Visibility of Al Surface Sites of γ-Alumina: A Combined Computational and Experimental Point of View

The nature of gamma-alumina (gamma-Al2O3) surface sites leaves many open questions today, and solid-state NMR spectroscopy has been proposed and used as a tool for assessing their structure. Here, we calculated Al-27 NMR parameters from first principles in periodic boundary conditions for a large number of Al sites with different coordination, potentially present on the alumina surface. The nature and accordingly the NMR parameters of these sites change with the level of hydroxylation and thereby the pretreatment temperature of gamma-Al2O3. While the Al chemical shift is little affected by hydroxylation, the magnitude of the quadrupolar interaction at the Al nucleus is strongly correlated to hydroxylation, with high to very high quadrupolar coupling constant (C-Q) values (20-34 MHz) on the weakly hydrated major (110) termination, which contains highly reactive Lewis acidic defect sites, and gradual lowering to bulklike C-Q values of around 5 MHz on highly hydrated surfaces. In addition, we studied the effects of local symmetry on the EFG tensor of Al atoms and the effects of hydrating neighboring Al sites. These calculation were combined with {H-1}Al-27 cross-polarization NMR experiments, carried out at high magnetic field (20 T) and high magic angle spinning (MAS) frequency (30 kHz) on gamma-Al2O3 pretreated at different temperatures. We clearly show that this method is mainly sensitive to strongly hydrated surface sites besides the much more abundant bulk Al atoms, and that catalytically important high-C-Q Al centers (Lewis acid sites) and even some types of hydroxylated Al sites are not visible in the spectrum. Therefore, even current high-field NMR experiments are not (yet) able to provide a complete picture of the structure of gamma-Al2O3 and much caution should be exercised when interpreting Al-27 NMR spectra.