Chemical absorption using aqueous amine-based solvents appears to be the most promising near-term technology for CO2 capture. It is now well known that amine carbamate and protonated amine ion pairs are preferentially formed over neutral carbamic acid during CO2 capture, due to polarity in aqueous solution. However, there is a lack of fundamental understanding about the role of surrounding water molecules in CO2 absorption and amine regeneration. In this talk, we will present how the CO2 capture process can be governed by the local arrangement and dynamics of water molecules solvating amines, using monoethanolamine (MEA), a benchmark solvent, as an example. Based on ab initio molecular dynamics coupled with metadynamics, we have found that the regeneration process is predominantly initiated by deprotonation of protonated MEA (MEAH+) to O in carbamate (MEACOO-) forming carbamic acid (MEACOOH), rather than directly to N in MEACOO- forming zwitterion (MEA+COO-), due primarily to kinetic preference. We have also identified subsequent proton transfer from O to N site in MEACOOH, forming MEA+COO- which may further release CO2 and yield MEA. Our further analysis also suggests that the regeneration mechanism depends on the amine basicity and CO2 loading, which will be discussed in this talk. This work highlights the importance of accurate description of the arrangement and dynamics of water molecules surrounding amines which may largely govern the CO2 capture process.

On the critical role of arrangement and dynamics of water molecules in CO2 capture by aqueous amines

ACS Fall 2021

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