(537b) Tuning of Carbon Molecular Sieve Membranes for Separation of the C3H6-C3H8 Pair

Carbon molecular sieve (CMS) membranes display exceptional separation properties for gas-pairs such as C₃H₆-C₃H₈, which cannot be addressed using conventional polymers. Such CMS membranes are often formed by controlled pyrolysis of asymmetric polymer precursor fibers. This study considers non-standard pyrolysis protocols for an important polyimide, 6FDA:BPDA-DAM that offers attractive CMS properties for the C₃H₆-C₃H₈ pair. Specifically, effects of heating ramp rate, maximum pyrolysis temperature, post-pyrolysis thermal soak time were considered. Fundamental bases for optimization of each of these parameters are offered. The heating ramp profile controls the time spent in each individual stage of the pyrolysis processâ??especially the period the precursor spends beyond its decomposition temperature. For a given precursor, this critical period sets the micropore and ultramicropore dimensions in the CMS fiber as well as their distributions. Subsequent thermal soak times enable equilibration and rearrangement of the carbon domains, thereby providing tools to tune the CMS fine structure. Somewhat surprisingly, an increase in ramp rates led to a reduction in C₃H₆ permeance, with the highest ramp rate providing a considerable gain in C₃H₆-C₃H₈ selectivity. For a given constant ramp rate, thermal soak time reductions led to discernible improvements in the C₃H₆ permeance with a moderate compromise in C₃H₆-C₃H₈ selectivity. Decreased soak time reduce processing time for CMS membrane fabrication and will be considered in the context of ultimate scale-up of these membranes. Preliminary data on the impact of the above process variables other gas-pair separation will also be considered.