Beyond tetrahedral coordination in zeolite-type materials - A computational approach
Final Report Abstract
Zeolites and zeolite-like materials (zeotypes) are porous inorganic materials that are widely used in various applications, most prominently catalysis, ion exchange, and separation. Their crystal structures consist of a three-dimensional framework of corner-sharing TO4 tetrahedra (where T = Si, Al, P…). While this implies that the T atoms have a coordination number of 4, there are instances where a coordination of additional ions or molecules results in an increase of the coordination number of some T atoms to 5 or 6 without affecting the tetrahedral connectivity of the framework. In this project, computational chemistry methods were employed to investigate the structures of zeolites and zeotypes with such “higher-coordinated” T sites, as well as the dynamic behaviour of the coordinated species and the framework as a whole. The calculations were carried out in the theoretical framework of dispersion-corrected density functional theory (DFT), including DFT-based ab initio molecular dynamics simulations (AIMD). The incorporation of fluoride anions in all-silica zeolites is of particular interest in this regard. Many of these zeolites can be synthesised only in the presence of fluoride, and it is known that fluoride anions are often bonded to individual Si atoms, forming [SiO4F]- units. A DFT-based comparison of a variety of possible fluoride sites in different zeolites showed that the preference for a particular position is governed by a complex interplay of factors. Moving from the static to the dynamic picture, AIMD simulations were employed to study the dynamic disorder of fluoride, i.e., the occurrence of “jumps” between different Si sites. These simulations demonstrated that the local structure (geometry of the fluoride-containing cage) usually determines whether fluoride anions are dynamically disordered or not. In special cases, however, attractive interactions with the organic structure-directing agents residing in the larger pores may suppress the dynamic disorder. Further investigations addressed zeolites and zeotypes with fluoride-containing cube-like double four-ring cages, with most emphasis on silicogermanates. In addition to investigating the bonding and dynamics of the encapsulated fluoride anions, which vary widely depending on the local environment, the analysis also gave insights into the energetically preferred germanium distributions. A coordination of adsorbed water molecules to framework Al atoms is often observed in hydrated aluminophosphate zeotypes (AlPOs). The impact of the adsorption of water on the local structure and the dynamic behaviour was investigated for the specific case of AlPO-11, where it was observed that the dynamic disorder of some bridging oxygen atoms disappears upon hydration. The silicoaluminophosphate (SAPO) ECR-40 is another interesting system, as it contains AlO4 tetrahedra linked by a common oxygen atom, which are usually assumed to be unstable (“Löwenstein’s rule”). AIMD simulations of hydrated ECR-40 pointed to a preferential coordination of water to the Al atoms of these linkages, resulting in irreversible structural changes. These findings confirmed the reduced stability of Al−O−Al links in the presence of water, as well as explaining why hydrated ECR-40 decomposes upon dehydration. Altogether, the calculations performed in this project delivered unprecedented insights into the local structure and dynamic behaviour of complex porous solids, resulting in an in-depth understanding that would, in many cases, not be possible on the basis of experimental results alone. Expanding upon the present work, computational methods could be combined with diffraction and spectroscopic experiments to achieve a comprehensive characterisation of the structure (covering both long-range order and local environments) and dynamics of zeolites or zeotypes. Beyond furthering fundamental understanding, the results from this project should also spur predictive uses of high-level computational methods, for example, to predict the phase selectivity during zeolite synthesis, or to propose suitable AlPO adsorbents for thermal energy storage applications.
Publications
- Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework, J. Phys. Chem. C 2019, 123, 1852-1865
M. Fischer
(See online at https://doi.org/10.1021/acs.jpcc.8b10770) - Proton Acidity and Proton Mobility in ECR-40, a Silicoaluminophosphate that Violates Löwenstein's Rule, Chem. Eur. J. 2019, 25, 13579-13590
M. Fischer
(See online at https://doi.org/10.1002/chem.201902945) - Influence of Organic Structure-Directing Agents on Fluoride Dynamics in As- Synthesized Silicalite-1, J. Phys. Chem. C 2020, 124, 5690-5701
M. Fischer
(See online at https://doi.org/10.1021/acs.jpcc.9b11608) - Fluoride Anions in All-Silica Zeolites: Studying Preferred Fluoride Sites and Dynamic Disorder with Density Functional Theory Calculations, J. Phys. Chem. C 2021, 125, 8825–8839
M. Fischer
(See online at https://doi.org/10.1021/acs.jpcc.1c01440) - Local Distortions in a Prototypical Zeolite Framework Containing Double Four‐Ring Cages: The Role of Framework Composition and Organic Guests, ChemPhysChem 2021, 22, 40–54
M. Fischer, L. Freymann
(See online at https://doi.org/10.1002/cphc.202000863) - Revisiting the structure of calcined and hydrated AlPO-11 with DFT-based molecular dynamics simulations, ChemPhysChem 2021, 22, 2063–2077
M. Fischer
(See online at https://doi.org/10.1002/cphc.202100486)
