(446f) Synthesis Methods to Influence Framework Al Arrangements in CHA Zeolites and Consequences for NOx Selective Catalytic Reduction

The use of organic and inorganic cations as structure-directing
agents (SDAs) in zeolite crystallization provides a mechanism to influence the arrangement
of anionic framework Al centers (Al−O(−Si−O)x−Al)
in isolated (x ≥ 3) or paired (x = 1, 2) configurations. We discuss this
within the context of chabazite (CHA), a high-symmetry framework comprised of one
crystallographically-distinct T-site, which facilitates complementary
experimental and theoretical approaches to probe how the molecular and
electronic structure of SDAs influence Al arrangement. Titration protocols to
quantify the number of paired Al sites in CHA zeolites were developed using
divalent Co²⁺ and Cu²⁺ cations, which were validated by measuring
UV-Visible spectra and quantifying residual protons that remain after cation exchange.
These experimental characterizations are consistent with calculations of
divalent cation siting energies at different framework Al arrangements in the
CHA framework. CHA zeolites (Si/Al = 15) crystallized from mixtures containing only
low charge-density organic N,N,N-trimethyl-1-adamantylammonium
cations (TMAda⁺) as the SDA contain predominantly
isolated Al sites, because adamantyl groups (~0.7 nm
diam.) impose steric constraints that limit the occupation of each CHA cage (~0.7
nm diam.) to only one TMAda⁺ molecule. The
partial replacement of TMAda⁺ with high charge-density
Na⁺ cations in crystallization media, holding other variables
constant, results in the formation of paired Al sites in amounts that correlate
with the amount of Na⁺ incorporated within crystalline CHA products.
This cooperation between Na⁺ and TMAda⁺
during CHA crystallization reflects incorporation of Na⁺ near the quaternary
ammonium group of a TMAda⁺ molecule occluded
within the CHA cage, which preserves dispersive contacts between non-polar
siloxane regions of the zeolite lattice and the hydrophobic adamantyl
group of TMAda⁺. Density functional theory
calculations, performed on a 36 T-atom CHA unit cell (Si/Al = 17), were used to
compute the energies of different Na⁺ and H⁺
configurations as a function of Al-Al proximity. DFT-derived energies indicate
that Na⁺ siting is preferred when two Al are located
in the same six-membered ring, suggesting that the occlusion of Na⁺
during CHA crystallization preferentially directs the formation of paired Al
sites. We also discuss our recent work combining experiment and theory to
develop alternate crystallization routes using mixtures of other SDAs to influence
framework Al arrangements in CHA, and discuss the consequences of these site
distributions for NOx selective catalytic reduction (SCR) with ammonia. These findings
provide further insight into the cooperative and competitive interactions
between organic and inorganic SDAs on the arrangement of Al atoms in CHA zeolites.