Ultrathin carbon molecular sieve membrane for propylene/propane separation

Ultrathin (down to 300 nm), high quality carbon molecular sieve (CMS) membranes were synthesized on mesoporous γ-alumina support by pyrolysis of defect free polymer films. The effect of membrane thickness on the micropore structure and gas transport properties of CMS membranes was studied with the feed of He/N₂ and C₃H₆/C₃H₈ mixtures. Gas permeance increases with constant selectivity as the membrane thickness decreases to 520 nm. The 520-nm CMS membrane exhibits C₃H₆/C₃H₈ mixture selectivity of ∼31 and C₃H₆ permeance of ∼1.0 × 10^-8 mol m^-2 s^-1 Pa^-1. Both C₃H₈ permeance and He/N₂ selectivity increase, but the permeance of He, N₂, and C₃H₆ and the selectivity of C₃H₆/C₃H₈ decrease with further decrease in membrane thickness from 520 to 300 nm. These results can be explained by the thickness-dependent chain mobility of the polymer film which yields thinner final CMS membranes with reduction in pore size and possible closure of C₃H₆-accessible micropores.