Solvation Dynamics of CO₂(g) by Monoethanolamine at the Gas-Liquid Interface: A Molecular Mechanics Approach

Insights into Aerosol Emission Control in the Postcombustion CO2 Capture Process: From Cluster Formation to Aerosol Growth

Aerosols produced in the amine carbon capture process can lead to secondary environmental pollution. This study employs molecular dynamics (MD) simulations to investigate cluster formation, amine behavior, and aerosol growth of amines, essential for reducing amine aerosol emissions. Results showed that the cluster evolution process can be divided into cluster formation and growth in terms of molecular content, and the nucleation rate for the present systems was estimated in the order of 1028 cm-3 s-1. CO2 absorption was observed alongside successful nucleation, with CO2 predominantly localizing in the cluster's outer layer postabsorption. Monoethanolamine (MEA) exhibited robust electrostatic interactions with other components via hydrogen bonding, leading to its migration toward regions where CO2 and H2O coexisted within the cluster. While MEA presence markedly spurred cluster formation, its concentration had a marginal effect on the final cluster size. Elevating water content can augment the aerosol growth rate. However, altering the gas saturation is possible only within narrow confines by introducing vapor. Contrarily, gas cooling introduced dual, opposing effects on aerosol growth. These findings, including diffusion coefficients and growth rates, enhance theoretical frameworks for predicting aerosol formation in absorbers, aiding in mitigating environmental impacts of amine-based carbon capture.

Stability of a Monoethanolamine-CO2 Zwitterion at the Vapor/Liquid Water Interface: Implications for Low Partial Pressure Carbon Capture Technologies

The need to chemically convert CO₂ at the interface of aqueous amine solutions has become particularly relevant for the development and the broad distribution of cost-effective and near-future devices for direct air capture working at low (e.g., ambient) partial pressure. Here, we have determined the stability of a CO₂-monoethanolamine zwitterion and its chemical conversion into carbamate at the vapor/liquid water interface by first-principles molecular dynamics simulations coupled with a recently introduced enhanced sampling technique. Contrary to the bulk water case, our results show that both the zwitterion and carbamate ions are poorly stable at the vapor/amine aqueous interface, further stating the differences between the homogeneous and heterogeneous CO₂ chemical conversion. The design of novel and cost-effective capture systems, such as those offered by amine-based scrubbing solutions, working at low (e.g., ambient) CO₂ partial pressure should explore the use of novel solvents, different from aqueous mixtures, to overcome the limits of the current absorbents.

Tuning CO2 Capture at the Gas/Amine Solution Interface by Changing the Solvent Polarity

Carbon dioxide scrubbing by aqueous amine solution is considered as a promising technology for post-combustion CO₂ capture, while mitigating climate change. The lack of physicochemical details for this process, especially at the interface between the gas and the condensed phase, limits our capability in designing novel and more cost-effective scrubbing systems. Here, we present classical and first-principles molecular dynamics results on CO₂ capture at the gas/amine solution interfaces using solvents of different polarities. Even if it is apolar, carbon dioxide is absorbed at the gas/monoethanolamine (MEA) aqueous solution interface, forming stable and interfacial [CO₂·MEA] complexes, which are the first reaction intermediate toward the chemical conversion of CO₂ to carbamate ions. We report that the stability of the interfacial [CO₂·MEA] precomplex depends on the nature and polarity of the solution, as well as on the conformer population of MEA. By changing the polarity of the solvent, using chloroform, we observed a shift in the interfacial MEA population toward conformers that form more stable [CO₂·MEA] complexes and, at the same time, a further stabilization of the complex induced by the solvent environment. Thus, while lowering the polarity of the solvent could decrease the solubility of MEA, at the same time, it favors conformers that are more prone to CO₂ capture and mineralization. The results presented here offer a theoretical framework that helps in designing novel and more cost-effective solvents for CO₂ scrubbing systems, while shedding further light on the intrinsic reaction mechanisms of interfacial environments in general.

The effect of CO2 loading on alkanolamine absorbents in aqueous solutions

Post-combustion carbon capture by amine scrubbing is the most frequently used process to remove CO2 from pulverized coal-fired power plants and also biogas flue gas streams. The quest for novel absorbents for CO2 capture with improved properties requires insight into the properties of the CO2-loaded mixed solutions. A comparative molecular dynamics study of the product state solutions, with chemically-bound CO2 of standard monoethanolamine (MEA) and the new alternative 4-diethylamino-2-butanol (DEAB) at various CO2-loadings yields solvent properties in good agreement with experimental data. The concentration of all post-reaction species in solution was based on experimental equilibria distributions. The data generated provide detailed insight into the properties of reactive mixed alkanolamine solutions. The liquid structure of aqueous MEA solutions undergoes only minor changes when absorbing CO2. The diffusion coefficients of all molecular species, however, decrease significantly with increasing CO2-loadings. The large hydrophobic clusters formed in the reactant state by DEAB molecules in water prior to CO2 binding significantly decrease in size and structure upon CO2 absorption. The diffusion coefficients of all components decrease with increasing CO2-loading, whereas the pre-reaction alkanolamine DEAB shows an increase in diffusion coefficient. This structural and kinetic information supports the molecular design and further development of novel compounds and provides data for a global process simulation and optimization.