Effect of Isomorphous Metal Substitution in Zeolite Framework on

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Ind. Eng. Chem. Res. 2010, 49, 809–816

809

Effect of Isomorphous Metal Substitution in Zeolite Framework on Pervaporation Xylene-Separation Performance of MFI-Type Zeolite Membranes Jessica O’Brien-Abraham and Y. S. Lin* Department of Chemical Engineering Arizona State UniVersity, Tempe, Arizona 85287

MFI-type zeolite membranes have shown good selectivity for separation of p-xylene from its isomers. The major problem with the MFI-type zeolite membrane is that the MFI-type zeolite framework loses its size/ shape selectivity under high loadings of p-xylene because of the significant framework distortion experienced by the pore structure and as a result observed high selectivities are not stable over time. This paper proposes changing the interaction of the xylene isomers with MFI-type framework to address this problem. MFI-type zeolite membranes with aluminum and boron isomorphously substituted into the framework were synthesized and subjected to multicomponent xylene separation via pervaporation. It is found that by performing this substitution, slight changes to both surface chemistry and framework flexibility can be introduced. Essentially, the interaction of the xylene molecules with the MFI structure is modified to limit p-xylene loading, as well as diffusion pathway access to o-xylene. As a result improvement in xylene separation performance over silicalite was observed. The boron-substituted membranes demonstrated the highest selectivities for p-xylene under a wide range of feed compositions; the highest selectivity observed was ∼55 (feed, 5% p-xylene; 95% o-xylene). This is higher than any previously reported xylene separation selectivity for pervaporation through MFI-type zeolite membranes. However, the performance stability of substituted membranes over time was also investigated, and it was found that, over a period of 96 h, a reduction in selectivity of about an order of magnitude was observed. 1. Introduction The three xylene isomers, p-xylene (PX), o-xylene (OX), and meta-xylene (MX) are used as industrial solvents and as chemical intermediates.1 Specifically, PX is highly desirable because of its use in the synthesis of terephthalic acid, which is a precursor used in the manufacture of polyester resins and fibers.2 Continuous membrane processes provide an alternative to complex and energy intensive techniques, such as selective adsorption (Parex process, ExxonMobil), azeotrophic distillation, or cryogenic distillation, and also pave the way for integration of separation and reactive processes in xylene production.1 Membrane pervaporation provides three advantages: (1) reduced energy demand because of the lower temperature requirements given that only a fraction of the feed needs to be vaporized, (2) continuous operation, and (3) higher driving force as a result of the downstream vacuum.3 The success of such technology depends on the development of membrane materials that provide high performance in terms of stability and flux and are stable under the desired operating conditions.2 In recent years, there has been significant focus on the development of MFI-type zeolite membranes for xylene separation via pervaporation.2 Supported MFI-type zeolite membranes are expected to be able to selectively pass PX (kinetic diameter kd ) 0.59 nm), while excluding the bulkier OX and MX isomers (kd ) 0.68 nm). The pore structure of MFI-type zeolite consists of two channels: straight channels with circular openings of 0.54 nm × 0.56 nm along the b-axis and sinusoidal channels with elliptical openings of 0.51 nm × 0.55 nm along the a-axis. Transport is also possible through the channel intersections along the c-axis.1 Table 1 summarizes vapor permeation and pervaporation results obtained for MFI-type zeolite membranes synthesized with different growth methods. Modifying the synthesis method causes differences in microstructure, orientation, degree * To whom correspondence should be addressed. E-mail: jerry.lin@ asu.edu.

of intergrowth, and defect concentration, which can affect the resulting membrane performance. Previous work3-5 on zeolite catalysis suggest that significant PX selective performance of MFI type zeolites is indeed possible and detailed transport models have been developed. In many of these studies the state of the system uses vapor phase xylenes that are well below saturation at temperatures above 150 °C in order to facilitate isomerization reactions. Under such conditions the assumption that diffusivity is independent of concentration is valid and the system becomes diffusion controlled.6 Under diffusion controlled conditions, the relative diffusivities of each component drive the selectivity.7 Membrane operation under low vapor pressures using this concept has led to significant separation capabilities for MFItype zeolite membranes (see Table 1). Overall, higher selectivity is observed under vapor permeation conditions where the partial pressures are generally low (0.27-2.5 kPa, ∼Pactual/Psatd < 1%). Xomeritakis et al.8 reported that the PX selectivity of MFItype zeolite membranes dropped significantly as the vapor pressure of the feed was increased. Increasing the adsorption loading by increasing the xylene partial pressure results in a Table 1. Previous Results for the Separation of Xylene Isomers through MFI-Type Zeolite Membranes by Vapor Permeation and Pervaporation Methods separation (50PX/50OX)

growth method

vapor permeation secondary growth vapor permeation secondary growth vapor permeation secondary growth (defect sealing) vapor permeation secondary growth (defect sealing) vapor permeation in situ pervaporation secondary growth (template-free) pervaporation in situ pervaporation secondary growth pervaporation secondary growth

10.1021/ie900926t  2010 American Chemical Society Published on Web 12/02/2009

orientation

permselectivity (-)

b-oriented c-oriented h,0,h-oriented

500 2-39.5 20-300

37 38 39

h,0,h-oriented

35-278

38

random random

18-73 7-40

9 26

random c-oriented h,0,h-oriented

1-1.2 2-3.6 2-2.4

26 18, 25 18, 25

ref

810

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shift of the controlling transport mechanism to be adsorption rather than diffusion controlled.7 When the system is adsorption controlled, it is known that the shape/size selectivity of the MFItype zeolite membrane is highly dependent on the PX loading. At high loadings of PX the MFI-type zeolite framework experiences distortion which changes the pore shape and ultimately allowing greater mobility of the larger isomers as observed by numerous researchers.8-11 At low partial pressures (i.e., low loadings), the deformation can be avoided and the effect of the PX-framework interaction is negligible. However, operation at low partial pressures can lead to impractically low fluxes of xylene through the membrane.8-10 Previous studies on pervaporation separation of xylene isomers suggest membrane performance is dependent on the quality of the membrane (governed by synthesis conditions and, microstructure) and on the sorbate-sorbate interactions under saturated conditions.11,12 By its nature, pervaporation yields operation at saturated adsorption conditions given the presence of a liquid feed. Below 4 molecules per unit cell (muc), the ORTHO phase of silicalite exists where the PX molecules reside solely in the channel intersections. At a loading of 4 muc and up (saturation ) 7.8 muc13), the silicalite undergoes a transition from the ORTHO phase to the PARA phase. At this point, the channel intersections are filled to capacity, and the PX molecules must fit themselves into the sinusoidal channels causing displacement of the MFI-type zeolite framework ions.14 These dimensional changes are large enough to significantly change the adsorption properties of the xylene isomers within the MFI zeolite.15,16 Molecular simulations of single component PX and MX in the two phases (ORTHO and PARA) of silicalite show that in the ORTHO phase MX has very limited access to the pores of the zeolite. In the PARA phase, MX will be able to enter the channel intersections as well as the sinusoidal channels.17 When framework distortion is induced at high PX loadings and both PX and OX must compete for transport through the zeolitic pores, the separation mechanism is no longer simple size/shape selectivity. At this point energetic and entropic effects become very important and ultimately determine membrane performance.15 Our group recently demonstrated the effect of this phenonomon on separation performance of MFI-type zeolite membrane by investigating multicomponent pervaporation through membranes of varying orientation.18 It was found that the observed performance (selectivity and PX flux) was highly dependent on the relative concentration of isomers in the feed: the higher the PX concentration the lower the observed selectivity. Despite observing high selectivities (>18) at low concentration of PX in the feed, these values were not stable over time, and this instability was independent of any microstructural orientation.18 Additionally, the relationship between microstructure and defect sealing, single component pervaporation molecular probing with tri-isopropylbenzene (TIPB, dk ) 0.86 nm) was conducted. The size of TIPB excludes it from entering the zeolite pores, even when silicalite is most distorted in the PARA phase. TIPB pervaporation measurements were taken before and after exposure to PX to determine how the swelling of the crystal affected the ability of the molecule to pass through the defects of the membrane. A reduction of approximately 60% of the original flux observed in randomly oriented MFI-type zeolite membrane indicates that changes to the zeolite structure actually cause effective defect sealing.18 The crystal experiences an overall expansion of 0.39% with a- and c- axes expansion of 0.09% and 0.52%, respectively.19 The b-axis experiences contraction of 0.23%.19 If a 1 µm crystal were to experience

these changes isotropically, an overall expansion could seal defects up to 4 nm. Other MFI-sorbate systems are known to exhibit similar behavior. In a recent study, Yu et al.20 reported that at saturated loadings of n-hexane a single MFI unit cell has an overall volume expansion of 2.3%, which can correlate to shrinkage in nonzeolitic pores up to 7 nm (1 µm crystal if isotropic expansion assumed). It has been demonstrated that even in membranes with a large number of defects, the crystallite swelling can cause the membrane to achieve significant separation between n-hexane and trimethylbenzene, iso-octane, and 2,2-dimethylbutane using pervaporation.20 To circumvent the structural changes caused by xylene exposure to MFI-type zeolite membranes, this work investigates the use of isomorphous substitution to introduce framework rigidity to enhance separation performance at high concentration/ extended duration. It is well-known that substitution into the MFI-type zeolite framework can be used to tune the zeolite pore size by choosing the appropriate molecule for a desired separation.21 Noble and co-workers successfully incorporated Al, Fe, B, and Ge into MFI-type zeolite membranes.21 They found that the substituted ion can play a substantial role in the separation of a number of mixtures including n-C4H10/i-C4H10, n-C4H10/H2, and H2/i-C4H10. In the present work, Al and B will be incorporated in the MFI-type zeolite framework to form AlZSM5 and B-ZSM5, respectively. Both are trivalent cations and are expected to cause differences in acidity and surface activity as compared to silicalite.22 Also, by replacing Si (cation diameter ) 0.042 nm) with cations of different sizes (Al, 0.051 nm; B, 0.023 nm), alterations in the crystal structure arise due to changes in the T-O-T angle and T-O length (T ) Si or substituted ion).22 The inevitable unit cell differences may be enough to induce framework rigidity and circumvent the deformation at high loading of PX during pervaporation. The objective of this work is to tailor MFI-type zeolite membranes through the use of isomorphous substitution to improve the performance of the MFI-type zeolite membrane for the separation of xylene isomers via pervaporation. Multicomponent xylene separation and stability tests were performed on silicalite, B-ZSM5, and Al-ZSM5 membranes to determine if the surface or structural changes caused by the framework modification are able to induce a more resilient MFI-type zeolite membrane. 2. Experimental Section 2.1. Preparation and Characterization of MFI-Type Zeolite Membranes. MFI-type zeolite membranes were synthesized on homemade R-Al2O3 disks, 20 mm in diameter and 2 mm thick (average pore diameter: 0.2 µm, porosity 45%),23 made with calcined alumina powder (Alcoa, A-16). Details of support preparation can be found in previous publications.24,25 Supports were polished with a mechanical grinder (Metaserve 2000) using no. 500 and no. 800 SiC paper until shiny. Polished supports were dip coated in a silicalite seed solution with initial composition 10 SiO2: 2.4 TPAOH: 1NaOH: 110 H2O synthesized hydrothermally at 125 °C for 8 h and diluted to 1-2 wt % in deionized water. After dip-coating, supports were dried at 40 °C at 40% relative humidity for 48 h. Procedure was repeated three times to ensure adequate coverage of seed layer as demonstrated by our group.23 Table 2 indicates the solution compositions for secondary growth of different membranes synthesized in this work. The silicalite and Al-ZSM5 membranes were synthesized using the template-free method developed by Lin and co-workers,26,27 with NaOH (97%, Aldrich) used as the stabilizing cation and fumed

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ponent i over component j in a mixture, represented by Rij, is defined as

Table 2. Secondary Growth Conditions for Synthesis of MFI Membranes of Varying Composition; Me ) Substituted Metal membrane type

Si/Me

temperature (°C), duration (hr)

silicalite B-ZSM5

∞ 10 (B)

180, 8 180, 24

50 (B) 100 (B) Al-ZSM5

20 (Al)

180, 8

gel composition

811

ref

1.67SiO2/0.4NaOH/58.3H2O 11 1SiO2/0.077TPAOH/ 15 0.1B(OH)3/22.46H2O 1SiO2/0.077TPAOH/ 0.02B(OH)3/22.46H2O 1SiO2/0.077TPAOH/ 0.01B(OH)3/22.46H2O 38 6SiO2/1.4NaOH/200H2O/ 0.3NaAlO2

silica (particle size 0.007 µm, Aldrich) as the silica source. Table 2 lists the solution compositions for the membranes synthesized in this work. The working state for the MFI-type zeolites prepared in this work is in Na+ form. The silica source for the B-ZSM5 membranes was colloidial silica (Ludox AS40, 40 wt % in H2O). B-ZSM5 synthesis utilized TPAOH (1 M solution in H2O, Aldrich) as templating agent, which was required to obtain good quality B-ZSM5 membrane. In each case, clear solution and seeded supports were added to Teflon-lined stainless steel autoclaves for hydrothermal syntheses. The autoclaves were placed in an oven at the 180 °C for 8 h for silicalite and Al-ZSM5 membranes. The synthesis duration was 24 h for B-ZSM5 to obtain a membrane that was impermeable to He prior to calcination. Upon completion of the synthesis, the autoclave was cooled to room temperature; the membranes were removed from the autoclave and washed with distilled water. After they were dried for two days, the synthesis of the template-free membranes was complete. Those synthesized with a template were tested for He permeance and were found to be gastight prior to template removal; the experimental procedure is described elsewhere.23 The templated membranes were then calcined at 525 °C for 8 h at a heating/ cooling rate of 0.3 °C min-1. Characterization of membrane microstructure was evaluated by X-ray diffraction (XRD) (Bruker AXS-D8, Cu KR radiation). XRD measurements were performed by stepwise scanning (2θ step-size, 0.015°; 5° < 2θ < 45°). Thickness and morphology were examined by scanning electron microscopy (SEM) (Philips, FEI XL30). Elemental analysis was conducted using electron dispersive spectroscopy (EDS) (Philips, FEI XL30 EDAX, 15 kV). 2.2. Pervaporation Experiments. Single and binary pervaporation experiments were performed with pure component, PX (99%, Aldrich), OX (99%, Aldrich), and tri-isopropylbenzene (TIPB, 99%, Aldrich). The pervaporation apparatus used in this work was described in detail in a previous publication.25 The membrane was sealed in the vertical stainless steel cell with top layer facing upward. The liquid feed was contained in a reservoir above, while vacuum was applied to the downstream side. Permeate vapors were captured in liquid nitrogen cold traps and measurements were taken by weighing the cold trap before and after each run. In experiments with binary mixtures, a 1 µL sample was removed from the cold trap, and diluted in 10 µL of toluene (99.5%, Mallinckrodt). Compositional analysis was conducted using a gas chromatograph (6890N, Agilent) with mass spectrometer (5973N, Agilent) fitted with a DB-Waxetr capillary column (J&W Scientific-Agilent Technologies) using ultrahigh purity He as a carrier gas. Feed composition was evaluated in the same manner postrun in order to ensure that the component concentrations remained constant.9 Ideal selectivity was determined by taking the ratio of the pure component fluxes. The membrane selectivity for com-

Rij )

yi /yj xi /xj

(1)

where yi and yj are the mole fractions of components i and j in the permeate and xi and xj are the mole fractions of i and j in the feed. All measurements were performed at 25 °C and atmospheric pressure; sample times were 2 h each unless otherwise indicated. Membrane regeneration was performed between experiments by heat-treating at 150 °C for 24 h in a vacuum oven (Precision model 6500, Thermo Electron Corporation).18 3. Results and Discussion 3.1. Membrane Morphology, Microstructure, and Composition. Representative XRD spectra for the membranes synthesized in this work are shown in Figure 1 and confirm the presence of randomly oriented MFI-type zeolite film. SEM images of the membrane cross sections for silicalite and Al-ZSM5 given in Figure 2a and b show similar thickness (about 5 µm) and membrane morphology. Figure 2c shows the SEM cross-section of a typical B-ZSM5 membrane synthesized in this work. The thickness of the B-ZSM5 (25 µm) is approximately five times that of the silicalite and AlZSM5 (5 µm) with a very different morphology because of (1) the use of a templating agent and (2) longer synthesis times for a quality layer. Elemental analysis using EDS was conducted to confirm that Al and B were incorporated into the membrane microstructure. It was found that the as-synthesized membrane compositions varied from gel compositions. For the silicalite membranes, the Si/Al ratio found was ∼170, which indicates the incorporation of a significant amount of aluminum from the R-Al2O3 support. Likewise, the Al-ZSM5 membrane showed higher aluminum content with a Si/Al of 13 (gel composition of 20). Dissolution of the R-Al2O3 support into the synthesis solution where it is subsequently incorporated into MFI-type zeolite structure has been reported previously for membranes synthesized with the template-free method.23 For B-ZSM5, the final membrane composition Si/B was 20 despite the initial gel composition of Si/B of 10. Previous studies have commonly used in situ crystallization for B-ZSM5 membrane synthesis adapting the original method by Taramasso et al.28 Tuan et al.21 synthesized B-ZSM5 membranes with Si/B ratios of 12-50 both with and without NaOH. Similar to the results presented here, they reported a decreased B content in the MFI-type zeolite crystals

Figure 1. XRD spectra for silicalite, B-ZSM5, and Al-ZSM5; the astrisk denotes peak from R-Al2O3 support.

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Ind. Eng. Chem. Res., Vol. 49, No. 2, 2010 Table 3. Relative Size and Percent Changes in Bond Length and Angles from Unmodified Silicalite as Reported by Valerio et al.31a percent change substituted cation

relative size (Si/Me)

dMe-Si

Me-O-Si

dMe-O

Al B

0.82 1.83

1.3 0.3

-5.2 0.6

7.7 -0.6

a

Figure 2. SEM cross-section for MFI type zeolite membranes: (a) silicalite, (b) Al-ZSM5, and (c) B-ZSM5.

when compared to the synthesis gel composition.21,22 For a given synthesis, it appears that equilibrium is likely reached between the B concentration in the mother liquor and the zeolite, which favors reduced content in the MFI-type zeolite crystals. Tuan et al.22 also reported that the membranes with the highest n-/i-C4H10 selectivities had increased B content and were synthesized without Na as the framework stabilizing cation. The method generally used for synthesis of these membranes involves using a series of growth steps: one 24-48 h initial growth to form a crystallized layer and subsequent growths to form the continuous membrane.2,21,22,29 A natural result of such an approach is a very thick final membrane layer, ∼80-90 µm.21 In the present work, an initial seed layer was deposited, and a single hydrothermal growth formed a continuous mem-

Me ) substituted metal.

brane layer. As a result, much thinner B-ZSM5 membrane layer was obtained in this work with much shorter synthesis time. On the basis of the comparison of the XRD spectra reported previously and the current work for B-ZSM-5 zeolite membranes, there appears to be little difference in resulting membrane morphology.21,22 3.2. Performance of Al-ZSM5 and B-ZSM5 Membranes in Xylene Separation. The most desired modification in isomorphous substitution is to induce unit cell volume differences by adjusting T-O-T angles and T-O bond lengths (T ) Si) upon substitution in the hopes that the slight structural changes may enhance the rigidity of the MFI-type zeolite framework enough to prevent or mitigate the effects of PX adsorption. Al and B exhibit opposite effects on crystallite framework because of their size differences relative to Si; subsequent bond angle and length changes as compared to silicalite are listed in Table 3 as reported by Valerio et al.30 The values were calculated through their modeling of substituted zeolites using quantum mechanical calculations.30 When Al replaces smaller Si in the framework an overall expansion of ∼1.3% occurs as a result of the large increase in dSi-Al and dAl-O bond lengths. This is accompanied by a reduction in the Si-O-Al angle, which may help to induce framework rigidity.30 Temperature programmed desorption studies on the saturation capacity of PX in various zeolites conducted by Chen and Rees31 suggest that this is the case. They found that for Al-ZSM5 with a Si/Al ) 15 the saturation capacity of PX is between 5.96 and 5.76 muc, which is substantially less than that of silicalite (∼7.6 muc).31 A 1.11% contraction of the MFItype zeolite unit cell is induced by B substitution that is accompanied by slight expansion and decreased flexibility in the Si-O-B angle.32 The small size of B (as compared to Al) relative to Si limits the preferential sites in the crystal tetrahedra that it will occupy. In a study by Vetrivel,33 they found that B prefers to situate itself where the T-O distance is minimized, and the T-O-T bond angle is maximized, which tends to have the effect of reducing the diameter of the straight and sinusoidal channels. Figure 3 shows the multicomponent pervaporation results at 25 °C through Al-ZSM5 and B-ZSM5 membranes; silicalite is shown as a reference. For all membranes tested there was no selectivity improvement at very high concentrations (mass fraction >80% PX) of PX. As the amount of PX in the feed is reduced, an improvement in the selectivity for PX over OX is observed for the substituted membranes over silicalite. Al-ZSM5 demonstrates higher selectivity than silicalite until 5% of PX is reached, whereas B-ZSM5 shows substantial improvement for most compositions. Figures 4-6 show how the flux of each component changes with the mass fraction of PX in the feed. The data for each point was collected in two hour intervals starting from high PX concentration to low PX concentration, which was demonstrated as a reproducible technique in our previous publication.18 Overall, under most conditions the flux of PX decreases as the amount of PX in the feed decreases. Observation of these phenomena indicates that the improvement in selectivity at low

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Figure 3. Selectivity of B-ZSM5 (solid line), silicalite (dotted line), and Al-ZSM5 (dashed line) as a function of mass fraction of PX in feed; balance feed composition OX.

Figure 4. Component flux (PX, solid line; OX, dotted line) as a function of changing mass fraction of PX in the feed through silicalite; balance feed composition OX.

PX concentrations is not due to a gradual accumulation of PX in the membrane as experimentation continues but rather it is a consequence of the current conditions being experienced by the membrane. In the silicalite membrane (Figure 4), OX dominates the transport at almost every feed composition. At 5% PX in the feed, the smaller isomer is finally able to break through and permeates faster than OX. The question of whether or not OX is simply traveling through intercrystalline defects indicates a need to examine the flux behavior of the larger molecule. If the OX was traveling freely through the defects we would expect its flux to increase proportionally with the amount of OX in the feed because of an increase in the driving force across the membrane. In this case, an opposite trend occurs and the OX flux decreases as a function of its feed concentration. Additionally, it is known that the defect sealing is observed for these membranes at elevated loadings of PX; this sealing has been shown to be proportional to the degree of PX exposure.18 If a membrane possessed a significant number of defects, one would expect little to no selectivity at low concentrations of PX because this is where defect sealing effects are the minimal. Because the PX selectivity at 5% is ∼20, it can be safely determined the effects are a result of crystallographic changes rather than that of transport through intercrystalline gaps. For Al-ZSM5 (Figure 5) and B-ZSM5 (Figure 6), the PX permeates at a faster rate than OX regardless of the feed composition; which is not the case for silicalite as observed here and in previous work.18 Initially, both the Al-ZSM5 and

813

Figure 5. Component flux (PX, solid line; OX, dotted line) as a function of changing mass fraction of PX in the feed through Al-ZSM5; balance feed composition OX.

Figure 6. Component flux (PX, solid line; OX, dotted line) as a function of changing mass fraction of PX in the feed through B-ZSM5; balance feed composition OX.

B-ZSM5 membranes demonstrate approximately the same amount of PX flux (between 0.14-0.16 kg.m-2.hr-1) at 90% PX in the feed. Over time, the Al-ZSM5 membrane exhibits a steady increase in OX flux, which was not observed for either silicalite or B-ZSM5. This indicates that the flux behavior through this membrane is likely being affected by the presence of defect transport pathways. Molecular simulation studies of silicalite by Chempath et al.16 found that for single-component MX within the PARA configuration (full saturation), the molecules fill the intersections and the sinusoidal channels equally while the PX molecules preferred the intersections over the sinusoidal channels. PX molecules have slight diffusion through the straight channels but this is highly unfavorable. When mixtures of PX and MX were simulated in the PARA configuration, it was found that when PX occupies the intersections, the adsorption of MX in the sinusoidal channels is enhanced.16 Under saturated conditions, OX (which will behave similarly to MX) dominates the transport by occupying the majority of the sinusoidal channels, PX access to channel intersections is reduced, and it is left to travel in less favorable pathways (i.e., straight channels), which reduces the PX selectivity.18 The highlighted difference in transport behavior between the Al and B substituted MFI-type zeolite membranes and the silicalite membrane is the ability of PX to travel at a faster rate than OX under high loading conditions. It is believed this is a function of the cation presence in the substituted MFI-type zeolite structures. Specifically, for B-ZSM5, preferential sites for the B substitution have the effect of restricting the size of the sinusoidal channels, which limits movement in this direc-

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Figure 7. Selectivity of B-ZSM5 membranes with varying composition as a function of mass fraction of PX in the feed: Si/B ) 10 (solid line), Si/B ) 50 (dotted line), and Si/B ) 100 (dashed line).

tion.33 PX is still able to pack more efficiently than OX in the intersection sites (the last transport option available to OX) because of its smaller size, which increases it overall transport through the membrane and enhances the observed selectivity.33 3.3. Effects of Degree of Boron Substitution on Membrane Stability. To determine how the degree of substitution affects the MFI-type zeolite membrane performance for xylene separation via pervaporation, B-ZSM5 membranes with varying compositions were investigated. The degree of substitution into the MFI-type zeolite framework can be controlled by manipulating the SiO2/B2O3 molar ratio in the synthesis sol;34 this technique was applied in this work. The decrease in the volume of the unit cell is directly proportional to amount of B substituted in the framework;29 therefore, by decreasing the boron content, it is possible to observe changes in membrane performance as the structure is relaxed. The multicomponent pervaporation results for B-ZSM5 membranes with as-synthesized Si/B ratios of 10, 50, and 100 are shown in Figure 7. For each membrane the PX flux dominates the total flux as the OX flux decreases with increased OX in the feed, confirming the performance is not determined by flow through intercrystalline gaps or defects (data not shown). As the B content decreases (relaxes the membrane microstructure) the membrane selectivity (at feed compositions less than 65% PX) also decreases. At high feed concentrations of PX, each membrane remains OX selective and there is no improvement even when the maximum concentration is shifted from ∼93% to ∼83% PX. Similar behavior for B-ZSM5 membranes was found for vapor permeation studies conducted by Noble and co-workers. In a study comparing the xylene vapor separation performance of silicalite, Al-ZSM5, and B-ZSM5 membranes, it was found that B-ZSM5 (Si/B ) 100) exhibited the highest vapor permeation selectivity for PX with a value of 60, whereas the separation selectivities of silicalite and Al-ZSM5 were 4.6 and 5.5, respectively (50/50 feed, 2.1 kPa for each isomer, 400-450K).12 Similar to the findings of Xomeritakis et al.,8 the authors reported a drop in B-ZSM5 selectivity as the partial pressure of xylenes increased from 0.4 to 2.5 kPa.12 These results indicate that even under low loadings (vapor permeation conditions) the effect of PX framework distortion on the B-ZSM5 membrane was still observed. Our results show that increasing the B content of the MFI-type zeolite membrane can reduce the effect. 3.4. Steady-State Performance of Al-ZSM5 and B-ZSM5 Membranes in Xylene Separation. Despite observing high selectivities at low concentrations of PX it is known that these values are not stable over time. The initial low PX concentration

Figure 8. Selectivity of B-ZSM5 (solid line), silicalite (dotted line), and Al-ZSM5 (dashed line) as a function of time for multicomponent feed consisting of 5% PX, balance OX.

Figure 9. Component flux (PX, solid line; OX, dotted line) as a function of time through B- ZSM5 at a feed composition of 5% PX, balance OX. Table 4. Steady-State Pervaporation Performance Values for MFI-Type Zeolite Membranes Subjected to Feed Composition of 5% PX/95% OX at 25°C component flux (×10-3 kg m-2 h-1) selectivity (5% PX, membrane PX OX 95% OX) Silicalite Al-ZSM5-1 Al-ZSM5-2 B-ZSM5-1 (Si/B B-ZSM5-2 (Si/B B-ZSM5-3 (Si/B B-ZSM5-4 (Si/B

) ) ) )

10) 10) 50) 100)

3 7 7 4.7 6.5 5.4 7.0

28 143 156 108 134 288 160

2.38 0.89 1.23 5.66 5.41 3.22 1.04

in the feed prevents saturation of the membrane from occurring but as the loading of PX increases over time, OX has greater mobility and the selectivity of the membrane suffers.18 Figure 8 shows the results of pervaporation (5% PX/95% OX at 25°) through silicalite, Al-ZSM5 and B-ZSM5 membranes for approximately 96 h. Each membrane exhibits a dramatic decrease in selectivity with time, reaching a steady state value within the time frame investigated. Component flux data is shown for B-ZSM5 in Figure 9 and is representative of what was observed for all membranes. A reverse in the PX/OX selectivity is observed within the first 48 h and is accompanied by a drop in PX flux which is presumably where the PX loading in the membrane has reached a high enough level to induce the structural deformation required for OX transport within the MFI pore network as seen in previous work.18 Table 4 summarizes the steady-state performance values for MFI-type zeolite membranes. There appears

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to be no correlation between the total xylene flux and membrane thickness (recall, the B-ZSM5 membrane is two times as thick as the silicalite and Al-ZSM5). Given the apparent morphology differences (because of synthesis conditions), it is speculated that the B-ZSM5 membrane possesses intercrystalline pathways that run perpendicular to the support surface, which are providing effective “short-cuts” for molecular diffusion. This would be similar to what has been previously observed for oriented silicalite membranes.28 Overall, while the framework distortion could not be completely prevented using isomorphous substitution, the B-ZSM5 (Si/B ) 10) yielded the best steady-state results with a final selectivity of PX over OX of 5.7. While substitution with Al and B into MFI-type zeolite structure, both result in enhanced xylene isomer separation capability, the B-ZSM5 membrane performance is better than Al-ZSM5. By substituting trivalent cations B3+ and Al3+ into tetravalent positions occupied by the Si4+, the zeolite becomes more acidic. The Brønsted acid strength increases in the order silicalite < B-ZSM5 < Al-ZSM5, and this has the possibility of changing the adsorption behavior of the isomers within the framework.21 While no correlation between membrane performance and acidity of the substituted membranes was found in our work, it is known that the intracrystalline diffusivity of PX will change when acid sites are present.35 Interaction corresponding to hydrogen bonding between aromatic group of xylene and the hydroxyl group of the zeolites is enhanced at acid sites. Two specific sites in ZSM5 include the Bronsted bridge hydroxyl groups (SiOHAl) and adjacent silanol groups (SiOH); the former being stronger. Masuda and co-workers35 proposed that as a PX molecule travels from intersection to intersection its size will induce local pore distortions, which provide one form of diffusion resistance. Additionally, when the molecule happens upon an intersection that contains a SiOHAl group, the strong interaction will be held for a particular time, which provides additional resistance. This retention time results in a lower diffusivity for PX traveling through MFI-type zeolite with acid sites as compared to the Al-free silicalite. At low loadings in a substituted membrane PX molecules will preferentially adsorb on intersections that contain a substituted ion (for example, SiOHAl groups); only at higher loadings will molecules be located in intersections where these bridging groups do not exist.36 When Al replaces smaller Si in the framework an overall expansion of ∼1.3% occurs as a result of the large increase in dSi-Al and dAl-O bond lengths. This is accompanied by a reduction in the Si-O-Al angle, which may help to induce framework rigidity in the local area.30 These minor structural changes are only beneficial at lower loadings; as the PX in the framework is increased, intersections without resistance to distortion are filled, and the same detriment to the selectivity is observed. This believed to be the reason for the lack of performance stability found for B-ZSM5 and Al-ZSM5 membranes. 4. Conclusions MFI-type zeolite membranes with Al and B isomorphously substituted into the framework were synthesized and subjected to multicomponent xylene separation via pervaporation. It is found that by performing this substitution, slight changes to both surface chemistry and framework flexibility can be introduced. This has the effect of changing the interaction of the xylene molecules with the MFI structure which limits PX loading and diffusion pathway access to OX. The multicomponent pervaporation results shown here demonstrate that the microstructural

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changes enable better pervaporation performance over silicalite membrane for the B-ZSM5 and Al-ZSM5 membranes. There was no apparent effect of surface chemistry changes playing a substantial role in the competitive adsorption of either isomer despite differences in terms of acidity. The unit cell changes associated with Al and B substitution appeared to significantly enhance the ability of the MFI framework to resist deformation at high loadings of PX. The B-ZSM5 membranes demonstrated the highest selectivities for p-xylene under a wide range of feed compositions; the highest selectivity observed was ∼55 (feed, 5% p-xylene/95% o-xylene). This is higher than any previously reported xylene separation selectivity for pervaporation through MFI-type zeolite membranes. The stability of the membrane performance over silicalite was only moderately improved (RPX/OX ∼5.5, 5% PX/95% OX, at 96 h). Our work demonstrates that to substantially improve the performance of MFI-type zeolite membranes for pervaporation the fundamental interaction between the xylene isomers and the MFI framework needs to be improved upon. Acknowledgment The authors would like to thank the Department of Energy for their support (DE-PS36-03GO93007). Note Added after ASAP Publication: The version of this paper that was published online December 2, 2009 was missing several minor text changes. The corrected version was reposted to the Web December 7, 2009. Literature Cited (1) Lin, Y. S.; Kumakiri, I.; Nair, B. N.; Alsyouri, H. Microporous inorganic membranes. Sep. Purif. Technol. 2002, 31, 229–379. (2) Bowen, T. C.; Noble, R. D.; Falconer, J. L. Fundamentals and applications of pervaporation through zeolite membranes. J. Membr. Sci. 2004, 245, 1–33. (3) Brandini, S.; Jama, M.; Ruthven, D. Diffusion, self-diffusion, and counter diffusion of benzene and paraxylene in silicalite. Microporous Mesoporous Mater. 2000, 36-36, 283–300. (4) Hedlund, J.; Ohrman, O.; Msimang, V.; Van Steen, E.; Bohringer, W.; Sibya, S.; Moller, K. Synthesis and testing of thin film ZSM-5 Catalysts. Chem. Eng. Sci. 2004, 59, 2647–2657. (5) Wei, J. A mathematical theory of enhanced paraxylene selectivity in molecular sieve catalysts. J. Catal. 1982, 76, 433–439. (6) Ruthven, D. Principles of Adsorption and Adsorption Processes; Wiley: New York, 2004. (7) Caro, J.; Noack, M.; Kolsch, P. Zeolite membranes: From the laboratory scale to technical applications. Adsorption 2005, 11, 215–227. (8) Xomeritakis, G.; Tsapatsis, M. Permeation of aromatic isomer vapors through oriented MFI-type membranes made by secondary growth. Chem. Mater. 1999, 11, 875–878. (9) Gu, X.; Dong, J.; Nenoff, T. M.; Ozokwelu, D. E. Separation of p-xylene from multicomponent vapor mixtures using tubular MFI zeolite membranes. J. Membr. Sci. 2006, 280, 624–633. (10) Hedlund, J.; Sterte, J.; Anthonis, M.; Bons, A. J.; et al. High-flux MFI membranes. Microporous Mesoporous Mater. 2002, 52, 179–189. (11) Matsufuji, T.; Nishiyama, N.; Matsukata, M.; Ueyama, K. Separation of butane and xylene isomers with MFI-type zeolitic membrane synthesized by a vapor-phase transport method. J. Membr. Sci. 2000, 178, 25–34. (12) Gump, C. J.; Tuan, V. A.; Noble, R. D.; Falconer, J. L. Aromatic permeation through crystalline molecular sieve membranes. Ind. Eng. Chem. Res. 2001, 40, 565–577. (13) Bonilla, G.; Vlachos, D. G.; Tsapatisis, M. Simulations and experiments on the growth and microstructure of zeolite MFI films and membranes made by secondary growth. Microporous Mesoporous Mater. 2001, 42, 191–203. (14) van Koningsveld, H.; Tuinstra, F. The location of p-xylene in a single crystal of zeolite H-ZSM5 with a new, sorbate-induced, orthorhombic framework symmetry. Acta Crystallogr. 1989, B45, 423–431. (15) Mohanty, S.; McCormick, A. V. Prospects for principles of size and shape selective separations using zeolites. Chem. Eng. J. 1999, 74, 1–14.

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ReceiVed for reView June 10, 2009 ReVised manuscript receiVed September 29, 2009 Accepted November 18, 2009 IE900926T