Solvation and Dynamics of CO2 in Aqueous Alkanolamine Solutions

Carbon dioxide sequestration from flue gases by chemical absorption is the most versatile process. The molecular engineering of novel high-performance biogas upgrading alkanolamine compounds requires detailed information about their properties in mixed solution. The liquid structure properties of four representative alkanolamine molecules (monoethanolamine (MEA) as a reference and standard, 3-aminopropanol (MPA), 2-methylaminoethanol (MMEA), and 4-diethylamino-2-butanol (DEAB)) in the presence of CO2 were investigated over a wide range of solvent alkanolamine/water mixture compositions and temperature. In aqueous solution, for the alkanolamine molecules MEA, MPA, and MMEA, hydrogen bonding with solvent water molecules is dominating over CO2 interactions. Analysis of the liquid structure reveals that carbon dioxide shows no preference of approaching the alkanolamine but is rather displaced by water molecules as the water content increases. CO2 dissolved in aqueous DEAB, however, accumulates within clusters of DEAB molecules almost devoid of water. The calculated carbon dioxide diffusion coefficients for all four molecules agree well with experiment where available and are obtained for all mixture compositions and as a function of temperature. The solute diffusion correlates with the mobility of the alkanolamines in water at various ternary mixture compositions. Kinetic aspects of the CO2-alkanolamine interactions are described by characteristic residence times of CO2. The hydrophobic interaction of carbon dioxide with the alkanolamine has a lifetime on the order of tens of picoseconds whereas polar interactions are about 1 order of magnitude shorter. The tertiary amine DEAB displays many favorable features for an efficient CO2 chemisorption process. Molecular engineering of novel compounds for absorptive sequestration has to take into account not only the thermodynamics and chemical reactivity but also the liquid structure properties, the dynamics of CO2 diffusion, and the kinetics of interactions in complex ternary solutions.