Synthesis and characterization of Al3+, Cr3+, Fe3+ and Ga3+ hydroxyfluorides: correlations between structural features, thermal stability and acidic properties

International audienceThe synthesis of aluminium, chromium, iron and gallium hydroxyfluorides in their hexagonal tungsten bronze (HTB)-form has been undertaken by sol precipitation followed by thermal treatments. These solids, which could be used in heterogeneous catalysis, have been firstly characterized by chemical analysis, X-ray diffraction and FTIR spectroscopy in order to determine their composition and structural features. In the HTB hydroxyfluorides series, there is competition between the formation of M–F and M–OH bonds, which depends on the type of cation, the nature of precursor and the route of synthesis. FTIR spectroscopy study has shown the presence of both free- and linked- OH– groups. The nature of cations, the decomposition kinetics of the M(H2O)63+ aquo-complex and the size of tunnels in the HTB framework account for the thermal stability of these compounds. For instance, a comparison between Al3+ and Fe3+ hydroxyfluorides shows that the Al–(F,OH) bond is more stable than the Fe–(F,OH) bond, with a difference of more than 200 K in their thermal stabilities..