Enhanced dispersibility of metal–organic frameworks (MOFs ...

RSC Advances

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View Article OnlineView Journal | View Issue

Enhanced disper

aSchool of Chemical Engineering and Tech

Tianjin 300130, China. E-mail: ghliu@hebubTianjin Key Laboratory of Chemical Proces

Tianjin 300130, China

† Electronic supplementary informa10.1039/c9ra09672h

Cite this: RSC Adv., 2020, 10, 4045

Received 19th November 2019Accepted 12th January 2020

DOI: 10.1039/c9ra09672h

rsc.li/rsc-advances

This journal is © The Royal Society o

sibility of metal–organicframeworks (MOFs) in the organic phase via surfacemodification for TFN nanofiltration membranepreparation†

Hengrao Liu,a Min Zhang,a Hao Zhao,a Yanjun Jiang, a Guanhua Liu *ab

and Jing Gao*a

The nanosized UiO-66-NH2 metal–organic framework (MOF) material was synthesized and modified by

palmitoyl chloride to enhance the dispersibility and restrain the aggregation of MOF particles in the

organic phase. Then the above nanomaterial was introduced into interfacial polymerization to prepare

thin film nanocomposite (TFN) nanofiltration membranes. The prepared membranes displayed “ridge-

valley” shaped Turing structure surface morphology with membrane thickness around 380 nm. The FE-

SEM, ATR-FTIR and XPS characterization showed the polyamide layer was fabricated on the substrate

surface. The TFN membranes showed higher hydrophobicity, zeta potential and roughness than TFC

membranes. Due to the introduction of MOF and the formation of MOF/polyamide interfacial

passageways, the TFN membranes showed higher water permeability but slightly lower rejection

properties than TFC membranes. Compared with the TFN membranes prepared from pristine UiO-66

and UiO-66-NH2, the TFN membrane prepared from modified UiO-66-NH2 showed better rejection

properties because of its superior dispersibility in the organic phase.

1 Introduction

Separation is one of the most crucial processes in the chemicalindustry. Conventional separation technologies, such as distil-lation, adsorption, and extraction, consume vast amounts ofenergy or chemical materials. So developing an energy-savingand economical separation technology is of great signicancein the chemical industry. Membrane separation is a promisingcandidate that has emerged in recent decades by reason of nophase transition, energy-efficiency, simple equipment require-ment and easy operation process.1–3

Nanoltration is a kind of membrane separation technology,possessing a membrane pore size about 1 nm.4 Polyamide (PA)thin lm composite (TFC) membrane was wildly used innanoltration and reverse osmosis such as desalination, dyeseparation and waste water treatment, but the “trade-off” effectbetween permeability and selectivity is a common problem inTFCmembrane.5 Researchers have made great effort to improvethe permeability of TFC membrane without losing soluteselectivity. Hoek and his coworkers brought forward the idea of

nology, Hebei University of Technology,

t.edu.cn; [email protected]

s Safety, Hebei University of Technology,

tion (ESI) available. See DOI:

f Chemistry 2020

fabricating thin lm nanocomposite (TFN) membranes byincorporating zeolite nanoparticles within the polyamidematrix.6 In their research, the TFN membranes showedincreased permeability with the increase of ller loading andstill maintained high rejection property. From then on, manykinds of inorganic nanomaterials (such as SiO2, TiO2, grapheneoxide and carbon nanotube) were introduced into interfacialpolymerization process to prepare TFN membranes.7 Theincorporated nanomaterials could create additional passage-ways for solvent transfer through their interconnected nano-pores or nanomaterial–polyamide interfaces.

Metal–organic frameworks (MOFs) are a series of porousorganic–inorganic hybrid materials, which consist of inorganicmetal nodes linked with organic ligands by coordinationbonds.8 The intrinsic nanopores of MOFs can be employed asthe solvent transfer passageways in nanoltration process.9

Furthermore, the size of particle/pore, and the property ofsurface/pore are highly tunable to meet the demand ofmembrane preparation.10 In addition, the presence of organicligands results in better compatibility with polymer matrix. SoMOFs are regarded as ideal materials to prepare TFNmembranes. In traditional ways, if MOFs are dispersed inaqueous phase in interfacial polymerization process to prepareTFN membrane, a large amount of MOFs will be removedtogether with the excess amine solution, leaving only very littleMOFs loaded on the substrate surface, which restricts the effect

RSC Adv., 2020, 10, 4045–4057 | 4045

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http://orcid.org/0000-0003-1470-2102

http://orcid.org/0000-0002-0824-9738

http://creativecommons.org/licenses/by-nc/3.0/


https://doi.org/10.1039/c9ra09672h

https://pubs.rsc.org/en/journals/journal/RA

https://pubs.rsc.org/en/journals/journal/RA?issueid=RA010007

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of the MOFs.11 While one of the greatest challenge to disperseMOF nanoparticles in organic phase is the particle aggrega-tion,12 which is ascribed to the poor dispersibility of nano-materials in membrane casting solution especially in organicphase. The aggregation tendency of inorganic nanoparticles insolution was an intrinsic challenge because of the high surfaceenergy, large specic surface area and strong interactionbetween nanoparticles. Under these factors, nanoparticles areeasy to aggregate together to reduce the total surface energy ofparticles which eventually reaches an equilibrium state. Theserious aggregation of nanoparticles may bring defects into TFNmembrane's selective layer, which can lead to signicantdecrease of rejection property. To restrain the aggregation ofMOF nanoparticles, Li et al. prepared high-performance MOFbased nanoltration membranes by coordination-driven in situself-assembly method,13,14 the prepared membranes showedoutstanding nanoltration performance. Zhu et al. modiedZIF-8 MOF particles with poly(sodium 4-styrenesulfonate) andintroduced it into interfacial polymerization.15 The preparedTFN nanoltration membrane showed obvious increase inwater permeability and achieved analogous rejection property atlow ZIF-8 loading amount. Guo et al. synthesized UiO-66-NH2

and modied the surface by dodecyl aldehyde to prepare TFNmembranes for organic solvent nanoltration.16 The TFNmembrane exhibited both high methanol permeance and highrejection to tetracycline. From the above instances, surfacemodication is a promising method to improve dispersibilityand reduce aggregation of nanoparticles in the preparationprocess of TFN membranes.

Among various MOFs, zirconium MOFs aroused greatconcern of researchers due to the high stability, diverse organicligands with changeable function groups (for example –NH2,–OH, –COOH and so on),17,18 and ease of modication to changethe surface property. UiO-66-NH2 was a kind of Zr-based MOFs,which was made up of Zr ions linked by 2-aminobenzene-1,4-dicarboxylic acid organic ligands. The highly reactive –NH2

groups situated in the internal pores and outside surface of UiO-66-NH2, which made it possible to be easily modied by variouschemical reagents to change its surface property and increasethe dispersity in casting solution.

For this reason, UiO-66-NH2 nanomaterial was synthesizedand the outside surface was modied by palmitoyl chloride ina facile way, then the modied UiO-66-NH2 nanomaterial wasdispersed in TMC/cyclohexane organic phase and introducedinto interfacial polymerization process to fabricate TFNmembranes. On the one hand, the long alkyl chain of palmitoylchloride on the outside surface can increase the compatibilitybetween UiO-66-NH2 nanoparticles and nonpolar organicsolvent because of their similar polarity, and hence decrease theaggregation tendency between nanoparticles. On the otherhand, compared with n-hexane used in most researches, the useof cyclohexane as organic solvent in this study can lead to betterdispersity of nanometerial in organic phase. For the above-mentioned reasons, the modied UiO-66-NH2 nanoparticlescan get good dispersibility in organic phase to form stablesuspension of nanoparticles during interfacial polymerizationto inhabit the formation of nonselective defects in TFN

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membranes. The morphology, ATR-FTIR, XPS, hydrophilicity,zeta potential, and surface roughness of the preparedmembranes were characterized. The inuence of the modiedUiO-66-NH2-PC concentration in organic phase on membraneproperty and the long-term stability of the membrane wasinvestigated in detail.

2 Experimental section2.1 Materials and reagents

Commercial polyacrylonitrile (PAN) ultraltration membrane(MWCO: 50 000) was bought from Beijing Separate EquipmentCo. Ltd. NaOH, Na2SO4, MgSO4, NaCl, n-hexane, cyclohexane,trichloromethane (CHCl3) triethylamine, and acetic acid glacialwere bought from Tianjin Fengchuan Chemical Reagent Tech-nologies Co. Ltd. Zirconium chloride (ZrCl4), piperazine (PIP),N,N-dimethylformamide (DMF), and benzene-1,3,5-tricarbonyltrichloride (TMC) were bought from Aladdin BiochemicalTechnology Co. Ltd. Palmitoyl chloride was bought fromMacklin Biochemical Co. Ltd. 2-Aminoterephthalic acid (NH2-BDC) was bought from Dibai Chemical Technology Co. Ltd.

2.2 Synthesis and modication of UiO-66-NH2 nanomaterial

Zr-based MOFs UiO-66-NH2 nanomaterial was synthesized asfollows: ZrCl4 (0.686 mmol) and NH2-BDC (0.686 mmol) weredissolved in 40 mL DMF. Then 20 mL H2O and 1.2 mL acetic acidglacial were added. Acetic acid glacial could decrease growthrate of Zr-based MOF crystals and hence increase the crystal-linity of MOF nanoparticles. The OH� of water molecules couldbe employed as the composition of secondary building units inUiO-66-NH2 crystal structure. The above mixture was sonicatedand stirred to dissolve completely and then heated at 100 �C for24 h. The resulting product was centrifuged and washed 3 timesby DMF and then washed 2 times by ethanol. 1 mL UiO-66-NH2

suspension was dried thoroughly to conrm the weight andconcentration of UiO-66-NH2.

The above puried UiO-66-NH2 nanoparticles (2 g) weredispersed in the mixture of 40 mL CHCl3 and 2.4 mL palmitoylchloride. 1.4 mL triethylamine was added aerwards. Theresultant mixture was sonicated to disperse and stirred at roomtemperature overnight tomake UiO-66-NH2 react with palmitoylchloride. The product was washed by CHCl3 3 times andcyclohexane 3 times and then re-dispersed in cyclohexane. Themodied UiO-66-NH2 was named as UiO-66-NH2-PC. Themodication scheme was displayed in Fig. S1† and the methodof calibrating UiO-66-NH2-PC concentration was similar to thatof UiO-66-NH2.

2.3 Hydrolyzation of PAN substrate membrane

The commercially available PAN ultraltration membrane wasused as the substrate to prepare nanoltration membranes.Prior to the preparation process, the PAN ultraltrationmembrane was put in NaOH solution (1 mol L�1) at 50 �C for 1 hto make –CN hydrolyze to –COOH, and the resultant membranewas denoted as HPAN. The existence of ionized –COOH canadsorb electropositive amine monomer in the following

This journal is © The Royal Society of Chemistry 2020




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interfacial polymerization process and increase the hydrophi-licity of the substrate.

2.4 Preparation of TFC and TFN membranes

The TFN membranes were prepared by interfacial polymeriza-tion. Before the preparation process, the UiO-66-NH2-PCnanoparticles with different concentration (0.05%w/v, 0.10%w/v, and 0.15% w/v) were dispersed into TMC/cyclohexane organicsolution (0.15% w/v). The HPAN substrate membrane wasimmersed in PIP aqueous solution (0.15% w/v) for 20 min. Thenthe substrate was taken out from PIP solution and removed theexcess solution on the substrate surface by dustless bibulouspaper. Aerwards, the substrate membrane with PIP solutioninside the pores was immersed into the above organic solutionfor 2 min. The membranes prepared with the UiO-66-NH2-PCconcentration of 0.05% w/v, 0.10% w/v, and 0.15% w/v inorganic solution were named as TFN-0.05, TFN-0.10 and TFN-0.15, respectively. The concentration of UiO-66-NH2-PC inorganic solution could be conrmed by different volume of UiO-66-NH2-PC suspension. The schematic diagram was shown inFig. 1.

The TFC membrane was prepared by the similar method ofTFN membranes without adding UiO-66-NH2-PC nanoparticlesin organic phase.

2.5 Characterization methods

Field emission scanning electron microscopy (FE-SEM FEI NovaNanoSEM450) was employed to observe the morphologies ofUiO-66-NH2 and UiO-66-NH2-PC. The surface and cross-sectionmorphologies of TFC and TFN membranes were also observedby FE-SEM. Fourier transform infrared (FTIR) spectra were ob-tained through a Bruker Vertex 70 spectrometer to characterizethe chemical constitutions of UiO-66-NH2, UiO-66-NH2-PC and

Fig. 1 Schematic diagram of the preparation process of TFNmembranes.


the prepared membranes. Water contact angles of TFC and TFNmembranes were measured by an optical contact anglemeasuring system (KRUSS DSA-100). The crystallinities ofsynthesized UiO-66-NH2 and UiO-66-NH2-PC were conrmed bythe X-ray diffractometer (XRD, Bruker AXS D8 Focus). Zetapotential of the prepared membranes was tested by SurPASSelectrokinetic analyzer (Anton Paar GmbH), and the roughnesswas measured by Atomic Force Microscope (AFM, Bruker).

2.6 Measurement of nanoltration performance

The permeability and rejection properties of the membraneswere measured using a dead-end ltration equipment withmagnetic stirring function to evaluate the nanoltrationperformance. Before measurement, the as-preparedmembranes was cut into a rotundity with specic diameterand then the membrane was placed into a ltration cell. Themembranes was pre-ltrated at 4 bar for 50 min to reacha stable condition. Then the pure water permeability (PWP) andrejection properties were measured with the duration of 3 min.

PWP could be calculated as follows:

J ¼ V

A� t� DP

where V (L) represents the volume of pure water passingthrough the membrane; A (m2) is the effective area of themeasured membranes; t (h) is the ltration time and DP (bar) isthe operation pressure. The unit of J is L (m�2 h�1 bar�1) orLMH bar�1.

Rejection properties were measured by divalent and mono-valent salt solution (MgSO4, Na2SO4 and NaCl, 0.1% w/v), therejection for each salt solution could be determined as follows:

Rð%Þ ¼ 1� Cp

Cf

where Cp is the salt concentrations at permeate side and Cf isthe concentration at feed side. The conductivity of each saltsolution could be measured by a conductivity meter. The saltconcentration was calculated by solution conductivity andstandard curves plotted in advance.

3 Results and discussion3.1 Characterization of UiO-66-NH2 and UiO-66-NH2-PC

The morphologies of the synthesized UiO-66-NH2 and UiO-66-NH2-PC were shown in Fig. 2. The regular-octahedral appear-ance could be clearly seen in the gures. The particle size ofUiO-66-NH2 was about 80–90 nm. Aer modication, theparticles size enlarged a little to about 100 nm.

To further characterize the well-synthesized UiO-66-NH2

nanoparticles, the XRD patterns were applied to analyze thestructure of synthesized material. In Fig. 3, the measureddiffraction pattern of UiO-66-NH2 was in consistent with thereported literature,12,19which conrmed the great crystallinity ofthe synthesized UiO-66-NH2. The sharp characteristic peaks at2q ¼ 7.3�, 8.4� and 25.7� were ascribed to (1 1 1), (2 0 0) and (60 0) crystal planes, respectively. Aer modied by palmitoylchloride, the XRD pattern of UiO-66-NH2-PC remained

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Fig. 2 FE-SEM images of (a) UiO-66-NH2 and (b) UiO-66-NH2-PC.

Fig. 3 XRD patterns of UiO-66-NH2 and UiO-66-NH2-PC.

Fig. 4 FT-IR spectra of UiO-66-NH2 and UiO-66-NH2-PC.

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unchanged, indicating the crystal structure was maintained.The regular-octahedral morphology, nanosized particles andthe accurate XRD pattern revealed the successful synthesis ofUiO-66-NH2 nanoparticles.

Fig. 4 shows the FT-IR spectra of the synthesized UiO-66-NH2

and UiO-66-NH2-PC. The absorption band at about 3200–3600 cm�1 was ascribed to the stretching vibration of –NH2. Theabsorption peaks at 1574 and 1257 cm�1 were ascribed to the

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stretching vibration of C–O and C–N in NH2-BDC, respectively.The absorption peak at 771 cm�1 was ascribed to the Zr–Obonds of metal nodes in the framework.12

The new absorption peaks at 2925 and 2854 cm�1, whichwere appeared only in UiO-66-NH2-PC, were ascribed to C–Hstretching vibration of the modied palmitoyl chloride. And thenew absorption peak at 1033 cm�1 was ascribed to C–Cstretching vibration of palmitoyl chloride.16 These 3 newabsorption peaks indicated that the surface of UiO-66-NH2 wassuccessfully modied by palmitoyl chloride.

The dispersibility of nanoparticles in solvent depends on theinteraction within particles, and the interaction betweenparticle and solvent. The surface modication of UiO-66-NH2 bypalmitoyl chloride could increase the compatibility betweenparticle and solvent, which enhanced the dispersibility innonpolar solvent (such as n-hexane and cyclohexane). In orderto conrm the above assumption, UiO-66-NH2 and UiO-66-NH2-PC nanoparticles were dispersed in n-hexane and cyclohexaneby ultrasonication and then stood for specic time. Theunmodied UiO-66-NH2 particles precipitated immediately infew seconds in n-hexane indicating the strong interactionbetween particles in n-hexane, which was in consistent with thephenomenon of other researchers.16 This phenomenon revealedthat it’s impossible to disperse UiO-66-NH2 in n-hexane. Fig. 5showed the dispersibility of UiO-66-NH2 and UiO-66-NH2-PC inn-hexane and cyclohexane. Aer still standing for 30 min, theunmodied UiO-66-NH2 particles precipitated completely incyclohexane, and the UiO-66-NH2-PC particles began toprecipitate in n-hexane, but the UiO-66-NH2-PC particles stillwell suspended in cyclohexane. Aer still standing for 12 h, onlyUiO-66-NH2-PC particles could suspend in cyclohexane. Thedispersibility experiment indicated that cyclohexane (ratherthan n-hexane) was better for the nanoparticle dispersion in thisstudy, and surface modication of UiO-66-NH2 by palmitoylchloride could signicantly increase the dispersibility of thesynthesized nanoparticles in nonpolar organic solvent. It couldbe deduced reasonably that the use of cyclohexane as organicsolvent and surface modication of nanoparticles could lead tothe best dispersion state of UiO-66-NH2 in organic phase during





Fig. 5 Dispersibility of UiO-66-NH2 and UiO-66-NH2-PC in cyclohexane and n-hexane after standing for (a) 30 min and (b) 12 h.

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the preparation process of TFN membranes by interfacialpolymerization.

3.2 Characterization of the prepared membranes

3.2.1 Morphologies of TFC and TFN membranes. Thesurface morphologies of TFC and TFN membranes wereobserved by FE-SEM, and the SEM images were shown in Fig. 6.

The prepared TFC membrane showed typical “ridge-valley”morphology. The white and bright stripe was the convex ridge,and the dark background was the valley region. This striped

Fig. 6 FE-SEM images of surface morphologies of TFC (a), TFN-0.05 (b


morphology was a kind of Turing structure, which was differentfrom the spotted morphology in other literature.20–22 Themorphology of nanoltration membranes prepared by interfa-cial polymerization depended on many factors: the property ofthe substrate, the diffusion rate of PIP at water/organic inter-face, and the interaction between amine monomer and thesubstrate, etc. Generally, the initial interfacial polymerizationreaction proceeds at water/organic interface and the PA selec-tive layer grows towards the organic phase side along with thePIP monomers diffusing from water to organic phase because of

), TFN-0.10 (c), TFN-0.15 (d) membranes.

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the disadvantageous partition coefficient of the acid chloridemonomers in water. The diffusion rate of the two monomershas a remarkable impact on the morphology of TFCmembranes. Under most conditions, the diffusion rate of PIP isless than that of TMC, but the diffusion rate disparity of the twomonomers is not very signicant (within the same order ofmagnitude), which leads to spotted morphology in the poly-amide membrane. When the diffusion rate disparity of the twomonomers comes up to orders of magnitude leading todiffusion-driven instability, the striped Turing structure couldappear.23 In this study, the HPAN substrate contained abundantnegative charged carboxyl groups that could interact with PIPmonomers via electrostatic force and hydrogen bonds, whichdecreased the diffusion rate of PIP molecules during the inter-facial polymerization process. So in this study, the diffusion rateof PIP was much lower than that of TMC molecules, whichresulted in the striped Turing structure morphology of the TFCmembrane in Fig. 6(a). However, the morphology of TFCmembrane in another literature was different,15 which might beascribed to the different hydrolytic degree of PAN substrate.

Fig. 7 FE-SEM images of cross-section morphologies of HPAN substrat

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As shown in Fig. 6(b–d), the surfaces of TFN membrane alsodisplayed the striped Turing structure morphology with UiO-66-NH2-PC loading on themembrane surfaces. The UiO-66-NH2-PCnanoparticles distributed more densely on membrane surfaceswith the increase of UiO-66-NH2-PC concentration in organicphase.

The cross-section morphologies of the prepared membraneswere shown in Fig. 7. Compared with HPAN substrate, theprepared nanoltration membranes presented poroussubstrate with PA selective layer tightly covering on the topsurface. And no interfacial defects between the selective layerand support layer could be found, which could be attributed tothe electrostatic interaction between the substrate and PIPindicating the good interfacial compatibility. The thickness ofthe PA selective layer for each membrane was around 380 nm,which was marked by the red parallel lines in the images.

3.2.2 Chemical characterization of the preparedmembranes. The ATR-FTIR spectra of the prepared membraneswere presented in Fig. 8. The absorption band from 3200 to3600 cm�1 was ascribed to the stretching vibration of unreactedamino groups of PIP monomer.24 The absorption peak at

e (a), TFC (b), TFN-0.05 (c), TFN-0.10 (d), TFN-0.15 (e) membranes.





Fig. 8 ATR-FTIR spectra of the prepared membranes.

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2933 cm�1 appeared in all membranes, which was related to thestretching vibration of –OH in carboxyl groups,25 because thesubstrate membrane contained abundant carboxyl groups aerhydrolysis and the prepared nanoltration membranes werealso the same due to the hydrolyzation of unreacted TMCmonomers. The absorption peak at 2243 cm�1 was attributed to–C^N in HPAN substrate.25 The absorption peaks appeared at1623 and 1385 cm�1 in TFC and TFN membranes were relatedto –C]O and –C–N in amide groups, respectively,12,26 whichindicated the PA layer was successfully fabricated on HPANsubstrate aer interfacial polymerization. The absorption peakat 771 cm�1 due to Zr–O could only be observed in TFNmembranes,12 indicating the existence of UiO-66-NH2-PC onTFN membrane surfaces.

XPS measurement was used to further characterize thechemical structure of TFN membranes. The XPS survey spectraof TFN-0.10 were shown in Fig. 9. As shown in Fig. 9(a), thechemical elements of TFN-0.10 included carbon, oxygen,nitrogen and zirconium. The high-resolution XPS spectrum ofeach element was shown in Fig. 9(b–e). The XPS spectrum of C1s could be resolved into 3 peaks at 287.8, 285.8, and 284.6 eV,which were attributed to C]O, C–N or C–O, and C–C, respec-tively.27,28 The N 1s spectrum could be tted to two peaks at399.9 and 399.3 eV which were assigned to C–N in PIP monomerand N–C]O in polyamide, respectively indicating the poly-amide selective layer successfully formed. The O spectrumcould be tted to 3 peaks at 531.7, 531.3, and 530.5 eV,respectively. The peak at 531.7 eV was ascribed to C–O ofcarboxyl groups from MOF nanoparticles and hydrolyzed acylchloride groups in TMC. The peak at 531.3 eV was ascribed toZr–O of MOF nanoparticles, and the peak at 530.5 eV wasascribed to C]O from acylamino, carboxyl groups and MOFnanoparticles in selective layer. The Zr element showed twopeaks at 184.7 and 182.3 eV. From Fig. 9(d and e), the existenceof Zr element and Zr–O bonds revealed the UiO-66-NH2-PCnanoparticles loaded on the membrane surface aer interfacialpolymerization process.

3.2.3 XRD patterns of the prepared membranes. XRDpatterns of HPAN substrate and the prepared membranes were


shown in Fig. 10. All the membranes displayed three conspic-uous peaks within 15� to 30� which were from HPAN substrate.The two strong characteristic peaks of UiO-66-NH2-PC at 7.3�

and 8.4� could be observed in TFNmembranes, which indicatedthe successful loading of UiO-66-NH2-PC in TFN membranes.

3.2.4 Hydrophilicity of the prepared membranes. Theinuence of UiO-66-NH2-PC on the hydrophilicity of TFC andTFN membranes was characterized by water contact angle. Asshown in Fig. 11, TFC membrane displayed the lowest contactangle, and with the increase of UiO-66-NH2-PC content, thecontact angle of TFN membranes presented a slight upwardtrend, indicating the ascending hydrophobicity of TFNmembranes. In order to increase the dispersibility of nano-particles in organic phase, the surface of nanoparticles neededto hold the character of nonpolarity, low surface energy so as toincrease the compatibility with organic solvent owing to thenonpolar intrinsic character of organic phase.16,29 The modi-cation of UiO-66-NH2 by palmitoyl chloride could increase thecompatibility between UiO-66-NH2 and cyclohexane due to theexistence of nonpolar alkyl which possessed the similar char-acter of hexane, and thus UiO-66-NH2-PC could get good dis-persibility. But it was for this reason that the hydrophilicity ofthe modied nanoparticles declined. Aer loading UiO-66-NH2-PC on membrane surface, the hydrophilicity of the membranesurface would also decline, so the water contact angle of TFNmembranes increased compared with TFC membrane. But theincrement was not signicant under UiO-66-NH2-PC loadingamount within 0.10% w/v. As shown in Fig. 11, the water contactangle was 41.9 � 2.2� for TFC membrane and increased to 46.7� 6.0� for TFN-0.10.

3.2.5 Surface potential of TFC and TFN membranes.According to the separation mechanism of nanoltrationmembranes, the charged membrane surface can interact withions and solute by electrostatic force so as to exclude oppositeelectrical substances, which is called the Donnan effect. Thesurface charge of the prepared membrane was characterized byzeta potential with pH value spanning from 3 to 10 and theresults were displayed in Fig. 12. It could be seen that all themembrane showed higher zeta potential at low pH value, andthe zeta potential decreased with the pH value increasing. Onlyat pH ¼ 3, the membranes possessed slightly positive charge,which could be ascribed to the protonation of unreacted aminogroups in PIP monomer and the protonation of amido bonds inpolyamide, because N atoms could combine H+ by coordina-tion. When the pH value was larger than 3, the coordination ofamino groups and amido bonds would disintegrate to a certainextent leading to the loss of positive charge. And simulta-neously, the –COOH formed by the hydrolysis of unreacted–COCl of TMC monomers gradually deprotonated, which madethe membrane surface turn into negatively charged.30 Underneutral conditions (pH ¼ 7), all the TFC and TFN membraneswere negatively charged, which was ascribed to the unreacted–COCl of TMC monomers of the polyamide nanoltrationmembranes. However, the zeta potential of TFN membraneswas higher around neutral conditions with the UiO-66-NH2-PCloading amount increasing. This could be explained by thepositively charged character of UiO-66-NH2-PC nanoparticles

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Fig. 9 XPS survey spectra of TFN-0.10 membrane.

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due to the existence of amino groups and zirconium ions in theframework structures.31,32

3.2.6 Surface roughness of TFC and TFN membranes. Thesurface roughness of TFC and TFN membranes was studied byAFM measurement. The three-dimensional surface topographyof the membranes was displayed in Fig. 13. The root averagearithmetic roughness (Ra) and root mean surface roughness (Rq)of the prepared membranes were listed in Table 1.

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The surface topography images of the prepared membranesalso displayed “ridge and valley” morphologies, which was inaccordance with FE-SEM images of the membrane surfaces inFig. 6. As shown in Table 1, the Ra and Rq value increased withthe increase of UiO-66-NH2-PC loading amount indicating themembranes became rougher aer loading nanoparticles onmembrane surface. The same phenomena about other nano-materials were also achieved in the previous researches.33,34 The





Fig. 10 XRD patterns of UiO-66-NH2-PC, HPAN substrate and theprepared membranes.

Fig. 11 Contact angle of TFC and TFN membranes.

Fig. 12 Zeta potential of TFC and TFN membranes.

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rougher membrane surface could provide larger surface area formass transfer to obtain a higher permeability.

3.3 Nanoltration performance of the prepared membranes

3.3.1 Determination of pre-ltration time. The nano-ltration performance of the prepared TFC and TFN


membranes were tested at 4 bar. Before test, the preparedmembrane needed to be wetted in water to dissolve unreactedreagents distributed within the polyamide layer,35 and alsocompacted to get a stable separation performance.36 So theprepared membranes were pre-ltrated at operation pressure of4 bar for a specic period to determine the needed pre-ltrationtime. The pre-ltration process was carried out using Na2SO4

solution (0.1% w/v), and the solution ux and Na2SO4 rejectionproperty were shown in Fig. 14. With the ltration time goingon, the Na2SO4 rejection increased continuously and thepermeation ux decreased continuously. The reason was that,the unreacted monomer reagents within the polyamidemacromolecule chains were washed out and the polyamideselective layer became denser under ltration pressure due tocompaction and rearrangement of polyamide chains duringpre-ltration process, which led to the increase of rejectionproperty and decrease of permeability. When the pre-ltrationtime reached 50 min the rejection property and permeationux came up to a nearly stable level. According to the abovediscussion, the pre-ltration time was set at least 50 min in thefollowing experiments.

3.3.2 Inuence of UiO-66-NH2-PC loading amount. Thepermeability of the prepared membranes was measured byultrapure water and the rejection properties were measured byNa2SO4, MgSO4, and NaCl solution. The concentrations of the 3kinds of typical salt solution were all 0.1% w/v. Beforemeasurement, all the membranes were pre-ltrated for at least50 min to get a stable performance and the results were shownin Fig. 15.

As displayed in Fig. 15, the pure water permeability (PWP)increased from 8.1 LMH bar�1 for TFC membrane to 12.4 LMHbar�1 for TFN-0.15. The increased PWP was largely ascribed tothe following factors: rstly, the adding of nanomaterialincreased the membrane free volume because the addednanomaterial could interfere with polyamide chain packing;37

secondly, the intrinsic nanopores of the UiO-66-NH2-PC mate-rial and the formation of nanomaterial/polyamide interfacescould supply additional passageways for water passing throughthe membrane.9 Due to the above reasons, the permeabilityincreased continuously with the increase of UiO-66-NH2-PCloading amount.

The rejection properties of the prepared membrane slightlydecreased continuously with the increase of UiO-66-NH2-PCloading amount. That was ascribed to the formation of thenanomaterial/polyamide interfaces, which led to the leakage ofsolute passing through the membrane selective layers. Thedecrease of rejection properties in TFN membranes is a generalphenomenon that was reported in other literature.38–40 It shouldbe noted that all the prepared membranes exhibited the rejec-tion property order of MgSO4 > Na2SO4 > NaCl, which could beexplained by the rejection mechanism of nanoltrationmembranes. It was known that the membrane rejection prop-erty was based on both size sieving effect and Donnan effect.41

The hydrated diameters of the salt ions were Mg2+ (0.86 nm) >Na+ (0.72 nm), SO4

2� (0.76 nm) > Cl� (0.66 nm).42 All themembranes showed negatively zeta potential and thus couldrepulse the same charged SO4

2� and Cl�, but the divalent ions

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Fig. 13 Three-dimensional surface topography of TFC and TFN membranes.

Table 1 Roughness of the prepared membranes

Membrane TFC TFN-0.05 TFN-0.10 TFN-0.15

Ra (nm) 15.0 24.6 30.6 37.8Rq (nm) 19.0 32.1 39.1 46.9

Fig. 14 Influence of pre-filtration time on separation performance.

Fig. 15 Influence of UiO-66-NH2-PC loading amount on separationperformance.

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were easy to be rejected than monovalent ions due to thestronger electronegativity and larger hydrated diameter. So themembranes exhibited higher rejection property of sulfate thanchlorate. The rejection of MgSO4 > Na2SO4 was due to the largerhydrated diameter of Mg2+, which was in accordance with theprevious research in other literature.10

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When the UiO-66-NH2-PC loading amount was within 0.10%w/v, the MgSO4 rejection was about 100% and NaCl rejectionwas above 30%, so the UiO-66-NH2-PC loading amount of 0.10%w/v was an acceptable value.

3.3.3 Inuence of operation pressure on TFN membraneproperty. The TFN-0.10 was used to study the inuence ofoperation pressure on nanoltration performance. The nano-ltration experiment was carried out at the operation pressureof 2, 4, 6 and 8 bar using Na2SO4 solution with the concentra-tion of 0.1% w/v. The rejection property and solution perme-ation ux were shown in Fig. 16. As exhibited in the results, theux rose linearly with the operation pressure increasing. From





Fig. 16 Influence of operation pressure. Fig. 18 Long-term stability of TFN-0.10 membrane.

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the classical solution diffusion model, the theoretical perme-ation ux could be given as follows:

Jw ¼ A(Dp � Dp)

The Jw represents water ux and A represents permeabilityparameter. Dp and Dp refer to the operation pressure andosmotic pressure, respectively. It could be found that Jw was thefunction of Dp and there was a linear relationship between Jwand Dp.43 It was worth noting that the permeation ux at theoperation pressure of 4 bar was 28.7 LMH, which was muchlower than the pure water permeability of 11.3 LMH bar�1. Thiswas attributed to the existence of osmotic pressure (Dp) causedby the concentration polarization between feed side and thepermeation side of the membrane.

The operation pressure also inuenced the rejection prop-erty of TFN-0.10 membrane. With the increase of operationpressure the Na2SO4 rejection decreased continuously, whichwas in accordance with the reported literature.15 Thisphenomenon could be explained by the formation of voids atnanoparticle/polyamide interfaces.32 Considering the mobilitydifference between rigid inorganic nanoparticles and exibleorganic polyamide matrix, high pressure might tear and weakenthe interfacial interaction between nanoparticles and poly-amide matrix, because rigid inorganic nanoparticles could notmove exibly and change shape with polyamide chain

Fig. 17 Nanofiltration performance of TFN membranes prepared withdifferent MOF nanomaterials.


synchronously and hence led to the formation of voids at theirinterfaces.

3.3.4 Comparison of nanoltration performance. As dis-cussed above, the modied nanomaterial could get better dis-persibility and hence reduce aggregation in the interfacialpolymerization process to prepare high performance nano-ltrationmembranes. In order to conrm the above conclusion,the TFN membranes prepared with pristine UiO-66-NH2 andUiO-66 were also measured to compare with that prepared withmodied UiO-66-NH2-PC and the results were shown in Fig. 17.The TFN membranes prepared with UiO-66-NH2 and UiO-66nanomaterials showed higher water permeability but lowerrejection properties than that prepared with UiO-66-NH2-PC.The results was due to the poor dispersibility of unmodiednanomaterials in organic phase which led to the seriousaggregation of nanoparticles on membrane surfaces, whichcould be seen in Fig. S2.† The results also revealed that UiO-66-NH2-PC was more preferable to prepare high rejection TFNmembrane due to its better dispersibility in organic phase thanUiO-66-NH2 and UiO-66.

3.3.5 Long-term stability of TFN-0.10 membrane. Tomeasure the long-term stability of the prepared TFNmembranes, TFN-0.10 membrane was employed to test themembrane property at 4 bar. As shown in Fig. 18, the pure waterpermeability decreased slightly with the increase of Na2SO4

rejection at early stage due to the compaction and rearrange-ment of polyamide chains.44 And then the permeability andrejection property reached a relatively stable level and keptnearly unchanged during the test. The result revealed a stableseparation performance of TFN-0.10 membrane. The surfacemorphology of TFN-0.10 membrane aer 72 h ltration wasshown in Fig. S3.†

4 Conclusion

In this study, the surface modied UiO-66-NH2 nanoparticleswere dispersed in organic phase and introduced into thepreparation process of TFN membranes. The surface modi-cation could increase the dispersibility of the synthesized UiO-66-NH2 nanoparticles and hence reduce the aggregation ofnanoparticles in organic phase to prepare high performance

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nanoltration membranes. The prepared TFC and TFNmembranes displayed typical “ridge-valley” shaped Turingstructure surface morphology. The SEM, ATR-FTIR and XPScharacterization demonstrated the polyamide layer was fabri-cated on HPAN substrates and the modied UiO-66-NH2

nanoparticles were loaded on the membrane surfaces. Hydro-phobicity, surface roughness and zeta potential of TFNmembrane increased compared with TFC membrane. The TFNmembranes showed higher water permeability and slightlylower rejection properties with the increase of UiO-66-NH2-PCconcentration in organic phase. Compared with the TFNmembranes prepared with pristine UiO-66 and UiO-66-NH2, theTFN membrane prepared with modied UiO-66-NH2 displayedhigher salt rejection properties due to the better dispersibilityaer modication. Furthermore, the membrane performancewas stable in the long-term operation test.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 21908040 and 21878068), the NaturalScience Foundation of Hebei Province (B2017202056), theProgram for Top 100 Innovative Talents in Colleges andUniversities of Hebei Province (SLRC2017029) and Hebei HighLevel Personnel of Support Program (A2016002027).

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