Ab Intio Calculations on Reaction Process of Piperazine Absorbing CO₂

The increasing amount of CO₂ emitted to the atmosphere has a deep impact on the environment. It is urgent to take effective measures to reduce CO₂ emissions from its intensive emission source. Chemical absorption is an effective way to cover the large reduction of CO₂, and piperazine (PZ) is one of the amines that have good performance on CO₂ capture. To make a deep understanding of the chemical mechanism of PZ absorbing CO₂, computational studies on PZ chemically absorbing CO₂ are discussed in this work using Density functional theory (DFT). Possible reaction pathways of PZ absorbing CO₂ are analyzed. Due to the special chemical structure of PZ, the reaction processes can be divided into two parts, the process of (PZ+CO₂)+PZ and (PZ+2CO₂)+PZ. To make a good understanding of the reaction process, a comparison of that between PZ and MEA are discussed. The results show that PZ has lower forward and backward energy barriers than MEA in the zwitterion formation, but in the intermolecular hydrogen transfer process, PZ has higher forward and backward barriers than MEA. For the reaction process of PZ+2CO₂, it can be stated as a two-step zwitterion formation with two transition states. For the following intermolecular hydrogen transfer of PZ(COO)₂+PZ, the atom movements are similar to that of (PZCOO+PZ). All the discussions provide theoretical information of PZ chemical absorbing CO₂.