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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
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Processes of Syringol-Type Monolignol Compounds
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Yuki FURUTANI1, Yuki DOHARA1, Shinji KUDO2, Jun-ichiro HAYASHI2,3, and
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Koyo NORINAGA*4
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1
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Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan.
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2
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Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan.
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1, Institute for Materials Chemistry and Engineering, Kyushu University, 6-1,
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3
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Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan.
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4
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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
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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
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derived from the primary pyrolysis of lignin. These monolignol-derived compounds are
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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
28
phenol types include 1-(4-hydroxyphenyl)prop-2-en-1-one (HPP), p-coumaryl alcohol,
29
3-hydroxy-1-(4-hydroxyphenyl)propan-1-one
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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
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sinapyl
alcohol, (HHDPP),
(HHPP),
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The Journal of Physical Chemistry
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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
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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,
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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
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broken-symmetry unrestricted open-shell singlet calculation with the “guess = mix”
9
keyword was employed for biradicals.
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High-pressure limit rate constants for all elementary reaction steps were calculated
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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
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channels without any pronounced energy barriers along the reaction coordinates.42 In
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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.
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Differences of bond dissociation energy (BDE) of O-CH3 and O-H in syringol among
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several ab-initio methods such as B3LYP36–38, M06-2X44, and CBS-QB345 were
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evaluated as described in Figure S1 of Supporting Information. Figure S3 indicated that
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B3LYP calculation underestimated the BDE in comparison with M06-2X, and
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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
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CBS-QB3 based on the molecular structure data in Supporting Information. Kim et al.46
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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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The Journal of Physical Chemistry
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Gwinn approximation.47,48
2 3 4
3. Results and Discussion
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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
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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.
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The Journal of Physical Chemistry
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
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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.
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The Journal of Physical Chemistry
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
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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
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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
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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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The Journal of Physical Chemistry
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
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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
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recalculated
at
the
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The Journal of Physical Chemistry
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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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
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The Journal of Physical Chemistry
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
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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
23 24 25
References:
26
(1)
of Phenolic β-Ether Type of Lignin Dimer. J. Wood Chem. Technol. 2007, 27, 113– 120.
27 28 29
(2)
32
Kawamoto, H.; Horigoshi, S.; Saka, S. Pyrolysis Reactions of Various Lignin Model Dimers. J. Wood Sci. 2007, 53, 168–174.
30 31
Kawamoto, H.; Saka, S. Role of Side-Chain Hydroxyl Groups in Pyrolytic Reaction
(3)
Czernik, S.; Bridgwater, A. V. Overview of Applications of Biomass Fast Pyrolysis Oil. Energy and Fuels 2004, 18, 590–598.
18 ACS Paragon Plus Environment
Page 18 of 24
Page 19 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
The Journal of Physical Chemistry
(4)
Bridgwater, A. V. Review of Fast Pyrolysis of Biomass and Product Upgrading. Biomass and Bioenergy 2012, 38, 68–94.
2 3
(5)
Kawamoto, H. Lignin Pyrolysis Reactions. J. Wood Sci. 2017, 63, 117–132.
4
(6)
Goyal, H. B.; Seal, D.; Saxena, R. C. Bio-Fuels from Thermochemical Conversion of Renewable Resources: A Review. Renew. Sustain. Energy Rev. 2008, 12, 504– 517.
5 6 7
(7)
Collard, F. X.; Blin, J. A Review on Pyrolysis of Biomass Constituents: Mechanisms
8
and Composition of the Products Obtained from the Conversion of Cellulose,
9
Hemicelluloses and Lignin. Renew. Sustain. Energy Rev. 2014, 38, 594–608.
10
(8)
Paper Mills: A Critical Review. Bioresour. Technol. 2010, 101, 8001–8015.
11 12
(9)
(10)
Klein, M. T.; Virk, P. S. Modeling of Lignin Thermolysis. Energy and Fuels 2008, 22, 2175–2182.
(11)
Nunn, T.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Product Compositions and
15 16
Mu, W.; Ben, H.; Ragauskas, A.; Deng, Y. Lignin Pyrolysis Components and Upgrading-Technology Review. Bioenergy Res. 2013, 6, 1183–1204.
13 14
Naqvi, M.; Yan, J.; Dahlquist, E. Black Liquor Gasification Integrated in Pulp and
17
Kinetics in the Rapid Pyrolysis of Milled Wood Lignin. Ind. Eng. Chem. Process
18
Des. Dev. 1985, 24, 844–852.
19
(12)
Macromolecules 1984, 17, 161–169.
20 21
(13)
23
(14)
Pasquali, C. E. L.; Herrera, H. Pyrolysis of Lignin and IR Analysis of Residues. Thermochim. Acta 1997, 293, 39–46.
25 26
Caballero, J. A.; Font, R.; Marcilla, A. Study of the Primary Pyrolysis of Kraft Lignin at High Heating Rates: Yields and Kinetics. J. Anal. Appl. Pyrolysis 1996, 36, 159–178.
22
24
Petrocelli, F. P.; Klein, M. T. Model Reaction Pathways in Kraft Lignin Pyrolysis.
(15)
Yang, H.; Kudo, S.; Hazeyama, S.; Norinaga, K.; Mašek, O.; Hayashi, J.-I. Detailed
27
Analysis of Residual Volatiles in Chars from the Pyrolysis of Biomass and Lignite.
28
Energy and Fuels 2013, 27, 3209–3223.
29 30
(16)
Crombie, K.; Mašek, O. Investigating the Potential for a Self-Sustaining Slow Pyrolysis System under Varying Operating Conditions. Bioresour. Technol. 2014, 19 ACS Paragon Plus Environment
The Journal of Physical Chemistry 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
162, 148–156.
1 2
(17)
Pathways. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 173–183.
3 4
(18)
Hosoya, T.; Kawamoto, H.; Saka, S. Secondary Reactions of Lignin-Derived Primary Tar Components. J. Anal. Appl. Pyrolysis 2008, 83, 78–87.
5 6
Jegers, H. E.; Klein, M. T. Primary and Secondary Lignin Pyrolysis Reaction
(19)
Zhou, S.; Garcia-Perez, M.; Pecha, B.; McDonald, A. G.; Kersten, S. R. A.;
7
Westerhof, R. J. M. Secondary Vapor Phase Reactions of Lignin-Derived Oligomers
8
Obtained by Fast Pyrolysis of Pine Wood. Energy and Fuels 2013, 27, 1428–1438.
9
(20)
Decomposition of Kraft Lignin. J. Anal. Appl. Pyrolysis 1996, 38, 131–152.
10 11
(21)
(22)
(23)
Ranzi, E.; Cuoci,
a; Faravelli, T.; Frassoldati, a; Migliavacca, G.; Pierucci, S.;
Sommariva, S. Chemical Kinetics of Biomass Pyrolysis. Energy & Fuels 2008, 22, 4292–4300.
16 17 18
Faravelli, T.; Frassoldati, A.; Migliavacca, G.; Ranzi, E. Detailed Kinetic Modeling of the Thermal Degradation of Lignins. Biomass and Bioenergy 2010, 34, 290–301.
14 15
Hough, B. R.; Schwartz, D. T.; Pfaendtner, J. Detailed Kinetic Modeling of Lignin Pyrolysis for Process Optimization. Ind. Eng. Chem. Res. 2016, 55, 9147–9153.
12 13
Caballero, J. A.; Font, R.; Marcilla, A. Kinetic Study of the Secondary Thermal
(24)
Furutani, Y.; Kudo, S.; Hayashi, J.-I.; Norinaga, K. Predicting Molecular
19
Composition of Primary Product Derived from Fast Pyrolysis of Lignin with
20
Semi-Detailed Kinetic Model. Fuel 2018, 212, 515–522.
21
(25)
Yang, H.-M.; Appari, S.; Kudo, S.; Hayashi, J.-I.; Norinaga, K. Detailed Chemical
22
Kinetic Modeling of Vapor-Phase Reactions of Volatiles Derived from Fast
23
Pyrolysis of Lignin. Ind. Eng. Chem. Res. 2015, 54, 6855–6864.
24
(26)
Thimthong, N.; Appari, S.; Tanaka, R.; Iwanaga, K.; Kudo, S.; Hayashi, J.-I.; Shoji,
25
T.; Norinaga, K. Kinetic Modeling of Non-Catalytic Partial Oxidation of Nascent
26
Volatiles Derived from Fast Pyrolysis of Woody Biomass with Detailed Chemistry.
27
Fuel Process. Technol. 2015, 134, 159–167.
28
(27)
Norinaga, K.; Yang, H.; Tanaka, R.; Appari, S.; Iwanaga, K.; Takashima, Y.; Kudo,
29
S.; Shoji, T.; Hayashi, J.-I. A Mechanistic Study on the Reaction Pathways Leading
30
to Benzene and Naphthalene in Cellulose Vapor Phase Cracking. Biomass and 20 ACS Paragon Plus Environment
Page 20 of 24
Page 21 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
The Journal of Physical Chemistry
Bioenergy 2014, 69, 144–154.
1 2
(28)
Norinaga, K.; Shoji, T.; Kudo, S.; Hayashi, J.-I. Detailed Chemical Kinetic
3
Modelling of Vapour-Phase Cracking of Multi-Component Molecular Mixtures
4
Derived from the Fast Pyrolysis of Cellulose. Fuel 2013, 103, 141–150.
5
(29)
Furutani, Y.; Kudo, S.; Hayashi, J.-I.; Norinaga, K. Theoretical Study on Reaction
6
Pathways Leading to CO and CO2 in the Pyrolysis of Resorcinol. J. Phys. Chem. A
7
2017, 121, 631–637.
8
(30)
Study of Unimolecular Decomposition of Catechol. J. Phys. Chem. A 2010, 114, 1060–1067.
9 10 11
Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. Theoretical
(31)
Liu, H.; Chen, J.; Wang, F.; Wang, Z.; Wang, L. Theoretical Study on the
12
Thermodynamic Properties and Stability of Polybrominated Diphenyl Sulfide Catena.
13
Acta Chim. Sin. 2010, 68, 11751–11760.
14
(32)
Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. Thermochemical
15
Properties and Decomposition Pathways of Three Isomeric Semiquinone Radicals. J.
16
Phys. Chem. A 2010, 114, 1098–1108.
17
(33)
Khachatryan, L.; Asatryan, R.; McFerrin, C.; Adounkpe, J.; Dellinger, B. Radicals
18
from the Gas-Phase Pyrolysis of Catechol. 2. Comparison of the Pyrolysis of
19
Catechol and Hydroquinone. J. Phys. Chem. A 2010, 114, 10110–10116.
20
(34)
from Phenoxy Radicals. Int. J. Chem. Kinet. 2011, 44, 75–89.
21 22
(35)
24
(36)
27
(37)
Becke, A. D. Density‐functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648–5652.
(38)
Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy
29 30
Becke, A. D. Density‐functional Thermochemistry. IV. A New Dynamical Correlation Functional and Implications for Exact‐exchange Mixing. J. Chem. Phys. 1996, 104, 1040–1046.
26
28
Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. . et al. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.
23
25
Carstensen, H.-H.; Dean, A. M. A Quantitative Kinetic Analysis of CO Elimination
21 ACS Paragon Plus Environment
The Journal of Physical Chemistry 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
Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785–789.
1
3
Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self‐consistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys.
4
1980, 72, 650–654.
2
5
(39)
(40)
Frisch, M. J.; Pople, J. A.; Binkley, J. S. Self‐consistent Molecular Orbital Methods 25. Supplementary Functions for Gaussian Basis Sets. J. Chem. Phys. 1984, 80, 3265–3269.
(41)
Gonzalez, C.; Schlegel, H. B. An Improved Algorithm for Reaction Path Following.
6 7 8
J. Chem. Phys. 1989, 90, 2154–2161.
9 10
(42)
Miyoshi, A. GPOP software, Home Page: http://akrmys.com/gpop/.
11
(43)
Garrett, B. C.; Truhlar, D. G. Semiclassical Tunneling Calculations. J. Phys. Chem. 1979, 83, 2921–2926.
12 13
(44)
Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group
14
Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited
15
States, and Transition Elements: Two New Functionals and Systematic Testing of
16
Four M06-Class Functionals and 12 Other Function. Theor. Chem. Acc. 2008, 120, 215–241.
17 18
(45)
Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. A Complete
19
Basis Set Model Chemistry. VII. Use of the Minimum Population Localization
20
Method. J. Chem. Phys. 2000, 112, 6532–6542.
21
(46)
Kim, S.; Chmely, S. C.; Nimlos, M. R.; Bomble, Y. J.; Foust, T. D.; Paton, R. S.;
22
Beckham, G. T. Computational Study of Bond Dissociation Enthalpies for a Large
23
Range of Native and Modified Lignins. J. Phys. Chem. Lett. 2011, 2, 2846–2852.
24
(47)
Pitzer, K. S.; Gwinn, W. D. Energy Levels and Thermodynamic Functions for
25
Molecules with Internal Rotation I. Rigid Frame with Attached Tops. J. Chem. Phys.
26
1942, 10, 428–440.
27
(48)
Separability of Rotational Degrees of Freedom. J. Phys. Chem. A 1998, 102, 3916– 3922.
28 29 30
Knyazev, V. D. Density of States of One-Dimensional Hindered Internal Rotors and
(49)
Ervin, K. M.; Ho, J.; Lineberger, W. C. A Study of the Singlet and Triplet States of 22 ACS Paragon Plus Environment
Page 22 of 24
Page 23 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
The Journal of Physical Chemistry
1
Vinylidene by Photoelectron Spectroscopy of H2C=C-, D2C=C-, and HDC=C-.
2
Vinylidene-Acetylene Isomerization. J. Chem. Phys. 1989, 91, 5974–5992.
3
(50)
Huang, J.; Liu, C.; Ren, L.; Tong, H.; Li, W.; Wu, D. Studies on Pyrolysis
4
Mechanism of Syringol as Lignin Model Compound by Quantum Chemistry. J.
5
FUEL Chem. Technol. 2013, 41, 657–666.
6
(51)
Asmadi, M.; Kawamoto, H.; Saka, S. Thermal Reactivities of Catechols/pyrogallols
7
and Cresols/xylenols as Lignin Pyrolysis Intermediates. J. Anal. Appl. Pyrolysis
8
2011, 92, 76–87.
9
(52)
as Lignin Model Aromatic Nuclei. J. Anal. Appl. Pyrolysis 2011, 92, 88–98.
10 11
(53)
14 15
Liu, C.; Zhang, Y.; Huang, X. Study of Guaiacol Pyrolysis Mechanism Based on Density Function Theory. Fuel Process. Technol. 2014, 123, 159–165.
12 13
Asmadi, M.; Kawamoto, H.; Saka, S. Thermal Reactions of Guaiacol and Syringol
(54)
Britt, P. F.; Buchanan, A. C.; Cooney, M. J.; Martineau, D. R. Flash Vacuum Pyrolysis of Methoxy-Substituted Lignin Model Compounds. J. Org. Chem. 2000, 65, 1376–1389.
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