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

Area of Science:

• Materials Science
• Chemical Engineering
• Adsorption Science

Background:

• Separating nitrogen (N₂) from methane (CH₄) is crucial for natural gas purification but challenging due to their similar properties.
• Existing methods often struggle with efficiency and selectivity, necessitating advanced adsorbent materials.

Purpose of the Study:

• To develop and evaluate a novel metal-organic framework (MOF), Cu-MFU-4l, for selective N₂/CH₄ separation.
• To investigate the role of open Cu(I) sites in enhancing the separation performance.

Main Methods:

• Synthesis and characterization of Cu-MFU-4l using X-ray absorption spectroscopy (XAS), in situ CO-adsorbed infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS).
• Gas sorption isotherm measurements to determine N₂ and CH₄ uptake capacities and selectivities.
• Breakthrough experiments for evaluating binary and multicomponent gas mixture separation performance.
• In situ infrared spectroscopy and computational modeling to elucidate the mechanism of selective adsorption.

Main Results:

• Cu-MFU-4l exhibits a high N₂/CH₄ uptake ratio (1.94) and kinetic selectivity (2.20), surpassing previously reported MOFs.
• Achieved record-high breakthrough selectivity (2.43) and CH₄ productivity (0.47 mmol g⁻¹) for binary N₂/CH₄ separation.
• Demonstrated the capability to produce high-purity CH₄ (99.99%) from complex gas mixtures in a single step.
• Confirmed that open Cu(I) sites are key to distinguishing between N₂ and CH₄ molecules.

Conclusions:

• Cu-MFU-4l is a highly effective adsorbent for selective N₂/CH₄ separation, offering significant advantages over existing technologies.
• The open Cu(I) sites in Cu-MFU-4l play a critical role in the preferential adsorption of N₂ over CH₄.
• This MOF presents a promising pathway for efficient natural gas upgrading and purification.