Directly measured pyrolysis temperature dependence of sorption selectivity for carbon dioxide/methane in Matrimid® polyimide-derived carbon molecular sieve membranes

Abstract

In this study, we augment our analysis of pyrolysis temperature dependence of carbon molecular sieve (CMS) membranes derived from a polyimide, Matrimid® useful for advanced CMS membranes. Our direct sorption results prove that the strong pyrolysis temperature dependence of permselectivity for the CO2/CH4 gas pair is promoted by an unusual trend in sorption selectivity reported here. This trend complements conventional diffusion selectivity trends for this pair with increasing pyrolysis temperatures. Our results show that the elimination of pyridinic and pyrrolic groups over the same pyrolysis temperature range occurs. Elimination of such CO2-philic pyridinic and pyrrolic groups typically would be expected to cause a trend opposite to that actually measured directly for CO2/CH4 selectivity. The resulting trend for pyrolysis temperatures between 550 °C and 900 °C, however, can be explained in terms of microporous and ultramicroporous properties of the CMS for increasing pyrolysis temperatures. Specifically, apparent thermodynamic exclusion selectivity of the 0.5 Å larger CH4 vs. the slim 3.3 Å CO2 provides a consistent explanation of the trends. This discovery provides additional options to tune permselectivity of CMS membranes to complement molecular sieving based on diffusion contributions.