Fluorination of Silica Nanoparticles by Aqueous NH₄F Solutions

Abstract

Fluorination of a precipitated silica (supplied by Rhodia) by aqueous solutions of NH 4 F (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, 19 F (Hahn echo sequence), 29 Si and 1 H MAS NMR, and 1 H- 29 Si and 19 F- 29 Si CPMAS NMR spectroscopy. The 19 F NMR allows both a qualitative and quantitative determination of the various detected fluoride-containing (in particular, fluosilicate) species, whereas the 1 H 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 opening of siloxane bonds, leading in particular to the formation of octahedral (hydroxy)fluosilicate species. The effect of the calcination, involving a secondary fluorination process through the decomposition of the octahedral fluosilicate species is also demonstrated. Finally, four kinds of fluosilicate species are determined, corresponding to isolated O 3/2 SiF groups (δ 19 F = −156 ppm/CFCl 3; δ 29 Si = −105 ppm/TMS), pentacoordinated O 4/2 SiF groups (δ 19 F = −153 ppm; δ 29 Si = −119 ppm), O 3/ 2 SiF species close to other groups of the same type (δ 19 F = −147 ppm; δ 29 Si = −105 ppm), and octahedral SiF 6 2- or Si(OH) 1 - 2 F 5 - 4 2- ions (δ 19 F = −127 or −129 ppm, respectively; δ 29 Si = −188 ppm).