In this presentation, I will describe the methods to evaluate the thermodynamic and kinetic parameters in polyimide-derived carbon molecular sieve (CMS) membranes, synthesized under varying processing conditions, that govern the transport of two same-sized gas molecules – CO and C2H4 (0.375 nm). CO is the intermediate product formed during the electrochemical reduction of CO2 to C2H4 and the separation of CO from C2H4 could facilitate high-purity C2H4 production. CMS membranes are synthesized by the high temperature (>500°C) pyrolysis of polyimide precursors under inert environments and the pyrolysis parameters help to tune the CMS structural and transport properties. For the purpose of this work, we focused on a specific pyrolysis condition – 550°C / inert environment, which has provided reasonable selectivities between CO and ethylene, primarily providing sorption-based separation between these two entities. Using this unique separation case, we probe the two distinct sorption and transport domains within these membranes – Langmuir (L) and continuous (C), and our results show that these same-sized entities have varying access to these two domains. We have also performed permeation and sorption experiments at three different temperatures, to evaluate the key diffusion and sorption intrinsic constants (e.g. activation energy, heat of sorption etc.). Comparing the performance of these CMS membranes under pure gas and mixed gas conditions, we were also able to estimate the competitive sorption effects and the diffusion coefficients within each of the individual morphological domains. My talk will highlight how the dual mode transport and sorption model provides molecular level insights into the CMS structure which are unavailable via any other known material characterization experiments and inform the design of novel materials for challenging gas pairs separation.
