Tailoring amine activators in MDEA/NHD/H2O biphasic absorbents for energy-efficient CO2 capture: Mechanistic insights into polarity-driven phase separation and multi-component synergy

Amine-based CO₂ capture faces significant challenges due to high regeneration energy requirements (3.50–4.50 GJ/tCO₂) and the trade-off between viscosity and liquid–liquid phase separation. In this work, we designed a biphasic absorbent by optimizing a ternary methyldiethanolamine (MDEA)/polyethylene glycol dimethyl ether (NHD)/water system, synergized with activators including monoethanolamine (MEA), diethanolamine (DEA), 2-amino-2-methyl-1-propanol (AMP), and piperazine (PZ) to address these limitations. Our results reveal that incorporating 15 wt% MEA yielded the best performance, enhancing absorption capacity to 2.04 mol/L–90 % higher than the MDEA/NHD/H₂O baseline–while achieving a sevenfold increase in the initial absorption rate and a 34 % reduction in regeneration energy (2.50 GJ/tCO₂) compared to 30 wt% MEA solutions. DEA and PZ showed lower capacity enhancements (24 % and 90 %, respectively), while PZ led to viscosity spikes, and AMP caused impractical solid–liquid separation. The optimized 15 wt% MEA + 15 wt% MDEA + 50 wt% NHD + 20 wt% H2O (3M3M10N4H) system exhibited polarity-driven liquid–liquid phase separation, where high-dipole absorption products (e.g., MEACOO⁻ with a dipole moment of 21.99 D) concentrated in the aqueous phase, while low-polarity NHD formed a distinct phase. Density functional theory (DFT) calculations revealed ion–dipole interactions as the thermodynamic driver of liquid–liquid phase separation. Mechanistic studies further demonstrated that MDEA reduced the energy barrier for zwitterionic intermediate conversion to carbamate via proton transfer (12 kcal/mol), enhancing both absorption kinetics and capacity through synergistic interactions with MEA. Desorption at 120 °C achieved 93 % CO₂ release efficiency with 92 % capacity retention over five cycles, maintaining stable lean/rich phase ratios. Based on these insights, we propose a functional division strategy integrating an “activator-main amine-phase separation agent” framework, elucidating the multi-component synergy mechanisms governing absorption, phase separation, and regeneration. This study highlights a rational design framework for energy-efficient and stable liquid–liquid phase change absorbents for CO₂ capture.