Building Additional Passageways in Polyamide ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38877-38886

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Building Additional Passageways in Polyamide Membranes with Hydrostable Metal Organic Frameworks To Recycle and Remove Organic Solutes from Various Solvents Xiquan Cheng,†,‡,§ Xu Jiang,† Yanqiu Zhang,† Cher Hon Lau,*,∥ Zongli Xie,*,‡ Derrick Ng,‡ Stefan J. D. Smith,‡ Matthew R. Hill,‡,⊥ and Lu Shao*,† †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P.R. China ‡ CSIRO Manufacturing, Private Bag 10, Clayton South, Victoria 3169, Australia § School of Marine Science and Technology, Sino-Europe Membrane Technology Research Institute, Harbin Institute of Technology, Weihai 264209, P.R. China ∥ School of Engineering, The University of Edinburgh, The King’s Buildings, Robert Stevenson Road, Edinburgh EH9 3FB, U.K. ⊥ Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia S Supporting Information *

ABSTRACT: Membrane separation is a promising technology for extracting temperature-sensitive organic molecules from solvents. However, a lack of membrane materials that are permeable toward organic solvents yet highly selective curtails large-scale membrane applications. To overcome the trade-off between flux and selectivity, additional molecular transportation pathways are constructed in ultrathin polyamide membranes using highly hydrostable metal organic frameworks with diverse functional surface architectures. Additional passageways enhance water permeance by 84% (15.4 L m−2 h−1 bar−1) with nearly 100% rose bengal rejection and 97.6% azithromycin rejection, while showing excellent separation performance in ethyl acetate, ketones, and alcohols. These unique composite membranes remain stable in both aqueous and organic solvent environments. This immediately finds application in the purification of aqueous mixtures containing organic soluble compounds, such as antibiotics, during pharmaceutical manufacturing. KEYWORDS: UiO-66, nanofiltration, polyamide, high flux, dye antibiotics removal

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challenging applications of molecular separations9−13 include the introduction of additional molecular transportation pathways in low flux polymers9,10,13 and increasing the tortuosity of high flux polymers to improve selectivity.11,12 Adopting the former approach, Karan, Jiang, and Livingston reported current state-of-the-art ultrathin PA OSN membranes with additional nanochannels that were created by etching an underlying sacrificial layer of Cd(OH)2 nanostrands, demonstrating an efficient route to enhance solvent permeation.10 The solvent permeances/flux of these OSN membranes are several to thousands of times higher than pristine polymer membranes.10 However, Cd(OH)2 etching generates Cd2+ waste solution, which is carcinogenic and detrimental to the environment. Metal organic frameworks (MOFs), a class of porous materials with ultrahigh specific surface areas, can be used in

INTRODUCTION Compared to conventional liquid purification methods such as distillation, recrystallization, and extraction,1−4 polymer membranes are preferred for purifying temperature-sensitive molecules such as active pharmaceutical molecules and catalysts from organic solvents. This is due to the lack of phase transition, a low risk of product loss, minimal energy consumption, and low spatial requirements.5−8 Nanofiltration (NF) membranes for separating solid molecules from water or organic solvents are commercially available. Commercial NF membranes are typically fabricated using rubbery polymers such as polydimethylsiloxane (PDMS) and glassy polymers like polyamides (PA) and polyimides. These cost-effective polymers are highly selective and stable in organic solvents. However, the flux of these cost-effective materials remain insufficient to handle large solvent quantities that are normally associated with industrial-scale separation processes.5−8 Strategies to overcome the low flux of polymeric membranes while maintaining their high solute retention performance in © 2017 American Chemical Society

Received: May 24, 2017 Accepted: October 12, 2017 Published: October 12, 2017 38877

DOI: 10.1021/acsami.7b07373 ACS Appl. Mater. Interfaces 2017, 9, 38877−38886

Research Article

ACS Applied Materials & Interfaces gas separation, heavy metal removal, drug delivery, and many other applications.14−19 The intrinsic pores of MOFs can provide additional molecular transportation pathways that can improve the solvent/water permeances of NF or reverse osmosis (RO) membranes.20−27 ZIF-8, a zinc-based MOF, is commonly used to enhance the water permeances of NF or RO membranes.20,21 A limitation of using ZIF-8 to enhance molecular transport in membranes is a restricted flux enhancement effect, particularly in NF membranes. This is due to the similar sizes between the intrinsic pores of ZIF-8 (0.32−0.34 nm)23,28,29 and water molecules. This effect is more pronounced when ZIF-8 composite membranes are used for organic solvent NF as the diameters of organic solvent molecules are larger.30 MOFs with larger pores like MIL-101 (Cr), MIL-101 (Al), and HKUST have been used to improve the transport of organic solvents.22,23,26,27 However, the synthesis of MIL-101 (Cr) and MIL-101 (Al) requires high temperatures and pressures, thereby increasing membrane production costs.22,23 Meanwhile, the poor hydrostability of HKUST confined the application of resultant nanocomposite membranes to only operate in organic solvent environments.26,27 Clearly, a judicious MOF selection is critical for imbuing additional pathways that can enhance solvent permeance to polymer membranes for solvent purification. An ideal MOF candidate for enhancing both water and solvent permeance in membranes is UiO-66, a Zr-based MOF. This is due to its subnanometer pore sizes and high hydro- and solvent stability, even in NaOH and HCl solutions.31−35 In addition, the ability to scale up the production of UiO-66 with flow microtechnology36 and tailor UiO-66 pore sizes and functionality to suit application requirements31−33 render them ideal for improving the separation performances of NF membranes. Moreover, the large number of defects (due to high coordination numbers) in UiO-66 MOFs may increase pore volume, surface areas, and adsorption, hence promoting water transportation.37−41 Solvent transport through UiO-66 pores can also be enhanced when functionalized ligands are used to during the synthesis of UiO-66.42−44 The benefits of UiO-66 MOFs have been exploited in membranes for water or solvent purification via desalination34 and pervaporation.35 However, the benefits of UiO-66 on flux enhancements in these membranes are not fully optimized. This could be ascribed to thick selective layers and compact aggregation of UiO-66 molecules that increased the mass transfer resistance. Clearly, key to optimizing the advantages of UiO-66 MOFs in mixed matrix membranes to yield excellent separation performances is to build a thin selective layer while reducing the MOF agglomeration selective layer. Here, we report the construction of additional solvent passageways in ultrathin PA NF membranes through the incorporation of hydrostable UiO-66 MOFs (Figure 1). The effects of functionalized ligands (UiO-66-(CH3)2, UiO-66NH2) and postsynthetic functionalization (UiO-66(Ti)) on pore size, structure, and defects that consequently impacted membrane molecular transport were also studied for the first time here. The intrinsic porosity of MOFs enhanced water and solvent permeation of PA membranes. The pure water permeances of PA/UiO-66 membranes were as high as 15.4 L m−2 h−1 bar−1, while rejecting 100% of Rose Bengal and 97.6% azithromycin dissolved in water. Ascribed to the hydrostability of the UiO-66 MOFs, the high water permeance of the PA/UiO-66 membrane was maintained over 100 h of continuous operation. Functionalized UiO-66 MOFs studied

Figure 1. (A) Solvent fluxes of conventional PA membranes are insufficient to meet the industrial demands of molecular separations. However, these materials are preferred for their ability to separate solid molecules from solvents effectively. (B) The inherent porosity of MOFs provided additional molecular transport passageways in PA membranes, drastically enhancing solvent flux without compromising the material’s high dye/solvent selectivity.

here also imbued excellent separation performances to PA membranes during operation in organic solvents such as alcohols, ketones, and ethyl acetate. Added functionalities and defects in UiO-66 tailored pore sizes and structures consequently impacted the additional passageways for water and solvent transport, and membrane separation efficiency. This indicated that permeance enhancements of these mixed matrix membranes are highly dependent on the MOF surface area.

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EXPERIMENTAL SECTION

Chemicals and Materials. Analytical grade methanol (MeOH), ethanol (EtOH), isopropanol (IPA), acetone (DMK), methyl ethyl ketone (MEK), ethyl acetate (EA), rose bengal (RB), crystal violet (CV), terephthalic acid, 2-aminoterephthalic acid, 2,5-dimethyl terephthalic acid, ZrCl4, concentrated HCl, TiCl4, Thiazol Yellow G (TY), Safranine O (SO), and methyl orange (MO) were purchased from Sigma-Aldrich and used without further purification. Branched polyethylenimine (PEI) was purchased from Sigma-Aldrich (Mw 10 kDa, 95% purity), and PAN powder was supplied by Qilu Petrochemical Company, both of which were used without purification. Trimesoyl chloride (TMC, 98.0% analytical grade) was supplied by Alfa Aesar. Analytical grade dimethylformamide (DMF), tetrahydrofuran (THF), and n-hexane were purchased from Merck and used as supplied. Azithromycin was supplied by Dalian Meilun Biology Technology Co., Ltd. The chemical structures of the dyes and the properties of the solvents used in this work are shown in Figure 2 and Table 1, respectively. Synthesis of MOFs. UiO-66 and functionalized UiO-66 were synthesized32 to yield powders with a Brunauer−Emmett−Teller (BET) surface area ranging from 767 to 1447 m2/g (Table 2). Briefly, a glass bottle was loaded with 1.25 g (5 mmol) of ZrCl4, 50 mL of DMF, and 10 mL of concentrated HCl before being sonicated for 20 min until fully dissolved. A total of 5 mmol of ligand and 100 mL of DMF were then added, and the mixture was sonicated an additional 20 min before being heated at 80 °C overnight. The resulting solid was then filtered over a fine frit and washed first with DMF (one time) and then with MeOH (three times). The sample was filtered for several hours to remove all residual solvent. With the exception of dried samples, the samples were activated by first heating to 90 °C under vacuum until a pressure of 100 mTorr was reached. The postsynthetic exchange of UiO-66-(Ti) was performed as reported before.14,31 TiCl4(THF)2 was first synthesized according to 38878


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the patent of Francesco et al.45 A total of 5.0 g (26.4 mmol, 2.8968 mL) of TiCl4 was dissolved in 50 mL of dichloroethane in a 250 mL RBF under inert conditions. Then, 7.62 g (105.8 mmol, 8.57 mL) of dried tetrahydrofuran (THF) was added at 0 °C to form a yellow solution. Next, 100 mL of sodium-dried n-hexane was used to precipitate a yellow powder, which was filtered and washed with nhexane (2 × 50 mL). It was dried for 2−3 h over a Schlenk line and used immediately for postsynthetic exchange. Postsynthetic exchange of UiO-66 with TiCl4(THF)2 was carried out according to the work of Cohen and co-workers.46 A mass of 0.17 g (0.3 mmol) of TiCl4(THF)2 was dissolved in 10 mL of DMF, and then, 0.14 g of UiO-66 was added. The mixture was incubated for 5 days. The solids were separated from the solvent via centrifugation and washed with fresh DMF (at least six times). The washed solids were immersed in a methanol bath for 3 days. The methanol was replaced every 24 h. The solids were dried at 40 °C in a vacuum for 24 h. The structures of these MOFs were characterized with XRD (Figure S1). Preparation of PA/UiO-66 Block Solid Composite Materials. The fabrication of PA/UiO-66 composite materials is illustrated in Figure 3. The composite materials were fabricated using interfacial polymerization. These materials were formed as block solid composite materials. We first developed PA-based materials to investigate the dispersion of UiO-66 in PA composite membranes and the compatibility between UiO-66 and PA. A total of 2.0 g of PEI (and 0.2 g of UiO-66) were added into 100 mL of water (water phase). To promote PA polymerization, 0.1 g of TEOA and 0.1 g of SDS were added to a PEI (UiO-66) solution. The mixture was stirred over 24 h to ensure fine dispersion of UiO-66 and to obtain a uniform aqueous solution. A mass of 0.15 g of TMC was added to 100 mL of n-hexane to form a uniform organic phase. The water phase mixture was mixed with the organic phase and stirred for 10 min to form PA composite materials. Residual monomers and solvents were removed from the system through filtration. The composite polymer product was washed thrice with ethanol and water. Finally, the composite polymer was immersed in 60 °C water for 30 min and dried under 60 °C in a vacuum oven before characterization. Preparation of PA/UiO-66 Composite Membrane. To generate PA thin film composite membranes, PAN substrates were first fabricated by phase inversion as reported elsewhere.47,48 Specifically, 18 wt % of PAN, 2 wt % of PEG-800, and 80 wt % of NMP were stirred under 70 °C for 4 h to form a uniform solution. Then, the solution degassed overnight at 70 °C. Prior knife-casting, the solution was allowed to cool down to room temperature. The solution was first poured onto a glass plate and knife-cast into a thin membrane using a 150 μm metal blade and immersed in 25 °C deionized water

Figure 2. Chemical structures of the dyes and the antibiotic used in this study.

Table 1. Solvent Properties Used in This Work basic properties

solvents

molecular weight (g mol−1)

molar volume (cm3 mol−1)

viscosity (cP)

surface tension (dyn/cm)

MeOH EtOH IPA DMK MEK EA

32.0 46.1 60.1 58.1 72.1 88.1

40.7 58.5 76.9 75.1 91.6 98.0

0.55 1.10 2.00 0.30 0.58 0.44

22.6 21.9 21.3 18.8 21.0 23.5

Table 2. BET Surface Areas of Different Kinds of UiO Type MOFs type

surface area (m2/g)

UiO-66 UiO-66-NH2 UiO-66-(CH3)2 UiO-66-(Ti)

1310 ± 46 820 ± 27 767 ± 39 1447 ± 31

Figure 3. (A) Structure of UiO-66 and its functionalized analogues. (B) The structure of monomers and the synthesis of the MOFs/PA composite materials through interfacial polymerization. 38879


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Figure 4. Morphologies of (A) PA and (B) PA with higher magnification indicate no nanoparticle structure appears in the pristine PA; (C) PA/UiO66 materials and (D) PA/UiO-66 materials with higher magnification; the EDX of PA/UiO-66 materials: (E) whole element distribution and (F) Zr distribution. for phase inversion to form the PAN ultrafiltration support. The PAN substrate was immersed in fresh deionized water for 24 h before use. The fabrication process of PA/UiO-66 composite materials is shown in Figure S2. PAN substrates were immersed in 100 mL of water phase (containing 2.0 g of PEI and 0.2 g of UiO-66) for a stipulated time and air-dried in a fume hood. Subsequently, these PAN substrates were immersed for 90 s in 100 mL of organic phase containing 0.15 g of TMC to form a PA selective layer. After interfacial polymerization, the membranes were washed with water three times (20 mL) and immersed in deionized water at 60 °C for 30 min. The final TFC membrane was immersed in fresh deionized water before characterization or performance testing. Characterization. BET surface areas of MOFs studied here were calculated from nitrogen adsorption isotherms at 77 K using a Micromeritics ASAP 2020MC instrument. Samples were activated at 120 °C under vacuum overnight prior to analysis. The surface areas were list in Table 2. The sizes of MOFs studied here were characterized using a nanoparticle analyzer (Zetasizer Nano, Malvern Instruments Ltd.). The average diameter of UiO-66-(Ti), UiO-66, UiO-66-NH2, and UiO-66-(CH3)2 was ∼360 nm. A Bruker D8 Advance X-ray Diffractometer operating under Cu Kα radiation (40 kV, 40 mA) equipped with a LynxEye detector was deployed to obtain the XRD patterns (Figure S1). Samples were scanned over the 2θ range of 5 to 40° with a step size of 0.02° and a count time of 1.6 s per step. These XRD patterns confirmed that all UiO-type MOFs studied here were crystalline. The viscosities of the polymer solutions were characterized using a capillary viscometer. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra were obtained using a Bruker TENSOR 27 spectrometer equipped with an ATR sampling accessory (GladiATR, PIKE Technologies) at room temperature. The chemical composition of the sample surface was analyzed using an X-ray photoelectron spectrometer (XPS) (VG ESCALAB 220i-XL, UK) equipped with a twin crystal monochromated Al Kα X-ray source, which emitted a photon energy of 1486.6 eV at 10 kV and 22 mA. Differential scanning calorimetry (DSC) was performed using a 2920 Modulated DSC (TA Instruments). Each sample was heated and cooled from −25−300 °C twice, at a heating rate of 10 °C min−1. The cross section and surface morphologies of PEI/UiO-66 composite, PAbased composite materials were characterized with a FESEM (MERLIN Compact, Zeiss Company). Membrane samples were cryofractured and coated with iridium before characterization. Evaporated PEI/UiO-66 solutions on a silicon wafer were also characterized to determine MOF dispersion within a polymer matrix.

After evaporation, the samples was coated by iridium before characterization. An advanced field emission scanning electron microscope (JEM-2100F, Germany) was used to image membrane thicknesses. Gatan ES500W Erlangshen (model 782) and MultiScan MSC 600HP (Model 794) CCD cameras were used for wide range TEM and high-resolution TEM (HRTEM) imaging of microtomed membrane samples, respectively. Membranes were first embedded in an Araldite resin and sliced into 60−100 nm thin membrane films. These film samples were collected and placed on a carbon/collodion covered copper grid. A contact angle measuring system (G10 Kruss, Germany) was used to measure the static water contact angle of membranes. In each measurement, an approximate 2 μL droplet was dispensed onto the substrates. A deionized water droplet was placed on a dry, flat membrane surface, and the contact angle was obtained after 30 s of stabilization. The reported contact angle value was an average of more than five measurements at different sites of the same sample. The permeances or fluxes of NF membranes were measured using a self-made, stainless steel dead-end pressure cell with a membrane area of 12.57 cm2. The feed solution was pressurized with nitrogen to 5 bar at room temperature. During filtration, the feed solution was stirred at 11.66 Hz (700 rpm) to minimize concentration polarization. Permeate samples were collected in flasks as a function of time, and then, they were weighed and analyzed. The water/solvent flux and water/solvent permeance were calculated using the following equations

F = V /(A × t )

(1)

permeance = F /ΔP

(2) −2

−1

where F represents the solvent flux (L m h ), V (L) is the volume of the solvent (or solution) passing through the membrane, A is the effective membrane area (m2), t is the operation time (h); and ΔP is transmembrane pressure (bar). Permeances of membranes were tested in the order of water > methanol (MeOH) > ethanol (EtOH) > isopropanol (IPA) > acetone (DMK) > methyl ethyl ketone (MEK) > ethyl acetate (EA). The solute rejections of NF membranes were calculated using eq 3

R = (1 − C P/Cf )

(3)

where Cp and Cf are the solute concentrations in the permeate and the feed solution, respectively. Dye concentrations in IPA were measured with a UV−VIS CINTRA20-GBC apparatus (λmax of RB = 548 nm, λmax of CV = 588 nm, λmax of TY = 548 nm, λmax of SO = 548 nm, λmax of MO = 463 nm, and λmax of azithromycin = 482 nm). Each data 38880


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Figure 5. TEM images of (A) PA composite membranes; (B) PA/UiO-66 composite membranes.

Figure 6. Morphologies of samples: (A) PEI molecules; (B) PEI/UiO-66; (C) PEI/UiO-66-NH2; (D) PAN substrates; (E) PA composite membranes; (F) PA/UiO-66 composite membranes; cross section of (G) PAN substrates; (H) PA composite membranes; (I) PA/UiO-66 composite membranes. point is an average of three repetitions of each test, with ±5% standard deviation.

when the PEI/MOF/water mixture was added to the TMC/nhexane solution. XPS and FT-IR confirmed that MOFs were successfully incorporated into the PA matrix (ESI). The BET surface areas of various UiO-66 MOFs are shown in Table 1. The surface area of UiO-66 studied here was 10% higher than those reported elsewhere. This was due to the presence of defects.40,49 Postsynthetic exchange of Zr (IV) ions with Ti ions generated more defects, increasing the surface area further.14,31,39 Meanwhile, functional groups on the ligands partially occupied the free volume within MOF pores, accounting for lower surface areas.32,44,50,51 UiO-66 MOFs were covered by PA with no obvious aggregation (Figure 4). EDX analysis of these films (Figure 4E,F) revealed a uniform distribution of Zr, inferring well-dispersed UiO-66 MOFs within the PA matrix. The fine dispersion of UiO-66 was also affirmed through intrinsic viscosity measurements of PEI/

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RESULTS AND DISCUSSION Preparation and Characterization of PA/UiO-66 Composite Materials. The chemical structures of MOFs, monomers, and PA are shown in Figure 3. PA/MOF block solid composite materials were fabricated using interfacial polymerization (IP) of branched polyethylenimine (PEI) and trimesoyl chloride (TMC). These block solid composite materials were used as control samples to determine the feasibility of this approach. PEI was dissolved in water, while TMC was dissolved in n-hexane. MOFs (0.2 wt %) were added to the PEI/water solution and sonicated and stirred for 24 h to ensure uniform dispersion. An ultrathin layer of the PA/MOF composite was formed instantaneously at the solvents interface 38881


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Figure 7. Separation performance of advanced membranes: (A) permeance of different dyes in aqueous solution, dye concentration = 0.1 g L−1; (B) separating RB from organic solvents; (C) different dye rejections in the aqueous solutions; (D) RB rejections in different kind of solvents.

MOF/water solutions (Figure S3). The addition of nonporous nanoparticles tended to increase polymer solution viscosities,52−54 while porous nanoparticles like porous aromatic frameworks55−57 and hyper-cross-linked polymers56−58 can reduce solution viscosity. The latter trend was observed in this work with the incorporation of MOFs into PEI/water solutions. The reduced viscosities of MOF/PEI/water mixtures could be caused by nonbonding interactions between MOFs and PEI chains underpinned by the threading of PEI chains into MOF pores56,57,59 and the wrapping of PEI chains around MOF nanoparticles.60 Additional free volume content61 from the intrinsic porosity of MOF nanoparticles could also contribute to lower solution viscosities. Increments in glass transition temperature (Tg) of PA/MOF composites (ESI) also indicated the intimate interactions between MOFs and the PA matrix. −COOH functional groups in both the PA matrix and on the ligands of UiO-66 series MOFs facilitated uniform dispersion of MOFs, akin to methyl groups on the archetypal porous aromatic framework, PAF-1, and hyper-cross-linked polymers that promoted their uniform dispersion in a methylated polymer matrix.55,58,62,63 Consequently, the excellent dispersion of MOFs in PA provides effective passageways for solvent transport, greatly enhancing the flux of thinfilm composite PA membranes. Fabricating PA/UiO-66 Composite NF Membranes. We deployed a sequential dip coating IP technique to form a thin selective layer of PA/MOF TFC membrane on a PAN substrate. From TEM micrographs (Figure 5), we observed that the average thickness of PA membranes was about 205 nm, while the average thickness of PA/UiO-66 membranes was 10% higher (230 nm). This increment of membrane thickness was due to the addition of MOFs during interfacial polymerization. SEM images of membrane cross sections in Figure 6 revealed a typical composite structure. Finger-like pores were observed in PAN substrates, while a lack of an apparent dense layer on the PAN substrate indicated formation of a thin selective layer that is highly compatible with the substrate. The deposition of PA selective layers led to a rough surface. Different from block solids of PA/UiO-66 control material (Figure 4), UiO-66 nanoparticles could be clearly observed in FESEM images of thin film composite membranes. The deposition of 200 nm thin PA layers was insufficient to fill up the depths between 360 nm

MOF nanoparticles to yield a smooth surface. It is important to point out that the approach of interfacial polymerization enabled the wrapping of PA chains around MOF additives. This approach inhibited the formation of cavities and defects around additives that could lead to a loss in rejection rates. Effects of MOF Functionalization on Membrane Separation Performance. Dye molecules of different sizes were used to demonstrate the separation performance of our PA/UiO-66 membranes. The pure water permeance of the pristine PA TFC membrane was 8.3 L m−2 h−1 bar−1 (Figure 7). Addition of 0.2 wt % UiO-66 in PEI aqueous solution enhanced water permeance by 84% to 15.4 L m−2 h−1 bar−1, without compromising the solute rejection rate. It is noteworthy to mention that the incorporation of UiO-66 increased membrane thickness. Typically, thicker membranes with similar pores demonstrate lower solvent permeances based on the Hagen−Poiseuille equation.7,8 This inferred that enhanced permeances of PA/UiO-66 were due to the intrinsic porosity of MOFs. This was also observed for the permeance of organic solvents, particularly for isopropanol (IPA). IPA permeance of PA/UiO-66 membrane was almost 2 times higher than that of PA/cyclodextrin composite membranes.14 The pore sizes of UiO-66 and its functionalized analogues (0.6−1.0 nm)32−34 are ideal for NF while also rejecting dyes. As such, the MOF pores provided additional transport routes for both water and organic solvents without compromising the rejection rate for the dye species.7,23,27 The relationship between BET surface areas of MOFs and membrane performance was also elucidated. Higher BET surface areas in MOFs lead to higher porosity.31,32 Among all functionalized UiO MOFs studied here, defects generated by the postsynthetic exchange of Zr with Ti ions in UiO-6614,31,39 yielded MOFs with the highest BET surface areas studied here. Hence, the incorporation of UiO-66-(Ti) led to the most drastic enhancement in PA solvent permeances with nearly 100% rose bengal and 96.7% Thiazol Yellow G rejections. Defects in the crystalline structures of UiO-66 MOFs also promote water/solvent adsorption,37,38,41 further enhancing molecular transportation. The functionalization of MOFs with −NH2 and (CH3)2 groups reduced BET surface areas,32,44,50,51 reducing molecular transportation pathways. Therefore, the incorporation of UiO-66-NH2 and UiO-66-(CH3)2 MOFs lowered the solvent permeances of PA membranes. Given that 38882


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ACS Applied Materials & Interfaces UiO-66-NH2 demonstrated better affinity to water than UiO66-(CH3)2,42,43 PA/UiO-66-(CH3)2 membranes exhibited the lowest permeance enhancement. The incorporation of porous nanoparticles into polymer hosts could have introduced additional free volume at the interfaces56,57 between porous nanoparticles and the selective layer (PEI wrapping around the MOFs), leading to a significant increase of water/solvent permeance without any loss of dye rejection as well. Among all UiO-66 MOFs studied here, UiO66 (Ti) interacted best with PEI, thereby promoting better MOF dispersion. When compared to pristine UiO-66, UiO-66NH2, and UiO-66-(CH3)2, where PEI-MOF interaction is less pronounced, this might enhance the generation of additional free volume at the PEI-UiO-66 (Ti) interface, resulting in faster water permeation. As UiO-66-NH2 is more hydrophilic than UiO-66-(CH3)2, the water permeance of PEI-UiO-66-NH2 membranes is higher than that of those loaded with UiO-66(CH3)2 (Figure S3). MOF functionalization also altered pore sizes that impacted interactions with PEI. Like other porous nanoparticles,56,57 PEI polymer chains may thread into a portion of UiO-66 and UiO-66 (Ti) pores via a single locality on the PEI chain, leaving other pores intact for enhancing solvent/water transport. Multiple interactions between a single PEI chain and MOF nanoparticle are difficult due to steric hindrance. The membranes studied here remained stable in alcohols, ketones, and ethyl acetate. RB rejection remained over 96% when applied in solvents. Solvent permeances are shown in Figure 7B. In general, the order of solvent permeances increments was dependent on viscosity and surface tension as reported by Bhanushali et al.64 The solvent properties are listed in Table 1. Dye rejections of the composite NF membranes were shown in Figure 7C,D. The incorporation of MOFs that were well-dispersed within the PA matrix did not create obvious defects that could have compromised the rejection of PA membranes. For the same dye, all PA-based composite NF membranes exhibited similar rejection. The order of dye rejection is RB > TY > CV > SO > MO, indicating a dependence on dye size (molecular weight). When applied in different organic solvents, the rejections of RB were different, which is attributed to different solute−solvent interactions.65 Compared with pure PA membranes, the incorporation of UiO-66 MOFs improved permeance stability even under high pressures (Figure S7). Different from other MOF-based mixed matrix membranes,20−27 the chemical stability and hydrostability of UiO66 MOFs ensured that the additional molecular transport pathways were retained within PA TFC NF membranes. After 100 h of continuous operation at 10 bar, the pure water permeances of PA/MOF TFC membranes declined by only 4− 5%, representing a 10% improvement when compared to pristine PA membranes (Figure 8A). The more significant decrease in water permeance of pristine PA membranes could be attributed to film compaction where pores within the ultrathin PA matrix collapsed.66−68 MOFs reduced compaction impact in PA membranes, yielding mechanically robust TFC membranes. Generally, the decline of membrane permeances was evidently less pronounced in PA/UiO-66 composites. This could be attributed to limited PA chain movements in the presence of UiO-66 additives. Defects in postsynthetic exchanged UiO-66(Ti) MOFs could enhance the attraction with PA polymer chains, leading to stronger interactions62 that resulted in PA composite membranes with the least permeance

Figure 8. (A) Long-term separation performance in aqueous solution; (B) the cycle RB solution permeances and RB rejection changing with operation time; the zero point on the x axis represents the pure water permeances of the fabricated membranes.

decline with operation time. Meanwhile, the infiltration of PA chains into the smaller pores of UiO-66-(CH3)2 and UiO-66NH232,42,48,49 would be more difficult, potentially leading to less robust membranes. Moreover, the reduction in intrinsic viscosity of solutions containing UiO-66-(CH3)2 were the lowest among all MOFs studied here (Figure S3), indicative of poorer compatibility between UiO-66-(CH3)2 and PEI. The rose bengal rejection rates of our PA/MOF membranes in all solvents were higher than 96%, implying that our membranes can retain stable separation performances in solvents that are commonly deployed in industry. Membrane fouling caused by dye build-up also contributed to lower solvent permeances over time (Figure 8B). A simple cleaning procedure allowed the recovery of more than 98% of the flux in these PA/MOF TFC membranes, indicating reversible fouling. Notably, the RB solution permeance of PA/UiO-66 membranes was only 10% lower than that of PA/ UiO-66(Ti) membranes. Considering the extra steps required for the postsynthetic exchange of UiO-66(Ti) MOFs, PA/UiO66 membranes represented a more promising option for organic solute separations at large scale. When applied for the separation of other dyes, the water/solvent permeances of PA/ UiO-66 TFC NF membranes outperformed those of other NF membranes (Figure 9).10,24,48,69−77 Membranes with high fluxes are usually prone to concentration polarization where increased dye concentration could lower both the rejection and permeance.1,2 To simulate this process, we performed NF separation experiments with solutions with increasing RB concentrations (Figure 10). The RB rejection rate of our PA/ MOF membranes remained 100% even as the RB concentration increased from 200−1000 ppm. This indicated that our high flux membranes are immune to the detrimental effects of concentration polarization. We also demonstrated the removal of antibiotics (30 ppm of azithromycin (748.9 g mol−1)) from aqueous solutions at low operating pressures using our PA/ MOF composite membranes. The incorporation of UiO-66 MOFs drastically enhanced membrane flux. The membrane flux was 67.2 L m−2 h−1 under 5 bar of operating pressure with a 38883



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07373. FTIR; water contact angle; the viscosity of the PEI/UiO66 solutions; and some separation performance of the PA/UiO-66 membranes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

Figure 9. Comparison of separation performances of our membranes with the state-of-the-art NF membranes:10,24,48,69−77 BPEI/GO membranes,69 PU,70 PASS,71 and activated GO membranes72 removing RB from water; polyarylate,73 ZIF-11/PA,74 ZIF-8,24 thin PA/xP84, and commercial membranes removing RB from methanol;10 PPy,75 GP/PPy,48 PEs,76 and PEEK77 membranes removing RB from IPA; solid star = PA/UiO-66 membranes; open star = PA membranes.

*E-mail: [email protected] (L.S.) *E-mail: [email protected] (C.H.L) *E-mail: [email protected] (Z.X.) ORCID

Xiquan Cheng: 0000-0001-8050-8805 Cher Hon Lau: 0000-0003-1368-1506 Stefan J. D. Smith: 0000-0001-7465-0565 Lu Shao: 0000-0002-4161-3861 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21676063, U1462103), the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute Technology) (No. 2017DX07), and the HIT Environment and Ecology Innovation Special Funds (HSCJ201619). M.R.H., K.K., and C.H.L. acknowledge the Science and Industry Endowment Fund (SIEF). M.R.H. and A.J.H. acknowledge the generous support of the CSIRO Office of the Chief Executive science team. M.R.H. acknowledges FT 130100345.

Figure 10. Effects of concentration of RB feed solutions on the separation performance of the PA/UiO-66 membranes.

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rejection rate of 97.6%. The ability to achieve high flux at low pressures is key to lowering energy requirements of fast and effective pharmaceutical recovery/removal.

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REFERENCES

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CONCLUSION

Through the intrinsic porosity of hydrostable MOFs such as UiO-66, we were able to create additional passageways to enhance the permeable properties of thin PA selective layers fabricated with interfacial polymerization. Nonbonding, intimate interactions between MOFs and PA chains were tailored through a judicious choice of organic ligands and postsynthetic exchange to ensure uniform MOF dispersion within the PA selective layer. Among all MOFs from the UiO family studied here, the intrinsic pores (0.6−1.0 nm) of UiO-66 MOFs are most beneficial for enhancing PA water permeance by 84% (15.4 L m−2 h−1 bar−1) with high rejections to dyes−RB (100%) and pharmaceuticals−AH (97.6%). Most importantly, the water permeances of PA/UiO-66 composite membranes were 200% higher than those of pristine PA membranes during long-term continuous operation, showing excellent anticompaction performances. Taken together, PA membranes incorporated with UiO-66 MOFs are exciting candidates for a diverse range of separation applications including concentrating organic solutes, purifying solvents or water, and recycling organic solutes. 38884


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