Access to Highly Fluorinated Silica by Direct F2 Fluorination: Chemical

The direct F2 gas fluorination of a mesoporous silica gel has been shown to be a unique method leading to very high levels of fluorination (up to 13 w...
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J. Phys. Chem. C 2008, 112, 10943–10951

10943

Access to Highly Fluorinated Silica by Direct F2 Fluorination: Chemical Compositions and FTIR Investigations E. Lataste,† A. Demourgues,*,† H. Leclerc,‡ J.-M. Goupil,‡ A. Vimont,‡ E. Durand,† C. Labruge`re,† H. Benalla,† and A. Tressaud† ICMCB-CNRS, UniVersite´ Bordeaux 1, 87 AVenue du Dr. Albert Schweitzer, 33608 Pessac Cedex, France, and Laboratoire Catalyse et Spectrochimie, ENSICAEN, UniVersite´ de Caen, CNRS, 6 BouleVard Mare´chal Juin, F-14050 Caen Cedex, France ReceiVed: NoVember 12, 2007; ReVised Manuscript ReceiVed: April 15, 2008

The direct F2 gas fluorination of a mesoporous silica gel has been shown to be a unique method leading to very high levels of fluorination (up to 13 wt % F in the bulk). The final powders are homogeneous with a controlled amount of grafted fluorine. In this study, various conditions of fluorination were tested, such as duration, temperature, F2 gas concentration of the fluorinating gas, or an annealing pretreatment. The content of grafted fluorine on silica was quantified by XPS and by the Seel method. Infrared spectroscopy measurements accounted for the consumption of different types of hydroxyl groups, that is, isolated (3740 cm-1), terminal (3715 cm-1), and bound (3520 cm-1), and also for the presence of unreacted internal hydroxyl groups inaccessible to D2O molecules and so to F2. Results showed that an F/OH substitution occurs during the fluorination process and that the grafted amount depends on the F2 concentration of the fluorinating gas and on the concentration of surface hydroxyl groups and physisorbed water trapped on starting silica. Elemental analyses and FTIR data led to the bulk composition of fluorinated silicas: SiO2-x-y(OH)2xF2y. Finally, on a quantitative basis, the elimination of silanol groups parallels the grafting of fluorine for low fluorine content. At higher fluorine contents, a reaction path takes place involving the Si-O-Si opening. Redox processes involving O2/OH- and F2/F- couples explain the wide range of reached F/OH substitution rates without formation of SiF4 or SiF62- species which are observed in classical routes with aqueous fluorinating agents. 1. Introduction The properties of amorphous oxides and nanoparticles with high specific surface area depend largely on the chemistry of the surface. Silica has been extensively investigated in both heterogeneous catalysis and chemical separations.1–5 Because of high surface area and porous structure, mesoporous silica properties are strongly dependent on the surface reactivity of silanol species (Si-OH) and their interactions via hydrogen bonds with other molecules such as water.6,7 Silica surface exhibits low Brønsted acidity without any Lewis acidity. However, surface modification reactions can be performed by using the Brønsted acidic sites of silica gels, and such processes have been carried out to prepare thin coatings with a wide variety of organic groups and to tune hydrophobic character.8 Several studies have shown that the substitution of fluoride (F-) for hydroxyl groups onto the silica improves the surface acidity, as illustrated by some cumene cracking catalysts,9,10 as well as its hydrophobic character. As far as the synthesis routes are concerned, in nonaqueous (organic) media,11 fluoro-organosilanes have been used as precursors in sol-gel synthesis,12–14 whereas in aqueous media, alkaline fluorides such as KF, NaF, or NH4F at various pH15 are generally employed. All these routes involving organic or aqueous solvents and counterions are complex11,15 because of the wide variety of intermediates species that often lead to the decomposition of silica or SiO2 etching16 with a subsequent formation of [SiF6]2- octahedral species.11,15 A recent study using aqueous medium and more especially * Corresponding author. E-mail: [email protected]. † Universite ´ Bordeaux 1. ‡ Universite ´ de Caen.

NH4F as fluorinating agent15 showed that small amounts of fluorine are chemisorbed on silica or substituted for hydroxyls, but less than 1 wt % F can be obtained without the formation of (NH4)2SiF6. Nevertheless, most fluorinated silica exhibits also a poor thermal stability because of its high reactivity with moisture.17 Furthermore, fluorine insertion decreases the electronic polarizability, the dielectric constant, and the refractive index of silica18 and improves its optical properties by increasing the transparency in visible range without parasitic optical absorption in transmission region from UV to near infrared.19,20 Reactive ion etching involving CF4 or SiF421,22 as well as plasmaenhanced chemical vapor deposition (PECVD) with SF6, CF4, C2F6, SiF4, or HF23–27 have been developed. In the case of SiF4-PECVD, a 3.3 wt % content of fluorine could be reached. Various [SiF6]2- octahedral and [O(4-n)/2SiFn] (1 e n e 3) tetrahedral species or even 5-fold coordinated [O4/2SiF] environments have been identified by NMR spectroscopy (1H, 19F, 29Si) and FTIR spectroscopy.11,15,25 Actually, it was very difficult to definitively conclude about the attribution of such species because in the used synthesis conditions the Si-F content remained too small at the silica surface. In the present study, elemental fluorine F2, the most reactive and strongest oxidizing agent (Eo(F2/F-) ) 2.87 V/SHE), was used for the silica treatment. Actually, the half-reaction 1/2 F2 + e- f F- is favored by the high electron affinity of fluorine and the low binding energy of the F2 molecule. Such a fluorination route has been scarcely investigated, because only a few laboratories possess the equipment and the expertise for research involving F2 gas.28 One goal of the present study is to obtain a wide range of SiOnFm species to get reference IR and

10.1021/jp710790e CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

10944 J. Phys. Chem. C, Vol. 112, No. 29, 2008 RMN spectra useful for identification purposes. XPS and global titration of fluorine were used to determine the F/Si atomic ratios. Scanning electron microscopy (SEM) and nitrogen physisorption analysis were undertaken to get information about the morphology, nanoparticle sizes, surface area, and porosity. Water, isolated silanols, and H-bonded hydroxyls were identified and quantified by FTIR spectroscopy. A forthcoming article is devoted to the (1H, 19F, 29Si) NMR investigations39 of these highly fluorinated silica as well as their chemical and thermal stabilities. 2. Experimental Section 2.1. Starting Material and Fluorination Experiments. A commercial mesoporous silica gel from Fluka (silica gel for column chromatography 60, CAS no. 112926-00-8, Fluka reference 60741) composed of agglomerates between 60 and 200 µm with 500 m2 g-1 surface area was used for direct F2 gas fluorination. The starting material was used as received or annealed at various temperatures under argon before fluorination experiments to remove water (T ) 200 °C) and a part of silanol groups (T ) 400 °C, T ) 600 °C). Direct F2 gas fluorination process was performed in dedicated fluorine equipment using special handling procedures previously described.28 The sample was set in a passivated nickel boat. F2 gas was diluted in argon (Air Products). F2 volume percentages (F2%) in the fluorinating gas were between 10 and 80% for reactions carried out at room temperature and between 10 and 50% for reactions at 100 °C. The reaction was carried out in a closed vessel at pressure around 1 bar, generally for two hours (some experiments were conducted for half an hour and 18 h for testing purpose). At the end of the experiment, excess F2 gas was eliminated by reaction on soda lime pellets and the reaction chamber was filled with argon. It should be noted that even if the F2 gas was carefully introduced in the chamber, small amounts of SiF4 could evolved. 2.2. Physicochemical Characterizations. Elemental Analysis. The amounts of fluorine and silicon elements were determined by the Service Central d’Analyze of the CNRS (Vernaison, France) and at ICMCB-CNRS. Fluorine was quantified by the Seel method.40 The sample was first dissolved in a K2CO3/Na2CO3 mixture by being heated in a platinum boat. The molten solution was cooled to room temperature and dissolved in a small amount of distilled water. About 1 g of silica and 20 mL of 98% H2SO4 were slowly added to the solution. This soution was then distilled under a water vapor flow at 250 °C to favor the formation of H2SiF6 and its evaporation. H2SiF6 was condensed, and the fluorine content was determined with a fluoride ion-specific electrode. X-ray Photoelectron Spectroscopy. XPS was used to determine the amounts of fluorine (F1s), silicon (Si2p), oxygen (O1s), and carbon (C1s) elements fixed onto the silica surface (analysis depth ) 5-10 nm). The analysis of the binding energies coupled to a deconvolution of the peaks could not allow identification of the signal of the Si-F chemical bonds.12,29 Measurements were performed with VG 220 i-XL ESCALAB equipment with a nonmonochromatized Mg source (1253.6 eV) at 100 W. The analyzed area was around 200 µm in diameter. The insulating character of silica needed low-energy (4-6 eV) electron compensation. Survey and high resolution spectra were recorded with pass energy of 150 and 20 eV, respectively. Quantification of F, Si, O, and C elements was obtained with an Eclipse processing program provided by Vacuum Generators. ThermograWimetric Analysis. A SETARAM MTB 10-8 system was used for recording the silica thermogravimetric

Lataste et al. curve. The experiments were carried out under N2 flow (180 mL min-1) in corundum crucibles containing around 40 mg of sample with a constant heating rate of 3 K min-1 up to 900 °C. Scanning Electron Microscopy. High-resolution SEM investigations were carried out with a Hitachi 4500-I apparatus fitted with a field emission gun (SEM-FEG) operating at 3 kV (low accelerating voltage) with a spatial resolution of roughly 5 nm. The size of nanoparticles present at the surface of the agglomerates was analyzed. Nitrogen Physisorption Measurements. N2 adsorption measurements were undertaken to get information about the morphology of materials. Adsorption-desorption isotherms were acquired at 77 K using an ASAP 2000 instrument from Micromeritics. A powdered sample of mass around 150 mg was evacuated overnight at 150 °C before adsorption. The total pore volume, Vporous, was calculated from the volume of nitrogen adsorbed at relative pressure P/P° ) 0.99. The specific surface area SBET was calculated according to the Brunauer-Emmett-Teller (BET) method applied in the P/P° (0.06-0.25) range, using the usual value of 0.162 nm2 for the molecular area of N2. The pore size distribution was obtained taking the Barrett-Joyner-Halenda (BJH) method using the adsorption branch.30 Infrared Spectroscopy. Infrared spectra were acquired (i) to identify the different bonds present in fluorinated silica, (ii) to quantify the water and hydroxyl group contents, (iii) to characterize the accessibility of Si-OH groups, and (iv) to estimate the thermal stability of hydroxyl groups and water trapped in the network.31 A powdered sample was pressed (∼107 Pa) into a self-supported disk (2 cm2 area). It was placed into a quartz cell equipped with KBr windows. A movable quartz sample holder allowed adjustment of the pellet in the infrared beam for spectra recording and displacement into a furnace at the top of the cell for outgassing. The cell was connected to a vacuum line for evacuation (Presidual≈ 10-4 Pa) and introduction of heavy water reactant. A Nicolet Nexus IR spectrometer equipped with an MCTA detector and an extended KBr beam splitter was used for the acquisition of spectra recorded at room temperature in the 600-5500 cm-1 range. The resolution of the spectra was 4 cm-1, and 256 scans were accumulated for each spectrum. 3. Results and Discussion 3.1. Characteristics of Pristine Silica. The final applications of silica are indeed dependent on the morphology, structure, and chemical bonding of the pristine material. Information about the surface organization of the starting commercial mesoporous silica gel, the powder morphology (particle size, surface area), and the nature and content of hydroxyl groups was acquired. High-resolution SEM analysis of the pristine silica revealed the occurrence of nanoparticles around 10-20 nm in diameter, packed in uncircular agglomerates between 100 and 200 µm (Figure 1a). It should be noted that the 500 m2 g-1 surface area specified by the supplier is in agreement with the measured SBET. IR spectra of starting silica before and after evacuation under vacuum at 150 °C are presented in Figure 2. Such a ν(OH) pattern is characteristic of silica having a heavily hydroxylated surface.31,32 The sharp ν(OH) band at 3737 cm-1 corresponds to isolated Si-OH, whereas the broad band centered at about 3550 cm-1 reveals the silanol H-bonded in a chain.33 Two others minor features evidence ν(OH) bands of terminal (3715 cm-1) and internal (3660 cm-1) Si-OH groups. The latter species is inaccessible to D2O because it is located in small cavities or in

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Figure 1. SEM photographs of pristine silica (a) and silica fluorinated at room temperature, for two hours, and with various F2 dilutions: 10 (b), 20 (c), and 50% (d).

Figure 3. Thermogravimetric analysis of pristine mesoporous silica up to 900 °C under Ar flow. Figure 2. Infrared spectra (transmission mode) of pristine silica in the 1400-1600 cm-1 range with possible SiOH groups. Dotted line: Spectrum recorded at room atmosphere. Full line: Spectrum recorded after outgassing the sample under vacuum for 3 h at 100 °C.

narrow interspace between grains. A schematic description of species is drawn in Figure 2. Infrared spectroscopy cannot differentiate isolated and geminated hydroxyl groups. Valuable information regarding these latter species will be described in a forthcoming article using the MAS NMR technique. The thermal stability of hydroxyl groups described above has been followed by thermogravimetric analysis up to 900 °C (Figure 3). The total weight loss of the pristine silica is about 9.2 wt %. A rapid weight loss occurring as soon as the temperature rises (30-100 °C) corresponds to the removal of physically absorbed water molecules from the silica surface.

The weight loss following the plateau observed around 150 °C is attributed to the condensation of H-bonded silanol groups into siloxane bonds with the subsequent elimination of water. At highest temperatures, the weight loss corresponds to the elimination of isolated Si-OH.6,34 3.2. Direct F2 Gas Fluorination of Silica. Fluorine has been brought in contact with the silica via a gas-solid method using F2 gas. This route was preferred to the conventional HF liquid one because of high reactivity of F2 (i.e., weak binding energy ∆HF2 ) 155 kJ mol-1). Conversely, HF exhibits a higher binding energy (∆HHF ) 565 kJ mol-1) and, furthermore, easily decomposes silica into SiF4 gas by an etching phenomena (SiO2 + 4HF f SiF4 + 2H2O). To limit an increase of temperature during the exothermic fluorination process, fluorine was diluted in a neutral argon carrier. 3.2.1. Quantification of Grafted Fluorine. Surface-grafted fluorine content was evaluated by XPS and for the global rate

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TABLE 1: Synthesis Conditions and Elemental Analysis of Fluorinated Silicaa chemical composition measured by experimental conditions sample notation SiO2 10F2-25 20F2-25 30F2-25 40F2-25 50F2-25 60F2-25 70F2-25 80F2-25 10F2-100 20F2-100 30F2-100 40F2-100 50F2-100

fluorination temp (°C)

25

100

XPS

bulk F titration

F2 (vol %)

F (at %) ((0.5)

F/Si ((0.04)

O/Si ((0.2)

F (wt %) ((0.02)

F/Si ((0.01)

(F/Si)bulk/(F/Si)XPS ratio ((0.05)

0 10 20 30 40 50 60 70 80 10 20 30 40 50

0.0 8.9 9.8 11.5 13.3 12.1 13.7 14.1 14.3 10.1 12.5 13.8 14.3 15.2

0.00 0.30 0.33 0.37 0.41 0.41 0.48 0.48 0.48 0.34 0.42 0.48 0.48 0.51

2.1 2.0 1.9 1.8 1.6 1.9 1.9 1.8 1.8 2.0 1.9 1.9 1.8 1.8

0.00 3.64 6.81 7.67 8.46 7.99 12.69 11.71 12.95 7.54 10.99 9.67 9.70 10.51

0.00 0.13 0.24 0.26 0.29 0.27 0.44 0.40 0.45 0.25 0.38 0.33 0.33 0.36

0.43 0.73 0.70 0.71 0.66 0.92 0.83 0.94 0.74 0.90 0.69 0.69 0.71

a Influence of the F2 dilution and fluorination temperature on chemical compositions of surface (XPS) and bulk (F-titration by specific electrode). The fluorination duration was two hours for all samples.

Figure 4. Comparison of surface (2 F2-100 °C XPS, [ F2-25 °C XPS) and bulk (∆ F2-100 °C global F-titration, ] F2-25 °C global F-titration) F/Si atomic ratios in fluorinated silica versus F2 dilution. The fluorination has been conducted during two hours at room temperature for all samples.

by elemental analysis (called F-titration throughout this article). The F2 percentage in the gaseous mixture induces significant changes in the grafted fluorine content. The results of fluorinations conducted at 25 and 100 °C are given in Table 1 and illustrated in Figure 4 as F/Si atomic ratios calculated from the titration results. As observed in most gas-solid reactions, the grafted fluorine content is clearly higher in the superficial layer of primary nanoparticles than in the bulk. The systematic difference between XPS and F-titration values observed for low F2 content in the fluorination gas (F2% < 50%) corresponds to a ratio (F/Si)bulk/(F/Si)XPS close to 0.7. However, considering the small particle size (10-20-nm diameter) and the analysis depth of the XPS technique (5-10 nm), one should have to notice that the XPS analysis account for averaged values intermediate between the hydroxyl-rich surface and the core of a nanoparticle. As expected, the fluorine grafting increases with increasing F2 content in the fluorinating mixture. A rapid ascent below F2% ) 10% is followed by a slight increase in both XPS and total fluorine content up to F2% ) 50%. For samples fluorinated at 25 °C, a gap in the F-content is noticed between 50 and 60% more clearly in the case of F-titration than for XPS analysis. The two curves tend to merge to a saturation F-content (F/Si ) about 0.45) for highest F2 contents in the reaction gas,

which corresponds indeed to a deeper fluorination of the agglomerates. Because of the fair reproducibility of the results, the fluorine content can be controlled with F at % varying between 0 and 15% (i.e., for F/Si values between 0 and 0.5). The rather close values obtained for higher fluorination levels on both surface and bulk bring to the fore that the fluorination proceeds thoroughly within the agglomerates. Only one experimental point, corresponding to a fluorination procedure involving 20% F2 in Ar carried out at T ) 100 °C, has an unusual variation for which the F-titration and F-XPS contents are almost identical (Table 1). Finally, the fair agreement between the variation of F/Si atomic ratios determined by XPS (surface) and F-titration (bulk) shows that F2 direct fluorination is homogeneous. It should be added that experiences conducted for different fluorination times (up to 18 h) gave similar F/Si values whatever the duration, at least for low F2/Ar ratios. For example, two experiments undertaken for 0.5 and 2 h with 10% F2 in Ar at room temperature exhibit F/Si (XPS) ratios of 0.31 and 0.30, respectively. It means that the reaction takes place rapidly and no kinetic limitation has to be considered in this duration range. In this study, samples were fluorinated for two hours at room temperature (RT) with F2% in the gas mixture lower than 50% to ensure a complete reaction. 3.2.2. Effect of Hydroxyl Groups on the Fluorination Process. The key role of hydroxyl groups was established by modulating their number on the surface. For this purpose, pristine silica was annealed at various temperatures (200, 400, and 600 °C) under argon atmosphere before the fluorination treatment. The analysis results are given in Table 2, and the evolution of F/Si ratios is illustrated in Figure 5 for samples fluorinated for two hours at room temperature using two different fluorine percentages: 10 and 50% (noted 10F2 and 50F2, respectively). The F/Si ratios determined by XPS measurements are higher than those in the bulk. However, this difference varying between 0.10 and 0.14 is roughly the same as the one previously mentioned in the case of fluorination of silica without pretreatment. More importantly, Figure 5 accounts for a significant reduction of the F/Si ratio as the annealing is performed above T ) 200 °C, which corresponds to the beginning of dehydroxylation. For annealing at T > 200 °C, a significant elimination of hydroxyl groups occurs in starting silica without modification

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TABLE 2: Dependence of the Grafted Fluorine Content on the Annealing Temperature of Silicaa chemical composition measured by experimental conditions

XPS

bulk F titration

annealing temp of silica (°C)

F2 (vol %)

F/Si ((0.04)

O/Si ((0.2)

F/Si ((0.01)

(F/Si)bulk/(F/Si)XPS ratio ((0.05)

25 200 400 600 25 200 400 600

10

0.30 0.30 0.28 0.25 0.41 0.42 0.40 0.27

2.0 2.0 2.1 1.9 1.9 2.1 2.0 2.0

0.13 0.20 0.16 0.06 0.27 0.39 0.29 0.11

0.43 0.66 0.57 0.24 0.66 0.93 0.73 0.41

a

50

Silica samples were fluorinated at room temperature for two hours.

Figure 5. Influence of the thermal pretreatment of silica on the surface ([, 2) and bulk (], ∆) values of F/Si atomic ratios. Samples were fluorinated at room temperature for two hours with 10% F2 and 50% F2, respectively.

of the texture,35 and a large decrease in fluorination level takes place (Figure 5) with a drastic diminution of both surface area and pore volume. These observations confirm that the fluorination reaction can be directly connected to the physisorbed water content as well as to the different types of Si-OH chemical bonds present in the starting material. 3.3. Modification of the Physicochemical Features of Fluorinated Silicas. SEM images are given in Figure 1 for various F2%. Within the SEM resolution range, the morphology of fluorinated silica appears identical to the one of pristine silica. The 10-20-nm nanoparticles constituting the agglomerates are still present after fluorination in those conditions. Nitrogen physisorption allowed investigation of the surface area and pore size distribution of samples fluorinated at 25 °C for two hours. Figure 6 shows N2 adsorption isotherms of pristine and fluorinated silicas. No change in the isotherm type is observed after fluorination, proving that the mesopore system is conserved. Fluorination of a pristine silica with SBET ) 508 m2 g-1 leads to a pronounced decrease of the specific surface area as the fluorine content increases (i.e., down to 273 m2 g-1 for the 80F2-25 sample). Moreover, we should point out that the surface area of fluorinated silica treated with 60% F2/Ar is surprisingly higher than that of the one treated with 50% F2/ Ar, then decreases, showing that a new mechanism takes place at this gas ratio as demonstrated in the forthcoming part. A further decrease in the pore volume from 0.76 to 0.61 cm3 g-1 is observed. This phenomenon can illustrate a compaction of the particles with a gumming effect of the surface asperities by material transport. The BJH pore size distribution suffers only slight modifications. The mean pore diameter, reported as the top of the dV/d

Figure 6. N2 adsorption isotherms of pristine and fluorinated silicas. Full symbols correspond to the adsorption branch and empty symbols to the desorption branch. Samples were fluorinated for two hours at room temperature.

Figure 7. Pore size distribution of pristine and fluorinated silicas as deduced from the adsorption branch (f ) 0) of the N2 isotherms.

log D curve, shifts slightly from 7.8 to 8.4 nm. The prominent feature is the decrease in the volume of the smallest pores with intensive fluorination (Figure 7). We can assume that the pore size measured by nitrogen physisorption corresponds to the intergrain distances between the nanoparticles forming agglomerates imaged by SEM (Figure 1). Table 3 includes weight loss corresponding to water release during the degassing at 150 °C before the isotherm acquisition. For as-prepared fluorinated silica, the weight loss is drastically lowered as the F2 content of the gas mixture increases. Indeed, the fluorination yields a decrease of the physisorbed water amount. Whereas starting silica contains 8 wt % water, for

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TABLE 3: Effect of Fluorination on Some Textural Properties (N2 Adsorption Results): Surface Area, Porous Volume, Pore Size, and Weight Loss at 150 °Ca sample SiO2 10F2-25 20F2-25 50F2-25 60F2-25 70F2-25 80F2-25

SBET porous volume pore size weight loss (m2 g-1) ((1) (cm3 g-1) ((0.01) (nm) (wt %) 508 436 364 300 337 287 273

0.76 0.73 0.71 0.64 0.69 0.61 0.61

7.8 7.8 7.9 8.7 8.3 8.1 8.5

8.0 1.5 1.5 2.0 2.1 2.3 0.9

a As-received silica samples were fluorinated at room temperature for two hours without any pre-annealing.

TABLE 4: Determination by Infrared Spectroscopy of Adsorbed Water in Pristine Silica and in Samples Fluorinated at Room Temperature with Various F2/Ar Molar Ratiosa sample

bulk H2O (wt %)

n H2O (µmol g-1)

n H2O/nm2

SiO2 10F2-25 20F2-25 50F2-25 60F2-25 70F2-25 80F2-25

8.0 2.9 1.0 1.2 0.8 1.2 1.7

4800 1650 550 650 450 650 950

5.7 2.3 0.9 1.3 0.8 1.4 2.1

a

Fluorination conditions as in Table 3.

fluorination carried out with an F2 percentage superior to 20% this amount rapidly decreases to around 1 wt %. The forthcoming determination from the FTIR analysis leads again to an 8.0 wt % value that is in between TGA and FTIR data (Figure 3 and Table 4). Such differences are fairly due to the dependence of water content on atmosphere conditions rather than on the analysis methods. FTIR spectra of pristine and silicas fluorinated at RT for two hours using F2 percentage up to 80% were recorded at room temperature in the 600-5500 cm-1 range. In the IR spectrum of the starting silica (Figure 2), two characteristic bands of water at about 1630 and 5270 cm-1 are assigned to δ(H2O) and ν + δ(H2O), respectively. Knowledge of the integrated molar absorption coefficients of the ν + δ (H2O) band (1.53 cm µmol-1)34 allowed determination of the water concentration for all samples; values are given in Table 4. Although starting silica contains about 8 wt % water, this amount rapidly decreases upon fluorination. Taking into account the specific surface area, the concentration of molecules at the surface (n H2O/nm2) can be calculated (Table 4). The decrease is less abrupt and leads to the conclusion that the fluorination improves the hydrophobic character of silica. Figure 8 shows IR spectra for dried silica after being outgassed under vacuum of 10-4 Pa at 150 °C for 3 h. The intensity of the ν(OH) band of the isolated Si-OH group at 3737 cm-1 becomes weaker upon fluorination, showing the lower concentration of isolated hydroxyl groups. For the highest F contents, the ν(OH) band of isolated silanol group finally vanishes, whereas a small part of one of the H-bonded species (3660 cm-1) persists. Quantification of the dependence of the residual content of surface hydroxyl groups upon the F2 percentage was obtained from the area of the characteristic ν + δ(OH) band of silanol situated in the 4520-4560 cm-1 range (Figures 2 and 8), because its integrated intensity is less sensitive to the type of silanol than the one corresponding to the ν(OH) band.36 An

Figure 8. IR spectra of pristine silica and silicas fluorinated at room temperature for different F2/Ar gas ratios. Full line: Spectra recorded after outgassing the sample under vacuum for 3 h at 150 °C. Dotted line: Spectra recorded after H/D exchange by D2O treatment. Spectra were normalized for 10 mg of sample.

integrated molar absorption coefficient εν+δ(OH) equal to 0.16 cm µmol-1 was taken from ref.34 For such a band located at high wavenumbers, deviation from the Beer-Lambert law was taken into account through evaluation of an enhancement factor due to high diffusion of these samples.36 The silanol contents are reported in Table 5 as the concentration (C in µmol/gram of dry silica) and the number (n/nm2 of dry surface) of OH groups. The concentration of OH groups varies in the following order: SiO2 > 10F2-25 > 20F2-25 g 50F2-25 > 60F2-25. We can conclude that the higher the F content, the lower the OH content. However, for F2 percentages in the gas mixture higher than 20%, the influence of the concentration of fluorine in the gas phase becomes limited. Then the formation of fluorinated surface, which limits the diffusion of reactive gas phase through the outer layer as well as the occurrence of residual Si-OH unaffected by fluorination, can be considered. Differentiation between accessible and inaccessible OH was performed by hydrogen/deuterium (H/D) exchange with D2O. This process can be followed by IR spectroscopy thanks to the downward shift of the stretching mode from 3750 to 3500 cm-1 (νOH) to 2760-2600 cm-1 (νOD). At first, all accessible silanols were exchanged with heavy water vapor several times (in contact with reagent excess, followed by evacuation in vacuum). After each exchange, an IR spectrum was recorded and the process was considered to be completed when there was no difference between two consecutive spectra. Modification of IR spectra for dehydrated samples by D2O treatment is illustrated in Figure 8, and the quantification of OH is given in Table 5. Dehydrated pristine silica shows a strong decrease of intensity in the νOH range and a subsequent

Access to Fluorinated Silica by Direct F2 Fluorination

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TABLE 5: Determination of Silanol Content from Infrared Dataa COH total µmol/g of dry silica

OHtotal per Si

nOH total/nm2

sample

of silica

COH acc. D2O µmol/g of dry silica

OHacc per Si

nOH acc. D2O/nm2

of silica

fraction of SiOH acc. D2O

OHinternal per Si

SiO2 10F2-25 20F2-25 50F2-25 60F2-25 70F2-25 80F2-25

6550 4150 2800 2650 1400 2050 2000

0.42 0.27 0.18 0.17 0.09 0.13 0.13

7.8 5.7 4.6 5.3 2.5 4.3 4.4

5150 2750 1500 1350 400 950 850

0.33 0.18 0.10 0.09 0.03 0.06 0.06

6.1 3.8 2.4 2.7 0.7 2.0 1.9

0.78 0.67 0.53 0.52 0.27 0.47 0.42

0.09 0.09 0.09 0.08 0.07 0.07 0.08

a C and n are considered concentration and site number, respectively. Calculations were based on the following chemical formula: SiO2-x-y(OH)2xF2y. Fluorination conditions as in Table 3.

Figure 9. Infrared spectra of silica samples diluted in KBr (1 mg of sample in 100 mg of KBr).

formation of a νOD band in the 2750-2600 cm-1 range. In addition, the similar profiles of the ν(OH) and ν(OD) bands before and after H/D exchange, respectively, indicate that no supplementary hydroxyl groups are formed by the heavy water treatment. After D2O treatment, a broad maximum centered at 3660 cm-1 remains. Such a band reveals unaffected internal Si-OH groups located in the interparticle space or at inner surfaces. Furthermore, the intensity of the 3660 cm-1 band of the internal Si-OH is not affected by F2 treatment. The content of internal Si-OH remains constant at about 1600 µmol g-1, and the OH/Si ratio (Table 5) remains equal to 0.09, whatever the fluorination treatment at room temperature. Several significant bands of low intensity appearing in the 600-1400 cm-1 range are illustrated in Figure 9.37 On the pristine spectra, the band at 975 cm-1 corresponds to a νSi-OH vibration mode and the bands around 1100 and 800 cm-1 are related to two Si-O-Si stretching modes of silica network.38 On the fluorinated materials, the pristine Si-OH band diminishes and two absorption bands already reported in the literature11 grow at its expanse. The 980 cm-1 band was tentatively attributed to O3/2SiOH, O2/2SiF2, and O1/2SiF3 tetrahedral species. The band at 935 cm-1 indicates the replacement of O3/2SiOH species by O3/2SiF ones. Moreover, a band at 746 cm-1 appears immediately at the lowest F content. Controversial interpretations11 exist. Most authors mention that this band is due to octahedral species with general formula [F6-nSi(OH)n]2-, but our recent (29Si,19F) MAS NMR investigations39 do not reveal

Figure 10. (a) Variation of the total ([) and accessible (]) silanol contents (deduced from IR experiments) versus the bulk grafted fluorine content (quantified by global titration) for various F2 dilution in the fluorination gas. (b) Variation of the F/Si ratio obtained from the global F-titration ([) and of the area of the IR 935 cm-1 band taken as an indicator of the Si-F bond content (∆) versus the F2 concentration in the fluorination gas. The drawn dotted line is a naive picture of the occurrence of the reaction path (1) associated to the redox process involving F2/F- and O2/OH- couples.

the kind of species that are indeed unstable on mesoporous silica treated under F2 gas. Then taking into account the nearest Si-O-Si stretching mode band, the new band at 746 cm-1 could be due to O4/2SiF species intermediate between tetrahedral and octahedral entities or could correspond to another Si-O-Si stretching mode for which the bond is relaxed because the occurrence of strongly polarized Si-F bonds in its vicinity induces a decrease of the wavenumber. Figure 10a clearly illustrates the relationship between the decrease of silanol content and the increase of the grafted fluorine content obtained from bulk F-titration. At low fluorine content, a linear relationship between silanol consumption and the amount of grafted fluorine is observed. Therefore, direct

10950 J. Phys. Chem. C, Vol. 112, No. 29, 2008 TABLE 6: Chemical Compositions of Bulk Pristine and Some Fluorinated Silicas As Deduced from FTIR and F-Titration Dataa sample

chemical compositions SiO2-x-y(OH)2xF2y

bulk fluorine F-titration (wt %)

SiO2 10F2-25 20F2-25 50F2-25 60F2-25 70F2-25 80F2-25

SiO1.79(OH)0.42F0.00 SiO1.80(OH)0.27F0.12 SiO1.79(OH)0.18F0.23 SiO1.78(OH)0.17F0.27 SiO1.74(OH)0.09F0.43 SiO1.73(OH)0.13F0.40 SiO1.71(OH)0.13F0.44

0.00 3.64 6.81 7.99 12.69 11.71 12.95

a

Fluorination conditions as in Table 3.

replacement of a hydroxyl group by a fluoride takes place at the beginning of the fluorination, and the key role of hydroxyl groups in the fluorination mechanism is then evidenced. At high fluorine contents, the amount of grafted fluorine reaches an upper value about 6500 µmol g-1 while, as noted previously, internal silanols remain unreacted. This suggests that for high F2 gas concentration a second reaction path involving the opening of Si-O-Si siloxane bridges occurs. Table 6 gives estimates of the chemical compositions SiO2-x-y(OH)2xF2y of fluorinated silica deduced for each sample from F-titration and FTIR data, respectively. Such formulation clearly accounts for the occurrence of higher rates of grafted fluorine for lower hydroxyl group contents. It should be pointed out that the amount of grafted fluorine is very high when compared with conventional methods ranging from 3.6 to 12.7 wt % for F2/Ar ratios varying from 10/90 to 60/40. The high fluorination level is obviously in a straight line with the initial high content of hydroxyl due to the nanoparticles morphology of the used silica. The area of the Si-F band at 935 cm-1 allows us to follow the increasing number of Si-F groups in the fluorinated silica. In Figure 10b, this area and the F/Si obtained from F-titration are drawn versus the F2 percentage in the fluorination gas. Both follow a similar trend, exhibiting a knee at nearby F2% ) 50. Considering the F/Si atomic ratio determined by F-titration, we should point out that, for F2/Ar ratios lower than 50/50, the F + OH sum lies always around 0.4. For instance, for 10F2-25 the sum is equal to 0.41, thus confirming the partial substitution of OH groups by fluoride, as shown in Table 6. For higher F2 percentage, the fact that this sum is superior to 0.5 confirms also that siloxane bridges have been partially opened and that one Si-O-Si bridge has been replaced by two terminal Si-F bonds. 3.4. Elemental F2 Reaction on Mesoporous Silica: A Strong Oxidizing Treatment. Observations, especially on the basis of Figure 10, may be explained through the hypothesis of two reactions paths. The first one operates yet at low F2 concentration and involves the formation of Si-F bonds at the expanse of silanol groups and appears to be nearly achieved when F2 ) 20%. The second reaction is proposed to start only when the F2 concentration in the gas phase is higher, starting from F2 > 50%. Probably the two reaction paths compete but in both mechanisms the formation of HF can not be excluded. The second reaction with more demanding breakdown of internal siloxane bridge will take place only at high fluorine concentration and is probably kinetically or thermodynamically activated. From a thermodynamic point of view, one can assume that the first reaction could be associated with a redox process involving the two half-redox equilibria with F2, O2, and H2O

Lataste et al. as gas phases as well as silanol and Si-F chemical bondings in solid-state phases. The elimination of physisorbed water trapped in silica and the occurrence of high hydroxyl content and F2 concentration contribute to a right shift of the first reaction and the creation of Si-F chemical bonds. Thus, a fluorination treatment at room temperature with 50% F2/Ar of silica outgassed at T ) 200 °C, or the fluorination at T ) 100 °C with 20% F2/Ar of not-outgassed as-received silica, yields a maximum content of grafted fluorine (F/Si ) 0.39). This first process could be thus associated with an exothermic redox reaction involving F2 gas and O2 in addition with H2O gas evolution. This result differs from what generally was observed with aqueous organic fluorination routes which imply in a simple manner SiO2 and H2SiF6 as well as Brønsted acid/base equilibrium. Considering this redox process, it is then clear why the elemental F2 fluorination allows control of the F/Si atomic ratio by decreasing the water content or increasing the OH concentration, the OH- groups being systematically removed, leading probably to the O2 evolution. However, it has been pointed out in the Experimental Section that, under fluorinated atmospheres, the decomposition of silica into SiF4 could not be totally ignored. In particular, it could be assumed that, in highly reactive conditions (high F2 concentration), the formation of volatile SiF4 occurs in more noticeable amounts. The subsequent volatilization of this gaseous species breaks numerous siloxane bridges, thus opening new paths to the core of the agglomerates. The created anionic vacancies are immediately filled with fluorine, which is present in large amounts in the reactive atmosphere, giving rise to very high levels of bulk fluorination, as shown for instance by the values of F/Si rates: 12-13 wt % F observed for 60-80% F2 atmospheres, even at room temperature (Table 1). This behavior is also clearly illustrated by the drastic change in the slope of bulk F/Si ratio starting from 50% F2 in Figure 10. 4. Conclusions The present work has demonstrated that large contents of fluorine (i.e., from 4 to 13 wt %) can be substituted for hydroxyl groups present in mesoporous silica gel by suitable fluorination treatments using elemental F2. Pretreatments of starting silica-outgassing or annealing at higher temperature under Ar, which lead either to removal of water (T ) 200 °C) or to largely decreasing the hydroxyl content (T ) 400-600 °C)-allow the amount of Si-F grafting during the fluorination process to be tuned. The other key parameters of this synthesis route are the fluorination temperature (25 and 100 °C) and the F2/Ar rate in the gas mixture. However, whatever the synthesis conditions, an amount of 0.11 OH groups per Si atom, nonaccessible to elemental F2 and corresponding to internal hydroxyls mainly located in the silica framework, is systematically present whatever the fluorination treatment. The chemical composition identified on the basis of F-titration and FTIR analysis, SiO2-x-y(OH)2xF2y, varies from SiO1.75OH0.49 to SiO1.75(OH)0.23F0.27 (50% F2/Ar) and SiO1.70(OH)0.15F0.44 (60% F2/ Ar) compositions. Despite the reduction of surface area after the fluorination treatment, the mesoporous nature of the material remains almost identical. The F content is higher at the surface of the agglomerates of the mesoporous silica than in the bulk, and this difference is governed by the surface area showing that F2 direct fluorination proceeds mainly at the surface. However, for specific pretreatment temperatures as well as fluorination temperatures, bulk and surface F/Si atomic ratios tend almost to be the same value, demonstrating that the F2 direct route leads also to rather homogeneous materials. For instance, if the fluorination

Access to Fluorinated Silica by Direct F2 Fluorination treatment occurs at T ) 100 °C, or if silica is pretreated at T ) 200 °C followed by a direct fluorination at RT with 50% F2/Ar, the Si-F species are equally distributed between surface and bulk. Actually, the limitation of physisorbed and structural water trapped into mesoporous silica, as well as the occurrence of a large amount of isolated, terminal, and bound OH groups, allows an increase in the number of Si-F chemical bonds. Taking into account a redox process involving O2/OH- and F2/F- couples, the Le Chatelier law perfectly accounts for the key role of H2O and OH- and for the large content of stable Si-F bonds obtained through this process. This redox mechanism proceeded up to 50% F2/Ar gas mixture; for higher F2 gas contents in the gaseous mixture, the SiO2 etching takes place with a partial breaking of siloxane bridges and becomes dominant. In this article, we have pointed out the key role of OH-silanols for F incorporation into mesoporous silica. However, several questions remain, and we will report some answers in a forthcoming article dealing with the chemical and thermal stabilities of fluorinated silica, as well as the identification of the local Si environments by (1H, 19F, 29Si) MAS NMR investigations.39 The first question concerns the thermal stability of the fluorinated silicas, their affinity with water, and the hydrophobic character of highly fluorinated materials. The second question turns around the nature of Si-F chemical bonds involved in tetrahedral coordination or even more complex environments between tetrahedral and octahedral [SiF6]2- species. Finally, highly fluorinated silica could produce Brønsted acidity in the presence of water as mentioned in the literature, and these materials are potential catalysts for various reactions. Moreover, the Al3+ partial substitution for Si4+ would also induce some Lewis acidity, and the combination between both behaviors should strongly affect the reactivity of these solids. Acknowledgment. We gratefully acknowledge the European Community for the financial support in the STREP FUNFLUOS (FUNctionalized FLUOrideS) network (NMP3-CT-2004-505575). We also thank Dr. J. P. Gallas for fruitful discussions. References and Notes (1) Barly, D. Silicas. In Characterization of Powder Silicas: With Special Reference to Pigments and Fillers; Parfitt, G. D., Sing, K. S. W., Eds.; Academic Press: New York, 1976. (2) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; Wiley-Interscience: New York, 1979. (3) Bossaert, W. D.; De Vosq, D. E.; Van Rhijn, W. M.; Bullen, J.; Grobet, P. J.; Jacobs, P. A. J. Catal. 1999, 182, 156. (4) Chapman, I. D.; Hair, M. L. J. Catal. 1963, 2, 145. (5) Duke, C. V. A.; Miller, J. M.; Clark, J. H.; Kybett, A. P. Spectrochim. Acta, Part A 1990, 46, 1381. (6) Branda, M. M.; Montani, R. A.; Castellani, N. J. Surf. Sci. 2000, 446, L89.

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