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Demand for energy-efficient gas separations exists across many industrial processes, and membranes can aid in meeting this demand. Carbon molecular sieve (CMS) membranes show exceptional separation performance and scalable processing attributes attractive for important, similar-sized gas pairs. Herein, we outline a mathematical and physical framework to understand these attributes. This framework shares features with dual-mode transport theory for glassy polymers; however, physical connections to CMS model parameters differ from glassy polymer cases. We present evidence in CMS membranes for a large volume fraction of microporous domains characterized by Langmuir sorption in local equilibrium with a minority continuous phase described by Henry’s law sorption. Using this framework, expressions are provided to relate measurable parameters for sorption and transport in CMS materials. We also outline a mechanism for formation of these environments and suggest future model refinements.

Main text paragraph: Carbon molecular sieve (CMS) membranes combine outstanding and tunable separation performance, thereby making them applicable to many different important applications [1]. The ability of such membranes to discriminate between similarly sized penetrants based on subtle physical differences has been well documented [2]. This discussion focuses on polyimide-derived CMS membranes, whose transformation process from flexible polymer coils into rigid molecular sieves has been described previously [3]. Unlike other rigid molecular sieves, CMS membranes can be configured into flexible, asymmetric hollow …