10.1021/jp503277m.s001
Raphael Wischert
Raphael
Wischert
Pierre Florian
Pierre
Florian
Christophe Copéret
Christophe
Copéret
Dominique Massiot
Dominique
Massiot
Philippe Sautet
Philippe
Sautet
Visibility
of Al Surface Sites of γ‑Alumina:
A Combined Computational and Experimental Point of View
American Chemical Society
2014
Al chemical shift
Lewis acid sites
2O
MAS
bulk Al atoms
EFG
Al Surface Sites
bulklike CQ values
Al sites
hydroxylated Al sites
27 Al NMR parameters
27 Al NMR spectra
2014-07-17 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Visibility_of_Al_Surface_Sites_of_Alumina_A_Combined_Computational_and_Experimental_Point_of_View/2273599
The
nature of γ-alumina (γ-Al<sub>2</sub>O<sub>3</sub>) 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 <sup>27</sup>Al 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 γ-Al<sub>2</sub>O<sub>3</sub>. 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 (<i>C</i><sub>Q</sub>) values (20–34
MHz) on the weakly hydrated major (110) termination, which contains
highly reactive Lewis acidic “defect sites”, and gradual
lowering to bulklike <i>C</i><sub>Q</sub> 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
{<sup>1</sup>H}<sup>27</sup>Al cross-polarization NMR experiments,
carried out at high magnetic field (20 T) and high magic angle spinning
(MAS) frequency (30 kHz) on γ-Al<sub>2</sub>O<sub>3</sub> 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-<i>C</i><sub>Q</sub> 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 γ-Al<sub>2</sub>O<sub>3</sub> and much caution should be exercised when interpreting <sup>27</sup>Al NMR spectra.