Browsing by Author "Basson, Jeanice Letitia"

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    Investigation of the carbon dioxide sorption properties of selected organic macrocycles
    (Stellenbosch : Stellenbosch University, 2018-03) Basson, Jeanice Letitia; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The inclusion of guest molecules within host compounds has been an ever-growing interest in supramolecular chemistry. Host-guest chemistry has attracted much attention due to potential applications in drug delivery, catalysis and the separation and storage of natural gases. As a result, a large variety of porous systems such as porous coordination polymers (PCPs), porous molecular systems, covalent organic frameworks (COFs) and supramolecular organic frameworks (SOFs) have been developed. In this study, three well-known organic macrocycles, namely p-tert-butylcalix[4]arene (TBC4), cucurbit[6]uril (CB[6]) and cucurbit[8]uril (CB[8]) were investigated as hosts for gaseous carbon dioxide (CO2). The aim of this study was to gain insight into the properties of these hosts and gain a better understanding of the inclusion of CO2 in these materials. The first section discusses TBC4 as a host for CO2. The guest-free low density polymorph of TBC4 contains discrete pockets (zero-dimensional porosity) and captures CO2 in a stepwise fashion. The system undergoes a gas-induced single-crystal to single-crystal (SC-SC) transformation to a more porous phase with an increased CO2 loading capacity. The CO2 inclusion compounds were investigated in-situ using single-crystal X-ray diffraction (SCXRD). The gas-induced phase transformation was further investigated using variable-pressure powder X-ray diffraction (VP-PXRD) and pressure-ramped differential scanning calorimetry (P-DSC). Interestingly, this porous phase is preserved when the CO2 molecules are removed. However, the original guest-free phase can be regenerated under mild activation conditions. The study was extended by investigating CO2 uptake by the high density polymorph of TBC4. Despite being a close-packed structure, CO2 molecules are able to diffuse through the host to instigate a gas-induced transformation at high pressure, and a possible mechanism is discussed. The second section describes the porosity of the well-known host, CB[6]. This host undergoes various phase transformations in order to produce a framework with permanent one-dimensional porosity. The various phases were subjected to thermal and structural analysis, where possible. This host, like most molecular organic hosts, produces a more close-packed phase when activated, and it expands upon CO2 loading. The CB[6] host framework displays a large affinity for CO2 and does not undergo a structural change at high CO2 pressure. In-situ SCXRD was used to investigate the CO2 inclusion compounds at various gas pressures to study the host-guest interactions. The final section discusses CB[8] as a CO2 adsorbent. Due to the high CO2 affinity demonstrated by CB[6], CB[8] was studied as a solid-state host. This host undergoes a phase transformation during desolvation to produce a more stable polycrystalline phase of CB[8]. Although the host could not be characterized structurally, it was evident from the CO2 sorption analysis that the host framework is porous. CB[8] displays stepwise uptake and release of CO2 molecules and has a larger affinity for CO2 as compared to methane and nitrogen gas. VP-PXRD was used to investigate the dynamic nature of the host with respect to CO2 uptake and release. Several attempts at preserving the single crystallinity of CB[8] during desolvation are also discussed.