Gel evolution in a FAU-type zeolite yielding system at 90 °C

The nanoscale events in an organic-template-free gel leading to the formation of a FAU-type zeolite at 90 °C were investigated by transmission electron microscopy (TEM). The study was complemented by ex situ X-ray diffraction (XRD), infrared (IR), 23Na 3Q-MAS NMR, N2 adsorption measurements and chemical analyses. It was found that the critical factor that triggers the growth process and mass transformation from 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-type zeolite composition. This period was also marked by the formation and development of voids in the gel particles, containing highly hydrated material transformed later on into negative crystals. The evolution of these areas has been found to be an integral and noteworthy part of the chemical transformation of the gel that preceded the nucleation in the system. The observed transformations in the solid part of the system were induced by sodium, which progressively penetrated in the aluminosilicate matrix, breaking its structure and reorganizing the aluminosilicate species. The sodium behaviour was tracked down by a combination of local and bulk chemical analyses complemented by 23Na 3Q-MAS NMR study. During the initial stage of reaction a growth via propagation through the amorphous network co-existed with the typical solution-assisted hydrothermal crystallization. The later stages of development of the system followed the classical for zeolite-yielding systems crystallization that could be described by the autocatalytic nucleation.