Routes of synthesis of nepheline-type polymorphs: An influence of Na-LTA bulk composition on its thermal transformations

In this work we investigated the conditions under which pure basic hexagonal nepheline phase is formed by the zeolite Na-LTA thermal transformation route. The influence of small variations in Na-LTA bulk stoichiometry on the rate and course of the thermal transformation process was examined. The starting materials were three Na-LTA samples with different Na2O: Al2O3: SiO2 ratios: 0.99Na2O: 1.00Al2O3: 2.00 SiO2, 1.05Na2O: 1.06Al2O3: 2.00SiO2 and 1.05Na2O: 1.00Al2O3: 2.05SiO2. Upon heat treatment, the zeolite samples showed different conversion routes. At temperatures above 950 °C all three Na-LTA samples transformed into nepheline-type polymorphs. While the Na-LTA sample with the composition close to ideal stoichiometry gave pure basic hexagonal nepheline, the Na-LTA samples containing sodium, aluminum and silica excess formed mixtures of basic hexagonal nepheline and monoclinic trinepheline. The synthesized nepheline-type polymorphs were investigated by X-ray powder diffraction, Raman and NMR spectroscopy.