An ultramicropore Co-MOF constructed from mixed N/O-bifunctional organic ligand for highly efficient C2H6/C2H4 separation

Abstract

The separation of ethane (C₂H₆) from ethylene (C₂H₄) is a crucial but energy-intensive process in the petrochemical industry. Developing adsorbents that preferentially capture C₂H₆ over C₂H₄ (inverse selectivity) enables one-step production of high-purity C₂H₄. Herein, we report a novel ultramicroporous cobalt-based metal–organic framework, 1, constructed from a mixed N/O-bifunctional ligand strategy using 4-(4-carboxyphenyl)-1,2,4-triazole (HCPT) and isonicotinic acid (HINA). Single-crystal X-ray diffraction reveals that 1 features one-dimensional rhombic channels with pore dimensions of 4.48–5.81 Å, fully coordinated Co(II) centers, and pore surfaces decorated with aromatic rings and uncoordinated O atoms. The activated framework (1a) exhibits a Brunauer–Emmett–Teller surface area of 517 m²/g and shows preferential C₂H₆ adsorption over C₂H₄ at 298 K, with C₂H₆ uptake of 3.9 mmol/g at 1 bar. Ideal adsorbed solution theory selectivity for a C₂H₆/C₂H₄ (10/90) mixture reaches 1.81 at 298 K and 1 bar, and the separation potential is 1.1 mmol/g. Grand canonical Monte Carlo simulations and density functional theory calculations reveal that C₂H₆ forms multiple C–H···O and C–H···π interactions with the framework, resulting in a higher binding energy (−12.22 kcal/mol) than C₂H₄ (−10.61 kcal/mol). This work demonstrates that the mixed N/O-donor ligand strategy is effective for designing inverse-selective MOFs for energy-efficient ethylene purification.