A stable ultramicroporous Cd(II)-MOF with accessible oxygen sites for efficient separation of light hydrocarbons with high methane production

Metal-organic frameworks (MOFs) have garnered increasing attention for their effective separation of light hydrocarbons owing to their prominent separation selectivity and energy-efficient adsorption process. Here, we constructed a robust stable ultramicroporous Cd(II)-MOF ([Cd₅(NTA)₄(H₂O)₂] (Me₂NH₂⁺)₂·10H₂O (1)) with abundant accessible oxygen sites and investigated its adsorption performance for recovering high-purity methane (CH₄) from natural gas (NG) including C₁(CH₄)/C₂(C₂H₆)/C₃(C₃H₈) mixtures. At ambient conditions, the theoretical equilibrium separation selectivity of 1 for C₂H₆/CH₄ (v/v = 10/85) and C₃H₈/CH₄ (v/v = 5/85) were found to be 34.3 and 223.8, respectively. The CH₄/C₂H₆/C₃H₈ (v/v/v = 85/10/5) mixture breakthrough experiments for 1, conducted at 298 K, demonstrated effective separation performance with breakthrough times of up to 136 and 280 min·g⁻¹ for C₂H₆ and C₃H₈. Particularly, the CH₄ productivity (purity > 99.9 %) with 9.8 mmol·g⁻¹ ranked the third in reported literatures, lower to the reported maximum value of 13.28 mmol·g⁻¹ for Ni(TMBDC)(DABCO)_0.5. Furthermore, Grand Canonical Monte Carlo (GCMC) simulations and first-principles density functional theory (DFT) calculations revealed that the high uptake and selectivity for C₃H₈ and C₂H₆ can be attributed to the abundant oxygen sites present in the pores. The dynamic breakthrough experiments comprehensively demonstrated that the proposed MOF can be an effective potential adsorbent for the practical separation of CH₄/C₂H₆/C₃H₈ mixtures.