(554b) A Mechanistic Study of the Monoethanolamine+CO2 Reaction in Aqueous Solution: Is the Zwitterion a Necessary Intermediate?

Monoethanolamine (MEA) has been used to capture CO2 from gas phase streams. The capture mechanism is a chemical reaction between CO2 and two MEA molecules, forming a carbamate product. The rate-determining step for the absorption of CO2 by MEA in aqueous solution is considered to be the carbamate formation in solution. Unfortunately, there is a controversy about the reaction mechanism, particularly regarding the intermediate steps in the reaction. In this paper, detailed theoretical investigations at the B3LYP/6-311++G(d,p) level have been performed for the reaction of MEA+CO2, covering three different reaction channels suggested by previous experimental studies. In contrast to previous studies, we have included implicit solvent effects in both the geometry optimizations and frequency calculations for this reaction. A two-step reaction channel that proceeds via a zwitterion intermediate to form the carbamate is found to be most favorable. However, other reaction channels predicted by previous theoretical studies are kinetically much less competitive. In addition, all reaction channels considered in this paper must proceed through zwitterion intermediates and therefore it is a vital intermediate in the process of reaction. Calculations with MP2 and B3LYP methods combined with various base sets affirm that the zwitterion geometry is a true minimum (has no imaginary frequencies). Furthermore, NBO charge analysis confirms the zwitterionic character of the complex. We have included explicit water molecules in addition to an implicit solvent model in order to identify explicit solvent effects on the reaction. We find that inclusion of explicit water lowers the reaction barriers.