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.