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Title: Computational study of layered zeolite MFI and titanium-substituted zeolite LTA and their applications in catalysis
Author: Hernandez Tamargo, Carlos Ernesto
ISNI:       0000 0004 7226 2325
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2017
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In this thesis, we have examined the layered morphology of zeolite MFI and the titanium substitution of zeolite LTA by using a combination of density functional theory and interatomic potential-based calculations. Layered MFI zeolite allows a straightforward hierarchization of the pore system which accelerates mass transfer and increases its lifetime as a catalyst. We have performed a theoretical study of the structural features of the pure-silica and aluminium-substituted MFI nanosheets. We have analysed the effects of aluminium substitution on the vibrational properties of silanols as well as the features of protons as counter-ions. The formation of the two-dimensional system did not lead to appreciable distortions within the framework, whilst dehydration of aluminium-substituted silanols is both kinetic and thermodynamically favoured. In addition, we have analysed the strength of Brønsted acid (BA) sites located at the internal and external surfaces of zeolite MFI by adsorbing trimethylphosphine oxide (TMPO) as a probe molecule and correlating structural information to experimental 31P NMR data. We have been able to provide a possible explanation to the variable strength of the BA sites probed by TMPO by considering the basicity of the centres sharing the acid proton. In addition, we have examined the possible role of the Lewis acid sites located at the external surface of zeolite MFI to catalyse the tautomerization of phenolic-type compounds. The tautomerization has been conceived as a three-step process involving two protons transfers between the molecule and the zeolite, and the rotation of a dihedral angle. The energy barrier of each step is lower than 55 kJ/mol, suggesting that this transformation is easily accessible under standard reaction conditions. Finally, we have studied the structural, electronic and mechanical properties of the pure-silica zeolite LTA, as well as the single and double titanium-substituted material. The energetics of the titanium distribution within the zeolite framework suggest that the inclusion of a second titanium atom with configurations Ti-(Si)0-Ti, Ti-(Si)1-Ti and Ti-(Si)2-Ti is more favourable than the mono-substitution. The energetics of the dissociation of water on these Lewis acid sites indicate that this process is only favoured when two titanium atoms form a two-membered ring (2MR) sharing both hydroxy groups, Ti-(OH)2-Ti, showing that the presence of water may tune the distribution of titanium atoms within the framework of zeolite LTA.
Supervisor: De Leeuw, N. H. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available