Ind. Eng. Chem. Res. 1997, 36, 1385-1390
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Thermal Decomposition of Substituted Phenols in Supercritical Water Christopher J. Martino and Phillip E. Savage* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136
The thermal decomposition of cresols, hydroxybenzaldehydes, nitrophenols, and benzenediols was studied in dilute aqueous solutions and in the absence of oxygen at 460 °C and 250 atm for residence times around 10 s. Thermolysis under these conditions produced conversions of less than 10% for o-, m-, and p-cresol, whereas hydroxybenzaldehydes and nitrophenols were much more reactive. Global rate expressions are reported for the thermolysis of each hydroxybenzaldehyde and nitrophenol isomer. Phenol was a major product from the decomposition of all of the substituted phenols studied. For a given substituent, ortho-substituted phenols reacted more rapidly than the other isomers. For a given substituted position, nitrophenols reacted more rapidly than hydroxybenzaldehydes, which in turn reacted more rapidly than cresols. These results demonstrate that the treatment of CHO- and NO2-substituted phenols by oxidation in supercritical water will involve the oxidation of thermal decomposition products in addition to the oxidation of the original compounds. Introduction Supercritical water (SCW) oxidation is a process technology that holds promise for safely and economically destroying organic compounds in aqueous waste streams. Investigations of the reaction kinetics and reaction pathways for model wastes and pollutants in water above its critical point (Tc ) 374 °C, Pc ) 218 atm) will aid in the eventual design, optimization, and control of commercial SCW oxidation facilities. The types of reactions that can occur under SCW oxidation conditions include pyrolysis, hydrolysis, and oxidation. The first two reactions can proceed even in the absence of an added oxidant. If the rates of these reactions are sufficiently rapid, then the compounds that are ultimately oxidized could be the hydrolysis and pyrolysis products of the target compound rather than the compound itself. Clearly, an understanding of both the oxidative and nonoxidative decomposition reactions of organic compounds in SCW would facilitate the safe treatment of wastes by SCW oxidation. In this paper, we focus attention on the decomposition of CH3-, CHO-, NO2-, and OH-substituted phenols in SCW. These types of compounds are ubiquitous in industrial waste waters, including those from coal conversion processes (Yen et al., 1982; Jevtitch and Bhattacharyya, 1986). There is a rich and voluminous literature on the pyrolysis and hydrolysis of organic compounds in supercritical water, and these topics are fully discussed in a recent review article (Savage et al., 1995). Most of these previous studies (e.g., Li and Houser, 1992; Tsao et al., 1992; Houser et al., 1993; Townsend and Klein, 1985; Townsend et al., 1988; Wu et al., 1991) employed compounds that mimicked structural elements in coal, oil, or biomass. Moreover, they tend to focus on temperatures near the critical point (Tc ) 374 °C) and long reaction times (minutes to hours). In contrast, the * Corresponding author. E-mail:
[email protected]. Phone: (313) 764-3386. Fax: (313) 763-0459. S0888-5885(96)00698-7 CCC: $14.00
important nonoxidative thermal decomposition reactions relevant to SCW oxidation technology will occur at higher temperatures and much shorter reaction times. There have been some previous studies of hydrolysis in SCW under these conditions. Harradine et al. (1993) investigated the destruction of trinitrotoluene (TNT) in SCW. At temperatures of 500 and 600 °C, a pressure of 340 bar, and no added oxidant, they obtained destruction efficiencies of >99.98% with >40% of the carbon remaining in the aqueous effluent. Although no external oxidant was present, we note that the NO2 groups can serve as oxidants and hence these reactions of TNT cannot be strictly classified as hydrolysis. Lee and Gloyna (1992) determined the hydrolysis kinetics for acetamide at supercritical conditions and cast the results in the form of a power-law rate expression. They also developed an expression for the parallel occurrence of both oxidation and hydrolysis of acetamide and found that hydrolysis proceeded more rapidly than oxidation. Tester and co-workers studied the hydrolysis of glucose (Holgate et al, 1995), acetic acid (Meyer et al., 1995), and dichloromethane (Marrone et al., 1995) in SCW. Although the reactivity of a large number of different organic compounds in SCW has been explored, the literature dealing with the hydrolysis or pyrolysis of phenolics in water at elevated temperatures and pressures is limited. Thornton (1991), who investigated the thermal decomposition of phenol in SCW at conditions of 380 °C, 278 atm, and 1000 mg/L phenol, found no measurable destruction of phenol for holding times up to 4 h. Katritzky et al. (1990a,b,c) studied the extents of reaction and products formed from the pyrolysis and hydrolysis of anisole, cresols, and several alkyl-substituted phenols in water at subcritical conditions. They found that o- and m-cresol were stable at 250 °C for times up to 264 h, whereas p-cresol experienced a modest amount of decomposition (2.1% conversion at 72 h), which was not due to dealkylation. Klein and co© 1997 American Chemical Society
1386 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997
workers reacted guaiacol in SCW (Lawson and Klein, 1985; Townsend et al., 1988; Huppert et al., 1989). They reported first-order rate constants and found that catechol was a major product whereas phenol and o-cresol formed in smaller quantities. Martino et al. (1995) reported that phenol was the major product from o-cresol pyrolysis in SCW, but they provided no quantitative data. The present report expands upon these previous reaction studies and further broadens the field by reporting on the thermal decomposition of methylphenols, hydroxybenzaldehydes, nitrophenols, and benzenediols in SCW. None of these phenolic compounds appear to have been studied previously in quantitative detail under conditions (temperatures above 450 °C, reaction times around a few seconds) relevant to SCW oxidation processes. Experimental Section The organic reactants were obtained either from Aldrich Chemical Co. or Eastman Chemical Co. The nominal purities were at least 99% for o-, m-, and p-cresol, o- and p-nitrophenol, catechol, resorcinol, and hydroquinone, 98% for o- and p-hydroxybenzaldehyde, and 97% for m-hydroxybenzaldehyde and p-nitrophenol. The presence of impurities was taken into account when calculating concentrations in reactor feedstocks and calibrating analytical equipment. Aqueous stock solutions on the order of 100 or 1000 ppm were made for each of these compounds. These solutions served as the reactor feed. The nitrophenols were dissolved in water heated to 60-80 °C because they have a more limited solubility in ambient-temperature water. The present thermolysis studies employed the flow reactor system and analytical protocol described previously (Martino et al., 1995; Thornton and Savage, 1990). This tubular reactor system consisted of a preheat portion and a reactor portion immersed in a temperature-controlled fluidized bath of aluminum oxide particles. A feed stream containing the organic compound dissolved in degassed water was fed through the preheat line and then into the reactor tube. There were two reactor assemblages used in this work, one with a 1 m long preheat line and a 1 m long reactor tube, the other with a 2 m long preheat line and a 4 m long reactor tube. The different lengths were required to explore a wide range of residence times. In both systems, the preheat tubes were 1/16 in. (1.59 mm) o.d. and 1.08 mm i.d., and the reactor tubes were 1/8 in. (3.18 mm) o.d. and 1.40 mm i.d. All materials in contact with supercritical water were composed of Hastelloy C276 alloy. The reactors were operated in the plug flow regime, and the nominal pressure and temperature were constant during each reaction experiment. The reactor effluent was cooled, depressurized, and then separated into gas and liquid phases at ambient conditions. The flow rate of gaseous products was too low to measure or quantify. The liquid-phase reactor effluent, on the other hand, was analyzed by reversephase high-performance liquid chromatography (HPLC) and by gas chromatography with a mass selective detector. The HPLC system includes a Waters model 600S controller, model 626 pump, model 717 autosampler, model 996 photodiode array detector, and Millennium 2010 chromatography software. The photodiode array detector provides a UV and visible absorbance spectrum for each peak observed and thereby permits identification of the wavelength of maximum UV absorbance for the reactants and byproducts. We used the
Table 1. Summary of o-Cresol Thermolysis Experiments in SCW at 460 °C and 250 atm product yields residence concn o-cresol conversion o-HB time (s) (µmol/L) (%) (%) 1.3 3.0 6.6 8.0 15 38 a
257 257 258 267 268 267
1.8 3.5 3.8 1.4 9.8 3.1
n.d.a n.d. n.d. n.d. n.d. n.d.
phenol carbon (%) tally (%) 1.6 3.1 5.5 2.1 1.3 2.5
99.6 99.2 100.9 100.4 91.3 99.1
n.d. ) not determined.
Table 2. Summary of m-Cresol Thermolysis Experiments in SCW at 460 °C and 250 atm product yields residence concn m-cresol conversion m-HB phenol carbon time (s) (µmol/L) (%) (%) (%) tally (%) 1.4 3.1 6.7
94.6 94.5 94.7
0.6 2.3 2.9
0.0 0.0 0.3
0.0 0.0 0.0
99.4 97.7 97.3
chromatogram for the most strongly absorbed wavelength, typically between 195 and 250 nm, to determine the reactant concentrations. By comparing the retention times and complete absorbance spectra for compounds in the reactor effluent with the spectra obtained for standard solutions of pure compounds with known identity, we positively identified some reaction products and judged the purity of overlapping peaks. The reactant conversion was calculated by direct comparison of the reactant concentration in the feed and in the reactor effluent. Molar yields of products were calculated as the molar flow rate of the product in the reactor effluent divided by the molar flow rate of reactant into the reactor. We estimate the run-to-run variability in the molar yields to be about (15%. We also calculated a carbon tally as the percentage of carbon atoms in the feed that appear in quantified products in the reactor effluent. The residence times for these experiments were calculated as the reactor volume divided by the volumetric flow rate at reaction conditions. This approach provides a consistent basis for the residence time, but it is a lower bound because it neglects the amount of time the fluid spends in the preheat line as it reaches the reaction temperature. We explore the effect of using this lower bound on the kinetics results later in this article. Results and Discussion We conducted thermolysis reactions of several phenolic compounds in SCW at the nominal reaction conditions of 460 °C and 250 atm. We used cresols, hydroxybenzaldeydes, nitrophenols, and dihydroxybenzenes as reactants, substituted phenols which have but a single substituent located at either the ortho-, meta-, or para-position relative to the OH group. Cresols. Tables 1-3 summarize the experimental results for o-, m-, and p-cresol. The cresol thermolysis products that were identified and quantified are phenol and the corresponding hydroxybenzaldehyde. Consistent with our earlier report (Martino et al., 1995), the thermal decomposition of o-cresol led to conversions of m- = p-hydroxybenzaldehyde. Now that a rate equation is available for thermal decomposition of hydroxybenzaldehydes, one can write the net rate of reaction when oxygen is present as the sum of the pyrolysis and oxidation rates. For such an endeavor, we would use a rate expression such as
rate ) ratepyrolysis + rateoxidation ) kp[organic]ap + ko[organic]ao[O2]bo (6) Martino and Savage (1997) report results from the use of eq 6 to model the disappearance of hydroxybenzaldehydes during SCW oxidation. Nitrophenols. Tables 8-10 contain the results of the SCW thermolysis of o-, m-, and p-nitrophenol. Each of the nitrophenols exhibited significant levels of con-
version. These high conversions are consistent with previous results for other NO2-containing compounds (Harradine et al., 1993) and may be partially attributed to the oxidizing power of the nitro group. Phenol was the only product consistently present in sufficiently high yields to quantify. Table 8 shows that o-nitrophenol attained complete conversion at a residence time of only 6 s. Although phenol was formed in yields of only around 10%, no other products were detected in the HPLC analysis of the aqueous reactor effluent. We did detect additional liquid-phase products through gas chromatography analysis with a mass selective detector (GC-MS), but only after sample preparation by extraction into dichloromethane and the subsequent evaporative concentration of the sample using the protocol described by Martino et al. (1995). This effort allowed us to identify products such as p-nitrophenol, benzoxazole, 2-methylbenzoxazole, indanone, dibenzofuran, and 2-phenoxyphenol in trace levels. Since the concentrations of these compounds was very low, however, they are not likely to account for a significant portion of the carbon atoms reacted. Moreover, the o-nitrophenol reactions did not produce a measurable gas flow. This absence of detectable reaction products in the reactor effluent, coupled with nearly complete conversion of o-nitrophenol, caused the carbon tally to fall far short of 100%. It ranged from 70% at the shortest residence time of 1.3 s to 8.7% at the longest residence time of 6 s. p-Nitrophenol also had high conversions in SCW without the addition of oxygen. Over the residence time range of 1.1-6.2 s, the conversion ranged from 14.7% to 62.4%. The phenol yields from these p-nitrophenol reactions were roughly the same as those observed from the thermolysis of o-nitrophenol, in spite of the pnitrophenol conversions being lower. Through GC-MS analysis, we were able to identify o-nitrophenol and dibenzofuran as products in the aqueous effluent from the reactor. As was the case for the o-nitrophenol reactions, most of the reacted carbon in p-nitrophenol did not appear in aqueous-phase products, which caused consistently low values for the carbon tally. Some of this reacted carbon appeared to form solid products that remained in the reactor system, for we detected a gray/ black powder on the filter located downstream of the reactor between the heat exchanger and the backpressure regulator. We recovered some of this material and dissolved a portion of it in dichloromethane and analyzed it through GC-MS. It contained several compounds of greater molecular weight than the nitrophenol reactant. These compounds include dibenzofuran, 2-nitrodibenzofuran, and 4-phenoxyphenol. The formation of these high-molecular-weight products and their subsequent deposition in the reactor system might account for the balance of the reacted carbon in these nitrophenol experiments. m-Nitrophenol displayed the lowest reactivity of the nitrophenols in SCW. With three of the six experiments having conversions that exceed 10%, however, the rate of pyrolysis appears to be great enough to be significant at SCW oxidation conditions. Phenol formed from the thermolysis of m-nitrophenol, but not to a sufficient extent to account for all of the reacted carbon. pNitrophenol was the only additional liquid-phase product identified with GC-MS. As with the p-nitrophenol reactions, m-nitrophenol thermolysis formed a gellike solid residue on the product filter. This residue con-
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1389 Table 11. Regression Statistics for the Thermolysis of Nitrophenols in SCW k′
σ2k′
a
σ2a
2 σk′,a
o-NP -0.6114 0.9238 6.52 × 10-2 4.05 × 10-3 1.62 × 10-2 m-NP -2.817 0.6322 8.59 × 10-2 5.68 × 10-3 2.19 × 10-2 p-NP -0.4017 1.147 1.18 × 10-2 8.30 × 10-4 3.10 × 10-3
Summary and Conclusions
Table 12. Summary of Resorcinol Thermolysis Experiments in SCW at 460 °C and 250 atm residence time (s)
concn resorcinol (µmol/L)
converison (%)
1.4 1.3 3.2 3.2 6.9 7.2
105.4 1001 106.3 1001 1000 105.4
1.2 10.1 6.8 10.1 5.9 6.8
tained dibenzofuran and 3-nitrodibenzofuran along with other unidentified compounds. Because the nitrophenols are reactive under the conditions investigated, we used the experimental data to develop global reaction rate laws for the thermal decomposition of nitrophenols in SCW. The data analysis followed the same procedure outlined above for the hydroxybenzaldehydes. The global rate expressions that best describe the experimental data in Tables 8-10 are
roNP ) 10
[oNP]
-0.61(0.81
0.92(0.24
conditions. Stock solutions of these compounds significantly decomposed in the approximately 24 h of time it took to complete the reaction and perform the effluent analysis. This behavior made it difficult to obtain reliable results from experiments with these compounds.
(7)
rmNP ) 10-2.82(0.81[mNP]0.63(0.21
(8)
rpNP ) 10-0.40(0.30[pNP]1.15(0.25
(9)
Table 11 gives a summary of the nitrophenol regression analysis. We can compare the relative reactivity of the three nitrophenols by calculating pseudo-first-order rate constants from eqs 7-9 at a fixed nitrophenol concentration. For example, at a nitrophenol concentration of 250 µmol/L, the pseudo-first-order rate constants are 0.460 ( 0.109, 0.032 ( 0.009, and 0.117 ( 0.012 s-1, for o-, m-, and p-nitrophenol, respectively. The uncertainty given with each rate constant is the 95% confidence interval, as calculated from the variances and covariances in Table 11. This comparison shows that the order of reactivity is o- > p- > m-nitrophenol. This ranking is consistent with that observed for the hydroxybenzaldehydes in that the ortho-substituted phenol is the most reactive in both cases. The pseudo-firstorder rate constants for the nitrophenols exceed those for the corresponding hydroxybenzaldehyde, which reveals that NO2-substituted phenols are more reactive in SCW than CHO-substituted phenols. Benzenediols. Table 12 displays the results from the thermolysis of resorcinol, 1,3-dihydroxybenzene, in SCW. With residence times ranging from 1.3 to 7.2 s, the greatest reactant conversion observed was 10.1%. This level of pyrolysis is not likely to be significant when compared to the oxidation rate expected at this temperature. Thus, no further experiments or data analysis was deemed necessary. We were unable to identify any products from the reactions of resorcinol in SCW under these conditions. We also attempted thermolysis experiments with catechol (1,2-dihydroxybenzene) and hydroquinone (1,4dihydroxybenzene), but these compounds appeared to be unstable in aqueous solutions even at ambient
Cresols are largely stable in SCW at 460 °C and 250 atm for residence times up to about 30 s. In contrast, hydroxybenzaldehydes and nitrophenols are reactive under these conditions. Thus, treatment of these compounds by SCW oxidation will involve a significant purely thermal component, which implies that much of the oxidation will be of the thermal reaction products (such as phenol) rather than the substituted phenol itself. For phenols with CHO and NO2 substituents, the ortho-substituted isomer is the most reactive in SCW. For a given substituent location, nitrophenols are more reactive than hydroxybenzaldehydes, which are much more reactive than cresols. Thus, there are clearly substituent effects on the reactivity of substituted phenols in SCW. This observation suggests that quantitative structure-reactivity relations might be available for this class of compounds. Acknowledgment This work was performed under Department of Energy Grant DE-FG22-92PC92536. The HPLC system was purchased with equipment Grant CTS-9311300 from the National Science Foundation. Nomenclature a ) global reaction order for the organic compound CR ) cresol (methylphenol) HB ) hydroxybenzaldehyde i.d. ) inner diameter k ) reaction rate constant k′ ) log(k) m ) metaNP ) nitrophenol o.d. ) outer diameter P ) pressure o ) orthop ) parar ) rate T ) temperature X ) reactant conversion Greek Letters σ2 ) variance or covariance τ ) residence time Subscripts 0 ) at reactor entrance o ) oxidation p ) pyrolysis c ) critical value
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Received for review November 4, 1996 Revised manuscript received January 31, 1997 Accepted February 3, 1997X IE960698I
X Abstract published in Advance ACS Abstracts, April 1, 1997.
