Theoretical Study on Reaction Pathways Leading to CO and CO2 in

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Theoretical Study on Reaction Pathways Leading to CO and CO in the Pyrolysis of Resorcinol 2

Yuki Furutani, Shinji Kudo, Jun-ichiro Hayashi, and Koyo Norinaga J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05168 • Publication Date (Web): 01 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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The Journal of Physical Chemistry

Theoretical Study on Reaction Pathways Leading to CO and CO2 in the Pyrolysis of Resorcinol Yuki Furutani,† Shinji Kudo,†, ‡ Jun-ichiro Hayashi, †, ‡, § and Koyo Norinaga*,†, ‡ † Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, 816-8580, Japan. ‡ Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, 8168580, Japan. § Research and Education Centre of Carbon Resources, Kyushu University, Kasuga, 8168580, Japan. * Corresponding author: Koyo Norinaga, Tel: +81 92 583 7794, Fax: +81 92 583 7793, Email: [email protected].

Abstract Possible pathways for the pyrolysis of resorcinol with the formation of CO and CO2 as final products were proposed and evaluated using ab initio calculations. Our experimental study revealed that large quantities of CO2 are generated in the pyrolysis of 1,3dihydroxybenzene (resorcinol) while the pyrolysis of the dihydroxybenzene isomers 1,2dihydroxybenzene (catechol) and 1,4-dihydroxybenzene (hydroquinone) produces little CO2. The fate of oxygen atoms in catechol and hydroquinone was essentially the formation of CO. In the proposed pathways, the triplet ground state m-benzoquinone was generated initially from simultaneous cleavage of the two O-H bonds in resorcinol. Subsequently, the direct cleavage of a C-C bond of the m-benzoquinone diradical yields 2-oxidanylcyclopenta-2,4-dien-1-yl-methanone, which can be converted via two channels: release of CO from the aldehyde radical group, and combination of the ketone radical and carbon atom in the aldehyde radical group to form the 6oxabicyclo[3.2.0]hepta-2,4-dien-7-one resulted in the release of CO2. Potential energy surfaces along the proposed reaction pathways were calculated employing the CBS-QB3 method, and the rate constants at the high pressure limit were also evaluated based on transition state theory to assess the feasibility of the proposed reaction pathways.

1. Introduction Phenolic compounds are well-documented by-products of the thermal degradation of biomass, particularly the degradation of lignin.1,2 For example, dihydroxybenzene 1 ACS Paragon Plus Environment


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isomers present in cigarette smoke are biologically active and induce DNA damage, thereby causing serious health problems, such as lung cancer, heart disease, and oxidative stress.3-8 As a model for the structural groups present in lignin, the pyrolysis of dihydroxybenzene isomers has been studied both experimentally and theoretically. For example, Thomas et al.9 examined the formation of polycyclic aromatic hydrocarbons from the pyrolysis of 1,2-dihydroxybenzene (catechol). The formation of persistent semiquinone radicals (hydroxyl-substituted phenoxy radicals) during the pyrolysis of catechol and 1,4dihydroxybenzene (hydroquinone) was observed by low-temperature matrix isolation electron paramagnetic resonance spectroscopy.10–12 These semiquinone radicals contribute to the formation of notorious dibenzo-p-dioxins and dibenzofuran.13,14 Recently, Yang et al.15 performed pyrolysis experiments on catechol, hydroquinone, and 1,3-dihydroxybenzene (resorcinol) in a tubular reactor with a residence time of up to 3.6 s at 650–950 °C. Investigation of the product distribution showed that CO was the major product formed from catechol, resorcinol, and hydroquinone, with maximum yields of 27.3 wt%, 19.4 wt%, and 20.0 wt% at 950 °C over 0.3 s, respectively. Notably, the amount of CO2 (15.5 wt%) generated from resorcinol was significantly higher than that generated from hydroquinone (1.0 wt%) and catechol (0.8 wt%). In addition to experimental studies, theoretical investigations using first-principles computational chemistry have been conducted to analyze the thermal decomposition pathways of dihydroxybenzene isomers. Alsoufi et al.16 evaluated self-coupling reactions as the first step in the formation of dioxins from semiquinone radicals in terms of enthalpy and Gibbs free energy. In addition, unimolecular decomposition pathways on the potential energy surface of dihydroxybenzene isomers to yield CO have been investigated.11,17-19 Indeed, CO formation via phenoxyl and semiquinone radicals to give cyclopentadienyl (cyc-C5H5) and hydroxycyclopentadienyl radicals (cyc-C5H4OH), respectively, has been reported.17-19 In addition, Khachatryan et al.11 suggested that the ortho-benzoquinone generated from the O-H bond dissociation of ortho-semiquinone could form hexa-2,4dienedial via ring-fission, resulting in the release of CO. The fate of oxygen atoms in catechol and hydroquinone is essentially the formation of CO, while in resorcinol it is the formation of both CO2 and CO as revealed by the experiments.15 However, to date, the resorcinol decomposition pathways leading to CO2 formation upon pyrolysis are not yet to be theoretically identified. We herein describe investigations into the pyrolysis of resorcinol via CO2 and CO expulsion using first-principles computational chemistry. Initially, thermal decomposition pathways were proposed, focusing on the stages following the formation of the m-benzoquinone diradical. Finally, the potential 2 ACS Paragon Plus Environment

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energy surfaces (PES) along the proposed reaction pathways were calculated using the CBS-QB3 procedure, and the high-pressure limiting rate constants were evaluated based on transition state theory. 2. Computational Methods All electronic structure calculations were performed using the Gaussian 09 computational chemistry software.20 The PES of the proposed pathways were calculated using a series of high accuracy methods, including a complete basis set extrapolation, namely the CBSQB3 composite method.21 In this CBS-QB3 method, geometries and frequencies were calculated at the B3LYP/6-311G(2d,d,p) level, which was important for the correct localization of transition structures. To approximate higher order contributions, the CBSQB3 method uses two additional calculations, i.e., MP4(SDQ)/6-31+G(d(f),p) and CCSD(T)/6-31+G†. The spin contamination and size consistency were also corrected. In addition, by combining the results of several electronic energy calculation steps, the CBSQB3 offers greater accuracies within 1 kcal/mol.21 Structures of transition state on the PES were confirmed by determination of only one imaginary frequency along the specific reaction coordinate through analysis of the vibration frequency. Intrinsic reaction coordinate (IRC)22 calculations were performed to connect the related reactants and products. Reaction rate constants at the high-pressure limit were calculated according to transition state theory (TST) using the GPOP program.23 One-dimensional semiclassical tunneling corrections were included by assuming the asymmetric Eckart potential.24 The variational TST (VTST) was used for the calculation of the rate constants for barrierless channels.23 Energies along the reaction coordinates were evaluated by restricting the bond lengths and relaxing all the other geometric parameters. 3. Results and Discussion 3.1. Proposal of Reaction Pathways for Resorcinol Pyrolysis Scheme 1 depicts the mechanisms of various reaction pathways that lead to CO and CO2 formation during the pyrolysis of resorcinol, R. The reaction pathways present in the pyrolysis of m-semiquinone radical M1 have been mapped out where the aromatic ring contracts to form a fused bicyclic compound containing a five-membered and a threemembered ring, followed by breaking of a C-C bond in the three-membered ring, and subsequent generation of both a hydroxycyclopentadienyl radical and CO.18,19 Based on this theoretical study, reaction pathways 1 and 2 for resorcinol pyrolysis were proposed, as shown in Scheme 1. However, previous studies did not touch on the indicated reaction 3 ACS Paragon Plus Environment


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pathways following the formation of m-benzoquinone involving M6 with triplet state and M6 (Singlet) with singlet state, which was formed by the O-H bond dissociation of M1. The possible pathways are: one is along with the singlet-state pathway following the formation of M6 (Singlet); the other is along with the triplet-state pathway following the formation of m-benzoquinone M6. Kayembe et al.28 theoretically determined the most stable bicyclo[3.1.0]quinone generated from singlet-ground state m-benzoquinone M6 (Singlet), while using the B3LYP/6-311+G(2d,p) method, Roithová et al.29 considered the reaction pathways for the decarbonylation of m-benzoquinone that lead to CO release. By applying these theoretical approaches in identifying the thermal decomposition routes for CO release, we proposed the singlet-state pathway (pathways 4, Scheme 1) leading to CO formation. Analogous to a study on the decomposition of the phenoxyl radical,30,31 we proposed the triplet-state reaction pathway 5, where the direct cleavage of a C-C bond of the six-membered ring in M6 occurs to give M8, which undergoes cyclization to form the five-membered ring intermediate M11. Moreover, based on experimental product distributions detected by online gas chromatography, Yang et al.15 reported resorcinol pyrolysis pathways leading to CO2 formation. Therefore, we proposed two additional triplet-state pathways involving CO2 release (reaction pathways 6 and 7), as shown in Scheme 1. Following the formation of M11, the ketone radical combines with the carbon atom of the aldehyde radical group to form 6-oxabicyclo[3.2.0]hepta-2,4-dien-7-one M12. M13 is then generated by C-C bond breakage in the four-membered ring, resulting in the release of CO2. More specifically, hydroxycyclopentadienyl radical M5 and cyclopenta-2,4-dienone M10 can be decomposed to form ethyne or vinylacetylene with the concurrent release of CO.32,33 In addition, the five-membered ring diradical M14 is expected to capture a H atom in the gas phase, and this is followed by the formation of either naphthalene or cyclopentadiene.34,35 As expected, C2, C4 and C5 hydrocarbons and naphthalene were indeed detected experimentally.15 Although the other reaction pathways (e.g. bimolecular decomposition) might exist, we assume that the unimolecular decomposition reactions proceed along the proposed Scheme 1.

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SCHEME 1. Proposal of Reaction Pathways for CO and CO2 Formation during Resorcinol Pyrolysis.

Reaction Pathway 1: Step 1 Reaction Pathway 2: Step 1 Reaction Pathway 3: Step 1 Reaction Pathway 4: Step 1 Reaction Pathway 5: Step 1 Reaction Pathway 6: Step 1 Reaction Pathway 7: Step 1

→ → → → → → →

Step 2 Step 2 Step 2 Step 9 Step 8 Step 2 Step 8

→ → → → → → →

Step 3 → Step 4 Step 5 → Step 6 Step 3 → Step 7 → Step 14 Step 12 → Step 13 Step 10 → Step 11 → Step 14 Step 3 → Step 7 → Step 15 → Step 16 → Step 17 Step 10 → Step 11 → Step 15 → Step 16 → Step 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.2. Optimized Resorcinol Configurations We then considered the optimized configurations of resorcinol, R. Various experimental36–39 and theoretical studies40–42 have been carried out to determine the configuration of R, and in particular the relative orientation of the two hydroxyl groups. Figure 1 shows the optimized configurations at the B3LYP/6-311G(2d,d,p) level of theory along with the relative energies at the CBS-QB3 level for the three possible conformers. Relative energies of these three configurations were within computational accuracy of 1kcal/mol. We herein selected the anti-anti conformer of R for all subsequent calculations.

Resorcinol [syn-syn]

Resorcinol [syn-anti]

Resorcinol [anti-anti], R

0 [kcal/mol]

-0.64508 [kcal/mol]

-0.64571 [kcal/mol]

Figure 1. Optimized resorcinol structures at the B3LYP/6-311G(2d,d,p) level of theory. Distances are measured in Å. Relative energies (kcal/mol) of the three resorcinol configurations at the CBS-QB3 level are also given with reference to the syn-syn configuration. 3.3. Potential Energy Surfaces in the Pyrolysis Process for Each Reaction Pathway Two possible pathways exist for cleavage of the O-H bond in R, M1, and M3, as depicted in Scheme 1. These pathways are fission of the O-H bond, or H-abstraction by a H atom generated during pyrolysis. The possibility of either process taking place depends on the H atom concentration in the gas phase. For O-H bond fission, the PES along the seven proposed reaction pathways calculated at the CBS-QB3 level are described in Figure 2 (reaction pathways 1–5, CO release) and Figure 3 (pathways 6, 7, CO2 release).


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250

TS10 + 2H. 230.1

Relative Energy (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TS11 + 2H. M8 + 2H. 230.5 222.6

TS13 + 2H. M11 + 2H. 197.2 M10 + CO + 2H. TS12 + H. 190.3 189.9 190.1 187.1 179.8 . . M7 + 2H M6 + 2H 172.4 M10 (Singlet) + CO + 2H. 165.7 . TS5 + 2H. 152.8 TS3 + H . TS2 + H M2 + H. . 140.4 141.1 . 134.9 132.5 M3 + H. M4 + H.TS4 + H TS6 + H 125.2 123.4 120.7 120.3 M5 + CO + H.

. M6(Singlet) + 2H. TS9 + 2H

200

150

100

M1 + H. 86.2

106.4

50 triplet state singlet state doublet state

R 0

0 0

2

4

6 8 Reaction Process

10

12

Figure 2. PES diagram calculated at 0 K with the CBS-QB3 level of theory for the formation of CO without H-abstraction. (Reaction Pathway 1: R → M1 + H → TS2 + H → M2 + H → TS3 + H → M3 + H →TS4 + H → M5 + CO + H; Reaction Pathway 2: R → M1 + H → TS2 + H → M2 + H → TS5 + H → M4 + H → TS6 + H → M5 + CO + H; Reaction Pathway 3: R → M1 + H → TS2 + H → M2 + H → TS3 + H → M3 + H → M11 + 2H → TS13 + 2H → M10 + CO + 2H; Reaction Pathway 4: R → M1 + H → M6 (Singlet) + 2H → TS9 + 2H → M7 + 2H → TS12 + 2H → M10 (Singlet) + CO + 2H; Reaction Pathway 5; R → M1 + H → M6 + 2H → TS10 + 2H → M8 + 2H → TS11 + 2H → M11 + 2H → TS13 + 2H → M10 + CO + 2H).



250

. TS10 + 2H

.

. TS16 + 2H . 243.5 TS15 + 2H . 227.5 225.6 M12 + 2H . M13 + 2H M14 + CO . 175.4 M11 + 2H

TS11 + 2H 230.1 M8 + 2H230.5 222.6

Relative Energy (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

200 M6 + 2H

.

.

TS2 + H M2 + H 134.9

.

. TS14 + 2H

199.0

190.3

172.4

150

132.5

. TS3 + H. 140.4

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M3 + H

2

+ 2H

.

195.6

.

120.7

. M1 + H

100

86.2

50 triplet state doublet state

R

0

0

0

2

4

6 8 Reaction Process

10

12

14

Figure 3. PES diagram calculated at 0 K with the CBS-QB3 level of theory for the formation of CO2 without H-abstraction. (Reaction Pathway 6: R → M1 + H → TS2 + H → M2 + H → TS3 + H → M3 + H → M11 + 2H → TS14 + 2H → M12 + 2H → TS15 + 2H → M13 + 2H → TS16 + 2H → M14 + CO2 + 2H; Reaction Pathway 7; R → M1 + H → M6 + 2H → TS10 + 2H → M8 + 2H → TS11 + 2H → M11 + 2H → TS14 + 2H → M12 + 2H → TS15 + 2H → M13 + 2H → TS16 + 2H → M14 + CO2 + 2H).


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According to Figure 2, the ring contraction of the m-semiquinone radical M1 in step 2 (see Scheme 1) had a significantly lower activation energy (37.5 kcal/mol) than the O-H bond cleavage in step 8. Comparison of the energy barriers of steps 4 and 7 shows that the activation energy of step 7 was 65.1 kcal/mol higher than that of step 4. Thus, reaction pathways 1 and 2 were kinetically more favorable than the other reaction pathways, and their steps requiring the highest energy corresponds to the initial O-H bond homolysis of R (step 1), which had an energy barrier of 86.2 kcal/mol (experimental value = 89.0 ± 2.0 kcal/mol27). Comparison of the energy of steps 8 and 9 demonstrated that the triplet-state pathways were more kinetically favorable than the singlet-state pathways. From the data shown in Figure 3, it appears that reaction pathway 6, with a lower activation energy in step 2, was a kinetically favorable route to CO2 expulsion, with the step requiring the highest energy corresponding to step 1 (as for pathways 1 and 2).

3.4. High-Pressure Limiting Rate Constants 3.4.1. Rate Constants for Step 1, 7, 8 and 9 Comparison of the activation energies of the various reaction steps shown in Figure 2 and 3 indicates that steps 1, 7, 8 and 9 had the high energy barriers (i.e., 86.2 kcal/mol, 69.6 kcal/mol, and 86.2 kcal/mol, respectively, experimental value for step 8 = 90.0 ± 3.6 kcal/mol27). Steps 1, 8 and 9 have comparable high O-H bond dissociation energies, whereas step 7 has a smaller one probably due to the destabilization of strained five-membered ring system in M3.43 The high-pressure limiting rate constants of steps 1, 7, 8 and 9 were calculated between 300 K and 1500 K at intervals of 100 K (temperature ranging from 650 to 950 °C in pyrolysis experiment15), as shown in Figure 4. Steps 1, 7, 8 and 9 have no pronounced barrier, thus the rate constants were calculated by using variational transition state theory (VTST) based on the calculated potential energy curves as a function of the O-H bond distance (see Figure S1 in Supporting Information). Figure 4 demonstrates that step 1 is the rate-determining reaction at all temperatures among reaction pathways 1 – 3 and 5 – 7, whereas step 9 is at reaction pathway 4.



20

Step 1 Step 7 Step 8 Step 9

0 -20

ln(k[1/s])

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-40 -60 -80 -100 -120 -140 0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000/T (1/K)

Figure 4. High-pressure limiting rate constants for Step 1, 7, 8 and 9 in the temperature range of 300 – 1500 K. (Step 1: R → M1 + H; Step 7: M3 → M11 + H; Step 8: M1 → M6 + H; Step 9: M1 → M6 (Singlet) + H). As shown in Figure 2 and 3, the homolytic cleavage of O-H bond could be the initial reaction channel and result in H radical generation. O-H bond dissociation of R, M1 and M3 would be expected to occur by abstraction reactions by H atoms formed in bond scission reactions. We then consider step 1, 7, and 8 in terms of the H-abstraction. Using the B3LYP/6-311G(2d,d,p) level of theory, structures of transition state for these steps were determined and are depicted in Figure 5. In addition, Table 1 provides the calculated activation energies both with and without H-abstraction, evaluated at the CBS-QB3 level. Abstraction of the hydroxyl H atom is associated with lower activation energies, and might be compatible to steps 1, 7 and 8, which depends on H radical concentration. Figure 6 provides the calculated rate constants at high-pressure limit for these three H-abstraction steps.


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TS1

TS7

TS8

Figure 5. Optimized structure of transition state at B3LYP/6-311G(2d,d,p). (TS1, TS7 and TS8 for H abstraction of step 1, 7 and 8, respectively). Distances are measured in Å. TABLE 1: CBS-QB3 calculated activation energies Ea (kcal/mol) at 0 K for OH bond scission and H-abstraction reactions from the hydroxyl group by H atoms. Ea

Ea

Step 1

R → M1 + H

86.24

Step 19

R + H → TS1 → M1 + H2

10.56

Step 7

M3 → M11 + H

69.66

Step 21

M3 + H →TS7 →M11 + H2

6.70

Step 8

M1 → M6 + H

86.19

Step 20

M1 + H →TS8 →M6 + H2

8.67

ln(k[cm3/mol/s])

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Step 19 Step 20 Step 21

60

40

20 0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000/T (1/K)

Figure 6. High-pressure limiting rate constants for Step 19, 20 and 21 in the temperature range of 300 – 1500 K. (Step 19: R + H → TS1 → M1 + H2, Step 20: M1 + H →TS8 →M7 + H2, and Step 21: M3 + H →TS7 →M10 + H2).



3.4.2. Rate Constants for Step 2 and 8 To evaluate if M1 is mainly consumed via the M2 or the M6 reaction pathway, we compared the high-pressure limiting rate constants between step 2 and 8 as shown in Figure 7. The formation of M2 was expected to be the major exit channel for M1 since step 2 had a larger rate constant than step 8 at all temperatures, whereas with H-abstraction step 8 tended to take place as discussed in section 3.4.1.

0

Step 2 Step 8

-20

ln(k[1/s])

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-40 -60 -80 -100 0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000/K (1/K) Figure 7. High-pressure limiting rate constants for Step 2 and 8 in the temperature range of 300 – 1500 K.

3.4.3. Rate Constants for Step 14 and 15 The rate constants of step 14 decomposing into CO and step 15 into CO2 were calculated as shown in Figure 8. The rate constant of step 14 was significantly larger than step 15 at all temperatures, thus CO formation was dominant within the proposed unimolecular decomposition pathways in Scheme 1. As the other possible pathways responsible for CO2 formation, the produced CO and M10 might recombine to generate M13 through step 21 as shown in Scheme 2. The reverse reactions of step 14 and 15 are also considered as the CO and M10 recombination. When comparing the rate constant for step 21 with those of the reverse reactions as shown in Figure 9, step 21 had a lower rate constant at all temperatures and was likely the minor reaction channel. CO2 formation routes remain unresolved theoretically, and further 12 ACS Paragon Plus Environment

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insight into this aspect is left to future work. Modified Arrhenius parameters for all steps (step 1 to 22) are given in Table S1 in the Supporting Information for future research of reaction kinetic modeling. SCHEME 2: Recombination of M10 and CO generating M13.

30 20

ln(k[1/s])

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 Step 14 Step 15

0 -10 -20 -30 0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000/K (1/K) Figure 8. High-pressure limiting rate constants for Step 14 and 15 in the temperature range of 300 – 1500 K.



40

Step 21 Reverse of Step 14

ln(k[cm3/mol/s])

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

0

-20 0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000/T (1/K)

Figure 9. High-pressure limiting rate constants for Step 21 and reverse reaction of step 14 in the temperature range of 300 – 1500 K.

4. Conclusions We herein proposed seven reaction pathways for resorcinol pyrolysis that led to the formation of CO and CO2. m-Benzoquinone had a reactive ground-state triplet, which indicated the existence of post-reaction pathways leading to the formation of 2oxidanylcyclopenta-2,4-dien-1-yl. This aldehyde radical group is separated to generate CO, or combines with the ketone radical group to form 6-oxabicyclo[3.2.0]hepta-2,4dien-7-one which releases CO2 by a subsequent C-C bond breakage in the four-membered ring. The PESs of the proposed reaction pathways were calculated at the CBS-QB3 composite level. The rate constants at the high pressure limit were also evaluated based on transition state theory to assess the feasibility of the proposed reaction pathways. The results suggested that CO2 formation was not dominant within the range of the proposed reaction pathways, though the experiment of the resorcinol pyrolysis15 indicated that the formations of CO and CO2 were competitive. Further explorations for the CO2 formation mechanism in the resorcinol pyrolysis are required. Acknowledgement This research was financially supported by JST. All the computations in this study were performed on the PC cluster systems in our group and the high-performance computing 14 ACS Paragon Plus Environment

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system at the Research Institute for Information Technology, Kyushu University. Supporting Information Modified Arrhenius parameters for all steps (step 1 to 21) are given in Table S1. The potential energy along the reaction coordinate of O-H fission is shown in Figure S1. The key atomic distances of the optimized benzoquinone isomers, intermediate, and transition structures are given in Figures S2 and S3. Cartesian coordinates of all structures are available. References and Notes (1) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. Occurrence and Reactivity of Phenoxyl Linkages in Lignin and Low Rank Coal. J. Anal. Appl. Pyrolysis 2000, 54, 153–192. (2) Dellinger, B.; Pryor, W. A.; Cueto, R.; Squadrito, G. L.; Hegde, V.; Deutsch, W. A. Role of Free Radicals in the Toxicity of Airborne Fine Particulate Matter. Chem. Res. Toxicol. 2001, 14, 1371–1377. (3) Flickinger, C. W. The Benzenediols: Catechol, Resorcinol and Hydroquinone — A Review of the Industrial Toxicology and Current Industrial Exposure Limits. Am. Ind. Hyg. Assoc. J. 2010, 37, 596–606. (4) Van Duursen, M. B. M.; Sanderson, J. T.; De Jong, P. C.; Kraaij, M.; Van Den Berg, M. Phytochemicals Inhibit Catechol-O-Methyltransferase Activity in Cytosolic Fractions from Healthy Human Mammary Tissues: Implications for Catechol Estrogen-Induced DNA Damage. Toxicol. Sci. 2004, 81, 316–324. (5) William A. P. Mechanisms of Radical Formation from Reactions of Ozone with Target Molecules in the Lung. 2014, 17, 451–465. (6) Leanderson, P.; Tagesson, C. Cigarette smoke-induced DNA damage in cultured human lung cells: Role of hydroxyl radicals and endonuclease activation. Chemi. Biol. Interactions. 1992, 686, 197-208. (7) Halliwell, B. B.; Poulsen, H. E. Chapter 16. In Cigarette Smoke and Oxidative Stress, Ed.; Springer-Verlag: Berlin, 2006; p 387. (8) Pryor, W. A.; Stone, K.; Zang, L. Y.; Bermúdez, E. Fractionation of Aqueous Cigarette Tar Extracts: Fractions That Contain the Tar Radical Cause DNA Damage. Chem. Res. Toxicol. 1998, 11, 441–448. (9) Thomas, S.; Wornat, M. J. Polycyclic Aromatic Hydrocarbons from the Co-Pyrolysis of Catechol and 1,3-Butadiene. Proc. Combust. Inst. 2009, 32, 615–622. 15 ACS Paragon Plus Environment


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Theoretical Study on Reaction Pathways Leading to CO and CO2 in

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