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

Abstract

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.