Separating carbon monoxide (CO) from ethylene (C2H4) is a challenge due to their identical kinetic diameters (~ 0.375 nm). This separation is crucial when electrochemically reducing CO2 to C2H4 to get high purity C2H4 and remove intermediates like CO. Carbon molecular sieve (CMS) membranes, derived through the high-temperature pyrolysis of polyimides, offer a promising solution because of their ability to discriminate between similarly sized gas molecules. 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. Our prior work has 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 have 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.). These prior studies reveal that sorption selectivity and diffusion selectivity show opposing trends at higher testing temperatures, which are relevant for the electrochemical conversions. Specifically, sorption selectivity is expected to rise with temperature, while diffusion selectivity will likely decrease. This suggests lower separation factors will be observed between these gas molecules under these conditions, which could make the overall performance unattractive. However, the fundamental analyses done on the base material (550°C / inert environment) suggests that optimal tuning of pyrolysis conditions, which could help to maintain the diffusion selectivities even at high temperatures, will be useful for this scenario. My talk, therefore, will focus on the effects of pyrolysis parameters on the performance of CMS membranes for both single gas and mixed gas cases. These pyrolysis parameters include final pyrolysis temperatures, ramp rates and cooling rates. We will carefully identify how different pyrolysis protocols influence the internal structure of CMS membranes and how those structural differences impact their ability to separate CO from C2H4. Using this particularly interesting same-sized gas pair as a test case, we uncover how membrane structure and processing conditions converge to shape performance - insights that cannot be captured by traditional material characterization methods alone.
