The ethylene glycol-mediated sol–gel synthesis of nano AlF3: structural and acidic properties after different post treatments

In this work, the structure and surface acidity of nano AlF3 prepared by the ethylene glycol-mediated sol-gel process, followed by different post treatments including post-fluorination and calcination, were systematically investigated. FT-IR, elemental analysis, XPS and TG-DTA-MS results indicate the ethylene glycol strongly interacts with the as-prepared AlF3 precursor, thus stabilizing the formed nano particles. MAS NMR spectroscopy combined with in situ FT-IR and HRTEM techniques reveal that AlF6, AlO6-xFx and AlO6 species are present in the resulting X-ray amorphous nano AlF3. The fraction of AlF6 species formed after post-fluorination significantly increases, whereas more AlO6-xFx species are formed just after calcination. After a comparable post-fluorination treatment, the CHClF2 dismutation activity at room temperature indicates that nano AlF3 prepared according the ethylene glycol-mediated route does not possess the same super-strong acidity as HS-AlF3 prepared by the fluorolytic sol-gel method, although NH3-TPD and N2-sorption results indicate larger BET surface areas and high concentration of acid sites for the former as compared to the latter. This might be rationalized based on the absence of terminal fluorine species and the presence of a significant number of AlO6-xFx and AlO6 species in the resulting nano AlF3 as revealed by 27Al, 19F MAS NMR and HRTEM. An interesting consequence is that these oxygen-containing species stabilize the microstructure of AlF3 formed, resulting in improved thermal stability of these phases as compared to "classically" prepared HS-AlF3.