(329e) Molecular Insights into the Enhanced Rate of CO2 Absorption to Produce Bicarbonate in Aqueous 2-Amino-2-Methyl-1-Propanol (AMP)

Carbon dioxide (CO₂) capture from the flue gases of fossil fuel-fired power plants is needed to curb rising anthropogenic greenhouse gas emissions and mitigate global warming.¹ Wet scrubbing using aqueous amine-based solvents currently appears the most suitable method to remove CO₂ from flue gas because of their ability to chemically absorb CO₂ at low partial pressure and release the absorbed CO₂ with adequate kinetics.^2–4 Monoethanolamine (MEA) has been considered the benchmark solvent, but its widespread implementation remains hindered by high costs associated with the parasitic energy consumption during solvent regeneration in addition to degradation and corrosion issues.⁵ Tertiary amines such as methyldiethanolamine (MDEA) may be more resistant to degradation and corrosion in addition to exhibiting higher CO₂ absorption capacities than MEA, but they suffer from slow absorption rates.^6,7 Sterically hindered amines, including 2-amino-2-methyl-1-propanol (AMP), typically have similar advantageous qualities as tertiary amines while exhibiting relatively much higher CO₂ absorption rates.^8–11 However, the molecular mechanisms underlying the enhanced absorption rate of sterically hindered amines relative to tertiary amines remains unclear.

Primary/secondary amines including MEA predominantly form carbamate, which is now widely accepted to occur via a two-step mechanism with a zwitterion intermediate; for instance, 2MEA + CO₂ → MEA⁺COO^- → MEACOO^- + MEAH⁺, limiting the loading capacity to 0.5 mol CO₂/mol MEA.^12,13 Tertiary amines predominantly form bicarbonate (HCO₃^-), which may occur via base-catalyzed hydration of CO₂, e.g. MDEA + H₂O + CO₂ → MDEAH⁺ + HCO₃^-.^6,14 Experimental measurements show that CO₂ capture in tertiary amines tends to follow the Bronsted relationship where the logarithm of the absorption rate is linearly dependent on the pKa value.^15,16 However, sterically hindered amines typically exhibit much higher rates than expected from their basicities. For example, AMP (pKa = 9.70) has a similar basicity to diethylethanolamine (DEEA, pKa = 9.81), but the former (sterically hindered primary amine) forms bicarbonate an order of magnitude faster than the latter (tertiary amine).^{7,17–19}

Due to this significant rate increase, it has been often speculated that AMP may first react with CO₂ to form carbamate, but the rather unstable carbamate due to the presence of bulky methyl groups may subsequently undergo hydrolysis to form bicarbonate, i.e., AMPCOO^- + H₂O → AMPH⁺ + HCO₃^-).^8,19 On the contrary, recent first-principles calculations predicted that AMP carbamate can be as stable as MEA carbamate in aqueous solution, and thus the former would not undergo hydrolysis more readily relative to the latter.²⁰ Very recently, through comparative theoretical study of AMP and MEA for CO₂ capture we have found that bicarbonate formation seems to be kinetically more probable in aqueous AMP while carbamate is more likely to form in aqueous MEA. It appears that the preferred bicarbonate formation is largely attributed to the relatively strong interaction between N (in AMP) and H (H₂O) which suppresses the reaction with CO₂ to form carbamate.²⁰

This theoretical work focuses on elucidating the molecular mechanisms responsible for the enhanced rate of bicarbonate formation during CO₂ capture in aqueous AMP in comparison to DEEA.²¹ Ab initio (Car-Parrinello) MD simulations in combination with metadynamics were employed to construct the free energy surfaces for CO₂ hydration catalyzed by AMP and DEEA. Our simulations unequivocally predict the free energy barrier to be lower in aqueous AMP (= 8.1 kcal mol^-1) compared to DEEA (= 11.4 kcal mol^-1). The amine-catalyzed hydration of CO₂ is found to require a lower barrier than that for its reaction with a free hydroxide anion (OH^-) in aqueous medium (=13.4 kcal mol^-1), which can be well explained by the stronger hydration of free OH^- in pure aqueous conditions. Analysis of MD simulation trajectories hints that the free energy barrier can largely be influenced by reorganization of H₂O molecules around the basic N site to stabilize the transition state involving OH^- formation and CO₂ polarization.

We further examined the entropic effects by evaluating the arrangement of H₂O molecules around AMP and DEEA in aqueous solution based on corresponding radial distribution functions (RDFs) calculated from AIMD simulations. The neighboring H₂O molecules around the NofDEEA (N_DEEA), relative to the AMP case, are more ordered and more hindered from facile rearrangement. The suppressed thermal displacement is largely attributed to the stronger hydrogen bonding of H₂O with N_DEEA than N_AMP in addition to the presence of bulky ethyl groups on N_DEEA. The hindered reorganization of neighboring H₂O molecules causes an increase in the free energy barrier for proton abstraction by DEEA compared to AMP due to a relatively lower stability of the transition state involving OH^- creation, and prevents the approach of CO₂ to the reaction region near the N_DEEA site, relative to N_AMP, as confirmed by spatial distribution analysis from classical MD simulations.

This study highlights that both mechanisms and rates of CO₂ absorption into aqueous amines can be strongly influenced by the entropic effects arising from steric constraints on surrounding H₂O molecules imposed by the geometry and chemical affinity of amines. The improved understanding at the molecular level may provide valuable guidance in the design of cost-effective amine-based solvents for CO₂ capture and utilization.

References

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