(85a) Understanding the Crystallization Mechanism of Heteroatom-Incorporated Zeolites

Zeolites are porous aluminosilicate frameworks, widely used in industries for applications spanning from catalysis to adsorption and separations. Controlling the synthesis of zeolites to attain desired physicochemical properties is challenging, owing to their complex growth media, which can hinder the judicious selection of synthesis parameters. While organic structure-directing agents (OSDAs) and the incorporation of heteroatoms offer potential solutions to tailor the physicochemical properties of zeolites, their utilization often raises concerns regarding economic viability, environmental impact and require harsh synthesis conditions. Incorporation of metals as inorganic structure-directing agents (ISDAs) or growth modifiers can be an alternative method. Determining the effects of metals on zeolite crystallization is complicated due in large part to the difficulty associated with characterizing diverse precursor species and their role in zeolite crystallization. We have pioneered methods to probe zeolite growth in situ using high temperature atomic force microscopy (AFM), which enables visualization of zeolite surface growth in real time at a near molecular level. We have demonstrated that zeolite crystallization primarily occurs through nonclassical pathways, such as crystallization by particle attachment, with classical layer-by-layer growth via monomer incorporation playing a minor role.

In this presentation, we will discuss our recent studies of two commercially-relevant zeolites: faujasite (FAU) and chabazite (CHA). Our findings have shown that OSDA-free synthesis of zeolite FAU in the presence of inorganics (e.g., zinc) can alter surface growth relative to conventional synthesis conditions [1]. Previously, we have seen how the stability and catalytic properties of FAU are improved with the incorporation of Zn [2]. Here, we will discuss our investigation into Zn-FAU crystallization using in situ AFM to explore the inhibitory effect of metal incorporation. Additionally, we will showcase the OSDA-free accelerated crystallization of zeolite CHA using a binary pair of inorganic cations, which function as inorganic structure-directing agents (ISDAs). Our studies have elucidated how the solvated structure and hydrated ionic radii of ISDAs operate cooperatively to influence the process of crystallization [3].

[1] Mallette, Adam J., et al. "Heteroatom manipulation of zeolite crystallization: stabilizing Zn-FAU against interzeolite transformation." JACS Au 2 (2022): 2295-2306

[2] Parmar, Deependra, et al. "Direct synthesis of highly siliceous ZnO-FAU zeolite with enhanced performance in hydrocarbon cracking reactions." ACS Materials Letters 5 (2022): 202-208

[3] Mallette, Adam J., et al. "Highly efficient synthesis of zeolite chabazite using cooperative hydration-mismatched inorganic structure-directing agents." Chemical Science 15 (2024): 573-583