Role of Pentacoordinated Al3+ Ions in the High Temperature Phase Transformation of γ-Al2O3

In this work, the structural stability of γ-alumina (γ-Al2O3) was investigated by a combination of XRD and high-resolution solid-state 27Al MAS NMR at an ultrahigh magnetic field of 21.1 T. XRD measurements show that γ-Al2O3 undergoes a phase transition to θ-Al2O3 during calcination at 1000 °C for 10 h. The formation of the θ-Al2O3 phase is further confirmed by 27Al MAS NMR; additional 27Al peaks centered at 10.5 and ∼78 ppm were observed in samples calcined at this high temperature. Both the XRD and NMR results indicate that, after calcination at 1000 °C for 10 h, the ratio of the θ-Al2O3 phase to the total alumina in samples modified by either BaO or La2O3 is significantly reduced in comparison with γ-Al2O3. 27Al MAS NMR spectra revealed that the reduction in the extent of θ-Al2O3 formation was highly correlated with the reduction in the amount of pentacoordinated aluminum ions, measured after 500 °C calcination, in both BaO- and La2O3-modified γ-Al2O3 samples. These results strongly suggest that the pentacoordinated aluminum ions, present exclusively on the surface of γ-Al2O3, play a critical role in the phase transformation of γ-Al2O3 to θ-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 °C) calcination.