Facile Carbamic Acid Intermediate Formation in Aqueous Monoethanolamine and Its Vital Role in CO2 Capture Processes

Abstract

Monoethanolamine (MEA) is the most studied and used to be considered as benchmark solvent for CO2 capture. CO2 absorption in aqueous MEA is well known to produce ion pairs such as carbamate (MEACOO–) and protonated amine (MEAH+), following a stepwise reaction mechanism involving a zwitterionic intermediate (MEA+COO–). Contrastingly, thermal degradation of MEA has been thought to occur through carbamic acid (MEACOOH) formation under stripper conditions. This raises a fundamental question regarding the MEACOOH formation mechanism and its relative concentration in CO2-loaded aqueous MEA. Using ab initio molecular dynamics and metadynamics simulations, we have determined probable routes and free-energy barriers for proton transfer reactions associated with the MEA+COO– ↔ MEACOO–/MEAH+ conversion. Our results clearly demonstrate that the lowest free-energy path involves MEACOOH formation in a 30 wt % MEA solution. A zero-dimensional model based on the predicted energetics illustrates that the concentration ratio of MEACOOH to MEACOO– increases with CO2 loading and temperature; the estimated ratio reaches up to 25% at high CO2 loadings (∼0.5) and high stripping temperatures (∼413 K). While carbamic acid formation tends to be a function of amine type, the quantitative understanding helps explain experimental observations and also greatly assists in designing amine-based solvents that have enhanced resistance to thermal degradation.