Design of polyhedral cage-engineered MOFs for efficient methane purification from natural gas

The development of adsorbents with balanced adsorption capacity and selectivity is critical for achieving efficient adsorptive separation of CH4 from natural gas, but it still remains challenging. The formation of large pores composed of polyhedral cages provides sufficient space for gas admission to ensure adsorption capacity, and creates pore confinement to maximize guest-host interactions for enhanced selectivity. Herein, the two iso-structured metal-organic frameworks were synthesized, which feature tailored cage dimensions and functionalized pore surfaces integrating N-containing groups and sulfate anions. At 298 K and 100 kPa, both MOFs exhibit high uptake for C3H8 and C2H6 but a negligible adsorption for CH4. Breakthrough tests demonstrate that both MOFs enable effective separation of C3H8/C2H6/CH4 from the home-made gas mixture simulating raw natural gas, yielding CH4 with a purity of > 99.9%. Crystal structure characterization and computational modeling demonstrate that the optimal cage size and polar surface microenvironment of Co-TPT-TATBA, induced by nitrogen-containing moieties and sulfate anions, facilitate favorable intermolecular interactions with C3H8 and C2H6 molecules—thereby directly underpinning its exceptional gas adsorption and separation capabilities. Furthermore, the good chemical stability renders the obtained MOFs an efficient adsorbent for CH4 purification.