Nanostructured Al-based Fluoride-Oxide Materials with a Core-Shell Morphology

The investigation of the thermal behavior of β-AlF 2.6 OH 0.4 compound which adopts the hexagonal tungsten bronze network shows that this structure is stable up to 873 K. As revealed by a detailed analysis combining high resolution transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and 19 F and 27 Al MAS NMR, the sample annealed at 873 K exhibits an unusual core−shell like morphology. The shell which is amorphous contains some AlO 6, AlO 5, and AlO 4 species which have been identified and quantified by 27 Al NMR. Contrarily, the core part of the material is crystallized and has been identified as a phase closely related to the hydroxyl free compound β-AlF 3 in agreement with the dehydroxylation process. The annealing treatment lowers the Lewis acidity, but strong Brønsted acid sites have been created as evidenced by pyridine adsorption. These strong Brønsted sites arise from the reaction between water and strongly under-coordinated species formed upon annealing. It should be pointed out that such core−shell morphology had never been observed in fluoride or oxide-fluoride systems, so far.