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Effects of ultrasound on zeolite A synthesis
Microporous and Mesoporous Materials 79 (2005) 225–233 www.elsevier.com/locate/micromeso
Effects of ultrasound on zeolite A synthesis ¨ zlem Andac¸ a, Melkon Tatlıer a, Ahmet Sirkeciog˘lu a, Isßık Ece b, O Aysße Erdem-Sß enatalar a,* a b
Department of Chemical Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey Department of Geological Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey Received 1 July 2004; received in revised form 6 November 2004; accepted 8 November 2004 Available online 6 January 2005
Abstract Synthesis of zeolite A from a clear-to-the-eye sodium aluminosilicate solution was carried out in the presence of ultrasound at different temperatures and times and the results were compared to those obtained by performing conventional static syntheses under similar conditions. It was possible to obtain highly crystalline zeolite A in the presence of ultrasound, which influenced the types and stability areas of the phases that formed during metastable phase transformations. Nucleation and crystallization rates, as well as the yield of zeolite A increased as a result of the application of ultrasound. Thermogravimetric analyses and water adsorption experiments revealed that the development of micropore adsorption capacity, which started very early in the X-ray amorphous samples obtained from this so-called ‘‘clear’’ solution, was also speeded up with the application of ultrasound. 2004 Elsevier Inc. All rights reserved. Keywords: Ultrasound; Zeolite A; Crystallization; Adsorption; Thermogravimetric analysis
1. Introduction The use of ultrasound of 20 kHz to 2 MHz frequency in synthetic chemistry has led to a branch termed sonochemistry [1,2]. The effects of ultrasound have been investigated for different cases, including polymerization reactions and the syntheses of various amorphous and crystalline materials. Significant changes have been commonly observed in the processes and properties of the reaction products in the presence of ultrasound. It was established that the use of ultrasound in polymer synthesis can provide rate and yield enhancements and can allow for a large degree of control over the polymer structure, especially in determining the molecular weight and polydispersity [3,4]. Higher ultrasonic intensity was
*
Corresponding author. Tel.: +90 2122856896; fax: 2122852925. E-mail address: [email protected] (A. Erdem-S ß enatalar).
+90
1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.11.007
determined to result in an increase in the size of amorphous silica spheres prepared from TEOS [5]. Ultrasound seems to also influence the physicochemical phenomena related to nucleation and crystal growth, occurring during crystallization. For instance, it was observed that ultrasonic waves decrease the supersaturation limits and modify the morphology of the potash alum crystals [6]. In this case, the average crystal size 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 [7]. 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 [8,9] 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. Studies
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have been carried out to explain the effects of cavitation in terms of the Ôhot spotÕ [10] and electrical theories [11]. Due to the diverse conventional and emerging applications of synthetic zeolites, which are prepared by hydrothermal crystallization, the degree of control over their preparation and properties bears importance. In this respect, the nucleation and crystal growth rates, and the resulting particle size distribution are especially important to tailor. The molar ratios of the reactants, and the synthesis time and temperature are the main parameters in determining the type and properties of the zeolite formed. Using seeding techniques [12], aging the reaction mixture [13,14] and performing the synthesis under stirring [15] have also been observed to influence the crystallization process and the final properties of the zeolite product. Application of ultrasound, clearly offers potential for affecting the hydrothermal crystallization process and some of the final properties of the synthetic zeolite samples. The significance of using sonication in zeolite synthesis has been shown previously for the preparation of zeolite A from kaolin [16]. In this study, the effects of ultrasound on the synthesis of zeolite A from a so-called ‘‘clear’’ sodium aluminosilicate solution, was investigated. For this purpose, synthesis experiments were conducted in an ultrasonic water bath for various synthesis times and temperatures, by using a reaction mixture composition known to yield pure zeolite A. The products were compared to those obtained by performing conventional static synthesis in a water bath under similar conditions.
2. Experimental Zeolite A in sodium form (NaA) was prepared from sodium aluminosilicate solutions with the composition of 50Na2O:Al2O3:5SiO2:1000H2O. This composition has been reported previously to produce zeolite A both in the bulk [17,18] and on substrates [19,20]. Sodium silicate solution (27 wt.% SiO2, Riedel-de Haen), granular sodium aluminate (Riedel-de Haen), anhydrous NaOH pellets (Riedel-de Haen) and deionized water were used in the synthesis experiments. NaOH was dissolved in deionized water, initially sodium aluminate and finally sodium silicate were added to the solution. Additions were made to clear solutions cooled to below 27 C, and the final reaction mixture, which was also clear to the eye, was stirred for 15 min at this temperature prior to the syntheses. Two different sets of experiments were performed in which zeolite A was synthesized in an ultrasonic water bath (35 kHz) and in a conventional water bath under static conditions, respectively. The hydrothermal crystallization was carried out at temperatures of 50 C and 60 C for various periods of time ranging from 2
to 15 h. The temperature of the ultrasonic water bath was controlled, by using a heater and a temperature controller. The product was filtered initially using a filter paper with fine mesh size. The filtrate was then refiltered through a 0.1 lm PVDF membrane. The mass of the collected solids was determined after washing thoroughly with deionized water and drying overnight in an oven at 65 C. X-ray diffraction (Phillips 1040 XRD) run with Cu Ka1 radiation in the 2h range of 5–50 was applied to all samples for the characterization of the phases present, and for the determination of the crystallinities of the zeolite A samples. The peak areas of the characteristic peaks of zeolite A, which do not overlap with the peaks of the other phases that formed as a result of metastable phase transformations, were used for this purpose. A commercial zeolite A sample (Degussa), was assumed to have 100% crystallinity, while the crystallinities for zeolite A of the synthesized samples were determined by taking into account the ratios of the sum of the peak areas to that pertaining to the commercial zeolite A sample. A rough estimation of the crystallization rates was made from the slopes of the crystallization curves at about 50% crystallinity. Induction periods were also estimated from the crystallization curves. The samples with the highest crystallinities were investigated by scanning electron microscopy (JEOL JSM-840 SEM) for the determination of the morphology of the zeolite A crystals. Particle size measurements for the same samples with the highest crystallinities, were performed by Laser Light Scattering Technique (Malvern Mastersizer 2000). From the particle size distribution curves obtained, the sizes for which the cumulative percentage undersize is equal to 10%, 50%, and 90% of the particles in the samples (d(0.1), d(0.5), and d(0.9), respectively) by volume and by number, could be determined. d(0.5) represents the median of the respective size distribution. The size span value, which is a measure of the width of the size distribution, and which is smaller for narrower distributions, was estimated by the help of the following equation: span ¼ ðdð0:9Þ dð0:1ÞÞ=dð0:5Þ
ð1Þ
Thermogravimetric analyses and adsorption experiments were carried out to further characterize the samples, especially those formed at the relatively earlier periods of synthesis, which were identified as amorphous or of low or partial crystallinity by XRD. Both methods were also applied to the commercial zeolite NaA sample (Degussa), in addition to some of the samples prepared in this study in the presence or absence of ultrasound. Prior to the thermogravimetric analyses, the samples were saturated in a controlled humidity atmosphere. The thermogravimetric analyses (Shimadzu TA-503) were carried out between ambient temperature and
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350 C, under a nitrogen flow of 20 cc/min. A heating rate of 20 C/min was used to raise the temperature of the zeolite samples to 350 C, after which the temperature was kept constant at this value for 30 min. Adsorption isotherms for water at 300 K were determined by using a constant volume apparatus. The samples were activated at 400 C for 6 h under a vacuum of 105 mbar, prior to the adsorption experiments.
Table 1 Phase composition from XRD of the solid phase synthesized at 50 C
No ultrasound
Ultrasound
6 7 8 9 10 11
100% Am 100% Am 17.3% A + 82.7% Am 33.5% A + 66.5% Am 43% A + 57% Am 55.8% A + 44.2% Am
3. Results and discussion
12
–
Zeolite NaA synthesis from clear solution was carried out with and without the application of ultrasound at two different temperatures and various times. XRD analysis was applied to all of the samples and the crystalline zeolite phases were identified accordingly. At early synthesis times, the samples were X-ray amorphous while the initial crystalline phase was identified to be zeolite NaA in all of the cases investigated. The nature of the X-ray amorphous phases will be discussed later in this study. It was possible to obtain highly crystalline zeolite A in the presence of ultrasound, as well as in its absence. At extended synthesis times, zeolite NaA started to transform into other phases, such as hydroxysodalite (HS), zeolite X and losod and the samples consisted of different combinations of these phases. The X-ray diffractograms of two of the samples prepared at the synthesis temperature of 50 C under ultrasound are shown in Fig. 1. The diffractograms given in Fig. 1(a) and (b) correspond to the samples obtained at synthesis times of 10 h and 12 h, respectively. It may be seen that at this temperature, conversion to other phases, namely, HS and losod, has started when the synthesis was extended to longer than 10 h. The solid phase compositions based on XRD, obtained from the reaction mixture at different synthesis times at 50 C and 60 C are shown in Tables 1 and 2,
13 14 15
72.7% A + 27.3% Am 69.1% A + 30.9% (X + Am) 66.6% A + 33.4% (X + Am)
100% Am 15.9% A + 84.1% Am 37.2% A + 62.8% Am 59.1% A + 40.9% Am 74% A + 26% Am 87.6% A + 12.4% (S + L + Am) 76.8% A + 23.2% (S + L + Am) – – –
Fig. 1. X-ray diffractograms of the samples obtained at 50 C in the presence of ultrasound after (a) 10 h, and (b) 12 h of synthesis (L: losod, S: hydroxysodalite).
Time (h)
Phase composition
Am: amorphous aluminosilicate; A: zeolite A; X: zeolite X; S: hydroxysodalite; L: losod.
Table 2 Phase composition from XRD of the solid phase synthesized at 60 C Time (h)
Phase composition No ultrasound
Ultrasound
2 3 4 5 6 7 8
100% Am 100% Am 21.7% A + 78.3% Am 47.9% A + 52.1% Am 69.6% A + 30.4% Am 84.8% A + 15.2% Am 94.3% A + 5.7% Am
9
79.1% A + 20.9% (S + L + Am)
100% Am 18.7% A + 81.3% Am 48.4% A + 51.6% Am 76.5% A + 23.5% Am 94.2% A + 5.8% Am 95.8% A + 4.2% Am 84% A + 16% (X + S + L + Am) –
Am: amorphous aluminosilicate; A: zeolite A; X: zeolite X; S: hydroxysodalite; L: losod.
respectively. It may be observed that applying ultrasound to zeolite synthesis led to some changes in the stability areas of the different zeolite phases. The appearance of zeolite A followed the formation of Xray amorphous material at both temperatures, irrespective of whether ultrasound was applied or not. However, a movement towards earlier synthesis times was observed for the synthesis in the presence of ultrasound. Another difference observed when ultrasound was applied to zeolite synthesis seemed to be in the type(s) of the zeolites formed when zeolite A began to transform into other phases. Zeolite A transformed into zeolite X and into HS + losod mixture at 50 C and 60 C, respectively, when no ultrasound was applied. In the presence of ultrasound, on the other hand, HS + losod mixture and X + HS + losod mixture replaced zeolite A at the lower and higher temperatures, respectively. Zeolites are known to transform into thermodynamically more stable phases as time proceeds [21,22]. Under the experimental conditions used, zeolite A was seen to be transformed into zeolites HS, X, and losod, with
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overlapping crystallization fields. The phase composition is a function of the relative rates of nucleation and crystal growth, determined by temperature and stirring conditions, of the competing zeolite types in the reaction mixture. Performing the synthesis with stirring has been observed to result in the formation of different zeolite phases when all the other synthesis parameters, including temperature and time, were kept constant [23]. The effect of ultrasonic stirring on the phase composition of the product, in this respect, is not surprising. The crystallinities of the samples with respect to zeolite A, calculated from XRD analyses of the samples, given in Tables 1 and 2, are plotted against the synthesis times in Fig. 2, in order to show that the crystallization curves allow the estimation of induction times and crystallization rates with reasonable accuracy. Table 3 lists the induction times and crystallization rates estimated from the XRD data as explained in the previous section. It may be observed clearly from the results presented that in the presence of ultrasound, induction times decreased and the crystallinity of zeolite A started to develop at earlier synthesis times, indicating an increase in the nucleation rates in the presence of ultrasound. The effect of ultrasound on the nucleation rate is expected to be related to cavitation. It has been suggested that each cavitational collapse may create a nucleation site analogous to that from a trace particle or a surface imperfection [8]. It may also be seen from Fig. 2 and Table 3 that
100
Zeolite A (%)
80
60
40 50°C No Us 50°C Us
20
the application of ultrasound led to a significant increase in the estimated crystallization rates of zeolite A. This effect was more apparent at the lower synthesis temperature of 50 C, where the relative rate of growth with respect to nucleation is smaller than that at the higher temperature, while the decrease in induction time was more pronounced at the higher temperature. The increase observed in the crystallization rate is an indication of an increase in the rate of crystal growth with ultrasound application. Ultrasound is known to result in acoustic streaming providing enhanced mass transfer close to the crystal surface [9], which is expected to increase the crystal growth rate. The increase obtained in the nucleation rate in the presence of ultrasound, however, may also have contributed to the increase in the crystallization rate to an extent, since it is known that nucleation can prevail even at the later periods of synthesis, where crystal growth is generally the dominating process. The effect of ultrasound on the yield of zeolite A (in g zeolite A per g reaction mixture, estimated from product mass · crystallinity from XRD) is shown in Fig. 3, for the synthesis temperatures of 50 C and 60 C. It may be observed from the figure that, at both temperatures, zeolite A yield increased significantly when ultrasound was applied to the reaction mixture; but the effect was much more pronounced at the lower temperature of 50 C. Zeolite A yield at 50 C after 10 h, for example, seems to have increased by more than 3 times when ultrasound was applied. At the higher temperature of 60 C, on the other hand, it seems that the rapid transformation of zeolite A into other phases limited the amount of this zeolite obtained under the given conditions. Fig. 4 shows the SEM pictures of the samples with the highest crystallinities, and having zeolite A as the only crystalline phase, prepared at 50 C, with and without the application of ultrasound. Fig. 4(a) corresponds to the sample prepared after 13 h in the absence of ultra-
60°C No Us 60°C Us
0
0.005
0
2
4
6
8 Time (h)
10
12
141
16
Table 3 Estimated induction times and crystallization rates of zeolite A at two temperatures in the absence and presence of ultrasound Temperature (C)
50 60
Induction period (h)
Crystallization rate (h1)
No ultrasound
Ultrasound
No ultrasound
Ultrasound
6.6 3.2
6.3 2.5
11.4 21.3
22.2 24.2
Yield (g/g)
0.004
Fig. 2. Variation of crystallinity as estimated from XRD analyses with respect to synthesis time at 50 C and 60 C in the absence and presence of ultrasound.
0.003
0.002 50°C No Us 50°C Us
0.001
60°C No Us 60°C Us 0 0
2
4
6
8
10
12
14
16
Time (h)
Fig. 3. Variation of the zeolite A yield of the samples with respect to synthesis time at 50 C and 60 C in the absence and presence of ultrasound.
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Fig. 4. SEM pictures of the zeolite A samples obtained at 50 C (a) after 13 h of synthesis in the absence of ultrasound, and (b) after 10 h of synthesis in the presence of ultrasound.
Fig. 5. SEM pictures of the zeolite A samples obtained at 60 C (a) after 8 h of synthesis in the absence of ultrasound, and (b) after 7 h of synthesis in the presence of ultrasound.
sound, and Fig. 4(b) to that prepared after 10 h of synthesis in the presence of ultrasound, respectively. The difference between the average particle sizes of these samples, for which the crystallinity values estimated from XRD are very close, 72.7% and 74%, respectively, can be easily inferred from the figures. On the other hand, the SEM pictures of the zeolite A samples with the highest crystallinities obtained at 60 C are given in Fig. 5. Fig. 5(a) and (b) correspond to the samples synthesized in the absence of ultrasound after 8 h (crystallinity 94.3%) and in the presence of ultrasound after 7 h (crystallinity 95.8%), respectively. Though less pronounced at this higher temperature, the particles in the sample prepared in the presence of ultrasound were again larger, in general. Aggregation is seen to take place both in the absence and presence of ultrasound, and is especially evident at the higher temperature from the SEM pictures. The particle size distributions (PSDs) of the same samples with the highest crystallinities estimated from XRD, and having zeolite A as the only crystalline phase
(for which SEM pictures were shown), prepared at 50 C and 60 C are given in Figs. 6 and 7, respectively. These experiments were repeated for several times and reproducibility was confirmed. In parallel with the SEM observations, in the presence of ultrasound, PSDs shifted to the right to larger particle sizes, as a result of decreased induction times and increased crystal growth rates. The size span values (by number) decreased from 1.137 to 1.096 at 50 C, and from 2.088 to 1.601 at 60 C, hence somewhat narrower PSDs were obtained with the application of ultrasound. The XRD data presented above indicate that an amorphous phase is formed prior to nucleation and crystal growth, from this so-called ‘‘clear solution’’. However, nuclei were implied to form and grow from the clear sodium aluminosilicate solution with the same composition as the one used here, in a previous study [18]. Since it is known that sufficiently long range order is necessary for crystallinity to be detected by XRD, the X-ray amorphous and partly crystalline solids were characterized further by adsorption and TGA analyses.
¨ zlem Andac¸ et al. / Microporous and Mesoporous Materials 79 (2005) 225–233 O 20
18
18
16
16
14
14 Number (%)
20
12 10 8
8 6 4
2
2 0.1
(a)
1
10
0 0.01
100
Particle size (µm)
20
18
18
16
16
14
14
12 10 8
4
2
2 0 0.01
100
Particle size (µm)
Fig. 6. Particle size distributions by number, corresponding to the samples obtained at 50 C (a) after 13 h of synthesis in the absence of ultrasound, and (b) after 10 h of synthesis in the presence of ultrasound.
The adsorption isotherms of water at 300 K for the commercial zeolite NaA sample and some other samples prepared in this study are shown in Fig. 8. The samples investigated were those prepared at synthesis times of 3 h, 5 h, 7 h and 10 h in the presence of ultrasound, and 5 h, 7 h, 9 h and 13 h in the absence of ultrasound, all at 50 C. These samples were identified as either amorphous or as zeolite A of low or partial crystallinity by XRD. The shapes of the adsorption isotherms varied and the adsorption capacities increased gradually with the synthesis time as the amorphous materials were transformed into zeolite A. The X-ray amorphous materials at the relatively earlier periods of synthesis, especially those prepared with ultrasound, seemed to exhibit step-wise isotherms similar to those of mesoporous materials, with the steps less sharp and at higher P/ P0 ratios, indicating the presence of wider pore size distributions and larger pore sizes, respectively. The steps in the isotherms of the X-ray amorphous materials gradually disappeared and the isotherms became more flattened with increased synthesis times. The isotherms of the samples determined to be partly crystalline by XRD, resembled more the Type I isotherm obtained
10
100
8 6
10
100
10
4
1
10
12
6
0.1
1 Particle size (µm)
20
0 0.01
0.1
(a)
Number (%)
Number (%)
10
4
0 0.01
(b)
12
6
0.1
1
(b)
Particle size (µm)
Fig. 7. Particle size distributions by number, corresponding to the samples obtained at 60 C (a) after 8 h of synthesis in the absence of ultrasound, and (b) after 7 h of synthesis in the presence of ultrasound.
25
20 mmol H 2O/g-zeolite
Number (%)
230
15 No Us 5h No Us 7h No Us 9h No Us 13h Us 3h Us 5h Us 7h Us 10h Commer..
10
5
0 0
0.2
0.4
0.6
0.8
1
1.2
P/ P0
Fig. 8. Adsorption isotherms for water at 300 K of the commercial zeolite A sample and the samples obtained at 50 C in the absence and presence of ultrasound.
for the commercial sample, but were somewhat less rectangular, exhibited lower adsorption capacities and higher rates of increase in the amount adsorbed with
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Table 4 Crystallinity and microporosity values of the samples that are synthesized at 50 C, estimated from XRD, TGA and adsorption data Sample
% Crystallinity from XRD
% Microporosity from TGA
% Microporosity from adsorption
No Us, 5 h No Us, 7 h No Us, 9 h No Us, 13 h Us, 3 h Us, 5 h Us, 7 h Us, 10 h
0 0 33.5 72.7 0 0 15.9 74
17.1 41.4 73.2 87.8 14.6 20.7 79.9 92.6
12.5 32.5 76.3 88.1 6.3 13.1 67.5 95.2
0.001
0 0
50
100
150
200
25 250
300
-0.001 DTG (mg/sec)
respect to the relative pressure, above the relative pressures corresponding to the filling of micropores. This indicates the presence of still larger external surface areas—and larger pores—in these samples, when compared to those in the fully crystalline sample. Micropores with diameters comparable to the size of the molecules adsorbed, are known to be filled at low relative pressures (P/P0 < 0.01) [24]. Larger micropores (up to 2 nm) are filled in the relative pressure region between P/P0 0.01–0.2. Since the t-plot method, which is conventionally used for the determination of micropore volume could not be used in this study due to the lack of statistical thickness values with respect to pressure for water on materials similar to zeolite A, the ratio of the adsorption capacity at the P/P0 value of 0.01, of each sample to that of the commercial NaA was used as an estimate of microporosity from the adsorption data, assuming that the commercial sample is fully crystalline. The % microporosity values estimated from the adsorption isotherms are listed in Table 4, together with those estimated from thermogravimetric analyses as will be discussed below, and with XRD crystallinities, for comparison. The results presented clearly indicate that micropore development starts earlier to take place in the X-ray amorphous phase, and is speeded up with the application of ultrasound. The samples prepared in the presence of ultrasound systematically exhibited higher adsorption capacities at low relative pressures— and seemingly higher micropore volumes—when compared to those obtained in the absence of ultrasound at the same synthesis times. Thermogravimetric analyses (TGA) were carried out to further inspect the X-ray amorphous and partly crystalline samples. The derivative TGA curves (DTG curves) of the samples synthesized at 50 C after 5 h, 7 h, and 10 h of reaction in the presence of ultrasound are given in Fig. 9. It may be observed from the figure that the curve for the amorphous sample was quite different than those of the crystalline and partly crystalline samples. In all curves, there was a minimum below or at about 100 C, which corresponds to the removal of loosely held moisture. The intensity of this lower tem-
231
-0.002 Us 5h -0.003
Us 7h Us 10h
-0.004
Commercial -0.005 Temperature (°C)
Fig. 9. DTG curves of the commercial zeolite A sample and the samples obtained at 50 C in the presence of ultrasound.
perature minimum was observably higher for the amorphous sample. All the DTG curves exhibited a second minimum at or above 150 C, which has been related [25] to the presence of ordered structural units having a channel/void system similar to that of zeolites in the sample. The intensities of the higher temperature minima on the DTG curves of the partly and fully crystalline samples were much higher when compared to that of the amorphous sample. The ratio of the intensity of the higher temperature minimum to that of the lower temperature minimum increased parallel to the X-ray crystallinity of the sample. The % microporosity values estimated from the TGA results are listed in Table 4. The ratio of the percent weight lost by each sample, above the temperature corresponding to the inflection point following the low temperature minimum of the DTG curve, to that of the commercial NaA sample was used in the estimations. In accordance with the results obtained from adsorption experiments, TGA results also indicate microporosity values higher than those implied by XRD crystallinity for all samples, and higher microporosity values for the samples prepared in the presence of ultrasound than for those prepared in its absence. It is clear that the processes taking place early in the amorphous phase, leading to the development of microporosity, are speeded up by the application of ultrasound. The agreement between the microporosities estimated from adsorption and TGA is seen to improve as the amount of crystalline material increases in the samples. Above X-ray crystallinities of about 30%, the agreement seems to be very good. Below X-ray crystallinities of about 30%, TGA estimates an even higher microporosity than adsorption. The microporosities estimated from adsorption are plotted against synthesis times in Fig. 10. The figure implies that the development of microporosity may be starting at or very close to the beginning of synthesis at 50 C both in the absence and presence of ultrasound, and is speeded up by sonication.
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232 100 90 80
Capacity (%)
70 60 50 40 30
No Us
20
Us
10 0 0
3
6
9
12
15
Time (h)
Fig. 10. Estimated microporosity development with respect to synthesis time for the samples obtained at 50 C in the absence and presence of ultrasound.
As was mentioned above, and as can be seen from Table 4, the microporosities estimated from adsorption and TGA experiments were systematically and significantly higher than those implied by the crystallinities obtained from XRD. Since the micropore development in the Xray amorphous phase is expected to be strongly related to the nucleation and growth of zeolite A crystals, it is possible that zeolite crystals may be nucleating starting from quite early times of synthesis, and may be contributing to the adsorption capacity as they grow in number and size, while they remain undetected by XRD below certain concentration and size limits. In this case, nucleation and growth start much earlier than that estimated by XRD, and the induction times estimated from XRD may have a limited meaning, and may only be used on a comparative basis. On the other hand, the differences between the XRD crystallinities and the microporosities estimated by adsorption and TGA experiments are so large in some cases (see for example, the significant microporosities implied by the isotherms of the samples obtained after 7 h at 50 C in the absence and presence of ultrasound) that it also seems highly possible that the amorphous material might also have contributed to the microporosity of the samples, which may be implying the formation (rather than the initial presence in the amorphous phase, due for example to aggregation of the 1 nm particles observed to be present in the synthesis solutions [18]) of microporous precursor structures/domains in the amorphous material prior to nucleation. Although the results show clearly that microporosity starts to develop in the samples much earlier than that implied by XRD crystallinity, they also document the presence of amorphous material during early times of synthesis, as opposed to the implications of the results of the previous study carried out with the same reaction mixture composition, mentioned above [18]. Small particles could be detected in the referred study by QELSS
at 60 C when their average sizes were about 200 nm, and these particles were observed to grow later with linear rates. Induction time was obtained by extrapolating the growth curve to zero particle size and was less than an hour at 60 C with no aging. Presence of an X-ray amorphous phase during any stage of synthesis at this temperature was not mentioned. However, it was mentioned that light-scattering spectra could not be obtained until after 65 min at the beginning. It was also reported that the distribution of the particle sizes obtained at synthesis times up to 85 min was wide, but decreased with increasing synthesis time. The formation of a precursor phase at room temperature was however evident in the same study. Our observation of the presence of an amorphous phase for extended periods from the same initial composition at 60 C must be related to the differences in the reactants and in the procedures used during the experiments. That the reaction mixtures were filtered through 0.2 lm membranes prior to the syntheses in the referred study, and that we have stirred the initial mixture for 15 min at room temperature, seem to be some of the important differences to us. One explanation of the events taking place in the sodium aluminosilicate solution initially clear to the eye, used in this study and the previous one [18], that may also explain the difference in the observations made in these studies, may be that no matter what their initial sizes are, or forced to be by filtration, X-ray amorphous precursor particles are involved in this zeolite A synthesis system. They form at room temperature. Their sizes and size distribution depend on the reactants and procedures used for the preparation of the reaction mixture. Filtration of the reaction mixture through membranes prior to the syntheses in the referred study [18] might have decreased the maximum size of these particles to 200 nm at the beginning of the heating period. Nucleation and then growth processes, with their rates depending on the temperature, start early, within these particles and continue during the heating and synthesis stages. Dissolution and reorganization of the X-ray amorphous phase is also accelerated with temperature. This stage may be completed shortly after heating to the reaction temperature or even earlier, in case the initial sizes of the amorphous material are small [18], or may last longer as was observed in the present study.
4. Conclusions Highly crystalline zeolite A could be synthesized from a clear-to-the-eye aluminosilicate solution in the presence of ultrasound. Sonocrystallization has been demonstrated to offer the possibilities of increasing the nucleation and crystallization rates of zeolites, improving the yield and particle size distribution of the product crystals, and directing the synthesis towards different
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crystal phases. Investigating the effects of varying the frequency and power of ultrasound and applying sonocrystallization to different zeolite synthesis systems are necessary in order to develop a better understanding of the range of possibilities offered by the technique. Thermogravimetric analyses and water adsorption experiments revealed that the development of micropore adsorption capacity started very early in the X-ray amorphous samples obtained from this so-called ‘‘clear’’ solution, and that the processes taking place in the Xray amorphous phase leading to the increase in microporosity, were also speeded up with the application of ultrasound.
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[7] N. Lyczko, F. Espitalier, O. Louisnard, J. Schwartzentruber, Chem. Eng. J. 86 (2002) 233. [8] L.J. McCausland, P.W. Cains, P.D. Martin, Chem. Eng. Prog. 97 (2001) 56. [9] G.J. Price, Chem. Eng. Prog. 93 (1997) 34. [10] K.S. Suslick, S.J. Doktycz, E.B. Flint, Ultrasonics 28 (1990) 280. [11] M.A. Margulis, I.M. Margulis, Ultrason. Sonochem. 9 (2002) 1. [12] V. Valtchev, S. Mintova, V. Dimov, A. Toneva, D. Radev, Zeolites 15 (1995) 193. [13] J.D. Cook, R.W. Thompson, Zeolites 8 (1988) 322. [14] H.J. Ko¨rog˘lu, A. Sarıog˘lan, M. Tatlıer, A. Erdem-S ß enatalar, ¨ .T. Savasßc¸ı, J. Crystal Growth 241/4 (2002) 481. O [15] S. Gontier, A. Tuel, Zeolites 16 (1996) 184. [16] J. Park, B.C. Kim, S.S. Park, H.C. Park, J. Mater. Sci. Lett. 20 (2001) 531. [17] P. Wengin, S. Ueda, M. Koizumi, in: Y. Murakami, A. Iijima, J.W. Ward (Eds.), Stud. Surf. Sci. Catal. 28 (1986) 177. [18] L. Gora, K. Streletzky, R.W. Thompson, G.D.J. Phillies, Zeolites 18 (1997) 119. ¨ rgen, Micropor. Mesopor. [19] A. Erdem-S ß enatalar, M. Tatlıer, M. U Mater. 32 (1999) 331. [20] T. C ¸ etin, M. Tatlıer, A. Erdem-S ß enatalar, U. Demirler, M. ¨ rgen, Micropor. Mesopor. Mater. 47 (2001) 1. U [21] M. Tassopoulos, R.W. Thompson, Zeolites 7 (1987) 243. [22] J.A. Kostinko, in: G.D. Stucky, F.G. Dwyer, J.W. Ward (Eds.), Intrazeolite Chemistry (1983) 3. [23] F. Di Renzo, Catal. Today 41 (1998) 37. [24] S. Storck, H. Bretinger, W.F. Meier, Appl. Catal. A: Gen. 174 (1998) 137. [25] I. Krznaric, T. Antonic, B. Subotic, V. Babic-Ivancic, Thermochim. Acta 317 (1998) 73.
Effects of ultrasound on zeolite A synthesis ¨ zlem Andac¸ a, Melkon Tatlıer a, Ahmet Sirkeciog˘lu a, Isßık Ece b, O Aysße Erdem-Sß enatalar a,* a b
Department of Chemical Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey Department of Geological Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey Received 1 July 2004; received in revised form 6 November 2004; accepted 8 November 2004 Available online 6 January 2005
Abstract Synthesis of zeolite A from a clear-to-the-eye sodium aluminosilicate solution was carried out in the presence of ultrasound at different temperatures and times and the results were compared to those obtained by performing conventional static syntheses under similar conditions. It was possible to obtain highly crystalline zeolite A in the presence of ultrasound, which influenced the types and stability areas of the phases that formed during metastable phase transformations. Nucleation and crystallization rates, as well as the yield of zeolite A increased as a result of the application of ultrasound. Thermogravimetric analyses and water adsorption experiments revealed that the development of micropore adsorption capacity, which started very early in the X-ray amorphous samples obtained from this so-called ‘‘clear’’ solution, was also speeded up with the application of ultrasound. 2004 Elsevier Inc. All rights reserved. Keywords: Ultrasound; Zeolite A; Crystallization; Adsorption; Thermogravimetric analysis
1. Introduction The use of ultrasound of 20 kHz to 2 MHz frequency in synthetic chemistry has led to a branch termed sonochemistry [1,2]. The effects of ultrasound have been investigated for different cases, including polymerization reactions and the syntheses of various amorphous and crystalline materials. Significant changes have been commonly observed in the processes and properties of the reaction products in the presence of ultrasound. It was established that the use of ultrasound in polymer synthesis can provide rate and yield enhancements and can allow for a large degree of control over the polymer structure, especially in determining the molecular weight and polydispersity [3,4]. Higher ultrasonic intensity was
*
Corresponding author. Tel.: +90 2122856896; fax: 2122852925. E-mail address: [email protected] (A. Erdem-S ß enatalar).
+90
1387-1811/$ - see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.11.007
determined to result in an increase in the size of amorphous silica spheres prepared from TEOS [5]. Ultrasound seems to also influence the physicochemical phenomena related to nucleation and crystal growth, occurring during crystallization. For instance, it was observed that ultrasonic waves decrease the supersaturation limits and modify the morphology of the potash alum crystals [6]. In this case, the average crystal size 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 [7]. 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 [8,9] 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. Studies
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have been carried out to explain the effects of cavitation in terms of the Ôhot spotÕ [10] and electrical theories [11]. Due to the diverse conventional and emerging applications of synthetic zeolites, which are prepared by hydrothermal crystallization, the degree of control over their preparation and properties bears importance. In this respect, the nucleation and crystal growth rates, and the resulting particle size distribution are especially important to tailor. The molar ratios of the reactants, and the synthesis time and temperature are the main parameters in determining the type and properties of the zeolite formed. Using seeding techniques [12], aging the reaction mixture [13,14] and performing the synthesis under stirring [15] have also been observed to influence the crystallization process and the final properties of the zeolite product. Application of ultrasound, clearly offers potential for affecting the hydrothermal crystallization process and some of the final properties of the synthetic zeolite samples. The significance of using sonication in zeolite synthesis has been shown previously for the preparation of zeolite A from kaolin [16]. In this study, the effects of ultrasound on the synthesis of zeolite A from a so-called ‘‘clear’’ sodium aluminosilicate solution, was investigated. For this purpose, synthesis experiments were conducted in an ultrasonic water bath for various synthesis times and temperatures, by using a reaction mixture composition known to yield pure zeolite A. The products were compared to those obtained by performing conventional static synthesis in a water bath under similar conditions.
2. Experimental Zeolite A in sodium form (NaA) was prepared from sodium aluminosilicate solutions with the composition of 50Na2O:Al2O3:5SiO2:1000H2O. This composition has been reported previously to produce zeolite A both in the bulk [17,18] and on substrates [19,20]. Sodium silicate solution (27 wt.% SiO2, Riedel-de Haen), granular sodium aluminate (Riedel-de Haen), anhydrous NaOH pellets (Riedel-de Haen) and deionized water were used in the synthesis experiments. NaOH was dissolved in deionized water, initially sodium aluminate and finally sodium silicate were added to the solution. Additions were made to clear solutions cooled to below 27 C, and the final reaction mixture, which was also clear to the eye, was stirred for 15 min at this temperature prior to the syntheses. Two different sets of experiments were performed in which zeolite A was synthesized in an ultrasonic water bath (35 kHz) and in a conventional water bath under static conditions, respectively. The hydrothermal crystallization was carried out at temperatures of 50 C and 60 C for various periods of time ranging from 2
to 15 h. The temperature of the ultrasonic water bath was controlled, by using a heater and a temperature controller. The product was filtered initially using a filter paper with fine mesh size. The filtrate was then refiltered through a 0.1 lm PVDF membrane. The mass of the collected solids was determined after washing thoroughly with deionized water and drying overnight in an oven at 65 C. X-ray diffraction (Phillips 1040 XRD) run with Cu Ka1 radiation in the 2h range of 5–50 was applied to all samples for the characterization of the phases present, and for the determination of the crystallinities of the zeolite A samples. The peak areas of the characteristic peaks of zeolite A, which do not overlap with the peaks of the other phases that formed as a result of metastable phase transformations, were used for this purpose. A commercial zeolite A sample (Degussa), was assumed to have 100% crystallinity, while the crystallinities for zeolite A of the synthesized samples were determined by taking into account the ratios of the sum of the peak areas to that pertaining to the commercial zeolite A sample. A rough estimation of the crystallization rates was made from the slopes of the crystallization curves at about 50% crystallinity. Induction periods were also estimated from the crystallization curves. The samples with the highest crystallinities were investigated by scanning electron microscopy (JEOL JSM-840 SEM) for the determination of the morphology of the zeolite A crystals. Particle size measurements for the same samples with the highest crystallinities, were performed by Laser Light Scattering Technique (Malvern Mastersizer 2000). From the particle size distribution curves obtained, the sizes for which the cumulative percentage undersize is equal to 10%, 50%, and 90% of the particles in the samples (d(0.1), d(0.5), and d(0.9), respectively) by volume and by number, could be determined. d(0.5) represents the median of the respective size distribution. The size span value, which is a measure of the width of the size distribution, and which is smaller for narrower distributions, was estimated by the help of the following equation: span ¼ ðdð0:9Þ dð0:1ÞÞ=dð0:5Þ
ð1Þ
Thermogravimetric analyses and adsorption experiments were carried out to further characterize the samples, especially those formed at the relatively earlier periods of synthesis, which were identified as amorphous or of low or partial crystallinity by XRD. Both methods were also applied to the commercial zeolite NaA sample (Degussa), in addition to some of the samples prepared in this study in the presence or absence of ultrasound. Prior to the thermogravimetric analyses, the samples were saturated in a controlled humidity atmosphere. The thermogravimetric analyses (Shimadzu TA-503) were carried out between ambient temperature and
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227
350 C, under a nitrogen flow of 20 cc/min. A heating rate of 20 C/min was used to raise the temperature of the zeolite samples to 350 C, after which the temperature was kept constant at this value for 30 min. Adsorption isotherms for water at 300 K were determined by using a constant volume apparatus. The samples were activated at 400 C for 6 h under a vacuum of 105 mbar, prior to the adsorption experiments.
Table 1 Phase composition from XRD of the solid phase synthesized at 50 C
No ultrasound
Ultrasound
6 7 8 9 10 11
100% Am 100% Am 17.3% A + 82.7% Am 33.5% A + 66.5% Am 43% A + 57% Am 55.8% A + 44.2% Am
3. Results and discussion
12
–
Zeolite NaA synthesis from clear solution was carried out with and without the application of ultrasound at two different temperatures and various times. XRD analysis was applied to all of the samples and the crystalline zeolite phases were identified accordingly. At early synthesis times, the samples were X-ray amorphous while the initial crystalline phase was identified to be zeolite NaA in all of the cases investigated. The nature of the X-ray amorphous phases will be discussed later in this study. It was possible to obtain highly crystalline zeolite A in the presence of ultrasound, as well as in its absence. At extended synthesis times, zeolite NaA started to transform into other phases, such as hydroxysodalite (HS), zeolite X and losod and the samples consisted of different combinations of these phases. The X-ray diffractograms of two of the samples prepared at the synthesis temperature of 50 C under ultrasound are shown in Fig. 1. The diffractograms given in Fig. 1(a) and (b) correspond to the samples obtained at synthesis times of 10 h and 12 h, respectively. It may be seen that at this temperature, conversion to other phases, namely, HS and losod, has started when the synthesis was extended to longer than 10 h. The solid phase compositions based on XRD, obtained from the reaction mixture at different synthesis times at 50 C and 60 C are shown in Tables 1 and 2,
13 14 15
72.7% A + 27.3% Am 69.1% A + 30.9% (X + Am) 66.6% A + 33.4% (X + Am)
100% Am 15.9% A + 84.1% Am 37.2% A + 62.8% Am 59.1% A + 40.9% Am 74% A + 26% Am 87.6% A + 12.4% (S + L + Am) 76.8% A + 23.2% (S + L + Am) – – –
Fig. 1. X-ray diffractograms of the samples obtained at 50 C in the presence of ultrasound after (a) 10 h, and (b) 12 h of synthesis (L: losod, S: hydroxysodalite).
Time (h)
Phase composition
Am: amorphous aluminosilicate; A: zeolite A; X: zeolite X; S: hydroxysodalite; L: losod.
Table 2 Phase composition from XRD of the solid phase synthesized at 60 C Time (h)
Phase composition No ultrasound
Ultrasound
2 3 4 5 6 7 8
100% Am 100% Am 21.7% A + 78.3% Am 47.9% A + 52.1% Am 69.6% A + 30.4% Am 84.8% A + 15.2% Am 94.3% A + 5.7% Am
9
79.1% A + 20.9% (S + L + Am)
100% Am 18.7% A + 81.3% Am 48.4% A + 51.6% Am 76.5% A + 23.5% Am 94.2% A + 5.8% Am 95.8% A + 4.2% Am 84% A + 16% (X + S + L + Am) –
Am: amorphous aluminosilicate; A: zeolite A; X: zeolite X; S: hydroxysodalite; L: losod.
respectively. It may be observed that applying ultrasound to zeolite synthesis led to some changes in the stability areas of the different zeolite phases. The appearance of zeolite A followed the formation of Xray amorphous material at both temperatures, irrespective of whether ultrasound was applied or not. However, a movement towards earlier synthesis times was observed for the synthesis in the presence of ultrasound. Another difference observed when ultrasound was applied to zeolite synthesis seemed to be in the type(s) of the zeolites formed when zeolite A began to transform into other phases. Zeolite A transformed into zeolite X and into HS + losod mixture at 50 C and 60 C, respectively, when no ultrasound was applied. In the presence of ultrasound, on the other hand, HS + losod mixture and X + HS + losod mixture replaced zeolite A at the lower and higher temperatures, respectively. Zeolites are known to transform into thermodynamically more stable phases as time proceeds [21,22]. Under the experimental conditions used, zeolite A was seen to be transformed into zeolites HS, X, and losod, with
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overlapping crystallization fields. The phase composition is a function of the relative rates of nucleation and crystal growth, determined by temperature and stirring conditions, of the competing zeolite types in the reaction mixture. Performing the synthesis with stirring has been observed to result in the formation of different zeolite phases when all the other synthesis parameters, including temperature and time, were kept constant [23]. The effect of ultrasonic stirring on the phase composition of the product, in this respect, is not surprising. The crystallinities of the samples with respect to zeolite A, calculated from XRD analyses of the samples, given in Tables 1 and 2, are plotted against the synthesis times in Fig. 2, in order to show that the crystallization curves allow the estimation of induction times and crystallization rates with reasonable accuracy. Table 3 lists the induction times and crystallization rates estimated from the XRD data as explained in the previous section. It may be observed clearly from the results presented that in the presence of ultrasound, induction times decreased and the crystallinity of zeolite A started to develop at earlier synthesis times, indicating an increase in the nucleation rates in the presence of ultrasound. The effect of ultrasound on the nucleation rate is expected to be related to cavitation. It has been suggested that each cavitational collapse may create a nucleation site analogous to that from a trace particle or a surface imperfection [8]. It may also be seen from Fig. 2 and Table 3 that
100
Zeolite A (%)
80
60
40 50°C No Us 50°C Us
20
the application of ultrasound led to a significant increase in the estimated crystallization rates of zeolite A. This effect was more apparent at the lower synthesis temperature of 50 C, where the relative rate of growth with respect to nucleation is smaller than that at the higher temperature, while the decrease in induction time was more pronounced at the higher temperature. The increase observed in the crystallization rate is an indication of an increase in the rate of crystal growth with ultrasound application. Ultrasound is known to result in acoustic streaming providing enhanced mass transfer close to the crystal surface [9], which is expected to increase the crystal growth rate. The increase obtained in the nucleation rate in the presence of ultrasound, however, may also have contributed to the increase in the crystallization rate to an extent, since it is known that nucleation can prevail even at the later periods of synthesis, where crystal growth is generally the dominating process. The effect of ultrasound on the yield of zeolite A (in g zeolite A per g reaction mixture, estimated from product mass · crystallinity from XRD) is shown in Fig. 3, for the synthesis temperatures of 50 C and 60 C. It may be observed from the figure that, at both temperatures, zeolite A yield increased significantly when ultrasound was applied to the reaction mixture; but the effect was much more pronounced at the lower temperature of 50 C. Zeolite A yield at 50 C after 10 h, for example, seems to have increased by more than 3 times when ultrasound was applied. At the higher temperature of 60 C, on the other hand, it seems that the rapid transformation of zeolite A into other phases limited the amount of this zeolite obtained under the given conditions. Fig. 4 shows the SEM pictures of the samples with the highest crystallinities, and having zeolite A as the only crystalline phase, prepared at 50 C, with and without the application of ultrasound. Fig. 4(a) corresponds to the sample prepared after 13 h in the absence of ultra-
60°C No Us 60°C Us
0
0.005
0
2
4
6
8 Time (h)
10
12
141
16
Table 3 Estimated induction times and crystallization rates of zeolite A at two temperatures in the absence and presence of ultrasound Temperature (C)
50 60
Induction period (h)
Crystallization rate (h1)
No ultrasound
Ultrasound
No ultrasound
Ultrasound
6.6 3.2
6.3 2.5
11.4 21.3
22.2 24.2
Yield (g/g)
0.004
Fig. 2. Variation of crystallinity as estimated from XRD analyses with respect to synthesis time at 50 C and 60 C in the absence and presence of ultrasound.
0.003
0.002 50°C No Us 50°C Us
0.001
60°C No Us 60°C Us 0 0
2
4
6
8
10
12
14
16
Time (h)
Fig. 3. Variation of the zeolite A yield of the samples with respect to synthesis time at 50 C and 60 C in the absence and presence of ultrasound.
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229
Fig. 4. SEM pictures of the zeolite A samples obtained at 50 C (a) after 13 h of synthesis in the absence of ultrasound, and (b) after 10 h of synthesis in the presence of ultrasound.
Fig. 5. SEM pictures of the zeolite A samples obtained at 60 C (a) after 8 h of synthesis in the absence of ultrasound, and (b) after 7 h of synthesis in the presence of ultrasound.
sound, and Fig. 4(b) to that prepared after 10 h of synthesis in the presence of ultrasound, respectively. The difference between the average particle sizes of these samples, for which the crystallinity values estimated from XRD are very close, 72.7% and 74%, respectively, can be easily inferred from the figures. On the other hand, the SEM pictures of the zeolite A samples with the highest crystallinities obtained at 60 C are given in Fig. 5. Fig. 5(a) and (b) correspond to the samples synthesized in the absence of ultrasound after 8 h (crystallinity 94.3%) and in the presence of ultrasound after 7 h (crystallinity 95.8%), respectively. Though less pronounced at this higher temperature, the particles in the sample prepared in the presence of ultrasound were again larger, in general. Aggregation is seen to take place both in the absence and presence of ultrasound, and is especially evident at the higher temperature from the SEM pictures. The particle size distributions (PSDs) of the same samples with the highest crystallinities estimated from XRD, and having zeolite A as the only crystalline phase
(for which SEM pictures were shown), prepared at 50 C and 60 C are given in Figs. 6 and 7, respectively. These experiments were repeated for several times and reproducibility was confirmed. In parallel with the SEM observations, in the presence of ultrasound, PSDs shifted to the right to larger particle sizes, as a result of decreased induction times and increased crystal growth rates. The size span values (by number) decreased from 1.137 to 1.096 at 50 C, and from 2.088 to 1.601 at 60 C, hence somewhat narrower PSDs were obtained with the application of ultrasound. The XRD data presented above indicate that an amorphous phase is formed prior to nucleation and crystal growth, from this so-called ‘‘clear solution’’. However, nuclei were implied to form and grow from the clear sodium aluminosilicate solution with the same composition as the one used here, in a previous study [18]. Since it is known that sufficiently long range order is necessary for crystallinity to be detected by XRD, the X-ray amorphous and partly crystalline solids were characterized further by adsorption and TGA analyses.
¨ zlem Andac¸ et al. / Microporous and Mesoporous Materials 79 (2005) 225–233 O 20
18
18
16
16
14
14 Number (%)
20
12 10 8
8 6 4
2
2 0.1
(a)
1
10
0 0.01
100
Particle size (µm)
20
18
18
16
16
14
14
12 10 8
4
2
2 0 0.01
100
Particle size (µm)
Fig. 6. Particle size distributions by number, corresponding to the samples obtained at 50 C (a) after 13 h of synthesis in the absence of ultrasound, and (b) after 10 h of synthesis in the presence of ultrasound.
The adsorption isotherms of water at 300 K for the commercial zeolite NaA sample and some other samples prepared in this study are shown in Fig. 8. The samples investigated were those prepared at synthesis times of 3 h, 5 h, 7 h and 10 h in the presence of ultrasound, and 5 h, 7 h, 9 h and 13 h in the absence of ultrasound, all at 50 C. These samples were identified as either amorphous or as zeolite A of low or partial crystallinity by XRD. The shapes of the adsorption isotherms varied and the adsorption capacities increased gradually with the synthesis time as the amorphous materials were transformed into zeolite A. The X-ray amorphous materials at the relatively earlier periods of synthesis, especially those prepared with ultrasound, seemed to exhibit step-wise isotherms similar to those of mesoporous materials, with the steps less sharp and at higher P/ P0 ratios, indicating the presence of wider pore size distributions and larger pore sizes, respectively. The steps in the isotherms of the X-ray amorphous materials gradually disappeared and the isotherms became more flattened with increased synthesis times. The isotherms of the samples determined to be partly crystalline by XRD, resembled more the Type I isotherm obtained
10
100
8 6
10
100
10
4
1
10
12
6
0.1
1 Particle size (µm)
20
0 0.01
0.1
(a)
Number (%)
Number (%)
10
4
0 0.01
(b)
12
6
0.1
1
(b)
Particle size (µm)
Fig. 7. Particle size distributions by number, corresponding to the samples obtained at 60 C (a) after 8 h of synthesis in the absence of ultrasound, and (b) after 7 h of synthesis in the presence of ultrasound.
25
20 mmol H 2O/g-zeolite
Number (%)
230
15 No Us 5h No Us 7h No Us 9h No Us 13h Us 3h Us 5h Us 7h Us 10h Commer..
10
5
0 0
0.2
0.4
0.6
0.8
1
1.2
P/ P0
Fig. 8. Adsorption isotherms for water at 300 K of the commercial zeolite A sample and the samples obtained at 50 C in the absence and presence of ultrasound.
for the commercial sample, but were somewhat less rectangular, exhibited lower adsorption capacities and higher rates of increase in the amount adsorbed with
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Table 4 Crystallinity and microporosity values of the samples that are synthesized at 50 C, estimated from XRD, TGA and adsorption data Sample
% Crystallinity from XRD
% Microporosity from TGA
% Microporosity from adsorption
No Us, 5 h No Us, 7 h No Us, 9 h No Us, 13 h Us, 3 h Us, 5 h Us, 7 h Us, 10 h
0 0 33.5 72.7 0 0 15.9 74
17.1 41.4 73.2 87.8 14.6 20.7 79.9 92.6
12.5 32.5 76.3 88.1 6.3 13.1 67.5 95.2
0.001
0 0
50
100
150
200
25 250
300
-0.001 DTG (mg/sec)
respect to the relative pressure, above the relative pressures corresponding to the filling of micropores. This indicates the presence of still larger external surface areas—and larger pores—in these samples, when compared to those in the fully crystalline sample. Micropores with diameters comparable to the size of the molecules adsorbed, are known to be filled at low relative pressures (P/P0 < 0.01) [24]. Larger micropores (up to 2 nm) are filled in the relative pressure region between P/P0 0.01–0.2. Since the t-plot method, which is conventionally used for the determination of micropore volume could not be used in this study due to the lack of statistical thickness values with respect to pressure for water on materials similar to zeolite A, the ratio of the adsorption capacity at the P/P0 value of 0.01, of each sample to that of the commercial NaA was used as an estimate of microporosity from the adsorption data, assuming that the commercial sample is fully crystalline. The % microporosity values estimated from the adsorption isotherms are listed in Table 4, together with those estimated from thermogravimetric analyses as will be discussed below, and with XRD crystallinities, for comparison. The results presented clearly indicate that micropore development starts earlier to take place in the X-ray amorphous phase, and is speeded up with the application of ultrasound. The samples prepared in the presence of ultrasound systematically exhibited higher adsorption capacities at low relative pressures— and seemingly higher micropore volumes—when compared to those obtained in the absence of ultrasound at the same synthesis times. Thermogravimetric analyses (TGA) were carried out to further inspect the X-ray amorphous and partly crystalline samples. The derivative TGA curves (DTG curves) of the samples synthesized at 50 C after 5 h, 7 h, and 10 h of reaction in the presence of ultrasound are given in Fig. 9. It may be observed from the figure that the curve for the amorphous sample was quite different than those of the crystalline and partly crystalline samples. In all curves, there was a minimum below or at about 100 C, which corresponds to the removal of loosely held moisture. The intensity of this lower tem-
231
-0.002 Us 5h -0.003
Us 7h Us 10h
-0.004
Commercial -0.005 Temperature (°C)
Fig. 9. DTG curves of the commercial zeolite A sample and the samples obtained at 50 C in the presence of ultrasound.
perature minimum was observably higher for the amorphous sample. All the DTG curves exhibited a second minimum at or above 150 C, which has been related [25] to the presence of ordered structural units having a channel/void system similar to that of zeolites in the sample. The intensities of the higher temperature minima on the DTG curves of the partly and fully crystalline samples were much higher when compared to that of the amorphous sample. The ratio of the intensity of the higher temperature minimum to that of the lower temperature minimum increased parallel to the X-ray crystallinity of the sample. The % microporosity values estimated from the TGA results are listed in Table 4. The ratio of the percent weight lost by each sample, above the temperature corresponding to the inflection point following the low temperature minimum of the DTG curve, to that of the commercial NaA sample was used in the estimations. In accordance with the results obtained from adsorption experiments, TGA results also indicate microporosity values higher than those implied by XRD crystallinity for all samples, and higher microporosity values for the samples prepared in the presence of ultrasound than for those prepared in its absence. It is clear that the processes taking place early in the amorphous phase, leading to the development of microporosity, are speeded up by the application of ultrasound. The agreement between the microporosities estimated from adsorption and TGA is seen to improve as the amount of crystalline material increases in the samples. Above X-ray crystallinities of about 30%, the agreement seems to be very good. Below X-ray crystallinities of about 30%, TGA estimates an even higher microporosity than adsorption. The microporosities estimated from adsorption are plotted against synthesis times in Fig. 10. The figure implies that the development of microporosity may be starting at or very close to the beginning of synthesis at 50 C both in the absence and presence of ultrasound, and is speeded up by sonication.
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232 100 90 80
Capacity (%)
70 60 50 40 30
No Us
20
Us
10 0 0
3
6
9
12
15
Time (h)
Fig. 10. Estimated microporosity development with respect to synthesis time for the samples obtained at 50 C in the absence and presence of ultrasound.
As was mentioned above, and as can be seen from Table 4, the microporosities estimated from adsorption and TGA experiments were systematically and significantly higher than those implied by the crystallinities obtained from XRD. Since the micropore development in the Xray amorphous phase is expected to be strongly related to the nucleation and growth of zeolite A crystals, it is possible that zeolite crystals may be nucleating starting from quite early times of synthesis, and may be contributing to the adsorption capacity as they grow in number and size, while they remain undetected by XRD below certain concentration and size limits. In this case, nucleation and growth start much earlier than that estimated by XRD, and the induction times estimated from XRD may have a limited meaning, and may only be used on a comparative basis. On the other hand, the differences between the XRD crystallinities and the microporosities estimated by adsorption and TGA experiments are so large in some cases (see for example, the significant microporosities implied by the isotherms of the samples obtained after 7 h at 50 C in the absence and presence of ultrasound) that it also seems highly possible that the amorphous material might also have contributed to the microporosity of the samples, which may be implying the formation (rather than the initial presence in the amorphous phase, due for example to aggregation of the 1 nm particles observed to be present in the synthesis solutions [18]) of microporous precursor structures/domains in the amorphous material prior to nucleation. Although the results show clearly that microporosity starts to develop in the samples much earlier than that implied by XRD crystallinity, they also document the presence of amorphous material during early times of synthesis, as opposed to the implications of the results of the previous study carried out with the same reaction mixture composition, mentioned above [18]. Small particles could be detected in the referred study by QELSS
at 60 C when their average sizes were about 200 nm, and these particles were observed to grow later with linear rates. Induction time was obtained by extrapolating the growth curve to zero particle size and was less than an hour at 60 C with no aging. Presence of an X-ray amorphous phase during any stage of synthesis at this temperature was not mentioned. However, it was mentioned that light-scattering spectra could not be obtained until after 65 min at the beginning. It was also reported that the distribution of the particle sizes obtained at synthesis times up to 85 min was wide, but decreased with increasing synthesis time. The formation of a precursor phase at room temperature was however evident in the same study. Our observation of the presence of an amorphous phase for extended periods from the same initial composition at 60 C must be related to the differences in the reactants and in the procedures used during the experiments. That the reaction mixtures were filtered through 0.2 lm membranes prior to the syntheses in the referred study, and that we have stirred the initial mixture for 15 min at room temperature, seem to be some of the important differences to us. One explanation of the events taking place in the sodium aluminosilicate solution initially clear to the eye, used in this study and the previous one [18], that may also explain the difference in the observations made in these studies, may be that no matter what their initial sizes are, or forced to be by filtration, X-ray amorphous precursor particles are involved in this zeolite A synthesis system. They form at room temperature. Their sizes and size distribution depend on the reactants and procedures used for the preparation of the reaction mixture. Filtration of the reaction mixture through membranes prior to the syntheses in the referred study [18] might have decreased the maximum size of these particles to 200 nm at the beginning of the heating period. Nucleation and then growth processes, with their rates depending on the temperature, start early, within these particles and continue during the heating and synthesis stages. Dissolution and reorganization of the X-ray amorphous phase is also accelerated with temperature. This stage may be completed shortly after heating to the reaction temperature or even earlier, in case the initial sizes of the amorphous material are small [18], or may last longer as was observed in the present study.
4. Conclusions Highly crystalline zeolite A could be synthesized from a clear-to-the-eye aluminosilicate solution in the presence of ultrasound. Sonocrystallization has been demonstrated to offer the possibilities of increasing the nucleation and crystallization rates of zeolites, improving the yield and particle size distribution of the product crystals, and directing the synthesis towards different
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crystal phases. Investigating the effects of varying the frequency and power of ultrasound and applying sonocrystallization to different zeolite synthesis systems are necessary in order to develop a better understanding of the range of possibilities offered by the technique. Thermogravimetric analyses and water adsorption experiments revealed that the development of micropore adsorption capacity started very early in the X-ray amorphous samples obtained from this so-called ‘‘clear’’ solution, and that the processes taking place in the Xray amorphous phase leading to the increase in microporosity, were also speeded up with the application of ultrasound.
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