Highly fluorinated silica obtained by direct F2-gas fluorination: stability and unprecedented fluorosilicate species revealed by solid state NMR investigations
Résumé
The direct F2-gas fluorination of mesoporous silica is a unique method leading to high fluorinated (up to 13 wt % F) and homogeneous powder with a controlled amount of grafted fluorine. Thermogravimetric analysis coupled with mass spectrometry for water, hydroxyl, and fluorine groups have allowed concluding that low fluorine-grafted silicas are thermically stable up to 550 °C whereas high fluorine-grafted silicas start to decompose from 250 °C with departure of SiF4 species. Water adsorption measurements have demonstrated the hydrophobic character of fluorinated silica and proved that direct fluorination is a way to control the hydrophilic−hydrophobic balance of silica. Pristine and fluorinated silicas have been studied by 19F and 1H MAS, and 1H−29Si and 19F−29Si CP-MAS NMR spectroscopies. NMR measurements have revealed various tetrahedral (O2/2SiF2, O3/2SiF) and pentahedral (O4/2SiF) fluorosilicate species previously observed in moderately fluorinated silica, and two unprecedented pentahedral species occurring in these highly fluorinated silica: O3/2SiF2 and O2/2SiF3 (δiso(19F) = −143.5 and −136.5 ppm; δiso(29Si) = −124 and −132 ppm, respectively). The occurrence of these unusual species, thanks to the coupling between the redox mechanism and an etching phenomenon provoked by direct F2-gas fluorination, should explain the thermal stability as well as the water affinity of fluorinated silica.