Spectroscopic investigations and molecular dynamics studies of cationic exchanged EMT zeolites

Significant Raman and infrared spectra of the fully dehydrated and bare hex agonal M20EMT zeolites (M = Li+, Na+, K+, Rb+, Cs+, NH4+, and H+) have been obtained particularly in the low-frequency region, at room and low tempera ture. The vibrational properties of M20EMT were found to be analogous to th ose of cubic M(40)FAU with identical aluminum content. However, significant changes in position and intensity of the bands assigned to the framework v ibrations were observed according to the nature of the extraframework charg e-balancing cation of the exchanged zeolites. Molecular dynamics calculatio ns were used to model the vibrational features,of the MnEMT (n = 0 or 20; M = Na+ or K+) framework and extraframework cations, The main features of bo th the IR and Raman spectra were reproduced in the mid as well as in the lo w-frequency ranges. The results of the calculations demonstrate that the ca tion vibrations significantly participate in the spectral range from 20 to 250 cm(-1) for all cation sites and are in accurate agreement with the infr ared and Raman features assigned to the cation motions. The data of the mol ecular dynamics simulations of the window fluctuations in the MnEMT zeolite s 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 spectra l densities of the window fluctuations show the most prominent peaks around 100 cm(-1) and are in agreement with the low-frequency Raman features obse rved around 100 cm(-1).