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 al uminum sec-butoxide as the precursor. FTIR, TG, XRD, and crystalline struct ure refinement were used to analyze the phase transformations and their cry stalline structures. When samples were annealed at 200 degrees C, they had two nanocrystalline phases gamma-boehmite and gamma*-boehmite, which had th e same crystalline structure but different lattice parameters and defect nu mbers in the structure. After sample annealing above 400 degrees C, the gam ma-boehmite and gamma*-boehmite phases were transformed into nanocrystallin e gamma-Al2O3 and theta-Al2O3. The gamma-alumina crystalline structure cont ained hydroxyls that brought about cationic defects in aluminum sites, main ly in those of octahedral symmetry. In the Al-27 NMR spectra of the samples calcined at 400, 600, and 800 degrees C, a chemical shift around 33 ppm wa s 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 s ymmetry by hydroxyl groups.