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Skeletal isomerization of 1-butene to isobutene over Mg-ZSM-22
~
A PA LE IY D CP AT L SS I A: GENERAL
ELSEVIER
Applied Catalysis A: General 164 (1997) 291-301
Skeletal isomerization of 1-butene to isobutene over Mg-ZSM-22 Sung Hyeon
Baeck, Wha
Young Lee*
Department of Chemical Engineering, Seoul National Universi~, Shinlim-Dong. Kwanak-Ku, Seoul 151-742, South Korea
Received 2 December 1996; received in revised form 12 April 1997, accepted 21 April 1997
Abstract ZSM-22s, ion-exchanged with various cations, were prepared and examined as catalysts with respect to the skeletal isomerization of 1-butene to isobutene in a continuous flow reactor. Mg-ZSM-22 gave the best yield for isobutene. The order of the catalysts with respect to the yield for isobutene was proportional to the ratio of Lewis acidity to Br6nsted acidity. The Lewis acid site was found to be responsible for improvement in catalytic activity, as well as suppression of coke formation. The role of Lewis acidity in the skeletal isomerization of l-butene was confirmed by experiments involving the addition of water to the feed. The effect of impregnated boron or phosphorus was clearly observed. Phosphorus poisoned not only the strong acid sites, but also the active sites responsible for the skeletal isomerization of l-butene, while boron mainly poisoned the strong acid sites, which resulted in a decrease in side reactions and an increase in isobutylene selectivity. ~ 1997 Elsevier Science B.V. Keywords." Skeletal isomerization; Isobutene; Mg-ZSM-22; Lewis acid site
1. Introduction C4 raffinates are composed of more than 10 components including n-butenes and isobutene [1]. After the extraction of isobutene for the synthesis of MTBE, the remainder of the C4 raffinates is largely used as fuel. Since the demand for isobutene for commercial processes such as synthesis of methyl t-butyl ether and t-butyl alcohol [2,3] has continuously increased, considerable interest has been focused on the skeletal isomerization of n-butene to isobutene. A number of catalysts, such as 7-alumina [4,5], a variety of zeolites [6-8], and non-zeolitic molecular sieves [9,10] have been tested for their effectiveness in this conversion. These studies have shown that the *Corresponding author. Tel.: +82 2 880 7404; fax: +82 2 888 7295; e-mail: [email protected]. 0926-860X/97/$17.00 ~, 1997 Elsevier Science B.V. All rights reserved. PI1 S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 1 80-4
yield for isobutene is limited by oligomerization reaction and catalyst deactivation due to coke deposition. It is known that acidity is very important [11]. Both pore size and catalyst structure also play an important role in this reaction. For example, zeolite Y which contains a supercage, is rapidly deactivated by coking [12]. Recently, Thomas [13] suggested that 10-member ring zeolites with a constrained channel system might effectively depress side reactions such as oligomerization, cracking and H-transfer, and as a result, enhance the skeletal isomerization of n-butene into isobutene. ZSM-22 which was used in this study has an orthorhombic framework consisting of 10-member tings (5.5 x4.5 A) and a unidimensional channel with an elliptical cross-section [14]. As compared to other zeolites with medium pores, such as ZSM-5 which has
292
S.H. Baeck, W.E Lee~Applied Catalysis A: General 164 (1997) 291-301
two intersecting channel systems (5.3 x5.6 A, 5.1 x 5.5 * ) or ferrierite/ZSM-35 which contains cavities about 6-7 ,~ in size, ZSM-22 may have unique coke-resistant and antideactivation properties for the skeletal isomerization of n-butene. In this study, ion-exchanged ZSM-22s and a MgZSM-22 prepared by impregnation of B or P were examined for their effectiveness in the skeletal isomerization of 1-butene. The effects of temperature and water addition to the feed on the catalytic activity were investigated in detail.
2. Experimental
as Mg 2+, Mn 2+, Cu 2+, Ca 2+ in 0.01 M aqueous metal nitrate solutions at 80°C for 12h. H-ZSM-22 was prepared by ion-exchange in NH4C1 solution. 100 ml of each aqueous solution was used per gram of K-ZSM-22. The resultant metal-ZSM-22 was filtered, washed three times, and then calcined at 500°C for 4 h under a 30 ml/min He flow. In order to modify the acidic characteristics, MgZSM-22 was impregnated with boric acid (referred to, herein, as B/Mg-ZSM-22) or phosphoric acid (referred to, herein, as P/Mg-ZSM-22) by varying the level of impregnation (feeding amount: 1, 3, 5 wt%) by the incipient method. The impregnated catalysts were dried at 110°C and calcined at 500°C for 4 h, respectively.
2.1. Catalyst preparation 2.2. Characterization K-ZSM-22 was prepared by a hydrothermal method described in the literature [ 15], using 1,6-diaminohexane as a templating agent. The synthesized K-ZSM-22 was then ion exchanged three times with cations such
The crystallinity of the prepared catalysts was confirmed by XRD (Rigaku, D/Max 1 l-A) analysis using Cu Ks as a radiation beam.
3530-
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>..
2o
c15.
.O O ¢n
10.
.
0
H
-
Ca
~ Cu
Mn
Mg
Cation Fig. 1. The activities of ion-exchanged ZSM-22 at 400°C (WHSV=6 h 1):(11) initial yield; ([~) yield after 10 h reacUon.
S.H. Baeck, W.Y. Lee~Applied Catalysis A." General 164 (1997) 291-301
The surface area of the catalysts was measured with a BET surface analyzer (Micromeritics ASAP-2000). The acid amounts and strengths of the catalysts were determined by TPD of ammonia and "y-collidine(trimethyl pyridine). The TPD experimental pro-
293
cedures are as follows: the catalysts were first heated from room temperature to 500°C at a rate of 10°C/min, then further heated at 500°C for 2 h under a helium flow of 30 ml/min, and cooled to room temperature. A m m o n i a or -y-collidine as an adsorbate was then
Table 1 Physicochemical properties of ZSM-22s ion exchanged and the yield of isobutene with various cations Cation
BET surface area (mZ/g)
Ion exchange level (%)
The amount of coke deposition (wt%)
LfB a
yb
Ca Cu Mn Mg
134.3 135.3 172.5 186.1
41.04 55.04 60.24 66.12
3.98 4.12 2.87 2.53
0.18 0.38 0.98 2.27
2.12 3.24 9.45 29.85
aL/B=amount of Lewis acid/amount of Br6nsted acid L/B ratio was calculated by measuring the peak intensity at 1450 cm ~ (Lewis acid) and at 1540 cm -1 (Br6nsted acid) in the pyridine adsorbed FI'-IR analysis. by represents the yield of isobutene after 10 h reaction.
40
o~ 30 -o >-
20
[]
10 360
400
440
480
520
Temperature(°(3) Fig. 2. Trans-2, cis-2-butene and isobutylene yield after 10 h reaction with reaction temperature (WHSV=6 h-l): ( 0 ) isobutylene yield; (D) trans-2-butene yield; ( I ) cis-2-butene yield.
294
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
dosed. The sample was evacuated at 100°C and purged with helium for 1 h, in order to remove physisorbed species. The temperature was then ramped at a rate of 10°C/min from 100°C to 550°C and the TPD signal was monitored with TCD. FT-IR analyses were performed for pyridine adsorbed on the samples using an in situ IR cell. A wafer was prepared from 30 mg of catalyst and was pretreated at 450°C in vacuum, and then cooled to 150°C, followed by pyridine adsorption. Physisorbed pyridine was removed by degassing for 1 h in vacuum. The quantities of Br6nsted acid sites and Lewis acid sites were then measured in the range of 17001350 cm -1. The amount of carbon deposited on the used catalysts was determined by the CHNO analysis method [16]. Elemental analyses were carried out on a PerkinElmer P-40 inductively coupled plasma atomic emission spectrophotometer (ICP-AES). The samples
were dissolved in hydrofluoric acid, heated by microwave treatment, and then digested in a teflon-lined chamber. 2.3. R e a c t i o n e x p e r i m e n t s
The reaction was carried out in a fixed-bed flow reactor at atmospheric pressure. The catalyst was heated from room temperature to 500°C at a rate of 10°C/min under a helium flow of 20 ml/min and then pretreated at 500°C for 1 h. After the temperature was reset to the desired reaction temperature, 1-butene was fed to the reactor at the rate of 5 ml/min. The reaction products were analyzed by gas chromatography using FID with a HP-PLOT/'~-alumina capillary column. For purposes of this study, trans- and c i s - 2 - b u t e n e were regarded as reactants, because an equilibrium between 1-butene and 2-butenes was established at the reaction temperature range.
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~o
40
~o
20
0
I
I
I
I
I
I
5
10
15
20
25
30
35
Time on Stream (hr.) Fig. 3. 1-Butene conversion and isobutylene selectivity with respect to time on stream over Mg-ZSM-22 at 450°C (WHSV=6 h 1): (©) 1butene conversion; (Q) isobutylene selectivity.
295
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
with Mn 2+, Cu 2+, and Ca 2+ showed lower activity and rapid deactivation. The order of isobutene yield after a 10h reaction was Mg-ZSM-22>H-ZSM-22>MnZSM-22>Cu-ZSM-22>Ca-ZSM-22. Table 1 represents the physicochemical properties of ZSM-22s, which were ion exchanged three times with various divalent cations. The order of the affinity sequence of the exchanged cations is Mg2+> Mn2+>Cu2+>Ca 2+. The extent of cation exchange is closely related to the acidity, and thus to catalytic behavior. The trends of isobutene yield, BET surface area, and ion-exchange level are in good agreement with the affinity sequence, whereas the amount of coke deposited on the catalyst is inversely proportional to the affinity sequence. A particularly interesting point is that the order of ion-exchanged ZSM-22s for isobutene yield parallels the ratio of L/B (Lewis acidity/ Br6nsted acidity). As the L/B ratio increased, isobutene yield increased and coke deposition decreased. These results clearly show that the Lewis acidity of the
3. Results and discussion
3.1. Catalytic activity of ion-exchanged ZSM-22; catalyst screening
Mother K-ZSM-22 was ion-exchanged with various cations (H +, Mg 2+, Mn 2+, Cu 2+, Ca 2÷) and their catalytic activities for the skeletal isomerization of 1-butene to isobutene were examined. Because both cation-exchange level and isobutylene yield increased with the number of ion exchanges up to three batch-wise treatments, all ZSM-22s tested in this work were ion-exchanged three times with Me(NO3)2 solution. Fig. 1 shows the yield for isobutene at the initial stage and after 10 h over the cation-exchanged ZSM22 catalysts. Among the catalysts tested, H-ZSM-22 and Mg-ZSM-22 initially showed the higher yields for isobutene than other catalysts, with yields of 27% and 32%, respectively, while ZSM-22s cation exchanged
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,
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I
200
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300
i
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400
,
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500
,
600
Temperature (°C) Fig. 4. Temperatureprogrammeddesorptionof NH3: (a) Fresh Mg-ZSM-22;(b) Mg-ZSM-22after 10 h reaction.
296
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
cation-exchanged ZSM-22 is closely related to the formation of isobutene from n-butenes and the suppression of coke formation on the catalyst. The results coincide with the proposals by Gielgens et al. [9], who reported selective isomerization of n-butene by crystalline aluminophosphates. Cheng and Ponec [17], who used C2Hs-SH as a poison to probe the mechanism also proposed that the skeletal isomerization of n-butene took place on Lewis acid sites. A possible intermediate on Lewis acid sites seems to be 7r-allylic form. Two different intermediates have been proposed for the 7r-allylic mechanism with Lewis acid sites. One is the 7r-allylic cation [18,19], and the other is 7r-allylic carbanion [9,20,21] generated from an olefin on the Lewis acid and base pair site, which would be more reasonable. The essence of this mechanism is as follows: the proton is split off from the olefin adsorbed on the A13+ site, and it is simultaneously attached to the basic oxygen site. In this way, an allylic anion is formed on the A13+ site.
3.2. Reaction characteristics o f Mg-ZSM-22
The effect of reaction temperature on catalytic activity was examined for the case of Mg-ZSM-22. The yield for trans- and cis-2-butene decreased with increasing reaction temperature from 350°C to 500°C, while the yield for isobutene increased with increasing reaction temperature and reached the maximum point at 450°C, as shown in Fig. 2. This suggests that 1butene is preferentially converted into trans- or cis-2butene at low temperatures, but is converted into oligomers and subsequently into coke by way of cracked products at high temperatures. Therefore, an optimum temperature exists for the skeletal isomerization of 1-butene into isobutylene. Considering the fact that the yield for 2-butenes decreases and that for isobutene increases with reaction temperature, the activation energy for the skeletal isomerization of 1butene into isobutene is presumably larger than that for the double bond shift of 1-butene into trans- and cis-2-butene.
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0
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100
I
200
i
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~
300
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400
i
500
Temperature(°C) Fig. 5. Temperatureprogrammeddesorption of "7-collidine: (a) Fresh Mg-ZSM-22; (b) Mg-ZSM-22after 10 h reaction.
297
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
sent the TPD spectra of Mg-ZSM-22, using ammonia and "~-collidine(trimethyl pyridine) as adsorbates, respectively. Since the molecular size of "7-collidine is much larger than that of ammonia, it is difficult for "~-collidine to diffuse into the pores of Mg-ZSM-22. The amount of-y-collidine (0.19/unit cage) adsorbed was much less than that of ammonia (0.48/unit cage). Therefore, -y-collidine appears to be adsorbed mainly on the external surface of Mg-ZSM-22. Comparing the NH3-TPD peak of fresh Mg-ZSM22 with that of used Mg-ZSM-22 in Fig. 4, it is evident that the large number of strong acid sites in the fresh catalyst are reduced after a 10 h reaction. The ,~collidine-TPD profile in Fig. 5 also shows that peak area around 370°C decreased significantly after a 10 h reaction. This suggests that a considerable portion of strong acid sites are located on the external surface of Mg-ZSM-22 and the amount of these decrease as a result of coke deposition after the reaction. The difference in the acid strength between external and
Fig. 3 s h o w s that the conversion of 1-butene decreases rapidly at the beginning of the reaction and then maintains a constant value, while the selectivity to isobutene rapidly increases at the beginning and thereafter remains unchanged. These data suggest that, at the early stage of the reaction, 1-butene is mainly converted into by-products via oligomerization, followed by cracking reactions, thus increasing the conversion and lowering the selectivity to isobutene. As the reaction time passes, the conversion of 1butene decreases and the selectivity for isobutene increases as a result of reduction in side reactions. The main byproducts w e r e C 3 and C5 hydrocarbons, which were probably formed after dimerization and subsequent cracking. The dimerizing and cracking steps are assumed to be carbonium ion catalyzed. The carbonium ion is assumed to be formed on the Br6nsted acid sites [17]. The TPD experiments indicate that these results are closely related to coke formation. Figs. 4 and 5 repre-
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Time on Stream (hr.) Fig. 6. The amountof coke deposition and isobutyleneselectivityover Mg-ZSM-22at 450°C with respect to time on stream: (O) the amount of coke deposition; (11) isobutylene selectivity.
298
S.H. Baeck, W.X Lee~Applied Catalysis A: General 164 (1997) 291-301
internal sites appears to be due to the difference of aluminum concentration between them. It has been suggested that minor differences in the local site environment may significantly affect acid strength [22]; for example, a low local aluminum content may give rise to enhanced acidity. Fig. 6 shows the amount of coke deposition and selectivity to isobutene over Mg-ZSM-22 with respect to time on stream. Coke deposition rapidly increased at the early stage of reaction and then remained constant, and selectivity to isobutene also paralleled coke deposition. This result indicates that coke formation occurs largely on the same acid sites on which side reactions, such as oligomerization and cracking, take place. It can be concluded that coke is mainly formed on the strong acid sites which exist mostly on the external surface of Mg-ZSM-22 and side reactions also take place on the same sites. These conclusions with respect to acid site distribution and coke deposition on the strong acid sites are in agreement with results reported by Deruane [23], who
proposed that coking on zeolites with medium pore sizes, such as H-ZSM-5, takes place mainly on the external surface. Some papers have also been reported that the inactivation of the external surface of zeolite improved the catalyst life for many reactions, e.g., the cracking of hexane and 2,2-dimethyl butane [24], the alkylation of toluene with methanol [25,26], and the alkylation of toluene with ethylene [27]. Xu et al. [28] also proposed that strong acidity of a zeolite leads to severe dimerization of olefin, and the majority of these dimers crack on such strong acid sites. Strong acid sites also result in rapid coke formation, which may be an important factor for the deactivation of zeolites in olefin processes.
3.3. The effect of boron or phosphorus impregnation The activities of Mg-ZSM-22 modified by impregnation by boron (referred, herein, as B/Mg-ZSM-22) or phosphorus (referred, herein, as P/Mg-ZSM-22) with respect to the feeding amount of impregnation
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400
i
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500
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600
Temperature (°C) Fig. 7. Temperature programmed desorption of NH3: (a) Mg-ZSM-22; (b) B/Mg-ZSM-22; (c) P/Mg-ZSM-22.
299
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
100
>
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60 0 r, O w L,,
4.0 n-
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> C 0 =
0 0
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=
10
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Time on stream(hr.) Fig. 8. The variation of activities over B/Mg-ZSM-22 with time on stream at 450°C: ((3) 1-butene conversion; (Q) isobutylene selectivity.
were examined. The conversion of 1-butene over P/ Mg-ZSM-22 decreased with increasing the amount of phosphorus, while the selectivity to isobutylene increased slightly. When 3 wt% or more phosphorus was impregnated, the catalytic activity was so poor that 1-butene conversion was less than 10%. When B/Mg-ZSM-22 was used as catalyst, the conversion of 1- butene decreased and selectivity to isobutene increased in proportion to the amount of boron impregnated. However, the rate of decrease for 1-butene conversion over B/Mg-ZSM-22 was much slower than that over P/Mg-ZSM-22. These catalytic behaviors of B/Mg-ZSM-22 and P/Mg-ZSM-22 can be explained based on the results of the NH3-TPD experiment in Fig. 7. In the case of P/Mg-ZSM-22, even though only a small amount of P was impregnated, it poisoned not only the external strong acid sites that led to the side reactions but also the active sites that led to the skeletal isomerization of 1-butene and existed largely within the pores. In the case of B/ Mg-ZSM-22, however, the boron mainly poisoned the strong acid sites which led to side reactions, so that the
selectivity to isobutene was enhanced without significant reduction in 1-butene conversion. Fig. 8 shows the conversion of 1-butene and the selectivity to isobutene as a function of time on stream over 3 wt%-B/Mg-ZSM-22. The pattern of catalytic activities in Fig. 8 is quite different from that of MgZSM-22, as shown in Fig. 3. B/Mg-ZSM-22 exhibited higher selectivity to isobutene and lower conversion of 1-butene than Mg-ZSM-22 from the early stage of reaction. It also showed constant conversion and 90% or higher selectivity to isobutene for 25 h. This is probably due to the fact that boron poisoned the stronger acid sites, which caused side reactions. Therefore, when boron is properly impregnated on Mg-ZSM-22, side reactions can be suppressed and high selectivity to isobutene can be achieved. 3.4. Effect o f added water in the f e e d
In order to investigate the effect of water in the feed, water vapor of 36 Torr was admixed with the standard feed. The effect of water addition on the conversion of
300
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
80
:~ 60 11)
40 tt
0 m
T T mT
t._
(9 20 > tO
-H20
0
,
0 0
-H20
,
10
°
20
,
30
Time on Stream (hr.) Fig. 9. The effect of the water addition into the feed on 1-butene conversion and isobutylene selectivity: (Q) 1-butene conversion; (O) isobutylene selectivity.
1-butene and the selectivity to isobutene is shown in Fig. 9. The conversion of 1-butene increased but the selectivity to isobutylene decreased in the presence of water vapor. However, the catalytic activities were recovered to the original level by the removal of water vapor from the feed, thus indicating that the effect of added water in the feed is essentially reversible. If water is added to zeolites, Lewis acid sites can be converted to Br6nsted acid sites and if it is dehydrated, the Br6nsted acid sites are reversibly converted to Lewis acid sites [29]. When water vapor is admixed with the feed, side reactions can easily take place on Br6nsted acid sites. Accordingly, the conversion of 1butene increases and the selectivity to isobutene decreases. On the contrary, when the amount of Lewis acid sites is high, as a result of the removal of water from the feed, side reactions are suppressed so that the conversion of l-butene is lowered and selectivity to isobutene is enhanced. The formation of and the amounts of Lewis acid sites play an important role
in isobutene formation via the conversion of 1-butene, as shown in Table 1.
4. Conclusion ZSM-22 ion exchanged with various cations were prepared and tested as catalysts for the skeletal isomerization of 1-butene into isobutylene in a continuous flow fixed-bed reactor. It was found that the order of isobutylene yield was consistent with the order of the ratio of the amounts of Lewis acid to the amounts of Br6nsted acid and that Lewis acid sites may well improve the catalytic activity and suppress coke deposition. This was also confirmed by the addition of water to the feed. When catalytic activities were examined as a function of temperature, the yields for trans- and cis-2butene decreased with increasing temperature, while the yield for isobutene passed through a maximum at 450°C. These results suggest that the activation energy
S.H. Baeck, W.Y Lee~Applied Catalysis A: General 164 (1997) 291-301
for the skeletal isomerization of 1-butene into isobutene is larger than that for the double bond shift reaction of 1-butene into 2-butene. Coke deposition mainly occurred on the stronger acid sites on the external surface of Mg-ZSM-22 which induced side reactions, as confirmed by TPD analyses. The effect of impregnated boron or phosphorus was clearly observed. Phosphorous poisoned not only the strong acid sites but also the active sites for the skeletal isomerization of 1-butene, while boron mainly poisoned the strong acid sites.
Acknowledgements The authors thank the Yokong Ltd. for financial support.
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30l
[8] A. Thangaraj, S. Sivasanker, R Ratnasamy, J. Catal. 130 (1991) 1. [9] L.H. Gielgens, I.H.E. Veenstra, V. Ponec, Catal. Lett. 32 (1995) 195. [10] L.E. Iton, I. Choi, O.A. Desjardins, V.A. Maroni, Zeolite 9 (1989) 535. [11] W.-Q. Xu, Y.-G. Yin, S.L. Suib, C.-L. O'Young, J. Catal. 150 (1994) 34. [12] B.W. Wojciechovski, A. Corma, Catalytic Cracking: Catalysis, Chemistry and Kinetics, Marcel Dekker, New York, 1986, p. 100. [13] J.M. Thomas, Scientific American, April 1992, p. 112. [14] R. Szostak, Handbook of Molecular Sieves, Van Nostrand Reinhold, New York, 1994, p. 538. [15] S. Ernst, J. Weitkamp, S.A. Martens, C.O. Bennett, S.L. Suib, J. Catal. 48 (1989) 137. [16] Y. Yoshimura, E. Furimsky, Appl. Catal. 23 (1986) 157. [17] Z.X. Cheng, V. Ponec, J. Catal. 148 (1994) 607. [18] H.E Leftine, Proceedings of Third International Congress on Catalysis, Technip, Paris, 1961, p. 1065. [19] A. Corado, A. Kis, H. Kntizinger, H.D. Muller, J. Catal. 37 (1975) 68. [20] J.B. Peri, J. Phys. Chem. 69 (1965) 231. [21] H.R. Gerberich, W.K. Hall, J. Catal. 5 (1966) 99. [22] R. Szostak, Molecular Sieves Principles of Synthesis and Identification, Van Nostrand Reinhold, New York, 1988, p. 33. [23] E.G. Deruane, Stud. Surf. Sci. Catal. 5 (1980) 5. [24] J.A. Anderson, K. Fager, T. Mole, R.A. Rajadhyaksba, J.V. Sanders, J. Catal. 58 (1979) 114. [25] T. Yashima, Y. Sakaguchi, S. Namba, Stud. Surf. Sci. Catal. 7 (1981) 739. [26] C.S. Lee, T.J. Park, W.Y. Lee, Appl. Catal. 96 (1993) 151. [27] L.D. Rollman, US Patent, 4 300 011 (1981 ). [28] W.-Q. Xu, Y.-G. Yin, S.L. Suib, C.-L. O'Young, J. Phys. Chem. 99 (1995) 758. [29] C.N. Satterfield, Heterogeneous Catalysis in industrial Practice, 2nd ed., Mcgraw-Hill, New York, 1991, p. 210.
A PA LE IY D CP AT L SS I A: GENERAL
ELSEVIER
Applied Catalysis A: General 164 (1997) 291-301
Skeletal isomerization of 1-butene to isobutene over Mg-ZSM-22 Sung Hyeon
Baeck, Wha
Young Lee*
Department of Chemical Engineering, Seoul National Universi~, Shinlim-Dong. Kwanak-Ku, Seoul 151-742, South Korea
Received 2 December 1996; received in revised form 12 April 1997, accepted 21 April 1997
Abstract ZSM-22s, ion-exchanged with various cations, were prepared and examined as catalysts with respect to the skeletal isomerization of 1-butene to isobutene in a continuous flow reactor. Mg-ZSM-22 gave the best yield for isobutene. The order of the catalysts with respect to the yield for isobutene was proportional to the ratio of Lewis acidity to Br6nsted acidity. The Lewis acid site was found to be responsible for improvement in catalytic activity, as well as suppression of coke formation. The role of Lewis acidity in the skeletal isomerization of l-butene was confirmed by experiments involving the addition of water to the feed. The effect of impregnated boron or phosphorus was clearly observed. Phosphorus poisoned not only the strong acid sites, but also the active sites responsible for the skeletal isomerization of l-butene, while boron mainly poisoned the strong acid sites, which resulted in a decrease in side reactions and an increase in isobutylene selectivity. ~ 1997 Elsevier Science B.V. Keywords." Skeletal isomerization; Isobutene; Mg-ZSM-22; Lewis acid site
1. Introduction C4 raffinates are composed of more than 10 components including n-butenes and isobutene [1]. After the extraction of isobutene for the synthesis of MTBE, the remainder of the C4 raffinates is largely used as fuel. Since the demand for isobutene for commercial processes such as synthesis of methyl t-butyl ether and t-butyl alcohol [2,3] has continuously increased, considerable interest has been focused on the skeletal isomerization of n-butene to isobutene. A number of catalysts, such as 7-alumina [4,5], a variety of zeolites [6-8], and non-zeolitic molecular sieves [9,10] have been tested for their effectiveness in this conversion. These studies have shown that the *Corresponding author. Tel.: +82 2 880 7404; fax: +82 2 888 7295; e-mail: [email protected]. 0926-860X/97/$17.00 ~, 1997 Elsevier Science B.V. All rights reserved. PI1 S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 1 80-4
yield for isobutene is limited by oligomerization reaction and catalyst deactivation due to coke deposition. It is known that acidity is very important [11]. Both pore size and catalyst structure also play an important role in this reaction. For example, zeolite Y which contains a supercage, is rapidly deactivated by coking [12]. Recently, Thomas [13] suggested that 10-member ring zeolites with a constrained channel system might effectively depress side reactions such as oligomerization, cracking and H-transfer, and as a result, enhance the skeletal isomerization of n-butene into isobutene. ZSM-22 which was used in this study has an orthorhombic framework consisting of 10-member tings (5.5 x4.5 A) and a unidimensional channel with an elliptical cross-section [14]. As compared to other zeolites with medium pores, such as ZSM-5 which has
292
S.H. Baeck, W.E Lee~Applied Catalysis A: General 164 (1997) 291-301
two intersecting channel systems (5.3 x5.6 A, 5.1 x 5.5 * ) or ferrierite/ZSM-35 which contains cavities about 6-7 ,~ in size, ZSM-22 may have unique coke-resistant and antideactivation properties for the skeletal isomerization of n-butene. In this study, ion-exchanged ZSM-22s and a MgZSM-22 prepared by impregnation of B or P were examined for their effectiveness in the skeletal isomerization of 1-butene. The effects of temperature and water addition to the feed on the catalytic activity were investigated in detail.
2. Experimental
as Mg 2+, Mn 2+, Cu 2+, Ca 2+ in 0.01 M aqueous metal nitrate solutions at 80°C for 12h. H-ZSM-22 was prepared by ion-exchange in NH4C1 solution. 100 ml of each aqueous solution was used per gram of K-ZSM-22. The resultant metal-ZSM-22 was filtered, washed three times, and then calcined at 500°C for 4 h under a 30 ml/min He flow. In order to modify the acidic characteristics, MgZSM-22 was impregnated with boric acid (referred to, herein, as B/Mg-ZSM-22) or phosphoric acid (referred to, herein, as P/Mg-ZSM-22) by varying the level of impregnation (feeding amount: 1, 3, 5 wt%) by the incipient method. The impregnated catalysts were dried at 110°C and calcined at 500°C for 4 h, respectively.
2.1. Catalyst preparation 2.2. Characterization K-ZSM-22 was prepared by a hydrothermal method described in the literature [ 15], using 1,6-diaminohexane as a templating agent. The synthesized K-ZSM-22 was then ion exchanged three times with cations such
The crystallinity of the prepared catalysts was confirmed by XRD (Rigaku, D/Max 1 l-A) analysis using Cu Ks as a radiation beam.
3530-
25-
>..
2o
c15.
.O O ¢n
10.
.
0
H
-
Ca
~ Cu
Mn
Mg
Cation Fig. 1. The activities of ion-exchanged ZSM-22 at 400°C (WHSV=6 h 1):(11) initial yield; ([~) yield after 10 h reacUon.
S.H. Baeck, W.Y. Lee~Applied Catalysis A." General 164 (1997) 291-301
The surface area of the catalysts was measured with a BET surface analyzer (Micromeritics ASAP-2000). The acid amounts and strengths of the catalysts were determined by TPD of ammonia and "y-collidine(trimethyl pyridine). The TPD experimental pro-
293
cedures are as follows: the catalysts were first heated from room temperature to 500°C at a rate of 10°C/min, then further heated at 500°C for 2 h under a helium flow of 30 ml/min, and cooled to room temperature. A m m o n i a or -y-collidine as an adsorbate was then
Table 1 Physicochemical properties of ZSM-22s ion exchanged and the yield of isobutene with various cations Cation
BET surface area (mZ/g)
Ion exchange level (%)
The amount of coke deposition (wt%)
LfB a
yb
Ca Cu Mn Mg
134.3 135.3 172.5 186.1
41.04 55.04 60.24 66.12
3.98 4.12 2.87 2.53
0.18 0.38 0.98 2.27
2.12 3.24 9.45 29.85
aL/B=amount of Lewis acid/amount of Br6nsted acid L/B ratio was calculated by measuring the peak intensity at 1450 cm ~ (Lewis acid) and at 1540 cm -1 (Br6nsted acid) in the pyridine adsorbed FI'-IR analysis. by represents the yield of isobutene after 10 h reaction.
40
o~ 30 -o >-
20
[]
10 360
400
440
480
520
Temperature(°(3) Fig. 2. Trans-2, cis-2-butene and isobutylene yield after 10 h reaction with reaction temperature (WHSV=6 h-l): ( 0 ) isobutylene yield; (D) trans-2-butene yield; ( I ) cis-2-butene yield.
294
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
dosed. The sample was evacuated at 100°C and purged with helium for 1 h, in order to remove physisorbed species. The temperature was then ramped at a rate of 10°C/min from 100°C to 550°C and the TPD signal was monitored with TCD. FT-IR analyses were performed for pyridine adsorbed on the samples using an in situ IR cell. A wafer was prepared from 30 mg of catalyst and was pretreated at 450°C in vacuum, and then cooled to 150°C, followed by pyridine adsorption. Physisorbed pyridine was removed by degassing for 1 h in vacuum. The quantities of Br6nsted acid sites and Lewis acid sites were then measured in the range of 17001350 cm -1. The amount of carbon deposited on the used catalysts was determined by the CHNO analysis method [16]. Elemental analyses were carried out on a PerkinElmer P-40 inductively coupled plasma atomic emission spectrophotometer (ICP-AES). The samples
were dissolved in hydrofluoric acid, heated by microwave treatment, and then digested in a teflon-lined chamber. 2.3. R e a c t i o n e x p e r i m e n t s
The reaction was carried out in a fixed-bed flow reactor at atmospheric pressure. The catalyst was heated from room temperature to 500°C at a rate of 10°C/min under a helium flow of 20 ml/min and then pretreated at 500°C for 1 h. After the temperature was reset to the desired reaction temperature, 1-butene was fed to the reactor at the rate of 5 ml/min. The reaction products were analyzed by gas chromatography using FID with a HP-PLOT/'~-alumina capillary column. For purposes of this study, trans- and c i s - 2 - b u t e n e were regarded as reactants, because an equilibrium between 1-butene and 2-butenes was established at the reaction temperature range.
100
o~ v >,
80
Q)
l,..
6O
o E:
._o (/) Q;
~o
40
~o
20
0
I
I
I
I
I
I
5
10
15
20
25
30
35
Time on Stream (hr.) Fig. 3. 1-Butene conversion and isobutylene selectivity with respect to time on stream over Mg-ZSM-22 at 450°C (WHSV=6 h 1): (©) 1butene conversion; (Q) isobutylene selectivity.
295
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
with Mn 2+, Cu 2+, and Ca 2+ showed lower activity and rapid deactivation. The order of isobutene yield after a 10h reaction was Mg-ZSM-22>H-ZSM-22>MnZSM-22>Cu-ZSM-22>Ca-ZSM-22. Table 1 represents the physicochemical properties of ZSM-22s, which were ion exchanged three times with various divalent cations. The order of the affinity sequence of the exchanged cations is Mg2+> Mn2+>Cu2+>Ca 2+. The extent of cation exchange is closely related to the acidity, and thus to catalytic behavior. The trends of isobutene yield, BET surface area, and ion-exchange level are in good agreement with the affinity sequence, whereas the amount of coke deposited on the catalyst is inversely proportional to the affinity sequence. A particularly interesting point is that the order of ion-exchanged ZSM-22s for isobutene yield parallels the ratio of L/B (Lewis acidity/ Br6nsted acidity). As the L/B ratio increased, isobutene yield increased and coke deposition decreased. These results clearly show that the Lewis acidity of the
3. Results and discussion
3.1. Catalytic activity of ion-exchanged ZSM-22; catalyst screening
Mother K-ZSM-22 was ion-exchanged with various cations (H +, Mg 2+, Mn 2+, Cu 2+, Ca 2÷) and their catalytic activities for the skeletal isomerization of 1-butene to isobutene were examined. Because both cation-exchange level and isobutylene yield increased with the number of ion exchanges up to three batch-wise treatments, all ZSM-22s tested in this work were ion-exchanged three times with Me(NO3)2 solution. Fig. 1 shows the yield for isobutene at the initial stage and after 10 h over the cation-exchanged ZSM22 catalysts. Among the catalysts tested, H-ZSM-22 and Mg-ZSM-22 initially showed the higher yields for isobutene than other catalysts, with yields of 27% and 32%, respectively, while ZSM-22s cation exchanged
250
480
v
.IQ
260
0
0 ~
e-
o
E E <
,
100
I
200
,
I
300
i
I
400
,
I
500
,
600
Temperature (°C) Fig. 4. Temperatureprogrammeddesorptionof NH3: (a) Fresh Mg-ZSM-22;(b) Mg-ZSM-22after 10 h reaction.
296
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
cation-exchanged ZSM-22 is closely related to the formation of isobutene from n-butenes and the suppression of coke formation on the catalyst. The results coincide with the proposals by Gielgens et al. [9], who reported selective isomerization of n-butene by crystalline aluminophosphates. Cheng and Ponec [17], who used C2Hs-SH as a poison to probe the mechanism also proposed that the skeletal isomerization of n-butene took place on Lewis acid sites. A possible intermediate on Lewis acid sites seems to be 7r-allylic form. Two different intermediates have been proposed for the 7r-allylic mechanism with Lewis acid sites. One is the 7r-allylic cation [18,19], and the other is 7r-allylic carbanion [9,20,21] generated from an olefin on the Lewis acid and base pair site, which would be more reasonable. The essence of this mechanism is as follows: the proton is split off from the olefin adsorbed on the A13+ site, and it is simultaneously attached to the basic oxygen site. In this way, an allylic anion is formed on the A13+ site.
3.2. Reaction characteristics o f Mg-ZSM-22
The effect of reaction temperature on catalytic activity was examined for the case of Mg-ZSM-22. The yield for trans- and cis-2-butene decreased with increasing reaction temperature from 350°C to 500°C, while the yield for isobutene increased with increasing reaction temperature and reached the maximum point at 450°C, as shown in Fig. 2. This suggests that 1butene is preferentially converted into trans- or cis-2butene at low temperatures, but is converted into oligomers and subsequently into coke by way of cracked products at high temperatures. Therefore, an optimum temperature exists for the skeletal isomerization of 1-butene into isobutylene. Considering the fact that the yield for 2-butenes decreases and that for isobutene increases with reaction temperature, the activation energy for the skeletal isomerization of 1butene into isobutene is presumably larger than that for the double bond shift of 1-butene into trans- and cis-2-butene.
370
V
'1o ..Q t,=.
0 ¢/)
350
"o ° ~
'1o ° i i
0
I
100
I
200
i
I
~
300
I
400
i
500
Temperature(°C) Fig. 5. Temperatureprogrammeddesorption of "7-collidine: (a) Fresh Mg-ZSM-22; (b) Mg-ZSM-22after 10 h reaction.
297
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
sent the TPD spectra of Mg-ZSM-22, using ammonia and "~-collidine(trimethyl pyridine) as adsorbates, respectively. Since the molecular size of "7-collidine is much larger than that of ammonia, it is difficult for "~-collidine to diffuse into the pores of Mg-ZSM-22. The amount of-y-collidine (0.19/unit cage) adsorbed was much less than that of ammonia (0.48/unit cage). Therefore, -y-collidine appears to be adsorbed mainly on the external surface of Mg-ZSM-22. Comparing the NH3-TPD peak of fresh Mg-ZSM22 with that of used Mg-ZSM-22 in Fig. 4, it is evident that the large number of strong acid sites in the fresh catalyst are reduced after a 10 h reaction. The ,~collidine-TPD profile in Fig. 5 also shows that peak area around 370°C decreased significantly after a 10 h reaction. This suggests that a considerable portion of strong acid sites are located on the external surface of Mg-ZSM-22 and the amount of these decrease as a result of coke deposition after the reaction. The difference in the acid strength between external and
Fig. 3 s h o w s that the conversion of 1-butene decreases rapidly at the beginning of the reaction and then maintains a constant value, while the selectivity to isobutene rapidly increases at the beginning and thereafter remains unchanged. These data suggest that, at the early stage of the reaction, 1-butene is mainly converted into by-products via oligomerization, followed by cracking reactions, thus increasing the conversion and lowering the selectivity to isobutene. As the reaction time passes, the conversion of 1butene decreases and the selectivity for isobutene increases as a result of reduction in side reactions. The main byproducts w e r e C 3 and C5 hydrocarbons, which were probably formed after dimerization and subsequent cracking. The dimerizing and cracking steps are assumed to be carbonium ion catalyzed. The carbonium ion is assumed to be formed on the Br6nsted acid sites [17]. The TPD experiments indicate that these results are closely related to coke formation. Figs. 4 and 5 repre-
5
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y
n
__
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v
60
I1} 11} Or)
V
"-"
3
m
=o ,~
A v
A W
A v
2
t-
40 /
!
O 20
O~
I
I
1
I
0
5
10
15
20
.0 0
0 25
Time on Stream (hr.) Fig. 6. The amountof coke deposition and isobutyleneselectivityover Mg-ZSM-22at 450°C with respect to time on stream: (O) the amount of coke deposition; (11) isobutylene selectivity.
298
S.H. Baeck, W.X Lee~Applied Catalysis A: General 164 (1997) 291-301
internal sites appears to be due to the difference of aluminum concentration between them. It has been suggested that minor differences in the local site environment may significantly affect acid strength [22]; for example, a low local aluminum content may give rise to enhanced acidity. Fig. 6 shows the amount of coke deposition and selectivity to isobutene over Mg-ZSM-22 with respect to time on stream. Coke deposition rapidly increased at the early stage of reaction and then remained constant, and selectivity to isobutene also paralleled coke deposition. This result indicates that coke formation occurs largely on the same acid sites on which side reactions, such as oligomerization and cracking, take place. It can be concluded that coke is mainly formed on the strong acid sites which exist mostly on the external surface of Mg-ZSM-22 and side reactions also take place on the same sites. These conclusions with respect to acid site distribution and coke deposition on the strong acid sites are in agreement with results reported by Deruane [23], who
proposed that coking on zeolites with medium pore sizes, such as H-ZSM-5, takes place mainly on the external surface. Some papers have also been reported that the inactivation of the external surface of zeolite improved the catalyst life for many reactions, e.g., the cracking of hexane and 2,2-dimethyl butane [24], the alkylation of toluene with methanol [25,26], and the alkylation of toluene with ethylene [27]. Xu et al. [28] also proposed that strong acidity of a zeolite leads to severe dimerization of olefin, and the majority of these dimers crack on such strong acid sites. Strong acid sites also result in rapid coke formation, which may be an important factor for the deactivation of zeolites in olefin processes.
3.3. The effect of boron or phosphorus impregnation The activities of Mg-ZSM-22 modified by impregnation by boron (referred, herein, as B/Mg-ZSM-22) or phosphorus (referred, herein, as P/Mg-ZSM-22) with respect to the feeding amount of impregnation
480
:5 a/
V
"0
0 "(3
250
"E
4150
c)j
E
'
<
i
100
I
200
i
I
300
i
I
400
i
I
500
i
600
Temperature (°C) Fig. 7. Temperature programmed desorption of NH3: (a) Mg-ZSM-22; (b) B/Mg-ZSM-22; (c) P/Mg-ZSM-22.
299
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
100
>
i m
80
60 0 r, O w L,,
4.0 n-
w m
> C 0 =
0 0
I
=
10
20
Time on stream(hr.) Fig. 8. The variation of activities over B/Mg-ZSM-22 with time on stream at 450°C: ((3) 1-butene conversion; (Q) isobutylene selectivity.
were examined. The conversion of 1-butene over P/ Mg-ZSM-22 decreased with increasing the amount of phosphorus, while the selectivity to isobutylene increased slightly. When 3 wt% or more phosphorus was impregnated, the catalytic activity was so poor that 1-butene conversion was less than 10%. When B/Mg-ZSM-22 was used as catalyst, the conversion of 1- butene decreased and selectivity to isobutene increased in proportion to the amount of boron impregnated. However, the rate of decrease for 1-butene conversion over B/Mg-ZSM-22 was much slower than that over P/Mg-ZSM-22. These catalytic behaviors of B/Mg-ZSM-22 and P/Mg-ZSM-22 can be explained based on the results of the NH3-TPD experiment in Fig. 7. In the case of P/Mg-ZSM-22, even though only a small amount of P was impregnated, it poisoned not only the external strong acid sites that led to the side reactions but also the active sites that led to the skeletal isomerization of 1-butene and existed largely within the pores. In the case of B/ Mg-ZSM-22, however, the boron mainly poisoned the strong acid sites which led to side reactions, so that the
selectivity to isobutene was enhanced without significant reduction in 1-butene conversion. Fig. 8 shows the conversion of 1-butene and the selectivity to isobutene as a function of time on stream over 3 wt%-B/Mg-ZSM-22. The pattern of catalytic activities in Fig. 8 is quite different from that of MgZSM-22, as shown in Fig. 3. B/Mg-ZSM-22 exhibited higher selectivity to isobutene and lower conversion of 1-butene than Mg-ZSM-22 from the early stage of reaction. It also showed constant conversion and 90% or higher selectivity to isobutene for 25 h. This is probably due to the fact that boron poisoned the stronger acid sites, which caused side reactions. Therefore, when boron is properly impregnated on Mg-ZSM-22, side reactions can be suppressed and high selectivity to isobutene can be achieved. 3.4. Effect o f added water in the f e e d
In order to investigate the effect of water in the feed, water vapor of 36 Torr was admixed with the standard feed. The effect of water addition on the conversion of
300
S.H. Baeck, W.Y. Lee~Applied Catalysis A: General 164 (1997) 291-301
80
:~ 60 11)
40 tt
0 m
T T mT
t._
(9 20 > tO
-H20
0
,
0 0
-H20
,
10
°
20
,
30
Time on Stream (hr.) Fig. 9. The effect of the water addition into the feed on 1-butene conversion and isobutylene selectivity: (Q) 1-butene conversion; (O) isobutylene selectivity.
1-butene and the selectivity to isobutene is shown in Fig. 9. The conversion of 1-butene increased but the selectivity to isobutylene decreased in the presence of water vapor. However, the catalytic activities were recovered to the original level by the removal of water vapor from the feed, thus indicating that the effect of added water in the feed is essentially reversible. If water is added to zeolites, Lewis acid sites can be converted to Br6nsted acid sites and if it is dehydrated, the Br6nsted acid sites are reversibly converted to Lewis acid sites [29]. When water vapor is admixed with the feed, side reactions can easily take place on Br6nsted acid sites. Accordingly, the conversion of 1butene increases and the selectivity to isobutene decreases. On the contrary, when the amount of Lewis acid sites is high, as a result of the removal of water from the feed, side reactions are suppressed so that the conversion of l-butene is lowered and selectivity to isobutene is enhanced. The formation of and the amounts of Lewis acid sites play an important role
in isobutene formation via the conversion of 1-butene, as shown in Table 1.
4. Conclusion ZSM-22 ion exchanged with various cations were prepared and tested as catalysts for the skeletal isomerization of 1-butene into isobutylene in a continuous flow fixed-bed reactor. It was found that the order of isobutylene yield was consistent with the order of the ratio of the amounts of Lewis acid to the amounts of Br6nsted acid and that Lewis acid sites may well improve the catalytic activity and suppress coke deposition. This was also confirmed by the addition of water to the feed. When catalytic activities were examined as a function of temperature, the yields for trans- and cis-2butene decreased with increasing temperature, while the yield for isobutene passed through a maximum at 450°C. These results suggest that the activation energy
S.H. Baeck, W.Y Lee~Applied Catalysis A: General 164 (1997) 291-301
for the skeletal isomerization of 1-butene into isobutene is larger than that for the double bond shift reaction of 1-butene into 2-butene. Coke deposition mainly occurred on the stronger acid sites on the external surface of Mg-ZSM-22 which induced side reactions, as confirmed by TPD analyses. The effect of impregnated boron or phosphorus was clearly observed. Phosphorous poisoned not only the strong acid sites but also the active sites for the skeletal isomerization of 1-butene, while boron mainly poisoned the strong acid sites.
Acknowledgements The authors thank the Yokong Ltd. for financial support.
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30l
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