Development of CO₂ Separation Membranes Using Diamine-based Ionic Liquid Mixtures

Abstract

To support carbon neutrality goals by 2050, the development of efficient CO₂ separation and capture technologies is essential. Among the available methods, membrane separation offers notable energy efficiency advantages. Ionic liquids (ILs) have attracted attention as functional materials for CO₂ separation in absorption, adsorption, and membrane-based systems. By introducing ether-functionalized carboxylate anions, the authors tuned both chemical and physical CO₂ absorption properties. In addition to designing single ILs, this study focuses on mixed IL systems for developing high-performance CO₂ separation materials. Mixing two ILs with complementary functions enabled the simultaneous enhancement of CO₂ capture and release—an achievement not possible with single-component ILs. These mixed ILs exhibited novel CO₂ absorption mechanisms, confirmed via NMR and IR analyses. When impregnated into porous membranes, the mixed ILs demonstrated CO₂ permeability and CO₂/N₂ selectivity values that significantly exceeded those of conventional polymer membranes. The study also explored a variety of diamine-based IL structures and found that hydroxyethyl modifications further improved both gas permeability and selectivity, highlighting the potential for fine structural control in IL design. In addition, the authors developed an accelerated degradation testing method to evaluate the oxidative stability of ILs. The results revealed that although degradation occurs under oxidative conditions, the membrane’s CO₂ separation performance remained stable up to a certain level of IL decomposition. This suggests sufficient operational durability under practical use. These findings provide a promising strategy for designing IL-based materials tailored to low-concentration CO₂ sources such as Direct Air Capture (DAC), offering both high performance and long-term stability under real conditions.