Performance of tertiary amines as the absorbents for CO2 capture: Quantum mechanics and molecular dynamics studies

Quantum mechanical (QM) methods were utilized to study the effect of substitution on the tertiary amines, for their application as the efficient absorbents, to capture CO2, known globally, as an undesirable greenhouse gas. For this purpose, by using density functional theory, the tertiary amine absorbents 1-M-2PPE, DEA-1,2-PD, DEA-1P and MDEA were examined by calculating the transition state activation energies of their absorption reactions. By considering the results obtained, the mechanism of the reaction kinetics, for CO2 absorption reaction, on these tertiary amines was proposed. The absorption results of QM calculations and the related interpretations were compared with the reported experimental results and those obtained by molecular dynamics (MD) simulations for similarly simplified systems. The pK(a) values for the tertiary amines were evaluated by QM calculations and their effects on the rate of CO2 absorption, were examined. The results of CO2 absorption on the studied tertiary amines aqueous solutions were compared with those obtained for conventional tertiary amine absorbent MDEA, which is used widely in various industries. It was found that for CO2 absorption, the tertiary amines 1M-2PPE and DEA-1,2-PD have higher reaction rate, along with higher diffusion coefficient and solubility, compared with MDEA. Therefore, it would be advantageous to use 1M-2PPE, DEA-1,2-PD in industrial units for CO2 capture in respect to both absorption performance, and energy consumption considerations.