Synthesis of zeolitic imidazolate framework-78 molecular-sieve membrane: Defect formation and elimination

Metal-organic frameworks (MOFs) are ideal micro- and mesoporous materials for molecular separation. A defect-free MOF membrane supported on a porous substrate is required for high separation performance, however it is rather difficult to eliminate the micro-defects or intercrystalline gaps in the membranes. In this work, a ZIF-78 membrane was synthesized on a porous ZnO support. The defect formation mechanism in the membrane was illustrated by in situ thermal expansion analysis. A novel strategy was proposed to eliminate not only the macroscopic defects but also the intercrystalline gaps in the membrane by controlling the diffusion of solvent molecules through the channels of the ZIF-78 crystal. The ZIF-78 membrane exhibited high performance in separating H₂. The ideal selectivity and mixture separation factor of H ₂-CO₂ are 11.0 and 9.5, respectively. The approach reported in this paper offers an efficient and universal strategy for the facile synthesis of high-quality MOF membranes on porous supports.