Tuning Fluorination of Carbon Molecular Sieve Membranes with Enhanced Reverse‐Selective Hydrogen Separation From Helium

Abstract Membrane technology has been explored for separating helium from hydrogen in natural gas reservoirs, a process that remains extremely challenging due to the sub‐Ångstrom size difference between H 2 and He molecules. Reverse‐selective H 2 /He separation membranes offer multiple advantages over conventional helium‐selective membranes, which, however, suffer from low H 2 /He selectivity. To address this hurdle, a novel approach is proposed to tune the ultra‐micropores of carbon molecular sieves (CMS) membranes through fluorination of the polymer precursor. By incorporating ‐CF 3 units into the backbone of Tröger's base polymers, the microporosity of CMS is tailored and reverse‐selective H 2 /He CMS membranes are deployed with remarkable separation performance, surpassing most reported membranes. These CMS membranes exhibit a H 2 permeability of 1505.2 Barrer with a notable H 2 /He selectivity of 3.8. Barometric sorption tests reveal preferential sorption of H 2 over He in the fluorinated CMS membranes, which also demonstrate a significantly higher H 2 /He diffusion selectivity compared to unfluorinated samples. Material studio calculations indicate that the “slim” hydrogen molecule penetrates ultra‐micropores more readily than the spherical He molecule, thus achieving reverse H 2 /He selectivity. This design approach offers a promising pathway for developing molecularly sieving membranes to tackle the challenging helium separation from natural gas.