Computational study on CO2 absorption mechanism and structure–activity relationship of biphasic solvent

Abstract

The biphasic solvents are conducive to improving the efficiency of CO2 capture and reducing the energy consumption caused by regeneration. This study explored their reaction process at the molecular level based on density functional theory (DFT): selection of diamines such as active amines with distinct amino groups, 1,4-butanediamine (BDA) and N-(2-aminoethyl)ethanolamine (AEEA), and with typical phase separation promoters, 2-(diethylamino)ethanolamine (DEEA), and N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA). This study systematically explores the potential reaction paths in CO2 capture to quantitatively assesse the structure–activity relationships among different amines in different stages of absorption, the influence of reaction sequence on reaction rates, and further elucidates the characteristics of product formation and its impact on the phase separation process. The results indicated that the zwitterion mechanism is widely applicable and remains the dominant explanation for the biphasic solvent CO2 capture mechanism. The intramolecular proton transfer reaction is the key to influencing the absorption rate of CO2, but in the case of interproton transfer, the rate-determining step needs to be judged in conjunction with the type of amino groups on it. And the amine groups on AEEA react with CO2 in different orders, forming distinct zwitterions that have similar activity in later intramolecular proton transfers. Under the zwitterion mechanism, primary amino groups exhibit stronger proton affinity than secondary ones. In solvents approaching saturation stage, DEEA reacts with AEEA and CO2 earlier than PMDETA. This study reveals the reaction mechanism of biphasic solvents and the key paths in different phases, providing insights for developing new solvents.