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The well-known frameworks of the type M2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) have numerous potential applications in gas storage and separations, owing to their exceptionally high concentration of coordinatively unsaturated metal surface sites, which can interact strongly with small gas molecules such as H2. Employing a related meta-functionalized linker that is readily obtained from resorcinol, we now report a family of structural isomers of this framework, M2(m-dobdc) (M = Mg, Mn, Fe, Co, Ni; m-dobdc(4-) = 4,6-dioxido-1,3-benzenedicarboxylate), featuring exposed M(2+) cation sites with a higher apparent charge density. The regioisomeric linker alters the symmetry of the ligand field at the metal sites, leading to increases of 0.4-1.5 kJ/mol in the H2 binding enthalpies relative to M2(dobdc). A variety of techniques, including powder X-ray and neutron diffraction, inelastic neutron scattering, infrared spectroscopy, and first-principles electronic structure calculations, are applied in elucidating how these subtle structural and electronic differences give rise to such increases. Importantly, similar enhancements can be anticipated for the gas storage and separation properties of this new family of robust and potentially inexpensive metal-organic frameworks.

Hydrogen adsorption in high surface area nanoporous coordination polymers has attracted a great deal of interest in recent years due to the potential applications in energy storage. Here we present combined inelastic neutron scattering measurements and detailed first-principles calculations aimed at unraveling the nature of hydrogen adsorption in HKUST-1 (Cu3(1,3,5-benzenetricarboxylate)2), a metal-organic framework (MOF) with unsaturated metal centers. We reveal that, in this system, the major contribution to the overall binding comes from the classical Coulomb interaction which is not screened due to the open metal site; this explains the relatively high binding energies and short H2-metal distances observed in MOFs with exposed metal sites as compared to traditional ones. Despite the short distances, there is no indication of an elongation of the H-H bond for the bound H2 molecule at the metal site. We find that both the phonon and rotational energy levels of the hydrogen molecule are closely similar, making the interpretation of the inelastic neutron scattering data difficult. Finally, we show that the orientation of H2 has a surprisingly large effect on the binding potential, reducing the classical binding energy by almost 30%. The implication of these results for the development of MOF materials for better hydrogen storage is discussed.

A series of isostructural metal-organic framework polymers of composition [Cu2(L)(H2O)2] (L= tetracarboxylate ligands), denoted NOTT-nnn, has been synthesized and characterized. Single crystal X-ray structures confirm the complexes to contain binuclear Cu(II) paddlewheel nodes each bridged by four carboxylate centers to give a NbO-type network of 64.82 topology. These complexes are activated by solvent exchange with acetone coupled to heating cycles under vacuum to afford the desolvated porous materials NOTT-100 to NOTT-109. These incorporate a vacant coordination site at each Cu(II) center and have large pore volumes that contribute to the observed high H2 adsorption. Indeed, NOTT-103 at 77 K and 60 bar shows a very high total H2 adsorption of 77.8 mg g(-)- equivalent to 7.78 wt% [wt% = (weight of adsorbed H2)/(weight of host material)] or 7.22 wt% [wt% = 100(weight of adsorbed H2)/(weight of host material + weight of adsorbed H2)]. Neutron powder diffraction studies on NOTT-101 reveal three adsorption sites for this material: at the exposed Cu(II) coordination site, at the pocket formed by three {Cu2} paddle wheels, and at the cusp of three phenyl rings. Systematic virial analysis of the H2 isotherms suggests that the H2 binding energies at these sites are very similar and the differences are smaller than 1.0 kJ mol-1, although the adsorption enthalpies for H2 at the exposed Cu(II) site are significantly affected by pore metrics. Introducing methyl groups or using kinked ligands to create smaller pores can enhance the isosteric heat of adsorption and improve H2 adsorption. However, although increasing the overlap of potential energy fields of pore walls increases the heat of H2 adsorption at low pressure, it may be detrimental to the overall adsorption capacity by reducing the pore volume.

The graphite intercalation compound KC24 adsorbs hydrogen gas at low temperatures up to a maximum stoichiometry of KC24(H2)2, with a differential enthalpy of adsorption of approximately -9 kJ mol-1. The hydrogen molecules and potassium atoms form a two-dimensional condensed phase between the graphite layers. Steric barriers and strong adsorption potentials are expected to strongly hinder hydrogen diffusion within the host KC24 structure. In this study, self-diffusion in a KC24(H2)0.5 sample is measured experimentally by quasielastic neutron scattering and compared to values from molecular dynamics simulations. Self-diffusion coefficients are determined by fits of the experimental spectra to a honeycomb net diffusion model and found to agree well with the simulated values. The experimental H2 diffusion coefficients in KC24 vary from 3.6 × 10-9 m2 s-1 at 80 K to 8.5 × 10-9 m2 s-1 at 110 K. The measured diffusivities are roughly an order of magnitude lower that those observed on carbon adsorbents, but compare well with the rate of hydrogen self-diffusion in molecular sieve zeolites. [ABSTRACT FROM AUTHOR]

In situ neutron inelastic scattering experiments on hydrogen adsorbed into a fully deutrated tetrahydrofuran-water ice clathrate show that the adsorbed hydrogen has three rotational excitations (transitions between J=0 and 1 states) at approximately 14 meV in both energy gain and loss. These transitions could be unequivocally assigned since there was residual orthohydrogen at low temperatures (slow conversion to the ground state) resulting in an observable J=1→0 transition at 5 K (kT=0.48 meV). A doublet in neutron energy loss at approximately 28.5 meV is interpreted as J=1→2 transitions. In addition to the transitions between rotational states, there are a series of peaks that arise from transitions between center-of-mass translational quantum states of the confined hydrogen molecule. A band at approximately 9 meV can be unequivocally interpreted as a transition between translational states, while broad features at 20, 25, 35, and 50–60 meV are also interpreted to as transitions between translational quantum states. A detailed comparison is made with a recent five-dimensional quantum treatment of hydrogen in the smaller dodecahedral cage in the SII ice-clathrate structure. Although there is broad agreement regarding the features such as the splitting of the J=1 degeneracy, the magnitude of the external potential is overestimated. The numerous transitions between translational states predicted by this model are in poor agreement with the experimental data. Comparisons are also made with three simple exactly solved models, namely, a particle in a box, a particle in a sphere, and a particle on the surface of a sphere. Again, there are too many predicted features by the first two models, but there is reasonable agreement with the particle on a sphere model. This is consistent with published quantum chemistry results for hydrogen in the dodecahedral 512 cage, where the center of the cage is found to be energetically unfavorable, resulting in a shell-like confinement for the hydrogen molecule wave function. These results demonstrate that translational quantum effects are very significant and a classical treatment of the hydrogen molecule dynamics is inappropriate under such conditions. [ABSTRACT FROM AUTHOR]

Herein we present a detailed study of the hydrogen adsorption properties of Cu‐BTTri, a robust crystalline metal–organic framework containing open metal‐coordination sites. Diffraction techniques, carried out on the activated framework, reveal a structure that is different from what was previously reported. Further, combining standard hydrogen adsorption measurements with in‐situ neutron diffraction techniques provides molecular level insight into the hydrogen adsorption process. The diffraction experiments unveil the location of four D2 adsorption sites in Cu‐BTTri and shed light on the structural features that promote hydrogen adsorption in this material. Density functional theory (DFT), used to predict the location and strength of binding sites, corroborate the experimental findings. By decomposing binding energies in different sites in various energetic contributions, we show that van der Waals interactions play a crucial role, suggesting a possible route to enhancing the binding energy around open metal coordination sites. The correct structure of a well‐known metal–organic framework (MOF), Cu‐BTTri, has been defined by means of in‐situ diffraction techniques. The in‐situ neutron diffraction technique, coupled with detailed DFT studies, has led to a deep understanding of the chemical and structural parameters influencing hydrogen adsorption in MOFs. [ABSTRACT FROM AUTHOR]

Highlights: [•] D2 adsorbed in Ni2(dobdc) exhibits a close metal-D2 distance of 2.20(1)Å. [•] H2 quantum rotational levels determined for 3 adsorption sites. [•] Layering of H2 over D2 reveal the transitions of H2 at the second adsorption site. [ABSTRACT FROM AUTHOR]

The hydrogen storage properties of Mg2(dobdc) (dobdc4−= 2,5-dioxido-1,4-benzenedicarboxylate), a metal–organic framework possessing hexagonal one-dimensional channels decorated with unsaturated Mg2+coordination sites, have been examined through low- and high-pressure adsorption experiments, infrared spectroscopy, and neutron scattering studies. [ABSTRACT FROM AUTHOR]

The well-known frameworks of the type M2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) have numerous potential applications in gas storage and separations, owing to their exceptionally high concentration of coordinatively unsaturated metal surface sites, which can interact strongly with small gas molecules such as H2. Employing a related meta-functionalized linker that is readily obtained from resorcinol, we now report a family of structural isomers of this framework, M2(m-dobdc) (M = Mg, Mn, Fe, Co, Ni; m-dobdc4- = 4,6-dioxido-l,3-benzenedicarboxylate), featuring exposed M2+ cation sites with a higher apparent charge density. The regioisomeric linker alters the symmetry of the ligand field at the metal sites, leading to increases of 0.4—1.5 kJ/mol in the H2 binding enthalpies relative to M2(dobdc). A variety of techniques, including powder Xray and neutron diffraction, inelastic neutron scattering, infrared spectroscopy, and firstprinciples electronic structure calculations, are applied in elucidating how these subtle structural and electronic differences give rise to such increases. Importantly, similar enhancements can be anticipated for the gas storage and separation properties of this new family of robust and potentially inexpensive metal—organic frameworks. [ABSTRACT FROM AUTHOR]

Use of the tritopic bridging ligand 1,3,5-benzenetristetrazolate (BTT3-) enables formation of [Mn-(DMF)6]3[(Mn4CI)3(BTT)8(H2O)12]2•42DMF•11H2O•20CH3OH, featuring a porous metal-organic framework with a previously unknown cubic topology. Crystals of the compound remain intact upon desolvation and show a total H2 uptake of 6.9 wt % at 77 K and 90 bar, which at 60 g H2/L provides a storage density 85% of that of liquid hydrogen. The material exhibits a maximum isosteric heat of adsorption of 10.1 kJ/mol, the highest yet observed for a metal-organic framework. Neutron powder diffraction data demonstrate that this is directly related to H2 binding at coordinatively unsaturated Mn2+ centers within the framework. [ABSTRACT FROM AUTHOR]