Zeolite infrared assignments

The observed bands were classified by Flanigen et al. [112] into two types, namely internal modes of the TO4 tetrahedra ( intra-tetrahedral modes, cf. Sect 5.2) and external modes ( inter-tetrahedral modes) of the zeolite fi-amework. Table 1 summarizes the zeolite infrared assignment according to the FKS correlation. The internal vibrations represent structure-insensitive modes, and no distinction has been made between the modes of Si04 and AIO4 tetrahedra. The bands of external modes were observed to be sensitive to the structure, and their positions in the spectra are shifted in dependence on the framework topology and on the nsi/n i ratio. Even though this classification proved to be very successful in many applications, from a theoretical point of view such a division into... 

Table 1. Zeolite infrared assignments foUowing the FKS correlation [112]...

Figure 7. Infrared assignments illustrated with the spectrum of zeolite Y, Si/Al of 2.5...

D correlation analysis is a powerful tool applicable to the examination of data obtained from infrared spectroscopy. The correlation intensities, displayed in the form of 2D maps, allow us to correlate the shift induced by CO adsorption and acidity of sites in dealuminated zeolites. Results are in accordance with previous results, obtained using only IR measurements, proving the validity of this technique. New correlations allowed the assignment of very complex groups of bands, and 2D correlation revealed itself as a great help for understanding acidity in dealuminated zeolites. 2D correlation has allowed us to validate the model obtained by NMR. 

They concluded that the infrared spectrum contained vibrational modes from both structure insensitive internal tetrahedra and structure sensitive external linkages. The exact frequency of these bands depends on the structure of the zeolite as well as its silicon to aluminum raho (Si/Al). A typical framework IR spectrum for a Y zeolite sample is shown in Figure 4.17. The accepted band assignments and frequency ranges are shown on the figure. 

Jacobs, P.A. and Uytterhoevin, J.B. (1973) Assignment of the hydroxyl bands in the infrared spectra of zeolites X and Y part 2. After different treatments. /. Chem. Soc. Faraday Trans. 

That these force constants are not far from a reasonable estimate is apparent from the assignment of a force constant 1.22x10 dyn/ cm for the symmetric vibration in Ca -zeolite in the feu infrared study by Butler et al (26). However, less them fully symmetric inodes must be used in the analysis of Jahn-Teller distortions, and so the AsF analogy will have to suffice. With these values, the distortion Reg is represented by -Q 2 0.05-0.06 A and... 

Infrared spectra of nitriles adsorbed on zeolites have been reported by Angell and Howell (252), by Karge (238), by Ratov et al. (237), and by Butler and Poles (253). Some Raman spectra are also available (254, 255). Besides H-bond-ing, the nitriles appear to interact primarily with the exchangeable cations. Acetonitrile, CH3CN, has been used frequently. However, assignment of the v2 mode (C=N stretch) in coordination compounds is quite confused due to the occurrence of Fermi resonance between the v2 mode and the (v3 + i>4) combination mode. This problem has recently been dealt with by Knozinger and Krietenbrink (255). 

Introduction of aluminium into a zeolite lattice broadens the lattice modes, but also introduces additional bands in the Raman spectra at low frequencies due to cation vibrations, completely analogous to the far infrared bands described in section 3.3. Figure 18 shows, for example, Raman spectra taken from the work of Bremard and Le Maire [53] of zeolite Y exchanged with different alkali metal cations. The arrows indicate bands assigned to translational modes of the cations these move to lower frequency as the mass of the cations increases, just as in the far infared spectra. 

Assuming an ionic-like interaction potential between the framework atoms and cations, Smirnov et al. performed an MD study to assess the cation dynamics in zeolite A. The calculation of the power spectra for Na" and cations at each site revealed that no specific spectral pattern can be attributed to a particular cation position. Vibrations of cations in all sites occur over a frequency region of 30-300 cm In addition, the spectra calculated with a flexible framework showed a substantial coupling between the cationic and lattice degrees of freedom. The results of this work °° have brought into question the assignments proposed for the bands in the far-infrared spectra of cationic forms of zeolites. 

The model is consistent with various infrared data. Many workers (see references in Ref. 48) found that H-Type Y yields infrared stretching frequencies at about 3750, 3650, and 3550 cm"1. The highest frequency band has been found for all types of zeolites, and has been ascribed to hydroxyls completing the surface of individual crystallites or, perhaps more likely, to Si(OH)4 occluded in the zeolite (I). The 3550 band is not perturbed by sorption of most molecules which cannot pass into the sodalite unit, and hence reasonably was ascribed to H2 protected inside the hexagonal prism. The 3650 band, which is perturbed by large sorbed molecules, was ascribed to HI. Other considerations supported this assignment (48, pp. 230-231). 

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