Predicting the Mechanism and Products of CO₂ Capture by Amines in the Presence of H₂O

Abstract

An extensive correlated molecular orbital theory study of the reactions of CO2 with a range of substituted amines and H2O in the gas phase and aqueous solution was performed at the G3­(MP2) level with a self-consistent reaction field approach. The G3­(MP2) calculations were benchmarked at the CCSD­(T)/CBS level for NH3 reactions. A catalytic NH3 reduces the energy barrier more than a catalytic H2O for the formation of H2NCOOH and H2CO3. In aqueous solution, the barriers to form both H2NCOOH and H2CO3 are reduced, with HCO3 – formation possible with one amine present and H2NCOO– formation possible only with two amines. Further reactions of H2NCOOH to form HNCO and urea via the Bazarov reaction have high barriers and are unlikely in both the gas phase and aqueous solution. Reaction coordinates for CH3NH2, CH3CH2NH2, (CH3)2NH, CH3CH2CH2NH2, (CH3)3N, and DMAP were also calculated. The barrier for proton transfer correlates with amine basicity for alkylammonium carbamate (ΔG ‡ aq < 15 kcal/mol) and alkylammonium bicarbonate (ΔG ‡ aq < 30 kcal/mol) formation. In aqueous solution, carbamic acids, carbamates, and bicarbonates can all form in small amounts with ammonium carbamates dominating for primary and secondary alkylamines. These results have implications for CO2 capture by amines in both the gas phase and aqueous solution as well as in the solid state, if enough water is present.

Review on CO2 Capture Using Amine-Functionalized Materials

“…The advance in spectroscopy, in situ techniques, and quantum mechanical modeling in the past decade promoted mechanistic investigations to unveil the underlying mechanism of CO 2 adsorption and desorption. Commonly used techniques in this field are infrared spectroscopy, ,− nuclear magnetic resonance (NMR), − and density functional theory (DFT). ,− This chapter briefly summarizes what information can be gained by each technique and major findings related to CO 2 adsorption mechanisms.…”

“…DFT calculations contributed considerably to understanding what species is favorably formed and what reaction path is reasonable for the reaction of amines with CO 2 . ,− Early stage research was reported by Mebane et al that the formation of a zwitterion was unstable in a polar environment of anhydrous PEI . However, a dielectric medium under humid conditions stabilizes the zwitterion.…”

Acid Gas Capture by Nitrogen Heterocycle Ring Expansion

“…20 Carbamic acid formation can be catalyzed by H 2 O, decreasing the enthalpy barrier by 30 kcal/mol and the free energy barrier by 12 kcal/mol. 20 The catalytic effect of H 2 O on cyclic carbamic acid derivatives will be explored in future work.…”

“…The mechanisms for the capture and conversion of CO 2 with a range of substituted amines in the gas phase and in aqueous systems has been studied using correlated molecular orbital theory with the results compared to experiment and to other computational studies. 20 Incorporation of nitrogen into 3-and 4-membered rings, forming nitrogen heterocycles, changes the method of gas capture compared to standard amines. The ability of such Nheterocycles to trap acid gases, primarily CO 2 , has been demonstrated.…”

Recent advances in direct air capture by adsorption

“…Previous studies have indicated that H 2 O affects the adsorption of CO 2 , but the presence of CO 2 does not significantly affect the adsorption of H 2 O. 112,278,400,563 While several studies on the development of new adsorbent materials have explored their adsorption properties in the presence of H 2 O, little is known about the detailed mechanism of the interaction between H 2 O and CO 2 .…”