Vol. 135, No. 8
POLYCRYSTALLINE
27. S. Menezes, L. F. Schneemeyr, and H. J. Lewerenz,
Appl. Phys. Lett., 38, 949 (1981).
28. L. Peraldo Bicelli and G. Razzini, Surf. Technol., 22,
115 (1984).
29. R. Noufi, R. Axton, C. Herrington, and S. K. Deb, Appl.
CuInSe2 ELECTRODES
1939
Phys. Lett., 45, 668 (1984).
30. L. Fornarini and B. Scorsati, Electrochim. Acta, 28, 667
(1983).
31. O. P. Bahl, E. L. Evans, and Y. M. Thomas, Surf. Sci., 8,
473 (1967).
High-Pressure Electrochemical Oxidation of Benzene at a Lead
Dioxide Electrode in Aqueous Bisulfate Solutions at 25 ~ to
250~
Allen J. Bard*
Department of Chemistry, University of Texas, Austin, Texas 78712
William M. Flarsheim** and Keith P. Johnston
Department of Chemical Engineering, University of Texas, Austin, Texas 78712
ABSTRACT
The oxidation of benzene at a lead dioxide electrode which produces predominantly benzoquinone, maleic acid, and
carbon dioxide, has been investigated in aqueous NaHSO4 solutions as a function of temperature up to 250~ An increase
in the benzene concentration does not increase the concentration of benzoquinone formed at high temperature, which is
different from the behavior at 25~ The formation of biphenyl at high temperature was also discovered. A novel type of
single-pass flow reactor for studying high temperature electrochemistry is described.
The oxidation of benzene to benzoquinone at a lead
dioxide electrode in aqueous sulfate solutions is limited by
the low solubility of benzene in water. Clarke et al. have
determined a reaction order with respect to benzene of 2
(1). Benzene solubility in water increases significantly with
increasing temperature; the two fluids b e c o m e miscible at
297~ at a pressure of 243 bar (2). This study examines the
effect of increasing temperature and increasing benzene
concentration on the electrochemical oxidation of benzene
on lead dioxide in the temperature range 25~176
Supercritical water has a combination of low dielectric
constant and high dipole m o m e n t that make it useful for a
n u m b e r of novel chemical applications. Water has a dipole
m o m e n t of 1.84 debye, and at ambient conditions it is extensively hydrogen bonded. This gives rise to a high dielectric constant, 78 at 25~ making water an excellent solvent for ions and a poor one for nonpolar species. High
temperature disrupts the hydrogen bonding and correlated orientation of water molecules (3) and decreases the
density and thereby lowers the dielectric constant to 5.2 at
the critical point (374~ 221 bar) (4). Because of the low dielectric constant, water in the critical region is an excellent
solvent for organic molecules. The water dipoles still orient in the vicinity of an ion, however, and produce a locally
high dielectric constant, so strong electrolytes remain dissolved and dissociated in supercritical water. The simultaneous solvation of both polar and nonpolar molecules
makes supercritical water a powerful solvent. The
MODAR process takes advantage of this superior solvent
pow e r in using supercritical water as a reaction m e d i u m
for the destructive oxidation of organic waste (5). Other
workers have investigated the use of supercritical water at
milder conditions to clean and liquify coal (6), and to hydrolyze plant material into useful fuels (7).
In a previous paper, we described electroanalytical
m e a s u r e m e n t s made in supercritical water at temperatures up to 390~ (8). An electrochemical cell was developed to operate at the extreme temperature and pressure of supercritical water. In addition, the voltammetric
behavior Of water, KBr, KI, and hydroquinone was
studied, and the diffusion coefficients of I- and hydroquinone were measured. Building on this experimental back*Electrochemical Society Active Member.
**Electrochemical Society Student Member.
ground, the present work is an expansion into the area of
high-temperature electro-organic synthesis, specifically
benzene oxidation. In supercritical water, high concentrations of nonpolar materials can be combined with strong
electrolytes in a one-phase system, and this presents the
possibility of synthetic routes that are not hindered by
mass transfer across a phase boundary. Although temperatures above 250~ are not reported here, the apparatus and
experimental procedures described could be used up to
375~ with only minor modification.
The electrochemical oxidation of benzene was first
studied near the end of the last'century (9) and has been
discussed frequently in the literature (1, 10-14). Work
through 1976 has been summarized by Clarke et al. (1). The
desired product from the oxidation is benzoquinone,
which can then be reduced to hydroquinone. Hydroquinone is used as an anti-oxidant in the manufacture of rubber and other products. Other major products from the oxidation are maleic acid and carbon dioxide.
The specific overall electrode reactions involved in oxidizing benzene are shown in Fig. 1. The complete conversion of benzene to carbon dioxide requires the removal of
30 electrons and the participation of 12 water molecules.
This full oxidation can occur readily at a platinum electrode (15). Lead dioxide is the only reported electrode material that produces large quantities of benzoquinone or
maleic acid. A lead dioxide electrode will perform the first
stage oxidation to benzoquinone readily in acidic sulfate
media; however, the yield and current efficiency vary
greatly d e p e n d i n g on the specific conditions of electrolysis. As the concentration of benzoquinone builds in the
solution, the benzoquinone is usually oxidized further to
maleic acid, accompanied by two moles of carbon dioxide.
Clarke et al. (1) report that at very low over-voltage, the
benzoquinone is not oxidized further. The one-step oxidation of benzene to maleic acid was studied extensively by
Ito et al. (13). This pathway is important under most reaction conditions. Few previous workers analyzed the carbon dioxide generated during oxidation, and no one has
discussed the pathway by which it is formed. This study
shows that single-step oxidation of benzene to carbon
dioxide can occur on lead dioxide, especially at high temperature. (The term single-step is used here to mean that
once benzoquinone or maleic acid is formed and desorbed
Downloaded 02 Feb 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp
J. Electrochem. Sac.: E L E C T R O C H E M I C A L
1940
SCIENCE
AND TECHNOLOGY
A u g u s t 1988
ENDELOCK
~*2H;z~
-6X *-6e-
0
ELECTROLYTE
VESSEL
,,
.
LINE
HEATER
~-D
GAS
/
X
."%/
~
~kE
COkk~'nON
LIQUID
I
\
I
FEEDSATURATOR
Fig. 3. Schematic of apparatus for high-temperature, high-pressure
electrosynthesis.
stirred t a n k reactor. T h e saturator can be h e a t e d to 200~
in o r d e r to increase the c o n c e n t r a t i o n of b e n z e n e in the
feed. T h e flow is controlled by a Milton Roy M i n i p u m p
(Model 2396), w h i c h p r o v i d e s fresh electrolyte to the saturator.
6CO 2
Fig. 1. Oxidation reactions of benzene on a lead dioxide electrode.
Adapted from Ito et el. (13).
f r o m the electrode, t h e y are not further oxidized. No
doubt, the o x i d a t i o n of b e n z e n e a d s o r b e d on the e l e c t r o d e
i n v o l v e s m a n y e l e m e n t a r y steps. This w o r k h o w e v e r , is
o n l y c o n c e r n e d w i t h t h e n e t result o f each e n c o u n t e r bet w e e n a b e n z e n e m o l e c u l e and the electrode.)
This w o r k differs f r o m p r e v i o u s studies of b e n z e n e oxid a t i o n in t h a t a single-pass flow reactor is used instead of a
stirred t a n k and the t e m p e r a t u r e r a n g e is m u c h larger
(12, 13). T h e reactant v o l u m e u s e d in t h e s e p r e v i o u s studies was relatively large, e.g., 90-1500 c m 3, so l o n g runs w e r e
n e c e s s a r y to build up m e a s u r a b l e quantities of product.
T h e p r o d u c t s in solution w e r e continually e x p o s e d to furt h e r oxidation. In t h e p r e s e n t apparatus, the r e s i d e n c e
t i m e of t h e electrolyte in the electrode r e g i o n is a b o u t 2s.
The e l e c t r o d e area is large e n o u g h so that m e a s u r a b l e conv e r s i o n is a c h i e v e d in a single pass. P r o d u c t s f r o m the initial o x i d a t i o n are q u i c k l y r e m o v e d f r o m the e l e c t r o d e w i t h
little o p p o r t u n i t y to be r e a d s o r b e d and u n d e r g o f u r t h e r
oxidation. Thus, analysis of t h e results is s o m e w h a t simplified, since t h e p r o d u c t s are p r o b a b l y f o r m e d p r e d o m i n a n t l y b y single-step o x i d a t i o n of benzene; only t h r e e of
t h e six reactions s h o w n in Fig. 1 n e e d be considered.
Experimental
Apparatus.--The e x p e r i m e n t s w e r e p e r f o r m e d in a
h i g h - t e m p e r a t u r e and -pressure, t w o - e l e c t r o d e flow cell
(Fig. 2). The cell b o d y is a 0.635 c m (1/4 in.)-od by 0.318 c m
(% in.)-id, 30 cm (12 in.)-long a l u m i n a t u b e (99.8% A1203).
A1203 was c h o s e n to avoid corrosion and spurious electric
fields w i t h i n the cell. The t u b e is c o n n e c t e d to t h e endblocks using stainless steel S w a g e l o k fittings w i t h Teflon
ferrules. The electrode leads pass t h r o u g h Teflon c o m p r e s sion fittings, w h i c h h a v e been d e s c r i b e d p r e v i o u s l y (8). All
m e t a l parts are either 316 stainless steel or H a s t e l l o y Alloy
C.
T h e c o m p l e t e e x p e r i m e n t a l system, i n c l u d i n g t h e p u m p
and feed vessels, is s h o w n s c h e m a t i c a l l y in Fig. 3. The cell
is fed f r o m the saturator, a one-liter A u t o c l a v e E n g i n e e r s '
INLET
~\\\\\\x-,\xx,,.,xx,.-. ~ "~ - ,
o/
\
\
\
\
~
T
OUTLET
Fig. 2. Diagram of high-temperature cell for electrosynthesis: (A) V4
in.-od alumina tube; (B) Hastelloy Alloy C endblock; (C) 316 SS SwageIok fittings; (D) Pb working electrode; (E) Pt counterelectrode and type
K thermocouple; (F) main cell heater (Nichrome wire coil); (G) preheater (Nichrome wire coil); (H) endblock heater (cartridge heater).
Electrodes.--The w o r k i n g e l e c t r o d e was c o n s t r u c t e d by
w e l d i n g a 1 c m piece of p l a t i n u m onto a t a n t a l u m wire.
T h e p l a t i n u m was w e l d e d to and w r a p p e d w i t h 0.05 c m
t h i c k lead foil (99.9995% P b f r o m A E S A R division of J o h n son Matthey), so that the p l a t i n u m was c o m p l e t e l y
covered. The lead was sealed and t r i m m e d with a soldering iron. T h e resulting e l e c t r o d e was 1.8 cm long, 0.3 c m
wide, and 0.15 c m thick. Before use the electrode was
rinsed in 5N nitric acid.
I n t h e cell, the c u r r e n t path is along the axis of the cell,
and b e c a u s e of solution resistance, only a portion of t h e
w o r k i n g e l e c t r o d e participates in the oxidation. W h e n t h e
w o r k i n g e l e c t r o d e is o x i d i z e d in the cell, lead d i o x i d e
forms f r o m t h e tip to a location a b o u t 0.5 c m further a w a y
f r o m the c o u n t e r e l e c t r o d e . Therefore, this is the area
w h i c h is active d u r i n g electrolysis, w h i l e the r e m a i n i n g
p o r t i o n passes v e r y little current.
T h e c o u n t e r e l e c t r o d e consisted of a p l a t i n u m w i r e
w e l d e d to the I n c o n e l 600 sheath of an u n g r o u n d e d therm o c o u p l e . P l a c e m e n t of t h e t h e r m o c o u p l e in the flowing
s t r e a m was n e c e s s a r y to achieve a c c u r a t e and stable temp e r a t u r e control. S i n c e t h e c o u n t e r e l e c t r o d e was t h e red u c i n g e l e c t r o d e in t h e s e e x p e r i m e n t s , there was minim u m c o r r o s i o n of the t h e r m o c o u p l e sheath d u r i n g the
project.
F o r cyclic v o l t a m m e t r y , a lead electrode (0.15 c m diam)
sealed in glass was used. A silver wire e l e c t r o d e was inserted into the h e a t e d portion of the cell as a r e f e r e n c e
electrode, and a c o u n t e r e l e c t r o d e was a d d e d d o w n s t r e a m .
Details of this p r o c e d u r e h a v e b e e n discussed p r e v i o u s l y
(8).
Procedure.--The electrolyte vessel was filled w i t h 290 mt
of 0.2M NaHSO4 solution saturated w i t h b e n z e n e at r o o m
t e m p e r a t u r e . D u r i n g long e x p e r i m e n t s , a second electrolyte v e s s e l c o u l d be filled and e x c h a n g e d w i t h t h e first o n e
w i t h o n l y a b r i e f i n t e r r u p t i o n of the electrolysis. T h e saturator was filled w i t h 300 m l of b e n z e n e and 700 m l of 0.2M
NaHSO4 solution. The NaHSO4 was r e a g e n t grade and t h e
b e n z e n e was s p e c t r o s c o p i c grade. B o t h c h e m i c a l s w e r e
u s e d as received. All water was purified w i t h a Millipore,
Milli-Q r e a g e n t w a t e r system.
I n p r e p a r a t i o n for e x p e r i m e n t s w i t h the feed b e n z e n e
c o n c e n t r a t i o n h i g h e r t h a n the 25~ saturation value, t h e
saturator was h e a t e d to the desired t e m p e r a t u r e , and the
c o n t e n t s m i x e d to saturate the a q u e o u s phase. T h e phases
w e r e t h e n allowed to separate before filling t h e cell. The
flow of electrolyte into the saturator was small c o m p a r e d
to the saturator v o l u m e , so b e n z e n e in the feed was not diluted d u r i n g an e x p e r i m e n t a l run.
T h e f e e d p u m p was set to the desired flow rate, and t h e
o u t l e t v a l v e was a d j u s t e d to give the desired b a c k pressure. B e c a u s e of t h e low flow rate, 1.3-1.4 ml/min, the outlet v a l v e had to be o p e r a t e d v e r y n e a r s h u t o f f w h e r e its
p e r f o r m a n c e was not consistent. The s y s t e m p r e s s u r e had
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O X I D A T I O N OF B E N Z E N E
Vol. 135, No. 8
to be continually monitored, and the outlet valve adjusted.
A ny system pressure that was at least 7 bar above the solm
tion saturation pressure and relatively stable was considered acceptable. After the cell had been flushed with a
few volumes of solution, the effluent was analyzed to insure that the feed contained no impurities that would interfere with the m ea s u r e m e n t of benzoquinone or maleic
acid.
All experiments were done at constant current. This
m a d e calculation of current efficiency possible without the
use of a coulometer. The narrow current path between the
two electrodes gives rise to a large cell resistance, so that
constant voltage experiments would have little meaning.
The cell potential was monitored during the electrolysis
and was relatively constant at any particular set of conditions. A value of 7.5V was typical at 25~ while it dropped
to about 6V at 250~ due to lower solution resistance.
The temperature of the solution at the working electrode
was maintained with an Autoclave Engineers (Erie, PA)
proportional-integral temperature controller (Model 520).
The temperatures at the preheater, endblock heater, and
transfer line heater, were monitored with thermocouples
and controlled with Powerstat variable transformers. In
e xpe r i m en t s conducted with the saturator at room temperature, the preheater was adjusted to between 10~ and
25~ below the desired electrolysis temperature and the
e n d b l o c k heater approximately 25~ below the preheater.
This minimized the heat load in the reaction zone, and
helped stabilize the temperature at the working electrode.
In experiments conducted with the saturator heated, the
heaters b et ween the saturator and the reaction zone were
adjusted to temperatures between that of the saturator and
the cell.
After all of the experimental parameters had been set,
the system was given time to reach equilibrium. Three
samples were collected to determine the reaction products: (i) a small liquid sample for immediate analysis of
benzoquinone and maleic acid; (ii) a timed liquid sample,
frozen in a Dry Ice-isopropyl alcohol bath for determination of benzene concentration and verification of flow rate;
and (iii) a timed gas sample for determination of CO2.
The operation of the saturator was tested by using it to
me a s u re the solubility of benzene in pure water. 'The results are compared with the measurements of Tsonopoulos and Wilson in Fig. 4 (6). The average deviation bet w e e n these measurements and the literature values is
20%. This confirmed that the saturator was mixing the
aqueous and organic phases well without entrainment of
benzene in the feed.
Analysis.--The concentrations of the soluble liquid
products were measured using a high-pressure liquid
1 0 "2
'
I
X'
i
1941
chromatograph (HPLC) with a C-18 stationary phase and a
variable wavelength detector. Tertiary butyl a m m o n i u m
h y d r o x i d e was added to the mobile phase to ion pair with
the maleic acid.
Benzene concentration in the reactor effluent was also
determined by HPLC. A v o l u m e of 5.0 ml of methanol was
added to the frozen samples before bringing them to room
temperature. The use of methanol induces total miscibility
and lowers the partial pressure of benzene above the solution, which limits the loss of benzene during handling. The
added methanol contained 1.96 mM nitrobenzene, which
served as an internal standard. The concentrations of benzene and nitrobenzene in the m i x ed sample were determined. F r o m these data, the v o l u m e and concentration of
benzene in the original frozen sample were calculated.
Since the samples were timed, the sample v o l u m e could
be used to calibrate the flow rate of the feed pump.
Analysis of carbon dioxide proved difficult because
m u c h of the carbon dioxide was dissolved in the liquid effluent. The chosen procedure is as follows. The samples
were collected in inverted, 25 ml, graduated centrifuge
tubes. The displaced liquid was 2.12M NaHSO4, which is
sufficiently dense so that all liquid and gas from the reactor were collected. The tubes were capped with a rubber
septum, inverted and stored. The duration of sample collection was set so that the sample tube contained an equal
v o l u m e of liquid effluent and the original 2.12M NaHSO4
solution. After mixing, the liquid in the sample tube was a
1.22M solution of NaHSO4. The solubility of carbon dioxide in this liquid is k n o w n to be 0.5 ml/ml (17). After allowing the gas and liquid to reach equilibrium, the a m o u n t of
CO2 in the gas phase was determined by gas chromatography, and the total amount of CO2 in the sample was calculated from Henry's law.
Results and Discussion
Feed saturated at room temperature.--The amounts of
benzoquinone, maleic acid, and carbon dioxide generated
by oxidation of benzene on a lead electrode at various temperatures are shown in Fig. 5. These are data from a single
days run with feed saturated at 25~ The benzene concentration in the feed was 8.5 raM. The data presented in this
figure, as well as results from other days, are given in Table
I.
There is some variability in the amount of product produced on the different days. These runs were made with
different electrodes. Though every effort was made to construct identical electrodes, the amount of benzoquinone
and maleic acid produced at 25~ varied between electrodes. In examining the variation in yield of these partial
oxidation products with changing temperature, there were
definite trends in all sets of data, and it is from these trends
that conclusions may be made.
The production of benzoquinone was always highest at
25~ There was a steady decline up to about 150~ after
which the amount levels off at a minimal value. Clarke
et al. (1) postulate that the oxidation of benzene to ben160
800
N
CO
r"
.o
120
10 "3
U.
__j,i'~
o
J
t
and Wilson
11983)
'~ ~ I ~
600
80
400
40
200
0
X This Work
0
<>Maleic Acid
10 .4
0
I
100
,
T
I
200
~
Fig. 4, Solubility of benzene in water vs. temperature, Data obtained
with this apparatus are compared to the empirical relation derived by
Tsonopoulosand Wilson (16).
0
=
I
100
=
I
200
0
300
T~
Fig. 5. Productsfrom the oxidation of benzene at varioustemperatures on lead dioxide, Feed: 0.2M NoHS04 saturated with benzene at
25~ (8.5 mM benzene). Flow rate = 1.3 mi/min. Current = 10 mA.
Downloaded 02 Feb 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp
J. E l e c t r o c h e m . Soc.: E L E C T R O C H E M I C A L
1942
SCIENCE
A u g u s t 1988
AND TECHNOLOGY
Table I. Products from the oxidation of benzene at various temperatures. Concentrations are based on feed volume. Feed: 0 . 2 M NaHSO4,
flow rate = 1.3 ml/min, current = 10 mA
Oxidation products (~M relative to liquid feed)
T~C
Benzene
concentration
= 9.2 mM
Rising
temperature
Falling
temperature
Benzene
concentration
= 8.5 mM
Rising
temperature
Benzoquinone
25
100
150
200
250
150
100
25
25
80
140
200
250
43 •
25 •
7•
7•
6•
7•
30 •
28 •
86 •
80 •
36 •
28 •
26 •
4
2
1
1
1
1
3
9
4
3
Maleic acid
CO2
85 • 9
111 • 11
59 • 6
47 • 5
43 • 4
56 • 6
102 • 10
62 • 6
42• 4
93 • 9
71• 7
26 • 3
26 • 3
zoquinone proceeds through a chemical reaction between
b e n z e n e a n d lead d i o x i d e . I n a d d i t i o n , Clarke et al. prop o s e t h a t t h e o t h e r o x i d a t i o n p r o d u c t s are p r o d u c e d w h e n
b e n z e n e is a t t a c k e d b y s o m e t y p e o f o x y g e n radical o n t h e
e l e c t r o d e . W a b n e r a n d G r a m b o w h a v e f o u n d t h a t relatively h i g h c o n c e n t r a t i o n s o f h y d r o x y l r a d i c a l s (OH.) are
f o r m e d d u r i n g o x y g e n e v o l u t i o n at a lead d i o x i d e elect r o d e (18), so OH. is p r o b a b l y t h e r e a c t i v e s p e c i e s called for
in t h e C l a r k e m o d e l . S i n c e l e a d d i o x i d e is k n o w n to dec o m p o s e in air w h e n h e a t e d a b o v e 300~ (19), this m o d e l
m a y e x p l a i n t h e d e c r e a s e in b e n z o q u i n o n e f o r m a t i o n w i t h
increasing temperature.
A series o f cyclic v o l t a m m o g r a m s w a s m a d e to s t u d y
l e a d d i o x i d e f o r m a t i o n in s o l u t i o n as a f u n c t i o n o f t e m p e r a t u r e . T h e r e are t w o e l e c t r o c h e m i c a l r e a c t i o n s t h a t
t a k e p l a c e as t h e p o t e n t i a l is s c a n n e d positive. T h e s u r f a c e
o f t h e e l e c t r o d e is c o n v e r t e d to lead d i o x i d e
210 •
420 •
590•
650•
470•
720 •
640 •
250 •
570•
480•
530 •
510•
700•
Current efficiency
Benzoquinone
Maleic acid
All products
(%)
(%)
(%)
40
80
120
130
140
140
130
50
6
3
1
1
1
1
4
4
11
10
5
3
3
100
110
140
140
34
44
23
19
17
22
40
25
16
35
27
10
10
44
69
76
81
60
90
92
42
77
76
72
61
81
t u r e s , total o x i d a t i o n to c a r b o n d i o x i d e b e c o m e s fast, a n d
t h e a m o u n t o f m a l e i c acid p r o d u c e d d r o p s .
T h e r e is n o d i s c u s s i o n in t h e l i t e r a t u r e as to w h y m a l e i c
acid should be the major significant product, and others,
e.g. p h e n o l , are a l m o s t a b s e n t . T h e c a r b o x y l i c acid g r o u p s
c o u l d b e o x i d i z e d u n d e r t h e c o n d i t i o n s n e c e s s a r y to b r e a k
o p e n t h e ring. A p o s s i b l e j u s t i f i c a t i o n for t h e lack o f furt h e r o x i d a t i o n o f m a l e i c acid is t h a t this s p e c i e s is less
s t r o n g l y h e l d t h a n b e n z e n e , b e n z o q u i n o n e (or o t h e r aromatics) on the electrode surface and desorbs. At temperat u r e s a b o v e 100~ t h e o x i d a t i o n rate o f o r g a n i c m o l e c u l e s
n e a r t h e s u r f a c e b e c o m e s fast e n o u g h to o v e r c o m e t h i s
I
100 ~A
'
410
25=C
q. bar
'
0.0
3.w0
PbSO4 + 2H20 -~ PbO2 + SO42= + 4H § + 2ea n d w a t e r is o x i d i z e d to o x y g e n
2H20 ~ 02 + 4H § + 4eA s c a n b e s e e n i n Fig. 6, at 25~ t h e s u r f a c e o x i d a t i o n
f o r m s a d i s t i n c t w a v e at a b o u t 2.0V w h e r e t h e rate o f oxyg e n e v o l u t i o n is small. T h e r e f o r e , b e n z e n e o x i d i z e d at t h i s
t e m p e r a t u r e will e n c o u n t e r PbO2 o n t h e e l e c t r o d e surface.
A t 100 ~ a n d 200~ t h e s u r f a c e o x i d a t i o n w a v e m o v e s v e r y
far p o s i t i v e a n d m e r g e s into t h e o x y g e n e v o l u t i o n c u r r e n t .
T h i s is a r e s u l t o f PbO2 d e c o m p o s i t i o n t h r o u g h t h e following reaction
I
500 ~A
1
100~
10 bar
I
4,0
I
3
~
0.0
PbO2 + SO42- + 2H § -~ PbSO4 + 1/2 02 + H20
T h e r e a c t i o n is v e r y s l o w at 25~ b u t b e c o m e s i m p o r t a n t
at h i g h e r t e m p e r a t u r e s . S i n c e t h e PbO2 is c o n t i n u a l l y
breaking down, the surface oxidation reaction m u s t be
d r i v e n at a faster rate (i.e., m o r e p o s i t i v e potential) to c o v e r
m o s t o f t h e s u r f a c e w i t h P b Q a n d p r o d u c e a " p e a k " in t h e
v o l t a m m o g r a m . T h e i m p l i c a t i o n o f t h i s w i t h r e g a r d to b e n z e n e o x i d a t i o n is t h a t at h i g h t e m p e r a t u r e , t h e e l e c t r o d e
s u r f a c e is n o t c o m p l e t e l y c o v e r e d b y l e a d d i o x i d e . T h e r e fore t h e r e is less PbO2 available for t h e p r o d u c t i o n o f b e n zoquinone, while the other reactions involving oxygen
species adsorbed on the electrode surface continue.
U p o n cooling, t h e e l e c t r o d e r e g a i n s m o s t o f its selectivity t o w a r d b e n z o q u i n o n e , as s h o w n in T a b l e I. It is n o t
k n o w n w h y t h e activity o f t h e e l e c t r o d e is n o t c o m p l e t e l y
r e s t o r e d at l o w e r t e m p e r a t u r e , b u t t h e f o r m a t i o n o f a film
o v e r p a r t o f t h e e l e c t r o d e is c o n s i d e r e d likely.
T h e g e n e r a t i o n of m a l e i c acid f r o m b e n z e n e is e n h a n c e d
b y i n c r e a s i n g t h e t e m p e r a t u r e a b o v e 25~ T h e m a x i m u m
y i e l d o f m a l e i c acid o c c u r s a r o u n d 100~ A f u r t h e r inc r e a s e i n t e m p e r a t u r e l o w e r s t h e y i e l d u n t i l a m i n i m u m is
r e a c h e d a s y m p t o t i c a l l y at 200~ T h e f o r m a t i o n o f m a l e i c
a c i d i n v o l v e s b r e a k i n g t h e a r o m a t i c r i n g w h i c h stabilizes
benzene. A considerable amount of activation energy m u s t
b e s u p p l i e d for o x y g e n o n t h e e l e c t r o d e to b r e a k t h e ring.
T h i s a c c o u n t s for t h e r e g i o n o f i n c r e a s i n g maleic acid form a t i o n w i t h i n c r e a s i n g t e m p e r a t u r e . At h i g h e r t e m p e r a -
I
i
1000 I~A
200~
30
t
4.
f
j
bar
i
.
i
i
.
j
i
I
0.0
Fig. 6. Cyclic voltammograms of a lead electrode in 0.2M NaHSO4.
Electrode area = 1.8 • 10 -2 cm 2. Scan rate = 1 V/s.
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Vol. 135, No. 8
OXIDATION OF BENZENE
rapid desorption, and the net production of maleic acid
drops.
As stated above, the m e a s u r e m e n t of carbon dioxide was
the most difficult of the analyses to perform. The analysis
was necessary in order to establish a current efficiency for
the consumption of benzene. As can be seen in Table I, this
current efficiency was always above 42%, indicating that
benzene oxidation was always one of the major electrode
reactions in the experiments. Table I also shows individual
current efficiencies for the production of benzoquinone
and maleic acid and reemphasizes the effect of temperature on their formation.
Though there is significant uncertainty in some of the
carbon dioxide measurements, a general trend can be seen
in the data of Fig. 5 and Table I. Carbon dioxide production rises with increasing temperature. This fits the hypothesis presented for the behavior of maleic acid with
temperature, and the knowledge that high temperature
should favor complete oxidation of benzene.
Feed saturated at high temperature.--Successful operation of the apparatus with the saturator at high temperature proved to be very difficult. The solution in the saturator becomes m u c h more compressible when heated. This
made it m u c h harder to adjust the outlet valve to maintain
the system at a stable flow rate, which was necessary for
the electrochemical processes to reach steady state. Still
sufficient data were obtained to conclude that with the
electrode at high temperature, increasing the benzene concentration of the feed did not increasse the production of
benzoquinone. Figure 7 compares the results of runs using
feed saturated with benzene at 25~ with runs that used
feed saturated at higher temperatures. In both cases, with
the cell at 140 ~ and at 200~ the amount of benzoquinone
p r o d u c e d drops slightly at the higher benzene concentration, though at 200~ the change is of the order of the accuracy of the measurement. The reason for this drop is not
known, but it is clear that increasing the benzene concentration does not increase benzoquinone yield at high temperature.
Apparently, at 140~ and above, the electrode character
has changed so that the reaction products are not dependent on benzene concentration. The cyclic voltammetry
shows that PbO2 is unstable at these temperatures, so the
predominant reaction mechanism would be direct attack
of benzene by oxygen radicals on the electrode surface. It
was hoped that high benzene concentrations would saturate the electrode surface and limit the degree of oxidation
so that m o r e benzoquinone would be produced, but this is
not the case.
Biphenyl.--A series of preliminary experiments were
performed with a two-phase flow of benzene and water.
The benzene layer was analyzed by gas chromatography
(GC), and a peak with higher molecular weight than ben-
Fig. 7. Effect of benzene concentration and temperature on product
d~stribution from oxidation on lead dioxide. Feed: 0.2M NariS04. Flow
rate
1.3 ml/min. Current 10 mA.
=
=
1943
Table II. Condttions necessaryfor the formation of biphenyl from
benzene in 0.2M NaS04 solution. (+) Biphenylformed at these
conditions. ( - ) Biphenyl not formed. (Blank) No data
Experimental conditions
Electrolysis of a two-phase
mixture of benzene and 0.2M
NaSO4.
Electrolysis of 0.2M NaSO4
saturated with benzene.
Two-phase mixture of benzene
and 0.2M NaSO4 saturated with
air at 30 bar. Electrodes removed
from cell.
Single-phase solution of
0.2M NaSO4 saturated with
benzene and air at 1 bar.
No current.
25~
Temperature
140~ 200~
-
+
-
+
+
-
-
300~
+
-
zene was noted. The peak was identified as biphenyl using
GC/mass spectrometry~ An analytical procedure for determination of biphenyl in aqueous or organic solutions
using H P L C was then established.
A series of experiments was performed to determine
what conditions were necessary for the formation of biphenyl and a possible mechanism for the reaction. A summary of the results is given in Table II. It is apparent from
the data that the reaction does not proceed below 200~
The other important point is that the reaction is not electrochemical, but involves oxygen in solution. Based on
this, the following reaction is proposed
2C6H6 + 1/2 02 --->C12H10 + H20
The reaction may take place in solution, or may be catalyzed by the alumina wails of the cell. No products of
higher molecular weight, such as terphenyl, were found.
Since this project was primarily concerned with electrochemical reactions, there was no further investigation of
this reaction.
Conclusions
As temperature is increased, PbO2 decreases its selectivity toward partial oxidation of benzene. The first stage oxidation product, benzoquinone, is most efficiently produced at 25~ Current efficiencies from 6 to 10% were
found at 25~ under the conditions described in this
study. Higher efficiencies have been reported previously
(1), but the electrochemical cells and experimental conditions used were not suitable for high temperature work.
Production of maleic acid, the second major product
from the oxidation of benzene, is enhanced by increasing
the cell temperature above ambient. In the range 80 ~ 100~
current efficiency for the production of maleic acid averaged 40%, vs. 25% at 25~ Increasing the cell temperature
above 100~ causes a drop in the amount of maleic acid recovered, as the rate of oxidation to CO~ proceeds more
quickly.
At higher temperatures, 140~176
the oxidation of benzene on a lead dioxide electrode proceeds rapidly to CO2.
Only small amounts of benzoquinone and maleic acid are
recovered in the liquid products. Increasing the concentration of benzene in the feed, by saturating the feed at higher
temperatures, did not enhance, and may have been detrimental to, the formation of benzoquinone and maleic acid.
At ambient temperature, the oxidation of benzene to benzoquinone is strongly dependent on benzene concentration (1), but at high temperature, the effect of increased
benzene concentration is more than offset by the reduction in electrode selectivity.
In addition to the electrochemical reactions studied, a
noneleetroehemical reaction was discovered. At 200~ and
above, in the presence of oxygen, benzene is coupled to
produce biphenyl. The proposed mechanism is a partial
oxidation of benzene to produce biphenyl and water. No
higher molecular weight products such as terphenyl were
identified, but since these are practically insoluble in
aqueous solution, it is u n k n o w n whether any are formed.
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1944
J. Electrochem. Soc.: E L E C T R O C H E M I C A L
Though operation at high temperature was found to be
detrimental to the yield of benzoquinone from benzene oxidation, the electrochemical e q u i p m e n t described here can
be used to study a wide range of high temperature aqueous
processes, both electrochemically induced or monitored.
The potential of chemical syntheses involving high concentrations of nonpolar organics in water has received little attention and holds m u c h promise for future discoveries.
Acknowledgments
W.M.F. is grateful to the Shell Foundation for support.
We are indebted to Yu-Min Tsou for his assistance in this
work and to Isaac Trachtenberg for his comments and advice. This research was funded by the Office of Naval Research and The University of Texas Separations Research
Program.
Manuscript submitted Aug. 10, 1987; revised manuscript
received Dec. 7, 1987.
REFERENCES
1. J. S. Clarke, R. E. Ehigamusoe, and A. T. Kuhn, J. Electroanal. Chem., 7@, 333 (1976).
2. J. P. Connolly, J. Chem. Eng. Data, 11, 13 (1966).
3. E. U. Franck, "Struct. Water Aqueous Solutions, Proc.
Int. Symp.," 49-73 (1974).
4. M. Uem at s u and E. U. Franck, J. Phys. Chem. Ref.
SCIENCE AND TECHNOLOGY
A u g u s t 1988
Data, 9, 1291 (1980).
5. M. Modell, U.S. Pat. 4,338,119.
6. S. H. Townsend and M. T. Klein, Fuel, 64, 635 (1985).
7. M. J. Antal, C. P. DeAmeida, W.S. Mok, S.V. Ramayya, and J. C. Roy, Paper presented at the 109th
National Meeting of the American Chemical Society,
Chicago, S e p t e m b e r 1985.
8. W. M. Flarsheim, Y. Tsou, I. Trachtenberg, K. P.Johnston, and A. J. Bard, J. Phys. Chem., 90, 3857 (1986).
9. A. Renard, Compt. Rend., 91, 175 (1880).
10. F. Fichter a n d R . Stocker, Ber., 47, 2003 (1914).
11. A. Seyewetz and G. Miodon, Bull. Soc. Chim. Ft., 33,
449 (1923).
12. K. S. Udupa, G. S. Subramanian, and H. V. K. Udupa,
Bull. Acad. Pol. Sci., 9, 45 (1961).
13. S. Ito, K. Sasaki, Y. Murakami, and H. Shiba, Denki
Kagaku, 4@, 733 (1972).
14. B. Fleszar and J. Ploszynska, Electrochim. Acta, 30, 31
(1985).
15. J. O' M. Bockris, H. Wroblow, E. Gileadi, and B . J .
Piersma, Trans. Faraday Soc., 61, 2531 (1965).
16. C. Tsonopoulos and G. M. Wilson, AIChE J., 29, 990
(1983).
17. M. K. Shchennikova, G. G. Devyatykh, and I.A.
Korshunov, J. Appl. Chem. USSR, 30, 1149 (1957).
18. D. Wabner and C. Grambow, J. Electroanal. Chem.,
195, 95 (1985).
19. M. I. Gillibrand and B. Halliwell, J. Inorg. Nucl. Chem.,
34, 1143 (1972).
The Anodic Oxidation of Galena in a Cationic
Surfactant-Aqueous Sodium Hydroxide Emulsion
Thomas C. Franklin,* Remigius Nnodimele, William K. Adeniyi, and David Hunt
Chemistry Department, Baylor University, Waco, Texas 76798
ABSTRACT
A study was made of the electro-oxidation of galena in Hyamine 2389 surfactant systems. It was found that during the
oxidation, one could observe four oxidation waves in voltammetric curves. These were concluded to be the oxidation of
the galena to (i) PbO and S, (ii) PbO and SO3 =, (iii) PbSO4, and (iv) PbO2 and SO4=: A study of the effect of the relation between the amount of galena in a suspension and the height of the highest peak in the voltammetric curve showed a complex relationship. At low concentrations of galena, the height varied with a regular periodicity indicating the formation on
the electrode of multilayers of the Hyamine-galena micelle co m p l ex with the layers arranged in a manner similar to
Langmuir-Blodgett films, alternating in nature from hydrophobic to hydrophilic to hydrophobic, etc. At higher concentrations of galena, it behaves as if one has simple monolayer adsorption.
In a recent study of the electro-oxidation of iron pyrite in
a cationic surfactant-styrene-aqueous sodium hydroxide
emulsion (1) it was shown that the filming of the electrode
allowed one to observe a series of reactions not observable
in normal aqueous solutions. In addition it was shown that
the surfactant solubilized the iron pyrite making it possible to obtain a wave that was sensitive to the concentration
of iron pyrite (2). However, because of the formation of
multilayers of surfactant on the electrode, the height of the
voltammetric peak was a complicated function of the concentration.
This is a report on a similar study of the oxidation of galena (lead sulfide) made in order to determine whether
these techniques are of general use for the study of sulfide
minerals. In connection with studies of mineral flotation
processes, there have been a variety of studies of the electro-oxidation of a n u m b e r of sulfide minerals. These studies have been reviewed and discussed in the papers by
Hayes et al. (3) and by Richardson et al. (4, 5). Their summaries of the results for the sulfide minerals including galena are that initial oxidation removes lead from the surface
producing a sulfur rich film which upon further oxidation
eventually produces sulfur. Electrochemical studies in
* Electrochemical Society Active Member.
basic solutions (6-10) have led to the conclusion that the
first stable products are sulfur and PbO, Pb(OH)2, or
HPbO2-, with secondary reactions that produced thiosulfate and sulfate.
In this investigation, similar to the iron pyrite work,
studies were made in a surfactant induced suspension of
the galena in order to determine the relation between the
a m o u n t of sulfide mineral and the height of the voltammetric peak. Studies were also made in a sandwich cell to
delineate the reactions that are occurring.
Experimental
Surfactant suspensions in a beaker cell.--Similar to previous studies (11) the emulsion contained 35 ml o f a 2N sodium hydroxide solution, containing 3.5 ml of styrene
and 3.5 mls of Hyamine 2389 (predominantly methyldodecylbenzyltrimethylammonium chloride) obtained from
R o h m and Haas. Weighed samples of naturally occurring
galena which had been ground in a mortar and sieved in a
279 (53 ~m) mesh screen were added to the emulsion after
the residual voltammetric curves had become constant.
The suspension was stirred with a magnetic stirrer and the
sample was allowed to stand for 25 m i n in the stirred suspension before obtaining the voltammetric curve. S o m e
voltammetric studies were done with lead sulfide prepared
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