Structure evolution of aluminosilicate sol and its structure-directing effect on the synthesis of NaY zeolite

Understanding the growth behaviour of an aluminosilicate sol during ageing is necessary for the design of the sol and the synthesis of NaY zeolite. Herein, aluminosilicate sols with three different SiO₂/Al₂O₃ ratios were prepared and aged for varying times at 293 K. The sol was then introduced as a structure-directing agent in the feedstock gel to generate NaY zeolite. The structure evolution of the sol species during the ageing process was studied by in situ small-angle X-ray scattering (SAXS) using synchrotron radiation. SAXS data, in combination with transmission electron microscopy images, Raman spectra and X-ray diffraction patterns, present a detailed description of the sol species, revealing an interplay between the fractal structure and reactivity to generate NaY zeolite. The SiO₂/Al₂O₃ ratios in the sol play a critical role in the structure evolution of the aluminosilicate species during the ageing, particularly with respect to their size distributions and the fractal dimensions. The species with suitable size and compactness in the sol are found to be an active precursor for achieving a highly crystalline NaY zeolite. The sol with an SiO₂/Al₂O₃ ratio of 20 possesses active species with a mass fractal dimension D_m of 2.0–2.6 after ageing, which leads to the formation of a well crystallized NaY zeolite. However, the high-silica sol-25, with an SiO₂/Al₂O₃ ratio of 25, and alumina rich sol-10, with a ratio of 10, experience growth at either slow or fast rates. In these cases the active species have smaller D_m (1.1–1.9) or larger D_m (2.6–2.8), respectively, displaying low/poor activity to generate NaY zeolite. The mechanisms regulating the growth behaviour of the sols during ageing are proposed.