(100a) Solventless Synthesis of Zeolitic-Imidazole Framework ZIF-8 Membranes Via Crystal-Specific Sintering Phenomenon

ZIFs, constructed by interconnecting divalent metal nodes (e.g., Zn, Co, Cd or their combinations) and imidazole linkers in periodic manners, have been one of the most intensively studied metal-organic frameworks (MOFs) primarily due to their exceptional thermal/chemical stabilities compared to other MOFs.^[1] In combination with the ZIFsâ?? stabilities, their pore apertures with molecular level dimension have led them to be considered as promising material candidates for energy-efficient membrane-based separations.^[2]

When prepared, ZIF membranes have polycrystalline nature like zeolite membranes. Therefore their performances, considering the materials are properly chosen, depend on membrane microstructures, most importantly grain boundary structure. Since the membrane microstructures are generally governed by membrane processing technique, current research focus on ZIF membranes has been mainly about developing new processing techniques.^[3] As results, newly designed methods, which are broadly classified into in-situ and secondary growth, enabled significant improvements in membrane performance, synthesis simplification, and potential scalability.^[2]In spite of all these advancements, however, since the majority of the current techniques are still solution-based syntheses, there are still issues inevitably encountered such as producing wastes (e.g., environmentally harmful solvent, excessively-used expensive ligand, metal precursor) and putting a limitation on reducing membrane thickness which is very important for large quantity gas processing. Therefore, there are still needs of developing synthesis protocols which are sustainable and allow membranes with thin thickness.

Here, we report a new synthesis strategy which allows the formation of continuous ultrathin ZIF-8 films / membranes (~300 nm) in a solventless condition. This new approach is on the basis of our new discovery that intentionally-designed defective ZIF-8 crystals display sintering phenomenon without crystal degradation under specific vapor environment, leading to crystal merging and subsequently grain growth.

[1] K. S. Park, Z. Ni, A. P. Côté, J. Y. Choi, R. Huang, F. J. Uribe-Romo, H. K. Chae, M. Oâ??Keeffe, O. M. Yaghi, Proceedings of the National Academy of Sciences 2006, 103, 10186-10191.

[2] aJ. Yao, H. Wang, Chemical Society Reviews 2014, 43, 4470-4493; bB. Chen, Z. Yang, Y. Zhu, Y. Xia, Journal of Materials Chemistry A 2014, 2, 16811-16831.

[3] H. T. Kwon, H.-K. Jeong, A. S. Lee, H. S. An, J. S. Lee, Journal of the American Chemical Society 2015, 137, 12304-12311.