Anomalous Facile Carbamate Formation at High Stripping Temperatures from Carbon Dioxide Reaction with 2-Amino-2-methyl-1-propanol in Aqueous Solution

Abstract

Based on first-principles simulations, we present that carbamate formation can be kinetically more favorable than bicarbonate formation at high stripping temperatures (>400 K) from the reaction between CO2 and 2-amino-2-methyl-1-propanol (AMP) in aqueous solution, while the latter tends to be predominant during CO2 capture at low absorber temperatures (<330 K). This finding offers explanation for the intriguing observation of oxazolidinone formation as the major product of AMP degradation, which is known to occur via carbamate, as also seen from thermal degradation of aqueous monoethanolamine (MEA) in CO2 capture processes. From ab initio molecular dynamics simulations coupled with metadynamics sampling, the free-energy barrier for carbamate formation is predicted to substantially decrease from 11.7 to 5.5 kcal/mol with increasing temperature from 313 to 413 K in 25 wt % AMP solution whereas that for bicarbonate formation increases from 9.6 to 12.4 kcal/mol. Likewise, the predicted free-energy barrier for carbamate formation in aqueous MEA also decreases with temperature but is significantly less compared to the AMP case. Further analysis demonstrates that the increase of temperature results in enhancing the disruption of the hydrogen bond network around the basic nitrogen atom of AMP (or MEA), allowing more facile CO2 access to form carbamate. Our work provides new insight on the strong temperature dependence of the CO2 capture mechanism and kinetics in aqueous solutions of amines, arising from changes in the hydrogen bond structure and dynamics around amines.

An experimental/computational study of steric hindrance effects on CO2 absorption in (non)aqueous amine solutions

“…First, we calculated the energy barrier for MEA‐zwitterion in aqueous solution (water as solvate) to verify the reliability of our simulations. The energy barrier was predicted to be 7.54 kcal/mol, which is was very close to the energy barrier calculated by Yoon et al 42 (7.9 kcal/mol) and by Kim et al 51 (7.67 kcal/mol). Thereafter, the energy barrier for the MEA‐zwitterion formation in nonaqueous (ethanol as solvate) was also predicted to be 5.77 kcal/mol, as shown in Figure 8.…”

“…To assess the mechanism and the factors that affect the reaction between CO 2 and AMP in aqueous solutions, we calculated the energy barrier of direct HCO As shown in Figure 9, the protonation of AMP and the HCO 3 À formation may occur simultaneously. The predicted FE barrier for the HCO 3 À formation through the base-catalyzed hydration mechanism in aqueous AMP solutions, was 10.3 kcal/mol, which is very close to the 9.6 kcal/mol calculated by Yoon et al 42 and Yamada et al, 21 and also compatible to the 8.1 kcal/mol calculated by Stowe et al 39 The energy barrier in aqueous AMP solution (10.3 kcal/mol) is predicted to be higher than that in aqueous MEA solution (7.54 kcal/mol), which agrees well with the higher activation energy of CO 2 absorption in aqueous AMP solution (10.31 kcal/mol from Saha et al 53 ) than that in aqueous MEA solution (9.56 kcal/mol from Leder 54 ) predicted by experiments. Together, our results confirm the reliability and accuracy of our simulations.…”

“…Compared to the energy barrier for the formation of AMP‐zwitterion in aqueous systems, estimated in the literature (11.7 kcal/mol), 42 the predicted energy barrier for the HCO 3 − formation was slightly lower. It should be pointed out that in aqueous AMP solutions the base‐catalyzed hydration of CO 2 is more kinetic favorable than the zwitterion formation, which can be explained by the fact that the steric effect of AMP dominates the reaction by affecting the nucleophilicity of N atom 55 , suppressing the thermal rearrangement of adjacent H 2 O molecules 39 .…”

“…Compared to the energy barrier for the formation of AMPzwitterion in aqueous systems, estimated in the literature (11.7 kcal/ mol), 42 the predicted energy barrier for the HCO Our CPMD-metadynamics simulations estimated the free energy (FE) barrier of the base-catalyzed hydration of CO 2 in aqueous MDEA solutions to be 14.20 kcal/mol; the corresponding FE surface for the reaction was also illustrated in Figure 10. The higher energy barrier in aqueous MDEA solutions than that in aqueous AMP solutions was consistent with the lower reaction constant in aqueous MDEA solutions (K C AMP,aq ¼ 314:4 1=s, K C MDEA,aq ¼ 5:646 1=s) calculated from the above kinetic model (as shown in Figure 7).…”

“…The potential hills were colloected every 100 timesteps (1 timestep = 0.17 fs) over a trajectory of about 100 ps. Other details of our simulations can be found in the literatures 39, 40, 42, 43.…”