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J. Phys. Chem. 1996, 100, 7676-7679
Permeation of Aromatic Hydrocarbon Vapors through Silicalite-Zeolite Membranes Chelsey D. Baertsch, Hans H. Funke, John L. Falconer,* and Richard D. Noble UniVersity of Colorado, Department of Chemical Engineering, Boulder, Colorado 80309-0424 ReceiVed: January 17, 1996X
Vapor permeances of pure aromatics through silicalite zeolite membranes follow the relative order m-xylene > p-xylene > benzene ≈ toluene > ethylbenzene ≈ o-xylene near 380 K with permeance ratios as high as 20. This trend does not follow the size order of the kinetic diameters, and therefore the kinetic diameter is not the determining factor for the permeation. Activation energies for the aromatics varied from 13 to 57 kJ/mol. In binary and ternary mixtures, the faster permeating compounds were slowed to rates similar to slower permeating molecules, and thus no separation was obtained for any mixture. These findings are consistent with single file transport, where all molecules have equal chances of entering the pores, but they cannot pass each other in the narrow channels and the slowest species determines the permeation rates.
Introduction Zeolite membranes are a new class of membranes that provide an opportunity to greatly expand the use of membrane technology. Zeolites are inorganic crystalline structures with pores of the same size as small molecules. They can operate at high temperatures and are resistant to chemical degradation. The wide spectrum of available zeolite structures offers potentially many new membrane materials, but the number of different types of membranes that have been fabricated is still limited. The most-studied zeolite membranes use MFI-type zeolites such as silicalite. Silicalite is a pure silica zeolite. It has a system of straight channels with elliptical pore openings of 0.51 × 0.57 nm interconnected by zigzag channels with a nearly circular cross section of about 0.54 nm diameter.1 Silicalite is hydrophobic and preferentially adsorbs organic molecules that are small enough to enter the pore openings. Recent studies indicate that zeolite membranes have a high separation potential for a variety of organic mixtures including xylenes [e.g., refs 2-14]. In this study, we investigated the vapor separation properties of silicalite composite membranes for xylene isomers, ethylbenzene, toluene, and benzene. Experimental Procedures The silicalite layers were synthesized on the inside of commercially available porous γ-alumina tubes (US Filters, 5-nm pores). The zeolite synthesis was performed hydrothermally from a gel. After two synthesis steps, the membranes were washed, dried, and calcined at 733 K. The detailed synthesis procedure is described elsewhere.2,3 SEM photographs show a zeolite layer thickness between 2 and 10 µm, and XRD indicates that the membrane surface consists of pure silicalite.2 In the interfacial region between the zeolite layer and the alumina support, however, some aluminum might be incorporated into the silicalite structure. No catalytic conversions that would indicate acidic aluminum sites in the membrane layer were detected when the pure compound permeances were measured at elevated temperatures. The membranes were characterized by pure gas permeation at room temperature. The selectivity for N2 (kinetic diameter of 0.36 nm) over SF6 (0.55 nm) was used as a standard measure of the quality of the membrane. The membrane used in this study had a N2/SF6 permeance ratio of 86 and a N2 permeance of 2.0 × 10-6 mol/ (m2 s Pa). X
Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(96)00226-2 CCC: $12.00
Figure 1. Permeance of pure hydrocarbon vapors through a silicalite membrane as function of temperature (organic feed concentrations: p-xylene ∼17 mol %, m-xylene ∼13 mol %, o-xylene ∼15 mol %, ethylbenzene ∼8 mol %, toluene ∼17 mol %, benzene ∼20 mol %, balance He).
The permeation experiments were performed in a continuous flow system with a He carrier gas on both feed and permeate sides. The membrane was sealed with graphite gaskets. The organic compounds were fed as liquids that were completely evaporated and mixed with He prior to entering the module. Feed, retentate, and permeate streams were analyzed on-line with a GC equipped with a FID. All experiments were performed at ambient pressure on feed and permeate side. The experimental setup is described in detail elsewhere.3 The aromatics were all Alltech analytical grades. Results Single Compound Permeation. The temperature dependencies of the permeances of the aromatic compounds are shown in Figure 1. The organic feed concentration was maintained between 8 and 20 mol %, and He was used as carrier gas. Except for p-xylene, permeances steadily increased with increasing temperature, indicating an activated transport mechanism. The permeances of p-xylene and m-xylene were significantly higher than the permeances of ethylbenzene, toluene, and benzene throughout the temperature range. At the lower temperatures, the permeance of o-xylene was similar to those © 1996 American Chemical Society
Permeation of Aromatic Hydrocarbon Vapors
J. Phys. Chem., Vol. 100, No. 18, 1996 7677
TABLE 1: Physical Properties of Aromatic Hydrocarbons, Diffusivities in Silicalite Crystals, Pure Compound Permeances, and Permeation Activation Energies permeances ×109 [mol/(m2 s Pa)] boiling kinetic point diameter [K] [nm] benzene toluene ethylbenzene p-xylene o-xylene m-xylene a
353 384 409
0.585 0.585 0.600
411 417 412
0.585 0.680 0.680
diffusivity at ∼380 K15 m2/s] ∼10-13 ∼2 × 10-13 ∼5 × 10-13 ∼4 × 10-14
T) 380 K
T) 480 K
activation energy [kJ/mol]
10a 7.5 4.0
50 25 25
26 ( 4b 17 ( 9 31 ( 9
30 2.5 50
85 90 110
15 ( 4 57 ( 9 13 ( 1
Extrapolated. b Standard error.
of ethylbenzene and toluene. The benzene permeance was slightly higher than the ethylbenzene and toluene permeances. However, at the higher temperatures, the o-xylene permeance approached those of p-xylene and m-xylene. Table 1 lists permeances of the aromatics obtained near 380 and 480 K. Extrapolation was used to estimate the permeance of benzene at 380 K. Permeance ratios between the faster and slower permeating compounds were as high as 20 at 380 K, whereas the highest permeance ratio was only 4.4 at 480 K. The data in Figure 1 were fit to Arrhenius plots with activation energies between 13 and 57 kJ/mol. These energies are listed in Table 1 along with kinetic diameters, boiling points, and diffusivities of the aromatics in silicalite crystals.15 Mixed Feed Permeation. Binary mixtures of p-xylene/oxylene, p-xylene/ethylbenzene, p-xylene/toluene, and m-xylene/ ethylbenzene and one ternary mixture of benzene, toluene, and p-xylene were used as feeds. The concentration of organics in the feed was maintained between 6 and 14 mol %, and equal molar concentrations of each compound were used in all mixtures. Since the total organic feed concentrations were similar to or lower than the concentrations in the pure component experiments, the concentration of each component in the mixtures was lower. Permeances, however, are not dependent on feed concentration in the concentration range where the experiments were carried out as shown previously for C6- and C8-alkanes.3 No separation was achieved for any of these mixtures in the temperature range investigated as shown in Figures 2-6. The permeance of each compound in the mixture was the same for the p-xylene/o-xylene binary mixture (Figure 2), and the permeances were lower than the permeances of either pure component. For the other systems, the permeation rates of the faster-permeating species (when run as pure components) were slowed to the rate of the slowest species in the mixtures. That is, the permeance curves in Figure 3-5 for each compound in the mixture are identical with each other and identical with the pure component permeance curve for the slowest components. The permeances in the ternary mixture were slightly faster than the slowest species as shown in Figure 6. The mixed feed permeances increased with temperature with the same dependence observed for the slower pure compound. Discussion Single Compound Permeation. Although p-xylene, toluene, benzene, and ethylbenzene have the same kinetic diameter, the pure compounds permeated through the membrane at different rates. Similarly, m-xylene and o-xylene have the same kinetic diameters but different permeances. The compounds with similar permeances have similar molecule shapes. Molecules with two alkyl groups that are not adjacent (p- and m-xylene)
Figure 2. Permeances of p-xylene and o-xylene vapors through a silicalite membrane as function of temperature for pure compounds (same as in Figure 1) and for the same compounds in mixtures (feed concentration: ∼3 mol % p-xylene, 3 mol % o-xylene, 94 mol % He).
Figure 3. Permeances of p-xylene and ethylbenzene vapors through a silicalite membrane as a function of temperature for pure compounds (same as in Figure 1) and for the same compounds in mixtures (feed concentration: ∼6 mol % p-xylene, 6 mol % ethylbenzene, 88 mol % He).
permeated more quickly through the silicalite pores than the molecules with two adjacent alkyl groups (o-xylene), one alkyl group (toluene, ethylbenzene), or no alkyl group (benzene). Diffusivities of the aromatics in silicalite crystals, as listed in Table 1, show similar trends for p-xylene, benzene, and o-xylene.15 The lowest diffusivity was reported for o-xylene, which was the slowest permeating compound at 380 K. The fast permeating p-xylene also has high diffusivities in silicalite crystals. Ethylbenzene did not follow this order and has a high diffusion coefficient in crystals even though it permeated more slowly through the membrane than other aromatics. Diffusivities or molecule size and shape apparently are not sufficient to predict pure compound permeances. Mixed Feed Permeation. Although no separation was achieved with any mixture in the temperature range investigated, the change in permeance from pure compounds to mixtures gives some insight into the transport process through silicalite membranes. Mixed compound adsorption and diffusion studies in MFI-zeolite crystals at conditions similar to those used for our zeolite membrane are not available. In H- and Na-ZSM-5 crystals, Namba16 found high adsorption selectivities for pxylene from liquid mixtures with o- and m-xylene between 283 and 313 K. Since membrane transport is a combination of
7678 J. Phys. Chem., Vol. 100, No. 18, 1996
Figure 4. Permeances of m-xylene and ethylbenzene vapors through a silicalite membrane as function of temperature for pure compounds (same as in Figure 1) and for the same compounds in mixtures (feed concentration: ∼7 mol % m-xylene, 7 mol % ethylbenzene, 86 mol % He).
Figure 5. Permeances of p-xylene and toluene vapors through a silicalite membrane as function of temperature for pure compounds (same as in Figure 1) and for the same compounds in mixtures (organic feed concentration: ∼5.5 mol % p-xylene, 5.5 mol % toluene, 89 mol % He).
sorption and diffusion processes and strongly dependent on the experimental conditions, permeances cannot be predicted solely from adsorption data obtained at different temperature- and concentration regimes. The mixed feed permeances of all compounds coincided with the pure compound permeance of the slowest compound in the mixtures except for the p-xylene/o-xylene binary mixture where the mixed feed permeances were even lower. This behavior suggests that single-file diffusion through the silicalite pores limits transport. In single-file transport, the channels are not large enough for molecules to pass one another so that the molecule with the slowest permeation rate limits diffusion and slows the other species down to its own rate. This effect has been studied both experimentally and theoretically for a variety of crystalline materials with small pores.17-21 Few studies were performed with aromatic systems. van den Broeke22 used Monte Carlo techniques and found that a single file transport model predicted transient uptake profiles of benzene/ethylbenzene mixtures in ZSM-5 crystals. Quershi et al.23,24 studied co- and counterdiffusion of benzene and toluene in ZSM-5 crystals and found good agreement with a single file diffusion model.
Baertsch et al.
Figure 6. Permeances of p-xylene, toluene, and benzene vapors through a silicalite membrane as a function of temperature for pure compounds (same as in Figure 1) and for the same compounds in mixtures (feed concentration: ∼4 mol % p-xylene, 4 mol % toluene, 4 mol % benzene, 88 mol % He).
Zeolite membrane systems, however, have not been used to discuss single file transport limitations of aromatic systems. Entering the pores is apparently equally probable for all compounds in the concentration regime studied with our system since no selectivity was observed. Our results are also consistent with pervaporation data obtained with silicalite membranes by Sano et al.,9 who found that a liquid feed of p- and m-xylene gave similar permeances for both compounds and no separation was found. Vroon14 was able to separate p- and o-xylene vapor mixtures with selectivities of 25 at 473 K using silicalite membranes and feed concentrations that were about 1 order of magnitude lower than in our experiments. This indicates that the concentration regime is an important variable that determines the transport mechanism. Vroon’s membrane was significantly different from our silicalite membranes since his single gas permeance ratio for H2/SF6 at 298 K was ∼11. Hydrogen flows more than 200 times faster than SF6 through our membranes at the same temperature. Apparently the pure gas selectivities for low boiling compounds cannot be used to predict separation properties for aromatic vapor mixtures. Single file diffusion is not controlling for all molecules at high hydrocarbon concentrations and thus high selectivities can be obtained with silicalite membranes. Funke et al.3 found that the same silicalite membranes can separate n-octane from the larger isooctane (2,2,4-trimethylpentane, kinetic diameter ) 0.62 nm) in mixtures with high selectivities, whereas pure isooctane permeated faster than pure n-octane. The n-octane fluxes were similar to pure p-xylene. Apparently the n-octane molecules prevent the isooctane molecules from entering the zeolite pores in a mixture. It is obvious that permeances of organic mixtures through silicalite membranes cannot be predicted based on pure compound permeances, and subtle differences in molecule size and shape can result in opposite trends in permeation behavior for otherwise similar systems. Summary Transport of pure aromatic vapors through silicalite composite membranes is not determined by molecule size since the permeances do not follow the size order of the kinetic diameters. Activation energies for the permeances are between 13 and 57 kJ/mol. o-Xylene has the lowest and m-xylene the highest permeances near 380 K, whereas at 480 K the o-xylene permeance is similar to the p- and m-xylene permeances. The
Permeation of Aromatic Hydrocarbon Vapors highest pure compound permeance ratio was 20 for m-xylene over o-xylene near 380 K. In binary and ternary mixtures, however, single-file transport seems to take place where the faster permeating compounds are slowed to rates similar to the slower molecules and no separations are obtained. Apparently all molecules have equal chances of entering the pores but cannot pass each other in the narrow channels and the slowest species determines the permeation rates. This effect has been widely discussed for adsorption in zeolite crystals zeolite but never observed directly with membranes. Acknowledgment. We gratefully acknowledge support by Chevron Research and Technology Co., and support for C.D.B. by the NSF-REU Program (NSF Grant EEC-9300435). References and Notes (1) Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature 1978, 271, 512 (2) Jia, M.; Chen, B.; Noble, R. D.; Falconer, J. L. J. Membr. Sci. 1994, 90, 1 (3) Funke, H. H.; Falconer, J. L.; Noble, R. D. Ind. Eng. Chem. Res., in press. (4) Bakker, W. J. W.; Zheng, G.; Kapteijn, F.; Makkee, M.; Moulijn, J. A.; Geus, E. R.; van Bekkum, H. Precision Process Technology; Weijnen, M. P. C., Drinkenburg, A. A. H., Eds.; Kluwer Academic Publishers: Dordrecht, 1993; p 425. (5) Geus, E. R.; den Exter, M. J.; van Bekkum, H. J. Chem Soc., Faraday Trans. 1992, 88, 3101. (6) Geus, E. R.; van Bekkum, H.; Bakker,W. J. W.; Moulijn, J. A. Microporous Mater. 1993, 1, 131. (7) Ko¨lsch, P.; Venzke, D.; Noack, M.; Toussaint, P.; Caro, J. J. Chem. Soc., Chem. Commun. 1994, 21, 2491.
J. Phys. Chem., Vol. 100, No. 18, 1996 7679 (8) Noack, M.; Ko¨lsch, P.; Venzke, D.; Toussaint, P.; Caro, J. Microporous Mater. 1994, 3, 201. (9) Sano, T.; Hasegawa, M.; Kawakami, Y.; Kiyozumi, Y.; Yanagishata, H.; Kitamoto, D.; Mitzsukami, F. Stud. Surf. Sci. Catal. 1994, 84, 1175. (10) Tsikoyiannis, J. G.; Haag, W. O. Zeolites 1992, 12, 126. (11) Vroon, Z. A. P. E.; Keizer, K.; Verweij, H.; Burggraaf, A. J. 3rd International Conference on Inorganic Membranes, Worcester, MA; July 10-14, 1994. (12) Xiang S.; Ma, Y. H. 3rd International Conference on Inorganic Membranes, Worcester, MA; July 10-14, 1994. (13) Yan, Y.; Tsapatsis, M.; Gavalas, G. R.; Davis, M. E. J. Chem. Soc., Chem. Commun. 1995, 227. (14) Vroon, Z. A. E. P. Synthesis and transport studies of thin ceramic supported zeolite (MFI) membranes. Ph.D. Thesis, University of Twente, Netherlands, 1995. (15) Ka¨rger, J.; Ruthven, D. M. Diffusion in zeolites and other microporous materials; Wiley-Interscience: New York, 1992. (16) Namba, S.; Kanai, Y.; Shoji, H.; Yashima, T. Zeolites 1984, 4, 77. (17) Ka¨rger, J.; Pfeifer, H. Proceedings of the 9th international zeolite conference, Montreal 1992; von Ballmoos, R., et al., Eds.; ButterworthHeinemann: Boston, 1993, 129. (18) Ka¨rger, J.; Petzold, M.; Pfeifer, H.; Ernst, S.; Weitkamp, J. J. Catal. 1992, 136, 283. (19) van den Broeke, L. J. P.; Nijhuis, S. A.; Krishna, R. J. Catal. 1992, 136, 463. (20) Shen, D.; Rees, L. V. C. J. Chem. Soc., Faraday Trans. 1994, 90, 3017. (21) Ka¨rger, J.; Keller, W.; Pfeifer, H.; Ernst, S.; Weitkamp, J. Microporous Mater. 1995, 3, 401. (22) van den Broeke, L. J. P. AIChE J. 1995, 44, 2399. (23) Qureshi, W. R.;Wei, J. J. Catal. 1990, 126, 126. (24) Qureshi, W. R.; Wei, J. J. Catal. 1990, 126, 147.
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