Metal-Organic Frameworks have received considerable attention in recent years because they provide extremely high specific surface areas exceeding traditional adsorbents such as zeolites and activated carbon. Especially mesoporous MOFs provide a wide range of options for further functionalization and even chiral groups can be incorporated rendering such materials as potential candidates for gas storage, enantioselective separation, catalysis and sensing. In recent years, exciting developments have pushed the limits of materials performance to ever higher surface areas up to 7000 m2/g and pore sizes unattained so far in traditional porous solids such as zeolites or activated carbons. DUT-49 (DUT = Dresden University of Technology) is a mesoporous flexible MOF composed of connected Metal-Organic Polyhedra (MOPs) and has the highest gravimetric methane uptake among all known MOFs (308 mg/g at 298 K). Chiral mesoporous MOFs were synthesized. A unique phenomenon observed only in a limited number of materials is porosity switching in the crystalline solid state. Such flexibility was predicted 1998 for MOFs by Kitagawa and later termed “3rd Generation MOFs”. Despite these early discoveries, among the about 20.000 coordination network structures only few compounds reveal substantial switching or breathing transitions or related stimuli responsive properties. In order to characterize the adsorbate-induced structural transformations, it is necessary to capture local and global structural information under variable, externally applied gas pressures in situ. The development of such in situ characterization techniques is essential (EXAFS, XRD, NMR, EPR) to monitor switching during adsorption/desorption cycling. A new phenomenon recently encountered by in situ methods is Negative Gas Adsorption (NGA), for the first time detected in a switchable (breathing) mesoporous MOF named DUT-49. While the mechanism could be explained by the aid of theoretical DFT and GCMC calculations, yet the underlying principles to predict such a phenomenon in other MOFs are unknown. For technological applications this effect could have wide implications as it represents a new counterintuitive phenomenon that could be used for pressure amplification.
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