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A preliminary investigation on kinetics of zeolite A crystallisation using optical diagnostics
Materials Chemistry and Physics 66 (2000) 120–125
A preliminary investigation on kinetics of zeolite A crystallisation using optical diagnostics D. Caputo a,∗ , B. De Gennaro a , B. Liguori a , F. Testa b , L. Carotenuto c , C. Piccolo c a
Department of Materials and Production Engineering, University of Napoli, Federico II, Napoli, Italy b Department of Chemistry and Materials Engineering, University of Calabria, Calabria, Italy c MARS Center, Napoli, Italy
Abstract Preliminary data concerning the monitoring in situ of the zeolite A crystallisation from clear solutions are reported. Experiments were performed at 60 and 70◦ C with fresh and aged solutions having the following composition: 8.6Na2 O·0.18SiO2 ·Al2 O3 ·150H2 O. The formation of early phases in the crystallisation process were followed using diagnostics such as dynamic light scattering (DLS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The comparison of the available data seems to indicate the formation of a precursor phase (like gel) in the case of not aged solution. Conversely, crystallisation in aged solution seems to proceed without intermediate phases. At the same time, an investigation on the kinetics aspects of the zeolite A crystallisation was carried out. As far as the reproducibility test is concerned, the chemical system selected showed good results on a large number of tests. © 2000 Published by Elsevier Science S.A. Keywords: Zeolite synthesis; Dynamic light scattering
1. Introduction Zeolites make up a group of microporous compounds which play an important role in several technological fields, mainly catalysis, ion exchange and molecular sieving [1]. As a consequence, there is a great interest in studying the synthesis process, whose understanding could yield a number of advantages such as: (a) optimisation of industrial production of zeolites; (b) development of new production techniques and (c) production of new zeolites, tailored for specific applications. The zeolite formation occurs in systems very far from equilibrium and the synthesis evolution is very sensitive to small variations of the control parameters [2]. For these reasons the understanding of the process is still incomplete. Up to now only integrated models are available, which describe the average composition of the system as a function of time. On the other hand, nucleation and growth are driven by the local conditions inside the solution. Therefore, transport phenomena have to be considered for a better understanding of the process. In fact, the interplay between reaction kinetics and transport phenomena can lead to non-negligible compositional non-uniformity of the solution that would influence the final product of the synthesis. ∗ Corresponding author. Tel.: +39-817684550; fax: +39-817682394. E-mail address: [email protected] (D. Caputo).
0254-0584/00/$ – see front matter © 2000 Published by Elsevier Science S.A. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 1 0 - 2
On the ground, transport is dominated by natural convection, driven by the density gradient induced around the growing crystals. In addition, the crystals, being denser than the feeding solution, settle soon after nucleation. Their sedimentation causes non-uniform growth conditions and could promote secondary nucleation in the solution. Under microgravity conditions, the transport is mainly diffusive and sedimentation should be strongly suppressed [3]. For this reason, it is expected that a systematic experimentation both on the ground and in space would give insights on the process dynamics. The use of in situ non-invasive diagnostics would allow collecting reference data for the validation of detailed models of the synthesis. The results reported here concern the preliminary ground-based investigation having the aim of deepening the understanding of crystallisation kinetics by correlation between measurements performed in situ with that ones performed on the final products of the synthesis. 2. Diagnostics Dynamic light scattering (DLS) is one of the main diagnostic tools identified for in situ measurements of the nucleation and the early phases of the growth of zeolites in clear solutions [4–6]. The DLS technique is based on the analysis of the time fluctuations of the intensity of scattered light. These fluctua-
D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
121
Fig. 1. A schematic representation of the laser light scattering data acquisition system.
tions are due to the relaxation of the sample that can be studied by analysing the autocorrelation function of the scattered intensity. In the case of Brownian motion, the characteristic decay time τ of the autocorrelation function is related to the average diffusion coefficient D of the scattering particles τ=
1 2Dq2
where q is the scattering vector: q = (4πn/λ)sin(θ/2), λ being the wavelength of the incident beam in vacuum, θ the scattering angle, and n the refraction index of the solution.
For dilute suspensions of spherical particles the hydrodynamic radius rh can be evaluated from the value of D using the Stokes–Einstein relation D=
kB T 6π µrh
where kB is the Boltzmann’s constant, T is the absolute temperature and µ is the dynamic viscosity of the liquid. Details of the DLS technique are reported in [7]. Fig. 1 shows a schematic representation of the laser light scattering data acquisition system. This technique is
Fig. 2. Typical X-ray diffraction patterns of zeolite A obtained on solid product collected inside the couvette showed on the left.
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D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
integrated with optical microscopy in order to monitor, in real time, the synthesis process. The apparatus is described in detail in [8]. A morphology study of the crystals was carried out by scanning electron microscopy (SEM) using a Leica-Cambridge S440 microscope. The X-ray powder diffraction analysis (XRD), using a Philips PW 1710 diffractometer with Cu K␣1 radiation (1.5406 Å), was performed to identify the phases obtained in the synthesis product. XRD was used to estimate the yield of the synthesis of zeolite A employing a quantitative analysis based on the ‘peak summation’ procedure. Accordingly, the areas under the most intense peaks, corresponding to the crystallographic planes (1 0 0), (1 1 0), (3 1 1), (3 2 1) and (4 1 0, 3 2 2), were evaluated by the Philips PC-APD analytical software and summed to each other. The sample, which presented the maximum value of total area (14 h synthesis) was selected as reference for evaluating the crystallinity percentage. The relative yield at the different synthesis times was evaluated with respect to this reference value.
3. Materials and methods A homogeneous synthesis system (clear solution) for testing the in situ monitoring techniques, was prepared with the following batch composition: 8.6Na2 O·0.18SiO2 ·Al2 O3 · 150H2 O used to obtain zeolite A [9]. Synthesis systems were prepared using: sodium silicate hydrate (Aldrich, 14% of NaOH and 27% of SiO2 ); sodium aluminate hydrate (Carlo Erba, minimum 54% of Al2 O3 ); sodium hydroxide (Sodio Idrato Gocce RPE-ACS, Carlo Erba) and double distilled water. Synthesis runs were carried out using sealed polypropylene bottles (maximum capacity 50 ml) and couvettes (maximum capacity 5 ml). In the first case 2.469 g of sodium silicate hydrate, 0.377 g of sodium aluminate hydrate, 7.157 g of sodium hydroxide and 28.080 g of double distilled water were used. For the synthesis runs in couvettes 10 divided the above amounts. The reactants were mixed at room temperature as follows: 1. sodium hydroxide was dissolved in double distilled water and the solution was cooled to room temperature; 2. sodium aluminate was added slowly to the sodium hydroxide solution until it completely dissolved; 3. sodium silicate was added rapidly to the sodium aluminate solution under continuous stirring. After mixing of all components, the liquid appeared transparent and homogeneous. During injection in the DLS cell, the batch solution was filtered using 0.5 m PTFE filters, to remove contaminant particles which, even at low concentration, would affect nucleation and light scattering. The clear solution obtained according to the above recipe was treated hydrothermally under static conditions for different times, ranging from 4 to 14 h.
Fig. 3. X-ray diffraction pattern of the synthesis products at 60◦ C after (a) 4 h; (b) 5 h; (c) 6 h and (d) 14 h.
Fig. 4. Crystallisation curve of zeolite Na-A at 60◦ C.
D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
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Fig. 5. Characteristic decay time τ of the autocorrelation function and the corresponding counting rate, obtained during an experiment carried out at 70◦ C.
Fig. 6. Effect of the time of ageing (ta) of the synthesis mixture on the growth rate and the induction time.
Reaction products were filtered, washed with double distilled water, dried overnight at 60◦ C, and stored in an environment with controlled humidity (about 50% RH). 4. Preliminary results To compare the results obtained in situ with DLS technique and on the products with XRD technique, preliminary
tests were needed, in order to verify, whether the differences in the experimental set-up would affect the results of the synthesis. The experimental set-up used in this study employed different vessels and thermostatic systems. Experiments dedicated to XRD analysis of the products were performed by using an oven and large vessels, to obtain a sufficient amount of products even at short synthesis times. On the other hand, DLS required a dedicated heating system
Fig. 7. Temporal evolution of DLS data for the experiment with 4 days and 21 h of ageing. Both counting rate and decay time increase, indicating particle growth.
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D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
Fig. 8. SEM observations of solid products obtained from fresh (a) and aged (b) solutions after 3 h of synthesis at 60◦ C.
and transparent vessels with a reduced volume, to optimise the thermal uniformity inside the sample. In fact thermal gradients would drive convective flows which would affect DLS measurements. Synthesis experiments were repeated using PMA couvettes (5 ml) and polypropylene bottles with solution volume in the range 30–50 ml. The experiments were performed at 60 and 70◦ C using both an oven and a circulating water temperature control system (needed for DLS measurements). Each test was repeated and the results reproducibility was verified using the ‘peak summation’ procedure. The reproducibility was satisfactory (data dispersion of the order of 4%). The XRD analysis has shown no relevant difference in the synthesis product obtained by the different methods and vessels. As example, Fig. 2 shows the XRD pattern of the
solid products collected inside the couvette (also shown in Fig. 2) after 5 h at 60◦ C. The pattern exhibits typical peaks of zeolite A as observed also in the other experiments. As described above, the DLS measurements require the filtering of the initial solutions. Comparison of XRD analysis of the product obtained with and without filtering showed that this operation does not influence appreciably the synthesis results. These preliminary tests confirmed the possibility to compare in situ DLS measurements and XRD analysis of the final products, to obtain insights on the synthesis kinetics. Experiments were performed on fresh solutions prepared as described above. Fig. 3 reports the diffraction patterns of solid products obtained at different times. The comparison shows the increase
D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
of the crystalline phase with the reaction time. In particular, the pattern in Fig. 3(a) denotes the presence of amorphous precipitate that could be correlated with the presence of a gel phase. Kinetics of crystallisation (shown in Fig. 4) points out that zeolite A formation starts after about 3 h and is essentially completed after 8–10 h. Induction time was estimated by extrapolation because of the very scarce amount of solid produced for synthesis times shorter than 4 h. Fig. 5 shows the characteristic decay time τ of the autocorrelation function and the corresponding counting rate, measured with DLS on a fresh sample. First detectable particles appeared at a very early time (about 12 min), not comparable with the induction time measured by XRD. In the first phase of the synthesis (0–50 min), τ increases and counting rate remains almost constant. This could be an indication of gel formation inside the solution [10]. Successively (at about 1 h), data dispersion increases and also the counting rate exhibits a continuous rise. This could indicate the onset of a different process, for instance, the beginning of crystal nucleation. The final decrease of both τ values and counting rate is attributed to sedimentation and/or gel dissolution. In fact, at these times (about 150 min) the solution appears again completely transparent in the upper part (where the laser beam is focused). The opacity is localised close to the bottom of the cell, where the solid precipitate begins to accumulate. The induction time measured by XRD, as well as the very low degree of crystallinity reached in 4 h, support the hypothesis that the particle growth detected by DLS in the first hour does refer not to a crystalline phase but mainly to an amorphous gel phase, as also supported by the SEM analysis reported below. Further DLS experiments were performed using aged solutions. Typical results are shown in Fig. 6; different data sets correspond to different ageing times. The appearance of detectable particles occurs some time (about 40 min) after the beginning of the hydro-thermal processing. Further, decay time of the autocorrelation function increases systematically with the experimental elapsed time, and this corresponds to a size increase of the scattering particles. The data show that ageing induces an increase of the growth rate and a decrease of the induction time; these results are in agreement with previous experiments [6]. An example of the temporal evolution of DLS data is shown in Fig. 7. The increment of particle size corresponds to an increment of the counting rate. These observations seem to indicate the absence of gel formation, in the case of aged solutions. This hypothesis is supported by the SEM analysis. Fig. 8 compares typical SEM observations of the solids obtained by fresh (Fig. 8a) and aged solutions (Fig. 8b) after 3 h of synthesis at 60◦ C. In the first case, particles
125
are smaller and more clustered than those obtained by aged solutions. The clustering could be attributed to the presence of an amorphous aluminosilicate phase. Further investigations are in progress to elucidate the mechanisms that could lead to the formation of gel in the early phases of the synthesis. 5. Conclusion Synthesis experiments have been performed on the system Na2 O·Al2 O3 ·SiO2 ·H2 O using in situ optical diagnostics (DLS). XRD analysis of the synthesis products has provided data on the crystallisation kinetics. The comparison of DLS results with the XRD analysis and SEM observation of the products seems indicated that, even if the sample is initially homogeneous (clear solution), during the synthesis crystallisation is preceded by the formation of a precursor phase (gel) in the case of not aged solutions. No evidence of gel formation was found in the case of aged solutions. The decrease of apparent particle size in the later stages of the synthesis can be attributed to a gravity effect. Differentiated settling of particles of different sizes prevented a quantitative analysis of growth process using DLS. These problems could be eliminated in microgravity conditions. Acknowledgements We would like to acknowledge the Italian Space Agency (ASI) for its support that allowed the realisation of this research. References [1] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974, p. 771. [2] R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982, p. 360. [3] E.N. Coker, J.C. Jansen, J.A. Martens, P.A. Jacobs, F. DiRenzo, F. Fajula, A. Sacco, Microporous Mesoporous Mater. 23 (1998) 119– 136. [4] T.A.M. Twomey, M. Mackay, H.P.C.E. Kuipers, R.W. Thompson, Zeolite 14 (1994) 162–168. [5] B.J. Schoeman, O. Regev, Zeolite 17 (1996) 447–456. [6] L. Gora, K. Streletzky, R.W. Thompson, G.D.J. Phillies, Zeolite 18 (1997) 119–131. [7] R. Pecora, Dynamic Light Scattering: Application of Photon Correlation Spectroscopy, Plenum Press, New York, 1985. [8] L. Carotenuto, C. Piccolo, Micogravity Quarterly, 2000, in press. [9] R. Aiello, F. Testa, J.B. Nagy, L. Maiorino, Influence of the ageing on the crystallization of zeolite A from clear solutions, Micogravity Quarterly, 2000, in press. [10] V. Lesturgeon, T. Nicolai, D. Durand, The Eur. Phys. J. B 9 (1999) 71–82.
A preliminary investigation on kinetics of zeolite A crystallisation using optical diagnostics D. Caputo a,∗ , B. De Gennaro a , B. Liguori a , F. Testa b , L. Carotenuto c , C. Piccolo c a
Department of Materials and Production Engineering, University of Napoli, Federico II, Napoli, Italy b Department of Chemistry and Materials Engineering, University of Calabria, Calabria, Italy c MARS Center, Napoli, Italy
Abstract Preliminary data concerning the monitoring in situ of the zeolite A crystallisation from clear solutions are reported. Experiments were performed at 60 and 70◦ C with fresh and aged solutions having the following composition: 8.6Na2 O·0.18SiO2 ·Al2 O3 ·150H2 O. The formation of early phases in the crystallisation process were followed using diagnostics such as dynamic light scattering (DLS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The comparison of the available data seems to indicate the formation of a precursor phase (like gel) in the case of not aged solution. Conversely, crystallisation in aged solution seems to proceed without intermediate phases. At the same time, an investigation on the kinetics aspects of the zeolite A crystallisation was carried out. As far as the reproducibility test is concerned, the chemical system selected showed good results on a large number of tests. © 2000 Published by Elsevier Science S.A. Keywords: Zeolite synthesis; Dynamic light scattering
1. Introduction Zeolites make up a group of microporous compounds which play an important role in several technological fields, mainly catalysis, ion exchange and molecular sieving [1]. As a consequence, there is a great interest in studying the synthesis process, whose understanding could yield a number of advantages such as: (a) optimisation of industrial production of zeolites; (b) development of new production techniques and (c) production of new zeolites, tailored for specific applications. The zeolite formation occurs in systems very far from equilibrium and the synthesis evolution is very sensitive to small variations of the control parameters [2]. For these reasons the understanding of the process is still incomplete. Up to now only integrated models are available, which describe the average composition of the system as a function of time. On the other hand, nucleation and growth are driven by the local conditions inside the solution. Therefore, transport phenomena have to be considered for a better understanding of the process. In fact, the interplay between reaction kinetics and transport phenomena can lead to non-negligible compositional non-uniformity of the solution that would influence the final product of the synthesis. ∗ Corresponding author. Tel.: +39-817684550; fax: +39-817682394. E-mail address: [email protected] (D. Caputo).
0254-0584/00/$ – see front matter © 2000 Published by Elsevier Science S.A. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 1 0 - 2
On the ground, transport is dominated by natural convection, driven by the density gradient induced around the growing crystals. In addition, the crystals, being denser than the feeding solution, settle soon after nucleation. Their sedimentation causes non-uniform growth conditions and could promote secondary nucleation in the solution. Under microgravity conditions, the transport is mainly diffusive and sedimentation should be strongly suppressed [3]. For this reason, it is expected that a systematic experimentation both on the ground and in space would give insights on the process dynamics. The use of in situ non-invasive diagnostics would allow collecting reference data for the validation of detailed models of the synthesis. The results reported here concern the preliminary ground-based investigation having the aim of deepening the understanding of crystallisation kinetics by correlation between measurements performed in situ with that ones performed on the final products of the synthesis. 2. Diagnostics Dynamic light scattering (DLS) is one of the main diagnostic tools identified for in situ measurements of the nucleation and the early phases of the growth of zeolites in clear solutions [4–6]. The DLS technique is based on the analysis of the time fluctuations of the intensity of scattered light. These fluctua-
D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
121
Fig. 1. A schematic representation of the laser light scattering data acquisition system.
tions are due to the relaxation of the sample that can be studied by analysing the autocorrelation function of the scattered intensity. In the case of Brownian motion, the characteristic decay time τ of the autocorrelation function is related to the average diffusion coefficient D of the scattering particles τ=
1 2Dq2
where q is the scattering vector: q = (4πn/λ)sin(θ/2), λ being the wavelength of the incident beam in vacuum, θ the scattering angle, and n the refraction index of the solution.
For dilute suspensions of spherical particles the hydrodynamic radius rh can be evaluated from the value of D using the Stokes–Einstein relation D=
kB T 6π µrh
where kB is the Boltzmann’s constant, T is the absolute temperature and µ is the dynamic viscosity of the liquid. Details of the DLS technique are reported in [7]. Fig. 1 shows a schematic representation of the laser light scattering data acquisition system. This technique is
Fig. 2. Typical X-ray diffraction patterns of zeolite A obtained on solid product collected inside the couvette showed on the left.
122
D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
integrated with optical microscopy in order to monitor, in real time, the synthesis process. The apparatus is described in detail in [8]. A morphology study of the crystals was carried out by scanning electron microscopy (SEM) using a Leica-Cambridge S440 microscope. The X-ray powder diffraction analysis (XRD), using a Philips PW 1710 diffractometer with Cu K␣1 radiation (1.5406 Å), was performed to identify the phases obtained in the synthesis product. XRD was used to estimate the yield of the synthesis of zeolite A employing a quantitative analysis based on the ‘peak summation’ procedure. Accordingly, the areas under the most intense peaks, corresponding to the crystallographic planes (1 0 0), (1 1 0), (3 1 1), (3 2 1) and (4 1 0, 3 2 2), were evaluated by the Philips PC-APD analytical software and summed to each other. The sample, which presented the maximum value of total area (14 h synthesis) was selected as reference for evaluating the crystallinity percentage. The relative yield at the different synthesis times was evaluated with respect to this reference value.
3. Materials and methods A homogeneous synthesis system (clear solution) for testing the in situ monitoring techniques, was prepared with the following batch composition: 8.6Na2 O·0.18SiO2 ·Al2 O3 · 150H2 O used to obtain zeolite A [9]. Synthesis systems were prepared using: sodium silicate hydrate (Aldrich, 14% of NaOH and 27% of SiO2 ); sodium aluminate hydrate (Carlo Erba, minimum 54% of Al2 O3 ); sodium hydroxide (Sodio Idrato Gocce RPE-ACS, Carlo Erba) and double distilled water. Synthesis runs were carried out using sealed polypropylene bottles (maximum capacity 50 ml) and couvettes (maximum capacity 5 ml). In the first case 2.469 g of sodium silicate hydrate, 0.377 g of sodium aluminate hydrate, 7.157 g of sodium hydroxide and 28.080 g of double distilled water were used. For the synthesis runs in couvettes 10 divided the above amounts. The reactants were mixed at room temperature as follows: 1. sodium hydroxide was dissolved in double distilled water and the solution was cooled to room temperature; 2. sodium aluminate was added slowly to the sodium hydroxide solution until it completely dissolved; 3. sodium silicate was added rapidly to the sodium aluminate solution under continuous stirring. After mixing of all components, the liquid appeared transparent and homogeneous. During injection in the DLS cell, the batch solution was filtered using 0.5 m PTFE filters, to remove contaminant particles which, even at low concentration, would affect nucleation and light scattering. The clear solution obtained according to the above recipe was treated hydrothermally under static conditions for different times, ranging from 4 to 14 h.
Fig. 3. X-ray diffraction pattern of the synthesis products at 60◦ C after (a) 4 h; (b) 5 h; (c) 6 h and (d) 14 h.
Fig. 4. Crystallisation curve of zeolite Na-A at 60◦ C.
D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
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Fig. 5. Characteristic decay time τ of the autocorrelation function and the corresponding counting rate, obtained during an experiment carried out at 70◦ C.
Fig. 6. Effect of the time of ageing (ta) of the synthesis mixture on the growth rate and the induction time.
Reaction products were filtered, washed with double distilled water, dried overnight at 60◦ C, and stored in an environment with controlled humidity (about 50% RH). 4. Preliminary results To compare the results obtained in situ with DLS technique and on the products with XRD technique, preliminary
tests were needed, in order to verify, whether the differences in the experimental set-up would affect the results of the synthesis. The experimental set-up used in this study employed different vessels and thermostatic systems. Experiments dedicated to XRD analysis of the products were performed by using an oven and large vessels, to obtain a sufficient amount of products even at short synthesis times. On the other hand, DLS required a dedicated heating system
Fig. 7. Temporal evolution of DLS data for the experiment with 4 days and 21 h of ageing. Both counting rate and decay time increase, indicating particle growth.
124
D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
Fig. 8. SEM observations of solid products obtained from fresh (a) and aged (b) solutions after 3 h of synthesis at 60◦ C.
and transparent vessels with a reduced volume, to optimise the thermal uniformity inside the sample. In fact thermal gradients would drive convective flows which would affect DLS measurements. Synthesis experiments were repeated using PMA couvettes (5 ml) and polypropylene bottles with solution volume in the range 30–50 ml. The experiments were performed at 60 and 70◦ C using both an oven and a circulating water temperature control system (needed for DLS measurements). Each test was repeated and the results reproducibility was verified using the ‘peak summation’ procedure. The reproducibility was satisfactory (data dispersion of the order of 4%). The XRD analysis has shown no relevant difference in the synthesis product obtained by the different methods and vessels. As example, Fig. 2 shows the XRD pattern of the
solid products collected inside the couvette (also shown in Fig. 2) after 5 h at 60◦ C. The pattern exhibits typical peaks of zeolite A as observed also in the other experiments. As described above, the DLS measurements require the filtering of the initial solutions. Comparison of XRD analysis of the product obtained with and without filtering showed that this operation does not influence appreciably the synthesis results. These preliminary tests confirmed the possibility to compare in situ DLS measurements and XRD analysis of the final products, to obtain insights on the synthesis kinetics. Experiments were performed on fresh solutions prepared as described above. Fig. 3 reports the diffraction patterns of solid products obtained at different times. The comparison shows the increase
D. Caputo et al. / Materials Chemistry and Physics 66 (2000) 120–125
of the crystalline phase with the reaction time. In particular, the pattern in Fig. 3(a) denotes the presence of amorphous precipitate that could be correlated with the presence of a gel phase. Kinetics of crystallisation (shown in Fig. 4) points out that zeolite A formation starts after about 3 h and is essentially completed after 8–10 h. Induction time was estimated by extrapolation because of the very scarce amount of solid produced for synthesis times shorter than 4 h. Fig. 5 shows the characteristic decay time τ of the autocorrelation function and the corresponding counting rate, measured with DLS on a fresh sample. First detectable particles appeared at a very early time (about 12 min), not comparable with the induction time measured by XRD. In the first phase of the synthesis (0–50 min), τ increases and counting rate remains almost constant. This could be an indication of gel formation inside the solution [10]. Successively (at about 1 h), data dispersion increases and also the counting rate exhibits a continuous rise. This could indicate the onset of a different process, for instance, the beginning of crystal nucleation. The final decrease of both τ values and counting rate is attributed to sedimentation and/or gel dissolution. In fact, at these times (about 150 min) the solution appears again completely transparent in the upper part (where the laser beam is focused). The opacity is localised close to the bottom of the cell, where the solid precipitate begins to accumulate. The induction time measured by XRD, as well as the very low degree of crystallinity reached in 4 h, support the hypothesis that the particle growth detected by DLS in the first hour does refer not to a crystalline phase but mainly to an amorphous gel phase, as also supported by the SEM analysis reported below. Further DLS experiments were performed using aged solutions. Typical results are shown in Fig. 6; different data sets correspond to different ageing times. The appearance of detectable particles occurs some time (about 40 min) after the beginning of the hydro-thermal processing. Further, decay time of the autocorrelation function increases systematically with the experimental elapsed time, and this corresponds to a size increase of the scattering particles. The data show that ageing induces an increase of the growth rate and a decrease of the induction time; these results are in agreement with previous experiments [6]. An example of the temporal evolution of DLS data is shown in Fig. 7. The increment of particle size corresponds to an increment of the counting rate. These observations seem to indicate the absence of gel formation, in the case of aged solutions. This hypothesis is supported by the SEM analysis. Fig. 8 compares typical SEM observations of the solids obtained by fresh (Fig. 8a) and aged solutions (Fig. 8b) after 3 h of synthesis at 60◦ C. In the first case, particles
125
are smaller and more clustered than those obtained by aged solutions. The clustering could be attributed to the presence of an amorphous aluminosilicate phase. Further investigations are in progress to elucidate the mechanisms that could lead to the formation of gel in the early phases of the synthesis. 5. Conclusion Synthesis experiments have been performed on the system Na2 O·Al2 O3 ·SiO2 ·H2 O using in situ optical diagnostics (DLS). XRD analysis of the synthesis products has provided data on the crystallisation kinetics. The comparison of DLS results with the XRD analysis and SEM observation of the products seems indicated that, even if the sample is initially homogeneous (clear solution), during the synthesis crystallisation is preceded by the formation of a precursor phase (gel) in the case of not aged solutions. No evidence of gel formation was found in the case of aged solutions. The decrease of apparent particle size in the later stages of the synthesis can be attributed to a gravity effect. Differentiated settling of particles of different sizes prevented a quantitative analysis of growth process using DLS. These problems could be eliminated in microgravity conditions. Acknowledgements We would like to acknowledge the Italian Space Agency (ASI) for its support that allowed the realisation of this research. References [1] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974, p. 771. [2] R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982, p. 360. [3] E.N. Coker, J.C. Jansen, J.A. Martens, P.A. Jacobs, F. DiRenzo, F. Fajula, A. Sacco, Microporous Mesoporous Mater. 23 (1998) 119– 136. [4] T.A.M. Twomey, M. Mackay, H.P.C.E. Kuipers, R.W. Thompson, Zeolite 14 (1994) 162–168. [5] B.J. Schoeman, O. Regev, Zeolite 17 (1996) 447–456. [6] L. Gora, K. Streletzky, R.W. Thompson, G.D.J. Phillies, Zeolite 18 (1997) 119–131. [7] R. Pecora, Dynamic Light Scattering: Application of Photon Correlation Spectroscopy, Plenum Press, New York, 1985. [8] L. Carotenuto, C. Piccolo, Micogravity Quarterly, 2000, in press. [9] R. Aiello, F. Testa, J.B. Nagy, L. Maiorino, Influence of the ageing on the crystallization of zeolite A from clear solutions, Micogravity Quarterly, 2000, in press. [10] V. Lesturgeon, T. Nicolai, D. Durand, The Eur. Phys. J. B 9 (1999) 71–82.