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

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.