Electrostatic Potential Optimization by Precise Tailoring of Amino Site Arrays in Isostructural Metal–Organic Frameworks for High-Efficiency CH₄ Purification

The modulation of binding sites and pore channel surfaces in adsorbents for advancing adsorptive separation is attracting considerable attention. In natural gas purification, the challenge of discriminating between homologous alkanes (C₂H₆ and C₃H₈) at low concentrations has been persistent, limiting the adsorption capacity and selectivity required for efficient CH₄ separation. By leveraging the highly positive electrostatic potential of H atoms in C₂H₆ and C₃H₈, a range of frameworks is synthesized in this study, and it is demonstrated that efficient CH₄ separation can be achieved by optimizing the negative electrostatic potential of pore surfaces. The isostructural metal–organic framework, namely, Co-3-AIN, features an asymmetric rhombic cross-sectional pore with inwardly directed amino sites, yielding exceptional C₂H₆ and C₃H₈ binding affinities (62 and 69 kJ mol⁻¹). This design achieves outstanding low-pressure uptakes of alkanes (58.7 cm³ g⁻¹ C₂H₆ at 0.1 bar; 56.4 cm³ g⁻¹ C₃H₈ at 0.05 bar) and unprecedented selectivities (C₂H₆/CH₄ = 188; C₃H₈/CH₄ = 2953) at 298 K. Breakthrough experiments using a ternary CH₄/C₂H₆/C₃H₈ mixture (85:10:5, v/v/v) demonstrate the exceptional CH₄ purification capability of Co-3-AIN, resulting in a high-purity CH₄ yield of 207.6 L/kg in a single separation cycle. The alkane sorption mechanism is clarified using in situ IR spectroscopy, DFT calculation, and Hirshfeld surface analysis.