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Title: Gas separation exploiting molecular trapdoors in small pore zeolites
Author: Alexander, Thomas
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2019
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Zeolite RHO has recently been identified as a promising candidate for the separation of CO2 and CH4. This is due to the presence of extraframework cations in the eight membered rings (8MR) which occupy the spaces between cages and act as gatekeepers, selectively allowing the uptake of CO2 but restricting the uptake of CH4. The mechanism by which the cations move to allow the passage of CO2 molecules is not fully understood and computationally has only been studied at the quantum level. This does not allow the gating phenomenon to be observed directly and so the aim of this work is to use faster classical simulations to gain insight into the separation mechanism. Zeolite RHO is a particularly flexible zeolite and on gas loading undergoes both a phase transition and cell expansion. This makes these simulations particularly challenging to model correctly. One of the first stages in this work is therefore to ensure that the behaviour framework is reproduced adequately. Using an optimised set of force field parameters, two mechanisms are found for CO2 diffusion. The first occurs when a gate-keeping 8MR cation is pushed through a double eight ring (D8R) by a CO2 molecule and the second, less common mechanism, occurs when the D8R is completely unoccupied by a cation. The work focuses mainly on the diffusion of CO2 but other gases are also examined. Studies of the diffusion of noble gases through Na-RHO show that Xe and Kr are blocked by Na? cations, whilst Ar shows low diffusivity. He shows very high diffusivity due to its small size. The gas diffusion rates through RHO can be tuned by adjusting the Si/Al ratio as well as the choice of cation. For mixed Li/Na-RHO systems, increasing the Na? content increases CO2 equilibrium uptake but leads to a drop in diffusivity. Higher silicon content frameworks have larger limiting pore diameters, giving faster diffusion rates.
Supervisor: Duren, Tina ; Herdes Moreno, Carmelo ; Ting, Valeska Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available