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

Boehmite (γ-AlOOH) and γ-Al2O3 were ground for 60 min and the changes in their structure and thermal reactions were monitored by X-ray powder diffraction, thermal analysis and 27Al MAS NMR. Grinding does not cause the boehmite to dehydrate, but causes a decrease in the intensity of its X-ray reflections, and the formation of an amorphous phase containing tetrahedral Al and another Al site with a resonance at 36 ppm (sometimes attributed to pentacoordinated Al). The ground material thermally transforms to corundum at 940°C via γ-Al2O3, by contrast with the unground control, which forms corundum at 1195°C via γ and δ-Al2O3. Grinding γ-Al2O3 decreases the intensity of its X-ray reflections and induces the formation of some corundum without further heating. Complete transformation to corundum occurs at 927°C, compared with 1193°C in the unground material which transforms via δ-Al2O3. The relationship between the mechanochemical development of the 36 ppm 27Al NMR resonance in aluminas and their hydroxyl and adsorbed water content is discussed.