Dissociation Constants of Amine Solvents Used in CO2 Capture: Titrimetric Estimation and Density Functional Theory Calculation

Abstract

Amine solvents are basic in nature and used for the absorption of acidic CO2. The dissociation constant of protonated amine solvents significantly influences their CO2 absorption, and it varies with temperature. It is thus important to estimate the temperature-dependent dissociation constant of new protonated amine solvents before using them for CO2 absorption purposes. In the present work, the procedures and protocols for estimating the dissociation constant (Ka) of protonated amine solvents using the potentiometric titration method are optimized in the laboratory. The validation of the optimized potentiometric methodologies is confirmed by estimating precisely the reported −ln Ka values of two different amine solvents, namely, monoethanolamine (MEA) and 2-amino-2-methyl-1-propenol (AMP). Subsequently, the −ln Ka of the new amine solvent bis(3-aminopropyl)amine (APA) is determined by the potentiometric titration method at different temperatures (298–318 K). The −ln Ka value estimated for APA at 298 K is 21.73, which is comparable to those of amine solvents known for efficient CO2 capture application. Electronic structure calculations are carried out to explain the influence of the methyl group (−CH3) and alcoholic group (−OH) on the −ln Ka values of the studied amine solvents. The scaffolds for the studied amine solvents are optimized by density functional theory (DFT). Natural bond orbital (NBO) calculations are performed to estimate the bond lengths as well as charge distributions on nitrogen and other elements in the optimized geometries of amine solvents. The experimentally obtained −ln Ka values of the studied amines are validated through DFT calculations.