LOW-FREQUENCY RAMAN-SPECTRA OF DEHYDRATED FAUJASITIC ZEOLITES

Significant Raman spectra of the fully dehydrated zeolites of the cubi c faujasitic family have been obtained particularly in the low-frequen cy 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 zeol ites Na3-DY, Na5-DY, and Na9-DY and the as-synthesized zeolites Na41-Y , Na56-Y, and Na85-X, respectively. The incorporation of aluminum atom s 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 zeol ite. This broadening is attributed to the disorder in the Al distribut ion. Upon incorporation of aluminum atoms at high level, marked change s in intensity as well as slight frequency shifts were observed for th e most Raman-active bands. Significant changes in position and intensi ty of the bands attributed to the framework vibrations were observed a ccording to the nature of the extraframework cation of zeolites obtain ed 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 spectro scopy enables us to assign some bands below 120 cm-1 to translational motions of the intrazeolite charge-balancing extraframework cations. T he dependence of the Raman spectrum on the water adsorption was found to be weak for the framework bands, except those between 400 and 300 c m-1, whereas the broadening of the bands corresponding to the translat ional motions of the cations, below 120 cm-1, is in good agreement wit h the change in the cation distribution and coordination of water mole cules which occurs in the hydrated zeolites.