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Persistent URL http://purl.org/net/epubs/work/45111282
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Record Id 45111282
Title Zeolites fit for a crown
Abstract Zeolite crystallisation involves the use of structure directing agents (SDAs), which stabilise the formation of specific framework structures. These are typically metal cations and organic additives, of which the latter is gaining pervasive use for the production of new zeolites. However, little is understood concerning the nature of the organic-framework interactions that direct structure during the crystallisation process. This project sought to gain a more coherent appreciation of the role of organic additives throughout crystallisation. This was achieved by investigating a specific case study - this being 18-crown-6 ether (18C6) which is capable of propagating the formation of the four different zeolites: Na-X, EMC-2, RHO and ZK-5. Moreover, questions arise as to how the additive is differentiating between which zeolite is crystallised. This research was carried out by taking three primary approaches. The first of these was the structural approach, whereby the location and orientation of the occluded additive was determined using high resolution X-ray diffraction and structural refinement. This was followed by the synthetic approach, in which the crystallisation field of the four zeolites were explored. Lastly, the dynamic approach was taken, whereby the organic-framework interactions between the occluded 18C6 and the zeolite host were probed. This was accomplished by studying the decomposition of the additive with thermogravimetry, the molecular vibrations with Inelastic Neutron Scattering (INS) spectroscopy and how the 18C6 impacts the dynamics of the zeolite framework with high pressure X-ray diffraction. Collaboratively, the approaches determined coherent distinctions in the relationship between 18C6 and the four zeolites. The crystallisation of zeolites Na-X and EMC-2 were observed to be intertwined, whereby the 18C6/Na2O ratio of the hydrogel was the primary defining factor in determining which crystallises. It was seen that the 18C6 behaves as a spacefilling species for zeolite Na-X, but expresses behaviour reminiscent of a true template in the assembly of zeolite EMC-2. Concerning zeolite RHO, the 18C6 is evaluated to be a structure directing agent that negates cocrystallisation of impurity phases. Finally, the 18C6 was seen to behave as a space-filling species that enhances the crystal growth rate of zeolite ZK-5. In all four cases, it is observed that 18C6 is only involved in the crystal growth stage of crystallisation. The initial accretion of structure during nucleation is instead determined by the metal cations present in the hydrogel.
Organisation ISIS , ISIS-TOSCA , STFC
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Language English (EN)
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Thesis PhD, University of Bath, 2019. https://researchp…esis_main_antony.pdf 2019