Nanostructured aluminium hydroxyfluorides derived from b-AlF3
Résumé
Microwave-assisted synthesis has been applied for the preparation of aluminum hydroxyfluorides derived from β-AlF3. In the course of obtaining high-surface-area and homogeneous nanocrystallites, several key parameters have been adjusted. A relationship between the OH/F molar ratio and the stabilized allotropic forms has been pointed out. The surface area has been monitored depending on the water/isopropanol volume ratio. The microstructural properties of the as-prepared material have been thoroughly investigated highlighting X-ray line broadening arising from strains and size effects. It has been concluded from X-ray diffraction data refinements and TEM results that the prepared crystallites possess a platelet shape with an average particle size of 15 nm associated with a high surface area of 82 m2 g−1. The occurrence of hydroxyl groups inside the network has been detected by FTIR spectroscopy and considered angles and bond distances are probably located in peculiar crystallographic sites, i.e., 8f (F1) and 16h (F2) Wyckoff positions. 19F NMR as well as 27Al high-field NMR spectroscopies are powerful tools to probe and quantify the various AlF6−x(OH)x environments and confirm the preferential location of OH groups in 8f (F1) and 16h (F2) Wyckoff positions. A preliminary study using NH3 adsorption revealed the occurrence of strong Lewis acid sites on the surface of β-AlF3−x(OH)x.