DFT Study on Thermodynamic Parameters of CO₂ Absorption into Aqueous Solution of Aliphatic Amines

Using density functional theory (DFT), the thermodynamic parameters of the CO₂ absorp-tion into the aqueous solution of the aliphatic amines including some alkylamines, alka-nolamines, diamines and a type of aminoamide were calculated. The geometry optimiza-tion and the calculation of vibrational frequencies in the gas phase were performed at the B3LYP/6-311+ G(d,p) level of theory. Moreover, the standard solvation free energies of the studied species were computed upon the solution phase optimized geometries through the latest continuum solvation model (SMD/IEF-PCM) at the HF/6-31G(d) level of theory. With this approach, two important properties of the CO₂ absorption into the aqueous so-lutions of the studied amines were evaluated: the acid dissociation constant (pK_a) of the parent amines and the standard enthalpy change (ΔH^?_abs) related to the CO₂ absorption process. A roughly linear relationship was observed between pK_a and ΔH^?_abs. This finding suggests that a raise in the basicity of an amine leads to an increase in the heat released in CO₂ absorption and consequently in the required energy for the regeneration of amine.