High-flux thin-film composite polyelectrolyte hydrogel membranes for ethanol dehydration by pervaporation

Author’s Accepted Manuscript High-flux thin-film composite polyelectrolyte hydrogel membranes for ethanol dehydration by pervaporation Jinpeng Liu, Roy Bernstein www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)30490-8 http://dx.doi.org/10.1016/j.memsci.2017.04.018 MEMSCI15182

To appear in: Journal of Membrane Science Received date: 17 February 2017 Revised date: 17 March 2017 Accepted date: 11 April 2017 Cite this article as: Jinpeng Liu and Roy Bernstein, High-flux thin-film composite polyelectrolyte hydrogel membranes for ethanol dehydration by pervaporation, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.04.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High-flux thin-film composite polyelectrolyte hydrogel membranes for ethanol dehydration by pervaporation Jinpeng Liu and Roy Bernstein* Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 8499000, Israel Abstract A thin-film composite (TFC) membrane for ethanol dehydration by pervaporation with a very high flux and a moderately high separation factor was fabricated using a strong polyelectrolyte hydrogel as an active layer. The thin-film hydrogel was graft-polymerized with vinyl sulfonic acid (VSA) as the polymer monomer and N, N′-methylenbisacrylamide (MBAA) as the cross-linker monomer, employing the UV photo-initiation method on a polyethersulfone ultrafiltration support. The successful grafting was confirmed by ATR-FTIR, and AFM and SEM studies showed that TFC membranes with an active layer of 100-300 nm were fabricated. The IEC of the active layer increased up to ca. 3 meq/g when the cross-linker fraction was changed from 1% to 5% MBAA and then decreased at 10% MBAA, probably due to the high cross-linker fraction in the active layer. The degree of swelling decreased with the cross-linker concentration and with the duration of the UV irradiation. The pervaporation performance of the TFC membranes was studied with a 90% ethanol solution at 50 °C. The separation factor coincided with the increase in the IEC, which is attributed to the higher sorption of water molecules to the active layer. The degree of swelling had a lesser influence on the membrane performance than the IEC, but greater swelling decreased the performance for similar IEC 1

values. The optimal membrane with a 7.5 kg m-2 h-1 flux and a separation factor of 313 was obtained at the highest IEC and a limited degree of swelling, making it one of the membranes with the highest pervaporation separation index reported to date. However, the stability of the membrane should be improved, as its high performance decreased after 24-48 h and reached a plateau after a few days of operation.

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1. Introduction Pervaporation is one of the most effective techniques for separating miscible liquids due to its low cost and the ability to separate azeotropic mixtures and liquids with close boiling points [1]. In addition, it is environmentally friendly and does not discharge hazardous chemicals or generate heat [2]. The three main applications of pervaporation are solvent dehydration, organic–organic solvent separation, and removal of solvents (e.g., volatile organic carbon) from aqueous solutions; of these, solvent dehydration is the most developed application, and it is being employed today on an industrial scale [3]. Pervaporation was commercialized for the dehydration of organic solvents in 1983. This was accomplished with a composite membrane with polyvinyl alcohol as the active layer [4]. Since then, numerous attempts have been made to improve the efficiency of the process and many other polymerbased membranes have been investigated [4], including membranes made of modified poly(vinyl alcohol) [5, 6], polyimides [7], polyamides [8], polysaccharides [9], polyelectrolytes [10-13], mixedmatrix membranes with graphene oxide [14], carbon nanotube [6, 15], zeolites [16], metal organic frameworks [17], and inorganic–organic hybrid membranes [18]. Nevertheless, improving the performance of the membrane, namely, its separation factor and permeability, is still a major challenge [3]. The accepted mechanism underlying the transport in pervaporation is solution-diffusingevaporation [4]. Therefore, an adequate membrane performance for dehydration requires (a) a high solution of water to the active layer; (b) preferable diffusion of the water molecules – relative to the molecules of the organic solvent – through the membrane to the permeate side through the fractional free volume (FFV) of the swollen active layer, although the swelling should be restricted to prevent high permeation of the organic solvent; and (c) a thin active layer, which will allow high permeability [1, 4]. One type of material that can be used for fabricating such thin-film membranes and combine high water sorption and limited swelling is polyelectrolytes. As polyelectrolytes have dissociated 3

(negatively or positively) charged groups on the polymer backbone, their water sorption is relatively high [11, 19]. Although polyelectrolytes usually swell to a high degree in water solutions, which could reduce the membrane performance [20], such swelling can be controlled by fabricating a cross-linked polyelectrolyte, multilayer polyelectrolyte membranes, or complex polyelectrolyte membranes. Several methods can be used for this process, including chemical crosslinking by adding a monomeric cross-linker, physical crosslinking through a thermal treatment, ionic crosslinking, layer-by-layer deposition, and more [14, 21-23]. Finally, polyelectrolytes can be produced as very thin layers to increase membrane permeability [24]. As mentioned above, the preferential solution of water over ethanol to a polyelectrolyte active layer is due to the charge moiety of the polyelectrolyte (also known as the active center); thus, an increase in the ion-charge density can increase the membrane separation factor. Furthermore, it was found that both the chemical nature of the charged group and the counter ion influence the water sorption [1]. For example, the use of strong sulfonate groups as an effective active center has attracted much attention due to the high hydrophilicity and hydration capabilities of these groups [11, 25-27]. Composite membranes with various polyelectrolytes as the active layer, which were either grafted or coated, have been investigated for solvent dehydration by pervaporation [1]. However, the separation factor of these membranes was usually insufficient, and most polyelectrolytes were not cross-linked. In the current study, a cross-linked polyvinyl sulfonic acid (pVSA) was used as the thinfilm active layer of a composite membrane, which was assessed for dehydration of ethanol by pervaporation. pVSA has a very high ion exchange capacity and is a highly hydrophilic polyelectrolyte, and the cross-linking was used to limit its swelling. The polymer was grafted onto a polyethersulfone (PES) ultrafiltration (UF) membrane by UV–photo-irradiation and the degree of grafting, ion exchange capacity (IEC), degree of swelling, and morphological characteristics of the composite membrane were measured. The performance of the composite membrane in dehydrating a 90% ethanol solution was then evaluated and correlated with the membrane characteristics.

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2. Experimental 2.1. Materials Commercial UF PES membranes (150 kDa, as reported by the manufacturer) were supplied by Microdyn-Nadir (Wiesbaden, Germany). Prior to the modification, the membranes were washed in a 50% ethanol/water solution for at least 12 h, then thoroughly washed again with Milli-Q water (Millipore, USA) and left in double distilled water (DDW) until used. Vinyl sulfonic acid sodium salt (VSA; 25%, Tokyo Chemical Industry Co., Ltd) was used as the polymer monomer and N,N′methylenbisacrylamide (MBAA, Sigma-Aldrich, Israel) was used as the cross-linker monomer. To obtain a monomer solution with 42% VSA, the 25% VSA solution was concentrated under reduced pressure (42 mbar) at 50 °C. The monomer concentration was 42% when the solution density was 1.37 g/L. Ethanol was purchased from Bio-Lab Ltd. (Jerusalem, Israel). All reagents were of analytical grade and were used as received. All experiments were conducted using purified water from a Milli-Q system. 2.2. Modification procedure The modification solution was prepared by dissolving the cross-linker (0–10 % mole, relative to the VSA concentrations) in the aqueous VSA solution. For the 5% and 10% fractions, a high concentration of MBAA was dissolved in 50% ethanol and then a small volume was added to the VSA solution. A water-wet membrane (9×4 cm) was placed in a holder, leaving only the outer surface of the membrane in contact with the solution. The water was wiped off with a tissue paper and the surface of the membrane was covered with 12 mL of the modification solution (which was first degassed with nitrogen for 1 min). The sample was immediately placed inside a UV system (Intelli-Ray 400, Uvitron International Inc., USA) and irradiated at an intensity of I = 65 mW/cm2 (measured with a UV power puck II radiometer, EIT, Sterling, VA), for various durations (2.5–15 min) at 30-35 °C. The UV 5

wavelength was narrowed to λ > 315 nm by using a filter glass to avoid membrane degradation [28]. At the end of the modification process, the membrane was washed with a 50% ethanol/water solution for at least 12 h at 45 °C to remove any ungrafted polymers, and it was left in DDW until tested. 2.3. Degree of grafting The degree of grafting (DG) was calculated using the gravimetric method. Pristine and modified membranes were cut with a punch-hole (24 mm diameter), dried for 24 h at 40 °C in a vacuum oven, and left in a desiccator for 1 h before weighing. For each condition, the weight of six different samples was measured with a microbalance (XP6, Mettler Toledo). The DG was calculated by using the following equation: DG =

mmodified −𝑚𝑃𝐸𝑆 A

(1)

where mmodified is the weight of the modified membrane, mPES is the weight of the membrane before modification, and A is the surface area of the membrane. 2.4 Membrane characterization 2.4.1. Ion-Exchange Capacity (IEC) Modified membranes with known DG values and pristine membranes were submerged in a 1M H2SO4 solution for 24 h, during which time the solution was replaced twice to ensure complete conversion of the sulfonic group to its acid form. The membranes were then washed with Milli-Q water until a neutral pH was obtained, and, afterwards, immersed for 24 h in 10 mL of a 2M NaCl solution to convert the sulfonic acid to its Na+ form. The solution was replaced five times, such that a total of 50 mL of solution was collected. The solution was then titrated with 0.001 N NaOH by using a T90 titrator (Mettler Toledo). The IEC [meq g-1] was calculated by using:

IEC =

(V−V0 )∗C

(2)

w

6

where V [mL] is the volume of NaOH required for the back-titration, V0 [mL] is the average volume required for the titration of five PES control samples, C [M] is the NaOH concentration, and w (g) is the weight of the modification layer, measured as the difference between the weight of the modified and pristine PES membranes following the DG experiments. 2.4.2. Degree of swelling The membranes were dried for 24 h at 40 °C in a vacuum oven to remove any absorbed moisture, weighted (Wdry), and then immersed in Milli-Q water for 48 h at 30 °C. The fully swollen membranes were carefully wiped with tissue paper to remove the excess solution and immediately weighted (Wwet). The degree of swelling was calculated as:

Swelling degree (%) =

Wwet −Wdry Wdry

∗ 100

(3)

2.4.3. Scanning electron microscopy (SEM) The cross-sectional images of the membranes were observed with a field-emission scanning electron microscope (FESEM). Prior to photographing, all samples were fractured in liquid nitrogen and then sputtered with gold. To better differentiate the newly grafted active layers from the pristine membranes, the membranes were stained with Ag nanoparticles by soaking them in a 100 mM AgNO3 solution for 24 h to exchange Na+ to Ag+, washing them with a diluted (10:1 and then 100:1) AgNO3:Milli-Q water solution, then with Milli-Q water to remove the residual Ag+ , and, finally, soaking them in a 10 mM NaBH4 solution in an ice bath for 10 min to convert Ag+ to Ag nanoparticles [29]. The silver element distribution in the cross section of the composite membrane was recorded by energy-dispersive X-ray spectroscopy (EDX) equipped on the FESEM. 2.4.3. Atomic force microscopy (AFM) The thickness of the dry active layer was also evaluated by using AFM. First, the PES UF membranes were cast by using the non-solvent phase inversion method. PES (15% w/w, Ultrason E3010P, kindly provided by BASF) was dissolved in N-Methyl-2-pyrrolidone (NMP) and cast on a polypropylene 7

non-woven surface by using an automatic casting device (Braive Instruments, Belgium) at a 200 µm wet thickness. The polymer film was then immersed in deionized water at 20 °C and kept in the water until used. The membranes were modified as described in Section 2.2. The polypropylene support of both a pristine and a modified membrane was then removed (the non-woven support could not be separated from the commercial membranes, hence the in-house membrane casting) and PES-grafted layer films were soaked with NMP in a glass Petri dish until the PES completely dissolved, leaving only the grafted layer floating in the NMP. The floating hydrogel layer was carefully mounted on a clean silicon wafer and the solvent was evaporated to leave a dry hydrogel film on the wafer. Finally, the sample was left for 12 h at 60 °C and 100% humidity to allow the film to swell and expand, and the dry film thickness was measured by AFM using MFP-3D-Bio (Asylum Research, Oxford Instruments) with a AC160TS probe (Olympus) in AC mode. 2.5. Pervaporation experiments The ability of the membranes to separate water from ethanol was characterized by using 90:10 (g/g) ethanol/water mixtures. The pervaporation measurements were performed using a lab-made apparatus, shown in Figure 1. The membrane cell dimensions were 8×3×0.5 cm (L×W×H), with 24 cm2 as the membrane effective area. The feed was circulated at 80 L/h and the temperature was controlled by an electronic control thermometer. The vacuum at the downstream pressure was maintained using a vacuum pump (Edwards E2M5). First, the system was operated for a duration of 1 h for equilibration. Then, the liquid was collected twice, using a liquid nitrogen trap, for approximately 2 h each time (a total of 5 h for each sample); the difference between the two successive measurements was always lower than 5%. The total permeate flux was determined by measuring the weight of the collected liquid membrane, and the permeate composition of the collected permeate was determined by the refractive index (ATR-W2, Schmidt+Haensch, Germany). The permeation flux (J), separation factor (α) [30, 31], and pervaporation separation index (PSI) [32] were calculated by using the following equations:

8

Q

𝐽 = At

(4)

PW ⁄PE FW ⁄FE

(5)

PSI = 𝐽(𝛼 − 1)

(6)

𝛼=

where Q (kg) is the weight of permeate in the operation duration t (h), A (m2) is the effective membrane area, and P and F represent the mass fractions of water (W) and ethanol (E) in the permeate solution and feed solution, respectively.

Figure 1. Experimental apparatus for pervaporation 3. Results and discussion 3.1. FTIR The ATR-FTIR spectra of the pristine and modified membranes with 25 wt% VSA as the monomer and with different MBAA fractions as the cross-linker (% mole, relative to the VSA concentrations) are shown in Figure 2. A new peak emerged for the modified membranes at 1040 cm-1, corresponding to the stretching vibration of the sulfone acid group and confirms the successful grafting of the pVSA [33]. As expected, the intensity of the sulfonic peak increased with increasing cross-linker concentration [33]. The other two new peaks shown in Figure 2 are the absorption peak of the amide I 9

(C=O) and the amide II (N-H), at 1662 cm-1 and at 1543 cm-1, respectively, which indicate that the cross-linker fraction in the grafting (active) layer increases with increasing MBAA fraction in the monomer solution.

1662 cm -1

1040 cm -1

1543 cm -1

Absorbance

10%MBAA

5%MBAA

2.5%MBAA

1%MBAA

Pristine PES

1800

1600

1400

1200

1000

800

Wave number (cm-1) Figure 2. ATR-FTIR spectrum of the pristine and modified membranes prepared with various crosslinker concentrations. Grafting polymerization conditions were 25% VSA, t = 7.5 min, I = 65 mW/cm2. 3.2. Degree of grafting The DG was evaluated at various cross linker–monomer ratios (Figure 3a) and for different durations of irradiation (Figure 3b) by using either 25% or 42% VSA, at otherwise constant polymerization conditions. We investigated a similar system in previous studies [33, 34], but the cross linker fraction in the current study was much higher (up to 10% MBAA) than that used in our previous work (up to 2.5% MBAA). Without a cross-linker, no grafting was detected under any of the examined conditions. However, a monotonic increase in DG was observed when the cross linker fraction was increased up to the maximum concentration of 10 mole% (Figure 3a), which was the maximum solubility of the cross-linker in the ethanol–VSA solution (without a significant change in the VSA concentration). The DG was higher when 42% VSA was used than when 25% VSA was used at the same cross-linker concentration. Only two cross-linker-to-monomer ratios were studied due to the low solubility of the cross-linker in the 42% VSA. 10

The DG also increased almost linearly with irradiation time for the two cross-linker fractions (Figure 3b). The increase in the DG with the cross-linker fraction and with irradiation time was also reported in our previous studies [33, 34] and by others [35]. Although the cross-linker fraction in the current study was much higher than the one used in our previous studies, the general trends of the DG are similar.

500

800

(a)

(b) 600

DG (g/cm2)

DG (g/cm2)

400 300 200

400

200

100

25%VSA

5%MBAA

42%VSA 0 0.0

10%MBAA 0

2.5

5.0

7.5

10.0

12.5

0

MBAA concentration (mole%)

5

10

15

20

Irradiation time (min)

Figure 3. (a) Degree of grafting (DG) at different cross-linker fractions (mole %) with 25% and 42% VSA, t = 7.5 min. (b) Effect of the UV irradiation duration on the DG at two cross-linker fractions (5% and 10% MBAA) with 25% VSA. Grafting polymerization conditions were I = 65 mW/cm2. n ≥ 5. 3.3. Ion Exchange Capacity (IEC) and degree of swelling The effect of the different graft polymerization conditions on the IEC – a key parameter in the pervaporation-mediated dehydration – is shown in Figure 4. Figure 4a shows that the IEC values (for 25% VSA) reached a maximum of 2.99 at 5% MBAA and dropped to 1.66 at 10% MBAA. This finding can be attributed to the adverse effect of the cross-linker fraction in the monomer solution on the ratio between the VSA (and, thus, the IEC) and the cross-linker fraction in the grafted layer. In our previous work, we showed that an increase in the cross-linker fraction in the monomer solution increases the VSA grafting [33, 34]. This phenomenon was attributed to the low reactivity of the VSA as compared with the cross-linker monomer during the co-polymerization, as was reported by others 11

[36, 37]. However, the cross-linker fraction in the grafted layer also increases. It seems that, at high cross-linker fractions (10% for 25% VSA and 2.5% for 42% VSA), the increase in the cross-linker fraction in the grafting layer exceeds the increased grafting of the VSA.

4.0

3.0 2.5 2.0 1.5 1.0 25%VSA

0.5 0.0 0.0

(b)

3.5

IEC (mequiv g -1)

IEC (mequiv g -1)

3.5

4.0

(a)

3.0 2.5 2.0 1.5 1.0 5%MBAA

0.5

42%VSA

10%MBAA

0.0 2.5

5.0

7.5

10.0

12.5

0

MBAA concentration (mole%)

5

10

15

20

Irradiation time (min)

Figure 4. (a) Ion exchange capacity (IEC) of modified membranes prepared at different cross-linker fractions (mole %) with 25% and 42% VSA, t = 7.5 min. (b) Effect of the irradiation duration on IEC values at two cross-linker fractions (5% and 10% MBAA) with 25% VSA. Grafting polymerization conditions were I = 65 mW/cm2, n ≥ 5. This phenomenon was also observed in the change of the IEC with the duration of irradiation for the 5% cross-linker fractions (Figure 4b). The IEC reaches its maximum value at 7.5 min for both crosslinker fractions and drops at longer irradiation durations. Figure 4b also shows that the IEC values were lower at all durations of irradiation for the 10% cross-linker fraction than for the 5% cross-linker fraction, indicating that the cross-linker fraction in the grafted layer is higher for the 10% MBAA fraction. Furthermore, the change in the IEC with increasing durations of irradiation was very small when the 10% cross-linker fraction was used (except a small increase from 5 to 10 min), implying that the 10% MBAA fraction is probably too high for an effective pervaporation separation. Notably, although the IEC values are lower than the theoretical value of pVSA (9.2 meq/g) in all conditions, and due to the high crosslinking fraction in the active layer [34, 38], the IEC values are much higher than those of composite membranes that were fabricated by using other polyelectrolytes as the active layer [39-42]. The degree of swelling of the active layer under various polymerization conditions is presented in 12

Figure 5. The degree of swelling decreased with increasing MBAA fraction (Figure 5a) and duration of irradiation (Figure 5b). A decrease in swelling with increasing cross-linker fractions due to an increase in the mechanical strength of the matrix is a well-known phenomenon [43]. However, the decrease in swelling with increasing irradiation duration was somewhat unexpected and can be explained based on the swelling of a thin-film hydrogel tethered to a surface. The swelling of a thin film is one-dimensional normal to the substrate plane [44]; as the duration of irradiation increases, the film becomes thicker (Figure 3b), while the volumetric expansion increases to a lesser extent because it is one-dimensional. Therefore, the ratio between the dry and swollen volumes, i.e., the degree of swelling, decreases with increasing irradiation duration. 25

60 25%VSA

(b)

42%VSA

Swelling degree (g/g)

Swelling degree (g/g)

(a) 40

20

20 15 10 5 5%MBAA

0 0.0

0 2.5

5.0

7.5

10.0

0

12.5

5

10

15

20

Irradiation time (min)

MBAA concentration (mole%)

Figure 5. (a) Degree of swelling of modified membranes prepared at different cross-linker fractions (mole %) with 25% and 42% VSA, t = 7.5 min. (b) Effect of the duration of irradiation on the degree of swelling at 5% MBAA. Grafting polymerization conditions were 25% VSA, I = 56 mW/cm2, n ≥ 5. 3.4. Thickness of the active layer In previous research, we demonstrated that, following the functionalization of a PES membrane with 25% VSA and at a high cross-linker fraction (2.5% MBAA), the surface of the membrane is covered completely and a composite membrane is fabricated [33, 34] . In the current study, the cross-section morphology of the composite polyelectrolyte membrane was characterized by FESEM to evaluate the thickness of its active layer, which has a considerable influence on membrane performance [45]. The

13

images in Figures 6a-d demonstrate that the thickness of the active layer of the membrane slightly increases with the increase in cross-linker fraction, namely, from ca. 90 nm in the pristine membrane (Figure 6a) up to 208 nm (i.e., ca. 118 nm grafted layer) for the 10% MBAA membrane (Figure 6d).

Figure 6. Cross-section morphology of polyelectrolyte hydrogel PES membrane, observed by FESEM. The thickness of the active layer (an average of ten different points) is shown for (a) pristine membrane and (b-d) membranes modified with (b) 2.5% (c) 5%, and (d) 10% cross-linker fractions (mole %). Polymerization conditions were 25% VSA, t = 7.5 min, I = 65 mW/cm2. Because it is very difficult to distinguish between the grafted hydrogel (the active layer) on top of the supporting PES membrane in the SEM images, both a pristine membrane and a modified membrane (25% VSA, 5% MBAA, 7.5 min irradiation) were stained with Ag and were then detected by using EDX measurements, which were taken from the active layer and from the middle of the cross section. Figure SI1 (Supporting Information) shows the exact positions on the pristine and

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modified membranes that were measured with EDX. Table SI1 (Supporting Information) demonstrates that Ag could be detected only at the active layer of the modified membrane, where the Ag+ ions were successfully exchanged with the grafted sulfone groups, while no measurable Ag was found at the other points, including at the ‘active layer’ of the pristine PES membrane. These findings confirm that the dense layer is, indeed, the newly grafted layer. The dry thickness of the hydrogel layer with 5% MBAA (25% VSA, 7.5 min irradiation) was also measured by AFM. The complete dissolution of the PES membrane and the presence of the isolated pVSA hydrogel layer on the silicone surface were confirmed by measuring the FTIR spectra, using the reflection mode (FT-IR Microscope Bruker HYPERION 1000); no peaks of the PES membrane were observed (data not shown). The bright areas in the image of the height profile of the isolated film (Figure 7) represent the highest points of the hydrogel layer, while the dark regions indicate the surface of the silicon wafer.

Figure 7. AFM images of a VSA film on a silicon surface (grafting conditions were 25% VSA, 5% MBAA, and 7.5 min of UV irradiation). As shown in Figure 7, the average thickness of the membrane that was fabricated by using 5% MBAA is approximately 270 nm. This thickness is somewhat higher than the thickness measured by using

15

SEM (ca. 90 nm), which may indicate some intrusion of the hydrogel to the membrane pores. Nevertheless, the active layer can still be considered a thin-film hydrogel. 3.5. Pervaporation performance 3.5.1 Effect of cross-linker concentration and irradiation duration The effect of the cross linker-to-monomer ratio for 25% VSA (7.5 min irradiation) and of the irradiation duration (for 25% VSA and a 5% cross–linker fraction) on the membrane pervaporation performance (separation factor and total flux) are presented in Figure 8a-b. Separation factor-25%VSA

Separation factor-5%MBAA

Permeate-25%VSA

400

16

(b)

(a)

8 200 6 4

100

2

2.5

5.0

7.5

10.0

Separation factor

Separation factor

10

14

400

12 10

300

8 200

6 4

100

2

0 12.5

0

0 0

MBAA concentration (mole%)

Permeation flux (kg/m 2h)

300

Permeation flux (kg/m2h)

12

0 0.0

Permeate-5%MBAA

500

14

5

10

15

20

Irradiation time (min)

Figure 8. (a) The effect of cross-linker concentration on membrane pervaporation performance (separation factor and flux). (b) The effect of irradiation duration on membrane pervaporation performance for 5%. Graft polymerization conditions were t = 7.5 min, I = 65 mW/cm2, n ≥ 3. Figure 8a demonstrates that the pervaporation separation factor increases significantly with increasing cross-linker concentration of up to a 5% fraction, and then drops; in contrast, the total flux gradually decreases with increasing cross-linker fraction. A similar trend was observed for the effect of irradiation duration (Figure 8b), but with a small variation: the total permeate flux slightly increased at a long duration of irradiation (15 min). The changes in the pervaporation separation factor with increasing cross-linker concentrations and irradiation durations correlate with the changes observed in the IEC (compare Figure 4 and Figure 8), probably due to the higher sorption of water molecules to the active layer [46]. The separation factor

16

increases from 24 to 303 when the IEC increases from 2.27 to 2.99 meq/gr, and then decreases to 94 as the IEC is reduced to 1.66 meq/gr. The increased swelling with increasing cross-linker fraction can also enhance the separation factor due to faster diffusion of the water molecules to the permeate side. The high swelling also increases the total flux, but it can lower the separation factor due to the enlargement of the free volume in the hydrogel, which enables the penetration of ethanol molecules [16]. Therefore, as seen in Figure 8a-b, there is an optimum performance that occurs at the optimal swelling–IEC value corresponding to a high IEC value and a limited degree of swelling. Furthermore, the changes in the trade-off between the separation factor and permeability can also result from a thicker active layer, which is expected to improve membrane separation factor but to reduce total flux [47]. The separation factor of a composite membrane, fabricated by using 42% VSA and 1% MBAA, was only 114 (and a total flux of 6.1 kg/m2h) – lower than a membrane with similar IEC values that was fabricated by using 25% VSA and 5% MBAA (IEC ≈ 3 meq/gr, Figure 4a). This is probably due to the much greater water swelling of the former (Figure 5a). 3.5.2 Effect of feed temperature It is well-known that the feed temperature is a key parameter that influences the permeation of water molecules through the membrane [11]. The effect of temperature (ranging from 303 K to 333 K) on the water and ethanol fluxes was studied for the membranes that presented the optimal performance (25% VSA, 5% MBAA, and 7.5 min irradiation duration; see Figures SI3, Supporting Information). The trend of permeation flux versus temperature and the apparent activation energy can be described by the Arrhenius equation: 𝐽𝑃 = 𝐴𝑃 ∗ exp(−𝐸𝑃 ⁄𝑅𝑇)

(7)

where AP, JP, EP, R, and T represent the pre-exponential factor, permeation flux (kg/m2h), apparent activation energy (kJ/mol), gas constant, and feed temperature (K), respectively. 17

Figure 9 shows that the ln(Jp) versus 1000/T curve is linear, indicating that the permeability followed the Arrhenius relation. The calculated apparent activation energy of water (29.93 KJ/mol) was higher than that of ethanol (6.31 KJ/mol), which is similar to the other sulfone-based composite membranes [48], indicating that the water permeation has a higher temperature sensitivity than ethanol, resulting in an increase of the separation factor [49, 50]. 10 Water 9

In J P

8

Ethanol EP,W=29.93kJ/mol

7 6

EP,E=6.31kJ/mol

5 4 2.9

3.0

3.1

3.2

3.3

3.4

-1

1000/T (K )

Figure 9. Arrhenius plots of permeation flux for separating an ethanol/water mixture. Grafting polymerization conditions were 25% VSA, 5% MBAA, t = 7.5 min, I = 65 mW/cm2. 3.6. Long-term operational stability To investigate their long-term stability, modified polyelectrolyte membranes were continuously operated with a 90 wt% ethanol aqueous solution at 323 K for up to 120 h. However, since the flux was very high, it was impossible to operate the system continuously under filtration mode. Therefore, the system was operated constantly in a cross-flow mode (without filtration) and two samples were taken daily, as described in Section 2.5. As presented in Figure 10, the separation factor and the permeation flux decreased during the first few days and stabilized afterward (at a separation factor of approximately 90), indicating that the long-term operation stability should be improved. The performance reduction was observed within 24–48 h from the beginning of the long-term experiments. The reason for the decrease in membrane performance is not completely understood. The ATR-FTIR spectra before and after the long-term operation were very similar (data not shown),

18

and degradation of the hydrogel would have resulted in an increase, rather than a decrease, in the permeate flux. Thus, it can be assumed that the hydrogel remains intact. It can be speculated that the reduction in performance is due to the aging of the hydrogel, as is usually observed when ethanol is removed from aqueous solutions by pervaporation using PTMS membranes [51]. Nevertheless, the long-term stability can be improved by using various approaches, such as adding nanoparticles [52] or coating the membrane with a thin-film of protective layer [53]; both are currently being explored.

100 80

Flux (kg/m2h)

6

60 4 40 2

20

Flux Water content 0 0

24

48

72

96

120

0 144

Water content in permeate (wt%)

8

Operation time (h) Figure 10. Pervaporation performance stability of a polyelectrolyte hydrogel membrane in dehydrating 10 wt% water/ethanol at 50 °C. 3.7. Comparison of pervaporation performance of various membranes Figure 11 summarizes the pervaporation performance of various types of polymeric membranes for the dehydration of an ethanol solution, as reported in the literature (see Table SI2, Supporting Information). The VSA hydrogel composite membrane demonstrates a moderately high separation factor, but has the highest permeation flux of all examined membranes. Furthermore, as compared with the other polyelectrolyte composite membranes, a very high separation factor was obtained. The pervaporation separation index (Equation 6), which enables a comparison between the performance of membranes with different separation factors and permeability [54], is very high, as compared with

19

other polymeric membranes [7, 10-13, 24, 39, 50, 55-64] (Table SI2, Supporting Information). 10000

PVA

Separation factor

HPA/SA SA/HAA GACS/PCP

1000

PAA/CS CS/CMC

100

10

(PAH/PSS)60 (PEI/PVS)60 Nexar™ /PEI b (PAA/PEI)4 S-PVS/CS-3

PDDA/SCMC

PECSM-20 SCMC-PDDA

Sulfonated SiO2/CS

g-PSS-Na+

Sulfonated PSF

PVA/SSA-Na+

DETA/TMC SCMC/CS

Nafion 811-K+ PEI/PAA This w ork

MPD/TMC BPDA-ODA/DABA Alg/DNA-Ca2+

1 0.01

PERVAP 2210 a

0.1

1

10

-2

Flux (kg.m .h)

Figure 11. Graphic representation of the performance of polyelectrolyte and composite membranes from the data presented in Table SI2 (Supporting Information). 4. Conclusions A composite membrane with a polyvinyl sulfonic acid hydrogel as the active layer on a PES membrane was successfully fabricated by using the UV grafting polymerization method. The degree of grafting monotonically increased with increasing cross-linker concentration (1-10%) and the duration of UV irradiation (2.5–15 min). The IEC also increased with increasing cross-linker concentration and the duration of irradiation until it reached a maximum value, after which it decreased with increasing cross-linker concentrations (10% for 25% VSA and 2.5% for 42% VSA) and irradiation durations (15 min). This behavior was attributed to the increase in the cross-linked fraction in the hydrogel. The degree of swelling decreased with increasing cross linker-concentrations (1-10%) and UV irradiation duration (2.5–15 min). It is speculated that the lower swelling observed in longer durations of irradiation is due to the one-dimensional swelling of the tethered hydrogel. It was also found that the composite membrane has a thin-film hydrogel active layer of approximately 100300 nm. The separation factor of the composite membrane significantly increased with increasing IEC values, probably due to the increase in water sorption. However, high swelling resulted in a lower separation

20

factor. The extensive swelling was successfully limited by means of cross-linking. The optimal pervaporation performance for ethanol dehydration (separation factor = 303; flux = 7.6 kg/m2h) was obtained at conditions that combined a high IEC value (2.99 meq/gr) and a limited degree of swelling (5% MBAA, 7.5 min). The very high flux was also due to the thin active layer. As expected, the performance of the membrane further improved by increasing the temperature of the feed solution (separation factor = 410; flux = 9.89 kg/m2h), and the change in the flux with the temperature correlated with an Arrhenius-type equation. The membrane presented in this report displays a high separation factor, as compared with other composite polyelectrolyte-based membranes that were studied for pervaporation, and a very high permeability and PSI factor, as compared with other polymeric membranes used for pervaporation mediated ethanol dehydration. The membrane stability still needs to be improved, as its performance decreased within 24-48 h, after which it reached a plateau. The decrease in performance with duration might be the result of an ageing effect and an increase in the ethanol permeation, which reduces the separation factor and swelling (and, thus, the flux). This caveat can be improved by using various methods, including the fabrication of a hybrid inorganic-organic membrane, which often improves the mechanical stability of polyelectrolytes.

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Highlights 

A thin-film composite membrane with a vinyl sulfonic hydrogel as the active layer was fabricated for dehydration by pervaporation.



The ion-exchange capacity and the swelling degree of the TFC membrane were governed by the ratio between the cross linker and the VSA monomer in the modification solution.



The ion-exchange capacity and the swelling degree significantly influenced the membrane performance for ethanol dehydration by pervaporation.



The membrane with optimal IEC and swelling degree exhibited very high flux and moderate high selectivity for ethanol dehydration by pervaporation.

26