Solid-state NMR investigation of disordered metal-organic frameworks

Abstract

This thesis focuses on synthetic studies for isotopic enrichment and characterisation using solid-state nuclear magnetic resonance (NMR) spectroscopy of metal-organic frameworks (MOFs). These materials, composed of metal cations and organic linkers, form porous structures which have a wide range of applications, and there is an increasing need to understand their structure-property relationships. Solid-state NMR spectroscopy, with its sensitivity to the local, atomic-scale environment makes it a powerful technique for the study of disordered materials. Combined with density functional theory (DFT) calculations, powder X-ray diffraction (PXRD), electron microscopy and energy dispersive X-ray (EDX) spectroscopy detailed structural information on these materials can be obtained. The low natural abundance of ¹⁷O (0.037%) hinders routine study of MOFs by ¹⁷O NMR spectroscopy and thus several methods for cost-effective and atom-efficient ¹⁷O isotopic enrichment of MOFs have been explored both during and post synthesis including dry-gel conversion (DGC), hydrothermal exchange and slurrying. Firstly, the breathing behaviour of mixed-metal MIL-53 is explored. ¹⁷O enrichment of these materials allows for high-resolution ¹⁷O NMR spectra to be acquired which indicate the types of pore forms adopted depending on composition and provides information on the cation distribution. DFT calculations help to better understand the energetic preferences for different cation arrangements and compositions. Secondly, post-synthetic ion-exchange methods are investigated as an alternative route to the synthesis of mixed-metal MOFs. By controlling when and how ¹⁷O enrichment occurs detailed information on the structure and metal distribution of (Al,Ga)-MIL-53 materials can be obtained using ¹⁷O NMR spectroscopy. An initial study of Al³⁺ exchange into Sc₂BDC₃ is also explored. Finally, anionic substitution at the secondary-building unit (SBU) of GUF-1 and Sc-fumarate is investigated. Here ¹³C NMR spectroscopy provides bulk measurements of the amount of μ₂-OCH₃⁻ exchange taking place under different temperature and pressure conditions, supported by ²H NMR spectroscopy and 2D correlation experiments.