progress in modeling aromatic combustion kinetics

PROGRESS IN MODELING AROMATIC COMBUSTION KINETICS 1

Gabriel da Silva, Joseph W. Bozzelli 1

2

Department of Chemical and Biomolecular Engineering The University of Melbourne Parkville, Victoria 3010, Australia [email protected] 2

Department of Chemistry and Environmental Science New Jersey Institute of Technology Newark, New Jersey 07102, USA

ABSTRACT Results are reported for important developments in modeling the combustion and oxidation

kinetics of aromatic compounds, using theoretical techniques in

computational chemistry and statistical rate theory.

Aromatic hydrocarbons are

important fuel components and atmospheric pollutants, and are also involved in the formation of unwanted soot and PCDD/Fs.

First, unimolecular and bimolecular

reactions of the resonantly stabilised benzyl radical are considered. Novel, low-energy pathways are presented for the thermal decomposition of benzyl to fulvenallene + H, and kinetic modeling results are in good agreement with the measured rate of benzyl decomposition. The proposed reaction should be important at higher temperatures, but at lower temperatures bimolecular associations reactions with radical species like HO2 and OH are expected to dominate. The benzyl + HO2 reaction process is modeled, and is shown to lead to benzoxyl + OH formation. Several rapid reaction pathways are available from the benzoxyl radical, to benzaldehyde + H, phenyl + CH2O, and benzene + HCO. Finally, the reaction of substituted phenyl radicals with O2(3P) is considered. The results of this study provide a greater fundamental understanding of the reaction processes and products in aromatic combustion and oxidation, as well as offering improved input parameters for kinetic models.

G. da Silva, J. W. Bozzelli INTRODUCTION Aromatic hydrocarbons are of wide occurrence and importance in combustion and atmospheric chemistry. Aromatics are one of the major components of petroleumderived fuels, including gasoline, jet fuels, and to a lesser extent diesel, while they are also present as functional groups in the solid fuels coal and biomass. Aromatic compounds like benzene (a suspected carcinogen) and soot form as unwanted combustion products, while the super-toxic polychlorinated dibenzo dioxins and dibenzo furans (PCDD/Fs) which can form in combustion contain an aromatic backbone. As fuels, aromatic hydrocarbons demonstrate high knock resistance (i.e., high octane rating), and have found increased use in gasoline since the phasing out of alkyl lead octane boosters. Furthermore, current efforts to reduce benzene levels in gasoline have resulted in greater use of substituted aromatics like toluene and other methyl- and alkyl-benzenes. Despite the near universal importance of alkyl-benzenes in gas phase chemistry, significant uncertainties remain with regard to the fundamental chemical processes involved in their oxidation and thermal decomposition. In particular, the chemical reactions involved in the ignition of alkylbenzenes are not understood, and their relative reactivity has not been sufficiently explained. Recently, we have been investigating several aspects of aromatic combustion, particularly related to toluene, using ab initio computational chemistry and statistical rate theory. Here, we highlight recent results that have led to the identification of several important new pathways and products in the toluene oxidation mechanism

METHODOLOGY A variety of theoretical methods have been employed to study aromatic combustion chemistry. A brief overview of these methods is provided here, with detailed descriptions available elsewhere. Ab initio and density functional theory (DFT) calculations have been performed in the Gaussian 03 software package (Frisch et al., 2004). The B3LYP DFT method has generally been used for geometry optimizations. High-accuracy molecular energies are obtained using composite theoretical methods such as G3B3 and G3X. These methods typically offer mean deviations from experiment of 1 kcal mol-1 or below. Thermochemical properties, including heats of formation (∆fH°298), entropies (S°298), and heat capacities (Cp(T)) have been obtained for all species reported here. Standard heats of formation have been determined using either atomization or isodesmic work reactions. Atomization work reactions utilise the accurately known enthalpies of the atomic elements in calculating heats of formation. Isodesmic reactions reduce several systematic error sources present in the atomization approach by conserving bonding across the work reaction. When accurate heats of formation are available for reference species in the work reaction, the use of isodesmic reactions can reduce mean errors from around 1 to 0.5 kcal mol-1. Following the evaluation of ∆fH°298, this value is used with calculated geometrical and vibrational parameters to obtain thermochemical

Chemeca 2008 Conference, Newcastle City Hall, New South Wales, Australia, 28 September - 1 October

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G. da Silva, J. W. Bozzelli properties as a function of temperature according to standard statistical mechanical techniques. Low-frequency internal rotational modes are treated as hindered rotors. Elementary rate constants in the high pressure limit are evaluated using canonical transition state theory (CTST), in conjunction with thermochemical properties for reactants, products, and transition states obtained using the above methods. Reactions involving movement of H atoms (i.e., hydrogen abstraction, intramolecular hydrogen shift, and R—H bond dissociation reactions) are corrected for quantum mechanical tunnelling through the potential energy surface. Rate constants for barrierless association reactions (and their reverse dissociations) are treated using canonical variational transition state theory (CVTST). The general CVTST procedure involves locating optimised structures along the reaction coordinate and then evaluating elementary rate constants for each structure (da Silva and Bozzelli, 2008). Rate constants are then minimised as a function of position along the reaction coordinate in order to obtain the variational reaction rate. Apparent rate constants in chemically activated bimolecular association mechanisms, and in multi-well thermal decomposition processes, are evaluated in k(E) using RRKM theory with master equation analysis for pressure fall-off, as implemented in the ChemRate program (Mokrushin et al., 2006). The methylphenyl + O2 reaction is treated using quantum RRK theory (QRRK), which uses a set of three reduced vibrational frequencies to accurately model densities of states (Sheng et al., 2002). In all cases an exponential-down model is used to describe collisional energy transfer.

RESULTS AND DISCUSSION The initial stage in the combustion of benzene is the loss of a ring hydrogen atom through bimolecular abstraction reactions or unimolecular dissociation, producing the phenyl radical. Phenyl C—H bonds are relatively strong (~ 113 kcal mol-1), and their abstraction reactions are generally slow. However, reactive phenyl radicals are rapidly oxidised, especially via reaction with O2. In toluene, combustion is complicated by the presence of a methyl side-chain. Bond dissociation energies (BDEs) in toluene are illustrated in Scheme 1, and by far the weakest bond here is on the benzyl carbon (C6H5CH2—H). Abstraction reactions from benzyl sites are much more rapid than from the phenyl sites, and benzyl radicals therefore dominate the initial stages of toluene combustion. Toluene pyrolysis also leads predominantly to benzyl radical formation (+ H), although C—C bond scission to phenyl + CH3 is also important, especially at higher temperatures (Oehlschlaeger et al., 2006a). Benzyl is a resonantly stabilised radical, and unlike the phenyl radical it reacts slowly with molecular oxygen. As a result, thermal decomposition of the benzyl radical, as well as bimolecular reactions with radical species like OH and HO2, assume increased importance. The low reactivity of benzyl also provides a role for the more active methylphenyl and phenyl radicals. We have recently studied several important reaction processes in the combustion of benzyl and methylphenyl radicals, in an effort to improve existing kinetic models for toluene oxidation. Here we present kinetic modeling results for the thermal


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G. da Silva, J. W. Bozzelli decomposition of benzyl, for the benzyl + HO2 reaction, and also for the reaction of methylphenyl radicals with O2. Thermal Decomposition of the Benzyl Radical As demonstrated above, the resonantly stabilised benzyl radical is the major initial product in the oxidation of toluene. In toluene pyrolysis, the benzyl radical can self-react to form stilbene + H, re-associate with H atoms to return toluene, or undergo unimolecular thermal decomposition. Experimentally, benzyl is known to decompose with an activation energy of around 80 – 90 kcal mol-1, making it relatively stable at temperatures of ca. 1500 K and below (Baulch et al., 1992; Oehlschlaeger et al., 2006b). However, while the kinetics are well established, there is some disagreement over the decomposition products. Earlier studies have suggested products including C5H5 + C2H2 and C4H4 + C3H3, although more recent results have identified that the decomposition products are a C7H6 fragment plus H (Fröchtenicht et al., 1994; Eng et al., 2002). The structure of this C7H6 species, however, remains unknown. We propose that the unidentified C7H6 fragment in benzyl decomposition is fulvenallene. Figure 1 illustrates a potential energy diagram for decomposition of benzyl to fulvenallene + H, featuring G3X heats of formation. In the proposed mechanism benzyl decomposition is initiated by contraction of the C6 ring to form a bicyclic species, which subsequently ring opens to a cyclopentadiene-ethenyl radical. This process demonstrates a barrier of about 70 kcal mol-1 above the benzyl radical. From here, the lowest energy pathway is provided by isomerization to the cyclopentenylallene radical, followed by barrier-less C—H bond dissociation to fulvenallene + H. These products can also be formed directly via H loss from the cyclopentadiene-ethenyl radical, in a somewhat higher energy process. Decomposition of the benzyl radical to fulvenallene + H requires a barrier of 84.9 kcal mol-1, making it the lowest energy pathway considered. High-pressure limit rate constants have been calculated for each of the elementary reaction steps depicted in Figure 1. The C—H bond dissociation reaction in the cyclopentenyl-allene radical has been treated variationally, with k [s-1] = 1.02×1013T0.342exp(-23500/T). Apparent rate constants have been evaluated as a function of temperature and pressure in this multi-channel multi-well reaction system according to RRKM reaction rate theory. In order to accurately reproduce fall-off at higher temperatures, a ‹∆Edown› value of 2000 cm-1 was required. Calculated rate constants for benzyl decomposition are plotted in Figure 2, along with the experimental results of Baulch et al. (1992) and Oehlschlaeger et al. (2006b). We find that agreement between experiment and theory is excellent, and well within the uncertainty of our calculations. We therefore conclude that fulvenallene is the previously unidentified C7H6 product in benzyl decomposition. We are currently studying the further reactions of fulvenallene in combustion systems. Kinetics and Mechanism of the Benzyl + HO2 Reaction While the overall reaction kinetics for the benzyl + O2 reaction are slow, benzyl does react rapidly with radicals such as OH and HO2. In toluene combustion, these


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G. da Silva, J. W. Bozzelli reactions are expected to dominate following establishment of the radical pool. The hydroperoxyl radcial (HO2) is one such species, and it is known to be of particular importance in low-temperature phernomena such as autoignition and cool flame phenomena. In toluene combustion, HO2 will form by abstraction reactions between O2 and hydrocarbon radicals. HO2 will continue to react in association reactions, or by abstracting a further hydrogen atom to form H2O2. H2O2 is an important chain-branching species in combustion, promoting ignition by HO—OH bond scission to form two reactive OH radicals. There is considerable uncertainty as to the rate at which benzyl reacts with HO2, and the products that form. Accordingly, we have undertaken to study this reaction process. A potential energy diagram for the benzyl + HO2 reaction on the singlet potential energy surface, calculated at the G3B3 level of theory, is presented here in Figure 3. A barrierless association reaction produces an activated benzylhydroperoxide adduct, formed with over 60 kcal mol-1 of excess energy. Several low-energy pathways are available for the dissociation of this adduct, with the lowest energy pathway leading to formation of the benzoxyl radical (C6H5CH2O) + OH. Elementary rate constants in Figure 3 have again been calculated from transition state theory, with CVTST for the barrierless radical + radical associations. Rate constants in the chemically activated benzyl + HO2 mechanism have been determined using RRKM theory, and the results at 1 atm pressure are reported here in Figure 4. We find that at temperatures of around 800 K and greater the only important reaction channel is for the production of benzoxyl radicals + OH, with rate constant of about 5×1012 cm3 mol-1 s-1. At lower temperatures, the formation of quenched benzylhydroperoxide molecules also becomes important, and the further reactions of this species will also need to be considered. Current oxidation mechanisms for toluene consider decomposition of benzoxyl radicals to benzaldehyde + H, and in some instances to phenyl + CH2O. We have calculated barriers of 30.5 and 17.6 kcal mol-1 for these respective reactions, at the G3X level. Additionally, a molecular elimination reaction leading to benzene + HCO has also been identified, with a barrier of 20.9 kcal mol-1. Methylphenyl Radical Oxidation Methylphenyl radicals are formed in toluene combustion by the abstraction of ring hydrogen atoms by reactive radicals like OH. While benzyl radicals still dominate, formation of methylphenyl radicals is important, even at low to moderate temperatures (Uc et al., 2006). Pathways to methylphenyl radicals also exist in the oxidation of polymethylbenzenes, as depicted in Scheme 2 (where the methylbenzoxyl precursor can be formed by the methylbenzyl + HO2 reaction) (da Silva et al., 2007). An energy diagram for isomerization of the three methylphenyl radicals with the benzyl radical (with G3X level heats of formation) is shown in Figure 5. We find that the m- and p-methylphenyl radicals are relatively stable, with barriers to isomerization of around 60 kcal mol-1. The o-methylphenyl radical is less stable, isomerising to benzyl with a barrier of around 50 kcal mol-1. Additionally, methylphenyl radicals will exchange an H atom with toluene, forming toluene + benzyl with a relatively low


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G. da Silva, J. W. Bozzelli barrier. Irrespective, once formed, methylphenyl radicals are expected to exist for significant lifetimes in combustion environments, especially at low to moderate temperatures. Phenyl radicals are known to associate with O2 without a barrier, forming phenylperoxy adducts with around 50 kcal mol-1 of excess energy (da Silva and Bozzelli, 2008). The barrierless phenyl + O2 association reaction, as well as the dissociation of phenylperoxy to phenoxy + O, have been treated previously using CVTST (da Silva and Bozzelli, 2008). Rate constants in the o-methylphenyl + O2 mechanism (Figure 6) have been modelled using QRRK theory (da Silva et al., 2007), and are plotted in Figure 7. Results for the m- and p-methylphenyl radicals are similar, except for the absence of the o-QM pathway. In Figure 7 we find that, for temperatures over ca. 1000 K the production of methylphenoxy radicals + O dominates. At lower temperatures, the most significant reactions are for the formation of methyl-dioxohexadienyl radicals and o-quinone methide + OH. Kinetic modeling is currently being performed in order to ascertain the relative importance of methylphenyl radical oxidation, especially in the early stages of combustion.

REFERENCES Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just, T.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J. Phys. Chem. Ref. Data 1992, 21, 411. da Silva, G.; Bozzelli, J. W. J. Phys. Chem. A 2008, 112, 3566. da Silva, G.; Chen, C.-C.; Bozzelli, J. W. J. Phys. Chem. A 2007, 111, 8663. Eng, R. A.; Gebert, A.; Goos, E.; Hippler, H.; Kachiani, C. Phys. Chem. Chem. Phys. 2002, 4, 3989. Frisch, M. J., et al. Gaussian 03, Revision D.01, Gaussian, Inc., Wallingford CT, 2004. Fröchtenicht, R.; Hippler, H.; Troe, J.; Toennies, J. P. J. Photochem. Photobiol. A: Chem. 1994, 80, 33. Mokrushin, V.; Bedanov, V.; Tsang, W.; Zachariah, M.; Knyazev, V. ChemRate, Version 1.5.2, National Institute of Standards and Testing, Gaithersburg, MD, 2006. Oehlschlaeger, M. A.; Davidson, D. F.; Hanson, R. K. Comb. Flame 2006a, 147, 195. Oehlschlaeger, M. A.; Davidson, D. F.; Hanson, R. K. J. Phys. Chem. A 2006b, 110, 6649. Sheng, C.; Chang, A. Y.; Dean, A. M.; Bozzelli, J. W. J. Phys. Chem. A 2002, 106, 7276. Uc, V. H.; Alvarez-Idaboy, J. R.; Galano, A.; García-Cruz, I.; Vivier-Bunge, A. J. Phys. Chem. A 2006, 110, 10155.

BRIEF BIOGRAPHY OF PRESENTER Dr Gabriel da Silva is a Lecturer in the Department of Chemical and Biomolecular Engineering at the University of Melbourne. Dr da Silva studied for his B.Eng. and Ph.D. in Chemical Engineering at the University of Newcastle, completing his dissertation with Profs. Dlugogorski and Kennedy on the mechanism and kinetics of nitrosation reactions. Following his Ph.D., Dr da Silva accepted a postdoctoral position at New Jersey Institute of Technology, in the field of combustion and atmospheric chemistry, before returning to Australia in 2007 to take up his current appointment at the University of Melbourne.


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G. da Silva, J. W. Bozzelli Diagrams: 91.7 ± 1.0 H H2C 103.6 ± 0.5 H

112.9 ± 0.5

Scheme 1: Important bond dissociation energies in toluene (in kcal mol-1).

CH2O

CHO

CO

-H

-H

- CH2O

- CO

- HCO

Scheme 2: Pathways to methylphenyl radicals from poly-methylbenzenes. CH2 C 140.7

+H 137.3

122.7

128.8 CH2

113.4

CH2

H CH2 C 110.6

H2C

103.1 90.6

CH2

52.4

Fig. 1: Energy diagram for thermal decomposition of the benzyl radical (with G3X heats of formation).


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G. da Silva, J. W. Bozzelli

106 This study Baulch et al. Oehlschlaeger et al.

105

k (s-1)

104

103

102

101 0.50

0.55

0.60

0.65

0.70

0.75

-1

1000/T (K )

Fig. 2: Comparison of theoretical and experimental rate constants (k, s-1) for decomposition of the benzyl radical (P = 1.5 bar). CH 2 + HO2 52.4

55.4

H 2C

O 48.8 + OH

41.1 40.5 + CH2 O + H 2O 27.2

H2 C

45.5

O OH H

HC

OH

+ OH 24.3 20.4

H 2C

OOH

-5.0

HC

O + H 2O

-66.1

Fig. 3: Energy diagram for reaction of the benzyl radical with HO2 (with G3B3 heats of formation).


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1015 1014

k (cm3 mol-1 s-1)

1013 1012 1011

Benzylhydroperoxide Benzoxyl + OH (concerted) Benzyl + HO2

1010

α-Hydroxybenzyl + OH Benzaldehyde + H2O Benzyne + CH2O + H2O

109

Benzoxyl + OH (step-wise)

108 107 106 300

600

900

1200

1500

1800

2100

Temperature (K)

Fig. 4: RRKM rate constants (k, cm3 mol-1 s-1) for the benzyl + HO2 reaction. CH 3

CH 3

CH 3 136.9

135.9

122.8

75.9

75.4

CH 2

75.4 52.4

Fig. 5: Energy diagram for isomerization of the methylphenyl and benzyl radicals (G3X heats of formation).


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TS6 CH3 +O2 TS1

TS10

CH3

74.5

TS3

O

O O

CH3 6 3.3

63.3

TS9

60.3 TS4

54.2

O CH 2

O CH3 OO

CH 3 O O

TS8

OOH

63.5

TS2 CH3

CH3 TS7

+O

TS5

CH3 O O

OOH TS11 CH2

3 7.7

O + OH

31.4 25.8

21.6 TS12

O

O

O

+H

TS14 O

TS13

2 2.1

CH3

CH3

CH 3

O

O -0.2

-2.1 O

H3C O

O

O CH 3

-25.4

-22.8 CH3 O OH -47.9

Fig. 6: Energy diagram for reaction of the o-methylphenyl radical with O2.

13

-1

logk (cm mol s )

12

3

-1

2-methylphenoxy + O 2- and 6-methyl-1,6-dioxo-2,4-hexadienyl ortho-quinone methide + OH 2-methylphenyl + O2

11

3-methyl-1,2-benzoquinone + H 2-methylphenylperoxy 10

9 0.38

0.61

0.84

1.07

1.30

-1

1000/T (K )

Fig. 7: QRRK rate constants (k, cm3 mol-1 s-1) for the o-methylphenyl radical + O2. reaction.


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