Some Selected Examples of Modeling Zeolite Vibrational Spectra

Some Selected Examples of Modeling Zeolite Vibrational Spectra 

The application of vibrational spectroscopic techniques in zeolite structure research is closely connected with the question as to whether the experimental spectra manifest any structure-specific normal modes. While for organic molecules this question can easily be answered in terms of characteristic group frequencies, the answer for zeolites is not straightforward due to the complexity of zeohtic systems and the variety of framework types with diverse structural features. At first we will focus on some selected problems of framework dynamics. 

From the detailed comparison of abundant experimental infrared spectra in the region between 1300 and 200 cmFlanigen, Khatami, and Szymanski derived a correlation between observed bands and structural features of zeolite frameworks, known since that time as the FKS correlation [112]. 

This correlation has gained significant importance in practice of zeolite research and will, therefore, be described and illustrated in some detail in Sect 5.2, in spite of serious objections outlined in the following paragraph. 

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 

---