Cation Vibrations Affected by Adsorption

It has been well-known for a long time that besides their cation sensitivity, far-infrared spectra of microporous materials can imdergo remarkable changes upon adsorption of probe molecules [366]. Disregarding the far-infrared normal modes of the adsorbed molecules themselves, the expectations with respect to this spectral region are twofold. 

On the other hand, the interaction with adsorbed molecules is indicated by frequency shifts of the cation modes. In most of the earlier studies downward-shifts of the cation bands were observed and explained by both the delocalization of the framework charge and the increase of the effective mass of the oscillator by solvent interaction upon adsorption. Nevertheless, an unambiguous interpretation of the adsorption-induced changes in the far-infrared spectra is not straightforward due to at least three reasons  

To illustrate the changes in the far-infrared spectra upon adsorption. Fig. 18 shows in an exemplary way the spectra of zeoUtes Na-Y and K-Y before and after adsorption of the N-heterocycles pyrrole [371] and pyrroUdine [372]. 

The framework vibrations of AIPO4-5 and the isostructuxal SSZ-24 as well as the effect of protonation and cation exchange on framework vibrations upon re-ammoniation of H-Y and dehydration of Ni-Y were demonstrated by Jacobs et al. [373] via in-situ IR experiments and, at the same time, related to theoretical studies of the correspondingly relaxed structures, using a valence force field model. 

In summary, the far-infrared region reveals valuable information about host/guest interactions in molecular sieves, but many of the results reported so far should presently be considered as in an initial stage. We have mentioned some aspects from which we suggest to look very carefully at the interpretation of adsorption-induced changes, especially in this spectral range. In particular, a 

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