Synthesis of LTA zeolites with controlled crystal sizes by variation of synthetic parameters: Effect of Na+ concentration, aging time, and hydrothermal conditions

Abstract

Comprehensive synthetic studies of crystalline microporous LTA zeolite with controlled crystal sizes were carried out in this work. The sol–gel-mediated crystallization of an aluminosilicate solution can be affected by changes in the synthetic parameters including the Na+ concentration, which depends on the tetramethylammonium hydroxide (TMAOH)/NaOH ratio, the aging time of the synthesis solution, and the hydrothermal conditions. Modifying these synthetic parameters was found to significantly influence the crystal size and uniformity of the LTA zeolites. Variation of the TMAOH/NaOH ratio had the strongest effect on the crystal size. The higher the TMAOH/NaOH ratio, the smaller the average crystal size produced. The average sizes of the LTA zeolite crystals could be tuned from less than 100 nm to 1 μm with a uniform distribution by simply varying the TMAOH/NaOH ratio. For a given gel composition, variation of the aging time (0 h–72 h) of the synthesis solution had a significant influence on the average size and uniformity of the LTA zeolite crystals. As the aging time was increased, the resultant LTA zeolite crystals became much smaller and exhibited higher uniformity. When the hydrothermal temperature and time were varied, longer hydrothermal treatment at higher temperature produced larger crystals. However, with sufficiently long aging, the change of hydrothermal condition had a much less significant effect on the crystal size. Although the comprehensive synthetic studies produced LTA zeolites with controlled crystal sizes, all the zeolites were stable, showing no structural collapse upon ion-exchange to various metal cations and thermal treatment up to 700 °C. The present synthetic studies and characterizations may provide important insight into the sol–gel-mediated control of crystal sizes of LTA zeolite.