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

Piperazine (PZ) and its blends have emerged as attractive solvents for CO 2 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 + CO 2 reaction. PZCOO − is found to directly react with CO 2 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 CO 2 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 CO 2 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 CO 2 capture processes. In this theoretical study, we elucidate molecular mechanisms underlying CO 2 absorption in aqueous piperazine (PZ) solution.