NMR Speciation of Aqueous MAPA, Tertiary Amines, and Their Blends in the Presence of CO2: Influence of pKa and Reaction Mechanisms

The diamine 3-(methylamino)­propylamine (MAPA) and eight tertiary amines were studied as single and blended aqueous CO2 absorbents at different concentrations and ratios to investigate their reaction mechanisms. After absorption (40 °C) and desorption (80 °C) experiments, quantitative 13C NMR experiments were performed on each liquid sample. After absorption, the following CO2-derived species were identified and quantified: (bi)­carbonate in each amine system, primary and secondary MAPA carbamate and MAPA dicarbamate in MAPA systems, and tertiary amine carbonate in tertiary amine systems. Concerning desorption, the main CO2 species removed was (bi)­carbonate, followed in the blends by MAPA dicarbamate. Since, after CO2 absorption, the concentration of MAPA dicarbamate was negligible in single MAPA and, in the blends, mainly increased at increasing pKa of the tertiary amines, it is concluded that the tertiary amines increase the availability of MAPA species to react further with CO2. MAPA and tertiary amines appear to influence each other also for the CO2 release.