Effects of polyimide pyrolysis conditions on carbon molecular sieve membrane properties

In previous research, carbon molecular sieve (CMS) membranes for gas separations have been produced using either a vacuum pyrolysis or an inert purge pyrolysis technique on a precursor which is often polymeric. This study compares both techniques using the same polyimide precursor material. Additional pyrolysis variables included the type of ''inert'' purge gas (argon, helium, and carbon dioxide), purge flow rate, and temperature. Vacuum pyrolysis produced more selective but less productive CMS membranes than the inert purge pyrolyzed membranes. ''High'' purge gas flow rates (i.e., 200 standard cubic centimeters per minute or cm(3)(STP)/min) produced a much higher permeability, but lower selectivity membrane compared to those produced in a ''low'' purge flow rate (20 cm(3)(STP)/min). By raising the pyrolysis temperature from 550 to 800 degrees C, the effective pore size was reduced, thereby making the CMS membranes more selective but less productive. Mixed gas tests using oxygen/nitrogen and hydrogen/nitrogen mixtures were used to evaluate membrane performance.