RAMAN-SPECTROSCOPIC STUDIES OF THE SYNTHESIS OF FAUJASITIC ZEOLITES - COMPARISON OF 2 SILICA SOURCES

In this study, we have examined the influence of two common commercial silica sources, Ludox and N-Brand, on the formation of faujasitic zeolites. The preparation of zeolite X is readily accomplished from both these sources. The Raman spectra indicate that the gel preceding the formation of crystals is composed of a solid phase with disordered four-membered aluminosilicate rings and soluble monomeric and dimeric silicate species. The Ludox preparation has a more rapid crystallization rate than does N-Brand, and zeolite X crystals are observed in the Raman spectrum at earlier times than by X-ray diffraction, indicating more extensive nucleation. Synthesis of zeolite Y was only possible with Ludox, whereas preparation with N-Brand as the silica source and identical in composition as that of Ludox led to the formation of zeolite P. Raman spectroscopy in this composition indicated that the structure of the solid phase in the initial stages of aging is different for the Ludox and N-Brand preparations of zeolite Y. At later stages, the spectra are dominated in both cases by soluble silicate species. It is possible to direct the N-Brand preparation to zeolite Y either by addition of tetramethyl-ammonium ions or by creating conditions under which zeolite X nuclei can form in the initial stage.