Structure, Location, and Spatial Proximities of Hydroxyls on γ‑Alumina Crystallites by High-Resolution Solid-State NMR and DFT Modeling: Why Edges Hold the Key

The atomic-scale characterization of surface active sites
on γ-alumina
still represents a great challenge for numerous catalytic applications.
Here, we combine advanced density functional theory (DFT) calculations
with one- and two-dimensional proton solid-state NMR experiments to
identify the exact location and the spatial proximity of hydroxyl
groups on γ-alumina crystallites. Our approach relies on revisited
models for the (100), (111), basal (110)b, and lateral
(110)l facets of γ-alumina, as well as for the edges
at their intersections. Notably, we show that the ≃0 ppm AlTd-μ1-OH protons are predominantly located
on edges, where these are free from the H-bond network. The proximities
among the AlTd-μ1-OH as well as with μ2-OH groups are revealed by 1H–1H dipolar correlation experiments and analyzed in the light of the
DFT calculations, which identify their location on the basal (110)b facet and on the (110)b/(100) and (110)b/(110)l edges. Using chlorine atoms to probe the presence
of hydroxyls, we show that the chlorination occurs selectively by
exchanging μ1-OH located on edges and on lateral
(110)l facets. By contrast, the basal (110)b and lateral (111) facets are not probed by Cl. This exchange explains
the disappearance of the ≃0 ppm peak and of the correlations
involving AlTd-μ1-OH species. Moreover,
after chlorination, a deshielding of the AlTd is observed
on high-resolution 27Al NMR spectra. More subtle effects
are visible on the proton correlation spectra, which are attributed
to the disruption of the H-bond network in the course of chlorination.