Environ. Sci. Technol. 2005, 39, 5831-5836
Temperature-Dependent Formation of Polychlorinated Naphthalenes and Dibenzofurans from Chlorophenols DO HYONG KIM AND JAMES A. MULHOLLAND* Environmental Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0512
To investigate the gas-phase formation of polychlorinated naphthalenes (PCNs) and dibenzofurans (PCDFs) from chlorinated phenols in combustion exhaust gas, experiments were performed with each of the three chlorophenols in a laminar flow reactor over the range of 550-750 °C under oxidative conditions. Maximum PCN and PCDF yields were observed between 625 and 725 °C. The degree of chlorination of naphthalene and dibenzofuran products decreased as temperature increased, and on average, the naphthalene congeners were less chlorinated than the dibenzofuran congeners. Congener distributions are consistent with proposed PCN and PCDF formation pathways, both involving phenoxy radical coupling at unchlorinated orthocarbon sites to form a dihydroxybiphenyl keto tautomer intermediate. Tautomerization of this intermediate and subsequent fusion via H2O loss results in PCDF formation, whereas CO elimination and subsequent fusion with hydrogen and/or chlorine loss leads to PCN formation. PCDF isomer distributions were found to be weakly dependent on temperature. PCN isomer distributions were found to be more temperature sensitive, however, with selectivity to particular isomers decreasing with increasing temperature. These results contribute to the understanding of PCN and PCDF formation in combustion and provide information on how to predict and minimize these emissions.
Introduction It has been reported that polychlorinated naphthalenes (PCNs) are formed along with polychlorinated dibenzo-pdioxins (PCDDs) and dibenzofurans (PCDFs) from chlorinated phenols both in gas-phase reactions at high temperatures in the combustion zone (1, 2) and in fly-ash-mediated pathways at low temperatures (3-6). Despite substantial improvements in the understanding of thermal PCDD/F formation processes in recent years, the mechanism of PCN formation in incinerators is not well understood. Recent studies, however, indicate that a strong correlation between several PCN and PCDF isomers may exist in municipal waste incinerator (MWI) fly ash, suggesting that the reaction pathways for the formation of PCNs and PCDFs might be very similar (5-7). In gas-phase PCDF formation, the most direct precursors among the aromatic compounds found in incinerator gas emissions are known to be chlorinated phenols (8). Chlorinated phenols readily form chlorinated phenoxy radicals that are capable of reacting at ortho- and para-carbon sites * Corresponding author phone: (404)894-1695; fax: (404)894-8266; e-mail:
[email protected]. 10.1021/es050576u CCC: $30.25 Published on Web 06/25/2005
2005 American Chemical Society
FIGURE 1. PCDF and PCN formation pathways from chlorophenols. as well as the oxygen site. The combination of phenoxy radicals and tautomerization to form o,o′-dihydroxybiphenyl (DOHB) is recognized as an intermediate in PCDF formation from phenol and chlorinated phenols in slow combustion processes (9). The formation of naphthalene in high-temperature combustion processes has largely been attributed to the hydrogen abstraction/acetylene addition (HACA) mechanism (10-12); however, in postcombustion gas under conditions in which PCDDs and PCDFs are formed from chlorinated phenols, PCNs may be formed from the decomposition of chlorinated phenoxy radical. On the basis of the observed simultaneous formation of PCNs and PCDFs from chlorinated phenols in this laboratory (13, 14), we performed experiments at 600 °C to identify PCN congeners produced by slow combustion of the three chlorophenols (15) and the six dichlorophenols (16). On the basis of those works, PCN formation pathways were proposed that build on the dihydrofulvalene-tonaphthalene mechanism of Melius et al. (17). Proposed overall reaction pathways for PCDF and PCN formation from chlorophenols are depicted in Figure 1; detailed reaction schemes for each chlorophenol are presented in the Supporting Information (Figures S1-S3). Chlorophenols can lose hydrogen or chlorine to produce chlorophenoxy or phenoxy (with hydrogen migration) radicals. These resonance-stabilized radicals can couple at unchlorinated ortho-carbon sites to produce the diketo tautomer of DOHB (step 1 in Figure 1). In PCDF formation, this intermediate undergoes keto-enol tautomerization to form DOHB (step 2); subsequent elimination of H2O yields the PCDF product (step 3). Alternatively, dihydrofulvalene is formed by CO elimination from the diketo-dimer intermediate (step 4). Elimination of CO from both stable and radical systems of phenol has been studied (18-20). Subsequent fusion of dihydrofulvalene to form naphthalene products is depicted by step 5. Unlike PCDF formation, PCN formation from the diketo-dimer intermediate may involve chlorine loss. On the basis of the detailed mechanisms presented in the Supporting Information, a priori hypotheses were developed for the major PCDF and PCN products from the three chlorophenols. In this paper, we test our a priori hypotheses and investigate the effect of temperature on gas-phase PCN and PCDF formation from chlorophenols to gain further mechanistic insight on these processes. These experimental data are needed to help develop and test congener-specific mechanisms of PCN formation. Because of the large number of reaction schemes possible in PCN formation involving resonance-stabilized radical intermediates, computational studies will also be required to elucidate detailed mechanisms. The predictive understanding that will be developed has potential uses for monitoring PCN emissions and for diagnosing PCN formation processes in combustion systems. VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Ratio of total naphthalene to total dibenzofuran product yields from chlorophenols. PCN congeners were identified based on the published relative retention time and the elution order of PCNs in Halowax 1001, 1014, and 1051 standards (AccuStandard, Inc.) (21-26). Monochloronaphthalene isomers, 1- and 2-monochlronaphthalenes, were separated, and seven peaks were separated for the 10 dichloronaphthalene isomers. Coeluting dichloronaphthalene isomers in our analytical system were as follows: 1,5-, 1,6-, and 1,7-dichloronaphthalenes and 2, 6- and 2,7-dichloronaphthalenes. Naphthalene was used as a universal response factor to estimate PCN product yields. Procedures for identifying PCDF congeners have been published previously (13-15). In addition, CO measurements in the exhaust gas were performed with a Hewlett-Packard 5890 II series gas chromatograph coupled to a thermal conductivity detector with a HP-MOLSIV column (15 m, 0.53 mm i.d., 0.25 µm film thickness). FIGURE 2. Chlorophenol recovery and product yields from 2-CP (top), 3-CP (middle), and 4-CP (bottom). Note that CP reactant and CO yields are 10 times higher than shown by the y-axis labels.
Experimental Section A laminar flow, isothermal quartz tube reactor (40 cm in length and 1.7 cm in diameter) was used to study PCDF and PCN formation from three chlorophenols (CPs). Two of the chlorophenols, 2-CP and 3-CP, were injected by a syringe pump into a heated vaporizer at a temperature of 180 ( 5 °C and mixed with a 92% nitrogen and 8% oxygen gas stream, maintaining a constant inlet concentration of 0.9% molar CP vapor. 4-CP, too viscous to be fed by syringe, was heated directly in the glass vessel and vaporized at approximately the same rate as the syringe-fed liquid reactants were fed. The total gas flow rate was controlled to yield a residence time of 10 s (nominal). Experiments were conducted at temperatures from 550 to 750 °C in 25 °C increments. The entire gas stream was immediately quenched at the outlet of the reactor and passed through in a dual ice-cooled dichloromethane trap. Experiments for all three reactants were performed in triplicate at 600 °C; duplicate experiments for 2-CP were conducted at 700, 725, and 750 °C. Isomer-specific analysis of PCDF and PCN congeners was accomplished with a Hewlett-Packard 6890 series gas chromatograph with a HP-5MS column (30 m, 0.25 mm i.d., 0.25 µm film thickness) coupled to a Hewlett-Packard 5973 mass spectrometer. The column oven temperature was programmed from 38 to 80 °C at a rate of 3 °C/min, 180 to 250 °C at a rate of 5 °C/min, 250 to 280 °C at a rate of 6 °C/min, and a final hold time of 3 min. For identification, the mass spectrometer was operated in the scan mode, and for quantification in selective ion mode at the two most intensive and characteristic ion masses. Naphthalene, dibenzo-p-dioxin, and dibenzofuran products are formed from the two phenol reactants. Therefore, expected products in these experiments were congeners containing up to two chlorine atoms. 5832
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Results and Discussion Chlorophenol Recovery and Overall Product Distributions. Chlorophenol reactant recovery and yields of CO, phenol (Ph), total naphthalene (labeled PCN and includes unchlorinated naphthalene), total dibenzofuran (labeled PCDF and includes unchlorinated dibenzofuran), and total dibenzop-dioxin (DD) products are shown in Figure 2 as percent yields of carbon feed. Unreacted CP was 51%, 36%, and 35.5% for 2-CP, 3-CP, and 4-CP, respectively, at 550 °C and less than 1% at 750 °C. Unsubstituted phenol was one of the major products from all three chlorophenols; its yield was a maximum at 675 °C, with yields of 1.7% from 2-CP, 0.5% from 3-CP, and 2% from 4-CP. The results demonstrate that hydrodechlorination is greatest for ortho and para chlorine in the parent phenol. The formation of phenol leads to other, less chlorinated naphthalene and furan products, as will be discussed. Yields of CO from the decomposition and oxidation of chlorophenol were a maximum at 700 °C, with peak yields of 63% for 2-CP, 43% for 3-CP, and 60% for 4-CP. At higher temperatures, CO oxidation to CO2 occurs (27). The total yields of dibenzofuran products peaked between 625 and 675 °C, with maximum yields of 0.21%, 2.2% and 1.3% for 2-CP, 3-CP, and 4-CP, respectively. Dibenzo-p-dioxin (DD) was observed only from 2-CP, with a peak yield of 0.05% observed at 650 °C. These results confirm that dibenzofuran formation is favored by meta substitution in the parent chlorophenol (14) and that ortho chlorine is necessary for dibenzo-p-dioxin formation (28). Maximum yields of naphthalene products were observed between 675 and 725 °C, with yields of 0.91%, 0.39%, and 0.94% from 2-CP, 3-CP, and 4-CP, respectively. Thus, ortho and para substitution on the parent phenol favored naphthalene product formation. On the basis of the low yield of benzene (not shown) and styrene (not detected), naphthalene formation via phenol decomposition to two- or three-carbon species and subsequent growth by HACA pathways was not significant under our experimental conditions.
FIGURE 4. PCDF (left) and PCN (right) homologue distributions from 2-CP (top), 3-CP (middle), and 4-CP (bottom). The ratio of naphthalene to dibenzofuran products was greatest for 2-CP and least for 4-CP (Figure 3). As the temperature increased, this ratio increased for each of the CP reactants. These results suggest that CO elimination (step 4 in Figure 1) that leads to naphthalene formation becomes increasingly favored relative to tautomerization (step 2 in Figure 1), which leads to dibenzofuran formation as the temperature increases. PCDF and PCN Homologue Distributions. Total yields of naphthalene (N), monochloronaphthalenes (MCNs), dichloronaphthalenes (DCNs), dibenzofuran (DF), monochlorodibenzofurans (MCDFs), and dichlorodibenzofurans (DCDFs) are shown in Figure 4 as a percent of carbon feed basis. At lower temperatures, DCDF products are favored. As the temperature increases, the total yields of MCDF and DF become larger fractions of the total dibenzofuran product yield. At 750 °C, DF was the major dibenzofuran product. The PCN homologue distributions show a similar trend with temperature as the PCDF homologue distributions, with the unchlorinated congener becoming increasingly the dominant product at high temperatures. Unlike the dibenzofuran homologue distribution, however, the total yields of monochlorinated and unchlorinated congeners were greatest in the case of naphthalene formation even at the lowest temperature studied (550 °C). For all three CPs, the high yields of DCDF and DCN products at low temperatures, relative to DCDF and DCN
yields at high temperatures, are consistent with ortho-ortho carbon coupling of two chlorophenoxy radicals. As the temperature increases, organic chlorine decreases. Increases in MCDFs and DF relative to DCDFs and MCNs and N relative to DCNs suggest coupling of the chlorophenoxy radical with the phenoxy radical. The identification of DCDF and DCN isomers, discussed in the next section, confirms this conclusion. Another inference can be drawn from examination of the PCN and PCDF homologue distributions. PCN products from all three chlorophenols were less chlorinated than PCDF products. DCDF yields were a couple of orders of magnitude higher than DCN yields, MCDF yields were similar to MCN yields, and dibenzofuran yields were an order of magnitude lower than naphthalene yields. The presence of chlorine may suppress CO elimination in the ring systems by withdrawing electron density, leading to higher DCDF yields than DCN yields. PCDF and PCN Isomer Distributions. Naphthalene and dibenzofuran products predicted by our hypotheses and observed in these experiments are listed in Table 1. Predicted isomers are listed in order of most favored to least, based on the number of alternative routes (Figures S1-S3 in the Supporting Information); observed peaks are listed in order of largest to smallest. All expected dibenzofuran and naphthalene congeners produced by the coupling of two chlorophenoxy radicals, one chlorophenoxy radical and phenoxy VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Predicted and Observed PCN and PCDF Products from Chlorophenols
radical, and two phenoxy radicals were observed, except for 1,8-DCN. Trace amounts of 1,8-DCN were observed from 2-CP, with yields of less than 0.0001%. In addition, the largest observed peaks for DCN, MCN, DCDF, and MCDF isomers corresponded to those products expected to be favored. For example, 1-MCN is predicted to be favored over 2-MCN from 2-CP based on the number of alternative routes (Figure S2), 2-MCN is predicted to be favored over 1-MCN from 4-CP (Figure S1), and the number of reaction paths to each isomer is the same from 3-CP (Figure S3). These predictions agree with the observations. PCDF and PCN product yields at each temperature are provided in Table S1 of the Supporting Information. In Figure 5, ratios of MCDF and DCDF isomers produced from 3-CP are shown as a function of temperature. The results show that the MCDF and DCDF isomer distributions are only weak functions of temperature. In Figure 6, ratios of MCN and DCN isomer peaks produced from each of the chlorophenols are shown as a function of temperature. The results show that the MCN and DCN isomer distributions from 2-CP and 4-CP are strong functions of temperature, whereas the MCN and DCN isomer distributions from 3-CP are not. The selectivity to particular PCN isomers decreased as the temperature increased; that is, the ratios tended to go toward 1. Different ortho-ortho phenoxy radical couplings, which are only possible in the case of 3-CP, result in the formation of different PCDF isomers and different sets of PCN isomers. The distributions of these isomers were found to be weakly dependent on temperature, consistent with a low activation energy process for phenoxy radical recombination. The distributions of different PCN isomers formed from 2- and 5834
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FIGURE 5. Ratios of MCDF and DCDF isomers from 3-CP. 4-CP, however, were found to be much more sensitive to temperature, suggesting that the alternative fusion routes from the same dihydrofulvalene intermediate have different activation energies. The gas-phase formation of highly chlorinated PCNs from phenols is likely inhibited relative to the formation of less chlorinated PCNs. This assertion is based on two factors: (1) the predominance less chlorinated precursors (i.e., phenols and benzenes) in the gas phase of combustion exhaust at these temperatures (29) and (2) the lower yields
Literature Cited
FIGURE 6. Ratios of MCN and DCN isomers from chlorophenols. of PCDF products from more chlorinated phenols in gasphase experiments (30). Highly chlorinated PCNs and PCDFs can, however, be formed from less chlorinated congeners produced in the gas phase by chlorination via metal-mediated reactions in particle collection devices (31). Alternative routes of PCN formation are possible. A detailed computational study was performed on the ketoenol tautomerization of phenol and 2,4-cyclohexadienone, with CO elimination from the keto tautomer (2,4-cyclohexadienone) to form cyclopentadiene via an acyclic intermediate (20). A heat of reaction of 20 kcal/mol for the formation of naphthalene from the 2-chlorophenol radicals via the recombination of cyclopentadiene radicals has been reported (32, 33). Another possible reaction sequence that may affect PCN formation from chlorophenols is chlorine migration. Intramolecular rearrangement of chlorine by a 1,5-sigmatropic shift in the cyclopentadiene ring has a similar energy barrier as that of hydrogen (34, 35), suggesting that chlorine can move as easily as hydrogen. Such chlorine migration would lead to the formation of additional PCN isomers. In summary, this work addresses the formation of PCNs and PCDFs from chlorophenols in postcombustion gas streams. The experimental results presented here support the hypothesis that PCNs and PCDFs are formed from a common intermediate produced by ortho-ortho carbon coupling of phenoxy radicals. The distributions of PCN and PCDF products indicate that phenoxy radical couplings containing fewer chlorine substituents tend to form naphthalenes rather than dibenzofurans. PCN isomer distributions were found to be temperature sensitive, with selectivity to particular isomers decreasing with increasing temperature. These experimental results provide information that can be used to develop and test detailed mechanisms for the formation of PCNs from phenols in combustion exhaust gas. In addition, these results suggest that control of PCNs and PCDFs produced by this mechanism can be achieved by minimizing the release of precursors (e.g., benzene and phenols) from the flame zone and by rapid quench of the exhaust gas through the temperature window 600-800 °C.
Acknowledgments This work was supported under National Science Foundation Grant No. CTS-0210089.
Supporting Information Available Detailed reaction pathways of PCDF and PCN formation and PCDF and PCN product yields from each of the three CPs. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review March 23, 2005. Revised manuscript received June 1, 2005. Accepted June 3, 2005. ES050576U
