Condensation and evolution of silicate and aluminate groups during aging of coal gangue-based Na-A zeolite

This study investigates the evolution of silicate and aluminate species during aging in the synthesis of Na-A zeolite from high-alumina coal gangue, aiming to clarify the structural changes that govern nucleation. X-ray diffraction (XRD), Raman, NMR, Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to track the transformation of Si-Al species from the liquid phase to the solid phase. Two critical transition points were identified at 195 s and 795 s. At 195 s, dissolved precursor species transformed into an amorphous sodium aluminosilicate gel, marking the onset of Si-Al co-condensation. At 795 s, a crystalline precursor with a more ordered aluminosilicate framework emerged, indicating the transition from disordered gel species to framework-bearing structural units. Overall, aging involved progressive condensation, structural reorganization, and precursor ordering, providing mechanistic insight into early nucleation and a basis for regulating crystallization.