Molecular elucidation of sub‐3 Å gas transport in ultramicroporous carbon molecular sieve membranes

Carbon molecular sieve membranes (CMSMs) are highly desirable for industrial gas separation. However, a molecular‐level understanding of their gas transport characteristics and separation mechanisms, governed by ultramicropores, remains challenging, especially for gas molecules with sub‐3 Å dimensions. Herein, we investigate the transport behavior of sub‐3 Å gas molecules (H 2 , He, and Ne) in the ultramicroporous CMSM using experimental‐based atomic modeling and simulations. Results demonstrate that the H 2 /He selectivity exceeding that of Knudsen diffusion arises from kinetic configurational confinement imposed by ultramicropores, specifically due to the slender geometry of H 2 and its smaller critical diameter (2.38 Å versus 2.79 Å for He). Unlike H 2 and He, Ne exhibits preferential adsorption within CMSM, which induces significant ultramicropore shrinkage during transport, especially at low operating temperatures, leading to remarkably enhanced He/Ne sieving capability. These results provide an in‐depth and molecular perspective on efficient sub‐3 Å gas separation using CMSMs.