(293a) Controlled Demolition and Reconstruction of Zeolitic Imidazolate Frameworks Via Solvent Assisted Crystal Redemption (SACRed)

Zeolitic Imidazolate Frameworks (ZIFs) exhibit many interesting functional properties and relatively good thermal and chemical stability amongst MOF materials. This favors their potential applications in technological processes such as gas and liquid separations, catalysis, chemical sensing, and drug delivery. However, recent studies (e.g., [1]) have shown that many ZIFs are degraded by acid gases such as CO₂, SO₂, or NO₂ under humid conditions. The degradation is caused by the cleavage of Zn-N coordination bonds by the acidic species - leading to pore blockage, reduced crystallinity, and loss of function.

Until recently, it was believed that acid-gas induced degradation in MOFs was irreversible. We have demonstrated recently [2] that the surface area, porosity, crystallinity, and functional performance of ZIFs such as ZIF-8 can be restored by a solvent-based treatment process named Solvent Assisted Crystal Redemption (SACRed). This involves treating the degraded ZIF in a solution of its native linker under conditions similar to its original synthesis. Using deuterium-substituted linkers and ²H-NMR spectroscopy, we showed that the mechanism of this process proceeds via the replacement of the dangling linker-acid species within the ZIF crystal with a fresh linker, and this restores the material into its pristine state. Based on linker exchange studies in ZIF materials that we conducted recently [3], we have also established that this process is limited by the diffusion of linker species in and out of the framework.

In this talk, we will show that SACRed can be used as an interesting new tool with wide applicability to engineer ZIF structures and synthesis materials that are otherwise difficult/not possible to obtain by other techniques. Particularly, we use acid gas treatment to “demolish” a ZIF structure to a controlled extent, and then reconstruct the ZIF by inserting new linkers that are not native to the ZIF; thereby creating mixed-linker structures that are not easily synthesized via de novo routes or solvent assisted linker exchange (SALE). For example, by treating acid-gas exposed ZIF-8 particles with benzimidazole linker (ZIF-7) we were able to successfully synthesize a hybrid material that had high levels of benzimidazole incorporation but still retained the original SOD-topology of ZIF-8. The extent of linker incorporation in this material can be tuned by controlling the level of acid-gas exposure. De novo synthesis cannot access most of the compositions enabled by SACRed and also cannot produce the SOD topology. We will also demonstrate the introduction of other bulky imidazole-based linkers into the ZIF-8 framework after degrading it with acid gases, giving rise to unusual structures and material properties.

References

[1] Interactions of SO₂-Containing Acid Gases with ZIF-8: Structural Changes and Mechanistic Investigations; S. Bhattacharyya, S. H. Pang, M. R. Dutzer, R. P. Lively, K. S. Walton, David S. Sholl, S. Nair; J. Phys. Chem. C.; 2016, 120(48), 27221-27229

[2] Recovery of Acid-Gas Degraded Zeolitic Imidazolate Frameworks by Solvent-Assisted Crystal Redemption (SACRed); K.C. Jayachandrababu, S. Bhattacharyya, Y. Chiang, D.S. Sholl, S. Nair; ACS Appl. Mater. Interfaces; 2017, 9(40), 34597-34602

[3] Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange (SALE) and de novo Routes; K.C. Jayachandrababu, D.S. Sholl, S. Nair; J. Am. Chem. Soc.; 2017; 139(16), 5906-5915