Polyimide Mixed Matrix Membranes

Article pubs.acs.org/IECR

Surface Cross-Linking of ZIF-8/Polyimide Mixed Matrix Membranes (MMMs) for Gas Separation Sumudu N. Wijenayake, Nimanka P. Panapitiya, Saskia H. Versteeg, Cindy N. Nguyen, Srishti Goel, Kenneth J. Balkus, Jr., Inga H. Musselman, and John P. Ferraris* Department of Chemistry, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States S Supporting Information *

ABSTRACT: The demand for cost-efficient separations requires membranes with high gas flux and high selectivity which opens the path for further improvements. Mixed matrix membranes (MMMs) made from 33.3 wt % ZIF-8 in 6FDA-durene were tested at 35 °C and 3.5 atm. At 33.3 wt % loading of ZIF-8, H2, N2, O2, and CH4 gas permeabilities increased approximately 400%. Cross-linking the surface of this MMM, by reacting with ethylenediamine vapor, yielded a 10-fold increase in H2/CO2, H2/N2, and H2/CH4 selectivities with respect to 6FDA-durene, preserving 55% of the H2 permeability of 6FDA-durene. The permselective properties of the cross-linked skin of the MMM fall above the most recent permeability−selectivity trade-off lines (2008 Robeson upper bounds) for H2/CO2, H2/N2, and H2/CH4 separations. To the best of our knowledge, this is the first example of a cross-linked ZIF/polymer MMM for gas separation.

1. INTRODUCTION Membrane based gas separation1−5 has gained attention due to low capital cost, reliability, and high energy efficiency6 compared to other customary methods such as pressure swing adsorption7 and cryogenic distillation.8 Some of the main commercially relevant separations include oxygen enrichment of air (O2/N2), hydrogen recovery from the ammonia purge gas (H2/N2) in the Haber−Bosch process, natural gas sweetening (CO2/CH4), and hydrogen separation for fuel cell applications (H2/CO2).9 The membranes used for gas separation can be categorized as (1) polymeric, (2) inorganic, (3) supported liquid, and (4) mixed matrix hybrids.10 The challenge faced by current membranes for gas separation is to obtain simultaneously a high gas selectivity and a high permeability.1 Polymeric gas separation membranes are inexpensive and easy to fabricate but are limited by the trade-off between permeability and selectivity.1 In contrast, porous inorganic membranes such as zeolites offer high gas flux and high selectivities, in addition to high chemical and thermal stability, but possess poor mechanical properties and high manufacturing costs.11 Mixed matrix membranes (MMMs) comprising porous inorganic materials dispersed in a polymer matrix can combine the favorable properties of both inorganic and polymer membranes.12−15 Different additives have been used in the fabrication of MMMs including zeolites,16,17 carbon molecular sieves,18−20 activated carbon,21 carbon nanotubes,22 mesoporous materials,23−25 and most recently, metal− organic frameworks (MOFs).26−31 One of the main challenges in the fabrication of MMMs is to obtain good interfacial contact between the continuous polymer phase and inorganic particles.12,15 Poor polymer−particle contact leads to nonselective voids at the interface, causing a loss of selectivity.12,32 Several approaches have been developed to improve interfacial properties of zeolite-containing MMMs. Koros and coworkers used silanation to increase zeolite−polymer adhesion.16 Other approaches include the use of a compatibilizer and creation of surface roughness on the particle.33 Although these modifications © XXXX American Chemical Society

improved the interfacial properties, a significant permselective enhancement was not observed. This result may reflect, in part, the relatively low loading of the additive used in the polymer. Therefore, new strategies are required to improve the interfacial adhesion of the polymer and the inorganic phase as well as to increase the loading of the additive. MOFs are a novel class of highly porous materials that offer tunable pore size and chemical functionality.34 The presence of organic linkers in MOFs results in better compatibility with organic polymers as compared to those in other additives, such as zeolites, which enables the addition of a higher loading of additives in MMMs.29 Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs which have structures similar to zeolites.35,36 The gas separation properties of several pure ZIF membranes37−41 and, recently, a number of ZIF−polymer mixed matrix membranes have been reported,42,43 with ZIF-8 being the most widely used filler. Ordoñez et al. incorporated ZIF-8 into Matrimid where a transition from a polymer-dominant to a ZIF-8 controlled gas transport was observed at 50% (w/w) ZIF-8 loading, with increased H2/CO2, H2/CH4, and CO2/CH4 gas selectivity over Matrimid.29 Diaz and co-workers added up to 30% (w/w) ZIF-8 to poly(1,4-phenylene ether−ether sulfone) (PEES) and observed increased permeability for all gases with improved H2/N2 and O2/N2 selectivity.44 Basu et al. fabricated MMMs using ZIF-8, HKUST-1, and MIL-53 in Matrimid for CO2/CH4 and CO2/N2 separations.45 Zornoza and co-workers embedded ZIF-8 in polysulfone (PSF) and tested gas permeation for H2/CH4, CO2/ CH4, and CO2/N2 separations.46 Zhang et al. explored ZIF-8/ 6FDA-DAM MMMs and observed a significantly improved propylene/propane separation as compared to the case of the unfilled polymer.47 Dai et al. reported improved CO2/N2 separation Received: January 14, 2013 Revised: April 22, 2013 Accepted: April 25, 2013

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dx.doi.org/10.1021/ie400149e | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 1. Chemical structures of 6FDA-durene and PDMC.

in ZIF-8/Utlem 1000 hollow fiber membranes.48 Song and coworkers observed increased H2, N2, O2, CO2, and CH4 permeabilities with essentially no change in gas selectivity in ZIF-8/ Matrimid MMMs.49 Recently, Yang et al. showed increased H2 permeability in ZIF-8/polybenzimidazole (PBI) MMMs without a substantial reduction in H2/CO2 selectivity.50 The different gas transport properties reported for the same filler−polymer combination demonstrate that the fabrication method may play an important role in membrane performance. Apart from ZIF-8, few other ZIFs have been used in producing MMMs. Bae et al. demonstrated enhanced CO2/CH4 selectivity and CO2 permeability in ZIF-90/6FDA-DAM MMMs, due to the good matching of the transport properties of the polymer and sieve.30 Seoane and co-workers obtained an increased O2/N2 selectivity in ZIF-20/PSF MMMs.51 Recently Yang et al. produced ZIF-7/PBI MMMs with improved H2/CO2 permeability and H2 selectivity which surpassed the Robeson upper bound.52 Although numerous research groups have studied ZIF-containing MMMs, there are only a few published studies on cross-linked MMMs for gas separation. Koros’ group synthesized cross-linkable copolyimides and cross-linked them through the pendant carboxylic group using diols, for CO2/CH4 separations.53,54 The same research group fabricated MMMs containing zeolite SSZ-13 and the crosslinkable polyimide, PDMC (Figure 1), which was cross-linked thermally at temperatures above 150 °C.16 Recently, polymers of intrinsic microporosity (PIM) and polybenzimidazole (PBI) have been cross-linked using azide55 and dichloride cross-linkers, respectively.5 In the pioneering work of Shao and coworkers, the surface of 6FDA-durene (Figure 1) membranes were cross-linked with ethylenediamine (EDA) vapor, which resulted in a very high selectivity of 102 for the H2/CO2 separation at room temperature.56 Following Hayes at Dupont, who first reported on cross-linking polyimides using diamines,57 a number of studies have been conducted on surface modification of polyimide membranes using diamines of different lengths.5,58−64 After EDA vapor modification, the hydrogen permeability of 6FDA-durene membranes decreased from 600 to 32 barrers.56 The H2/CO2 selectivity of 102 is one of the best reported for a cross-linked polymer, but the hydrogen permeability is too low to be commercially viable.65 To remediate this loss in gas permeability, we added ZIF-8 to 6FDA-durene to increase gas permeability and then cross-linked the surface of this mixed

matrix membrane to increase selectivity for H2/CO2, H2/N2, H2/CH4, and O2/N2 separations. ZIF-8 is commercially available as Basolite Z1200 (Sigma-Aldrich) and is the most studied ZIF to date. In ZIF-8, methylimidazolate (MIm) ligands coordinate to Zn(II) ions through the N atoms forming a tetrahedral framework with sodalite topology.35 ZIF-8 features a small pore aperture of 0.34 nm,66 chemical and thermal stability (>400 °C), high surface area (1630 m2/g),35 and a hydrogen storage capacity of 4.2 wt %.67 A recently reported pure ZIF-8 membrane exhibited a H2 permeance of 0.604 × 10−7 mol cm−2 s−1 Pa−1 and an ideal H2/CO2 selectivity of 4.5.68 6FDA-based polyimides provide a robust polymer matrix for the incorporation of a porous additive.16,30 In particular, 6FDA-durene is a thermally stable polymer with a high H2 permeability of 600 barrers56 and a glass transition temperature (Tg) of 422 °C69 and has been well studied in dense membrane69,70 and asymmetric hollow fiber71,72 forms. In this study, we have fabricated MMMs adding 33.3 wt % (or 50% w/w) nanocrystalline ZIF-8 to 6FDA-durene to increase gas permeability. The surfaces of these MMMs are then cross-linked by reacting with EDA vapor to improve gas selectivity, and the gas transport properties of the MMMs before and after EDA cross-linking are studied. The effect of thermal treatment (up to 150 °C) of the cross-linked MMM on H2/CO2 separation is also investigated. The resistance model is applied to obtain the permselective properties of the crosslinked skin to compare the performances on the Robeson plot. The membranes are routinely characterized utilizing SEM, XRD, TGA, and ATR-FTIR. To the best of our knowledge, this is the first study of a cross-linked ZIF/polymer MMM for gas separation.

2. EXPERIMENTAL SECTION 2.1. Materials. Anhydrous dimethyl acetamide (DMAc, 99.8% purity) was purchased from EMD Chemicals Inc. 4,4(Hexafluoroisopropylidene) diphthalic anhydride (6FDA, >99% purity) was acquired from Akron Polymer Systems and was dried under a vacuum at 150 °C prior to use. 2,3,5,6Tetramethyl-1,3-phenyldiamine (durene-diamine, 99% purity), triethylamine (TEA, 99% purity), and acetic anhydride (Ac2O) were obtained from Sigma-Aldrich. Durene-diamine was purified by recrystallization from methanol. B



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laminar flow hood using a Sheen (1133N) automatic casting table equipped with a doctor blade. The cast membranes were immediately covered with a watch glass (to slow solvent evaporation) and were allowed to dry overnight at room temperature. The membranes were then removed from the substrate and were annealed in a vacuum oven with a heating rate of 2 °C/min, at 50 °C for 24 h, 100 °C for 12 h, 150 °C for 12 h, and 210 °C for 24 h. After annealing, the MMMs were allowed to cool to room temperature naturally prior to the removal from the vacuum oven. To anneal the mixed matrix membranes, the temperature was increased gradually from 50 to 210 °C to prevent the formation of defects in the membrane due to the fast evaporation of the solvent.75 2.4. EDA Vapor Cross-Linking. EDA cross-linking was carried out following procedures published by Shao et al.56 EDA (20 mL) was equilibrated in a tightly capped 1000 mL glass jar for 6 h, and annealed ZIF-8/6FDA-durene MMMs were suspended in the chamber containing EDA vapor. Care was taken to prevent EDA liquid from contacting the membranes. The MMMs were reacted with EDA vapor inside the closed chamber for 40 min at ∼35 °C and then immediately washed with deionized water to remove any unreacted ethylenediamine. Finally, these MMMs were dried at 70 °C for 24 h under a vacuum. To test the stability of ZIF-8 to EDA vapor, ZIF-8 nanocrystals were reacted with EDA vapor for 40 min and dried for 12 h under a vacuum at 70 °C. Shao et al.56 observed a 10 μm thick layer after cross-linking the pure polymer for 10 min. We monitored the thickness of the cross-linked layer in the MMM with cross-linking time, which increased with cross-linking time and observed an ∼10 μm thick cross-linked layer after crosslinking for 40 min. 2.5. Characterization. 2.5.1. Characterization of 6FDADurene. The chemical structure of 6FDA-durene (Mw = ∼60 000, PDI = 1.8) was confirmed by 1H NMR using a JEOL FX-270 MHz spectrometer with a TMS internal standard. Molecular weight (Mw) was determined on a gel permeation chromatography (GPC) system equipped with a Viscotek TDA 302 Triple Array Detector and two ViscoGEL I-Series (I-MBHMW 3078, Viscotek) columns. Data were analyzed using Viscotek OmniSEC software, version 3.0.2.145 with Multidetectors option. THF at a flow rate of 1 mL/min was used as the eluent, and polystyrene standards were used for calibration. 2.5.2. Characterization of MMMs (XRD, SEM, TGA, ATRFTIR). X-ray diffraction (XRD) patterns were acquired using a Rigaku Ultima-IV diffractometer with a Cu (Kα) target (λ = 0.154 nm). The samples were measured from 2θ = 5° to 40° in 1°/min steps. Scanning electron microscope (SEM) images were collected on a Zeiss SUPRA40 SEM having a field emission gun operating at 10 keV. Membrane cross-sections for SEM imaging were prepared by freeze-fracture of the samples after immersion in liquid nitrogen. These samples were coated using a Denton Vacuum Desk II sputter coater equipped with a gold/ palladium target prior to imaging. The thicknesses of membranes used in permeability studies were measured by SEM. Thermogravimetric analysis (TGA) was done under nitrogen using a PerkinElmer Pyris 1 TGA instrument operating from 100 to 1000 °C at a heating rate of 10 °C/min. Fourier transform infrared (FTIR) spectra were obtained on a Nicolet 360 FTIR spectrophotometer with a single bounce attenuated total reflectance (ATR) accessory (diamond crystal). The mechanical properties of the membranes were tested at room temperature using a 584B Instron micro tester. 2.6. Permeability Testing. Gas permeability testing was carried out using a custom built permeameter reported previously by Reid et al.24,74 All pressure monitoring and valve actuations

Commercially available ZIF-8 (Basolite Z1200) was purchased from Sigma-Aldrich and was activated at 100 °C under a vacuum for 24 h. Chloroform (CHCl3) and methanol (MeOH) were acquired from Fisher Scientific. All solvents were dried over activated 4A molecular sieves purchased from Sigma-Aldrich prior to use. Ethylenediamine (EDA, 99.5%) was obtained from Sigma-Aldrich. Mylar A92 thin films, purchased from Active Industries, were used as the substrate for membrane casting. For permeability testing, H2, N2, O2, CH4, and CO2 gases were obtained from Air Liquide. The purity of the gases was greater than 99.99% except for O2 and CH4 which had a purity of >99.5% 2.2. Synthesis of 6FDA-Durene. The synthesis of 6FDAdurene (Figure 2) was carried out using chemical imidization

Figure 2. Synthesis scheme for 6FDA-durene.

(in DMAc) following literature procedures.59,69,73 Care was taken to minimize the amount of water in the reaction mixture by drying all glassware prior to use and conducting the reaction under a flow of nitrogen. 6FDA (0.003 mol, 1.33g) was added to durene-diamine (0.003 mol, 0.49g) in DMAc (7.8 mL) in a three-necked round-bottom flask fitted to a condenser to obtain a 20 wt % monomer concentration. The mixture was stirred at 50 °C for 2 h under a nitrogen purge to obtain polyamic acid. Next, a 1:1 molar mixture of triethylamine (0.012 mol, 1.7 mL) and acetic anhydride (0.012 mol, 1.2 mL) was added (four times the number of moles of 6FDA). The mixture was stirred at 50 °C for 1 h, 75 °C for 1 h, and 100 °C for 30 min under a nitrogen purge. The polymer was precipitated in methanol, washed several times with methanol, and dried at 150 °C under a vacuum for 24 h. 2.3. Membrane Fabrication. The 33.3 wt % ZIF-8/6FDAdurene MMMs were prepared using 0.20 g of 6FDA-durene and 0.10 g of ZIF-8 in CHCl3. The polymer solution and ZIF-8 dispersion were prepared separately in two vials using 3 and 7 mL of chloroform, respectively, and stirred and sonicated for 4 h (alternating 1 h stirring followed by 1 h sonication). Sonication was done in an ultrasonication water bath operating at 120 W and 40 kHz. After 10% of the polymer solution was added to the ZIF-8 dispersion (precoating), the mixture was stirred and sonicated for 4 h. Then, the remaining polymer was added and the mixture was further stirred and sonicated for 4 h and finally stirred for 24 h. Next, the polymer-ZIF dispersion was mixed in a Resodyn LabRam Acoustic Mixture for 10 min to break any aggregations and was concentrated to about 10 wt % by evaporating chloroform using a nitrogen purge to obtain a viscosity suitable for casting. The mixture was further sonicated for 10 min to remove any trapped air bubbles and was cast onto a Mylar substrate in a C



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were controlled using LabVIEW 7.1 software (National Instruments). In a typical experiment, 1 cm2 of a membrane was mounted inside a stainless steel cell, which separates the upstream side with a feed pressure of 2000 Torr from the downstream side, which is connected to a vacuum line (1 mTorr). Both upstream and downstream sides were evacuated for at least 6 h followed by a leak rate test before starting the experiments. Upstream and downstream pressures were recorded by pressure transducers. The steady state slope of the downstream pressure vs time was used for the permeability calculations using the solution diffusion model.3,4 The permeability was evaluated from the last 50% of the data in the steady state region. Ideal selectivities (αi/j) were calculated using the ratio of the permeabilities (Pi/Pj) of gases. Permeability testing was done for pure gases at 35 °C and 3.5 atm, and the permeability of a gas through an individual membrane was measured four times. The average of the last three runs was reported as a single reading. The gas permeabilities and ideal selectivities reported are the average of two membranes from two different castings and their standard deviations.

Figure 4. ATR-FTIR spectra of (a) ZIF-8 dispersed in chloroform for 24 h, (b) ZIF-8 exposed to EDA for 40 min, and (c) ZIF-8 dried at 70 °C for 12 h under a vacuum.

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. Characterization of ZIF-8/ 6FDA-Durene MMMs before and after EDA Vapor Modification. The stability of ZIF-8 in the presence of water at 50 °C for 7 days, in 0.1 M NaOH, and in other organic solvents including methanol and benzene is reported.35 However, the dimensions of the EDA molecule (0.310 nm in diameter and 0.555 nm in length)61 allow its absorption into the ZIF-8 (pore aperture of 0.34 nm)66 especially given the flexibility of the ZIF-8 framework.76 Therefore, the stability of ZIF-8 in EDA vapor was tested. XRD data of the ZIF-8 crystals reacted with EDA vapor for 40 min (Figure 3) suggest that the

Figure 5. SEM images of (A) a cross-section of 33.3 wt % ZIF-8/ 6FDA-durene and (B) the same at higher magnification.

smooth ZIF-8 clusters with no defects have not had a detrimental impact on the selectivity of the membranes.47 After the MMM reacts with EDA vapor for 40 min, SEM reveals a dense skin of ∼10 ± 0.8 μm thickness on both sides of the MMM (Figure 6), which can be attributed to cross-linking

Figure 3. XRD patterns of (a) ZIF-8 and (b) ZIF-8 exposed to EDA for 40 min dried at room temperature for 3 h and 70 °C for 12 h under a vacuum.

crystal structure of ZIF-8 is intact after EDA exposure. Stability of ZIF-8 to chloroform and EDA vapor was verified by the similar ATR-FTIR spectra of ZIF-8 crystals before and after exposure to chloroform and EDA vapor (Figure 4). SEM images of the cross-sections of a 33.3 wt % ZIF-8/ 6FDA-durene MMM are shown in Figure 5 at different magnifications. The polymer veins surrounding ZIF particles evident in Figure 5A are an indication of good polymer− particle contact.27,29 In the same figure, apart from welldispersed ZIF-8 particles, submicrometer and micrometer size smooth ZIF-8 clusters can be seen. A similar morphology with larger size clusters was reported by Zhang et al. in ZIF-8/ 6FDA-DAM MMMs and explained that the morphology of

Figure 6. SEM images of cross-sections of (A) un-cross-linked MMM, (B) un-cross-linked MMM zoomed in at the edge, (C) cross-linked MMM, and (D) cross-linked MMM zoomed in at the edge showing the cross-linked skin.

(vide infra, section 3.1.2). For comparison, the SEM images of the cross-linked 6FDA-durene showing a ∼10 ± 0.5 μm thick cross-linked layer are included (Figure 7). TGA plots of 6FDA-durene and the MMMs prior to reaction with EDA vapor show thermal stability up to >400 °C (Figure 8). However, thermal reversibility of diamine cross-linking has been reported at temperatures >150 °C.58,77 A weight loss of ∼5% from D



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Figure 7. SEM images of cross-sections of (A) un-cross-linked 6FDAdurene, (B) un-cross-linked 6FDA-durene zoomed in at the edge, (C) cross-linked 6FDA-durene, and (D) cross-linked 6FDA-durene zoomed in at the edge showing the cross-linked skin.

Figure 9. ATR-IR spectra of (a) 50% (w/w) ZIF-8/6FDA-durene MMM, (b) 50% (w/w) ZIF-8/6FDA-durene MMM treated with EDA for 40 min and annealed at 250 °C, (c) EDA treated MMM after annealing at 150 °C, and (d) EDA treated MMM after annealing at 70 °C. (All membranes were annealed for 24 h.)

Figure 8. Thermogravimetric analysis of (a) 6FDA-durene, (b) 50% (w/w) ZIF-8/6FDA-durene MMM, and (c) 50% (w/w) ZIF-8/6FDAdurene after treatment with EDA for 40 min.

Figure 10. Chemical reaction during EDA modification.

150 to 300 °C is observed for the EDA cross-linked MMM due to the elimination of diamine. Assuming a cross-linking molar ratio of 1:2 6FDA-durene:EDA, the EDA cross-linked MMM would have a 3.4% weight loss (Supporting Information) when EDA is lost from the 10 μm thick layers at the surfaces of the MMM upon thermal treatment, which is quite close to what is observed. 3.1.2. Chemical Modification during EDA Cross-Linking of MMMs. The chemical modification occurring during EDA vapor modification at the surface of the membranes is examined using ATR-FTIR spectroscopy. The ZIF-8/6FDA-durene MMM (Figure 9) shows distinctive bands at 1721 cm−1 (imide CO symmetric stretch), 1781 cm−1 (imide CO asymmetric stretch), and 1353 cm−1 (imide C−N stretch) due to the imide ring in 6FDA-durene. When the MMM is exposed to EDA vapor for 40 min (Figure 9), these bands disappear, and peaks characteristic of the amide group appear at 1671 cm−1 (amide CO stretch) and 1524 cm−1 (amide CN stretch). The band at 1253 cm−1 (CF stretch) remains unchanged. Shifts in these IR bands are similar to those observed for EDA crosslinked 6FDA-durene56 and demonstrate that the chemical reaction taking place during EDA vapor reaction is the ring-opening of the imide group of the 6FDA-durene to an amide group followed by cross-linking (Figure 10).58,61

3.1.3. Thermal Stability of the EDA Cross-Linked MMMs. The thermal stability of EDA cross-linking in the MMM was also evaluated. EDA-treated MMMs that were annealed at 150 °C for 24 h under a vacuum revealed low intensity bands corresponding to the imide CO bond, showing the thermal reversibility of the cross-linking reaction (Figure 9). When the MMMs were further annealed at 250 °C for 24 h (Figure 9), the intensity of the imide CO bands increased, and the bands corresponding to amide CO and amide CN completely disappeared, indicating that, at 250 °C, the cross-linking, at least on the surface, was completely lost. Similar thermal reversibility has been reported for cross-linked 6FDA-durene.58 This thermal reversion of the EDA cross-linking limits the application of this membrane to gas separations below 150 °C. To test the effect of thermal reversibility on gas permeability, preliminary tests were conducted on surface cross-linked 33.3 wt % ZIF-8/6FDA-durene MMMs annealed at 150 °C (section 3.2). Also, the possibility of chain scission during diamine modification,77 which may affect long-term membrane performance, cannot be overlooked and needs further investigation. 3.1.4. Mechanical Properties of the Membranes. The mechanical properties (Table 1) of the membranes were tested at room temperature using a 584B Instron micro tester. Aberg et al. studied the mechanical properties of 1,3-diaminopropane E



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(Table 2). Gas transport in polymers is governed by two factors, namely diffusivity and solubility.3 Gas transport in MMMs also mainly occurs by this solution diffusion mechanism due to the larger fraction of the polymer matrix. Generally, transport of less condensable gases such as H2, N2, O2, and CH4 is governed by diffusivity where the gas molecules adsorbed in to the membrane at the feed side diffuse through the free volume (space unoccupied by polymer chains) in the polymer matrix. Transport of condensable gases such as CO2 mainly occurs through solubility due to interactions between the gas molecules and the polymer matrix.3,4 The pore size of ZIF-8 is 3.4 Å, and ideally an increase in H2 and CO2 (kinetic diameters smaller than the pore size of ZIF-8) permeabilities is expected with a drop in permeability of other gases, if molecular sieving takes place. However, sharp size selectivity for gases with a smaller diameter than the pore size of ZIF-8 is not observed owing to the flexibility of the ZIF-8 framework.76 The selectivity values for the 33.3 wt % ZIF-8 in 6FDA-durene MMM are comparable to that of the unfilled polymer. When the polymer−particle interface contains defects (i.e., sieve in cage), an increase in permeability with a decrease in selectivity is normally observed.32 The enhanced permeability likely arises from the porosity or the free volume introduced by ZIF-8 filler, which results in increased gas diffusivity as observed by Perez et al.27 Yang et al. monitored the free volume in PBI with increased ZIF-7 loading and concluded that adding the filler increases the free volume fraction in the polymer phase, leading to increased gas permeability.52 This increase in permeability was also observed by Ordoñ ez et al. who reported a large increase in gas permeability with no significant change in ideal gas selectivity for 40% (w/w) ZIF-8 loadings in Matrimid.29 Nevertheless, due to the significant enhancement in gas permeability, the gas transport properties of the 33.3 wt % ZIF-8/6FDA-durene MMM surpass the Robeson 2008 upper bound for H2/N2, H2/CO2, and H2/CH4 separations. 3.2.2. Theoretical Permeability Prediction of the MMM. The volume fraction (øD) of ZIF-8 in the MMM can be defined as

Table 1. Measured Mechanical Properties of the Membranes sample

tensile strength (MPa)

Young’s modulus (GPa)

6FDA-durene 6FDA-durene (cross-linked) 33.3 wt % ZIF-8 MMM 33.3 wt % ZIF-8 MMM (cross-linked)

51.6 50.6 37.8 11.0

2.5 2.8 2.7 3.2

cross-linked Matrimid membranes and observed that the tensile moduli (E) and tensile strengths were comparable in magnitude for time periods up to 1 h.62 The observed tensile strength value for the cross-linked 6FDA-durene follows this trend, where the tensile strength of cross-linked 6FDA-durene is comparable to that of pure 6FDA-durene. Cross-linking 6FDAdurene increased the Young’s modulus similar to that observed in p-xylenediamine cross-linked 6FDA-NDA/DABA (9:1) membranes,82 which is in general due to the increased intermolecular interaction and compactness of cross-linked membranes.82 Adding ZIF-8 to 6FDA-durene improved the modulus, whereas a drop in tensile strength was observed. Ordoñez et al.29 and Zhang et al.25 also observed similar mechanical properties in ZIF-8/Matrimid and ZSM-5/Matrimid MMMs. The increase in Young’s modulus was correlated with good interfacial adhesion between the sieve and the polymer, whereas poor interfacial properties would lead to a lower modulus.25 The drop in the tensile strength of the MMM is due to the rigidification of the polymer matrix upon addition of ZIF-8.29 Cross-linking the MMM again increased the modulus, which can be attributed rigidification of the polymer chains at the surface due to cross-linking. Cross-linking the MMM results in a significant loss in tensile strength which we attribute to the loss of efficient polymer− particle contact that occurs with cross-linking of polymer chains at the surface. Although this accounts for the lower flexibility of MMMs after cross-linking, the cross-linked MMMs still possess reasonable flexibility, allowing them to be easily handled in permeability testing. 3.2. Gas Transport Properties. 3.2.1. Gas Transport Properties of the MMM. The gas permeabilities measured here for 6FDA-durene agree with the published values.70 The kinetic diameters of the tested gases increase in the order H2 (2.89 Å) < CO2 (3.3 Å) < O2 (3.46 Å) < N2 (3.64 Å) < CH4 (3.8 Å). 6FDA-durene is a glassy polyimide,69 and the permeabilities of gases in 6FDA-durene increase with decreasing kinetic diameter (Table 2). Adding a 33.3 wt % (50% w/w) loading of ZIF-8 to 6FDA-durene resulted in permeability increases for all gases (412% for H2, 393% for N2, 418% for O2, 468% for CH4, and 332% for CO2) as compared to the case of 6FDA-durene

øD =

(mD /ρD ) (mD /ρD ) + (mC /ρC )

In the above equation, m and ρ are mass and density respectively. D is the dispersed phase (ZIF-8) and C is the continuous phase (6FDA-durene). When the void volume is negligible, this apparent volume fraction can be approximated to the true volume fraction of ZIF-8 in the membrane.49 The reported density of 6FDA-durene is 1.33 g cm−3, and the theoretical density of ZIF-8 is about 0.95 g cm−3.35,69 The values are included in the

Table 2. Permeability (barrers) Values for 6FDA-Durene, 6FDA-Durene Cross-Linked by Reacting with EDA for 12 min, ZIF-8, 33.3 wt % ZIF-8/6FDA-durene MMM, MMM Cross-Linked by Reacting with EDA for 40 min, and the Cross-Linked Skin of the MMM at 35 °C and 3.5 atm and the Theoretically Calculated Permeability of the MMM permeability (barrers) membrane

H2

CO2

N2

CH4

O2

6FDA-durene cross-linked 6FDA-durene ZIF-849,68 MMM cross-linked MMM cross-linked skin of MMMb MMM (theoretical)

518.5 52.1 5411 2136.6 ± 189.9 283.5 ± 33.0 89.4 1220.9

468.5 0.4 1192 1552.9 ± 138.4 23.7 ± 2.8 6.8 696.3

34.9 a 466 137.3 ± 17.6 2.0 ± 0.17 0.58 86.4

30.0 a 430 140.3 ± 38.4 1.40 ± 0.13 0.40 75.3

107.7 2.0 932 449.7 ± 100.2 16.9 ± 2.7 5.0 245

a

Permeability was too low to be detected. bPermeability was calculated using the resistance model (section 3.3). F



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this approach and will present this work in the future. In this study, we investigate option b because the stability of ZIF-8 in EDA vapor (40 min) was verified. Cross-linking the MMM with EDA vapor resulted in a drop in permeability for all gases. The resultant H2 and O2 permeabilities of the cross-linked MMM are 283.4 and 16.9 barrers, respectively, which is 55% of the H2 permeability and 16% of the O2 permeability of 6FDAdurene. The observed CO2, CH4, and N2 permeabilities are 5.5%, 4.7%, and 5.1%, respectively, of those observed for 6FDAdurene (Table 2). Cross-linking leads to 1 order of magnitude increase in selectivities for H2/CO2 (10.9-fold), H2/N2 (9.5-fold), and H2/CH4 (11.4-fold) separations, whereas O2/N2 selectivity was increased by a factor of 2.7 in the cross-linked MMM, as compared to that in 6FDA-durene (Table 3). This shows a size

above equation to get a ZIF-8 volume fraction (øD) of 0.41 for the 33.3 wt % ZIF-8/6FDA-durene MMM. In order to predict the gas permeability properties of the MMMs, Maxwell model was used. This is the simplest model used to predict gas transport properties in polymer/molecular sieve composite systems. It is valid for spherical particles in dilute suspensions where the particle−particle interaction is negligible.14 ⎡ P + 2PC − 2øD(PC − PD) ⎤ Peff = PC⎢ D ⎥ ⎣ PD + 2PC + øD(PC − PD) ⎦

Peff is the effective permeability of the MMM. PC and PD are the gas permeability of the continuous (6FDA-durene) phase and the dispersed (ZIF-8) phase, respectively. The gas permeation values of the pure ZIF-8 membrane reported by Bux et al.68 and converted to barrer units by Song et al.49 (listed in Table 2) are used in the calculation. Using the Peff calculated for different gases, the ideal selectivity is also calculated. As listed in the table, the predicted selectivities match well with the experimental data, although there appears to be an underprediction of the permeability for all the gases. Song et al. also observed that the Maxwell model predictions match well with the experimental data up to 30 wt % ZIF-8 nanocrystals in Matrimid, although there appears to be a systematic underprediction of the permeability for all the gases, which is attributed to the enhanced polymer free volume with the addition of ZIF-8.49 Recently, Yang et al. reported that addition of ZIF-7 nanoparticles into a polybenzimidazole (PBI) matrix enhances the permeability of H2 and selectivity over CO2, which is also higher than that predicted by the Maxwell model. They attributed the increase of selectivity to the favorable interaction of ZIF-7 with PBI.52 A similar behavior was observed by Hao et al. for ionic liquid ZIF-8 systems, where experimental permeability values are slightly higher than the Maxwell predictions.83 It is hypothesized that this is due to the additional free volume contributed by ZIF-8 nanoparticles and also nanoparticle intercalation where a small portion of ZIF-8 agglomerates together, allowing gas molecules to preferentially diffuse through these areas.83 It is reasonable to surmise that the larger than predicted permeabilities observed in this study could arise from both additional free volume contribution by ZIF-8 and the agglomeration of some ZIF-8 nanoparticles as evident from SEM. 3.2.3. Gas Transport Properties of the Cross-Linked MMM. Approaches to further enhance permselectivities of the MMM include (a) increasing the loading of ZIF-8 and/or (b) crosslinking the MMM (Figure 11). Option a is limited by the

Table 3. Ideal Gas Selectivities for 6FDA-Durene, 6FDADurene Cross-Linked by Reacting with EDA for 12 min, ZIF-8,49,68 33.3 wt % ZIF-8/6FDA-Durene MMM, MMM Cross-Linked by Reacting with EDA for 40 min, and the Cross-Linked Skin of the MMM at 35 °C and 3.5 atm ideal gas selectivity membrane

H2/CO2

H2/N2

6FDA-durene 1.1 14.9 cross-linked 6FDA-durene 144 4.5 11.6 ZIF-849,68 MMM 1.4 ± 0.0 16.0 ± 3.1 cross-linked MMM 12.0 ± 0.07 141.4 ± 4.7 13.1 155.0 cross-linked skin of MMMa Knudsen factor 4.7 3.7 MMM (Maxwell model) 1.7 14.13 a

O2/N2 3.1

H2/CH4 17.9

2.0 12.6 3.3 ± 0.14 15.3 ± 0.56 8.5 ± 2.1 203.3 ± 3.7 8.6 222.1 0.9 2.83

2.8 16.2

Gas selectivity was calculated using the resistance model (section 3.3).

selectivity favoring the small gas molecules. Although the size of CO2 is smaller than that of O2, CO2 permeability drops significantly because solubility is the main factor governing CO2 transport. This suggests that the main transport mechanism in the cross-linked MMM is through diffusion. These results are comparable to the molecular sieving reported by Liu et al.59 for the diamine modification of unfilled polyimides, which has been attributed to the reduction in the d spacing between polymer chains upon cross-linking, as confirmed by XRD.59 3.2.4. Gas Transport Properties of the Cross-Linked MMMs Annealed at 150 °C. Because H2/CO2 separation is normally carried out at high temperatures in water−gas shift reactors,78 the separation properties of the cross-linked MMM annealed at higher temperatures were evaluated. When the EDA-treated MMM was annealed under a vacuum at 150 °C, a decrease in H2/CO2 selectivity from 12.0 to 7.7 was observed accompanied by a relatively small decrease in permeability from 283 to 232 barrers. As discussed in section 3.1.2, ATR-IR of the EDA cross-linked MMM shows reconversion of some of the amides functionality to imides (Figure 9) at 150 °C. Therefore, the decrease in H2/CO2 selectivity could be due to the breaking of some amide cross-links, which reduces the size selectivity of the membrane. In this case, an increase in hydrogen permeability would normally be expected, but a small decrease in hydrogen permeability was observed. As shown by Shao et al., the reason behind this lowering of permeability at 150 °C is due to densification of membranes due to the formation of charge transfer complexes (CTCs).58 Because of

Figure 11. Schematic representation of the formation of the crosslinked skin in 33.3 wt % ZIF-8/6FDA-durene MMM upon reaction with EDA vapor.

highest gas selectivity observed for a pure ZIF-8 membrane for respective gas pairs.68 It can be assumed that adding more ZIF-8 could increase the H2 permeability further and reach a limit where molecular sieving will be observed with a corresponding enhancement in gas selectivity. We are currently investigating G



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the 2008 Robeson upper bound for H2/CO2, H2/N2, and H2/CH4 separations and is close to the upper bound for O2/N2 separation. It shows higher H2 and O2 permeabilities compared to that of the EDA treated 6FDA-durene. As emphasized by Merkel et al., achieving a radically high selectivity is not the primary objective of large scale membrane separations.81 Membranes with a moderate selectivity and a high permeability are favored for industrial applications over those with ultrahigh selectivity and low gas permeability. The high selectivities and moderate gas permeability of the surface cross-linked ZIF-8/6FDA-durene membrane could make it attractive for large scale H2/N2, H2/ CH4, and O2/N2 separations. 3.3. Future Direction. The cross-linked ZIF hybrid membrane demonstrates the feasibility of developing dense multilayer membranes by the combination of cross-linking and mixed-matrix formulation. Separation properties of the membrane as well as mechanical properties may be improved by minimizing the undesirable ZIF particle agglomeration. Recently Song et al. has shown the use of colloidal, 60 nm size ZIF-8 nanoparticles in membrane fabrication, to minimize particle aggregation and improve polymer particle contact.49 Using colloidal ZIF suspension in membrane fabrication can be outlined as one future direction to improve the performance of cross-linked hybrid membranes. Also it is important to use the correct combination of highly permeable polymers with a highly selective MOF to obtain enhanced separation properties.30 Because thin asymmetric membrane structure is the most efficient in separations, the ultimate goal would be to extend this approach to fabricate thin layers of ZIF hybrid on a porous support or hollow fibers (using the small particle size of ZIFs may enable fabrication of thin layers) and perform cross-linking.

the thermal reversibility of the EDA cross-linking, this membrane can be used for separations operating at temperatures

Polyimide Mixed Matrix Membranes

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