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Persistent URL http://purl.org/net/epubs/work/24506623
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Record Id 24506623
Title The structure-property relations of zeolitic imidazolate framework 7 for carbon dioxide capture
Abstract CO2 capture is not only a promising solution for global warming but also an important component of many gas separation and purification processes. Metal-organic frameworks (MOFs) are competitive candidates for efficient CO2 capture because of their large capacity and adjustable affinity. This dissertation studies a specific MOF, zeolitic imidazolate framework 7 (ZIF-7), for the relationship between its stimuli-responsive structure and distinctive CO2 capture properties under varying conditions. The phase transition observed in ZIF-7 upon CO2 adsorption has been investigated. The crystal structure of the unsaturated phase, ZIF-7-II, has been determined and compared with the saturated ZIF-7(-I) structure. The “gate-opening” behaviour of ZIF-7 primary guest-hosting pore (pore A) for CO2 adsorption is visualised for the first time. An irreversible phase transition from ZIF-7-I or ZIF-7-II framework to a dense layer structure, ZIF-7-III, is observed when water is present. The crystal structures and dynamic behaviours of ZIF-7 (CO2) system have been studied by diffraction and spectroscopic analysis. CO2 is found to have an adsorption preference for a geometrically-open pore in ZIF-7 (pore B) over the primary guest-hosting pore. This preference is responsible for the fast CO2 transport diffusion in the framework. The rotation of bIm ligands in the geometrically-open pore gives rise to the cooperative CO2 adsorption in the primary guest-hosting pore, which is reflected by the step-shape of the adsorption isotherm. The rotation of bIm linkers also induces the ZIF-7-II to ZIF-7-I phase transition upon increasing CO2 pressure. The vibrational modes of bIm ligands have been indicated to have a strong influence on the CO2 accessibility of the primary guest-hosting pore. The pressure-dependent structural behaviour of ZIF-7 is described in the final chapter. ZIF-7 is shown to be able to withstand high hydrostatic pressure whilst retaining its porosity and structural integrity. Its structural response to pressure is also influenced by bIm rotation and includes a displacive structural transition to a distorted but stable and topologically invariant high-pressure phase, ZIF-7-IV. Additional efforts have been made to understand ZIFs in a broader context. The adsorption of CH4 and H2 in ZIF-7 are briefly discussed.
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Language English (EN)
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Thesis PhD, University of Cambridge, 2015. http://eprints.esc.cam.ac.uk/3430/ 2015