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

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-Ne_LTA, ps-Ne_FAU, ps-Ne_GIS). By X-ray diffraction analysis of intermediaries, we found that ps-Ne_LTA and ps-Ne_FAU were obtained as polymorphous transformation products from carnegieite phases and ps-Ne_GIS was synthesized directly from amorphous zeolite precursor. Stoichiometric ps-Ne_LTA {Na₈Al₈Si₈]₁₆O₃₂} and non-stoichiometric ps-Ne_FAU {(Na_7.20□_0.80)₈Al_7.20Si_8.80]₁₆O₃₂} and ps-Ne_GIS {(Na_6.41□_1.59)₈Al_6.41Si_9.59]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 T_G and T_S 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.