Doctoral Degrees (Chemistry and Polymer Science)

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Browsing Doctoral Degrees (Chemistry and Polymer Science) by Author "Aggarwal, Himanshu"

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    Switching of degree of interpenetration and its effects on porosity of metal-Organic Frameworks (MOFs)
    (2015-12) Aggarwal, Himanshu; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science
    ENGLISH ABSTRACT: of solvent molecules from the channels. The primary objective of this work was to reinvestigate these systems in order to gain an understanding of the reasons behind loss of porosity in otherwise seemingly highly porous frameworks. The work provides a detailed account of switching of degree of interpenetration and its effects on porosity of MOFs. The first section describes a well-known doubly-interpenetrated framework, [Zn2(ndc)2(bpy)] (ndc = 2,6-naphthalenedicarboxylate and bpy = 4,4′-dipyridyl) which has been studied for loss of porosity upon activation. The zinc-based pillared-layered structure possesses minimal porosity when activated and the framework was thought to collapse upon desolvation, leading to unexpected sorption results. In the present study, it is shown that the structure does not collapse, but converts to its triply-interpenetrated analogue upon desolvation and the transformation occurs in a single-crystal to single-crystal manner under ambient conditions. A mechanism has also been proposed for the conversion and is supported by computational methods. In the second section, the work has been further extended to more robust and entirely different systems. Two known Cd(II) non-interpenetrated doubly-pillared MOFs, [Cd(tp)(4,4′-bpy)] (tp = terphthalate) and [Cd(atp)(4,4′-bpy)] (atp = 2-aminoterephthalate), have been studied for switching of degree of interpenetration. Both of these systems have been reported to form non-interpenetrated as well as doubly-interpenetrated structures. However, the possibility of inter-conversion has not been suggested. In the present study, these MOFs are shown to undergo a change in degree of interpenetration upon loss of solvent molecules from the channels. The transformation in these cases takes place at much higher temperatures as compared to the [Zn2(ndc)2(bpy)] case. In the final section of this thesis, the effect of switching of degree of interpenetration on the porosity of MOFs is demonstrated using a previously reported system, [Co2(ndc)2(bpy)], where an intermediate structure has been successfully isolated by modifying the activation conditions. This framework has also been reported to lose porosity upon desolvation. It has been found that the doubly-interpenetrated structure converts to its triply-interpenetrated form when activated at 120 °C, whereas the same material converts to an intermediate empty doubly-interpenetrated structure when activated under milder conditions. Sorption analysis using the intermediate twofold structure and the converted threefold structure shows a clear difference in the porosities of the two forms. Interestingly, all the transformations occur in single-crystal to single-crystal fashion.