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Alcohol and water adsorption and capillary condensation in MFI zeolite membranes
Journal of Membrane Science 334 (2009) 23–29
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Alcohol and water adsorption and capillary condensation in MFI zeolite membranes Begum Tokay, John L. Falconer ∗ , Richard D. Noble Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0424, United States
a r t i c l e
i n f o
Article history: Received 30 November 2008 Received in revised form 10 February 2009 Accepted 14 February 2009 Available online 3 March 2009 Keywords: MFI membrane Permporosimetry Capillary condensation Pervaporation 2-Propanol/water separations
a b s t r a c t A boron-substituted MFI membrane was selective for 2-propanol/water separations during pervaporation in part because 2-propanol expanded the MFI crystals and thus shrank the size of the membrane defects. Permporosimetry measurements show that adsorption of 2-propanol, 1-butanol, and ethanol decreased helium flow through the defects, apparently because these alcohols swell MFI crystals. In contrast, neither methanol nor water adsorption appeared to change the defect size. Helium flow through membrane defects was also blocked by capillary condensation, but this was only observed when the pressure drop across the membrane was less than about 20 kPa. Thus defect blockage by crystal expansion was observed without capillary condensation by carrying out permporosimetry with a pressure drop of 89 kPa. The defect sizes were estimated from capillary condensation to be approximately 2 nm. Published by Elsevier B.V.
1. Introduction Silicalite membranes selectively separate alcohols from water [1–4]. Lin et al. reported a separation factor of 106 for ethanol/water separation at 333 K; the flux was 14 mol/m2 h for 5 wt.% ethanol in the feed [5]. Tuan et al. obtained a separation factor of 49 for methanol/water separation for 5 wt.% methanol feed with a B-ZSM5 membrane, but this membrane had poor separation selectivities for ethanol (2.1) and 2-propanol (1.6) [1]. Their Ge-ZSM-5 membrane had separation factors of 29 and 16 for 5 wt.% ethanol and 2-propanol in water, respectively. These separation factors were 25 and 43 for B-ZSM-5 membranes on monolith supports at 303 K [4]. Matsuda et al. modified the surface of a silicalite-1 membrane with a silicone coating to increase the ethanol/water separation factor from 51 to 125 [6]. Modifying the surface with a silane coupling agent raised the flux to 22 mol/m2 h but decreased the separation factor to 44 [7]. These studies found that increasing the alcohol percentage in the feed decreased the water flux, and separation was attributed to preferential adsorption of alcohols. Sano et al. observed that the separation factor decreased significantly as the ethanol feed concentration increased for a silicalite membrane [8]. Since ethanol (0.43 nm) and water (0.265 nm) are both smaller than MFI pore (∼0.6 nm), preferential adsorption was considered responsible for the selectivity. Nomura et al. also found that ethanol inhibited water
∗ Corresponding author. Tel.: +1 303 492 8005; fax: +1 303 492 4341. E-mail address: [email protected] (J.L. Falconer). 0376-7388/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.memsci.2009.02.010
permeation, but water only slightly affected ethanol permeation [9]. Similarly Liu et al. reported increased methanol (0.38 nm) flux with increasing methanol concentration at high methanol concentrations [10]. The highest separation factor they obtained was 14 for 16.5 wt.% methanol in the feed with a silicalite-1 membrane at room temperature. In the current paper, the possibility was investigated that alcohol adsorption expands MFI crystals and shrinks the size of the defects, and that this, in addition to preferential adsorption, affects selective removal of alcohols from water with MFI membranes. We recently reported that n-alkanes (C3 -C8 ) and SF6 swell MFI crystals and shrink membrane defects [11–14]. Yu et al. [13,14] showed that single-component pervaporation fluxes of smaller molecules (n-hexane: 0.43 nm; acetone: 0.47 nm) were significantly lower than the fluxes of larger molecules (2,2-dimethylbutane (DMB), 1,3,5-trimethylbenzene (TMB), i-octane, benzene) in some MFI membranes, even though DMB (0.63 nm), TMB (0.75 nm), and ioctane (0.7 nm) are too large to adsorb at a significant rate in MFI pores. 2,2-Dimethylbutane can adsorb in MFI pores, but its diffusivity is orders of magnitude lower than the n-hexane diffusivity. For a silicalite-1 membrane, the DMB transit time was an order of magnitude shorter than the n-hexane transit time at 313 K because DMB diffused through defects, whereas these defects were mostly unavailable to n-hexane, which diffused mainly through MFI pores [15]. Thus, DMB had a higher flux than n-hexane in this membrane during pervaporation. Adding only 0.2% n-hexane to the feed reduced the DMB flux by two orders of magnitude [14]. Yu et al. concluded that n-hexane swelled the crystals, and this decreased the size of the defects and thus decreased the flux through the
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defects. Benzene (0.585 nm) did not swell the crystals so it diffused through the defects and it had a higher flux than n-hexane [11]. X-ray diffraction measurements showed that MFI crystals expand when n-hexane adsorbs but not when benzene adsorbs [16]. A saturation loading of n-hexane increased the MFI unit cell dimensions by about 0.4% in each direction at room temperature, and the unit cell volume increased by 1.2%. For 1-m crystals, a linear expansion of 0.4% would close off 4-nm pores if the overall membrane layer could not expand much because it was attached to the support. In contrast, the unit cell volume increased less than 0.01% for benzene [16]. Yu et al. directly showed with temperature-programmed desorption that the pore volume of the MFI membrane layer decreased to less than half its value when n-hexane adsorbed in the MFI pores. This intercrystalline pore volume included defects that were transport pathways as well as dead-end pores, and it decreased as the MFI crystals swelled when n-hexane adsorbed [17]. The behavior observed in these previous studies may not occur for membranes with different microstructures, which result from different preparation methods. For example, for c-oriented membranes prepared by secondary growth, Tsapatsis and coworkers [18–20] reported that p-xylene adsorption significantly increased the flow through defects, as indicated by increased o-xylene flux. They conjectured that p-xylene adsorption expanded the MFI crystals and distorted the structure so that the intercrystalline porosity increased. For membranes with a different microstructure, because their preparation was different and cracks were sealed with a silica sol, the flux through defects did not increase when p-xylene adsorbed [20]. In the current study, we investigated the contribution of MFI crystal swelling to selectivities of MFI membranes for alcohol/water separation by pervaporation. The membrane used had the majority of its helium flux through defects at room temperature. Although this is not expected to be the best membrane for alcohol/water separations, it allows the changes in flux through defects to be readily detected, and the same behavior is expected for membranes with less flow through defects. Permporosimetry measurements showed that some alcohols dramatically decreased the helium flux through the membrane defects. We conclude that the flux decreased because defects shrank due to adsorption, not because of capillary condensation. Indeed, crystal swelling decreased the helium flux at much lower activities than where capillary condensation was observed. Capillary condensation is shown to strongly depend on the pressure drop across the membrane, and was only observed at low activities (indicating small defects) when the trans-membrane pressure drop was low. Vapor permeation measurements with i-octane, which can only diffuse through defects that are larger than the MFI pores, also showed that alcohols shrank the size of defects. Pervaporation measurements were consistent with the permporosimetry measurements: the separation selectivity for 2-propanol/water mixtures was five orders of magnitude higher than the ideal selectivity for certain concentration ranges because of crystal swelling due to alcohol adsorption. Thus, both crystal swelling and preferential adsorption can contribute to the ability of MFI membranes to separate alcohol/water mixtures.
with about 2 mL of synthesis gel. The other end was then plugged with a Teflon cap and left overnight at room temperature while the porous support soaked up most of the gel. The tube was again filled with synthesis gel, plugged with a Teflon cap, and put into an autoclave for hydrothermal synthesis at 458 K for 24 h. The membrane was then brushed, washed with DI water, and dried. The same synthesis procedure was repeated, except that the tube was not soaked overnight, and the membrane’s vertical orientation in the autoclave was reversed. The resulting membrane was impermeable to N2 at room temperature. It was calcined at 700 K for 8 h, with heating and cooling rate of 0.6 and 0.9 K/min, respectively. An XRD pattern for crystals collected from the bottom of the autoclave confirmed the MFI structure. 2.1. Permporosimetry Adsorption branch porosimetry or permporosimetry was used in an effort to estimate the percentage of flow through defects [21–23]. During permporosimetry, the helium flux was measured as a function of the activity of a second compound in the feed stream. The activity was calculated as the ratio of the partial pressure of the compound to its saturation pressure. The membrane was sealed in a stainless steel module using silicone o-rings. In all measurements, the normalized helium flux is presented, where the flux was normalized by the helium flux at zero activity and at the same pressure drop across the membrane. The n-hexane (>99.5%, Fluka), benzene (99+%, Sigma–Aldrich), water, methanol (>99.8%, Sigma–Aldrich), ethanol (>99.8%, Sigma–Aldrich), 2propanol (99.5+%, Sigma–Aldrich) and 1-butanol (99.9%, Fisher) were added to the feed by bubbling helium through two liquid bubblers in series and mixing the resulting saturated stream with pure helium. The activity was adjusted by changing the temperature of the bubblers and the ratio of the helium streams. The permeate pressure was 84 kPa, which is atmospheric pressure in Boulder, CO. For most measurements the feed pressure was maintained at 185 kPa by a back pressure regulator, but permporosimetry was also carried out for lower feed pressures. Because the feed pressure was kept constant, as the activity of the adsorbing molecule increased, the helium partial pressure decreased. Thus, the helium driving force decreased as the activity increased, and this caused the helium flux to decrease. Therefore, the normalized helium fluxes were corrected for this decreased driving force. Measurements indicated that the helium flux was linear in pressure drop across the membrane. This was not a large correction. Also, for some measurements, instead of measuring the helium flux as a function of activity of an alcohol or water, the activity was kept constant and the feed pressure was increased from 104 to 185 kPa. The activity was kept constant by increasing the pressure drop across the membrane and keeping the partial pressure of the organic constant. In that case, the normalized flux was the measured flux divided by the flux at zero activity and at the same pressure drop. A mass flow meter and a bubble flow meter were used to measure the helium permeate flow rate. An activated-carbon and molecular sieve (MS 13X Dunniway) trap on the permeate line removed the alcohols and water from the helium stream before permeate flow measurements, and the trap was periodically baked out.
2. Experimental methods 2.2. Pervaporation A membrane containing isomorphously substituted boron in the framework (B-ZSM-5) was synthesized by in situ crystallization onto the inside of a tubular ␣-alumina support (0.2-m pores, Pall Corp.) using procedures described previously [11]. The synthesis gel had a molar composition of 4.44 TPAOH:19.5 SiO2 :1.55 B(OH)3 :500 H2 O. The resulting gel was aged at room temperature for at least 6 h. One end of the support tube was wrapped with Teflon tape and plugged with a Teflon cap, and the inside of the support was filled
The apparatus used for pervaporation has been described in detail previously [24]. The membrane was sealed in a stainless steel module using silicone o-rings. A centrifugal pump circulated liquid feed through the inside of the membrane tube and through a feed reservoir at approximately 1 L/min to minimize concentration polarization. All measurements were at room temperature. The permeate side pressure was kept below 0.23 kPa using a liquid nitrogen
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trap and a mechanical vacuum pump. Steady-state fluxes were measured by condensing the permeate in the liquid nitrogen trap, and permeate concentrations were analyzed with a GC with a thermal conductivity detector. The permeate liquid was collected after 2 h of pervaporation and then it was collected for 4–5 h. 2.3. Vapor permeation Vapor permeation measurements were carried out at room temperature in a continuous flow system, as described in detail elsewhere [25]. The membrane was mounted in a stainless steel module using silicone o-rings. The flux of i-octane, which can only diffuse through defects that are larger than the MFI pores, was measured as a function of the activity of 2-propanol. These measurements directly observed the change in flux through defects as 2-propanol adsorbed in the MFI pores. A syringe pump injected a liquid into a preheated helium carrier stream, which then passed through a heated zone at ∼383 K for complete vaporization. A second liquid could be fed to the system using two liquid bubblers and it was mixed with the first stream at the heating zone. A helium sweep stream was used on the permeate side. Both the feed and permeate streams were analyzed on-line by an Agilent 6890 GC with a flame ionization detector. A bypass line allowed analysis of the feed before entering the module. Bubble flow meters were used to measure feed and permeate flow rates. Pressure was kept at 110 kPa on both sides of the membrane. The feed activities were adjusted by varying the syringe pump feed rate, bubbler temperature, and helium flow. 3. Results and discussion 3.1. Membrane characterization During permporosimetry, the helium flux through the B-ZSM-5 membrane decreased when n-hexane and benzene adsorbed, and the changes with activities were similarly to those observed by Yu et al. [11] and Lee et al. [12] for other MFI membranes. Benzene only blocked 35% of the helium flux, even at activities close to one (Fig. 1a), indicating that approximately 65% of the helium flow was through defects at room temperature. That is, benzene adsorbed in the MFI pores and blocked flow through them, but it did not appear to decrease the helium flux through defects. In contrast, as shown in Fig. 1a and b, at a n-hexane activity below 0.0035, the helium flux decreased to 0.01% of its original value. Based on our previous studies [12,15], this dramatic difference in behavior results because n-hexane swells MFI crystals (approximately 0.4% in each direction at saturation loading), but benzene does not [16]. Because the helium flux was almost completely blocked by n-hexane swelling of crystals, the defects must be small. As shown in Fig. 2, the four alcohols studied exhibited permporosimetry behavior that was between that for benzene and n-hexane. At an activity of 0.01, 2-propanol (kinetic diameter = 0.47 nm) reduced the helium flux to about 1% of its original value, whereas ethanol (0.43 nm) and 1-butanol (0.49 nm) only reduced the helium flux to 1% of its original value at higher activities. Methanol had significantly less effect on the helium flux, and even at a methanol activity of 0.85, 7% of the helium flux remained. Water decreased the helium flux even less than methanol. As shown in Fig. 3, even at an activity of 0.95, only 60% of the helium flux was blocked. As shown in the next section, the decreases in helium flux with adsorption are not due to capillary condensation in the membrane defects. The permporosimetry measurements in Figs. 1–3 were made for a pressure drop across the membrane of 89 kPa, but capillary condensation was only observed for much smaller pres-
Fig. 1. Normalized helium flux as a function of benzene and n-hexane activities during permporosimetry for a B-ZSM-5 membrane at room temperature and a pressure drop across the membrane of 96 kPa. Two activity ranges are shown: (a) 0–1 and (b) 0–0.005.
sure drops. Also, the four alcohols would be expected to exhibit similar capillary condensation behavior but the difference in behavior of the helium flux for 2-propanol and methanol is dramatic. Also, 1-butanol would be expected to preferentially adsorb over ethanol, but as shown in Fig. 2, for activities below 0.2, ethanol decreases the helium flux more than 1-butanol. Thus, the large decreases in helium flux when ethanol, 2-propanol, and 1-butanol
Fig. 2. Normalized helium flux as a function of alcohol activity during permporosimetry for a B-ZSM-5 membrane at room temperature and a pressure drop across the membrane of 89 kPa.
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Fig. 3. Normalized helium flux as a function of benzene and water activities during permporosimetry for a B-ZSM-5 membrane at room temperature. The pressure drop across the membrane was 96 kPa for benzene and 89 kPa for water.
Fig. 5. Normalized helium flux as a function of MFI crystal loading for alcohols and water for a B-ZSM-5 membrane at room temperature and a pressure drop across the membrane of 89 kPa.
adsorb are attributed MFI crystal swelling. These molecules blocked flow through the MFI pores by adsorption in those pores, and they blocked most of the flow through the defects because swelling the MFI crystals decreased the defect sizes. Recent XRD measurements on silicalite single crystals found that 2-propanol increased the MFI unit cell dimension by approximately 0.35% for a and c directions and 0.23% for b direction, whereas ethanol increased the unit cell 0.15% in each direction. Thus, the unit cell volume increased 0.95% for 2-propanol and 0.47% for ethanol [26]. If the overall length of the MFI membrane layer could not increase because it was attached to the support, a 0.35% increase in unit cell dimension would seal a 3.5 nm defect if the MFI crystals were 1 m in diameter. At low alcohol activities, the helium flux during permporosimetry was quite different for each alcohol, as shown in Fig. 4. The helium flux dropped dramatically for small changes in 2-propanol activity. At a 2-propanol activity of 0.003, the helium flux decreased 45% from its original value, whereas at a methanol activity of 0.003, the helium flow only decreased 23%. In contrast, 1-butanol and ethanol had no effect on the helium flux at that activity. At an activity of 0.013, 99% of the helium flow through the defects was blocked by 2-propanol, and 90% was blocked by ethanol. Adsorption isotherms for ZSM-5 (Si/Al = 990) from the literature [27] were used to plot the helium flux dependence on loadings (Fig. 5). The ZSM-5 (Si/Al = 990) and B-ZSM-5 zeolites were assumed to have similar adsorption behavior. As the alcohol size increased, the loading (molec./u.c.) required to decrease the helium flux more
than an order of magnitude decreased. The helium fluxes exhibited large decreases for small increases in alcohol loadings. In contrast to the alcohols, high loading of water were not obtained for the activities used, and this may explain why water did not exhibit the same behavior [8]. The water loading were probably higher than indicated in Fig. 5 because boron substitution makes MFI zeolites less hydrophobic [1].
Fig. 4. Normalized helium flux as a function of alcohol activity during permporosimetry for a B-ZSM-5 membrane at room temperature and a pressure drop across the membrane of 89 kPa.
Fig. 6. Normalized helium flux, as a function of water activity, during permporosimetry for a B-ZSM-5 membrane at room temperature. Measurements were made for the indicated pressure drops across the membrane.
3.2. Effect of pressure on permporosimetry The decreases in helium flux during permporosimetry measurements in Figs. 1 and 2 are not due to capillary condensation in the membrane defects. Indeed, as shown in Fig. 1b, benzene does not condense, even for an activity 220 times higher than the n-hexane activity where the helium flux dramatically decreased. Instead, the activity where molecules condense in the defects strongly depends on the pressure drop across the membrane. Capillary condensation was not observed for a pressure drop across the membrane of 89 kPa, but molecules did condense, and at lower activities, when lower pressure drops were used. As shown in Fig. 6, the helium flux decreased two to three orders of magnitude, as the water activity increased, when the pressure drop was 21 kPa or smaller. As the pressure drop decreased, the activity where water condensed in the membrane defects decreased, so that at a pressure drop of approximately 1.4 kPa and an
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Table 1 i-octane and 2-propanol vapor permeation fluxes at room temperature and zero pressure drop. Flux (mol/m2 h)
Single component Mixture
Fig. 7. Normalized helium flux as a function of pressure drop at the indicated water activities during permporosimetry for a B-ZSM-5 membrane at room temperature.
activity of 0.11, the flux dropped almost four orders of magnitude. The lowest activity that used at this pressure drop was 0.05, and the flux dropped only 10% from its original value. Pressure drops below 1.4 kPa were not used because the helium flux decreased as the pressure drop decreased, so accurate measurements were not possible at lower pressure drops when molecules condensed in the pores. These results show that water did not condense in the defects when the pressure drop was 89 kPa, and thus the alcohols are also not expected to condense at this pressure drop. Indeed, the 2-propanol activity where the helium flux dropped orders of magnitude (Fig. 4) was an order of magnitude lower than the lowest water activity where water condensed, and the pressure drop was 64 times higher. Thus, the large decreases in helium flux at low activities for n-hexane and alcohols in Figs. 1, 2 and 4 are not due to capillary condensation, but instead appear to be due to crystal swelling. Permporosimetry measurements were also carried out by increasing the feed pressure, and thus the pressure drop, at constant water activity. As shown in Fig. 7, the helium flux decreased dramatically as the pressure drop decreased below 10 kPa. Below 2 kPa pressure drop, water condensation blocked almost all the flux through the defects at a water activity of 0.11 (Fig. 7). Benzene exhibited similar behavior, as shown in Fig. 8, but the helium flux decreased with pressure over a wider range of pressure drop. The helium flux dependence on pressure drop for DMB in the feed was similar to that seen for the water and benzene. As shown
i-octane (0.8 activity)
2-propanol (0.08 activity)
0.47 0.013
0.32 0.09
in Fig. 8, at higher pressure drops across the membrane, DMB decreased the normalized helium flux by 10–15%. A measurable amount of DMB does not adsorb in the MFI pores at these conditions [15], but this decrease could be due to DMB adsorption on the external surface of the MFI crystals, and this adsorption could slow helium entrance into the MFI pores. At a trans-membrane pressure drop of 3.5 kPa, the normalized helium flux exhibited the same behavior observed with the water and benzene (Figs. 7 and 8), indicating that DMB condensed in the defects and decreased the helium flux by 80%. Figs. 6–8 indicate that capillary condensation were only observed at low pressure drops across the membrane; apparently water in the defects cannot support a pressure drop of more than 21 kPa, and the highest pressure drop where a condensed layer remains is probably similar for other molecules. Uchytil et al. [28] reported n-butane condensed in Vycor glass pores (average radius = 4 nm) when the n-butane pressure on both sides of the pores was close to its condensation pressure and the pressure drop across the membrane was small. They did not observe condensation at high-pressure drops even when the n-butane pressure was close to saturation. 3.3. Vapor permeation Because i-octane is larger than MFI pores, its flux is a measure of permeation through defects that are larger than the MFI pores. Thus, i-octane was added to the feed at an activity of 0.8, and after steady-state was reached, 2-propanol was added at an activity that was two orders of magnitude lower (0.08). The large difference in activities minimized the effects of possible preferential adsorption of 2-propanol in the defects. When 2-propanol was added to the feed, the i-octane flux decreased to 3% of its original value; the 2-propanol flux was 29% of its pure-component flux at the same activity (Table 1). The lower i-octane flux through the defects is attributed to swelling of the MFI crystals by 2-propanol. The i-octane flux did not decrease the same percentage that the helium flux decreased during permporosimetry (for the same 2propanol activity, the helium flux decreased to 1% of its original value) for at least two reasons. First, helium also permeated through the MFI pores and 2-propanol adsorption in the MFI pores blocked helium flux through them. Second, some defects may be too small for i-octane but large enough to permeate helium. Those smaller defects decrease in size with crystal swelling, as do larger pores. Thus the helium flux is expected to decrease a larger percent than the i-octane flux. 3.4. Pervaporation
Fig. 8. Normalized helium flux as a function of pressure drop at three benzene activities and at a DMB activity of 0.01 during permporosimetry for a B-ZSM-5 membrane at room temperature.
The single-component pervaporation fluxes through the membrane are plotted in Fig. 9 as a function of kinetic diameter. The water flux was almost three orders of magnitude higher than the n-hexane flux and 200 times the 2-propanol and 1-butanol fluxes. The smaller water molecule might be expected to diffuse much faster than n-hexane and the alcohols. However, benzene permeated 35 times faster than n-hexane, eight times faster than 2propanol and five times faster than 1-butanol, even though benzene is the larger molecule. More significantly, DMB, which only diffuses
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tivity for a 0.045 mol% 2-propanol concentration is 830, which is more than five orders of magnitude higher than the ideal selectivity. The selectivity drops significantly at higher concentrations of 2-propanol because the defects do not shrink more and the higher concentration does not result in a higher 2-propanol flux. The selectivity also drops at lower 2-propanol concentration because the defects are too large. Thus, this type MFI membrane, with much of its flow through defects, would not be useful for 2-propanol/water separations. However, the same crystal swelling would be expected in membranes with less flow through defects, and thus crystal swelling would contribute to selective separations of 2-propanol from water. 3.5. Defect sizes Fig. 9. Single component pervaporation fluxes through a B-ZSM-5 membrane at room temperature as a function of kinetic diameter.
through the defects at room temperature, permeated nine times faster than n-hexane and twice as fast as 2-propanol. As reported previously for n-hexane in other MFI membranes, its lower flux is attributed to swelling of MFI crystals. That is, DMB and benzene diffuse through defects, most of which are not available to n-hexane diffusion because crystal swelling by n-hexane decreased their size. The single-component pervaporation fluxes indicate that 2-propanol and 1-butanol also swell MFI crystals and decrease the size of defects, and this is why their fluxes are lower than expected for their size. Since 2-propanol permeates slower than 1-butanol, apparently 2-propanol swells the crystals more than 1-butanol. Pervaporation of 2-propanol/water mixtures also indicates that 2-propanol swells the MFI crystals and shrinks the defects. The 2propanol/water ideal selectivity is 0.005; as shown in Fig. 9, the water flux was 200 times the 2-propanol flux. Pervaporation was carried out for low 2-propanol concentrations in water (Table 2), so that the 2-propanol activities were similar to those where the helium flux dropped dramatically during permporosimetry for small changes in 2-propanol activity (Fig. 4). The 2-propanol concentrations of 0.014 and 0.045 mol% in the pervaporation feed correspond to 2-propanol activities of 0.005 and 0.011, respectively. These liquid activities were calculated by the UNIFAC model. As shown in Table 2, 0.014 mol% 2-propanol decreased the water flux almost an order of magnitude, and 0.045 mol% 2-propanol decreased the water flux more than another order of magnitude. Further increase in 2-propanol concentration did not change the water flux significantly. The decrease in water flux to 0.5% of its original value, when the 2-propanol concentration was only 0.045 mol%, does not appear to be due to preferential adsorption of 2-propanol. Instead, these results correlate with the decreased helium flux in permporosimetry and the decreased i-octane flux in vapor permeation and appear to be due to crystal expansion that decreased the size of the defects. As a result the mixture selecTable 2 Pervaporation fluxes of 2-propanol/water mixtures at room temperature through B-ZSM-5 membrane. 2-Propanol (mol%)
0 0.014 0.045 4.0 100 a
Ideal selectivity.
Flux (mol/m2 h) 2-Propanol
Water
– 0.06 0.22 0.24 0.528
107 12.4 0.53 0.53 –
2-Propanol/water separation factor
0.005a 24 830 11 0.005a
The lowest water activity where condensation was observed (at the lowest pressure drop across the membrane) was used in the Kelvin equation and the Horvath-Kawazoe (H-K) potential function for slits to estimate defect sizes. Benzene and DMB activities were also used to estimate the defect sizes for the lowest pressure drop used. In the Kelvin equation, contact angles were assumed as 0◦ [29], and surface tensions against air at room temperature were used [30]. The thickness of the adsorbed layer, which was estimated by assuming hexagonal packing, was added to the Kelvin radius to get the pore radius [31]. The adsorbed layer thickness was 0.44 nm for water and 0.5 nm for benzene and DMB. The defect diameter was estimated from the Kelvin equation to be 2.4, 1.9, and 1.9 nm from benzene, DMB, and water condensation, respectively. Because this is the lower limit of where the Kelvin equation is applicable, and because the contact angles and the thickness of the adsorbed layer were estimated, these values are not accurate to two significant figures. Similar diameters were obtained from condensation of the three molecules, however, and pores of this size could certainly be closed off by crystal expansion. 2-Propanol would expand 1 m MFI crystals by 2.3–3.5 nm, depending on which unit cell direction is considered. The defect diameter was also estimated from H–K equation to be between 0.7 and 1.2 nm for small activities of water, benzene and DMB, but pores these sizes are too small for condensation. 4. Conclusions Adsorption of 2-propanol, 1-butanol, and ethanol swells MFI crystals and shrinks the size of the defects in a MFI membrane. In contrast, water and methanol do not have a large effect on the defect size. This crystal swelling may contribute to high selectivities for alcohol/water separations during pervaporation for some MFI membranes. Permporosimetry, vapor permeation of ioctane through defects, and pervaporation all indicate the same behavior. Capillary condensation was not observed in the membrane defects when the pressure drop across the membrane was great than 21 kPa. At lower pressure drops, however, benzene, DMB, and water condensed in the defects. The average size of the defects estimated from the Kelvin equation was about 2 nm. Apparently the condensed layers were not stable at higher pressure drops, and thus permporosimetry at higher pressure drops allowed crystal swelling to be detected in the absence of capillary condensation. Acknowledgments We gratefully acknowledge support by the National Science Foundation CTS 0340563. We also thank Justin B. Lee for his assistance with the permporosimetry measurements.
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References [1] V.A. Tuan, S. Li, J.L. Falconer, R.D. Noble, Separating organics from water by pervaporation with isomorphously substituted MFI zeolite membranes, J. Membr. Sci. 196 (2002) 111. [2] S. Li, V.A. Tuan, R.D. Noble, J.L. Falconer, ZSM-11 membranes: characterization and pervaporation performance, AIChE J. 48 (2002) 269. [3] T.C. Bowen, S. Li, R.D. Noble, J.L. Falconer, Driving force for pervaporation through zeolite membranes, J. Membr. Sci. 225 (2003) 165. [4] T.C. Bowen, H. Kalipcilar, J.L. Falconer, R.D. Noble, Pervaporation of organic/water mixtures through B-ZSM-5 zeolite membranes on monolith supports, J. Membr. Sci. 215 (2003) 235. [5] X. Lin, X. Chen, H. Kita, K.-I. Okamoto, Synthesis of silicalite tubular membranes by in situ crystallization, AIChE J. 49 (2003) 237. [6] H. Matsuda, H. Yanagishita, H. Negishi, D. Kitamoto, T. Ikegami, K. Haraya, T. Nakane, Y. Idetomo, N. Koura, T. Sano, Improvement of ethanol selectivity of silicalite membrane in pervaporation by silicone rubber coating, J. Membr. Sci. 210 (2002) 433. [7] T. Sano, M. Hasegawa, S. Ejiri, Y. Kawakami, H. Yanagishita, Improvement of the pervaporation performance of silicalite membranes by modification with a silane coupling reagent, Micropor. Mater. 5 (1995) 179. [8] T. Sano, H. Yanagishita, Y. Kiyomuzi, F. Mizukami, Separation of ethanol/water mixture by silicalite membrane on pervaporation, J. Membr. Sci. 95 (1994) 221. [9] M. Nomura, T. Yamaguchi, S.-I. Nakao, Ethanol/water transport through silicalite membranes, J. Membr. Sci. 144 (1998) 161. [10] Q. Liu, R.D. Noble, J.L. Falconer, H.H. Funke, Organics/water separation by pervaporation with a zeolite membrane, J. Membr. Sci. 117 (1996) 163. [11] M. Yu, J.L. Falconer, R.D. Noble, Characterizing nonzeolitic pores in MFI membranes, Ind. Eng. Chem. Res. 47 (2008) 3943. [12] J.B. Lee, H.H. Funke, R.D. Noble, J.L. Falconer, High selectivities in defective MFI membranes, J. Membr. Sci. 321 (2008) 309. [13] M. Yu, J.L. Falconer, T.J. Amundsen, M. Hong, R.D. Noble, A controllable nanometer-sized valve, Adv. Mater. 19 (2007) 3032. [14] M. Yu, T.J. Amundsen, J.L. Falconer, R.D. Noble, Flexible nanostructure of MFI zeolite membranes, J. Membr. Sci. 298 (2007) 182. [15] M. Yu, J.C. Wyss, R.D. Noble, J.L. Falconer, 2,2-Dimethylbutane adsorption and diffusion in MFI zeolite, Micropor. Mesopor. Mater. 111 (2008) 24. [16] S.G. Sorenson, J.R. Smyth, M. Kocirik, A. Zikanova, J.B. Lee, J.L. Falconer, R.D. Noble, Adsorbate-induced MFI zeolite crystal expansion, Ind. Eng. Chem. Res. 47 (2008) 9611.
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[17] M. Yu, J.L. Falconer, R.D. Noble, Characterizing non-zeolitic pore volume of MFI membranes by temperature-programmed desorption, Micropor. Mesopor. Mater. 113 (2008) 224. [18] G. Xomeritakis, S. Nair, M. Tsapatsis, Transport properties of alumina supported MFI membranes made by secondary (seeded) growth, Micropor. Mesopor. Mater. 38 (2000) 61. [19] S. Nair, M. Tsapatsis, Synthesis and properties of zeolitic membranes, in: S.M. Auerback, K.A. Carrado, P.K. Dutta (Eds.), CRC Handbook of Zeolite Science and Technology, Marcel Dekker, New York, 2003, pp. 1106– 1164. [20] S. Nair, Z. Lai, V. Nikolakis, G. Xomeritakis, G. Bonilla, M. Tsapatsis, Separation of close-boiling hydrocarbon mixtures by MFI and FAU membranes made by secondary growth, Micropor. Mesopor. Mater. 48 (2001) 219. [21] J. Hedlund, F. Jareman, A.J. Bons, M. Anthonis, A masking technique for high quality MFI membranes, J. Membr. Sci. 222 (2003) 163. [22] G.T.P. Mabande, M. Noack, A. Avhale, P. Kolsch, G. Georgi, W. Schwieger, J. Caro, Permeation properties of bi-layered Al-ZSM-5/silicalite-1 membranes, Micropor. Mesopor. Mater. 98 (2007) 55. [23] J. Hedlund, J. Sterte, M. Anthonis, A.J. Bons, B. Carstensen, N. Corcoran, D. Cox, H. Deckman, W. De Gijnst, P.P. De Moor, F. Lai, J. McHenry, W. Mortier, J. Reinoso, J. Peters, High flux MFI membranes, Micropor. Mesopor. Mater. 52 (2002) 179. [24] T.C. Bowen, J.C. Wyss, R.D. Noble, J.L. Falconer, Measurements of diffusion through a zeolite membrane using isotopic-transient pervaporation, Micropor. Mesopor. Mater. 71 (2004) 199. [25] H.H. Funke, M.G. Kovalchick, J.L. Falconer, R.D. Noble, Separation of hydrocarbon isomer vapors with silicalite zeolite membranes, Ind. Eng. Chem. Res. 35 (1996) 1575. [26] S.G. Sorenson, J.R. Smyth, J.L. Falconer, R.D. Noble, unpublished data, 2008. [27] V.S. Nayak, J.B. Moffat, Sorption and diffusion of alcohols in heteropoly oxometalates and ZSM-5 zeolite, J. Phys. Chem. 92 (1988) 7097. [28] P. Uchytil, R. Petrickovic, A. Seidel-Morgenstern, Study of capillary condensation of butane in a Vycor glass membrane, J. Membr. Sci. 264 (2005) 27. [29] F.W. Sears, M.W. Zemansky, University Physics, 2nd ed., Addison-Wesley, Massachusetts, 1955. [30] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Cleveland, OH, 1977. [31] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area, and Porosity, Academic Press, New York, 1967.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Alcohol and water adsorption and capillary condensation in MFI zeolite membranes Begum Tokay, John L. Falconer ∗ , Richard D. Noble Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0424, United States
a r t i c l e
i n f o
Article history: Received 30 November 2008 Received in revised form 10 February 2009 Accepted 14 February 2009 Available online 3 March 2009 Keywords: MFI membrane Permporosimetry Capillary condensation Pervaporation 2-Propanol/water separations
a b s t r a c t A boron-substituted MFI membrane was selective for 2-propanol/water separations during pervaporation in part because 2-propanol expanded the MFI crystals and thus shrank the size of the membrane defects. Permporosimetry measurements show that adsorption of 2-propanol, 1-butanol, and ethanol decreased helium flow through the defects, apparently because these alcohols swell MFI crystals. In contrast, neither methanol nor water adsorption appeared to change the defect size. Helium flow through membrane defects was also blocked by capillary condensation, but this was only observed when the pressure drop across the membrane was less than about 20 kPa. Thus defect blockage by crystal expansion was observed without capillary condensation by carrying out permporosimetry with a pressure drop of 89 kPa. The defect sizes were estimated from capillary condensation to be approximately 2 nm. Published by Elsevier B.V.
1. Introduction Silicalite membranes selectively separate alcohols from water [1–4]. Lin et al. reported a separation factor of 106 for ethanol/water separation at 333 K; the flux was 14 mol/m2 h for 5 wt.% ethanol in the feed [5]. Tuan et al. obtained a separation factor of 49 for methanol/water separation for 5 wt.% methanol feed with a B-ZSM5 membrane, but this membrane had poor separation selectivities for ethanol (2.1) and 2-propanol (1.6) [1]. Their Ge-ZSM-5 membrane had separation factors of 29 and 16 for 5 wt.% ethanol and 2-propanol in water, respectively. These separation factors were 25 and 43 for B-ZSM-5 membranes on monolith supports at 303 K [4]. Matsuda et al. modified the surface of a silicalite-1 membrane with a silicone coating to increase the ethanol/water separation factor from 51 to 125 [6]. Modifying the surface with a silane coupling agent raised the flux to 22 mol/m2 h but decreased the separation factor to 44 [7]. These studies found that increasing the alcohol percentage in the feed decreased the water flux, and separation was attributed to preferential adsorption of alcohols. Sano et al. observed that the separation factor decreased significantly as the ethanol feed concentration increased for a silicalite membrane [8]. Since ethanol (0.43 nm) and water (0.265 nm) are both smaller than MFI pore (∼0.6 nm), preferential adsorption was considered responsible for the selectivity. Nomura et al. also found that ethanol inhibited water
∗ Corresponding author. Tel.: +1 303 492 8005; fax: +1 303 492 4341. E-mail address: [email protected] (J.L. Falconer). 0376-7388/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.memsci.2009.02.010
permeation, but water only slightly affected ethanol permeation [9]. Similarly Liu et al. reported increased methanol (0.38 nm) flux with increasing methanol concentration at high methanol concentrations [10]. The highest separation factor they obtained was 14 for 16.5 wt.% methanol in the feed with a silicalite-1 membrane at room temperature. In the current paper, the possibility was investigated that alcohol adsorption expands MFI crystals and shrinks the size of the defects, and that this, in addition to preferential adsorption, affects selective removal of alcohols from water with MFI membranes. We recently reported that n-alkanes (C3 -C8 ) and SF6 swell MFI crystals and shrink membrane defects [11–14]. Yu et al. [13,14] showed that single-component pervaporation fluxes of smaller molecules (n-hexane: 0.43 nm; acetone: 0.47 nm) were significantly lower than the fluxes of larger molecules (2,2-dimethylbutane (DMB), 1,3,5-trimethylbenzene (TMB), i-octane, benzene) in some MFI membranes, even though DMB (0.63 nm), TMB (0.75 nm), and ioctane (0.7 nm) are too large to adsorb at a significant rate in MFI pores. 2,2-Dimethylbutane can adsorb in MFI pores, but its diffusivity is orders of magnitude lower than the n-hexane diffusivity. For a silicalite-1 membrane, the DMB transit time was an order of magnitude shorter than the n-hexane transit time at 313 K because DMB diffused through defects, whereas these defects were mostly unavailable to n-hexane, which diffused mainly through MFI pores [15]. Thus, DMB had a higher flux than n-hexane in this membrane during pervaporation. Adding only 0.2% n-hexane to the feed reduced the DMB flux by two orders of magnitude [14]. Yu et al. concluded that n-hexane swelled the crystals, and this decreased the size of the defects and thus decreased the flux through the
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defects. Benzene (0.585 nm) did not swell the crystals so it diffused through the defects and it had a higher flux than n-hexane [11]. X-ray diffraction measurements showed that MFI crystals expand when n-hexane adsorbs but not when benzene adsorbs [16]. A saturation loading of n-hexane increased the MFI unit cell dimensions by about 0.4% in each direction at room temperature, and the unit cell volume increased by 1.2%. For 1-m crystals, a linear expansion of 0.4% would close off 4-nm pores if the overall membrane layer could not expand much because it was attached to the support. In contrast, the unit cell volume increased less than 0.01% for benzene [16]. Yu et al. directly showed with temperature-programmed desorption that the pore volume of the MFI membrane layer decreased to less than half its value when n-hexane adsorbed in the MFI pores. This intercrystalline pore volume included defects that were transport pathways as well as dead-end pores, and it decreased as the MFI crystals swelled when n-hexane adsorbed [17]. The behavior observed in these previous studies may not occur for membranes with different microstructures, which result from different preparation methods. For example, for c-oriented membranes prepared by secondary growth, Tsapatsis and coworkers [18–20] reported that p-xylene adsorption significantly increased the flow through defects, as indicated by increased o-xylene flux. They conjectured that p-xylene adsorption expanded the MFI crystals and distorted the structure so that the intercrystalline porosity increased. For membranes with a different microstructure, because their preparation was different and cracks were sealed with a silica sol, the flux through defects did not increase when p-xylene adsorbed [20]. In the current study, we investigated the contribution of MFI crystal swelling to selectivities of MFI membranes for alcohol/water separation by pervaporation. The membrane used had the majority of its helium flux through defects at room temperature. Although this is not expected to be the best membrane for alcohol/water separations, it allows the changes in flux through defects to be readily detected, and the same behavior is expected for membranes with less flow through defects. Permporosimetry measurements showed that some alcohols dramatically decreased the helium flux through the membrane defects. We conclude that the flux decreased because defects shrank due to adsorption, not because of capillary condensation. Indeed, crystal swelling decreased the helium flux at much lower activities than where capillary condensation was observed. Capillary condensation is shown to strongly depend on the pressure drop across the membrane, and was only observed at low activities (indicating small defects) when the trans-membrane pressure drop was low. Vapor permeation measurements with i-octane, which can only diffuse through defects that are larger than the MFI pores, also showed that alcohols shrank the size of defects. Pervaporation measurements were consistent with the permporosimetry measurements: the separation selectivity for 2-propanol/water mixtures was five orders of magnitude higher than the ideal selectivity for certain concentration ranges because of crystal swelling due to alcohol adsorption. Thus, both crystal swelling and preferential adsorption can contribute to the ability of MFI membranes to separate alcohol/water mixtures.
with about 2 mL of synthesis gel. The other end was then plugged with a Teflon cap and left overnight at room temperature while the porous support soaked up most of the gel. The tube was again filled with synthesis gel, plugged with a Teflon cap, and put into an autoclave for hydrothermal synthesis at 458 K for 24 h. The membrane was then brushed, washed with DI water, and dried. The same synthesis procedure was repeated, except that the tube was not soaked overnight, and the membrane’s vertical orientation in the autoclave was reversed. The resulting membrane was impermeable to N2 at room temperature. It was calcined at 700 K for 8 h, with heating and cooling rate of 0.6 and 0.9 K/min, respectively. An XRD pattern for crystals collected from the bottom of the autoclave confirmed the MFI structure. 2.1. Permporosimetry Adsorption branch porosimetry or permporosimetry was used in an effort to estimate the percentage of flow through defects [21–23]. During permporosimetry, the helium flux was measured as a function of the activity of a second compound in the feed stream. The activity was calculated as the ratio of the partial pressure of the compound to its saturation pressure. The membrane was sealed in a stainless steel module using silicone o-rings. In all measurements, the normalized helium flux is presented, where the flux was normalized by the helium flux at zero activity and at the same pressure drop across the membrane. The n-hexane (>99.5%, Fluka), benzene (99+%, Sigma–Aldrich), water, methanol (>99.8%, Sigma–Aldrich), ethanol (>99.8%, Sigma–Aldrich), 2propanol (99.5+%, Sigma–Aldrich) and 1-butanol (99.9%, Fisher) were added to the feed by bubbling helium through two liquid bubblers in series and mixing the resulting saturated stream with pure helium. The activity was adjusted by changing the temperature of the bubblers and the ratio of the helium streams. The permeate pressure was 84 kPa, which is atmospheric pressure in Boulder, CO. For most measurements the feed pressure was maintained at 185 kPa by a back pressure regulator, but permporosimetry was also carried out for lower feed pressures. Because the feed pressure was kept constant, as the activity of the adsorbing molecule increased, the helium partial pressure decreased. Thus, the helium driving force decreased as the activity increased, and this caused the helium flux to decrease. Therefore, the normalized helium fluxes were corrected for this decreased driving force. Measurements indicated that the helium flux was linear in pressure drop across the membrane. This was not a large correction. Also, for some measurements, instead of measuring the helium flux as a function of activity of an alcohol or water, the activity was kept constant and the feed pressure was increased from 104 to 185 kPa. The activity was kept constant by increasing the pressure drop across the membrane and keeping the partial pressure of the organic constant. In that case, the normalized flux was the measured flux divided by the flux at zero activity and at the same pressure drop. A mass flow meter and a bubble flow meter were used to measure the helium permeate flow rate. An activated-carbon and molecular sieve (MS 13X Dunniway) trap on the permeate line removed the alcohols and water from the helium stream before permeate flow measurements, and the trap was periodically baked out.
2. Experimental methods 2.2. Pervaporation A membrane containing isomorphously substituted boron in the framework (B-ZSM-5) was synthesized by in situ crystallization onto the inside of a tubular ␣-alumina support (0.2-m pores, Pall Corp.) using procedures described previously [11]. The synthesis gel had a molar composition of 4.44 TPAOH:19.5 SiO2 :1.55 B(OH)3 :500 H2 O. The resulting gel was aged at room temperature for at least 6 h. One end of the support tube was wrapped with Teflon tape and plugged with a Teflon cap, and the inside of the support was filled
The apparatus used for pervaporation has been described in detail previously [24]. The membrane was sealed in a stainless steel module using silicone o-rings. A centrifugal pump circulated liquid feed through the inside of the membrane tube and through a feed reservoir at approximately 1 L/min to minimize concentration polarization. All measurements were at room temperature. The permeate side pressure was kept below 0.23 kPa using a liquid nitrogen
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trap and a mechanical vacuum pump. Steady-state fluxes were measured by condensing the permeate in the liquid nitrogen trap, and permeate concentrations were analyzed with a GC with a thermal conductivity detector. The permeate liquid was collected after 2 h of pervaporation and then it was collected for 4–5 h. 2.3. Vapor permeation Vapor permeation measurements were carried out at room temperature in a continuous flow system, as described in detail elsewhere [25]. The membrane was mounted in a stainless steel module using silicone o-rings. The flux of i-octane, which can only diffuse through defects that are larger than the MFI pores, was measured as a function of the activity of 2-propanol. These measurements directly observed the change in flux through defects as 2-propanol adsorbed in the MFI pores. A syringe pump injected a liquid into a preheated helium carrier stream, which then passed through a heated zone at ∼383 K for complete vaporization. A second liquid could be fed to the system using two liquid bubblers and it was mixed with the first stream at the heating zone. A helium sweep stream was used on the permeate side. Both the feed and permeate streams were analyzed on-line by an Agilent 6890 GC with a flame ionization detector. A bypass line allowed analysis of the feed before entering the module. Bubble flow meters were used to measure feed and permeate flow rates. Pressure was kept at 110 kPa on both sides of the membrane. The feed activities were adjusted by varying the syringe pump feed rate, bubbler temperature, and helium flow. 3. Results and discussion 3.1. Membrane characterization During permporosimetry, the helium flux through the B-ZSM-5 membrane decreased when n-hexane and benzene adsorbed, and the changes with activities were similarly to those observed by Yu et al. [11] and Lee et al. [12] for other MFI membranes. Benzene only blocked 35% of the helium flux, even at activities close to one (Fig. 1a), indicating that approximately 65% of the helium flow was through defects at room temperature. That is, benzene adsorbed in the MFI pores and blocked flow through them, but it did not appear to decrease the helium flux through defects. In contrast, as shown in Fig. 1a and b, at a n-hexane activity below 0.0035, the helium flux decreased to 0.01% of its original value. Based on our previous studies [12,15], this dramatic difference in behavior results because n-hexane swells MFI crystals (approximately 0.4% in each direction at saturation loading), but benzene does not [16]. Because the helium flux was almost completely blocked by n-hexane swelling of crystals, the defects must be small. As shown in Fig. 2, the four alcohols studied exhibited permporosimetry behavior that was between that for benzene and n-hexane. At an activity of 0.01, 2-propanol (kinetic diameter = 0.47 nm) reduced the helium flux to about 1% of its original value, whereas ethanol (0.43 nm) and 1-butanol (0.49 nm) only reduced the helium flux to 1% of its original value at higher activities. Methanol had significantly less effect on the helium flux, and even at a methanol activity of 0.85, 7% of the helium flux remained. Water decreased the helium flux even less than methanol. As shown in Fig. 3, even at an activity of 0.95, only 60% of the helium flux was blocked. As shown in the next section, the decreases in helium flux with adsorption are not due to capillary condensation in the membrane defects. The permporosimetry measurements in Figs. 1–3 were made for a pressure drop across the membrane of 89 kPa, but capillary condensation was only observed for much smaller pres-
Fig. 1. Normalized helium flux as a function of benzene and n-hexane activities during permporosimetry for a B-ZSM-5 membrane at room temperature and a pressure drop across the membrane of 96 kPa. Two activity ranges are shown: (a) 0–1 and (b) 0–0.005.
sure drops. Also, the four alcohols would be expected to exhibit similar capillary condensation behavior but the difference in behavior of the helium flux for 2-propanol and methanol is dramatic. Also, 1-butanol would be expected to preferentially adsorb over ethanol, but as shown in Fig. 2, for activities below 0.2, ethanol decreases the helium flux more than 1-butanol. Thus, the large decreases in helium flux when ethanol, 2-propanol, and 1-butanol
Fig. 2. Normalized helium flux as a function of alcohol activity during permporosimetry for a B-ZSM-5 membrane at room temperature and a pressure drop across the membrane of 89 kPa.
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Fig. 3. Normalized helium flux as a function of benzene and water activities during permporosimetry for a B-ZSM-5 membrane at room temperature. The pressure drop across the membrane was 96 kPa for benzene and 89 kPa for water.
Fig. 5. Normalized helium flux as a function of MFI crystal loading for alcohols and water for a B-ZSM-5 membrane at room temperature and a pressure drop across the membrane of 89 kPa.
adsorb are attributed MFI crystal swelling. These molecules blocked flow through the MFI pores by adsorption in those pores, and they blocked most of the flow through the defects because swelling the MFI crystals decreased the defect sizes. Recent XRD measurements on silicalite single crystals found that 2-propanol increased the MFI unit cell dimension by approximately 0.35% for a and c directions and 0.23% for b direction, whereas ethanol increased the unit cell 0.15% in each direction. Thus, the unit cell volume increased 0.95% for 2-propanol and 0.47% for ethanol [26]. If the overall length of the MFI membrane layer could not increase because it was attached to the support, a 0.35% increase in unit cell dimension would seal a 3.5 nm defect if the MFI crystals were 1 m in diameter. At low alcohol activities, the helium flux during permporosimetry was quite different for each alcohol, as shown in Fig. 4. The helium flux dropped dramatically for small changes in 2-propanol activity. At a 2-propanol activity of 0.003, the helium flux decreased 45% from its original value, whereas at a methanol activity of 0.003, the helium flow only decreased 23%. In contrast, 1-butanol and ethanol had no effect on the helium flux at that activity. At an activity of 0.013, 99% of the helium flow through the defects was blocked by 2-propanol, and 90% was blocked by ethanol. Adsorption isotherms for ZSM-5 (Si/Al = 990) from the literature [27] were used to plot the helium flux dependence on loadings (Fig. 5). The ZSM-5 (Si/Al = 990) and B-ZSM-5 zeolites were assumed to have similar adsorption behavior. As the alcohol size increased, the loading (molec./u.c.) required to decrease the helium flux more
than an order of magnitude decreased. The helium fluxes exhibited large decreases for small increases in alcohol loadings. In contrast to the alcohols, high loading of water were not obtained for the activities used, and this may explain why water did not exhibit the same behavior [8]. The water loading were probably higher than indicated in Fig. 5 because boron substitution makes MFI zeolites less hydrophobic [1].
Fig. 4. Normalized helium flux as a function of alcohol activity during permporosimetry for a B-ZSM-5 membrane at room temperature and a pressure drop across the membrane of 89 kPa.
Fig. 6. Normalized helium flux, as a function of water activity, during permporosimetry for a B-ZSM-5 membrane at room temperature. Measurements were made for the indicated pressure drops across the membrane.
3.2. Effect of pressure on permporosimetry The decreases in helium flux during permporosimetry measurements in Figs. 1 and 2 are not due to capillary condensation in the membrane defects. Indeed, as shown in Fig. 1b, benzene does not condense, even for an activity 220 times higher than the n-hexane activity where the helium flux dramatically decreased. Instead, the activity where molecules condense in the defects strongly depends on the pressure drop across the membrane. Capillary condensation was not observed for a pressure drop across the membrane of 89 kPa, but molecules did condense, and at lower activities, when lower pressure drops were used. As shown in Fig. 6, the helium flux decreased two to three orders of magnitude, as the water activity increased, when the pressure drop was 21 kPa or smaller. As the pressure drop decreased, the activity where water condensed in the membrane defects decreased, so that at a pressure drop of approximately 1.4 kPa and an
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Table 1 i-octane and 2-propanol vapor permeation fluxes at room temperature and zero pressure drop. Flux (mol/m2 h)
Single component Mixture
Fig. 7. Normalized helium flux as a function of pressure drop at the indicated water activities during permporosimetry for a B-ZSM-5 membrane at room temperature.
activity of 0.11, the flux dropped almost four orders of magnitude. The lowest activity that used at this pressure drop was 0.05, and the flux dropped only 10% from its original value. Pressure drops below 1.4 kPa were not used because the helium flux decreased as the pressure drop decreased, so accurate measurements were not possible at lower pressure drops when molecules condensed in the pores. These results show that water did not condense in the defects when the pressure drop was 89 kPa, and thus the alcohols are also not expected to condense at this pressure drop. Indeed, the 2-propanol activity where the helium flux dropped orders of magnitude (Fig. 4) was an order of magnitude lower than the lowest water activity where water condensed, and the pressure drop was 64 times higher. Thus, the large decreases in helium flux at low activities for n-hexane and alcohols in Figs. 1, 2 and 4 are not due to capillary condensation, but instead appear to be due to crystal swelling. Permporosimetry measurements were also carried out by increasing the feed pressure, and thus the pressure drop, at constant water activity. As shown in Fig. 7, the helium flux decreased dramatically as the pressure drop decreased below 10 kPa. Below 2 kPa pressure drop, water condensation blocked almost all the flux through the defects at a water activity of 0.11 (Fig. 7). Benzene exhibited similar behavior, as shown in Fig. 8, but the helium flux decreased with pressure over a wider range of pressure drop. The helium flux dependence on pressure drop for DMB in the feed was similar to that seen for the water and benzene. As shown
i-octane (0.8 activity)
2-propanol (0.08 activity)
0.47 0.013
0.32 0.09
in Fig. 8, at higher pressure drops across the membrane, DMB decreased the normalized helium flux by 10–15%. A measurable amount of DMB does not adsorb in the MFI pores at these conditions [15], but this decrease could be due to DMB adsorption on the external surface of the MFI crystals, and this adsorption could slow helium entrance into the MFI pores. At a trans-membrane pressure drop of 3.5 kPa, the normalized helium flux exhibited the same behavior observed with the water and benzene (Figs. 7 and 8), indicating that DMB condensed in the defects and decreased the helium flux by 80%. Figs. 6–8 indicate that capillary condensation were only observed at low pressure drops across the membrane; apparently water in the defects cannot support a pressure drop of more than 21 kPa, and the highest pressure drop where a condensed layer remains is probably similar for other molecules. Uchytil et al. [28] reported n-butane condensed in Vycor glass pores (average radius = 4 nm) when the n-butane pressure on both sides of the pores was close to its condensation pressure and the pressure drop across the membrane was small. They did not observe condensation at high-pressure drops even when the n-butane pressure was close to saturation. 3.3. Vapor permeation Because i-octane is larger than MFI pores, its flux is a measure of permeation through defects that are larger than the MFI pores. Thus, i-octane was added to the feed at an activity of 0.8, and after steady-state was reached, 2-propanol was added at an activity that was two orders of magnitude lower (0.08). The large difference in activities minimized the effects of possible preferential adsorption of 2-propanol in the defects. When 2-propanol was added to the feed, the i-octane flux decreased to 3% of its original value; the 2-propanol flux was 29% of its pure-component flux at the same activity (Table 1). The lower i-octane flux through the defects is attributed to swelling of the MFI crystals by 2-propanol. The i-octane flux did not decrease the same percentage that the helium flux decreased during permporosimetry (for the same 2propanol activity, the helium flux decreased to 1% of its original value) for at least two reasons. First, helium also permeated through the MFI pores and 2-propanol adsorption in the MFI pores blocked helium flux through them. Second, some defects may be too small for i-octane but large enough to permeate helium. Those smaller defects decrease in size with crystal swelling, as do larger pores. Thus the helium flux is expected to decrease a larger percent than the i-octane flux. 3.4. Pervaporation
Fig. 8. Normalized helium flux as a function of pressure drop at three benzene activities and at a DMB activity of 0.01 during permporosimetry for a B-ZSM-5 membrane at room temperature.
The single-component pervaporation fluxes through the membrane are plotted in Fig. 9 as a function of kinetic diameter. The water flux was almost three orders of magnitude higher than the n-hexane flux and 200 times the 2-propanol and 1-butanol fluxes. The smaller water molecule might be expected to diffuse much faster than n-hexane and the alcohols. However, benzene permeated 35 times faster than n-hexane, eight times faster than 2propanol and five times faster than 1-butanol, even though benzene is the larger molecule. More significantly, DMB, which only diffuses
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B. Tokay et al. / Journal of Membrane Science 334 (2009) 23–29
tivity for a 0.045 mol% 2-propanol concentration is 830, which is more than five orders of magnitude higher than the ideal selectivity. The selectivity drops significantly at higher concentrations of 2-propanol because the defects do not shrink more and the higher concentration does not result in a higher 2-propanol flux. The selectivity also drops at lower 2-propanol concentration because the defects are too large. Thus, this type MFI membrane, with much of its flow through defects, would not be useful for 2-propanol/water separations. However, the same crystal swelling would be expected in membranes with less flow through defects, and thus crystal swelling would contribute to selective separations of 2-propanol from water. 3.5. Defect sizes Fig. 9. Single component pervaporation fluxes through a B-ZSM-5 membrane at room temperature as a function of kinetic diameter.
through the defects at room temperature, permeated nine times faster than n-hexane and twice as fast as 2-propanol. As reported previously for n-hexane in other MFI membranes, its lower flux is attributed to swelling of MFI crystals. That is, DMB and benzene diffuse through defects, most of which are not available to n-hexane diffusion because crystal swelling by n-hexane decreased their size. The single-component pervaporation fluxes indicate that 2-propanol and 1-butanol also swell MFI crystals and decrease the size of defects, and this is why their fluxes are lower than expected for their size. Since 2-propanol permeates slower than 1-butanol, apparently 2-propanol swells the crystals more than 1-butanol. Pervaporation of 2-propanol/water mixtures also indicates that 2-propanol swells the MFI crystals and shrinks the defects. The 2propanol/water ideal selectivity is 0.005; as shown in Fig. 9, the water flux was 200 times the 2-propanol flux. Pervaporation was carried out for low 2-propanol concentrations in water (Table 2), so that the 2-propanol activities were similar to those where the helium flux dropped dramatically during permporosimetry for small changes in 2-propanol activity (Fig. 4). The 2-propanol concentrations of 0.014 and 0.045 mol% in the pervaporation feed correspond to 2-propanol activities of 0.005 and 0.011, respectively. These liquid activities were calculated by the UNIFAC model. As shown in Table 2, 0.014 mol% 2-propanol decreased the water flux almost an order of magnitude, and 0.045 mol% 2-propanol decreased the water flux more than another order of magnitude. Further increase in 2-propanol concentration did not change the water flux significantly. The decrease in water flux to 0.5% of its original value, when the 2-propanol concentration was only 0.045 mol%, does not appear to be due to preferential adsorption of 2-propanol. Instead, these results correlate with the decreased helium flux in permporosimetry and the decreased i-octane flux in vapor permeation and appear to be due to crystal expansion that decreased the size of the defects. As a result the mixture selecTable 2 Pervaporation fluxes of 2-propanol/water mixtures at room temperature through B-ZSM-5 membrane. 2-Propanol (mol%)
0 0.014 0.045 4.0 100 a
Ideal selectivity.
Flux (mol/m2 h) 2-Propanol
Water
– 0.06 0.22 0.24 0.528
107 12.4 0.53 0.53 –
2-Propanol/water separation factor
0.005a 24 830 11 0.005a
The lowest water activity where condensation was observed (at the lowest pressure drop across the membrane) was used in the Kelvin equation and the Horvath-Kawazoe (H-K) potential function for slits to estimate defect sizes. Benzene and DMB activities were also used to estimate the defect sizes for the lowest pressure drop used. In the Kelvin equation, contact angles were assumed as 0◦ [29], and surface tensions against air at room temperature were used [30]. The thickness of the adsorbed layer, which was estimated by assuming hexagonal packing, was added to the Kelvin radius to get the pore radius [31]. The adsorbed layer thickness was 0.44 nm for water and 0.5 nm for benzene and DMB. The defect diameter was estimated from the Kelvin equation to be 2.4, 1.9, and 1.9 nm from benzene, DMB, and water condensation, respectively. Because this is the lower limit of where the Kelvin equation is applicable, and because the contact angles and the thickness of the adsorbed layer were estimated, these values are not accurate to two significant figures. Similar diameters were obtained from condensation of the three molecules, however, and pores of this size could certainly be closed off by crystal expansion. 2-Propanol would expand 1 m MFI crystals by 2.3–3.5 nm, depending on which unit cell direction is considered. The defect diameter was also estimated from H–K equation to be between 0.7 and 1.2 nm for small activities of water, benzene and DMB, but pores these sizes are too small for condensation. 4. Conclusions Adsorption of 2-propanol, 1-butanol, and ethanol swells MFI crystals and shrinks the size of the defects in a MFI membrane. In contrast, water and methanol do not have a large effect on the defect size. This crystal swelling may contribute to high selectivities for alcohol/water separations during pervaporation for some MFI membranes. Permporosimetry, vapor permeation of ioctane through defects, and pervaporation all indicate the same behavior. Capillary condensation was not observed in the membrane defects when the pressure drop across the membrane was great than 21 kPa. At lower pressure drops, however, benzene, DMB, and water condensed in the defects. The average size of the defects estimated from the Kelvin equation was about 2 nm. Apparently the condensed layers were not stable at higher pressure drops, and thus permporosimetry at higher pressure drops allowed crystal swelling to be detected in the absence of capillary condensation. Acknowledgments We gratefully acknowledge support by the National Science Foundation CTS 0340563. We also thank Justin B. Lee for his assistance with the permporosimetry measurements.
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