Electrochimica Acta 54 (2009) 1630–1637

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Electrochemical membrane reactor: Synthesis of quaternary ammoniumhydroxide from its halide by in situ ion substitution

Mahendra Kumar, Bijay P. Tripathi, Arunima Saxena, Vinod K. Shahi ∗

Electro-membrane Processes Division, Central Salt & Marine Chemicals Research Institute,Council of Scientific & Industrial Research (CSIR), G. B. Marg Bhavnagar 364002 (Gujrat) India

a r t i c l e i n f o

Article history:Received 10 April 2008Received in revised form19 September 2008Accepted 24 September 2008Available online 4 October 2008

Keywords:Anion-exchange membraneElectrochemical membrane reactorElectrodialysis

a b s t r a c t

Electrochemical membrane reactors (EMRs) with two compartments (EMR-2: anion-exchange membrane(AEM) separated catholyte and anolyte) and three compartments (EMR-3: three compartments separatedby two AEMs to avoid contact between the product and the electrodes) were developed for the synthesisof tetrabutylammonium hydroxide (TBAOH) from tetrabutylammonium bromide (TBABr) by in situ ionsubstitution. In house prepared AEM with good physicochemical, electrochemical properties and excellentstabilities was used. Schematic diagrams are presented for the possible synthesis of TBAOH from TBABr byin situ ion substitution in EMR-2 and EMR-3. Synthesis of TBAOH using EMR-2 and EMR-3 was achievedunder different experimental conditions and process parameters (rate of synthesis, current efficiency(CE) and energy consumption) were estimated. In EMR-2, relatively slow synthesis of TBAOH with lowrecovery was explained due to Hofmann elimination of TBAOH in contact with the electrode. While in

Quaternary ammonium hydroxideC

EMR-3, relatively faster rate of TBAOH synthesis with its high recovery and current efficiency indicatede dev

1

awsTasthsTotdngIfs

Ha

(

Aot

caimptr

0d

urrent efficiency practical application of thadditives or reagents.

. Introduction

Tetrabutylammonium hydroxide (TBAOH) is not easily obtain-ble as a pure compound; rather it is employed as a solution inater or alcohols [1]. Relative to more conventional inorganic bases,

uch as KOH and NaOH, TBAOH is highly soluble in organic solvents.BAOH is a strong base, which readily deprotonates the carboxyliccid moiety of amino acid and organic acids to form a carboxylatealt and water. Generally, TBAOH preparation has been achieved byhe exchange of OH− by halide ions by the addition of alkali withalide salt as a byproduct. Further separation of TBAOH and halidealt is very difficult due to their high water solubility. In addition,BAOH behaves as highly conducting ionic liquids and bulky naturef the tetrabutylammonium cation reduces intermolecular interac-ions [2]. Pure TBAOH has been used as phase transfer catalysts foriverse chemical transformations [1,3–5], as template for preparinganoparticles [6], for transesterification [7], in thermochemolysis-

as chromatography [8], and for other commercial processes [9].n all these processes, TBAOH purity enhances its catalytic per-ormance. Attempts were made to isolate pure TBAOH by ionubstitution of tetrabutylammonium halide [10], which induced

∗ Corresponding author. Tel.: +91 278 2569445; fax: +91 278 2567562/2566970.E-mail addresses: vkshahi@csmcri.org, vinodshahi1@yahoo.com (V.K. Shahi). T

013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2008.09.049

eloped process for the efficient synthesis of TBAOH without the use of any

© 2008 Elsevier Ltd. All rights reserved.

ofmann elimination as follows, leading to the formation of tertiarymine and 1-butene:

C4H9)4N+OH− → (C4H9)3N + CH3-CH2-CH CH2 + H2O.

lso, further separation of halogen acids is a difficult problem. Nother chemical method is available for synthesizing TBAOH frometrabutylammonium halide.

Among the possible alternatives, membrane technologies, espe-ially the electrochemical membrane reactor (EMR), offer manydvantages for achieving separation and in situ ion substitutionn different chemical transformations [11–19]. In this process,

embrane electrolysis (ME) combines with ion transport across aermselective ion exchange membrane and electrochemical reac-ions at electrodes. During water electrolysis following electrodeeactions will occur.

Anode reaction:

1/2H2O → 1/4O2 + H+ + e−

Cathode reaction:

H2O + e− → 1/2H2 + OH−

Thus, complete electrochemical process for converting TBABr toBAOH consists of electro-substitution of ions and may be written

mica A

a

(

Tccuioa

btbmtTcnwt(toerehq

tpwpodwceE

2

2

sdSTbrs

coafeafswg

tcrcmt

2

N3E(wamwearr

ho2aalau

W

wlptTw

otwbctpcsfM

E

wtt

2

M. Kumar et al. / Electrochi

s follows:

C4H9)4N+Br− + OH− → (C4H9)4N+OH− + Br−

he main industrial application of this technology is found inhlor-alkali production [20], production of acid and base fromorresponding salt [21], conversion of sodium phenoxides intondissociated phenols [22], etc. During the past years ME was stud-

ed to generate concentrated mineral acids [21,23–25], conversionf sodium lactate into lactic acid [14,26], production of ammoniand nitric acid from ammonium nitrate wastewater [27], etc.

Water-splitting electrodialysis with bipolar membrane has alsoeen proposed as an effective separation and alkali productionechnique [28,29]. However, excessive salt diffusion across theipolar membrane is a serious problem, which reduces perfor-ance and product purity. Thus, an electrochemical route similar to

he chlor-alkali process will be useful for the efficient synthesis ofBAOH with high recovery. An electro-electrodialysis process usingation-exchange membrane was employed for producing quater-ary ammonium hydroxides from their respective halides [10]. Thisas achieved by transporting tetrabutylammonium cations from

he anolyte to the catholyte through a cation-exchange membraneNafion) followed by ion pairing with OH− (formed due to reduc-ive water splitting at the cathode) [10]. Slow electro-transportationf bulky tetrabutylammonium cation through the dense cation-xchange membrane was observed, which gave a low synthesisate of TBAOH. In this case, an electrochemical process using anion-xchange membrane will be suitable in view of the small size andigh transport number of the anion (halide) associated with theuaternary ammonium group used for synthesizing TBAOH.

In this investigation, an OH−/Br− selective AEM was developedo achieve efficient synthesis of TBAOH from TBABr, based on therinciple of electro-electrodialysis in EMR-2. Synthesis of TBAOHas achieved with moderate current efficiency (CE) but its lowerroduct recovery was assigned to the Hofmann elimination becausef degradable nature of TBAOH. EMR-3 using the same AEM was alsoeveloped, in which the product stream was not in direct contactith the electrodes and thus avoids degradation of TBAOH. It was

oncluded that under optimized experimental conditions, fast andfficient synthesis of TBAOH with high recovery was possible usingMR-3.

. Experimental

.1. Materials and membrane preparation

Polyvinyl alcohol (PVA; MW: 125,000), hydrochloric acid,odium hydroxide, sodium chloride, ammonium persulfate andimethyl sulfate of Analytical Reagent grade were obtained from.D. fine Chemicals, India and used without any further purification.etraethylorthosilicate (TEOS), 4-vinylpyridine (4-VP), and divinyl-enzene were received from Sigma Aldrich Chemicals and used aseceived. Distilled water was used for the preparation of all theolutions.

50 g of PVA was dissolved in 500 ml of hot deionized water underonstant stirring to obtain a homogeneous solution. Then 15 gf 4-vinylpyridine (monomer), 0.60 g divinylbenzene, and 0.60 gmmonium persulfate were added at 70 ◦C, the mixture was keptor 1 h to get the semi-interpenetrating polymer network. Furtherquivalent amount of the dimethyl sulfate with respect to 4-VP was

dded to quaternize the pyridinium group. The mixture was stirredor 2 h at room temperature (30 ◦C) to obtain a clear homogeneousolution. Then 10 ml of TEOS was added at 30 ◦C and the mixtureas kept under stirred condition for 24 h to get a gel. The resulting

el was cast on a clean glass plate and dried at room tempera-

T

Ep

cta 54 (2009) 1630–1637 1631

ure to obtain a film. Dried membrane was immersed in a solutionontaining formaldehyde (54.1 g), sodium sulfate (150.0 g), sulfu-ic acid (125.0 g), and water (470.0 g) for 2 h at 60 ◦C for effectiveross-linking. The obtained membrane was conditioned by treat-ent with 1 M HCl and 1 M NaOH successively and then washed

horoughly with distilled water before its use.

.2. Investigations on membrane electrochemical properties

Membrane conductivity measurements in equilibration withaCl, TBABr and TBAOH solutions of different concentrations at0 ◦C were carried out using a clip cell as reported earlier [30].xperimental cell composed of two black graphite electrodes1.0 cm2) fixed on the plexiglass plates. The equilibrated membraneas sandwiched between the electrodes and secured in place withset of screws. Membrane conductivity was measured in terms ofembrane resistance (Rm), which was estimated by Rm = Rcell − Rsol,here Rcell is the total cell resistance and Rsol is the resistance of

quilibrating solution. Membrane resistance was measured withdigital conductivity meter (Century, model CC601; conductance

ange, 0–200 mS; frequency, 1–50 kHz; AC current) with ±0.01 mSeproducibility.

The thickness of the wet membrane was measured with theelp of micrometer up to 0.1 �m accuracy. For the determinationf water content, membranes were immersed in distilled water for4 h. Then their surface moisture was mopped with tissue papernd the wet membrane weighed. Wet membranes were then driedt a fixed temperature of 333 K until constant weight. Due to pro-onged heating (about 6 h) at temperature 333 K, membrane lostbsorbed water. Water content of the membrane was calculatedsing the following equation:

ater content = Ww − Wd

Wd× 100 (1)

here Ww and Wd are the weights of the membrane at the equi-ibrium swelling (wet) and dry state, respectively. The degradationrocess and the thermal stability of the membranes were inves-igated using thermogravimetric analysis (TGA) (Mettler ToledoGA/SDTA851e with stare software), under nitrogen atmosphereith a heating rate of 10 ◦C/min from 50 to 600 ◦C.

For the estimation of ion-exchange capacity, the desired piecef AEM was equilibrated in NaOH solution under 8 h constant agi-ations for converting into the OH− form and then it was washedith distilled water to remove excess of NaOH. The washed mem-

rane was then equilibrated with 50 ml of 0.50 M NaCl solution. Theoncentration of liberated OH− ions was estimated by acid–baseitration [31]. Counter-ion transport number in the membranehase (tm− ) in equilibration with NaOH solution of 0.055 M meanoncentration was determined by membrane potential (Em) mea-urements. Using membrane potential data (tm− ) was estimatedrom the membrane potential measurements using TMS (Teorell,

eyer, and Sievers) approach [32,33]:

m = (2tm− − 1)

RT

Fln

a1

a2(2)

here a1 and a2 are the activities of electrolyte solutions contactingwo surfaces of the membrane, R is the gas constant, T is the absoluteemperature, and F is the Faraday constant.

.3. Experimental procedure for the synthesis of TBAOH from

BABr in EMR-2

Schematic diagram of the experimental setup employed forMR-2 is presented in Fig. 1(A). The system consisted two tanks,eristaltic pumps, and an adjustable DC power supply. The mem-

1632 M. Kumar et al. / Electrochimica Acta 54 (2009) 1630–1637

Fa

bwaera(fws1dadiapaatotaaratw

2

ttmhwcstawa

Table 1Physicochemical and electrochemical properties of the anion-exchange membrane.

Property Value

Thickness (�m) 150Water content (%) 25.0Ion-exchange capacity (mequiv./g of dry membrane) 1.89Counter-ion transport numbera (tm

− ) 0.96Membrane conductivityb (S cm−1) 2.0 × 10−3

a m

i

t

Lcmr

3

3

pPicgoc

tnsmamdbiTi

For developing an electrochemical membrane reactor, theknowledge of membrane conductivity in operating conditions isan essential parameter. Membrane conductivities were recordedin equilibration with TBABr and TBAOH solutions of different

ig. 1. Schematic diagrams of: (a) EMR-2 and (b) EMR-3 for synthesis of quaternarymmonium hydroxides from respective halides.

rane reactor was made of polytetraflouroethylene (PTFE), inhich two compartments viz. catholyte and anolyte were sep-

rated by the AEM. Distance between both electrodes and theffective membrane area was 1.10 × 10−2 m and 8.0 × 10−3 m2,espectively. Two expanded TiO2 sheets with 1.5 mm thicknessnd 8.0 × 10−3 m2 area, coated with a triple precious metaltitanium–ruthenium–platinum) oxide (6 �m thickness), obtainedrom Titanium Tantalum Products (TITAN, India), Madras, India,ere served as electrodes. All experiments were carried out at con-

tant applied current using constant DC power supply (model L285, Aplab, Mumbai, India), while voltage was measured usingigital multimeter (model 435, Systronics, India) connected in par-llel. Two storage tanks and pumps were used to continuouslyeliver anolyte and catholyte in recirculation mode of operation

nto membrane reactor with 0.006 m3/h constant flow rate, to cre-te high turbulence in all compartments. The whole setup waslaced in an environment at room temperature (30 ◦C) without anydditional temperature control. Initially 300 cm3 of distilled waternd TBABr solution of known concentration were recirculatedhrough anolyte and catholyte, respectively. Under the influencef current, OH− produced at cathode by reductive water split-ing converted TBABr into TBAOH and Br− migrated towards anodend produced HBr. Current flowing in the system under constantpplied current and the pH values of catholyte and anolyte wereecorded as function of time by the pH sensors kept on the catholytend anolyte. TBAOH concentration in the catholyte was also moni-ored regularly. In all cases, equal volumes of anolyte and catholyteere taken to study the feasibility of the separation process.

.4. Synthesis of TBAOH in EMR-3

Fig. 1(B) shows schematic diagram of EMR-3. Dimensions andhicknesses of each compartments and electrodes were kept similaro the EMR-2. In this case, EMR was divided into three compart-

ents viz. central compartment, catholyte and anolyte with theelp of two AEMs. Electrode material, power supply and flow rateere same as EMR-2. Initially 300 cm3 distilled water was recir-

ulated through catholyte and anolyte, separately. 300 cm3 TBABr

olution of known concentration was recirculated through cen-ral compartment using peristaltic pumps. Under the influence ofpplied current densities, OH− produced at cathode by reductiveater splitting migrated to the central compartment through AEM

nd in situ substituted Br− leading to the formation of TBAOH.

(t− ) was estimated from membrane potential measurements for a membranen equilibrium with NaOH solutions of 0.10 and 0.01 M concentrations.

b Membrane conductivity was measured in equilibration with 0.10 M NaCl solu-ion.

iberated Br− migrated to the anolyte and formed HBr. TBAOHoncentration in the central compartment and pH of all compart-ents were also monitored regularly by pH sensors placed in the

espective compartments.

. Results and discussion

.1. Properties of anion-exchange membranes

The physicochemical and electrochemical properties of AEMrepared and used in this investigation are included in Table 1.repared anion-exchange membrane showed good water content,on-exchange capacity, anion selectivity, and specific membraneonductivity. Mechanical strength of the membrane was investi-ated by the dynamic mechanical analysis (DMA) and no breakingr elongation in the membrane was observed under the testingonditions.

The thermal stability of the membrane was illustrated by itshermogravimetric analysis study. The TGA curve under flowingitrogen is presented in Fig. 2. Curve shows two main degradationtages arising from desolvation and thermal oxidation of the poly-er matrix. The first weight loss occurred below 100 ◦C and was

ttributed to the loss of absorbed water molecules in the membraneatrix. The second weight loss region (300–400 ◦C) corresponds to

ecomposition of main chain of PVA. In addition, AEM was kept inoiling water for a prolonged time and no loss either in weight or

n ion exchange capacity and dimensional changes was observed.hus, developed AEM was suitable for the use in EMR because ofts thermal, mechanical and dimensional stabilities.

Fig. 2. TGA curve for anion-exchange membrane.

M. Kumar et al. / Electrochimica Acta 54 (2009) 1630–1637 1633

FT

ce

�

w1ciwvditHiciiq

3r

bcttroipaaPHdedw

Ea(

Fh

tccadochatwfAscftaacat constant applied current density (15.0 mA cm−2), respectively.Initially, anolyte offered pH 7 because water was passed in thebeginning, and afterwards it was decreased due to the formation ofH+ or HBr and attained almost limiting value after sufficient amount

ig. 3. Specific membrane conductivity of AEM in equilibration with TBABr andBAOH solutions of different concentrations.

oncentrations. The specific membrane conductivity (�m) wasstimated by the relation:

m = �x

ARm(1)

here �x is the thickness of the wet membrane, A its area and/Rm is its electrical conductivity. Variation of specific membraneonductivity of AEM with TBABr and TBAOH solution concentrations presented in Fig. 3. Membrane conductivity initially increased

ith TBABr or TBAOH concentration and attained almost limitingalue beyond 0.50 M. Also, membrane conductivity was highlyependent on the ionic concentration in the membrane/solution

nterfacial zone. This variation may be attributed to the rela-ively low dissociation of TBABr/TBAOH at higher concentration.owever, membrane showed comparable conductivity values

n equilibration with either TBABr or TBAOH solution of equaloncentration. Furthermore, comparable membrane conductivitiesn equilibration with either TBABr/TBAOH or NaCl (data presentedn Table 1 and Fig. 3) reveal suitability of AEM for operation inuaternary ammonium hydroxide/bromide environment.

.2. Synthesis of quaternary ammonium hydroxide fromespective bromide in EMR-2

Synthesis of quaternary ammonium hydroxide from respectiveromide was carried out by ion substitution reaction in the cathodehamber of EMR-2 as described in Fig. 1(a). Single step ion substitu-ion process involved two stages: (i) the formation of OH− and H+ athe cathode and anode by reductive and oxidative water splitting,espectively; (ii) ion substitution of Br− by OH− and the formationf HBr by combining H+ present in the anolyte. Thus in this process,on substitution was achieved by the combined effect of electrodeolarization and migration of Br− through AEM from catholyte tonolyte. A schematic representation of possible electro-migrationnd ion substitution in both compartments is presented in Fig. 4.roton produced at the anode was partly consumed for producingBr, while rest was transferred at the cathode through the AEMue to their fast mobility (protons can cross the AEM under appliedlectric field due to their fast mobility). Simultaneously, OH− pro-uced at cathode also might have transferred partly towards anode,

hile rest was consumed for producing hydroxide in the catholyte.

Experiments for the synthesis of TBAOH was carried out inMR-2 at different applied current density ranging between 10nd 20 mA cm−2 using TBABr solutions of different concentrations0.1–1.0 M) as initial feed of catholyte, while water was fed in

Fha

ig. 4. Schematic representation of the possible synthesis of quaternary ammoniumydroxide from respective bromide by in situ ion substitution in EMR-2.

he anolyte in recirculation mode of operation. The variations ofell voltage with time at applied current density and 0.1 M TBABratholyte feed are presented in Fig. 5, as a representative case. Forchieving constant current density, initially cell voltage was highue to high electrical resistance of EMR because of high resistancef anode compartment (electrical resistance offered by AEM andatholyte was very low) through which water was passed offeredigh electrical resistance. In this case, two compartments (catholytend anolyte) and AEM acted as a resistor in series and only resis-ance of anolyte was dominated initially. Progressively, cell voltageas reduced, because of increase in ionic concentration due to the

ormation of H+ and migration of Br− from catholyte to anolyte.fter certain time, resistance offered by anolyte was reducedubsequently and attained almost constant value. During electro-hemical process, applied current density was responsible for theormation of OH− and H+ and migration of Br−, resulting varia-ions of pH of both compartments. Variations of pH of catholytend anolyte with Coulombs passed are presented in Fig. 6(A and B)t varied applied current density with 0.1 M TBABr as initial feed ofatholyte and also at varied initial feed concentration of catholyte

ig. 5. Variation of current with time during synthesis of quaternary ammoniumydroxide in EMR-2 and catholyte feed was 0.10 M TBABr solution at differentpplied current densities.

1634 M. Kumar et al. / Electrochimica Acta 54 (2009) 1630–1637

F R-2: (c of TBA

o(som

rtaBct

J

wtpiTtsTrswcwsctaie

mac

ppkE

W

weif

C

wtpcRt

T

w

F1

ig. 6. Variation of anolyte and catholyte pH values with applied coulombs in EMatholyte; (b) at 15.0 mA cm−2 applied current density and different concentration

f Coulombs was passed. pH of catholyte was initially about 6.0–6.5depending on the concentration of TBABr), which was increasedubstantially due to formation of OH− and thus TBAOH. Variationf pH in both the compartments supports the basic electrochemicalechanism proposed for EMR-2.Synthesis rate of TBAOH (J) in the EMR can be defined as the

ate of Br− transport from catholyte to anolyte or rate of forma-ion of HBr in the anolyte. Estimation of Br− concentration in thenolyte with time was difficult because partial Br− was oxidized tor2 and rest formed HBr. Thus, J values were estimated from OH−

oncentration in the catholyte. Considering negligible mass (water)ransport through AEM, as in this case, J may be defined as [13,34]:

= Va

A

Ct − C0

�t(2)

here C0 and Ct are the initial and final concentrations of TBAOH inhe catholyte (mol m−3), �t the time allowed for electro-membranerocess (s), Va the total volume of anolyte (0.30 × 10−3 m3), and A

s the effective membrane area (8.0 × 10−3 m2). Synthesis rate ofBAOH (J) in the EMR-2 are presented in Fig. 7(A and B) as a func-ion of time at different applied current densities with 0.1 M TBABrolution as initial feed of catholyte and at varied concentrations ofBABr solution at constant applied current density (15.0 mA cm−2),espectively. For all applied current density, nature of curves wasame. At the beginning of the process, J values increased linearlyith time and negligible back diffusion of Br− from anolyte to

atholyte due to extremely low concentration of Br− in the anolyteas observed. For this process, simultaneous migration and sub-

titution of Br− is necessary for maintaining the electro-neutrality

onditions in both compartments. Synthesis rate of TBAOH attainedhe highest value (the case when Br− concentration both in anolytend in catholyte was equal) because Br− concentration increasedn the anolyte and afterwards reduced progressively because ofnhanced back diffusion of Br− from anolyte to catholyte. Further-

fiot

t

ig. 7. Variation of synthesis rate of TBAOH (J) with time in EMR-2: (a) at different app5.0 mA cm−2 applied current density and different concentration of TBABr solution as in

a) at different applied current density with 0.1 M TBABr solution as initial feed ofBr solution as initial feed of catholyte.

ore, TBAOH synthesis rate was not only highly dependent onpplied current density but also strongly depended on initial feedoncentration of the catholyte.

Energy consumption and current efficiency are importantarameters for assessing the suitability of any electrochemicalrocess for practical applications. The energy consumption (W,Wh kg−1 of TBAOH produced) for the synthesis of TBAOH in theMR may be obtained as follows [15]:

(kWh kg−1) =∫ t

0

VI dt

m(3)

here V is the cell voltage, I the current, t the time allowed for thelectrochemical process, and m is the weight of TBAOH synthesizedn the catholyte. The overall current efficiency was defined as theraction of Coulombs utilized for the synthesis of TBAOH:

E (%) = mnF

MQ× 100 (4)

here F is the Faraday constant, M the molecular weight of TBAOH, nhe stoichiometric number (n = 1 in this case), and Q is the Coulombassed (As). An electrochemical process must not only be techni-ally feasible, but should also be less expensive and green in nature.ecovery of the product is also an important parameter to examinehe economic feasibility of any process, which may be defined as

BAOH recovery (%) = CtPVtP

C0F V0F× 100 (5)

here V0F is the initial volume of TBABr feed stream, and VtP is the

nal volume of the product stream, C0F is the initial concentrationf TBABr feed stream and CtP is the final concentration of TBAOH inhe product stream.

To evaluate the technical and economic feasibility for the syn-hesis of TBAOH in EMR-2, specific energy consumption, CE (%)

lied current density with 0.1 M TBABr solution as initial feed of catholyte; (b) atitial feed of catholyte.

M. Kumar et al. / Electrochimica Acta 54 (2009) 1630–1637 1635

Table 2Two compartments membrane reactor: current efficiency, energy consumption and TBAOH recovery data under different experimental conditions.

Applied currentdensity (mA cm−2)

Coulomb passed(×103 C)

TBABr feed incatholyte (M)

Current efficiency(%)

W (kWh kg−1) ofTBAOH produced

Recovery ofTBAOH (%)

10.0 7.20 0.10 72.36 1.65 54.015.0 7.60 0.10 57.01 1.81 44.920.0 7.80 0.10 44.04 1.98 35.622.5 8.20 0.10 33.70 2.12 28.715.0 10.10 0.20 51.59 2.26 27.0

50.88 2.25 14.546.71 2.13 9.5

A applied current, feed of catholyte: TBABr (300 cm3); anolyte: deionized water (300 cm3).

adww0mstwdnwtiHcbtwcto

dawaHspue

3r

sEatccaHFdpdwcd

Fig. 8. Schematic representation of the possible synthesis of quaternary ammoniumhydroxide from respective bromide by in situ ion substitution in EMR-3.

Fig. 9. Variation of current with time during synthesis of quaternary ammoniumhydroxide in EMR-3 at different applied current densities.

15.0 13.50 0.4015.0 15.70 0.80

ll experiments were carried out in recirculation mode of operation under constant

nd recovery (%) of TBAOH data under different experimental con-itions are presented in Table 2. Energy consumption increased,hile current efficiency and recovery of TBAOH were decreasedith an increase in the applied current density, for initial feed of

.1 M TBABr solution. At relatively high applied current density, for-ation of OH− will be quite large because of the increased water

plitting, which would have enhanced the formation of TBAOH, buthis was not the case. Observed reduction in the current efficiencyith applied current density may be because of either (i) partialegradation of TBAOH (Hofmann elimination) occurred since it isot very inert to the electrode reaction or (ii) leakage of OH− alongith Br− from catholyte to anolyte. Under strong alkaline condi-

ion (at high applied current density), TBAOH seems to be degradednto tertiary amine and 1-butene because of Hofmann elimination.ofmann elimination reaction of TBABr/TBAOH in contact withathode was confirmed by recording FTIR spectra of feed streamefore and after the electrode reaction. Formation of alkenes dueo Hofmann elimination reaction of TBABr/TBAOH in the catholyteas confirmed from the existence of additional peaks, which indi-

ated the formation of alkenes (supporting information). It seemshat both factors were responsible for observed low CE and recoveryf TBAOH in EMR-2.

TBAOH recovery at higher applied current density was reducedue to Hofmann elimination, and resulted low current efficiencynd high energy consumption. Also TBAOH recovery was decreasedith the increase in initial concentration of TBABr in the catholyte

t constant applied current density (15.0 mA cm−2). This meansofmann elimination reaction was not only sensitive to alkaline

trength but it also depends on TBAOH concentration. To avoid thisroblem, EMR was modified in such a way that feed TBABr (prod-ct stream) should not have any contact to the electrode, which willliminate its degradation.

.3. Synthesis of quaternary ammonium hydroxide fromespective bromide in EMR-3

Single step in situ production of OH−, its separation and ionubstitution for the synthesis TBAOH from TBABr was achieved byMR-3, in which central compartment was separated from anolytend catholyte by two AEMs, as shown in Fig. 8. This process involvedhree stages: (i) generation of OH− by reductive water splitting atathode and its migration through AEM from catholyte to centralompartment; (ii) substitution of Br− by OH−; (iii) migration of Br−

cross AEM from central compartment to anolyte and formation ofBr/Br2. Cell voltage vs. time curves for EMR-3 are presented inig. 9, which are similar to that obtained for EMR-2 (Fig. 5). Voltagerop across the EMR-3 was relatively high because of three com-

artments, of which two compartments were initially very resistiveue to water feed. For a constant current, cell voltage in this caseas comparatively high. Thus in EMR-3, we applied slightly higher

urrent density (20–25 mA cm−2) in comparison to EMR-2 for pro-ucing TBAOH. Fig. 10 shows the variation of pH of the anolyte

Fig. 10. Variation of anolyte and central compartment pH values with appliedcoulombs in EMR-3 at different applied current density with 0.1 M TBABr solutionas initial feed of central compartment.

1636 M. Kumar et al. / Electrochimica Acta 54 (2009) 1630–1637

Fig. 11. Variation of synthesis rate of TBAOH (J) for EMR-2 and EMR-3: (a) at different applied current density with 0.1 M TBABr solution as initial feed of product stream; (b)at 15.0 mA cm−2 applied current density and different concentration of TBABr solution as initial feed of product stream.

Table 3Three compartments membrane reactor: current efficiency, energy consumption and TBAOH recovery data under different experimental conditions.

Applied currentdensity (mA cm−2)

Coulomb passed(×103 C)

TBABr feed in centralcompartment (M)

CE (%) W (kWh kg−1) ofTBAOH produced

Recovery ofTBAOH (%)

20.0 10.80 0.10 81.26 1.78 91.022.5 10.80 0.10 79.23 1.97 88.725.0 10.80 0.10 76.16 2.03 85.222.5 19.99 0.20 82.65 2.17 85.622.5 25.97 0.40 85.47 2.29 57.522

A t appw

attgoatEfTEciidT

dtrwwin1shtemiv

4

t

cEbcastaterc

etdtatate

A

CaDRa

2.5 29.84 0.602.5 34.42 0.80

ll experiments were carried out in recirculation mode of operation under constanater (300 cm3); anolyte: deionized water (300 cm3).

nd central compartment for EMR-3 under different experimen-al conditions. The pH of the central compartment increased dueo the formation of TBAOH, while for anolyte it was reduced pro-ressively with the passage of the electricity due to the formationf HBr. Also, pH of both the compartments was dependant on thepplied current density and initial feed concentration of TBABr inhe central compartment. Synthesis rate of TBAOH for EMR-2 andMR-3 was compared under similar experimental conditions as aunction of applied current density and initial feed concentration ofBABr in the central compartment. It was observed that J values forMR-3 were about two times higher than those for EMR-2 underomparable conditions (Fig. 11). Also, rate of synthesis for TBAOHn EMR-3 increased with applied current density, while for EMR-2t was reduced. This variation may be explained by high extent ofegradation of TBAOH in EMR-2 in comparison to EMR-3, whereBAOH was not in direct contact with the electrodes.

Process efficiency, energy consumption and TBAOH recoveryata for EMR-3 are presented in Table 3 for different experimen-al conditions. It was observed that current efficiency and TBAOHecovery was decreased while energy consumption was increasedith the increase in applied current density. All these parametersere very sensitive to the operating conditions of EMR-3 along with

nitial feed concentration of TBABr. In this case, Hofmann elimi-ation of TBAOH leading to the formation of tertiary amine and-butene was insignificant in comparison with EMR-2, because ofeparate electrode and product chambers. Furthermore, extremelyigh current efficiency and recovery of TBAOH in EMR-3 indicatehe suitability of the process for the industrial exploitation. How-ver, one has to completely optimize the process for obtainingaximum current efficiency, high recovery of TBAOH along with

ts fast synthesis rate, all of which has high impact on economiciability of this process.

. Conclusions

AEM with good physicochemical and electrochemical proper-ies, high selectivity for OH−/Br−and high mechanical, thermal and

A

i

86.13 2.31 44.487.02 2.37 38.8

lied current, feed of central compartment: TBABr (300 cm3); catholyte: deionized

hemical stabilities was prepared and employed for developingMR-2 and EMR-3 for efficient synthesis of TBAOH from TBABrased on the principles of electro-electrodialysis without any use ofhemicals or further separation. Although, EMR-2 showed moder-te current efficiency at low applied current density and low TBABrolution initial feed concentration, but low recovery of TBAOH dueo its Hofmann elimination was a serious drawback. EMR-3 waslso developed in which TBABr was not in direct contact with elec-rode and thus avoided degradation of TBAOH up to maximumxtent. Comparatively high recovery (85–90%) with excellent cur-ent efficiency (80–85%) was obtained for selective experimentalonditions.

The obtained results indicate the suitability of EMR-3 for thefficient synthesis of TBAOH in an economically viable manner. Fur-hermore, all these process parameters were estimated in lab scaleepends upon the nature, stability and durability of AEM and reac-or design. Thus, complete optimization of the EMR is necessary forchieving efficient synthesis of TBAOH. Above all, this novel elec-rochemical membrane reactor did not consume any chemicals andll separation and ion substitution occurred through water split-ing, providing eco-friendly and economically viable route for thefficient synthesis of TBAOH.

cknowledgements

One of the authors (Mahendra Kumar) is grateful to theSIR, India, for providing a Junior Research Fellowship. Wecknowledge the support and encouragement of Dr. P.K. Ghosh,irector, CSMCRI, Bhavnagar. Helpful discussions with Dr. G.amachandraiah, Head, EMP Division, CSMCRI, are also gratefullycknowledged.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.electacta.2008.09.049.

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