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 H₂ and He molecules. Reverse-selective H₂/He separation membranes offer multiple advantages over conventional helium-selective membranes, which, however, suffer from low H₂/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 -CF₃ units into the backbone of Tröger's base polymers, the microporosity of CMS is tailored and reverse-selective H₂/He CMS membranes are deployed with remarkable separation performance, surpassing most reported membranes. These CMS membranes exhibit a H₂ permeability of 1505.2 Barrer with a notable H₂/He selectivity of 3.8. Barometric sorption tests reveal preferential sorption of H₂ over He in the fluorinated CMS membranes, which also demonstrate a significantly higher H₂/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 H₂/He selectivity. This design approach offers a promising pathway for developing molecularly sieving membranes to tackle the challenging helium separation from natural gas.