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Shape-Selective Catalysis Using Modified Ferris Ilicate Molecular Sieves
.i.w. Ward (Editor). Catalysis ]987
© 1988 Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
SHAPE-SELECTIVE CATALYSIS USING MODIFIED FERRISILICATE MOLECULAR SIEVES V. Nair I,2 R. Szostak 1, P. K. Agrawal 2, T. L. Thomas 1 lZeolite Research Program, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332 2S chool of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 ABSTRACT Ferrisilicate molecular sieves have been synthesized with the ZSM-5 structure having Si02/Fe203 molar ratios ranging from 25 to infinity. Thermal and/or hydrothermal treatment causes some of the iron is to be removed from the tetrahedral framework sites and forms a secondary iron oxide phase. These ferrisilicates have been characterized using dehydrogenation of para- and ortho-ethyltoluenes. The para-ethyltoluene reacts on the iron oxide present within the pores and on the surface of the crystal. Whereas, ortho-ethyltoluene reacts only on the iron oxide present on the surface of the crystal. The results obtained using the catalytic reactions in conjunction with those obtained through the physico-chemical techniques aid in understanding the nature and location of iron in these ferrisilicate and modified ferrisilicate molecular sieves. INTRODUCT ION Recent advances in molecular sieve synthesis include preparation of molecular sieves which contain iron, gallium, boron, phosphorous and other transitional metals in the framework sites. The ferrisilicate molecular sieves comprise a class of molecular sieves in which, besides Si, only Fe (III) occupies tetrahedral framework sites[I-7J. It has been shown [IJ that when the ferrisilicate molecular sieve analog of the zeolite ZSM-5 is subjected to thermal or hydrothermal treatment, the Fe (III) is removed from its lattice sites. Once iron is removed from the framework it can be present in two possible locations: highly dispersed within the pores of the sieve or on the surface of the crystal. Most of the characterization of ferrisilicate molecular sieves thus far[I-7J has been devoted to determination of the state of the iron in these molecular sieves and whether or not they incorporate into the framework sites. In this study, shape-selective catalysis is used to determine the distribution of non-framework iron. As iron oxide is an active catalyst for the conversion of ethyl benzene to styrene[8J, the location of the iron oxide within the pores and on the surface of the crystal can be determined by studying the dehydrogenation of the ortho and para isomers of the
209
210
methyl-substituted ethylbenzene, methyltoluene. In the ZSM-5 structure, the para-isomer has access to the internal pores of the crystal to react with any iron oxide present within those pores. The ortho-isomer is not appreciably adsorbed into the pores and therefore will only react with any iron oxide present on the surface of the crystals. By examining these two reactions, the iron oxide present on the outside of the pores can be distinguished from the iron oxide within the pores. In addition, the migration of the iron from the pores of the crystal to the surface under various thermal and hydrothermal treatments can be monitored. The advantage of using these probe molecules is that there are chemically very similar. The results obtained using the probe reactions have been correlated with those obtained with physico-chemical techniques. EXPERIMENTAL PROCEDURE Sample preparation: The synthesis and modification procedure used for preparing ferrisilicate molecular sieves with the ZSM-5 structure have been explained in detail earlierCl,7J. Ferrisilicate molecular sieves with the ZSM-5 type structure having SiOZ/FeZ03 ratios of 175, 9Z, and 54 were synthesized using a stirred autoclave, then calcined in nitrogen followed by air, after which they were ammonium-exchangedCl,7J. A portion of the ammonium-exchanged sample was calcined and then potassium-exchanged with KOH to a pH of 8.0Cl,7J. Portions of the ammonium-exchanged samples were hydrothermally treated in 100% steam for 1 hour, Z hours, ad 4 hours (at 500°C) respectively. Each of these hydrothermally modified ferrisilicate molecular sieves was then potassium-exchanged by titration with KOH to a pH of 8.0Cl,7J. Samples in this study are denoted by a notation FeZSM-5(XX); where, FeZSM-5 implies that the material is a ferrisilicate molecular sieve with a ZSM-5 type structure and XX denotes the SiOZ/FeZ03 molar ratio of the materials. For example, FeZSM-5 (175) denotes a ferrisilicate molecular sieve with a ZSM-5 type structure with a SiOZ/FeZ03 molar ratio of 175. Union Carbide's silicalite S-115LE was used for comparison. Unsupported iron oxide was obtained as ferric oxide from Fisher Scientific. It was calcined in dry air at 500°C for 16 hours before using it as catalyst. Basic Characterization: X-ray diffraction patterns of all the molecular sieves were obtained using Rigaku X-ray diffractometer. The range of two theta values over which the diffraction pattern was obtained was 5° to 40°; the scan was conducted at a rate of 5°/min. The adsorption capacities of molecular sieves were examined using a McBain-Bakr adsorption
211
system. The oxygen adsorption capacities were determined at -196°C and 75 torr pressure. The n-hexane adsorption capacities were determined at room temperature and 70 torr pressure. The adsorption capacities for paraethyltoluene and orthoethyltoluene over ferrisilicate molecular sieves were determined at room temperature and at a pressure of 1 torr. Chemical analysis of the samples was done by atomic absorption spectroscopy. The results of the SEM analysis, infrared spectroscopic study. adsorption properties, ion-exchange ability, and Mossbauer spectroscopic study have been described elsewhereC1,7J. Reactor System: The details of the reactor system used and the procedure for conducting the reactions are described elsewhere[9J. The products and the reactants are analyzed by a gas chromatograph using a 10' X 1/8" Alltech column filled with liquid phase of AT-1200 + 5% Bentone on chromosorb WAW, 100/1210 mesh (carrier flow 25cclmin. and column temperature of 100°C). The catalysts were run in the potassium-exchanged form to supress any possible acid-catalyzed cracking side reactions which could occur over any acid sites within the material. The main reaction that takes place is the dehydrogenation of paraethyltoluene and orthoethyltoluene to methylstyrenes. Some side reactions of cracking to form small amounts of benzene, toluene and xylenes were observed; however, the product selectivity to methylstyrenes was over 90%. The reaction results reported are for 0.5 hours on stream.
RESULTS The X-ray diffraction patterns of all the ferrisilicate molecular sieves show that the molecular sieves are highly crystalline and that they possess a lSM-5 type structure. Even after severe hydrothermal treatment (excess steam, 4 hours, 550°C) no significant changes are observed indicating that these materials retain their lSM-5 type structure even on severe steaming. The oxygen and n-hexane adsorption capacities of all molecular sieves used in this study (including that of silicalite) was determined to be around 22 (g oxygen adsorbed/100g sample) and 11.6 (g n-hexane/100g sample) indicating little change in pore volume under various treatments. The paraethyltoluene adsorption capacity of the ferrisilicate molecular sieves was found to be 15 wt%. The orthoethyltoluene adsorption capacity was less than 1 wt%; confirming the selectivity of these materials for the para-form.
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The %Fe and %Si oDtained using atomic absorption were used in calculating the bulk SiOZ/FeZ03 ratios for each sample (Table 1). The mole ratio of K/Fe in the potassium-exchanged samples is used as a measure of the fraction of the total iron present in the framework (Table 1). Based on these exchange results, for calcined FeZSM-5(175) it is seen that essentially all of the iron is present in the framework sites. On the other hand, for FeZSM-5(54) hydrothermally treated for 4 hours, only 18% of the total iron is present in the framework sites. The other ferrisilicates have intermediate fractions of the total iron in the framework sites. The exchange study results provide a quantitative measure of the degree of TABLE 1 Chemical Composition of all ferrisilicate molecular sieves Sample Name
SiOZ Fez03
Calcined
K+/Fe Mole Ratio Steamed at 550°C 1 hour Z hours 4 hours
FeZSM-5 (ZOO)
17b
0.96
0.71
0.53
0.43
FeZSM-5 (90)
92
0.88
0.43
0.33
0.31
FeZSM-5 (50)
54
0.44
O.ZO
0.19
0.18
framework iron incorporation which agrees reasonably well with those obtained using Mossbauer spectroscopy[15J. The dehydrogenation reactions of paraethyltoluene and orthoethyltoluene were conducted separately over silicalite and unsupported iron oxide (Table Z). It is clear from these results that the silicalite matrix is not active for the dehydrogenation of ethyltoluenes and that it has a very low activity for production of methylstyrene. On the other hand, unsupported iron oxide acts as a good catalyst for the dehydrogenation of both paraethyltoluene and othoethyltolune to form methylstyrenes. In addition, paraethyltoluene dehydrogenates more easily than orthoethyltoluene. The intrinsic activity of iron oxide sites to dehydrogenate paraethyltoluene to methylstyrenes is 1.53 times that at which orthoethyltoluene dehydrogenates to methylstyrenes. Ferrislicate molecular sieves containing three bulk SiOZ/Fez03 ratios of 54, 92 ana 175 were examined. The results obtained are shown in Table 3. The external mass-transfer effects for these catalysts were negligible. The paraethyltaluene as feed showed no internal mass-transfer effects, however, orthoethyltoluene as feed showed a weak internal mass-transfer effect.
213
FeZSM-5(175): In this very silica rich sample (approximately 1 iron per unit cell), most of the iron is retained in the framework of the molecular sieve after thermal treatment (Table 1). The calcined form also has a very low activity for formation of methylstyrene from orthoethyltoluene and paraethyltoluene (Table 3). With mild hydrothermal treatment (steamed 1 hour), a large increase in the rate of methylstyrene produced from orthoethyltoluene and paraethyltoluene is observed (Table 3) as more of the iron is removed from the framework sites (Tables 1). Further steaming (2 and 4 hours) causes a marginal increase in the fraction of iron removed from the framework of the molecular sieve (Table I), which in turn also causes a small increase in the rates of methylstyrenes produced from orthoethyltoluene and paraethyltoluene (Table 3) over these materials. Upon severely steaming the FeZSM-5(175) for four hours at 550°C, it is seen that only 43% of the total iron is retained in the framework of the molecular TABLE 2 Reaction results obtained over silicalite and unsupported iron oxide Sample Name
+
Feed
Conversion (%)
g moles of MS produced X 1010 g cat - s
Si 1ical ite
PET+ OET*
1.3 1.3
5.8 5.8
Iron Oxide
PET OET
15.0 9.4
113 74
PET denotes paraethyltoluene; * OET denotes Orthoethyltoluene
sieve. Tne iron removea from the framework sites is deposited as iron oxide on the outside of the framework[7]. This hydrothermally treated material has approximately seven-fold activity for formation of methylstyrene from orthoethyltoluene and paraethyltoluene (Table 3) as compared to the calcined FeZSM-5(175). Both the calcined and the steamed materials have the same amount of total iron content present in them, the only difference between them is that the calcined material has almost all of the iron in the framework sites, whereas, the steamed material has nearly half of its total iron content present as iron oxide phase in the non-framework sites indicating that the framework iron does not participate in the production of
214
methylstyrene. It is the iron present in the non-framework sites which is active for the dehydrogenation of ethyltoluenes to methylstyrenes. This is in agreement with the results stated earlier. Orthoethyltoluene is a molecule which cannot enter the pores of the ferrisilicate molecular sieves and therefore the rate of methylstyrene produced from orthoethyltoluene is a reflection on the amount of iron oxide present on the outside of the crystal (as non-framework iron). The rate of methylstyrene produced from orthoethyltoluene increases with steaming of the FeZSM-5(175) (Table 3) showing that the iron migrates from the pores of the molecular sieve to the surface of the crystal. The results obtained with compositional analysis of the ferrisilicate molecular sieves (and verified by Mossbauer spectroscopy[7]) are used to determine the fraction of the total iron present in the framework sites of the molecular sieve as shown in Figure 1. The zero time of steaming in Figure 1 corresponds to the calcined material. The rest of the iron which is present in the non-framework sites is distributed within the pores and on the surface of the crystal. By calculating the ratio of methylstyrene TABLE 3
Reaction results obtained over ferrisilicate molecular seives Sample Name
Feed Calcined
g moles of MS produced X 1010 9 cat. - s Steamed at 550°C 1 hours 2 hours 4 hours
FeZSM-5(l75)
PET+ OET*
22 13
104 50
158 90
162 98
FeZSM-5(92)
PET+ OET*
192 79
253 108
294 133
295 136
FeZSM-5 (54)
PET+ OET*
409 121
406 160
399 167
398 167
Silicalite
PET+ OET*
5.8 5.8
+ denotes paraethyltoluene; * OET denotes orthoethyltoluene
215
produced using paraethyltoluene in the feed to that obtained with orthoethyltoluene (Table 3) and considering that any value above the ratio of 1.53 (the intrinsic activity of methylstyrene formation from paraethyltoluene to that from orthoethyltoluene over unsupported iron oxide) is due to the iron oxide present in the pores of the molecular sieve, the fraction of total iron present in the pores of the molecular sieve and the fraction of the total iron present outside the crystal of the molecular sieve was determined. The results obtained are shown in Figure 1. It is clearly seen from the Figure 1 that as the FeZSM-5(175) is hydrothermally treated, the iron is removed from the framework of the molecular sieve and it migrates to the surface of the crystal. It is also seen that after an initial increase in the amount of iron in the pores, the fraction of iron in the pores does not change with increased time of steaming. FeZSM-5(92): In a sample containing, on the average two framework iron per unit cell (in the as-synthesized form), significant rates of methylstyrene production is observed for both orthoethyltoluene and paraethyltoluene over the calcined form. From the compositional analysis (Table 1) it is seen that 88% of the total iron occupies th framework sites and that 12% of the total iron occupies the non-framework sites. Therefore, the methylstyrene produced could be due to the non-framework iron oxide being finely distributed throughout the molecular sieves. The presence of such iron oxide has been reported earlier[7J. With mild steaming (1 hour) over 57% of the total iron is removed from the framework (Table 1). This is accompanied by increased rates of methylstyrene production from both paraethyltoluene and orhtoethyltoluene. With increased amount of hydrothermal treatment (2 and 4 hours) a stabilization is observed in the amount of iron removed (at about 70%, Table 1) and the rates of methylstyrene produced from both paraethyltolue~e and orthoethyltoluene remain a constant. It is also seen that the rate of methylstyrene produced from orthoethyltoluene increases with increasing time of hydrothermal treatment of the FeZSM-5(92), showing that the hydrothermal treatment causes iron to migrate from the pores of the molecular sieve to the surface of the crystal. The distribution of the iron in the FeZSM-5(92) was calculated (similar to the procedure use for determining the distribution of iron in FeZSM-5(175)) using the compositional analysis and the rates of production of the methylstyrenes. The results obtained are shown in Figure 2. In the calcined FeZSM-5(92) about 88% of the total iron is in the framework sites.
216
1.0 c
0.8
0
0.6
-=Cii 0
l-
a c
nco 0
u::
. -o- . . -
1.0
in-the -ores· -:
-- ----0- -
c
-= Cii
0.8
0
0.6
c
0.4
a
0.4
Q
0
outside the pores
co
u::
in the framework sites
0.0 1.0
2.0
3.0
4.0
0.2 0.0
5.0
in the framework sites
0.0
Distribution of iron in FeZSM-5 (175)
1.0
2.0
3.0
4.0
5.0
Time of Steaming (Hours)
Time of Steaming (Hours)
Figure 1.
----'-<0
0
I-
0.0
~_
in the pores
outside the pores
0.2
- - - - 0 - - ____
Figure 2.
Distribution of iron in FeZSM-5(92)
The rest (12%) is almost equally divided between the inside of the pores and outside of the crystal surface. Upon hydorthermal treatment of the FeZSM-5(92) more iron is removed from the framework of the molecular sieve and migrates to the surface of the crystal. The amount of iron (in the non-framework sites) present on the surface of the crystal increases with increasing time of hydrothermal treatment. It is also seen that after an initial increase in the amount of iron in the pores of the molecular sieves, the fraction of the total iron in the pores remains constant and does not change with increasing time of hydrothermal treatment. A comparison of Figures 1 and 2 shows that with decreasing Si02/Fe203 ratio the fraction of the total iron in the pores increases. FeZSM-5(54): In a sample containing on the average, 4 framework iron ions per unit cell (in the as-synthesized form), the rates of methy l styr ene
217
production over the calcined material were significantly higher compared to any other Si02/Fe203 ratios. Unlike other materials, mild hydrothermal treatment (1 hour steaming) does not cause the rate of methylstyrene production from paraethyltoluene to change. From the compositional analysis
. 10
Cii
o
I ~
- - - - -0- - - - - -0- - - - - - -
0.8
1
0.6
I 1
---0
in the pores
i
j
I-
o
1
0.4~
~
0.2
0.0
outside the pores
...., - - . . . - - -_ _
!
in the framework sites
+1--~---,------r--.,..-----,
00
1.0
20
3.0
4.0
5.0
Time of Steaming (Hours)
Figure 3.
Distribution of iron in FeZSM-5(54)
(Table 1) it is seen that about 56% of the iron in calcined FeZSM-5(54) is non-framework iron oxide is finely in the non-framework sites. This dispersed throughout the molecular sieve[7]. The high rates of methylstyrene production from paraethyltoluene in the calcined FeZSM-5(54) is due to this high dispersion. On mild hydrothermal treatment (1 hours of steaming), although more iron comes out of the framework (Table 1) there is no change in the rate of paramethylstyrene produced (Table 3). This is attributed to hydrothermal treatment that causes non-framework iron oxide to agglomerate[7] and thereby does not increase rate of paramethylstyrene production, even though there is more non-framework iron oxide phase. On the other hand, as the FeZSM-5(54) is hydrothermally treated under mild conditions there is an increase in the rate of paramethylstyrene. The mild hydrothermal treatment (1 hour of steaming) at 550°C increases the amount of iron on the surface of the crystal, and is attributed to the migration of the iron oxide phase from the inside of the pores to the surface of the
218
crystal. Prolonged hydrothermal treatment (2 and 4 hours of steaming) does not cause any further removal of iron from the framework nor do the rates of methylstyrene production from either paraethyltoluene or orthoethyltoluene change (Table 3) indicating that the fraction of the total iron in the framework sites, in the pores and outside the surface of the crystal stabilizes after 1 hour of hydrothermal treatment. The distribution of the iron in the FeZSM-5(54) was calculated (similar to the procedure used for determining the distribution of iron in FeZSM-5(1751. The results obtained are shown in Figure 3. It is seen from Figure 3 that in the calcined FeZSM-5(54) about 45% of the total iron is in the framework sites, about 30% of the total iron is in the pore in the molecular sieve and Z5% of the total iron is located on the surfaces of the crystal. It is also seen that the fraction of the iron in the pores does not change with time of hydrothermal treatment. Comparison of Figures 1, Z and 3 shows that with decreasing SiOZ/Fez03 ratio the fraction of the total iron in the pores increases. Conclusions: Modifications of ferrisilicate molecular sieves offers a unique method of dispersing iron oxide within the pores of the ZSM-5 type structure. The results of this study clearly show that, when there is little iron contained in the molecular sieve (SiOZ/Fez03 = 175), in the calcined form almost all of the iron remains in the framework sites. As the material becomes more iron rich (SiOZ/FeZ03=9Z and 54), upon calcination more iron is removed from the framework of the molecular seive and is very finely dispersed throughout the molecular sieve. Upon hydrothermal treatment, initially the amount of iron in the pores increases, however, on increasing the steaming time the amount of iron in the pores of the ferrisilicate molecular sieves does not change further. The fraction of the total iron in the pores of the molecular sieves increases with increasing iron content (or decreasing SiOZ/FeZ03 ratio). Hydrothermal treatment of ferrisilicate molecular seives causes migration of iron oxide to the surface of the crystal. The fraction of the total iron on the outside of the crystal initially increases with increasing time of hydrothermal treatment; however, it reaches a steady state with time. The higher the iron content (or lower the SiOZ/FeZ03 ratio), the less the time it takes to reach an equilibrium value.
219
From this study, it has been shown that shape-selective probe reactions are a very useful tool in determining the distribution of iron oxide present in the ferrisilicate molecular sieves.
The shape-selective catalytic
reactions can be used to monitor the location of the non-framework iron in the ferrisilicates. Such information would be difficult to obtain using other characterization techniques. REFERENCES 1. Szostak, R., Nair, V., ana Thomas, T. L., J. Chem. Soc., Faraday transactions, 83 (1987) 487.
2.
Szostak, R., and Thomas, T. L., J. Catal., 100 (1986) 555.
3. Ratnasamy, P., Borade, R. E., Sivasanker, S., Shiralkar, V. P., and Hedge, S. G., Acta Phys. Chem., 31(1-2) (1985) 137. 4. Kotasthane, A. N., Shiralkar, V. P., Hedte, S. G., and Kulkarni, S. B., Zeolites, 6 (1986) 253. 5. Calis, G., Frenken, P., de Boer, E., Swolfs, A., and Hefni, M. A., Zeolites, 7 (1987)319. 6. lone, K. G., Vostrikova, L. A., Petrova, A. V., and Mastikhim, V. M., 'Structure and Reactivity of Modified Zeolites', Jacobs, ed., Elsevier, Amsterdam, 1984, p151. 7.
Meagher, A., Nair, V., and Szostak, R., Zeolites, accepted.
8. Satterfield, C. N., "Heterogeneous Catalysis in Pr act ive ", McGraw Hill, New York, 1980, p268. 9. Nair V., Ph.D. Thesis, Georgia Institute of Technology, Atlanta, Georgia, (1987).
© 1988 Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
SHAPE-SELECTIVE CATALYSIS USING MODIFIED FERRISILICATE MOLECULAR SIEVES V. Nair I,2 R. Szostak 1, P. K. Agrawal 2, T. L. Thomas 1 lZeolite Research Program, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332 2S chool of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 ABSTRACT Ferrisilicate molecular sieves have been synthesized with the ZSM-5 structure having Si02/Fe203 molar ratios ranging from 25 to infinity. Thermal and/or hydrothermal treatment causes some of the iron is to be removed from the tetrahedral framework sites and forms a secondary iron oxide phase. These ferrisilicates have been characterized using dehydrogenation of para- and ortho-ethyltoluenes. The para-ethyltoluene reacts on the iron oxide present within the pores and on the surface of the crystal. Whereas, ortho-ethyltoluene reacts only on the iron oxide present on the surface of the crystal. The results obtained using the catalytic reactions in conjunction with those obtained through the physico-chemical techniques aid in understanding the nature and location of iron in these ferrisilicate and modified ferrisilicate molecular sieves. INTRODUCT ION Recent advances in molecular sieve synthesis include preparation of molecular sieves which contain iron, gallium, boron, phosphorous and other transitional metals in the framework sites. The ferrisilicate molecular sieves comprise a class of molecular sieves in which, besides Si, only Fe (III) occupies tetrahedral framework sites[I-7J. It has been shown [IJ that when the ferrisilicate molecular sieve analog of the zeolite ZSM-5 is subjected to thermal or hydrothermal treatment, the Fe (III) is removed from its lattice sites. Once iron is removed from the framework it can be present in two possible locations: highly dispersed within the pores of the sieve or on the surface of the crystal. Most of the characterization of ferrisilicate molecular sieves thus far[I-7J has been devoted to determination of the state of the iron in these molecular sieves and whether or not they incorporate into the framework sites. In this study, shape-selective catalysis is used to determine the distribution of non-framework iron. As iron oxide is an active catalyst for the conversion of ethyl benzene to styrene[8J, the location of the iron oxide within the pores and on the surface of the crystal can be determined by studying the dehydrogenation of the ortho and para isomers of the
209
210
methyl-substituted ethylbenzene, methyltoluene. In the ZSM-5 structure, the para-isomer has access to the internal pores of the crystal to react with any iron oxide present within those pores. The ortho-isomer is not appreciably adsorbed into the pores and therefore will only react with any iron oxide present on the surface of the crystals. By examining these two reactions, the iron oxide present on the outside of the pores can be distinguished from the iron oxide within the pores. In addition, the migration of the iron from the pores of the crystal to the surface under various thermal and hydrothermal treatments can be monitored. The advantage of using these probe molecules is that there are chemically very similar. The results obtained using the probe reactions have been correlated with those obtained with physico-chemical techniques. EXPERIMENTAL PROCEDURE Sample preparation: The synthesis and modification procedure used for preparing ferrisilicate molecular sieves with the ZSM-5 structure have been explained in detail earlierCl,7J. Ferrisilicate molecular sieves with the ZSM-5 type structure having SiOZ/FeZ03 ratios of 175, 9Z, and 54 were synthesized using a stirred autoclave, then calcined in nitrogen followed by air, after which they were ammonium-exchangedCl,7J. A portion of the ammonium-exchanged sample was calcined and then potassium-exchanged with KOH to a pH of 8.0Cl,7J. Portions of the ammonium-exchanged samples were hydrothermally treated in 100% steam for 1 hour, Z hours, ad 4 hours (at 500°C) respectively. Each of these hydrothermally modified ferrisilicate molecular sieves was then potassium-exchanged by titration with KOH to a pH of 8.0Cl,7J. Samples in this study are denoted by a notation FeZSM-5(XX); where, FeZSM-5 implies that the material is a ferrisilicate molecular sieve with a ZSM-5 type structure and XX denotes the SiOZ/FeZ03 molar ratio of the materials. For example, FeZSM-5 (175) denotes a ferrisilicate molecular sieve with a ZSM-5 type structure with a SiOZ/FeZ03 molar ratio of 175. Union Carbide's silicalite S-115LE was used for comparison. Unsupported iron oxide was obtained as ferric oxide from Fisher Scientific. It was calcined in dry air at 500°C for 16 hours before using it as catalyst. Basic Characterization: X-ray diffraction patterns of all the molecular sieves were obtained using Rigaku X-ray diffractometer. The range of two theta values over which the diffraction pattern was obtained was 5° to 40°; the scan was conducted at a rate of 5°/min. The adsorption capacities of molecular sieves were examined using a McBain-Bakr adsorption
211
system. The oxygen adsorption capacities were determined at -196°C and 75 torr pressure. The n-hexane adsorption capacities were determined at room temperature and 70 torr pressure. The adsorption capacities for paraethyltoluene and orthoethyltoluene over ferrisilicate molecular sieves were determined at room temperature and at a pressure of 1 torr. Chemical analysis of the samples was done by atomic absorption spectroscopy. The results of the SEM analysis, infrared spectroscopic study. adsorption properties, ion-exchange ability, and Mossbauer spectroscopic study have been described elsewhereC1,7J. Reactor System: The details of the reactor system used and the procedure for conducting the reactions are described elsewhere[9J. The products and the reactants are analyzed by a gas chromatograph using a 10' X 1/8" Alltech column filled with liquid phase of AT-1200 + 5% Bentone on chromosorb WAW, 100/1210 mesh (carrier flow 25cclmin. and column temperature of 100°C). The catalysts were run in the potassium-exchanged form to supress any possible acid-catalyzed cracking side reactions which could occur over any acid sites within the material. The main reaction that takes place is the dehydrogenation of paraethyltoluene and orthoethyltoluene to methylstyrenes. Some side reactions of cracking to form small amounts of benzene, toluene and xylenes were observed; however, the product selectivity to methylstyrenes was over 90%. The reaction results reported are for 0.5 hours on stream.
RESULTS The X-ray diffraction patterns of all the ferrisilicate molecular sieves show that the molecular sieves are highly crystalline and that they possess a lSM-5 type structure. Even after severe hydrothermal treatment (excess steam, 4 hours, 550°C) no significant changes are observed indicating that these materials retain their lSM-5 type structure even on severe steaming. The oxygen and n-hexane adsorption capacities of all molecular sieves used in this study (including that of silicalite) was determined to be around 22 (g oxygen adsorbed/100g sample) and 11.6 (g n-hexane/100g sample) indicating little change in pore volume under various treatments. The paraethyltoluene adsorption capacity of the ferrisilicate molecular sieves was found to be 15 wt%. The orthoethyltoluene adsorption capacity was less than 1 wt%; confirming the selectivity of these materials for the para-form.
212
The %Fe and %Si oDtained using atomic absorption were used in calculating the bulk SiOZ/FeZ03 ratios for each sample (Table 1). The mole ratio of K/Fe in the potassium-exchanged samples is used as a measure of the fraction of the total iron present in the framework (Table 1). Based on these exchange results, for calcined FeZSM-5(175) it is seen that essentially all of the iron is present in the framework sites. On the other hand, for FeZSM-5(54) hydrothermally treated for 4 hours, only 18% of the total iron is present in the framework sites. The other ferrisilicates have intermediate fractions of the total iron in the framework sites. The exchange study results provide a quantitative measure of the degree of TABLE 1 Chemical Composition of all ferrisilicate molecular sieves Sample Name
SiOZ Fez03
Calcined
K+/Fe Mole Ratio Steamed at 550°C 1 hour Z hours 4 hours
FeZSM-5 (ZOO)
17b
0.96
0.71
0.53
0.43
FeZSM-5 (90)
92
0.88
0.43
0.33
0.31
FeZSM-5 (50)
54
0.44
O.ZO
0.19
0.18
framework iron incorporation which agrees reasonably well with those obtained using Mossbauer spectroscopy[15J. The dehydrogenation reactions of paraethyltoluene and orthoethyltoluene were conducted separately over silicalite and unsupported iron oxide (Table Z). It is clear from these results that the silicalite matrix is not active for the dehydrogenation of ethyltoluenes and that it has a very low activity for production of methylstyrene. On the other hand, unsupported iron oxide acts as a good catalyst for the dehydrogenation of both paraethyltoluene and othoethyltolune to form methylstyrenes. In addition, paraethyltoluene dehydrogenates more easily than orthoethyltoluene. The intrinsic activity of iron oxide sites to dehydrogenate paraethyltoluene to methylstyrenes is 1.53 times that at which orthoethyltoluene dehydrogenates to methylstyrenes. Ferrislicate molecular sieves containing three bulk SiOZ/Fez03 ratios of 54, 92 ana 175 were examined. The results obtained are shown in Table 3. The external mass-transfer effects for these catalysts were negligible. The paraethyltaluene as feed showed no internal mass-transfer effects, however, orthoethyltoluene as feed showed a weak internal mass-transfer effect.
213
FeZSM-5(175): In this very silica rich sample (approximately 1 iron per unit cell), most of the iron is retained in the framework of the molecular sieve after thermal treatment (Table 1). The calcined form also has a very low activity for formation of methylstyrene from orthoethyltoluene and paraethyltoluene (Table 3). With mild hydrothermal treatment (steamed 1 hour), a large increase in the rate of methylstyrene produced from orthoethyltoluene and paraethyltoluene is observed (Table 3) as more of the iron is removed from the framework sites (Tables 1). Further steaming (2 and 4 hours) causes a marginal increase in the fraction of iron removed from the framework of the molecular sieve (Table I), which in turn also causes a small increase in the rates of methylstyrenes produced from orthoethyltoluene and paraethyltoluene (Table 3) over these materials. Upon severely steaming the FeZSM-5(175) for four hours at 550°C, it is seen that only 43% of the total iron is retained in the framework of the molecular TABLE 2 Reaction results obtained over silicalite and unsupported iron oxide Sample Name
+
Feed
Conversion (%)
g moles of MS produced X 1010 g cat - s
Si 1ical ite
PET+ OET*
1.3 1.3
5.8 5.8
Iron Oxide
PET OET
15.0 9.4
113 74
PET denotes paraethyltoluene; * OET denotes Orthoethyltoluene
sieve. Tne iron removea from the framework sites is deposited as iron oxide on the outside of the framework[7]. This hydrothermally treated material has approximately seven-fold activity for formation of methylstyrene from orthoethyltoluene and paraethyltoluene (Table 3) as compared to the calcined FeZSM-5(175). Both the calcined and the steamed materials have the same amount of total iron content present in them, the only difference between them is that the calcined material has almost all of the iron in the framework sites, whereas, the steamed material has nearly half of its total iron content present as iron oxide phase in the non-framework sites indicating that the framework iron does not participate in the production of
214
methylstyrene. It is the iron present in the non-framework sites which is active for the dehydrogenation of ethyltoluenes to methylstyrenes. This is in agreement with the results stated earlier. Orthoethyltoluene is a molecule which cannot enter the pores of the ferrisilicate molecular sieves and therefore the rate of methylstyrene produced from orthoethyltoluene is a reflection on the amount of iron oxide present on the outside of the crystal (as non-framework iron). The rate of methylstyrene produced from orthoethyltoluene increases with steaming of the FeZSM-5(175) (Table 3) showing that the iron migrates from the pores of the molecular sieve to the surface of the crystal. The results obtained with compositional analysis of the ferrisilicate molecular sieves (and verified by Mossbauer spectroscopy[7]) are used to determine the fraction of the total iron present in the framework sites of the molecular sieve as shown in Figure 1. The zero time of steaming in Figure 1 corresponds to the calcined material. The rest of the iron which is present in the non-framework sites is distributed within the pores and on the surface of the crystal. By calculating the ratio of methylstyrene TABLE 3
Reaction results obtained over ferrisilicate molecular seives Sample Name
Feed Calcined
g moles of MS produced X 1010 9 cat. - s Steamed at 550°C 1 hours 2 hours 4 hours
FeZSM-5(l75)
PET+ OET*
22 13
104 50
158 90
162 98
FeZSM-5(92)
PET+ OET*
192 79
253 108
294 133
295 136
FeZSM-5 (54)
PET+ OET*
409 121
406 160
399 167
398 167
Silicalite
PET+ OET*
5.8 5.8
+ denotes paraethyltoluene; * OET denotes orthoethyltoluene
215
produced using paraethyltoluene in the feed to that obtained with orthoethyltoluene (Table 3) and considering that any value above the ratio of 1.53 (the intrinsic activity of methylstyrene formation from paraethyltoluene to that from orthoethyltoluene over unsupported iron oxide) is due to the iron oxide present in the pores of the molecular sieve, the fraction of total iron present in the pores of the molecular sieve and the fraction of the total iron present outside the crystal of the molecular sieve was determined. The results obtained are shown in Figure 1. It is clearly seen from the Figure 1 that as the FeZSM-5(175) is hydrothermally treated, the iron is removed from the framework of the molecular sieve and it migrates to the surface of the crystal. It is also seen that after an initial increase in the amount of iron in the pores, the fraction of iron in the pores does not change with increased time of steaming. FeZSM-5(92): In a sample containing, on the average two framework iron per unit cell (in the as-synthesized form), significant rates of methylstyrene production is observed for both orthoethyltoluene and paraethyltoluene over the calcined form. From the compositional analysis (Table 1) it is seen that 88% of the total iron occupies th framework sites and that 12% of the total iron occupies the non-framework sites. Therefore, the methylstyrene produced could be due to the non-framework iron oxide being finely distributed throughout the molecular sieves. The presence of such iron oxide has been reported earlier[7J. With mild steaming (1 hour) over 57% of the total iron is removed from the framework (Table 1). This is accompanied by increased rates of methylstyrene production from both paraethyltoluene and orhtoethyltoluene. With increased amount of hydrothermal treatment (2 and 4 hours) a stabilization is observed in the amount of iron removed (at about 70%, Table 1) and the rates of methylstyrene produced from both paraethyltolue~e and orthoethyltoluene remain a constant. It is also seen that the rate of methylstyrene produced from orthoethyltoluene increases with increasing time of hydrothermal treatment of the FeZSM-5(92), showing that the hydrothermal treatment causes iron to migrate from the pores of the molecular sieve to the surface of the crystal. The distribution of the iron in the FeZSM-5(92) was calculated (similar to the procedure use for determining the distribution of iron in FeZSM-5(175)) using the compositional analysis and the rates of production of the methylstyrenes. The results obtained are shown in Figure 2. In the calcined FeZSM-5(92) about 88% of the total iron is in the framework sites.
216
1.0 c
0.8
0
0.6
-=Cii 0
l-
a c
nco 0
u::
. -o- . . -
1.0
in-the -ores· -:
-- ----0- -
c
-= Cii
0.8
0
0.6
c
0.4
a
0.4
Q
0
outside the pores
co
u::
in the framework sites
0.0 1.0
2.0
3.0
4.0
0.2 0.0
5.0
in the framework sites
0.0
Distribution of iron in FeZSM-5 (175)
1.0
2.0
3.0
4.0
5.0
Time of Steaming (Hours)
Time of Steaming (Hours)
Figure 1.
----'-<0
0
I-
0.0
~_
in the pores
outside the pores
0.2
- - - - 0 - - ____
Figure 2.
Distribution of iron in FeZSM-5(92)
The rest (12%) is almost equally divided between the inside of the pores and outside of the crystal surface. Upon hydorthermal treatment of the FeZSM-5(92) more iron is removed from the framework of the molecular sieve and migrates to the surface of the crystal. The amount of iron (in the non-framework sites) present on the surface of the crystal increases with increasing time of hydrothermal treatment. It is also seen that after an initial increase in the amount of iron in the pores of the molecular sieves, the fraction of the total iron in the pores remains constant and does not change with increasing time of hydrothermal treatment. A comparison of Figures 1 and 2 shows that with decreasing Si02/Fe203 ratio the fraction of the total iron in the pores increases. FeZSM-5(54): In a sample containing on the average, 4 framework iron ions per unit cell (in the as-synthesized form), the rates of methy l styr ene
217
production over the calcined material were significantly higher compared to any other Si02/Fe203 ratios. Unlike other materials, mild hydrothermal treatment (1 hour steaming) does not cause the rate of methylstyrene production from paraethyltoluene to change. From the compositional analysis
. 10
Cii
o
I ~
- - - - -0- - - - - -0- - - - - - -
0.8
1
0.6
I 1
---0
in the pores
i
j
I-
o
1
0.4~
~
0.2
0.0
outside the pores
...., - - . . . - - -_ _
!
in the framework sites
+1--~---,------r--.,..-----,
00
1.0
20
3.0
4.0
5.0
Time of Steaming (Hours)
Figure 3.
Distribution of iron in FeZSM-5(54)
(Table 1) it is seen that about 56% of the iron in calcined FeZSM-5(54) is non-framework iron oxide is finely in the non-framework sites. This dispersed throughout the molecular sieve[7]. The high rates of methylstyrene production from paraethyltoluene in the calcined FeZSM-5(54) is due to this high dispersion. On mild hydrothermal treatment (1 hours of steaming), although more iron comes out of the framework (Table 1) there is no change in the rate of paramethylstyrene produced (Table 3). This is attributed to hydrothermal treatment that causes non-framework iron oxide to agglomerate[7] and thereby does not increase rate of paramethylstyrene production, even though there is more non-framework iron oxide phase. On the other hand, as the FeZSM-5(54) is hydrothermally treated under mild conditions there is an increase in the rate of paramethylstyrene. The mild hydrothermal treatment (1 hour of steaming) at 550°C increases the amount of iron on the surface of the crystal, and is attributed to the migration of the iron oxide phase from the inside of the pores to the surface of the
218
crystal. Prolonged hydrothermal treatment (2 and 4 hours of steaming) does not cause any further removal of iron from the framework nor do the rates of methylstyrene production from either paraethyltoluene or orthoethyltoluene change (Table 3) indicating that the fraction of the total iron in the framework sites, in the pores and outside the surface of the crystal stabilizes after 1 hour of hydrothermal treatment. The distribution of the iron in the FeZSM-5(54) was calculated (similar to the procedure used for determining the distribution of iron in FeZSM-5(1751. The results obtained are shown in Figure 3. It is seen from Figure 3 that in the calcined FeZSM-5(54) about 45% of the total iron is in the framework sites, about 30% of the total iron is in the pore in the molecular sieve and Z5% of the total iron is located on the surfaces of the crystal. It is also seen that the fraction of the iron in the pores does not change with time of hydrothermal treatment. Comparison of Figures 1, Z and 3 shows that with decreasing SiOZ/Fez03 ratio the fraction of the total iron in the pores increases. Conclusions: Modifications of ferrisilicate molecular sieves offers a unique method of dispersing iron oxide within the pores of the ZSM-5 type structure. The results of this study clearly show that, when there is little iron contained in the molecular sieve (SiOZ/Fez03 = 175), in the calcined form almost all of the iron remains in the framework sites. As the material becomes more iron rich (SiOZ/FeZ03=9Z and 54), upon calcination more iron is removed from the framework of the molecular seive and is very finely dispersed throughout the molecular sieve. Upon hydrothermal treatment, initially the amount of iron in the pores increases, however, on increasing the steaming time the amount of iron in the pores of the ferrisilicate molecular sieves does not change further. The fraction of the total iron in the pores of the molecular sieves increases with increasing iron content (or decreasing SiOZ/FeZ03 ratio). Hydrothermal treatment of ferrisilicate molecular seives causes migration of iron oxide to the surface of the crystal. The fraction of the total iron on the outside of the crystal initially increases with increasing time of hydrothermal treatment; however, it reaches a steady state with time. The higher the iron content (or lower the SiOZ/FeZ03 ratio), the less the time it takes to reach an equilibrium value.
219
From this study, it has been shown that shape-selective probe reactions are a very useful tool in determining the distribution of iron oxide present in the ferrisilicate molecular sieves.
The shape-selective catalytic
reactions can be used to monitor the location of the non-framework iron in the ferrisilicates. Such information would be difficult to obtain using other characterization techniques. REFERENCES 1. Szostak, R., Nair, V., ana Thomas, T. L., J. Chem. Soc., Faraday transactions, 83 (1987) 487.
2.
Szostak, R., and Thomas, T. L., J. Catal., 100 (1986) 555.
3. Ratnasamy, P., Borade, R. E., Sivasanker, S., Shiralkar, V. P., and Hedge, S. G., Acta Phys. Chem., 31(1-2) (1985) 137. 4. Kotasthane, A. N., Shiralkar, V. P., Hedte, S. G., and Kulkarni, S. B., Zeolites, 6 (1986) 253. 5. Calis, G., Frenken, P., de Boer, E., Swolfs, A., and Hefni, M. A., Zeolites, 7 (1987)319. 6. lone, K. G., Vostrikova, L. A., Petrova, A. V., and Mastikhim, V. M., 'Structure and Reactivity of Modified Zeolites', Jacobs, ed., Elsevier, Amsterdam, 1984, p151. 7.
Meagher, A., Nair, V., and Szostak, R., Zeolites, accepted.
8. Satterfield, C. N., "Heterogeneous Catalysis in Pr act ive ", McGraw Hill, New York, 1980, p268. 9. Nair V., Ph.D. Thesis, Georgia Institute of Technology, Atlanta, Georgia, (1987).