrared Spectra. IR absorption spectra in the zeolitic framework vibration region (1400-400 cm-1) were recorded using a Nicolet MX-1 Fourier transform spectrometer and the conventional KBr disc technique. 

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 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... 

In order to know whether the Pd ions or complexes are anchored to the zeolite framework or not, the IR framework vibrations of Pd-H-ZSM-5(0.49) were investigated (Figure 5). After activation under O2, a weak band at 930 cm" forms. Upon NO adsorption, the 930 cm band disappear while a new band appears at 980 cm". These bands are attributed to asymmetric internal stretching vibrations of T-O-T bonds (T = Si or Al) perturbed by Pd ions. The higher the perturbation, the lower the frequency. Therefore, the 930 cm band could be related to anchored Pd(II) ions or complexes formed upon decomposition of exchanged complexes, and the 980 cm band could be due to Pd(I) nitrosyl entities formed upon NO contact. Similar observations were found on Cu-ZSM-5 catalysts (34). 

Most of the physico-chemical measurements (XRD, SEM, N2 adsorption, framework vibrations) show little difference between fresh and hydrothermally treated (air + 10 % H2O mixture) Cu-ZSM-5 solids. There is no clear destruction of the zeolite framework the decrease in micropore volume remains moderate and it is difficult to observe dealumination in the aged solids, even after treatment at 1073 or 1173 K. Significant changes in catalytic activity are observed, however, even after treatment at 923 K, and the activity becomes negligible after treatment at 1073 or 1173 K. 

R spectroscopy is one of the most useful techniques for the characterization of zeolitie materials. It provides information on the nature of OH-groups in the materials and on the framework vibrations. Even more useful is the IR analysis of adsorbed species. Basic probe molecules, such as pyridine, ammonia, or benzene, allow the analysis of acidic sites. CO adsorbed at low temperature also helps to analyze acidic sites, but can also be used for the analysis of noble metal particles in zeolites. 

The far-infrared spectra of hydrated and dehydrated Naj -A are shown in Figure 2,(24). Framework vibrations ol the unit cell that are rather insensitive to the nature of the metal cation guest occur between 350-250 cm l. The hydrated sample exhibits four well defined cation related absorptions which shift slightly upon dehydration of the zeolite. The largest change occurs for the vibration at 133 cm l shifting to 142 cm I upon removal of water. Crystallographic determinations of site locations for hydrated and dehydrated Na-A are compiled in Table I. 

The band at 2350 cm is attributed to CO2 present in the beam path within the IR microscope. The spectrum of the freshly activated sample exhibits IR bands at 2007, 1882, and 1644 cm which may be attributed to overtones of zeolite framework vibrations [52,53]. The broad feature at 3500 cm is due to Si(OH) groups of lattice defects [44]. After equilibration of the sample with 3.1 mbar of n-hexane, the positions and relative intensities of the IR bands mentioned remain essentially unchanged. Additionally, the spectrum shows the IR bands characteristic of n-hexane, i.e., the asymmetric vibrations of the CH3 and CH2 groups at v = 2960 and 2930 cm respectively, and the symmetric counterparts at v = 2890 and 2870 cm (see Vol. 4, Chap. 1 of the present series). 

It is worthy to note that IR spectroscopy was also relatively early employed to identify and investigate framework vibrations (vide supra, cf. [112, 114] and Sect. 5.2). In these experiments, usually the so-called KBr-technique was used (cf. Sects. 2.5,4.2). Relationships between the vibration modes and, e.g., the nsi/n i ratio of the zeolite fi-amework were disclosed and discussed. 

The suggestions of band assignments to framework vibrations advanced in Refs. [112,114] were based on earlier IR investigations of silica and non-zeolitic aluminosilicate frameworks [238, 239]. These assignments are illustrated in Fig. 11. 

Fig. 11. Zeolite framework vibrations according to the interpretation of Flanigen et al. (adopted from [ 112 ]) 1, solid lines intra-tetrahedral vibrations 2, broken lines inter-tetrahedral vibrations...

Fig. 12. Wavenumbers of various framework vibrations as a function of the mole fraction of aiuminum in tetrahedral (T) sites of the zeolite framework s indicates the slope, i.e., the decrease of the wavenumber (cm ) per 0.1 atom fraction of A1 ion substitution (adopted from [112]). D6R means double six-membered ring, and stand for symmetric and asymmetric stretching modes, respectively (cf. text and list of abbreviations)...

Falabella et al. [250] included IR framework spectra in their report on structural and acidity properties of RE-Y (where RE=La, Nd, Sm, Gd, Dy). Ballivet et al. [251] obtained IR spectra of La, Na-Y zeolites in the region of framework vibrations as well as in the OH stretching region (cf. Sect. 5.4.1.1) and used the spectral features in the range 1300-400 cm in dependence on the activation temperature for structural and stability determinations. 

Yu et al. [252] successfully employed UV Raman laser spectroscopy for the characterization of the framework vibration range of zeolites A,X, Y, MOR, L, and Beta. UV Raman laser spectroscopy proved to be advantageous in that it was much less disturbed by fluorescence than the conventional Raman laser technique. The authors claimed that x-membered rings (xMR) in the structures were manifested by absorptions in the wavenumber ranges (in cm 0 470-530 (4MR), 370-430 (5MR), 290-410 (6MR) and 220-280 (8MR). The method was also used for the characterization of TS-1, [Fe]ZSM-5, [V]MCM-41 and in synthesis studies (vide infra). 

IR framework spectra were used as a diagnostic tool by Occelli et al. [260] in detecting the presence of offretite (via a band at 600-610 cm ) and erionite (bands at 410-425,550-610,655-685 cm ) in mixtures of these two structures. Roessner et al. [261 ] considered, in their IR spectroscopic work on the cation distribution in dehydrated calcium-exchanged erionite, also the framework vibrations of Ca-erionite besides OD vibrations, CO adsorption and DRIFT spectroscopy in the NIR region. They were able to show that the Ca + cations were selectively located in the supercages in front of the six-membered rings. Similar to the features encountered with Y-type zeolites and mordenite (vide supra), also with offretite a sufficiently linear relationship was found between the wave-numbers of the asymmetric and symmetric T-0 vibrations and the number of framework Al atoms per unit cell [262]. 

Using the IR/KBr pellet technique, Ernst and Weitkamp [275] obtained framework vibration spectra of ZSM-35 (cf. also [276]) and, for the st time, ZSM-57 (MFS). Both zeolites have two characteristic absorbances around 1230 cm, which were assigned to vibrations of five-membered rings. While ZSM-5 exhibited a single band at about 600 cm, ZSM-57 showed a doublet. The additional band was attributed to the presence of four-membered rings in the structure of ZSM-57. 

The influence of exchangeable cations on zeolite framework vibrations was investigated by Maxwell and Baks [285], and, similar to the studies of Flanigen et al. [112], they derived linear relationships between the reciprocal of the sum of the cation radii and oxygen anion radius, l/frcation+fo -) and the wavenumbers of framework vibrations of zeoHtes A and X. 

The FTIR/KBr method was, inter aha, applied by Maache et al. [306] for the characterization of various samples of H-Beta dealuminated by acid leaching. In the case of framework vibrations of zeolite Beta, a good Hnear correlation was found between the wavenumber of the asymmetric O-T-O vibration, Vas(O T-o)> and the aluminum mole fraction, nAi/(nsj+nAi) [263]. Ponthieu et al. [307] attempted to find a correlation between the wavenumbers of framework vibrations of ojfretite and zeolite Omega and the degree of dealumination (aluminum mole fraction) similar to those observed with faujasites and mordenites. However, Hnear correlations were hardly obtained. 

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