6634
J. Phys. Chem. C 2007, 111, 6634-6644
Fluorination of Silica Nanoparticles by Aqueous NH4F Solutions Geoffrey Hartmeyer,† Claire Marichal,*,† Be´ ne´ dicte Lebeau,† Philippe Caullet,† and Julien Hernandez‡ Laboratoire de Mate´ riaux a` Porosite´ Controˆ le´ e, CNRS UMR 7016, ENSCMu-UHA, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France, and Centre de Recherches d’AuberVilliers, RHODIA, 52 rue de la Haie Coq, 93308 AuberVilliers, France ReceiVed: December 8, 2006; In Final Form: February 13, 2007
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 opening of siloxane bonds, leading in particular to the formation of octahedral (hydroxy)fluosilicate species. The effect of the calcination, involving a secondary fluorination processsthrough the decomposition of the octahedral fluosilicate speciessis also demonstrated. Finally, four kinds of fluosilicate species are determined, corresponding to isolated O3/2SiF groups (δ19F ) -156 ppm/CFCl3; δ29Si ) -105 ppm/TMS), pentacoordinated O4/2SiF groups (δ19F ) -153 ppm; δ29Si ) -119 ppm), O3/2SiF species close to other groups of the same type (δ19F ) -147 ppm; δ29Si ) -105 ppm), and octahedral SiF62- or Si(OH)1-2F5-42- ions (δ19F ) -127 or -129 ppm, respectively; δ29Si ) -188 ppm).
1. Introduction Reaction with fluorine is well known to modify the chemical and physical properties of silica through the replacement of surface hydroxyl groups by fluorine atoms.1 This results, for instance, in geochemistry, in the modification of the viscosity, density, and solubility of the silicon oxides in the magmas. Fluorination treatments present great interest in a technological and industrial point of view with several application fields such as metallurgy,2-4 optics,5,6 electronics,7 and catalysis.8-10 In addition to the rise in catalyst activity,11 the fluorination treatment also improves the hydrophobic properties of the carrier.12 Both the chemical vapor deposition and the liquid-phase deposition processes are used for fluorination of silicon oxides in high-technology industry such as telecommunication, microelectronics, and so forth. The method of chemical vapor deposition is, however, specifically adapted for the fluorination surface treatment of macroscopic substrates and/or to allow the deposition on the substrate (film, fibers, etc.) of a silica-based coating with good control of the thickness and composition.13-15 The liquid-phase deposition method is more suitable for fluorination surface treatment of divided solids. Fluorinated oxides for catalysis are thus usually prepared by reaction with solutions (water, organic solvent, or water/organic solvent * Corresponding author. E-mail:
[email protected]. Fax: 00 33 3 89 33 68 85. Tel: 00 33 3 89 33 67 31. † Laboratoire de Mate ´ riaux a` Porosite´ Controˆle´e. ‡ Centre de Recherches d’Aubervilliers.
mixture) of fluoride-containing salts (e.g., NH4F, NaF, NaBF4, Na2SiF6, (NH4)2SiF6) according to two different routes. The first one consists of the total immersion of the material to be fluorinated, with a volume solution, Vsol, much larger than the total pore volume of the substrate Vp (Vsol . Vp). According to the second route, the material to be fluorinated is impregnated, with close Vsol and Vp values. For both methods, fluorination treatment is usually ended by a calcination of the solid phase. The changes that occur upon fluorination can be studied by many techniques, of which infrared and 1H and 19F NMR spectroscopies are most widely used. As shown by Duke et al.,10 the fluorination of silica by aqueous KF or NH4F solutions leads to the formation of surface O3/2Si-F bonds and SiF62- anions. Later, Clark et al.16 reported that the treatment of silica with different fluorinating agents in aqueous solution may lead to the formation of various [FxSi(OH)6-x]2- species rather than simply SiF62- anions. In addition, they showed that heating at 300 °C causes the decomposition of the hydroxyfluoride species and the subsequent formation of additional tetrahedral O3Si-F species. More recently, Barabash et al.17 studied the effect of the solvent to avoid excessive fluorination leading to octahedral SiF62- species. They evidenced the formation of two types of surface-fluorinated silicon species: surface-bonded tetrahedral Ox-Si(OH)4-x-yFy and octahedral [FxSi(OH)6-x]2- species. The thermal decomposition of [FxSi(OH)6-x]2- leads to the removal of corresponding silicon fluorides and hydroxyfluorides, and surface-bonded O3/2Si-F and O1/2-SiF3 can be additionally formed. Heating to 500-600 °C causes the surface migration
10.1021/jp068450m CCC: $37.00 © 2007 American Chemical Society Published on Web 04/19/2007
Fluorination of Silica Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6635
TABLE 1: Textural Properties and Fluoride Content of the Fluorinated Silica Samples sample
precipitated silicaa
SBET (m2 g-1) F- wt %theor F- wt %exp a
174
S-0.05
S-0.1
S-0.25
S-0.5 ab
S-1.0
S-2.0
145 0.5 0.5
145 0.9 0.9
n.d.c 2.2 2.1
130 4.2 3.5
105 7.7 6.1
89 13.3 14.6
Untreated silica. b Four samples S-0.5 a-d were prepared (see the text). c n.d.: not determined.
TABLE 2: Total F- wt % Values and F- wt % Values Corresponding to the Fluorinated Species Detected in the Three S-0.5 Samples sample ref total F- wt %
δa (F- wt %)
a
S-0.5a
S-0.5b
S-0.5c
3.5
3.4
3.4
-127 (0.1)
-127 (0.1)
-127 (0.1)
-128 (0.5)
-128 (0.2)
-128 (0.3)
-140 (0.1)
-140 (0.1)
-140 (0.1)
-147 (0.9)
-147 (0.7)
-147 (0.9)
-153 (0.7)
-153 (0.9)
-153 (0.8)
-155 (1.2)
-155 (1.4)
-155 (1.2)
2. Experimental Section
In ppm/CFCl3.
of fluorine in O3/2Si-F over siloxane bridges and the formation of silicon fluorides and their subsequent removal from the surface at higher temperatures (>600 °C). In these two latter studies, fluorinated species have been identified and it was clearly showed that the fluorination processes are dependent on the type of fluorinating agent used and the experimental conditions. However, the quantitative determination of these fluorinated species was not achieved. The present work deals with the surface modification of precipitated silica nanoparticles by (i) impregnation with aqueous ammonium fluoride solutions (0.05 to 2 mol L-1) and (ii) optional calcination at 110, 150, 200, 400, or 600 °C for 5 or 24 h. The initial and fluorinated samples are characterized by nitrogen adsorption manometry (BET surface area measurements), transmission electron microscopy (TEM), and 29Si, 19F, and 1H NMR spectroscopy. It must be emphasized that the 19F NMR technique allows not only a qualitative but also a quantitative determination of the various detected fluoridecontaining (in particular, fluosilicate) species. On the basis of the whole of the characterization data, assignments of the 19F and 29Si MAS NMR signals relative to fluosilicate species are proposed and the fluorination mechanisms are described in detail.
2.1. Characteristics of the Used Silica. The parent silica, referred to as precipitated silica, was supplied by Rhodia18 and is made of small aggregates of almost spherical nanometric particles (diameter close to 16 nm). The specific surface area, measured from the N2 adsorption isotherm, is equal to 174 m2 g-1. The total porous volume measured by Hg porosimetry is close to 3.8 cm3 g-1. The number of silanol groups per square nanometer, calculated from either the weight loss recorded by thermogravimetry in the 200-1200 °C range or the molar proportion of Q3-type silicon atoms determined by decomposition of the 29Si MAS NMR spectrum, is estimated to be about 8. According to the X-ray fluorescence analysis, the precipitated silica contains some impurities in minor amounts, mainly Ca, Cl, Na, Al, and Mg, corresponding, respectively, to about 0.14, 0.09, 0.09, 0.08, and 0.04 wt %. 2.2. Fluorination Procedure. The aqueous NH4F solution (15 cm3), with concentrations ranging from 0.05 to 2 mol L-1, are added drop by drop at room temperature to about 3.15 g of wet silica. This corresponds to 3.0 g of dry SiO2 (according to the weight loss recorded by heating up to 1200 °C), the molar composition of the final mixture thus being 1 SiO2/0.015-0.6 NH4F/∼16.8 H20. The mixture, which is prepared in a glass crystallizer, is then, after a careful homogenization by mixing, aged for 24 h at room temperature (the vessel is covered by a plastic film to avoid water evaporation) and finally dried for 5 days at 70 °C in an oven. In some particular experiments, the drying process is performed at room temperature (for 1 or 10 days) or at 70 °C but for 15 or 24 h. Samples of the silica fluorinated by the 0.5 mol L-1 NH4F solution are calcined under air at 110, 150, or 200 °C for 5 or 24 h, and at 400 or 600 °C for 5 h. All of the fluorinated samples are stored in an exsiccator over silica gel before characterization. 2.3. Characterization Methods. 2.3.1. Structural InVestigation Techniques. NMR Spectroscopy. 19F liquid-state NMR is mainly used to determine the total fluoride content in the silica samples. Fluorinated silica are thus first dissolved by 4 g of NaOH (SDS) 10 mol L-1 in a closed polypropylene tube. Once complete solubilization of the sample is achieved by sonication, a precisely known amount of trifluoroacetic acid solution
TABLE 3: SBET Values, Fluoride Content, and F- wt % Values Assigned to Each Detected Species in the As-Made and Calcined Fluorinated S-0.5 Silica Samples sample ref calc. temp.(˚C) calc.time (h) SBET (m2 g-1) total F- wt % F- wt %
a
S-0.5
S-0.5a
S-0.5b
110
-107b -127b -129b -140b -147b -153b -156b
132 3.4a un. 0.1a 0.3a 0.1a 0.8a 0.8a 1.3a
5 134 3.6 un. 0.1 0.2
