A first-principle study of CO2 binding by monoethanolamine and mono-n-propanolamine solutions

- CO2 binding by MEA and MPA were simulated by first-principle molecular dynamic.
- MPA binds CO2 better due to its additional CH2 to reduce the electronic repulsion.
- Bimolecular pathway is more favored than trimolecular one as the [CO2] increases.
- Lone pairs of N are blocked at high [CO2] because inter-solvent OH⋯N HB was formed.
- Adding water molecules hinders the access of CO2 because OH2⋯N HB was formed.

Monoethanolamine (MEA) and mono-n-propanolamine (MPA) molecules were investigated for CO2 binding using Density Functional Theory. MPA was predicted to bind CO2 better than MEA along the bimolecular and trimolecular pathways. The additional CH2 in MPA provided additional polarization to reduce the electrostatic repulsion for the charge-separated zwitterionic intermediates (ZW) as shown in the Polarizable Continuum Model calculations; also became more polar solvent to stabilize ZW. 25% and 50% CO2 loading at 400 K were studied by first-principle molecular dynamic simulations. With including the explicit solvation effect, CO2 in alcoholamines favored a reduced-hydrogen-bonding (HB) environment. The probability of identifying the HB precursors-(MEA)2 and (MPA)2 for the subsequent trimolecular pathway decreased. Moreover, higher CO2 uptake accompanied with more OH⋯N HB, and the lone pairs of N were blocked to CO2. Water also preferred to form intermolecular OH⋯N HB so that the accesses of CO2 were hindered.