Beyond tetrahedral coordination in zeolite-type materials - A computational approach

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Zeolites are a class of crystalline inorganic materials consisting of a three-dimensional framework of corner-sharing tetrahedra. By virtue of their intrinsic porosity, zeolites and related materials with zeolite-type topologies (zeotypes) find use in various large-scale applications, e.g. in gas and liquid separation, catalysis, and ion exchange. Ideal zeolites correspond to a perfect framework of tetrahedrally coordinated atoms (T atoms) linked by oxygen atoms. However, there are many examples of actual zeolite structures where some T atoms have a coordination number (CN) that is larger than 4 because additional non-bridging species are bonded to these sites.The present project explores such zeolite-type materials with 'higher-coordinated' T atoms by means of electronic structure calculations in the framework of dispersion-corrected density-functional theory (DFT). The focus will be on two groups of materials, namely (1) fluoride-containing all-silica zeolites and (2) hydrated aluminophosphates (AlPOs). In the former group, fluoride anions are covalently bonded to framework Si atoms, forming trigonal-bipyramidal SiO4F- units. In hydrated AlPOs, the coordination of water to framework Al atoms leads to the formation of five- or six-coordinated aluminium. While the existence of higher-coordinated T atoms is well-known for both groups, it remains largely unclear why their formation occurs preferentially at certain positions. To address this, DFT calculations will be employed to elucidate the crystal-chemical factors that determine which T atoms in a structure are most susceptible to assume a coordination number beyond 4. These structural investigations will be complemented by a DFT-based prediction of the vibrational properties to gain further insights into the host-guest interactions. Furthermore, these calculations will serve to identify fingerprint modes that indicate the presence of higher-coordinated T atoms in the vibrational spectra. For fluoride-containing all-silica zeolites, additional Molecular Dynamics calculations will be used to study fluoride anion disorder. Finally, the bonding and dynamics of fluoride in germanium-containing zeotypes will be investigated, as there are some conflicting observations regarding the existence of GeO4F- units in these systems.It is the primary aim of the project to further the understanding of zeolite-type systems with higher-coordinated T atoms on a fundamental level. Nevertheless, it can be anticipated that the findings will also have a certain relevance to applications. For example, new insights into the structure-directing properties of fluoride anions may aid the rational development of new synthesis routes, and a better atomic-level understanding of the framework-water interaction in hydrated AlPOs can help to explain the different degree of water stability of these materials, which is a crucial property for various applications.