Ind. Eng. Chem. Res. 1997, 36, 83-87
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Stacking Faults Effects on Shape Selectivity of Offretite J. F. Bengoa, S. G. Marchetti, N. G. Gallegos, A. M. Alvarez, M. V. Cagnoli, and A. A. Yeramian* Centro de Investigacio´ n y Desarrollo en Procesos Catalı´ticos (CINDECA),† Calle 47 No. 257, 1900 La Plata, Repu´ blica Argentina
The cocrystallization of erionite during the synthesis of tetramethylammonium-offretite produces faults in the crystalline structure known as stacking faults. The presence of such stacking faults is studied as a function of the amount of tetramethylammonium (TMA) bromide template added. Simultaneously the effect of the stacking faults on the shape selectivity of toluene disproportionation is also studied. Characterization of the catalytic material is carried out by XRD, SEM, and toluene disproportionation used as the test reaction. I. Introduction
Table 1. TMA-SiO2 Molar Ratios Present in the Parent Gels
It was established in the literature (Gard and Tait, 1972; Chen et al., 1984; Bennett and Gard, 1967) that erionite and offretite belong to the same family of zeolites, and intergrowth was frequently observed among them. Experimental conditions under which this intergrowth occurs are well-known and were already described in several reports (Howden, 1987a,b; Lillerud and Raeder, 1986; Occelli et al., 1987; Aiello and Barrer, 1970), and methods to evaluate such intergrowth were also provided (Lillerud and Raeder, 1986; Aiello and Barrer, 1970; Whyte et al., 1971). The offretite framework shows two types of channels, one of them of 6.4 Å diameter constituted by a 12membered oxygen ring, located parallel to the c axis. The other type of channel is built by 8-membered oxygen rings, located perpendicular to the former with a size of 5.2 × 3.8 Å (Naccache and Ben Taarit, 1980). The 12-membered oxygen ring channels in erionite are blocked every 15 Å in the c direction as a result of rotating successive layers of cancrinite cages by 60°. Thus, the erionite only presents channels of 8-membered oxygen rings open in the a direction (Gard and Tait, 1972; Chen et al., 1984; Bennett and Gard, 1967; Schlenker et al., 1977; Jenkins, 1971; Whyte et al., 1971; Mirodatos and Barthomeuf, 1979). The channel shape and diameter, added to the fact that the offretite possesses strong acid sites, contribute to the high coking tendency shown by this zeolite (Naccache and Ben Taarit, 1980; Fernandez et al., 1986). The offretite industrial application is not common at the moment, but some catalytic processes using offretite have been proposed, such as elimination of high molecular weight hydrocarbons (oil dewaxing) (Chen et al., 1984) or naphthalene cracking (Hernandez et al., 1984). Other authors have found that the intergrowth of erionite in offretite blocks the wide channels, forcing the reactants through the channels of lower diameter (Chen et al., 1984; Occelli et al., 1987). Therefore, intergrowth of erionite should lead to the selective formation of p-xylene in the toluene disproportionation reaction. The amount of structural faults known as “stacking faults” produced by intergrowth should depend strongly on the ratio of TMA/SiO2 in the initial * Author to whom correspondence should be addressed. FAX: 54-21-254277. † Dependent of the Facultad de Ciencias Exactas de la Universidad Nacional de La Plata, associated with CONICET.CIC. S0888-5885(96)00237-0 CCC: $14.00
zeolite
TMA-SiO2 (mole ratio)
ZO-0 ZO-1 ZO-2 ZO-3 ZO-4
0.000 00 0.009 97 0.011 96 0.013 96 0.015 95
zeolite
TMA-SiO2 (mole ratio)
ZO-5 ZO-6 ZO-7 ZO-8
0.017 94 0.019 94 0.023 92 0.039 87
solution precursor of TMA-offretite since the TMA controls the erionite intergrowth (Howden, 1987a,b). The purpose of this paper is to verify the hypothesis of the influence of TMA on the amount of offretite stacking faults and to confirm possible effects of these faults on the shape selectivity on the reaction carried out in the pores of this zeolite. II. Experimental Section II.a. Preparation of the Zeolites. Reacting mixtures of silica gel 40 Merck, Al2(SO4)3‚18H2O, NaOH, and KOH (both 99%) were prepared. TMA bromide was used as a lattice template. Data provided on the silica-alumina-alkali system (Jenkins, 1971) and the effects of the silica-alumina ratio allowed one to choose the following precursor mixture in order to prepare the zeolites:
Na2O:K2O ) 2.88:1 (SiO2 + Al2O3):(Na2O + K2O) ) 2.42:1 H2O:(Na2O + K2O) ) 35.83:1 SiO2:Al2O3 ) 17.45:1 These mixtures were placed in Teflon vessels and were aged for 48 h before adding the template. Considering the important effect of the template on the erionite growth in the offretite (Howden, 1987a,b), the amount of TMA bromide was varied in order to obtain offretites with different amounts of erionite. The TMA-SiO2 molar ratios are presented in Table 1. Once the template was added, the mixtures were vigorously stirred and placed in an autoclave for the reaction period. The reaction conditions, where the initial precursors are kept at 100 °C without stirring, were maintained for 6 days. Quaternary ammonium salts were removed by calcination at 630 °C for 12 h. The zeolite acid form was obtained by ion exchange of © 1997 American Chemical Society
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Figure 1. X-ray diffraction pattern of ZO-0.
the calcinated crystals with 3 M NH4NO3 at 80 °C followed by heating in air at 600 °C for 12 h. II.b. X-ray Diffraction Analysis (XRD) and Scanning Electron Microscopy (SEM). X-ray diffraction was used to identify and characterize the zeolites prepared. A Philips PN 1732/10 diffractometer and Cu KR radiation were used. The 2θ ) 9.3° peak was used in order to determine the amount of existing erionite. The particle size and morphology of the solid products were determined by SEM. II.c. Catalytic Tests. In order to measure the activity and selectivity of the obtained zeolites, a fixedbed stainless steal reactor of 4 mm inner diameter, heated by an electric furnace, was used. The reactor was provided with an axial thermocouple. A total of 250 mg of catalytic material supported on a quartz wool plug was used for all measurements. The toluene was fed by means of a peristaltic pump. The tests were carried out at atmospheric pressure. Vapors from the reactor were collected in a melting ice trap, and the liquid products were analyzed by means of gas chromatography. A 4.38 m and 5 mm inner diameter column filled with Chromosorb W 100200 impregnated with 5% diisodecyl phthalate and 5% bentone 34 was used. The absence of noncondensing products was verified. Once the steady state had been reached, one sample of reaction products per hour on stream was collected for analysis and material balance calculations.
Figure 2. X-ray diffraction patterns of the synthesized zeolites: (a) ZO-1, (b) ZO-2, (c) ZO-3, (d) ZO-4, (e) ZO-5, (f) ZO-6, (g) ZO-7, (h) ZO-8.
III. Results III.a. X-ray Analysis and Scanning Electron Microscopy. The XRD and SEM analysis of the different samples revealed that higher amounts of the template in the parent gels lead to an increase of the crystallinity, a change of shape and size of the crystals, and a decrease of the amount of erionite present. When no template was added, chabazite (Figure 1) was obtained, which coincides with the results reported by Moudafi et al. (1987). In Figure 2, XRD patterns of the synthesized zeolites are shown. As the template is added to the parent gel, the presence of particles which consist of small crystal agglomerates (Figure 3) starts to be noticed. According to the report of Jenkins (1971) zeolites synthesized following this method are very similar to type T zeolites in regards to the shape of the crystals and the percent-
Figure 3. Scanning electron micrograph of ZO-1. Scale represents 1 µm.
age of erionite present (ZO-1, ZO-2, ZO-3). A decrease of the amount of erionite present, with increasing amounts of template, is observed in the respective XRD patterns since 2θ ) 9.3° peak intensity decreases following the mentioned sequence. In the ZO-4 the presence of different forms should be stressed. On the one hand, small crystal agglomerates still appear, but, on the other hand, one can notice the presence of some hexagonal crystals (Figure 4). The amount of erionite revealed by the X-ray diffraction pattern is lower than those in ZO-1, ZO-2, and ZO-3.
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Figure 6. Scanning electron micrograph of ZO-6. Scale represents 1 µm. Table 2. Toluene Disproportionation over Different Catalystsa xylene composition (%) zeolite ZO-1 ZO-2 ZO-3 ZO-4 ZO-5 ZO-6 ZO-7 Figure 4. Scanning electron micrographs of ZO-4. Scale represents 1 µm.
ZO-8 H-ZSM-5 offretite
t (h)b
xφ-CH3 (%)c
para
meta
ortho
0-1 1-2 0-1 1-2 0-1 1-2 0-1 1-2 0-1 1-2 0-1 1-2 0-1 1-2 0-1 1-2 0-1 1-2 0-1 1-2
2.0 2.0 2.0 2.0 3.3 2.5 3.5 2.7 6.2 3.8 12.1 8.4 13.3 9.1 16.1 7.0 40.0 41.0 22.0 9.0
28 35 28 37 34 41 45 46 40 41 30 35 28 38 27 33 26 26 26 32
49 45 48 46 45 43 47 41 49 45 50 44 50 41 49 47 50 50 50 44
22 20 24 17 21 16 8 13 11 14 20 21 22 21 24 20 24 24 24 24
a WHSV: 2.5 h-1 (weight of toluene feed per hour unit weight of catalyst). Reaction at atmopsheric pressure. Temperature: 600 °C. b t (h): time on stream (h). c Xφ-CH3 (%): toluene conversion (%).
Figure 5. Scanning electron micrograph of ZO-5. Scale represents 1 µm.
The samples ZO-5, ZO-6, ZO-7, and ZO-8 show the presence of well-defined hexagonal crystals (Figures 5 and 6), with a bigger size than ZO-1 to ZO-4. Besides, a progressive decrease of the amount of erionite occurs, as it turns out from the corresponding XRD and SEM. III.b. Catalytic Test. Measurements of the activity and selectivity of the different samples of zeolites are shown in Table 2. These measurements are compared to results obtained under similar conditions using the acid form of ZSM-5 and pure offretite.
The activity in the toluene disproportionation reaction decreases as the erionite percentage in the sample increases. Although a pronounced fall of the activity with time on stream is noticed, all catalysts maintain an appreciable activity during 2 h of reaction under these conditions. H-ZSM-5 activity and stability are higher compared to the values obtained when offretite is the used catalyst. A sample of offretite, prepared according to the method of Aiello and Barrer (1970), is also tested for the disproportionation reaction. Results are also presented in Table 2 and are very similar to those observed for ZO-8. Comparing the selectivities to p-xylene obtained from different offretites, a maximum is reached, as can be observed in Figure 7. Values close to those of the thermodynamical equilibrium (24% of p-xylene, 52% of m-xylene, and 24% of o-xylene) are obtained during the first reaction hour, when zeolites are prepared with the maximum and minimum amount of template. The selectivity to p-xylene increases markedly during the second operation hour, except for the samples with the highest para-selectivity. Instead, the H-ZSM-5 zeolite presents a constant selectivity for periods of 2 h.
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Figure 7. p-Xylene selectivity of the catalysts at 2 times on stream.
The quality of the catalytic test results is insured to carry out the present study, since the results obtained with H-ZSM-5 are very similar to those found by Kaeding et al. (1981). IV. Discussion The material synthesized shows a direct dependence between the amount of TMA template used and the erionite and offretite intergrowth. There is an important morphological change of the crystals from a crystalline agglomerate to well-defined hexagonal crystals at a certain value of the amount of TMA added. Both morphologies appear for the zeolite ZO-4. The lack of stirring of the reacting media during the synthesis would lead to an uneven distribution of the TMA template. This distribution, which allows the existence of different template concentration zones, should be considered the reason for the appearance of such a product. Zones of higher template concentration favor the crystalline growth by molecular addition. This argument would apparently contradict the model proposed by Howden (1987b), but in our work the TMA concentrations are always lower compared to those in the system studied by Howden (1987b). The TMA highest concentration used in our work is located below the lower limit of the system considered by Howden. Thus, the fact that an increase of the amount of TMA in the parent gel favors hexagonal crystal formation does not contradict Howden’s proposal. According to the results observed, there is no doubt that TMA is an important structural directing unit and, present in a sufficient amount, avoids the erionite growth. If it is considered that each gmelinite cage contains a TMA unit and the amount of TMA in excess is located in the channels, then the filling of the channels of offretite by the template must hinder the growth of erionite. This consideration confirms that, for determined synthesis media and low TMA concentrations, there is a direct relation between the amount of TMA and the erionite percentage present in the product. This intergrowth is verified by different techniques: XRD, SEM, and measurements of selectivity and activity effects on selected reaction. Considering XRD, it becomes obvious that the relative intensity of the peak which identifies erionite presence, 2θ ) 9.3° (corresponding to an odd 101 line), decreases progressively as the amount of TMA in the synthesis media increases.
Scanning electron microscopy shows a gradual variation of the morphology of crystals as the TMA concentration increases in the initial mixtures. The crystals vary from initial agglomerates similar to those observed for zeolite T to well-defined hexagonal crystals characteristics of offretite with low TMA content (Howden, 1987b). The increase of activity in the samples with a higher amount of TMA confirms, in an indirect way, the lower percentage of erionite in these materials. The lower percentage of erionite allows the unblocking of the channels conformed by the 12-membered oxygen rings, and reactants are able to contact a major amount of active sites. On the other hand, lower TMA concentrations in the synthesis media lead to a higher percentage of erionite, and most of the active sites of easy access for the reactants are on the external surface and the activity decreases markedly. The variation of the selectivity with the amount of TMA added to the synthesis media is observed in Table 2 and Figure 7. In order to analyze the selectivity data, one must consider that there are two effects superposed: (a) the increase of stacking faults blocking channels; (b) the coke formation with the time on stream, narrowing the channels diameter. Both effects would increase the selectivity to p-xylene. Therefore, to understand the selectivity results, the yields of p-xylene in the first hour and in the second hour on stream must be considered separately. In the ZO-8 zeolite, which has its channels almost free of erionite, reactants and products go through the channels without sterical restrictions and do not show shape selectivity. On the other hand, in the zeolite ZO-1, with channels completely blocked by erionite, only the external surface is catalytically active; therefore, no shape selectivity is observed again. The blocking of the wide channels in zeolites with intermediate composition would force the reactants through the narrow channels, increasing the p-xylene selectivity. There is a difference between the critical size of toluene and p-xylene (5.85 Å) (Richter et al., 1989) and the diameter of the 8-membered oxygen ring channels (5.2 × 3.8 Å) which should not allow their access to these pores. However, one must consider that the zeolite lattice is not rigid and will expand as the temperature increases (Fraenkel, 1981). Demontis and Suffriti (1994) have demonstrated, by extensive molecular dynamics simulations, that the pore diameters of a zeolite A increase 0.3 Å at 87 °C once the lattice vibrations are incorporated to the model. Besides, Misk et al. 1996) determined that isobutene is able to diffuse rapidly through a window opening of 4.2 Å of 5A zeolite. These results were obtained at 350 °C. These authors also claimed that isobutane (kinetic diameter ≈ 5 Å) can diffuse into the same zeolite. The critical size of o- and m-xylene (6.8 Å for both compounds) is larger than that of toluene and p-xylene, so they would still have sterical problems in diffusing into the channels. Also, as mentioned, the selectivity observed in the second hour on stream shows an increasing amount of p-xylene. The coke formation would lead to an additional shape selectivity due to channel narrowing. As stated by Weitkamp et al. (1989), the three possibilities to obtain information on pore size related to shape selectivity are (1) crystallographic structure, (2) selectivities in the adsorption of a set of probe molecules, and (3) results of certain catalytic test
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reactions. The use of this last method seems more appropriate since the materials are characterized under conditions close to its further application. The materials produced are useful as alternate catalysts showing activity and intrinsic selectivity toward reactions affected by shape selectivity as toluene disproportionation. V. Conclusions It has been established for this synthesis media that there is a direct relation between the amount of TMA added and the erionite quantity present in the product. This relation enables the intergrowth control and establishes a technique to prepare an alternative catalyst for toluene disproportionation reaction with distinct levels of selectivity but with a lower activity compared to H-ZSM-5. Literature Cited Aiello, R.; Barrer, R. M. J. Chem. Soc. A 1970, 1470. Bennett, J. M.; Gard, J. A. Nature 1967, 214, 1005. Chen, N. Y.; Schlenker, J. L.; Garwood, W. E.; Kokotailo, G. T. J. Catal. 1984, 86, 24. Demontis, P.; Suffriti, G. B. Zeolites and Related Microporous Materials: State of the Art 1994. In Studies in Surface Science and Catalysis; Weitkamp, J., Karge, H. G., Pfeifer, H., Ho¨lderlich, W., Eds.; Elsevier: New York, 1994; Vol. 84, p 2107. Fernandez, C.; Grosmangin, J.; Szabo, G.; Vedrine, J. C. Appl. Catal. 1986, 27, 335. Fraenkel, D. CHEMTECH 1981, 1, 60. Gard, J. A.; Tait, J. M. Acta Crystallogr. 1972, B28, 825. Hernandez, F.; Moudafi, L.; Fajula, F.; Figueras, F. Proc. 8th Int. Congr. Catal. 1984, 2, 447.
Howden, M. G. Zeolites 1987a, 7, 255. Howden, M. G. Zeolites 1987b, 7, 260. Jenkins, E. E. U.S. Patent 3,578,398, 1971. Kaeding, W. W.; Chu, C.; Young, L. B.; Butter, S. A. J. Catal. 1981, 69, 392. Lillerud, K. P.; Raeder, J. H. Zeolites 1986, 6, 474. Mirodatos, C.; Barthomeuf, D. J. Catal. 1979, 57, 136. Misk, M.; Joly, G.; Magnoux, P.; Jullian, S.; Guisnet, M. Zeolites 1996, 16, 265. Moudafi, L.; Massiani, P.; Fajula, F.; Figueras, F. Zeolites 1987, 7, 63. Naccache, C.; Ben Taarit, Y. In Proceedings of the 5th International Conference on Zeolites, Napoli, Italy, 1980; Rees, L. V. C., Ed.; Heyden & Sons: London, 1980; p 592. Occelli, M. L.; Innes, R. A.; Pollack, S. S.; Sanders, J. V. Zeolites 1987, 7, 265. Richter, M.; Fiebig, W.; Jerschkewitz, H. G.; Lischke, G.; O ¨ hlmann, G. Zeolites 1989, 9, 238. Schlenker, J. L.; Pluth, J. J.; Smith, J. V. Acta Crystallogr. B 1977, 33, 3265. Weitkamp, J.; Ernst, S.; Chen, C. Y. Zeolites: Facts, Figures, Future. In Studies in Surface Science and Catalysis; Jacobs, P. A., van Santen, R. A., Eds.; Elsevier: New York, 1989; Vol. 49B, p 1115. Whyte, T. E., Jr.; Wu, E. L.; Kerr, G. T.; Venuto, P. B. J. Catal. 1971, 20, 88.
Received for review April 26, 1996 Revised manuscript received October 9, 1996 Accepted October 16, 1996X IE960237X
X Abstract published in Advance ACS Abstracts, December 1, 1996.
