Thermal decomposition of mechanically activated gibbsite

Gibbsite (AlOH)(3) was mechanically activated by grinding for 20 h, and the changes in its structure were studied by thermal analysis, X-ray powder di ffraction and Al-27 MAS NMR. Grinding caused the rupture of a significant p roportion of the Al-OH bonds, but the resulting molecular water was immedia tely adsorbed on to the activated surfaces from which it could be endotherm ically desorbed at 125 degrees C. Grinding causes the gibbsite to become X- ray amorphous, containing octahedral and tetrahedral sites, and another sit e at about 34 ppm sometimes ascribed to Al in 5-fold coordination; the rela tive site occupancies suggest that this phase is similar to rho-Al2O3. The amorphous phase converts via gamma-Al2O3 to alpha-Al2O3 (corundum) at 900 d egrees C, by contrast with unactivated gibbsite which transforms to corundu m via gamma and theta-Al2O3 at least 400 degrees C higher (C) 1999 Elsevier Science B.V. All rights reserved.