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 CO 2 /N 2 separation performance of composite membranes composed of a porous organic cage (CC3) surface coated with four distinct imidazolium-based ionic liquids (ILs): [BMIM][BF 4 ], [BMIM][PF 6 ], [BMIM][Tf 2 N], and [BMIM][SCN]. The results indicate that the CC3/[BMIM][BF 4 ] composite membrane, with a thickness of 8 Å, demonstrates the optimal CO 2 /N 2 selectivity (20.6) coupled with a significant CO 2 permeance (1.59 × 10 4 GPU). Mechanistic analysis reveals that the strength of CO 2 -anion interactions follows the order [Tf 2 N] − > [PF 6 ] − > [BF 4 ] − > [SCN] −, consistent with the order of CO 2 permeance in the composite membranes. Additionally, the interaction between CO 2 and the composite membrane is markedly stronger than that observed for N 2 . Further examination of gas transport behavior suggests that different solubility of CO 2 and N 2 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.