Microstructural properties of boehmite formed under hydrothermal conditions

The microstructural properties of boehmite formed in an alkaline medium under hydrothermal conditions were investigated using X-ray diffraction (XRD), Fourier transform IR (FT-IR) spectroscopy, transmission electron microscopy (TEM) and microelectrophoresis. The bayerite formed at the beginning of the precipitation process converted to boehmite by the dissolution/reprecipitation mechanism. Boehmite crystallites were in the nanosize range and of anisotropic shape, as determined by XRD. A tendency toward an increase in the crystallite size with prolonged duration of autoclaving was evident both at 160 and 120°C. The boehmite particles produced by autoclaving at 160°C for 10 h looked like thin microfoils, i.e. different from the boehmite morphology usually obtained (rods, fibrils and very small plates). The factors that influence the pHiep of boehmite particles are discussed. The FT-IR spectra of bayerite and boehmite are interpreted.