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 SiO 2:Al 2 O 3:(TMA) 2 O:NaOH:H 2 O ( 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.