6FDA-DETDA: DABE polyimide-derived carbon molecular sieve hollow fiber membranes: Circumventing unusual aging phenomena

Abstract

Transport properties are reported for asymmetric carbon molecular sieve (CMS) hollow fiber membranes based on polyimide precursors derived from a butanol esterified diamino benzoic acid (DABA) based polyimide, 6FDA-DETDA:DABE. Precursor fiber pretreatment with 10% Ethenyl(trimethoxy)silane [vinyltrimethoxysilane (VTMS)] solution in hexane followed by pyrolysis at 550°C in ultra-high purity argon created asymmetric CMS fibers with CO2 permeance above 1000 GPU and CO2/CH4 selectivities > 25. Storage of the as-made modules for 72 days in 7bar CO2 suppressed undesirable aging typically seen under vacuum or atmospheric pressure air and provided CO2 permeance and CO2/CH4 selectivity of 780 GPU and 48 respectively. These results are in contrast to significant losses in CO2 permeance and CO2/CH4 selectivity for CMS created under equivalent pyrolysis conditions from non-esterified 6FDA-DETDA: DABA variant, even under CO2 storage. The 6FDA-DETDA: DABA-derived CMS results were surprising, since dense film CMS samples from the same precursors did not show the undesirable permeance and selectivity changes. Moreover, other non-DABA containing precursors in dense film and asymmetric fiber forms can be effectively stabilized under CO2 storage. Such results suggest that high residual stress in the freshly formed thin selective layers of asymmetric structures in DABA-derived CMS causes undesirable micro-scale rearrangements. In dense films and even asymmetric skins of non-DABA derived CMS membranes, evidence of such undesirable rearrangements are not apparent. DABA-derived cross-links are presumably the cause of these effects. Mono-esterification of 6FDA-DETDA: DABA to form 6FDA-DETDA: DABE controls such crosslinking-derived stresses and resultant instabilities in the CMS fibers, thereby providing stable high performance gas separation properties under CO2 storage or use in mixed gas feeds.