Unraveling the relationship between microstructure of CMS membrane and gas transport property using molecular simulation

Carbon molecular sieve (CMS) membranes are attractive for energy-efficient gas separations. A challenge with the fabrication of a high-performance CMS membrane is fine-tuning its microstructure for precise and efficient separation. This necessitates a molecular-scale analysis to understand its microstructure–performance relationship. Herein, molecular simulations were performed to unravel the relationships between four similar-sized CMS matrices with different microstructural characteristics (e.g., chemical composition and micromorphology) and their gas transport properties. Results show that the disordered packing of carbon layers, leading to the formation of ultramicropore (2–7 Å), originates from stereoscopic sp³ hybridized carbon atoms rather than non-carbon (oxygen) atoms. The size-sieving ability of CMS depends positively on ultramicroporosity; the adsorption capacity is strengthened and then weakened with the increase of ultramicroporosity. Competitive effects are observed in binary-mixture transport, and it is expected that the separation performance can be optimized by a reasonable distribution of ultramicropores combined with the affinity of oxygen-containing species.