Mechanism Study of Carbon Dioxide Capture from Ambient Air by Hydration Energy Variation

Abstract:

Hydration of neutral and ionic species on solid interfaces plays an important role in a wide range of natural and engineered processes within energy systems as well as biological and environmental systems. Various chemical reactions are significantly enhanced, both in the rate and the extent of the reaction, because of water molecules present or absent at the interface. A novel technology for carbon dioxide capture, driven by the free energy difference between more or less hydrated states of an anionic exchange resin is studied for a new approach to absorb CO₂ from ambient air. For these materials the affinity to CO₂ is dramatically lowered as the availability of water is increased. This makes it possible to absorb CO₂ from air in a dry environment and release it at two orders of magnitude larger partial pressures in a wet environment. While the absorption process and the thermodynamic properties of air capture via ion exchange resins have been demonstrated, the underlying physical mechanisms remain to be understood. In order to rationally design better sorbent materials, the present work elucidates through molecular dynamics and quantum mechanical modeling the energy changes in the carbonate, bicarbonate and hydroxide ions that are induced by hydration, and how these changes affect sorbent properties.

A methodology is developed to determine the free energy change during carbonate ion hydrolysis changes with different numbers of water molecules present. This makes it possible to calculate the equilibrium in the reaction

CO₃^--∙nH₂O ↔ HCO₃^- ∙ m₁H₂O + OH^- ∙ m₂H₂O + (n – 1 – m₁ – m₂)H₂O

Molecular dynamics models are used to calculate free energies of hydration for the CO₃^2- ion, the HCO₃^- ion, and the OH^- ion as function of the amount of water that is present. A quantum mechanical model is employed to study the equilibrium of the reaction

Na₂CO₃ + H₂O ↔ NaHCO₃ + NaOH

in a vacuum and at room temperature. The computational analysis of the free energy of hydration reveals that in an ionic exchange resin the equilibrium between carbonate, bicarbonate and hydroxide favors a combination of bicarbonate and hydroxide over the formation of carbonate ions. In the case of low water content, the presence of a large number of hydroxide ions increases the affinity of the resin to CO₂.