High-Flux Graphene Oxide Membranes Intercalated by MetalâOrganic

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High-Flux Graphene Oxide Membranes Intercalated by MOF with Highly Selective Separation of Aqueous Organic Solution Yunpan Ying, Dahuan Liu, Weixin Zhang, Jing Ma, Hongliang Huang, Qingyuan Yang, and Chongli Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14371 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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ACS Applied Materials & Interfaces

High-Flux Graphene Oxide Membranes Intercalated by MOF with Highly Selective Separation of Aqueous Organic Solution Yunpan Ying, Dahuan Liu*, Weixin Zhang, Jing Ma, Hongliang Huang, Qingyuan Yang and Chongli Zhong* State Key Laboratory of Organic-Inorganic Composites, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

ABSTRACT: Graphene oxide (GO) membranes assembled by single-atom thick GO nanosheets have displayed huge potential application both in gas and liquid separation process due to its facile and large-scale preparation resulting from various functional groups, such as hydroxyl, carboxyl and epoxide groups. Taking advantage of these characters, GO membranes intercalated by super-hydrophilic metal-organic frameworks (MOFs) as strengthening separation fillers were prepared on modified polyacrylonitrile (PAN) support by a novel pressure-assisted self-assembly (PASA) filtration technique instead of traditional vacuum filtration method for the first time. The synthesized MOF@GO membranes were characterized with several spectroscopic techniques

ACS Paragon Plus Environment

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including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) as well as scanning electron microscope (SEM). Compared with GO membrane, these MOF@GO membranes combine the unique properties of MOF and GO and thus have significant enhancements of pervaporation (PV) permeation flux and separation factor simultaneously for ethyl acetate/water mixtures (98/2, w/w) through PV process, which are also superior to the reported other kinds of membranes. Especially, for [email protected] membrane (corresponding MOF loading: 23.08 wt. %), the increments are 159 % and 244 % respectively at 303 K, and the permeate water content can reach as high as 99.5 wt % (corresponding separation factor: 9751) with a high permeation flux of 2423 g·m-2·h-1. Moreover, the procedures of both the synthesis of MOF and membranes preparation are environmentally friendly that only water was used as solvent. Such naonosized MOF-intercalating approach may be also extended to other laminated membranes, providing valuable insights in designing and developing of advanced membranes for effective separation of aqueous organic solution through nanostructure manipulating of the nanomaterials.

KEYWORDS: Dehydration, Graphene Oxide, Membrane Separations, Metal-Organic Frameworks, Pervaporation


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INTRODUCTION Dehydration of organic solvents is one of essential chemical separation processes from both scientific and practical points of view all the time.1 Conventional separation processes like distillation (azeotropic distillation and extractive distillation) and adsorption by molecular sieves are always considered to be energy intensive, operation complex, and high safety requirements. 2,3

Pervaporation (PV), a liquid separation technology using membrane as a barrier material, has

attracted tremendous attention recently because of its many advantages including low cost, high efficiency, energy saving, safe operation, and eco-friendliness.4-10 This process is especially suitable for the separation of low concentration of organic solvent or water system, and azeotropic mixtures. 11, 12 Generally, a solution-diffusion mechanism could be used to understand the PV process.

13

There are three consecutive steps: (a) sorption of liquid component from the

mixed solution to the surface of membrane, (b) diffusion of the component across the membrane, and (c) desorption of vapor component from the membrane surface to the vacuum phase.14 The synergic results of these three steps are responsible for the PV separation performance. Therefore, the successful development of advanced membrane materials is the key to realize excellent PV separation performance. There are numerous studies on the application of inorganic, polymeric, and hybrid membranes for PV process so far.4,

15-18

Despite the good PV dehydration separation performance, these

membranes still suffer from several problems. For example, polymeric membranes have the trade-off restriction between flux and separation factor, high swelling after long-term operation and weak mechanical strengths, thus resulting in a significant decrease in separation performance.19 Inorganic membranes, such as zeolites

20

, carbon molecular sieves,

21-23

and

metal-organic frameworks (MOFs) 24, 25 are difficult to prepare and reproduce, tend to form non-


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selective cracks, and even are very expensive. In contrast, graphene oxide (GO) have recently received great attention as a functionalized graphene derivative.

26

The large-scale and facile

producing capability in solution combining the abundant polar functional groups including hydroxyl, epoxide, and carboxyl groups endows such material with promising applications in membrane science.

27-29

Assembled by single-atom thick graphene oxide (GO) nanosheets, GO

membranes exhibit extraordinary characteristics, like excellent mechanical strength and flexibility, and are considered as a promising candidate especially in liquid mixtures separation process. For instance, Geim and co-workers

30

found that GO membranes with the thickness of

submicrometer scale were hardly permeable to gases, liquids and vapors, whereas allowed the unimpeded permeation of water due to a low-friction flow through two dimensional capillaries constructed from densely spaced graphene sheets. Joshi and co-workers

31

prepared micrometer-

thick GO membranes for precise and ultrafast molecular sieving separation for ions in solution. Jin and co-workers reported GO membranes and CS@GO membranes with selective water permeation from a water-dimethyl carbonate mixture and water-butanol mixture through a pervaporation process. 13, 32 Membranes with high permeation flux are very important for organics dehydration and water purification processes. The process economics relies heavily on the performance of high flux, separation factor, and long-term stability of the membranes. Although these breakthroughs have previously achieved, the permeation flux of GO-based membrane is still needed to be further improved due to the narrow interlayer spacing. In addition, effective molecular transport nanochannels within well-defined GO membranes are also desired to achieve higher separation factor at the same time. One promising strategy is to intercalate nanomaterials between GO nanosheets to enrich their transport channels.29, 31 Several kinds of materials like nanostrands,


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carbon nanotubes, and carbon dots have been intercalated into GO/rGO sheets for the nanofiltration of macromolecules and dyes.33-36 However, the enhancement of separation performance in these membranes is not evident and even worse, inducing high demand of seeking other appropriate embedded materials. As a new kind of inorganic-organic nanoporous crystalline materials, MOFs have attracted extensive interests during the past decades because of the highly adjustable and designable pore structures and chemical functionalities.37-43 Compared to other traditional nanomaterials, they possess excellent separation performance for both gas molecules and large adsorbates44, which may provide a great opportunity to be an ideal “regulator” to intensify the separation performance of GO membranes. Thus, it is expected that high-performance GO based hybrid membranes could be formed by intercalating MOFs as strengthening fillers into the GO laminates. Unfortunately, the related study is still not reported so far, to the best of our knowledge; though a small amount of GO nanosheets has been used to facilitate the preparation of MOF membranes. 45, 46 Herein, to meet the aforementioned problems and challenges, we aim to choose superhydrophilic MOF UiO-66(Zr)-(COOH)2 to intercalate into GO laminates to prepare novel hybrid membranes (denoted as MOF@GO membranes) for the dehydration separation of ethyl acetate (EA) aqueous solution (2 wt. % water content) through a PV process for the first time. EA is a widely used chemical solvent and raw material for the preparation of drugs, plasticizers, and perfumes, etc. To well control the micro-structure of the membranes and the resulting separation performance, a novel pressure-assisted self-assembly (PASA) filtration technique instead of traditional vacuum filtration method was applied to prepare the MOF@GO membranes.

47

A

schematic of preparation procedure is illustrated in Scheme 1. As expected, the as-prepared membranes exhibit significant enhancement of PV separation performance in comparison to the


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pristine GO membrane. It is worth noting that only water was used in the procedures of both the synthesis of MOF and membranes preparation, which is environmentally friendly. Therefore, this work may provide valuable insights in the design and development of advanced membranes aiming to the effective separation of aqueous organic solution through nanostructure manipulating of the nanomaterials. EXPERIMENTAL SECTION Materials. Zirconium tetrachloride (ZrCl4, 98 %) and 1, 2, 4, 5-benzenetetracarboxylic acid (H4BTEC, 98%) were bought from J&K Scientific Co. Ltd and Beijing HWRK Chem. Co. Ltd, respectively. Single-layered GO powder obtained using the modified Hummer’s method was purchased from Nanjing JCNANO Tech Co., Ltd, China. The polyacrylonitrile (PAN) ultrafiltration membranes (the molecular weight cut off is 400, 000) were supported from AMFOR INC, China. Ethyl acetate (EA, > 99.5%) was provided by Sinopharm Chemical Reagent Co., Ltd, China. Milli-Q deionized water (18.1 MΩ·cm at 298 K) was used in this study. All chemical reagents were used without further purification, which are commercially available. Synthesis of MOF nanocrystals. UiO-66(Zr)-(COOH)2 was prepared according to our previous reported literature.48 Briefly, reflux condenser and magnetic stirrer were equipped in a round bottom flask. 2.3 g (10 mmol) of ZrCl4 and 4.3 g (17 mmol) of H4BTEC were dissolved in water (50 ml, 2.778 mol) with stirring at room temperature, followed by being heated under reflux (~373 K) for one day. The obtained white gel was centrifuged and washed using abundant deionized water for the sake of removing free acid as much as possible. The preliminary solid was dispersed in about 50 ml of deionized water and underwent another reflux process for 16 hours. The MOF gel was again centrifuged and washed using deionized water at least four times.


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Finally the wet solids were dispersed again in the deionized water for further MOF@GO membranes preparation. Preparation of MOF@GO membranes. A pressure-assisted self-assembly (PASA) filtration technique was used to prepare the MOF@GO membranes according reported literature

47

with

slight modification. The PAN supports were pre-treated as described in the Section 3 in the Supporting Information in detail. Single layered GO powder was dispersed into deionized water and underwent a sonication process of 30 min to form a concentration of 0.1 mg/ml GO dispersion liquid. A certain volume (0.2-1.0 ml) of UiO-66(Zr)-(COOH)2 dispersion liquid (0.5 mg/ml) and 10 mL of GO dispersion (0.1 mg/ml) were mixed in 300 ml of deionized water by sonicating for 0.5 h. The pre-treated PAN support and dispersion liquid were installed in the membrane-preparation device (Figure S1 in the Supporting Information). PASA filtration processes were operated at a constant pressure difference (∆P = 2.0 bar). The membranes were dried in a vacuum oven at 313 K for one day after being dried overnight at room temperature. Various weights loaded MOF@GO membranes were prepared using the same procedure. Corresponding composite membranes designated as [email protected] representing 0.3 mg of MOF was loaded into 1.0 mg of GO sheets (corresponding MOF loading: 23.08 wt. %). The GO membranes by PASA filtration method were prepared using the same procedure except for the absence of MOF dispersion liquid.

Scheme 1 Schematic diagram of MOF@GO membranes prepared by PASA filtration technique.


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Characterizations. X-ray diffraction (XRD) testes of dry UiO-66(Zr)-(COOH)2, GO membrane and MOF@GO membranes were conducted on an X-ray diffractometer (Bruker D8 ADVANCE) in reflection mode with Cu Kα radiation (λ = 1.5406 Å) at 2θ range of 5° to 50° at 298 K. N2 adsorption-desorption isotherm at 77 K of UiO-66(Zr)-(COOH)2 powders was measured on Autosorb-iQ-MP (Quantachrome Instruments) with BET surface area analysis. The sample was degassed at 423 K overnight before measurement. Fourier transform infrared spectroscopy (FTIR) characterization of MOF and membranes were carried out on a FTIR spectrophotometer (Nicolet 6700, Thermo Fisher). Transmission electron microscope (TEM) was used to observe the morphologies and particles size of MOF nanocrystals and GO nanosheets by Tecnai G2 20 S-TWIN (FEI). The morphologies of the membranes were studied by scanning electron microscopy (SEM, JEM-7800F). The membrane samples were fractured in liquid N2 in order to obtain the sharp and clear cross-sectional SEM images. To increase the conductivity, all of the samples except that for the Energy Dispersive X-Ray Spectroscopy (EDXS) mapping were mounted by conductive tape and coated by Pt layer with a thickness of 1.5–2.0 nm. The measurements were performed under 2-10 kV acceleration voltage. Crosssectional SEM image with Zr elemental EDXS mapping was operated on Bruker XFlash 6160 and the samples were not coated with Pt layer. The static water contact angles of UiO-66(Zr)(COOH)2 nanocrystals and membranes were analyzed at 298 K using a contact angle Dropmeter (OCA20, Dataphysics Instrument, Germany). The result for each sample was calculated by averaging at least five values obtained from different areas of the sample. The elemental information of the membranes surface was characterized by X-ray photoelectron spectroscopy (XPS) using ESCALAB 250 (Thermo VG).


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Figure 1 Water contact angel of UiO-66(Zr)-(COOH)2 (a), TEM image of MOF nanocrystals (b) and GO sheets (c) and N2 adsorption-desorption isotherm of UiO-66(Zr)-(COOH)2 at 77 K with BET surface area (d). Pervaporation experiment. Pervaporation separation experiments were conducted using a pervaporation device assembled by us, as described in our previous literature. 49 In the permeate side, a porous sintered stainless steel was used to support the tested PV membranes. The PV cell has an effective area of 7.0 cm2. To ensure the weight concentration of feed aqueous organic solution is constant during the PV measurement, the feed liquid mixture (EA/H2O = 98/2, w/w) with a volume of 2.5 L in the feed tank was cycled in the membrane cell. Water bath was also cycled in the outer cavity, which was used to maintain the stability of PV experiment temperature. The permeate was collected in a condenser pipe by condensing vapor in a liquid N2 cold trap. The vapor pressure at the permeate side was maintained below 0.5 kPa (monitored by a digital vacuum gauge) using a vacuum pump. Each measurement was stabilized for at least 2 h and then conducted for a certain time (h). The value of flux can be obtained by weighing the condensing permeate vapor in the condenser pipe after being warmed to room temperature. To


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confirm the reproducibility of PV separation data, more than three identical membranes were prepared and used to test for each operational condition. The water content of the collected permeate was analyzed by a gas chromatography-mass spectrometer (GC-MS, ISQ Trace1300, Thermo Fisher). Total flux and separation factor can be obtained by Eq. (1) and Eq. (2):

J=

m A⋅t

(1)

and

α=

y（1 - y） x（1 - x）

(2)

where J and α are the total flux (g·m-2·h-1) and the separation factor of EA-water aqueous organic solution, respectively; m is the weight of the gathered permeate sample (g); A is the effective area of the membrane (m2); t is the collecting sample time (h); x (y) is the weight content of water in the feed (permeate) (wt. %). To ensure the separation performance was not mainly affected by the support, control PV experiment of pure modified PAN support was also conducted. The results indicated that the PAN support has nearly no separation ability with very high flux (> 10000 g·m-2·h-1). RESULTS AND DISCUSSION Design and Preparation of MOF@GO membranes. Nanosized UiO-66(Zr)-(COOH)2 developed by our group 48 was selected mainly due to the super-hydrophilicity and the small nanoparticle size. The super-hydrophilicity can be observed by the contact angle dropmeter (Figure 1(a)). Meanwhile, the nanoparticle size of the used MOF was about 20-30 nm as illustrated by the TEM graph in Figure 1(b). On the one hand, the super-hydrophilicity can make


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the membranes adsorb more water molecules on the membrane surface to impede other organic molecules. On the other hand, small particles size guarantees the well dispersion of MOF nanoparticles in solution with few non-selective voids occurred between MOF and GO sheets. It should be mentioned that UiO-66(Zr)-(COOH)2 should be re-dispersed into water without being dried in the preparation process after the synthesis in water under refluxing condition, in order to avoid aggregation occurred during the drying process. The determination of MOF dispersion concentration is described in detail in the Supporting Information (Section 1). A UiO-66(Zr)(COOH)2 dispersion liquid of 0.5 mg/ml was used by diluting the denser dispersion liquid in the preparation process. GO dispersion liquid with sheets lateral size of about 2.0 µm as shown in Figure 1(c) were prepared by exfoliating the single-layer GO powder in deionized water through ultra-sonication, forming dispersion liquid with concentration of 0.1 mg/ml. It is believed that the membranes preparation methods may largely affect the micro-structure of the as-prepared membranes as well as the separation performance. 47, 50 In this work, we used PASA filtration technique, a novel developed preparation method, instead of traditional vacuum filtration self-assembly. PASA is a filtration process with constant-pressure, while vacuum filtration self-assembly is a variable-pressure process as demonstrated in Hung’s work. 47 The constant-pressure process may lead to the denser and tighter membranes. To balance the preparation time and membranes quality, PASA filtration was operated at constant pressure difference ∆P = 2.0 bar in this work. In view of the uniformity and stability of the prepared membranes, the optimum volume of GO dispersion liquid (0.1 mg/ml) is selected as 10 ml and the mixtures of MOF and GO dispersion liquid are diluted in 300 mL of deionized water for the PASA preparation process.


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Characterizations. XRD patterns in simulation and experiment confirm the successful synthesis of UiO-66(Zr)-(COOH)2 (Figure 2). The N2 adsorption-desorption isotherm and the calculated BET surface area (517.9 m2/g) demonstrated the porosity of the used MOF nanocrystals (Figure 1 (d)). These are all in good consistent with the results reported in literature.48

Support

Intensity

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GO MOF@GO 2 θ =10.8° d = 0.82 nm

UiO-66-(COOH)2 Exp. UiO-66-(COOH)2 Sim.

10

20

30

40

50

2 θ (°) Figure 2 XRD patterns of PAN support, GO membrane, MOF@GO membrane and UiO-66(Zr)(COOH)2. The XRD characterizations of the prepared membranes were conducted and the results are shown in Figure 2. As is known, the d-spacing of layered materials could be calculated according to the position (angle) of diffraction peak. It is observed that there is no peak position shift, demonstrating both the d-spacing of GO and MOF@GO membrane are about 0.82 nm, which is different from the intercalation of some other nanomaterials with a slight change of d-spacing.33 This may be resulted from the usage of PASA technology which is a constant-pressure filtration method. The compact structure of GO nanosheets was still remained after the intercalation of


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MOFs. Thus, the d-spacing of MOF@GO membrane changes little compared with that of GO membrane (0.82 nm). Besides, the characteristic peaks of UiO-66(Zr)-(COOH)2 also existed in XRD pattern of MOF@GO membrane in Figure 2, indicating the presence of MOF in the hybrid membranes. The FTIR spectrograms of PAN support, MOF nanocrystals, GO membrane and MOF@GO membrane are shown in Figure 3. For the spectrogram of MOF@GO membrane, two obvious peaks around 1407 cm-1 and 1583 cm-1 were observed, corresponding to those in the spectrum of UiO-66(Zr)-(COOH)2. From XPS results in Figure 4, Zr component peaks at about 181 ev and 179 ev which corresponding to 3d3/2 and 3d5/2 emerged after the intercalation of MOF into GO sheets, indicating the presence of MOF in the surface within 10 nm (the investigation depth is less than 10 nm). These results further confirmed that MOF nanocrystals were successfully intercalated into the GO sheets. 51

UIO-66(Zr)-(COOH)2

[email protected]

GO

Support

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Figure 3 FTIR of UiO-66(Zr)-(COOH)2, GO membrane, MOF@GO membrane and bare PAN support.


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Counts

GO [email protected]

192 189 186 183 180 177 174 Binding Energy (ev)

Counts

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Zr

1400

1200

1000

800

600

400

200

0

Binding Energy (ev) Figure 4 XPS of GO membrane and MOF@GO membrane (the insert is the magnification area of Zr 3d3/2 and 3d5/2 peaks). The morphologies of the PAN support and MOF@GO composite membranes on PAN support were characterized by SEM, as depicted in Figure 5. The nanopores of PAN support observed in Figure 5 (a) were uniformly distributed at the upper surface. The surface morphologies of GO and MOF@GO membranes in Figure 5(b) and 5(c) demonstrate little non-seletive voids under this magnification of SEM observation. The densely stacked MOF@GO membrane exhibits a good affinity with the modified support (Figure 5 (d)). The laminates were not easily removed from the PAN substrate, which is probably due to the strong hydrogen bonds between -COOH in modified PAN support and the functional groups in GO like carboxyl and hydroxyls.

52, 53

Meanwhile, the thickness of the formed membrane is about 1 µm as observed in Figure 5(d). The cross-sectional SEM image with Zr elemental EDXS mapping of [email protected] membrane given in Figure S3 (Supporting Information) investigated the distribution of UiO-66-(COOH)2 nanoparticles, which also confirming the existence of MOF particles below the GO surface. The


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hydrophilicity of membrane surface is vital to the separation performance. As shown in Figure 6(a) and 6(b), the hydrophilicity was increased after the introduction of super-hydrophilic UiO66(Zr)-(COOH)2, revealed by the decrease of water contact angles of GO and MOF@GO membrane surface from 63.8° to 46.8°.

Figure 5 Top view SEM images of PAN support (a), GO membrane (b), [email protected] membrane (c) and cross-sectional SEM image of [email protected] membrane (d).

Figure 6 Water contact angles of GO membrane (a) and [email protected] membrane (b) surface.


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Dehydration Performance. The PV performances in MOF@GO membranes with different MOF loadings (wt. %) at 303 K are shown in Figure 7. Obviously, the total flux increases with increasing MOF weight loadings, from 937 g·m-2·h-1 for pristine GO membrane to 3302 g·m-2·h-1 for [email protected] membrane (corresponding MOF loading: 33.33 wt. %). This is attributed to the increased preferential diffusion paths for water molecules that offered by porous MOF. Moreover, a great enhancement of separation factor and water content in the permeate of MOF@GO membranes is obviously achieved compared to the GO membrane with the help of such super-hydrophilic MOF until a loading weight of 0.3 mg. The water content in the permeate side increases from 98.3 wt. % to 99.5 wt. %, corresponding to the increase of separation factor from 2833 to 9751 (increased by 244 %) at 303 K. In addition, the largest average relative error in the measurements on the same sample is less than 6 %, indicating the good reproducibility. In fact, the hydrophilicity of MOF@GO membranes also increases with increasing the MOF loading, which can be confirmed by the decrease of water contact angle in Figure 6 (a) and (b). The increased hydrophilicity of MOF@GO membranes surface could also contribute to the enhancement of PV separation performance. However, the separation factor decreases when the MOF loading weight is above 0.3 mg. This may be due to the formation of large UiO-66(Zr)(COOH)2 aggregates resulting from excess small sized nanoparticles as well as the corresponding non-selective voids between MOF and GO sheets (Figure S4, Supporting Information), which become low selective transport paths. Molecules transport paths through GO membranes are considered to be mainly dependent on the d-spacing between GO sheets and structural defects within GO flakes.13, 54 The d-spacing between GO sheets in the GO membrane is normally located in the range from 0.6 nm to 1.0 nm with different preparation methods. 50, 55 Different from the previous work that the d-spacing


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increased with the intercalation of nanomaterials in GO membrane 33, the d-spacing (0.82 nm) of MOF@GO membrane is almost the same as that of GO in this work (Figure 2), indicating the micro-structure of MOF@GO membrane is still dense and compact. Corresponding to the solution-diffusion mechanism that PV process is strongly determined by the sorption capability of the membrane surface and the diffusivity across the membrane, the intercalation of porous super-hydrophilic MOF could strengthen both of solution and diffusion aspects simultaneously. In detail, the MOF@GO membrane surface shows preferential sorption of water, inducing that the water molecules are prone to aggregate on the membrane surface and block EA molecules. From another aspect, the optimized water diffusion transport paths by adding porous MOF make the diffusion of water molecules in the interlayer d-spacing and/or structural defects is much faster than that of EA molecules due to the smaller kinetic diameter of water and its low-friction flow. Thanks to these synergistic effects, excellent separation performance was achieved.

(a)

5000

100

Flux Water content in permeate

4000

99

[email protected]

3000

2

Flux (g/m h)

[email protected]

98

[email protected] [email protected]

2000

97 [email protected]

1000

GO

96

0

Water content in permeate (wt.%)

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


95

0

28.57 33.33 9.09 16.67 23.08 UiO-66(Zr)-(COOH)2 loading(wt.%)


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(b)10000 Separation factor 8000

Separation factor

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

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6000

4000

2000

0

0

9.09 16.67 23.08 28.57 33.33 UiO-66(Zr)-(COOH)2 loading(wt.%)

Figure 7 PV dehydration (a) flux, water content in permeate and (b) separation factor of MOF@GO membranes with different UiO-66(Zr)-(COOH)2 weight loadings (wt.%) at 303 K. Error bars correspond to the standard deviation. Effect of PV temperature. In PV separation process, the operational conditions like temperature have a significant effect. Therefore, PV experiments of GO and [email protected] membrane at different temperatures were further performed, ranging from 303 K to 343 K with an interval of 10 K. Figure 8 (a) and (b) show that the total flux increases and the separation factor decreases with increasing temperature within the studied PV operational temperature range. However, the water content in permeate side still can reach as high as 99.1 wt. % at 343 K with a high permeation flux of 4165 g·m-2·h-1. The diffusion of both water and EA across the membrane can be accelerated at higher operational temperature. According to the previous work, the difference in the partial vapor pressures of the feed and the permeate side is considered as the driving force for PV process.53 With increasing the temperature on the feed side, the driving force increases resulting from the exponentially increased vapor pressure of water here, while the


18

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water vapor pressure is almost constant on the permeate side. Thus, the water flux is evidently increased. In comparison, the permeation flux of EA increases much more than that of water as PV temperature increases since EA permeation flux is more sensitive to the variation of PV temperature, inducing the water content decreasing in permeate side and the corresponding decrease of separation factor. Although the diffusivity could be enhanced by the increase of PV operational temperature, the sorption capacity would be weakened, which would result in a decrease of the separation factor. Therefore, the separation factor is a little lower at higher PV operational temperature.

(a) 5000 Flux (GO) Flux ([email protected]) Water content (GO) Water content ([email protected])

99

3000

98

2000

97

1000

96

0

95

2

Flux (g/m h)

4000

100

303

323 313 333 Temperature (K)

Water content in permeate (wt.%)

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


343


19


(b) 10000

Separation factor

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

Page 20 of 32

GO [email protected]

8000

6000

4000

2000

0

303

313

323

333

343

Temperature (K) Figure 8 The effect of PV temperature on (a) flux, water content and (b) separation factor of GO and MOF@GO membranes. Arrhenius equation can be used to describe the influence of the operational temperature on flux on basis of the solution-diffusion mechanism.13,56 Figure S6 in the Supporting Information shows the Arrhenius plots of lnJ versus PV operational temperature in GO and [email protected] membrane. The results reveal that there is a linear relationship between flux and the reciprocal of absolute PV operational temperature. The Arrhenius activation energy (Ea) of feed liquid through the membranes can be obtained from the slope of the Arrhenius plots, as listed in Table S1. We can find that Ea of [email protected] membrane (11.56 kJ/mol) shows a 10.9 % decrease in energy barrier in comparison to that of GO membrane (12.97 kJ/mol), demonstrating the permeation of feed mixture across the MOF@GO membrane is of less resistance than that of GO membrane. The relatively low Ea may be resulted from the optimized micro-structure of the MOF@GO membranes. Water would diffuse through the passages formed by the comprehensive effect of


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porous MOF and GO sheets easier at higher temperature. As a result, the increased flux is obtained as PV operational temperature increased. The long-term stability is of great importance for the practical membrane separation process. Thus, the corresponding PV experiments were conducted and the result of [email protected] membrane is shown in Figure S7 (Supporting Information) as an example. It can be seen that after a few hours for stabilization, the flux and water content in permeate side remain almost unchanged during the test period as long as 120 h at 303 K, indicating the good operational stability of the obtained MOF@GO membranes. Comparison with other membranes for EA dehydration. The PV separation performances obtained in this work were also compared with those in other materials. As shown in Table 1, the MOF@GO membranes show a superior PV separation performance than other kinds of membranes for the EA-water (98/2, w/w) mixture at different temperatures. The permeation flux is higher than that in most of the other polymeric membranes, due to the fact that the polymeric membranes are usually several micrometers in thickness, which are much thicker than the MOF@GO membranes (~ 1µm). Although PVA-CS membrane has quite high separation factor (> 10000), the permeation flux is very low and the value in [email protected] membrane is about 15 times higher than that of it at 323 K. The microporous nature of MOFs was different from the polymer matrix and the competitive separation performance of MOF@GO membranes in this work was responsible for the hydrophilicity of strengthening fillers and the usage of the PASA technology, where the microporous nature facilitates the overall permeation of water molecules that is adsorbed preferentially within the selective MOF.


21


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Table 1 Comparison with other membranes for EA dehydration.

Membrane

Temperature (K)

Flux (g·m-2·h-1)

Separation factor

Ref.

PVA/PAN

313

34.5

7270

57

PVA

323

22

5000

58

PVA−CS

323

200

>10000

59

CS/PAN

313

336

6270

60

Perfluorosulfonic acid - TEOS/PAN

313

205

496

61

PBI/PEI dual-layer

333

820

2478

62

GO

303

937

2833

This work

[email protected]

303

2423

9751

This work

[email protected]

313

2806

8118

This work

[email protected]

323

3233

6951

This work

[email protected]

333

3632

6076

This work

CONCLUSIONS In conclusion, novel composite membranes based on MOF nanoparticles and GO nanosheets (MOF@GO membranes) were successfully prepared by PASA filtration technique for the first time. Further characterizations such as XRD, FTIR, XPS and SEM analysis confirm the continuous and dense of MOF@GO membranes and the successful intercalation of MOFs. These membranes exhibited excellent water permeation for EA/water mixtures (98/2, w/w) through a PV process. At 303 K, the permeate water content can reach 99.5 wt % with a high permeation flux (2423 g·m-2·h-1, a 159 % enhancement compared with pristine GO membrane) when 1.0 mg GO was loaded by 0.3 mg of wet UiO-66(Zr)-(COOH)2 (corresponding MOF loading: 23.08


22

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wt. %). The excellent PV dehydration separation performance could be ascribed to: (i) the sufficient small size of inserts can avoid the presence of non-selective voids between MOF and GO sheets and meanwhile ensure the homogeneous dispersion of nanoparticles; (ii) the increased hydrophilicity of membranes increase the preferential water sorption ability; (iii) constructed fast water diffusion transport paths between GO sheets and MOF strengthen the diffusivity; (iv) advanced PASA technique guarantees the compact and dense micro-structure of membranes. More importantly, only water was used in the procedures of both the synthesis of MOF and membranes preparation, which is environmentally friendly. Simple preparation process combing the high flux and separation factor makes such MOF@GO composite membranes be promising candidates for chemical separation applications. Such naonosized MOF-intercalating approach can be also applied to other laminated membranes, providing potential application in improving the separation performance. ASSOCIATED CONTENT Supporting Information. The determination of MOF dispersion concentration, preparation of GO dispersion liquid, pre-treatment of the PAN supports, photograph of membranes preparation device, PASA process details, relevant characterization results, activation energy (Ea) and the long-term stability supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (D. H. Liu)


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* E-mail: [email protected] (C. L. Zhong) Present Addresses State Key Laboratory of Organic-Inorganic Composites, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Basic Research Program of China (“973”) (grant number: 2013CB733503) and the National Natural Science Foundation of China (grant numbers: 21136001, 21536001 and 21576009). REFERENCES (1) Hua, D.; Shi, G.; Fang, C.; Chung, T.-S. Teflon AF2400/Ultem Composite Hollow Fiber Membranes for Alcohol Dehydration by High-temperature Vapor Permeation, AIChE J. 2016, 62, 1747-1757. (2) Salehian, P.; Yong, W. F.; Chung, T.-S. Development of High Performance Carboxylated PIM-1/P84 Blend Membranes for Pervaporation Dehydration of Isopropanol and CO2/CH4 Separation. J. Membr. Sci. 2016, 518, 110-119. (3) Liu, Z.; Wang, W.; Xie, R.; Ju, X.-J.; Chu, L.-Y. Stimuli-responsive Smart Gating Membranes. Chem. Soc. Rev. 2016, 45, 460-475 (4) Fan, H.; Shi, Q.; Yan, H.; Ji, S.; Dong, J.; Zhang, G. Simultaneous Spray Self-assembly of


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H.-K.;

Xu,

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Tetraethoxysilane/Polyacrylonitrile

Shi,

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Ma,

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(PFSA-TEOS/PAN)

Perfluorosulfonic

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Composite

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