A facile two-step synthesis of hollow MOF-74 for enhanced dynamic Xe/Kr separation.

Metal-organic frameworks (MOFs) with high-density uncoordinated open metal sites have been intensively investigated in Xe/Kr separation because of their strong and selective interaction with Xe. However, the dynamic Xe/Kr separation behavior of these MOFs is often unsatisfactory in practical applications due to slow diffusion kinetics. This work presents a facile two-step method to synthesize hollow Ni-MOF-74 particles with short diffusion lengths to enhance dynamic Xe/Kr separation. Unlike conventional sacrificial template approaches, where a crystalline MOF layer is directly grown on to the template surface, this method first rapidly deposits a metal-ligand complex layer under mild reaction conditions while the template undergoes simultaneous degradation. These poorly crystalline yet well-faceted hollow capsules are then reconstructed into crystalline hollow Ni-MOF-74 particles of the same morphology. Xe adsorption kinetics analyses show that the Xe diffusion rate of hollow Ni-MOF-74 was 1.5 times faster than that of solid Ni-MOF-74 despite their identical Xe and Kr adsorption capacity and selectivity. As a result, the enhanced diffusion kinetics of the hollow structure resulted in a steeper breakthrough curve and a 17% increase in breakthrough time than its solid counterpart during column separation of a Xe/Kr mixture.