In this paper we study how the methane adsorption properties of the ionic MOF-5 are derived from the local structure of its coordinated metal-cluster. Density functional theory is used to study the adsorption process and identify the key interactions which drive it at ambient temperatures. A detailed adsorption model which represents the adsorption process is derived and used to extract thermodynamic properties from previously reported adsorption isotherms. We find that after adsorption of a single molecule to the face of the metal cluster, a nanostructured surface is formed which enables adsorption of additional CH4 molecules at reduced entropic penalty thanks to on-surface hopping motions and retention of significant translational freedom. Binding of the CH4 molecules to the MOF is dominated by electrostatic interactions with negatively charged carboxylate groups, while CH4-CH4 dispersion interactions are important only at high pressures. Last, the MOF-specific adsorption model is compared against the single-site Langmuir model. (Graph Presented).

HNgY molecules are chemically-bound compounds of a noble-gas atom (Ng) with a hydrogen and with an electronegative group Y. There is considerable current interest in the stability of these species in different types of media. The kinetic stability of several compounds, HXeOH, HXeOXeH, HXeBr and HXeCCH, in water clusters is explored by ab initio calculations. It is found that the kinetic stability of the compounds is reduced by the water environment, generally falling off with the number of H2O molecules. For a relatively modest number of water molecules, the compounds decompose spontaneously. Implications of the results for storage of HNgY in molecular media are discussed.

In order for hydrogen gas to be used as a fuel, it must be stored in sufficient quantity on board the vehicle. Efforts are being made to increase the hydrogen storage capabilities of metal-organic frameworks (MOFs) by introducing unsaturated metal sites into their linking element(s), as hydrogen adsorption centers. In order to devise successful hydrogen storage strategies there is a need for a fundamental understanding of the weak and elusive hydrogen physisorption interaction. Here we report our findings from the investigation of the weak intermolecular interactions of adsorbed hydrogen molecules on MOF-linkers by using cluster models. Since physical interactions such as dispersion and polarization have a major contribution to attraction energy, our approach is to analyze the adsorption interaction using energy decomposition analysis (EDA) that distinguishes the contribution of the physical interactions from the charge-transfer (CT) "chemical" interaction. Surprisingly, it is found that CT from the adsorbent to the σ*(H2) orbital is present in all studied complexes and can contribute up to approximately -2 kJ/mol to the interaction. When metal ions are present, donation from the σ(H2) → metal Rydberg-like orbital, along with the adsorbent → σ*(H2) contribution, can contribute from -2 to -10 kJ/mol, depending on the coordination mode. To reach a sufficient adsorption enthalpy for practical usage, the hydrogen molecule must be substantially polarized. Ultimately, the ability of the metalated linker to polarize the hydrogen molecule is highly dependent on the geometry of the metal ion coordination site where a strong electrostatic dipole or quadrupole moment is required.

To store natural gas (NG) inexpensively at adequate densities for use as a fuel in the transportation sector, new porous materials are being developed. This work uses computational methods to explore strategies for improving the usable methane storage capacity of adsorbents, including metal-organic frameworks (MOFs), that feature open-metal sites incorporated into their structure by postsynthetic modification. The adsorption of CH₄ on several open-metal sites is studied by calculating geometries and adsorption energies and analyzing the relevant interaction factors. Approximate site-specific adsorption isotherms are obtained, and the open-metal site contribution to the overall CH₄ usable capacity is evaluated. It is found that sufficient ionic character is required, as exemplified by the strong CH₄ affinities of 2,2'-bipyridine-CaCl₂ and Mg, Ca-catecholate. In addition, it is found that the capacity of a single metal site depends not only on its affinity but also on its geometry, where trigonal or "bent" low-coordinate exposed sites can accommodate three or four methane molecules, as exemplified by Ca-decorated nitrilotriacetic acid. The effect of residual solvent molecules at the open-metal site is also explored, with some positive conclusions. Not only can residual solvent stabilize the open-metal site, surprisingly, solvent molecules do not necessarily reduce CH₄ affinity, but can contribute to increased usable capacity by modifying adsorption interactions.

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.