An International Laboratory Comparison Study of Volumetric and Gravimetric Hydrogen Adsorption Measurements.

In order to determine a material's hydrogen storage potential, capacity measurements must be robust, reproducible, and accurate. Commonly, research reports focus on the gravimetric capacity, and often times the volumetric capacity is not reported. Determining volumetric capacities is not as straight-forward, especially for amorphous materials. This is the first study to compare measurement reproducibility across laboratories for excess and total volumetric hydrogen sorption capacities based on the packing volume. The use of consistent measurement protocols, common analysis, and figure of merits for reporting data in this study, enable the comparison of the results for two different materials. Importantly, the results show good agreement for excess gravimetric capacities amongst the laboratories. Irreproducibility for excess and total volumetric capacities is attributed to real differences in the measured packing volume of the material.

Performance of van der Waals Corrected Functionals for Guest Adsorption in the M2(dobdc) Metal-Organic Frameworks.

Small-molecule binding in metal-organic frameworks (MOFs) can be accurately studied both experimentally and computationally, provided the proper tools are employed. Herein, we compare and contrast properties associated with guest binding by means of density functional theory (DFT) calculations using nine different functionals for the M2(dobdc) (dobdc4- = 2,5-dioxido,1,4-benzenedicarboxylate) series, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn. Additionally, we perform Quantum Monte Carlo (QMC) calculations for one system to determine if this method can be used to assess the performance of DFT. We also make comparisons with previously published experimental results for carbon dioxide and water and present new methane neutron powder diffraction (NPD) data for further comparison. All of the functionals are able to predict the experimental variation in the binding energy from one metal to the next; however, the interpretation of the performance of the functionals depends on which value is taken as the reference. On the one hand, if we compare against experimental values, we would conclude that the optB86b-vdW and optB88-vdW functionals systematically overestimate the binding strength, while the second generation of van der Waals (vdW) nonlocal functionals (vdw-DF2 and rev-vdW-DF2) correct for this providing a good description of binding energies. On the other hand, if the QMC calculation is taken as the reference then all of the nonlocal functionals yield results that fall just outside the error of the higher-level calculation. The empirically corrected vdW functionals are in reasonable agreement with experimental heat of adsorptions but under bind when compared with QMC, while Perdew-Burke-Ernzerhof fails by more than 20 kJ/mol regardless of which reference is employed. All of the functionals, with the exception of vdW-DF2, predict reasonable framework and guest binding geometries when compared with NPD measurements. The newest of the functionals considered, rev-vdW-DF2, should be used in place of vdW-DF2, as it yields improved bond distances with similar quality binding energies.

Critical Factors Driving the High Volumetric Uptake of Methane in Cu3(btc)2.

A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu3(btc)2 (btc(3-) = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu3(btc)2, and its isostructural analogue Cr3(btc)2, show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu(2+) or Cr(2+) cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu3(btc)2 are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu3(btc)2 itself.

On the importance of a precise crystal structure for simulating gas adsorption in nanoporous materials.

We show that simulation of gas adsorption in nanoporous sorbents may be highly sensitive to accurate crystallographic coordinates, even for frameworks anticipated to have low flexibility.

M2(m-dobdc) (M = Mg, Mn, Fe, Co, Ni) metal-organic frameworks exhibiting increased charge density and enhanced H2 binding at the open metal sites.

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.

Nanoporous metal formates for krypton/xenon separation.

Metal(II) formates (Co and Ni) show a significantly larger heat of adsorption for xenon than krypton across all loadings due to size selectivity in the primary adsorption site.

Hydrogen storage in a highly interpenetrated and partially fluorinated metal-organic framework.

A partially fluorinated metal-organic framework, Zn(bpe)(tftpa)·cyclohexanone [bpe = 1,2-bis(4-pyridyl)ethane; tftpa = tetrafluoroterephthalate], has been synthesized, and its H2 storage properties are reported. The structure is highly interpenetrated yet contains templating cyclohexanone molecules, which can be easily removed to give a porous material with fluorine atoms exposed to the pore surface. The material adsorbs 1.04 wt % H2 at 77 K and 1 atm with an adsorption enthalpy (Qst) of 6.2 kJ/mol, which represents a slight enhancement in the binding strength due to the presence of fluorine atoms over comparable metal-organic frameworks, which bind through purely physisorptive methods.

Structure and magnetic field-induced transition in a one-dimensional hybrid inorganic-organic chain system, Co2(4,4'-bpy)(tfhba)2.4,4'-bpy (4,4'-bpy = 4,4'-bipyridine; tfhba = 2,3,5,6-tetrafluoro-4-hydroxybenzoate).

Two isostructural hybrid inorganic-organic frameworks, M(2)(4,4'-bpy)(tfhba)(2).4,4'-bpy, (M = Co (1), Zn (2); 4,4'-bpy = 4,4'-bipyridine; tfhba = 2,3,5,6-tetrafluoro-4-hydroxybenzoate) have been synthesized and their structures elucidated using single crystal X-ray diffraction. These materials are the first hybrid structures synthesized using the tfhba ligand. They contain a one-dimensional chain of corner-sharing MO(4)N trigonal bipyramids that has not been observed to date for Co(2+) units. The magnetic properties of 1 have been investigated and indicate ferromagnetic ordering along the chains, with weak antiferromagnetic interactions between them. Overall the system is antiferromagnetic with a Neel temperature of 3 K, but undergoes a field-induced magnetic phase transition at 3 kOe.

Ionothermal synthesis of inorganic-organic hybrid materials containing perfluorinated aliphatic dicarboxylate ligands.

Two new hybrid inorganic-organic frameworks containing perfluorinated dicarboxylate ligands have been synthesized ionothermally from a mixture of 1-ethyl-3-methylimidazolium bromide (emim-Br), and triflimide (emim-Tf(2)N). The cobalt(ii) tetrafluorosuccinate, [emim](2)[Co(H(2)O)(2)(O(2)CCF(2)CF(2)CO(2))(2)] (), is a two-dimensional coordination polymer containing anionic framework layers separated by emim cations. When hexafluoroglutaric acid is used in similar reactions, [emim](2)[Co(3)(O(2)CCF(2)CF(2)CF(2)CO(2))(4)(H(2)O)(4)] () forms. Like , this phase contains anionic layers of cobalt and the fluorinated carboxylate separated by emim cations. These new phases represent the first examples of ionothermally synthesized materials using perfluorinated aliphatic carboxylates.

Synthesis and characterization of AU2Se6 (A = K, Cs).

Two new ternary uranium selenides, AU(2)Se(6) (A = K, Cs), were prepared using the reactive flux method. Single crystal X-ray diffraction was performed on single crystals. The compounds crystallize in the orthorhombic Immm space group, Z = 2. CsU(2)Se(6) has cell parameters of a = 4.046(2) A, b = 5.559(3) A, and c = 24.237(12) A. KU(2)Se(6) has cell parameters of a = 4.058(3) A, b = 5.556(4) A, and c = 21.710(17) A. The compounds are isostructural to the previously reported KTh(2)Se(6). The two-dimensional layered structure is related to ZrSe(3) with the alkali metals residing in the interlayer space. The oxidation states of uranium and selenium were evaluated using X-ray photoelectron spectroscopy (XPS). Uranium was found to be tetravalent, while selenium was found to be in two oxidation states, one of which is -2. The other oxidation state is similar to that found in a polyselenide network. While this structure is known, our work examines how the structure changes through the transactinide series.