Study of MOFs water stability by NMR of isotopes exhibiting close Larmor frequencies – H2O-MOF-NMR

The project consists of five distinct tasks. The first task concerns the management of the project. The second is relative to the synthesis of MOFs and the water stability tests. The third task deals with the manufacturing of high performance diplexers with improved efficiency to allow the observation of CLF isotopes, such as 13C-27Al. The fourth task focuses on the development of advanced NMR methods suitable for CLF nuclei. This task involves the understanding of spin dynamics using Average Hamiltonian Theory and numerical simulations with SIMPSON or SPINEVOLUTION software43 as well as NMR and DNP/NMR experiments on model compounds at different magnetic fields (9.4 and 18.8 T). The fifth task aims at (i) identifying the most stable MOFs and (ii) determining the structural modifications caused by water adsorption using solid-state NMR spectroscopy. The five tasks are listed below: - Task 1: Management of the project, - Task 2: Synthesis and water stability tests, 1) Synthesis of the MOFs 2) Water stability tests and conventional characterization - Task 3: Development of high-performance diplexer, - Task 4: Development of (DNP)-NMR methods for close Larmor frequencies (CLF) nuclei, 1) Recoupling and decoupling between CLF isotopes 2) Wideband excitation 3) DNP adapted to CLF nuclei - Task 5: NMR characterization of water stability and structure alteration caused to MOFs by water. 1) Characterization of MOFs by conventional NMR pulse sequences 2) Characterization of MOFs using CLF nuclei. DNP/NMR measurements

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Microporous hybrid materials, such as metal-organic frameworks (MOFs), are promising for numerous applications (energy, health, environment), since they combine high surface areas and tunable pore dimensions. However, little is known about the water stability of the MOFs, even if it represents a critical issue for many applications, including gas storage, radionuclides capture, drug delivery or water purification. The rational improvement of MOFs water stability requires the development of suitable characterization methods since diffraction techniques are not able to detect local defects or change in the organization of extra-framework species caused by hydration.
This project aims at analysing water stability of MOFs through advanced solid-state NMR characterization of structural modifications caused by water. By its local character, solid-state NMR is a promising method to probe the structural alterations of MOFs resulting from water adsorption. The final deliverables of this project are (i) standardized NMR protocols to assess water stability of MOFs and (ii) the identification of the most water stable MOFs. In a first step, we will investigate Al containing MOFs, since they benefit from high water and thermal stability (up to 500°C for MIL-53) as well as low cost, density and toxicity. Furthermore, they are promising systems for the capture of radionuclides and heterogeneous catalysis. In a second step, scandium and gallium containing MOFs will be investigated in order to determine the influence of the metal on the water stability.
A major stumbling block for the NMR characterization of MOFs is the inability to probe 13C-27Al, 13C-45Sc and 13C-69,71Ga using common solid-state NMR probes since these isotopes exhibit close Larmor frequencies. In this project, we will develop high-performance diplexers and new NMR sequences to circumvent this limitation. These new diplexers will benefit from higher sensitivity and extended tuning range with respect to the existing diplexers, while these devices must be fully compatible with commercial NMR probes. These instrumental developments will be conducted in close collaborations with NMR Service Company and are in line with the strategy of UCCS to contribute to high-field NMR instrumentation for the future installation of 1.2 GHz NMR spectrometer at the University of Lille 1. In this project, we will also develop advanced heteronuclear NMR methods suitable for isotopes of close Larmor frequencies and compatible with the use of diplexers. These NMR methods include two- and three-dimensional heteronuclear correlation experiments to probe 13C-27Al, 13C-45Sc and 13C-69,71Ga proximities. We will also use high magnetic field and/or Dynamic Nuclear Polarization (DNP) to improve the sensitivity of these experiments. The combination of instrumental and methodological developments with conventional NMR characterization (1H, 13C, 17O) will allow determining structural alterations caused by water adsorption and clarifying the mechanisms and the kinetics of the processes involved in water adsorption in MOFs.
Besides MOFs, this project is expected to have a broad impact on solid-state NMR and materials science. The developed diplexers will open new avenues for NMR of other isotopes with close Larmor frequencies (31P-7Li, 1H-19F...), which are present in important systems, such as glasses, polymers, soils, biomolecules, organometallics…