  While organic structure directing agents (OSDAs) are well known to have a
directional influence on the topology of a crystallizing zeolite, the
relationship between OSDA charge and siting of aliovalent ions on a primarily
siliceous framework is unclear. Here, we explore the relationship between OSDA
orientation, Al3+ siting, and lattice energy, taking as a model system CHA
zeolite occluded with N,N,N-trimethyl-1-adamantyl ammonium (TMAda+) at an Si/Al
ratio of 11/1. We use density functional theory calculations to parametrize a
fixed-charge classical model describing van der Waals and electrostatic
interactions between framework and OSDA. We enumerate and explore all possible
combinations of OSDA orientation and Al location (attending to Lowenstein's
rule) within a 36 T-site supercell. We find that interaction energies vary over
60 kJ/double-six-ring-unit (d6r). Further, analysis of configurations reveals
that energies are sensitive to Al-Al proximity, such that low energies are
associated with Al3+ pairs in 8-membered rings and higher energies associated
with Al3+ pairs in smaller 6- and 4-membered rings. Comparisons with Al siting
inferred from CHA zeolite crystallized with TMAda+ suggests that these computed
interaction energies are useful reporters of observed Al siting in CHA
synthesized with TMAda+.
