Mechanism of Temperature Effect on CH 4 /N 2 Separation by HKUST-1 in Low-Concentration Coalbed Methane

The efficient separation of CH 4 from binary CH 4 /N 2 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 CH 4, N 2, and their mixed gases under different temperatures and pressures. Single-component adsorption experiments revealed that the adsorption capacity of HKUST-1 for CH 4 was almost twice that of N 2 . The adsorption capacity of HKUST-1 for CH 4 or N 2 is negatively correlated with temperature, and there is a notably higher isosteric heat of adsorption for CH 4 than for N 2 . In addition, grand canonical Monte Carlo (GCMC) simulations confirmed that both CH 4 and N 2 preferentially occupy the binding sites in T1 pores of HKUST-1 and then migrate to L2 and L3 pores. Differently, CH 4 molecules form denser packing at the high-energy metal sites in HKUST-1, while N 2 distributes more diffusely. The difference is due to the higher binding energy between CH 4 and the Cu node (CH 4 is −14.99 kJ/mol, while N 2 is −7.83 kJ/mol). Leveraging ideal adsorbed solution theory (IAST) and a breakthrough curve, the superior adsorption selectivity of HKUST-1 for CH 4 was evaluated in binary CH 4 /N 2 mixtures. The total adsorption capacity of HKUST-1 for CH 4 /N 2 mixtures is positively correlated with the CH 4 concentration, and the adsorption selectivity is better at low CH 4 concentrations or low temperatures. Meanwhile, HKUST-1 exhibits good moisture resistance and cycling stability. This research underscores the promise of HKUST-1 for efficient CH 4 /N 2 separation and provides molecular-level insights for next-generation adsorbent design.