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Theoretical Study on the Kinetics of Thermal Decomposition of Guaiacol and Catechol Yuki Furutani, Yuki Dohara, Shinji Kudo, Jun-ichiro Hayashi, and Koyo Norinaga J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08112 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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The Journal of Physical Chemistry

Theoretical Study on the Kinetics of Thermal Decomposition of Guaiacol and Catechol Yuki FURUTANI1, Yuki DOHARA1, Shinji KUDO2, Jun-ichiro HAYASHI2,3, and Koyo NORINAGA*4 1 Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1, Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan. 2 Institute for Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan. 3 Research and Education Centre of Carbon Resources, Kyushu University, 6-1, Kasugakoen, Kasuga, Fukuoka, 816-8580, Japan. 4 Department of Chemical Systems Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan. *Corresponding author: Koyo NORINAGA, Tel +81 52 789 3618, Fax +81-52-789-3272 Email: [email protected] Abstract The theoretical aspects of the development of a chemical kinetic model for guaiacol and catechol pyrolysis are presented to describe the pyrolysis behaviors of the individual lignin-derived components. The possible pyrolysis pathways involving both unimolecular and bimolecular decomposition were investigated by the potential energy surfaces (PES) calculated at CBS-QB3 level. The high-pressure limiting rate constants of each elementary reaction step were evaluated based on the transition state theory (TST) to determine the dominant pyrolysis pathways. The kinetic analysis results predicted the most favorable catechol unimolecular decomposition pathways, where catechol isomerization to 2-hydroxycyclohexa-2,4-dien-1-one occurred via migration of the hydroxyl H atom, followed by decomposition into 1,3-butadiene, acetylene, and CO. In the case of the bimolecular reaction of catechol, a hydrogen radical is coupled to the carbon atom in the benzene ring, leading to the formation of phenol and a hydroxyl radical through dehydroxylation. On the other hand, guaiacol is likely to form catechol and phenol via the O-CH3 homolysis and coupling of a hydrogen radical to the carbon atom with the methoxyl group, respectively.

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1. Introduction Lignin has received broad attention as a sustainable source of aromatic polymers composed of three phenyl propane monomers (guaiacyl, syringyl, and p-hydroxylphenyl units).1,2 Their compositions are quite different in softwood and hardwood lignins; softwood lignins have only guaiacyl-type unit, whereas hardwood lignins have both types.3,4 Pyrolysis-based technologies (fast pyrolysis and gasification) are promising methods for converting lignin into gas, liquid fuels, and valuable aromatic compounds.5– 11 A better understanding of the mechanism underlying lignin pyrolysis at the molecular level is crucial for optimizing these methods. Our group has established a detailed chemical kinetic model (DCKM) consisting of thousands of elementary reaction steps and hundreds of chemical species, which predicts the vapor-phase reactions of volatiles derived from the fast pyrolysis of biomass at the molecular level and provides detailed information on the molecular composition of the products.12–15 However, empirical kinetic parameters estimated to minimize the gaps between the simulation and experimental results has been used in the DCKM because of the lack of a kinetic database on the elementary decomposition steps of several ligninderived compounds such as catechol, guaiacol, syringol, and their derivatives.12 Thus, the DCKM was not sufficient to adapt to the change in operation parameters, such as temperature, gasifying agent and lignin type. In order to exclude the empirical parameters and enhance the versatility of the DCKM, it is necessary to estimate the rate constants of elementary decomposition steps of individual lignin-derived compounds. Since guaiacol is the predominant species generated from lignin pyrolysis16–19, many experimental studies have focused on identifying the molecular products derived from the thermal decomposition of catechol20–25 and guaiacol.4,26–32 For instance, Ledesma et al.22 carried out the pyrolysis of catechol in a tubular flow reactor to quantify the pyrolytic products ranging from gases (especially CO and CH4) to heavy polyaromatic hydrocarbons. Hosoya et al.31 revealed that the main products generated from guaiacol pyrolysis at 600 °C were phenolic compounds, such as catechol and o-cresol. In addition to experimental studies, theoretical investigations using first-principles computational chemistry have been conducted to analyze the thermal decomposition pathways of catechol33–37 and guaiacol.32,38 For instance, Altarawneh et al.35 investigated unimolecular decomposition pathways of catechol leading to the formation of experimentally reported major products (CO, 1,3-C4H6, and cyclo-C5H6) based on the potential energy surfaces (PES) calculated with the density functional theory (DFT). Liu 2 ACS Paragon Plus Environment

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et al.38 theoretically predicted that the most energetically favorable thermal decomposition pathway of guaiacol involved coupling a hydrogen radical to the carbon atom with the methoxyl group, followed by demethoxylation. However, to date, the rate constants of the individual decomposition steps have not yet been estimated theoretically. In this study, the high-pressure limiting rate constants for the elementary steps of catechol and guaiacol decomposition are evaluated using highlevel ab initio methods. First, we proposed possible unimolecular and bimolecular decomposition pathways on the PES calculated with a suitably accurate method, i.e., “complete basis set” CBS-QB3 method. Second, the high-pressure limiting rate constants for each elementary reaction step were calculated using the transition state theory (TST) to assess the feasibility of the proposed reaction pathways. The theoretically determined rate constants of these elementary reaction steps will aid the development of a DCKM that can describe the behavior of lignin pyrolysis.

2. Computational Method Ab initio calculations were performed with the Gaussian09 (G09) software package.39 In this study, PES along the selected pathways were calculated using the CBS-QB3 method40, which offers greater accuracy within 1 kcal/mol. This highly accurate method includes a ﬁve-step calculation starting with a geometry optimization and frequency calculation at B3LYP/6-311G(2d,d,p) level of theory41,42, followed by a single-point calculation at CCSD(T)/6-31+G(d’), MP4SDQ/CBSB4 and MP2/CBSB3 level, and a complete basis set extrapolation to correct the total energy. Analytical frequency calculations at the same level of theory, namely, B3LYP/6-311G(2d,d,p), were performed to verify one imaginary frequency for a transition structure and all positive frequencies for a stable structure. Subsequently, intrinsic reaction coordinate (IRC) calculations43 were carried out to establish the correct connections between the reaction intermediates. High-pressure limiting rate constants were calculated based on the TST using the GPOP program.44 The partition functions and the density/sum of states for internal degrees of freedom were calculated at B3LYP/6-311G(2d,d,p) level by assuming harmonic vibrational frequencies. The internal rotation of the hydroxyl and methoxyl groups in catechol and guaiacol was treated as hindered internal rotors by using the Pitzer–Gwinn approximation.45,46 In this program, the 1D semiclassical tunneling effects were corrected by assuming the asymmetric Eckart potential.47 The variational TST (VTST) was used for the calculation of rate constants for barrierless channels.44



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3. Results and Discussion 3.1. Proposal of Pyrolysis Pathways on Potential Energy Surfaces Altarawneh et al.34,35 investigated the unimolecular decomposition pathways of catechol (o-C6H4(OH)2) based on the PES calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6311+G(d,p) level. Following the theoretical study of Altarawneh et al.34,35, we provided the unimolecular decomposition pathways of o-C6H4(OH)2, involving both singlet and doublet states, as shown in Figure 1. All energy values on the PES were calculated at the CBS-QB3 level and are given relative to o-C6H4(OH)2. According to Figure 1, there exist five possible initial decomposition channels of C6H4(OH)2 (Rxn-1, Rxn-2, Rxn-5, Rxn-7, and Rxn-16). Rxn-1 corresponds to concerted dehydrogenation of the two hydroxyl H atoms, resulting in the production of o-benzoquinone with H2 release via the transition state TS1. Previous studies have suggested that o-benzoquinone decomposes into carbon monoxide (CO) and cyclopentadienone (C5H4O) via a ring-opening reaction.35,36,48,49 Direct self-expulsion of H2 and H2O (Rxn-2 and Rxn-5) forms intermediates IM1 and IM3, which decomposes into cyclopentadienone (C5H4O) and cyclopenta-1,2,4-triene (C5H4), with the release of CO by decarbonylation (Rxn-3, Rxn-4 and Rxn-6), respectively.50 It is expected that C5H4O contracts to form a fused bicyclic compound with a four-membered and three-membered ring and dissociates into 1,3-butadiene (C4H4), acetylene (C2H2), and CO32,51–53, whereas C5H4 captures a H radical in the gas phase to form either naphthalene or cyclopentadiene.54,55 Rxn-7 contributes to the formation of intermediate IM4 through the migration of the hydroxyl H to a carbon bearing a H atom via TS5. IM4 can branch into three channels (Rxn-8, Rxn-9, and Rxn-13). Rxn-8 associated with the O–H bond dissociation might be energetically unfavorable due to its exceedingly high potential barrier of 78.1 kcal/mol. Hydroxycyclopentadienyl radical (C5H4OH) might be formed with CO release as a result of the condensed ring reaction (Rxn-9 -> Rxn-10 -> Rxn-11 -> Rxn-12) or ring-opening reaction (Rxn-13 -> Rxn-14 -> Rxn-15). C5H4OH is converted into C5H4O via homolysis of the O–H bond56, followed by decomposition into 1,3-butadiene (C4H4), acetylene (C2H2), and CO as described earlier. After the O–H bond dissociation at Rxn-16, there exist four possible doublet state routes (Rxn-17, Rxn-22, Rxn-26, and Rxn-27). Rxn-17 and Rxn-22 involve intramolecular transfer of the hydroxyl H to the carbon site via TS11 and TS16 to form intermediates IM11 and IM14, respectively, both of which are capable of converting to a 1,3-butadiene radical (CH2CHCHCH) or propenal radical (CHCHCHO) with the release of CO. The fate of CH2CHCHCH is characterized by its dissociation into C2H2 + C2H357,58, whereas 4 ACS Paragon Plus Environment

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the fate of CH2CHCHO is characterized by its dissociation into CO + C2H259. The intermediate IM17 containing an aldehyde radical is formed through an aromatic ring contraction to a five-membered ring at Rxn-27, subsequently decomposing into C5H4OH via the expulsion of CO.60



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Figure 1: Reaction pathways for the unimolecular decomposition of catechol (oC6H4(OH)2) on singlet and doublet PES. All energy values are in kcal/mol computed relative to o-C6H4(OH)2 at 0 K with the CBS-QB3 method. 6 ACS Paragon Plus Environment

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The inclusion of bimolecular reactions, either with the radical pool (mostly H in pyrolysis) or with intermediates, could provide more facile routes for the formation of experimentally detected major products, such as phenol (C6H5OH) and benzene (C6H6).22,23 Figure 2 depicts the possible pathways for the H-addition reaction with catechol on the doublet PES. After coupling of a H radical to the carbon atom in the sixmembered ring (via Rxn-28, Rxn-29, and Rxn-30), the added H atom is transferred to a carbon-bearing hydroxyl group to form intermediate IM20, which converts to C6H5OH and OH radical by dehydroxylation (via Rxn-33). C6H5OH is expected to decompose into c-C5H6 and CO through the isomerization to 2,4-cyclohexadienone 50,61–64 or convert to C6H6 by the displacement of an OH species with H.22

Figure 2: Reaction pathways for H-addition with catechol (o-C6H4(OH)2) on doublet PES. All energy values are in kcal/mol computed relative to o-C6H4(OH)2 + H at 0 K with the CBS-QB3 method.

Figure 3 shows the H-abstraction channels for catechol by several radicals (H, CH3, OH, and IM10) on doublet PES. The self-reaction of catechol leading to the formation of IM4 on singlet PES is presented in Figure 4. Note that the PES for Rxn-37 and Rxn-38 had to be calculated with B3LYP/6-311++G(d,p) level in order to shorten the single-point calculation time for large transition structures TS30 and TS31 as depicted in Figure 5.



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Figure 3: H-abstraction channels for catechol (o-C6H4(OH)2) by radicals (H, CH3, OH, IM10) on doublet PES. Energy values for Rxn-37, Rxn-38, and Rxn-39 in kcal/mol are computed relative to the reactant states at 0 K with the CBS-QB3 method, otherwise at the B3LYP/6-311++G(d,p) level.

Figure 4: Self-reaction of catechol on singlet PES. Energy values in kcal/mol are computed relative to o-C6H4(OH)2 + o-C6H4(OH)2 at 0 K at the B3LYP/6-311++G(d,p) level.


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Figure 5: Optimized structures for the transition state of (a) TS30 and (b) TS31 calculated at the B3LYP/6-311++G(d,p) level. Distances are measured in Å. The possible pyrolytic pathways of guaiacol (CH3OC6H4OH) were proposed with an emphasis on the reactivity of the methoxyl group, as shown in Figure 6. Rxn-39 corresponds to the O–C bond scission reactions in the methoxyl group, which leads to the formation of IM10 and CH3 radicals. The hydroxyl H atom is transferred to a carbonbearing methoxyl group through Rxn-41 to form intermediate IM21, which results in its decomposition into phenoxy (C6H5O) and methoxyl radicals (OCH3) by demethoxylation through Rxn-41. At Rxn-42 and Rxn-43, the formation of intermediate IM22 is achieved by coupling a hydrogen radical to the carbon atom in the benzene ring through Rxn-42, and then, it is converted to C6H5OH and methoxyl radical (OCH3) through demethoxylation via TS34. Rxn-44 represents the methoxyl CH3 abstraction by a H radical.



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Figure 6: Reaction pathways of guaiacol pyrolysis on PES. All energy values are in kcal/mol computed at 0 K with the CBS-QB3 method.

3.2.

High-Pressure Limiting Rate Constants Calculated With Transition State Theory As shown in Figure 7, the high-pressure limiting rate constants of Rxn-1, Rxn-2, Rxn-5, Rxn-7, and Rxn-16 were calculated with TST in the range of 300–1500 K to determine the dominant unimolecular channel of catechol. Since Rxn-16 involves barrierless O–H bond fission in catechol (as shown in Figure S1 in Supporting Information), the VTST was applied to obtain its rate constant. Figure 7 indicated that Rxn-7 has a large rate constant at all temperatures; thus, hydroxyl H migration to a carbon site bearing a H atom is probably the most facile channel. With increasing temperature, the rate constants of Rxn-5 and Rxn-7 approach one another, but do not intersect one another. Even in the lowpressure limiting rate constants obtained with Rice-Ramsperger-Kassel-Marcus (RRKM) theory65–68, Rxn-7 has larger values as shown in Figure S3 of the Supporting Information. Horn et al.69 also concluded from time-resolved measurements with a shock tube that catechol decomposition was initiated by the hydroxyl H migration to the carbon site. 10 ACS Paragon Plus Environment

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40 Rxn-1

0

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


Rxn-2 -40

Rxn-5

-80

Rxn-7 Rxn-16

-120 -160 0

1

2

3

4

1000/T [1/K] Figure 7: High-pressure limiting rate constants for Rxn-1, Rxn-2, Rxn-5, Rxn-7, and Rxn16 calculated with TST in the temperature range of 300−1500 K.

Figure 8 shows a comparison of the high-pressure limiting rate constants among the three channels (Rxn-8, Rxn-9, and Rxn-13) and indicated that Rxn-21 and Rxn-25, both of which lead to the formation of C4H4, C2H2, and CO as described in the previous section, were likely to be the major channels. Thus, the unimolecular decomposition pathways would be pass through the singlet state (Rxn-7 -> Rxn-9 -> Rxn-10 -> Rxn-11 -> Rxn-12 or Rxn-7 -> Rxn-13 -> Rxn-14 -> Rxn-15). Ledesma et al. 22 also supported the formation of C4H4, C2H2, and CO from catechol pyrolysis based on an experimental observation with a quartz tubular-flow reactor.



ln (k [cm3/mol/s])

20 0

Rxn-8

-20

Rxn-9

-40

Rxn-13

-60 -80 -100 -120 0

1

2

3

4

1000/T [1/K] Figure 8: High-pressure limiting rate constants for Rxn-8, Rxn-9 and Rxn-13 calculated with TST in the temperature range of 300−1500 K.

In order to evaluate if the H radical contributes to Rxn-28, Rxn-29, Rxn-30, or Rxn-34, the high-pressure limiting rate constants of these channels were plotted in Figure 9. According to this figure, H-addition into a carbon site with the hydroxyl group, namely, Rxn-30, was least likely to occur at all temperatures. Thus, it is reasonable to conclude that H-addition into a carbon bearing a H atom (Rxn-28 and Rxn-29) or H-abstraction (Rxn-34) tends to take place more easily. 35

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

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Rxn-28

30

Rxn-29 25

Rxn-30

20

Rxn-34

15 10 0

1

2

3

4

1000/T [1/K] Figure 9: High-pressure limiting rate constants for Rxn-28, Rxn-29, Rxn-30, and Rxn-34 12 ACS Paragon Plus Environment

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in the temperature range of 300−1500 K.

Figure 10 illustrates the calculated high-pressure limiting rate constants for the initial channels of guaiacol unimolecular decomposition (Rxn-39 and Rxn-40). A comparison of these two rate constants suggested that Rxn-39, which produces precursor IM10 for the formation of catechol, has a higher rate constant and plays an important role in guaiacol pyrolysis. Hosoya et al.31 reported that a large amount of catechol was observed from the guaiacol pyrolysis experiment with total-ion chromatograms in the GC–MS analysis. 40 Rxn-39

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


0

Rxn-40

-40 -80 -120 0

1

2

3

4

1000/T [1/K] Figure 10: High-pressure limiting rate constants for Rxn-39 and Rxn-40 calculated with TST in the temperature range of 300−1500 K. Figure 11 is a graphic representation for comparing the high-pressure limiting rate constants between Rxn-42 and Rxn-44. As indicated in this figure, H-addition (Rxn-42) would be more susceptible than H-abstraction (Rxn-44) when focusing on the bimolecular reaction of guaiacol with a H radical.



30

ln (k [cm3/mol/s])

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Rxn-42

20

Rxn-44 10 0 -10 -20 0

1

2

3

4

1000/T [1/K] Figure 11: High-pressure limiting rate constants for Rxn-42 and Rxn-44 calculated with TST in the temperature range of 300−1500 K. Modified Arrhenius parameters for all reaction channels, including both the forward and reverse rate constants, are given in Table 1.


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Table 1: Modified Arrhenius Parameters (k=ATnexp(-E/RT)) for Each Elementary Channel, Forward (f) ((cm3/mol)/s and kcal/mol) and Reverse (r) ((cm3/mol)/s and kcal/mol) Channel

Reaction

A(f)

n(f)

E(f)

A(r)

n(r)

E(r)

10.54

65.33

7.73×10-28

11.32

20.16

1.44×105

2.65

57.93

0.00

2.88

Rxn-1

o-C6H4(OH)2

→

o-C6H4O2

+ H2

1.61×10-23

Rxn-2

o-C6H4(OH)2

→

IM1

+ H2

5.94×108

Rxn-3

IM1

→

IM2

Rxn-4

IM2

→

C5H4O

Rxn-5

o-C6H4(OH)2

→

IM3

Rxn-6

IM11

→

C5H4

Rxn-7

o-C6H4(OH)2

→

IM4

Rxn-8

IM4

→

IM10

Rxn-9

IM4

→

IM5

5.54×10

Rxn-10

IM5

→

IM6

2.89×1012

Rxn-11

IM6

→

IM7

+ CO

6.31×10

11

Rxn-12

IM7

→

C5H4OH

+ H

3.40×108

Rxn-13

IM4

→

IM8

Rxn-14

IM8

→

IM9

Rxn-15

IM9

→

C5H4OH

Rxn-16

o-C6H4(OH)2

→

IM10

Rxn-17

IM10

→

IM11

8.50×10

Rxn-18

IM11

→

IM12

7.50×1011

Rxn-19

IM12

→

IM13

12

0.35

Rxn-20

IM13

→

CH2CHCHCH + CO

1.87×1012

0.13

+ CO

6.43×10

16

1.13×109

2.28 109.38

15

0.00

93.11

6.46×10

1.14

2.69

8.62×107

1.63

79.41

1

3.22

46.93

0.00

0.00

17.74

19.98

+ H2O

1.02×10

12

1.18

82.22

7.82×10

+ CO

3.76×1015

0.00

77.65

2.62×1017

1.15×10 + H

5.69×106 8

17.60

35.21

1.51×10

2.18

78.90

8.61×105

2.30

15.39

1.18

38.14

9

0.96

7.22

0.05

8.16

3.97×10

3.58×1012

0.05

4.72

3

3.86

52.37

1.46

1.42

0.12

4.06

0.61

27.58

1.37×10

1.66

77.81

7.62×107

0.67

44.19

8.99×10

1.76×1012

0.31

35.39

8.67×104

+ H

8.33×10

13

0.00

+ H

1.95×1014

0.00

+ CO

-48

11

2.90×10 + CO

-47

4.91×10


-55

10

3.98

66.20

77.10

2.43×10

12

0.00

1.56

84.42

1.40×1013

0.00

7.50

19.72

4.33

-55

19.90

40.22

2.39×10

0.54

36.07

2.45×1011

0.10

4.33

7.20

1.61×10

5

3.45

16.59

7.18

6.95×105

3.73

4.80


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

IM14

→

Rxn-22

IM10

→

IM14

2.59×10

Rxn-23

IM14

→

IM15

3.66×1012

Rxn-24

IM15

→

IM16

+ CO

6.25×10

12

Rxn-25

IM16

→

CHCHCHO

+ C2H2

2.58×1011

Rxn-26

IM10

→

o-C6H4O2

Rxn-27

IM10

→

IM17

Rxn-28

o-C6H4(OH)2

+ H

→

Rxn-29

o-C6H4(OH)2

+ H

→

+ H

Rxn-21

2.20×108

IM11

+ H

-12

1.30 7.40

61.81

0.18

2.13

0.62

27.89

4.25

3.14×109

1.24

27.31

1.75

6.08

8

1.48

25.75

18.98

14.74

2.56×10-52

18.98

14.60

16.63

19.69

1.54×10

-44

16.68

16.30

18.88

17.29

2.41×104

2.33

-0.96

1.63

3.82

1.96×10

5

2.36

29.57

4.01

1.75

2.34×10-3

5.03

27.67

IM19

2.63×10-52 -44

5.45×10-52

Rxn-32

IM19

→

IM20

Rxn-33

IM20

→

C6H5OH

Rxn-34

o-C6H4(OH)2

+ H

→

IM10

+ H2

1.00×10

Rxn-35

o-C6H4(OH)2

+ CH3

→

IM10

+ CH4

9.51×10-2

1.54×10

8

Rxn-36

o-C6H4(OH)2

+ OH

→

IM10

+ H2O

1.97×10

Rxn-37

o-C6H4(OH)2

+ IM10

→

IM10

+ IM4

1.23×10-8

+ o-C6H4(OH)2

→

o-C6H4(OH)2

+ IM4

2.58×10

-2

IM10

+ CH3

1.61×1014

Rxn-41

IM21

→

C6H5O

Rxn-42

CH3OC6H4OH

→

IM22

Rxn-43

IM22

→

C6H5OH

6.06×10 + OCH3

4

-69

1.42×1013 5.09×10

+ OCH3

1.84

1.30

1.73×10

IM21

1.61

1.76×10

IM20

→

2.81

4.68

→

CH3OC6H4OH

2.50

8

16.98

7

Rxn-40

4.39×104

0.34

1.57

→

284.18

9

1.87×108

→

7

4.02×1012


5.51

3.66

5.36×10

1.02×1011

IM18

6.99

14.80

61.20

Rxn-31

40.73

0.07

0.87

o-C6H4(OH)2

1.53

4

1.04×1011

IM19

CH3OC6H4OH

1.64×1011

2.87×10

1.62

Rxn-39

5.38×10

-12

72.77

1.02×10

o-C6H4(OH)2

2.98×107

1.55

1.17×10

IM18

Rxn-38

34.52

1.11 321.99

8

+ OH

6.25

10

Rxn-30

+ H

Page 16 of 27

2.91×10

1

2.29

0.30

5.48×10

5.65

20.05

2.02×10-9

5.94

6.90

3.20

27.20

4.23×10

-3

3.50

14.06

0.00

50.65

3.11×1011

0.00

0.00

24.05

16.72

-69

3.08 4763.39

24.00

44.34

1.55×10

0.00

41.72

175×1011

0.00

0.00

1.62

5.08

7.84×10

8

1.35

25.83

0.29

16.28

1.11×10-1

3.51

2.54

Page 17 of 27

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


Rxn-44

CH3OC6H4OH

+ H

→

IM10

+ CH4

2.16×102


3.38

21.76

7.74×10-7

5.60

67.66


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

4. Conclusions The possible pathways of catechol and guaiacol pyrolysis, including bimolecular reaction and unimolecular decomposition, were proposed. Subsequently, PES were calculated by the CBS-QB3 method. The high-pressure limiting rate constants for the proposed pyrolysis channels were estimated based on the TST. The kinetic analysis suggested that the catechol unimolecular decomposition starts with H migration to a carbon site bearing a H atom to produce 2-hydroxycyclohexa-2,4-dien-1-one, which finally decomposes into C4H4, C2H2, and CO. On the other hand, guaiacol unimolecular decomposition seems to give rise to homolysis of the O–CH3 bond, followed by the formation of catechol. In the case of bimolecular reaction with a H radical, the formation of phenol is kinetically the most favorable in both the catechol and guaiacol pyrolysis pathways. In order to establish the capability of the DCKM for describing the complex phenomena related to lignin pyrolysis more accurately, further work is required so that the rate constants for the elementary decomposition steps of other individual lignin-derived compounds, such as syringol, coniferyl, sinapyl, and p-coumaryl alcohols, can be determined. Supporting Information Geometries, frequencies, spin multiplicities, and Gaussian outputs of all stable structure and transition state are described in Supporting Information. Acknowledgements This research was in part financially supported by KAKENHI (Grant-in-Aid for Scientific Research (B): 17H03454). The authors are also grateful to the support by the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. All the computations in this study were performed on the PC cluster systems in our group and the high-performance computing system at the Research Institute for Information Technology, Kyushu University. References: (1)

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(2)

Kawamoto, H.; Horigoshi, S.; Saka, S. Pyrolysis Reactions of Various Lignin Model Dimers. J. Wood Sci. 2007, 53 (2), 168–174. 18 ACS Paragon Plus Environment

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(4)

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(5)

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(6)

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(7)

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(8)

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(9)

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TOC graphic


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TOC Graphic 40 Rxn-1 0

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


Rxn-2 -40

Rxn-5 Rxn-7

-80

Rxn-16

-120 -160 0

1

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1000/T [1/K] ACS Paragon Plus Environment

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Theoretical Study on the Kinetics of Thermal ... - ACS Publications

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