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Proceedings of the Combustion Institute 32 (2009) 615–622

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CombustionInstitute

Polycyclic aromatic hydrocarbons from theco-pyrolysis of catechol and 1,3-butadiene

Shiju Thomas, Mary J. Wornat *

Louisiana State University, Department of Chemical Engineering, South Stadium Drive, Baton Rouge, LA 70803, USA

Abstract

In order to better understand the reactions responsible for the formation and growth of polycyclic aro-matic hydrocarbons (PAH) from solid fuels, we have performed pyrolysis experiments in an isothermallaminar-flow reactor (at temperatures of 600–1000 �C and a fixed residence time of 0.3 s) with catechol,a model fuel representative of the aromatic moieties in coal and biomass fuels; 1,3-butadiene, a major prod-uct of biomass pyrolysis; and with catechol and 1,3-butadiene together (in a catechol-to-1,3-butadienemolar ratio of 0.83). No PAH of P3 rings are produced at temperatures <700 �C, but PAH productionbecomes significant at temperatures P800 �C. Analysis of the higher-temperature reaction products byhigh-pressure liquid chromatography with diode-array ultraviolet-visible absorbance detection has led tothe identification of over 100 PAH (ranging in size to 10 fused aromatic rings) – 47 of which have neverbefore been reported as products of any phenol-type fuel. Quantification of the product yields shows thata much higher percentage of fed carbon is converted to PAH in the catechol-only pyrolysis experimentsthan in the 1,3-butadiene-only pyrolysis experiments – a result attributable to catechol’s relatively labileO–H bond and capacity for generating oxygen-containing radicals, which accelerate both fuel conversionand the pyrolysis reactions leading to 1- and 2-ring aromatics and PAH. When the two fuels are co-pyro-lyzed, the percentage of the total fed carbon converting to PAH is more than two times higher than theamount calculated for the hypothetical case of the two fuels together behaving as a linear combinationof the two fuels individually. This elevated production of PAH from the co-pyrolysis experiments reflectsnot only the reaction-accelerating role of the oxygen-containing radicals but also the efficacy, as growthagents, of the C2 – and especially the C4 – species abundantly present in the catechol/1,3-butadiene co-pyrolysis environment.� 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Polycyclic aromatic hydrocarbons; Pyrolysis; Catechol; 1,3-Butadiene

1. Introduction

Within the diffusion flames of solid fuel com-bustion systems, pyrolytic reactions of fuel com-ponents and devolatilization products can lead

1540-7489/$ - see front matter � 2009 The Combustion Institdoi:10.1016/j.proci.2008.05.043

* Corresponding author. Fax: +1 225 578 1476.E-mail address: mjwornat@lsu.edu (M.J. Wornat).

to polycyclic aromatic hydrocarbons (PAH), animportant class of environmental pollutants.Since some PAH are mutagenic [1–3] and/or car-cinogenic [3,4] and since PAH are known precur-sors to soot [5,6], it is important to understandthe pyrolytic reactions responsible for PAHformation.

In our attempts to study the formation of PAHfrom solid fuels, catechol (ortho-dihydroxybenzene)

ute. Published by Elsevier Inc. All rights reserved.

616 S. Thomas, M.J. Wornat / Proceedings of the Combustion Institute 32 (2009) 615–622

has emerged as a very useful model compoundfuel, since: (1) catechol represents aromatic moie-ties within coal [7], wood [8], and other plant-based biomass fuels [9]; (2) catechol is a majorcomponent of biomass tars [10]; and (3) thePAH produced from catechol pyrolysis arecompositionally very similar to those producedfrom coal volatiles pyrolyzed at similar conditions[11].

Our previous studies [11–15] on catechol pyro-lysis and fuel-rich oxidation have investigated theeffects of temperature, residence time, and oxygenconcentration on catechol product compositionand yields. The abundance of C1 to C5 species incatechol’s thermal decomposition products[12,14], the wide variety of compound classes rep-resented in catechol’s product PAH [11,15], andthe high dependence of mutagenic and carcino-genic activity on PAH structure [1–4] haveprompted us to pursue three questions of rele-vance both to catechol pyrolysis and to solid fuelpyrolysis and combustion in general: (1) Which ofthe C1–C5 thermal decomposition products arethe most effective participants in PAH formationand growth? (2) Are there linkages between partic-ular growth species and particular structural clas-ses of PAH? (3) How does temperature influencethe findings for (1) and (2)?

As a start in addressing these questions, for thecurrent study, we have chosen to conduct catecholpyrolysis, 1,3-butadiene pyrolysis, and catechol/1,3-butadiene co-pyrolysis experiments. The rea-sons for choosing 1,3-butadiene (C4H6) are sev-eral-fold: (1) 1,3-butadiene is a product of thepyrolysis and/or combustion of biomass [16,17],wood [18,19], and coal [20,21]; (2) 1,3-butadieneis the highest-yield hydrocarbon product of cate-chol pyrolysis at temperatures <900 �C [12,14];(3) 1,3-butadiene is an important source of theC2 growth species vinyl radical [22] as well as eth-ylene and acetylene [23,24]; (4) 1,3-butadiene hasbeen reported as a major contributor to the for-mation of 1- and 2-ring aromatics [25–28] andPAH [28–30].

The catechol pyrolysis, 1,3-butadiene pyroly-sis, and catechol/1,3-butadiene co-pyrolysis exper-iments are each conducted in an isothermallaminar-flow quartz tube reactor at temperaturesof 600–1000 �C and a fixed residence time of0.3 s. The products are analyzed by gas chroma-tography as well as high-pressure liquid chroma-tography (HPLC) with diode-array ultraviolet-visible (UV) absorbance detection, a techniqueideally suited for the isomer-specific analysis ofPAH. A companion paper [26] reports the resultsof these experiments for each of the C1–C10 prod-ucts; the current paper presents the results for thePAH. In the following, we report the identities ofthe 111 individual products identified by HPLC/UV and compare PAH product yields, as func-tions of temperature, for the different sets of

experiments. Differences in PAH production forthe co-pyrolysis experiments, in comparison tothe pyrolysis experiments with the individualfuels, are discussed in terms of the two fuels’ struc-tures and products of decomposition. Effects onPAH of different ring number and structural classare also examined.

2. Experimental equipment and procedures

The catechol pyrolysis, 1,3-butadiene pyroly-sis, and catechol/1,3-butadiene co-pyrolysis exper-iments are conducted in a reactor system,described elsewhere [11–15,26], that consists of afuel vaporizer, isothermal laminar-flow quartz-tube reactor, and product collection system. Cate-chol pyrolysis experiments are carried out byloading catechol particles (>99.5% pure) into aPyrex tube fixed within the vaporizer, a con-stant-temperature oven held at 85 �C, for slightvaporization of the catechol. A flowing streamof ultra-high purity nitrogen picks up the vapor-phase catechol, resulting in a 0.65 mol% carbonloading in the reactor feed gas. Upon exiting thevaporizer, the vaporized catechol/nitrogen mix-ture enters the quartz tube reactor, which is insu-lated at both ends and maintained at uniformtemperature by a three-zone electrically heatedfurnace. The reactor is operated at temperaturesof 600–1000 �C and at a fixed residence time of0.3 s.

The 1,3-butadiene pyrolysis experiments arecarried out exactly like the catechol pyrolysisexperiments except for two differences: (1) Insteadof a pure nitrogen carrier gas, the carrier gas isnitrogen containing 1300 ppm of 1,3-butadiene.(2) The carrier gas is passed through the by-passline in the fuel vaporizer so that no catechol ispicked up. The feed to the reactor in this case isthus nitrogen with 0.52 mol% carbon loadingfrom 1,3-butadiene.

The catechol/1,3-butadiene co-pyrolysisexperiments are carried out exactly like the cat-echol pyrolysis experiments except instead of thepure nitrogen carrier gas, the carrier gas isnitrogen containing 1300 ppm of 1,3-butadiene.The feed to the reactor in this case is thusnitrogen with 0.65 mol% carbon loading fromcatechol and 0.52 mol% carbon loading from1,3-butadiene. Therefore 55.5% of the total car-bon fed to the reactor in the co-pyrolysis exper-iments is from catechol; 44.5% is from 1,3-butadiene. (The corresponding ratio of molesof catechol to moles of 1,3-butadiene in the feedis 0.83, a level chosen to ensure that the 1,3-butadiene provided in the feed is several timesthe level produced by catechol during theexperiments.)

For each of the pyrolysis experiments, theproducts exiting the reactor are quenched to room

S. Thomas, M.J. Wornat / Proceedings of the Combustion Institute 32 (2009) 615–622 617

temperature. The gas-phase products are analyzedas described elsewhere [14,26]. The condensed-phase products are dissolved in dichloromethane,concentrated in a Kuderna-Danish evaporator,and analyzed by gas chromatography with massspectrometry (for the 1- and 2-ring aromatics)[14,15] and by HPLC with diode-array UV absor-bance detection (for PAH of P3 rings). Details ofthe HPLC chromatographic separation methodare given elsewhere [11,15,31]. The individualPAH products are unequivocally identified bymatching each component’s retention time andUV absorbance spectrum with those of the appro-priate reference standard. Quantification of theidentified PAH comes from extensive calibrationof the HPLC/UV instrument with referencestandards.

Product yields obtained from repeat experi-ments under the same conditions give very repro-ducible results. The masses of carbon quantified inthe collected C1–C10 products (reported elsewhere[26]), PAH, oxygen-containing aromatics, andunconverted fuel account for all of the carbonfed to the reactor. A carbon balance is thus closedon this reactor system.

Fig. 1. HPLC chromatogram of the products of catechol/1,3-baseline at 63 min corresponds to a change in mobile-phase coproducts are color-coded according to structural class: benzenobenzologues (dark blue), cyclopenta-fused PAH (red), ethynylblue), bi-aryls (gray), and oxygen-containing aromatics (brownby class, in Table S1 in the Supplemental Material. Portions ofthe HPLC sample-preparation procedure, so these species are

3. Results and discussion

Catechol pyrolysis, 1,3-butadiene pyrolysis,and catechol/1,3-butadiene co-pyrolysis experi-ments have been conducted in the isothermal lam-inar-flow reactor at a fixed residence time of 0.3 sand at eight temperatures within the range of 600–1000 �C. No PAH of P3 rings are produced insignificant quantities at temperatures <700 �C.

All of the species identified by HPLC/UV inthe condensed-phase products of the experimentsin this study are present in the products of cate-chol/1,3-butadiene co-pyrolysis at 1000 �C, sothe HPLC chromatogram of that product mixtureappears in Fig. 1. Included in the chromatogramare the structures (color-coded by compoundclass) of the 111 products unequivocally identifiedby their UV spectra: 47 benzenoid PAH, in black;14 indene benzologues, in green; 13 fluoranthenebenzologues, in dark blue; 10 cyclopenta-fusedPAH, in red; 8 ethynyl-substituted species (triacet-ylene and 7 ethynyl-substituted aromatics), inpurple; 10 methylated aromatics, in light blue;2 bi-aryls, in gray; and 7 oxygen-containingaromatics, in brown. The names and structures

butadiene co-pyrolysis at 1000 �C and 0.3 s. The rise inmposition to UV-absorbing dichloromethane. Identifiedid PAH (black), indene benzologues (green), fluoranthene-substituted species (purple), methylated aromatics (light). The names and structures of all products are presented,the 1- and 2-ring products are lost by evaporation duringquantified by gas chromatography.

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weighted sum

1,3-butadiene pyrolysis

% F

ed C

as

C in

PA

H o

f

3 R

ings

Temperature (oC)

catechol and 1,3-butadiene co-pyrolysis

catechol pyrolysis

Fig. 2. Yields, as functions of temperature, of summedPAH of P3 rings. Experiments: catechol pyrolysis (j);1,3-butadiene pyrolysis (�); catechol/1,3-butadiene co-pyrolysis (N). Weighted sum values (M) calculated fromEq. (1).

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of all 111 products are presented, by class, inTable S1 in the Supplemental Material.

Of the 111 products of Fig. 1, 61 have beenreported as products of catechol pyrolysis in ourearlier studies [11,15], and the UV spectralmatches documenting the identifications of 11 ofthese 61 PAH that required specially synthesizedreference standards have been presented [11].The remaining 50 products in Fig. 1 (47 PAH; 3oxygen-containing aromatics) have never beforebeen reported as products of the pyrolysis of cat-echol, catechol plus 1,3-butadiene, or any otherphenol-type fuel. The UV spectra documentingthe 47 new PAH product identifications are pre-sented in three forthcoming papers: one on theC24H14 PAH [31], one on the indene benzologues[32], and one on the large-ring-number PAH.

The 111 products shown in Fig. 1 for catechol/1,3-butadiene co-pyrolysis at 1000 �C are the verysame 111 products that are produced by catecholpyrolysis alone at this same temperature. Thechromatogram for the 1000-�C catechol-onlypyrolysis products thus looks similar to Fig. 1and is not presented here.

HPLC analysis of the products of 1,3-butadi-ene pyrolysis at 1000 �C reveals, however, thatpyrolysis of 1,3-butadiene alone at 1000 �C pro-duces only about two-thirds of the productsshown in Fig. 1 for catechol/1,3-butadiene co-pyrolysis (or catechol-only pyrolysis) at thistemperature. Because 1,3-butadiene pyrolysis pro-duces no PAH that are not also in Fig. 1, theHPLC chromatogram of the products of 1,3-buta-diene pyrolysis at 1000 �C is presented in the Sup-plemental Material as Fig. S1. Since there is nosource of oxygen in the 1,3-butadiene-only pyroly-sis environment, the oxygen-containing productsof catechol pyrolysis or catechol/1,3-butadieneco-pyrolysis, shown in brown in Fig. 1, are notpresent in the 1,3-butadiene pyrolysis productsof Fig. S1. Also, many of the larger-ring-numberPAH in Fig. 1, which are formed by catecholpyrolysis or catechol/1,3-butadiene co-pyrolysisat 1000 �C, are either not present or are presentin much smaller amounts in the 1,3-butadienepyrolysis products of Fig. S1. This difference is asign that, for a given temperature and residencetime, the pyrolysis reactions leading to PAH for-mation and growth in the catechol pyrolysis orcatechol/1,3-butadiene co-pyrolysis environmentprogress to a much greater extent than they doin the 1,3-butadiene-only pyrolysis environment.

To further explore this difference, the yields ofall of the PAH shown in Fig. 1 have been quanti-fied for all of the experiments of this study. (Theyield/temperature profiles for 26 individualhigher-yield PAH appear in Figs. S2–S8 of theSupplemental Material.) For each pyrolysis tem-perature, the yields of all of the PAH of P3 ringshave been summed, and the results are plotted, asfunctions of temperature, in Fig. 2 for each exper-

iment set: catechol pyrolysis (filled squares), 1,3-butadiene pyrolysis (filled circles), and catechol/1,3-butadiene co-pyrolysis (filled triangles).Because different fuels are used in the differentexperiment sets, the total PAH yields of Fig. 2are expressed as ‘‘% fed carbon as carbon inPAH of P3 rings.” For the catechol pyrolysisexperiments, the ‘‘fed carbon” is the carbon inthe fed catechol; for the 1,3-butadiene experi-ments, it is the carbon in the fed 1,3-butadiene;for the catechol/1,3-butadiene co-pyrolysis exper-iments, it is the carbon in the fed catechol and 1,3-butadiene. The fourth symbol plotted in Fig. 2 isthe open triangle, which corresponds to a calcu-lated ‘‘weighted sum,” for use in comparison tothe catechol/1,3-butadiene co-pyrolysis experi-mental data. For any given temperature, theweighted sum is calculated from:

weighted sum ¼ 0:555yc þ 0:445yb ð1Þwhere 0.555 and 0.445 are the fractions of the feedcarbon in the co-pyrolysis experiment from cate-chol and 1,3-butadiene, respectively, and yc andyb are the yield values from the catechol pyrolysisexperiment (filled square) and 1,3-butadiene pyro-lysis experiment (filled circle), respectively. Thisweighted sum represents what the catechol/1,3-butadiene co-pyrolysis experiment would yield ifthe two fuels together simply behaved as a linearcombination of the two fuels individually.

Consistent with the observed lower proportionof larger-ring-number PAH in the 1,3-butadieneproducts of Fig. S1, Fig. 2 shows that – compared

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1,3-butadiene pyrolysis

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catechol pyrolysis

% F

ed C

as

C in

C1-C

5H

ydro

carb

ons

(Exc

ept C

4H6)

Temperature (oC)

Fig. 4. Yields, as functions of temperature, of summedC1 to C5 hydrocarbons (excluding 1,3-butadiene).Experiments: catechol pyrolysis (j); 1,3-butadiene pyro-lysis (�); catechol/1,3-butadiene co-pyrolysis (N).Weighted sum values (M) calculated from Eq. (1). 1,3-butadiene is excluded since it is supplied as one of thefuels to the experiments.

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rom

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HC

s

weighted sum

catechol pyrolysis

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to catechol pyrolysis or catechol/1,3-butadiene co-pyrolysis – 1,3-butadiene pyrolysis requires highertemperatures to produce PAH, but, even at thehigher temperatures, produces a much loweramount of PAH. To explain these observations,we turn to our data on catechol and 1,3-butadieneconversion, in Fig. 3, and on the other hydrocar-bon product yields: summed C1–C5 hydrocarbons,in Fig. 4, and summed 1- and 2-ring aromatichydrocarbons, in Fig. 5. (Our companion paper[26] presents and discusses the yield/temperaturebehavior of each of the C1–C10 hydrocarbonproducts individually as well as of CO and phenol,two major products of catechol whose yields areunaffected by the addition of 1,3-butadiene tothe catechol pyrolysis environment [26].)

We first consider just the results for the 1,3-butadiene-only pyrolysis experiments and the cat-echol-only pyrolysis experiments, represented bythe circles and squares, respectively, in Figs. 2–5.As Fig. 3 demonstrates, catechol conversion risesrapidly with temperature between 750 and850 �C, reaching 87% at 850 �C and 100% at900 �C. 1,3-butadiene conversion, in contrast,goes from 6% to 39% in the 750–850 �C tempera-ture interval and requires temperatures in excessof 900 �C to achieve 90% conversion. We there-fore see from Fig. 3 that for any given tempera-ture below 950 �C, the extent of catecholconversion in the catechol pyrolysis experimentsis much higher than the extent of 1,3-butadieneconversion in the 1,3-butadiene experiments – aresult consistent with the much higher productionof PAH from catechol pyrolysis than from 1,3-butadiene pyrolysis, in Fig. 2.

Since the main hydrocarbon product ofcatechol decomposition in the 750–850 �C temper-ature range (where catechol conversion dramaticallyrises) is 1,3-butadiene itself [14] and since theother C1–C6 hydrocarbon products of catechol

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% C

onve

rsio

n

catechol conversion

from catechol pyrolysis

1,3-butadiene conversion from 1,3-butadiene

pyrolysis

Fig. 3. Conversion, as a function of temperature, forcatechol in the catechol pyrolysis experiments (j) andfor 1,3-butadiene in the 1,3-butadiene pyrolysis experi-ments (�).

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Fig. 5. Yields, as functions of temperature, of summed1- and 2-ring aromatic hydrocarbons. Experiments:catechol pyrolysis (j); 1,3-butadiene pyrolysis (�);catechol/1,3-butadiene co-pyrolysis (N). Weighted sumvalues (M) calculated from Eq. (1).

pyrolysis – ethylene, acetylene, vinylacetylene,benzene, cyclopentadiene, propyne, propadiene,propene, and methane – are the very C1–C6 prod-ucts of 1,3-butadiene pyrolysis [26], the facilitated

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conversion of catechol, relative to that of 1,3-butadiene, must be linked to the oxygen that cat-echol contains in the form of its two hydroxylgroups. The bond dissociation energy (BDE) ofthe O–H bond in catechol, the weakest bond inthe catechol molecule, is 81.2 kcal/mol [33], whichis lower than 87.5 kcal/mol [34], the experimen-tally determined BDE for the weakest bond in1,3-butadiene, the C–H bond of an internal C.Catechol’s lower BDE enables the radical-initia-tion step to take place at a lower temperaturefor catechol than for 1,3-butadiene. Furthermore,once catechol pyrolysis reactions start, OH andother vigorous oxygen-containing radicals aregenerated, further propelling the pyrolysis reac-tions that lead to 1- and 2-ring aromatics andeventually PAH. When 1,3-butadiene is not inthe presence of catechol, not only does it have ahigher energy threshold to overcome to initiatepyrolysis reactions, but there are also no oxy-gen-containing radicals around like OH, whichare extremely effective at accelerating pyrolysisreactions. The 1,3-butadiene conversion curve ofFig. 3 is therefore not only shifted to higher tem-peratures, relative to that of catechol, but it is alsoless steep in slope.

As for the hydrocarbon products – since the1,3-butadiene-only pyrolysis environment is voidof the oxygen-containing radicals that acceleratethe reactions leading to aromatic ring formationand growth – Figs. 4 and 5, respectively, showthat: (1) the proportion of fuel carbon ending upas C1–C5 hydrocarbons is roughly two timeshigher for 1,3-butadiene pyrolysis than for cate-chol pyrolysis and (2) the proportion of fuel car-bon ending up as 1- and 2-ring aromatichydrocarbons is roughly two times higher for cat-echol pyrolysis than for 1,3-butadiene pyrolysis at850–950 �C. (At temperatures P950 �C, it falls toless than twice, as the 1- and 2-ring aromaticsfrom catechol become vulnerable to oxidation[14,15].) Since these 1- and 2-ring aromatics arethe kernels for PAH formation and growth[25,30], catechol pyrolysis produces a much higheryield of PAH than does 1,3-butadiene pyrolysis,just as Fig. 2 portrays.

Having considered the results from the cate-chol-only and 1,3-butadiene-only experiments,we now examine the co-pyrolysis of catechol and1,3-butadiene. Results from our companion paper[26] show that when catechol and 1,3-butadieneare pyrolyzed together, the catechol conversion/temperature profile is the same as when catecholis pyrolyzed by itself. The results also show [26]that 1,3-butadiene conversion is enhanced when1,3-butadiene is in the presence of catechol, com-pared to when 1,3-butadiene is pyrolyzed by itself– and that this enhancement begins at tempera-tures >750 �C, the temperature at which catecholconversion really starts to ‘‘take off.” These obser-vations are fully consistent with the oxygen-asso-

ciated conversion-enhancement effects describedabove: Since catechol already contains the rela-tively labile O–H bond and is able to generateoxygen-containing radicals, adding 1,3-butadienedoes not influence catechol conversion. Since1,3-butadiene, however, has only higher-BDEbonds and no source of oxygen, adding catecholto its pyrolysis environment significantly increases1,3-butadiene’s conversion.

The oxygen-containing radicals from catecholnot only increase 1,3-butadiene’s conversion inthe co-pyrolysis environment but they also permitthe C1–C5 hydrocarbon products of 1,3-butadienepyrolysis to participate as effectively as the C1–C5

hydrocarbon products of catechol in the reactionsleading to the 1- and 2-ring aromatics and PAH.Evidence of this effect is found in the summedhydrocarbon product yield graphs of Figs. 4 and5 – particularly in the comparison of the actualco-pyrolysis experiment yields (filled triangles)with the calculated ‘‘weighted sum” values (opentriangles), which represent the hypothetical yieldsthe co-pyrolysis experiments would produce if thetwo fuels together simply behaved as a linear com-bination of the two fuels individually. As Figs. 4and 5 show, the summed C1–C5 hydrocarbonyields from the catechol/1,3-butadiene co-pyroly-sis experiments are lower than the weighted sumvalues in Fig. 4, and the 1- and 2-ring aromatichydrocarbon yields from the co-pyrolysis experi-ments are much higher (by a factor of 1.3–1.6)than the weighted sum values in Fig. 5. The effectis even more dramatic for the PAH in Fig. 2: At850–1000 �C, PAH yields from the co-pyrolysisexperiments are 2.1–2.5 times greater than theweighted sum values. We therefore see – fromcomparison of the co-pyrolysis and weighted-sum values in Figs. 2, 4 and 5 – that in thecatechol/1,3-butadiene co-pyrolysis environment,catechol’s relatively labile O–H bonds and capac-ity to generate oxygen-containing radicals effec-tively facilitate conversion of feed carbon toPAH carbon, whether the feed carbon is from cat-echol or 1,3-butadiene.

The enhanced PAH production in the co-pyro-lysis experiments is due not only to the reaction-promoting effects of the oxygen-containingradicals, however, but also to the effectiveness,as growth species, of the small hydrocarbons gen-erated by catechol and 1,3-butadiene during pyro-lysis. Of the C1–C5 hydrocarbons, it is the C2

species ethylene and acetylene and the C4 species1,3-butadiene and vinylacetylene that, by far, arethe most abundant in the catechol/1,3-butadieneco-pyrolysis environment [14,26]. Our experi-ments with catechol/acetylene co-pyrolysis andcatechol/ethylene co-pyrolysis (unpublished data),however, show that neither acetylene nor ethylenehas any more than a moderate effect in increasingPAH yields in the temperature range of ourexperiments (600–1000 �C). For example, at no

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temperature is the PAH yield from the catechol/acetylene co-pyrolysis experiment any more than29% higher than the corresponding weightedsum for that system. Since, in contrast, PAHyields from catechol/1,3-butadiene co-pyrolysisare more than twice their corresponding weightedsum – and since vinyl acetylene is produced insubstantial amounts from both 1,3-butadienepyrolysis [26] and catechol pyrolysis [14] – we con-clude that it is the C4 species, and not the C2 spe-cies, that are the more effective growth agents inthe catechol/1,3-butadiene co-pyrolysis environ-ment – at least at temperatures up to 1000 �C,the maximum temperature of our experiments.Consistent with our findings, several works [35–39] have cited C4 as an important growth speciesfor PAH.

Having established that catechol and 1,3-buta-diene produce more than twice the yield of PAHwhen they are pyrolyzed together than when theyare pyrolyzed in the same amounts separately – wenow examine whether the synergism preferentiallyaffects PAH of particular sizes or structural clas-ses. When the yields of PAH from catechol/1,3-butadiene co-pyrolysis experiments are summedaccording to ring number and compared, at eachtemperature, with the corresponding weightedsums, the ratios of the co-pyrolysis experimentyield to the weighted sum are found (Table S2)not to vary much with temperature or with ringnumber. (For the 3-ring PAH, the ratio is2.1 ± 0.2; for the 4- and 5-ring PAH, 2.5 ± 0.3.)This observation implies that PAH of all ringnumbers benefit fairly equally from the synergisticeffects associated with pyrolyzing the catechol and1,3-butadiene together. When the same type ofdata are compiled (Table S3) for the PAHgrouped by structural class, however, two otherobservations emerge. First – similar to resultsfrom coal pyrolysis experiments [40] – PAH yieldsfrom the catechol/1,3-butadiene co-pyrolysisexperiments are highest for the cyclopenta-fusedPAH and the benzenoid PAH, the two groupsthat contain the most potent mutagens producedin our experiments: cyclopenta[cd]pyrene, ben-zo[a]pyrene, dibenzo[a,i]pyrene, naphtho[2,3-a]pyrene, naphtho[2,1-a]pyrene [1,2]. Second, theratio of the co-pyrolysis experiment yield to theweighted sum varies some with PAH structuralclass. For the benzenoid PAH and the indene ben-zologues, the ratio is constant at 2.3 ± 0.2, but itis a little lower for the cyclopenta-fused PAH(1.7 ± 0.2) and quite a bit higher for the fluo-ranthene benzologues (3.2 ± 0.4).

Even though these differences between struc-tural classes are not huge, they serve to substanti-ate that it is indeed the C4 species, and not the C2

species, that are the most important growthspecies for PAH in the catechol/1,3-butadieneco-pyrolysis environment. Since PAH growthoccurs through the bonding of a hydrocarbon

growth species to two adjacent peripheral carbons(each initially attached to a hydrogen) of analready-existing PAH structure, C4 addition to abenzenoid PAH, indene benzologue, or fluoranth-ene benzologue would produce another benzenoidPAH, indene benzologue, or fluoranthene benzo-logue, respectively. C4 addition to a cyclopenta-fused PAH, however, would produce anothercyclopenta-fused PAH only if the addition occursto a six-membered ring. If the addition occurs tothe cyclopenta ring – the more favored locationsince the electrons associated with the two externalcarbons of the cyclopenta ring are more localized[41] than those associated with six-membered rings– a fluoranthene benzologue results. We thereforesee that a dominance of C4-addition reactions inthe co-pyrolysis environment would explain the rel-ative enhancement of fluoranthene benzologuesand relative depletion of cyclopenta-fused PAHobserved in the PAH product distributions fromour co-pyrolysis experiments. In contrast, a domi-nance of C2-addition reactions would yield resultsthat are inconsistent with our experimental find-ings, since, in that case, a preferential enhancementof the cyclopenta-fused PAH would result (from C2

addition to benzenoid PAH, fluoranthene benzo-logues, or other cyclopenta-fused PAH).

4. Conclusions

Catechol’s relatively labile O–H bond andcapacity for generating oxygen-containing radi-cals accelerate fuel conversion and the pyrolysisreactions leading to 1- and 2-ring aromatics andPAH. Since 1,3-butadiene has no such internalsource of oxygen, for a given temperature, a muchhigher percentage of fed carbon is converted toPAH in the catechol-only pyrolysis experimentsthan in the 1,3-butadiene-only pyrolysis experi-ments. However, when the two fuels are pyrolyzedtogether, 1,3-butadiene and its decompositionproducts also benefit from the reaction-promotingeffects of the oxygen-containing radicals from cat-echol: For any given temperature, the percentageof the total fed carbon converting to PAH in theco-pyrolysis experiment is more than two timeshigher than the amount calculated for the hypo-thetical case of the two fuels together behavingas a linear combination of the two fuels individu-ally. This elevated production of PAH from theco-pyrolysis experiments reflects not only thereaction-accelerating role of the oxygen-contain-ing radicals but also the efficacy, as growth agents,of the C2 and C4 species abundantly present in thecatechol and 1,3-butadiene pyrolysis environment.The observation that yields of fluoranthene ben-zologues are preferentially enhanced in theco-pyrolysis experiments, at the expense of thecyclopenta-fused PAH, suggests that C4 speciesare the more effective growth agents in the cate-

622 S. Thomas, M.J. Wornat / Proceedings of the Combustion Institute 32 (2009) 615–622

chol/1,3-butadiene pyrolysis environment – atleast at temperatures up to 1000 �C, the maximumtemperature of our experiments. This finding iscorroborated by the fact that our catechol/acety-lene co-pyrolysis and catechol/ethylene co-pyroly-sis experiments each produce PAH yields that areonly moderately higher than those from catecholpyrolysis alone. Since the hydrocarbons and oxy-gen-containing radicals of the catechol/1,3-buta-diene co-pyrolysis experiments are also abundantin the reaction environments of actual solid fuels,the results presented here illustrate the kind ofsynergistic effects that can occur during PAH for-mation from solid fuel pyrolysis and combustion.

Acknowledgements

The authors gratefully acknowledge PhilipMorris USA for financial support of this research.For PAH reference standards and UV spectra, theythank Dr. Arthur Lafleur and Ms. Elaine Plummer,of the Massachusetts Institute of Technology;Dr. John Fetzer, of FETZPAHS; Dr. AlbrechtSeidel, of the Biochemical Institute for Environ-mental Carcinogens; and Dr. Dianne Poster, ofthe National Institute of Standards and Technol-ogy. They also express their appreciation to Dr.Davis Maliakal, Dr. Jorge Ona, and Mr. NimeshPoddar for helpful technical contributions.

Appendix A. Supplementary data

Supplementary data associated with this articlecan be found, in the online version, atdoi:10.1016/j.proci.2008.05.043.

References

[1] J.L. Durant, W.F. Busby Jr., A.L. Lafleur, B.W.Penman, C.L. Crespi, Mutat. Res. 371 (1996) 123–157.

[2] J.L. Durant, A.L. Lafleur, W.F. Busby Jr., L.L.Donhoffner, B.W. Penman, C.L. Crespi, Mutat.Res. 446 (1999) 1–14.

[3] J.B. Howard, J.P. Longwell, J.A. Marr, et al.,Combust. Flame 101 (1995) 262–270.

[4] A. Dipple, R.C. Moschel, C.A.H. Bigger, ChemicalCarcinogens, American Chemical Society, Washing-ton, DC, 1984, pp. 41–164.

[5] M.J. Wornat, A.F. Sarofim, J.P. Longwell, EnergyFuels 1 (1987) 431–437.

[6] B.S. Haynes, H.G. Wagner, Prog. Energy Combust.Sci. 7 (1981) 229–273.

[7] B.M. Lynch, R.A. Durie, Aust. J. Chem. 13 (1960)567–581.

[8] S. Lee, Alternative Fuels, Taylor & Francis, Wash-ington, DC, 1996.

[9] R.L. Stedman, Chem. Rev. 68 (1968) 153–207.

[10] D.C. Elliot, Pyrolysis Oils from Biomass, AmericanChemical Society, Washington, DC, 1988.

[11] M.J. Wornat, E.B. Ledesma, N.D. Marsh, Fuel 80(2001) 1711–1726.

[12] E.B. Ledesma, N.D. Marsh, A.K. Sandrowitz,M.J. Wornat, Proc. Combust. Inst. 29 (2002)2299–2306.

[13] N.D. Marsh, E.B. Ledesma, A.K. Sandrowitz,M.J. Wornat, Energy Fuels 18 (2004) 209–217.

[14] S. Thomas, E.B. Ledesma, M.J. Wornat, Fuel 86(2007) 2581–2595.

[15] S. Thomas, M.J. Wornat, Fuel 87 (2008) 768–781.[16] G. Barrefors, G. Petersson, J. Chromatogr. A. 710

(1995) 71–77.[17] A.N. Garcia, M.M. Esperanza, R. Font, J. Anal.

Appl. Pyrol. 68–69 (2003) 577–598.[18] R.J. Evans, T.A. Milne, Energy Fuels 1 (1987) 123–

137.[19] M. Olsson, O. Ramnas, G. Petersson, J. Anal. Appl.

Pyrol. 71 (2004) 847–854.[20] W.H. Calkins, Fuel 63 (1984) 1125–1129.[21] K.R. Doolan, J.C. Mackie, M.F.R. Mulcahy, R.J.

Tyler, Proc. Comb. Inst. 19 (1982) 1131–1138.[22] J.H. Kiefer, H.C. Wei, R.D. Kern, C.H. Wu, Int. J.

Chem. Kinet. 17 (1985) 225–253.[23] G.B. Skinner, E.M. Sokoloski, J. Phys. Chem. 64

(1960) 1028–1031.[24] M.B. Colket, Pyrolysis of 1,3-Butadiene in a Single-

Pulse Shock Tube, The Combustion Institute, East-ern States Section Fall Meeting, Providence, RhodeIsland, 1983.

[25] C.S. McEnally, L.D. Pfefferle, B. Atakan, K.Kohse-Hoinghaus, Prog. Energy Combust. Sci. 32(2006) 247–294.

[26] S. Thomas, M.J. Wornat, Energy Fuels, inpreparation.

[27] P. Dagaut, M. Cathonnet, Combust. Sci. Technol.140 (1998) 225–257.

[28] N. Olten, S. Senkan, Combust. Flame 125 (2001)1032–1039.

[29] W.J. Catallo, Chemosphere 37 (1998) 143–157.[30] H. Richter, J.B. Howard, Prog. Energy Combust.

Sci. 26 (2000) 565–608.[31] S. Thomas, M.J. Wornat, Int. J. Environ. Anal.

Chem. 88 (2008) 825–840.[32] S. Thomas, M.J. Wornat, Polycyclic Aromat.

Compd., in preparation.[33] Y.-R. Luo, Handbook of Bond Dissociation Energies

in Organic Chemistry, CRC Press, Boca Raton, FL,2003, p. 185.

[34] L.A. Gribov, I.A. Novakov, A.I. Pavlyuchko, I.V.Kuchurov, J. Struct. Chem. 47 (2006) 629–634.

[35] S. Fascella, C. Cavallotti, R. Rota, S. Carra, J.Phys. Chem. A 108 (2004) 3829–3843.

[36] M.B. Colket, Proc. Comb. Inst. 21 (1986) 851–864.[37] H. Wang, H. Yang, W. Chuang, X. Ran, Q.

Shi, Z. Wen, J. Mol. Graph. Modell. (2007) 824–830.

[38] J.A. Cole, J.D. Bittner, J.P. Longwell, J.B. Howard,Combust. Flame 56 (1984) 51–70.

[39] B. Atakan, A. Lamprecht, K. Kohse-Hoinghaus,Combust. Flame 133 (2003) 431–440.

[40] M.J. Wornat, B.A. Vernaglia, A.L. Lafleur, et al.,Proc. Combust. Inst. 27 (1998) 1677–1686.

[41] P.P. Fu, F.A. Beland, S.K. Yang, Carcinogenesis 1(1980) 725–727.