The structural evolution of hollow boehmite particles induced by citric acid

Spherical boehmite particles with hollow interiors were synthesized in the hydrothermal condition using aluminum nitrate as Al3+ source, urea as precipitator and citric acid as structure directing agent, which was further transformed to hollow γ-Al2O3 products after simply heating post-treatment under 700 °C for 4 h. The crystal phase, morphologies and microstructure of the sample were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. It is suggested that the as-synthesized boehmite powder possesses hollow interior with ca.1 μm in diameter and shells with 100 nm in thickness, and the calcinated sample have γ crystal phase without morphological changes after heat post-treatment. The interesting hollow structure evolution was further investigated by comparative experiments and a reasonable evolution mechanism was proposed, in which the chelation of citric acid with Al3+ species and the dissolution of boehmite play the critical role in forming the hollow structure of boehmite.