Framework band assignments

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. 

Figure 4.17 Framework spectrum of FAU (Si/Al2 = 5.5) with band assignments as per Flanigen et al. [91].

As it could be expected, for D-NH4Y the increase of activation temperature caused a reduction of the concentration of Brensted acid sites with a simultaneous increase in the amount of "true" Lewis acid sites (Table 1). The disappearance of the h.f and l.f bands upon activation at 975 K showed that framework Bronsted acidity is practically eliminated at this temperature. However, the existence of a tiny band assigned to pyridinium ions (1540 cm l). 

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. 16. (a) Extra-framework cation sites in X- and Y-type zeolites, (b) Far- infrared spectrum of Na-Y with band assignments to cation sites according to [232]. (c) Experimental IR spectrum in comparison to simulated spectra calculated according to the shell model and occupancy of different cation sites, (d) Experimental spectrum in comparison to power spectra simulated by MD at occupancy of different cation sites (parts c and d from [79] with permission)... 

The isomorphous substitution of tetrahedral A1 in zeolite structure with elements such as gallium, boron or iron has been described in a large number of papers. In the case of Fe for instance, the presence of Fe h jp the framework gives rise to new infrared bands assigned to Si-O-Fe bonds (8). Mossbauer spectroscopy, which is a very valuable tool in this case,... 

An important objective of the efforts to understand the structure of the optical RC spectra is to obtain a framework for the interpretation of the data of kinetic spectroscopy. It is clear that without e.g. a correct band assignment it is not possible to draw conclusions from the kinetics on the pathways of electron transport. Especially for this purpose, exciton calculations may be useful. For example, while the long-wavelength absorption band (at about... 

On the basis of these assignments, the two bands must be associated with the presence of isolated Ti atoms in tetrahedral coordination within the silicalite framework. Consequently, a quantitative linear correlation between the Ti02 content and the intensities of both the infrared and Raman bands at 960 cm-1 is expected—and this is indeed observed, as shown in Fig. 9b. 

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