Yields of Polycyclic Aromatic Hydrocarbons from the Pyrolysis of

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Yields of Polycyclic Aromatic Hydrocarbons from the Pyrolysis of Catechol [ortho-Dihydroxybenzene]: Temperature and Residence Time Effects Nathan D. Marsh, Elmer B. Ledesma, Alyssa K. Sandrowitz, and Mary J. Wornat* Princeton University, Department of Mechanical and Aerospace Engineering, Princeton, New Jersey 08544-5263 Received November 6, 2001

To understand in more detail the formation of polycyclic aromatic hydrocarbons (PAH) from complex fuels, we have performed pyrolysis experiments in a laminar-flow reactor, using the model fuel catechol (ortho-dihydroxybenzene), a phenol-type compound representative of structural entities in tobacco, coal, and wood. Catechol pyrolysis at temperatures of 700 to 1000 °C and residence times of 0.4 to 1 s produces a range of aromatic products, which have been analyzed by gas chromatography with flame-ionization detection and by high-pressure liquid chromatography with diode-array ultraviolet-visible absorption detection. Of the 64 aromatic products identified, yields are reported for the 30 species whose yields are at least 0.005% of the mass of fed catechol, over a significant temperature range. The quantified products fall into 8 structural categories: benzene, benzenoid PAH, indene benzologues, fluoranthene benzologues, cyclopenta-fused PAH, ethynyl-substituted aromatics, alkyl-substituted aromatics, and vinyl-substituted aromatics. In general, the more prominent products within a particular structural class are more prominent at all temperatures examined, and the most prominent product in a class is usually 10 times more prevalent than other compounds in the same class. The product quantifications show that at 900-950 °C, the aromatic products account for up to 22% of the mass of fed catechol. At these higher temperatures, and all the way up to 1000 °C, there are also more quantifiable products that are the result of multiple ring-buildup steps (e.g., the larger benzenoid PAH) as well as an increase in the number and relative quantity of ethynyl-aromatics and cyclopenta-fused PAH. At the lower temperatures, indene is produced at especially high yield, indicative perhaps of a particular facility for the formation of indene from catechol. Also readily formed from catechol is benzene, the aromatic product of highest yield for the entire temperature range investigated. Experiments at different residence times show that at 800 °C the 0.4-1 s time interval is one of mostly increasing yields; at 1000 °C this same span of residence times sees mostly decreasing yields. The data reported here represent one of the most extensive quantifications of aromatic products from any fuel, and the only one for catechol.

Introduction In the practical combustion of solid organic fuels such as coal, wood, biomass, and tobacco, each solid material is burned in a diffusion flame configuration, leading to high-temperature, oxygen-deficient zones where the solid fuel undergoes devolatilization and the gasified volatiles undergo pyrolysis reactions. In combustion systems, pyrolysis reactions are recognized as the primary source of polycyclic aromatic hydrocarbons (PAH), many of which are known1 to exhibit carcinogenic and mutagenic activity. It is therefore desirable to determine both qualitatively and quantitatively the distribution of PAH products from solid organic fuels, because the inherent chemical structures of these fuels, coupled with their fuel-rich pyrolysis environments, * Author to whom correspondence should be addressed at Louisiana State University, Department of Chemical Engineering, South Stadium Drive, Baton Rouge, Louisiana 70803. Tel: (225)-578-7509. Fax: (225)578-1476. E-mail: [email protected]. (1) Durant, J. L.; Busby, W. F., Jr.; Lafleur, A. L.; Penman, B. W.; Crespi, C. L. Mutation Res. 1996, 371, 123-157.

make them particularly prone to PAH production. Furthermore, sufficiently detailed quantification of PAH products from any fuel or configuration can give insight into specific chemical pathways for their formation. However, because solid organic fuels are in fact complex mixtures containing a variety of weakly bound chemical units, it is extremely difficult to attribute specific products to specific chemical pathways and source compounds. This difficulty is surmounted by using model compounds,2-18 pure compounds that are (2) Wornat, M. J.; Sarofim, A. F.; Lafleur, A. L. Proc. Combust. Inst. 1992, 24, 955-963. (3) Wornat, M. J.; Lafleur, A. L.; Sarofim, A. F. Polycyclic Aromatic Compd. 1993, 3, 149-161. (4) Marsh, N. D.; Zhu, D.; Wornat, M. J. Proc. Combust. Inst. 1998, 27, 1897-1905. (5) Bruinsma, O. S. L.; Moulijn, J. A. Fuel Proc. Technol. 1988, 18, 213-236. (6) Badger, G. M. Prog. Phys. Org. Chem. 1965, 3, 1-40. (7) Badger, G. M.; Donnelly, J. K.; Spotswood, T. M. Australian J. Chem. 1964, 17, 1147-1156. (8) Bruinsma, O. S. L.; Tromp, P. J. J.; deSauvage Nolting, H. J. J.; Moulijn, J. A. Fuel 1988, 67, 327-333. (9) Kinney, C. R.; Del Bel, E. Ind. Eng. Chem. 1954, 46, 548-556.

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either structurally similar to molecular units in more complex fuels, or significant primary pyrolysis products of these fuels. We have chosen catechol (ortho-dihydroxybenzene) as a model compound for solid organic fuels for four reasons: (1) catechol is a structural unit in flavonoids such as rutin and quercetin, as well as in chlorogenic acidscompounds frequently found in leafy plant material;19,20 (2) catechol is also representative of structures found in coal,21 as well as lignin, a major component of wood;22 (3) catechol has been identified as the most abundant phenolic compound in tobacco smoke;23 and (4) the existing aromatic ring in catechol, as in other phenolic compounds, is more likely to contribute to aromatic products than are purely aliphatic constituents. We expect, therefore, that catechol pyrolysis should characterize much of the production of aromatics by the more complex fuels. To gain understanding of both catechol decomposition and the subsequent production of PAHseven to the point of developing and testing kinetic modelssit is instructive to examine the effects of pyrolysis temperature and residence time on the pyrolysis product distributions. To this end, we have carried out catechol pyrolysis experiments in a laminar flow reactor at wellcontrolled temperatures and residence times that are relevant to practical applications of tobacco, coal, and wood. The aromatic products are quantified by gas chromatography (GC) with flame-ionization detection (FID) as well as by high-pressure liquid chromatography (HPLC) with diode-array ultraviolet-visible (UV) absorption detection. Due to the multi-faceted nature of our study and the plethora of species produced from catechol pyrolysis, our results to date are reported in four papers. The first24 documents the identification of over sixty aromatic products of catechol pyrolysissfifty of which had never before been identified as pyrolysis products of any pure phenol-type compound. The second25 reports the temperature-dependent yields of twelve C1 to C6 products of catechol’s thermal decomposition and suggests how these small hydrocarbons and their radicals may par(10) Lang, K. F.; Buffleb, H. Chem. Ber. 1957, 90, 2894-2898. (11) Lang, K. F.; Buffleb, H.; Kalowy, J. Chem. Ber. 1957, 90, 28882893. (12) Lang, K. F.; Buffleb, H.; Kalowy, J. Chem. Ber. 1960, 93, 303309. (13) Zander, M.; Haase, J.; Dreeskamp, H. Erdo¨ l Kohle Erdgas Petrochemie 1982, 35, 65-69. (14) Lewis, I. C.; Edstrom, T. J. Org. Chem. 1963, 28, 2050-2057. (15) Wornat, M. J.; Vriesendorp, F. J. J.; Lafleur, A. L.; Plummer, E. F.; Necula, A.; Scott, L. T. Polycyclic Aromatic Compd. 1999, 13, 221-240. (16) Wornat, M. J.; Mikolajczak, C. J.; Vernaglia, B. A.; Kalish, M. A. Energy Fuels 1999, 13, 1092-1096. (17) Wornat, M. J.; Ledesma, E. B. Polycyclic Aromatic Compd. 2000, 18, 129-147. (18) Marsh, N. D.; Wornat, M. J. Proc. Combust. Inst. 2000, 28, 2585-2592. (19) Stedman, R. L. Chem. Rev. 1968, 68, 153-207. (20) Bimer, J.; Given, P. H.; Raj, S. Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symposium Series 71; American Chemical Society: Washington, DC, 1978; pp 86-99. (21) Lynch, B. M.; Durie, R. A. Australian J. Chem. 1960, 13, 567581. (22) Lee, S. Alternative Fuels; Taylor & Francis: Washington, DC, 1996. (23) Hoffmann, D.; Hoffmann, I. Beitra¨ ge zur Tabakforschung International 1998, 18, 49-52. (24) Wornat, M. J.; Ledesma, E. B.; Marsh, N. D. Fuel 2001, 80, 1711-1726. (25) Ledesma, E. B.; Marsh, N. D.; Sandrowitz, A. K.; Wornat, M. J. Proc. Combust. Inst. 2002, 29, 2299-2306.

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ticipate in ring-growth reactions leading to PAH. The present paper reports detailed product yields, as functions of pyrolysis temperature and residence time, of the PAH (and certain monocyclic aromatics) produced from catechol pyrolysis. The fourth paper26 presents the firstorder global kinetic rate parameters for the formation of PAH from catecholsboth for individual species as well as for PAH grouped by structural class and by ring number. It is the purpose of the present paper to examine the effects of pyrolysis temperature and residence time on the yields of PAH and monocyclic aromatics produced by catechol pyrolysis. In the following, we describe the pyrolysis experiments and the quantification of the aromatic products. We report the yields of thirty PAH and monocyclic aromatics across the temperature range of 700-1000 °C, at a fixed residence time of 0.4 s. (We have determined that catechol begins to decompose at 500 °C,25 but that PAH production does not become significant until 700 °C.) In addition, we present the yields of the most prominent aromatics as a function of residence time at the fixed temperatures of 800 °C and 1000 °C. We discuss the trends observed, as well as the implications for detailed reaction kinetics. Experimental Equipment and Procedures The catechol pyrolysis experiments are performed in a laminar flow reactor shown in schematic form in Figure 1.27 The system consists of three parts, indicated in Figure 1: a fuel vaporizer, an isothermal quartz reactor where the actual pyrolysis takes place, and a collection system for condensedphase products. The reactor has been designed so that laminar flow is maintained and so that the flow conditions meet Lee’s criteria28 for idealized plug flow, allowing the attribution of a specific residence time, without any corrections for mixing. Each of the componentssthe fuel vaporizer, the reactor, and the product collection systemsis described below. Fuel Vaporizer. Catechol (>99.5% pure, purchased from Aldrich Chemical Co.), a solid powder at room temperature, is loaded into a Pyrex tube capped at both ends by a stainless steel mesh that prevents the solid material from being entrained in the gas flow. Installed in an isothermal oven (the fuel vaporizer), the fuel tube is plumbed to the inlet of the quartz reactor tube, as shown in Figure 1. The oven is run at 85 °C in order to provide a small amount of catechol vapor, which is taken up by an ultrahigh purity (grade 6.0) nitrogen stream, resulting in a reactor feed gas that is 0.7 mol-% carbon. The fuel vaporizer also has a bypass line, also shown in Figure 1, so that catechol-free nitrogen can be run through the reactor before and after an experiment. Reactor. The reactor section itself is a 4-ft length of 2-mm I.D. quartz tubing supported in the centerline of an electrically heated Lindberg/Blue tube furnace. The electric power is supplied in three independent zones along the length of the furnace, each separately regulated to ensure a uniform temperature throughout the reactor length. As shown in Figure 1, the fore end of the reactor tube passes through a heated insulating plug, into the adjacent fuel vaporizer; the other end passes through a heated insulating plug and attaches to the collection system. The furnace has been calibrated between 500 and 1000 °C with a thermocouple placed at various (26) Ledesma, E. B.; Marsh, N. D.; Sandrowitz, A. K.; Wornat, M. J. Energy Fuels 2002, 16, 1331-1336. (27) Ledesma, E. B.; Marsh, N. D.; Wornat, M. J. Poster presentation 2-E14 at the Twenty-Eighth International Symposium on Combustion, Edinburgh, Scotland, August, 2000. (28) Lee, J. C. Y. Ph.D. Thesis, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, 1996.

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Figure 1. Schematic of the laminar flow reactor system. Table 1. Quantified Products of Catechol Pyrolysis

a

These compounds are quantified by GC-FID.

locations along the centerline to ensure that the temperature profile is uniform and drops off sharply at both endssso that the reaction zone is well-characterized. Residence time is controlled by varying the flow-rate of nitrogen between 30 and

100 standard cm3 per minute, allowing residence times between 0.4 and 1 s. Product Collection. Benzene, the most volatile of the aromatic products reported here, is quantified by gas chro-

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matographic analysis of catechol pyrolysis products collected in gas sampling bags.25 The rest of the aromatic products reported here are collected in a condensed-phase product collection system, which consists of three parts in series: an unheated, removable quartz arm on which a majority of the products condense, a Balston Teflon filter on which some lighter products and any particulates are collected, and a dichloromethane (DCM) solvent trap intended to capture the lightest condensed-phase products. The masses of the condensed-phase products (including any unconverted catechol) collected from these three pieces, along with the masses of the gas-phase products (reported elsewhere25), account for all of the mass of catechol fed to the reactor. A mass balance is thus closed on this reactor system. Procedure. A pyrolysis experiment consists of heating the reactor to the desired temperature while flushing thoroughly with nitrogen, then switching the fuel vaporizer from the bypass to the fuel chamber. Fuel is fed into the reactor for 75 min, followed by five minutes of nitrogen purge. At the conclusion of an experiment, the three components of the product-collection systemsthe quartz arm, Balston filter, and DCM trapsare removed and thoroughly flushed with DCM. The quartz arm is also sonicated in DCM; the filter is three times filled with DCM, sonicated, and flushed. All of the DCM product solutions from these steps are consolidated, and 10% of the total volume (representing 10% of the total products collected) is set aside for GC analysis.26 The remaining 90% is prepared for HPLC analysis by concentration in a KudernaDanish evaporator, followed by solvent-exchange into dimethyl sulfoxide (DMSO). In this final step, the remaining DCM is evaporated under a stream of nitrogen. The DMSO product solution is analyzed by HPLC on a Hewlett-Packard Model 1050 high-pressure liquid chromatograph, coupled to a diode-array UV detector. The separation method2,3 uses a reverse-phase Vydac 201-TP octadecylsilica column, with a solvent program beginning with a 60:40 mixture of water and acetonitrile, ramped over 40 min to 100% acetonitrile, then ramped over an additional 40 min to DCM. The recorded spectra from the UV detector are used to identify sample products; in previous work24 we have documented the identification of the products discussed here. Products can usually be detected and successfully identified at yields as low as 0.0005% of the mass of fed catechol, when they are not masked by a more prominent coeluting compound. For purposes of examining yield trends with temperature, we report here only those species whose yields are at least 0.005% of catechol fedswhich are those whose quantifications are the most reliable. For most of these compounds, the response of the detector has been calibrated using solutions of known concentrations of these compounds. In the few cases where the compound is not readily available to us in pure form, we have used calibrations for structurally similar compounds, a technique that introduces little error.29 Experiments are conducted at 10 different temperatures over a range of 500-1000 °C, and at four residence times over a range of 0.4-1 s. HPLC and GC analyses of the products from repeat experiments at the same pyrolysis temperature and residence time demonstrate that the reactor system, product workup procedures, and product analysis techniques all give very reproducible results.

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Figure 2. Temperature dependence of unsubstituted PAH yields from the pyrolysis of catechol at 0.4 s residence time. Individual PAH yields summed by structural class.

Results and Discussion Collectively, our analyses24-26 of the catechol pyrolysis products have led to the identification of 11 C1 to C6 nonaromatic light gases and 64 individual aromatic species. The aromatic products comprise 11 structural classes: benzene, benzenoid PAH, indene benzologues,

Figure 3. Temperature dependence of substituted PAH yields from the pyrolysis of catechol at 0.4 s residence time. Individual PAH yields summed by structural class.

(29) Lafleur, A. L.; Monchamp, P. A.; Plummer, E. F.; Wornat, M. J. Anal. Lett. 1987, 20, 1171-1192.

fluoranthene benzologues, cyclopenta-fused PAH (CPPAH), ethynyl-substituted aromatics, vinyl-substituted



Figure 4. Temperature dependence of yields of benzene and common PAH from the pyrolysis of catechol at 0.4 s residence time. Benzene, naphthalene, phenanthrene, anthracene, fluoranthene, and pyrene. Yields of benzene and naphthalene are indicated on the right-hand axis.

Figure 5. Temperature dependence of yields of indene and indene benzologues from the pyrolysis of catechol at 0.4 s residence time. Indene, fluorene, benz[f]indene, benzo[a]fluorene. Yields of indene are indicated on the right-hand axis.

aromatics, methyl-substituted aromatics, bi-aryls, oxygenated aromatics, and aromatics with ring oxygen. Of the total 64 identified aromatic products, the 30 that exceed the 0.005% threshold (described in the Experimental Section) are listed in Table 1, along with their structural classifications. The yields of these species, collectively and/or individually, are presented as functions of temperature in Figures 2-9. Temperature Effects. Figure 2 summarizes the yields of the unsubstituted aromatic classes, expressed as mass percent of fed catechol, from 700 to 1000 °C. We have presented benzene separately because of its large yield and because its behavior with respect to temperature does not coincide with that of the larger benzenoid PAH, as can be seen in the figure. Also evident in Figure 2 is that benzene and the indene benzologues have peak yields around 900 °C; whereas the CP-PAH, fluoranthene benzologues, and benzenoid PAH have peak yields around 950 °C. It should be noted that apart from benzene, at temperatures less than 850 °C, the indene benzologues are the most prominent class of species, whereas at higher temperatures benzene and the benzenoid PAH dominate the product distribution. Furthermore, all the classes follow an exponential increase vs temperature, consistent with pseudo-firstorder global reaction kinetics.26 Finally, at their respective peak yields, each class of PAH accounts for 1-5% of the mass of fed catechol, contributing, at 900 °C, to an overall maximum aromatic product yield of 22% of the mass of fed catechol. Figure 3 shows the behavior of the substituted aromatic classes. We observe that the yields of the alkyl

and vinyl aromatics are nearly identical, both peaking around 900 °C with maximum values of 1.1 and 1.2% of fed catechol, respectively. Ethynyl aromatics, on the other hand, have a considerably higher temperature of peak yield, and also a higher peak yield of 1.8% of fed catechol. Figure 4 presents the yields, versus temperature, of benzene and several commonly observed PAH of two to four rings. As noted earlier from Figure 2 (and here again from Figure 4), benzene yield peaks at 900 °C, at a value of 6.7% of fed catechol. As Figure 4 illustrates, naphthalene accounts for the majority of the yield of benzenoid PAH (3.5% of fed catechol); the next most prominent compound in that class, phenanthrene, has a peak yield of 0.54%. The larger benzenoid PAH appear to have peak yields at temperatures that shift more toward 950 °C. Figure 4 also shows that the yield of fluoranthene, the only nonbenzenoid PAH in Figure 4, closely follows the trends of its isomer pyrene, with respect to both magnitude and temperature dependence. Figure 5 shows the temperature profiles for indene and the indene benzologues. As with the benzenoid PAH, the most common compound, in this case indene, accounts for the majority of collected compounds from the class, 3.1% for indene vs 0.44% for fluorene, the next most prominent compound in the class. Indene appears to peak around 900 °C, and is nearly unique in that it does not exhibit the same exponential trend of increasing yield with temperature. We also observe that although fluorene and benzo[a]fluorene clearly have peak yields at 900 °C, the remaining compound benz[f]indene has a much flatter profile with a peak yield

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Figure 6. Temperature dependence of yields of fluoranthene and fluoranthene benzologues from the pyrolysis of catechol at 0.4 s residence time. Fluoranthene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[j]fluoranthene, indeno[1,2,3-cd]pyrene. Yields of fluoranthene are indicated on the right-hand axis.

Figure 7. Temperature dependence of yields of cyclopentafused PAH from the pyrolysis of catechol at 0.4 s residence time. Acenaphthylene, cyclopenta[cd]pyrene, acephenanthrylene, cyclopent[hi]acephenanthrylene, cyclopenta[cd]fluoranthene, dicyclopenta[cd,jk]pyrene. Yields of acenaphthylene are indicated on the right-hand axis.

between 825 and 900 °C. This contrast may be indicative of different chemical mechanisms resulting in indene and benz[f]indene vs fluorene and benzo[a]fluorene. Both the fluoranthene benzologues and the CP-PAH, shown in Figure 6 and Figure 7, respectively, behave much more uniformly than the indene benzologues. As Figures 6 and 7 depict, the peak yield of each fluoranthene benzologue and CP-PAH is clearly at 950 °C, followed by a rapid decrease. Previously,15,18,24,30,31 we have suggested that CP-PAH in general are formed by a C2 addition reaction to a parent PAH, followed by ring closure into the five-membered ring. The uniform yield/ temperature behavior followed by the six CP-PAH in Figure 7 lends support to that earlier assertion. However, although fluoranthene and its benzologues also contain five-membered rings, these rings are internal rather than external, and therefore cannot be formed by the same C2 addition reaction as the CP-PAH. The similar profiles of the fluoranthene benzologues and the CP-PAH might alternately be explained by enhanced thermal stability provided by the five-membered ring, common to both classes of structures. In both the fluoranthene benzologue class and the CPPAH class, as with the other classes of aromatic products, the most prominent compound in the class

accounts for a majority of the yield from the class. For instance, fluoranthene, shown in Figure 6, has a maximum yield of 0.36%, compared to the maximum yield of the next most prominent fluoranthene benzologue benzo[b]fluoranthene at only 0.054%. Similarly, acenaphthylene, shown in Figure 7, accounts for most of the total yield of the CP-PAHs2.0% for acenaphthylene, versus the maximum yield of the next most prominent compound cyclopenta[cd]pyrene, which has a maximum yield of 0.31%. The three methylated aromatics shown in Figure 8 also behave very uniformly with temperature. All three have peak yields at 900 °C and follow nearly identical profiles that simply adhere to different scales, with toluene peaking at 0.89% and 2- and 1-methylnaphthalene peaking at 0.13 and 0.070%, respectively. We see virtually the same behavior in Figure 8 for the two vinyl-substituted aromatics, which also peak at 900 °C and have similar temperature profiles. In the case of the vinyl-aromatics, styrene has a maximum yield of 1.2% compared to 0.090% for 2-vinylnaphthalene. As mentioned previously, the ethynyl-aromatics do not follow the trend that the other substituted aromatics do. Except for phenylacetylene (ethynylbenzene), the ethynyl-aromatics (shown in Figure 9) have peak yields at 950 °C. This behavior is very similar to the behavior of the CP-PAH, which would be consistent with our previous argument18 that ethynyl-aromatics result from the same kind of C2 addition reactions that produce CPPAH. The slightly lower-temperature peak of phenyl-

(30) Ledesma, E. B.; Kalish, M. A.; Wornat, M. J.; Nelson, P. F.; Mackie, J. C. Energy Fuels 1999, 13, 1167-1172. (31) Wornat, M. J.; Vernaglia, B. A.; Lafleur, A. L.; Plummer, E. F.; Taghizadeh, K.; Nelson, P. F.; Li, C.-Z.; Necula, A.; Scott, L. T. Proc. Combust. Inst. 1998, 27, 1677-1686.



Figure 8. Temperature dependence of yields of vinyl- and methyl-aromatics from the pyrolysis of catechol at 0.4 s residence time. Styrene, toluene, 2-methylnaphthalene, 2-vinylnaphthalene, 1-methylnaphthalene. Yields of styrene and toluene are indicated on the right-hand axis.

Figure 9. Temperature dependence of yields of ethynylaromatics from the pyrolysis of catechol at 0.4 s residence time. Phenylacetylene, 2-ethynylnaphthalene, 1-ethynylacenaphthylene, 2-ethynylphenanthrene, 3-ethynylphenanthrene. Yields of phenylacetylene are indicated on the right-hand axis.

acetylene may be explained by the large quantity of benzene observed at the lower temperature of 900 °C and its lower yield at 950 °C. It is possible that the prominence of the parent compound (benzene or a phenyl radical) at lower temperatures somewhat offsets the unfavorable energetics of C2 formation or addition. Residence Time Effects. Because the product yields exhibit a strong temperature dependence, and because this dependence results in both increasing and decreasing yields, we also examine the effects of residence time on catechol product yields. Residence time effects for the 7 most prominent PAH are examined at two temperaturessone where yields are all increasing with temperature, 800 °C, and one where yields are all decreasing with temperature, 1000 °C. Figure 10 shows the yields of indene, benzene, naphthalene, toluene, styrene, ethynylbenzene, and acenaphthylene at 800 °C. Residence times lower than 0.4 s are below the operating range of our experimental apparatus, so we have interpolated the yields in Figure 10 down to zero at t ) 0. Although this treatment necessarily fails to capture the early inception of these compounds, it does reveal that the aromatic product yields grow monotonically up to nearly 0.8 s. More significant in Figure 10 is the falloff of some compoundss indene, naphthalene, and toluene particularlysabove 0.8 s. This change in production rate indicates that there are long-time-scale phenomena operating on the order of 1 s. Figure 11 shows the yields of the seven most prominent compounds at 1000 °C: naphthalene, acenaphth-

ylene, phenylacetylene, fluorene, cyclopenta[cd]pyrene, phenanthrene, and 2-ethynylnaphthalene. (Benzene is excluded because of a measurement problem at some residence times.) Figure 11 reveals that within the time interval of 0.4 to 1.0 s, the yields of all the PAH decrease with increasing residence time, so no attempt is made to interpolate the yields down to t ) 0 at 1000 °C. However, we can infer that at this temperature, some time between 0 and 0.4 s the aromatic products reach their peak yields, just as indene, naphthalene, and toluene did at 800 °C and 0.8 s (Figure 10). In other words, by raising the temperature from 800 °C to 1000 °C, the 0.4-1.0 s regime has changed from one of primarily aromatics production to one of primarily aromatics consumption. (It is perhaps worth noting that the only two compounds in Figure 11 whose yields rise slightly in the 0.4- to 0.6-s range are CP-PAH, PAH whose production requires higher temperatures than the other species of Figure 11sas noted earlier from Figure 2.) Reaction Mechanism Implications. Comparison of yield/temperature profiles of different structural classes can advance a qualitative assessment of the respective chemical mechanisms. For example, compounds with higher peak yields at higher temperatures must be more thermally stable than those with lower-temperature peak yields. If the thermally stable compounds have relatively low yields at lower temperatures, compared to other compounds, then they must result from a higher energy formation mechanism that only becomes significant at the higher temperatures. Furthermore, it is

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Figure 10. Yields of the most prevalent aromatics at 800 °C, as functions of residence time. Indene, benzene, naphthalene, toluene, styrene, ethynylbenzene, and acenaphthylene. Yields of benzene and indene are indicated on the right-hand axis.

likely that compounds with virtually identical yield profiles are formed by similar, if not identical, mechanisms. We have identified the following trends in yield vs temperature: the ethynyl- and CP-PAH have their maximum yields at 950 °C and have sharp peaks at that temperature; the vinyl- and methyl-aromatics have their maximum yields at 900 °C and have sharp peaks at that temperature; benzene also has a sharp peak at 900 °C; indene appears to peak at 900 °C, but is also, along with benzene, one of the two most prominent compounds from 700 °C to 850 °C. From these observations, it seems reasonable that the ethynyl- and CP-PAH are formed by similar mechanisms, probably involving C2 addition to a parent PAH.18,32,33 The similarity of the methyl and vinyl aromatic profiles is more perplexing, but implies that either the methyl and vinyl addition reactions are quite similar to one another, or that the larger species in these classes are built up by some other mechanism such as ring growth on an existing substituted aromatic. One fact that is clear is that the temperature of peak yield of the methyl- and vinyl-aromatics is significantly lower than that of the ethynyl- and CP-PAH, indicating that the reactions forming the methyl- and vinyl-aromatics are lower energy reactions, and that these compounds are not thermally stable, i.e., they are more readily consumed as temperature increases. (32) Wang, H.; Frenklach, M. J. Phys. Chem. 1994, 98, 1146511489. (33) Richter, H.; Mayzar, O. A.; Sumathi, R.; Green, W. H.; Howard, J. B.; Bozelli, J. W. J. Phys. Chem. A 2001, 105, 1561-1573.

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Figure 11. Yields of the most prevalent aromatics at 1000 °C, as functions of residence time. Naphthalene, acenaphthylene, phenylacetylene, fluorene, cyclopenta[cd]pyrene, phenanthrene, and 2-ethynylnaphthalene. Yields of acenaphthylene and naphthalene are indicated on the right-hand axis.

The large quantity of benzene is not particularly surprising; it is relatively easy to form from phenol via hydroxyl displacement by H atom,34,35 and similar reaction energetics should apply for its formation from catechol as well.25 The prominence of indene at the lower temperatures also indicates a relatively lowenergy pathway from catechol. Previously,24 we have proposed both CO elimination from naphthoxy radical and propargyl addition to benzene as two plausible facile routes for the formation of indene in the catechol pyrolysis environment. The presence of appreciable levels of propyne and other C3 hydrocarbons,25 in addition to benzene, however, suggests that the latter of these two routes is the one more likely responsible for the prominent yield of indene at lower temperatures. Likewise, propargyl addition to indene and indene benzologues, followed by isomerization, may explain the relatively high yields of ethynyl-PAH observed from catechol, compared to other fuels. Conclusions In order to examine the aromatic products of catechol pyrolysis, we have used a laminar flow reactor to pyrolyze dilute vapor-phase catechol in nitrogen under well-controlled isothermal conditions with well-defined residence times. The collected condensed-phase products (34) Horn, C.; Roy, K.; Frank, P.; Just, T. Proc. Combust. Inst. 1998, 27, 321-328. (35) Brezinski, K.; Pecullan, M.; Glassman, I. J. Phys. Chem. A 1998, 102, 8614-8619.


have been analyzed and quantified by HPLC-UV and GC-FIDsallowing us to generate yield/temperature and yield/residence time profiles for a large number of aromatic pyrolysis products. These data comprise one of only a few extensive sets of PAH product yields,36-41 and furthermore cover a larger number of PAH than the previous works. In addition, the data presented here are potentially more relevant to the longer-time-scale pyrolysis conditions found in some practical applications. We have found that at 900-950 °C the aromatic products account for up to 22% of the mass of fed catechol. At these higher temperatures, and all the way up to 1000 °C, there are a larger number of quantifiable products that are the result of multiple ring-buildup steps (e.g., the larger benzenoid PAH) as well as an increase in the number and relative quantity of ethynyland CP-PAH. This trend is consistent with the supposition that both ring growth and the production of ethynyl- and CP-PAH depend on relatively high temperatures to proceed, and reinforces the significance of the temperature field on product distributions in practical systems. The quantified products fall into 8 structural categories: benzene, benzenoid PAH, indene benzologues, fluoranthene benzologues, cyclopenta-fused PAH, ethynyl-substituted aromatics, methyl-substituted aromatics, and vinyl-substituted aromatics. In general, the more prominent products within a particular structural class are more prominent at all temperatures examined, and the most prominent product in a class is usually 10 times more prevalent than other compounds in the same class. This finding suggests that the most promi(36) Bockhorn, H.; Fetting, F.; Wenz, H. W. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1067-1073. (37) Harris, S. J.; Weiner, A. M.; Blint, R. J.; Goldsmith, J. E. M. Proc. Combust. Inst. 1987, 21, 1033-1045. (38) Harris, S. J.; Weiner, A. M.; Blint, R. J. Combust. Flame 1988, 72, 91-109. (39) Westmoreland, P. R.; Dean, A. M.; Howard, J. B.; Longwell, J. P. J. Phys. Chem. 1989, 93, 8171-8180. (40) Grieco, W. J.; Lafleur, A. L.; Swallow, K. C.; Richter, H.; Taghizadeh, K.; Howard, J. B. Proc. Combust. Inst. 1998, 27, 16691675. (41) Bittner, J. D.; Howard, J. B. Proc. Combust. Inst. 1981, 18, 1105-1116.


nent compound might be used as an indicator for other compounds in the same class that might be present at significantly lower levels but which may be of particular interest due to unique properties, either in reaction mechanisms or as environmental pollutants. Our results reveal that, over the range of conditions investigated, benzene is the highest-yield aromatic product of catechol pyrolysis. We have also found that indene (and to a much lesser extent benz[f]indene) is an unusually prominent product of low-temperature catechol pyrolysis. It is possible that indene’s prominence is a result of a particular facility for the formation of indene by propargyl addition to benzene. Similarly, propargyl addition to indene and indene benzologues may account for the high propensity of catechol to form ethynyl-PAH. By examining the effect of residence time at two temperatures, we have determined that at 800 °C, reactions are occurring, on a time scale of order 1 s, that lead to the net consumption of some of the aromatic products of catechol pyrolysissas evidenced by maxima of indene, naphthalene, and toluene yields occurring at 0.8 s. Furthermore, by raising the temperature to 1000 °C, this time scale is shortened to less than 0.4 s, reinforcing the significance of temperature in the reactions leading to aromatic pyrolysis products. Currently, there is no kinetic model for catechol decomposition or PAH production from catechol. The most extensive general models for PAH growth42,43 have been tested primarily on data from premixed laminar flames burning light hydrocarbons such as ethylene.36-41 The results presented in this work, however, in concert with those from the analyses of gas-phase catechol products,25 should provide critical input for the development of a detailed kinetic model for both pyrolytic catechol decomposition and PAH formation and growth. Acknowledgment. The authors gratefully acknowledge Philip Morris, Inc., and the National Science Foundation for support of this research. EF010263U (42) Wang, H.; Frenklach, M. Combust. Flame 1997, 110, 173-221. (43) Richter, H.; Grieco, W. J.; Howard, J. B. Combust. Flame 1999, 119, 1-22.

Yields of Polycyclic Aromatic Hydrocarbons from the Pyrolysis of

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