Characterization of micro- and nano-structured materials by NMR: enhanced sensitivity by electron-nuclei coherence transfers – MatDNP

In this project, we investigated different types of nanostructured materials, such as mesoporous silica and alumina, microporous metal-organic framework (MOF) and nanoparticles (clay nanodisks, fibrous silica nanoparticles, alumina nanoparticles). These materials were characterized by DNP-NMR at 9.4 T under Magic-Angle Spinning (MAS). For these studies, nitroxide radicals were introduced into the investigated samples. Electron Paramagnetic Resonance (EPR) spectroscopy was employed to control the incorporation of radicals into the materials and to determine their localization and their concentration. The sensitivity of DNP-NMR experiments was compared to that of conventional NMR ones at room temperature. In order to obtain new structural information, DNP-NMR was combined with advanced NMR sequences, which were developed on conventional NMR spectrometers.

We demonstrated that DNP-NMR permits us to probe the structure of the surfaces of alumina (mesoporous Al2O3 or nanoparticles of ?-alumina) and of nanocatalysts made of nitridated fibrous silica. This project led to partnerships with companies (Bruker BioSpin, NMR Service…) and academic research teams located in France and in foreign countries (projects funded by ANR and CEFIPRA). This project also contributed to the selection of the University of Lilly as a host site for the 1.2 GHz NMR spectrometer, which will be the first one installed in France.

This project contributed to prove the potential of DNP-NMR for the characterization of materials. This technique opens new avenues for the study of a large panel of materials (pharmaceutical formulations, polymers, catalysts, energy materials…). In particular, this project will lead to new developments. We will notably employ DNP-NMR to characterize the stability of MOF in the presence of water vapor in the frame of ANR project H2O-MOF-NMR. In the frame of CEFIPRA project, we will also use DNP-NMR to study the structure of catalysts supported on fibrous silica nanoparticles.

This project led to 34 publications in peer-reviewed international journals, notably in those having the highest impact factors in our field of research, chemistry (Angew. Chem. Int. Ed., J. Am. Chem. Soc., Chem. Commun, Chem. Mater., J. Phys. Chem. Lett.…). The coordinator of the project was also awarded Magnetic Resonance in Chemistry Award for Young Scientists by scientific committee of EUROMAR in 2013. The results of this project were also presented in 59 communications at international conferences.

NMR spectroscopy provides useful information on the structure and dynamics of non-crystalline solids, such as heterogeneous catalysts or glasses. However, it is plagued by a lack of sensitivity. Such a limitation prevents solving important chemical questions, such as the structural and dynamic investigation of interfaces (crucial in catalysis, electrochemistry or nanomaterials) or the observation of diluted species (reaction intermediates, defects).
In this project, we propose to improve significantly the sensitivity of NMR by using the dynamic nuclear polarization (DNP). In principle, DNP can enhance the NMR sensitivity by 2 or 3 orders of magnitude. Recently the Griffin group at MIT demonstrated that DNP is compatible with high field and magic angle spinning. However, DNP/NMR at high field was applied mainly for 13C and 15N nuclei of organic and biological compounds in frozen solutions.
In the present project, we will explore high field DNP/NMR of heterogeneous catalysts. The first stage of the project aims at demonstrating the feasibility of DNP/NMR for model heterogeneous catalysts, the zeolites. In particular, we will determine the DNP signal enhancement for different isotopes contained in zeolite (1H, 29Si, 27Al, 23Na). During the second stage of the project, we will show that DNP signal enhancement allows gaining new insights into the structure and the dynamics of heterogeneous catalysts. A first application will consist in identifying the reaction intermediates and products inside the heterogeneous catalysts. A second application is the structure determination of molybdenum active sites in catalysts containing supported MoO3.
DNP/NMR will rapidly become a method of choice for structural characterization of materials. Besides catalysis, the present project is expected to have a major impact on the surface studies in porous materials, nanomaterials or battery materials. It would also have applications in the field of glasses, irradiated materials and semiconductors.