Recently, we have described the metal-organic framework Ni(2)(2,6-ndc)(2)(dabco), denoted as DUT-8(Ni) (1) (DUT = Dresden University of Technology, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). Upon adsorption of molecules such as nitrogen and xenon, this material exhibits a pronounced gate-pressure effect which is accompanied by a large change of the specific volume. Here, we describe the use of high-pressure in situ (129)Xe NMR spectroscopy, i.e., the NMR spectroscopic measurements of xenon adsorption/desorption isotherms and isobars, to characterize this effect. It appears that the pore system of DUT-8(Ni) takes up xenon until a liquid-like state is reached. Deeper insight into the interactions between the host DUT-8(Ni) and the guest atom xenon is gained from ab initio molecular dynamics (MD) simulations. van der Waals interactions are included for the first time in these calculations on a metal-organic framework compound. MD simulations allow the identification of preferred adsorption sites for xenon as well as insight into the breathing effect at a molecular scale. Grand canonical Monte Carlo (GCMC) simulations have been performed in order to simulate adsorption isotherms. Furthermore, the favorable influence of a sample pretreatment using solvent exchange and drying with supercritical CO(2) as well as the influence of repeated pore opening/closure processes, i.e., the "aging behavior" of the compound, can be visualized by (129)Xe NMR spectroscopy.

The primary adsorption sites in two representative zeolite imidazolate frameworks (ZIF), ZIF-8 and ZIF-11, have been identified by molecular dynamics (MD) techniques. MD simulations reveal two symmetry-independent adsorption sites in ZIF-8. The first adsorption site is located above the imidazolate ring, in proximity of the CC bond. The second one is in the pore channel. In ZIF-11, an additional adsorption site located on top of the benzene ring is identified.

The outstanding ability of carbon to form a variety of structures (graphite, diamond, lonsdaleite, fullerenes, nanotubes etc .) with quite different physical properties stimulated a lot of work on carbon-based materials in the last decades. [ 1 ] To mention a few actively developing fi elds, we note the search for superhard materials, materials with valuable transport properties, and hydrogen storage media. [ 2 ] Many hypothetical as well as experimentally characterized carbon materials with promising mechanical, electronic or transport properties are known. [ 2 ] On the contrary, carbon-based materials proposed so far as hydrogen storage media [3][4][5][6] (graphite intercalated with fullerenes, carbon foams, nanotube bundles, etc.) show quite moderate performance. [ 7a ] Hydrogen uptake of such materials [ 6 , 7b ] usually amounts up to ∼ 3.0-7.0 wt.% at 77 K which is quite far from the target value set by the US Department of Energy (6 wt.% at nearly ambient conditions) and makes carbon materials poor candidates for hydrogen storage applications. As a result, nowadays the research focus is shifted to other materials such as metal-organic frameworks (MOFs) or covalent-organic frameworks (COFs) which allow for much higher storage capacities and are capable of easy chemical functionalization. [ 8 ] In this communication we show that the potential of carbon materials is far from being exhausted and propose for the fi rst time energetically and mechanically stable packings of carbon nanotubes with outstanding hydrogen storage capacities.

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