NMR Speciation of Aqueous MAPA, Tertiary Amines, and Their Blends in the Presence of CO₂: Influence of pK_a 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 pK_a 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.