Bioresource Technology 123 (2012) 324–331
Contents lists available at SciVerse ScienceDirect
Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech
FeCl3 and acetic acid co-catalyzed hydrolysis of corncob for improving furfural
production and lignin removal from residue
Liaoyuan Mao, Lei Zhang, Ningbo Gao, Aimin Li ⇑
Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2,
Dalian 116024, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" FeCl3 and acetic acid were used to
Acetic acid and FeCl3 co-catalyzed hydrolysis of corncob not only promoted the furfural yield, but also
enhanced the lignin removal. In addition, FeCl3 was more effective in cellulose degradation while acetic
acid favored the lignin removal. A maximum furfural yield of 67.89% was obtained within 30 min, combined with a lignin and cellulose removal rates of 54.79% and 25.71%, respectively, at 3% of acetic acid and
20 mM of FeCl3 and 180 °C of reaction temperature.
co-catalyze the hydrolysis of
corncob.
" Lignin could be removed by the
solvation effect of acetic acid in FeCl3
medium.
" Destroyed fibrous structure of
corncob leads to a great pressure
drop in the system.
a r t i c l e
i n f o
Article history:
Received 9 May 2012
Received in revised form 13 July 2012
Accepted 16 July 2012
Available online 25 July 2012
Keywords:
Acetic acid
Corncob
FeCl3
Furfural
Mass heat transfer
a b s t r a c t
In order to increase furfural yield and lignin removal, both FeCl3 and acetic acid were used to co-catalyze
the hydrolysis of corncob. A series of experiments were carried out to investigate the effects of acetic acid,
FeCl3 concentrations and temperatures on furfural production and residue characteristics. The results
showed that high FeCl3 concentrations caused serious cellulose degradation while acetic acid was more
effective for lignin removal. A maximum furfural yield of 67.89% (35.74% higher than that in conventional
sulfuric acid-catalyzed process) was obtained at 180 °C in the presence of 20 mM of FeCl3 and 3% of acetic
acid. Simultaneously, lignin removal reached 54.79%, and 74.29% of the cellulose was remained for further utilization. Acetic acid and FeCl3 co-catalyzed hydrolysis was not only a high efficiency and environmental friendly technique, but also provided a possibility to utilize the furfural residue for ethanol
production and other industries.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
As one of the most important and valuable chemicals, furfural is
produced from lignocellulosic biomass by dehydrating pentoses
⇑ Corresponding author. Tel.: +086 0411 84707448; fax: +086 0411 84706679.
E-mail address: [email protected] (A. Li).
0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.biortech.2012.07.058
which are present in the hemicellulose of the agriculture residues,
such as corncob, corn stock, rice hull, and olive stones (Lichtenthaler,
1998; Zeitsch, 2000). The current output of furfural is about
700,000 t yr1 around the world with a market price of around
$2000 t1. Commercially, furfural is produced using sulfuric acid
as homogeneous catalyst. First, the feedstock is mixed with an aqueous solution of sulfuric acid and loaded to the digester. The system is
L. Mao et al. / Bioresource Technology 123 (2012) 324–331
maintained at the desired reaction temperature by injecting steam
to the digester. Xylan is hydrolyzed to xylose and other monosaccharide, and further dehydration reactions of the pentoses yield furfural. Furfural is continuously stripped out from the reaction system
by steam distillation (Montané et al., 2002). Although this process
technology has been commonly used in more than 400 furfural
plants in China for decades, some problems were encountered, such
as large amount of furfural loss (nearly account for 40–50% of the potential furfural yield), corrosion of pipelines and valves at high temperature, safety issues and environmental problems due to toxic
waste effluents (Telleria et al., 2011). For those reasons, the development of appropriate technology remains great interests for lignocellulose hydrolysis and the growth of furfural industry.
In last decades, plenty of efforts have been made to improve the
furfural yield. Although the furfural yield higher than 80% was
achieved when pure xylose was used as a substrate (Dias et al.,
2005; Montané et al., 2002), the furfural yield rarely exceeded
60% when it was produced from lignocellulosic biomass via one
step process (Montané et al., 2002; Yemisß and Mazza, 2011). In
previous studies, many acidic heterogeneous catalysts (i.e. NaCl,
MgCl2, CaCl2, Fe(acac)3, FeCl3 6H2O, FeSO4 7H2O, FeCl2 4H2O,
MnCl2, Cu(OAc)2, and CuCl22H2O) were applied to obtain a mild
and homogeneous hydrolysis reaction in the dehydration of xylose
(Garrote et al., 1999; Xing et al., 2011), and FeCl3 6H2O was found
to display a superior performance on hydrolysis of lignocellulosic
biomass (Bayramoglu et al., 2012; Bu et al., 2011; Montané et al.,
2002; Weingarten et al., 2010). Furthermore, it was also observed
that the reaction of xylose catalyzed by FeCl3 was much faster than
that by strong acid (i.e. HCl) and other chlorine salts (i.e. CaCl2,
NaCl, KCl and KBr) (Marcotullio and Jong, 2010). So far, there are
limited researches focusing on both promoting furfural production
and the residue characteristics of lignocellulosic biomass, which
may contain large amount of cellulose for bioethanol production
and paper industry.
An integrated use of cellulose, hemicellulose and lignin is very
important for economical feasibility of biorefining industry. Due
to technology and cost barriers (Chandra et al., 2007), currently,
only hemicellulose (xylan) that contained in the raw stuff was utilized and converted to furfural in industrial process, and the waste
residue was often used for solid fuel after drying. It was estimated
that about 12–15 t of hydrolysis residue was discharged if 1 t of
furfural is produced (Bu et al., 2011; Sun et al., 2008). The furfural
residue contained large portion of cellulose that could be converted into glucose for ethanol fermentation (Bu et al., 2011), single cell protein production (Parajó et al., 1995) or for paper pulp
(Grethlein and Converse, 1991). The largest obstacle to utilizing
cellulosic biomass or furfural residue was cost-effectively to release sugars from recalcitrant lignocelluloses (Bu et al., 2011; Lynd
et al., 2008; Rollin et al., 2011). It was rather difficult to digest the
furfural residue with a low loading of enzymes because of the presence of a large amount of lignin (Bu et al., 2011). Therefore, a pretreatment to remove lignin is a crucial step for an effective
enzymatic hydrolysis (Bu et al., 2011). However, there is little research about the optimization of the catalysts and hydrolysis conditions for promoting furfural production and economically
utilizing the furfural residue.
Acetic acid as a modest, low corrosive and environmental
friendly catalyst was proved effective in catalyzing lignocellulosic
biomass to produce furfural (Dallinger and Kappe, 2007; Garrote
et al., 1999; Lowry, 1927; Zeitsch, 2000). In those studies, acetic
acid was mixed lignocellulosic biomass in aqueous phase for
hydrolysis and furfural production (Abad et al., 1997; Yemisß and
Mazza, 2011), but there was limited information about utilizing
acetic acid steam as catalyst for furfural production. In addition,
acetic acid was found to be an effective agent for fractionation of
lignocellulosic biomass. For example, it caused an extensive delig-
325
nification with simultaneous hemicellulose degradation at a high
pulp yield (Dallinger and Kappe, 2007; Ligero et al., 2008; Vanderghem et al., 2011; Zhao and Liu, 2010).
In this study, FeCl3 and acetic acid (in steam form) were used to
co-catalyze the hydrolysis of corncob for promoting furfural production. Another special focus was put on the effects of FeCl3 and
acetic acid on cellulose recovery and delignification, which is
important for value-added utilization of furfural residue. Specifically, we systematically investigated the separate and combined
effect of FeCl3 and acetic acid on furfural yield, cellulose and lignin
removals at different concentrations and temperatures. Based on
experimental results, the mechanism of FeCl3 and acetic acid stimulating the furfural production was tentatively proposed.
2. Methods
2.1. Materials
Corncob was obtained from Changtu County Furfural Plant, Liaoning Province, China. The corncob was sieved to a size between 5
and 10 mm, air-dried, homogenized in a single lot, and stored for
further use. Acetic acid, furfural standard, FeCl36H2O, sulfuric acid
(Tianjin Kewei Co., China) used in the experiments were analytical
reagents without further purification. Distilled water was used to
prepare the solutions and dilute the samples.
2.2. Reactor system and experiment procedure
A series of experiments were carried in a semi-batch tubingbomb reactor system (see Fig. 1), which was designed according
to industrial furfural production process. The reactor was a
47 mm of I.D. (Internal Diameter) stainless-steel vessel with a volume of 2.1 L. K-type thermocouple and pressure transducer were
mounted inside the reactor to record the temperature and pressure, respectively. The feedstock was firstly mixed with varying
concentrations of FeCl3 and then loaded to the digester. Varying
concentrations of acetic acid solution was firstly loaded into a tailor-made electric boiler (Jinxin Petrochemical Equipment Co., Ltd.,
Weihai, China) and then heated to a desired pressure. The highpressure steam that contained acetic acid was continuously injected into the reactor to supply catalysts and maintain the system
at the desired reaction temperature. Furfural was continuously extracted from the digester by steam distillation. In order to collect
the liquid products completely, the exhausted steam was condensed by the liquid nitrogen.
In each experiment, the reactor was loaded with an amount of
corncob equivalent to 400 g of dry matter. In order to reduce interstitial water and ensure the feeding steam distribute homogeneously into the granular corncob, a liquid: solid ratio of 0.6:1
(slightly below saturated water content of corncob) was used.
Based on our previous optimization (data not shown), the reaction
time was chosen to be 30 min. It must be noted that, in order to obtain a constant and homogeneous content of acetic acid in the reaction system, acetic acid was continuously carried into the reactor
by feeding steam rather than directly mixing with feedstock before
the reaction occurred.
2.3. Product analysis
The main fractions (cellulose, hemicelluloses and lignin) were
analyzed after a quantitative acid hydrolysis under standard conditions (Parajó et al., 1993). The main compositions of the corncob
were 34 ± 1% of cellulose, 31 ± 1% of hemicelluloses, and 18 ± 1%
of lignin. Liquid was extracted and filtered through a 0.22 lm syringe filter. The residues were firstly immersed in 1% of NaOH solu-
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L. Mao et al. / Bioresource Technology 123 (2012) 324–331
Y-type
filter
Steam valve
Ball valve
Condenser
Electric thermocouple
Electronic
balance
Electric
boiler
Pressure transducer
Hydolyzer
Stop check valve
Fig. 1. Schematic of the batch system used for the furfural production from corncob.
tion for half an hour and then washed with fresh distilled water
until neutral. The samples were dried at 50 °C for 12 h and milled
to a size under 40 meshes. The xylan was determined using NREL
LAP ‘‘Determination of Structural Carbohydrates and Lignin in Biomass’’ (Sluiter et al., 2008). For this method, a two-step acid hydrolysis was used to hydrolyze the remaining xylan into forms of
xylose, which is then quantified by HPLC. The concentrations of
furfural in the liquid were also analyzed by HPLC with a Bio-Rad
HPX 87H column at 40 °C. The mobile phase was a mixture of
0.005 M H2SO4 and HPLC-grade acetonitrile (84%/16% vol/vol) at
a flow rate of 0.5 mL min1 (Yuan and Chen, 1999).
The furfural yield, remaining cellulose and xylan, and xylan loss
were calculated according to the following equations, respectively.
Theoretical yield of furfural yield was obtained by multiplying the
amount of xylan by 0.7272 (Zeitsch, 2000).
Furfural yield ¼
furfural yield ½gin concentrate
theoretical production of furfural ½g
100%
Remaining cellulose ¼
ð1Þ
cellulose ½g in residue
cellulose ½g in dry; untreated corncob
100%
ð2Þ
Remaining xylan ¼
xylan ½g in residue
xylan ½g in dry; untreated corncob
100%
Xylan loss ¼ 1 Remaining xylan—Furfural yield
ð3Þ
ð4Þ
Corncob and residue were submitted to SEM (Scanning Electron
Microscopy) analysis. The samples were prepared by adhibiting
them on a specimen stub using a double-coated tape, and were
sputter coated with AuPd prior to imaging with a KYKY-2800 scanning electron microscope (KYKY Technology Development Ltd.,
China) using 15 kV accelerating voltage.
3. Results and discussion
3.1. Impact of acetic acid or FeCl3 on hydrolysis of corncob
Fig. 2A shows the effects of acetic acid concentrations on furfural yield, hemicellulose degradation, and lignin and cellulose removals. It was observed that the furfural yields firstly increased from
17.45% to 41.29% as acetic acid concentrations in the feeding steam
increased from 1% to 3%, and then declined to 38.98% at 4% of acetic
acid concentration. The xylan loss was found to continuously increase from 24.88% to 43.59%, indicating that high concentrations
of acetic acid caused more xylan loss. Acetic acid exerted different
effects on the lignin and cellulose removals during furfural process.
For instance, the removals of lignin and cellulose increased from
7.82% and 5.83% to 41.54% and 18.02%, respectively, as acetic acid
concentrations in the feeding steam increased from 1% to 4%. These
results suggested that acetic acid exerted stronger effect on lignin
removal than that of cellulose when using as catalyst for corncob
hydrolysis.
Fig. 2B shows the effects of FeCl3 as a substitute of strong acid
on furfural production, xylan loss, and lignin and cellulose removals during furfural production process. It was observed that furfural yield firstly increased from 32.47% to 55.98% as FeCl3
concentrations increased from 20 to 60 mM, but further increase
of FeCl3 concentration to 100 mM caused the lower yield
(44.67%). Similarly as acetic acid, the xylan loss continuously increased from 21.55% to 53.95% when FeCl3 concentrations increased from 20 to 100 mM. These results indicated that
increasing FeCl3 concentrations favored the furfural formation,
but an excessive FeCl3 caused more furfural loss. Liu and Wyman
(2006) also reported that FeCl3 not only catalyzed furfural formation but also accelerated furfural resinification and condensation.
The lignin removal increased from 6.81% to 19.14%, while the cellulose removal increased from 9.71% to 51.06% as FeCl3 concentrations increased from 20 to 100 mM. Based on these results, it was
concluded that FeCl3 was more effective in the cellulose degradation than that of lignin removal.
3.2. Effect of temperatures on the xylan degradation, lignin and
cellulose removals in the presence of FeCl3 and acetic acid
When high yield of furfural was recovered as the primary product from corncob hydrolysis, it was expected to obtain high deligninification and low cellulose degradation for the residue. So,
furfural residue can be further utilized in bioethanol production
or paper industry. Fig. 3 illustrates the effects of temperatures, concentrations of acetic acid and FeCl3 on the furfural yield, lignin and
cellulose degradation during the hydrolysis of corncob at diverse
and combination conditions.
3.2.1. Experiments at 170 °C
Fig. 3A shows the effects of FeCl3 and acetic acid concentrations
on the xylan degradation, lignin and cellulose removals at 170 °C.
At 1% of acetic acid concentration, the furfural yields increased
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L. Mao et al. / Bioresource Technology 123 (2012) 324–331
80
80
70
70
60
60
50
50
100
B
90
80
80
70
70
60
60
50
50
40
40
30
30
40
40
30
30
20
20
20
20
10
10
10
10
0
0
0
1
2
3
40
60
80
100
FeCl 3 concentration (mM)
remaining xylan
xylan loss
furfural yield
Acetic acid concentration (wt. %)
remianing xylan
xylan loss
furfural yield
remaining cellulose
0
20
4
remaining lignin or cellulose (%)
90
90
potential xylan (%)
potential xylan (%)
90
100
100
A
remaining lignin or cellulose (%)
100
remaining lignin
remaining lignin
remaining cellulose
Fig. 2. Influences of acetic acid or FeCl3 concentration on xylan degradation, lignin removal and remaining cellulose. (A) Only acetic acid was used and (B) only FeCl3 was
used.
90
80
80
70
70
60
60
50
50
40
40
B 100
80
70
70
60
60
50
50
40
40
30
30
20
20
10
20
10
10
10
0
0
acetic acid concentration (%)
xylan loss
C 100
(1) (2) (3) (4)
(1) (2) (3) (4)
acetic acid concentration (%)
furfural yield
(1) (2) (3) (4)
100
90
90
80
80
70
70
60
60
50
50
40
40
30
30
D 100
80
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
acetic acid concentration (%)
remaining xylan
remaining cellulose
xylan loss
furfural yield
remaining lignin
100
70
10
3
(1) (2) (3) (4)
80
10
2
(1) (2) (3) (4)
90
20
1
(1) (2) (3) (4)
furfural yield
remaining lignin
90
20
0
3
2
remaining xylan
xylan loss
remaining cellulose
remaining lignin
remaining cellulose
0
1
potential xylan (%)
remaining xylan
100
80
20
3
(1) (2) (3) (4)
90
30
2
(1) (2) (3) (4)
90
30
1
(1) (2) (3) (4)
remaining lignin or cellulose (%)
100
90
0
potential xylan (%)
(1) (2) (3) (4)
remaining lignin or cellulose (%)
(1) (2) (3) (4)
potential xylan (%)
(1) (2) (3) (4)
remaining lignin or cellulose(%)
100
remaining lignin or cellulose (%)
potential xylan (%)
A
0
1
3
2
acetic acid concentration (%)
remaining xylan
remaining cellulose
xylan loss
furfural yield
remaining lignin
Fig. 3. Effects of FeCl3 and acetic acid concentration on xylan degradation, lignin removal and remaining cellulose. (A) 170 °C; (B) 180 °C; (C) 190 °C; and (D) 200 °C. (1)
20 mM of FeCl3; (2) 40 mM of FeCl3; (3) 60 mM of FeCl3; (4) 80 mM of FeCl3.
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L. Mao et al. / Bioresource Technology 123 (2012) 324–331
from 32.25% to 52.19% as FeCl3 concentrations increased from 20 to
80 mM. Simultaneously, the remaining xylan deceased from
41.27% to 8.98%, and the xylan loss increased from 26.48% to
38.83%. Remaining cellulose and lignin decreased from 87.54%
and 92.19% to 69.98% and 80.37%, respectively. These results suggested that FeCl3 not only promoted the hydrolysis of hemicellulose but also favored the condensation of furfural and
intermediates causing the loss of furfural (Liu et al., 2009). When
acetic acid was 2%, the furfural yields first increased from 48.59%
to 61.27% as the FeCl3 concentrations increased from 20 to
60 mM, and then decreased to 57.98% at 80 mM of FeCl3. At the
same time, xylan loss first decreased from 31.74% to 25.85% as
FeCl3 concentrations increased from 20 to 40 mM, and then increased to 36.41% at 80 mM of FeCl3. The remaining lignin and cellulose continuously decreased from 76.24% and 85.22% to 49.85%
and 65.54%, respectively, when FeCl3 concentrations increased
from 20 to 80 mM. Interestingly, at acetic acid concentration of
3%, the furfural yields continuously decreased from 64.27% to
50.24% as the FeCl3 concentrations increased from 20 to 80 mM.
This difference might be due to the fact that at high acetic acid concentrations, the increased FeCl3 concentrations caused much stronger side reactions between furfural, cellulose, lignin and glucose.
Simultaneously, the xylan loss also increased from 31.52% to
48.77%, and the remaining lignin and cellulose decreased from
47.28% and 84.27% to 32.25% and 63.44%, respectively, as the FeCl3
concentrations increased from 20 to 80 mM. These results confirmed that acetic acid played an essential role on lignin removal,
and the high FeCl3 caused serious cellulose degradation. In addition, the combination of acetic acid and FeCl3 showed synergistic
effects on furfural yield and xylose loss.
3.2.2. Experiments at 180 °C
Fig. 3B shows the effects of FeCl3 and acetic acid concentrations
on the xylan degradation, lignin and cellulose removals at 180 °C.
The results were similar with those obtained at 170 °C. At acetic
acid concentration of 1%, furfural yields increased from 36.98% to
59.64% as the FeCl3 concentrations increased from 20 to 60 mM,
and further increase (80 mM) caused a lower yield (53.67%). Xylan
loss also increased from 25.43% to 42.12% as above. At acetic acid
concentration of 2%, the furfural yield first increased from 51.59%
to 63.69% as FeCl3 concentrations increased from 20 to 40 mM,
and then declined to 59.45% at 80 mM of FeCl3. Similarly, the xylan
loss showed first decreasing trend, then increasing with the minimum value of 29.44% at 40 mM of FeCl3. At acetic acid concentration of 3%, the furfural yield dropped from 67.89% to 55.21% and
the xylan loss increased from 31.14% to 44.79% as FeCl3 concentrations increased from 20 to 80 mM. The largest difference between
the results at 180 °C and those at 170 °C was the degree of cellulose
degradation. As the temperatures increased from 170 to 180 °C,
much more cellulose was removed (Fig. 3A and B). For instance,
at 3% of acetic acid concentration and 80 mM of FeCl3, the remaining cellulose was decreased from 63.44% to 29.98%. In addition, it
was found that the granular corncob was destroyed and converted
to muddy fluid under these conditions. There was a similar result
in a previous work (Liu et al., 2009), where the remaining cellulose
decreased from 73% to 20% as the temperatures increased from 160
to 180 °C at 100 mM of FeCl3 concentration. By contrast, no significant change on lignin removal was found as the temperatures increased from 170 to 180 °C.
3.2.3. Experiments at 190 °C
The effects of FeCl3 and acetic acid concentration on the xylan
degradation, lignin and cellulose removal at 190 °C are shown in
Fig. 3C. As the temperatures increased from 180 to 190 °C, the furfural yield generally increased when the acetic acid concentration
was 1%. For instance, an increasing trend of furfural yield (from
59.64% to 62.35%) and a minor reduction of xylan loss (form
31.39% to 31.11%) were obtained as temperatures increased from
180 to 190 °C when acetic acid and FeCl3 concentration were 1%
and 60 mM, respectively. This result indicated that furfural loss
caused by side reactions could be reduced by increasing temperature in a mild condition. But when the acetic acid concentrations
were 2% and 3%, the furfural yield generally declined with increasing temperatures. For example, xylan loss increased from 29.44% to
36.04% and furfural yield declined from 63.69% to 56.98% as temperatures increased from 180 to 190 °C when acetic acid and FeCl3
concentration were 2% and 40 mM, respectively. In addition, the
remaining lignin and cellulose gradually declined as the temperature increased from 180 to 190 °C. Although high lignin removal
was achieved (20.37% of the lignin remained in the residue) at
190 °C at the conditions of 3% of acetic acid and 80 mM of FeCl3,
only 18.01% of the cellulose was remained in the residue. The high
cellulose loss was not desirable for further utilization of cellulose
for bioethanol plant and other industries.
FeCl3 not only promoted furfural formation, but also caused the
cellulose degradation especially at high temperatures, which was
the main cause of the cellulose loss during the pretreatment (Liu
et al., 2009). Based on our experimental results, the concentrations
of FeCl3 should not exceed 20 mM so as to promote the furfural
yield and simultaneously obtain a high cellulose yield in the
residue.
3.2.4. Experiments at 200 °C
Fig. 3D showed a general uptrend of xylan loss and corresponding downtrend of furfural yield as the temperatures increased from
190 to 200 °C. For instance, furfural yield declined from 64.57% to
45.61% and xylan loss increased from 31.16% to 52.29% as temperatures increased from 190 to 200 °C at 2% of acetic acid and 60 mM
of FeCl3. Riera et al. (1991) also obtained a similar result that furfural yields decreased from 8.7 to 4.1 g/100 g dried corncob as
the temperatures increased from 180 to 200 °C at 0.16 mM of
H2SO4. These results indicated that many side reactions (condensation of furfural with intermediates and self-resinification of furfural) occurred at high temperatures especially in the presence of
strong catalysts. Other side reactions, such as the esterification of
cellulose with acetic acid and the polymerization of furfural with
lignin could occur (Lam et al., 2009). The products, a kind of black,
viscous polymerized resin attached to the surface of particle corncob, which would prevent the diffusion of furfural and the further
hydrolysis of corncob. On the other hand, the viscous aqueous
phase prevent the stripping out of furfural causing the long residence time, which further facilitated more side reactions.
In short, above results showed the differential effects of acetic
acid and FeCl3 on furfural yield, xylan loss, and lignin and cellulose
degradation at different temperatures. In general, the combination
of high temperature with high concentration of FeCl3 in acetic acid
media could not only accelerated cellulose and hemicellulose solubilization, but also promoted cellulose and xylose degradation.
Therefore, the FeCl3 could be employed but the concentrations
should not exceed 20 mM for enhanced furfural production and a
high level of remaining cellulose in the residue. Based on above results, balancing the furfural yield, xylan loss, lignin removal and
cellulose degradation, the optimized conditions were 20 mM of
FeCl3 and 3% of acetic acid and 180 °C of reaction temperature.
At the optimum conditions, a high furfural yield of 67.89% was
achieved together with high lignin removal of 54.79% and high
remaining cellulose of 74.29%.
3.3. Mechanism of FeCl3 and acetic acid promoting furfural yield
In previous studies, the ‘‘proton transfer’’ theory of acid (Lowry,
1927), ‘‘stabilizing effect’’ (Marcotullio, 2010) and ‘‘salting out’’ ef-
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L. Mao et al. / Bioresource Technology 123 (2012) 324–331
B
A
xylan
xylan
xylan
xylan
xylan
H+
xyl an
xylan
H+
xy l o se
xylan
furfu ral
H+
xylose
H+
furfural
xyl an
xylan
xylan
x y l an
xylan
x y l an
steam
distributed
H , CH 3COOH
liquid
steam
+
steam
corncob
H+ , CH 3COOH
distributed
liquid
steam
Fig. 4. Simplified model of furfural formation and stripping. (A) Corncob particle that was severely damaged with high interior porosity; (B) minor damaged corncob particle
with low interior porosity.
18000
(a)
(b)
(c)
(d)
16000
pressure drop (Pa)
14000
12000
10000
8000
6000
4000
2000
0
5
10
15
20
25
30
time (min)
Fig. 5. Pressure drops in the reactor at different times. (a) Catalyzed by pure water
without any foreign acid, and the final furfural yield was 11.89%; (b) 1% of acetic
acid and 20 mM of FeCl3 were used, and the final furfural yield was 36.25%; (c) 3% of
acetic acid and 20 mM of FeCl3 were used, and furfural yield was 67.89%; (d) 3%
acetic acid and 80 mM of FeCl3 were used; and the furfural yield was 55.24%. Other
conditions were same for all trials: the liquid/solid ratio of 0.6 and reaction
temperature of 180 °C.
fect of chlorine salts were adopted to explain the phenomena in
hydrolysis of lignocellulosic materials, furfural production, degradation of lignin and cellulose. In this study, the theory of mass
and heat transfer was tentatively employed to elucidate the mechanisms of strong promoting effect of FeCl3 and acetic acid on the
furfural production and other performances, such as xylose loss,
lignin removal, and cellulose degradation.
Classical one-step method for the furfural production in a batch
process involves six processes (Gámez et al., 2006): (I) diffusion of
protons through the wet lignocellulosic matrix; (II) solvation of xylan from hemicellulose; (III) acid catalyzed hydrolysis of xylan to
xylose; (IV) dehydration of xylose to form furfural catalyzed by
acid; (V) diffusion of furfural in the liquid phase; (VI) distillation
and stripping of furfural by feeding steam. Ideally, the theoretical
yield of furfural was 100%, where xylose could be converted to furfural (Zeitsch, 2000). But in practice, the maximum furfural yields
were between 45% and 55% of the potential (Montané et al., 2002).
The relative low furfural yield could be mainly caused by two reasons. One was the low hydrolysis of lignocellulosic materials, and
the other was ascribed to the furfural loss caused by the side
reactions.
Previous results proved that the furfural yield was higher from
smaller particle size due to the bigger surface area per unit volume
of reacting solids (Riera et al., 1991). Since more energy was consumed to produce smaller particles, in industrial furfural plant,
the granule was not too small (generally 1–2 cm). In this study,
corncob particles were further destroyed to small size during the
hydrolysis process when FeCl3 and acetic acid were used as catalysts. SEM observation of microscopic structure of hydrolysis residues clearly showed that the corncob particles were greatly
destroyed in acetic acid and FeCl3 medium (data not shown). By
contrast, an intact fibrous structure of the residue was observed
when corncob was hydrolyzed by pure water. Furthermore, there
were clear images indicating that crash severity of corncob increased with increasing the concentrations of FeCl3 and acetic acid
(data not shown). Consequently, the interior porosity of the corncob dramatically increased as the fibrous structure was damaged
and fragmented during steps I-III. As a result, the dissolution and
diffusion rate of xylan increased and the catalysts (Fe3+, Cl, H+
and CH3COOH) distributed more homogeneously into the liquid
phase as schematically shown in Fig. 4. In addition, the entirely destroyed fibrous structure resulted in an increased specific surface
area of the granular corncob. Therefore, the contacting area between distributed liquid and steam increased as shown in
Fig. 4A, which also promoted the mass and heat transfer, and favored the furfural recovery by steam stripping. All these contributed to a milder and more homogeneous hydrolysis and
dehydration reaction in the presence of FeCl3 and acetic acid.
After reactions of steps I–III, the furfural was formed by dehydration of xylose catalyzed by acid and/or FeCl3 in step VI. Furfural
contains three unsaturated bonds with the highly reactive characteristics. Chemically, furfural participates in the same kinds of
reactions as other aldehydes and other aromatic compounds,
which caused furfural loss. So, the step VI was very important for
controlling the furfural recovery and furfural loss, where furfural
was continuously extracted from the reactor by steam distillation.
A short residence time of furfural in the aqueous phase was the key
to avoid the side reactions. Ideally, 100% of the furfural could be
330
L. Mao et al. / Bioresource Technology 123 (2012) 324–331
recovered, once all furfural formed from xylose was transferred
into gas phase from the aqueous phase. In an ampoule experiment,
100% of the potential furfural was achieved by reducing the pressure of the system continuously to obtain a subsequent boiling
reaction medium (Zeitsch, 2000). The principal difference between
the industrial processes of less than 60% yield and the analytical
furfural process of 100% yield lied in the fact that in the first case
the reaction medium was boiling while in the second case it was
not boiling (Zeitsch, 2000). However, in order to maintain the reaction medium in a state of boiling, the initial reaction temperature
must be maintained at 230–240 °C (3–3.5 MPa) (Dahlgren, 1978).
A series of problems such as corrosion of reactor and pipelines,
safety issues limit the development of this method. More importantly, all the cellulose in the raw stuff might be catalytically
decomposed by strong acid at the high temperatures.
The distillation and removal rate of furfural from liquid phase
mainly was depended on the mass and heat transfer efficiency of
the reaction system (Tamir, 1994). In addition, the two-phase
transfer efficiency is mainly decided by the steam-liquid interfacial
area, pressure and temperature gradients in the system. Therefore,
the methods to increase pressure drop and obtain a subsequent
increment of mass transfer driving force could be effective in accelerating the stripping of furfural and obtaining a higher furfural
yield. In this study, the influences of pressure gradient on the furfural yield were analyzed quantitatively.
It was reported that the pressure drop Dp is depends on particle
diameter dp, porosity e, particle shape and fluid property (density
qf and viscosity lf ) and flow velocity of the fluid (x0 ) (Ergun,
1952). The pressure drop in a packed bed of porous medium can
be calculated according to Eq. 5:
Dp
ð1 eÞ2 lf x0
1 e qf x20
¼ 150 þ 1:75 3
L
e3 ðUdp Þ2
e Udp
ð5Þ
where, L is the height of the reactor. U is the shape factor. Obviously, the pressure drop Dp is inversely proportional to porosity e
and particle diameter dp. During the hydrolysis of corncob, the corncob particles were destroyed which led to a diminishing of e and dp.
Consequently, the pressure drop Dp increases. In order to investigate the relationship of pressure drop and the furfural yield, the
pressure drop and the corresponding furfural yield at 180 °C with
different catalysts were determined, and the experimental results
are shown in Fig. 5.
It was found that the pressure drop was dramatically affected
by the crash degree of corncob, which was catalyzed by different
catalysts. When steam was used without any foreign acid, the fibrous structure in the residue remained intact and the pressure
drop was 114 Pa. By contrast, the pressure drop reached
13,873 Pa at the conditions of 3% of acetic and 20 mM of FeCl3.
Additionally, when the pressure drop increased from 4785 Pa (at
the conditions of 3% of acetic and 20 mM of FeCl3, reaction temperature of 180 °C) to 13,873 Pa, the furfural yield increased from
36.25% to 67.89%. Simultaneously, the percentage of xylan loss of
the hydrolyzed corncob also decreased from 54.91% to 32.91%.
These results indicated that the pressure drop could reflect the furfural yield and furfural loss, and the high pressure drop correlated
severe crash of corncob catalyzed by catalysts.
4. Conclusion
Acetic acid and FeCl3 co-catalyzed hydrolysis of corncob not
only promoted the furfural production, but also enhanced the lignin removal. The increased acetic acid concentrations exhibited
higher lignin removal than that of cellulose removal, while the
higher FeCl3 exerted stronger effect on cellulose degradation than
that of lignin removal. A maximum furfural yield of 67.89% was ob-
tained within 30 min at 180 °C together with a 54.79% of lignin removal and a 25.71% of cellulose removal in the presence of 3% of
acetic acid and 20 mM of FeCl3.
Acknowledgement
The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC) (No.
51006018), National Water Pollution Control and Management
Technology Major Projects (No. 2012ZX07202003-004), and Special Fund of Environmental Protection Research for Public Welfare
of China (201109035).
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