Are Nanoparticles Aggregated Oligomers or Silica Particles?

Nov 12, 2009 - 45, AVenue des Etats-Unis, 78035 Versailles Cedex, France, Michael Barber Centre for Mass Spectrometry,. Manchester Interdisciplinary ...
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J. Phys. Chem. C 2009, 113, 20827–20836

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Connectivity Analysis of the Clear Sol Precursor of Silicalite: Are Nanoparticles Aggregated Oligomers or Silica Particles? David P. Petry,†,‡ Mohamed Haouas,*,‡ Stephen C. C. Wong,§ Alexander Aerts,| Christine E. A. Kirschhock,| Johan A. Martens,| Simon J. Gaskell,§ Michael W. Anderson,† and Francis Taulelle*,‡ Centre for Nanoporous Materials, School of Chemistry, The UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K., Tectospin, Institut LaVoisier de Versailles, UMR CNRS 8180, UniVersite´ de Versailles St.-Quentin, 45, AVenue des Etats-Unis, 78035 Versailles Cedex, France, Michael Barber Centre for Mass Spectrometry, Manchester Interdisciplinary Biocentre, The UniVersity of Manchester, 131 Princess Street, Manchester M1 7DN, U.K., and Centre for Surface Chemistry and Catalysis, Katholieke UniVersiteit, LeuVen, Kasteelpark Arenberg 23, B-3001 HeVerlee, Belgium ReceiVed: July 3, 2009; ReVised Manuscript ReceiVed: October 2, 2009

The very first stages of the incorporation of tetraethoxysilane (TEOS) into aqueous tetrapropylammonium hydroxide (TPAOH) by hydrolysis are investigated to clarify the formation of silicate species in solution: oligomers and nanoparticles. Silicate speciation of both oligomers and nanoparticles were characterized using quantitative 29Si NMR, electrospray ionization mass spectrometry (ESI-MS), dynamic light scattering (DLS), and 1H diffusion-ordered NMR spectroscopy (DOSY). The main parameter measured for following this formation with the advancement of hydrolysis of TEOS is the evolution of silicon connectivity, in oligomers as well as in nanoparticles. At the beginning of TEOS hydrolysis, small oligomers are formed, which grow in number and size as the reaction progresses, with an average connectivity going from 0 to 2.1. At Si/ TPAOH ratio of 1.0 and above, nanoparticles form through aggregation of oligomers with some additional condensation. Their connectivity varies from 2.4 to 3.1. These nanoparticles cannot be confused with condensed silica particles. 1. Introduction The understanding of zeolites synthesis is still today a highly topical matter since the pioneering work of Breck.1 Zeolites are crystalline materials with nanoporous structures, which can be designed for different purposes such as catalysis, adsorption, and ion exchange.2-5 Silicalite-1 (pure siliceous MFI framework type) synthesis has for a long time been used as a model for mechanistic studies of zeolite crystallization with a simpler chemical composition compared to corresponding aluminosilicate systems.6,7 Silicalite-1 precursor is a transparent sol avoiding the formation of a gel.8 The TEOS:TPAOH:H2O (TEOS, tetraethoxysilane; TPAOH, tetrapropylammonium hydroxide) sol has been described for a long time as a homogeneous and clear solution consisting of silicate oligomeric species.9 The oligomers are in dynamic equilibrium between them, interconverting, leading to complex nuclear magnetic resonance (NMR) and mass spectrometry (MS) spectra.10,11 Silicate solutions have been studied since the work of Harris,12,13 and the number of silicate anions being structurally elucidated has incrementally increased for alkaline-metal silicate solutions.14-16 For practical reasons quaternary ammonium silicate solutions have received less attention than the alkaline-metal silicate ones. It is claimed that no specific difference exists for the species in alkalinemetal silicate and the quaternary ammonium silicates solution. * To whom correspondence should be addressed. Tel.: +33 1 39 25 42 54 (M.H.); +33 1 39 25 44 77 (F.T.). Fax: +33 1 39 25 44 76 (F.T.). E-mail: [email protected] (M.H.); [email protected] (F.T.). † School of Chemistry, The University of Manchester. ‡ Universite´ de Versailles St.-Quentin. § Manchester Interdisciplinary Biocentre, The University of Manchester. | Katholieke Universiteit.

Some are pointing out that the distribution of the species are not the same, and larger molecular species are present. The limit of the size of such species is correlated to the limit of detection of the method used. Schu¨th et al. have detected species of high nuclearity by electron spray ionization mass spectrometry (ESIMS).17 Because silicate speciation is highly sensitive to subtle differences of the chemical system, the nature of the organic ammonium salt can alter the chemical equilibria between oligomers, their dynamics, and their distribution.11 Alkyl ammonium cations are known to stabilize the large silicate cages of double four-ring (D4R) or double three-ring (D3R) type.18,19 Moreover, ethanol released by the hydrolysis of TEOS changes the dielectric and acido-basicity properties of the solutions. The D5R silicate anion, though its existence is surprisingly still under discussion, is favored after the addition of a polar organic solvent.20-22 The presence of additional alcohol enhances the reactivity of the precursor sol with respect to the zeolite synthesis.23 Aerts et al. studied transparent sol precursors of silicalite-1 using NMR, SAXS (small-angle X-ray scattering), and DLS (dynamic light scattering), varying the amount of TPA in the system.9 In the latter study, it is suggested that the transparent sol could be a mixture of oligomers and oligomer aggregates. Most of the silicon atoms are in the nanoparticles. 29 Si{1H} and 13C{1H} cross-polarization at magic-angle spinning (CPMAS) indicates the spatial proximity between the organic template and the silicate network in nanoparticles as well as in crystals.24 Very few studies investigated the first chemical stages of hydrolysis of TEOS in TPAOH/water solution.25-27 In a first attempt of quenching the hydrolysis reaction at these early stages, Kirschhock et al. separated the organic/water phase,

10.1021/jp906276g CCC: $40.75  2009 American Chemical Society Published on Web 11/12/2009

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rinsing the water phase with octane to remove any trace of unhydrolyzed TEOS, and tried to evidence the existence of early secondary building units in the water phase.26 A theoretical approach has been performed by simulating the hydrolysis of TEOS, in dissolved silica in TPAOH/water solutions and added ethanol,28 and has also been modeled by Provis and Vlachos.27 Extraction of nanoparticles has been attempted, in order to investigate the TPA to silicate species interaction.26,29,30 The different techniques of extraction consisted of quenching at low temperature and pumping out water, “salting out”, and ultracentrifugation. Actually, the nanoparticles separation method used prejudices the possible stability of the nanoparticles as isolated species and may therefore bias the results obtained. The main focus of the other contributions has rather been on studying the post-transformation of these nanoparticles into silicalite-1 materials, something beyond the scope of this study.6,8,26,30-33 It is therefore not clear if the solutions obtained are really representative of actual TEOS hydrolyzed solutions, and this led us to study in situ the formation of the sol and the characterization of the nanoparticles. The idea that silicalite-1 growth might occur by the sequential addition of secondary building units (SBU) has been a subject of passionate debates for several years.26,30,32,34 The proposal of large secondary building units26 put higher demands on the task with regard to the limits of the techniques available at that time for such a purpose, NMR and MS. On the other hand, the role of nanoparticles as possible precursors of silicalite-1 formation27 imposes on one to know more about the nanoparticles themselves, particularly their formation, their inner structure, and their evolution. In this work, NMR and MS have been used to follow the earliest stages of the hydrolysis of TEOS for the 25:9:152 TEOS:TPAOH:H2O system. MS has been recently used10 and is prone to provide information on species having a low concentration, as well as species exhibiting a high degree of nuclearity.35 Diffusion measurements by DLS and by DOSY NMR measurements will add information on the size of the nanoparticles and on their interaction with the rest of the solution. It is therefore the aim of this paper to investigate the hydrolysis of TEOS at its very first stages to follow the formation of oligomers in solution and the appearance of nanoparticles and to address the question asked by many: are the nanoparticles aggregates of oligomers or are they silica particles? 2. Materials and Methods Nine solutions with different degrees of TEOS hydrolysis, and with the final molar composition 25:9:152 TEOS:TPAOH: H2O, were prepared in polypropylene bottles by addition of an appropriate quantity of TEOS (98%, Acros) to a known amount of aqueous TPAOH solution (40 wt %, Alfa) at room temperature (see the Supporting Information for details). The resulting solutions were mixed by vigorous magnetic stirring, for different reaction times, after which the solutions were immediately quenched in an ice bath. For short reaction times, before completion of the hydrolysis, the aqueous phase was carefully separated from the organic phase by decantation to avoid further hydrolysis of TEOS. According to 29Si NMR, the organic phase consists of unreacted TEOS and some traces of partially hydrolyzed TEOS (mainly Si(OEt)4-n(OH)n, n ) 1 or 2). Since the hydrolysis process is exothermic (Figure 1), the solution temperature is highly dependent on the extent of the reaction. Moreover, other parameters strongly influence the progress of the hydrolysis, including mixing rate, mixing duration, vessel shape, and sample volume, etc. Although all

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Figure 1. Sample temperature and h-1 as a function of mixing time during the room-temperature hydrolysis of TEOS with TPAOH in the 25:9:152 TEOS:TPAOH:H2O system. The final sample amount was 10 g, and the stirring rate was 1100 rpm. The hydrolysis ratio h was determined by 1H NMR from the EtOH/TPA signal ratio.

samples were prepared in identical experimental conditions, reproducibility is very difficult to achieve. However, these solutions could be qualified precisely by their degree of TEOS hydrolysis, which can be easily determined from the 1H NMR signal ratio of EtOH/TPA, reflecting the EtOH fraction released in solution. In this way, the sample series could be compared by knowing how to measure accurately the hydrolysis level of each solution prepared, as it is difficult to reproduce strictly identical solutions. The TEOS incorporation into water by hydrolysis of the organic phase can therefore be quantified by the silicon concentration or the related ethanol concentration obtained in the resulting aqueous phase. However, there is also a need to measure in the aqueous phase the ratio [TPAOH]/ [Si] which is commonly called h, the hydrolysis ratio of the aqueous phase. The ratio h-1 ) [Si]/[TPAOH] gives at the same time the amount of silicon incorporated in the aqueous phase and by inversion, h, the hydrolysis ratio of the aqueous solution. The variation of h-1 spans from 0 to 2.78 when complete incorporation of 25 TEOS in 9 TPAOH and 152 H2O has been reached. The pH was above 14 along the hydrolysis process, decreasing progressively to reach a final value of 13.9 at the end of the hydrolysis. All samples were stored at room temperature, without stirring, for 3 weeks prior to measurement, except for two solutions, which were measured by NMR immediately following preparation to determine if the aging of the solutions had any effect. No effect due to storage has been observed. The 1D 29Si NMR experiments were carried out on a Bruker Avance 500 spectrometer with 29Si resonating at 99.353 MHz. In a modified background free probe, 10 mm quartz tubes were used to avoid the strong background signal of glass. The spectra were recorded with single-pulse acquisition at room temperature (24 °C) using a pulse of 9.4 µs (45°), a recycle delay of 5 s, and an acquisition time of 1.6 s and accumulating 1024 scans. To account for the longer relaxation delays required for Q3 and Q4 sites in nanoparticles,36 correction factors of 1.14 and 2.17, respectively, have been applied to the corresponding signal area. Indeed, knowing the spin-lattice relaxation times for each signal, the different magnetizations obtained for a given relaxation delay can be derived. 29Si NMR quantification was performed by spectral deconvolution. Simulation of the narrow lines was conducted with Lorenzian shape, while the broad bands with Lorenzo-Gaussian shape to better simulate the chemical shifts distribution using the NMRnotebook software program. The diffusion NMR experiments were carried out at room temperature on a Bruker AMX 400 spectrometer, at 9.4 T and

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Figure 2. 1D 29Si NMR spectra of successive aqueous phases obtained during TEOS hydrolysis with TPAOH in the 25:9:152 TEOS:TPAOH:H2O system, at various h-1: (a) 0.11, (b) 0.25, (c) 0.36, (d) 0.58, (e) 0.83, (f) 1.03, (g) 1.28, (h) 1.83, and (i) 2.78. The main signals due to the most known symmetric oligomers are shown as M for monomer, D for the dimer, 3R for the single three-membered ring, 4R for the four-membered ring, D3R for the double three-membered ring, D4R for the double four-membered ring, and D5R for the double five-membered ring, as well as, the main Qn bands of the nanoparticles. Vertical expansion of the spectra is presented in the right side showing the broad signals of nanoparticles. No line broadening (LB) prior to Fourier transform was applied to spectra in the left side, while a 5 Hz LB was used in the right side spectra.

resonating frequency of 400.13 MHz for 1H, using a BBI Bruker 5 mm gradient probe. To enable field-frequency locking, external D2O solvent was employed. Spectra were recorded with a pulsed-field gradient stimulated echo (PFGSTE) sequence,37 using bipolar gradients and a 90° pulse duration of 10.0 µs. Mass spectra were recorded on a Bruker 9.4 T Apex III FTICR mass spectrometer equipped with an Apollo ESI source. The solutions were introduced by direct injection at a flow rate of 100 µL/h and measured, accumulating 150 scans with a capillary voltage of 4500 V, a desolvation temperature of 150 °C, and a CapExit voltage of -120 V. Each acquisition contained 500k data points with lower and upper mass limits of 70 m/z and 3500 m/z, respectively. When the concentration of silicate is too high (at high hydrolysis levels), dilution with H2O at the 25% (v/v) level of the original solution was necessary to enable mass spectrometry measurements. However, no dilution effects were observed on silicate speciation in the mass spectra of samples at a low value of h-1, allowing direct comparison between the overall series of samples whatever the dilution state of the sample. With the FT-ICR mass spectrometer it is possible to obtain high mass accuracy up to about 1-5 ppm. An external calibration has been carried out using a reference mixture containing compounds with m/z from 431 to 2834. This high mass accuracy permits one to attribute, with the help of the isotopic distribution if necessary, unequivocally a specific silicate species to the different m/z observed. For this, a macro written in Microsoft Visual Basic has been used which screened all the possibilities for silicate species with 1-50 Si atoms and TPA-silicate clusters with up to seven TPA. From the isotopic distribution of the species signals it was apparent that mainly singly charged species were present; thus only silicate anions or silicate clusters with TPA having a total charge of -1 have been searched. The m/z difference tolerance was up to 0.03 between the theoretical and the experimental m/z. Dynamic light scattering was performed with an ALV CGS-3 equipped with a multiangle goniometer with a HeNe laser having a wavelength of 632.8 nm with 22 mW output power. Samples were measured at 25.0 °C.

Figure 3. Si distribution in organic phase (TEOS), aqueous phase (oligomers), and nanoparticles during the hydrolysis of TEOS with TPAOH in the 25:9:152 TEOS:TPAOH:H2O system according to 29Si NMR, at various h-1. The red lines represent the expected amounts of oligomers (broken) and nanoparticles (solid) according to Iler’s model based on the solubility limit of silica particles.

3. Results 3.1. 29Si NMR Spectroscopy. The 1D 29Si NMR spectra of the different aqueous phases obtained during the TEOS hydrolysis with TPAOH in the 25:9:152 TEOS:TPAOH:H2O system are shown in Figure 2. In the first moments of the hydrolysis, the spectra show only sharp lines, characteristic for dissolved silicate oligomers (line width ∼1 Hz). As the hydrolysis progresses, broader bands appear, caused by the distributed local silicon environment of nanosized particles.9,25,28,35,38,39 The spectra can be quantitatively analyzed in order to determine the relative amount of nanoparticles and oligomers (see Figure 3). Before reaching a hydrolysis ratio h-1 of about 1.0, no nanoparticles are present. Between values of h-1 ) 0-1.0, the amount of oligomers increases as TEOS is being hydrolyzed. When h-1 exceeds the value of 1.0, i.e., the total chemical composition passes 9:9:152:36 Si:TPAOH:H2O:EtOH, nanoparticles appear, and at the same time, the relative fraction of silicon in oligomers decreases. This discontinuity at h-1 ) 1 is fully consistent with the conductivity measurements carried out by Vlachos et al.25 Figure 4 shows the quantitative evolution of the different Qn sites of the oligomers and nanoparticles. The chemical shift ranges are as follows: from -71 to -72 ppm for Q0, from -79 to -83

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Figure 4. Quantitative distribution of the silicate Qn sites in the (a) oligomers and (b) nanoparticles during the hydrolysis of TEOS with TPAOH in the 25:9:152 TEOS:TPAOH:H2O system according to 29Si NMR, at various h-1. Qn∆ means a three-membered ring Si site.

ppm for Q1/2∆, from -86 to -91 ppm for Q2/3∆, from -92 to -100 ppm for Q3, and from -100 to -108 ppm for Q4. In the earliest stages of the hydrolysis, the amount of each Qn site of the oligomers increases, and especially Q2 reaching the highest value. When the nanoparticles begin to appear, the amount of Q2 sites in the oligomers decreases, while the amount of Q3 sites increases until h-1 of over 1.28. Above this level, the most abundant Qn sites in the oligomers are the Q3 sites, while they dominate the distribution in the nanoparticles from h-1 ) 1.03. The dominant oligomers at h-1 ) 2.78 are D4R and D5R, which correlate with the major MS peaks for Si8O20H7- and Si10O25H9- species (see Table 1 in the next section). At the end of the hydrolysis, the fraction of Q3 sites is about 55% in nanoparticles and 5% in oligomers. A comparison of the global degree of condensation of the different fractions irrespective of the detailed connectivity pattern is estimated by the weighted average 〈n〉 in Qn that could be calculated as follows.

〈n〉 )

∑ n(Qn/ ∑ Qn)

(1)

The 〈n〉 value can range from 0 to 4, where a 〈n〉 value of 0 is the silicate monomer and 4 is a fully four-connected silicate. The evolution of the 〈n〉 value for the range of samples studied is shown in Figure 5. The oligomers are initially slightly condensed, with a 〈n〉 value of 0.4 at the very beginning of the hydrolysis. As the hydrolysis progresses, the degree of condensation increases for the oligomers, as well as for the nanoparticles. At the end of the hydrolysis, the oligomers have a 〈n〉 value of 2.8 and the nanoparticles a value of 3.1. Thus, as the hydrolysis progresses, the silicate centers become more and more interconnected in both the oligomeric species and nanoparticles. 3.2. MS. If quantification in MS is not obvious, the technique has been shown to be at least semiquantitative in such silicate solutions.40 The nanoparticles cannot however be precisely monitored by this method, as they are too large.41 Figure 6 shows the ESI-MS spectra of successive aqueous phases obtained during

TEOS hydrolysis with TPAOH in the 25:9:152 TEOS:TPAOH: H2O system. The most dominant m/z peaks were at 233, 413, and 551 for low (h-1 ) 0.36), intermediate (0.36 < h-1 < 1.28), and extended (h-1 > 1.03) hydrolysis levels, respectively (Table 1). These weights correspond to Si3O9H5-, Si6O15H5-, and Si8O20H7anions, respectively. These results are in quite good agreement with the NMR spectra (Figure 2), which showed that at these hydrolysis levels the main species were actually the single three-ring (3R), the double three-ring (D3R), and the double four-ring (D4R) species, respectively. A table of all identified m/z peaks with their intensities and species assignment is provided as Supporting Information. In some cases TPA remains attached to oligomers as previously reported.40 Figure 7 shows the evolution of the different Sin-mer species monitored by mass spectrometry. For h-1 ) 0.36, there is a small amount of species containing mainly between three and six silicon atoms, with a maximum for tetramers. For h-1 ) 1.03, there is an increased amount of larger oligomers, consistent with the NMR results. The species present in the highest quantities are hexamers and heptamers for this solution. At h-1 ) 1.83, NMR showed that there is already over 58% of silicon in nanoparticles and less than 8% in oligomers, the remaining silicon being the unreacted TEOS. This is reflected in the mass spectra, which also indicate fewer oligomers at h-1 ) 1.83 than at h-1 ) 1.03. The major species in the clear sol at h-1 ) 1.83 contains seven to nine silicon atoms. At the end of the hydrolysis, the system has reached a distribution of species ranging from the trimer to the 18-mer with a maximum at the octamer. The most abundant species in the mass spectrum, containing eight silicon atoms, may correspond to the cubic octamer D4R oligomer as indicated by NMR to be the predominant species together with the D5R silicate. 3.3. DLS. The original undiluted samples have been studied by DLS. Figure 8 shows the evolution of the autocorrelation functions (ACFs) g1(t) of the solutions with changing h-1. For diffusive processes, the decay time τ of the ACFs is inversely proportional to the diffusion constant D of the scattering species, via the relation

1/τ ) Dq2

(2)

where q is the scattering vector. The ACFs for h-1 below 1.03 vary only slightly from one another and showed rapid decay indicative of fast diffusing oligomers. The ACF’s decay times become longer from h-1 ) 1 onward in agreement with the presence of nanoparticles. Inverse Laplace transformation (ILT) of the ACFs is usually used to determine the decay time distributions. However, the ILT is very sensitive to the signalto-noise of the data. The ILT analysis with the actual signalto-noise did not seem to capture the evolution of the ACFs toward slower decay times with increasing h-1. To quantify the average decay time, stretched exponential functions were used to model the ACFs (see Supporting Information). The stretched exponential has the following form:

g1(t) - 1 ) B + A exp(-(t/τs)β)

(3)

where B refers to the baseline correction factor and A to signal amplitude and is a value between 0 and 1. The average decay time is given by

τs,av ) τs /βΓ(1/β)

(4)

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TABLE 1: Main Oligomeric Silicate Species for the Major MS m/z Peaks Observed during the Hydrolysis of TEOS with TPAOH in 25:9:152 TEOS:TPAOH:H2O intensity (%) m/z 214.9141 232.9247 274.8809 292.8914 310.9020 352.8582 370.8688 412.8250 430.8355 490.8023 496.1164 508.8128 550.7690 628.7464 676.0166 688.7131 748.6799 753.9940 808.6466

species 1-

Si3O8H3 Si3O9H51Si4O10H31Si4O11H51Si4O12H71Si5O13H51Si5O14H71Si6O15H51Si6O16H71Si7O18H71Si4O12H62- + 1 TPA+ Si7O19H91Si8O20H71Si9O23H91Si7O18H62- + 1 TPA+ Si10O25H91Si11O27H91Si8O21H82- + 1 TPA+ Si12O29H91-

h-1 ) 0.36

h-1 ) 0.58

h-1 ) 0.83

h-1 ) 1.03

h-1 ) 1.28

h-1 ) 1.83

h-1 ) 2.78

6.22 17.15 7.12 15.33 6.59 1.84 15.87 8.11 3.37 0.31 4.62 1.75 0.36

1.88 3.61 7.15 7.81 0.89 6.66 10.24 35.06 2.43 14.39 0.08 1.03 3.50 0.95 0.37 0.11 0.26 0.38 0.16

0.40 1.27 1.87 3.95 1.42 1.30 12.46 26.98 9.31 2.38 0.82 10.27

1.23 2.69 2.78 5.24 1.22 1.76 11.25 12.52 8.67 7.59 0.30 10.92 9.70 3.37 3.16 0.68 0.28 3.43 0.09

0.20 0.33 1.05 1.03 0.24 0.58 4.94 11.07 3.34 2.16 0.07 11.10 14.61 7.54 2.75 5.55 3.94 2.58 3.35

0.39 0.66 1.03 1.78 0.34 0.57 5.44 6.59 3.38 1.79 0.06 12.62 12.33 7.47 1.55 3.89 1.81 3.58 1.13

0.38 0.32 1.40 0.74

0.30

where Γ is the gamma function. The stretched exponential function is typically used to empirically describe complex exponential decay processes. A lower β means a broader distribution of decay times. The average decay times from the stretched exponential function fits well the increase with h-1 and are consistent with the static scattered intensity, which should increase when nanoparticles are formed (Figure 9). The DLS data could be analyzed further by noting that ACFs show biexponential decay for h-1 values exceeding 1, with characteristic decay times around 0.005 and 0.1 ms, respectively. The biexponential decay reflects the dual oligomer-nanoparticle nature of the samples. The fast decay is also present at lower h-1 and is therefore assigned to the oligomers, whereas the slow decay is due to nanoparticles. With a biexponential fit, the average decay from the stretched fit is decomposed into these two components. The fit equation is

g1(t) - 1 ) B + Aoligomers exp(-t/τoligomers) + Aparticles exp(-t/τparticles) (5) Examples of biexponential fits as a function of hydrolysis level are provided as Supporting Information. The decay time of the oligomers was of similar magnitude in the absence (below

Figure 5. Average 〈n〉 coefficient in Qn notation for oligomers and nanoparticles during the hydrolysis of TEOS with TPAOH in the 25: 9:152 TEOS:TPAOH:H2O system according to 29Si NMR, at various h-1.

0.58 3.61 0.23 0.15 0.72 0.13

0.51 3.99 7.04 1.11 2.08 8.96 16.04 10.42 0.48 8.24 6.01 1.26 5.06

h-1 ) 1) and in the presence of nanoparticles. The decay time of nanoparticles was an order of magnitude larger than that of oligomers and increased only slightly for h-1 above 1. The amplitude of the two exponential modes is proportional to the intensity of light scattered by each of the diffusing species. The proportion of the intensity scattered by nanoparticles increases while that of oligomers decreases continuously after h-1 above 1 (see the Supporting Information). The hydrodynamic radius of nanoparticles is calculated from the decay times using the Stokes-Einstein equation, assuming an average viscosity between ca. 0.011 and 0.013 Ns/m2 observed for solutions before and after complete hydrolysis and gives a value in the range of 0.8-1.2 nm (Figure 10), in agreement with previously published data.38 3.4. 1H DOSY NMR Spectroscopy. The 1H NMR spectra of the aqueous phases obtained at different levels of TEOS hydrolysis exhibit six signals as expected due to water (singlet at ∼5.0 ppm), ethanol produced during the hydrolysis (narrow triplet at ∼1.2 ppm and narrow quadruplet at ∼3.7 ppm), and TPA species (broad triplet at ∼1.0 ppm and unresolved multiplets at ∼1.7 and 3.2 ppm). The corresponding DOSY spectra were processed using an adapted algorithm based on the Inverse Laplace Transform, and Maximum Entropy Maximisation (ILT MEM), to extract a diffusion coefficient D for each detected peak.42 The DOSY spectrum was then constructed by taking the NMR line shape of the individual separated species in the NMR (horizontal) dimension centered on the fitted diffusion coefficient D in the diffusion (vertical) dimension. The diffusion profile for each species is obtained from the diffusion dimension on the respective NMR chemical shift position. Selected spectra are provided as Supporting Information. The evolution of diffusion coefficients, for each species obtained in these solutions, as a function of h-1 is plotted in Figure 11. The relative low values of observed diffusion coefficients (D ) 100-400 µm2 s-1) compared to those of free solvated species in diluted solutions, usually ca. 2100 µm2 s-1, are due to the large viscosity of the medium, 0.013 Ns/m2 instead of 0.001 Ns/m2 for pure water. The progressive increase of the diffusion coefficient observed for ethanol with increasing h-1 is an interesting result considering the stationary values of viscosity within the range of 0.011-0.013 Ns/m2.

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Figure 6. ESI-MS spectra of successive aqueous phases obtained during TEOS hydrolysis with TPAOH in the 25:9:152 TEOS:TPAOH:H2O system, at various h-1: (a) 0.36, (b) 0.58, (c) 0.83, (d) 1.03, (e) 1.28, (f) 1.83, and (g) 2.78. The spectra are presented on the same absolute vertical scale except in b, where a factor of 1/2 is applied, allowing direct quantitative comparison.

4. Discussion Among the many studies on silicalite syntheses, the question of nanoparticles formation has been addressed by studying the first stages of the synthesis of silicalite, either by a theoretical description27 or by progressive addition of silica to TPAOH aqueous solutions.25 In this work, the direct incorporation of TEOS into TPAOH water solution has been performed and oligomer as well as nanoparticle connectivity established. The results presented above show clearly that the hydrolysis of TEOS into an aqueous solution of TPAOH has two distinct regions before and after a h-1 value of approximately 1. For h-1 smaller than 1, TEOS hydrolyses forming a clear solution of silicate oligomers. For h-1 greater than 1, nanoparticles are formed. This phenomenon is often referred to as the silica

solubility limit. If this would be the case, because the oligomers are the soluble species, their concentration would stay constant as soon as precipitation of silica starts, i.e., at h-1 ) 1. This is not the case; when h-1 ) 1 is reached, the amount of oligomers decreases, and the increase of silicon inside the nanoparticles increases much more than the added silicon beyond h-1 ) 1. The expected behavior for reaching silica solubility is plotted in Figure 3, and the data reported in this study do not match this thermodynamic expectation. One may object that when a value of h-1 ) 1 is reached, the situation could be a supersaturation, explaining why the fraction of soluble oligomers decreases afterward. This does not agree either with the observed data. Actually, if the silicon concentration decreases due to precipitation, the condensation state of oligomers should be

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Figure 10. Average particle radii from stretched exponential fits and separate radii of nanoparticles and oligomers from biexponential fits during the hydrolysis of TEOS with TPAOH in 25:9:152 TEOS: TPAOH:H2O system, at various h-1 using the Stokes-Einstein formula assuming an average viscosity of 0.012 ( 0.001 Ns/m2.

Figure 7. Silicate species distribution in solution at various h-1, during the hydrolysis of TEOS with TPAOH in the 25:9:152 TEOS:TPAOH: H2O system according to Si atom number determined by mass spectrometry. The top panel corresponds to the beginning of TEOS hydrolysis, from h-1 from 0 to 1 and the bottom to higher values of h-1.

Figure 11. Diffusion coefficients of water, ethanol, and TPA species according to 1H DOSY NMR, and of nanoparticles according to DLS, during the hydrolysis of TEOS with TPAOH in the 25:9:152 TEOS: TPAOH:H2O system, at various h-1.

SCHEME 1

Figure 8. Autocorrelation functions (θ ) 105°) of successive aqueous phases obtained at various h-1, during the hydrolysis of TEOS with TPAOH in the 25:9:152 TEOS:TPAOH:H2O system.

Figure 9. Comparison of average decay times from stretched exponential fits (left) with static scattered intensity (right) during the hydrolysis of TEOS with TPAOH in the 25:9:152 TEOS:TPAOH:H2O system, at various h-1.

reduced. This is not the case; the connectivity of oligomers after h-1 ) 1 increases. Therefore neither a silica solubility limit nor a supersaturation of silica can explain the data established in this study. Therefore, one has to consider the critical evolution of parameters, the fraction of soluble oligomers, and the fraction of nanoparticles, as well as their connectivity. The connectivity

of oligomers increases all along the TEOS incorporation, the connectivity of nanoparticles also, as soon as nanoparticles are formed. The difference between the connectivity of oligomers and nanoparticles is small, about 0.3 units. Additionally, the nanoparticles connectivity is far from being close to 4.0 as it would be expected for silica particles. At h-1 ) 1 the connectivity of nanoparticles is 2.4 to finish at 3.1 at the end of TEOS incorporation, for h-1 ) 2.78. The global evolution of silicon all along this incorporation into TPAOH/water solution is in agreement with the generic reactions presented in Scheme 1. The first reactions describe the oligomers formation; the higher the silicon concentration the higher the nuclearity of the oligomer will be. As expected, the hydrolysis of TEOS proceeds initially by producing monomer and small silicate anions in aqueous solution. From the 29Si NMR and MS studies the most abundant silicate species present in aqueous phase in the early stages of hydrolysis are monomers, dimers, and single-ring species. As the hydrolysis progresses, the concentrations of these species decrease rapidly, leading to more and more

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condensed species, especially double rings. The Qn distribution of the oligomers changes quickly, and the dominant Si environment becomes Q3 between h-1 ) 1 and the end of the hydrolysis. According to the MS data, the majority of silicate anions present in solution during this final period of hydrolysis contain six to nine Si atoms, with some silicate species with higher Si atom numbers (up to 18 Si), in lower abundance. The second reaction describes the aggregation of oligomers into nanoparticles. For simplicity of the typographical notation, Sik is an average value of the distribution of the oligomers in the aggregates. The medium contains therefore both small silicate oligomers and stable colloidal nanoparticles simultaneously. The nanoparticles are composed of Q2, Q3, and Q4 silicate sites9,30,38,39 and follow an increase in condensation that parallels exactly the condensation evolution of oligomers. While aggregation occurs, the rate of nanoparticles formation is much higher than the simple addition of silicon in excess to the silicon added after h-1 ) 1, as can be observed on Figure 3, with a slope “fraction of silicon to h-1” about three times greater for nanoparticle formation than for oligomer formation in the range of h-1 values from 0 to 0.8. One can sum up the succession of steps as follows. First a critical concentration of silicon exists, as already suggested,25 that depends on the concentration of TPAOH in the aqueous phase and of the pKa of Si(OH)4 in the final medium containing TPAOH, water, and ethanol. At this critical concentration, aggregation takes place, and nanoparticles are formed by aggregation of oligomers. Some extra-condensation takes also place, though being not the dominant aspect of the nanoparticles formation. In the early stage of nanoparticle formation within the period 1 < h-1 < 1.8, the particles’ size is roughly constant, or increasing only slowly, with an average radius of ca. 0.9 nm (Figure 10) and in the same h-1 range the internal Q3 and Q4 fractions increase (Figure 4). Provis and Vlachos proposed a model to form nanoparticles by addition of monomers,27 while our results indicate that they are formed directly from aggregation of all types of oligomers. At h-1 ) 1 the titration equivalent point of TPAOH by Si(OH)4 introduced in (TPAOH-water solution is reached. Because the liquid is far from being an infinitely diluted solution but is a mixture of components in comparable amounts, the titration cannot be simply described by the usual terms. When TPAOH titrates Si(OH)4, neither TPAOH nor Si(OH)4 species are completely solvated. Therefore, the titration may be more consistently described as TPAOH + Si(OH)4 ) TPA(OH · · · H)OSi(OH)3, i.e., the formation of an inner ion pair. The two pKa of the monomeric silicic acid are 9.8 and 13.2. At about pH 14, the silicic acid occurs mainly as Si(OH)3O- next to some Si(OH)2O22-. A small change at the equivalent point Si:TPAOH ) 1:1 should be observed in diluted solution. In concentrated systems, as in the present case, the pH may not change much, despite being quantitative due to the occurrence of ion pairing. However, a strong change in activities of components can be expected to take place. Beyond h-1 ) 1.0 any addition of TEOS would be an addition of Si(OH)4. Any oligomerization goes through an oxolation reaction of the general type Si(OH) + (OH)Si ) Si-O-Si + H2O. The latter does not affect the acido-basicity of the equilibria. Beyond h-1 ) 1, the addition of a zero charged species Si(OH)4 is concomitant with the appearance of nanoparticles. Ion-paired oligomers as well as Si(OH)4 can condense and aggregate. An additional phenomenon takes place within the nanoparticles with the apparition of Q4 sites, at contrast to the oligomeric anions that do not display any noticeable amount of Q4 silicon sites. It implies that aggregation cannot explain alone the average

Petry et al. connectivity, but some specific condensation takes place within the nanoparticles. Actually, connecting the last Si-O-Si bond to form a Q4 site implies two aspects. The enthalpic part of the Si-O-Si bond formation is probably equivalent to what happens for a Si-O-Si formation for an oligomer. The entropic part is probably not the same at all. Forming a Q4 site implies the accessibility of the reacting bonds. Accessibility requires being able to break and form bonds quite easily in order to reach the proper conformation that will allow the condensation reaction. If this were not the case, only the bonds that are spontaneously accessible would form, not all that are energetically possible. The Q4 sites formation is probably what makes the difference between nanoparticles and oligomers. In oligomers the last reacting Si(OH) is probably not favored, from an enthalpic point of view; but from an entropic point of view, forming a Q4 site, it may be favored, but this may require a much longer time to break and form bonds many times to reach the proper accessible geometry. At the end of the TEOS hydrolysis, the average silicon site is of Si(OH)0.54(OSi)3.1(OTPA)0.36. This ratio TPA/Si is much higher than expected for an amorphous “proto-silicalite” that would contain about 10% Q3 and their associated TPAOH and 90% Q4, as the crystalline compound does.30 It is consistent with the observation of Martens et al. of TPA included into the nanoparticles,43 or with those of Vlachos et al.,30 which are having nanoparticles of about the same nature as that in this work. The nanoparticles observed by Fyfe et al.29 are obtained after about 40 h at 90 °C and separated by ultracentrifugation; they were at a much later stage of condensation, with, probably, almost all silicon sites in the Q4 state; they cannot contain any longer much TPA, which is their conclusion. The formation of nanoparticles by aggregation/condensation is also supported by the DLS data, indicating a stationary particle size along the early stage of their formation, while their concentration during hydrolysis increases rapidly, starting at h-1 ) 1.0 as well as their connectivity. The unexpected diffusion coefficients of water, ethanol, and TPA, much lower than their value when diluted in water, have been attributed to the very high viscosity and to the strong interactions taking place between the components of the mixture, water, oligomers, ethanol, and TPA, differing significantly from pure water. Indeed, in extra 1H DOSY experiments with simplified systems, diffusion coefficients changed depending on the solution composition and concentration. For water and ethanol, D of about 1200 and 830 µm2 s-1 were found respectively in a mixture of water-ethanol with respective molar ratio of 4:1. However, in 9:400 TPA: H2O solution, diffusion coefficients of water and TPA were surprisingly low, ca. 530 and 120 µm2 s-1, respectively, in the same order of magnitude as the values observed in the studied solutions (Figure 11). Dilution with increasing water amount led progressively to an increase of both diffusion coefficients up to ca. 2100 and 570 µm2 s-1, respectively, for 9:1200 TPA: H2O solution (see Supporting Information). Nevertheless, water is definitely affected by the appearance of nanoparticles and seems to be slowed down in association with them. The stationary evolution in TPA diffusion coefficients during the course of hydrolysis would be due to a weaker interaction of TPA with water and ethanol. Indeed, the measured values correspond to an average behavior of all kinds of TPA species within the NMR time scale, a fast exchange process between TPA in solution, and TPA interacting with nanoparticles. The progressive increase of the diffusion coefficient of ethanol upon hydrolysis reaching the value of water at the end of the process would indicate implication of water and ethanol in a cooperative

Connectivity of Silicates in Solution for Silicalite-1 cosolvent network. Due to its lower dielectric constant, it would favor a weak ion pair between TPA and silicate oligomers such as clathrates as it has been previously proposed.44 However, one has to keep in mind that EtOH is progressively generated during hydrolysis, leading to a continuous increase of the relative amount of “free EtOH” that could also affect the dynamic phenomena occurring in these solutions. 5. Conclusion Several studies have tried to describe the silicate nanoparticles in the TEOS-TPAOH aqueous system using various investigation techniques and approaches.26,29,30 Early stages did not have for all these studies the same meaning. Here, we are capturing directly the first stages with no separation of the nanoparticles from solution and no thermal treatment at about 95 °C as in the classical silicalite synthesis. The actual work is in full consistency with the previous studies concerning the stages of hydrolysis though providing a much more detailed view of the evolution of the connectivity of the silicate centers throughout the hydrolysis process. The earliest stages captured in this study demonstrate condensation of oligomers of an average number of 8 silicon atoms/ oligomer and with a maximum of about 30 silicon atoms/ oligomer, incorporation of TPA, before further interoligomer condensation within the nanoparticles. As a consequence, TPA is correlatively eliminated when connectivity increases. 29 Si NMR monitors the evolution of the Qn sites distribution in oligomers and nanoparticles over the hydrolysis process of TEOS in aqueous TPAOH. The results correlate well with MS. Both techniques showed an increase in the size of the oligomers during the hydrolysis. At the end of the hydrolysis process, MS shows that the most abundant species contains eight Si, and according to NMR, the cubic octamer D4R oligomer is a predominant species together with the D5R silicate anions. As the hydrolysis progresses, nanoparticles appear when the Si content in aqueous solution exceeds a critical value corresponding to the Si/OH ratio, h-1 ) 1, exhibiting increasing Si-O-Si connectivity with increasing h-1. The formation of nanoparticles occurs according to a two-step process: an oligomer aggregation and additional restructuring to form Q4 sites. Increase in connectivity occurs for oligomers as well as for nanoparticles with an average connectivity of nanoparticles being about 0.3 units above oligomers. DLS measurements support the observations made by NMR and MS that the nanoparticles appear at a very specific h-1 value close to 1.0. With further hydrolysis up to completion, the number of nanoparticles increases, but their size remains approximately constant. The dynamics of the system as observed by 1H DOSY NMR would suggest implication of water and ethanol in a cooperative cosolvent network favoring ion-pair interaction between TPA and silicate species. The results presented in this study confirm and extend the original ideas of Iler about the silica particles.45 The nanoparticles reported here have a structure that depends on pH, on Si(OH)4/TPAOH ratio, and on ethanol content. More generally they will also depend on the nature of the tetraalkyl amonium used. As they form colloidal particles by aggregation of oligomers, with partial condensation their behavior is probably more complex than what had been foreseen by Iler and need further differentiation. The term silica particles for the nanoparticles formed in this study would be misleading. A new term is demanded. Maybe nanoparticles of aggregated oligomers could do, or why not a short version: “silicate nanoaggregates”?

J. Phys. Chem. C, Vol. 113, No. 49, 2009 20835 Acknowledgment. The authors acknowledge the Engineering and Physical Sciences Research Council (EPSRC) and ExxonMobil Research and Engineering for funding within the framework of the international “Nanogrowth” project. A.A. is grateful to the Flemish FWO for a postdoctoral scholarship. Supporting Information Available: Experimental details of samples preparation, stretched exponential fits to field ACFs, examples of biexponential fits to field ACFs, relative amplitudes of the oligomers and nanoparticles fraction from stretched biexponential fits, 1H DOSY NMR spectra, diffusion coefficients of water and TPA species according to 1H DOSY NMR, and accurate determined m/z. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J. Am. Chem. Soc. 1956, 78, 5963. (2) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 156. (3) Snyder, M. A.; Tsapatsis, M. Angew. Chem., Int. Ed. 2007, 46, 7560. (4) Bjorgen, M.; Joensen, F.; Lillerud, K. P.; Olsbye, U.; Svelle, S. Catal. Today 2009, 142, 90. (5) Bein, T. Curr. Opin. Solid State Mater. Sci. 1999, 4, 85. (6) Cundy, C. S.; Lowe, B. M.; Sinclair, D. M. J. Cryst. Growth 1990, 100, 189. (7) Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature 1978, 271, 512. (8) Persson, A. E.; Schoeman, B. J.; Sterte, J.; Otterstedt, J.-E. Zeolites 1994, 14, 557. (9) Aerts, A.; Follens, L. R. A.; Haouas, M.; Caremans, T. P.; Delsuc, M.-A.; Loppinet, B.; Vermant, J.; Goderis, B.; Taulelle, F.; Martens, J. A.; Kirschhock, C. E. A. Chem. Mater. 2007, 19, 3448. (10) Bussian, P.; Sobott, F.; Brutschy, B.; Schrader, W.; Schu¨th, F. Angew. Chem., Int. Ed. 2000, 39, 3901. (11) Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1988, 27, 4259. (12) Knight, C. T. G.; Harris, R. K. Magn. Reson. Chem. 1986, 24, 872. (13) Harris, R. K.; Kimber, B. J. Appl. Spectrosc. ReV. 1975, 10, 117. (14) Cho, H.; Felmy, A. R.; Craciun, R.; Keenum, J. P.; Shah, N.; Dixon, D. A. J. Am. Chem. Soc. 2006, 128, 2324. (15) Haouas, M.; Taulelle, F. J. Phys. Chem. B 2006, 110, 3007. (16) Knight, C. T. G.; Balec, R. J.; Kinrade, S. D. Angew. Chem., Int. Ed. 2007, 46, 1. (17) Schaack, B. B.; Schrader, W.; Schu¨th, F. Angew. Chem., Int. Ed. 2008, 47, 9092. (18) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4278. (19) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4272. (20) Auner, N.; Ziemer, B.; Herrschaft, B.; Ziche, W.; John, P.; Weis, J. Eur. J. Inorg. Chem. 1999, 1087. (21) Boxhoorn, G.; Sudmeijer, O.; Vankasteren, P. H. G. J. Chem. Soc., Chem. Commun. 1983, 1416. (22) Groenen, E. J. J.; Kortbeek, A.; Mackay, M.; Sudmeijer, O. Zeolites 1986, 6, 403. (23) Cheng, C. H.; Shantz, D. F. J. Phys. Chem. B 2005, 109, 19116. (24) Click, C. A.; Assink, R. A.; Brinker, C. J.; Naik, S. J. J. Phys. Chem. B 2000, 104, 233. (25) Fedeyko, J. M.; Rimer, J. D.; Lobo, R. F.; Vlachos, D. G. J. Phys. Chem. B 2004, 108, 12271. (26) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965. (27) Provis, J. L.; Vlachos, D. G. J. Phys. Chem. B 2006, 110, 3098. (28) Cheng, C. H.; Shantz, D. F. J. Phys. Chem. B 2006, 110, 313. (29) Fyfe, C. A.; Darton, R. J.; Schneider, C.; Scheffler, F. J. Phys. Chem. C 2008, 112, 80. (30) Kragten, D. D.; Fedeyko, J. M.; Sawant, K. R.; Rimer, J. M.; Vlachos, D. G.; Lobo, R. F.; Tsapatsis, M. J. Phys. Chem. B 2003, 107, 10006. (31) Davis, T. M.; Drews, T. O.; Ramanan, H.; He, C.; Dong, J. S.; Schnablegger, H.; Katsoulakis, M. A.; Kokkoli, E.; McCormick, A. V.; Penn, R. L.; Tsapatsis, M. Nat. Mater. 2006, 5, 400. (32) Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.; Houssin, C. J. Y.; Mojet, B. L.; van Santen, R. A.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. Angew. Chem., Int. Ed. 2001, 40, 2637. (33) Tokay, B.; Somer, M.; Erdem-Senatalar, A.; Schuth, F.; Thompson, R. W. Microporous Mesoporous Mater. 2009, 118, 143. (34) Knight, C. T. G.; Wang, J. S.; Kinrade, D. Phys. Chem. Chem. Phys. 2006, 8, 3099.

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