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27Al MAS NMR Studies of HBEA Zeolite at Low to High Magnetic Fields

Hu, Jian Zhi, Wan, Chuan, Vjunov, Aleksei, Wang, Meng, Zhao, Zhenchao, Hu, Mary Y., Camaioni, Donald M., Lercher, Johannes A.
The Journal of Physical Chemistry C 2017 v.121 no.23 pp. 12849-12854
aluminum, magnetic fields, nuclear magnetic resonance spectroscopy, physical chemistry, silicon, spectral analysis, stable isotopes, zeolites
²⁷Al single pulse (SP) MAS NMR spectra of HBEA zeolites with high Si/Al ratios of 71 and 75 were obtained at three magnetic field strengths of 7.05, 11.75, and 19.97 T. High field ²⁷Al MAS NMR spectra acquired at 19.97 T show significantly improved spectral resolution, resulting in at least two well-resolved tetrahedral-Al NMR peaks. Based on the results obtained from ²⁷Al MAS and MQMAS NMR acquired at 19.97 T, four different quadrupole peaks are used to deconvolute the ²⁷Al SP MAS spectra acquired at various fields by using the same set of quadrupole coupling constants, asymmetric parameters and relative integrated peak intensities for the tetrahedral Al peaks. The line shapes of individual peaks change from typical quadrupole line shape at low field to essentially symmetrical line shapes at high field. We demonstrate that, for fully hydrated HBEA zeolites, the effect of second-order quadrupole interaction can be ignored, and quantitative spectral analysis can be performed by directly fitting the high field spectra using mixed Gaussian/Lorentzian line shapes. Also, the analytical steps described in our work allow direct assignment of spectral intensity to individual Al tetrahedral sites (T-sites) of zeolite HBEA. Finally, the proposed concept is suggested to be generally applicable to other zeolite framework types, thus allowing a direct probing of Al distributions by NMR spectroscopic methods in zeolites with high confidence.