The “fluoride route”, synthesis in the presence of fluoride anions, has been established as a versatile strategy for the synthesis of all-silica zeolites. Experimental investigations of as-synthesised zeolites using diffraction and NMR methods have shown that the fluoride anions are often located in small cages and bonded to one Si atom at a cage corner, with dynamic or static disorder occurring in some, but not all systems. It has also been observed that the dynamic disorder can be modulated through a variation of the organic structure-directing agent (OSDA) used in the zeolite synthesis. As relatively little is known about the underlying factors determining the preferred fluoride location(s) in a given cage and the dynamic behaviour, the present work employs a combination of density functional theory (DFT) calculations and DFT-based ab-initio molecular dynamics (AIMD) simulations to contribute to the understanding of these systems.
A systematic comparison of different fluoride positions in four all-silica zeolites with different topologies (IFR, NON, STF, STT) shows excellent agreement with experiment for all systems except STF. A remarkable result is obtained for STT, where the DFT calculations predict three distinct sites to be very close in energy, in perfect correspondence with the experimentally observed disorder over these sites. The AIMD simulations do not only reproduce the dynamic disorder of fluoride anions in IFR and STT, and its absence at room temperature in NON and STF, but also allow to develop an explanation for the qualitative differences between these systems. The role of the OSDA in influencing the dynamics of the fluoride anions is investigated for MFI-type Silicalite-1. AIMD simulations for Silicalite-1 models containing different alkylammonium OSDAs show that the introduction of asymmetric organic cations with one short alkyl chain leads to a strong reduction of the fluoride mobility, agreeing with experimental observations. Further analysis shows that the heterogeneous charge distribution of these OSDAs, together with their restricted freedom of movement in the channels, enhances electrostatic interactions with the fluoride anions, reducing the dynamic motion. While the primary aim of the present work is an improved fundamental understanding, similar computational approaches may, in the future, be exploited to aid the targeted synthesis of zeolites in fluoride-containing media.
Funding by the Deutsche Forschungsgemeinschaft (DFG project no. 389577027) is gratefully acknowledged.
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