In Situ Nuclear Magnetic Resonance Mechanistic Studies of Carbon Dioxide Reactions with Liquid Amines in Aqueous Systems: New Insights on Carbon Capture Reaction Pathways

A series of closely related primary, secondary, and tertiary alkanolamine model compounds were monitored in real time in aqueous solution via in situ nuclear magnetic resonance (NMR) spectroscopy while purging CO2-rich gas through the solution over a range of temperatures. The real-time in situ spectroscopic monitoring of this reaction chemistry provides new insight about reaction pathways through identification of primary products and their transformations into secondary products. New mechanistic pathways were observed and elucidated. The effects of CO2 loadings, relative absorption and desorption kinetics, pH, temperature, and other critical features of the amine/CO2 reaction system are discussed in detail. The effect of amine basicity and structure on these parameters was further elucidated by studying complementary electron-rich and -poor amines (pK(a) similar to 4.5-11) and guanidines (pK(a) similar to 14-15). While tertiary amines act only as simple proton acceptors, primary and secondary amines function as both bases and nudeophiles to form carbamates and (bi) carbonates, whose product ratio is a function of both reaction conditions and amine steric and electronic properties. Water is also acting as a Lewis base by hydrolysis of carbamate species into bicarbonate, which results in a more beneficial 1:1 CO2/amine ratio. Primary and secondary amines tend to react with CO2 similarly at different CO2 partial pressures, showing weak pressure dependence upon CO2 loading; in contrast, reaction efficiencies of tertiary amines, which generally form less stable carbonate and bicarbonate products, are a strong function of CO2 pressure. Primary and secondary amines capture significantly less CO2 per mole of amine than tertiary amines (lower CO2 loading capacities) because of the formation of carbarnate species. Their faster reaction rates with CO2 and high capture efficiencies at low CO2 partial pressures are advantageous. In contrast, tertiary amines more effectively react with CO2 at lower temperatures, capturing up to 1 CO2 per amine; initially and unexpectedly, carbonate and bicarbonate species are initially formed simultaneously. Even at high pH, carbonates evolve into a final bicarbonate product. The secondary benefit of forming bicarbonates is their lower thermal stability (greater ease of desorption). Unexpectedly, guanidines do not form bicarbonates directly; the reaction proceeds via exclusive initial formation of the guanidinium carbonate. In summary, varying amine basicity leads to significant changes in the carbamate/(bi)carbonate equilibrium and stability of reaction products.