Crystallization process of LTA zeolite from alkoxides assisted by microwave heating

Microwave heating (MH) is a promising way for zeolite synthesis in terms of rapid and uniform reaction. However, detail process of the synthesis reaction and mechanism of its promotion by microwave are still under investigation. In the present study, LTA (Linde Type-A) zeolite was hydrothermally synthesized by 300 W of MH from hydrolyzed Al and Si alkoxides. The formation process of LTA was elucidated by nuclear magnetic resonance spectroscopy (²⁹Si NMR), small angle X-ray scattering (SAXS), X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS) and scanning electron microscopy (SEM). ²⁹Si NMR and SAXS analyses suggest that linear polycondensates of aluminosilicate in the precursor solution was immediately transformed into densely packed structure by MH. TEM observations reveals that crystalline phase with several nanometers was already formed in amorphous aluminosilicate nanoparticles prior to the MH while XRD shows that the amorphous phase had been transformed to LTA phase until 400 min of MH. It is basically consistent with the previous studies among zeolite synthesis assisted by both of conventional heating and MH. The aluminosilicate/LTA particle size was measured by DLS and SEM image as well as the yield was calculated from weight of the reaction products. The particle gradually grew up to 242 nm in diameter until 36 h while the yield sharply increased to nearly 100 % during 6.7 h and 12 h. In other words, all aluminosilicate in the solution once had polycondensed to form insoluble particle until 12 h followed by particle growth by dissolution-reprecipitation or agglomeration until 36 h.