Quantitative Relationship between CO2 Absorption Capacity and Amine Water System: DFT, Statistical, and Experimental Study

Computational screening methodology identifies effective solvents for CO2 capture

Carbon capture and storage technologies are projected to increasingly contribute to cleaner energy transitions by significantly reducing CO₂ emissions from fossil fuel-driven power and industrial plants. The industry standard technology for CO₂ capture is chemical absorption with aqueous alkanolamines, which are often being mixed with an activator, piperazine, to increase the overall CO₂ absorption rate. Inefficiency of the process due to the parasitic energy required for thermal regeneration of the solvent drives the search for new tertiary amines with better kinetics. Improving the efficiency of experimental screening using computational tools is challenging due to the complex nature of chemical absorption. We have developed a novel computational approach that combines kinetic experiments, molecular simulations and machine learning for the in silico screening of hundreds of prospective candidates and identify a class of tertiary amines that absorbs CO₂ faster than a typical commercial solvent when mixed with piperazine, which was confirmed experimentally.

Quantitative Kinetic Model of CO2 Absorption in Aqueous Tertiary Amine Solvents

Aqueous tertiary amine solutions are increasingly used in industrial CO₂ capture operations because they are more energy-efficient than primary or secondary amines and demonstrate higher CO₂ absorption capacity. Yet, tertiary amine solutions have a significant drawback in that they tend to have lower CO₂ absorption rates. To identify tertiary amines that absorb CO₂ faster, it would be efficacious to have a quantitative and predictive model of the rate-controlling processes. Despite numerous attempts to date, this goal has been elusive. The present computational approach achieves this goal by focusing on the reaction of CO₂ with OH^- forming HCO₃^-. The performance of the resulting model is demonstrated for a consistent experimental data set of the absorption rates of CO₂ for 24 different aqueous tertiary amine solvents. The key to the new model's success is the manner in which the free energy barrier for the reaction of CO₂ with OH^- is evaluated from the differences among the solvation free energies of CO₂, OH^-, and HCO₃^-, while the pK_a of the amines controls the concentration of OH^-. These solvation energies are obtained from molecular dynamics simulations. The experimental value of the free energy of reaction of CO₂ with pure water is combined with information about measured rates of absorption of CO₂ in an aqueous amine solvent in order to calibrate the absorption rate model. This model achieves a relative accuracy better than 0.1 kJ mol^-1 for the free energies of activation for CO₂ absorption in aqueous amine solutions and 0.07 g L^-1 min^-1 for the absorption rate of CO₂. Such high accuracies are necessary to predict the correct experimental ranking of CO₂ absorption rates, thus providing a quantitative approach of practical interest.