KineticModelingandMechanismsofAcid-CatalyzedDelignificationofSugarcaneBagasse

1. See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/236681120
Kinetic Modeling and Mechanisms of Acid-Catalyzed Delignification of
Sugarcane Bagasse by Aqueous Acetic Acid
Article in BioEnergy Research · June 2013
DOI: 10.1007/s12155-012-9265-4
CITATIONS
21
READS
881
2 authors:
Some of the authors of this publication are also working on these related projects:
Production of 5-HMF from sustainable bioresource View project
Xuebing Zhao
Tsinghua University
97 PUBLICATIONS 3,916 CITATIONS
SEE PROFILE
Dehua Liu
Chinese Academy of Sciences
190 PUBLICATIONS 7,566 CITATIONS
SEE PROFILE
All content following this page was uploaded by Xuebing Zhao on 05 June 2014.
The user has requested enhancement of the downloaded file.

2. Kinetic Modeling and Mechanisms of Acid-Catalyzed
Delignification of Sugarcane Bagasse by Aqueous Acetic Acid
Xuebing Zhao & Dehua Liu
# Springer Science+Business Media New York 2012
Abstract Organosolv pretreatment of lignocellulose pertains
to a biomass fractionation process to obtain cellulosic pulp,
high-purity lignin, and hemicellulosic syrup. In the present
work, sugarcane bagasse was delignified by aqueous acetic
acid (AcH) under atmospheric pressure with addition of sulfu-
ric acid (SA) as a catalyst. Based on the multilayered structure
of plant cell wall and the inhibitive effect of dissolved lignin on
delignification rate, a novel pseudo-homogeneous kinetic mod-
el was proposed by introducing the concept of “potential degree
of delignification (dD)” into the model. It was found that
delignification rate was a first-order reaction with respect to
SA concentration, while AcH concentration showed a high
reaction order to delignification rate. The activation energy
for delignification was determined to be 64.41 kJ/mol. The
relationships of kinetic constants and dD with reaction temper-
ature, AcH, and SA concentrations were determined according
to experimental data. Mechanism analysis indicated that cleav-
age of α-aryl ethers bonds were mainly responsible for the
formation of lignin fragments. AcH concentration affected the
solubility parameter (δ value) of AcH solution and the ability to
form hydrogen bonds with lignin fragments. Therefore, the
driving force for solubilizing lignin fragments increased with
AcH concentration, and thus AcH concentration had a very
significant influence on delignification rate.
Keywords Lignocellulosic biomass . Acetic acid
delignification . Kinetic modeling . Potential degree of
delignification . Delignification mechanism
Introduction
Lignocellulose, such as agricultural and forestry residue, is
the most abundant organic material, and offers an immense
potential for the production of biofuels and chemicals in a
sustainable fashion [1]. However, during the bioconversion
of lignocellulose, the feedstock usually should undergo pre-
treatment to overcome plant cell wall recalcitrance, which is
a multiscale phenomenon spanning several orders of mag-
nitude encompassing both macroscopic and microscopic
barriers [2]. Organosolv pretreatment has been found to
effectively increase the cellulose digestibility by cellulase
enzymes. Moreover, organosolv pretreatment also pertains
to a biomass fractionation process by which the biomass can
be fractionated to lignin, hemicellulosic sugars, and a rela-
tively pure cellulose fraction as shown in Fig. 1. All of these
products show promises for further production of biofuels
and biochemicals in a biorefinery concept [3]. It is found
that during organosolv pretreatment, large parts of lignin
and hemicellulose are dissolved, thus increasing the expo-
sure of cellulose. Among the various organic solvents used
for pretreating lignocellulosic biomass, acetic acid (AcH)
showed some advantages: (1) it has a Hildebrand solubility
parameter (δ value) around 11 cal1/2
/cm−3/2
, thus showing a
good solvency to lignin fragments [4]; (2) lignin can be well
removed under atmospheric pressure by addition of mineral
acids (H2SO4 or HCl) as a catalyst [5]; and (3) the AcH formed
Electronic supplementary material The online version of this article
(doi:10.1007/s12155-012-9265-4) contains supplementary material,
which is available to authorized users.
D. Liu
Institute of Applied Chemistry,
Department of Chemical Engineering, Tsinghua University,
Haidian District,
Beijing 100084, China
Present Address:
X. Zhao (*)
Institute of Applied Chemistry,
Department of Chemical Engineering, Tsinghua University,
Haidian District,
Beijing 100084, China
e-mail: zhaoxb@mail.tsinghua.edu.cn
Bioenerg. Res.
DOI 10.1007/s12155-012-9265-4

3. by deacetylation of hemicellulose during pretreatment can be as
a supplement of solvent. AcH has been well employed for
pulping of lignocellulosic biomass at relatively low temper-
atures with addition of mineral acids as catalysts, which is
known as Acetosolv process invented by Nimz et al. [6–8]. In
our previous work, it was found that the Acetoline pretreatment
based on AcH delignification could greatly increase the cellu-
lose digestibility of sugarcane bagasse due to the removal of
physical barrier constructed by lignin [9]. The lignin products
obtained by AcH delignification also had a relatively high purity
for further modification and applications [3, 10]. Therefore,
understanding the delignification kinetics of AcH pretreatment
can be helpful for further controlling and optimizing the process.
Some papers have been published to describe the kinetics of
acid-catalyzed delignification of lignocellulosic biomass by
organic acids [11–18]. In these works, the process was consid-
ered as a pseudo-homogeneous reaction system, and lignin
solubilization was found to be a first-order reaction with respect
to the residual lignin concentration. Vázquez et al. [12–14]
proposed a model of consecutive first-order reactions involving
lignin solubilization followed by lignin condensation reactions.
The experimental results could be satisfactorily explained by
the model when 70 % or 90 % (w/w) AcH solution was used
for pulping of woody biomass with HCl as a catalyst.
Villaverde et al. [18] proposed a kinetic model of two parallel
first-order reactions by regarding lignin as fast- and slow-
reacting fractions. All of these models showed good accuracy
to predict the experimental results. However, the relationship
between delignification rate and organic acid concentration
was not investigated in these works. In our experiments, we
have found that the degree of delignification is also strongly
dependent on the reaction severity, but it is always hard to
completely remove lignin from cell wall. Vázquez et al. [13]
also found that there was 5 % of the lignin fraction that could
not be eliminated under the condition studied. Moreover, we
also have found that the dissolved lignin showed some inhi-
bition to the rate and degree of delignification, and this
inhibitive effect should be considered in the kinetic models
to more accurately characterize the kinetic behavior of deligni-
fication by organic acid. The objective of this paper is thus to
propose a novel kinetic model by introducing the concept of
“potential degree of delignification (dD)”. The relationships
between rate constants, dD, and reaction severity (catalyst
concentration, AcH concentration, and temperature) were de-
termined according to experimental data. The delignification
mechanisms were further investigated based on kinetic results.
Materials and Methods
Raw Materials
Sugarcane bagasse used in the present work was obtained
from Guangxi Zhuang Autonomous Region in South China.
It was air-dried and screened. The part that could not pass
through 20-mesh sieve was collected for AcH delignification.
The main components of the bagasse were determined to be
42.1 % glucan, 23.5 % xylan, 1.2 % araban, 2.4 % acetyl
group, 24.7 % klason lignin, and 1.0 % acid-soluble lignin.
The standard compounds used for HPLC calibration, includ-
ing glucose, xylose, and arabinose, were purchased from
Sigma-Aldrich (Shanghai agent). For preparation of the stan-
dard AcH lignin (AcL), sugarcane bagasse was delignified
with 70 % AcH, and crude lignin was isolated after the spent
liquor was concentrated followed by water precipitation. The
crude lignin was lyophilized and further dissolved in a 90 %
(v/v) AcH solution and then 10 vol of water was added. The
precipitated lignin was centrifuged off, washed with deionized
water, and lyophilized to obtain the purified AcL.
Delignification Process
The delignification process was carried out in a 1,000-ml
three-neck glass flask heated by electric jacket or water bath
Fig. 1 Organosolv
pretreatment of lignocellulose
to produce cellulosic pulp,
lignin, and hemicellulosic syrup
Bioenerg. Res.

4. under atmospheric pressure with one of the necks connected
with a condenser. Thirty grams of screened bagasse was
packed into the flask followed by addition of 300 ml 60–
90 wt.% AcH solution with 0.05–0.4 wt.% sulfuric acid
(SA, based on liquid). Electrical stirring with a Teflon
paddle was used at 300 rpm to keep the system as homoge-
neous as possible. After delignification, the mixture was
filtered. The obtained solid was first washed with 300 ml
60–90 wt.% AcH solution and then filtered under pressure
to remove as much liquid as possible. The solid was then
washed with water until neutrality and dried for further
analysis.
Analytical Methods
The main components of the bagasse and delignified solid
were determined according to NREL’s Laboratory Analytical
Procedure [19]. The monosaccharides and AcH concentra-
tions were determined by Shimadzu (Tokyo, Japan) HPLC
(LC-10AT) system as described in our previous work [20]. In
terms of the dissolved lignin, its concentration can be deter-
mined by UV absorption. However, the presence of AcH and
sugar degradation products such as furfural might interfere
with the absorbance. We thus recorded the UV spectra of
several samples as shown in Fig. 2a. It can be known that
AcH had an absorption peak at about 245 nm and furfural
showed strong absorption at 280 nm. The spent liquor sample
and standard AcH lignin showed the same spectra and three
apparent absorption peaks were observed at 205, 280, and
315 nm, respectively. Therefore, 205 and 315 nm were used to
determine the dissolved lignin concentration. Two respective
standard curves were obtained as shown in Fig. 2b. Before
measuring the UV absorbance, the spent liquor was centri-
fuged at 14,000 rpm and diluted with 70 % acetic acid. The
reported dissolved lignin concentration was the average of the
data measured at above two wavelengths.
Data Processing
The kinetic constants were regressed according to the ex-
perimental data by a simplex optimization method using
Matlab 6.5 software to minimize the objective function
(fobjective), which was the quadratic sum of the difference
between calculated data [f(xi)] and experimental data (yi), as
shown in the following expression:
fobjective ¼
X
n
i¼1
d2
i ¼
X
n
i¼1
f xi
ð Þ yi
½ 2
ð1Þ
The four-order Runge–Kutta method was used for the
numerical solution of differential equations.
Results and Discussion
Kinetic Model Development
In the present work, the liquid-to-solid ratio was 10:1 (v/m,
l/kg) and the delignification process was assumed to occur
in a pseudo-homogeneous system. The reactions mainly
include the solubilization of lignin from solid phase into
liquid phase and the condensation of dissolved lignin to
form the condensed lignin as shown in following consecu-
tive reactions:
LR
!
Hþ
; AcH; kL1obs
LS
!
Hþ
; AcH; kL2obs
LP ð2Þ
where LR, LS and LP denote residual lignin in solid,
dissolved lignin in liquid phase and condensed lignin,
respectively; kL1obs and kL2obs are observed rate constants for
delignification and condensation reactions, respectively. Be-
ing similar to the reported works on the kinetics of organosolv
delignification, in the present work first-order reactions were
used to describe the kinetics of delignification and lignin
condensation. Therefore, the rates of lignin solubilization into
Fig. 2 UVabsorption for determination of dissolved lignin in the spent
liquor. a UV spectra of several samples; b standard curves at 205 and
315 nm
Bioenerg. Res.

5. liquid phase and formation of dissolved lignin can be
expressed as follows:
dCLR
dt
¼ kL1obsCLR ð3Þ
dCLS
dt
¼ kL1obsCLR kL2obsCLS ð4Þ
where CLR and CLS are residual solid lignin and dissolved
lignin concentrations (g/l) in the pseudo-homogeneous sys-
tem, respectively. Defining the degree of delignification (D)
as follows:
D ¼
CLR0 CLR
CLR0
ð5Þ
where CLR0 are the initial solid lignin concentration, we thus
can express the rate of delignification (Eq. (3)) as:
dD
dt
¼ kL1obs 1 D
ð Þ ð6Þ
and D00 at time 0. This equation indicates that the degree of
delignification is a first-order reaction with respect to the un-
solubilized lignin fraction. Furthermore, Eq. (6) also indi-
cates that in any cases the final degree of delignification
would reach 1 (100 %). Namely, all of the lignin can be
solubilized under any reaction conditions. The integration
form of Eq. (6) is
D ¼ 1 exp kL1obst
ð Þ ð7Þ
Therefore, if the delignification reaction follows the
kinetics described by Eq. (6), a line can be obtained by
plotting ln(1−D) with reaction time t. However, accord-
ing to experimental results, the plots of ln(1−D) versus
reaction time t actually showed an apparent deviation
from linear relationship (see Supplementary Fig. S1).
This discrepancy indicates that kinetic model (6) could
not be employed to accurately describe the kinetics of
delignification by aqueous AcH. Therefore, a novel
kinetic model is necessary to more accurately describe
the kinetic behaviors of lignin solubilization by acid-
catalyzed AcH delignification.
In our previous work, a novel kinetic model was pro-
posed to describe glycan solubilization and formation of
monosaccharides during dilute-acid hydrolysis of sugarcane
bagasse by introducing the concept of “potential hydrolysis
degree” (hd) [20]. The model could very well predict the
experimental results. Similarly, in the present work the
concept of “potential degree of delignification (dD)” are
introduced to the kinetic model for delignification, which
is defined as the maximum solubilization ratio of solid
lignin in biomass under a certain delignification severity.
The concept of dD is proposed based on the following facts.
1. The plant cell wall has a multilayered structure. It has
been known that the plant cell wall generally consists of
three types of layers, middle lamella (ML), primary wall (P),
and secondary wall (S), and the secondary wall also consists
of three layers, which are called S1, S2, and S3 lamellae,
respectively. Cellulose, hemicellulose, and lignin have dif-
ferent distribution in these layers. Lignin is found to be the
dominant composition in the outer portion of the compound
middle lamellae. The concentration of lignin in the ligno-
cellulosic matrix decreases with increasing distance into the
fiber cell wall. The concentrations of lignin in the primary
wall and in the S1 layer of the secondary wall are much
higher than those in the S2 and S3 sections. Cell corner (CC)
is found to have the highest lignin concentration on average
[21]. It has been found that when eucalyptus wood was
delignified by organosolv process, delignification proceeded
from the lumen wall toward the middle lamella, and then
reaching the cell corners [22]. Chen investigated the mech-
anisms of formic acid delignification of wheat straw by
SEM–EDXA and found that lignin removal followed the
order of SMCC [23]. Wang et al. also obtained the same
conclusions when Taiwania (Taiwania cryptomerioides) and
eucalyptus (Eucalyptus grandis) chips were delignified by
ethanol pulping [24]. However, Paszner and Behera showed
that the topochemistry of delignification during the fiber
liberation stage of organosolv pulping of softwood was
limited to preferential removal of lignin from the cell corner
and middle lamella regions [25]. It seems that the mecha-
nisms on organosolv delignification have not been com-
pletely clear yet with respect to lignin redistribution in the
cell wall structure. However, it is clear that not all the parts
of lignin in cell wall have the same reactivity. Therefore, due
to the complex structure of plant cell wall, it is proposed that
not all of the lignin is “reactive” for delignification. There is
a maximum degree of delignification depending on reaction
severity such as temperature, concentrations of AcH, and
catalyst. Namely, under a certain reaction severity, deligni-
fication just can happen into a certain cell wall layer, and the
other part of lignin displays as a hard-to-remove fraction.
Vázquez et al. [13] also found that there was 5 % of the
lignin fraction that could not be eliminated when Eucalyptus
globulus wood was delignified by 70 % AcH with HCl as
catalyst.
2. The dissolved lignin showed some inhibition to
delignification. In our work, we found that the residual
lignin content increased with the initial AcH lignin concen-
tration as shown in Fig. 3, which demonstrated that the
dissolved lignin actually showed some inhibition to both
rate and degree of delignification. Thus, this inhibitive effect
also should be considered in the delignification kinetics.
Bioenerg. Res.

6. Therefore, by introducing dD into the kinetic model, the
rates of residual lignin solubilization into liquid phase and
formation of dissolved lignin can be expressed as:
dD
dt
¼ kL1obs dD D
ð Þ ð8Þ
dCLS
dt
¼ CLR0kL1obs dD D
ð Þ kL2obsCLS ð9Þ
The observed kinetic constants kL1obs and kL2obs are
functions of reaction temperature (T), SA concentration
(CSA), and AcH concentration (CAcH). It should be noted
that acetic acid can dissociate H+
in its aqueous solution.
However, when SA is added the dissociation of AcH is
inhibited and the H+
in the system can be regarded as being
only from the dissociation of SA [26, 27].
Determination of Kinetic Constants
Sugarcane bagasse was delignified with 60–90 wt.% AcH
with addition of 0.05–0.4 wt.% SA as catalysts. After
delignification for a certain time, the solid was firstly
washed with AcH solution to dissolve the possible precipi-
tated lignin on fiber surface, and then washed with water
until neutrality. Degree of delignification was calculated
according to solid yield and lignin content of the obtained
pulp. As shown in Figs. 4 and 5, the rate and extent of
delignification and dissolved lignin concentration were sig-
nificantly affected by SA and AcH concentrations. The rate
of lignin solubilization was dramatically increased with SA
and AcH concentrations. For instance, when using 70 %
AcH to treat bagasse for 2 h, increasing SA concentration
from 0.0051 M to 0.0408 M resulted in the degree of
delignification from 0.357 to 0.765. When SA concentration
was fixed at 0.0204 M, after delignification for 2 h the
degrees of delignification at AcH concentrations of 60 %,
70 %, 80 %, and 90 % were 0.537, 0.660, 0.753, and 0.797,
respectively. Generally, the dissolved lignin concentration
continually increased with reaction time. However, depend-
ing on SA and AcH concentrations, the dissolved lignin
Fig. 3 Effect of dissolved lignin on acid-catalyzed delignification of
sugarcane bagasse by 80 % AcH
Fig. 4 Experimental and model
predicted data for degree of
delignification at different AcH
concentrations. a 60 % AcH; b
70 % AcH; c 80 % AcH; d
90 % AcH
Bioenerg. Res.

7. concentration slightly decreased after reaching a maximum
due to the condensation reaction. Nevertheless, this reaction
is not significant compared with lignin solubilization. Fig-
ures 4 and 5 also show that the model could very well
Fig. 5 Experimental and model
predicted data for concentration
of dissolved lignin (CLS) at
different AcH concentrations. a
60 % AcH; b 70 % AcH; c
80 % AcH; d 90 % AcH
Fig. 6 Experimental and model
predicted data for degree of
delignification and
concentration of dissolved
lignin (CLS) at different reaction
temperatures. a, c 70 % AcH for
delignification; b, d 90 % AcH
for delignification
Bioenerg. Res.

8. predict the experimental data. Statistical analysis showed
that the determination coefficients for the data regression
by the kinetic model were in the range of 0.92–0.99. It
indicated the developed model was good enough to describe
the kinetics of lignin solubilization and condensation. Sim-
ilarly, the delignification process was performed at different
temperatures under atmospheric pressure (Fig. 6). As shown
in the figure, reaction temperature also significantly affected
the rate and extent of delignification. The model calculated
data also could well predict the experimental results.
The regressed kinetic constants are summarized in
Tables 1 and 2. It is clear that the observed rate constants
for delignification (kL1obs) significantly increased with SA
and AcH concentrations. However, when SA concentration
was low (for example 0.0051 M), kL1obs was found to
decrease with AcH concentration. This is probably because
more H+
can be dissociated from AcH at a lower AcH
concentration (higher water concentration) when a small
amount of SA was added. Plotting kL1obs versus CSA at
different AcH (see Supplementary Fig. S2) showed that
there was a good linear relationship between kL1obs and
CSA with corresponding determination coefficients (R2
) in
the range of 0.955–0.989. It indicated that lignin solubiliza-
tion from solid into liquid phase was a first-order reaction
with respect to SA concentration. Vázquez et al. [13, 14]
also found that AcH delignification of several woody bio-
mass feedstocks was a first-order reaction with respect to
HCl concentration. However, a nonlinear relationship be-
tween kL1obs and CAcH was observed (see Supplementary
Fig. S2), and determining the reaction order of delignifica-
tion with respect to AcH concentration is one of the aims of
this work. Therefore, kL1obs can be correlated with T, CSA,
and CAcH by the following equation:
kL1obs ¼ kL1 exp
EL
RT
CSACa
AcH ð10Þ
where kL1, EL, and α are pre-exponential factor, activation
energy for delignification, and reaction order with respect to
CAcH, respectively. By taking logarithm on both sides, Eq.
(10) can be expressed as
ln
kL1obs
CSA
¼ ln kL1
EL
RT
þ a ln CAcH ð11Þ
kL1, EL, and α thus can be determined by plotting ln(kL1obs/
CSA) versus 1/T and CSA with multivariate linear regression,
and the results are shown in Table 3 and Fig. 7. Therefore,
kL1obs can be calculated by the following expression:
kL1obs ¼ 1:825 108
exp
64; 410
RT
CSAC2:23
AcH ð12Þ
The determined activation energy was 64.41 kJ/mol, which
was a little lower than those determined by Vázquez et al. [13,
14]. In terms of dD, it is the function of reaction severity.
Table 1 Kinetic constants for aqueous AcH delignification of sugarcane bagasse at different SA and AcH concentrations (T0107 °C)
CSA
(mol/l)
AcH concentration (%)
60 70 80 90
kL1obs
(h−1
)
dD kL2obs
(h−1
)
kL1obs
(h−1
)
dD kL2obs
(h−1
)
kL1obs
(h−1
)
dD kL2obs
(h−1
)
kL1obs
(h−1
)
dD kL2obs
(h−1
)
0.0051 0.6397 0.4643 0.0070 0.5356 0.5591 0.0097 0.4506 0.5919 0.0060 0.5055 0.6072 0.0081
0.0102 0.7748 0.5481 0.0088 0.5951 0.6778 0.0143 0.5942 0.6963 0.0091 1.1712 0.7108 0.0018
0.0204 0.8384 0.6709 0.0176 1.1527 0.7599 0.0113 2.0318 0.8006 0.0089 3.0159 0.8182 0.0049
0.0306 1.2311 0.7228 0.0181 1.7600 0.7616 0.0178 2.8432 0.8081 0.0038 4.2895 0.8736 0.0020
0.0408 1.5006 0.7391 0.0269 1.8617 0.7856 0.0085 3.4804 0.8349 0.0059 5.3222 0.8817 0.0083
Table 2 Kinetic constants for
aqueous AcH delignification of
sugarcane bagasse at different
temperatures under atmospheric
pressure (CSA00.0306 M)
Temperature (°C) AcH concentration (%)
70 90
kL1obs (h−1
) dD kL2obs (h−1
) kL1obs (h−1
) dD kL2obs (h−1
)
80 0.3602 0.3903 0.0006 1.0214 0.6236 0.0012
90 0.7354 0.5161 0.0173 1.5440 0.7204 0.0081
99 1.0093 0.6683 0.0109 2.6898 0.7785 0.0031
107 1.7600 0.7616 0.0178 4.2895 0.8736 0.0020
Bioenerg. Res.

9. According to our previous work [20], the temperature-
dependent severity factor (R0) is defined as
R0 ¼ exp
T0
100
14:75
ð13Þ
where T′ is reaction temperature in °C. dD increases
with T′, CSA and CAcH, and it tends to be 1 (100 %)
when reaction condition is severe enough. Therefore,
the relationship of dD with CSA, CAcH and R0 is pro-
posed as following equation:
dD ¼ 1
A
Cm
SACn
AcHRq
0
ð14Þ
Similarly, by taking logarithm on both sides, Eq. (14) can
be expressed as
ln
1
1 dD
¼ ln A þ m ln CSA þ n ln CAcH þ q
ln R0 ð15Þ
and corresponding parameters were determined by mul-
tivariate linear regression as shown in Table 3 and
Fig. 7. dD thus can be calculated by the following
equation:
dD ¼ 1
3:506
C0:41
SA C1:56
AcHR0:55
0
ð16Þ
The integral form of Eq. (8) is
D ¼ dD 1 exp kL1obst
ð Þ
½ ð17Þ
Therefore, the degree of delignification at time t can be
calculated by the following equation:
D ¼ 1 3:506
C0:41
SA
C1:56
AcH
R0:55
0
1 e 1:825108
expð64410
RT ÞCSAC2:23
AcHt
ð Þ
h i ð18Þ
From Tables 1 and 2, it can be known that kL2obs
were smaller than kL1obs by two to three orders of
magnitude, which indicated that lignin condensation re-
action was much slower than lignin solubilization reac-
tion. It demonstrates that under the studied conditions,
the condensation reaction of lignin was so insignificant
that it could be neglected. The work of Vázquez et al. [13]
also showed that lignin condensation and precipitation
were only significant under severe conditions (160 °C,
0.027 M HCl). However, kL2obs generally decreased with
increase of AcH, indicating that high AcH concentration
were advantageous to inhibit lignin condensation. There-
fore, when the condensation reaction of lignin can be
neglected, the dissolved lignin concentration in liquid
phase can be calculated by the following equation:
CLS ¼ CLR0D ¼ CLR0 1 3:506
C0:41
SA
C1:56
AcH
R0:55
0
1 e 1:825108
exp 64410
RT
ð ÞCSAC2:23
AcHt
ð Þ
h i
ð19Þ
Mechanism Analysis of Acid-Catalyzed Delignification
by Aqueous Acetic Acid
Effect of Sulfuric Acid
It has been known that lignin is an amorphous polymer
consisting of phenylpropane units. These units are mainly
Table 3 Determination of ki-
netic constants by multivariate
linear regression
For kL1obs
kL1 EL (kJ/mol) α R2
F value P value
1.826×108
64.41 2.23 0.926 45.8099 0.0001
For dD
A m n q R2
F value P value
3.506 0.41 1.56 0.55 0.9574 74.4481 0.0001
Fig. 7 Comparison of experimental data and model-predicted data for
kL1obs and dD
Bioenerg. Res.

10. linked by carbon–carbon bonds and ether bonds. Ether link-
ages of various types are over two thirds (70–75 %) of all the
intermonomeric linkages, and carbon–carbon linkages are less
than one third (25–30 %). Among the ether linkages, the β-
aryl ether linkages are the most frequent ones (48/100 C9),
while α-aryl ether linkages account for about 15–20/100 C9
[28]. Therefore, cleavages of ether bonds mainly contribute to
lignin degradation during organosolv delignification. It has
been observed that α-aryl ether bonds were hydrolyzed more
easily than β-aryl ether bonds, especially when they occurred
in a lignin structural unit containing a free phenolic hydroxyl
group in the para position [29]. Moreover, it is believed that
solvolytic splitting of α-ether linkages are the responsible
reactions for lignin solubilization during organosolv pulping
of lignocellulose under acidic conditions [29]. According to
the kinetic modeling results, the rate and degree of delignifi-
cation significantly increased with sulfuric acid concentration,
which illustrated that delignification reaction was an H+
-cata-
lyzed process. According to literatures [29], the possible re-
action mechanisms for acid-catalyzed cleavages of α-aryl
ether and β-aryl ether bonds during acetic acid delignification
are shown in Fig. 8. For both cleavage reactions, the first step
is protonation to form activated intermediums. Therefore,
increasing sulfuric acid concentration accelerates the forma-
tion of activated intermediums, thus increasing delignification
rate. However, it has been found that α-aryl ether bonds are
hydrolyzed generally 100 times faster than β-aryl ether bonds
[28], and the activation energy for the hydrolysis of α-aryl
ethers of lignin model compounds were found to be 80 to118
kJ/mol depending on the substituent, while the activation
energy reported for β-aryl ether hydrolysis (150 kJ/mol) was
considerably higher [13]. In the present work, the activation
energy for delignification was 64.41 kJ/mol, indicating that
the hydrolysis of α-aryl ethers bonds were mainly responsible
for delignification. However, since the β-aryl ether linkages
are the most frequent ones in the ether linkages of phenyl-
propane units, according to Fig. 8b, the cleavages of β-aryl
ether bonds lead to formation of new phenolic hydroxyl group
(Ph–OH). Phenolic hydroxyl groups can also be formed from
the cleavage of α-aryl groups. We thus determined the Ph–OH
content of the lignins obtained by 80 % AcH delignification
under the catalysis of 0.3 % SA. As shown in Table 4, Ph–OH
content continually decreased with increase of delignification
time, indicating that cleavages of β-aryl ether bonds did not
easily happen during AcH delignification. Contrarily, the de-
crease of Ph–OH content was probably due to the condensa-
tion reaction of lignin fragments to form larger molecules as
shown in Fig. 9a. However, since the delignification reaction
was conducted at low temperature with a relatively low SA
concentration, the degree of lignin condensation was not
significant observed as found in kinetic study. Table 4 also
OR
OCH3
R' O
R''
OR
OCH3
R'
P
(II)
H+
OR
OCH3
R' O
R''
H
+
HP
-R''OH
-H+
OH
OCH3
R' O
R''
O
OCH3
H R'
OH
OCH3
R'
HP
P
(I)
H+
OR
OCH3
R' O
R''
OR
OCH3
R'
P
H+
OR
OCH3
R' O
R''
H
+
OR
OCH3
R'
-R''OH
+
HP (III)
O
OCH3
R' O
R''
H
+
-R''OH
H
OR
OCH3
R' O
R''
H
+
OH
COH
HC
C
O
OCH3
+H+
, -H2O
OCH3
OH
HC+
CH
C
O
OCH3
OCH3
OH
C
C
C
O
OCH3
OCH3
H+
,
H2O
OH
H2C
HOC
C
O
OCH3
OCH3
OH
CH2
C
C
OH
OCH3
OCH3
O
+
a
b
Fig. 8 Possible reaction mechanisms for acid-catalyzed cleavages of
α-aryl ether and β-aryl ether bonds during acetic acid delignification. a
Cleavages of α-aryl ether bonds, R0H or CH3; P0HO– or CH3COO–;
b cleavages of β-aryl ether bonds [29]
Table 4 Phenolic hydroxyl
group (Ph–OH), sugars, and
acetyl-group contents of lignins
obtained by delignification with
80 % AcH under catalysis of
0.3 % SA
Delignification time (h) Ph–OH (%) Glucose
(%)
Xylose (%) Arabinose
(%)
Acetyl group (%)
0.5 4.58±0.21 1.74±0.18 6.27±0.09 2.19±0.02 6.68±0.04
1.0 3.80±0.18 1.62±0.03 6.93±0.13 2.59±0.18 7.49±0.01
1.5 3.30±0.11 1.29±0.01 5.71±0.03 2.30±0.04 7.05±0.01
2.0 2.81±0.01 1.03±0.01 3.75±0.03 1.92±0.01 6.11±0.05
Bioenerg. Res.

11. shows that the obtained lignin had a high acetyl-group con-
tent, which illustrated that acetylation reaction happened dur-
ing delignification. A possible reaction mechanism for lignin
acetylation is shown in Fig. 9b, by which an aliphatic hydrox-
yl group reacts with AcH, resulting in the introduction of
acetyl group into lignin structure. The decrease in Ph–OH
content can also be due to their acetylation reactions. More-
over, the solvolytic cleavage of α-aryl ether bonds (Fig. 8a)
also can cause acetylation of lignin fragments. It also can be
known that sugar contents of the lignin products decreased
with increase of delignification time, demonstrating that the
cleavage of the linkage between lignin and carbohydrate
became more thorough as delignification reaction proceeded.
Effect of Acetic Acid
The kinetic results showed that AcH concentration had very
significant influence on rate and degree of delignification,
which can be reflected by the fact that the delignification
rate had a high reaction order with AcH concentration.
During delignification, although AcH might react with lig-
nin, its concentration was not found to significantly decrease
(see Supplementary Fig. S3). It indicated that AcH mainly
played a role to dissolve lignin fragments formed by cleav-
ages of α-aryl ether bonds of original lignin polymer. Gen-
erally, the enthalpy of polymer solution (ΔHm) can be
calculated by Hildebrand equation (Eq. 20):
ΔHm ¼ Vmf1f2
ΔE1
~
V1
1 2
=
ΔE2
~
V2
1 2
=
#2
¼ Vmf1f2 d1 d2
ð Þ2
ð20Þ
where Vm is the total volume of mixture; ϕ1 and ϕ2 are
volume fractions of solvent and solute (polymer), respec-
tively; E1
~
V1
and E2
~
V2
are the cohesive energy densities of
solvent and solute (polymer), respectively; and d1 and d2 are
solubility parameters, respectively. Therefore, when a sol-
vent and a polymer have close solubility parameters, the
polymer would have a good solubility in the solvent. The δ
values for pure AcH and water are 10.1 and 23.4 cal1/2
cm−3/
2
, respectively [30]. The δ values of AcH solution at differ-
ent temperatures thus can be calculated as shown in Table 5.
The δ value continually decreases with the increase in AcH
concentration and temperature. In terms of lignin structure,
it has been known that the three basic monomeric units
constituting herbaceous plants lignin are p-hydroxyphenyls
(H), guaicyls (G), and syringyls (S) [21], and according to
Feng and Glasser [31] the isolated milled lignin of sugar-
cane bagasse has a weight ratio of H/G/S as 0.66:1:0.91. Ni
and Hu [32] calculated the cs of G, S, and H groups as 13.5,
14.2, and 14.1 cal1/2
cm−3/2
, respectively. Therefore, it can
be known that the δ value of sugarcane bagasse was
13.9 cal1/2
cm−3/2
. According to Table 4, AcH solutions with
70–80 % AcH concentration are of high solvency to dis-
solve lignin fragments since they have close δ value to that
of bagasse lignin. We further conducted experiments to
determine the solubility of bagasse AcH lignin in different
AcH solutions as shown in Fig. 10. It can be known that
AcH lignin indeed had the highest solubility in 70–80 %
C
OH
+
C
C
MeO
C
OH
C
C
OMe
C
O
C
C
MeO
C
OH
C
C
OMe
a
H+
b
H+, CH3COOH
O
HO
HO
OMe
OMe
OMe
H
O
CH3
O
OH
O
OMe
OMe
OMe
H
Fig. 9 Possible condensation and acetylation reactions of dissolved
lignin during AcH delignification. a condensation reaction [29]; b
acetylation reaction
Table 5 Calculated δ values of
AcH solution at different
temperatures
T (°C) AcH concentration (v/v, %)
30 40 50 60 70 80 90 95 100
30 20.36 19.22 18.12 16.85 15.46 13.92 12.15 11.16 10.06
50 19.81 18.70 17.53 16.27 14.92 13.44 11.77 10.83 9.82
70 19.45 18.35 17.18 15.94 14.60 13.13 11.48 10.56 9.55
80 19.27 18.17 17.01 15.77 14.44 12.98 11.34 10.42 9.43
90 19.07 17.98 16.82 15.59 14.26 12.81 11.18 10.27 9.28
95 18.97 17.88 16.72 15.49 14.17 12.73 11.10 10.19 9.21
Bioenerg. Res.

12. AcH solution. Therefore, with the increase of AcH concen-
tration, the difference between saturated and actual lignin
concentrations, which can be regarded as the driving force
for lignin solubilization, becomes increased to enhance the
delignification rate. However, the ability of solvent to dis-
solve lignin also increased with their capacity to form hy-
drogen bonds [4]. Therefore, increasing AcH concentration
naturally enhances the formation of hydrogen bonds be-
tween AcH and lignin fragments, thus increasing the
delignification rate.
The aforementioned inhibition of dissolved lignin on
delignification rate thus can be explained by similar mech-
anisms. With the increase of dissolved lignin concentration,
the driving force for lignin solubilization decreases. More-
over, since a part of AcH molecule combines with dissolved
lignin by hydrogen bonds, the other part which can form
hydrogen bonds with new lignin fragments becomes less
when dissolved lignin concentration increases. Therefore,
dissolved lignin showed an observed inhibition on deligni-
fication rate and degree during aqueous AcH treatment of
sugarcane bagasse.
Conclusions
Sugarcane bagasse was delignified with aqueous AcH under
the catalysis of SA. Based on the multilayered structure of
plant cell wall and the inhibitive effect of dissolved lignin on
delignification rate, a novel pseudo-homogeneous kinetic
model was proposed by introducing the concept of “poten-
tial degree of delignification (dD)”. It was found that the
model could well predict the degree of delignification and
dissolved lignin concentration. Delignification rate was a
first-order reaction with respect to SA concentration, while
AcH concentration showed a high reaction order to deligni-
fication rate. The activation energy for delignification was
determined to be 64.41 kJ/mol. dD increased with reaction
temperature, SA, and AcH concentrations. A mathematical
model was proposed to correlate dD with reaction parame-
ters. The model showed satisfactory accuracy to predict
experimental results. Under the studied conditions, the con-
densation reaction of lignin was so insignificant that it could
be neglected, but high AcH concentration was found to be
advantageous to inhibit lignin condensation. Mechanism
analysis indicated that the hydrolysis of α-aryl ethers bonds
were mainly responsible for the formation of lignin frag-
ments, and cleavages of β-aryl ether bonds did not easily
happen during AcH delignification. Acetylation of lignin
fragments was also observed. The solubility parameter (δ
value) of AcH solution decreased with increase of AcH
concentration. Furthermore, 70–80 % AcH solution had a
similar δ value to that of bagasse lignin, thus showing the
highest solvency to lignin fragments. It is hypothesized that
the solubilization driving force for lignin fragments
increases with AcH concentration, and thus AcH concentra-
tion has a very significant influence on delignification rate.
Acknowledgments The authors are grateful for the support of this
work by the National Natural Science Foundation of China (No.
21106081), National Basic Research Program of China (973 Program)
(No. 2011CB707406), and International Cooperation Project of the
Ministry of Science and Technology of China (No. 2010DFB40170).
References
1. Lau MW, Bals BD, Chundawat SPS, Jin M, Gunawan C, Balan V
et al (2012) An integrated paradigm for cellulosic biorefineries:
utilization of lignocellulosic biomass as self-sufficient feedstocks
for fuel, food precursors and saccharolytic enzyme production.
Energy Environ Sci. doi:10.1039/C2EE03596K
2. Chundawat SPS, Donohoe BS, LdC S, Elder T, Agarwal UP, Lu F
et al (2011) Multi-scale visualization and characterization of plant
cell wall deconstruction during thermochemical pretreatment. En-
ergy Environ Sci 4(3):973–984
3. Zhao X, Cheng K, Liu D (2009) Organosolv pretreatment of
lignocellulosic biomass for enzymatic hydrolysis. Appl Microbiol
Biotechnol 82(5):815–827
4. Pan XJ, Sano Y (1999) Atmospheric acetic acid pulping of rice
straw IV: physico-chemical characterization of acetic acid lignins
from rice straw and woods. Part 1. Physical characteristics.
Holzforschung 53(5):511–518
5. Pan XJ, Sano Y (1999) Acetic acid pulping of wheat straw under
atmospheric pressure. J Wood Sci 45(4):319–325
6. Nimz HH, Granzow C, Berg A (1986) Acetosolv pulping. Holz
Roh Werkstoff 44:362
7. Nimz HH, Berg A, Granzow C, Casten R, Muladi S (1989)
Pulping and bleaching by the acetosolv process. Papier 43
(l0A):Vl02–Vl08
8. Nimz HH, Schöne M (1993) Non waste pulping and bleaching
with acetic acid. In: Proc. 7th International Symposium on Wood
and Pulping Chemistry, Vol. I. Beijing. pp. 258–265
9. Zhao X, Liu D (2011) Fractionating pretreatment of sugarcane
bagasse for increasing the enzymatic digestibility of cellulose.
Chin J Biotechnol 27(3):384–392
10. Zhao X, Liu D (2010) Chemical and thermal characteristics of
lignins isolated from Siam weed stem by acetic acid and formic
acid delignification. Ind Crops Prod 32(3):284–291
Fig. 10 Solubility of bagasse AcH lignin in different AcH solutions
Bioenerg. Res.

13. 11. Parajo JC, Alonso JL, Santos V (1995) Kinetics of eucalyptus
wood fractionation in acetic acid–HCl–water media. Bioresour
Technol 51(2):153–162
12. Vázquez G, Antorrena G, González J (1994) Kinetics and
mechanism of acetic acid pulping of detannined Pinus
pinaster bark. Wood Sci Technol 28(6):403–408
13. Vázquez G, Antorrena G, González J (1995) Kinetics of acid-
catalysed delignification of Eucalyptus globulus wood by acetic acid.
Wood Sci Technol 29(4):267–275
14. Vázquez G, Antorrena G, González J, Freire S, Lopez S (1997)
Acetosolv pulping of pine wood. Kinetic modelling of lignin solubi-
lization and condensation. Bioresour Technol 59(2–3):121–127
15. Dapía S, Santos V, Parajó JC (2002) Study of formic acid as an
agent for biomass fractionation. Biomass Bioenergy 22(3):213–
221
16. Fan Y (2007) Delignification mechanism and chemical reactions in
the formic acid pulping process of wheat straw. PhD thesis, Beijing
Forestry University
17. Tu Q, Fu S, Zhan H, Chai X, Lucia LA (2008) Kinetic modeling of
formic acid pulping of bagasse. J Agric Food Chem 56(9):3097–
3101
18. Villaverde JJ, Ligero P, de Vega A (2010) Formic and acetic acid as
agents for a cleaner fractionation of Miscanthus × giganteus. J
Cleaner Prod 18(4):395–401
19. Sluiter A, Hames B, Ruiz R, Scarlata C, Sluiter J, Templeton D et
al (2008) Laboratory Analytical Procedure (LAP), National
Renewable Energy Laboratory
20. Zhao X, Zhou Y, Liu D (2012) Kinetic model for glycan hydrolysis
and formation of monosaccharides during dilute acid hydrolysis of
sugarcane bagasse. Bioresour Technol 105:160–168
21. Zhao X, Zhang L, Liu D (2012) Biomass recalcitrance. Part I: the
chemical compositions and physical structures affecting the
enzymatic hydrolysis of lignocellulose. Biofuels Bioprod Biorefin.
doi:10.1002/bbb.1333
22. Muurinen E (2000) Organosolv pulping—a review and distillation
study related to peroxyacid pulping. PhD thesis, University of
Oulu
23. Chen N (2004) Studies on delignification mechanism and kinetics
of formic acid based wheat straw cooking processes. Master thesis,
Beijing Forestry University
24. Wang IC, Kuo ML, Ku YC (1993) Alcohol pulping of plantation
Taiwania and Eucalyptus part II. Scanning electron microscope
observations. Bull Taiwan Forest Res Inst 8:177–185
25. Paszner L, Behera NC (1989) Topochemistry of softwood
delignification by alkali earth metal salt catalyzed organosolv
pulping. Holzforschung 43:159–168
26. Zhao X, Zhang T, Zhou Y, Liu D (2007) Preparation of peracetic
acid from hydrogen peroxide. Part I: kinetics for peracetic acid
synthesis and hydrolysis. J Mol Catal A Chem 271(1–2):246–252
27. Zhao X, Cheng K, Hao J, Liu D (2008) Preparation of peracetic
acid from hydrogen peroxide, part II: kinetics for spontaneous
decomposition of peracetic acid in the liquid phase. J Mol Catal
A Chem 284(1–2):58–68
28. Sama KB (1998) Lignin depolymerization and condensation
during acidic organosolv delignification of Norway spruce
(Picea abies). PhD thesis, State University of New York
29. McDonough TJ (1993) The chemistry of organosolv delignification.
TAPPI J 76(8):186–193
30. Xu F, Sun RC, Zhan HY (2004) Progress of organosolv pulping.
Trans Chin Pulp Pap 19(2):152–156
31. Feng J, Glasser WG (1986) The chemical structure of bagasse
lignin. Chin Pulp Pap 5(3):10–18
32. Ni Y, Hu Q (1995) Alcell lignin solubility in ethanol–water
mixtures. J Appl Polym Sci 57(12):1441–1446
Bioenerg. Res.
View publication stats
View publication stats