Zeolites are attractive catalytic materials, offering both expansive structural diversity associated with the many zeolite polymorphs and compositional diversity associated with the amount and location of isomorphic substitutions on the primarily siliceous framework. Al are common isomorphic substitutes for Si, and the charge imbalance associated with this replacement introduces a capacity for accommodating extra-framework cations, such as protons or transition metal ions, which are frequently the centers relevant to catalytic function. The interaction energy between a chargeless OSDA and framework is a direct reporter on the OSDAs affinity to crystallize that zeolite phase. However, these models neglect the influence of the structure directing agents on Al siting and Al configurations. Previous work has shown that OSDAs can electrostatically direct the incorporation of Al substitution to specific sites, which in turn, can be used as a predictive tool to exploit mixtures of OSDAs in order to influence Al arrangements in different zeolite topologies.<br/>This body of evidence suggests an opportunity to use OSDA-Al interactions as a reporter of Al siting in a framework. We test this hypothesis on the CHA framework. We start by considering a family of OSDAs known to crystallize CHA and apply a combination of static sampling and molecular dynamics to characterize the conformational spaces available to the OSDA. We find N,N,N-trimethyl-1-adamantyl ammonium (TMAda+) is distinct in occupying only one of two stable orientations within the CHA cage. Focusing on this OSDA, we parameterize a classical forcefield against first-principles-derived charges, and then use this classical model to explore the relationship between TMAda+ orientations, Al distribution on the lattice, and lattice total energies. We find that low energy structures have a large number of 8 membered ring (MR) Al pairs and isolated Al. High energy structures include primarily 6MR Al-pairs. The results also show that the lattice energy is highly dependent on Al/TMAda+ distribution.

Exploring the relationship between organic structure directing agents and Al distribution in CHA

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