- Email: [email protected]
cyclohexane mixtures
Journal Pre-proof Ionic liquid copolymerized polyurethane membranes for pervaporation separation of benzene/cyclohexane mixtures
Tao Xi, Yingying Lu, Xinyu Ai, Lin Tang, Lulu Yao, Wentao Hao, Peng Cui PII:
S0032-3861(19)30954-1
DOI:
https://doi.org/10.1016/j.polymer.2019.121948
Reference:
JPOL 121948
To appear in:
Polymer
Received Date:
31 July 2019
Accepted Date:
26 October 2019
Please cite this article as: Tao Xi, Yingying Lu, Xinyu Ai, Lin Tang, Lulu Yao, Wentao Hao, Peng Cui, Ionic liquid copolymerized polyurethane membranes for pervaporation separation of benzene /cyclohexane mixtures, Polymer (2019), https://doi.org/10.1016/j.polymer.2019.121948
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Ionic liquid copolymerized polyurethane membranes for pervaporation separation of benzene/cyclohexane mixtures
Tao Xi, Yingying Lu, Xinyu Ai, Lin Tang, Lulu Yao, Wentao Hao*, Peng Cui* School of Chemistry & Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Materials Chemical Engineering, Hefei University of Technology, Hefei, 230009 P.R. China *Email: [email protected]; [email protected], Tel & Fax: 8655162902661 ABSTRACT: Benzene and cyclohexane are both important raw materials in polymer chemical industry. However, they are hard to be separated via the traditional distilling methods. In this work, ionic liquid copolymerized waterborne polyurethane (IL-co-PU) membranes were proposed for the pervaporation separation of benzene/cyclohexane mixtures. The results, including swelling rate, diffusion coefficients, partial flux, permeance and selectivity, showed that the IL-co-PU membranes could preferentially absorb and transfer benzene molecules rather than cyclohexane. The ILs might have functioned as facilitated transporter in the membranes. The permeation flux and separation factor of the IL-co-PU membranes increased simultaneously with the increase of IL contents. That is, these membranes had no trade-off between permeability and selectivity, which most of the polymer membranes suffered. With consideration of the good performance and easy handling property of the IL-co-PU membranes, they are of great potential in pervaporation separation of benzene/cyclohexane mixtures. KEYWORDS: polyurethane; membrane; ionic liquid; copolymer; pervaporation; benzene
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1. INTRODUCTION Benzene and cyclohexane are both important raw materials in polymer chemical industry. The products of benzene include styrene-acrylate latex and polyurethane, etc. The cyclohexane is used to produce polyamides. However, the benzene and cyclohexane are hardly separated via the traditional methods, including azeotropic distillation and extraction distillation [1,2]. Moreover, the traditional methods are highly energyconsuming and inefficient. As a promising alternative, pervaporation based on membrane technology has attracted much attention in recent years for their high efficiency and low costs [3-5]. According to the solution-diffusion theory, the pervaporation membranes selectively absorb benzene molecules and diffuse them through the membranes. Therefore, those membranes have aromatic components can have better permeability. In the past decades, membranes from aromatic polyetheramides have been widely researched [6,7]. The permeation flux of these membranes was impressive. However, their separation selectivity was relatively poor because the free volume of polymers was enlarged by swelling during the pervaporation. Alternatively, polyimides were adopted to fabricate the pervaporation membranes [8,9]. The polyimide membranes showed much better separation selectivity than the polyetheramides because they had densely packed chains and therefore high ability to resist swelling. Nevertheless, the permeation flux dropped. Crosslinking is effective in reducing swelling, and in improving selectivity. However, the permeation flux often goes controversially [10]. The trade-off between permeation flux and selectivity is always a problem for the polymer membranes [11].
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In recent years, the hybrid membranes were intensively investigated [12-15]. The fillers in hybrid membranes functioned as facilitated transport reagents, which greatly favored the diffusion of benzene in the membranes [16]. For example, Wu and coworkers reported sliver nanoparticle@carbon nanotubes (Ag-MWCNTs) incorporated polyurethane membranes [13]. The Ag-MWCNTs interacted with benzene molecules via complexation, facilitating the selective transportation of benzene. Ji and coworkers reported metal-organic frameworks (MOFs) embedded polyvinyl alcohol (PVA) membranes [15]. The MOFs have specific interactions with benzene molecules that the PVA/MOFs hybrid membranes showed higher separation factors, as well as greater permeation flux than the pristine PVA membrane. However, the nanofillers, like CNTs and MOFs, have strong tendency to agglomerate in polymer membranes because of their large specific area and high surface energy [17]. Great challenges are there in fabricating pervaporation membranes for the efficient separation of benzene/cyclohexane mixtures. Ionic liquids (ILs) have attracted much attention in recent years because of their great potential in extraction separation of benzene/cyclohexane mixtures [18-22]. The ILs are polar molecules with planar structure that they preferentially absorb the polarizable benzene rather than the non-polarizable cyclohexane [23,24]. Unlike the nanofillers, the ILs are tiny organic molecules that can be chemically incorporated into the polymer chains [25], therefore randomly dispersed in the polymer matrix. The IL copolymer membranes will have good affinity to the benzene as well as those hybrid membranes. However, the undesired agglomeration is effectively avoided. Rationally, the IL copolymer membranes are very promising in pervaporation separation of benzene/cyclohexane mixtures. Nevertheless, there was rare report at present.
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Herein, we reported a kind of IL copolymerized polyurethane (IL-co-PU) membrane. The ILs with vinyl groups were chemically bonded to the prepolymer of waterborne polyurethane with acryloyl end groups via free radical polymerization. Because the ILs had strong affinity to the benzene, the IL-co-PU membranes preferentially absorbed benzene rather than cyclohexane. Moreover, these membranes showed much higher diffusion coefficient for the benzene than the cyclohexane. Both the selectivity and permeability of these membranes increased with the IL contents, suggesting that they overcame the tradeoff between selectivity and permeability, which was suffered by most of the polymer membranes. With consideration of their high performances and easy handling property, the IL-co-PU
membranes
have
great
potential
in
pervaporation
separation
of
benzene/cyclohexane mixtures. 2. EXPERIMENTAL 2.1 Materials. Poly-1,4-butylene adipate glycol (PBA) (Mn = 2000) were supplied by Qingdao Xinyutian Chemical Co., LTD. (China). Trimethylol propane (TMP) was obtained from Hefei Anke Fine Chemicals Co., LTD. (China). Toluene diisocyanate (TDI), 2-hydroxyethyl acrylate (HEA) was provided by Shanghai Aladdin Bio-Chem Technology Co., LTD. (China). Dimethylolpropionic acid (DMPA) was provided from Xiya Regent Co., LTD. (China). Triethylamine (TEA), ethylenediamine (EDA), dibutyltindilaurate (DBTDL), butanone, azobisisobutyronitrile (AIBN), benzene, cyclohexane were purchased from Sinopharm Chemical Regents Co., LTD. (China). The 1-butyl-3vinylimidazolium hexafluorophosphate ([bvim][PF6]) was obtained from Shanghai ChengJie Chemical Co., LTD. (China). The PBA was dried in a vacuum oven at 120 °C for 12 h before use. All the chemicals were used as received.
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2.2 Membranes preparation. The synthetic route of IL-co-PU was shown in Scheme 1. A predetermined amount of TDI was added drop-wise to the PBA in butanone, reacting in a three-necked round bottom flask at 70 °C for 1 h. Subsequently, appropriate amounts of DMPA and TMP were added and the reaction was performed for 2 h in the presence of a DBTDL catalyst to obtain the NCO-end capped PU. HEA was then added in to introduce acrylate groups onto the PU chains. Thereafter, the IL monomer ([bvim][PF6]) was added and stirred for 0.5 h. AIBN as initiator was added dropwise within 0.5 h. Then, the mixture was maintained at 70 °C for another 2 h. The IL copolymerized PU was cooled to 30 °C. Lastly, TEA was added in accompanying a large amount of deionized water under high speed shearing. The obtained IL-co-PU emulsion had a total solid content of 10 wt%. A series of IL-co-PU emulsions were prepared with IL concentrations of 0/100, 3/100, 5/100 and 10/100 with respect to the solid contents of PU. The emulsions were cast onto glass plates to obtain membranes. The membranes were dried in air at first, and then dried in a vacuum oven at 50 °C for 8-10 h. The thickness of the membranes was about 70-90 m. The membranes were named as PU, PU-3, PU-5, and PU-10 with respect to the IL contents. 2.3 Characterization. The particle sizes of IL-co-PU emulsion were characterized on a MS2000 laser light scattering machine (Malvern). Reaction between IL and PU was confirmed by FTIR spectrometer (Nicolet 67, ThermoFisher Scientific). Thermal properties of pristine PU and IL-co-PU membranes were studied using a DSC instrument (DSC 214, Netzsch) and a DMA instrument (Q800, TA Instruments). A scanning electron microscope (SEM) (SU-8020, Hitachi) was used to study the cross-section morphology of membranes.
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Elemental composition of membranes was determined using the same SEM equipped with an energy dispersive X-ray spectrometer (EDX).
Scheme 1. Schematic diagram of preparation of IL-co-PU
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2.4 Swelling and Sorption Measurements. A weight-gain method was used to determine the degree of swelling (DS). Dry samples were immersed in pure solvents or in benzene/cyclohexane mixtures at 50 °C. At specific time intervals, the samples were taken out, wiped with tissue paper and weighed. These experiments were continued until sorption equilibrium was reached, at which no further weight gains of the samples occurred. The DS values were defined as:
DSt
Mt M0 100% M0
(1)
Where M0 and Mt are the weight of pristine membranes and swollen samples at time t, respectively. The composition of liquid mixture inside membrane at sorption equilibrium was determined by desorbing liquid on a vacuum line and condensing in a cold trap. The composition was analyzed by gas chromatography in the same way as for pervaporation. The sorption selectivity was defined as:
sorp
X M / 1 X F X F / 1 X M
(2)
Where XF and XM are the weight fractions of benzene in the feed solution and membrane, respectively. 2.5 Pervaporation experiments. The pervaporation experiment apparatus was shown in Scheme 2. Pervaporation experiments were performed at 50 °C on a homemade device. The effective surface area of the membrane was 20.2 cm2, in contacting with the feed mixture. Benzene composition in the feed mixture was 50 wt%. In the downstream side of the apparatus, vacuum was maintained at -0.1009 MPa. After steady state was attained, the
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permeate liquid was collected in cold traps immersed in liquid nitrogen. Permeation rate was determined by measuring the weight of permeates. Composition of each phase was measured by gas chromatography (GC-2008 equipped with flame ionization detector, Shenyang Guangzheng Analytical Instruments Co., LTD.). Efficiency of the membranes was assessed in terms of permeation flux (J), separation factor (αPV) and pervaporation separation index (PSI).
W At
(3)
X P / 1 X F X F / 1 X P
(4)
J
PV
PSI J PV 1
(5)
Where W represents the mass of permeates (g); A is the effective membrane area (m2); t is the permeation time (h); XF and XP are the weight fractions of benzene in feed and permeate, respectively.
Scheme 2. Pervaporation setup used in this work.
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1- Funnel; 2- Feed Tank; 3- Feed pump; 4- Flowmeter; 5- Plate membrane module; 6&7Cold traps; 8- Vacuum pump; 9- Emptying valve. Permeances of membranes for benzene and cyclohexane were calculated according to the following equation [15,26]:
Pi ji 0 l xi i Pi yi Pp
(6)
Where Pi is the permeability of component i. l is the membrane thickness. xi and yi are molar concentration of aromatic and aliphatic compounds in the feed and permeate. i is the activity coefficients for each component. P0i and Pp are the saturated vapor pressure of component i and the total pressure at the permeate side. ji (cm3(STP) cm-2 s-1) is the molar flux for each component, which is calculated from:
ji
J i viG mi
(7)
Where vGi is the molar volume of gas I (22.4 L (STP) mol-1) and mi is the molecular weight of component i. Ji is the experimental partialflux for each component, which is calculated as: J i J yi
(8)
The permeance is normally expressed in GPU units (1 GPU = 110-6 cm3 (STP)/(cm2 s cmHg)). The selectivity of the membrane is calculated from the ratio between permeance, which is defined as:
i/ j
Pi / l Pj / l
(9)
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3. RESULTS AND DISCUSSION 3.1 Characterization of the IL-co-PU emulsion and the IL-co-PU membranes The IL-co-PU emulsion was prepared by a two-step procedure. Firstly, the polyurethane prepolymer was synthesized majorly from PBA, TDI, DMPA and HEA. The HEA was used to introducing double bond onto the polyurethane chains (shown in Scheme 1). Secondly, the IL monomer with vinyl group ([bvim][PF6]) was added into the prepolymer along with the AIBN as initiator. The free radical polymerization initiated by AIBN produced polyimidazole segments, which were linked to the PU chains via C-C bonds (Scheme 1). Such a procedure was often adopted in synthesis of polyurethanepolyacrylates copolymers [27,28]. Thereafter, the IL-co-PU polymers were mixed with large amount of water to produce emulsion. The DMPA on the PU chains stabilized the emulsion particles.
Figure 1. Particle size distribution of the IL-co-PU emulsion
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The particle size distribution of IL-co-PU emulsion was characterized by light scattering. The results were shown in Figure 1. As indicated, the particle sizes were in the range from 40 nm to 100 nm. The pristine PU emulsion had the smallest particle size, while PU-10 emulsion had the largest one. The PU chains became longer after attaching the polyimidazole segments. Moreover, the polyimidazole segments are rigid. The rigid chains have more extended conformation that the particle size were enlarged. The rigidity of polyimidazole segments comes from the strong repellence between the dangling imidazole groups. PU-IL copolymer had been reported recently [29-34]. The reported PU-IL copolymers were all synthesized via urethane bonding. For example, Gin and coworkers reported PUIL copolymer/IL composite films [29] using ammonium-diol and -triol ionic liquid (IL) as monomers to react with diisocynanates. However, the PU-IL copolymer synthesized through free radical polymerization was not reported yet. This work is the first report on PU-IL copolymer obtained from free radical polymerization. Moreover, large amounts of organic solvents were often used during the synthesis of PU-IL copolymers. Only in few report, the water dispersible PU-IL copolymer was reported [35]. Nevertheless, the present IL-co-PU copolymers used large amount of water as dispersion reagent. They are ecofriendly.
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Figure 2. FTIR spectra of (a) IL-co-PU (PU-10), (b) PU membranes and (c) difference. The FTIR spectrum of a typical IL-co-PU membrane (PU-10) was shown in Figure 2, with the pristine PU as control. The difference between spectra of these two samples was shown either. On the spectrum of PU-10, there were characteristic vibration peaks of PU, such as 3330 cm-1, 2956 cm-1and 2873 cm-1, 1728 cm-1 and 1167 cm-1, which were attributed to vibration of –NH, -CH2-, and >C=O groups from polyester-polyurethane [36]. The data of vibration peaks was collected in Table 1. It was noticed that there was a peak at 811 cm-1 for the pristine PU, which was characteristic absorption of vinyl groups from 2-hydroxyethyl acrylate [37]. However, this peak disappeared on the spectrum of PU-10. It indicated that the IL monomers had reacted with the acryloyl PU prepolymer.
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Table 1. Spectral data and band assignments of FTIR analysis presented in Figure 2. PU
IL-co-PU
Assignment
References
Wavenumber (cm-1)
Wavenumber (cm-1)
3330
3329
NH
36,43,44
2956/2873
2955/2873
CH2
44
1727
1728
>C=O
44
1532
1536
C-N-H
45,46,47
1456
1462
CH2
47,48
1369
1368
CH2
49
1258
1256
C-O in NHC(O)O,
46,50
1165
1167
CO
51
1061
1062
COC
47,48
959
959
C-H
52
C=C
37,53
PF6
41,42,54
810 843
The difference of FTIR spectra could show the characteristics of IL-co-PU membrane more clearly. As it could be seen, there were peaks located at 1641 cm-1, 1570 cm-1, 1306 cm-1 and 843 cm-1. These peaks were attributed to the C=C and C=N stretching vibration of imidazole ring [38,39], in plane bending of the ring [40] and PF6 ions [41,42]. These results indicated that the ILs have been chemically bonded to the PU. The appearance of IL-co-PU membranes was distinguishable from that of the pristine PU membrane. As seen in Figure 3, the IL-co-PU membranes were yellowish while the pristine PU membrane was colorless. The yellow color of IL-co-PU membranes became deeper and deeper with IL contents increasing. However, these membranes remained good
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transparency. Notably, all the IL-co-PU membranes were flexible that they could be folded up. Compared to the SILMs based on ceramic membranes [55], the flexible IL-co-PU membranes were more profitable. They are adaptable to fit in various types of pervaporation devices, either the planar or the tube-type.
Figure 3. Appearance of pristine PU and IL-co-PU membranes. From (a) to (d), the samples are PU-0, PU-3, PU-5 and PU-10. Microstructure of the IL-co-PU membranes was observed by SEM, as indicated in Figure 4. The cross section of the pristine PU membrane was smooth, indicating that there was no macroscopic phase separation. However, the surface of IL-co-PU membranes was rough, and the surface roughness increased with the IL contents. In the previous report, the surface roughness was attributed to the incompatibility between ILs and polymer matrix [56,57]. The SEM observation indicated that phase separation had occurred in the IL-co-PU membranes. That is, the ILs were dispersed in the PU matrix as isolated phases.
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Figure 4. SEM photographs of cross section of pristine PU and IL-co-PU membranes: (a) PU-0, (b) PU-3, (c) PU-5, (d) PU-10 Although there was phase separation, the EDX results suggested that the ILs were homogeneously dispersed in the IL-co-PU membranes. In Figure 5, the bright dots represented the fluorine elements from the PF6 anions, which located nearby the imidazoles. As shown, there was no apparent agglomeration of the bright dots. This result was much different from that in the IL blended PU membranes [57]. In those membranes, the ILs migrated to places near the membrane surface for the lack of the strong interactions between ILs and PUs. In the IL-co-PU membranes, however, the imidazoles were chemically bonded onto the PU chains and were dispersed in membranes randomly. Their
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migration was impeded due to the high viscosity of polymers. Therefore, the PF6 anions were homogeneously dispersed nearby the imidazoles.
Figure 5. Element mapping of the cross section of IL-co-PU membranes. Fluorine signals from: (left) PU-3, (middle) PU-5, (right) PU-10. DSC measurement was performed for the pristine PU and IL-co-PU membranes. The results were shown in Figure 6. It was found that there were melting peaks for all these samples. The melting peaks were located at around 50 C, very close to the melting point of PBA (47 C) reported previously [58]. This result indicated that the PBA segments on the PU macromolecules had strong tendency to agglomerate into ordered structure, i.e. to crystallize. To clearly demonstrate the influence of ILs on the crystallization behavior of PBA segments, melting peaks, the melting points, melting enthalpy and crystallinity were listed in Table 2. From PU-0 to PU-5, the melting points of IL-co-PU samples increased. Moreover, the melting enthalpy increased too. These results suggested that the ILs were able to promote the growth of PBA crystals. Or it could be said that the molecular motion had been activated by the ILs.
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Figure 6. DSC curves of PU and IL-co-PU membranes Table 2. Crystallization data of PU and IL-co-PU membranes
Sample
Tm (C)
Hm (J/g)
Relative Crystallinity (%)
PU-0
45.07
39.33
32.5
PU-3
45.58
50.93
42.1
PU-5
50.25
70.63
58.4
PU-10
45.89
33.69
30.6
The ILs used in this work are composed of cationic imidazoles and anionic PF6 counter ions. As shown in Scheme 1, the polyimidazole segments were chemically bonded onto the polyurethane chains. Due to the strong repellence between imidazole groups, the single bonds in polyimidazole segments are of poor rotation ability. It is not possible for the rigid polyimidazole segments to activate the molecular motion. Therefore, there must be some other reasons. In the previous reports, the enhanced molecular motion was attributed to the
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phase separation caused by ILs [59,60]. Singha and coworkers found that the melting enthalpy and the crystallinity of soft segments of PU-IL were higher than that of the PU. They attributed it to the phase separation in PU-IL copolymer, because the ILs on the hard segments led to greater incompatibility with the soft segments [59]. In the report of Einloft and coworkers, they suggested that the counter ions of ILs functioned as a driving force in microphase separation, subsequently improved molecular flexibility and facilitated the molecular motion [60]. Here in this work, the phase separation is possible. The polyimidazole segments can form - interactions with benzene rings (from TDI) on the hard segments of PU, but they have rare structural similarity to the PBA soft segments, resulting in poorer compatibility. However, the phase separation was not always observed [57]. In that report, the imidazoles could hardly form - interaction with aliphatic rings of isophorone diisocyanate on the hard segments. From Figure 6 and Table 2, it was learned that the PU-10 sample had not shown further improvement in melting point and enthalpy. Its melting point and enthalpy were lower than that of PU-5 sample. The experimental results from DSC measurement seems not able to be fully explained by phase separation. Dynamic mechanical analysis (DMA) is a sensitive tool to detect molecular motion, which determines the overall performances of polymer materials [61,62]. As seen in Figure 7a, the storage moduli of PU-5 and PU-10 membranes were lower than that of the PU membrane when the temperature was more than -45 C. It indicated that the IL-co-PUs have higher molecular mobility than the pristine PU. This result was coincided with the results from DSC measurement. Correspondingly, the peaks at -45 C on tan curves of
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PU-5 and PU-10 were remarkably larger than that of the pristine PU as shown in Figure 7b. Furthermore, there were several peaks located in the high temperature ranges from 90 C to 135 C. They were attributed to the molecular motion of the hard segments. It was shown that the PU-5 and PU-10 had much lower transition temperature of hard segments than the pristine PU. These were also evidences that the IL-co-PUs had higher molecular mobility. The enhanced molecular motion could be partially attributed to the plasticizing effect of counter ions of ILs [31,63], except for the possible phase separation in IL-co-PU membranes. The counter ions of ILs are PF6 ions, locating nearby the cationic polyimidazole segments. As the polyimidazole segments were uniformly distributed in the polymer matrix, the PF6 ions were distributed evenly too. The PF6 ions have small molecular weight that they are of stronger molecular mobility than the polymer segments. It is highly possible that they functioned as plasticizers to activate the molecular motion of polyurethane in the IL-co-PU membranes [63]. As a result, the tan values for glass transition of soft segments increased, and the glass transition temperature of hard segments decreased. The plasticizing effect could also explain the lowered melting point and enthalpy of PU-10 sample. In PU-10 sample, the ILs contents were the highest that there were a lot of PF6 ions. They isolated the PBA segments, and therefore impeded their agglomeration to form large size crystals.
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Figure 7. DMA results of pristine PU and IL-co-PU membranes: (a) storage modulus vs. temperature; (b) tan vs. temperature. 3.3 Swelling behavior of IL-co-PU membranes Sorption is a crucial procedure for the pervaporation. The sorption potential of pervaporation membranes can be characterized by the swelling test results [7]. In this study, the IL-co-PU membranes were immersed in 100% benzene and 100% cyclohexane respectively. During the test, data indicating swelling rate were collected carefully and were shown in Figure 8 and Table 3.
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Figure 8. Swelling processes of pristine PU and IL-co-PU membranes (a) in benzene and (b) in cyclohexane. As seen in Figure 8a, the slope at the initial stage of swelling curves of IL-co-PU membranes in benzene was much larger than that of the pristine PU membrane. It indicated that the IL-co-PU membranes swelled faster than the pristine PU membrane. However, the swelling rate of IL-co-PU membranes in cyclohexane was very close to that of the pristine PU membrane, except for the PU-10 membrane (Figure 8b). These results indicated that the IL-co-PU membranes selectively absorbed benzene, but had little affinity to
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cyclohexane. The preferential absorption of these membranes to benzene was undoubtedly contributed by the ILs because of their strong affinity to aromatic compounds [18-24]. As shown in Table 3, the swelling degree at equilibrium (DS) of IL-co-PU membranes increased remarkably from PU-0 to PU-10 when they were immersed in benzene. On the contrary, the DS values decreased when the membranes were immersed in cyclohexane. Although the molecular motion of IL-co-PUs was stronger than that of the pristine PU, which always suggested larger free volume, the IL-co-PU membranes were not swelled in cyclohexane, but shrank. The IL-co-PU membranes had preferential sorption to benzene rather than cyclohexane. As a result, the sorption selectivity (sorp) for benzene increased drastically from PU-0 to PU-10. Table 3. Swelling and sorption behavior of the membranes Swelling Degree at Sorption equilibrium (DS) Sample Selectivity /% (αsorp.) Benzene Cyclohexane
Diffusion Coefficient /μm2s-1 Benzene
Cyclohexane
PU-0
113
3.5
3.32
1.11
0.21
PU-3
184
3.2
3.62
3.87
0.34
PU-5
229
1.3
4.68
6.16
0.86
PU-10
256
1.6
6.55
9.27
6.05
From the swelling curves shown in Figure 8, the diffusion coefficients were calculated according to the following equation [64], and the results were shown in Table 3 either.
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h D Diffusion coefficient 4 4Q
2
(6)
Where θD is slope of the initial portion of the plots of Qt, Q is the mol% uptake at equilibrium. In this equation, the θD/Q equals to slope of the initial portion of the plots of DSt, and h (m) is thickness of the membranes. The IL-co-PU membranes showed much larger diffusion coefficients than the pristine PU membrane, which had diffusion coefficient comparable to that in the previous report [65]. Nevertheless, the diffusion coefficients of IL-co-PU membranes for cyclohexane were very small, except for the PU-10 sample. The enlarged diffusion coefficient for benzene was not only contributed by the higher molecular mobility in IL-co-PU membranes, but also by the preferential transport of benzene. In the previous section, it was shown that the ILs were homogeneously dispersed in the membranes. They could function as carriers. The benzene were diffused from one IL phases to another as well as in the polymer matrix. However, the ILs made the diffusion pathways of cyclohexane twisted and longer. The exception of PU-10 sample might be the result of excessively enlarged free volume in this membrane [66]. 3.4 Pervaporation performance of IL-co-PU membranes The pervaporation performance of IL-co-PU membranes was tested and the total permeation flux and separation factor of IL-co-PU membranes were shown in Figure 9. The permeation flux increased constantly from PU-0 to PU-10. For the PU-10 sample, its flux was 150% that of the pristine PU membrane. Interestingly, the separation factor of ILco-PU membranes increased simultaneously. The separation factor of PU-10 sample was
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240% that of the pristine PU membrane. That is, there is no trade-off effect for the IL-coPU membranes in the IL concentrations investigated. Generally, the polymer membranes suffered from their intrinsic trade-off between permeability and selectivity [67,68]. The reports that solved trade-off problem were always about hybrid membranes [69]. The ILco-PU membrane was one of the a few polymer membranes that overcame the trade-off effect.
Figure 9. Pervaporation performances for benzene/cyclohexane mixtures (50/50 w/w) at 50 °C The partial flux (Ji) is defined as J i J t yi , where Jt is total flux, and yi is the weight fraction of component i in the permeate [70]. The Ji values of benzene and cyclohexane were shown in Figure 10. It was found that the partial flux of benzene increased constantly from PU-0 to PU-10. However, the partial flux of cyclohexane decreased. The partial flux values showed that the IL-co-PU membranes preferentially permeate the benzene. Similar results were found for the ILs incorporated supporting membranes [55,71].
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Figure 10. Partial flux of IL-co-PU membrane for benzene and cyclohexane
Figure 11. Permeance and selectivity of IL-co-PU membranes for benzene and cyclohexane Compared to the permeation flux, permeance (Pi/l) is more suitable to describe the intrinsic property of a membrane [72]. As shown in Figure 11, the permeance of IL-co-PU membranes for benzene increased constantly from PU-0 to PU-10. Nevertheless, the permeance for cyclohexane decreased. Consequently, the selectivity (Pbenzene/Pcyclohexane)
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permeance of IL-co-PU membranes increased drastically. The incorporation of ILs greatly improved the preferential permeation of benzene in the IL-co-PU membranes. The preferential permeation of benzene by the ILs was similar to the facilitated transportation of the hybrid membranes. The ILs functioned as facilitated transporter to accelerate the transfer of benzene molecules, in addition to the Fickian diffusion [16,69], as illustrated in Figure 12. On the contrary, the ILs impeded the transportation of cyclohexane. Therefore, both the permeability and separation selectivity of IL-co-PU membranes for benzene were remarkably increased.
Figure 12. Proposed diffusion of benzene and cyclohexane in the IL-co-PU membrane. The pervaporation performance of IL-co-PU membranes for benzene/cyclohexane mixtures was compared with that of the reported membranes, as shown in Table 4. The separation factor of IL-co-PU membrane was 8.3, which is among the best of the reported polymer membranes. Moreover, the PSI (PSI=J*(-1)) was one of the best too.
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Table 4. Comparison of separation performances (benzene/cyclohexane) of various membranes Membrane
Bz/Chx (w/w)
Feed temp. (°C)
Separation factor,
Permeation flux, J (gm-2h-1)
PSI
Reference
(gm-2h-1)
Crosslinked PEBA@Ceramic
50/50
40
4.2
92.0
294.4
[73]
PUU
50/50
40
6.3
277.0
1468.1
[74]
P(AN-b-MA)
50/50
30
10.5
70.0
665.0
[75]
BTAPPPI–IA (PEA I)
50/50
50
6.2
430.0
2236.0
[76]
BTAPPI–TIPA (PEA I)
50/50
50
4.7
563.0
2083.1
[6]
BTAPPI–TA (PEA I)
50/50
50
6.9
320.0
1888.0
[7]
SILM ([C4mim]PF6AAOM)
50/50
45
34.2
12.2
405.0
[55]
IL-co-PU
50/50
50
8.3
310.0
2263.0
This study
4. CONCLUSIONS In this study, IL-co-PU copolymers were synthesized and the IL-co-PU membranes were employed in pervaporation separation of benzene/cyclohexane mixtures. The IL-coPU membranes preferentially absorb and permeate benzene rather than cyclohexane. With the IL contents increasing, both the permeability and the selectivity were improved. That is, there was no trade-off effect in these membranes. The improved performances of the IL-co-PU membranes were attributed to the ILs, which have strong affinity to benzene and were homogenously dispersed in the polymer matrix. The usage of ILs instead of
27
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nanofillers leads to pseudo hybrid membranes, which obey the same facilitated transport rules, but with no worries of agglomeration. This work might provide a new insight into the design and fabrication of high performance membranes for pervaporation separation of benzene/cyclohexane mixtures. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant number 21476055) and the Natural Science Foundation of Anhui Province (grant number 1808085ME112). REFERENCES (1) H.X. Liu, N. Wang, C. Zhao, S. Ji, J.R. Li, Membrane materials in the pervaporation separation of aromatic/aliphatic hydrocarbon mixtures - A review, Chin. J. Chem. Eng. 26 (2018), 1-16. (2) J.P.G. Villaluenga, A. Tabe-Mohammadi, A review on the separation of benzene/cyclohexane mixtures by pervaporation processes, J Membrane Sci. 169 (2000), 159-174. (3) T. Mahdi, A. Ahmad, M.M. Nasef, A. Ripin, State-of-the-art technologies for separation of azeotropic mixtures, Sep. Purif. Rev. 2015, 44, 308-330. (4) P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membrane Sci. 2007, 287, 162-179. (5) B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of organic-organic mixtures by pervaporation - a review, J. Membrane Sci. 2004, 241, 1-21.
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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof Highlights 1. The IL-co-PU membranes were among those polymer membranes with best performances. 2. Overcome trade-off between permeability and selectivity of polymer membranes; 3. Have no agglomeration suffered by hybrid membranes;
Tao Xi, Yingying Lu, Xinyu Ai, Lin Tang, Lulu Yao, Wentao Hao, Peng Cui PII:
S0032-3861(19)30954-1
DOI:
https://doi.org/10.1016/j.polymer.2019.121948
Reference:
JPOL 121948
To appear in:
Polymer
Received Date:
31 July 2019
Accepted Date:
26 October 2019
Please cite this article as: Tao Xi, Yingying Lu, Xinyu Ai, Lin Tang, Lulu Yao, Wentao Hao, Peng Cui, Ionic liquid copolymerized polyurethane membranes for pervaporation separation of benzene /cyclohexane mixtures, Polymer (2019), https://doi.org/10.1016/j.polymer.2019.121948
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Ionic liquid copolymerized polyurethane membranes for pervaporation separation of benzene/cyclohexane mixtures
Tao Xi, Yingying Lu, Xinyu Ai, Lin Tang, Lulu Yao, Wentao Hao*, Peng Cui* School of Chemistry & Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Materials Chemical Engineering, Hefei University of Technology, Hefei, 230009 P.R. China *Email: [email protected]; [email protected], Tel & Fax: 8655162902661 ABSTRACT: Benzene and cyclohexane are both important raw materials in polymer chemical industry. However, they are hard to be separated via the traditional distilling methods. In this work, ionic liquid copolymerized waterborne polyurethane (IL-co-PU) membranes were proposed for the pervaporation separation of benzene/cyclohexane mixtures. The results, including swelling rate, diffusion coefficients, partial flux, permeance and selectivity, showed that the IL-co-PU membranes could preferentially absorb and transfer benzene molecules rather than cyclohexane. The ILs might have functioned as facilitated transporter in the membranes. The permeation flux and separation factor of the IL-co-PU membranes increased simultaneously with the increase of IL contents. That is, these membranes had no trade-off between permeability and selectivity, which most of the polymer membranes suffered. With consideration of the good performance and easy handling property of the IL-co-PU membranes, they are of great potential in pervaporation separation of benzene/cyclohexane mixtures. KEYWORDS: polyurethane; membrane; ionic liquid; copolymer; pervaporation; benzene
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1. INTRODUCTION Benzene and cyclohexane are both important raw materials in polymer chemical industry. The products of benzene include styrene-acrylate latex and polyurethane, etc. The cyclohexane is used to produce polyamides. However, the benzene and cyclohexane are hardly separated via the traditional methods, including azeotropic distillation and extraction distillation [1,2]. Moreover, the traditional methods are highly energyconsuming and inefficient. As a promising alternative, pervaporation based on membrane technology has attracted much attention in recent years for their high efficiency and low costs [3-5]. According to the solution-diffusion theory, the pervaporation membranes selectively absorb benzene molecules and diffuse them through the membranes. Therefore, those membranes have aromatic components can have better permeability. In the past decades, membranes from aromatic polyetheramides have been widely researched [6,7]. The permeation flux of these membranes was impressive. However, their separation selectivity was relatively poor because the free volume of polymers was enlarged by swelling during the pervaporation. Alternatively, polyimides were adopted to fabricate the pervaporation membranes [8,9]. The polyimide membranes showed much better separation selectivity than the polyetheramides because they had densely packed chains and therefore high ability to resist swelling. Nevertheless, the permeation flux dropped. Crosslinking is effective in reducing swelling, and in improving selectivity. However, the permeation flux often goes controversially [10]. The trade-off between permeation flux and selectivity is always a problem for the polymer membranes [11].
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In recent years, the hybrid membranes were intensively investigated [12-15]. The fillers in hybrid membranes functioned as facilitated transport reagents, which greatly favored the diffusion of benzene in the membranes [16]. For example, Wu and coworkers reported sliver nanoparticle@carbon nanotubes (Ag-MWCNTs) incorporated polyurethane membranes [13]. The Ag-MWCNTs interacted with benzene molecules via complexation, facilitating the selective transportation of benzene. Ji and coworkers reported metal-organic frameworks (MOFs) embedded polyvinyl alcohol (PVA) membranes [15]. The MOFs have specific interactions with benzene molecules that the PVA/MOFs hybrid membranes showed higher separation factors, as well as greater permeation flux than the pristine PVA membrane. However, the nanofillers, like CNTs and MOFs, have strong tendency to agglomerate in polymer membranes because of their large specific area and high surface energy [17]. Great challenges are there in fabricating pervaporation membranes for the efficient separation of benzene/cyclohexane mixtures. Ionic liquids (ILs) have attracted much attention in recent years because of their great potential in extraction separation of benzene/cyclohexane mixtures [18-22]. The ILs are polar molecules with planar structure that they preferentially absorb the polarizable benzene rather than the non-polarizable cyclohexane [23,24]. Unlike the nanofillers, the ILs are tiny organic molecules that can be chemically incorporated into the polymer chains [25], therefore randomly dispersed in the polymer matrix. The IL copolymer membranes will have good affinity to the benzene as well as those hybrid membranes. However, the undesired agglomeration is effectively avoided. Rationally, the IL copolymer membranes are very promising in pervaporation separation of benzene/cyclohexane mixtures. Nevertheless, there was rare report at present.
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Herein, we reported a kind of IL copolymerized polyurethane (IL-co-PU) membrane. The ILs with vinyl groups were chemically bonded to the prepolymer of waterborne polyurethane with acryloyl end groups via free radical polymerization. Because the ILs had strong affinity to the benzene, the IL-co-PU membranes preferentially absorbed benzene rather than cyclohexane. Moreover, these membranes showed much higher diffusion coefficient for the benzene than the cyclohexane. Both the selectivity and permeability of these membranes increased with the IL contents, suggesting that they overcame the tradeoff between selectivity and permeability, which was suffered by most of the polymer membranes. With consideration of their high performances and easy handling property, the IL-co-PU
membranes
have
great
potential
in
pervaporation
separation
of
benzene/cyclohexane mixtures. 2. EXPERIMENTAL 2.1 Materials. Poly-1,4-butylene adipate glycol (PBA) (Mn = 2000) were supplied by Qingdao Xinyutian Chemical Co., LTD. (China). Trimethylol propane (TMP) was obtained from Hefei Anke Fine Chemicals Co., LTD. (China). Toluene diisocyanate (TDI), 2-hydroxyethyl acrylate (HEA) was provided by Shanghai Aladdin Bio-Chem Technology Co., LTD. (China). Dimethylolpropionic acid (DMPA) was provided from Xiya Regent Co., LTD. (China). Triethylamine (TEA), ethylenediamine (EDA), dibutyltindilaurate (DBTDL), butanone, azobisisobutyronitrile (AIBN), benzene, cyclohexane were purchased from Sinopharm Chemical Regents Co., LTD. (China). The 1-butyl-3vinylimidazolium hexafluorophosphate ([bvim][PF6]) was obtained from Shanghai ChengJie Chemical Co., LTD. (China). The PBA was dried in a vacuum oven at 120 °C for 12 h before use. All the chemicals were used as received.
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2.2 Membranes preparation. The synthetic route of IL-co-PU was shown in Scheme 1. A predetermined amount of TDI was added drop-wise to the PBA in butanone, reacting in a three-necked round bottom flask at 70 °C for 1 h. Subsequently, appropriate amounts of DMPA and TMP were added and the reaction was performed for 2 h in the presence of a DBTDL catalyst to obtain the NCO-end capped PU. HEA was then added in to introduce acrylate groups onto the PU chains. Thereafter, the IL monomer ([bvim][PF6]) was added and stirred for 0.5 h. AIBN as initiator was added dropwise within 0.5 h. Then, the mixture was maintained at 70 °C for another 2 h. The IL copolymerized PU was cooled to 30 °C. Lastly, TEA was added in accompanying a large amount of deionized water under high speed shearing. The obtained IL-co-PU emulsion had a total solid content of 10 wt%. A series of IL-co-PU emulsions were prepared with IL concentrations of 0/100, 3/100, 5/100 and 10/100 with respect to the solid contents of PU. The emulsions were cast onto glass plates to obtain membranes. The membranes were dried in air at first, and then dried in a vacuum oven at 50 °C for 8-10 h. The thickness of the membranes was about 70-90 m. The membranes were named as PU, PU-3, PU-5, and PU-10 with respect to the IL contents. 2.3 Characterization. The particle sizes of IL-co-PU emulsion were characterized on a MS2000 laser light scattering machine (Malvern). Reaction between IL and PU was confirmed by FTIR spectrometer (Nicolet 67, ThermoFisher Scientific). Thermal properties of pristine PU and IL-co-PU membranes were studied using a DSC instrument (DSC 214, Netzsch) and a DMA instrument (Q800, TA Instruments). A scanning electron microscope (SEM) (SU-8020, Hitachi) was used to study the cross-section morphology of membranes.
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Elemental composition of membranes was determined using the same SEM equipped with an energy dispersive X-ray spectrometer (EDX).
Scheme 1. Schematic diagram of preparation of IL-co-PU
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2.4 Swelling and Sorption Measurements. A weight-gain method was used to determine the degree of swelling (DS). Dry samples were immersed in pure solvents or in benzene/cyclohexane mixtures at 50 °C. At specific time intervals, the samples were taken out, wiped with tissue paper and weighed. These experiments were continued until sorption equilibrium was reached, at which no further weight gains of the samples occurred. The DS values were defined as:
DSt
Mt M0 100% M0
(1)
Where M0 and Mt are the weight of pristine membranes and swollen samples at time t, respectively. The composition of liquid mixture inside membrane at sorption equilibrium was determined by desorbing liquid on a vacuum line and condensing in a cold trap. The composition was analyzed by gas chromatography in the same way as for pervaporation. The sorption selectivity was defined as:
sorp
X M / 1 X F X F / 1 X M
(2)
Where XF and XM are the weight fractions of benzene in the feed solution and membrane, respectively. 2.5 Pervaporation experiments. The pervaporation experiment apparatus was shown in Scheme 2. Pervaporation experiments were performed at 50 °C on a homemade device. The effective surface area of the membrane was 20.2 cm2, in contacting with the feed mixture. Benzene composition in the feed mixture was 50 wt%. In the downstream side of the apparatus, vacuum was maintained at -0.1009 MPa. After steady state was attained, the
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permeate liquid was collected in cold traps immersed in liquid nitrogen. Permeation rate was determined by measuring the weight of permeates. Composition of each phase was measured by gas chromatography (GC-2008 equipped with flame ionization detector, Shenyang Guangzheng Analytical Instruments Co., LTD.). Efficiency of the membranes was assessed in terms of permeation flux (J), separation factor (αPV) and pervaporation separation index (PSI).
W At
(3)
X P / 1 X F X F / 1 X P
(4)
J
PV
PSI J PV 1
(5)
Where W represents the mass of permeates (g); A is the effective membrane area (m2); t is the permeation time (h); XF and XP are the weight fractions of benzene in feed and permeate, respectively.
Scheme 2. Pervaporation setup used in this work.
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1- Funnel; 2- Feed Tank; 3- Feed pump; 4- Flowmeter; 5- Plate membrane module; 6&7Cold traps; 8- Vacuum pump; 9- Emptying valve. Permeances of membranes for benzene and cyclohexane were calculated according to the following equation [15,26]:
Pi ji 0 l xi i Pi yi Pp
(6)
Where Pi is the permeability of component i. l is the membrane thickness. xi and yi are molar concentration of aromatic and aliphatic compounds in the feed and permeate. i is the activity coefficients for each component. P0i and Pp are the saturated vapor pressure of component i and the total pressure at the permeate side. ji (cm3(STP) cm-2 s-1) is the molar flux for each component, which is calculated from:
ji
J i viG mi
(7)
Where vGi is the molar volume of gas I (22.4 L (STP) mol-1) and mi is the molecular weight of component i. Ji is the experimental partialflux for each component, which is calculated as: J i J yi
(8)
The permeance is normally expressed in GPU units (1 GPU = 110-6 cm3 (STP)/(cm2 s cmHg)). The selectivity of the membrane is calculated from the ratio between permeance, which is defined as:
i/ j
Pi / l Pj / l
(9)
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3. RESULTS AND DISCUSSION 3.1 Characterization of the IL-co-PU emulsion and the IL-co-PU membranes The IL-co-PU emulsion was prepared by a two-step procedure. Firstly, the polyurethane prepolymer was synthesized majorly from PBA, TDI, DMPA and HEA. The HEA was used to introducing double bond onto the polyurethane chains (shown in Scheme 1). Secondly, the IL monomer with vinyl group ([bvim][PF6]) was added into the prepolymer along with the AIBN as initiator. The free radical polymerization initiated by AIBN produced polyimidazole segments, which were linked to the PU chains via C-C bonds (Scheme 1). Such a procedure was often adopted in synthesis of polyurethanepolyacrylates copolymers [27,28]. Thereafter, the IL-co-PU polymers were mixed with large amount of water to produce emulsion. The DMPA on the PU chains stabilized the emulsion particles.
Figure 1. Particle size distribution of the IL-co-PU emulsion
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The particle size distribution of IL-co-PU emulsion was characterized by light scattering. The results were shown in Figure 1. As indicated, the particle sizes were in the range from 40 nm to 100 nm. The pristine PU emulsion had the smallest particle size, while PU-10 emulsion had the largest one. The PU chains became longer after attaching the polyimidazole segments. Moreover, the polyimidazole segments are rigid. The rigid chains have more extended conformation that the particle size were enlarged. The rigidity of polyimidazole segments comes from the strong repellence between the dangling imidazole groups. PU-IL copolymer had been reported recently [29-34]. The reported PU-IL copolymers were all synthesized via urethane bonding. For example, Gin and coworkers reported PUIL copolymer/IL composite films [29] using ammonium-diol and -triol ionic liquid (IL) as monomers to react with diisocynanates. However, the PU-IL copolymer synthesized through free radical polymerization was not reported yet. This work is the first report on PU-IL copolymer obtained from free radical polymerization. Moreover, large amounts of organic solvents were often used during the synthesis of PU-IL copolymers. Only in few report, the water dispersible PU-IL copolymer was reported [35]. Nevertheless, the present IL-co-PU copolymers used large amount of water as dispersion reagent. They are ecofriendly.
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Figure 2. FTIR spectra of (a) IL-co-PU (PU-10), (b) PU membranes and (c) difference. The FTIR spectrum of a typical IL-co-PU membrane (PU-10) was shown in Figure 2, with the pristine PU as control. The difference between spectra of these two samples was shown either. On the spectrum of PU-10, there were characteristic vibration peaks of PU, such as 3330 cm-1, 2956 cm-1and 2873 cm-1, 1728 cm-1 and 1167 cm-1, which were attributed to vibration of –NH, -CH2-, and >C=O groups from polyester-polyurethane [36]. The data of vibration peaks was collected in Table 1. It was noticed that there was a peak at 811 cm-1 for the pristine PU, which was characteristic absorption of vinyl groups from 2-hydroxyethyl acrylate [37]. However, this peak disappeared on the spectrum of PU-10. It indicated that the IL monomers had reacted with the acryloyl PU prepolymer.
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Table 1. Spectral data and band assignments of FTIR analysis presented in Figure 2. PU
IL-co-PU
Assignment
References
Wavenumber (cm-1)
Wavenumber (cm-1)
3330
3329
NH
36,43,44
2956/2873
2955/2873
CH2
44
1727
1728
>C=O
44
1532
1536
C-N-H
45,46,47
1456
1462
CH2
47,48
1369
1368
CH2
49
1258
1256
C-O in NHC(O)O,
46,50
1165
1167
CO
51
1061
1062
COC
47,48
959
959
C-H
52
C=C
37,53
PF6
41,42,54
810 843
The difference of FTIR spectra could show the characteristics of IL-co-PU membrane more clearly. As it could be seen, there were peaks located at 1641 cm-1, 1570 cm-1, 1306 cm-1 and 843 cm-1. These peaks were attributed to the C=C and C=N stretching vibration of imidazole ring [38,39], in plane bending of the ring [40] and PF6 ions [41,42]. These results indicated that the ILs have been chemically bonded to the PU. The appearance of IL-co-PU membranes was distinguishable from that of the pristine PU membrane. As seen in Figure 3, the IL-co-PU membranes were yellowish while the pristine PU membrane was colorless. The yellow color of IL-co-PU membranes became deeper and deeper with IL contents increasing. However, these membranes remained good
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transparency. Notably, all the IL-co-PU membranes were flexible that they could be folded up. Compared to the SILMs based on ceramic membranes [55], the flexible IL-co-PU membranes were more profitable. They are adaptable to fit in various types of pervaporation devices, either the planar or the tube-type.
Figure 3. Appearance of pristine PU and IL-co-PU membranes. From (a) to (d), the samples are PU-0, PU-3, PU-5 and PU-10. Microstructure of the IL-co-PU membranes was observed by SEM, as indicated in Figure 4. The cross section of the pristine PU membrane was smooth, indicating that there was no macroscopic phase separation. However, the surface of IL-co-PU membranes was rough, and the surface roughness increased with the IL contents. In the previous report, the surface roughness was attributed to the incompatibility between ILs and polymer matrix [56,57]. The SEM observation indicated that phase separation had occurred in the IL-co-PU membranes. That is, the ILs were dispersed in the PU matrix as isolated phases.
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Figure 4. SEM photographs of cross section of pristine PU and IL-co-PU membranes: (a) PU-0, (b) PU-3, (c) PU-5, (d) PU-10 Although there was phase separation, the EDX results suggested that the ILs were homogeneously dispersed in the IL-co-PU membranes. In Figure 5, the bright dots represented the fluorine elements from the PF6 anions, which located nearby the imidazoles. As shown, there was no apparent agglomeration of the bright dots. This result was much different from that in the IL blended PU membranes [57]. In those membranes, the ILs migrated to places near the membrane surface for the lack of the strong interactions between ILs and PUs. In the IL-co-PU membranes, however, the imidazoles were chemically bonded onto the PU chains and were dispersed in membranes randomly. Their
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migration was impeded due to the high viscosity of polymers. Therefore, the PF6 anions were homogeneously dispersed nearby the imidazoles.
Figure 5. Element mapping of the cross section of IL-co-PU membranes. Fluorine signals from: (left) PU-3, (middle) PU-5, (right) PU-10. DSC measurement was performed for the pristine PU and IL-co-PU membranes. The results were shown in Figure 6. It was found that there were melting peaks for all these samples. The melting peaks were located at around 50 C, very close to the melting point of PBA (47 C) reported previously [58]. This result indicated that the PBA segments on the PU macromolecules had strong tendency to agglomerate into ordered structure, i.e. to crystallize. To clearly demonstrate the influence of ILs on the crystallization behavior of PBA segments, melting peaks, the melting points, melting enthalpy and crystallinity were listed in Table 2. From PU-0 to PU-5, the melting points of IL-co-PU samples increased. Moreover, the melting enthalpy increased too. These results suggested that the ILs were able to promote the growth of PBA crystals. Or it could be said that the molecular motion had been activated by the ILs.
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Figure 6. DSC curves of PU and IL-co-PU membranes Table 2. Crystallization data of PU and IL-co-PU membranes
Sample
Tm (C)
Hm (J/g)
Relative Crystallinity (%)
PU-0
45.07
39.33
32.5
PU-3
45.58
50.93
42.1
PU-5
50.25
70.63
58.4
PU-10
45.89
33.69
30.6
The ILs used in this work are composed of cationic imidazoles and anionic PF6 counter ions. As shown in Scheme 1, the polyimidazole segments were chemically bonded onto the polyurethane chains. Due to the strong repellence between imidazole groups, the single bonds in polyimidazole segments are of poor rotation ability. It is not possible for the rigid polyimidazole segments to activate the molecular motion. Therefore, there must be some other reasons. In the previous reports, the enhanced molecular motion was attributed to the
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phase separation caused by ILs [59,60]. Singha and coworkers found that the melting enthalpy and the crystallinity of soft segments of PU-IL were higher than that of the PU. They attributed it to the phase separation in PU-IL copolymer, because the ILs on the hard segments led to greater incompatibility with the soft segments [59]. In the report of Einloft and coworkers, they suggested that the counter ions of ILs functioned as a driving force in microphase separation, subsequently improved molecular flexibility and facilitated the molecular motion [60]. Here in this work, the phase separation is possible. The polyimidazole segments can form - interactions with benzene rings (from TDI) on the hard segments of PU, but they have rare structural similarity to the PBA soft segments, resulting in poorer compatibility. However, the phase separation was not always observed [57]. In that report, the imidazoles could hardly form - interaction with aliphatic rings of isophorone diisocyanate on the hard segments. From Figure 6 and Table 2, it was learned that the PU-10 sample had not shown further improvement in melting point and enthalpy. Its melting point and enthalpy were lower than that of PU-5 sample. The experimental results from DSC measurement seems not able to be fully explained by phase separation. Dynamic mechanical analysis (DMA) is a sensitive tool to detect molecular motion, which determines the overall performances of polymer materials [61,62]. As seen in Figure 7a, the storage moduli of PU-5 and PU-10 membranes were lower than that of the PU membrane when the temperature was more than -45 C. It indicated that the IL-co-PUs have higher molecular mobility than the pristine PU. This result was coincided with the results from DSC measurement. Correspondingly, the peaks at -45 C on tan curves of
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PU-5 and PU-10 were remarkably larger than that of the pristine PU as shown in Figure 7b. Furthermore, there were several peaks located in the high temperature ranges from 90 C to 135 C. They were attributed to the molecular motion of the hard segments. It was shown that the PU-5 and PU-10 had much lower transition temperature of hard segments than the pristine PU. These were also evidences that the IL-co-PUs had higher molecular mobility. The enhanced molecular motion could be partially attributed to the plasticizing effect of counter ions of ILs [31,63], except for the possible phase separation in IL-co-PU membranes. The counter ions of ILs are PF6 ions, locating nearby the cationic polyimidazole segments. As the polyimidazole segments were uniformly distributed in the polymer matrix, the PF6 ions were distributed evenly too. The PF6 ions have small molecular weight that they are of stronger molecular mobility than the polymer segments. It is highly possible that they functioned as plasticizers to activate the molecular motion of polyurethane in the IL-co-PU membranes [63]. As a result, the tan values for glass transition of soft segments increased, and the glass transition temperature of hard segments decreased. The plasticizing effect could also explain the lowered melting point and enthalpy of PU-10 sample. In PU-10 sample, the ILs contents were the highest that there were a lot of PF6 ions. They isolated the PBA segments, and therefore impeded their agglomeration to form large size crystals.
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Figure 7. DMA results of pristine PU and IL-co-PU membranes: (a) storage modulus vs. temperature; (b) tan vs. temperature. 3.3 Swelling behavior of IL-co-PU membranes Sorption is a crucial procedure for the pervaporation. The sorption potential of pervaporation membranes can be characterized by the swelling test results [7]. In this study, the IL-co-PU membranes were immersed in 100% benzene and 100% cyclohexane respectively. During the test, data indicating swelling rate were collected carefully and were shown in Figure 8 and Table 3.
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Figure 8. Swelling processes of pristine PU and IL-co-PU membranes (a) in benzene and (b) in cyclohexane. As seen in Figure 8a, the slope at the initial stage of swelling curves of IL-co-PU membranes in benzene was much larger than that of the pristine PU membrane. It indicated that the IL-co-PU membranes swelled faster than the pristine PU membrane. However, the swelling rate of IL-co-PU membranes in cyclohexane was very close to that of the pristine PU membrane, except for the PU-10 membrane (Figure 8b). These results indicated that the IL-co-PU membranes selectively absorbed benzene, but had little affinity to
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cyclohexane. The preferential absorption of these membranes to benzene was undoubtedly contributed by the ILs because of their strong affinity to aromatic compounds [18-24]. As shown in Table 3, the swelling degree at equilibrium (DS) of IL-co-PU membranes increased remarkably from PU-0 to PU-10 when they were immersed in benzene. On the contrary, the DS values decreased when the membranes were immersed in cyclohexane. Although the molecular motion of IL-co-PUs was stronger than that of the pristine PU, which always suggested larger free volume, the IL-co-PU membranes were not swelled in cyclohexane, but shrank. The IL-co-PU membranes had preferential sorption to benzene rather than cyclohexane. As a result, the sorption selectivity (sorp) for benzene increased drastically from PU-0 to PU-10. Table 3. Swelling and sorption behavior of the membranes Swelling Degree at Sorption equilibrium (DS) Sample Selectivity /% (αsorp.) Benzene Cyclohexane
Diffusion Coefficient /μm2s-1 Benzene
Cyclohexane
PU-0
113
3.5
3.32
1.11
0.21
PU-3
184
3.2
3.62
3.87
0.34
PU-5
229
1.3
4.68
6.16
0.86
PU-10
256
1.6
6.55
9.27
6.05
From the swelling curves shown in Figure 8, the diffusion coefficients were calculated according to the following equation [64], and the results were shown in Table 3 either.
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h D Diffusion coefficient 4 4Q
2
(6)
Where θD is slope of the initial portion of the plots of Qt, Q is the mol% uptake at equilibrium. In this equation, the θD/Q equals to slope of the initial portion of the plots of DSt, and h (m) is thickness of the membranes. The IL-co-PU membranes showed much larger diffusion coefficients than the pristine PU membrane, which had diffusion coefficient comparable to that in the previous report [65]. Nevertheless, the diffusion coefficients of IL-co-PU membranes for cyclohexane were very small, except for the PU-10 sample. The enlarged diffusion coefficient for benzene was not only contributed by the higher molecular mobility in IL-co-PU membranes, but also by the preferential transport of benzene. In the previous section, it was shown that the ILs were homogeneously dispersed in the membranes. They could function as carriers. The benzene were diffused from one IL phases to another as well as in the polymer matrix. However, the ILs made the diffusion pathways of cyclohexane twisted and longer. The exception of PU-10 sample might be the result of excessively enlarged free volume in this membrane [66]. 3.4 Pervaporation performance of IL-co-PU membranes The pervaporation performance of IL-co-PU membranes was tested and the total permeation flux and separation factor of IL-co-PU membranes were shown in Figure 9. The permeation flux increased constantly from PU-0 to PU-10. For the PU-10 sample, its flux was 150% that of the pristine PU membrane. Interestingly, the separation factor of ILco-PU membranes increased simultaneously. The separation factor of PU-10 sample was
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240% that of the pristine PU membrane. That is, there is no trade-off effect for the IL-coPU membranes in the IL concentrations investigated. Generally, the polymer membranes suffered from their intrinsic trade-off between permeability and selectivity [67,68]. The reports that solved trade-off problem were always about hybrid membranes [69]. The ILco-PU membrane was one of the a few polymer membranes that overcame the trade-off effect.
Figure 9. Pervaporation performances for benzene/cyclohexane mixtures (50/50 w/w) at 50 °C The partial flux (Ji) is defined as J i J t yi , where Jt is total flux, and yi is the weight fraction of component i in the permeate [70]. The Ji values of benzene and cyclohexane were shown in Figure 10. It was found that the partial flux of benzene increased constantly from PU-0 to PU-10. However, the partial flux of cyclohexane decreased. The partial flux values showed that the IL-co-PU membranes preferentially permeate the benzene. Similar results were found for the ILs incorporated supporting membranes [55,71].
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Figure 10. Partial flux of IL-co-PU membrane for benzene and cyclohexane
Figure 11. Permeance and selectivity of IL-co-PU membranes for benzene and cyclohexane Compared to the permeation flux, permeance (Pi/l) is more suitable to describe the intrinsic property of a membrane [72]. As shown in Figure 11, the permeance of IL-co-PU membranes for benzene increased constantly from PU-0 to PU-10. Nevertheless, the permeance for cyclohexane decreased. Consequently, the selectivity (Pbenzene/Pcyclohexane)
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permeance of IL-co-PU membranes increased drastically. The incorporation of ILs greatly improved the preferential permeation of benzene in the IL-co-PU membranes. The preferential permeation of benzene by the ILs was similar to the facilitated transportation of the hybrid membranes. The ILs functioned as facilitated transporter to accelerate the transfer of benzene molecules, in addition to the Fickian diffusion [16,69], as illustrated in Figure 12. On the contrary, the ILs impeded the transportation of cyclohexane. Therefore, both the permeability and separation selectivity of IL-co-PU membranes for benzene were remarkably increased.
Figure 12. Proposed diffusion of benzene and cyclohexane in the IL-co-PU membrane. The pervaporation performance of IL-co-PU membranes for benzene/cyclohexane mixtures was compared with that of the reported membranes, as shown in Table 4. The separation factor of IL-co-PU membrane was 8.3, which is among the best of the reported polymer membranes. Moreover, the PSI (PSI=J*(-1)) was one of the best too.
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Table 4. Comparison of separation performances (benzene/cyclohexane) of various membranes Membrane
Bz/Chx (w/w)
Feed temp. (°C)
Separation factor,
Permeation flux, J (gm-2h-1)
PSI
Reference
(gm-2h-1)
Crosslinked PEBA@Ceramic
50/50
40
4.2
92.0
294.4
[73]
PUU
50/50
40
6.3
277.0
1468.1
[74]
P(AN-b-MA)
50/50
30
10.5
70.0
665.0
[75]
BTAPPPI–IA (PEA I)
50/50
50
6.2
430.0
2236.0
[76]
BTAPPI–TIPA (PEA I)
50/50
50
4.7
563.0
2083.1
[6]
BTAPPI–TA (PEA I)
50/50
50
6.9
320.0
1888.0
[7]
SILM ([C4mim]PF6AAOM)
50/50
45
34.2
12.2
405.0
[55]
IL-co-PU
50/50
50
8.3
310.0
2263.0
This study
4. CONCLUSIONS In this study, IL-co-PU copolymers were synthesized and the IL-co-PU membranes were employed in pervaporation separation of benzene/cyclohexane mixtures. The IL-coPU membranes preferentially absorb and permeate benzene rather than cyclohexane. With the IL contents increasing, both the permeability and the selectivity were improved. That is, there was no trade-off effect in these membranes. The improved performances of the IL-co-PU membranes were attributed to the ILs, which have strong affinity to benzene and were homogenously dispersed in the polymer matrix. The usage of ILs instead of
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nanofillers leads to pseudo hybrid membranes, which obey the same facilitated transport rules, but with no worries of agglomeration. This work might provide a new insight into the design and fabrication of high performance membranes for pervaporation separation of benzene/cyclohexane mixtures. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant number 21476055) and the Natural Science Foundation of Anhui Province (grant number 1808085ME112). REFERENCES (1) H.X. Liu, N. Wang, C. Zhao, S. Ji, J.R. Li, Membrane materials in the pervaporation separation of aromatic/aliphatic hydrocarbon mixtures - A review, Chin. J. Chem. Eng. 26 (2018), 1-16. (2) J.P.G. Villaluenga, A. Tabe-Mohammadi, A review on the separation of benzene/cyclohexane mixtures by pervaporation processes, J Membrane Sci. 169 (2000), 159-174. (3) T. Mahdi, A. Ahmad, M.M. Nasef, A. Ripin, State-of-the-art technologies for separation of azeotropic mixtures, Sep. Purif. Rev. 2015, 44, 308-330. (4) P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membrane Sci. 2007, 287, 162-179. (5) B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of organic-organic mixtures by pervaporation - a review, J. Membrane Sci. 2004, 241, 1-21.
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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof Highlights 1. The IL-co-PU membranes were among those polymer membranes with best performances. 2. Overcome trade-off between permeability and selectivity of polymer membranes; 3. Have no agglomeration suffered by hybrid membranes;