Improving hydrostability of ZIF-8 membranes via surface ligand exchange

Zeolitic imidazolate frameworks (ZIFs) are promising materials for gas and liquid separations due to their unique pore structures and tunable surface properties. Recent studies show that ZIF crystalline powders, especially membranes, are not stable in aqueous solutions. The present work shows the improvement of ZIF-8 crystal and membrane stability in water through utilization of a ligand exchange post-modification method which replaces methylimidazole ligands on the outer surface of ZIF-8 crystals or membranes by the more hydrophobic, bulkier 5,6-dimethylbenzimidazole. Ligand exchange modification does not change the crystal structure, morphology or gas permeance as shown ZIF-8 powder and membrane characterization experiments. The modified ZIF-8 powders and membranes retain identical morphologies and crystallinities after static and dynamic water immersion. The modified ZIF-8 membrane exhibits stable water pervaporation flux controlled by the ZIF-8 layer while the unmodified ZIF-8 membrane experiences dissolution of the ZIF-8 layer during water pervaporation. This ligand exchange strategy enables hydrostable ZIF-8 membranes for practical applications involving aqueous solutions.