A molecular dynamics simulation study on hydrocarbon ladder polymer membranes for gas separation†

Abstract

To address global environmental challenges and support the transition of energy systems, the study of CO₂ capture and separation is at the forefront of scientific research. Utilizing membranes based on polymers of intrinsic microporosity (PIMs) for CO₂ separation presents a promising approach. However, the mechanisms of CO₂ separation in PIMs are not fully understood. In this study, an isobaric model combined with molecular dynamics (MD) simulation was used to explore the adsorptive and diffusive behaviors of CO₂ and N₂ in PIM membranes. We elucidated the gas separation mechanism by analyzing three critical aspects: microporous structure, adsorptive selectivity, and diffusive selectivity. The findings reveal that PIM membranes exhibit advantageous separation characteristics due to their large Brunauer–Emmett–Teller (BET) surface areas and Pore Limiting Diameters (PLDs) that are more compatible with the size of CO₂ molecules. Additionally, the difference in solvation free energy and diffusion rates between the two gases within the membranes significantly contributes to their selectivity. Specifically, CO₂ diffuses within the membrane primarily through a hopping mechanism supplemented by diffusive motion, whereas N₂ relies mainly on diffusion with less hopping. Since dissolution often takes precedence over diffusion in the separation process, it can sometimes lead to less effective diffusion for gas molecules. Moreover, the simulation results indicate that the diffusion behavior of the CO₂/N₂ mixture in PIM membranes is governed by a solubility-driven separation mechanism. This work provides a theoretical foundation for understanding gas transport and separation mechanisms in PIM membranes.