Enhancing selectivity of ZIF-8 membranes by short-duration postsynthetic ligand-exchange modification

Post-synthesis modification of zeolitic imidazolate framework (ZIF) membranes offers new opportunities to tailor ZIF structures and improve their separation performance. This work utilizes membrane surface ligand exchange (MSLE) to replace methylimidazole (MeIM) with 5,6-dimethylbenzimidazole (DBIM) and investigates its effect on the gas separation properties of ZIF-8 membranes containing varying amounts of intercrystalline defects (perfect and imperfect membranes). Upon short time (0.5 h) MSLE modification, the pristine ZIF-8 membrane shows a significant enhancement in H₂/C₂H₆ selectivity (12–22) and C₃H₆/C₃H₈ selectivity (150–188) with small declines in gas permeance (29.5% for H₂, 6.5% for C₃H₆). MSLE modification also improves the separation performance of imperfect ZIF-8 membranes, but to a lesser extent (C₃H₆/C₃H₈ selectivity increasing from 37.5 to 63.3). Some of the MeIM in the ZIF-8 membrane crystals are replaced by bulkier DBIM, facilitated by the presence of the intercrystalline pores resulting in more exchange on the imperfect membrane. The short-duration ligand exchange reduces intracrystalline pore diffusivity, to a greater extent for larger molecules as compared to smaller molecules, leading to a reduction in gas permeance and an increase in separation factor. However, long-duration (5 h) MSLE modification on a perfect ZIF-8 membrane lowers the C₃H₆/C₃H₈ selectivity due to an increase in the intercrystalline transport for bulkier C₃H₈ with an increase in C₃H₆ permeance due to enhanced solubility within the modified ZIF-8 structure.