Environ. Sci. Technol. 1997, 31, 542-547
Mechanisms of the Formation of Polychlorinated Benzenes and Phenols by Heterogeneous Reactions of C2 Aliphatics KENNETH L. FROESE† AND OTTO HUTZINGER* Ecological Chemistry and Geochemistry, University of Bayreuth, 95440 Bayreuth, Germany
Persistent aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo-pdioxins and furans (PCDD/F), are formed during thermal reactions in combustion systems, such as municipal waste incineration (MWI). Aliphatic products of incomplete combustion (PICs) can react to form simple aromatics that are intermediates or precursors to PAHs and PCDD/F. Products from 600 °C heterogeneous combustion reactions of C2 aliphatics, benzene, and hexachlorobenzene were compared in order to initiate further understanding of the mechanisms of aromatic formation from simple aliphatics. Reaction results (1) suggest that the C2 aliphatics share related thermal decomposition pathways; (2) confirm that Cu plays a critical role in the catalytic action of MWI fly ash; and (3) suggest that Al2O3 plays a unique catalytic role in the formation of aromatic rings from aliphatics. Comparisons of congener distribution patterns of chlorinated benzenes and chlorinated phenols and additional volatile and semivolatile compounds provided the basis for a proposed set of pathways that we believe are important in the formation of aromatics from short-chain aliphatics. CuO and fly ash promote the chlorination of aliphatic intermediates followed by ring formation, whereas Al2O3 selectively catalyzes the formation of nonchlorinated aromatics followed by chlorination.
Introduction Combustion processes, including municipal waste incineration (MWI), are a primary source of chlorinated aromatic hydrocarbons, including polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) to the environment (1-4). Formation of PCDD/F and related compounds in MWI systems have been attributed to de novo synthesis from particulate carbon (5-10) and formation from aromatic precursors such as pentachlorophenol (10-14). The importance of simple chlorinated aromatics, such as chlorinated phenols, as precursors or intermediates in the fly ashcatalyzed formation of PCDD/F is widely accepted. However, there is a critical lack of mechanistic information regarding the formation of these compounds. Products of incomplete combustion (PICs) include C1 and C2 alkyl olefins and radicals (3, 15, 16) that can react and recombine in the postcombustion zone. The formation of aromatic compounds from such short-chain aliphatic PICs is an important process * To whom correspondence may be addressed. † Present address: Department of Public Health Sciences, University of Alberta, 13-103 Clinical Sciences Bldg., Edmonton, Alberta, T6G 2G3 Canada.
542
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 2, 1997
in combustion systems (2, 3, 17-20). However, there remains a critical lack of knowledge regarding the mechanisms of formation of the first aromatic rings in combustion systems, particularly in the heterogeneous environments found in MWI systems. Previously, we reported the results for heterogeneous acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) reactions (18, 19). These investigations indicated that the formation of chlorinated benzenes from the C2 aliphatics increased with reaction temperature to 600 °C. We postulate that simple aromatics and chlorinated aromatics are formed in the higher temperature region in the immediate post-combustion zone of an MWI system. These compounds can behave as precursors for PCDD/F formation in the 300 °C region, further downstream of the incineration zone. This paper is the third part in a series examining the formation of chlorinated benzenes (ClxBz) and chlorinated phenols (ClxPh) in heterogeneous combustion reactions of the nonchlorinated C2 aliphatics. The catalytic effects of MWI fly ash and fly ash model compounds (SiO2, SiO2/CuO, SiO2/ Al2O3) were compared. (Hereafter SiO2/CuO and SiO2/Al2O3 will be referred to as CuO and Al2O3, respectively.) More detailed introduction to the investigation, description of the experimental design, and results for acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) reactions are provided elsewhere (18, 19). The dominance of hexachlorobenzene (HCB) in many of the reactions and the observation of benzene in some reactions prompted us to do some experiments with benzene and HCB to gain information on these compounds as potential intermediates or first rings. Finally, we will use the information gained from these experiments and those done previously to postulate a set of reaction pathways for the formation of ClxBz and ClxPh from C2 aliphatics.
C2 Aliphatic Reactions From the investigations of heterogeneous reactions of C2 aliphatics reported previously (18, 19), three primary observations could be made. These observations, together with the experimental results from the benzene and HCB reactions, provide the basis for the proposed reaction pathways. The congener distribution patterns for ClxBz and ClxPh were similar among the three reagents for each catalyst investigated. The implication of such an observation is that the pathways for formation of these aromatic compounds from the three C2 aliphatics are mechanistically related. There were similar trends in congener distribution patterns for the CuO and the fly ash-catalyzed reactions, with HCB and Cl5Bz dominating the ClxBz products. Additional products identified in the reactions consisted primarily of perchlorinated aliphatics and aliphatic olefins (e.g., hexachlorobutadiene). The similarities between products of the CuO and fly ash reactions suggests that Cu is an important component of fly ash for heterogeneous combustion reactions of aliphatic PICs, under the reaction conditions as outlined elsewhere (18, 19). Previous investigations regarding de novo synthesis and heterogeneous catalyzed reactions of aromatic precursors have demonstrated that Cu is an important catalyst for the thermal formation of PCDD/F (6, 7, 10, 21-23). The ClxBz and ClxPh patterns from Al2O3-catalyzed reactions were consistently different from those produced on the other catalyst materials: These patterns were dominated by Cl2 congeners, with only trace amounts of the higher chlorinated congeners produced. Major additional products identified in the Al2O3 reactions included benzene and nonchlorinated aromatics and alkyl substituted aromatics (e.g., naphthalene, methyl biphenyl) (18, 19). These observations suggest that Al2O3 has a unique catalytic role in the
S0013-936X(96)00425-7 CCC: $14.00
1997 American Chemical Society
TABLE 1. Experimental Conditions for Heterogeneous Combustion Reactions of Benzene with Various Catalysts C6H6
c6h6 catalyst fly ash 600 °C fly ash 600 °C SiO2 600 °C SiO2/CuO 600 °C SiO2/CuO 600 °C
HCl
conditions
total C6H6
rxn res time
temp corrected
(tot (tot (mmol/ (mL/ (mmol/ air O2 (air) N2 temp rxn time (mmol) (mg) catalyst free vol res flow res mg) time) h) min) h) (mL/min) (mmol/h) (air) (°C) (min) (cm) (mL) time (s) (mL/min) time (s) 480 460 480 460 460
23.0 22.3 23.0 22.3 22.3
16.0 15.8 16.0 15.8 15.8
3.0 3.0 3.0 3.0 3.0
7.5 7.5 7.5 7.5 7.5
56 55 56 55 55
28 28 28 28 27
110 110 110 110 110
600 600 600 600 600
8 5 10 5 5
2.1 1.3 2.7 1.3 1.3
280 170 350 170 170
2 2 2.5 2.5 2.5
2.5 2.5 2.4 2.4 2.4
2.6 2.6 2.4 2.4 2.4
170 170 170 170 170
0.9 0.9 0.8 0.8 0.8
TABLE 2. Experimental Conditions for Heterogeneous Combustion Reactions of HCB with Various Catalysts HCl
C6Cl6 catalyst fly ash 600 °C fly ash 600 °C SiO2 600 °C SiO2/CuO 600 °C SiO2/CuO 600 °C
conditions
total HCB
rxn res time
temp corrected
HCB (mL/ mmol/ air O2 (air) N2 (air) temp rxn time (mmol) (µg) catalyst free vol res flow res (tot µg) min) h) (mL/min) (mmol/h) (mmol/h) (°C) (min) (cm) (mL) time (s) (mL/min) time (s) 2.0 2.0 2.0 2.0 2.0
>3.0 >7.5 3.0 7.5 3.0 7.5 3.0 7.5 3.0 7.5
56 56 57 56 56
28 28 28 28 28
110 110 110 110 110
600 600 600 600 600
15 15 15 15 15
7.0E-06 7.0E-06 7.0E-06 7.0E-06 7.0E-06
2 2 2 2 2
2 2 2.5 2.5 2.5
2.5 2.5 2.4 2.4 2.4
2.6 2.6 2.4 2.4 2.4
160 160 160 160 160
1.0 1.0 0.9 0.9 0.9
combustion reactions of C2 aliphatic compounds. However, despite the greater Al content (3.7%) than Cu (0.12%) (17) in the fly ash used, the catalytic characteristics of Cu appear to dominate in the fly ash.
Experimental Section Benzene and HCB reactions were performed using conditions described in detail previously (17, 18). Briefly, the reaction apparatus consisted of a glass chamber for mixing reagent gases and a quartz reaction tube suspended vertically in an electronically controlled oven. About 0.5 g of extracted and annealed fly ash, SiO2, or SiO2/CuO was held in place in the reaction tube using quartz wool. Benzene was introduced into the reaction system by passing the carrier gas (80% N2; 20% O2) through a cooled impinger containing 2-3 mL of liquid benzene; a mixture of gas-phase benzene, air, and HCl gas was transported through the heated (600 °C) reaction tube and the catalyst. HCB, due to its relatively high vapor pressure, was deposited on quartz wool in the reaction tube, just above the catalyst material, and transported with the air/HCl gas as it vaporized. Reaction conditions are provided in Tables 1 and 2. Gas-phase reaction products were collected on Carbotrap sorbent tubes. Following a reaction, the quartz reaction tube and the Carbotrap tube were eluted with toluene and a combination of hexane and dichloromethane, respectively. 13C6-labeled internal standards were added, and the eluates were concentrated, fractionated, and derivatized (ClxPh). Analysis of the ClxBz and ClxPh fractions was done using a Hewlett Packard 5890/5970 gas chromatograph/mass selective detector (GC/MSD) system.
Results We reacted benzene with HCl over SiO2, SiO2/CuO, and fly ash to determine whether catalytic chlorination of nonchlorinated benzene is instrumental in the formation of ClxBz from C2 compounds. The gas-phase concentration of benzene during the reaction was a factor of 4 greater than generally used in the C2 reactions. Additionally, if nonchlorinated benzene is formed as a “first aromatic” from the C2 compounds, it is likely present in a different state or has a different relationship with the catalyst than the pure benzene brought into the reaction system with the carrier gas. It therefore would likely react in a different manner. With these variables in mind, based on a single set of results of the benzene reactions, one cannot verify the role that nonchlorinated benzene might play in the formation of ClxBz from C2 compounds. However, critical information for use in further investigations may be obtained.
FIGURE 1. ClxBz isomer patterns from 600 °C reactions of benzene/ HCl with various catalysts; (a) n ) 1; (b) dx(CuO) ) 26%, n ) 2; (c) dx(flyash) ) 25%, n ) 2. The observation of the lower chlorinated homologues (Figures 1 and 2), as expected in the direct chlorination of benzene, does not correspond with the results obtained on CuO or fly ash in the C2 reactions. A notable observation in the benzene reactions over CuO and fly ash is the change in the relative ratios of 1,3,5-Cl3Bz:1,2,3-Cl3Bz. The ratios are reversed in the two samples, with 1,3,5-Cl3Bz < 1,2,3-Cl3Bz in the CuO sample and > 1,2,3-Cl3Bz in the fly ash sample. In the C2 reactions, as a rule, 1,3,5-Cl3Bz was not detected or
VOL. 31, NO. 2, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
543
FIGURE 3. Cluster analysis dendrogram (SPSS Software) comparing Al2O3-catalyzed reactions with CuO and fly ash (Ac ) C2H2, Eth ) C2H4, E ) C2H6, Bz ) C6H6, TCE ) C2HCl3, Fa ) fly ash). Two primary groupings are formed, separating Al2O3 from the CuO and fly ash reactions (as expected) and confirming the similarity of the SiO2catalyzed benzene/HCl reaction with the Al2O3 patterns. The Al2O3catalyzed TCE reaction (17) was not grouped together with the remaining Al2O3 samples because of the dominant production of Cl5Bz and HCB. The remaining TCE reactions were not included in the comparison for this reason.
FIGURE 2. ClxPh isomer patterns from 600 °C reactions of benzene/ HCl with various catalysts; (a) n ) 1; (b) dx(CuO) ) 19%, n ) 2; (c) dx(flyash) ) 19%, n ) 2. was observed only as a minor peak, as in the CuO and SiO2 reactions with benzene. Since different chlorination mechanisms are required to produce the different Cl3Bz isomers, specific component(s) of the fly ash must be responsible for this effect. The fact that 1,3,5-Cl3Bz was not observed as a dominant Cl3Bz congener in the C2 reactions decreases the likelihood of nonchlorinated benzene as a primary intermediate in the fly ash-catalyzed formation of aromatic compounds from aliphatic units. In contrast, the SiO2 and CuO patterns are very similar to those obtained in C2H2/HCl reactions over SiO2 (18) and all C2 reactions over Al2O3. In the ClxPh products, the isomer pattern in the SiO2 reaction is similar to those obtained in the C2 reactions over Al2O3, providing further support for the hypothesis that nonchlorinated benzene might be formed as a critical intermediate in the Al2O3 reactions. Reactions with HCB over SiO2, SiO2/CuO, and fly ash were completed to test whether a fully chlorinated benzene might be the “first aromatic” formed in the C2 reactions, followed by catalytic dechlorination to account for the characteristic isomer distribution patterns. As in the reactions with benzene, one must keep in mind that HCB introduced intact into the reaction system will interact differently with the catalytic surface than HCB that might form in the course of catalytic C2 reactions. There is nearly equal distribution of HCB between the products eluted from the Carbotrap and those eluted from the catalytic material. The total amount of HCB recovered in the CuO sample is approximately one-third to one-half of the total amounts observed in the SiO2 or fly ash samples, suggesting that CuO causes a higher rate of HCB decomposition. Quantifiable amounts of Cl4Bz and Cl5Bz as dechlorination products were detected in the CuO and fly ashcatalyzed reactions. For ClxPh production on CuO and fly ash, 2,4,6-Cl3Ph and PCP are present in similar relative amounts (2,4,6-Cl3Ph:PCP ratios of 0.32 and 0.2, respectively), whereas 2,4,6-Cl3Ph is absent in the SiO2 sample. PCP was
544
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 2, 1997
detected in essentially the same amounts (SiO2: 1.6 × 107, CuO: 1.4 × 107, fly ash: 1.1 × 107); therefore, no direct catalytic effect by fly ash or CuO is apparent. Overall, the results from the HCB reactions indicate that dechlorination of HCB alone is not responsible for the ClxBz and ClxPh congener patterns observed in the C2 reactions. Statistical Comparison of Results. We compared the isomer distribution patterns of the various reactions using a hierarchical cluster analysis method (SPSS Software). This method compares the groups of data and orders them in clusters of nearest similarities. We employed a “between neighbors” analysis using squared Euclidean distance (24). Cluster analysis (Figure 3) of the congener distribution patterns (normalized to total ClxBz and ClxPh) further verified the relationship between the benzene + SiO2 reaction and the C2Hx + Al2O3 reactions, and their differences from the CuO and fly ash reactions.
Mechanistic Considerations Background. Under heterogeneous combustion conditions, one must consider gas-surface and surface-surface interactions in addition to those mechanisms outlined above. The role of surface catalysis in promoting radical species formation is potentially a critical factor in the gas-phase formation of aromatic and chlorinated aromatic species from C1 and C2 hydrocarbons. Recently, many heterogeneous reactions of the C1-C4 hydrocarbons have been investigated regarding their relevance to environmentally important processes such as municipal waste incineration (8, 25, 26). Additionally, through theoretical (modeling) or experimental investigations of the formation of PCDD/F in combustion systems, many investigators have recognized the need to examine fast heterogeneous reactions in order to match observed rates of dioxin formation with those predicted from gas-phase reactions or long-residence time reactions/slow heterogeneous reactions (3, 9, 27, 28). If HCl is present in a combustion system, various equilibrium reactions may occur to provide chlorination agents for reactions with PICs (3, 21, 22, 29, 30). For example, the Deacon process can take place to provide Cl2 for the reaction environment.
2HCl + 2O2 h H2O + Cl2
Deacon process
Above 400 °C, Cl2 will dissociate into Cl radicals. Thus, aside from HCl and Cl2 addition reactions, radical chlorination reactions are possible at each step in the pathways listed above. The resultant mono- and polychloroaliphatics may undergo thermal reactions. Such reactions are reviewed and considered in some detail (3). Chloroacetylenes are likely to
be formed as fundamental decomposition intermediates (3) and were observed in a number of reactions in our study. They subsequently undergo condensation type reactions, ultimately leading to chlorinated and nonchlorinated aromatics. The work by Aubrey and van Wazer (31) provides important background information for the thermal decomposition of chlorinated Cx aliphatics (where x > 2) and highlights the significance of dichloroacetylene. From results obtained in the 350 °C pyrolysis of octachloropropane (C3Cl8), they proposed a radical disproportionation mechanism involving chlorinated radical and olefinic intermediates (eqs i-v) (1, 31):
of formation of benzene-type compounds was based on a 1,3-butadienyl radical (•C4H5) reacting with an acetylenic molecule. The possible key reactions leading to the formation of the butadienyl radical in the C2 system are given in eqs 1-3, and the proposed sequence of radical propagation and condensation is provided in eqs 4-6.
C2H3 + •C2H3 h C4H6
(1)
C4H6 + •H h •C4H5 + H2
(2)
C2H3 + C2H2 h •C4H5
(3)
C4H5 + C2H2 h •C6H7
(4)
C6H7 h c-•C6H7
(5) (6)
•
•
C3Cl8 h •C2Cl5 + •CCl3
(i)
C2Cl5 + •CCl3 h C2Cl4 + CCl4
(ii)
2C2Cl4 h C2Cl2 + C2Cl6
(iii)
c-•C6H7 h C6H6 + •H
C2Cl4 + C2Cl6 h C2Cl2 + 2CCl4
(iv)
3C2Cl2 h C6Cl6
(v)
This radical reaction sequence could perhaps occur in a parallel way with chlorinated aliphatic compounds. Such parallel comparisons, however, must be considered cautiously since it is not clear whether chlorinated aliphatic radicals, especially perchlorinated compounds, will behave in a similar manner under radical reaction conditions. The presence of dichloroacetylene in the radical reaction mechanisms, parallel to eq 3, is supported by our observation of dichloroacetylene in many of the C2 reactions (35) and in the thermal decomposition of octachloropropane (31). Aubrey and van Wazer (31) also proposed dichloroacetylene as an intermediate. Radical dimerization of C3 aliphatics may also account for some ring formation. Mulder and Jarmohamed (36), through propene/HCl reactions over fly ash, suggest that benzene ring formation occurs preferentially via dimerization reactions of C3 aliphatics and that C2 aliphatics play only a minor role, if any. Trimerization and Fischer-Tropsch Synthesis. The condensation of three acetylenic species to form a benzene molecule is perhaps related to the radical propagation and condensation pathway; however, this process is likely to be more dependent on interaction with a suitable catalytic surface (37). We assume that radical gas-phase/adsorbedphase chlorination may occur simultaneously with the trimerization process. The trimerization of chlorinated acetylenes likely occurs as well. As proposed by Aubrey and van Wazer (31) for the thermal decomposition mechanism of octachloropropane, dichloroacetylene could trimerize to form HCB. Spontaneous, noncatalyzed trimerization of dichloroacetylene has also been observed (38). Heterogeneous Fischer-Tropsch polymerization processes may also occur. Acetylenic or vinylic intermediates, observed in C2 reactions, chemisorb to the fly ash or catalyst surface and undergo propagation reactions with other surfaceadsorbed species (39, 40). Catalyst-specific interactions may account for the difference in the catalytic effect observed with Al2O3 from those seen in the CuO and fly ash reactionssi.e., Al2O3 inhibits chlorination of chemisorbed aliphatic units whereas Cu (in CuO or fly ash) promotes aliphatic chlorination. Diels-Alder Reactions. In the C2 thermal system, butadiene could combine with ethylene or acetylene in DielsAlder type reactions to eventually form benzene. The reactions should also occur with the intermediate compounds at various stages of chlorination, leading directly to polychlorinated benzenes. Hexachloro-1,3-butadiene was observed in many of the reactions and supports the possibility of such a reaction occurring. A Diels-Alder condensation with hexachloro-1,3-butadiene and chloroacetylene or dichloroacetylene should lead to Cl5Bz and HCB. It should be noted
•
Potential Pathways from C2 Aliphatics to Aromatics. It is clear that no single mechanism or reaction pathway can be accountable for the formation of aromatic molecules from aliphatic precursors in a heterogeneous combustion system. Selected pathways and potential mechanisms that we believe are instrumental are discussed briefly. Our proposed overall pathways for the formation of ClxBz and ClxPh in heterogeneous combustion reactions of C2 compounds are shown in Figure 4. These pathways build upon the observations made from heterogeneous combustion reactions of C2 aliphatics, benzene, and HCB. After the initial thermal decomposition of the C2 compounds, the aromatic formation process is divided into two primary catalytic pathways: that promoted by Al2O3 and the effect seen in the CuO and fly ash reactions (Figure 4). In the CuO and fly ash reactions, chlorinated aliphatics were, as a rule, observed as the major gas-phase reaction products. In contrast, the major gas-phase and catalyst-adsorbed products in the Al2O3 reactions were nonchlorinated aromatic ring structures, such as naphthalene, biphenyl, alkyl biphenyls, and alkyl benzenes. These compounds were similar to the major products observed in reactions of benzene/HCl over SiO2. Additionally, the ClxBz and ClxPh isomer patterns of the Al2O3 reactions were remarkably similar to that produced in the benzene/HCl reaction. These results led to the tentative conclusion that Al2O3 promotes the formation of nonchlorinated aromatic structures (through a benzene intermediate), whereas CuO and fly ash catalyze the chlorination of the aliphatic intermediates, leading more directly to the formation of a mixture of chlorinated benzenes. It must be noted that grouping CuO together with fly ash in Figure 4 is not meant to imply that their behavior was identical in all cases, only that a relationship between the two exists and that their behavior was very different from Al2O3. Thermal Decomposition of Aliphatics. A potential pathway for C2 thermal decomposition is highlighted in the top portion of the schematic in Figure 4. Though conceived for methane (16), consideration of potential reactions (3, 16, 32, 33) confirms the plausibility of such a sequence in the thermal decomposition of alkenes and alkynes. The sequence shows particularly how the reaction mechanisms of the C2 aliphatics might be interrelated in a thermal reaction environment. Radical Reactions: Propagation and Condensation. Cole et al. (34) proposed a radical reaction mechanism based on experimental work for the formation of benzene rings in the flame combustion of 1,3-butadiene with various acetylenic species. The mechanism that best explains the observed rate
•
•
VOL. 31, NO. 2, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
545
FIGURE 4. Proposed overall pathway for the formation of ClxBz and ClxPh in 600 °C heterogeneous combustion reactions of C2 aliphatics. that hexachloro-1,3-butadiene is likely one of the most stable forms of the butadienes; therefore, it will be the most noticeable in the reaction products. Nevertheless, such
546
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 2, 1997
intermediates at varying degrees of chlorination were observed in some reactions (18, 19). A Diels-Alder type reaction may explain the formation of C6Cl8, which was tentatively
identified as octachloro-1,4-cyclohexadiene. A 4+2 cycloaddition reaction of hexachloro-1,3-butadiene with dichloroacetylene could result in such a structure. Cole et al. (34) determined that the Diels-Alder mechanism is too slow to account for the formation of benzene compounds in an aliphatic flame. Their proposed mechanism is discussed above. Dechlorination of HCB/Chlorination of Benzene. The investigations of HCB and benzene are really somewhat beyond the central problem of aromatic ring formation. However, it is important to determine if one or the other is an obvious first ring in the C2 combustion reactions. The results obtained in the CuO and fly ash reactions did not provide conclusive information; however, some clues do appear. It appears that dechlorination of HCB under oxidative conditions is too slow to account for the observed patterns. Furthermore, it seems that conversion to phenolic type compounds occurs in the HCB/HCl system more readily than dechlorination, i.e., under these conditions oxidation preferentially takes place. On the other hand, the combustion of benzene/HCl over SiO2 or CuO produced a ClxBz pattern surprisingly similar to that observed for all of the C2Hx + Al2O3 reactions and for ethylene and acetylene reactions over SiO2. For the ClxPh results of the benzene reactions, the pattern produced over CuO no longer resembles that of the SiO2 samplesa different reaction process occurs in this case. Nevertheless, the fact that the SiO2 results resemble those from C2Hx + Al2O3 in both ClxBz and ClxPh products suggests that nonchlorinated benzene might be an important intermediate in the mechanism of formation of ClxBz and ClxPh from nonchlorinated C2 aliphatics. A thorough mechanistic investigation of this aspect of the reaction is required.
Conclusions Based on observations from heterogeneous combustion reactions of C2 aliphatics, we have proposed a set of reaction pathways that describe the potential pathways of ClxBz and ClxPh formation in combustion systems, contributing to a fundamental understanding of the formation of aromatic compounds during post-combustion reactions. We provided evidence to support an interrelated thermal reaction mechanism for the formation of simple aromatics from C2 aliphatics. With respect to waste incineration, we confirmed the importance of Cu in the fly ash matrix for the formation of ClxBz and ClxPh. We also demonstrated the unique catalytic action of Al2O3 on the formation of aromatics from aliphatics. Such information is critical to initiate further understanding of the formation of persistent aromatic hydrocarbons in combustion systems.
Acknowledgments We would like to thank the Natural Science and Engineering Research Council of Canada for their financial support of K.L.F. through this project. We also thank D. Lenoir, A. Hauk, and H. Kaupp and the Journal reviewers for their constructive criticisms during the preparation of this manuscript.
Literature Cited (1) Choudhry, G. G.; Hutzinger, O. Mechanistic aspects of the thermal formation of halogenated organic compounds including polychlorinated dibenzo-p-dioxins; Current Topics in Environmental and Toxicological Chemistry; Gordon and Breach Science Publishers: New York, 1983. (2) Altwicker, E. R. Sci. Total Environ. 1991, 104, 47-72. (3) Tsang, W. Combust. Sci. Technol. 1990, 74, 99-116.
(4) Hagenmaier, H.; Kraft, M.; Brunner, H.; Haag, R. Environ. Sci. Technol. 1987, 21, 1080-1084. (5) Vogg, H.; Stieglitz, L. Chemosphere 1986, 15, 1373-1378. (6) Stieglitz, L.; Zwick, G.; Beck, J.; Roth, W.; Vogg, H. Chemosphere 1989, 18, 1216-1226. (7) Stieglitz, L.; Vogg, H.; Zwick, G.; Beck, J.; Bautz, H. Chemosphere 1991, 23, 1255-1264. (8) Milligan, M. S.; Altwicker, E. R. Environ. Sci. Technol. 1995, 29, 1353-1358. (9) Milligan, M. S.; Altwicker, E. Environ. Sci. Technol. 1993, 27, 1595-1601. (10) Addink, R.; Olie, K. Environ. Sci. Technol. 1995, 29, 1425-1435. (11) Karasek, F. W.; Dickson, L. C. Science 1987, 237, 754-756. (12) Dickson, L. C.; Lenoir, D.; Hutzinger, O. Environ. Sci. Technol. 1992, 26, 1882-1888. (13) Milligan, M. S.; Altwicker, E. R. Environ. Sci. Technol. 1996, 30, 225-229. (14) Altwicker, E. R.; Kumar, R.; Konduri, N. V.; Milligan, M. S. Chemosphere 1990, 20 (10-12), 1935-1944. (15) Chigier, N. Energy, Combustion, and Environment; McGrawHill Book Company: New York, 1981. (16) Hucknall, D. J. Chemistry of Hydrocarbon Combustion; Chapman and Hall Ltd.: London, 1985. (17) Froese, K. L.; Hutzinger, O. Chemosphere 1994, 28, 1977-1987. (18) Froese, K. L.; Hutzinger, O. Environ. Sci. Technol. 1996, 30, 9981008. (19) Froese, K. L.; Hutzinger, O. Environ. Sci. Technol. 1996, 30, 10091013. (20) Ballschmiter, K.; Swerev, M. Fresenius Z. Anal. Chem. 1987, 328, 125-127. (21) Bruce, K. R.; Beach, L. O.; Gullett, B. K. Waste Manage. Res. 1991, 11, 97-102. (22) Gullett, B. K.; Bruce, K. R.; Beach, L. O. Waste Manage. Res. 1990, 8, 203-214. (23) Bruce, K. R.; Gullett, B. K.; Beach, L. O. Copper-based organic catalysis in formation of PCDD/PCDF in municipal and hazardous waste incineration. In Proceedings of the 1991 Incineration Conference, May 13-17, 1991; EPA/RREL: Knoxville, TN, 1991. (24) Backhaus, K.; Erichson, B.; Plinke, W.; Schuchard-Ficher, Chr.; Weiber, R. Multivariate Analysemethoden: Eine anwendungsorientierte Einfu ¨ hrung; Springer-Verlag: Berlin, 1989. (25) Stromberg, B. Chemosphere 1991, 23, 5151-1525. (26) Eklund, G.; Pedersen, J.; Stromberg, B. Chemosphere 1988, 17, 575-589. (27) Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983, 17, 721730. (28) Faengmark, I.; van Bavel, B.; Marklund, S.; Stromberg, B.; Berge, N.; Rappe, C. Environ. Sci. Technol. 1993, 27, 1602-1610. (29) Hoffman, R. V.; Eiceman, G. A.; Long, Y.-T.; Collins, M. C.; Lu, M.-C. Environ. Sci. Technol. 1990, 24, 1635-1641. (30) Beard, A.; Naikwadi, K. P.; Karasek, F. W. Environ. Sci. Technol. 1993, 27, 1505-1511. (31) Aubrey, N. E.; Van Wazer, J. R. J. Am. Chem. Soc. 1964, 86, 43804383. (32) Gardiner, W. C., Jr. Combustion Chemistry; Springer-Verlag: New York, 1984. (33) Warnatz, J. Rate coefficients in the C/H/O system. In Combustion Chemistry; Gardiner, W. C., Jr., Ed.; Springer-Verlag: New York, 1984; Chapter 5, pp 197-360. (34) Cole, J. A.; Bittner, J. D.; Longwell, J. P.; Howard, J. B. Combust. Flame 1984, 56, 51-70. (35) Froese, K. L. Pathways in the formation of chlorinated aromatic compounds through heterogeneous combustion reactions of C2-hydrocarbons: Dissertation, University of Bayreuth, Bayreuth, Germany, 1994. (36) Mulder, P.; Jarmohamed, W. Organohalogen Compd. 1993, 11, 273-276. (37) March, J. Advanced Organic Chemistry, 4th ed.; John Wiley & Sons, Inc.: New York, 1992. (38) Ott, E.; Dittus, G. Chem. Ber., 1943, 76, 80-84. (39) Freemantle, M. Chem. Eng. News 1996, 77 (33), 31-33. (40) Maitlis, P.; Long, H. C.; Quyoum, R.; Turner, M. L.; Wang, Z.-Q. Chem. Commun. 1996, 1, 1-8.
Received for review May 13, 1996. Revised manuscript received September 13, 1996. Accepted September 16, 1996.X ES960425E X
Abstract published in Advance ACS Abstracts, November 15, 1996.
VOL. 31, NO. 2, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
547
