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

Low temperature (333 K) 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 3 h. Its crystal size was found to be around 40 nm over synthesis periods from 3 h to 96 h. 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 473 K.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.

SEM images of the product after synthesis at 333 K for 3 h with addition of TEA..Figure optionsDownload as PowerPoint slideHighlights
► Synthesis of Na2CO3 enclathrated intermediate phase between SOD and CAN at 333 K.
► A strong effect of TEA on crystal morphology was found.
► With TEA INT appeared as aggregates of spherical crystals in form of big prisms.
► This is a result of interaction between Na2CO3, TEA, and aluminosilicate.
► Thermal stability of INT higher as of CAN whereas hydrothermal stability is lower.