Highly Fluorinated Silica Obtained by Direct F2-Gas Fluorination: Stability and Unprecedented Fluorosilicate Species Revealed by Solid State NMR Investigations

The direct F-2-gas fluorination of mesoporous silica is a unique method leading to high fluorinated (up to 13 wt % F) and homogeneous powder with a coutrolled 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 degrees C whereas high fluorine-grafted silicas start to decompose from 250 degrees 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 F-19 and H-1 MAS, and H-1-Si-29 and F-19-Si-29 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 (delta(iso)(F-19) = -143.5 and -136.5 ppm; delta(iso)(Si-29) = -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 F-2-gas fluorination, should explain the thermal stability as well as the water affinity of fluorinated silica.