Inhibition by Inorganic Dopants of γ‐Alumina Chemical Weathering under Hydrothermal Conditions: Identification of Reactive Sites and their Influence in Fischer–Tropsch Synthesis

Abstract

Understanding how alumina degradation to boehmite AlOOH can be prevented under hydrothermal conditions is key to design more stable catalysts supported on alumina for Fischer–Tropsch synthesis. We compare the effect of four inorganic dopants to inhibit γ‐alumina chemical weathering under hydrothermal conditions: three metal ions: Mg2+, Zr4+, Ni2+, introduced by impregnation, and nonmetal Si, introduced by the grafting of tetraethyl orthosilicate. Boehmite is formed by a mechanism of dissolution–precipitation. A significant decrease of alumina weathering to boehmite is evidenced for all dopants. For metal ions, the thermal conversion of doped γ‐Al2O3 into an alumina‐rich mixed oxide or the coverage of the alumina surface by particles of a poorly soluble oxide such as NiO contribute to the stabilization of the solid. Alumina dissolution and degradation are only inhibited fully through Si grafting. IR spectra in the OH stretching region suggest that the most reactive alumina sites toward dissolution are basic Al−OH sites located on lateral facets that are blocked upon Si introduction. Fischer–Tropsch reactivity shows that Co catalysts prepared on Mg2+‐ and Si‐doped supports are less active or, at best, as active as a reference Co/γ‐Al2O3 catalyst, which is explained by a lower reducibility of CoO on doped supports. The Co catalyst prepared on Mg2+‐doped γ‐Al2O3 calcined at 900 °C exhibits the best combination of catalytic activity, ease of preparation, and hydrothermal stability.

Reactivity of shape-controlled crystals and metadynamics simulations locate the weak spots of alumina in water

“…Albeit not fully described at the atomic scale yet, the decomposition of γ-Al 2 O 3 was shown to proceed, at the macroscopic scale, through a sequential mechanism involving first dissolution of Al atoms, and then precipitation thereof into the undesired hydroxides 16,24 . These structural changes can be retarded by tuning the surface chemical composition: silica deposition 26 , presence of metallic nanoparticles 12 , or impregnation with metal ions 27 . They are also impacted by the content of the solution: pH 24 but also presence of polyols 28 or polyphenols 25 .…”

Selective Carbon Deposition on γ-Alumina Acid Sites: toward the Design of Catalyst Supports with Improved Hydrothermal Stability in Aqueous Media

“…These observations are once again in good agreement with previous works. 5,11 The initial evolution has been attributed to an increase of the roughness of the alumina surface due to the formation of AlOOH micro domains. As these micro domains grow, the morphology of the particles changes as it can be observed on TEM pictures (Figure 2) and the f .…”

Coupling Hydrogenation of Guaiacol with In Situ Hydrogen Production by Glycerol Aqueous Reforming over Ni/Al2O3 and Ni-X/Al2O3 (X = Cu, Mo, P) Catalysts

“…The XRD patterns of used catalysts are presented in Figure 5 . It is noticeable that a new crystalline phase, boehmite (AlOOH), was formed, showing main reflections 28.3°, 38.5°, 49.3°, 55.3°, 60.7°, 64.2° [JCPDS 21-1307] in all used catalysts except Ni-P/Al 2 O 3 , despite being reported [ 30 , 31 ] that the transformation of γ-Al 2 O 3 into boehmite could be retarded in the presence of metal particles. It can be seen that the crystallinity of Ni/γ-Al 2 O 3 and copper-modified Ni catalysts significantly increased, and the diffraction peaks related to metallic Ni were much sharper after the reaction, suggesting that the metal particles were potentially sintered.…”

“…The XRD patterns of used catalysts are presented in Figure 5. [30,31] that the transformation of γ-Al 2 O 3 into boehmite could be retarded in the presence of metal particles. It can be seen that the crystallinity of Ni/γ-Al 2 O 3 and copper-modified Ni catalysts significantly increased, and the diffraction peaks related to metallic Ni were much sharper after the reaction, suggesting that the metal particles were potentially sintered.…”