Adsorption selectivity studies of flexible metal-organic-frameworks by in situ and solid-state NMR spectroscopy

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The present project aims to further develop the fundamental understanding of the pronounced structural changes (switching) observed for special, flexible metal-organic frameworks (MOFs) under external stimuli (QP1). The planned investigations on flexible MOFs are part of QP2 within this DFG Research Unit. The main focus of the project in the 2nd funding period will be the investigation of flexible MOFs with respect to their adsorption selectivity from mixtures of gases as well as liquids (QP4). The results of these investigations will help exploring specific functions of flexible MOF model systems, e.g., for separation processes (QF1). Our studies will be performed on MOF samples synthesized within sub-projects S1 and S2. Furthermore, we aim to characterize catalysts based on flexible MOFs with integrated catalytic sites (QF2). Together with the results of in situ X-ray diffraction (S2) and in situ EPR spectroscopy (P2), the planned studies are likely to deliver a deeper and more comprehensive fundamental understanding of the switching process and its influence upon the adsorption selectivity. The project will substantially profit from the collaboration with theoretical groups (T1,T2). Investigations will include on in situ and solid-state NMR spectroscopy. In situ NMR spectroscopic adsorption/desorption studies will be performed using gas mixtures like 13CO2/13CH4, 13CO2/13CO, C3H8/C3H6 (13C and/or 2H labeled) and 129Xe/83Kr. Liquid-state adsorption – including liquids mixtures - on flexible MOFs will be investigated by solid-state NMR spectroscopy. Advanced solid-state NMR techniques will be applied in order to characterize the MOF lattice and its structural changes, e.g., by 13C MAS NMR, 1H-13C HETCOR (heteronuclear correlation) and other experiments. The state of the adsorbed molecules as well as their interactions with the MOF lattice will be studied by experiments like 2H NMR spectroscopy, cross-polarization experiments as well as distance-sensitive techniques such as REDOR (rotational echo double resonance) etc. The present research project thus aims to address the following questions:1) Does framework flexibility in general influence the adsorption selectivity? Answering this question will help designing highly selective MOFs with potential applications for separation processes, selective catalysis, and others.2) Which molecular interactions between flexible MOFs and adsorbed molecules are driving the structural transitions and does framework flexibility influence the state of the adsorbed molecules? These studies aim to understand the influence of the type of gases or liquids in mixtures upon the structural transitions and will also contribute to characterize specific interactions of these molecules with the MOF lattice.