Role of pentacoordinated Al3+ ions in the high temperature phase transformation of γ-Al2O3

In this work, the structural stability of gamma-alumina (gamma-Al2O3) was investigated by a combination of XRD and high-resolution solid-state Al-27 MAS NMR at an ultrahigh magnetic field of 21.1 T. XRD measurements show that gamma-Al2O3 undergoes a phase transition to theta-Al2O3 during calcination at 1000 degrees C for 10 h. The formation of the theta-Al2O3 phase is further confirmed by Al-27 MAS NMR; additional Al-27 peaks centered at 10.5 and similar to 78 ppm were observed in samples calcined at this high temperature. Both the XRD and NMR results indicate that, after calcination at 1000 degrees C for 10 h, the ratio of the theta-Al2O3 phase to the total alumina in samples modified by either BaO or La2O3 is significantly reduced in comparison with gamma-Al2O3. Al-27 MAS NMR spectra revealed that the reduction in the extent of theta-Al2O3 formation was highly correlated with the reduction in the amount of pentacoordinated aluminum ions, measured after 500 degrees C calcination, in both BaO- and La2O3-modified gamma-Al2O3 samples. These results strongly suggest that the pentacoordinated aluminum ions, present exclusively on the surface of gamma-Al2O3, play a critical role in the phase transformation of gamma-Al2O3 to theta-Al2O3. The role of the modifiers, in our case BaO or La2O3, is to convert the pentacoordinated aluminum ions into octahedral ones, thereby improving the thermal stabilities of the samples. Oxide additives, however, seem to have little, if any beneficial effect on preventing reductions in specific surface areas that occurred during high-temperature (<= 1000 degrees C) calcination.