Experimental and computational investigation on CO2 absorption kinetics of five piperazine-derived cyclic amines in aqueous/ether-water systems

Intrinsic CO 2 kinetics of cyclic amines are quantified in water-lean ethers. • Zwitterion mechanism is validated as dominant in aprotic ether-water mixtures. • Ether solvents impede CO 2 absorption by disrupting the dynamic H-bond network. • MD shows an ether-induced solvent cage effect restricting CO 2 access to amines. • Rigid C-substituted amines with high nucleophilicity suit ether-based systems. Piperazine-derived cyclic amines represent promising solutions for efficient CO 2 capture. However, fundamental understanding of their structure-activity relationships remains limited. This work combined experimental and computational approaches to elucidate the influences of cyclic structures and functional groups of piperazine-derived cyclic amines (PZ, 2MPZ, AEP, 1MPZ and 2HPZ) on their CO 2 capture performance and the structure-activity relationship in both pure water and water-lean environments. Key findings reveal that substituents reduce nitrogen electron density and impose steric hindrance, impeding CO 2 access to active sites. Stopped-flow measurements of the intrinsic kinetics of cyclic amines in aqueous and ether-water solvents revealed a reactivity trend: PZ > 2MPZ > AEP > 1MPZ > 2HPZ. The zwitterion mechanism demonstrates relatively good accuracy (AARD: 1.30%-2.66%) than base-catalyzed hydration (AARD: 2.62%-9.46%). DFT validates the zwitterion pathway, while MD simulations indicate that pure water solution maintains dynamic hydrogen-bond networks without outer-layer clustering. Ether addition disrupts hydrogen bonding and diminishes electron density at reactive sites, reducing absorption rates. These findings elucidate the underlying structure-activity relationships and solvation effects, providing a fundamental molecular-level understanding of the kinetic behaviors in water-lean amine systems. Download: Download high-res image (578KB) Download: Download full-size image