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The advancements of chemists, engineers, and material scientists has yielded an enormous and diverse library of high-performance materials with varying chemical and physical properties that can be used in a wide array of applications. A molecular-level understanding of the nature of gas–surface interactions is critical to the development of next generation materials for applications such as gas storage and separation, chemical sensing, catalysis, energy conversion, and protective coatings. Quartz crystal microbalance (QCM) and in situ infrared (IR) spectroscopic techniques were employed to probe how topological features of a material as well as structural differences of the analytes affect gas sorption. Detailed studies of the interactions of three categories of molecules: aromatic hydrocarbons, triatomic ambient gases, and chemical warfare agents, with metal-organic frameworks (MOFs) and polyurethane coatings were conducted to build structure–property relationships for the nature and energetics of gas sorption within each material. Differences in the molecular structure of the guest compounds were found to greatly influence how, and to what extent each molecule interacts with the MOF or polyurethane film. Specifically, IR studies revealed that transport of aromatic compounds within the zirconium-based MOF, UiO-66 was limited by steric restrictions as molecules passed through small triangular apertures within the pore environment of the MOF. In contrast, the smaller triatomic molecules, CO2, SO2, and NO2, were able to pass freely through the MOF apertures and instead reversibly adsorbed inside the MOF cavities. Specifically, SO2 and NO2 were observed to preferentially bind to undercoordinated zirconium sites located on the MOF nodes. In addition, uptake of CO2, SO2, and NO2 was also aided by dispersion forces within the confined pore environments and by hydrogen bond formation with μ3 OH groups of the MOFs. Dimethyl chlorophosphate (DMCP), a nerve agent simulant that contains several electronegative moieties, was also found to strongly adsorb to undercoordinated zirconium; however, unlike in the aromatic and triatomic molecule systems, DMCP remained permanently bound to the MOFs, even at high temperatures. Finally, QCM studies of mustard gas simulant uptake into polyurethane films of varying hard:soft segment compositions revealed that dipole-dipole and dipole-induced dipole interactions were responsible for favorable absorption conditions. Furthermore, the ratio of hard and soft segment components of the polyurethane had a minor impact on simulant adsorption. Higher hard-segment content resulted in a more crystalline film that reduced simulant uptake, whereas the rubbery, high soft segment polyurethane allowed for greater vapor absorption. Ultimately, molecular-level insight into how the chemical identity of a guest molecule impacts the mechanism and energetics of vapor sorption into both MOFs and polymeric films can be extended to other relevant systems and may help identify how specific characteristics of each material, such as size, shape, and chemical functionality impact their potential use in targeted applications. / Doctor of Philosophy / The nature in which specific gases interact with materials will largely dictate how the material can be utilized. By understanding where and how strongly gas molecules interact with a material, scientists and engineers can rationally design new and improved systems for targeted applications. In the research described in this thesis, we examined how the chemical structure of three different groups of compounds, which have relevance in many industrial, environmental, and defense-related applications, affected the type and strength of interaction between the gas and material of interest. From these studies, we have identified how key properties and features within the examined materials such as size, shape, and chemical composition, lead to significant differences in how vapor molecules interacted with the materials. For example, benzene, toluene, and xylene, which are incredibly important chemicals in industry, were found to be restricted by narrow passageways as they moved through materials with small pores. In contrast, small gases present in the environment from combustion exhaust such as CO₂, SO₂, and NO₂ were able to freely traverse through the passageways, and instead weakly interacted with specific chemical groups inside the cavities of the material. On the same material however, a third class of compounds, organophosphorus-containing chemical warfare agent mimics, irreversibly reacted with chemical groups of the surface, and remained bound even after exposure to high temperatures. Ultimately, the work presented in this thesis is aimed at providing key fundamental insights about specific classes of materials on how, and how strongly they interact with targeted hazardous vapors, which can be utilized by synthetic chemists to design next generation materials.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24587号 / 工博第5093号 / 新制||工||1975(附属図書館) / 京都大学大学院工学研究科合成・生物化学専攻 / (主査)教授 古川 修平, 教授 生越 友樹, 准教授 堀毛 悟史, 教授 松田 建児 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

A molecular-level understanding of gas-surface energy exchange and reaction mechanisms will aid in the prediction of the environmental fate of pollutants and enable advances toward catalysts for the decomposition of toxic compounds. To this end, molecular beam scattering experiments performed in an ultra-high vacuum environment have provided key insights into the initial collision and outcome of critical interfacial processes on model systems. Results from these surface science experiments show that, upon gas-surface collisions, energy transfer depends, in subtle ways, on both the properties of the gas molecules and surfaces. Specifically, model organic surfaces, comprised of long-chain methyl- and hydroxyl-terminated self-assembled monolayers (SAMs) have been employed to test how an interfacial hydrogen bonding network may affect the ability of a gas-phase compound to thermally accommodate (typically, the first step in a reaction) with the surfaces. Results indeed show that small organic compounds transfer less energy to the interconnected hydroxyl-terminated SAM (OH-SAM) than to the organic surface with methyl groups at the interface. However, the dynamics also appear to depend on the polarizability of the impinging gas-phase molecule. The π electrons in the double bond of ethene (C2H4) and the triple bond in ethyne (C2H2) appear to act as hydrogen bond acceptors when the molecules collide with the OH-SAM. The molecular beam scattering studies have demonstrated that these weak attractive forces facilitate energy transfer. A positive correlation between energy transfer and solubilities for analogous solute-solvent combinations was observed for the CH3-SAM (TD fractions: C2H6 > C2H4 > C2H2), but not for the OH-SAM (TD fractions: C2H6 > C2H2 > C2H4). The extent of energy transfer between ethane, ethene, and ethyne and the CH3-SAM appears to be determined by the degrees of freedom or rigidity of the impinging compound, while gas-surface attractive forces play a more decisive role in controlling the scattering dynamics at the OH-SAM. Beyond fundamental studies of energy transfer, this thesis provides detailed surface-science-based studies of the mechanisms involved in the uptake and decomposition of chemical warfare agent (CWA) simulants on or within metal-organic frameworks (MOFs). The work presented here represents the first such study reported in with traditional surface-science based methods have been applied to the study of MOF chemistry. The mechanism and kinetics of interactions between dimethyl methylphosphonate (DMMP) or dimethyl chlorophosphate (DMCP), key CWA simulants, and Zr6-based metal-organic frameworks (MOFs) have been investigated with in situ infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), and DFT calculations. DMMP and DMCP were found to adsorb molecularly (physisorption) to the MOFs through the formation of hydrogen bonds between the phosphoryl oxygen and the free hydroxyl groups associated with Zr6 nodes or dangling -COH groups on the surface of crystallites. Unlike UiO-66, the infrared spectra for UiO-67 and MOF-808, recorded during DMMP exposure, suggest that uptake occurs through both physisorption and chemisorption. The XPS spectra of MOF-808 zirconium 3d electrons reveal a charge redistribution following exposure to DMMP. Besides, the analysis of the phosphorus 2p electrons following exposure and thermal annealing to 600 K indicates that two types of stable phosphorus-containing species exist within the MOF. DFT calculations (performed by Professor Troya at Virginia Tech), were used to guide the IR band assignments and to help interpret the XPS features, suggest that uptake is driven by nucleophilic addition of a surface OH group to DMMP with subsequent elimination of a methoxy substituent to form strongly bound methyl methylphosphonic acid (MMPA). With similar IR features of MOF-808 upon DMCP exposure, the reaction pathway of DMCP in Zr6-MOFs may be similar to that for DMMP, but with the final product being methyl chlorophosphonic acid (elimination of the chlorine) or MMPA (elimination of a methoxy group). The rates of product formation upon DMMP exposure of the MOFs suggest that there are two distinct uptake processes. The rate constants for these processes were found to differ by approximately an order of magnitude. However, the rates of molecular uptake were found to be nearly identical to the rates of reaction, which strongly suggests that the reaction rates are diffusion limited. Overall, and perhaps most importantly, this research has demonstrated that the final products inhibit further reactions within the MOFs. The strongly bound products could not be thermally driven from the MOFs prior to the decomposition of the MOFs themselves. Therefore, new materials are needed before the ultimate goal of creating a catalyst for the air-based destruction of traditional chemical nerve agents is realized. / Doctor of Philosophy / A molecular-level understanding of gas-surface energy exchange and reaction mechanisms will aid in the prediction of the environmental fate of pollutants and enable advances toward catalysts for the decomposition of toxic compounds. Our gas-surface scattering experiments performed in an ultra-high vacuum environment have provided key insights into the outcome of critical interfacial processes on model systems. Results show that energy transfer upon gas-surface collisions depends on both the properties of the gas molecules and surfaces. Due to the formation of interfacial hydrogen bonding network in hydroxyl-terminated surface, the small organic compounds transfer less energy to it than to the organic surface with methyl groups at the interface. The dynamics also appear to depend on the properties of the impinging gas-phase molecule. The π electrons in the double bond of ethene and the triple bond in ethyne act as hydrogen bond acceptors when the molecules collide with the hydroxyl-terminated surface. The attractive forces facilitate energy transfer. A positive correlation between energy transfer and solubilities for analogous solute-solvent combinations was observed for the methyl-terminated surface, but not for the hydroxyl-terminated surface. The extent of energy transfer between ethane, ethene, and ethyne and the methyl-terminated surface appears to be determined by the degrees of freedom or rigidity of the gas, while gas-surface attractive forces play a more decisive role in controlling the scattering dynamics at the hydroxyl-terminated surface. Furthermore, this thesis provides detailed surface-science-based studies of the mechanisms involved in the uptake and decomposition of chemical warfare agent (CWA) simulants on or within metal-organic frameworks (MOFs). Dimethyl methylphosphonate (DMMP) and dimethyl chlorophosphate (DMCP), key CWA simulants, physisorbed to the MOFs through the formation of hydrogen bonds between the phosphoryl oxygen and the free hydroxyl groups associated with inorganic nodes or dangling -COH groups on the surface of crystallites. The infrared spectra for UiO-67 and MOF-808 suggest that uptake occurs through both physisorption and chemisorption. The XPS spectra of MOF-808 zirconium 3d electrons reveal a charge redistribution following exposure to DMMP. Besides, the analysis of the phosphorus 2p electrons following exposure and thermal annealing to 600 K indicates that two types of stable phosphorus-containing species exist within the MOF. DFT calculations suggest that uptake is driven by nucleophilic addition of a surface OH group to DMMP with subsequent elimination of a methoxy substituent to form strongly bound methyl methylphosphonic acid (MMPA). With similar IR features of MOF-808 upon DMCP exposure, the reaction pathway of DMCP in MOFs may be similar to that for DMMP, but with the final product being methyl chlorophosphonic acid (elimination of the chlorine) or MMPA (elimination of a methoxy group). The rates of product formation suggest that there are two distinct uptake processes. The rate constants for these processes were found to be nearly identical to the rates of physisorption, which suggests that the reaction rates are diffusion limited. Overall, this research has demonstrated that the final products inhibit further reactions within the MOFs. The strongly bound products could not be thermally driven from the MOFs prior to the decomposition of the MOFs themselves. Therefore, new materials are needed before the ultimate goal of creating a catalyst for the air-based destruction of traditional chemical nerve agents is realized.

Redox hopping is the dominant charge transport mechanism in many catalyst-modified metal-organic frameworks (MOFs). Previous studies have shown that ion diffusion is the rate-determining step of redox hopping, but the realization regarding to the fundamental mechanism of redox hopping in MOF is still infantile. In this dissertation, we will discuss the redox hopping process in MOFs from multiple perspectives, including how to use the Scholz model to analyze the coefficients in redox hopping, the influence of the type of carrier, the influence of electrolyte concentration, and the influence of temperature on redox hopping, so as to try to reveal the mechanism of the redox hopping process and make some constructive suggestions for the future design and application of MOF based on this topic. / Doctor of Philosophy / Redox hopping is a keyway that electron moves in many special materials called metal-organic frameworks (MOFs) used as catalysts. While we know that the movement of ion is always the bottleneck in MOF because of their huge volume when compared to tiny electrons, there's still a lot to learn about the basic details of how redox hopping works in MOFs. This thesis explores redox hopping in MOFs from various angles. It looks at how we can use a specific model (the Scholz model) to understand this process better and examines the effects of different factors like the type of carrier (electron or hole), the concentration and type of the electrolyte, and the temperature. The goal is to uncover the fundamental workings of redox hopping and gives helpful ideas or principles to guide the design and applications of MOFs in the future.

Thesis advisor: Matthias M. Waegele / Depositing metal-organic frameworks (MOFs) on the surfaces of metal nanoparticles (NPs) to enhance catalytic selectivity has recently attracted great attention; however, a solid understanding of how the NP-MOF interface promotes catalytic selectivity is lacking. In this thesis, we have conducted three fundamental studies and further applied the knowledge to other types of catalysts using enzymes. The first part of this thesis focuses on understanding the NP-MOF interfacial structures and their impact on catalytic performance. We have systematically probed the NP-MOF interface generated by three commonly used approaches by IR and Raman spectroscopy. We have revealed significant differences in interfacial chemical interactions between them, and have found that these differences in interfacial structure dramatically impact selectivity. For example, the interface generated by the coating approach contains trapped capping agents. This trapped capping agent reduces crotyl alcohol selectivity for the hydrogenation of crotonaldehyde. The second part of this thesis focuses on addressing the trapped capping agents at the NP-MOF interface. We developed an approach to creating a direct NP-MOF interface by utilizing weakly adsorbed capping agents during the MOF coating process. Their dynamic nature allows for their gradual dissociation from the NP surface with the assistance of the organic MOF linkers. Thus, direct chemical interactions can be built between NP and MOF, generating a clean and well-defined interface. Direct evidence on capping agent dissociation and formation of chemical interactions was obtained by Raman and IR spectroscopy. Combined with transmission electron microscopy and X-ray diffraction, we have revealed the relative orientation and facet alignment at the NP-MOF interface. The third part of this thesis investigates how various MOF components affect the selectivity of hydrogenation reactions catalyzed at the MOF-NP interface. We found that the replacement of Zr-oxo nodes with Ce-oxo nodes yields the highest selectivity for cinnamyl alcohol (~87%), whereas the functionalization of the terephthalic acid linker with -OH, CH3, -NO2 and NH2 groups only moderately modulates the selectivity relative to the Zr-UiO-66 (~58%). Reaction kinetics studies demonstrate that coating Pt NPs with Ce-UiO-66 increases the rate of C=O hydrogenation, which infrared spectroscopic observations suggest is due to the interaction of the C=O group with the Ce-oxo node. This work highlights the critical role of metal-oxo nodes in regulating the catalytic selectivity of metal NPs in specific reactions. The fourth part of this thesis extends the interface control to other catalysts involving enzymes. We compared the interfacial interactions of catalase in solid and hollow MOF microcrystals. The solid sample with confined catalase was prepared through a reported method. The hollow sample was generated by hollowing the MOFs crystal, sealing freestanding enzymes in the central cavities of the hollow MOF. By monitoring this hollowing process, we observed that the enzymes gradually changed from a confined form to a freestanding form. The freestanding enzymes in the hollow MOFs show higher activity in the decomposition of hydrogen peroxide, attributed to their lesser chemical interactions and confinement. This study highlights the importance of the freestanding state for the biological function of encapsulated enzymes. Taken together, the four sections in this thesis establish design rules for refining MOF-based catalyst design. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第25574号 / 理博第5075号 / 京都大学大学院理学研究科化学専攻 / (主査)教授 北川 宏, 教授 堀毛 悟史, 教授 畠山 琢次 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM