Effect of mechanochemical activation on the thermal reactions of boehmite (gamma-AlOOH) and gamma-Al2O3

Boehmite (gamma-AlOOH) and gamma-Al2O3 were ground for 60 min and the chang es in their structure and thermal reactions were monitored by X-ray powder diffraction, thermal analysis and Al-27 MAS NMR. Grinding does not cause th e boehmite to dehydrate, but causes a decrease in the intensity of its X-ra y reflections, and the formation of an amorphous phase containing tetrahedr al Al and another Al site with a resonance at 36 ppm (sometimes attributed to pentacoordinated Al). The ground material thermally transforms to corund um at 940 degrees C via gamma-Al2O3, by contrast with the unground control, which forms corundum at 1195 degrees C via gamma and delta-Al2O3. Grinding gamma-Al2O3 decreases the intensity of its X-ray reflections and induces t he formation of some corundum without further heating. Complete transformat ion to corundum occurs at 927 degrees C, compared with 1193 degrees C in th e unground material which transforms via delta-Al2O3, The relationship betw een the mechanochemical development of the 36 ppm Al-27 NMR resonance in al uminas and their hydroxyl and adsorbed water content is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.