Advanced fabrication of carbon molecular sieve membranes by nonsolvent pretreatment of precursor polymers

We have conducted an extensive study to investigate the effects of nonsolvent (methanol, ethanol, 1-propanol, and 1-butanol) pretreatment of polyimide precursor before carbonization on membrane structure and on the separation performance of carbon molecular sieve membranes (CMSMs). The measured gas separation properties show that carbon membranes prepared with nonsolvent pretreatment have a lower flux and higher selectivity, suggesting a narrower pore size distribution. XRD data show a slightly smaller d spacing in pretreated CMSMs. Positron annihilation lifetime spectroscopy results indicate smaller pores in the CMSMs pretreated with nonsolvents. The best separation efficiency is obtained by carbon membranes pyrolyzed from ethanol-treated polymers. The CO₂/CH₄ selectivity of Matrimd- and P84-derived carbon membrane increases significantly from 61 to 169 and 89 to 139, respectively, after ethanol pretreatment as compared to those carbon membranes when untreated. This study demonstrates for the first time that nonsolvent pretreatment of polyimide precursors can play an essential role in the transport properties of high-performance CMSMs.