High‐Field NMR, Reactivity, and DFT Modeling Reveal the γ‐Al₂O₃Surface Hydroxyl Network**

Abstract

Aluminas are strategic materials used in many major industrial processes, either as catalyst supports or as catalysts in their own right. The transition alumina γ-Al 2 O 3 is a privileged support, whose reactivity can be tuned by thermal activation. This study provides a qualitative and quantitative assessment of the hydroxyl groups present on the surface of γ-Al 2 O 3 at three different dehydroxylation temperatures. The principal [AlOH] configurations are identified and described in unprecedented detail at the molecular level. The structures were established by combining information from high-field 1 H and 27 Al solid-state NMR, IR spectroscopy and DFT calculations, as well as selective reactivity studies. Finally, the relationship between the hydroxyl structures and the molecular-level structures of the active sites in catalytic alkane metathesis is discussed.

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

“…None of these species provide 1 H chemical shift close to 0 ppm as observed in the 1 H DEPTH spectrum, except some μ 2 -OH species present on the (111)-P1_2 facet model (Table S13). This trend partially contradicts the recent assignment proposed by Merle et al 39 who proposed that the Al IV -μ 1 -OH group present on the (110) facet at low OH coverage was able to explain this NMR signal. According to ref 44, the same (110) l -A1 facet exhibits either strong donor μ 1 -OH, at high coverage, or only μ 2 -OH stable at low coverage (Table S6).…”

“…One of the main effects of adding chlorine on γ-alumina is the disappearance of the 0 ppm signal on the 1 H NMR spectrum (Figure 1a), as previously reported in the literature. 15,16,39,42 It can also be underlined that only for the 1.4 wt % Cl loading, the 0 ppm signal vanishes completely, while at 0.5 wt % Cl, some μ 1 -OH remain present at edges as previously discussed in refs 42 and 68. The impact of chlorine addition on the 1 H− 1 H DQ spectrum is more complex (Figure 2b).…”

“…22 More recently, 2D correlation experiments carried out at 18.8 T allowed the identification of spatial proximities between the various hydroxyl groups: Al x -μ n -OH with n = 1 to 3 and x = IV (tetracoordinated), V (pentacoordinated), VI (hexacoordinated), leading to a detailed description of the hydroxyl network structure. 39 Density functional theory (DFT) simulations were also employed to provide more relevant bulk 40 and surface 26−29 models with hydration states controlled by temperature and water pressure, which can be used jointly with experimental data to get insight into the structure of acid sites. DFT calculations of the 27 Al and 1 H chemical shifts 19,41,42 were notably employed to revisit some earlier empirical assignments, providing a rational bridge between experimental observations and Al x -μ n -OH architecture.…”

Resolving Transformative Reactions in Zeolites under Catalytic Hydrothermal Conditions

“…However, the different defect sites, surface irregularities and inhomogeneous distribution of active sites, collectively makes it challenging to characterize them at the atomic-scale resolution. , While it is essential to assess their impact on catalytic activity, a precise understanding of the local structures associated with reactive interfaces, the proximities therein, and its dynamic evolution under reaction conditions is still lacking. Conventional long-range methods including XRD fall short in providing insights, but advanced 2D ssNMR experiments are typically more suitable and intuitive. ,− Hence, we used ex situ 2D 27 Al– 1 H and 1 H– 1 H through-space correlation experiments to resolve the local structures of the reactive interfaces. In experiments of this type, surface and bulk sites coupled via through-space dipolar interactions produce 2D correlation peaks.…”