Probing the transformation paths from aluminum (oxy)hydroxides (boehmite, bayerite, and gibbsite) to metastable alumina: A view from high-resolution 27Al MAS NMR

Detailed knowledge of the structural evolution of diverse aluminum (oxy)hydroxides with varying temperatures up to similar to 300 degrees C provides insights into the dehydration processes involving transitions among metastable phases on Earth's surface and in the crust. Here, we report the high-resolution solid-state Al-27 NMR spectra for three different types of aluminum (oxy)hydroxides (i.e., boehmite, bayerite, and gibbsite) with varying annealing temperatures up to 300 degrees C, revealing the effect of distinct precursor minerals on the stability of metastable alumina. Al-27 MAS NMR results allow us to obtain the quantitative fractions and NMR parameters for each phase during transformation. The results demonstrate that each aluminum (oxy)hydroxide phase follows a unique transformation path to metastable alumina. The Al-27 MAS and 3QMAS NMR spectra of boehmite show that a minor but observable Al-[4] signal (similar to 2%) is detected at similar to 50 degrees C, and the Al-[4] fraction gradually increases up to 300 degrees C (similar to 16%), indicating that the phase transformation from boehmite to gamma-/eta-Al2O3 occurs at a temperature as low as similar to 50 degrees C, significantly lower than earlier estimations based on XRD. Together with the Al-[4] fraction, the Al-[5] fraction increases from <1% at 50 degrees C to similar to 2.3% at 300 degrees C, whereas the NMR results of bayerite and gibbsite do not show the presence of Al-[5]. In addition, the Al-27 3QMAS NMR spectra resolved the Al-[6] site in boehmite and that in gamma-/eta-Al2O3, which could not be uniquely determined from 1D NMR spectra. The population of bayerite abruptly decreases from 100% (at 150 degrees C), through similar to 47% (at 200 degrees C), to 0% (at 250 degrees C), indicating that the phase transition from bayerite to boehmite + gamma-/eta-Al2O3 occurs within a narrow temperature range. As for gibbsite, while Al-[4] is not observed in the spectra up to 200 degrees C, the Al-[4] fraction of similar to 2% is observed in the spectra for gibbsite annealed at 250 degrees C, and the Al-[4] fraction increases rapidly to similar to 15% as the annealing temperature increases to 300 degrees C, suggesting that the phase transformation into gamma-/eta-Al2O3 occurs at similar to 250 degrees C. The results confirm that the phase transformation paths (gradual vs. dramatic) depend on the type of precursor minerals. Particularly, the onset temperature of the phase transformation from boehmite to metastable alumina (similar to 50 degrees C) is lower than those from other precursor minerals (>150 degrees C). Furthermore, the phase transformation from boehmite to gamma-/eta-Al2O3 occurs gradually within broad temperature ranges from similar to 50 degrees C. This is due to their configurational disorder as evidenced by the presence of Al-[5]. The observed structural evolution in aluminum (oxy)hydroxides in the low-temperature range facilitates our understanding of the nature of phase transformation and dehydration of oxides and hydroxides in the Earth's surface environments.