Structural characterization of pure Na-nephelines synthesized by zeolite conversion route Original Research Article

Using the zeolite thermally induced transformation route and starting from Na-LTA, Na-FAU and Na-GIS zeolites as precursors, we synthesized pure sodium nephelines (ps-Ne) with Si/Al ratios between 1.0 and 1.5 (ps-NeLTA, ps-NeFAU, ps-NeGIS). By X-ray diffraction analysis of intermediaries, we found that ps-NeLTA and ps-NeFAU were obtained as polymorphous transformation products from carnegieite phases and ps-NeGIS was synthesized directly from amorphous zeolite precursor. Stoichiometric ps-NeLTA {Na8[Al8Si8]16O32} and non-stoichiometric ps-NeFAU {(Na7.20□0.80)8[Al7.20Si8.80]16O32} and ps-NeGIS {(Na6.41□1.59)8[Al6.41Si9.59]16O32} were obtained at temperatures 900–1200 °C. The crystal structures of these ps-Ne-s were refined in the P63 space group, from X-ray powder data using the Rietveld method. Mean T–O bond lengths decreases almost linearly with increasing the Si/Al ratio. These changes are induced by short-range order–disorder of Si, Al cations through the TG and TS sites. The increase of short-range disorder over T-sites, is followed and confirmed by 29Si MAS NMR and IR spectroscopy. Comparison of the Si/Al ordering of synthesized ps-Ne structures with the published data shows inverse segregation of framework cations (Si; T1=T4 and Al; T2=T3). That is probably a consequence of applied synthesis route.