(551i) Relevance of “Cross-over” Pores in CMS Membrane Transport Properties

So-called “cross-over pores” are believed to exist at imperfect joints between adjacent micropores in carbon molecular sieve (CMS) membranes, thereby providing unusually large ultramicropores between neighboring micropores. Ideally, CMS membranes can be described by a bimodal pore-size distribution curve comprising two different classes of pores - micropores (7 Å < d_micropore < 20 Å) and ultramicropores (d_{ultramicropore} <7 Å). While micropores are envisioned to regulate the sorption characteristics and diffusion jump lengths for penetrant gas molecules, the molecular sieving characteristics are attributed to the ultramicroporous slits in plates between micropores. Cross-over pores need not be undesirable and may provide additional tuning tools to engineer transport of penetrant molecules, especially for bigger hydrocarbon gas-pairs. Such pores do not contribute significantly to the sorption capacities of the CMS membranes, but double the diffusion jump lengths (λ) of the molecules for the idealized case of equal-sized micropores. In the direction of transport, crossover pores act “in-parallel” with standard ultramicropores within the micropore walls. The crossover or “defect” pores can be tightened by optimizing processing parameters during CMS formation post-formation tuning strategies. Beyond size-based sieving abilities, these pores appear to present opportunities for subtle entropic discriminations as well, by restricting the degrees of freedom for large, complex gas molecules.