The intermediate phase between sodalite and cancrinite: Synthesis of nano-crystals in the presence of Na2CO3/TEA and its thermal- and hydrothermal stability

Abstract

Low temperature (333K) crystallization of hyperalkaline carbonate containing aluminosilicate gels has been studied. Nanocrystalline carbonate enclathrated zeolite-type with intermediate structure between sodalite and cancrinite was observed already after 3h. Its crystal size was found to be around 40nm over synthesis periods from 3h to 96h. A further experimental series was performed under addition of triethanolamine to study its effect on crystallization kinetics, crystal size and morphology. Nucleation deceleration of aluminosilicates but recrystallization of large crystals of sodium carbonate could be revealed in the early stage of these reactions. As a result big elongated rectangular hollow plates of the intermediate phase composed of small spherical crystal aggregates consisting of numerous nanocrystallites were observed. Heterogeneous nucleation and growth of the intermediate phase on the surfaces of the previously formed big plate-like Na2CO3 crystals was responsible for this unusual particle aggregation.All products were characterized by X-ray powder diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and simultaneous thermal analysis. The intermediate phase were compared with microcrystalline carbonate–cancrinite and hydro-sodalite of common synthesis at 473K.The thermal and hydrothermal behavior of the nanocrystalline intermediate phase was further characterized and discussed in terms of its specific structural features. Compared with microcrystalline cancrinite remarkable thermal stability could be stated. In contrast a much higher decomposition rate of the intermediate phase under hydrothermal stress was observed.