Fluorination of Silica Nanoparticles by Aqueous NH4F Solutions

Abstract

Fluorination of a precipitated silica (supplied by Rhodia) by aqueous solutions of NH4F (0.1−2 mol L-1) is performed at room temperature (RT), the resulting fluorinated samples recovered after drying (RT or 70 °C) being possibly calcined (110−600 °C, for 5 or 24 h). The fluorinated silica samples are analyzed by TEM, nitrogen adsorption manometry, 19F (Hahn echo sequence), 29Si and 1H MAS NMR, and 1H-29Si and 19F-29Si CPMAS NMR spectroscopy. The 19F NMR allows both a qualitative and quantitative determination of the various detected fluoride-containing (in particular, fluosilicate) species, whereas the 1H MAS NMR leads to a complete speciation of the proton-containing species. The whole of the experimental data confirms clearly the existence of two kinds of fluorination mechanisms. For low fluoride concentrations, a nucleophilic substitution of OH groups by F- ions occurs, whereas, for higher fluoride concentrations, this substitution reaction is accompanied by a second mechanism implying the fluorinative...