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Factors Influencing the Synthesis of Zeolite ZSM-20
P.,J. Grobet et nl. (Editors) / Innovation in Zeolite Materials Science © Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
29
FACTORS INFLUENCING THE SYNTHESIS OF ZEOLITE ZSM-20 S. ERNST 1, G. 1. KOKOTAIL0 2 and J. WEITKAMP' 1University of Oldenburg, Department of Chemistry, Chemical Technology, Ammerlaender Heerstrasse 114-118, 0-2900 Oldenburg (Federal Republic of Germany) 2University of Karlsruhe, Institut fUr Chemische Verfahrenstechnik, Kaiserstrasse 12, 0-7500 Karlsruhe 1 (Federal Republic of Germany)
ABSTRACT For an investigation of the factors which influence the synthesis of zeolite ZSM-20, a system which yields ZSM-20 in a reproducible manner was subjected to controlled changes. These comprise the addition of salts (NaCl, KC1, TEA-Br), the influence of ageing and seeding on the crystallization kinetics and the crystal size, and the replacement of the formerly used tetraethylorthosilicate as silica source by its tetramethyl-, tetrapropyl- and tetrabutyl homologues. The crystalline phases obtained from these modified synthesis gels were characterized by XRO and SEM. Reliable and improved recipes for the synthesis of zeolite ZSM-20 emerge from this study. INTRODUCTION ZSM-20 is a Mobil proprietary zeolite first synthesized by Ciric (ref. 1). Its X-ray powder pattern can be indexed in the hexagonal system (refs. 1, 2), but the framework structure has not yet been published. However, from a comparison of the powder patterns of ZSM-20 and faujasite some structural similarities can be deduced (refs. 1- 3). In addition, the application of selected catalytic test reactions (viz., determination of the Spaciousness Index (refs. 4,5) and the isomerization and hydrocracking of long chain n-alkanes (ref. 5)) have shown, that the catalytic properties of ZSM-20 and Y-type zeolites are very s imil ar. The major difference between both zeol ites is a notably hi gher Si0 2/A1 203-ratio of the former one (Si0 2/A1 203 around 10) which results in an improved thermal stability (ref. 6). Therefore, ZSM-20 is an attractive material for catalytic applications in petroleum refining processes, e.g., in fluid catalytic cracking (FCC), and for the production of fine chemicals (refs. 7 -9). ZSM-20 crystallizes in the presence of tetraethylammonium cations (TEA+) (refs. 1,3). However, using TEA+ as templating agent, a variety of other zeolites may crystallize as well. Among them are zeolite Beta, mordenite, ZSM-12 and ZSM-25 (ref. 10). Therefore, the purity and crystallinity of the product from a synthesis aiming at ZSM-20 will strongly depend on the gel chemistry and
30
the synthesis conditions. It was the aim of the present study to investigate the influence of these parameters on the nature of the products. For this purpose, a system which was recently reported to yield ZSM-20 in a reproducible manner (ref. 6) was subjected to controlled changes and the products were characterized by X-ray powder diffraction and scanning electron microscopy. EXPERIMENTAL For the standard preparation of ZSM-20 an aqueous solution of sodium aluminate and tetraethylammoniumhydroxide (TEA-OH) was combined with tetraethylorthosilicate (TEO-Si) to yield a gel with the molar composition 1.25 Na 20 A1 203 - 22.2 Si0 2 - 22.6 TEA-OH - 258 H20. The exact procedure of the gel preparation is very critical with respect to the products. The recommended procedure is as follows: An aqueous solution of tetraethylammoniumhydroxide (3.9 n; obtained through concentrating a 20 wt.-% solution of TEA-OH supplied by Aldrich) is added under stirring to a solution of NaA10 2 (Riedel-de-Haen; 54 wt.-% A1 203, 41 wt.-% Na 20, 5 wt.-% H20) in distilled water. The resulting solution is quickly added to the silica supplying compound (tetramethylorthosilicate, TMO-Si, Fluka; tetraethylorthosilicate, TEO-Si, Fluka; tetrapropylorthosilicate, TPO-Si, Ventron; tetrabutylorthosilicate, TBO-Si, Ventron) in a 125 ml polypropylene bottle. The bottle is closed and shaken periodically. When the mixture becomes hot, it is cooled under flowing water. After about one hour, the bottle is opened, placed in a furnace at 100°C and kept there for about one day. The bottle is then tightly capped and held in the furnace at the same temperature. FollOWing the above procedure, small samples were withdrawn periodically, washed with distilled water, dried at 120°C and afterwards characterized by X-ray powder diffraction using CuKa -radiation with 40 rnA and 40 kV. The crystallinity of the samples was calculated as follows: The height of a selected, well resolved peak (20 ~ 20.5° for ZSM-20, 20 ~ 22.8° for zeolite Beta) was compared to the height of the same peak of a reference sample prepared in our laboratory. Scanning electron micrographs were taken on a Cambridge Stereoscan S4-10 instrument. RESULTS AND DISCUSSION The X-ray powder pattern of the product obtained from the standard gel composition after 14 days at 100°C is shown in Fig. 1. A well crystallized product free from impurity phases as, e.g. zeolite Beta, is obtained. The shape and size of the crystallites are equal to those reported earlier (ref. 6): multiply twinned, ball shaped particles with a size of ca. 0.5 ~m are formed.
31
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'II
I'
~~ ! ! :
I' :>
l(/)
Z W
I-
Z
42
38
34
30
26
ANGLE
2
22
e
18
deg
10
14
Fig. 1. X-ray powder pattern of ZSM-ZO crystallized after 14 days at 100 GC from a gel with the composition 1.Z5 NaZO - A1 Z0 3 - ZZ.Z SiO Z - ZZ.6 TEA-OH - Z58 HZO. It is indicated in the patent literature (ref. 3) that the Na+-content of the qel has to beminimized in order to obtain pure ZSM-ZO. Therefore, the influence of an addition of NaCl as well as of other salts (KC1. TEA-Br) was investigated. The initial gel compositions. the syntheses times and the products obtained are listed in Table 1. TABLE 1 Influence of the addition of NaCl. KCl and TEA-Br to the standard gel composition (T = 100 GC).
gel composition (molar basis)
time. d
product
1.Z5 NaZO - A1 Z03 - ZZ.Z SiO Z - ZZ.6 TEA-OH - Z58 HZO - 1 NaCl 1.Z5 NaZO - A1 Z03 - ZZ.Z 5iOZ - ZZ.6 TEA-OH - Z58 HZO - 1.3 KCl
ZO Z5
ZSM-ZO Beta + 10 % unid. component
1.Z5 Na ZO-A1 Z03-ZZ.Z SiO Z-ZZ.6 TEA-OH-Z58 HZO4.Z TEA-Br
Z5
lSM-ZO + ZO % Beta
Upon addition of NaCl to a gel with the standard composition, the crystallization product remains unchanged (ZSM-ZO). but the time required to obtain a
32
fully crystalline material increases by several days. The latter observation is also made when KCl or TEA-Br are added instead of NaCl. Moreover, the introduction of potassium or TEA cations brings about a significant change in the nature of the synthesis products: Upon addition of KC1, zeolite Beta and traces of an unidentified compound are formed. This observation can explain why zeolite Beta is often obtained from gels which are intended to yield ZSM-20: According to our experience, most of the commercially available TEA-OH solutions are contaminated with considerable amounts (typically 5 to 8 gil) of potassium. It is emphasized here that, in order to obtain pure ZS~1-20, the K+ -content of the TEA-OH solution must be as low as possible (less than 1 gil in this study). The addition of TEA-Br results in the formation of a mixture of about 80 % ZSM-20 and 20 % zeolite Beta. Three possible reasons can be envisaged: 1) A decrease in OH--concentration (an aqueous solution of TEA-Br is slightly acidic), ii) a decrease in the ratio Na+/TEA+ and, iii) an influence of bromide ions.
(
100
80
;!.
>- 60
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0
ageing. 1 h
0
ageing . 1 d
c:
ageing. 3 d
'V
seeding
(
o
Z
::::; -J
<{
I(f)
~o
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u
20
a
0
25
7.5
10
CRYSTALLIZATION
TIME. d
5
12.5
15
Fig. 2. Influence of ageing or seeding on the crystallization kinetics of gels with the standard composition at 100°C. The influence of ageing or seeding on the kinetics of ZSM-20 crystallisation was investigated with the aim to reduce the synthesis time. Mixtures with standard composition were held for one hour, one day or three days at room temperature before heating to 100°C. In the case of seed addition, 10 wt.-% of uncalcined ZSM-20 crystals (ca. 0.5 ~m in diameter) based on the silica in the gel
33
were added after one hour at room temperature. Immediately afterwards, the gel was placed in an oven at 100 0 e for crystallization. The results of the ageing and seeding experiments are depicted in Fig. 2. Increasing the ageing period from the standard value of one hour to one day shortens the time required for complete crystallization from about 14 days to about 7 days. Simultaneously, the mean size of the crystallites, as revealed by SEM, decreases from ca. 0.5 ~m to ca. 0.3 ~m. A further increase of the ageing period to three days at room temperature does not lead to a further enhancement of the crystallization rate, instead, crystallization time increases as compared to the standard preparation. From these results it may be concluded that the number of nuclei formed during the ageing period in dependence of the ageing time passes through a maximum. Probably, ZSM-20 nuclei are metastable in the highly alkaline mother liquor and start to redissolve after prolonged times at room temperature. Upon addition of ZSM-20 seed crystallites the induction period is considerably shortened, but not completely avoided as it could have been expected on the basis of literature data (ref. 11). At the moment, this is an unexpected result which cannot be explained satisfactorily. In the patent literature on ZSM-20 preparation, tetramethylorthosilicate (ref. 1) and tetraethylorthosilicate (ref. 3) are claimed as the silica sources, but higher tetraalkylorthosilicates are also mentioned (ref. 3). Therefore, the influence of the length of the alkyl chains was investigated with respect to crystallization kinetics and products. The results are presented in Fig. 3. The TEO-Si in the standard gel composition was replaced by equal molar amounts of TMO-Si, TPO-Si or TBO-Si. Therefore, the Si0 2/A1 203-ratio in the initial gel remains constant. With TMO-Si and TEO-Si, ISM-20 can be obtained in high yields and crystallinities. According to our results and in contradiction to what has been stated in the patent literature (ref. 3), TMO-Si is a much more appropriate silica source than TEO-Si with respect to the time required for complete crystallization: With TMO-Si, the synthesis of ZSM-20 is completed after about five days, whereas with TEO-Si about 14 days are required under standard synthesis conditions. The crystallites obtained with the former silica source are somewhat smaller than those from the latter one (ca. 0.3 ~m instead of ca. 0.5 ~m) pointing to a higher number of nuclei which are initially available for crystal growth. The use of TPO-Si as silica source results in the crystallization of zeolite Beta after prolonged synthesis times (21 to 27 days) at 100 0 e, whereas with TBO-Si, only amorphous material is obtained even after 28 days (Fig. 3). In both cases it cannot be ruled out that at least part of the alcohols (propanol or
34
100
80
-,
•
~
:>
~
(
(0 0
60
0
ZSM - 20
Z
-'
-' 40
:! en ~
u
20
0
0
5
~
J 10
silica - source T MO - Si
0
0
TEO - Si
A
TPO
v
TBO
-
15
CRYSTALLIZATION
Si Si
20 TIME.
25
30
d
Fig. 3. Influence of the silica source on the crystallization kinetics and the products obtained. butanol) formed during hydrolysis of the tetraalkylorthosilicates is withheld in the synthesis mixtures. Indeed, the boiling points of propanol and butanol are above 100 0e and both form azeotropes with water. As a whole, it is evident from the results of this study that, under the synthesis conditions applied here, the length of the alkyl chains of the tetraalkylorthosilicates has a stron~ influence on the resulting zeolite: TMO-Si and TEO-Si favor the formation of ZSM-ZO nuclei, while the use of TPO-Si as silica source leads to zeolite Beta. When TBO-Si is used, no crystalline phase is observed within one month. The influence of the SiOZ/Al z0 3-ratio and the temperature on the crystallization time and nature of the products is summarized in Table Z. With TMO-Si as the silica source and at 100 0 e , ZSM-20 can be obtained from gels with SiO Z/ A1Z0 3-ratios from about 20 to about 30. With increasing SiO Z/A1 203-ratio the crystallization time increases which is an observation frequently made in the synthesis of low silica zeolites. This phenomenon can be interpreted with an increasing viscosity of the gel which results in a slower rate of crystal growth. Using TEO-Si as the silica source and increasing the crystallization temperature from 10Qoe to lZ00e for a gel with the standard composition results in the formation of highly crystalline zeolite Beta with crystallite sizes of ca. 0.5 ~m. Keeping the temperature at 100 0e and increasing the SiO Z/A1 Z03-ratio to 30.0 leads to zeolite Beta as well. Even by addition of 20 wt.-% (based on the Si0 2-content of the gel) ZSM-20 seed crystallites it is not possible to synthe-
35
size ZSM-20 from a gel with Si0 2/A1 203 = 30 and TEO-Si as the silica source. Therefore, the results presented in Table 2 underline the importance of choosing the right Si0 2/A1 203-ratio and the right temperature in order to obtain pure and highly crystalline ZSM-20. If these synthesis parameters are too high, the system escapes into zeolite Beta which appears to be the thermodynamically more stable phase. TABLE 2 Influence of Si0 2/A1 203-ratio and temperature on crystallization time and products. silica source
Si0 2/A1 203
T, °C
time,d
product
TMO-Si
10.0 22.4 30.6
100 100 100
12 5 7
faujasite ZSM-20 ZSM-20
TEO-Si
22.2 22.2 30.0 30.0*
100 120 100 100
14 8 13 13
ZSM-20 Beta Beta Beta
* addition of ZSM-20 seeds (20 wt.-% based on the Si0 2-content of the gel). CONCLUSIONS A systematic study was undertaken to investigate the factors influencing the synthesis of zeolite ZSM-20. Pure ZSM-20 with a crystallite size of ca. 0.5 ~m can be obtained from a gel with the molar composition 1.25 Na 20 - A1 20322.2 Si0 2 - 22.6 TEA-OH - 258 H20 after 14 days at 100°C. The crystallization time can be considerably shortened by introducing either of the following modifications: i) ageing of the gel at room temperature for one day instead of one hour, ii) seeding with 10 wt.-% ZSM-20 crystallites (based on the Si0 2-content of the gel) and, iii) replacing the tetraethylorthosilicate by its tetramethyl homologue. It was another important observation that the purity and concentration of the reactants have to be carefully controlled in order to obtain pure ZSM-20: In particular, potassium must be kept from the synthesis mixture, otherwise zeolite Beta crystallizes. On the other hand, faujasite is formed if the ratio of Si0 2/A1 203 is not high enough. As a whole, several reliable and new recipes emerge from this study for the preparation of pure ZSM-20. Some of these recipes enable much shorter synthesis times than hitherto reported. Work is underway in our laboratories to investigate the structural and ion exchange properties of this very promising material.
36
ACKNOWLEDGEMENTS The authors thank Mrs. Sonja Hesselmann for her excellent technical assistance. Financial support by Deutsche Forschungsgemeinschaft, Max-Buchner-Forschungsstiftung and Fonds der Chemischen Industrie is gratefully acknowledged~ George T. Kokotailo acknowledges the Humboldt Senior US Scientist Award 1985. REFERENCES 1 J. Ciric, US Patent 3 972 983 (Aug. 3, 1976), assigned to Mobil Oil Corp. 2 E.G. Derouane, N. Dewaele, Z. Gabelica and J.B. Nagy, Appl. Catal. 28 (1986) 285-293. 3 E.W. Valyocsik, Europ. Patent Appl. 12572 (Jun. 25, 1980) assigned to Mobil Oil Corp. 4 J. Weitkamp, S. Ernst and R. Kumar, Appl. Catal. 27 (1986) 207-210. 5 J. Weitkamp, S. Ernst, V. Cortes Corberan and G.T. Kokotailo, 7th Intern. Zeolite Conf., Tokyo, Japan, 17./20. Aug. 1986, Preprints of Poster Papers, Japan Association of Zeolite, Tokyo, 1986, pp. 239-240. 6 S. Ernst, G.T. Kokotailo and J. Weitkamp, Zeolites 7 (1987) 180-182. 7 R.B. LaPierre, R.D. Partridge and S.S.F. Wong, Europ. Patent Appl. 94862 (Nov. 23, 1983), assigned to Mobil Oil Corp. 8 R.M. Dessau, J. Chem. Soc., Chem. Commun. 1986, 1167. 9 H. Litterer, Ger. Patent Appl. 3 334 673 (Apr. 11, 1985), assigned to Hoechst AG. 10 B.M. Lok, T.R. Cannan and C.A. Messina, Zeolites 3 (1983) 282-291. 11 R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, New York, 1982.
29
FACTORS INFLUENCING THE SYNTHESIS OF ZEOLITE ZSM-20 S. ERNST 1, G. 1. KOKOTAIL0 2 and J. WEITKAMP' 1University of Oldenburg, Department of Chemistry, Chemical Technology, Ammerlaender Heerstrasse 114-118, 0-2900 Oldenburg (Federal Republic of Germany) 2University of Karlsruhe, Institut fUr Chemische Verfahrenstechnik, Kaiserstrasse 12, 0-7500 Karlsruhe 1 (Federal Republic of Germany)
ABSTRACT For an investigation of the factors which influence the synthesis of zeolite ZSM-20, a system which yields ZSM-20 in a reproducible manner was subjected to controlled changes. These comprise the addition of salts (NaCl, KC1, TEA-Br), the influence of ageing and seeding on the crystallization kinetics and the crystal size, and the replacement of the formerly used tetraethylorthosilicate as silica source by its tetramethyl-, tetrapropyl- and tetrabutyl homologues. The crystalline phases obtained from these modified synthesis gels were characterized by XRO and SEM. Reliable and improved recipes for the synthesis of zeolite ZSM-20 emerge from this study. INTRODUCTION ZSM-20 is a Mobil proprietary zeolite first synthesized by Ciric (ref. 1). Its X-ray powder pattern can be indexed in the hexagonal system (refs. 1, 2), but the framework structure has not yet been published. However, from a comparison of the powder patterns of ZSM-20 and faujasite some structural similarities can be deduced (refs. 1- 3). In addition, the application of selected catalytic test reactions (viz., determination of the Spaciousness Index (refs. 4,5) and the isomerization and hydrocracking of long chain n-alkanes (ref. 5)) have shown, that the catalytic properties of ZSM-20 and Y-type zeolites are very s imil ar. The major difference between both zeol ites is a notably hi gher Si0 2/A1 203-ratio of the former one (Si0 2/A1 203 around 10) which results in an improved thermal stability (ref. 6). Therefore, ZSM-20 is an attractive material for catalytic applications in petroleum refining processes, e.g., in fluid catalytic cracking (FCC), and for the production of fine chemicals (refs. 7 -9). ZSM-20 crystallizes in the presence of tetraethylammonium cations (TEA+) (refs. 1,3). However, using TEA+ as templating agent, a variety of other zeolites may crystallize as well. Among them are zeolite Beta, mordenite, ZSM-12 and ZSM-25 (ref. 10). Therefore, the purity and crystallinity of the product from a synthesis aiming at ZSM-20 will strongly depend on the gel chemistry and
30
the synthesis conditions. It was the aim of the present study to investigate the influence of these parameters on the nature of the products. For this purpose, a system which was recently reported to yield ZSM-20 in a reproducible manner (ref. 6) was subjected to controlled changes and the products were characterized by X-ray powder diffraction and scanning electron microscopy. EXPERIMENTAL For the standard preparation of ZSM-20 an aqueous solution of sodium aluminate and tetraethylammoniumhydroxide (TEA-OH) was combined with tetraethylorthosilicate (TEO-Si) to yield a gel with the molar composition 1.25 Na 20 A1 203 - 22.2 Si0 2 - 22.6 TEA-OH - 258 H20. The exact procedure of the gel preparation is very critical with respect to the products. The recommended procedure is as follows: An aqueous solution of tetraethylammoniumhydroxide (3.9 n; obtained through concentrating a 20 wt.-% solution of TEA-OH supplied by Aldrich) is added under stirring to a solution of NaA10 2 (Riedel-de-Haen; 54 wt.-% A1 203, 41 wt.-% Na 20, 5 wt.-% H20) in distilled water. The resulting solution is quickly added to the silica supplying compound (tetramethylorthosilicate, TMO-Si, Fluka; tetraethylorthosilicate, TEO-Si, Fluka; tetrapropylorthosilicate, TPO-Si, Ventron; tetrabutylorthosilicate, TBO-Si, Ventron) in a 125 ml polypropylene bottle. The bottle is closed and shaken periodically. When the mixture becomes hot, it is cooled under flowing water. After about one hour, the bottle is opened, placed in a furnace at 100°C and kept there for about one day. The bottle is then tightly capped and held in the furnace at the same temperature. FollOWing the above procedure, small samples were withdrawn periodically, washed with distilled water, dried at 120°C and afterwards characterized by X-ray powder diffraction using CuKa -radiation with 40 rnA and 40 kV. The crystallinity of the samples was calculated as follows: The height of a selected, well resolved peak (20 ~ 20.5° for ZSM-20, 20 ~ 22.8° for zeolite Beta) was compared to the height of the same peak of a reference sample prepared in our laboratory. Scanning electron micrographs were taken on a Cambridge Stereoscan S4-10 instrument. RESULTS AND DISCUSSION The X-ray powder pattern of the product obtained from the standard gel composition after 14 days at 100°C is shown in Fig. 1. A well crystallized product free from impurity phases as, e.g. zeolite Beta, is obtained. The shape and size of the crystallites are equal to those reported earlier (ref. 6): multiply twinned, ball shaped particles with a size of ca. 0.5 ~m are formed.
31
II ,'II
'II
I'
~~ ! ! :
I' :>
l(/)
Z W
I-
Z
42
38
34
30
26
ANGLE
2
22
e
18
deg
10
14
Fig. 1. X-ray powder pattern of ZSM-ZO crystallized after 14 days at 100 GC from a gel with the composition 1.Z5 NaZO - A1 Z0 3 - ZZ.Z SiO Z - ZZ.6 TEA-OH - Z58 HZO. It is indicated in the patent literature (ref. 3) that the Na+-content of the qel has to beminimized in order to obtain pure ZSM-ZO. Therefore, the influence of an addition of NaCl as well as of other salts (KC1. TEA-Br) was investigated. The initial gel compositions. the syntheses times and the products obtained are listed in Table 1. TABLE 1 Influence of the addition of NaCl. KCl and TEA-Br to the standard gel composition (T = 100 GC).
gel composition (molar basis)
time. d
product
1.Z5 NaZO - A1 Z03 - ZZ.Z SiO Z - ZZ.6 TEA-OH - Z58 HZO - 1 NaCl 1.Z5 NaZO - A1 Z03 - ZZ.Z 5iOZ - ZZ.6 TEA-OH - Z58 HZO - 1.3 KCl
ZO Z5
ZSM-ZO Beta + 10 % unid. component
1.Z5 Na ZO-A1 Z03-ZZ.Z SiO Z-ZZ.6 TEA-OH-Z58 HZO4.Z TEA-Br
Z5
lSM-ZO + ZO % Beta
Upon addition of NaCl to a gel with the standard composition, the crystallization product remains unchanged (ZSM-ZO). but the time required to obtain a
32
fully crystalline material increases by several days. The latter observation is also made when KCl or TEA-Br are added instead of NaCl. Moreover, the introduction of potassium or TEA cations brings about a significant change in the nature of the synthesis products: Upon addition of KC1, zeolite Beta and traces of an unidentified compound are formed. This observation can explain why zeolite Beta is often obtained from gels which are intended to yield ZSM-20: According to our experience, most of the commercially available TEA-OH solutions are contaminated with considerable amounts (typically 5 to 8 gil) of potassium. It is emphasized here that, in order to obtain pure ZS~1-20, the K+ -content of the TEA-OH solution must be as low as possible (less than 1 gil in this study). The addition of TEA-Br results in the formation of a mixture of about 80 % ZSM-20 and 20 % zeolite Beta. Three possible reasons can be envisaged: 1) A decrease in OH--concentration (an aqueous solution of TEA-Br is slightly acidic), ii) a decrease in the ratio Na+/TEA+ and, iii) an influence of bromide ions.
(
100
80
;!.
>- 60
I-
0
ageing. 1 h
0
ageing . 1 d
c:
ageing. 3 d
'V
seeding
(
o
Z
::::; -J
<{
I(f)
~o
>a:::
u
20
a
0
25
7.5
10
CRYSTALLIZATION
TIME. d
5
12.5
15
Fig. 2. Influence of ageing or seeding on the crystallization kinetics of gels with the standard composition at 100°C. The influence of ageing or seeding on the kinetics of ZSM-20 crystallisation was investigated with the aim to reduce the synthesis time. Mixtures with standard composition were held for one hour, one day or three days at room temperature before heating to 100°C. In the case of seed addition, 10 wt.-% of uncalcined ZSM-20 crystals (ca. 0.5 ~m in diameter) based on the silica in the gel
33
were added after one hour at room temperature. Immediately afterwards, the gel was placed in an oven at 100 0 e for crystallization. The results of the ageing and seeding experiments are depicted in Fig. 2. Increasing the ageing period from the standard value of one hour to one day shortens the time required for complete crystallization from about 14 days to about 7 days. Simultaneously, the mean size of the crystallites, as revealed by SEM, decreases from ca. 0.5 ~m to ca. 0.3 ~m. A further increase of the ageing period to three days at room temperature does not lead to a further enhancement of the crystallization rate, instead, crystallization time increases as compared to the standard preparation. From these results it may be concluded that the number of nuclei formed during the ageing period in dependence of the ageing time passes through a maximum. Probably, ZSM-20 nuclei are metastable in the highly alkaline mother liquor and start to redissolve after prolonged times at room temperature. Upon addition of ZSM-20 seed crystallites the induction period is considerably shortened, but not completely avoided as it could have been expected on the basis of literature data (ref. 11). At the moment, this is an unexpected result which cannot be explained satisfactorily. In the patent literature on ZSM-20 preparation, tetramethylorthosilicate (ref. 1) and tetraethylorthosilicate (ref. 3) are claimed as the silica sources, but higher tetraalkylorthosilicates are also mentioned (ref. 3). Therefore, the influence of the length of the alkyl chains was investigated with respect to crystallization kinetics and products. The results are presented in Fig. 3. The TEO-Si in the standard gel composition was replaced by equal molar amounts of TMO-Si, TPO-Si or TBO-Si. Therefore, the Si0 2/A1 203-ratio in the initial gel remains constant. With TMO-Si and TEO-Si, ISM-20 can be obtained in high yields and crystallinities. According to our results and in contradiction to what has been stated in the patent literature (ref. 3), TMO-Si is a much more appropriate silica source than TEO-Si with respect to the time required for complete crystallization: With TMO-Si, the synthesis of ZSM-20 is completed after about five days, whereas with TEO-Si about 14 days are required under standard synthesis conditions. The crystallites obtained with the former silica source are somewhat smaller than those from the latter one (ca. 0.3 ~m instead of ca. 0.5 ~m) pointing to a higher number of nuclei which are initially available for crystal growth. The use of TPO-Si as silica source results in the crystallization of zeolite Beta after prolonged synthesis times (21 to 27 days) at 100 0 e, whereas with TBO-Si, only amorphous material is obtained even after 28 days (Fig. 3). In both cases it cannot be ruled out that at least part of the alcohols (propanol or
34
100
80
-,
•
~
:>
~
(
(0 0
60
0
ZSM - 20
Z
-'
-' 40
:! en ~
u
20
0
0
5
~
J 10
silica - source T MO - Si
0
0
TEO - Si
A
TPO
v
TBO
-
15
CRYSTALLIZATION
Si Si
20 TIME.
25
30
d
Fig. 3. Influence of the silica source on the crystallization kinetics and the products obtained. butanol) formed during hydrolysis of the tetraalkylorthosilicates is withheld in the synthesis mixtures. Indeed, the boiling points of propanol and butanol are above 100 0e and both form azeotropes with water. As a whole, it is evident from the results of this study that, under the synthesis conditions applied here, the length of the alkyl chains of the tetraalkylorthosilicates has a stron~ influence on the resulting zeolite: TMO-Si and TEO-Si favor the formation of ZSM-ZO nuclei, while the use of TPO-Si as silica source leads to zeolite Beta. When TBO-Si is used, no crystalline phase is observed within one month. The influence of the SiOZ/Al z0 3-ratio and the temperature on the crystallization time and nature of the products is summarized in Table Z. With TMO-Si as the silica source and at 100 0 e , ZSM-20 can be obtained from gels with SiO Z/ A1Z0 3-ratios from about 20 to about 30. With increasing SiO Z/A1 203-ratio the crystallization time increases which is an observation frequently made in the synthesis of low silica zeolites. This phenomenon can be interpreted with an increasing viscosity of the gel which results in a slower rate of crystal growth. Using TEO-Si as the silica source and increasing the crystallization temperature from 10Qoe to lZ00e for a gel with the standard composition results in the formation of highly crystalline zeolite Beta with crystallite sizes of ca. 0.5 ~m. Keeping the temperature at 100 0e and increasing the SiO Z/A1 Z03-ratio to 30.0 leads to zeolite Beta as well. Even by addition of 20 wt.-% (based on the Si0 2-content of the gel) ZSM-20 seed crystallites it is not possible to synthe-
35
size ZSM-20 from a gel with Si0 2/A1 203 = 30 and TEO-Si as the silica source. Therefore, the results presented in Table 2 underline the importance of choosing the right Si0 2/A1 203-ratio and the right temperature in order to obtain pure and highly crystalline ZSM-20. If these synthesis parameters are too high, the system escapes into zeolite Beta which appears to be the thermodynamically more stable phase. TABLE 2 Influence of Si0 2/A1 203-ratio and temperature on crystallization time and products. silica source
Si0 2/A1 203
T, °C
time,d
product
TMO-Si
10.0 22.4 30.6
100 100 100
12 5 7
faujasite ZSM-20 ZSM-20
TEO-Si
22.2 22.2 30.0 30.0*
100 120 100 100
14 8 13 13
ZSM-20 Beta Beta Beta
* addition of ZSM-20 seeds (20 wt.-% based on the Si0 2-content of the gel). CONCLUSIONS A systematic study was undertaken to investigate the factors influencing the synthesis of zeolite ZSM-20. Pure ZSM-20 with a crystallite size of ca. 0.5 ~m can be obtained from a gel with the molar composition 1.25 Na 20 - A1 20322.2 Si0 2 - 22.6 TEA-OH - 258 H20 after 14 days at 100°C. The crystallization time can be considerably shortened by introducing either of the following modifications: i) ageing of the gel at room temperature for one day instead of one hour, ii) seeding with 10 wt.-% ZSM-20 crystallites (based on the Si0 2-content of the gel) and, iii) replacing the tetraethylorthosilicate by its tetramethyl homologue. It was another important observation that the purity and concentration of the reactants have to be carefully controlled in order to obtain pure ZSM-20: In particular, potassium must be kept from the synthesis mixture, otherwise zeolite Beta crystallizes. On the other hand, faujasite is formed if the ratio of Si0 2/A1 203 is not high enough. As a whole, several reliable and new recipes emerge from this study for the preparation of pure ZSM-20. Some of these recipes enable much shorter synthesis times than hitherto reported. Work is underway in our laboratories to investigate the structural and ion exchange properties of this very promising material.
36
ACKNOWLEDGEMENTS The authors thank Mrs. Sonja Hesselmann for her excellent technical assistance. Financial support by Deutsche Forschungsgemeinschaft, Max-Buchner-Forschungsstiftung and Fonds der Chemischen Industrie is gratefully acknowledged~ George T. Kokotailo acknowledges the Humboldt Senior US Scientist Award 1985. REFERENCES 1 J. Ciric, US Patent 3 972 983 (Aug. 3, 1976), assigned to Mobil Oil Corp. 2 E.G. Derouane, N. Dewaele, Z. Gabelica and J.B. Nagy, Appl. Catal. 28 (1986) 285-293. 3 E.W. Valyocsik, Europ. Patent Appl. 12572 (Jun. 25, 1980) assigned to Mobil Oil Corp. 4 J. Weitkamp, S. Ernst and R. Kumar, Appl. Catal. 27 (1986) 207-210. 5 J. Weitkamp, S. Ernst, V. Cortes Corberan and G.T. Kokotailo, 7th Intern. Zeolite Conf., Tokyo, Japan, 17./20. Aug. 1986, Preprints of Poster Papers, Japan Association of Zeolite, Tokyo, 1986, pp. 239-240. 6 S. Ernst, G.T. Kokotailo and J. Weitkamp, Zeolites 7 (1987) 180-182. 7 R.B. LaPierre, R.D. Partridge and S.S.F. Wong, Europ. Patent Appl. 94862 (Nov. 23, 1983), assigned to Mobil Oil Corp. 8 R.M. Dessau, J. Chem. Soc., Chem. Commun. 1986, 1167. 9 H. Litterer, Ger. Patent Appl. 3 334 673 (Apr. 11, 1985), assigned to Hoechst AG. 10 B.M. Lok, T.R. Cannan and C.A. Messina, Zeolites 3 (1983) 282-291. 11 R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, New York, 1982.