The gas separation properties of carbon molecular sieve membranes derived from polyimides having carboxylic acid groups

Carbon molecular sieve (CMS) membranes were prepared by pyrolysis of polyimides containing carboxylic acid groups, in order to investigate their permeation properties for He, CO2, O2 and N2. The polyimides were synthesized with a varying number of carboxylic acid groups in the diamine. Wide-angle X-ray diffraction and nitrogen gas sorption isotherms were employed to characterize the carbon structures. From pure gas permeation experiments, it was found that the removal of carboxylic acid pendent side groups during the pyrolysis resulted in a large pore volume in the carbon matrix, which significantly affected the gas separation performance of the final CMS membranes. The gas permeabilities of the CMS membranes pyrolyzed at 700 °C improved with increasing carboxylic acid group content. The CMS membranes derived from polyimide containing 50 mol% diamines showed maximum gas permeability for O2 of 707 Barrers (1×10−10 cm3 (STP) cm/cm2 s cmHg) and an O2/N2 selectivity of 9 at 25 °C.