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

In this work, the structural stability of γ-alumina (γ-Al 2 O 3 ) was investigated by a combination of XRD and high-resolution solid-state 27 Al MAS NMR at an ultrahigh magnetic field of 21.1 T. XRD measurements show that γ-Al 2 O 3 undergoes a phase transition to θ-Al 2 O 3 during calcination at 1000 °C for 10 h. The formation of the θ-Al 2 O 3 phase is further confirmed by 27 Al MAS NMR; additional 27 Al 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-Al 2 O 3 phase to the total alumina in samples modified by either BaO or La 2 O 3 is significantly reduced in comparison with γ-Al 2 O 3 . 27 Al MAS NMR spectra revealed that the reduction in the extent of θ-Al 2 O 3 formation was highly correlated with the reduction in the amount of pentacoordinated aluminum ions, measured after 500 °C calcination, in both BaO- and La 2 O 3 -modified γ-Al 2 O 3 samples. These results strongly suggest that the pentacoordinated aluminum ions, present exclusively on the surface of γ-Al 2 O 3, play a critical role in the phase transformation of γ-Al 2 O 3 to θ-Al 2 O 3 . The role of the modifiers, in our case BaO or La 2 O 3, 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.