In situ small-angle and wide-angle X-ray scattering investigation on nucleation and crystal growth of nanosized zeolite A

The entire sequence of crystallization processes, starting with the formation of precursor particles, proceeding through the nucleation stage, and finishing with complete transformation into Linde type A (LTA) zeolite nanocrystals and crystal growth, from a clear solution using tetramethylammonium cation with the composition 11.25:1.8:13.4:x:700 SiO2:Al2O3:(TMA)(2)O:NaOH:H2O (x = 0.6, 0.9, 1.2, or 1.5) has been monitored by simultaneous in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). Primary units with a size of ca. 0.5 nm and precursor particles with a size of ca. 4.5 nm are formed during the crystallization. The precursor particles with a size of 4.5 nm play an important role during the nucleation process. The influences of Na+ on the crystallization are studied by varying the concentration of Na+ cations in the synthesis solution. It is found that the Na+ cations affect not only the nucleation process but also crystal growth process. When x < 1.5, the number density of the precursor particles increases with the concentration of Na+, which is due to the structure-making influence and charge-compensating role of Na+. However, when the concentration of Na+ is relatively high (x = 1.5), the 6 nm sized particles in the synthesis solution are observed at the beginning of the synthesis, which is due to the salt-outing effect of Na+. On the other hand, the studies on the structure of the precursor particles suggest that they do not contain long-range order, but some medium-range order related to the crystalline LTA structure, which become ordered during the course of hydrothermal treatment.