Impacts of MetalâOrganic Frameworks on Structure and Performance

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Impacts of Metal-organic Frameworks on Structure and Performance of Polyamide Thin-film Nanocomposite Membranes Yangying Zhao, Yanling Liu, Xiao-Mao Wang, Xia Huang, and Yuefeng F Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01923 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Applied Materials & Interfaces

Impacts of Metal-Organic Frameworks on Structure and Performance of Polyamide Thin-Film Nanocomposite Membranes Yang-ying Zhao a, Yan-ling Liu a, Xiao-mao Wang a,*, Xia Huang a,*, Yuefeng F. Xie a,b

a State

Key Joint Laboratory of Environment Simulation and Pollution Control, School

of Environment, Tsinghua University, Beijing 100084, China b

Environmental Engineering Programs, The Pennsylvania State University,

Middletown, PA 17057, USA

Keywords: Metal-organic frameworks (MOFs); Thin-film nanocomposite (TFN) membrane; Cross-linking degree; Membrane structure; Pharmaceuticals.

________________________________________ * Corresponding author: Dr. Xiao-mao Wang Tel.: +86-10-6278 1386; FAX: +86-10-6277 1472 Email: [email protected] * Corresponding author: Prof. Xia Huang Tel.: +86-10-6277 2324; FAX: +86-10-6277 1472 Email: [email protected] 1

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Abstract Metal-organic frameworks (MOFs), a class of hybrid organic-inorganic materials, have recently attracted tremendous interests in the fabrication of thin-film nanocomposite (TFN) membranes with exceptional permselectivity. However, the structureperformance relationship of such membranes, which is a function of both MOF type and membrane fabrication procedure, has not been elucidated in the literature. In this study, three types of hydro-stable MOFs, namely MIL-53(Al), NH2-UiO-66 and ZIF8, were used to fabricate TFN nanofiltration (NF) membranes via both blending (BL) and preloading (PL) interfacial polymerization (IP) methods. Results show that the incorporation of MOFs could enhance water permeability of TFN membranes to 7.2 L/m2·h·bar at most (TFNNH2-UiO-66-BL-0.10%), about 1.3 times of corresponding TFC membranes, without sacrificing their selectivity to reject NaCl (> 40%) and xylose (> 65%). Membrane characterization revealed that MOFs decreased the cross-linking degree while increasing the membrane thickness, surface negative charge and roughness of polyamide active layer. MIL-53(Al) were found to bind with polyamide via reacting with piperazine, while weaker polyamide-MOF interactions were observed for NH2-UiO-66 and ZIF-8. This difference, along with the hydrophilicity of MOF particles, explained the varied permselectivity of different TFN membranes. Compared to pristine polyamide membrane, the TFN membranes demonstrated higher or comparable efficiencies in removing a set of six pharmaceuticals (PhACs), which were determined by the molecular properties of PhACs and membrane structure. The findings of this study deepens our understanding of the roles that MOFs plays in regulating membrane performance, promoting molecular design of MOF-incorporated TFN membranes via precise control of MOF-polymer interactions. Keywords: Metal-organic frameworks (MOFs); Thin-film nanocomposite (TFN) membrane; Cross-linking degree; Membrane structure; Pharmaceuticals.

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1. Introduction Nanofiltration (NF) emerged initially as a continuation and development of reverse osmosis (RO) in the 1980s 1. After more than two decades of technical innovation, NF has been widely applied to various water and wastewater treatment processes 2. As compared to RO, NF has higher water permeability under lower operation pressure, while maintaining relatively high selectivity for solute rejection. Hence, NF is considered as an economic method to remove trace organic compounds, including pharmaceuticals, disinfection by-products, surfactants and other organic pollutants that could hardly be removed by conventional water and wastewater treatment processes 34. One of the major drawbacks associated with polyamide membrane, the start-of-theart membrane for NF, is the trade-off between the permeability and selectivity 5. This trade-off indicates that an improvement of water permeability (i.e., water productivity) is typically achieved at the expense of the removal efficiency of target pollutants. Since high water productivity and pollutant removal are both desirable in water and wastewater treatment, the development of novel membrane materials, which are able to break this trade-off, is highly desirable to promote the cost and energy efficiency of NF. Thin film nanocomposite (TFN) membrane is a type of composite membranes that are usually generated by incorporating nanoparticles (NPs) within the polyamide active layer via interfacial polymerization process 6. Several NPs, including TiO2, SiO2, Ag, zeolite and carbon nanotube, have been applied to generate TFN membranes 7-10. Previous studies show that the incorporated nanoparticles were able to remarkably improve the membrane permeability by providing water channels around the hydrophilic surfaces or within the nanoparticle pores 11-13. However, some problems still prevail to compromise the performance of TFN membranes, such as the aggregation of NPs, the incompatibility between rigid NPs and polymer matrix, as well as the non-selective voids that deteriorate membrane selectivity 14-15. Recently, metal-organic frameworks (MOFs) have emerged as a novel alternative to traditional NPs. MOFs are referred to a type of hybrid organic-inorganic solid materials consisting of metal ions or clusters coordinated by organic moieties 16. Most of MOFs possess one or several innate advantages such as large porosity, designable structure, controllable pore size, and exceptional affinity with the polymer matrix 17-18, rendering MOF materials the new favorites for the fabrication of TFN membranes. A series of MOFs, including ZIF-8, MIL-53(Al), NH2-MIL-53(Al) and MIL-101(Cr), were firstly tested on fabricating membranes for organic solvent nanofiltration (OSN), with their enhancement of methanol and tetrahydrofuran permeances verified 19. Later, more MOF types were applied to OSN, such as HKUST-1 20-21, UiO-66 22, ZIF-11 and MIL68(Al) 23. The MOF-doped NF membranes for water treatment began to emerge after Year 2014 24. These TFN membranes were tested for the removal of dyes 25-26, salts 2728, or organic solutes29-30, and results showed that the doping of MOF nanoparticles enhanced the membrane water permeability while basically maintaining the selectivity. Recently, Liu et al. 31 reported a facial solution casting method for fabricating UiO-66 nanocomposite thin films. The fabricated thin films were more mechanical robust and thermally stable, and demonstrated an increase of both water permeability and Na2SO4 3



rejection. However, the applications of MOF materials to membranes used in water treatment are limited and require careful selection of appropriate MOF properties. More fundamental knowledge needs to be gained to guide rational design of MOF-based membranes for water purification. The first barrier is associated with the selection of appropriate MOF types. An ideal MOF should have good stability in water, moderate particle size, appropriate window size, and high hydrophilicity 32. It should be also synthesized under mild conditions and nontoxic, in order to avoid health and environmental risks. Further, a comprehensive understanding of the role that MOFs play in determining membrane performance is still unavailable. The interactions of MOFs with membrane polymers, which are a function of both MOF type and membrane fabrication procedure, regulate the structure of active layer and the transport of both water and solute molecules. Closing this knowledge gap, therefore, will allow us to predict the properties of MOFbased TFN membranes, achieving smart membrane design based on both MOF and polymer functionalities 18. In this study, we synthesized three types of hydro-stable MOFs, MIL-53(Al), NH2-UiO66, and ZIF-8. These materials possess different inherent properties, and their particle sizes were intentionally controlled less than 50 nm. The MIL-53(Al) and NH2-UiO-66 were hydrophilic MOFs, while the ZIF-8 was hydrophobic. The window sizes were 0.86, 0.6 and 0.34 nm for MIL-53(Al), NH2-UiO-66 and ZIF-8, respectively. Due to their varied chemical structures and physicochemical properties, these MOFs would behave differently during the fabrication process of the polyamide active layer, resulting in TFN NF membranes with distinct properties. Further, two MOF-doping methods, blending and preloading interfacial polymerization, were applied to testing the influence of MOF nanoparticles on the polyamide cross-linking process. The MOF loading amounts were selected by optimizing both the water permeability and the rejections of solutes (NaCl and xylose). The interactions between different types of MOFs and the polymer/monomers were comprehensively evaluated, for the first time, through a set of characterization methods. Our results showed that the properties of the generated MOF-polymer TFN membranes were highly related to the properties of MOF nanoparticles, which were further reflected by the membrane performance in removing several pharmaceuticals. 2. Experimental Section 2.1 Materials and Chemicals Commercially available flat sheet polysulfone ultrafiltration (UF) membranes (MWCO = ~100 kDa) were kindly supplied by Beijing OriginWater Technology Co., Ltd (China). Aluminum nitrate nonahydrate (Al(NO3)3∙9H2O, 98%), zirconyl chloride octahydrate (ZrOCl2·8H2O, 98%), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%), terephthalic acid, 2-aminoterephthalic acid, 2-methylimidazole (Hmim), triethylamine (TEA), piperazine (PIP) and trimesoyl chloride (TMC) as well as a set of pharmaceuticals (PhAC, as described in Table S1) were all purchased from SigmaAldrich (USA). The stock solution of the PhAC mixture in methanol (10 mg/L for each) was kept in dark at -20 ºC before use. Acetonitrile, N,N-dimethylformamide (DMF), 4


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acetic acid, methanol, acetone, n-hexane, xylose and inorganic salts (NaCl and NaHCO3) were purchased from Aladdin Ltd. (Shanghai, China). 2.2 MOFs Synthesis The MOFs were synthesized using previously reported recipes 17, 19, 22 with some modifications. For the synthesis of MIL-53(Al), 1.6 g Al(NO3)3∙9H2O and 1.2 g terephthalic acid were dissolved in 30 mL DMF. After vigorous stirring for 1 h, 0.1 mL deionized (DI) water was added into the solution and the stirring was continued for another 5 min. The resultant solution was then transferred into a Teflon-lined stainless steel autoclave and heated in inert atmosphere (N2 gas) at 130 ℃ for 24 h. Afterwards the autoclave was allowed to naturally cool down to room temperature before opened up. For the synthesis of NH2-UiO-66, 0.21 g ZrOCl2·8H2O and 0.55 g 2aminoterephthalic acid were dissolved in 40 mL DMF. The mixture was stirred for 10 min prior to the addition of 3.7 g acetic acid. After ultrasonication for 10 min, the solution was transferred to an autoclave and treated at 90 ℃ for 18 h. Further, ZIF-8 was synthesized by mixing 0.15 g Zn(NO3)2·6H2O and 0.66 g 2-methylimidazole with 0.8 g TEA in 20 mL DI water. The mixture was stirred for 10 min before adding another 0.8 g TEA and stirring for 1 h. The products of previous procedures were collected by 5000 rpm centrifugation for 10 min. The supernatants were discarded and the solid residues were extensively washed with acetone for three times, with methanol for three times, and then dried under vacuum at 75 ℃ overnight. The building blocks of three MOFs are shown in Fig. S1(a). 2.3 Preparation of TFC and MOF-Polymer TFN Membranes The TFC NF membranes were manufactured by fabricating a polyamide layer on UF support membranes through interfacial polymerization. In this study, the UF membrane was fixed on a support plate with the polysulfone layer facing upward. The membrane was firstly immersed in an aqueous solution of 1.5% (w/w) PIP for 2 min, with the excess aqueous solution removed with a rubber roller. Then 0.15% (w/w) TMC in nhexane solution was added onto the membrane surface, and the reaction was lasted for 1 min before pouring off the excess solution. After the IP process, the membrane was cured at 60 °C for 5 min before thoroughly washed with DI water and stored at 4 ºC for further tests. The MOF-doped TFN membranes were fabricated through two methods, the blending interfacial polymerization and the preloading interfacial polymerization, as depicted in Fig. S1(b). Both methods involved the IP process between the monomers, PIP and TMC, to generate the polyamide active layer. The differences were that in the blending method, the MOF nanoparticles were pre-dispersed together with TMC in n-hexane and loaded within the active layer 14, 26-27, 33, while in the preloading method the nanoparticles were loaded directly onto the polysulfone support before the IP process 25, 34. The detailed methods were described below: The BL membranes were made by dispersing a certain amount of (0.05%, 0.10%, 0.15% and 0.20% w/w) MOF nanoparticles into the TMC solution and forming a MOFpolyamide composite layer instead of a pure polyamide layer. The TMC-MOFs mixture 5



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were pretreated by ultrasonication for 20 min before use. These membranes are denoted as TFNMOF-BL-x, where x (% w/w) indicates the MOF loadings. The PL membranes were prepared as follows. Firstly, 5 mg MOF nanoparticles was dispersed in 10 mL methanol. Then 0.3 mL, 0.6 mL and 0.9 mL of the MOF solution was added into 60 mL mixed solution of water and methanol (with a volume ratio of 1:1). The solutions were ultrasonicated for 10 min before being filtered through the UF membranes (39.6 cm2), resulting in a loading mass of 3.8, 7.6 or 11.4 μg MOF/cm2 membrane, respectively. After dried at 60 °C for 1 min, a polyamide layer was fabricated upon the MOF-loaded membrane surface through the same IP procedure as described above. These membranes are denoted as TFNMOF-PL-y, where y (μg/cm2) indicates the MOF loadings. 2.4 Filtration System and Operation A bench-scale cross-flow filtration system with three parallel CF016D (Sterlitech, USA) cells was used for membrane performance tests. The effective filtration area for each cell was 20.6 cm2 and the height of channel was 2 mm. During filtration, permeate and concentrate were recirculated back to the feed tank. Before each set of test, the membranes were compacted by filtering DI water at 15 bar for 2 h. All of the filtration tests were conducted at 20±1 ºC with a pressure of 10 bar and a cross-flow velocity of 0.32 m/s. The feed water contained 10 mmol/L NaCl and 0.1 mmol/L NaHCO3 as background electrolytes, as well as 20 mg/L xylose or 50 μg/L of each PhAC as the rejection targets. The feed water pH was adjusted to 7.4. At least 12 h of stable filtration was performed before sampling for PhACs concentration measurement. Taking the effect of concentration polarization into consideration, the real rejection ratio was calculated by R  1  c p / c m  1  c p /[c f exp( J w / k )]

(1)

where cp, cf and cm are the solute concentrations in the permeate, in the feed water and at the feed water−membrane interface, respectively. The mass transfer coefficient (k) in Equation (1) was obtained by the Sherwood (Sh) correlation kd  ud h Sh  h  1.195 D  

  

0.554

    D

  

0.371

 dh     L 

0.131

(2)

where dh is the hydraulic diameter of the filtration cell, D∞ is the solute diffusivity, ρ is the feed water density, μ is the feed water viscosity, u is the cross flow velocity, and L is the filtration cell length. 2.5 Analytical and Characterization Methods The aggregate size and zeta potential of the MOFs were analyzed using a DelsaNano C Particle Size and Zeta Potential Analyzer (Beckman Coulter Inc., USA), the measurements were carried out independently for three times for each type of MOF. An electrokinetic analyzer (Anton Paar, Graz, Austria) was used to measure the zeta potential of membrane surface. The crystalline structure of MOFs nanoparticles was analyzed by a X-ray diffractometer (Rigaku SmartLab, Japan) using Cu Kα radiation 6


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(λ= 1.54 Å) in the interval of 5°≤2θ≤50°. Simulated patterns were generated from CIF files for MIL-53(Al) 35, NH2-UiO-66 36, and ZIF-8 37. Functional groups were analyzed based on attenuated total reflectance-frontier transform infrared spectroscopy (FTIR) (Nicolet 6700, Thermo, USA). The elemental compositions of both MOFs and membranes were investigated by using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher, USA). The spectrum was shifted with C 1s at 284.8 eV as internal reference. The cross-linking degree of polyamide was obtained with the O:N ratio as described in the Supporting Information. Static water contact angle was measured according to the sessile drop method, by using a goniometer (Contact Angle System OCA20, Data Physics Instruments GmbH, Germany). Surface roughness of the membrane was determined by using an atomic force microscope (AFM, Dimension ICON, Bruker, Germany) with a scan area of 5×5 μm2 for each image. Membrane surface morphology was observed by field emission scanning electron microscopy (FESEM) (Hitachi S5500, Japan). Cross-section morphology of membrane was observed under a transmission electron microscopy (TEM) (Hitachi H7650, Japan). To make a TEM sample, the membrane was firstly embedded in Type 812 epoxy resin and cured at 60 °C overnight, and then sliced into 70 nm-thick pieces using a Leica EM UC6 Ultramicrotome (Leica, Wetzlar, Germany). The concentrations of xylose and NaCl were measured using a total organic carbon analyzer (Shimadzu TOC-VCPH, Japan) and an anion chromatograph (Metrohm 761 Compact IC, Switzerland), respectively. The concentrations of PhACs were determined according to USEPA Method 1694 by using an ultra-performance liquid chromatography-tandem mass spectrometry (Agilent LC1290/QQQ6460, USA) equipped with a ZORBAX Eclipse Plus C18 column (2.1 mm×50 mm, 1.8 μm, Agilent, USA). The mobile phases were acetonitrile (solvent A) and 0.1% formic acid in ultrapure water (v/v, solvent B). The flow rate was 0.35 mL/min, and the injection volume was 5 μL. 3. Results and Discussion 3.1 Characterization of MOF Nanoparticles The crystalline structures of the synthesized MOFs nanoparticles were revealed by the XRD patterns (Fig. 1(a)), which matched well with the theoretical patterns and data reported in the literatures 38-40. The peak broadening was attributed to the super-small size of crystal according to the Scherrer equation 41. The characteristic peaks of MIL53(Al) were at 2θ of 9.3°, 12.5°, 17.8° and 18.7°. The peaks at 7.3°, 8.4°, and 25.6° corresponded to the (111), (002), and (006) planes of NH2-UiO-66. For ZIF-8, the peaks at 7.4°, 10.4°, 12.8° and 18.2° were attributed to the (111), (002), (112) and (222) planes, respectively. FTIR spectra shown in Fig. 1(b) revealed the distinct chemical structures of the MOFs. For MIL-53(Al), the wide peak at 3400 cm-1 could be attributed to −OH stretching vibration. The peaks at 1590 and 1510 cm-1 came from –COO asymmetric stretching while the bands at 1442 and 1418 cm-1 were from –COO symmetric stretching. The absorption band at 1673 cm-1 was due to the –COOH group. For NH2-UiO-66, the two 7



peaks at 3460 and 3348 cm-1 were from the stretching vibration of −NH2. The peaks at 1623 and 1256 cm-1 were due to the bending of N−H and C−H. Adsorption bands at 3136 and 2930 cm-1, which could be only observed in the spectrum of ZIF-8, corresponded to the aromatic and the aliphatic C−H stretch of the imidazole, respectively. The peaks of C-N bond in the imidazole ring were at 1310, 1178 and 1146 cm-1. TEM images (Fig. 1(c)) showed the morphologies of the synthesized MOFs. The MIL53(Al) particles had a rod-like structure (62±7 nm in length and 21±3 nm in width, as averaged from at least 20 measurements) that was consistent with previous literature 42. The particles of NH2-UiO-66 were the smallest with size of 15±2 nm. Also, although in extra-small sizes, the particles of ZIF-8 showed the rhombic dodecahedron shapes, with sizes of 25±3 nm. More properties of the nanoparticles are demonstrated in Table 1. In accordance with results in the literatures 43-44, both MIL-53(Al) and NH2-UiO-66 were negatively charged in water solution, while ZIF-8 was positively charged under pH of 7. Noncoordinating carboxyl groups in the ligand of MOFs (as shown in the FTIR spectra) or unreacted residue precursors were the source of negative charges. On the other hand, the protonated –NH– (pKa 7.52 and 13.92) from the imidazole ring resulted in the positive charge of ZIF-8. The average aggregate size (in hexane) increased with higher MOF concentrations because of nanoparticle aggregation. When the MOF concentration reached 0.2% (w/v), the aggregates had average sizes larger than the thickness of the active layer of polyamide membrane (typically less than 200 nm) and were therefore inapplicable to the membrane preparation. The XPS analysis confirmed the existence of feature elements (i.e., Al, Zr, and Zn) of the three MOF materials. Inevitably, the elemental compositions were slightly deviated from theoretical compositions calculated from the chemical formula due to the structural defects and small quantities of residual solvents. 3.2 Membrane Filtration Performance In order to determine the influences of MOF doping on the membrane filtration performance, water permeability, sugar rejection, and salt rejection were tested and compared in Fig. 2. For the pristine polyamide TFC membrane without MOF as a control, its water permeability (J0) was 5.62±0.12 L/(m2·h·bar), and the rejection of xylose and NaCl were 65.3% (marked by the red dotted line) and 31.9% (marked by the blue dotted line), respectively. For the TFN membranes generated by both BL and PL methods, the water permeability increased with higher MOF doping amounts. According to the literature 29, the interfacial defects and/or inner voids brought by the accumulated nanoparticles contributed to the increased water permeability under high MOF loading. The existence of non-selective defects also provides explanation to the drastic decrease of NaCl and xylose rejections for BL-generated membranes with MOF loadings higher than 0.15% (Fig. 2(a)). Dispersed in the n-hexane along with TMC, the MOF particles in the BL method could directly influence the polymerization process. On one hand, the MOF 8


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particles would decrease the diffusion of PIP to the organic phase by increasing the hexane solution viscosity, leading to the formation of a low cross-linked polyamide layer 45. A low extent of polyamide cross-linking corresponds to large pore sizes and a higher quantity of –COOH groups. On the other hand, the hydration of hydrophilic MOF particles (i.e., MIL-53(Al) and NH2-UiO-66 in this study) could release heat and enhance the miscibility of aqueous and organic phases, which also affected both the morphology and cross-linking degree of the membrane active layer 46. A higher density of –COOH groups from a lower degree of cross-linking results in more negative surface charge, which would improve the rejection of NaCl via electrostatic repulsion. This explains the increased NaCl rejection of BL-generated membranes with MOF loadings lower than 0.15%. Besides, a low cross-linking degree also resulted in larger membrane pore sizes, which not only increase the water permeability but also decrease the rejection of neutral solute xylose. However, except for ZIF-8, the xylose rejection of 0.10% MIL-53(Al) and NH2-UiO-66 doped membranes were both no lower than that of the pristine membrane. The window sizes for MIL-53(Al), NH2-UiO-66 and ZIF-8 MOFs are 0.86 nm, 0.60 nm and 0.34 nm, respectively. Considering the size of xylose molecule (0.73 nm), the high xylose rejection of the TFN membranes might be due to size exclusion by the nanoparticles. Although the window size of ZIF-8 was larger than the water molecule (0.28 nm), the penetration of water molecules through ZIF-8 particles was not favorable due to their hydrophobicity. Instead, water molecules tended to transport around the ZIF-8 particles through the polyamide matrix. It was the difference in the properties of MOF nanoparticles that made the “optimal” MOF loadings, at which the fabricated membrane would sacrifice neither the water permeability nor the rejection, vary among the TFN membranes. When pre-loaded on the polysulfone layer, the MOFs only imposed indirect and limited influences on the cross-linking degree. As a result, the variation in NaCl rejection between MOF-incorporated TFN membranes and pristine polyamide membrane was not as distinct as those using BL method. The deposited nanoparticles might majorly alter the morphology of the polyamide layer, forming a more crumple surface structure that was beneficial to water permeability by providing more available area 47(as supported by the AFM results in Table 2). Compared to ZIF-8, the higher porosity and selectivity of MIL-53(Al) and NH2-UiO-66 endorsed the corresponding TFN membranes with both higher water permeabilities and xylose rejections. The results shown above indicate that we are able to fabricate TFN membranes with higher water permeability and uncompromised selectivity compared to pristine polyamide TFC membranes, via carefully selecting the optimal MOF loading. The MOF loadings of 0.10% and 7.6 μg/cm2 generally resulted in the optimal balance between permeability and rejection of the TFN membranes for the BL method and PL method, respectively. As a result, those loadings were adopted to fabricate TFN membranes for detailed membrane characterization (see Section 3.3), in order to understand the interactions between MOF particles and polyamide formation. 3.3 Characterization of Membranes Some important physicochemical properties, including surface roughness, water 9



contact angle, zeta potential, as well as water permeability and selectivity of the membranes, were measured and presented in Table 2. All the TFN membranes exhibited higher water permeability than the pristine polyamide TFC membrane (19-31% increase on average). So far, a majority of MOF-doped TFN membranes were fabricated through the IP process of m-phenylenediamine (MPD) and TMC which were the frequently-used monomers to generate RO membranes 14, 17, 25, 28, 33. The inherent nature, i.e., relatively high thickness and small pore size, of active layers of RO membranes endowed them with low water permeability (0.75-3.0 L/(m2·h·bar)) and high salt rejection (95-99.5% for NaCl). In comparison, for the NF membranes fabricated by the monomers PIP and TMC, the thinner active layers along with larger pore sizes led to higher water permeability (5.5-6.9 L/(m2·h·bar)) and lower salt rejection (24-31% for NaCl) 17, 48. The incorporation of MOF nanoparticles were reported to enhance the water permeability of the TFN membranes by 1.4-4.5 times compared with the corresponding TFC membranes, while maintaining 14, 27-28 or decreasing the selectivity 17, 49, which depended on the MOF type, the doping amount and membrane fabrication method. In this study, a MOF-doped TFN membrane was able to achieve higher (up to 130%) water permeability, and higher NaCl and xylose rejection ratios than the TFC membrane simultaneously, especially for MIL-53(Al) and NH2-UiO-66 doped membranes. The doping of MOF nanoparticles also increased the membrane surface roughness from 11.1 nm of the pristine TFC membrane to 16.5-27.7 nm for the TFN membranes, this trend of increase of surface roughness coincided with the observations in the literature 17, 25, 46-47. Among all the TFN membranes, the NH -UiO-66-polymer membrane showed 2 the lowest surface roughness, owning to the smallest particle size and highest hydrophilicity of NH2-UiO-66 among all the MOFs. The surfaces of PL-generated membranes were more uneven than those of BL-generated ones which could be partially explained by that during BL method, nanoparticles were “inserted” into the polyamide layer and after PL methods, nanoparticles were “buried” under the active layer. Besides the change of surface morphology, the doping of MOF nanoparticles could also cast impacts on membrane surface hydrophilicy, which was usually evaluated by static water contact angle. The contact angle of a pristine TFC membrane was commonly reported within 45-70°, and both increased 17, 25-26 and decreased 14, 27, 29 contact angles after MOF doping were observed, which were explained by the hydrophilicity of MOF particles, the interactions between MOF and polymer, and etc. In this work, the MOF doping imposed an insignificant effect on membrane hydrophilicity, as reflected by the negligible difference of water contact angles among the membranes. The MOF-incorporated TFN membranes were all more negatively charged compared to pristine TFC membrane. First, for TFN membranes incorporated with MIL-53 and NH2-UiO-66, the increase of negative charge density might be due to the negative charge of the MOF particles. However, this explanation could not explain the effect of positively charged ZIF-8. Therefore, the interaction between MOF and polyamide might play a role (e.g., more –COOH generated due to lower degree of crosslinking). The higher negative charge density of TFN membrane surface provided explanation to the slower decline of rejection of NaCl than that of xylose with a larger 10


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amount of MOF loading. Fig. S2 presents the FTIR spectra of the membranes, which reflect functionality of both the polyamide active layer and the polysulfone support layer. After normalizing the peaks at ~1230 cm-1 that is assigned to the S(=O)2 asymmetric vibrations of the polysulfone layer 31, the differences of the peaks at ~1630 cm-1, which is assigned to the C=O stretching vibrations (amide Ⅰ group), revealed that the doping of MOFs interrupted the cross-linking of the polyamide layer. As FTIR spectroscopy was unable to quantify the cross-linking degree of polymerization process, XPS analysis was performed to further characterize the chemical composition of the membranes (Table 3). Comparing the atomic percentages of metal elements in the nanoparticles and the MOFpolyamide layers, the amount of nanoparticles that were present on the membrane surface, rather than washed away with redundant precursors, were MIL-53(Al) > NH2UiO-66 > ZIF-8. For the membranes fabricated via the PL method, we would expect that the polyamide layer with a thickness of tens of nanometers completely covered the pre-loaded nanoparticles, leaving no detectable metal elements on the very surface. Nevertheless, the metal compositions in these membranes, although much lower than those in BL membranes, were apparent. This phenomenon indicated that the interaction between the pre-loaded nanoparticles and the polysulfone layer was not strong enough to completely prevent their migration to the polyamide layer during the IP process. The cross-linking degree of the pristine TFC membrane was 0.86, which was within the range of commercial polyamide NF membranes 50. The cross-linking degrees of TFN membranes were all below 0.8, consistent with the FTIR spectra as discussed above. The cross-linking degrees of PL membranes were higher than those of BL membranes. Since the MOF particles were mixed with TMC in the BL method, there were more interactions between MOFs and monomers of polyamide during the IP process. In previous studies 27, 51, it was reported that the Zn contents in the MOF-polymer layer were close to the detection limit of XPS (0.1%). Later, Van Goethem et al. 52 proposed that the strong acid HCl generated during the polymerization reaction of the RO membrane could dissolve ZIF-8 particles. The subsequent release of Zn2+ would increase the cross-linking degree and decrease the active layer thickness at the same time. However, neither an abnormally low quantity of Zn nor an increased cross-linking degree was observed in the XPS results of this study. There were some possible explanations. First, the unique method (evaporation controlled filler positioning) adopted by Van Geothem to fabricate the active layer made the nanoparticles more easily approachable by the strong acid HCl. In contrast, the MOF nanoparticles were less likely to be directly exposed to the reaction products in our study, because the reaction proceeded mainly on the organic side of the water-organic interface. Second, the organic phase monomer of NF membrane, piperazine (pKa 5.18 and 9.56), was a stronger Lewis base than the organic phase monomer of RO membrane, m-phenylene diamine (pKa 2.73 and 5.48). This difference results in a less acidic microenvironment after the cross-linking reaction. Further, only a small fraction of the nanoparticles was 11



degraded compared to the overall doping amount (0.1%). In order to better understand the interactions between MOFs and polyamide/monomers, the IP process was performed in solution with a MOF loading of 0.1% (w/w). The resultant products were collected and analyzed by FTIR spectroscopy (Fig. 3), in which the interference of the polysulfone layer was eliminated. The FTIR spectra of MOFdoped polymers had similar peak distributions with that of the pristine polymer. However, characteristic peaks of MOFs, such as the peaks of C-H or N-H out-of-plane vibration of the ring in all three MOFs (below 790 cm-1) and the peaks of C-N in ZIF8 (1310 and 1146 cm-1), appeared exclusively in the MOF-doped polymers, verifying the existence of MOF nanoparticles in the doped polymers. In the spectrum of MIL53(Al)-doped PA, a new peak at 815 cm-1 emerged, which was not observed either in the MIL-53(Al) nanoparticle or in the un-doped PA polymer. This was actually a blueshifted peak of C-H vibration of p-substituted benzene ring that originally lied at 825 cm-1 for MIL-53(Al). The peak proved that parts of the MIL-53(Al) could react with piperazine during the interfacial polymerization process, resulting in a more stable structure. The missing peak relating to –COO at 1510 cm-1 in the spectrum of PA/MIL53(Al) also suggested that these groups might eventually turn into –C(=O)N after crosslinking reaction. For the spectrum of PA/ NH2-UiO-66, the peak at 1510 cm-1 was also absent, but no discernable change of peak positions was observed in the range of 885-800 cm-1. This might be because of that the peaks of both tri-substituted (1, 2, 4for NH2-UiO-66 and 1, 3, 5- for polyamide) kinds of benzene rings overlapped and were indistinguishable; the concealed peak at 1510 cm-1 was only related to free carboxylic acid, and the very small portion of –COO that took part in the reaction was not traceable in the FTIR spectrum. As for ZIF-8 nanoparticles, no variation of peaks was prominent enough to prove their reaction with the monomers. The difference of interactions between MOF nanoparticles and the polymer monomers offered a clue to the difference of xylose rejections among TFN membranes formed by BL method. Since the MIL-53(Al) nanoparticles were able to bind tightly with the surrounding polymer, the possibility of forming non-selective voids decreased and water molecules were more likely to go through the windows of MOF nanoparticles, resulting membranes with higher selectivity. In comparison, the weak interaction between the polymer and NH2-UiO-66 nanoparticles allowed more voids to grow but the high selectivity of the nanoparticles themselves might compensate the overall rejection ratio. Since ZIF-8 could neither form tight bonds with the polymer nor permeate water molecules, water tended to flow through the non-selective voids of a loose active layer, which resulted in the lowest rejections of xylose. Figs. S3 and S4 present the surface morphologies of the MOF-polymer composite membranes fabricated by BL and PL methods with different MOF loadings. When the MOF loadings were low (i.e., 0.05% and 3.8 μg/cm2 for BL and PL methods, respectively), the surfaces of all the membranes synthesized with both methods showed nodular structure, which was similar to that of the pristine polyamide layer. As the MOF loading increased, however, the membrane surface morphologies differed among membranes created using different MOF types and fabrication methods. For the 12


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membrane incorporated with MIL-53(Al), which was found to interact with polyamide monomer PIP, a gelatinous structure appeared on the membrane surface. The NH2-UiO66 nanoparticles were considered to not, or only by a small portion, take part in the reaction. The ridged surface of TFNNH2-UiO-66-BL-0.2 was primarily because of the accumulation of the nanoparticles under high concentration (see Table 1). The ear-like structure (Fig. S4(b3)) was formed as a defect due to partially peeling off of the accumulated nanoparticles as well. As for ZIF-8, which was unable to react during polymerization process, the π-π interaction 53 between the imidazole ring and the benzene ring resulted in the adsorption of more monomer, which led to the formation of small floccules on the surface. The special structures seen in the SEM images were confirmed by the cross-section TEM images in Fig. 4. The thicknesses of polyamide active layer were higher for all the MOF-doped membranes than the pristine membrane. For MIL-53(Al) and NH2UiO-66, their hydration accelerated the diffusion of PIP into organic phase and generate an active layer with higher thickness but looser pore structure. The doping of ZIF-8 in the organic phase increased the thickness of the polyamide active layer mainly by forming the leaf-like structures. With a lower active layer thickness and more structural defects (as discussed above), it was reasonable for a ZIF-8 doped membrane to have a higher cross-linking degree and a higher filtration flux than a MIL-53(Al) or NH2-UiO66 doped membrane at the same time. Among all three kinds of MOFs, the MIL-53(Al) which built chemical bonds with the polyamide could be most clearly distinguished in the images and the reaction process seemed not to destroy the integrity of the nanoparticles. Some nanoparticles of NH2-UiO-66 and ZIF-8, although less than of MIL-53(Al), could also be seen (as noted by the red circles). Moreover, in the images of NH2-UiO-66 and ZIF-8 doped membranes, the voids left by the dropping off of the nanoparticles during sample preparation were more pronounced because of the loose attachment between the nanoparticles and the polymer phase. 3.4 Rejection of Pharmaceuticals The rejection rates of six pharmaceuticals with varied properties (Table S1) were tested with TFC and MOF-polymer composite membranes, with the results presented in Fig. 5. To eliminate the interference of experimental deviations in the following discussions, the differences between the rejections by MOF-polymer TFN and pristine TFC membranes were calculated by multivariate tests (one-way MANOVA) using an IBM SPSS statistic 20 software (Table S3). Generally, the rejection of pharmaceuticals by the hydrophilic MIL-53(Al) or NH2-UiO-66 doped membranes were higher than those by the pristine membrane, which was in accordance with the results of xylose rejection (Table 2). For the ZIF-8 doped membranes, unlike the lower rejections to xylose, their removal efficiencies for the pharmaceuticals were higher or comparable to those for the pristine membrane. These results proved that rather than bringing non-selective voids to the membrane, the doping of MOF nanoparticles under the selected synthetic conditions in this study increased the filtration flux primarily via forming an active layer with looser structure. Nevertheless, with larger pore sizes of the polymer, the variation of the rejections due to the effects of electrostatic interaction aggravated54. 13



The influence of electrostatic interaction on rejection was more remarkable for the solutes with lower molecular weights (MWs), taking the rejections of nalidixic acid and sulfamethoxazole by MIL-53(Al) doped membranes for examples. While the steric hindrance effect took over the predominant role on rejection for the solutes with higher MWs 55. Looking back to the neutral pharmaceutical phenacetine, its rejections by the MOFdoped membranes were lower than expected, considering their larger molecular sizes than xylose. This is explainable if taking the adsorption mechanism into account. Adsorption, which results in diffusive transport where the solute first bonds to the membrane surface via hydrophobic, electrostatic or hydrogen-bonding interactions, then migrates across the active layer 56, is disadvantageous to rejection. For phenacetin with the log D value 1.41, the adsorption between the molecules and the MOFs was strong enough to discount its total rejection ratio. The counteractive effect of adsorption on rejection was also seen for carbamazepine. Besides hydrophobic interaction, other specific and non-specific interactions, i.e., hydrogen bonding and π-π stacking, between the solute and membrane surface were also important for rejection. Taking sulfamethoxazole for example, each molecule contains two hydrogen donor and four hydrogen acceptor to easily form hydrogen bonds with the nanoparticles, especially with the –NH2 group in NH2-UiO-66 nanoparticles. Also, the isoxazole ring was found to be capable of forming strong π-π interaction (stronger than with benzene rings or that between the benzene rings) with the imidazole rings in ZIF-8 nanoparticles 57. These adsorption mechanisms were reflected on the lower-than-predicted rejection ratios. Fig.6 systematically illustrates the roles of MOFs in regulating the permeability and selectivity of polyamide membranes. The membranes without MOFs follow the wellknown permeability-selectivity trade-off, in which the increase of water permeability typically decreases the removal efficiencies of solutes. The presence of some MOFs is able to provide unique channels that facilitate the transport of water molecules, increasing the water permeability of TFN membranes. Meanwhile, the angstrom-sized windows in MOFs perform as a selective barrier that maintains the selectivity of the TFN membranes via size exclusion. Further, the incorporation of MOFs reduces the cross-linking degree of polyamide, providing more carboxyl functional groups that enhance the negative charge of the membrane surface. This feature improves the selectivity of negatively charged species via electrostatic repulsion. In our study, both MIL-53(Al) and NH2-UiO-66, two hydrophilic MOFs, enhanced the permselectivity via the above mechanisms. As compared to NH2-UiO-66, MIL-53(Al) binds to the polyamide membrane more tightly due to its reaction with piperazine. This structure difference reduces the formation of non-selective voids in the TFN membranes incorporated with MIL-53(Al), which exhibited the best permselectivity among the three TFN membranes. In contrast, hydrophobicity of ZIF-8 prohibits the permeation of water molecules. As a result, the effect of ZIF-8 in improving membrane permselectivity was negligible. Our study demonstrates that the improved permselectivity of MOF-incorporated TFN membranes is determined comprehensively by the MOF property and the interactions between MOFs and membrane polymers. 14


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4. Conclusions Three types of MOF nanoparticles, MIL-53(Al), NH2-UiO-66 and ZIF-8, with sizes up to 50 nm were applied to generate thin-film nanocomposite NF membranes through blending and preloading interfacial polymerization methods. Results showed that the incorporation of MOFs enhanced water permeability of TFN membranes by up to 30% without sacrificing their rejection performance. As revealed by a series of membrane characterization, MOFs altered membrane properties by decreasing the cross-linking degree while increasing the thickness as well as surface negative charge and roughness of the polyamide active layer. The influence of preloading method on the membrane cross-linking degree and other characteristics were limited compared to the blending method. MIL-53(Al) were able to bind with polyamide by reacting with PIP, whereas weaker interactions were observed between polyamide and the other two MOF materials. This difference in MOF-polyamide interaction, along with the differed hydrophilicity of MOF particles, was responsible for the varied permselectivity of different TFN membranes. In addition, the TFN membranes demonstrated higher or comparable efficiencies in removing a set of six PhACs, which were determined synergistically by the molecular properties of PhACs and membrane structure and resulting properties. The findings deepen our understanding of the roles that MOFs play in regulating membrane permselectivity, and highlight the potential of molecular design of MOFincorporated TFN membranes via precise control of MOF-polymer interactions. Associated Content Supporting Information Texts of calculations of the effect of concentration polarization and the membrane cross-linking degree. Figures showing schematic diagram of MOFs and TFN membranes; FTIR spectra of TFC and TFN membranes; and top-view SEM images of TFN membranes. Tables showing physicochemical properties of the PhACs; calculation results of concentration polarization; and results of multivariate tests between the PhACs rejection ratios of TFC and TFN membranes. Acknowledgements We acknowledge the fundings for this research provided by the National Natural Science Foundation of China (No. 51678331 and No. 51761125013).

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Figures & Tables

Impacts of Metal-Organic Frameworks on Structure and Performance of Polyamide Thin-Film Nanocomposite Membranes Yang-ying Zhao a, Yan-ling Liu a, Xiao-mao Wang a,*, Xia Huang a,*, Yuefeng F. Xie a,b

Figure 1. (a) measured and simulated (small sticks) XRD patterns, (b) FTIR spectra and (c) TEM images of prepared MOF nanoparticles. Figure 2. Water permeability, xylose rejection, and NaCl rejection of the TFN membranes at 10 bar with different MOF loadings generated by (a) BL method and (b) PL method. Figure 3. FTIR spectra of the polymers without (noted as PA) and with (noted as PA/MOF) nanoparticles synthesized by the interfacial polymerization process. Figure 4. Cross-section TEM images of (a) TFC membrane, (b1) TFNMIL-53(Al)-BL-0.1, (b2) TFNMIL-53(Al)-PL-7.6, (c1) TFNNH2-UiO-66-BL-0.1, (c2) TFNNH2-UiO-66-PL-7.6, (d1) TFNZIF-8-BL-0.1, and (d2) TFNZIF-8-PL-7.6. The apparent thicknesses were marked and measured. Figure 5. Concentration polarization corrected pharmaceutical rejections by TFC and MOF-polymer composite membranes. Figure 6. A brief schematic illustration of the filtration process by an ideal polyamide membrane without and with MOFs. Table 1. Major physicochemical properties of the MOFs. Table 2. Physicochemical properties of TFC and MOF-polymer composite membranes. Table 3. Surface elemental composition and calculated cross-linking degree of the membranes.

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Figure 1. (a) measured and simulated (small sticks) XRD patterns, (b) FTIR spectra and (c) TEM images of prepared MOF nanoparticles.

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Figure 2. Water permeability, xylose rejection, and NaCl rejection of the TFN membranes at 10 bar with different MOF loadings generated by (a) BL method and (b) PL method. The water permeability of each membrane has been normalized to that of pristine polyamide TFC membrane without MOFs. The red and blue dotted lines indicate the xylose rejection and NaCl rejection of pristine polyamide TFC membrane, respectively. Error bars were calculated from three independent tests.

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Figure 3. FTIR spectra of the polymers without (noted as PA) and with (noted as PA/MOF) nanoparticles synthesized by the interfacial polymerization process.

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Figure 4. Cross-section TEM images of (a) TFC membrane, (b1) TFNMIL-53(Al)-BL-0.1, (b2) TFNMIL-53(Al)-PL-7.6, (c1) TFNNH2-UiO-66-BL-0.1, (c2) TFNNH2-UiO-66-PL-7.6, (d1) TFNZIF-8-BL-0.1, and (d2) TFNZIF-8-PL-7.6. The apparent thicknesses were marked and measured.

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Figure 5. Concentration polarization corrected pharmaceutical rejections by TFC and MOF-polymer composite membranes. (The colors of tick labels of x-axis represented the charge of pharmaceuticals: black for neutral, blue for negatively charged, and red for positively charged ones. The asterisks (*) denote the rejections that are significantly different from the control. All rejection tests were operated under a same pressure of 10 bar, and the corresponding water permeability for each membrane is consistent with the respective value in Table 2.)

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Figure 6. A brief schematic illustration of the filtration process by an ideal polyamide membrane without and with MOFs.

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Table 1. Major physicochemical properties of the MOFs. MOF

Zeta potential (mV, 0.1%)

Mean aggregate size (nm) 0.02% 0.1% 0.2% (w/v) (w/v) (w/v)

Elemental composition (at%) C N O Al(2p)/Zr(3d) (1s) (1s) (1s) /Zn(2p)

MIL-53(Al)

-21.9±2.5

53±17

101±19

219±43

59.0

-

34.1

6.9

NH2-UiO-66

-39.5±1.0

27±11

62±26

242±41

54.7

6.3

33.4

5.6

ZIF-8

11.2±0.7

40±7

96±22

143±21

60.2

29.4

2.4

8.0

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Table 2. Physicochemical properties of TFC and MOF-polymer composite membranes. Membrane

Water permeability (L/m2·h·bar)

NaCl rejection (%)

Xylose rejection (%)

RMS Roughness (nm) a

Water contact angle (°)

TFC

5.62±0.12

31.9±1.1

65.3±3.8

11.1±2.5

54.7±2.5

Zeta potential (mV, pH 7) -26.1±0.4

6.91±0.11

40.4±2.1

67.0±1.1

21.3±2.9

52.1±3.0

-44.2±2.2

6.69±0.39

34.6±3.1

71.6±2.3

24.9±3.4

60.1±3.8

-34.8±0.7

7.19±0.17

42.2±0.6

65.2±2.1

16.5±2.6

49.9±3.3

-43.5±1.7

6.97±0.22

36.1±0.4

68.5±2.0

27.7±1.5

56.0±4.8

-38.1±0.3

7.13±0.34

35.4±1.5

60.9±1.6

19.6±1.6

58.8±1.6

-33.3±0.6

7.36±0.39

33.8±1.7

62.3±1.5

26.0±3.1

57.7±2.4

-30.2±1.4

TFNMIL-53(Al)BL-0.1 TFNMIL-53(Al)PL-7.6 TFN NH2-UiO-66BL-0.1 TFN NH2-UiO-66PL-7.6 TFNZIF-8-BL-0.1 TFNZIF-8-PL-7.6 a

Measured by AFM

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Table 3. Surface elemental composition and calculated cross-linking degree of the membranes. Surface elemental composition (at%) C(1s)

N(1s)

O(1s)

Al(2p)/Zr(3d)/Zn(2p)

Cross-linking degree

TFC

70

14.3

15.7

-

0.86

TFNMIL-53(Al)-BL-0.1

62.2

12.2

23.8

1.8

0.66

TFNMIL-53(Al)-PL-7.6

68.2

12.9

18.3

0.6

0.72

TFN NH2-UiO-66-BL-0.1

67.4

13.1

18.9

0.6

0.69

TFN NH2-UiO-66-PL-7.6

69.6

13.7

16.5

0.2

0.80

TFNZIF-8-BL-0.1

63.9

17.4

17.9

0.8

0.71

TFNZIF-8-PL-7.6

68.0

15.4

16.2

0.4

0.79

Membrane

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For Table of Contents Only

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Impacts of MetalâOrganic Frameworks on Structure and Performance

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