Sol–gel-fluorination synthesis of amorphous magnesium fluoride

The sol–gel fluorination process is discussed for the reaction of magnesium alkoxides with HF in non-aqueous solvents to give X-ray amorphous nano-sized magnesium fluoride with high surface areas in the range of 150–350 m2/g (HS-MgF2). The H2 type hysteresis of nitrogen adsorption–desorption BET-isotherms is indicative for mesoporous solids. A highly distorted structure causes quite high Lewis acidity, shown by NH3 temperature-programmed desorption (NH3-TPD) and catalytic test reactions. XPS data of amorphous and conventionally crystalline MgF2 are compared, both show octahedral coordination at the metal site. Thermal analysis, F-MAS NMR- and IR-spectroscopy give information on composition and structure of the precursor intermediate as well as of the final metal fluoride. The preparation of complex fluorides, M+MgF3−, by the sol–gel route is reported. From the magnesium fluoride gel of the above process thin films for optical application are obtained by, e.g., spin coating.

Graphical abstractAn amorphous high surface area magnesium fluoride (HS-MgF2) can be prepared via a non-aqueous sol–gel route. It differs from crystalline MgF2 remarkably in, e.g., exhibiting a substantial Lewis acidity as proved by NH3-TPD. Because of its low crystallisation temperature, moderate preheating reduced the Lewis acidity drastically.Figure optionsDownload full-size imageDownload as PowerPoint slide