Design and Performance Evaluation of 6FDA-Based Polyimide Membranes for Enhanced CO2/CH4 Separation: Insights from Molecular Dynamics Simulations

The development of high-performance polyimide (PI) membranes for efficient gas separation is of great importance, yet challenging. Herein, we design 17 6FDA-based PI membranes and employ molecular dynamics (MD) simulations to investigate their carbon dioxide (CO₂) /methane (CH₄) separation performance. The 6FDA-Durene membrane emerges as a standout candidate, with a mean pore size (MPS) of 5.12 Å, achieving a CO₂ permeability of 13,271.40 Barrer and a CO₂/CH₄ selectivity of 26.87, surpassing the 2019 upper bound. Further analysis reveals that the steric effect in ultramicropores is favorable for improving CO₂ selectivity, while the combination of outstanding gas diffusivity and high-effective gas solubility in larger pores enables this optimal membrane to exceed the upper bound. When the MPS is increased to 7.06 Å, the CO₂ permeability is improved by approximately 5-fold (up to 66,552.28 Barrer), while the CO₂/CH₄ selectivity dropped to 5.08 (below the 2008 upper bound). This reduction in selectivity is a consequence of the decline in both gas diffusivity and gas solubility. This study emphasizes the crucial relationship between membrane structures and separation efficiency, offering invaluable insights for the development of advanced PI membranes with enhanced selective transport performance.