Duration: 05/2018 to 05/2021
Project description:
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.
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