Transmission Electron Microscopy Study of the Formation of FAU-Type Zeolite at Room Temperature

FAU-type zeolite was synthesized at room temperature to study the mechanism of its formation. The sluggish crystal growth kinetics at ambient conditions permitted us to track down the entire sequence of crystallization events from the formation of the initial gel to the complete transformation into a zeolite-type material. The processes taking place at the nanometer scale were studied by transmission electron microscopy (TEM). The crystalline character of the particles observed during the induction period was determined by in situ synchrotron XRD analysis. The study was completed by ex situ X-ray diffraction (XRD), infrared (IR), dynamic light scattering (DLS), N2 adsorption measurements and chemical analyses. The combined TEM/synchrotron XRD analysis of the initial crystallization stage revealed that zeolite nuclei were formed during the homogenization (1.5 h) of the initial reactants. Nevertheless, a relatively long induction period (10 days) was observed. During this period, extensive exchange between the solid and liquid parts of the system took place. However, the changes in the chemical composition of the gel and mother liquor were not coupled with a detectible increase of the number of nuclei. It seems that the critical factor that triggers the growth process and mass transformation of amorphous into crystalline zeolite material is the attainment of a specific level of chemical evolution of the system, expressed in homogenization of the starting gel approaching stoichiometric FAU compositon. The process of zeolite formation can be divided into four general stages: (i) 0−1.5 h, formation of an amorphous gel with highly variable composition, local formation of stable zeolite nuclei, and nanometer sized metastable hydrated silica phase; (ii) 1.5 h to 10 days, chemical evolution of the gel composition that is coupled with structural rearrangements of the amorphous gel and further development of zeolite nuclei; (iii) 10−14 days, spontaneous mass transformation of the amorphous gel into spherical aggregates of small (10−20 nm) crystals around individual crystallization centers of viable nuclei and collapse of the metastable phases; (iv) 17−38 days, crystal growth involving agglomeration, dissolution, and regrowth of individual nanoparticles around spherical aggregates.