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

Membrane separation provides an energy-efficient route for separating 1,3-butadiene (C4H6) from C4 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 C4H6 permeability of 4353.0 Barrer, a C4H6/n-C4H10 selectivity of 141.9, and a C4H6/i-C4H10 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 C4H6 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 C4 separation and polymer-grade C4H6 purification.