NMR and SAXS Analysis of Connectivity of Aluminum and Silicon

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NMR and SAXS Analysis of Connectivity of Aluminum and Silicon Atoms in the Clear Sol Precursor of SSZ-13 Zeolite Einar A. Eilertsen,†,‡ Mohamed Haouas,‡ Ana B. Pinar,§ Nathan D. Hould,§ Raul F. Lobo,§ Karl P. Lillerud,† and Francis Taulelle*,‡ †

Center for Material Science and Nanotechnology/INGAP/Departement of Chemistry, University of Oslo, Norway. Tectospin, Institut Lavoisier de Versailles, University of Versailles Saint Quentin en Yvelines, France. § Catalytic Center for Energy Innovation, University of Delaware, Delaware, United States ‡

S Supporting Information *

ABSTRACT: We report the first study of the hydrolysis of tetraethyl orthosilicate (TEOS) in an aqueous solution of N,N,N-trimethyl-1-adamantammonium (TMAda) hydroxide, the clear sol precursor for the preparation of the high-silica zeolite SSZ-13 (CHA). The initial stages of the hydrolysis of TEOS were monitored by quantitative 29Si and 27Al nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS). 29Si NMR allowed quantitative characterization of Si in nanoparticles and dissolved oligomers and measuring the average Si−O−Si connectivity. The average Si connectivity increases when hydrolysis advances, and at a [Si]/[TMAdaOH] ratio of one, nanoparticles are detected. The average connectivity of nanoparticles reached 3.1. This is similar to what has been observed during TEOS hydrolysis with other organic bases, i.e., tetrapropylammonim hydroxide (TPAOH) and tetrabutylammonium hydroxide (TBAOH) used for silicalite-1 and silicalite-2 syntheses, confirming that it is a general phenomenon independent of the structure of the organocation. 27Al NMR shows that the connectivity of Al increases as well with increasing [Si]/[TMAdaOH] ratio. Aluminum atoms are in tetrahedral coordination to four silicate units SiO44− and are located exclusively in the nanoparticles. KEYWORDS: zeolite synthesis, NMR, SAXS, nanoparticles, nanoaggregate, in situ NMR, crystallogenesis, self-assembling, silicon connectivity

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and SAXS.6−14 During hydrolysis of TEOS in the basic SDA solution, small nanoaggregates form at critical aggregation concentration (cac) corresponding to a [Si]/[SDA+] ratio of 1.8,9,14 At lower ratios, the solution consists exclusively of monomers and small oligomers. The nanoaggregates, often called primary nanoparticles in the zeolite literature, are involved at the first stage of zeolite formation, through at least another condensation/aggregation stage forming secondary nanoparticles and to the zeolite through an aggregative nucleation process.15 MFI-type silicalite-1 has served as a model system for studying zeolite synthesis mechanisms because of its ease of preparation.3,6,7,10,14,16−26 The pure silica system was chosen because aluminosilicate complexes would lead to much more complex 29Si NMR spectra. The initial stages of hydrolysis of TEOS in TPAOH solutions for the preparation of the silicalite1 precursor were investigated by Petry et al.14 They showed, by calculating the average Si connectivity, that the nanoparticles in the system had to be mostly aggregates of oligomers rather than fully condensed silica-like particles.

INTRODUCTION Zeolites are microporous crystalline materials that have found commercial applications in the refining and petrochemical industries because of their well-defined porosity and acidic properties.1,2 Even though zeolites have been extensively investigated and a large number of different zeolites have been synthesized, zeolite crystallization is still an area where molecular understanding of the self-organization process is lacking. Zeolites are commonly synthesized in aqueous gels that contain a silica source, an alumina source, a structure directing agent (SDA), and a mineralizer (F− or OH−), all heated under autogenous pressure at high temperature (usually 90−200 °C). Zeolite synthesis gels are usually difficult to characterize with most analytical methods, but investigations of the formation of zeolites in dilute clear sols has attracted particular interest because it may yield experimental analysis and provide insight into the molecular processes that underlie zeolite formation.3−5 These clear sols are visually transparent and when prepared from tetraethylorthosilicate (TEOS), the SDA, and water, they contain both nanoparticles and oligomeric silicate species. It allows for the characterization of the reactive medium with methods such as NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), dynamic light scattering (DLS), © 2012 American Chemical Society

Received: October 31, 2011 Revised: January 11, 2012 Published: January 11, 2012 571

dx.doi.org/10.1021/cm2032515 | Chem. Mater. 2012, 24, 571−578

Chemistry of Materials

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nanoparticles,45 correction factors, in the range 1.1−1.4 and 2.2−2.9, respectively, have been applied. These values were determined by comparing two spectra of some selected samples acquired with two relaxation periods, i.e., short (5 s) and long (120 s) recycle delays using PTFE NMR tubes. Interpolated factors were applied for the other samples. 27Al NMR spectra were obtained by applying 15° pulses (8 μs pulse duration) and a recycle delay of 0.5 s, sufficient enough to allow a complete return of the magnetization to equilibrium. Chemical shifts were referenced with respect to tetramethylsilane for 29Si and to the [Al(H2O)6]3+ cation for 27Al. All solutions were clear to the eye at the time of recording the spectra (i.e., no turbid gel had been formed), except for some samples in the CHA system in the range Si/TMAdaOH = 0.66−1.33 exhibiting small crystals due to cyclosilicate hydrates. NMR quantification was performed by spectral analysis. Simulation of the narrow lines was conducted with a Lorenzian shape, while broad bands were with a Lorenzo-Gaussian shape to better simulate the chemical shifts distribution using the NMRnotebook software program.46 SAXS patterns were collected on a SAXSess instrument (Anton Paar) using line-collimated CuKα radiation (1.542 Å). A TCS120 sample holder fitted for liquid samples in a quartz capillary (1 mm diameter) was employed. The X-ray scattering patterns were measured for 40 min using a phosphor image plate, and scaled to I(q = 0) = 1. Deionized water background was then subtracted from the scattering pattern. Real-space information was obtained by calculating pair distance distribution functions (PDDFs) by the indirect Fourier transform (IFT) technique.47,48

Only few reports have investigated the role of the trivalent heteroatom (e.g., Al, B, Ga, Fe, etc.) substitution in zeolite crystallization.27−29 These heteroatoms are important in zeolites because they give rise to new chemical and structural properties of zeolites.2,30 In addition, many zeolites can only be synthesized in a narrow heteroatom concentration range. It is of practical and scientific value to understand the role of these heteroatoms in zeolite crystallization. Zeolite beta has been used as a model system to study the effects of B and Al heteroatom substitutions on zeolite nucleation because of the broad heteroatom synthesis concentration range and the variety of SDA’s available for its crystallization.31−33 A number of studies have investigated the crystallization for related zeotype materials,34−40 including the silicoaluminophosphate analogue of SSZ-13 (SAPO-34).41,42 We have recently developed a clear sol synthesis route for the SSZ-13 zeolite with Si/Al ratios from 12−100.43 The presence of Al in the clear sol precursor has proven to be essential to obtain the crystalline high-silica CHA phase. Still, the only way to obtain pure SiO2 CHA is from an almost dry gel synthesis (H2O/SiO2 = 3) with F− as the mineralizer.44 Here, we report the first study of the clear sol precursor to the zeolite SSZ-13 (CHA) in an effort to elucidate the effect of the organocation TMAda+ and the presence of Al on chemical composition, internal connectivity, and stability of precursor nanoparticles as well as soluble species. The initial stages of the hydrolysis of TEOS in an aqueous solution of TMAdaOH, with and without Al, were monitored by quantitative 29Si and 27Al NMR as well as SAXS. Furthermore, the results from the hydrolysis of TEOS in TMAdaOH are compared with results from hydrolysis of TEOS in TPAOH and TBAOH.

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RESULTS AND DISCUSSION Hydrolysis of TEOS in a Pure Silicate System. Ten solutions/sols with different degrees of TEOS hydrolysis by TMAdaOH were prepared and analyzed with 29Si NMR. Note that the hydrothermal treatment of the pure silicate sol results in an amorphous material, while hydrothermal treatment of the aluminosilicate system (Si/Al = 12−100) yields SSZ-13 (CHA). The 29Si NMR spectra of the clear solutions and sols at different stages of the hydrolysis as a function of increasing [Si]/[TMAdaOH] ratio are shown in Figure 1. In the initial

EXPERIMENTAL SECTION

Clear sols with compositions according to Table 1 were prepared by adding the appropriate amount of TEOS (Aldrich 98%) to a solution

Table 1. Molar Composition of Sols/Solutions Studieda

a

topology

SDA

SiO2

Al2O3

SDA

H2O

EtOH

CHA MFI MEL

TMAdaOH TPAOH TBAOH

x x x

y 0 0

7.5 9 13

438 152 156

4x 4x 4x

x = 0−25; y = 0−0.5.

of TMAdaOH (ZeoGen 2825 Sachem 1M), TPAOH (Acros 40 wt %), or TBAOH (Fluka 40 wt %) in D2O/H2O containing optionally dissolved Al(OH)3 (Aldrich 50−57.5 wt % as Al2O3) at room temperature. The resulting emulsion was vigorously stirred for 30 min until complete hydrolysis of TEOS, leading to a single aqueousalcoholic phase. The resulting solutions/sols appeared to be stable with time since no significant change of the 29Si NMR spectra was noticed over a period of ca. 24 h (see Supporting Information). Furthermore, 29Si NMR indicated that almost all TEOS was fully hydrolyzed, and only traces of (EtO)Si(OHn)33‑n were detected. Therefore, the samples were analyzed with 29Si, 27Al NMR, and SAXS within a few hours of their preparation. The 1D NMR experiments were carried out on a Bruker Avance 500 spectrometer, operating at 99.353 MHz for 29Si and 130.319 MHz for 27Al. In a modified background free probe, 10 mm quartz tubes were used in conjunction with relatively short repetition delays between pulses to avoid the background signal of quartz. The 29Si 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

Figure 1. Evolution of 29Si NMR spectra of clear solutions/sols obtained during the progress of TEOS hydrolysis in the TMAdaOH solution.

stages of the hydrolysis, the spectra show only sharp lines, characteristic of small dissolved silicate oligomers. As shown in the figure with the progressive appearance of upfield signals, the 572



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internal connectivity of Si in these oligomers increases with the advancement of the hydrolysis. Formation of nanoparticles, as determined by the presence of broad resonances in the spectra, is observed when the [Si]/[TMAdaOH] is 1 or higher. As the silica concentration increases, the intensity of the broad signals also increases, and there is a reduction in the absolute amount of silicon present as oligomers. This has also been observed with the silicalite-1 and silicalite-2 systems when TPAOH and TBAOH are used as SDA, and thus, this seems to be a general pattern for alkylammonium-based silicate sols.14 The formation of nanoparticles at the [Si]/[TMAdaOH] of 1 has also been observed by pH, conductivity, and SAXS measurements for the hydrolysis of TEOS in a number of alkylammonium hydroxides as well as inorganic hydroxides.8,9 The spectra can be quantitatively analyzed to determine the distribution of Si into oligomers and nanoparticles by decomposing the 29Si NMR spectra. Oligomer resonances are distinguished from those of nanoparticles based on their respective linewidths. Figure 2a (and Figure 1) shows that

Si MAS NMR revealed that both types are D4R silicate hydrates (Supporting Information). The D4R/TMAdaOH ratio is 1:6 for silicate hydrates of type I and 1:8 for silicate hydrates of type II determined by TGA. These crystals are obtained in samples with [Si]/[TMAdaOH] ratio between 0.33 and 1.33. For [Si]/[TMAdaOH] = 0.33, these crystals form after more than one day of aging at room temperature and do not affect the resolution of the NMR measurements. According to liquid state 29Si NMR (Figure 1), D4R oligomers are detected at [Si]/[TMAdaOH] = 0.66 and compete with D5R species when [Si]/[TMAdaOH] > 1.33. It is interesting to note that D4R-containing crystals form in large yield from a solution essentially free of D4R species in solution. One can therefore deduce that the D4R solubility is the lowest and that its interconversion rate with oligomers is reasonably fast compared to the crystallization rate. Figure 2b shows a comparison among hydrolysis of TEOS in TPAOH (MFI, silicalite-1), TBAOH (MEL, silicalite-2), and TMAdaOH (CHA, SSZ-13). For all systems, the same trend can be observed: similar processes regarding nanoparticles formation occurred within these systems independently of the organic SDA used. When [Si]/[SDAOH] exceeds 1, nanoparticles are formed via nanoaggregation and some additional interoligomer condensation: this appears to be a general phenomenon. At this stage, the organic SDA acts as the hydroxide ion source adjusting the pH of the medium, and affecting the stability of the silicate species present in solution by shifting the hydrolysis, condensation, and precipitation equilibria. In identifying the different Si species, we have employed the following chemical shift ranges: Q0 = −71 to −72 ppm, Q1 and Q2Δ = −79 to −83 ppm, Q2 and Q3Δ = −86 to −91 ppm, Q3 = −92 to −100 ppm, and Q4 = −100 to −108 ppm. QnΔ means a Si site with n connectivity present in a 3-ring site. Figure 3 follows the evolution of different Si sites for oligomers and nanoparticles during the hydrolysis. As seen in Figure 3a the monomer is the dominating Si site during the initial phase of the hydrolysis. As the hydrolysis progresses, the amount of each Qn sites increases, especially Q2. As nanoparticles start to form, the Q2 sites rapidly decrease in concentration leaving Q3 as the dominating site. By inspecting the most symmetrical oligomers, i.e., the Monomer (M), the Dimer (D), the cyclic trimer (3R) and tetramer (4R), and the cages D3R, D4R and D5R in Figure 3b, M is initially the dominating Si species and is followed by D, then D3R until [Si]/[TMAdaOH] = 1. After nanoparticle formation, the concentration of the D3R decreases rapidly, and the D4R is the dominating Si species. In the nanoparticles (Figure 3c) for [Si]/[TMAdaOH] > 1, Q3 sites dominate. As the hydrolysis progresses, the nanoparticles will be made up from mainly Q3 and Q4 sites. The SAXS patterns of pure-silica samples with increasing [Si]/[TMAdaOH] ratios (Figure 4) show clearly the presence of a population of nearly monodisperse nanoparticles in the sol at [Si]/[TMAdaOH] = 1.67. In the solutions investigated having [Si]/[TMAdaOH] ratios lower than 1, there is no evidence of any nanoparticle, in agreement with the 29Si NMR results. An increase in the concentration of nanoparticles with increasing [Si]/[TMAdaOH] ratios is observed, also consistent with the NMR observations. Note that the pH of the solutions/ sols (Figure 5) decreases rapidly when the [Si]/[TMAdaOH] ratio is increased from 0.67 to 1.67, fully consistent with the formation of the first nanoparticles at [Si]/[TMAdaOH] = 1.0. Further increase of the concentration of silica has only minor effects on the pH of the solutions, as has been reported before

Figure 2. (a) Si-distribution in TEOS (■), oligomers (□), and nanoparticles (●) during the hydrolysis of TEOS in TMAdaOH. The gray area represents the supersaturation domain with respect to the crystallization of D4R silicate hydrates. The dotted rings represent the samples where silicate hydrate of type I is crystallized. The full rings represent the samples where silicate hydrates of type II are crystallized. The solubility refers to that of D4R only and is dependent on the pH, i.e., Si/OH ratio. (b) Comparison of the Si distribution for CHA, MEL, and MFI systems. TEOS decreases as a straight line similar to that in panel a.

TEOS is gradually hydrolyzed to give oligomers until a [Si]/ [TMAdaOH] ratio of 1 is reached. At that point nanoparticles are detected, and the concentration of oligomers decreases, consequently increasing the concentration of nanoparticles. For four of the samples, crystallization of two different types of crystals depending on sample composition was observed: 573



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Figure 4. SAXS patterns of the synthesis solutions/sols with different [Si]/[TMAdaOH] ratios, in a pure silica system.

Figure 5. Change of the pH of the synthesis solutions/sols as a function of the silica concentration, in the absence (solid symbols) and in the presence (open symbols) of aluminum.

nanoaggregates from a [Si]/[TMAdaOH] ratio of 1 on defining therefore the “critical aggregation concentration”. At this stage, the nanoaggregates should not be confused with ulterior stages of nanoparticle formation, for which the zeolitic order has been reached.49−52 Pair distance distribution functions derived from the SAXS patterns (Figure 6) indicate that the particles increase slightly in size as the silica concentration is increased, but the changes in dimensions are small (∼2 Å). This is consistent with what has been reported for TPAOH solutions8 and also consistent with the changes in Q 4 /Q 3 ratio for the nanoparticles, which is nearly constant as a function of silica concentration. Hydrolysis of TEOS in the Aluminosilicate System. Eight solutions/sols similar to pure silicate case were prepared with small amounts of Al in the clear sol precursors ([Al2O3]/ [TMAdaOH] = 0.033). 29Si NMR spectra of these samples (not shown) show great resemblance to the spectra of Figures 1 and 3. The small amount of Al does not affect the 29Si NMR spectra to a great extent since only a small fraction of the silica is associated with Al. The 27Al NMR spectra (Figure 7) exhibit four resonances in the following chemical shift ranges: 77 to 73, 72 to 67, 66 to 62, and 61 to 54 ppm, assigned, respectively, to q0/q1 and q2, q3, and q4, where qn stands for Al directly connected to n silicate centers SiO4 via oxygen linkage, i.e., Al(OSiO3)n.53 The signal appearing between 73 and 77 ppm

Figure 3. Si-distribution in (a) all oligomers, (b) some representative selected oligomers, and (c) nanoparticles during the hydrolysis of TEOS in TMAdaOH. 100% represents all Si of the final system studied, i.e., oligomers, TEOS, and nanoparticles, assuming all Si is NMR visible. Some neglected amounts of Si invisible at Si/TMAdaOH = 1−1.33 are noticed due to silicate hydrates crystallization. QnΔ means Si site with n connectivity present in a 3-membered ring.

for TPAOH and TEAOH solutions.8 This confirms that at a ratio of 1 an equivalent acido-basic point between the weakly acidic Si(OH)4 acid and the strong OH− base is reached. This neutralization process drives the formation of nanoparticles/ 574



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Figure 6. PDDFs calculated from SAXS patterns with increasing [Si]/[TMAdaOH]ratios. (a) In the absence of aluminum; (b) in the presence of aluminum (Al2O3/TMAdaOH = 0.033).

nanoparticles are formed, Al is present within a four-connected environment (Al(OSi)4). At this stage of hydrolysis, silicate species are being increasingly negatively charged, and intersilicate polycondensation becomes the predominant process in order to compensate the species charge excess. Because they are present only in a small amount, Al species react quickly with the surrounding silicate anions and are tetrahedrally coordinated to silicates. Figure 8 shows the same

Figure 8. Quantitative distribution of the Al qn sites in oligomers and nanoparticles (q4 only for[Si]/[TMAdaOH] > 1).

trend for the Al connectivity as Figure 3 shows for the Si connectivity. In the beginning of the hydrolysis process, q0/q1 are the most important Al species. Then q2, q3, and q4 gradually take over until the point of nanoparticle formation. The global Al connectivity to silicates, increases similarly to the Si connectivity in oligomers until the formation of nanoparticles. Then all the aluminum atoms, within the detection limits of the technique, go to the nanoparticles as four-coordinated AlO4. A 5 ppm shift in the q4 peak position can be observed going from oligomeric species to nanoparticles (Figure 7b). This could indicate a drastic change in local q4 Al environment when passing from solution to nanoparticles, i.e., rigidity of the conformation or second coordination shell effect. Similar observations have been obtained when 27Al solution NMR was used to probe molecular changes occurring during the reaction of geopolymers.54 A signal at 61 ppm is detected in the early stage of reaction that transforms progressively into an upfield

Figure 7. Evolution of (a) 27Al NMR spectra of clear sols/solutions with the progress of TEOS hydrolysis. (b) Evolution of chemical shifts of qn Al sites with [Si]/[TMAdaOH] ratio.

corresponds to the resonance of both q0 and q1 species in fast chemical exchange with respect to the NMR time scale, and so its position is located at a weighted average chemical shift between those of the two separate species. As seen from the 27 Al NMR spectra in Figure 7a, the connectivity of Al also increases with increasing [Si]/[TMAdaOH]. This is a direct consequence of the progressive increase of silicate concentration in the system with fixed content of Al. After the 575



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shoulder at 57 ppm. Both signals are attributed to tetrahedral Al surrounded by four SiO4 groups, q4, but the former at peak position 61 ppm is due to dissolved mobile species and the later one at 57 ppm to a more rigid environment in a geopolymer network. Further evidence that Al is located inside the nanoparticles is given in the line width analysis of the 29Si NMR resonances of some oligomers and in particular the monomer. Indeed, NMR spectra of samples where [Si]/[TMAdaOH] is less than 1 experience significant line broadening when Al is present, while for [Si]/[TMAdaOH] higher than 1, no such “Al effect” is observed. Figure 9 shows that for [Si]/[TMAdaOH] = 0.67

Figure 10. SAXS patterns of the synthesis solutions/sols with different [Si]/[TMAdaOH] ratios, in the presence of aluminum. [Al2O3]/ [TMAdaOH] = 0.033.

correlation peak is monitored by comparing the tendency of Imax/I[q→0] vs [Si]/[TMAdaOH] ratio for samples prepared in the presence and in the absence of aluminum (Figure 11).

Figure 9. Linewidth of the monomer site in 29Si NMR spectra as function of Al concentration in solutions with no nanoparticles, at nanoparticle formation and in sols containing nanoparticles.

(only oligomers present), we observe broadening of the monomer peak by increasing the Al concentration. This is interpreted as being due to the slowing down of the interconversion exchange process between silicates provoked by the interferences of species with Al. This phenomenon is well known to occur in aluminosilicate solutions.55 When nanoparticles are present, we do not observe this effect because all aluminum atoms are four-connected and trapped inside the nanoparticles. Particles are observed at lower values of [Si]/[TMAdaOH] ratio when aluminum is added to the synthesis precursor (Figure 10) since some nanoparticles are already present at [Si]/[TMAdaOH] as low as 0.67 (see the hump in curve at q ∼2.7 nm−1). Examining the SAXS results for the aluminum containing cases one can observe with the increase of [Si]/[TMAdaOH] a phenomenon that has been observed in the case of dissolution of nanoparticles. The SAXS pattern exhibits two components, the intensity decay starting below q ∼1.5 nm−1 and, in addition, a “peak” located at about 2 nm−1. This peak has been assigned to the strongly correlated oligomers, in sols.19 The oligomers are in at composition beyond the cac, and can therefore be considered as “dressed oligomers”, i.e., solvated ion covered with their counter cations or shared-ion pair between one oligomer and its maximum of counter cations.56,57 The amount of nanoparticles increases with the [Si]/[TMAdaOH], while the correlation peak increases too, for the non-aluminum case (Figure 4). This relationship indicates that the correlated dressed oligomers are located inside the nanoaggregates and not in the sol. For the aluminum containing sol, such an increase of the correlation peak is not observed. The evolution of the

Figure 11. Maximum intensity (Imax) divided by the intensity in the limit q → 0 (Iq→0) plotted vs the [Si]/[TMAdaOH] ratio.

In the presence of aluminum, Imax/I[q→0] is closer to 1 and in general has lower values than those observed in the absence of aluminum. The position of the scattering peak assigned to oligomers does not change noticeably with the [Si]/ [TMAdaOH] ratio and with the presence and absence of aluminum, indicating an almost constant distance between interacting oligomers. However, with aluminum, this correlation peak is much weaker than without. Aluminum atoms have a connectivity of 4 when they enter nanoparticles, and all aluminum goes into nanoparticles (see below). Therefore, the different behavior of the smaller correlation peak in the SAXS experiment with Al indicates that the role of the aluminum is to connect oligomers within the nanoparticles into core shell elements. As aluminum exhibits a lower activation energy than Si for metal oxygen bond breaking reformation, it catalyzes the core shell networking within the nanoparticles.58 Internal Si and Al Connectivity in Nanoparticles. The global degree of condensation of the different fractions can be estimated by the weighted average ⟨n⟩ calculated from the 576



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where y = 0.010, 0.016, and 0.040. These observations are known from the literature.28 The 27Al spectra are only affected by the

following equation:

n = Σn(Qn/ΣnQn) for 29Si and n = Σn(q n/Σq n) for 27Al ⟨n⟩ can take values from 0 to 4, where ⟨n⟩ = 0 is the silicate or aluminate monomer, and ⟨n⟩ = 4 is the fully four-connected to silicate groups. Figure 12a shows the evolution of ⟨n⟩ for the

Figure 13. 27Al NMR spectra where Si/TMAdaOH is kept constant, and the Si/Al ratio is changed in the system: 0.4SiO2 − yAl2O3 − 0.6TMAdaOH − 35H2O −1.6EtOH, with y = 0.040 (bottom), 0.016 (middle), and 0.010 (top).

change in Si/TMAdaOH ratio and the connectivity of Si. This means that aluminate species behave like silicate species. One expects then in the presence of Al an occurrence of aluminosilicates oligomers similar to oligomeric silicates. The presence of such polyanions would help inter-oligomer condensation when critical concentration is reached, i.e., when the aluminum enters the nanoparticles. More interestingly, the connectivity of Al already reached its maximum value as soon as nanoparticles formed. This is a strong indication of the implication of Al cation in the formation process of nanoparticles. Figure 12b compares the average connectivity for the MFI, MEL, and the CHA systems. As seen from the figure, a general trend is observed in which the internal Si connectivity within the nanoparticles reaches a plateau value between 3 and 3.5.

Figure 12. (a) Average connectivity of Si in oligomers (◼) and in nanoparticles (◻), and of Al (▲) as function of TEOS hydrolysis progress. (b) Comparison of average Si connectivity in the oligomers and nanoparticles as a function of Si/SDA ratio during the hydrolysis of TEOS in MFI, MEL, and CHA systems.

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CONCLUSIONS The early stages of hydrolysis of TEOS in TMAdaOH were studied by 29Si, 27Al NMR, and SAXS. 29Si NMR spectroscopy gave quantitative information on the nature of silicate species and permitted us to separate oligomeric species from particles based on their respective line widths. The results for hydrolysis of TEOS in the CHA precursor sols showed behaviors of Si connectivity and nanoparticle formation during TEOS hydrolysis similar to what has been seen previously for silicalite-1 and silicalite-2, confirming that it is a general phenomenon independent of the nature of the organocation. It can also be seen that the Al is fully bound to four Si and thus located exclusively in the nanoparticles. Inside the nanoparticles, Al facilitates the integration of not yet connected dressed silicate oligomers into the fully connected core−shell part of the nanoparticles.

oligomers, nanoparticles, and Al sites during the hydrolysis of TEOS. In the beginning of the hydrolysis, the oligomers have a low degree of condensation with the monomer as the dominant species as also seen in Figure 3 and Figure 7. As the hydrolysis progresses, the Si connectivity increases to ⟨n⟩ = 2.5 for [Si]/ [TMAdaOH] = 1, where nanoparticles start to form. Upon further hydrolysis, the Si connectivity of the oligomers decreases slightly because the more connected oligomers go into nanoparticles to a greater extent than the monomer. At [Si]/[TMAdaOH] = 1, nanoparticles with a Si connectivity of 3.1 form according to 29Si NMR. The connectivity of Al evolves in a manner similar to that of Si before nanoparticle formation meaning that the local environment around Al in oligomeric aluminosilicates is almost comparable to Si centers in silicate and aluminosilicate oligomers. Further support of this can be found in the 27Al NMR spectra shown in Figure 13 of additional samples with fixed hydrolysis level and varied Si/Al ratio. Indeed, no change in the distribution of Al sites was observed when varying Al content in the systems: 0.4SiO2 − yAl2O3 − 0.6TMAdaOH − 35H2O − 1.6EtOH,

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ASSOCIATED CONTENT

* Supporting Information S

Room temperature 99.4 MHz 29Si NMR spectra of clear sol; crystallographic structural models of the two silicate hydrates synthesized in this work; and 1H 15 kHz MAS NMR spectra, 13 C{1H} 5 kHz CPMAS NMR spectra, and 29Si{1H} 5 kHz 577



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CPMAS NMR spectra of plate shape crystals type I and needle shape crystals type II of silicate hydrates D4R-TMAda (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Sachem Inc. for supplying us with TMAdaOH (ZeoGen 2825). We acknowledge the Norwegian Research council and the KOSK II program for financing E.A.E.'s Ph.D. The CNRS and the Region d’Ile de France are thanked for funding the NMR spectrometers used in this study, and for financial support of the members of Tectospin. The University of Versailles Saint-Quentin en Yvelines provided running costs for operating the spectrometers. We also thank David P. Petry and Pr. Michael W. Anderson, Center for Nanoporous Materials, School of Chemistry, University of Manchester, U.K., for the results plotted in Figure 12b concerning the evolution of the average silicon connectivities of MFI and MEL systems.

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REFERENCES

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