Diesel combustion of an alkylated polycyclic aromatic hydrocarbon

ELSEVIER

0016-2361(95)00300-2

FuelVol. 75, No. 6, pp. 717-723, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0016-2361/96 $15.00+ 0.00

Diesel combustion of an alkylated polycyclic aromatic hydrocarbon

Paul J. Tancell, M i c h a e l M. Rhead*, Robin D. Pemberton and Jim Braven Departmentof EnvironmentalSciences, Universityof Plymouth,Plymouth,DevonPL4 8AA, UK (Received 13 October 1994; revised 17 July 1995)

The combustion of an alkylated PAH in a direct-injection diesel engine has been investigated using a lowaromatic diesel fuel spiked with 2- and 3-ethylphenanthrene. Exhaust samples were collected using a total exhaust solvent stripping apparatus (TESSA). The 2- and 3-ethylphenanthrene were recovered in yields of 0.3 and 0.35 wt% respectively. The major product from the combustion of ethylphenanthrene was shown to be vinylphenanthrene, produced in a yield of 0.01 wt% of the total added ethylphenanthrene. Although there was no increase in the emission of phenanthrene or other PAH, a statistically significant increase in the emission of 3-methylphenanthrene was observed. These results suggest that for these isomers, under the engine conditions used, no significant dealkylation occurs. The results fit a model based on Dewar reactivity indices, which predicts the extent to which alkyl PAH isomers are dealkylated under pyrolysis conditions. Copyright © 1996 Elsevier Science Ltd. (Keywords: alkylated polycyclic aromatics; combustion;diesel engines)

Alkylated polycyclic aromatic hydrocarbons (PAH) are major aromatic components of diesel fuel. Methylated PAH are the most abundant, especially naphthalenes, phenanthrenes, dibenzothiophenes and fluorenes. PAH substituted with ethyl groups are also present. The chemical and biological reactions of alkyl PAH may be substantially different from those of the parent PAH. For example, it has been suggested that there is a greater tendency for aromatic molecules with alkyl side chains to form larger condensed aromatic structures during pyrolysis. These in turn are considered to be precursors in the formation of soot 1. The carcinogenicities of alkyl PAH may also differ from those of their parent PAH. Glatt et al. 2 proposed that certain alkylated PAH may be as potent as, or even more mutagenic and carcinogenic than, the corresponding parent molecule. Longwell3 has demonstrated the carcinogenicity of methylphenanthrenes. In view of the abundance of alkyl PAH in diesel fuel, the combined carcinogenicity of these compounds may be more significant than that of more highly carcinogenic but less abundant PAH (e.g. benzo[a]pyrene, typically < 1 ppm in diesel fuel). Clearly there is a need to understand the behaviour of alkylaromatic species in this context. Pyrolysis of alkyl aromatics has been shown to produce parent aromatics as products 4'5. It has also been proposed that dealkylation of fuel aromatics may take place in conditions of partial oxidation in combustion systems 3'6. The influence of the size and position of the alkyl group on the course of alkyl PAH pyrolysis has been investigated by Smith and Savage 5. These authors

pyrolysed a series of alkylated PAH and identified two major reaction pathways: either cleavage of the arylalkyl bond or cleavage of carbon-carbon bonds in the alkyl side chain. The preferred reaction pathway was affected not by the length of the alkyl side chain but by the position of substitution on the aromatic molecule. This effect was correlated with Dewar reactivity indices for certain positions on the PAH molecule and allowed predictions to be made regarding the ease of aryl-alkyl bond cleavage for other alkyl PAH species. It has been assumed that dealkylation must also take place in the extreme conditions of temperature and pressure encountered in a diesel combustion chamber. Various authors have noted a decrease in the abundance of methyl-substituted PAH relative to the parent unsubstituted PAH in diesel emissions, compared with the distribution in the fuel7'8. This may constitute evidence for the existence of a dealkylation pathway occurring during the diesel combustion process. Alternatively it may be a consequence of the greater relative combustibility of alkyl aromatics relative to the unsubstituted aromatics. In the current work a diesel fuel with a low aromatic content (7.0wt%), spiked with 2- and 3-ethylphenanthrene (2-EtPa and 3-EtPa, Figure 1), was burnt in a 2 L direct-injection Perkins Prima diesel engine. The fuel contained <0.1wt% triaromatics and was used to facilitate the observation of expected products of combustion: phenanthrene and/or methylphenanthrene (MePa). Ethylphenanthrene has not been identified in this diesel fuel; however, 1- and 2-ethylnaphthalenes (1-EtNp and 2-EtNp, Figure 1) are present in substantial

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Diesel combustion of an alkylated polycyclic aromatic hydrocarbon: P. J. Tancell et al.

1-methylnaphthalene (1-MeNp)

2-ethylnaphthalene (2-EtNp)

2-butylnaphthalene (2-BtNp)

2-vinylnaphthalene (2-ViNp)

3-ethylphenanthrene (3-EtPa)

3-vinylphenanthrene (3-ViPa)

Figure 1 Structures of representative alkyl aromatics

quantities (1460ppmw), as were methylethylnaphthalenes (4400 ppmw). EXPERIMENTAL

Chemicals All solvents were of h.p.l.c, grade (Rathburns) and were used as received. Chemicals used in the synthesis of EtPa were purchased from Aldrich and were of >98 wt% purity. Deuterated PAH internal standards--naphthalene (d8-Np), phenanthrene (dl0-Pa) and chrysene (dl2Ch)--were purchased from Aldrich.

Engine facility The engine used was a 2 L direct-injection Perkins Prima engine mounted on a test bed and controlled by a Borghi & Saveri FA100 eddy current dynamometer, controlled by a Test Automation Series Compact Controller.

Table 1 Diesel fuel specification Density at 15°C (kgdm -3) Sulfur content (ppmw) F.b.p. (°C) Distillate/residue/loss (vol.%) Cetane number (D613) Hydrogen content (wt%) Carbon content (wt%) Calorific value (MJ kg -1) Net Gross Aromatics contents (IP391) (vol.%) Mono Di Tri

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0.8145 510 327 97.5/1.5/0 63.6 14.0 86.2 43.16 46.07 6.45 0.46 <0.01

Exhaust gas sampling was performed using the total exhaust solvent stripping apparatus (TESSA) sampling system9. The TESSA system consists of a vertical stainless steel tower (~1.5 x 0.3m) connected by a short (50cm) heated transfer line to the engine. The tower comprises two sections. Collection of the exhaust organics occurs in the lower section, which is filled with graded glass tubing to maximize the area of contact between solvent and exhaust gases. The upper section is used to cool the exhaust effluent and contains a series of cooling baffles. The exhaust is sampled using a pressurecontrolled countercurrent flow of solvent (dichloromethane-methanol 1 : 1) which strips the organic species from the exhaust gases as they enter the apparatus. The exhaust sample is collected in a large flask (41) at the base of the tower. The fuel lines to the engine had been modified to enable the engine to be run on the test fuel. This was achieved by attaching a 51 reservoir to the main fuel line by means of two three-way valves which could be switched manually to allow the engine to receive exclusively the low-aromatic spiked fuel. The diesel fuel specifications are presented in Table 1. Fuel consumption during the experiment was determined using a 400ml graduated measuring burette.

Diesel exhaust sampling The engine was conditioned for 1 h at full power on the standard A2 diesel fuel before being conditioned for a further 15min under the test conditions of 3000rev min -1 and 10 N m on the test fuel. Previous research had established that dealkylation pathways, if in operation, were most likely under conditions of low load and high speed 1°. The exhaust was sampled for a period of 1 min. To minimize contamination of the test fuel with triaromatics from the standard diesel fuel, the fuel return lines were diverted to waste for several minutes during conditioning on the test fuel until there was no visible trace of the red diesel in the diesel return line. This required the system to be purged with ,-~ 21 of the test fuel. The fuel return line was then diverted back to the auxiliary reservoir. The sampling procedure consisted of collecting five exhaust samples from the engine operating on the test fuel without the 3-EtPa spike and five exhaust samples collected with the engine operating on the spiked test fuel. The purpose of the first five samples was to establish a baseline for emissions from the new fuel, with which to compare the emissions from the fuel with the 3-EtPa spike. Three fuel samples were collected at the beginning, middle and end of the two sets of samples to verify the homogeneity of the fuel in terms of concentration of the spike, which was determined as 3.7 wt% for 3-EtPa and 0.28 wt% for 2-EtPa in each fuel sample by g.c. analysis.

Synthesis of 3-ethylphenanthrene No commercial source of 3-EtPa could be found. Although pure 3-EtPa can be synthesized following the multi-step method of Haworth et al. 11, that degree of purity was not required for this experiment. Consequently ,,~60 g of material was synthesized in a two-step procedure. The first stage consisted of Friedel-Crafts acylation of phenanthrene following the method of Mosettig and van de Kemp !2. Acylation was performed using acetyl chloride in nitrobenzene with AIC13 catalyst, giving a mixture of 2- and 3-acetylphenanthrene (2- and

Diesel combustion of an alkylated polycyclic aromatic hydrocarbon: P. J. Tancell et al. C14

C15

C16

3-EtPa

2L

2-EtPa

J

J

i

i

i

L .

i

~g~e2 Gas chromatogram of diesel fuel spiked with 2- and 3-ethylphenanthrene 3-acetylPa), with the latter predominating. Several recrystallizations from ethanol yielded 3-acetylPa of 93 wt% purity as measured by g.c.-f.i.d., the remainder being 2-acetylPa. The mixture of 2- and 3-acetylphenanthrene was subsequently reduced to 2- and 3-EtPa by a WolffKishner reduction using hydrazine hydrate and potassium hydroxide in triethylene glycol. The identity of the product was verified by g.c.-m.s., i.r. spectroscopy and n.m.r, spectroscopy.

Sample preparation The exhaust sample was collected in 21 of methanolDCM (1 : 1); dl0-Np, dl2-Pa and dl4-Ch were added as internal standards to the solution prior to sample workup. The procedure for isolating the exhaust extract involved two stages. First, separation between the methanol and DCM was achieved by adding excess water. The aqueous methanol layer was separated and the DCM layer dried with anhydrous sodium sulfate. The DCM was reduced by low-pressure rotary evaporation to a small volume and was then transferred to a glass vial where the remaining solvent was removed by a slow stream of nitrogen. The residue was redissolved in hexane (500 #1). Clean-up of the exhaust sample was performed by silica gel column chromatography. The aliphatics were eluted with hexane and the aromatics were isolated by elution with DCM. The fractions were evaporated to dryness and redissolved in a known volume of solvent for g.c. and g.c.-m.s, analysis.

G.c.-f.i.d. and g.c.-m.s. The 2- and 3-EtPa isomers spiked in the fuel were quantified by g.c.-f.i.d. The 3-EtPa, spiked in the fuel at a concentration of 3.7 wt %, was the fourth mos( abundant compound in the fuel after C14, C15 and C]6 n-alkanes (Figure 2). G.c. analysis was performed on a Carlo Erba

HRGC Mega series 5160 gas chromatograph fitted with a J&W Scientific DB-5 capillary column (30 m x 0.32 mm i.d., 0.25 #m film thickness). In all g.c. analyses an initial oven temperature of 40°C was raised linearly to 300°C at 5 Kmin -1 and held at 300°C for 10min. G.c.-m.s. analysis was the preferred method of quantification for other PAH in the exhaust, since this allowed precise manual integration of the molecular ion isolated from the total ion chromatogram. Analysis was performed using a Hewlett Packard 5890 series II gas chromatograph equipped with a Hewlett Packard 7673 autosampler and Hewlett Packard 5970 series mass-selective detector (MSD). The MSD was operated in the scan mode with an ionizing potential of 70 eV and an ion source temperature of 300°C. The g.c., operated in the splitless injection mode with an injector temperature of 250°C, was fitted with an HP-5 capillary column (12m × 0.25mm i.d., 0.25#m phase thickness). The temperature programme used was the same as for g.c. analysis. RESULTS AND DISCUSSION The fate of alkyl PAH during diesel combustion is of interest owing to their abundance in the fuel, yet comparatively little research has been performed in this area. The behaviour of the alkyl PAH isomers 2- and 3-EtPa during diesel combustion was investigated by combusting a low-aromatic diesel fuel spiked with 2- and 3-EtPa and comparing the emissions with those collected from unspiked fuel. 3-EtPa was added to the fuel at a concentration of 3.7 wt% and 2-EtPa at a concentration of 0.28wt%. The total recoveries for these PAH amounted to 0.35 and 0.3% respectively. These are similar to the recoveries of other PAH determined in this laboratory ]°. The diesel enriched fuel technique (DEFT) allows the behaviour of individual diesel fuel molecules to be

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Diesel combustion of an alkylated polycycfic aromatic hydrocarbon." P. J. Tancell et al. (a) DI2-Pa

DI0-NP

DI4-Ch

I

I

I

I

I

I

I

I

t

I

I

(b) DI0.Np

Di2.Pa

DI4-Ch

3-EtPa

2-EtPa

(

I

I

I

I

I

I

I

I

I

Figure 3 Gas chromatogramsof the aromatic fraction of (a) baselineexhaust, (b) spiked exhaustshowing3-ViPa

studied in detail and is complementary to the radiolabelling techniques developed in this laboratory 13-15. A major consideration with the technique is to ensure that the physical and chemical properties of the spiked fuel vary as little as possible from those of the unspiked fuel so that the combustion characteristics of the two fuels remain as similar as possible. The fuel parameters having the greatest affect on combustion conditions are cetane number, density, full boiling point (f.b.p.) and aromatics

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content 16. No bulk analysis of the diesel fuel spiked with EtPa used in this experiment was possible, owing to the small quantity of synthesized EtPa available. However, in separate experiments in which a standard A2 diesel was spiked at 4 w t % with both 1- and 2-MeNp, the cetane number was found to vary by < 1 for both spiked fuels. Similarly, the variations in density, aromatics content and f.b.p, were <0.005kgm -3, <2.5wt% and <0.5°C respectively. It was concluded that the small

Diesel combustion of an alkylated polycyclic aromatic hydrocarbon. P. J. Tancell et al.

+R"

~

+RH

-~ ~

+H.

Figure4 Formationof2-vinylphenanthrenefrom2-ethylphenanthrene variation in the properties of spiked fuels compared with the standard fuel would result in negligible differences in the cylinder combustion conditions. The major identifiable combustion product was shown to be vinylphenanthrene (ViPa). Gas chromatograms of two exhaust samples collected from the spiked and unspiked fuel are shown in Figure 3a and b. ViPa, identified from mass-spectral data, is clearly visible at a retention time of 38 min, eluting after the 2- and 3-EtPa surviving combustion. ViPa was recovered in a yield of 0.01 wt% of the total added 2- and 3-EtPa. Formation of vinyl aromatics from alkyl-aromatic precursors has been observed by a number of workers in pyrolysis studies 4'5'17. The preference for alkylaromatics to form vinyl derivatives may be explained by the extra stability imparted to the system by the conjugation of the newly formed double bond with the aromatic ring 18'19. The initial stage in the formation of the vinyl group is the abstraction of a hydrogen atom from the a-carbon (Figure 4). The resulting alkylaromatic radical would then be expected to stabilize by loss of hydrogen to produce vinylphenanthrene 4. Further evidence from this laboratory in support of the formation of vinyl aromatics from alkylaromatic precursors has come from the identification of vinylnaphthalene (ViNp) in diesel exhaust emissions. The absence of ViNp in diesel fuel suggests that this compound is a product of combustion, formed presumably from ethylnaphthalenes (EtNp) present in diesel fuel, by a similar reaction pathway to that for the formation of ViPa from EtPa. The formation of ViNp has also been observed in this laboratory in experiments in which a low-aromatic diesel fuel spiked with 2-ethylnaphthalene (2-EtNp) and 2-butylnaphthalene (2-BtNp) was burnt in the Perkins Prima engine. The extents of conversion of the alkyl PAH spiked in the fuel to the vinyl derivative in these two experiments, corrected for the formation of ViNp from alkyl naphthalenes normally present in diesel fuel, were 0.015% for 2-EtNp and 0.085% for 2-BtNp. The extents of conversion of EtPa and EtNp to the vinyl derivatives are similar (0.01 and 0.015% respectively). However, the conversion of 2-BtNp to ViNp is significantly greater and may reflect a kinetically favoured pathway to the formation of the vinyl derivative from the longer butyl side chain. No comparable published data could be found for yields from the pyrolysis of PAH substituted with short alkyl side chains (C1-C4). These results suggest that the formation of vinyl aromatics from alkylaromatic precursors may be a common reaction pathway in diesel combustion. Styrene and similar structures have been identified as key intermediates in the formation or larger aromatic structures through combination with unsaturated aliphatics and acetylenic species 1'2°. It has been suggested that similar diaromatic intermediates such as the naphthalene radical and vinylnaphthalene may promote the formation of larger aromatic species and hence soot 21. The formation of vinyl aromatics may therefore

represent an important intermediate step in the formation of soot in diesel fuel combustion. One of the most likely alternative reactions that an alkyl PAH may undergo during diesel combustion is dealkylation. It has been proposed that dealkylation may proceed by a number of different reaction pathways, including dissociation of the alkyl-aryl carbon-carbon bond 22, preferential oxidation of the alkyl side chain 23 and H-atom substitution 24. Evidence for the dealkylation of EtPa in this research would be a relative increase in the emission of phenanthrene resulting from the combustion of a fuel spiked with EtPa, compared with that from the combustion of the unspiked fuel. Variations in phenanthrene emissions were measured by determining the ratio of the integrated areas of the phenanthrene molecular ion by comparison with that of the molecular ion for the dl0-Pa internal standard in each sample. Statistical analyses were performed on the two sets of data to establish any variation in emission rates. No significant variation in the emission of phenanthrene was observed between the two sets of exhaust samples. It is concluded that, for the 2- and 3-EtPa isomers, a dealkylation pathway does not exist, or at least is not significant, under these engine conditions. The same procedure was applied to the emission of methylphenanthrenes (MePa). Figure 5a and b show the integrated molecular ions for the MePa isomers in exhaust samples from spiked and unspiked fuel. Individual isomers were identified using retention indices, which agreed well with published data 25 and results from co-injection with standards. The elution order of the isomers was 2-MePa, 3-MePa, 9- and 4-MePa co-eluted, and finally 1-MePa. Figure 5 reveals an increase in 3MePa relative to 2-MePa in the spiked exhaust compared with the baseline exhaust. This was confirmed by measuring the ratio of the integrated peak areas of the molecular ion for the two isomers. Analysis of variance assigned the difference between the two sets of data a significance of 0.0009 at 95% C.I. No significant difference in the ratio between the 9- and 4-methyl and the 1-methyl isomers was detected in the exhaust samples. This was taken as a further indication that the increase in 3-MePa was a product of the combustion of 3-EtPa and not a chance result. The increase in the emission of 3-MePa however is equivalent to a conversion of <0.0004% of the 3-EtPa spike. These results were recently confirmed in experiments in which 2-ethylnaphthalene (2-EtNp, 2.7wt%) and synthesized 2butylnaphthalene (2-BtNp, 0.74wt%) spiked in a low aromatic diesel fuel were burnt in the Prima engine in separate experiments. No statistically significant increase in the emission of naphthalene was observed in either experiment. These results suggest that dealkylation reactions make a negligible contribution to emissions in the Prima engine under these engine conditions and for these isomers. The preferential cleavage of the alkyl C - C bond observed in this experiment, as opposed to the aryl-alkyl C - C bond, may be predicted in terms of bond energies. The alkyl-aryl bond is the strongest in

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Diesel combustion of an alkylated polycycfic aromatic hydrocarbon." P. J. Tancefl et al. (a) 2-MePa

1~,-I~,

t

3-MePa

9 & 4-MePa

70 l-MePa

@

10 0 i

,

i

,

i

,

,

,

i

,

t

•

,

(b) IN

3-MePa 9 & 4-MePa

7 t~,-t~, 2-MePa

70 l-MePa

0

Figure 5 G.c.-m.s. integrated molecular ions of methylphenanthrene isomers in (a) baseline exhaust, (b) exhaust from spiked fuel, illustrating an increase in emission of the 3-MePa isomer

the alkyl chain, and its bond dissociation energy has been estimated26 at 426.2 kJmol -l whereas the terminal C - C bond energy in ethylbenzene determined by Swarcz 17 was 264.6 kJ mo1-1. The results of the current work are also in agreement with those of the pyrolysis experiments performed by Smith and Savages. These authors pyrolysed a large number of alkyl PAH, and from detailed g.c.-m.s. analysis of the products proposed a reaction scheme for the pyrolysis of alkyl PAH. The importance of alkyl-aryl

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bond cleavage was found to be dependent not on the length of the alkyl side chain but on the position of substitution in ,the aromatic molecule. A relation between alkyl-aryl bond cleavage and the localization energy at the position of substitution, as measured by Dewar reactivity numbers, was determined. Dewar reactivity numbers are a measure of the reactivity of a particular carbon centre in an aromatic structure and are considered to represent the energy required to isolate an electron from the aromatic structure at the carbon

Diesel combustion of an alkylated polycyclic aromatic hydrocarbon." P. J. Tancell et al. centre 27. Reactivity numbers have been shown to correlate well with experimentally determined reactivities o f aromatic structures 28. Smith and Savage 5 used this model to predict that a l k y l - a r y l b o n d cleavage would occur only where alkyl substitution had taken place at c a r b o n centres with D e w a r reactivity numbers <1.33. F o r alkyl P A H with the substituent at a centre with a D e w a r reactivity n u m b e r >1.81, cleavage o f the C - C b o n d at the 3-position in the alkyl side chain was the d o m i n a n t reaction pathway. F o r P A H substituted at positions with intermediate D e w a r reactivity numbers, a mixture o f the two main p r o d u c t groups would be expected. The D e w a r n u m b e r for the 3-position in phenanthrene is 2.04 (ref. 27). On the basis o f the model proposed by Smith and Savage 5, a l k y l - a r y l b o n d cleavage would not be expected to occur. Instead, cleavage o f the C - C b o n d in the ethyl side chain would be preferred. The results from the current experiments support this hypothesis. Further investigations into the c o m b u s t i o n o f alkyl aromatics, specifically 14C-1-MeNp and l a c - 2 - M e N p , are planned using radiolabelled techniques developed in this laboratory specifically for diesel emissions research.

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACKNOWLEDGEM ENTS The authors would like to thank the Fuels Technology Unit, BP Oil, Sunbury, for supplying fuels. They would also like to acknowledge the advice given by Professor S. R o w l a n d during discussions on this work.

REFERENCES Crittenden, B. D. and Long, R. In 'Polynuclear Aromatic Hydrocarbons: Chemistry, Metabolism and Carcinogenesis' (Eds R. I. Freudenthal and P. W. Jones), Raven Press, New York, 1976

21 22 23 24 25 26 27 28

Glatt, H. et al. Environ. Health Perspect. 1990, 88, 43 Longwell,J. P. In 'Nineteenth Symposium (International) on Combustion', The Combustion Institute, Pittsburgh, 1982, p. 1339 Badger,G. M. and Spotswood, T. M. Aust. J. Chem. 1960, 13, 4420 Smith,C. M. and Savage, P. E. AIChE J. 1991, 37, 1613 Her|an, A. Combust. Flame 1978, 31, 297 Barbella,R., Ciajolo, A. and d'Anna, A. Fuel 1989, 68, 690 Trier, C. J., Rhead, M. M., Fussey, D. E., Ryder, D. and Graham, M. A. Proc. Inst. Mech. Eng. 1991, C433/010, 159 Petch,G. S., Trier, C. J., Rhead, M. M., Fussey, D. E. and Milward, G. E. Proc. Inst. Mech. Eng. 1987, C340/87, 97 Collier,T., Trier, C. J., Rhead, M. M. and Bell, M. A. Fuel 1995, 74, 362 Haworth, R. D. et al. J. Chem. Soc. 1933, 1012 Mosettig,E. and van de Kemp, J. J. Am. Chem. Soc. 1930, 52, 3704 Trier,C. J., Petch, G. S., Rhead, M. M. and Fussey, D. E. Proc. Inst. Mech. Eng. 1988, C64/88, 135 Trier,C. J., Rhead, M. M. and Fussey, D. E. Proc. Inst. Mech. Eng. 1990, C394/003, 53 Tancell,P. J., Rhead, M. M., Trier, C. J. Bell, M. A. and Fussey, D. E. Sci. Tot. Environ. 1995, 162, 179 Floysand, S. A., Kvinge, F. and Betts, W. E. SAE Paper 932683, 1993 Swarzc,M. J. Chem. Phys. 1949, 17, 431 Wiersum,U. E. Janssen Chim. Acta 1992, 10, 3 Stein,E. J. Phys. Chem. 1985, 89, 3714 Bittner,J. D. and Howard, J. B. In 'Eighteenth Symposium (International) on Combustion', The Combustion Institute, Pittsburgh, 1981, p. 1105 Glassman, I. In 'Twenty-second Symposium (International) on Combustion', The Combustion Institute, Pittsburgh, 1988 p. 295 Hamins, A., Anderson, D. T. and Miller, J. H. Combust. Sci. Technol. 1990, 71, 175 Brezinsky,K. Prog. Energy Combust. Sci. 1986, 12, 1 Freund, H., Manurro, M. G.,Olmsted, W. R. et al. Am. Chem. Soc. Div. Fuel Chem. Preprints 1990, 35, 496 Vassilaros,D. L., Kong, R. C., Later, D. W. and Lee, M. L. J. Chromatogr. 1982, 252, 1 McMillen,D. F. and Golden, D. M. Ann. Rev. Phys. Chem. 1982, 33, 493 Salem,L. 'Molecular Orbital Theory of Conjugated Systems', Benjamin, London, 1974 Dewar, M. J. S. J. Am. Chem. Soc. 1952, 74, 3357

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