O3-modified PDMS membranes

Separation and Purification Technology 100 (2012) 15–21

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Pervaporation separation of ethanol/water mixture by UV/O3-modified PDMS membranes Cheng-Lee Lai a, Ywu-Jang Fu b, Jung-Tsai Chen c, Quan-Fu An d, Kuo-Sung Liao c, Shih-Ching Fang c, Chien-Chieh Hu c,⇑, Kueir-Rarn Lee c a

Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan Department of Biotechnology, Vanung University, Chung-Li 32023, Taiwan R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan d Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalization, Ministry of Education, Zhejiang University, Hangzhou 310027, China b c

a r t i c l e

i n f o

Article history: Received 2 April 2012 Received in revised form 18 July 2012 Accepted 30 July 2012 Available online 5 September 2012 Keywords: UV/ozone modification PDMS membrane Pervaporation

a b s t r a c t A UV/O3 surface modification technology was used to develop high-performance dense polydimethylsiloxane (PDMS) pervaporation membranes for separating ethanol/water mixtures. Surface characteristics of PDMS membranes were evaluated by attenuated total reflectance – Fourier transform infrared spectroscopy, scanning electron microscopy – energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and water contact angle measurement. The effect of UV/O3 treatment time and working distance on the membrane physical and chemical properties and pervaporation performance were investigated. Results indicated that either longer treatment time or shorter working distance resulted in lower water contact angle and higher O/Si ratio, which implied higher conversion of PDMS into hydrophilic silica-like structure. The selectivity was greatly improved with longer treatment time and decreasing working distance. The UV/O3 modification technique effectively increased the pervaporation performance of PDMS, especially at high temperature conditions of the feed. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Pervaporation is a very useful energy-saving and cost-effective technique for the separation of azeotropic, close-boiling, or aqueous organic mixtures [1]. Pervaporation dehydration of ethanol/ water mixtures is very important for obtaining high-purity bioethanol [2]. Based on solution-diffusion model, the structure and hydrophilicity of membranes are key factors that affect pervaporation separation performance [3]. Generally, a hydrophilic polymer is chosen as a pervaporation dehydration membrane [4–7] to obtain high performance of the membrane. However, there is a big challenge in using hydrophilic membranes, because abundant hydrophilic moieties in polymer chains induce excessive swelling during the pervaporation dehydration process [8–13]. Therefore, a novel modified pervaporation membrane with moderate hydrophilicity was developed in this work. PDMS is considered as a potential material for gas and pervaporation separations of volatile organic compounds [2,14–19]. Because PDMS is a rubbery material and possesses large free volume, it exhibits very high permeability for many gases or organ-

⇑ Corresponding author. Tel.: +886 3 2654190; fax: +886 3 2654198. E-mail address: [email protected] (C.-C. Hu). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.07.018

ic vapors unlike most glassy polymers. In the past decades, PDMS has been widely used for removing organic compounds from dilute organic/water mixtures [2,20–23]. However, from the literature, there are very few studies using PDMS for the dehydration of concentrated water/ethanol mixtures, because of its very hydrophobic intrinsic property. The surface hydrophilicity of membranes can be improved by several techniques, such as chemical modification [24,25], plasma treatment [26–28], blending with amphiphilic copolymers [29,30], surface grafting with hydrophilic polymers [31–35], and surface crosslinking [36]. This work tried to modify PDMS membranes using a UV/O3 treatment technique to improve its hydrophilicity. According to our previous study [14], treatment by UV/O3 is an effective technique to change the structure of PDMS. Our results showed that the surface of PDMS membranes was converted to hydrophilic SiOx structures after the UV/O3 treatment. We also proved that the modified membrane had a denser structure compared to the pristine PDMS membrane, and the ideal O2/ N2 selectivity increased from 2.0 to 5.2 when the modified membrane was tested for pure gas separation of O2 and N2. In this study, we expected that the surface hydrophilicity and pervaporation performance of PDMS membranes could be greatly improved by using a UV/O3 treatment technique. The effect of UV/O3 treatment time and working distance on the surface properties and pervaporation performance were investigated.

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2. 1.Experimental 2.1. Preparation and modification of PDMS membranes

Si-OH

Transmittance

Silicon rubber (PDMS), crosslinking agent (SYL-OFF 7048), and catalyst (SYL-OFF 4000) were purchased from Dow Corning Corporation. Proper amounts of the crosslinking agent and catalyst were mixed with PDMS and stirred for 20 min. The resultant solution was cast onto a Teflon substrate with a 600-lm casting knife, and the resulting membrane was cured at 120 °C for 15 min. The thickness of the resulting membranes is about 350 lm. The PDMS membrane was then placed under a 1 kW UV lamp (wavelength is 254 nm) at a distance of 6.4–1.2 cm. Ozone was generated from atmospheric oxygen (concentration of ozone is 36.3 g/m3), and it flowed into the UV lamp chamber. The PDMS membrane was treated with UV/O3 for different periods of time.

pristine 10 min 20 min 30 min 45 min 4000

3600

3200

Si-CH3 Si-O-Si 2800

1300

1200

1100

1000

900

-1

Wavenumber (cm ) 2.2. Membrane characterization Attenuated total reflectance – Fourier transform infrared (ATRFTIR) spectra for PDMS membranes were measured using Perkin– Elmer FTIR spectrometer (Perkin Elmer Cetus Instruments, Norwalk, CT) in the wavenumber range of 600–4000 cm1. Water contact angle was measured by a face contact angle meter CA-D type (Kyowa Interface Science Co., Ltd.). Deionized water was used as probe liquid. All measurements were taken at ambient temperature. The reported contact angle values were the average of at least ten measurements, taken at different locations on the membrane surface. Chemical surface compositions of pristine and UV/ozonetreated membranes were analyzed using an energy-dispersive Xray microanalysis (EDX) system (Model S-3000, Hitachi) and a Thermo Fisher Scientific Thermo Scientific K-Alpha X-ray photoelectron spectrometer (XPS), equipped with a monochromatic Al Ka X-ray source (1486.6 eV). 2.3. Pervaporation measurement The apparatus for pervaporation experiment was described in a previous paper [37]. The effective surface area of the membrane in direct contact with the feed solution was 3.5 cm2. The permeation flux was determined as follows: the weight of the permeate collected for a period of operation time was measured, and then it was divided by the effective membrane area. The compositions of feed solution and permeate were measured with gas chromatography (GC China Chromatography 8700 T). Pervaporation experiments were conducted at 25 °C, using a mixture containing 10 wt.% water and 90 wt.% ethanol as feed solution. The effects of feed temperature were studied in the range of 25–75 °C. The separation factor, aH2 O=EtOH was calculated from the following equation:

Fig. 1. FTIR-ATR spectra for PDMS membranes treated with UV/O3 for different periods of time (working distance is fixed at 1.2 cm).

The Si–O–Si peak at 1000–1150 cm1 showed a significant change in shape and wavenumber, confirming that Si–OH groups were converted to an SiOx structure; similar results were observed by a previous study [14]. ATR-FTIR spectra for PDMS membranes treated at different working distances are shown in Fig. 2 It can be seen that the intensity of Si–CH3 groups slightly decreased with the working distance, and the wavenumber of Si–O–Si shifted to lower positions, indicating that more Si–CH3 groups were substituted for oxygen to form Si–OH or SiOx. From results of ATR-FTIR spectra (Figs. 1 and 2), they demonstrated that the longer the treatment time or the shorter the working distance, the higher the degree of siloxane converted into SiOx. XPS and SEM–EDX can give a quantitative analysis of the chemical composition of membranes. Fig. 3 shows atomic ratio as a function of UV/O3 treatment time. O/Si ratios obtained from XPS and SEM–EDX both increased with treatment time, demonstrating that more siloxanes were converted into an SiOx structure (theoretical O/Si is 1.0 for PDMS and 2.0 for SiO2). It is interesting to note that the O/Si ratio obtained from the XPS analysis was higher than that from the SEM–EDX. The measurement depth was 1–10 nm for XPS and 1–3 lm for SEM–EDX; the 1–10 nm layer of surface had a higher degree of conversion than the thicker layer of 1–3 lm. The higher O/Si ratio from XPS than from SEM–EDX demonstrated that

where X H2 O , X EtOH and Y H2 O , Y EtOH are the weight fractions of water and alcohol in the feed and permeate, respectively. 3. 1.Results and discussion 3.1. Characterization of PDMS membranes Chemical compositions of pristine and UV/O3-treated PDMS membranes were characterized by ATR-FTIR (Fig. 1). Bands at 1250 and 2800–3600 cm1 corresponded to Si–CH3 and OH, respectively. The intensity of Si–CH3 decreased with increasing UV/O3 treatment time due to substitution of –CH3 for oxygen atoms, indicating that the oxidation of PDMS took place. Si–CH3 groups were converted to Si–OH during the oxidation reaction.

Transmittance

aH2 O=EtOH ¼ ðY H2 O =Y EtOH Þ=ðX H2 O =X EtOH Þ

pristine 1.2 cm 3 cm 4.8 cm 6.4 cm 1400

1300

Si-CH3 Si-O-Si 1200

1100

1000

900

-1

Wavenumber (cm ) Fig. 2. ATR-FTIR spectra for PDMS membranes treated at different working distances (UV/O3 treatment time is 45 min).

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Table 1 Comparison of chemical compositions of PDMS membranes treated for different periods of time (working distance is 1.2 cm).

1.75

1.25

SiO2C2 (%)

SiO3C (%)

SiO4 (%)

100 32.7 8.0 11.7 0 0

0 46.1 39.5 30.7 44.1 34.0

0 21.2 52.6 57.5 55.9 66.0

1.00 XPS SEM-EDX

0.75

0

10

20

30

40

50

UV/O3 treatment time (min) Fig. 3. Atomic ratio of O/Si as function of UV/O3 treatment time (working distance is 1.2 cm).

the conversion degree of the silica-like structure decreased from top to bottom of PDMS membranes. This decrease was due to the decrease in both the energy of ultraviolet and the ozone concentration in the membrane with increasing membrane depth. Fig. 4 presents O/Si ratio as a function of working distance. Results showed that the O/Si ratio slightly increased with decreasing working distance. It is well known that the radiation intensity sharply increases with decreasing distance. Because of the high energy at a short working distance, there is a high conversion of PDMS into a silica-like structure. Comparing Fig. 1 to Fig. 4 it can be seen that the influence of UV/O3 treatment time on the surface chemical structure is more significant than that of working distance. Table 1 compares the chemical compositions of PDMS membranes treated for different periods of time. It is shown that the 10-min UV/O3-treated PMDS membrane had 46.1% SiO3C and 21.2% SiO4. For a treatment time from 10 to 30 min, the concentration of SiO3C decreased and the concentration of SiO4 increased. It is interesting to note that SiO2C2 was converted first into SiO3C, and then the SiO3C was converted into SiO4. When treatment time was longer than 40 min, SiO2C2 was completely converted into SiO3C or SiO4. The dose of UV radiation was a key factor for the UV/O3 modification. We varied the distance between UV lamp and membrane

Table 2 Comparison of chemical compositions of PDMS membranes treated at different working distances (treatment time is 45 min). Working distance (cm)/UV intensity (mW/ cm2)

Chemical composition SiO2C2 (%)

SiO3C (%)

SiO4 (%)

1.2/18 3.0/13 4.8/9 6.4/7

0 10.0 14.8 14.6

34.0 40.8 48.1 45.5

66.0 49.2 37.1 39.9

(a) 100

60 40 20

0

10

20

30

40

50

UV/O3 treatment time (min) 45 40

1.60 1.55 1.50

XPS SEM-EDX

1.45

1

2

3

4

5

6

Working distance (cm) Fig. 4. Atomic ratio of O/Si as function of UV/O3 working distance (treatment time is 45 min).

Water cantact angle (°)

1.65

1.40

80

0

(b)

1.70

Atomic ratio (O/Si)

Chemical composition

0 10 20 30 40 45

Water cantact angle (°)

Atomic ratio (O/Si)

Treatment time (min)

1.50

35 30 25 20 15 10

1

2

3

4

5

6

7

Working distance (cm) Fig. 5. Water contact angle as function of (a) UV/O3 treatment time or (b) working distance for PDMS membranes.

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260

95

Permeation flux (g/m 2h)

240 90 85

220

80 200

75 70

180 65 160

Water content in permeate (wt%)

100

60 0

10

20

30

40

50

UV/O3 treatment time (min) Fig. 6. Pervaporation performance as function of UV/O3 treatment time for PDMS membranes.

260

95

2

Permeation flux (g/m h)

240

90

220

85 200

80

180

75

160

70 65

140 0

1

2

3

4

5

6

7

Water content in permeate (wt%)

100

60

UV/O3 working distance (cm) Fig. 7. Pervaporation performance as function of working distance for PDMS membranes.

90 80

2

Permeation flux (g/m h)

600

500

70 60

400 50 300

40 30

200 20

30

40

50

60

70

80

Water content in permeate (wt%)

100

700

20

Feed temperature ( °C) Fig. 8. Effect of feed solution temperature on pervaporation performance of pristine PDMS ( ,

) and UV/O3-treated PDMS (1.2 cm, 45 min) membranes ( ,

).

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C.-L. Lai et al. / Separation and Purification Technology 100 (2012) 15–21

500

UV/O3 treated PDMS Pristine PDMS 20 18 16

Separation factor

Separation factor

400

300

200

14 12 10 8 6 4 2 0 20

30

40

50

60

70

80

o

Feed temperature ( C)

100

0 20

30

40

50

60

70

80

o

Feed temperature ( C) Fig. 9. The separation factor of PDMS ( ) and UV/O3-treated PDMS (1.2 cm, 45 min) membranes (j) as a function of feed temperature.

(working distance) to control the UV intensity. Si2p curve fitting results of PDMS membranes treated at different working distances are shown in Table 2 The amount of SiO2C2 decreased and that of SiO4 increased with decreasing working distance. A shorter working distance was related to a higher UV intensity. The shorter the working distance, the higher the UV intensity (show in Table 2). It demonstrated that a higher UV intensity facilitated the siloxane conversion into an SiOx structure. Hydrophilicity is an importment characteristic of pervaporation separation membranes. UV/O3 treatment, converting SiO2C2 into SiO3C or SiO4, should change the hydrophilicity of PDMS membrane surface. Fig. 5 presents the effect of UV/O3 treatment time and working distance on the water contact angle for PDMS membranes. A dramatic decrease in the water contact angle (from 100.9° to 34.4°) was observed when PDMS membranes were treated by UV/O3 for only 10 min (Fig. 7a). For a UV/O3-treatment time from 10 to 45 min, the water contact angle kept decreasing from 34.4° to 16.6°, indicating that the hydrophilicity of PDMS membrane surface was greatly improved. Hydrophilicity is an inherent characteristic of silica, so a higher degree of siloxane converted into a silica-like structure results in a lower water contact angle. The change in the water contact angle for PDMS membranes as a function of working distance is shown in Fig. 5b. It is evident that the water contact angle decreased with decreasing working distance. This result could be correlated to the change in surface chemical structure. The surface of PDMS was converted to a more silica-like structure when the working distance decreased.

3.2. Pervaporation performances of PDMS and UV/O3-treated PDMS membranes Figs. 6 and 7 depict the effect of UV/O3 treatment time and working distance on the pervaporation performance of UV/O3-treated PDMS membranes for dehydrating a 90 wt.% aqueous ethanol mixture at 25 °C. A significant increase in the concentration of water in permeate was obtained (from about 65 wt.% to 98 wt.%, 50% increase), while the permeation flux showed only slight decrease (from about 200 g/m2 h to 180 g/m2 h, 10% decrease) either when the UV/O3 treatment time increased from 0 to 45 min or the UV/O3 working distance decreased from 6.4 cm to 1.2 cm. These results might be a consequence of an increase in the UV/O3 treatment time, and a decrease in the UV/O3 working distance effectively increased the conversion of the chemical structure from siloxane to silica-like structure (Tables 1 and 2). The membrane with a higher silica-like composition became more hydrophilic (Fig. 5), and it had a denser structure [14]. On the basis of solution-diffusion model, an improvement in the selectivity can be explained by two reasons: (1) the increase of solubility of water molecules in membrane surface due to the increase of hydrophilicity (2) the increase of diffusion selectivity because of the denser structure provide higher diffusion hindrance for larger molecules (ethanol). Pervaporation results demonstrated that UV/O3 technique is a very effective method for improving the water selectivity of PDMS membranes. The pervaporation performance of pristine PDMS and UV/O3treated PDMS (treated at 1.2 cm for 45 min) membranes were studied at elevated temperatures. Changes in permeation flux and concentration of water in permeate for both membranes as a function of temperature are shown in Fig. 8 It shows that the permeation flux increased and the water content in permeate decreased with increasing feed solution temperature. This is due to increase in temperature, resulting in higher mobility of polymer chains and larger free volume in the membrane. Furthermore, the vapor pressure of feed components increased with the feed solution temperature, leading to higher driving force [38,39]. Thus, the permeation of permeating molecules was faster, leading to an increase in permeation flux and a decrease in water content in permeate [38–41]. The separation factor of PDMS and UV/O3-treated PDMS membranes (treated at 1.2 cm for 45 min) is shown in Fig. 9 It can be seen that the selectivity of UV/O3-treated PDMS membranes is much higher than that of PDMS membranes. The selectivity was increased by more than 25 times at 25 °C after modification. Both of the selectivity of PDMS and UV/O3-treated PDMS membranes decreases with increasing feed temperature. This is due to the increase of free volume in the membrane at high temperature, resulting in ethanol much easier passthrough the membrane than that of water. Furthermore, the increase of vapor pressure of ethanol is higher than that of water. These two reasons

Table 3 The membrane pervaporation performances for ethanol and isopropanol dehydration. Membrane (material)

Thickness (lm)

PDMS

350

UV/O3-PDMS

350

PDMS GA crosslinked Chitosan GA crosslinked CS/NaAlg blends PVA-TMC GA crosslinked PVA CS

87 20 10 50–60 22 22

T (°C)

Feed (wt%)

Flux (g/m2 h)

Separation factor

Refs.

25 40 25 40 50 40 60 40 40 60

EtOH (90) EtOH (90) EtOH (90) EtOH (90) EtOH (5) EtOH (90) EtOH (90) IPA (90) EtOH (70) EtOH (90)

230 360 180 220 236 130a 300 170a 2200a 800

16 11 450 130 25b 140a 200 400a 60a 35

This This This This [42] [6] [43] [44] [3] [45]

GA: glutaraldehyde, PVA: poly(vinyl alcohol), CS: chitosan, NaAlg: sodium alginate, TMC: trimesoyl chloride. a Data with  are estimations from the graph. b Ethanol selective.

work work work work

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led to the decrease in selectivity. A larger discrepancy between UV/ O3-treated membrane and pristine membrane is observed at high operating temperature. The effect of feed temperature on pristine PDMS membrane is larger than that of UV/O3-treated PDMS membrane. This result is due to the UV/O3-treated membrane has an inorganic silica-like structure which can exhibit a better stability at high temperature than that of pristine PDMS membrane. In the case of the UV/O3-treated membrane at 75 °C, the permeation flux and water concentration in permeate were both higher than those for the pristine PDMS at 25 °C. The permeation flux was increased by 35% and the selectivity was increased by 150%. This means that the improvement in pervaporation performance will be more evident when the operation temperature increases. Table 3 lists the membrane pervaporation performances for dehydrating alcohol/water mixtures. We can see that the UV/O3-treated PDMS membranes show right performance compare to other PDMS and hydrophilic membranes. The thickness of UV/O3-treated PDMS membranes is much thicker than that of hydrophilic membranes. Hence, a high permeation flux and separation factor composite membrane with thin UV/O3-treated PDMS active layer can be expected. 4. Conclusions In this study, PDMS membranes were modified by UV/O3 treatment. Pristine and modified membranes were used for separating a mixture of 90 wt.% aqueous ethanol. ATR-FTIR results showed that siloxane groups on the PDMS membrane surface were converted to silica-like structures after the UV/O3 treatment. XPS and SEM–EDX results demonstrated that with longer treatment time or decreasing working distance, higher conversion of siloxane to silica was obtained. Comparing the O/Si ratio obtained from XPS with that from SEM–EDX analysis, it could be found that the conversion decreased with increasing depth of UV/O3-treated PDMS membranes. Results of water contact angle demonstrated that the hydrophilicity of PDMS membranes surface was significantly improved due to the change in the structure from siloxane to silica-like. Pervaporation results indicated that both the treatment time and the working distance during the UV/O3 treatment were important variables for promoting the performance of PDMS membranes. The water content in permeate could be significantly increased from 64.2 wt.% to 98 wt.% when the treatment time lengthened from 0 to 45 min at a working distance of 1.2 cm. This UV/O3 treatment technique was a very effective method to increase the pervaporaton performance of PDMS membranes. This method is also possible used to crosslink the surface of other hydrophilic membranes, such as poly(vinyl alcohol), chitosan to enhance the water selectivity. Acknowledgements The authors wish to sincerely thank the Ministry of Economic Affairs (MOEAWRA1000089), the Ministry of Education Affairs, and the National Science and Technology Program-Energy from NSC of Taiwan for financially supporting this work. References [1] S.H. Huang, C.L. Li, C.C. Hu, H.A. Tsai, K.R. Lee, J.Y. Lai, Polyamide thin-film composite membranes prepared by interfacial polymerization for pervaporation separation, Desalination 200 (2006) 387–389. [2] T. Mohammadi, A. Aroujalian, A. Bakhshi, Pervaporation of dilute alcoholic mixtures using PDMS membrane, Chem. Eng. Sci. 60 (2005) 1875–1880. [3] M.N. Hyder, R.Y.M. Huang, P. Chen, Correlation of physicochemical characteristics with pervaporation performance of poly(vinyl alcohol) membranes, J. Membr. Sci. 283 (2006) 281–290.

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