Building Additional Passageways in Polyamide Membranes with

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07373 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Building Additional Passageways in Polyamide Membranes with Hydrostable Metal Organic Frameworks to Recycle and Remove Organic Solutes from Various Solvents Xiquan Cheng,a,b,c Xu Jiang,a Yanqiu Zhanga Cher Hon Lau,*,d Zongli Xie,*b Derrick Ng,b Stefan J. D. Smith,b Matthew R. Hill,b,e and Lu Shao*a a.

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 b.

c.

CSIRO Manufacturing, Private Bag 10, Clayton South VIC 3169, Australia

School of Marine Science and Technology, Sino-Europe Membrane Technology Research

Institute, Harbin Institute of Technology, Weihai 264209, P.R. China d

School of Engineering, The University of Edinburgh, The King’s Buildings, Robert Stevenson

Road, EH9 3FB, UK e

Department of Chemical Engineering, Monash University, Clayton VIC 3800, Australia

KEYWORDS: UiO-66, nanofiltration, polyamide, high flux, dye & antibiotics removal

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ABSTRACT

Membrane separation is a promising technology for extracting temperature-sensitive organic molecules from solvents. However, a lack of membrane materials that are permeable towards 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, whilst 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.


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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 challenging applications of molecular separations9-13 include the introduction of additional molecular transportation pathways in low flux polymers;9-10, 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 times higher



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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 ultra-high specific surface areas, can be used in 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. to the similar sizes between the intrinsic pores of ZIF-8 (0.32~0.34 nm)23,

This is due

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 sub-nanometre pore sizes and high hydro- and


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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 desalination,34 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 whilst reducing 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-66-NH2) and post-synthetic functionalization (UiO-66(Ti)) on pore size, structure, and defects that consequently impacted on 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. Ascribing to the hydrostability of UiO-66



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MOFs, the high water permeance of PA/UiO-66 membrane was maintained over 100 hours of continuous operation.

Functionalized UiO-66 MOFs studied 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 that consequently impacted on 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 MOF surface area.

Figure 1. (A) The 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


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provided additional molecular transport passageways in PA membranes; drastically enhance solvent flux without compromising the material’s high dye/solvent selectivity.



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Experimental 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 polyethyleneimine (PEI) was purchased from Sigma Aldrich (Mw 10 kDa, 95 % purity) and PAN powder was supplied by Qilu Petrochemical Company, which were used without purification. 98.0 % analytical grade trimesoyl chloride (TMC) 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 dyes and the properties of the solvents used in this work are shown in Figure 2 and Table 1, respectively.

Table 1. The solvent properties used in this work Basic properties Solvents MeOH EtOH IPA DMK MEK EA

Molecular

Molar volume

weight (g mol )

(cm mol )

Viscosity (cP)

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

-1

3

-1

Surface tension (dyne/cm) 22.6 21.9 21.3 18.8 21.0 23.5

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

Synthesis of MOFs UiO-66

and

functionalized

UiO-66

were

synthesised32

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) ZrCl4, 50 ml DMF, and 10 ml concentrated HCl before being sonicated for 20 minutes until fully dissolved. 5 mmol of ligand and 100 ml of the DMF were then added and the mixture was sonicated an additional 20 minutes 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. 9 ACS Paragon Plus Environment


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Table 2. The BET surface area of different kind of UiO type MOFs Type

Surface area (m2/g)

UiO-66

1310±46

UiO-66-NH2

820±27

UiO-66-(CH3)2

767±39

UiO-66-(Ti)

1447±31

The post-synthetic exchange of UiO-66-(Ti) was performed as reported before.14, 31 TiCl4(THF)2 was first synthesized according to the patent of Francesco et al.45 5.0 g (26.4 mmol, 2.8968 ml) of TiCl4 was dissolved in 50 ml of dichloroethane in a 250mL RBF under inert conditions. 7.62 g (105.8 mmol, 8.57 ml) of dried tetrahydrofuran (THF) was added at 0 °C to form a yellow solution. 100 ml of sodium dried n-hexane was used to precipitate a yellow powder, which was filtered and washed with n-hexane (2 x 50 ml). It was dried for 2-3 hours over a Schlenk line, and used immediately for post-synthetic exchange. Post-synthetic exchange of UiO-66 with TiCl4(THF)2 was carried out according to the work of Cohen and co-workers.46 0.17g (0.3 mmol) of TiCl4(THF)2 was dissolved in 10 ml of DMF. 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 6 times). The washed solids were immersed in a methanol bath for 3 days. The methanol was replaced every 24 hours. The solids were dried at 40 oC in a vacuum for 24 hours. 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 10


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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. 2.0 g of PEI (and 0.2 g UiO-66) were added into 100 ml water (water phase). To promote PA polymerization, 0.1 g TEOA and 0.1 g SDS were added to a PEI (UiO-66) solution. The mixture was stirred over 24 h to ensure fine dispersion of UiO-66 and uniform aqueous solutions. 0.15 g TMC was added to 100 ml n-hexane to form uniform the 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 these system through filtration. The composite polymer product was washed thrice with ethanol and water. Finally the composite polymer was immersed in 60 oC water for 30 min, and dried under 60 oC in a vacuum oven before characterization.



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Figure 3. (A)The 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.

Preparation of PA/UiO-66 composite membrane To generate PA thin film composite membranes, PAN substrates were first fabricated by phase inversion reported everywhere.47-48 Specifically, 18 wt.% of PAN, 2 wt.% of PEG-800 and 80 wt.% of NMP were stirred under 70 oC for 4 hours to form uniform solution. Then, the solution degassed overnight at 70 oC. Prior knife-casting, the solution was allowed to cool down to room temperature. The solution was first poured on to a glass plate and knife-cast into a thin membrane using a 150 µm metal blade, and immersed in 25 oC deionized water 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 water phase (containing 2.0 g PEI and 0.2 g UiO-66) for a stipulated time, and air-dried in a fumehood. Subsequently, these PAN substrates were immersed in 100 ml of organic phase containing 0.15 g TMC for 90 s 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 oC for 30 min. The final TFC membrane was immersed in fresh deionized water before characterization or performance testing.

Characterization 12 ACS Paragon Plus Environment

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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 Cukα radiation (40kV, 40mA) equipped with a LynxEye detector was deployed to obtain the XRD patterns (Figure S1). Samples were scanned over the 2θ range 5° to 40° with a step size of 0.02° and a count time of 1.6 second per step.

These XRD patterns confirmed that all UiO-type

MOFs studied here were crystalline. The viscosity of polymer solutions were characterized using a capillary viscometer. Attenuated total reflectance-Fourier transforminfrared (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

analysed using an X-ray photoelectron spectrometer (XPS) VG ESCALAB 220i-XL (UK) equipped with a twin crystal monochromated Al Ka 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 oC twice, at a heating rate of 10 oC min-1. The cross-section and surface morphologies of PEI/UiO-66 composite, PA-based composite materials were characterised with a FESEM (MERLIN Compact, Zeiss Company). Membrane samples were cryo-fractured 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 13



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evaporation, the samples was coated by iridium before characterization.

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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 seconds 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 pressurised

with nitrogen to 5 bar at room temperature. During filtration, the feed solution was stirred at 11.66 Hz (700 rpm) to minimise concentration polarization. Permeate samples were collected in flasks as a function of time, weighed and analysed. The water/solvent flux and water/solvent permeance were calculated using the following equations: F=V/(A×t)

(1)

Permeance=F/∆P

(2)

where F represents the solvent flux (L m-2 h-1), 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); 14 ACS Paragon Plus Environment

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and ∆P is trans-membrane 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-CP/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, and λmax of SO = 548 nm, λmax of MO=463 nm, λmax of azithromycin=482 nm).

Each data point is an average of three repetitions

of each test, with ± 5 % standard deviation.

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 polyethyleneimine (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. 0.2 wt. % of MOFs were added to the PEI/water solution and sonicated and stirred for 24 h to ensure uniform dispersion. An ultrathin layer of PA/MOF composite was formed instantaneously at the solvents interface when the PEI/MOF/water mixture was added to the TMC/n-hexane solution. XPS and FT-IR confirmed that MOFs were successfully incorporated into the PA matrix (ESI).



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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 Post-synthetic exchange of Zr (IV) ions with Ti ions generated more defects in, 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 occurred

(Figure 4). EDX analysis of these films (Figure 4E & Figure 4F) 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/MOF/water solutions (Figure S3). The addition of non-porous nanoparticles tend to increase polymer solution viscosities,52-54

while porous nanoparticles like porous aromatic

frameworks,55-57 and hypercrosslinked polymers

56-58

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 non-bonding interactions between MOFs and PEI chains underpinned by the threading of PEI chains into MOF pores,56-57, 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 archetypal porous aromatic framework, PAF-1, and hyper-crosslinked 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 16 ACS Paragon Plus Environment

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passageways for solvent transport, greatly enhancing the flux of thin-film composite PA membranes.

Figure 4. The morphologies of (A) PA and (B) PA with higher magnification indicates no nanoparticle structure appear in the pristine PA (C) PA/UiO-66 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

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 17 ACS Paragon Plus Environment


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observed in PAN substrates, while a lack of apparent dense layer on the PAN substrate indicated formation of a thin selective layer that is highly compatible with the substrate. of PA selective layers led to a rough surface.

The deposition

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.

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


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Figure 6. The 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 . 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 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 solute rejection rate. It is noteworthy to mention that the incorporation of UiO-66 increased membrane thickness. 19 ACS Paragon Plus Environment


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Typically, thicker membranes with similar pores demonstrate lower solvent permeances based on 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, whilst 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 Amongst all functionalized UiO MOFs studied here, defects generated by the post-synthetic exchange of Zr with Ti ions in UiO-66 with the highest BET surface areas studied here.

14, 31, 39

yielded MOFs

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, (CH3)2 groups reduced BET surface areas;32, reducing molecular transportation pathways.

44, 50-51

Therefore, the incorporation of UiO-66-NH2 and

UiO-66-(CH3)2 MOFs lowered the solvent permeances of PA membranes.

Given that

UiO-66-NH2 demonstrated better affinity to water than UiO-66-(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 20 ACS Paragon Plus Environment

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wrapping around the MOFs); leading to a significant increase of water/solvent permeance without any loss of dye rejection as well.

Amongst all UiO-66 MOFs studied here, UiO-66 (Ti)

interacted best with PEI; thereby promoting better MOF dispersion.

When compared to pristine

UiO-66, UiO-66-NH2 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 and 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). interactions with PEI.

MOF functionalization also altered pore sizes that impacted on

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 is 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 and Figure 7D. 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; attributing to different 21 ACS Paragon Plus Environment


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solute-solvent interactions.65 Compared with pure PA membranes, the incorporation of UiO-66 MOFs improved permeance stability even under high pressures (Figure S7).

Figure 7. The 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.

Different from other MOF-based mixed matrix membranes,20-27 the chemical, and hydrostability of UiO-66 MOFs ensured that the additional molecular transport pathways were retained within PA TFC NF membranes.

After 100 hours 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 the ultrathin PA matrix collapsed. 66-68 MOFs reduced compaction impact 22 ACS Paragon Plus Environment

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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

post-synthetic 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 decline with operation time.

Meanwhile, the infiltration of PA chains into the

smaller pores of UiO-66-(CH3)2 and UiO-66-NH2

32,42,48-49

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 amongst 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.



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


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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 PA/UiO-66(Ti) membranes. Considering the extra steps required for the post-synthetic exchange of UiO-66(Ti) MOFs, PA/UiO-66 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 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 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. MOFs drastically enhanced membrane flux.

The incorporation of UiO-66

The membrane flux was 67.2 L m-2 h-1 under 5 bar

of operating pressure with a 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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Figure 9. Comparison of separation performances of our membranes with the state-of-the-art NF membranes10, 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 PEs76 and PEEK77 membranes removing RB from IPA; solid star=PA/UiO-66 membranes; open star=PA membranes.


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

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. Non-bonding, intimate interactions between MOFs and PA chains were tailored through a judicious choice of organic ligands and post-synthetic exchange to ensure uniform MOF dispersion within the PA selective layer.

Amongst all MOFs

from the UiO family studied here, the intrinsic pores (0.6~1.0 nm) of UiO-66 MOFs are most 27 ACS Paragon Plus Environment


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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 anti-compaction 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.

Conflict of Interest: The authors declare no conflict of interest. Supporting Information Available: The FTIR, water contact angle, the viscosity of the PEI/UiO-66 solutions, and some separation performance of the PA/UiO-66 membranes are included in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Correspondence should be addressed to [email protected], [email protected] and [email protected] Acknowledgements This work was supported by National Natural Science Foundation of China (21676063, U1462103), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute 28 ACS Paragon Plus Environment

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Technology) (No. 2017DX07), and HIT Environment and Ecology Innovation Special Funds (HSCJ201619). MRH, KK and CHL acknowledge the Science and Industry Endowment Fund (SIEF).

MRH, and AJH acknowledge the generous support of the CSIRO Office of the Chief

Executive science team.

MRH acknowledges FT 130100345.

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