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Density functional theory investigation of gasification mechanism of a lignin dimer with β-5 linkage
Accepted Manuscript Density functional theory investigation of gasification mechanism of a lignin dimer with β-5 linkage
Zhang Hang, Deng Shengxiang, Cao Xiaolin PII:
S0960-1481(17)30854-6
DOI:
10.1016/j.renene.2017.08.095
Reference:
RENE 9193
To appear in:
Renewable Energy
Received Date:
03 March 2017
Revised Date:
14 July 2017
Accepted Date:
31 August 2017
Please cite this article as: Zhang Hang, Deng Shengxiang, Cao Xiaolin, Density functional theory investigation of gasification mechanism of a lignin dimer with β-5 linkage, Renewable Energy (2017), doi: 10.1016/j.renene.2017.08.095
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ACCEPTED MANUSCRIPT
We propose four decomposition and four subsequent reaction pathways.
We calculate bond dissociation energies of homolytic cleavage for lignin dimmer.
We calculate enthalpy changes and Gibbs free energy changes of each reaction.
The equilibrium geometries of stagnation points in the reaction are optimized.
ACCEPTED MANUSCRIPT
2
Density functional theory investigation of gasification mechanism of a lignin dimer with β-5 linkage
3
Zhang Hang 1*, Deng Shengxiang1, Cao Xiaolin2
1
4 5
1 College 2 Energy
of Energy Science and Engineering, Central South University, Changsha 410083, China
& Power Engineering College, Changsha University of Science & Technology, Changsha 410012, China
6
Abstract: Density functional theory was employed to investigate gasification mechanism of β-5
7
linkage lignin in high temperature steam at B3LYP/6-31G++(d,p) level. Four possible
8
decomposition reaction pathways and four successive reactions pathways were proposed. The
9
equilibrium geometries of the reactants, transition states, intermediates and products were
10
optimized. The activation energies, Gibbs free energy changes and enthalpy changes of each
11
reaction step in gasification pathways were analyzed. From the calculation results of ΔHθ, it is
12
concluded that the four decomposition reaction are endothermic below 1300 K, and exothermic
13
exceed 1500 K. Furthermore, the ΔHθ of each reaction decrease with temperature increasing. From
14
the ΔGθ calculation results, Path 2 and Path 3 occur spontaneously without temperature limited.
15
Path 1 and Path 4, however, are spontaneous reaction above 700 K. It is not ensure that Path 5,
16
Path 6, Path 7 and Path 8 can occur spontaneously. Comparison of activation energy of each
17
reaction pathway, Path 2 is optimal decomposition reaction without a competitive reaction. The
18
main products in decomposition reaction are 5-hydroxyphenyl-acetaldehyde (P5) and 12-
19
hydoxymethyl-phenol (P4). Path7 is the optimal subsequent reaction with a competitive reaction,
20
i.e., Path 5. The main products in subsequent reaction are CO, phenol and formaldehyde.
21
Keywords: Lignin dimer; Density function theory; Gasification; Thermodynamic; Dynamic
22
1. Introduction
23
The rising concerns about energy security and environmental degradation in newly
24
industrialized and transition economies has increased renewable energy demand and alternate
25
energy resources [1-2]. CO2 emissions, especially, are becoming a major problem all around
26
world. Biomass is viewed as a potentially significant source of renewable energy[3-4].
1
*Corresponding author. Address: School of Energy Science and Engineering, Central South University, Changsha 410083, China. Tel: +86 18508449270; e-mail: [email protected] (H. Zhang) 1
ACCEPTED MANUSCRIPT 27
Gasification of biomass in the high-temperature steam, being as a technology which is quite
28
promising[5-6], is greatly featured that the emission of CO2 can be lowered by 30%, 15-30% of
29
energy can be saved and the emission of NOX is lower than 100ppm.
30
Lignin, being as one of the three major components of biomass[7,8], can reflect the
31
gasification rules of biomass by its gasification features. Therefore, it is necessary to study lignin
32
pyrolysis mechanism in gasification process. For lignin, the polymerization is via the
33
dehydrogenation of monomer, which is made up of some chemical bonds (like C-C bond and C-O
34
bond) in an unordered way. For example, β-5 type lignin dimmer, a typical structure of lignin, is a
35
propyl structure consisting of two benzene rings with a Cβ-C5 bond linkage (see Fig. 1).
36
In recent years, Tan et al. [9]and Yue et al. [10] found that abundant phenol after the
37
decomposition reaction in their experiments. It is well known that phenol will not be produced
38
unless cleavage of Cα-Cβ and Cβ-C5. In fact, Beste et al. [11]studied that the initial step in the
39
thermal decomposition of PPE (β-O-4 type lignin dimer) is the hemolytic cleavage of the oxygen-
40
carbon bond, and that carbon-carbon bond cleavage in PPE could be a competitive initial reaction
41
under high-temperature pyrolysis conditions. According to the experiment result in reference [12],
42
it is concluded that phenol, p-hydroxybenzyl alcohol and ethyl alcohol are the main products of
43
Cα-Cβ bond cleavage and β-O-4 bond cleavage. Qin et al. [13] investigated that the saw powder
44
rich in lignin will produce oil tar after gasification, which exist large amounts of
45
aromatic compounds (side chain). They also found that the side chain cracking with experiment
46
temperature increasing. As we know, lignin are interconnected by a variety of linkages (β-O-4, α-
47
O-4, β-1, β-5). The β-O-4 bond is the major type of linkage which occupies 48-60% of the total
48
linkages depending on the type of wood. Regarding β-O-4 lignin, there are different views on its
49
mechanism of thermal cracking. For example, Elder and Beste [14]did a quantum chemical study
50
on pyrolysis mechanism of β-O-4 lignin, concluding that activation energy of concerted reaction is
51
lower than the bond dissociation energy of homolytic reaction, which reflects that homolytic
52
reaction plays a leading role among these reactions. On the contrary, Huang et al. [15]found that
53
concerted reaction and homolytic reaction are both the major reaction paths of lignin dimer, which
54
will be affected by the temperature. It is summarized that concerted reaction will play the leading
55
role during the pyrolysis at the low-temperature, but homolytic reaction will do at high-
56
temperature. Apart from β-O-4 lignin, there are other kinds of lignin, such as β-1 lignin (Huang et 2
ACCEPTED MANUSCRIPT 57
al. [16]once studied the pyrolysis mechanism); α-O-4 lignin (Wu et al. [17]studied the pyrolysis
58
process of lignin dimer with α-O-4 linkage). Moreover, Cao et al. [18]once studied the reaction
59
mechanism of high temperature steam gasification of β-1 lignin dimer in the water solvent
60
environment.
61
However, today, there are few related to high temperature steam gasification of β-5 linkage
62
lignin dimer in terms of its chemical reaction, evolution reaction between intermediates and
63
transition state. In order to gain a better understanding of the detailed mechanism of lignin
64
gasification and the temperature effects on gasify process, mechanism of β-5 linkage lignin dimer
65
gasification at different temperature steam have been investigated in this paper.
66
2. Density functional theory (DFT) calculation methods
67
As for the electric correlation effects, DFT will calculate the large molecular system, whose
68
calculation results are quite accurate and reliable[19,20].In this paper, the relevant calculation
69
work was relied on DFT method with B3LYP (Becke’s three parameter gradient corrected
70
exchange functional with the gradient corrected correlation function [21]) and 6-31++G(d, p) basis
71
set (“6-31G” is an expression of splitting valence bond basis set. “++” are addition diffuse
72
functions of heavy atom and hydrogen. “d” and “p” are addition polarization functions of carbon
73
and hydrogen，respectively [22]), which consists of optimizing the equilibrium geometries such
74
as reactant, intermediates, products and transition states. The reason for providing the polarized
75
function is that is better to describe the system. Transition states (TS) method was employed to
76
find the transition state[23]. Intrinsic reaction coordinate (IRC) calculations were applied to
77
identify the minimum energy path from a transition state to the two corresponding local minima.
78 79
Thermodynamic data such as enthalpy changes (ΔHθ) and Gibbs free energy changes (ΔGθ) were calculated to analyze priorities of the reaction pathways.
80
For reaction：
81
A B A· B·
82
Enthalpy change of the reaction are calculated by Eq. (2) [24]:
83
H H ( A) H ( B) H ( A B)
84
Enthalpy change are estimated by the following expression:
85
H E nRT
(1)
(2)
(3) 3
ACCEPTED MANUSCRIPT 86
E =E0 Etrans Erot Evib
87
E0 =Eelec ZPE
88
Which, ∆Eelec is internal energy, ZPE is zero point energy, ∆Etrans is translation energy, ∆Erot is
89
(4) (5)
rotation energy, and ∆Evib is vibration energy.
90
The gibbs free energy change calculation equation:
91
G H T S
92
Which, T is reaction temperature, ∆S is entropy change.
93
When ΔH < 0, it means that the reaction is exothermic, it is endothermic if ΔH > 0. When ΔG <
94
0, it indicated that the reaction can be carried out spontaneously, instead, it can not occur
95
spontaneously if ΔG > 0. Furthermore, the size of ΔG determines conversion rate of reaction
96
reaching equilibrium.
(6)
97
According to transition state theory, activation energy is the minimal potential-energy
98
difference between transition state and reactants in ground state[25]. Since temperature affecting
99
slightly on thermodynamic, it, normally, did not take into account in calculating activation
100
energy[26]. The authors used E0 to represents the activation energy of each species in the reaction.
101
For free-radical reactions, the bond dissociation energies are usually equal or almost equal to the
102
activation energies. All calculations were performed using the Gaussian 09 suites of programs[27].
103
3. Results and discussion
104
3.1 Bond dissociation energies for β-5 linkage lignin dimer
105
Bond dissociation energies (ΔE) are recognized as an important thermodynamic quantity that
106
can exemplify the reaction sensitivity of a hemolytic cleavage of the considered bond. Cao et al.
107
[28] investigated biomass pyrolysis and carbonization process by TG-FTIR experimental. The
108
results show that when the temperature is in the range of 773~1173K, C - C bond and C - H bond
109
were further fractured. Most of the benzene ring has been decomposition or aromatic to form
110
amorphous carbon. In lignin gasification process, the homolytic bond dissociation is an important
111
reaction step. Fig. 1 gives the homolytic cleavage paths for the β-5 type lignin dimmer model
112
compound and their bond dissociation energies. The lowest bond dissociation energy is that of Cγ-
113
Cβ. The order of ΔE is as follows: Cγ-Cβ < Cα-Cβ < Cα-C12 < C5-Cβ < Cγ-OH < Cα-OH < C4-OH < 4
ACCEPTED MANUSCRIPT 114
C9-OH, therefore, the homolytic cleavage of the Cγ-Cβ, Cα-Cβ, Cα-C12, and C5-Cβ bond could be
115
the initial step in the decomposition reaction of β-5 type lignin dimmer. 2
2 1
3
1
3
6
4
6
4
OH
OH
5
HO
OH
+ HO
OH
12
11
1
3
8
10
8
10
6
4
10
E =8 1.
9
7k
HO
cal /m ol
2 1
3
6
7
C
12
8
11 10
+
9
3
6
4
HO
HO
E=40.8 kcal/mol HO
5
OH
OH
CH
CH
7
10 9
116
HO
+
CH 2
.1
ol l/m a kc
.5
ol /m l a kc
11
8
1
10
2
4
C
1
3
6
4
OH
5
E=35.2 kcal/mol
HO
CH
7
10 9
HO
-5 linkage lignin dimer
E =3 7
11
8
OH
+
12
H2C
10 9
.6
HO
kca l/m ol
HO
2 1
3
6
HO
9
OH OH
+
HO
11
8
12
2 11
8
6 =4 E
OH
12 7
12
HO
4 5
2 1
7
4 =6 E
OH
5
HO
OH
HO
k
4
11
8
9
C
12 7
9
C
l mo al/ kc
2
7
.2
+
HO
+ HO
11
6 =8
HO
12
E
5
HC
7
3
6
cal /m ol
2 1
5
3
6
4 5
OH
5
HC
117
Fig.1.
118
3.2 Reaction pathways of belta-5 linkage lignin dimer gasification
OH OH
+
CH 12
7
11
8
10 9
HO
Homolytic cleavage paths of β-5 linkage lignin dimer and their bond dissociation enthalpies (ΔE).
119
From the above calculation results of bond dissociation enthalpies of β-5 linkage lignin dimer,
120
this paper proposes four decomposition reactions as initial gasification pathways: homolytic
121
cleavage of Cβ-C5 bond, Cα-Cβ bond, Cα-C12 bond and Cβ-Cγ bond (see Fig. 2) and four subsequent
122
reactions as secondary gasification pathways. From the decomposition reactions depicted in Fig.
123
2, it is inferred that the major decomposition products are phenolic compounds, no small molecule
124
gases. So the authors proposed that the four special products (P2, P4, P5 and P6) are utilized to
125
produce small molecule gases with water molecule in the subsequent reactions formed as
126
concerted reaction (see Fig. 3).
5
ACCEPTED MANUSCRIPT HO
CH
7
11
8
10
Path1
TS1
HO
OH
12
-H
7
11
8
10
9
9
HO
HO
+
IM1
2
2
1
3
8
4
HO
OH
HO
CH
12
TS2
7
11
8
10
3
6
4 5
OH
HC
OH
Path2
TS3 HO
+
IM4
-H
CH2
P1
2 1
3
6
4
OH
OH
5
HO
2
2
6
4
TS4 OH
5
1
3
1
3
6
4
6
4
OH
5
OH
O
11
8
10
+
12
H
7
11
8
10
9
9
HO
HO
P4
2
8
1
1
3
6
3
3
6
4
10
HO
9
OH
CH
+H
HO
Path3
OH
7
11 10
7
11
8
10
TS6
HO
+
-H
OH
Path4
3 4
OH
OH
C
12
8
11
IM3
10
8
10
3
6
4
HC
1
3 4
OH
+
OH
O
TS9
IM5
9
9
HO
HO
P6
10
2
2 3
6
4
OH
OH
5
3
1
TS4
1
OH
4
6
OH
5
O
IM2
-H
O
2
CH2
TS9b
11
8
+H OH
CH
12 7
9
1
3
6
4
P9
5
OH
O
10 9
P5
HO
P8
127 Fig.2.
Decomposition reaction pathways schematic of belta-5 linkage lignin dimer model compound gasification O
HO
CH3
12 7
11
8
10 9
+
H
OH
TS10
12
12 7
11
8
10 9
HO
+
CO
+
CH3OH
11
8
10
+
H
OH
Path5
TS11
12 7
11
8
10
9
9
HO
HO
P4
2 + 2H2 + CO
Path6
O
12 7
11
8
10 9
P5
Fig.3.
7
HO
P2
129 130
1
6
+H
12
8
5
11
HO
O
HO
128
O
-H
7
2
-H
+ 7
CH
5
5
TS8
P7
11
2
1
+ 12
6
OH
7
2
CH
5
P1
HO
OH
CH2
4
CH3
O
CH2
3
6
CH2
HO
9
9
1
12
8
TS5
4 5
12
R
+H
5 6
4 5
HO
2
1 2
2
11
OH O
P5
12 7
5
CH
7
P2
O
OH
3
HO
11 10 9
IM2
12
7 8
HO
CH2
1
+
11 10 9
OH
2
-H
7 8
HO
4
2
12
9
P3
1
TS7
CH3
HO
3
5
11 10
12
9
1
5
7 8
O
12
+H
CH2
O
CH2
HO
6
+H
C
OH
12
OH
+
H
OH
TS12
11
8
10 9
O
12 7
3OH + CH + CO
12 7
11
8
10
+
H
OH
TS13
HO
10
+
CO2
+ H2
HO
HO
P6
11
8 9
9
Path7
12 7
Path8
Subsequent reaction pathways schematic of belta-5 linkage lignin dimer model compound gasification 6
ACCEPTED MANUSCRIPT 131 132
3.3 Optimization of geometries and their key bonds length Table 1 shows the equilibrium geometries of the reactants, intermediates and partial transition
133
states after optimizing and lists the key bond length of the species.
134 135
Table 1． Geometric structure and key bond length optimization of the species in each reaction. Optimized structure
Bond length / (Å)
Optimized structure
Bond length / ( Å)
R(19,20)
1.5432
R(19,20)
1.5432
R(20,22)
1.5397
R(20,22)
1.5397
R(1,20)
1.5420
R(12,19)
1.5403
R(12,19)
1.5403
R(19,34)
1.4303
R(19,34)
1.4307
R(22,26)
1.4307
R(20,22)
1.5429
R(19,20)
1.5432
R(1,20)
1.5420
R(1,20)
1.4065
R(12,19)
1.5403
R(19,12)
1.3720
R(22,26)
1.0847
R(19,34)
1.3948
R(19,22)
1.0871
R(19,20)
1.4132
R(19,34)
1.4498
R(20,22)
1.3997
R(19,20)
1.3471
R(22,26)
1.3489
R(19,20)
1.2632
R(20,22)
1.8811
R(19,21)
1.3257
R(22,26)
1.2677
R(1,20)
1.8860
R(26,27)
1.4086
R(19,34)
1.4307
R(1,20)
1.5420
R(19,20)
1.8843
R(20,22)
1.2643
R(20,22)
1.2616
R(22,26)
1.2576
R(22,26)
1.4958
R(26,27)
1.4128
R(1,20)
1.5420
R(1,20)
1.5420
R(20,22)
1.8774
R(19,20)
1.8812
R(22,26)
1.2638
R(19,34)
1.2611
R(26,27)
1.4088
R(34,35)
1.4122
R(1,20)
1.5420
R(20,22)
1.5397
R(19,34)
1.2558
R(19,12)
1.8804
R(19,20)
1.3628
R(19,34)
1.2611
R(34,35)
1.1869
R(34,35)
1.4122
R(20,22)
1.5397
R(19,20)
1.5432
R(19,20)
1.8846
R(20,22)
1.3496
R(22,25)
1.3200
R(1,20)
1.5420
136 137
Table note: bond length of Cγ-Cβ, Cα-Cβ, Cα-C12, C5-Cβ, Cγ-OH and Cα-OH are marked as R(19,20), R(20,22), R(1,20), R(19,12),
138
3.4 Thermodynamics Analysis of the decomposition and subsequent reactions
R(19,34), R(22,26), respectively.
7
ACCEPTED MANUSCRIPT 139 140
Table A1 in appendix gives enthalpy changes ΔHθ and Gibbs free energy changes ΔGθ of each reaction pathway at different temperature.
141
Fig. 4 shows the ΔHθ (θ represents temperature) of each reaction at different temperature (700 -
142
1700 K). The results indicate that the ΔHθ of Path 1 decreases with an increase of temperature
143
(700 K to 1700 K). It is an endothermic reaction because ΔHθ is positive value. The ΔHθ of Path2
144
starts from 3.0 kcal/mol and reduces to -2.4 kcal/mol (at 1500K) with a change of endothermic to
145
exothermic . For Path 3, the ΔHθ is very closed to 0 kcal/mol at 1500 K, and would be a negative
146
value if temperature kept increasing. The ΔHθ of Path 4 are higher than zero, so it is endothermic.
147
The ΔHθ of Path 5 reduces from 12.3 to 1.6 kcal/mol and keeps endothermic reaction. The ΔHθ of
148
Path 6 is down to -1.1 kcal/mol and converts into exothermic reaction until temperature increased
149
to 1700 K. For Path 7, the ΔHθ decreases from 15.4 kcal/mol to 2.2 kcal/mol (at 700-1700 K) and
150
forms as endothermic. The ΔHθ of Path 8 decreases to -0.8 kcal/mol with changing from
151
endothermic to exothermic at 1700 K. (a) Decomposition reaction
kcalmol-1
40
Path1 Path2 Path3 Path4
30 20 10 0 -10 700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
kcalmol-1
Tempeature / (K)
152 153
16 14 12 10 8 6 4 2 0 -2
(b) Subsequent reaction
700
800
900
1000
1100
1200
1300
1400
Path5 Path6 Path7 Path8
1500
1600
1700
Tempeature / (K)
Fig.4. Enthalpy changes as function of temperature, (a) decomposition reaction, (b)subsequent reaction.
154
Fig. 5 indicates that the Path 2 and Path 3 are able to be carried out spontaneously because their
155
ΔGθ<0 (at 700 - 1700 K). Path 1 and Path 4, however, are spontaneous reaction until temperature
156
raised to 700 K. Through comparison of the ΔGθ of each reaction pathway, the authors can see
157
that the order of ΔGθ is Path 3
158
as : Path 3< Path 1
159
Path 5, Path 6, Path 7, Path 8 can not occur spontaneously with an order of ΔGθ: Path 8< Path 8
ACCEPTED MANUSCRIPT 160
6
G kcalmol
0 -20 -40 -60 -80 -100 -120
G kcalmol
700
800
900
1000
7 6 5 4 3 2 1 0
1100
700
163
1200
1300
1400
Tempeature / (K)
1500
1600
800
900
1000
1100
1200
1300
1400
1700
Path5 Path6 Path7 Path8
(b) Subsequent reaction
1500
1600
1700
Tempeature / (K)
161 162
Path1 Path2 Path3 Path4
Fig.5. Gibbs free energy changes as function of temperature, (a) decomposition reaction, (b)subsequent reaction.
3.5 Dynamics Analysis of the decomposition and subsequent reactions
164
Table A2 in appendix gives the activation energies of each specie in the reactions proposed.
165
Fig. 6 shows the sectional drawing of potential energy profile along decomposition reaction
166
pathways. In Path 1, β-5 type lignin dimer model compound decomposes into transition state TS1a
167
and TS1b through the homolytic cleavage of the C5-Cβ bond with an energy barrier of 46.1
168
kcal/mol. The TS1a converts into intermediate IM1 through a dehydrogenation, and the TS1b
169
undergoes a hydrogenation to form compound P3. The decomposition reaction absorbs an energy
170
of 24.4 kcal/mol. It can be verified in Table 1 that C5-Cβ bond length enlarges to 1.8860 Å, so it is
171
unstable to breaks. Cα gives a hydrogen atom to C5 and connects to Cβ with a short double bond
172
(1.2632 Å). Compound IM1 can further decompose into TS2a and TS2b through the hemolytic
173
cleavage of Cγ-Cβ bond with an energy barrier of 33.4 kcal/mol. The TS2a and TS2b convert into
174
Compound IM4 and P3 through a hydrogenation and a dehydrogenation, respectively. From Table
175
1, it can be seen that distance of Cγ-Cβ bond reaches 1.8811 Å and is highly possible to cracks. The
176
O34 atom loses a hydrogen atom and makes a connection to Cγ with a short double bond,
177
meanwhile Cβ absorbs the hydrogen atom to form IM4. The decomposition reaction takes an
178
energy of 10.3 kcal/mol. Compound IM4 tautomerize to form hydroxyphenyl-ethanone P2 via
179
TS7 with an energy barrier of 19.0 kcal/mol through intramolecular hydrogen, releasing an energy 9
ACCEPTED MANUSCRIPT 180
of 14.4 kcal/mol. The tautomerization reaction is exothermic and compound IM4 is more stable
181
than its isomers P2.
182
In Path 2, reactant R decomposes into TS3a and TS3b through the homolytic cleavage of the
183
Cα-Cβ bond with an energy barrier of 37.6 kcal/mol. TS3a and TS3b change to compound IM2 and
184
12-hydroxymethyl-phenol P3, respectively. It can be proved in Table 1 that bond distance of Cα-
185
Cβ bond is 1.8843 Å, and it is most likely to rupture. The decomposition reaction is supported by
186
an energy of 18.7 kcal/mol. Compound IM2 tautomerizes to form product P2 via TS7 with an
187
energy barrier of 44.1 kcal/mol through intramolecular hydrogen, releasing an energy of 14.1
188
kcal/mol. There is a transfer of a hydrogen atom from O26 to Cβ, and then product P5 is formed.
189
The tautomerization reaction is exothermic and the intermediate IM2 is more stable than its
190
isomers P5.
191
In Path 3, reactant R, similarly, decomposes into TS5a and TS5b through the homolytic
192
cleavage of the Cγ-Cβ bond (bond length 1.8874 Å) with an energy barrier of 35.2 kcal/mol. The
193
TS5a turns into intermediate IM3 through a hydrogenation, and the TS5b undergoes a
194
dehydrogenation to form product P1. The decomposition reaction absorbs an energy of 14.9
195
kcal/mol. Compound IM3 can further decompose into TS6a and TS6b through the hemolytic
196
cleavage of Cα-Cβ bond (bond length 1.8812 Å) with an energy barrier of 66.5 kcal/mol. These
197
two translate state species can be translated into product P6 and P7. The decomposition reaction
198
takes an energy of 36.2 kcal/mol.
199
In Path 4, reactant R decomposes into TS8a and TS8b through the homolytic cleavage of the
200
Cα-C12 bond (bond length 1.8804 Å)with an energy barrier of 40.8 kcal/mol. TS8a is conversion
201
into intermediate IM5 through a dehydrogenation, and TS8b can undergo a hydrogenation to form
202
product P8. The decomposition reaction absorbs an energy of 30.4 kcal/mol. Compound IM5 can
203
further decompose into TS9a and TS9b through the hemolytic cleavage of Cα-Cβ bond (bond
204
length 1.8846 Å) with an energy barrier of 73.9 kcal/mol. TS9a and TS9b convert into
205
intermediate IM2 and phenol P9 through a hydrogenation and a dehydrogenation, respectively.
206
The decomposition reaction takes an energy of 25.5 kcal/mol. Compound IM2 in Path 4 have a
207
same tautomerization reaction like Path 2, as well as bond length and energy barrier of the
208
reaction.
209
Comparing the energies barriers of each reaction step in the decomposition reactions such as 10
ACCEPTED MANUSCRIPT 210
homolytic cleavage of the Cγ-Cβ, Cα-Cβ, Cα-C12, and C5-Cβ bond, it is summarized that cleavage of
211
Cα-Cβ bond in Path 2 would be the optimal decomposition reaction without a competitive reaction.
212
Hence, the main products of β-5 type lignin dimer model compound in decomposition reaction are
213
5-hydroxyphenyl-acetaldehyde P5 and 12-hydoxymethyl-phenol P4. In reference[29], the
214
pyrolysis properties of lignin were studied by TG-FTIR method and Flynn-Wall-Ozawa
215
calculation method. The results showed that the activation energy of aromatic ring condensation
216
into carbon is much larger than the energy required for the fracture of the branched chain attached
217
to the benzene ring, which indicates that the benzene ring is more likely to break. 120
Path 1 Path 2 Path 3 Path 4
110 100
E0/(kcalmol-1)
90
TS4(100.0)
TS6(81.4)
80 70
TS2(57.8) IM2+P9(55.9)
60 50
TS8(40.8)
40
TS3(37.6)
30
TS5(35.2)
20
0
TS1(46.1)
TS7(62.1)
TS4(54.4) P6+P7(51.3) IM5+P8(30.4) IM1+P3(24.4)
IM4+P1(43.1)
P5(41.8) P2(28.7)
IM3+P1(14.9)
10
218 219
TS9(104.3)
R
IM2+P4(10.3) P5(-3.8)
-10
Fig.6. The potential energy profile along the decomposition reaction pathways
220
Fig. 7 shows the potential energy profile along subsequent reaction pathways. In Path 5, The
221
bonds of Cα-Cβ, Cα-C12, and H-OH in compound P2 are ruptured, meanwhile the Cα atom gets a
222
hydrogen atom and the Cβ attracts a functional group -OH. This hydrothermal reaction between P2
223
and water molecular, finally, forms CO, phenol and formaldehyde with an energies barrier of 15.9
224
kcal/mol and release an energy of 9.7 kcal/mol. In Path 6, Compound P4 and water molecular can
225
undergo a hydrothermal reaction to form CO2, phenol and double H2 with an energies barrier of
226
40.7 kcal/mol, absorbing an energy of 31.4 kcal/mol. In Path 7, the Cγ-Cβ and C5-Cβ bond in
227
compound P5 are broke in a hydrothermal reaction with water molecular. The reaction forms CO,
228
phenol and formaldehyde with an energies barrier of 11.9 kcal/mol, releasing an energy of 12.8
229
kcal/mol. In Path 8, the Cα-C12 bond in compound P6 is cracked, meanwhile a hydrogen atom is
230
transferred to C12 atom and a functional group -OH is attracted by Cα atom. Consequently the
231
reaction forms compounds of CO2, phenol and H2 with an energies barrier of 27.5 kcal/mol, 11
ACCEPTED MANUSCRIPT 232
releasing an energy of 20.8 kcal/mol.
233
Through comparisons of the activation energy of each subsequent reaction, it is concluded that
234
Path7 is the optimal reaction with a competitive reaction, i.e., Path 5. The main products of
235
subsequent reaction are CO, phenol and formaldehyde. 45 40 35 30
TS11(40.7)
Path 5 Path 6 Path 7 Path 8
TS13(27.5)
-1
E0 / (kcalmol )
25 20
TS10(15.9)
15 10 5 0
P3+H2+CO2(20.8)
TS12(11.9) R+ H20
-5
P3+CH3OH+CO(-9.7)
-10 -15
P3+CH3OH+CO(-12.8)
-20
236 237 238
P3+2H2+CO2(31.4)
Fig.7. The potential energy profile along the subsequent reaction pathways
4. Conclusions
239
In this paper, the gasification at high temperature steam mechanism and products formation of
240
lignin dimer with β-5 linkage was investigated by applying DFT method at B3LYP/6-31++G(d, p)
241
level. Four possible decomposition reaction pathways (homolytic cleavage of the Cγ-Cβ bond, and
242
Cα-Cβ bond, Cα-C12 bond, C5-Cβ bond) and four subsequent reaction pathways (hydrothermal
243
reaction of P2, P4, P5 and P6) were proposed. The ΔHθ and ΔGθ of each reaction and the effect of
244
temperature were studied. The activation energies of each reaction step were calculated. Here
245
below are the conclusions:
246
1.
From the ΔHθ calculation results, it is concluded that the four decomposition reaction are
247
endothermic below 1300 K, and exothermic above 1500 K. Furthermore, ΔHθ of each
248
reaction decrease with temperature increasing.
249
2.
Path 2 and Path 3 can occur spontaneously without temperature limited. Path 1 and Path 4,
250
however, are spontaneous reaction until temperature raised to 700 K. The calculation results
251
of the ΔGθ of each decomposition reaction give an order: Path 3
252
700 - 1100 K), and Path 3< Path 1
ACCEPTED MANUSCRIPT 253 254
pathways such as Path 5, Path 6, Path 7, Path 8 can not ensure to occur spontaneously. 3.
Path 2 is optimal decomposition reaction without a competitive reaction. The main products
255
in decomposition reaction are 5-hydroxyphenyl-acetaldehyde P5 and 12-hydoxymethyl-
256
phenol P4. Path7 is the optimal subsequent reaction with a competitive reaction, i.e., Path 5.
257
The main products in subsequent reaction are CO, phenol and formaldehyde.
258
Appendix
259
Table A1.
260
Thermodynamic parameters of reaction path 1, path2, path3 (unit: kcal/mol)
Path1 Path2 Path3 Path4 Path5 Path6 Path7 Path8
261 262
263
700K
900K
ΔHθ
36.8
35.4
ΔGθ
-13
ΔHθ ΔGθ ΔHθ ΔGθ ΔHθ
1100
1300K
1500K
1700K
33.3
27.4
21.7
11.1
-27.6
-40
-55.1
-65.5
-78.5
3.0
1.3
0.3
-2.4
-5.4
-12.1
-24.2
-33.4
-40.9
-50.6
-58.4
-64.1
10.6
6.7
4.6
2.2
0.6
-0.5
-56.0
-69.6
-83.0
-96.1
-108.5
-121.5
25.7
22.6
16.3
12.2
8.6
6.1
ΔGθ
-12.1
-21.9
-30.4
-41.8
-53.6
-63.7
ΔHθ
12.3
8.9
6.3
4.7
2.2
1.6
ΔGθ
6.0
5.1
4.5
3.8
3.2
2.6
ΔHθ
9.4
7.9
5.3
3.6
1.4
-1.1
ΔGθ
3.5
2.9
2.1
1.4
0.5
0.1
ΔHθ
15.4
11.4
8.5
4.6
3.1
2.2
ΔGθ
6.2
4.6
3.3
2.8
1.4
0.5
ΔHθ
8.2
6.6
4.7
2.6
1.3
-0.8
ΔGθ
2.5
1.3
0.6
0.5
0.2
0.1
K
Table A2. Total energy of species in gasification reaction paths(unit: kcal/mol ) Species
E0
Species
E0
Species
E0
Species
E0
R
- 553384.7
IM1
- 360474.2
IM2
- 288637.4
IM3
-481554.5
IM4
- 288640.2
IM5
- 360468.2
P1
-71815.3
P2
-288654.6
P3
-192886.1
P4
- 264737.0
P5
- 288651.5
P6
-264684.8
P7
- 216833.3
H20
-47949.9
H2
-717.1
CO
-71119.7
CO2
-118335.2
CH3OH
-72608.4
TS1
- 553338.6
TS2
- 360440.8
TS3
- 553347.1
TS4
- 288593.3
TS5
-553349.5
TS6
- 481488.0
TS7
-288621.2
TS8
- 553343.9
TS9
- 360394.3
TS10
-336588.6
TS11
-312646.2
TS12
-336589.5
TS13
-312607.2
Table note: Total energy of P8 and P9 are equal to P3 and P1, respectively.
13
ACCEPTED MANUSCRIPT 264
Acknowledgments
265
The authors thank the National Natural Science Foundation of China (No. 51276023) and Funds
266
for International Cooperation and Exchange of the National Natural Science Foundation of China
267
(No. 513101410) for supporting this work.
268
References
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15
Zhang Hang, Deng Shengxiang, Cao Xiaolin PII:
S0960-1481(17)30854-6
DOI:
10.1016/j.renene.2017.08.095
Reference:
RENE 9193
To appear in:
Renewable Energy
Received Date:
03 March 2017
Revised Date:
14 July 2017
Accepted Date:
31 August 2017
Please cite this article as: Zhang Hang, Deng Shengxiang, Cao Xiaolin, Density functional theory investigation of gasification mechanism of a lignin dimer with β-5 linkage, Renewable Energy (2017), doi: 10.1016/j.renene.2017.08.095
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ACCEPTED MANUSCRIPT
We propose four decomposition and four subsequent reaction pathways.
We calculate bond dissociation energies of homolytic cleavage for lignin dimmer.
We calculate enthalpy changes and Gibbs free energy changes of each reaction.
The equilibrium geometries of stagnation points in the reaction are optimized.
ACCEPTED MANUSCRIPT
2
Density functional theory investigation of gasification mechanism of a lignin dimer with β-5 linkage
3
Zhang Hang 1*, Deng Shengxiang1, Cao Xiaolin2
1
4 5
1 College 2 Energy
of Energy Science and Engineering, Central South University, Changsha 410083, China
& Power Engineering College, Changsha University of Science & Technology, Changsha 410012, China
6
Abstract: Density functional theory was employed to investigate gasification mechanism of β-5
7
linkage lignin in high temperature steam at B3LYP/6-31G++(d,p) level. Four possible
8
decomposition reaction pathways and four successive reactions pathways were proposed. The
9
equilibrium geometries of the reactants, transition states, intermediates and products were
10
optimized. The activation energies, Gibbs free energy changes and enthalpy changes of each
11
reaction step in gasification pathways were analyzed. From the calculation results of ΔHθ, it is
12
concluded that the four decomposition reaction are endothermic below 1300 K, and exothermic
13
exceed 1500 K. Furthermore, the ΔHθ of each reaction decrease with temperature increasing. From
14
the ΔGθ calculation results, Path 2 and Path 3 occur spontaneously without temperature limited.
15
Path 1 and Path 4, however, are spontaneous reaction above 700 K. It is not ensure that Path 5,
16
Path 6, Path 7 and Path 8 can occur spontaneously. Comparison of activation energy of each
17
reaction pathway, Path 2 is optimal decomposition reaction without a competitive reaction. The
18
main products in decomposition reaction are 5-hydroxyphenyl-acetaldehyde (P5) and 12-
19
hydoxymethyl-phenol (P4). Path7 is the optimal subsequent reaction with a competitive reaction,
20
i.e., Path 5. The main products in subsequent reaction are CO, phenol and formaldehyde.
21
Keywords: Lignin dimer; Density function theory; Gasification; Thermodynamic; Dynamic
22
1. Introduction
23
The rising concerns about energy security and environmental degradation in newly
24
industrialized and transition economies has increased renewable energy demand and alternate
25
energy resources [1-2]. CO2 emissions, especially, are becoming a major problem all around
26
world. Biomass is viewed as a potentially significant source of renewable energy[3-4].
1
*Corresponding author. Address: School of Energy Science and Engineering, Central South University, Changsha 410083, China. Tel: +86 18508449270; e-mail: [email protected] (H. Zhang) 1
ACCEPTED MANUSCRIPT 27
Gasification of biomass in the high-temperature steam, being as a technology which is quite
28
promising[5-6], is greatly featured that the emission of CO2 can be lowered by 30%, 15-30% of
29
energy can be saved and the emission of NOX is lower than 100ppm.
30
Lignin, being as one of the three major components of biomass[7,8], can reflect the
31
gasification rules of biomass by its gasification features. Therefore, it is necessary to study lignin
32
pyrolysis mechanism in gasification process. For lignin, the polymerization is via the
33
dehydrogenation of monomer, which is made up of some chemical bonds (like C-C bond and C-O
34
bond) in an unordered way. For example, β-5 type lignin dimmer, a typical structure of lignin, is a
35
propyl structure consisting of two benzene rings with a Cβ-C5 bond linkage (see Fig. 1).
36
In recent years, Tan et al. [9]and Yue et al. [10] found that abundant phenol after the
37
decomposition reaction in their experiments. It is well known that phenol will not be produced
38
unless cleavage of Cα-Cβ and Cβ-C5. In fact, Beste et al. [11]studied that the initial step in the
39
thermal decomposition of PPE (β-O-4 type lignin dimer) is the hemolytic cleavage of the oxygen-
40
carbon bond, and that carbon-carbon bond cleavage in PPE could be a competitive initial reaction
41
under high-temperature pyrolysis conditions. According to the experiment result in reference [12],
42
it is concluded that phenol, p-hydroxybenzyl alcohol and ethyl alcohol are the main products of
43
Cα-Cβ bond cleavage and β-O-4 bond cleavage. Qin et al. [13] investigated that the saw powder
44
rich in lignin will produce oil tar after gasification, which exist large amounts of
45
aromatic compounds (side chain). They also found that the side chain cracking with experiment
46
temperature increasing. As we know, lignin are interconnected by a variety of linkages (β-O-4, α-
47
O-4, β-1, β-5). The β-O-4 bond is the major type of linkage which occupies 48-60% of the total
48
linkages depending on the type of wood. Regarding β-O-4 lignin, there are different views on its
49
mechanism of thermal cracking. For example, Elder and Beste [14]did a quantum chemical study
50
on pyrolysis mechanism of β-O-4 lignin, concluding that activation energy of concerted reaction is
51
lower than the bond dissociation energy of homolytic reaction, which reflects that homolytic
52
reaction plays a leading role among these reactions. On the contrary, Huang et al. [15]found that
53
concerted reaction and homolytic reaction are both the major reaction paths of lignin dimer, which
54
will be affected by the temperature. It is summarized that concerted reaction will play the leading
55
role during the pyrolysis at the low-temperature, but homolytic reaction will do at high-
56
temperature. Apart from β-O-4 lignin, there are other kinds of lignin, such as β-1 lignin (Huang et 2
ACCEPTED MANUSCRIPT 57
al. [16]once studied the pyrolysis mechanism); α-O-4 lignin (Wu et al. [17]studied the pyrolysis
58
process of lignin dimer with α-O-4 linkage). Moreover, Cao et al. [18]once studied the reaction
59
mechanism of high temperature steam gasification of β-1 lignin dimer in the water solvent
60
environment.
61
However, today, there are few related to high temperature steam gasification of β-5 linkage
62
lignin dimer in terms of its chemical reaction, evolution reaction between intermediates and
63
transition state. In order to gain a better understanding of the detailed mechanism of lignin
64
gasification and the temperature effects on gasify process, mechanism of β-5 linkage lignin dimer
65
gasification at different temperature steam have been investigated in this paper.
66
2. Density functional theory (DFT) calculation methods
67
As for the electric correlation effects, DFT will calculate the large molecular system, whose
68
calculation results are quite accurate and reliable[19,20].In this paper, the relevant calculation
69
work was relied on DFT method with B3LYP (Becke’s three parameter gradient corrected
70
exchange functional with the gradient corrected correlation function [21]) and 6-31++G(d, p) basis
71
set (“6-31G” is an expression of splitting valence bond basis set. “++” are addition diffuse
72
functions of heavy atom and hydrogen. “d” and “p” are addition polarization functions of carbon
73
and hydrogen，respectively [22]), which consists of optimizing the equilibrium geometries such
74
as reactant, intermediates, products and transition states. The reason for providing the polarized
75
function is that is better to describe the system. Transition states (TS) method was employed to
76
find the transition state[23]. Intrinsic reaction coordinate (IRC) calculations were applied to
77
identify the minimum energy path from a transition state to the two corresponding local minima.
78 79
Thermodynamic data such as enthalpy changes (ΔHθ) and Gibbs free energy changes (ΔGθ) were calculated to analyze priorities of the reaction pathways.
80
For reaction：
81
A B A· B·
82
Enthalpy change of the reaction are calculated by Eq. (2) [24]:
83
H H ( A) H ( B) H ( A B)
84
Enthalpy change are estimated by the following expression:
85
H E nRT
(1)
(2)
(3) 3
ACCEPTED MANUSCRIPT 86
E =E0 Etrans Erot Evib
87
E0 =Eelec ZPE
88
Which, ∆Eelec is internal energy, ZPE is zero point energy, ∆Etrans is translation energy, ∆Erot is
89
(4) (5)
rotation energy, and ∆Evib is vibration energy.
90
The gibbs free energy change calculation equation:
91
G H T S
92
Which, T is reaction temperature, ∆S is entropy change.
93
When ΔH < 0, it means that the reaction is exothermic, it is endothermic if ΔH > 0. When ΔG <
94
0, it indicated that the reaction can be carried out spontaneously, instead, it can not occur
95
spontaneously if ΔG > 0. Furthermore, the size of ΔG determines conversion rate of reaction
96
reaching equilibrium.
(6)
97
According to transition state theory, activation energy is the minimal potential-energy
98
difference between transition state and reactants in ground state[25]. Since temperature affecting
99
slightly on thermodynamic, it, normally, did not take into account in calculating activation
100
energy[26]. The authors used E0 to represents the activation energy of each species in the reaction.
101
For free-radical reactions, the bond dissociation energies are usually equal or almost equal to the
102
activation energies. All calculations were performed using the Gaussian 09 suites of programs[27].
103
3. Results and discussion
104
3.1 Bond dissociation energies for β-5 linkage lignin dimer
105
Bond dissociation energies (ΔE) are recognized as an important thermodynamic quantity that
106
can exemplify the reaction sensitivity of a hemolytic cleavage of the considered bond. Cao et al.
107
[28] investigated biomass pyrolysis and carbonization process by TG-FTIR experimental. The
108
results show that when the temperature is in the range of 773~1173K, C - C bond and C - H bond
109
were further fractured. Most of the benzene ring has been decomposition or aromatic to form
110
amorphous carbon. In lignin gasification process, the homolytic bond dissociation is an important
111
reaction step. Fig. 1 gives the homolytic cleavage paths for the β-5 type lignin dimmer model
112
compound and their bond dissociation energies. The lowest bond dissociation energy is that of Cγ-
113
Cβ. The order of ΔE is as follows: Cγ-Cβ < Cα-Cβ < Cα-C12 < C5-Cβ < Cγ-OH < Cα-OH < C4-OH < 4
ACCEPTED MANUSCRIPT 114
C9-OH, therefore, the homolytic cleavage of the Cγ-Cβ, Cα-Cβ, Cα-C12, and C5-Cβ bond could be
115
the initial step in the decomposition reaction of β-5 type lignin dimmer. 2
2 1
3
1
3
6
4
6
4
OH
OH
5
HO
OH
+ HO
OH
12
11
1
3
8
10
8
10
6
4
10
E =8 1.
9
7k
HO
cal /m ol
2 1
3
6
7
C
12
8
11 10
+
9
3
6
4
HO
HO
E=40.8 kcal/mol HO
5
OH
OH
CH
CH
7
10 9
116
HO
+
CH 2
.1
ol l/m a kc
.5
ol /m l a kc
11
8
1
10
2
4
C
1
3
6
4
OH
5
E=35.2 kcal/mol
HO
CH
7
10 9
HO
-5 linkage lignin dimer
E =3 7
11
8
OH
+
12
H2C
10 9
.6
HO
kca l/m ol
HO
2 1
3
6
HO
9
OH OH
+
HO
11
8
12
2 11
8
6 =4 E
OH
12 7
12
HO
4 5
2 1
7
4 =6 E
OH
5
HO
OH
HO
k
4
11
8
9
C
12 7
9
C
l mo al/ kc
2
7
.2
+
HO
+ HO
11
6 =8
HO
12
E
5
HC
7
3
6
cal /m ol
2 1
5
3
6
4 5
OH
5
HC
117
Fig.1.
118
3.2 Reaction pathways of belta-5 linkage lignin dimer gasification
OH OH
+
CH 12
7
11
8
10 9
HO
Homolytic cleavage paths of β-5 linkage lignin dimer and their bond dissociation enthalpies (ΔE).
119
From the above calculation results of bond dissociation enthalpies of β-5 linkage lignin dimer,
120
this paper proposes four decomposition reactions as initial gasification pathways: homolytic
121
cleavage of Cβ-C5 bond, Cα-Cβ bond, Cα-C12 bond and Cβ-Cγ bond (see Fig. 2) and four subsequent
122
reactions as secondary gasification pathways. From the decomposition reactions depicted in Fig.
123
2, it is inferred that the major decomposition products are phenolic compounds, no small molecule
124
gases. So the authors proposed that the four special products (P2, P4, P5 and P6) are utilized to
125
produce small molecule gases with water molecule in the subsequent reactions formed as
126
concerted reaction (see Fig. 3).
5
ACCEPTED MANUSCRIPT HO
CH
7
11
8
10
Path1
TS1
HO
OH
12
-H
7
11
8
10
9
9
HO
HO
+
IM1
2
2
1
3
8
4
HO
OH
HO
CH
12
TS2
7
11
8
10
3
6
4 5
OH
HC
OH
Path2
TS3 HO
+
IM4
-H
CH2
P1
2 1
3
6
4
OH
OH
5
HO
2
2
6
4
TS4 OH
5
1
3
1
3
6
4
6
4
OH
5
OH
O
11
8
10
+
12
H
7
11
8
10
9
9
HO
HO
P4
2
8
1
1
3
6
3
3
6
4
10
HO
9
OH
CH
+H
HO
Path3
OH
7
11 10
7
11
8
10
TS6
HO
+
-H
OH
Path4
3 4
OH
OH
C
12
8
11
IM3
10
8
10
3
6
4
HC
1
3 4
OH
+
OH
O
TS9
IM5
9
9
HO
HO
P6
10
2
2 3
6
4
OH
OH
5
3
1
TS4
1
OH
4
6
OH
5
O
IM2
-H
O
2
CH2
TS9b
11
8
+H OH
CH
12 7
9
1
3
6
4
P9
5
OH
O
10 9
P5
HO
P8
127 Fig.2.
Decomposition reaction pathways schematic of belta-5 linkage lignin dimer model compound gasification O
HO
CH3
12 7
11
8
10 9
+
H
OH
TS10
12
12 7
11
8
10 9
HO
+
CO
+
CH3OH
11
8
10
+
H
OH
Path5
TS11
12 7
11
8
10
9
9
HO
HO
P4
2 + 2H2 + CO
Path6
O
12 7
11
8
10 9
P5
Fig.3.
7
HO
P2
129 130
1
6
+H
12
8
5
11
HO
O
HO
128
O
-H
7
2
-H
+ 7
CH
5
5
TS8
P7
11
2
1
+ 12
6
OH
7
2
CH
5
P1
HO
OH
CH2
4
CH3
O
CH2
3
6
CH2
HO
9
9
1
12
8
TS5
4 5
12
R
+H
5 6
4 5
HO
2
1 2
2
11
OH O
P5
12 7
5
CH
7
P2
O
OH
3
HO
11 10 9
IM2
12
7 8
HO
CH2
1
+
11 10 9
OH
2
-H
7 8
HO
4
2
12
9
P3
1
TS7
CH3
HO
3
5
11 10
12
9
1
5
7 8
O
12
+H
CH2
O
CH2
HO
6
+H
C
OH
12
OH
+
H
OH
TS12
11
8
10 9
O
12 7
3OH + CH + CO
12 7
11
8
10
+
H
OH
TS13
HO
10
+
CO2
+ H2
HO
HO
P6
11
8 9
9
Path7
12 7
Path8
Subsequent reaction pathways schematic of belta-5 linkage lignin dimer model compound gasification 6
ACCEPTED MANUSCRIPT 131 132
3.3 Optimization of geometries and their key bonds length Table 1 shows the equilibrium geometries of the reactants, intermediates and partial transition
133
states after optimizing and lists the key bond length of the species.
134 135
Table 1． Geometric structure and key bond length optimization of the species in each reaction. Optimized structure
Bond length / (Å)
Optimized structure
Bond length / ( Å)
R(19,20)
1.5432
R(19,20)
1.5432
R(20,22)
1.5397
R(20,22)
1.5397
R(1,20)
1.5420
R(12,19)
1.5403
R(12,19)
1.5403
R(19,34)
1.4303
R(19,34)
1.4307
R(22,26)
1.4307
R(20,22)
1.5429
R(19,20)
1.5432
R(1,20)
1.5420
R(1,20)
1.4065
R(12,19)
1.5403
R(19,12)
1.3720
R(22,26)
1.0847
R(19,34)
1.3948
R(19,22)
1.0871
R(19,20)
1.4132
R(19,34)
1.4498
R(20,22)
1.3997
R(19,20)
1.3471
R(22,26)
1.3489
R(19,20)
1.2632
R(20,22)
1.8811
R(19,21)
1.3257
R(22,26)
1.2677
R(1,20)
1.8860
R(26,27)
1.4086
R(19,34)
1.4307
R(1,20)
1.5420
R(19,20)
1.8843
R(20,22)
1.2643
R(20,22)
1.2616
R(22,26)
1.2576
R(22,26)
1.4958
R(26,27)
1.4128
R(1,20)
1.5420
R(1,20)
1.5420
R(20,22)
1.8774
R(19,20)
1.8812
R(22,26)
1.2638
R(19,34)
1.2611
R(26,27)
1.4088
R(34,35)
1.4122
R(1,20)
1.5420
R(20,22)
1.5397
R(19,34)
1.2558
R(19,12)
1.8804
R(19,20)
1.3628
R(19,34)
1.2611
R(34,35)
1.1869
R(34,35)
1.4122
R(20,22)
1.5397
R(19,20)
1.5432
R(19,20)
1.8846
R(20,22)
1.3496
R(22,25)
1.3200
R(1,20)
1.5420
136 137
Table note: bond length of Cγ-Cβ, Cα-Cβ, Cα-C12, C5-Cβ, Cγ-OH and Cα-OH are marked as R(19,20), R(20,22), R(1,20), R(19,12),
138
3.4 Thermodynamics Analysis of the decomposition and subsequent reactions
R(19,34), R(22,26), respectively.
7
ACCEPTED MANUSCRIPT 139 140
Table A1 in appendix gives enthalpy changes ΔHθ and Gibbs free energy changes ΔGθ of each reaction pathway at different temperature.
141
Fig. 4 shows the ΔHθ (θ represents temperature) of each reaction at different temperature (700 -
142
1700 K). The results indicate that the ΔHθ of Path 1 decreases with an increase of temperature
143
(700 K to 1700 K). It is an endothermic reaction because ΔHθ is positive value. The ΔHθ of Path2
144
starts from 3.0 kcal/mol and reduces to -2.4 kcal/mol (at 1500K) with a change of endothermic to
145
exothermic . For Path 3, the ΔHθ is very closed to 0 kcal/mol at 1500 K, and would be a negative
146
value if temperature kept increasing. The ΔHθ of Path 4 are higher than zero, so it is endothermic.
147
The ΔHθ of Path 5 reduces from 12.3 to 1.6 kcal/mol and keeps endothermic reaction. The ΔHθ of
148
Path 6 is down to -1.1 kcal/mol and converts into exothermic reaction until temperature increased
149
to 1700 K. For Path 7, the ΔHθ decreases from 15.4 kcal/mol to 2.2 kcal/mol (at 700-1700 K) and
150
forms as endothermic. The ΔHθ of Path 8 decreases to -0.8 kcal/mol with changing from
151
endothermic to exothermic at 1700 K. (a) Decomposition reaction
kcalmol-1
40
Path1 Path2 Path3 Path4
30 20 10 0 -10 700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
kcalmol-1
Tempeature / (K)
152 153
16 14 12 10 8 6 4 2 0 -2
(b) Subsequent reaction
700
800
900
1000
1100
1200
1300
1400
Path5 Path6 Path7 Path8
1500
1600
1700
Tempeature / (K)
Fig.4. Enthalpy changes as function of temperature, (a) decomposition reaction, (b)subsequent reaction.
154
Fig. 5 indicates that the Path 2 and Path 3 are able to be carried out spontaneously because their
155
ΔGθ<0 (at 700 - 1700 K). Path 1 and Path 4, however, are spontaneous reaction until temperature
156
raised to 700 K. Through comparison of the ΔGθ of each reaction pathway, the authors can see
157
that the order of ΔGθ is Path 3
158
as : Path 3< Path 1
159
Path 5, Path 6, Path 7, Path 8 can not occur spontaneously with an order of ΔGθ: Path 8< Path 8
ACCEPTED MANUSCRIPT 160
6
G kcalmol
0 -20 -40 -60 -80 -100 -120
G kcalmol
700
800
900
1000
7 6 5 4 3 2 1 0
1100
700
163
1200
1300
1400
Tempeature / (K)
1500
1600
800
900
1000
1100
1200
1300
1400
1700
Path5 Path6 Path7 Path8
(b) Subsequent reaction
1500
1600
1700
Tempeature / (K)
161 162
Path1 Path2 Path3 Path4
Fig.5. Gibbs free energy changes as function of temperature, (a) decomposition reaction, (b)subsequent reaction.
3.5 Dynamics Analysis of the decomposition and subsequent reactions
164
Table A2 in appendix gives the activation energies of each specie in the reactions proposed.
165
Fig. 6 shows the sectional drawing of potential energy profile along decomposition reaction
166
pathways. In Path 1, β-5 type lignin dimer model compound decomposes into transition state TS1a
167
and TS1b through the homolytic cleavage of the C5-Cβ bond with an energy barrier of 46.1
168
kcal/mol. The TS1a converts into intermediate IM1 through a dehydrogenation, and the TS1b
169
undergoes a hydrogenation to form compound P3. The decomposition reaction absorbs an energy
170
of 24.4 kcal/mol. It can be verified in Table 1 that C5-Cβ bond length enlarges to 1.8860 Å, so it is
171
unstable to breaks. Cα gives a hydrogen atom to C5 and connects to Cβ with a short double bond
172
(1.2632 Å). Compound IM1 can further decompose into TS2a and TS2b through the hemolytic
173
cleavage of Cγ-Cβ bond with an energy barrier of 33.4 kcal/mol. The TS2a and TS2b convert into
174
Compound IM4 and P3 through a hydrogenation and a dehydrogenation, respectively. From Table
175
1, it can be seen that distance of Cγ-Cβ bond reaches 1.8811 Å and is highly possible to cracks. The
176
O34 atom loses a hydrogen atom and makes a connection to Cγ with a short double bond,
177
meanwhile Cβ absorbs the hydrogen atom to form IM4. The decomposition reaction takes an
178
energy of 10.3 kcal/mol. Compound IM4 tautomerize to form hydroxyphenyl-ethanone P2 via
179
TS7 with an energy barrier of 19.0 kcal/mol through intramolecular hydrogen, releasing an energy 9
ACCEPTED MANUSCRIPT 180
of 14.4 kcal/mol. The tautomerization reaction is exothermic and compound IM4 is more stable
181
than its isomers P2.
182
In Path 2, reactant R decomposes into TS3a and TS3b through the homolytic cleavage of the
183
Cα-Cβ bond with an energy barrier of 37.6 kcal/mol. TS3a and TS3b change to compound IM2 and
184
12-hydroxymethyl-phenol P3, respectively. It can be proved in Table 1 that bond distance of Cα-
185
Cβ bond is 1.8843 Å, and it is most likely to rupture. The decomposition reaction is supported by
186
an energy of 18.7 kcal/mol. Compound IM2 tautomerizes to form product P2 via TS7 with an
187
energy barrier of 44.1 kcal/mol through intramolecular hydrogen, releasing an energy of 14.1
188
kcal/mol. There is a transfer of a hydrogen atom from O26 to Cβ, and then product P5 is formed.
189
The tautomerization reaction is exothermic and the intermediate IM2 is more stable than its
190
isomers P5.
191
In Path 3, reactant R, similarly, decomposes into TS5a and TS5b through the homolytic
192
cleavage of the Cγ-Cβ bond (bond length 1.8874 Å) with an energy barrier of 35.2 kcal/mol. The
193
TS5a turns into intermediate IM3 through a hydrogenation, and the TS5b undergoes a
194
dehydrogenation to form product P1. The decomposition reaction absorbs an energy of 14.9
195
kcal/mol. Compound IM3 can further decompose into TS6a and TS6b through the hemolytic
196
cleavage of Cα-Cβ bond (bond length 1.8812 Å) with an energy barrier of 66.5 kcal/mol. These
197
two translate state species can be translated into product P6 and P7. The decomposition reaction
198
takes an energy of 36.2 kcal/mol.
199
In Path 4, reactant R decomposes into TS8a and TS8b through the homolytic cleavage of the
200
Cα-C12 bond (bond length 1.8804 Å)with an energy barrier of 40.8 kcal/mol. TS8a is conversion
201
into intermediate IM5 through a dehydrogenation, and TS8b can undergo a hydrogenation to form
202
product P8. The decomposition reaction absorbs an energy of 30.4 kcal/mol. Compound IM5 can
203
further decompose into TS9a and TS9b through the hemolytic cleavage of Cα-Cβ bond (bond
204
length 1.8846 Å) with an energy barrier of 73.9 kcal/mol. TS9a and TS9b convert into
205
intermediate IM2 and phenol P9 through a hydrogenation and a dehydrogenation, respectively.
206
The decomposition reaction takes an energy of 25.5 kcal/mol. Compound IM2 in Path 4 have a
207
same tautomerization reaction like Path 2, as well as bond length and energy barrier of the
208
reaction.
209
Comparing the energies barriers of each reaction step in the decomposition reactions such as 10
ACCEPTED MANUSCRIPT 210
homolytic cleavage of the Cγ-Cβ, Cα-Cβ, Cα-C12, and C5-Cβ bond, it is summarized that cleavage of
211
Cα-Cβ bond in Path 2 would be the optimal decomposition reaction without a competitive reaction.
212
Hence, the main products of β-5 type lignin dimer model compound in decomposition reaction are
213
5-hydroxyphenyl-acetaldehyde P5 and 12-hydoxymethyl-phenol P4. In reference[29], the
214
pyrolysis properties of lignin were studied by TG-FTIR method and Flynn-Wall-Ozawa
215
calculation method. The results showed that the activation energy of aromatic ring condensation
216
into carbon is much larger than the energy required for the fracture of the branched chain attached
217
to the benzene ring, which indicates that the benzene ring is more likely to break. 120
Path 1 Path 2 Path 3 Path 4
110 100
E0/(kcalmol-1)
90
TS4(100.0)
TS6(81.4)
80 70
TS2(57.8) IM2+P9(55.9)
60 50
TS8(40.8)
40
TS3(37.6)
30
TS5(35.2)
20
0
TS1(46.1)
TS7(62.1)
TS4(54.4) P6+P7(51.3) IM5+P8(30.4) IM1+P3(24.4)
IM4+P1(43.1)
P5(41.8) P2(28.7)
IM3+P1(14.9)
10
218 219
TS9(104.3)
R
IM2+P4(10.3) P5(-3.8)
-10
Fig.6. The potential energy profile along the decomposition reaction pathways
220
Fig. 7 shows the potential energy profile along subsequent reaction pathways. In Path 5, The
221
bonds of Cα-Cβ, Cα-C12, and H-OH in compound P2 are ruptured, meanwhile the Cα atom gets a
222
hydrogen atom and the Cβ attracts a functional group -OH. This hydrothermal reaction between P2
223
and water molecular, finally, forms CO, phenol and formaldehyde with an energies barrier of 15.9
224
kcal/mol and release an energy of 9.7 kcal/mol. In Path 6, Compound P4 and water molecular can
225
undergo a hydrothermal reaction to form CO2, phenol and double H2 with an energies barrier of
226
40.7 kcal/mol, absorbing an energy of 31.4 kcal/mol. In Path 7, the Cγ-Cβ and C5-Cβ bond in
227
compound P5 are broke in a hydrothermal reaction with water molecular. The reaction forms CO,
228
phenol and formaldehyde with an energies barrier of 11.9 kcal/mol, releasing an energy of 12.8
229
kcal/mol. In Path 8, the Cα-C12 bond in compound P6 is cracked, meanwhile a hydrogen atom is
230
transferred to C12 atom and a functional group -OH is attracted by Cα atom. Consequently the
231
reaction forms compounds of CO2, phenol and H2 with an energies barrier of 27.5 kcal/mol, 11
ACCEPTED MANUSCRIPT 232
releasing an energy of 20.8 kcal/mol.
233
Through comparisons of the activation energy of each subsequent reaction, it is concluded that
234
Path7 is the optimal reaction with a competitive reaction, i.e., Path 5. The main products of
235
subsequent reaction are CO, phenol and formaldehyde. 45 40 35 30
TS11(40.7)
Path 5 Path 6 Path 7 Path 8
TS13(27.5)
-1
E0 / (kcalmol )
25 20
TS10(15.9)
15 10 5 0
P3+H2+CO2(20.8)
TS12(11.9) R+ H20
-5
P3+CH3OH+CO(-9.7)
-10 -15
P3+CH3OH+CO(-12.8)
-20
236 237 238
P3+2H2+CO2(31.4)
Fig.7. The potential energy profile along the subsequent reaction pathways
4. Conclusions
239
In this paper, the gasification at high temperature steam mechanism and products formation of
240
lignin dimer with β-5 linkage was investigated by applying DFT method at B3LYP/6-31++G(d, p)
241
level. Four possible decomposition reaction pathways (homolytic cleavage of the Cγ-Cβ bond, and
242
Cα-Cβ bond, Cα-C12 bond, C5-Cβ bond) and four subsequent reaction pathways (hydrothermal
243
reaction of P2, P4, P5 and P6) were proposed. The ΔHθ and ΔGθ of each reaction and the effect of
244
temperature were studied. The activation energies of each reaction step were calculated. Here
245
below are the conclusions:
246
1.
From the ΔHθ calculation results, it is concluded that the four decomposition reaction are
247
endothermic below 1300 K, and exothermic above 1500 K. Furthermore, ΔHθ of each
248
reaction decrease with temperature increasing.
249
2.
Path 2 and Path 3 can occur spontaneously without temperature limited. Path 1 and Path 4,
250
however, are spontaneous reaction until temperature raised to 700 K. The calculation results
251
of the ΔGθ of each decomposition reaction give an order: Path 3
252
700 - 1100 K), and Path 3< Path 1
ACCEPTED MANUSCRIPT 253 254
pathways such as Path 5, Path 6, Path 7, Path 8 can not ensure to occur spontaneously. 3.
Path 2 is optimal decomposition reaction without a competitive reaction. The main products
255
in decomposition reaction are 5-hydroxyphenyl-acetaldehyde P5 and 12-hydoxymethyl-
256
phenol P4. Path7 is the optimal subsequent reaction with a competitive reaction, i.e., Path 5.
257
The main products in subsequent reaction are CO, phenol and formaldehyde.
258
Appendix
259
Table A1.
260
Thermodynamic parameters of reaction path 1, path2, path3 (unit: kcal/mol)
Path1 Path2 Path3 Path4 Path5 Path6 Path7 Path8
261 262
263
700K
900K
ΔHθ
36.8
35.4
ΔGθ
-13
ΔHθ ΔGθ ΔHθ ΔGθ ΔHθ
1100
1300K
1500K
1700K
33.3
27.4
21.7
11.1
-27.6
-40
-55.1
-65.5
-78.5
3.0
1.3
0.3
-2.4
-5.4
-12.1
-24.2
-33.4
-40.9
-50.6
-58.4
-64.1
10.6
6.7
4.6
2.2
0.6
-0.5
-56.0
-69.6
-83.0
-96.1
-108.5
-121.5
25.7
22.6
16.3
12.2
8.6
6.1
ΔGθ
-12.1
-21.9
-30.4
-41.8
-53.6
-63.7
ΔHθ
12.3
8.9
6.3
4.7
2.2
1.6
ΔGθ
6.0
5.1
4.5
3.8
3.2
2.6
ΔHθ
9.4
7.9
5.3
3.6
1.4
-1.1
ΔGθ
3.5
2.9
2.1
1.4
0.5
0.1
ΔHθ
15.4
11.4
8.5
4.6
3.1
2.2
ΔGθ
6.2
4.6
3.3
2.8
1.4
0.5
ΔHθ
8.2
6.6
4.7
2.6
1.3
-0.8
ΔGθ
2.5
1.3
0.6
0.5
0.2
0.1
K
Table A2. Total energy of species in gasification reaction paths(unit: kcal/mol ) Species
E0
Species
E0
Species
E0
Species
E0
R
- 553384.7
IM1
- 360474.2
IM2
- 288637.4
IM3
-481554.5
IM4
- 288640.2
IM5
- 360468.2
P1
-71815.3
P2
-288654.6
P3
-192886.1
P4
- 264737.0
P5
- 288651.5
P6
-264684.8
P7
- 216833.3
H20
-47949.9
H2
-717.1
CO
-71119.7
CO2
-118335.2
CH3OH
-72608.4
TS1
- 553338.6
TS2
- 360440.8
TS3
- 553347.1
TS4
- 288593.3
TS5
-553349.5
TS6
- 481488.0
TS7
-288621.2
TS8
- 553343.9
TS9
- 360394.3
TS10
-336588.6
TS11
-312646.2
TS12
-336589.5
TS13
-312607.2
Table note: Total energy of P8 and P9 are equal to P3 and P1, respectively.
13
ACCEPTED MANUSCRIPT 264
Acknowledgments
265
The authors thank the National Natural Science Foundation of China (No. 51276023) and Funds
266
for International Cooperation and Exchange of the National Natural Science Foundation of China
267
(No. 513101410) for supporting this work.
268
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