Electrolysis Reaction Pathway for Lactic Acid in Subcritical Water

Dec 9, 2010 - Bioelectrics Research Center, Kumamoto UniVersity, 2-39-1 Kurokami, Kumamoto 860-8555, Japan. Electrolysis reactions of lactic acid were...
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Ind. Eng. Chem. Res. 2011, 50, 728–734

Electrolysis Reaction Pathway for Lactic Acid in Subcritical Water Asli Yuksel,† Mitsuru Sasaki,*,† and Motonobu Goto‡ Graduate School of Science and Technology, Kumamoto UniVersity, 2-39-1 Kurokami, Kumamoto 860-8555, Japan, and Bioelectrics Research Center, Kumamoto UniVersity, 2-39-1 Kurokami, Kumamoto 860-8555, Japan

Electrolysis reactions of lactic acid were studied using a 500 mL continuous flow reactor made of SUS 316 stainless steel. In this system, a titanium wall acted as a cathode and a titanium plate-layered type electrode was used as an anode in a subcritical reaction medium. The reactor wall (stainless steel) and the cathode (titanium) were separated from each other by a cylindrical ceramic wall. This hydrothermal electrolysis process provides an environmentally friendly route that does not use any organic solvents or catalysts to produce value-added chemicals from wastewater treatment. Reactions were conducted with a 30 min residence time at a pressure of 10 MPa at 280 °C via application of various direct currents ranging from 0.5 to 2 A. In addition, to improve our understanding of the reaction mechanism, we investigated the effects of initial lactic acid and electrolyte (NaOH) concentrations on the degradation of lactic acid and the product yields using continuous flow hydrothermal electrolysis. Acrylic acid, acetic acid, and acetaldehyde were detected as the main reaction products using high-performance liquid chromatography. Increasing the applied current increased the conversion of lactic acid and product yields. With a current of 2 A, an electrolysis time of 30 min, and the addition of 50 mM NaOH, a 55% conversion was achieved. The acetaldehyde yield increased almost linearly with current, and at 2 A, 24.73% of the acetaldehyde was produced compared to a 2.25% yield of acetic acid under the same conditions. For acrylic acid, at higher currents (1.5 and 2.0 A), the rate of generation of acrylic acid decreased (values of 0.82, 0.65, and 0.49% at 1.0, 1.5, and 2.0 A, respectively). Increasing the pH of the feed solution resulted in a drastic decrease in the yields of acrylic acid and acetaldehyde. Introduction Electrochemical methods for the oxidation of organic compounds and treatment of industrial wastewaters have recently been attracting more attention. Traditional methods pose some problems such as corrosion and emission of greenhouse gases if the operating conditions are not perfectly controlled. We propose a new hydrothermal electrolysis technique that offers promise for the treatment of organic wastes and their conversion into more valuable chemicals.1-6 The most important advantage of hydrothermal electrolysis is that it is environmentally friendly and no organic solvents are used as reaction media. All reactions are conducted in subcritical water without addition of any catalyst or additional oxidizer. Near the critical point (22 MPa and 374 °C), water becomes an attractive solvent with many unusual thermodynamic and transport properties. For instance, the dielectric constant is much lower and the ion product is ∼3 orders of magnitude larger than that of liquid ambient water.7,8 In addition, the current efficiency as a function of the reaction progress increases with increasing water temperature as it approaches the critical point, which enhances the economic feasibility of the electrochemical reactions.9 Because of the unusual properties of water below the critical point, we conducted hydrothermal electrolysis reactions of lactic acid in subcritical water in this study. The technique uses reactions that do not require any organic solvents or catalysts. Lactic acid is a product derived from biomass via fermentation or other chemical processes. A wide range of biomasses can be used as fermentation feed stock in the manufacture of lactic acid. Lactic acid also appears as a byproduct of many industrial * To whom correspondence should be addressed. Fax: +81-96-3423679. Telephone: +81-96-342-3666. E-mail: [email protected]. † Graduate School of Science and Technology, Kumamoto University. ‡ Bioelectrics Research Center, Kumamoto University.

carbohydrate-refining processes, such as the alkaline degradation of sugar.10 It can be an important intermediate for synthesizing other organic compounds11 and is a commercial fine chemical used mainly in the food industry. In addition, it is a main component in the production of many medical, pharmaceutical, and cosmetic products. Recently, much research has focused on the conversion of lactic acid to other useful chemicals. Mok et al.10 formed acrylic acid from lactic acid in supercritical water at 385 °C and 34.5 MPa with an initial lactic acid concentration of 0.1 M and a residence time of 30 s. After a series of experiments, they found that three major reaction pathways compete to convert lactic acid in supercritical water: heterolytic decarbonylation, hemolytic decarboxylation, and dehydration. The first pathway formed carbon monoxide, water, and acetaldehyde. In the second reaction, carbon dioxide, hydrogen, and acetaldehyde were produced, and in the final reaction, lactic acid was converted to acrylic acid and water. The yield of acrylic acid, which was the target product, reached its maximal value when the reaction was conducted in an alkaline medium produced by the addition of a small amount of base. Some work has focused on the catalytic conversion of lactic acid to acrylic acid, 1,2-propanediol, 2,3-pentanedione, etc. Odell et al.12 studied the conversion of lactic acid in the presence of group VIII metal complexes and found that acrylic, propanoic, pyruvic, and acetic acids were formed with a very low acrylic acid yield of 5%. In other studies, catalytic conversion of lactic acid over various sodium salt catalysts, support materials,13,14 and silica-supported copper15 has been conducted to determine the best conditions for the production of desired products, achieving >83% conversion of lactic acid. Some studies have investigated the oxidation and hydrolysis of lactic acid in near-critical water. Lira and McCrackin16 found that a temperature of 360 °C was optimal for the formation of acrylic acid with molar yields of 58%, based on conversion at

10.1021/ie101839r  2011 American Chemical Society Published on Web 12/09/2010

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Table 1. Conditions for HPLC Analysis HPLC column detectors eluent operating temperature detectors eluent

Figure 1. Continuous flow reactor for hydrothermal electrolysis: (1) pump, (2) preheater, (3) electric heaters, (4) reactor, (5) cooler, (6) back pressure regulator, (7) gas-liquid separator, (8) direct current supply, (9) electrodes, and (10) anode.

a pressure of 310 bar. Li et al.17 developed kinetics models with respect to concentrations of lactic acid and total organic carbon (TOC), resulting in a best-fit model for lactic acid oxidation with a 95% confidence limit. Results of oxidation of lactic acid at lower temperatures (50-90 °C) have shown that under acidic conditions the kinetics were second-order for lactic acid and first-order for Mn(VII).18 In previous work, we focused on the production of lactic acid from glycerol in both batch4 and continuous flow5 hydrothermal electrolysis by comparing our results with the conversion of glycerin and glycolaldehyde to lactic acid in the alkaline hydrothermal reaction studied by Kishida et al.19,20 After a series of experiments, we found that increasing the concentration of NaOH led to a great increase in the amount of lactic acid, with a yield of 34.7% after a 90 min reaction at 280 °C and an NaOH concentration of 50 mM. Therefore, in this study, we conducted hydrothermal electrolysis of lactic acid under alkaline conditions to improve our understanding of the continuous flow hydrothermal electrolysis reaction mechanism. Reactions were conducted in subcritical media without using any organic solvents or catalysts. An additional aim was to understand the effects of initial lactic acid and electrolyte (NaOH) concentrations on the lactic acid degradation and product yields. Experimental Procedure and Analysis Chemical Reagents. Wako Pure Chemicals Ind., Ltd. (Osaka, Japan), supplied the following chemicals for this study: sodium hydroxide (96%), methanol (99.7%), acetic acid (99%), formic acid (99%), lactic acid (85-92%), acrylic acid (98%), and acetaldehyde (90%). Equipment and Procedure. Aqueous electrolyte solutions were prepared by dissolving various amounts of NaOH in deionized water and adding a specified amount of lactic acid (0.05 and 0.1 M). A diagram of the sealed 500 mL continuous flow reactor, made of SUS 316 stainless steel, 180 mm in length with a 70 mm outer diameter, used for the electrolysis experiments is shown in Figure 1. The reactor was initially filled with distilled water to the sampling line (20 min). Next, the system pressure was set to 10 MPa via adjustment of the back pressure regulator. Four heaters, connected to the bottom and top of the reactor, were set to 280 °C. The length of preheater is around 3 m, and it is a quarter inch stainless steel tube in which the outlet temperature can be controlled. After the system reached the desired

Sugar SH1011 (Shodex) UV-vis (Jasco) and RI (Jasco) 3 mM HClO4 (0.5 mL/min) and BTB solution as a coloring reagent (1.0 mL/min) 60 °C UV-vis (Jasco) and RI (Jasco) 3 mM HClO4 (0.5 mL/min) and BTB solution as a coloring reagent (1.0 mL/min)

temperature, the aqueous electrolyte solution was fed into the system at a flow rate of 25 mL/min. Once the reactor was filled with the feed solution (∼30 min), the reaction was initiated and allowed to proceed for 30 min. During the reaction, the flow rate of the feed solution was set to 12.6 mL/min, determined by the temperature and residence time. For the hydrothermal electrolysis experiments, a constant direct current was passed through the electrodes; no current was applied for the hydrothermal degradation runs. In this system, a plate-layered type electrode (length of 124 mm, 12 layers, diameter of one layer of 40 mm) made of titanium (Akico) was used as an anode. A cylindrical wall (length of 140 mm, outer diameter of 54 mm), also made of titanium, acted as a cathode to protect the reactor from corrosion. The time needed for the solution to reach the bottom of the reactor from the pump was 25 s. Therefore, the residence time at the bottom of the reactor was assumed to be zero, and no conversion of lactic acid occurred. For this study, 0.05 and 0.1 M lactic acid and varying concentrations of NaOH (0, 15, 50, 75, and 100 mM) were used as feed materials. The reaction temperature and pressure were kept constant at 280 °C and 10 MPa, respectively. Various currents (0, 0.5, 1.0, 1.5, and 2.0 A) were passed through the electrodes during the 30 min electrolysis time. Product Analysis. Liquid products were quantified by highperformance liquid chromatography (HPLC) analysis. The HPLC analysis conditions are listed in Table 1. For the conversion calculations, the concentration of lactic acid consumed was divided by the initial lactic acid concentration. The selectivity of any product was defined as the yield of that product divided by the degree of conversion of lactic acid. Results and Discussion Major products from the hydrothermal electrolysis of lactic acid include acetaldehyde, acetic acid, and acrylic acid. The effects of applied current, electrolyte concentration (NaOH), reaction time, and initial lactic acid concentration were investigated, leading to the proposal of a lactic acid electrolysis reaction pathway for continuous flow hydrothermal electrolysis. Typical UV-vis spectra and RI chromatograms obtained from HPLC analysis of the liquid products are shown in panels a and b of Figure 2. As one can see, there were also some unidentified peaks. Effect of Electrolysis Current. To understand the effect of the applied current on the degradation of lactic acid and the product yields, we performed experiments in which we varied the applied current within the range of 0-2 A, holding the residence time at 30 min and the lactic acid concentration at 0.1 M, with 50 mM NaOH. The reaction temperature and pressure were 280 °C and 10 MPa, respectively. The results are shown in Figure 3. As illustrated in Figure 3, the degree of conversion of lactic acid increased with increasing direct current, except at 0.5 A. In the hydrothermal degradation run (0 A), the degree of

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Figure 2. (a) UV-vis spectra and (b) RI data of 0.1 M lactic acid after hydrothermal electrolysis for 30 min (280 °C, 2 A, and 50 mM NaOH).

conversion was 13.88%, which increased to 41.73% after a 2 A current had been passed through the titanium electrodes for 30 min. After the transition to hydrothermal electrolysis experiments, the degree of conversion decreased to 0.21% at 0.5 A. This might be because of the presence of some dead volume on the plate-layered type electrode. When current was applied for the first time after hydrothermal degradation, unstable conditions might have occurred. The acetaldehyde yield increased almost linearly with current, and at 2 A, 24.73% of the acetaldehyde was produced compared to a 2.25% yield of acetic acid under the same conditions. For acrylic acid, at higher currents (1.5 and 2.0 A), the rate of generation of acrylic acid decreased (0.82, 0.65, and 0.49% at 1.0, 1.5, and 2.0 A, respectively). This result indicates that acrylic acid and acetaldehyde were produced by lactic acid hydrothermal electrolysis through two different routes, and at higher currents, the acetaldehyde reaction was promoted. When electrolysis is conducted in the subcritical water reaction zone around the anode, vapor H2O molecules are ionized or activated and then bombard each other, transferring

charge. As a result, free radicals or ions and sometimes H atoms are generated.21 In addition, in the liquid phase reaction zone, several liquid H2O molecules are broken into H2 and O2. For reaction modeling, the mechanism is very complicated during hydrothermal electrolysis. Mainly, two mechanisms are effective: the ionic reaction and the free radical reaction. However, because the electrolysis reactions were conducted in subcritical water, the free radicals have a very short life. In addition, ionic reaction steps are preferred at higher pressures and/or lower temperatures, whereas free radical degradation dominates at lower pressures and/or higher temperatures.22,28,29 As a result, the product distribution is mainly determined by the ionic reaction mechanism in subcritical water. Effect of Electrolyte Concentration. To investigate the effect of alkali concentration on the conversion of lactic acid and product yields, various experiments in which 0, 50, 75, or 100 mM NaOH was added to a solution of 0.05 M lactic acid were conducted. The results are shown in Figure 4 for three different applied current conditions (0, 1.0, and 2.0 A). As shown in this figure, the degree of conversion of lactic acid increased with increasing current. Changing the NaOH concentration produced a similar effect. From our previous studies4,5 and other works,19,20,23 we know that alkali concentration greatly enhances the formation of lactic acid, especially from glycerol, and leads to a great increase in lactic acid yield. However, for lactic acid degradation, the situation is quite different; adding 100 mM NaOH and increasing the pH of the product solution to 13 resulted in difficulty in degrading the lactic acid, especially in the hydrothermal degradation run (Figure 4a) and when the applied current was 1.0 A (Figure 4b). The acetaldehyde yield was highest (32%) at an applied current of 2 A with no NaOH added. We then increased the pH by adding 50, 75, and 100 mM NaOH. With each pH increase, the acetaldehyde yield decreased, first to 13%, then to 9%, and finally to 7%. However, this decrease in the amount of acetaldehyde resulted in an increase in the yield of acetic acid. In an acidic medium, the yield of acetic acid was 6% at 2 A. When the reaction medium was no longer acidic, the yield started to increase until it reached 12%. This indicated that acetic acid was formed from the oxidation of acetaldehyde. This can also be understood from the observation that under highly alkali conditions, more lactic acid remained in the product solution24,25 but the level of acetic acid production did not decrease. In the case of acrylic acid, which was produced by dehydration of

Figure 3. Effect of current on the conversion of 0.1 M lactic acid and product yields with 50 mM NaOH at 280 °C and 10 MPa.

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Figure 4. Effect of electrolyte concentration on the conversion of 0.05 M lactic acid and product yields at 280 °C and 10 MPa: (a) 0, (b) 1.0, and (c) 2.0 A.

lactic acid, hydrothermal degradation in an acidic medium produced higher yields because after the addition of alkali, even at a concentration of only 50 mM, almost no acrylic acid was produced. Although in high-temperature and high-pressure electrolysis systems alkali addition causes some corrosion problems, our system did not encounter problems upon addition of up to 100 mM NaOH at 280 °C and a pressure of 10 MPa. Effect of Residence Time. The change in the conversion of 0.1 M lactic acid and the selectivities of products at different electrolysis times for a current of 1 A and 50 mM NaOH are shown in Figure 5. As expected, the degree of conversion of lactic acid increased with increasing electrolysis time and reached 37% at the end of 60 min. From 15 to 30 min, the selectivity of acetaldehyde continued to increase but then decreased from 63 to 34%. This might be explained by the

increase in acetic acid selectivity that was observed beginning at 30 min. Acrylic acid selectivity showed a profile similar to that of acetic acid, decreasing for the final 30 min. Effect of Initial Lactic Acid Concentration. Experimental results for initial lactic acid concentrations of 0.05 and 0.1 M are listed in Table 2. When the degrees of conversion were considered for both 0.05 and 0.1 M lactic acid, we recognized that at the same applied current, the degree of conversion decreased with increasing lactic acid concentration. It is accepted in published literature that hydrothermal electrolysis occurs via a free radical mechanism21,26,27 or ionic reaction22,28,29 as explained in Effect of Electrolysis Current. However, because all reactions were conducted in subcritical water, ionic reactions predominated. The results in Table 2 indicate that at high initial lactic acid concentrations, the amount of ions generated became insufficient for lactic acid conversion. This is because at a

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Figure 5. Effect of electrolysis time on the conversion of 0.1 M lactic acid and product selectivities (280 °C and 10 MPa with 50 mM NaOH and a 1 A current). Table 2. Experimental Results with Initial Lactic Acid Concentrations of 0.05 and 0.1 M 0.05 M lactic acid

0.1 M lactic acid

product yield (%) I (A) 0 0.5 1 1.5 2 0 0.5 1 1.5 2 0 0.5 1 1.5 2 0 0.5 1 1.5 2

[NaOH] (mM) 0

50

75

100

pH

conversion (%)

acrylic acid

acetic acid

acetaldehyde

3.2 3.2 3.2 3.2 3.5 5.2 6.9 8.5 7.8 8.3 11.7 9.9 9.6 9.3 8.6 12.9 12.8 12.6 11.9 10.8

5 17 30 45 54 7 19 31 46 55 8 20 32 47 55 5 17 30 45 54

1.1 1.1 1.4 1.5 1.6 0.2 0.3 0.3 0.3 0.3 0.2 0.3 0.3 0.3 0.2 0.1 0.2 0.2 0.2 0.3

0.3 1.9 3.7 4.2 5.9 0.2 1.8 4.6 7.2 11.1 0.1 2.2 5.7 9.0 11.9 0.3 2.5 5.5 8.7 11.8

3.0 8.9 20.0 28.8 31.8 3.6 7.8 11.3 14.5 13.4 3.9 5.1 6.6 8.7 9.4 3.9 5.3 4.3 7.5 7.0

constant current, ions would be generated at a constant rate. This case is true for all electrolysis runs conducted in both acidic and basic media, but for hydrothermal degradation (0 A), we can see that with 0.1 M lactic acid, the degree of conversion was higher than with 0.05 M lactic acid. In the case of product yields, the level of acrylic acid production was higher in low-pH solutions with 0.1 M lactic acid. However, in an acidic medium, the acrylic acid yield was almost the same for both initial concentrations of lactic acid. The most visible effect of lactic acid concentration was observed for the generation of acetic acid, which was produced from the oxidation of acetaldehyde. When the lactic acid concentration was increased to 0.1 M, the acetic acid yield decreased from 5.9 to 2.0% at a 2 A applied current and from 3.7 to 0.7% at a 1 A applied current, without addition of NaOH. A similar effect seemed to occur under alkaline conditions (50 mM NaOH) where the yield of acetic acid decreased from 11.1 to 2.3% when a 2 A current was applied. This explains the increase in the yield of acetaldehyde for the same conditions. As a result of an

product yield (%) [NaOH] (mM) 0

10

50

pH

conversion (%)

acrylic acid

acetic acid

acetaldehyde

2.9 2.8 2.8 2.8 2.8 3 2.9 3 3.1 3.2 3.6 3.7 3.9 4.1 4.4

18 12 21 32 43 12 10 21 32 43 14 9 21 34 42

1.0 1.4 1.3 1.2 1.4 0.8 1.5 1.3 1.3 1.2 0.6 0.5 0.8 0.7 0.5

0.2 0.4 0.7 1.4 2.0 0.2 0.5 1.4 2.4 3.3 0.1 0.4 0.7 1.6 2.3

1.2 5.4 13.2 21.6 25.7 1.8 7.9 10.1 14.0 17.0 3.8 7.2 14.6 18.7 24.7

increase in the initial lactic acid concentration, the decarboxylation reaction was favored while oxidation was obstructed. Hydrothermal electrolysis is advantageous because it does not require the use of catalysts or additional oxidizers. There have been several reports on the degradation of lactic acid in hot compressed water. Gunter et al.13 found that both the degree of conversion of lactic acid and selectivity of desirable products (2,3-pentanedione and acrylic acid) increased with increasing basicity of the phosphates. Over Na3PO4 on silica-alumina, a combined selectivity of acrylic acid and 2,3-pentanedione of 36% was observed at 350 °C. In a separate study, they achieved acrylic acid and acetaldehyde yields of only 15.2 and 10.2%, respectively, with Na2SiO3 catalyst at 300 °C and 0.5 MPa.14 Similarly, Lira and McCrackin16 used a Na2HPO4 catalyst and achieved 58% selectivity for acrylic acid from lactic acid in near critical water at 310 bar and 360 °C. The phosphate salt catalyst was observed to have little positive effect on the rate of acrylic acid formation. In another study, an oxidation experiment with lactic acid was performed at 300 °C with an

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Electrolysis Current, for reaction modeling, because electrolysis reactions were conducted in subcritical water, the product distribution was mainly determined by the ionic reaction mechanism. Conclusions

Figure 6. Proposed electrolysis reaction pathways for lactic acid by continuous flow hydrothermal electrolysis.

oxygen supply of 70%.24 Reaction pressures with and without H2O2 were 17 and 9 MPa, respectively. In that study, the acetic acid yield was high, reaching ∼40% on the carbon base. In contrast, by applying 2 A over 30 min at 280 °C and 10 MPa, we successfully converted 53.5% of the lactic acid without addition of NaOH and produced acetaldehyde, acrylic acid, and acetic acid in yields of 32, 2, and 6%, respectively. When the reaction was conducted under alkaline conditions (50 mM NaOH), the degree of conversion of lactic acid decreased to 41.7% with acetaldehyde, acrylic acid, and acetic acid yields of 24.7, 0.5, and 2.3%, respectively. In our study, all reactions were conducted in subcritical water medium and no additional oxidizer or catalyst was added, leading to a cost-effective and environmentally friendly process. Electrolysis Reaction Pathway. Possible electrolysis reaction pathways for the degradation of lactic acid by continuous flow hydrothermal electrolysis are shown in Figure 6. The first part (until the formation of lactic acid) was proposed in one of our previous studies5 from hydrothermal electrolysis of glycerol under alkaline conditions by using same experimental apparatus. As mentioned earlier, glyceraldehyde is an important intermediate in the generation of lactic acid by hydrothermal electrolysis.4,5 This also can be demonstrated by the decrease in the amount of glyceraldehyde with an increase in lactic acid yield as we investigated from hydrothermal electrolysis of glycerol.5 Via application of high currents during a long electrolysis time (>90 min), lactic acid production via glyceraldehyde oxidation becomes more favorable than that via the glycolaldehyde oxidation route. In excess NaOH, the generation of lactic acid as an oxidation product from glycerol was enhanced. To further understand the complete decomposition mechanism, we conducted a series of hydrothermal electrolysis experiments with lactic acid by using a continuous flow reactor that led us to propose a combined reaction pathway as illustrated in Figure 6. With hydrothermal electrolysis in a subcritical reaction medium, decomposition of lactic acid may follow two major pathways: dehydration and decarboxylation. When a high current was passed through electrodes under alkaline conditions, the route leading to the formation of acetaldehyde predominated. This route might begin with the elimination of carboxylic OH followed by a subsequent loss of the carbonyl group as CO to form acetaldehyde.10 It is then oxidized further to acetic acid under the same conditions. On the other hand, acrylic acid production was favored with hydrothermal degradation (0 A) in an acidic medium. The formation of acrylic acid was followed by a lactic acid dehydration reaction. As explained in Effect of

Hydrothermal electrolysis of lactic acid using a continuous flow reactor formed acrylic acid, acetaldehyde, and acetic acid as its main products. The applied current greatly affected the conversion of lactic acid. The maximal degree of lactic acid conversion (55%) was recorded when a 2 A current had been passed through electrodes for 30 min and 50 mM NaOH had been added. However, further increases in the pH of the solution, up to 13, resulted in difficulty in degrading the lactic acid, especially in the hydrothermal degradation runs. Increased currents and alkali concentrations caused a decrease in the yield of acrylic acid. On the other hand, acetaldehyde production was promoted at high current values and high concentrations of NaOH. It was then further converted into acetic acid by an oxidation reaction. Without a change in the applied current, increasing the initial concentration of lactic acid decreased the degree of conversion. This might be explained by insufficient formation of ions for higher concentrations of lactic acid. In a subcritical reaction medium after hydrothermal electrolysis, decomposition of lactic acid may follow two major pathways, dehydration and decarboxylation. When a high current was applied under alkaline conditions, the acetaldehyde formation pathway was predominant. In contrast, no applied current and an acidic medium favored the production of acrylic acid, which was formed by dehydration of lactic acid. Acknowledgment We thank the Kumamoto University Global Center of Excellence (COE) Program “Global Initiative Centre for Pulsed Power Engineering” for their financial support of this work and Hiromichi Koga for his help with experiments. Literature Cited (1) Sasaki, M.; Yamamoto, K.; Goto, M. Reaction Mechanism and Pathway for the Hydrothermal Electolysis of Organic Compounds. J. Mater. Cycles Waste Manag. 2007, 9, 40. (2) Yuksel, A.; Wahyudiono; Sasaki, M.; Goto, M. Hydrothermal Electrolysis of Organic Contaminants. Proceedings of the 07 AlChE Annual Meeting, 2007, Salt Lake City, UT. (3) Goto, M.; Koga, H.; Yuksel, A.; Sasaki, M.; Kuwahara, Y. Electrochemical Reactions of Alcohols in Sub-critical Water. Proceedings of the 08 AlChE Annual Meeting, 2008, Philadelphia. (4) Yuksel, A.; Koga, H.; Sasaki, M.; Goto, M. Electrolysis of Glycerol in Subcritical Water. J. Renewable Sustainable Energy 2009, 1, 033112. (5) Yuksel, A.; Koga, H.; Sasaki, M.; Goto, M. Hydrothermal Electrolysis of Glycerol Using a Continuous Flow Reactor. Ind. Eng. Chem. Res. 2010, 49, 1520. (6) Sasaki, M.; Wahyudiono; Yuksel, A.; Goto, M. Applications of Hydrothermal Electrolysis for Conversion of 1-Butanol in Wastewater Treatment. Fuel Process. Technol. 2010, 91, 1125. (7) Miller, D. J.; Hawthorne, S. B. Method for Determining the Solubility of Hydrophobic Organics in Sub-critical Water. Anal. Chem. 1998, 70, 1618. (8) Clifford, T. Fundamentals of Supercritical Fluids; Oxford University Press: New York, 1998; Vol. 23. (9) Asghari, F. S.; Yoshida, H. Electrodecomposition in Subcritical Water Using O-xylene as a Model for Benzene, Toluene, Ethylbenzene and Xylene Pollutants. J. Phys. Chem. A 2008, 112, 7402. (10) Mok, W. S.; Antal, M. J. Formation of Acrylic Acid from Lactic Acid in Supercritical Water. J. Org. Chem. 1989, 54, 4596. (11) Datta, R.; Tsai, S. P. Lactic-acid Production and Potential Uses: A Technology and Economics Assessment. ACS Symp. Ser. 1997, 666, 224.

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ReceiVed for reView September 2, 2010 ReVised manuscript receiVed November 7, 2010 Accepted November 19, 2010 IE101839R