Nitrogen Adsorption Sites with Low Polarizability for Benchmark N₂/CH₄ Separation

Selective adsorption of N₂ from CH₄ is an industrially promising but challenging process given their similar sizes and physicochemical properties. Herein, we report a robust metal-organic framework (Cu-MFU-4l) featuring open Cu(I) sites of low polarizability for benchmark N₂/CH₄ separation. The presence of open Cu(I) sites in the framework was confirmed by X-ray absorption spectroscopy (XAS), in situ CO-adsorbed infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS). Gas sorption isotherms revealed that Cu-MFU-4l exhibited a significant difference between N₂ and CH₄ uptakes, resulting in a high N₂/CH₄ uptake ratio (1.94) and kinetic selectivity (2.20), of which the N₂/CH₄ uptake ratio was the highest among all MOFs reported to date. Breakthrough experiments confirmed Cu-MFU-4l as the best porous adsorbent hitherto reported for binary N₂/CH₄ separation, based on the record-high breakthrough selectivity (2.43) and CH₄ productivity (0.47 mmol g^-1). High-purity CH₄ (99.99%) could also be obtained from ternary and even six-component CH₄ mixtures by a one-step separation process. In situ infrared spectroscopy and computational modeling studies revealed that the open Cu(I) sites could better distinguish N₂ and CH₄.