Carbon molecular sieve membranes derived from UV-irradiated polyimides for enhanced molecular separation and physical aging resistance

Abstract

Herein, we present an approach for adjusting the pore structure of carbon molecular sieve (CMS) membranes by controlling UV irradiation on polyimide (PI) precursors. CMS dense membranes were fabricated by pyrolyzing UV-crosslinkable PIs synthesized from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), and 2,4-diamino mesitylene (DAM). Increasing UV irradiation time enhanced molecular sieving performance while decreasing gas permeability. At the highest UV irradiation time of 40 min, the gas selectivities for H₂/CH₄ and CO₂/CH₄ improved 3-fold and 2-fold, respectively. Meanwhile, H₂ and CO₂ permeabilities decreased by ∼9 % and ∼30 % relative to the pristine CMS membrane, both deviating notably from the upper bound slope trends. Gas diffusivity and pore size distribution analyses suggested that these improvements stemmed from a reduction in larger ultramicropores. Furthermore, a prolonged CO₂/CH₄ mixed-gas permeation test (170 days) showed that pre-crosslinked CMS membranes demonstrated enhanced resistance to physical aging. After the aging period, the pre-crosslinked CMS membranes showed slightly higher mixed-gas CO₂ permeability and ∼2 times higher mixed-gas CO₂/CH₄ selectivity than pristine CMS. This study introduces a novel method for tuning the CMS microstructure to produce more selective and aging-resistant carbon membranes.