Browsing by Subject "metal-organic frameworks"

Now showing 1 - 4 of 4
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    Ab Initio Modeling of the Structures and Catalytic Properties of Selected Mononuclear, Dinuclear, Supramolecular, and Periodic Metal-Organic Complexes
    (2016-07) League, Aaron
    Numerous varieties of metal-organic compounds have been synthesized to date, ranging in size from complexes containing only one or two metal atoms to supramolecular clusters, cages, and periodic latices. Metal-organic compounds can be tuned for a multitude of applications by modifying organic ligands and metal centers to select for desired size, shape, and electronic properties. Because of the complicated nature of many such systems, characterization of them can be a challenge. Mechanisms of reactivity, too, are notoriously difficult to characterize experimentally; as the saying goes, "If you can isolate the species, it probably isn't important to the mechanism." Theory can be of assistance in both cases. Herein, we investigate the properties of a variety of metal-organic systems, including mononuclear and dinuclear ruthenium water oxidation catalysts, supramolecular Cu(I) and Fe(II) host-guest complexes, and a metal-organic framework modified with single-site Ni(II) and Co(II). In each case, we are able to use \textit{ab initio} methods to provide some valuable insight into the system: for the water oxidation catalysts, we examine the effects on catalytic efficacy of modifications to the ligand systems in order to advise the development of even better catalysts; for the host-guest complexes, we help to explain the binding trends observed between small molecules that incorporate into the internal cavities of the cages; and for the metal-organic framework, we take steps toward elucidating the structure resulting from deposition of Ni(II), as well as giving insight into the mechanisms by which the deposited metals catalyze ethylene hydrogenation and oligomerization.
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    Active Space Methods In Electronic Structure theory and Applications To Gas Separations In Metal-Organic Frameworks
    (2019-06) Stoneburner, Samuel
    Active space methods such as complete active space self-consistent field theory (CASSCF) are applied to many systems of interest with a focus on the challenges in choosing orbitals for active spaces. The systematic exploration of active spaces is considered from the standpoint of theoretical development, specifically the benchmarking of generalized active space self-consistent field theory and SplitGAS on a variety of systems. Additionally, a “correlated participating orbital” active space selection scheme is applied to CASSCF and restricted active space self-consistent field theory followed by second-order perturbation theory (CASPT2 and RASPT2, respectively) for singlet-triplet splittings of diradical organic molecules. “πCPO” is introduced as an effective and economical option for π-system excitation energies. It is also demonstrated that multiconfiguration pair-density functional theory (MC-PDFT) can provide good agreement with CASPT2 at a much lower computational cost. The computational affordability of MC-PDFT is also shown through the calculation of the full spin ladder of Fe2S2 compounds for which second-order perturbation theory could only be performed for high-spin states. The effects of including high local exchange (HLE) modifications to the MC-PDFT exchange and correlation energies for the relative spin-state energies of several other iron complexes is examined. The remainder of the work features gas separations in metal-organic frameworks (MOFs), beginning with CO2 capture in a copper paddle-wheel MOF and continuing to metal-catecholates, which are studied using Kohn-Sham density functional theory and CASPT2 in comparisons of different first-row transition metals for the capture of toxic NO and for O2/N2 separation. Finally, a screening study identifies specific MOF structures for metal-catecholate modification as synthetic targets for the purpose of O2/N2 separation.
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    Designing Metal-Organic-Frameworks For Selective Biomass Catalysis
    (2020-03) Dorneles de Mello, Matheus
    Metal-organic frameworks (MOFs) are microporous materials with a wide range of pore sizes and functionalities, making them attractive for a variety of potential applications in catalysis, separations, sensing, and gas storage. Associated with the global demand for clean energy sources to find alternatives to fossil fuels, their use as catalysts for biomass conversion to chemicals finds potential application. The performance of MOFs in these applications is dependent on how stable they are upon modifications to their tunable frameworks. Such modifications include acid treatment, ligand, and cluster functionalization that can be performed by direct synthesis or post-synthesis modification, as desired for optimum performance. This dissertation focuses on using synthetic methods that may enable the tailoring the microstructure of MOFs towards their use for catalysis of biomass. We discuss the use of a method called acid modulation to introduce missing-ligands defects and open Lewis acid sites into the framework of UiO-66 to make it an active and selective catalyst for glucose isomerization to fructose in alcohol media. We demonstrate that upon the alcohol choice, the selectivity of the reaction to fructose can change drastically by favoring other reaction pathways. Furthermore, we investigate the reaction mechanism of glucose isomerization into UiO-66 and identify that glucose reacts via a 1,2-hydride transfer mechanism similar to what was reported for Sn-zeolites. We report the synthesis and installation of phosphonic acid moieties into the ligands of UiO-66 and UiO-67 as a way to introduce Brønsted acidity to these materials by a post-synthesis ligand exchange method. The active sites of P-UiO-66 are elucidated by a combination of solid-state NMR and DFT calculations. P-UiO66 is reported to be active and selective for several acid-catalyzed reactions such as alcohol dehydration and furans dehydra-decyclization with site-time yields approaching that of highly selective phosphoric acid zeolites, holding promise for its use in this and other reactions for the biomass conversion to chemicals. The tunability of MOFs combined with the PSM method and synthesis of phosphonic acids can provide accurate control of the density of active sites with a uniform distribution throughout the framework.
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    Optimization of Porous Metal Oxides and Metal-Organic Frameworks for High Temperature Catalysis
    (2017-09) May, Camille
    The structural integrity of porous materials is critical to their application as heterogeneous catalysts. For high temperature catalysis, sintering and decomposition are common routes to structural destabilization and ultimately to irreversible deactivation of porous catalysts. This thesis describes the optimization of two types of porous catalysts, namely, porous metal oxides and metal-organic frameworks, for application in solar thermochemical fuel production and high temperature natural gas conversion. Thermochemical cycles can use the heat generated from solar thermal power to split H2O and CO2 into H2 and CO, both of which are valuable fuel and chemical feedstocks. These cycles can be catalyzed by metal oxides. In the first part of this thesis, wood-templating is demonstrated as an approach to generate a macroporous oxide structure that balances high thermal stability and accessible porosity for enhanced thermochemical cycling kinetics. The other part of this thesis describes the development of a silica nanocasting method for the thermal stabilization of metal-organic framework (MOF)-based catalytic metal sites. Nanocasting incorporates a thermally stable silica layer in the MOF pores, which serves as a scaffold for the metal active sites in the MOF after the organic linkers are removed at high temperatures. The work described here shows the applicability of the nanocasting method to MOFs with different pore sizes and compositions, and demonstrates that the method is capable of producing MOF-derived materials that retain their catalytic properties after exposure to high temperatures.