Reaction mechanisms of aqueous monoethanolamine with carbon dioxide: a combined quantum chemical and molecular dynamics study

Abstract

Aqueous monoethanolamine (MEA) has been extensively studied as a solvent for CO2 capture, yet the underlying reaction mechanisms are still not fully understood. Combined ab initio and classical molecular dynamics simulations were performed to revisit and identify key elementary reactions and intermediates in 25-30 wt% aqueous MEA with CO2, by explicitly taking into account the structural and dynamic effects. Using static quantum chemical calculations, we also analyzed in more detail the fundamental interactions involved in the MEA-CO2 reaction. We find that both the CO2 capture by MEA and solvent regeneration follow a zwitterion-mediated two-step mechanism; from the zwitterionic intermediate, the relative probability between deprotonation (carbamate formation) and CO2 removal (MEA regeneration) tends to be determined largely by the interaction between the zwitterion and neighboring H2O molecules. In addition, our calculations clearly demonstrate that proton transfer in the MEA-CO2-H2O solution primarily occurs through H-bonded water bridges, and thus the availability and arrangement of H2O molecules also directly impacts the protonation and/or deprotonation of MEA and its derivatives. This improved understanding should contribute to developing more comprehensive kinetic models for use in modeling and optimizing the CO2 capture process. Moreover, this work highlights the importance of a detailed atomic-level description of the solution structure and dynamics in order to better understand molecular mechanisms underlying the reaction of CO2 with aqueous amines.

On the origin of preferred bicarbonate production from carbon dioxide (CO2) capture in aqueous 2-amino-2-methyl-1-propanol (AMP)

“…44 During about 120 ps of simulation, we observed only the AMP + H 2 O -AMPH + + OH À reaction, as illustrated in Fig. For the aqueous AMP system, 30H 2 O, 2AMP, and 1CO 2 molecules, corresponding to 30 wt% aqueous AMP with CO 2 , were placed in a cubic simulation box of edge length 10.62 Å with periodic boundary conditions.…”

“…1; this reaction occurred in 7 out of 10 cases with different initial configurations. For the case of 25 wt% aqueous MEA with CO 2 (where 20H 2 O, 2MEA, and 1CO 2 molecules were placed in a cubic periodic box with side length of 9.278 Å), MEA is likely to undergo the reaction with CO 2 to form MEACOO À and MEAH + , i.e., 2MEA + CO 2 -MEACOO À + MEAH + according to our previously reported AIMD simulations at 400 K. 44 As seen in The AIMD results may suggest that carbamate formation would be more likely in aqueous MEA while the route to bicarbonate formation would be kinetically more probable in aqueous AMP. 45 The OH À may react with CO 2 to form bicarbonate, i.e., OH À + CO 2 -HCO 3 À .…”

CO 2 absorption mechanism in amine solvents and enhancement of CO 2 capture capability in blended amine solvent

“…This two process determines the major activation energy barrier. There are many theoretical analysis using higher calculation method (QM/MM or ab initio MD) about the reaction of amine and CO 2 (Guido et al, 2012;Han et al, 2011;Hwang et al, 2015;Sumon et al, 2014;Xie et al, 2014). Most of the studies considered the role of surrounding water molecules at the first step (formation of zwitterion) and then the additional amine molecule was added for deprotonation process.…”

Mechanisms of CO2 Capture into Monoethanolamine Solution with Different CO2 Loading during the Absorption/Desorption Processes

“…Using static quantum chemical calculations, Hwang et al 13 found that both the CO 2 capture by MEA and the solvent regeneration followed a zwitterionmediated two-step mechanism. Also, from the zwitterionic intermediate, the relative probability between deprotonation (carbamate formation) and CO 2 removal (MEA regeneration) largely depends on the interaction between the zwitterion and adjacent H 2 O molecules.…”