Impact of Temperature on Zeolitization Elucidated by <i>In Situ</i> High-Energy X-ray Total Scattering Measurement

Zeolites are generally synthesized by the hydrothermal method, where temperature is an important parameter. Previous studies have shown that aging can accelerate the synthesis of zeolites; however, while the phenomena have been well investigated, the medium-range structural evolution of the aluminosilicate species during zeolite synthesis has not yet been fully understood, leaving the underlying mechanism unclear. Herein, zeolite X (FAU-type framework) is selected as an example to reveal the role of temperature in zeolitization, and the structural evolutions during the induction and crystallization processes of zeolite X under different temperatures are comparatively investigated by the advanced <i>in situ</i> high-energy X-ray total scattering (HEXTS) technique, combined with UV-Raman spectroscopy and <i>ex situ</i> HEXTS. Zeolite X is synthesized in a homemade reactor, by aging the reactants at 70 °C prior to the hydrothermal treatment at 90 °C. At an aging temperature of 70 °C, 4-rings (4Rs) and double 6-ring (<i>d6r</i>) units are formed in the amorphous matrix, which are subsequently assembled to form ordered large structures (12Rs and <i>sod</i>-cages) at 90 °C, triggering the formation of a crystalline framework. In comparison, when the reactant is only treated at 90 °C without aging, an SOD-type framework is formed instead of the targeted FAU-type due to the lack of <i>d6r</i> units. Our findings demonstrate that a suitable aging temperature is crucial for zeolite synthesis, and temperature significantly influences the local structural evolutions during the formation of zeolites.