Permeability of Small Alcohols through ... - ACS Publications

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Permeability of Small Alcohols through Commercial Ion-Exchange Membranes Used in Electrodialysis Thomas Rottiers,† Bart Van der Bruggen,‡,§ and Luc Pinoy*,† †

Department of Chemical Engineering, Cluster Sustainable Chemical Process Technology, KU Leuven @ Technology campus Ghent, Gebroeders Desmetstraat 1, B-9000 Ghent, Belgium ‡ Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium § Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680, Pretoria, Gauteng 0001, South Africa ABSTRACT: The use of solvent mixtures composed of water and organic cosolvents enables electrodialysis to remove poor water-soluble organic compounds from solutions. The flux of organic cosolvents forms the main problem for these applications because of changing cosolvent concentrations. The present paper investigates the permeability of small linear alcohols, namely, methanol, ethanol, and 1-propanol, through commercial ion-exchange membranes. The membrane permeability decreases with increasing molecular size of the cosolvent. This indicates that the transport is limited by steric hindrance in the membrane. However, the use of 1-propanol leads to a high pressure drop and a high permeability in certain membranes. In addition, the effect of operating parameters on the cosolvent permeability was tested. A change in current density has only a minor influence on the cosolvent flux. The flux of organic cosolvents was found to be mainly determined by the concentration gradient of alcohol across the membrane. The cosolvent permeability decreases with an increase in the concentration gradient. A screening of several cation and anion-exchange membranes from PCA GmbH and Astom Corporation was performed and showed that all membranes tested have a lower permeability than Nafion membranes.

1. INTRODUCTION Electrodialysis is conventionally used as a technology to desalinate aqueous solutions.1 The potential of electrodialysis, however, may be substantially broader. The use of ion-exchange membranes in mixtures of organic cosolvents and water and, more specifically, in water and methanol mixtures is of interest in the use of direct methanol fuel cells2−7 and electrodialysis of slightly soluble organic compounds.8−11 Various fundamental studies have been conducted on membrane properties in the presence of organic cosolvents.12−16 For example, the physicochemical properties of two cation-exchange membranes in different methanol−water mixtures at different temperatures were studied by Chaabane et al.12 It was found that a sufficiently cross-linked ion-exchange membrane is only weakly dimensionally influenced by the methanol concentration. The same authors also investigated the change in conductivity of two cation-exchange membranes in relation to the methanol content and the lithium chloride concentration.13 It was found that the conductivity of a membrane that shows a strong swelling effect in the presence of methanol decreases much less than a membrane with a lower effect of (methanol) swelling due to methanol. The flux of organic cosolvents through ion-exchange membranes is in most cases an unwanted phenomenon and constitutes a problem for these applications. The transport of © 2016 American Chemical Society

solvent through ion-exchange membranes is caused by two phenomena. Electro-osmosis is the transport of solvent in the hydration shell of the transported ions.17 The second type of solvent transport originates from a concentration difference across the membrane; a water flux is caused by osmosis while a cosolvent flux is caused by diffusion.5 In the application of ion-exchange membranes in methanol fuel cells, the methanol crossover leads to a decrease in fuel cell voltage and efficiency due to the oxidation of methanol.2 As a consequence, the methanol flux in Nafion membranes is widely ́ et al. investigated in the literature.2−4,15,18−24 Garcia-Villaluenga investigated the osmotic transport of methanol−water electrolyte solutions.18 The results show that the osmotic volumetric flux decreases as a function of the methanol concentration and reaches a minimum at a methanol fraction of approximately 50 vol %. At higher methanol concentrations, an increase in volumetric flux was observed. Barragán et al. studied the electro-osmotic transport through a Nafion membrane.3 The influence of the methanol concentration on electro-osmotic transport at different KCl concentrations was investigated. The Received: Revised: Accepted: Published: 8215

May 18, 2016 July 6, 2016 July 7, 2016 July 7, 2016 DOI: 10.1021/acs.iecr.6b01915 Ind. Eng. Chem. Res. 2016, 55, 8215−8224

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Industrial & Engineering Chemistry Research results show that the electro-osmotic flow is predominantly determined by the methanol concentration. A second application of ion-exchange membranes in mixtures of organic cosolvents and water is the production of chemicals. Several studies have shown that the use of mixtures of organic cosolvents and water in electrodialysis8,9,25 and bipolar membrane electrodialysis10,11 is promising. For example, Xu et al. studied the behavior of several cation-exchange membranes in organic media. Subsequently, the optimal type of membrane was applied for the conversion of linear organic acid salts, with 7 to 15 carbon atoms, into their corresponding acids.8 Kameche et al. demonstrated the use of electrodialysis in water ethanol mixtures up to 50 vol % to convert the sodium salts of several organic acids ranging from propionate to octanoate without a significant increase in energy consumption.9 It can be concluded from the literature that the use of organic solvents or mixtures of organic cosolvents in water in electrodialysis offers different assets compared to aqueous solutions. More specifically, acidification of organic compounds by means of electrodialysis in water is not favored because of the low solubility of many organic acids, such as salicylic acid.26,27 In this view, mixtures of water and organic cosolvents can be used to increase the solubility of many organic compounds. However, the crossover of organic cosolvents leads to a change in cosolvent concentration and an increase in electrical resistance and sequentially energy costs. Therefore, the crossover limits the use of organic cosolvents for the production of chemicals by means of electrodialysis. This study aims to examine the transport of small alcohols through ionexchange membranes. To accomplish this purpose, the permeability of different organic cosolvents, namely, methanol, ethanol, and 1-propanol, is compared. Moreover, the influence of the current density and cosolvent concentration on the cosolvent permeability is investigated. In addition, the cosolvent flux and cosolvent permeability in different types of commercial cation and anion-exchange membranes are compared. To make an estimation of the methanol and ethanol blocking capabilities of the commercial membranes tested, the obtained results are compared with methanol permeabilities of Nafion membranes reported in the literature.

2.2. Experimental Setup. In all experiments, the alcohol flux was measured in a metathesis configuration. This setup consisted of two different compartments as the repeating unit. Two repeating units were used in one stack. One compartment contained a solvent mixture of water and an organic cosolvent (methanol, ethanol, or 1-propanol). The other compartment initially contained only water as the solvent. In all experiments, the cosolvent rich compartment contained a 0.5 mol L−1 salt solution, and the cosolvent free compartment contained a 1 mol L−1 salt solution. Unless otherwise mentioned, NaCl was used as the salt. The initial volume of all salt solutions was 0.5 L. To separate the stack configuration from the electrode rinsing solution, PC-SC membranes were used in every experiment. Before an experiment, membranes were placed overnight in a bath with an equal methanol, ethanol, or 1propanol concentration, as in the experiment. The organic cosolvent flux was measured as the increase in cosolvent concentration in the initial cosolvent free compartment. Every experiment was performed during 60 min. Samples of all vessels were taken in the beginning and after every time interval of 20 min. In the first part of this study, the influence of the current density and cosolvent gradient on the cosolvent flux of methanol and ethanol was investigated. This is done to determine which cosolvent and operating parameters are preferentially used to minimize the cosolvent flux in practical applications. These experiments were performed with either PC-SK or PC-SA membranes. These membranes are standard cation-exchange and anion-exchange membranes supplied by PCA GmbH. Either sulfonic acid or quaternary amines were used as functional groups. Polyester was used as a reinforcement material.28 An overview of the experimental conditions is shown in Table 1. Two levels of each variable were used. The Table 1. Overview of the Experimental Conditions in the First Set of Experiments low level current density [A m−2] cosolvent concentration [mol %] cosolvent

2. MATERIALS AND METHODS 2.1. Electrodialysis Equipment. Experiments were carried out with a PCCell ED 64-002 stack manufactured by PCCell GmbH, Heusweiler, Germany. Each membrane had an active surface area of 0.0064 m2; all membranes were separated by silicon/polypropylene spacers with a thickness of 0.5 mm. Spacers were supplied by PCA (PolymerChemie Altmeier GmbH, Heusweiler, Germany). The setup consisted of three vessels for two salt streams and the electrode rinsing solution. All experiments were carried out in batch mode. The flow rate of the two salt streams was 6 L h−1 per compartment, and the flow rate of the electrode rinsing solution was 150 L h−1. The flow rates correspond to a linear flow velocity of 4.2 × 10−2 m s−1 for the two salt solutions and 5.2 × 10−1 m s−1 for the electrode rinsing solution. A 1 mol L−1 sodium sulphamate solution was used as an electrode rinsing solution. This aqueous solution was renewed in every experiment to avoid contamination by alcohols. During each experiment, a constant current density was applied to achieve desalination. The current was set by using a DC adjustable power source (Delta Elektronika ES030-10, Zierikzee, The Netherlands).

150 10 methanol

intermediate level

high level

325 20

500 30 ethanol

(salt free) concentration of organic cosolvent was either 10 or 30 mol %, and the applied current density was chosen at 150 or 500 A m−2. One experiment with intermediate levels of both variables (20 mol % organic cosolvent and 325 A m−2) was performed and replicated once to test the repeatability of the results. In another set of experiments, the possibility of using 1propanol as cosolvent is investigated. The permeability of 1propanol is compared with the results of the previous experiments, where the permeability of both methanol and ethanol is determined. Other cosolvents such as 2-propanol were not investigated because 2-propanol is insoluble in salt solutions, such as an aqueous NaCl solution. These experiments were also performed with either PC-SK or PC-SA membranes. Furthermore, the experiments were performed at a cosolvent concentration of 10 mol % and a current density of 500 A m−2 and replicated once to determine the repeatability of the results obtained. Subsequently, the influence of the migration of a different cation on the permeability of methanol and ethanol was 8216

DOI: 10.1021/acs.iecr.6b01915 Ind. Eng. Chem. Res. 2016, 55, 8215−8224

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Industrial & Engineering Chemistry Research

measured. This time interval at the end of the experiment was chosen to allow the solvent composition in the membranes to reach a steady state and to obtain a constant voltage difference. This voltage drop is the sum of the voltage drop over the stack and the electrode rinsing solution. The voltage drop over the electrode rinsing compartment was measured during a separate experiment. In this experiment, the stack consisted of a single PC-SC membrane. This value was then subtracted from the total voltage drop over the stack. This yields the voltage drop over the stack. During all experiments, no significant pH changes occurred and thus the limiting current density was not exceeded. As a result, the experiments were performed in the Ohmic region. The stack resistance was calculated by the ratio of the voltage difference and the current. The permeability of cosolvents was calculated by eq 1,30 with j being the organic cosolvent flux [mol s−1 m−2], P being the permeability of the membrane [m2 s−1], L being the thickness of the membrane [m], and Δccosolvent being the organic cosolvent gradient across the membrane [mol m−3]. In this formula, several assumptions were made, namely, the absence of pressure differences; it was also assumed that the current density has no significant effect on the organic cosolvent flux and that there are no convective mass transfer phenomena such as concentration polarization. If the concentration of co-organic solvent is measured in a short time interval and if the concentration in the receiving compartment is not too high, a linear increase in cosolvent concentration as a function of time is obtained. Thus, the permeability can be calculated using eq 1.

investigated in PC-SK membranes. In view of the possible production of chemicals, the substitution of cations was previously found to be the most suitable configuration.10,11 Therefore, only the effect of the migration of cations was tested. As cation, Na+, K+, Ca2+, or H+ was used. These experiments were performed at a cosolvent concentration of 10 and 30 mol % and a current density of 500 A m−2. The experiment at a cosolvent concentration of 10 mol % was replicated once. In the last part of this study, the cosolvent permeability of different commercial ion-exchange membranes was compared. This was done to screen and select commercial membranes with low cosolvent permeability. As cosolvent, either methanol or ethanol was used. The cosolvent flux was measured at a cosolvent gradient of 10 and 30 mol %. The experiment at a cosolvent concentration of 10 mol % was replicated once. In every experiment, the current density was 500 A m−2. An overview of the characteristics of the membranes used is shown in Table 2. All membranes are nonporous ion-exchange Table 2. Overview of the Properties of the Used IonExchange Membranes28,29 membrane

PC-SK

PCMVK

Neosepta CMX

Neosepta CMS

thickness [mm] electric resistancea [Ω cm2] burst strength [MPa] pH range maximum temperature [°C]

0.16−0.20 2.5

0.10 na

0.17 3.0

0.15 1.8

0.4−0.5 0−11 50

0.3 na 40

≥0.40 0−10 40

≥0.10 0−10 40

Neosepta AMX

Neosepta ACS

membrane

PC-SA

PCMVA

thickness [mm] electric resistancea [Ω cm2] burst strength [MPa] pH range maximum temperature [°C]

0.18−0.22 1.8

0.11 20

0.14 2.4

0.13 3.8

0.4−0.5 0−9 60

0.20 0−9 40

≥0.25 0−8 40

≥0.15 0−8 40

a

j=

P × Δcco‐solvent L

(1)

The change in mass of an ion-exchange membrane, equilibrated in different cosolvents, was determined as follows. Rectangular membranes (30 × 30 mm) were equilibrated in a solution containing 0.5 M NaCl with different solvent for a minimum of 24 h. 100 mol % H2O, 30/70 mol % MeOH/H2O, or 30/70 mol % EtOH/H2O is used as the solvent. Surface water was removed by carefully wiping the membrane between two filter papers. Each experiment was performed eight times to minimize the experimental error caused by the removal of surface water. Membranes were weighed with an analytical balance. The dry mass of the membranes was determined after the membranes are stored in an oven at 50 °C for a minimum of 24 h. Then, the swelling degree is calculated using eq 2, where Wwet [g] and Wdry [g] are the mass of the wet and dry membrane, respectively.

Equilibrated with a 0.5 N NaCl solution at 25 °C.

membranes. The standard and monovalent selective cationexchange membranes of PCA (PC-SK and PC-MVK) and Astom Corporation (Neosepta CMX and Neosepta CMS) were compared. In case of anion-exchange membranes, the cosolvent flux in standard and monovalent selective anionexchange membranes of PCA GmbH (PC-SA and PC-MVA) and Astom Corporation (Neosepta AMX and Neosepta ACS) were compared. 2.3. Data Analysis. In all experiments, methanol, ethanol, or 1-propanol was the only organic compound in the different streams. Therefore, the organic cosolvent flux was monitored by the change in nonpurgeable organic carbon and was measured by a TOC Analyzer (TOC-Vcph Total Organic Carbon Analyzer, Shimadzu, Japan). The cosolvent flux was calculated by the increase in cosolvent concentration in the initial cosolvent free compartment. During all experiments, the electrical potential difference across the stack was monitored by using a multimeter (Fluke 117 True RMS Multimeter, Fluke Corporation, USA). The stack resistance was calculated as follows. A constant current density was applied during all experiments. The average voltage drop over the stack during the last 60 s of the experiment was

change in mass =

Wwet − Wdry Wdry

× 100% (2)

The thickness of a membrane was measured with a micrometer with a resolution of 1 μm (Mitutoyo, Japan). Before the analysis, membranes were equilibrated for a minimum of 24 h in 0.5 mol L−1 NaCl solution. The solvent was chosen to be identical to the different solvent mixtures used in the experiments. All data points represent the average of 10 measurements.

3. RESULTS AND DISCUSSION 3.1. Methanol and Ethanol Flux through IonExchange Membranes. First, the influence of current density and organic cosolvent was investigated. Figure 1 shows the methanol and ethanol flux through PC-SK membranes. A line 8217


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osmotic flux of methanol and ethanol is obtained in these dense ion-exchange membranes. The experiments with intermediate level of current density and organic cosolvent concentration, namely, 325 A m−2 and 20 mol %, are replicates. The average value of the cosolvent flux is shown; error bars are added and represent the standard deviation on both measurements. In the case of methanol, for example, the cosolvent flux amounts to 4.03 and 4.14 mmol s−1 m−2, respectively. In the case of ethanol, a cosolvent flux of 1.83 and 1.78 mmol s−1 m−2 was measured. In practice, the error bars are not visible because the standard deviation does not exceed the size of the markers used. This illustrates the repeatability of the experiments and the coherence of the procedure used. This set of experiments also indicates the presence of a curvature of the cosolvent flux with respect to the cosolvent concentration. In the methanol experiments, the center point proves that the increase in cosolvent flux is larger for variations in small cosolvent concentrations than for variations in large cosolvent concentrations. For ethanol, the increase in cosolvent flux is not significant anymore in the range of 10 to 30 mol % ethanol, but as the cosolvent flux is zero when no ethanol is present, the ethanol profile also indicates the presence of curvature. A more rapid stagnation of this ethanol flux is observed in comparison with methanol. Because the cosolvent flux is predominately driven by diffusion of the cosolvent, this more rapid stagnation is presumably caused by the larger molar volume of ethanol compared to methanol. In conclusion, the transport of organic cosolvents is mainly a concentration driven phenomenon which is limited at higher concentration gradients of organic cosolvents. Xu et al. determined the ethanol flux of Neosepta CMX in a Hittorf’s cell at different current densities with a cosolvent concentration of 23.6 mol % ethanol in the cosolvent rich compartment.8 At a current density of 100 and 300 A m−2, an ethanol flux of 2.0 and 1.8 mmol s−1 m−2 was obtained. Compared to the values in this paper (for example, 1.8 mmol s−1 m−2 in the PC-SK membrane at a current density of 325 A m−2 and cosolvent concentration of 20 mol %), similar fluxes are obtained by Xu et al. For the PC-SA membrane, an analogous set of experiments is performed. These results are shown in Figure 2. A similar result was obtained as for the PC-SK membranes. The variation in

Figure 1. Influence of current density and cosolvent gradient on methanol and ethanol flux through PC-SK membrane (a line was added as guide to the eye).

was added to the methanol and ethanol profile to guide the eye. For both the methanol and ethanol, a small increase in cosolvent flux can be observed when the current density is increased from 150 to 500 A m−2. For methanol at a cosolvent concentration of 10 mol %, for example, the methanol flux increases by 0.33 mmol s−1 m−2. The ion transport cannot be measured in this configuration because both salt solutions contain NaCl and no increase or decrease in bulk salt concentration is achieved. However, no pH changes occurred during the experiments, indicating that the limiting current density is not exceeded. Thus, all current is used for the transport of sodium through the PC-SK membranes. Assuming that the current efficiency is 100%, the increase in current density corresponds with an increase in ion flux (in this case a sodium flux) of 7.26 mmol s−1 m−2. The increase in ion flux is more than a factor 20 larger than the increase in cosolvent flux. Therefore, only a small amount of organic cosolvents is transported in the hydration shell of the transported sodium or is transported by drag due to the ion flux. The low cosolvent flux due to electro-osmosis is unexpected. Methanol and water have similar interactions with ions in solution. The first solvation shell of Na+ contains six ions; in pure methanol and ethanol, the average solvation number of Na+ is slightly lower. The lower solvation number is caused by the larger steric hindrance of methanol and ethanol in comparison with water.31 In solvent mixtures, for example, in water−methanol mixtures with 10 mol % methanol, Hawlicka and Swiatla-Wojcik found that both sodium and chloride are preferentially solvated by methanol.32 In aqueous solutions, the transport of water through ion-exchange membranes caused by an ion flux has been studied in the literature. It was found that the estimated amount of water molecules for each ion transported is lower than the amount of water molecules in the first solvation shell. This lower amount is mainly caused by steric hindrance; weakly bound water molecules leave the hydration shell of the ion when it enters the pore. Therefore, water transport due to electro-osmosis depends on the pore size of the membrane.33,34 The molar volume of water, methanol, and ethanol is 18.07, 40.7, and 58.4 cm3 mol−1, respectively.35,36 A bulkier molecule such as methanol or ethanol encounters more steric resistance than water when it enters the pores of the membrane. Due to size exclusion, it is unlikely that methanol and ethanol are transported through the membrane in the solvation shell. As a result, only a low electro-

Figure 2. Influence of current density and cosolvent gradient on methanol and ethanol flux through PC-SA membrane (a line was added as guide to the eye). 8218


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Industrial & Engineering Chemistry Research current density has only a small influence on the cosolvent flux, and a larger increase in cosolvent flux for lower cosolvent concentrations is observed. Therefore, the migration of the sodium cation through the PC-SK membranes has a similar effect as the migration of chloride in PC-SA membranes. However, a larger flux of both methanol and ethanol was observed for PC-SK membranes in comparison with PC-SA membranes. Figure 3 shows the permeability of methanol and ethanol through the PC-SA membrane and PC-SK membrane. For both

solutions.39 Polar solutes are therefore likely to dissolve in large amounts in solvents with a large relative permittivity. The relative permittivity for water, methanol, and ethanol at 298 K is 80.1, 33.0, and 25.3, respectively.38 This confirms that water is a highly polar solvent. Methanol and ethanol on the other hand are less polar solvents, with little difference in relative permittivity between both solvents. Because the relative permittivity is also related to the electrical properties of a solvent, it can also be used to interpret the stack resistance of solvent mixtures. An overview of the stack resistances with PC-SA and PC-SK membranes is given in Figure 4. For both experiments with methanol and ethanol as a cosolvent, an increase in stack resistance was observed with an increase in cosolvent concentration. With an increase in organic cosolvent concentration, the solvent with the highest relative permittivity, i.e., water, is replaced by a solvent with a lower dielectric constant. This can also be seen in the conductivity of the salt streams. The conductivity of the ethanol−water mixtures with 0.5 M NaCl decreases from 12.4 to 9.5 mS cm−1 if the ethanol concentration is increased from 10 to 30 mol %. Similarly, the conductivity decreases from 26.5 to 17.1 mS cm−1 in the methanol−water mixture with 0.5 M NaCl and with 10 and 30 mol % methanol, respectively. Moreover, the stack resistance in the experiments with ethanol as a cosolvent amounts to higher values than in the experiments with methanol. This difference is also caused by the difference in conductivity between the aqueous methanol and ethanol solutions. A large difference can be observed for a variation in current density. For both cosolvents and both concentrations, a higher stack resistance was encountered at 150 A m−2 than at 500 A m−2. As a result, in view of the applications, a high current density is preferentially used for both cosolvents to minimize the stack resistance. Furthermore, ethanol has the advantage of a lower cosolvent flux due to its larger molecular size but has a larger stack resistance due to the lower dielectric constant. 3.3. 1-Propanol Flux through Ion-Exchange Membranes. In this paragraph, the feasibility of using 1-propanol as cosolvent is investigated and compared with the results obtained with methanol and ethanol as cosolvent. The cosolvent permeability of these three alcohols in PC-SA and PC-SK membranes is shown in Figure 5. For PC-SA membranes, the cosolvent permeability decreases in the order of methanol > ethanol >1-propanol; i.e., a decrease in cosolvent permeability was observed with the chain length of the alcohol and molar volume. Although all these alcohols have a linear structure, these results confirm that the cosolvent transport is limited by steric hindrance. For PC-SK membranes, a different tendency was observed. The ethanol permeability is lower than the permeability of methanol, but the highest permeability was observed for 1propanol. Comparing the membrane thickness, the PC-SA membranes equilibrated in a 0.5 mol L−1 NaCl solution only show a small variation in thickness depending on the cosolvent (214 ± 1, 215 ± 1, and 219 ± 2 μm in 10 mol % MeOH, EtOH, and 1-PrOH, respectively). However, the PC-SK membrane has a large increase in thickness in the presence of 1-PrOH (140 ± 1, 142 ± 2, and 166 ± 1 μm in 10 mol % MeOH, EtOH, and 1-PrOH, respectively). This tendency was also observed in the pressure drop in the diluate compartment (cf. Figure 6). During the experiment with 1-propanol and PCSK membranes, a strong increase in pressure drop in the 1propanol rich compartment was noticed. This increase was not

Figure 3. Influence of current density and cosolvent gradient on the cosolvent permeability of methanol and ethanol in PC-SA membrane (a) and PC-SK membrane (b) (a line was added as guide to the eye).

cosolvents, the cosolvent permeability decreases with an increase in cosolvent gradient. This decrease was observed earlier by Cheryan and Parekh for the diffusion of glycerol.37 In accordance with the cosolvent flux, the ethanol permeability is lower than the methanol permeability. As a result, a high cosolvent concentration and the use of ethanol instead of methanol as a cosolvent are preferred to minimize the permeability of cosolvents. 3.2. Stack Resistance while Using Methanol and Ethanol. In order to compare the stack resistance when different cosolvents are used, it is useful to compare the dielectric constants or relative permittivity of all solvents. A substance with a large relative permittivity is polar or highly polarizable.38 A large relative permittivity of solvents can also be interpreted as the ability to shield the charges of ions in 8219


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Figure 4. Electrical resistance across the stack as a function of the cosolvent gradient and current density for different membranes and cosolvents (PC-SA membrane with methanol (a) and ethanol (b) and the PC-SK membrane with methanol (c) and ethanol (d)).

ated by the use of strongly cross-linked membranes and bulky cosolvents. For both PC-SK and PC-SA membranes, an increase in stack resistance was observed with alcohol chain length. This tendency is caused by the decrease in relative permittivity of larger alcohols. The relative permittivity of methanol, ethanol, and 1-propanol is 33.0, 25.3, and 20.8, respectively, at a temperature of 293.2 K.36 A larger alcohol has a larger nonpolar group and leads to a decrease in electrical conductivity of salt solutions with organic cosolvents. 3.4. Influence of the Migration of Different Cations on the Cosolvent Permeability. The influence of a different cation (sodium, potassium, calcium, or protons) on the cosolvent flux is studied in this paragraph. Because the current density has only a small influence on the cosolvent flux, this parameter is disregarded in these experiments. All experiments were performed at a fixed current density of 500 A m−2. A high current density is chosen to maximize the influence of the cation on the cosolvent flux. The cosolvent flux of methanol and ethanol is shown in Figure 7. Similarly as in Section 3.1, a strong increase in methanol flux is measured with a change in cosolvent concentration. On the other hand, the ethanol flux shows only a minor increase. This phenomenon was observed for all cations tested. With the use of methanol as cosolvent, an increase in cosolvent flux is observed in the order of Na+ < K+/Ca2+ ≪ H+. In the literature, it was demonstrated that the swelling of a Nafion membrane in aqueous methanol solutions depends on the cation sorbed. The highest swelling degrees were found for the ions with the smallest ion size.40,41 Generally, the diffusional flux increases as the membrane swells.42 Therefore, the highest

Figure 5. Comparison of the cosolvent permeability of methanol, ethanol, and 1-propanol through PC-SA and PC-SK membranes and the electrical resistance across the stack (i = 500 A m−2, c = 0.5 mol L−1 NaCl, 10 mol % organic cosolvent).

observed in any other experiments. Therefore, these experiments indicate that the use of 1-propanol leads to a strong swelling of the PC-SK membranes. This swelling presumably results in a higher permeability of 1-propanol in comparison with methanol and ethanol. Moreover, as the pressure drop in the experiments with PC-SA is not significantly influenced by the presence of 1-propanol, these measurements also indicate that the PC-SA membrane has a larger cross-linking density than the PC-SK membrane. In general, the cosolvent permeation through ion-exchange membranes can be attenu8220


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Figure 6. Pressure drop across the diluate compartment as a function of time and different cosolvents (methanol, ethanol, or 1-propanol) in the experiments with PC-SA membranes (a) and PC-SK membranes (b) (i = 500 A m−2, c = 10 mol % organic cosolvent).

Figure 7. Influence of the migration of sodium (a), calcium (b), potassium (c), or protons (d) on the methanol and ethanol flux through PC-SK membrane as a function of the cosolvent gradient (i = 500 A m−2).

Table 3. Overview of the Permeability of Methanol and Ethanol in Commercial Ion-Exchange Membranes membrane

permeability methanol [10−11 m2 s−1]

PC-SK PC-MVK PC-SA PC-MVA Neosepta CMX Neosepta CMS Neosepta AMX Neosepta ACS Nafion 117

7.78 ± 0.22 2.15 ± 0.05 6.33 ± 0.04 2.17 ± 0.01 1.85 ± 0.18 2.00 ± 0.06 3.18 ± 0.02 1.22 ± 0.03 6.9 ± 1.0 8.95

permeability ethanol [10−11 m2 s−1] 4.74 1.26 3.08 0.72 1.02 1.02 2.48 1.13

± ± ± ± ± ± ± ±

0.10 0.06 0.04 0.03 0.01 0.01 0.04 0.07

methanol flux is encountered when protons are used. The difference in ethanol flux is small if sodium, potassium, or calcium is used. This indicates that the difference in swelling and pore size as a consequence of the different cations is small compared to the molecular dimensions of ethanol. These experiments are not performed with protons as a cation because

conditions Δc Δc Δc Δc Δc Δc Δc Δc Δc Δc

= = = = = = = = = =

10 mol %, i = 500 A m−2, T = ambient 10 mol %, i = 500 A m−2, T = ambient 10 mol %, i = 500 A m−2, T = ambient 10 mol %, i = 500 A m−2, T = ambient 10 mol %, i = 500 A m−2, T = ambient 10 mol %, i = 500 A m−2, T = ambient 10 mol %, i = 500 A m−2, T = ambient 10 mol %, i = 500 A m−2, T = ambient 12.6 mol %, i = 0 A m−2, T = 20 °C 5.85 mol %, i = 0 A m−2, T = 25 °C

reference this this this this this this this this 20 22

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of the rapid increase in pressure drop in the ethanol rich compartment. This increase indicates the extensive swelling of the PC-SK membrane in the presence of protons in combination with ethanol. This increase in pressure drop was not encountered in analogous experiments with sodium, potassium, or calcium as a cation. As mentioned above, the 8221


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also observed by Chaabane et al. These authors argued that a reduction in the mass of the membrane was caused by a strong density of cross-links. When a membrane is immersed in a salt solution, solvents are sorbed in the membrane because of the concentration gradient between the membrane and the solution. Water occupies the zone with functional sides in the membrane. Meanwhile, organic solvents are concentrated in the interstitial volume. A high solvent uptake leads to an increase in the internal pressure of the membrane. In membranes with a high cross-linking density, it is argued that the increase in membrane thickness is limited. As methanol enters the membrane, a part of the water content of the membrane leaves to reduce the internal pressure.12 In addition, the study by Chaabane et al. also revealed a difference in behavior with membranes with no cross-links. The wet mass of the un-crosslinked Nafion membrane varies linearly with the methanol content in a water/methanol mixture. The total solvent uptake in the membrane increases with increasing methanol concentration of the solution. Therefore, a large methanol flux is obtained in un-cross-linked ion-exchange membranes.12 In Figure 9, the variation in wet mass of a membrane as a function of the cosolvent permeability is shown. In this case,

highest swelling degrees are encountered with protons compared to other metal ions. 3.5. Cosolvent Flux in Different Commercial IonExchange Membranes. In this paragraph, the cosolvent flux through different commercial cation and anion-exchange membranes is compared. The permeability of methanol and ethanol is calculated by using eq 1. Table 3 shows an overview of the permeability determined in this work and two references. The membrane thickness was measured as a function of the solvent mixture. To compare these permeabilities with literature values, the permeability of methanol in Nafion 117 membranes is also given in this table. Nafion membranes are the most commonly used membranes in direct methanol fuel cells.6 However, attention should be paid to the comparison because of the different properties and applications. The Nafion membranes have another pretreatment than the membranes in this work.20 Both methanol permeabilities of Nafion were measured in a diffusion cell. In this work, the permeability was measured in the presence of an electrical current. However, despite the differences in pretreatment and measuring conditions, a comparable value was found for the methanol permeability. Compared to the cation-exchange membranes only, the PC-SK membrane has a higher permeability. The lowest permeability was found for Neosepta CMX and Neosepta CMS membranes. With respect to the anionexchange membranes, all membranes tested have a lower permeability than Nafion membranes. Neosepta ACS membranes have the lowest permeability. In this view, the ionexchange membranes tested have a higher methanol blocking capability than Nafion membranes. Moreover, for all membranes tested, a lower permeability of ethanol was observed in comparison with methanol. To examine the difference in permeability of the ionmembranes, the variation in wet mass as a function of the solvent composition was investigated (cf. Figure 8). Mem-

Figure 9. Relative change of wet mass of several commercial ion exchange membranes (equilibrated in a 0.5 mol L−1 NaCl 30 mol % methanol or ethanol solution compared to a 0.5 mol L−1 NaCl 100% water solution) as a function of the cosolvent permeability.

the relative change in mass between a membrane equilibrated in a solvent mixture relative to a membrane equilibrated in an aqueous solution is given. An increase in cosolvent permeability was observed when the change in wet mass increases. If a negative change in mass is obtained, some organic cosolvent enters the membrane and expulses the water from the membrane. Moreover, the volume of the zone with active groups decreases and an increased volume of the interstitial volume is obtained. Therefore, the reduction in mass is accompanied by a high concentration of organic cosolvent in the membrane compared to the water content. Subsequently, the membranes with the highest decrease in mass (PC-SK and PC-SA membranes) have the highest cosolvent permeability. Moreover, in the literature, it is also shown that the chemical composition of the membranes is related to the methanol permeability. For example, Pu23 proposed that the formation of hydrogen bounds between methanol and the polymer matrix, in this case imidazole rings, reduces the methanol permeability. Therefore, the cosolvent permeability of an ion-exchange membrane depends both on the variation in wet mass and

Figure 8. Change in wet mass of several commercial ion-exchange membranes equilibrated in 0.5 mol L−1 NaCl solution with different cosolvents (100 mol % water, 30 mol % ethanol, or 30 mol % methanol).

branes were equilibrated in a 0.5 mol L−1 NaCl solution with varying solvent composition. For the PC-SK and PC-SA membrane, a lower wet mass was found in an aqueous methanol and ethanol mixture than in an aqueous solution. For all other membrane types, an increased or comparable wet mass is obtained. A reduction in the mass and thickness of the membrane in contact with high methanol concentrations was 8222


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(5) Godino, M. P.; Barragán, V. M.; Villaluenga, J. P. G.; Ruiz-Bauzá, C.; Seoane, B. Water and methanol transport in Nafion membranes with different cationic forms. J. Power Sources 2006, 160 (1), 181−186. (6) Heinzel, A.; Barragan, V. M. A review of the state-of-the-art of the methanol crossover in direct methanol fuel cells. J. Power Sources 1999, 84, 70−74. (7) Neburchilov, V.; Martin, J.; Wang, H.; Zhang, J. A review of polymer electrolyte membranes for direct methanol fuel cells. J. Power Sources 2007, 169 (2), 221−238. (8) Xu, F.; Innocent, C.; Pourcelly, G. Electrodialysis with ion exchange membranes in organic media. Sep. Purif. Technol. 2005, 43 (1), 17−24. (9) Kameche, M.; Xu, F.; Innocent, C.; Pourcelly, G. Electrodialysis in water-ethanol solutions - Application to the acidification of organic salts. Desalination 2003, 154, 9−15. (10) Zhang, F.; Huang, C.; Xu, T. Production of Sebacic Acid Using Two-Phase Bipolar Membrane Electrodialysis. Ind. Eng. Chem. Res. 2009, 48, 7482−7488. (11) Liu, X.; Li, Q.; Jiang, C.; Lin, X.; Xu, T. Bipolar membrane electrodialysis in aqua−ethanol medium: Production of salicylic acid. J. Membr. Sci. 2015, 482, 76−82. (12) Chaabane, L.; Bulvestre, G.; Innocent, C.; Pourcelly, G.; Auclair, B. Physicochemical characterization of ion-exchange membranes in water−methanol mixtures. Eur. Polym. J. 2006, 42 (6), 1403−1416. (13) Chaabane, L.; Dammak, L.; Nikonenko, V. V.; Bulvestre, G.; Auclair, B. The influence of absorbed methanol on the conductivity and on the microstructure of ion-exchange membranes. J. Membr. Sci. 2007, 298 (1−2), 126−135. (14) Ethève, J.; Huguet, P.; Innocent, C.; Bribes, J. L.; Pourcelly, G. Electrochemical and raman spectroscopy study of a nafion perfluorosulfonic membrane in organic solvent-water mixtures. J. Phys. Chem. B 2001, 105, 4151−4154. (15) Chaabane, L.; Dammak, L.; Grande, D.; Larchet, C.; Huguet, P.; Nikonenko, S. V.; Nikonenko, V. V. Swelling and permeability of Nafion®117 in water−methanol solutions: An experimental and modelling investigation. J. Membr. Sci. 2011, 377 (1−2), 54−64. (16) Hamann, C. H.; Theile, V.; Koter, S. Transport properties of cation-exchange membranes in aqueous and methanolic solutions. Diffusion and osmosis. J. Membr. Sci. 1993, 78, 147−153. (17) Strathmann, H. Ion-Exchange Membrane Separation Processes, First ed.; Elsevier: Amsterdam, 2004. (18) García-Villaluenga, J. P.; Seoane, B.; Barragán, V. M.; RuizBauzá, C. Osmotic behavior of a Nafion membrane in methanol− water electrolyte solutions. J. Colloid Interface Sci. 2003, 263 (1), 217− 222. (19) Xue, S.; Yin, G. Methanol permeability in sulfonated poly(etheretherketone) membranes: A comparison with Nafion membranes. Eur. Polym. J. 2006, 42 (4), 776−785. (20) Diaz, L. A.; Abuin, G. C.; Corti, H. R. Methanol sorption and permeability in Nafion and acid-doped PBI and ABPBI membranes. J. Membr. Sci. 2012, 411−412, 35−44. (21) Mukoma, P.; Jooste, B. R.; Vosloo, H. C. M. A comparison of methanol permeability in Chitosan and Nafion 117 membranes at high to medium methanol concentrations. J. Membr. Sci. 2004, 243 (1−2), 293−299. (22) Zhou, X.; Weston, J.; Chalkova, E.; Hofmann, M. A.; Ambler, C. M.; Allcock, H. R.; Lvov, S. N. High temperature transport properties of polyphosphazene membranes for direct methanol fuel cells. Electrochim. Acta 2003, 48 (14−16), 2173−2180. (23) Pu, H. Methanol permeation and proton conductivity of aciddoped poly(N-ethylbenzimidazole) and poly(N-methylbenzimidazole). J. Membr. Sci. 2004, 241 (2), 169−175. (24) Pivovar, B. S.; Wang, Y.; Cussler, E. L. Pervaporation membranes in direct methanol fuel cells. J. Membr. Sci. 1999, 154, 155−162. (25) Kameche, M.; Xu, F.; Innocent, C.; Pourcelly, G.; Derriche, Z. Characterisation of Nafion®117 membrane modified chemically with a conducting polymer: An application to the demineralisation of

subsequently the density of cross-links and the chemical composition of the polymer matrix.

4. CONCLUSION In this Article, the flux of small alcohols (methanol, ethanol, or 1-propanol) in ion-exchange membranes, used in electrodialysis, was investigated. It was observed that the organic cosolvent flux is mainly determined by the concentration gradient of the alcohol across the membrane. A change in current density has only a minor influence on the cosolvent fluxes. When ethanol is used, the cosolvent flux is limited to a maximum value at higher concentration gradients. To minimize the permeability of cosolvents, a high cosolvent gradient and low current density are preferred. Due to the larger steric hindrance of ethanol in comparison with methanol, the permeability of the former cosolvent is lower. For PC-SA membranes, the lowest cosolvent permeability is found with 1-propanol. In PC-SK membranes, the use of 1propanol results in a strong increase in pressure drop in the 1propanol rich compartment and a higher permeability compared to methanol and ethanol. The influence of the migration of cations on the cosolvent permeability of a PC-SK membrane was also investigated. An increase in methanol flux was observed in the order of Na+ < K+/Ca2+ ≪ H+. With the use of ethanol as cosolvent, the difference in cosolvent flux was found to be small comparing sodium, potassium, and calcium. Finally, the methanol and ethanol fluxes in various commercially available membranes from PCA GmbH and Astom Corporation were compared. The permeability of methanol and ethanol was compared with the permeability of methanol in Nafion membranes reported in the literature. It was found that, except for PC-SK membranes, all the membranes tested have a lower methanol permeability. Moreover, it was also found that the cosolvent permeability is correlated with the change in wet mass of the membrane. Membranes with a decrease in wet mass (solvent mixture relative to aqueous solution) showed the highest cosolvent permeability.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +32 (0)9 265 86 37. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support by Flanders Innovation & Entrepreneurship. REFERENCES

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