The Effect of Framework Structure and Chemical Functionality on Melting in Zeolitic Imidazolate Frameworks
Interest in the amorphous phases of metal–organic frameworks (MOFs) has increased in recent years. Special consideration has been given to melt-quenched MOF glasses: the first new category of glass discovered in 50 years. Zeolitic imidazolate frameworks (ZIFs) are the most common MOF family that have been found to undergo melt-quenching. They are composed of tetrahedrally coordinated metal ions connected to imidazolate linkers. The dynamic nature of the melting mechanism in ZIFs has been demonstrated, with melting occurring via de-coordination and re-coordination of the imidazolate linkers at high temperatures. A wide variety of ZIF crystal structures have been reported to date. However, at present, the number of ZIFs that can undergo melt-quenching remains limited. This thesis aims to provide a better understanding of melting in ZIFs as well as the different factors that control the melting process so that, ultimately, novel melt-quenched MOF glasses can be prepared. Initially, four closely related ZIFs were studied to systematically investigate how linker chemistry and framework structure influence the melting process. Importantly, dense framework structures — specifically those displaying the cag network topology — were found to be crucial for melting. Moreover, their presence could initiate melting in more open framework structures. As dense frameworks were found to be essential for melting, the thermal behaviour of ZIFs displaying ultra-high framework densities, specifically those exhibiting the zni network topology, were investigated. Melting in these ZIFs was found to occur at higher temperatures than in cag topology systems. Furthermore, melting was found to be highly sensitive to chemical composition, with a 0.25% change in linker composition capable of eliciting a 7 °C change in melting temperature. We then demonstrate, for the first time, the possibility of further altering the chemistry in a ZIF glass by post-synthetic modification (PSM). A novel amine-functionalised ZIF glass was prepared that would be an ideal candidate for PSM. As a proof of concept, this amine-functionalised ZIF glass was reacted with octyl isocyanate, resulting in a urea-functionalised glass surface and a change in its surface wetting behaviour from hydrophilic to hydrophobic. Finally, we further expand the possible chemistries that can be incorporated in ZIF glasses by the inclusion of purine in a novel ZIF structure. The resulting glass forming ZIF was found to have one of the lowest melting temperatures reported for any ZIF. This represents a reduction in the melting temperature of over 250 °C compared to some of the early reports of ZIF melting. Evidently, judicious control of both linker chemistry and framework structure can be utilised to alter the thermal behaviour of ZIFs and to prepare novel melt-quenched glasses.
Chemistry, Materials science, Metal-organic frameworks, Glass
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