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

Abstract

Detailed knowledge of the structural evolution of diverse aluminum (oxy)hydroxides with varying temperatures up to ~300 °C provides insights into the dehydration processes involving transitions among metastable phases in Earth's surface and crust. Here, we report the high-resolution solid-state 27 Al NMR spectra for three different types of aluminum (oxy)hydroxides (i.e., boehmite, bayerite, and gibbsite) with varying annealing temperatures up to 300 °C, revealing the effect of distinct precursor minerals on the stability of metastable alumina. 27 Al 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 the unique transformation path to metastable alumina. The 27 Al MAS and 3QMAS NMR spectra of boehmite show that a minor but observable [4] Al signal (~2%) is detected at ~50 °C, and the [4] Al fraction gradually increases up to 300 °C (~16%), indicating that the phase transformation from boehmite to -/-Al 2 O 3 occurs at a temperature as low as ~50 °C, significantly lower than earlier estimations based on XRD. Together with the [4] Al fraction, the [5] Al fraction increases from < 1% at 50 °C to ~2.3% at 300 °C, whereas the NMR results of bayerite and gibbsite do not show the presence of [5] Al. In addition, the 27 Al 3QMAS NMR spectra resolved the [6] Al site in boehmite and that in -/-Al 2 O 3 , which could not be uniquely determined from 1D NMR spectra. The population of bayerite abruptly decreases from 100% (at 150 ºC), through ~47% (at 200 ºC), to 0% (at 250 ºC), indicating that the phase transition from bayerite to boehmite + -/-Al 2 O 3 occurs within a narrow temperature range. As for gibbsite, while [4] Al is not observed in the spectra up to 200 °C, the [4] Al fraction of ~2% is observed in the spectra for gibbsite annealed at 250 °C, and the [4] Al fraction increases rapidly to ~15% as the annealing temperature increases to 300 °C, suggesting that the phase transformation into -/-Al2O3 occurs at ~250 ºC. The results confirm that the phase transformation paths (gradual vs. dramatic) depend on the type of precursor minerals. Particularly, the threshold temperature at which the onset of the phase transformation from boehmite to metastable alumina (~50 ºC) is lower than those from other precursor minerals (>150 ºC). Furthermore, the phase transformation from boehmite to -/-Al 2 O 3 occurs gradually within broad temperature ranges from ~50 °C. This is due to This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Thermally Activated Al(OH)3 Part II—Effect of Different Thermal Treatments

“…This step has a minimum point between 292.9 • C and 306.2 • C in all samples and again can be attributed to gibbsite dehydroxylation, with AlOOH most probably the final product. The results are in good agreement with literature data, which indicate two concurrently occurring processes at this stage, namely the formation of boehmite and the transformation of gibbsite in some amorphous or χ-Al 2 O 3 phase [41,42]. The boehmite will undergo transformation to γ-Al 2 O 3 above 320 • C. The main endothermic effect for this process was observed at around 500 • C. The final product represents ~64% of the initial mass for all the samples.…”

“…Among the desired properties for these applications are a specific pore size and pore size distribution [33,34], a high specific surface area [35][36][37], and the degree of crystallinity, which also plays an essential role [38][39][40][41]. When discussing the synthesis routes for α-Al 2 O 3 , the thermal transformation of aluminum oxy-hydroxides remains the most studied [42].…”

Geopolymers made using organic bases. Part I: Synthesis and atomic structure

“…These four sites were present in all spectra except for TMA WG + Al 2 O 3 , which showed only the Al(IV) site at 62 ppm along with the Al(VI) sites. The Al(VI) site at 5 ppm and the shoulder at −10 ppm were due to gibbsite, 128 but assignment of the other sites to particular phases was inhibited by a lack of information about the number of phases observed by XRD and the relation, if any, between the peaks and amorphous halos. The Al(IV) peak at 54 ppm consistently was much more prominent in the samples cured at 50 • C yet was still present in those samples cured at 20 • C that showed amorphous halos, so that site was attributed to one amorphous phase.…”

Generation and nature of water-tolerant Lewis acid sites in InxSn10−xOy/Al2O3 catalysts as active centers for the green synthesis of methyl lactate from glucose

“…However, it has been well recognized that Al 2 O 3 with tetrahedrally coordinated Al 3+ (Al IV ) and octahedrally coordinated Al 3+ (Al VI ) ions can be rehydrated under hydrothermal conditions to boehmite with only Al VI . 24,25 As LAS are associated with CUS, this explains why water-tolerant LAS are absent in Al 2 O 3 . Therefore, it is necessary to hydrophobize the Al 2 O 3 surface via reducing the surface OH groups thus to suppress the hydration of the catalyst during MLA synthesis.…”