(725c) Rapid Microwave-Assisted Synthesis of Hybrid Zeolitic-Imidazolate Frameworks with Mixed Metals and Mixed Linkers

Separating a crude component into a more valuable purer product is a major part of many industrial chemical plants. Typically these processes utilize distillation, which account to 10 – 15 % of world’s energy consumption.¹ One alternative solution that can reduce energy consumption up to 90 % relative to distillation is through membrane separation using porous solid material such as zeolites and metal organic frameworks (MOFs).² Zeolitic-imidazolate frameworks (ZIFs),³ a subclass of MOFs, in particular have attracted many attentions due to their chemical/thermal stabilities, their ultra-microporosities, and high surface area when compared to other MOFs material.³ ZIFs possess zeolite-like topologies in which the tetrahedral Si or Al and the bridging O in zeolite structure are replaced by transition metals (such as Zn,³ Co,⁴ and Cd⁵) and imidazolate-derived ligands. A common drawback for membrane gas separation is the limited availability of aperture pore size and functionalities of the porous materials. Studies have shown that through mixing metals^6,7 and ligands,^8,9 one can continuously tune the MOFs/ZIFs (termed hybrid ZIFs) properties to match with the characteristic of specific gas mixture. However, the common synthesis of hybrid ZIFs has generally been through slow diffusion techniques,^4,10 conventional hydrothermal,^7,11 and solvothermal methods,^9,12,13requiring several hours up to days of synthesis time. In addition, the yields of those synthesis protocols are often low, an economical issue for hybrid ZIFs to be used in a large industrial scale.

Herein we plan to present a new microwave-assisted synthetic strategy to rapidly prepare hybrid zeolitic-imidazolate frameworks (ZIFs): ZIFs with mixed metal centers and/or mixed linkers.¹⁴The microwave-based method significantly shortens synthesis time, produces higher yield, substantially reduces the amount of ligand, and eliminates the use of deprotonating agents. X-ray diffraction pattern reveals a mixed metal CoZn-ZIF-8 (i.e., ZIF-8 with both Co and Zn centers) maintains the sodalite (SOD) zeolitic topology from ZIF-8 parent. Elemental mapping using Energy-dispersive X-ray spectroscopy (EDS) and electronic/geometric information obtained from X-ray absorption spectroscopy (XAS) confirm the uniform distribution of tetrahedral Co and Zn metal centers within the same framework of the mixed-metal ZIF. The metal to nitrogen (M-N) stretching frequencies on IR band were observed to be systematically blue-shifted as the Co/Zn ratio in the mixed metal ZIF increases. Furthermore, for the first time, a hybrid ZIF with both mixed metal centers (Co and Zn) and mixed linkers (2-methylimidazolate and benzimidazolate) was prepared through one-step microwave synthesis. Finally, a mixed metal CoZn-ZIF-8 with the Co/Zn ratio of ~ 1 was grown as membranes on porous α-Al2O3 supports, showing higher propylene/ propane separation factor (~120) when compared to pure Zn-ZIF-8 membranes (~63) prepared with similar method.

References:

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7. Kaur, G.; Rai, R. K.; Tyagi, D.; Yao, X.; Li, P.-Z.; Yang, X.-C.; Zhao, Y.; Xu, Q.; Singh, S. K. Room-temperature synthesis of bimetallic Co-Zn based zeolitic imidazolate frameworks in water for enhanced CO2 and H2 uptakes. J. Mater. Chem. A 2016, 4, 14932-14938.

8. Eum, K.; Jayachandrababu, K. C.; Rashidi, F.; Zhang, K.; Leisen, J.; Graham, S.; Lively, R. P.; Chance, R. R.; Sholl, D. S.; Jones, C. W.; Nair, S. Highly Tunable Molecular Sieving and Adsorption Properties of Mixed-Linker Zeolitic Imidazolate Frameworks. J. Am. Chem. Soc. 2015, 137, 4191-4197.

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10. Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O’Keeffe, M.; Yaghi, O. M. Synthesis, Structure, and Carbon Dioxide Capture Properties of Zeolitic Imidazolate Frameworks. Acc. Chem. Res. 2010, 43, 58-67.

11. Roshan, R. K.; Babu, R.; Jeong, H.-M.; Hwang, G.-Y.; Jeong, G.-S.; Kim, M.-I.; Kim, D.; Park, D.-W. A solid solution zeolitic imidazolate framework as a room temperature efficient catalyst for the chemical fixation of CO2. Green Chem. 2016, 18, 6349-6356.

12. Wu, T.; Bu, X.; Zhang, J.; Feng, P. New Zeolitic Imidazolate Frameworks: From Unprecedented Assembly of Cubic Clusters to Ordered Cooperative Organization of Complementary Ligands. Chem. Mater. 2008, 20, 7377-7382.

13. Rashidi, F.; Blad, C. R.; Jones, C. W.; Nair, S. Synthesis, characterization, and tunable adsorption and diffusion properties of hybrid ZIF-7-90 frameworks. AIChE J. 2016, 62, 525-537.

14. Hillman, F.; Zimmerman, J. M.; Paek, S.-M.; Hamid, M. R. A.; Lim, W. T.; Jeong, H.-K. Rapid microwave-assisted synthesis of hybrid zeolitic-imidazolate frameworks with mixed metals and mixed linkers. J. Mater. Chem. A 2017, 5, 6090-6099.