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Effects of ultrasound on the synthesis of silicalite-1 nanocrystals
Ultrasonics Sonochemistry 19 (2012) 1108–1113
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Effects of ultrasound on the synthesis of silicalite-1 nanocrystals Hale Gürbüz a,⇑, Begüm Tokay b, Aysße Erdem-Sß enatalar a a b
Department of Chemical Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey Chemical Engineering Program, Middle East Technical University, Northern Cyprus Campus, Kalkanlı, Güzelyurt, KKTC, via Mersin 10, Turkey
a r t i c l e
i n f o
Article history: Received 29 May 2011 Received in revised form 19 January 2012 Accepted 19 January 2012 Available online 9 February 2012 Keywords: Silicalite-1 Nanoparticles Crystallization Synthesis Ultrasound
a b s t r a c t Application of power ultrasound, offers potential in the degree of control over the preparation and properties of nanocrystalline zeolites, which have become increasingly important due to their diverse emerging applications. Synthesis of silicalite-1 nanocrystals from a clear solution was carried out at 348 K in the absence and presence of ultrasound of 300 and 600 W, in an attempt to investigate the effects of sonication, in this respect. Variation of the particle size and particle size distribution was followed with respect to time using a laser light scattering device with a detector set to collect back-scattered light at an angle of 173°. Product yield was determined and the crystallinity was analyzed by X-ray diffraction for selected samples collected during the syntheses. Nucleation, particle growth and crystallization rates all increased as a result of the application of ultrasound and highly crystalline silicalite-1 of smaller average particle diameter could be obtained at shorter synthesis times. The particle size distributions of the product populations, however, remained similar for similar average particle sizes. The rate of increase in yield was also speeded up in the presence of ultrasound, while the final product yield was not affected. Increasing the power of ultrasound, from 300 to 600 W, increased the particle growth rate and the crystalline domain size, and decreased both the final particle diameter and the time required for the particle growth to reach completion, while its effect on nucleation was unclear. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Application of power ultrasound has been shown to be instrumental in improving the rates, yields and product properties of a variety of processes in synthetic chemistry [1–8]. In polymer synthesis, besides the rate and yield enhancements, a larger degree of control over the polymer structure, especially in determining the molecular weight and polydispersity could be achieved using ultrasound [3,4]. Higher ultrasonic intensity was determined to result in an increase in the size of amorphous silica spheres prepared from TEOS [5]. It was argued that the deposition of europeum oxide nanoparticles on amorphous silica microspheres was facilitated by the use of ultrasound via the enhancement of cross-condensation of surface silanols on the silica microspheres with the hydroxyls of the europium hydroxide nanoparticles [6]. Ultrasound was also observed to influence the physicochemical phenomena related to nucleation and crystal growth, occurring during crystallization. As an example, ultrasonic waves decreased the supersaturation limits and modified the morphology of the potash alum crystals [7], in which case the average crystal size
⇑ Corresponding author. E-mail address: [email protected] (H. Gürbüz). 1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2012.01.008
decreased with an increase in the ultrasonic power. Applying ultrasound was also shown to affect the primary nucleation of potassium sulphate, by leading to reductions in the induction time and the metastable zone width of this material [8]. The chemical effects of ultrasound originate from cavitation, a phenomenon that may be defined as the growth and explosive collapse of microscopic bubbles. It is thought [9,10] that cavitation also increases the secondary nucleation rates and the crystal purity during cooling crystallization, and the enhanced mass transfer, which results from the application of ultrasound, increases crystal growth rates. The degree of control over the preparation of synthetic zeolites by hydrothermal crystallization in a way to tailor their properties, bears special importance due to their diverse conventional and emerging applications, and the use of ultrasound, clearly offers potential in this respect. The effects of ultrasound on zeolite A synthesis from a clear-to-the-eye sodium aluminosilicate solution have been reported in a previous study [11]. Nucleation and crystallization rates, as well as the yield of zeolite A were observed to increase significantly, as a result of sonication. The application of ultrasound was also shown in another study [12], to allow the preparation of thinner and continuous zeolite A coatings on stainless steel substrates in significantly shorter synthesis times. Ultrasound-assisted aging was seen to shorten the crystallization time of analcime, and to lead to altered morphologies of the
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product particles in one study [13], and to broaden the crystallization range, to shorten the crystallization time and to decrease the amount of hexamethyleneimine (HMI) used for the syntheses of MCM-49 [14] and MCM-22 [15] in other studies. We report here, the effects of application of ultrasound on the synthesis of silicalite-1 nanocrystals. Silicalite-1 is the pure silica end member of MFI type zeolites, and has significant emerging potential applications for its nanocrystalline forms, especially benefiting from the high hydrophobicity and high thermal stability of its structure [16].Additionally, due to the absence of alumina and inorganic cations in its synthesis mixtures, and the wide composition range from which it can be synthesized, it has been commonly used as a prototype to investigate and systematically tailor zeolite crystallization [17–44]. In most of these studies, its synthesis from clear solutions has been pursued, since these synthesis systems allow non-invasive analytical techniques, such as light scattering, to follow the crystallization in situ. Even in these so-called clear solutions though, which generally had been filtered through membrane filters prior to synthesis, subcolloidal particles of a few nanometers have been observed to be present, by a variety of techniques such as dynamic light scattering (DLS), cryo-TEM, and SAXS [22–24]. Both the nature of these nanoparticles and their role in silicalite-1 crystallization have been the subject of much debate [19,20,26–33]. Recent studies have indicated that the initial nanoparticles existing in silicalite-1 synthesis solutions at the beginning of synthesis, consisted of silica-rich cores composed mostly of polymerized silicic acid, surrounded by shells composed of TPA+ and water [37–39], and that they were amorphous, although they did possess a degree of ordering greater than that in dense amorphous silica[40–43]. Furthermore, these particles were observed to aggregate at very early times of synthesis at 373 K, to yield a second population of slightly larger nanoparticles, which later aggregated once more during the synthesis [42–44]. This second aggregation was observed to accompany the nucleation of the silicalite-1 crystal structure, after which the particles of the third population formed could grow at a constant linear rate. The particle population growing at a constant linear rate was not fully crystalline during its growth period, but its crystallinity increased as the particles grew in size [42–44]. Synthesis of silicalite-1 nanocrystals from a clear solution was carried out in this study, at 348 K in the absence and presence of ultrasound of 300 W and 600 W, in an effort to investigate the effects of sonication. Variation of the particle size and particle size distribution (PSD) was followed with respect to time using a laser light scattering device with a detector set to collect back-scattered light at an angle of 173°. Product yield was determined and the crystallinity was analyzed by X-ray diffraction (XRD) for selected samples collected during the syntheses.
2. Experimental Silicalite-1 syntheses were carried out from synthesis solutions having a composition containing TPAOH:SiO2:H2O in the molar ratios of 9:25:1500. The reactants were tetraethyl ortho silicate (TEOS, Merck, 98%), tetrapropyl ammonium hydroxide (TPAOH, Fluka, 20% in water) and deionized water. TEOS was added under vigorous stirring to the solution obtained initially by adding water to TPAOH. Hydrolysis of the TEOS was carried out in a horizontal shaker at room temperature for 24 h. The resulting solution (about 150 mL) was then filtered through a membrane filter (Millipore Durapore, nominal pore size 0.1 lm) into a 250 mL polypropylene flask and placed either in a silicone oil bath or an ultrasonic bath, kept at 348 K. The ultrasonic bath had a frequency of 35 kHz and a peak power output of 600 W (Sonorex DK156 BP), which was adjustable in the range of 10–100%. Two power levels of 300 and
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600 W were tested in this study. All syntheses were carried out under reflux. Laser light scattering (DLS) was used to monitor the particle growth during the syntheses. A Malvern Zetasizer Nano S (5 mW He-Ne laser, 633 nm), with a detector set to collect back-scattered light at 173owasused for the purpose of estimating the variation of average particle size and PSD with respect to time. The term ‘‘particle size’’ will be used to denote the hydrodynamic particle diameter throughout the study, unless specified otherwise. All light scattering measurements were carried out at 298 K, using original or diluted mother liquors. Dilutions were continued until reproducible size results could be obtained from successive measurements for each sample. Each measurement was repeated at least three times. Additionally, all synthesis experiments had to be repeated, and reproducibility between syntheses was also checked, in order to be able to collect data for up to 82 h in some cases. The shortest synthesis experiments required data collection for about 60 h. The intensity PSDs were obtained from analysis of the correlation functions using the Non-Negative Least Squares (NNLS) algorithmin the Zetasizer software. Conversion of these intensity PSDs to volume and/or number distributions were accomplished using Mie theory. Due to the polydispersity of the samples, better fits of the correlation functions were obtained from the multimodal algorithm, when compared to the cumulants analysis. Products were separated from the mother liquor at selected times by centrifugation using a Beckman Coulter Centrifuge Avanti J25-I (75,600g – 25,000 rpm) or a Beckman Coulter Ultracentrifuge L-100 XP (802,400g – 100,000 rpm). A sufficient pelleting time, calculated by dividing the rotor’s friction coefficient (k) by the particles’ sedimentation coefficient (s), was allowed for. While calculating the pelleting time for a selected sample, particle size was chosen as the smallest size of the respective PSD estimated previously by DLS. For unknown density values, 1.6 g/cm3, based on the density range estimated previously for the nanoparticles [38], was used in the calculation of the sedimentation coefficient, s, as in previous studies [42–44]. When extremely long centrifugation times were calculated to be necessary at 25,000 rpm, respective samples were separated using the Ultracentrifuge L-100 XP at higher rpm values, in order to decrease the centrifugation times. After each centrifugation, the liquid phase was carefully decanted and the solid phase was redispersed in distilled water in an ultrasonic bath (BandelinSonorex RK 106, 35 kHz). Centrifugation was repeated once more and the product was dried at 323 K in an oven for 12 h. The samples were kept in a desiccator in the presence of a saturated NH4Cl solution for at least 16 h prior to the determination of the yields and to the analyses of their crystallinities by XRD using a Panalytical X’Pert Pro MPD diffractometer. XRD data were recorded over a 2h range of 5–50°, with a step size of 0.0167°. CuKa (k = 1.5418 Å) radiation was used. Crystallinities were calculated according to ASTM D3906.
3. Results and discussion A representative growth curve, showing the variation of the intensity-average particle diameter with time, during the crystallization of silicalite-1 in the absence of ultrasound at 348 K, is given in Fig. 1. The presence of three populations is clearly seen in the figure. Error bars for the data points of the third population are marked on the growth curve, while those of the first and second populations are shown in the insert at the bottom right of the figure. Although the second population, known to result from the aggregation of the particles of the first population [42,44], was seen to be present in the intensity-weighted distributions even at the very beginning of the synthesis, it was not present in the
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300
250 200 Ave. Par. Diam., nm
Average Particle Diameter, nm
350
150 100 50
20 15 10 5 0 0
5
10
15 20 25 Time, h
30
35
0 0
10
20
30
40 50 Time, h
60
70
80
90
Fig. 1. Variation of the intensity-average particle diameter with time during crystallization of silicalite-1in the absence of ultrasound at 348 K.
number-weighted distributions until 5 h. Since it is known that the scattered intensity is proportional to the sixth power of the particle diameter, and that this results in a significant magnification of the larger sized population in the intensity-weighted distributions, the results indicate that although the first aggregation had already started, there were still relatively few particles of the second population in the reaction mixture at very early times. The first population seemed to disappear and only the second population was seen to be present in the mixture after 5 h. As was also stated previously [42,44], DLS data is not reliable though, regarding the disappearance of a smaller sized population from the mixture, since DLS may be almost blind to the presence of a population of smaller particles in the presence of larger particles beyond a certain concentration, at all scattering angles. After about 25 h, the third population, which is known to be the product of the second aggregation [42–44] taking place during the synthesis, between the particles of the second population, emerged. This third population was seen to appear later, at about 30 h, in the volumeweighted distributions, and even more later, at 36 h, in the number-weighted distributions, indicating that aggregation initially resulted in the formation of relatively few particles, and time was required for the number density of the particles of this third population to be sensed by the number-weighted distributions.
This second aggregation leading to the formation of the third population is known [42–44] to accompany the nucleation of the silicalite-1 crystal structure, which was also verified in this study by the XRD analyses (Fig. 5a) that will be discussed later. It can be seen from Fig. 1 that the particles of the third population then began to grow at a constant linear rate, and that growth continued until about 74 h in this case of synthesis carried out in the absence of ultrasound. Since we will be comparing number-average particle diameters, growth rates calculated from number-average diameters, and PSDs weighted with respect to number, later in this study, in order to discuss the effects of sonication, it is worth noting here that generally, the average particle size values weighted with respect to intensity, were higher than those weighted with respect to number. Deconvolution of light scattering data yields intensityweighted PSDs, which can then be converted to volume- and number-weighted PSDs. For a sample containing 50% by number of each of two populations of spherical particles of 5 and 50 nm diameters, for example, the areas of the peaks of the two populations should be equal in the number-weighted distribution. Since the volume of a spherical particle is proportional to the third power of the particle diameter, in the volume-weighted distribution, the area of the peak for the larger particles will be (50/
Average Particle Diameter, nm
300 250 200 150 100 Static
50
US 300W US 600W
0 0
10
20
30
40 Time, h
50
60
70
80
Fig. 2. Variation of the number-average particle diameter with time in the constant linear growth region during crystallization of silicalite-1 in the absence and presence of ultrasound (300 W and 600 W) at 348 K.
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Growth rate, nm/h tind, h tc,int, h teg, h dp, nm dS, nm
Static
US 300 W
US 600 W
5.2 21.0 24 74 266.1 ± 2.0 55.6
6.2 15.2 20 55 241.7 ± 2.3 80.4
6.4 16.9 19 52 226.8 ± 2.9 97.9
a 4h
5h 44 h 54 h
Intensity
Table 1 Comparison of silicalite-1 syntheses carried out in the absence and presence of ultrasound.
72 h
tind: Induction time. tc,int: Time when first indications of nucleation appeared in the intensity distributions. teg: Time corresponding to the end of particle growth. dp: Final number-average particle diameter. dS: Crystalline domain size estimated from the Scherrer equation.
82 h
5
15
20
25 30 2 Theta ( )
35
40
45
50
b 5h 6h
Intensity
26 h 28 h 39 h
52 h
5
10
15
20
25 30 2 Theta ( )
35
40
45
50
c 4h 5h 28 h
Intensity
5)3 = 103 times larger with respect to that of the smaller particles. The ratio of the areas of the peaks for the larger to smaller particles will increase by another 103 times from the volume to intensityweighted distributions, since the intensity of scattering of a particle is proportional to the sixth power of its diameter, according to Rayleigh’s approximation. The strong size dependency of the intensities, understandably, results in a change not only in the areas under the peaks, but also in the shapes of the PSD peaks and thus, in the average sizes calculated for the intensity-, volume-, and number-weighted distributions. In general, number-average sizes were observed to be more representative of the sizes in the products, during microscopy studies [42], as would be expected from the above information. In the case of the synthesis carried out in the absence of ultrasound, the number-average diameter of the first population was 1.3 ± 0.1 nm at the beginning of the synthesis (t = 0 h). The number-average diameter of the second population fluctuated between 5.8 ± 0.3 and 10.4 ± 0.2 nm. The smallest number-average diameter sensed by the DLS instrument for the third population was 81.6 ± 1.3 nm and the final number-average diameter reached at 74 h was 266.1 ± 2.0 nm. Although the number-average diameters of the first and second populations obtained from the syntheses carried out in the presence of ultrasound of 300 and 600 W were similar to the above values obtained for unsonicated synthesis, there were other significant effects of sonication, which will be discussed below. Variation of the number-average particle diameter of the third population with time in the constant linear growth region, during crystallization of silicalite-1 in the absence and presence of ultrasound (300 and 600 W) at 348 K is shown in Fig. 2. The data indicate that the particle growth rate was increased by the application of ultrasound. The growth rates calculated from the lines drawn by linear regression are given in Table 1. Fig. 2 shows that the induction time estimated from the growth curve decreased in the presence of ultrasound, indicating that not only particle growth, but also nucleation was accelerated by sonication. The induction time values estimated from the growth curves are also listed in Table 1. The values in the table indicate that increasing the ultrasound power from 300 to 600 W somewhat increased the particle growth rate, but not the nucleation rate. Table 1 also lists the time values, when the first indications of aggregation between the particles of the second population, appeared in the intensity-weighted distributions. In case these values are used as measures of the respective induction times, the nucleation rate seems to have been slightly increased by the power of sonication. However, the differences in both cases, which showed opposite trends, are small, and hence the effect of the power of the ultrasound on the rate of nucleation remains unclear. The final number-average diameters of the crystalline products obtained in the absence and presence of ultrasound and the times
10
30 h
58 h
5
10
15
20
25 30 2 Theta ( )
35
40
45
50
Fig. 3. X-ray diffractograms of some selected samples obtained from synthesis, (a) in the absence of US and (b) in the presence of US of 300 W, and c) in the presence of US of 600 W.
required for the particle growth to come to completion are also given in Table 1. The values indicate that both the final particle diameter, and the time required for particle growth to be completed, decreased in the presence of ultrasound. In the following discussion of the XRD results, it will be shown that the final products were highly crystalline in all cases. As a result, it is clear that highly crystalline products of smaller average particle diameter could be obtained in shorter synthesis times as a result of sonication.
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30 Stat-40h (102 ± 2.2 nm) US 300W-31h (102 ± 2.2 nm) US 600W-33h (102 ± 2.2 nm) Stat-50h (148 ± 0.3 nm) US 300W-41h (148 ± 0.3 nm) US 600W-39h (148 ± 0.3 nm) Stat-62h (216 ± 3.5 nm) US 300W-48h (216 ± 3.5 nm) US 600W-51h (216 ±3.5 nm)
Frequency, % num
25 20 15
10 5 0 0
100
200
300 400 500 Particle Diameter, nm
600
700
800
Fig. 4. Comparison of some of the number-weighted PSDs of particle populations with the same number-average diameter.
0.045
Yield, g solid/g sol
0.040 0.035 0.030 0.025 0.020 0.015
static
0.010
US 300W
US 600W
0.005
0.000 0
10
20
30
40 50 Time, h
60
70
80
90
Fig. 5. Variation of yield with time during crystallization of silicalite-1 in the absence and presence of ultrasound (300 and 600 W) at 348 K.
The XRD results shown in Fig. 3 not only verify successful synthesis of silicalite-1 crystals, but similar to the results reported previously [42,44], for the syntheses carried out from several compositions at the higher temperature of 373 K, also demonstrate that the first indications of X-ray crystallinity were observed after the jump in particle diameter, resulting from the second aggregation. Additionally, the particle population growing at a constant rate was not fully crystalline during its growth period, but its crystallinity increased as the particles grew in size. These observations meaning that the change in size reflects the growth of the third population of aggregates, the crystallinity of which increase with time as they grow, are parallel to the previous results reported [42,44]. XRD patterns given in Fig. 3a–c indicate that the rate of crystallization was increased by the application of ultrasound. A significant effect of sonication was observed on the crystalline domain sizes estimated by the Scherrer equation. 23.6° peak, which is the highest intensity peak for silicalite-1, was used for this calculation. Instrumental broadening was also taken into account by measuring the profile of a standard material (LaB6). The values obtained for the final products are listed in Table 1. The domain size increased from 55.6 to 80.4 by the application of ultrasound of 300 W, and to 97.9 nm when the power of ultrasound was increased to 600 W. Fig. 4 shows a comparison of some of the number-weighted PSDs of particle populations with the same number-average diam-
eter for the products of syntheses obtained in the presence and absence of ultrasound. The PSDs given in the figure are seen to be almost identical for the same number-average diameter. It is clear that although the rates of nucleation, particle growth and crystallization increased, and smaller particles were obtained eventually at shorter synthesis times, the PSDs did not change for the same average diameters. Variation of the yield with time during crystallization of silicalite-1 in the absence and presence of ultrasound is shown in Fig. 5. It is clear that the rate of increase of the yield increased significantly with the application of ultrasound, while the final yield remained almost the same in all cases. The yield was also observed not to be sensitive to the power of ultrasound employed. Similar yields of smaller product crystals obtained at shorter times in the presence of ultrasound, when compared to the results obtained without sonication, imply that higher number densities of these smaller products crystals were collected, as would also be expected from the increased nucleation rates observed as a result of the application of ultrasound.
4. Conclusions Synthesis of silicalite-1 nanocrystals from a clear solution was carried out at 348 K in the absence and presence of ultrasound of
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300 and 600 W, in an effort to investigate the effects of sonication. Variation of the particle size and particle size distribution was followed with respect to time using a laser light scattering device with a detector set to collect back-scattered light at an angle of 173°. Product yield was determined and the crystallinity was analyzed by X-ray diffraction for selected samples collected during the syntheses. Nucleation, particle growth and crystallization rates all increased as a result of the application of ultrasound and highly crystalline silicalite-1 of smaller average particle diameter could be obtained at shorter synthesis times. The particle size distributions of the product populations, however, remained similar for similar average particle sizes. The rate of increase in yield was also speeded up in the presence of ultrasound, while the final product yield was not affected. Increasing the power of ultrasound, from 300 to 600 W, increased the particle growth rate and the crystalline domain size, and decreased both the final particle diameter and the time required for the particle growth to reach completion, while its effect on nucleation was unclear. The yield was also observed not to be sensitive to the power of ultrasound employed. Similar yields of smaller product crystals obtained at shorter times in the presence of ultrasound, when compared to the results obtained without sonication, imply that higher number densities of these smaller products crystals were collected, as would also be expected from the increased nucleation rates observed as a result of the application of ultrasound.
Acknowledgement Advanced Technologies in Engineering program of the State Planning Organization of Turkey (DPT) is gratefully acknowledged for the financial support provided.
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Vlachos, Zeolite growth by addition of subcolloidal particles: modeling and experimental validation, Chem. Mater. 12 (2000) 845–853. [33] C.T.G. Knight, S.D. Kinrade, Comment on ‘‘Identification of precursor species in the formation of MFI zeolite in the TPAOH TEOS H2O system’’, J. Phys. Chem. B 106 (2002) 3329–3332. [34] S. Mintova, N.H. Olson, J. Senker, T. Bein, Mechanism of the transformation of silica precursor solutions into Si-MFI zeolite, Angew. Chem. Int. Ed. 41 (2002) 2558–2561. [35] V. Nikolakis, M. Tsapatsis, D.G. Vlachos, Physicochemical characterization of silicalite-1 surface and its implications on crystal growth, Langmuir 19 (2003) 4619–4626. [36] T.M. Davis, T.O. Drews, H. Ramanan, C. He, J. Dong, H. Schnablegger, M.A. Katsoulakis, E. Kokkoli, A.V. McCormick, R. Lee Penn, M. Tsapatsis, Mechanistic principles of nanoparticle evolution to zeolite crystals, Nature Mater. 5 (2006) 400–408. [37] J.D. Rimer, D.G. Vlachos, R.F. Lobo, Evolution of self-assembled silica tetrapropylammonium nanoparticles at elevated temperatures, J. Phys. Chem. B 109 (2005) 12762–12771. [38] J.M. Fedeyko, D.G. Vlachos, R.F. Lobo, Formation and structure of selfassembled silica nanoparticles in basic solutions of organic and inorganic cations, Langmuir 21 (2005) 5197–5206. [39] J.M. Fedeyko, J.D. Rimer, R.F. Lobo, D.G. Vlachos, Spontaneous formation of silica nanoparticles in basic solutions of small tetraalkylammonium cations, J. Phys. Chem. B 108 (2004) 12271–12275. [40] J.D. Rimer, R.F. Lobo, D.G. Vlachos, Physical basis for the formation and stability of silica nanoparticles in basic solutions of monovalent cations, Langmuir 21 (2005) 8960–8971. [41] J.D. Rimer, O. Trofymluk, A. Navrotsky, R.F. Lobo, D.G. Vlachos, Kinetic and thermodynamic studies of silica nanoparticle dissolution, Chem. Mater. 19 (2007) 4189–4197. [42] B. Tokay, M. Somer, A. Erdem-Senatalar, F. Schüth, R.W. Thompson, Nanoparticle silicalite-1 crystallization from clear solutions: nucleation, Micropor. Mesopor. Mater. 118 (2009) 143–151. [43] B. Tokay, O. Karvan, A. Erdem-Senatalar, Nanoparticle silicalite-1 crystallization as monitored by nitrogen adsorption, Micropor. Mesopor. Mater. 131 (2010) 230–237. [44] B. Tokay, A. Erdem-Senatalar, Variation of particle size and its distribution during the synthesis of silicalite-1 nanocrystals, B. Tokay, A. Erdem-Senatalar, Micropor. Mesopor. Mater. 148(1) (2012) 43–52.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Effects of ultrasound on the synthesis of silicalite-1 nanocrystals Hale Gürbüz a,⇑, Begüm Tokay b, Aysße Erdem-Sß enatalar a a b
Department of Chemical Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey Chemical Engineering Program, Middle East Technical University, Northern Cyprus Campus, Kalkanlı, Güzelyurt, KKTC, via Mersin 10, Turkey
a r t i c l e
i n f o
Article history: Received 29 May 2011 Received in revised form 19 January 2012 Accepted 19 January 2012 Available online 9 February 2012 Keywords: Silicalite-1 Nanoparticles Crystallization Synthesis Ultrasound
a b s t r a c t Application of power ultrasound, offers potential in the degree of control over the preparation and properties of nanocrystalline zeolites, which have become increasingly important due to their diverse emerging applications. Synthesis of silicalite-1 nanocrystals from a clear solution was carried out at 348 K in the absence and presence of ultrasound of 300 and 600 W, in an attempt to investigate the effects of sonication, in this respect. Variation of the particle size and particle size distribution was followed with respect to time using a laser light scattering device with a detector set to collect back-scattered light at an angle of 173°. Product yield was determined and the crystallinity was analyzed by X-ray diffraction for selected samples collected during the syntheses. Nucleation, particle growth and crystallization rates all increased as a result of the application of ultrasound and highly crystalline silicalite-1 of smaller average particle diameter could be obtained at shorter synthesis times. The particle size distributions of the product populations, however, remained similar for similar average particle sizes. The rate of increase in yield was also speeded up in the presence of ultrasound, while the final product yield was not affected. Increasing the power of ultrasound, from 300 to 600 W, increased the particle growth rate and the crystalline domain size, and decreased both the final particle diameter and the time required for the particle growth to reach completion, while its effect on nucleation was unclear. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Application of power ultrasound has been shown to be instrumental in improving the rates, yields and product properties of a variety of processes in synthetic chemistry [1–8]. In polymer synthesis, besides the rate and yield enhancements, a larger degree of control over the polymer structure, especially in determining the molecular weight and polydispersity could be achieved using ultrasound [3,4]. Higher ultrasonic intensity was determined to result in an increase in the size of amorphous silica spheres prepared from TEOS [5]. It was argued that the deposition of europeum oxide nanoparticles on amorphous silica microspheres was facilitated by the use of ultrasound via the enhancement of cross-condensation of surface silanols on the silica microspheres with the hydroxyls of the europium hydroxide nanoparticles [6]. Ultrasound was also observed to influence the physicochemical phenomena related to nucleation and crystal growth, occurring during crystallization. As an example, ultrasonic waves decreased the supersaturation limits and modified the morphology of the potash alum crystals [7], in which case the average crystal size
⇑ Corresponding author. E-mail address: [email protected] (H. Gürbüz). 1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2012.01.008
decreased with an increase in the ultrasonic power. Applying ultrasound was also shown to affect the primary nucleation of potassium sulphate, by leading to reductions in the induction time and the metastable zone width of this material [8]. The chemical effects of ultrasound originate from cavitation, a phenomenon that may be defined as the growth and explosive collapse of microscopic bubbles. It is thought [9,10] that cavitation also increases the secondary nucleation rates and the crystal purity during cooling crystallization, and the enhanced mass transfer, which results from the application of ultrasound, increases crystal growth rates. The degree of control over the preparation of synthetic zeolites by hydrothermal crystallization in a way to tailor their properties, bears special importance due to their diverse conventional and emerging applications, and the use of ultrasound, clearly offers potential in this respect. The effects of ultrasound on zeolite A synthesis from a clear-to-the-eye sodium aluminosilicate solution have been reported in a previous study [11]. Nucleation and crystallization rates, as well as the yield of zeolite A were observed to increase significantly, as a result of sonication. The application of ultrasound was also shown in another study [12], to allow the preparation of thinner and continuous zeolite A coatings on stainless steel substrates in significantly shorter synthesis times. Ultrasound-assisted aging was seen to shorten the crystallization time of analcime, and to lead to altered morphologies of the
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product particles in one study [13], and to broaden the crystallization range, to shorten the crystallization time and to decrease the amount of hexamethyleneimine (HMI) used for the syntheses of MCM-49 [14] and MCM-22 [15] in other studies. We report here, the effects of application of ultrasound on the synthesis of silicalite-1 nanocrystals. Silicalite-1 is the pure silica end member of MFI type zeolites, and has significant emerging potential applications for its nanocrystalline forms, especially benefiting from the high hydrophobicity and high thermal stability of its structure [16].Additionally, due to the absence of alumina and inorganic cations in its synthesis mixtures, and the wide composition range from which it can be synthesized, it has been commonly used as a prototype to investigate and systematically tailor zeolite crystallization [17–44]. In most of these studies, its synthesis from clear solutions has been pursued, since these synthesis systems allow non-invasive analytical techniques, such as light scattering, to follow the crystallization in situ. Even in these so-called clear solutions though, which generally had been filtered through membrane filters prior to synthesis, subcolloidal particles of a few nanometers have been observed to be present, by a variety of techniques such as dynamic light scattering (DLS), cryo-TEM, and SAXS [22–24]. Both the nature of these nanoparticles and their role in silicalite-1 crystallization have been the subject of much debate [19,20,26–33]. Recent studies have indicated that the initial nanoparticles existing in silicalite-1 synthesis solutions at the beginning of synthesis, consisted of silica-rich cores composed mostly of polymerized silicic acid, surrounded by shells composed of TPA+ and water [37–39], and that they were amorphous, although they did possess a degree of ordering greater than that in dense amorphous silica[40–43]. Furthermore, these particles were observed to aggregate at very early times of synthesis at 373 K, to yield a second population of slightly larger nanoparticles, which later aggregated once more during the synthesis [42–44]. This second aggregation was observed to accompany the nucleation of the silicalite-1 crystal structure, after which the particles of the third population formed could grow at a constant linear rate. The particle population growing at a constant linear rate was not fully crystalline during its growth period, but its crystallinity increased as the particles grew in size [42–44]. Synthesis of silicalite-1 nanocrystals from a clear solution was carried out in this study, at 348 K in the absence and presence of ultrasound of 300 W and 600 W, in an effort to investigate the effects of sonication. Variation of the particle size and particle size distribution (PSD) was followed with respect to time using a laser light scattering device with a detector set to collect back-scattered light at an angle of 173°. Product yield was determined and the crystallinity was analyzed by X-ray diffraction (XRD) for selected samples collected during the syntheses.
2. Experimental Silicalite-1 syntheses were carried out from synthesis solutions having a composition containing TPAOH:SiO2:H2O in the molar ratios of 9:25:1500. The reactants were tetraethyl ortho silicate (TEOS, Merck, 98%), tetrapropyl ammonium hydroxide (TPAOH, Fluka, 20% in water) and deionized water. TEOS was added under vigorous stirring to the solution obtained initially by adding water to TPAOH. Hydrolysis of the TEOS was carried out in a horizontal shaker at room temperature for 24 h. The resulting solution (about 150 mL) was then filtered through a membrane filter (Millipore Durapore, nominal pore size 0.1 lm) into a 250 mL polypropylene flask and placed either in a silicone oil bath or an ultrasonic bath, kept at 348 K. The ultrasonic bath had a frequency of 35 kHz and a peak power output of 600 W (Sonorex DK156 BP), which was adjustable in the range of 10–100%. Two power levels of 300 and
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600 W were tested in this study. All syntheses were carried out under reflux. Laser light scattering (DLS) was used to monitor the particle growth during the syntheses. A Malvern Zetasizer Nano S (5 mW He-Ne laser, 633 nm), with a detector set to collect back-scattered light at 173owasused for the purpose of estimating the variation of average particle size and PSD with respect to time. The term ‘‘particle size’’ will be used to denote the hydrodynamic particle diameter throughout the study, unless specified otherwise. All light scattering measurements were carried out at 298 K, using original or diluted mother liquors. Dilutions were continued until reproducible size results could be obtained from successive measurements for each sample. Each measurement was repeated at least three times. Additionally, all synthesis experiments had to be repeated, and reproducibility between syntheses was also checked, in order to be able to collect data for up to 82 h in some cases. The shortest synthesis experiments required data collection for about 60 h. The intensity PSDs were obtained from analysis of the correlation functions using the Non-Negative Least Squares (NNLS) algorithmin the Zetasizer software. Conversion of these intensity PSDs to volume and/or number distributions were accomplished using Mie theory. Due to the polydispersity of the samples, better fits of the correlation functions were obtained from the multimodal algorithm, when compared to the cumulants analysis. Products were separated from the mother liquor at selected times by centrifugation using a Beckman Coulter Centrifuge Avanti J25-I (75,600g – 25,000 rpm) or a Beckman Coulter Ultracentrifuge L-100 XP (802,400g – 100,000 rpm). A sufficient pelleting time, calculated by dividing the rotor’s friction coefficient (k) by the particles’ sedimentation coefficient (s), was allowed for. While calculating the pelleting time for a selected sample, particle size was chosen as the smallest size of the respective PSD estimated previously by DLS. For unknown density values, 1.6 g/cm3, based on the density range estimated previously for the nanoparticles [38], was used in the calculation of the sedimentation coefficient, s, as in previous studies [42–44]. When extremely long centrifugation times were calculated to be necessary at 25,000 rpm, respective samples were separated using the Ultracentrifuge L-100 XP at higher rpm values, in order to decrease the centrifugation times. After each centrifugation, the liquid phase was carefully decanted and the solid phase was redispersed in distilled water in an ultrasonic bath (BandelinSonorex RK 106, 35 kHz). Centrifugation was repeated once more and the product was dried at 323 K in an oven for 12 h. The samples were kept in a desiccator in the presence of a saturated NH4Cl solution for at least 16 h prior to the determination of the yields and to the analyses of their crystallinities by XRD using a Panalytical X’Pert Pro MPD diffractometer. XRD data were recorded over a 2h range of 5–50°, with a step size of 0.0167°. CuKa (k = 1.5418 Å) radiation was used. Crystallinities were calculated according to ASTM D3906.
3. Results and discussion A representative growth curve, showing the variation of the intensity-average particle diameter with time, during the crystallization of silicalite-1 in the absence of ultrasound at 348 K, is given in Fig. 1. The presence of three populations is clearly seen in the figure. Error bars for the data points of the third population are marked on the growth curve, while those of the first and second populations are shown in the insert at the bottom right of the figure. Although the second population, known to result from the aggregation of the particles of the first population [42,44], was seen to be present in the intensity-weighted distributions even at the very beginning of the synthesis, it was not present in the
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300
250 200 Ave. Par. Diam., nm
Average Particle Diameter, nm
350
150 100 50
20 15 10 5 0 0
5
10
15 20 25 Time, h
30
35
0 0
10
20
30
40 50 Time, h
60
70
80
90
Fig. 1. Variation of the intensity-average particle diameter with time during crystallization of silicalite-1in the absence of ultrasound at 348 K.
number-weighted distributions until 5 h. Since it is known that the scattered intensity is proportional to the sixth power of the particle diameter, and that this results in a significant magnification of the larger sized population in the intensity-weighted distributions, the results indicate that although the first aggregation had already started, there were still relatively few particles of the second population in the reaction mixture at very early times. The first population seemed to disappear and only the second population was seen to be present in the mixture after 5 h. As was also stated previously [42,44], DLS data is not reliable though, regarding the disappearance of a smaller sized population from the mixture, since DLS may be almost blind to the presence of a population of smaller particles in the presence of larger particles beyond a certain concentration, at all scattering angles. After about 25 h, the third population, which is known to be the product of the second aggregation [42–44] taking place during the synthesis, between the particles of the second population, emerged. This third population was seen to appear later, at about 30 h, in the volumeweighted distributions, and even more later, at 36 h, in the number-weighted distributions, indicating that aggregation initially resulted in the formation of relatively few particles, and time was required for the number density of the particles of this third population to be sensed by the number-weighted distributions.
This second aggregation leading to the formation of the third population is known [42–44] to accompany the nucleation of the silicalite-1 crystal structure, which was also verified in this study by the XRD analyses (Fig. 5a) that will be discussed later. It can be seen from Fig. 1 that the particles of the third population then began to grow at a constant linear rate, and that growth continued until about 74 h in this case of synthesis carried out in the absence of ultrasound. Since we will be comparing number-average particle diameters, growth rates calculated from number-average diameters, and PSDs weighted with respect to number, later in this study, in order to discuss the effects of sonication, it is worth noting here that generally, the average particle size values weighted with respect to intensity, were higher than those weighted with respect to number. Deconvolution of light scattering data yields intensityweighted PSDs, which can then be converted to volume- and number-weighted PSDs. For a sample containing 50% by number of each of two populations of spherical particles of 5 and 50 nm diameters, for example, the areas of the peaks of the two populations should be equal in the number-weighted distribution. Since the volume of a spherical particle is proportional to the third power of the particle diameter, in the volume-weighted distribution, the area of the peak for the larger particles will be (50/
Average Particle Diameter, nm
300 250 200 150 100 Static
50
US 300W US 600W
0 0
10
20
30
40 Time, h
50
60
70
80
Fig. 2. Variation of the number-average particle diameter with time in the constant linear growth region during crystallization of silicalite-1 in the absence and presence of ultrasound (300 W and 600 W) at 348 K.
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Growth rate, nm/h tind, h tc,int, h teg, h dp, nm dS, nm
Static
US 300 W
US 600 W
5.2 21.0 24 74 266.1 ± 2.0 55.6
6.2 15.2 20 55 241.7 ± 2.3 80.4
6.4 16.9 19 52 226.8 ± 2.9 97.9
a 4h
5h 44 h 54 h
Intensity
Table 1 Comparison of silicalite-1 syntheses carried out in the absence and presence of ultrasound.
72 h
tind: Induction time. tc,int: Time when first indications of nucleation appeared in the intensity distributions. teg: Time corresponding to the end of particle growth. dp: Final number-average particle diameter. dS: Crystalline domain size estimated from the Scherrer equation.
82 h
5
15
20
25 30 2 Theta ( )
35
40
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50
b 5h 6h
Intensity
26 h 28 h 39 h
52 h
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40
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50
c 4h 5h 28 h
Intensity
5)3 = 103 times larger with respect to that of the smaller particles. The ratio of the areas of the peaks for the larger to smaller particles will increase by another 103 times from the volume to intensityweighted distributions, since the intensity of scattering of a particle is proportional to the sixth power of its diameter, according to Rayleigh’s approximation. The strong size dependency of the intensities, understandably, results in a change not only in the areas under the peaks, but also in the shapes of the PSD peaks and thus, in the average sizes calculated for the intensity-, volume-, and number-weighted distributions. In general, number-average sizes were observed to be more representative of the sizes in the products, during microscopy studies [42], as would be expected from the above information. In the case of the synthesis carried out in the absence of ultrasound, the number-average diameter of the first population was 1.3 ± 0.1 nm at the beginning of the synthesis (t = 0 h). The number-average diameter of the second population fluctuated between 5.8 ± 0.3 and 10.4 ± 0.2 nm. The smallest number-average diameter sensed by the DLS instrument for the third population was 81.6 ± 1.3 nm and the final number-average diameter reached at 74 h was 266.1 ± 2.0 nm. Although the number-average diameters of the first and second populations obtained from the syntheses carried out in the presence of ultrasound of 300 and 600 W were similar to the above values obtained for unsonicated synthesis, there were other significant effects of sonication, which will be discussed below. Variation of the number-average particle diameter of the third population with time in the constant linear growth region, during crystallization of silicalite-1 in the absence and presence of ultrasound (300 and 600 W) at 348 K is shown in Fig. 2. The data indicate that the particle growth rate was increased by the application of ultrasound. The growth rates calculated from the lines drawn by linear regression are given in Table 1. Fig. 2 shows that the induction time estimated from the growth curve decreased in the presence of ultrasound, indicating that not only particle growth, but also nucleation was accelerated by sonication. The induction time values estimated from the growth curves are also listed in Table 1. The values in the table indicate that increasing the ultrasound power from 300 to 600 W somewhat increased the particle growth rate, but not the nucleation rate. Table 1 also lists the time values, when the first indications of aggregation between the particles of the second population, appeared in the intensity-weighted distributions. In case these values are used as measures of the respective induction times, the nucleation rate seems to have been slightly increased by the power of sonication. However, the differences in both cases, which showed opposite trends, are small, and hence the effect of the power of the ultrasound on the rate of nucleation remains unclear. The final number-average diameters of the crystalline products obtained in the absence and presence of ultrasound and the times
10
30 h
58 h
5
10
15
20
25 30 2 Theta ( )
35
40
45
50
Fig. 3. X-ray diffractograms of some selected samples obtained from synthesis, (a) in the absence of US and (b) in the presence of US of 300 W, and c) in the presence of US of 600 W.
required for the particle growth to come to completion are also given in Table 1. The values indicate that both the final particle diameter, and the time required for particle growth to be completed, decreased in the presence of ultrasound. In the following discussion of the XRD results, it will be shown that the final products were highly crystalline in all cases. As a result, it is clear that highly crystalline products of smaller average particle diameter could be obtained in shorter synthesis times as a result of sonication.
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30 Stat-40h (102 ± 2.2 nm) US 300W-31h (102 ± 2.2 nm) US 600W-33h (102 ± 2.2 nm) Stat-50h (148 ± 0.3 nm) US 300W-41h (148 ± 0.3 nm) US 600W-39h (148 ± 0.3 nm) Stat-62h (216 ± 3.5 nm) US 300W-48h (216 ± 3.5 nm) US 600W-51h (216 ±3.5 nm)
Frequency, % num
25 20 15
10 5 0 0
100
200
300 400 500 Particle Diameter, nm
600
700
800
Fig. 4. Comparison of some of the number-weighted PSDs of particle populations with the same number-average diameter.
0.045
Yield, g solid/g sol
0.040 0.035 0.030 0.025 0.020 0.015
static
0.010
US 300W
US 600W
0.005
0.000 0
10
20
30
40 50 Time, h
60
70
80
90
Fig. 5. Variation of yield with time during crystallization of silicalite-1 in the absence and presence of ultrasound (300 and 600 W) at 348 K.
The XRD results shown in Fig. 3 not only verify successful synthesis of silicalite-1 crystals, but similar to the results reported previously [42,44], for the syntheses carried out from several compositions at the higher temperature of 373 K, also demonstrate that the first indications of X-ray crystallinity were observed after the jump in particle diameter, resulting from the second aggregation. Additionally, the particle population growing at a constant rate was not fully crystalline during its growth period, but its crystallinity increased as the particles grew in size. These observations meaning that the change in size reflects the growth of the third population of aggregates, the crystallinity of which increase with time as they grow, are parallel to the previous results reported [42,44]. XRD patterns given in Fig. 3a–c indicate that the rate of crystallization was increased by the application of ultrasound. A significant effect of sonication was observed on the crystalline domain sizes estimated by the Scherrer equation. 23.6° peak, which is the highest intensity peak for silicalite-1, was used for this calculation. Instrumental broadening was also taken into account by measuring the profile of a standard material (LaB6). The values obtained for the final products are listed in Table 1. The domain size increased from 55.6 to 80.4 by the application of ultrasound of 300 W, and to 97.9 nm when the power of ultrasound was increased to 600 W. Fig. 4 shows a comparison of some of the number-weighted PSDs of particle populations with the same number-average diam-
eter for the products of syntheses obtained in the presence and absence of ultrasound. The PSDs given in the figure are seen to be almost identical for the same number-average diameter. It is clear that although the rates of nucleation, particle growth and crystallization increased, and smaller particles were obtained eventually at shorter synthesis times, the PSDs did not change for the same average diameters. Variation of the yield with time during crystallization of silicalite-1 in the absence and presence of ultrasound is shown in Fig. 5. It is clear that the rate of increase of the yield increased significantly with the application of ultrasound, while the final yield remained almost the same in all cases. The yield was also observed not to be sensitive to the power of ultrasound employed. Similar yields of smaller product crystals obtained at shorter times in the presence of ultrasound, when compared to the results obtained without sonication, imply that higher number densities of these smaller products crystals were collected, as would also be expected from the increased nucleation rates observed as a result of the application of ultrasound.
4. Conclusions Synthesis of silicalite-1 nanocrystals from a clear solution was carried out at 348 K in the absence and presence of ultrasound of
H. Gürbüz et al. / Ultrasonics Sonochemistry 19 (2012) 1108–1113
300 and 600 W, in an effort to investigate the effects of sonication. Variation of the particle size and particle size distribution was followed with respect to time using a laser light scattering device with a detector set to collect back-scattered light at an angle of 173°. Product yield was determined and the crystallinity was analyzed by X-ray diffraction for selected samples collected during the syntheses. Nucleation, particle growth and crystallization rates all increased as a result of the application of ultrasound and highly crystalline silicalite-1 of smaller average particle diameter could be obtained at shorter synthesis times. The particle size distributions of the product populations, however, remained similar for similar average particle sizes. The rate of increase in yield was also speeded up in the presence of ultrasound, while the final product yield was not affected. Increasing the power of ultrasound, from 300 to 600 W, increased the particle growth rate and the crystalline domain size, and decreased both the final particle diameter and the time required for the particle growth to reach completion, while its effect on nucleation was unclear. The yield was also observed not to be sensitive to the power of ultrasound employed. Similar yields of smaller product crystals obtained at shorter times in the presence of ultrasound, when compared to the results obtained without sonication, imply that higher number densities of these smaller products crystals were collected, as would also be expected from the increased nucleation rates observed as a result of the application of ultrasound.
Acknowledgement Advanced Technologies in Engineering program of the State Planning Organization of Turkey (DPT) is gratefully acknowledged for the financial support provided.
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