Effect of Adsorption on Framework Vibrations

As recently demonstrated by Huang et al. [288,289], FT-Raman spectroscopy is also a useful tool to investigate phase transitions that many zeolite frameworks undergo upon adsorption of certain organic molecules at higher loadings. 

As already shown with a few examples, Raman scattering techniques provide an alternative to the extensively used IR spectroscopy for obtaining information about the dynamics of zeolite systems. In the following, we will focus on some selected studies deaUng with Raman spectroscopy of bare zeolite frameworks. 

As a typical appHcation of considerable interest, the dependence of the Raman spectra on the nsi/n i ratio and the influence of cations has been investigated in several studies, e.g., [246,247,290,291] and [292-296], respectively. Fig. 13 shows the FT-Raman spectra of two faujasites in the framework region in dependence on the nsi/nAi ratio and Fig. 14 on varying extra-framework cations. 

The spectra were taken from the hydrated samples without any chemical treatment. For excitation an Nd-YAG laser was used at 1064 nm with a laser power of 450 mW. As a general feature of all Raman spectra of zeolites, the stretching modes above 600 cm are of lower intensities and give less structured bands than the bending modes in the range below 600 cm k As in the corresponding infrared 

Attempts have been made to correlate this behavior with the ring sizes and average T-O-T angles present in zeolites of different topologies [300]. However, the observed frequency dependencies were not fairly uniform. In particular, for A-type zeolites a trend reversed to that for faujasites has been observed by changing the nsi/n i ratio systematically [290]. Hence, the frequency shifts obviously arise from different effects of structural changes, altered nature of normal modes, and electron density distribution and from combinations of these effects. Instead of the band shift observed for aliunimun-rich samples, for highly dealuminated faujasites the band width of the most prominent peak was discovered to reflect the aliuninum content at low levels [295]. 

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