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Research Article
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Coproduction of Furfural and Easily Hydrolyzable Residue from
Sugar Cane Bagasse in the MTHF/Aqueous Biphasic System:
Influence of Acid Species, NaCl Addition, and MTHF
Xing-kang Li,†,§ Zhen Fang,*,‡,† Jia Luo,† and Tong-chao Su†
†
Key Laboratory of Tropical Plant Resources and Sustainable Use, Xishuangbanna Tropical Botanical Garden, Chinese Academy of
Sciences, 88 Xuefulu, Kunming, Yunnan Province 650223, China
‡
Biomass Group, College of Engineering, Nanjing Agricultural University, 40 Dianjiangtai Road, Nanjing, Jiangsu 210031, China
§
University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China
ABSTRACT: In order to develop a process for the simultaneous
production of furfural and easily hydrolyzable cellulose, the degradation
of sugar cane bagasse in a single aqueous system and in a 2methyltetrahydrofuran (MTHF)/aqueous AlCl3 biphasic system was
studied. In single aqueous system, the influence of acid species (FeCl3,
HCl, and AlCl3) on furfural production and cellulose degradation was
investigated at 150 °C. FeCl3 and HCl promoted furfural production from
hemicellulose but with severe cellulose degradation. AlCl3 decreased
cellulose degradation with considerable furfural yield and high glucan
content in solid residues. The role of NaCl in furfural production and
cellulose decomposition was also investigated in the single aqueous system
using different acids as catalysts. Addition of NaCl significantly promoted
furfural yield but also accelerated cellulose decomposition when FeCl3 or
HCl was used as catalyst. In the AlCl3-catalyzed system, NaCl had less
influence on residue yield and its composition, although NaCl also promoted furfural production. The influence of MTHF on
furfural yield, residue composition, and enzymatic hydrolysis of residue was also studied. Under the best conditions (0.45 g of
bagasse, 9 mL of MTHF, 9 mL of water, 0.1 M AlCl3, 150 °C, 45 min, and 10 wt % NaCl), 58.6% furfural was obtained while
more than 90% of cellulose remained in the residue. The organic phase was separated from the aqueous phase directly by
decantation. After reuse of organic phase for 3 cycles, 11.5 g/L furfural was obtained. The catalyst-containing aqueous phase
could be reused directly after decantation of the organic phase without loss of activity. The obtained residue was easy to
hydrolyze and produced 89.3% glucose yield after 96-h enzymatic hydrolysis at low cellulase loading (30 FPU of cellulase/g
glucan).
KEYWORDS: Furfural, Glucose, Bagasse, Hydrolysis, 2-Methyltetrahydrofuran/aqueous, AlCl3, NaCl
■
biorefinery process.7 However, few studies have focused on
the coproduction of furfural and cellulose-enriched residue.
Nowadays, most industrial production of furfural is adapted
from the Quaker Oats process started in 19218 that uses strong
mineral acids such as H2SO4 as catalyst and is operated at high
temperature (>150 °C) causing complete degradation of
cellulose.4 Some metal chlorides (e.g., AlCl3 and CrCl3) have
both moderate Lewis and Bronsted acidity in water. Lewis
acidity catalyzes the isomerization of xylose to xylulose, and
Bronsted acidity catalyzes the dehydration of xylulose into
furfural.9,10 The combination of Lewis and Bronsted acidity
enables furfural formation under mild conditions that may
avoid severe decomposition of cellulose. It is found that AlCl3
INTRODUCTION
The dwindling reserve of fossil resources and environmental
concern associated with fossil fuel application spur research on
the conversion of lignocellulosic biomass to renewable fuels and
chemicals.1,2 Furfural is a platform chemical that has potential
to produce biofuels and attracts much attention from academia
and industry.3 It is produced from pentose (e.g., xylose and
arabinose) in hemicellulose that occupies 20−30 wt % of
typical terrestrial biomass such as wood. Cellulose, accounting
for another 30−50 wt % biomass, is normally hydrolyzed to
fermentable sugars for bioethanol.4,5 In lignocellulosic biomass,
cellulose is surrounded by hemicellulose and protected by
lignin. Pretreatment of lignocellulosic biomass is required to
remove hemicellulose and lignin and make cellulose highly
accessible to catalysts or cellulases.6 A chemical method that
could efficiently convert hemicellulose to furfural and
simultaneously produce cellulose-enriched residue is encouraged as it remarkably promotes the economics of the
© 2016 American Chemical Society
Received: August 3, 2016
Revised: August 25, 2016
Published: August 30, 2016
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Scheme 1. Furfural and Cellulosic Residue Production from Xylan Catalyzed by Lewis Acid/Bronsted Acid
similar enzymatic digestibility to Avicel cellulose.26 In this
process, lignin was extracted into the organic phase, and soluble
sugars were reserved in the aqueous phase (mainly from
hemicellulose) as byproducts simultaneously.
In this study, sugar cane bagasse, one of the commonly used
biomasses in the furfural industry,7 was chosen as raw material
for one-pot production of furfural and cellulose-enriched
residue using AlCl3, FeCl3, and HCl as catalysts. The influences
of AlCl3, NaCl, and MTHF on furfural yield and enzymatic
accessibility to residue were studied (Scheme 1).
can reduce the active energy of xylan degradation in the
organosolv pulping process.11
Normally, furfural is produced with very low yield from the
reactions of lignocelluloses in the single aqueous system.12
Adding salts such as NaCl can significantly influence Bronsted
acid-catalyzed furfural production from xylose in both the single
aqueous system13,14 and the organic/aqueous biphasic
system.15 However, the role of NaCl in Lewis-acid-catalyzed
furfural production from actual biomass has not been studied.
Adding NaCl also improves 5-hydroxymethylfurfural (HMF)
production from glucose over AlCl3 catalyst,16 and accelerates
cellulose degradation in aqueous organic acid solution17 and
subcritical water.18 Therefore, new methods that avoid the
degradation of cellulose during furfural production over Lewis
acid catalyst in NaCl solutions are required to develop.
Adding organic solvent considerably promotes furfural yield
by reactive extraction, while the optional organic solvents could
be ethanol, methanol, γ-valerolactone, tetrahydrofuran, 2methyltetrahydrofuran (MTHF), and alkylphenols.15,19,20
Among these solvents, MTHF deserves particular attention
due to its high immiscibility with water, stability under acidic
environment, and easy recycle (boiling point about 80 °C).21
FeCl322 and SnCl423 catalyzed furfural production in the
MTHF/water biphasic system, and the reaction with the
assistance of NaCl was studied. Furfural was efficiently
extracted into MTHF, with its yield promoted from 34−50%
to 54−78%. After reaction, the MTHF phase rich in furfural
was easily separated from the aqueous phase containing catalyst
by decantation. Furfural could be purified after distillation of
MTHF, and the aqueous phase was reused without the loss of
activity.22 However, these studies use pure xylose and xylan or
hemicellulose hydrolysate as substrate rather than actual
lignocelluloses that may have different furfural yields.24
MTHF also effectively removes lignin, the third abundant
component in raw lignocelluloses that greatly restrains the
activity of cellulases by competitive adsorption during cellulose
saccharification.25 One-step fractionation of biomass in aqueous
oxalic acid/MTHF biphasic system provided cellulose with
■
EXPERIMENTAL SECTION
Materials. Bagasse was purchased from Dehong Liangheliliang
Biological Food Ltd. (Yunnan). It was air-dried, milled, and passed
through a 20-mesh sieve (<0.85 mm). It was air-dried in an oven
(WFO-710, EYELA, Tokyo Rikakikai Co., Ltd.) again at 105 °C for 4
h before use. Its composition was determined with two repetitions
according to the National Renewable Energy Laboratory (NREL)
procedure27 with (wt %) 38.3 ± 1.0 glucan, 20.4 ± 1.5 xylan, 3.1 ± 0.4
arabinan, 3.0 ± 0.2 manan, 22.5 ± 0.9 lignin, 2.1 ± 0.2 ash, and 8.4 ±
0.3 extractives. A detailed analytical procedure was described in
previous work.28 Microcrystalline cellulose Avicel PH101 was
purchased from Sigma-Aldrich (Shanghai). MTHF (>99%), HMF
(>99%), furfural (≥99.5%), cellubiose (≥99%), D-(+)-glucose
(≥99.5%), D-(+)-xylose (99%), L-(+)-arabinose (≥99%), and Dmannose (≥99%) were bought from Aladdin Factory Co., Ltd.
(Shanghai). AlCl3·6H2O (≥99%) and FeCl3·6H2O (>99%) were from
the Xilong Chemical Factory Co., Ltd. (Shantou, Guangdong).
Concentrated HCl (36−38%) was bought from Shandian Medicine
Co., Ltd. (Kunming, Yunnan). Cellulase (Celluclast 1.5L from
Trichoderma reesei ATCC 26921, ≥700 Ug−1) and β-glucosidase
(Novozyme 188 from Aspergillus niger) were purchased from SigmaAldrich (Shanghai). The activities of cellulase and β-glucosidase were
88.8 FPU/mL and 514 cellobiase unit (CBU)/mL, respectively,
determined by the method established by Ghose et al.29
Acid-Catalyzed Reactions and Analysis of the Filtrate. Acidcatalyzed reactions of bagasse were carried out in a microwave reactor
(Monowave 300, Anton Paar, Graz, Austria). For a typical reaction, 0.3
g dried bagasse, appropriate amount of chlorides [e.g., 0.217 g of
AlCl3·6H2O (0.1 M) and 0.9 g of NaCl (10 wt %)], 9 mL aqueous
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solution and 9 mL MTHF were added into a 30 mL quartz tube with a
magnetic stir bar stirring at 600 rpm. The tube with sample was heated
by microwave irradiation to 130−170 °C (5.6−13.1 bar) within 3 min
for 30 min reaction. After reaction, the tube was cooled down to 40 °C
within 5−10 min by pressurized air. The product mixture was taken
out from the tube and filtered through a 0.22 μm membrane.
Deionized water was used to wash the residue and dilute the filtrate to
150 mL in a homogeneous phase (MTHF and water were miscible).
The filtrate was analyzed by HPLC (LC-20A, Shimadzu) equipped
with Bio-Rad Aminex HPX-87H column, and 50 mM H2SO4 was used
as mobile phase at flow rate of 0.6 mL min−1. Oven temperature was
set at 60 °C. Furfural and HMF were determined by UV detector at
284 nm, while pentose and hexose in aqueous product were
determined by refractive index (RI) detector. Standard calibrated
curves for these products were determined with five points (0.05, 0.1,
0.2, 0.4, and 0.8 mg/mL, R2 > 0.999).28 The yields of furfural, HMF,
glucose, and xylose were defined as their weights in the filtrate divided
by their weight equivalents contained in bagasse:19
partition coefficient = (product concentration in organic phase)
/(product concentration in aqueous phase)
(7)
Haan et al. reported separation of furfural from a liquid aqueous
phase comprising furfural and organic acids.31 However, in this study,
after the removal of MTHF by vacuum rotary evaporator (RE-52AA,
Yarong biological and chemical instrument factory, Shanghai) at 30 °C
for about 30 min, products in the MTHF phase from the R−Al−Na−
MTHF reaction system (MTHF/AlCl3−NaCl aqueous solution at 150
°C for 45 min) were dissolved in CDCl3 for 13C NMR analysis on
Avance III 600 spectrometer (600 MHz, Bruker BioSpin GmbH,
Karlsruher, Germany) at 22 °C.
The solid residue was washed with 30 mL of distilled water 3 times
and dried in a freeze dryer (PDU-1200, EYELA, Tokyo Rikakikai Co.,
Ltd.) for 48 h for weight (residue yield) and composition
determination (the same method for raw bagasse), and subsequent
enzymatic hydrolysis.
Characterization of Bagasse and Residues. SEM (ZEISS EVO
LS10, Cambridge, U.K.) operated at 10 keV was used to observe the
morphology of the solid samples. The FT-IR measurement was
performed on a Nicolet is10 spectrometer (Madison, WI) with a
resolution of 4 cm−1 at 500−4000 cm−1.
Enzymatic Hydrolysis of Residue. Enzymatic hydrolysis was
carried out in 20 mL glass vials containing 2500 μL of 0.1 M citrate
buffer, 50 μL of 2% sodium azide, 75 mg of solid residue, 5−30 FPU of
cellulase, and 20 CBU of β-glucosidase. Deionized water was
supplemented to bring the total volume of reaction solution to 5000
μL in the vials. Temperature was kept at 50 °C with rotation rate at
150 rpm for hydrolysis in a thermostat incubator (SPH-100B,
Shanghai Shiping Laboratory Equipment Co., Ltd.). During hydrolysis,
a 200 μL sample was taken out of the vials by a 1 mL pipettor at 3, 6,
12, 24, and 96 h for glucose and cellubiose analysis by LC-20A HPLC
equipped with RI detector, using the same conditions as the above
analysis for the sugars. Glucose yield (wt %) was defined as follows:
furfural yield (wt %) =
(furfural weight in filtrate)/(pentosan weight in bagasse
× 1.375) × 100%
(1)
HMF yield (wt %) = (HMF weight in filtrate)
/(hexosan weight in bagasse × 1.286)
× 100%
(2)
glucose yield (wt %)
= (glucose weight in filtrate)/(glucan weight in bagasse × 0.90)
(3)
× 100%
xylose yield (wt %) = (xylose weight in filtrate)
/(xylan weight in bagasse × 0.88) × 100%
(4)
glucose yield (wt %)
= 0.90 × (glucose weight in hydrolysate)
residue yield (wt %) = (weight of recovered residue)
/(weight of bagasse) × 100%
■
(5)
Under these definitions, theoretical yield of all products were 100%.
CSF (combined severity factor) used in this study is defined as
CSF = log{t exp[(TH − 100)/14.75]} − pH
/(glucan weight in residue) × 100%
(8)
RESULTS AND DISCUSSION
Furfural Production from Bagasse Using Different
Acids as Catalysts. Time evolution of furfural production
from bagasse (about 0.3 g) was studied in 9 mL of aqueous
phase at 150 °C using different chloride salts as catalyst (Figure
1). Fixed moles of AlCl3 (0.1 M, pH = 3.4), FeCl3 (0.1 M, pH =
1.6), and HCl (0.3 M, pH = 0.52) were used to keep the
concentration of chloride ions the same for different reactions.
Furfural yields of 49.2% and 54.8% were obtained at 90 min
with FeCl3 and at 45 min with HCl, respectively. Surprisingly,
AlCl3 gave furfural yield of 35.5% at 30 min, much lower than
those with FeCl3 and HCl. However, severe degradation of
cellulose was observed using FeCl3 or HCl as catalyst, with
glucose yield of more than 20%. This was attributed to their
strong Bronsted acidity for a long reaction time.
The CSF used for furfural production was much higher than
that for hemicellulose removal and lignocellulose pretreatment,
so when the furfural yield reached the peak value the
hemicellulose removal procedure has already finished. Thus,
furfural production from lignocellulose in one pot often led to
severe decomposition of cellulose. To obtain a high yield of
furfural and high recovery of cellulose in residue simultaneously, both furfural yield and degradation products of
cellulose such as glucose and HMF should be monitored. In
the FeCl3-catalyzed system, the yield of furfural increased from
(6)
where t is the reaction time of treatment in minutes, TH is the reaction
temperature in °C, and pH is the acidity of the aqueous solution
before reaction.30
All of the experiments were repeated 2 times unless otherwise
mentioned, and the errors were 0.1−4.3% for furfural, 0.8−8.0% for
HMF, 0−6.3% for glucose, 0.3−5.8% for xylose, and 0.5−3.4% for
residue, respectively.
Solvent Separation and Recycle. MTHF was insoluble in water;
two phases were formed after MTHF was added to aqueous AlCl3.
Adding NaCl further decreased MTHF solubility in the aqueous
phase. So, the liquid product was centrifuged at 3000 rpm (relative
centrifugal force of 2013) for 5 min in a refrigerated centrifuge
(TG18G-II, Kaida Scientific Instrument Co., Ltd., Hunan). Then, the
MTHF phase (about 8.5 mL) was drawn out directly with a 5 mL
syringe; the salt-containing aqueous phase was separated from the
residue by filtration with a 0.22 μm filter membrane and can be reused
directly. Several runs were done to obtain enough used organic/
aqueous phase. To examine the reusability of the organic phase, 9 mL
of used MTHF was added to 9 mL of fresh aqueous phase. Similarly, 9
mL of used aqueous phase was added to 9 mL of fresh MTHF to test
the reusability of the aqueous phase. Also, 5 mL of used organic/
aqueous phase after each recycle was diluted to 150 mL using
deionized water to determine product concentration in both phases
and calculate the partition efficiency as follows:
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0.58 for FeCl3 and 0.39 for HCl at 45 min); (ii) the shortest
time for the completion of furfural production (30 min vs 90
min for FeCl3 and 45 min for HCl); and (iii) moderate pH
(3.4) as compared to that for FeCl3 (1.6) and HCl (0.52).
Role of NaCl in Furfural Production in the AlCl3
System. NaCl was found to significantly increase furfural
yield from xylose in Bronsted-acid-catalyzed reactions;14
however, the influence of NaCl on furfural production with
Lewis acid has not been studied yet. The effect of NaCl on
furfural production from bagasse with different catalysts was
studied in 9 mL of aqueous phase at 150 °C (Figure 2). In the
Figure 1. Furfural production vs time from raw bagasse in aqueous
solution with (a) 0.1 M FeCl3, (b) 0.3 M HCl, and (c) 0.1 M AlCl3
(0.3 g bagasse, 9 mL aqueous solution and magnetic stirring rate of
600 rpm at 150 °C; ◊ xylose; □ glucose; △ furfural; ○ HMF).
26.3% to 49.2% as time was prolonged from 45 to 90 min, and
changed slightly within 47.2−49.2% as time rose further to 180
min (Figure 1a). The yield of xylose was 53.9% at 45 min, but
gradually dipped to 10.1% at 180 min. The yields of glucose
and HMF at 90 min were 19.9% and 1.0%, respectively, and
rose to 25.5% and 1.6% at 120 min.
In the HCl-catalyzed system, the yield of furfural increased
from 18.3% to 54.8% as time was prolonged from 5 to 45 min,
but remarkably decreased to 44.5% at 120 min (Figure 1b).
The yield of xylose sharply jumped from 73.3% to 6.5% as time
increased from 5 to 75 min, while the yields of glucose and
HMF were 21.3% and 1.3% at 45 min, and 29.8% and 1.8% at
90 min, respectively.
In the AlCl3-catalyzed system, the yield of furfural increased
from 13.0% to 35.5% as time was prolonged from 5 to 30 min,
and slightly declined to 31.6% at 75 min (Figure 1c).
Simultaneously, the yield of xylose remarkably decreased
from 38.9% to 5.1% as time increased from 5 to 30 min, and
decreased further to 1.2% at 75 min. The yields of glucose and
HMF at 30 min were 0.7% and 5.6%, respectively. AlCl3
seriously catalyzed the degradation of glucose to HMF, which
made the glucose yield much lower than the HMF yield.
When the highest furfural yield was obtained, CSF values
were 1.8, 2.6, and −0.45, respectively, for FeCl3-, HCl-, and
AlCl3-catalyzed reaction systems. In comparison, the CSF value
applied for acid-assisted lignocellulose pretreatment varied from
1.0 to 2.0.30,32 Although CSF for FeCl3-catalzyed furfural
production was similar to that for traditional acid-assisted
pretreatment, it did cause severe degradation of cellulose,
suggesting that appropriate CSF may be different when
different acid species were used as catalysts.33 When HCl was
used as catalyst, CSF was much higher than that for
pretreatment and also caused severe degradation of cellulose,
in agreement with phenomena found in industrial production
of furfural. A very interesting result was found when AlCl3 was
used as catalyst. AlCl3 facilitated hemicellulose removal and
furfural formation even at a low CSF value of −0.45. In
summary, AlCl3 was more suitable to coproduce furfural and
cellulosic residue from bagasse, because it had three advantages:
(i) considerable furfural yield but the least degradation of
cellulose (yield ratio of glucose to furfural was 0.02 for AlCl3,
Figure 2. Influence of NaCl on the degradation of raw bagasse in the
aqueous phase with (a) 0.1 M FeCl3, (b) 0.3 M HCl, and (c) 0.1 M
AlCl3 (0.3 g bagasse, 9 mL aqueous solution and magnetic stirring rate
of 600 rpm at 150 °C; ◊ xylose; □ glucose; △ furfural; ○ HMF).
FeCl3-catalyzed reaction at 150 °C for 90 min (Figure 2a),
furfural yield slightly increased from 49.2% to 52.5% as NaCl
increased from 0 to 10 wt %, but decreased to 46.2−47.2%
when NaCl exceeded 10 wt %. However, the yield of xylose
substantially decreased from 25.6% to 5.1% as NaCl increased
from 0 to 20 wt %. Cellulose degradation was also promoted
when NaCl increased to 20 wt %, with the yields of glucose and
HMF increasing from 19.9% and 0.9% to 29.4% and 3.0%,
respectively.
In HCl-catalyzed reaction at 150 °C for 45 min, NaCl with
less than 10 wt % showed a slight influence on furfural
production with furfural yield of 54.8−56.6% (Figure 2b).
However, when NaCl increased from 10 to 20 wt %, furfural
yield remarkably decreased from 55.9% to 35.6%. Simultaneously, the degradation of xylose was substantially accelerated.
Xylose yield decreased from 24.0% to 7.8% when NaCl
increased from 0 to 5 wt %. Marcotullio et al.13 proposed that
the Cl− anion could promote the formation of 1,2-enediol, an
intermediate from xylose dehydration. The yields of glucose
and HMF were also promoted from 21.3% and 1.3% to 30.1%
and 2.4%, respectively, when NaCl increased to 10 wt %.
In the AlCl3-catalyzed system at 150 °C for 30 min, with
NaCl increasing from 0 to 20 wt %, furfural yield almost linearly
rose from 35.5% to 56.3%, while xylose yield slightly declined
from 5.1% to 3.1% (Figure 2c). It demonstrated that adding
NaCl significantly improved furfural production from xylose
but relieved the degradation of furfural. When 20 wt % NaCl
was added, furfural yield of 56.3% was achieved, which was
comparable to the optimized furfural yields obtained using
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almost constant until 170 °C (Figure 3a). Cellulose in
lignocellulosic materials contains both crystalline and amorphous parts. Hydrolysis of amorphous cellulose could be easily
achieved at low temperature of 150 °C, but the degradation of
crystalline cellulose needs much more severe conditions (e.g.,
>160 °C).17,34 So, the increase of HMF yield with temperature
was mainly from the degradation of amorphous cellulose. On
the other hand, the yield of solid residue continuously
decreased from 62.7% to 39.9% with temperature growing
from 130 to 170 °C. The selected temperature was 150 °C.
Under the fixed conditions (150 °C and 0.1 M AlCl3 without
NaCl), prolonging the reaction time from 15 to 45 min
significantly promoted the yields of both furfural and HMF
from 25.7% and 3.2% to 46.0% and 6.3%, respectively, and
changed little until 75 min (Figure 3b). The yield of solid
residue decreased from 64.3% to 53.8% with time increasing
from 15 to 45 min, and changed slightly until 75 min. The
selected time was 45 min.
Under the fixed conditions (150 °C and 45 min without
NaCl), AlCl3 concentration increasing from 0.03 to 0.1 M
significantly promoted furfural yield from 31.9% to 46.0%, but
excessive AlCl3 slightly influenced furfural yield (Figure 3c).
The yields of HMF and solid residue were slightly affected by
the variation of AlCl3 concentration. So, 0.1 M AlCl3 was
selected as the optimized concentration.
The influence of NaCl on furfural yield was optimized at 150
°C with 0.1 M AlCl3 for 45 min (Figure 3d). NaCl increasing
from 5 to 10 wt % promoted furfural yield greatly by 24% (from
48.2% to 60.0%), while NaCl concentration of <5 wt % or >10
wt % had a slight influence on furfural yield. The highest
furfural yield (62.2%) was obtained at 20 wt % NaCl, which was
comparable to the results (furfural yield of 55−66%) obtained
from different lignocellulosic biomass in a microwave-assisted
tetrahydrofuran/aqueous saturated NaCl system at 160 °C for
60 min in previous studies.9 High NaCl concentration in
aqueous solution had slight effects on the variation of HMF and
residue yields. Concentration of 10 wt % NaCl was selected as
the best value.
The influence of MTHF and NaCl was studied at 150 °C for
45 min using 0.1 M AlCl3 (Table 1). The total volume of
reaction liquor (including aqueous and organic phases) was
adjusted to 18 mL to keep the liquid/biomass ratio fixed (0.45
g of bagasse). Table 1 lists different systems with results and
reaction conditions: R-Al (18 mL of 0.1 M AlCl3 aqueous
solution at 150 °C for 45 min, where R means reaction), R-AlNa (18 mL of water with 0.1 M AlCl3 and 10 wt % NaCl), RAl-MTHF (9 mL of MTHF and 9 mL of water with 0.1 M
AlCl3), and R-Al-Na-MTHF (9 mL of MTHF and 9 mL of
water with 0.1 M AlCl3 and 10 wt % NaCl). In the AlCl3catalyzed reaction without NaCl, adding MTHF significantly
FeCl3 (52.5% at 150 °C for 90 min in aqueous solution
containing 0.1 M FeCl3 and 10 wt % NaCl) or HCl (56.6% at
150 °C for 45 min in aqueous solution containing 0.3 M HCl
and 10 wt % NaCl). More importantly, both glucose and HMF
yields from the degradation of cellulose were not stimulated by
NaCl.
In summary, furfural yields in the FeCl3- or HCl-catalyzed
system at 150 °C declined remarkably as NaCl exceeded 10 wt
%, but the degradation of cellulose was promoted. However, in
the AlCl3-catalyzed system, furfural production was significantly
enhanced by adding NaCl without serious degradation of
cellulose as both glucose and HMF yields were still low (<2%
glucose yield and about 7% HMF yield).
Role of MTHF and Furfural Production in MTHF/
Aqueous AlCl3 Biphasic System. Addition of organic solvent
could benefit furfural production20 by reactive extraction, while
MTHF efficiently promoted furfural yield catalyzed by FeCl322
or SnCl4.23 Production of furfural from bagasse was studied in
an MTHF/aqueous AlCl3 biphasic system (volume ratio of 1:1)
at 130−170 °C (Scheme 1). The influence of temperature,
time, AlCl3 concentration, and NaCl usage on the yields of
furfural, HMF, and residue in MTHF/aqueous AlCl3 biphasic
solution was studied by single-factor experiment (Figure 3).
Figure 3. Furfural, HMF, and residue yields changed with (a)
temperature, (b) time, (c) AlCl3 concentration, and (d) NaCl
concentration (0.3 g of bagasse, 9 mL of MTHF, 9 mL of aqueous
solution, magnetic stirring rate of 600 rpm; × residue; △ furfural; ○
HMF).
Under the fixed conditions (60 min and 0.1 M AlCl3 without
NaCl), temperature increasing from 130 to 150 °C significantly
promoted the yields of both furfural and HMF from 8.7% and
1.1% to 45.8% and 6.6%, respectively, while both yields stayed
Table 1. Influence of NaCl and MTHF on the Yields of Furfural and HMF
reaction conditiona
a
reaction syst
AlCl3
R-Al
R-Al-Na
R-Al-MTHF
R-Al-Na-MTHF
18 mL 0.1 M
18 mL 0.1 M
9 mL 0.1 M
9 mL 0.1 M
NaCl
MTHF
10 wt %
10 wt %
partition coefficientb
product yield (%)
9 mL
9 mL
HMF
4.3
7.0
6.1
7.9
±
±
±
±
0.2
0.3
0.1
0.1
furfural
org(furfural)/aq(furfural)
org(HMF)/aq(HMF)
±
±
±
±
6.3 ± 0.4
8.7 ± 0.3
1.4 ± 0.1
2.2 ± 0.1
30.6
50.0
43.6
58.6
0.3
0.6
0.3
0.5
a
R-Al, AlCl3 aqueous solution; R-Al-Na, AlCl3 and NaCl; R-Al-MTHF, MTHF and water with AlCl3; R-Al-Na-MTHF, MTHF and water with AlCl3
and NaCl (0.45 g of bagasse, 150 °C, 45 min and stirring rate of 600 rpm). bPartition coefficient was defined as product concentration in the organic
phase divided by that in the aqueous phase.
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Table 3. Recycle of Organic and Aqueous Phasesa in
MTHF/Aqueous AlCl3 System
promoted furfural yield from 30.6% to 43.6% (Table 1, R-Al vs
R-Al-MTHF) because the reactive extraction of furfural into the
MTHF phase reduced the degradation of furfural in the
aqueous phase. The concentration of furfural in MTHF phase
was 6.3 times that in the aqueous phase. Similarly, in the AlCl3catalyzed reaction with 10 wt % NaCl, furfural yield grew from
50.0% to 58.6% as MTHF was added (Table 1, R-Al-Na vs RAl-Na-MTHF). Simultaneously, the yield of solid residue
decreased from 62.6−63.0% to 54.7−54.9% with a slight
growth in HMF yield.
The results of NaCl influence in Table 1 were in accordance
with the results in Figure 2c. Adding 10 wt % NaCl in aqueous
AlCl3 solution resulted in the furfural yield greatly increasing
from 30.6% to 50.0%, while the yield of residue changed little
(Table 1, R-Al vs R−Al-Na). In MTHF/aqueous AlCl3 solution
(Table 1, R-Al-MTHF vs R−Al-Na-MTHF), NaCl improved
furfural yield from 43.6% to 58.6% with almost constant residue
yield of 54.7% because of the enhanced partition coefficient of
furfural in organic phase (Table 1, R-Al-MTHF vs R−Al-NaMTHF).4 On the other hand, HMF formation was also
promoted by NaCl, especially in aqueous solution.
In summary, to simultaneously achieve high furfural and
cellulosic residue yields, the suggested conditions for bagasse
conversion in MTHF/aqueous AlCl3 biphasic system were 150
°C, 45 min, 0.1 M AlCl3, and 10 wt % NaCl with furfural yield
of 60.0% and residue yield of 53.9%.These conditions were
further used for the following optimization of solid loading.
Influence of Solid Loading and Solvent Recycle on
Furfural Production. In a practical process, a relatively high
ratio of solid biomass to reaction solvent was always required to
achieve better economics, although it might affect the efficiency
of mass and heat transfer. Here, the influence of solid loading
on furfural yield was tested in MTHF/AlCl3 aqueous solution
at 150 °C for 45 min (Table 2). Other conditions were 9 mL of
product yield in MTHF
phase (%)
HMF
fresh MTHF
recycle 1
recycle 2
fresh aqueous
recycle 1
recycle 2
furfural
residue
0.3
0.45
0.6
8.6 ± 0.3
7.9 ± 0.1
7.4 ± 0.5
60.0 ± 0.4
58.6 ± 0.5
54.0 ± 0.8
53.9 ± 0.4
54.7 ± 2.5
57.2 ± 1.0
0.1
0.3
0.3
0.1
0.6
0.4
58.6
55.6
53.8
58.6
60.8
61.2
±
±
±
±
±
±
0.5
0.5
0.7
0.5
0.8
1.5
4.4 ± 0.2
8.0 ± 0.1
11.5 ± 0.4
Spent MTHF containing a high concentration of furfural from the
latest reaction was directly used in the next recycle without any
pretreatment. Spent aqueous solution containing residual sugars from
the latest reaction was directly used in the next reaction without
supplement of AlCl3 and NaCl, but another 0.45 g of bagasse was
added (0.45 g of bagasse, 9 mL of MTHF, 9 mL of aqueous solution,
150 °C, 45 min, magnetic stirring rate of 600 rpm, 0.1 M AlCl3, and 10
wt % NaCl).
products. The recycle of the aqueous phase containing residual
sugars from the latest reaction slightly promoted the furfural
yield in the next reaction. Furfural yields were enhanced from
58.6% to 60.8% and 61.2% with the first and second recycle of
the aqueous phase. HMF yield remarkably increased from 7.9%
to 9.9% and 11.1%, respectively, for the first and second recycle
of aqueous solution.
Influence of MTHF and NaCl on the Composition and
Structure of Residues. Besides a high furfural yield, it is
important to obtain a high cellulose yield that has good
structure for easily enzymatic hydrolysis. Table 4 showed the
glucan and xylan contents in solid residues as well as lignin
content and residue yields after different reactions under the
same conditions in Table 1 (150 °C, 45 min, 0.1 M AlCl3),
while the conditions for MTHF extraction were 0.45 g of
bagasse, 18 mL of MTHF, 150 °C, and 45 min. After reaction
in aqueous AlCl3 or aqueous AlCl3−NaCl solution, xylan
content in residues from R-Al and R-Al-Na systems was sharply
reduced from 20.4% to 3.5% and 2.8%, respectively. Glucan
content remarkably increased from 38.3% to 52.1−53.3%
because hemicellulose and extractives were removed from solid
residues. Adding MTHF did not influence the degradation and
removal of hemicellulose. However, glucan content in residues
from R-Al-MTHF and R-Al-Na-MTHF rose further to 63.2−
64.1%, indicating that other chemical compositions such as
lignin in raw bagasse were removed by MTHF. Nuclear
magnetic resonance (NMR) analysis of the organic phase after
reaction in MTHF/aqueous AlCl3−NaCl solution (R-Al-NaMTHF) demonstrated that some aromatic products were
indeed extracted by MTHF (Figure 4). For subsequent
enzymatic hydrolysis, the removal of lignin and other impurities
in bagasse might be more important than the reduction of
cellulose crystallinity. The crystalline structure of cellulose only
hindered the contact of cellulosic substrate with enzyme
molecules, and retarded the efficiency of enzymatic hydrolysis.
However, competitive adsorption by lignin and other impurities
might result in irreversible damage of enzymes.25 Extraction of
bagasse by MTHF without the assistance of AlCl3 could hardly
remove hemicellulose or lignin from bagasse to increase
cellulose concentration in the residues. So, the combination
of MTHF and aqueous AlCl3 corporately removed hemicellulose and lignin to yield a cellulose-enriched residue. On the
product yield (%)a
HMF
±
±
±
±
±
±
furfural concentration in
organic phase (g L−1)
a
Table 2. Influence of Solid Loading on Furfural Yield in
MTHF/Aqueous AlCl3 System
bagasse loading (g)
7.9
7.3
6.5
7.9
9.9
11.1
furfural
9 mL of MTHF, 9 mL of aqueous solution, 150 °C, 45 min, magnetic
stirring rate of 600 rpm, 0.1 M AlCl3, and 10 wt % NaCl.
a
MTHF, 9 mL of aqueous solution, magnetic stirring rate of 600
rpm, 0.1 M AlCl3, and 10 wt % NaCl. When bagasse loading
increased from 0.3 to 0.6 g, the yield of furfural remarkably
decreased from 60.0% to 54.0%, while residue yield increased
from 53.9% to 57.2%. Increased bagasse loading led to higher
furfural concentration both in the organic phase and in the
acidic aqueous phase. Exposure of furfural to acidic aqueous
phase accelerated furfural degradation to humins. Polymeric
soluble lignin also greatly affected furfural yield.35
Organic and aqueous phases were recycled 2 times (Table 3).
Increasing concentration of furfural in the spent MTHF
decreased the efficiency of furfural production in the next
recycle. Furfural yield decreased from 58.6% to 55.6% and
53.8% for the first and second recycle. However, high furfural
concentration of 11.5 g L−1 (>1 wt %, based on the mass of
organic solvent) was realized in the MTHF phase after 2
recycles, which benefited the post-treatment of organic
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Table 4. Residue Yields and Glucan Contents in Residues after Reactions
residue composition (wt %)
reaction systems
raw bagasse
MTHF extraction
R-Al
R-Al-Na
R-Al-MTHF
R-Al-Na-MTHF
reaction conditions
glucan
0.45 g bagasse, 18 mL MTHF, 150 °C, 45 min
0.45 g bagasse, 18 mL water, 0.1 M AlCl3, 150 °C, 45 min
0.45 g bagasse, 18 mL water, 0.1 M AlCl3, 10 wt % NaCl, 150 °C, 45 min
0.45 g bagasse, 9 mL MTHF, 9 mL water, 0.1 M AlCl3, 150 °C, 45 min
0.45 g bagasse, 9 mL MTHF, 9 mL water, 0.1 M AlCl3, 10 wt % NaCl, 150 °C,
45 min
38.3
40.4
53.3
52.1
62.5
63.7
±
±
±
±
±
±
1.0
0.8
0.5
0.7
0.7
0.4
xylan
20.4
21.2
3.45
2.8
2.6
3.2
±
±
±
±
±
±
1.5
2.0
0.3
0.2
0.2
0.2
lignin
22.5
23.4
35.2
36.1
25.2
26.7
±
±
±
±
±
±
0.9
1.2
0.6
1.0
1.4
0.7
residue yield
(wt %)
94.8
63.0
62.6
54.9
54.7
±
±
±
±
±
0.2
1.0
0.9
0.7
2.5
Figure 4. 13C NMR spectra of organic phase after reaction from R-Al-Na-MTHF system (600 MHz, CDCl3, and 22 °C).
polysaccharides.36 It could be concluded that after reaction in
aqueous AlCl3 or MTHF/aqueous AlCl3 solution, the carbonyl
and carboxyl stretching at 1715 cm−1 for the solid residues
became weaker, proving the removal of hemicellulose (mainly
containing acetyl group).38 Simultaneously, absorptions at
1370−1205 and 1160−1030 cm−1 were remarkably strengthened, indicating the substantial increase of polysaccharides in
the solid residues after reactions.38
Enzymatic Digestibility of Residues. Enzymatic hydrolysis of bagasse and residues after different reactions was carried
out with low cellulase loading of 10 FPU (filter paper unit)
(Figure 7). Cellulose in raw bagasse was hard to hydrolyze, with
low glucose yield of 16.2% obtained after 96-h reaction, similar
to previous studies (8.6−19.3% for 72-h enzymatic hydrolysis).39,40 At the initial stage (<6 h), glucose was released
rapidly from all substrates, and significantly slowed down until
96 h passed.40 After reaction in AlCl3 aqueous solution (R-Al
system), the residue yielded a high glucose yield of 48.1% after
96-h hydrolysis, mainly due to the removal of hemicellulose.
Adding NaCl in the R-Al system suppressed glucose yield to
43.6% after 96 h. The negative effect of NaCl on enzymatic
digestibility of residues was caused by (i) acceleration of
pseudolignin formation via the condensation of furfural with
high concentration,41,42 and (ii) more recalcitrant structure of
other hand, 10 wt % NaCl in aqueous AlCl3 or MTHF/aqueous
AlCl3 solution showed less influence on the variation of residue
composition (Table 4), although the furfural production was
promoted (50.0−58.6% with NaCl vs 30.6−43.6% without
NaCl in Table 4). In conclusion, reaction of bagasse in MTHF/
AlCl3−NaCl aqueous solution produced not only high furfural
yield of 58.6%, but also led to a high glucan content of 63.7% in
the residue (residue yield 54.7%).
Scanning electron microscopy (SEM) images (Figure 5)
showed that, after treatment with aqueous AlCl3 or MTHF/
aqueous AlCl3 solution, the surface of residues became neater
with fewer deposits on the parallel cellulosic microfibers,
because of the removal of hemicellulose and lignin. Fourier
transform infrared spectroscopy (FT-IR, Figure 6) illustrated
that all samples had absorptions at 3410−3460 and 2896−2905
cm−1 corresponding to the stretching of the −OH group and
C−H bonds of alkyl group, respectively.36 On the basis of ref
37, the band around 1715 cm−1 was from carbonyl and carboxyl
stretching. The bands at 1605 and 1512 cm−1 were from the
skeletal and stretching vibration of benzene rings. Absorptions
at 1370−1205 cm−1 were mainly from the bending of C−H or
O−H bonding in polysaccharides, including cellulose and
hemicellulose; those absorptions at 1160−1030 cm−1 were
from the bending of C−O or C−O−C linkages in
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Figure 5. SEM images of original bagasse and residues after reactions.
Figure 6. FT-IR spectra of bagasse and residues after reactions (a, wavenumber at 400−4000 cm−1; b, wavenumber at 800−2000 cm−1).36
(58.4% vs 81.0%) than that from enzymatic hydrolysis of Avicel
PH101 cellulose, which might be attributed to high lignin
content in residues that inhibited enzymatic hydrolysis of
cellulose. Adsorption of cellulases by lignin decelerated
cellulose degradation.45 As compared with other studies
where bagasse was pretreated with the organosolv method,
this glucose yield was also lower. Agnihotri et al.46 reported
almost complete conversion of cellulose for bagasse pretreated
at 210 °C for 90 min in 50:50 (v/v)% ethanol/water media
lignin in the raw material formed during the chemical
degradation reaction.43,44 Adding MTHF to the AlCl3−NaClcatalyzed system further improved the enzymatic hydrolysis of
residue, with glucose yield enhanced to 58.4% after 96 h.
MTHF effectively removed lignin, which relieved the physical
barrier and nonproductive bonding of lignin to cellulases, and
increased glucan content in the pretreated residue. Lignin
content in residue decreased from 36.1% to 26.7% when
MTHF was added. However, this glucose yield was still lower
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53.8% for the second recycle of organic phase and 61.2% for the
second recycle of aqueous phases. Enzymatic hydrolysis of
cellulosic residue was also remarkably enhanced with the
highest glucose yield of 58.4% at 50 °C for 96 h [10 FPU of
cellulase and 20 CBU (cellubiase unit) of β-glucosidase], 3.6
times that from raw bagasse. After cycles, the organic phase was
decanted and distilled to separate furfural and lignin and obtain
purified MTHF. Lignin and furfural concentrations in the
aqueous phase were low; they tended to solubilize in the
organic phase. After many cycles, the aqueous phase could be
purified by extraction with the organic phase.
■
Figure 7. Enzymatic hydrolysis of cellulosic residues (75 mg solid
residue, 10 FPU cellulase, 20 CBU β-glucosidase, 50 °C, and rotation
rate of 150 rpm).
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Website: http://biomassgroup.njau.edu.cn/.
using pH 3.5 formic acid as catalyst. Considering the higher
enzyme loading applied in their study (30 FPU of cellulase, 32
pNPGU of β-glucosidase per gram of cellulose), we
investigated the influence of different cellulase dosage on the
enzymatic hydrolysis of residue. Increasing cellulase loading
often improved enzymatic hydrolysis rate.47 When cellulase
loading increased from 5 to 10, 15, and 30 FPU, glucose yield
sharply increased from 45.4% to 58.4%, 68.8%, and 89.3%,
respectively, after 96-h digestion (Figure 8), indicating that high
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors wish to acknowledge financial support from
Nanjing Agricultural University (68Q-0603), and the Yunnan
Provincial Government (Baiming Haiwai Gaocengci Rencai
Jihua).
■
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■
CONCLUSION
Conversion of bagasse in aqueous solution with FeCl3 and HCl
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DOI: 10.1021/acssuschemeng.6b01847
ACS Sustainable Chem. Eng. 2016, 4, 5804−5813