INFLUENCE OF FRAMEWORK SI/AL RATIO ON THE RAMAN-SPECTRA OF FAUJASITIC ZEOLITES

This paper examines the dependence of the Raman spectrum of the faujasitic family of zeolites on the Si/Al ratio. The vibrational spectrum of the completely siliceous faujasite is consistent with those of other silica polymorphs. Based on the correlation between the prominent Raman bands and the T-O-T angle, the strong, sharp bands at 488 and 510 cm-1 in the completely siliceous faujasite correspond to average Si-O-Si angles of 141-degrees and 147-degrees. For the framework with Si/Al = 1.0, the four bands in the 900-1250-cm-1 region are assigned to the Si-O stretching vibrations of Si attached to the four different oxygens of the framework. At intermediate Si/Al ratios, bands in the 900-1250-cm-1 region show an increase in frequency as the Si/Al ratio increases.