application of ion exchange membranes - electrochemical...
TRANSCRIPT
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Eleclrochemical Applications of Ion Exchange Alembranes ill OJemical Transformations 66
Chapter 3 Application of Ion Exchange Membranes -Electrochemical Organic Transformations
3.1. Introduction
E lectro-organic synthesis largely depends on the choice of a suitable
solvent and supporting electrolyte. Due to poor solubility of organic
compounds in aqueous media and poor conductivities of organic
solvents, in general, two-phase (aqueous-organic) is often employed. In
such cases, a phase transfer catalyst is used for improving the system efficiency.
However, this makes the electro-organic synthesis, on the whole, more complicated.
Solid polymer electrolyte i. e. ion-exchange membrane reactors (SPE) have been shown
to be promising for electro-organic transformations. The mobility of counter ion affects
the electrode reactions possible. However, the studies on SPE reactors are confined to
metal-membrane composite electrodes only. Besides few disadvantages such as the
involvement of metal-membrane composite electrodes, confinement of these studies to
very costly Nafion membranes and greater threat possed by the adhering capability of
the metal film of such composite electrodes during their function, they have been
reported to be used in literature.
3.1.1. Oxidation of methylsulfoxide to methyl sulfone. Dimethylsulfone (DMS02) is
useful as a high temperature solvent in extractive distillation, in electroplating baths, in
making ink, adhesives and many other substances of organic or inorganic nature!.
Molten DMS02 is a suitable electrolyte for lithium intercalation batteries2 It is found in
primitive plants and in the adrenal cortex of cattle. It has reportedly been used in the
preparation of epoxide catalysts for polymerization reactions]
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Electrochemical Applications of lmJ Exchange ,Uembranes in Chemical Transfonnations 67
The synthesis techniques, properties and numerous practical applications of methyl
sulfone and other sulfones have been reviewed earlier4 The oxidation of organo sulfur
compounds, in fact, is important as stated by Vedemikov and Maksimov5 from the view
point of effluent deodorization and utilization of sulfur compounds from the wastes of
pulp and paper industries.
Devis and Sorensen6 patented the process of preparation of methyl sulfone from
methyl sulfide by heating the later to 120-125°C in a bomb containing HN03 (6 N) or
N-oxides as oxidants and OS04 as catalyst. A yield of 71 % methyl sulfone was reported
at the end of three hours. They, further reported7 the synthesis of sulfones by the
disproportionation of sulfoxides in the presence of HN03 (6 N) or N-oxides and OS04 at
110-120°C. In this case methyl sulfoxide was reported to give 83% methyl sulfone.
Goheen and Bennett8, reported the oxidation of methyl sulfoxide to methyl sulfone in
hot HN03. About 86% of methyl sulfone was obtained in 0.64 equivalents of HN03 at
120-150°C.
Vedemikov and Maksinov5 synthesized methyl sulfone by passing methyl sulfide
vapors through 30% H2S04 at an elevated temperature (11 0 -120°C). Hydrogen
peroxide was added to catalyze the reaction. Seree de Roch and Menguy9 patented the
liquid phase oxidation of alkyl/aryl sulfides or sulfoxides using an alkyl hydroperoxides
at JO-150°C. Group IV-VII metal acids, oxides or salts were employed as catalysts.
Sulfones were reported to be formed in excess hydroperoxide at >50°C and sulfoxides
in excess sulfide at low temperatures. Methyl sulfone was not reported to be achieved
by this method. Boehme and Sitorus 10 in their communication, reported that ethanolic
solutions of thioethers produce methyl or alkyl/aryl sulfones when alkaline H20 2 was
added to it.
Thompson II reported the formation of stoichiometric amounts of methyl sulfone
when ozone in Et20 was added to methyl sulfide or methyl sulfoxide at -30 to -40°C in
the presence of triaryl phosphites. Akamatsu l2 e/ al. reported the ozone oxidation of
methyl sulfide derived from the pulp and claimed a yield of 25% sulfone and 75%
sulfoxide with 7% ozone. Akamatsu, Ueshima and Kimural3 patented the methyl
sulfoxide and methyl sulfone preparations by the oxidation of methyl sulfide with 0 3 at
O-IOO°C. According to them, the percentage of methyl sulfide should be between 2-19.7
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Electrochemical Applications of Ion Exchange Jlembranes in Chemical Tramfonnatio1Js 68
moles to avoid explosions. A mixture of 3:7 molar solution of Me2S and 20% 0 3 gave
85% of methyl sulfoxide and only < 15% of methyl sulfone at 30° C.
Mallievskii 14 et al. have reported the sulfides and sulfoxides oxidation to get
sulfones by atmospheric oxygen in the presence of variable valence metal compounds.
Acetic acid medium at 20-130° C was found to give sulfones in the purest form. While,
Bennett, Goheen and Mac Gregorl5 reported the formation of 10-30% methyl sulfone as
side product in the preparation of methyl sulfonyl chloride (MeS02CI) by the
chlorination of methyl sulfide under aqueous conditions at 25° C.
Methyl and other alkyl/aryl sulfoxides were converted photochemically to sulfones
by Schenck and Krauch 16 via photosensitized O-atom transfer mechanism. The yield of
methyl sulfone was around 55-99%, depending upon the type of photosensitizer used.
Hubenett l7 et al. patented an electrochemical process for the preparation of dialkyl
sulfones from sulfoxides. They used Pb02 electrodes as anodes and an aqueous solution
of 0.05-1.0 N salt/acid as an electrolyte to affect the reaction in a single compartment
cell at 20-40° C. About 79% of methyl sulfone was reported to be obtained by this
method from methyl sulfoxide at a current density of 40 rnA cm-2 in 0.2 N H2S04 at 20°
C. In an other patent, Bennett and Goheen l8 reported electrolytic preparation of methyl
sulfone from methyl sulfoxide. They used a high density graphite anode and carried out
the electrolysis at 5° C in 0.4 N HCI medium at a current density of 10 - 54 rnA cm-2
and voltage of 3-6 V. The yield of sulfone was reported to be improved to 100% and the
current efficiency has gone up to 76-93% when a salt or oxide of W, V, Mo or Se was
added as catalyst. Although, the yields of sulfone and current efficiencies in the above
processes are good, but the requirement of a supporting electrolyte, saturation of the
medium with the sulfone before electrolysis and the problems such as the separation of
supporting electrolyte and catalyst from the reaction mixture/product truly limit the
wide utility of these processes.
The limitations of the above methods are that they involve expensive catalysts,
corrosive chemicals, cumbersome procedures, or the use of OMS which has a pungent
odorl . In electrochemical methods, the yields of sulfone and current efficiencies in some
of the above processes are good, but the requirement of adding a supporting electrolyte,
saturation of the medium with the sulfone before electrolysis and the problems such as
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Electrochemical Applications of Ion Exchange Afembranes in Chemical TransfomlGtions 69
the separation of supporting electrolyte and catalyst from the reaction mixture/product
limit their wide utility.
3.1.2. Oxidation of benzyl alcohol to benzaldehyde. The electrooxidation of benzyl
alcohol was investigated by A. Kunugi 19 et at. employing a Pt-Nafion composite
electrode in both polar and non-polar solvents. The benzaldahyde formed in different
solvents was reported to be in the range of34 - 62% and benzoic acid in the range of 9 -
28%. S. M. Lin and T. C.Wen20 have reported the electrocatalytic oxidation of benzyl
alcohol in alkaline medium on Ru02-coated titanium electrode. J. S. D021 et at. have
studied the indirect anodic oxidation of benzyl alcohol in the presence of phase-transfer
catalyst in a continuous flow stirred-tank electrochemical reactor (CSTER).
3.1.3. Reduction of maleic acid to succinic acid. Succinic acid or butanedioic acid
(C4~04) is a constituent of plant and animal tissues. It is found in beer, molasses, meat,
eggs, peat, fruits, honey and urine. It has wide applications ranging from scientific, such
as radiation dosimetry and use in standard buffer solutions to agriculture, food,
medicine, plastics, cosmetics, textiles, plating, photography and waste-gas scrubbing 22
The hydrogenation of maleic acid has been reported in aqueous cobalt chloride and
potassium chloride solutions to give 41% succinic acid in the presence of Zn/Hg and
22% in its absence23. Succinic acid has also been prepared24 chemically by treating
maleic acid with 50% H3P02 at room temperature in aqueous NaOHIEtOH with an
average yield of 83%. It has been reported to be obtained in 99.5% yield when a
mixture of maleic acid, water and Pd/C was autoclaved at 1000 C while bubbling with
hydrogen containing
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Electrochemical App/icaliotU if /"" Exc/umge MemhrtllW. in o,emical TrumfQmlQlioIU 70
The yield of succinic acid in some of the above processes is good, but, the requirement
of adding a supporting electrolyte in the electrochemical processes, saturation of the
medium with succinic acid before electrolysis, requirement of costly catalysts or
reagents, and the problems associated with the separation of supporting electrolyte,
catalyst or unreacted reagents from the reaction mixture/product limits application.
3.2. Experimental
3.2.1. Materials. Interpolymer cation and anion exchange membranes (300 cm x 60 cm)
were either purchased from Nuchem Weir Ltd. Faridabad or prepared from styrene
divinyl benzene copolymer (prepared initially) by attaching with sulfonic acid (to
function as a cation exchange) or tertiary amine (to act as anion exchanger) as patented
from this laborato~. Dimethyl sulfoxide (S. D. Fine Chemicals Ltd., India), maleic
acid (S. D. Fine Chemicals Ltd., India) and benzyl alcohol (Sisco Research Laboratory)
were used as received. Expanded precious triple metal oxide coated electrode
(Dimensionally Stable Anode, DSA) was purchased from Titanium Tantalum PToducts
Limited (TITAN), Chennai, India. An expanded stainless steel sheet to serve as cathode
was procured from Hy-Tech Engineering, Bhavnagar.
3.2.2. General description of the fabrication of membrane flow cell. Fig. 3.1 shows a
View of the two Cathode Anode
compartmental
electrochemical
membrane flow cell.
The capacity of the
each electrode
compartment of the cell
was about 20-30 mi. A
dimensionally stable
anode which is an
expanded titanium
sheet with a precious
triple metal oxide
coating was used. A
Ph. D. Thew of Mr. Sanjfl}fumror S. Vo[!}lela
Fig.3.L A look oftlle membl3De flow reactor
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Electrochemical Applications of Ion Exchange Afembralles in O,emical Trans/omlGtiolls 71
fine stainless steel mesh or expanded sheet was served as the cathode. Both of these
electrodes were separately fitted inside the electrode chambers made of TeflonIPVC.
They were separated by about 4-6 mm by the conditioned indigenous anion/cation
exchange membrane between them. The anolyte or catholyte solution of desired
concentration was passed through the respective compartment at a regulated flow rate,
under gravity, while a solution of a desired electrolyte or distilled water depending on
the nature of the experiment carried out was circulated through the other compartment
at the rate equaling to that of the other side, with the aid of inlets and outlets provided to
the cell.
3.2.3. Methods. (a) Galvanostatic electrolysis. (i) Oxidation of Methyl sulfoxide to
methylsulfolle. Galvanostatic oxidation of methyl sulfoxide to methyl sulfone was
carried out in two ways viz. batch process in an undivided cell and a divided membrane
flow reactor. In batch process, the undivided cell had provisions for introducing
expanded DSA (area 48 cm2) and stainless steel cathode. 20 grams of DMSO (0.256
moles) was dissolved in 500 ml of 0.53 M H2S04 or 0.2 M Na2S04 as the case and
oxidized at current densities of25.0, 62.5 and 100.0 mAcm-2 till the charge equivalent
to two Faradays was passed.
A rectangular Perspex cell of 18 cm x 15 cm consisting of an expanded DSA (area
60 cm2) and a stainless steel cathode was used as a flow reactor. A cation exchange
membrane35•36 (an interpolymer of polyethylene and styrene-divinyl benzene copolymer
with sulfonic acid as functional group having aerial resistance, 1.5 Q cm-2, ion
exchange capacity, 1.8 m equiv g -I and moisture content, 29.8% after drying) of 100
cm2 exposed surface area was placed between the cathode and the anode, which are
separated by 3-4 mm, thus dividing the cell into two compartments each having an inlet
and an outlet. The anolyte consisted of 250 ml of aqueous solution containing 20 grams
of DMSO and the catholyte was 250 ml of 0.1 M H2S04. both of which were
recirculated through the respective compartments. The flow in both the cases was
regulated at 10 ml min - I The current passed corresponding to anode was of 35 rnA
cm -2 The cell voltage between the cathode and the anode, and the potential at the anode
vs Ag/ AgCI (3M NaCl) were monitored.
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Electrochemical Applications of Ion Exchange AJembranes in D,emical Transformations 72
The progress of the electrolysis was followed in both the cases, by recording UV-vis
or IH_NMR spectra of I ml sample withdrawn periodically and diluted suitably. After
the completion of the electrolysis, the solution was evacuated (after neutralization in
case of H2S04 as electrolyte) using rotary evaporator and the resulting solid was
recrystallized from ethanol. DMS02 thus obtained as white prisms, had melted at 109
dc. Cyclic voltammetry. Cyclic voltammetric studies were made in the conventional
three electrode cell with provisions for introducing the working, reference and counter
electrodes. Glassy carbon (area 0.0314 cm2), AglAgCl (0.222 V vs NHE) in 3 M NaCI
and a platinum wire served as the working, reference and counter electrodes,
respectively. Cyclic voltammograms for DMSO in the concentration range of 0.05-0.10
M, were recorded in acetonitrile-O.I M BU4NBF4 medium.
(ii) Reduction of Maleic acid to succinic acid A rectangular Teflon cell of area IS
cm x 10 cm fitted with a stainless steel, copper or lead cathode of area 40 cm2 and a
dimensionally stable anode (DSA) or lead anode was used as the flow cell (Fig. 3.1). An
interpolymer of polyethylene and styrene divinyl benzene copolymer attached with
sulfonic acid to function as a cation exchanger purchased from Nuchem Weir Ltd.,
Faridabad, India, was used as the conducting solid polymer electrolyte. It had dry
resistance 1.5 ohms cm-2, ion exchange capacity 1.8 meq g-l and moisture content
29.8% after drying. The cation exchange membrane, measuring about 66 cm2 exposed
surface area, was placed between the cathode and the anode at a distance of 2-4 mm,
thus dividing the cell into two compartments each having one inlet and one outlet. A
100 ml solution consisting of the desired quantity of maleic acid was used as the
catholyte, while a 100 ml of 0.1 M solution of H2S04, Na2S04 or NaOH was used as
anolyte. The quantity of current consumed was 2 faraday mole- l Both solutions were
recirculated at 10 ml min-1 through the respective compartments until the end of
electrolysis. Cathodic current densities in the range 18.7 - 31.2 rnA cm -2 were used. The
cell voltage between the anode and the cathode at a given current density was noted.
The electrolysis was continued beyond the calculated time and stopped corresponding
to the maximum yield of the product observed by regular analysis every minute after the
completion of the theoretical time.
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Electrochemical Applications 0/ Ion Exchange Membranes in Dlemical Trans/onnations 73
The progress of the reaction was followed by recording lH~NMR spectra of I mlof
the sample withdrawn periodically during electrolysis. After completion of electrolysis,
the solution was concentrated and allowed to crystallize as white prisms which melted
at 187°C.
Cyclic vo/lammetry. Cyclic voltammetric studies were made in a conventional three
electrode cell with provisions for introducing the working, reference and counter
electrodes. A hanging/static mercury drop electrode (HMDE/SMDE) (0.017 cm2), a
saturated calomel electrode (SCE) and a platinum wire served as the working, reference
and counter electrodes, respectively37 Cyclic voltammograms for maleic acid (1 mM)
were recorded in 1M H2S04, or 0.1 M CH3COONa-H2S04 and 0.1 M KH2P04-
Na2HP04 solutions in the pH range 1.0 -8.1.
(iii) Oxidalion of benzyl alcohol A rectangular Teflon cell of area 15 cm x 10 cm
fitted with a stainless steel, copper or lead cathode of area 40 cm2 and a dimensionally
stable anode (DSA) used as the flow cell (Fig. 3.1). An interpolymer of polyethylene
and styrene divinyl benzene copolymer attached with sulfonic acid to function as a
cation exchanger purchased from Nuchem Weir Ltd., Faridabad, India, was used as the
conducting solid polymer electrolyte. The cation exchange membrane, measuring about
66 cm2 exposed surface area, was placed between the cathode and the anode at a
distance of 2-4 mm, thus dividing the cell into two compartments each having one inlet
and one outlet. A 100 ml 1
solution consisting of the
desired quantity of benzyl
alcohol was used as the
anolyte, while a 100 ml of
0.1 M solution of H2S04
used as catholyte. The
quantity of current
consumed was 2 faraday
mole-1 Both solutions
were recirculated at 10 ml
'" U I: III -e 0.5 5l .c 0:(
O+-~--~----~----~----
o 0.2 0.4 0.6 0.8 Benzaldehyde, 1M] x 10-3
Fig.3.2. Plot of Absorbance against Concentration of Benzaldehyde in 0.1 % (v/v) benzyl alcohol.
Ph. D. Thesis of Mr. Sanjaykumar S. Vaghela Bhavnagar University, Reg/II. No. 654, 411012000
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Electrochemical Applications of Ion Exchange AJembranes in Olemicai Trans/onnalions 74
min- l through the respective compartments until the end of electrolysis. Anodic current
densities in the range 5-60 rnA em -2 were used. The cell voltage between the anode and
the cathode at a given current density was noted. The electrolysis was continued beyond
the calculated time and stopped corresponding to the maximum yield of the product
observed by regular analysis every minute after the completion of the theoretical time.
The amount of benzaldehyde was estimated by Uv vis spectrophotometry38 by
following the absorption band at Amax.= 283 nm, (t::, 664 ~l em-I; Fig. 3.2)
characteristic of benzaldehyde.
3.2..1. Instrumentation. Perkin Elmer Series II-2400, CHNS/O Analyzer for C, H,
N-analysis; Bruker Avance DPX-200-FT NMR-200 MHz spectrometer for IH_NMR
spectra and Perkin Elmer Spectrum (GX FTlR system) for IR data were used. Shimadzu
Uv vis spectrophotometer was used to record Uv spectra. All pH measurements were
made with Adair and Dutt digital pH meter.
EG&G P ARC Models PAR 174A Polarographic Analyzer and PAR 175 Universal
Programmer coupled to a high precision Houston X-V recorder were used to record all
sampled-de, differential pulse (DPP) and cyclic (cv) voltammograms. A three electrode
assembly, PAR 303 SMDEIHMDE comprising of a dropping mercury (DME, 3.85 mg
S-I ) or a hanging mercury drop (HMDE, 0.021 cm2) as working, platinum wire as
auxiliary and SCE (or Ag/AgCI) as reference electrodes was employed for recording all
electrochemical data.
EG&G PAR model 273A PotentiostatiGalvanostat coupled with three electrode cell
assembly and Gateway 2000 (4DX2-66) computer loaded with M 270 Research
Electrochemistry were utilized for electrokinetic measurements. A glassy carbon of area
0.0314 cm2 was served as the working electrode, while its potentials were measured
against Ag/AgCI (0.22 V vs NHE) in 3 M NaCI. A platinum wire served as a counter
electrode.
Controlled potential coulometry (CPC) was performed on EG&G PAR model 173
PotentiostatiGalvanostat coupled to models 179 digital coulometer. A coulolmetric cell
used here was assembled with a working Hg-pool (I in convex surface diameter)
electrode in the main compartment, a platinum mesh as counter and a SCE reference
electrodes. The solution was uniformly agitated throughout the experiment.
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Electrochemical Applications OJ/Oil Exchange Alembranes in Owmical Tranifomrations 75
An Aplab D. C. Power supply (model L 1285 and model L 1288SR) were employed
as the constant current source.
3.3. Results and discussion
3.3.1. Galvanostatic transformation of methyl sulfoxide to methylsulfone. The results of
galvanostatic oxidation studies on DMSO in different media at different current
densities and reactor types are given in Table 3.1. It can be seen that, under the batch
reactor conditions, the percentage yield ofDMS02 in 0.53 M H2S04 increased by 8.3%
when the current density was increased from 25.0 to 62.5 rnA cm-2, while the CE
rose by about 9%. When the current density was further enhanced to lOa rnA cm-2, the
yield ofNa2S04 medium, the yield ofDMS02 along with the CE was reduced, possibly
due to competing oxygen evolution. In 0.2 M increased slightly (4.2%) at 62.5 rnA
cm-2, though the cell voltage was relatively much higher (10 V) which is generally
Table 3.1. Data on the galvanostatic oxidation ofDMSO to DMSO, at DSA in membrane and batch flow reactors
DMSO Elcctrol)1e Anode Current Cell current 'Cell % Yield cCE 0/0** (g) (M) density (A) voltage of
(rnA ern-2) (V) DMSO,'
20 'H,S04(0.53) 25.0 1.21 4.0 71.0 78.0 20 'H,S04(0.53) 62.5 3.02 5.0 79.3 87.1 20 'H,S04(0.53) 100.0 4.83 6.0 71.1 78.2 20 'Na,S04(0.2) 62.5 3.02 10.0 83.5 91.9 20 b 35 2.20 2.5 '96.2 95.1 20 c 35 2.20 2.5 '97.5 98.1
'wlth respect to the Isolated "eld of DMSO" •• CE for DMSO, fonnatlOn'Batch reactor, bMembrane flow reactor, 0.1' M H,S04 as cathol)te, 'Membrane flow reactor, 0.1 M Na,S04 as cathol)tc.
not advisable. The cell voltage in all other experiments was between 4 - 6 V while the
potential at the anode maintained to about + 1.5 to +2.0 V vs Agi AgCI, depending on the
current density applied and the DMSO composition used. The percentage yield of
DMS02 and the CE were relatively high even at low current densities in flow reactor as
compared to those obtained in batch reactor (S. No.5, 6; Table 3.1) The yield and CE
values were further improved when Na2S04 was used as catholyte. However, the H2S04
medium is preferable as the cell voltage is less. In both the cases, the flow of counter
ion, W across the membrane occurs from the water auto decomposition equilibrium at
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Electrochemical Applications of Ion Exchange Membranes in Chemical Transformatiolls 76
the membrane-solution interface of the anode compartment into the cathode
compartment and carries the cell current.
Product analysis
The IH-NMR spectrum of DMSO exhibited a single sharp peak at 2.66 (5
corresponding to six protons of the two symmetric methyl groups. With the progress of
electrolysis, the intensity of this signal was reduced and a new peak at 3.05 (5
corresponding to six protons
of two symmetric methyl
groups was observed whose
intensity increased. The IH_
NMR spectrum of DMSO
(5%) after 7 h electrolysis is
depicted in Fig. 3.3. The low
intensity peak at 2.66 (5 and
the high intensity peak at ppm 3.00 250
3.05 (5 correspond to the Fig.3.3. IH-NMR spectrum showing the 96% conversion of (5%) unconverted (4%) and (CH3hSO to (CH3hS02 after 7 hr. electrolysis at 15 rnA crn-2
converted (96%) DMSO,
respectively. The isolated products obtained by using both the reactors showed a single
peak at 3.05 (5 indicating DMS02 as the sole product.
The FT-IR spectrum of the isolated DMS02 exhibited strong absorption bands at
1136 cm-I corresponding to symmetric S=O stretch and at 1298 and 1336 cm-I due to
asymmetric S=O stretch. Besides, the other absorption bands observed at 3020, 936,
763 and 699 cm-I are typical for DMS02 as reported in the literatureW The strong band
at 1030-1080 cm-I corresponding to S=O stretch ofDMSO was absent in these spectra.
DMSO showed absorption maximum at 208 nm (959 ~I em-I) while no such
absorption was found for the product in the uv-visible spectra. This is as per Iiterature40
reports that sulphone shows absorption due to IT-IT* transition in the far uv at
wavelength less than 180 nm but not in the near uv whereas sulphoxide shows weak
absorption in the near uv due to n-IT* transition at wavelengths longer than 200nm. The
Ph. D. Thesis afAlr. Sanjaykumar S. Vaghela Bhavnagar University, Regtn. No. 654, 411012000
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Electrochemical Applications of Ion Er:challge ~fembralles in O,emical Trans/ormations 77
elemental analysis of the purified product indicated % C as 24.74 and % H as 6.05
which are close to the calculated values 25.52 % and 6.42 %, respectively for DMS02.
Cyclic voltammetry and mechanism of electrooxidation. Typical cyclic
voltammograms obtained at a gc electrode for various amounts of DMSO in
acetonitrile-O.I M n-Bu4NBF4 are shown in Fig. 3.4. A single irreversible anodic peak
is observed at around 1.9V corresponding to the two-electron oxidation of DMSO to
DMS02 as follows.
•
Fig. 3.4a shows the cyclic
voltammetric response for the solvent-
supporting electrolyte system. It can be
seen that with increasing concentration
of the added DMSO to the acetonitrile-
0.1 M n-Bu4NBF4, a peak starts to
appear, whose height Increases,
indicating the oxidation of DMSO to
(3.1)
DMS02 (Fig. 3.4 b,c) Further 2 1.5 1 0.5 o
enhancement in the current was seen
with the addition of 2 mmoles of
water (Fig. 3.4d), indicating the
E, V vs Ag/AgCI
Fig.3.4. CV of CH3CN.o.! M Bu,NBF, containing (C(CH3hSO (a) 0.00 M, (b) 0.05 M, (c) 0.10 M, (d) 0.10 M with 2 mmols ofHoO at gc electrode
participation of trace quantities of water present in the solvent in the electrochemical
oxidation of sulfide to sulfone at + I. 9 V, as per equation 3.1 above.
3,3.2. Voltammetric studies all the reduction of maleic acid to slIccinic acid Cyclic
voltammetry. Cyclic voltammetric study was carried out in 1 M H2S04 and in different
buffer solutions. In buffers, the range of pH employed was 0.0 to 8.1. Fig. 3.SA, shows
the voltammetric responses of 1 mM maleic acid in 1 M H2S04 at different scan rates. It
showed only one peak in the reductive scans and no response in reverse scans. At 0.5
Vis (Fig. 3.SA(a)), it gave a sharp peak (E1I2-Ep = 24 mY) at Ep = -0.570 V which
shifted negligibly with the decrease in scan speed as seen in Fig. 3.SA(b-c) Similarly,
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Electrochemical Applications of 10 II Exchange Membranes in Chemical Transfonnations 78
the Ep!2-Ep values varied little in the range 25-35 mV. The plot, ip liS square root of scan
speed (Fig. 3.5B(a)) was linear and passed through the origin which together with the
above results revealed that the
reduction of maleic acid seen in
Fig. 3.5A, is an irreversible two-
electron diffusion controlled
process. The peak position, its
intensity and the slope of the
plateau of the peak
corresponding to the maleic acid
reduction at -0.570 V (Fig.
3.5A(a)) were altered
considerably when the
concentration of the acid was
reduced. The changes
·'-~'-'~~~-~. ..... ~/}c·· * ...... .
pH. Between pH 2.Y"'" 4.5; the· Fig.3.6. (A) Cyclic voltammetric response of ImM maleic acid at HMDE in buffered and unbuffered media
plateau of the peak appeared to be (a) 0 (b) 2 (c) 3 (d) 4 (e) 5 (I) 7 (g) 8.1 Scan rate = 0.1
split in two at slow scan speeds (,s; Vis (8) Plot of E"" vs pH
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Electrochemical Applications of Ion Exchange Membranes ill Glemical Transfonnations 79
0.2 Vis) as seen in Fig. 3.6A (c, d) which is explained in latter part of this discussion.
The decrease in peak intensity (ipe) in the pH range 0-2 (Fig. 3.6A(a, b» is explained
for the decrease in hydrogen ion activity which takes part in the electrochemical
reaction while that at pH ~ 8 is attributed to the slow participation of the deprotonated
maleic acid species and the protons of the medium. However, the overall peak height
(ipe) at all the pH values increased with the increase in the scan speed. The plots of ipe vs
square root of scan speed were linear and passed through the origin as seen in Figs. 3.5
B(b and c) for pH, 5.0 and 8.1, respectively. Moreover, the peak height at a given scan
speed and pH was increased linearly with the increase in the concentration of maleic
acid. This indicated that the electrode reaction at all pH investigated here was still under
diffusion controlled.
The ipe, and Ep/2-Ep data measured at three different scan speeds and pH pertinent to
Fig. 3.6A are summarized in Table 3.2. The ipe value at a given scan speed, was large at
pH = 0 and decreased with increase in the pH. On the other hand the Ep value at a given
pH, was nearly constant and shifted negligibly to positive potentials with decrease in
scan speed. But, at a given scan speed, the peak position shifted considerably to more
negative potentials with the increase in pH of the solution indicating the involvement of
hydrogen ions in the reduction process. The plot ofEpc vs pH for 0.1 Vis is shown in the
Fig. 3.6B. The plot is linear with an average slope of -0.125 VlpH.
Table 3.2. Cyclic voltammctric data of Maleic acid reduction at HMDE in different buffer solutions. 100 I
pH Scan i" -Ep Ep12-Ep" pH Scan i" -Ep Epl2-E~ 'peed (vI,)
(1lA) (V) (mY) speed (vIs)
(1lA) (V) (mY)
0.0 0.05 13.77 0.559 24 4.10 0.05 8.66 1.078 102 0.10 19.48 0.556 23 0.10 12.60 1.094 110 0.50 38.97 0.574 39 0.50 25.59 1.118 118
1.0 0.05 11.02 0.685 24 5.05 0.05 9.06 1.212 70 0.10 15.15 0.688 35 0.10 12.20 1.204 70 0.50 2933 0.696 32 0.50 2362 1.181 70
2.02 0.05 10.24 0.787 79 7.00 0.05 7.87 1.456 55 0.10 14.57 0.748 56 0.10 11.02 1.464 55 0.50 31.50 0.740 63 0.50 2205 1.496 87
3.00 0.05 8.27 0.881 70 8.10 0.05 4.72 1.543 55 0.10 11.42 0.905 87 0.10 5.91 1.551 63 0.50 24.41 0.960 134 0.50 9.45 1.574 86
Ph. D. Thesis o[}.dr. Sanjaykumar S. Vaghela Bhavnagar University. Regtn. No. 654, 4/10/2000
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Electrochemical Applications of Ion Exchange ~fembranes in Olemical Tramformations 80
It is noted from the data in Table 3.2 that, at all scan speeds, the EpI2-Ep values were
in the range of23-39 mV in 1 M H2S04 as well as in buffer solutions of pH = 00 - 1.5.
But, it increased to 55 - 87, 102-134 and 55 - 87 mV in the pH ranges 1.5-3.0, 3.0-4.5
and 5.0-8.1, respectively. This may be the resultant effect of decrease in hydrogen ion
activity and transformation of maleic acid into mono- (HOOCCH=CHCOO-) and di-
COOCCH=CHCOO- ) cations (discussed in the later part of this section) with the
increase in solution pH. The large Epl2-Ep values in the pH range 3.0-4.5 may be the
reduction of HOOCCH=CHCOOH and HOOCCH=CHCOO- at closely separated
potentials.
Product identification. The solutions after the electrolysis were studied by IH_NMR
spectra in D20 which showed the yield of -100 % succinic acid in 0.1 M H2S04 and
99.6% sodium succinate in 0.1 M Na2S04 at pH 8.1. It is interesting to note that the
product yield in both cases was nearly same.
The products from the electrolyzed solutions stated above were isolated and
characterized. The compound obtained from 0.1 M H2S04 showed a peak at Ii = 2.44
ppm corresponds to the protons of two symmetric methylene (-CH2) groups in IH_
NMR and a strong absorption band at 1695 cm-I corresponding to >C=O stretch, apart
from other bands in FT-IR spectra for succinic acid. The product obtained from 0.1 M
Na2S04 showed a peak at Ii = 2.18 ppm in IH_NMR spectra and a broad IR band
centered at 1563 cm -I corresponding to >C=O stretch for disodium salt of succinic acid.
The peak at Ii = 6.38 or 5.91 ppm in IH_NMR and the strong band at 1637 cm-I in FT-
IR corresponding to -HC=CH- in maleic acid were absent. The elemental analysis of
the purified product, succinic acid indicated % C as 40.55 and % H 5. \0 against the
calculated values 40.64 and 5.12, respectively.
Mechanism of electro reduction of maleic acid Both, the HOOCCHxCHxCOOH
(where x = I for maleic or 2 for succinic) acids are dibasic acids. They dissociate
stepwise in two well separated pH regions. Their acid dissociation constants (pK. and
pK2.) corresponding to the equilibria I and 2, are reported to be 1.75 and 5.82 for
maleic acid, and 3.92 and 5.12 for succinic acid, respectivel/1 These values reveal that
the species HOOCCHxCHxCOOH, HOOCCHxCHxCOO- and -OOCCHxCHxCOO- of
Ph. D. Thesis of Mr. SallJaykumar S. Vaghefa Bhavnagar University. Regtn. No. 654. 4/10/2000
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Electrochemical Applications 0/ Ion Exchange Alembralles in Olemieal Trans/annalians 81
both maleic and succinic acids all exist in equilibrium at pH < 7 but they convert into a
single species -OOCCHxCHxCOO- at pH > 7.
The split in the plateau of the peak seen in Fig. 2 A(c and d) could probably due to
the resolution of the reduction steps corresponding to viz. HOOCCH=CHCOOH and
HOOCCH=CHCOO- present at equilibrium (Eq. 3.2).
HOOCCHxCHxCOOH, • HOOCCHxCHxCOO- + H+ (3.2)
(3.3) .. Then, the reduction peak at low negative potentials may be attributed to the
involvement of HOOCCH=CHCOOH while the other at more negative potentials is due
to HOOCCH=CHCOO- in the electrode process. This assumption is supported by the
observation that a single reduction peak was observed at pH;?: 5 (Fig. 3.5A(e-g» due to
the complete conversion of HOOCCH=CHCOOH to HOOCCH=CHCOO-.
Comparison of pKa values also revealed that one of the carboxylic acid in both
maleic and succinic acids behave nearly as a strong acid while the other as a weak.
However, the more acidic carboxylic group of maleic acid is relatively stronger than
that of succinic acid while, the second carboxylic acid of maleic acid is relatively a
weak as compared to that of the succinic acid. Based on these facts and the cyclic
voltammetric (Ep12-Ep) data given in Table 3.2, the pH dependent nature of the cyclic
voltammetric peak at -0.570 V (Fig. 3.4A) is explained by the following mechanism of
electrode reactions.
pH 0.0-2.5
HOOCCH=CHCOOH + H+ + 2e-
HOOCCH2CH2COO- + H+
pH 2.5-4.5
HOOCCH=CHCOO- + 2fT' + 2e-
HOOCCH2CH2COO- + H'
pH 4.5-6.5
HOOCCH=CHCOO- + 2H+ + 2e-
Ph. D. Thesis a/Afro Sanjaykumar S. Vaghela
--.. HOOCCH2CH2COO-
HOOCCH2CH2COOH •
------c.~ HOOCCH2CH2COO- and/or
(3.4)
(3.5)
(3.6)
(3.7)
Bhavnagar University. Regtn. No. 654. 4/10/2000
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Electrochemical , .... pplications of Ion Exchange A-Iembranes in Olemical TrallsfomlGtions 82
-OOCCH=CHCOO- + 2H+ + 2e-
-00CCH2CH2COO- + H+
pH >6.5
-OOCCH=CHCOO- + 2H+ + 2e-
-~.. -00CCH2CH2COO-
HOOCCH2CH2COO--(3.8)
(3.9)
(3.10)
In 1 M H2S04 or in the pH range pH 0.0 - 2.5, maleic (HOOCCH=CHCOOH and/or
HOOCCH=CHCOO-) acid reduces at the electrode by taking 2e - from the electrode and
2W (one from its more acidic carboxylic acid group and the other from the medium) to
give HOOCCH2CH2COO- which rapidly protonates to succinic acid (Eqs. 3.4 and 3.5).
But, in the pH region 2-5-4.5, wherein HOOCCH=CHCOOH converts totally into
HOOCCH=CHCOO-, it reduces by taking 2e- from the electrode and 2H+ from the
medium to give HOOCCH2CH2COO- which again protonates to HOOCCH2CH2COOH
(Eqs. 3.6 and 3.7) by abstracting a proton from the solution. In the pH region 4.5-6.5
maleic acid mostly presents as a mixture of HOOCCH=CHCOO- and
-OOCCH=CHCOO-, and they under go 2e--2H+ change simultaneously at the electrode
producing HOOCCH2CH2COO- and -00CCH2CH2COO-, respectively (Eqs. 3.6 and
3.8). The latter product may be in equilibrium with the former one or may rapidly
proto nates to the former in a succeeding chemical step (Eq. 3.9). On the contrary, at all
pH > 6.5, where the maleic acid exists solely as -OOCCH=CHCOO- reduces by 2e--
2H+ to give a single product i.e. -00CCH2CH2COO- (Eq. 3.10).
Electro reduction of maleic acid can occur by (i) the direct electron transfer from the
electrode, (ii) the reaction of activated hydrogen ions and/or (iii) the reaction of
adsorbed hydrogen ions with it. The reaction (iii) will be more likely to occur at
reversible and low overpotential electrodes at which hydrogen is easily dissolved. In
case of mercury cathode, reaction (iii) was ruled out and hence, the reduction may take
place either by (1) or (2). But the very low values obtained for the transfer coefficient
indicated the possibility of the reduction by direct electron transfer. The electro-
reduction of maleic acid was concluded to take place through activated hydrogen ions.
3.3.3. Ga/vanostatic transformation of maleic acid to succinic acid using a membrane
flow cell. Excellent yields (95-99%) of succinic acid were obtained in the galvanostatic
Ph. D. Thesis o/AIr. Sanjaykumar S. Vaghela Bhavnagar University, Regtn. No. 654, 4/10/2000
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Electrochemical Applications of Ion exchange fl.,fembralles in Q,emical TransfomlOtions 83
reduction of maleic acid at SS, Cu and Pb cathodes vs DSA in the membrane cell. The
important data obtained under different experimental conditions with different electrode
pairs at different current densities are presented in Table 3.3. It can be seen in Table 3.3
that, at a given current density (I8.7 and 25 rnA cm-z) with O. I M HzS04-DSA anode,
the membrane flow cell yielded 0.4-3.3% more of succinic acid with Pb cathode than
with the other two electrodes, indicating the specificity of Pb-cathode for this process.
On the basis of succinic acid yields, the current efficiency improved marginally in the
order SS < Cu < Pb. However, the use of SS cathode is preferable as it is stable and
gives no harmful impurities. Moreover, the yield of succinic acid in all cases was found
to decrease as the current density increased from 18.75 to 31.25 rnA cm -z, which may
be accounted for by competing hydrogen evolution. The yield and coulombic
efficiencies were high when O. I M Na2S04 and 0.1 M NaOH instead of 0.1 M H2S04,
were used as anolytes. However, the H2S04 medium in the anode compartment is
preferable as the cell voltage is low. In both cases, the flow of counter ion -Na+ from
the anode compartment or H+ ion from the water auto decomposition equilibrium at the
membrane-solution interface of the anode compartment across the membrane to the
cathode compartment carry all the cell current.
Table 3.3. Data on the galvanostatic synthesis of succinic acid at different cathodes III a membrane flow cell
MA Anol}1e Cathode Cathodic Cell current Cell % Yield CE' fg (0.1 M) current fA Voltage of f%
fmA cm-2 N SA' 5 H2SO4 SS' 18.7 0.75 4 96.4 96.6
25.0 1.00 5 95.3 95.5 31.2 1.25 6 95.2 96.2
5 H2SO4 Cu' 18.7 0.75 4 98.4 98.6 25.0 1.00 5 96.1 96.3 31.2 1.25 6 95.4 95.5
5 H2SO4 Pb' 18.7 0.75 4 98.8 98.8 25.0 1.00 5 98.6 98.9 31.2 1.25 6 95.3 95.5
5 Na,S04 SS' 25.0 1.00 9-13 95.5 95.7 5 NaOH SS' 25.0 1.00 16-24 96.5 96.7 5 H,S04 SS' 25.0 1.00 5-6 93.8 93.9 5 Na,S04 SS' 25.0 1.00 9-13 96.6 96.9 5 NaOH SS" 25.0 1.00 15-20 80.4 80.5 , . " , With DSA as anode, \\Ith Pb as anode, calculated on the basiS of maXimum YICld of
product obtained.
Ph. D. Thesis ofJfr. Sanjaykumar S. Vaghela Bhavnagar University, Regtn. No. 654, 4/10/2000
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Electrochemical Application8 of Ion Exchange ,Membranes in a,emical Transformations 84
The percentage yield of succinic acid obtained at SS was negligibly affected when
DSA anode was replaced by Pb anode, in both the cases wherein H2S04 or Na2S04
were used as anolyte. The data indicated that, with Pb anode, the use of H2S04 as
anolyte is preferred over
Na2S04 as the rise in the
cell voltage in the case of
latter was objectionable.
On the other hand, the %
yield of succinic acid
formed at SS cathode and
the CE was considerably
decreased when Pb and
o I M NaOH anolyte were used. This may be
attributed to the drastic
increase in cell voltage.
Also a weak electro
dissolution of the anode of
I I I
J , _,---.-J "._ .. ~\'-. __ _
Fig.3.7. 1 H-NMR spectrum showing the 98.4% conversion of 5% Maleic acid to succinic acid after electrolysis
lead under these conditions and the migration of Pb2+ to the cathodic compartment is
likely to cause a decrease in the yield and current efficiency.
The reaction of maleic acid reduction at the cathode in the membrane flow cell is
simultaneously followed by recording the 'H_NMR at regular intervals. The untreated
solution exhibited a single sharp peak at 6.33 Ii corresponding to -CH=CH- (ethylenic)
protons. The intensity of this was reduced as the electrolysis was in progress.
Simultaneously, a new peak at 2.56 Ii corresponding to four protons of two methylene
groups (-CH2-CH2-) was observed whose intensity increased with the progress of the
reaction at the anode. The 'H_NMR spectrum of electrolyzed solution (containing
initially 5% maleic acid), after 138 minutes of electrolysis, is depicted in Fig. 3.6. The
low intensity peak at 6.33 Ii and the high intensity peak at 2.57 0 correspond to the
unconverted maleic acid (1.4%) and converted succinic acid (98.4%), respectively. The
Ph, D. Thesis ofAfr. Sanjaykumar S. Vaghela Bhavnagar University. Regln. No. 654, 411012000
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Electrochemical Applications of Ion £xchange }rlembranes ill Chemical Tramjomlatiolls 85
product isolated at the completion of electrolysis showed a single peak at 2.57 0
confirming the formation of succinic acid as exclusive product. The peak at 4.8 0
corresponds to protons of water.
3.3. -I. Galvanostatic transformation of benzyl alcohol to benzaldehyde. The
e1ectrooxidation of benzyl alcohol to benzaldehyde was carried out in a membrane cell
wherein the plain ion exchange membrane was placed between the cathode and the
anode at a minimum distance (2 -3 mm) to act as an SPE and make the oxidation
reaction feasible in the absence of supporting electrolyte. The data obtained for
galvanostatic oxidation with different percentages of benzyl alcohol in pure water at
different current densities are presented in Table 3.4. The product benzaldehyde was
estimated by UV -vis spectrophotometry and was compared with a standard sample.
Table 3.4. Data on the galvanostatic oxidation of benzyl alcohol at a dimensionally stable anode in a membrane flow cell.
% 'Catbol}1e Anode Anode Cell Cell % Yield of Benzyl (0.1 M) current current Voltage Benzaldehyde alcohol rnA cm-' fA N
I H2SO4 DSA 5 0.16 14-7 15.7 10 OJ3 41-13 15.6 20 0.65 54-26 15A 30 1.00 58-27 17.2 40 1.31 49-32 20A 50 1.65 61-32 33.7 60 1.97 51-42 29.0
2 H2SO4 DSA 30 1.0 57-18 9.5 3 H2SO4 DSA 30 1.0 53-18 13.6 4 H2SO4 DSA 30 1.0 71-15 12.7
• Catbode - Stamless Steel.
The Data in Table 3.4 reveals that the yields (10-29%) of benzaldehyde obtained in
the galvanostatic reduction of benzyl alcohol at DSA in the membrane cell were not
encouraging. However, the yield increased with the increase in current density
indicating the feasibility of the oxidation of benzyl alcohol to benzaldehyde at DSA, but
the cell voltages at all current densities were more than the normal values due to high
resistance caused by the substrate. Further, it is noticed that the yield of benzaldehyde
decreased as the % benzyl alcohol increased due to decrease in the solution
conductivity. Because of theses reasons, the studies were not further extended.
Ph. D. Thesis of AIr. Sanjaykumar S. Vaghela BhavnagarUniversity, Regtn. No. 65.f, 411012000
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Electrochemical Applications of/on E'Cchange Alembranes in a,emical Transformations 86
3.4. Conclusions
The results obtained in the present study indicate the potential applications of SPE
reactors with plain Membranes (Membrane flow cells) for the water based
transformation of some of the organic compounds. The advantages in specific example
exploited here are summarized below. They may be further investigated for their
commercial utilization.
Oxidation of methyl sulfoxide. The difficulties involving with expensive catalysts,
corrosive chemicals, cumbersome procedures, or the use of dimethyl sulfide which has
a pungent odor, the requirement of adding a supporting electrolyte, saturation of the
medium with the sulfone before electrolysis as required in the conventional methods
and the separation of supporting electrolyte and catalyst from the reaction
mixture/product are overcome by the use of membrane flow reactor.
Reduction of maleic acid. The present study finds a novel and simple method of
electrochemical preparation of succinic acid from maleic acid using an ion conducting
polymer electrolyte flow cell. It demonstrates the advantages of using a highly
conductive solid ion exchange membrane film between the two electrodes in the flow
cell wherein the use of supporting electrolyte is avoided. Thus, the additional separation
steps otherwise required in conventional methods are not required. The most advantage
is that the required hydrogen is generated from water.
Ph. D. Thesis ofAfr. Sanjaykumar S ~ aghela Bhavnagar University. Regtn. No. 654. 4; 10J 2000
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Electrochemical Applications of Ion Exchange Alemhranes ill O,emieal Transformations 87
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