(725b) Mixed-Linker MOF Synthesis and Remediation of Acid-Gas Degradation Using Linker-Exchange Techniques

Solvent Assisted Linker Exchange (SALE) is known as a useful tool to modify existing MOFs and thereby obtain otherwise difficult-to-synthesize structures, or improve the stability and functional properties of MOFs. By partially exchanging the native linker in a MOF, this technique can also be used to synthesize mixed-linker MOFs. However, the structural properties of such materials, including the spatial distributions of linkers in these mixed-linker materials and how they differ from mixed-linker MOFs made via de novo routes, are not well known.

This work consists of two main parts. First, we compare and contrast a series of mixed-linker ZIF-8-90 materials made via both routes. ¹H CRAMPS NMR spectroscopy, water and nitrogen adsorption, and native fluorescence confocal microscopy are used to elucidate the differences in the outcome of these syntheses routes. A generalizable mechanism for SALE in ZIFs is proposed by studying the impact of crystal size, temperature, and SALE duration. We observe that under moderate conditions, SALE in ZIFs is diffusion-limited, and results in a core-shell type structure. Harsher conditions lead to a deviation from this behavior including linker etching resulting in morphological changes to particles. Under appropriate reaction conditions, SALE techniques appear to be capable of producing controlled core-shell ZIF structures of good morphological quality that complement the well-mixed structures obtained by de novo routes.

Second, linker exchange was explored as a means of recovering crystallinity and porosity of ZIFs damaged by acid-gases. We developed a technique called Solvent Assisted Crystal Redemption (SACRED) wherein the damaged crystals of a ZIF are treated with a solution of its native linker. The structural damage to ZIFs induced by acid-gases were found to be partially or completely reversed by this treatment. The recovered material is shown to be chemically identical to the pristine material using FTIR spectroscopy and X-ray diffraction. Adsorption studies show that the functional performance of ZIF-8 is retained after the recovery. The use of deuterium-substituted linkers and ²H NMR spectroscopy provides mechanistic insight into the recovery process. This method has the potential to reduce costs and improve lifetime of ZIF-based adsorbents or membranes.

References

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[2] Structure Elucidation of Mixed-Linker Zeolitic Imidazolate Frameworks by Solid-State ¹H CRAMPS NMR Spectroscopy and Computational Modeling; K.C. Jayachandrababu, R.J. Verploegh, J. Leisen, R.C. Nieuwendaal, D.S. Sholl, S. Nair; J. Am. Chem. Soc.; 2016; 138(23); 7325-7336

[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; Just Accepted Manuscript