Remarkably enhanced molecular sieving effect of carbon molecular sieve membrane by enhancing the concentration of thermally rearranged precursors

A significant obstacle that persists in the field of membrane-based gas separation is the development of advanced membranes. In this study, a series of thermally rearranged (TR) membrane precursors (6FDA/ODA/6FAP) with gradient concentration of 6FAP was selected for carbon molecular sieve membranes (TR-CMS) formation. The investigation into the structural evolution from polyimide to TR precursor and finally to CMS membrane utilized various characterization techniques to manipulate the relationship between membrane structure formation and their separation performance. Results show that the structural properties of intermediate TR-polymers, strongly influenced by the –OH content, significantly affect the resultant CMS membrane performance. Specifically, a higher –OH concentration in pristine membrane at 450 °C increases chain rigidity and FFV, leading to a more open pore structure and enhanced gas permeability in TR membranes. Interestingly, TR-CMS derived from increased PBO content exhibits a marked decrease in BET surface area, which correlates with improved molecular sieving performance. This can be attributed to the increased PBO content in the precursor, facilitated degradation upon further heating, thus leads to more orderly structure of the resulting CMS, thereby significantly enhanced molecular sieving performance. The TR-CMS-0.5 sample exhibited the most favorable gas separation performance (PH2 = 3843 and PCO2 = 901 Barrer) as well as high H2/CH4 (6 1 0) and CO2/CH4 (1 4 3) selectivity, which exceeded the 2019 Upper bound line. In addition, the TR-CMS-0.5 demonstrates a remarkable mixed-gas CO2/CH4 (50:50) separating performance that far exceeding its 2018 Upper bound line. In conclusion, the adoption of gradient PBO precursors for preparation of CMS membrane materials show great potential for hydrogen recovery and CO2 removal from natural gas.