Article pubs.acs.org/IECR
Study on Acid/Base Formation by Using Sulfonated Polyether Ether Ketone/Aminated Polysulfone Bipolar Membranes in Water Splitting Electrodialysis Se Hwan Kwon and Ji Won Rhim* Department of New Materials and Chemical Engineering, Hannam University 1646 Yuseongdae-ro, Yuseong-gu, Daejeon 34054, Korea S Supporting Information *
ABSTRACT: The sulfonated polyether ether ketone (SPEEK) as a cation exchange polymer and aminated polysulfone (APSf) as an anion exchange membrane (AEM) were synthesized to prepare bipolar membranes (BPMs) through hot pressing. The water splitting electrodialysis (WSED) experiments were conducted to produce acids and bases by using the resulting SPEEK/ APSf BPM. For comparative performance evaluation of the commercialized ASTOM membrane and the newly synthesized membranes in our lab, the acid/base production rate and the voltage changes according to the constant current were measured. The acid and base formation rates of the commercialized membrane from ASTOM were 2.3 and 2.7 g/L/h, respectively, whereas the rates of the membranes we developed were 1.0−2.4 and 1.5−2.7 g/L/h, respectively, depending on the amine contents in the AEM. The performance of the synthesized BPM in our lab was equivalent or higher than that of the commercialized membrane from the viewpoint of the acid/base production rates. Tongwen et al.6 developed BPMs consisting of sulfonated and aminated polyphenylene oxide for the purpose of concentrating citric acid. It was found that the citric acid increased from 5 to 30 g/L. Peng et al.,7 used FeCl3 and palygorskite to form a catalytic layer inside a bipolar membrane to reduce the voltage depending on the constant current. It was concluded that the resistance of a bipolar membrane was lower in the combined use of FeCl3 and palygorskite than a single use of FeCl3. In the study of Yang et al.,8 NaCl and sodium sulfate were supplied to produce acids at current densities ranging from 34 to 57 mA/cm2. It was shown that the time required to produce acids was reduced as the current density increased. Additionally, it was suggested that WSED process can be used for pretreatment of seawater. And, for restoration and reproduction of waste acid or wastewater, the applicability of the WSED process has been investigated.9−18 In this study, the ratio of trimethylamine (TMA) to polysulfone (PSf) was increased from 1:1 to 2:1, 3:1, and 4:1 to synthesize 4 types of the anion-exchange membrane (AEM), and the polyether ether ketone (PEEK) was sulfonated to make the cation-exchange membrane (CEM). The BPMs were prepared by using hot pressing at 120 °C. The commercial membrane of ASTOM Corp. from Japan was used to compare the WSED performance of our synthesized membranes. The acid/base outputs, subsequent pH levels over time, and the voltage depending on the constant current as well were measured.
1. INTRODUCTION The current water splitting electrodialysis (WSED) process is widely used to wash organic and inorganic substances in chemical and biochemical industries. A WSED process has a few advantages in comparison with the electrodialysis process, of which it is easy to scale up and consumes less energy than that of the electrodialysis process. The WSED process, which uses bipolar membranes (BPMs), has been recently and consistently developed due to stability and environmentally friendly acid and base production.1,2 During the WSED process, when the cation exchange membrane (CEM) of a bipolar membrane is heading for a cathode and the anion exchange membrane (AEM) is heading toward an anode, water molecules break down into hydrogen ions and hydroxyl ions.3 Depending on the circulation of the solution, cations and anions selectively pass through their respective ion exchange membranes and, therefore, the concentration of acids and bases increase as time passes.4 The higher the ion exchange capacity (IEC) and ionic conductivity, the better the materials of the ion exchange membrane used for WSED need to be. However, the durability of a membrane must be preferentially considered because strong acids and bases may affect the stability of each ion-exchange membrane.5 Typical examples where the WSED process is applied include a highly concentrated sodium hypochlorite generating device, producing sodium hypochlorite for disinfection, and washing of ballast water for cargo shipment. If these chemicals were to be produced by equipment through WSED, instead of installing and storing HCl, NaOH, and chlorine gas in a tank, a spill of harmful substances can be prevented and human resources and costs required for facility maintenance can be reduced. Hence, it becomes critically important to note the applicability and the significance of the WSED. © 2016 American Chemical Society
Received: Revised: Accepted: Published: 2128
August 25, 2015 October 22, 2015 February 1, 2016 February 1, 2016 DOI: 10.1021/acs.iecr.5b03137 Ind. Eng. Chem. Res. 2016, 55, 2128−2133
Article
Industrial & Engineering Chemistry Research
2. EXPERIMENTAL PROCEDURE 2.1. Materials. PEEK 450F and Udel PSf were provided from Victrex and from Solvay Plastics, respectively. Methanol, dichloroethane (DCE), tin chloride (SnCl2), 1-methyl-2pyrrolidinone (NMP, 99.5%), and dimethylacetamide (DMAc) were purchased from Sigma-Aldrich Co. (Milwakee). Sulfuric acid, chlorotrimethylsilane, and trimethylamine (TMA) were purchased from Junsei Co., Japan, and chloromethyl ethyl ether (CMEE) was provided from Kanto, Japan. Hydrochloric acid (HCl, 35%), sodium hydroxide (NaOH), and sodium chloride (NaCl, 99.5%) were obtained from OCI, Korea. All reagents and solvents were used without further purification. 2.2. Membrane Preparation. 2.2.1. Cation-Exchange Membrane.19 Twenty grams of PEEK polymer was added to 400 mL of sulfuric acid and stirred vigorously in a thermostatic bath at 35 °C for 40 h. The solution was washed several times with distilled water at 10 °C until the pH level increased to 6− 7. Then, the solution was dried at 100 °C for 24 h until solidified. To produce a solution of SPEEK with a 10 wt %, the SPEEK polymers were dissolved in NMP and stirred for 12 h. Finally, the solution was cast onto glass plates, dried at 70 °C for 4 h, and stored in distilled water before use (Figure 1A). 2.2.2. Anion-Exchange Membrane. Aminated polysulfone (APSf) is produced through a two-step reaction: (1) in the presence of SnCl2, a Friedel−Crafts catalyst, the chloromethylation reaction between PSf and CMEE progresses then (2) amination of chloromethylated PSf is facilitated by the addition of TMA.20 Additionally, 20 g of PSf polymer in 180 g of DCE was dissolved to make a 10% solution. Next, 14.56 g of CMEE and 2 g of SnCl2 as a catalyst were added and stirred at 40 °C for 4 h for chloromethylation. The solution was washed several times with methanol and dried in a forced convection oven at 70 °C for 24 h. The dried PSf was then dissolved in 100 mL of DMAc to make an 8 wt % solution and was stirred for 4 h. Next, 13.3 g of TMA was added and stirred for 8 h. Finally, the aminated PSf solution was cast onto a glass plate after filtering with a 10 μm pore Teflon membrane, dried at 40 °C, and then stored in distilled water (Figure 1B). 2.2.3. Characterization of Synthesized Polymers. To characterize the electric and physical properties of the synthesized SPEEK and APSf, the swelling degree, ionexchange capacity, and ion conductivity were measured. Measurement procedures followed previously adopted standards.21,22 2.2.4. Bipolar Membranes. The surface of the CEM was sanded in a bipolar membrane uniformly. Then we immersed it in a 2 wt % FeCl3 solution which was treated as a catalyst, to decrease electric resistance. Next, it was washed with distilled water. After the moisture on the surfaces of both CEM (SPEEK) and the AEM (APSf) was removed, the dried membranes were prepared by using a hot press. At this time, the temperature of the top and bottom plates of the hot press was set at 120 °C and then the resulting membranes were stored in distilled water for further use. Typically, 13 × 8 cm membranes with a thickness of about 180 μm were prepared for WSED experiments. 2.3. WSED Experiments. The cell used in this study consisted of CEM, BPM, AEM, and arrayed in the order of CEM, BPM, AEM, and CEM. Each membrane was 13 × 8 cm, and a 2 mm silicon plate was inserted between each membrane to take the role of the flow channel. It was designed that the reaction occurred in the flow channel which had a volume of 5
Figure 1. Synthesis of ion-exchange polymers of (A) SPEEK and (B) APSf.
cm3 once the feed was injected in the cell. A solution of 0.2 N HCl, 2 N NaOH, and 5 wt % NaOH as the feed were circulated, and 3 wt % NaCl solution was also fed into the cell continuously. In detail, as shown in Figure 2, the 0.2 N NaOH solution was inserted into the gap (2 mm) between the CEM and the bipolar membrane while the 0.2 N HCl was added between the bipolar membrane and the AEM. During the operation, Na+ and OH− were produced from the CEM and BPM surfaces, respectively, by the contact with the 0.2 N NaOH solution; concomitantly, H+ and Cl− formed from the surfaces of the BPM and AEM from contact with the 0.2 N HCl solution. A 3.5 wt % NaCl solution was constantly supplied between the AEM and the CEM, and electrode water (NaOH 5 wt %) was continuously circulated inside the cell as well. Then, the solution was collected every hour from the chamber where the acids and bases were created, and the amount and concentrations of these concentrated acids and bases were measured and analyzed. The flow rate of all supplied fluids was 2129
DOI: 10.1021/acs.iecr.5b03137 Ind. Eng. Chem. Res. 2016, 55, 2128−2133
Article
Industrial & Engineering Chemistry Research
Figure 2. Schematic diagram of the overall experimental apparatus and the internal structure of the cell.
fixed at 20 mL/min and the current was also set at 0.5 A. All experimental results were collected under the steady state.
Table 1. IEC and Ion Conductivity of Synthesized IonExchange Membranes and Commercialized Membranes
3. RESULTS AND DISCUSSION The production performance of acid and base by WSED may be dependent on the number of exchange sites in the ionexchange polymers. Also, the ion-exchange membrane is known to be very hydrophilic because of hydrophilic ionic groups such as −SO3−, −COO−, and −PO32− at the CEM and −NH3+, −NRH2+, −NR2H+, −NR3+, and −PR3+ at the AEM. These hydrophilic functional groups stimulate dimensional changes in the ion-exchange membranes, so that the intramolecular spaces become wide enough for ions to pass into the membrane more freely at a certain desired adsorption time, depending on the IEC values. A note of caution is that the introduction of more exchange sites to polymer backbones may lead to undesirable breakage of the chain. In other words, lowering the molecular weight during the substitution reaction with the sulfuric acid, amine group, or other compound, may lead to chain breakage. Typical basic properties of the synthesized ion polymers and commercial membranes used for WSED are shown in Table 1. To investigate the effect of IEC on the formation of acids and bases, the IEC of the anion-exchange polymer, PSf, was controlled rather than that of the cation-exchange polymer, PEEK. The final proton conductivity of SPEEK was synthesized to be the same value of typical value of the Nafion membrane which is being widely used in various industries. On the other hand, for AEM, APSf was synthesized to have its ion conductivity range from 0.02 to 0.14 mequiv/g, of which the last value is close to that of the SPEEK’s. However, the ionexchange capacities of both SPEEK and APSf 4:1 were shown to have almost the same value of 2.1 or 2.2 mequiv/g. Conversely, for the commercial AEM and CEM, IEC values are a little bit lower than those of the synthesized membranes. Moreover, the manufacturer provided the area resistance
ion-exchange membranes
IEC (meq/g)
ion conductivity (S/cm) [area resistance (Ω cm2)]
SPEEKa APSf 1:1a APSf 2:1a APSf 3:1a APSf 4:1a ASTOM CEMb ASTOM AEMb
2.2 0.8 1.3 1.6 2.1 1.8 1.7
0.10 0.02 0.07 0.12 0.14 (1.8−2.3) (2.0−2.5)
a
Membrane thickness: 60−62 μm. bAdapted from the data sheet provided by manufacturer.
instead of the ion conductivity, and the values seem low so as to show very high ion conductivity. First, the commercial ASTOM membranes were investigated for performance for the production of the acid/base in the WSED process (Figure 3). The formation rates of HCl and NaOH for the duration of the experiment were 2.3 and 2.7 g/ L/h, respectively. The ratio of TMA to the PSf polymer was increased from 1:1 to 2:1, 3:1, and 4:1 to prepare the several grades of the bipolar membrane. The acid/base production in the test is shown in Figure 4 using these bipolar membranes. The results show that, the formation rates of HCl were 1.0, 1.3, 1.7, and 2.4 g/L/h when APSf membranes with the ratio of 1:1, 2:1, 3:1, and 4:1 were used, respectively. The formation rates of NaOH were shown to be 1.5, 1.7, 2.2, and 2.7 g/L/h as APSf membranes of the amination ratio increased correspondingly from 1:1 through 4:1. At lower amination ratio, the production rates are much lower than those of the commercialized membrane because the electrical properties are lower than those of the commercial membrane. The rate at the ratio of 4:1 was almost equivalent to the commercial membrane. 2130
DOI: 10.1021/acs.iecr.5b03137 Ind. Eng. Chem. Res. 2016, 55, 2128−2133
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Figure 3. Acid/base production concentration as a function of the operating time for the commercialized membrane of ASTOM, Japan (flow rate: 20 mL/min, current: 0.5 A at 25 °C).
Figure 5. Comparison of changes in voltage as a function of the operating time for the commercialized membrane of ASTOM and the self-prepared APSf (3:1)/SPPEK bipolar membrane (flow rate: 20 mL/min, current: 0.5 A at 25 °C).
increased resistance. Therefore, as the operating time elapses, the voltage increases, even though it appears stable. Another possible reason is that electrolytes, such as hydrogen ions and hydroxyl ions, increase due to water splitting in the intermediate layer of the bipolar membrane, so the electric resistance of the membrane increases and the voltage tends to rise over time.2 As seen in Figure 5, the voltage of the test membrane is higher than that of the commercial membrane. It may be due to the electrical properties used in this experiment such as IEC and ion conductivity being lower than those of the commercialized membrane. Because the lower electrical properties may cause more resistance, the voltage of the synthesized membrane becomes higher than that of the commercial membrane. As shown in Figure 6, the pH level of the NaCl solution changed from neutral to acidic as a result of the changes in pH level in each solution. The pH rise in the NaCl stream is because hydrogen ions are delivered to the NaCl solution by the diffusion of acids through the AEM according to the Grotthuss mechanism.23 The Grotthuss mechanism is a series of proton transfer reactions where protons are not transferred by diffusion but delivered along hydrogen bonds of the water molecule.23−25 This process generally arises through tunneling, and therefore protons can be easily transferred over long distance in the liquid form at low temperature.23 The results showed the same trends between the ASTOM and the selfprepared bipolar membrane. Neutral and acidic are adjective forms, “over” long distance.
Figure 4. Acid/base production concentration as a function of the amination degree for the APSf and SPEEK BPMs (flow rate: 20 mL/ min, current: 0.5 A at 25 °C).
Figure 5 shows the change in voltage depending on the constant current of the membrane for the commercialized and the self-prepared bipolar membranes. The voltage of the commercialized membrane started at 4.6 V and then stabilized near 5 V over the next several hours. On the contrary, the voltage of the self-prepared membrane started at 5.7 V and consistently increased from its initial value up to 6 V where it stabilized. The voltage and current are affected by the concentration and operating temperature. As the operating time transpires, the proton and hydroxyl ions increase; as a result, the transport of ions is more difficult because of 2131
DOI: 10.1021/acs.iecr.5b03137 Ind. Eng. Chem. Res. 2016, 55, 2128−2133
Industrial & Engineering Chemistry Research
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +82-42-629-8839. Fax: +82-42-629-8835. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This research was financed by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resources Training Project for Regional Innovation (Grant 2013H1B8A2032261).
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Figure 6. Changes of pH in each solution (HNU: self-prepared BPM, SPEEK/APSf 4:1).
4. CONCLUSION In this study, four types of APSf as AEMs according to the ratio of TMA to PSf and SPEEK as CEM were synthesized and then BPMs were prepared by hot pressing. The resulting BPMs were applied to yield acid/base production for the WSED process. To compare the performance of acid/base production, the commercial ASTOM membrane was investigated. IEC and ion conductivity of the synthesized SPEEK were 2.2 mequiv/g and 0.10 Ω/cm. Aditionally, 4 types of APSf as AEM were synthesized according to the amination degree, where IEC and ion conductivity ranged from 0.8−2.1 mequiv/g and 0.02− 0.14 Ω/cm, respectively. (1) The acid/base production rates for the commercial BPM were shown to be 2.3 and 2.7 g/L/h, respectively, whereas those of the self-prepared APSf/SPEEK were observed at 1.5, 1.7, 2.2, and 2.7 g/L/h, depending on the amination degree from 1:1 through 4:1. The performance of the synthesized BPM in our lab was equivalent or higher than that of the commercialized membrane from the viewpoint of the acid/base production rates. (2) The voltage at the constant current density of 0.5 A started at 5.7 V and gradually increased and stabilized at 6 V for the self-prepared BPM while the voltage for the commercialized BPM started at 4.6 V and stabilized near 5 V during the experiment. From this result, more energy consumption for the self-prepared BPM may be expected than for the commercialized BPM. (3) Finally, in order to prepare more durable, stable, and low-resistant BPMs, the surface fluorination and reinforcement with mesh must be considered in the next study.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03137. Diagram of production of HCl and NaOH using bipolar membranes by the water splitting electrodialysis process (PDF) 2132
DOI: 10.1021/acs.iecr.5b03137 Ind. Eng. Chem. Res. 2016, 55, 2128−2133
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DOI: 10.1021/acs.iecr.5b03137 Ind. Eng. Chem. Res. 2016, 55, 2128−2133
