Enhancing the C3 separation performances of polycrystalline ZIF-8 membranes by additive-assisted secondary growth

Polycrystalline zeolitic-imidazolate framework-8 (ZIF-8) membranes have shown very promising for propylene/propane (C3) separation. Though high-performance ZIF-8 membranes have been prepared by various synthesis methods, secondary growth method offers unique advantages due to its decoupling of a seeding step and a growth step, leading to membranes with better microstructures, thereby not only better separation performances but also higher reproducibility. However, there are only a few reports on ZIF-8 membranes with ultra-high C3 separation factors (i.e., >200) that are made by secondary growth method. Here, we report polycrystalline ZIF-8 membranes by a secondary growth method modified with an addition of 1,4-butanediol (Bdiol). The membranes showed significantly improved C3 separation factors up to ∼283 as compared to those prepared without modification. It was found that adding Bdiol in the secondary growth solution improved the microstructures of the ZIF-8 membranes, thereby enhancing their C3 separation performances. Furthermore, the presence of Bdiol in the growth solution enabled formation of well-intergrown ZIF-8 membranes even with a low linker-to-metal ratio (∼16), otherwise not possible to form ZIF-8 membranes. Interestingly, the resulting membranes showed a plate-like morphology with the highest separation factor of ∼283 among those prepared. All membranes in this work exhibited enhanced C3 separation performances as compared to those synthesized without modification.