Mechanism of Temperature Effect on CH4/N2 Separation by HKUST-1 in Low-Concentration Coalbed Methane

The efficient separation of CH4 from binary CH4/N2 mixtures is crucial for utilizing low-concentration coalbed methane but remains a significant challenge. Herein, we systematically investigated the adsorption and separation performance of the metal–organic framework HKUST-1 for CH4, N2, and their mixed gases under different temperatures and pressures. Single-component adsorption experiments revealed that the adsorption capacity of HKUST-1 for CH4 was almost twice that of N2. The adsorption capacity of HKUST-1 for CH4 or N2 is negatively correlated with temperature, and there is a notably higher isosteric heat of adsorption for CH4 than for N2. In addition, grand canonical Monte Carlo (GCMC) simulations confirmed that both CH4 and N2 preferentially occupy the binding sites in T1 pores of HKUST-1 and then migrate to L2 and L3 pores. Differently, CH4 molecules form denser packing at the high-energy metal sites in HKUST-1, while N2 distributes more diffusely. The difference is due to the higher binding energy between CH4 and the Cu node (CH4 is −14.99 kJ/mol, while N2 is −7.83 kJ/mol). Leveraging ideal adsorbed solution theory (IAST) and a breakthrough curve, the superior adsorption selectivity of HKUST-1 for CH4 was evaluated in binary CH4/N2 mixtures. The total adsorption capacity of HKUST-1 for CH4/N2 mixtures is positively correlated with the CH4 concentration, and the adsorption selectivity is better at low CH4 concentrations or low temperatures. Meanwhile, HKUST-1 exhibits good moisture resistance and cycling stability. This research underscores the promise of HKUST-1 for efficient CH4/N2 separation and provides molecular-level insights for next-generation adsorbent design.