The ethylene glycol-mediated sol-gel synthesis of nano AlF 3 : structural and acidic properties after different post treatments.

In this work, the structure and surface acidity of nano AlF 3 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 AlF 3 precursor, thus stabilizing the formed nano particles. MAS NMR spectroscopy combined with in situ FT-IR and HRTEM techniques reveal that AlF 6 , AlO 6- x F x and AlO 6 species are present in the resulting X-ray amorphous nano AlF 3 . The fraction of AlF 6 species formed after post-fluorination significantly increases, whereas more AlO 6- x F x species are formed just after calcination. After a comparable post-fluorination treatment, the CHClF 2 dismutation activity at room temperature indicates that nano AlF 3 prepared according the ethylene glycol-mediated route does not possess the same super-strong acidity as HS-AlF 3 prepared by the fluorolytic sol-gel method, although NH 3 -TPD and N 2 -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 AlO 6- x F x and AlO 6 species in the resulting nano AlF 3 as revealed by 27 Al, 19 F MAS NMR and HRTEM. An interesting consequence is that these oxygen-containing species stabilize the microstructure of AlF 3 formed, resulting in improved thermal stability of these phases as compared to "classically" prepared HS-AlF 3 .