Coupling Sol-Gel Synthesis and Microwave-Assisted Techniques: A New Route from Amorphous to Crystalline High-Surface-Area Aluminium Fluoride

A non‐aqueous sol–gel Al‐based fluoride has been subjected to the microwave solvothermal process. The final material depends on the temperature heat treatment used. Three types of material have been prepared: 1) for low temperature heat treatment (90 °C) X‐ray amorphous alkoxy fluoride was obtained; 2) for the highest temperature used (200 °C) the metastable form β‐AlF3 was obtained with a very large surface area of 125 m2 g−1. The mechanism of the amorphous⇄crystalline transformation has been rationalised by the occurrence of a decomposition reaction of the gel fluoride induced by the microwave irradiation. 3) Finally, at intermediate temperature (180 °C) a multi‐component material mixture exhibiting a huge surface area of 525 m2 g−1 has been obtained and further investigated after mild post‐treatment fluorination using F2 gas. The resulting aluminium‐based fluoride still possesses a high‐surface‐area of 330 m2 g−1. HRTEM revealed that the solid is built from large particles (50 nm) identified as α‐AlF3, and small ones (10 nm), relative to an unidentified phase. This new high‐surface‐area material exhibits strong Lewis acidity as revealed by pyridine adsorption and catalytic tests. By comparison with other materials, it has been shown that whatever the composition/structure of the Al‐based fluoride materials, the number of strong Lewis acid sites is related to the surface area, highlighting the role of surface reconstruction occurring on a nanoscopic scale on the formation of the strongest Lewis acid sites. New routes to aluminium fluorides: Nanosized aluminium fluorides were prepared by microwave treatment of an alkoxy fluoride gel, leading to amorphous or crystalline materials (see HRTEM image), depending on the reaction temperature.