Aluminum local environment and defects in the crystalline structure of sol-gel alumina catalyst

Nanocrystalline alumina was synthesized by using the sol-gel method with aluminum sec-butoxide as the precursor. FTIR, TG, XRD, and crystalline structure refinement were used to analyze the phase transformations and their crystalline structures. When samples were annealed at 200°C, they had two nanocrystalline phases γ-boehmite and γ*-boehmite, which had the same crystalline structure but different lattice parameters and defect numbers in the structure. After sample annealing above 400°C, the γ-boehmite and γ*-boehmite phases were transformed into nanocrystalline γ-Al₂O₃ and θ-Al₂O₃. The γ-alumina crystalline structure contained hydroxyls that brought about cationic defects in aluminum sites, mainly in those of octahedral symmetry. In the ²⁷Al NMR spectra of the samples calcined at 400, 600, and 800°C, a chemical shift around 33 ppm was observed, which occurred between that of aluminum in oxygen octahedra and aluminum in oxygen tetrahedra. Its origin can be explained by assuming the existence the substitution of some lattice oxygen ions in the octahedral symmetry by hydroxyl groups.