This work deals with the preparation of various forms of crystallized aluminium hydroxyfluorides exhibiting nanosized particles. In this aim, a new route has been developed based on microwave solvothermal process. By tuning the synthesis parameters, i.e. aluminium precursor, HF/Al molar ratio, three materials has been prepared : AlF1.7(OH)1.3 with the pyrochlore type structure (Fd-3m), AlF2.6(OH)0.4 derived from the hexagonal tungsten bronze type structure (Cmcm) and the first Al-based fluoride compound with cationic vacancies l0.82-0.18F2.46(H2O)0.54 derived from the ReO3 type structure (Pm-3m). Beside X-ray diffraction analysis, NMR spectroscopy has enabled the description of the framework based on AlF6-x(OH/H2O)x species. FTIR spectroscopy has been revealed to be a suitable technique for the characterization of OH groups and water molecules. A material exhibiting a huge surface area (330 m2.g-1) has been obtained by treating an aluminium alkoxyfluoride gel by microwave process followed by a fluorination using F2 gas. The acidic behaviour of these materials has been studied by adsorption of probe molecules detected by FTIR. It has highlighted the effect of fluoride ions on the Lewis character and also the occurrence of Brønsted acidic sites whose strength depends on the anionic nature of the site. Finally, it has been shown that the formation of stronger Lewis acid sites depends on surface area, i.e. are sizedependent, highlighting the effect of surface reconstruction occurring at the nanoscale. The investigation of the thermal behaviour of the HTB type structure AlF2.6(OH)0.4 has shown that upon annealing a core-shell like morphology build of an amorphous oxide shell and a crystallized AlF3 core can be prepared.
Toward new aluminium hydroxyfluorides : synthesis, structure and acidic (Lewis/Brønsted) properties