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-Br-2) 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-Br-2 exhibited the highest CH4/N-2 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/N-2 separation.