Structural investigations and acidic properties of high surface area pyrochlore aluminium hydroxyfluoride

Abstract

High surface area aluminium hydroxyfluoride exhibiting the pyrochlore-type structure has been synthesized using microwave hydrothermal process. The optimization of the crystallite size and surface area of the material has been performed by varying some synthesis parameters, i.e. the nature of the aluminium precursor, the choice of solvents and the addition of a small amount of ether. The as-prepared solid contains nanosized particles as calculated by XRD (average particle size: 12 nm) leading to a surface area around 140 m² g⁻¹. These results have been confirmed by TEM observations. The framework is built of AlF_6−x(OH)_x species which have been characterized and quantified by high field ²⁷Al NMR. A random distribution of F atoms and OH groups is evidenced to occur on the 48f sites. Inside the channels, water molecules interact with the framework by hydrogen bonding either with fluoride ions or OH groups present in their vicinity. The use of probe molecules has shown that the pyrochlore exhibits a Lewis acidity lying between that of γ-Al₂O₃ and that of the strong Lewis acid β-AlF₃ adopting the hexagonal tungsten bronze form. Surface OH groups have been characterized by IR spectroscopy and exhibit weak Brønsted acidity.