Journal of Membrane Science 215 (2003) 235–247
Pervaporation of organic/water mixtures through B-ZSM-5 zeolite membranes on monolith supports Travis C. Bowen, Halil Kalipcilar, John L. Falconer∗ , Richard D. Noble Department of Chemical Engineering, University of Colorado, Boulder, Campus Box 424, Boulder, CO 80309-0424, USA Received 26 August 2002; received in revised form 2 December 2002; accepted 10 December 2002
Abstract High-quality, boron-substituted ZSM-5 zeolite membranes were prepared on Al2 O3 -coated SiC multi-channel monolith supports. Monoliths have larger surface to volume ratios than tubular supports and are more practical for large-scale applications. Two types of supports with 66 channels (2 mm × 2 mm, 10.6 cm2 surface area cm−3 ) and 22 channels (4 mm × 4 mm, 7.2 cm2 cm−3 ) were used. The membranes effectively removed alcohols and acetone from 5 wt.% organic/water binary feeds by pervaporation over a temperature range of 303–333 K. The membranes were selective because acetone and the alcohols preferentially adsorbed and inhibited water transport, but diffusion differences caused by different adsorption strengths and molecular sizes were also important. Methods for calculating molecular kinetic diameters for polar molecules are compared. The acetone/water separation factor was 330 at 303 K, and it decreased to 220 at 333 K. Methanol, ethanol, 2-propanol, and 1-propanol separation factors were 8.4, 31, 42, and 75, respectively, at 333 K and were relatively independent of temperature. With the exception of methanol, these separation factors are significantly higher than those reported for a B-ZSM-5 tubular membrane. The fluxes at 333 K were 0.90, 0.16, 0.047, 0.071, and 0.22 kg m−2 h−1 for methanol, ethanol, 1-propanol, 2-propanol, and acetone, respectively, and increased with temperature. The methanol and ethanol fluxes are comparable to those for the B-ZSM-5 tubular membrane. © 2003 Elsevier Science B.V. All rights reserved. Keywords: MFI zeolite membrane; Multi-channel monolith; Pervaporation; Inorganic membranes
1. Introduction Membrane pervaporation has advantages over distillation for liquid separations because of lower operating temperatures and because azeotropes can be separated. Polymeric membranes have been used to separate numerous feed mixtures by pervaporation [1–4]. Membrane swelling and low chemical resistance, however, often limit the feed mixtures that can be used. ∗ Corresponding author. Tel.: +1-303-492-8005; fax: +1-303-492-4341. E-mail address:
[email protected] (J.L. Falconer).
The uniform, molecular-size pores and the adsorption properties of zeolites, plus their high thermal, chemical, and mechanical stability make zeolite membranes good candidates for pervaporation separations. Zeolite membranes have been used in pervaporation mostly to dehydrate organic solutions. High separation factors have been reported for water/ethanol separations using hydrophilic A-type membranes [5–7]. Kita et al. also reported preferential permeation of water over organics in Y- [8,9] and X-type [9] membranes. Organic/organic separations have been observed for both hydrophilic [8,10,11] and hydrophobic [12] zeolite membranes. Organics have been removed from water using MFI [13–17], MEL [17,18], and X-type
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(02)00617-8
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[10] membranes. Most zeolites contain Si and Al, but isomorphous substitution of other metals (B, Fe, and Ge), into the zeolite framework has improved the separation performance of zeolite membranes. Tuan et al. [19] observed that an MFI membrane with boron substituted into its framework had higher separation factors for n-C4 H10 /i-C4 H10 than Al-ZSM-5 and silicalite-1 membranes prepared by the same procedure. The B-ZSM-5 membrane also had higher separation factors than the Al-ZSM-5 and silicalite-1 membranes for removing methanol from water [20]. Previous studies have used zeolite membranes synthesized on either tubular or disc-shaped porous supports. We recently reported that a B-ZSM-5 membrane, prepared on a monolith support, separated butane isomer vapors [21]. We also characterized these monolith-supported membranes and observed that they separated n-hexane/2,2-dimethylbutane vapor mixtures with selectivities as high as 312 at 473 K [22]. Permeances and selectivities of the C6 isomers increased with temperature. The ZSM-5 structure was verified with XRD and SEM photographs of a membrane showed good crystalline intergrowth and a uniform zeolite layer on each channel wall, but the zeolite thickness varied for different monolith channels. The average thickness was approximately 100 m and the zeolite layer was only about 30 m on the outer channel walls. Monoliths are more practical than tubes or discs because they provide higher surface to volume ratios; this is an advantage industrially and also provides higher flow rates for laboratory studies. Spiral wound and hollow fiber modules dramatically improved polymeric membranes because of their higher surface to volume ratio. The multi-channel monolith geometry has the potential to benefit inorganic membranes in the same way. Monoliths are already used in chemical reactor applications, such as catalytic converters.
In the current study, B-ZSM-5 (Si/B = 100) zeolite membranes on alumina-coated, silicon carbide monolith supports were used to separate methanol, ethanol, 1-propanol, 2-propanol, and acetone from binary 5 wt.% organic/water feed mixtures by pervaporation over a temperature range of 303–333 K. 2. Experimental 2.1. Membrane synthesis The multi-channel monolith supports were provided by CeraMem Corp. and were porous silicon carbide with an average pore diameter of 11–15 m. The SiC channels were coated with two ␣-Al2 O3 layers to reduce the support pore size and allow uniform zeolite deposition. The first ␣-Al2 O3 layer had an average pore diameter of 0.5 m and a thickness of 100 m. The second layer was 10-m thick with 0.2-m pores. Membrane M1 had an additional 2-m thick ␥-Al2 O3 layer with 5-nm pores. Each Al2 O3 layer and the SiC had approximately 40% porosity. The monoliths were 2.5 cm OD × 4.6 cm long, with approximately 1 cm on each end glazed for sealing. Monoliths with 2 mm × 2 mm and 4 mm × 4 mm square channels were used (Table 1). The porous section of these monoliths had surface to volume ratios of 10.6 and 7.2 cm−1 , respectively. Zeolite layers were hydrothermally crystallized on the inside surface of the monolith support as described by Kalipcilar et al. [21]. The synthesis gel used 1 M tetrapropylammonium hydroxide (TPAOH) as the structure-directing template, Ludox AS40, and boric acid, and the molar composition was 1.6 TPAOH: 19.5 SiO2 : 0.2 B(OH)3 : 438 H2 O. Before synthesis, a Teflon spacer was placed on one end of the support and the end was covered with Teflon tape and sealed with a Teflon cap. Synthesis gel was
Table 1 Properties of monoliths and B-ZSM-5 membranes Membrane
Top layer of Al2 O3 on support
No. of channels
Channel size (mm)
Zeolite layers
Calcination temperature (K)
Permeable area (cm2 )
M1 M2 M3
␥ ␣ ␣
66 22 22
2×2 4×4 4×4
4 2 4
723 753 723
93 ± 8 63 ± 7 63 ± 7
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then injected into a single channel of the support and the other channels were filled from the bottom. The support was left overnight so the gel could soak into the pores. More gel was then added and the open end of the support was sealed with Teflon tape and a Teflon cap. The support was placed vertically into a Teflon-lined, stainless steel autoclave, which was placed in an oven at 458 K. The first zeolite layer was synthesized for 24 h and all other layers were synthesized for 48 h. Before deposition of each subsequent layer, the membrane was rinsed with deionized water and turned upside down in the autoclave to make the zeolite layer thickness more uniform. After the final layer was deposited, the membrane was washed with deionized water and dried in a vacuum oven at 373 K. The membranes were impermeable to N2 at 298 K with a 138 kPa trans-membrane pressure drop because the template filled the pores, indicating that no large defects were present. The template was then removed by calcining the membrane in air at 723 or 753 K (Table 1) for 8 h using a computer-controlled muffle furnace. Heating and cooling rates during calcinations were 0.01 and 0.03 K/s, respectively. 2.2. Membrane characterization The Si and Al contents of a similar membrane to the ones used in this study were measured with energy-dispersive X-ray (EDX) spectroscopy. Single gas permeances of H2 , CO2 , N2 , n-C4 H10 , and i-C4 H10 , and H2 /n-C4 H10 , H2 /i-C4 H10 , and n-C4 H10 /i-C4 H10 mixture permeances and separation selectivities were measured to determine the membrane quality. Ideal and separation selectivities are permeance ratios for single gas and gas mixture measurements, respectively. The gas permeances were measured at 298, 373, and 473 K and used a feed pressure of 223 kPa and a trans-membrane pressure drop of 138 kPa. Mixture separations used 50/50 feed mixtures and the log-mean pressure drop was used as the driving force for permeance calculations. 2.3. Pervaporation apparatus/procedure Organic/water binary liquid mixtures were separated using a pervaporation apparatus with a feed cir-
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culation system similar to that described by Liu et al. [15]. Each time the feed mixture was changed, the membrane was calcined in air at 573 K for 4 h to remove adsorbed molecules and then stored under vacuum at 373 K. Silicone o-rings sealed the membrane in a module made from a modified 2.5-cm stainless steel Swagelok cross. A centrifugal pump circulated the feed through the system at a flow rate of approximately 320 cm3 min−1 . Circulation has been shown to significantly improve membrane pervaporation performance because it reduces concentration polarization [20]. An Omega temperature controller maintained a constant feed temperature between 303 and 333 K by using a chromel-alumel thermocouple that measured the retentate temperature. The system was filled with approximately 420 cm3 of feed solution. At the start of the pervaporation experiments, a mechanical vacuum pump evacuated the permeate side of the membrane to about 1 kPa. The feed mixture was heated to 333 K. After about 2 h, the vacuum pump valve was partially closed and permeate was collected for 10–40 min in a liquid nitrogen trap. The pressure was kept between 0.25 and 1 kPa during permeate collection. Samples collected in a second cold trap, downstream of the valve, were less than 4% of the total collected, and their concentrations were approximately the same as the samples in the first trap. However, more than 2 h were required to collect a measurable amount in the second trap. Therefore, only the sample in the first trap was used for the flux and separation factor measurements. Successive permeate samples were collected until the flux and permeate concentration deviated from the previous sample by less than 5%. This is a pseudo-steady-state condition because the feed concentration changes, but it will be referred to as steady state because the change is small during each permeate collection. The propanol mixtures reached steady state in about 2 h at 333 K. Mixtures with higher flux reached steady state more quickly. Methanol, for example, reached steady state in about 1 h, which was the time required to heat the system. Steady state for 323, 313, and 303 K was reached by the time the feed cooled to the new temperature and stabilized, which took about an hour. The feed was sampled at the end of each permeate collection time. A GC (HP 5890) equipped with an Alltech Hayesep D column and a thermal conductivity detector measured the concentrations of the feed
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and permeate samples. The organic feed concentration decreased slightly as the organic preferentially permeated through the membrane. During the cooling period between each temperature measurement, the proper amount of organic was injected into the system so that a feed concentration between 4.5 and 5.3 wt.% organic was maintained. The average concentration of two samples collected after steady state was reached was used to calculate the separation factor, which is defined as: yorganic /ywater αorganic/water = (1) xorganic /xwater where y and x are the weight fractions of the permeate and feed, respectively.
3. Results 3.1. Membrane characterization The gas permeances depended on molecular size. The high H2 /i-C4 H10 and n-/i-C4 H10 ideal selectivities and n-/i-C4 H10 separation selectivities in Table 2 show that membrane M1 was of high quality with most of its flow through zeolite pores. Both the gas and pervaporation measurements in Table 2 show that each membrane separated the feeds but their qualities were different. The performance of membrane M1 was studied in more detail. Single gas permeances of H2 , CO2 , N2 , n-C4 H10 , and i-C4 H10 through membrane M1 are shown in Fig. 1 as a function of temperature. Permeances increased with increasing temperature for n-C4 H10 and i-C4 H10 but decreased for H2 , CO2 , and N2 . This follows the trends that Bakker et al. [23] observed for gas permeation through silicalite-1 membranes.
Surface diffusion increases with temperature, and for the strongly-adsorbing molecules this is the dominant factor because the coverage remains high over this temperature range. The light gases, on the other hand, have lower surface coverages, and their coverages decrease as temperature increases, so their permeances decrease. The permeances at 473 K are about four times higher than permeances through B-ZSM-5 membranes on stainless steel tubular supports [20]. 3.2. Organic/water pervaporation The monolith membranes effectively separated alcohol/water and acetone/water mixtures. At short times, the permeate concentration was near the feed concentration, but the organic selectively permeated after longer times. Pervaporation of acetone/water through membrane M1 at 303 K reached steady state after about 4–5 h, as shown in Fig. 2. The feed concentration decreased during these measurements because the total feed volume was only 200 cm3 , or about 0.5 of the volume used during the rest of the pervaporation measurements. Transient behavior was similar for the other feed mixtures and temperatures, but the time to reach steady state varied from 1 to 5 h. Successive measurements of the steady-state fluxes and permeate concentrations exhibited experimental variations that were less than or equal to 5%. Two pervaporation measurements made 17 days apart for a 5 wt.% methanol feed at 313 K had average fluxes and permeate concentrations within 5% of each other. The fluxes for pure methanol, water, and acetone, and the total fluxes for 5 wt.% organic/water mixtures increased with increasing temperature, as seen in Fig. 3. The total fluxes increased at all temperatures in the following order: 1-PrOH < 2-PrOH < EtOH < acetone < MeOH. Fig. 4 shows that the separation
Table 2 Gas and liquid selectivities and fluxes of monolith membranes Membrane
M1 M2 M3 a
Gas separation selectivity at 473 K
Pervaporation at 303 K (5 wt.% alcohol/water feed)
H2 /i-C4 H10 (ideal)
n-/i-C4 H10 (ideal)
n-/i-C4 H10 (separationa )
Separation factor MeOH
EtOH
77 18 26
34 8.8 14
35 17 3.7
8.5 12 –
24 – 14
50/50 feed mixture.
Total flux (kg m−2 h−1 )
0.21 (MeOH), 0.035 (EtOH) 0.12 0.063
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Fig. 1. Single gas permeances through B-ZSM-5 zeolite monolith membrane M1. The feed pressure was 223 kPa and the trans-membrane pressure drop was 138 kPa.
factors for the alcohols were relatively insensitive to temperature. Acetone preferentially permeated with a separation factor of 330 at 303 K and the separation factor decreased to 220 at 333 K. Separation factors were lower for the alcohols. The smallest separation factors for MeOH, EtOH, 1-PrOH, and 2-PrOH were 8.3, 24, 75, and 42, respectively. The separation factors were higher than vapor/liquid equilibrium
separations (Table 3) for all feeds and temperatures investigated. Even though water comprised 95% of the feed, the alcohols and acetone decreased the water flux dramatically below the pure water flux (Fig. 5). Methanol, ethanol, 2-propanol, acetone, and 1-propanol decreased the water flux by approximately 42, 95, 98, 98, and 99%, respectively.
Fig. 2. Total flux and feed and permeate concentrations as a function of time at 298 K for acetone/water feed through B-ZSM-5 zeolite monolith membrane M1.
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Fig. 3. Total fluxes as a function of temperature for pervaporation of 5 wt.% organic/water binary mixtures through B-ZSM-5 zeolite monolith membrane M1. The pure fluxes of acetone, methanol, and water are shown for comparison.
Fig. 4. Separation factors as a function of temperature for pervaporation of 5 wt.% organic/water binary mixtures through B-ZSM-5 zeolite monolith membrane M1. Some separation factors are multiplied by the indicated number for clarity.
4. Discussion 4.1. Membrane performance Diffusivity in zeolite membranes is a strong function of molecular diameter. Breck [24] compiled a list of kinetic diameters for non-polar molecules, including H2 (0.29 nm), CO2 (0.33 nm), N2 (0.36 nm),
n-C4 H10 (0.43 nm), and i-C4 H10 (0.50 nm). Most studies follow Breck’s method and use the characteristic length parameter of the Lennard-Jones [6–12] potential as the kinetic diameter of a spherical, non-polar molecule, and the diameter of the smallest cylinder that encompasses the van der Waals radii of the atoms in a molecule is used as the kinetic diameter of a non-spherical, non-polar molecule. Diameters
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Table 3 Properties of feed components Molecule
Methanol Ethanol 1-Propanol 2-Propanol Acetone Water a
Relative volatility, ␣, [41] for 5 wt.% in water with temperature (K) in parentheses
Diameter (nm)a From SVC data [25]
From phase-coexistence data [26]
7.40 (368.2) 10.5 (368.5) 16.3 (367.9) 20.9 (366.4) 42.7 (355.7) –
0.276 0.238 0.264 0.242 0.374 0.263
0.380 0.430 0.469 0.470 0.469 0.296
−Hads on H-ZSM-5 (kJ/mol)
115 [42] 130 [42] 145 [42] N/A 130 [43] 90 [42]
Assumes spherical molecule.
based on the van der Waals radii have also been used to estimate kinetic diameters for some polar molecules [7]. Dipole moments, however, increase attractive forces so that apparent diameters are smaller than those based on van der Waals radii. Breck used the characteristic length parameter of the Stockmayer potential function as the kinetic diameter of the polar molecules H2 O and NH3 , which are roughly spherical. The widely-accepted 0.263-nm kinetic diameter of water is the Stockmayer length parameter derived from second-virial-coefficient (SVC) data [25]. However, kinetic diameters based on Stockmayer parameters derived from SVC data unreasonably predict that ethanol and 2-propanol are smaller than methanol
and water and that 1-propanol is the same size as water (Table 3). Similar inconsistencies occur for other polar molecules using the SVC data [25]. This method predicts that methyl chloride is larger than chloroform, for instance. Stockmayer parameters have also been derived from gas-viscosity and phase-coexistence data. van Leeuwen [26] concluded that Stockmayer potential parameters based on phase-coexistence data are more physically realistic than those based on the gas-viscosity and SVC data. The kinetic diameters for water and alcohols based on phase-coexistence data (Table 3) increase in the following order: H2 O < MeOH < EtOH < 1-PrOH ∼ = 2-PrOH. This follows
Fig. 5. Water fluxes as a function of temperature for pervaporation of 5 wt.% organic/water binary mixtures through B-ZSM-5 zeolite monolith membrane M1. Some fluxes are multiplied by the indicated number for clarity. The pure water flux is shown for comparison.
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the expected order based on the size of the atoms in each molecule and the molecular structures. The other inconsistencies, such as that for methyl chloride and chloroform, are also resolved when the phase-coexistence data are used. Therefore, kinetic diameters for the polar molecules were based on phase-coexistence data because these diameters appear to be more realistic representations of molecular size than the kinetic diameters based on second-virial-coefficients. The Stockmayer potential function assumes the molecules are spherical, and the diameters of ethanol and 1-propanol are expected to be slightly lower than those shown in Table 3 because the molecules are shaped like cylinders. All molecules used in this study are smaller than the 0.51 nm×0.55 nm and 0.54 nm×0.56 nm pores (XRD size) [27] of the MFI structure. The highest total flux of the mixtures was for methanol/water, and methanol has the smallest diameter and the highest diffusivity of the organics. Ethanol is larger and had higher separation factors than methanol, and the propanols are larger and had higher separation factors than ethanol. However, acetone, 1-propanol, and 2-propanol have similar sizes but their separation factors differed dramatically. Adsorption is also important in permeation because molecules move through zeolite pores by surface diffusion. The organic molecules adsorb more strongly than water on the H+ form of Al-ZSM-5 zeolite (Table 3). Boron substituted ZSM-5 zeolite is similar to Al-ZSM-5 and although different magnitudes of −Hads are expected for the two zeolites, the trends should be similar. The heats of adsorption for the alcohols increase with the number of carbons due to increased van der Waals interactions [28]. This suggests that −Hads of 2-propanol is greater than that of ethanol, but its branched structure reduces its interactions with the zeolite structure and −Hads of 2-propanol is expected to be slightly lower than that of 1-propanol. The separation factors for the alcohols increased with increasing −Hads of the alcohols, but acetone did not follow this trend. The acetone heat of adsorption is the same as ethanol and its diameter is similar to 2-propanol, but the acetone fugacity is three to six times higher than the alcohol fugacities. Acetone, methanol, ethanol, 1-propanol, and 2-propanol fugacities calculated with HYSYS Distil 4.1 software for 5 wt.% aqueous solutions at 303 K were 7.1, 1.5, 1.3,
1.5, and 1.6 kPa, respectively. The fugacities increase with temperature and the order is the same at 333 K. We have observed a correlation between the organic fugacity in the feed and the organic/water separation factors for pervaporation through Ge-ZSM-5 zeolite membranes [29]. Adsorption depends on heat of adsorption, molecule size, and feed fugacity. The water fluxes were significantly reduced by organics (Fig. 5); the organics adsorbed in the zeolite pores and inhibited water transport. Water fluxes in the mixtures increased in the following order: 1-PrOH < acetone < 2-PrOH < EtOH < MeOH < pure water. Fig. 5 suggests that selective adsorption dominates the separations, but diffusivities of the molecules are also important because flux is proportional to the product of coverage gradient and diffusivity. 4.2. Comparison of monoliths and other membranes Many studies have used polymeric membranes to separate alcohol from alcohol/water mixtures [30–35]. The highest separation factor that we found for ethanol/water separations using a polymeric membrane with feed conditions similar to ours is 46 [34]. The feed for this separation was 8 wt.% ethanol at 303 K and the membrane was a styrene-heptadecafluorodecyl acrylate graft copolymer and polydimethylsiloxane composite membrane with a total flux of 0.005 kg m−2 h−1 . The highest flux reported for a polymer membrane that separated ethanol from water is 5.7 kg m−2 h−1 [35]. This was a microporous polytetrafluoroethylene membrane with a 5 wt.% ethanol feed at 303 K and a separation factor of 6.0. These values are higher than the flux and separation factor of the monolith, but no polymeric membrane with either higher flux at the same separation factor or higher separation factor at the same flux was found. In addition, zeolites are more resistant to degradation than polymers in the presence of some organics, making them attractive for specialized applications. Moreover, monolith membranes are in the early stages of development, and improvements to their performance are likely. The alcohol/water separation factors for the monolith membrane are lower than those reported by Sano et al. [14] for a silicalite-1 membrane (∼400-m thick) on a porous stainless steel tube. Their separation
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factors were 18, 65, 88, and 45 for MeOH, EtOH, 1-PrOH, and 2-PrOH, respectively, for 1 mol% feeds at 303 K and the same order was observed up to 333 K. This order is similar to that for the monolith and the total fluxes observed by Sano et al. increased in the same order seen in Fig. 3 but were two to three times higher. Their feed concentrations are lower than we used and some differences in pervaporation performance could be due to this. Also, a small amount of Al was incorporated into the zeolite framework because the ␥-alumina layer on the monolith partially dissolved during membrane synthesis. Measurements with EDX showed that the Si/Al ratio in the zeolite layer of a similar membrane on a monolith support was approximately 1000. The Al and B make the membrane more hydrophilic than silicalite, and therefore decrease the separation factor for organic/water mixtures [36]. Tuan et al. [20] reported separation factors of 200, 101, 3.6, 2.4, and 2.5 for acetone, MeOH, EtOH, 1-PrOH, and 2-PrOH, respectively, for a B-ZSM-5 membrane on a stainless steel tube at 303 K. The organic feed concentrations were 5 wt.% and the same separation factor order was seen at higher temperatures. This order for the alcohols is almost opposite of the order in this study and the order reported by Sano et al. for silicalite-1 membranes. The tubular B-ZSM-5 membrane had a n-/i-C4 H10 separation selectivity of 20 at 473 K, which is slightly lower than that for the monolith membrane (Table 2), suggesting few non-zeolite pores larger than the zeolite pores for either membrane. The differences between the organic/water separation factors for the B-ZSM-5 monolith and tube arise mainly from differences in the water fluxes through these membranes. Fig. 6 shows that the trends for the organic fluxes through the B-ZSM-5 monolith and tube are qualitatively similar, but the trends for the water fluxes are significantly different. The water fluxes through the B-ZSM-5 monolith membrane decreased dramatically as the size of the alcohol increased, as might be expected for inhibition within the channels. The larger and more strongly adsorbed alcohols would inhibit water more effectively. However, the water fluxes for the B-ZSM-5 tubular membrane are approximately the same for each 5 wt.% alcohol/water feed. This qualitative difference in behavior does not appear to be caused by a difference in membrane quality because the pure water fluxes are similar: 0.27 and
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0.24 kg m−2 h−1 at 303 K for the tubular and monolith membranes, respectively. A possible explanation for almost equal water fluxes through the tubular membrane for all the alcohol mixtures (Fig. 6b) is flow through non-zeolite pores that are smaller than the zeolite pores. Non-zeolite pores smaller than ∼0.43 nm would exclude ethanol and larger alcohols, but still allow water and methanol to permeate. All the alcohols inhibit water flow through zeolite pores, and larger alcohols are better at inhibiting water permeation in these pores. Methanol inhibits less effectively in the zeolite pores, but it could also inhibit permeation in the smaller pores, whereas the larger alcohols cannot. Thus the higher water flux through the tubular membrane, relative to the monolith, for the larger alcohols could be due to uninhibited flow through smaller pores. In contrast, methanol adsorbed in these smaller pores and inhibited water permeation through them. Methanol may inhibit water more effectively in the small pores than in the zeolite pores. The other alcohols could not enter the small pores, and thus the tubular membrane had less zeolite pores per area because the small non-zeolite pores took up some of the area. This is why the larger alcohol fluxes are lower for the tubular membranes but the water fluxes are higher. Higher water flux would decrease the selectivity of the membrane compared to a membrane with only zeolite pores. The non-zeolite pores are caused by intracrystalline boundaries, and therefore the membrane would have a distribution of small non-zeolite pores. Non-zeolite pores smaller than ∼0.46 would exclude propanols but allow water, methanol, and ethanol to permeate. Thus, a distribution of more, small non-zeolite pores in the tubular than in the monolith membrane could yield water fluxes that were similar for the four alcohols. Measurements of H2 /i-C4 H10 and H2 /n-C4 H10 separations, shown in Fig. 7, are also consistent with the explanation that the tubular membrane has more small non-zeolite pores than the monolith. Fig. 7a shows that the ratio of single gas H2 permeances to the H2 permeances in a H2 /i-C4 H10 mixture are significantly higher for the monolith than for the tubular membrane, indicating that i-C4 H10 inhibits H2 permeance more effectively in the monolith. However, n-C4 H10 inhibits H2 less than i-C4 H10 does in the monolith, but n-C4 H10 inhibits H2 permeation more than i-C4 H10 does in the tubular membrane, as shown in Fig. 7b.
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Fig. 6. Organic fluxes (a) and water fluxes (b) through monolith (M1) and tubular ([20]) B-ZSM-5 zeolite membranes. Feeds were 5 wt.% alcohol water mixtures at 303 K.
This is likely because i-C4 H10 is larger and inhibits H2 more effectively in zeolite pores than n-C4 H10 does, but i-C4 H10 cannot enter the small non-zeolite pores. Normal-butane inhibits H2 more in small pores than it does in zeolite pores and i-C4 H10 does not inhibit H2 in small pores. Comparisons to the silicalite-1 [14] and B-ZSM-5 [20] tubular membranes show that the B-ZSM-5 monolith membranes were good, but recent reports have suggested that the zeolite membrane synthesis can be improved. Lin et al. [37] reported ethanol/water
separations with high-quality, high-flux silicalite-1 membranes. They observed a 1.8 kg m−2 h−1 flux and a separation factor of 89 using an ␣-Al2 O3 tubular support and a 3.7 kg m−2 h−1 flux and a separation factor of 35 using a stainless steel tubular support. The feed concentration was 5 wt.% ethanol/water and the temperature was 333 K for these measurements. The zeolite layer in these membranes was only 10–30 m, which is thinner than ours. These fluxes and separation factors and those of Sano et al. are significantly higher than ours. However, our membranes are the
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Fig. 7. Single gas to mixture ratios of hydrogen permeances for monolith (M1) and tubular ([20]) B-ZSM-5 zeolite membranes with H2 /i-C4 H10 (a) and H2 /n-C4 H10 (b) mixtures.
first that have been prepared on monolith supports, and the monolith membrane quality and performance will be improved. Several factors may contribute to the low fluxes through the monolith membranes. Boron-ZSM-5 zeolite membranes are not as hydrophobic as silicalite membranes and, therefore, the driving force for organic permeation is lower than in silicalite membranes. Concentration gradients in the support can also reduce the driving force for transport through the membrane. Mass transfer resistance in 3- mm thick
porous stainless steel disc supports has been shown to have a measurable effect on gas permeances through zeolite membranes [38,39]. This effect will be much larger in the monolith membranes because molecules permeating through the center channels must travel up to 1.2 cm in the porous support before entering the vacuum line. Uneven distribution of flow into each channel may also decrease the fluxes through the monolith membranes. The outer channels may have stagnant feed or low feed flow rates so that some of the
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surface area is not used efficiently. Lastly, producing a thinner, continuous zeolite layer can dramatically increase flux without decreasing selectivity. Hedlund et al. [40] reported a method to synthesize 0.5-m thick silicalite-1 membranes on porous ␣-Al2 O3 disc supports with high reproducibility. These membranes separated n-C4 H10 /i-C4 H10 mixtures with permeances over 20 times higher than previous literature values for this separation. The selectivities were between 3 and 9, which are lower than the previous separations but higher than the Knudsen selectivity.
5. Conclusions High-quality B-ZSM-5 zeolite membranes on monolith supports effectively separated alcohols and acetone from water by pervaporation. Separations were based on selective adsorption of the organic, because the membrane is hydrophobic, and, to a lesser extent, on diffusion differences. The total fluxes for methanol and ethanol are similar to fluxes through B-ZSM-5 membranes on stainless steel tubes. Membrane performance may be limited by lower quality zeolite layers on the outside channels of the monolith and by mass transfer limitations in the support.
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