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Polyarylate membrane constructed from porous organic cage for high-performance organic solvent nanofiltration
Journal of Membrane Science 595 (2020) 117505
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Polyarylate membrane constructed from porous organic cage for high-performance organic solvent nanofiltration Zhe Zhai, Chi Jiang, Na Zhao, Wenjing Dong, Peng Li, Haixiang Sun, Q. Jason Niu * State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, 266580, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Organic solvent nanofiltration Porous organic cage Interfacial polymerization Molecular dynamics simulation
Preparation of highly permeable and selective organic solvent nanofiltration (OSN) membranes is desired for precise chemical separations. Although the emerging nanomaterials and novel preparation methods have significantly boosted the membrane performance, it still requires much effort to drive them toward scale-up production with lower cost and easier preparation process. Herein, we constructed a polyarylate membrane from one kind of porous organic cage, namely Noria through traditional interfacial polymerization technique. Under the catalysis of triethylamine, Noria in aqueous phase would react with terephthaloyl chloride (TPC) in hexane to form a dense nano-film. The inner cavity of Noria molecule provided nano-channel for both polar and non-polar solvents to transport through the membrane. Specially, the Noria þ TPC membrane exhibits a high permeance for methanol up to 18 L m 2h 1bar 1. This is resulted from the low interaction energy between methanol and Noria as revealed by the molecular dynamics (MD) simulation. In addition to the excellent per formance, the simple preparation process with low-cost materials suggests the great potential of Noria þ TPC membrane for practical application. Moreover, it would inspire the exploration of other molecules with tailored chemical structures for the assembly of high-performance membranes in future.
1. Introduction Organic solvent nanofiltration (OSN), also known as solvent resistant nanofiltration (SRNF) has developed rapidly in recent years and it is receiving more and more attention in chemical and pharmaceutical separations [1]. Compared with other purification processes, including adsorption [2], distillation and photocatalytic degradation [3,4], the membrane based separation was energy-efficient and easily scaled-up. The OSN could fulfill the separation of mixtures in organic solvents at molecular level, which shows great potential in purification, concen tration and solvent exchange [5,6]. In addition to the integrally skinned asymmetric (ISA) membranes [7,8], the OSN membranes with thin-film composite (TFC) structure prepared either through interfacial polymerization (IP) [9–12] or dip coating method [13–17] are also widely studied. Their performance could be tailored for specific applications by adjusting the properties of separation layer and porous support independently [18]. As a simple and efficient technique, IP enables the large-scale production of polymer films and other nanomaterials [19]. Especially, it has been widely employed for the preparation of nanofiltration (NF) membranes and
reverse osmosis (RO) membranes. For the IP on porous support, the polymerization reaction mainly takes place in the organic phase due to the poor solubility of organic monomer in water [20]. The resulting cross-linked thin film is mostly composed of polyamide, which could serve as an excellent candidate for OSN membrane benefitting from its good tolerance toward organic solvent [21]. For the preparation of highly permeable OSN membranes, various strategies have been developed to tailor the separation layers, including reducing the film thickness, incorporating nano-fillers and designing novel monomers for IP. Livingston and co-workers [22] creatively pre pared a sacrificial interlayer for the preparation of TFC membrane. The as-formed ultrathin polyamide film with sub-10 nm thickness signifi cantly promoted the membrane permeance. In addition, various nano materials, such as metal–organic framework (MOF) [23,24], carbon dot [25] and covalent organic framework (COF) [26] were employed as fillers in the polyamide layer to boost the OSN membrane performance. However, the permeance of these hybrid membranes for non-polar sol vents would be still unsatisfactory due to the limitation of polyamide as bulk material. Lai and co-workers [27] assembled 2D COF membrane on anodic aluminum support through Langmuir Blodgett (LB) method,
* Corresponding author. E-mail address: [email protected] (Q.J. Niu). https://doi.org/10.1016/j.memsci.2019.117505 Received 16 June 2019; Received in revised form 14 September 2019; Accepted 22 September 2019 Available online 23 September 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
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which exhibited high flux for both polar and non-polar solvents. Tang and co-workers [28] reported an OSN membrane prepared by conju gated microporous polymer (CMP) and its inner open and inter connected pores allowed the fast diffusion for both hexane and methanol. Shao and co-workers [29] constructed OSN membrane by β-cyclodextrin and polydopamine through the host-guest interactions and the membrane was also permeable for polar and non-polar solvents. Despite the excellent OSN performance of the above mentioned mem branes, the sophisticated preparation process and high-cost materials make it difficult for them toward practical application. Considering the advantages of IP technique, it is more appealing to prepare OSN mem branes through IP by molecularly designed monomers. For example, contorted phenol [30] and cyclodextrin [31,32] as monomers have been studied to improve the OSN membrane performance by increasing the microporosity of separation layers. Unfortunately, such kind of molecule with tailored structure employed for the preparation of high-performance OSN membrane is quite limited. Herein, for the first time, we reported a polyarylate membrane constructed from one kind of porous organic cage, namely Noria through IP for OSN. As a macrocyclic molecule with an inner cavity and multihydroxyl groups, Noria was simply synthesized by the condensation reaction of resorcinol and glutaraldehyde. Under the catalysis of trie thylamine, Noria in aqueous phase would react with acyl chloride in hexane to form a dense nano-film. The inner cavity of Noria provided paths for the transport of both polar and non-polar solvents, which efficiently promoted the membrane permeance. Meanwhile, the resor cinol was also employed to prepare membranes as control for compar ison. The physicochemical properties of the films formed on the porous support as well as at the free oil-water interfaces were systematically studied. The transportation behavior of various solvents, including water, methanol and ethanol through the molecular cavity was also investigated by molecular dynamics (MD) simulation. This work would not only enrich the available monomers for the preparation of highperformance OSN membranes, but also promote the understanding of molecular separation mechanism in the film with nano-cavity.
RES þ TMC were also synthesized at the bulk oil-water interface. After vigorous stirring, the obtained solid was collected and then washed by deionized water for several times. 2.3. Preparation of TFC membranes The TFC membranes were prepared on the PAN support through IP reaction and the process has been previously reported by our group [34, 35]. Briefly, the PAN support was fixed with the right side up and then covered with an aqueous solution containing 1.0 wt% Noria and 4.0 wt % triethylamine. After 3 min, the solution was removed and an air knife was applied to blowing off the visible liquid drops on the membrane surface. A TPC solution in hexane (0.2 wt%) was subsequently poured onto the above membrane for 5 min. Finally, the freshly prepared membrane was put in an oven at 60 � C for 3 min to complete the polymerization reaction. Other membranes, including Noria þ TMC, RES þ TPC and RES þ TMC were prepared under the same condition. 2.4. Characterizations Field-emission scanning electron microscopy (FESEM, Hitachi S4800, Japan) was employed to observe the membrane and powder morphology. The surface roughness of membrane was analyzed by Atomic force microscopy (AFM, Shimadzu SPM-9700, Japan). For the measurement of nano-film thickness, the film was firstly transferred onto a silicon wafer and then scratched by a sharp knife. The height difference between the nano-film and silicon wafer was recorded by AFM to be the film thickness. The element contents of PAN support and Noria þ TPC membrane were detected by X-ray photoelectron spec troscopy (XPS) (Escalab 250Xi, ThermoFisher). The IR spectrum of Noria as well as ATR-FTIR spectra of various membranes was also ob tained (Nicolet iS10, Thermo Scientific). The water contact angles (DSA30, Kruss GmbH, Germany) and zeta potentials (SurPASSTM3, Anton-Paar GmbH, Austria) were recorded to analyze the variation of membrane surface properties. The X-ray diffraction (XRD) patterns of Noria and polyarylate powders were obtained from an X’Pert PRO MPD diffractometer (PANalytical B.V., Netherlands). The UV–vis spectra (UV2700, Shimadzu, Japan) of dye solutions were analyzed to calculate the membrane rejection toward various dyes. CO2 sorption isotherm of polyarylate powder was collected at 273 K (Autosorb-iQ, Quantach rome, USA).
2. Experimental 2.1. Chemicals and materials The following chemicals were used without purification. Resorcinol (RES, >99.0%), terephthaloyl chloride (TPC, >99.0%), trimesoyl chlo ride (TMC, >98.0%), glutaraldehyde (24.0–26.0% in water), titan yel low (TY, 695.7 g mol 1), primuline (P, 475.5 g mol 1) and brilliant blue G (BBG, 854.0 g mol 1) were purchased from TCI. Rose Bengal (RB, 1017.6 g mol 1) was obtained from Aladdin. Other chemicals, such as methyl orange (MO, 327.3 g mol 1), triethylamine (>99.0%), methanol (>99.5%), ethanol (>99.7%), isopropyl alcohol (IPA, >99.5%), toluene (>99.5%), cyclohexane (>99.7%), hexane (>97.0%), hydrochloric acid (HCl, 36.0%), Na2SO4 (>99.0%), MgSO4 (>98.0%), MgCl2 (>99.0%) and NaCl (>99.5%) were all supplied by Sinopharm Chemical Reagent Co., Ltd. The polyacrylonitrile (PAN) ultrafiltration membrane employed for the preparation of TFC membranes was provided by Shandong Lanjing Membrane Technology and Engineering Co., Ltd.
2.5. Nanofiltration performance evaluation of various membranes The membrane rejections of different inorganic salts, including Na2SO4, MgSO4, NaCl and MgCl2 were evaluated under 4 bar in a crossflow testing cell with an effective area of 18.5 cm2 and the feed con centrations were all 2000 ppm. Meanwhile, the OSN performance of Noria þ TPC membrane was tested under the same condition by sepa rating various dyes in methanol solution (50 ppm). Prior to the test, the membrane was pre-compacted under 6 bar for 0.5 h. The membrane rejection could be calculated as follows: R ¼ (1- Cp / Cf) � 100%
(1)
Where R represents the membrane rejection. For inorganic salts and dyes, the values of Cp/Cf are acquired through the conductivity and UV–vis spectra of permeate and feed solution respectively. The permeance (Jw) was calculated by equation (2):
2.2. Preparation of free-standing polyarylate nano-films and powders Firstly, Noria was synthesized according to the literature [33] (Figs. S1 and S2). The Noria-TPC nano-film was prepared at the free oil-water interface. Briefly, the aqueous solution containing 1.0 wt% Noria and 4.0 wt% triethylamine was poured into a beaker. Then, the TPC solution of 0.2 wt% in hexane was gently added into the above beaker and the reaction time was allowed to be 1 min, 5 min, 10 min, 15 min, 20 min and 25 min respectively. After that, the two immiscible solutions were drained out to obtain the free-standing film. The poly arylate powders of Noria þ TPC, Noria þ TMC, RES þ TPC and
Jw ¼ Q / (P △T A)
(2)
Here, Q is the permeate volume at a given time (L), P stands for the testing pressure (bar), △T represents the testing period (h) and A is the area of membrane for test (m2). The measurement of membrane permeance toward various solvents, including methanol, ethanol, IPA, toluene, cyclohexane, hexane and 2
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Fig. 1. (a) Schematic illustration of the formation of polyarylate film by Noria and TPC (b) Photograph of free-standing Noria þ TPC film formed at the oil-water interface at a reaction time of 5 min (c) The AMF image and height profiles of the free-standing Noria þ TPC film at a reaction time of 5 min (d) The thicknesses of free-standing films synthesized at various time.
Fig. 2. (a) SEM image of Noria þ TPC membrane prepared on the PAN support (b) AFM topography image of Noria þ TPC membrane (c) XPS spectra of PAN support and Noria þ TPC membranes before and after washing (d) High resolution spectrum of O 1s for Noria þ TPC membrane after washing (e) Water contact angles of PAN support and Noria þ TPC membrane. 3
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Fig. 3. SEM images of (a) Noria þ TPC (b) Noria þ TMC (c) RES þ TPC (d) RES þ TMC powders (e) XRD patterns of various powders (f) CO2 sorption isotherm of Noria þ TPC (g) Pore size distribution of Noria þ TPC calculated from the CO2 sorption isotherm.
water was carried out in a dead-end unit cell pressurized by N2. As for the activation of membrane, 50 mL methanol was added into the cell and then filtered through the membrane. The solvent permeance was also obtained from equation (2).
the nascent PAN support (Fig. 2c) due to the formation of polyarylate membrane by Noria and TPC. In addition to aiding the dissolution of Noria, the triethylamine would also react with the generated HCl during IP as catalyst to promote the cross-linking reaction [38–40]. Considering the detection depth of XPS (~10 nm) [41], the observed N element for Noria þ TPC membrane before washing presumably derived from the triethylamine hydrochloride entrapped in the separation film. Due to the dissolution of triethylamine hydrochloride in water, the peak of N element for Noria þ TPC membrane disappeared after being washed by filtration of deionized water. As shown in Fig. 2d, the peak at about – C-OR in the high resolu 534.0 eV attributed to the ester oxygen of O– tion spectrum of O 1s indicated the formation of polyarylate. Besides, the detected C–O–C group at 1140 cm 1 of TFC membrane in the ATR-FTIR spectra further confirmed the reaction between Noria and TPC (Fig. S5). The water contact angle was commonly employed to evaluate the hydrophilicity of subject surface and it turned to be more hydrophobic with the increase of water contact angle [42,43]. Despite of the existence of hydroxyl and carboxyl groups, the TFC membrane turned to be more hydrophobic than the PAN support (Fig. 2e), which could be resulted from the formation of plenty of hydrophobic ester groups [44].
3. Results and discussion 3.1. The formation of polyarylate membrane by Noria and TPC The polyarylate membrane was prepared by traditional IP technique, which was schematically depicted in Fig. 1a. Though not soluble in pure water, Noria could dissolve well in the aqueous triethylamine solution (Fig. S3). The phenolic hydroxyl groups of Noria turned to deprotonate after the addition of triethylamine, resulting in the formation of reactive phenoxide ions. The reaction mechanism of Noria and TPC was depicted in Fig. S4, for which the deprotonated Noria reacted with TPC through nucleophilic substitution and then polymerized into polyarylate. As a macrocyclic molecule, β-cyclodextrin has been explored as aqueous monomer to prepare high-performance OSN membranes [31,32]. Due to the stronger electron-withdrawing ability of benzene ring than alkyl group, the formation of oxide ion participating in the polymerization reaction was much easier for Noria than β-cyclodextrin molecule. This made Noria free of careful regulation of IP conditions as well as support properties necessary for β-cyclodextrin in order to obtain a non-defective film [31]. Firstly, we synthesized free-standing poly arylate film at the bulk oil-water interface. After draining off the solu tions, an integrated film without visible cracks was obtained on the dish (Fig. 1b) and this suggested its high cross-linking degree. The resulting film was subsequently transferred onto silicon wafer to measure its thickness, which was nearly 252 nm characterized by AFM (Fig. 1c). Owing to the self-limiting trait of IP reaction [36,37], the diffusion of aqueous monomer would be held back when the initially formed film turned to be increasingly denser. Thus, the film thickness firstly increased with the reaction time and then experienced little change (Fig. 1d). Subsequently, the TFC membrane used for OSN was prepared on porous PAN support and the surface of as-prepared membrane was observably smooth (Fig. 2a and Fig. 2b). After the IP reaction, the ox ygen content of membrane surface obviously increased compared with
3.2. Polyarylate powders and membranes interfacially synthesized by various monomers For comparison, Noria þ TPC, Noria þ TMC, RES þ TPC and RES þ TMC powders were firstly prepared by IP reaction. As shown in Fig. 3c, the RES þ TPC powder appeared amorphous probably due to the linear polymer chain formed by the reaction of RES and TPC. However, the cross-linked structure of Noria þ TPC, Noria þ TMC and RES þ TMC endowed them with crystal-like morphology (Fig. 3a and b and Fig. 3d). The peak at about 10� attributed to the inner cavity of Noria (Fig. S6) was still detected for Noria þ TPC and Noria þ TMC powders (Fig. 3e), suggesting the structure of Noria was well retained during the IP reac tion. As a result, the Noria þ TPC depicted a steep adsorption of CO2 under the relative low pressure (Fig. 3f) and its surface area was calculated to be 73.4 m2 g 1, which was comparable to that synthesized by contorted monomers [30]. Meanwhile, the pore size of Noria þ TPC obtained with density functional theory (DFT) was mainly ranged in 4
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Fig. 4. Surface SEM images of (a) and (e) PAN support, (b) and (f) Noria þ TMC, (c) and (g) RES þ TPC, (d) and (h) RES þ TMC membrane (i) Water contact angles of various membranes (j) Zeta potentials of various membranes.
which the generated heat would induce interfacial instabilities and contribute to the production of a crumpled film [22,46]. Owing to the large molecular size, the diffusion rate of Noria in the aqueous phase toward the oil-water interface was much lower than RES and that resulted in the slower reaction rate of Noria with acyl chloride. Conse quently, the surface morphology of Noria based TFC membranes appeared flat due to the relatively less heat generated in the IP reaction at a given time. The AFM characterization (Fig. 5) confirmed the surface morphology variations of various TFC membranes detected by SEM images. Meanwhile, the surface roughness of RES þ TPC was visually higher than that of RES þ TMC membrane. For RES þ TPC, the formed linear polymer chains contributed to a loose film at the initial IP stage, which would allow the RES monomers to continuously travel through the membrane. This resulted in more RES monomers existing at the interface to react with TPC and it generated more heat than the cross-linked RES þ TMC membrane. The as-formed separation film thicknesses of various membranes were measured from their cross-section SEM images (Fig. S7). It could be seen that the separation layers of Noria based TFC membranes were all thicker than those con structed by RES. Compared with RES, it was more difficult for Noria to form a highly cross-linked film at the initial stage due to its bulky molecule volume. Thus, the Noria monomers would continuously diffused from the defects to react with acryl chloride in the organic phase, resulting in the increase of film thickness. In addition, the plenty of unreacted hydroxyl groups of Noria enhanced the hydrophilicity and negative charge of as-prepared TFC membranes as revealed by their water contact angles (Fig. 4i) and zeta potentials (Fig. 4j).
Fig. 5. AFM topography images of (a) PAN support (b) Noria þ TMC (c) RES þ TPC and (d) RES þ TMC membranes.
0.4–0.6 nm (Fig. 3g) corresponding with the portal diameter of Noria [45]. When prepared into membrane, the nano-cavity in Noria would not only serve as the primary sieving channel, but also efficiently pro mote the molecular transport through the TFC membrane. Afterwards, various TFC membranes were prepared on PAN support under the same condition. Although the membrane surfaces were all flat (Fig. 4a and Fig. 4b), the surface pores of PAN support (Fig. 4e) were covered with a dense film for Noria þ TMC (Fig. 4f) just as Noria þ TPC membrane. However, the surfaces of RES þ TPC (Fig. 4c and g) and RES þ TMC membranes (Fig. 4d and h) were filled with plate-like structure. As known to all, the IP was an exothermic reaction, during
3.3. Nanofiltration performance of TFC membranes The performance of Noria þ TPC, Noria þ TMC, RES þ TPC and RES þ TMC membranes in terms of permeance and rejection in aqueous solution were firstly tested. As seen in Fig. 6a, the permeance of Noria based membranes were significantly higher than RES based membranes despite of their thicker separation layers. The nano-cavity in Noria 5
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Fig. 6. (a) The Na2SO4 rejection and permeance of various membranes (b) The rejections of Noria þ TPC toward different inorganic salts.
Fig. 7. (a) Rejections of Noria þ TPC membrane toward various dyes in methanol (b) Pure solvent permeance of Noria þ TPC membrane for both polar and non-polar solvents (c) Pure solvent permeance variation as a function of their dkins (d) The positions of water, methanol and ethanol in Noria with the maximal interaction energy (e) The values of maximal interaction energy for water, methanol and ethanol.
molecule efficiently reduced the transport resistance of water (~0.28 nm) through the Noria based film. Due to the formed linear polymer chain, the Na2SO4 rejection of RES þ TPC was merely as much as 40%. Meanwhile, the reduced salt rejection for Noria þ TMC compared with Noria þ TPC was mainly resulted from its less crosslinked structure as a result of the high steric hindrance for the reac tion with TMC. Fig. 6b showed the rejections of Noria þ TPC toward various salts and it followed the sequence: Na2SO4> MgSO4 > NaCl > MgCl2. Due to the Donnan effect, the Noria þ TPC membrane with negative charge depicted a higher rejection toward Na2SO4 than that of MgSO4. Meanwhile, the positive Mg2þ could shield the negative charge on membrane surface and further contribute to the low rejection of MgCl2. The OSN performance of Noria þ TPC membrane was subsequently evaluated by separating various industrial dyes (Table S1) in methanol (Fig. S8). As seen in Fig. 7a, the rejections of Noria þ TPC membrane increased with the molecular weights of various dyes. The nano-cavity of Noria molecule was retained after IP, which provided pathways for
the transport of various solvents throughout the polyarylate film. Thus, the Noria þ TPC membrane was permeable for both polar and non-polar solvents (Fig. 7b). However, for the membrane prepared by m-phenyl enediamine and TMC, it was mainly consisted of hydrophilic polyamide and the amorphous film lacked inner pores for the molecular transport of non-polar solvent, resulting in its ultralow permeance [22]. Inter estingly, the permeance of toluene, cyclohexane and hexane markedly increased after the methanol activation, while it changed little for methanol, ethanol, IPA and water. This was mainly resulted from the blockage of nano-cavity of Noria by the formed triethylamine hydro chloride as discussed above, which further held back the molecular diffusion throughout the membrane. However, the solvents, such as methanol, ethanol, IPA and water could dissolve the triethylamine hy drochloride during the filtration, thus experiencing little change after activation. To explore the diffusion mechanism of various solvents in the polyarylate membrane, we regulated the permeance with their physi cochemical properties, including molecular kinetic diameters, viscos ities and solubility parameters. However, the result indicated the solvent 6
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Noria and TPC through traditional IP reaction on the PAN support. The nano-cavity structure of Noria was well retained after IP as revealed by XRD and CO2 adsorption experiment. Compared with RES based mem branes, the separation layer thicknesses of Noria based membranes were much thicker and their surfaces turned to be more hydrophilic and negatively charged. As a result of the molecular cavity, the prepared Noria þ TPC membrane was permeable for both polar and non-polar solvents. Except for methanol, the solvent permeance decreased grad ually with their dkins. The MD simulation result suggested the unex pected high methanol flux was mainly resulted from the low interaction energy with Noria when it passed through the nano-cavity. The work reported here would promote the study of other molecules with special structures for the preparation of high-performance membranes toward various separation applications. Conflicts of interest No conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all co-authors for publication.
Fig. 8. Performance comparison of Noria þ TPC with other OSN membranes reported previously in literatures.
Acknowledgements
permeance except that of methanol (dkin ¼ 0.38 nm) corrected well with their molecular kinetic diameters (Fig. 7c). As the molecular kinetic diameter increased, the solvent permeance turned to be lower. This phenomenon was obviously discriminated from the solution-diffusion theory and pore-flow model for membranes constructed with amor phous polymer [22] and inorganic nanomaterials [47,48] respectively, probably due to the specified pore size introduced by Noria in the polymer film. Just like the membrane prepared by cyclodextrin [32], the Noria þ TPC membrane also comprised hydrophilic regions surrounding the Noria molecule. It was more difficult for non-polar solvent to diffuse through them before having access to the Noria cavity. As a result, the permeance of hexane was lower than that of ethanol despite of their nearly identical molecular kinetic diameters. In order to figure out the mechanism responsible for the unexpected higher flux of methanol than water despite of its larger dkin, we carried out MD simulation to study the interaction energy variations between the solvent molecules, including water, methanol and ethanol with Noria as they diffused through the nano-cavity of Noria (Fig. S9). When the methanol and ethanol molecules positioned at the center of Noria, the interaction energy for them turned to be the maximum, while it was the highest at the portal for water (Fig. 7d) due to the strong hydrogenbond interaction between water and the unreacted hydroxyl groups in the periphery of Noria, indicating it was more difficult for water to enter into the cavity. This resulted in the higher maximal interaction energy (ΔEmax) of water than methanol (Fig. 7e) and further contributed to the relatively lower water permeance. However, the highest ΔEmax of ethanol with the lowest flux among these three solvents was mainly resulted from its large molecular size. Thus, the small molecular size combined with weak interaction with the hydroxyl groups surrounding Noria jointly endowed methanol with excellent ability to penetrate through the Noria þ TPC membrane. The performance comparison of Noria þ TPC with other membranes reported in the literatures was depicted in Fig. 8 [7,49–55]. It could be seen that the Noria þ TPC membrane was among the best-performance membranes for the separation of RB dyein organic solvents. Besides, the Noria þ TPC membrane depicted good stability in the long-term test (Fig. S10). The high-performance polyarylate membrane constructed from porous organic cage here would inspire the exploration of other molecules with inner pore for the preparation of TFC membranes for diverse applications in future.
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4. Conclusion In summary, a high-performance OSN membrane was prepared by 7
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Polyarylate membrane constructed from porous organic cage for high-performance organic solvent nanofiltration Zhe Zhai, Chi Jiang, Na Zhao, Wenjing Dong, Peng Li, Haixiang Sun, Q. Jason Niu * State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, 266580, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Organic solvent nanofiltration Porous organic cage Interfacial polymerization Molecular dynamics simulation
Preparation of highly permeable and selective organic solvent nanofiltration (OSN) membranes is desired for precise chemical separations. Although the emerging nanomaterials and novel preparation methods have significantly boosted the membrane performance, it still requires much effort to drive them toward scale-up production with lower cost and easier preparation process. Herein, we constructed a polyarylate membrane from one kind of porous organic cage, namely Noria through traditional interfacial polymerization technique. Under the catalysis of triethylamine, Noria in aqueous phase would react with terephthaloyl chloride (TPC) in hexane to form a dense nano-film. The inner cavity of Noria molecule provided nano-channel for both polar and non-polar solvents to transport through the membrane. Specially, the Noria þ TPC membrane exhibits a high permeance for methanol up to 18 L m 2h 1bar 1. This is resulted from the low interaction energy between methanol and Noria as revealed by the molecular dynamics (MD) simulation. In addition to the excellent per formance, the simple preparation process with low-cost materials suggests the great potential of Noria þ TPC membrane for practical application. Moreover, it would inspire the exploration of other molecules with tailored chemical structures for the assembly of high-performance membranes in future.
1. Introduction Organic solvent nanofiltration (OSN), also known as solvent resistant nanofiltration (SRNF) has developed rapidly in recent years and it is receiving more and more attention in chemical and pharmaceutical separations [1]. Compared with other purification processes, including adsorption [2], distillation and photocatalytic degradation [3,4], the membrane based separation was energy-efficient and easily scaled-up. The OSN could fulfill the separation of mixtures in organic solvents at molecular level, which shows great potential in purification, concen tration and solvent exchange [5,6]. In addition to the integrally skinned asymmetric (ISA) membranes [7,8], the OSN membranes with thin-film composite (TFC) structure prepared either through interfacial polymerization (IP) [9–12] or dip coating method [13–17] are also widely studied. Their performance could be tailored for specific applications by adjusting the properties of separation layer and porous support independently [18]. As a simple and efficient technique, IP enables the large-scale production of polymer films and other nanomaterials [19]. Especially, it has been widely employed for the preparation of nanofiltration (NF) membranes and
reverse osmosis (RO) membranes. For the IP on porous support, the polymerization reaction mainly takes place in the organic phase due to the poor solubility of organic monomer in water [20]. The resulting cross-linked thin film is mostly composed of polyamide, which could serve as an excellent candidate for OSN membrane benefitting from its good tolerance toward organic solvent [21]. For the preparation of highly permeable OSN membranes, various strategies have been developed to tailor the separation layers, including reducing the film thickness, incorporating nano-fillers and designing novel monomers for IP. Livingston and co-workers [22] creatively pre pared a sacrificial interlayer for the preparation of TFC membrane. The as-formed ultrathin polyamide film with sub-10 nm thickness signifi cantly promoted the membrane permeance. In addition, various nano materials, such as metal–organic framework (MOF) [23,24], carbon dot [25] and covalent organic framework (COF) [26] were employed as fillers in the polyamide layer to boost the OSN membrane performance. However, the permeance of these hybrid membranes for non-polar sol vents would be still unsatisfactory due to the limitation of polyamide as bulk material. Lai and co-workers [27] assembled 2D COF membrane on anodic aluminum support through Langmuir Blodgett (LB) method,
* Corresponding author. E-mail address: [email protected] (Q.J. Niu). https://doi.org/10.1016/j.memsci.2019.117505 Received 16 June 2019; Received in revised form 14 September 2019; Accepted 22 September 2019 Available online 23 September 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
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which exhibited high flux for both polar and non-polar solvents. Tang and co-workers [28] reported an OSN membrane prepared by conju gated microporous polymer (CMP) and its inner open and inter connected pores allowed the fast diffusion for both hexane and methanol. Shao and co-workers [29] constructed OSN membrane by β-cyclodextrin and polydopamine through the host-guest interactions and the membrane was also permeable for polar and non-polar solvents. Despite the excellent OSN performance of the above mentioned mem branes, the sophisticated preparation process and high-cost materials make it difficult for them toward practical application. Considering the advantages of IP technique, it is more appealing to prepare OSN mem branes through IP by molecularly designed monomers. For example, contorted phenol [30] and cyclodextrin [31,32] as monomers have been studied to improve the OSN membrane performance by increasing the microporosity of separation layers. Unfortunately, such kind of molecule with tailored structure employed for the preparation of high-performance OSN membrane is quite limited. Herein, for the first time, we reported a polyarylate membrane constructed from one kind of porous organic cage, namely Noria through IP for OSN. As a macrocyclic molecule with an inner cavity and multihydroxyl groups, Noria was simply synthesized by the condensation reaction of resorcinol and glutaraldehyde. Under the catalysis of trie thylamine, Noria in aqueous phase would react with acyl chloride in hexane to form a dense nano-film. The inner cavity of Noria provided paths for the transport of both polar and non-polar solvents, which efficiently promoted the membrane permeance. Meanwhile, the resor cinol was also employed to prepare membranes as control for compar ison. The physicochemical properties of the films formed on the porous support as well as at the free oil-water interfaces were systematically studied. The transportation behavior of various solvents, including water, methanol and ethanol through the molecular cavity was also investigated by molecular dynamics (MD) simulation. This work would not only enrich the available monomers for the preparation of highperformance OSN membranes, but also promote the understanding of molecular separation mechanism in the film with nano-cavity.
RES þ TMC were also synthesized at the bulk oil-water interface. After vigorous stirring, the obtained solid was collected and then washed by deionized water for several times. 2.3. Preparation of TFC membranes The TFC membranes were prepared on the PAN support through IP reaction and the process has been previously reported by our group [34, 35]. Briefly, the PAN support was fixed with the right side up and then covered with an aqueous solution containing 1.0 wt% Noria and 4.0 wt % triethylamine. After 3 min, the solution was removed and an air knife was applied to blowing off the visible liquid drops on the membrane surface. A TPC solution in hexane (0.2 wt%) was subsequently poured onto the above membrane for 5 min. Finally, the freshly prepared membrane was put in an oven at 60 � C for 3 min to complete the polymerization reaction. Other membranes, including Noria þ TMC, RES þ TPC and RES þ TMC were prepared under the same condition. 2.4. Characterizations Field-emission scanning electron microscopy (FESEM, Hitachi S4800, Japan) was employed to observe the membrane and powder morphology. The surface roughness of membrane was analyzed by Atomic force microscopy (AFM, Shimadzu SPM-9700, Japan). For the measurement of nano-film thickness, the film was firstly transferred onto a silicon wafer and then scratched by a sharp knife. The height difference between the nano-film and silicon wafer was recorded by AFM to be the film thickness. The element contents of PAN support and Noria þ TPC membrane were detected by X-ray photoelectron spec troscopy (XPS) (Escalab 250Xi, ThermoFisher). The IR spectrum of Noria as well as ATR-FTIR spectra of various membranes was also ob tained (Nicolet iS10, Thermo Scientific). The water contact angles (DSA30, Kruss GmbH, Germany) and zeta potentials (SurPASSTM3, Anton-Paar GmbH, Austria) were recorded to analyze the variation of membrane surface properties. The X-ray diffraction (XRD) patterns of Noria and polyarylate powders were obtained from an X’Pert PRO MPD diffractometer (PANalytical B.V., Netherlands). The UV–vis spectra (UV2700, Shimadzu, Japan) of dye solutions were analyzed to calculate the membrane rejection toward various dyes. CO2 sorption isotherm of polyarylate powder was collected at 273 K (Autosorb-iQ, Quantach rome, USA).
2. Experimental 2.1. Chemicals and materials The following chemicals were used without purification. Resorcinol (RES, >99.0%), terephthaloyl chloride (TPC, >99.0%), trimesoyl chlo ride (TMC, >98.0%), glutaraldehyde (24.0–26.0% in water), titan yel low (TY, 695.7 g mol 1), primuline (P, 475.5 g mol 1) and brilliant blue G (BBG, 854.0 g mol 1) were purchased from TCI. Rose Bengal (RB, 1017.6 g mol 1) was obtained from Aladdin. Other chemicals, such as methyl orange (MO, 327.3 g mol 1), triethylamine (>99.0%), methanol (>99.5%), ethanol (>99.7%), isopropyl alcohol (IPA, >99.5%), toluene (>99.5%), cyclohexane (>99.7%), hexane (>97.0%), hydrochloric acid (HCl, 36.0%), Na2SO4 (>99.0%), MgSO4 (>98.0%), MgCl2 (>99.0%) and NaCl (>99.5%) were all supplied by Sinopharm Chemical Reagent Co., Ltd. The polyacrylonitrile (PAN) ultrafiltration membrane employed for the preparation of TFC membranes was provided by Shandong Lanjing Membrane Technology and Engineering Co., Ltd.
2.5. Nanofiltration performance evaluation of various membranes The membrane rejections of different inorganic salts, including Na2SO4, MgSO4, NaCl and MgCl2 were evaluated under 4 bar in a crossflow testing cell with an effective area of 18.5 cm2 and the feed con centrations were all 2000 ppm. Meanwhile, the OSN performance of Noria þ TPC membrane was tested under the same condition by sepa rating various dyes in methanol solution (50 ppm). Prior to the test, the membrane was pre-compacted under 6 bar for 0.5 h. The membrane rejection could be calculated as follows: R ¼ (1- Cp / Cf) � 100%
(1)
Where R represents the membrane rejection. For inorganic salts and dyes, the values of Cp/Cf are acquired through the conductivity and UV–vis spectra of permeate and feed solution respectively. The permeance (Jw) was calculated by equation (2):
2.2. Preparation of free-standing polyarylate nano-films and powders Firstly, Noria was synthesized according to the literature [33] (Figs. S1 and S2). The Noria-TPC nano-film was prepared at the free oil-water interface. Briefly, the aqueous solution containing 1.0 wt% Noria and 4.0 wt% triethylamine was poured into a beaker. Then, the TPC solution of 0.2 wt% in hexane was gently added into the above beaker and the reaction time was allowed to be 1 min, 5 min, 10 min, 15 min, 20 min and 25 min respectively. After that, the two immiscible solutions were drained out to obtain the free-standing film. The poly arylate powders of Noria þ TPC, Noria þ TMC, RES þ TPC and
Jw ¼ Q / (P △T A)
(2)
Here, Q is the permeate volume at a given time (L), P stands for the testing pressure (bar), △T represents the testing period (h) and A is the area of membrane for test (m2). The measurement of membrane permeance toward various solvents, including methanol, ethanol, IPA, toluene, cyclohexane, hexane and 2
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Fig. 1. (a) Schematic illustration of the formation of polyarylate film by Noria and TPC (b) Photograph of free-standing Noria þ TPC film formed at the oil-water interface at a reaction time of 5 min (c) The AMF image and height profiles of the free-standing Noria þ TPC film at a reaction time of 5 min (d) The thicknesses of free-standing films synthesized at various time.
Fig. 2. (a) SEM image of Noria þ TPC membrane prepared on the PAN support (b) AFM topography image of Noria þ TPC membrane (c) XPS spectra of PAN support and Noria þ TPC membranes before and after washing (d) High resolution spectrum of O 1s for Noria þ TPC membrane after washing (e) Water contact angles of PAN support and Noria þ TPC membrane. 3
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Fig. 3. SEM images of (a) Noria þ TPC (b) Noria þ TMC (c) RES þ TPC (d) RES þ TMC powders (e) XRD patterns of various powders (f) CO2 sorption isotherm of Noria þ TPC (g) Pore size distribution of Noria þ TPC calculated from the CO2 sorption isotherm.
water was carried out in a dead-end unit cell pressurized by N2. As for the activation of membrane, 50 mL methanol was added into the cell and then filtered through the membrane. The solvent permeance was also obtained from equation (2).
the nascent PAN support (Fig. 2c) due to the formation of polyarylate membrane by Noria and TPC. In addition to aiding the dissolution of Noria, the triethylamine would also react with the generated HCl during IP as catalyst to promote the cross-linking reaction [38–40]. Considering the detection depth of XPS (~10 nm) [41], the observed N element for Noria þ TPC membrane before washing presumably derived from the triethylamine hydrochloride entrapped in the separation film. Due to the dissolution of triethylamine hydrochloride in water, the peak of N element for Noria þ TPC membrane disappeared after being washed by filtration of deionized water. As shown in Fig. 2d, the peak at about – C-OR in the high resolu 534.0 eV attributed to the ester oxygen of O– tion spectrum of O 1s indicated the formation of polyarylate. Besides, the detected C–O–C group at 1140 cm 1 of TFC membrane in the ATR-FTIR spectra further confirmed the reaction between Noria and TPC (Fig. S5). The water contact angle was commonly employed to evaluate the hydrophilicity of subject surface and it turned to be more hydrophobic with the increase of water contact angle [42,43]. Despite of the existence of hydroxyl and carboxyl groups, the TFC membrane turned to be more hydrophobic than the PAN support (Fig. 2e), which could be resulted from the formation of plenty of hydrophobic ester groups [44].
3. Results and discussion 3.1. The formation of polyarylate membrane by Noria and TPC The polyarylate membrane was prepared by traditional IP technique, which was schematically depicted in Fig. 1a. Though not soluble in pure water, Noria could dissolve well in the aqueous triethylamine solution (Fig. S3). The phenolic hydroxyl groups of Noria turned to deprotonate after the addition of triethylamine, resulting in the formation of reactive phenoxide ions. The reaction mechanism of Noria and TPC was depicted in Fig. S4, for which the deprotonated Noria reacted with TPC through nucleophilic substitution and then polymerized into polyarylate. As a macrocyclic molecule, β-cyclodextrin has been explored as aqueous monomer to prepare high-performance OSN membranes [31,32]. Due to the stronger electron-withdrawing ability of benzene ring than alkyl group, the formation of oxide ion participating in the polymerization reaction was much easier for Noria than β-cyclodextrin molecule. This made Noria free of careful regulation of IP conditions as well as support properties necessary for β-cyclodextrin in order to obtain a non-defective film [31]. Firstly, we synthesized free-standing poly arylate film at the bulk oil-water interface. After draining off the solu tions, an integrated film without visible cracks was obtained on the dish (Fig. 1b) and this suggested its high cross-linking degree. The resulting film was subsequently transferred onto silicon wafer to measure its thickness, which was nearly 252 nm characterized by AFM (Fig. 1c). Owing to the self-limiting trait of IP reaction [36,37], the diffusion of aqueous monomer would be held back when the initially formed film turned to be increasingly denser. Thus, the film thickness firstly increased with the reaction time and then experienced little change (Fig. 1d). Subsequently, the TFC membrane used for OSN was prepared on porous PAN support and the surface of as-prepared membrane was observably smooth (Fig. 2a and Fig. 2b). After the IP reaction, the ox ygen content of membrane surface obviously increased compared with
3.2. Polyarylate powders and membranes interfacially synthesized by various monomers For comparison, Noria þ TPC, Noria þ TMC, RES þ TPC and RES þ TMC powders were firstly prepared by IP reaction. As shown in Fig. 3c, the RES þ TPC powder appeared amorphous probably due to the linear polymer chain formed by the reaction of RES and TPC. However, the cross-linked structure of Noria þ TPC, Noria þ TMC and RES þ TMC endowed them with crystal-like morphology (Fig. 3a and b and Fig. 3d). The peak at about 10� attributed to the inner cavity of Noria (Fig. S6) was still detected for Noria þ TPC and Noria þ TMC powders (Fig. 3e), suggesting the structure of Noria was well retained during the IP reac tion. As a result, the Noria þ TPC depicted a steep adsorption of CO2 under the relative low pressure (Fig. 3f) and its surface area was calculated to be 73.4 m2 g 1, which was comparable to that synthesized by contorted monomers [30]. Meanwhile, the pore size of Noria þ TPC obtained with density functional theory (DFT) was mainly ranged in 4
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Fig. 4. Surface SEM images of (a) and (e) PAN support, (b) and (f) Noria þ TMC, (c) and (g) RES þ TPC, (d) and (h) RES þ TMC membrane (i) Water contact angles of various membranes (j) Zeta potentials of various membranes.
which the generated heat would induce interfacial instabilities and contribute to the production of a crumpled film [22,46]. Owing to the large molecular size, the diffusion rate of Noria in the aqueous phase toward the oil-water interface was much lower than RES and that resulted in the slower reaction rate of Noria with acyl chloride. Conse quently, the surface morphology of Noria based TFC membranes appeared flat due to the relatively less heat generated in the IP reaction at a given time. The AFM characterization (Fig. 5) confirmed the surface morphology variations of various TFC membranes detected by SEM images. Meanwhile, the surface roughness of RES þ TPC was visually higher than that of RES þ TMC membrane. For RES þ TPC, the formed linear polymer chains contributed to a loose film at the initial IP stage, which would allow the RES monomers to continuously travel through the membrane. This resulted in more RES monomers existing at the interface to react with TPC and it generated more heat than the cross-linked RES þ TMC membrane. The as-formed separation film thicknesses of various membranes were measured from their cross-section SEM images (Fig. S7). It could be seen that the separation layers of Noria based TFC membranes were all thicker than those con structed by RES. Compared with RES, it was more difficult for Noria to form a highly cross-linked film at the initial stage due to its bulky molecule volume. Thus, the Noria monomers would continuously diffused from the defects to react with acryl chloride in the organic phase, resulting in the increase of film thickness. In addition, the plenty of unreacted hydroxyl groups of Noria enhanced the hydrophilicity and negative charge of as-prepared TFC membranes as revealed by their water contact angles (Fig. 4i) and zeta potentials (Fig. 4j).
Fig. 5. AFM topography images of (a) PAN support (b) Noria þ TMC (c) RES þ TPC and (d) RES þ TMC membranes.
0.4–0.6 nm (Fig. 3g) corresponding with the portal diameter of Noria [45]. When prepared into membrane, the nano-cavity in Noria would not only serve as the primary sieving channel, but also efficiently pro mote the molecular transport through the TFC membrane. Afterwards, various TFC membranes were prepared on PAN support under the same condition. Although the membrane surfaces were all flat (Fig. 4a and Fig. 4b), the surface pores of PAN support (Fig. 4e) were covered with a dense film for Noria þ TMC (Fig. 4f) just as Noria þ TPC membrane. However, the surfaces of RES þ TPC (Fig. 4c and g) and RES þ TMC membranes (Fig. 4d and h) were filled with plate-like structure. As known to all, the IP was an exothermic reaction, during
3.3. Nanofiltration performance of TFC membranes The performance of Noria þ TPC, Noria þ TMC, RES þ TPC and RES þ TMC membranes in terms of permeance and rejection in aqueous solution were firstly tested. As seen in Fig. 6a, the permeance of Noria based membranes were significantly higher than RES based membranes despite of their thicker separation layers. The nano-cavity in Noria 5
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Fig. 6. (a) The Na2SO4 rejection and permeance of various membranes (b) The rejections of Noria þ TPC toward different inorganic salts.
Fig. 7. (a) Rejections of Noria þ TPC membrane toward various dyes in methanol (b) Pure solvent permeance of Noria þ TPC membrane for both polar and non-polar solvents (c) Pure solvent permeance variation as a function of their dkins (d) The positions of water, methanol and ethanol in Noria with the maximal interaction energy (e) The values of maximal interaction energy for water, methanol and ethanol.
molecule efficiently reduced the transport resistance of water (~0.28 nm) through the Noria based film. Due to the formed linear polymer chain, the Na2SO4 rejection of RES þ TPC was merely as much as 40%. Meanwhile, the reduced salt rejection for Noria þ TMC compared with Noria þ TPC was mainly resulted from its less crosslinked structure as a result of the high steric hindrance for the reac tion with TMC. Fig. 6b showed the rejections of Noria þ TPC toward various salts and it followed the sequence: Na2SO4> MgSO4 > NaCl > MgCl2. Due to the Donnan effect, the Noria þ TPC membrane with negative charge depicted a higher rejection toward Na2SO4 than that of MgSO4. Meanwhile, the positive Mg2þ could shield the negative charge on membrane surface and further contribute to the low rejection of MgCl2. The OSN performance of Noria þ TPC membrane was subsequently evaluated by separating various industrial dyes (Table S1) in methanol (Fig. S8). As seen in Fig. 7a, the rejections of Noria þ TPC membrane increased with the molecular weights of various dyes. The nano-cavity of Noria molecule was retained after IP, which provided pathways for
the transport of various solvents throughout the polyarylate film. Thus, the Noria þ TPC membrane was permeable for both polar and non-polar solvents (Fig. 7b). However, for the membrane prepared by m-phenyl enediamine and TMC, it was mainly consisted of hydrophilic polyamide and the amorphous film lacked inner pores for the molecular transport of non-polar solvent, resulting in its ultralow permeance [22]. Inter estingly, the permeance of toluene, cyclohexane and hexane markedly increased after the methanol activation, while it changed little for methanol, ethanol, IPA and water. This was mainly resulted from the blockage of nano-cavity of Noria by the formed triethylamine hydro chloride as discussed above, which further held back the molecular diffusion throughout the membrane. However, the solvents, such as methanol, ethanol, IPA and water could dissolve the triethylamine hy drochloride during the filtration, thus experiencing little change after activation. To explore the diffusion mechanism of various solvents in the polyarylate membrane, we regulated the permeance with their physi cochemical properties, including molecular kinetic diameters, viscos ities and solubility parameters. However, the result indicated the solvent 6
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Noria and TPC through traditional IP reaction on the PAN support. The nano-cavity structure of Noria was well retained after IP as revealed by XRD and CO2 adsorption experiment. Compared with RES based mem branes, the separation layer thicknesses of Noria based membranes were much thicker and their surfaces turned to be more hydrophilic and negatively charged. As a result of the molecular cavity, the prepared Noria þ TPC membrane was permeable for both polar and non-polar solvents. Except for methanol, the solvent permeance decreased grad ually with their dkins. The MD simulation result suggested the unex pected high methanol flux was mainly resulted from the low interaction energy with Noria when it passed through the nano-cavity. The work reported here would promote the study of other molecules with special structures for the preparation of high-performance membranes toward various separation applications. Conflicts of interest No conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all co-authors for publication.
Fig. 8. Performance comparison of Noria þ TPC with other OSN membranes reported previously in literatures.
Acknowledgements
permeance except that of methanol (dkin ¼ 0.38 nm) corrected well with their molecular kinetic diameters (Fig. 7c). As the molecular kinetic diameter increased, the solvent permeance turned to be lower. This phenomenon was obviously discriminated from the solution-diffusion theory and pore-flow model for membranes constructed with amor phous polymer [22] and inorganic nanomaterials [47,48] respectively, probably due to the specified pore size introduced by Noria in the polymer film. Just like the membrane prepared by cyclodextrin [32], the Noria þ TPC membrane also comprised hydrophilic regions surrounding the Noria molecule. It was more difficult for non-polar solvent to diffuse through them before having access to the Noria cavity. As a result, the permeance of hexane was lower than that of ethanol despite of their nearly identical molecular kinetic diameters. In order to figure out the mechanism responsible for the unexpected higher flux of methanol than water despite of its larger dkin, we carried out MD simulation to study the interaction energy variations between the solvent molecules, including water, methanol and ethanol with Noria as they diffused through the nano-cavity of Noria (Fig. S9). When the methanol and ethanol molecules positioned at the center of Noria, the interaction energy for them turned to be the maximum, while it was the highest at the portal for water (Fig. 7d) due to the strong hydrogenbond interaction between water and the unreacted hydroxyl groups in the periphery of Noria, indicating it was more difficult for water to enter into the cavity. This resulted in the higher maximal interaction energy (ΔEmax) of water than methanol (Fig. 7e) and further contributed to the relatively lower water permeance. However, the highest ΔEmax of ethanol with the lowest flux among these three solvents was mainly resulted from its large molecular size. Thus, the small molecular size combined with weak interaction with the hydroxyl groups surrounding Noria jointly endowed methanol with excellent ability to penetrate through the Noria þ TPC membrane. The performance comparison of Noria þ TPC with other membranes reported in the literatures was depicted in Fig. 8 [7,49–55]. It could be seen that the Noria þ TPC membrane was among the best-performance membranes for the separation of RB dyein organic solvents. Besides, the Noria þ TPC membrane depicted good stability in the long-term test (Fig. S10). The high-performance polyarylate membrane constructed from porous organic cage here would inspire the exploration of other molecules with inner pore for the preparation of TFC membranes for diverse applications in future.
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