Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Computational screening of metal-organic framework (MOF) materials for selective oxygen adsorption from air is used to identify new sorbents for oxyfuel combustion process feedstock streams. A comprehensive study on the effect of MOF metal chemistry on gas binding energies in two common but structurally disparate MOFs has been undertaken. Dispersion-corrected density functional theory (DFT) methods were used to calculate the oxygen and nitrogen binding energies with each of 14 metals, respectively, substituted into two MOF series, M2(dobdc) and M3(btc)2. The accuracy of DFT methods was validated by comparing trends in binding energy with experimental gas sorption measurements. A periodic trend in oxygen binding energies was found, with greater oxygen binding energies for early transition-metal-substituted MOFs compared to late transition metal MOFs; this was independent of MOF structural type. The larger binding energies were associated with oxygen binding in a side-on configuration to the metal, with concomitant lengthening of the O-O bond. In contrast, nitrogen binding energies were similar across the transition metal series, regardless of both MOF structural type and metal identity. Taken together, these findings suggest that early transition metal MOFs are best suited to separating oxygen from nitrogen and that the MOF structural type is less important than the metal identity.

Here we describe the homogeneous substitution of Mn, Fe and Co at various levels into a prototypical metal-organic framework (MOF), namely Cu-BTC (HKUST-1), and the effect of that substitution on preferential gas sorption. Using a combination of density functional theory (DFT) calculations, postsynthetic metal substitutions, materials characterization, and gas sorption testing, we demonstrate that the identity of the metal ion has a quantifiable effect on their oxygen and nitrogen sorption properties at cryogenic temperatures. An excellent correlation is found between O₂/N₂ selectivities determined experimentally at 77 K and the difference in O₂ and N₂ binding energies calculated from DFT modeling data: Mn > Fe > Co > Cu. Room temperature gas sorption studies were also performed and correlated with metal substitution. The Fe-exchanged sample shows a significantly higher nitrogen isosteric heat of adsorption at temperatures close to ambient conditions (273 K - 298 K) as compared to all other metals studied, indicative of favorable interactions between N₂ and coordinatively unsaturated Fe metal centers. Furthermore, differences in gas adsorption results at cryogenic and room temperatures are evident; they are explained by comparing experimental results with DFT binding energies (0 K) and room temperature Grand Canonical Monte Carlo simulations.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Metal organic frameworks (MOF) are a recently discovered class of nanoporous, defect-free crystalline materials that enable rational design and exploration of porous materials at the molecular level. MOFs have tunable monolithic pore sizes and cavity environments due to their crystalline nature, yielding properties exceeding those of most other porous materials. These include: the lowest known density (91% free space); highest surface area; tunable photoluminescence; selective molecular adsorption; and methane sorption rivaling gas cylinders. These properties are achieved by coupling inorganic metal complexes such as ZnO4 with tunable organic ligands that serve as struts, allowing facile manipulation of pore size and surface area through reactant selection. MOFs thus provide a discovery platform for generating both new understanding of chemistry in confined spaces and novel sensors and devices based on their unique properties. At the outset of this project in FY06, virtually nothing was known about how to couple MOFs to substrates and the science of MOF properties and how to tune them was in its infancy. An integrated approach was needed to establish the required knowledge base for nanoscale design and develop methodologies integrate MOFs with other materials. This report summarizes the key accomplishments of this project, which include creation of a new class of radiation detection materials based on MOFs, luminescent MOFs for chemical detection, use of MOFs as templates to create nanoparticles of hydrogen storage materials, MOF coatings for stress-based chemical detection using microcantilevers, and "flexible" force fields that account for structural changes in MOFs that occur upon molecular adsorption/desorption. Eight journal articles, twenty presentations at scientific conferences, and two patent applications resulted from the work. The project created a basis for continuing development of MOFs for many Sandia applications and succeeded in securing $\$$2.75 M in funding from outside agencies to continue the research.

We introduce a nonbonded three-body harmonic potential energy term for Mg-O-H interactions for improved edge surface stability in molecular simulations. The new potential term is compatible with the Clayff force field and is applied here to brucite, a layered magnesium hydroxide mineral. Comparisons of normal mode frequencies from classical and density functional theory calculations are used to verify a suitable spring constant (k parameter) for the Mg-O-H bending motion. Vibrational analysis of hydroxyl librations at two brucite surfaces indicates that surface Mg-O-H modes are shifted to frequencies lower than the corresponding bulk modes. A comparison of DFT and classical normal modes validates this new potential term. The methodology for parameter development can be applied to other clay mineral components (e.g., Al, Si) to improve the modeling of edge surface stability, resulting in expanded applicability to clay mineral applications. © 2014 American Chemical Society.

Abstract not provided.

Molecular dynamics simulations were used to investigate trends in noble gas (Ar, Kr, Xe) diffusion in the metal-organic frameworks HKUST-1 and ZIF-8. Diffusion occurs primarily through inter-cage jump events, with much greater diffusion of guest atoms in HKUST-1 compared to ZIF-8 due to the larger cage and window sizes in the former. We compare diffusion coefficients calculated for both rigid and flexible frameworks. For rigid framework simulations, in which the framework atoms were held at their crystallographic or geometry optimized coordinates, sometimes dramatic differences in guest diffusion were seen depending on the initial framework structure or the choice of framework force field parameters. When framework flexibility effects were included, argon and krypton diffusion increased significantly compared to rigid-framework simulations using general force field parameters. Additionally, for argon and krypton in ZIF-8, guest diffusion increased with loading, demonstrating that guest-guest interactions between cages enhance inter-cage diffusion. No inter-cage jump events were seen for xenon atoms in ZIF-8 regardless of force field or initial structure, and the loading dependence of xenon diffusion in HKUST-1 is different for rigid and flexible frameworks. Diffusion of krypton and xenon in HKUST-1 depends on two competing effects: the steric effect that decreases diffusion as loading increases, and the “small cage effect” that increases diffusion as loading increases. Finally, a detailed analysis of the window size in ZIF-8 reveals that the window increases beyond its normal size to permit passage of a (nominally) larger krypton atom.

Abstract not provided.

Anthropogenic activities have led to an increased concentration of uranium on the Earth’s surface and potentially in the subsurface with the development of nuclear waste repositories. Uranium is soluble in groundwater, and its mobility is strongly affected by the presence of clay minerals in soils and in subsurface sediments. We use molecular dynamics simulations to probe the adsorption of aqueous uranyl (UO₂²⁺) ions onto the basal surface of muscovite, a suitable proxy for typically ultrafine-grained clay phases. Model systems include the competitive adsorption between potassium counterions and aqueous ions (0.1 M and 1.0 M UO₂Cl₂ , 0.1 M NaCl). We find that for systems with potassium and uranyl ions present, potassium ions dominate the adsorption phenomenon. Potassium ions adsorb entirely as inner-sphere complexes associated with the ditrigonal cavity of the basal surface. Uranyl ions adsorb in two configurations when it is the only ion species present, and in a single configuration in the presence of potassium. Finally, the majority of adsorbed uranyl ions are tilted less than 45° relative to the muscovite surface, and are associated with the Si₄Al₂ rings near aluminum substitution sites.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.