First-Principles Comparison of Proton and Divalent Copper Cation Exchange Energy Landscapes in SSZ-13 Zeolite

The
relative proximity of Al atoms substituted in zeolite lattices
is an important parameter that influences both hydrothermal stability
and catalytic function, but the underlying chemistry that governs
Al site proximity is not well understood. Here, we examine relationships
between exchanged countercations and different Al–Al arrangements
in a chabazite (SSZ-13) zeolite lattice. We report periodic supercell
density functional theory (DFT) calculations for structures and energies
of SSZ-13 lattices with systematically enumerated and varied Al–Al
proximity, both charge-uncompensated and charge-compensated by either
proton pairs (H+/H+) or divalent copper cations
(Cu2+). Al–Al interactions are electrostatically
repulsive without charge compensation, but the relative energies of
certain Al–Al site arrangements change upon compensation by
countercations. Al–Al interactions are uniformly attractive
when compensated by H+/H+ pairs but are attractive
at long and repulsive at short Al–Al distances when compensated
by Cu2+, highlighting the role of the countercation in
stabilizing different Al–Al arrangements. Through descriptor
analysis, we find that the Cu2+ energy landscape can be
described by models consisting of electrostatics and a binary term
that specifies whether or not Cu2+ resides in the six-membered
ring (6MR). The H+/H+ and Cu2+ energy
landscapes together imply that Cu2+ prefers to reside at
6MR Al–Al pairs. These results shed light on how countercations
influence Al distribution and rearrangement during synthesis and postsynthetic
treatments of the SSZ-13 zeolite, which potentially influences its
susceptibility to dealumination during hydrothermal aging. The systematic
DFT computation workflow and descriptor analysis reported here are
promising approaches that can be applied generally to examine other
combinations of ions and zeotypes of interest.