Improving Water-Treatment Performance of Zirconium Metal-Organic

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Improving Water-Treatment Performance of Zirconium MetalOrganic Framework Membranes by Post-Synthetic Defect Healing Xuerui Wang, Linzhi Zhai, Yuxiang Wang, Ruitong Li, Xuehong Gu, Yi Di Yuan, Yuhong Qian, Zhigang Hu, and Dan Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12750 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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ACS Applied Materials & Interfaces

Improving Water-Treatment Performance of Zirconium Metal-Organic Framework Membranes by Post-Synthetic Defect Healing Xuerui Wang,†,§ Linzhi Zhai,† Yuxiang Wang,† Ruitong Li,† Xuehong Gu,‡ Yi Di Yuan,† Yuhong Qian,† Zhigang Hu,†,ǁ and Dan Zhao*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4

Engineering Drive 4, 117585 Singapore ‡

College of Chemistry and Chemical Engineering, Nanjing Tech University, No. 5 Xinmofan

Road, Nanjing 210009, P. R. China

KEYWORDS Metal-Organic Frameworks, Polycrystalline Membranes, Hollow Fibers, UiO-66, Post-Synthetic Defect Healing, Water Treatment

ACS Paragon Plus Environment

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ABSTRACT Microporous metal-organic frameworks (MOFs) as building materials for molecular sieving membranes offer unique opportunities to tune the pore size and chemical property. The recently reported polycrystalline Zr-MOF membranes have greatly expanded their applications from gas separation to water treatment. However, Zr-MOFs are notoriously known for their intrinsic defects caused by ligand/cluster missing, which may greatly affect the molecular sieving property of Zr-MOF membranes. Herein, we present the mitigation of ligand-missing defects in polycrystalline UiO-66(Zr)-(OH)2 membranes by post-synthetic defect healing (PSDH), which can help to increase the membranes’ Na+ rejection rate by 74.9 %. Intriguingly, the membranes also exhibit excellent hydrothermal stability in aqueous solution (> 600 h). Our study proves the feasibility of PSDH in improving the performance of polycrystalline Zr-MOF membranes for water-treatment applications.


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INTRODUCTION Metal-organic frameworks (MOFs), a type of porous and crystalline materials, are constructed with metal ions/clusters and organic ligands via coordination bonds.1-2 MOFs feature welldefined and highly-tunable porous structures because of the vast types of secondary building unit, ligand topology, connectivity, and chemical functionality.3 Therefore, MOFs are promising building materials for molecular sieving membranes that can separate mixtures based on molecular size and shape of individual components.4-5 Over the past decade, intensive efforts have been made to fabricate MOF-based polycrystalline or composite membranes for gas separation.6-9 However, the practical application of MOF-based membranes for liquid separation, especially those involving water, is severely suppressed by the poor hydrothermal stability of MOFs.10 For example, polycrystalline MOF-5 membranes are stable in organic solvents,11 but MOF-5 crystals would degrade upon exposure to moist ambient air.12 Polycrystalline ZIF-8 membranes have been demonstrated competent to reject ions in seawater through pervaporation.13 However, a recent study unveils the dynamic degradation of ZIF-8 in aqueous solutions through continuous Zn2+ leaching, indicating the challenge of using ZIF-8 membranes for separations involving water.14-15 The hydrothermal stability of MOFs is especially dominated by the strength of metal-ligand bonds.16-17 Thus, researchers are looking for more robust coordination bonds such as Hf-O (802 kJ mol-1) or Zr-O (776 kJ mol-1) to construct MOF-based membranes.18 Recently, Li and coworkers pioneered the synthesis of polycrystalline UiO-66(Zr) membranes by in-situ solvothermal approach, and explored their applications in seawater desalination and pervaporation dehydration.19-20 Subsequently, hydrophobic UiO-66(Zr)-(CH3)2 membrane was developed for CO2/N2 separation.21 The aperture size of polycrystalline UiO-67(Zr) membrane


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was tailored by loading azobenzene molecules to afford a H2/CO2 separation factor of 14.7 by Knebel et al.22 Just recently, the hydrothermal stability of UiO-66(Zr)-NH2 membrane was experimentally demonstrated with seawater desalination by pervaporation at 75 °C.23 Despite their superior water stability, Zr-MOFs are increasingly found to have intrinsic defects caused by ligand missing, leading to dramatically enhanced porosity and aperture size that are unfavorable for molecular sieving separation.24-26 Zhou et al. confirmed that, on average, one out of 12 ligands are missing from UiO-66(Zr) framework based on neutron powder diffraction study.24 The defective structure was subsequently identified in molecular-level precision by Yaghi et al.25 The 8 % coordination vacancies generated by the missing bridging ligands from Zr6O4(OH)4 clusters are mainly charge-compensated by water molecules, evidenced by X-ray diffraction of the in-situ activated single crystal.25 Similar defective structure has also been discovered in the isoreticular counterpart UiO-67(Zr) by Farha et al.26 Such intrinsic defects in Zr-MOFs will, unfortunately, be inherited by the polycrystalline Zr-MOF membranes. For example, the rejection rate of monovalent ions (e.g., Na+ with a hydrated diameter of 7.16 Å) in UiO-66(Zr) membrane is relatively lower than the molecular sieving selectivity determined based on the theoretical 6.0 Å aperture of UiO-66(Zr) framework, indicating a defective structure of the membrane.19, 27-28 In addition, the CO2/N2 selectivity is merely 2.2 for the polycrystalline UiO-66(Zr)-(CH3)2 membrane, which is much lower than the expected result assuming a perfect crystalline membrane layer.21 Theoretically, the non-bridging groups, such as water molecules in UiO-66(Zr),25 can be fully displaced by fitting dicarboxylates, which can seal the framework defects by bridging two adjacent clusters together.29-31 Denny Jr. and Cohen firstly demonstrated the feasibility of in-situ ligand exchange for MOFs incorporated in mixed-matrix membranes.30 Recently, Yaghi et al.


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achieved mechanically robust MOF-520-BPDC by precisely replacing the formate ligand with 4,4’-biphenyldicarboxylate (BPDC) as a “girder” in the mechanically unstable MOF-520.31 Herein, we report the first study of mitigating the intrinsic defects of polycrystalline UiO-66(Zr)(OH)2 membranes by post-synthetic defect healing to afford highly selective membranes for water treatment. EXPERIMENTAL SECTION Preparation of polycrystalline UiO-66(Zr)-(OH)2 membranes The polycrystalline UiO-66(Zr)-(OH)2 membranes were fabricated on the outer surface of porous Al2O3 hollow fibers (O.D.: 1.96 mm, length: 50 mm, porosity: ca. 40 %, average pore size: 1.6 µm) by secondary growth method. The hollow fibers were fabricated according to the previous report.32 The supports were firstly seeded with UiO-66(Zr)-(OH)2 crystals by in-situ solvothermal synthesis in the hydrous mother solution with a molar composition of 1 ZrCl4: 1 DOBDC: 1 H2O: 500 dimethylformamide (DMF): 100 formic acid. The crystallization was conducted at 120 °C for 1 day in a Teflon-lined stainless steel autoclave. Prior to in-situ seeding, both ends of the Al2O3 hollow fibers were capped with Teflon tape to ensure no seed formation on the inner surface. After cooling to room temperature, the seeded Al2O3 hollow fibers were intensively washed with DMF, ethanol, and then dried at room temperature. The crystals within the membrane layer were further fused together to form a continuous and well-intergrown polycrystalline UiO-66(Zr)-(OH)2 membrane by secondary growth in another anhydrous mother solution with a molar composition of 1 ZrCl4: 1 DOBDC: 500 DMF: 100 formic acid. The synthesis was conducted at 120 °C for 3 days. The membranes were intensively activated by soaking in fresh DMF for 12 h and repeating for 5 times. After that, the residual ligands and


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DMF were completely exchanged with hot ethanol using Soxhlet extraction. Before membrane performance tests, the membrane was dried at room temperature overnight. Water-treatment performance test The separation performance of UiO-66(Zr)-(OH)2 membranes were tested with aqueous solutions at a pressure difference of 3 bar using a home-built membrane filtration apparatus at room temperature (Figure S1). One end of the membrane was sealed with silicone and the other open end was assembled in the module. The effective length of the membrane is approximately 40 mm. The water flux and rejection of metal ions and methyl blue were tested using DI water, 2000 ppm aqueous salt solutions (i.e., FeCl3, CrCl3, ZnCl2, MgCl2, NaCl, H3BO3) and 100 ppm aqueous methyl blue solution. To attain stable separation performance, each test was initially stabilized for 12 h to eliminate the adsorption effects, then the feed and permeate samples were collected every 12 h for three times. Meanwhile, the feed solution was renewed every 12 h by the needle valve connected with the membrane module. Water flux (F, kg m-2 h-1), water permeance (P, kg m-2 h-1 bar-1), and rejection (R, %) were calculated as follows: w A∆t

(1)

F p f - p p - ∆π

(2)

F=

P=

R=

C f - Cp Cf

×100%

(3)

where w is the weight of water collected from permeate side, kg; A is the effective membrane layer, m2; ∆t is the collecting time, h; Pf and Pp are the pressure in the feed and the permeate, bar; ∆π is the difference of osmosis pressures between the feed and permeate solutions, bar; Cf and Cp are the ion/methyl blue concentration in the feed and the permeate, respectively, which are determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) or UV-


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Vis spectrometry. The water flux and rejection were obtained from the averaged value of three data points. Regarding the acid resistance test, the polycrystalline UiO-66(Zr)-(OH)2 membrane was soaked in aqueous methyl blue solution (50 mL) under acidic conditions (pH = 1) for 7 days and washed with DI water for further characterization. Post-synthetic defect healing The as-synthesized hollow fiber polycrystalline UiO-66(Zr)-(OH)2 membrane was soaked in a mother solution with a molar composition of 2 DOBDC: 500 DMF. Post-synthetic defect healing was performed at 120 °C for 1 day in a Teflon-lined stainless steel autoclave. The UiO-66(Zr)(OH)2 crystal particles were also solvothermally treated in the same condition to prepare the compared sample. All the membranes and crystal particles were completely activated as mentioned above. Characterization The morphology of Al2O3 hollow fiber, seeded Al2O3 hollow fiber, and the as-synthesized polycrystalline UiO-66(Zr)-(OH)2 membrane was observed by field emission scanning electron microscopy (FESEM, JSM-7610F, JEOL). The crystal phases were measured by powder X-ray diffraction (PXRD, MiniFlex 600, Rigaku) equipped with a Cu sealed tube (λ = 0.154178 nm) with a scan rate of 0.04 deg·s-1 in the 2θ range of 5-50°. The sample (50 mg) with the strongest peak intensity at 7.4° was denoted as 100 % crystallinity, and the relative crystallinity was obtained by calculating the ratio of the peak intensity according to the previous report.33 X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos AXIS Ultra XPS system (Kratos Analytical Ltd) with a monochromated Al Kα radiation (hλ = 1486.6 eV). Fourier transform infrared spectroscopy (FTIR) spectra were obtained with a Nicolet 6700 FTIR spectrometer. Thermogravimetric analysis (TGA) were performed using a Shimadzu DTG-60AH.


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UiO-66-(OH)2 samples (ca. 8 mg) were added to platinum crucibles before measurement. Each TGA run was made in two different heating stages under a simultaneous feed of air (20 mL min1

). First step, the samples were heated in the temperature range of 20 °C to 100 °C, and kept for

30min; second step, the samples continued to be heated to 950 °C at a rate of 5 °C min-1. RESULTS AND DISCUSSION Preparation and characterization of UiO-66(Zr)-(OH)2 membranes UiO-66(Zr)-(OH)2 is isoreticular to UiO-66(Zr) with the ligand benzenedicarboxylate (BDC) being replaced by 2,5-dihydroxy benzenedicarboxylate (DOBDC). The extra hydroxy groups help to reduce the aperture size to around 4.0 Å in UiO-66(Zr)-(OH)2,34 which should be more beneficial toward the rejection of monovalent ions during seawater desalination. The membranes were fabricated on porous ceramic hollow fiber substrates (SEM shown in Figure 1a, and photo shown in Figure S2), which could endow high module packing density and reduced capital investment for practical applications.35 Formic acid was used as the modulator to promote the nucleation (Figure S3a, S3b). Initially, we attempted to prepare the membrane by in-situ growth from the anhydrous mother solution with a molar composition of 1 Zr: 1 DOBDC: 500 DMF: 100 formic acid at 120 °C for 3 days. Unfortunately, we could not observe any crystals grown on the hollow fiber surface (Figure S4b, S4g). However, the UiO-66(Zr)-(OH)2 powder collected from the bottom of the autoclave exhibits a relative crystallinity of 26 % (Figure S3c, S3d) and reasonable Brunauer-Emmett-Teller (BET) surface area (564 m2 g-1, Figure S5), suggesting that the UiO-66(Zr)-(OH)2 crystals prefer homogeneous nucleation from solution rather than crystal growth on substrates, possibly due to the limited heterogeneous nucleation sites on substrates. Well-intergrown membranes could be obtained after 9-day reaction in the anhydrous solution (Figure S4g, S4h, S4i, and S4j). Interestingly, we found that adding a trace amount of water in


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the reaction media could dramatically shorten the induction period to 1 day (Figure S4k). This is probably due to the easier formation of Zr4+ oxo/hydroxo clusters favored by water.36 Another possible reason is the facilitated attachment of nucleus onto the substrate by water as previously demonstrated in zeolite membrane.37 However, visible cracks can be easily found from the membrane surface (Figure S4l, S4m), which can be attributed to the shrinkage of Zr6O4(OH)4 clusters once the charge-compensating water molecules are removed by high-energy electron beam under the high vacuum condition (1.91×10-4 Pa) during SEM observation. Previously, Yaghi et al. announced a 0.55 % shrinkage of the unit cell of UiO-66(Zr) under vacuum.25 A video visually demonstrates the process of crack formation in the membranes under SEM observation (Video S1).

Figure 1. Scanning electron microscope (SEM) images of porous alumina hollow fiber (a), seeds layer (b, c), and membrane layer (d, e). The red and green lines in (e) indicate the intensity of Zr and Al elements by EDS line analyses, respectively. (f) Energy dispersive X-ray (EDX) elemental mapping of the cross-section shown in (e).


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It has been well-documented that the nucleation and growth of polycrystalline membrane can be well balanced by secondary growth method.5, 7 Here, we propose to first sow the substrates with UiO-66(Zr)-(OH)2 crystal seeds using the hydrous solution (kinetically favorable condition), and then grow crack-free membrane by heating the seeded substrates in the anhydrous solution (thermodynamically favorable condition, illustration of the process shown in Figure S6). Monolayered irregular crystal seeds, determined by almost identical crystal size (0.75 µm) and layer thickness (0.88 µm), were obtained on the substrates after 1-day reaction in the hydrous solution (Figure 1b, 1c, and Figure S4k). These crystals were proven to be UiO66(Zr)-(OH)2 by powder X-ray diffraction (PXRD, Figure S7c). The in-situ synthesized seeds effectively prevent the peeling of seed layer compared to that obtained by external seeding techniques, such as dip-coating, rubbing, and vacuum coating.38 The same conclusion was obtained by Jin et al. as well, who pioneered a reactive seeding method wherein the substrate acts as the precursor of metal ions for nucleation and crystal growth.39 Intriguingly, wellintergrown and crack-free polycrystalline UiO-66(Zr)-(OH)2 membranes can be obtained after 3day secondary growth in the anhydrous solution (Figure 1d, 1e, and PXRD shown in Figure S7d). The crystal grain size increases to 2.15 ± 0.29 µm through the epitaxial growth of the previously nucleated irregular seeds, and the membrane thickness is 3.5 µm. Homogeneous distribution of Zr element is clearly demonstrated by EDX line and mapping analyses (Figure 1e, 1f, and Figure S8). The relative round crystal shape on the membrane surface, which is different from the typically well-defined octahedral shape,19 further proves the slow nucleation and growth in anhydrous solution during 2nd step to prevent the formation of cracks. The slow growth could minimize generation of membrane defects inside the membrane layer.21 Separation performance


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The integrity of the membrane was initially evaluated by pure water permeation under a pressure difference of 3 bar. The fresh UiO-66(Zr)-(OH)2 membranes prepared by in-situ growth and 2nd growth exhibited a water flux of 2.40 kg m-2 h-1 and 6.72 kg m-2 h-1, respectively, which are much lower than those of the pristine and seeded alumina hollow fiber substrates (Table 1). These results indicate the formation of a continuous membrane layer on the porous alumina hollow fibers in line with the SEM image (Figure 1e).

Table 1. Pure water flux of substrates and membranes Sample

Synthesis condition

Fluxǁ/ kg m-2 h-1

Substrate

N.A.

9700 ± 150

Seeded substrate

120 °C for 1 d

376 ± 72

Membrane#

120 °C for 3 d

6.72

Membrane*

120 °C for 9 d

2.40

ǁ

pressure difference of 3 bar; # 2nd growth in the anhydrous solution; * in-situ growth in the anhydrous solution.

Separation performance of the membrane was further evaluated by pressure-driven permeation of aqueous solutions containing metal salts or methyl blue. To our surprise, the water fluxes drastically decrease down to 1.05, 0.69, 0.72, and 1.06 kg m-2 h-1 for the aqueous salt solutions containing 2000 ppm FeCl3, CrCl3, ZnCl2, or NaCl, respectively (Part 1 in Figure 2). A similar phenomenon was observed in the membrane synthesized by in-situ solvothermal synthesis for 9 days (Table S2). Considering the difference of osmotic pressure among pure water and the aqueous salt solutions, we calculated the water permeance based on normalized driving force.40 The permeance of pure water is 2.24 kg m-2 h-1 bar-1, which is still much higher than those of salt


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solutions (0.45, 0.28, 0.28, 0.40 kg m-2 h-1 bar-1 for FeCl3, CrCl3, ZnCl2, and NaCl solution, respectively).

4

0

80

Pure water

60 40

Rejection/ %

Methyl blue Na+

Mg2+

1

H3BO3

2+

Zn

Cr3+

Fe3+

+

Zn2+

Fe3+

Cr3+

-2

3 2

100

Part 2

Na Pure water

-1

Part 1

Flux/ kg m h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 0

0

200 400 Operation time/ h

600

Figure 2. Separation performance of UiO-66(Zr)-(OH)2 membrane for the removal of metal ions and methyl blue in aqueous solutions for a continuous operation period of 600 h. Feed: Pure water, 2000 ppm salt solutions (FeCl3, CrCl3, ZnCl2, MgCl2, NaCl, H3BO3), or 100 ppm methyl blue solution; Test condition: 3 bar at room temperature. Insert: photo of feed solution (left) and the permeate (right) for FeCl3 and methyl blue. After 170 h of continuous operation, the membrane was thoroughly washed with pure water to remove the adsorbed metal ions. However, the water flux of washed membrane stabilized at 1.26 kg m-2 h-1, which is only 18.75 % of the original value (Table 1 and Part 2 in Figure 2). Considering that the Stokes diameter of hydrated Fe3+ can be as high as 8.12 Å,41 it would be difficult to permeate through the UiO-66(Zr)-(OH)2 membrane layer which theoretically should have an aperture size of approximate 4.0 Å. Thus, we speculate that the Fe3+ might be irreversibly trapped in the defects of UiO-66(Zr)-(OH)2 crystals caused by ligand or cluster


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missing.24, 42 This agrees well with the blocking effects of MFI-type zeolite membrane by Fe3+ cations, which causes a decrease in water flux from 5.6 kg m-2 h-1 to 1.2 kg m-2 h-1 because of the rigid ion blocking effect.43-44 However, it is worth noting that the metal ions might metallate with the hydroxyl groups of DOBDC during the separation process, leading to the reduced water flux compared to the initial pure water flux. Previously, Cohen and co-workers reported that 2,3dihydroxyterephthalate exchanged UiO-66 films could metallate with Fe3+ by soaking in FeCl3 solution at room temperature for 24 h.45 In order to test this hypothesis, UiO-66(Zr)-(OH)2 crystals were soaked in an aqueous FeCl3 solution with the condition identical to that of the membrane tests. Interestingly, Fe could be clearly detected by XPS even after thorough washing as reported by Cohen and co-worker (Figure 3a and Figure S9).46 More interestingly, a small cavity of 4.65 Å was identified by N2 sorption tests in UiO-66(Zr)-(OH)2 crystals treated with FeCl3 solution (highlight in cyan, Figure 3b, 3c). However, the Fourier transform infrared spectroscopy (FTIR) spectra were identical between the pristine and FeCl3-treated MOFs (Figure 3d). Therefore, we believe that the Fe3+ ions are just physical adsorbed or trapped within the defects of polycrystalline UiO-66(Zr)-(OH)2 membrane resulting in the reduced water permeance and the slight increase of rejection ratio (Part 2 in Figure 2). The separation performance of the as-synthesized UiO-66(Zr)-(OH)2 membrane is quite stable during the 600 h operation as mentioned above, indicating great potential for practical application. The acid resistance of the membrane was further evaluated by soaking the membrane in 100 ppm methyl blue solution with a pH value of 1. After 7-day soaking, the octahedral UiO-66(Zr)-(OH)2 crystals could still be clearly identified from the membrane (Figure S10), indicating its superior water stability even in acidic conditions.


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Figure 3. (a) XPS Fe 2p spectra of FeCl3-treated UiO-66(Zr)-(OH)2 crystals. (b-c) N2 sorption isotherms at 77 K and pore size distribution based on DFT calculation. (d) FTIR spectra of the pristine and the FeCl3-treated UiO-66(Zr)-(OH)2. Fresh UiO-66(Zr)-(OH)2 crystals were treated in 0.2 wt.% FeCl3 solution under 3 bar for 24 h. Separation mechanism The membrane selectivity is largely dependent on the diameter of probe ions or molecules (Figure 4a). The rejection rate is 26 % for Na+ (hydrated diameter: 7.16 Å), 42.5 % for Zn2+ (8.60 Å), 54.7 % for Fe3+ (9.14 Å), and 98.7 % for methyl blue (21 Å × 13.6 Å × 8.1 Å). Meanwhile, the water flux is in the order of pure water (0.81 kg m-2 h-1) > methyl blue solution (0.77 kg m-2 h-1) > Na+ solution (0.73 kg m-2 h-1) > Fe3+ solution (0.56 kg m-2 h-1) > Zn2+ solution (0.45 kg m-2 h-1). Thus, we speculate that the effective pore size of the membrane is between 9.1


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and 12.6 Å, which is almost identical to the defective aperture size of UiO-66(Zr)-(OH)2 crystals caused by ligand missing. A wider micropore of 11.79 Å could be clearly observed from the pore size distribution data calculated from N2 sorption isotherms (Figure 3c). The pores with sizes ranging from 10 to 11 Å were also detected from the defective UiO-66(Zr).47 Therefore, hydrated Na+ ions (7.16 Å) can diffuse through the porous membrane layer without much resistance in this case, leading to a poor Na+ rejection (26 %). The relatively low water flux for aqueous solutions containing multivalent metal cations (highlighted in Figure 4a) can be attributed to the delayed transport of water molecules by the slow diffusion of hydrated Mg2+, Zn2+, or Fe3+ ions because of the single-file diffusion within the microporous channels (Figure 4b).48 Notably, the water permeance for Na+ solution is 0.286 kg m-2 h-1 bar-1, which is two times higher than that of the reported polycrystalline UiO-66(Zr) membranes (0.14 kg m-2 h-1 bar-1).19 This enhancement can be attributed to the highly hydrophilic channels caused by the hydroxyl groups of DOBDC ligands. In addition, the defective pores of UiO-66(Zr)-(OH)2 membranes with wider aperture size may be another reason.


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Figure 4. Separation performance of polycrystalline UiO-66(Zr)-(OH)2 membrane for the removal of metal ions and methyl blue in aqueous solutions (a) and illustration of single-file diffusion of water molecule and hydrated Mn+ ion (b). Test condition: 2000 ppm salt solutions and 100 ppm methyl blue solution at 3 bar and room temperature. Post-synthetic defect healing Compared to attaining higher water permeance, a more important target for water-treatment membrane is to attain improved selectivity for water over solutes. For example, NaCl rejection could be as high as >99.5 % for polymeric seawater reverse osmosis membranes.49 We reason that the poor monovalent salt rejection rate of our membrane is due to the defects caused by


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ligand missing. Thermogravimetric analyses (TGA) were conducted on UiO-66(Zr)-(OH)2 crystals to quantify these defects: each Zr6O4(OH)4 cluster is coordinated with 4.68 DOBDC ligands on average, equivalent to 21.7 % coordinative defects (Figure 5a). This is almost two times higher than the defects reported in UiO-66(Zr) (~10 %),24-25 possibly due to the different ligand reactivity and synthetic condition. The coordinative defects are normally occupied by mono-coordinated modulators,24 water molecules,25 hydroxide ions,26 chloride ions,50 etc. We hypothesize that by replacing two monocoordinated moieties with one bridging dicarboxylate ligand, a process we called post-synthetic defect healing (PSDH, Figure 5b), the defective large aperture size can be reduced affording better separation performance of the resultant membranes. Briefly, the as-synthesized membrane was solvothermally treated in DOBDC solution at 120 °C for 1 day. As expected, the coordinative defects can be mitigated after PSDH: each Zr6O4(OH)4 cluster is now coordinated with 5 ligands on average (Figure 5a), meaning 24 % of the coordinative defects have been mitigated by re-coordinating with DOBDC ligands. Meanwhile, this causes a slight decrease in the surface area (from 883.2 m2/g to 703.2 m2/g) and pore volume (from 0.4102 cm3/g to 0.3487 cm3/g, Table S3). The result is consistent with the previous investigation on defective UiO-66NH2.51 The improved separation performance of membranes after PSDH was confirmed by water-treatment tests using aqueous solutions containing Na+ or methyl blue (Figure 5c). The rejection rate of Na+ is now 45 % (74.9 % increase compared to pristine membranes), while that of methyl blue reaches as high as 99.8 %, which is even higher than the current polymeric membranes.52 Meanwhile, there is only a minor decrease of the water flux after PSDH (from 0.77 kg m-2 h-1 to 0.69 kg m-2 h-1, 10.4 % decrease). The water flux of the UiO-66-(OH)2


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membrane after PSDH is still higher than that of the polycrystalline zeolite membranes (e.g., SOD and MFI, Table S4).

Figure 5. (a) TGA curves of UiO-66(Zr)-(OH)2 crystals before and after PSDH; the weight was normalized with respect to the ZrO2 residue left after heating up to 650 °C in air. (b) Scheme of PSDH by relinking two adjacent Zr6O4(OH)4 clusters (blue polyhedron) by one DOBDC ligand. The yellow area indicates the effective aperture size for molecular sieving separation; the cyan area indicates the location of the re-coordinated DOBDC within the framework. (c) Separation performance of the membrane before and after PSDH. CONCLUSIONS In summary, we present an efficient approach to heal the intrinsic defects in water-stable polycrystalline UiO-66(Zr)-(OH)2 membrane by post-synthetic defect healing. The healed membranes exhibit NaCl rejection of 45 % with water permeance of 0.285 kg m-2 h-1 bar-1, and


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methyl blue rejection of 99.8 % with water permeance of 0.23 kg m-2 h-1 bar-1. Our approach is based on the dynamic feature of coordination bonds in MOFs, and is reminiscent of the solventassisted linker exchange reactions in bulk MOF crystals.29 This approach offers a novel platform for further optimization and functionalization of polycrystalline MOF membranes from molecular levels. In the future, intensive research work would be highly desired to optimize the membrane structure as well as to expand the applications in niche markets. ASSOCIATED CONTENT Supporting Information. XPS, PXRD, gas sorption isotherms, SEM image. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

[email protected]

Present Addresses §

Catalysis Engineering, Chemical Engineering Department, Delft University of Technology,

Van der Maasweg 9, 2629 HZ Delft, The Netherlands ǁ

Department of Chemical Engineering and Biotechnology, University of Cambridge, West

Cambridge Site, Philippa Fawcett Drive, Cambridge, CB3 0AS Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ORCID Xuerui Wang: http://orcid.org/0000-0003-2220-7531 Linzhi Zhai: http://orcid.org/0000-0001-9785-0973 Yuxiang Wang: http://orcid.org/0000-0002-6945-6431 Ruitong Li: http://orcid.org/0000-0002-4787-582X Yi Di Yuan: http://orcid.org/0000-0002-7905-8218 Yuhong Qian: http://orcid.org/0000-0003-3820-8731 Zhigang Hu: http://orcid.org/0000-0003-1916-6484 Dan Zhao: http://orcid.org/0000-0002-4427-2150 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is supported by National University of Singapore (CENGas R-261-508-001-646), Ministry of Education - Singapore (MOE AcRF Tier 1 R-279-000-472-112), and Agency for Science, Technology and Research (PSF R-279-000-475-305, IRG R-279-000-510-305). REFERENCES 1.

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Improving Water-Treatment Performance of Zirconium Metal-Organic

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