Aqueous piperazine (PZ) has been considered as an attractive solvent for CO2 capture, but its thermal degradation behavior is not fully understood yet. In this talk, we will present molecular mechanisms responsible for thermal degradation of aqueous PZ based on first-principles based studies. From ab initio molecular dynamics (AIMD) simulations coupled with metadynamics, we have first identified that the formation of an intermediate, 1-[2-(2-aminoethyl)amino]ethyl] piperazine (AEAEPZ), from the reaction between PZ and protonated PZ (PZH+) becomes kinetically more facile as a result of free-energy barrier reduction at high stripping temperatures (&gt; 400 K). This findings is consistent with experimental observations that PZ thermal degradation is enhanced with increasing temperature. Our calculations reveal that the following reaction between AEAEPZ and PZ may lead to final degradation products and also that the formation of N-(2-aminoethyl)piperazine (AEP), a major product, is kinetically more favorable than that of 1,1’-(1,2-ethanediyl)bis-piperazine (PEP) plus ethylenediamine (EDA), minor products, despite their similar thermodynamic stability. Our further analyses demonstrate that solvation around PZ and the intermediates may strongly influence the intermolecular interactions between amine molecules, which may in turn explain the fundamental reasons for the preferential reaction paths and mechanisms for PZ thermal degradation. This work features the significance of understanding temperature-dependent local structure and dynamics of solvent molecules during thermal degradation of CO2-loaded aqueous amine solution.

First-principles assessment of thermal degradation mechanisms of aqueous piperazine (PZ) for CO2 capture

ACS Spring 2022

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