Cation Vibrations in Pure Zeolites

WAVENUMBER [cm ] Fig. 1S. Far-infrared (FIR) spectra of alkali-metal cation-exchanged faujasites and H-Y zeolite 

It can be seen in Fig. 15 that within the series of alkali-metal cation-exchanged faujasites the bands are shifted to lower wavenumbers with increasing cation mass. This phenomenon was recognized very early by Brodskii et al. [358,359]. In an attempt to correlate the observed band positions with physical characteristics of the cations they found the relation 

However, further investigations are necessary to prove unequivocally the vahdity of two approximations made  

To verify the correlation between site occupancies and far-infrared (FIR) bands, at first we have to distinguish between bands arising from cations and those originating from framework modes. This can be achieved by isotope substitution techniques. LabeHng of ZSM-5 zeoHtes with the isotopes O and yielded sig- 

As can be seen, two bands at 102 and 73 cm arise. However, a reliable interpretation of this spectrum is quite difficult. The key problem in interpreting far-infrared spectra of silicon-rich zeolites such as ZSM-5 is connected with the fact that, due to the low cation concentration, structural information about cation sites are so far rather scarce. Under the mentioned conditions it would certainly be a substantial progress if the vibrational assignment in the far-infrared region could be assisted by other suitable cation-sensitive techniques which provide additional information. One way, as chosen in Ref. [363], is to start from X-ray absorption spectroscopy (XAS) giving access to the local environment of the cations and their coordination spheres. For the dehydrated Ba-ZSM-5 sample a six-fold oxygen coordination at a distance of 2.75 A was obtained for Ba ions by EXAFS analysis of the XAS spectrum. In a second step, positions in the unit cell of ZSM-5, which fulfill these criteria, were searched by computer simulation 

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