High-Pressure Electrochemical Oxidation of Benzene at a Lead Dioxide Electrode in Aqueous Bisulfate Solutions at 25 ~ to 250~

Vol. 135, No. 8 POLYCRYSTALLINE CuInSe2 ELECTRODES 1939

27. S. Menezes, L. F. Schneemeyr, and H. J. Lewerenz, Phys. Lett., 45, 668 (1984). Appl. Phys. Lett., 38, 949 (1981). 30. L. Fornarini and B. Scorsati, Electrochim. Acta, 28, 667 28. L. Peraldo Bicelli and G. Razzini, Surf. Technol., 22, (1983). 115 (1984). 31. O. P. Bahl, E. L. Evans, and Y. M. Thomas, Surf. Sci., 8, 29. R. Noufi, R. Axton, C. Herrington, and S. K. Deb, Appl. 473 (1967).

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 ground, the present work is an expansion into the area of dioxide electrode in aqueous sulfate solutions is limited by high-temperature electro-organic synthesis, specifically the low solubility of benzene in water. Clarke et al. have benzene oxidation. In supercritical water, high concentra- determined a reaction order with respect to benzene of 2 tions of nonpolar materials can be combined with strong (1). Benzene solubility in water increases significantly with electrolytes in a one-phase system, and this presents the increasing temperature; the two fluids become miscible at possibility of synthetic routes that are not hindered by 297~ at a pressure of 243 bar (2). This study examines the mass transfer across a phase boundary. Although tempera- effect of increasing temperature and increasing benzene tures above 250~ are not reported here, the apparatus and concentration on the electrochemical oxidation of benzene experimental procedures described could be used up to on lead dioxide in the temperature range 25~176 375~ with only minor modification. Supercritical water has a combination of low dielectric The electrochemical oxidation of benzene was first constant and high dipole moment that make it useful for a studied near the end of the last'century (9) and has been number of novel chemical applications. Water has a dipole discussed frequently in the literature (1, 10-14). Work moment of 1.84 debye, and at ambient conditions it is ex- through 1976 has been summarized by Clarke et al. (1). The tensively hydrogen bonded. This gives rise to a high di- desired product from the oxidation is benzoquinone, electric constant, 78 at 25~ making water an excellent sol- which can then be reduced to hydroquinone. Hydroqui- vent for ions and a poor one for nonpolar species. High none is used as an anti-oxidant in the manufacture of rub- temperature disrupts the hydrogen bonding and corre- ber and other products. Other major products from the ox- lated orientation of water molecules (3) and decreases the idation are maleic acid and carbon dioxide. density and thereby lowers the dielectric constant to 5.2 at The specific overall electrode reactions involved in oxi- the critical point (374~ 221 bar) (4). Because of the low di- dizing benzene are shown in Fig. 1. The complete conver- electric constant, water in the critical region is an excellent sion of benzene to carbon dioxide requires the removal of solvent for organic molecules. The water dipoles still ori- 30 electrons and the participation of 12 water molecules. ent in the vicinity of an ion, however, and produce a locally This full oxidation can occur readily at a platinum elec- high dielectric constant, so strong electrolytes remain dis- trode (15). Lead dioxide is the only reported electrode ma- solved and dissociated in supercritical water. The simulta- terial that produces large quantities of benzoquinone or neous solvation of both polar and nonpolar molecules maleic acid. A lead dioxide electrode will perform the first makes supercritical water a powerful solvent. The stage oxidation to benzoquinone readily in acidic sulfate MODAR process takes advantage of this superior solvent media; however, the yield and current efficiency vary power in using supercritical water as a reaction medium greatly depending on the specific conditions of electrol- for the destructive oxidation of organic waste (5). Other ysis. As the concentration of benzoquinone builds in the workers have investigated the use of supercritical water at solution, the benzoquinone is usually oxidized further to milder conditions to clean and liquify coal (6), and to hy- maleic acid, accompanied by two moles of carbon dioxide. drolyze plant material into useful fuels (7). Clarke et al. (1) report that at very low over-voltage, the In a previous paper, we described electroanalytical benzoquinone is not oxidized further. The one-step oxida- measurements made in supercritical water at tempera- tion of benzene to maleic acid was studied extensively by tures up to 390~ (8). An electrochemical cell was de- Ito et al. (13). This pathway is important under most reac- veloped to operate at the extreme temperature and pres- tion conditions. Few previous workers analyzed the car- sure of supercritical water. In addition, the voltammetric bon dioxide generated during oxidation, and no one has behavior Of water, KBr, KI, and hydroquinone was discussed the pathway by which it is formed. This study studied, and the diffusion coefficients of I- and hydroqui- shows that single-step oxidation of benzene to carbon none were measured. Building on this experimental back- dioxide can occur on lead dioxide, especially at high tem- *Electrochemical Society Active Member. perature. (The term single-step is used here to mean that **Electrochemical Society Student Member. 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 1940 J. Electrochem. Sac.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY August 1988

ENDELOCK

~*2H;z~ -6X *-6e- 0

ELECTROLYTE LINE ~-D VESSEL HEATER . ,,

GAS ~ ~kE COkk~'nON LIQUID /X I ."%/ \ I FEEDSATURATOR Fig. 3. Schematic of apparatus for high-temperature, high-pressure electrosynthesis.

stirred tank reactor. The saturator can be heated to 200~ in order to increase the concentration of benzene in the feed. The flow is controlled by a Milton Roy Minipump (Model 2396), which provides fresh electrolyte to the satu- 6CO 2 rator. Fig. 1. Oxidation reactions of benzene on a lead dioxide electrode. Electrodes.--The working electrode was constructed by Adapted from Ito et el. (13). welding a 1 cm piece of platinum onto a tantalum wire. The platinum was welded to and wrapped with 0.05 cm from the electrode, they are not further oxidized. No thick lead foil (99.9995% Pb from AESAR division of John- doubt, the oxidation of benzene adsorbed on the electrode son Matthey), so that the platinum was completely involves many elementary steps. This work however, is covered. The lead was sealed and trimmed with a solder- only concerned with the net result of each encounter be- ing iron. The resulting electrode was 1.8 cm long, 0.3 cm tween a benzene molecule and the electrode.) wide, and 0.15 cm thick. Before use the electrode was This work differs from previous studies of benzene oxi- rinsed in 5N nitric acid. dation in that a single-pass flow reactor is used instead of a In the cell, the current path is along the axis of the cell, stirred tank and the temperature range is much larger and because of solution resistance, only a portion of the (12, 13). The reactant volume used in these previous stud- working electrode participates in the oxidation. When the ies was relatively large, e.g., 90-1500 cm 3, so long runs were working electrode is oxidized in the cell, lead dioxide necessary to build up measurable quantities of product. forms from the tip to a location about 0.5 cm further away The products in solution were continually exposed to fur- from the counterelectrode. Therefore, this is the area ther oxidation. In the present apparatus, the residence which is active during electrolysis, while the remaining time of the electrolyte in the electrode region is about 2s. portion passes very little current. The electrode area is large enough so that measurable con- The counterelectrode consisted of a platinum wire version is achieved in a single pass. Products from the ini- welded to the Inconel 600 sheath of an ungrounded ther- tial oxidation are quickly removed from the electrode with mocouple. Placement of the thermocouple in the flowing little opportunity to be readsorbed and undergo further stream was necessary to achieve accurate and stable tem- oxidation. Thus, analysis of the results is somewhat sim- perature control. Since the counterelectrode was the re- plified, since the products are probably formed predomi- ducing electrode in these experiments, there was mini- nantly by single-step oxidation of benzene; only three of mum corrosion of the thermocouple sheath during the the six reactions shown in Fig. 1 need be considered. project. For cyclic voltammetry, a lead electrode (0.15 cm diam) Experimental sealed in glass was used. A silver wire electrode was in- Apparatus.--The experiments were performed in a serted into the heated portion of the cell as a reference high-temperature and -pressure, two-electrode flow cell electrode, and a counterelectrode was added downstream. (Fig. 2). The cell body is a 0.635 cm (1/4 in.)-od by 0.318 cm Details of this procedure have been discussed previously (% in.)-id, 30 cm (12 in.)-long alumina tube (99.8% A1203). (8). A1203 was chosen to avoid corrosion and spurious electric fields within the cell. The tube is connected to the end- Procedure.--The electrolyte vessel was filled with 290 mt blocks using stainless steel Swagelok fittings with Teflon of 0.2M NaHSO4 solution saturated with benzene at room ferrules. The electrode leads pass through Teflon compres- temperature. During long experiments, a second electro- sion fittings, which have been described previously (8). All lyte vessel could be filled and exchanged with the first one metal parts are either 316 stainless steel or Hastelloy Alloy with only a brief interruption of the electrolysis. The satu- C. rator was filled with 300 ml of benzene and 700 ml of 0.2M The complete experimental system, including the pump NaHSO4 solution. The NaHSO4 was reagent grade and the and feed vessels, is shown schematically in Fig. 3. The cell benzene was spectroscopic grade. Both chemicals were is fed from the saturator, a one-liter Autoclave Engineers' used as received. All water was purified with a Millipore, Milli-Q reagent water system. In preparation for experiments with the feed benzene INLET concentration higher than the 25~ saturation value, the saturator was heated to the desired temperature, and the contents mixed to saturate the aqueous phase. The phases ~\\\\\\x-,\xx,,. ,xx,. -.~ "~- ,\\\~ were then allowed to separate before filling the cell. The flow of electrolyte into the saturator was small compared o/ \ T to the saturator volume, so benzene in the feed was not di- OUTLET luted during an experimental run. Fig. 2. Diagram of high-temperature cell for electrosynthesis: (A) V4 The feed pump was set to the desired flow rate, and the in.-od alumina tube; (B) Hastelloy Alloy C endblock; (C) 316 SS Swage- outlet valve was adjusted to give the desired back pres- Iok fittings; (D) Pb working electrode; (E) Pt counterelectrode and type sure. Because of the low flow rate, 1.3-1.4 ml/min, the out- K thermocouple; (F) main cell heater (Nichrome wire coil); (G) pre- let valve had to be operated very near shutoff where its heater (Nichrome wire coil); (H) endblock heater (cartridge heater). performance was not consistent. The system pressure had

Downloaded 02 Feb 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp Vol. 135, No. 8 OXIDATION OF BENZENE 1941

to be continually monitored, and the outlet valve adjusted. chromatograph (HPLC) with a C-18 stationary phase and a Any system pressure that was at least 7 bar above the solm variable wavelength detector. Tertiary butyl ammonium tion saturation pressure and relatively stable was con- hydroxide was added to the mobile phase to ion pair with sidered acceptable. After the cell had been flushed with a the maleic acid. few volumes of solution, the effluent was analyzed to in- Benzene concentration in the reactor effluent was also sure that the feed contained no impurities that would in- determined by HPLC. A volume of 5.0 ml of methanol was terfere with the measurement of benzoquinone or maleic added to the frozen samples before bringing them to room acid. temperature. The use of methanol induces total miscibility All experiments were done at constant current. This and lowers the partial pressure of benzene above the solu- made calculation of current efficiency possible without the tion, which limits the loss of benzene during handling. The use of a coulometer. The narrow current path between the added methanol contained 1.96 mM nitrobenzene, which two electrodes gives rise to a large cell resistance, so that served as an internal standard. The concentrations of ben- constant voltage experiments would have little meaning. zene and nitrobenzene in the mixed sample were deter- The cell potential was monitored during the electrolysis mined. From these data, the volume and concentration of and was relatively constant at any particular set of condi- benzene in the original frozen sample were calculated. tions. A value of 7.5V was typical at 25~ while it dropped Since the samples were timed, the sample volume could to about 6V at 250~ due to lower solution resistance. be used to calibrate the flow rate of the feed pump. The temperature of the solution at the working electrode Analysis of carbon dioxide proved difficult because was maintained with an Autoclave Engineers (Erie, PA) much of the carbon dioxide was dissolved in the liquid ef- proportional-integral temperature controller (Model 520). fluent. The chosen procedure is as follows. The samples The temperatures at the preheater, endblock heater, and were collected in inverted, 25 ml, graduated centrifuge transfer line heater, were monitored with thermocouples tubes. The displaced liquid was 2.12M NaHSO4, which is and controlled with Powerstat variable transformers. In sufficiently dense so that all liquid and gas from the reac- experiments conducted with the saturator at room tem- tor were collected. The tubes were capped with a rubber perature, the preheater was adjusted to between 10 ~ and septum, inverted and stored. The duration of sample col- 25~ below the desired electrolysis temperature and the lection was set so that the sample tube contained an equal endblock heater approximately 25~ below the preheater. volume of liquid effluent and the original 2.12M NaHSO4 This minimized the heat load in the reaction zone, and solution. After mixing, the liquid in the sample tube was a helped stabilize the temperature at the working electrode. 1.22M solution of NaHSO4. The solubility of carbon diox- In experiments conducted with the saturator heated, the ide in this liquid is known to be 0.5 ml/ml (17). After allow- heaters between the saturator and the reaction zone were ing the gas and liquid to reach equilibrium, the amount of adjusted to temperatures between that of the saturator and CO2 in the gas phase was determined by gas chromatogra- the cell. phy, and the total amount of CO2 in the sample was calcu- After all of the experimental parameters had been set, lated from Henry's law. the system was given time to reach equilibrium. Three samples were collected to determine the reaction prod- Results and Discussion ucts: (i) a small liquid sample for immediate analysis of Feed saturated at room temperature.--The amounts of benzoquinone and maleic acid; (ii) a timed liquid sample, benzoquinone, maleic acid, and carbon dioxide generated frozen in a Dry Ice-isopropyl alcohol bath for determina- by oxidation of benzene on a lead electrode at various tem- tion of benzene concentration and verification of flow rate; peratures are shown in Fig. 5. These are data from a single and (iii) a timed gas sample for determination of CO2. days run with feed saturated at 25~ The benzene concen- The operation of the saturator was tested by using it to tration in the feed was 8.5 raM. The data presented in this measure the solubility of benzene in pure water. 'The re- figure, as well as results from other days, are given in Table sults are compared with the measurements of Tsono- I. poulos and Wilson in Fig. 4 (6). The average deviation be- There is some variability in the amount of product pro- tween these measurements and the literature values is duced on the different days. These runs were made with 20%. This confirmed that the saturator was mixing the different electrodes. Though every effort was made to con- aqueous and organic phases well without entrainment of struct identical electrodes, the amount of benzoquinone benzene in the feed. and maleic acid produced at 25~ varied between electrodes. In examining the variation in yield of these partial Analysis.--The concentrations of the soluble liquid oxidation products with changing temperature, there were products were measured using a high-pressure liquid definite trends in all sets of data, and it is from these trends that conclusions may be made. 10 "2 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 ' I X' i et al. (1) postulate that the oxidation of benzene to ben-

160 800

N CO r" 120 '~ ~I~ 600 .o 10 "3 J and Wilson U. o __j,i'~ t 11983) 80 400 0 X This Work

0 40 200 <> Maleic Acid 10 .4 I , I 0 = I = I 0 0 100 200 100 200 300 T ~ T ~ Fig. 4, Solubility of benzene in water vs. temperature, Data obtained Fig. 5. Productsfrom the oxidation of benzene at various tempera- with this apparatus are compared to the empirical relation derived by tures on lead dioxide, Feed: 0.2M NoHS04 saturated with benzene at Tsonopoulos and Wilson (16). 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 1942 J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY August 1988

Table I. Products from the oxidation of benzene at various temperatures. Concentrations are based on feed volume. Feed: 0.2M NaHSO4, flow rate = 1.3 ml/min, current = 10 mA

Oxidation products (~M relative to liquid feed) Current efficiency Benzoquinone Maleic acid All products T~C Benzoquinone Maleic acid CO2 (%) (%) (%)

Benzene Rising 25 43 • 4 85 • 9 210 • 40 6 34 44 concentration temperature 100 25 • 2 111 • 11 420 • 80 3 44 69 = 9.2 mM 150 7 • 1 59 • 6 590• 120 1 23 76 200 7 • 1 47 • 5 650• 130 1 19 81 250 6 • 1 43 • 4 470• 140 1 17 60 Falling 150 7• 1 56 • 6 720 • 140 1 22 90 temperature 100 30 • 3 102 • 10 640 • 130 4 40 92 25 28 • 62 • 6 250 • 50 4 25 42 Benzene Rising 25 86 • 9 42• 4 570• 11 16 77 concentration temperature 80 80 • 93 • 9 480• 100 10 35 76 = 8.5 mM 140 36 • 4 71• 7 530 • 110 5 27 72 200 28 • 26 • 3 510• 140 3 10 61 250 26 • 3 26 • 3 700• 140 3 10 81

zoquinone proceeds through a chemical reaction between tures, total oxidation to carbon dioxide becomes fast, and benzene and lead dioxide. In addition, Clarke et al. pro- the amount of maleic acid produced drops. pose that the other oxidation products are produced when There is no discussion in the literature as to why maleic benzene is attacked by some type of oxygen radical on the acid should be the major significant product, and others, electrode. Wabner and Grambow have found that rela- e.g. phenol, are almost absent. The carboxylic acid groups tively high concentrations of hydroxyl radicals (OH.) are could be oxidized under the conditions necessary to break formed during oxygen evolution at a lead dioxide elec- open the ring. A possible justification for the lack of fur- trode (18), so OH. is probably the reactive species called for ther oxidation of maleic acid is that this species is less in the Clarke model. Since lead dioxide is known to de- strongly held than benzene, benzoquinone (or other aro- compose in air when heated above 300~ (19), this model matics) on the electrode surface and desorbs. At tempera- may explain the decrease in benzoquinone formation with tures above 100~ the oxidation rate of organic molecules increasing temperature. near the surface becomes fast enough to overcome this A series of cyclic voltammograms was made to study lead dioxide formation in solution as a function of temperature. There are two electrochemical reactions that 100 ~A 25=C take place as the potential is scanned positive. The surface I q. bar of the electrode is converted to lead dioxide ' 410 ' 3.w0 0.0 PbSO4 + 2H20 -~ PbO2 + SO42= + 4H § + 2e- and water is oxidized to oxygen 2H20 ~ 02 + 4H § + 4e- As can be seen in Fig. 6, at 25~ the surface oxidation forms a distinct wave at about 2.0V where the rate of oxygen evolution is small. Therefore, benzene oxidized at this temperature will encounter PbO2 on the electrode surface. 500 ~A 100~ I 10 bar I At 100 ~ and 200~ the surface oxidation wave moves very 1 I far positive and merges into the oxygen evolution current. 4,0 3 ~ 0.0 This is a result of PbO2 decomposition through the following reaction PbO2 + SO42- + 2H § -~ PbSO4 + 1/2 02 + H20 The reaction is very slow at 25~ but becomes important at higher temperatures. Since the PbO2 is continually breaking down, the surface oxidation reaction must be driven at a faster rate (i.e., more positive potential) to cover most of the surface with PbQ and produce a "peak" in the voltammogram. The implication of this with regard to benzene oxidation is that at high temperature, the electrode surface is not completely covered by lead dioxide. There- 1000 I~A 200~ fore there is less PbO2 available for the production of ben- I 30 bar j j i t f i i i i I zoquinone, while the other reactions involving oxygen 4. . . 0.0 species adsorbed on the electrode surface continue. Upon cooling, the electrode regains most of its selectivity toward benzoquinone, as shown in Table I. It is not known why the activity of the electrode is not completely restored at lower temperature, but the formation of a film over part of the electrode is considered likely. The generation of maleic acid from benzene is enhanced by increasing the temperature above 25~ The maximum yield of maleic acid occurs around 100~ A further increase in temperature lowers the yield until a minimum is reached asymptotically at 200~ The formation of maleic acid involves breaking the aromatic ring which stabilizes benzene. A considerable amount of activation energy must be supplied for oxygen on the electrode to break the ring. This accounts for the region of increasing maleic acid for- Fig. 6. Cyclic voltammograms of a lead electrode in 0.2M NaHSO4. mation with increasing temperature. At higher tempera- Electrode area = 1.8 • 10 -2 cm 2. Scan rate = 1 V/s.

Downloaded 02 Feb 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp Vol. 135, No. 8 OXIDATION OF BENZENE 1943

rapid desorption, and the net production of maleic acid Table II. Condttions necessary for the formation of biphenyl from drops. benzene in 0.2M NaS04 solution. (+) Biphenyl formed at these As stated above, the measurement of carbon dioxide was conditions. (-) Biphenyl not formed. (Blank) No data the most difficult of the analyses to perform. The analysis was necessary in order to establish a current efficiency for Temperature the consumption of benzene. As can be seen in Table I, this Experimental conditions 25~ 140~ 200~ 300~ current efficiency was always above 42%, indicating that benzene oxidation was always one of the major electrode Electrolysis of a two-phase - + + reactions in the experiments. Table I also shows individual mixture of benzene and 0.2M current efficiencies for the production of benzoquinone NaSO4. and maleic acid and reemphasizes the effect of tempera- Electrolysis of 0.2M NaSO4 - - + ture on their formation. saturated with benzene. Though there is significant uncertainty in some of the Two-phase mixture of benzene - + and 0.2M NaSO4 saturated with carbon dioxide measurements, a general trend can be seen air at 30 bar. Electrodes removed in the data of Fig. 5 and Table I. Carbon dioxide produc- from cell. tion rises with increasing temperature. This fits the hy- Single-phase solution of - - pothesis presented for the behavior of maleic acid with 0.2M NaSO4 saturated with temperature, and the knowledge that high temperature benzene and air at 1 bar. should favor complete oxidation of benzene. No current. Feed saturated at high temperature.--Successful operation of the apparatus with the saturator at high tempera- zene was noted. The peak was identified as biphenyl using ture proved to be very difficult. The solution in the satura- GC/mass spectrometry~ An analytical procedure for deter- tor becomes much more compressible when heated. This mination of biphenyl in aqueous or organic solutions made it much harder to adjust the outlet valve to maintain using HPLC was then established. the system at a stable flow rate, which was necessary for A series of experiments was performed to determine the electrochemical processes to reach steady state. Still what conditions were necessary for the formation of bi- sufficient data were obtained to conclude that with the phenyl and a possible mechanism for the reaction. A sum- electrode at high temperature, increasing the benzene con- mary of the results is given in Table II. It is apparent from centration of the feed did not increasse the production of the data that the reaction does not proceed below 200~ benzoquinone. Figure 7 compares the results of runs using The other important point is that the reaction is not elec- feed saturated with benzene at 25~ with runs that used trochemical, but involves oxygen in solution. Based on feed saturated at higher temperatures. In both cases, with this, the following reaction is proposed the cell at 140 ~ and at 200~ the amount of benzoquinone produced drops slightly at the higher benzene concentra- 2C6H6 + 1/2 02 ---> C12H10 + H20 tion, though at 200~ the change is of the order of the ac- The reaction may take place in solution, or may be cata- curacy of the measurement. The reason for this drop is not lyzed by the alumina wails of the cell. No products of known, but it is clear that increasing the benzene concen- higher molecular weight, such as terphenyl, were found. tration does not increase benzoquinone yield at high tem- Since this project was primarily concerned with electro- perature. chemical reactions, there was no further investigation of Apparently, at 140~ and above, the electrode character this reaction. has changed so that the reaction products are not dependent on benzene concentration. The cyclic voltammetry Conclusions shows that PbO2 is unstable at these temperatures, so the As temperature is increased, PbO2 decreases its selectiv- predominant reaction mechanism would be direct attack ity toward partial oxidation of benzene. The first stage oxi- of benzene by oxygen radicals on the electrode surface. It dation product, benzoquinone, is most efficiently pro- was hoped that high benzene concentrations would satu- duced at 25~ Current efficiencies from 6 to 10% were rate the electrode surface and limit the degree of oxidation found at 25~ under the conditions described in this so that more benzoquinone would be produced, but this is study. Higher efficiencies have been reported previously not the case. (1), but the electrochemical cells and experimental condi- Biphenyl.--A series of preliminary experiments were tions used were not suitable for high temperature work. performed with a two-phase flow of benzene and water. Production of maleic acid, the second major product The benzene layer was analyzed by gas chromatography from the oxidation of benzene, is enhanced by increasing (GC), and a peak with higher molecular weight than ben- the cell temperature above ambient. In the range 80 ~ 100~ current efficiency for the production of maleic acid aver- aged 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 reduc- tion 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 Fig. 7. Effect of benzene concentration and temperature on product higher molecular weight products such as terphenyl were d~stribution from oxidation on lead dioxide. Feed: 0.2M NariS04. Flow identified, but since these are practically insoluble in

rate = 1.3 ml/min. Current = 10 mA. aqueous solution, it is unknown whether any are formed.

Downloaded 02 Feb 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp 1944 J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY August 1988

Though operation at high temperature was found to be Data, 9, 1291 (1980). detrimental to the yield of benzoquinone from benzene ox- 5. M. Modell, U.S. Pat. 4,338,119. idation, the electrochemical equipment described here can 6. S. H. Townsend and M. T. Klein, Fuel, 64, 635 (1985). be used to study a wide range of high temperature aqueous 7. M. J. Antal, C. P. DeAmeida, W.S. Mok, S.V. Ra- processes, both electrochemically induced or monitored. mayya, and J. C. Roy, Paper presented at the 109th National Meeting of the American Chemical Society, The potential of chemical syntheses involving high con- Chicago, September 1985. centrations of nonpolar organics in water has received lit- 8. W. M. Flarsheim, Y. Tsou, I. Trachtenberg, K. P.John- tle attention and holds much promise for future discov- ston, and A. J. Bard, J. Phys. Chem., 90, 3857 (1986). eries. 9. A. Renard, Compt. Rend., 91, 175 (1880). 10. F. Fichter andR. Stocker, Ber., 47, 2003 (1914). Acknowledgments 11. A. Seyewetz and G. Miodon, Bull. Soc. Chim. Ft., 33, W.M.F. is grateful to the Shell Foundation for support. 449 (1923). We are indebted to Yu-Min Tsou for his assistance in this 12. K. S. Udupa, G. S. Subramanian, and H. V. K. Udupa, work and to Isaac Trachtenberg for his comments and ad- Bull. Acad. Pol. Sci., 9, 45 (1961). vice. This research was funded by the Office of Naval Re- 13. S. Ito, K. Sasaki, Y. Murakami, and H. Shiba, Denki search and The University of Texas Separations Research Kagaku, 4@, 733 (1972). Program. 14. B. Fleszar and J. Ploszynska, Electrochim. Acta, 30, 31 (1985). Manuscript submitted Aug. 10, 1987; revised manuscript 15. J. O' M. Bockris, H. Wroblow, E. Gileadi, and B.J. received Dec. 7, 1987. Piersma, Trans. Faraday Soc., 61, 2531 (1965). 16. C. Tsonopoulos and G. M. Wilson, AIChE J., 29, 990 REFERENCES (1983). 1. J. S. Clarke, R. E. Ehigamusoe, and A. T. Kuhn, J. Elec- 17. M. K. Shchennikova, G. G. Devyatykh, and I.A. troanal. Chem., 7@, 333 (1976). Korshunov, J. Appl. Chem. USSR, 30, 1149 (1957). 2. J. P. Connolly, J. Chem. Eng. Data, 11, 13 (1966). 18. D. Wabner and C. Grambow, J. Electroanal. Chem., 3. E. U. Franck, "Struct. Water Aqueous Solutions, Proc. 195, 95 (1985). Int. Symp.," 49-73 (1974). 19. M. I. Gillibrand and B. Halliwell, J. Inorg. Nucl. Chem., 4. M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. 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 complex 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 basic solutions (6-10) have led to the conclusion that the a cationic surfactant-styrene-aqueous sodium hydroxide first stable products are sulfur and PbO, Pb(OH)2, or emulsion (1) it was shown that the filming of the electrode HPbO2-, with secondary reactions that produced thio- allowed one to observe a series of reactions not observable sulfate and sulfate. in normal aqueous solutions. In addition it was shown that In this investigation, similar to the iron pyrite work, the surfactant solubilized the iron pyrite making it possi- studies were made in a surfactant induced suspension of ble to obtain a wave that was sensitive to the concentration the galena in order to determine the relation between the of iron pyrite (2). However, because of the formation of amount of sulfide mineral and the height of the voltam- multilayers of surfactant on the electrode, the height of the metric peak. Studies were also made in a sandwich cell to voltammetric peak was a complicated function of the con- delineate the reactions that are occurring. centration. Experimental This is a report on a similar study of the oxidation of galena (lead sulfide) made in order to determine whether Surfactant suspensions in a beaker cell.--Similar to pre- these techniques are of general use for the study of sulfide vious studies (11) the emulsion contained 35 ml ofa 2N so- minerals. In connection with studies of mineral flotation dium hydroxide solution, containing 3.5 ml of styrene processes, there have been a variety of studies of the elec- and 3.5 mls of Hyamine 2389 (predominantly methyldode- tro-oxidation of a number of sulfide minerals. These stud- cylbenzyltrimethylammonium chloride) obtained from ies have been reviewed and discussed in the papers by Rohm and Haas. Weighed samples of naturally occurring Hayes et al. (3) and by Richardson et al. (4, 5). Their sum- galena which had been ground in a mortar and sieved in a maries of the results for the sulfide minerals including gal- 279 (53 ~m) mesh screen were added to the emulsion after ena are that initial oxidation removes lead from the surface the residual voltammetric curves had become constant. producing a sulfur rich film which upon further oxidation The suspension was stirred with a magnetic stirrer and the eventually produces sulfur. Electrochemical studies in sample was allowed to stand for 25 min in the stirred suspension before obtaining the voltammetric curve. Some * Electrochemical Society Active Member. voltammetric studies were done with lead sulfide prepared

Downloaded 02 Feb 2009 to 146.6.143.190. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp