CH₄ Purification by a Carborane-Based Metal-Organic Framework with Rare Tetranuclear Copper Nodes

The development of efficient methane (CH₄) purification technology from natural gas is crucial to its utilization. Adsorptive separation using porous metal-organic frameworks (MOFs) offers a promising and energy-efficient strategy. Herein, a novel carborane hybrid MOF, Cu-CB-DMTPA, was solvothermally synthesized by using Cu²⁺, p-C₂B₁₀H₁₀-(COOH)₂, and 2,6-dimethyl-N,N-di(pyridin-4-yl)pyridin-4-amine (DMTPA). This material features electronegative pore surfaces rich in B-H^δ- groups and unique one-dimensional (1D) channels with dual pore sizes (large: 6.5 × 4.3 Ų; small: 3.4 × 2.8 Ų). Gas adsorption studies revealed significantly higher uptake of C₃H₈ (29.9 cm³ g^-1) and C₂H₆ (31.3 cm³ g^-1) over CH₄ (9.9 cm³ g^-1) at 298 K and 1.0 bar. Ideal adsorbed solution theory (IAST) selectivity calculations demonstrated outstanding performance for equimolar C₃H₈/CH₄ (61.2) and C₂H₆/CH₄ (14.4) separations, outperforming many popular materials. The separation mechanism, probed via in situ single-crystal X-ray diffraction (SCXRD) and Grand Canonical Monte Carlo (GCMC) simulations, is attributed to multiple cooperative weak interactions between the framework and hydrocarbons, with stronger binding affinity for larger and more polarizable molecules. Dynamic breakthrough experiments confirmed efficient one-step purification of CH₄ (>99.99% purity) from a CH₄/C₂H₆/C₃H₈ (85:10:5) mixture at 298 K, achieving a high CH₄ productivity of 81.53 L kg^-1. Cu-CB-DMTPA also maintains excellent recyclability and humidity resistance, highlighting its potential for industrial natural gas upgrading.