Acces to highly fluorinated silica by direct F2 fluorination : chemical compositions and FTIR investigations
Résumé
The direct F2 gas fluorination of a mesoporous silica gel has been shown to be a unique method leading to very high levels of fluorination (up to 13 wt % F in the bulk). The final powders are homogeneous with a controlled amount of grafted fluorine. In this study, various conditions of fluorination were tested, such as duration, temperature, F2 gas concentration of the fluorinating gas, or an annealing pretreatment. The content of grafted fluorine on silica was quantified by XPS and by the Seel method. Infrared spectroscopy measurements accounted for the consumption of different types of hydroxyl groups, that is, isolated (3740 cm−1), terminal (3715 cm−1), and bound (3520 cm−1), and also for the presence of unreacted internal hydroxyl groups inaccessible to D2O molecules and so to F2. Results showed that an F/OH substitution occurs during the fluorination process and that the grafted amount depends on the F2 concentration of the fluorinating gas and on the concentration of surface hydroxyl groups and physisorbed water trapped on starting silica. Elemental analyses and FTIR data led to the bulk composition of fluorinated silicas: SiO2−x−y(OH)2xF2y. Finally, on a quantitative basis, the elimination of silanol groups parallels the grafting of fluorine for low fluorine content. At higher fluorine contents, a reaction path takes place involving the Si−O−Si opening. Redox processes involving O2/OH− and F2/F− couples explain the wide range of reached F/OH substitution rates without formation of SiF4 or SiF62− species which are observed in classical routes with aqueous fluorinating agents.