First-principles assessment of CO₂ capture mechanisms in aqueous piperazine solution

Piperazine (PZ) and its blends have emerged as attractive solvents for CO₂ capture, but the underlying reaction mechanisms still remain uncertain. Our study particularly focuses on assessing the relative roles of PZCOO^- and PZH⁺ produced from the PZ + CO₂ reaction. PZCOO^- is found to directly react with CO₂ forming COO^-PZCOO^-, whereas PZH⁺ will not. However, COO^-PZCOO^- appears very unlikely to be produced in thermodynamic equilibrium with monocarbamates, suggesting that its existence would predominantly originate from the surface reaction that likely occurs. We also find production of H⁺PZCOO^- to be more probable with increasing CO₂ loading, due partly to the thermodynamic favorability of the PZH⁺ + PZCOO^- → H⁺PZCOO^- + PZ reaction; the facile PZ liberation may contribute to its relatively high CO₂ absorption rate. This study highlights an accurate description of surface reaction and the solvent composition effect is critical in thermodynamic and kinetic models for predicting the CO₂ capture processes.