(346b) On the Origin of Preferred Bicarbonate Production from Carbon Dioxide (CO2) Capture into Aqueous 2-Amino-2-Methyl-1-Propanol (AMP)

Chemical absorption by aqueous amine-based solvents appears the most promising near-term solution for CO₂ capture from flue gas because of their ability to capture CO₂ at low pressure with adequate absorption/desorption kinetics.^1,2 Monoethanolamine (MEA) is one of the most widely used and studied alkanolamines, but its expansion for commercial use tends to be limited due to degradation problems and high parasitic energy consumption during solvent regeneration.^3-4 2-amino-2-methyl-1-propanol (AMP) and other sterically hindered primary amines have been suggested as an alternative because they exhibit both high CO₂ loading capacities and relatively fast absorption rates, in addition to low degradation and corrosion rates.^5–8 However, the mechanism underlying the reaction between CO₂ and aqueous AMP still remains controversial.

It is now well adopted that CO₂ capture by most primary amines including MEA in an aqueous solution takes place via a two-step mechanism involving a zwitterion intermediate.^9-10 The zwitterion may subsequently undergo deprotonation by another amine to form a carbamate ion and a protonated amine, leading to a loading capacity of 0.5 mol CO₂/mol amine. Conversely, the primary products of the reaction of aqueous AMP with CO₂ have been found to be bicarbonate (HCO₃^-) and protonated AMP (AMPH⁺), with carbamate (AMPCOO^-) but whose concentration is about an order of magnitude lower than HCO₃^-.¹¹ It is also known that bicarbonate formation predominantly takes place in aqueous tertiary amines such as methyldiethanolamine (MDEA), yielding the theoretical maximum capacity of 1 mol CO₂/mol amine.¹² In the tertiary amine system, bicarbonate can be formed by the base-catalyzed hydration of CO₂, e.g., MDEA + H₂O + CO₂ → MDEAH⁺ + OH^- + CO₂ ® MDEAH⁺ + HCO₃^-.¹³

However, according to previous experimental observations, the absorption rate of CO₂ in aqueous AMP tends to be about two orders of magnitude faster as compared to the MDEA case.¹⁴ On the basis of the much higher absorption rate of AMP relative to tertiary amines, it has been speculated that first AMP and CO₂ react to form carbamate (AMPCOO^-), which undergoes hydrolysis to form bicarbonate due to its possible instability (i.e., AMPCOO^- + H₂O → HCO₃^- + AMPH⁺).^5,15 However, previous theoretical studies have predicted free energy barriers for the hydrolysis of both AMPCOO^- and MEACOO^- to HCO₃^- to be high and also comparable, implying that not only MEACOO^- but also AMPCOO^- are relatively stable and may not easily undergo hydrolysis to form HCO₃^-.^16–19 Rather, it would be more likely that the carbamates would revert back to the amine and CO₂.²⁰ This gives a hint that preferred bicarbonate formation in the AMP-H₂O-CO₂ system might not be due to carbamate being less stable than the MEA case.

In this work, we investigate the factors underlying the preferred production of bicarbonate over carbamate from CO₂ absorption in aqueous AMP, with comparison to the MEA case where carbamate is predominantly formed, by explicitly taking into account both thermodynamic and kinetic contributions. First, we evaluated the thermodynamic favorability for bicarbonate and carbamate formation by calculating the changes in free energy (and enthalpy) using static QM with an implicit solvent model (and AIMD with explicit solvent). Our calculations show that both CO₂ capture mechanisms are thermodynamically favorable with similar exothermicities in aqueous AMP and MEA solutions. In addition, AIMD simulations demonstrate that AMP carbamate (AMPCOO^-) can be as stable as MEA carbamate (MEACOO^-) in an aqueous solution, which is consistent with previous theoretical studies predicting large and similar energy barriers for carbamate hydrolysis to form bicarbonate in both cases^19,17, i.e., AMPCOO^- (MEACOO^-) + H₂O → HCO₃^- + AMPH⁺ (MEAH⁺); the comparable stability between AMPCOO^- and MEACOO^- is also well demonstrated by analysis of the N-C (in CO₂) bonding interaction. This may suggest that the relative concentrations of carbamate and bicarbonate could not be predicted only by their thermodynamic favorability, especially since it may take too long to reach equilibrium at typical operating conditions.

From AIMD simulations, we observed that the AMP + H₂O → AMPH⁺ + OH^- reaction frequently occurs, rather than carbamate formation (i.e., 2AMP + CO₂ → AMPCOO^- + AMPH⁺) which tends to occur readily in the case of MEA. If OH^- does not abstract a proton to form H₂O, bicarbonate can be formed via the OH^- + CO₂ → HCO₃^- reaction.^21,22 This can also occur in one step, via the amine-catalyzed hydration of CO₂ proposed by Donaldson and Nguyen¹³, (i.e., AMP + H₂O + CO₂ → AMPH⁺ + HCO₃^-). These results suggest that bicarbonate formation may be more kinetically favorable relative to carbamate formation in aqueous AMP. The enhanced AMP protonation reaction is apparently related to its high basicity. Our electronic structure analysis shows that the steric hindrance of the CH₃ groups of AMP causes a more planar-like configuration around the central C atom (C_N) which is attached to N. The tendency of sp²-like hybridization results in charge redistribution in which the C_N atom is more positively charged and the N atom is more negatively charged. This in turn strengthens the interaction between the N in AMP and surrounding H₂O molecules while suppressing the accessibility of CO₂ to the N site to form carbamate, relative to the MEA case, as confirmed by analysis of radial/spatial distribution functions from MD simulations.

Based on our calculation results, we attribute the preferential formation of bicarbonate in CO₂ absorption into an aqueous AMP solution largely to kinetic factors. As discussed above, the strong interaction between N (in AMP) and H (in H₂O) suppresses the reaction with CO₂ and promotes the protonation reaction, while the bicarbonate and carbamate reaction routes exhibit similar thermodynamic favorability. This study highlights that not only thermodynamic but also kinetic factors should be considered in describing CO₂ capture by aqueous amines.

 

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