Theoretical Study on Elementary Reaction Steps in Thermal

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Theoretical Study on Elementary Reaction Steps in Thermal Decomposition Processes of Syringol-Type Monolignol Compounds Yuki Furutani, Yuki Dohara, Shinji Kudo, Jun-ichiro Hayashi, and Koyo Norinaga J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09450 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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The Journal of Physical Chemistry

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Theoretical Study on Elementary Reaction Steps in Thermal Decomposition

2

Processes of Syringol-Type Monolignol Compounds

3 4

Yuki FURUTANI1, Yuki DOHARA1, Shinji KUDO2, Jun-ichiro HAYASHI2,3, and

5

Koyo NORINAGA*4

6

1

7

Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan.

8

2

9


Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1, Institute for Materials Chemistry and Engineering, Kyushu University, 6-1,

10

3

11


12

4

13

Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan.

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*Corresponding

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+81-52-789-3272

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Email: [email protected]

Research and Education Centre of Carbon Resources, Kyushu University, 6-1, Department of Chemical Systems Engineering, Graduate School of Engineering, author:

Koyo

NORINAGA,

Tel

+81

52

789

3618,

Fax

17

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Abstract This paper theoretically investigated a large number of reaction pathways and kinetics

20

to describe the vapor-phase pyrolytic behavior of several syringol-type monolignol

21

compounds

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1-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-en-1-one

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3-hydroxy-1-(4-hydroxy-3,5-dimethoxyphenyl)propan-1-one

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1-(4-hydroxy-3,5-dimethoxyphenyl)propane-1,3-diol (HDPPD), and syringol. The

25

possible pyrolytic pathways involving unimolecular decomposition, addition, and

26

abstraction reactions were investigated by comparing the energy barriers calculated at

27

the B3LYP/6-311++G(d,p) level. In the proposed pathways, all syringol-type

28

monolignols containing a side chain undergo its cleavage to form syringol through the

29

formation of syringaldehyde or 4-vinylsyringol. Syringol is then converted into two

30

products: (a) pyrogallol via the homolysis of the O-CH3 bond and hydrogenation; or (b)

31

guaiacol via addition of an H atom with a carbon bearing methoxyl group in syrignol

32

and the subsequent demethoxylation. The pyrolytic pathways of pyrogallol are

33

classified into two processes: (a) the concerted dehydrogenation of the two hydroxyl H

34

atoms and the unimolecular decomposition to produce acetylene (C2H2), ethynol

35

(C2HOH), and CO; or (b) the displacement of an OH with H to produce catechol and

36

resorcinol.

18

that

are

Additionally,

derived

from

the

HDPP undergoes

primary

pyrolysis

(HDPP),

O–CH3

1

ACS Paragon Plus Environment

bond

sinapyl

of

lignin: alcohol,

(HHDPP),

cleavage

to

form

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1

but-1-en-3-yne. The high-pressure limit rate constants for all the proposed elementary

2

reaction steps were evaluated based on transition state theory.

3

5

1. Introduction Lignin, which mainly consists of three phenylpropane monomers (syringyl, guaiacyl,

6

and p-hydroxylphenyl units), has received considerable attention as a renewable source

7

of aromatics.1,2 Pyrolysis is considered one of the most promising methods for

8

converting lignin solids into valuable chemicals, as well as syngas and transportation

9

fuels.3–9 Pyrolysis is generally distinguished by two processes: the primary pyrolysis

10

with volatile generation from solids;10–16 and secondary vapor-phase reactions with

11

cracking, combination, or condensation of the generated volatiles.17–20 A better

12

understanding of lignin pyrolysis processes at the molecular level is indispensable for

13

developing process control strategies to optimize the product formation.

14

A detailed chemical kinetic model (DCKM) based on elementary reaction steps can

15

elucidate the underlying pyrolytic mechanism, and provide much information on the

16

molecular composition of products. The integration of the DCKMs of primary pyrolysis

17

and secondary vapor-phase reactions is crucial for predicting the detailed pyrolysis

18

behavior of lignin.

19

Several researchers explored a semi-DCKM of primary pyrolysis, where solid lignin

20

structures are characterized with three model compounds.21–23 Approximately 100

21

species and 400 reactions are included in the kinetic data.21,22 This kinetic model can

22

provide information on the molecular composition of monolignol volatile products

23

derived from the primary pyrolysis of lignin. These monolignol-derived compounds are

24

mainly divided into the syringol types and phenol types. The syringol types include

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1-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-en-1-one

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3-hydroxy-1-(4-hydroxy-3,5-dimethoxyphenyl)propan-1-one

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1-(4-hydroxy-3,5-dimethoxyphenyl)propane-1,3-diol (HDPPD), and syringol. The

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phenol types include 1-(4-hydroxyphenyl)prop-2-en-1-one (HPP), p-coumaryl alcohol,

29

3-hydroxy-1-(4-hydroxyphenyl)propan-1-one

30

1-(4-hydroxyphenyl)propane-1,3-diol (HPPD), and phenol. These molecular structures

31

are shown in Scheme 1. With the semi-DCKM developed by Hough et al.21, recently,

32

our group predicted the gas and tar yields derived from fast pyrolysis of lignin with a

33

two-stage tubular reactor setting a residence time of 0.1 s for secondary vapor-phase

34

reactions.24 However, this model prediction overestimated the tar yield and

35

underestimated the CO yield above 1023 K, probably due to a lack of the DCKM of the

36

secondary vapor-phase reactions of monolignol-derived compounds.

4

(HDPP),

2


sinapyl

alcohol, (HHDPP),

(HHPP),

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1 2

SCHEME 1. Chemical Structures of (a) Syringol-Type and (b) Phenol-Type

3

Monolignol Compounds

4 5

On the other hand, our group has established a DCKM for secondary vapor-phase

6

reactions including hundreds of chemical species and thousands of elementary reaction

7

steps.25–28 However, the kinetic database in this DCKM has been limited to the

8

elementary

9

dihydroxybenzenes (such as catechol, resorcinol, and hydroquinone) obtained through

10

ab initio calculations and transition state theory (TST).29–34 Establishing a kinetic

11

database for the thermal decomposition processes of both syringol and phenol-type

12

monolignol compounds is a prerequisite for the integration with the DCKM of primary

13

pyrolysis.

14

In this research, we theoretically derive a kinetic database of elementary decomposition

15

steps of syringol-type monolignol compounds. Initially, the thermal decomposition

16

pathways were proposed focusing on unimolecular decomposition and bimolecular

17

reactions involving the addition and abstraction by H and CH3 radicals. Finally, the

18

energy barrier for each proposed elementary reaction step was calculated at the

19

B3LYP/6-311++G(d,p) level, and the high-pressure limit rate constants were evaluated

20

based on TST.

reaction

constants

of

the

thermal

decomposition

processes

of

21 22

24

2. Computational Method All quantum chemical calculations were conducted using the Gaussian09 (G09)

25

software package.35 In this study, the geometries of the reactants, intermediates,

23

3



1

transition states, and products were optimized using the Becke 3-parameter

2

Lee-Yang-Parr (B3LYP) functional of the density functional theory (DFT)36–38 with the

3

basis set of 6-311++G(d,p).39,40 Analytical frequency calculations at the same level of

4

theory were performed to correct for zero-point energy (ZPE), and to verify the

5

existence of one imaginary frequency (for a transition structure) or all positive

6

frequencies (for a stable structure). Subsequently, intrinsic reaction coordinate (IRC)

7

calculations41 were carried out to connect the related reactants and products. The

8

broken-symmetry unrestricted open-shell singlet calculation with the “guess = mix”

9

keyword was employed for biradicals.

10

High-pressure limit rate constants for all elementary reaction steps were calculated

11

based on the TST with B3LYP/6-311++G(d,p) level of theory using the GPOP

12

program.42 The density/sum of states and the partition functions were calculated by

13

treating all vibrations as harmonic vibrational frequencies. The 1D semiclassical

14

tunneling effects are corrected by assuming the asymmetric Eckart potential.43 The

15

variational TST (VTST) was applied to the calculation of rate constants for the reaction

16

channels without any pronounced energy barriers along the reaction coordinates.42 In

17

the VTST, the vibrational frequencies were obtained from the potential energy surfaces

18

along the reaction coordinates, which were calculated by the geometry optimization

19

restricting the bond lengths and relaxing all of the other geometric parameters. Standard

20

enthalpies of formation for all chemical species were obtained based on the atomization

21

energies with B3LYP/6-311++G(d,p) level of theory, and entropies, heat capacities for

22

all chemical species were also calculated with the GPOP program.42 These

23

thermodynamic properties were described in Table S1 of Supporting Information.

24

Differences of bond dissociation energy (BDE) of O-CH3 and O-H in syringol among

25

several ab-initio methods such as B3LYP36–38, M06-2X44, and CBS-QB345 were

26

evaluated as described in Figure S1 of Supporting Information. Figure S3 indicated that

27

B3LYP calculation underestimated the BDE in comparison with M06-2X, and

28

CBS-QB3 methods. For further quantitative evaluation, it seems to be necessary to

29

reconstruct the kinetic database with high-level ab-initio methods, such as M06-2X, and

30

CBS-QB3 based on the molecular structure data in Supporting Information. Kim et al.46

31

also supported the importance of the quantitative examination with M06-2X and

32

CBS-QB3 from the BDE calculation of four model dimers with the β-O-4 linkage. In

33

this study, the high-pressure limit rate constants for key elementary reaction steps

34

(potential candidates for branching reaction steps and rate-determining steps) were

35

recalculated based on the TST by using M06-2X/6-311++G(d,p) level of theory and

36

treating low-frequency torsional modes as hindered rotors under assumption of Pitzer– 4


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1

Gwinn approximation.47,48

2 3 4

3. Results and Discussion

5 6

3.1. Reaction Pathways and Kinetic Database of HDPP Pyrolysis The possible thermal decomposition processes of HDPP are as follows: one is started

7

from the conversion of the side-chain –C(=O)CH=CH2, and the other is started from

8

O-CH3 bond cleavage. The proposed side-chain conversion reaction pathways of HDPP

9

are shown in Figure 1. For each proposed elementary reaction step, energy barriers

10

calculated at the B3LYP/6-311++G(d,p) level are also described in Figure 1. We

11

considered eight initial reaction channels (steps 1, 3, 6, 11, 18, 19, 21, 24, and 27). Both

12

steps 1 and 3 are associated with the bond dissociation in HDPP to produce the radical

13

A2, which is converted into syringol through the CO release and hydrogenation (via

14

steps 4 and 5). Steps 6 and 11 represent a hydrogen transfer reaction simultaneously

15

with release of vinylidene (CH2=C) and CO to form syringaldehyde (A4) and

16

4-vinylsyringol (A6), respectively. The CH2=C is converted into the stable acetylene

17

C2H2 through a hydrogen transfer reaction (step 23) with an energy barrier of 2.8

18

kcal/mol (experimental value: 1.3–2.0 kcal/mol obtained from lifetime broadening

19

measurements49). Both A4 and A6 finally decompose into syringol through

20

unimolecular decomposition (steps 8, 12, and 13), addition of an H atom (steps 9 and

21

14), and abstraction by an H atom (steps 25 and 26) and by a CH3 radical (steps 28 and

22

29). Step 18 involves the direct conversion to syringol with the release of

23

propa-1,2-dien-1-one (C2H2CO). Among the initial unimolecular decomposition

24

channels (steps 1, 3, 6, 11, and 18), steps 3, 6, 11, and 18 with an energy barrier in the

25

range of 80–90 kcal/mol seem to be the dominant channels compared with step 1 (108.5

26

kcal/mol). Compared with the unimolecular decomposition, H-addition (steps 19 and

27

21) and abstractions by H and CH3 radicals (steps 24 and 27) can effectively lower

28

energy barriers of the initial reaction channels, whose values are in the range of 2–11

29

kcal/mol.

30

Figure 2 shows the reaction pathways starting from the homolytic cleavage of O-CH3

31

bond in HDPP, which were developed based on the pyrolytic pathways of syringol

32

proposed by Huang et al.50 and the pyrolytic pathways of pyrogallol proposed by

33

Asmadi et al.51 In Figure 2, A12 and methyl radicals are firstly generated through the

34

homolytic cleavage of O-CH3 bond in syringol with an energy barrier of 48.3 kcal/mol

35

(step 30), and then A12 undegoes the hydrogenation reaction (step 31) to form A13.

36

Through the same processes (steps 32 and 33), A13 is converted into A15. Then, A16 is 5



1

generated through a concerted dehydrogenation process of the two hydroxyl H atoms in

2

A15 by conquering an energy barrier of 76.5 kcal/mol (step 34), and then undergoes a

3

ring-opening reaction with an energy barrier of 34.6 kcal to form A17 (step 35). A17

4

finally decomposes into but-1-en-3-yne (C4H4) with the formation of CO and ethynol

5

(C2HOH) via steps 36–39. HDPP unimolecular decomposition is likely to start from the

6

homolytic cleavage of O-CH3 bond rather than the side-chain conversion as shown in

7

Figure 1. Steps 40 and 41 correspond to abstractions by an H atom in HDPP and A13. In

8

the future, the reaction pathways as described in Figure S2 of Supporting Information

9

also need to be discussed in more detail.

10

Modified Arrhenius parameters for all elementary reaction steps in Figure 1 and 2 are

11

given in Table S2 in Supporting Information.

6


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OH O

OH

Step2

O

H3C

O

H3C

O

CO

CH3

TSA1 (22.8)

O

H

CH3

O

H3C

CH3

(0.0) Syringol

A3

O

Step22

OH

Step5 O

H3C

TSA3 (28.6)

O

TSA18 (20.1)

OH

Step4

O

C2H2

CH3

A2

C2H4

Step8

A1 Step1 H

(108.5)

(87.1)

C2H3

CO

OH

Step21

O

H3C

CH3

O

H

OH

Step6

O

H3C

H

H2C C

CH3

TSA17 (2.0)

O

TSA15 Step19 (4.3) H

H

CH3

H CH O

O

A4

HDPP

O

H3C

TSA6 (4.4)

TSA10 (3.1)

H2C C

TSA12 (25.2)

A5

Step15 C2H3

Step11

TSA14 Step18 (83.1) C2H2CO

TSA8 (88.0)

CO

OH

OH O

H3C

Step20

CH3

O

OH

O

H3C

CH3

CH3

Syringol

A7

(107.3)

TSA11 (5.3)

H

H

Step17

OH O

HC CH

O

H3C

H

OH

OH O

C2H2

CH3

C H

Step14

Step16

Step23

CH3

H2C

A6

(0.0) Step5

O

H3C

TSA9 (104.5)

A10

TSA22 (2.8)

O

Step12

O

H3C

TSA16 (9.9)

H C O

O

OH

C2H3CO

H2C C

O

H

CH3

O

A11

OH

Step9 O

H3C

TSA4 (88.6)

O

O

TSA7 Step10 (13.8) HCO

TSA5 (100.0)

Step13

OH O

Step7

Step3

TSA2 (83.9)

O

O

H3C

O

H3C

CH3

CH3

TSA13 (42.9) A3

H2C

A9

A8

OH O

CH H

OH O

H3C

O CH3

O

H3C

CH3

Step24 H2

H

TSA19 (10.6)

O

O

HDPP

A1

OH O

OH O

H3C

O CH3

Step25

O

H3C

CH3

H

O

H

H2

TSA20 (0.0)

O

A2

A4

OH

OH O

O

O

H3C

CH3

Step26

O

H3C

CH3

H

H2

TSA21 (10.2) A6

A8

OH O

OH O

H3C

O CH3

O

H3C

CH3

Step27 CH3

O

CH4

TSA23 (15.7)

O

HDPP

A1

OH O

OH O

H3C

O CH3

Step28

O

H3C

CH3

CH3

O

H

CH4

TSA24 (6.6)

O

A2

A4

OH

OH O

O

O

H3C

CH3

Step29

O

H3C

CH3

CH3

1

CH4

TSA25 (15.7) A6

A8

2

Figure 1: Side-chain conversion reaction pathways of HDPP. All energy barriers are in

3

kcal/mol and computed at the B3LYP/6-311++G(d,p) level of theory. 7



1

2 3

Figure 2: Reaction pathways staring from the homolytic cleavage of O-CH3 bond in

4

HDPP. All energy barriers are in kcal/mol and computed at the B3LYP/6-311++G(d,p)

5

level of theory.

8


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1 2

3.2. Reaction Pathways and Kinetic Database for the Pyrolysis of HHDPP, Sinapyl

3 4

Alcohol, and HDPPD Figure 3 shows the proposed side-chain conversion reaction pathways of (a) sinapyl alcohol,

5

(b) HHDPP, and (c) HDPPD; and they involve unimolecular decomposition (steps 42, 43,

6

44, 46, 48, 49, 50, 52, and 54), addition of an H atom (steps 45 and 51) and abstraction by

7

an H atom (steps 47, 53, and 55). All energy barriers for each proposed elementary reaction

8

step were calculated at the B3LYP/6-311++G(d,p) level and described in Figure 3. Sinapyl

9

alcohol decomposes into A6 or A8 as shown in Figure 3(a), while both HHDPP and

10

HDPPD decompose into A2 or A4 as shown in Figure 3(b) and (c). According to Figure

11

3(a) and (b), the direct bond dissociation conversion to A8 and A2 with energy barriers of

12

87.5 and 73.2 kcal/mol via steps 42 and 48 is energetically more favorable than the indirect

13

conversion (via steps 43, 44 and 49, 50) with the expulsion of the H atom, respectively.

14

Figure 3(c) indicated that HDPPD is converted into B4 by the expulsion of the hydroxyl H

15

atom (via steps 54 or 55). The pyrolytic pathways of intermediates A2, A4, A6, and A8 are

16

already described in Figure 1. Modified Arrhenius parameters for all elementary reaction

17

steps in Figure 3 are given in Table S3 in Supporting Information. It remains a challenge for

18

future research to theoretically estimating the rate constants of the elementary

19

decomposition processes associated with O–CH3 bond scission, H-addition into the double

20

bond, and abstraction by a CH3 radical in sinapyl alcohol, HHDPP, and HDPPD.

21 22

9 ACS Paragon Plus Environment


1 2

Figure 3: Side-chain conversion reaction pathways of (a) sinapyl alchol, (b) HHDPP, and

3

(c) HDPPD. All energy barriers are in kcal/mol computed at the B3LYP/6-311++G(d,p)

4

level. 10 ACS Paragon Plus Environment

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1

3

3.3. Reaction Pathways and Kinetic Database of Syringol Pyrolysis Figure 4 depicts the pyrolytic pathways of syringol leading to the formation of pyrogallol,

4

catechol, and guaiacol, which were proposed based on the theoretical approach of Huang et

5

al.50 Asmadi et al.52 also suggested the formation of these products from syringol pyrolysis

6

based on experimental observation with a closed ampoule reactor. All energy barriers for

7

each proposed elementary reaction step were calculated at the B3LYP/6-311++G(d,p) level

8

and also described in Figure 4. We considered five initial channels: steps 56 and 66 (bond

9

dissociation), step 74 (addition of an H atom), and steps 76 and 77 (abstraction by an H

10

atom). At step 56, C1 and methyl radical are generated through the homolytic cleavage of

11

O-CH3 bond in syringol with an energy barrier of 47.3 kcal/mol. Then, C1 is converted into

12

3-methoxycatechol (C2) through a hydrogenation reaction (step 57). C2 further undergoes

13

the demethylation (step 58) and hydrogenation (step 59) to form pyrogallol, or the

14

expulsion of the methoxyl H atom (step 60) and isomerization (step 61) to form the radical

15

C5, which is converted into catechol through either the release of CO + H (via steps 62 and

16

63) or the elimination of a formyl radical (via steps 64 and 65). Pyrogallol pyrolysis

17

pathways were described in next section. Pyrogallol tends to be formed more easily

18

compared with catechol, because the energy barrier of step 58 (51.9 kcal/mol) is nearly half

19

of that of step 60 (93.4 kcal/mol). This is consistent with a pyrolysis experiment of syringol

20

which observed a larger amount of pyrogallol rather than catechol.52

21

Step 66, which has an energy barrier of 77.5 kcal/mol (76.9 kcal/mol at

22

B3LYP/6-31G++(d,p) level50) and is associated with the expulsion of the hydroxyl H atom

23

in syringol, is less likely to occur compared with step 56 (47.3 kcal/mol). After passing

24

through step 66, the generated radical C8 undergoes isomerization (via steps 67, 68, and

25

69) to form radical C11, which is converted into guaiacol through the release of CO + H

26

(via steps 70 and 71) or the elimination of a formyl radical (via steps 72 and 73). However,

27

guaicol is rarely detected in pyrolysis experiments of syringol,52 probably due to the higher

28

energy barrier at step 66. In step 74, an H atom is coupled with syringol to form the radical

29

C14 with an energy barrier of 4.3 kcal/mol (5.1 kcal/mol at B3LYP/6-31G++(d,p) level50),

30

followed by the formation of guaiacol through the CH3O release (via step 75). Modified

31

Arrhenius parameters for all elementary reaction channels in Figure 4 are given in Table S4

32

in Supporting Information.

33

Liu et al.53 proposed five possible pyrolytic pathways of guaiacol with an emphasis on the

2



1

reactivity of the methoxyl group. In addition to this previous study53, we obtained a kinetic

2

database for the thermal decomposition processes of guaiacol leading to the formation of

3

phenol, as described in Figure S3 and Table S5 in Supporting Information. Although the

4

formation routes of o-cresol from C8 was proposed by Britt et al.54, the elementary rate

5

constants have not yet been estimated theoretically; and thus, further insight into this aspect

6

is left to future work.

7 8 9


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1 2

Figure 4: Pyrolysis pathways of syringol. All energy barriers are in kcal/mol and computed

3

at the B3LYP/6-311++G(d,p) level. 13 ACS Paragon Plus Environment


Page 14 of 24

2

3.4. Reaction Pathways and Kinetic Database of Pyrogallol Pyrolysis The proposed pyrolytic pathways of pyrogallol are shown in Figure 5. All energy barriers

3

for each proposed elementary reaction step calculated at the B3LYP/6-311++G(d,p) level

4

are also described in Figure 5. Altarawneh et al.30 theoretically investigated the catechol

5

decomposition producing o-benzoquinone with dehydrogenation of the two hydroxyl H

6

atoms at B3LYP/6-311+G(d,p) level of theory. Khachatryan et al.33 studied the ring-fission

7

of o-benzoquinone at the B3LYP/6-31G(d,p) level. Steps 80 and 81 were proposed by

8

applying the theoretical approaches investigated by Altarawneh et al.30 and Khachatryan et

9

al.33 in identifying the pyrogallol unimolecular decomposition routes. Steps 82–84 were

10

developed following the reaction pathways proposed by Asmadi et al.51 Dehydrogenation of

11

the two hydroxyl H atoms (via step 80) forms 3-hydroxy-o-benzoquinone (D1), which

12

finally decomposes into C2H2 and C2HOH with CO release via a ring-opening reaction

13

(step 81) and bond dissociation (steps 82, 83, and 84). The energy barriers of 75.4 and 37.7

14

kcal/mol at step 80 and 81 are in good agreement with those of 75.5 kcal/mol30 (at

15

B3LYP/6-311+G(d,p) level) and 38.1 kcal/mol33 (at B3LYP/6-31G(d,p) level) via a

16

dehydrogenation of the two hydroxyl H atoms in catechol and a ring-opening reaction in

17

o-benzoquinone, respectively. Altarawneh et al.30 also proposed the addition/elimination

18

reaction in catechol to replace an OH species with H. Steps 85 and 86 were developed

19

based on its theoretical approach30 and involve the addition of an H atom with the carbon

20

atom bearing hydroxyl group to form radicals D5 and D6, respectively; both of which could

21

be converted into catechol or resorcinol through dehydroxylation (via steps 88 or 89).

22

Modified Arrhenius parameters for all reaction channels in Figure 5 are given in Table S6 in

23

Supporting Information.

24

Table 1 lists modified Arrhenius parameters for key elementary reaction steps of

25

unimolecular decomposition in Figures 1 – 4 recalculated based on the TST with

26

M06-2X/6-311++G(d,p) level of theory. The corresponding energy barriers and the

27

thermodynamic

28

M06-2X/6-311++G(d,p) level were also given in Table S7 and S8 of Supporting

29

Information, respectively. A comparison of the energy barriers between the B3LYP

30

calculation (in Figures 1 – 4) and the M06-2X calculation (in Table S7) shows a trend

31

toward the underestimation of the B3LYP calculation.

1

properties

for

key

chemical

species

32


recalculated

at

the

Page 15 of 24 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


33 34

Figure 5: Pyrolysis pathways of pyrogallol. All energy barriers are in kcal/mol and

35

computed at the B3LYP/6-311++G(d,p) level.

36 37 38 39


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1 2

Page 16 of 24

Table 1: Modified Arrhenius Parameters (k = ATnexp(-E/RT)) for key elementary reaction steps of unimolecular decomposition in Figures 1 – 4 recalculated based on the TST with M06-2X/6-311++G(d,p) level of theory. Step Step1 Step3 Step6 Step8 Step11 Step12 Step18 Step30 Step34 Step42 Step43 Step48 Step49 Step54 Step56 Step58 Step60 Step66 Step80

Reaction HDPP HDPP HDPP A4 HDPP A6 HDPP HDPP A15 Sinapyl alcohol Sinapyl alcohol HHDPP HHDPP HDPPD Syringol C2 C2 Syringol Pyrogallol

→ A1 → A2 → A4 → Syringol → A6

A [1/s] + H

→ A12 → A16 → A8 → B1 → A2 → B3 → B4 → C1 → C3 → C4 → C8 → D1

E [kcal/mol]

0.00

108.5

14

1.18

92.9

10

0.86

92.0

6

2.38

98.9

8

1.46

87.8

-6

5.51

94.1

5

2.07

85.8

10

1.66

59.5

-35

13.86

63.1

16

0.00

94.5

13

0.00

100.8

16

0.00

83.8

14

0.00

92.7

14

0.00

103.3

9

2.62

56.9

9

2.79

62.7

14

0.00

95.2

12

0.00

82.4

-40

15.34

59.4

6.13×10

+ C2H3

1.04×10

+ CH2=C + CO

2.85×10

2.21×10

+ CO

5.28×10

→ A7 → Syringol

n [-]

14

6.73×10 + C2H2CO + CH3

9.44×10 4.42×10

+ H2

3.97×10

+ CH2OH + H

2.49×10

8.35×10

+ CH2CH2OH + H

1.84×10

5.63×10

+ H

7.62×10

+ CH3

2.69×10

+ CH3

4.13×10

+ H

8.58×10

+ H

6.61×10

+ H2

1.09×10

3

Note that A factors at steps 1, 42, 43, 48, 49, 54, 60,60 were calculated based on VTST with B3LYP/6-311++G(d,p) and

4

activation energies E at these steps were calculated with M06-2X/6-311++G(d,p). 16 ACS Paragon Plus Environment

Page 17 of 24 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


1

3

4. Conclusions We proposed the possible pyrolytic pathways of syringol-type monolignol compounds

4

(HDPP, sinapyl alcohol, HHDPP, HDPPD, and syringol) produced during lignin primary

5

pyrolysis, including unimolecular decomposition, addition, and abstraction reactions.

6

HDPP pyrolytic pathway is divided into two processes, the side-chain conversion to form

7

syringol and O–CH3 bond cleavage to decompose into but-1-en-3-yne. Sinapyl alcohol,

8

HHDPP, HDPPD are assumed to be converted to syringol through the side-chain

9

conversion. Then, the generated syringol decomposes into pyrogallol, catechol, or guaiacol.

10

Among these three products, the formation of pyrogallol is likely to be the major channel.

11

Pyrogallol is further converted into C2H2 and C2HOH through unimolecular decomposition,

12

or into catechol and resorcinol by the displacement of an OH with H. Energy barriers along

13

the proposed reaction pathways were calculated at the B3LYP/6-311++G(d,p) level, and the

14

high-pressure limit rate constants for each proposed elementary reaction steps were

15

evaluated based on TST. Future work should examine the pyrolytic pathways involving O–

16

CH3 bond scission and abstraction by a CH3 radical in sinapyl alcohol, HHDPP, and

17

HDPPD, and should apply high level ab-intio calculations to further verify our results.

18

Additionally, Further estimating the elementary reaction rate constants of the thermal

19

decomposition processes of phenol-type monolignols (HPP, p-coumaryl alcohol, HHPP,

20

and HPPD) would help to establish the DCKM for predicting lignin pyrolysis behavior with

21

both primary pyrolysis and secondary vapor-phase reactions.

2

22 23 24 25 26

Supporting Information Figure S1: Bond dissociation energy (kcal/mol) of O-CH3 and O-H in syringol for several ab-initio methods.

28

Figure S2: Proposed possible initial pyrolytic steps and H-addition into the double bond in HDPP other than those in Figure 1 and 2.

29

Figure S3: Proposed pyrolysis pathways of guaiacol.

30 31

Table S1: Standard enthalpies of formation, entropies, heat capacities for all chemical species calculated with the GPOP program42 based on the B3LYP/6-311++G(d,p) level of

32

theory.

33

Tables S2 – S6: Modified Arrhenius Parameters (k=ATnexp(-E/RT)) for Each Elementary

27



1

Channel in Figures 1 – 5 and Figure S2, Forward (f) and Reverse (r).

2

Table S7: Energy barrier for key elementary reaction steps recalculated based on the M06-2X/6-311++G(d,p) level of theory.

3

5

Table S8: Standard enthalpies of formation, entropies, heat capacities for key chemical species recalculated based on the M06-2X/6-311++G(d,p) level of theory.

6

(1) Geometries, frequencies, spin multiplicities (2S+1), and total energies (with ZPE

7

correction) of all stable structures and transition states calculated at the

8

B3LYP/6-311++G(d,p) level of theory. Note that the geometries, frequencies, and total

9

energies of biradicals (such as A7, A18, A19, D3, D4, TSA2, TSA4, TSA5, TSA9,

10

TSA10, TSA22, TSA26, TSA27, TSA30, TSA31, TSA32, TSC1, TSC2, TSD3, TSD4,

11

and TSD5) were calculated at open-shell singlet states.

4

12

(2) Geometries, frequencies, spin multiplicities (2S+1), and total energies (with ZPE

13

correction) of key chemical species recalculated at the M06-2X/6-311++G(d,p) level of

14

theory.

15

17

Acknowledgements This research was in part financially supported by KAKENHI (Grant-in-Aid for Scientific

18

Research (B): 17H03454). The authors are also grateful to the support by the Cooperative

19

Research Program of “Network Joint Research Center for Materials and Devices”. All the

20

computations in this study were performed on the PC cluster systems in our group and the

21

high-performance computing system at the Research Institute for Information Technology,

22

Kyushu University.

16

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Theoretical Study on Elementary Reaction Steps in Thermal

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