Jian Zhi Hu
Pacific Northwest National Laboratory
Richland, WA, United States
In Situ (Operando) Magic Angle Spinning NMR for Harsh Experimental Conditions
High resolution magic angle spinning (MAS) NMR is a powerful technique for studying structure and dynamics in a heterogeneous system containing a mixture of e.g., solid, semi-solid, liquid, and gaseous phases. Due to its intrinsic advantage of probing local structure at molecular level, MAS NMR is an attractive tool for in situ (operando) investigations of reaction mechanisms, including but not limited to the identification of active centers, intermediates, and the reaction dynamics associated with material synthesis or chemical reactions using solid catalysts, the adsorption and desorption of molecules on surfaces of porous materials. However, the commercially available reusable-MAS rotors are rarely capable of achieving 100% seal under harsh experimental conditions of, e.g., significantly elevated temperature and pressure, or significantly cold temperature with high pressure. To address these challenges and advance the application of MAS NMR under harsh experimental conditions, we have developed a perfectly sealed, powerful, in situ MAS NMR rotor that is capable of sealing a heterogeneous sample under extreme experimental conditions of combined high pressure and high temperature.1 The same technology is equally applicable to low temperature and high pressure operations. The in situ MAS rotor is constructed using an integrated high mechanical strength zirconia rod except an O-ring and a spin tip. Herein, we will first report the latest advancements associated with high field and fast sample spinning where in situ MAS rotor with outside diameters of 4 and 3.2 mm have been successfully developed. To illustrate the power of in situ MAS NMR, examples of application in material synthesis and catalytic reactions, including but not limited to (a) Mechanisms of phenol-cyclohexanol alkylation in zeolite H-BEA studied by 13C and 1H NMR,2-3 (b) The crystallization of AlPO4-5 by 1H, 27Al, and 31P NMR,4 and (c) Genesis and stability of hydronium ions in zeolite channels by 1H and 1H-29Si CP NMR,5 and (d) Zeolite framework stability and degradation in hot water studied by high field and fast spinning 27Al MAS NMR.
1. Hu, J. Z.; Hu, M. Y.; Zhao, Z. C.; Xu, S. C.; Vjunov, A.; Shi, H.; Camaioni, D. M.; Peden, C. H. F.; Lercher, J. A., Sealed rotors for in situ high temperature high pressure MAS NMR. Chem Commun 2015, 51 (70), 13458-13461.
2. Zhao, Z. C.; Shi, H.; Wan, C.; Hu, M. Y.; Liu, Y. S.; Mei, D. H.; Camaioni, D. M.; Hu, J. Z.; Lercher, J. A., Mechanism of Phenol Alkylation in Zeolite H-BEA Using In Situ Solid-State NMR Spectroscopy. J Am Chem Soc 2017, 139 (27), 9178-9185.
3. Liu, Y. S.; Barath, E.; Shi, H.; Hu, J. Z.; Camaioni, D. M.; Lercher, J. A., Solvent-determined mechanistic pathways in zeolite-H-BEA-catalysed phenol alkylation. Nature Catalysis 2018, 1 (2), 141-147.
4. Zhao, Z. C.; Xu, S. C.; Hu, M. Y.; Bao, X. H.; Hu, J. Z., In Situ High Temperature High Pressure MAS NMR Study on the Crystallization of AlPO4-5. J Phys Chem C 2016, 120 (3), 1701-1708.
5. Wang, M.; Jaegers, N. R.; Lee, M.-S.; Wan, C.; Hu, J. Z.; Shi, H.; Mei, D.; Burton, S. D.; Camaioni, D. M.; Gutiérrez, O. Y.; Glezakou, V.-A.; Rousseau, R.; Wang, Y.; Lercher, J. A., Genesis and Stability of Hydronium Ions in Zeolite Channels. J Am Chem Soc 2019, 141 (8), 3444-3455.
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