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-performant 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, hydrogen bonding with solvent water molecules are dominating over CO2 interactions for MEA, MPA and MMEA. Analysis of the liquid structure reveals that carbon dioxide shows no preference of approaching the alkanolamine but is rather displaced by water molecules. CO2 dissolved in aqueous DEAB, however, accumulates within clusters of DEAB molecules devoid of water. The carbon dioxide diffusion coefficients for all four molecules agree well with experiment for 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 hydrophic interaction of carbon dioxide with the alkanolamine has a lifetime of the order of tens of picoseconds whereas polar interactions are about one order of magnitude shorter. The tertiary amine DEAB displays many favorable features for an efficient CO2 chemisorption process. Molecular engineering of novel compounds for absortive sequestration have to take into account not only the thermodynamics and chemical reactivity but also liquid structure properties and the dynamics and kinetics of interactions in complex ternary solutions.