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DNP Surface Enhanced Quadrupolar NMR Spectroscopy – SEQUANS

DNP Surface Enhanced Quadrupolar NMR Spectroscopy

Quadrupolar nuclei comprise nearly 70% of the NMR active nuclei in the periodic table and many areas of nanotechnology and new materials, where probe nuclei are often quadrupolar in nature. Because their NMR lines considerably sharpen at very high magnetic fields, with gains in resolution and sensitivity proportional to the square of B0, they would highly benefit from DNP at high magnetic field.

General objectives of the project

The SEQUANS project aims at implementing new spectroscopic approaches for the atomic-scale structural characterization of challenging surfaces by exploring new frontiers of DNP surface enhanced quadrupolar NMR spectroscopy at very high magnetic field and fast MAS. The project tackles several issues in basic research in the field of characterization tools and in materials science.

it will address the following challenges: i) the introduction of innovative sample preparation strategies for high-field DNP that are expected to outperform today’s protocols. Formulation protocols will be notably optimized for Overhauser Effect (OE), a polarization scheme that has recently emerged as a promising route for very high-field MAS DNP. In parallel, new approaches will be probed to amplify the micro-waves and to formulate the sample in the absence of a glassy solvent. ii) the implementation under DNP conditions of advanced solid-state quadrupolar NMR methods. Many correlation experiments essential for the structural characterization of surface species having quadrupolar probe nuclei have not yet been implemented and explored in DNP conditions and high magnetic field. This will concern the implementation and the in-depth performance analysis of both J-based and dipolar based heteronuclear (1H-27Al / 29Si-27Al) and homonuclear (27Al-27Al) correlation techniques, as well as the development of proton-detected experiments.

The following results have been achieved during the reporting period:
1. We introduced a series of new hybrid biradicals, that consist of a narrow line radical BDPA, tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron-electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand were established. With the best radical in this series, dubbed HYTek2, enhancements of up to 185 were reported at 18.8 T (800 MHz 1H resonance frequency) and 40 kHz MAS. Enhancement factors of over 60 in 3.2 mm sapphire rotors were also reported at 21.1 T (900 MHz 1H resonance frequency), the highest magnetic field available today for DNP. We are now working on the development of new binitroxide radicals tailored for DNP at very high magnetic field (18.8 and 21.1 T) in aqueous solution.
2. The potential of these new polarizing agents for Surface DNP enhanced quadrupolar NMR spectroscopy has been demonstrated on model aluminosilicates and on (Al)MCM-41-supported Ni(II) single-site catalysts, converting ethene to propene. Their activity and stability strongly depend on the specific location of aluminum sites that are introduced in the catalyst. DNP-enhanced 27Al NMR and the formulation developed during the project was essential to selectively observe the aluminum surface atoms in these materials.

The effectiveness of the developed approaches will be tested with the characterization of activated aluminas and amorphous silica-alumina, for which the structure of the so far invisible but catalytically important highly reactive Al surface sites remains a matter of debate in the field of heterogeneous catalysis.

1. Wisser, D.; Karthikeyan, G.; Lund, A.; Casano, G.; Karoui, H.; Yulikov, M.; Menzildjian, G.; Pinon, A. C.; Purea, A.; Engelke, F.; Chaudhari, S. R.; Kubicki, D.; Rossini, A. J.; Moroz, I. B.; Gajan, D.; Copéret, C.; Jeschke, G.; Lelli, M.; Emsley, L.; Lesage, A.; Ouari, O. BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields Up to 21.1 T, J. Am. Chem. Soc. 2018, 140 (41), 13340–13349.
2. Moroz; Lund, I.; Kaushik, A.; Sévery, M.; Gajan, L.; Fedorov, D.; Lesage, A.; Copéret, A.; Christophe. Specific Localization of Aluminum Sites Favors Ethene-to-Propene Conversion on (Al)MCM-41-Supported Ni(II) Single Sites. ACS Catal. 2019, in press.

NMR spectroscopy (often in conjunction with diffraction methods) is the method of choice for characterizing the atomic-scale structure of surfaces whenever possible, but the detection limit of NMR is far too low to allow many modern materials to be examined. Because it provides dramatic sensitivity enhancement, solid-state Dynamic Nuclear Polarization (DNP) NMR is currently emerging as a powerful tool to study samples previously inaccessible to NMR, and notably to selectively enhance the NMR signals from surfaces using an approach called DNP SENS (DNP Surface Enhanced NMR Spectroscopy). This approach provides remarkable signal enhancements (currently up to ~ 250) at moderate magnetic fields (9.4 T, 400 MHz proton frequency) and consequently leads to large reduction in experimental times making complete characterization of surface species possible. Most modern DNP experiments rely on cross-effect, a polarization transfer scheme from electrons to nuclei which is efficient for nitroxide-based polarizing agents, but whose efficiency scales with the inverse of the magnetic field strength B0 and is significantly reduced at fast Magic Angle Spinning (MAS). Drastic reductions in the amplification factors (by more than a factor of ten) are usually observed at 18.8 T (800 MHz proton frequency) and sizable DNP enhancements are difficult to obtain on surfaces at this field. This currently strongly limits a broad applicability of DNP SENS in materials science.
Quadrupolar nuclei comprise nearly 70% of the NMR active nuclei in the periodic table. Because their NMR lines considerably sharpen at very high fields, with gains in resolution and sensitivity proportional to the square of B0, they would greatly benefit from DNP at high magnetic field.
The SEQUANS project aims at implementing new spectroscopic approaches for the atomic-scale structural characterization of challenging surfaces by exploring new frontiers of DNP surface enhanced quadrupolar NMR spectroscopy at very high magnetic field and fast MAS. The project tackles several basic research issues in the field of characterization tools and in materials science. In particular, it will address the following challenges: i) the introduction of innovative sample preparation strategies for high-field DNP that are expected to outperform today’s protocols. Formulation protocols will be notably optimized for Overhauser Effect (OE), a polarization scheme that has recently emerged as a promising route for very high-field MAS DNP. In parallel, new approaches will be probed to amplify the micro-waves and to formulate the sample in the absence of a glassy solvent. ii) the implementation of advanced solid-state quadrupolar NMR methods tailored for DNP. Many correlation experiments essential for the structural characterization of surface species having quadrupolar probe nuclei have not yet been implemented and explored in DNP conditions and at high field. This will concern the implementation and the in-depth performance analysis of both J-based and dipolar based heteronuclear (1H-27Al / 29Si-27Al) and homonuclear (27Al-27Al) correlation techniques, as well as the development of proton-detected experiments. The developed approaches will be applied to the characterization of activated aluminas and amorphous silica-alumina, for which the structure of the so far invisible but catalytically important highly reactive Al surface sites remains a matter of debate in the field of heterogeneous catalysis.
This proposal will lean on a unique DNP instrumentation available in Lyon at 18.8 T, and on a first-class consortium in solid-state NMR and materials science. The project will directly extend the areas of application of this spectroscopy, and substantially contribute to fundamental knowledge in catalysis.

Project coordinator

Madame Anne LESAGE (Institut des Sciences Analytiques)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

CNRS-CEMHTI UPR 3079 Conditions Extrêmes et Matériaux : Haute température et Irradiation
ISA-CNRS Institut des Sciences Analytiques

Help of the ANR 341,549 euros
Beginning and duration of the scientific project: December 2017 - 36 Months

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