Anion exchange hybrid membranes from PVA and multi-alkoxy silicon copolymer tailored for diffusion dialysis process

Journal of Membrane Science 356 (2010) 96–104

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Anion exchange hybrid membranes from PVA and multi-alkoxy silicon copolymer tailored for diffusion dialysis process Cuiming Wu a , Yonghui Wu b , Jingyi Luo b , Tongwen Xu b,∗ , Yanxun Fu b a b

School of Chemical Engineering, Hefei University of Technology, Hefei 230009, PR China Lab of Functional Membranes, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China

a r t i c l e

i n f o

Article history: Received 28 January 2010 Received in revised form 18 March 2010 Accepted 20 March 2010 Available online 25 March 2010 Keywords: PVA Anion exchange membrane Hybrid membrane Diffusion dialysis Acid recovery

a b s t r a c t Anion exchange hybrid membranes have been prepared from quaternization and sol–gel reaction of multi-alkoxy silicon copolymer (poly(vinylbenzyl chloride-␥-methacryloxypropyl trimethoxy silane) (poly(VBC-co-␥-MPS)) in presence of poly(vinyl alcohol) (PVA). The membranes are of high thermal stability, mechanical strength, and swelling resistance. Thermal degradation temperatures (Td , defined as the temperature at 5% weight loss) are in the range of 263–283 ◦ C. Tensile strength (TS) ranges from 24.5 MPa to 59.2 MPa and elongation at break (Eb ) is in the range of 25.4–101.2%. The membranes can remain integrity in 65 ◦ C hot water for more than 25 days and the swelling behavior is better suppressed as poly(VBC-co-␥-MPS) content increases. The membranes are tested for diffusion dialysis (DD) recovery of hydrochloric acid (HCl) from the mixture of HCl and ferrous chloride (FeCl2 ). The temperature for DD process ranged from 20 ◦ C to 60 ◦ C, revealing that the separation factor (S) values are in the range of 12.1–35.7, comparable to the values of commercial DF-120 membranes. When the temperature increases from 20 ◦ C to 30 ◦ C, the separation performances are increased. Further increase of temperature increases the dialysis coefficients, but decreases the S value. The membrane structure and properties are correlated with the DD performances. © 2010 Elsevier B.V. All rights reserved.

1. Introduction During the last two decades, organic–inorganic hybrid membranes have been developed rapidly into a fascinating new field of research. Among these, ion exchange membranes have been investigated for a variety of applications such as water treatment, chemical separation, electrochemical sensing, and fuel cells [1–3]. For the preparation of ion exchange hybrid membranes, different polymers are utilized as the starting materials, such as poly(vinylidene fluoride) (PVDF) [4,5], poly(ether ether ketone) (PEEK) [6,7] and poly(phenylene oxide) (PPO) [8]. These polymers are generally of high price and their sulfonation or quaternization is complex process. Hence, cheaper material and milder preparation process are quite desirable. Poly(vinyl alcohol) (PVA) is a low cost polymer and has the unique characterizations of good resistance to organic solvent, excellent flexibility and high adhesion to various surfaces. However, the main chain of PVA cannot undergo sulfonation or quaternization reaction. Besides, PVA is highly swelling and even dissoluble (at higher temperature) in water. Therefore, incorporation of ion exchange components and suppress of PVA chains

∗ Corresponding author. Tel.: +86 551 360 1587. E-mail address: [email protected] (T. Xu). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.03.035

swelling are the two main concerns for the preparation of PVAbased ion exchange membranes. The former can be achieved by the addition of acid or base fillers [9,10], so that heterogeneous ion exchange membranes are obtained. And the latter is generally achieved through crosslinking of PVA with aldehyde [11] or small alkoxysilane, such as tetraethoxysilane (TEOS) [12]. The crosslinking effect depends on the reaction conditions, and the structure of the aldehyde or alkoxysilane. In our previous work, copolymer from small alkoxysilane (␥-methacryloxypropyl trimethoxy silane (␥-MPS)) and organic monomer (glycidylmethacrylate (GMA)) has been synthesized [13]. The copolymer contains multi-alkoxy silicon (–SiOR) groups and multi-epoxy group. Hence we have tried it as a novel crosslinking agent in the sol–gel reaction system of PVA and charged alkoxysilane (N-triethoxysilylpropyl-N,N,Ntrimethylammonium iodine) (A-1100(+)) [14]. The significantly higher crosslinking ability of the copolymer was confirmed through systematic comparison with small alkoxysilanes, such as TEOS, ␥-glycidoxypropyltrimethoxysilane (GPTMS), or monophenyltriethoxysilane (EPh). Unfortunately, the membranes are still not stable enough and the performances need to be improved further. For instance, the hybrid membranes lose 12–25 wt.% during IEC measurement and the practical IEC values are much lower than the theoretical ones [14]. This indicates that part of the charged component (A-1100(+)) can still

C. Wu et al. / Journal of Membrane Science 356 (2010) 96–104

be lost and its crosslinking with PVA cannot be achieved completely. The present work will be a further development of our previous effort of PVA–SiO2 anion exchange hybrid membranes. Similarly, one copolymer with multi-alkoxy silicon groups will be used as the crosslinking agent with PVA. However, we will select vinylbenzyl chloride (VBC), rather than GMA as the organic monomer for preparation of the copolymer. The chloromethyl groups (–CH2 Cl) groups in vinylbenzyl chloride (VBC) can be easily turned to quaternary ammonium groups through reaction with trimethylamine [15], so that no additional ion exchange fillers such as A-1100(+) need to be added. In this way, the problem of loss of the charged component can be effectively resolved and the electrical performances and stabilities improved. The membrane preparation procedures are also simplified and more environmental friendly, since the hazardous CH3 I for preparation of A-1100(+) is prevented. The multi-alkoxy silicon (–SiOR) groups in VBC and ␥-MPS copolymer (poly(VBC-co␥-MPS)) are expected to crosslink highly with the large amount of –OH groups from PVA, so that the homogeneity between the organic and inorganic components and between the non-charged PVA and the charged ammonium components can be simultaneously guaranteed. The membrane structure and properties, including thermal, mechanical and chemical stabilities will be fully characterized. Moreover, as an illustration of the application, they will be tried for recovery of acid from mixture of HCl and FeCl2 by diffusion dialysis (DD). As well known, DD is such a process that the solutes pass through the ion exchange membrane from the high concentration side to the low concentration side [16,17]. Since the hybrid membranes contain large amount of –OH groups (from PVA and –SiOH groups) as well as the quaternary ammonium (–N+ ) groups, transport of H+ ions through the membranes should be more facilitated [18], which has been confirmed by our previous trial about DD running [14]. Furthermore, different DD running temperatures will be attempted. In this way, the competing transfer of the Fe2+ and H+ ions across the membranes, and the stability of the membranes in aqueous solutions can be further investigated. 2. Experimental 2.1. Materials Poly(vinyl alcohol) (PVA) was supplied by Shanghai Yuanli Chemical Co. (Shanghai, China). The average degree of polymerization was 1750 ± 50. The other reagents were from domestic chemical reagents company and of analytical grade. Ethanol, toluene, n-hexane and dimethyl formamide (DMF) were kept in molecular sieve before use. Trimethylamine was collected with ethanol to form 1.97 mol/L solution from its 33% aqueous solution. Azobisisobutyronitrile (AIBN) was dissolved in warm methanol (35 ◦ C), recrystallized in an ice bath, and then dried in a vacuum oven at room temperature. The commercial polymeric membrane DF-120 for diffusion dialysis was kindly supplied by Tianwei Membrane Co. Ltd., Shandong of China and used as a comparison membrane. The main properties of the membranes are: ion exchange capacity (IEC) = 1.96 mmol/g; water content (WR ) = 42%; transport number (t) = 0.98; membrane area resistance (Rm ) < 3.0  cm2 ; thickness = 0.32 mm. 2.2. Preparation of the VBC and -MPS copolymer Copolymerization of vinylbenzyl chloride (VBC) and ␥methacryloxypropyl trimethoxy silane (␥-MPS) was carried out by similar procedures as our previous work [15]. The reaction is shown in step 1 of Scheme 1. Under nitrogen atmosphere, VBC (0.04 mol) and ␥-MPS (0.16 mol) in 140 mL toluene were reacted at

97

Table 1 Weights of the starting materials during the preparation of hybrid membranes A–F. Membrane

Poly(VBC-co-␥-MPS) (g) Additional water (g) PVA in 3.33 wt% solution (g)

A

B

C

0.5 0

0.67 0

1 0

D 1.33 5 1

E

F

2 8.4

4 20

70 ◦ C for 24 h, with 1.6 mmol AIBN as initiator. Poly(VBC-co-␥-MPS) was obtained after purification by toluene dissolution/hexane precipitation. Before use, it was kept under low temperature (0–4 ◦ C). 2.3. Preparation of the hybrid membranes Pre-weighed PVA was immersed in water at room temperature for 1 day, then heated to 120 ◦ C at the rate of 10 ◦ C/h and kept at 120 ◦ C for around 4 h. The homogenous and transparent solution (3.33 wt%) was cooled to 60 ◦ C before use. The hybrid membranes were prepared from quaternization and sol–gel process of poly(VBC-co-␥-MPS) in presence of PVA (step 2 and step 3 in Scheme 1). (1) Quaternization step. Poly(VBC-co-␥-MPS) of 24 g was dissolved in DMF to get a concentration of 0.32 g/mL. Then ethanol solution of trimethylamine was added under violent stirring within 10 min to get a homogeneous solution. The molar ratio of VBC in poly(VBC-␥-MPS) to trimethylamine was set at 1:3.6. More ethanol was then added to get a total volume of 240 mL, followed by stirring for 2.5 h. (2) Sol–gel process. The solution from above step was added within ∼40 min into water solution of PVA under violent stirring. Additional water was also added in some cases to keep the homogeneity of the mixture solution. The weight ratios of different starting materials are shown in Table 1. After stirring at 60 ◦ C for 18 h, the solution was cast onto glass plate. Then it was dried at room temperature for 2 days, heated from 50 ◦ C to 130 ◦ C at the rate of 10 ◦ C/h, and kept at 130 ◦ C for 4 h. The ratio of poly(VBC-co-␥-MPS) to PVA was increased, so that membranes A–F were gotten, as shown in Table 1. For comparison, only poly(VBC-co-␥-MPS) membrane without addition of PVA was also prepared from the quaternization and sol–gel procedures of poly(VBC-co-␥-MPS). 2.4. Membrane characterizations Fourier transform infrared spectroscopy (FTIR) of the hybrid membrane samples was recorded using FTIR spectrometer (Vector 22, Bruker) with a resolution of 2 cm−1 and a spectral range of 4000–400 cm−1 . 1 H NMR spectra of poly(VBC-␥-MPS) were recorded on a Bruker DMX-300 NMR instrument at 300 Hz. CDCl3 was used as solvent, and tetramethylsilane as internal standard. 29 Si NMR investigations were performed with an Infinity Plus-300 (Varian Inc., USA) instrument, and the samples were ground into powder before test. Thermogravimetry analysis (TGA) was conducted on a Shimadzu TGA-50H analyzer under air flow, with a heating rate of 10 ◦ C/min. The morphologies of hybrid membranes were observed with a scanning electron microscopy (XT30 ESEMTMP PHILIP). Before observation, the cross-sections of membranes were coated with gold. For determining practical ion exchange capacities (IECp ), dry membranes were accurately weighed and converted to Cl− form in 1.0 M NaCl for 2 days. Excessive NaCl was washed off and then the membrane was immersed in 0.5 M Na2 SO4 for 2 days. Anion exchange capacities were obtained by determining the amount of the exchanged Cl− through titration with 0.1 mol/L AgNO3 .

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C. Wu et al. / Journal of Membrane Science 356 (2010) 96–104

Scheme 1. The synthesis route of poly(VBC-co-␥-MPS) and the hybrid membranes.

Water uptake (WR ) was measured to investigate the hydrophilicities of membranes: the dried membrane samples were weighed and immersed in distilled water at 25 ◦ C for one and a half days. Surfaces of the wet membranes were then carefully dried and the membranes weighed. WR was calculated as the relative weight gain per gram of the dry membrane sample. Swelling degree of the membranes was measured by their swelling behavior in 65 ◦ C hot water. Pre-weighed membrane was immersed in 65 ◦ C hot water, then taken out at different times. The surface was quickly wiped dry with filter paper and the wet membrane weighed. Weight gain per gram of dry membrane was recorded over the time. The tensile properties were measured using an Instron universal tester (Model 1185) at 25 ◦ C with dumbbell shape specimens. The crosshead speed during elongation was 25 mm/min, while the initial gauge length was set as 20 mm. Tensile strength (TS) and elongation at break (Eb ) were recorded.

The membrane electrical resistances (Rm ) were determined by similar method as our previous work [14]. One clip cell and the CS350 of CorrTest were used. The membrane samples were pretreated with 0.5 mol/L NaCl for 24 h, and then inserted into the clip cell (effective area 0.196 cm2 ). The current was supplied by CS350 of CorrTest (Electrochemical workstation, Wuhan, China) with a frequency of 100 kHz. The magnitude of impedance, |Z|, and the phase angle of impedance, , of the membranes were converted into MER ( cm2 ) according to the equation: MER = |Z| cos  × area. Three times of measurement were conducted for each membrane and the average value was calculated. 2.5. Diffusion dialysis Diffusion dialysis (DD) running of the hybrid membranes was carried out using our previous method [19] and briefly summarized here. The commercial DF-120 membrane has already been

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Fig. 1. The 1 H NMR spectrum of poly(VBC-co-␥-MPS).

measured [19] and therefore its data will be directly cited in this work. For the measurement, the membrane sample was in between two-compartment cell, which was maintained by a water bath at different temperatures (20 ◦ C, 30 ◦ C, 40 ◦ C, 50 ◦ C or 60 ◦ C). The feed side of the cell was filled with the mixture solution of HCl–FeCl2 (FeCl2 26.03 g/L, HCl 30.9 g/L), and the permeate side with distilled water. Diffusion was allowed for 45 min and then the concentration of HCl and FeCl2 in feed/permeate solutions was determined by titration. The separation factor (S) with respect to one species over another is given as the ratio of dialysis coefficients (U) of the two species present in the solution. U can be calculated by the following formula: U=

M At C

where M is the amount of component transported in moles, A is the effective area in square meters, t is the time in hours, and C is the logarithm average concentration between the two chambers in moles per cubic meters. C was defined as below:



Cf0 − Cft − Cdt



C = ln





Cf0 − Cdt /Cft



Fig. 2. The FTIR spectra of hybrid membranes A–F.

From Fig. 2(a)–(f), the intensity of –Si–O–Si– (∼1100 cm−1 ) absorption band follows a general increasing trend. This should be ascribed to the increasing silica content in the membranes, since the dosages of poly(VBC-co-␥-MPS) gradually increase from membranes A to F. Solid-state 29 Si NMR spectra of membranes A and F were observed and shown in Fig. 3 to explore further the chemical structure. In the spectra, the chemical shift at −48 ppm, −57 ppm, and −66 ppm are ascribed to mono-, di-, and tri-substituted siloxanes, respectively [20]. For both membranes A and F, the di- and tri-substituted silicates are dominant. Hence the multi-alkoxy silicon (–SiOR) groups from poly(VBC-co-␥-MPS) have undergone crosslinking with –OH groups of PVA or with each other to form inorganic–organic network relatively completely. The peaks at −57 ppm and −66 ppm for membrane F are stronger than those for membrane A. Besides, in membrane F, the relative intensity of −66 ppm peak as compared with the peak at −57 ppm is also stronger. Therefore, silica crosslinking is strengthened in membrane F as poly(GMA-co-␥-MPS) content increases.

where Cf0 and Cft are the feed concentrations at time 0 and t, respec-

tively, and Cdt is the dialysate concentration at time t. 3. Results and discussions 3.1.

1H

NMR, FTIR and 29 Si NMR spectra

1H

NMR spectrum of poly(VBC-␥-MPS) is shown in Fig. 1. The copolymer composition can be estimated from the ratio of the integrated area of the CH2 Cl (4.5 ppm) protons to CH3 (1.0 ppm) and CH2 Si protons (0.6 ppm). The result shows that the molar percent of VBC in the copolymer is 21.7%. For preparation of the hybrid membranes, the –CH2 Cl and Si(OCH3 )3 groups in the copolymer were subject to quaternization and sol–gel reaction in presence of PVA. FTIR spectra of the hybrid membranes are shown in Fig. 2. All the spectra show a large band in the range of 3200–3600 cm−1 , which is mainly due to the stretching vibration of –OH groups from PVA or Si–OH groups. The band at ∼1720 cm−1 is attributed to the carbonyl stretching vibration in ester (C O ), while peaks at ∼1100 cm−1 are characteristic of C( O)–O–C and –Si–O–Si– stretching [13].

Fig. 3. Solid-state 29 Si NMR spectra of hybrid membranes A and F.

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Fig. 4. The IEC and WR values of hybrid membranes A–F in which IECP is the practical IEC, and IECT is the theoretical IEC values.

3.2. Practical ion exchange capacity (IECP ), water content (WR ) and electrical resistances (Rm ) The practical ion exchange capacities (IECP ), together with the theoretical IEC values (IECT ) are shown in Fig. 4. As the content of poly(VBC-co-␥-MPS) increases from membranes A to F, IECP values increase gradually from 0.34 mmol/g to 0.76 mmol/g, while IECT change in the range of 0.35–0.90 mmol/g. The ratios of IECP /IECT are in the range of 77–96%. Therefore, most of the –CH2 Cl groups in poly(VBC-co-␥-MPS) have been successfully transformed to anion exchange quaternary amine (–N+ R3 ) groups. Compared with our previous PVA–SiO2 membranes (IECP 0.52–1.01 mmol/g, IECP /IECT ratio 45–66%) [14], the hybrid membranes here exhibit similar IECP values, and the IECP /IECT ratio has been significantly improved. Improvement should be due to the modification of the preparation routes: previously, PVA and charged alkoxysilane (A-1100(+)) were used as the starting material, with poly(GMA-co-␥-MPS) as the crosslinking agent. The content of poly(GMA-co-␥-MPS) is relatively low and the crosslinking between A-1100(+) and PVA is insufficient. Hence part of the A-1100(+) content may be lost during the IECP measurement [14]. Meanwhile, in our present work, the charged groups (quaternary ammonium) are from the poly(VBC-co-␥-MPS) chains and therefore can remain higher stability after crosslinking with each other or with PVA chains. WR values of the membranes A–F, as shown in Fig. 4, follow a decreasing trend from 53.4% to 31.9%, indicating the gradual decrease of the membrane hydrophilicity as the IECP values increase. This trend is contrary to that for common organic polymer ion exchange membranes, hydrophilicity of which generally increases with increasing of IECP because of the high hydrophilicity of the ion exchange groups (quaternary ammonium, sulfuric acid groups, etc.). The abnormal change of the hydrophilicity of our membranes is quite inspiring for preparation of membranes with high IECP but low swelling behavior. As shown in Scheme 1, poly(VBC-co-␥-MPS) is the source of the silica component as well as the source of ion exchange groups. Silica network from poly(VBC-co-␥-MPS) can crosslink partially with PVA chains through the Si–O–C bonding. Therefore, the number of the highly hydrophilic –OH groups is decreased. More important, the membranes structure becomes more compact and the adsorption of water is restrained, which leads to the decreasing hydrophilicity of the membranes [12].

Fig. 5. TGA diagrams of hybrid membranes A–F.

Membranes A–D have been conducted Rm measurements further. Membranes E and F are more brittle and their surface become rough when immersed in NaCl solution, and hence Rm measurements cannot be well conducted. The Rm values of membranes A–D decrease from 4.77  cm2 , 3.99  cm2 , 3.85  cm2 to 3.36  cm2 , which is in accordance with their gradually increasing IEC values (0.34 mmol/g, 0.39 mmol/g, 0.46 mmol/g and 0.49 mmol/g). In our previous work [14], the membranes with the highest IEC (0.76 mmol/g and 1.01 mmol/g) have Rm values of 3.2  cm2 and 3.5  cm2 . Therefore, the Rm values here are acceptable considering their lower IEC values. 3.3. Thermal stability TGA diagrams are shown in Fig. 5. Initial decomposition temperature (IDT) and thermal degradation temperature (Td , defined as the temperature at 5% weight loss) are determined and shown in Table 2. The IDT values are in the range of 255–275 ◦ C and Td values in the range of 263–283 ◦ C. Compared with PVA or other PVAbased ion exchange hybrid membranes [21,22], our membranes have significantly higher thermal stability. The high thermal stabilities indicate the advantages of the use of poly(VBC-co-␥-MPS). Poly(VBC-co-␥-MPS) contains a long polyolefine main chain and a number of –Si(OR)3 groups. Our previous research has revealed that the membrane from its quaternization and sol–gel processes can have high IDT and Td values (247.2 ◦ C and 291.6 ◦ C) [15]. In the present study, part of the –OH groups of PVA can take part in the sol–gel reaction. Silica network can be formed to induce crosslinking between PVA and the polymer main chain. Therefore, the matrix of the membranes is strengthened to resist relatively high thermal treatment.

Table 2 Initial decomposition temperature (IDT) and thermal decomposition temperature (Td ) of hybrid membranes A–F. Membrane

IDT (◦ C)a Td (◦ C)b

A

B

C

D

E

F

264 272

255 263

275 283

263 270

268 277

264 267

a IDT is the initial decomposition temperature determined from TGA thermograms. b The thermal degradation temperatures (Td ) are defined as the temperature at which the weight loss reaches 5 wt%.

C. Wu et al. / Journal of Membrane Science 356 (2010) 96–104

Fig. 6. The swelling degree of membranes A, C, E, F and poly(VBC-␥-MPS) membrane in 65 ◦ C water. Swelling degree is defined as weight gain per gram of dry membrane.

101

Fig. 7. The fixed group concentration (CR ) of the hybrid membranes A, C, E and F at room temperature and 65 ◦ C.

3.5. Morphology of the hybrid membranes 3.4. Swelling resistance in 65 ◦ C hot water and fixed group concentration (CR ) Since PVA can be readily swelled or even dissolved in hot water, the stability of the membranes in water will be a major concern. The swelling degree defined as weight gain of the membranes (A, C, E and F) in 65 ◦ C water has been collected at different times and results are shown in Fig. 6. For comparison, performances of membrane from only PVA and only poly(VBC-co-␥-MPS) have also been investigated. PVA membrane can be quickly swelled and expanded in 65 ◦ C water and becomes gel-like after around 1 day. The weight change data cannot be easily obtained and hence not shown in Fig. 6. Poly(VBC-co-␥-MPS) membrane, on the other hand, is very stable in 65 ◦ C water. The swelling degree reaches the highest value of 8.9% after 12 h, then remains relatively stable. After 100 h, the swelling degree somewhat decreases, which may be due to dissolve of some unstable components. For the hybrid membranes A, C, E and F, the swelling degree increases rapidly in the initial 48 h to 120%, 130%, 112% and 62%, respectively. Thereafter, the values keep relatively stable in the following 25 days (600 h) and membranes still keep physical integrity after the measurement. These indicate that membranes have long-term stability in 65 ◦ C water, which is quite meaningful for practical applications. As for the different hybrid membranes, the swelling degree generally follows a decreasing trend from membranes A, C, E to F. Therefore, the incorporation of poly(VBC-co-␥-MPS) can effectively hinder the swelling behavior of PVA matrix, which confirms further the WR results in Section 3.2. As the swelling of the membranes occur mostly during the initial 48 h, the swelling degree at 48 h is taken approximately as the WR values of the membranes at 65 ◦ C. Hence the fixed group concentration (CR ), defined as the ratio of IECP to WR , can be calculated. The data are shown together with the CR values at room temperature in Fig. 7. CR generally increases as the silica content increases from membrane A to F. Meanwhile, with an increase in temperature from room temperature to 65 ◦ C, CR of all the membranes decreases significantly (1/3–1/2 of the value at room temperature). According to the previous researches [23,24], the change of CR can affect the DD performances of the membranes distinctively, which will be discussed in the following sections.

Morphologies of the cross-sections of the membranes have been observed through SEM, and the micrographs of membranes A, C and F are shown as examples in Fig. 8. Membrane A is compact and smooth, with no obvious aggregation of different components. Membrane C is generally homogeneous with only sporadic particles of 0.01–0.05 ␮m. Membrane F shows aggregations and particles throughout, indicating the excessive use of poly(VBC-co-␥-MPS) is disadvantageous to membrane homogeneity and may cause phase separation. Nevertheless, phase separation of the membranes here is still much less serious than our previous PVA–SiO2 hybrid membranes, in which large particles (2–8 ␮m) are dispersed [14]. Improvement of the homogeneity is due to the difference of the preparation method: previously, the copolymer, charged alkoxysilane (A-1100(+)) and other small alkoxysilane (EPh) went on pre-sol–gel reaction for one week before mixing with PVA [14]. Therefore, silica-rich aggregations with large size are more likely to be formed. Here in this work, the copolymer went on sol–gel reaction in presence of PVA and no additional small alkoxysilanes have been added. Hence phase separation between the inorganic and organic components is restrained. 3.6. Mechanical properties of the hybrid membranes In our previous work [15], the membranes from poly(VBC-co␥-MPS) are brittle and needs PETEX woven screening fibers (PET fibers) as the support for practical usage. In the present work, all the membranes are free-standing without support because of the presence of highly flexible PVA component. Tensile strength (TS) and elongation at break (Eb ) of the hybrid membranes A–D are measured and the data collected in Table 3. The TS values vary in the range of 24.5–59.2 MPa and Eb in the range of 25.4%–101.2%. Membranes A and B perform better than membranes C and D, as Table 3 The tensile strength (TS) and elongation at break (Eb ) of hybrid membranes A–F. Membrane

TS (MPa) Eb (%)

A

B

C

D

Ea

Fa

46.8 94.7

59.2 101.2

24.5 25.4

34.0 44.8

– –

– –

a Membranes E and F are brittle and not suitable for the testing, and therefore no data can be gotten.

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Fig. 9. Dialysis coefficient of hydrochloric acid (HCl) vs. temperature for hybrid membranes A–D and commercial DF-120 membrane.

ing of the temperature. The increase of UFe of membranes A–D is especially significant when the temperature increases from 50 ◦ C to 60 ◦ C. Among the different hybrid membranes, membrane A is most sensitive to temperature changing. Its UH and UFe values increase sharply when the temperature increases from 50 ◦ C to 60 ◦ C. Membranes C and D with higher poly(VBC-co-␥-MPS) content exhibit relatively lower UH and UFe values all through the different temperatures. As for commercial DF-120 membrane, the increasing trend of its UH and UFe values is the least significant. In our previous work of PPO–SiO2 membranes for DD running, similar increasing trend of UH and UFe values with increasing temperature has been observed [19]. This trend has been explained by the change of the WR and CR values, as summarized in the following [19,24–26]: for the hybrid membranes, the WR values increase, and CR values decrease with increase of the temperature. The higher WR favors the diffusion of ions, whereas the lower CR decreases electrostatic repulsion of the membrane to H+ or Fe2+ . These changing trend, in addition to the elevated diffusivity of the H+ and Fe2+ inside the membrane, leads to the increasing diffusion of both Fe2+ and H+ . Therefore, UH and UFe values increase with temperature in Figs. 9 and 10. Previous research of PVA–SiO2 hybrid membranes has revealed similar phenomenon of expanding of polymer (PVA) chains after incorporation of silane(aminopropyl triethoxysilane). The so-called “size exclusion” effect can increase the transport of small molecules like water and alcohol [27]. Meanwhile, commerFig. 8. The SEM graphs of hybrid membrane A (a), membrane C (b) and membrane F (c).

reflected by their higher TS and Eb values. Therefore, membrane strength and flexibility decrease with higher poly(VBC-co-␥-MPS) content. Flexibility of membranes E and F is decreased further, so that no dumbbell shape samples can be gotten for tensile properties measurement. 3.7. Diffusion dialysis results 3.7.1. H+ and Fe2+ dialysis coefficients (UH and UFe ) Hybrid membranes A–D have been used for the DD tests at different temperatures (20–60 ◦ C). The membranes E and F are more brittle and DD measurements cannot be well conducted. Figs. 9 and 10 show the effects of different temperatures on the acid (H+ ) and ferrous (Fe2+ ) dialysis coefficients (UH and UFe ) of the membranes. For comparison, data from our previous work [19] about commercial DF-120 membrane are also shown. UH and UFe values of the membranes generally increase with the increas-

Fig. 10. Dialysis coefficient of ferrous chloride (FeCl2 ) vs. temperature for hybrid membranes A–D and commercial DF-120 membrane.

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Fig. 11. Separation factor (S) vs. temperature for hybrid membranes A–D and commercial DF-120 membrane.

cial DF-120 membrane matrix is based on PPO and should be less sensitive to temperature of the solution [19,24]. Hence the increasing trend of Fe2+ and H+ diffusion is less significant. As for the difference of membranes A–D in UH and UFe changing, the silica network should play a vital role. Membrane A has the least poly(VBC-co-␥-MPS) content and the silica crosslinking with PVA is the least. Therefore, the swelling at higher temperature is the most serious and the transport of H+ or Fe2+ would be the most facilitated. Membranes C and D, on the opposite, have more compact structure and can resist the transport of H+ or Fe2+ more. Their UH and UFe values increase more slowly than the other membranes. For membrane D, the UH value even decreases as temperature increases from 50 ◦ C to 60 ◦ C, while the UFe value increases. The abnormity should be at least partially due to the competition between Fe2+ and H+ transport in the membranes: Fe2+ has bigger size and higher valence state than H+ . Hence, the structure loosening and WR increasing of membrane D has more significant influence on Fe2+ transport, inducing higher UFe value at 60 ◦ C than 50 ◦ C. Transport of H+ is suppressed accordingly, leading to lower UH value. 3.7.2. Separation factors (S) Separation factors (S) are the ratio of UH to UFe . The S values of hybrid membranes A–D at different temperatures are shown in Fig. 11. Data of commercial DF-120 membrane are cited from our previous work [19] and also shown. S values of the hybrid membranes A–D are in the range of 12.1–35.7, while the values of membrane DF-120 are in the range of 17.4–32.0. Therefore, the hybrid membranes can show comparable acid recovery capability, despite the much lower IECP values (only ∼1/5–2/5 IECP values of DF-120). Presence of the large amount of –OH groups from PVA and the multi-alkoxy silicon (–SiOR) should be responsible for the facilitated transport of the H+ ions [16]. This point would be quite meaningful for further development of PVA-based DD membranes for acid recovery. Compared with PPO, PVA is very low cost and common polymer, hence a cheaper anion exchange membrane can be expected from the idea of this work. Effect of the temperature change on the S values of the hybrid membranes A–D are as following: the values increase first from 20 ◦ C to 30 ◦ C, then follow a slight decreasing trend from 30 ◦ C to 50 ◦ C. As the temperature increases further to 60 ◦ C, S values of all the A–D membranes (especially membrane D) decrease significantly. Among the different membranes, membranes A–C exhibit similar S values at all the temperatures. Membrane D has the high-

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est S values at 20–50 ◦ C. However, at 60 ◦ C, its S value decreases to the lowest. In our previous work of PPO–SiO2 membranes, the influence of the temperature and the inorganic silica content on the membrane structure has been utilized for explanation of the S values changing trend [19]. Similar influence can also be found in our membranes: there are organic (PVA) and inorganic (silica from poly(VBC-co-␥MPS)) components in the membranes. The interphase between the different phases can be more expanded as silica content increases from membrane A to D, or as the temperature increases from 20 ◦ C to 30 ◦ C. Hence, the transport of H+ and Fe2+ can be both facilitated. H+ is more facilitated due to its smaller size, lower valence state and weaker interaction with the membrane, which results into increasing S values. Nevertheless, when temperature increases further (higher than 30 ◦ C, especially 65 ◦ C), interphase between PVA and silica is loosened and expanded to a great extent. Therefore, hindrance for Fe2+ diffusion is weak and priority of H+ diffusion is reduced, which leads to a gradual decrease of the S value. The effect of temperature is the most significant for membrane D with the highest silica content. Especially, as the temperature increases from 50 ◦ C to 60 ◦ C, the increase of Fe2+ diffusion and suppress of H+ diffusion (as reflected by UH and UFe values in Section 3.7.1) leads to the lowest S value.

4. Conclusions Anion exchange hybrid membranes are prepared from quaternization and sol–gel reaction of poly(vinylbenzyl chloride␥-methacryloxypropyl trimethoxy silane) (poly(VBC-co-␥-MPS)) with poly(vinyl alcohol) (PVA). The practical anion exchange capacities (IECP ) of the membranes are in the range of 0.34–0.76 mmol/g. Water content gradually decreases from 53.4% to 31.9% as the membrane IECP increases. This trend is in contrary to common organic polymer ion exchange membranes and valuable for development of membranes with high IECP but low swelling property. The membranes have good thermal stabilities and the Td values are in the range of 263–283 ◦ C. All the membranes can remain integrity in 65 ◦ C hot water for more than 25 days. The swelling degree is decreased as the poly(VBC-co-␥-MPS) increases. Diffusion dialysis (DD) running of the hybrid membranes is conducted at 20–60 ◦ C for hydrochloric acid (HCl) recovery from the mixture of HCl and ferrous chloride (FeCl2 ). Acid dialysis coefficient (UH ) and ferrous dialysis coefficient (UFe ) generally increase with increase of the temperature, which is mainly due to increase of WR , decrease of fixed group concentration (CR ) of the membranes, and elevated diffusivity of the ions. The separation factors (S) increase as the temperature increases from 20 ◦ C to 30 ◦ C, and keep balanceable at 30–50 ◦ C. The S values are comparable to those of commercial DF-120 and reported PPO–SiO2 membranes with higher IEC values, indicating the advantages of PVA-based hybrid membranes in DD process.

Acknowledgements This project was supported in part by the National Science Foundation of China (Nos. 20974106 and 20636050), Specialized Research Fund for Doctors of Hefei University of Technology (No. GDBJ2009-040), the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KSCX2-YW-G-075-25), Specialized Research Fund for the Doctoral Program of Higher Education (No. 200803580015), the National Basic Research Program of China (973 program, No. 2009CB623403) and the Significant and Key Provincial Programs of Anhui Province for Universities Science Research (Nos. ZD2008002, KJ2009A003 and KJ2010A265).

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