Influence of Organic Structure-Directing Agents on
Fluoride Dynamics in As-Synthesized Silicalite‑1
Posted on 2020-02-28 - 21:44
Silicalite-1, an
all-silica zeolite having the MFI topology, can
be synthesized in the presence of fluoride ions, which balance the
charge of the cationic organic structure-directing agents (OSDAs;
such as tetrapropylammonium). In as-synthesized Silicalite-1, the
fluoride anions occupy well-defined positions in the 41·52·62 cages where they are covalently
bonded to framework Si atoms. Previous experimental studies showed
that the fluoride anions are dynamically disordered at room temperature.
The dynamic disorder can be suppressed by cooling to cryogenic temperatures
and also through a variation of the OSDA, pointing to an interplay
between framework and extra framework dynamics. In the present work,
ab initio molecular dynamics (AIMD) simulations in the framework of
density functional theory (DFT) were employed to study the influence
of the OSDA on the fluoride dynamics, comparing a total of eight alkylammonium
OSDAs having either four alkyl chains of equal length (symmetric)
or one short and three longer chains (asymmetric). In addition to
time-averaged quantities, such as atomic root mean square displacements,
the number of dynamic events in which fluoride ions “jump”
from one Si atom to a neighboring one was analyzed. While the timescale
accessible to the AIMD simulations was found to be too short to observe
significant fluoride dynamic disorder at room temperature, it was
clearly detectable when increasing the temperature to 373 K. For systems
incorporating symmetric OSDAs, increasing the OSDA size correlated
with decreasing freedom of motion of the framework atoms. In line
with previous experimental findings, a drastic reduction of the fluoride
dynamic disorder was observed for systems containing tributylammonium-based
OSDAs having one short (methyl or ethyl) chain. Besides reproducing
the experimental observations, the computational approach also helped
to explain this suppression of the dynamic disorder on the basis of
the size and shape of the OSDA.
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Fischer, Michael (2020). Influence of Organic Structure-Directing Agents on
Fluoride Dynamics in As-Synthesized Silicalite‑1. ACS Publications. Collection. https://doi.org/10.1021/acs.jpcc.9b11608
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