The Transition States for CO2 Capture by Substituted Ethanolamines

Quantum chemical studies are used to understand the electronic and steric effects on the mechanisms of the reaction of substituted ethanolamines with CO2. SCS-MP2/6-311+G(2d,2p) calculations are used to obtain the activation energy barriers and reaction energies for both the carbamate and bicarbonate formation. Implicit solvent effects are included with the universal solvation model SMD. Carbamate formation is more favorable than bicarbonate formation for monoethanolamine (MEA) both kinetically and thermodynamically. Increase of the steric hindrance on the C atoms around the N atom in substituted ethanolamines favors bicarbonate formation over carbamate formation with lower activation barriers and thereby higher reaction rates. In contrast, substitution by an N-methyl or N-ethyl group on MEA leads to a lower activation barrier for both carbamate formation and bicarbonate formation. As a result, higher reaction rates are expected as compared to MEA, and therefore these compounds have significant potential as industrial CO2 capturing solvents.