Elucidating the Molecular Mechanism of CO2 Capture by Amino Acid Ionic Liquids

Amino acid ionic liquids have received increasing attention as ideal candidates for the CO2 chemisorption process. However, the underlying molecular mechanisms, especially those involving proton transfer, remain unclear. In this work, we elucidate the atomistic-level reaction mechanisms responsible for carbamate formation during CO2 capture by amino acid ionic liquids through explicit ab initio molecular dynamics augmented by well-tempered metadynamics. The resulting ab initio free-energy sampling reveals a two-step reaction pathway in which a zwitterion, initially formed from the reaction between the anion of serine and CO2, undergoes a kinetically facile intermolecular proton transfer to the O atom of the COO- moiety in the nearby serine. Further analysis reveals that the significantly reduced free-energy barriers are attributed to enhanced intermolecular interaction between the zwitterion and serine, thus facilitating the kinetic favorability of the proton transfer, which governs the overall CO2 capture mechanism. This work provides valuable insight into the important mechanistic and kinetic features of these reactions from explicit condensed phase ab initio MD free-energy sampling of the CO2 capture process.