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

Abstract

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, AlO_6−xF_x 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 AlO_6−xF_x 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 AlO_6−xF_x 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.