Browsing by Author "Ye, Jiajia"

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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.