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‐Al2O3 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‐Al2O3 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 1H and 27Al 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 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 , 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 a), as previously reported in the literature. ,,, 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 and . The impact of chlorine addition on the 1 H– 1 H DQ spectrum is more complex (Figure b).…”

“…The coordination of O atoms involved in OH groups as well their nonrandom spatial proximities were also unambiguously identified at high magnetic field . 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 (penta-coordinated), VI (hexacoordinated), leading to a detailed description of the hydroxyl network structure …”

Non-Oxidative Propane Dehydrogenation by a Well-Defined Ga Catalyst Prepared by Surface Organometallic Chemistry

“…After heat treatment at 300 °C, the DRIFT spectrum closely resembles that of γ-alumina dehydroxylated at 300 °C (Figure 2d,e, bottom right). 35 1 H MAS NMR spectra recorded during thermal treatment of (I) show a similar evolution (Figure 2, bottom left). The prominent signal at 1.3 ppm, attributed to the methyl groups of tert-butoxide ligands, diminished in intensity with increasing temperatures and nearly disappeared by 300 °C.…”

“…These signals are shifted by approximately 6 ppm from the unperturbed terminal and bridging hydroxyl groups signals, at 0 and 1.5 ppm, respectively. 35 Despite the low density of such sites, as indicated by the 1 H NMR MAS spectrum (Figure 3c), some of the Ga centers must interact with hydroxyl groups, generating specific Ga−O(H)-Al sites. Pyridine adsorption studies failed to reveal strong Brønsted acidic sites in material (II) (Figure S4).…”