In Situ High-Energy Synchrotron Radiation Study of Boehmite Formation, Growth, and Phase Transformation to Alumina in Sub- and Supercritical Water

• Published in: Chemistry : a European journal 2009-12, Vol.15 (48), p.13381-13390
• Main Authors: Lock, Nina, Bremholm, Martin, Christensen, Mogens, Almer, Jonathan, Chen, Yu-Sheng, Iversen, Bo B.
• Format: Article
• Language: English
• Subjects: alumina; boehmite; nanoparticles; supercritical fluids; X-ray scattering
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200901269

Boehmite (AlOOH) nanoparticles have been synthesized in subcritical (300 bar, 350 °C) and supercritical (300 bar, 400 °C) water. The formation and growth of AlOOH nanoparticles were studied in situ by small‐ and wide‐angle X‐ray scattering (SAXS and WAXS) using 80 keV synchrotron radiation. The SAXS/WAXS data were measured simultaneously with a time resolution greater than 10 s and revealed the initial nucleation of amorphous particles takes place within 10 s with subsequent crystallization after 30 s. No diffraction signals were observed from Al(OH)3 within the time resolution of the experiment, which shows that the dehydration step of the reaction is fast and the hydrolysis step rate‐determining. The sizes of the crystalline particles were determined as a function of time. The overall size evolution patterns are similar in sub‐ and supercritical water, but the growth is faster and the final particle size larger under supercritical conditions. After approximately 5 min, the rate of particle growth decreases in both sub‐ and supercritical water. Heating of the boehmite nanoparticle suspension allowed an in situ X‐ray investigation of the phase transformation of boehmite to aluminium oxide. Under the wet conditions used in this work, the transition starts at 530 °C and gives a two‐phase product of hydrated and non‐hydrated aluminium oxide. Opening of the black box: Anisotropic nanocrystalline boehmite (AlOOH) is formed within minutes from a simple metal oxide precursor in sub‐ and supercritical water. Particle growth was monitored in situ by WAXS and SAXS by using high‐energy synchrotron radiation (see figure). At elevated temperatures AlOOH dehydrates to aluminium oxide.