Atmospheric Oxidation Mechanism of Benzene. Fates of Alkoxy

The fate of alkoxy radicals formed in the atmospheric oxidation of benzene initiated by OH radical is investigated by using quantum chemistry and kine...
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Atmospheric Oxidation Mechanism of Benzene. Fates of Alkoxy Radical Intermediates and Revised Mechanism Liming Wang,*,†,‡ Runrun Wu,† and Cui Xu† †

School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China Guangdong Provincial Key Laboratory of Atmospheric Environmental and Pollution Control, South China University of Technology, Guangzhou 510006, China

‡

S Supporting Information *

ABSTRACT: The fate of alkoxy radicals formed in the atmospheric oxidation of benzene initiated by OH radical is investigated by using quantum chemistry and kinetics calculations. The two alkoxy radicals (R2 and R3), formed from the commonly accepted bicyclic radical intermediates, are found to undergo ring-closure preferentially, in addition to the ring-breakage, as suggested in previous studies. The ratio between the ring-closure and ring-breakage is ∼2:1. The ringclosure route will lead to equal amounts of glyoxal and 2,3epoxybutandial, while the ring-breakage route leads to glyoxal and butenedial. Overall, the new mechanism suggests the yield of glyoxal to be three times that of butenedial, consistent with the previous experimental measurements. The new mechanism calls for the search of the newly proposed product 2,3-epoxybutandial.

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INTRODUCTION Aromatic hydrocarbons are recognized as the important precursors for ozone and secondary organic aerosol (SOA) formation in the urban atmosphere.1−7 The atmospheric aromatic hydrocarbons are emitted mainly from anthropogenic sources and are oxidized to phenolic compounds and αdicarbonyl compounds, which contribute significantly to SOA formation. There has been a wealth of studies on the oxidation mechanism of aromatic hydrocarbons. However, uncertainty persists on the formation mechanism and yields for the ringopening products. As the smallest aromatic hydrocarbon, benzene has been well studied for its oxidation mechanism, and the results have been incorporated into the atmospheric chemistry models such as the Master Chemical Mechanism (MCM)7,8 and SAPRC-07/11.9,10 In the atmosphere, the oxidation of benzene is initiated by OH addition. The benzene−OH adduct thus formed will react with the atmospheric O2 to form peroxyl radicals by addition or to form phenol by direct H-abstraction.7,11−21 Scheme 1 shows the oxidation mechanism of benzene taken from MCM 3.1/3.2,7,8 and a similar mechanism is available from SAPRC-07/11,9,10 even though trivial discrepancy between them exists. MCM eliminates the role of peroxyl radicals R0, while SAPRC-11A10 assumes the formation of muconaldehyde via the reduction of R0 to R1. Theoretical study19 suggested that the peroxyl radicals would cyclize exclusively to the bicyclic intermediates (R26), while the reactions of R0 with the atmospheric trace species (NOx, HOx, ROx, etc.) are too slow, supporting the MCM mechanism. The bicyclic radical will either break the −O−O− to © 2013 American Chemical Society

form epoxy alkoxy radicals (R2) or combine with the atmospheric O2 to eventually form bicyclic alkoxy radicals (R3), which would break down to glyoxal (GLY) and coproducts butenedial. The mechanism thus predicts the same yields for GLY and butenedial as the coproduct. However, the yield of GLY was always measured to be higher than that of butenedial in the experimental studies of benzene oxidation.11,15,21,22 To be in accord with the experimental fact, MCM splits the C4H4O2 coproduct equally into butenedial and 2(3H)-furanone. The latter, however, has never been experimentally observed, even though a similar product, 3,5-dimethyl2(3H)-furanone, was tentatively assigned in a chamber study of 1,3,5-trimethylbenzene.23 Similarly, SAPRC-119,10 splits butenedial into symbolic AFG1 and AFG2, which are photolyzed to radical and stable products, respectively, and forces the total yield of AFG1 (0.205) and AFG2 (0.105) to be equal to that of GLY (0.310). The MCM mechanism in Scheme 1 identifies two important alkoxy radical intermediates R2 and R3. For radicals R2 and R3, previous studies24−29 have naturally assumed C−C bond breakage by analogizing to the fates of alkoxy radicals encountered in the atmospheric oxidation of alkanes and alkenes.30−32 Ring-breakage of R2 leads to the formation of epoxymuconaldehyde (P1 in Scheme 1), and that of R3 leads to equal amounts of GLY and butenedial. However, it is noticed that Received: October 14, 2013 Revised: November 30, 2013 Published: December 2, 2013 14163

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Scheme 1. Atmospheric Oxidation Mechanism of Benzene from MCM 3.2

atmosphere at ∼2.5 × 105 s−1 if assuming rate constants of ∼5 × 10−14 cm3 molecule−1 s−1 as for other alkoxy radicals.30,40 Transition states have been identified at the BH&HLYP level for ring-closure and ring-breakage, and Figure 1 shows the Gibbs

R2 and R3 are structurally different from the “normal” alkoxy radicals by possessing a neighboring >CC< bond; therefore, they may behave differently. Previous theoretical study33 indeed suggested that radical R1 would instead cyclize to epoxidecontaining radicals, and our recent studies on the oxidation of naphthalene and 2,7-dimethyl naphthalene34,35 also suggested that the alkoxy radicals similar to R1 would also close the ring to epoxide-containing radicals. This casts doubts as to whether R2 and R3 would cyclize to epoxide-containing radicals as well. This paper examines the fates of R2 and R3 using quantum chemistry and kinetics calculations and proposes a revised mechanism for benzene oxidation in the atmosphere.

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COMPUTATIONAL METHODS The molecular structures are optimized at the BH&HLYP/6311++G(2df,2p) level, which has been found to perform satisfactorily for kinetics study.36 The transition states are confirmed by viewing the displacement vector of the transition modes. Because of the heavy spin contamination that is typical for delocalized aromatic rings, the optimized structures are subject to single-point energy calculation using the restricted open-shell complete basis set model chemistry (ROCBSQB3),37 for which the uncertainty in the predicted energy is ∼4 kJ/mol, as estimated from the mean absolute deviation of ROCBS-QB3 for a set of open-shell radicals. All of the quantum chemistry calculations are carried out using the Gaussian 09 suite of programs.38 The rates for the unimolecular reactions at their high-pressure limit are estimated using kUni = kBT/h·exp(−ΔrG‡/ RT).39

Figure 1. The Gibbs energy diagram of the R2-syn radical (298 K, in kJ/ mol).

energies for these processes. Detailed reaction energetics are listed in Table 1. The barrier for ring-closure is ∼24 kJ/mol lower than that for ring-breakage. Consequently, radical R2 will undergo ring-closure to radical R21 exclusively. To be more quantitative, the fate of R2-syn is modeled as

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RESULTS AND DISCUSSION Fate of Radicals R2. Radicals R2 are formed from the ringbreakage of the bicyclic radical R26 (Scheme 1). Previous theoretical study19 has found that R26 was formed almost exclusively as the syn conformer, that is, the −OH and −OO− groups are added to the benzene ring from the same side. Therefore, radical R2 is formed as the syn conformer as well. R2syn is subject to ring-closure to R21, ring-breakage to R22, and Habstraction by O2, of which the latter will be slow in the 14164

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Table 1. Reaction Energetics (in kJ/mol) at the ROCBS-QB3//BH&HLYP/6-311++G(2df,2p) Level and the High-Pressure Limit Rate Constants for the Forward and Reverse Unimolecular Processes (kF and kR, in s−1) reactions

ΔE0K

ΔG298K

ΔE0K‡

ΔG298K‡

kF

kR

R2-syn → R21 R2-syn → R22 R2O-syn → R24 R2O-anti → R24 R3-anti → R32 R3-anti → R33-anti R3-syn → R32 R3-syn → R33-syn R3O-anti-5O-syn → C5C6-breakage R3O-anti-5O-syn → C4C5-breakage R3O-anti-5O-anti → C5C6-breakage R3O-anti-5O-anti → C4C5-breakage

−2.0 −8.6 ∼−120 ∼−120 ∼−180 0.1 ∼−180 12.5

−1.5 −13.5 ∼−130 ∼−130 ∼−190 0.8 ∼−190 13.0

18.5 45.0 63.1 51.6 32.1 17.5 34.2 30.6 26.9 67.4 27.1 59.4

19.4 43.8 59.6 48.9 31.1 19.1 33.3 31.7 26.2 65.9 26.5 58.2

2.49 × 109 1.29 × 105 2.24 × 102 1.71 × 104 2.19 × 107 2.84 × 109 9.01 × 106 1.71 × 107 1.62 × 108 1.75 × 101 1.41 × 108 3.90 × 102

1.34 × 109 5.64 × 102 slow slow slow 3.97 × 109 slow 3.24 × 109 slow slow slow slow

where both ring-breakage and ring-closure are considered as being reversible, and the radicals R21 and R22 are also drained away by the atmospheric O2 at rates of ∼108 s−1. Here, the bimolecular rate constants with O2 (kb1 and kb1) are assumed as ∼10−11 cm3 molecule−1 s−1 by referring kb1 to that for reaction between cyclohexyl and O2 and kb2 to that between CH3CHOH and O2 at 298 K.41−43 If using the high-pressure limit rates for the unimolecular processes (Table 1, 2.5 × 109, 1.3 × 109, 1.3 × 105, and 5.6 × 102 s−1 for k1, k−1, k2, and k−2, respectively, at 298 K), the effective rates of R2 removal by ring-closure and ringbreakage could be estimated as kEff,RC =

k1k b1[O2 ] k k [O ] ≈ 1 b1 2 ≈ 2.5 × 108 s−1 (k −1 + k b1[O2 ]) k −1

kEff,RB =

k 2k b2[O2 ] ≈ k 2 = 1.3 × 105 s−1 (k −2 + k b2[O2 ])

Fate of Radicals R3. Radicals R3 are formed from addition of O2 to R26-syn, followed by reactions with NOx, HOx, ROx, and so forth. Transition states are identified for O2 addition to R26-syn from the syn and anti directions, with the barrier for anti addition being lower by 7.6 (ΔE0K‡) or 8.4 kJ/mol (ΔG298K‡), resulting in a ratio of ∼32:1 for R3-anti to R3-syn. Figure 2 shows the Gibbs energies for the R3 radicals and the unimolecular processes. Being similar to R2, ring-breakage to

Therefore, R2-syn will be removed by ring-closure almost exclusively, and the ultimate product will be R21-OO-a/s, where a/s = anti/syn is relative to the −OH group. Formation of epoxyl muconaldehyde (P1 in Scheme 1) is not expected from R2-syn even if R2-syn is formed. In the atmosphere, R21-OO-s/a will react with NO or HOx and be partially reduced to R2O-s/a, which is also subject to ring-breakage and bimolecular Habstraction by O2

Figure 2. The Gibbs energy diagram of R3 radicals (298 K, in kJ/mol).

form R31/R32 and ring-closure to form R33 are available for R3, and only the ring-closures are reversible. Reaction of R3 with O2 through H-abstraction is again negligibly slow at rates of ∼105 s−1, and radicals R33 are assumed to recombine with the atmospheric O2 at rates of ∼108 s−1, forming peroxyl radical R33anti-OO. Assuming a high-pressure limit for the unimolecular processes and applying analysis similar to R2, we estimate the effective rates of R3 removal by ring-breakage and ring-closure as 9.0 × 106 and ∼4 × 105 s−1 for R3-syn and 2.2 × 107 and ∼5 × 107 s−1 for R3-anti, all being much higher than the H-abstraction by O2. Therefore, the minor conformer R3-syn will undergo ringbreakage, forming GLY and butenedial, while the dominant conformer R3-anti will undergo ring-breakage and ring-closure with a somewhat arbitrary ratio of 1 to 2 (2.2 to ∼5). Radical R33-anti-OO would also react with NO and HOx and be reduced to alkoxy radicals R3O-anti-5O

Formation rate for P2 is again ∼105 s−1. Ring-breakage will lead directly to R24 formation because our calculations show that the process from R23 to R24 is highly exothermic by more than 120 kJ/mol and no transition barrier can be found for the process. For the ring-breakage, the Gibbs barriers at 298K are 48.9 and 59.6 kJ/mol, rendering their high-pressure limit rates of 1.7 × 104 and 2.2 × 102 s−1 for R2O-anti and R2O-syn, respectively. Therefore, the products expected from R2 include P2 (C6H6O4) from R2O and the GLY and 2,3-epoxy-butandial (P3) from R24 14165

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Scheme 2. Revised Atmospheric Oxidation Mechanism of Benzene

identified in the benzene oxidation studies. The appearance of 2,3-epoxymuconaldehyde in the model mechanisms arose likely from the theoretical energy calculations of the epoxidecontaining radical intermediates in toluene oxidation by Bartolotti and Edney,44 the observation of epoxide compounds from the oxidation of toluene in a smog chamber by Yu and Jeffries,45 and the observation of 2,3-epoxymuconaldehyde in the oxidation of benzene oxide/oxepin by Klotz et al.46 On the other hand, the RRKM-ME study by Glowacki et al.19 found only ∼1% of prompt formation of R2 from R26-syn under the atmospheric pressure. Furthermore, our calculations show that the ultimate product from R2-syn is P2 (C6H6O4), not muconaldehyde or epoxymuconaldehyde. Therefore, the current mechanism revises the fraction for prompt formation of R2 to a smaller value of ∼5%. For O2 addition to R26-syn, 92% is assigned to O2 addition from the anti direction and 3% from the syn direction (a ratio of ∼32:1). With the new mechanism in Scheme 2, the yields of 0.38 and 0.15 are predicted for GLY and butenedial, respectively, if assuming that the conversion of all peroxy radicals to alkoxy radicals is all due to NO at a fixed yield of 0.918 as in MCM. The values agree with the observed yields of ∼31% for GLY and ∼10% for butenedial.11,15,21,22 The new mechanism predicts high yield for 2,3-epoxybutandial and calls for the experimental hunting for this compound in benzene oxidation. Note that the photolysis of 2,3-epoxybutandial should be slow (∼6 × 10−5 s−1 for noon at 40° N at 0.5 km altitude if assuming the same photolysis rate as glycidaldehyde) compared to that of butenedial (∼10−3 s−1),1,47 but high reactivity with water is expected because of the epoxide group. 2,3-Epoxybutandial would also be removed rapidly by OH radical if considering the rate constant of 1.7 × 10 −11 cm3 molecule−1 s −1 for reaction between glycidaldehyde and OH radical.47 The route to 2,3-epoxybutandial involves an additional NO2 formation in converting R3anti to R3O-anti-5O; therefore, the new mechanism may predict a different ozone formation potential for benzene, though a quantitative prediction on NOx-dependent ozone formation cannot be derived from the current mechanism. The formation of 2,3-epoxybutandial may also change the predicted SOA yield.

which are subject to C4−C5 and C5−C6 bond breakages and to H-abstraction by O2 at a rate of ∼105 s−1. The reaction energies and rates for these bond breakages are listed in Table 1. Obviously, the fate of R3O-anti-5O radicals is to break the C5− C6 bond at rates > 108 s−1, forming GLY and 2,3-epoxybutandial

Therefore, the dominant conformer R3-anti will be transformed as ring-breakage and ring-closure with a ratio of ∼1 to 2. The products after ring-breakage are GLY and butenedial, while the products after ring-closure are GLY and 2,3-epoxybutandial. Revised Atmospheric Oxidation Mechanism of Benzene. Our calculations show that the fates of R2 and R3 are different from the previous assumptions. Current and previous theoretical results are incorporated into the existing oxidation mechanism of benzene in MCM 3.2. The new mechanism proposed is shown in Scheme 2. As in MCM, we first eliminate the involvement of radical R1 from the atmospheric oxidation mechanism of benzene, even though SAPRC-11A still assigns ∼6% of muconaldehyde (AFG3) production via R1. Only in the laboratory study at [NO] of ∼1014 molecules cm−3 (∼10 ppm) could radicals R1 be formed from the reaction of C6H6−OH−OO with NO and subsequently play a role.12 Radicals R2 come from R26 by breaking the −O−O− bond. MCM assumes a yield of ∼12% for 2,3-epoxymuconaldehyde via the R26 → R2 route, while SAPRC-11 assumes only ∼3.1% for 2,3-epoxymuconaldehyde (AFG5). However, neither muconaldehyde nor 2,3-epoxymuconaldehyde has been positively 14166

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ASSOCIATED CONTENT

S Supporting Information *

The Gaussian-09 output files for important intermediates and products, containing the BH&HLYP/6-311++G(2df,2p) geometries, dipole, vibrational frequencies, and infrared intensities, are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from NSF China (No. 21177041) and the service support from SCUTGrid from Information Network Engineering and Research Center of South China University of Technology.

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