Ind. Eng. Chem. Res. 1999, 38, 1775-1783
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Supercritical Water Oxidation Kinetics and Pathways for Ethylphenols, Hydroxyacetophenones, and Other Monosubstituted Phenols Christopher J. Martino and Phillip E. Savage* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136
We examined the decomposition of o-, m-, and p-ethylphenol and o-, m-, and p-hydroxyacetophenone in dilute aqueous solutions at 460 °C and 25.3 MPa, both in the presence and absence of added oxygen. In the absence of oxygen, the ethylphenols produced vinylphenols as the major product and the hydroxyacetophenones produced phenol, benzendiols, and hydroxybenzaldehydes. In the presence of oxygen, ethylphenols and hydroxyacetophenones reacted through two major parallel paths and one minor path. The major primary paths for ethylphenols were to vinylphenols and to ring-opening products and ultimately CO2. The minor path was to phenol. For hydroxyacetophenones, the major primary paths were to phenol and to ring-opening products and ultimately CO2. The minor path was to hydroxybenzaldehydes. The relative rates of these parallel paths were sensitive to the location of the substituent. Although reactions did occur in the absence of oxygen, the disappearance rates were much slower than those observed during oxidation. Power-law global rate expressions were developed for reactant disappearance during oxidation. These rate laws were used along with rate laws previously reported for other monosubstituted phenols to examine the relative oxidation rates for different phenols. All of the substituted phenols oxidized more quickly than phenol itself. The oxidation rates for the substituted phenols were functions of both the identity and location of the substituent. For a given substituent, the reactivity was always in the order ortho > para > meta for all of the substituted phenols examined. Introduction Supercritical water oxidation (SCWO) is a wastetreatment technology wherein organic compounds are oxidized to CO2 and H2O in an aqueous reaction medium at conditions that exceed the critical point of water (Tc ) 374 °C, Pc ) 22.1 MPa). The design, optimization, control, and evaluation of a SCWO process is facilitated by knowledge of the kinetics and potential byproducts formed from the oxidation of real pollutants. Knowing the reaction kinetics allows one to calculate the reactor residence times required for a desired destruction and removal efficiency. Knowing the identities and yields of byproducts and the reaction networks allows one to identify processing conditions that minimize the production of undesired products of incomplete oxidation. For these reasons, investigations into the SCWO reaction kinetics and reaction pathways for representative pollutants assist the continued development of SCWO process technology. Phenols are a class of organic compounds present in industrial wastewater streams. Their prevalence in wastewaters can be attributed to phenols possessing high water solubilities and being used in or byproducts of processes for the production of chemicals, steel, and forest, paper, and petroleum products. The ubiquity of phenols in wastewaters has led to SCWO of phenol itself being studied extensively.1-12 Catalytic oxidation of phenol in supercritical water has also been explored.13-16 Although phenol has been in the spotlight, the SCWO of substituted phenols has received much less scrutiny. * Corresponding author. E-mail:
[email protected]. Phone: (734) 764-3386. Fax: (734) 763-0459.
Prior to our initial investigations with substituted phenols, the literature provided only a few accounts and these were for the SCWO of chlorophenols.17,18 More recently, there have been accounts of SCWO of a dichlorophenol19 and a dihydroxybenzene.20 The importance of substituted phenols and the lack of information regarding their reactivity in SCWO motivated our study of the reaction kinetics, products, and pathways for monosubstituted phenols in supercritical water. We have previously reported on the reactions of -CH3-, -CHO-, -OH-, -OCH3-, and -NO2-substituted phenols.21-24 This article presents experimental results for the SCWO of phenols with -C2H5 and -COCH3 substituents. Neither of these two compounds (ethylphenols and hydroxyacetophenones) have been studied before under SCWO conditions. Moreover, in this article we compare the reaction kinetics and reaction networks for the seven different monosubstituted phenols we have investigated to date and use these comparisons to make some generalizations regarding structure and reactivity. Experimental Section All chemicals used in this investigation were obtained commercially and used as received. The water was distilled, deionized, and degassed prior to use. As noted in the Introduction section, the work presented herein is part of a broader study of the reactivity of seven different substituted phenols under SCWO conditions. Because 21 different individual compounds (o-, m-, and p-isomers of seven different phenols) were investigated, we performed experiments at only a single temperature of 460 °C and a single pressure of 25.3 MPa, which fall within the range of normal SCWO operating conditions.
10.1021/ie980575t CCC: $18.00 © 1999 American Chemical Society Published on Web 02/20/1999
1776 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
Figure 1. Molar yields of products from SCWO of o-ethylphenol at 460 °C and 25.3 MPa.
Figure 2. Molar yields of products from SCWO of m-ethylphenol at 460 °C and 25.3 MPa.
Figure 3. Molar yields of products from SCWO of p-ethylphenol at 460 °C and 25.3 MPa.
Experiments were performed both in the presence and absence of added oxygen so that the potential effects of nonoxidative decomposition could be evaluated. The oxidation experiments were designed such that the initial concentrations (at reaction conditions) of the reactant, [organic]0 and oxygen [O2]0, and the reactor residence time, τ , varied by an order of magnitude, by greater than a factor of 2, and by at least a factor of 5, respectively. All experiments were done in an isothermal, isobaric, tubular flow reactor that is described in detail elsewhere.21,25 Two aqueous streams, one saturated with oxygen and the other containing an organic compound, were preheated separately to the desired reaction temperature, mixed together, and then fed into the flow reactor. The reactor effluent was then cooled and depressurized so that separate vapor and liquid streams at ambient conditions emerged from the reactor system. The flow rates of both the gas and liquid streams were measured. The gas phase was analyzed by an on-line gas chromatograph (GC) with a thermal conductivity detector. The liquid-phase effluent was sampled and analyzed with a high-performance liquid chromatograph (HPLC) with a UV detector. These analyses allowed us to determine the reactant conversion, X, and the molar
yields, Yi, of CO, CO2, and any phenolic products formed from incomplete oxidation. We also used liquid-liquid extraction to concentrate the organic compounds in the reactor effluent and a GC with a mass selective detector (MS) to identify several of the aqueous-phase products. Results In this section, we report the results from reactions of ethylphenols and hydroxyacetophenones in supercritical water at 460 °C and 25.3 MPa. The identities and yields of major SCWO products are given in Figures 1-6. Additionally, tables containing the specific reaction conditions and more detailed results for o-, m-, and p-ethylphenol and for o-, m-, and p-hydroxyacetophenone are available in Martino’s thesis25 and in the Supporting Information that accompanies this article. We include one of these tables here as Table 1 to provide a representative example. One of the tables in the Supporting Information also includes the results of an uncertainty analysis for all of the calculated quantities. These uncertainties are the 95% confidence intervals. They were determined by using the propagation of errors’ formula along with the estimated uncertainties in the individual experimental measurements (e.g., flow
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1777
Figure 4. Molar yields of products from SCWO of o-hydroxyacetophenone at 460 °C and 25.3 MPa.
Figure 5. Molar yields of products from SCWO of m-hydroxyacetophenone at 460 °C and 25.3 MPa.
Figure 6. Molar yields of products from SCWO of p-hydroxyacetophenone at 460 °C and 25.3 MPa. Table 1. Results from Reactions of p-Hydroxyacetophenone in Supercritical Water at 460 °C and 25.3 MPa τ (s)
[p-COCH3]0 (µmol/L)
[O2]0 (mmol/L)
X (%)
YCO2 (%)
YCO (%)
Yphenol (%)
Yp-CHO (%)
Yp-CH3 (%)
Yo-OH (%)
Yp-OH (%)
C tally
1.37 1.38 3.09 3.14 5.84 5.90 0.55 0.67 0.67 0.68 0.68 0.78 1.55 1.80 1.81 1.82 1.82 2.97
787 79.9 79.7 787 787 79.7 119 38.3 384 31.0 206 121 114 31.7 210 377 37.6 119
0.00 0.00 0.00 0.00 0.00 0.00 5.12 7.08 7.08 3.02 3.02 5.08 5.37 2.95 2.98 7.25 7.25 5.17
3.8 ( 3.9 5.2 ( 2.9 3.8 ( 3.6 5.8 ( 9.1 4.7 ( 1.6 9.2 ( 3.9 25.3 ( 0.5 61.7 ( 1.4 27.7 ( 0.4 43.6 ( 18.9 21.3 ( 10.0 30.6 ( 2.0 47.3 ( 1.2 74.6 ( 2.4 33.3 ( 3.5 51.1 ( 2.4 87.5 ( 0.3 66.6 ( 1.1
0.0 0.0 0.0 0.0 0.0 0.0 9.6 35.0 14.4 19.2 6.4 10.9 24.8 37.0 13.7 33.4 64.9 36.0
0.0 0.0 0.0 0.0 0.0 0.0 0.6 2.0 1.0 1.3 0.5 0.8 2.4 3.2 1.4 3.0 5.2 3.9
0.1 0.0 0.0 0.2 0.3 0.7 4.2 10.4 6.9 8.4 3.2 5.1 10.4 13.6 7.0 12.2 13.2 13.5
0.1 0.0 0.5 0.1 0.1 0.6 2.1 4.6 2.9 3.4 1.5 2.4 4.4 4.3 3.1 4.7 4.2 4.9
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.3 0.0 0.0 0.0 0.7 0.0 0.2 0.5 0.0 0.7
0.1 0.8 1.9 0.2 0.1 2.1 0.1 0.0 0.5 0.0 0.3 0.3 0.5 1.2 0.6 0.9 1.1 0.6
0.0 0.0 0.0 0.0 0.0 0.0 1.3 3.4 1.0 1.0 0.9 1.5 2.6 1.7 1.3 1.4 2.7 2.0
96 95 98 95 96 93 91 90 97 87 90 88 94 82 92 101 99 90
rates, HPLC areas, etc.). Martino25 provides the details of this uncertainty analysis. Product molar yields, Yi, were calculated as the molar flow rate of product i in the reactor effluent divided by the reactant molar flow rate at the reactor entrance.
Furthermore, the molar yields of CO and CO2 were normalized by dividing by the number of carbon atoms in each reactant. With this approach, complete conversion of the phenolic reactant to CO2 leads to a CO2 molar yield of unity (or 100%). The carbon tally is the percent
1778 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
of the carbon in the feed that appears in quantified products in the reactor effluent. The first several rows of each table correspond to experiments that were performed without the addition of oxygen. With only a few exceptions, less than 10% conversion was obtained in these thermolysis experiments. Our previous work24 showed that this modest level of nonoxidative reactivity proceeds at a rate that is at most 5% of the oxidation rate. Accordingly, we investigated the kinetics of the nonoxidative reactions no further. The products from the thermolysis of hydroxyacetophenones were phenol, benzenediols, and the corresponding hydroxybenzaldehyde. Phenol was the major product from o-hydroxyacetophenone, m-hydroxybenzaldehyde was the major product from m-hydroxyacetophenone, and the three products were present in comparable yields from p-hydroxyacetophenone. The products detected from the thermolysis of ethylphenols were vinylphenols and phenol. For both o- and p-ethylphenol, the yield of the corresponding vinylphenol was always at least an order of magnitude higher than the yield of phenol. For m-ethylphenol, the yields of phenol and m-vinylphenol were comparable. Our identification of the vinylphenols is based on GC-MS results as described in the following paragraphs. These compounds are not commercially available, so we could not measure their HPLC detector response factors. Therefore, the yields of the vinylphenols were calculated from the HPLC analysis by assuming that their detector response factors are the same as that for phenol. This assumption might be in error, so the values reported for the vinylphenol yields and the carbon tallies for the ethylphenol experiments should be taken as semiquantitative estimates. Thermolysis of each of the three ethylphenols produced a compound, which we identify as the corresponding vinylphenol, that eluted from the GC shortly after the ethylphenol reactant. The three products formed from the three different ethylphenols eluted with three different retention times. This evidence indicates that these products are three chemically distinct compounds. Additionally, the peak with the highest massto-charge ratio (m/z) appearing in the mass spectrum was a large peak at m/z ) 120, which we take as the molecular weight of the product because aromatic compounds usually exhibit strong molecular ion peaks.26 For reference, the molecular weight of ethylphenol is 122. Thus, it is likely that these thermolysis products are the three isomers (o-, m-, and p-) of the same disubstituted aromatic compound. The mass spectra for the three products were not identical, which again indicates that they are chemically distinct, but they were similar in that the peaks at m/z ) 120 and 91 were always the two highest. For the products from m- and p-ethylphenol, the peak at m/z ) 120 was about twice the height of the m/z ) 91 peak. For the product from o-ethylphenol, the peak at m/z ) 91 was slightly higher than the peak at m/z ) 120. The height of the peak at m/z ) 119 was always less than one-third of that of the peak at m/z ) 120. There was no peak at m/z ) 105, which eliminates any multiply substituted alkylbenzenes from consideration, because these all have high peaks at m/z ) 105. This elimination of exclusively hydrocarbon compounds and the molecular weight of 120 suggested a molecular formula of C8H8O for the products.
Figure 7. Free-radical reaction steps for vinylphenol formation.
We compared the mass spectra of the reaction products with the mass spectrum of every C8H8O compound stored in the GC-MS computer library and in the NIST WebBook.27 The only C8H8O compounds in these databases that existed as o-, m-, and p-isomers were methylbenzaldehydes. The mass spectra for the methylbenzaldehydes did not provide a good match with the products because the peak height at m/z ) 119, which corresponds to loss of the aldehydic hydrogen atom, exceeds that at m/z ) 120. These relative peak heights are not consistent with the spectra for the reaction products. Other compounds examined and rejected are acetophenone, phenylacetaldehyde, ethenyloxybenzene, phenyloxirane, phthalan, and 2,3-dihydrobenzofuran. The mass spectra of the products compared favorably with the library spectrum for 2,3-dihydrobenzofuran, but the GC and HPLC retention times for 2,3-dihydrobenzofuran did not match those of the reaction products. One possibility that remained for the identity of the products is that they are ethene-substituted phenols, or vinylphenols. We could not make this identification absolutely certain because no complete mass spectra were available for vinylphenols, and these compounds are not commercially available. We did find a published report, however, that provided some limited information on the mass spectra of vinylphenols.28 According to this source, the two prominent peaks in the mass spectrum for vinylphenols are at m/z ) 120 and 91. The ratio of the 91/120 peak heights for o-, m-, and p-vinylphenol are 0.78, 0.47, and 0.31. These features of the mass spectra for vinylphenols are consistent with the features observed in the mass spectra of the ethylphenol thermolysis products. Therefore, we conclude that vinylphenols are products of ethylphenol thermolysis in SCW. Figure 7 shows a set of elementary free-radical reaction steps that can account for the formation of vinylphenol. The first step is homolytic dissociation of the benzylic C-H bond, and the second step is β-scission of the radical to form vinylphenol. Zhou and Crynes29 pointed out that the first reaction is a possible initiation step for o-ethylphenol thermolysis. The thermal decomposition of o-ethylphenol has been examined previously,29 but not in supercritical water and not at the short reaction times considered here. Thermolysis of o-ethylphenol as a 5 wt % solution in dodecane produced heavier oxygen-containing compounds and hydrocarbons as the main products at 400 °C and reaction times on the order of 103 s. Cresols, other ethylphenols, phenol, and substituted benzenes also appeared in the product spectrum but in lower yields. Vinylphenols were not reported as products. Given the reactivity of o-vinylphenol observed in the present study (see Figure 1) for reaction times on the order of a few seconds, we suspect that any o-vinylphenol formed during thermolysis in dodecane would be converted to other products and hence no longer present after reaction for thousands of seconds. Comparing the o-ethylphenol disappearance kinetics in dodecane and in supercritical water, we find that the apparent first-order rate constant at 460 °C is about 50-
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1779 Table 2. Gas-Phase and Aqueous-Phase Phenolic Products from the SCWO of Substituted Phenols reactant (C6H4OH)-CH3
gas-phase products CO2 > CO . CH4
(C6H4OH)-CH2CH3 CO2 > CO . CH4, C2H6 (C6H4OH)-COCH3 CO2 > CO (C6H4OH)-CHO CO2 > CO (C6H4OH)-OH CO2 > CO (C6H4OH)-OCH3 CO2 > CO . CH4 (C6H4OH)-NO2 CO2 ≈ CO
phenolic products -CHO, phenol, -OH, dimers, etc. -CHdCH2, phenol phenol, -CHO, -OH phenol, -OH none -OH phenol, dimers
100 times higher in SCW. This difference in apparent reactivity could be due to the disappearance reaction not being truly first-order. The o-ethylphenol concentrations in our experiments are much lower than that of the 5 wt % used in the dodecane experiments. Moreover, the experimental data show that, for comparable residence times, the conversion increases as the o-ethylphenol concentration decreases, which is indicative of a reaction order less than unity. Of course, another potential explanation for the difference in reactivity is that the reaction medium influences the kinetics.
phenol,4,8 o-chlorophenol,18 p-chlorophenol,17 and hydroquinone.20 The yield of CO2 consistently exceeding the yield of CO for all phenolic compounds studied to date indicates that CO is not an important intermediate in the reaction network for CO2 production from phenolic compounds. Rather, the main path for CO2 formation must bypass CO. We believe that decarboxylation of carboxylic acid intermediates, which are formed via ring-opening reactions, is the major path to CO2 formation. We identified numerous aqueous-phase phenolic products from SCWO of the substituted phenols. Other investigators2,3,9,18,30 also reported rich aqueous-phase product spectra arising from the incomplete oxidation of aromatic and cyclic compounds in SCW. Table 2 shows the most abundant aqueous-phase oxidation products for the seven substituted phenols used in this study. Loss of the substituent, either from the reactant itself or from an oxidation product, to form phenol appears to be common for these substituted phenols. Dihydroxybenzenes are also ubiquitous oxidation products. The next section uses these observations and other data to develop a reaction network for the SCWO of monosubstituted phenols.
Oxidation Products Ethylphenols. Figures 1-3 contain the product data for the SCWO of the ethylphenols. The products that are formed from the ethylphenols in the highest yields are typically CO2 and the corresponding vinylphenols. CO and phenol are also formed and are present in comparable yields. Smaller amounts of dihydroxybenzenes were detected for SCWO of o-ethylphenol. The carbon tallies for o-ethylphenol are consistently low, possibly because other, unidentified aqueous-phase products of incomplete oxidation are present. In addition to the presence of unquantified products, a deviation of the carbon tallys from 100% could be caused by the assumption made regarding the HPLC response factor for the vinylphenols. Hydroxyacetophenones. Figures 4-6 contain the product yield data for the SCWO of hydroxyacetophenones. Phenol and CO2 are the oxidation products typically formed in the highest yields from o- and p-hydroxyacetophenone. CO2 and m-hydroxybenzaldehyde are the major oxidation products from m-hydroxyacetophenone. p-Hydroxybenzaldehyde was also present in moderate yields from p-hydroxyacetophenone. Cresols and dihydroxybenzenes were also formed, but in lower yields. The dihydroxybenzenes were also oxidation products from most of the phenolics we had previously investigated,21-24 but their yields had not been quantified. Monosubstituted Phenols. In this section we compare and contrast the identities and relative yields of products from the SCWO of the seven different substituted phenols we have investigated. These reactants include cresols and hydroxybenzaldehydes,21,23 nitrophenols, methoxyphenols, benzenediols,24 and the ethylphenols and hydroxyacetophenones examined herein. Table 2 summarizes the relative yields of gas-phase and aqueous-phase products formed from the SCWO of substituted phenols. The gas-phase SCWO products were always CO2 and CO, with an occasional trace of CH4 and C2H6 from the alkyl-substituted phenols. The yield of CO was always lower than the yield of CO2, even at the lowest conversions we examined. This behavior is identical to that reported in earlier work with
Oxidation Pathways Ethylphenols. Figures 1-3 show that the yields of CO and CO2 increase with the ethylphenol conversion, as expected for complete oxidation reactions. The yield of o-vinylphenol decreases with increasing conversion at the high o-ethylphenol conversions (>35%) investigated. The yield of m-vinylphenol increases with conversion at the lower conversions ( para > meta for all of the substituted phenols investigated. This trend can be attributed to the effect of the -OH group on the electron densities and hence bond strengths of substituents at the different positions. The relative rates in Table 5 are consistent with the results of previous aqueous-phase oxidation studies of substituted phenols at lower temperatures. For example, Joglekar et al.31 found the reactivities for a set of monosubstituted phenols at 150-190 °C to be in the order p-OCH3 > o-CH2CH3 > o-CH3 > m-CH3 > o-Cl > H. Kirso et al.32 reported that ortho- and para-substituted phenols are comparatively easy to oxidize and that cresols reacted more rapidly than phenol at temperatures between 25 and 80 °C. Implications for Structure-Reactivity Relationships. Having obtained a large set of rate data for the SCWO of substituted phenols, it is natural to consider whether one can generalize the results in the form of a quantitative structure-reactivity relationship. These empirical relationships use some structure-dependent reactivity index (e.g., heat of formation, Hammet parameters, etc.) to correlate the reactivities of members in a family of compounds undergoing the same reaction. If successful, such a correlation would allow one to use the experimental database to predict the reactivity of substituted phenols not yet investigated experimentally. It is clear from the reaction network in Figure 10 that the substituted phenols investigated in this study do not react through the same reaction pathways, as is required for the existence of chemically meaningful structure-reactivity relationships. A structure-reactivity relationship is only available for a group of X,Ysubstituted reactants, where Y is common to all reactants, if the substituent Y participates in the reaction while X remains unaffected.33 For the case of the
1782 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999
substituted phenols, -OH is the Y group common to all reactants, but the SCWO reactions were not confined exclusively to the -OH group. Indeed, it was typically the X substituents that underwent reactions in our study of the SCWO of substituted phenols. Figure 10 indicates that the -OH group remains unaffected while reactions proceed through conversion or elimination of the -CH2CH3, -CH3, -COCH3, -CHO, -OCH3, and -NO2 groups. Because the reactions of the substituted phenols we used did not meet this requirement of a common substituent being exclusively attacked, chemically meaningful linear free-energy relationships will not exist for the rate data in Tables 4 and 5. Summary and Conclusions The thermal decomposition of ethylphenols in supercritical water at 460 °C produces vinylphenols as the major products. The thermal decomposition of hydroxyacetophenones under similar conditions produces phenol, benzenediols, and hydroxybenzaldehydes, with the relative yields of the three products being a function of the specific locations of the -OH and -COCH3 substituents in the reactant. The disappearance of ethylphenols and hydroxyacetophenones was much faster in the presence of added oxygen. The oxidation kinetics for each compound were correlated with a global power-law rate expression and then compared to the oxidation kinetics for other substituted phenols. All of the substituted phenols for which the literature now provides SCWO kinetics data react more quickly than phenol itself. Additionally, phenol is a common byproduct from the thermal and/or oxidative decomposition of substituted phenols in supercritical water. These observations point out the value of phenol as a good “worst-case” model pollutant for SCWO studies. The oxidation rates for the substituted phenols are strong functions of the identity and location of the substituent. For a given substituent, the reactivity was always in the order ortho > para > meta. The oxidation of substituted phenols in supercritical water typically proceeds through multiple parallel primary paths. One path involves ring opening and the subsequent rapid formation of CO2. Other paths involve conversion or elimination of the substituent to form other phenolics. The relative rates of these parallel paths depend on the identity and location of the substituent. Acknowledgment This work was supported by the U. S. Department of Energy University Coal Research Program (DE-FG2292PC97536) and the National Science Foundation (CTS9521698, CTS-9311300). Supporting Information Available: Six tables of results of o-, m-, and p-ethylphenol and o-, m-, and p-hydroxyacetophone reactions in SCW at 460 °C and 25.3 MPa. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Wightman, T. J. Studies in Supercritical Wet Air Oxidation, M.S. Thesis, University of California, Berkeley, CA, 1981. (2) Thornton, T. D.; Savage, P. E. Phenol Oxidation in Supercritical Water. J. Supercrit. Fluids 1990, 3, 240.
(3) Thornton, T. D.; LaDue, D. E.; Savage, P. E. Phenol Oxidation in Supercritical Water: Formation of Dibenzofuran, Dibenzo-p-dioxin, and Related Compounds. Environ. Sci. Technol. 1991, 25, 1507. (4) Thornton, T. D.; Savage, P. E. Kinetics of Phenol Oxidation in Supercritical Water. AIChE J. 1992, 38, 321. (5) Thornton, T. D.; Savage, P. E. Phenol Oxidation Pathways in Supercritical Water. Ind. Eng. Chem. Res. 1992, 31, 2451-2456. (6) Gopalan, S.; Savage, P. E. Reaction Mechanism for Phenol Oxidation in Supercritical Water. J. Phys. Chem. 1994, 98, 2646. (7) Gopalan, S.; Savage, P. E. Phenol Oxidation in Supercritical Water: From Global Kinetics and Product Identities to an Elementary Reaction Model. In Innovations in Supercritical Fluids; Hutchenson, K. W., Foster, N. R., Eds.; ACS Symposium Series 608; American Chemical Society: Washington, DC, 1995; pp 217-231. (8) Gopalan, S.; Savage, P. E. A Reaction Network Model for Phenol Oxidation in Supercritical Water. AIChE J. 1995, 41, 1864-1873. (9) Krajnc, M.; Levec, J. On the Kinetics of Phenol Oxidation in Supercritical Water. AIChE J. 1996, 42, 1977-1984. (10) Koo, M.; Lee, W. K.; Lee, C. H. New Reactor System for Supercritical Water Oxidation and Its Application on Phenol Destruction. Chem. Eng. Sci. 1997, 52, 1201-1214. (11) Rice, S. F.; Steeper, R. R. Oxidation Rates of Common Organic Compounds in Supercritical Water. J. Hazard. Mater. 1998, 59, 261-278. (12) Oshima, Y.; Hori, K.; Toda, M.; Chommanad, T.; Koda, S. Phenol Oxidation Kinetics in Supercritical Water. J. Supercrit. Fluids 1998, 13, 241-246. (13) Krajnc, M.; Levec, J. Catalytic Oxidation of Toxic Organics in Supercritical Water. Appl. Catal. B: Environ. 1994, 3, L101L107. (14) Krajnc, M.; Levec, J. Oxidation of Phenol over a TransitionMetal Oxide Catalyst in Supercritical Water. Ind. Eng. Chem. Res. 1997, 36, 3439-3445. (15) Ding, Z.; Aki, S. N. V. K.; Abraham, M. A. Catalytic Supercritical Water Oxidation: Phenol Conversion and Product Selectivity. Environ. Sci. Technol. 1995, 29, 2748-2753. (16) Zhang, X.; Savage, P. E. Fast Catalytic Oxidation of Phenol in Supercritical Water. Catal. Today 1998, 40, 333-342. (17) Yang, H. H.; Eckert, C. A. Homogeneous Catalysis in the Oxidation of p-Chlorophenol in Supercritical Water. Ind. Eng. Chem. Res. 1988, 27, 2009. (18) Li, R.; Savage, P. E.; Szmukler, D. 2-Chlorophenol Oxidation in Supercritical Water: Global Kinetics and Reaction Products. AIChE J. 1993, 39, 178. (19) Lin, K. S.; Wang, H. P.; Li, M. C. Oxidation of 2,4Dichlorophenol in Supercritical Water. Chemosphere 1998, 36, 2075-2083. (20) Thammanayakatip, C.; Oshima, Y.; Koda, S. Inhibition Effect in Supercritical Water Oxidation of Hydroquinone. Ind. Eng. Chem. Res. 1998, 37, 2061-2063. (21) Martino, C. J.; Kasiborski, J.; Savage, P. E. Kinetics and Products from o-Cresol Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1995, 34, 1941-1951. (22) Martino, C. J.; Savage, P. E. Thermal Decomposition of Substituted Phenols in Supercritical Water. Ind. Eng. Chem. Res. 1997, 36, 1385-1390. (23) Martino, C. J.; Savage, P. E. Supercritical Water Oxidation Kinetics, Products, and Pathways for CH3- and CHO-Substituted Phenols. Ind. Eng. Chem. Res. 1997, 36, 1391-1400. (24) Martino, C. J.; Savage, P. E. Oxidation and Thermolysis of Methoxy-, Nitro-, and Hydroxy-Substituted Phenols in Supercritical Water. Ind. Eng. Chem. Res. 1999, 38, 1784-1791. (25) Martino, C. J. Supercritical Water Oxidation of Monosubstituted Phenols: A Comparative Study of Reaction Kinetics and Products. Ph.D. Thesis, University of Michigan, 1997. (26) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John Wiley and Sons: New York, 1991. (27) Stein, S. E. IR and Mass Spectra. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Mallard, W. G., Linstrom, P. J., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, March 1998; (http://webbook.nist.gov).
Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 1783 (28) Yoshizawa, Y.; Kawaii, S.; Kanauchi, M.; Chida, M. Chavicol and Related Compounds as Nematocides. Biosci. Biotechnol. Biochem. 1993, 57, 1572-1574. (29) Zhou, P.; Crynes, B. L. Thermolytic Reactions of oEthylphenol. Ind. Eng. Chem. Process. Des. Dev. 1986, 25, 898907. (30) Crain, N.; Tebbal, S.; Li, L.; Gloyna, E. F. Kinetics and Reaction Pathways of Pyridine Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1993, 32, 2259. (31) Joglekar, H. S.; Samant, S. D.; Joshi, J. B. Kinetics of Wet Air Oxidation of Phenol and Substituted Phenols. Water Res. 1991, 25, 135-145.
(32) Kirso, U. E.; Gubergrits, M. Y. Kinetics and Macromechanism of Oxidation of Phenols of Various Structures by Molecular Oxygen in Aqueous Alkali. Zh. Prikl. Khim. 1972, 45, 835-839. (33) March, J., Advanced Organic Chemistry: Reactions Mechanisms, and Structure, 4th ed.; John Wiley & Sons: New York, 1992; pp 278-286.
Received for review September 8, 1998 Revised manuscript received November 30, 1998 Accepted December 17, 1998 IE980575T
