Design, Synthesis and Evaluation of Liquid-like Nanoparticle Organic Hybrid Materials for Carbon Dioxide Capture

摘要

Given the rapid increase in atmospheric concentration of CO2, the development of efficient CO2 capture technologies is critical for the future of carbon-based energy. Currently, the most commonly employed approach to capture CO2 is amine scrubbing in which amine-based solvents react with gaseous CO2 to form carbamate. Although the amine-based solvents such as monoethanolamine (MEA) exhibit high CO2 capture capacity, their high volatility results in corrosive fumes and energy-intensive regeneration process. Therefore, there is an urgent need to develop alternative CO2 capture media that can be efficient and environmentally sustainable. To achieve this goal, a new class of CO2 capture media named Nanoparticle Organic Hybrid Materials (NOHMs) has been formulated. A unit of NOHMs consists of a surface-functionalized nanoparticle as a core to which selected polymers are tethered to form a canopy. Such a configuration prevents loss of polymers and enables NOHMs to exhibit near zero vapor pressure. As the canopy is tethered to the core, it has been theorized that CO2 can be captured not only by the enthalpic effect via reactions with functional groups along the polymeric canopy but also by the entropic means via introduction of small gaseous molecules such as CO2 to reduce the free energy of the frustrated canopy. This study represents the first attempt to investigate CO2 capture using NOHMs. In this dissertation, NOHMs were designed, synthesized and evaluated for CO2 capture properties. Characterization of NOHMs was conducted by employing various spectroscopic tools, such as ATR FT-IR, Raman and NMR, to confirm successful synthesis of NOHMs. Thermal stability and nanoscale configuration of NOHMs were measured using TGA and TEM, respectively. NOHMs with various chemical and structural parameters, including bonding types, functional groups, chain lengths, core sizes, and core fractions, were prepared. The effects of these parameters on CO2 capture relevant properties such as thermal stability, thermally-induced swelling, CO2-induced swelling, CO2 packing behavior and CO2 capture capacity were explored in detail. In comparison to the unbound polymers, NOHMs exhibited enhanced thermal stability. Such an enhancement allows NOHMs to be used in a wide-range of operational temperatures. While an unbound polymer degraded 80 wt% after a 100-cycle temperature swing, there was no significant loss in its corresponding NOHMs. Elevated temperatures also caused NOHMs to swell but the degree of thermally-induced swelling of NOHMs was less than that of the unbound polymers due to restriction on movement of the tethered polymer chains. CO2 capture capacity studies revealed that NOHMs can capture 0.1 - 0.4 mmol/g-solvent depending on partial pressure of CO2 and temperatures. The CO2 capture mechanism was also revealed as a Lewis acid-base interaction between CO2 and ether groups which were the most common functional groups of the polymers selected for the NOHMs synthesis (e.g. NOHM-I-HPE, NOHM-I-tPE and NOHM-I-PEG). The effect of functional groups on CO2 capture was far more significant. When amines were incorporated in NOHMs (e.g. 2.2 mmol/g-solvent in NOHM-I-PEI), as expected, the presence of amines enhanced CO2 capture capacity. While the enthalpic effect was pronounced, the entropic effect from NOHMs' unique structural nature would allow CO2 to be captured more effectively. In order to explore the entropic effect, NOHMs were synthesized to minimize the enthalpic effect for the most of structural studies, such as studies of CO2-induced swelling and interaction of CO2 with the canopy. For the CO2-induced swelling behavior, NOHMs exhibited notably less swelling than the unbound polymers at a given CO2 capture capacity. NOHMs comprised of shorter polymer chains exhibited even less swelling than NOHMs having longer polymer chains at a given CO2 capture capacity. This may be due to conformational differences between NOHMs and the unbound polymers which allow more CO2 molecules to pack within polymer chains. Such conformational differences were further pronounced by lowering the grafting densities in NOHMs. These differences were attributed to specific structural configuration of a NOHMs' canopy in which polymer chains were tethered onto inorganic nanoparticle cores causing more "rigid" arrangements than in a bulk polymer. In order to facilitate the implementation of NOHMs for CO2 capture, several aspects were also investigated, including impact of SO2, viscosity of NOHMs and CO2 diffusivity in NOHMs. It was found that no significant amount of SO2 was captured in NOHMs at low concentration (200 ppm of SO2 in N2), while a considerable amount of SO2 was captured by NOHMs at 3010 ppm of SO2 in N2. As N2 is almost insoluble in NOHMs, NOHMs showed a high selectivity toward SO2 capture over N2 capture. This behavior enables NOHMs to be a potential candidate for SO2 removal. About 10 - 30% of CO2 capture capacity was reduced after NOHMs exposed to SO2 due to unavailability of some capture sites in NOHMs, which were occupied by SO2. The result of the simultaneous removal of CO2 and SO2 showed that at low SO2 concentration, NOHMs did not exhibit a noticeable selectivity toward SO2 over CO2. CO2 capture capacity and CO2-induced swelling were also measured with a mixture of CO2/SO2 to explore the effect of SO2 on CO2-induced swelling and packing behaviors of NOHMs. Similar swelling behaviors were observed under pressurization with CO2/SO2 and pure CO2 at low pressures. However, swelling behaviors of two cases deviated at higher pressures. This may be attributed to the distinct packing patterns in NOHMs under the pressurization with CO2/SO2 compared to the pressurization with pure CO2. The viscosity and CO2 diffusivity in NOHMs with various structural parameters were also measured. The effect of core size was not pronounced on viscosity and CO2 diffusivity as the core fraction was fixed. In contrast, a higher core fraction in NOHMs resulted in a significantly higher viscosity and a lower CO2 diffusivity. The effect of temperature was also notable on CO2 diffusion in NOHMs. However, a higher temperature can have a negative impact on the CO2 capture capacity of NOHMs. To obtain an improved CO2 diffusivity for CO2 capture, the optimal operation temperature ranging from 40 to 70 ºC was determined. Finally, if the viscosity of NOHMs could be appropriately lowered by manipulating core sizes and core fractions at an optimal operation temperature, fluid NOHMs could be used in a spraying tower to capture CO2. For relatively viscous NOHMs, such as amine functionalized NOHMs, a supported liquid membrane system could be used by coating or filling NOHMs inside the membrane to increase contact area for CO2 capture. NOHMs could also be even functionalized to serve as dual-purpose smart materials for CO2 capture and photocatalytic conversion of CO2 to alcohols. A significantly amount of energy could be saved from solvent regeneration and the product would improve the process economics. In summary, NOHMs were designed and synthesized to investigate the effects of chemical and structural parameters on key factors affecting CO2 capture, including thermal stability, thermally-induced swelling, CO2-induced swelling, CO2 packing behavior, selectivity, viscosity and CO2 diffusivity. The fundamental knowledge gained in this study not only became a basis for the optimal design of NOHMs for CO2 capture but also provided important information on how to design nanoscale hybrid materials for other advanced environmental and energy technologies.