Highly Fluorinated Silica Obtained by Direct F2-Gas Fluorination

Oct 7, 2009 - CNRS FR 2575, UniVersité du Maine, AVenue OliVier Messiaen, 72085 Le Mans Cedex 9, France, and. Laboratoire de Physique de l'Etat ...
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18652

J. Phys. Chem. C 2009, 113, 18652–18660

Highly Fluorinated Silica Obtained by Direct F2-Gas Fluorination: Stability and Unprecedented Fluorosilicate Species Revealed by Solid State NMR Investigations Emilie Lataste,† Christophe Legein,*,§ Monique Body,‡ Jean-Yves Buzare´,‡ Alain Tressaud,† and Alain Demourgues*,† ICMCB-CNRS, 87 AVenue du Dr. Albert Schweitzer, 33608 Pessac Cedex, France, Laboratoire des Oxydes et Fluorures, CNRS UMR 6010, Institut de Recherche en Inge´nierie Mole´culaire et Mate´riaux Fonctionnels, CNRS FR 2575, UniVersite´ du Maine, AVenue OliVier Messiaen, 72085 Le Mans Cedex 9, France, and Laboratoire de Physique de l’Etat Condense´, CNRS UMR 6087, Institut de Recherche en Inge´nierie Mole´culaire et Mate´riaux Fonctionnels, CNRS FR 2575, UniVersite´ du Maine, AVenue OliVier Messiaen, 72085 Le Mans Cedex 9, France ReceiVed: March 2, 2009

The direct F2-gas fluorination of mesoporous silica is a unique method leading to high fluorinated (up to 13 wt % F) and homogeneous powder with a controlled 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 °C whereas high fluorine-grafted silicas start to decompose from 250 °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 19F and 1H MAS, and 1H-29Si and 19F-29Si 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 (δiso(19F) ) -143.5 and -136.5 ppm; δiso(29Si) ) -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 F2-gas fluorination, should explain the thermal stability as well as the water affinity of fluorinated silica. 1. Introduction Numerous applications of oxides are related to hydrophobichydrophilic balance and to surface reactivity which obviously depend on the surface chemistry. Mesoporous silica has been largely investigated in this field of research. The F- ions, generating ionic bonds, have been largely used to modify the surface chemistry of silica.1-6 Fluorination is considered to be an important treatment for catalysts to improve their catalytic activity. It is known to modify surface acidity and subsequently the reactivity. For example, fluorinated silica has a higher cumene cracking rate than nonfluorinated silica.7 Recently, F2-gas has been employed for the first time to obtain highly fluorinated silica. The following is a summary of results already published.8 High contents of fluorine were reached (up to 13 wt % F in the bulk). Elemental analysis and FTIR data allowed the determination of the bulk composition of fluorinated silica. Gradual variations of F and OH contents in the SiO2-x-y(OH)2xF2y samples as a function of fluorination conditions were found with 0.065 e x e 0.21 and 0 e y e 0.22 (Table 1). An amount of 0.11 OH groups per Si atom, nonaccessible to elemental F2, is systematically present whatever the fluorination treatment. These species correspond to internal * To whom correspondence should be addressed. E-mail: demourg@ icmcb-bordeaux.cnrs.fr and [email protected]. † ICMCB-CNRS. § Laboratoire des Oxydes et Fluorures, Institut de Recherche en Inge´nierie Mole´culaire et Mate´riaux Fonctionnels. ‡ Laboratoire de Physique de l’Etat Condense´, Institut de Recherche en Inge´nierie Mole´culaire et Mate´riaux Fonctionnels.

TABLE 1: Synthesis Conditions (F2/Ar Ratio),a Chemical Formula As Deduced from FTIR and F-Titration data, Surface Area (SBET in m2 · g-1), and Percent Weight of Fluorine in Pristine and Fluorinated Silica8 samples

F2/Ar (%)

formula

SBET ((1)

% wt F

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

0 10 20 50 60 70 80

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)0.13F0.43 SiO1.71(OH)0.13F0.44

508 436 364 300 337 287 273

0 3.64 6.81 7.99 12.69 11.71 12.95

a Fluorination was carried out for 2 h at room temperature for all samples.

hydroxyls, connected (or not) through H bonding, and are mainly located in the silica framework. The fluorination reaction has been explained in terms of an F/OH substitution as well as an etching phenomenon with the opening of siloxane bonds. The grafted amount depends on the F2 concentration in the gas mixture, the content of the silanols, and finally the physisorbed water trapped on pristine silica. Redox processes involving F2/ F- and O2/OH- couples explain the wide range of F grafting without formation of fluosilicate species SiF62-. At low fluorine contents, a linear relationship between silanol consumption and the amount of grafted fluorine obtained from bulk F-titration was observed. Actually, for F2/Ar ratios lower than 50/50, the F + OH sum lies always around 0.4 (Table 1). For instance, for the sample treated with 10% F2 at room temperature, the

10.1021/jp9087628 CCC: $40.75  2009 American Chemical Society Published on Web 10/07/2009

Stability and NMR of Highly Fluorinated Silica sum is equal to 0.39, thus confirming the partial substitution of fluoride ion for OH- groups. Direct replacement of a hydroxyl group by a fluoride ion took place and the key role of hydroxyl groups in the fluorination mechanism was then evidenced. For higher F2 percentages, the amount of grafted fluorine reached an upper value. The fact that the F + OH sum is larger than 0.5 shows that siloxane bridges Si-O-Si have been partially opened and replaced by terminal Si-F bonds. The subsequent volatilization of SiF4 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 values of F contents as high as 13 wt % observed for 60-80% F2 atmospheres, even at room temperature (Table 1). However this etching phenomenon cannot be ruled out to occur besides the F/OH substitution, whatever the F2 concentration in the gas mixture. In the preceding paper, FTIR signals were tentatively assigned to tetrahedral species such as O3/2SiOH, O3/2SiF, O2/2SiF2, and O1/2SiF3, as well as pentahedral species O4/2SiF. Solid state nuclear magnetic resonance (NMR) spectroscopy is essential to investigate the pristine silica and the changes that occur upon fluorination. For nuclear spins 1/2 like 1H, 19F, and 29 Si, the species are identified through the position of their NMR signals on the spectrum. Nevertheless, cross-polarization9,10 (CP) is routinely used in NMR of spin I ) 1/2 as a means of enhancing sensitivity by magnetization transfer between dipolar coupled abundant (1H and 19F) and diluted nuclei (29Si). The CP technique is furthermore a valuable spectral editing tool that can be used to determine heteronuclear spatial proximities. Therefore in this paper, 1H and 19F magic angle spinning (MAS) and 1H-29Si and 19F-29Si CP-MAS NMR experiments have been performed to probe the local environments of H, F, and Si atoms. In addition, the chemical (water uptake) and thermal stabilities of these highly fluorinated silica have been investigated. 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 fluorination at room temperature, using a F2/Ar gas mixture with various ratios. The dilution of F2-gas (Comurhex) in Ar was set between 10% and 80%. The reactions were carried out below 1 bar, over 2 h. The starting material was used as received or annealed at various temperatures under argon before fluorination experiments (F2/Ar ratios equal to 10/90 and 50/50) to remove water (T ) 200 °C) and a portion of the silanol groups (T ) 400 and 600 °C). Fluorination experiments, elemental analysis (F titration and XPS), as well as morphological characterizations and FTIR measurements have been described in detail previously.8 The corresponding results are summarized in Table 1 for pristine silica and as generated fluorinated samples. The nomenclature for the samples is xF2 for the x percentage of F2-gas in Ar at room temperature and xF2-T for the x percentage of F2-gas in Ar with previous annealing of silica gel at T °C. 2.2. Physicochemical Characterization. 2.2.1. Thermal Stability. A SETARAM symmetric thermoanalyser 2400 Skimmer system equipped with a quadrupole mass spectrometer BALZERS Thermostar GSD 300 was used for recording the thermoanalytical curves (T, TG, DTG) together with the ionic current (IC) curves in the multiple ion detection (MID) mode.

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18653 A TG sample carrier system with corundum plate crucibles (17 mm diameter) and Pt/PtRh10 thermocouples was used for the pulsed experiments. A constant purge gas flow of 180 mL · min-1 argon and a constant heating rate of 3 deg · min-1 were applied to samples of about 15 mg. For thermoanalytical measurements uncoupled with mass spectroscopy, a simpler SETARAM MTB 10-8 system was used. The experiments were carried out under N2 in corundum crucibles containing around 40 mg of sample with a constant heating rate of 3 deg · min-1 up to 900 °C. 2.2.2. Water Uptake. The hydrophobic character of fluorinated silica was estimated by water adsorption.11 Around 250 mg of pristine silica and F-silica material were set in an atmosphere with a 75% controlled humidity level obtained with a NaCl saturated solution. The evolution of water uptake was obtained by weighting the amount of water adsorbed at room temperature. 2.2.3. Solid-State NMR. All the solid state NMR experiments were performed with an Avance 300 Bruker spectrometer (magnetic field of 7 T) operating at Larmor frequencies of 300.1, 282.2, and 59.62 MHz for 1H, 19F, and 29Si, respectively. 1H and 29Si chemical shifts were referenced to tetramethylsilane (TMS) and 19F chemical shifts were referenced to CFCl3. 1 H and 19F MAS NMR spectra were recorded at spinning frequencies of 15 and 25 kHz, using 1H/X and 19F/X probe heads with 4 and 2.5 mm diameter ZrO2 rotors, respectively, and employing the Hahn echo sequence in order to remove the probe signal (to avoid baseline distortion). The 19F Hahn-echo spectra were acquired in 256 scans, using a 2.5 µs 90° pulse and an interpulse delay synchronized with the rotor period. The recycle delay was taken to 10 s. The 1H Hahn-echo spectra were acquired in 512 scans, using a 3.75 µs 90° pulse and an interpulse delay synchronized with the rotor period. The recycle delay was taken equal to 1 s. 1 H-29Si CP-MAS NMR spectra were acquired, under 1H decoupling, at a spinning frequency of 5 kHz, using a double bearing probehead with a 4 mm diameter ZrO2 rotor, a square amplitude sequence, a 5 ms contact time, and a repetition delay of 5 s. The numbers of transients range from 4096 to 32768. 19 F-29Si CP-MAS NMR spectra were acquired, under 19F decoupling, with the same experimental parameters except for a repetition delay of 10 s and contact times which were chosen equal to 0.5 and 5 ms successively. The number of transients is 8192 for all the 19F-29Si CP-MAS NMR spectra. As-received and fluorinated silica (10F2, 50F2, and 60F2) samples were studied by solid state NMR. Only the most significant spectra are shown in the following. They were reconstructed with use of the DMFit software.12 3. Results and Discussions 3.1. Stability of Fluorinated Silica. 3.1.1. Thermal Stability. A thermogravimetric analysis (TGA) has been undertaken on fluorinated silica samples to estimate their thermal stability and to compare it to those of nonfluorinated samples. This technique has been coupled with a mass spectrometer in order to determine the various species released at critical temperatures and to identify the hydroxyl sites concerned by fluorination. Figure 1 illustrates the weight loss of fluorinated compounds measured by TGA during a heating cycle from 25 to 900 °C under argon at 5 deg/min-1. Figure 2 illustrates the ion current corresponding to masses m ) 18, 19, 85, 86, and 104 correlated to the departure of H2O, F, and SiF4 (three fragments), respectively.13 The comparison of these TGA curves with that of initial silica shows that fluorinated silica contains less water than pristine silica (weak loss between 25 and 100 °C), proving

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Figure 1. Thermal stability of pristine and various fluorinated silica measured by TGA up to 900 °C.

that the quantity of water physically absorbed is substantially lowered by fluorine introduction. The hydroxyl loss between 100 and 250 °C is almost the same for starting silica and

Lataste et al. fluorinated silicas. The first effect of fluorination appears at 250 °C when the curve of silica fluorinated with an F2/Ar g 20% gas mixture exhibits a strong weight loss, correlated to the departure of fluorinated fragments in the mass spectrometry analysis (Figure 2b). At 450 °C this phenomenon is slightly increased and results in a higher weight loss in the TGA curve (Figure 1) and a higher ion current on the mass spectrometry curve (Figure 2b). The fluorination by F2 leads to a significant fluorination that makes it possible to clean the surface of Si-OH groups stable at low temperatures. All the types of hydroxyl seem to be involved in the reaction of fluorination. Furthermore, for the 10F2 sample, between 100 and 550 °C, the TGA curve exhibits exactly the same weight loss as the unfluorinated silica indicating that the fluorinated groups are stable up to fairly high temperature with less than 3% weight loss at 550 °C. From 550 °C an important weight loss (Figure 1) occurs followed by a departure of fluorinated fragments13 (Figure 2a), the same as observed at 250 and 450 °C for highly fluorinated samples (Figure 2b). 3.1.2. Aging. Previous studies conducted on fluorinated silica obtained via various synthesis routes pointed out that these materials exhibit some drawbacks like strong sensitivity to moisture and weak durability and thermal stability. The deter-

Figure 2. Thermal stability of fluorinated silicas followed by mass spectrometry for (a) 10F2 and (b) 50F2 samples. Ion current curves for m/z 18, 19, 85, 86, and 104 correspond to the departure of water, fluorine, and SiF4 species under various forms.

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Figure 3. Water affinity for pristine and fluorinated silica at room temperature deduced from weight uptake up to 50 days in an atmosphere containing 75% humidity. Silica samples were fluorinated for 2 h at room temperature for different F2/Ar gaseous ratios varying between 10% and 60%.

mination of the fluorine content in fluorinated silica was undertaken for samples (10F2 and 50F2) stored up to 50 days in an atmosphere containing 75% humidity (saturated NaCl). Surprisingly, the resulting fluorine contents are identical with the initial ones. For instance, the sample 50F2 exhibits an F/Si ratio of 0.40 after 50 days in 75% relative humidity, versus 0.42 just after the synthesis. Moreover, the 10F2 sample exhibits the same F/Si ratio after being stored in laboratory conditions for up to six months after synthesis or after five days under ultra-high vacuum in the XPS chamber. Moreover the other samples with higher F contents exhibit also a rather good stability under ultra-high vacuum in the XPS chamber. These very attractive features illustrate the high fluorine stability in silica fluorinated by direct F2-gas, even in extreme conditions (moisture, ultra-high vacuum). 3.1.3. Water Affinity. The hydrophobic character has been evaluated by a test of water adsorption in a 75% humidity atmosphere. The results obtained for silica fluorinated with various F2/Ar gaseous ratios are illustrated in Figure 3. After 50 days in such atmosphere all fluorinated silicas have a lower water adsorption (between 2% and 13%) than unfluorinated silica (around 30%). This result can easily be correlated to the lower hydroxyl content in fluorinated silica, these groups being at the origin of the surface water adsorption. However, the phenomenon appears more complex when the content of grafted fluorine is considered. Indeed the higher the fluorine content, the higher the water adsorption. For example, 20F2 absorbs around 8% of water, whereas 60F2 holds around 13% of water. Moreover, concerning the 10F2 sample, after a weight increase of 5% up to the seventh day (170 h), a weight loss starts until the end of the measurement (1200 h). This unexpected phenomenon could be explained by the coexistence of two mechanisms involving the water adsorption due to the relative humidity on one hand and the weight loss due to the departure of HF on the other hand. This removal could be compared with the mechanism of HF/H2O chemical etching of SiO2.14 Actually, as far as the F2 treatments with F2 contents higher than 10% are concerned, even if these samples contain a higher F amount and a lower OH content than the 10F2 sample, the uptake due to water is always larger than the HF departure, leading to a monotonous variation of global uptake with time.

Figure 4. Water affinity for silica fluorinated at room temperature deduced from weight uptake up to 50 days in an atmosphere containing 75% humidity in (a) 50% F2/Ar and (b) 10% F2/Ar, either nonannealed (50F2 and 10F2) or preannealed samples at various temperatures: 200, 400, and 600 °C. The weight uptake is measured up to 50 days in an atmosphere containing 75% humidity.

To understand the effect of the surface groups (-OH, -F, water) on the water affinity, the weight uptake of F-silica pretreated at various temperatures has been studied (Figure 4). Annealing temperatures of 200, 400, and 600 °C have been chosen considering the shapes of the TGA curves with respect to the different phenomena occurring during the heat treatment. Concerning the annealed samples followed by fluorination (50F2-T conditions), the water uptake decreases with increasing annealing temperatures without changing the shape of the curve. After 50 days the weight uptake follows the sequence 50F2200 > 50F2 > 50F2-400 > 50F2-600, which is in agreement with the phenomena already described above and the reduction of surface area as a function of annealing temperature. Until 200 °C the removal of the water from the silica surface is the main phenomenon that reduces the hindrance at the silica surface. After this dehydration step, the dehydroxylation becomes the most important process of the sample annealing, reducing the amount of hydroxyl groups content at the silica surface and therefore limiting the fluorine grafting. This point has been developed in our previous paper.8 As far as the slight increase of water uptake in the case of the 50F2-200 sample compared to the initial 50F2 sample is concerned, it is due to the fact that only water is removed from silica during the annealing under Ar at T ) 200 °C without strongly modifying the surface area and porosity. The same trend occurs for samples

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Lataste et al.

Figure 5. 1H MAS (15 kHz) Hahn-echo NMR spectra of pristine (SiO2 sample) and fluorinated silica (10F2 and 50F2 samples). The double arrows indicate the chemical shift ranges of non-hydrogen-bonded silanols, physisorbed water, and hydrogen-bonded silanols.

fluorinated in 10F2 conditions with a lower weight uptake after 50 days. However, the peculiar character of nonannealed silica, fluorinated at room temperature with F2/Ar ) 10%, is again pointed out in Figure 4b, with a weight loss from the seventh day (170 h). In fact this observation can be accounted for by two simultaneous and competitive processes occurring in humid environment: the initial water uptake due to the high humidity level (75%), then the departure of HF molecules at the surface of samples due to the reaction between the absorbed water molecules and some unstable grafted fluorine groups, as will be shown below. Finally for both fluorination treatments the weight uptake of fluorinated silica samples after 50 days lies between 1% and 4% for 10% F2/Ar, and between 1% and 14% for 50% F2/Ar fluorinations, whereas pristine silica absorbs 30% in the same time (Figure 3). It should be pointed out that the weight uptake is as low as ∼1% when silica has been preannealed at 600 °C. 3.2. Determination of Nucleus Environments by Solid State NMR. 3.2.1. Nucleus EnWironment of the Pristine Silica. Surface hydroxyl groups can be classified according to their coordination (single O3/2SiOH and geminal O2/2Si(OH)2 silanols) and their hydrogen bonding (isolated and bound silanols, the latter divided between silanols bridged to other silanols and silanols hydrogen bonded to physisorbed water). Despite severe line broadening due to a combination of strong homonuclear dipolar coupling and small chemical shift dispersion, previous 1 H NMR studies of silicas allowed identification of the resonances of water, bound silanols, and isolated silanols. The 1 H NMR signals ranging from 5 to 3 ppm correspond to physisorbed water as well as to labile rapidly exchanging weakly hydrogen-bonded hydroxyl groups. The broad peak ranging from 8 to 2 ppm corresponds to silanol protons in a variety of hydrogen-bonding environments (the lower the isotropic chemical shift, δiso, the weaker the hydrogen bond) and the sharp peaks ranging from ∼1 to ∼2 ppm were assigned to non-hydrogenbonded silanol.15-29 Moreover, in a recent study on precipitated silica nanoparticles,30 the 1H resonance at 1.1 ppm was tentatively assigned to inaccessible isolated silanol groups. The 1 H MAS spectrum of pristine silica is shown in Figure 5. No annealing was undertaken to remove water. Therefore, the contribution of the absorbed water, at δiso ) 4 ppm, is the largest one. The occurrence of physically absorbed water molecules on the silica surface was shown by their removal, i.e., the weight loss that occurs as soon as the temperature rises (30-100 °C)

Figure 6. Experimental and calculated1H-29Si CP-MAS (5 kHz) NMR spectra of pristine (SiO2 sample) and fluorinated silicas (10F2, 50F2, and 60F2 samples). The numbers of transients are equal to 4096, 16384, 16384, and 32768, respectively. For each calculated spectrum, the individual contributions are shown.

(Figure 1). Owing to this large peak, the hydrogen-bonded silanols, which correspond to a distribution of chemical shifts between 2.5 and 8 ppm, cannot be easily seen. The NMR lines at 1.1 and 2.2 ppm are assigned to non-hydrogen-bonded silanols. The 1H-29Si CP-MAS NMR spectrum (Figure 6) of pristine silica exhibits the well-established three lines previously observed in amorphous silicas.17-20,22,24,29,31-39 The peak at ca. -101 ppm is due to silicon with only one hydroxyl group, O3/2SiOH, often referred to as Q3 (Qn represents the SiO4 tetrahedron of the amorphous network, which forms n bonds with neighboring tetrahedra). The resonance at ca. -111 ppm is assigned to siloxanes, O4/2Si (Q4), whose 29Si nuclei are crosspolarized by nearby protons leading to a resonance on the 1 H-29Si CP-MAS NMR spectrum. The peak at ca. -92 ppm is assigned to silicon atoms having two geminated hydroxyl groups attached: O2/2Si(OH)2 (Q2). The isotropic chemical shifts, the line widths, and the relative intensities of these resonances are gathered in Table 2. Whereas IR spectroscopy cannot differenti-

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TABLE 2: Isotropic Chemical Shifts (δiso, ppm), Line Widths (LW, ppm), and Relative Intensities (Int., %) As Deduced from the Reconstruction of the 1H-29Si CP-MAS NMR Spectra of Pristine and Fluorinated Silica and Line Assignment SiO2

10F2

δiso

LW

Int.

-92.0

7.5

10

-101.0

6.0

50

-111.0

8.0

40

50F2

60F2

δiso

LW

Int.

δiso

LW

Int.

δiso

LW

Int.

assignment

-95.5 -101.5 -106.5 -112.0 -117.5

8.0 6.5 5.5 8.0 4.5

10 45 6 35 4

-96.5 -101.5 -107.0 -112.0 -118.5 -124.5

8.0 7.0 6.5 7.5 7.0 4.5

9 22 8 54 5 1

-96.5 -101.5 -106.5 -112.0 -118.5 -124.5 -132.0

7.5 7.5 6.5 7.5 8.0 7.0 7.5

4 22 7 51 9 3 4

O2/2Si(OH)2 O2/2SiF2 O3/2SiOH O3/2SiF O4/2Si O4/2SiF O3/2SiF2 O2/2SiF3

ate isolated and geminated hydroxyl groups,8 the occurrence of these latter species is clearly evidenced on the 1H-29Si CPMAS NMR spectra. On the other hand, IR spectroscopy allows differentiating isolated, terminal, and internal Si-OH groups.8 These NMR experiments confirm the IR spectroscopy observations concerning the heavily hydroxylated surface of this pristine silica:8 the studied sample presents more Q3 and Q2 species than the silica studied by Hartmeyer et al.29,30 or Barabash et al.35 This may partially explain the high fluorine ratios obtained after fluorination, since a proposed fluorination mechanism consists of a substitution of F- ions for OH- groups.8 3.2.2. Effect of Fluorination on the Nucleus EnWironment. The 1H MAS NMR spectra of 10F2 and 50F2 fluorinated silica are shown in Figure 5. It may be outlined that the spectra of this figure were recorded by using the same scan number and are presented with the same amplitude. The decrease of the signal-to-noise ratio may be related to the decrease of the amount of protons when the F content increases from pristine silica to 50F2 sample. The same trend is observed up to the 60F2 sample (not shown). Moreover, the intensity of the peak at 4 ppm, corresponding to the absorbed water, clearly decreases. As previously shown by IR spectroscopy,8 the amount of water rapidly decreases upon fluorination leading to the conclusion that fluorination improves the hydrophobic character of silica. The 1H MAS NMR spectra also show the decrease of the amount of both the hydrogen-bonded and non-hydrogen-bonded silanols as the amount of grafted fluorine increases (as expected from the corresponding sample formulas given in Table 1). IR spectroscopy, which allows the differentiation of isolated silanol groups from internal (nonaccessible) silanol groups, revealed internal Si-OH groups unaffected by F2 treatment (3660 cm-1 band) whereas the intensity of the ν(OH) band of the isolated Si-OH groups becomes weaker upon fluorination and finally vanishes for the highest F contents.8 From the 1H MAS NMR spectra, the F/OH substitution that occurs during the fluorination process affects both the hydrogen-bonded (from 2.5 to 8 ppm) and non-hydrogen-bonded silanols (at 1.1 and 2.2 ppm). This result is surprising as the 1H resonance at 1.1 ppm was assigned to nonaccessible isolated silanol groups in a recent study on precipitated silica nanoparticles.30 Indeed, the absences of both decomposition of the 1H MAS NMR spectra and calculation of the absolute number of H corresponding to each signal prevent definitive conclusion about this point. Finally, in agreement with the FTIR study and the F-titration data8 (Table 1), silanol species (hydrogen-bonded and non-hydrogen-bonded silanols as shown by NMR, and hydrogen-bonded and internal silanols as evidenced by FTIR) remain even for the highest F contents. The main issue is now to characterize which fluorosilicate species appear upon fluorination. Relevant 29Si and 19F NMR experiments are described in the following. Owing to the low signalto-noise ratio due to the low natural abundance of 29Si, 1H-29Si and 19F-29Si CPMAS NMR experiments have been recorded.

Moreover, it is the only way to give evidence for hydroxysilicate and fluorosilicate species and to differentiate these species. Figure 6 displays the 1H-29Si CP-MAS NMR experimental and reconstructed spectra of fluorinated silica. It may be outlined that the number of transients was increased with the fluorination level in order to keep constant the signal-to-noise ratio. This mirrors the large decrease of the intensity showing the decrease of the silanol content from SiO2 to 60F2 samples in good agreement with the 1H NMR results. Even at the lowest fluorine content (10F2), the Q2 geminated hydroxyl groups (ca. -92 ppm) disappear. On the contrary, the Q3 species (ca. -101 ppm) are still present but their relative intensity decreases when the F content increases. These data indicate the lower stability of the Q2 species upon fluorination. The reconstruction of the spectrum of the 10F2 sample was achieved with three extra NMR lines at ca. -95, ca. -106, and ca. -118 ppm (Figure 6 and Table 2). According to Hartmeyer et al.,29 who studied silica fluorinated by aqueous NH4F solution, these three lines correspond to O2/2SiF2, O3/2SiF species, and pentacoordinated O4/2SiF groups, respectively. These assignments were deduced from a detailed NMR investigation, and supported by previous studies about fluorine-doped silica glasses,36,37 silica fluorinated by a nonaqueous solution of NH4F,35 and fluorinated zeolites.40 For the highest fluorine contents (50F2 and 60F2), the reconstructions need two supplementary lines at ca. -124 and ca. -132 ppm (Figure 6 and Table 2), which have not yet been reported in the literature. These two fluorosilicate species are related to the high fluorine content of the studied samples. The 19F-29Si CP-MAS NMR spectra of fluorinated silica, recorded with two contact times equal to 5 and 0.5 ms, are shown in Figures 7 and 8, respectively. Their reconstructions, achieved by using the above-described procedure, confirm the occurrence of five kinds of fluorosilicate species, and especially the two unprecedented species corresponding to the lines at ca. -124 and ca. -132 ppm. The relative intensities of these five lines increase from the 1H-29Si to the 19F-29Si CP-MAS NMR spectra confirming their attribution to fluorosilicate species (Tables 2-4). The same conclusion may be drawn from the comparison between the two 19F-29Si CP-MAS NMR experiments at different contact times (Figures 7 and 8): the shorter the contact time, the higher the fluorosilicate species relative intensity (Tables 3 and 4). The occurrence of these new species can be related to the high fluorine level reached in these samples, compared to those measured by Hartmeyer et al. (from 0.5 to 14.6 wt % but with a large amount of (NH4)2SiF6 for the high fluorine levels)29 or Barabash et al. (from the low signal-to-noise ratio of the recorded spectra).35 What could be the formula for these two new species? Four species are possible a priori: one tetracoordinated O1/2SiF3 species (mentioned by Barabash et al.35) and three pentacoordinated species O3/2SiF2, O2/2SiF3, and O1/2SiF4. The former O1/2SiF3 and the latter O1/2SiF4 present a low stability

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Lataste et al. TABLE 3: Isotropic Chemical Shifts (δiso, ppm), Line Widths (LW, ppm), and Relative Intensities (Int., %) As Deduced from the Reconstruction of the 19F-29Si CP-MAS NMR Spectra of Fluorinated Silica (Contact Time 5 ms) and Line Assignment 10F2 δiso -95.0 -102.0 -106.5 -112.0 -118.5 -124.0

50F2

LW Int. 7.5 8.0 7.0 6.0 8.5 8.0

5 20 18 20 27 10

δiso -94.5 -101.5 -106.0 -112.5 -119.0 -124.5 -132.0

60F2

LW Int. 8.0 7.5 7.0 7.0 8.0 7.0 8.0

4 15 17 29 26 7 2

δiso -95.0 -102.0 -106.0 -112.0 -118.5 -124.5 -132.0

LW Int. assignment 7.5 8.0 7.0 6.5 8.0 7.5 8.0

5 15 15 30 23 8 4

O2/2SiF2 O3/2SiOH O3/2SiF O4/2Si O4/2SiF O3/2SiF2 O2/2SiF3

TABLE 4: Isotropic Chemical Shits (δiso, ppm), Line Widths (LW, ppm), and Relative Intensities (Int., %) As Deduced from the Reconstruction of the 19F-29Si CP-MAS NMR Spectra of Fluorinated Silica (Contact Time 0.5 ms) and Line Assignment 50F2

Figure 7. Experimental and calculated 19F-29Si CP-MAS (5 kHz) NMR spectra of fluorinated silicas (10F2, 50F2, and 60F2 samples) recorded with a contact time of 5 ms. For each calculated spectrum, the individual contributions are shown.

60F2

δiso

LW

Int.

δiso

LW

Int.

assignment

-95.0 -101.5 -106.0 -112.0 -119.0 -124.5 -131.5

9.0 8.0 8.5 7.0 8.0 8.5 8.0

7 15 23 11 29 13 2

-95.0 -102.0 -106.0 -112.0 -119.0 -124.5 -132.0

10.0 7.0 7.5 6.0 7.0 8.0 8.0

9 13 21 19 25 11 2

O2/2SiF2 O3/2SiOH O3/2SiF O4/2Si O4/2SiF O3/2SiF2 O2/2SiF3

rosilicate species, such as O3/2SiOHF or O2/2SiOHF2 species for instance, is unlikely because of the strong probability to form HF with two adjacent F and OH ligands. Then the most probable formula for these new species are O3/2SiF2 and O2/2SiF3. Figure 9 displays the 19F MAS NMR spectra of two fluorinated silicas. Each spectrum is reconstructed with seven lines (Table 5). The method used for 1H-29Si and 19F-29Si was also applied, i.e., starting the reconstruction with the well-known and previously assigned 19F NMR lines. The resonance at

Figure 8. Experimental and calculated 19F-29Si CP-MAS (5 kHz) NMR spectra of two fluorinated silicas (50F2 and 60F2 samples) recorded with a contact time of 0.5 ms. For each calculated spectrum, the individual contributions are shown.

and would easily lead to SiF4 or SiF62-. Moreover, it seems difficult to conceive the occurrence of O1/2SiF4 without forming O3/2SiF2 and O2/2SiF3 species. The occurrence of hydroxyfluo-

Figure 9. Experimental and calculated 19F MAS (25 kHz) NMR spectra of two fluorinated silicas (10F2 and 50F2 samples). For each calculated spectrum, the individual contributions are shown.

Stability and NMR of Highly Fluorinated Silica

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18659

TABLE 5: Isotropic Chemical Shifts (δiso, ppm), Line Widths (LW, ppm), and Relative Intensities (Int., %) of the NMR Resonances As Deduced from the Reconstruction of the 19F MAS NMR Spectra of Fluorinated Silica, Line Assignment, and Relative Amount (mol %) of the Fluorosilicate Species 10F2

50F2

δiso

LW

Int.

δiso

LW

Int.

-160.0 -156.0 -153.0 -149.0 -143.5 -136.5 -127.0

3.0 5.0 4.9 7.0 7.3 8.0 4.0

4.3 31.0 37.8 17.3 7.2 1.4 1.0

-160.0 -156.0 -153.0 -148.5 -143.5 -136.5 -128.5

3.0 5.0 5.1 7.2 7.3 8.0 4.6

4.5 19.1 27.6 30.5 16.1 1.7 0.5

assignment 10F2 50F2 O2/2SiF2 O3/2SiFa O4/2SiF O3/2SiFb O3/2SiF2 O2/2SiF3 SiF62-

2.3 33.6 40.9 18.7 3.9 0.5 0.2

2.6 21.6 31.3 34.6 9.1 0.7 0.1

a Isolated O3/2SiF species. b O3/2SiF species close to other groups of the same type.

δiso(19F) ) -127 ppm can be assigned to SiF62- species4,29,35 and its relative intensity indicates a very low amount, ca. 0.1 to 0.2 mol % (Table 5). This explains why this species could not be detected on the 19F-29Si CP-MAS NMR spectra (part of spectra not shown, δiso(29Si) ) -188 ppm4,29) nor by IR spectroscopy.8 The three resonances at δiso(19F) ) -156, -153, and -149 ppm were previously observed in fluorinated silica29,35 and fluorinated zeolites.40,41 According to Hartmeyer et al.,29 these resonances are assigned to isolated O3/2SiF groups, pentacoordinated O4/2SiF groups, and O3/2SiF species close to other groups of the same type, respectively (Table 5). On the other hand, no 19F NMR line was assigned to the O2/2SiF2 species. From the 1H-29Si and 19F-29Si CPMAS spectra, this species appears at low fluorine content simultaneously with the disappearance of the O2/2Si(OH)2 species and its relative amount remains roughly unchanged (Table 3). On the 19F NMR spectra, the resonance at -160 ppm offers the same trend. Moreover, ab initio 19F chemical shift calculations performed by Liu et al.42 to investigate the nature of possible species in aluminosilicate glasses lead to a δiso(19F) value of -155 ppm for O2/2SiF2. Therefore the line at δiso(19F) ) -160 ppm is assigned to O2/2SiF2. The two remaining lines at δiso(19F) ) -143.5 and -136.5 ppm should then correspond to the two unprecedented species, O3/2SiF2 and O2/2SiF3. Given that the less fluorinated species presents the highest abundance, the higher relative intensities of the 29Si lines at ca. -124 ppm and of the 19F lines at -143.5 ppm allow assigning these lines to the O3/2SiF2 species. Consequently, the 29Si line at ca. -132 ppm and the 19 F line at -136.5 ppm are assigned to the O2/2SiF3 species (Tables 2-5). These assignments are supported by the ab initio 19 F chemical shift calculations of Liu et al. which give δiso values ranging between -128 and -132 ppm for O2/2SiF3.42 The 19F NMR spectra being quantitative, the relative amount of each species has been calculated (Table 5). As expected, the relative amount of O3/2SiF species as a close neighbor of other O3/2SiF groups increases whereas the relative amount of isolated O3/2SiF species decreases when the F2 content increases. Actually, the relative amount of O2/2SiF2 species which correspond formally to neutral entities remains quasiconstant whatever the F2 content in the gas mixture whereas the relative amount of isolated O3/2SiF and pentahedral O4/2SiF species globally decreases and the relative amount of pentahedral O3/2SiF2 and O2/2SiF3 species as well as O3/2SiF species close to other groups of the same type increases with the F2 concentration. The F2-direct fluorination with higher F2 contents allows the concentration of isolated monofluorinated species to

be limited in favor of di- or trifluorinated species, as well as interactions between O3/2SiF species. However, it seems surprising to stabilize more pentahedral O3/2SiF2 species than O2/2SiF2 species. This means that multi Si-F bonds up to 3 can be stabilized around each Si atom when the number of siloxane bridges is equal to 2 or 3. Moreover no O1/2SiF3 species have been detected. Furthermore, one must point out that, for increasing F2 concentration, the relative amount of O4/2SiF species decreases whereas that of O3/2SiF2 species increases as the variation of the relative amounts of isolated O3/2SiF and O3/2SiF species close to other groups of the same type. Then, in addition to the redox mechanism involving F2/F- and O2/OH- couples, the high oxidizing power of F2-gas allows the breaking of one Si-O-Si bridge replaced by an additional Si-F bond. The occurrence of pentahedral species such as O2/2SiF3, O3/2SiF2, or O4/2SiF can be easily explained by taking into account the stabilization of [SiO4/2] tetrahedra and [SiF6]2octahedra in oxides and fluorides, respectively. Finally for the following entities [O2/2SiF3]-, [O3/2SiF2]-, and [O4/2SiF]-, the effective charge is always equal to -1, intermediate between the [SiO4/2] tetrahedron and the [SiF6]2- octahedron. The polarization of such pentahedral species should strongly increase with the number of fluorines due to the coexistence of various ligands with different electronegativities. Such species are less stable than the other ones. This explains the decrease of the thermal stability for highly fluorinated silica leading to the formation of SiF4 as well as the increase of the water affinity with the F2 content due to the high concentration of highly polar species. 4. Conclusion In silica samples fluorinated by using F2-gas, NMR measurements reveal various fluorosilicate species previously observed in moderately fluorinated silica such as O2/2SiF2, O3/2SiF, and O4/2SiF and two unprecedented species occurring for highly fluorinating conditions. The 19F (δiso ) -143.5 ppm and -136.5 ppm) and 29Si (δiso ) -124 ppm and -132 ppm) NMR resonances of these two species have been tentatively assigned to O3/2SiF2 and O2/2SiF3 based on the relative intensities of 19F and 19F-29Si NMR lines and previous ab initio 19F isotropic chemical shift calculations. This study demonstrates that the physicochemical properties of silica powder can be modified by fluorine substitution. The F2-gas direct fluorination of silica leads to a substitution of Si-OH groups by Si-F and SiF2 species, and to unprecedented pentahedral species O2/2SiF3 and O3/2SiF2 by breaking various siloxane bridges. The high oxidizing power of F2-gas allows reaching such unusual species thanks to a coupling between a redox reaction and an etching phenomenon, as developed in our preceding paper.8 This original gas-solid method allows grafting large amounts of fluorine at the SiO2 surface up to 13 wt %. Fluorination conditions lead to fluorinated products stable up to 400 °C. Moreover such fluorinated silica exhibit a lower water uptake than untreated silica in highly humid environment (75% relative humidity) for long duration times (up to 50 days), a property that can be correlated to its hydrophobic character. It has been established that a reduction of the concentration of silanol groups on the silica surface is generally accompanied by an increase of their hydrophobic character. In molecular adsorption on a solid surface, the chemical nature of the solid surface plays an important role. Especially, hydrophilic-hydrophobic properties are important ones in order to control the adsorption for polar/ nonpolar molecules.

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