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

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 l imited 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 become 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 Supercrit ical water has a combination of low dielectric constant and high dipole moment that make it useful for a number of novel chemical applications. Water has a dipole moment 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 power in using supercritical water as a reaction medium 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 measurements 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 Iand 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 depending 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 1940 J. Electrochem. 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