9066
J. Phys. Chem. C 2007, 111, 9066-9071
Speciation of Silanol Groups in Precipitated Silica Nanoparticles by 1H MAS NMR Spectroscopy Geoffrey Hartmeyer,† Claire Marichal,*,† Be´ ne´ dicte Lebeau,† Se´ verinne Rigolet,† Philippe Caullet,† and Julien Hernandez‡ Laboratoire de Mate´ riaux a` Porosite´ Controˆ le´ e, ENSCMu, UniVersite´ de Haute Alsace, UMR 7016, 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: February 22, 2007; In Final Form: April 11, 2007
Speciation of the various proton species present in precipitated silica samples by 1H high-spinning frequency magic-angle spinning (MAS) NMR (25 kHz) is achieved. External standards are used for the calibration of the absolute amount of protons, the relevance of the proton spin counting being checked by comparison with the data deduced from classical thermogravimetric (TG) and 29Si MAS NMR experiments. The method also allows to follow postsynthesis modifications of silica, corresponding for instance to fluorination or deuteration treatments. In particular, a 1H resonance occurring at 1.1 ppm/TMS and corresponding to a constant proton amount whatever the applied treatment is tentatively assigned for the first time to inaccessible isolated silanol groups.
1. Introduction 1H MAS (magic-angle spinning) NMR is a powerful tool to characterize materials such as silica, catalysts supports, or metal oxides, owing to its 99.98% natural abundance and the very high sensitivity of this nucleus. It has thus been widely used to investigate silicas,1-20 since their numerous applications depend on the hydrophilic/hydrophobic balance and the surface reactivity, which in turn are conditioned by the nature (isolated or H-bonded), the number, and the accessibility of the silanol groups (noted hereafter as Si-OH groups). 1H NMR experiments that allow simultaneously the quantification and the identification of hydrogen species in silica, but also in other materials, are thus highly desirable. However, they are subjected to two main difficulties, namely, the chemical shift anisotropy (CSA) and the H-H dipolar coupling.21-22 Thanks to the improvement of the NMR probes, during the last 10 years, highspinning frequencies up to 35 kHz are now routinely reached, which leads to an averaging of both the chemical shift anisotropy and the H-H dipolar couplings and results in a significant enhancement of the resolution.23-25 Recently, Wang et al.26 and Kennedy et al.27 reported two closely related methods to perform the quantification of protons by 1H MAS NMR in silicas. Both groups carried out their NMR experiments with a 4 mm MAS probe using a 10-15 kHz spinning frequency. Wang et al.26 described in detail a procedure for the quantitative determination of the total H concentration by using poly(dimethylsiloxane) (PDMS) as an internal reference. These authors underlined the fact that the sample volume has to be restricted to the middle region of the rotor to avoid effects of the radio frequency (rf) field inhomogeneity, which decreases significantly the accuracy. However, the question of the confinement of the sample inside the rotor is still a matter
* To whom correspondence should be addressed. Phone: 00 33 3 89 33 67 31. Fax: 00 33 3 89 33 68 85. E-mail:
[email protected]. † Universite ´ de Haute Alsace. ‡ Centre de Recherches d’Aubervilliers.
of debate, since Kennedy et al.27 proposed a simplified method for the quantitative determination of the hydrogen types in solids, in which they did not mention the necessary confinement of the sample volume to the middle region of the rotor. Besides, they used PDMS as an external standard to prevent any reaction with the sample under study. The original feature of our work consists in adapting the previously described methodologies to 2.5 mm MAS NMR rotors to take advantage of the increased resolution of the 1H MAS NMR spectra that was observed for some of our silica samples because of the higher accessible spinning frequencies (up to 35 kHz). This resolution improvement, thanks to fast MAS, was already observed for mesoporous silica.20,28 Figure 1 a-c illustrates the disappearance of the spinning sidebands when increasing the spinning frequency from 8 to 25 kHz and the resolution improvement of the isotropic part of the proton spectrum of a precipitated silica supplied by Rhodia. As deconvolution is a delicate operation, such resolution improvement is found interesting. External standards were used, first because of possible reactions of an internal standard with the material of interest, and second, because of the difficulty of weighing accurately the small otherwise required quantities of internal standards, that is, a few micrograms in the 2.5 mm rotor. Calibration was performed with three reference samples, namely, hexamethylbenzene (HMB), camphor, and PDMS. The method presented in this paper allows both the identification and the quantification of the various proton species by 1H fast MAS NMR. Applied to a precipitated silica,29 the method proves to be reliable, as the proton concentration deduced from the 1H MAS NMR spectra is in very good agreement with the concentrations calculated from thermogravimetric (TG) and 29Si MAS NMR experiments. The method is also applied to the corresponding fluorinated sample30 and provides valuable information about the number and the type of protons involved upon fluorination of nanoparticles of precipitated silica. Finally, the assignment of the proton resonances to “accessible” or “inaccessible” isolated silanols is also discussed, on the basis
10.1021/jp071490l CCC: $37.00 © 2007 American Chemical Society Published on Web 06/07/2007
Speciation of Silanol Groups by 1H MAS NMR
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9067
Figure 1. 1H MAS NMR spectra of a precipitated silica at a spinning frequency of (a) 25 kHz, (b) 15 kHz, (c) 8 kHz, (d) probe background at 8 kHz, and (e) before background subtraction at 8 kHz.
of the spectra of the untreated silica sample and the corresponding fluorinated and deuterated samples. 2. Experimental Section A. Samples. Poly(dimethylsiloxane) (PDMS, [-Si(CH3)2O-]n, Ha¨cke), camphor (C10H16O, Fluka), and hexamethylbenzene (HMB, C12H18 synthesized at the Laboratoire de Chimie Organique et Bio-organique, ENSCMu, Mulhouse, France) were used as references for 1H NMR quantification. The precipitated silica, referred to as P3, was supplied by Rhodia.29 This silica is made of small aggregates of almost spherical primary nanometric particles, the diameter of which is close to 16 nm. The specific surface area, determined from N2 adsorption isotherm, is equal to 174 m2 g-1. The total interparticle porous volume of 3.8 cm3 g-1, measured thanks to Hg porosimetry, corresponds to macropores (≈3 cm3 g-1) and mesopores (≈0.8 cm3 g-1). For proton quantification, the material under study must be dehydrated properly prior to the 1H NMR measurement to remove the water which prevents the observation of the hydrogen species of interest.6,31,32 For the precipitated P3 silica studied in this work, several activation procedures were tested according to the literature. Comparison of the 1H MAS NMR spectra of the dehydrated samples enabled the selection of the activation procedure (allowing the elimination of as much physisorbed water as possible while avoiding dehydroxylation reactions), which consists of heating the silica at 107 °C overnight under a reduced pressure of 2 × 10-3 Torr.31 The dehydrated samples were then transferred to a glovebox and were packed in the 2.5 mm rotor under argon. The P3 silica sample was fluorinated29,30 at room temperature by drop-by-drop addition of 3.8 cm3 of a 0.1 mol dm-3 (P3F1) or a 0.5 mol dm-3 (P3-F2) aqueous NH4F solution to about 3.15 g of nondehydrated silica (corresponding to ca. 3 g of dry
silica). After a careful homogenization by mechanical mixing, the mixture was aged for 24 h at room temperature (the vessel was covered by a plastic film to prevent evaporation) and finally was dried for 5 days at 70 °C in an oven. The final F weight percents 0.7 and 3.4 wt % for P3-F1 and P3-F2 silica, respectively, were determined by 19F liquid-state NMR.30 To characterize the accessibility to the silanol groups in the P3 silica, a proton exchange with deuterium was undertaken according to the procedure given by Chuang et al.33 Typically, 1.0 g of dehydrated P3 silica was immersed in 5 cm3 of D2O under stirring for 1 day at room temperature. The solution was then evacuated under vacuum for 3 h at 40 °C. The “dry” deuterated silica was further heated at 100 °C overnight under a reduced pressure of 2 × 10-3 Torr and was stored in a glovebox under Ar. The deuterated P3 silica sample is referred to as P3-D. B. Thermogravimetric Measurements. Thermogravimetric (TG) analyses were used to determine the amount of physisorbed water and to quantify the silanol groups in P3 silica from the weight loss associated with the dehydroxylation process. TG measurements were performed on a Setaram TG/DSC Labsys thermoanalyzer by heating the material under air at a rate of 5 °C min-1 up to 1250 °C. Prior to the measurements, the silica was dehydrated for 15 h at 70 °C and then was rehydrated for 24 h at room temperature in a closed vessel under an 80% relative humidity to set the hydration degree. C. NMR Measurements. 29Si (I ) 1/2) MAS NMR spectra were recorded at room temperature on a Bruker DSX 400 spectrometer operating at B0 ) 9.4 T (Larmor frequency ν0 ) 79.49 MHz). Single-pulse experiments were performed with a 7 mm Bruker MAS probehead, a spinning frequency of 4 kHz, π/6 pulse duration of 2 µs, and 80 s recycling delay. These recording conditions ensure the quantitative determination of the proportions of the Qn Si species.34
9068 J. Phys. Chem. C, Vol. 111, No. 26, 2007
Hartmeyer et al. TABLE 1: 1H Content (nH) of the Untreated P3 Silica Sample Determined by Thermogravimetric Analysis (TG), 29Si MAS NMR, and 1H MAS NMR H content in 10-4 mol g-1 of silica
1
TG P3
Figure 2. Area of the 1H resonances of the reference samples (PDMS, HMB, camphor) as a function of the theoretical number of protons contained in PDMS (5.39 ( 0.02 mg), HMB (4.39 ( 0.04 mg), and camphor (4.84 ( 0.02 mg). The dashed line is the linear regression best fit.
(I ) 1/2) NMR experiments were performed at room temperature on the same Bruker DSX 400 spectrometer (Larmor frequency ν0 ) 400.13 MHz). Single-pulse experiments were recorded with a 2.5 mm Bruker MAS probe, a spinning frequency of 25 kHz, and a π/2 pulse duration of 3.5 µs. 1H spin lattice relaxation times (T1) were estimated with the inversion-recovery pulse sequence for all silica and reference samples, indicating that a 35 s recycle delay was more than enough to avoid saturation whatever the sample. Typically, 72 scans were recorded. Chemical shifts reported hereafter are relative to tetramethylsilane for both 1H and 29Si nuclei. 1H MAS NMR spectra were first recorded on a rotor containing a known amount of each of the three reference samples (HMB, camphor, and PDMS) to check the reliability of the determined proton amount. It turned out that under these conditions, the proton concentration was actually underestimated by 8 to 15%. Consequently, Kel-F spacers were specially designed by Bruker to restrict the sample volume to the middle region of the 2.5 mm MAS rotor so that the available space is one-third of the initial volume. 1H MAS NMR spectra of a known amount of the reference samples, confined in the middle region of the rotor, were then again recorded. Figure 2 shows the experimental area of the proton resonances for each reference sample as a function of the expected number of protons calculated from the chemical formula and the weight. A linear relationship going through zero is obtained and is characterized by a very good correlation coefficient value (R2 ) 0.99). The uncertainty on the 1H concentration in each of the three standards thus does not exceed 1.5%. Therefore, in agreement with Wang et al.,26 we found that a reliable proton concentration can only be deduced from the NMR data when a restricted sample volume is used. The used amounts of samples were very small (between 1.5 and 6 mg requiring a high precision balance ((10-5 g)), which is not a difficulty because of the high sensitivity of the 1H nucleus. The different steps of our procedure can be finally summarized as follows. After an accurate weighing of the empty rotor, a 1H MAS NMR spectrum of the empty rotor (with two 2 mm long spacers and caps) is recorded at the chosen spinning frequency (25 kHz). This spectrum evidences a broad probe background signal as shown Figure 1d that cannot be neglected because of the small amount of sample under study. As already mentioned before, to prevent rehydration of the silica materials, the 2.5 mm rotor is then filled in the glovebox with the dehydrated sample, which is centered in the middle part of the rotor between the two spacers. After an accurate weighing of the rotor with spacers, caps, and sample, a 1H MAS NMR 1H
29
1
Si MAS NMR
23.3
H MAS NMR
20.1
20.4
spectrum is recorded with exactly the same parameters (Figure 1e). An identical processing of the two 1H MAS NMR spectra is performed, followed by a subtraction of the probe background signal, leading to a “corrected” 1H spectrum presented in Figure 1c. Deconvolution of the corrected 1H MAS NMR spectrum with the dmfit software35 gives the relative proportion of each component. The absolute quantity is obtained by comparison of the area of each resonance with the area of the external standards (PDMS, camphor, and HMB in our case) recorded under the same conditions taking into account the sample weight. 3. Results and Discussion This section is divided into three parts. The first one concerns the validation of the chosen methodology for recording quantitative high-spinning frequency 1H MAS NMR spectra by comparing the proton concentration in P3 silica obtained from 1H MAS NMR with the concentrations deduced from thermogravimetric analysis and 29Si NMR. The second part is devoted to the application of 1H MAS NMR for the quantification and the speciation of the various proton species present in the fluorinated P3 silica sample. Finally, the last section deals with the differentiation of the inaccessible and accessible isolated SiOH in precipitated P3 silica on the basis of the 1H MAS NMR spectra of the P3, P3-F1, P3-F2, and P3-D samples. 3.1. Determination of the Proton Concentration in P3 Silica. To check the validity of the proposed protocol, the proton concentration of P3 silica was determined by three different methods, that is, thermogravimetric analysis and 29Si and 1H MAS NMR. The thermal behavior of P3 silica was investigated by TG thermal analysis (thermograms not shown). The total weight loss occurs in two steps. The first (w1), below 200 °C (5.2 wt %), corresponds to the loss of physisorbed water. The second weight loss (w2), between 200 and 1250 °C (2.1 wt %), is associated with the removal of water arising from dehydroxylation reactions.31,36 The proton number (nH) in P3 silica reported in Table 1 can be estimated from eq 1 where MH2O is the molar mass of water.
nH (mol g-1 of silica) )
2 × w2(%) MH2O × 100
(1)
The proton amount in P3 silica can also be estimated from MAS NMR. Indeed, the 29Si MAS NMR spectrum of P3 silica (not displayed) gives the relative proportions of Q3 (x ) 13%) and Q4 (y ) 87%) units corresponding to the area of the resonances assigned to silanol groups (SiO1.5OH of molar mass MQ3 ) 69 g; δ ∼ -102 ppm/TMS) and siloxane bridges (SiO2 of molar mass MQ4 ) 60 g; δ ∼ -112 ppm/TMS), respectively. The proton concentration calculated according to eq 2 is also reported in Table 1. 29Si
(
)
w1(%) 100 nH (mol g-1 of silica) ) (MQ3x(%) + MQ4y(%)) x(%) 1 -
(2)
Speciation of Silanol Groups by 1H MAS NMR
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9069 TABLE 2: Total Proton Content, 1H Chemical Shift (δ), and Content (nH) for Each Silanol Species in the P3, P3-F1, P3-F2, and P3-D Samples nH content in 10-4 mol g-1 δ 1H in ppm
total
for each species
P3
1.8 2.1 2.8-3.0 4.5 1.1 1.8 2.2
20.4
2.0 2.0 11.3 5.1 0.3 1.1 1.1
P3-F1
2.8-3.0 3.9 4.5 6.6 1.1.
15.4
8.6 1.1 3.1 0.1 0.4
P3-F2
1.8 2.2 2.8-3.0 3.9 4.5 5.4 6.6 1.1 1.8
21.5
0.0 0.7 5.0 4.3 0.0 4.8 6.3 0.4 0.4
P3-D
2.1 2.8-3.0 4.5
10.8
0.5 4.9 4.6
sample
Figure 3. 1H MAS NMR spectra of (a) P3 silica (1.6 ( 0.02 mg), (b) fluorinated P3-F1 silica (2.36 ( 0.02 mg), (c) fluorinated P3-F2 silica, and (d) deutered P3-D silica (2.10 ( 0.02 mg) together with their decompositions (from top to bottom).
Figure 3a shows the “corrected” 1H MAS NMR spectrum of P3 silica together with its decomposition which will be discussed below (see section 3.2). The proton amount of P3 silica reported in Table 1 was obtained using the linear relation given in Figure 2 of the experimental part. Whatever the method (TG, 29Si MAS NMR, 1H MAS NMR), the proton amounts given in Table 1 are in very good agreement with each other, indicating that the chosen protocol for quantitative 1H MAS NMR is suitable for determining the total proton content of a sample. Obviously, the main advantage of the latter technique in comparison with the other two is that, in addition to the total proton content, speciation of the different protons can be achieved, which is clearly of prime importance for the study of chemical reaction mechanisms. Thus, the 1H MAS NMR method allowed us to characterize postsynthesis surface modifications of P3 silica, as described in the following sections. 3.2. Study of the P3-F Silica Sample. The corrected 1H MAS NMR spectrum of the P3-F1 sample is given in Figure 3b together with its decomposition. The total number of protons determined experimentally for P3-F1 silica is nH ) 15.4 × 10-4 mol g-1 of silica. The slight observed decrease (around 20%) of the number of protons in comparison with the parent P3 silica (nH ) 20.4 × 10-4 mol g-1 of silica) is due to the fluorination treatment. With the dilute NH4F solution (0.1 mol l-1) used here, one may assume that the fluorination process involves only a nucleophilic substitution of the Si-OH groups by the F- ion (Si-OH + F- T Si-F + OH-) with no fluorinative opening of siloxane bonds.30 On this basis, the number of protons left after fluorination (P3-F1 sample) can be calculated by subtracting from the initial number of protons in the P3 silica sample the number of incorporated F atoms,
which is close to 3.7 10-4 mol g-1 of silica (0.007 g F- per g divided by 19, the molar mass of F). The number of protons left is thus expected to be 16.7 × 10-4 mol g-1 of silica, which is reasonably close to the nH value measured from 1H quantitative high-spinning frequency MAS NMR for P3-F1 silica. At this stage, it is interesting to concentrate on the decomposition of the 1H MAS NMR spectra of P3 and P3-F1 silica shown in Figure 3a and 3b. At least four resonances are needed to reproduce properly the experimental 1H MAS NMR spectrum of P3 silica. According to the experimental line width at halfheight of the resonances) and the 1H chemical shifts reported in the literature,5,9,10,14 the two sharp components at 1.8 and 2.1 ppm are assigned to isolated silanol groups. The resonances at 2.8 and 4.5 ppm are broader, indicating a stronger interaction between hydrogen species. They probably correspond to SiOH hydrogen-bonded to other Si-OH groups and water, respectively. Actually, despite the dehydration performed before recording the 1H spectra, the presence of a small amount of water cannot be excluded. Indeed, there is no clear border between dehydration and dehydroxylation, so entirely removing water would probably alter the silica surface. The 1H MAS NMR spectrum of P3-F1 silica shown in Figure 3b exhibits three additional resonances at 1.1, 3.8, and 6.6 ppm. The discussion on the assignment of the signal at 1.1 ppm is postponed to paragraph 3.3, which is especially devoted to this point. According to the literature,26,32 the resonance at 6.6 ppm is due to the ammonium cation (NH4+), whereas the one at 3.8 ppm is probably attributable, according to the changes of the proton contents upon fluorination reported in Table 2, to Si-OH groups close to a fluorine nucleus. Indeed, the appearance of this new resonance detected at 3.8 ppm is accompanied by a decrease of the number of protons associated with the resonance at 2.8 ppm (hydrogen-bonded silanols). Furthermore, the silica surface becomes also more hydrophobic after fluorination,37 with a decrease of the number of residual adsorbed water molecules and of the related 4.5 ppm resonance. The previous changes upon fluorination and relative to the resonances at 2.8,
9070 J. Phys. Chem. C, Vol. 111, No. 26, 2007 3.8, and 4.5 ppm are more pronounced in the fluorinated P3F2 silica sample characterized by a markedly higher fluorine content (3.4 wt %) (Figure 3c), which strengthens the proposed assignments. Finally, the resonance at 5.4 ppm (Figure 3c) is assigned to silanol groups of octahedral hydroxyfluosilicate species as the presence of (NH4)2SiFx(OH)6-x salts is evidenced in the highly fluorinated samples in particular by 19F MAS NMR spectroscopy.30 This assignment is confirmed by the fact that upon calcination of the fluorinated silica,30 a drastic decrease of this signal (together with the corresponding 19F NMR signal) is observed. The number of protons corresponding to isolated silanols (resonances at 1.8 and 2.2 ppm) decreases strongly (by ca. 50%) upon fluorination in the P3-F1 sample confirming the replacement of part of the silanols according to the nucleophilic substitution mechanism mentioned above. Both types of isolated silanols are affected in a similar way indicating that they both should correspond to “accessible” silanols. This observation is clearly at variance with the 1H MAS and 29Si CP MAS NMR results of Liu and Maciel,13 who suggested that one of these two signals could correspond to inaccessible silanols, the other one corresponding to accessible silanol groups. The next paragraph 3.3 is devoted to the discussion on the differentiation between accessible and inaccessible silanols and thus concerns specifically the assignment of the 1.8 and 2.1 ppm resonances, attributed to isolated silanols, and also that of the 1.1 ppm signal. 3.3. Discrimination between Accessible and Inaccessible Isolated Silanol Groups in Silica. To clear up this point, a deuterated P3 silica sample (noted P3-D) was prepared (see Experimental Section for details) to exchange the accessible hydrogen atoms and to discriminate the inaccessible silanols. The 1H MAS NMR spectrum of P3-D silica is presented in Figure 3d together with its decomposition. The total amount of protons in the P3-D sample is equal to 10.8 × 10-4 mol g-1 of silica instead of 20.4 × 10-4 mol g-1 in the untreated silica. Five resonances at 1.1, 1.8, 2.1, 3.0, and 4.5 ppm are observed, which obviously indicate an only partial proton exchange and possibly the presence of inaccessible Si-OH sites. The proton amounts of each detected species are reported in Table 2. Both components at 1.8 and 2.1 ppm, which correspond to isolated Si-OH, decrease drastically and in similar proportions (by 7080%) after deuterium exchange. This is in agreement with the observations made in the previous paragraph upon fluorination and confirms that these two 1H resonances correspond probably to accessible isolated Si-OH. After deuterium exchange, only 45% of the amount of protons corresponding to hydrogenbonded silanols (3.0 ppm) are left, whereas the water content (at 4.5 ppm) is not significantly affected. What is worth underlining is the presence on the 1H MAS NMR spectrum of P3-D silica (Figure 3d) of a fifth resonance at 1.1 ppm, as already observed in the case of both fluorinated P3-F1 and P3-F2 samples. According to the chemical shift and the line width (around 240 Hz in the present work), this resonance is also assigned to isolated non-hydrogen-bonded SiOH in the literature.32 Besides, the amount of protons corresponding to this species remains the same (ca. 0.4 × 10-4 mol g-1) for the P3-F1, P3-F2, and P3-D samples (Table 2). To summarize, whatever the postsynthesis treatment performed on the P3 silica sample, a 1H resonance at 1.1 ppm is observed corresponding to a constant amount, close to 0.4 × 10-4 mol g-1. Consequently, this resonance should correspond to inaccessible isolated silanol groups. These silanol groups might be located inside or in-between the primary silica nanoparticles. Unfortunately, this proton species cannot be detected in the as-
Hartmeyer et al. synthesized P3 silica because of the weak intensity of the associated line (related to the small quantity), which is probably hidden in the tail of the large resonance at 2.8 ppm (see Figure 3a). Consequently, the number of protons corresponding to this resonance might be overestimated by 0.4 10-4 mol g-1 which represents an error of 3.5%. 4. Conclusions A 1H high-spinning frequency MAS NMR methodology has been developed to quantify the total number of protons in silica sample (precipitated P3 silica) together with the speciation of the various proton/silanol species. The main advantage of this technique is that information about the amount and the speciation is available in a “one-shot” experiment. The relevance of the measurements was first established through calibration with three reference samples. The proton spin counting results deduced from quantitative 1H high-spinning frequency MAS NMR on a silica sample were compared to data obtained from TG and 29Si MAS NMR. The very good agreement between the results obtained with the three methods validates the protocol. Finally, we showed that combining 1H high-spinning frequency MAS NMR with quantification provides valuable results. In particular, the accurate determination, using our methodology, of the amount of each silanol species in different P3 silica samples, that is, untreated, fluorinated, and deuterated, revealed for the first time that inaccessible isolated silanol groups may correspond to the 1H resonance at 1.1 ppm. We anticipate that this method will help in the understanding of the elementary steps in surface chemistry. Acknowledgment. We are grateful to Dr. F. Aussenac from Bruker Company and to Dr. P. Reinheimer for providing the spacers for the 2.5 mm probe and the polydimethylsiloxane sample, respectively. This work was supported by Rhodia. References and Notes (1) Bermudez, V. M. J. Phys. Chem. 1970, 74 (23), 4160. (2) Freude, D.; Hunger, M.; Pfeifer, H. Chem. Phys. Lett. 1982, 91 (4), 307. (3) Freude, D.; Hunger, M.; Pfeifer, H.; Schwieger, W. Chem. Phys. Lett. 1986, 128 (1), 62. (4) Ratcliffe, C. I.; Ripmeester, J. A.; Tse, J. S. Chem. Phys. Lett. 1985, 120 (4-5), 427. (5) Bronniman, C. E.; Chuang, I. S.; Hawkins, B. L.; Maciel, G. E. J. Am. Chem. Soc. 1987, 109, 1562. (6) Bronniman, C. E.; Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1988, 110, 2023. (7) Eckert, H.; Yesinowski, J. P.; Silver, L. A.; Stolper, E. M. J. Phys. Chem. 1988, 92, 2055. (8) Yesinowski, J. P.; Eckert, H.; Rossman, G. R. J. Am. Chem. Soc. 1988, 110, 1367. (9) Legrand, A. P.; Hommel, H.; Tuel, A.; Vidal, A.; Balard, H.; Papirer, E.; Levitz, P.; Czernichowski, M.; Erre, R.; Van Damme, H.; Gallas, J. P.; Hemidy, J. F.; Lavalley, J. C.; Barres, O.; Burneau, A.; Grillet, Y. AdV. Colloid Interface Sci. 1990, 33, 91. (10) Haukka, S.; Lakomaa, E-L.; Root, A. J. Phys. Chem. 1993, 97, 5085. (11) Fleischer, U.; Kutzelnigg, W.; Bleiber, A.; Sauer, J. J. Am. Chem. Soc. 1993, 115, 7833. (12) Haukka, S.; Root, A. J. Phys. Chem. 1994, 98, 1695. (13) Liu, C. C.; Maciel, G. E. J. Am. Chem. Soc. 1996, 118, 5103. (14) D’Espinose de la Caillerie, J. B.; Aimeur, M. R.; El Kortobi, Y.; Legrand, A. P. J. Colloid Interface Sci. 1997, 194, 434. (15) Turov, V. V.; Leboda, R. AdV. Colloid Interface. Sci. 1999, 79, 173. (16) Shantz, D. F.; Schmedt auf der Gu¨nne, J.; Koller, H.; Lobo, R. F J. Am. Chem. Soc. 2000, 122, 6659. (17) Robert, E.; Whittington, A.; Fayon, F.; Pichavant, M.; Massiot, D. Chem. Geol. 2001, 174, 291. (18) Sizun, C.; Raya, J.; Intasiri, A.; Boos, A.; Elbayed, K. Microporous Mesoporous Mater. 2003, 66, 27.
Speciation of Silanol Groups by 1H MAS NMR (19) Pawsey, S.; McCormick, M.; De Paul, S.; Graf, R.; Lee, Y. S.; Reven, L.; Spiess, H. W. J. Am. Chem. Soc. 2003, 125, 4174. (20) Tre´bosc, J.; Wiench, J. W.; Huh, S.; Lin, V. S.-Y.; Pruski M. J. Am. Chem. Soc. 2005, 127 (9), 3057-3068, 7587-7593. (21) Brown, S. P.; Spiess, H. W. Chem. ReV. 2001, 101, 4125. (22) Canet, D. La RMN: Concepts et me´ thodes; InterEdition: Paris, 1991. (23) Dec, S. F.; Wind, A.; Maciel, G. E. J. Magn. Reson. 1986, 70, 355. (24) Schnell, I.; Lupulescu, A.; Hafner, S.; Demco, D. E.; Spiess, H. W. J. Magn. Reson. 1998, 133, 61. (25) Schnell, I.; Spiess, H. W. J. Magn. Reson. 2001, 151, 153. (26) Wang, X.; Coleman, J.; Jia, X.; White, J. L. J. Phys. Chem. B 2002, 106, 4941. (27) Kennedy, G. J.; Afeworki, M.; Calabro, D. C.; Chase, C. E.; Smiley, R. J., Jr. Appl. Spectrosc. 2004, 58 (6), 698. (28) Alonso, B.; Massiot, D. J. Magn. Reson. 2003, 163, 347-352. (29) Rhodia. French Patent no. 05 02783, 2005. (30) Hartmeyer, G.; Marichal, C.; Lebeau, B.; Caullet, P.; Hernandez, J. J. Phys. Chem. C 2007, 111, 6634-6644. The used procedure for the determination of the fluoride content is the following: about 100 mg of
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9071 silica is weighed accurately and is dissolved in about 4 g of a 10 M aqueous NaOH solution; to the resulting solution is then added a precise mass (in the 100-200 mg range) of a 1 wt % aqueous CF3COOH solution; part of the resulting mixture is introduced in an NMR PTFE tube and is added with a similar amount of D2O; spectra are recorded on a Bruker Avance 400 NMR spectrometer under the following conditions: Larmor frequency ν0 ) 376. 5 MHz, π/6 pulse duration of 3 µs, 40 s recycling delay, number of scans: 40. (31) Zhuravlev, L. T. Colloids Surf. 2000, 173, 1. (32) Dore´mieux-Morin, C.; Heeribout, L.; Dumousseaux, C.; Fraissard, J.; Hommel, H.; Legrand, A. P. J. Am. Chem. Soc. 1996, 118, 13040. (33) Chuang, I. S.; Kinney, D. R.; Maciel, G. E. J. Am. Chem. Soc. 1993, 115, 8695. (34) Engelhardt, G.; Michel, D. High-Resolution Solid State NMR of Silicates and Zeolites; John Wiley & Sons: Chichester, U.K., 1987. (35) Massiot, D. J. Magn. Reson. 1996, 122, 240. (36) Ek, S.; Root, A.; Peussa, M.; Niinisto¨, L. Thermochim. Acta 2001, 379, 201. (37) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979.
