Towards processable metal-organic frameworks

Abstract

In this work, porous metal-organic framework (MOF) materials were investigated towards processability. Processability of porous materials is an advantageous ability for industrial applications, where it is often mandatory to form materials into certain shapes, such as pellets or thin films. Materials can be formed if they are in a liquid/melted state and their films are accessible e.g. during solution-processing (i.e. ’liquid-casting’). Melting of MOFs was investigated for the formation of porous MOF glasses, which are currently an emerging field in MOF chemistry. Fundamentals of the melting behaviour of MOFs were systematically investigated by deriving relations between the inorganic building units (metal ions) and organic building units (linkers) of melting MOFs, their structural features and their thermal properties (e.g. melting temperature and glass transition temperature). Most importantly, it was demonstrated that the melting point of a melting mixed-linker MOF system (i.e. MOFs containing two different types of linkers) can be systematically controlled by adjusting the linker ratio. These in-depth studies also allow to derive structural and compositional requirements for MOF melting, which set the stage for the targeted search and discovery of new meltable MOFs. Further, it was shown that porosity appears to be an intrinsic feature of MOF glasses based on imidazolate linkers, a feature, which was so far unknown. Adjusting their compositions allows even to vary the porosity features (i.e. pore sizes and volumes) and thus tune their gas sorption properties. Solution-processability of MOFs was realized by the development of a new class of water-processable MOFs, termed amphiphile salt frameworks (ASFs). ASFs are based on systematically designed amphiphilic molecules linked by kinetically labile sodium ions, forming honeycomb-like structures. These well-defined framework materials reversibly assemble from an aqueous solution by evaporation of water and disassemble again by dissolution in water. Remarkably, ASFs are shown to be porous towards sorption of several gases (N2, CO2, propane, propylene, and n-butane) and also feature high thermal and mechanical stability. In summary, the results of this thesis provide new insights towards processable MOFs addressed by two different approaches (i.e. melting and solution-processability). Overall, this aids for a better understanding of how the macrostructure of MOF materials can be influenced and adjusted in the future.