Coarsening and Spinodal Decomposition of Zeolite Linde Type A Precursor Gels Aged at Low Temperatures

We monitored the effect of different gel aging temperatures (from -20 to 40 °C) and gel aging times (from 7 to 21 days) on the particle size and crystalline structure of template-free Linde type A zeolites through scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. We demonstrate the synthesis of zeolite LTA with average particle sizes of 0.45 ± 0.07 μm by preliminary heat treatment of the precursor gel at -8 °C followed by crystallization at 100 °C. Here, we found that aging the precursor gels for 2 weeks at 40 °C decreases the size of particles by 59% compared with particles formed from unaged gels, and aging gels for 2 weeks at -8 °C results in particles that have a 95% smaller diameter compared with particles formed from the unaged gel. We hypothesize that decreasing the precursor aging temperature below 0 °C leads to the occurrence of spinodal decomposition at which the nucleation barrier vanishes. Consequently, a very large number of nuclei form. Decreasing the gel aging temperature from 40 to 0 °C leads to an increase in the average size of zeolite particles by the Ostwald ripening phenomena (coarsening). Additionally, we found aging the precursor gels for 2 weeks at 40 °C leads to the formation of 27% zeolite X (an undesirable product), while aging at -8 °C for 2 weeks leads to the formation of only 2% zeolite X. The primary purpose of our paper is to explore the two experimentally observed phenomena that occur during low temperature aging of zeolite gel precursors that result in significant effects on particle size.