Water effect on CO2 absorption mechanism and phase change behavior in [N1111][Gly]/EtOH anhydrous biphasic absorbent: In density functional theory and molecular dynamics view

Abstract

Ionic liquids-based anhydrous biphasic absorbents have the potential to lower the energy consumption for CO2 capture. Recent experimental studies have revealed that the presence of water in [N1111][Gly]/EtOH system leads to generation of a new CO2-product – bicarbonate and affects the phase change behavior. However, the reaction pathway for bicarbonate formation remains unclear. In this work, the reaction pathways were investigated in depth through quantum chemical calculations employing density functional theory, with reaction rate constants determined via transition state theory. Among the three possible reaction pathways, the base-catalyzed CO2 hydration reaction prevailed, i.e. a one-step reaction involving [Gly]-, H2O, and CO2. An H atom in H2O was transferred to [Gly]-’s amino group, forming protonated [Gly]-, and hydroxide combined with CO2 to generate bicarbonate. Additionally, molecular dynamics simulations were conducted to understand the phase change behaviors with varying water content. When water content exceeded 5 wt%, an increased water content led to reduced hydrogen bonding among products and enhanced hydrogen bonding of products-solvent with the solvent, diminishing product self-aggregation. No phase change behavior were observed at water contents up to 80 wt%. This study could well explain the experimental phenomena of water effect on CO2 absorption in [N1111][Gly]/EtOH, providing theoretical references for application of ILs-based anhydrous biphasic absorbent.