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

Abstract

The synthesis of aluminium, chromium, iron and gallium hydroxyfluorides in their hexagonal tungsten bronze (HTB) β-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. The substitution of Fe³⁺ by Cr³⁺, which gives rise to an increase in the content of H₂O/OH groups preferentially around Cr³⁺, allows the improvement of the M–F bonding stability. The template effect of water has also been pointed out. The acidic character of these solids has been evaluated by FTIR analysis using probe molecule adsorption and leads to the conclusion that the strongest Lewis acidity is found in Al³⁺ and Ga³⁺ homologous compounds with respect to that of iron hydroxyfluoride. These characteristics can be directly related to the strength of the M–(F,OH) chemical bond and the thermal stability of these solids. The use of the ratio χ/r² between the electronegativity χ and the ionic radius r, which can be ascribed to an electrical field gradient around the cation, has been proposed. This parameter allows a more accurate approach of both the acidic strength and the thermal stability in the hydroxyfluoride series and accounts for the experimentally observed sequence.