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Summary: The separation of H2/CO2 is technologically important to produce the next generation fuel source, hydrogen, from synthesis gas. However, the separation efficiency achieved by polymeric membranes is usually very low because of both unfavourable diffusivity selectivity and solubility selectivity between H2 and CO2. A series of novel diamino‐modified polyimides has been discovered to enhance the separation capability of polyimide membranes especially for H2 and CO2 separation. Both pure gas and mixed gas tests have been conducted. The ideal H2/CO2 selectivity in pure gas tests is 101, which is far superior to other polymeric membranes and is well above the Robeson's upper‐bound curve. Mixed gas tests show an ideal selectivity of 42 for the propane‐1,3‐diamine‐modified polyimide. The lower selectivity is a result of the sorption competition between H2 and the highly condensable CO2 molecules. However, both pure gas and mixed gas data are better than other polymeric membranes and above the Robeson's upper‐bound curve. It is evident that the proposed modification methods can alter the physicochemical structure of polyimide membranes with superior separation performance for H2 and CO2 separation.

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 CO 2 /CH 4 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.

Novel approaches to fabricate carbon molecular sieve membranes based on chemical modified and solvent treated polyimides Tin, P.; Chung, T-S.; Kawi, S.; Guiver, Michael AbstractTwo brand-new modification technologies were developed for pyrolyzing the carbon molecular sieve membranes (CMSMs) with excellent separation efficiency. The modifications were performed on polymeric precursors. It is believed that the space filling effect by these modifications could considerably alter the separation performance of resultant carbon membranes. Firstly, a cross-linking modification was performed on polymeric precursors at room temperature before pyrolysis. The effectiveness of chemical crosslinking technology in improving gas separation capability of CMSMs was investigated. In this study, the permeation properties of carbon membranes derived from cross-linked Matrimid were characterized as a function of cross-linking density. Results demonstrated that the permeability of modified CMSMs decreased with increasing in cross-linking density. Detailed examination reveals that cross-linking modification increased the selectivity at a low degree of cross-linking but reduced the selectivity at a higher degree of cross-linking. The improvement of separation efficiency at low degree of cross-linking is presumably related to the swelling of polymer chains by methanol during cross-linking modification. Consequently, the second extremely simple modification method by using pure methanol immersion was developed. It was found that the CMSMs derived from methanol-treated precursors exhibited superior transport properties. Methanol treatment yielded CMSMs with higher selectivities if compared to CMSMs based on untreated and cross-linked Matrimid. Therefore, it can be concluded that the swelling of polymer chains by methanol appears to be an effectual modification method to produce the CMSMs with excellent separation properties.