(169d) Choreographing Zeolite Crystallization: It's Elementary

Zeolites are composed of negatively-charged aluminosilicate frameworks balanced by counterions typically introduced as structure-directing agents. The crystallization of zeolites is governed by complex physicochemical interactions over a broad range of length scales. During hydrothermal treatment, anionic aluminosilicate oligomers interact with cations that can either be organic (e.g., tetraethylammonium) or inorganic (e.g., Na⁺) species. Organic structure-directing agents (OSDAs) are generally governed by heuristics used to predict how molecular structures will influence resultant zeolite properties. Owing to the hazardous and/or costly nature of most OSDAs, it is preferable to instead use inorganic structure-directing agents (ISDAs) to generate zeolites; however, when using the latter, it is more challenging to predict how cation(s) selection will alter synthesis progress. Selection of elements as heteroatoms, which can be incorporated into the zeolite framework, is an additional topic of interest. During zeolite synthesis, charged heteroatom complexes interact in solution with (alumino)silicate species and can potentially accelerate, interrupt, or otherwise alter crystallization. Collectively, by understanding the effects of diverse elements forming cationic or anionic species in zeolite growth solutions, the rational design of zeolite crystals becomes a more attainable goal.

In this presentation, we highlight various strategies for introducing additives or controlling composition to tune zeolite crystallization towards desirable outcomes. Examples include the promotion of certain zeolite frameworks using mixtures of inorganic cations, including some unconventional species (e.g., Li⁺ and Ca²⁺), wherein the judicious selection of composition can lead to dramatically reduced synthesis temperatures. Conversely, addition of zinc to zeolite growth mixtures is beneficial owing to its ability to suppress the formation of undesired impurity phases. We also observe that zeolites synthesized in the presence of germanium have smaller crystal dimensions, which demonstrate enhanced performance and greater longevity as catalysts with reduced diffusion limitations. Overall, elucidating the role of individual species present in zeolite growth mixtures assists in developing a more holistic picture of zeolite crystallization mechanisms. Our research provides several influential insights into the critical phenomena that should be considered for future advancements in zeolite engineering and design.