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Two-dimensional MXene membrane for ethanol dehydration
Journal of Membrane Science 590 (2019) 117300
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Two-dimensional MXene membrane for ethanol dehydration 1
1
T
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Yi Wu , Li Ding , Zong Lu, Junjie Deng, Yanying Wei
School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou, 510640, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane separation Two-dimensional MXene Ethanol dehydration Pervaporation
Two-dimensional MXene membranes have been successfully used in separation technology. With controllable interlayer nanochannels, abundant surface-terminating groups and hydrophilicity, MXene membranes also exhibit the potential in alcohol dehydration. Here, 2-μm-thick MXene membranes stacked by Ti3C2Tx nanosheets were fabricated and applied in ethanol dehydration for the first time. Effects of the feed ethanol concentration and operating temperature on the ethanol dehydration performance of the MXene membrane were investigated through a pervaporation process. The water/ethanol separation factor of the MXene membrane increased with increasing feed ethanol concentration. Additionally, the MXene membrane exhibited better ethanol dehydration performance at room temperature compared with that at elevated temperature. It gave a water/ethanol separation factor of 135.2 with a total flux of 263.4 g m−2 h−1 at room temperature for the azeotrope dehydration (95% ethanol mixed with 5% water). MXene membranes are promising in applications of pervaporation dehydration and solvent separation.
1. Introduction Considering that bioethanol is regarded as a kind of promising green energy and sustainable energy for replacing fossil fuels [1], where a certain amount of water would be generated during its production. So ethanol dehydration is a crucial process to obtain ethanol with high purity for energy applications [2]. Distillation is a conventional method to concentrate ethanol, but it is an energy-intensive process [3]. Moreover, once the concentration of ethanol reached 95.6% (by weight), an azeotrope with water would be formed, which made it impossible to separate ethanol-water by traditional distillation. In terms of energy efficiency, membrane separation technology is a promising alternative for ethanol dehydration [4,5]. In recent years, two-dimensional (2D) lamellar membranes have attracted worldwide attention due to their easy processing [6], tunable pore size [7], mechanical robustness [8] and high performance [9]. Plenty of 2D nanomaterials have been successfully fabricated to 2D lamellar membranes, such as graphene [10], molybdenum disulfide [11] and hexagonal boron nitride [12]. These 2D membranes have shown promising performance in gas separation [13], ion sieving [14], water purification [15] etc. For example, Yang et al. reported a kind of ultrathin graphene oxide membrane, which exhibited fast water and organic solvent permeation with high rejection towards ions and dye molecules [9]. Chen et al. reported an amino functionalized boron
nitride based membrane, which performed a high water flux and good rejection for dye molecules in both aqueous and organic solvents [12]. Huang et al. present an integrated and continuous graphene oxide membrane by vacuum filtration method, which exhibited high separation performance for dimethyl carbonate dehydration [16]. Wang et al. assembled a g-C3N4 membrane with novel self-supported structure, which showed an excellent separation performance in water purification [17,18]. Apart from these 2D materials mentioned above, 2D transition metal carbides, carbonitrides and nitrides (MXenes), as a kind of young 2D material, was first synthesized by Gogotsi and Barsoum [19]. MXene nanosheets with atomic thickness can be obtained by selectively etching the layers of A-element atoms from the laminar bulk precursor of MAX ternary phase, where M refers to the early transition metal elements, like Ti, Ni, Mo and so on; X represents nitrogen and/or carbon; A-elements are mostly in the group 13 and 14 of the periodic table. There are more than 19 different MXene compositions successfully synthesized [20] and Ti3C2Tx is the most studied one. Prepared by selectively etching method using HF or the combination of LiF and HCl, MXene has abundant surface terminations (represented as T in Ti3C2Tx) like –F, =O, –OH etc [21]. Previous reports have demonstrated that MXene possesses excellent conductivity, mechanical stability and hydrophilic surface, making them applicable for lithium ion battery [22], supercapacitor [23] electromagnetic shielding devices [24] and NH3
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Corresponding author. E-mail address: [email protected] (Y. Wei). 1 Yi Wu and Li Ding contributed equally to this work. https://doi.org/10.1016/j.memsci.2019.117300 Received 10 April 2019; Received in revised form 20 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 590 (2019) 117300
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2.3. Ethanol dehydration experiment
electrosynthesis [25]. Moreover, stacked MXene nanosheets also showed obvious edge when used in membrane separation. First of all, MXene nanosheets have great dispersibility in water and other solvent [26], which means they can be easily fabricated to 2D membrane by vacuum filtration or coating and so on. In addition, MXene membranes have hydrophilcity and tunable interlayer spacing, which make them available for water permeation and molecule rejection. MXene membranes have been successfully applied in gas separation [27,28], ion sieving [29] and water purification [30,31]. For example, Ding et al. reported a kind of MXene membrane which showed favorable rejection for dye molecules in water [30]. Recently, ultrathin MXene membranes were implemented for pervaporation desalination and they can effectively reject NaCl at 65 °C with a high water flux [32]. Besides, MXene has also been used to optimize chitosan (CS) membrane for solvent dehydration, where the separation factor of the CS membrane was notably improved with 3% addition of MXene [33]. However, to the best of our knowledge, there was no report on the pristine MXene membrane used for alcohol dehydration till now. In this work, a kind of 2-μm-thick MXene membrane stacked by monolayer Ti3C2Tx nanosheets was fabricated and applied to separate ethanolwater mixture through a pervaporation process. Effects of the feed concentration and operating temperature on the ethanol dehydration performance were investigated in detail. With the concentration of ethanol in feed solution changing from 75% to 95%, the water/ethanol separation factor increased. The MXene membrane exhibited a water/ ethanol separation factor of 135.2 with a total flux of 263.4 g m−2 h−1 at room temperature using 95% ethanol concentration as feed solution. This performance indicates a promising future of MXene membranes for the dehydration of ethanol and other solvents via pervaporation process.
The ethanol dehydration performance of the MXene membrane was measured with a pervaporation apparatus (Fig. 1b). The MXene membrane was loaded into the membrane cell with an effective area of 1.77 cm2. The feed solution was circulated between the feed tank and the membrane cell by a peristaltic pump to avoid concentration polarization. The feed tank was immersed in a thermostatic oil bath to keep at a constant temperature. The pressure on the permeate side was maintained at −0.1 MPa by a vacuum pump. Permeates were collected by a cold trap immersed in liquid nitrogen. The mass of permeates in the cold trap was weighted by an analytical balance. And compositions of permeates were analyzed by a gas chromatography (Angilent 7890A, PLOT-Q capillary column) with a flame ionization detector (FID) detector. The total flux (J) is calculated by the following equation:
J=
M At
(1)
where M (g) is the mass of the permeate, t (h) is the operation time, A (m2) represents the effective area of membrane. The separation factor (α) of water/ethanol is defined as blow: Xwater
α=
Ywater
Xethanol Yethanol
(2)
where Xwater and Xethanol are the weight fractions of water and ethanol in the permeate, Ywater and Yethanol are the weight fractions of water and ethanol in the feed, respectively. 2.4. Characterizations Scanning electron microscopy (SEM) images of the MXene nanosheets and membranes were taken with Hitachi SU8220 device. Energy-dispersive X-ray spectroscopy (EDXS) was also carried out on Hitachi SU8220 device. The Atomic force microscope (AFM) investigation of the MXene nanosheets was performed on Bruker Multi Mode 8 scanning probe microscope (SPM, VEECO). Transmission electron microscopy (TEM) images were obtained by JEOL JEM-2100F microscope with an acceleration voltage of 200 kV. Elemental mapping in TEM was conducted by the Bruker EDS System. X-ray diffraction (XRD) measurements were conducted on Bruker D8 Advance with filtered Cu-Kα radiation. Fourier transform infrared spectroscopy (FTIR) was obtained by using Bruker VERTEX 33. X-ray photoelectron spectra (XPS) analysis was performed on an ESCALAB 250 spectrometer (Thermo Fisher Scientific) with monochromated Al-Kα radiation (1486.6 eV).
2. Experiment 2.1. Materials Ti3AlC2 particles were obtained from Beijing Jinhezhi Materials Co. Ltd. LiF (99%) was purchased from Aladdin Industrial Corporation, Shanghai China. HCl (12 M) was received from Guangzhou Chemical Reagent Co. Ltd. Nylon microfiltration membranes with pore size of 0.2 μm and diameter of 50 mm were purchased from Jinteng Experimental Equipment Co. Ltd. Tianjin China. Ethanol and isopropanol were purchased from Nanjing Chemical Reagent Co. Ltd. 2.2. Preparation of MXene membrane Synthesis of Ti3C2Tx MXene nanosheets: 5 g LiF was dissolved in 130 mL HCl solution (9 M) with stirring at 35 °C for 5 min. Then 5 g Ti3AlC2 particles were slowly added into the mixture solution over 10 min with continuous magnetic stirring at 35 °C. The reaction was allowed to proceed for 24 h. After reaction, the mixture was transferred to centrifugation at 3500 rpm for 5 min, then decanted the supernatant and washed by distilled water. To remove the residual acid, this centrifugation and washing operation was repeated for several times until the pH of the supernatant was about 6. The final clay-like sediment was collected and re-dispersed in 500 ml distilled water. Then the suspension was followed by sonication for 1 h for further delamination. After that, the suspension was centrifuged at 3500 rpm for 1 h to remove the un-exfoliated Ti3C2Tx particles. Finally, the supernatant, a colloidal solution of Ti3C2Tx MXene nanosheets, was collected and preserved. Preparation of Ti3C2Tx MXene membrane: A certain amount of MXene solution was diluted by distilled water. The MXene membrane was prepared by vacuum-assisted filtration of the diluted MXene solution on a substrate of Nylon microfiltration membrane (50 mm diameter) with a pore size of 0.2 μm. The obtained membranes were then dried in vacuum at room temperature for 12 h. The apparatus for vacuum-assisted filtration is displayed in Fig. 1a.
3. Result and discussion 3.1. Characterizations of MXene nanosheets The precursor Ti3AlC2 particles showed a compact laminar-like bulk structure as displayed in Fig. 2a. After selectively etching the Al element of the precursor, sonication and centrifugation, the MXene nanosheets solution was obtained as shown in Fig. 2b. The obvious Tyndall effect of the MXene solution indicated it was a stable colloidal solution with good dispersion of MXene nanosheets in water. The morphology of the resulting MXene nanosheets was characterized by SEM, AFM and TEM. From the SEM images shown in Fig. 2c and d, it can be seen that the lateral size of the as-synthesized MXene nanosheets were around 1–2 μm, in accordance with our previous work [27]. The MXene nanosheets dropped upon an anodic aluminum oxide (AAO) substrate were almost transparent under the electron beam (Fig. 2d), showing an ultrathin and atomic thickness of Ti3C2Tx nanosheets. Further, the AFM image present in Fig. 3a revealed that the thickness of Ti3C2Tx nanosheets was around 1.5 nm, indicating almost single2
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Fig. 1. (a) Vacuum-assisted filtration apparatus and (b) pervaporation apparatus for the ethanol dehydration performance measurement.
Ti3C2OH–H2O moieties, and a pair of weak peaks at 460.0 eV and 465.0 eV was attributed to the Ti–F moiety, which is in accordance with previous reports [27,35]. Apart from XPS, FTIR was also conducted to analyze the functional groups terminating on the surface of MXene. As displayed in Fig. 6, the FTIR spectrum of MXene exhibited two typical representative peaks at 3445 cm−1 and 1645 cm−1, corresponding to the vibration of –OH [36]. Terminating by these functional groups endows MXene hydrophobicity to some extent, which also allows the MXene nanosheets to form a stable colloidal solution in water. 3.2. Characterizations of MXene membranes The MXene membranes were obtained by vacuum filtration of the MXene nanosheets solution on the Nylon porous substrate (Fig. 7a). As shown in Fig. 7b, the surface of the MXene membrane stacked by Ti3C2Tx nanosheets showed a typical wave-like structure, similar to the graphene oxide membrane [37]. No obvious defects and pinholes can be seen on the membrane surface. The water contact angle of the asprepared MXene membrane was 60° shown in the inset of Fig. 7b, indicating the relatively hydrophilicity of the MXene membrane due to the abundant oxygen functional groups. The cross-sectional SEM image of the MXene membrane was present in Fig. 7c, indicating that the thickness of the MXene membrane was about 2 μm. And a regular layerby-layer stacked structure of the MXene membrane can be clearly observed in this image. XRD characterizations of the Ti3AlC2 particles and the MXene membranes were also carried out. As shown in Fig. 7d, the peak for (104) planes located at 39° disappeared and the diffraction peak for (002) planes shifted to lower angles after etching, which verified that Ti3AlC2 was successfully converted into Ti3C2Tx. The d-spacing of the MXene membrane can be calculated as 1.37 nm according to the location of (002) peak (2θ = 6.4°) based on the Bragg's law. Taking account of a monolayer MXene thickness of 1 nm [34,38], the empty space between two neighboring monolayer MXene nanosheet should be 0.37 nm. Further, the removal of Al element in MXene membrane was also confirmed by EDXS. As listed in Table 2, the atomic content of Al element was negligible (0.86%) in the MXene membrane.
Fig. 2. (a) SEM image of the Ti3AlC2 particles. (b) Photograph of the as-synthesized MXene colloidal solution. (c) and (d) SEM images of the dispersive Ti3C2TX nanosheets with different magnitudes.
Fig. 3. (a) AFM image of the Ti3C2TX nanosheets. (b) TEM image of the Ti3C2TX nanosheets. Insert is a selected-area electron diffraction (SAED) image of the Ti3C2TX nanosheets.
layered MXene nanosheets were obtained [34]. The hexagonal structure of the Ti3C2Tx plane was detected by the selected-area electron diffraction (SAED) patterns (Fig. 3b), demonstrating the high crystallinity of the as-synthesized Ti3C2Tx MXene nanosheets. The chemical properties of the MXene nanosheets were further investigated by the elemental mapping of TEM, XPS and FTIR characterizations. The elemental mapping in TEM (Fig. 4) demonstrated the uniform distribution of Ti, C, O, F elements on MXene nanosheets. XPS was carried out to further study the surface terminal groups of Ti3C2Tx nanosheets. As shown in Fig. 5a, the XPS spectra confirmed the existence of Ti, O, C and F component in MXene. Corresponding peaks of Ti 2p, O 1s, C1s and F 1s were shown in Table 1. Typically, in the XPS spectrum of Ti 2p (Fig. 5b), the peaks of Ti, Ti2+ and Ti3+ were dominant, belong to the mixture of C–Ti-OX, C–Ti-(OH)X, and/or
3.3. Ethanol dehydration performance 3.3.1. Effect of the feed composition on ethanol dehydration performance Subsequently, the ethanol dehydration performance of the as-prepared MXene membranes were measured via a pervaporation process. First, the effect of the membrane thickness on ethanol dehydration performance of the MXene membranes was investigated, as shown in Fig. 8. It can be noted that the total flux decreases with increasing membrane thickness due to the prolonged mass transfer pathway, while the separation factor rises. For a general consideration of flux and selectivity, the 2-μm-thick MXene membranes were chosen for following 3
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Fig. 4. TEM image of the MXene nanosheets and the corresponding elemental mapping distribution images.
separation factor increased to 135.2 with the total flux of 263.4 g m−2 h−1. Moreover, the molecules lager than ethanol were also studied, such as isopropanol (IPA) dehydration, as shown in Fig. 9b, the MXene membrane exhibited similar trends as ethanol dehydration in both separation factor and total flux when increasing the feed isopropanol concentration. To further investigate the separation mechanism of this process, XRD was carried out to detect the changes in d-spacing of the MXene membranes before and after immersion in different solvents. In detail, the MXene membranes were immersed in liquid water, ethanol and isopropanol for 12 h, respectively. Then these wet MXene membranes were conducted by XRD characterization immediately after scooping from the solvent. The results were present in Fig. 10a. As mentioned above, the dry MXene membrane exhibited an XRD peak at 2θ = 6.4°, corresponding to a d-spacing of 1.37 nm. After immersing in water, the XRD peak of the wet MXene membrane shifted to 5.7°, indicating an increase of 0.23 nm in d-spacing. In addition, the d-spacing of MXene
study. In order to check the effect of the porous support on the MXene membrane separation performance, the separation performance of the nylon microfiltration membrane was investigated. The total flux and separate factor of the substrate for ethanol dehydration were 4179.3 kg m−2.h−1 and 1.2, respectively, feeding with 95% ethanol. Compared to the performance of the MXene membrane supported on nylon substrate, it can be confirmed that the substrate almost has no contribution on the separation performance of MXene membrane. Further, the mixture of ethanol-water with different concentrations was used as the feed solution, ranging from 75% to 95% (ethanol content by weight). Fig. 9a exhibits the total flux and the water/ethanol separation factor of the MXene membrane under varying ethanol concentration at room temperature. It can be observed that the separation factor increased as the feed ethanol content increased and the total flux fluctuated around 250 g m−2 h−1. Feeding with 75% ethanol, the total flux through the MXene membrane was 234.3 g m−2 h−1 and the separation factor was 14.1. While feeding with 95% ethanol, the
Fig. 5. (a) XPS spectra for MXene nanosheets of the (b) Ti 2p, (c) C 1s, (d) O 1s and (e) F 1s. 4
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Table 1 XPS peak fitting results for the Ti3C2Tx MXene nanosheets. Region
Position(eV)
FWHM(eV)
Assigned to
Ti 2p
455.7(462.2) 456.2(461.8) 457.2(463.3) 458.8(464.5) 460.0(465.0)
0.7(1.8) 1.2(1.6) 1.85(1.8) 1.8(1) 1.6(1)
Ti–C Ti2+ Ti3+ Ti–O Ti–F
C 1s
282.5 284.9 289.8 286.5
0.72 1.76 0.6 0.5
C–Ti C–C C–O O–C]O/C–F
F 1s
687.3 685.7
1.65 1.185
F–C F–Ti
530.4 531.2 532.2 533.8
0.9 1.25 2 1.65
O–Ti O–Ti/OH O–C/OH H2O
O 1s
Fig. 8. The ethanol dehydration performance of the MXene membranes with different thickness feeding with 90% ethanol aqueous.
membranes with different immersing time were also recorded when MXene membranes were immersed in water and ethanol. As shown in Fig. 10b, the d-spacing of the MXene membrane increased 0.2 nm after immersed in water for 2 h. Moreover, after immersion in water for even 5 days, the increase in d-spacing was still ~0.2 nm compared with that of the dry membrane, which means this increase in d-spacing won't be infinite, implying a relatively stability of the MXene membrane in liquid solution. As is known, many 2D layered membranes, such as GO membranes, are easy to disintegrate in aqueous environment due to the strong electrostatic repulsion forces between nanosheets. However, different from those membranes, MXene membranes were proved to show stronger van der Waals attraction force and weaker electrostatic repulsion force compared to that of GO membranes, which gives MXene better stability in water, according to the report by Lao et al. [39]. For the MXene membranes soaked in ethanol, the XRD peak did not shift notably compared with that of the dry one, indicating a much better anti-swelling property in such solvents. For the MXene membrane soaked in water, the expansion of d-spacing was due to the intercalation of water molecules between nanosheets, which was already confirmed by other reports [29]. With higher feed ethanol concentration, a better anti-swelling property enables MXene membrane to keep the empty space of ~0.42 nm. The dynamic diameter of water and ethanol molecules are 0.29 nm and 0.45 nm, respectively, which demonstrates that such MXene membrane can separate water and ethanol by molecular sieving mechanism. When the concentration of water in the feed solution was relatively high, an expansion of the MXene nanochannels caused by the intercalation of H2O will weaken the ethanol rejection ability of the MXene membrane, thus cause a decrease in water/ethanol separation factor subsequently. This is why the separation factor of the MXene membrane increased a lot with increasing ethanol concentration in the feed solution as shown in Fig. 9a.
Fig. 6. FTIR spectra of the MXene membrane.
3.3.2. Effect of operating temperature on ethanol dehydration performance The effect of the operating temperature on the ethanol dehydration performance of the MXene membrane were further investigated. The total flux and water/ethanol separation factor as a function of operation temperature feeding with 90% ethanol are shown in Fig. 11. In general, when the operating temperature elevated from the room temperature (25 °C) to 70 °C, the total flux showed a slightly increasing trend, while the separation factor decreased. Similar phenomena can also be found for other 2D membranes. For example, Jin et al. reported a kind of graphene oxide membrane for dimethyl carbonate dehydration, which showed similar trend at elevated operating temperature, because the organic solvent permeated faster than water with operating temperature increasing [16]. Furthermore, the time dependent separation performance of the MXene membrane in ethanol dehydration was also tested feeding with 95% ethanol at room temperature. As shown in Fig. 12, during the 48 h-
Fig. 7. (a) SEM image of the porous Nylon 6 substrate. (b) SEM image of the MXene membrane surface. Insert is the water contact angle of the MXene membrane. (c) Cross-sectional SEM image of the MXene membrane. Inset is a photograph of the MXene membrane. (d) XRD patterns of the Ti3AlC2 particles and the MXene membrane.
Table 2 EDX analysis (atomic %) of MXene membrane. Element
Ti
C
O
F
Al
Content (atomic %)
36.85
24.28
21.38
13.43
0.86
5
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Fig. 9. (a) Effect of the feed composition on the ethanol dehydration performance of the MXene membrane at room temperature. (b) The column chart shows the separation factor of MXene membrane for ethanol and IPA dehydration, while the symbol and line show corresponding total flux. Blue: ethanol dehydration, gray: IPA dehydration. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
operation, the water/ethanol separation factor and the total flux through the MXene membrane exhibit some fluctuation without obvious reduction. The microstructure of the MXene membrane after separation was investigated and shown in Fig. 13. Compared with the surface SEM image of the as-prepared MXene membrane present in Fig. 7b, there is no obvious change after separation test, which demonstrates its good structure stability. Compared with other reported zeolite membranes for ethanol dehydration as shown in Table 3, we have to admit that the dehydration performance of the MXene membrane is not so good as that of the commercial zeolite membranes, such as NaA, which can be explained by following points. i) Much longer mass transfer path in 2D MXene membranes gives lower flux. In most 2D lamellar membranes, the mass transfer path is the amount of nanochannels between the neighboring nanosheets, which is always tortuous curves along each 2D layers. Such special microsturcture inside 2D membrane endows it much prolonged mass transfer path with increased mass transfer resistance, which counts against high flux. That is why the water flux through the MXene membrane is lower than that of zeolite membranes, which possess much shortened pathway.
Fig. 11. Effect of the operating temperature on the ethanol dehydration performance of the MXene membrane feeding with 90% ethanol.
where h is the membrane thickness, ε and τ represent the porosity and tortuosity of the pathway, D and K represent the diffusivity and the solubility. In other words, the permeance depends on the membrane thickness, surface property (affecting the solubility), pore size (affecting the diffusivity), as well as the porosity and tortuosity of two pathways in the membrane. For the MXene membranes prepared in our work, the semiquantative analysis of the permeance can be performed using above model. Considering that there were no obvious defects on the fabricated MXene nanosheets as proved in our previous work [27], so the permeance contribution of pathway B can be neglected. The porosity (εA) of the MXene membrane in this work can be approximately estimated as the ratio of the empty interlayer space (0.37 nm) to d-spacing (1.37 nm), which is 0.27. The tortuosity (τA) of the MXene membrane can be approximated as the ratio of the MXene nanosheets length to thickness, which is about 1500. So the ratio of εA and τA is approximately 1.8 × 10−4 which is significantly smaller than that of the zeolite
For the mass transfer in special 2D membranes, Ibrahim and Lin [48,49] reported the inter-sheet and inner-sheet mass transport model for semiquantative analysis. Two pathways can be found in 2D structure, where the inter-sheet pathway A stands for the interlayer space between stacked 2D nanosheets and randomly distributed nanochannels at wrinkles, while the inner-sheet pathway B constitutes 2D nanosheets structure defects. And the permeance through the 2D membrane can be described by the formula below:
1 ε ε F = ⎛ ⎞ ⎧ ⎛ A ⎞ DA KA + ⎛ B ⎞ DB KB ⎫ ⎬ τ τ ⎝ h ⎠⎨ A ⎝ B⎠ ⎩⎝ ⎠ ⎭ ⎜
⎟
⎜
⎟
Fig. 10. (a) XRD patterns and corresponding d-spacing of the MXene membranes immersed in different solvents. (b) d-spacing of MXene membranes as a function of immersing time when soaked in water and ethanol. 6
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presents a much better anti-swelling property in ethanol. Therefore, it is impossible to avoid membrane swelling during water/ethanol separation. Once the interlayer spacing of MXene membrane expands up higher than the dynamic diameter of ethanol (0.45 nm), the water/ethanol separation factor will go down sharply. That is why higher separation factor can be obtained with higher feed ethanol concentration as shown in Fig. 9, where membrane swelling was limited to some extent. For the zeolite membrane, there is no swelling when immersed in water/ethanol solution, the constant pore size endows it higher separation factor. iii) Some defects in layered structure also make the separation factor sacrificed. It is not easy to prepare a perfect 2D membrane without any defect, where the size sieving pore is only the nanochannels between neighboring nanosheets. But in fact, the defects existing in the membrane are also used as the pore for mass transfer. Thus, the defects with size bigger than 0.45 nm will reduce the water/ethanol separation factor of membrane. Although there are several problems as mentioned above, it is still a new try to utilize the MXene membrane for ethanol dehydration, which extends the application field for 2D membranes. In order to enhance the total permeance through the MXene membranes, several ways can be tried. (a) Decreasing the membrane thickness. (b) Improving the solubility by chemical modification of the membrane surface or MXene nanosheets. (c) Increasing the diffusivity by designing the 2D nanochannel structure, and/or operating at elevated pressure or temperature. (d) 2D membrane with bigger empty interlayer space presents faster mass transfer, but the separation factor should be concerned. (e) MXene membrane stacked of nanosheets with smaller radial size will also show better permeance. (f) Punching some nanopores or sub-nanopores on the nanosheets will contribute more pathway, benefiting for enhancing the permeance. In order to obtain better separation performance, defect-free ultra-thin 2D membranes with better anti-swelling property are required, and more work are on the way.
Fig. 12. Dehydration performance of a 2-μm-thick MXene membrane as a function of operating time feeding with 95% ethanol at room temperature.
Fig. 13. The surface SEM image of the MXene membrane after separation test. Table 3 Comparison of the ethanol dehydration performance between the MXene membrane and some zeolite membranes. Membrane material
Feed ethanol content (wt%)
Operation temperature (oC)
Flux (g·m−2·h−1)
Separation factor
Reference
NaA NaA FAU CHA Cu-LTA Silica Mordenite zeolite X MXene MXene
90 90 90 90 90 90 90 90 90 95
75 75 75 75 75 80 150 60 25 25
19700 9500 9100 12000 3520 1000 160 3370 258.8 263.4
> 80000 11,100 190 10000 3591 800 139 296 92.8 135.2
[40] [41] [42] [43] [44] [45] [46] [47] This work This work
4. Conclusion Ti3AlC2 particles were successfully exfoliated to 2D Ti3C2Tx MXene nanosheets and 2 μm-thick MXene membranes were fabricated with the as-synthesized MXene nanosheets by vacuum filtration. For the first time, the MXene membranes are applied in the ethanol dehydration considering its advantage of abundant oxygen functional groups terminated on the 2D MXene nanosheets. This kind of MXene membrane exhibited a good performance at room temperature with relatively higher ethanol feed concentration (such as the alcohol water azeotrope with concentration of 95%). Typically, the as-prepared MXene membrane showed a water/ethanol separation factor of 135.2 with a total flux of 263.4 g m−2 h−1 at room temperature for 95% ethanol dehydration. Furthermore, the MXene membrane showed a better antiswelling property when soaked in such solvent, which is beneficial for obtaining higher water/ethanol separation factor, demonstrating a promising alternative for ethanol dehydration process compared with the traditional distillation. Of course, the performance of the MXene membrane applied in the alcohol dehydration process still needs to be improved compared with other pervaporation membranes, and more efforts will be done in the future.
membranes, for example the ratio of εA and τA of MFI-type zeolite membrane is 0.25 as reported in Ref. [50]. Therefore, it is easy to understand the low flux through the 2D MXene membrane, which is mainly resulted from the three orders of magnitude smaller ratio of εA to τA, comparing with the traditional zetolite membranes.
Notes ii) Swelling of MXene membrane in water/ethanol mixture makes larger interlayer spacing, which decreases the separation factor. Normally, water (dynamic diameter of 0.29 nm) and ethanol (dynamic diameter of 0.45 nm) molecules can be separated via size sieving by the MXene membrane with small empty spacing of ~0.42 nm (soaked in ethanol). As shown in Fig. 10, the d-spacing of MXene membrane increases when immersed in water, although it
The authors declare no competing financial interest. Acknowledgements We gratefully acknowledge the funding from the Natural Science Foundation of China (21606086 and 21861132013), Guangdong Natural Science Funds for Distinguished Young Scholar 7
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(2017A030306002), the Guangzhou Technology Project (no. 201707010317) and Fundamental Research Funds for the Central Universities.
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Two-dimensional MXene membrane for ethanol dehydration 1
1
T
*
Yi Wu , Li Ding , Zong Lu, Junjie Deng, Yanying Wei
School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou, 510640, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane separation Two-dimensional MXene Ethanol dehydration Pervaporation
Two-dimensional MXene membranes have been successfully used in separation technology. With controllable interlayer nanochannels, abundant surface-terminating groups and hydrophilicity, MXene membranes also exhibit the potential in alcohol dehydration. Here, 2-μm-thick MXene membranes stacked by Ti3C2Tx nanosheets were fabricated and applied in ethanol dehydration for the first time. Effects of the feed ethanol concentration and operating temperature on the ethanol dehydration performance of the MXene membrane were investigated through a pervaporation process. The water/ethanol separation factor of the MXene membrane increased with increasing feed ethanol concentration. Additionally, the MXene membrane exhibited better ethanol dehydration performance at room temperature compared with that at elevated temperature. It gave a water/ethanol separation factor of 135.2 with a total flux of 263.4 g m−2 h−1 at room temperature for the azeotrope dehydration (95% ethanol mixed with 5% water). MXene membranes are promising in applications of pervaporation dehydration and solvent separation.
1. Introduction Considering that bioethanol is regarded as a kind of promising green energy and sustainable energy for replacing fossil fuels [1], where a certain amount of water would be generated during its production. So ethanol dehydration is a crucial process to obtain ethanol with high purity for energy applications [2]. Distillation is a conventional method to concentrate ethanol, but it is an energy-intensive process [3]. Moreover, once the concentration of ethanol reached 95.6% (by weight), an azeotrope with water would be formed, which made it impossible to separate ethanol-water by traditional distillation. In terms of energy efficiency, membrane separation technology is a promising alternative for ethanol dehydration [4,5]. In recent years, two-dimensional (2D) lamellar membranes have attracted worldwide attention due to their easy processing [6], tunable pore size [7], mechanical robustness [8] and high performance [9]. Plenty of 2D nanomaterials have been successfully fabricated to 2D lamellar membranes, such as graphene [10], molybdenum disulfide [11] and hexagonal boron nitride [12]. These 2D membranes have shown promising performance in gas separation [13], ion sieving [14], water purification [15] etc. For example, Yang et al. reported a kind of ultrathin graphene oxide membrane, which exhibited fast water and organic solvent permeation with high rejection towards ions and dye molecules [9]. Chen et al. reported an amino functionalized boron
nitride based membrane, which performed a high water flux and good rejection for dye molecules in both aqueous and organic solvents [12]. Huang et al. present an integrated and continuous graphene oxide membrane by vacuum filtration method, which exhibited high separation performance for dimethyl carbonate dehydration [16]. Wang et al. assembled a g-C3N4 membrane with novel self-supported structure, which showed an excellent separation performance in water purification [17,18]. Apart from these 2D materials mentioned above, 2D transition metal carbides, carbonitrides and nitrides (MXenes), as a kind of young 2D material, was first synthesized by Gogotsi and Barsoum [19]. MXene nanosheets with atomic thickness can be obtained by selectively etching the layers of A-element atoms from the laminar bulk precursor of MAX ternary phase, where M refers to the early transition metal elements, like Ti, Ni, Mo and so on; X represents nitrogen and/or carbon; A-elements are mostly in the group 13 and 14 of the periodic table. There are more than 19 different MXene compositions successfully synthesized [20] and Ti3C2Tx is the most studied one. Prepared by selectively etching method using HF or the combination of LiF and HCl, MXene has abundant surface terminations (represented as T in Ti3C2Tx) like –F, =O, –OH etc [21]. Previous reports have demonstrated that MXene possesses excellent conductivity, mechanical stability and hydrophilic surface, making them applicable for lithium ion battery [22], supercapacitor [23] electromagnetic shielding devices [24] and NH3
*
Corresponding author. E-mail address: [email protected] (Y. Wei). 1 Yi Wu and Li Ding contributed equally to this work. https://doi.org/10.1016/j.memsci.2019.117300 Received 10 April 2019; Received in revised form 20 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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2.3. Ethanol dehydration experiment
electrosynthesis [25]. Moreover, stacked MXene nanosheets also showed obvious edge when used in membrane separation. First of all, MXene nanosheets have great dispersibility in water and other solvent [26], which means they can be easily fabricated to 2D membrane by vacuum filtration or coating and so on. In addition, MXene membranes have hydrophilcity and tunable interlayer spacing, which make them available for water permeation and molecule rejection. MXene membranes have been successfully applied in gas separation [27,28], ion sieving [29] and water purification [30,31]. For example, Ding et al. reported a kind of MXene membrane which showed favorable rejection for dye molecules in water [30]. Recently, ultrathin MXene membranes were implemented for pervaporation desalination and they can effectively reject NaCl at 65 °C with a high water flux [32]. Besides, MXene has also been used to optimize chitosan (CS) membrane for solvent dehydration, where the separation factor of the CS membrane was notably improved with 3% addition of MXene [33]. However, to the best of our knowledge, there was no report on the pristine MXene membrane used for alcohol dehydration till now. In this work, a kind of 2-μm-thick MXene membrane stacked by monolayer Ti3C2Tx nanosheets was fabricated and applied to separate ethanolwater mixture through a pervaporation process. Effects of the feed concentration and operating temperature on the ethanol dehydration performance were investigated in detail. With the concentration of ethanol in feed solution changing from 75% to 95%, the water/ethanol separation factor increased. The MXene membrane exhibited a water/ ethanol separation factor of 135.2 with a total flux of 263.4 g m−2 h−1 at room temperature using 95% ethanol concentration as feed solution. This performance indicates a promising future of MXene membranes for the dehydration of ethanol and other solvents via pervaporation process.
The ethanol dehydration performance of the MXene membrane was measured with a pervaporation apparatus (Fig. 1b). The MXene membrane was loaded into the membrane cell with an effective area of 1.77 cm2. The feed solution was circulated between the feed tank and the membrane cell by a peristaltic pump to avoid concentration polarization. The feed tank was immersed in a thermostatic oil bath to keep at a constant temperature. The pressure on the permeate side was maintained at −0.1 MPa by a vacuum pump. Permeates were collected by a cold trap immersed in liquid nitrogen. The mass of permeates in the cold trap was weighted by an analytical balance. And compositions of permeates were analyzed by a gas chromatography (Angilent 7890A, PLOT-Q capillary column) with a flame ionization detector (FID) detector. The total flux (J) is calculated by the following equation:
J=
M At
(1)
where M (g) is the mass of the permeate, t (h) is the operation time, A (m2) represents the effective area of membrane. The separation factor (α) of water/ethanol is defined as blow: Xwater
α=
Ywater
Xethanol Yethanol
(2)
where Xwater and Xethanol are the weight fractions of water and ethanol in the permeate, Ywater and Yethanol are the weight fractions of water and ethanol in the feed, respectively. 2.4. Characterizations Scanning electron microscopy (SEM) images of the MXene nanosheets and membranes were taken with Hitachi SU8220 device. Energy-dispersive X-ray spectroscopy (EDXS) was also carried out on Hitachi SU8220 device. The Atomic force microscope (AFM) investigation of the MXene nanosheets was performed on Bruker Multi Mode 8 scanning probe microscope (SPM, VEECO). Transmission electron microscopy (TEM) images were obtained by JEOL JEM-2100F microscope with an acceleration voltage of 200 kV. Elemental mapping in TEM was conducted by the Bruker EDS System. X-ray diffraction (XRD) measurements were conducted on Bruker D8 Advance with filtered Cu-Kα radiation. Fourier transform infrared spectroscopy (FTIR) was obtained by using Bruker VERTEX 33. X-ray photoelectron spectra (XPS) analysis was performed on an ESCALAB 250 spectrometer (Thermo Fisher Scientific) with monochromated Al-Kα radiation (1486.6 eV).
2. Experiment 2.1. Materials Ti3AlC2 particles were obtained from Beijing Jinhezhi Materials Co. Ltd. LiF (99%) was purchased from Aladdin Industrial Corporation, Shanghai China. HCl (12 M) was received from Guangzhou Chemical Reagent Co. Ltd. Nylon microfiltration membranes with pore size of 0.2 μm and diameter of 50 mm were purchased from Jinteng Experimental Equipment Co. Ltd. Tianjin China. Ethanol and isopropanol were purchased from Nanjing Chemical Reagent Co. Ltd. 2.2. Preparation of MXene membrane Synthesis of Ti3C2Tx MXene nanosheets: 5 g LiF was dissolved in 130 mL HCl solution (9 M) with stirring at 35 °C for 5 min. Then 5 g Ti3AlC2 particles were slowly added into the mixture solution over 10 min with continuous magnetic stirring at 35 °C. The reaction was allowed to proceed for 24 h. After reaction, the mixture was transferred to centrifugation at 3500 rpm for 5 min, then decanted the supernatant and washed by distilled water. To remove the residual acid, this centrifugation and washing operation was repeated for several times until the pH of the supernatant was about 6. The final clay-like sediment was collected and re-dispersed in 500 ml distilled water. Then the suspension was followed by sonication for 1 h for further delamination. After that, the suspension was centrifuged at 3500 rpm for 1 h to remove the un-exfoliated Ti3C2Tx particles. Finally, the supernatant, a colloidal solution of Ti3C2Tx MXene nanosheets, was collected and preserved. Preparation of Ti3C2Tx MXene membrane: A certain amount of MXene solution was diluted by distilled water. The MXene membrane was prepared by vacuum-assisted filtration of the diluted MXene solution on a substrate of Nylon microfiltration membrane (50 mm diameter) with a pore size of 0.2 μm. The obtained membranes were then dried in vacuum at room temperature for 12 h. The apparatus for vacuum-assisted filtration is displayed in Fig. 1a.
3. Result and discussion 3.1. Characterizations of MXene nanosheets The precursor Ti3AlC2 particles showed a compact laminar-like bulk structure as displayed in Fig. 2a. After selectively etching the Al element of the precursor, sonication and centrifugation, the MXene nanosheets solution was obtained as shown in Fig. 2b. The obvious Tyndall effect of the MXene solution indicated it was a stable colloidal solution with good dispersion of MXene nanosheets in water. The morphology of the resulting MXene nanosheets was characterized by SEM, AFM and TEM. From the SEM images shown in Fig. 2c and d, it can be seen that the lateral size of the as-synthesized MXene nanosheets were around 1–2 μm, in accordance with our previous work [27]. The MXene nanosheets dropped upon an anodic aluminum oxide (AAO) substrate were almost transparent under the electron beam (Fig. 2d), showing an ultrathin and atomic thickness of Ti3C2Tx nanosheets. Further, the AFM image present in Fig. 3a revealed that the thickness of Ti3C2Tx nanosheets was around 1.5 nm, indicating almost single2
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Fig. 1. (a) Vacuum-assisted filtration apparatus and (b) pervaporation apparatus for the ethanol dehydration performance measurement.
Ti3C2OH–H2O moieties, and a pair of weak peaks at 460.0 eV and 465.0 eV was attributed to the Ti–F moiety, which is in accordance with previous reports [27,35]. Apart from XPS, FTIR was also conducted to analyze the functional groups terminating on the surface of MXene. As displayed in Fig. 6, the FTIR spectrum of MXene exhibited two typical representative peaks at 3445 cm−1 and 1645 cm−1, corresponding to the vibration of –OH [36]. Terminating by these functional groups endows MXene hydrophobicity to some extent, which also allows the MXene nanosheets to form a stable colloidal solution in water. 3.2. Characterizations of MXene membranes The MXene membranes were obtained by vacuum filtration of the MXene nanosheets solution on the Nylon porous substrate (Fig. 7a). As shown in Fig. 7b, the surface of the MXene membrane stacked by Ti3C2Tx nanosheets showed a typical wave-like structure, similar to the graphene oxide membrane [37]. No obvious defects and pinholes can be seen on the membrane surface. The water contact angle of the asprepared MXene membrane was 60° shown in the inset of Fig. 7b, indicating the relatively hydrophilicity of the MXene membrane due to the abundant oxygen functional groups. The cross-sectional SEM image of the MXene membrane was present in Fig. 7c, indicating that the thickness of the MXene membrane was about 2 μm. And a regular layerby-layer stacked structure of the MXene membrane can be clearly observed in this image. XRD characterizations of the Ti3AlC2 particles and the MXene membranes were also carried out. As shown in Fig. 7d, the peak for (104) planes located at 39° disappeared and the diffraction peak for (002) planes shifted to lower angles after etching, which verified that Ti3AlC2 was successfully converted into Ti3C2Tx. The d-spacing of the MXene membrane can be calculated as 1.37 nm according to the location of (002) peak (2θ = 6.4°) based on the Bragg's law. Taking account of a monolayer MXene thickness of 1 nm [34,38], the empty space between two neighboring monolayer MXene nanosheet should be 0.37 nm. Further, the removal of Al element in MXene membrane was also confirmed by EDXS. As listed in Table 2, the atomic content of Al element was negligible (0.86%) in the MXene membrane.
Fig. 2. (a) SEM image of the Ti3AlC2 particles. (b) Photograph of the as-synthesized MXene colloidal solution. (c) and (d) SEM images of the dispersive Ti3C2TX nanosheets with different magnitudes.
Fig. 3. (a) AFM image of the Ti3C2TX nanosheets. (b) TEM image of the Ti3C2TX nanosheets. Insert is a selected-area electron diffraction (SAED) image of the Ti3C2TX nanosheets.
layered MXene nanosheets were obtained [34]. The hexagonal structure of the Ti3C2Tx plane was detected by the selected-area electron diffraction (SAED) patterns (Fig. 3b), demonstrating the high crystallinity of the as-synthesized Ti3C2Tx MXene nanosheets. The chemical properties of the MXene nanosheets were further investigated by the elemental mapping of TEM, XPS and FTIR characterizations. The elemental mapping in TEM (Fig. 4) demonstrated the uniform distribution of Ti, C, O, F elements on MXene nanosheets. XPS was carried out to further study the surface terminal groups of Ti3C2Tx nanosheets. As shown in Fig. 5a, the XPS spectra confirmed the existence of Ti, O, C and F component in MXene. Corresponding peaks of Ti 2p, O 1s, C1s and F 1s were shown in Table 1. Typically, in the XPS spectrum of Ti 2p (Fig. 5b), the peaks of Ti, Ti2+ and Ti3+ were dominant, belong to the mixture of C–Ti-OX, C–Ti-(OH)X, and/or
3.3. Ethanol dehydration performance 3.3.1. Effect of the feed composition on ethanol dehydration performance Subsequently, the ethanol dehydration performance of the as-prepared MXene membranes were measured via a pervaporation process. First, the effect of the membrane thickness on ethanol dehydration performance of the MXene membranes was investigated, as shown in Fig. 8. It can be noted that the total flux decreases with increasing membrane thickness due to the prolonged mass transfer pathway, while the separation factor rises. For a general consideration of flux and selectivity, the 2-μm-thick MXene membranes were chosen for following 3
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Fig. 4. TEM image of the MXene nanosheets and the corresponding elemental mapping distribution images.
separation factor increased to 135.2 with the total flux of 263.4 g m−2 h−1. Moreover, the molecules lager than ethanol were also studied, such as isopropanol (IPA) dehydration, as shown in Fig. 9b, the MXene membrane exhibited similar trends as ethanol dehydration in both separation factor and total flux when increasing the feed isopropanol concentration. To further investigate the separation mechanism of this process, XRD was carried out to detect the changes in d-spacing of the MXene membranes before and after immersion in different solvents. In detail, the MXene membranes were immersed in liquid water, ethanol and isopropanol for 12 h, respectively. Then these wet MXene membranes were conducted by XRD characterization immediately after scooping from the solvent. The results were present in Fig. 10a. As mentioned above, the dry MXene membrane exhibited an XRD peak at 2θ = 6.4°, corresponding to a d-spacing of 1.37 nm. After immersing in water, the XRD peak of the wet MXene membrane shifted to 5.7°, indicating an increase of 0.23 nm in d-spacing. In addition, the d-spacing of MXene
study. In order to check the effect of the porous support on the MXene membrane separation performance, the separation performance of the nylon microfiltration membrane was investigated. The total flux and separate factor of the substrate for ethanol dehydration were 4179.3 kg m−2.h−1 and 1.2, respectively, feeding with 95% ethanol. Compared to the performance of the MXene membrane supported on nylon substrate, it can be confirmed that the substrate almost has no contribution on the separation performance of MXene membrane. Further, the mixture of ethanol-water with different concentrations was used as the feed solution, ranging from 75% to 95% (ethanol content by weight). Fig. 9a exhibits the total flux and the water/ethanol separation factor of the MXene membrane under varying ethanol concentration at room temperature. It can be observed that the separation factor increased as the feed ethanol content increased and the total flux fluctuated around 250 g m−2 h−1. Feeding with 75% ethanol, the total flux through the MXene membrane was 234.3 g m−2 h−1 and the separation factor was 14.1. While feeding with 95% ethanol, the
Fig. 5. (a) XPS spectra for MXene nanosheets of the (b) Ti 2p, (c) C 1s, (d) O 1s and (e) F 1s. 4
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Table 1 XPS peak fitting results for the Ti3C2Tx MXene nanosheets. Region
Position(eV)
FWHM(eV)
Assigned to
Ti 2p
455.7(462.2) 456.2(461.8) 457.2(463.3) 458.8(464.5) 460.0(465.0)
0.7(1.8) 1.2(1.6) 1.85(1.8) 1.8(1) 1.6(1)
Ti–C Ti2+ Ti3+ Ti–O Ti–F
C 1s
282.5 284.9 289.8 286.5
0.72 1.76 0.6 0.5
C–Ti C–C C–O O–C]O/C–F
F 1s
687.3 685.7
1.65 1.185
F–C F–Ti
530.4 531.2 532.2 533.8
0.9 1.25 2 1.65
O–Ti O–Ti/OH O–C/OH H2O
O 1s
Fig. 8. The ethanol dehydration performance of the MXene membranes with different thickness feeding with 90% ethanol aqueous.
membranes with different immersing time were also recorded when MXene membranes were immersed in water and ethanol. As shown in Fig. 10b, the d-spacing of the MXene membrane increased 0.2 nm after immersed in water for 2 h. Moreover, after immersion in water for even 5 days, the increase in d-spacing was still ~0.2 nm compared with that of the dry membrane, which means this increase in d-spacing won't be infinite, implying a relatively stability of the MXene membrane in liquid solution. As is known, many 2D layered membranes, such as GO membranes, are easy to disintegrate in aqueous environment due to the strong electrostatic repulsion forces between nanosheets. However, different from those membranes, MXene membranes were proved to show stronger van der Waals attraction force and weaker electrostatic repulsion force compared to that of GO membranes, which gives MXene better stability in water, according to the report by Lao et al. [39]. For the MXene membranes soaked in ethanol, the XRD peak did not shift notably compared with that of the dry one, indicating a much better anti-swelling property in such solvents. For the MXene membrane soaked in water, the expansion of d-spacing was due to the intercalation of water molecules between nanosheets, which was already confirmed by other reports [29]. With higher feed ethanol concentration, a better anti-swelling property enables MXene membrane to keep the empty space of ~0.42 nm. The dynamic diameter of water and ethanol molecules are 0.29 nm and 0.45 nm, respectively, which demonstrates that such MXene membrane can separate water and ethanol by molecular sieving mechanism. When the concentration of water in the feed solution was relatively high, an expansion of the MXene nanochannels caused by the intercalation of H2O will weaken the ethanol rejection ability of the MXene membrane, thus cause a decrease in water/ethanol separation factor subsequently. This is why the separation factor of the MXene membrane increased a lot with increasing ethanol concentration in the feed solution as shown in Fig. 9a.
Fig. 6. FTIR spectra of the MXene membrane.
3.3.2. Effect of operating temperature on ethanol dehydration performance The effect of the operating temperature on the ethanol dehydration performance of the MXene membrane were further investigated. The total flux and water/ethanol separation factor as a function of operation temperature feeding with 90% ethanol are shown in Fig. 11. In general, when the operating temperature elevated from the room temperature (25 °C) to 70 °C, the total flux showed a slightly increasing trend, while the separation factor decreased. Similar phenomena can also be found for other 2D membranes. For example, Jin et al. reported a kind of graphene oxide membrane for dimethyl carbonate dehydration, which showed similar trend at elevated operating temperature, because the organic solvent permeated faster than water with operating temperature increasing [16]. Furthermore, the time dependent separation performance of the MXene membrane in ethanol dehydration was also tested feeding with 95% ethanol at room temperature. As shown in Fig. 12, during the 48 h-
Fig. 7. (a) SEM image of the porous Nylon 6 substrate. (b) SEM image of the MXene membrane surface. Insert is the water contact angle of the MXene membrane. (c) Cross-sectional SEM image of the MXene membrane. Inset is a photograph of the MXene membrane. (d) XRD patterns of the Ti3AlC2 particles and the MXene membrane.
Table 2 EDX analysis (atomic %) of MXene membrane. Element
Ti
C
O
F
Al
Content (atomic %)
36.85
24.28
21.38
13.43
0.86
5
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Fig. 9. (a) Effect of the feed composition on the ethanol dehydration performance of the MXene membrane at room temperature. (b) The column chart shows the separation factor of MXene membrane for ethanol and IPA dehydration, while the symbol and line show corresponding total flux. Blue: ethanol dehydration, gray: IPA dehydration. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
operation, the water/ethanol separation factor and the total flux through the MXene membrane exhibit some fluctuation without obvious reduction. The microstructure of the MXene membrane after separation was investigated and shown in Fig. 13. Compared with the surface SEM image of the as-prepared MXene membrane present in Fig. 7b, there is no obvious change after separation test, which demonstrates its good structure stability. Compared with other reported zeolite membranes for ethanol dehydration as shown in Table 3, we have to admit that the dehydration performance of the MXene membrane is not so good as that of the commercial zeolite membranes, such as NaA, which can be explained by following points. i) Much longer mass transfer path in 2D MXene membranes gives lower flux. In most 2D lamellar membranes, the mass transfer path is the amount of nanochannels between the neighboring nanosheets, which is always tortuous curves along each 2D layers. Such special microsturcture inside 2D membrane endows it much prolonged mass transfer path with increased mass transfer resistance, which counts against high flux. That is why the water flux through the MXene membrane is lower than that of zeolite membranes, which possess much shortened pathway.
Fig. 11. Effect of the operating temperature on the ethanol dehydration performance of the MXene membrane feeding with 90% ethanol.
where h is the membrane thickness, ε and τ represent the porosity and tortuosity of the pathway, D and K represent the diffusivity and the solubility. In other words, the permeance depends on the membrane thickness, surface property (affecting the solubility), pore size (affecting the diffusivity), as well as the porosity and tortuosity of two pathways in the membrane. For the MXene membranes prepared in our work, the semiquantative analysis of the permeance can be performed using above model. Considering that there were no obvious defects on the fabricated MXene nanosheets as proved in our previous work [27], so the permeance contribution of pathway B can be neglected. The porosity (εA) of the MXene membrane in this work can be approximately estimated as the ratio of the empty interlayer space (0.37 nm) to d-spacing (1.37 nm), which is 0.27. The tortuosity (τA) of the MXene membrane can be approximated as the ratio of the MXene nanosheets length to thickness, which is about 1500. So the ratio of εA and τA is approximately 1.8 × 10−4 which is significantly smaller than that of the zeolite
For the mass transfer in special 2D membranes, Ibrahim and Lin [48,49] reported the inter-sheet and inner-sheet mass transport model for semiquantative analysis. Two pathways can be found in 2D structure, where the inter-sheet pathway A stands for the interlayer space between stacked 2D nanosheets and randomly distributed nanochannels at wrinkles, while the inner-sheet pathway B constitutes 2D nanosheets structure defects. And the permeance through the 2D membrane can be described by the formula below:
1 ε ε F = ⎛ ⎞ ⎧ ⎛ A ⎞ DA KA + ⎛ B ⎞ DB KB ⎫ ⎬ τ τ ⎝ h ⎠⎨ A ⎝ B⎠ ⎩⎝ ⎠ ⎭ ⎜
⎟
⎜
⎟
Fig. 10. (a) XRD patterns and corresponding d-spacing of the MXene membranes immersed in different solvents. (b) d-spacing of MXene membranes as a function of immersing time when soaked in water and ethanol. 6
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presents a much better anti-swelling property in ethanol. Therefore, it is impossible to avoid membrane swelling during water/ethanol separation. Once the interlayer spacing of MXene membrane expands up higher than the dynamic diameter of ethanol (0.45 nm), the water/ethanol separation factor will go down sharply. That is why higher separation factor can be obtained with higher feed ethanol concentration as shown in Fig. 9, where membrane swelling was limited to some extent. For the zeolite membrane, there is no swelling when immersed in water/ethanol solution, the constant pore size endows it higher separation factor. iii) Some defects in layered structure also make the separation factor sacrificed. It is not easy to prepare a perfect 2D membrane without any defect, where the size sieving pore is only the nanochannels between neighboring nanosheets. But in fact, the defects existing in the membrane are also used as the pore for mass transfer. Thus, the defects with size bigger than 0.45 nm will reduce the water/ethanol separation factor of membrane. Although there are several problems as mentioned above, it is still a new try to utilize the MXene membrane for ethanol dehydration, which extends the application field for 2D membranes. In order to enhance the total permeance through the MXene membranes, several ways can be tried. (a) Decreasing the membrane thickness. (b) Improving the solubility by chemical modification of the membrane surface or MXene nanosheets. (c) Increasing the diffusivity by designing the 2D nanochannel structure, and/or operating at elevated pressure or temperature. (d) 2D membrane with bigger empty interlayer space presents faster mass transfer, but the separation factor should be concerned. (e) MXene membrane stacked of nanosheets with smaller radial size will also show better permeance. (f) Punching some nanopores or sub-nanopores on the nanosheets will contribute more pathway, benefiting for enhancing the permeance. In order to obtain better separation performance, defect-free ultra-thin 2D membranes with better anti-swelling property are required, and more work are on the way.
Fig. 12. Dehydration performance of a 2-μm-thick MXene membrane as a function of operating time feeding with 95% ethanol at room temperature.
Fig. 13. The surface SEM image of the MXene membrane after separation test. Table 3 Comparison of the ethanol dehydration performance between the MXene membrane and some zeolite membranes. Membrane material
Feed ethanol content (wt%)
Operation temperature (oC)
Flux (g·m−2·h−1)
Separation factor
Reference
NaA NaA FAU CHA Cu-LTA Silica Mordenite zeolite X MXene MXene
90 90 90 90 90 90 90 90 90 95
75 75 75 75 75 80 150 60 25 25
19700 9500 9100 12000 3520 1000 160 3370 258.8 263.4
> 80000 11,100 190 10000 3591 800 139 296 92.8 135.2
[40] [41] [42] [43] [44] [45] [46] [47] This work This work
4. Conclusion Ti3AlC2 particles were successfully exfoliated to 2D Ti3C2Tx MXene nanosheets and 2 μm-thick MXene membranes were fabricated with the as-synthesized MXene nanosheets by vacuum filtration. For the first time, the MXene membranes are applied in the ethanol dehydration considering its advantage of abundant oxygen functional groups terminated on the 2D MXene nanosheets. This kind of MXene membrane exhibited a good performance at room temperature with relatively higher ethanol feed concentration (such as the alcohol water azeotrope with concentration of 95%). Typically, the as-prepared MXene membrane showed a water/ethanol separation factor of 135.2 with a total flux of 263.4 g m−2 h−1 at room temperature for 95% ethanol dehydration. Furthermore, the MXene membrane showed a better antiswelling property when soaked in such solvent, which is beneficial for obtaining higher water/ethanol separation factor, demonstrating a promising alternative for ethanol dehydration process compared with the traditional distillation. Of course, the performance of the MXene membrane applied in the alcohol dehydration process still needs to be improved compared with other pervaporation membranes, and more efforts will be done in the future.
membranes, for example the ratio of εA and τA of MFI-type zeolite membrane is 0.25 as reported in Ref. [50]. Therefore, it is easy to understand the low flux through the 2D MXene membrane, which is mainly resulted from the three orders of magnitude smaller ratio of εA to τA, comparing with the traditional zetolite membranes.
Notes ii) Swelling of MXene membrane in water/ethanol mixture makes larger interlayer spacing, which decreases the separation factor. Normally, water (dynamic diameter of 0.29 nm) and ethanol (dynamic diameter of 0.45 nm) molecules can be separated via size sieving by the MXene membrane with small empty spacing of ~0.42 nm (soaked in ethanol). As shown in Fig. 10, the d-spacing of MXene membrane increases when immersed in water, although it
The authors declare no competing financial interest. Acknowledgements We gratefully acknowledge the funding from the Natural Science Foundation of China (21606086 and 21861132013), Guangdong Natural Science Funds for Distinguished Young Scholar 7
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(2017A030306002), the Guangzhou Technology Project (no. 201707010317) and Fundamental Research Funds for the Central Universities.
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