Molecular Dynamics Insights into CO₂/N₂ Separation by a Porous Organic Cage-Supported Ultrathin Imidazolium Ionic...

This study employed molecular dynamics simulations to systematically evaluate the CO₂/N₂ separation performance of composite membranes composed of a porous organic cage (CC3) surface coated with four distinct imidazolium-based ionic liquids (ILs): [BMIM][BF₄], [BMIM][PF₆], [BMIM][Tf₂N], and [BMIM][SCN]. The results indicate that the CC3/[BMIM][BF₄] composite membrane, with a thickness of 8 Å, demonstrates the optimal CO₂/N₂ selectivity (20.6) coupled with a significant CO₂ permeance (1.59 × 10⁴ GPU). Mechanistic analysis reveals that the strength of CO₂-anion interactions follows the order [Tf₂N]^- > [PF₆]^- > [BF₄]^- > [SCN]^-, consistent with the order of CO₂ permeance in the composite membranes. Additionally, the interaction between CO₂ and the composite membrane is markedly stronger than that observed for N₂. Further examination of gas transport behavior suggests that different solubility of CO₂ and N₂ within the IL phase, together with their distinct diffusion behavior at the IL-CC3 interface, synergistically facilitate effective gas separation. Comparative assessments involving related POC materials, including CC1 and CC2, demonstrate that the CC3-based composite membrane achieves superior separation performance. This investigation elucidates the molecular-level mechanisms underlying gas separation in CC3/IL composite membranes, offering theoretical insights to inform the design and optimization of advanced gas separation membranes.