Influence of N,N,N-trimethyl-1-adamantyl ammonium (TMAda+) Structure Directing Agent on Al Pair Distributions and Features in Chabazite Zeolite

TL;DR

This study investigates how the structure of TMAda+ influences aluminum pair distribution and stability in CHA zeolite, using computational modeling to relate OSDA orientation to Al siting and lattice energy.

Contribution

It provides a detailed computational analysis linking OSDA structure and charge to Al distribution and lattice energy in CHA zeolite, enhancing understanding of zeolite synthesis.

Findings

Al-Al pair energies vary over 60 kJ/d6r

Low energies correlate with Al pairs in 8-membered rings

Computed energies align with experimental Al siting in CHA

Abstract

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+.