Highly rapid mechanochemical synthesis of a pillar-layer metal-organic framework for efficient CH4/N2 separation

Efficient capture of methane (CH4) from nitrogen (N-2) by adsorption separation remains a challenge. In this work, we demonstrate the first solvent-free mechanochemical synthesis of a zinc-based pillar-layer MOF Zn-2(5-aip)(2)(bpy) (1) within minutes, making 1 accessible via a rapid and green route. By rational control of grinding time, the resulting product of 1-4 (which stands for grinding 4 min) has the optimal porosity confirmed by using CO2 as the probe gas. For the recovery of CH4 from N-2 at 5.0 bar and 298 K, the adsorption uptakes of CH4 and N-2 for 1-4 are 1.10 and 0.27 mmol/g, exhibiting preferential adsorption CH4 over N-2 with high CH4/N-2 selectivity of 7.0. The high selectivity of CH4/N-2 in 1-4 is mainly ascribed to the difference in affinity interactions between the pore walls and gas molecules, confirmed by experimental adsorption heat and molecular simulation. Two CH4 molecules can enter in the same pores, but most of the pores can only adsorb one N-2 molecule. Furthermore, the dynamic separation performance for CH4/N-2 mixture in 1-4 is verified by simulated breakthrough curves. The unprecedentedly rapid solvent-free mechanochemical synthesis within minutes makes 1 easily accessible at the gram scale via an environmentally friendly method for accelerating potential application of CH4/N-2 separation.