Highly water-permeable and stable hybrid membrane with asymmetric covalent organic framework distribution

Author’s Accepted Manuscript Highly water-permeable and stable hybrid membrane with asymmetric covalent organic framework distribution Hao Yang, Hong Wu, Fusheng Pan, Zhen Li, He Ding, Guanhua Liu, Zhongyi Jiang, Peng Zhang, Xingzhong Cao, Baoyi Wang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)30775-X http://dx.doi.org/10.1016/j.memsci.2016.08.022 MEMSCI14672

To appear in: Journal of Membrane Science Received date: 24 June 2016 Revised date: 15 August 2016 Accepted date: 16 August 2016 Cite this article as: Hao Yang, Hong Wu, Fusheng Pan, Zhen Li, He Ding, Guanhua Liu, Zhongyi Jiang, Peng Zhang, Xingzhong Cao and Baoyi Wang, Highly water-permeable and stable hybrid membrane with asymmetric covalent organic framework distribution, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly water-permeable and stable hybrid membrane with asymmetric covalent organic framework distribution Hao Yanga,b, Hong Wua,b, Fusheng Pana,b, Zhen Lia,c, He Dinga,b, Guanhua Liua,b, Zhongyi Jianga,b*, Peng Zhangd, Xingzhong Caod, Baoyi Wangd a

Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072, China

c

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China. d

Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

*Corresponding author: School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P R China, Tel: 86-22-23500086. Fax: 86-22-23500086. E-mail:[email protected]

Abstract Although covalent organic frameworks (COFs) have received tremendous attention in 1

recent years due to their unique advantages, their application in membrane processes for liquid mixture separation has not been explored. In this study, COF-based hybrid membranes were fabricated by incorporating COF SNW-1 into sodium alginate (SA) matrices and used for ethanol dehydration. Due to the remarkable density difference between SNW-1 and the membrane casting solvent, majority of SNW-1 nanoparticles floated to the top of the membrane and minority of SNW-1 nanoparticles were entrapped in SA during the slow evaporation of the solvent, resulting in asymmetric distributions. The surface hydrophilicity and water sorption capacity of the hybrid membrane are enhanced by the enriched SNW-1. And the porous structure of SNW-1 could render additional selectivity and free volume cavities for water molecules transfer. As a result, the hybrid membrane shows high separation factor and permeation flux of 1293 and 2397 g/m2h, respectively. Moreover, the incorporation of SNW-1 also endows membranes with enhanced thermal and mechanical stability, good anti-swelling properties and desirable long-term operation stability. This study offers a novel approach to design high performance hybrid membranes by incorporating fillers with the density different from that of the membrane casting solvent.

Keywords: Covalent organic framework; SNW-1; Asymmetric distribution; Hybrid membrane; Ethanol dehydration

1. Introduction 2

Recently, water-selective membranes have aroused immense interest for their advantages of high energy efficiency, acceptable cost and environmental sustainability[1, 2]. They can be utilized for a great deal of membrane applications such as seawater desalination, biofuel production and organic solution dehydration[3-7]. The ideal water-selective membranes are supposed to have high water permeability, selectivity as well as stability. Currently, the applications of polymeric membrane materials are dramatically limited by the well-known tradeoff effect, which states that the permeability and selectivity cannot be simultaneously enhanced[8]. To address this issue, hybrid membranes have been employed by incorporating fillers into polymeric matrices. Among the various fillers, metal organic frameworks (MOFs) comprised of metal ions and organic ligands have desirable compatibility with polymer[9]. They also feature the merits of high sorption capacity, appropriate pore size and numerous pathways with relatively low resistance, which are favourable for the improvement of both the permeability and selectivity[10, 11]. Several kinds of MOFs like ZIF-7[12], ZIF-8[13], ZIF-71[14-16] and NH2-MIL-125(Ti)[17] have been incorporated into polymers to fabricate hybrid membranes with both enhanced permeability and selectivity in pervaporation dehydration or recovery of bio-alcohol. However, the water stability of most MOFs is considered to be a great challenge especially for the water capture applications[18, 19]. MOFs are constructed by coordination bonds between metal ions and organic ligands, forming a cage structure that can easily collapse when exposed to moisture or high temperature due to the weakened coordination bonds[20]. Therefore, there is much scope for obtaining novel membrane properties by incorporating alternative porous materials with not only tunable pore size and hydrophilicity, but high thermal and 3

water stability. Besides the types of the fillers, the distribution of fillers in polymer matrices is also of great importance to the performance of hybrid membranes. However, studies associated with filler distribution in hybrid membranes have been rarely concerned. When there is a remarkable density difference between filler and membrane casting solvent, dual-layer hybrid membranes with one filler-rich layer and one polymer-rich layer could be fabricated by slow evaporation of the solvent due to the asymmetric distributions of fillers, and the two layers can be fully connected through the polymer[21, 22]. The asymmetric distributions of the fillers endow the hybrid membranes with different properties from top to bottom, and such hybrid membranes could be expected to show superiorities in some membrane separation processes such as pervaporation dehydration process based on the analysis of solution-diffusion mechanism: water molecules are initially adsorbed on the membrane surface, then diffuse through the active layer and eventually desorb on the downstream side[23]. Thus the dual-layer hybrid membrane bearing a top hydrophilic filler-rich layer and a bottom polymer-rich layer, may be very useful in pervaporation dehydration process according to the solution-diffusion mechanism. The filler-rich layer containing high-concentration hydrophilic fillers could guarantee sufficient hydrophilicity and large water sorption capacity, in favour of enhancing the solubility selectivity. And the entrapped fillers in polymer-rich layer could improve the free volume characteristics of polymer, which is beneficial for enhancing diffusivity selectivity. To achieve such asymmetric distributions of hydrophilic fillers in hybrid membranes by utilizing alternatives to MOFs, covalent organic frameworks (COFs) could be the competitive candidate. 4

COFs, a new class of MOF-like porous materials, have spurred tremendous interest among researchers since they were first synthesized by Yaghi group in 2005[24]. Unlike MOFs, COFs are covalently linked by light elements (C, H, O, N, and B) without metal ions[25]. COFs can be used for gas adsorption[26], photoelectricity[27, 28], catalysis[29, 30] owing to their appealing properties such as highly ordered and low-density framework, high specific surface area, tunable functionality and excellent thermal stability[25, 31]. COFs with low-density feature could be expected to form asymmetric distributions in polymer bulk when used to fabricate hybrid membranes. In recent years, some kinds of COFs have been used for preparing gas separation membranes with enhanced selectivity and permeability due to their well-defined pores and the interactions with gas molecules[32-34]. There are two major types of COFs: one is boron-containing COFs, the other is triazine-based frameworks (CFTs). Compared with the boron-containing COFs, the CTFs usually feature lower crystallinity yet excellent chemical and thermal stability[25]. The water adsorption characteristics of CFTs were first studied by Hug et al, using two typical samples bipy-CTF500 and pym-CTF500[35]. The pym-CTF500 reveals a water uptake of 115 cm3 g-1 at P/P0=0.1, which is higher than that of bipy-CTF500 and any other hydrophilic porous carbon materials. And the higher nitrogen content of pym-CTF500 rendered highly hydrophilic nature. Quite recently, Biswal et al[36] synthesized a series of COFs and evaluated their water adsorption ability as well as water stability. They found that not only the hydrogen bonding interaction between COFs and water molecules, but also the pore size and surface area of COFs are decisive to the water adsorption behaviour. The typical COF TpPa-1 was found to have excellent water adsorption ability of 368 cm3 g-1 at P/P0=0.3, which is comparable with 5

much of carbon materials and MOFs. Moreover, the as-synthesized COFs show outstanding hydrolytic stability and recyclability. In short, the outstanding water adsorption ability and water stability of COFs demonstrate their promising potential for fabricating highly water-selective membranes. To date, to the best of our knowledge, such utilizations of COFs have not been exploited. Herein, SNW-1, a kind of CFTs with microporous structure and high nitrogen content, was synthesized by one-step polycondensation reaction of cheap and abundant materials[37]. And the as-synthesized SNW-1 nanoparticles were then incorporated into hydrophilic sodium alginate (SA) matrices to fabricate dual-layer hybrid membranes for pervaporation dehydration of ethanol aqueous solution. SA is a widely used pervaporation membrane material due to its good membrane forming property and desirable separation performance, and it can be readily cross-linked with Ca2+ to form a stable membrane.[38]. However, the pure SA membrane suffers from tradeoff effect between permeability and selectivity. Its hydrophilicity also needs to be further improved to achieve better dehydration performances. The influence of SNW-1 on the morphology, crystallinity, free volume characteristics, hydrophilicity, water or ethanol uptake, swelling degree, and stability of the hybrid membranes was investigated. And the effects of SNW-1 content, along with operation temperature and water concentration in feed on the membrane performance were also extensively evaluated.

2. Experimental 2.1 Materials and chemicals Melamine

and

terephthalaldehyde

were 6

purchased

from

TCI

(Shanghai)

Development Co., Ltd. Dimethyl sulfoxide (DMSO, 99.8 wt%) was purchased from Tianjin Heowns Biochemical Technology Co., Ltd. Sodium alginate (SA) was received from Bright Moon (Qingdao) Seaweed Group Co., Ltd. Polyacrylonitrile (PAN) membranes used for ultrafiltration (molecular weight cut-off is 100,000) were supplied by Shandong Megavision Membrane Engineering and Technology Co., Ltd. Ethanol (99.8

wt%),

methanol,

Calcium

chloride

dehydrate

(CaCl2·2H2O),

N,N-

dimethylformamide (DMF) and tetrahydrofuran (THF) were purchased from Guangfu (Tianjin) Technology Development Co., Ltd. All the reagents were of analytical grade and used as received without further purification. Deionized water was produced by Millipore Elix® Advantage 5 and used for the entire experiment.

2.2 Preparation of SNW-1

Highly cross-linked COF SNW-1 was synthesized by a polycondensation reaction (Scheme 1) of abundant and cheap materials and without any catalyst[37]. In detail, melamine (6.26 g), terephthalaldehyde (10 g) and DMSO (310 mL) were filled into a Schlenk flask, which was fitted with a condenser and a magnetic stirring. The mixture was heated to 180 oC for 72 h under an argon atmosphere. After the reaction, the mixture was cooled to ambient temperature and the off-white powders were isolated by filtration over a Büchner funnel. The products were successively washed with excess DMF and THF, and then sequentially purified by Soxhlet extraction with methanol, THF and methanol. The purified products were dried in a vacuum oven at 120 oC for 12 h to obtain

7

the final SNW-1.

Scheme 1. The synthesis protocol and chemical structure of SNW-1 2.3 Preparation of membranes A certain amount of SNW-1 nanoparticles were added into 25 mL of deionized water, and the suspension was then treated by sonication for 30 min to ensure the uniform dispersion of SNW-1 in water. Subsequently, 0.38 g of SA was added into the dispersion prepared above with stirring for 5 h at 30 oC, and the well-distributed casting membrane solution was obtained. The as-prepared solutions with different SNW-1 contents were then spin-coated on PAN substrates. After naturally drying at room temperature for 24 h, the hybrid membranes were then cross-linked by CaCl2 solution (0.5 M) and rinsed by substantial deionized water. After drying at room temperature for 24 h again, the hybrid membranes were successfully fabricated and designated as SA-SNW-1(X)/PAN, where X (=5, 10, 15, 20, 25, or 30) is the mass percentage of the SNW-1 to SA. The pure SA membrane was fabricated in a similar manner without incorporating SNW-1, and denoted 8

as SA/PAN. In addition, the free-standing membranes (SA-SNW-1(X)) were also prepared for the use of characterizations and performance tests.

For comparison, the hybrid membranes with homogenous SNW-1 distribution are fabricated by fast evaporation of the casting solvent at 80 oC for 2h, and the remainder procedures are as same as that of the hybrid membranes fabricated by slow evaporation of the solvent.

2.4 Characterizations

The microstructure of SNW-1 nanoparticles and hybrid membranes was observed by field emission scanning electron microscopy (FESEM) (Nanosem 430, 10 eV). The chemical compositions of SNW-1 and hybrid membranes were analyzed by Fourier transform infrared spectra (FTIR) (BRUKER Vertex 70, 4000-500 cm-1). Solid-state 13C NMR measurement (Varian InfinityPlus, 300MHz) was employed to verify the successful polycondensation of melamine and terephthalaldehyde. The elemental analysis was carried out on a VarioMICRO elemental analyzer and used to detect the elemental contents of SNW-1. The N2 sorption isotherms of SNW-1 were recorded on Quantachrome Autosorbe-1 analyzer at 77 K, and the sample was degassed at 393 K for 24 h prior to the measurement. The surface area, micropore volume and pore size distribution

of

SNW-1

were

calculated

from

N2

sorption

isotherms

using

Brunauer-Emmet-Teller (BET), t-plot and NL-DFT methods. The density of SNW-1 was

9

measured by 3H-2000TD series full-auto density analyzer. X-ray diffraction (XRD) (D/MAX-2500, CuKα) was used to investigate the crystallization properties of hybrid membranes. The hydrophilicity of the membrane surface was tested by static contact angles (POWEREACH®, JC2000D2M). The glass transition temperature (Tg) of the membranes was determined by a differential scanning calorimetry (DSC) (NETZSCH instrument, 204 F1). The thermal stability of the membranes was carried out on a thermal gravimetric analyzer (TGA) (NETZSCH instrument, 209 F3), the samples were in nitrogen atmosphere and heated from 40 oC to 600 oC at a heating rate of 10 oC min-1. The mechanical properties of the membranes were characterized by an electronic stretching machine (Zhongke, WDW-02) at a strain rate of 5 mm/min.

The free volume characteristics of the hybrid membranes were carried out by a high resolution positron annihilation lifetime spectroscopy (PALS, EG&G, USA) with a fast-slow coincidence system. Membrane samples with dimensions of 1 cm×1 cm×0.5 μm were prepared, and the positron source (22Na, 13 μci) was sandwiched in two same membrane samples. The detectors of PALS are a pair of BaF2 probes with a resolution of 210ps. Each spectrum was recorded at least 2×106 coincidences and resolved with the LT 9.0 program. The radius of the free volume cavities (r3) and free volume (fapp) of the hybrid membranes were calculated by Eqs (1) and (2)[39]:

1 2

 3  [1 

r3 2 r3 1 1  ( )sin( )] r3  r 2 r3  r

(1)

10

f app 

4 3 r3 I 3 3

(2)

where τ3,I3 and Δr are the ortho-positronium (o-Ps) pickoff lifetime, the intensity of o-Ps and the thickness of the electron layer (0.1656 nm), respectively. 2.5 Uptake and swelling degree

The dry free-standing membrane samples were tailored into square shape (around 10 cm×10 cm), and the water or ethanol uptake and swelling degree were calculated based on the weight increase after complete absorption in water or ethanol (30 oC, 48 h) and ethanol aqueous solution (90 wt%, 76 oC, 48 h), respectively. The measurements were repeated more than three times to ensure the error was less than ±5%, and the water uptake, ethanol uptake and swelling degree were calculated according to Eqs (3) and (4):

Uptake(%) 

MW  MD 100 MD

Swelling degree(%) 

(3)

MS  M D 100 MD

(4)

where MD , MW, MS are the weight of the dry membranes, wet membranes in water or ethanol, and swollen membranes in ethanol aqueous solution, respectively.

2.6 Pervaporation experiments

The pervaporation experiments were carried out on a membrane module (CM-Celfa AG Co., Switzerland) with an effective area of 37.3 cm2. The feed solution was circulated 11

at a rate of 60 L/h using a gear pump, and the down-stream was maintained below 0.3 kPa by a vacuum pump. The detailed pervaporation processes were described previously[40]. The gas chromatography (3420, Beifen-Ruili Analytical Instrument Co., Ltd.) was utilized to determine the component concentration. The permeation flux (J, g/m2h) and separation factor (α) were calculated by Eqs (5) and (6):

J

Q A t

(5)



PW / PE FW / FE

(6)

where Q is the weight of permeate (g) during an interval of time (t, h), A is the effective membrane area (m2), F and P represent the mass fractions of water (subscript W) or ethanol (subscript E) in the feed and permeate, respectively. To decouple the effects of thickness and driving forces, the permeance ((P/l), GPU, 1GPU=7.501×10-12 m3 (STP)/m2 s Pa) and selectivity (β) were calculated by Eqs (7) and (8): ( P / l )i 



Ji Ji  pi 0  pil  i 0 xi 0 pisat 0  pil

( P / l )W (P / l)E

(7)

(8)

where l is the thickness of membrane (m), Ji, is the permeation flux (g/m2h), pi0 and pil (pil is approximated to zero for the high vacuum degree) are the partial pressures of component i on the feed and permeate side (Pa), respectively. γi0 and xi0 are the activity 12

coefficient (calculated via Aspen simulation) and mole fraction of component i in the feed, respectively. pisat is the saturated vapor pressure of component i calculated through 0 Antoine equation. The permeance of water and ethanol should be calculated by transforming the permeation fluxes into the volumes under standard temperature and pressure (STP): 1 kg of water vapor at STP = 1.245106 cm3 (STP), and 1 kg of ethanol vapor at STP = 0.487106 cm3 (STP)[41]. Furthermore, in order to ensure the repeatability and reliability of data, three duplicated samples for each membrane were prepared under the same condition and method, and the pervaporation experiments for each sample were repeated three times and error bars are shown in each Figure.

3. Results and discussion

3.1 Characterization of SNW-1

The SEM image of the as-prepared SNW-1 nanoparticles is shown in Fig. 1(a), from which spherical shape is observed for SNW-1 nanoparticles and the diameter of each one is 50-70 nm. The FTIR spectrum of SNW-1 is shown in Fig. 1(b). The band at 3419 cm-1 corresponds to the stretching vibration of N-H. And the bands at 1552 cm-1 and 1473 cm-1 are ascribed to the quadrant and semicircle stretching of the triazine ring[37]. In addition, the inexistence of band at 1690 cm-1 corresponding to carbonyl group indicates the complete polycondensation of terephthalaldehyde and melamine. As shown in Fig. 1(c),

13

the 13C NMR spectrum of SNW-1 demonstrates four resonances at 167, 135, 127 and 52 ppm, which is consistent with the result in literature[37]. The resonance at 167 ppm corresponds to the carbon atoms present in the triazine ring. The resonances at 135 and 127 ppm are ascribed to the carbon atoms of benzene ring. And the resonance at 52 ppm reveals the tertiary carbon atoms, further indicating that the polycondensation reaction is completed. The C, N and H element contents were found to be 40.45%, 39.07% and 3.945%, respectively, which is consistent with the element analysis result in literature[37]. In Fig. 1(d), the N2 sorption isotherms demonstrate a steep N2 uptake at low relative pressures, followed by a flat course at the intermediate section, which reflects the microporous nature of SNW-1. The BET surface area and micropore volume of SNW-1 are 858 m2 g-1 and 0.27 cm3 g-1, respectively, which also indicates a high degree of crosslinking of monomers. The pore size distribution curve of SNW-1 is shown in the inset of Fig. 1(d), which reveals a major pore size of nearly 5 Å. The intrinsic small micropores in SNW-1 are appropriate for water molecules (2.6 Å) transfer and –NH or – NH2 groups in SNW-1 are beneficial for water recognition, thus selective diffusion of water molecules through the micropores would be expected.

14

Transmittance

(b)

3419

1552

4000

3500

3000

2500

2000

1473

1500

1000

500

-1

Wavenumber/cm

Fig. 1 (a) SEM image, (b) FTIR spectrum, (c) 13C NMR spectrum and (d) N2 sorption isotherms and pore size distribution curve (insert) of SNW-1.

3.2 Characterization of membranes

The cross-section morphologies of the pure SA and hybrid membranes were observed by FESEM and shown in Fig. 2. The SA/PAN membrane reveals a void-free active layer with a thickness of around 500 nm and the active layer tightly adheres to the PAN layer (Fig. 2(a)). With the incorporation of SNW-1, the hybrid membrane demonstrates dual-layer structures (Fig. 2(b)), in which there is a top SNW-1-rich layer and a bottom SA-rich layer, and these two layers are well connected through SA. To 15

further investigate the distributions of SNW-1 in SA matrices, the FESEM cross-section images of free-standing membranes were observed and displayed in Fig. 2(c-f). The internal microstructure of the hybrid membranes is smooth and void-free with the SNW-1 content increasing from 5 to 25 wt%, indicating good interfacial compatibility. However, the aggregation of SNW-1 can be seen in Fig. 2(f) when the SNW-1 content increases to 30 wt%. The EDX N-mapping of SA-SNW-1(25) is shown in Fig. S1(b), indicating that the N element content in the upper layer is higher than that in the bottom layer. The density of SNW-1 is determined to be 0.45 g cm-3, which is much lower than that of water solvent (1.0 g cm-3). Due to the low-density of the SNW-1 nanoparticles, majority of them could float to the membrane surface during the solvent evaporation, and minority of them are entrapped in the SA by the slow evaporation of the solvent, and thus asymmetric distributions of SNW-1 nanoparticles in membrane have been achieved. For comparison, SA-SNW-1(25) membrane dried via fast evaporation of the solvent was also fabricated, and its morphology and EDX N-mapping are shown in Fig. S1(c) and (d). It can be seen that the membrane fabricated by fast evaporation of the solvent shows homogeneous SNW-1 distributions, and therefore the SNW-1 distributions in the membrane could be adjusted by altering the rate of solvent evaporation. The microstructure of the membrane surface was observed by FESEM and shown in Fig. 3(a-d). The SA/PAN shows a dense and smooth surface, and the SNW-1-rich layer of the hybrid membrane can also be observed from Fig. 3(b-d). When the SNW-1 content is 25 wt%, the SNW-1-rich layer almost overspreads the membrane surface as shown in Fig. 16

3(c). And when the SNW-1 content is up to 30 wt%, the excessive aggregation of SNW-1 nanoparticles results in their uneven distribution on membrane surface (Fig. 3(d)).

Fig. 2 FESEM cross-section images of (a) SA/PAN, (b) SA-SNW-1(25)/PAN, (c) SA, (d) SA-SNW-1(15), (e) SA-SNW-1(25) and (f) SA-SNW-1(30).

17

Fig. 3 FESEM images of the surface of (a) SA/PAN, (b) SA-SNW-1(15)/PAN, (c) SA-SNW-1(25)/PAN and (d) SA-SNW-1(30)/PAN.

The FTIR spectra of the surface of the hybrid membranes are shown in Fig. 4. The pure SA membrane demonstrates three characteristic bands at 3352, 1605 and 1429 cm -1, which correspond to the stretching vibration of hydroxyl group, symmetric and asymmetric stretching vibration of carboxylate group, respectively[5]. With the increase in SNW-1 contents, the intensity of the three characteristic bands for SA gradually declines. The change of peak intensity indicates the existence of strong interfacial interaction, mainly attributed to the electrostatic interaction and intermolecular hydrogen bonds between the –NH or –NH2 groups of SNW-1 and the –COOH or –OH groups of SA[42]. Additionally, when the SNW-1 content is more than 25 wt%, the characteristic bands for triazine ring (1552 and 1773 cm-1) gradually appear and the bands for –OH group almost disappear, indicating the high concentration of SNW-1 on the membrane surface.

18

1473

1552 SA-SNW-1(30)/PAN

Transmittance

SA-SNW-1(25)/PAN SA-SNW-1(15)/PAN SA-SNW-1(5)/PAN SA/PAN

3352 1605 4000

3500

3000

2500

2000

1429 1500

1000

-1

Wavenumber (cm )

Fig. 4 FTIR spectra of SA/PAN and SA-SNW-1(X)/PAN membranes. The crystalline structures of the SNW-1 and hybrid membranes were evaluated by XRD and shown in Fig. S2. The pure SA membrane reveals two characteristic peaks, one is a sharp peak at 2θ=22o corresponding to the crystalline region, and the other is a broad peak at 2θ=15o belonging to amorphous region[5]. With the SNW-1 content increasing from 5 to 30 wt%, the influence of SNW-1 on the crystallinity of SA is not obvious. The DSC results of the hybrid membranes are shown in Fig. S3, and the change trend of the Tg for all hybrid membranes is also not obvious. The XRD and DSC results indicate that the incorporation of SNW-1 has almost no influence on the main structure of SA, which can be attributed to the good compatibility between SA and SNW-1.

The representative results of the free volume characteristics of the membranes are listed in Table 1. The SA-SNW-1(25) reveals a maximum fapp value of 0.674, increasing by 23% compared with that for pure SA membrane. The increase in the fractional free volume for the hybrid membranes could be explained by the corporation of SNW-1. Due 19

to the microporous nature, the SNW-1 possesses a large number of free volume cavities and thus the I3 values are increased. On the other hand, there is little change of r3 due to the good compatibility between SA and SNW-1 and the appropriate pore size of SNW-1. Therefore, the calculated fractional free volume for hybrid membranes is higher than that for pure SA. It is worth pointing out that the increased fractional free volume is expected to achieve high permeation flux. Table 1. Free volume characteristics of the membranes Membrane SA SA-SNW-1 (15) SA-SNW-1 (25) SA-SNW-1 (30)

I3 (%) 8.38 9.11 9.72 9.18

τ3 (ns) 1.646 1.668 1.692 1.733

r3 (nm) 0.250 0.252 0.255 0.259

fapp 0.548 0.613 0.674 0.668

3.3 Thermal and mechanical stability of the membranes

The thermal stability of SNW-1 and the membranes is determined by TGA as shown in Fig. 5. The TGA curve of SA reveals a typically three-stage weight loss: the first stage from 40 to 208 oC is attributed to the evaporation of bound water, the second stage between 200 to 393 oC corresponds to thermal decomposition of carboxyl and hydroxyl groups, and the third stage over 393 oC is due to the degradation of backbone of SA[39]. Compared with SA, the SNW-1 shows lower weight loss when the temperature is below 500 oC. The SNW-1 is endowed with highly thermal stability because it is formed by covalent bonds with high bond energy, and its thermal stability surpasses that of most MOFs[25, 43-46]. Therefore, with the incorporation of SNW-1, the hybrid membranes 20

are more thermally stable than pure SA membrane. The TGA results also reveal that the hybrid membranes can meet the requirement of the practical pervaporation operation (generally less than 80 oC).

100 SNW-1 SA SA-SNW-1(25) SA-SNW-1(30)

90

Weight (%)

80 70 60 50 40 30 20 100

200

300

400

500

600

o

Temperature ( C)

Fig. 5 TGA curves of SA and SA-SNW-1(X) membranes. The mechanical properties of the membranes are shown in Fig. 6. With the SNW-1 content increasing from 5 to 25 wt%, both the tensile strength and tensile modulus are enhanced compared with the pure SA membrane. The SA-SNW-1(25) demonstrates a maximum tensile strength of 106 MPa (increased by 34.2%) and a maximum tensile modulus of 4543 MPa (increased by 111.5%). The simultaneous increase in tensile strength and tensile modulus for hybrid membranes are mainly ascribed to the strong interfacial interactions between the SNW-1 nanoparticles and SA chains[47]. When the SNW-1 content increases to 30 wt%, the mechanical property of SA-SNW-1(30) starts going down, which could be caused by the aggregation of SNW-1[48].

21

120

5000 Tensile Strength Tensile Modulus

4000

80 3000 60 2000 40 1000

20 0

Tensile modulus (MPa)

Tensile strength (MPa)

100

0 0

5

10

15

20

25

30

SNW-1 content (wt%)

Fig. 6 Mechanical properties of SA and SA-SNW-1(X) membranes. 3.4 Hydrophilicity and swelling properties of the membranes

To identify the affinity for water, the water contact angle of SA/PAN and SA-SNW-1(X)/PAN were measured. As shown in Fig. 7, the SA/PAN shows a water contact angle of 38.9o. With the incorporation of SNW-1, the water contact angle of the hybrid membranes significantly decreases and can be as low as 9.2 o. The low water contact angle could be attributed to the formed SNW-1-rich layer of the hybrid membrane. The abundant –NH and –NH2 groups of SNW-1 could provide ultralow hydration energy for water molecules and rapidly combine with water molecules by hydrogen bonds[49]. Therefore, the hydrophilicity of the surface of the hybrid membranes is significantly enhanced.

22

Fig. 7 The contact angle of SA/PAN and SA-SNW-1(X)/PAN membranes. The swelling degree of the membranes is shown in Fig. 8. All swelling degree values of the membranes in feed solution are lower than 6%, demonstrating excellent anti-swelling properties. When incorporating SNW-1 of 5 wt%, the swelling degree of the membrane decreases from 2.2% to 1.0%, which can be attributed to interactions between the SNW-1 and SA chains[48]. However, when the SNW-1 content increases from 10 to 25 wt%, the swelling degree gradually increases. On account of the high specific surface area of SNW-1, the adsorption capacity of the hybrid membranes is enhanced. Moreover, the decline of swelling degree for SA-SNW-1(30) is attributed to the aggregation of SNW-1 nanoparticles.

23

Swelling degree (%)

7 6 5 4 3 2 1 0 0

5

10

15

20

25

30

Filler content (wt%)

Fig. 8 The swelling degree of SA and SA-SNW-1(X) membranes. To identify the difference between water and ethanol adsorption ability of the membranes, the water uptake and ethanol uptake were tested and shown in Fig. 9. The water uptake initially increases and then decreases with the SNW-1 content increasing from 5 to 30 wt%, and this trend is consistent with that of the swelling degree. The SA-SNW-1(25) shows a maximum water uptake of 72.8%, which is increased by 32.8% compared with that for the pure SA membrane. However, the ethanol uptake for the hybrid membrane is considerably lower than water uptake. Due to the hydrogen bonds between SNW-1 and water molecules, the hybrid membranes are endowed with improved affinity toward water molecules while repelling ethanol molecules. Therefore, the hybrid membranes are expected to show high water adsorption selectivity for water/ethanol mixtures.

24

80

3

Water uptake (%)

70 60

2

50 40 30

1

20

Ethanol uptake (%)

Water uptake Ethanol uptake

10 0

0

5

10

15

20

25

30

0

SNW-1 content (wt%)

Fig. 9 The water uptake and ethanol uptake of the membranes. 3.5 Pervaporation performance of the membranes

3.5.1

Effect of SNW-1 content

The separation performance of the hybrid membranes was studied by measuring the pervaporation dehydration of ethanol aqueous solution (90 wt%) at 76 oC. As shown in Fig. 10, the separation factor of the hybrid membrane obviously increases with the SNW-1 content increasing from 5 to 25 wt%. And the SA-SNW-1(25)/PAN membrane reveals a separation factor of 1293, which is approximately three times larger than that of the pure SA membrane (328). Meanwhile, a desirable permeation flux of 2397 g/m2h is achieved for the SA-SNW-1(25) membrane, which is increased by 60% compared with that of the pure SA membrane. The simultaneous enhancement of separation factor and permeation flux for the hybrid membrane indicates that the tradeoff effect has been successfully overcome. In addition, both the permeation flux and separation factor start to decrease with the SNW-1 content further increasing. 25

1500

2000

1200

1500

900

1000 600

Separation factor

Permeation flux (g/m2h)

2500

500 300 0 0

5

10

15

20

25

30

SNW-1 content (wt%)

Fig. 10 Separation performance of the membranes with different SNW-1 contents.

The remarkable improvement of separation performance is principally attributed to the dual-layer structures of the hybrid membrane and the intrinsic characteristics of SNW-1. The structure of the hybrid membrane and presumable molecule transfer mechanisms through the membrane are explicitly demonstrated in Scheme 2. Due to the formation of the SNW-1-rich layer, the surface hydrophilicity of the hybrid membranes is significantly enhanced (Fig. 7). And from the results of water uptake and ethanol uptake (Fig. 9), the hybrid membranes reveal high water adsorption selectivity. Thus the hydrophilic top layer could preferentially capture an extraordinary number of water molecules from feed mixtures, increasing the driving force for water molecules transfer. Furthermore, the increased fractional free volume by the incorporation of SNW-1 (Table 1) makes the membranes more permeable to both water and ethanol molecules, and thus resulting in the enhancement of permeation flux. Generally, the selectivity for membrane 26

Scheme 2. Schematic diagrams of water and ethanol molecules transport through SA-SNW-1(X)/PAN and interfacial interaction between SNW-1 and SA.

would be sacrificed caused by the increase in fractional free volume, because the permeation of molecules with large size is more sensitive to the increase in interchain spacing. However, the separation factor for the hybrid membrane is also increased, which can be attributed to the water permselectivity of SNW-1. As shown in Fig. 2(b), the high concentration of SNW-1 on membrane surface can provide continuous water-selective channels. On one hand, the large number of SNW-1 nanoparticles can rapidly combine with water molecules by hydrogen bonds and adsorb a large amount of water molecules by their large specific surface area. On the other hand, in couple with the molecular sieving effect, once the water molecules are penetrating through the micropores (5 Å) of SNW-1, the diffusion of ethanol molecules (4.5 Å) is blocked to a large extent[32]. As a result, the separation factor of the hybrid membranes is also enhanced with the SNW-1 27

content increasing from 5 to 25 wt%. When the SNW-1 content increases to 30 wt%, the decrease in both permeation flux and separation factor is attributed to the aggregation of SNW-1 on the membrane surface and in the SA matrices. For comparison, the separation performance of the hybrid membranes fabricated by fast evaporation of the solvent was also evaluated and shown in Fig. S4. With the SNW-1 content increasing from 5 to 25 wt%, both the permeation flux and separation factor are increased compared with that of the pure SA. However, the optimum performance of the hybrid membrane with homogenous SNW-1 distribution is inferior to that of the hybrid membrane with asymmetric SNW-1 distribution. The lower separation performance could be attributed to that the membrane without SNW-1-rich layer has both low adsorption selectivity and adsorption capacity for water, and therefore both the permeation flux and separation factor are decreased. The comparison result further verifies that the hybrid membrane with asymmetric SNW-1 distribution is more favorable for the pervaporation processes.

3.5.2

Effect of operation temperature

The pervaporation dehydration for 90 wt% of ethanol aqueous solution at different feed temperatures was carried out using SA-SNW-1(25)/PAN membrane. As shown in Fig. 11(a), the total flux dramatically increases with the increase in feed temperature, which can be explained as follows. 1) The partial pressure in the upstream side of membrane increases due to the increased temperature, and the downstream side is almost 28

under vacuum. Thus the increased difference of partial pressure results in the enhancement of mass transfer driving force[50]. 2) The motion of polymer chains is sensitive to the variation of temperature, so the free volume of SA increases with the increased temperature[51]. 3) With increasing temperature, the molecule transfer rate is accelerated due to the enhanced thermal motion[52]. Therefore, both of the water flux and ethanol flux increase with the increase in feed temperature as shown in Fig. 11(b). The apparent activation energy (Ep) of water and ethanol through membrane were calculated by the Arrhenius equation (Eq (9)):

J i  A0i exp(

E pi RT

)

(9)

where Ji, A0i, and Epi are the permeation flux (g/m2h), pre-exponential factor and apparent activation energy (kJ/mol). R is the gas constant (8.314 J/(mol K)) and T is the temperature of feed (K). The fitting Arrhenius curves are shown in Fig. 11(c), it can be seen that the Ep of water (50.3 kJ/mol) is higher than that of ethanol (31.0 kJ/mol), suggesting that the water flux increases more rapidly than that of the ethanol flux with increasing operation temperature[53]. Thus both of the separation factor and permeation flux increase with increasing temperature. The permeance and selectivity were calculated by normalizing the driving force and membrane thickness, and shown in Fig. 11(d). With increasing operation temperature, the water permeance maintains increasing while the ethanol permeance reveals a decline trend. On one hand, the increase in temperature is beneficial to improving the diffusion rates of water and ethanol molecules. And on the 29

other hand, the increased temperature inevitably leads to the reduced adsorption of the components. Due to the hydrogen interactions between the membrane surface and water molecules, the reduction of ethanol adsorption is more dominant than that of water adsorption[54]. In addition, the increased temperature can weaken the coupling effect between water and ethanol, which further inhibiting the ethanol diffusion together with water molecules[55]. Therefore, the selectivity significantly increases with increasing operation temperature, manifesting that high temperature is desired for achieving high pervaporation performance. 2500

1000 1500 800 1000 600 400

500

18

2000

16 14

1500 12 10 1000 8 6

500

4

200 0

0 40

50

60

70

2

Water flux (g/m2h)

1200

Separation factor

Total flux (g/m2h)

2000

20

(b)

1400

Ethanol Flux (g/m h)

2500

(a)

80

40

50

60

70

2 80

o

o

Temperature ( C)

Temperature ( C)

Fig. 11 Effect of feed temperature on pervaporation performance of SA-SNW-1(25)/PAN: (a) total flux and separation factor, (b) water flux and ethanol flux, (c) Arrhenius curves and apparent activation energy for water and ethanol, and 30

(d) water permeance, ethanol permeance and selectivity. 3.5.3

Effect of water concentration in feed

The effect of water concentration in feed on the pervaporation performance of SA-SNW-1(25) was investigated at 76 oC. As shown in Fig. 12(a), the total flux increases from 1131 to 4870 g/m2h with the water concentration in feed increasing from 5 to 30 wt%. The water flux and ethanol flux as a function of water concentration in feed are demonstrated in Fig. 12(b). Although the increased water concentration in feed could enhance the driving force for water, the ethanol flux shows an exponential growth while the water flux shows a logarithmic growth. With increasing water concentration in feed, the plasticization effect results in considerable membrane swelling[56]. Thus the swollen membrane is more permeable to both water and ethanol. Due to the larger size of ethanol molecule, the improvement of ethanol flux is more dominant than that of water flux. From Fig. 12(c), with the normalization of driving force, the ethanol permeance increases more quickly than water permeance when the water concentration in feed increases from 5 to 30 wt%, further confirming the swelling effect on permeation of water and ethanol. Consequently, the high permeation flux could be obtained by increasing water concentration in feed, but the selectivity of membranes would be sacrificed.

31

4000

900

3000

2000

600

80 3000 60 2000 40

2

1200 4000

100

Ethanol flux (g/m h)

(b) 5000 Water flux (g/m2h)

1500

Separation factor

Total flux (g/m2h)

(a) 5000

1000

1000

300

0 10

15

20

25

0

30

20 5

Water concentration in feed (wt%)

(c) 10000 9000

10

20

25

30

1000 (p/l)W

8000

900

(p/l)E

7000

Permeance (GPU)

15

Water concentration in feed (wt%)

800

6000

700

5000 4000

600

25

500

20

Selectivity

5

400

15 10

300

5 0

200 5

10

15

20

25

30

Water concentration in feed (wt%)

Fig. 12 Effect of water concentration in feed on (a) total flux and separation factor, (b) water flux and ethanol flux, and (c) permeance and selectivity of SA-SNW-1(25).

3.5.4

Long-term operation stability

To assess the long-term operation stability of the SA-SNW-1(25)/PAN membrane, the pervaporation dehydration for 90 wt% of ethanol aqueous solution was continuously carried out at 76 oC. As shown in Fig. 13, both of the permeation flux and separation factor maintain stable during the long time operation. The SA-SNW-1(25)/PAN membrane reveals desired permeation flux of 2625 g/m2h and water concentration of 99.2 wt% in permeate after operating for more than 250 h, demonstrating the excellent long-term stability as well as the great potential for practical applications. The results also 32

indicate that the SNW-1 possesses high water stability, and these COF-based hybrid membranes show longer lifetime than MOF-based hybrid membranes reported in

100

2400

80

1800

60

1200

40

600

20

2

Permeation flux (g/m h)

3000

0

0

50

100

150

200

250

0

Water concentration in permeate (wt%)

literatures[13, 17].

Operation time (h)

Fig. 13 Long-term operation stability of the SA-SNW-1(25)/PAN membrane.

3.5.5

Comparison of performance with reported hybrid membranes based on porous

fillers.

The separation performance of the SA-SNW-1/PAN membrane and other reported porous materials based hybrid membranes applied in pervaporation dehydration are shown in Fig. 14. It can be clearly seen that SA-SNW-1/PAN displays superior separation performance to other porous materials-based membranes (including MOF-based hybrid membranes).

33

Fig. 14 Comparison of separation performance with porous fillers-based hybrid membranes as reported in literature: SA-Zeolite 4A[57], SA-AC[58], SA-Zeolite bate[59], SA-Al-MCM-41[60], PVA-ZIF8[13], CS-ZIF7[12], SA-NH2-MIL125[17], SA-AlPO4-5[61], and PVA-Zeolite 4A[62]. 4. Conclusions

COF-based hybrid membrane SA-SNW-1/PAN was fabricated and for the first time used for separation of liquid mixture. The SA-SNW-1(25)/PAN membrane with a SNW-1 loading of 25 wt% achieves the optimal separation performance with the permeation flux of 2397 g/m2h and separation factor of 1293 for ethanol dehydration. This work provides a new approach to design high performance hybrid membranes with asymmetric filler distribution. The high separation performance and stability of the hybrid membranes based on SNW-1 indicate their promising application potential, and it 34

can be also speculated that a variety of COF-based hybrid membranes could be designed and fabricated for much broader separation processes.

Appendix A. supplementary materials

Cross-section SEM images and EDX N-mapping of the membranes (Fig. S1); XRD patterns of SNW-1 and the membranes (Fig. S2); DSC curves of the membranes (Fig. S3) and separation performance of the hybrid membranes fabricated by fast evaporation of the solvent (Fig. S4).

Acknowledgments We genuinely appreciate the National Natural Science Foundation of China (No. 21490583), the Program for New Century Excellent Talents in University (NCET-10-0623), the National Science Fund for Distinguished Young Scholars (21125627), Tianjin Application Foundation and Research in Cutting-edge Technology Plan (15JCQNJC43300) and the Programme of Introducing Talents of Discipline to Universities (No. B06006).

35

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Graphical abstract

Highlights 

Hybrid membranes with asymmetric COF SNW-1 distribution were fabricated.



COF-based hybrid membranes were used for pervaporation for the first time.



The SNW-1 provided highly selective channels for water transfer.



The hybrid membranes revealed high hydrophilicity and water sorption capacity.

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The hybrid membranes showed high water permeation and long-term stability. 45