Article pubs.acs.org/IECR
Pervaporation Separation of Organic Mixtures by MOF‑5 Membranes Amr Ibrahim and Y. S. Lin* School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, Arizona 85287-6006, United States ABSTRACT: Metal organic framework (MOF) membranes have received much attention for gas separation applications, with however limited information about their liquid separation properties. This paper reports a study of permeation and separation of liquid organics by a MOF-5 membrane in pervaporation mode. Multiple high quality MOF-5 membranes were reproducibly prepared by the secondary growth method for various experimental runs. The pervaporation of pure toluene, oxylene, and 1,3,5-triisopropylbenzene (TIPB) and the separation of their binary mixtures were studied. The permeation flux and separation factors decrease with pervaporation on-stream time and steady state permeation flux could not be reached even after 10 h of pervaporation. The fouling effects do not change the crystalline structure of the MOF-5 membrane. The pervaporation flux with the mixture feed is lower than the pure component flux, and the reduction in the flux decreases with decreasing affinity of the permeating species with MOF-5. The mixture maximum separation factors for toluene/TIPB and o-xylene/TIPB are respectively about 26.7 and 14.6, significantly higher than the pure component ideal separation factor. The fluxes and separation factors cannot be restored to their original values upon membrane activation at 100 °C in vacuum.
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in the flux for the two larger molecules, as summarized in Table 1. Also, it was found that MOF-5 membranes are structurally stable upon pervaporation of organic liquids.
INTRODUCTION Metal−organic frameworks (MOFs) are a group of crystalline, microporous materials consisting of metal ions linked together by organic ligands.1 One representative MOF is IRMOF-1 (or more commonly referred to as MOF-5) with a highly crystalline cubic structure consisting of a network of zinc oxide tetrahedra connected by terephthalic acid linkers creating inner cavities of about 12 and 15 Å in diameter and aperture opening of 8 Å in width.2 MOF-5 has been widely studied as adsorbents for gas storage, gas purification, and separation applications as well as heterogeneous catalysis.3 MOF-5 membranes were also prepared and studied for gas separation.4−6 However, MOF-5 is known to be unstable in humid air,7−9 which hinders its gasphase applications involving a trace amount of water vapor. MOF-5 may find applications in which the contact with the humid atmosphere is negated, such as adsorption10−12 and pervaporation of organic liquids.6,13 Recent studies have shown that MOF-5 can separate three pentane isomers10 and some aromatics of different molecular size.14 The adsorption strength of aromatics on MOF-5 decreases in the order p-xylene > ethylbenzene > toluene > benzene.14 Experimental data showed that a fixed-bed MOF-5 adsorber can separate a xylene isomer from ethylbenzene but not the three xylene isomers.15 The MOF-5 crystals are structurally stable in organic liquids. Lin and co-workers6 studied pervaporation of several organic liquids including some aromatics with molecular sizes close to the pore size of MOF-5 membranes. They found that the pervaporation fluxes for a 14 μm thick MOF-5 membrane decreases with increasing kinetic diameter of the permeating organic compounds, with a sharp (2 order of magnitude) drop © 2016 American Chemical Society
Table 1. Pervaporation Fluxes of Pure Organic Compounds through a MOF-5 Membrane6 permeating molecule p-xylene o-xylene tri-isopropylbenzene 1,3-di-tert-butylbenzene 2-dicyclohexyl-phosphino-2′-(N,Ndimethylamino)-biphenyl
kinetic diameter (Å)
flux (10−4 mol/(m2·s))
5.8 6.8 8.4 11 12
9.00 7.25 2.08 1.71 0.014
The data in Table 1 show molecular sieving separation characteristics of the MOF-5 membrane for the molecules with the size significantly larger than the aperture size of MOF-5 crystals. Molecules with a size smaller or slightly larger than the MOF-5 pore size can permeate through the membrane. Similar results were found for some of zeolitic imidazolate framework materials. For example, from crystallographic data, ZIF-8 pore opening is estimated to be 3.4 Å. However, several studies show that even molecules with a size larger than that of ZIF-8 pores, Received: Revised: Accepted: Published: 8652
May 21, 2016 July 16, 2016 July 21, 2016 July 21, 2016 DOI: 10.1021/acs.iecr.6b01965 Ind. Eng. Chem. Res. 2016, 55, 8652−8658
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Industrial & Engineering Chemistry Research
made macroporous α-alumina support (2 mm thick, 20 mm in diameter, average pore diameter, 120 nm; porosity, 40%) was immersed in the prepared MOF-5 suspension for 4 s then left to dry under vacuum at 50 °C for 6 h. For the secondary growth, 0.416 g of zinc nitrate hexahydrate and 0.088 g of terephthalic acid were added to 40 mL of DMF in a capped vial. Once completely dissolved, N-ethyldiisopropylamine, CH3CH2N[CH(CH3)2]2 (0.069 g, 99.5%, Acros Organic) was slowly added. After that, the seeded support, held vertically by a Teflon holder, was placed in the solution. The vial was then tightly capped and placed in an oil bath held at 130 °C for 3 h, then removed from the oil bath and left to cool to room temperature. The membrane was placed in chloroform for 2 h, then removed and dried under vacuum for 4 h at 50 °C. Second secondary growth was then conducted with the same procedure, and then the membrane was held in chloroform for 2 days. The membrane was dried under vacuum overnight and finally activated at 100 °C for 6 h to be ready for characterization and use. MOF-5 crystals and membranes were characterized by X-ray diffraction (XRD) for phase structure and crystallinity using a Bruker D8 ADVANCE X-ray diffractometer. Scans were run at room temperature from 5° to 40°, with 0.05° step using Cu Kα radiation (λ = 1.542 Å) at 40 kV and 40 mA. Scannine electron microscopy (SEM) (Philips FEI XL-30) was used to image the surface morphology and cross-section of the membranes. The steady state helium permeance was measured at room temperature for the alumina support and for the as synthesized MOF-5 membrane at various upstream pressures using the setup schematically indicated in the literature.13 The relative accuracy of the gas permeance reported in this work is about ±2.0%. 1.2. Membrane Liquid Permeation and Liquid Sorption Studies. Pervaporation tests were performed using the setup described in ref 13. The physical properties of the studied organic liquids are summarized in Table 2. A liquid was
such as CH4 (kinetic diameter 3.8 Å)16 and p-xylene (5.8 Å),17 can enter the ZIF-8 pore network. Caro and co-workers18 studied the pervaporation of n-hexane (4.3 Å), benzene (5.8 Å), and mesitylene (8.4 Å) liquids through a supported ZIF-8 membrane. Experimental results showed that both n-hexane and benzene could permeate through the ZIF-8 membrane, with the pure component ideal separation factor of 25 for nhexane/benzene. Mesitylene with much larger molecular size cannot permeate through the ZIF-8 membrane. It was suggested that the framework flexibility of MOF crystals allows penetration of the larger organic molecules into the pores.6,18 Mixture pervaporation of the large organic molecules through MOF membranes may provide useful data for gaining better insight into transport properties of large molecules in MOF materials. Caro and co-workers18 studied pervaporation of equimolar-binary n-hexane/benzene through the ZIF-8 membrane and found that the mixture separation factor is only 8.4 as compared to 23 for the ideal separation factor (based on pure species permeation). They suggested that the lower mixture separation factor was due to reduction in nhexane flux blocked by the larger, less mobile benzene in mixture pervaporation. However, no other organic liquid mixture separation data were reported to shed more light on mixture pervaporation separation by MOF membranes. It is important to study the mechanism of organic liquid pervaporation separation by MOF membranes as pervaporation separation is a molecular probing tool to characterize the quality of MOF membranes. Furthermore, if MOF membranes indeed show molecular sieving characteristics for organic liquids, they offer potential for use in pervaporation processes for separation of liquid organic mixtures. With the proven structural stability of MOF-5 membranes for pure organic liquid pervaporation as reported by Kasik and Lin,13 the objective of this work was to understand the pervaporation separation mechanism of large hydrocarbon molecules by MOF-5 membranes via an experimental study on pervaporation separation of mixture aromatics (toluene, o-xylene, and 1,3,5triisopropylbenzene) with MOF-5 membranes.
Table 2. Summary of the Properties of the Studied Pervaporation Liquids
1. EXPERIMENTAL SECTION 1.1. Synthesis and Characterization of MOF-5 Powder and Membranes. MOF-5 seeds were synthesized via solvothermal synthesis procedures adopted by Lin and coworkers.4,6,13 Zinc nitrate hexahydrate, Zn(NO3)2·6H2O (1.664 g, 99%, Sigma-Aldrich) and terephthalic acid, C6H4(CO2H)2(0.352 g, + 99%, Acros Organics) were added to a vial containing N,N-dimethylformamide, DMF, HCON(CH3)2(40 mL, 99.9%, Alfa Aesar). Once the precursors were fully dissolved, the vial was tightly capped and immersed in an oil bath held at 130 °C. After 3 h the vial was removed from the oil bath, and allowed to cool naturally to room temperature. After that, DMF was decanted and the crystals were kept immersed in chloroform, CHCl3 (99.8%, Alfa Aesar) for 2 days. Eventually, the crystals were dried under vacuum overnight and activated at 100 °C for 6 h. The produced MOF-5 crystals were reduced in size by ball milling (Across International PQ-N04 Planetary) in a Teflon container charged with chloroform and alumina balls at 200 rpm for 4 h. The ball-milled seeds after drying overnight were added at a concentration of 2 wt % to DMF and ultrasonically agitated to prepare a stable MOF-5 suspension. MOF-5 membranes were synthesized via the dip-coating and secondary growth method. The polished surface of a home-
toluene [TOL]
o-xylene [OX]
1,3,5-triisopropylbenzene [TIPB]
99.8% SigmaAldrich 92.14
98%, SigmaAldrich 106.17
95% Alfa Aesar
−95 0.556
−25.2 0.756
−7.4 3.4
28.4
6.60
0.0416
5.8
6.8
8.4
molecules grade molecular weight (g/mol) melting point, °C viscositya (cp) at 25 °C vapor pressureb, mmHg, 25 °C kinetic diameterc, Å
204.35
a
Viscosity information for toluene and o-xylene from ref 20, that for TIPB from ref 6. bVapor pressure for toluene and o-xylene from ref 21, that for TIBP from ref 22. cKinetic diameter information for toluene, o-xylene, TIPB from ref 23.
fed to the upstream membrane surface, and the permeate as vapor was collected in the cold trap for flux calculation. Pervaporation runs were conducted using pure organic liquids and equal mass composition binary mixtures. The first set of pervaporation runs were conducted for 10 h, and then the membrane was dried under vacuum overnight, followed by activation in vacuum at 100 °C for 6 h to remove organics in 8653
DOI: 10.1021/acs.iecr.6b01965 Ind. Eng. Chem. Res. 2016, 55, 8652−8658
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Industrial & Engineering Chemistry Research the membrane pores prior to the second set of pervaporation runs. XRD scans were done in between pervaporation/ activation cycles to assess any change in membrane structure. Sorption of liquid organics on MOF-5 was measured by the static method. About 0.5 g of activated MOF-5 crystals was added to about 1 g of each of the three liquid mixtures in vials that were used as the feed in pervaporation experiments. The liquid concentrations in the vials, before and after they were firmly closed and airtight for 24 h at room temperature, were measured to calculate adsorbed amounts. Compositional analysis in binary pervaporation and liquid sorption studies was conducted using a gas chromatograph (Agilent Technologies, 7890A) equipped with a flame ionization detector and fitted with Agilent J&W HP-5 GC capillary column (30 m long, 0.32 mm diameter, 0.25 μm film). Ultrahigh purity helium was used as the carrier gas. The flux of a pure component was calculated on the basis of the molar mass of the organic liquid for that species collected in the cold trap, the permeation area of the membrane, and permeation time. The binary mixture separation factor is calculated from the ratio of the molar fraction in the permeate to that in the feed.19 The accuracy is ±3.0% for the permeation flux of pure components, and ±3.8% for binary mixture separation data.
Figure 2. SEM images of as-synthesized MOF-5 membrane [A,B] surface morphology, [C] cross-sectional view, and [D] membrane morphology after toluene pervaporation.
2. RESULTS AND DISCUSSION 2.1. Membrane Characteristics. The XRD patterns of the synthesized MOF-5 powder and membrane are given in Figure 1. The typical dominant peaks for randomly oriented MOF-5
Figure 3. Steady state helium permeance versus average pressure of produced α-alumina support and MOF-5 membrane.
The membrane helium permeance is about 3.5 × 10−7 mol/(m2 s Pa). The permeance for the MOF-5 top layer was calculated by adopting the resistance-in-series model with the help of support permeation data. Being independent of pressure, the MOF-5 top layer permeance indicates that the membrane is free of large defects or pinholes. Permeance for different gases through the membranes is plotted against the reciprocal of the square root of its molecular weight in Figure 4. The data can be correlated by a straight line going through the origin, indicating that the permeation is governed by Knudsen-like diffusion through microspores (typical pore diameter range: 1−10 nm) rather than viscous flow (>100 nm) through large defects, further confirming good quality of the membranes.4 As will be shown next, multiple MOF-5 membranes are needed for permeation and separation tests because of irreversible fouling of the MOF-5 membrane by permeating molecules after each pervaporation experiment. For this work, a total of seven MOF-5 membranes were synthesized under the same synthesis conditions. The helium permeance of these membranes was measured, and the results are given in Table 3. These seven MOF-5 membranes have an average helium permeance of 3.9 × 10−7 mol/(m2 s Pa) with ±14% variation in
Figure 1. XRD pattern of as synthesized MOF-5 membrane on αAl2O3 support.
film match well with the published XRD pattern for MOF-5.2 Figure 2 shows SEM images of the top view and cross-section of the membrane. The surface of the membrane looks homogeneous with no visible cracks, pinholes, or other defects. The neat cubic crystals indicated in Figure 2B range in size from 5 to 10 μm. The cross-section view of the membrane indicates a sufficiently thin MOF-5 top- layer of 14 μm (Figure 2C). This continuous top film is densely grown and adheres strongly to top of the α-Al2O3 support. These results replicate what was reported previously in the literature6,13 although secondary growth was conducted twice here in this work to ensure high quality of the MOF-5 membrane. The quality of the MOF-5 membrane was further tested using gas permeation data. Steady state helium permeance data for the alumina support and the alumina supported MOF-5 membrane at various upstream pressures are given in Figure 3. 8654
DOI: 10.1021/acs.iecr.6b01965 Ind. Eng. Chem. Res. 2016, 55, 8652−8658
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After 10 h on-stream pervaporation, the flux decreased to around 17.00 × 10−4 mol/(m2 s), about 25% reduction compared to the initial flux, and the steady state flux was not reached, as shown in Figure 5. Kasik and Lin13 reported a decline of the p-xylene pervporation flux through the MOF-5 membrane, and the flux reached a steady state after about 18 h of on-stream pervaporation. When the MOF-5 membrane is in contact with toluene during the pervaporation experiment, the microspores are gradually filled by the toluene molecules. It is possible that some toluene molecules might get trapped in the pores and become immobile, which limits effective passage of other toluene molecules causing the reduction in pervaporation flux. Amirjalayer et al.,24 in their molecular dynamics simulation of benzene diffusion in MOF-5, have shown that the benzene molecules accumulate in distinct binding sites or “pockets” in the corners of the MOF-5 unit cells. The MOF-5 membrane after the first round of pervaporation was dried in vacuum overnight and activated in vacuum at 100 °C for 6 h, then another set of toluene pervaporation runs was conducted on the membrane. The results are also given in Figure 5. The initial flux increased after the activation but did not reach the initial flux of the fresh membrane, and a continuous flux decline was noticed as a function of time. For many membranes, activation in vacuum may allow for a complete removal of retained molecules and flux could return to its initial state.25 However, this was not observed in our MOF-5 membrane, which indicates that the membrane fouling was not fully reversible in the MOF-5 membrane. Kasik and Lin13 reported that p-xylene initial flux could not be fully restored even after 48 h of membrane activation at 100 °C. Retention of xylene isomers in the MOF-5 crystals after activation was also confirmed using Fourier transform infrared spectroscopy in our previous study.13 It is possible that the retained xylene molecules could be removed from the MOF-5 membrane through activation at higher temperatures in vacuum, like zeolite membranes. However, MOF materials are generally less thermally stable than zeolites, so high temperature activation was not conducted in this work because of the concern about the thermal stability of the MOF-5 membrane layer. The XRD patterns of the membrane after toluene pervaporation/activation cycles are shown in Figure 6. The αalumina diffraction peaks remain unchanged. All XRD spectra
Figure 4. Permeance of simple gases through the as-synthesized MOF5 membrane.
permeance, indicating high reproducibility of membrane synthesis. Table 3. Summary of Helium Permeance of MOF-5 Membrane Samples Synthesized in This Study for Chacterization and Pervaporation Studies membrane
He permeance ×10−7 mol/(m2sPa)
MEM-A MEM-B MEM-C MEM-D MEM-E
3.5 3.8 3.5 3.9 4.8
MEM-F
4.4
MEM-G
3.4
usage in this work XRD−SEM−gas characterization toluene pervaporation o-xylene pervaporation TIPB Pervaporation toluene/TIPB mixture pervaporation o-xylene/TIPB mixture pervaporation toluene/o-xylene mixture pervaporation
2.2. Pure Component Pervaporation and Fouling. The organic liquid flux through the as-synthesized MOF-5 membrane was found to decrease with pervaporation onstream time, as shown in Figure 5 for toluene. The MOF-5 membrane initially has a toluene flux of 23.6 × 10−4 mol/(m2s).
Figure 5. Pervaporation fluxes of toluene through as-synthesized MOF-5 membrane and after membrane activation, and o-xylene and TIPB fluxes on as-synthesized membranes.
Figure 6. XRD spectra of synthesized MOF-5 membrane after toluene pervaporation/activation cycle. 8655
DOI: 10.1021/acs.iecr.6b01965 Ind. Eng. Chem. Res. 2016, 55, 8652−8658
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Industrial & Engineering Chemistry Research show no discernible difference in peak location, indicating MOF-5 remains in the same structure. However, using the αalumina diffraction peak as reference, the diffraction peak intensity for MOF-5 decreases after pervaporation. Membrane activation could restore peak intensity but not to the value of the as-synthesized membrane. This indicates that the membrane activation could not remove all the trapped toluene molecules. Hafizovic et al.26 reported that MOF-5 pore filling effects are responsible for the pronounced variations in XRD peak intensities. Also, the SEM image in Figure 2D after toluene pervaporation shows the same morphology and microstructure compared to the as-synthesized MOF-5 membrane. The pervaporation fluxes versus time for o-xylene and TIPB on as-synthesized MOF-5 membranes are also given in Figure 5. Again o-xylene and TIPB exhibit a decrease in the pervaporation flux as a function of time similar to that described for toluene. The molecules smaller than 8 Å, such as toluene (5.8 Å) and o-xylene (6.8 Å), can pass through the MOF-5 membrane easily at high fluxes. The molecules with size close to the MOF-5 pore, such as TIPB (8.4 Å) can still pass through the intracrystalline microspores of the MOF-5 membrane at a certain value of flux due to the MOF-5 framework flexibility. The MOF-5 membranes prepared in this work are of similar quality as those reported in previous studies for MOF-5 membranes6,13 as reflected by the consistent pervaporation flux (8.75 × 10−4 mol/(m2 s) and 2.21 × 10−4 mol/(m2 s) for o-xylene and TIPB on fresh membrane in this work versus 5.3−7.3 × 10−4 mol/(m2 s) and 2.0−2.4 × 10−4 mol/(m2 s) in the previous work6,13). The permeation fluxes measured in this work are larger than benzene pervaporation flux through a smaller pore MOF (ZIF-8) (6 × 10−5 mol/(m2 s)).18 Permeation through a dense polymer membrane is generally described by the solution-diffusion model with the vapor 27 pressure difference (Psat Dividing i − Pi) as the driving force. the permeation flux by the vapor pressure difference gives an initial permeance of 6.24, 9.94, and 398 (10−7 mol/(m2 s Pa)) for toluene, o-xylene, and TIPB. The permeance for TIPB appears very large and cannot be explained from its large molecule size. It was also suggested that the large molecule may permeate through the membrane defects via a viscous flow mechanism with flux inversely proportional to viscosity.28 The flux multiplied by the viscosity for each of the components does not yield a constant value in this work. Our gas permeation data show that the membranes are of high quality. Thus, the mechanism of pervaporation of these molecules through the MOF-5 membranes is not clear at this stage. 2.3. Mixture Pervaporation. Figure 7 shows pervaporation fluxes and separation factors for three 50:50 (mass composition) binary mixtures (toluene/TIPB, o-xylene/TIPB, and toluene/o-xylene) through three respective MOF-5 membranes versus permeation time. Both permeation flux and separation factor decrease with pervaporation time for all three mixtures. The reduction in permeation flux due to membrane fouling is consistent with the pure component permeation data described in section 2.2. The reduction in the separation factor as a function of time could be explained by the gradual blockage of the membranes microspores though which selective flow could be achieved. These membranes after initial rounds of pervaporation separation tests with results shown in Figure 7 were activated to remove residual organics and the pervaporation separation
Figure 7. Pervaporation separation of 50:50 binary mixtures through MOF-5 membranes: [I] toluene/TIPB, [II] o-xylene/TIPB, and [III] toluene/o-xylene.
results for these membranes after the activation are compared with the fresh membranes in Figure 8. As shown, the initial fluxes of the activated membranes could not be restored to the values for the as-synthesized membranes under the used activation conditions. Both pervaporation flux and separation factor for the activated membranes decrease with permeation time. The presence of entrapped molecules within the structure makes the hosting cavity and possibly also adjacent cavities inaccessible and thus efficiently reduces the pore volume of the membrane. Furthermore, the secondary interactions between retained molecules and those passing through the membrane could reduce the permeation flux. The fouling by the permeating molecules impedes the permeation and separation performance of the membrane. However, XRD analysis of the membranes after the binary mixture pervaporation and activation cycles shows that the crystalline structure of the MOF-5 membrane remained unchanged, the same as noticed with the toluene pervaporation in Figure 6. 8656
DOI: 10.1021/acs.iecr.6b01965 Ind. Eng. Chem. Res. 2016, 55, 8652−8658
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Table 4. Comparison of Initial Flux and Separation Factor for Pure Component and Binary Mixture Pervaporation pervaporation flux, ×10−4 (mol m−2 s−1) species
pure
mixture
purea
mixture
toluene/TIPB
toluene TIPB o-xylene TIPB toluene o-xylene
23.6 2.21 8.75 2.21 23.6 8.75
11.7 0.190 7.55 0.270 11.0 8.50
10.7
27.7
o-xylene/TIPB toluene/o-xylene a
separation factor
mixture system
3.96 2.70
14.6 1.09
Ratio of permeation flux of component to the other.
it is clear that the reduction in permeation flux by the presence of the second component decreases in the order TIPB > toluene > o-xylene. The equilibrium mass fractions for the (50:50) binary mixtures in the adsorption experiments are 49:51 (toluene/TIPB), 48.5:51.5 (o-xylene/TIPB), and 50.5/ 49.5 (toluene/o-xylene). This indicates that the adsorption affinity for MOF-5 decreases in the order: o-xylene > toluene > TIPB. The pervaporation results showed that in the binary mixture the permeation flux for the more adsorptive component is closer to the flux of the pure component. For the binary mixture of toluene/TIPB and o-xylene/TIPB, the more adsorptive component appears to inhibit permeation of TIPB. Thus, the mixture separation factor is larger than the ideal separation factor. The kinetic diameter of the permeating species also affects the separation results. The higher total flux and separation factor in the toluene/TIPB mixtures as compared to that for the o-xylene/TIPB mixture is attributed to the smaller kinetic diameter of toluene as compared to that of o-xylene. The absence of the large TIPB allowed for higher pervaporation flux in the toluene/o-xylene mixture. Experiments with the pure components of toluene and oxylene showed significant mass transfers for both species, and toluene flux was three times higher than that for o-xylene. For pure components, toluene is expected to permeate faster than o-xylene because of a smaller kinetic diameter and higher saturation pressure. However, in the real mixture the separation factor was almost 1. This nonselective pervaporation property of the toluene/o-xylene binary mixtures is attributed to the higher affinity of o-xylene to MOF-5 and to the fact that toluene and oxylene can flow through the MOF-5 membrane microspores which are nonselective for either of the two liquids.
Figure 8. Pervaporation separation of 50:50 binary mixtures through as synthesized MOF-5 membranes [A], and after membrane activation [B], for the binary mixtures: [I] toluene/TIPB, [II] o-xylene/TIPB, and [III] toluene/o-xylene.
The initial fluxes and separation factor for these binary mixtures are compared with the pure permeation (and ideal separation) data in Table 4. It should be pointed out that these pervaporation and separation data were measured on fresh assynthesized membranes of different samples. However, Table 3 shows that all these membranes have similar transport resistance (with variation within ±14%). Thus, the difference in pervaporation fluxes between the binary mixture (one membrane sample) and pure component (two different membrane samples) should be due to the different transport properties of permeating molecules and effects of the intermolecular interaction in the case of the binary mixture permeation. As shown in Table 4, in all cases, the permeation flux in the binary feed is lower than that of the pure component feed. This is due to reduced driving force for the component in the mixture as compared to that for the pure component. However,
3. CONCLUSIONS MOF-5 membranes can be reproducibly prepared by the secondary growth method under controlled experimental conditions. The results of pervaporation permeation and separation of pure and binary mixture of toluene, o-xylene, and 1,3,5-triisopropylbenzene show a continuous decrease of permeation flux and a separation factor with pervaporation onstream time due to presence of immobile permeating molecules in the MOF-5 framework. However, MOF-5 is structurally stable in organic liquids. The pervaporation flux with the mixture feed is lower than the pure component flux, and the reduction in the permeation flux decreases with decreasing affinity of the permeating species with MOF-5. The mixture separation factor for toluene/TIPB and o-xylene/TIPB mixture is significantly higher than the pure component ideal separation 8657
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Two Zn−Terephthalate Metal−Organic Frameworks. J. Phys. Chem. C 2010, 114 (1), 311−316. (16) Zhou, W.; Wu, H.; Udovic, T. J.; Rush, J. J.; Yildirim, T. QuasiFree Methyl Rotation in Zeolitic Imidazolate Framework-8. J. Phys. Chem. A 2008, 112 (49), 12602−12606. (17) Peralta, D.; Chaplais, G.; Simon-Masseron, A.; Barthelet, K.; Chizallet, C.; Quoineaud, A.-A.; Pirngruber, G. D. Comparison of the Behavior of Metal−Organic Frameworks and Zeolites for Hydrocarbon Separations. J. Am. Chem. Soc. 2012, 134 (19), 8115−8126. (18) Diestel, L.; Bux, H.; Wachsmuth, D.; Caro, J. Pervaporation Studies of n-hexane, Benzene, Mesitylene and Their Mixtures on Zeolitic Imidazolate Framework-8 Membranes. Microporous Mesoporous Mater. 2012, 164 (0), 288−293. (19) Wijmans, J. G.; Baker, R. W. A simple predictive treatment of the permeation process in pervaporation. J. Membr. Sci. 1993, 79 (1), 101−113. (20) Al-Kandary, J. A.; Al-Jimaz, A. S.; Abdul-Latif, A.-H. M. Viscosities, Densities, and Speeds of Sound of Binary Mixtures of Benzene, Toluene, O-xylene, M-xylene, P-xylene, and Mesitylene with Anisole at (288.15, 293.15, 298.15, and 303.15) K. J. Chem. Eng. Data 2006, 51 (6), 2074−2082. (21) Pitzer, K. S.; Scott, D. W. The thermodynamics and molecular structure of benzene and its methyl derivatives1. J. Am. Chem. Soc. 1943, 65 (5), 803−829. (22) Verevkin, S. P. Thermochemical properties of iso-propylbenzenes. Thermochim. Acta 1998, 316 (2), 131−136. (23) Gu, X.; Dong, J.; Nenoff, T. M.; Ozokwelu, D. E. Separation of P-xylene From Multicomponent Vapor Mixtures Using Tubular MFI Zeolite Mebranes. J. Membr. Sci. 2006, 280 (1−2), 624−633. (24) Amirjalayer, S.; Tafipolsky, M.; Schmid, R. Molecular Dynamics Simulation of Benzene Diffusion in MOF-5: Importance of Lattice Dynamics. Angew. Chem., Int. Ed. 2007, 46 (3), 463−466. (25) Faibish, R. S.; Cohen, Y. Fouling and Rejection Behavior of Ceramic and Polymer-Modified Ceramic Membranes for Ultrafiltration of Oil-in-Water Emulsions and Microemulsions. Colloids Surf., A 2001, 191 (1−2), 27−40. (26) Hafizovic, J.; Bjørgen, M.; Olsbye, U.; Dietzel, P. D. C.; Bordiga, S.; Prestipino, C.; Lamberti, C.; Lillerud, K. P. The Inconsistency in Adsorption Properties and Powder XRD Data of MOF-5 isRationalized by Framework Interpenetration and the Presence of Organic and InorganicSpecies in the Nanocavities. J. Am. Chem. Soc. 2007, 129 (12), 3612−3620. (27) Wijmans, J. G.; Baker, R. W. The solution-diffusion model: a review. J. Membr. Sci. 1995, 107 (1−2), 1−21. (28) Kasik, A.; James, J.; Lin, Y. S. Synthesis of ZIF-68 Membrane on a ZnO Modified α-Alumina Support by a Modified Reactive Seeding Method. Ind. Eng. Chem. Res. 2016, 55 (10), 2831−2839.
factor. Fouling is a critical issue that needs to be addressed for application of MOF membranes for pervaporation separation of organic liquids.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support of NSF (CBET1160084) on this project. Amr Ibrahim is grateful for the scholarship provided by the Egyptian Government through the Cultural Affairs and Missions Sector.
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DOI: 10.1021/acs.iecr.6b01965 Ind. Eng. Chem. Res. 2016, 55, 8652−8658
