High-Temperature Hydrogen Storage of Multiple Molecules: Theoretical Insights from Metalated Catechols.

Insertion of open metal sites (OMS) into metal-organic frameworks (MOFs) is a promising strategy for preparation of physical adsorbents that enable H2 storage at room temperature. Density functional theory (DFT) calculations are reported on a promising paradigm for adsorption of multiple hydrogen molecules to a single OMS attached to an MOF linker via a catechol or thiocatechol. The interactions between adsorbed H2 and the OMS are characterized with special attention to their degrees of freedom and thermal properties. By combining the present calculations with experimental data, some of these materials are predicted to have usable capacities close to the Department of Energy (DOE) 2020 target of 40 gr L-1 marking them as important synthetic targets. Surprisingly, calculations suggest that a Ca-catechol OMS retains the ability to bind up to two hydrogens even in the presence of residual solvent.

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.

Hydrogen physisorption on metal-organic framework linkers and metalated linkers: a computational study of the factors that control binding strength.

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.

Destabilization of noble-gas hydrides by a water environment: calculations for HXeOH@(H2O)n, HXeOXeH@(H2O)n, HXeBr@(H2O)n, HXeCCH@(H2O)n.

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.

Stability of noble-gas hydrocarbons in an organic liquid-like environment: HXeCCH in acetylene.

Noble-gas hydrides such as HXeCCH are prepared in cryogenic noble-gas matrices where they are stable. Molecular dynamics simulations reported here predict that HXeCCH is chemically stable in clusters of acetylene, and that stability prevails for temperatures of at least 150 K, at which the clusters are liquid-like. The HXeCCH(C(2)H(2))(n) clusters are studied for sizes up to n = 7. Ab Initio Molecular Dynamics trajectories of 10 ps duration are computed using BLYP-D DFT potential. The liquid-like nature of the system at 150 K is reflected in large amplitude motion of intermolecular distances and orientations. In addition, structures, energetics, NBO charges and bonding analysis at equilibrium are also reported. Complexation is found to be energetically favorable, and to increase the stability of the HXeCCH molecule. The significance of the existence of stable liquid-like complexes of noble-gas hydrides is discussed.

Predicted compounds of radon with acetylene and water.

It is found that HRnCCH and HRnOH are metastable, chemically bound compounds of radon. These molecules are studied by multi-reference ab initio methods. Equilibrium geometry, NBO partial charges and bond orders, harmonic frequencies, are calculated. Intrinsic life-times are obtained by calculating the dissociation barriers and related partition functions, and by applying transition state theory. HRnCCH and HRnOH are found to be protected by an energy barrier of 2.1 and 0.79 eV, respectively. Using transition state theory, HRnOH is predicted to have a half-life of 1 h at about 230 K. HRnCCH is found to be kinetically stable at room temperature with its lifetime limited by the lifetime of the radioactive Rn atom. The significance of compound formation of radon with acetylene and water is discussed.

Direct visualization of the H-Xe bond in xenon hydrides: xenon isotopic shift in the IR spectra.

IR spectra of xenon hydrides (HXeCCH, HXeCC, and HXeH) obtained from different xenon isotopes ((129)Xe and (136)Xe) exhibit a small but detectable and reproducible isotopic shift in the absorptions assigned to H-Xe stretching (by 0.17-0.38 cm(-1)). To our knowledge, it is the first direct experimental evidence for the H-Xe bond in HXeY type compounds. The shift magnitude is in good agreement with quantum-chemical calculations.

Properties of stable organic bond-stretched non-Lewis molecules.

The conditions required for the existence of a stable bond-stretched singlet isomer of hetero derivatives of bicyclo[2.1.0]pentane (which is a cyclopentane-1,3-diyl derivative) are discussed. Such species are non-Lewis systems with a ruptured C-C bond (formally diradicals), in which two electrons occupy the nonbonding orbital. A high-level calculation shows that in contrast with the carbon substituted compounds, in which the open form is a transition state between two classical-bonded closed bicyclic forms, in the heterosubstituted molecules, the open form is calculated to be a stable minimum. The ionization potentials of the open forms are considerably lower than those of their bicyclic isomers and also of regular organic radicals/diradicals. Nitrogen atoms are found to be more effective than oxygen or sulfur in stabilizing the open isomer. In this case, the open isomer is calculated to be a little more stable than the bicyclic compound, and a barrier of approximately 40 kcal/mol is computed for the ring closing reaction. Thus, the open isomer is both thermodynamically and kinetically stable. This result rationalizes some experimental observations that indicated the existence of non-Lewis singlet species.

A computational study of CH4 storage in porous framework materials with metalated linkers: connecting the atomistic character of CH4 binding sites to usable capacity.

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.

Hetero-bimetallic metal-organic polyhedra.

Porous metal-organic polyhedra (MOPs), constructed from heterometallic Pd(II)-M(II) (M = Cu, Ni, Zn) paddlewheel nodes and 5-tert-butyl-1,3-benzenedicarboxylate organic links, were prepared in which the Pd(II) ions preferentially line the inner surface of the cage molecules. Careful activation produces co-ordinatively unsaturated 3d transition metal sites on the external MOP surfaces giving rise to H2 adsorption enthalpies in excess of -12 kJ mol(-1).