LOW-FREQUENCY RAMAN-SPECTRA OF DEHYDRATED FAUJASITIC ZEOLITES

Significant Raman spectra of the fully dehydrated zeolites of the cubic faujasitic family have been obtained particularly in the low-frequency region, at room and low temperatures. The experimental Si/Al ratios (44, 32, 20, 3.8, 2.49, and 1.26) correspond to the dealuminated zeolites Na3-DY, Na5-DY, and Na9-DY and the as-synthesized zeolites Na41-Y, Na56-Y, and Na85-X, respectively. The incorporation of aluminum atoms at low level in the zeolite framework broadens the sharp Raman lines observed at 298, 312, 492, and 510 cm-1 for the purely siliceous zeolite. This broadening is attributed to the disorder in the Al distribution. Upon incorporation of aluminum atoms at high level, marked changes in intensity as well as slight frequency shifts were observed for the most Raman-active bands. Significant changes in position and intensity of the bands attributed to the framework vibrations were observed according to the nature of the extraframework cation of zeolites obtained by exchange of the Na+ cations of Na56-Y and Na85-X with H+, Li+, K+, Rb+, Cs+, Tl+, Ca2+, and Mg2+ cations. The Raman scattering spectroscopy enables us to assign some bands below 120 cm-1 to translational motions of the intrazeolite charge-balancing extraframework cations. The dependence of the Raman spectrum on the water adsorption was found to be weak for the framework bands, except those between 400 and 300 cm-1, whereas the broadening of the bands corresponding to the translational motions of the cations, below 120 cm-1, is in good agreement with the change in the cation distribution and coordination of water molecules which occurs in the hydrated zeolites.