NaA zeolite cubic crystal formation and deformation: cubes with crystalline core, simultaneous growth of surface and core crystals, and layer-by-layer destruction

Abstract

The entire sequence of NaA zeolite crystal growth mechanism, involving the formation of initial inorganic precursor gel, NaA zeolite nucleation, primary intermediate crystalline building block formation followed by their successive oriented aggregation and then finishing with cubic NaA zeolite crystal formation, has been monitored by light scattering analysis (LSA), X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and N₂ adsorption–desorption. The presented crystal growth mechanism in this work not only provides a prominent feature in NaA zeolite cubic crystal formation but also in the deformation of cubes. The deformation of micron-size NaA zeolite cubes first involves layer-by-layer destruction followed by complete breakage of cubes due to core–shell crystal expansion, resulting in nanosize cubic crystals having the same characteristics as the original NaA zeolite before destruction. Therefore, the present study provides a first demonstration about an occurrence of multiple phenomena during the NaA zeolite crystal growth process such as formation of intermediate crystalline building blocks which are structurally similar to NaA nuclei, multiple nucleation (crystalline island formation), heat facilitated fast oriented aggregation, formation of chamfered edged cubic crystals, successive layer-by-layer destruction and finally breakage of cubic crystals. Furthermore, LSA, SEM and TEM results reveals the formation of nanocrystals with size ranging from 50 to 500 nm during the crystal growth process.