Department of Chemistry and Polymer Science

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Browsing Department of Chemistry and Polymer Science by browse.metadata.advisor "Barbour, Leonard James"

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    Expansion of a porous organic molecular material induced by gas pressure
    (Stellenbosch : Stellenbosch University, 2024-03) Mathada, Gundo; Barbour, Leonard James; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The interplay between external stimuli and the structural properties of crystalline materials holds immerse potential for the design of novel functional materials. In this study, the intriguing phenomenon of reversible elongation of an organic crystal in response to gas pressure (carbon dioxide and ethylene) was investigated. Through a comprehensive experimental approach combining series in situ crystallography, gas sorption isotherm measurements, and photomicroscopy, we elucidate the molecular-level mechanisms underlying this behaviour. The findings of this study reveal that gas molecules infiltrate the intricate 0D cavities within the crystal lattice, prompting subtle adjustments in molecular arrangement to accommodate the guest species. The molecular rearrangement (with no phase changes), manifested as an elongation of the crystallographic c axis, is observed across varying gas pressures and molecular environments. Notably, the empirical modelling of gas-induced elongation using the Sips equation provides valuable insights into the relationship between gas pressure and crystal expansion. The in-house instruments were utilized throughout this study to investigate the observed phenomena in the crystals
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    Guest inclusion in porous metal-organic crystals and high-pressure single-crystal X-ray diffraction analysis at low temperatures
    (Stellenbosch : Stellenbosch University, 2024-03) Kutama, Aluwani; Barbour, Leonard James; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Porous metal-organic materials are an intriguing class of compounds that are capable of adsorbing guest molecules, such as gases, into their cavities. This ability of these materials has stimulated research across various domains, including purification, gas storage, separation, and drug delivery. In this study, we analyzed two well-known porous metal-organic compounds namely, [Zn2(L1)(OBA)2] and [Cu2(L2)2(Cl)4]. The metal-organic framework (MOF), [Zn2(L1)(OBA)2], was successfully synthesized and fully characterized in preparation for gas sorption studies. This characterization included single crystal Xray diffraction (SCXRD), thermogravimetric analysis (TGA), and Fourier-transform infrared (FT-IR) spectroscopy. Gas sorption studies were thereafter carried out using volumetric sorption analysis and pressure-ramped differential scanning calorimetry (P-DSC) up to 50 bar using CO2 and C2H4. The sorption profiles with CO2 indicated the presence of shape memory, while the sorption profiles with C2H4 were indicative of Type 1 isotherms. In addition, the metallocycle [Cu2(L2)2(Cl)4], was synthesized using a layering methodology that required the use of three solvents (namely EtOAc, DMF and EtOH), and further studies revealed that EtOAc was the solvent that was included in the crystal structure and hence responsible for the structural channel formation. The characterization of [Cu2(L2)2(Cl)4] involved using powder X-ray diffraction, SCXRD, TGA, FT-IR, and thereafter the activated crystals were pressurized up to 20 bar with CO2 and characterized using, gravimetric sorption analysis and P-DSC. In this work a novel method for obtaining high-pressure SCXRD data at low temperatures is also described for the first time and utilized primarily to reduce the thermal motion of the included guest molecules for structural modelling purposes. This was carried out with CO2 (20 bar) and Xe (10 bar). The results revealed acceptable molecular geometrical parameters for the included CO2 guest molecule, and that the cavity of [Cu2(L2)2(Cl)4] increased in volume to accommodate the large Xe molecule, indicating the flexible nature of this metallocycle.
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    Investigating mechanical responses to structural changes in crystalline materials
    (Stellenbosch : Stellenbosch University, 2023-11) van Rijn, Raymond Michael; Barbour, Leonard James; Loots, Leigh-Anne; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Previous work has established that the compound 2,7-dimethylocta-3,5-diyne-2,7-diol crystallizes to produce a range of inclusion compounds – the crystals of which exhibit elastic flexibility. In an attempt to tailor the Young’s modulus of the system, substitution of guest species, using both polar and apolar compounds, was carried out. Ultimately, little difference in elasticity was observed, regardless of the included guest species. Analysis of crystal structures revealed that little interaction takes place between guest molecules, or between host and guest. Thus, hydrogen bonding in the host framework is concluded to be the determining factor in crystal flexibility. Several other elastically flexible crystals were subsequently investigated to determine how their elastic moduli would change when temperature was varied. A relationship was identified between the change in elasticity and thermal expansion of the crystals. As the bending axis of a crystal expands in length, the Young’s modulus decreases. Thus, for crystals exhibiting positive thermal expansion, elasticity is reduced as temperature is decreased. Conversely, for a material displaying negative thermal expansion, decrease in temperature was found to produce an increase in elasticity. Greater intermolecular spacing likely allows for a greater degree of molecular reorientation that must occur to facilitate mechanical bending of crystals. Two of the compounds subjected to variable-temperature flexibility studies, Pd(acac)2 and Cu(acac)2, are isostructural and isomorphous, yet exhibit opposite thermal expansion characteristics along their crystallographic b axes. As such, they were identified as promising candidates to form solid solutions, whose thermal expansion behaviours could be tuned. A mixed crystal displaying near-zero thermal expansion was successfully produced, demonstrating the applicability of solid solutions in tailoring the thermal properties of molecular compounds. A reaction between 2,7-dimethylocta-3,5-diyne-2,7-diol and iodine was observed to produce a novel cumulene-type compound in high yield. It was established that light is required for the reaction to proceed, and can also be used to isomerize the cumulene in a cis to trans manner. Several other diyne compounds were found to react in analogous ways under the novel reaction conditions, provided they featured hydroxyl functional groups. Thus, it is proposed that formation of hydrogen-bonded adducts is responsible for halting the halogenation reaction upon formation of a cumulene.
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    Solid-liquid separation of xylene isomers using metallocycles
    (Stellenbosch : Stellenbosch University, 2022-04) Ye, Jiajia; Barbour, Leonard James; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Host-guest chemistry is an important concept in supramolecular chemistry. Porous hosts may be able to selectively accommodate guests, making them attractive candidates for potential applications such as sorption, storage, separation, drug delivery, etc. In this study, two porous materials were investigated for their solid-liquid xylene separation ability. These materials are the metallocycles MC1 and MC2. Xylenes are an important chemical feedstock to produce consumer products. However, they are always produced as a mixture of isomers and thus their separation is essential. The first material investigated was MC1·2(MeOH). Its phase purity could not be established directly by comparison of the simulated and experimental powder patterns. This is because the MeOH guest readily escapes from the host at ambient temperatures. However, the phase purity can be confirmed after the material is fully activated (MC1a). Solvent exchanges of the as-synthesized MC1·2(MeOH) with xylenes, as well as liquid sorption of MC1a with xylenes were attempted. Both investigations showed that MC1 is not a suitable material for xylene separation. The host structure remained unchanged after exposure to xylene. Further investigation revealed that the kinetic diameters of the xylenes are too large for these molecules to fit in the host cavity without a phase transformation. The second material explored was MC2·2(DMSO). It can undergo multiple single- crystal to single-crystal transformations upon guest exchange and removal and exhibits solvatochromism. The activation of MC2 by different methods results in two possible activated forms (open form 3 and collapsed form 4). In this study, the main obstacle was to obtain phase pure sample of either activated form. The problem was resolved by trialing variations of temperature and time. Vapor and liquid sorption of xylenes were carried out using form 3. Form 3 is able to separate xylenes in the order of preference of para-xylene >> meta-xylene > ortho-xylene under mild conditions. The pockets within form 3 merge and become channels to accommodate the xylenes, as established by single-crystal X-ray diffraction analysis. The selectivity trend was rationalized as being dependent on the kinetic diameter of the respective xylene isomers. The non-porous form 4 is not suitable for xylene separations even at high temperature. Thus, as the phase purity problem has been solved, porous form 3 with its ability to change pore sizes, is a worthy substrate for xylene purification, and this also has implications for future studies involving gas sorption and storage.