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Ind. Eng. Chem. Res. 1997, 36, 2990-2995
Catalytic Activity of ZSM-22 Zeolites in the Skeletal Isomerization Reaction of 1-Butene Rune Byggningsbacka, Lars-Eric Lindfors,* and Narendra Kumar Laboratory of Industrial Chemistry, Faculty of Chemical Engineering, A˙ bo Akademi University, Biskopsgatan 8, FIN-20500 A˙ bo, Finland
Skeletal isomerization of 1-butene was carried out over ZSM-22 zeolites in a fixed-bed reactor system at near atmospheric pressure. The ZSM-22 zeolites obtained after the synthesis and subsequent calcination showed high activity in the transformation of 1-butene even without ion-exchange procedures. There was a great difference in the catalytic activity between catalysts calcined in a separate calcination oven and catalysts calcined using high flows in the reactor. When a selective isomerization catalyst was prepared using calcination in air, a high flow was required in order to efficiently remove steam and heat. Calcination without any possibility of steam formation could be performed in a flow of nitrogen or even in the reactant flow at temperatures as low as 400 °C. The effect of temperature, time on stream, weight hour space velocity (WHSV), and partial pressure of 1-butene on the skeletal isomerization reaction of 1-butene was studied over the prepared ZSM-22 catalysts. ZSM-22 demonstrated high activity in the skeletal isomerization of 1-butene at extremely high WHSV, even at temperatures as low as 400 °C. Introduction Skeletal isomerization of n-butenes to isobutene has received a great deal of attention in recent years, mainly because of the increasing importance of MTBE (methyl tert-butyl ether) as an octane-boosting additive in gasoline. The production of MTBE, in the reaction between isobutene and methanol, is at present limited by the supply of isobutene. Skeletal isomerization of n-butenes over acid catalysts is a promising way of increasing the source of isobutene. A large variety of acid catalysts have been reported as being active in isomerization reactions. Catalysts such as halogenated aluminas (Cheng and Ponec, 1994), zeolites (Asensi et al., 1996; Bianchi et al., 1994; Mooiweer et al., 1994; Simon et al., 1994; Woo et al., 1996; Xu et al., 1994, 1995), and aluminophosphates (Gielgens et al., 1995; Yang et al., 1994; Zubowa et al., 1993) have all been studied in the skeletal isomerization of n-butenes. Zeolites are acid crystalline materials providing the shape selectivity required for the selective isomerization of 1-butene to isobutene. Promising results have been achieved, especially with ten-membered-ring zeolites. ZSM-22, the zeolite used in this work, has the Theta-1 structure type and consists of a one-dimensional, tenmembered-ring pore system with channel diameters of 4.5 × 5.5 Å (Kokotailo et al., 1985). Properties such as number of acid sites and acid strength are also of great importance in hydrocarbon reactions over zeolites. It has been proposed that the acid strength required for acid-catalyzed conversion of hydrocarbons can be ranked as follows: cracking ∼ oligomerization > skeletal isomerization . double-bond isomerization (O’Young et al., 1994). The acidity of zeolites can be modified by changing the Si/Al ratio or by replacing aluminum in the framework with elements such as iron, gallium, and boron. Improvements in isobutene selectivity have been reported when boron has been substituted for aluminum in ZSM-5 and ZSM-11 zeolites (Bianchi et al., 1994). There has been much speculation in the literature whether the reaction mechanism in the production of * Author to whom correspondence should be addressed. E-mail:
[email protected]. S0888-5885(96)00641-0 CCC: $14.00
isobutene is monomolecular or bimolecular. While some authors favored a monomolecular mechanism (Asensi et al., 1996; Gielgens et al., 1995; Woo et al., 1996; Xu et al., 1994), others have strongly suggested a bimolecular mechanism mainly because the monomolecular mechanism would require a very unstable primary carbenium ion as an intermediate (Guisnet et al., 1996). On the other hand, it is obvious that the main byproductsspropene, pentenes, and octenessare produced in bimolecular reactions. The product distribution obtained in the transformation of 1-butene over zeolites is fairly complex because reactions such as dimerization, cracking, isomerization, aromatization, coke formation, and hydride transfer are always present. Although ZSM-22 has previously been studied as a catalyst for 1-butene transformation (Simon et al., 1994), there are still unresolved questions regarding how the preparation of a ZSM-22 catalyst to be used in selective skeletal isomerization of 1-butene should be performed. Our experiments showed that the method of calcination was of great importance. Higher selectivity to isobutene was achieved after calcination of ZSM22 in a high flow of air or nitrogen through the reactor system as compared to separate calcination in a calcination oven with limited heat and mass transfer. No ion exchange was needed for introduction of acidity. The ZSM-22 catalyst obtained as such after the synthesis and subsequent calcination demonstrated high activity in the transformation of 1-butene. The main advantage of using ZSM-22 in the skeletal isomerization of 1-butene, as compared to ZSM-35/ferrierite catalysts, would be that extremely high weight hour space velocity (WHSV) can be used. Similar high yields of isobutene at high WHSV of 1-butene have previously been reported only for ZSM-23 catalysts (Xu et al., 1994). Experimental Section Synthesis of ZSM-22. ZSM-22 was synthesized in accordance with the method employed by Ernst et al. (1988), with some modification. Two solutions, A and B, were prepared as follows: A by diluting the silica source, Ludox AS-40, in distilled water and B by © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2991
dissolving the aluminum source, Al2(SO4)3‚18H2O (Merck), in distilled water. The organic template 1,6diaminohexane (Fluka) and KOH (Merck) were added to solution B. The solutions were stirred for 15 min after which solution B was transferred to solution A under conditions of vigorous stirring. Mixing the two solutions at ambient temperature (25 °C) resulted in formation of a white gel having a pH of 12, which was stirred for an additional 30 min. The crystallization took place at hydrothermal pressure in a polytetrafluoroethylene cup contained in an autoclave rotating in an oven heated to 160 °C. The crystallization process was completed in 3 days. The white crystalline product was filtered, washed, and finally dried at 110 °C for 12 h. Calcination. Two different calcination methods were used in order to remove the organic template. Some of the catalysts were calcined in a high flow of air or nitrogen (200 mL/min per gram of catalyst) in the same reactor as used in the reaction with 1-butene. Before the calcination in the reactor system took place, the zeolite powder was pressed, crushed, and sieved to 0.125-0.250 mm particle size. The catalysts were packed in a quartz microreactor and placed in an oven (Carbolite) connected to a temperature controller. The temperature of the catalyst bed was measured using a thermocouple inside the bed. The heating rate of the catalyst bed was 20 °C/min from 25 to 400 °C and 10 °C/min from 400 °C to the final calcination temperature. Catalysts calcined in the reactor system were compared with catalysts calcined in a separate oven for 15 h at 550 °C. Because the K-ZSM-22 catalysts obtained directly after calcination showed high activity in the transformation of 1-butene, the effect of different calcination methods was studied more thoroughly. The effect of the calcination temperature and time on the catalytic properties of ZSM-22 in the isomerization of 1-butene was studied after calcination in a flow of air or nitrogen in the reactor system. Finally, a larger batch was calcined at 550 °C in the reactor for 6 h in a flow of nitrogen followed by 8 h in a flow of air (flow of nitrogen and air was 200 mL/min per gram of catalyst). The calcined catalyst was used in order to study the influence of different reaction conditions on the catalytic activity of ZSM-22 in the isomerization reaction of 1-butene to isobutene. Ion Exchange with NH4+. Some of the K-ZSM-22 zeolites were ion-exchanged for 48 h in a 1 M NH4Cl solution in order to remove any potassium remaining from the synthesis. The ion-exchanged zeolites were washed free of chloride ions and dried for 12 h at 110 °C. H-ZSM-22 was obtained after the NH4+ ions had been decomposed in a calcination step. This calcination was performed in the reactor system or in the oven using similar procedures to those employed when the organic template was removed. Characterization of ZSM-22. The phase purity and structure of ZSM-22 were determined by X-ray powder diffraction (Philips pw 1830) using Cu KR radiation. The bulk Si/Al ratio of ZSM-22 was measured by X-ray fluorescency (X-MET 880). The relative amounts of Lewis and Bro¨nsted acid sites were determined by a FTIR spectrometer (ATI Mattson infinity spectrometer) equipped with a Deuterated Triglycine Sulfate (DTGS) detector. The in situ FTIR experiments of adsorbed pyridine on a 1/1 (weight ratio) mixture of ZSM-22 and KBr powder were performed in a diffuse reflectance accessory equipped with a standard
Figure 1. X-ray powder diffraction pattern of K-ZSM-22.
controlled environmental chamber (Spectra-Tech, Model 0030-103). The zeolites were evacuated in vacuum (10-5 atm) at 500 °C for 2 h before the pyridine adsorption at 100 °C took place. The ZSM-22 samples were kept at 100 °C in a flow of helium for 1 h in order to allow the pyridine to penetrate the samples. The zeolites were evacuated at 200 °C for 2 h before the spectra were recorded at ambient temperature. Catalyst Testing. The catalytic properties of ZSM22 zeolites in the isomerization of 1-butene (99.3% purity, AGA) to isobutene were studied in a fixed-bed microreactor system at near atmospheric pressure. The amount of catalyst was varied between 0.05 and 0.1 g and the flow of 1-butene between 21 and 87 mL/min in order to obtain the different WHSV used in the experiments. The reactant 1-butene was diluted with a 0-127 mL/min flow of nitrogen (99.999% purity, AGA) in order to achieve different partial pressures of 1-butene. The products from the reactor were analyzed by a gas chromatograph (Varian 3700) equipped with an FID detector. A capillary column (50 m × 0.32 mm i.d. fused-silica PLOT Al2O3-KCL) was used for product separation. The three n-butene isomers, 1-butene, cis2-butene, and trans-2-butene, were considered reactants; thus conversion, yield, and selectivity were defined as follows:
conversion (mass %) ) (Qm)1-butene,in - (Qm)n-butenes,out × 100 (1) (Qm)1-butene,in yield of isobutene (mass %) )
(Qm)isobutene,out × 100 (Qm)1-butene,in (2)
selectivity to isobutene (mass %) ) (Qm)isobutene,out × 100 (3) (Qm)1-butene,in - (Qm)n-butenes,out The following relationship exists between conversion, yield, and selectivity: yield of isobutene ) conversion × selectivity to isobutene. Results and Discussion Characterization. The structure of the prepared catalyst was confirmed by means of X-ray diffraction. Figure 1 shows the X-ray diffraction patterns obtained for the K-ZSM-22 catalyst used in the experiments. The X-ray diffraction pattern was similar to those reported
2992 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 2. Conversion, Yield of Isobutene, and Selectivity to Isobutene over K-ZSM-22 Catalysts Calcined in the Reactor in a High Flow of Air or Nitrogen Using Different Calcination Temperature and Timea calcination
conversion (%)
yield (mass %)
selectivity (mass %)
1 h in air at 550 °C 1 h in N2 at 550 °C 15 h in N2 at 450 °C 15 h in N2 at 550 °C 15 h in N2 at 650 °C 15 h in air at 550 °C 15 h in air at 650 °C
45.4 45.6 48.1 47.1 37.2 47.6 28.8
27.8 27.9 29.0 29.1 23.4 28.4 17.9
61.2 61.2 60.2 61.9 63.0 59.6 62.1
a The experiments were carried out at 0.5 atm partial pressure and 150 h-1 WHSV of 1-butene. Products were analyzed after 30 min TOS.
Figure 2. IR spectra of pyridine adsorbed after thermal treatment at 200 °C on (A) K-ZSM-22 calcined in the reactor, (B) H-ZSM-22 calcined in the reactor, (C) K-ZSM-22 calcined in the oven, and (D) H-ZSM-22 calcined in the oven. Table 1. Conversion, Yield of Isobutene, and Selectivity to Isobutene over K-ZSM-22 and H-ZSM-22 Catalysts Calcined in the Reactor System and in a Separate Ovena catalyst
calcination
conversion (%)
yield (mass %)
selectivity (mass %)
K-ZSM-22 H-ZSM-22 K-ZSM-22 H-ZSM-22
oven oven reactor reactor
46.3 56.0 45.9 45.4
17.5 14.0 27.7 26.7
37.8 25.0 60.4 58.8
a The experiments were carried out at 0.5 atm partial pressure and 150 h-1 WHSV of 1-butene. Products were analyzed after 30 min TOS.
in the literature by Ernst et al. (1988). The Si/Al ratio measured by X-ray fluorescency was found to be 53:1. A more thorough characterization of the ZSM-22 catalyst used in the experiments can be found in the article by Kumar et al. (1996). The relative amounts of Lewis and Bro¨nsted acid sites were measured using FTIR spectroscopy. Figure 2 shows the spectra recorded for pyridine adsorbed on different ZSM-22 zeolites. All catalysts showed peaks associated with pyridine adsorbed on Lewis acid sites (1453 and 1598 cm-1) and Bro¨nsted acid sites (1548 and 1640 cm-1), as well as a peak at 1491 cm-1 associated with adsorption on both Lewis and Bro¨nsted acid sites. FTIR spectra of pyridine proved that a large number of Bro¨nsted acid sites were present in the K-ZSM-22 zeolites even without any ion exchange having taken place. The ratio between Bro¨nsted and Lewis acid sites, calculated from the peak areas at 1548 and 1453 cm-1, was found to be 5:1 after calcination in the oven and 11:1 after calcination in the reactor. These results indicate that the amount of aluminum in the zeolite framework was reduced due to dealumination when the calcination was carried out in a separate calcination oven. The steam produced when the organic template was burned off in combination with the limited heat and mass transfer conditions present in the oven was obviously sufficient to cause partial dealumination of the zeolite. Effect of Calcination Methods on the Catalytic Activity. The catalytic activity of catalysts calcined in the reactor system and in the oven in the isomerization reaction of 1-butene to isobutene is compared in Table 1. The partial pressure and WHSV of 1-butene in the experiments carried out at 400 °C were 0.5 atm and 150 h-1, respectively. The products were analyzed after 30
min on stream (TOS). There was a great difference in the catalytic activity between catalysts calcined in the reactor system and in the oven. The yield of isobutene as well as selectivity to isobutene was much higher when the catalysts had been calcined in a high flow of air and nitrogen in the reactor system. The lower selectivity to isobutene after calcination in the oven could be explained by the dealumination observed in the FTIR experiments in combination with generation of acid sites with increased acid strength. Enhancement of acid strength as a result of interaction between Lewis and Bro¨nsted acid sites has been proposed as an explanation for the increased activity in different hydrocarbon reactions after mild steaming of zeolites (Corma et al., 1996). On the other hand, high selectivity to isobutene in the isomerization of 1-butene requires only acid sites of medium acid strength. The difference between the ion-exchanged H-ZSM-22 zeolites and K-ZSM-22 zeolites is also compared in Table 1. There was almost no difference in conversion, yield, and selectivity between the ion-exchanged H-ZSM-22 catalyst and the K-ZSM-22 catalyst when the calcination was carried out in the reactor. On the other hand, K-ZSM-22 showed a much higher selectivity to isobutene as compared to that of H-ZSM-22 when the calcination was carried out in a separate oven because K-ZSM-22 was calcined only once in unfavorable conditions as compared to the two calcination steps required for H-ZSM-22. Results from experiments after calcination in a flow of air or nitrogen at different calcination temperatures and using different calcination times are compared in Table 2. The partial pressure and WHSV of 1-butene in the experiments carried out over K-ZSM-22 catalysts at 400 °C were 0.5 atm and 150 h-1, respectively. The products were analyzed after 30 min TOS. The activity of the catalysts decreased with increasing calcination temperature. Calcination for 1 h at 550 °C was sufficient to remove most of the organic template because the conversion increased only slightly when the calcination time was increased from 1 to 15 h. The organic template could also be easily removed without air treatment in a flow of nitrogen even at temperatures as low as 450 °C. Although the differences between calcination in air and nitrogen were small, because of the high flows used in these experiments, benefits might be obtained if calcination is performed without the presence of air in order to completely eliminate the possibility of steam formation and dealumination. Calcination could also be performed in the reactant flow. Activation of the catalyst at 400 °C by the removal of the organic template in the reactant flow is presented as a function of time on stream in Figure 3. The
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2993 Table 3. Conversion, Yield of Isobutene, and Selectivity to Different Products at Different Reaction Conditions over K-ZSM-22 Calcined in the Reactor 150 270 400 0.5
150 10 400 0.5
150 270 500 0.5
240 270 400 0.5
150 270 400 1.0
ethene propane propene butanes isobutene pentenes hexenes heptenes octenes
0.1 0.2 5.9 4.9 63.3 6.6 0.7 1.8 16.7
0.2 0.4 7.6 5.2 58.5 9.3 1.1 2.8 14.9
0.2 0.1 5.4 2.7 81.4 5.5 0.9 0.4 3.4
0.1 0.2 6.3 5.4 59.6 7.0 0.1 2.2 19.1
0.2 0.5 7.4 6.0 48.9 9.9 1.5 4.3 21.3
yield of isobutene conversion
26.8 42.4
27.9 47.7
34.7 42.6
24.4 40.9
26.5 54.2
WHSV, h-1 TOS, min temperature, °C 1-butene pressure, atm
Figure 3. Effect of time on stream on (A) conversion, (B) yield of isobutene, and (C) selectivity to isobutene at 400 and 500 °C over K-ZSM-22 catalysts calcined in the reactor and at 400 °C over K-ZSM-22 not calcined at all. The experiments were carried out at 0.5 atm partial pressure and 150 h-1 WHSV of 1-butene.
selectivity to isobutene at 400 °C over the catalyst not calcined was even higher than that of the calcined catalyst at the same reaction temperature, probably for the same reasons as coke formation had a positive effect on isobutene selectivity. The difference between the calcined catalyst and the catalyst not calcined decreased with TOS, as the calcined catalyst became deactivated. Effect of TOS. The partial pressure and WHSV of 1-butene in the deactivation experiments carried out over K-ZSM-22 at 400 and 500 °C were 0.5 atm and 150 h-1, respectively. Results from the deactivation experiments can be seen in Figure 3. The rate of deactivation
due to coke formation on the catalyst was similar in experiments carried out at 400 and 500 °C. The deactivation reached a more stable level after approximately 100 min TOS, after a fast initial deactivation of the fresh catalyst had taken place. Selectivity to isobutene increased as the catalyst became deactivated. This was, as can be seen from the product distributions in Table 3, mainly due to a reduced formation of propene and pentenes. The reasons for the increased selectivity to isobutene over ZSM-22 with increasing TOS might be the same as proposed by Xu et al. (1995) for ZSM-35: a selective blocking of the strong acid sites by coke deposits, an improved shape selectivity since the zeolite channels are modified by the coke, or reduced total contact time since coke buildup may confine the reaction zone for isomerization close to the crystalline surface. Effect of WHSV. The partial pressure of 1-butene was 0.5 atm in the experiments carried out over K-ZSM22 at 300, 400, and 500 °C in order to investigate the effect of WHSV. The results from the experiments using slightly deactivated catalysts (TOS over 100 min) in order to minimize the effect of deactivation are presented in Figure 4. The K-ZSM-22 catalysts demonstrated excellent isobutene production at 400 and 500 °C, even at the extremely high WHSV used in the experiments. This can be compared to ZSM-35 (unpublished results) which under identical reaction conditions requires at least 10 times longer contact time for similar activity. The main difference in product distribution at different WHSV over K-ZSM-22 catalysts at 400 °C was, as can be seen in Table 3, in selectivity to isobutene and octenes. The selectivity to isobutene decreased while the selectivity to octenes increased with increasing WHSV. This would indicate that cracking of octenes could be the rate-limiting step in the formation of isobutene at 400 °C, if isobutene is produced in a bimolecular reaction mechanism. This was not observed at 500 °C because the cracking reactions were much faster. At 500 °C there was even an increased selectivity to isobutene with increasing WHSV at the expense of propene and pentenes. The main difference in product distribution between 400 and 500 °C was in the selectivity to octenes and isobutene. When the reaction temperature was increased from 400 to 500 °C the selectivity to isobutene increased, while selectivity to octenes decreased. The conversion was also higher at 400 °C, as compared to that at 500 °C, because dimerization reactions were favored over cracking reactions.
2994 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997
Figure 4. Effect of 1-butene WHSV on (A) conversion, (B) yield of isobutene, and (C) selectivity to isobutene at 300, 400, and 500 °C over K-ZSM-22 catalysts calcined in the reactor. The experiments were carried out at 0.5 atm partial pressure of 1-butene.
Figure 5. Effect of 1-butene partial pressure on (A) conversion, (B) yield of isobutene, and (C) selectivity to isobutene at 400 and 500 °C over K-ZSM-22 catalysts calcined in the reactor. The experiments were carried out at 150 h-1 WHSV of 1-butene.
Effect of Partial Pressures. The WHSV of 1-butene was 150 h-1 in the experiments carried out over K-ZSM22 catalysts at 400 and 500 °C in order to investigate the effect of 1-butene partial pressure. Results from the experiments using a slightly deactivated catalyst (TOS over 100 min) are presented in Figure 5. Conversion increased with increasing 1-butene partial pressure. As can be seen in Table 3, the increase in conversion was mainly due to an increased formation of byproducts since the yield of isobutene and selectivity to isobutene decreased with increasing 1-butene partial pressure. Bimolecular dimerization reactions were favored over
cracking reactions at high partial pressures of 1-butene, increasing the selectivity to heavier hydrocarbons such as pentenes, hexenes, heptenes, and octenes. Summary K-ZSM-22 can be used as a selective skeletal isomerization catalyst for 1-butene transformation, if a high flow of air is used in order to efficiently remove steam and heat produced during calcination. Calcination can also be performed in a flow of nitrogen, or even in the reactant flow, in order to eliminate any possibility of
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2995
steam formation. High activity in the skeletal isomerization of 1-butene was achieved if the catalysts were calcined at temperatures below 550 °C. The catalyst obtained after calcination could be used as such because no ion exchange was needed in order to introduce acidity. FTIR results of adsorbed pyridine showed that a large number of Bro¨nsted acid sites were already present in the catalyst after the synthesis and calcination. No improvements in the yield of isobutene or selectivity to isobutene could be observed after ion exchange with NH4+ and subsequent calcination. The prepared ZSM-22 catalyst demonstrated high activity in the isomerization of 1-butene at extremely high WHSV even at temperatures as low as 400 °C. The main advantage of using ZSM-22 as compared to ZSM35 in a skeletal isomerization process would be that a much smaller reactor could be used. Nomenclature K-ZSM-22 ) ZSM-22 zeolites prepared without any ionexchange procedures H-ZSM-22 ) ZSM-22 zeolites after ion-exchange with NH4+ and calcination WHSV ) weight hour space velocity ) flow of 1-butene/ weight of catalyst TOS ) time on stream (Qm)i ) mass flow of component i
Acknowledgment Financial support from the Finnish Technology Development Center (TEKES) and the Graduate School in Chemical Engineering (GSCE) is gratefully acknowledged. Literature Cited Asensi, M. A.; Corma, A.; Martinez, A. Skeletal Isomerization of 1-Butene on MCM-22 Zeolite Catalyst. J. Catal. 1996, 158, 561. Bianchi, D.; Simon, M. W.; Nam, S. S.; Xu, W.-Q.; Suib, S. L.; O’Young, C.-L. Kinetic Studies of the Isomerization of n-Butenes over Boroaluminosilicate Zeolites. J. Catal. 1994, 145, 551. Cheng, Z. X.; Ponec, V. Selective Isomerization of Butene to Isobutene. J. Catal. 1994, 148, 607. Corma, A.; Martinez, A.; Arroyo, P. A.; Monteiro, J. L. F.; SousaAguiar, E. F. Isobutane/2-butene alkylation on zeolite beta: Influence of post-synthesis treatments. Appl. Catal. 1996, 142, 139.
Ernst, S.; Kokotailo, G. T.; Kumar, R.; Weitkamp, J. Shape selective catalysis in zeolites ZSM-22 and ZSM-23: Influence of pore shapes on reaction selectivity. Proceedings of the 9th International Congress on Catalysis, Calgary, 1988; Chemical Institute of Canada: Ottawa, 1988; Vol. 1, p 388. Gielgens, L. H.; Veenstra, I. H. E.; Ponec, V. Selective isomerization of n-butene by crystalline aluminophosphates. Catal. Lett. 1995, 32, 195. Guisnet, M.; Andy, P.; Gnep, N. S.; Benazzi, E.; Travers, C. Skeletal Isomerization of n-Butenes. J. Catal. 1996, 158, 551. Kokotailo, G. T.; Schlenker, J. L.; Dwyer, F. G.; Valyocsik, E. W. The framework topology of ZSM-22: A high silica zeolite. Zeolites 1985, 5, 349. Kumar, N.; Lindfors, L. E.; Byggningsbacka, R. Synthesis and characterization of H-ZSM-22, Zn-H-ZSM-22 and Ga-H-ZSM22 zeolite catalysts and their catalytic activity in aromatization of n-butane. Appl. Catal. 1996, 139, 189. Mooiweer, H. H.; de Jong, K. P.; Kraushaar-Czarnetzki, B.; Stork, W. H. J.; Krutzen, B. C. H. Skeletal isomerization of olefins with the zeolite Ferrierite as catalyst. Stud. Surf. Sci. Catal. 1994, 84, 2327. O’Young, C.-L.; Xu, W.-Q.; Simon, M.; Suib, S. L. Skeletal isomerization of n-butenes on zeolite catalysts: Effects of acidity. Stud. Surf. Sci. Catal. 1994, 84, 1671. Simon, M. W.; Suib, S. L.; O’Young, C.-L. Synthesis and Characterization of ZSM-22 Zeolites and Their Catalytic Behavior in 1-Butene Isomerization Reactions. J. Catal. 1994, 147, 484. Woo, H. C.; Lee, K. H.; Lee, J. S. Catalytic skeletal isomerization of n-butenes to isobutene over natural clinoptilolite zeolite. Appl. Catal. 1996, 134, 147. Xu, W.-Q.; Yin, Y. G.; Suib, S. L.; O’Young, C.-L. Selective Conversion of n-Butene to Isobutylene at Extremely High Space Velocities on ZSM-23 Zeolites. J. Catal. 1994, 150, 34. Xu, W.-Q.; Yin, Y.-G.; Suib, S. L.; Edwards, J. C.; O’Young, C.-L. n-Butene Skeletal Isomerization to Isobutylene on Shape Selective Catalysts: Ferrierite/ZSM-35. J. Phys. Chem. 1995, 99, 9443. Yang, S. M.; Guo, D. H.; Lin, J. S.; Wang, G. T. 1-Butene Conversion Over SAPO-11 and MeAPO-11. Stud. Surf. Sci. Catal. 1994, 84, 1677. Zubowa, H.-L.; Richter, M.; Roost, U.; Parlitz, B.; Fricke, R. Synthesis and catalytic properties of substituted AlPO4-31 molecular sieves. Catal. Lett. 1993, 19, 67.
Received for review October 11, 1996 Revised manuscript received January 29, 1997 Accepted February 4, 1997X IE960641W
X Abstract published in Advance ACS Abstracts, June 15, 1997.
