We
combine experiment and theory to investigate the cooperation
or competition between organic and inorganic structure-directing agents
(SDAs) for occupancy within microporous voids of chabazite (CHA) zeolites
and to rationalize the effects of SDA siting on biasing the framework
Al arrangement (Al–O(−Si–O)
x
–Al, x = 1–3) among CHA zeolites
of essentially fixed composition (Si/Al = 15). CHA zeolites crystallized
using mixtures of TMAda+ and Na+ contain one
TMAda+ occluded per cage and Na+ co-occluded
in an amount linearly proportional to the number of 6-MR paired Al
sites, quantified by Co2+ titration. In contrast, CHA zeolites
crystallized using mixtures of TMAda+ and K+ provide evidence that three K+ cations, on average, displace
one TMAda+ from occupying a cage and contain predominantly
6-MR isolated Al sites. Moreover, CHA crystallizes from synthesis
media containing more than 10-fold higher inorganic-to-organic ratios
with K+ than with Na+ before competing crystalline
phases form, providing a route to decrease the amount of organic SDA
needed to crystallize high-silica CHA. Density functional theory calculations
show that differences in the ionic radii of Na+ and K+ determine their preferences for siting in different CHA rings,
which influences their energy to co-occlude with TMAda+ and stabilize different Al configurations. Monte Carlo models confirm
that energy differences resulting from Na+ or K+ co-occlusion promote the formation of 6-MR and 8-MR paired Al arrangements,
respectively. These results highlight opportunities to exploit using
mixtures of organic and inorganic SDAs during zeolite crystallization
in order to more efficiently use organic SDAs and influence framework
Al arrangements.
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.
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