γ-Al2O3 is intensively used as catalyst or support in many industrial processes such as alcohol dehydration, propane dehydrogenation, isomerization, alkylation and catalytic cracking. The oxygen speciation (e.g., hydroxyl or defects) and coordination significantly impact the property (i.e., acidity/basicity) and local environment of γ-Al2O3. Thus, the determination of oxygen species in γ-Al2O3 is prerequisite for understanding its structure and physicochemical property. 17O magic-angle spinning (MAS) NMR is emerging as a method of choice for characterizing oxide-based materials. However, the 17O NMR spectroscopy often suffers from a combination of the quadrupolar nature of the 17O nucleus (I =5/2), relatively low gyromagnetic ratios, and low 17O abundance (0.037%), resulting in low sensitivity and spectral resolution. 

Around five years ago, we took advantage of dynamic nuclear polarization (DNP) technique to enhance ¹⁷O NMR sensitivity on γ-Al₂O₃¹, enabling the detection of surface oxygen sites with (e.g., aluminols and adsorbed water) or without (bare oxygen) bound protons. The next challenge is to reveal the spatial proximity/connectivity between different oxygen sites from the surface to the bulk, which is essential to better understanding the local structure of γ-Al₂O₃. The conventional NMR approach is hampered by the small ¹⁷O-¹⁷O interactions due to the low γ of the ¹⁷O nucleus and the dilution from insufficient ¹⁷O isotope enrichment.

The higher the better. The availability of high magnetic field which is up to 35.2 Tesla (National High Magnetic Field Laboratory, Tallahassee) offer new opportunities to explore the challenging low γ nuclei. In this contribution, we proposed a strategy to unambiguously determine oxygen structure of γ-Al₂O₃ with the aid of ultrahigh magnetic field and state-of-the-art pulse sequences.

Over the past ten years, we have been working on the development of NMR method to be applied in catalysts for a deep understanding of local structure and chemical reactions². An efficient dipolar recoupling pulse sequence (BR2¹₂) was developed by Dr. Qiang Wang and successfully applied on ²⁷Al-²⁷Al homonuclear correlation experiments in the study of distribution of Al species in zeolites^3,4. We envisioned that we could use this NMR method at high magnetic field to tackle the ¹⁷O NMR challenge.

As expected, the remarkable gain in sensitivity as well as resolution at 35.2 T field allow us to achieve the first 2D ¹⁷O homonuclear correlation MAS NMR experiment on γ-Al₂O₃ with moderate ¹⁷O labelling (ca. 20%). Importantly, the 2D correlation maps can be achieved in several hours. 4- and 3-coordinated oxygen sites are clearly identified and their spatial proximities between different oxygen species from the surface to the bulk are revealed. Furthermore, 2D proton-detected ¹H-¹⁷O heteronuclear correlation experiments allows the discrimination of (sub-)surface oxygen species including bare oxygen, hydroxyl groups and adsorbed water. A non-random distribution of oxygen species and a non-spinel structure in γ-Al₂O₃ is determined.

The detailed insights into the oxygen sites provide a basis for tuning the property of γ-Al₂O₃ and rational design of improved oxide-based catalysts. Our present work demonstrates the tremendous potential of the advanced NMR technique combing hardware with novel pulse sequences to provide atomic-scale information in solid materials of technical importance.

The manuscript titled, " Mapping the oxygen structure of γ-Al₂O₃ by high-field solid-state NMR spectroscopy" can be found here for further details: https://www.nature.com/articles/s41467-020-17470-4#citeas

References

(1)  Li, W.; Wang, Q.; Xu, J.; Aussenac, F.; Qi, G.; Zhao, X.; Gao, P.; Wang, C.; Deng, F. Probing the surface of γ-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy. Phys. Chem. Chem. Phys. 2018, 20, 17218-17225.

(2)  Xu, J.; Wang, Q.; Deng, F. Metal Active Sites and Their Catalytic Functions in Zeolites: Insights from Solid-State NMR Spectroscopy. Accounts. Chem. Res. 2019, 52, 2179-2189.

(3)  Wang, Q.; Hu, B.; Lafon, O.; Trébosc, J.; Deng, F.; Amoureux, J. P. Double-quantum homonuclear NMR correlation spectroscopy of quadrupolar nuclei subjected to magic-angle spinning and high magnetic field. J. Magn. Reson. 2009, 200, 251-260.

(4)  Yu, Z. W.; Zheng, A. M.; Wang, Q. A.; Chen, L.; Xu, J.; Amoureux, J. P.; Deng, F. Insights into the Dealumination of Zeolite HY Revealed by Sensitivity-Enhanced Al-27 DQ-MAS NMR Spectroscopy at High Field. Angew. Chem. Int. Edit. 2010, 49, 8657-8661.