Intriguing Thermal Degradation Behavior of Aqueous Piperazine for Carbon Dioxide Capture: A First-Principles Assessment

Thermal degradation of aqueous piperazine (PZ) for CO2 capture is experimentally known to yield a major product N-(2-aminoethyl)piperazine (AEP) and a minor product pair 1,1′-(1,2-ethanediyl)bis-piperazine (PEP) plus ethylenediamine (EDA), although the two reactions exhibit no substantial difference in thermodynamic favorability. This raises a question on factors affecting the rates of key reactions involved in PZ thermal degradation. Herein, we present the underlying mechanisms of PZ degradation and the relative rates of key steps involved based on ab initio metadynamics simulations. We have identified the reaction between PZ and protonated PZ resulting in an intermediate, 1-[2-(2-aminoethyl)amino]ethyl] piperazine (AEAEPZH+), and the subsequent reaction of AEAEPZH+ with PZ leading to AEP or PEP/EDA. Our simulations demonstrate that the free-energy barriers for these intermolecular reactions are largely determined by the local solvation structure and dynamics of the amine species involved. We also discuss the strong effects of temperature and molecular structure on the solvation environments. While offering an explanation for preferred AEP formation in 30 wt % PZ, our work sheds light on the possibility that both the mechanism and rate of the PZ degradation reaction can be altered with changes in temperature and amine concentration.