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

Abstract: 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.