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PIM-1 pore-filled thin film composite membranes for tunable organic solvent nanofiltration
Journal of Membrane Science 601 (2020) 117951
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
PIM-1 pore-filled thin film composite membranes for tunable organic solvent nanofiltration Jiaqi Li a, Mengxiao Zhang a, Weilin Feng a, Liping Zhu a, *, Lin Zhang b a
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China b Key Laboratory of Biomass Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymers of intrinsic microporosity (PIMs) Pore-filling Thin film composite membranes Solvent vapor annealing Organic solvent nanofiltration
Herein, a novel thin film composite (TFC) membrane pore-filled with polymer of intrinsic microporosity, PIM-1, has been developed for organic solvent nanofiltration (OSN). The TFC membranes were facilely prepared by dipcoating of PIM-1/chloroform solution onto polyacrylonitrile support membranes followed by a solvent vapor annealing (SVA) process. The relationship between preparation conditions, selective layer morphologies and OSN performances of the prepared TFC membranes were studied systematically. It was found PIM-1 was evidently filled into the near surface pores of support membranes due to the infiltration of PIM-1 solution. With the elongation of SVA time, the thickness of the PIM-1 layer decreased and the PIM-1/PAN interface gradually disappeared, which is advantageous to enhance the interfacial bonding. The prepared TFC membranes were used to reject organic molecules in organic solvents and exhibited tunable solvent permeability and solute rejection. Especially, the membranes were suitable for the removal of neutral molecules and anionic dyes from ethanol. The typical membrane demonstrated an ethanol permeance of 4.3 L m 2 h 1 bar 1 with a rejection of 93.7% towards Methyl Orange (327 Da). By comparing the permeance of various solvents, it is found that the closer the Hansen solubility parameter of solvent is to that of PIM-1, the higher the solvent permeance. This indicates the affinity between solvent and membrane plays an important role in solvent permeability. This work offers a novel and convenient strategy to prepare highly permeable membranes with controllable microstructures and tunable permselectivity to meet the diverse demands of molecular separation in organic solvents.
1. Introduction Separation process plays a vital role in chemical and pharmaceutical industry. Traditional separation processes like distillation and evapo ration, suffer from high energy consumption due to the latent heat of vaporization. Membrane technology has been widely used in separation because of nearly less 90% of energy consumption than traditional distillation or rectification [1–3]. Many industrial processes in phar maceuticals and specialty chemicals require separation of solutes from organic solvents, which give rise to the development of organic solvent nanofiltration (OSN) [4–7]. As a novel technology that allows efficient and sustainable separation of solute molecules with molecular weights of 200-1000 g mol 1 from different organic solvents, OSN process can perform concentration, purification and solvent exchange by applying a pressure gradient across a membrane. However, low solvent perme ability is currently one of the major obstacles that limits the
development and application of OSN technique [8]. Thus, materials with high solvent permeability are highly desirable to achieve time-saving and energy-efficient separation. In recent years, new microporous materials with pore size less than 2 nm have shown great promise and attracted much attention in OSN, such as polymers of intrinsic microporosity (PIMs) [9–11], metal organic frameworks [12–15] and covalent organic framework materials [16–19]. PIMs are a class of polymers that synthesized from a series of mol ecules with twisted or rigid structures via dibenzodioxane, imidization or amidization reactions [20]. These twisted and rigid structures allow the polymer segments to have low flexibility in spatial conformation, which results in the formation of interconnected pores when polymer segments are stacked. The interconnected pore is defined as “intrinsic microporosity” [21–24]. The intrinsic micropores can act as channels that allow small molecules to pass quickly through, which makes PIMs be potential in membrane-based molecular separation such as gas
* Corresponding author. E-mail address: [email protected] (L. Zhu). https://doi.org/10.1016/j.memsci.2020.117951 Received 26 August 2019; Received in revised form 11 January 2020; Accepted 8 February 2020 Available online 10 February 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.
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Journal of Membrane Science 601 (2020) 117951
Fig. 1. Schematic diagram of PIM-1 TFC membrane preparation.
separation, pervaporation as well as OSN [25–29]. Recently, some work on the design and preparation of PIM-1 thin film composite (TFC) membranes for molecular separation has been reported. Fritsch et al. [30] reported a technique to fabricate PIMs TFC membranes, in which PIMs and the crosslinker were blended in solution and then dip-coated onto the surface of polyacrylonitrile (PAN) support membrane fol lowed by a thermal crosslinking process. The typical TFC membrane exhibited about 40 times higher permeance than Starmem240 (a com mercial polyimide-based OSN membrane) for alkanes. Livingston’s group prepared PIMs TFC membranes by spin-coating and roll-to-roll dip-coating, respectively and the obtained membranes exhibited two orders of magnitude higher n-heptane flux than Starmem240 as well as high separation factor for linear and branched C16 alkanes [31,32]. Gao et al. used a modified PIM-1 to prepare TFC membranes by spin-coating, and the PIM-1 selective layer was further crosslinked with trimesoyl chloride to improve its solvent resistance [33]. Recently, in Budd’s group, the pore size and surface porosity of polyvinylidene fluoride (PVDF) supports used for PIM-1/PVDF TFC membrane were optimized [34]. Furthermore, a poly[1-trimethylsilyl-1-propyne] gutter layer was introduced as a middle layer between PIM-1 and porous backing mate rial to enhance gas selectivity of the TFC membranes [35]. These efforts show that PIM-1 is a promising material for the fabrication of composite membranes with high solvent or gas permeability. However, due to the limited processability of PIM-1 (dissolved only in a small number of solvents), some problems need to be further addressed in the PIM-1 coating process, such as unavoidable surface defects, weak bonding of selective layer with support membrane, trade-off effect between selec tivity and permeability, etc. The PIM-1 TFC membranes can be fabricated by spin-coating or dipcoating technique, wherein the latter is more suitable for continuous production in practical industry. In the dip-coating process, immersion and dwelling, deposition and drainage, and evaporation are three important technical steps that affect the structures and performance of coating [36]. The solvents for PIM-1 like tetrahydrofuran (THF) or chloroform (CHCl3) have low boiling points and are easy to evaporate in air, which often results in a rough film with uncontrolled defects due to rapid solvent evaporation. Fortunately, the solvent evaporation process can be effectively controlled by solvent vapor annealing (SVA) process [37,38]. Therefore, in this work, we intend to exploit a novel strategy, dip-coating followed by SVA process, to fabricate PIM-1 TFC membranes with tunable OSN permeation and separation performances. The effects of dipping concentration and SVA time on the morphologies and per formances of the PIM-1 TFC membranes were investigated in detail.
Equipment (China) and used as the supports of TFC membranes. 1,4Dicyanotetrafluorobenzene (DCTB), potassium carbonate (K2CO3) and 5,50 ,6,60 -Tetrahydroxy-3,3,30 ,30 -tetramethyl-1,10 -spirobisindane (TTSBI) were purchased from Energy Chemical (Shanghai, China). Prior to use, DCTB was purified by sublimation at 150 � C, K2CO3 was dried at 110 � C overnight and TTSBI was dissolved in methanol and reprecipitated from dichloromethane. Dimethylacetamide (DMAc), chlo roform (CHCl3), toluene, ethanol (EtOH), acetone and n-heptane were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China). Isatin, Basic Orange 2, Azure B, Methyl Orange, Riboflavin, Crystal Vi olet, Food Yellow 3, Basic Blue 26, Acid Black 1, Tetrazolium Blue Chloride, Acid Blue 90 and Acid Red 94 were purchased from the Aladdin Industry Co. (Shanghai, China). Vitamin B12 (VB12) was pur chased from Sigma-Aldrich Trading Co. Ltd. (Shanghai, China). All re agents were of analytical grade and used without further purification except for those specifically mentioned.
2. Experimental
2.4. Preparation of PIM-1 TFC membrane
2.1. Materials
The PIM-1 TFC membrane was fabricated by dip-coating method followed by an SVA process (Fig. 1). Typically, the PAN support mem brane was rinsed in CHCl3 for 30 s. Then, the membrane was transferred to another container having a PIM-1/CHCl3 dipping solution inside and
2.2. Synthesis of PIM-1 PIM-1 was synthesized as described by Satilmis et al. [39]. TTSBI (17.02 g, 0.05 mol), DTCB (10.01 g, 0.05 mol), K2CO3 (20.7 g, 0.15 mol), DMAc (100 mL) and toluene (50 mL) were added to a dry 500 mL of three necked round bottom flask equipped with a Dean-stark trap and mechanical stirrer. Then the device was filled with nitrogen and placed in an oil bath pre-heated to 160 � C, and the reaction was carried out for 50 min under reflux. After cooling, the obtained highly viscous solution was dissolved in CHCl3, filtered and re-precipitated from methanol. The product was washed alternately with water and methanol and then dried at 110 � C overnight to obtain purified PIM-1. 2.3. Characterization of PIM-1 Gel permeation chromatography (GPC, Water-515, USA) was used to analyze number-average molecular weight (Mn) and polymer dispersity index (PDI) of PIM-1. The polymer was dissolved in THF to obtain a 0.3 wt% of solution. It was then run through GPC with THF as the eluent was at 1 mL min 1. Using polystyrene as a standard, the effluent time of the THF mobile phase was measured at a test temperature of 40 � C and the obtained data were integrated to calculate the molecular weight of PIM1. The chemical structures of PIM-1 were analyzed by attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 6700, USA) and solution-state 1H nuclear magnetic resonance spec troscopy (NMR, Bruker Advance III 500, Germany).
A PAN flat-sheet ultrafiltration (UF) membrane with molecular weight cut-off of 50 kDa was purchased from Beijing Separation 2
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Journal of Membrane Science 601 (2020) 117951
held at the upper end of container without contact with the dipping solution. Afterwards, the container was turned upside down to completely dip the support membrane in the solution for 20 s, and then the container was reset upright. The soaked membrane was kept in CHCl3 vapor to conduct a SVA step for a pre-designed period. Finally, the membrane was taken out and dried in air to obtain a PIM-1 TFC mem brane. The above-mentioned PIM-1/CHCl3 dipping solutions with various concentrations were beforehand prepared and filtered through an 0.45 μm PTFE filter before use. Both of dip-coating and SVA steps were carried out at room temperature (18 � 5 � C). 2.5. Characterizations of TFC membranes The surface chemistry of the prepared TFC membranes were analyzed by ATR-FTIR and X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA system, USA). The surface morphologies of the investigated membranes were observed by field-emission scanning electron micro scopy (FE-SEM, Hitachi S-4800, Japan), atomic force microscopy (AFM, SPI3800 N, Seiko Instrumental) and transmission electron microscopy (TEM, HT-7700, Japan). The sample photographs were taken by a smartphone.
Fig. 2. ATR-FTIR spectra and sample photographs of PIM-1 TFC membrane (4.0 wt% of dipping concentration and 4 min of SVA time) and PAN sub strate surface. Table 1 Elemental mole percentages of PIM-1 polymer, the PIM-1 TFC membranes sur face and the PAN substrate detected by XPS analysis.
2.6. OSN experiments
Sample
OSN performance tests were carried out using a dead-end solvent resistant stirred cell (Millipore Co., USA) with an effective area of 15 cm2. The pressure in the stirred cell was controlled by a pressure regu lator. In a typical test, a membrane was pre-pressured under 0.5 MPa with pure solvent as the feed for 0.5 h and then tested under 0.4 MPa. The permeation flux (F, L m 2 h 1) was calculated using the following equation: F¼
V At
PAN substrate PIM-1 powder PIM-1 TFC membrane #1 PIM-1 TFC membrane #2
O
N
F
70.9 81.8 79.5 81.6
24.9 14.1 16.6 14.9
4.0 3.9 3.5 3.2
0.2 0.2 0.4 0.3
Herein, PIM-1 solutions with CHCl3 as solvent were used to dip-coat PAN UF membrane to construct thin PIM-1 selective layers. SVA process was employed to control solvent evaporation rate and eliminate the small pits on PIM-1 layer. The coating of PIM-1 onto the PAN substrate was confirmed by ATR-FTIR, XPS, and sample photographs (Fig. 2 and Table 1). Compared with PAN substrate, obvious characteristic peaks in the range of 1273–1248 cm 1 and 1510-1400 cm 1 corresponding to aromatic ether and benzene ring of PIM-1, respectively, appear in the ATR-FTIR spectrum of the PIM-1 dip-coated membrane (Fig. 2). More over, the spectrum is very similar to the FTIR of PIM-1 (Fig. S1). From the elemental mole percentages detected by XPS (Table 1), it can be seen that the surface chemical compositions of PIM-1 TFC membranes are nearly identical to that of PIM-1. Moreover, it can be seen obviously that the membrane color changes from white to yellow after dip-coating (Fig. 2). These results show evidently PIM-1 was successfully coated onto the PAN UF support. In the design and preparation of TFC membranes, thin and defectfree dense selective layer is generally desirable to achieve high separa tion performances, which are inevitably dominated by intrinsic trade-off between permeability and selectivity. In this work, the dipping solution concentration and SVA time are two key parameters to tune the thick ness and uniformity of PIM-1 layer. The cross-sectional morphologies of PIM-1 TFC membranes prepared with various dipping concentrations (SVA time was set at 3 min) were observed by FE-SEM and the typical images are shown in Fig. 3. The higher dipping concentration resulted in the thicker PIM-1 layer. As the concentration is 0.5 wt%, the thickness of PIM-1 layer coated onto PAN substrate is relatively thin (about 200 nm). But this may lead to the generation of defects and thus low solute rejection. With the increase of the dipping concentration to 4.0 wt%, the thickness of PIM-1 layer reaches up to ~600 nm and the defects are effectively avoided. However, too thick selective layer often results in low solvent flux. Therefore, 2.0 wt% was used as the typical dipping concentration in the following study.
(1)
Cp � 100% Cf
C
#1: 2.0 wt% of dipping concentration, 1 min of SVA time; #2: 2.0 wt% of dip ping concentration, 3 min of SVA time.
where V(L) is the volume of permeated solvent during the operating time t(h), and A (m2) is the effective membrane area. A serial of solute molecules (anionic dyes, cationic dyes and neutral molecules, Table S1) were used as the probes to evaluate solute rejection performances of the prepared TFC membranes. Ethanol was used as the solvent of the feed solutions. The solute rejection tests were performed in the dead-end cell loaded about 500 mL of solution and the operation pressure was locked at 0.4 MPa. A small volume of the permeate at the initial stage of filtration was collected and the concentration was determined using a UV/vis spectrophotometer (UV5500-PC, shanghai Jingke Instrument, China). The rejection (R, %) was calculated by the following equation: R¼1
Atomic percentage (%, mole concentration)
(2)
where Cp and Cf represent the solute concentrations in the permeate and the feed (the original solution), respectively. All filtration tests were performed at room temperature (25 � 1 � C). All of the reported data are the averages of three paralleled measurements and the standard de viations were also reported. 3. Results and discussion 3.1. Characterizations of the PIM-1 selective layer The successful synthesis of PIM-1 was proved by the ATR-FTIR and H NMR spectra as shown in Fig. S1 and Fig. S2 of Supplementary In formation. The Mn of PIM-1 was about 50,000 Da with a PDI value of 1.6 as characterized by GPC with polystyrene as standards and THF as sol vent (Fig. S3). The molecular weight of PIM-1 here is enough high to form uniform separation layer on PAN UF supports by dip-coating.
1
3
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Journal of Membrane Science 601 (2020) 117951
Fig. 3. Cross-sectional FE-SEM images for PIM-1 TFC membranes prepared with different dipping concentrations (a) 0.5 wt%; (b) 1.0 wt%; (c) 2.0 wt%; (d) 4.0 wt% (SVA time was set at 3 min).
Fig. 4. Cross-sectional and surface FE-SEM images for PIM-1 TFC membranes prepared with different SVA times (a) 0 min; (b) 1 min; (c) 2 min; (d) 3 min; (e) 4 min; (f) 5 min (dipping concentration was fixed at 2.0 wt%).
The rapid evaporation of low boiling point solvents often leads to small pits in polymer coatings in dip-coating process due to the gener ation of solvent bubbles. Although the depth of these pits is only 30–60 nm (analyzed by AFM, Figs. S4 and S5) and don’t span all over whole cross-section of PIM-1 layer (200–800 nm of thickness), they may become gradually defects during long-term separation operation. Therefore, in this work, a SVA process was used to control the evapo ration rate of CHCl3 and eliminate the small pits in PIM-1 layer. The influences of SVA time on the cross-sectional and surface morphologies of PIM-1 TFC membranes are shown in Fig. 4. It can be seen that, as the SVA time is less than 1 min, many small pits appear on the PIM-1 layers because of rapid solvent evaporation (Fig. 4 (a) and (b)). As the SVA time
increases, the pits of PIM-1 layer surfaces are significantly suppressed and more smooth membrane surfaces are obtained. More importantly, it is found that the thickness of PIM-1 layer can be regulated by SVA process. As the PIM-1 TFC membrane was prepared by single dip-coating without SVA step (namely SVA time was 0 min), the PIM-1 precipitated rapidly from the dipping solution film with solvent evaporation and a PIM-1 layer was created on the support membrane (Fig. 4(a)). An obvious interface is observed between PIM-1 layer and the substrate, and the thickness of PIM-1 layer is about ~800 nm. With the elongation of SVA time, the thickness of surface layer decreases due to the infil tration of PIM-1 solution into surface pores. When the SVA time reaches 5 min, the thickness of surface layer is only ~200 nm and the interface 4
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Journal of Membrane Science 601 (2020) 117951
Fig. 5. Ethanol permeation flux and VB12 rejection for the PIM-1 pore-filled TFC membranes prepared with (a) different dipping concentrations (SVA time was fixed at 3 min) and (b) different SVA time (dipping concentration was fixed at 2.0 wt%). The trans-membrane pressure was 0.4 MPa.
become obscure. The yellow color of the TFC membranes gradually become lighter (Fig. S6). The TEM and AFM images of the interface between PIM-1 layer and PAN support (S7 and S8) further indicate the PIM-1 was evidently filled into the near surface pores of PAN support membrane. Just as discussed above, too thin surface layer may lead to the generation of defects and thus low solute rejection. Therefore, in this work, the upper limit of SVA time was set at 5 min. The occurrence of filling phenomenon is advantageous to the firm attachment of PIM-1 atop the support membrane and avoid the peeling off in long-term separation operation. In addition, the surface layer thickness can be conveniently regulated by SVA time and thus the permeation and sep aration performances can be tuned. In this work, considering the filling of PIM-1 inside the near surface pores of PAN support membrane, the obtained TFC membranes are called as PIM-1 pore-filled TFC membranes. The thickness and unifor mity of PIM-1 layer can be regulated and controlled by two key pa rameters including the dipping concentration and the SVA time. These results show it is possible to develop TFC membranes with tunable OSN performances by the combination of dip-coating and SVA process.
Fig. 6. The trade-off between ethanol flux and VB12rejection for the PIM-1 pore-filled TFC membranes prepared with different SVA time in the cases of 1.0 or 2.0 wt% of dipping concentration.
membranes, and results in decreased solute rejection. These results indicate that tunable permeability and selectivity can be achieved through the SVA process. To clarify the relationship between permeability and selectivity for the investigated PIM-1 pore-filled TFC membranes more clearly, the VB12 rejection was plotted against the ethanol flux, as shown in Fig. 6. A typical trade-off effect was observed between the ethanol flux and the VB12 rejection. Obviously, with the increase of dipping concentration in the preparation of TFC membranes, the trade-off line shifts towards right. With the increase of ethanol flux, the VB12 rejection decreases for both dipping concentrations 1.0 or 2.0 wt%. Moreover, at same ethanol flux level, the TFC membranes prepared with 2.0 wt% of dipping con centration demonstrate higher rejection than those prepared with 1.0 wt %. Similarly, the membranes prepared with 2.0 wt% have higher fluxes than those prepared with 1.0 wt%. As the dipping concentration is 1.0 wt%, shorter SVA time (e.g. direct evaporation in air without SVA process, namely 0 min of SVA time, Point 1 in Fig. 6) is required to achieve a high rejection. The TFC membrane displays thick PIM-1 layer and lots of small pits (Fig. S10). As the dipping concentration rises to 2.0 wt% and the SVA time increases to 3 min (Point 2 in Fig. 6), the obtained TFC membrane exhibits high flux (17.2 L m 2 h 1) as well as high rejection (93.9%) because a defect-free and thinner PIM-1 layer is generated (Fig. 4 (d)). It is worth noting that too high dipping concen tration (>2.0 wt%) results in a thick PIM-1 layer even if a long SVA process is conducted. Therefore, an appropriate collocation of the dip ping concentration and the SVA time is critical to achieve high perme ation and separation performances for PIM-1 pore-filled TFC
3.2. Tunable separation performances of PIM-1 TFC membranes The prepared PIM-1 pore-filled TFC membranes were used for or ganics rejection in organic solvent. The effects of the dipping concen tration and SVA time on the ethanol permeation flux and the VB12 rejection were investigated, respectively. As shown in Fig. 5 (a), with the increase of dipping concentration, the ethanol flux gradually decreases while the VB12 rejection increases. This can be attributed to the increase of PIM-1 layer thickness and the elimination of defects, which has been discussed in Section 3.1. In the case of 0.5 wt% of dipping concentration, the VB12 rejection is only ~20% since the PIM-1 layer is too thin and possibly defective. When the dipping concentration increases up to 2.0 wt%, the rejection reaches 93%, indicating the PIM-1 layer is defect-free and capable of separating effectively VB12 molecules from ethanol. Nevertheless, the further increase of dipping concentration to 4.0 wt% leads to a low ethanol flux (only ~5 L m 2 h 1), which is disadvanta geous to achieve time-saving and energy-efficient separation. Therefore, in this work, based on a compromise between solvent permeation flux and VB12 rejection, 2.0 wt% was a preferential dipping concentration. As shown in Fig. 5 (b) (2.0 wt% of the dipping concentration), with the elongation of SVA time, the ethanol flux increases obviously while the VB12 rejection decrease to some extent. As the SVA time reaches 3 min, the flux increases significantly from 2.9 to 17.2 L m 2 h 1 and the rejection keeps more than 93%. Similar phenomenon occurred in the case of 1.0 wt% as the dipping concentration (Fig. S9). However, too long SVA time (>4 min) may bring some defects to PIM-1 layer due to excessive infiltration of PIM-1 solution into the interior pores of support 5
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Journal of Membrane Science 601 (2020) 117951
permeance reaches up to 31.7 L m 2 h 1 bar 1 since the HSP of acetone (20.3 J0.5 cm 1.5) is very close to that of PIM-1. This phenomenon shows the solvent permeance is greatly influenced by the affinity of solvent with PIM-1. Except for solubility parameter, the viscosity and molecular size of solvents are associated with their permeability [8]. It can be seen from Fig. 7, although both acetone and toluene have similar HSPs with PIM-1 (20.3 and 18.2 J0.5 cm 1.5, respectively), their permeances in the TFC membrane are obviously different (only 18.0 L m 2 h 1 bar 1 for toluene). This can be attributed to their great difference in viscosity (0.32 � 10 3 Pa s for acetone and 0.55 � 10 3 Pa s for toluene at 25 � C). More interestingly, the fluxes of toluene and heptane are remarkably higher than that of ethanol, which is different from the reported phe nomenon [8]. This may be attributed to the difference in membrane materials (crosslinked polyamide in the reference, but linear PIM-1 in our work) which have different swelling characteristics in solvents. However, more detailed mechanism keeps unknown and should be studied in next work. The rejections towards various solutes in ethanol including anionic and cationic dyes and neutral molecules with a serial of molecular weights were examined and the results are shown in Fig. 8. The mo lecular weight cut-off (MWCO) for a membrane is often defined by the molecular weight with 90% of rejection. The MWCO towards both neutral molecules and cationic dyes is about 530 Da while that towards anionic dyes is lower than 320 Da. In other words, the membrane ex hibits higher rejection towards anionic dyes than cationic ones with a similar molecular weight. This phenomenon was also reported in other work [33,41]. This indicates that the solute rejection is determined by not only membrane pore size and solute molecular size, also the inter action between solute and membrane [42]. The solvent permeances and MWCOs for the typical membrane developed in this work are compared with the membranes reported in literatures, as shown in Table 2. It is found that the PIM-1 pore-filled TFC membrane demonstrates higher permeation and separation performances.
Fig. 7. The solvent permeances plotted against their corresponding HSPs for the typical PIM-1 pore-filled TFC membranes (prepared with 2.0 wt% of dip ping concentration, 3 min of SVA time).
4. Conclusions
Fig. 8. Rejection of TFC membrane for different molecules (20 ppm in ethanol solution, 2.0 wt% of dipping concentration, 3min of SVA time).
A PIM-1 pore-filled TFC membrane was successfully developed by dip-coating followed by a SVA process. The introduction of SVA process can effectively reduce the generation of the small pits and defects on the PIM-1 layer. Long SVA time is advantageous to form a thin PIM-1 porefilled layer, but too long would bring defects to the TFC membranes. As the SVA time increases, the ethanol flux increases while the VB12 rejection decreases for the obtained TFC membranes. An appropriate collocation of the dipping concentration and the SVA time is critical to achieve high permeation and separation performances. The PIM-1 porefilled TFC membranes display remarkable rejection performance to wards dyes and neutral molecules, especially towards anionic dye in ethanol. Typically, the prepared TFC membrane exhibited an ethanol permeance of 4.3 L m 2 h 1 bar 1 with a rejection of 93.7% towards Methyl Orange (327 Da). The solvent permeability is higher than that of the PIM-1 TFC membranes reported previously.
membranes. 3.3. Solvent permeance and molecular separation Considering the interaction between solvent and membrane plays an important role in solvent permeation, the solvent permeances are related to their corresponding Hansen solubility parameters (HSPs) [31]. The solvent permeances were obtained by examining the fluxes of different solvents permeating through a typical PIM-1 pore-filled TFC membranes (prepared with 2.0 wt% of dipping concentration and 3 min of SVA time) under 0.4 MPa, and the results are shown in Fig. 7. It can be seen that, the closer the HSP of the solvent is to that of PIM-1 (~19.4 J0.5 cm 1.5 [40]), the higher the solvent permeance. For instance, the ethanol permeance is only 4.3 L m 2 h 1 bar 1 while the acetone
Table 2 The Comparisons in solvent permeances and MWCOs between the PIM-1 TFC membranes reported in literatures with the typical membrane developed in this work. PIM-1 TFC membranes PIM-1 (dip-coating) PIM-1/20%PEI PIM1-CO1-50/20%PEI PIM-1 (dip-coating) PIM-1 (spin coating) Thioamide-PIM-1 PIM-1 (dip-coating)
Solvent permeance (L m
2
h
1
bar
1
)
N-heptane
Ethanol
Acetone
Toluene
4.2 / / 7 18 / 14.7
/ / / / / 3.4 4.3
/ / / / / 12.4 31.7
/ 3.1 0.9 11 / / 18.2
/represents no data in the references. 6
Molecular weight cut-off (Da)
Ref.
290 460 350 ~700 ~500 ~600 ~600
[30] [30] [30] [31] [32] [33] This work
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Declaration of competing interests
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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. CRediT authorship contribution statement Jiaqi Li: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft. Mengxiao Zhang: Writing - review & editing, Resources. Weilin Feng: Investigation. Lin Zhang: Funding acquisition, Supervision, Project administration, Writing - review & editing. Acknowledgments The authors are grateful for the financial supports from the National Natural Science Foundation of China (Grant No. 51973185, 51773175 and 51828301) and the Fundamental Research Funds for the Central Universities (Grant No. 2019QNA4062). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2020.117951. References [1] P. Marchetti, M.F. Jimenez Solomon, G. Szekely, A.G. Livingston, Molecular separation with organic solvent nanofiltration: a critical review, Chem. Rev. 114 (2014) 10735–10806, https://doi.org/10.1021/cr500006j. [2] D.S. Sholl, R.P. Lively, Seven chemical separations to change the world, Nature 532 (2016) 435–437, https://doi.org/10.1038/532435a. [3] P. Vandezande, L.E.M. Gevers, I.F.J. Vankelecom, Solvent resistant nanofiltration: separating on a molecular level, Chem. Soc. Rev. 37 (2008) 365–405, https://doi. org/10.1039/B610848M. [4] M.G. Buonomenna, J. Bae, Organic solvent nanofiltration in pharmaceutical Industry, Separ. Purif. Rev. 44 (2015) 157–182, https://doi.org/10.1080/ 15422119.2014.918884. [5] G. Sz�ekely, J. Bandarra, W. Heggie, B. Sellergren, F.C. Ferreira, Organic solvent nanofiltration: a platform for removal of genotoxins from active pharmaceutical ingredients, J. Membr. Sci. 381 (2011) 21–33, https://doi.org/10.1016/j. memsci.2011.07.007. [6] J.P. Sheth, Y. Qin, K.K. Sirkar, B.C. Baltzis, Nanofiltration-based diafiltration process for solvent exchange in pharmaceutical manufacturing, J. Membr. Sci. 211 (2003) 251–261, https://doi.org/10.1016/S0376-7388(02)00423-4. [7] G.C. Vougioukalakis, Removing ruthenium residues from Olefin metathesis reaction products, Chem. Eur J. 18 (2012) 8868–8880, https://doi.org/10.1002/ chem.201200600. [8] S. Karan, Z. Jiang, A.G. Livingston, Sub-10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation, Science 348 (2015) 1347–1351, https://doi.org/10.1126/science.aaa5058. [9] N.B. McKeown, P.M. Budd, Exploitation of intrinsic microporosity in polymerbased materials, Macromolecules 43 (2010) 5163–5176, https://doi.org/10.1021/ ma1006396. [10] N. Du, G.P. Robertson, J. Song, I. Pinnau, M.D. Guiver, High-performance carboxylated polymers of intrinsic microporosity (PIMs) with tunable gas transport properties y, Macromolecules 42 (2009) 6038–6043, https://doi.org/10.1021/ ma9009017. [11] P. Budd, N. Mckeown, B. Ghanem, K. Msayib, D. Fritsch, L. Starannikova, N. Belov, O. Sanfirova, Y. Yampolskii, V. Shantarovich, Gas permeation parameters and other physicochemical properties of a polymer of intrinsic microporosity: polybenzodioxane PIM-1, J. Membr. Sci. 325 (2008) 851–860, https://doi.org/ 10.1016/j.memsci.2008.09.010. [12] J. Campbell, J.D.S. Burgal, G. Szekely, R.P. Davies, D.C. Braddock, A. Livingston, Hybrid polymer/MOF membranes for Organic Solvent Nanofiltration (OSN): chemical modification and the quest for perfection, J. Membr. Sci. 503 (2016) 166–176, https://doi.org/10.1016/j.memsci.2016.01.024. [13] J. Campbell, G. Sz�ekely, R.P. Davies, D.C. Braddock, A.G. Livingston, Fabrication of hybrid polymer/metal organic framework membranes: mixed matrix membranes versus in situ growth, J. Mater. Chem. A. 2 (2014) 9260–9271, https://doi.org/ 10.1039/C4TA00628C. [14] A.J. Howarth, Y. Liu, P. Li, Z. Li, T.C. Wang, J.T. Hupp, O.K. Farha, Chemical, thermal and mechanical stabilities of metal–organic frameworks, Nat. Rev. Mater. 1 (2016) 15018, https://doi.org/10.1038/natrevmats.2015.18.
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[39] B. Satilmis, P.M. Budd, Base-catalysed hydrolysis of PIM-1: amide versus carboxylate formation, RSC Adv. 4 (2014) 52189–52198, https://doi.org/ 10.1039/C4RA09907A. [40] S. Thomas, I. Pinnau, N. Du, M.D. Guiver, Pure- and mixed-gas permeation properties of a microporous spirobisindane-based ladder polymer (PIM-1), J. Membr. Sci. 333 (2009) 125–131, https://doi.org/10.1016/j. memsci.2009.02.003.
[41] Z. Wang, S. Guo, B. Zhang, L. Zhu, Hydrophilic polymers of intrinsic microporosity as water transport nanochannels of highly permeable thin-film nanocomposite membranes used for antibiotic desalination, J. Membr. Sci. 592 (2019) 117375, https://doi.org/10.1016/j.memsci.2019.117375. [42] S. Tsarkov, V. Khotimskiy, P.M. Budd, V. Volkov, J. Kukushkina, A. Volkov, Solvent nanofiltration through high permeability glassy polymers: effect of polymer and solute nature, J. Membr. Sci. 423–424 (2012) 65–72, https://doi.org/10.1016/j. memsci.2012.07.026.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
PIM-1 pore-filled thin film composite membranes for tunable organic solvent nanofiltration Jiaqi Li a, Mengxiao Zhang a, Weilin Feng a, Liping Zhu a, *, Lin Zhang b a
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China b Key Laboratory of Biomass Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymers of intrinsic microporosity (PIMs) Pore-filling Thin film composite membranes Solvent vapor annealing Organic solvent nanofiltration
Herein, a novel thin film composite (TFC) membrane pore-filled with polymer of intrinsic microporosity, PIM-1, has been developed for organic solvent nanofiltration (OSN). The TFC membranes were facilely prepared by dipcoating of PIM-1/chloroform solution onto polyacrylonitrile support membranes followed by a solvent vapor annealing (SVA) process. The relationship between preparation conditions, selective layer morphologies and OSN performances of the prepared TFC membranes were studied systematically. It was found PIM-1 was evidently filled into the near surface pores of support membranes due to the infiltration of PIM-1 solution. With the elongation of SVA time, the thickness of the PIM-1 layer decreased and the PIM-1/PAN interface gradually disappeared, which is advantageous to enhance the interfacial bonding. The prepared TFC membranes were used to reject organic molecules in organic solvents and exhibited tunable solvent permeability and solute rejection. Especially, the membranes were suitable for the removal of neutral molecules and anionic dyes from ethanol. The typical membrane demonstrated an ethanol permeance of 4.3 L m 2 h 1 bar 1 with a rejection of 93.7% towards Methyl Orange (327 Da). By comparing the permeance of various solvents, it is found that the closer the Hansen solubility parameter of solvent is to that of PIM-1, the higher the solvent permeance. This indicates the affinity between solvent and membrane plays an important role in solvent permeability. This work offers a novel and convenient strategy to prepare highly permeable membranes with controllable microstructures and tunable permselectivity to meet the diverse demands of molecular separation in organic solvents.
1. Introduction Separation process plays a vital role in chemical and pharmaceutical industry. Traditional separation processes like distillation and evapo ration, suffer from high energy consumption due to the latent heat of vaporization. Membrane technology has been widely used in separation because of nearly less 90% of energy consumption than traditional distillation or rectification [1–3]. Many industrial processes in phar maceuticals and specialty chemicals require separation of solutes from organic solvents, which give rise to the development of organic solvent nanofiltration (OSN) [4–7]. As a novel technology that allows efficient and sustainable separation of solute molecules with molecular weights of 200-1000 g mol 1 from different organic solvents, OSN process can perform concentration, purification and solvent exchange by applying a pressure gradient across a membrane. However, low solvent perme ability is currently one of the major obstacles that limits the
development and application of OSN technique [8]. Thus, materials with high solvent permeability are highly desirable to achieve time-saving and energy-efficient separation. In recent years, new microporous materials with pore size less than 2 nm have shown great promise and attracted much attention in OSN, such as polymers of intrinsic microporosity (PIMs) [9–11], metal organic frameworks [12–15] and covalent organic framework materials [16–19]. PIMs are a class of polymers that synthesized from a series of mol ecules with twisted or rigid structures via dibenzodioxane, imidization or amidization reactions [20]. These twisted and rigid structures allow the polymer segments to have low flexibility in spatial conformation, which results in the formation of interconnected pores when polymer segments are stacked. The interconnected pore is defined as “intrinsic microporosity” [21–24]. The intrinsic micropores can act as channels that allow small molecules to pass quickly through, which makes PIMs be potential in membrane-based molecular separation such as gas
* Corresponding author. E-mail address: [email protected] (L. Zhu). https://doi.org/10.1016/j.memsci.2020.117951 Received 26 August 2019; Received in revised form 11 January 2020; Accepted 8 February 2020 Available online 10 February 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram of PIM-1 TFC membrane preparation.
separation, pervaporation as well as OSN [25–29]. Recently, some work on the design and preparation of PIM-1 thin film composite (TFC) membranes for molecular separation has been reported. Fritsch et al. [30] reported a technique to fabricate PIMs TFC membranes, in which PIMs and the crosslinker were blended in solution and then dip-coated onto the surface of polyacrylonitrile (PAN) support membrane fol lowed by a thermal crosslinking process. The typical TFC membrane exhibited about 40 times higher permeance than Starmem240 (a com mercial polyimide-based OSN membrane) for alkanes. Livingston’s group prepared PIMs TFC membranes by spin-coating and roll-to-roll dip-coating, respectively and the obtained membranes exhibited two orders of magnitude higher n-heptane flux than Starmem240 as well as high separation factor for linear and branched C16 alkanes [31,32]. Gao et al. used a modified PIM-1 to prepare TFC membranes by spin-coating, and the PIM-1 selective layer was further crosslinked with trimesoyl chloride to improve its solvent resistance [33]. Recently, in Budd’s group, the pore size and surface porosity of polyvinylidene fluoride (PVDF) supports used for PIM-1/PVDF TFC membrane were optimized [34]. Furthermore, a poly[1-trimethylsilyl-1-propyne] gutter layer was introduced as a middle layer between PIM-1 and porous backing mate rial to enhance gas selectivity of the TFC membranes [35]. These efforts show that PIM-1 is a promising material for the fabrication of composite membranes with high solvent or gas permeability. However, due to the limited processability of PIM-1 (dissolved only in a small number of solvents), some problems need to be further addressed in the PIM-1 coating process, such as unavoidable surface defects, weak bonding of selective layer with support membrane, trade-off effect between selec tivity and permeability, etc. The PIM-1 TFC membranes can be fabricated by spin-coating or dipcoating technique, wherein the latter is more suitable for continuous production in practical industry. In the dip-coating process, immersion and dwelling, deposition and drainage, and evaporation are three important technical steps that affect the structures and performance of coating [36]. The solvents for PIM-1 like tetrahydrofuran (THF) or chloroform (CHCl3) have low boiling points and are easy to evaporate in air, which often results in a rough film with uncontrolled defects due to rapid solvent evaporation. Fortunately, the solvent evaporation process can be effectively controlled by solvent vapor annealing (SVA) process [37,38]. Therefore, in this work, we intend to exploit a novel strategy, dip-coating followed by SVA process, to fabricate PIM-1 TFC membranes with tunable OSN permeation and separation performances. The effects of dipping concentration and SVA time on the morphologies and per formances of the PIM-1 TFC membranes were investigated in detail.
Equipment (China) and used as the supports of TFC membranes. 1,4Dicyanotetrafluorobenzene (DCTB), potassium carbonate (K2CO3) and 5,50 ,6,60 -Tetrahydroxy-3,3,30 ,30 -tetramethyl-1,10 -spirobisindane (TTSBI) were purchased from Energy Chemical (Shanghai, China). Prior to use, DCTB was purified by sublimation at 150 � C, K2CO3 was dried at 110 � C overnight and TTSBI was dissolved in methanol and reprecipitated from dichloromethane. Dimethylacetamide (DMAc), chlo roform (CHCl3), toluene, ethanol (EtOH), acetone and n-heptane were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China). Isatin, Basic Orange 2, Azure B, Methyl Orange, Riboflavin, Crystal Vi olet, Food Yellow 3, Basic Blue 26, Acid Black 1, Tetrazolium Blue Chloride, Acid Blue 90 and Acid Red 94 were purchased from the Aladdin Industry Co. (Shanghai, China). Vitamin B12 (VB12) was pur chased from Sigma-Aldrich Trading Co. Ltd. (Shanghai, China). All re agents were of analytical grade and used without further purification except for those specifically mentioned.
2. Experimental
2.4. Preparation of PIM-1 TFC membrane
2.1. Materials
The PIM-1 TFC membrane was fabricated by dip-coating method followed by an SVA process (Fig. 1). Typically, the PAN support mem brane was rinsed in CHCl3 for 30 s. Then, the membrane was transferred to another container having a PIM-1/CHCl3 dipping solution inside and
2.2. Synthesis of PIM-1 PIM-1 was synthesized as described by Satilmis et al. [39]. TTSBI (17.02 g, 0.05 mol), DTCB (10.01 g, 0.05 mol), K2CO3 (20.7 g, 0.15 mol), DMAc (100 mL) and toluene (50 mL) were added to a dry 500 mL of three necked round bottom flask equipped with a Dean-stark trap and mechanical stirrer. Then the device was filled with nitrogen and placed in an oil bath pre-heated to 160 � C, and the reaction was carried out for 50 min under reflux. After cooling, the obtained highly viscous solution was dissolved in CHCl3, filtered and re-precipitated from methanol. The product was washed alternately with water and methanol and then dried at 110 � C overnight to obtain purified PIM-1. 2.3. Characterization of PIM-1 Gel permeation chromatography (GPC, Water-515, USA) was used to analyze number-average molecular weight (Mn) and polymer dispersity index (PDI) of PIM-1. The polymer was dissolved in THF to obtain a 0.3 wt% of solution. It was then run through GPC with THF as the eluent was at 1 mL min 1. Using polystyrene as a standard, the effluent time of the THF mobile phase was measured at a test temperature of 40 � C and the obtained data were integrated to calculate the molecular weight of PIM1. The chemical structures of PIM-1 were analyzed by attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 6700, USA) and solution-state 1H nuclear magnetic resonance spec troscopy (NMR, Bruker Advance III 500, Germany).
A PAN flat-sheet ultrafiltration (UF) membrane with molecular weight cut-off of 50 kDa was purchased from Beijing Separation 2
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held at the upper end of container without contact with the dipping solution. Afterwards, the container was turned upside down to completely dip the support membrane in the solution for 20 s, and then the container was reset upright. The soaked membrane was kept in CHCl3 vapor to conduct a SVA step for a pre-designed period. Finally, the membrane was taken out and dried in air to obtain a PIM-1 TFC mem brane. The above-mentioned PIM-1/CHCl3 dipping solutions with various concentrations were beforehand prepared and filtered through an 0.45 μm PTFE filter before use. Both of dip-coating and SVA steps were carried out at room temperature (18 � 5 � C). 2.5. Characterizations of TFC membranes The surface chemistry of the prepared TFC membranes were analyzed by ATR-FTIR and X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA system, USA). The surface morphologies of the investigated membranes were observed by field-emission scanning electron micro scopy (FE-SEM, Hitachi S-4800, Japan), atomic force microscopy (AFM, SPI3800 N, Seiko Instrumental) and transmission electron microscopy (TEM, HT-7700, Japan). The sample photographs were taken by a smartphone.
Fig. 2. ATR-FTIR spectra and sample photographs of PIM-1 TFC membrane (4.0 wt% of dipping concentration and 4 min of SVA time) and PAN sub strate surface. Table 1 Elemental mole percentages of PIM-1 polymer, the PIM-1 TFC membranes sur face and the PAN substrate detected by XPS analysis.
2.6. OSN experiments
Sample
OSN performance tests were carried out using a dead-end solvent resistant stirred cell (Millipore Co., USA) with an effective area of 15 cm2. The pressure in the stirred cell was controlled by a pressure regu lator. In a typical test, a membrane was pre-pressured under 0.5 MPa with pure solvent as the feed for 0.5 h and then tested under 0.4 MPa. The permeation flux (F, L m 2 h 1) was calculated using the following equation: F¼
V At
PAN substrate PIM-1 powder PIM-1 TFC membrane #1 PIM-1 TFC membrane #2
O
N
F
70.9 81.8 79.5 81.6
24.9 14.1 16.6 14.9
4.0 3.9 3.5 3.2
0.2 0.2 0.4 0.3
Herein, PIM-1 solutions with CHCl3 as solvent were used to dip-coat PAN UF membrane to construct thin PIM-1 selective layers. SVA process was employed to control solvent evaporation rate and eliminate the small pits on PIM-1 layer. The coating of PIM-1 onto the PAN substrate was confirmed by ATR-FTIR, XPS, and sample photographs (Fig. 2 and Table 1). Compared with PAN substrate, obvious characteristic peaks in the range of 1273–1248 cm 1 and 1510-1400 cm 1 corresponding to aromatic ether and benzene ring of PIM-1, respectively, appear in the ATR-FTIR spectrum of the PIM-1 dip-coated membrane (Fig. 2). More over, the spectrum is very similar to the FTIR of PIM-1 (Fig. S1). From the elemental mole percentages detected by XPS (Table 1), it can be seen that the surface chemical compositions of PIM-1 TFC membranes are nearly identical to that of PIM-1. Moreover, it can be seen obviously that the membrane color changes from white to yellow after dip-coating (Fig. 2). These results show evidently PIM-1 was successfully coated onto the PAN UF support. In the design and preparation of TFC membranes, thin and defectfree dense selective layer is generally desirable to achieve high separa tion performances, which are inevitably dominated by intrinsic trade-off between permeability and selectivity. In this work, the dipping solution concentration and SVA time are two key parameters to tune the thick ness and uniformity of PIM-1 layer. The cross-sectional morphologies of PIM-1 TFC membranes prepared with various dipping concentrations (SVA time was set at 3 min) were observed by FE-SEM and the typical images are shown in Fig. 3. The higher dipping concentration resulted in the thicker PIM-1 layer. As the concentration is 0.5 wt%, the thickness of PIM-1 layer coated onto PAN substrate is relatively thin (about 200 nm). But this may lead to the generation of defects and thus low solute rejection. With the increase of the dipping concentration to 4.0 wt%, the thickness of PIM-1 layer reaches up to ~600 nm and the defects are effectively avoided. However, too thick selective layer often results in low solvent flux. Therefore, 2.0 wt% was used as the typical dipping concentration in the following study.
(1)
Cp � 100% Cf
C
#1: 2.0 wt% of dipping concentration, 1 min of SVA time; #2: 2.0 wt% of dip ping concentration, 3 min of SVA time.
where V(L) is the volume of permeated solvent during the operating time t(h), and A (m2) is the effective membrane area. A serial of solute molecules (anionic dyes, cationic dyes and neutral molecules, Table S1) were used as the probes to evaluate solute rejection performances of the prepared TFC membranes. Ethanol was used as the solvent of the feed solutions. The solute rejection tests were performed in the dead-end cell loaded about 500 mL of solution and the operation pressure was locked at 0.4 MPa. A small volume of the permeate at the initial stage of filtration was collected and the concentration was determined using a UV/vis spectrophotometer (UV5500-PC, shanghai Jingke Instrument, China). The rejection (R, %) was calculated by the following equation: R¼1
Atomic percentage (%, mole concentration)
(2)
where Cp and Cf represent the solute concentrations in the permeate and the feed (the original solution), respectively. All filtration tests were performed at room temperature (25 � 1 � C). All of the reported data are the averages of three paralleled measurements and the standard de viations were also reported. 3. Results and discussion 3.1. Characterizations of the PIM-1 selective layer The successful synthesis of PIM-1 was proved by the ATR-FTIR and H NMR spectra as shown in Fig. S1 and Fig. S2 of Supplementary In formation. The Mn of PIM-1 was about 50,000 Da with a PDI value of 1.6 as characterized by GPC with polystyrene as standards and THF as sol vent (Fig. S3). The molecular weight of PIM-1 here is enough high to form uniform separation layer on PAN UF supports by dip-coating.
1
3
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Fig. 3. Cross-sectional FE-SEM images for PIM-1 TFC membranes prepared with different dipping concentrations (a) 0.5 wt%; (b) 1.0 wt%; (c) 2.0 wt%; (d) 4.0 wt% (SVA time was set at 3 min).
Fig. 4. Cross-sectional and surface FE-SEM images for PIM-1 TFC membranes prepared with different SVA times (a) 0 min; (b) 1 min; (c) 2 min; (d) 3 min; (e) 4 min; (f) 5 min (dipping concentration was fixed at 2.0 wt%).
The rapid evaporation of low boiling point solvents often leads to small pits in polymer coatings in dip-coating process due to the gener ation of solvent bubbles. Although the depth of these pits is only 30–60 nm (analyzed by AFM, Figs. S4 and S5) and don’t span all over whole cross-section of PIM-1 layer (200–800 nm of thickness), they may become gradually defects during long-term separation operation. Therefore, in this work, a SVA process was used to control the evapo ration rate of CHCl3 and eliminate the small pits in PIM-1 layer. The influences of SVA time on the cross-sectional and surface morphologies of PIM-1 TFC membranes are shown in Fig. 4. It can be seen that, as the SVA time is less than 1 min, many small pits appear on the PIM-1 layers because of rapid solvent evaporation (Fig. 4 (a) and (b)). As the SVA time
increases, the pits of PIM-1 layer surfaces are significantly suppressed and more smooth membrane surfaces are obtained. More importantly, it is found that the thickness of PIM-1 layer can be regulated by SVA process. As the PIM-1 TFC membrane was prepared by single dip-coating without SVA step (namely SVA time was 0 min), the PIM-1 precipitated rapidly from the dipping solution film with solvent evaporation and a PIM-1 layer was created on the support membrane (Fig. 4(a)). An obvious interface is observed between PIM-1 layer and the substrate, and the thickness of PIM-1 layer is about ~800 nm. With the elongation of SVA time, the thickness of surface layer decreases due to the infil tration of PIM-1 solution into surface pores. When the SVA time reaches 5 min, the thickness of surface layer is only ~200 nm and the interface 4
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Journal of Membrane Science 601 (2020) 117951
Fig. 5. Ethanol permeation flux and VB12 rejection for the PIM-1 pore-filled TFC membranes prepared with (a) different dipping concentrations (SVA time was fixed at 3 min) and (b) different SVA time (dipping concentration was fixed at 2.0 wt%). The trans-membrane pressure was 0.4 MPa.
become obscure. The yellow color of the TFC membranes gradually become lighter (Fig. S6). The TEM and AFM images of the interface between PIM-1 layer and PAN support (S7 and S8) further indicate the PIM-1 was evidently filled into the near surface pores of PAN support membrane. Just as discussed above, too thin surface layer may lead to the generation of defects and thus low solute rejection. Therefore, in this work, the upper limit of SVA time was set at 5 min. The occurrence of filling phenomenon is advantageous to the firm attachment of PIM-1 atop the support membrane and avoid the peeling off in long-term separation operation. In addition, the surface layer thickness can be conveniently regulated by SVA time and thus the permeation and sep aration performances can be tuned. In this work, considering the filling of PIM-1 inside the near surface pores of PAN support membrane, the obtained TFC membranes are called as PIM-1 pore-filled TFC membranes. The thickness and unifor mity of PIM-1 layer can be regulated and controlled by two key pa rameters including the dipping concentration and the SVA time. These results show it is possible to develop TFC membranes with tunable OSN performances by the combination of dip-coating and SVA process.
Fig. 6. The trade-off between ethanol flux and VB12rejection for the PIM-1 pore-filled TFC membranes prepared with different SVA time in the cases of 1.0 or 2.0 wt% of dipping concentration.
membranes, and results in decreased solute rejection. These results indicate that tunable permeability and selectivity can be achieved through the SVA process. To clarify the relationship between permeability and selectivity for the investigated PIM-1 pore-filled TFC membranes more clearly, the VB12 rejection was plotted against the ethanol flux, as shown in Fig. 6. A typical trade-off effect was observed between the ethanol flux and the VB12 rejection. Obviously, with the increase of dipping concentration in the preparation of TFC membranes, the trade-off line shifts towards right. With the increase of ethanol flux, the VB12 rejection decreases for both dipping concentrations 1.0 or 2.0 wt%. Moreover, at same ethanol flux level, the TFC membranes prepared with 2.0 wt% of dipping con centration demonstrate higher rejection than those prepared with 1.0 wt %. Similarly, the membranes prepared with 2.0 wt% have higher fluxes than those prepared with 1.0 wt%. As the dipping concentration is 1.0 wt%, shorter SVA time (e.g. direct evaporation in air without SVA process, namely 0 min of SVA time, Point 1 in Fig. 6) is required to achieve a high rejection. The TFC membrane displays thick PIM-1 layer and lots of small pits (Fig. S10). As the dipping concentration rises to 2.0 wt% and the SVA time increases to 3 min (Point 2 in Fig. 6), the obtained TFC membrane exhibits high flux (17.2 L m 2 h 1) as well as high rejection (93.9%) because a defect-free and thinner PIM-1 layer is generated (Fig. 4 (d)). It is worth noting that too high dipping concen tration (>2.0 wt%) results in a thick PIM-1 layer even if a long SVA process is conducted. Therefore, an appropriate collocation of the dip ping concentration and the SVA time is critical to achieve high perme ation and separation performances for PIM-1 pore-filled TFC
3.2. Tunable separation performances of PIM-1 TFC membranes The prepared PIM-1 pore-filled TFC membranes were used for or ganics rejection in organic solvent. The effects of the dipping concen tration and SVA time on the ethanol permeation flux and the VB12 rejection were investigated, respectively. As shown in Fig. 5 (a), with the increase of dipping concentration, the ethanol flux gradually decreases while the VB12 rejection increases. This can be attributed to the increase of PIM-1 layer thickness and the elimination of defects, which has been discussed in Section 3.1. In the case of 0.5 wt% of dipping concentration, the VB12 rejection is only ~20% since the PIM-1 layer is too thin and possibly defective. When the dipping concentration increases up to 2.0 wt%, the rejection reaches 93%, indicating the PIM-1 layer is defect-free and capable of separating effectively VB12 molecules from ethanol. Nevertheless, the further increase of dipping concentration to 4.0 wt% leads to a low ethanol flux (only ~5 L m 2 h 1), which is disadvanta geous to achieve time-saving and energy-efficient separation. Therefore, in this work, based on a compromise between solvent permeation flux and VB12 rejection, 2.0 wt% was a preferential dipping concentration. As shown in Fig. 5 (b) (2.0 wt% of the dipping concentration), with the elongation of SVA time, the ethanol flux increases obviously while the VB12 rejection decrease to some extent. As the SVA time reaches 3 min, the flux increases significantly from 2.9 to 17.2 L m 2 h 1 and the rejection keeps more than 93%. Similar phenomenon occurred in the case of 1.0 wt% as the dipping concentration (Fig. S9). However, too long SVA time (>4 min) may bring some defects to PIM-1 layer due to excessive infiltration of PIM-1 solution into the interior pores of support 5
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Journal of Membrane Science 601 (2020) 117951
permeance reaches up to 31.7 L m 2 h 1 bar 1 since the HSP of acetone (20.3 J0.5 cm 1.5) is very close to that of PIM-1. This phenomenon shows the solvent permeance is greatly influenced by the affinity of solvent with PIM-1. Except for solubility parameter, the viscosity and molecular size of solvents are associated with their permeability [8]. It can be seen from Fig. 7, although both acetone and toluene have similar HSPs with PIM-1 (20.3 and 18.2 J0.5 cm 1.5, respectively), their permeances in the TFC membrane are obviously different (only 18.0 L m 2 h 1 bar 1 for toluene). This can be attributed to their great difference in viscosity (0.32 � 10 3 Pa s for acetone and 0.55 � 10 3 Pa s for toluene at 25 � C). More interestingly, the fluxes of toluene and heptane are remarkably higher than that of ethanol, which is different from the reported phe nomenon [8]. This may be attributed to the difference in membrane materials (crosslinked polyamide in the reference, but linear PIM-1 in our work) which have different swelling characteristics in solvents. However, more detailed mechanism keeps unknown and should be studied in next work. The rejections towards various solutes in ethanol including anionic and cationic dyes and neutral molecules with a serial of molecular weights were examined and the results are shown in Fig. 8. The mo lecular weight cut-off (MWCO) for a membrane is often defined by the molecular weight with 90% of rejection. The MWCO towards both neutral molecules and cationic dyes is about 530 Da while that towards anionic dyes is lower than 320 Da. In other words, the membrane ex hibits higher rejection towards anionic dyes than cationic ones with a similar molecular weight. This phenomenon was also reported in other work [33,41]. This indicates that the solute rejection is determined by not only membrane pore size and solute molecular size, also the inter action between solute and membrane [42]. The solvent permeances and MWCOs for the typical membrane developed in this work are compared with the membranes reported in literatures, as shown in Table 2. It is found that the PIM-1 pore-filled TFC membrane demonstrates higher permeation and separation performances.
Fig. 7. The solvent permeances plotted against their corresponding HSPs for the typical PIM-1 pore-filled TFC membranes (prepared with 2.0 wt% of dip ping concentration, 3 min of SVA time).
4. Conclusions
Fig. 8. Rejection of TFC membrane for different molecules (20 ppm in ethanol solution, 2.0 wt% of dipping concentration, 3min of SVA time).
A PIM-1 pore-filled TFC membrane was successfully developed by dip-coating followed by a SVA process. The introduction of SVA process can effectively reduce the generation of the small pits and defects on the PIM-1 layer. Long SVA time is advantageous to form a thin PIM-1 porefilled layer, but too long would bring defects to the TFC membranes. As the SVA time increases, the ethanol flux increases while the VB12 rejection decreases for the obtained TFC membranes. An appropriate collocation of the dipping concentration and the SVA time is critical to achieve high permeation and separation performances. The PIM-1 porefilled TFC membranes display remarkable rejection performance to wards dyes and neutral molecules, especially towards anionic dye in ethanol. Typically, the prepared TFC membrane exhibited an ethanol permeance of 4.3 L m 2 h 1 bar 1 with a rejection of 93.7% towards Methyl Orange (327 Da). The solvent permeability is higher than that of the PIM-1 TFC membranes reported previously.
membranes. 3.3. Solvent permeance and molecular separation Considering the interaction between solvent and membrane plays an important role in solvent permeation, the solvent permeances are related to their corresponding Hansen solubility parameters (HSPs) [31]. The solvent permeances were obtained by examining the fluxes of different solvents permeating through a typical PIM-1 pore-filled TFC membranes (prepared with 2.0 wt% of dipping concentration and 3 min of SVA time) under 0.4 MPa, and the results are shown in Fig. 7. It can be seen that, the closer the HSP of the solvent is to that of PIM-1 (~19.4 J0.5 cm 1.5 [40]), the higher the solvent permeance. For instance, the ethanol permeance is only 4.3 L m 2 h 1 bar 1 while the acetone
Table 2 The Comparisons in solvent permeances and MWCOs between the PIM-1 TFC membranes reported in literatures with the typical membrane developed in this work. PIM-1 TFC membranes PIM-1 (dip-coating) PIM-1/20%PEI PIM1-CO1-50/20%PEI PIM-1 (dip-coating) PIM-1 (spin coating) Thioamide-PIM-1 PIM-1 (dip-coating)
Solvent permeance (L m
2
h
1
bar
1
)
N-heptane
Ethanol
Acetone
Toluene
4.2 / / 7 18 / 14.7
/ / / / / 3.4 4.3
/ / / / / 12.4 31.7
/ 3.1 0.9 11 / / 18.2
/represents no data in the references. 6
Molecular weight cut-off (Da)
Ref.
290 460 350 ~700 ~500 ~600 ~600
[30] [30] [30] [31] [32] [33] This work
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Declaration of competing interests
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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. CRediT authorship contribution statement Jiaqi Li: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft. Mengxiao Zhang: Writing - review & editing, Resources. Weilin Feng: Investigation. Lin Zhang: Funding acquisition, Supervision, Project administration, Writing - review & editing. Acknowledgments The authors are grateful for the financial supports from the National Natural Science Foundation of China (Grant No. 51973185, 51773175 and 51828301) and the Fundamental Research Funds for the Central Universities (Grant No. 2019QNA4062). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2020.117951. References [1] P. Marchetti, M.F. Jimenez Solomon, G. Szekely, A.G. Livingston, Molecular separation with organic solvent nanofiltration: a critical review, Chem. Rev. 114 (2014) 10735–10806, https://doi.org/10.1021/cr500006j. [2] D.S. Sholl, R.P. Lively, Seven chemical separations to change the world, Nature 532 (2016) 435–437, https://doi.org/10.1038/532435a. [3] P. Vandezande, L.E.M. Gevers, I.F.J. Vankelecom, Solvent resistant nanofiltration: separating on a molecular level, Chem. Soc. Rev. 37 (2008) 365–405, https://doi. org/10.1039/B610848M. [4] M.G. Buonomenna, J. Bae, Organic solvent nanofiltration in pharmaceutical Industry, Separ. Purif. Rev. 44 (2015) 157–182, https://doi.org/10.1080/ 15422119.2014.918884. [5] G. Sz�ekely, J. Bandarra, W. Heggie, B. Sellergren, F.C. Ferreira, Organic solvent nanofiltration: a platform for removal of genotoxins from active pharmaceutical ingredients, J. Membr. Sci. 381 (2011) 21–33, https://doi.org/10.1016/j. memsci.2011.07.007. [6] J.P. Sheth, Y. Qin, K.K. Sirkar, B.C. Baltzis, Nanofiltration-based diafiltration process for solvent exchange in pharmaceutical manufacturing, J. Membr. Sci. 211 (2003) 251–261, https://doi.org/10.1016/S0376-7388(02)00423-4. [7] G.C. Vougioukalakis, Removing ruthenium residues from Olefin metathesis reaction products, Chem. Eur J. 18 (2012) 8868–8880, https://doi.org/10.1002/ chem.201200600. [8] S. Karan, Z. Jiang, A.G. Livingston, Sub-10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation, Science 348 (2015) 1347–1351, https://doi.org/10.1126/science.aaa5058. [9] N.B. McKeown, P.M. Budd, Exploitation of intrinsic microporosity in polymerbased materials, Macromolecules 43 (2010) 5163–5176, https://doi.org/10.1021/ ma1006396. [10] N. Du, G.P. Robertson, J. Song, I. Pinnau, M.D. Guiver, High-performance carboxylated polymers of intrinsic microporosity (PIMs) with tunable gas transport properties y, Macromolecules 42 (2009) 6038–6043, https://doi.org/10.1021/ ma9009017. [11] P. Budd, N. Mckeown, B. Ghanem, K. Msayib, D. Fritsch, L. Starannikova, N. Belov, O. Sanfirova, Y. Yampolskii, V. Shantarovich, Gas permeation parameters and other physicochemical properties of a polymer of intrinsic microporosity: polybenzodioxane PIM-1, J. Membr. Sci. 325 (2008) 851–860, https://doi.org/ 10.1016/j.memsci.2008.09.010. [12] J. Campbell, J.D.S. Burgal, G. Szekely, R.P. Davies, D.C. Braddock, A. Livingston, Hybrid polymer/MOF membranes for Organic Solvent Nanofiltration (OSN): chemical modification and the quest for perfection, J. Membr. Sci. 503 (2016) 166–176, https://doi.org/10.1016/j.memsci.2016.01.024. [13] J. Campbell, G. Sz�ekely, R.P. Davies, D.C. Braddock, A.G. Livingston, Fabrication of hybrid polymer/metal organic framework membranes: mixed matrix membranes versus in situ growth, J. Mater. Chem. A. 2 (2014) 9260–9271, https://doi.org/ 10.1039/C4TA00628C. [14] A.J. Howarth, Y. Liu, P. Li, Z. Li, T.C. Wang, J.T. Hupp, O.K. Farha, Chemical, thermal and mechanical stabilities of metal–organic frameworks, Nat. Rev. Mater. 1 (2016) 15018, https://doi.org/10.1038/natrevmats.2015.18.
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