Spectroscopic Investigations and Molecular Dynamics Studies of Cationic Exchanged EMT Zeolites

Significant Raman and infrared spectra of the fully dehydrated and bare hexagonal M<INF>20</INF>EMT zeolites (M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH<INF>4</INF>⁺, and H⁺) have been obtained particularly in the low-frequency region, at room and low temperature. The vibrational properties of M<INF>20</INF>EMT were found to be analogous to those of cubic M<INF>40</INF>FAU with identical aluminum content. However, significant changes in position and intensity of the bands assigned to the framework vibrations were observed according to the nature of the extraframework charge-balancing cation of the exchanged zeolites. Molecular dynamics calculations were used to model the vibrational features of the M<INF>n</INF><INF></INF>EMT (n = 0 or 20; M = Na⁺ or K⁺) framework and extraframework cations. The main features of both the IR and Raman spectra were reproduced in the mid as well as in the low-frequency ranges. The results of the calculations demonstrate that the cation vibrations significantly participate in the spectral range from 20 to 250 cm^-1 for all cation sites and are in accurate agreement with the infrared and Raman features assigned to the cation motions. The data of the molecular dynamics simulations of the window fluctuations in the M<INF>n</INF><INF></INF>EMT zeolites show that the aluminum content and extraframework cations affect both the size and the amplitude of the fluctuations of the windows in the zeolite, although the cations are not located in the windows. The calculated spectral densities of the window fluctuations show the most prominent peaks around 100 cm^-1 and are in agreement with the low-frequency Raman features observed around 100 cm^-1.