Molecular Dynamics Study of the Solution Structure, Clustering, and Diffusion of Four Aqueous Alkanolamines

Abstract

CO sequestration from anthropogenic resources is a challenge to the design of environmental processes at a large scale. Reversible chemical absorption by amine-based solvents is one of the most efficient methods of CO removal. Molecular simulation techniques are very useful tools to investigate CO binding by aqueous alkanolamine molecules for further technological application. In the present work, we have performed detailed atomistic molecular dynamics simulations of aqueous solutions of three prototype amines: monoethanolamine (MEA) as a standard, 3-aminopropanol (MPA), 2-methylaminoethanol (MMEA), and 4-diethylamino-2-butanol (DEAB) as potential novel CO absorptive solvents. Solvent densities, radial distribution functions, cluster size distributions, hydrogen-bonding statistics, and diffusion coefficients for a full range of mixture compositions have been obtained. The solvent densities and diffusion coefficients from simulations are in good agreement with those in the experiment. In aqueous solution, MEA, MPA, and MMEA molecules prefer to be fully solvated by water molecules, whereas DEAB molecules tend to self-aggregate. In a range from 30/70-50/50 (w/w) alkanolamine/water mixtures, they form a bicontinuous phase (both alkanolamine and water are organized in two mutually percolating clusters). Among the studied aqueous alkanolamine solutions, the diffusion coefficients decrease in the following order MEA > MPA = MMEA > DEAB. With an increase of water content, the diffusion coefficients increase for all studied alkanolamines. The presented results are a first step for process-scale simulation and provide important qualitative and quantitative information for the design and engineering of efficient new CO removal processes.

Solvation and Dynamics of CO2 in Aqueous Alkanolamine Solutions

“…At the studied CO 2 concentrations of 0.1-0.2 M, the simulated liquid densities of alkanolamine/water/CO 2 ternary mixtures do not significantly differ from those of the previously simulated two component solvents (alkanolamine/water) 22 and are in good agreement with experiment 29 (see Table 1). This shows that the flow and solvation of CO 2 at common loadings hardly affect the system's bulk properties.…”

“…Above a weight fraction of 50%, the temperature-dependence of the diffusion coefficient deviates 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 24 from linearity and the system deviates from ideal behaviour. 10 The same effect is also observed for the diffusion the alkanolamine molecules in water 22 . A comparison of CO 2 diffusion in the different alkanolamine solutions at a specific temperature of 313 K is given in Figure 11.…”

“…Representative snapshots from MD simulations showing the dominant association channels of carbon dioxide approaching DEAB molecule.On average, each nitrogen atom of DEAB molecule forms about 0.97 hydrogen bonds with water at a composition of 7.5/92.5 DEAB/W (w/w) and 0.47 hydrogen bonds at 80/20 DEAB/W (w/w) 22. …”

The effect of CO2 loading on alkanolamine absorbents in aqueous solutions

“…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.…”

Solubility prediction in mixed solvents: A combined molecular simulation and experimental approach

“…Classical molecular simulation offers direct prediction of thermodynamic, kinetic [6], and structural properties [7,8] for mixed solvents of arbitrary composition.…”