Submicrometer Zeolite A Crystals Formation: Low-Temperature Crystallization Versus Vapor Phase Gel Transformation

In the present study, low-temperature hydrothermal crystallization and vapor phase gel transformation have been employed to synthesis LTA-type zeolite crystals of submicrometer size. The crystal growth curves under hydrothermal conditions at 35, 50, and 65 °C were obtained. From these curves, the induction periods and the times for reaching maximum crystallinity for respective temperatures were determined. A set of characterization methods, including XRD, FTIR, SEM, TEM, and DLS, was employed to obtain complementary information. The inspection of the crystalline solids obtained under hydrothermal conditions showed that the size of the crystals is a function of the crystallization temperature. The largest crystals (300 × 300 nm) were obtained at 65 °C, while much smaller particles were synthesized at lower temperatures. Second approach involved vapor-phase transformation of the solid part of initial gel. This approach has also yielded submicrometer-sized zeolite crystallites. The characteristics of crystalline particles synthesized by two methods were compared. Both synthetic methods showed relatively high conversion (70–80%) of the initial gel into LTA-type zeolite, which is substantial advantage in respect to nanozeolite synthesis from clear solutions.