Nanocrystalline Sodalite from Al₂O₃ Pillared Clay by Solid-Solid Transformation

Nanocrystalline sodalite has been synthesized by a non-hydrothermal route from a solid gel mixture of Al₂O₃ pillared clay and NaOH at 80 °C. The nucleation and crystal growth mechanism for the solid-solid transformation were studied using X-ray diffraction (XRD), ²⁷Al and ²⁹Si MAS NMR, SEM, and HRTEM. Compared with hydrothermal synthesis of zeolites, the microporous Al₂O₃ pillared clay is rapidly transformed into three-dimensional zeolitic sodalite without undergoing a noticeable X-ray amorphous phase. Likewise, ²⁷Al and ²⁹Si NMR obviously reveal that the local environments around Al and Si rapidly evolve into a zeolite Al-O-Si framework at the initial stage of reaction. Scanning electron micrographs indicate that the resulting products are composed of nanometer-sized sodalite particles (ca. 30-50 nm) with the lamellar character of pristine layered clay. HRTEM images clearly demonstrate both formations of sodalite nuclei through localized solid-solid transformation of an aluminosilicate matrix and crystal growth by rearrangement of delocalized nuclei. The results clearly explain that sodalite crystallization takes place by the solid-solid transformation without involving any solution or amorphous-mediated process.