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

Abstract

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 {Na 8 [Al 8 Si 8 ] 16 O 32 } and non-stoichiometric ps-Ne FAU {(Na 7.20 □ 0.80 ) 8 [Al 7.20 Si 8.80 ] 16 O 32 } and ps-Ne GIS {(Na 6.41 □ 1.59 ) 8 [Al 6.41 Si 9.59 ] 16 O 32 } were obtained at temperatures 900-1200 °C. The crystal structures of these ps-Ne-s were refined in the P6 3 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 29 Si 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. © 2004 Elsevier Ltd. All rights reserved.