Robust Multilayer Graphene-Organic Frameworks for Selective

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Robust Multilayer Graphene-Organic Frameworks for Selective Separation of Monovalent Anions Yan Zhao, Jiajie Zhu, Jian Li, Zhijuan Zhao, Sebastian Ignacio Charchalac Ochoa, Jiangnan Shen, Congjie Gao, and Bart Van der Bruggen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03839 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Applied Materials & Interfaces

Robust Multilayer Graphene-Organic Frameworks for Selective Separation of Monovalent Anions ∥

Yan Zhao,†,‡ Jiajie Zhu,† Jian Li,‡ Zhijuan Zhao,‡,§ Sebastian Ignacio Charchalac Ochoa,‡, Jiangnan Shen,*,† Congjie Gao† and Bart Van der Bruggen*,‡, †

⊥

Center for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang University of

Technology, Hangzhou 310014, P. R. China ‡

Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

§

Beijing Engineering Research Center of Process Pollution Control, Division of Environment Technology and

Engineering, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ∥

Division of Engineering Sciences, CUNOC, University of San Carlos of Guatemala, Modulo G, Calle Rodolfo

Robles 29-99 Zona 1, Quetzaltenango, Guatemala ⊥

Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680,

Pretoria 0001, South Africa

ABSTRACT: The chemical and mechanical stability of graphene nanosheets was used in this work to design a multilayer architecture of graphene, grafted with sulfonated 4,4’- diaminodiphenyl sulfone (SDDS). Quaternized poly (phenylene oxide) (QPPO) was synthesized and mixed with SDDS (rGO-SDDS-rGO@QPPO), yielding a multilayer graphene-organic framework (MGOF) with positive as well as negative functional groups that can be applied as a versatile electro-driven membrane in electrodialysis (ED). Multilayer graphene-organic frameworks are a new class of multilayer structures, with an architecture having a tunable interlayer spacing connected by cationic polymer material. MGOF membranes were demonstrated to allow for an excellent selective separation of monovalent anions in aqueous solution. Furthermore, different types of rGO-SDDS-rGO@QPPO membranes were found to have a good mechanical strength, with a tensile strength up to 66.43 MPa. The membrane (rGO-SDDS-rGO@QPPO-2) also has a low surface electric resistance (2.79 Ω·cm2), a low water content (14.5%) and swelling rate (4.7%). In addition, the selective separation between Cl- and SO42- of the MGOF membranes could be as high as 36.6%.

KEYWORDS: graphene oxide, selective separation of monovalent anions, multilayer graphene-organic frameworks, monovalent anion exchange membrane, quaternized

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polyphenylene oxide

1. INTRODUCTION In view of future sustainable supply of non-renewable scarce resources, the extraction of useful materials such as ions, from oceans and salt lakes may be a promising source to meet their growing demand.1,

2

For this purpose, some techniques have been

explored, including the extraction of lithium ions from salt-lake brines,3 molecular sieving of magnesium ions4 and sieving of alkaline earth cations.5 Particularly, selective separation of monovalent ions in salt mixtures, such as separation of monovalent/multivalent anions is attracting much attention.6-8 Traditional techniques to achieve this include solvent extraction of Li+, precipitation, salting out and adsorption to extract monovalent ions.3, 9 Membrane separation using ion exchange membranes (IEMs) is an economical and environmentally friendly separation method for aqueous solutions. However, its current limitations in terms of selective separation ability and efficiency hinder its application for separation of monovalent ions. Recently, various types of metal-organic frameworks (MOFs) have been studied for selective separation of monovalent ions.10-12 However, the mechanical instability of MOFs, still makes them impractical. 13-17 Graphene is a flexible two-dimensional material with good chemical stability and mechanical strength, and has been used in water desalination and purification for the selective separation of monovalent ions and for gas separation.10, 18-24 The tunable nanoscale pores in a layer of graphene oxide (GO), which is considered as a highly efficient material for selective separation of monovalent ions, yields a multilayer structure for selective separation of monovalent ions in electrodialysis (ED).25 However, the difficulty to control the interlayer spacing and the easy exfoliation of layered graphite when immersed in aqueous solution limit its application in selective separation of monovalent ions.25, 26 Fortunately, the relatively easy functionalization of GO may allow to design special architectures for selective separation of monovalent ions. For example, the epoxy and functional groups grafted on the GO sheets can be used to act as structural nodes in MOFs and show a potential chemical stability and selective separation of monovalent anions.27-30 However, MOF-based separation materials (including membranes based on MOFs) are difficult to develop because of the

poor chemical stability and


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mechanical strength.31 Those are the main reasons why their application is limited in the industry. In membrane preparation, GO sheets are used to improve the rejection of multivalent ions or to provide special ion channels for selective separation of monovalent ions.32-34 However, the randomized distribution layout of the graphene sheets may limit the efficiency of ion separation.16, 35-37 Thus, a controlled separation between each layer of graphene among the multilayer structure of graphene is essential in selective separation of monovalent ions. Inspired by the special structure of MOFs, multilayer graphene grafted with organic polymers is thought to be applicable for selective separation of monovalent ions. In our previous work, a symmetrical multilayer architecture with a tunable interlayer spacing was prepared by grafting sulfonated 4,4’-diaminodiphenyl sulfone (SDDS)

to

the

graphene,

graphene-SDDS-graphene

which

resulted

(rGO-SDDS-rGO).38

in The

a

stable resulting

structure

of

membrane,

rGO-SDDS-rGO with its negatively charged ion channels of ~0.5 nm (wet), showed an excellent selective separation of lithium. In order to construct rGO-SDDS-rGO to a functional anion exchange membrane (AEM), the selectivity and the stability of the membrane was lacking. Quaternized polyphenylene oxide (QPPO) is a polymer with characteristics that may be useful to fulfill both requirements. The QPPO polymer contains a large number of -N+(CH3)3 groups and a controllable degree of quaternization that could be applied to prepare of an anion exchange membrane (AEM).39-42 Therefore, we chose QPPO to be coalesced with the multilayer rGO-SDDS-rGO structure, and to form a hybrid multilayer graphene organic framework (MGOF). According to theory, the -N+(CH3)3 groups of the QPPO are expected to graft with the-SO3- groups of rGO-SDDS-rGO through either ion-dipole or H-bond interaction or in some cases ionic bonding or other Coulomb interactions, leading to coating the surface of the membrane. The anions would cross the QPPO freely and the monovalent anions would selectively pass through the independent rGO-SDDS-rGO in the MGOF. The QPPO, owing to the -N+(CH3)3 groups, would prevent the cations from crossing the MGOF membrane. In summary, MGOFs as a novel membrane was designed for selective separation of monovalent anions. In order to better understand and to optimize the performance of the MGOFs, several experiments were performed to investigate the effect of the mass ratio between rGO-SDDS-rGO and QPPO on the resulting membranes. This included the evaluation of the tensile strength with the MGOF, which showed an ACS Paragon Plus Environment


excellent

mechanical

strength

and

tensile

strength

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of

66.43

MPa

(rGO-SDDS-rGO@QPPO-1). In addition, the selective separation of monovalent anions measurement showed that the high selectivity efficiency value and with the low surface electric resistance.

2. EXPERIMENTAL SECTION 2.1.

Materials.

4,4’-Diaminodiphenylsulfone

(DDS,

90%),

potassium

permanganate (KMnO4, A.R.) and hydrogen peroxide (H2O2, 30%) were purchased from Lingfeng Chemical Reagent Co. Ltd.

(Guangdong, China). Sulfanilic acid

(A.R.) was purchased from Macklin Biochemical Co. Ltd. (Shanghai, China). Graphite

powder

(99.95%),

Potassium

persulfate

(K2S2O8,

A.R.),

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO, with a molecular weight of 20 000 g/mol), H2SO4, NaCl, Na2SO4 and all other chemicals were analytical reagent and obtained from Aladdin Industrial Co. Ltd. (Shanghai, China) and used without pretreatment. Polycarbonate (PC) membrane (Whatman) with pore size of 200 nm, and effective diameters of 19 mm were used for the preparation of the composite paper by vacuum filtration. 2.2. Synthesis of rGO-SDDS-rGO. The GO sheets were prepared using the modified Hummer’s method as described elsewhere.20, 43-47 To prepare the multilayer architecture of graphene, we firstly synthesized the sulfonated 4,4’-diaminodiphenyl sulfone (SDDS) monomers. Then, the miltilayer rGO-SDDS-rGO was synthesised just as the previous work.38 (The steps of synthesis of sulfonated 4,4’diaminodiphenyl sulfone and rGO-SDDS-rGO were also shown in Supporting Information) 2.3. Synthesis of QPPO. The derived QPPO from BPPO (brominated PPO), as reported by several papers have reported on its synthetization (The steps of synthesis of QPPO were shown in Supporting Information)39-42, 48. Once the fibrous BPPO was obtained, we re-dissolved it in chloroform (60 mL) and poured into the excess ethanol and repeat this step for 3 times, in order to obtain purified BPPO. Then, it was dried in a vacuum drying oven (60 ℃) for 24 h. Afterwards, BPPO (1g) was completely dissolved in 1-methyl-2-pyrrolidone (NMP, 6 mL) solution and trimethylamine (TMA, 1 mL). It was kept under magnetic stirring at 450 rpm for 24h, and then the QPPO solution was obtained.


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2.4.

Preparation

of

rGO-SDDS-rGO@QPPO

membrane.

The

rGO-SDDS-rGO was fully dispersed in dimethyl sulfoxide（DMSO）solution by 40-kHz ultrasound. Then, an aliquot was added to the QPPO solution, the volume was controlled to obtain four different weight ratios (0, 0.25%, 0.50% and 1.00%, respectively) and allowed to react for the following 24 h. Then, each mixture was cast on a flat glass plate and first dried at 80 ℃ for 12 h and then dried at 80 ℃ under vacuum for 24 h. In Scheme 1, the synthetic route to the multilayer graphene-organic frameworks and named as rGO-SDDS-rGO@QPPO-0, rGO-SDDS-rGO@QPPO-1, rGO-SDDS-rGO@QPPO-2 and rGO-SDDS-rGO@QPPO-3, respectively and listed in Table 1.

Scheme 1 Synthetic route to the multilayer graphene-organic frameworks. Table 1. Denotation of different types of multilayer graphene-organic frameworks. MGOFs’ named

Mass of rGO-SDDS-rGO (mg)

Mass of QPPO (g)

rGO-SDDS-rGO@QPPO-0*

0

1

rGO-SDDS-rGO@QPPO-1

2.5

1

rGO-SDDS-rGO@QPPO-2

5.0

1

rGO-SDDS-rGO@QPPO-3

10.0

1

* rGO-SDDS-rGO@QPPO-0 is the QPPO. In order to illustrate the MGOFs, we named it as a MGOF.

2.5. Materials characterization. GO, rGO, rGO-SDDS-rGO and MGOFs were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photo-electron spectroscopy (XPS), X-ray diffraction (XRD), scanning electronic microscopy (SEM) and transmission electron microscopy (TEM) (the details are shown in Supporting Information). 2.6. Evaluation of electrochemical performance. The details and results information of rGO-SDDS-rGO electrochemical impedance spectroscopy (EIS) and current-voltage (CV) curve are shown in Supporting Information. Given that, the



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MGOFs detailed in this work are applied in ED for selective separation of monovalent anions, the surface electric resistance of MGOFs is one of the most important electrochemical properties to be characterized. It was carried out separately in 0.50 M NaCl solution at room temperature by a custom-designed cell (described in Supporting Information). Surface electric resistance of MGOFs is calculated as

R=

×S

(1)

where R is the membrane surface electric resistance of MGOFs and expressed in Ω·cm2, U is the voltage of the MGOFs and U0 is the voltage of the blank expressed in V, I is the constant current through the MGOFs (0.004 A), and S is the MGOFs effective area (7.065 cm2). The ion exchange capacity (IEC) is another crucial electrochemical property of MGOFs; it was measured by acid-base titration. The details of the measurement are shown in Supporting Information. The IEC was calculated as:

IEC =

∙

(2)

where cAg is the concentration of AgNO3 and expressed in mmol·L; VAg is the volume of consumed AgNO3 expressed in L; mDry is the weight of the dry MGOFs and expressed in g.

2.7. Evaluation of the physical and mechanical properties. The water content (WC) and swelling rate (SR) of MGOFs are calculated by measuring the change of weight and length when being hydrated. Prior to the measurement, the absolute dryness of MGOFs is attained bydrying the samples in a vacuum oven at 60 °C for 24 h. After that, MGOFs were immersed in water and the change of weight and length was measured. WC and SR were calculated as

W (%) =

S (%) =

× 100%

× 100%

(3)

(4)

where mWet and LWet are the weight and length of wet MGOFs samples, mDry and LDry are the weight and length of wet MGOFs. The mechanical strength of the membrane is one of the most important materials


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for selective separation of monovalent anions and measured with a tensile strength instrument (CTM2500, SANS Company, China). The MGOFs samples in rectangle-shape were prepared with a size of 4mm (width) and 25mm (length). The experiments were performed with the constant separating speed of 25 mm·min-1 under ambient atmosphere. In this work, the tensile strength (TS) and elongation at break (EB) was measured and used to evaluate the mechanical strength of MGOFs. 2.8. Evaluation of separation performance. The performance in selective separation of monovalent anions was measured in a four-compartment module (shown in Supporting Information). The current density utilized for these tests was 5.00 mA/cm2, which is in the suitable current range for MGOFs. The effective area of the rGO-SDDS-rGO membranes was 20.00 cm2 and the approximate thickness of the MGOFs was 20 µm. Initially, a mixed solution of 0.05 M of both NaCl and Na2SO4 was applied in both compartments in contact with the MGOFs. The concentration of anions was measured by Anion Chromatography (Dionex ICS-1600, the United States) in the dilute compartment every 10mins. To compare the separation efficiency, we followed the method proposed by Van der Bruggen et al7, where the separation efficiency, S, between component a and b is evaluated as ((! (#))/(! (&) ))((! (#))/(!' (&) ))

' S(t) = (((!" (#))/(!" (&) )))((((! "

"

' (#))/(!' (&) ))

× 100%

(5)

where S(t) is the separation efficiency; ca(0) and cb (0) are the initial concentration of SO42- and Cl-, respectively;

ca(t) and cb(t) are the concentration of

SO42- and Cl- at time t.

3. RESULTS AND DISCUSSION 3.1. Synthesis of rGO-SDDS-rGO. The synthetic route of the symmetrical structure polyelectrolyte of SDDS and rGO-SDDS-rGO is illustrated in Supporting Information. The ATR-FTIR spectrum and the XPS spectra are used to characterize the chemical groups and elemental content in rGO-SDDS-rGO, respectively (in Supporting Information). The transmission electron microscope (TEM) images of as-prepared GO and rGO-SDDS-rGO are shown in Figure 1a and 1b. Due to the excellent dispersion of GO in aqueous solution, a single nano sheet can be clearly distinguished. When the rGO-SDDS-rGO is fully dispersed in the aqueous solution, as shown in Figure 1b, the rGO-SDDS-rGO still keeps a multilayer structure. The structure of the cross-section can be clearly distinguished in the TEM images, which



illustrated the stability of multilayer architecture and the average interlayer spacing (wet) is 0.48 nm. X-ray diffraction (XRD) patterns (show in Figure 1c) further confirm that the average interlayer spacing of dry rGO-SDDS-rGO (~3.70 nm) is close to rGO (~3.85 nm), suggesting close links of each nano sheets. This further confirms the stability of the multilayer. The top view and cross-sectional SEM images of rGO and rGO-SDDS-rGO are shown in Figure 1d, e, f and g. The flat surface can be observed in Figure 1d, and the cross-sectional SEM image show as in Figure 1e. In contrast, in Figure 1f, the rGO-SDDS-rGO surface shows wrinkles and in Figure 1g, an ultrathin graphene multilayer can be clearly distinguished from the cross-sectional SEM image. Electrochemical impedance spectroscopy (EIS) and current-voltage (CV) curve measurements are the two important variables to investigate the electrical resistance and characterize the CV change performance of rGO-SDDS-rGO were show in Figure S8 and Figure S9, respectively.

Figure 1. TEM images and the cross-sectional TEM images of the GO (a) and rGO-SDDS-rGO(b); XRD patterns of GO, rGO and rGO-SDDS-rGO (c); Surface (d), cross-section (e) SEM images of GO and Surface (f), cross-section (g) SEM images of rGO-SDDS-rGO

3.2. Preparation of MGOFs. The presence of rGO-SDDS-rGO in MGOFs (rGO-SDDS-rGO@QPPO-0, rGO-SDDS-rGO@QPPO-1, rGO-SDDS-rGO@QPPO-2


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and rGO-SDDS-rGO@QPPO-3 membranes) was identified by ATR-FTIR, as shown in Figure 2. In comparison with the rGO-SDDS-rGO@QPPO-0, the existence of the multilayer graphene peaks, which only exist inMGOFs, is in the range of 1500~1600 cm-1. Additionally, the peaks at about 1370 cm-1 (symmetric stretching vibrations of the two S=O of -SO3H), 1150 cm-1 (S=O symmetric stretching vibrations) and 870 cm-1 (C-N symmetric stretching vibrations)3, 8, 16 confirm that the functional groups and stable multilayer architecture of the rGO-SDDS-rGO in MGOFs. Figure 3I shows a comparison of the four different MGOF membranes that were prepared, based on an increasing mass ratio of rGO-SDDS-rGO to QPPO. A steep reduction in transparency of MGOFs is observed, while the color distribution is rather homogeneous in all four cases. However, once the mass ration of rGO-SDDS-rGO surpasses 1.0%, the color distribution of the MOGFs shows some irregular distribution, such as shown in photos of the 1.5% and 2.0% of MOGFs in Figure S10. The morphology of the MGOFs surfaces is shown in Figure 3II. According to the SEM image of the rGO-SDDS-rGO@QPPO-0 membrane (Figure 3IIa), the rough and granulated

surface

of

QPPO

can

be

clearly

seen,

while

the

other

rGO-SDDS-rGO@QPPO membranes have a smooth surface. Furthermore, it seems that the surface smoothness is in par with the increasing rGO-SDDS-rGO content in MGOFs. This phenomenon illustrates that by grafting QPPO on the surface of the rGO-SDDS-rGO multilayer structure, a controlled assembly layout of the multilayer graphene organic framework can be achieved, forming a smooth and homogeneous surface. Figure 3III shows the cross-sectional SEM for the membranes. The multilayer structure of rGO-SDDS-rGO can be observed clearly in Figure 3IIIb, c and d, which correspond to rGO-SDDS-rGO@QPPO-1, rGO-SDDS-rGO@QPPO-2 and rGO-SDDS-rGO@QPPO-3, respectively. A clear difference is observed in comparison to the cross-section of rGO-SDDS-rGO@QPPO-0 (as shown in Figure 3IIIa), which shows only the granulated cross-section. Furthermore, MGOFs show the general orientation of multilayer graphene. That is probably because the effect of interfacial tension grafts the QPPO on the surface of rGO-SDDS-rGO but not inside the structure (only 0.48 nm) and the MGOFs are formed, they follow the orientation of the rGO-SDDS-rGO membranes.



Figure 2. ATR-FTIR spectra of rGO-SDDS-rGO@QPPO-0, rGO-SDDS-rGO@QPPO-1, rGO-SDDS-rGO@QPPO-2 and rGO-SDDS-rGO@QPPO-3.


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Figure 3. MGOFs photos (Zone I), SEM surface images (Zone II) and Cross-sectional SEM & HRSEM images in inset (Zone III) of rGO-SDDS-rGO@QPPO-0 (a), rGO-SDDS-rGO@QPPO-1 (b), rGO-SDDS-rGO@QPPO-2 (c) and rGO-SDDS-rGO@QPPO-3 (d).

3.3. Performances of MGOFs. Electrochemical performance is a key element for MGOFs in ED. Thus, characterization of surface electric resistance is an important performance variable for MGOFs when used as electro-driven membranes. As shown in Figure 4a, the surface electric resistance of rGO-SDDS-rGO@QPPO-0 is 1.32



Page 12 of 21

Ω·cm2, which is considered a low surface electric resistance value and therefore of high efficiency in ED process. As we increased the

rGO-SDDS-rGO mass content,

so did the value of the resulting area resistance. However, the surface electric resistance

of

rGO-SDDS-rGO@QPPO-1,

rGO-SDDS-rGO@QPPO-2

and

2

rGO-SDDS-rGO@QPPO-3 (2.55, 2.79 and 3.45 Ω·cm ) is still suitable to for industrial application. The ion exchange capacity (IEC) (shown in Figure 4a) is also a crucial property of electro-driven membranes. Inspite of the negatively charged ion channels’ multilayer of rGO-SDDS-rGO, the IEC values will increase with the rGO-SDDS-rGO mass content in MGOFs. This anomalous phenomenon may be explained by the multilayer structure of rGO-SDDS-rGO in MGOFs, which increases the relative surface area and increases the anion exchange capacity with QPPO. The water content (WC) and swelling rate (SR) of MOGFs are shown in Figure 4b. With an increase of rGO-SDDS-rGO mass content, the value of WC decreases from 16.5% (rGO-SDDS-rGO@QPPO-0) to 15.5% (rGO-SDDS-rGO@QPPO-1) 14.5% (rGO-SDDS-rGO@QPPO-2) and 14.1% (rGO-SDDS-rGO@QPPO-3). This is because of the stronger stability of MGOFs limits the WC. Correspondingly, the SR value decrease from 6.5% to 5.4%, 4.7% and 2.6%, respectively. This phenomenon can be explained by the stability of the multilayer structure of rGO-SDDS-rGO in the MGOFs and the stable coalescence between rGO-SDDS-rGO and QPPO in MGOFs. Thus, the MGOFs will likely maintain a stable size and shape in aqueous separation system. The mechanical strength of MGOFs, the tensile strength (TS) and elongation at break (EB) values are the two important parameters to show the stability in relation to application in ED. In Figure 4c, rGO-SDDS-rGO@QPPO-0 exhibits the TS (61.28 MPa) and EB values (34.17%). However, the TS value of rGO-SDDS-rGO@QPPO-1 increases to 66.43 MPa, because of the stability of the multilayer structure and the high mechanical strength of graphene nano sheets. The stability of rGO-SDDS-rGO connected with QPPO is also influential in this property. In addition, the EB value of rGO-SDDS-rGO@QPPO-1 decreases to 27.20%. It illustrates that the mechanical strength of rGO-SDDS-rGO@QPPO-1 is enhanced by the stability of the multilayer structure and high mechanical strength of graphene nano sheets. However, for MGOFs with higher rGO-SDDS-rGO content, both TS and EB values will decrease. Because less QPPO is grafted on MGOFs and the less ion bonds are generated, the mechanical strength of MGOFs also decreases. ACS Paragon Plus Environment

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(a)

2.2

-1

2.0 3 1.8 2 1.6

1.4

1 0

1

2

3

rGO-SDDS-rGO@QPPO membrane types 18

10

8

Water content (%)

16

6 14 4 12

Swelling rate (%)

(b)

Ion exchange capacity (mmol·g )

2

Membrane surface resistance (Ω·cm )

4

2

0

10 0

1

2

3

rGO-SDDS-rGO@QPPO membrane types

(c)

80

60 66.43

50

61.28

60 40 50

43.83

34.17

40

30 27.20

30

33.57

20

19.81 13.82

20 0

1

2

Elongation at break (%)

70

Tensile strength (MPa)

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


10

3

rGO-SDDS-rGO@QPPO membrane types

Figure 4. Surface electric resistance and ion exchange capacity of MGOFs (a); Water content and swelling rate of MOGFs (b); Tensile strength and elongation at break (c).

3.4. Evaluation of separation performance. In order to investigate the selective separation of monovalent anions, NaCl and NaSO4 (Cl- and SO42-) were chosen as the monovalent and multivalent anions, respectively. As shown in Figure 5, the concentration between Cl- and SO42- change in the dilute compartment of different types of MGOFs. As expected, rGO-SDDS-rGO@QPPO-0 had a poor performance in the separation of monovalent anions, while the MGOFs showed an excellent selective



separation of monovalent anions. With an increasing of rGO-SDDS-rGO content in the MGOFs, the retention of SO42- is improved. However, the concentration of total anions in the dilute compartment decreased slowly, which is the reason of the electrostatic repulsion on the membrane surface.49 In order to illustrate this phenomenon clearly, the separation efficiency values between Cl- and SO42- were chosen as the evaluation parameter. In Figure 6a, the schemes of selective separation of monovalent anions (Cl- and SO42-) measurement show clearly in ED. In Figure 6b, the separation efficiency of rGO-SDDS-rGO@QPPO-0 is lower than others. Especially during the 30 min, the average separation efficiency value of rGO-SDDS-rGO@QPPO-0 is only 1.9%. This phenomenon suggests that the QPPO membrane by itself has no selective separation of monovalent anions and cannot be used for monovalent anions separation. In contrast, MGOFs showed an excellent selective separation of monovalent anions. At the beginning of 30 min, the average separation

efficiency

value

of

rGO-SDDS-rGO@QPPO-1,

rGO-SDDS-rGO@QPPO-2 and rGO-SDDS-rGO@QPPO-3 is 22.4%, 36.6% and 24.2%, respectively. The excellent selective separations of monovalent anions of MGOFs show the potential in extraction or selective separation of monovalent anions in ED.


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Figure 5 The concentration change in the dilute compartment of different types of MGOFs.

(a)

MGOF

SO42-

CEM Na2SO4/ NaCl Na2SO4

CEM Na2SO4/ NaCl

Na2SO4

(b) 50 rGO-SDDS-rGO@QPPO-0 rGO-SDDS-rGO@QPPO-1

36.56

40

Anions selectivity (%)

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


rGO-SDDS-rGO@QPPO-2

30

rGO-SDDS-rGO@QPPO-3

24.21

20

22.39 10

1.88 0 0

20

40

60

80

100

120

Time (min)

Figure 6 Schemes of selective separation of monovalent anions measurement (a) and the selectivity of MGOFs (b).

4. CONCLUSION Novel membranes described as multilayer graphene-organic frameworks (MGOFs) are a new approach to pursue a selective separation of monovalent anions. We combined a multilayer graphene oxide sheet grafted with SDDS with negative groups and coalesced it with quaternized poly (phenylene oxide) with numerous positive groups, which yielded a functionalized membrane applicable for selective separation of

monovalent

anions

in

electrodialysis.

We


evaluated

a

series

of


rGO-SDDS-rGO@QPPO membranes, with varying mass ratio of 0, 0.25, 0.5 and1.0%. The resulting membranes showed a high mechanical strength and excellent electrochemical properties for monovalent ion separation. The highest tensile strength, elongation at break and membrane surface resistance was achieved with a mass ratio of 1% of rGO-SDDS-rGO to QPPO, which yielded 66.43 MPa, 27.2% and 2.55Ω·cm2, respectively. The best separation efficiency was achieved with the configuration of mass ratio at 0.5%, which showed 36.6% separation efficiency when using Cl- and SO42-in electrodialysis rocess. The outstanding performance of the MGOFs membrane makes it a very promising material for the efficient extraction of monovalent anions.

■ ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. The theories of hydrated ion and hydrated anions cross the multilayer of graphene and multilayer graphene-organic frameworks (MGOFs) for anions sieving; Details of some steps of synthesis of MGOFs membrane and the results of electrochemical properties of the MGOFs.

■ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (Jiangnan Shen) *E-mail: [email protected] (Bart Van der Bruggen)

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS Yan Zhao would like to acknowledge the support provided by the China Scholarship Council (CSC) of the Ministry of Education, P. R. China (CSC NO. 201708330281). The research was also supported by National Natural Science Foundation of China


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(No.

21676249),

National

Key

Research

and

Development

Plan

(NO.

2017YFC0403700) and National High Technology Resrearch and Development Program 863 (No. 2015AA030502).

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Abstract graphic: The novel multilayer graphene-organic frameworks (MGOFs) are designed as a special electro-driven membrane and used for selective separation of monovalent anions.


Robust Multilayer Graphene-Organic Frameworks for Selective

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