LOCAL ENVIRONMENT FINE-STRUCTURE IN THE SI-29 NMR-SPECTRA OF FAUJASITE ZEOLITES

The Si-29 NMR spectra of FAU framework (X and Y) zeolites exhibit a we ll-resolved splitting into five major bands Si(nAl), where n is the nu mber of first shell aluminum neighbors about silicon. It is well-estab lished that the intensities Si(nAl) reflect the local order resulting from ''Al-Al avoidance'' in the placement of first-shell (Loewenstein' s rule) and second-shell (''Dempsey's rule'') aluminum neighbors. Stud y of framework metals ordering in FAU materials has been limited by th e information content of the intensities Si(nAl) and by the range of S i/Al compositions available by direct synthesis. Recent synthetic adva nces have greatly increased the composition range of available materia ls. In this paper we report Si-29 NMR spectra of 15 directly synthesiz ed FAU zeolites ranging in Si/Al ratios from similar to 1.3 to similar to 5.3. Careful study of the Si-29 NMR spectra reveals considerable i nformation beyond the intensities Si(nAl), manifested in the complex c ompositional dependence of the band positions and heterogeneous band s hapes. The basic premise of this paper is that the Si-29 NMR spectra o f FAU zeolites are superpositions of many components arising from the multiple local environments for a silicon atom, these local environmen ts differing in the number and types of first- and second-shell Al sub stituents. Compositional dependence arises because the relative popula tions of these local environments are a function of composition. This paper discusses the process by which this additional information has b een retrieved and its implications concerning the building units invol ved in FAU crystallization. The major conclusion is that the immediate precursor to the FAU lattice is the hexagonal prism tertiary building unit and that the secondary unit is the single 4-ring.