Triazine Crosslinking Enables Nitrogen‐Enriched Carbon Molecular Sieve Membranes for Precision 1,3‐Butadiene/n‐Butane Separation

ABSTRACT Membrane separation provides an energy‐efficient route for separating 1,3‐butadiene (C 4 H 6 ) from C 4 hydrocarbon mixtures, but limited permeability and selectivity remain key barriers. Here, nitrogen‐enriched carbon molecular sieve (CMS) membranes were fabricated from triazine‐crosslinked fluorene‐based copolyimide precursors. Pendant carboxyl sites enabled triazine crosslinking and anchored nitrogen‐rich moieties within the precursor matrix. During pyrolysis, pre‐dispersed triazine units carbonized with the polymer backbone, contributing to the formation of pyridinic‐N and pyrrolic‐N active sites while narrowing ultramicropores to approximately 4.79 Å. The optimized CL‐CMS membrane exhibited a C 4 H 6 permeability of 4353.0 Barrer, a C 4 H 6 /n‐C 4 H 10 selectivity of 141.9, and a C 4 H 6 /i‐C 4 H 10 selectivity of 1817.2, surpassing current upper bounds. Mechanistic analysis revealed a synergistic interplay between nitrogen‐enriched pore chemistry and tailored ultramicropore confinement: the former preferentially enhanced C 4 H 6 affinity, while the latter dominated molecular discrimination through diffusion‐controlled sieving, increasing sorption and diffusion selectivities by up to 35% and 97%, respectively. This work establishes triazine‐crosslinking‐assisted precursor design as a promising strategy to couple adsorption chemistry with ultramicropore architecture, highlighting its potential for highly selective C 4 separation and polymer‐grade C 4 H 6 purification.