Nitrogen Adsorption Sites with Low Polarizability for Benchmark N 2 /CH 4 Separation

Abstract Selective adsorption of N 2 from CH 4 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 N 2 /CH 4 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 N 2 and CH 4 uptakes, resulting in a high N 2 /CH 4 uptake ratio (1.94) and kinetic selectivity (2.20), of which the N 2 /CH 4 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 N 2 /CH 4 separation, based on the record‐high breakthrough selectivity (2.43) and CH 4 productivity (0.47 mmol g −1 ). High‐purity CH 4 (99.99%) could also be obtained from ternary and even six‐component CH 4 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 N 2 and CH 4 .