Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework

In
silicogermanate zeolites containing double four-ring (d4r) building units, the germanium atoms preferentially
occupy the corners of these cube-like units, but the absence of long-range
order precludes the determination of the preferred arrangements of
Si and Ge atoms by means of crystallographic methods. If fluoride
anions are present during the synthesis, they are incorporated into
these cages. Because of the sensitivity of the 19F chemical
shift to the local environment, NMR experiments can provide indirect
insights into the predominant (Si,Ge) arrangements. However, conflicting
interpretations have been reported, both with regard to the preference
for, or avoidance of, Ge–O–Ge linkages, and concerning
the equilibrium position of fluoride inside the cage, where fluoride
might either occupy the cage center or participate in a partly covalent
Ge–F bond. In order to shed light on the energetically preferred
local arrangements, periodic dispersion-corrected density functional
theory (DFT) calculations were performed for the AST framework, which
is synthetically accessible across the range of (Si1–n,Gen)­O2 compositions
(0 ≤ n ≤ 1). DFT structure optimizations
for (Si,Ge)-AST systems containing fluoride anions and organic cations
revealed that arrangements of Si and Ge which maximize the number
of Ge–O–Ge linkages are energetically preferred and
that fluoride tends to form relatively short (∼2.2–2.4
Å) bonds to Ge atoms surrounded by Ge–O–Ge linkages.
The preference for Ge–O–Ge linkages disappears in the
absence of fluoride. DFT-based molecular dynamics calculations were
performed for selected AST models to analyze the dynamics of fluoride
anions confined to d4r cages. These calculations
showed that the freedom of movement of fluoride varies depending on
the local environment and that it correlates with the average Ge–F
distance. An analysis of the Ge–F radial distribution functions
provided no evidence for a coexistence of separate local energy minima
at the cage center and in the proximity of a germanium atom. The computational
approach pursued in this work provides important new insights into
the local structure of silicogermanate zeolites with d4r units, enhancing the atomic-level understanding of these materials.
In particular, the findings presented here constitute valuable complementary
information that can aid the interpretation of experimental data.