Theoretical inorganic chemistry Corundum inertness

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Corundum α-Al2O3 is a thermodynamically stable modification of aluminum oxide in the entire temperature range, has a fully ordered structure with the densest hexagonal packing, unlike other polymorphic modifications of aluminum oxide. All these modifications , depending on their stability during heat treatment, turn into α-Al2O3.

@nknh716 Nevertheless, it is interesting how it turns out that the crystal structure changes the properties of a substance, in fact, the bonds remain the same as in other forms Al2O3

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The change in the structure of the crystal lattice is associated with heat treatment at different temperatures. During this, cations and hydroxyl groups move in the lattice. This means that there is a heterogeneity of acid centers (Lews and Brensted) on the surface of aluminum oxide. They affect the properties, namely the activity of aluminum oxide. Catalytically active modifications of OA having a metastable state are characterized by the presence of Al3+ ions in their structure in both tetra- and octahedral positions, with which the acidic and catalytic properties of OA are associated. During the transition to high-temperature modifications of aluminum oxide (δ-, κ-, θ-Al₂O₃), the number of Al3+ ions in tetrahedral coordination decreases, which means that Lewis acid centers decrease. In the crystal lattice of corundum α-Al2O3, Al3+ ions occupy only octahedral positions on the surface of a corundum single crystal, almost only oxygen anions and a small amount of hydroxyls are present.

Thus, the chemical activity of synthetic aluminum oxide decreases greatly with an increase in the temperature of its production.
Natural and artificial (formed above 1200 ° C) corundum in air under normal conditions are chemically inert and non-hygroscopic. Corundum, obtained at a temperature of about 1000 ° C, intensively interacts with alkalis and carbonates of alkali metals. Reacts slowly with SiO₂ and acidic slags to form aluminosilicates. Corundum formed from diaspores at 500-600 ° C interacts with solutions of acids and alkalis. Alumogel and Al₂O₃ obtained by firing Al hydroxides at ~ 550 ° C (low-temperature modication) are very hygroscopic and chemically active, react with solutions of acids and alkalis.

I hope this will be useful information for you.

Thank you for the question, I am ready to discuss this topic further.