Solvation and Dynamics of CO₂ in Aqueous Alkanolamine Solutions

Abstract

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-2butanol (DEAB)) in the presence of CO 2 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 CO 2 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. CO 2 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 CO 2alkanolamine interactions are described by characteristic residence times of CO 2 . The hydrophobic 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 CO 2 chemisorption process. Molecular engineering of novel compounds for absorptive sequestration has to take into account not only the thermodynamics and chemical reactivity but also liquid structure properties, the dynamics of CO 2 diffusion and the kinetics of interactions in complex ternary solutions.

The effect of CO2 loading on alkanolamine absorbents in aqueous solutions

“…In CO 2 -loaded solution, the primary amine MEA molecules are fully solvated by water and the CO 2 molecules are equally distributed between water and alkanolamine molecules. 17 A simulation snapshot of an entire system at 0.3 CO 2 -loading is provided in Fig. S1 (ESI †).…”

“…The CO 2 molecules show a preferred localization within these hydrophobic aggregates. 14,17 In contrast to MEA, absorption of CO 2 in aqueous solutions by DEAB significantly affects the liquid structure. The effect of increasing CO 2 loading on DEAB cluster formation can be seen in Fig.…”

“…32 The accuracy of the use of these forcefields for the simulation of CO 2 solvation in aqueous alkanolamines was validated before. 14,17 Partial charges for the carbamate MEACOO À and protonated MEAH + were taken from literature, 18 and for the protonated DEABH + from ref. 30.…”

“…Recently, a wealth of information about liquid structure, intermolecular interactions and diffusion properties of aqueous alkanolamines, as well as solvation and mobility of CO 2 molecules within such solvents was obtained computationally. [7][8][9][10][11][12][13][14][15][16][17][18] However, all these studies were limited to the investigation of the pre-reaction state of CO 2 in aqueous amine solutions. During the chemical CO 2 absorption, carbamate and carbonate ions together with protonated alkanolamines are formed as products of the amine reaction with CO 2 , which then would eventually change the complex mixed solvent properties.…”

“…22 Here, we focus on the primary monoethanolamine (MEA), the standard compound currently mostly used for CO 2 capture, and the novel tertiary alkanolamine 4-diethylamino-2-butanol (DEAB) which were both fully characterized in binary aqueous alkanolamine solutions 14 and in the presence of CO 2 . 17 The choice of these two compounds is motivated by their difference in mechanism upon CO 2 absorption and the formation of different intermediates in solution. MEA is fully hydrated and when chemically absorbing one CO 2 molecule the carbamate anion MEACOO À and the protonated MEAH + are generated.…”