Pressure-confined fluorination engineering of carbon molecular sieve membranes for tunable propylene/propane separation

The separation of propylene from propane (C 3 H 6 /C 3 H 8 ) demands membranes with refined tuning of ultramicropore dimensions to discriminate molecules differing by only 0.3 Å in kinetic diameter. Here, we report a pressure-confined fluorination strategy for engineering the ultramicroporous structure of carbon molecular sieve (CMS) membranes derived from trifluoromethyl-containing polyimides. Mild external pressure (0–0.1 bar) applied during pyrolysis kinetically retards CF x volatilization, enabling retained fluorine species to introduce steric defects and expand inter-strand spacing via in situ secondary reactions. This enables pressure-dependent tuning of ultramicropore accessibility and molecular-sieving behavior. At 0.03 bar, the membrane achieves a C 3 H 6 permeability of 2400 ± 200 Barrer with a C 3 H 6 /C 3 H 8 selectivity of 17 ± 1; at 0.1 bar, a selectivity of 33 ± 3 is obtained alongside a C 3 H 6 permeability of 1900 ± 200 Barrer, surpassing the upper bound for C 3 H 6 /C 3 H 8 separation. Excellent CO 2 /CH 4 separation performance further demonstrates the broader applicability of this approach.