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An in Situ Energy-Dispersive X-ray Diffraction Study of the Hydrothermal Crystallization of Zeolite A. 1. Influence of Reaction Conditions and Transformation into Sodalite Richard I. Walton,† Franck Millange, and Dermot O’Hare* Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QR, U.K.
Andrew T. Davies, Gopinathan Sankar, and C. Richard A. Catlow The Royal Institution of Great Britain, 21 Albemarle Street, London, W1X 4BS, U.K. ReceiVed: July 30, 2000
The hydrothermal crystallization of sodium zeolite A from amorphous aluminosilicate gels at 80-120 °C has been studied by time-resolved in situ energy-dispersive X-ray diffraction. This has allowed the course of crystallization to be followed in greater detail than previously possible. Quantitative crystallization curves have been obtained, and these kinetic data are analyzed using the Avrami-Erofe’ev nucleation growth model to allow a simple means of determining the crystallization rate. We observe that the choice of silica starting material can affect the course of reaction; the use of fumed silica gives rise to a distinctive two-stage growth curve. This unusual crystallization behavior is also dependent on both NaOH concentration and temperature. At the highest NaOH concentrations and when the amount of water is low, zeolite A is only present for a short time and hydroxosodalite is the sole product on continued heating. We discuss our new in situ observations in relation to previous quenching studies of zeolite crystallizations, and their implications in understanding zeolite formation mechanism.
Introduction Microporous solids are the focus of intense research activity because of their huge industrial and commercial value.1,2 The archetypal microporous materials are the aluminosilicate zeolites, which are widely used as catalysts (for example in hydrocarbon cracking processes, vital for the manufacture of petroleum products), as ion exchangers (used in detergents to soften water by removal of calcium and magnesium ions) and as selective gas sorbers (for example in the separation and purification of nitrogen from oxygen).3,4 Although zeolites have been used in these applications for over 50 years and more recently other large families of open-framework materials have been discovered and widely studied,2,5 there is still a lack of knowledge about the mechanism of formation of microporous materials. There is a pressing need to understand the formation mechanisms of open-framework inorganic materials under hydrothermal conditions so that the rational design of new materials with properties appropriate for a specific application will be possible.6 A detailed understanding of zeolite crystallization mechanism would also allow the preparation of microporous materials with specific particle size or morphology, a vital consideration in the optimization of the performance of materials in commercial applications.7 Zeolites and other microporous solids are usually prepared using the hydrothermal method, whereby a mixture of solid and liquid reagents is heated in aqueous conditions in a sealed autoclave to temperatures of up to 200 °C. The determination of reaction kinetics is often the first step in determining the mechanism of any reaction, but the fact that hydrothermal reactions take place in sealed stainless steel containers has in * Corresponding author. E-mail:
[email protected]. † Present address: School of Chemistry, Stocker Road, University of Exeter, Exeter, EX4 4QD, UK.
the past meant that following physical changes, and hence determining kinetic information during the course of a reaction, has been extremely difficult. Crystallization curves for such reactions have traditionally been measured using quenching methods, whereby a series of identical reaction mixtures were heated to a specific temperature and after a given period of time each reaction is rapidly cooled and the solid products are examined, often by diffraction methods, to gauge how far crystallization has proceeded. The method is effective, but timeconsuming and requires large quantities of often expensive starting materials, which perhaps explains the small number of data points (when plotted as a function of time under isothermal conditions) presented for the majority of zeolite crystallization curves in the literature. In addition to the lack of time resolution, experiments based on arresting the reaction (filtering and drying before any form of characterization is performed) assume that the solid examined after quenching is the same as the solid phase present under reaction conditions. This is not necessarily the case, indeed in some cases intermediate phases are observed which are not present when the products were filtered and dried.8,9 In recent years, techniques that allow hydrothermal crystallizations to be monitored in situ as they proceed have been devised and are continuing to be developed10,11 Such methods overcome the problems of quenching experiments, providing an effective means of continuously monitoring reactions under real conditions. Diffraction methods are perhaps one of the most powerful methods to monitor crystallizations since the degree of crystallinity in the sample can immediately be observed. Recently, some spectroscopic methods have been employed, such as NMR,12 EXAFS,13 and Raman spectroscopy,14 as well as small-angle neutron and X-ray scattering methods,15-17 but it was always necessary to complement the observations from
10.1021/jp002711p CCC: $20.00 © 2001 American Chemical Society Published on Web 12/13/2000
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Figure 1. Representations of the structures of sodium zeolite A (LTA structure type) and hydroxosodalite (SOD structure type), showing how they are constructed from the sodalite cage. The lines represent AlO-Si linkages. Occluded sodium cations, hydroxide anions, and water molecules are not shown for clarity.
these experiments with diffraction methods to interpret the spectroscopic results obtained before, during, and after crystallization. Both angular dispersive and energy-dispersive X-ray diffraction methods have been shown to be useful in monitoring the crystallization of microporous materials; however, the energy-dispersive X-ray diffraction (EDXRD) method is more appropriate, since the powerful white radiation can penetrate the walls of the stainless steel autoclaves thereby allowing the use of laboratory-sized reaction vessels.18-20 We have previously used the EDXRD technique to study the hydrothermal crystallization of open-framework gallium fluorophosphates,8,9 of cobalt-substituted aluminophosphates21-23of layered porous tin sulfides,24 and presented some preliminary results of a study of zeolite A crystallization.23 In the present study we have performed a detailed investigation of the hydrothermal formation of sodium zeolite A and its transformation into hydroxosodalite. We have used timeresolved energy-dispersive X-ray diffraction to perform a kinetic study in situ under operating laboratory conditions. Zeolite A (International Zeolite Association code, LTA25) was the first synthetic zeolite to be prepared,26 and finds widespread application in both laboratories and industries. For these purposes it is manufactured industrially on a greater scale than any other zeolite.27 The relatively simple structures of sodium zeolite A and the related hydroxosodalite (SOD), Figure 1, and the ease of preparation (involving inexpensive chemicals and requiring only few hours to produce highly crystalline materials under moderate temperatures) makes the compounds an attractive system to study as a model for more complex zeolite structures. The formation of zeolite A from an amorphous aluminosilicate precursor and its subsequent collapse into hydroxosodalite is an elegant example of Ostwald’s rule; the first polymorph of a compound formed from solution is the least thermodynamically stable and is then replaced in succession by more thermodynamically stable polymorphs.4 Such successive transformations are very common in zeolite chemistry, making the LTA/SOD system particularly worthy of detailed study to understand the stability of a specific microporous structure. In this first paper we will discuss the crystallization of zeolite A and its subsequent transformation to hydroxosodalite, concentrating on the effect of choice of starting materials, reagent concentration and temperature on the reaction. In the following paper28 some of us describe an investigation of the effect of deuteration on the kinetics of crystallization of zeolite A and relate this and the results of this first paper to descriptions of zeolite crystallization mechanism in the literature.
Laboratory Studies. The crystallization of zeolite A and sodalite has been investigated previously by many workers (see for example refs 29-35) but there are many reaction variables that can subtly affect the rate of crystallization. A laboratory study using quenching methods was performed to estimate the time scale of a typical reaction with the reagents and conditions we chose to use. The alumina source used was poorly crystalline Al2O3 prepared by calcination of amorphous hydrated Al(OH)3 (supplied by Aldrich) at 700 °C for 4 h. The same batch of alumina was used for all the experiments we describe here, to ensure that reaction conditions remained as similar as possible throughout. The silica source was fumed SiO2 (particle size 0.007 µm, 99.8%) supplied by Aldrich, and the sodium source sodium hydroxide solution freshly prepared by dissolving sodium hydroxide pellets (BDH) in distilled water. The concentration of the sodium hydroxide solutions was determined accurately by titration against standardized hydrochloric acid solution. Synchrotron beam time being expensive and limited, we chose reaction conditions in such a way that a given crystallization would be complete within ca. 3 h. Highly concentrated sodium hydroxide solution was therefore used and a small amount of water, since it is well-known that under these conditions zeolite A crystallization rate is considerably enhanced.31 Crystallization from gels of composition Al2O3:2SiO2:3.5NaOH:20H2O was studied in the laboratory. The Al2O3 (1 g) and SiO2 (1.2 g) were shaken together to achieve intimate mixing and the required quantity of sodium hydroxide solution was added. The mixture was stirred by hand to produce a thick paste which was transferred to a Teflon vessel sealed in a 23 mL Parr hydrothermal autoclave, and heated to 100 °C. A set of reactions was performed concurrently, and after certain periods of time an autoclave was removed and allowed to cool and the solid recovered by suction filtration, washed with distilled water and then acetone, and allowed to dry in air on the pump. X-ray diffraction patterns of the recovered solids were obtained using Philips PW1729 X-ray diffractometer operating with Cu KR radiation. The in Situ EDXRD Experiment. Energy-dispersive X-ray diffraction experiments were performed on Station 16.4 of the Daresbury SRS using an apparatus previously described.19 The synchrotron source operates with an average stored current of 200 mA and a typical beam energy of 2 GeV. Station 16.4 is illuminated with radiation from a 6 T superconducting wiggler and receives X-rays over an range energy range 5-120 keV with a maximum X-ray flux of 3 × 1010 photons/s at around 13 keV. The position of this energy maximum is shifted by the absorption of lower energy photons by the apparatus so that in practice X-rays with energies above ∼30 keV are useful. Briefly, the hydrothermal cell is a similar volume to those available commercially for laboratory use (∼30 cm3), but has a thinner stainless steel outer wall (0.4 mm) to minimize absorption of X-rays. A series of experiments was performed on zeolite A and hydroxosodalite crystallizations using gels of nominal composition Al2O3:2SiO2:xNaOH:20H2O; x ) 2.5, 2.75, 3, 3.25, 3.5, 3.75, and 4) and Al2O3:2SiO2:xNaOH:80H2O (x ) 3, 3.25, 3.5, 3.75, and 4). The NaOH solutions were accurately weighed and the amount of NaOH used was then calculated from their predetermined concentrations. For the majority of reactions, fumed silica was used, as described above, but some attention was also given to the choice of silica source used in the crystallizations. For these reactions a gel of composition Al2O3: 2SiO2:2NaOH:17.5 H2O was studied at 115 °C, and three
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different silica sources used: Cab-O-Sil M5 fumed silica, Ludox-HS40 stabilized colloidal silica, and the mesoporous silica MCM-41. For all reactions performed, the reagents were mixed manually to give a homogeneous gel and transferred into the Teflon liner of the cell. A constant fill volume was used for a given series of crystallization studies (in the region of ∼30 for the more concentrated gels and ∼50% for the dilute series) depending on the amount of water used in the gel). The time between mixing reagents, sealing them in the autoclave, and beginning data collection was always less than 5 min, and although introducing the cold cell causes cooling of the heating block, constant desired temperature was attained in around 5 min. X-ray diffraction data were collected from close to the bottom of the cell, and the high viscosity of the reaction mixtures meant that the solid did not settle out during the time of the experiment. X-ray diffraction patterns were recorded every 30 s by a three-element solid-state detector, recently described by Colston et al.36 Each detector element is separated by an angle of ∼3° so that a d spacing range of greater than 20 Å may be observed in a given experiment. In the current work the lower detector was always set at 2θ ∼ 2.0°, so that the strong high d spacing Bragg reflections of the zeolite A and hydroxosodalite (at 12.83 and 6.33 Å, respectively) appeared in the region of the optimum energy position of the energy-dispersive spectrum. For the energy-dispersive diffraction experiment, E(keV) ) 6.199 26/d sin θ for a Bragg refection arising from a plane of d Å. Five strong, well-resolved Bragg reflections of premade powders of sodium zeolite A and hydroxosodalite were used to determine accurately the angle of each detector element. Data Analysis. Variation in the areas of individual Bragg reflections observed in the EDXRD experiments with time were determined using an automated Gaussian-fitting routine.37 Typically, a region of 4 keV above and below the center of each reflection of interest was selected and a background function and Gaussian profile were calculated. In order to extract quantitative information about the kinetics of crystallization, we used the nucleation-growth model described by Avrami and Erofe’ev, eq 1.38-41 This model has been previously been used to model the crystallization curves of zeolites.42
R ) 1 - exp{-(k(t - t0))n}
(1)
R is the extent of reaction scaled from zero at the beginning of reaction and unity at the end, t the time coordinate, k the rate constant, and n the Avrami exponent. The value of n contains information about the mechanism of the process studied. Hulbert, for example, analyzed various possible ideal reaction situations and tabulated expected values of n for each, taking into account a variety of factors that might influence crystal growth.43 The interpretation of the Avrami exponent is, however, often far from straightforward and usually independent experimental information is required to establish a mechanism, since a given value of n does not always unequivocally allow different types of reaction mechanisms to be distinguished. The AvramiErofe’ev expression, in this very simple form, does not take into account the fact that different stages of crystal growth may take place by different mechanisms but the expression, and other similar exponential functions have in the past been used successfully to model zeolite crystal growth.4,42,44 More complex models have been developed,45 but in the present context the Avrami-Erofe’ev expression provides a simple means for rate constant determination and for a series of crystallization under varying conditions, which can then be directly compared. The most straightforward means of extracting the kinetic parameters from a crystallization curve is to use the method of Sharp and
Hancock;46 rearranging the Avrami-Erofe’ev expression and taking logarithms twice gives eq 2.
ln[-ln(1 - R)] ) n ln(t) + n ln(k)
(2)
Thus, a plot of ln(-ln(1 - R)) vs ln(t) will yield a straight line of gradient n and intercept n ln(k) if the Avrami-Erofe’ev nucleation-growth model is valid. Sharp and Hancock also showed that for a variety of classical solid-state crystallization mechanisms, linear plots will be produced at low values of R, and so the method allows different types of reaction mechanism to be distinguished.46 Considering the large error involved in fitting the peaks having very low intensity during the onset of crystallization and the reproducibility (between repetitive experiments) of the area under the peak, we estimated the error involved in the determination of the onset of crystallization to be (5 min. Errors on the kinetic parameters were estimated by considering the statistical error associated with peak-area determination and the scatter associated with data points at the end of a crystallization. The statistical error of the Gaussian fit was found to be small, resulting in a maximum error on R of (0.01 (largest at the beginning of crystallization when the Bragg reflection are least intense and the signal:noise ratio of the data lowest) but the largest error is due to experimental factors, the scatter of data at R ) 1 giving a maximum error of (0.03. Taking this error and the error on determining t0 discussed above, the linear regression used in extracting rate constants was weighted to give associated statistical errors. This resulted in maximum errors in rate constant of (3 × 10 -4 s-1 and maximum error in n of (0.5. Results Comparison of ex Situ and in Situ Results. Powder X-ray diffraction patterns of the data collected of the samples obtained by quenching reactions performed using the gel composition Al2O3:2SiO2:3.5NaOH:20H2O are shown in Figure 2a after three heating periods. The powder diffraction pattern of the solid product after 30 min of heating can largely be matched to that of sodium zeolite A, Na12Al12Si12O48‚27H2O,47,48 and that of the material produced after 2 h of heating, hydroxosodalite, Na8Al6Si6O24(OH)2‚4H2O.49,50 The Miller indices shown were derived from previous structure determinations of the compounds.48,50 The diffraction data first demonstrate that the reaction takes place on a time-scale suitable for synchrotron studies, and second that zeolite A and sodalite exhibit several nonoverlapping Bragg reflections whose intensity could be monitored in situ during the course of crystallization even when both the phases are present. A noteworthy point of the diffraction data of the quenched materials is that the broad feature seen at low angle in the early stages of reaction (at ∼5.81°; 15.20 Å) was never observed in our in situ studies (see below). We suggest that this might be due to the quenching process and have arisen from the transformation of the unreacted amorphous starting materials into a poorly crystalline material. The fact that only one reflection is seen at low angle indicates that the material might be layered; this is a reasonable assumption since some previous quenching studies of zeolite syntheses have produced layered materials in the early stages of synthesis.51 Figure 2b shows in situ diffraction patterns collected during a typical crystallization at selected times from the low-angle detector (2θ 1.80°). High d spacing Bragg reflections of each phase are well-resolved, even with the intrinsic low resolution of the solid-state detector. One important feature of the data is that course and time scale of crystallization is similar to that
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Figure 3. Normalized crystallization curves for (a) zeolite A and (b) hydroxosodalite determined by Bragg peak area determination during heating of the gel Al2O3:2SiO2:3.25NaOH:20H2O at 100 °C. Figure 2. (a) Diffraction data measured in the laboratory from the products of heating a gel of composition Al2O3:2SiO2:3.5NaOH:20H2O at 100 °C for various periods of time. (b) Synchrotron X-ray energydispersive diffraction data (30 s aquisition time) measured from the hydrothermal apparatus from the same reaction mixture, after selected periods of time. Bragg reflections of zeolite A (LTA) and hydroxosodalite (SOD) used later to produce crystallization curves are identified.
observed in the laboratory (Figure 2a). Note that different apparatus was used for these two series of experiments, so the rate of heat transfer to the reaction mixture is likely to be very different, but the rate of reaction is obviously very similar. A typical result of peak integration obtained for one of the crystallization studies is shown in Figure 3 for several Bragg reflections representing LTA and SOD phases. The diffraction pattern of sodium zeolite A exhibits many Bragg reflections; thus it was relatively straightforward to select individual peaks for area determination, but the pattern of hydroxosodalite exhibits fewer Bragg reflections and it was found that only two of them did not overlap with reflections of LTA. All Bragg peaks are observed to appear at the same time within the errors of the experiment and the growth curves shown in Figure 3 have similar shapes, suggesting that crystal growth is isotropic.23 Effect of Silica Source on Crystallization. Figure 4 shows normalized growth curve data determined by studying the strong LTA (222) reflection for the reactions performed using three different silica sources. In this case, conditions were chosen so that only the initial growth of zeolite A was studied. It is apparent that the choice of silica source can have a great influence on not only the induction time for crystallization, and the rate of crystallization, but also the course of the zeolite formation. When fumed silica source was used, the growth curves had a distinctive step midway through reaction, where crystallization appeared to halt for a short period before continuing again. The growth curve produced when using
Figure 4. Crystallization curves of zeolite A from gels of composition Al2O3:2SiO2:2NaOH:20H2O heated at 115 °C, for three different silica sources. Normalization was performed using the maximum intensity of the LTA Bragg reflection.
mesoporous MCM-41 also has a distinctively different shape. All later studies were performed using the fumed silica, being the most readily available silicon source, to investigate further whether the shape of growth curve was real or an artifact of the experiment. Effect of NaOH Concentration on the LTA-SOD Transformation. Figure 5 shows growth and decay curves of LTA and SOD obtained during crystallization from gels of composition Al2O3:2SiO2:xNaOH:20H2O with 2.5 < x < 4. at 100 °C. The crystallization curves have been normalized to the ultimate intensity of the sodalite Bragg reflection, so that the relative amount of LTA can be seen in each case. For all reactions studied, over a range of NaOH concentrations, a similar reaction pathway is observed. After a time when no Bragg reflections are seen, LTA crystallizes, and its Bragg reflections increase in intensity. Before the Bragg reflections of LTA reach their maximum intensity, SOD begins to crystallize, and continues
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Figure 6. Crystallization curves of zeolite A from gels of composition Al2O3:2SiO2:3.5NaOH:80H2O heated at five different temperatures.
Figure 5. Crystallization curves of zeolite A (LTA) and hydroxosodalite (SOD) from gels of composition Al2O3:2SiO2:xNaOH:20H2O heated at 100 °C for (a) x ) 4, (b) x ) 3.5, and (c) x) 3. The data were normalized to the intensity of the final hydroxosodalite reflection to allow the relative amount of zeolite A to be seen in each case.
to increase in intensity while the amount of LTA passes through a maximum and eventually decays totally. At high sodium hydroxide concentrations, LTA is present for only a short period of time but as the concentration is reduced, LTA is present longer. The time between LTA and SOD crystallization also varies with NaOH concentration; at the highest concentration studied here the two aluminosilicates appear at exactly the same time and as the concentration is reduced, the time for SOD to appear after LTA increases. Reactions were also performed in more dilute gels, in order to study the growth curve of LTA alone without the competing formation of SOD. Gels of composition Al2O3:2SiO2:xNaOH: 80H2O with 3 < x < 4 were used. In this composition regime, experiments in the lab showed hydroxosodalite did not crystal-
lize unless periods of heating in excess of 12 h were used. Although less viscous than the first series of gels studied, the solid was never observed to have settled out of the beam after heating; the material recovered after opening the bomb showed was similar in appearance to before heating, with no visible sign of the gellike material separating into solid and liquid components. The effect of temperature was investigated in order to shed light on the step seen in the growth curves earlier, and Figure 6 shows normalized growth curves obtained from the LTA (222) reflection for one gel composition. This again clearly shows the distinctive step in the growth curve, and also shows that it is dependent on reaction conditions, and therefore most unlikely to be due to the experimental technique. Kinetic Study of the Formation of LTA. The first simple kinetic data that can be extracted from the growth curves is the time for onset of crystallization of each zeolite, i.e., the time taken for the first Bragg reflection of the zeolite to be detected. Estimated errors on this time were derived by considering the results of repeated runs at each concentration (these experiments were performed over a period of months during several visits to the synchrotron source). The measured time for the onset of crystallization, when using a given batch of alumina, silica, and the same premade sodium hydroxide solution, was highly reproducible. Sharp-Hancock analysis was performed for the initial growth of zeolite A from the concentrated gels of composition Al2O3:2SiO2:xNaOL:20L2O. The growth curves were normalized to the maximum intensity of the zeolite A Bragg reflection to produce the extent of reaction data. Figure 7 shows typical Sharp-Hancock plots for a number of NaOH concentrations, over the range 0.2 < R < 0.9. Table 1 contains the kinetic parameters derived from the Sharp-Hancock plots by linear regression, along with the measured induction time. For the crystallizations performed from the more dilute gels, where the growth curve is not smooth, there is clearly more than one region of crystal growth, and Sharp-Hancock analysis is thus complicated. Table 2 shows kinetic parameters extracted by study of the first period of crystal growth; in all cases analysis was performed over the region 0.1 < R < 0.5 and the plots are shown in Figure 7b. Discussion From the present study it is obvious that in situ methods provide much greater detail on the crystallization of microporous solids compared to information obtained by quenching studies.
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Figure 7. Linear Sharp-Hancock plots obtained from crystallization curves describing the crystallization of zeolite A from gels of composition Al2O3:2SiO2:xNaOH:20H2O.
TABLE 1: Kinetic Parameters Determined for the Growth of Zeolite A from Gels of Composition Al2O3:2SiO2:xNaOH:20H2O at 100 °Ca NaOH concn/mol dm-3
t0/s
n
k/10-4 s-1
11.21 10.48 9.75 9.07 8.43 8.32 7.68 6.85
900 780 1110 1050 1290 1620 1560 3240
3.12 4.86 3.54 2.28 2.40 2.07 2.78
32.8 27.5 25.7 24.0 11.5 9.1 3.9
a
The induction time (t0) was determined by inspection. n is the Avrami coefficient and k the rate constant for crystallization. The growth curve for the highest concentration contained too few data points to extract meaningful parameters.
TABLE 2: Kinetic Parameters for the Growth of Zeolite A from Gels of Composition Al2O3:2SiO2:3.5NaOH:80H2O at a Variety of Temperaturesa temp/°C 80 90 100 a
t0/s
n
6500 2.42 4920 2.03 4050 1.69
k/10-4 s-1 temp/°C 1.64 1.92 2.31
110 120
t0/s
n
3120 1.70 2280 1.78
k/10-4 s-1 3.27 5.34
Legend as for Table 1.
The present study clearly shows the instability of zeolite A and its conversion to the sodalite phase when higher amounts of sodium hydroxide are used in the synthesis gel. In addition, it was possible to follow the rate of formation and dissolution of zeolite A with sodium hydroxide concentration. The rate at which the LTA is formed and its dissolution increases with
NaOH concentration. With a given silica source, in particular with fumed silica, there appears to be a point at which the LTA formation is halted momentarily before the growth proceeds further. This is particularly the case for a dilute gel mixture when higher amounts of water are used. With the quality of data and the time resolution, it was possible to obtain a detailed kinetic curve for the crystallization of zeolite A, for some of the gel compositions. The Avrami-Erofe’ev nucleation-growth model, which has been widely used to describe the growth curves of zeolites, clearly models our new kinetic data for zeolite A formation well. The simple nucleation growth model is expected to be valid only at the early stages of crystal growth when the assumption that reagent concentration is constant (i.e., not continually changing as reactants are used) is true.42 For the data presented here, the Sharp-Hancock plots are linear for a large part of the crystallization. Avrami exponents of ∼3 are produced and such values have previously been observed for zeolite crystallization. As well as implying that the nucleation-growth model is appropriate for the crystallization, the value of n ) 3 suggests that nucleation sites for crystallization continue to form for a considerable time after crystallization begins.42 Other similar exponential growth models that have been applied to zeolite crystallizations have used similar values of exponent; for example one of the earliest kinetic studies of zeolite A crystallization by Cirac used the Johnson-Avrami expression,30 essentially the Avrami-Erofe’ev expression with n fixed at 4, to model the early stages of crystal growth. Gualtieri et al. performed an in situ angular dispersive study of the formation of zeolite A from kaolinites and used the Avrami-Erofe’ev expression to obtain rate constants, finding that values of n between 2 and 6 were necessary to model the data.52 As we mentioned above, since the Avrami-Erofe’ev expression is an empirical expression, the most important parameter we extract is the rate constant which can be compared directly from run to run. We observe that the rate of reaction is greater if the concentration of NaOH is increased. Considering all the data from the crystallizations, we suggest that the rate of crystallization is directly proportional to the concentration of NaOH, i.e., the rate of crystallization is first order with respect to sodium hydroxide concentration. This relationship between NaOH concentration and rate of crystallization of zeolites was one of the earliest features of zeolite crystallizations to be studied,29,30 and our new results are consistent with the previous studies. A new observation is that the crystallization curves of a zeolite are not necessarily smooth and continuous; this has been made possible through the use of a technique that continually monitors the extent of reaction with high time resolution. Previous studies using quenching methods could have missed the pause in crystallization we observe, since far fewer points were generally measured to describe zeolite crystallizations. We observe reproducibly that at the lowest NaOH concentrations part way through the zeolite growth curve, crystallization stops for a short period before commencing again. The fact that the step has a marked temperature dependence and its appearance also depends on the nature of the silica source used strongly implies that it is a real feature and not due to the nature of the experimental measurement. One plausible reason for this pause in crystallization is that solution species necessary for crystal growth to continue, and indeed for the formation of new nucleation sites, are depleted and a critical concentration of the silica and alumina anions must be again reached for crystallization to continue. This in turn implies that dissolution of the starting materials is
Hydrothermal Crystallization of Zeolite A. 1 the rate-determining step of the zeolite formation, i.e., that the formation of nucleation sites and the crystallization of the zeolite from the solution species are much more rapid than the production of anionic solution species. Another possible model for zeolite crystallization, whose kinetics also depend critically on the rate of dissolution of the amorphous precursors, is one in which “autocatalytic nucleation” is postulated. This concept was first proposed almost 20 years ago and had been widely discussed in the literature since.53,54 The autocatalytic model invokes a situation where nucleation sites for crystal growth are formed not only in solution, but also in the amorphous aluminosilicate gel. For these “hidden” dormant nuclei to be released and crystal growth to take place, dissolution of the amorphous gel must take place. Thus as the reaction proceeds and more gel dissolves, increasingly more nuclei are released, accelerating crystal growth.53,54 The two-step growth curves we observe could be arise from a situation being reached where all solution species have been depleted and no new nucleation sites are available for crystal growth, so that more starting material, even late into the crystallization, must dissolve to provide more nucleation sites and more material for crystallization. We will discuss this hypothesis in the following paper.28 The transformation of zeolite A into hydroxosodalite has been previously investigated due to its importance in understanding the stability of dense aluminosilicates over large-pore zeolites. Subotic et al. performed a comprehensive study of the dissolution of zeolites in concentrated NaOH and of transformation of zeolites55-58 and paid some attention to the transformation of zeolite A into sodalite.59,60 One immediate observation from our experiments is that the Bragg reflections of the two zeolites can appear at almost exactly the same time under certain conditions (within the time resolution of the experiment). This suggests that two distinct nucleation sites for crystallization are formed, rather than LTA being an intermediate necessary for the formation of SOD. The latter deduction is consistent with the fact that if higher NaOH concentrations or higher temperatures are used then hydroxosodalite will crystallize directly without any appearance of zeolite A.59,60 Subotic’s model for the transformation of zeolite A into hydroxosodalite involves complete dissolution of the initial zeolite A.59,60 Our new results show that the decay of zeolite A and the continued growth of sodalite in the concentrated gels we studied are highly correlated. For instance, Figure 4c shows that once the amount of zeolite A has reached its maximum value and it begins to decay, the growth rate of sodalite is reduced rapidly. Then the growth of SOD and decay of LTA are almost linear in nature, mirroring each other until the reaction is complete. Conclusions In this first paper we have illustrated how the use of timeresolved energy-dispersive X-ray diffraction provides an excellent method of following zeolite crystallization under genuine laboratory conditions in real time, with unrivalled time resolution. Very high quality growth curves can be obtained with far superior time resolution to those previously determined by quenching studies. This allows us to determine a more accurate description of the course of crystal growth. This is particularly important for the rapid crystallizations we have studied here where quenching studies would be most difficult to perform. In the following paper,28 some of us will present further experimental results and relate our observations to possible models for zeolite crystallization. Acknowledgment. We are grateful to the EPSRC for financial support and access to synchrotron radiation facilities.
J. Phys. Chem. B, Vol. 105, No. 1, 2001 89 We thank Dr. D. Taylor, Mr. A. Neild, and Dr. S. M. Clark of Daresbury Laboratory for their assistance with using Station 16.4. References and Notes (1) Zones, S. I.; Davis, M. E. Curr. Opin. Solid State Mater. Chem. 1996, 1, 107. (2) Cheetham, A. K.; Ferey, G.; Loiseau, T. Angew. Chem., Int. Ed. Engl. 1999, 38, 3268. (3) Breck, D. W. Zeolite Molecule SieVes: Structure, Chemistry and Use; Wiley and Sons: London, 1974. (4) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (5) Weller, M. T.; Dann, S. E. Curr. Opin. Solid State Mater. Sci. 1998, 3, 137-143. (6) Davis, M. E. Stud. Surf. Sci. Catal. 1995, 97, 35. (7) Renzo, F. D. Catal. Today 1998, 41, 37. (8) Francis, R. J.; O’Brien, S.; Fogg, A. M.; Halasyamani, P. S.; O’Hare, D.; Loiseau, T.; Ferey, G. J. Am. Chem. Soc. 1999, 121, 1002. (9) Walton, R. I.; Loiseau, T.; O’Hare, D.; Ferey, G. Chem. Mater. 1999, 3201, 1. (10) Cheetham, A. K.; Mellot, C. F. Chem. Mater. 1997, 9, 2269. (11) Francis, R. J.; O’Hare, D. J. Chem. Soc., Dalton Trans. 1998, 3133. (12) Geradin, C.; In, M.; Allouche, L.; Haouas, M.; Taulelle, F. Chem. Mater. 1999, 11, 1285. (13) Sankar, G.; Thomas, J. M.; Rey, F.; Greaves, G. N. J. Chem. Soc., Chem. Commun. 1995, 2549. (14) Twu, J.; Dutta, P. K.; Kresge, C. T. J. Phys. Chem. 1991, 95, 5267. (15) Watson, J. N.; Brown, A. S.; Iton, L. E.; White, J. W. J. Chem. Soc., Faraday Trans. 1998, 94, 2181. (16) Kirschhock, C. E. A.; Ravishankar, R.; Looveren, L. V.; Jacobs, P. A.; Martins, J. A. J. Phys. Chem. B 1999, 103, 4972. (17) Moor, P.-P. E. A. d.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; Santen, R. A. v. Chem. Eur. J. 1999, 5, 2083. (18) Munn, J.; Barnes, P.; Hausermann, D.; Axon, S. A.; Klinowski, J. Phase Transitions 1992, 39, 129. (19) Evans, J. S. O.; Francis, R. J.; O’Hare, D.; Price, S. J.; Clarke, S. M.; Flaherty, J.; Gordon, J.; Nield, A.; Tang, C. C. ReV. Sci. Instrum. 1995, 66, 2442. (20) Clark, S. M.; Nield, A.; Rathbone, T.; Flaherty, J.; Tang, C. C.; Evans, J. S. O.; Francis, R. J.; O’Hare, D. Nucl. Instrum. Methods B 1995, 97, 98. (21) Rey, F.; Sankar, G.; Thomas, J. M.; Barrett, P. A.; Lewis, D. W.; Catlow, C. R. A.; Clark, S. M.; Greaves, G. N. Chem. Mater. 1995, 7, 1435. (22) Rey, F.; Sankar, G.; Thomas, J. M.; Barrett, P. A.; Lewis, D. W.; Catlow, C. R. A.; Clark, S. M.; Greaves, G. N. Chem. Mater. 1996, 8, 590. (23) Davies, A. T.; Sankar, G.; Catlow, C. R. A.; Clark, S. M. J. Phys. Chem. B 1997, 101, 10115. (24) Francis, R. J.; Price, S. J.; Evans, J. S. O.; O’Brien, S.; O’Hare, D.; Clark, S. M. Chem. Mater. 1996, 8, 2102. (25) Meier, W. Atlas of Zeolite Structure Types, 2nd ed.; Butterworth: London, 1987. (26) Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J. Am. Chem. Soc. 1956, 78, 5962. (27) Dyer, A. An Introduction to Zeolite Molecular SieVes; Wiley: Chichester, UK, 1988. (28) Walton, R. I.; O’Hare, D. J. Phys. Chem. B, following paper in this issue. (29) Kerr, G. T. J. Phys. Chem. 1966, 4, 1047. (30) Cirac, J. J. Colloid Interface Sci. 1968, 28, 315. (31) Kostinko, J. A. ACS Symp. Ser. 1983, 218, 1. (32) Dutta, P. K.; Shieh, D. C. J. Phys. Chem. 1986, 90, 2331. (33) Chen, W. H.; Hu, H. C.; Lee, T. Y. Chem. Eng. Sci. 1993, 48, 3683. (34) Shi, J. M.; Anderson, M. W.; Carr, S. W. Chem. Mater. 1996, 8, 369. (35) Antonic, T.; Subotic, B.; Stubicar, N. Zeolites 1997, 18, 291-300. (36) Colston, S. L.; Jacques, S. D. M.; Barnes, P.; Jupe, A. C.; Hall, C. J. Synchrotron Radiat. 1998, 5, 112. (37) Clark, S. M. J. Appl. Crystallogr. 1995, 28, 646. (38) Avrami, M. J. Chem. Phys. 1939, 7, 1103. (39) Avrami, M. J. Chem. Phys. 1940, 8, 212. (40) Avrami, M. J. Chem. Phys. 1941, 9, 177. (41) Erofe’ev, B. V. C. R. Dokl. Acad. Sci. URSS 1946, 52, 511. (42) Thompson, R. W. Zeolites 1992, 12, 681. (43) Hulbert, S. F. J. Br. Ceram. Soc. 1969, 6, 11. (44) Thompson, R. W.; Dyer, A. Zeolites 1985, 5, 202. (45) Thompson, R. W.; Yer, A. Zeolites 1985, 5, 293.
90 J. Phys. Chem. B, Vol. 105, No. 1, 2001 (46) Sharp, J. D.; Hancock, J. H. J. Am. Ceram. Soc. 1972, 55, 74. (47) JCPDS International Center for Diffraction Data, Swarthmore, USA, 1997; Card No. 39-0222. (48) Gramlich, V.; Meier, W. Z. Kristallogr., Kristallgeom., Kristallphys., Kristallchem. 1971, 133, 134. (49) JCPDS International Center for Diffraction data, Swarthmore, USA, 1997; Card No. 41-0009. (50) Felsche, J.; Luger, S. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 731. (51) Tuel, A. Chem. Mater. 1999, 11, 1865. (52) Gualtieri, A.; Norby, P.; Artioli, G.; Hansen, J. Phys. Chem. Miner. 1997, 24, 191. (53) Subotic, B. ACS Symp. Ser. 1989, 398, 110.
Walton et al. (54) Gonthier, S.; Gora, L.; Guray, I.; Thompson, R. W. Zeolites 1993, 13. (55) Subotic, B.; Smit, I.; Madzija, O.; Sekovanic, L. Zeolites 1982, 2, 135. (56) Cizmek, A.; Komunjer, L.; Subotic, B.; Siroki, M.; Roncevic, S. Zeolites 1991, 11, 258. (57) Cizmek, A.; Komunjer, L.; Subotic, B.; Siroki, M.; Roncevic, S. Zeolites 1991, 11, 810. (58) Cizmek, A.; Komunjer, L.; Subotic, B.; Siroki, M.; Roncevic, S. Zeolites 1992, 12, 190. (59) Subotic, B.; Skritic, D.; Smit, I.; Sekovanic, L. J. Cryst. Growth 1980, 50, 498. (60) Subotic, B.; Sekovanic, L. J. Cryst. Growth 1986, 75, 561.
