Master of Science / Department of Chemistry / Tendai Gadzikwa / Molecular architecture involves the assembly of molecular building blocks to form supramolecular structures and the decoration of their interiors. The evolution and gathering of molecular building blocks into supramolecular constructs include examples such as co-crystals, micelles, nanoparticles, etc. These cases offer novel and advantageous pathways for research in supramolecular chemistry, however, a class of materials known as metal-organic frameworks (MOFs) materials has emerged as a prime candidate for molecular construction and interior design. MOFs are highly tunable materials because they can be synthesized from a wide range of metals cations and organic linkers. The organic linkers can also be functionalized after the MOF material has been synthesized through a process known as post-synthetic modification (PSM). These materials can be synthesized using two different organic linkers, resulting in a mixed-ligand MOF. If these ligands are modifiable and react independently, the resulting MOF structure will be orthogonally functionalized. Upon PSM we hypothesize that our porous, mixed-ligand MOF will contain homogenous bifunctionality as a blueprint for the construction of a uniformly orthogonally functionalized MOF. The synthesis of the first metal-organic framework, KSU-1, is the first of its kind to be developed at Kansas State University. PSM strategies used in this research show successful functionalization of each organic linker leading to uniform bifunctionality throughout our material. Characterization studies commonly used with MOFs verifies the synthesis and PSM of KSU-1.

The theme of this work is the observation and understanding of the effects of pore functionalization on adsorption properties of stable Metal-Organic Frameworks (MOFs). Over the first two sections of this work, sets of materials with representative pore sizes and functional groups are studied for adsorption properties. Observed trends are used to identify the best pore properties achievable via functionalization for adsorption systems. The third section of this work provides perspective on MOF materials and proposes target pore features for an efficient adsorbent for carbon dioxide capture from flue gas. First, the highly stable UiO-66 series of materials was selected for a pure-component adsorption study. The selectivity and capacity for CO2 can be best enhanced with the smallest, most polar functional group, such as an amino group, but significantly enhance water adsorption. Large, non-polar groups do not yield a completely hydrophobic material, but may be useful for humid gas separations as pore filling with water is inhibited. Next, a breakthrough study was conducted using CO2:CH4 and CO2:N2 mixtures on a set of stable MOFs. UiO-66-NH2 and UiO-66-DM, where DM=dimethyl, outperform predictions based on published isotherms and have dynamic CO2:CH4 selectivity comparable to zeolite NaY. UiO-66-DM may be a good candidate for further study due to the combination of partial hydrophobicity and good selectivity. Finally, by combining a review of literature with observations made in this work, a perspective on MOFs as efficient humid gas separation materials is provided. The presence of water vapor prevents use of current high performance adsorbents, but several MOF pore features show promise for these separations. The designable nature of MOFs allows for targeted design of size-matched pores and single-molecule traps which can selectively or cooperatively adsorb CO2 in the presence of water. Also, many MOF materials would be well suited for advanced pressure swing adsorption cycles and engineered sorbents, which enables greater material utilization and system efficiency.

"The investigation through design and synthesis of metal-organic frameworks was conducted in an effort to create new types of porous crystalline solids. The supramolecular chemistry and crystal structures of six novel frameworks (1-Cd, 2Cd, 1-Cu, 2a-Cu, 2b-Cu, 3-Cu) are reported. We are targeting porous solids composed of the transition metals Cu2+ and Cd2+ with three related families of organic molecules: Ethyl 4-(1H-imidazol-1-yl)benzoate, 4-(1H-benzo[d]imidazol-1-yl)benzoic acid, and Ethyl 4-(2-methyl-1H-imidazol-1-yl)benzoate. These molecular building blocks self assemble via metal coordination into coordination polymers that form a variety of 1-D, 2-D, and 3-D architectures. The networks are comprised of M•••O and M•••N bonds that coordinate into different geometrical arrangements dependent on steric hindrance and the metal ions that are used. The frameworks synthesized display porous behavior using weight measurements that are also seen to be reversible in some cases using atmospheric reuptake of guest molecules from growth solution. The uptake of rhodamine b was examined for the framework 3-Cu."

Thesis advisor: Chia-Kuang Tsung / Incorporating shape and size controlled metal nanoparticles (NPs) into metal-organic-frameworks (MOFs) shows great potential in heterogeneous catalysis. The combination of ordered nanoporous structure of MOFs and the well-defined surfaces of metal NPs provides a new tool to modulate the catalysis on the metal surface. Due to the large pore size, framework flexibility and selective interaction with gas molecules, MOFs have been widely used for gas storage with high selectivity. Among which have been developed to date, Zeolitic Imidazolate Frameworks-8 (ZIF-8) and UiO-66 show advantageous properties. The solvent resistivity and high thermal stability makes them stand out to be good candidates as shell materials in core shell catalysts. In our work, we developed an efficient way to create a yolk-shell structure of Pd nanoparticles in ZIF-8 and, at the same time, a method to incorporate the shape/size controlled Pt nanoparticles into well-defined octahedral UiO-66 nanocrystals with the control of concentration and dispersion. The formation mechanisms of both yolk-shell and core-shell structures were also studied in the work. / Thesis (MS) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

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Luminescent metal organic frameworks (MOFs) are promising new materials with applications as sensors, photocatalysts, and other luminescent devices. Although MOFs retain the chemical and physical properties of their constituents, the properties of the MOF are often altered from those of its building blocks, making rational design and synthesis difficult. Anthracene is a polyaromatic hydrocarbon whose photophysical properties have been found to be easily tuned through structural modifications. The tunability of anthracene makes it an ideal candidate for use in luminescent devices, such as photoprobes and organic light emitting diodes. MOFs designed with π conjugated molecules like anthracene ligands possess similar photophysical properties such as absorption and fluorescence in the UV and visible spectrum. In hopes of better understanding how the photophysical properties of the organic ligand is altered upon incorporation into a MOF, the spectroscopic properties of anthracenedicarboxylic acids were studied before and after integration into zinc based MOFs. Steady state and time resolved measurements were performed on three anthracenedicarboxylic acids: 9,10-anthracenedicarboxylic acid, 2,6-anthracendicarboxylic acid, and 1,4-anthracenedicarboxylic acid. The position of the carboxylic acid groups on anthracene was found to effect the position and structure of the absorption and emission spectra. The difference in the spectra is attributed to the perturbation by the acid groups on certain electronic transitions with dipole moments across two of the three axes of anthracene. The position of the acid groups had different effects on the fluorescence quantum yields and lifetimes of the three anthracenic acids studied. Two of the linkers were synthesized into MOFs through a solvothermal reaction with zinc nitrate, to form PCN-13, from 9,10-anthracenedicarboxylic acid, and [Zn(C₁₆H₈O₄)(H₂O)]_n, from 2,6-anthracenedicarboxylic acid. The luminescent properties of the two MOFs were studied and compared to those of the free based linker. Incorporation of the luminescent anthracenedicarboxylic acids into Zn based MOFs were found to either increase or decrease the luminescent properties of the ligands. / Master of Science

Flexible electronics have garnered much interest over the past several decades. Hybrid organic-inorganic materials, such as metal-organic frameworks, offer a unique opportunity to encompass the effective electronic properties of the inorganic material and the flexible nature of the organic with the potential of enhancing other desirable properties, such as the contributing multiferroicity. Using a first principles approach, the goal of this thesis is to serve as a guide for identifying potential ferroelectric and multiferroic metal-organic frameworks. This is done through a screening method of metal-organic frameworks based on their geometry; certain symmetry operators cannot be present in a ferroelectric material. We report the theoretical spontaneous polarization for several dozens of MOFs in which ferroelectricity has not previously been tested, and further we discuss the likelihood that these materials could be engineered to have either increased polarization or added ferromagnetism, the latter of which would lead to multiferroicity.

A growing field of research has evolved around the design and synthesis of a variety of porous metal-organic framework (MOF) materials. Some of the most promising areas for which these materials are potentially useful candidates include gas-separation, heterogeneous catalysis, and gas-storage, and all of these applications involve placing the MOF under pressure. There is clearly a need to understand the structural response of MOFs to applied pressure. Nevertheless, hitherto there are very few published investigations dedicated to determining the behaviour of porous hybrid materials under pressure. Through the use of high-pressure single-crystal X-ray diffraction studies, a series of MOF materials have been studied. Here we present the effect of pressure on a series of MOFs. In chapter 2, the effect of pressure on the prototypical MOF called MOF-5 was studied experimentally from ambient pressure to 3.2 GPa. Here, application of pressure was driven by the hydrostatic medium being forced into the pores of the MOF, which altered the mechanical properties of MOF-5, in particular, medium inclusion delayed the onset of amorphization. Complementary computational analysis was also performed to elucidate further the effect of medium inclusion on compressive behaviour. Detailed structural data was also collected as a function of pressure on the MOF Cu-btc. Application of pressure caused solvent to be squeezed into the pores (like MOF-5) until a phase transition occurred, driven by the sudden compression and expansion of equatorial and axial Cu–O bonds. High-pressure post-synthetic modification of a MOF is reported for the first time. On application of pressure of 0.2 GPa to the Cu-based MOF called STAM-1, a ligand exchange reaction takes place resulting in a change in pore size, shape, and hydrophilicity of the resulting pores. Here, we also demonstrate the ability to force hydrophilic molecules into hydrophobic pores using pressure, counteracting the hydrophobic effect. A high-pressure combined experimental and computational study has been carried to probe the effect of pressure on ‘breathing’ mechanisms in a zeolitic imidazolate framework (or ZIF) called ZIF-8. The penetration of guest molecules and the accommodation of pressure are shown to be inextricably linked to the rotation of methylimidazolate groups in the structure. Finally, the application of pressure to the MOF Sc₂BDC₃ and the nitro functionalized derivative Sc₂(NO₂-BDC)₃ was also studied. Here, the effect of chemical modification of the organic ligand, whilst maintaining framework topology, has been investigated as it pertains to compressibility. Directionality of compression is observed and this is rationalized with respect to the framework topology and medium inclusion/exclusion.

Metal-organic frameworks (or coordination polymers) are a recently-identified class of porous polymeric materials, consisting of metal ions or clusters linked together by organic bridging ligands. The major advantage of MOFs over other traditional materials, such as zeolites or activated carbons, is that their synthesis methods have provided an extensive class of crystalline materials with high stability, tunable metrics, and organic functionality. The ability to modify the physical environment of the pores and cavities within MOFs allow tuning of the interactions with guest species, and serves as a route to tailor the chemical stability and/or reactivity of the frameworks for specific applications. The classical way to incorporate functional groups into a MOF is the modification of the organic precursor with specific substituents before synthesizing the MOF itself; we call this approach pre-functionalization method. Functionalization of organic precursors is the initial and necessary step to obtaining functionalized isostructural MOFs and also provides the possibility for the post-synthetic modification of MOFs. However, in some cases, the functional groups may interfere with MOF synthesis and alter the topology of desired MOF. The goal of this proposed research is to explore the possibilities of metal-organic frameworks (MOFs) as novel porous structures, to study the effect of functional groups on the topologies and adsorption behavior of MOFs, and to understand how the synthesis conditions affect the phase purity and the in-situ reaction of ligands.

Adsorption in porous materials plays a significant role in industrial separation processes. Here, the host-guest interaction and the pore shape influence the distribution of products. Metal-organic frameworks (MOFs) are promising materials for separation purposes as their diversity due to their building block synthesis from metal corners and organic linker gives rise to a wide range of porous structures. The selectivity differs from MOF to MOF as the size and shapes of their pores are tuneable by altering the organic linkers and thus changing the host-guest interactions in the pores. Using mainly molecular simulation techniques, this work focuses on three types of separations using MOFs. Firstly, the experimental incorporation of calix[4]arenes in MOFs as a linker to create additional adsorption sites is investigated. For a mixture of methane and hydrogen, it is shown that in the calix[4]arene-based MOFs, methane is adsorbed preferentially over hydrogen with much higher selectivities compared to other MOFs in the literature. Remarkably, it was shown that extra voids created by calix[4]arene-based linkers, were accessible to only hydrogen molecules. Secondly, the strong correlation between different pore sizes and shapes in MOFs and their capabilities to separate xylene isomers were investigated for a number of MOFs. Finally, the underlying molecular mechanism of enantioseparation behaviour in a homochiral MOF for a number of chiral diols is presented. The simulation results showed good agreement with experimental enantioselectivity values. It was observed that high enantioselectivity occurs only at high loadings and when a perfect match in terms of size and shape exists between the pore size and the adsorbates. Ultimately, the information obtained from molecular simulations will further our understanding of how network topology, pore size and shape in MOFs influence their performance as selective adsorbents for desired applications.