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Si NMR Relaxation of Silicated Nanoparticles in Tetraethoxysilane-Tetrapropylammonium Hydroxide-Water System (TEOS-TPAOH-H2O) Mohamed Haouas,*,† David P. Petry,†,‡ Michael W. Anderson,‡ and Francis Taulelle† Institut LaVoisier de Versailles, UniVersite´ de Versailles-St. Quentin en YVelines, Versailles, France, and Centre for Nanoporous Materials, School of Chemistry, The UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K. ReceiVed: April 15, 2009; ReVised Manuscript ReceiVed: May 20, 2009
Silicon-29 longitudinal (T1) and transverse (T2) NMR relaxation times have been measured in the clear solution precursor of silicalite-1 of composition 25 TEOS-5 TPAOH-400 H2O. The nanoparticles as well as the silicate oligomers are giving rise to observable resonances. An unusually long T1 relaxation time of 126 s is observed for Q4 in nanoparticles. Proper care for acquisition is therefore required for quantifying the distribution of Qn of the nanoparticles, an essential measurement to follow the nanoparticles connectivity evolution. Introduction Understanding how zeolites nucleate and grow is crucial to optimizing catalytic and molecular-sieving properties of these important materials, especially when tailoring crystal size and shape. In this study, silicalite-1 is synthesized from a clear mixture of tetraethoxysilane (TEOS), tetrapropylammonium hydroxide (TPAOH) and water, providing a model system for zeolite formation. The mixture is actually a colloidal suspension of nanoparticles in water-ethanol with many coexisting silicate oligomers. A comprehensive silicate speciation is far from being completed1,2 and the existence of some structures such as the double-five ring, a key unit for mobile type five (MFI) topology, is still not evidenced.3,4 Even though the presence of the nanoparticles as an amorphous-like structure is agreed upon, their precise role in the synthesis remains a subject of debate.5-8 In particular, the question of whether these nanoparticles contain occluded TPA templates or are simply TPA-free silica entities, is still open.9,10 Even though a core-shell structure,11 with a silica core surrounded by a shell of organocations, appears now to be the most accepted description, the chemical composition, as well as the local structure are not elucidated yet, due to a lack of an appropriate characterization technique. For instance, X-ray diffraction (XRD) cannot be used because of the lack of periodicity and the small size of particles below the coherence domain12 and 29Si NMR provides very poorly resolved spectra with severe signal broadening.13-18 Nevertheless, valuable information on internal Si connectivities can be provided by quantification of 29Si Qn distribution, especially the Q4 peak,13,17,18 since it influences strongly the interpretation on the internal degree of Si condensation. In a recent paper, Vlachos et al. have pointed out that a much longer recycle delay than the usual values considered for silicate solutions containing nanoparticles, is to be used for Q4 resonances.18 The spectra were however of * To whom correspondence should be addressed. Fax: (+33)-139254476. Tel: (+33)-1-39254254. E-mail:
[email protected]. † Universite´ de Versailles-St. Quentin en Yvelines. ‡ The University of Manchester.
10.1021/jp903454f CCC: $40.75
poor spectral resolution, detected only the broad signals of nanoparticles but not the sharp resonances of oligomers and exhibited a considerable overlap of lines. The relaxation time measurements of nanoparticles 29Si signals in TEOS-TPAOHH2O system have indeed never been measured, though for aqueous silicates solutions T1 relaxation times have been studied by Kinrade et al. and are in the range of 3-35 s.19-23 In order to clarify the quantification issue of the different sites for oligomers as well as for nanoparticles, T1 and T2 relaxation time have been measured in this study. Experimental Methods NMR measurements were performed on a clear solution containing nanoparticles prepared following the procedure of Aerts et al.,13 which consists of a mixture of 25 SiO2-5 TPAOH-400 (H2O/D2O)-100 EtOH. First, TEOS (Acros, 98%) was hydrolyzed in aqueous TPAOH solution (Alfa, 40 wt %). D2O (Cortecnet, 99.9%) was added subsequently to obtain the desired molar ratio. In this system, 98% of Si are in nanoparticles and only 2% are oligomers. Reasonable NMR signal-to-noise ratios were obtained on both the very broad signals of nanoparticles and the sharp signals oligomers. In order to avoid the strong signal background of the probe, all glass supports of coils in the probe-head were replaced with equivalent parts machined from silicon-free materials. Two kinds of tubes were tested, FEP and quartz tubes, both exhibited no silicon background with a better resolution for the quartz tubes. Quartz has a broad signal at around -107 ppm. The long T1 of quartz glass in the order of 3-6 h allows to eliminate by presaturation this components.24 Indeed, a train of 32 π/2 pulses was used for saturation prior to magnetization recovery delay. This ensured a flat spectral baseline. All NMR experiments were carried out at room temperature (27 °C) on a Bruker Avance 500 spectrometer at 11.7 T at the resonating frequency of 99.35 MHz for 29Si, using a BBO Bruker 10 mm probe. Longitudinal relaxation times were measured by the saturation-recovery method using variable recycle delay in the range 2-242 s. The 2009 American Chemical Society
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Figure 1. Relaxation delay dependence of 29Si (99.35 MHz) spectra in T1 saturation-recovery experiment for a solution with a molar ratio of 25:5:400:100 of SiO2-TPAOH-H2O/D2O-EtOH. As an example, the simulation of the last spectrum of the series is shown in blue with its components in red and the difference after subtracting the computed spectrum in green.
transverse relaxation times were determined using presaturation pulses and 85 s recovery delay and Hahn-echo sequence. Results and Discussion Figure 1 displays the successive NMR spectra in the saturation-recovery experiment with various relaxation delays. The spectrum exhibits four broad signals due to Qn silicate centers in the nanoparticles at δ(Q1) ∼ -80.1 ppm, δ(Q2/3∆) ∼ -89.7 ppm, δ(Q3) ∼ -99.4 ppm, and δ(Q4) ∼ -110.3 ppm, as well as some superimposed narrow signals accounting for oligomeric silicates. Only the strongest signals, corresponding to the well-established symmetric silicates, were considered here for the oligomeric species. Thus, seven sharp signals can be easily distinguished at -71.70, -80.50, -81.96, -88.55, -88.79, -98.78, and -99.40 ppm assigned to the monomer (M), dimer (D), single 3-membered ring (3R), single 4-membered ring (4R), double 3-membered ring (D3R), double 5-membered ring D5R, and double 4-membered ring (D4R), respectively. These assignments are consistent with the literature.1,2,25 Only one signal at -98.78 ppm is ambiguously assigned to the tetrahedral tetramer2,26 or to the D5R. These assignments are still a matter of discussion.25,27 We prefer the assignment to D5R silicate anions, which is most likely much more stabilized in tetraalkylammonium hydroxide silicate solutions.28,29 Signals of silicate anions with large cage structures (DnR, n ) 3, 4, or 5) are enhanced in tetraalkylammonium solutions,3,19,28,30 while in alkaline silicate solutions the signal of D5R is not detected.4,25 Data were analyzed using NMRnotebook software.31 Spectra were fitted by Lorentzo-Gaussian shape to determine the magnetization of each resonance. An example of the simulation of individual broad resonances together with the narrow signals is given in Figure 1 on the last spectrum of the series as well as the difference spectrum between the experimental and model spectra. T1 values were obtained by fitting the experimental exponential dependence of NMR magnetization as a function of the relaxation delay shown in Figure 2 and summarized in Table 1. T1 values of oligomeric species, containing Q0 to Q3 sites, were found in the range of 8-18 s, typical for aqueous silicates solutions.20,22,26 In alkaline solutions, T1 for small oligomers were found in the 3-8 s range, shorter than the larger silicates cages in the range of 10-16 s.20,22 The trend is inverted when organic cations are present19 with small oligomers in the 10-20 s range, longer than the larger silicates cages in the range of 4-8 s. This is the result of the stabilization of silicate cages with organocations forming an hydrophobic layer. The hydrated
Figure 2. Relaxation delay dependence of 29Si magnetization in T1 saturation-recovery experiment for silicates species present in (a) symmetric oligomers (M, monomer; D, dimer; 3R, single 3-membered ring; 4R, single 4-membered ring; D3R, double 3-membered ring; D5R, double 5-membered ring; D4R, double 4-membered ring) and (b) nanoparticles (Qn silicon).
TABLE 1: Longitudinal (T1) and Transverse (T2) Relaxation Times for the 29Si Resonances of Silicate Species in Symmetric Oligomers and Particles Occurring in a Solution With a Molar Ratio of 25:5:400:100 of SiO2-TPAOH-H2O/ D2O-EtOH oligomers
particles
Si species
T1 (s)
T2 (s)
∆ν1/2 (Hz)
M D 3R 4R D3R D4R D5R Q1 Q2 Q3 Q4
18 ( 1 18 ( 2 17 ( 1 17 ( 2 13 ( 2 7.5 ( 0.3 8.4 ( 0.7 18 ( 2 15 ( 1 22 ( 1 126 ( 4
1.5 ( 0.1 0.80 ( 0.18 2.6 ( 0.5 2.9 ( 0.7 2.3 ( 0.5 3.4 ( 0.6 2.6 ( 0.2 2.6 ( 0.3 0.24 ( 0.04 0.23 ( 0.01 0.61 ( 0.16
0.8 1.1 0.7 1.3 0.9 1.1 1.1 64 340 520 770
anions encaged in the organic cations surrounding them forms a water-silicate heteronetwork clathrate.19,32-35 Our results confirm for TPA+ the trend observed for the other tetraalkylammonium ions for more condensed silicate cages and in particular D4R and D5R. For Qn sites in nanoparticles, the value of T1 varies according to the following trend: Q1 ∼ Q2 < Q3 , Q4 respectively 18, 15, 22, 126 s. For oligomers as well as for the nanoparticles, the extreme narrowing condition does not hold for such solutions,20,22 as the large differences between T1 and T2 confirm (Table 1). This observation was already done for solutions of oligomers only20 and is not surprising for the nanoparticles for which the rotational diffusion is expected to be much slower than the oligomers. However, if the line broadening increases gradually with increasing Qn (see Table 1), no regular trend is visible for T2 variation. As no obvious relation holds between line widths and T2 values, the origin of the line broadening cannot then be
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assigned only to rotational diffusion of the particles. The increase in line width reflects an increasing range of chemical shifts distribution for Qn centers in the nanoparticles. This increased distribution of chemical shifts is an indication of a smaller conformational exchange of the different sites environments. Most likely a similar silicate anion encaged within an organic layer may have several possible conformations coexisting within the time scale of the NMR measurement. Exchange spectroscopy (EXSY) experiments have shown that slow chemical interconversion occurs between Qn sites within the nanoparticles,36 including exchange between Q4 and Q3 resonances. A progressive decrease of the chemical exchange rates involving an increasing number of bond breaking reformation of Si-O-Si bonds would lead probably to a higher contrast between sites. This is definitely true for Q4 and hardly marked for the other silicate sites. The high T1 value for Q4 sites in nanoparticles, more than 1 order of magnitude, compared to that of all other sites points toward an even more dramatic lowering of the interaction leading to relaxation or to slower fluctuations of the interaction or a combination of both.37,38 To our knowledge, no T1 have been measured for oligomers containing Q4 sites, though they are few such silicate anions containing Q4.39,40 From the experimental conditions used in the latter references, one may deduce that the T1 of these sites are in the range of the other silicate centers, that is, in the range of 10-20 s. T1 has been reported for as synthesized silicalite, that is, still containing TPA occluded.38 In their work, the authors are measuring T1 as a function of Si/Al ratio. For Si/Al of 5000, the T1 value obtained is 145 s, while we find 126 s for Q4 sites in the nanoparticles. When the silicalite is calcined, T1 drops to 6.5 s due to the influence of paramagnetic oxygen of the air diffusing through the pores, while the T1 are as high as 230-255 s when the calcined silicalite has been flushed with nitrogen or argon.41,42 It was also demonstrated37 that the dipole-dipole coupling between protons of guest molecules as adsorbate and framework silicon atoms in silicalite does not contribute significantly to the 29Si T1 relaxation time. Si-O-Si distance and angle fluctuations when absent do not allow any 29Si T1 either.24 For Qn in nanoparticles, T1 is close to what is observed for oligomers. For T2, values for Qn in nanoparticles are about an order of magnitude lower. No definitive relaxation mechanism is claimed in the literature, for silicate oligomers and for the silicalite-as synthesized. However, the interaction that might be at the origin of the relaxation must be, as stated by Kinrade et al., longrange and indirect.19 The hypothesis of having silicate anions surrounded by a rather rigid water shell and then by a tetraalkylammonium shell19 has been further supported by molecular dynamics calculations.32-35 Such low charged or even neutral objects, may aggregate as soon as a critical aggregation concentration is reached. If this would be so, it would be consistent with the fact that the T1 relaxation behavior of nanoparticles and oligomers is close for all sites of Q1 to Q3, and T2 would decrease by an order of magnitude due to the formation of much larger objects with a longer rotational correlation time. For the Q4 in nanoparticles, which are the critical resonances we were interested in, with a long-range and indirect interaction the much longer T1 values are indicating an even weaker interaction responsible of the relaxation and additionally less fluctuations of it. The similarities of T1 and decrease of T2 between relaxation of oligomers and Q1-Q3 sites in the nanoparticles would suggest that oligomers dressed with water and TPA cations are aggregated. With time, the Q4 sites appear and increase in number, indicating therefore that the water and TPA shells have been open to allow silicate sites to
Letters connect with rearrangement of the water and TPA shell. As at the end of the condensation process, water would have been eliminated and very few TPA would remain inside the silicalite framework, the intermediate state of connections leave room for progressive water expulsion from the nanoparticles and progressive expulsion of TPA. Therefore the Q4 containing parts of the nanoparticles may have even weaker relaxation paths through the same sort of interaction that would be responsible of relaxation for oligomers or the Q1-Q3 sites in the nanoparticles. Conclusion In conclusion, the nanoparticles in the 25 TEOS-5 TPAOH400 H2O system are most likely formed by aggregation of oligomers probably dressed with water and TPA under a form of ion-pairs and present additional condensation with Q4 sites not present in the oligomers themselves. Counting the Qn distribution is a primary importance to reveal the nature and evolution of the nanoparticles. In the actual example, this distribution is Q1/Q2/Q3/Q4 ∼ 1:6:44:49. The description of these nanoparticles can be thought of as an aggregation of oligomers ion-paired with TPA for which the fraction of Q4 would be negligible. In addition, when aggregated, connections between oligomers lead to the apparition of Q4 sites and progressive removal of water and TPA from those regions. However, the analogy for T1 and difference for T2 relaxation between oligomers and silicon resonances in the nanoparticles suggest that they do contain silicate sites in very similar environment than those of oligomers. TPA would therefore be around and in the nanoparticles.43 Finally, to measure quantitatively the distribution of Qn connectivity in the nanoparticles, we have shown in this study that the T1 of Q4 are unexpectedly longer than Q1 to Q3 and this could not be guessed only from the main trend known from solutions, or from a simple analogy with amorphous silica gels, as it is often assumed that the nanoparticles are. Special care has to be taken in the future for careful protocols of quantification in this area. Acknowledgment. The authors are grateful to Drs. A. Aerts and CEA Kirschhock for the many helpful and stimulating discussions. Financial support for this work was provided by the EPSRC Grant and ExxonMobil Research and Engineering of the Nanogrowth consortium, as well as the CNRS and the University of Versailles Saint Quentin en Yvelines. References and Notes (1) Haouas, M.; Taulelle, F. J. Phys. Chem. B 2006, 110, 22951. (2) Knight, C. T. G.; Balec, R. J.; Kinrade, S. D. Angew. Chem., Int. Ed. 2007, 46, 8148. (3) Boxhoorn, G.; Sudmeijer, O.; Vankasteren, P. H. G. J. Chem. Soc., Chem. Commun. 1983, 1416. (4) Knight, C. T. G.; Kirkpatrick, R. J.; Oldfield, E. J. Chem. Soc., Chem. Commun. 1989, 919. (5) 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. (6) Jorge, M.; Auerbach, S. M.; Monson, P. A. J. Am. Chem. Soc. 2005, 127, 14388. (7) Liang, D.; Follens, L. R. A.; Aerts, A.; Martens, J. A.; Van Tendeloo, G.; Kirschhock, C. E. A. J. Phys. Chem. C 2007, 111, 14283. (8) Patis, A.; Dracopoulos, V.; Nikolakis, V. J. Phys. Chem. C 2007, 111, 17478. (9) Houssin, C. J. Y.; Kirschhock, C. E. A.; Magusin, P.; Mojet, B. L.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Phys. Chem. Chem. Phys. 2003, 5, 3518. (10) Rimer, J. D.; Kragten, D. D.; Tsapatsis, M.; Lobo, R.; Vlachos, D. Stud. Surf. Sci. Catal. 2004, 154A, 317. (11) Fedeyko, J. M.; Vlachos, D. G.; Lobo, R. F. Langmuir 2005, 21, 5197.
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