Molecular structure-derived stability mechanism for sustainable amine-based CO2 capture

Abstract

Solvent degradation is the most critical challenge of amine-based CO₂ capture technology, which necessitates the development of chemically stable amine/amine blends to mitigate the absorbent degradation for industrial application. This study experimentally examined fourteen structurally different amines, namely the relatively stable cyclic, sterically hindered, and tertiary amines, to reveal the relationship between amine structure and chemical stability. The cyclic amines with a symmetrical ring structure greatly promote their oxidative stability, whilst the asymmetrical ring structure exhibited poor performance due to its relatively high C-N bond flexibility. The potential intramolecular interaction between the parent ring and the branched chain also reduced the stability of cyclic amines. While sterically hindered amines showed desirable oxidative stability thanks to the C-N bond protection by substitutions adjacent to amino group, their stability was very sensitive to the number of chain and the type of functional group surrounding to the amino group. Thermal stability of sterically hindered amines was also less satisfactory. It is observed that the amines with a rigid C-N bond, a longer branched chain connected to parent ring, and a preferable steric hinderance substantially promote the oxidative stability, with approximately 20 times less degradation compared to the benchmark MEA. Quantum calculation of bond dissociation energies (BDEs) of hydrogen-containing bonds were performed to reveal the atom position of radical formation that initiates the amine degradation. The potential degradation pathways were further predicted based on the positions of radical formation and the calculation of reaction Gibbs energy for possible degradation reactions. The analysis of heat stable salts products and the nitrogen distribution in the degraded solutions confirmed the bond cleavage position induced by radical formation and the reaction pathways, which clarified the potential amine oxidation mechanism. This work is anticipated to provide the generic principles to evaluate and develop the chemically stable amine solvent towards long-term operation of the amine process.