Bimetallic Metal–Organic Framework Featuring Nitrogen/Oxygen Sites for Superior Methane/Nitrogen Adsorption Separation

Abstract

The development of advanced adsorbents for methane (CH₄) and nitrogen (N₂) separation is critical for the efficient utilization of coal-bed methane (CBM), which is a key alternative energy resource. Herein, we report a bimetallic copper–indium-based metal–organic framework (MOF), CuIn(3-ain)₄, featuring high-density nitrogen and oxygen sites distributed along its pore surface. These negatively charged atom sites, derived from CuN₄ and InO₈ clusters, form multiple hydrogen-bonding interactions with CH₄, significantly enhancing its adsorption affinity. CuIn(3-ain)₄ demonstrates a high CH₄ uptake of 1.56 mmol g^–1 and excellent CH₄/N₂ selectivity under ambient conditions, outperforming many reported materials. Dynamic breakthrough experiments confirm that over 90% purity methane can be obtained in a single adsorption–desorption cycle. Grand Canonical Monte Carlo (GCMC) simulations and density functional theory (DFT) calculations reveal that the nitrogen and oxygen sites play a crucial role in selectively recognizing CH₄ over N₂, leading to a superior separation efficiency. Moreover, the material exhibits excellent cyclic stability and scalability, making CuIn(3-ain)₄ a promising candidate for practical CH₄/N₂ separation in CBM purification. This study provides valuable insights into the design of MOFs for challenging gas separations, emphasizing the role of polar sites in facilitating selective adsorption.