Nanocrystalline sodalite grown from superalkaline NaCl bearing gels at low temperature (333 K) and the influence of TEA on crystallization process

Crystallization of aluminosilicate gels under addition of high amounts of NaOH and NaCl was investigated within reaction periods of 1–96 h at low temperature of 333 K. Nanoparticles of NaCl-sodalite could be successfully synthesized already after 3 h reaction time.The nanocrystalline products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and simultaneous thermal analysis (STA). Crystal sizes not exceeding 40 nm and amorphous parts up to 50% were found within the products.The nanocrystallites were compared with microcrystalline sodalite synthesized under the well known conditions of hydrothermal transformation of kaolinite in NaOH at 473 K for 48 h.In further experiments triethanolamine (TEA) was added to study the influence on phase formation in superalkaline salt bearing gels. Apart from a very small amount of a crystalline byproduct sodalite could be obtained as the major phase and only a marginal influence of TEA on crystal size and crystallinity of sodalite was detected. In contrast a strong effect of TEA on crystal morphology was found. Sodalite appeared in form of unusual big cuboctahedral hollow blocks of small spheres which in turn consist of numerous nanocrystalline particles. Additional experiments in the TEA containing system without washing of the products could reveal that heterogeneous nucleation and growth of sodalite on the surfaces of previously formed big cubes of NaCl crystals is responsible for this unusual particle aggregation. A simple four step mechanism is proposed for the observed behaviour.

SEM-images of the nanocrystalline product of the 3 h experiment from gel synthesis and addition of TEA.Figure optionsDownload as PowerPoint slideResearch highlights
► Nanoparticles of NaCl-sodalite could be synthesized already after 3 h at 333 K.
► Addition of TEA shows a strong effect on crystal morphology by TEA/NaCl interactions.
► In the presence of TEA sodalite appeared in form of big cuboctahedral hollow blocks.
► These blocks are composed of small spheres, made up of numerous nanocrystallites.