Characterization of Metal-organic Frameworks via Wideline and High-resolution Solid-state NMR Spectroscopy

Wanli Zhang, Western University

Abstract

Metal-organic frameworks (MOFs) are a class of porous materials composed of metal centers and organic linkers. MOFs have diverse applications, including gas storage/separation, catalysis and energy storage, due to their unique properties such as high specific surface area, tunable topologies and good stabilities. Understanding the relationship between MOF structure and properties is crucial for enhancing performance and developing new materials. In this thesis, solid-state NMR (SSNMR) spectroscopy combined with theoretical calculations have been utilized to characterize the local structure of MOFs and adsorbed guest molecules.

In the first part of this thesis, the local structures of several representative MOFs were studied with wideline NMR. The local environments of Cu(I) ions in Cu-MOFs regarding the coordination numbers, phase changes, oxidation states, and anion exchanges, were characterized using ^63/65Cu SSNMR. The Zr centers in Zr-MOFs were examined with ⁹¹Zr SSNMR at high magnetic field of 35.2 T and 19.6 T, providing insights into the local site symmetry and short-range ordering around Zr centers. ⁹¹Zr NMR is very sensitive to structural variations in MOFs caused by guest molecules, linker substitution, and post-synthetic treatments. The ²⁰⁹Bi and ¹²⁷I SSNMR spectra of bismuth- and iodine-containing MOFs were acquired at magnetic fields up to 36 T, with breadths ranging from 8 to 50 MHz, pushing the boundaries of ultra-wideline NMR. Cl-containing MOFs with different metals and organic linkers were studied with ³⁵Cl SSNMR. The correlation between ³⁵Cl NMR parameters and both local bond lengths and bond angles were obtained. ³⁵Cl SSNMR was also used to identify an unknown product from chemical reactions. MOFs can be tailored for applications such as Xe and Kr separation. The co-adsorption behavior of Xe and Kr in five MOFs including the adsorption locations, binding strength, guest-host interactions and exchange dynamics was investigated with ¹²⁹Xe and ⁸³Kr SSNMR aided by molecular simulations.

The second part of this thesis features two examples of high resolution SSNMR applied to MOFs. (i) We demonstrate that combining new cryogenic MAS probe technology and performing NMR experiments at a high magnetic field leads to significant signal enhancement for ⁶⁷Zn SSNMR. The multiple non-equivalent Zn sites with very similar local environments in two MOFs, ZIF-4 and α-Zn₃(HCOO)₆, were well-resolved by natural abundance ⁶⁷Zn 3QMAS NMR technique. (ii) The structure of a defective MOF MIL-120(Al) was investigated by multinuclear (¹H, ¹³C, and ²⁷Al) SSNMR spectroscopy. The local structure around defective Al sites was directly probed by ²⁷Al 1D and 3QMAS NMR.