Exploration of Al-based metal-organic frameworks with multiple linkers and functionalities - microscopic structuring and proton conduction

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Metal-organic frameworks represent a versatile class of porous materials, whose properties can be controlled by varying the framework topology, the organic linker molecules and by introducing guest molecules. Aluminium, which combines low weight, easy availability and low toxicity together with a high thermal and chemical stability of the Al-MOFs, is one of the most appealing ions to build such materials. Its tendency to lead to microcrystalline materials, however, often hamper the determination of structure-property relationships. This project aims at exploring Al-based MOFs based on polycarboxylate linkers, their functionalised derivatives and mixtures thereof in both aqueous and non-aqueous synthesis conditions. In particular, by varying number and arrangement of carboxylic acid groups, we expect to encourage the formation of MOFs with new inorganic building units and frameworks. Special emphasis will be placed on introducing free, acidic proton bearing groups to create materials potentially prone to proton conduction. The incorporation of amphoteric guest molecules shall help to establish efficient proton conduction pathways. To develop a better understanding for the complex interplay between the proton donor density, the incorporated guest molecules and the influence of the particle size on the proton mobility and transport, dynamical processes of framework, guests and the protons themselves will be studied systematically within this project.To accomplish this, we brought together the research groups of N. Stock and J. Senker, adding their complementary skills in synthesis and characterisation of structural and dynamical properties of Al-MOFs. For a thorough and efficient synthetic screening of new MOF topologies, the Stock group will develop two high-throughput reactors allowing to work with temperature gradients and to carry out vapour phase crystallisation reactions. For selected systems, synthetic strategies towards Al-MOF nanoparticles with controlled particle sizes will be established. For the structure elucidation, including order parameters in isomorphous MOFs and host-guest connectivities, the Senker group will rely on NMR-crystallographic approaches combining solid-state NMR spectroscopy and powder diffraction. Furthermore, solid-state NMR spectroscopy will play an essential role for analysing local dynamical processes of linkers, guests and acidic protons over a broad temperature and frequency range. To discriminate between proton mobility at the outer vs. the inner particle surfaces of the MOF nanoparticles, hyperpolarisation techniques will be used. The NMR data shall be supported by temperature dependent impedance spectroscopic investigations which will be carried out in close cooperation with Profs. Janek, Papastavrou and Wark. For most materials we will rely on the behaviour of the DC conductivity and complement these data with simulations of the AC conductivity based on equivalent circuits for selected systems.