Carbon molecular sieve gas separation membranes from crosslinkable bromomethylated 6FDA-DAM polyimide

Tuning the relationship between the microporous structure of carbon molecular sieve (CMS) membranes and the corresponding gas permeability performance enables optimization for gas separation applications. The pre-crosslinked structure of the precursor plays a key role in improving the performance of the CMS membrane. Herein, the thermally crosslinkable brominated 6FDA-based polyimide (BMPI) by debromination is prepared and selected as the precursor to fabricate high performance CMS gas separation membranes. By heat-treating the BMPI membrane at 350 °C for 10 h, the debromination-induced crosslinking exhibited higher permeability over the uncrosslinked precursors, as it forms a pre-crosslinked structure that is more stable at 550 °C. A representative BMPI-350/10h-550 °C CMS membranes pyrolyzed at 550 °C had CO2 and O2 permeabilities of 11169 and 2182 Barrer with a CO2/CH4 and O2/N2 ideal selectivity of 26.5 and 5.4. These values are much higher than that of the un-crosslinkable precursor (6FDA-DAM) derived CMS membrane (PCO2 = 3465, αCO2/CH4 = 21.1). Even at the high carbonization temperature of 800 °C, the as-obtained CMS membrane still show a promising CO2 permeability of 1119 Barrer and a CO2/CH4 ideal gas selectivity of 41.2. Importantly, the excellent gas separation performance of CMS membranes obtained in this work exceeds the Robeson's upper-bound. This work outlines a new protocol to tailor CMS membrane microstructures to meet high permeability performance needs. These CMS membrane materials hold great potential in corrosive natural gas purification applications.