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Blocking defects of zeolite membranes with WS2 nanosheets for vapor permeation dehydration of low water content isopropanol
Journal of Membrane Science xxx (xxxx) xxx
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Blocking defects of zeolite membranes with WS2 nanosheets for vapor permeation dehydration of low water content isopropanol Yuting Zhang a, Peng Du a, Rui Shi a, Zhou Hong b, Xinfeng Zhu a, Bing Gao a, Xuehong Gu a, * a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, 211816, Jiangsu, China b Nanjing Membrane Materials Industrial Technology Research Institute Co, Ltd, Nanjing, 211800, Jiangsu, China
A R T I C L E I N F O
A B S T R A C T
Keywords: WS2 nanosheets Zeolite membrane Defect Vapor permeation dehydration
It is difficult for zeolite membrane to behave good separation selectivity in dehydration of low water content isopropanol (IPA) by vapor permeation (VP) because capillary condensation in small intercrystalline defects is usually insufficient for excluding IPA molecules and adsorbate-induced contraction of zeolite crystals open new defects at low water content. To address this issue, a facile and high-efficiency modification strategy was pro posed to block defects of NaA zeolite membranes on hollow fibers by using WS2 nanosheets. Effect of nanosheets thickness on VP separation performance of modified membranes was investigated. It showed that the separation factor of membrane was improved by more than two orders of magnitude (from 60 to >10000) after modified with the few-layer WS2 nanosheets while the loss in water flux was only 18.6% in VP dehydration of 10 wt% water/IPA mixture at 373 K. Moreover, the modified membrane still maintained a high separation factor of ~5000 rather than that of pristine membrane (336) even when the feed water content lowered to 0.5 wt%, indicating that not only intrinsic defects but also the crystal contraction-induced defects were blocked simul taneously by the thin WS2 nanosheets.
1. Introduction Isopropanol (IPA) is an important solvent and cleaning medium widely used in chemical, electronics, and semiconductor industries. Dehydration of IPA is a significant process in production and recycling of IPA. It is complicated and energy-intensive when using conventional dehydration processes because IPA and water forms an azeotropic mixture when the water content is about 14% [1]. Alternatively, membrane-based separation processes like pervaporation (PV) and vapor permeation (VP) show advantages of low energy consumption, zero emission and simple operation for liquid separation especially for azeotropic and close-boiling mixtures. The main difference between PV and VP is the state of feed mixture. For energy utilization, VP mode is often adopted followed with distillation where the feed mixture is in vapor state [2] Among the VP membranes, zeolite membranes (e.g. LTA, FAU, MOR membranes) have been attractive for their good separation performance and robust thermal stability [1,3,4]. Particularly, sodium LTA (NaA) zeolite membrane has strong hydrophilicity and well-defined pore size of 0.42 nm, which is suitable for dehydration of IPA [5–8]. Due to polycrystalline structure, zeolite membrane usually
inevitably contains intercrystalline defects that are larger than its zeolitic pores. The existence of any defect affects its inherent molecularsieving property. It was found that although NaA zeolite membrane had some small defects, the capillary condensation of water molecules in these defects was still able to exclude organic solvent molecules from entering the defects when the feed mixture had enough water content [3]. However, once the feed water content was lowered to a certain degree (<5 wt%) especially by VP at high temperature, the capillary condensation in defects was always insufficient. In that case, large organic solvent molecules would pass through the defects and then the separation selectivity fell a lot. Such phenomenon was also observed on other zeolite membranes for dehydration of low water content organic solvents [4,9,10]. Another significant concern is the crystal flexibility of NaA zeolite membrane related to its adsorption amount of water molecules, which results in shrinkage/enlargement behaviors of intercrystalline defects [11]. For example, when the membrane was used in VP dehydration of IPA, it behaved good separation factor at high feed water content because large adsorption amount of water molecules in zeolitic pores expanded zeolite crystals, which therefore shrank the defects. But, once
* Corresponding author. E-mail address: [email protected] (X. Gu). https://doi.org/10.1016/j.memsci.2019.117625 Received 7 July 2019; Received in revised form 17 October 2019; Accepted 30 October 2019 Available online 2 November 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yuting Zhang, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117625
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Fig. 1. Schematic diagram of WS2 nanosheets modification on NaA zeolite membrane.
the feed water content lowered to below 5 wt%, the separation factor fell sharply because low adsorption amount of water molecules in zeolitic pores contracted the crystals, which enlarged the defects consequently. Such variation in crystal size of NaA zeolite membrane makes it difficult to be used in dehydration of low water content organic solvents such as IPA and n-butanol despite that the membrane is defect-free [12]. Until now, many post-treatment methods have been developed to minimize defects of zeolite membranes, such as chemical liquid or fluid deposition (CLD or CFD) [13,14], surface coating [15], chemical vapor deposition (CVD) [16–18], coke deposition [19,20] and so on. Most of them improved separation selectivity efficiently but also caused large decline in permeation flux because zeolitic pores of membranes were simultaneously blocked during the treatment. In rare cases, the treat ment did not result in considerable reduction in membrane permeance, but it was conducted on the membrane that had a low initial permeance [21]. Recently, two-dimensional nanosheets of transitional metal disulfides (TMDs), such as MoS2, WS2, TiS2, MoSe2, have been an emerging class of nanomaterials [22–24]. Similar to graphene oxide nanosheets, WS2 nanosheets have lamellar structure with single or few layers atomic thickness. With virtues of nanometer size, good wettability and high flexibility, WS2 nanosheets can also be constructed as an effective barrier for water filtration [25–27]. Herein, we proposed a facile and high-efficiency modification strategy to block intercrystalline defects of NaA zeolite membranes on hollow fibers by using WS2 nanosheets. The membrane qualities were evaluated by VP dehydration of water/IPA mixture under the feed water content ranging from 10 to 0.5 wt%. Effect of nanosheets thickness on separation performance of modified membranes was investigated. Both pristine and modified membranes were characterized extensively in terms of SEM, XRD, dynamic contact angle, permporosimetry and XPS for comparison.
and N-vinylpyrrolidone (NVP, �99%) were purchased from Sigma-Aldrich. An ultrasonicator (Scientz-IID, Ningbo Scientz Biotech, China) was used to exfoliate the powders. 30 mg of WS2 powders were mixed with 10 mL of NVP in a 50 mL flask. The solution was ultra sonicated for 4 h at 400 W. After ultrasonication, the samples were centrifuged for 10 min at 10000 rpm. The top layer, which contained the exfoliated WS2 layers, was pipetted off, and the NVP was removed by washing with IPA. The final products were dried at 303 K in a vacuum oven, and the obtained nanosheets were redispersed in IPA solvent for further measurement and membrane modification. The concentration of WS2 nanosheets in the dispersion was about 0.2 mg mL-1. 2.3. Membrane modification NaA zeolite membranes on hollow fibers were modified by vacuum inhalation method using as-prepared WS2 nanosheets/IPA dispersion. The pristine membrane was mounted in a stainless steel module and both ends of it were sealed with silicon O-rings. The inner side of membrane was evacuated by a vacuum pump through a vacuum line, which maintained a downstream pressure below 200 Pa. The outer surface of membrane was fed with a circular stream pumped from a tank containing 50 mL WS2 nanosheets/IPA dispersion. The temperature of the dispersion was kept at 313 K. After vacuumed for a certain time (1–8 h), the membrane was dried in an oven at 343 K for 1 h and then tested in VP separation. Fig. 1 illustrates the schematic diagram of WS2 nanosheets modification on NaA zeolite membrane. When the mem brane was immersed in the dispersion, zeolite crystals of the membrane contracted in the IPA solvent which contained trace amount of water [12]. Under the force of vacuum inhalation, both intrinsic and crystal contraction-induced defects of the membrane were pasted by WS2 nanosheets.
2. Experimental
2.4. Membrane characterization
2.1. Synthesis of NaA zeolite membranes
The morphologies of the membranes were examined via FESEM (S4800, Hitachi, Japan). The working parameters were a voltage (HV) of 5 kV and a work distance (WD) of 8 mm. With EDX spectroscopy, the working parameters were a voltage (HV) of 20 kV and a WD of 15 mm. The crystal phases of the samples were determined by XRD with Cu Kα radiation (Bruker, model D8 Advance). The size and surface morphology of WS2 nanosheets were obtained by AFM (XE-100, Park SYSTEMS, Korea) and transmission electron microscopy (TEM, JEOL-2100F, Japan). The dynamic contact angles of water were analyzed using a contact-angle Dropmeter (A100P, MAIST Vision Inspection and Mea surement Co., Ltd.). X-ray photoemission spectroscopy (XPS) spectra were measured using a Thermo ESCALAB 250Xi surface analysis system. Permporosimetry test was conducted to characterize the distribution of defects in NaA zeolite membrane [31–34]. The detailed test system was described in our previous publication [12]. Prior to the permpor osimetry test, the membrane mounted in a stainless steel module was heated at 383 K for 24 h in a flowing helium stream to remove any adsorbate in zeolitic and nonzeolitic pores. The operating temperature
NaA zeolite membranes were hydrothermally synthesized on outersurface of α-Al2O3 hollow fibers by the secondary growth method as described in our previous work [28]. Prior to membrane synthesis, ball-milled NaA zeolite seeds (average particle size: ~220 nm) were planted on external surface of the supports by dip-coating method. The synthesis gel was prepared by dissolving sodium aluminate, sodium hydroxide and water glass into deionized water with the molar ratio of 1Al2O3: 2SiO2: 2Na2O: 120H2O. All the chemicals were industrial grade and purchased from commercial companies in China. The hydrothermal crystallization was performed at 373 K for 4 h. 2.2. Preparation of WS2 nanosheets WS2 nanosheets were prepared by the liquid sonication exfoliation method as described by Nguyen et al. [29,30], which is feasible for large-scale production. WS2 powders (99%, average particle size: 2 μm) 2
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Fig. 2. Schematic diagram of experimental apparatus for VP dehydration separation.
was then fixed at 298 K for permporosimetry experiment. Dried helium was used as the noncondensable gas and vapor was employed as the condensable gas. Water was loaded in a saturator for producing vapor. The feed stream was obtained by mixing a pure helium stream with another helium stream saturated with vapor. The vapor relative pressure (the ratio of partial vapor pressure to its saturation pressure, P/Psat) was varied by adjusting the ratio of the two helium flows. The feed pressure was maintained at 201 kPa and the permeate pressure was kept at at mospheric pressure (101 kPa). The defect width in NaA zeolite mem brane was calculated from vapor relative pressure using Kelvin equation and Halsey adjusted for the adsorbed layer thickness (t) as described by Noack et al. [34].
separation factor (α), which were respectively defined as follows:
2.5. VP dehydration separation
3.1. WS2 nanosheets characterization
VP separation performance of NaA zeolite membranes were inves tigated with dehydration of binary mixtures (IPA/water and ethanol/ water). The VP experiment was reported in our previous work [12]. The tested membrane had an effective membrane area of ca. 6 cm2. Feed steam was continuously pumped into the shell side of a membrane module and the permeate product was removed from the lumen by a vacuum pump, which maintained a downstream pressure below 300 Pa. The permeated vapor was collected with a cold trap cooled by liquid nitrogen (Fig. 2). Both of samples at the feed and permeate sides were analyzed by a gas chromatograph (GC-2014A, Shimadzu) equipped with a thermal conductivity detector and a packed column of Parapak-Q. The membrane performance was determined by permeation flux (J) and
Fig. 3a shows the photograph of as-prepared WS2 nanosheets dispersed in IPA solvent. The dispersion is clear and stable for over hundreds of hours. We also performed high-resolution transmission electron microscopy (HRTEM) on the dispersion. It can also be seen from Fig. 3b that the sizes of nanosheets are 50–70 nm. The selected-area electron diffraction (SAED) pattern shown in Fig. 3c confirms that the as-prepared WS2 nanosheets exhibit hexagonal lattice structure, indi cating that the sonication process did not degrade the hexagonal struc ture of WS2. The lattice constant of WS2 measured from TEM images is 0.32 nm, which is consistent with those in other literatures [29,30]. Fig. 4 shows XRD patterns of WS2 powders and exfoliated WS2
m A⋅t
(1)
� yi yj αi=j ¼ � xi xj
(2)
J¼
Where m is the total mass of the permeate product, kg; A is the effective area of the membrane, m2, t is the elapsed time, h; yi/yj is the weight fraction ratio of water over alcohol in the permeate and xi/xj is the corresponding ratio in the feed. 3. Results and discussion
Fig. 3. Photograph, HRTEM and SAED images of WS2 nanosheets. 3
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water content occurred on the membrane which was modified with WSIII. The permeate water content of membrane reached to about 97.5 wt% in 3 h and then kept it stably. On the other side, the water flux decreased most largely from 12.5 to 7.4 kg m-2 h-1. When it comes to the membrane that was modified with WS-II, the permeate water content was improved to 97.5 wt% in 8 h while the water flux decreased moderately from 10.7 to 7.8 kg m-2 h-1. When the membrane was modified with WS-I, the permeate water content increased most slowly, but it achieved to nearly 100 wt% finally after modified for 9 h (the corresponding separation factor was >10000), indicating that all of non-selective defects of the membrane had been blocked with the thinnest WS2 nanosheets. At the same time, it is very interesting to find that the water flux decreased only by 18.6% from 10.2 to 8.3 kg m-2 h-1. Since such modification method is simple and controllable, it can be also seen from Table 1 that a good reproducibility was obtained on separation improvement of other hol low fiber supported NaA zeolite membranes. Fig. 7 shows surface and cross-section SEM images of pristine and different WS2 nanosheets modified NaA zeolite membranes. The modi fication time of modified membranes agreed with that shown in Fig. 6. It is observed from Fig. 7a that the pristine membrane had a wellintergrown zeolite layer with uniform thickness of ~7 μm. The mem brane surface appears to be free of defects such as cracks and pinholes, but grain boundaries are clearly seen. After modified with WS-I, the membrane surface was coated with small scale-like WS2 nanosheets. The EDX map scan results in Fig. 8 confirm that WS2 nanosheets were distributed on membrane surface evenly. The insert figure in Fig. 7c displays that many WS2 nanosheets aggregated near grain boundaries of the membrane as the vacuum inhalation force was stronger there during the modification. The thickness of modified membrane shown in Fig. 7b was similar to that of pristine membrane, no element of tungsten and sulfur was even detected on the cross section of modified membrane in the EDX analysis, indicating that the thickness of coated WS2 nanosheets was very thin. We suggest that the ultrathin lamellar WS2 nanosheets, like GO nanosheets, were also able to provide fast paths for water transport [27], so the coating of ultrathin WS2 nanosheets on membrane surface did not cause large loss in water flux. It is seen from Fig. 7e–h that thickness of WS2 nanosheets coating on membrane surface was more conspicuous after modified with larger and thicker nanosheets, which resulted in larger loss in water flux. On the other side, there were also some agglomerated particles, formed from the folding of WS2 nanosheets, on the surfaces of modified membranes especially that was modified with WS-III. The large agglomerated particles might detach from the membrane surface when the vapor stream flowed around the membrane surface during the VP separation. In addition, the WS-II and WS-III with a lot of layers were more rigid than WS-I, which might not be adhered to membrane surfaces closely and block small defects completely. As a result, the separation factors were improved less notably than that for WS-I. Fig. 9 shows XRD patterns of pristine and different modified NaA zeolite membranes. As consistent with observation in Fig. 7, the XRD pattern of pristine NaA zeolite membrane matched well with the
Fig. 4. XRD patterns of WS2 powders and nanosheets.
nanosheets. The WS2 powders exhibit numerous peaks, indicating that they have many layers. After sonication and removal of large WS2 ma terials, the nanosheets only exhibit one weaker (002) peak. 3.2. Membrane modification The as-prepared WS2 nanosheets (denoted as WS-I) dispersed in IPA solvent was further used for membrane modification. For comparison, another two types of WS2 nanosheets with different thicknesses were prepared via sonication treatment at 100 W. After centrifuged, the su pernatant was collected as nanosheets with medium thickness (denoted as WS-II). Meanwhile, the sediment was collected as the thickest WS2 nanosheets (denoted as WS-III). Fig. 5 shows their thicknesses and lateral sizes measured by atomic force microscopy (AFM). It is seen that the WS-I had the thinnest thickness of 1.6–2.3 nm, indicating the fewlayer structure of WS2 nanosheets. Its lateral size was also the smallest of ~50 nm. By contrast, the thicknesses of WS-II and WS-III were 42 (~60 layers) and 100–170 nm (143–243 layers) respectively. And their lateral sizes were as large as 200 nm and 700 nm, respectively. The WS2 nanosheets/IPA dispersions were further used to modify three NaA zeolite membranes on hollow fibers with similar quality for comparison. The concentrations of WS2 nanosheets in dispersions were controlled at 0.2 mg mL-1. Fig. 6 shows VP separation results of the membranes as a function of modification time in dehydration of 10 wt% water/IPA mixture at 373 K. Although the pristine membranes exhibited high separation factors of >5000 in PV dehydration of 10 wt% water/ ethanol mixture at 348 K, their permeate water contents in VP dehy dration of water/IPA mixture were only 88 wt% (namely the corre sponding separation factors were about 60). As the modification time increased, all the permeate water contents climbed up initially and then tended to plateau. It is observed that the fastest increment in permeate
Fig. 5. AFM images of different WS2 nanosheets: (a) WS-I, (b) WS-II and (c) WS-III. 4
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characteristic peaks of NaA zeolite besides of α-Al2O3 signals of the support. A very weak WS2 nanosheets (002) peak was observed on the WS-I/NaA composite membrane. As the WS2 nanosheets were larger and thicker, the intensities of NaA zeolite characteristic peaks in composite membrane decreased gradually while that of WS2 nanosheets (002) peak became more obvious. Particularly, the characteristic peaks of NaA zeolite nearly vanished in the WS-III/NaA composite membrane. Instead, two strong WS2 characteristic peaks were observed at 14� and 33� respectively, indicating that the coated WS-III still contained many multi-crystalline WS2 particles. 3.3. Membrane characterization In order to explore the role of WS-I in membrane modification, the dynamic contact angle was first tested on both pristine and modified NaA zeolite membranes. As shown in Fig. 10, the final steady contact angle of membrane surface decreased from 76.6� to 72.1� after modifi cation, indicating the improvement in hydrophilicity of membrane surface resulted by the coating of highly wettable WS2 nanosheets. It is also noted that the dynamic contact angles of modified membrane declined slower than those of pristine membrane. Such result could be owed to the effective elimination in defects of the membrane after modification, which slowed down the penetration of water droplet into the modified membrane. Generally, it is difficult to observe microporous and small meso porous defects in zeolite membrane from conventional SEM image, permporometry technique is a useful way to illustrate the distribution of defects in zeolite membrane. In this technique, helium permeance through the membrane is measured as a function of vapor relative pressure (P/Psat) in the feed. As P/Psat increases, the vapor diluted in helium flow condensates first in zeolitic pores and then in increasing larger defects, therefore the helium permeance decreases gradually. As shown in Fig. 11, for the pristine NaA zeolite membrane, the helium permeance at P/Psat ¼ 0 was 2.38 � 10-7 mol m-2 s-1 Pa-1, which repre sented the permeance through both zeolitic pores and defects [31]. At higher P/Psat of about 0.03 where zeolitic pores of the membrane had been filled with condensed water, the helium permeance represented the flow through the defects which were larger than zeolitic pores. The further decrease in helium permeance in the range of P/Psat from 0.03 to 0.76 indicated that the increasing larger defects in the membrane were gradually blocked. After modification, the helium permeance of mem brane at P/Psat ¼ 0 was lowered to 4.5 � 10-9 mol m-2 s-1 Pa-1. The heli um permeance at P/Psat ¼ 0.03 that represented the flow through defects reduced from 1.82 � 10-7 to 2.4 � 10-9 mol m-2 s-1 Pa-1. The helium permeances at higher P/Psat were nearly zero, confirming that most of nano-sized defects in the membrane had been blocked by WS2 nano sheets. By comparison, the helium permeances of WS-II and WS-III modified NaA zeolite membranes at P/Psat ¼ 0.11 are 7.7 � 10-9 and 1.8 � 10-8 mol m-2 s-1 Pa-1 respectively (Fig. S1 in Supporting informa tion), suggesting that they still had more or larger nano-sized defects. Unlike helium permeance, the loss in water flux of membrane in VP separation was only a little after modification. We suggest that such difference was probably due to that the stacking WS2 nanosheets on zeolite membrane swelled slightly under humidity during the VP sepa ration, which provided fast and highly-selective paths for water diffu sion and thus avoided large loss in water flux. In contrast, helium gas was excluded by the narrow spacing distance between WS2 nanosheets and zeolite membrane as it was nearly non-adsorptive and hardly able to swell the nanosheets. The XPS spectra in Fig. 12 also confirms the adhesion of WS2 nano sheets on membrane surface. For the spectra of WS-I, two predominant peaks of W4f7/2 and W4f5/2 at 33.1 and 35.2 eV (Fig. 12a) corresponding to the 2H phase are observed [35]. The weak and broad W5p3/2 peak at 38.7 eV is attributed to the slight oxidation of WS2 nanosheets [36]. The doublet peaks of S2p3/2 and S2p1/2 (Fig. 12b) located at 162.7 and 163.9 eV are assigned to divalent sulfide ions (S2 ) [37]. After coated on
Fig. 6. VP results as a function of modification time of different NaA zeolite membranes modified with WS-I, WS-II and WS-III: (a) permeate water content, (b) separation factor and (c) water flux.
Table 1 VP separation results of different hollow fiber supported NaA zeolite membranes before and after modified with WS-I in VP dehydration of 10 wt% water/IPA mixture at 373 K. Membranes M1 M2 M3
Before modification
After modification
J (kg m-2 h-1)
α
J (kg m-2 h-1)
α
10.2 9.7 9.9
60 643 2307
8.3 8.3 8.6
>10000 >10000 >10000
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Fig. 7. Surface and cross-section SEM images of (a and b) pristine and different modified NaA zeolite membranes: (c and d) WS-I, (e and f) WS-II and (g and h) WS-III.
the surface of NaA zeolite membrane, the W4f peaks shifted to lower binding energies (31.1 and 34.7 eV), which could be owed to the 2H to 1T phase transition of WS2 nanosheets [38]. Similar shift in binding energy also occurred on the S2p spectra, implying the electronic inter action between WS2 nanosheets and NaA zeolite membrane [35]. Furthermore, a new broad peak appeared at 168.4 eV, suggesting the presence of SO24 or S2O23 . It could be produced via the oxidation of sulfur in air [36] or the sharing S–O bonds between WS2 nanosheets and hydroxide groups of zeolite membrane [39]. Fig. S2 in Supporting in formation shows XPS survey spectra of pristine and WS-I modified NaA zeolite membranes. The corresponding binding energies and surface ratio results are listed in Table S1 in Supporting information. It is seen that the Na1s, Al2p peaks of NaA zeolite membrane shift to lower en ergies after modification. Such shift might indicate the partial substi tution of O by S that is less electronegative. The O1s shift from 531.84 to 532.12 eV demonstrates that some of O–H and O–Na bonds might be
took place with O–W bonds, confirming the strong electronic interaction between WS2 nanosheets and zeolite membrane. On the other side, the reduction in Na/Si surface ratio could be caused by the dissolving of Na cations in the WS2 nanosheets dispersion from zeolite membrane. Meanwhile, the increase in Al/Si surface ratio could be explained by the interference of Al element from the hollow fiber support. 3.4. VP dehydration separation 3.4.1. VP dehydration of low water content IPA Both pristine and WS-I modified NaA zeolite membranes were car ried out into VP dehydration of low water content IPA for comparison. As shown in Fig. 13a, the water content in feed mixture decreased continuously from ~10 to 0.5 wt% due to the selective removal of water through the membrane. At the same time, the water flux declined gradually due to the continuous reduction in driving force of water 6
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Fig. 8. Surface elemental mapping images of WS-I modified NaA zeolite membrane.
Fig. 10. Dynamic contact angles of pristine and WS-I modified NaA zeolite membranes.
Fig. 9. XRD patterns of pristine and different WS2 nanosheets modified NaA zeolite membranes.
decreased more obviously from 10.4 to 7.3 kg m-2 h-1 after modification while the separation factor was improved only a little from 611 to 1310 (Fig. 13b). When the feed water content lowered from 10 to 0.5 wt%, the separation factor of modified membrane declined more than a half from 1310 to 502, indicating that the coating of rigid WS2 powders might also have blocked some large defects in the membrane but failed to restrain the small defects induced by crystal contraction at low water content.
permeation. For the pristine membrane, the water/IPA separation factor fell from 643 to only 336, which was mainly caused by crystal contraction at low feed water content as mentioned above. By contrast, the modified membrane still maintained a high separation factor of ~5000 at the low feed water content of 0.5 wt%, indicating that the coating of WS2 nanosheets on membrane surface had blocked the small defects induced by crystal contraction. On the other hand, it is inter esting to find that only a little loss in water flux occurred on the modified membrane in the whole range of feed water content. As shown in Fig. 7b, grain boundaries of the modified membrane were filled with small WS2 nanosheets. When the feed water content was very low and zeolite crystals contracted, the highly flexible WS2 nanosheets were attached to defects more closely under the enhanced capillary force in enlarged defects, so the IPA molecules were still excluded [26]. In order to demonstrate it more clearly, WS2 powders were dispersed in IPA solvent with similar content and used to modify another NaA zeolite membrane for comparison. It can be seen that the water flux of membrane
3.4.2. Separation stability The WS-I modified NaA zeolite membrane was further tested in VP dehydration of 10 wt% water/IPA mixture at 373 K for over 50 h. As shown in Fig. 14, the water flux and separation factor maintained at about 8.2 kg m-2 h-1 and >10000 respectively during the whole period, indicating that the modified membrane had good long-term separation stability. We consider that the thin WS2 nanosheets were suppressed on membrane surface firmly under the high vapor pressure on the feed side. Furthermore, according to the XPS results, the nanosheets were adhered stably onto membrane surface for the strong electronic interaction 7
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Fig. 11. Permporosimetry results of pristine and WS-I modified NaA zeolite membranes.
Fig. 13. VP separation results of NaA zeolite membrane before and after modified with (a) WS-I and (b) WS2 powders in dehydration of water/IPA mixtures under different water contents at 373 K.
Fig. 14. VP separation stability of WS-I modified NaA zeolite membrane in dehydration of 10 wt% water/IPA mixture at 373 K.
between them, which ensured the membrane to behave good and stable separation performance. In contrast with WS-I, the separation factors of WS-II and WS-III modified membranes decreased by about 16% and 22% respectively after continuous VP dehydration separation for 10 h (Fig. S3 in Supporting information). Simultaneously, their water fluxes increased by about 0.6% and 17% respectively. The WS-II and WS-III are expected to have weaker adhesion with NaA zeolite membranes due to their much higher thicknesses. When the membranes were used in VP separation, some of them might peel off from membrane surface and thus led to the decline in separation factors as well as the increase in
Fig. 12. XPS spectra of (a) W 4f and (b) S 2p for WS-I and WS-I modified NaA zeolite membrane.
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org/10.1016/j.memsci.2019.117625. References [1] K. Sawamura, T. Furuhata, Y. Sekine, E. Kikuchi, B. Subramanian, M. Matsukata, Zeolite membrane for dehydration of isopropylalcohol–water mixture by vapor permeation, ACS Appl. Mater. Interfaces 7 (2015) 13728–13730. [2] J. Zuo, D. Hua, V. Maricar, Y.K. Ong, T.-S. Chung, Dehydration of industrial isopropanol (IPA) waste by pervaporation and vapor permeation membranes, J. Appl. Polym. Sci. 135 (2017) 45086. [3] K. Okamoto, H. Kita, K. Horii, K. Tanaka, M. Kondo, Zeolite NaA membrane: Preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures, Ind. Eng. Chem. Res. 40 (2001) 163–175. [4] C. Chen, Y. Cheng, L. Peng, C. Zhang, Z. Wu, X. Gu, X. Wang, S. Murad, Fabrication and stability exploration of hollow fiber mordenite zeolite membranes for isopropanol/water mixture separation, Microporous Mesoporous Mater. 274 (2019) 347–355. [5] Y. Liu, Z. Yang, C. Yu, X. Gu, N. Xu, Effect of seeding methods on growth of NaA zeolite membranes, Microporous Mesoporous Mater. 143 (2011) 348–356. [6] Z. Yang, Y. Liu, C. Yu, X. Gu, N. Xu, Ball-milled NaA zeolite seeds with submicron size for growth of NaA zeolite membranes, J. Membr. Sci. 392 (2012) 18–28. [7] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Separ. Purif. Technol. 25 (2001) 251–260. [8] C. Zhang, L. Peng, J. Jiang, X. Gu, Mass transfer model, preparation and applications of zeolite membranes for pervaporation dehydration: a review, Chin. J. Chem. Eng. 25 (2017) 1627–1638. [9] X. Wang, Y. Chen, C. Zhang, X. Gu, N. Xu, Preparation and characterization of highflux T-type zeolite membranes supported on YSZ hollow fibers, J. Membr. Sci. 455 (2014) 294–304. [10] J. Jiang, X. Wang, L. Peng, X. Wang, X. Gu, Batch-scale preparation of hollow fiber supported CHA zeolite membranes and module for solvents dehydration, Microporous Mesoporous Mater. 250 (2017) 18–26. [11] S.G. Sorenson, E.A. Payzant, W.T. Gibbons, B. Soydas, H. Kita, R.D. Noble, J. L. Falconer, Influence of zeolite crystal expansion/contraction on NaA zeolite membrane separations, J. Membr. Sci. 366 (2011) 413–420. [12] F. Qu, R. Shi, L. Peng, Y. Zhang, X. Gu, X. Wang, S. Murad, Understanding the effect of zeolite crystal expansion/contraction on separation performance of NaA zeolite membrane: a combined experimental and molecular simulation study, J. Membr. Sci. 539 (2017) 14–23. [13] Z. Hong, C. Zhang, X. Gu, W. Jin, N. Xu, A simple method for healing nonzeolitic pores of MFI membranes by hydrolysis of silanes, J. Membr. Sci. 366 (2011) 427–435. [14] B. Zhang, C. Wang, L. Lang, R. Cui, X. Liu, Selective defect–patching of zeolite membranes using chemical liquid deposition at organic/aqueous interfaces, Adv. Funct. Mater. 18 (2008) 3434–3443. [15] H. Matsuda, H. Yanagishita, H. Negishi, D. Kitamoto, T. Ikegami, K. Haraya, T. Nakane, Y. Idemoto, N. Koura, T. Sano, Improvement of ethanol selectivity of silicalite membrane in pervaporation by silicone rubber coating, J. Membr. Sci. 210 (2002) 433–437. [16] X. Gu, Z. Tang, J. Dong, On-stream modification of MFI zeolite membranes for enhancing hydrogen separation at high temperature, Microporous Mesoporous Mater. 111 (2008) 441–448. [17] Z. Tang, J. Dong, T.M. Nenoff, Internal surface modification of MFI-type zeolite membranes for high selectivity and high flux for hydrogen, Langmuir 25 (2009) 4848–4852. [18] Z. Hong, Z. Wu, Y. Zhang, X. Gu, Catalytic cracking deposition of methyldiethoxysilane for modification of zeolitic pores in MFI/α-Al2O3 zeolite membrane with Hþ ion exchange pretreatment, Ind. Eng. Chem. Res. 52 (2013) 13113–13119. [19] Y. Hirota, K. Watanabe, Y. Uchida, Y. Egashira, K. Yoshida, Y. Sasaki, N. Nishiyam, Coke deposition in the SAPO-34 membranes for examining the effects of zeolitic and non-zeolitic pathways on the permeation and separation properties in gas and vapor permeations, J. Membr. Sci. 415–416 (2012) 176–180. [20] D. Korelskiy, P. Ye, M.S. Nabavi, J. Hedlund, Selective blocking of grain boundary defects in high-flux zeolite membranes by coking, J. Mater. Chem. 5 (2017) 7295–7299. [21] Y. Zhang, A.M. Avila, B. Tokay, H.H. Funke, J.L. Falconer, R.D. Noble, Blocking defects in SAPO-34 membranes with cyclodextrin, J. Membr. Sci. 358 (2010) 7–12. [22] J.N. Coleman, M. Lotya, 1 A. O’Neill, S.D. Bergin, P.J. King, U. Khan, K. Young, A. Gaucher, S. De, R.J. Smith, I.V. Shvets, S.K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G.T. Kim, G.S. Duesberg, T. Hallam, J.J. Boland, J.J. Wang, J.F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R.J. Nicholls, J.M. Perkins, E.M. Grieveson, K. Theuwissen, D.W. McComb, P.D. Nellist, V. Nicolosi, Two-dimensional nanosheets produced by liquid exfoliation of layered materials, Science 331 (2011) 568–571. [23] M. Chhowalla, Z. Liu, H. Zhang, Two-dimensional transition metal dichalcogenide (TMD) nanosheets, Chem. Soc. Rev. 44 (2015) 2584–2586. [24] H. Zhang, Ultrathin two-dimensional nanomaterials, ACS Nano 9 (2015) 9451–9469. [25] E.S. Philippe, K. Chow, B.C. Viana, J. Gao, J. Luo, J. Li, Z. Lin, A.L. Elias, Y. Shi, Z. Wang, M. Terrones, N. Koratkar, Wetting of mono and few-layered WS2 and MoS2 films supported on Si/SiO2 substrates, ACS Nano 9 (2015) 3023–3031. [26] X. Peng, L. Peng, C. Wu, Y. Xie, Two dimensional nanomaterials for flexible supercapacitors, Chem. Soc. Rev. 43 (2014) 3303–3323.
Fig. 15. VP separation results of NaA zeolite membrane before and after modified with WS-I in dehydration of water/ethanol mixtures under different feed water contents at 373 K.
water fluxes. 3.4.3. VP dehydration of ethanol Not only IPA, such modification method was also adopted to modify a new NaA zeolite membrane for VP dehydration of low water content ethanol. As shown in Fig. 15, the water/ethanol separation factor of membrane was improved from 715 to >10000 while the water flux declined from 7.4 to 6.1 kg m-2 h-1 in dehydration of 10 wt% water/ ethanol mixture at 373 K. When the feed water content lowered to 0.5 wt %, the modified membrane still possessed a very high separation factor of ~9500. We believe that the modification strategy can be applied broadly to other hydrophilic zeolite membranes for deep dehydration of different organic solvents. 4. Conclusions A facile and high-efficiency modification strategy was proposed to block defects of NaA zeolite membrane with WS2 nanosheets for VP dehydration of low water content IPA. Owing to good wettability and high flexibility of WS2 nanosheets, the hydrophilicity of membrane surface was improved after modification. Not only intrinsic defects but also the defects induced by crystal contraction at low feed water content were blocked. As a result, the separation factor of membrane was enhanced more than two orders of magnitude while the loss in water flux was only 18.6%. Furthermore, the modified membrane still behaved a high separation factor even when the feed water content lowered to 0.5 wt%. Such achieved results suggest great potential to develop defectfree zeolite membranes with high reproducibility for deep dehydration of organic solvents in industry. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work is sponsored by the National Natural Science Foundation of China (21490585 and 21776128), the “333 Talent Project” and Young Fund (BK20170132) of Jiangsu Province, State Key Laboratory of Ma terials- Oriented Chemical Engineering (ZK201602). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 9
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[27] L. Sun, Y. Ying, H. Huang, Z. Song, Y. Mao, Z. Xu, X. Peng, Ultrafast molecule separation through layered WS2 nanosheet membranes, ACS Nano 8 (2014) 6304–6311. [28] D. Liu, Y. Zhang, J. Jiang, X. Wang, C. Zhang, X. Gu, High-performance NaA zeolite membranes supported on four-channel ceramic hollow fibers for ethanol dehydration, RSC Adv. 5 (2015) 95866–95871. [29] T.P. Nguyen, S. Choi, J.-M. Jeon, K.C. Kwon, H.W. Jang, S.Y. Kim, Transition metal disulfide nanosheets synthesized by facile sonication method for the hydrogen evolution reaction, J. Phys. Chem. C 120 (2016) 3929–3935. [30] T.P. Nguyen, W. Sohn, J.H. Oh, H.W. Jang, S.Y. Kim, Size-dependent properties of two-dimensional MoS2 and WS2, J. Phys. Chem. C 120 (2016) 10078–10085. [31] J. Hedlund, D. Korelskiy, L. Sandstr€ om, J. Lindmark, Permporometry analysis of zeolite membranes, J. Membr. Sci. 345 (2009) 276–287. [32] J.B. Lee, H.H. Funke, R.D. Noble, J.L. Falconer, High selectivities in defective mfi membranes, J. Membr. Sci. 321 (2008) 309–315. [33] M. Yu, J.L. Falconer, Richard D. Noble, Characterizing nonzeolitic pores in MFI membranes, Ind. Eng. Chem. Res. 47 (2008) 3943–3948. [34] M. Noack, P. K€ olsch, A. Dittmar, M. St€ ohr, G. Georgi, M. Schneider, U. Dingerdissen, A. Feldhoff, J. Caro, Proof of the ISS-concept for LTA and FAU
[35]
[36] [37]
[38] [39]
10
membranes and their characterization by extended gas permeation studies, Microporous Mesoporous Mater. 102 (2007) 1–20. Y. Pi, B. Liu, Z. Li, Y. Zhu, Y. Li, F. Zhang, G. Zhang, W. Peng, X. Fan, TiO2 nanorod arrays decorated with exfoliated WS2 nanosheets for enhanced photoelectrochemical water oxidation, J. Colloid Interface Sci. 545 (2019) 282–288. T. Lei, W. Chen, J. Huang, C. Yan, H. Sun, C. Wang, W. Zhang, Y. Li, J. Xiong, Multi-functional layered WS2 nanosheets for enhancing the performance of lithium–sulfur batteries, Adv. Energ. Mater. 7 (2017) 1601843. J. Yang, D. Voiry, S.J. Ahn, D. Kang, A.Y. Kim, M. Chhowalla, H.S. Shin, Twodimensional hybrid nanosheets of tungsten disulfide and reduced graphene oxide as catalysts for enhanced hydrogen evolution, Angew. Chem. Int. Ed. 52 (2013) 13751–13754. M.A. Lukowski, A.S. Daniel, C.R. English, F. Meng, A. Forticaux, R.J. Hamers, S. Jin, Highly active hydrogen evolution catalysis from metallic WS2 nanosheets, Energy Environ. Sci. 7 (2014) 2608–2613. X. Wang, C. Chi, J. Tao, Y. Peng, S. Ying, Y. Qian, J. Dong, Z. Hu, Y. Gu, D. Zhao, Improving the hydrogen selectivity of graphene oxide membranes by reducing nonselective pores with intergrown ZIF-8 crystals, Chem. Commun. 52 (2016) 8087–8090.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Blocking defects of zeolite membranes with WS2 nanosheets for vapor permeation dehydration of low water content isopropanol Yuting Zhang a, Peng Du a, Rui Shi a, Zhou Hong b, Xinfeng Zhu a, Bing Gao a, Xuehong Gu a, * a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, 211816, Jiangsu, China b Nanjing Membrane Materials Industrial Technology Research Institute Co, Ltd, Nanjing, 211800, Jiangsu, China
A R T I C L E I N F O
A B S T R A C T
Keywords: WS2 nanosheets Zeolite membrane Defect Vapor permeation dehydration
It is difficult for zeolite membrane to behave good separation selectivity in dehydration of low water content isopropanol (IPA) by vapor permeation (VP) because capillary condensation in small intercrystalline defects is usually insufficient for excluding IPA molecules and adsorbate-induced contraction of zeolite crystals open new defects at low water content. To address this issue, a facile and high-efficiency modification strategy was pro posed to block defects of NaA zeolite membranes on hollow fibers by using WS2 nanosheets. Effect of nanosheets thickness on VP separation performance of modified membranes was investigated. It showed that the separation factor of membrane was improved by more than two orders of magnitude (from 60 to >10000) after modified with the few-layer WS2 nanosheets while the loss in water flux was only 18.6% in VP dehydration of 10 wt% water/IPA mixture at 373 K. Moreover, the modified membrane still maintained a high separation factor of ~5000 rather than that of pristine membrane (336) even when the feed water content lowered to 0.5 wt%, indicating that not only intrinsic defects but also the crystal contraction-induced defects were blocked simul taneously by the thin WS2 nanosheets.
1. Introduction Isopropanol (IPA) is an important solvent and cleaning medium widely used in chemical, electronics, and semiconductor industries. Dehydration of IPA is a significant process in production and recycling of IPA. It is complicated and energy-intensive when using conventional dehydration processes because IPA and water forms an azeotropic mixture when the water content is about 14% [1]. Alternatively, membrane-based separation processes like pervaporation (PV) and vapor permeation (VP) show advantages of low energy consumption, zero emission and simple operation for liquid separation especially for azeotropic and close-boiling mixtures. The main difference between PV and VP is the state of feed mixture. For energy utilization, VP mode is often adopted followed with distillation where the feed mixture is in vapor state [2] Among the VP membranes, zeolite membranes (e.g. LTA, FAU, MOR membranes) have been attractive for their good separation performance and robust thermal stability [1,3,4]. Particularly, sodium LTA (NaA) zeolite membrane has strong hydrophilicity and well-defined pore size of 0.42 nm, which is suitable for dehydration of IPA [5–8]. Due to polycrystalline structure, zeolite membrane usually
inevitably contains intercrystalline defects that are larger than its zeolitic pores. The existence of any defect affects its inherent molecularsieving property. It was found that although NaA zeolite membrane had some small defects, the capillary condensation of water molecules in these defects was still able to exclude organic solvent molecules from entering the defects when the feed mixture had enough water content [3]. However, once the feed water content was lowered to a certain degree (<5 wt%) especially by VP at high temperature, the capillary condensation in defects was always insufficient. In that case, large organic solvent molecules would pass through the defects and then the separation selectivity fell a lot. Such phenomenon was also observed on other zeolite membranes for dehydration of low water content organic solvents [4,9,10]. Another significant concern is the crystal flexibility of NaA zeolite membrane related to its adsorption amount of water molecules, which results in shrinkage/enlargement behaviors of intercrystalline defects [11]. For example, when the membrane was used in VP dehydration of IPA, it behaved good separation factor at high feed water content because large adsorption amount of water molecules in zeolitic pores expanded zeolite crystals, which therefore shrank the defects. But, once
* Corresponding author. E-mail address: [email protected] (X. Gu). https://doi.org/10.1016/j.memsci.2019.117625 Received 7 July 2019; Received in revised form 17 October 2019; Accepted 30 October 2019 Available online 2 November 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yuting Zhang, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117625
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Fig. 1. Schematic diagram of WS2 nanosheets modification on NaA zeolite membrane.
the feed water content lowered to below 5 wt%, the separation factor fell sharply because low adsorption amount of water molecules in zeolitic pores contracted the crystals, which enlarged the defects consequently. Such variation in crystal size of NaA zeolite membrane makes it difficult to be used in dehydration of low water content organic solvents such as IPA and n-butanol despite that the membrane is defect-free [12]. Until now, many post-treatment methods have been developed to minimize defects of zeolite membranes, such as chemical liquid or fluid deposition (CLD or CFD) [13,14], surface coating [15], chemical vapor deposition (CVD) [16–18], coke deposition [19,20] and so on. Most of them improved separation selectivity efficiently but also caused large decline in permeation flux because zeolitic pores of membranes were simultaneously blocked during the treatment. In rare cases, the treat ment did not result in considerable reduction in membrane permeance, but it was conducted on the membrane that had a low initial permeance [21]. Recently, two-dimensional nanosheets of transitional metal disulfides (TMDs), such as MoS2, WS2, TiS2, MoSe2, have been an emerging class of nanomaterials [22–24]. Similar to graphene oxide nanosheets, WS2 nanosheets have lamellar structure with single or few layers atomic thickness. With virtues of nanometer size, good wettability and high flexibility, WS2 nanosheets can also be constructed as an effective barrier for water filtration [25–27]. Herein, we proposed a facile and high-efficiency modification strategy to block intercrystalline defects of NaA zeolite membranes on hollow fibers by using WS2 nanosheets. The membrane qualities were evaluated by VP dehydration of water/IPA mixture under the feed water content ranging from 10 to 0.5 wt%. Effect of nanosheets thickness on separation performance of modified membranes was investigated. Both pristine and modified membranes were characterized extensively in terms of SEM, XRD, dynamic contact angle, permporosimetry and XPS for comparison.
and N-vinylpyrrolidone (NVP, �99%) were purchased from Sigma-Aldrich. An ultrasonicator (Scientz-IID, Ningbo Scientz Biotech, China) was used to exfoliate the powders. 30 mg of WS2 powders were mixed with 10 mL of NVP in a 50 mL flask. The solution was ultra sonicated for 4 h at 400 W. After ultrasonication, the samples were centrifuged for 10 min at 10000 rpm. The top layer, which contained the exfoliated WS2 layers, was pipetted off, and the NVP was removed by washing with IPA. The final products were dried at 303 K in a vacuum oven, and the obtained nanosheets were redispersed in IPA solvent for further measurement and membrane modification. The concentration of WS2 nanosheets in the dispersion was about 0.2 mg mL-1. 2.3. Membrane modification NaA zeolite membranes on hollow fibers were modified by vacuum inhalation method using as-prepared WS2 nanosheets/IPA dispersion. The pristine membrane was mounted in a stainless steel module and both ends of it were sealed with silicon O-rings. The inner side of membrane was evacuated by a vacuum pump through a vacuum line, which maintained a downstream pressure below 200 Pa. The outer surface of membrane was fed with a circular stream pumped from a tank containing 50 mL WS2 nanosheets/IPA dispersion. The temperature of the dispersion was kept at 313 K. After vacuumed for a certain time (1–8 h), the membrane was dried in an oven at 343 K for 1 h and then tested in VP separation. Fig. 1 illustrates the schematic diagram of WS2 nanosheets modification on NaA zeolite membrane. When the mem brane was immersed in the dispersion, zeolite crystals of the membrane contracted in the IPA solvent which contained trace amount of water [12]. Under the force of vacuum inhalation, both intrinsic and crystal contraction-induced defects of the membrane were pasted by WS2 nanosheets.
2. Experimental
2.4. Membrane characterization
2.1. Synthesis of NaA zeolite membranes
The morphologies of the membranes were examined via FESEM (S4800, Hitachi, Japan). The working parameters were a voltage (HV) of 5 kV and a work distance (WD) of 8 mm. With EDX spectroscopy, the working parameters were a voltage (HV) of 20 kV and a WD of 15 mm. The crystal phases of the samples were determined by XRD with Cu Kα radiation (Bruker, model D8 Advance). The size and surface morphology of WS2 nanosheets were obtained by AFM (XE-100, Park SYSTEMS, Korea) and transmission electron microscopy (TEM, JEOL-2100F, Japan). The dynamic contact angles of water were analyzed using a contact-angle Dropmeter (A100P, MAIST Vision Inspection and Mea surement Co., Ltd.). X-ray photoemission spectroscopy (XPS) spectra were measured using a Thermo ESCALAB 250Xi surface analysis system. Permporosimetry test was conducted to characterize the distribution of defects in NaA zeolite membrane [31–34]. The detailed test system was described in our previous publication [12]. Prior to the permpor osimetry test, the membrane mounted in a stainless steel module was heated at 383 K for 24 h in a flowing helium stream to remove any adsorbate in zeolitic and nonzeolitic pores. The operating temperature
NaA zeolite membranes were hydrothermally synthesized on outersurface of α-Al2O3 hollow fibers by the secondary growth method as described in our previous work [28]. Prior to membrane synthesis, ball-milled NaA zeolite seeds (average particle size: ~220 nm) were planted on external surface of the supports by dip-coating method. The synthesis gel was prepared by dissolving sodium aluminate, sodium hydroxide and water glass into deionized water with the molar ratio of 1Al2O3: 2SiO2: 2Na2O: 120H2O. All the chemicals were industrial grade and purchased from commercial companies in China. The hydrothermal crystallization was performed at 373 K for 4 h. 2.2. Preparation of WS2 nanosheets WS2 nanosheets were prepared by the liquid sonication exfoliation method as described by Nguyen et al. [29,30], which is feasible for large-scale production. WS2 powders (99%, average particle size: 2 μm) 2
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Fig. 2. Schematic diagram of experimental apparatus for VP dehydration separation.
was then fixed at 298 K for permporosimetry experiment. Dried helium was used as the noncondensable gas and vapor was employed as the condensable gas. Water was loaded in a saturator for producing vapor. The feed stream was obtained by mixing a pure helium stream with another helium stream saturated with vapor. The vapor relative pressure (the ratio of partial vapor pressure to its saturation pressure, P/Psat) was varied by adjusting the ratio of the two helium flows. The feed pressure was maintained at 201 kPa and the permeate pressure was kept at at mospheric pressure (101 kPa). The defect width in NaA zeolite mem brane was calculated from vapor relative pressure using Kelvin equation and Halsey adjusted for the adsorbed layer thickness (t) as described by Noack et al. [34].
separation factor (α), which were respectively defined as follows:
2.5. VP dehydration separation
3.1. WS2 nanosheets characterization
VP separation performance of NaA zeolite membranes were inves tigated with dehydration of binary mixtures (IPA/water and ethanol/ water). The VP experiment was reported in our previous work [12]. The tested membrane had an effective membrane area of ca. 6 cm2. Feed steam was continuously pumped into the shell side of a membrane module and the permeate product was removed from the lumen by a vacuum pump, which maintained a downstream pressure below 300 Pa. The permeated vapor was collected with a cold trap cooled by liquid nitrogen (Fig. 2). Both of samples at the feed and permeate sides were analyzed by a gas chromatograph (GC-2014A, Shimadzu) equipped with a thermal conductivity detector and a packed column of Parapak-Q. The membrane performance was determined by permeation flux (J) and
Fig. 3a shows the photograph of as-prepared WS2 nanosheets dispersed in IPA solvent. The dispersion is clear and stable for over hundreds of hours. We also performed high-resolution transmission electron microscopy (HRTEM) on the dispersion. It can also be seen from Fig. 3b that the sizes of nanosheets are 50–70 nm. The selected-area electron diffraction (SAED) pattern shown in Fig. 3c confirms that the as-prepared WS2 nanosheets exhibit hexagonal lattice structure, indi cating that the sonication process did not degrade the hexagonal struc ture of WS2. The lattice constant of WS2 measured from TEM images is 0.32 nm, which is consistent with those in other literatures [29,30]. Fig. 4 shows XRD patterns of WS2 powders and exfoliated WS2
m A⋅t
(1)
� yi yj αi=j ¼ � xi xj
(2)
J¼
Where m is the total mass of the permeate product, kg; A is the effective area of the membrane, m2, t is the elapsed time, h; yi/yj is the weight fraction ratio of water over alcohol in the permeate and xi/xj is the corresponding ratio in the feed. 3. Results and discussion
Fig. 3. Photograph, HRTEM and SAED images of WS2 nanosheets. 3
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water content occurred on the membrane which was modified with WSIII. The permeate water content of membrane reached to about 97.5 wt% in 3 h and then kept it stably. On the other side, the water flux decreased most largely from 12.5 to 7.4 kg m-2 h-1. When it comes to the membrane that was modified with WS-II, the permeate water content was improved to 97.5 wt% in 8 h while the water flux decreased moderately from 10.7 to 7.8 kg m-2 h-1. When the membrane was modified with WS-I, the permeate water content increased most slowly, but it achieved to nearly 100 wt% finally after modified for 9 h (the corresponding separation factor was >10000), indicating that all of non-selective defects of the membrane had been blocked with the thinnest WS2 nanosheets. At the same time, it is very interesting to find that the water flux decreased only by 18.6% from 10.2 to 8.3 kg m-2 h-1. Since such modification method is simple and controllable, it can be also seen from Table 1 that a good reproducibility was obtained on separation improvement of other hol low fiber supported NaA zeolite membranes. Fig. 7 shows surface and cross-section SEM images of pristine and different WS2 nanosheets modified NaA zeolite membranes. The modi fication time of modified membranes agreed with that shown in Fig. 6. It is observed from Fig. 7a that the pristine membrane had a wellintergrown zeolite layer with uniform thickness of ~7 μm. The mem brane surface appears to be free of defects such as cracks and pinholes, but grain boundaries are clearly seen. After modified with WS-I, the membrane surface was coated with small scale-like WS2 nanosheets. The EDX map scan results in Fig. 8 confirm that WS2 nanosheets were distributed on membrane surface evenly. The insert figure in Fig. 7c displays that many WS2 nanosheets aggregated near grain boundaries of the membrane as the vacuum inhalation force was stronger there during the modification. The thickness of modified membrane shown in Fig. 7b was similar to that of pristine membrane, no element of tungsten and sulfur was even detected on the cross section of modified membrane in the EDX analysis, indicating that the thickness of coated WS2 nanosheets was very thin. We suggest that the ultrathin lamellar WS2 nanosheets, like GO nanosheets, were also able to provide fast paths for water transport [27], so the coating of ultrathin WS2 nanosheets on membrane surface did not cause large loss in water flux. It is seen from Fig. 7e–h that thickness of WS2 nanosheets coating on membrane surface was more conspicuous after modified with larger and thicker nanosheets, which resulted in larger loss in water flux. On the other side, there were also some agglomerated particles, formed from the folding of WS2 nanosheets, on the surfaces of modified membranes especially that was modified with WS-III. The large agglomerated particles might detach from the membrane surface when the vapor stream flowed around the membrane surface during the VP separation. In addition, the WS-II and WS-III with a lot of layers were more rigid than WS-I, which might not be adhered to membrane surfaces closely and block small defects completely. As a result, the separation factors were improved less notably than that for WS-I. Fig. 9 shows XRD patterns of pristine and different modified NaA zeolite membranes. As consistent with observation in Fig. 7, the XRD pattern of pristine NaA zeolite membrane matched well with the
Fig. 4. XRD patterns of WS2 powders and nanosheets.
nanosheets. The WS2 powders exhibit numerous peaks, indicating that they have many layers. After sonication and removal of large WS2 ma terials, the nanosheets only exhibit one weaker (002) peak. 3.2. Membrane modification The as-prepared WS2 nanosheets (denoted as WS-I) dispersed in IPA solvent was further used for membrane modification. For comparison, another two types of WS2 nanosheets with different thicknesses were prepared via sonication treatment at 100 W. After centrifuged, the su pernatant was collected as nanosheets with medium thickness (denoted as WS-II). Meanwhile, the sediment was collected as the thickest WS2 nanosheets (denoted as WS-III). Fig. 5 shows their thicknesses and lateral sizes measured by atomic force microscopy (AFM). It is seen that the WS-I had the thinnest thickness of 1.6–2.3 nm, indicating the fewlayer structure of WS2 nanosheets. Its lateral size was also the smallest of ~50 nm. By contrast, the thicknesses of WS-II and WS-III were 42 (~60 layers) and 100–170 nm (143–243 layers) respectively. And their lateral sizes were as large as 200 nm and 700 nm, respectively. The WS2 nanosheets/IPA dispersions were further used to modify three NaA zeolite membranes on hollow fibers with similar quality for comparison. The concentrations of WS2 nanosheets in dispersions were controlled at 0.2 mg mL-1. Fig. 6 shows VP separation results of the membranes as a function of modification time in dehydration of 10 wt% water/IPA mixture at 373 K. Although the pristine membranes exhibited high separation factors of >5000 in PV dehydration of 10 wt% water/ ethanol mixture at 348 K, their permeate water contents in VP dehy dration of water/IPA mixture were only 88 wt% (namely the corre sponding separation factors were about 60). As the modification time increased, all the permeate water contents climbed up initially and then tended to plateau. It is observed that the fastest increment in permeate
Fig. 5. AFM images of different WS2 nanosheets: (a) WS-I, (b) WS-II and (c) WS-III. 4
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characteristic peaks of NaA zeolite besides of α-Al2O3 signals of the support. A very weak WS2 nanosheets (002) peak was observed on the WS-I/NaA composite membrane. As the WS2 nanosheets were larger and thicker, the intensities of NaA zeolite characteristic peaks in composite membrane decreased gradually while that of WS2 nanosheets (002) peak became more obvious. Particularly, the characteristic peaks of NaA zeolite nearly vanished in the WS-III/NaA composite membrane. Instead, two strong WS2 characteristic peaks were observed at 14� and 33� respectively, indicating that the coated WS-III still contained many multi-crystalline WS2 particles. 3.3. Membrane characterization In order to explore the role of WS-I in membrane modification, the dynamic contact angle was first tested on both pristine and modified NaA zeolite membranes. As shown in Fig. 10, the final steady contact angle of membrane surface decreased from 76.6� to 72.1� after modifi cation, indicating the improvement in hydrophilicity of membrane surface resulted by the coating of highly wettable WS2 nanosheets. It is also noted that the dynamic contact angles of modified membrane declined slower than those of pristine membrane. Such result could be owed to the effective elimination in defects of the membrane after modification, which slowed down the penetration of water droplet into the modified membrane. Generally, it is difficult to observe microporous and small meso porous defects in zeolite membrane from conventional SEM image, permporometry technique is a useful way to illustrate the distribution of defects in zeolite membrane. In this technique, helium permeance through the membrane is measured as a function of vapor relative pressure (P/Psat) in the feed. As P/Psat increases, the vapor diluted in helium flow condensates first in zeolitic pores and then in increasing larger defects, therefore the helium permeance decreases gradually. As shown in Fig. 11, for the pristine NaA zeolite membrane, the helium permeance at P/Psat ¼ 0 was 2.38 � 10-7 mol m-2 s-1 Pa-1, which repre sented the permeance through both zeolitic pores and defects [31]. At higher P/Psat of about 0.03 where zeolitic pores of the membrane had been filled with condensed water, the helium permeance represented the flow through the defects which were larger than zeolitic pores. The further decrease in helium permeance in the range of P/Psat from 0.03 to 0.76 indicated that the increasing larger defects in the membrane were gradually blocked. After modification, the helium permeance of mem brane at P/Psat ¼ 0 was lowered to 4.5 � 10-9 mol m-2 s-1 Pa-1. The heli um permeance at P/Psat ¼ 0.03 that represented the flow through defects reduced from 1.82 � 10-7 to 2.4 � 10-9 mol m-2 s-1 Pa-1. The helium permeances at higher P/Psat were nearly zero, confirming that most of nano-sized defects in the membrane had been blocked by WS2 nano sheets. By comparison, the helium permeances of WS-II and WS-III modified NaA zeolite membranes at P/Psat ¼ 0.11 are 7.7 � 10-9 and 1.8 � 10-8 mol m-2 s-1 Pa-1 respectively (Fig. S1 in Supporting informa tion), suggesting that they still had more or larger nano-sized defects. Unlike helium permeance, the loss in water flux of membrane in VP separation was only a little after modification. We suggest that such difference was probably due to that the stacking WS2 nanosheets on zeolite membrane swelled slightly under humidity during the VP sepa ration, which provided fast and highly-selective paths for water diffu sion and thus avoided large loss in water flux. In contrast, helium gas was excluded by the narrow spacing distance between WS2 nanosheets and zeolite membrane as it was nearly non-adsorptive and hardly able to swell the nanosheets. The XPS spectra in Fig. 12 also confirms the adhesion of WS2 nano sheets on membrane surface. For the spectra of WS-I, two predominant peaks of W4f7/2 and W4f5/2 at 33.1 and 35.2 eV (Fig. 12a) corresponding to the 2H phase are observed [35]. The weak and broad W5p3/2 peak at 38.7 eV is attributed to the slight oxidation of WS2 nanosheets [36]. The doublet peaks of S2p3/2 and S2p1/2 (Fig. 12b) located at 162.7 and 163.9 eV are assigned to divalent sulfide ions (S2 ) [37]. After coated on
Fig. 6. VP results as a function of modification time of different NaA zeolite membranes modified with WS-I, WS-II and WS-III: (a) permeate water content, (b) separation factor and (c) water flux.
Table 1 VP separation results of different hollow fiber supported NaA zeolite membranes before and after modified with WS-I in VP dehydration of 10 wt% water/IPA mixture at 373 K. Membranes M1 M2 M3
Before modification
After modification
J (kg m-2 h-1)
α
J (kg m-2 h-1)
α
10.2 9.7 9.9
60 643 2307
8.3 8.3 8.6
>10000 >10000 >10000
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Fig. 7. Surface and cross-section SEM images of (a and b) pristine and different modified NaA zeolite membranes: (c and d) WS-I, (e and f) WS-II and (g and h) WS-III.
the surface of NaA zeolite membrane, the W4f peaks shifted to lower binding energies (31.1 and 34.7 eV), which could be owed to the 2H to 1T phase transition of WS2 nanosheets [38]. Similar shift in binding energy also occurred on the S2p spectra, implying the electronic inter action between WS2 nanosheets and NaA zeolite membrane [35]. Furthermore, a new broad peak appeared at 168.4 eV, suggesting the presence of SO24 or S2O23 . It could be produced via the oxidation of sulfur in air [36] or the sharing S–O bonds between WS2 nanosheets and hydroxide groups of zeolite membrane [39]. Fig. S2 in Supporting in formation shows XPS survey spectra of pristine and WS-I modified NaA zeolite membranes. The corresponding binding energies and surface ratio results are listed in Table S1 in Supporting information. It is seen that the Na1s, Al2p peaks of NaA zeolite membrane shift to lower en ergies after modification. Such shift might indicate the partial substi tution of O by S that is less electronegative. The O1s shift from 531.84 to 532.12 eV demonstrates that some of O–H and O–Na bonds might be
took place with O–W bonds, confirming the strong electronic interaction between WS2 nanosheets and zeolite membrane. On the other side, the reduction in Na/Si surface ratio could be caused by the dissolving of Na cations in the WS2 nanosheets dispersion from zeolite membrane. Meanwhile, the increase in Al/Si surface ratio could be explained by the interference of Al element from the hollow fiber support. 3.4. VP dehydration separation 3.4.1. VP dehydration of low water content IPA Both pristine and WS-I modified NaA zeolite membranes were car ried out into VP dehydration of low water content IPA for comparison. As shown in Fig. 13a, the water content in feed mixture decreased continuously from ~10 to 0.5 wt% due to the selective removal of water through the membrane. At the same time, the water flux declined gradually due to the continuous reduction in driving force of water 6
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Fig. 8. Surface elemental mapping images of WS-I modified NaA zeolite membrane.
Fig. 10. Dynamic contact angles of pristine and WS-I modified NaA zeolite membranes.
Fig. 9. XRD patterns of pristine and different WS2 nanosheets modified NaA zeolite membranes.
decreased more obviously from 10.4 to 7.3 kg m-2 h-1 after modification while the separation factor was improved only a little from 611 to 1310 (Fig. 13b). When the feed water content lowered from 10 to 0.5 wt%, the separation factor of modified membrane declined more than a half from 1310 to 502, indicating that the coating of rigid WS2 powders might also have blocked some large defects in the membrane but failed to restrain the small defects induced by crystal contraction at low water content.
permeation. For the pristine membrane, the water/IPA separation factor fell from 643 to only 336, which was mainly caused by crystal contraction at low feed water content as mentioned above. By contrast, the modified membrane still maintained a high separation factor of ~5000 at the low feed water content of 0.5 wt%, indicating that the coating of WS2 nanosheets on membrane surface had blocked the small defects induced by crystal contraction. On the other hand, it is inter esting to find that only a little loss in water flux occurred on the modified membrane in the whole range of feed water content. As shown in Fig. 7b, grain boundaries of the modified membrane were filled with small WS2 nanosheets. When the feed water content was very low and zeolite crystals contracted, the highly flexible WS2 nanosheets were attached to defects more closely under the enhanced capillary force in enlarged defects, so the IPA molecules were still excluded [26]. In order to demonstrate it more clearly, WS2 powders were dispersed in IPA solvent with similar content and used to modify another NaA zeolite membrane for comparison. It can be seen that the water flux of membrane
3.4.2. Separation stability The WS-I modified NaA zeolite membrane was further tested in VP dehydration of 10 wt% water/IPA mixture at 373 K for over 50 h. As shown in Fig. 14, the water flux and separation factor maintained at about 8.2 kg m-2 h-1 and >10000 respectively during the whole period, indicating that the modified membrane had good long-term separation stability. We consider that the thin WS2 nanosheets were suppressed on membrane surface firmly under the high vapor pressure on the feed side. Furthermore, according to the XPS results, the nanosheets were adhered stably onto membrane surface for the strong electronic interaction 7
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Fig. 11. Permporosimetry results of pristine and WS-I modified NaA zeolite membranes.
Fig. 13. VP separation results of NaA zeolite membrane before and after modified with (a) WS-I and (b) WS2 powders in dehydration of water/IPA mixtures under different water contents at 373 K.
Fig. 14. VP separation stability of WS-I modified NaA zeolite membrane in dehydration of 10 wt% water/IPA mixture at 373 K.
between them, which ensured the membrane to behave good and stable separation performance. In contrast with WS-I, the separation factors of WS-II and WS-III modified membranes decreased by about 16% and 22% respectively after continuous VP dehydration separation for 10 h (Fig. S3 in Supporting information). Simultaneously, their water fluxes increased by about 0.6% and 17% respectively. The WS-II and WS-III are expected to have weaker adhesion with NaA zeolite membranes due to their much higher thicknesses. When the membranes were used in VP separation, some of them might peel off from membrane surface and thus led to the decline in separation factors as well as the increase in
Fig. 12. XPS spectra of (a) W 4f and (b) S 2p for WS-I and WS-I modified NaA zeolite membrane.
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org/10.1016/j.memsci.2019.117625. References [1] K. Sawamura, T. Furuhata, Y. Sekine, E. Kikuchi, B. Subramanian, M. Matsukata, Zeolite membrane for dehydration of isopropylalcohol–water mixture by vapor permeation, ACS Appl. Mater. Interfaces 7 (2015) 13728–13730. [2] J. Zuo, D. Hua, V. Maricar, Y.K. Ong, T.-S. Chung, Dehydration of industrial isopropanol (IPA) waste by pervaporation and vapor permeation membranes, J. Appl. Polym. Sci. 135 (2017) 45086. [3] K. Okamoto, H. Kita, K. Horii, K. Tanaka, M. Kondo, Zeolite NaA membrane: Preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures, Ind. Eng. Chem. Res. 40 (2001) 163–175. [4] C. Chen, Y. Cheng, L. Peng, C. Zhang, Z. Wu, X. Gu, X. Wang, S. Murad, Fabrication and stability exploration of hollow fiber mordenite zeolite membranes for isopropanol/water mixture separation, Microporous Mesoporous Mater. 274 (2019) 347–355. [5] Y. Liu, Z. Yang, C. Yu, X. Gu, N. Xu, Effect of seeding methods on growth of NaA zeolite membranes, Microporous Mesoporous Mater. 143 (2011) 348–356. [6] Z. Yang, Y. Liu, C. Yu, X. Gu, N. Xu, Ball-milled NaA zeolite seeds with submicron size for growth of NaA zeolite membranes, J. Membr. Sci. 392 (2012) 18–28. [7] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Separ. Purif. Technol. 25 (2001) 251–260. [8] C. Zhang, L. Peng, J. Jiang, X. Gu, Mass transfer model, preparation and applications of zeolite membranes for pervaporation dehydration: a review, Chin. J. Chem. Eng. 25 (2017) 1627–1638. [9] X. Wang, Y. Chen, C. Zhang, X. Gu, N. Xu, Preparation and characterization of highflux T-type zeolite membranes supported on YSZ hollow fibers, J. Membr. Sci. 455 (2014) 294–304. [10] J. Jiang, X. Wang, L. Peng, X. Wang, X. Gu, Batch-scale preparation of hollow fiber supported CHA zeolite membranes and module for solvents dehydration, Microporous Mesoporous Mater. 250 (2017) 18–26. [11] S.G. Sorenson, E.A. Payzant, W.T. Gibbons, B. Soydas, H. Kita, R.D. Noble, J. L. Falconer, Influence of zeolite crystal expansion/contraction on NaA zeolite membrane separations, J. Membr. Sci. 366 (2011) 413–420. [12] F. Qu, R. Shi, L. Peng, Y. Zhang, X. Gu, X. Wang, S. Murad, Understanding the effect of zeolite crystal expansion/contraction on separation performance of NaA zeolite membrane: a combined experimental and molecular simulation study, J. Membr. Sci. 539 (2017) 14–23. [13] Z. Hong, C. Zhang, X. Gu, W. Jin, N. Xu, A simple method for healing nonzeolitic pores of MFI membranes by hydrolysis of silanes, J. Membr. Sci. 366 (2011) 427–435. [14] B. Zhang, C. Wang, L. Lang, R. Cui, X. Liu, Selective defect–patching of zeolite membranes using chemical liquid deposition at organic/aqueous interfaces, Adv. Funct. Mater. 18 (2008) 3434–3443. [15] H. Matsuda, H. Yanagishita, H. Negishi, D. Kitamoto, T. Ikegami, K. Haraya, T. Nakane, Y. Idemoto, N. Koura, T. Sano, Improvement of ethanol selectivity of silicalite membrane in pervaporation by silicone rubber coating, J. Membr. Sci. 210 (2002) 433–437. [16] X. Gu, Z. Tang, J. Dong, On-stream modification of MFI zeolite membranes for enhancing hydrogen separation at high temperature, Microporous Mesoporous Mater. 111 (2008) 441–448. [17] Z. Tang, J. Dong, T.M. Nenoff, Internal surface modification of MFI-type zeolite membranes for high selectivity and high flux for hydrogen, Langmuir 25 (2009) 4848–4852. [18] Z. Hong, Z. Wu, Y. Zhang, X. Gu, Catalytic cracking deposition of methyldiethoxysilane for modification of zeolitic pores in MFI/α-Al2O3 zeolite membrane with Hþ ion exchange pretreatment, Ind. Eng. Chem. Res. 52 (2013) 13113–13119. [19] Y. Hirota, K. Watanabe, Y. Uchida, Y. Egashira, K. Yoshida, Y. Sasaki, N. Nishiyam, Coke deposition in the SAPO-34 membranes for examining the effects of zeolitic and non-zeolitic pathways on the permeation and separation properties in gas and vapor permeations, J. Membr. Sci. 415–416 (2012) 176–180. [20] D. Korelskiy, P. Ye, M.S. Nabavi, J. Hedlund, Selective blocking of grain boundary defects in high-flux zeolite membranes by coking, J. Mater. Chem. 5 (2017) 7295–7299. [21] Y. Zhang, A.M. Avila, B. Tokay, H.H. Funke, J.L. Falconer, R.D. Noble, Blocking defects in SAPO-34 membranes with cyclodextrin, J. Membr. Sci. 358 (2010) 7–12. [22] J.N. Coleman, M. Lotya, 1 A. O’Neill, S.D. Bergin, P.J. King, U. Khan, K. Young, A. Gaucher, S. De, R.J. Smith, I.V. Shvets, S.K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G.T. Kim, G.S. Duesberg, T. Hallam, J.J. Boland, J.J. Wang, J.F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R.J. Nicholls, J.M. Perkins, E.M. Grieveson, K. Theuwissen, D.W. McComb, P.D. Nellist, V. Nicolosi, Two-dimensional nanosheets produced by liquid exfoliation of layered materials, Science 331 (2011) 568–571. [23] M. Chhowalla, Z. Liu, H. Zhang, Two-dimensional transition metal dichalcogenide (TMD) nanosheets, Chem. Soc. Rev. 44 (2015) 2584–2586. [24] H. Zhang, Ultrathin two-dimensional nanomaterials, ACS Nano 9 (2015) 9451–9469. [25] E.S. Philippe, K. Chow, B.C. Viana, J. Gao, J. Luo, J. Li, Z. Lin, A.L. Elias, Y. Shi, Z. Wang, M. Terrones, N. Koratkar, Wetting of mono and few-layered WS2 and MoS2 films supported on Si/SiO2 substrates, ACS Nano 9 (2015) 3023–3031. [26] X. Peng, L. Peng, C. Wu, Y. Xie, Two dimensional nanomaterials for flexible supercapacitors, Chem. Soc. Rev. 43 (2014) 3303–3323.
Fig. 15. VP separation results of NaA zeolite membrane before and after modified with WS-I in dehydration of water/ethanol mixtures under different feed water contents at 373 K.
water fluxes. 3.4.3. VP dehydration of ethanol Not only IPA, such modification method was also adopted to modify a new NaA zeolite membrane for VP dehydration of low water content ethanol. As shown in Fig. 15, the water/ethanol separation factor of membrane was improved from 715 to >10000 while the water flux declined from 7.4 to 6.1 kg m-2 h-1 in dehydration of 10 wt% water/ ethanol mixture at 373 K. When the feed water content lowered to 0.5 wt %, the modified membrane still possessed a very high separation factor of ~9500. We believe that the modification strategy can be applied broadly to other hydrophilic zeolite membranes for deep dehydration of different organic solvents. 4. Conclusions A facile and high-efficiency modification strategy was proposed to block defects of NaA zeolite membrane with WS2 nanosheets for VP dehydration of low water content IPA. Owing to good wettability and high flexibility of WS2 nanosheets, the hydrophilicity of membrane surface was improved after modification. Not only intrinsic defects but also the defects induced by crystal contraction at low feed water content were blocked. As a result, the separation factor of membrane was enhanced more than two orders of magnitude while the loss in water flux was only 18.6%. Furthermore, the modified membrane still behaved a high separation factor even when the feed water content lowered to 0.5 wt%. Such achieved results suggest great potential to develop defectfree zeolite membranes with high reproducibility for deep dehydration of organic solvents in industry. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work is sponsored by the National Natural Science Foundation of China (21490585 and 21776128), the “333 Talent Project” and Young Fund (BK20170132) of Jiangsu Province, State Key Laboratory of Ma terials- Oriented Chemical Engineering (ZK201602). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 9
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[27] L. Sun, Y. Ying, H. Huang, Z. Song, Y. Mao, Z. Xu, X. Peng, Ultrafast molecule separation through layered WS2 nanosheet membranes, ACS Nano 8 (2014) 6304–6311. [28] D. Liu, Y. Zhang, J. Jiang, X. Wang, C. Zhang, X. Gu, High-performance NaA zeolite membranes supported on four-channel ceramic hollow fibers for ethanol dehydration, RSC Adv. 5 (2015) 95866–95871. [29] T.P. Nguyen, S. Choi, J.-M. Jeon, K.C. Kwon, H.W. Jang, S.Y. Kim, Transition metal disulfide nanosheets synthesized by facile sonication method for the hydrogen evolution reaction, J. Phys. Chem. C 120 (2016) 3929–3935. [30] T.P. Nguyen, W. Sohn, J.H. Oh, H.W. Jang, S.Y. Kim, Size-dependent properties of two-dimensional MoS2 and WS2, J. Phys. Chem. C 120 (2016) 10078–10085. [31] J. Hedlund, D. Korelskiy, L. Sandstr€ om, J. Lindmark, Permporometry analysis of zeolite membranes, J. Membr. Sci. 345 (2009) 276–287. [32] J.B. Lee, H.H. Funke, R.D. Noble, J.L. Falconer, High selectivities in defective mfi membranes, J. Membr. Sci. 321 (2008) 309–315. [33] M. Yu, J.L. Falconer, Richard D. Noble, Characterizing nonzeolitic pores in MFI membranes, Ind. Eng. Chem. Res. 47 (2008) 3943–3948. [34] M. Noack, P. K€ olsch, A. Dittmar, M. St€ ohr, G. Georgi, M. Schneider, U. Dingerdissen, A. Feldhoff, J. Caro, Proof of the ISS-concept for LTA and FAU
[35]
[36] [37]
[38] [39]
10
membranes and their characterization by extended gas permeation studies, Microporous Mesoporous Mater. 102 (2007) 1–20. Y. Pi, B. Liu, Z. Li, Y. Zhu, Y. Li, F. Zhang, G. Zhang, W. Peng, X. Fan, TiO2 nanorod arrays decorated with exfoliated WS2 nanosheets for enhanced photoelectrochemical water oxidation, J. Colloid Interface Sci. 545 (2019) 282–288. T. Lei, W. Chen, J. Huang, C. Yan, H. Sun, C. Wang, W. Zhang, Y. Li, J. Xiong, Multi-functional layered WS2 nanosheets for enhancing the performance of lithium–sulfur batteries, Adv. Energ. Mater. 7 (2017) 1601843. J. Yang, D. Voiry, S.J. Ahn, D. Kang, A.Y. Kim, M. Chhowalla, H.S. Shin, Twodimensional hybrid nanosheets of tungsten disulfide and reduced graphene oxide as catalysts for enhanced hydrogen evolution, Angew. Chem. Int. Ed. 52 (2013) 13751–13754. M.A. Lukowski, A.S. Daniel, C.R. English, F. Meng, A. Forticaux, R.J. Hamers, S. Jin, Highly active hydrogen evolution catalysis from metallic WS2 nanosheets, Energy Environ. Sci. 7 (2014) 2608–2613. X. Wang, C. Chi, J. Tao, Y. Peng, S. Ying, Y. Qian, J. Dong, Z. Hu, Y. Gu, D. Zhao, Improving the hydrogen selectivity of graphene oxide membranes by reducing nonselective pores with intergrown ZIF-8 crystals, Chem. Commun. 52 (2016) 8087–8090.