Novel approaches to fabricate carbon molecular sieve membranes based on chemical modified and solvent treated polyimides

Two 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 cross-linking 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.