Ultramicropore Tightening in Carbon Molecular Sieve Membranes Enables Entropy‐Dominated Propylene/Propane Separation

Membrane‐based propylene/propane separation demands molecular discrimination at the sub‐ångström scale. Carbon molecular sieve (CMS) membranes offer rigid molecular‐sieving ultramicropores which exploit the subtle differences in molecular size and configuration between planar C 3 H 6 and three‐dimensional C 3 H 8 . However, precisely tailoring these ultramicropores while retaining rapid transport remains challenging. Here, fluorinated copolyimide precursors with tunable concentrations of bulky ─C(CF 3 ) 2 ─ units enable composition‐controlled ultramicropore tightening. Increasing the concentration of these units raises precursor free volume and enhances volatile evolution during pyrolysis. Regulating precursor composition and pyrolysis temperature produces CMS structures with tighter ultramicropores and preserved transport accessible microporosity. After 65 days of stabilization, the optimized membrane achieves a mixed‐gas C 3 H 6 permeability of 55.8 Barrer and a C 3 H 6 /C 3 H 8 selectivity of 68.6 under an equimolar feed at 60 psia and 35°C, surpassing the reported mixed‐gas upper bound. Selectivity remains close to 70 over 35–55°C and feed pressures of 60–180 psia. Sorption‐diffusion analysis identifies diffusivity selectivity as the principal contributor to separation. The enhanced apparent entropic selectivity is consistent with greater configurational restrictions on three‐dimensional C 3 H 8 than on planar C 3 H 6 . These findings demonstrate ultramicropore tightening in CMS membranes as a means of leveraging configurational entropy for challenging sub‐ångström molecular separations.