Mechanochemical approaches to MOF glasses and the thermal response of amorphous MOFs

Abstract

This thesis focuses on the newly emerging class of materials known as metal–organic framework (MOF) glasses. These materials represent the fourth category of glassy materials alongside inorganic, organic and metallic glasses, and are the first new class of glassy materials discovered since the 1970s. Melting a crystalline MOF isomorphic to the zeolitic imidazolate framework (ZIF) known as ZIF-62, (cag topology, consisting of M2+ and imidazolate linkers) results in the formation of a ZIF liquid. Cooling of this liquid leads to vitrification of the ZIF, i.e. production of an amorphous ZIF, or more specifically ZIF glass. Here we demonstrate how mechanosynthesis, i.e. solid state grinding, can be employed to not only scale up the production of glass forming crystalline ZIFs, but also to produce ZIFs with specific chemical composition. This leads us to the production of a wide variety of ZIF glass formulations, with varying metal and organic com positions. We then exploit the scale-up and chemical specificity possible with mechanosynthesis to investigate bulk physical properties of these new glassy materials. From this, correlations between chemical composition and thermal, surface and mechanical properties can be drawn. Further to the large scale production of ZIF glasses, we also investigate how mechanochemistry can be used to directly form amorphous ZIFs which exhibit glassy behaviour, without undergoing a melt-quenching process. This not only allows for the by-passing of the melt-quenching process, but also allows for the expansion of the glass forming ZIF chemical space. We study the local structure and properties of mechanochemically produced and melt-quenched ZIF glasses, showing the similarities of materials produced by both routes. Beyond the work on glass forming ZIFs, we investigate the thermal response of mechanochemically amorphised ZIFs. Specifically we study ZIFs that do not undergo melting in the crystalline state. This results in the formation of a crystalline polymorph of the prototypical ZIF known as ZIF-8, from the mechanochemically amorphised phase. We solve the structure of this polymorph and investigate its porosity and thermal behaviour. The main points highlighted in this thesis include the importance of conducting thorough thermal analysis on ZIFs, and employing appropriate supplementary techniques to identify phase changes upon heating. Also we highlight the importance and validity of the amorphous phase of ZIFs, and the importance of investigating the thermal behaviours of amorphous ZIFs. Both of these aspects are rarely investigated or considered in MOF literature, even though there is a rich and unexplored landscape of materials and phase changes to investigate.