Two-stage interaction performance of CO2 absorption into biphasic solvents: Mechanism analysis, quantum calculation and energy consumption

The CO2 capture capabilities of amine-based biphasic solvents have been extensively investigated. However, the mechanism of CO2 absorption into the biphasic solvents has not been adequately examined, even if it is very important for the solvent screening, composition optimization, efficiency improvement and heat duty reduction. In this work, a two-stage interaction mechanism is proposed through CO2 absorption performance, 13C nuclear magnetic resonance analysis, and quantum calculation. The experimental results and quantum calculation revealed that the shift of the absorption mechanism depended on the active amine group in the biphasic solvents. In the first stage, for a high concentration of the active amine group, a proton was transferred from zwitterion to tertiary amines with the primary amines as intermediates. The CO2 absorption rate was controlled by reaction kinetics, which resulted in a high absorption rate and inconspicuous phase change. In the second stage, for a low concentration of the active amine group, the thermodynamics-controlled absorption rate decreased sharply (by 65.2% for 1,4-butanediamine/N,N-dimethylcyclohexylamine) because the proton transferred from zwitterion to tertiary amines directly. Kinetic analysis revealed that, with an increase in the temperature from 298 to 333 K, the reaction rate constants increased by 285–362% at the first stage. However, the equilibrium constants decreased by 17.7–21.3% at the second stage. Taking both the kinetics and energy penalty into account, the triethylenetetramine/N,N-dimethylcyclohexylamine solution with a lean loading of 0.4 mol mol−1 is promising to achieve the high absorption rate and low-energy consumption.