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

Abstract

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.

Polymorph Selection in Zeolite Synthesis Occurs after Nucleation

“…The vanishing nucleation barriers make the crystallization of the zeolites a spinodal-like decomposition of the amorphous phase. We note that spinodal-like decomposition of the amorphous phase into a zeolite has been previously hypothesized for the synthesis of LTA at temperatures below 0 °C on the basis of the experimental evolution of the size of the crystals as a function of aging of the synthesis mixture . The radii of critical zeolite nuclei we determine for the synthesis of silicalite-1 and BEA are consistent with a previous estimation of 0.4 to 0.8 nm for the synthesis of zeolite A using experimental solubility and kinetics of crystallization .…”

“…We note that spinodal-like decomposition of the amorphous phase into a zeolite has been previously hypothesized for the synthesis of LTA at temperatures below 0 °C on the basis of the experimental evolution of the size of the crystals as a function of aging of the synthesis mixture. 53 The radii of critical zeolite nuclei we determine for the synthesis of silicalite-1 and BEA are consistent with a previous estimation of 0.4 to 0.8 nm for the synthesis of zeolite A using experimental solubility and kinetics of crystallization. 54 We note that the critical nuclei are much smaller than the unit cell of zeolites (96 SiO 2 for silicalite-1 39 and 64 for BEA 8 ) and even smaller than the structural building units of these crystals.…”

Stepwise Gel Preparation for High-Quality CHA Zeolite Synthesis: A Common Tool for Synthesis Diversification

“…With this in mind, the creation of a suitable starting system that provides aluminosilicates with the desired structural order and distribution will lead to an additional sharable key strategy for the further development of zeolite syntheses. The charge-density mismatch (CDM) approach, − the use of two-dimensional zeolite precursors, − and the careful optimization of classic inorganic-synthesis systems − is also useful for the rational construction of zeolite frameworks. However, existing intentional synthesis routes focusing on the building-block intermediate, including the above-mentioned “zeolite synthesis from zeolite” method, still requires special additives.…”

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization

“…320,340,341 Lind and co-workers pointed out that although there are mathematical differences between the theories of SD and nucleation, there are physical similarities regarding their barriers for nucleation (i.e., a small thermodynamic barrier for nucleation and none for SD phase separation). 342 SD has been used to explain how solid precipitates form in complex crystallization systems, such as calcium carbonate, 343 alloys, 344 amino acids, 290 and minerals. 345 De Yoreo and co-workers observed liquid−liquid phase separation in calcium carbonate systems at a lower ion activity product than required for the solid−liquid phase separation of the SD pathway.…”