Graphene OxideâPolyaniline as a Water Dissociation Catalyst in the

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Graphene Oxide-Polyaniline as a Water Dissociation Catalyst in the Interfacial Layer of Bipolar Membrane for Energy-Saving Production of Carboxylic Acids from Carboxylates by Electrodialysis Murli Manohar, and Vinod Kumar Shahi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03734 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Sustainable Chemistry & Engineering

Graphene Oxide-Polyaniline as a Water Dissociation Catalyst in the Interfacial Layer of Bipolar Membrane for Energy-Saving Production of Carboxylic Acids from Carboxylates by Electrodialysis Murli Manohar, †,‡ and Vinod K. Shahi*†,‡ ‡

†

Electro-Membrane Processes Division, and Academy of Scientific and Innovative Research, CSIR-Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research, Gijubhai Badheka Marg, Bhavnagar 364 002, Gujarat, India

Fax: +91-0278-2566970; Tel: +91-278-2569445; E-mail: [email protected]; [email protected]

ABSTRACT: Bipolar membrane (BPM) was prepared by layer-by-layer casting of cation exchange layer (CEL; sulphonated poly(2,6–dimethyl–1,4–phenylene oxide) (SPPO), interfacial layer (IL), and anion exchange layer (AEL; quaternized PPO) in same solvent to achieve the good adhesion. Graphene oxide-polyaniline composite (GO-PANI) was introduced in the IL of BPM as water dissociation (WD) catalyst. Under applied reverse bias, water molecules in IL zone -

OH

dissociate, useful

and

H+

generate

electro-synthesis

by

and BPM

electrodialysis (BPMED). Prepared BPM was assessed by current–voltage (i–V) curves and performance homologues

of

BPMED

carboxylates

for

converting into

their

corresponding acid and base. Under the operating conditions in BPMED, 71-63% recovery of the different carboxylic acids was recorded with 92-97% CE and 0.90-0.90 kWh kg-1 energy consumption. Negligible co-ion leakage across the BPM also revealed its efficient nature and product (carboxylic acid) purity. Additionally, low and stable Vdiss (0.82-1.12 V) in equilibrium with different carboxylates responsible for the low energy consumption and making viable high-performance BPM. KEYWORDS: Bipolar membrane, Water dissociation, Electrodialysis, Graphene oxide

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INTRODUCTION Bipolar membrane (BPM) consists three laminated layers namely cation-exchange layer (CEL), interfacial layer (IL), and anion-exchange layer (AEL), respectively.1–4 The specific phenomenon of BPM electrodialysis (BPMED) is water dissociation (WD) in the interfacial zone under applied potential to produce H+/OH- for converting salt into corresponding acid and base, has great potential for industries.5,6 Acidic and alkaline functional groups on the CEL and AEL, facilitate water diffusion from bulk solution zone to the interfacial zone and transfer the H+/OH.7,8 This led to the increase in concentration (pH) gradient in both adjacent compartments across the BPM-solution interfaces. Thus, BPM has been considered to be useful for the proton-coupled electro-synthesis cell, and electrochemical conversion of feed stock into value-added products for pH-dependent redox reactions.9 An efficient BPM should have good adhesion between different layers, mechanical strength, ability to operate at high current density, high permselectivity, and low potential drop.10 Generally, AEL are prepared via chloromethylation of polysulfone, poly(etherimide), poly(2,6dimethyl-1,4-phenylene oxide) (PPO), and Cardo poly(ether sulfone) based polymer using chloromethyl ether (CME) followed by quaternization.11-15 These preparative procedures are complicated and particular CME is carcinogenic and potentially harmful to human health.16,17 Recently, we reported safe route of chloromethylation followed by amination for preparing quaternized PPO based anion exchange membrane, which was used as AEL in this case to avoid the use of brominating agent and CME.18 Further, sulfonated PPO based CEL was used here to achieve good adhesion between AEL and CEL.19 Across the BPM under applied potential gradient, WD in the IL zone significantly depends on nature and characteristics of CEL, AEL and IL.20-22 Different types of ion-exchange layers were 2 ACS Paragon Plus Environment

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extensively studied for different applications. An efficient WD catalyst in the IL zone controls water dissociation potential close to ~ 2.0 V. Different types of inorganic materials, such as metal alkoxides, metallic salts, heavy metal ion complexes etc,23-26 and hydrophilic organic polymers ((poly(4-vinylpyridine), poly(ethylene glycol), poly(vinyl alcohol), poly(amidoamine, poly(aniline), etc),27-29 were reported as WD catalyst in the IL of the BPM. Graphene oxide (GO) was explored as WD electro-catalyst for the oxygen reduction reaction in the fuel cells, because presence of sp3-hybridized carbon sites and various functional groups.30,31 Due to good thermal, electrical conductivity, and electron mobility, GO is a potential candidate for the preparation of nanocomposite materials.32,33 However, lack of fine control over aggregation of nanoparticles in the polymer matrix (size-dependent advantage) is a serious obstacle.34 Polyaniline (PANI) is a low-dimensional organic conductor with high surface area and investigated intrinsically conducting polymer due to good electrical conductivity, unique redox chemistry.35 The GOPANI nanocomposite provide a synergetic effect to improve the conductivity of PANI and to mitigate the GO aggregation, thus may be of great interest WD catalyst in the IL of BPM. Herein, we demonstrate the preparation of polyaniline (PANI) wrapped aminated GO as a potential WD catalyst in the IL of the BPM. The reported graphene-polyaniline (GO-PANI) catalyst exhibits excellent proton transport associated oxidation/reduction property of PANI with improved conductivity. Also the reported BPM was used in BPMED to develop a sustainable technology for converting sodium carboxylates into their corresponding carboxylic acids with relatively high current efficiency and low energy consumption. EXPERIMENTAL SECTION Chemicals. Poly(2,6–dimethyl–1,4–phenylene oxide) (PPO) was obtained from SigmaAldrich. Chloroform, Methanol, stannic chloride, aniline and trimethyl chlorosilane (TMCS),



para formaldehyde, sulfuric acid, chlro-sulfonic acid, sodium hydroxide, and sodium chloride of Ar grade was obtained from RANKEM, India. Sodium acetate, sodium tartrate, sodium oxalate, sodium citrate, N-methyl pyrrolidone (NMP), and trimethylamine (TMA), etc were received from SD Fine Chem, India, and used without further purification. Deionized water was used for all the experiments. Preparation of CEL and AEL. To synthesize the CEL, sulfonation of PPO was carried out in a chlorobenzene solvent using concentrated H2SO4 as a sulfonating agent under continuous stirring at room temperature (30 oC) for 3 hours.36 Obtained reaction mass was precipitated in 2propanol and washed with DI water vigorously to remove excess of acid and impurities. For preparing AEL, chloromethylation of PPO was carried out in chlorobenzene solvent in presence of stannic chloride and para formaldehyde, by drop-wiser addition of chlorobenzene under stirred condition for 24 h at 45 °C-50 °C.18 After completion of reaction, reaction mass was precipitated in 2-propanol, washed with several times with solvent and finally with water to remove the unreacted catalyst and impurities. Obtained chloro methylated PPO (CMPPO) was dried in vacuum oven, and degree of chloromethylation was estimated by 1H NMR spectra (Figure S1). Reaction schemes for the preparation of CEL and AEL have been depicted in Scheme 1. Synthesis of GO-polyaniline (GO-PANI) composite. Graphite oxide (GtO) was synthesized using purified natural flake graphite (Gt) powder by Hummers method, according to the earlier reported method.37 GtO was used to exfoliate ultra-sonication for preparing graphene oxide (GO). To prepare GO-PANI composite, HCl (37%, v/v) was added to known volume of aniline to synthesize the positively charged anilinium salt by protonation. Obtained GO was directly added to anilinium salt solution (equal mole numbers of aniline) and mixture was heated at 80 oC


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under constant stirring for 1 hour, followed by mixture was cooled at 30 oC. By charge-charge interactions, anilinium cation will attached with GO under thermal annealing conditions. Purified and re-crystalized ammonium persulfate (APS) crystals were added to the above mixture as redox-polymerization agent under continuous stirring and obtained dispersion was centrifuged, dried under vacuum at 60 oC and resulted dark green solid powder (GO-PANI) and reaction scheme presented in Scheme 2. Preparation of Bipolar Membrane. BPM was prepared by layer-by-layer casting method. To cast the CEL, desired amount of sulphonated PPO was dissolved into NMP under vigorous stirring for 12-14 h to get a clear homogeneous solution. The CEL was casted onto flat cleaned glass-surface and dried under IR lamps for 3-4 h. Followed by IL was casted onto the top of CEL using GO-PANI composite dispersion (10%, w/w) in NMP and dried under IR lamps. Further, to cast AEL, CMPPO was dissolved in NMP with slow addition of trimethyl amine (TMA) under stirred conditions for 12 h at 60-65 oC. Obtained solution was transformed into the thin film onto the top of IL and dried under the IR lamps. Same solvent (NMP) was used to cast the different layers one onto other, because of good adhesion all between layers obtained intact composite BPM film was named as BPM-GO(P). Similar method was adopted to prepare pristine BPM with GO as IL (BPM-GO), separate CEL and AEL used in BPMED. Characterization. 1H NMR spectra were recorded by Brucker 200 MHz spectrometer using DMSO-d6 or CDCl3 as a solvent (Figure S1). FTIR-ATR spectra for GO and GO-PANI were recorded by spectrum GX series 49387 spectrometer in the range of 4000–600 cm−1 (Figure S2). Scanning electron microcopy (SEM) images for AEL, CEL and BPM-GO(P) (cross-section) were obtained by LEO Instruments (H.K.) Ltd., Kowloon, Hong Kong, after gold sputter coatings. Thermal analysis was carried out using thermo gravimetric analyzer (TGA) (Mettler 5 ACS Paragon Plus Environment


Toledo TGA /SDTA851e with starc software) under N2 atmosphere at 10 oC/min heating rate (30-550 oC) (Figure S3). The dynamic mechanical analysis (DMA) was carried out by dynamic mechanical analyzer (Mettler Toledo; DMA) 861c instrument with starc software under N2 atmosphere between 30–350 oC with 10 oC/min heating rate DMA (Figure S4). Bursting strength measured (Kg cm ) of the prepared composite membranes was measured by Burst Strength Tester machine provided (Table S1) by Test Techno Consultant, Vadodara, Gujarat, India. For water content measurements, membranes were dried in vacuum oven at 100 oC until a constant dry weight (Wdry) was obtained. Then membranes were immersed in H2O (30 oC) 24 h to obtain the constant wet weight (Wwet). The water content (%) was calculated as following equation. Water content (% ) =

Wwet − Wdry Wdry

×100

(1)

The ion-exchange capacity (IEC) was determined by the classical titration. Membrane samples, of known dry weight, were equilibrated in 1.0 M NaOH/HCl solution for converting all ionic sites into OH-/Cl- form. The membranes were then thoroughly washed with double distilled water to remove the last trace of acid/base. Then they were equilibrated in 50 mL of 0.10 M NaCl solution for 24 h to replace the OH- by Cl-. Exchanged H+/OH- were back titrated using phenolphthalein as an indicator. The IEC value (in mequi/g) was obtained by following equation. IEC =

V NaOH / HCl × C NaOH / HCl Wdry

(2)

Conductivity of equilibrated membranes was measured by using potentiostat /galvanostat frequency response analyzer (Auto Lab, Model PGSTAT 30, Eco Chemie, B.V. Utrecht, The Netherlands). The membrane was sandwiched between two in house made circular electrodes (4.0 cm2) of stainless steel. Direct current (dc) and sinusoidal alternating currents (ac) were 6 ACS Paragon Plus Environment

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applied for recording the Nyquist plot at a scanning rate of 1 µA/s within a frequency range of 106-100 Hz. The spectrum of the short-circuited cell was also recorded and data was subtracted (as a series circuit) from each recorded spectra to eliminate cell and wire resistances and inductances. The membrane resistance (R) was determined from Nyquist plots using the Fit and Simulation method and membrane conductivity (κm) was estimated by following equation.

κ m (S cm −1 ) =

L R× A

(3)

where, L is the membrane thickness, and A is the effective membrane area. Detailed experimental methods for the measurements of counter-ion transport number and permselectivity are included in supporting information (section S1). Methods for estimating oxidative, hydrolytic, and acid-base stabilities are included in supporting information (section S2). The i–V characteristics of BPM were recorded in a two-compartment glass cell (50 cm3) separated by membrane. Both compartments were regularly fed with different equilibrating solutions of known concentrations. Thus, there is no possibility of concentration change across the membrane interface during recording of i–V curves. A known current density (0-100 mA cm2

) was applied (DC power supply; Aplab India, model L1285) in steps across using Pt electrodes

fitted in both compartments (25 cm2) and resultant membrane potential was recorded using digital potentiometer with the help of saturated calomel electrodes (Ag/AgCl reference electrode) and salt bridges (Figure 1). BPMED Experiments for Conversion of Carboxylate Salt into their Corresponding AcidBase. BPMED experiments were carried out in a cell consisting 5 compartments separated by two pieces of BPM, CEL and AEL (Figure 2). Parallel-cum-series flow arrangement was used to feed compartments by carboxylate salts (sodium chloride, sodium acetate, sodium citrate, sodium tartarate, and sodium oxalate) solutions (1000 cm3) using peristaltic pumps in a recirculation 7 ACS Paragon Plus Environment


mode with constant flow rate (0.0063 m3/h). Precious metal oxide coated titanium sheet (TiO2 sheet coated with a triple precious metal oxide (titanium–ruthenium–platinum); of 6.0 µm thickness, and 8.0×10−3 m2 effective area) were obtained from TITAN, Chennai, India, and used as cathode and anode. Constant voltage across the electrodes was imposed using DC power supply (Aplab India, model L1285) and resultant variation in current was recorded as a function of time using a digital multimeter. The whole setup was placed under ambient conditions (30 o

C). The 0.10 M Na2SO4 solution was recirculated through electrode as electrode wash (EW)

compartments, while desired NaCl solution was initially fed into compartment 1, and distilled water was fed into compartments 2 and 3. The pH and conductivity of each output stream were regularly monitored. RESULTS AND DISCUSSION Membrane Preparation. BPM studied herein was prepared by layer-by-layer (l-b-l) casting of CEL (SPPO), IL (GO-PANI or GO) and AEL (QPPO), in a same solvent (DMAC) to achieve good adhesion between all three layers responsible for stability of BPM. Characteristics of the individual ion exchange membrane (CEL and AEL) used in electrodialysis (BPMED) are included in Table 1. In view of comparable essential properties (ion exchange membrane (IEC), membrane conductivity (κm), and counter-ion transport number ( tim )) of both, CEL and AEL, their thickness was kept equal (200 µm), and thus reported BPM may considered as symmetric due to nearly comparable transport properties of both layers. Under hydrated conditions, CEL and AEL showed 12.7 and 11.5% swelling ratio, respectively. Thus, in absence of excessive swelling of both membranes, delamination of different layer in BPM structure has been completely ruled out. The properties of pristine BPM-GO and BPM-GO(P) are also summarized in Table 1. The dry thickness of CEL, AEL and BPM, predicted 18 µm thickness of IL, thus


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thickness of contact IL was ~10% of the thickness of the individual membrane. It was observed that unlike individual membrane, swelling ratio for pristine BPM-GO and BPM-GO(P) is relatively high (15.2 &14.8%, respectively), which implies significant swelling of the BPM under hydrated conditions may be due to physical rearrangement in IL zone. The fixed charge groups of CEL and AEL provide suitable environment for the interaction of water in the IL zone, thus IL acts as water reservoir.23,38 To achieve good water dissociation performance of the BPM excellent compaction/adhesion between three layers (namely CEL, IL, and AEL) is an urgent requirement. The SEM images (surface) for the CEL, AEL showed smooth and homogenous dense surface without any cracks, holes, or phase separation (Figure 3). The cross-section image of BPM-GO(P) shows good adhesion between all three layers, and clearly demonstrates intact structure, may be same nature of solvent/polymer backbone used for the layer-by-layer casting. The membrane conductivity (κm) for pristine BPM-GO (5.3 × 10-2 S cm-1) increased to 6.01× 10-2 S cm-1 for BPM-GO(P) may be due to highly conductive nature of GO-PANI composite in compare with pristine GO (Table 1). Because of high conductivity, BPM-GO(P) was further assessed for the BPMED. The κm values of CEL, AEL and BPM-GO(P) increased with the NaCl concentrations (Figure 4). The high κm values for CEL or AEL in compare with BPM-GO(P) under similar experimental conditions, was attributed to the relatively high thickness of the BPM and thus comparatively low charge density (fixed charge concentration). Further, nearly similar conductivity for CEL and AEL reduces the possibility of the co-ion leakage across the BPM. Under electrodialytic conditions, the CEL will transport H+ (or any cation), while AEL will predominantly transport OH- (or any other anion). Nearly similar conductivity of CEL and AEL are not surprising, but may be due to well controlled functionalization (sulphonation or



chloromethylation of PPO). In addition the thicknesses of both individual membranes are also same (200 µm), which may also lead to significant reduction in co-ion leakage cross the BPM. In BPMED, water dissociation occurs at the IL and different types of hydrophilic-hydrophobic martials were used for the IL.39,40 In the IL zone, water dissociation occurs may be due to: (i) applied electrical field across the BPM junction (IL), (ii) catalytic water dissociation reactions, and/or (iii) increased water activity due to interfacial wetting ability.41 BPM Stability. Mechanical stability for CEL, AEL and BPM was assessed by burst strength data included in Table S1 (supporting information). Burst strength values for individual membranes (CEL & AEL) (5-7 Kg cm-2) were significantly increased to 12.55 Kg cm-2 in case of BPM due to sandwiched structure. Acid-alkaline stabilities test revealed about 3-5% weight loss of these membranes, thus membranes are quite stable under acidic and alkaline conditions. Oxidative stability of individual membranes and BPM was assessed under treatment with Fenton's reagent. Individual membranes as well as BPM showed about 2- 3% weight loss and ~1.15% IEC loss Table S2 (supporting information). This observation confirmed oxidativeresistant nature of both individual membranes and BPM may be high stable PPO polymeric matrix. Hydrolytic stability for CEL, AEL and BPMs was also analysed under pressurized stream at 140 oC for 192 h by recoding the loss for weight and IEC (%). These membranes were found to be hydrolytic stable in nature with about ~ 3% weight loss and ~1.13% IEC loss. The i-V Curves. The i-V performance of the ion exchange membranes is an important technique to assess for their transport charactristics.42,43 Under applied potential gradient across the IEM, concentration polarization occurs due to un-equal ion transport in the bulk and membrane phase, and responsible for water splitting in the membrane-solution interfacial zone. iV curves for prepared BPM-GO(P) in equilibration with 0.10 M solution of NaCl and different


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carboxylates (Figure 5). Different parameters such as first limiting current density (Ilim1), second limiting current density (Ilim2), dissociation potential (Vdiss) and operating potential (Vop) were estimated from i-V curves according to the procedure entertain in previously, and included in Table 2.44 Below Ilim1 (Ohmic region), current varied due to ionic transportation.45,46 At Ilim1, depletion of counter-ions at the interfacial zone leads the increase in electrical resistance. Thus, Ilim1 may be considered as a measure of co-ion leakage across BPM.45 The Ilim1 values demonstrate that co-ion leakage is significant for NaCl and acetate dissociation, while in case of tartrate, oxalate and citrate co-ion leakage was negligible. Further, incorporation of GO-PANI as IL, contributes towards ion transport and is responsible for steep increase in current above Ilim1, and allowed catalytic water dissociation (Vdiss).46 The voltage, where water dissociation is started (Vop), the current is predominately carried by H+/OH-. It was observed that BPM-GO(P) under different operating conditions showed 2.38-2.88 V, operating potential (Vop), which higher than the thermodynamic potential of water splitting (1.23 V).38 Prepared BPM-GO(P) showed better i-V parameters in equilibration with NaCl or different carboxylates solution, and has been considered as promising candidate for converting homologues carboxylates into their corresponding acid and base. Effect of GO-PANI IL. Across the BPM, water dissociation efficiency is accelerated by catalytic IL, which promotes transport of H+/OH- between solution-membrane interfacial zone of the BPM (fixed charge groups) and IL.23 According to model proposed by Simons et al,24 protonation-deprotonation reaction between fixed charge groups and water molecules occurred in following manner, and responsible for the formation of H+ and OH-.



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A − + H 2 O ⇔ AH + OH − AH + H 2 O ⇔ A − + H 3 O + or B + + H 2 O ⇔ BOH + H + BOH + H + ⇔ B + + H 2 O

A- is acidic group fixed in the CEL, while B+ referred as alkaline group fixed in the AEL. AH and BOH is neutral acid and base. Thus, the water dissociation efficiency of a BPM is governed by fixed charge density of CEL/AEL, and availability water molecule in IL zone. Synergized property such as high ionic/electron mobility of GO along with conductivity and electrochemical reversibility of PANI, are significant advantages for incorporation of GO-PANI nano-composite as IL.47-51 This statement was also verified by recording membrane conductivity of pristine BPM-GO (5.3 × 10-2 S cm-1) and BPM-GO(P) (6.01× 10-2 S cm-1) (Table 1). Relatively high conductivity of BPM-GO(P) was attributed to good synergy between GO and PANI. In addition, good dispersion of GO-PANI and formation of uniform layer are also important characteristics for a BPM IL.52,53 The contact formed between CEL-IL-AEL was very good with a clear interface due to good adhesion layer of GO-PANI with CEL or AEL. This may be attributed to the balanced hydrophilic-hydrophobic property of GO-PANI in dry state, and thus better harmonization with CEL/AEL. Electro-dialysis experiment observed that the applied potential 10 V sufficient for water molecule dissociation under the best conditions and increases the rate of water splitting due to GO-PANI hybrid electro-catalyst. Conversion of Salt into their Corresponding Acid and Base.

To convert sodium

carboxylate salts into their corresponding acid and base, BPMED experiments were carried out under the different applied potentials (6.0–12.0 V). During the BPMED experiments, both electrode compartments were interconnected and recirculated with Na2SO4 solution (0.10 M), while acid/base compartments were recirculated with deionized water, and sodium carboxylate 12 ACS Paragon Plus Environment

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solutions of known concentration and volume were recirculated in two adjacent compartments (Figure 2). Constant potential (8.0 V) was applied across the electro-dialyser and resulting increased progressively, while after attaining the maximum value and it further decreased with time. Observed voltage-drop across the membrane may be explained by Donnan and diffusion potentials at solution–membrane interface, and solution resistance. Initially, the compartments fed by deionized water were offered high electrical resistance, due to high voltage drop. With onset of BPMED experiments, increase ionic molality in these compartments controlled the variation in current. Conversion of sodium salts of carboxylates (acetic acid, citric acid, oxalic acid, tartaric acid) and NaCl into their corresponding acid and base (NaOH) was achieved by BPMED, and variation in different acid concentrations with electricity passed (A s) have been included in Figure 6. It was observed that concentrations of all acids are increased almost linearly with electricity passed and followed the order: HCl > acetic acid > oxalic acid > tartaric acid > citric acid. Effective radii of these carboxylic anions are very close (~ 4.5-5.0 A), and thus size base exclusion of these anions will be similar.54 It seems that pKa values of these carboxylates are determining factor and control the formation of carboxylic acids. The pKa values for carboxylates (C2H3O2-, C2O42-, C4H4O62-, and C6H5O73-) are 4.5, 1.3, 2.9, and 3.1, respectively. These carboxylate anions require 1, 2, or 3 H+ to produce one mole of acids. Thus, under similar experimental condition amount of CH3COOH produced will be high in compare with other bi-basic or tri-basic carboxylic acids. Rate of water dissociation (J) across the BPM during BPMED, was obtained by following equation assuming negligible mass (water) transfer across the membrane.55

J =

Va C t − C 0 δ ∆t

(4)



The C0 and Ct are the initial and final concentration of base (NaOH) or carboxylic acid (RH; R-: Cl− or C2H3O2-, C2O42-, C4H4O62-, and C6H5O73-) concentration (mol m-3), ∆t is the time allowed for BPMED (s), Va is the total volume of electrolyte solution (1.0 ×10-3 m3), and δ is the effective membrane area (8.0×10-3 m2). The J vs electricity passed (A s) curves (Figure 7) also follow the similar pattern as acid concentration vs electricity passed curves (Figure 6). To assess the suitability of BPMED with proposed BPM membrane, energy consumption (W) and current efficiency (CE) of the process are important parameters for industrial validation. The energy consumption (E; kWhkg−1 for producing acid or base) in BPMED process may be obtained by using following equation:

(

)

t

W dt m 0

(5)

E k Wh kg −1 = ∫

where W is the watt (VI), V is the applied potential (V), I is the current, t is the time allowed for BPMED, and m is the weight of NaOH or RH (R-: Cl−, or C2H3O2-, C2O42-, C4H4O62-, and C6H5O73-) produced. The current efficiency (η) was estimated as:

η=

F Vs ∆C ×100 NIt

(6)

where F is the Faraday constant, M is the molecular weight of the base or acid, n is the stoichiometric number (n = 1, 2, or 3 in this case). Feasibility for the production of acid and base from salt was evaluated in terms of energy consumption, and CE (%), and relevant data for different salts are included in Table 3. The CE (%) and energy consumption value for NaCl solution (1.0 M) were found to be 97.57% and 0.904 kWh kg-1. In EDBPM, relatively low energy consumption, and high CE may be attributed to the higher water splitting at the interfacial zone of BPM and simultaneously ionic migration


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through CEL and AEL. Thus, major portion of electricity passed to the system was consumed for water splitting as well as for ionic migration, rather than co-ion leakage.56,57 Recovery of the product (carboxylic acid in this case) is an important parameter to examine the economic feasibility of any process, which may be defined as: Acid re cov ery =

C tA VtA ×100 C 0 S V0 S

(7 )

where CtA and VtA are the final concentration and volume of carboxylic acid in the acid compartment, while C0S and V0S are the initial concentration and volume of sodium carboxylate salt in the respective compartment. Recoveries of different carboxylic acids were also estimated and relevant data are included in Table 3. Under the operating conditions in BPMED, 71-63% recovery of the different carboxylic acids was obtained. These Data also revealed that CE and acid recovery were reduced, while energy consumption increased with the increase in the molecular weight of carboxylate anions. This may be attributed to the lowering in ionic transport by electro-diffusion of the carboxylate anions (X-) across the AEL. Availability of X- in the organic acid compartment may also control the water splitting at the interfacial zone of BPM, to maintain the electro-neutrality conditions. Furthermore, in view of fast mobility of H+, there may be possibility for the leakage of H+ towards anode side in the sodium carboxylate compartment, and similar phenomena may be possible with OH- on the other side. To reduce this possibility, care was rendered to balance the charge nature of the both, AEL and CEL. Therefore, the leakage of H+/OH- in opposite direction does not seems to be feasible, in case of available co-ion for the ion-substitution. Dissociation of sodium carboxylates (availability of ions) decreased with the increase in the carbon chain length (molecular weight). Further, low recovery and CE of carboxylic acid with relatively large carbon chain (molecular weight), may be attributed to: (i) low degree of dissociation for higher homologues of sodium carboxylates; the low 15 ACS Paragon Plus Environment


permselectivity across the AEL; and (ii) relatively slow migration along with low co-ion leakage of bulkier carboxylate anions. Co-ion leakage across the BPM was monitored by measuring Na+ content in the organic acid compartments and found to be negligible. Thus, reported BPM is efficient in nature due to negligible co-ion leakage under the operating conditions, which affects the product purity. Data revealed that performance of BPMED for converting homologous sodium carboxylates into their corresponding carboxylic acid, decreased slightly with the increase in carbon chain or molecular weights of the carboxylate anions (Table 4). Both the CE and energy consumption were significantly affected by characterises of CEL, AEL along with BPM. It seems, proposed scheme of BPMED for water/salt splitting ionic-migration is feasible due to low energy consumption, negligible possibility of co-ion leakage, which affects the product purity and lowered the CE. Excellent characteristics of CEL, AEL as well as BPM, and extremely low co-ion leakage across the BPM revealed the suitability of BPM with GO-PANI IL for the conversion of homologues sodium carboxylates into their corresponding acids. To assess the suitability of reported BPM for converting different sodium carboxylates into their corresponding acid and base by BPMED, CE and energy consumption values were compared with commercially available membranes under nearly similar experimental conditions (Table 5).58-60 Especially for converting sodium tartrate into tartaric acid and NaOH by BPMED, FT-BP (FuMA-Tech GmbH, Germany) BPM relatively low CE and high enrgy consumption value in compare with reported BPM. Here, it is worthy to state that both CE and energy consumption depends on physicochemical and electrochemical properties of CEL, AEL and BPM along with cell configuration of BPMED unit. For efficient water/salt dissociation by


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BPMED, reported BPM seems to be attractive due to low energy consumption and negligible possibility of co-ion leakage. CONCLUSIONS PPO was modified by grafting of –SO3H and –N+R3 groups to prepare CEL and AEL thin film membrane. GO-PANI composite was synthesized by chemical oxidation polymerization process and used as IL of the BPM due to high electro-active characteristics, and water dissociation catalyst. BPMs with GO-PANI as IL was architected by layer-by-layer solution casting method of CEL, IL, and AEL, respectively in same solvent to obtain good adhesion between different layers. Prepared BPM-GO(P) showed good water content, high membrane conductivity and IEC. Prepared BPM was drastically studied for mechanical, acid-base, oxidative, and hydrolytic stabilities, and found to be suitable for operating in the harassed experimental conditions. The IL provides a synergetic effect to improve the conductivity of PANI and to mitigate the GO aggregation, thus may be of great interest WD catalyst in the IL of BPM. Also BPM-GO(P) exhibited superior CVC parameters in compare with pristine BPM-GO in equilibration with NaCl or different carboxylates solutions and considered as promising candidate for converting homologues carboxylates into their corresponding acid and base by BPMED. During BPMED, the concentrations of acid produced linearly increased with electricity passed and followed the order: HCl > acetic acid > oxalic acid > tartaric acid > citric acid. Under the operating conditions in BPMED, 71-63% recovery of the different carboxylic acids was recorded with 92-97% CE and 0.90-0.90 kWh kg-1 energy consumption. Negligible co-ion leakage across the BPM also revealed its efficient nature and product (carboxylic acid) purity. These data were obtained with laboratory scale BPMED cell, in case of commercial BPMED



unit with many cells and membranes the performance of BPM may be slightly altered and should be carefully analysed. Further, these preliminary data may encourage the membrane researches to fabricate BPM with well hydrophilic-hydrophobic balanced IL (such as GO-PANI) and diversified CEL/AEL for efficient water splitting. ASSOCIATED CONTENT s Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental procedure for counter-ion transport number measurements, method for oxidative, hydrolytic, acid and base stability assessment and relevant data, 1H NMR for PPO and chlromethylated PPO; ATR-FTIR for GO and GO-PANI; TGA curves for different BPMs. AUTHOR INFORMATION Corresponding Author ∗

E-mail: [email protected] or [email protected]. Fax: +91-0278-2566970. Tel.: +91-2782569445.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Manuscript registration number: CSIR-CSMCRI- 013/2018. Instrumental support received from Analytical Science Division, CSIR-CSMCRI, is also gratefully acknowledged. REFERENCES (1) Kemperman, A. J. B. Handbook on bipolar membrane technology, Twente University Press, Enschede, 2000, 17–46.


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Table 1. Physicochemical and Electrochemical properties of CEL, AEL and BPM Properties Thickness(µm) Water Content (%) Ion- Exchange Capacity (meq./of dry membrane) Counter-Ion Transport Number ( ) Membrane Conductivity (S cm ) Swelling ratio (%)

CEL 200 17 1.41

AEL 200 21 1.38

BPM-GO 414 27 -

BPM-GO(P) 418 30 -

0.87

0.85

-

-

4.85 ×10-2

4.72 × 10-2 5.3 × 102

6.01 ×10-2

12.7

11.5

14.8

15.2

Table 2. Different Parameters Estimated from CVCs for BPM-GO(P) in Equilibration with 0.10M Different Salt solution of Electrolytes Salt Acetate Tartrate oxalate Citrate

( ) 9.32 1.78 3.32 2.88

( ) 21.21 6.23 12.44 8.85

(V) 0.77 1.17 1.02 1.12

() 2.38 2.12 2.81 2.88

Table 3 Current Efficiency (CE%), Base Recovery and Energy Consumption (E), Data for Converting Sodium Salts (0.1M) into Corresponding Acid and Base by EDBPM using BPM-GO(P). Salt

CE%

NaCl Acetate Tartrate oxalate Citrate

97.6 96.6 80.3 95.0 82.9

Recovery % 75.0 71.0 64.5 66.9 63.0

E (KWh/Kg) 0.90 0.95 0.97 0.98 0.98



Page 28 of 35

Table 4. Water Splitting and Flux (JNaOH) for Different Salts Solutions (0.1M) during EDBPM using BPM-GO(P). Salt

NaCl Acetate Titrate oxalate Citrate

Rate of water splitting (mL h-1) (Calculated) 0.265 0.201 0.251 0.221 0.254

JNaOH × 10-3 (mol m-2 s-1) 6.23 5.45 6.01 5.89 6.07

Table 5. Comparision of BPM Performance in EDBPM for the Conversion of Sodium Salts into their Corresponding Acids and Base. Bipolar membrane BPM-3400a FT-BP (FuMA-Tech GmbH, Germany)b BP-1 (Tokuyama Soda), JapanC BP-1 (Neosepta)d BPM-GO(P)e

CE (%) 88.8 92 30 72 93.3

a

W (kWh kg-1) 2.03 12-96 4.1 4.3-16.6 0.97

Reference 22 58 59 60 In this work

BPM used for producing NaOH and HCl from 0.5 M NaCl. Commercial BPM used for producing tarteric acid from its sodium salt. c Commercial BPM used for the producing p-toluenesulfonic acid from its sodium salt. d Commercial BPM used for the producing formic acid from its sodium salt. e Reported BPM used for producing tarteric acid from its sodium salt. b


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CHCl3, (HCHO)n, TMCS, SnCl4 40-45oC, 24h

NMP, TMA 60-65oC Chloromethylated PPO Quaternized PPO (AEL)

Chlorobenzene, H2SO4 3h, RT Sulphonated PPO (CEL)

PPO Scheme 1. Preparation of AEL and CEL.

Scheme 2. Schematic route for preparation of GO-PANI composite.



Figure 1. Two-compartment glass cell (50 cm3) separated by BPM for i-V, 1: anode; 2: cathode; 3: magnetic stirrers; 4: Haber-Luggin capillaries; 5: BPM; and 6: membrane holder

Figure 2. Schematic persentation for conversion of carboxylates into carboxylic acid by BPMED.


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Figure 3. SEM images of (A) AEL, (B) CEL and (C) cross-section image of BPM-GO(P).

Figure 4. Membrane conductivity (κm) values for CEL, AEL and BPM-GO(P), in equilibrium with NaCl solution of different concentrations.



Figure 5. The i-V curves for BPM-GO(P) in equilibrium with different sodium carboxylates and NaCl solutions (0.10 M concentration).

Figure 6. Formation of different acids as function of electricity passed by splitting of corresponding salt in EDBPM using BPM-GO(P).


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Figure 7. Variation of J with electricity passed (A s) during BPMED using BPM-GO(P) for converting different salts into their acid and base.



For Table of Contents Use Only

Under applied potential, water molecules dissociate in the IL zone of the BPM (optical image), and generate H+ and OH-.

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For Table of Contents Use Only

Under applied potential, water molecules dissociate in the IL zone of the BPM (optical image), and generate H+ and OH-.

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