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The Location and Effect of Aluminium in Zeolites

Fletcher, Rachel E.; (2019) The Location and Effect of Aluminium in Zeolites. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

A systematic DFT study has been carried out to investigate the thermodynamic driving force of Al distribution in zeolites and the effect of Al position on catalytic reactions. Much of this study is concerned with small-pore zeolite SSZ-13, which exhibits a CHA-type framework, although Al distribution is also examined in a range of other zeolite framework-types, including RHO, LTA, ABW, AEI and MOR. Al distribution across framework T-sites was examined in zeolite frameworks at different Si/Al ratios, where Al is compensated by different counter-cation species, including alkali metal cations and protons. The most thermodynamically favourable Al distribution in protonated zeolites was found to be that which violates Löwenstein’s rule of Al avoidance. On the contrary, alkali metal-containing frameworks favour Löwenstein Al arrangements, demonstrating the influence of cation identity on Al distribution in zeolites. On investigation of the effect of the organic template on these distributions, it was found that whilst the template has some Al directing ability, in the presence of Na+ and H+ counter-cations the influence of the SDA on Al arrangement is greatly reduced. The diminished influence of the SDA over Al distribution compared to the Na+ and H+ counter-cations is primarily due to sterics and the decreased charge/size ratio of the SDA. Furthermore, introducing water to sodium-containing frameworks screens the charge of the cation, causing Dempsey ordered Al distributions, driven by Al-Al repulsions, to become more thermally accessible in SSZ-13. To examine the distribution of Al throughout the zeolite crystal, SSZ-13 slab structures with (001) and (011) terminating faces were examined. Surface enrichment of Al was found to be favourable in H-SSZ-13 slab structures, whilst well distributed Al was favoured in Na-SSZ-13 structures. Finally, the effect of Al on Mo speciation during methane dehydroaromatisation was examined in Mo/MFI catalysts. The evolution of the catalytically active Mo species was investigated in both silicalite and ZSM-5, these simulations were informed by experiment. Improved binding between the MFI framework and the Mo species was observed in ZSM-5, indicating that the presence of Al is necessary to prevent catalyst deactivation by migration of the active species to the surface of the zeolite.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The Location and Effect of Aluminium in Zeolites
Event: UCL (University College London)
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2019. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Chemistry
URI: https://discovery.ucl.ac.uk/id/eprint/10066394
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