Molecular Dynamic 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 fore front of scientific research. Utilizing membsranes 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 (PLD) 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 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.