Molecular Dynamics Insights into CO 2 /N 2 Separation by a Porous Organic Cage-Supported Ultrathin Imidazolium Ionic Liquid Layer

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.