Nitrogen Adsorption Sites with Low Polarizability for Benchmark N2/CH4 Separation
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Using porous materials for N2/CH4 separation is promising but remains a great challenge. Here, a metal-organic framework with Cu(I)-based nitrogen adsorption sites of low polarizability is proposed. It affords a new benchmark N2/CH4 uptake ratio and the highest breakthrough selectivity surpassing those of current adsorbents.
Abstract
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.
