Temperature-dependent CH4/N2 selective adsorption mechanisms of HKUST-1 in low-concentration coalbed methane leveraged by experimental and theoretical approaches

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 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 is almost twice that of N2. The adsorption capacity of HKUST-1 for CH4 or N2 is negatively correlated with temperature, and 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 forms denser packing at the high-energy metal sites in HKUST-1, while N2 distributes more diffusely. Leveraging ideal adsorbed solution theory (IAST), the superior adsorption selectivity of HKUST-1 for CH4 was evaluated in binary CH4/N2 mixtures, and moderately elevated temperatures were found to enhance the separation performance by accelerating the adsorption kinetics. Moreover, the total adsorption capacity of HKUST-1 for CH4/N2 mixtures is positively correlated with CH4 concentration, and the adsorption selectivity is better at low CH4 levels. This research underscores the promise of HKUST-1 for efficient CH4/N2 separation, and provides molecular-level insights for next-generation adsorbent design.