Magic angle spinning oxygen-17 NMR of aluminum oxides and hydroxides

Abstract

The authors have obtained magic angle spinning {sup 17}O nuclear magnetic resonance spectra of {sup 17}O-labeled {alpha}-Al{sub 2}O{sub 3}, boehmite (AlO(OH)), bayerite (Al(OH){sub 3}), and several transitional aluminas, including {gamma}-, {eta}-, {delta}-, and {theta}-Al{sub 2}O{sub 3}. Cross polarization from {sup 1}H to {sup 17}O has been used to enhance the hydroxyl oxygen resonances of boehmite and bayerite and to allow the observation of hydroxyl groups on the surface of {gamma}-Al{sub 2}O{sub 3}. The spectra yield nuclear quadrupole coupling constants, electric field gradient tensor asymmetry parameters, and isotropic chemical shifts for each chemically distinct oxygen site. The quadrupole coupling constants for tetrahedrally coordinated oxide sites (OAl{sub 4}) range from 1.2 to 2.2 MHz, while those of trigonally coordinated hydroxide sites (Al{sub 2}OH) are in the 5-6-MHz range. Trigonally coordinated oxide sites (OAl{sub 3}), arising as a result of cation vacancies in the transitional aluminas, are estimated to have quadrupole coupling constants of approximately 4 MHz. It is shown that the observed quadrupole coupling constants and asymmetry parameters are in generally good agreement with the results of electric field gradient calculations based on a point charge model.