Molecular-Level Characterization of Oxygen Local Environments in a Pristine and Post-Synthetically Modified Metal–Organic Framework via ¹⁷O Nuclear Magnetic Resonance Spectroscopy

Porous metal–organic frameworks (MOFs) have found many technological applications in fields such as carbon capture and storage, catalysis, and selective guest adsorption. Post-synthetic modification (PSM) approaches can influence MOF properties by introducing new functional groups or metals. MIL-121 is a prototypical aluminum MOF containing free uncoordinated carboxylic acid groups, which can act as adsorption sites for metal exchange as they are accessible to guests from within the pores. The introduction of metal species has been proven to enhance the gas adsorption capacity and catalytic properties of MIL-121. A deeper understanding of how MOF carboxylic acid groups interact with metals is imperative for the development of advanced industrially relevant materials. In this work, we demonstrate the remarkable capability of 17O solid-state NMR at 35.2 and 19.6 T to assign each 17O resonance in MIL-121 to its chemical/crystallographic oxygen site, provide site-specific structural information, and probe both the location and binding mode of metal guests within the framework. A series of 1D and 2D 17O NMR experiments on 17O-enriched MIL-121 supported by computational methods have been employed to study changes in the local environments of oxygen upon the activation of the material as well as upon metal loading. These results clearly show that the high spectral resolution achieved via high-field magic-angle spinning (MAS), REDOR (rotational-echo double-resonance), multiple-quantum MAS (MQMAS), and D-HMQC (dipolar heteronuclear multiple-quantum coherence) experiments yields unprecedented insight into this MOF. The 17O NMR parameters provide molecular-level information regarding the local oxygen environment, intermolecular interactions, and host–guest connectivity. This experimental approach can be applied to a wide variety of oxygen-containing MOFs.