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Nucleation Gels for the Synthesis of Faujasite Type Zeolites
J. Weitkamp, H.G. Karge, H. Pfeifer and W. HOlderich (Eds.) 2olites and Related Microporous Materials: Stale of the Arr 1994 Studies in Surface Science and Caklysis, Vol. 84 0 1994 Elsevier Scicnce B.V. All rights rcscrved.
147
Nucleation Oels for the Synthesis of Faujasite Type Zeolites H. Lechert, P. Staelin, M. Wrobel and U. Schimmel I n s t i t u t e of Physical Chemistry, University of Hamburg Bu ndesstr .4S, 20146 Hamburg, Germany
1. ABSTRACT
For a controlled application of seeding agents in zeolite synthesis the knowledge of the average radii of the nuclei is e x t r e m e l y useful as we w e r e able t o show i n a series of e a r l i e r p a p e r s C1-41. In the p r e s e n t p a p e r the radii of the n u c l e i in commonly used seeding gels f o r Y-zeolites could be determined by kinetic experim e n t s by a comparison of t h e crystallization c u r v e s of the seeding gel with c u r v e s f r o m crystalline seeds. The radii are dependent o n the time of ageing and on temperature. They lie between S O and 120 nm. The radii of the crystalline seeds a r e about 230 nm. The gel can be s e p a r a t e d into a solid p a r t with nuclei of about 200 nm radius and a liquid p a r t with n u c l e i of about SO nm radius.
2. INTRODUCTION
The kinetics of faujasite growth obeys f o r a wide range of batch compositions a relation k
= xo(--
r0
t +
q3
[l-31 w h e r e x is the fraction of formula units CNaA10, n SiO,] of aluminosilicate p r e s e n t in the batch in the crystalline state a t time t. xo is the respective number a t t = 0, which is given by the f r a c tion added to the batch a s s e e d s o r nucleation gel. x is determined as a relative number by X-ray diffraction, which c a n be calibrated by a comparison with a series of mixtures of a fully crystalline sample with amorphous material. k is the rate constant of linear growth in nm/hour, ro is the radius of the s e e d s in nm. The time t is given in hours. Elliott and M c Daniel [Sl p r e p a r e d aluminosilicate gels which are efficient nucleation agents f o r the crystallization of faujasites. These gels have a high alkalinity and a high Si/Al-ratio.
148
The nucleation r a t e of faujasite has been found to be proportional to the square of the alkali content [6]. The Si/Al-ratio of the nuclei in these gels lies between 1.0 and 1.2 [7,81. Due t o the high alkalinity in the nucleation gels the r a t e of nucleation is high and a l a r g e number of n u c l e i is a l r e a d y formed at the ageing a t room t e m p e r a t u r e . The nucleation stops when the aluminate in the b a t c h is exhausted. Because of the l a r g e number of nuclei this occurs a t a r a t h e r small particle size. Following Eq.1, this explains the effectivity of these gels. The assumption t h a t almost all of the aluminate is incorporated in the nuclei allows a good estimation of xo in the nucleation gels. The values of k can be determined f r o m crystallization e x p e r i ments with crystalline s e e d s of known radii by an analysis of Eq.1.
3. Experimental
3.1. Preparation of the Synthesis C3els. The crystallization gels in the following experiments had the compositions giving comparatively small Si/Al-ratios b u t not too long crystallization times. NaAl0, 3-5 SiO, 0,6-5 NaOH * 195-390 QO In the given range of batch compositions no nucleation of faujasite occurs. Without seeds only P zeolite crystallizes. The batches w e r e p r e p a r e d by adding Na-aluminate solutions to w a t e r glass ( d = 1.37 ) . A f t e r the gel formation, the mixture was homogenized f o r 0.5 to 1.5 hours. The crystallization was c a r r i e d out in new polyethylene vessels to avoid disturbances f r o m earlier experiments. Into this mixture seeds o r nucleation gel was added and then homogenized f o r another hour. The reaction mixtures w e r e then heated to 90 "C f o r crystallization. For kinetic studies samples w e r e taken a t suitable time intervals. The samples w e r e filtered, washed to near neutrality and d r i e d . 3.2. Preparation of Cryatallhe Seeds Seeds of NaX with a n average radius of 230 nm [l-31 w e r e p r e p a r e d f r o m a s t a r t i n g gel with the composition NaAlO, * 4 SiO, * 17,6 NaOH * 390 H,O After homogenization the mixture was crystallized f o r 6 hours a t 90 OC and f i l t e r e d . The solid p a r t was washed t o n e a r n e u t r a l i t y and s t o r e d a s an aqueous suspension. The radii of the c r y s t a l s w e r e determined by a scanning microscope.
149
3.3. Preparation of the Nucleation C3el
Nucleation activity could be observed f o r gels with the composition NaAl0, * 7,s SiO, * 1.5 NaOH * 1.5.5 4 0 The aluminate w a s added into a silicate solution containing 0.3 Mole SiO, and 0.63 Mole NaOH in 100 g of solution. This mixture was homogenized u n t i l a completely c l e a r solution was obtained. For ageinghis mixture was s t o r e d a t room t e m p e r a t u r e . A f t e r a f e w d a y s a slight turbidity a p p e a r e d , indicating roughly the beginning of the nucleation activity. The experiments w e r e r e p e a t e d a t higher ageing t e m p e r a t u r e s .
3.4. A d p L of the Products
The crystallinity of the samples has been determined by X-ray analysis comparing t h e intensities of the 642 and the 5.55 reflections with those of a well crystallized faujasite sample ( Linde SK 40 ). This procedure can be calibrated with good accuracy by a comparison with a series of mixtures of this sample with amorphous aluminosilicate. The Si/Al-ratios w e r e determined by EDX analysis.
4. RESULTS AND DISCUSSION 4.1. Influence of the Agsing of the Nucleation &la
For the demonstration of the activity of the nucleation gels f o r different aging times a t f i r s t a batch with the composition NaAlO, 14 SiO, * 9.1 NaOH 400 H,O has been used. From this batch composition products with high Si/Al-ratios of 2.9 - 3.0 are obtained, crystallizing usually only with long crystallization times bearing the danger of the formation of P-zeolite a s by-product. Fig. 1 shows that an ageing time of the nucleation gel of 4 days a t 2S°C leads with xo = 0.09 t o fully crystalline products a f t e r 3 days of crystallization a t 90 OC. For s h o r t e r ageing times the activity is lower. Fig. 2 shows f o r two different batch compositions a comparison of the kinetic c u r v e s obtained with a nucleation gel aged f o r 4 days a t 2 5 OC and the same amount of s e e d s with an average ro = 230 nm I t can be seen t h a t the cystallization times can be appreciably reduced, even if lower xo are applied. In c o n t r a s t t o t h e p r o d u c t s with the seeds, the products with the nucleation gels show a r a t h e r broad distribution of p a r t i c l e size.
150
100
Crystallization time
80 60 40 20
0 1
0
4
3
2
5
Ageing Period [days] Fig. 1 The crystallinity of faujasite samples from batches NaA102 * 14 SiO, 9.1 NaOH 400 %O and xo ~ 0 . 0 9 of nucleation gel of. xo = 0.09 ( 4,5 wt%) in dependence on the ageing period of the gel f o r different crystallization times. 100
80
0
0
10
20
30
40
50
60
70
Crystallization Time [hl
+Si/AI=2;
-0-
Na-AI/AI=0.6 Nucl. Gel
Si/AI=2; Na-AI/AI=0.6 Seeds Si/AI=3; Na-AI/AI=2.1 Nucl. Gel Si/AI=3: Na-AI/AI=2.1 Seeds
Fig2 Comparison of crystallization c u r v e s of faujasites using xo = 0.03 crystalline seeds with ro = 230 nm and a nucleation gel aged for 4 days a t 25OC.
151
The period of 4 - 5 days a f t e r which the final activity of the nucleation gel is obtained coincides with the appearance of a turbidity of the gel, which has been mentioned to be an indication of the full activity ( see also C9l). Increasing the temperature, the time of the appearance of the turbidity is reduced. The indication of the turbidity has been controlled with a photometer ( see C91) 35k 2 d ; 6 0 k 2.5 h ; 80 k 0.5 h Fig. 3 shows the crystallization c u r v e s obtained with the standard batch composition for xo = 0.03 of the gels a t the time of turbidity. I t can be seen that the gel aged for 1 year a t Oo C has by f a r the highest activity expressing itself in a short induction time and in a comparatively higher r a t e of growth. The activity f o r the other gels'decrease with t h e temperature of ageing. The gel aged at 80 k shows no activity a t all For the gel aged a t 0°C turbidity was observed a f t e r 7 months. Ageing CondC tions
100
-.o- 0
80
*
60
.E
40
--
-0- 20 'C --P- 35 ' C
c
0-
'C 1s
+60 ' C
Q
CI
Seeds
0 5
10
15
20
25
30
35
40
Crystallization Time [hl
Fig.3. Kinetics of crystallization of Y-zeolite from batches with NaAlO, * 5 SiO, * 4 NaOH 390 %O and xo = 0.03 Aging times of the nucleation gels: 0 OC 1 year; 20 O C 6d; 35 O C 2d j 60k 2.5 h ;
The most important processes of the formationj of nuclei seem t o take place preferably fefore the appearance of the turbidity. Gels aged e.g. f o r 4 days a t 2 5 k and left f o r another 4 weeks a t 40 "c showed no appreciable change of the crystallization curves. This is in agreement with the general principles discussed above f o r the formation of the nuclei in the gel.
152
4.2. Evaluation of the Average Radius of the Nuclei in Nucleation Gels 4.2.1. Determination of the Rate Constant k for Different Batch Compositiona The r a t e constants k were determined f o r different batch compositions by the u s e of ~0 0.03 seeds with known average radius of 230 nm. The batches w e r e heated to 90°C f o r crystallization From the x at different time t k can then be calculated from Eq.1. These values can be found in Table 1. The k obtained of our experiments are in fairly good agreement with those obtained in [l-31. 4.2.2. Crystallization Experiments with the Nucleation ael
The kinetic experiments w e r e repeated with the nucleation gel as seeding agent ( see also Fig2 1. A s mentioned, the products show a rather broad particle size distribution in contrast to the observations made with the seeds. For a determination of the radii of the nuclei in the seed gel the have been estimated by the aluminate content of the gels.. e k f o r the different batches can be taken from Table 1. B y application of Eq. 1 values of ro of about 100 nm can be obtained, which is less than a half of the ro of the crystalline seeds. Tab. 2 contains results from aging experiments at different temperatures. Table 1 Determination of the radii ro of the nuclei in the standard nucleation gel f o r different batch compositions. The rate constants k have been taken from experiments with crystalline seeds. The Si/Al- ratios of the products are given in the last column. Si/Al Na-Al/A1 H,O/Al 2 2 2 2 3 3 3 3 5
5
0.6 0.8 1. o 1.2 1.2 1.5 1.8 2.1 4.0 5.0
195 195 195 195 260 260 260 260 390 390
xo[%l 1.66 1.66 1.65 1.65 2.20 2.19 2.19 2.19 1 .Ol 1.01
k Cnm/hl 8.0
10.1 12.3 14.2 7.0 10.3 14.7 19.0 12.5 25.9
ro[nml 102 101 105 101 95 97 104 129 94 89
Si/A1 Prod. 1.79 1.74 1.67 1.65 2.33 2.15 2.06 2.05 2.20 1.95
153
Table 2 Radii of the Nuclei in Oels Aged at Different Temperatures n = 4, k = 12 nm/h
C nrnl
Ageing Temp.C°CI 0 20 3s 60 60 s t i r r e d
n = 5, k = 25 nm/h
ro,calc C nml
69 94 113 136 118
64 89 116 119 123
Synthesis Ctel : S SiO, * n NaOH *NaA10,-390 H,O An analysis of the particle size nucleation gel using the particle size distribution of the final products and an extrapolation by r = k tend+ro (2) a s reported in 161 fails because the exact time tend of the complete crystallization can be determined only with poor accuracy. Further, the solid phase of the nucleation gel has been separated from the liquid by centrifugation. Both fractions have, then, be used separately as seeding agent for the crystallization of zeolite Nay. In similar studies Robson [lo] has shown that the liquid phase o f the nucleation gel is much more active than the solid p a r t . A standard nucleation gel aged for 8 days was centrifuged until only a slight opalescence was observed in the liquid. To estimate XO, a p a r t of the liquid was heated to 90°C f o r 2 days. A solid was formed containing all of the aluminatelwhich was shown by careful analyses of the remaining liquid phase. About 20 36 of the total aluminate was present in the liquid. A d i r e c t analysis of the aluminate e.g. by AAS is difficult because of the high silicate concentration. In the crystallization experiments the liquid phase showed a simil a r activity as the standard gel, because of the lower xo a smaller average radius of t h e nuclei near SO nm was calculated . The res u l t s a r e summarized in Table 3. For the solid p a r t a ro near that of the crystalline seeds was found. In the standard gel both effects a r e superimposed, explaining the broad distribution of particle size in the final product. A s can be seen from the foregoing discussions, the problem of the calclulation of ro lies in an exact estimation of xo. A s xo is usually estimated too high for the gels, the obtained ro give an upper limit.
154
Table 3 Radii of the Nuclei in the Liquid and the Solid Phase of a Seed Gel n = 4, k = 12 nm/h T0,calc
Nucleation Gel Liquid Phase Solid Phase
1.0 0.2 6 .O
94 43 227
n = 5,k = 25 nm/h
Cnml 89 52
__
Synthesis Gel : 5 SiO, * n NaOH *NaA102*390 H,O
5. C O n c h i O M The crystallization kinetics of Y-zeolites with different preparations of nucleation gels gives the average radii of the nuclei in these gels f r o m an analysis of the crystallization c u r v e s by a gener a l theory of the zeolite growth ( see Eq.1). These informations are important f o r a calculation of the crystallization curves in technical processes. 6. Acknowledgements The authors thank the Deutsche Forschungsgemeinschaft f o r the support of their work.
Reference6 1. H. Kacirek and H. Lechert, J. Phys. Chem. 79 (1975) 1584. 2. H. Kacirek and H. Lechert, J. Phys. Chem. 80 (1976) 1291. 3. H. Kacirek, Ph. D. Thesis, University of Hamburg, 1974. 4. J. Wienecke, Ph. D. Thesis, University of Hamburg, 1985. 5. C.H. Elliot and C.V. Mc. Daniel U.S. P a t 3.639.099. 6. H. Lechert and H. Kacirek, Zeolites 13 ( 1993) 192. 7. S.P. Zdhanov and N.N. Samulevich in "Proceedings of the 5th. International Conference on Zeolites, L.V.C. R e e s (ed .) Heyden, London 1980, p. 75. 8 . H. Lechert, H. Kacirek and H. Weyda in "Molcular Sieves" ,
M.L. Ocelli and H.E.Robson (eds.) Van Nostrand, Reinhold, New York , 1992, p. 494. 9. S. Kasahara, K. ltabashi and K. Igawa, in " New Developments in Zeolite Science and Technology" Y. Murakami, A . Iima and J.W. Ward (eds.) , Kodansha, Elsevier , 1986, p. 185. 10. H.E.Robson in *'Zeolite Synthesis" ACS. SyrnpSeries 398 (1989) 436.
147
Nucleation Oels for the Synthesis of Faujasite Type Zeolites H. Lechert, P. Staelin, M. Wrobel and U. Schimmel I n s t i t u t e of Physical Chemistry, University of Hamburg Bu ndesstr .4S, 20146 Hamburg, Germany
1. ABSTRACT
For a controlled application of seeding agents in zeolite synthesis the knowledge of the average radii of the nuclei is e x t r e m e l y useful as we w e r e able t o show i n a series of e a r l i e r p a p e r s C1-41. In the p r e s e n t p a p e r the radii of the n u c l e i in commonly used seeding gels f o r Y-zeolites could be determined by kinetic experim e n t s by a comparison of t h e crystallization c u r v e s of the seeding gel with c u r v e s f r o m crystalline seeds. The radii are dependent o n the time of ageing and on temperature. They lie between S O and 120 nm. The radii of the crystalline seeds a r e about 230 nm. The gel can be s e p a r a t e d into a solid p a r t with nuclei of about 200 nm radius and a liquid p a r t with n u c l e i of about SO nm radius.
2. INTRODUCTION
The kinetics of faujasite growth obeys f o r a wide range of batch compositions a relation k
= xo(--
r0
t +
q3
[l-31 w h e r e x is the fraction of formula units CNaA10, n SiO,] of aluminosilicate p r e s e n t in the batch in the crystalline state a t time t. xo is the respective number a t t = 0, which is given by the f r a c tion added to the batch a s s e e d s o r nucleation gel. x is determined as a relative number by X-ray diffraction, which c a n be calibrated by a comparison with a series of mixtures of a fully crystalline sample with amorphous material. k is the rate constant of linear growth in nm/hour, ro is the radius of the s e e d s in nm. The time t is given in hours. Elliott and M c Daniel [Sl p r e p a r e d aluminosilicate gels which are efficient nucleation agents f o r the crystallization of faujasites. These gels have a high alkalinity and a high Si/Al-ratio.
148
The nucleation r a t e of faujasite has been found to be proportional to the square of the alkali content [6]. The Si/Al-ratio of the nuclei in these gels lies between 1.0 and 1.2 [7,81. Due t o the high alkalinity in the nucleation gels the r a t e of nucleation is high and a l a r g e number of n u c l e i is a l r e a d y formed at the ageing a t room t e m p e r a t u r e . The nucleation stops when the aluminate in the b a t c h is exhausted. Because of the l a r g e number of nuclei this occurs a t a r a t h e r small particle size. Following Eq.1, this explains the effectivity of these gels. The assumption t h a t almost all of the aluminate is incorporated in the nuclei allows a good estimation of xo in the nucleation gels. The values of k can be determined f r o m crystallization e x p e r i ments with crystalline s e e d s of known radii by an analysis of Eq.1.
3. Experimental
3.1. Preparation of the Synthesis C3els. The crystallization gels in the following experiments had the compositions giving comparatively small Si/Al-ratios b u t not too long crystallization times. NaAl0, 3-5 SiO, 0,6-5 NaOH * 195-390 QO In the given range of batch compositions no nucleation of faujasite occurs. Without seeds only P zeolite crystallizes. The batches w e r e p r e p a r e d by adding Na-aluminate solutions to w a t e r glass ( d = 1.37 ) . A f t e r the gel formation, the mixture was homogenized f o r 0.5 to 1.5 hours. The crystallization was c a r r i e d out in new polyethylene vessels to avoid disturbances f r o m earlier experiments. Into this mixture seeds o r nucleation gel was added and then homogenized f o r another hour. The reaction mixtures w e r e then heated to 90 "C f o r crystallization. For kinetic studies samples w e r e taken a t suitable time intervals. The samples w e r e filtered, washed to near neutrality and d r i e d . 3.2. Preparation of Cryatallhe Seeds Seeds of NaX with a n average radius of 230 nm [l-31 w e r e p r e p a r e d f r o m a s t a r t i n g gel with the composition NaAlO, * 4 SiO, * 17,6 NaOH * 390 H,O After homogenization the mixture was crystallized f o r 6 hours a t 90 OC and f i l t e r e d . The solid p a r t was washed t o n e a r n e u t r a l i t y and s t o r e d a s an aqueous suspension. The radii of the c r y s t a l s w e r e determined by a scanning microscope.
149
3.3. Preparation of the Nucleation C3el
Nucleation activity could be observed f o r gels with the composition NaAl0, * 7,s SiO, * 1.5 NaOH * 1.5.5 4 0 The aluminate w a s added into a silicate solution containing 0.3 Mole SiO, and 0.63 Mole NaOH in 100 g of solution. This mixture was homogenized u n t i l a completely c l e a r solution was obtained. For ageinghis mixture was s t o r e d a t room t e m p e r a t u r e . A f t e r a f e w d a y s a slight turbidity a p p e a r e d , indicating roughly the beginning of the nucleation activity. The experiments w e r e r e p e a t e d a t higher ageing t e m p e r a t u r e s .
3.4. A d p L of the Products
The crystallinity of the samples has been determined by X-ray analysis comparing t h e intensities of the 642 and the 5.55 reflections with those of a well crystallized faujasite sample ( Linde SK 40 ). This procedure can be calibrated with good accuracy by a comparison with a series of mixtures of this sample with amorphous aluminosilicate. The Si/Al-ratios w e r e determined by EDX analysis.
4. RESULTS AND DISCUSSION 4.1. Influence of the Agsing of the Nucleation &la
For the demonstration of the activity of the nucleation gels f o r different aging times a t f i r s t a batch with the composition NaAlO, 14 SiO, * 9.1 NaOH 400 H,O has been used. From this batch composition products with high Si/Al-ratios of 2.9 - 3.0 are obtained, crystallizing usually only with long crystallization times bearing the danger of the formation of P-zeolite a s by-product. Fig. 1 shows that an ageing time of the nucleation gel of 4 days a t 2S°C leads with xo = 0.09 t o fully crystalline products a f t e r 3 days of crystallization a t 90 OC. For s h o r t e r ageing times the activity is lower. Fig. 2 shows f o r two different batch compositions a comparison of the kinetic c u r v e s obtained with a nucleation gel aged f o r 4 days a t 2 5 OC and the same amount of s e e d s with an average ro = 230 nm I t can be seen t h a t the cystallization times can be appreciably reduced, even if lower xo are applied. In c o n t r a s t t o t h e p r o d u c t s with the seeds, the products with the nucleation gels show a r a t h e r broad distribution of p a r t i c l e size.
150
100
Crystallization time
80 60 40 20
0 1
0
4
3
2
5
Ageing Period [days] Fig. 1 The crystallinity of faujasite samples from batches NaA102 * 14 SiO, 9.1 NaOH 400 %O and xo ~ 0 . 0 9 of nucleation gel of. xo = 0.09 ( 4,5 wt%) in dependence on the ageing period of the gel f o r different crystallization times. 100
80
0
0
10
20
30
40
50
60
70
Crystallization Time [hl
+Si/AI=2;
-0-
Na-AI/AI=0.6 Nucl. Gel
Si/AI=2; Na-AI/AI=0.6 Seeds Si/AI=3; Na-AI/AI=2.1 Nucl. Gel Si/AI=3: Na-AI/AI=2.1 Seeds
Fig2 Comparison of crystallization c u r v e s of faujasites using xo = 0.03 crystalline seeds with ro = 230 nm and a nucleation gel aged for 4 days a t 25OC.
151
The period of 4 - 5 days a f t e r which the final activity of the nucleation gel is obtained coincides with the appearance of a turbidity of the gel, which has been mentioned to be an indication of the full activity ( see also C9l). Increasing the temperature, the time of the appearance of the turbidity is reduced. The indication of the turbidity has been controlled with a photometer ( see C91) 35k 2 d ; 6 0 k 2.5 h ; 80 k 0.5 h Fig. 3 shows the crystallization c u r v e s obtained with the standard batch composition for xo = 0.03 of the gels a t the time of turbidity. I t can be seen that the gel aged for 1 year a t Oo C has by f a r the highest activity expressing itself in a short induction time and in a comparatively higher r a t e of growth. The activity f o r the other gels'decrease with t h e temperature of ageing. The gel aged at 80 k shows no activity a t all For the gel aged a t 0°C turbidity was observed a f t e r 7 months. Ageing CondC tions
100
-.o- 0
80
*
60
.E
40
--
-0- 20 'C --P- 35 ' C
c
0-
'C 1s
+60 ' C
Q
CI
Seeds
0 5
10
15
20
25
30
35
40
Crystallization Time [hl
Fig.3. Kinetics of crystallization of Y-zeolite from batches with NaAlO, * 5 SiO, * 4 NaOH 390 %O and xo = 0.03 Aging times of the nucleation gels: 0 OC 1 year; 20 O C 6d; 35 O C 2d j 60k 2.5 h ;
The most important processes of the formationj of nuclei seem t o take place preferably fefore the appearance of the turbidity. Gels aged e.g. f o r 4 days a t 2 5 k and left f o r another 4 weeks a t 40 "c showed no appreciable change of the crystallization curves. This is in agreement with the general principles discussed above f o r the formation of the nuclei in the gel.
152
4.2. Evaluation of the Average Radius of the Nuclei in Nucleation Gels 4.2.1. Determination of the Rate Constant k for Different Batch Compositiona The r a t e constants k were determined f o r different batch compositions by the u s e of ~0 0.03 seeds with known average radius of 230 nm. The batches w e r e heated to 90°C f o r crystallization From the x at different time t k can then be calculated from Eq.1. These values can be found in Table 1. The k obtained of our experiments are in fairly good agreement with those obtained in [l-31. 4.2.2. Crystallization Experiments with the Nucleation ael
The kinetic experiments w e r e repeated with the nucleation gel as seeding agent ( see also Fig2 1. A s mentioned, the products show a rather broad particle size distribution in contrast to the observations made with the seeds. For a determination of the radii of the nuclei in the seed gel the have been estimated by the aluminate content of the gels.. e k f o r the different batches can be taken from Table 1. B y application of Eq. 1 values of ro of about 100 nm can be obtained, which is less than a half of the ro of the crystalline seeds. Tab. 2 contains results from aging experiments at different temperatures. Table 1 Determination of the radii ro of the nuclei in the standard nucleation gel f o r different batch compositions. The rate constants k have been taken from experiments with crystalline seeds. The Si/Al- ratios of the products are given in the last column. Si/Al Na-Al/A1 H,O/Al 2 2 2 2 3 3 3 3 5
5
0.6 0.8 1. o 1.2 1.2 1.5 1.8 2.1 4.0 5.0
195 195 195 195 260 260 260 260 390 390
xo[%l 1.66 1.66 1.65 1.65 2.20 2.19 2.19 2.19 1 .Ol 1.01
k Cnm/hl 8.0
10.1 12.3 14.2 7.0 10.3 14.7 19.0 12.5 25.9
ro[nml 102 101 105 101 95 97 104 129 94 89
Si/A1 Prod. 1.79 1.74 1.67 1.65 2.33 2.15 2.06 2.05 2.20 1.95
153
Table 2 Radii of the Nuclei in Oels Aged at Different Temperatures n = 4, k = 12 nm/h
C nrnl
Ageing Temp.C°CI 0 20 3s 60 60 s t i r r e d
n = 5, k = 25 nm/h
ro,calc C nml
69 94 113 136 118
64 89 116 119 123
Synthesis Ctel : S SiO, * n NaOH *NaA10,-390 H,O An analysis of the particle size nucleation gel using the particle size distribution of the final products and an extrapolation by r = k tend+ro (2) a s reported in 161 fails because the exact time tend of the complete crystallization can be determined only with poor accuracy. Further, the solid phase of the nucleation gel has been separated from the liquid by centrifugation. Both fractions have, then, be used separately as seeding agent for the crystallization of zeolite Nay. In similar studies Robson [lo] has shown that the liquid phase o f the nucleation gel is much more active than the solid p a r t . A standard nucleation gel aged for 8 days was centrifuged until only a slight opalescence was observed in the liquid. To estimate XO, a p a r t of the liquid was heated to 90°C f o r 2 days. A solid was formed containing all of the aluminatelwhich was shown by careful analyses of the remaining liquid phase. About 20 36 of the total aluminate was present in the liquid. A d i r e c t analysis of the aluminate e.g. by AAS is difficult because of the high silicate concentration. In the crystallization experiments the liquid phase showed a simil a r activity as the standard gel, because of the lower xo a smaller average radius of t h e nuclei near SO nm was calculated . The res u l t s a r e summarized in Table 3. For the solid p a r t a ro near that of the crystalline seeds was found. In the standard gel both effects a r e superimposed, explaining the broad distribution of particle size in the final product. A s can be seen from the foregoing discussions, the problem of the calclulation of ro lies in an exact estimation of xo. A s xo is usually estimated too high for the gels, the obtained ro give an upper limit.
154
Table 3 Radii of the Nuclei in the Liquid and the Solid Phase of a Seed Gel n = 4, k = 12 nm/h T0,calc
Nucleation Gel Liquid Phase Solid Phase
1.0 0.2 6 .O
94 43 227
n = 5,k = 25 nm/h
Cnml 89 52
__
Synthesis Gel : 5 SiO, * n NaOH *NaA102*390 H,O
5. C O n c h i O M The crystallization kinetics of Y-zeolites with different preparations of nucleation gels gives the average radii of the nuclei in these gels f r o m an analysis of the crystallization c u r v e s by a gener a l theory of the zeolite growth ( see Eq.1). These informations are important f o r a calculation of the crystallization curves in technical processes. 6. Acknowledgements The authors thank the Deutsche Forschungsgemeinschaft f o r the support of their work.
Reference6 1. H. Kacirek and H. Lechert, J. Phys. Chem. 79 (1975) 1584. 2. H. Kacirek and H. Lechert, J. Phys. Chem. 80 (1976) 1291. 3. H. Kacirek, Ph. D. Thesis, University of Hamburg, 1974. 4. J. Wienecke, Ph. D. Thesis, University of Hamburg, 1985. 5. C.H. Elliot and C.V. Mc. Daniel U.S. P a t 3.639.099. 6. H. Lechert and H. Kacirek, Zeolites 13 ( 1993) 192. 7. S.P. Zdhanov and N.N. Samulevich in "Proceedings of the 5th. International Conference on Zeolites, L.V.C. R e e s (ed .) Heyden, London 1980, p. 75. 8 . H. Lechert, H. Kacirek and H. Weyda in "Molcular Sieves" ,
M.L. Ocelli and H.E.Robson (eds.) Van Nostrand, Reinhold, New York , 1992, p. 494. 9. S. Kasahara, K. ltabashi and K. Igawa, in " New Developments in Zeolite Science and Technology" Y. Murakami, A . Iima and J.W. Ward (eds.) , Kodansha, Elsevier , 1986, p. 185. 10. H.E.Robson in *'Zeolite Synthesis" ACS. SyrnpSeries 398 (1989) 436.