| Summary: | Research Doctorate - Doctor of Philosophy (PhD) Post Combustion Capture (PCC) of carbon dioxide (CO₂) involving reversible chemical absorption using aqueous amine solvents is at the forefront of technologies for the immediate abatement of runaway anthropogenic emissions from the coal-fired generation of electricity. It is clear from our current knowledge and information that improvements must be made to the current technology platform in order to facilitate successful upscale and deployment to the industrial process. One of the key aspects of PCC is the selection of the reactive solvent solution and the chemical pathways for CO₂in these solutions. A fundamental understanding of the chemical behaviour at the molecular level will contribute to significant reductions in the initial capital cost investment since the physical size of the PCC plant is directly influenced by the solvent chemistry, and improvements in the capture rates and efficiencies of the process which will ultimately reduce the ongoing energy penalty to the power plants associated with the PCC process. The culmination of work embodied in this thesis is focussed on a fundamental understanding of the reaction mechanisms and the kinetic interactions of CO₂(aq) with amines in aqueous solutions, at the molecular level, with the core focussing on the extended relationship between the structure of the amine and the chemical properties of the amine that are relevant for PCC applications. Since the formation of the carbamate product via the reaction of CO₂ with non-hindered, primary and secondary monoamines is detrimental to the cyclic capacity of the solutions in that the total amount of CO₂ chemically bound and released per absorption/desorption cycle is limited by this pathway, its formation is an undesirable part of the interaction. However, the favorable kinetics associated with the carbamate pathway make it highly attractive for PCC purposes. Hence, a deeper understanding of the reactions leading to the carbamate product are critical in any endeavour for the deployment of the highest efficiency amine process. The rate and equilibrium constants for the formation of carbamates/carbamic acids from the reaction of CO₂(aq) with amines, and the coupled relationships between the structure of the amine with this reactivity, can be used to guide the selection of the ideal amine solvent. Furthermore, the resulting chemical constants can be used in the detailed modelling of the PCC process and in the validation of engineering data acquired in pilot plant investigations. Additionally, the chemical constants can be used in the ranking of alternative solvents against the current industry standard solvent, monoethanolamine (MEA). Beginning with a complete mechanism to formally describe the complete series of reversible chemical reactions of CO₂(aq) in aqueous solutions, including those with water (H₂O), hydroxide (OH⁻), and amine (RNH₂), to form carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻) and carbamic acid (RNHCOOH), respectively, the mechanism has been applied and verified experimentally throughout this work in the determination of kinetic and equilibrium constants, including the protonation constant of the carbamate, for a comprehensive series of amines. A critical aspect of this work involved the development of analytical techniques that generate the measurement data in order to define the reaction mechanism, as well as the analysis protocols. The reaction of CO₂(aq) with primary and secondary amines to form the carbamate is typically a fast reaction. To investigate the rapid kinetics of this reaction, required a suitably fast technique. Stopped-flow spectrophotometric kinetic measurements in the presence of coloured acid-base indicators was employed to follow the progression of the fast reactions. Such reactions involve the formation of acids (i.e carbonic and carbamic acid) or bases (i.e. OH⁻, CO₃²⁻, HCO₃⁻, and amine) resulting in pH changes, and hence instantaneous changes in colour, in solution, of the indicators, which can be monitored spectrophotometrically. In addition to the fast stopped-flow studies, the progress of the slower parallel reaction between bicarbonate (HCO₃⁻) and amine was followed here for several of the amines by 1H-NMR spectroscopy. The 1H NMR technique was also employed in the quantitative determination of chemical speciation in equilibrium solutions of carbamates. A complete overview of the stopped-flow and 1H NMR methods including a selection of stopped-flow spectrophotometric absorbance measurements and 1H-NMR spectrum is detailed in the publications in chapters (2) to (7) of this thesis. Advanced data analysis was also an important element of this work since the entire suite of chemical reactions, i.e. the reactions of CO₂(aq) with water (H₂O), hydroxide (OH⁻) and amine (RNH₂), occur simultaneously in solution and are intimately coupled via the solution pH. Global analysis of a series of stopped-flow and 1H-NMR kinetic measurements acquired at the same temperatures and different concentrations was used to determine the corresponding kinetic rate constants and equilibrium constants for the complete mechanism.
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