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 gamma-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 gamma-alumina crystallites. Our approach relies on revisited models for the (100), (111), basal (110)(b), and lateral (110)(l) facets of gamma-alumina, as well as for the edges at their intersections. Notably, we show that the similar or equal to 0 ppm AlTd-mu(1)-OH protons are predominantly located on edges, where these are free from the H-bond network. The proximities among the Al-Td-mu(1)-OH as well as with mu(2)-OH groups are revealed by H-1-H-1 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 mu(1)-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 similar or equal to 0 ppm peak and of the correlations involving Al-Td-mu(1)-OH species. Moreover, after chlorination, a deshielding of the Al-Td is observed on high-resolution Al-27 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.

Automated summary

We use DFT calculations and proton solid-state NMR experiments to determine the exact location and spatial proximity of hydroxyl groups on gamma-alumina crystallites. We find that the hydroxyl groups are predominantly located on edges, free from the H-bond network. Chlorination selectively occurs on edges and lateral facets, resulting in the disappearance of the hydroxyl groups and disruption of the H-bond network.