Relative Aromaticity/Aliphaticity Steered Pore Structure in Polyamide-Derived Ultramicroporous Carbons for Efficient C3H6/C3H8 Separation.

Carbon molecular sieves (CMS) with a tunable pore structure hold significant promise for efficient C₃H₆/C₃H₈ separation. However, understanding the relationship between a precursor's carbon framework and the microstructure of carbonized products is still ambiguous and requires further investigation. Herein, a relative aliphaticity/aromaticity regulated strategy was proposed to tailor the carbon skeleton of the polyamide precursor, aiming to fine tune the CMS pore size between the kinetic diameter of C₃H₆ (4.68 Å) and C₃H₈ (5.11 Å). The relative aliphaticity/aromaticity of the precursor was rationally modulated by replacing aromatic rings in diamine monomers with aliphatic chains of different lengths. Results indicated that polyamide precursors with higher relative aliphaticity exhibited increased susceptibility to fragmentation during carbonization. Thus, a higher degree of carbon layer restructuring arising from the degradation of aliphatic chains promoted the formation of orderly graphitized structures with sub 5 Å ultramicropores. The ETDA-derived CMS pyrolyzed at 700 °C (ETDA700) exhibited outstanding sieving performance in separating C₃H₆ from C₃H₈, with C₃H₆ uptakes of up to 2.33 mmol/g, while propane adsorption capacity was negligible. This work may provide valuable insights for the design of sieving carbonaceous material by rationally tuning precursor properties for the efficient separation of gas mixtures with similar sizes.