Molecular insights into the CO₂ absorption mechanism by superbase protic ionic liquids by a combined density functional theory and molecular dynamics approach

Protic ionic liquids (PILs) are emerging as a new class of sustainable and efficient solvents for CO2 capture, requiring a fundamental understanding of their properties for their optimal design. To obtain a molecular-level understanding of the mechanism behind CO2 absorption in this class of absorbents, we selected four novel and high-efficient PILs prepared from superbase 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]-5-nonene (DBN) as cations, with imidazole (Im) and pyrazole (Pyr) as anions. Density functional theory (DFT) and molecular dynamics (MD) simulations were used to quantify their interactions and reaction mechanisms as well as the dynamics of the CO2 absorption process. Results indicate that CO2 primarily interacts with the anions of PILs through van der Waals forces, while the cations and anions of PILs mainly engage in strong hydrogen-bonding interactions. Additionally, the anions primarily serve as the absorption reaction sites for CO2, with their molecule centers of mass being the closest. Meanwhile, reaction with CO2 requires overcoming a relatively low energy barrier (i.e., similar to 35-40 kJmol(-1)), making them more favorable for regeneration than benchmark solvents. Notably, MD simulations have also shown that CO2 molecules are preferentially accumulating at the gas/PILs interfaces and that chemisorption is leading the CO2 capture at low pressures in these PILs. Among the studied systems [DBUH][Pyr] is the most reacting system with CO2, while [DBUH][Pyr] shows the lower regeneration energy. The findings would shed more light on understanding and designing PILs for CO2 capture.