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Nucleation and Growth of NH4-ZSM-5 Zeolites
Nucleation and Growth of NH 4-ZSM-5 Zeolites Liang-Yuan Hou, Leonard B. Sand, and Robert W. Thompson Department of Chemic-a1 Engineering, Worcester Polytechnic Institute, 01609 USA
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The aim of this study was to improve the understanding of nucleation and crystal growth of NH4-ZSM-5 zeolites having very high silica content, using the (TPA)20-(NH4)20-A1203-Si02-H20 system. Our data show that: (a) the concentration of [OH-] promotes the dissolution rate of amorphous gel and the crystal growth rate, (b) the presence of A1203 strongly inhibits crystal nucleation and enhances the final crystal size, (c) both the final crystal size and the maximum peak size of the crystal size distribution decrease with increasing TPA content, and (d) the crystal growth rate is not strongly temperature dependent in this AI-free system. INTRODUCTION In order to use ZSM-5 as a catalyst, such as in the conversion of methyl alcohol to hydrocarbons, frequently the hydrogen form is desired [1]. Making H-ZSM-5 directly from ammonium hydroxide without using any alkali metal cation was reported by Bibby, et al. [2]. The development of a mathematical model for hydrothermal zeolite synthesis based on the population balance was published previously [3]. The computer simulation results provided an interpretation of the nucleation and growth of zeolites in relation to the crystal size distribution of the final product. Several investigations have reported on a systematic study of the influence of individual factors on the course of crystallization [4-6], however, none report any results related to crystal size distribution. In highly silicious ZSM-5 synthesis the amorphous gel in intermediate samples can be dissolved away selectively, while leaving the crystals unaffected. This feature permitted us to study NH4-Z5M-5 crystallization by analyzing the evolution of the crystal size distribution. The aim of the present study was to use the analysis of the crystal size distribution to understand the nucleation and the crystal growth of NH4-ZSM-5 as a function of reagent compositions and temperature. EXPERIMENTAL The NH4-Z5M-5 zeolites were synthesized hydrothermally from supersaturated alkali solution using Ludox silica solution, aluminum hydroxide, ammonium hydroxide, tetrapropylammonium bromide, and deionized water. The control batch formula studied was: 4(TPA) 20-60(NH4) 20-90Si02-750H20. Three reagent concentrations [5i02. A1203' (TPA)20] were varied at a temperature of 453°K. A control batch composition also was reacted at 423°K and 438°K. The reaction times were form 4 to 480 hours in modified Morey type autoclaves at autogeneous pressure without agitation. The X-ray crystallinity was calculated from the sum of areas of the five peaks between 28= 22 to 28 =25° [7]. The average crystal size and the crystal size distributions were measured with a Particle Data Electrozone Colloscope model 80XY. In order to avoid counting amorphous gel particles. the gel was dissolved in a 1.5M NaOH solution from several minutes to several hours. depending on the crystallinity of zeolite. In independent tests we determined that even more severe caustic trea~ ments did not affect the crystal size distribution. 5EM photomicrographs were made of selected samples. 239
240 (SY-12-2) RESULTS AND DISCUSSION The crystalline product obtained from each system was always ZSM-5 without other crystalline phases. The crystals were large lath- shape well- developed and double terminated single crystals similar to those observed in mixtures containing NH4+ ions crystallizing under "dilute" conditions [8]. The crystals usually grow at different rates in three dimensions, thus it is impossible to determine lineAr growth rate of those crystals. For different sy£tems, the relative lengths of the three coordinate axes differ, thus it is difficult to compare crystal sizes from different systems. When using the particle size analyzer, the equivalent spherical diameter was reported for crystal size [9]. This particle size analysis was more accurate than using optical or electron microscope techniques, since more particles were measured and particle orientation was not a concern. Figure 1 shows the crystallization curves obtained on starting gel compositions 4(TPA)20-60(NH4)20-XSiOZ-750HZO for X=30, 60, 90 and 108. It can be seen that the induction period is short and the % crystallization rate is rapid at low Si02 content. Figure Z shows the average crystal size as a function of time. The final average crystal size increases with increasing concentration of SiOZ' It should be noted that XRD meaAurpmpnts arp based on the volume or mass of solid product. The induction period is defined as the time necessary for a sufficient amount of crystal to be formed so as to allow detection by X-ray diffractometer scan. In other words, the "X-ray amorphous" solid may contain either a numerous quantity of tiny crystals or a few large crystals, both representing a small mass of the product mixture [10, 11]. From Figure 1 and 2 at X = 108, while only 1% is crystallized after Z4 hr., the average crystal size is about 5~m. From optical microscope observations we noted that only several large crystals were present in the sample. Thus, it appears the NH4-ZSM-5 zeolite belongs to the latter case.
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80 60 40 20 1Z0 TIME (hours) Fig. 1. % Crystallinity vs. time at 180 aC for 4(TPA)ZO-60(NH4)ZOXSi02-750HZO. X = 30(_), 60(4), 90(_), 108(.). Note also in Figures 1 and 2 that for X = 30 the XRD measurements suggest that 100% crystallinity is reached at about 40 hours. From that time to ZOO hours the average crystal size went from about 15]J m to ZZ]J m, While that represents about a 1.5 fold increase in the average linear dimension, it suggests more than a tripling of the mass of crystals during that time. These results can only be understood in the context of a solution facilitated crystal growth mechanism. Similar results were noted for many experiments. The crystal properties of starting gel compositions with different SiOZ content are compared in Table I. The (OH/SiOZ) was calculated from: (number of NH40H moles) (number of SiOZ moles)
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Effect of Si02 content on product properties
X Si02 OH/Si02 Initial PH Final crystal size Total crystal Linear growth rate Max peak of PSD
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2 1.33 1.11 11.2 11.4 11.0 22.34 28.48 39.42 41.43 3) 1.04xl0 6 9.59xl0 5 5.82xl0 5 5.33xl0 5 0.6 0.45 0.38 0.31 24.84 29.67 39.91 39.91 4 1l.8
The total crystal number was estimated by: Weight of Si02/Total volume of reagent mixtures Volume of final crystal X crystal density (1.81) The rate of crystal growth is an autocatalytic process. The decrease in the rate of crystallization observed above 50% conversion is caused by the decrease in reactant concentration in the solution. even though the cumulative crystal surface is still increasing. The average crystal growth rate is defined here as: Average crystal size at 50% crystallinity Reaction time at 50% crystallinity With increasing concentration of Si02, the linear growth rate and the total crystal number strongly decreased. but the maximum peak size of the PSD increased, resulting from longer crystal growth time. Those phenomena, due to the initial value of pH or ratio of (OH/Si02), decreased as a result of the slower dissolution rate of amorphous gel. The final crystal size increased with increasing silica content, which depends primarily on the pH of the solution [12, 13]. The average crystal size of the NH4 system is much larger than that of the alkali (Na+, K+) system ([14, 15]. In aqueous solution, the interaction of NH4+ ion with the surrounding water molecules is weak so that the "hydrated structure" of NH4+ value is 1.54A, which is much smaller than that of the alkali cations [16]. The interaction between the aluminosilicate anions and the positively charged species in the synthesis mixture will be the most important in the case of NH4+ ions, with respec.t to far bigger TPA+. As a result, the probability of TPA+-aluminosilicate associations occurring will be reduced. Very few nuclei will be formed and they will grow slowly, yielding very large crystallites at the end of the process, as observed.
Z4Z (SY-lZ-Z) Plotted in Figure 3 are the crystalli7ation curves obtained with starting compositions 4(TPA)ZO-60(NH4)ZO-XAlZ03-90SiOZ-750HZO for X = 0, 0.5, 1.0, Z.O and Z.5. Figure 4 shows the average crystal size as a function of time. The crystal properties of different AlZ03 contents are given in Table Z. The increase in AlZ03 content caused a sharp decrease in crystal growth rate and total crystal number, but the final average crystal size and induction time increased with increasing amount 'of AlZ03' This is opposite to results of Romannikou et al. [17] and Debras et al. [18]. In these two systems, the concentrations of Na+ and TPA+ cations were increasing AlZ03 content. Both cations facilitate nucleation [19], many nuclei were formed, and the final crystal size decreased.
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TIME (hours) Fig. 4. Average crystal size vs. time for same experiments as in Figure 3. AlZ03 content strongly inhibited the rate of nucleation of zeolite, because of OH--Al interactions and reduction of the ability of OH- to depolymerize polys ilicates. The result was formation of few nuclei, yielding very large crystals at the end of the process. Higher AlZ03 content resulted in slower nucleation rate and longer growth time, thus forming a broader crystal size distribution. TPA cations are recognized to form complexes with a aluminosilicate and silicate species, and subsequently to cause replication of the framework structure via stereospecific interaction [ZO]. This phenomena is called a templating effect. During the process, TPA ions are incorporated and stabilized within the zeolite framework
L.-Y. Hou, LoB. Sand and R.W. Thompson Table 2.
243
Effect of A1203 content on product properties
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X A1203
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Si02/A1203 11.3 Initial PH Final crystal size ( u) 39.42 Total crystal number (l1/cm3) 5.82xl0 5 Linear growth rate ( u/hr ) 0.38 Max peak of PSD (lJ) 39.91
1.0
180 11.2 45.74 3.76xl0 5 0.33 47.1
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90 11.2 47.14 1. 8xl0 5 0.26 49.34
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45 36 11.1 11.0 42.59(93%) 37.23(40%) 0.12 50.12
0.16 51. 69
[20,21]. Figure 5 shows the crystallization curves of ZSM-5 from reaction mixtures with different TPA contents. Figure 6 shows the average crystal size as a function of time. It can be seen that the rate of % crystallinity, linear growth rate, and the induction time strongly depend on TPA content only for TPA/Si02 ratios below 0.09, while for the ratio above 0.09 the rate of % crystallinity is not affected by TPA content. Howden et al. [23] have shown that a value of 0.08 is required to fill the whole zeolite pore volume. The presence of nearly 4 TPA cations per unit cell of crystalline ZSM-5 has been confirmed experimentally [24]. From Table 3 both the final crystal size and the maximum peak size of the crystal size distribution decrease with increasing TPA content. The abundance of submicron crystals produced from starting compositions with high TPA content shows the effect of rapid nucleation. The larger number of nuclei formed consume the nutrients in the mixture very rapidly, so a large number of small crystals are formed, preventing the growth of a smaller number of larger crystals. 1001_----.
120
180
240
300
TIME (hours) Fig. 5. 750H20.
% Crystallinity vs. time at 180°C for X(TPA)20-60(NH4)20-90Si02X 0.5(0), 1.0CA), 2.0(a), 4.0(&), 6.0(_), 8.0(e), 10.0(.).
Table 3. X(TPA)20
Effect of (TPA)20 content on product properties 1.0
2.0
4.0
6.0
8.0
10.0
0.022 0.048 0.088 0.133 0.178 0.222 TPA!Si02 Initial PH 11.4 11.3 11.3 11.2 11.2 11.1 25.14 29.56 39.42 Final crystal size (u ) 36.29 34.66 28.37 Total crystal number (11/c,3) 5.82xl0 5 7.54x10 5 8.66xl0 5 1. 57x10 6 0.08 0.16 0.38 0.41 0.40 Linear growth rate (lJ/hr) 0.39 Max peak of PSD (lJ) 30.74 32.44 39.91 36.55 34.63 28.88 To study the effect of temperature on nucleation and crystal growth rate of zeolite NH4-ZSM-5, the reaction mixture: 4(TPA)20-60(NH4)20-90Si02-750H20 was reacted
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360 TIME (hours) Fig. 6. Average crystal size vs. time for same experiments as Figure 5. at three different temperatures. Crystallization curves from this composition at 423°K, 438°K, and 453°K are presented in Figure 7. Figure 8 shows the average crystal size as a function of time. It can be noticed that with increasing temperature the induction period decreased, resulting from faster dissolution rate of amorphous gel. From Figure 8 and Table 4 both the % crystallinity rate and the crystal growth rate are not strong functions of temperature, which is opposite to results from alkali systems [14, 25]. The concentration of silicate anion increases with increasing temperature and should accelerate the crystal growth. However, the NH4+ ions interact more strongly with silicate anions at higher temperature, thus the probability of TPA+ silicate interaction occuring will be reduced. Thus, NH4+ ion inhibits nucleation and crystal growth. These two opposite effects compete with each other, therefore temperature changes only influence crystal growth rate in a minor way. The final crystal size is larger and crystal number is less at higher temperature.
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Effect of temperature on product properties
Temperature
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165
150
Final crystal size 39.42 37.66 36.29 3) 5.82xl0 5 6.67xl0 5 7. 45xl0 5 Total crystal 0.38 Linear Growth rate (u /hr ) 0.36 0.33 Max peak of PSD 39.91 36.97 34.96
ACKNOWLEDGEMENTS Support of the National Science Foundation (grant CBT-8500828) and of WPI Department of Chemical Engineering is great fully acknowledged. This manuscript is dedicated to the memory of Professor Leonard B. Sand who passed away during the course of this study. REFERENCES 1. C.D. Chang and A.J. Silvestri, J. Cayal. 41, 249 (1977). 2. D.M. Bibby, N.B. Milestone and L.P. Aldridge, Naute, 285, 30 (1980). 3. R.W. Thompson, and A. Dyer, Zeolite, 5, 292 (1985. 4. M. Ghamami and L.B. Sand, Zeolite, 3,-155 (1983). 5. A. Nastro, Rend. Ace. Sci. Fis. Mat~, Napoli, Italy, 50, 211 (1984). 6. Z. Gabelica, N. Blom and E.G. Derouane, Appl. Catal. 5, 227 (1983). 7. K.J. Chao, T.C. Tasi and M.S. Chen, J. Chern. Soc. Faraday Trans. I, 77, 547 (1981). 8. R. Von Ballmoos and W.M. Meier, Nature, 289, 782 (1981). 9. R.F. Karuhn and R.H. Berg, "Practical Aspects of Electrozone Size Analysis" Particle Data Lab., Ltd., (1982). 10. A. Ausoux, H. Dexpert, C. Ledeng and J.C. Vedine, Appl. Catal. ~, 95 (1983). 11. P.A. Jacobs, J.C.S. Chern. Comm. 591, (1981). 12. J.L. Casci and B.M. Lowe, Zeolite, 1, 186 (1983). 13. S.G. Fegan and B.M. Lowe, J.C.S. Chern. Comm., 437 (1984). 14. L.Y. Hou and L.B. Sand, Proc. Sixth IntI. Conf. on Zeolites, p. 887 (1983). 15. R. Mostowicz and L.B. Sand, Zeolite, 3, 219 (1983). 16. Y. Marcus, "Introduction to Liquid State Chemistry", Wiley, New York (1983). 17. V.N. Romannikov and V.M. Mastilhin, Zeolite, 3, 311 (1983). 18. G. Debras, A. Gourgue and J.B. Nagy, Zeolite,-5, 369 (1985). 19. Z. Gabelica, N. Blom and E.G. Derouane, Appl. Catal. ~, 227 (1983).
245
246 (5Y-12-2) 20. 21. 22. 23.
E.G. Derouane, J.B. Nagy, Z. Gabelica and N. Blom, Zeolites, 2, 299 (1981). L.D. Romann in "Inorganic Compounds with Unusual Properties" Vol. II, p , 387. E.M. Flanigen, Pure Appl. Chern., 52, 2191 (1980). M.G. Howden, "The Role of Tetrapropylarnoniurn Template in the Synthesis of ZSM-5", CSIR Report CENG 413, Pretoria, (982). 24. J.B. Nagy, Z. Gabelica and E.G. Derouane, Zeolite, 3, 43 (1983). 25'. V. Lecluze, M.S. Thesis, Worcester Polytechnic Institute, U.S.A. (1979).
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The aim of this study was to improve the understanding of nucleation and crystal growth of NH4-ZSM-5 zeolites having very high silica content, using the (TPA)20-(NH4)20-A1203-Si02-H20 system. Our data show that: (a) the concentration of [OH-] promotes the dissolution rate of amorphous gel and the crystal growth rate, (b) the presence of A1203 strongly inhibits crystal nucleation and enhances the final crystal size, (c) both the final crystal size and the maximum peak size of the crystal size distribution decrease with increasing TPA content, and (d) the crystal growth rate is not strongly temperature dependent in this AI-free system. INTRODUCTION In order to use ZSM-5 as a catalyst, such as in the conversion of methyl alcohol to hydrocarbons, frequently the hydrogen form is desired [1]. Making H-ZSM-5 directly from ammonium hydroxide without using any alkali metal cation was reported by Bibby, et al. [2]. The development of a mathematical model for hydrothermal zeolite synthesis based on the population balance was published previously [3]. The computer simulation results provided an interpretation of the nucleation and growth of zeolites in relation to the crystal size distribution of the final product. Several investigations have reported on a systematic study of the influence of individual factors on the course of crystallization [4-6], however, none report any results related to crystal size distribution. In highly silicious ZSM-5 synthesis the amorphous gel in intermediate samples can be dissolved away selectively, while leaving the crystals unaffected. This feature permitted us to study NH4-Z5M-5 crystallization by analyzing the evolution of the crystal size distribution. The aim of the present study was to use the analysis of the crystal size distribution to understand the nucleation and the crystal growth of NH4-ZSM-5 as a function of reagent compositions and temperature. EXPERIMENTAL The NH4-Z5M-5 zeolites were synthesized hydrothermally from supersaturated alkali solution using Ludox silica solution, aluminum hydroxide, ammonium hydroxide, tetrapropylammonium bromide, and deionized water. The control batch formula studied was: 4(TPA) 20-60(NH4) 20-90Si02-750H20. Three reagent concentrations [5i02. A1203' (TPA)20] were varied at a temperature of 453°K. A control batch composition also was reacted at 423°K and 438°K. The reaction times were form 4 to 480 hours in modified Morey type autoclaves at autogeneous pressure without agitation. The X-ray crystallinity was calculated from the sum of areas of the five peaks between 28= 22 to 28 =25° [7]. The average crystal size and the crystal size distributions were measured with a Particle Data Electrozone Colloscope model 80XY. In order to avoid counting amorphous gel particles. the gel was dissolved in a 1.5M NaOH solution from several minutes to several hours. depending on the crystallinity of zeolite. In independent tests we determined that even more severe caustic trea~ ments did not affect the crystal size distribution. 5EM photomicrographs were made of selected samples. 239
240 (SY-12-2) RESULTS AND DISCUSSION The crystalline product obtained from each system was always ZSM-5 without other crystalline phases. The crystals were large lath- shape well- developed and double terminated single crystals similar to those observed in mixtures containing NH4+ ions crystallizing under "dilute" conditions [8]. The crystals usually grow at different rates in three dimensions, thus it is impossible to determine lineAr growth rate of those crystals. For different sy£tems, the relative lengths of the three coordinate axes differ, thus it is difficult to compare crystal sizes from different systems. When using the particle size analyzer, the equivalent spherical diameter was reported for crystal size [9]. This particle size analysis was more accurate than using optical or electron microscope techniques, since more particles were measured and particle orientation was not a concern. Figure 1 shows the crystallization curves obtained on starting gel compositions 4(TPA)20-60(NH4)20-XSiOZ-750HZO for X=30, 60, 90 and 108. It can be seen that the induction period is short and the % crystallization rate is rapid at low Si02 content. Figure Z shows the average crystal size as a function of time. The final average crystal size increases with increasing concentration of SiOZ' It should be noted that XRD meaAurpmpnts arp based on the volume or mass of solid product. The induction period is defined as the time necessary for a sufficient amount of crystal to be formed so as to allow detection by X-ray diffractometer scan. In other words, the "X-ray amorphous" solid may contain either a numerous quantity of tiny crystals or a few large crystals, both representing a small mass of the product mixture [10, 11]. From Figure 1 and 2 at X = 108, while only 1% is crystallized after Z4 hr., the average crystal size is about 5~m. From optical microscope observations we noted that only several large crystals were present in the sample. Thus, it appears the NH4-ZSM-5 zeolite belongs to the latter case.
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80 60 40 20 1Z0 TIME (hours) Fig. 1. % Crystallinity vs. time at 180 aC for 4(TPA)ZO-60(NH4)ZOXSi02-750HZO. X = 30(_), 60(4), 90(_), 108(.). Note also in Figures 1 and 2 that for X = 30 the XRD measurements suggest that 100% crystallinity is reached at about 40 hours. From that time to ZOO hours the average crystal size went from about 15]J m to ZZ]J m, While that represents about a 1.5 fold increase in the average linear dimension, it suggests more than a tripling of the mass of crystals during that time. These results can only be understood in the context of a solution facilitated crystal growth mechanism. Similar results were noted for many experiments. The crystal properties of starting gel compositions with different SiOZ content are compared in Table I. The (OH/SiOZ) was calculated from: (number of NH40H moles) (number of SiOZ moles)
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Effect of Si02 content on product properties
X Si02 OH/Si02 Initial PH Final crystal size Total crystal Linear growth rate Max peak of PSD
30
60
90
108
2 1.33 1.11 11.2 11.4 11.0 22.34 28.48 39.42 41.43 3) 1.04xl0 6 9.59xl0 5 5.82xl0 5 5.33xl0 5 0.6 0.45 0.38 0.31 24.84 29.67 39.91 39.91 4 1l.8
The total crystal number was estimated by: Weight of Si02/Total volume of reagent mixtures Volume of final crystal X crystal density (1.81) The rate of crystal growth is an autocatalytic process. The decrease in the rate of crystallization observed above 50% conversion is caused by the decrease in reactant concentration in the solution. even though the cumulative crystal surface is still increasing. The average crystal growth rate is defined here as: Average crystal size at 50% crystallinity Reaction time at 50% crystallinity With increasing concentration of Si02, the linear growth rate and the total crystal number strongly decreased. but the maximum peak size of the PSD increased, resulting from longer crystal growth time. Those phenomena, due to the initial value of pH or ratio of (OH/Si02), decreased as a result of the slower dissolution rate of amorphous gel. The final crystal size increased with increasing silica content, which depends primarily on the pH of the solution [12, 13]. The average crystal size of the NH4 system is much larger than that of the alkali (Na+, K+) system ([14, 15]. In aqueous solution, the interaction of NH4+ ion with the surrounding water molecules is weak so that the "hydrated structure" of NH4+ value is 1.54A, which is much smaller than that of the alkali cations [16]. The interaction between the aluminosilicate anions and the positively charged species in the synthesis mixture will be the most important in the case of NH4+ ions, with respec.t to far bigger TPA+. As a result, the probability of TPA+-aluminosilicate associations occurring will be reduced. Very few nuclei will be formed and they will grow slowly, yielding very large crystallites at the end of the process, as observed.
Z4Z (SY-lZ-Z) Plotted in Figure 3 are the crystalli7ation curves obtained with starting compositions 4(TPA)ZO-60(NH4)ZO-XAlZ03-90SiOZ-750HZO for X = 0, 0.5, 1.0, Z.O and Z.5. Figure 4 shows the average crystal size as a function of time. The crystal properties of different AlZ03 contents are given in Table Z. The increase in AlZ03 content caused a sharp decrease in crystal growth rate and total crystal number, but the final average crystal size and induction time increased with increasing amount 'of AlZ03' This is opposite to results of Romannikou et al. [17] and Debras et al. [18]. In these two systems, the concentrations of Na+ and TPA+ cations were increasing AlZ03 content. Both cations facilitate nucleation [19], many nuclei were formed, and the final crystal size decreased.
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TIME (hours) Fig. 3. % Crystallinity vs. time at 180°C for X(TPA)ZO60(NH4)ZO-XAlZ03-90SiOZ-750HZO. X = O(e), 0.5(_), 1.0(1rt) , Z.O(_), Z.5(.).
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TIME (hours) Fig. 4. Average crystal size vs. time for same experiments as in Figure 3. AlZ03 content strongly inhibited the rate of nucleation of zeolite, because of OH--Al interactions and reduction of the ability of OH- to depolymerize polys ilicates. The result was formation of few nuclei, yielding very large crystals at the end of the process. Higher AlZ03 content resulted in slower nucleation rate and longer growth time, thus forming a broader crystal size distribution. TPA cations are recognized to form complexes with a aluminosilicate and silicate species, and subsequently to cause replication of the framework structure via stereospecific interaction [ZO]. This phenomena is called a templating effect. During the process, TPA ions are incorporated and stabilized within the zeolite framework
L.-Y. Hou, LoB. Sand and R.W. Thompson Table 2.
243
Effect of A1203 content on product properties
°
X A1203
0.5
Si02/A1203 11.3 Initial PH Final crystal size ( u) 39.42 Total crystal number (l1/cm3) 5.82xl0 5 Linear growth rate ( u/hr ) 0.38 Max peak of PSD (lJ) 39.91
1.0
180 11.2 45.74 3.76xl0 5 0.33 47.1
2.0
90 11.2 47.14 1. 8xl0 5 0.26 49.34
2.5
45 36 11.1 11.0 42.59(93%) 37.23(40%) 0.12 50.12
0.16 51. 69
[20,21]. Figure 5 shows the crystallization curves of ZSM-5 from reaction mixtures with different TPA contents. Figure 6 shows the average crystal size as a function of time. It can be seen that the rate of % crystallinity, linear growth rate, and the induction time strongly depend on TPA content only for TPA/Si02 ratios below 0.09, while for the ratio above 0.09 the rate of % crystallinity is not affected by TPA content. Howden et al. [23] have shown that a value of 0.08 is required to fill the whole zeolite pore volume. The presence of nearly 4 TPA cations per unit cell of crystalline ZSM-5 has been confirmed experimentally [24]. From Table 3 both the final crystal size and the maximum peak size of the crystal size distribution decrease with increasing TPA content. The abundance of submicron crystals produced from starting compositions with high TPA content shows the effect of rapid nucleation. The larger number of nuclei formed consume the nutrients in the mixture very rapidly, so a large number of small crystals are formed, preventing the growth of a smaller number of larger crystals. 1001_----.
120
180
240
300
TIME (hours) Fig. 5. 750H20.
% Crystallinity vs. time at 180°C for X(TPA)20-60(NH4)20-90Si02X 0.5(0), 1.0CA), 2.0(a), 4.0(&), 6.0(_), 8.0(e), 10.0(.).
Table 3. X(TPA)20
Effect of (TPA)20 content on product properties 1.0
2.0
4.0
6.0
8.0
10.0
0.022 0.048 0.088 0.133 0.178 0.222 TPA!Si02 Initial PH 11.4 11.3 11.3 11.2 11.2 11.1 25.14 29.56 39.42 Final crystal size (u ) 36.29 34.66 28.37 Total crystal number (11/c,3) 5.82xl0 5 7.54x10 5 8.66xl0 5 1. 57x10 6 0.08 0.16 0.38 0.41 0.40 Linear growth rate (lJ/hr) 0.39 Max peak of PSD (lJ) 30.74 32.44 39.91 36.55 34.63 28.88 To study the effect of temperature on nucleation and crystal growth rate of zeolite NH4-ZSM-5, the reaction mixture: 4(TPA)20-60(NH4)20-90Si02-750H20 was reacted
244 (SY-l2-2) 40 ~
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~
30
;>l
N .....
'" ....:l
20
~
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>< '" po: u
360 TIME (hours) Fig. 6. Average crystal size vs. time for same experiments as Figure 5. at three different temperatures. Crystallization curves from this composition at 423°K, 438°K, and 453°K are presented in Figure 7. Figure 8 shows the average crystal size as a function of time. It can be noticed that with increasing temperature the induction period decreased, resulting from faster dissolution rate of amorphous gel. From Figure 8 and Table 4 both the % crystallinity rate and the crystal growth rate are not strong functions of temperature, which is opposite to results from alkali systems [14, 25]. The concentration of silicate anion increases with increasing temperature and should accelerate the crystal growth. However, the NH4+ ions interact more strongly with silicate anions at higher temperature, thus the probability of TPA+ silicate interaction occuring will be reduced. Thus, NH4+ ion inhibits nucleation and crystal growth. These two opposite effects compete with each other, therefore temperature changes only influence crystal growth rate in a minor way. The final crystal size is larger and crystal number is less at higher temperature.
e-:
100 >< E-< .....
80
H
60
Z
....:l
~E-<
en
~
... u
• • ;;./'
40 20
~
.Ii . .. /..
...,6:/ / ~.
00
30
60
90
120
150
TIME (hours) Fig. 7. % Crystallinity vs. time for 4(TPA)20-60(NH4)20-90Si02750H20 at 150°C(A), 165°C(I), 180°C(e).
L.-Y. Hou, L.B. Sand and R.W. Thompson
40
-;:
""'N H
30
'" ....,
20
'" >< cG
10
«: 1-< u
00
80
160
240
320
400
TIME (hours) Fig. 8. Average crystal size vs. time for same experiments as in Figure 7. Table 4.
Effect of temperature on product properties
Temperature
180
165
150
Final crystal size 39.42 37.66 36.29 3) 5.82xl0 5 6.67xl0 5 7. 45xl0 5 Total crystal 0.38 Linear Growth rate (u /hr ) 0.36 0.33 Max peak of PSD 39.91 36.97 34.96
ACKNOWLEDGEMENTS Support of the National Science Foundation (grant CBT-8500828) and of WPI Department of Chemical Engineering is great fully acknowledged. This manuscript is dedicated to the memory of Professor Leonard B. Sand who passed away during the course of this study. REFERENCES 1. C.D. Chang and A.J. Silvestri, J. Cayal. 41, 249 (1977). 2. D.M. Bibby, N.B. Milestone and L.P. Aldridge, Naute, 285, 30 (1980). 3. R.W. Thompson, and A. Dyer, Zeolite, 5, 292 (1985. 4. M. Ghamami and L.B. Sand, Zeolite, 3,-155 (1983). 5. A. Nastro, Rend. Ace. Sci. Fis. Mat~, Napoli, Italy, 50, 211 (1984). 6. Z. Gabelica, N. Blom and E.G. Derouane, Appl. Catal. 5, 227 (1983). 7. K.J. Chao, T.C. Tasi and M.S. Chen, J. Chern. Soc. Faraday Trans. I, 77, 547 (1981). 8. R. Von Ballmoos and W.M. Meier, Nature, 289, 782 (1981). 9. R.F. Karuhn and R.H. Berg, "Practical Aspects of Electrozone Size Analysis" Particle Data Lab., Ltd., (1982). 10. A. Ausoux, H. Dexpert, C. Ledeng and J.C. Vedine, Appl. Catal. ~, 95 (1983). 11. P.A. Jacobs, J.C.S. Chern. Comm. 591, (1981). 12. J.L. Casci and B.M. Lowe, Zeolite, 1, 186 (1983). 13. S.G. Fegan and B.M. Lowe, J.C.S. Chern. Comm., 437 (1984). 14. L.Y. Hou and L.B. Sand, Proc. Sixth IntI. Conf. on Zeolites, p. 887 (1983). 15. R. Mostowicz and L.B. Sand, Zeolite, 3, 219 (1983). 16. Y. Marcus, "Introduction to Liquid State Chemistry", Wiley, New York (1983). 17. V.N. Romannikov and V.M. Mastilhin, Zeolite, 3, 311 (1983). 18. G. Debras, A. Gourgue and J.B. Nagy, Zeolite,-5, 369 (1985). 19. Z. Gabelica, N. Blom and E.G. Derouane, Appl. Catal. ~, 227 (1983).
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246 (5Y-12-2) 20. 21. 22. 23.
E.G. Derouane, J.B. Nagy, Z. Gabelica and N. Blom, Zeolites, 2, 299 (1981). L.D. Romann in "Inorganic Compounds with Unusual Properties" Vol. II, p , 387. E.M. Flanigen, Pure Appl. Chern., 52, 2191 (1980). M.G. Howden, "The Role of Tetrapropylarnoniurn Template in the Synthesis of ZSM-5", CSIR Report CENG 413, Pretoria, (982). 24. J.B. Nagy, Z. Gabelica and E.G. Derouane, Zeolite, 3, 43 (1983). 25'. V. Lecluze, M.S. Thesis, Worcester Polytechnic Institute, U.S.A. (1979).