Predicted Influence of Organic Structure Directing Agents on Al Distributions in CHA Zeolites

The distribution of Al on a zeolite framework can have a determining influence on chemical and catalytic properties. Those distributions are typically determined during synthesis as influenced by cationic organic structure directing agents (OSDAs). Here we use density functional theory (DFT) to compare the Al directing influence of N,N,N-trimethyl-1-adamantyl ammonium (TMAda+), an OSDA commonly used to crystallize CHA zeolite, with four isomers that retain the adamantyl structure but relocate the nitrogen center. Low energy configurations balance electrostatics-driven maximization of Al–Al and minimization of Al–N separations, the latter more important in OSDAs with more accessible charge centers. Statistical thermodynamics are used to predict thermal equilibrium Al distributions and corresponding Al pair features as a function of OSDA. All distributions differ from those predicted through naive application of Löwenstein's rule, disfavor the placement of two Al second-nearest-neighbor or within a single six-ring, and introduce different biases toward eight-membered rings and more remote Al. Results illustrate the potential to influence Al location through OSDA selection.