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Ind. Eng. Chem. Res. 2004, 43, 3000-3007
Gas and Organic Vapor Permeation through b-Oriented MFI Membranes Zhiping Lai and Michael Tsapatsis* Department of Chemical Engineering and Materials Science, 151 Amundson Hall, 421 Washington Avenue SE, Minneapolis, Minnesota 55455-0132
Siliceous ZSM-5 zeolite membranes oriented with their straight 0.55-nm pores perpendicular to the support and exhibiting high separation factors for p-xylene over o-xylene were reported recently using secondary growth of an oriented seed monolayer and employing organic structuredirecting agents as crystal shape modifiers (Lai et al., Science 2003, 300, 456). Here, we report additional permeation results for this type of membrane. High separation factors, as well as high permeances, were observed for benzene/cyclohexane and benzene/2,2-dimethylbutane, although the separation factors for n-/i-butane and N2/SF6 were small. The membrane stability is satisfactory. Reproducibility of the permeation properties at 220 °C is demonstrated. 1. Introduction Recent contributions of George R. Gavalas in the field of experimental reaction and materials engineering include pioneering developments in the area of dense or ultramicroporous oxide1,2 and microporous molecular sieve3,4 membranes. In collaboration with Mark E. Davis at Caltech in the early 1990s, he initiated a research effort that evolved into one of the most influential zeolite membrane programs. Although in these “early days” of zeolite membrane research,5 the goal was to show feasibility by demonstrating selective separations, Gavalas and co-workers were among the first to emphasize the importance of film microstructure and attempt to correlate it with permeation properties.4 We are grateful to G. R. Gavalas for his contributions in the field, and we like to think that his scholarship has permeated into our work in the field of zeolite membranes. We are glad to contribute this manuscript that contains the first permeation measurements of several gases and organic vapors through b-oriented MFI membranes. Recently, our group developed a seeded growth procedure that allowed us to fabricate the first functional b-oriented membrane of the siliceous form of the zeolite ZSM-5 (structure type MFI).6 Synthesis of b-oriented MFI membranes had been attempted for more than a decade because superior performance had been predicted on the basis of the orientation of straight channels (pore openings of 0.55 nm) across the membrane.7 However, for reasons described elsewhere,6 growth of such a microstructure had not been demonstrated until recently. Our method consists of growing the crystals of an oriented seed layer into a well-intergrown film by avoiding events that lead to loss of preferred orientation, such as twin overgrowths and random nucleation. Organic polycations are used as zeolite crystal shape modifiers to enhance relative growth rates along the desirable out-of-plane direction. Changes in relative growth rates not only change the membrane orientation, but also influence the membrane thickness, grain boundary structure, and robustness upon calcination. * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 612-626-0920. Fax: 612-6267246.
As a result, a thin, compact, crack-free, and b-oriented MFI membrane was successfully obtained. This microstructurally optimized film showed a significant improvement in separation performance for xylene isomers. Here, we report additional single-component and binary-mixture permeation data for this microstructurally optimized MFI membrane and demonstrate separations of close-boiling hydrocarbon mixtures. 2. Experimental Section 2.1. Membrane Synthesis. Supported and b-oriented MFI membranes were prepared through a modified seeded growth procedure. First, a homemade porous R-alumina support (pore size ) 200 nm)8 was coated with an ultrathin, smooth mesoporous silica layer (pore size ≈ 2 nm) using the sol-gel technique developed by Brinker and co-workers.9 On top of the silica layer, a b-oriented silicalite-1 seed monolayer was prepared by using flat-shaped seeds and a chemical-reaction-based deposition method that was developed by Yoon and coworkers.10 Flat seeds with approximate particle dimensions of 500 × 200 × 100 nm were synthesized by hydrothermal growth at 130 °C for 12 h from a mixture of deionized water, tetrapropylammonium hydroxide (TPAOH), and tetraethyl orthosilicate (TEOS) with a molar composition of 5 SiO2:1 TPAOH:500 H2O:20 EtOH. The seeded support was then exposed to the synthesis solution at 175 °C for 24 h to allow secondary growth to occur. The synthesis solution was prepared from a mixture of deionized water, potassium hydroxide, structure-directing agent (SDA), and TEOS. The molar composition was 40 SiO2:5 SDA:9500 H2O:8 KOH:160 EtOH. The trimer TPA was used as the SDA to enhance the growth rate along the b axis compared to those along the a and c axes. Its synthesis can be found in ref 11. The synthesized membrane was then washed, dried, and calcined at 480 °C to remove the occluded organic SDA. 2.2. Permeation Setup. Binary permeation and single-component vapor permeation experiments were carried out in the Wicke-Kallenbach mode. An illustration of the setup can be found in our previous report.8 Supported membranes were fixed into a custom-made stainless steel permeation cell and sealed with an O-ring (Viton). The entire permeation cell was then placed into
10.1021/ie034096s CCC: $27.50 © 2004 American Chemical Society Published on Web 03/02/2004
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 3001 Table 1. Molecular Kinetic Diameters from Ref 14 molecule dk (nm) H2 O2 N2 CH4 CO2 He
0.289 0.346 0.364 0.38 0.33 0.26
molecule dk (nm)
molecule dk (nm)
n-C4 i-C4 n-C6 2,2-DMB p-xylene
o-xylene C3H8 cyclo-C6 benzene SF6
0.43 0.5 0.43 0.62 0.585
0.68 0.43 0.6 0.585 0.55
Figure 1. Single-component gas permeation setup.
a box furnace (Thermolyne). The membrane side of the supported membranes was flushed with the feed gas stream, and the support side was flushed with helium sweep gas. The composition of the permeate stream was analyzed by GC. The flow rates of the feed and sweep streams were controlled with mass flow controllers (MKS Instruments). The total pressure in either side of the membrane was 1 atm. The detection limit of the permeation system is on the order of 10-8 mol‚m-2‚s-1. The setup for single-component gas (not vapor) permeation is schematically shown in Figure 1. We use permeation cells similar to those used in the binary permeation setup, but one outlet in the permeate side is capped, and the other is connected to a bulb, which is subsequently connected to a vacuum pump. The volume of the bulb and all connection lines is accurately measured. The pressure of the bulb is monitored by computer. System leaking is checked by capping the other two outlets in the feed side and then measuring the pressure change after the system is first evacuated and then isolated from the vacuum pump. During permeation experiments, the feed side is flushed with feed gas while the permeate side is evacuated to a vacuum of about 1.0 Torr. Then, the system is isolated from the vacuum pump. The pressure in the bulb is recorded as a function of time. The permeance through the zeolite membrane can thus be calculated. Permeance, K, is the flux of the permeating gas/vapor, J, normalized for the partial pressure difference, ∆P, across the membrane. The separation factor, SF, is the concentration ratio of two species in the permeate side to that in the feed side. 3. Results and Discussion The microstructure of the membranes was described in our previous work,6 and more details will be given in a forthcoming publication. Here, it suffices to mention that the membranes are crack-free, approximately 1 µm thick, and contain mostly b-oriented grains of siliceous ZSM-5, i.e., straight uninterrupted pores with pore openings of approximately 0.55 nm are extended across the membrane thickness. Examination using fluorescence confocal optical microscopy12 suggests that the grains are well intergrown, although the presence of intercrystalline openings at the grain boundaries cannot be excluded because of resolution limitations. No evidence for a well-defined grain boundary pore network
Figure 2. Single-component permeation results. Filled bars, permeation through the zeolite membrane; open bars, permeation through the silica-coated support. The permeances of He, H2, CO2, O2, N2, CH4, C3H8, and SF6 were measured in the setup shown in Figure 1, whereas the permeances of benzene and 2,2-DMB were measured in the Wicke-Kallenbach mode.
is obtained, as was the case for the c-oriented membranes reported by our group previously.8,12,13 Permeation studies of several gases and organic vapors were performed. The kinetic diameters, dk, of all molecules investigated in this study are listed in Table 1.14 3.1. Single-Component Permeation Results. Single-component permeation results for some common gases/vapors are shown in Figure 2. In the figure, the filled bars indicate the permeances through the membrane, whereas the open bars represent the permeances through the support (coated with a very thin mesoporous silica layer). The investigated sample has a separation factor of about 450 for xylene isomers at 220 °C. For each gas, the permeation measurements were performed proceeding from 220 to 25 °C. Repeated measurements were taken to ensure that steady-state permeances were established. During experiments, the residual of the previously measured gas or vapor inside the membrane might have some effects on the permeation of the next. A thorough discussion of such effects can be found in ref 15. To minimize such effects, after the completion of a gas or vapor permeation measurement, the membrane was softly regenerated before the introduction of the next permeate by being flushed with helium at 180 °C until the permeance of helium was restored to its original value. The results in Figure 2 show that the gas permeance generally decreases with increasing molecular size. This trend is more dramatic when the kinetic diameter is close to or larger than the nominal size of the zeolitic pores. However, the permeances of H2, He, CO2, O2, N2, CH4, and C3H8 are close to the corresponding values through the support, indicating that, for these rapidly diffusing molecules, the resistance through the support might be limiting. To exploit the separation potential
3002 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004
Figure 3. Benzene/2,2-DMB single-component (open symbols) and binary (solid symbols) permeation results. Permeances and separation factors (SFs) are plotted vs temperature. The dashed lines indicate the permeance levels through the silica-coated support. Feed partial pressures for single-component measurements: benzene, 12.5 kPa; 2,2-DMB, 42 kPa. Feed partial pressures for binary measurements: benzene, 6.25 kPa; 2,2-DMB, 21.2 kPa. Adsorption isotherms of benzene and 2,2-DMB are given in the three insets at the top of the figure. FPP indicates feed partial pressure; PPP indicates permeate partial pressure as determined by GC. The solid symbols are from the references, whereas the open symbols are low-pressure-regime extrapolations obtained using Henry’s law. Adsorption isotherm data sources: for benzene 30 °C, ref 16; 100 °C, ref 17; and for 2,2-DMB 27 °C, ref 18; 100 °C, ref 19; 205 °C, ref 20.
of the thin b-oriented MFI layer for small-molecule separations, more permeable supports should be used. 3.2. Binary Permeation Results. Case 1: Benzene/ 2,2-Dimethylbutane (2,2-DMB). The single-component (open symbols) and binary (solid symbols) permeation results for benzene/2,2-DMB are shown in Figure 3. The dashed lines in the figure indicate the permeance levels of benzene and 2,2-DMB through the silica-coated support. The single-component adsorption isotherms of benzene and 2,2-DMB on silicalite-1 at different temperatures are given in the insets at the top of the figure. On the adsorption isotherms, the solid symbols indicate data obtained from the cited references, and the open symbols indicate values extrapolated to low pressure using Henry’s law. The adsorption isotherms are included to indicate the driving force in terms of loadings at the feed and permeate sides for single-component transport. As shown in Figure 3, the single-component permeance of benzene is up to 4 times larger than the corresponding binary value, but the single-component and binary permeances are almost the same for 2,2DMB, indicating that the diffusion of benzene is hindered by 2,2-DMB in binary permeation. In both cases, the permeance of benzene increases with temperature,
whereas the permeance of 2,2-DMB shows a minimum between 125 and 150 °C. As a result, the binary separation factor reaches a maximum of about 25 at 150 °C. Subsequently, the separation factor does not change much in the investigated temperature range. Diffusion through zeolite membranes can be described by the Maxwell-Stefan formulation.21,22 From the singlecomponent permeation results and the Langmuir adsorption isotherm, one can calculate the MaxwellStefan diffusivity (see, for example, ref 23) using the equation
DMS )
JL 1 + bPF Fqsat ln 1 + bPP
(
)
(1)
where J is the flux; L is the membrane thickness; b and qsat are the Langmuir parameters; F is the density of zeolite MFI; and PF and PP are the partial pressures in the feed and permeate sides, respectively. Some results for benzene and 2,2-DMB are listed in Table 2. These values were obtained using a membrane thickness of 1 µm. The thickness is determined by SEM
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 3003 Table 2. Single-Component Maxwell-Stefan Diffusivities of Benzene and 2,2-DMB
benzene 2,2-DMB 2,2-DMB 2,2-DMB
DMS (m2/s)
temp (°C)
expta
literatureb
100 27 100 205
6.7 × 10-14 6.4 × 10-15 2.8 × 10-15 1.4 × 10-14
(a) 5.0 × 10-14, (b) 4 × 10-13 6.0 × 10-21 9.8 × 10-19 9.8 × 10-17
a Calculated from experimental permeation data using singlecomponent adsorption isotherms and eq 1 (see text). b Benzene diffusivity data from (a) ref 24 and (b) ref 17. 2,2-DMB diffusivity data from ref 22.
imaging of cross sections, as reported elsewhere.6 Corresponding diffusivities reported in the literature are also listed in Table 2. As shown in Table 2, the calculated diffusivity for benzene is within the range of values reported in the literature, whereas the calculated diffusivity values for 2,2-DMB from our permeation data are several orders of magnitude larger than the literature values. Moreover, the diffusivity of 2,2-DMB calculated from the permeation measurements first decreases and then increases with temperature. This analysis of the permeation results suggests that membrane defects, rather than intracrystalline transport, dominate the permeance of 2,2-DMB. The effect of defects appears to decrease as the permeation temperature increases. Possible reasons for this result could be that grain boundary defects become smaller and/or the ratio of the transport rate though zeolitic pores vs that through defects increases as the temperature increases. However, even for the 100 and 200 °C, measurements the calculated diffusivities still exceed the literature diffusivity values, and the apparent activation energy for diffusion (24 kJ/mol) is considerably lower than the reported value of 65 kJ/mol. Thus, even at high temperature, the membrane is still not defect-free for 2,2-DMB. However, defects do not much affect the diffusion of benzene, apparently because of its faster intracrystalline transport. It appears that, for separations such as benzene/2,2DMB, there is still room for improving the selectivities of these b-oriented membranes by further reducing the contribution of grain boundary and other defects. It is also worth noting that the permeance of benzene is about 1 order of magnitude lower than the value we reported earlier for p-xylene (see also below for p-xylene data in this paper) in this type of membrane. This result can be understood by considering that p-xylene diffuses faster than benzene in silicalite-1.17 Case 2: Benzene/Cyclohexane. The binary permeation behavior of benzene/cyclohexane is shown in Figure 4. The dashed line in the figure indicates the permeance level of benzene through the silica-coated support. The permeance of benzene increases with temperature, whereas the permeance of cyclohexane shows a minimum between 75 and 100 °C. The separation factor reaches maximum of about 20 at 100 °C. At higher temperature, the separation factor slightly decreases to 15 at 220 °C, whereas the permeance of benzene increases. Separation of benzene/cyclohexane is an important process in the petrochemical industry.25 Cyclohexane is produced by the catalytic hydrogenation of benzene. Azeotropic distillation and extractive distillation, although feasible and in use in industry, suffer from high energy consumption. Thus, as an alternative, membrane-based separations have attracted significant attention. Relevant literature can be found in ref 25 and
Figure 4. Benzene/cyclohexane binary permeation results. Feed partial pressure: benzene, 6.25 kPa; cyclohexane, 6.45 kPa. The dashed line shows the permeance of benzene through the silicacoated support, whereas the open symbols show the singlecomponent permeance of benzene through the membrane for a feed partial pressure of 12.5 kPa.
Figure 5. n-Butane/isobutane binary permeation results. Feed partial pressure: n-butane, 50 kPa; isobutane, 50 kPa. The dashed line in the figure indicates the permeance of n-butane through the silica-coated support.
references therein. Most of these reports are on polymeric membranes. For MFI membranes, Keizer et al.26 reported that the selectivity of benzene over cyclohexane was about 5 at room temperature and 2.5 at 200 °C. The permeance for benzene in the study of Keizer et al. was low ( benzene/2,2-DMB (∼25 at T > 150 °C) > benzene/ cyclohexane (15-20 at T > 100 °C). The data clearly show that this microstructurally optimized ZSM-5 membrane is able to efficiently separate species on the basis of small size/shape differences. The experimental results also show that the separation performances for butane isomers and N2/SF6 are generally poor. 3.3. Membrane Stability and Reproducibility. As we stated before, the investigated sample was first tested for p-xylene/o-xylene separation before the permeation studies. After completion of the permeation tests with gases and vapors other than xylene isomers, i.e., after the sample experienced almost 100 heating and cooling cycles, the sample was tested for p-xylene/ o-xylene separation again. The separation factor did not change from the original value, and the permeance was only slightly reduced (∼10%). Regarding reproducibility, Table 4 shows xylene isomer separation results for three membranes, benzene/ cyclohexane separation for two membranes, and butane separation results for three membranes. Membrane reproducibility appears to be satisfactory from a practical standpoint, although further improvements might be possible.
Figure 7. Effect of n-hexane on binary permeation of p-xylene/ o-xylene. Solid symbols, binary permeation results from a sample not exposed to n-hexane vapor; open symbols, permeance of p-xylene from another sample after exposure to n-hexane vapor (the permeance of o-xylene for this sample is below the detection limit). Feed partial pressures: p-xylene, 0.5 kPa; o-xylene, 0.45 kPa.
3.4. Effect of Adsorbed Molecules on Xylene Isomer Permeation. Funke et al.15 reported on the effect of adsorbed molecules on the permeation properties of silicalite-1 membranes. Their results showed that the gas permeances could be either increased or decreased upon exposure of the membrane to strongly adsorbing compounds. Such effects were not easy to eliminate by either heating or evacuation. We found a similar effect that is reproducible and might be of practical significance. Figure 7 shows the binary pxylene/o-xylene permeation results obtained on b-oriented MFI membranes before and after exposure to n-hexane vapor. Unlike the procedure described in our previous work,8 in which n-hexane was introduced into the system as a third component, here, the sample was simply exposed to the vapor of n-hexane, and after that, it was softly regenerated (i.e., flushed with helium at 180 °C). Interestingly, the permeance of o-xylene on the treated sample was reduced to levels below the detection limit even at room temperature, whereas the permeance of p-xylene was only reduced by a smal factor (a factor of 2 at the high end of the investigated temperature range). The single-component permeance of p-xylene (not shown) is very close to the binary permeation value, whereas the single-component permeance of o-xylene is below the detection limit. Thus, for this n-hexanetreated sample, the permeance of o-xylene is not blocked by p-xylene, but its low value is due to the n-hexane treatment. Adsorbed n-hexane was apparently trapped
3006 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004
so strongly that, even after the sample had been softly regenerated several times, the effect on the separation of xylene isomers remained almost the same. Except for n-hexane, all of the other gases and vapors used in this study did not exhibit such a strong effect. At this stage, it is not possible to determine whether n-hexane adsorption affects intrazeolitic defect sites or grain boundary defects. The answer to this question might point to methods for further improving the selectivity of boriented MFI membranes. 4. Conclusions Thin, crack-free, and b-oriented MFI membranes were investigated in this study by gas and vapor permeation. The single-component permeation results show that the permeances of some common gases, such as H2, He, O2, N2, CO2, and C3H8, are close to their values through the support. Including our previously reported results on o-xylene/p-xylene separation, the separation performance of the membrane was demonstrated using five binary mixtures: p-xylene/o-xylene, benzene/2,2-DMB, benzene/cyclohexane, n-/i-butane, and N2/SF6. High permeances, close to the limits set by the support, as well as high separation factors were reproducibly achieved for three of the mixtures. The separation factors in these three mixtures at 220 °C were in the order of p-xylene/o-xylene (200-450) > benzene/2,2DMB (∼25) > benzene/ cyclohexane (15-20), correlating with the corresponding size differences. For n-/i-butane and N2/SF6, only low separation factors were achieved, suggesting that the use of these mixtures as the only indicator of membrane quality might be misleading. Treatment with n-hexane can have a great influence on membrane permeation properties. For the case of xylene isomers, the permeance of o-xylene was reduced below the detection limit, whereas the effect on the p-xylene permeance was small. The beneficial blocking effect persists upon repeated mild regeneration (He purge at 180 °C). Good membrane stability and reproducibility were demonstrated as well. Acknowledgment Funding for this work was provided by NSF (CTS0091406 and CTS-0103010) and NASA-Microgravity (98 HEDS-05-218). Literature Cited (1) Gavalas, G. R.; Megiris, C. E.; Nam, S. W. Deposition of H2 Permselective SiO2 Films. Chem. Eng. Sci. 1989, 44, 1829. (2) Tsapatsis, M.; Kim, S. J.; Nam, S. W.; Gavalas, G. R. Synthesis of Hydrogen Permselective SiO2, TiO2, Al2O3, and B2O3 Membranes from the Chloride Precursors. Ind. Eng. Chem. Res. 1991, 30, 2152. (3) Yan, Y.; Tsapatsis, M.; Gavalas, G. R.; Davis, M. E. Zeolite ZSM-5 Membranes Grown on Porous R-Al2O3. Chem. Commun. 1995, 2, 227. (4) Yan, Y.; Davis, M. E.; Gavalas, G. R. Preparation of Zeolite ZSM-5 Membranes by in Situ Crystallization on Porous R-Al2O3. Ind. Eng. Chem. Res. 1995, 34, 1652. (5) Davis, M. E. The Early Days of Zeolite Membrane Synthesis. In Proceedings of the Annual AIChE Meeting; American Institute of Chemical Engineers (AIChE): New York, 2002; p 371a. (6) Lai, Z.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Microstructural Optimization of a Zeolite Membrane for Organic Vapor Separation. Science 2003, 300, 456.
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Received for review August 27, 2003 Revised manuscript received November 25, 2003 Accepted December 1, 2003 IE034096S
