Improved methane/nitrogen separation properties of zirconium-based metal–organic framework by incorporating highly polarizable bromine atoms

The separation of methane, a commercially important fuel and greenhouse gas, from nitrogen is a very important and challenging issue. To address this problem, we systematically incorporated various functional groups (-H, -NH₂, -NO₂, -Br, and -Br₂) into the pores of a hydrothermally stable zirconium-based metal–organic framework (MOF). Various adsorption experiments and molecular simulations confirmed that the CH₄ uptake at low pressures could be clearly enhanced by incorporating functional groups due to the effect of the pore size and the functional group polarizability. Remarkably, UiO-66-Br₂ exhibited the highest CH₄/N₂ selectivity (5.06 and 5.63 at 1 bar and 15 bar, respectively) under both vacuum-swing adsorption (VSA) and pressure-swing adsorption (PSA) conditions. Moreover, the UiO-66-Br₂ showed efficient separation of CH₄ from N₂ under dynamic mixture flow conditions, good cyclic CH₄ adsorption–desorption profile for 15 cycles and easy regeneration under mild conditions without increasing temperature. These results suggest that incorporation of functional groups with high polarizability, such as bromine, into the pores of MOFs is an efficient strategy for developing adsorbents for CH₄/N₂ separation.