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 Al_Td-μ₁-OH protons are predominantly located on edges, where these are free from the H-bond network. The proximities among the Al_Td-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 Al_Td-μ₁-OH species. Moreover, after chlorination, a deshielding of the Al_Td 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.