Oxidation Reaction of High Molecular Weight Carboxylic Acids in

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Environ. Sci. Technol. 2003, 37, 3220-3231

Oxidation Reaction of High Molecular Weight Carboxylic Acids in Supercritical Water F A N G M I N G J I N , * ,† TAKEHIKO MORIYA,‡ AND HEIJI ENOMOTO† Department of Geoscience and Technology, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan, and Research and Development Center, Tohoku Electric Power Co., Inc., Sendai 981-0952, Japan

Stearic acid, being a model compound of high molecular weight carboxylic acids, was oxidized in a batch reactor by changing the oxygen supply with an insufficient oxygen supply at a constant reaction time at 420 °C. On the basis of the intermediate products identified by GC/MS, NMR, and HPLC analyses and the free-radical reaction mechanism, the oxidation pathways of high molecular weight carboxylic acids in supercritical water are discussed. The reaction of carboxylic acids in supercritical water proceeds with the consecutive oxidation of higher molecular weight carboxylic acids to lower molecular weight carboxylic acids through several major pathways. The attack of the hydroxyl radical occurs not only at the carbons in R-, β-, γ-positions to a -COOH group but also at the carbons ((ω-1)-carbon and/or ω-carbon) far in the alkyl chain from a -COOH group, which may lead to the formation of dicarboxylic acids.

Introduction As there is a growing interest in applying supercritical water oxidation (SCWO) to the treatment of organic wastes, studying its reaction pathways, kinetics, and mechanisms has become increasingly important. Recently, many attempts have been made to study reaction mechanisms for SCWO of simple substances such as carbon monoxide, methanol, ethanol, methane, ammonia (1-5), p-chlorophenol (6), 2-dichlorophenol (7), phenol (8), pyridine (9), 5-nitro-o-toluenesufonic acid (10), acetic acid (11-12), and glucose (13). However, little has been reported about the oxidation mechanisms of complex substances such as garbage. Our research group began studying oxidation of garbage in supercritical water using a batch reactor system, to develop an effective and acceptable garbage treatment process and to take advantage of the generated heat as well. The results of a previous study (14) showed that fatty meats, particularly beef suet, were most difficult to decompose in all the selected garbage. Thus, the knowledge of the SCWO reaction pathways and mechanisms is of critical importance for the design and development of a SCWO garbage process. In the present study, high molecular weight carboxylic acids were selected as test materials for SCWO of beef suet. As for reaction mechanisms of wet oxidation (WO) for carboxylic acids, including hydrothermal reaction, Day et al. * Corresponding author phone: +81-22-217-7385; fax: +81-22217-7385; e-mail: [email protected]. † Tohoku University. ‡ Tohoku Electric Power Co., Inc. 3220

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(15) and Williams et al. (16) studied WO pathways for propionic acid and butyric acid, on the basis of intermediate products identified by GC analysis. Recently, Brill et al. (1719) studied kinetics and mechanisms of decarboxylation of formic acid, malonic acid, and acetic acid derivatives at hydrothermal conditions by IR spectroscopy. However, little has been reported about SCWO mechanisms of high molecular carboxylic acids. For the oxidation mechanism, WO of organic compounds is generally considered to proceed via free-radical mechanisms (20, 21). The general scheme for the free-radical mechanism of organic compounds is currently believed to follow a branching free-radical mechanism:

RH + O2 f R• + HO2•

(1)

RH + HO2• f R• + H2O2

(2)

H2O2 f 2HO•

(3)

RH + HO• f R• + H2O

(4)

R• + O2 f ROO•

(5)

ROO• + RH f ROOH + R•

(6)

ROOH f alcohols V ROOH f ketones f acids

(7)

It is now being recognized that SCWO also most likely follows a free-radical reaction mechanism because the density of water is sufficiently low at a common supercritical state, making an ionic mechanism unlikely (22). Several reports have proposed that SCWO of organic materials may proceed via a free-radical reaction mechanism. For example, Tester and colleagues (1-3) interpreted the SCWO mechanisms of methanol, methane, carbon monoxide, and ethanol based on a combustion theory and proposed kinetic models of these compounds. Yang and Eckert (6) also explained SCWO mechanisms of p-chlorophenol using free-radical mechanisms. In their model, oxygen was not involved in the initial steps but was considered to participate only in the propagation steps to produce paraphenol peroxy radicals. Similarly, Lee et al. (11, 23) proposed a number of reaction pathways for the oxidation of acetamide and acetic acid by hydrogen peroxide. The hydroxyl radical (HO•) was considered to be a dominant oxidizing species. Hydrogen abstraction was postulated to be involved in the initial steps. Furthermore, Li et al. (24) reported that a free-radical reaction mechanism appeared to account for the destructive oxidation of organic compounds in both subcritical and supercritical water. Iyer et al. (25) recently modeled the experimental data for pure component alcohols and their mixtures using a branching free-radical reaction mechanism. In this paper, on the basis of intermediate products identified by GC/MS, NMR, and HPLC analyses, the oxidation pathways of high molecular weight carboxylic acids in supercritical water are discussed in terms of free-radical mechanisms. In general, for the oxidation of carboxylic acids, an attack on the carbon near a -COOH group by hydroxyl radicals easily occurs (15, 16, 20, 21). In this paper, both the mechanisms for the oxidation at carbons near the -COOH 10.1021/es026418+ CCC: $25.00

 2003 American Chemical Society Published on Web 06/12/2003

TABLE 1. Experimental Conditions for SCWO sample no.

oxygen supply, %

time,a s

temp,b °C

water fill,c %

1 2 3 4

7 50 70 100

30 30 30 30

420 420 420 420

30 30 30 30

a Residential time that the reactor was kept in the salt bath. Temperature of the salt bath. c Water fill was defined as the ratio of the volume of H2O2 aqueous solution put into the reactor and the inner volume of the reactor. At a water fill of 30% the pressure was approximately 40 MPa at 420 °C reaction temperature. b

group and carbons away from the -COOH group, which may lead to the formation of dicarboxylic acids, are proposed.

Experimental Section SCWO Experiments. Stearic and oleic acids (analytical reagent grade), which are the main carboxylic acids corresponding to beef suet, were chosen as test materials. H2O2 diluted with water was used as an oxidant. The stoichiometric demand of oxygen for complete oxidation of stearic acid and oleic acid to CO2 and H2O (molar ratio 26:1 for C18H36O2) was defined as the 100% oxygen supply. SCWO experiments were performed using a batch reactor system which was convenient for solid test materials. The experimental setup and method for oxidation employed in this study have been described in detail elsewhere (26), and only a brief description is given below. The desired amount of stearic acid or oleic acid as a test material and a H2O2 aqueous solution of an appropriate concentration were added to the reactor (5.9 mL inner volume), which was then sealed. The reactor was placed in a salt bath, which had been preheated to 420 °C, for 30 s. The typical heat-up period required to raise the temperature of reaction medium from 20 to 385 °C was about 15 s (11 s to 350 °C and 20 s to 410 °C), so the time used for the reaction in supercritical water would be around 15 s. The reactor was agitated during reaction. After the desired reaction time, the reactor was taken out of the salt bath and put into a cold water bath to quench the reaction. Liquid samples were taken out of the reactor by washing with an extraction solution (hexane, dichloromethane or acetone) for analyses. To avoid concerns of the residue contaminating successive experiments, the reactor was cleaned first with THF, then with detergent by hand, and finally in an ultrasonic cleaning tank for 20 min. All experiments were performed with undegassed water and without purging the reactor. Experimental conditions for SCWO are summarized in Table 1. All experiments were carried out at a constant salt bath temperature of 420 °C and during the 30 s that the reactor was kept in the salt bath. Oxygen supply was changed in the range for insufficient oxygen supply. This may help to detect as many intermediate oxidation products as possible. Analytical Methods. Gas chromatography/mass spectroscopy (GC/MS) and NMR spectroscopy were used to analyze all test samples. HPLC was also used in the case of identifying dicarboxylic acids. GC/MS Analysis. A liquid sample obtained after reaction was separated into a water sample and a sample extracted with hexane, dichloromethane, or acetone for analysis. A Hewlett-Packard model 5890 Series II Gas Chromatograph equipped with a model 5890B Mass Selective Detector was used. The conditions for GC/MS analysis are listed in Table 2. Identification of intermediate products was made with the total and selected ion chromatograms with the aid of a computer library and data books (27-29) as well as the GC retention times of products and authentic compounds.

NMR Analysis. A liquid sample obtained after SCWO was separated into a water sample and a CDCl3 sample. These samples were then put into 5 mm i.d. NMR tubes for 1H NMR. Chemical shifts for CDCl3 samples and water samples are relative to the TMS signal (δ ) 0.0 ppm) and DDS signal (δ ) 0.0 ppm) as references, respectively. 1H NMR spectra were recorded on a JEOL JNM-LA300 spectrometer operating at 300 MHz. 1H NMR spectra were obtained with the conditions as follows: spectral width, 6009.6 Hz; repetition time, 4 s; data points, 32768; pulse width, 6.05 ms (90°); pulse delay, 1.5474; and number of scans, 160 for the CDCl3 samples and 640 for the water samples. In general, identification of intermediate products with NMR is obtained by comparing the spectrum of a test sample with the spectrum of the authentic compound. However, there are many intermediate products in the test samples and it is difficult to obtain all of their authentic compounds. For assignment by 1H NMR analysis, first of all, a series of typical authentic compounds, which correspond to the compounds identified by GC/MS, were recorded to test if the resonances of pure compounds shift by mixing. Results showed that the resonance for each authentic compound in the mixture samples almost agreed with literature data shown in Table 4 (29-31) where CDCl3 was used as a solvent. When water was used as a solvent, however, many authentic compounds, particularly some low molecular weight compounds, gave resonances that disagreed with data from literatures where CDCl3 was the solvent. Therefore, for CDCl3 samples only, the assignment of intermediate products was made by comparing chemical shift data available in the literature shown in Table 4, when the authentic compound was not available. For water samples, because almost all data from literature are obtained with CDCl3 as a solvent, assignment of intermediate products was made by comparing the spectrum of the test sample with that of the authentic compound. HPLC Analysis. To confirm the presence of dicarboxylic acids, some liquid samples were also analyzed with HPLC (Waters) equipped with a Ionpak KC-811 column (Shodex). Peak identification was accomplished by comparison of sample peak retention times with those of standard solutions of pure compounds.

Results and Discussion Identification of Intermediate Products. GC/MS Identification. Figure 1 shows the variation of GC/MS total ion chromatograms of extracted samples with hexane (called hexane samples hereafter) and water samples for stearic acid oxidation with an oxygen supply. First, it can be seen from Figure 1(A) that many peaks appear, and the peak of stearic acid becomes gradually smaller with increasing oxygen. The peaks labeled from C4 to C18 were identified easily as carboxylic acids with a carbon number of 4-18, where C represents carboxylic acids and Arabic subscript numbers show the carbon number of carboxylic acids. It is particularly notable that although C15 and C17 peaks among these carboxylic acids in the figure did not appear, further analyses performed by changing the extraction solvent to acetone showed that peaks of carboxylic acids of C15 and C17 appeared. In Figure 1(B), carboxylic acids of C2-C5 were also identified. Therefore, all saturated carboxylic acids under 18 carbon atoms, except formic acid, were found. Organic compounds greater than C18 were not found. Peaks a′-d′ in Figure 1(B) can be also identified as shown in Table 3. Additionally, in Figure 1(A), there is a group of peaks from 1#-7# for which the computer library provided no good matches with reference spectra. The mass spectra for these products have peaks at m/z ) 41, 55, and 69, which indicates the presence of unsaturated hydrocarbon groups because they are characteristic fragments (R-CHdCH-CH2)+ formed by β-cleavVOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Conditions for GC/MS Analysis extracted samples instrument column flow rate injection temp detection temp MS source temp interface temp method injection

water samples

GC: Hewlett-Packard 5890 Series II Plus MS: Hewlett-Packard 5890B Series II Plus HP-INNOWAX (30 m × 0.25 mm × 0.25 µm) HP-INNOWAX (30 m × 0.25 mm × 0.25 µm) HP-1 (30 m × 0.25 mm × 0.25 µm) HP-INNOWAX (30 m × 0.25 mm × 0.5 µm) 0.7 mL/min 230 °C 250 °C 200 °C 250 °C E (1),a E(2)b W (1),c W(2),d W(3)e splitless

a Begin at 40 °C, ramped at 10 °C/min to 250 °C, and then hold for 20 min. b Begin at 40 °C, ramped at 7 °C/min to 230 °C, and then hold for 20 min. c Begin at 40 °C, ramped at 7 °C/min to 230 °C, and then hold for 20 min. d Begin at 40 °C, ramped at 2 °C/min to 150 °C, and then hold for 20 min. e Begin at 40 °C, ramped at 10 °C/min to 100 °C, then ramped at 2 °C/min to 150 °C, then ramped at 3 °C/min to 230 °C, and then hold for 20 min.

TABLE 3. Summary of Intermediate Products Identification by GC/MS extracted samples

water samples

peak no.

identification

comments

peak no.

identification

comments

1 2 3 4 5 6 7 8 9 10 11 12 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ 10′ 11′

2-octanone octanal 2-nonanone nonanal 2-decanone decanal 2,5-hexanedione γ-heptanolactone γ-pentanolactone 2,6-heptadione γ-hexanolactone 7-oxo-octanoic acid 1-butanol 3-heptanone 2-heptanone cyclopentanone 2-pentanol 1-pentanol cyclohexanone 1-hexanol γ-octanolactone γ-nonanolactone γ-decanolactone

library match library match library match library match library match library match positive identificationa library match library match positive identification library match library match positive identification positive identification positive identification positive identification positive identification positive identification positive identification library match library match library match library match

a′ b′ c′ d′ a′′ b′′ c′′ a# b# c#

1-hydroxy-2-propanon γ-pentanolactone γ-butyrolactone 2-propanoic acid 2-hexanedione 3-butenonic acid 4-oxopentanoic acid formic acid 4-pentanoic acid 5-hexenoic acid

positive identification library match library match positive identification positive identification library match positive identification positive identification library match library match

a

Both retention time and mass spectrum of a product match those of an authentic compound.

age for unsaturated hydrocarbons (27-29). They also exhibit a strong peak at m/z ) 60, which indicates the presence of the carboxylic group in these compounds, because β-cleavage with the transfer of a γ-hydrogen gives a m/z ) 60 oxygencontaining fragment when a chain of three or more carbon atoms is attached to the carboxylic group (27-29). Therefore, these compounds are likely to be unsaturated carboxylic acids containing 5 carbon atoms and more. The discussion above shows that the intermediate products identified in Figure 1 are mainly carboxylic acids, and it can be also seen from Figure 1 that separation of some peaks is poor and some peaks are too small. Thus, further analyses were performed by decreasing the column temperature ramp for both extracted samples and water samples and changing the extraction solvent from hexane to dichloromethane. Results are shown in Figures 2 and 3, respectively. Many new peaks appeared in either dichloromethane samples (Figure 2) or water samples (Figure 3). Peaks labeled with 1-12 in Figure (A) and 1′-11′ in Figure (B) for dichloromethane samples (Figure 2) and labeled with a′′-c′′ in Figure (A) and a#-c# in Figure (B) for water samples (Figure 3) represent the compounds which could be identified among new peaks. The results of identification are summarized in 3222

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Table 3. From Table 3, not only many ketones, aldehydes, and alcohols but also γ-keto acid and γ-lactones were identified. These identifications were mainly obtained by matching both the mass spectrum and the GC retention time for each compound with those of the authentic compounds. When authentic standards were not commercially available, identification was performed only by matching the mass spectrum. In these cases, a good match between the mass spectra of products and reference spectra stored in the computer library was obtained. It should be noted that the peaks d′, b′′, b#, and c# were identified as unsaturated carboxylic acids of C3:1, C4:1, C5:1, and C6:1 with a double bond at the end of the straight-chain alkene, because these peaks showed characteristic mass fragments for unsaturated carboxylic acids and also matched well with the peaks in mass spectra and those of reference spectra stored in the computer library. As described later, 1H NMR spectroscopy study of intermediate products in CDCl3 samples also indicated the presence of this kind of unsaturated carboxylic acids. So, for the peaks 1#-7# shown in Figure 1(A), their possible compounds are unsaturated carboxylic acids of C5:1, C6:1, C7: 1, C8:1, C9:1, C10:1, and C11:1 having a double bond lying at the end of the straight-chain alkene.

TABLE 4. Chemical Shifts of Various Organic-Function Groups for 1H-NMR

Likewise, intermediate products in the oxidation reaction of oleic acid were identified by GC/MS analysis. Results show that intermediate products in oleic acid oxidation (not shown) were fundamentally in agreement with those in stearic acid oxidation. However, there are two differences, i.e., one is that the peaks of high molecular weight carboxylic acids containing 9 carbon atoms and above were not observed, and the other is that a few low molecular weight carboxylic acids remained when the oxygen supply was 100%, indicating that oleic acid is easier to oxidize than stearic acid. Only intermediate products for stearic acid are described in detail hereafter. NMR Identification. Figure 4 shows the variation of 1H NMR spectra of CDCl3 and water samples after oxidation of stearic acid with an oxygen supply. In Figure 4(A), a spectrum of standard stearic acid is also given for comparison. The spectrum of standard stearic acid is composed of four peaks, which are assigned according to the literature data shown

in Table 4. Peaks signals of methyl protons CH3-, middle chain methylene protons -CH2-, and R and β methylene protons -CH2-COOH and -CH2-CH2-COOH are readily found at 0.90, 1.26, 2.35, and 1.60 ppm, respectively. Assigned results are also shown for respective peaks in Figure 4(A). The signal at 7.27 ppm is caused by residual protons in the CDCl3 solvent. Assignment of samples after oxidation of stearic acid by comparison with the spectrum of standard stearic acid shows that higher molecular weight carboxylic acids still remain in the CDCl3 samples. In spectra of water samples (Figure 4(B)), two singlets at 2.07 and 8.25 ppm were easily assigned to the methyl protons of acetic acid and the proton of formic acid, respectively. Further, in the expanded spectra (see Figure 5) at 0.8-3.0 ppm for Figure 4(B) with 7% and 50% oxygen supplies, a, b1, and c of CaH3Cb1H2CcH2COOH and triplet a1 of Ca1H3CH2COOH were observed. Thus, low molecular weight carboxylic acids having 1-4 carbon atoms were VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Variations of GC/MS chromatograms of hexane samples (A) and water samples (B) with an oxygen supply for stearic acid oxidation (method E (1) for hexane samples and method W (1) for water samples, column: HP-INNOWAX (30 m × 0.25 mm × 0.25 µm)).

FIGURE 2. GC/MS chromatograms of dichloromethane samples (method E (2), column: HP-INNOWAX (30 m × 0.25 mm × 0.25 µm)). considered to be identified in water samples. Small signals of a and b1 suggest that the amount of carboxylic acids with more than 4 carbon atoms is small in water samples. In fact, carboxylic acids with more than 4 carbon atoms are almost insoluble in water. It is notable that for water samples, many low molecular weight compounds did not give resonances 3224

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in agreement with those in CDCl3 samples, so that their assignments were made by comparing the spectra of test samples with those of the authentic compounds as described before. Moreover, primary alcohols were considered to be identified, because the following characteristic signals of

FIGURE 3. GC/MS chromatograms of water samples after oxidation of stearic acid with a 7% oxygen supply ((A): method W (2), column: HP-INNOWAX (30 m × 0.25 mm × 0.25 µm); (B): method W (3), column: HP-INNOWAX (30 m × 0.25 mm × 0.50 µm)).

FIGURE 4. Variation of 1H NMR spectra of CDCl3 samples (A) and water samples (B) after oxidation of stearic acid with an oxygen supply. primary alcohols were observed in Figures 5 and 6 which are expanded spectra of Figure 4(B),(A) at 0.8-4.0 ppm and 2.07.5 ppm, respectively: (1) signal t at 3.65 ppm in both Figure 5 and Figure 6(A), which was caused by the protons attached to a carbon atom bearing the OH group of primary alcohols;

(2) signal s at 3.35 and 3.50 ppm in Figure 5 and Figure 6(A), which was the methyl protons of methanol; (3) signals at 3.2 ppm in Figure 6(A) and 2.9 ppm in Figure 6(B), which were caused by the OH proton of CH3(CH2)nCH2OH (n ) 0-1) and CH3(CH2)nCH2OH (n ) 2, 3), respectively. VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Expanded spectra of Figure 4(B) at 0.8-4.0 ppm for samples with a 7% oxygen and 50% oxygen supply.

FIGURE 6. Expanded 1H NMR spectra of Figure 4(A) at 2.0-3.0 ppm (B) and 3.0-7.5 ppm (A) for samples with a 7% oxygen supply and a 50% oxygen supply. Next, not only resonances caused by methyl protons next to the carbonyl group of ketones at 2.20 ppm in Figure 5 and at 2.17 ppm in Figure 6 but also signals (f, f ′, g, g′, h) of methyl and methylene protons of 2,5-hexanedine and 4-oxopentanoic acid were found in both Figures 5 and 6. Thus, ketones, including γ-keto acid were considered to be identified. It should be noted that a larger signal of g′ in the 7% oxygen sample in Figure 5 may be caused by the signal of the DSS methylene used as a reference. For γ-lactones, although the assignment of the middle methylene protons w within a five-membered ring was not completed, because protons w resonated near 2.4 ppm, in the same way as the methylene protons of ketones d, characteristic multiplets of γ-lactones u, corresponding to the protons of the methylene 3226

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group adjacent to the γ-lactone oxygen as well as another signal of a methylene proton attached to the carbonyl group v are observed clearly in Figure 6. So, γ-lactones were considered to be identified. Finally, in Figure 4(A), a downfield signal at 9.76 ppm was easily assigned to the aldehydic proton of aldehydes, -CHO. As mentioned in the section on GC/MS analyses, identification of unsaturated carboxylic acids, particularly the positions of the double bond, was not completely resolved by GC/MS analysis, so identification of unsaturated carboxylic acids was performed by NMR analysis. As for unsaturated compounds, resonances caused by olefinic protons usually appear in the downfield of 5.0-6.0 ppm. As shown in Figure 6(A), many peaks are observed at 5.0-6.0 ppm. First, a

multiplet n near 5.0 ppm may correspond to olefinic protons of unsaturated carboxylic acids having a double bond at the end of the alkyl group, CnH2dCH-(CH2)nCOOH, which resonate at 4.9-5.2 ppm depending on the alkyl chain length according to literature (see Table 4). Signals of one of the other olefinic protons CH2dCoH-(CH2)nCOOH, labeled o, which resonate in the region of 5.63-5.80 ppm depending on the alkyl chain length as described in the literature (see Table 4), are also observed between 5.7 and 5.8 ppm. Next, a multiplet l can be seen near the signal of residual protons of CDCl3. As mentioned above, olefinic protons give downfield peaks at 5.0-6.0 ppm. When a function group with electronegative atoms attaches to a double bond carbon, however, the olefinic protons will shift far downfield. So, a multiplet like l at 7.15 ppm may correspond to the proton of R, β-unsaturated carboxylic acid,

which resonates at 7.10-7.20 ppm as described in the literature (see Table 4). The multiplet was due to splitting by adjacent proton and methylene protons. According to the discussion above, the signal of another olefinic proton -CHd CmH-COOH of R, β-unsaturated carboxylic acid would appear at 5.9 ppm as a doublet. As expected, a signal m near 5.85 ppm is observed as the expected doublet. These data may indicate the presence of R, β-unsaturated carboxylic acids. Furthermore, signals, p, q, and r, at 6-6.5 ppm were assigned to the olefinic protons of CpqH2dCrH-COOH, which gave signals at 5.92, 6.50, and 6.15 ppm in the literature (see Table 4). This gives one more indication of the presence of R, β-unsaturated carboxylic acids. From the discussion above, the intermediate products identified by GC/MS and NMR analyses are carboxylic acids including unsaturated carboxylic acids, ketones, aldehydes, alcohols, and γ-lactones. Major Intermediate Products (Carboxylic Acids). GC/ MS chromatograms in Figures 1 and 2 show that the peaks of carboxylic acids are much higher than those of other compounds, and the peak of acetic acid is overwhelmingly high. This indicates that carboxylic acids are the major intermediate products, and acetic acid is a key refractory intermediate product, that is, oxidation reactions to form a lower molecular weight carboxylic acid occur consecutively with the production of acetic acid as a byproduct. The quantification of carboxylic acids was carried out by measuring the average chain length of mixed carboxylic acids with NMR, while acetic acid was quantified by GC/MS analysis. The average chain length of mixed carboxylic acids was obtained by comparing peak areas of the middle methylene protons (-CH2-) of carboxylic acids with those of the terminal methyl protons (-CH3) in 1H NMR spectra. The molar ratio of the middle methylene group and the terminal methyl group, R, (-CH2-/CH3-) can be calculated as follows:

R ) Ms/Mp

(8)

Ms ) [-CH2-] integrate/2

(9)

Mp ) [CH3] integrate/3

(10)

where Ms is the molar concentration of a middle methylene group, and Mp is the molar concentration of a terminal methyl group. Table 5 shows the results of the average chain length of mixed carboxylic acids after oxidation of stearic acid as well as the chain length of standard stearic acid. Ms and Mp were obtained by integrating the signals at 1.26 ppm (b) and 0.90 ppm (a) in Figure 4. The chain length calculated by this method for stearic acid is 14.5, which is almost in

TABLE 5. Average Chain Length of Mixed Carboxylic Acids after SCWO of Stearic Acid and Standard Stearic Acid SCWO conditions sample no.

oxygen supply, %

time, s

temp, °C

Mp/Ms

1 2 standard stearic acid

7 50

30 30

420 420

10 2.7 14.5

FIGURE 7. Variation of acetic acid concentration with oxygen supply. agreement with the result according to the structure of stearic acid (CaH3(CbH2)14Cb1H2CcH2COOH), that is 14. This may suggest that this method is successful. It can be seen in Table 5 that the average chain length of mixed carboxylic acids decreases greatly with an increase in the oxygen supply, the average chain length becomes as low as 2.7 when the oxygen supply increase to 50%. This results indicates that the oxidative decomposition reaction proceeds rapidly and the higher molecular weight carboxylic acids decompose into lower molecular weight carboxylic acids, as the oxidation reaction proceeds, as mentioned before. Additionally, it should be noted that when determining the average chain length of mixed carboxylic acids, the signals at 1.26 and 0.90 ppm also contain the middle methylene protons and terminal methyl protons of higher grade ketones, alcohols, and aldehydes. Considering that the amount of carboxylic acids was much higher than that of higher grade ketones, alcohols, and aldehydes, the error can be considered to be negligible. The peak area ratio of carboxylic acids to all ketones, alcohols, and aldehydes over 6 carbon atoms was about 13:1 in the case shown in Figure 2(A). The result of quantification of acetic acid is shown in Figure 7. Acetic acid concentration increased with an increase in the oxygen supply. Again, it indicates that acetic acid appears to be a stable intermediate product. Mechanism of Oxidation at Carbons near a -COOH Group. For free-radical mechanisms, the hydrogen abstraction by hydroxyl radicals HO• from organic compounds is a key step, and weaker C-H bonds of the oxidized organic compounds are considered to be easily attacked. For high molecular weight carboxylic acids, a probable attack of hydroxyl radicals is generally considered to occur easily at carbons near a -COOH group because these carbons have high acidity. Oxidation at r-Carbon. Because R-hydrogen of high molecular weight carboxylic acids is the most acidic, one of the most probable attacks by hydroxyl radicals is at the R-carbon. Day et al. (15) and Williams et al. (16), in their studies on WO of propionic and butyric acids, reported the VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Reaction pathways proposed for the oxidation at r-carbon of high molecular weight carboxylic acids (n is the number of C atoms in the initial high molecular weight carboxylic acids).

FIGURE 9. Reaction pathways proposed for the oxidation at β-carbon of high molecular weight carboxylic acids (n is the number of C atoms in the initial high molecular weight carboxylic acids). attack of hydroxyl radicals at the R-carbon. According to the WO mechanism for propionic and butyric acids, the oxidation of stearic acid may lead to the production of saturated monocarboxylic acids with all carbon numbers under 18. The existence of carboxylic acids having a carbon numbers of 17 may strongly suggest the oxidation at an R-carbon because R-oxidation should give a carboxylic acid with one less carbon atom. Therefore, for high molecular weight carboxylic acids, the mechanism by the attack of hydroxyl radicals at an R-carbon may be proposed as shown in Figure 8. First, a hydroxyl radical HO• abstracts an R-hydrogen from a high molecular weight carboxylic acid, CH3(CH2)n-2COOH, and yields a R• radical, CH3(CH2)n-3C(•)HCOOH (n is the number of carbon atoms in the initial carboxylic acid). Since the R• radical is extremely active, it adds a molecule of oxygen andisconvertedtoaperoxyradical,CH3(CH2)n-3C(OO•)HCOOH, which then abstracts a hydrogen from an organic compound in the reaction mixture and forms a hydroperoxide, CH3(CH2)n-3C(OOH)HCOOH. Subsequently, the decomposition of hydroperoxide proceeds through the pathway shown in Figure 8(B). The radical CH3(CH2)n-3C(O•)HCOOH formed by the decomposition of hydroperoxide abstracts a hydrogen atom from an organic compound in the reaction mixture, leading to the formation of an R-hydroxy acid. The R-hydroxy acid is unstable and is readily eliminated to form aldehyde and formic acid. The aldehyde is further oxidized to form a carboxylic acid with one less carbon atom, CH3(CH2)n-3COOH. 3228

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Therefore, the R-oxidation should give rise to a mixture of all carboxylic acids and aldehydes from C1 to Cn-1. Although aldehydes are oxidized easily, not all respective aldehydes are found, but various aldehydes are identified as well as saturated monocarboxylic acids of all carbon numbers under 18. Oxidation at β-Carbon. Although the acidity of β-hydrogen is lower than that of R-hydrogen for carboxylic acids, Williams et al. (16) suggested that a hydroxyl radical might react on either an R- or β-carbon to form a hydroxy carboxylic acid for WO of butyric acid. Therefore, for high molecular weight carboxylic acids, an attack on the β-position by a peroxy radical is the next most favored possibility. A possible mechanism may be represented as shown in Figure 9. The first stage of oxidation of high molecular weight carboxylic acids, shown in Figure 9(A), follows the mechanism for low molecular weight carboxylic acids suggested by Day et al. (15) and Williams et al. (16). That is, a hydroperoxide, CH3(CH2)n-4C(OOH)HCH2COOH, is first formed via β-hydrogen abstraction by a hydroxyl radical, followed by addition of an oxygen molecule and abstraction of a hydrogen from another compound. This hydroperoxide then undergoes decomposition, yielding a β-keto acid via β-hydroxy acid. The β-keto acid is decarboxylated extremely easily, and the loss of carbon dioxide from β-keto acid yields a β-ketone. The end product of this pathway for butyric acid is acetone, which is easily oxidized to carbon dioxide and water.

FIGURE 10. Reaction pathways proposed for the oxidation at γ-carbon of high molecular weight carboxylic acids (n is the number of C atoms in the initial high molecular weight carboxylic acids). For higher molecular weight carboxylic acids, further reactions, in the second stage of oxidation shown in Figure 9(B), would be initiated by the oxidation of β-ketone. For β-ketone, several studies on the mechanism of WO suggest that the oxidation by hydroxyl radicals occurred at both the R- and β-carbons of ketones (20, 21). As shown in Figure 9(B), the oxidation of each of the R-, β-, and γ-carbons of β-ketone would result in the formation of acetic acid and higher molecular weight carboxylic acids (two carbon atoms for R-oxidation and three carbon atoms for β-oxidation are eliminated) or β-keto acid (for γ-oxidation), via diketones. From the above discussions, it is understandable that the β-oxidation of carboxylic acids would result in the formation of β-keto acids, β-ketones, diketones, various kinds of carboxylic acids, and aldehydes. In other words, β-keto acids, β-ketones, diketones, and a large amount of acetic acid among many intermediate products would be characteristic of the β-oxidation. Since β-keto acids, diketones, and β-ketones were found as shown in Table 3 and a large amount of acetic acid was produced as described before, the oxidation at the β-carbon is considered to exist. Also, as shown in Figure 9(A), dehydration of the β-hydroxy acid formed by the β-hydrogen abstraction by a hydroxyl radical may take place in parallel with the decarboxylation of β-hydroxy acid and give an R, β-unsaturated carboxylic acid, since a β-hydroxy acid loses water easily to yield an R, β-unsaturated carboxylic acid, R-CHdCH-COOH. Therefore, characteristic intermediate products for β-oxidation of carboxylic acids would also include R, β-unsaturated carboxylic acids. As described before, R, β-unsaturated carboxylic acids were also identified as intermediate products. This may be further support of the presence of β-oxidation, although Day et al. (15) and Williams et al. (17) did not report the formation of an R, β-unsaturated carboxylic acid. Oxidation at γ-Carbon. To the best of our knowledge, there are no reports concerning γ-oxidation for carboxylic acids, because γ-oxidation for low molecular weight carboxylic acids does not necessarily play an important role. However, several studies (20, 21) on the WO of ketones showed that both R- and β-oxidations were far from being the only mechanism and the mechanism of the rupture of a γ-C-C bond paralleled them. The presence of γ-oxidation of ketones cannot directly imply the presence of γ-oxidation for carboxylic acids, because acidity of the γ-hydrogen of high molecular weight carboxylic acids is lower than that of ketones. The ease of hydrogen abstraction increases with an increase in acidity of hydrogen. However, some intermediate products, which were expected when the oxidation at the γ-carbon occurred, were identified. These are γ-keto acid and γ-lactones, and thus the oxidation at a γ-carbon is also considered to exist. A reaction scheme which appears most likely to explain the formation of γ-keto acid and γ-lactone is shown in Figure 10. First, the attack at a γ-carbon by a hydroxyl radical yields a γ-hydroxy acid, which may then

undergo two side reactions. A first side reaction is that a γ-hydroxy acid is further oxidized, forming a γ-keto acid, and a second side reaction yields a cyclic ester known as a γ-lactone, because a γ-hydroxy acid easily loses water spontaneously to yield a cyclic ester. According to the above discussion for SCWO of high molecular weight carboxylic acids, oxidation occurs not only at the R- and β-carbons but also at the γ-carbon to a -COOH group. Figure 11 summarizes the reaction networks for SCWO of high molecular weight carboxylic acids. Through the attack of a hydroxy radical at either an R-, β-, or γ-carbon, the reaction proceeds to reduce the carbon number of a higher molecular weight carboxylic acid to a lower molecular weight carboxylic acid via either hydroxy acids, keto acids, aldehydes or ketones, with the direct production of acetic acid as well as carbon dioxide. Finally, formic acid and acetic acid are further oxidized into carbon dioxide. Oxidation at a Carbon far from a -COOH Group. Although it is understood that oxidation of a carboxylic acid occurs at carbons near a COOH group, as discussed and summarized in Figure 11, oxidation at a carbon far from a -COOH group may occur. As described before, unsaturated carboxylic acids having a double bond at another end of the carbon chain were identified. Since the double bond lies at a position away from a -COOH group, it may not come from the oxidation at carbons near the -COOH group. This may be caused by the reaction of an alkyl group near the end away from a -COOH group. Two hypothetical mechanisms would lead to the formation of a double bond at the end of the carbon chain. One would be that an alkyl group loses two hydrogen atoms (primary and secondary hydrogen atoms to the methyl group) to form a double bond as reaction

However, this is unlikely since unsaturated carboxylic acids of this kind were not identified when there was little oxygen supply. Another mechanism would be that the oxidation at a carbon away from a -COOH group ((ω-1)-carbon or ω-carbon) takes place. If so, the oxidation reactivity of an alkyl group may be governed by the relative ease with which the different classes of hydrogen atoms could be abstracted. For straight chain alkanes, it is known that the ease of abstraction of hydrogen atoms follows the sequence of secondary hydrogen > primary hydrogen. Several studies concerning the WO of straight chain alkanes have also suggested that hydrogen is most easily removed from the β-carbon atom RCH3βCH2(CH2)nβCH2RCH3 (20, 21, 32). Therefore, a hydroxy acid having a hydroxy group at (ω-1)-carbon ((ω-1)-hydroxy acid) may be formed by abstraction of hydrogen at the (ω-1)-carbon. It then loses water to form a double bond, because the order of the reaction of secondary VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 11. Reaction network for high molecular weight carboxylic acid oxidation in supercritical water (n is the number of C atoms in the initial carboxylic acids; /, compounds identified in this study).

FIGURE 12. Reaction pathways proposed for the ω-oxidation and (ω-1)oxidation. alcohol toward dehydration is primary hydrogen > secondary hydrogen. An unsaturated carboxylic acid having a double bond at the end of the carbon chain may be formed by the (ω-1)oxidation. A possible mechanism is shown in Figure 12(A). This explanation agrees well with the identification of 7-oxooctanoic acid as well as unsaturated carboxylic acids having a double bond at the end of the carbon chain, as shown in Table 3. Therefore, (ω-1)-oxidation may be considered to occur. Moreover, although in the case of the WO of straight chain alkanes, the hydrogen is most easily removed from the β-carbon atom, it has been reported that R-oxidation also occurrs (20, 21). So, ω-oxidation may also take place, as shown in Figure 12(B). However, in this study, any intermediate product directly supporting the presence of ω-oxidation, for example, ω-hydroxy acid, was not detected. This is possibly due to the instability of the ω-hydroxy acid formed. Either (ω-1)-oxidation or ω-oxidation may lead to the formation of dicarboxylic acids. To test if dicarboxylic acids were formed, the samples obtained with oxygen supplies of 7% and 50% were analyzed by HPLC. Results showed that dicarboxylic acids with carbon numbers of 2-10 were identified (data not shown). The identification of these dicarboxylic acids may be confirmed by the identification of cyclopentanone and cyclohexanone by GC/MS (see Table 3) because dicarboxylic acids with carbon number of 6 and 7 easily lose carbon dioxide and water to form ketones with respective fivemembered or six-membered rings. The identification of the dicarboxylic acids including cyclopentanone and cyclohexanone may give one more indication of the presence of (ω-1)-oxidation and/or ω-oxidation. For SCWO of high 3230

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molecular weight carboxylic acids, the oxidization occurs not only at the carbons in the R-, β-, γ-positions to a -COOH group but also at carbons far in the alkyl chain from a COOH group.

Acknowledgments This work was funded in part by a grant-in-aid for Scientific Research (1996-1998) from the Ministry of Education, Science and Culture, Japan.

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Received for review December 13, 2002. Revised manuscript received April 9, 2003. Accepted April 29, 2003. ES026418+

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