(90d) Multivalent Cations Function As Accelerants and Structure-Directing Agents of Zeolite Crystallization

Zeolites are nanoporous aluminosilicates widely used as commercial adsorbents, heterogeneous catalysts, and ion-exchange materials due to their unique porosity, acidity, and (hydro)thermal stability. There are currently over 245 known zeolite framework types, and the vast majority of these structures are prepared synthetically under hydrothermal conditions in alkaline media, and often in the presence of an organic structure-directing agent (OSDA) that directs the assembly of diverse porous networks. Due to economic and environmental disadvantages of OSDAs, it is often more desirable to synthesize zeolites in the absence of organics; however, less than 15% of zeolite structures have been reportedly synthesized in OSDA-free media. Among these cases, alkali metals are most commonly employed as inorganic structure-directing agents. Many zeolites derived solely from alkali metals are usually prepared under moderate conditions using temperatures less than 100 °C and synthesis times on the order of hours.[1] One notable exception is zeolite chabazite (CHA framework), which is prepared from potassium ions and requires atypically long crystallization times on the order of weeks. Shortened synthesis times require alternative approaches, such as the use of zeolite crystal seeds. In this presentation, we will summarize our studies of zeolite synthesis where alkali metals are partially replaced by multivalent ions. Examples include chabazite synthesis from growth solutions prepared with potassium and small quantities of strontium. Our findings reveal that these conditions yield fully crystalline chabazite within hours without the need for either organics or crystal seeds.[2] Similar studies of strontium inclusion in seed-assisted syntheses also reveal shorter crystallization times. Notably, we show that strontium has a pronounced impact on the kinetics of chabazite formation, leading to 14- and 3-fold reductions in crystallization time compared to pure potassium syntheses in the absence and presence of crystal seeds, respectively. Using a combination of ²⁹Si and ²⁷Al MAS NMR spectroscopy, we also show that strontium functions as a structure-directing agent based on its ability to alter Q⁴(nAl)²⁹Si speciation, thereby redistributing tetrahedral Al sites in the framework. Given the widespread application of chabazite in adsorption and separation processes, designing facile and efficient synthesis approaches with concomitant control of physicochemical properties is commercially relevant. Details of these studies along with other analyses of multivalent/alkali metal combinations will be discussed in this presentation.

References

[1] Oleksiak, M.D. and Rimer, J.D., Rev. Chem. Eng. 30 (2014) 1-49

[2] Liang, Y., Jacobson, A.J., Rimer, J.D., ACS Mater. Lett. 3 (2021) 187-192.