17478
J. Phys. Chem. C 2007, 111, 17478-17484
Investigation of Silicalite-1 Crystallization Using Attenuated Total Reflection/Fourier Transform Infrared Spectroscopy Angelos Patis,†,‡ Vassilios Dracopoulos,† and Vladimiros Nikolakis*,† Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation for Research and Technology Hellas, Stadiou Str., Platani, P. O. Box 1414, 26504 Patras, Greece, and Department of Chemical Engineering, UniVersity of Patras, 26500 Patras, Greece ReceiVed: June 14, 2007; In Final Form: September 15, 2007
The crystallization of silicalite-1 in aqueous sols has been investigated using ATR/FTIR spectroscopy as a function of sol composition. The existence of oligomeric siliceous species has been verified for all compositions after the end of tetraethoxysilane (TEOS) hydrolysis. On the other hand, more condensed and highly crosslinked siliceous structures have been observed only at the sols of high SiO2 concentrations. These structures have been considered to be the “primary” nanoparticles reported by other researchers, and their IR spectra revealed that their structures are different from that of the silicalite-1 crystals. The evolution of the nanoparticle structure as a function of crystallization time has been monitored. It has been found that, upon heating of the synthesis sols, the “primary” nanoparticles transform to a new form that has an increased number of SiOSi bonds. These “intermediate” particles do not have the structure of silicalite-1, and they are present for most of the crystallization. Finally, once the silicalite-1 nuclei are formed, they grow in a suspension of these “intermediate” particles.
Introduction The elucidation of zeolite nucleation and growth mechanisms has attracted the interest of many researchers worldwide. The conclusions of all these efforts have been critically summarized in a recently published review.1 Among the numerous zeolite structures, the one of silicalite-1 has received a lot of attention during the past 15 years. The reasons for the interest partially stem from the need to control the texture and minimize the defects of silicalite-1 membranes as well as from the large number of experimental conditions known to crystallize silicalite-1 crystals. Silicalite-1 crystals are usually synthesized hydrothermally from mixtures of tetraethoxysilane (TEOS), tetrapropylammonium hydroxide (TPAOH), and water. Small-angle X-ray scattering (SAXS),2-7 small-angle neutron scattering (SANS),4,5,8-10 and dynamic light scattering (DLS)11-16 indicate that the apparently clear solutions formed immediately after the end of TEOS hydrolysis are in reality suspensions of small colloidal particles having estimated sizes in the range of 2-10 nm. The processes taking place within the nanoparticle suspension lead to the crystallization of silicalite-1. Crystals can be synthesized from very dilute sols (i.e., 20 mol of SiO2 per 9500 mol of H2O)7,11 as well as from highly concentrated sols (i.e., 25 mol of SiO2 per 480 mol of H2O).12,13,17 Furthermore, crystallization can proceed even at room temperature;7 however, the nucleation and growth rates are significantly accelerated at elevated temperatures. As a result, silicalite-1 synthesis usually takes place at temperatures that can be as high as 175 °C. It is noteworthy that for an extended period of time, which is usually called induction time, it is not possible to detect the existence * Corresponding author. Telephone: ++302610965242. Fax: ++302610965223. E-mail:
[email protected]. † Foundation for Research and Technology Hellas. ‡ University of Patras.
of any crystalline material. During that time only small variations of the size of the primary nanoparticles have been observed.3-7 However, once the first nuclei are formed, their size increases linearly with time without any new crystals being formed. The complexity of the reactions taking place has been underscored by in situ calorimetric studies of crystallization, with simultaneous pH monitoring.17-19 These experiments showed that crystallization shifts from exothermic to endothermic with a concomitant increase of the pH of the reaction sol. Despite the numerous studies published, the structure and role of the siliceous nanoparticles are still subjects of scientific discussion. Analysis of the synthesis mixtures with 1H-29Si cross-polarization magic angle spinning (CP MAS) NMR indicates that the primary nanoparticles are preorganized inorganic-organic composite structures.20-22 Kirschhock et al.23,24 proposed that silicate polyions in the presence of TPA cations form species containing 33 Si atoms that occlude TPA. These species aggregate to form nanoslabs having the MFI structure and dimensions of 1.0 × 4.0 × 4.0 nm,23,25 and can be considered as the observed nanoparticles. The same authors also suggested that growth proceeds via an oriented aggregation of the nanoslabs. More recent investigations9,10 suggest that the SiO2 nanoparticle formation is a more general phenomenon once the amount of silica added to the system is above the value of the critical aggregation concentration (CAC). In all cases examined the nanoparticles appeared to be ellipsoid with a monodisperse size, which was mainly affected by the pH rather than by the total SiO2 content. Comparison of SAXS and SANS experiments showed that at room temperature the nanoparticles have a silica core surrounded by a TPA+ shell. The exact structure of the core or shell has not been identified; however, 29Si MAS NMR data26 revealed that the nanoparticles are comprised by combination of silicate species having different degrees of condensation (Q2, Q3, and Q4). In particular, it is stated that the
10.1021/jp074600x CCC: $37.00 © 2007 American Chemical Society Published on Web 10/31/2007
ATR/FTIR Spectroscopy of Silicalite-1 nanoparticles must have open shapes and a considerable number of internal defects. Furthermore, analysis of transmission electron microscopic (TEM) and atomic force microscopic (AFM) images of nanoparticles extracted from the synthesis sols revealed that the suspensions contain mainly amorphous silica nanoparticles.27 The previous experimental findings have been confirmed by Jorge et al.,28 who used Monte Carlo simulations to show that the observed nanoparticles are a metastable phase that is stabilized mainly by electrostatic interactions and a layer of TPA cations. Even though the core-shell structure is maintained upon heating, structural changes take place and TPA molecules gradually get embedded in the core, resulting in nanoparticles having more of a zeolite-like structure.29 The importance of the structural transformations of the nanoparticles, which take place during the induction period, has also been highlighted in a series of recent publications.7,30,31 The authors proposed a crystal growth mechanism that considered oriented aggregation of metastable primary nanoparticles having structures different from the one of the final crystal. In such a mechanism, the evolution of the primary nanoparticles is considered to take place in a series of reactions where several intermediate “species” are formed. Finally, B. J. Schoeman14 suggested that nucleation takes place via a cluster-cluster aggregation mechanism, while crystal growth proceeds via the addition of monomeric species. This mechanism is deemed as more probable by other researchers as well.1 Vibrational spectroscopy has been used by several researchers to characterize not only silicalite-1 but also a large number of zeolite crystals.32-36 Several research groups have tried to investigate silicalite-1 nucleation and growth using IR spectroscopy. However, in order to avoid the interference from the IR absorption bands of water, the nanoparticles had to be extracted from the mother liquor before the spectra were measured.15,24,26,37 In addition to the need for using one additional experimental step, it is not known if the sample preparation techniques caused any structural changes to the samples and as a result to the measured spectra as well. The only in situ investigation of a similar system, known to the authors, is the study of TEOS hydrolysis in concentrated TPAOH solutions24 using in situ DRIFT spectroscopy. The present paper intends to give insight into structural changes taking place during silicalite-1 nucleation and growth using attenuated total reflection/Fourier transform infrared spectroscopy (ATR/FTIR). ATR/FTIR enables the acquisition of spectra from aqueous solutions or suspensions. It is a noninvasive and nondestructive experimental technique and has already been used for the in situ investigation of mesoporous molecular sieve synthesis38,39 and to investigate the hydrolysis and condensation alkoxysilanes in water-rich acidic solutions.40 In this publication we show that it can also be used for measuring the evolution of the IR spectra of the siliceous species present during the crystallization of silicalite-1. Spectra were collected from sols having 10 different compositions. These compositions have been selected because they have been used in earlier studies.9-13,17,24 The analysis of the spectral evolution can provide information about differences in the structure of the primary nanoparticles and final silicalite-1 nanocrystals, as well as the nature of the possible transformations taking place during nucleation and growth. Experimental Section Sample Preparation. Ten apparently clear (transparent to the naked eye) silica sols were prepared by the hydrolysis of
J. Phys. Chem. C, Vol. 111, No. 47, 2007 17479 TABLE 1: Molar Composition of the Sols Examined code name
molar composition
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11
5 SiO2/9 TPAOH/9500 H2O/20 EthOH 10 SiO2/9 TPAOH/9500 H2O/40 EthOH 20 SiO2/9 TPAOH/9500 H2O/80 EthOH 40 SiO2/9 TPAOH/9500 H2O/160 EthOH 80 SiO2/9 TPAOH/9500 H2O/320 EthOH 25 SiO2/9 TPAOH/1450 H2O/100 EthOH 25 SiO2/9 TPAOH/680 H2O/100 EthOH 25 SiO2/9 TPAOH/480 H2O/100 EthOH 25 SiO2/5.6 TPAOH/251 H2O/100 EthOH 40 SiO2/9 NaOH/9500 H2O/160 EthOH 1 SiO2/4.3 NH4OH/51.7 EthOH
tetraethoxysilane (Aldrich, TEOS 98%) in alkaline solutions of tetrapropylammonium hydroxide (Aldrich, TPAOH 1 M solution in water). Amorphous Sto¨ber silica nanoparticles were also prepared from a sol having composition S11. The molar compositions of the sols are shown in Table 1. Sols S1-S9 were prepared using the following procedure. An appropriate amount of base (TPAOH) was mixed with distilled water. A prespecified amount of TEOS was then added, and the mixture was stirred at room temperature until a clear solution was formed. For sol S10 NaOH pellets were used instead of TPAOH. The hydrolyzed sols were tightly sealed in polypropylene vials and were placed in an oven preheated at 85 °C. The sols gradually became cloudy. Upon further heating their turbidity increased and in some cases a white powder precipitate could be seen at the bottom of the vials. The Sto¨ber silica nanoparticles were prepared by adding 2 mL of TEOS in a solution containing 1.5 mL of NH4OH (2830% as NH3, Aldrich) and 25 mL of ethanol (absolute, Riedelde Hae¨n). The sol was stirred at room temperature for several hours until it became cloudy. It was then boiled to dryness, washed with distilled water several times, and dried at room temperature. Characterization Methods. After the end of the crystallization, the crystals were separated from the solution via centrifugation at 15 000 rpm for 40 min (Eppendorf Centrifuge 5804). The centrifugation was repeated several times until the pH of the supernatant liquid was less than 9. The white powder was then air-dried and was characterized using X-ray diffraction (XRD; NONIUS-ENRAF, equipped with an INEL CPS-120 detector). Attenuated Total Reflectance/Fourier Transformed Infrared (ATR/FTIR) Spectroscopy. For each composition, the evolution of the siliceous species structure as a function of time was monitored using ATR/FTIR spectroscopy. Samples were withdrawn from each vial using glass micropipettes, were placed in Eppendorf (Aldrich, Plastibrand PP Tubes, 1.5 mL) microcentifuge tubes, and were cooled at room temperature. The ATR/ FTIR spectrum of each sample was obtained using a Varian Excalibur spectrometer, equipped with a single reflection diamond ATR attachment of Golden Gate. The ATR attachment had KRS-5 lenses, thus enabling the acquisition of spectra from 400 cm-1 wavenumber. For each composition, the spectrum of a reference solution that contained the equivalent amounts of H2O, TPAOH, and ethanol was measured and was used as background. Furthermore, in order to account for the difference in the penetration depth as a function of the wavenumber, the spectra were corrected using the following equation:41
17480 J. Phys. Chem. C, Vol. 111, No. 47, 2007
absorbance(ν)corrected ) dp )
Patis et al.
absorbance(ν)ATR dp
10000 2πν(np2 sin2 θ - ns2)1/2
where dp is the penetration depth, ν is the wavenumber in cm-1, np is the refractive index of diamond (2.47), ns is the refractive index of the sample, and θ is the angle of incidence of the beam at the surface of the internal reflection element (45°). The spectra of silicalite-1 powders were corrected using the refractive index of the crystals (ns ) 1.54). The spectra of the synthesis sols were corrected assuming that their refractive index is equal to that of water (ns ) 1.33). For each sol, the overall procedure was repeated at least three times and a large number of spectra were recorded in order to ensure reproducibility. To maintain clarity and simplicity, only a portion of them are presented in the figures of this paper. Results and Discussion After ∼50 h of crystallization a white powder was collected from sols S3-S9. The sols S1, S2, and S10 remained transparent even after prolonged heating at 85 °C (several months), and it was not possible to collect any powder from them. The XRD patterns of the powders (Figure 1) indicate that in all cases MFI crystals were formed.42 The ATR/FTIR spectra of the Sto¨ber silica, of the silicalite-1 powder, and of the silicalite-1 aqueous suspension are shown in Figure 2. In the same figure the assignment of each absorption band to particular vibrations is shown.32-35 It is clear that the well-defined absorption bands at ∼1200 and 550 cm-1 are observed only in the silicalite-1 samples and are not present in the Sto¨ber silica particles. It is also seen that the energies of all bands of silicalite-1 powder and aqueous suspension lie on similar wavenumbers, with the exception of the intense band of the antisymmetric SiOSi stretching modes, which shifts to higher wavenumbers when the crystals are suspended in water. This shift can be attributed to the interaction of water with the framework. Effect of Sol Composition on the Structure of Siliceous Species Formed After the End of TEOS Hydrolysis. The ATR/IR spectra of sols S1-S9 immediately after the end of TEOS hydrolysis are shown in Figure 3. For sols S1-S6, absorption peaks were observed only in the region of the antisymmetric Si-O-Si stretching (1000-1300 cm-1). For sols S7-S9, however, it was possible to observe one peak in the region between 400 and 500 cm-1. A less intense broad band between 550 and 600 cm-1 seems to appear in these sols as well. The acquisition of spectra at this region is very difficult because of the presence of water that also absorbs IR radiation in the same region. The existence of this band is rather uncertain because its absorption intensity is very close to the detection limits of our technique. A comparison of the IR absorption bands of the sols immediately after TEOS hydrolysis with those of MFI crystals (Figure 2) indicates that the initial structure of the siliceous species is not the same as that of the MFI crystals. The most striking differences are the absence of well-defined absorption peaks in the regions of ∼550 and ∼1230 cm-1. It is also interesting to compare the spectra of sol S9 with those of the extracted nanoparticles shown in Figures 1Ba and 1Bb of the publication by Kirschhock et al.24 Shifts in the frequency and relative intensities of bands P2-P4 can be observed. Furthermore, the absorption band at 590 cm-1 is not seen in
Figure 1. X-ray diffraction patterns of the powders collected from sols S3-S9. (The stars denote the positions of the X-ray diffraction peaks of silicalite-1.42)
Figure 2. ATR/FTIR spectra of Sto¨ber silica, of silicalite-1 powder, and of silicalite-1 suspension in water.
our spectra. These differences are probably due to structural changes induced by the nanoparticle extraction procedure. A broad band at ∼1022 cm-1 (P1) was observed in the spectrum of each sol. Three additional absorption bands, at ∼1065 (P2), ∼1109 (P3), and ∼1150 cm-1 (P4), were observed in the spectra of sols S3-S9. It can also be seen that the wavenumber difference between bands P3 and P4 increased from ∆ν ∼ 40 cm-1 at sol with composition S3 to ∆ν ∼ 90 cm-1 at sol with composition S9. The above observations indicate that several structural changes occur in the sols as the concentration of SiO2 changes. Moreover, the data show that when the composition changed from S2 to S3 the structure of the siliceous species changed drastically and new species, associated with the appearance of the P2, P3, and P4 bands, were formed. In the next paragraphs the observed changes in the frequencies of SiOSi antisymmetric stretching will be discussed. To elaborate about the possible groups of SixOy species formed in each sol, it is necessary to compare our spectra with those of other siliceous species that can be found in the literature. For example, studies using methylpolysiloxanes established correlations between several different SixOy species and their IR absorption wavenumbers.43-46 The correlations for linear and
ATR/FTIR Spectroscopy of Silicalite-1
J. Phys. Chem. C, Vol. 111, No. 47, 2007 17481
Figure 4. ATR/FTIR spectra of sols S4 and S10 collected immediately after the end of TEOS hydrolysis.
Figure 3. ATR/FTIR spectra of sols (a) S1-S6 and (b) S6-S9 collected immediately after the end of TEOS hydrolysis.
TABLE 2: Infrared Absorption Bands (cm-1) of Cyclic and Linear Methylpolysiloxanes43 methylpolysiloxanes dimer trimer tetramer pentamer hexamer heptamer octamer
cyclic
linear
1018 1076 1081 1068 1060 1056
1054 1046 1038 1033 1029 1027 1023
cyclic species are presented in Table 2.43 Based on the data shown in this table, we can conclude that the band P1 of Figure 3 most likely appears due to the existence of siliceous oligomeric species like isolated cyclic trimers, linear hexamers, heptamers, and octamers or a combination of such cyclic and linear species. This conclusion is in agreement with that of Orel et al.,47 who studied the aprotic condensation of trialkoxysilanes, and has also assigned the ∼1020 cm-1 absorption band to the existence of oligomeric siliceous species. The appearance of the P2, P3, and P4 bands in the IR spectra of the SiO2-rich sols (S3-S9) indicates the appearance of new silicate species. In Figure 4, the spectrum of sol S4 is compared with that of sol S10. Composition S10 does not lead to the crystallization of silicalite-1; however, it is known to form siliceous nanoparticles.10 The overall features of the spectra in the 1045-1200 cm-1 region are similar to those observed in silica gels and/or porous silica xerogels48-50 as well as those of sol S10, indicating the existence of similar chemical species. In particular, the band positions and shapes of bands P3 and P4 seem to be correlated with the transverse optics and longitudinal optics (TO-LO) bands in silica gels and glasses. This implies that in sols S3-S9 silicate clusters having different symmetries
Figure 5. Absorption intensity of absorption bands P1 and P3 as a function of SiO2 concentration, for sols S1-S5. (In the cases shown in the figure the concentration of TPAOH was 0.05 ( 0.002 M.)
from that of sols S1 and S2 are present. Recently, Fedeyko et al.9,10 reported that the formation of silica nanoparticles in basic solutions of tetralkylammonium or alkali cations is a spontaneous phenomenon that occurs once the concentration of SiO2 is above a critical value. For the alkalinity of compositions S1S5, they determined this value to be close to composition S2. Thus it is reasonable to attribute these bands to the absorbance from the siliceous nanoparticles. We also examined the effect of SiO2 concentration on the intensity of the absorption bands P1 and P3 (Figure 5). In order to make a meaningful comparison, the data shown in this figure are limited to the results from sols with the same total alkalinity (S1-S5). As seen in Figure 5, the intensity band P3 has a linear dependence on SiO2 concentration. Furthermore, it crosses the x-axis at 0.05-0.06 M SiO2, indicating that this is the critical concentration for the formation of the siliceous nanoparticles. The linear dependence of band P3 on SiO2 concentration indicates that the concentration of the nanoparticles increases as well. It must be noted that the intensity of this band is proportional to the concentration of SiOSi bonds and it will be proportional to the concentration of the nanoparticles only if the additional SiO2 does not cause changes in their size and structure. On the other hand, the dependence of the P1 intensity band on SiO2 concentration changes at the point of critical concentration. It increases with the SiO2 concentration both below and above the critical concentration. However, below the critical concentration it has a more pronounced dependence on
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Patis et al.
Figure 6. Evolution of ATR/FTIR spectra of sols (a) S4, (b) S6, (c) S8, and (d) S9 as a function of crystallization time.
SiO2 concentration. This result indicates that further increase of SiO2 concentration above its critical value contributes primarily to the increase of the siliceous nanoparticle concentration. All of our observations presented above are in agreement with the conclusions reported by Fedeyko et al.9,10 further justifying the assignment of P2, P3, and P4 bands to the siliceous nanoparticles. Our results also indicate that the structure of the nanoparticles is affected by composition. As the SiO2 concentration increases, the P3-P4 band splitting slightly increases. In silica xerogels,48 in ormosils,50 and in SiO2 glasses49 this splitting was attributed to the enhancement of the long-range Coulombic forces in silica network, which is correlated to the formation of more crosslinked structures. This conclusion is further supported by the increase of the ratio of the intensity of band P2 over that of P3. Earlier studies on fully condensed silica gel films or glasses showed that the dominant high frequency antisymmetric stretching band is near 1070 cm-1. This frequency is also very close to the primary absorption band of the Sto¨ber silica nanoprticles, which are also comprised from fully condensed silicon atoms. As a result, band P2 can be assigned to the antisymmetric stretching of SiOSi units when both Si atoms are fully condensed. Thus the increase of the intensity of band P2 over that of P3 as the SiO2 concentration increases indicates that nanoparticles having a more cross-linked structure are formed. Based on the above, we can conclude that SiO2 concentration most likely affects both the concentration and the structure of the siliceous nanoparticles.
Unfortunately, it is not easy to assign the absorption bands P2, P3, and P4 with specific symmetry and get detailed information about the local structure of the SixOy units in the nanoparticles. This is somewhat expected because a broad range of different SixOy units probably exist in the primary nanoparticles. However, based on the shape and location of these bands and the data reported in the literature,44-46 we can infer the nonexclusive existence of T7(OH)x and/or T8(OH)x species (T denotes a trifunctional unit of siloxane structure45). Evolution of ATR/FTIR Spectra during Crystallization. The spectra of all sols have been recorded as a function of crystallization time. On sols S3-S5 changes have been observed only in the SiOSi antisymmetric region. On the other hand, on sols S6-S9 changes have been observed in the whole frequency range of interest. Sols with compositions S6-S9 were concentrated enough to enable the observation of the silicate species absorption in this region after the subtraction of the solvent spectra. For the reasons mentioned above, the discussion that follows is based on changes observed in the spectra of sols S4, S6, S8, and S9 that are presented in Figure 6. The spectra of sols S3, S5, and S7 can be seen in the Supporting Information. In sol S6, after approximately 2 h of crystallization a broad band between 400 and 480 cm-1 appears. As mentioned earlier, this region has been associated with the bending vibration of the SiOSi or OSiO bonds and is expected to be found in both amorphous silica and MFI crystals. As the crystallization progresses, this band becomes slightly narrower and its intensity slightly increases.
ATR/FTIR Spectroscopy of Silicalite-1 In the sol with composition S4, it is observed (Figure 6a) that between 0 and 6 h of crystallization the absorption bands P1 and P2 remained unchanged in terms of shape and energy, while bands P3 and P4 shifted to higher wavenumbers. Furthermore, the intensity of band P3 increased and finally reached a constant value. This increase can be attributed to the formation of new SiOSi bonds either as a result of structural reorganization of the nanoparticles (i.e., condensation of silanol neighboring groups) or by the addition of monomeric species to the nanoparticles. The latter is highly unlikely because the intensity and position of band P1 that has been assigned to the siliceous oligomeric species remained unaffected. Thus the structural reorganization of the nanoparticles is considered to be the most likely explanation for the observed shifts. The reorganization of the nanoparticles’ structure at the first stages of crystallization has been observed by other research groups.29 and it has been suggested to be part of the silicalite-1 nucleation mechanism.7,30 The main features of the changes in the SiOSi antisymmetric region are similar for all compositions. However, the observed changes are more pronounced in the diluted sols (sols S3-S5). This result is somewhat expected because, immediately after TEOS hydrolysis, the primary particles of sols S6-S9 are more cross-linked than those of sols S3-S5. It is noteworthy that the absorption intensity of the band P3 reached a maximum value after several hours of crystallization (6-15 h depending on composition). Then, for an extended period of time (20-30 h), it was not possible to observe any changes in the ATR/FTIR spectra. However, during that time the sols became cloudy, indicating that a population of larger particles had been formed. Based on the results of previously published studies on silicalite-1 crystallization at similar compositions, these larger nanoparticles must have the MFI structure.7,12,30 We believe that it is not possible to detect changes in the spectra of the sols at this stage because the number and size of these crystals are very small, and as a result the ATR/ FTIR spectra are dominated by that of the primary nanoparticles. The intensity of the scattered light is proportional to the sixth power of the particle radius, and as a result a very small number of larger particles will cause a significant increase in the turbidity of the sol. On the other hand, the intensity of the IR absorption can be considered to be proportional to the number of the SiOSi bonds, which is independent of the particle size. Furthermore, this observation also indicates that after the initial period of structural rearrangements the structure of the nanoparticles remains practically unchanged and that the nucleated crystals grow in a suspension of nanoparticles that do not have the silicalite-1 structure. As crystallization continued, two new absorption bands appeared, one at ∼544 cm-1 and one at ∼1218 cm-1. The intensity of both bands increased with crystallization time. Furthermore, band P3 shifted to lower wavenumbers with a parallel increase of its intensity and a decrease of its full width at half-maximum (fwhm). Due to the energy shift and intensity enhancement, at the end of crystallization P3 become superimposed with P2 and the splitting between the bands P3 and P4 increased. The above observations can be attributed to the condensation between the silanol groups of the nanoparticles. This is consistent with the crystal formation since the increase of number of Si-O bonds in the nanoparticles enhances the Coulombic forces between the atoms. It must be noted, however, that close to the end of crystallization the intensity of the absorption bands can also increase due to the precipitation of the larger silicalite-1 crystals. Such precipitation has not been
J. Phys. Chem. C, Vol. 111, No. 47, 2007 17483
Figure 7. ATR/FTIR spectra of the three characteristic structures observed during the crystallization of silicalite-1.
observed for the sols with compositions S6-S9; however, it was very pronounced in the sols with compositions S3-S5. It is also noteworthy that, even though the spectra of the sols at the end of crystallization are slightly different (Figure 6), the spectra of all powders are the same as that of the silicalite-1 crystal shown in Figure 2, indicating that the observed differences are due to the noncrystalline siliceous species of the sols. Based on the results presented above, we can argue that silicalite-1 crystallization, under the conditions studied, proceeds via the following mechanism: (i) Upon the completion of TEOS hydrolysis, nanoparticles having a structure different from that of silicalite-1 are formed. (ii) Upon heating of the sol, the structure of the nanoparticles changes to one having a larger number of SiOSi bonds and fewer SiOH groups. The structure of these “intermediate” nanoparticles is again different from that of MFI. (iii) Gradually some of these nanoparticles transform to new ones having the structure of MFI (nucleation), possibly by an Oswald ripening process. (iv) The nuclei formed in step iii grow in a suspension that contains the “intermediate” nanoparticles. In Figure 7 the ATR/FTIR spectra of the three characteristic structures observed (nanoparticles immediately after the TEOS hydrolysis, “intermediate” nanoparticles, and silicalite-1 crystal suspension) are compared. It must be pointed out that the results of our investigation cannot discriminate whether silicalite-1 growth proceeds via an oriented aggregation mechanism7,30 or via addition of monomeric/oligomeric species.14 However, if the mechanism of oriented aggregation is true, then the concentration of the nanoparticles having the appropriate structure (denoted as particles Bm in refs 7 and 31) for contributing to growth must be very small. Nevertheless, it is clear that the mechanism proposed by Kirschhock et al.6,23,51 cannot explain our experimental results. Conclusions The crystallization of silicalite-1 in aqueous sols has been investigated as a function of SiO2 concentration using ATR/ FTIR spectroscopy. The effect of composition on the structure of the siliceous species formed after the TEOS hydrolysis has been examined. The existence of oligomeric siliceous species has been verified for all compositions. On the other hand, more condensed and highly cross-linked siliceous structures have been observed only in the sols of high SiO2 concentrations. The differences of the IR spectra between these structures and those
17484 J. Phys. Chem. C, Vol. 111, No. 47, 2007 of silicalite-1 crystals indicate that the silicate units do not have the MFI connectivity. Furthermore, these structures have been considered to be the primary nanoparticles observed by several researchers in the past. The evolution of the ATR/FTIR spectra with synthesis time revealed the possible transformations in the primary nanoparticle structure that take place during silicalite-1 crystallization. In particular, it has been found that upon heating of the synthesis sols the primary nanoparticles transform to a new form that has an increased number of SiOSi bonds. This transformation is pronounced in the dilute sols. These “intermediate” particles do not have the structure of silicalite-1 and are present for most of the crystallization. Finally, once the silicalite-1 nuclei are formed, they grow in the suspension of these “intermediate” particles. Acknowledgment. The authors thank the laboratory of Food Biotechnology, Department of Chemistry, University of Patras, for providing access to their X-ray diffractometer. Supporting Information Available: Evolution of the ATR/ FTIR spectra of sols S3, S5, and S7 during crystallization. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Cundy, C. S.; Cox, B. G. Microporous Mesoporous Mater. 2005, 82, 1. (2) Cheng, C. H.; Shantz, D. F. Curr. Opin. Colloid Interface Sci. 2005, 10, 188. (3) de Moor, P.; Beelen, T. P. M.; vanSanten, R. A. Microporous Mater. 1997, 9, 117. (4) Dougherty, J.; Iton, L. E.; White, J. W. Zeolites 1995, 15, 640. (5) Watson, J. N.; Iton, L. E.; Keir, R. I.; Thomas, J. C.; Dowling, T. L.; White, J. W. J. Phys. Chem. B 1997, 101, 10094. (6) Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021. (7) Davis, T. M.; Drews, T. O.; Ramanan, H.; He, C.; Dong, J.; Schnablegger, H.; Katsoulakis, M. A.; Kokkoli, E.; McCormick, A. V.; Penn, R. L.; Tsapatsis, M. Nat. Mater. 2006, 5, 400. (8) Dokter, W. H.; Beelen, T. M. P.; van Garderen, H. F.; Rummens, C. P. J.; van Santen, R. A.; Ramsay, J. D. F. Colloids Surf., A: Physicochem. Eng. Aspects 1994, 85, 89. (9) Fedeyko, J. M.; Rimer, J. D.; Lobo, R. F.; Vlachos, D. G. J. Phys. Chem. B 2004, 108, 12271. (10) Fedeyko, J. M.; Vlachos, D. G.; Lobo, R. F. Langmuir 2005, 21, 5197. (11) Nikolakis, V.; Kokkoli, E.; Tirrell, M.; Tsapatsis, M.; Vlachos, D. G. Chem. Mater. 2000, 12, 845. (12) Schoeman, B. J. Zeolites 1997, 18, 97. (13) Schoeman, B. J. Microporous Mater. 1997, 9, 267. (14) Schoeman, B. J. Microporous Mesoporous Mater. 1998, 22, 9. (15) Schoeman, B. J.; Regev, O. Zeolites 1996, 17, 447. (16) Twomey, T. A. M.; MacKay, M.; Kuipers, H. O. C. E.; Thompson, R. W. Zeolites 1994, 14, 162. (17) Yang, S. Y.; Navrotsky, A.; Wesolowski, D. J.; Pople, J. A. Chem. Mater. 2004, 16, 210.
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