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Stimuli-responsive polymers have attracted great attention due to their unique responses in physicochemical properties to changes in external environment and stimuli variations (e.g. pH, thermo, light, electric and magnetic field, etc.). Introduction of gas triggers has greatly expanded the family of stimuli-responsive polymers since the first work on a CO2-switchable polymer was reported in 2006. Although significant progresses have been achieved over the past decade, studies on gas-switchable polymers are still at an early stage, with many challenges in both fundamental and application areas remaining to be tackled. This thesis focuses on synthesis and application development of O2 and CO2 switchable polymers. The thesis starts from investigating O2-switchable thermo-responsive fluorinated monomers and their linear random copolymers, which undergo LCST shifting induced by O2 treatment. It provides good insight and general guidance for design and screening of monomer candidates for the synthesis of O2-switchable polymers. The second part of the thesis presents development of the first O2 and CO2 dual gas-switchable microgel system and building of the microgel-colloidosomes with O2 and CO2 tailored shell permeability. The latter is evaluated as microcapsules for hierarchical control-release of water-soluble cargo molecules upon respective O2 and CO2 treatment. The last part of the thesis explores preparation of various porous polymer systems from high internal phase emulsion (HIPE) templates. In particular, a highly porous polyHIPE membrane with "open-cell" structure exhibiting CO2-switchable surface wettability is promising for smart separation applications. This thesis represents a significant progress and a solid step forward in the development of gas-switchable polymers from monomer design, polymer preparation, and advanced application of polymer systems. / Thesis / Doctor of Philosophy (PhD) / Polymer materials are used in every aspect of our daily life, from clothing, furniture to construction. This thesis work is to develop functional polymers that can feel variations in environment (e.g. pH, thermo, light, magnetic, etc) and give out changes in properties. Such materials are smart for stimuli-responsive applications. The first report of carbon dioxide (CO2)-switchable polymers in 2006 was marked as the start of gas-responsive polymers. Gas triggers interact with specific functionalities in polymer chains and change their physicochemical properties such as chain structure, architecture, hydrophilicity and polarity. Oxygen (O2) and CO2 are explored in this work to reversibly trigger changes in volume, shell permeability and surface wettability, of different polymer systems. Potential applications include drug control-release, smart oil/water separation, and so on. This work presents a systematic study on the development of O2 and/or CO2-switchable polymers from monomer design, polymer preparation, and advanced application of polymer systems. It represents a significant progress and a solid step forward in the study of gas-switchable polymers.

In the pursuit of reliable, efficient, and cost-effective life support systems for NASA's deep space exploration missions, this study explores an innovative variable conductance radiator (VCR) as an alternative CO2 deposition method, capitalizing on phase change temperatures of air components to selectively remove CO2. The research objectives encompass the design, construction, and comprehensive assessment of a prototype VCR. Initial experimental characterization of the VCR prototype, employing water as the working fluid and an ice/water bath as a heat sink, demonstrates its variable conductance capabilities in both capture and recovery modes. The impact of flow rate and inlet fluid temperatures on heat transfer efficiency were studied, offering insights into system optimization. Results are presented detailing thermal performance under various conditions. The study also involves numerical characterization of the VCR prototype employing SolidWorks flow simulation. Comparison of the experimental and numerical results show an acceptable agreement, indicate likely sources of deviation, and encourages further utilization of numerical approach for optimization and scaling. Additionally, an analytical analysis is conducted to find the energy (heat absorption) involved in CO2 deposition, to evaluate various design aspects including radiator panel size and the equivalent thermal resistance of the system, and finally to do a rough optimization of the full-scale radiator design parameters considering heat transfer and pressure loss effects. As part of the investigative process, a preliminary design for the full-size radiator is developed.

Silylated amines, also known as reversible ionic liquids (RevILs), have been designed and structurally modified by our group for potential use as solvents for CO₂ capture from flue gas. An ideal CO₂ capture ionic liquid should be able to selectively and reversibly capture CO₂ and have tolerance for other components in flue gas, including SO₂, NO₂, and O₂. In this project, we study the reactivity, selectivity, uptake capacity, and reversibility of RevILs in the presence of pure SO₂ and mixed gas streams tosimulate flue gas compositions. Tripropylsilylamine (TPSA), a candidate CO₂ capture RevIL, reacts with pure SO₂ to form an ionic liquid consisting of an ammonium group and a salfamate group, supported by IR and NMR results. The resulting IL with pure SO₂ partially reverses when heated to temperatures of upto 500 C in the TGA. TGA analysis of the ionic liquid formed from a 4 vol% SO₂ in CO₂ mixture indicates a possible reversal temperature in the 86-163 C range.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22467号 / 工博第4728号 / 新制||工||1738(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 田中 庸裕, 教授 江口 浩一, 教授 佐藤 啓文 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Atmospheric carbon dioxide (CO₂) concentrations have increased rapidly in recent decades due to the burning of fossil fuels, deforestation, and other industrial practices. The excessive accumulation of CO₂ in the atmosphere leads to global warming, ocean acidification, and other environmental imbalances, which may ultimately have wider societal implications. One potential solution to closing the carbon cycle is utilizing CO₂, rather than fossil fuels, as the carbon source for fuels and chemicals production. This lowers atmospheric CO₂ levels while simultaneously providing an economic incentive for capturing and converting CO₂ into more valuable products. This dissertation includes studies on three hybrid catalytic reactor systems coupling electrochemistry, thermochemistry, and plasma chemistry for the conversion of CO₂ into value-added oxygenates, such as methanol and C3 oxygenates (propanal and 1-propanol). First, a tandem two-stage system is described where CO₂ is electrochemically reduced into syngas followed by the thermochemical methanol synthesis reaction. The work here specifically focuses on the electrochemical CO₂ reduction reaction to produce syngas with tunable H₂/CO ratios. Using a combination of electrochemical experiments, in-situ characterization, and density functional theory calculations, palladium-, gold-, and silver-modified transition metal carbides and nitrides were found to be promising catalysts for enhancing electrochemical activity while reducing the overall precious metal loading. Second, another tandem two-stage system is demonstrated where CO₂ is electrochemically reduced into ethylene and syngas followed by the thermochemical hydroformylation reaction to produce propanal and 1-propanol. The CO₂ electrolyzer was evaluated with Cu catalysts containing different oxidation states and with modifications to the gas diffusion layer hydrophobicity, while the hydroformylation reactor was tested over a Rh₁Co₃/MCM-41 catalyst. The tandem configuration achieved a C₃ oxygenate selectivity of ~18%, representing over a 4-fold improvement compared to direct electrochemical CO₂ conversion to 1-propanol in flow cells. Third, a hybrid plasma-catalytic system is investigated where CO₂ and ethane are directly converted into multi-carbon oxygenates in a one-step process under ambient conditions. Oxygenate selectivity was enhanced at lower plasma powers and higher CO₂ to C₂H₆ ratios, and the addition of a Rh₁Co₃/MCM-41 catalyst increased the oxygenate selectivity at early timescales. Plasma chemical kinetic modeling, isotopically-labeled CO₂ experiments, and in-situ spectroscopy were also used to probe the reaction pathways, revealing that alcohol formation occurred via the oxidation of ethane-derived activated species rather than a CO₂ hydrogenation pathway. It is critical to assess whether the proposed CO₂ conversion strategies consume more CO₂ than they emit. A comparative analysis of the energy costs and net CO₂ emissions is conducted for various reaction schemes, including four hybrid pathways (thermocatalytic-thermocatalytic, plasma-thermocatalytic, electrocatalytic-thermocatalytic, and electrocatalytic-electrocatalytic) for converting CO₂ into C₃ oxygenates. The hybrid processes can achieve a net reduction in CO₂ provided that low-carbon energy sources are used, however further catalyst improvements and engineering optimizations are necessary. Hybrid catalytic systems can provide an alternative approach to traditional processes, and these concepts can be extended to other chemical reactions and products, thereby opening new opportunities for innovative CO₂ utilization technologies.

This thesis aims to provide different aspects to make the Direct Air Capture Dual Function Materials (DAC-DFM) project more applicable in commercialization and large-scale deployment to directly address the global warming problem caused by anthropogenic CO₂. Dual function materials are comprised of nano dispersed alkaline sorbents and a methanation catalyst to capture CO₂ from ambient air and convert it to CH₄ upon the addition of green H₂. Two sub-projects named “Ru thrifting project” and “hydrophobicity/surface modification project” were performed to study the potential optimization and tradeoff when modifying the DFM components. The major accomplishment in this thesis has been the thrifting of Ru from its original value of 5% to 1% and finally 0.25% with no sacrifice in stability but with some decrease in capacity for CO₂ capture and methanation. Given the Ru unit price (around 14 USD/g, flexible market, in March 2024), this approach would greatly reduce the overall production cost. In the hydrophobicity/surface modification project, Al₂O₃ the high surface area carrier for the DFM components, was treated using 3 different methods (high temperature calcination, acid treatment, and metal oxide doping). The goal of this study was to reduce the surface water uptake in the Al₂O₃ from humidity, present in ambient air, by increasing hydrophobicity to reduce the energy evaporation cost during temperature swing in the DFM CO₂ desorption/methanation process. Samples treated after these 3 methods showed a significant decrease in water uptake. ZrO₂ doped Na₂CO₃-Al₂O₃ sample showed a low H₂O uptake and the highest CO₂ adsorption, twice that of the CO₂ capacity of the Na₂CO₃-Al₂O₃ samples treated by calcination and acid. Such hydrophobicity study would be used to further optimize the components of DFM to meet requirements in real world applications. Future outlook for DFMs is also briefly discussed with focuses on (1) using pre-heated H₂ to increase the temperature of inner DFM coatings for increased energy efficiency, (2) increased CO₂ capture, utilization, and conversion to CH₄, as well as (3) practical acceptance of DAC hubs and CO₂ taxes/credits. A new approach of moisture swing DAC sorbents as an alternate technology to the thermal swing DFM is suggested as a future project. With loaded CO₃²⁻ on both organic ion exchange particles/membranes and inorganic silica-based granules, the CO₂ adsorption/desorption effect can be controlled by the moisture level (relative humidity) from ambient air in the inlet sample chamber.