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

Abstract

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.

Consequences of exchange-site heterogeneity and dynamics on the UV-visible spectrum of Cu-exchanged SSZ-13

“…Discrimination within these sets is more challenging. Samples prepared to contain only Z 2 Cu species potentially contain two geometrically distinct sites distinguished by the location of the charge-compensating Al, 80 and such sites are predicted here to have distinct UV-Vis spectra if computed at one minimum energy structure. Accounting for the nite temperature uctuations in Cu location between local minima and associated uctuations in Cu coordination environment, however, attenuates these differences, such that the two sites become spectroscopically indistinguishable.…”

Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

“…here: as-made zeolites, containing OSDA, in equilibrium with their 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 synthesis liquor). 44,57 Zeotype materials kept within their synthesis liquor are known to be non-rigid: e.g. Si-islanding is a well-documented phenomenon in silicoaluminophoshates.…”

“…43 Computational studies demonstrate the presence of 25 symmetrydistinct combinations within the CHA unit cell containing two Al atoms (Si/Al=17) and 210 possible combinations to counterbalance a divalent cation on these 25 configurations, due to the four possible acidic O F (framework oxygen) neighboring each Al. 44 The most stable combination, at least in certain conditions, is often found to be a divalent cation in square planar coordination on two opposite Al tetrahedra in a 6MR in SSZ-13 materials (Cu 2+ , 45 Co 2+ 43 and Fe 2+ 27-28 ). Di Iorio and Gounder pioneered synthetic control over the Aldistribution in SSZ-13 in the context of acid-catalyzed reactions (methanol to dimethylether).…”

The Operating Cycle of NO Adsorption and Desorption in Pd–Chabazite for Passive NOx Adsorbers

“…Schneider and co-workers performed similar calculations and reported that, in general, the 4NN configuration is the least stable followed by 3NN and 2NN. 31 The geometries for 6MR-2NN and 6MR-3NN are shown in Figure 2. Since the energies of 8MR-2NN, 8MR-3NN, and 8MR-4NN are nearly the same, the structure of only one of them (8MR-4NN) is shown in Figure 2.…”