Computational studies of the structures and properties of microporous materials

Stacey, Edward 2024. Computational studies of the structures and properties of microporous materials. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis applies a range of computational modelling methodologies to different problems in the science of microporous materials. We present an assessment of different interatomic potential methodologies for the modelling of pure silica zeolites and consider the extent charge transfer plays in accurate determination of lattice energies. The assessment of these parameters enabled the selection of a model that is compatible with Quantum Mechanical methods to provide a suitably accurate methodology for modelling the molecular mechanical (MM) region in hybrid quantum mechanical molecular mechanical (QM/MM) calculations. These same silica structures are also modelled using quantum mechanical methods based on Density Functional Theory to calculate zeolite cohesive energies. The results of both the lattice and cohesive energies are compared with published thermochemical data. Zeolite energies were produced with a high level of accuracy using DFT when compared to experimental values, with energy values from interatomic potential methods reproducing the same trends across a large variety of microporous structures. The formal charge shell model is determined to be superior to the partial charge rigid ion model due to the inherent modelling of polarisability which prevents the erroneous calculation of linear bond angels sometimes present in the rigid ion model. Furthermore, the same approach is then applied to study microporous alumino-phosphates (AlPOs) to assess the ability of the different methodologies to reproduce energies in comparison to experimental data. The calculation of these AlPOs was shown to be more reliable with the shell model approach than with DFT. We then focus on the zeolite ZSM-5 where we use hybrid Quantum Mechanical Molecular Mechanical (QM/MM) methods, to model the properties of cation exchanged systems in the context of their use in oxidation catalysis. Molecular oxygen is introduced to determine whether these cations form either an activated oxo species or an activated super-oxo species. We demonstrate that Fe(II), Mn(II), Mn(III), Cu(I), Zn(I), Ti(II), Ti(III), Mo(I) and Mo(II) systems could activate molecular oxygen and form a super-oxide species upon coordination to the metal cation as indicated by the oxygen bond length extending to 1.3 Å. 7 Furthermore, both Au(I), Cu(I) structural types yield positive binding energies along with b-type Na(I) and Zn(II) structures, indicating oxygen is unable to bind to these cationic centres. The abstraction of a hydrogen atom by these oxo/super-oxo species is subsequently modelled in order to predict a viable catalyst for the activation of methane via a novel reaction pathway. Mo(I) cations are shown to break the oxygen-oxygen bond of the coordinating molecule to form a hydroxyl and thus is unlikely to act as a catalyst. Using a valence bond approach we demonstrate that the [Zn(I)-ZSM-5]-super-oxo complex has a sufficiently low Δacid suggesting it would be a viable alternative for the catalytic activation of methane, with a low bond dissociation energy that is comparable to the currently used μ-nitrido-bridged diiron-oxo porphyrin. Whilst Cu(I) and Na(I) form super-oxo species with similar basicities to the natural porphyrin alternative, they cannot bind molecular oxygen as demonstrated by their positive binding energies. The final part of this thesis aims to model comprehensively the behaviour of water within the pores of two different ZSM-5 structures at different temperatures. Both H-ZSM-5 and Na-ZSM-5 systems are simulated with loadings of 1-3 water molecules at two temperatures (298 K and 400 K) for a simulated timeframe of 10 ps using Ab-Initio Molecular Dynamics (AIMD) techniques. Water clusters of significance from the AIMD are modelled using hybrid Quantum Mechanical Molecular Mechanical (QM/MM) methods and their corresponding vibrational spectra are calculated using the Generalised Vibrational Perturbation Theory (GVPT2). Water clustering behaviour is analysed and compared between the H-ZSM-5 and Na-ZSM-5 systems to elucidate the effect the cation has on the anharmonicity of water clusters, and how the two temperatures influence the dynamics of the water clusters. The general trends from the resultant spectra show similar dynamics with a loading of one water molecule with it coordinating to either cation at both temperatures illustrating no anharmonic cross-coupling behaviour. At a loading of two water molecules, we reveal a difference in clustering dynamics between the two cationic systems, with hydrogen bonding present in the H-ZSM-5 system resulting in significant shifts of the asymmetric vibrations. Hydrogen bonding was not present in Na-ZSM-5 system between the water molecules, but a coordinating water’s hydrogens can potentially interact with oxygens in the zeolite’s pore wall causing a slight shift to the observed vibrational frequencies. Finally at loadings of three water 8 molecules we demonstrate a propensity for the H-ZSM-5 system to form hydroxonium ions, with a greater frequency of formation at the higher temperatures with significant shifting behaviour of the anharmonic vibrations. Na-ZSM-5 is able to form hydrogen bonds between water molecules but do not form hydroxonium ions, instead forming weak clusters involving only two of the waters as the large cation size prevents formation of larger stabilising water clusters. The large Na cation exerts a minor influence over the cluster that can shift the frequencies down-field. The key results are highlighted which will form the foundation of future work analysing experimental 2D-IR spectra. The thesis highlights the role of a range of computational techniques in probing different aspects of the properties of microporous materials, and provides new insights into our understanding of structure, stability, adsorption, and reactivity in these widely investigated materials.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Date of First Compliant Deposit:	24 January 2025
Last Modified:	24 Jan 2025 17:00
URI:	https://orca.cardiff.ac.uk/id/eprint/175571

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