Conversion of C5 carbohydrates into furfural catalyzed by a Lewis acidic ionic liquid in renewable γ-valerolactone

Article abstract of DOI:10.1039/c7gc01298e

For the purpose of building a green reaction system to produce furfural (FF), the conversion of two important pentoses from hemicellulose, namely xylose and arabinose, was investigated in an aqueous reaction system including a Lewis acidic ionic liquid as a catalyst and renewable γ-valerolactone (GVL) as a co-solvent. The results showed that the introduction of GVL greatly improved the reactivity of pentose and inhibited the secondary decomposition reaction of FF compared to a pure-water reaction system. NMR analysis suggested that the composition of pentose conformers was greatly altered towards a reactive distribution. The highest FF yields were 79.76% (from xylose) and 58.70% (from arabinose), which were obtained at 140 °C. The influence of reaction parameters on pentose conversion was also studied. A comparison between different reaction conditions suggested that arabinose had less reactivity than xylose, leading to its lower conversion rate and FF yield. Furthermore, xylan and real biomass materials were tested in the proposed reaction system, and decent FF yields of up to 69.66% (from xylan) and 47.96% (from corn stalk) were obtained.

Full text of DOI:10.1039/c7gc01298e

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Conversion of C5 carbohydrates into furfural catalyzed by a Lewis
acidic ionic liquid in renewable γ-valerolactone
Received 00th January 20xx,
Accepted 00th January 20xx
*
a
a
a
a
a
a
Shurong Wang, Yuan Zhao, Haizhou Lin, Jingping Chen, Lingjun Zhu and Zhongyang Luo
DOI: 10.1039/x0xx00000x
For the purpose of building a green reaction system to produce furfural (FF), the conversion of two important pentoses
from hemicellulose, namely xylose and arabinose, was investigated in an aqueous reaction system including Lewis acidic
ionic liquid as catalyst and renewable γ-valerolactone (GVL) as co-solvent. The results showed that the introduction of GVL
greatly improved the reactivity of pentose and inhibited the secondary decomposition reaction of FF compared to pure-
water reaction system. NMR analysis suggested that the composition of pentose conformers was greatly altered towards a
reactive distribution. The highest FF yields were 79.76% (from xylose) and 58.70% (from arabinose), which were obtained
at 140 °C. The influence of reaction parameters on pentose conversion was also studied. Comparison between different
reaction conditions suggested that arabinose had less reactivity than xylose, leading to its lower conversion rate and FF
yield. Furthermore, xylan and real biomass materials were tested in the proposed reaction system, and decent FF yields of
up to 69.66% (from xylan) and 47.96% (from corn stalk) were obtained.
www.rsc.org/
formation of FF, because the solvent effects have great
influence on the thermodynamics and kinetics of the reaction
Introduction
1
1
process. Water is a typical green solvent that is extensively
utilized in large-scale FF production as a reaction medium.
However, water is also a polar protic solvent, which can easily
Biomass is a renewable carbon-containing resource in nature.
It is abundant in source and can be converted into various
liquid fuels. Therefore, the development of biomass energy is
1
2
1
-3 lead to the occurrence of side reactions and low FF yield. In
order to solve this problem, biphasic solvent system including
water and water-insoluble organic solvent is adopted to
prevent the consumption of FF in side reactions by means of
simultaneously extracting FF from water to the organic
very important for the substitution of traditional fossil fuels.
Hemicellulose is one of the three major components in
4
biomass, of which it accounts for about 15-30%. Xylose and
arabinose are important pentoses existing in the monomeric
5
composition of hemicellulose, and they make up over 50%
1
3
phase. In recent years, single phase solvent made up of
water and organic solvent is receiving more and more
attention. In particular, the introduction of renewable organic
solvent further promotes the development of green reaction
and around 10% of hemicellulose for most biomass,
6
respectively. A typical dehydration product of these two
pentoses is furfural (FF), which can be used to produce a series
of high value-added chemicals and novel liquid fuels like γ-
valerolactone (GVL), tetrahydrofuran (THF) and 2-methylfuran.
Hence FF was listed as one of the value-added platform
1
3, 14
system.
GVL is a downstream product from FF, and it is
widely recognized as a novel and renewable solvent, attracting
1
5-17
7
-9
the attention from a number of researchers.
In addition,
chemicals produced from biomass. In current commercial FF
production technology, mineral acids such as sulfuric acid and
hydrochloric acid are usually chosen as catalysts, and the
process still has several problems including low FF yield and
the boiling point of GVL (207°C) is higher than that of FF
(
162°C), so FF can be easily isolated from GVL with the method
1
of (vacuum) distillation.
1
1
0
Recently, ionic liquids were selected as reaction media for
the production of FF and excellent results were achieved,
because ionic liquids have good solubility for biomass
carbohydrates, and they can also effectively stabilize certain
intermediates during the reaction process. Thus the
equipment corrosion. For this reason, the development of
green catalytic reaction system with high efficiency for the
production of FF has been the focus of much attention in the
past few years.
Reaction medium is an important factor that affects the
1
8
conversion rate and FF yield were greatly improved. Much
research suggested that FF yield from xylose could reach up to
1
80-90% when ionic liquids were used as solvent. For example,
8
a.
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou,
310027, China.
E-mail: srwang@zju.edu.cn
Tel: +86 571 87952801; Fax: +86 571 87951616
Zhang et al. selected AlCl as catalyst and [bmim]Cl as solvent
3
19
and carried out xylose conversion experiments. They finally
†
Electronic Supplementary Information (ESI) available: [details of any obtained a remarkable FF yield of 84%. In spite of this, ionic
supplementary information available should be included here]. See
liquids have high prices and viscosity, and it is pretty difficult
DOI: 10.1039/x0xx00000x
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for the products to be separated from the ionic liquid solvent. from a local timber mill in Zhejiang province, China. The
Therefore, the large-scale utilization of ionic liquids as a sources of all these three kinds of biomass are widely
reaction medium is still limited. Nevertheless, ionic liquid can distributed in China. The raw biomass was washed and dried at
behave as not only a solvent but also a catalyst. For instance, 110 °C for 24 h. Prior to the experiments, all the biomass
Lima et al. performed the conversion of xylose in [emim]HSO
4
samples were ground and sieved to 60–80 mesh.
ionic liquid without additional catalysts and a FF yield of 62% Compositional analysis (cellulose, hemicellulose, lignin,
0
2
was obtained. The use of ionic liquids as catalysts will extractives, and acid-insoluble ash) was performed with a fiber
significantly decrease the reaction cost compared with when it analyzer (ANKOM 200i Fiber Analyzer, ANKOM, USA) according
2
7
is used as solvent. Besides, ionic liquids possess satisfactory to the Van Soest method.
stability and low corrosivity as catalysts. Matsagar et al.
Procedure for Conversion of Carbohydrates to FF
system using several kinds of ionic liquid as catalysts. Among In a typical experiment for FF production, 1 mmol sugar and 5
all the catalysts [C SO Hmim][HSO showed better mL solvent were charged into a glass pressure reactor (15 mL,
performance and a maximum FF yield of 81% was obtained at Synthware) with a catalyst dosage of 0.1 M. The reactor was
70 °C when the reaction lasted for 5 h. The above research heated in an oil bath, and the magnetic stirring rate was
reported the conversion of xylose in water-MIBK solvent
2
1
3
3
4
]
1
suggests a promising strategy for building a sustainable green maintained at 600 rpm. When the reaction temperature and
reaction system, in which FF is produced from xylose through time reached preset values, the glass reactor was taken out
catalytic dehydration in a renewable organic solvent with ionic from the oil bath and was cooled by air flow. The solution in
liquids as catalysts.
the glass reactor was filtered through a 0.22-μm syringe filter
Lewis acidic ionic liquids received much in-depth prior to high-performance liquid chromatography (HPLC)
investigation in recent years due to its fine physicochemical analysis. Each experiment was repeated three times.
2
2
properties. This kind of ionic liquid is produced by mixing
metallic Lewis acid and organic halide salt (usually an
High Performance Liquid Chromatography (HPLC) Analysis
imidazolium or pyridinium halide salt) in certain ratios to form The filtered solution was analyzed on a Dionex HPLC system
23
the required composition. Among all of the Lewis acidic ionic equipped with a Bio-Rad Aminex HPX-87H column and an RI
liquids, those with imidazolium group are the most suitable for 2000 refractive index detector. A H SO solution (pH 2.5) was
FF production and attracted lots of attention from many used as the mobile phase, and its flow rate and corresponding
2
4
1
8
scholars . Some researchers studied the xylose conversion column temperature were kept at 0.6 mL/min and 60 °C,
process to produce FF with AlCl as a Lewis acidic catalyst and respectively. The concentrations of xylose, arabinose, and FF
3
1
9, 24
decent results were achieved.
3
However, AlCl is much were determined by comparison against standard calibration
2
5
harder to be recycled compared with ionic liquid catalysts. In curves. The conversion rate of pentose (xylose and arabinose)
our previous study, we have already investigated the and the yield of FF were calculated as follows:
dehydration reaction of pentoses using Brønsted acidic ionic
2
6
moles of pentose reacted
liquids as catalysts and obtained decent results. In this paper,
we further investigated the application of Lewis acidic ionic
liquid as catalyst for the conversion of two important pentoses
from hemicellulose, namely xylose and arabinose, in a green
solvent system of GVL and water. The whole research aims at
revealing the effects of solvent composition, reaction
temperature, catalysts dosage and substrate concentration on
the conversion of the pentoses and the yield of FF.
pentose conversion rate=
×100%
moles of starting pentose
moles of FF formed
FF yield (from sugar)=
×100%
moles of starting pentose
moles of FF formed
FF yield (from biomass)=
×100%
moles of pentose in biomass
1
3
C-NMR Analysis
Experimental Section
The tautomeric composition of D-xylose and D-arabinose was
1
3
analyzed by
C-NMR on an Agilent 600 MHz DD2
Materials
spectrometer. Tetramethylsilane was dissolved in D O and
2
Xylose, arabinose, FF and GVL were purchased from Aladdin
filled into a sealed capillary. The sealed capillary was then put
into the NMR tube for the purpose of internal calibration and
1
3
Industrial Corporation. 1- C-labelled xylose and arabinose
were purchased from Shanghai ZZBIO. Co. Ltd. (Shanghai,
China). The Lewis acidic ionic liquid 1-butyl-3-
1
the lock of magnetic field. Solutions of 0.10 M 1- C-labelled
3
1
3
xylose and 1- C-labelled arabinose were prepared in the
solvent mixture with a GVL-water ratio of 4:1. Then the
solutions were injected into different NMR tubes for following
3
methylimidazolium chloroaluminate ([bmim]Cl-AlCl , IL) was
purchased from Shanghai Chengjie Chemical Corporation. All
of these chemicals were analytical grade and used without
further purification. Xylan (from beechwood, >90% xylose
residues) were purchased from Sigma–Aldrich. Corn stalk (CS)
was obtained from a local farm in Zhejiang province, China.
Eucalyptus saligna (ES) and Tsuga chinensis(TC) were obtained
1
3
analysis. C-NMR chemical shifts at 105.36 ppm, 100.09 ppm,
8.92 ppm, 95.38 ppm were assigned to the C1 anomeric
9
carbon in β-xylofuranose, β-xylopyranose, α-xylofuranose and
α-xylopyranose, respectively. The chemical shifts at 104.96
ppm, 100.03 ppm, 98.69 ppm and 95.73 ppm were addressed
2
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to the C1 anomeric carbon in α-arabinofuranose, α- relatively high activation energy (approximately 30 kcal/mol)
arabinopyranose, β-arabinofuranose and β-arabinopyranose, that the conversion from xylose to FF needed in pure-water
3
0
respectively. The NMR spectra were described in ESI.
reaction system with acidic catalysts. After the addition of
GVL into the solvent, it was obvious that the conversion of
xylose was significantly promoted along with the increase of
Characterization of Pentoses in Biomass
The pentoses in the biomass samples were released with a GVL-water ratio. When the reaction lasted for 30 min, the
2
8, 29
modified Saeman hydrolysis method.
building blocks of the biomass were released by successive However, in the meantime, the conversion rate of xylose
hydrolysis in 72% H SO for 3 h at 20 °C and in 1 mol/L H SO reached 94.62% at a GVL-water ratio of 19:1. This was owing
for 2 h at 100 °C. The obtained sugar solution was treated with to the higher reactivity of acidic proton relative to the
The monosaccharide conversion rate of xylose was as low as 69.13% at ratio 1:1.
2
4
2
4
2
-
Ba(OH)
2
until neutral pH was reached so as to precipitate SO
4
protonated transition states with the addition of GVL
to avoid its influence on the following measurements. The compared to pure-water solvent, and therefore the apparent
obtained neutral monosaccharides were quantitatively activation energy of the conversion from xylose to FF was
detected by HPLC described above.
greatly reduced, leading to an accelerated reaction rate for the
3
1
acid-catalyzed xylose conversion. When the reaction time
prolonged to 240 min, xylose nearly completely decomposed
at each GVL-water ratio, with all of the conversion rates higher
than 99%.
Results and Discussion
Effect of Solvent Composition
The presence of GVL also had profound effects on FF
formation. In a certain range of GVL-water ratio, the FF yield
increased when the ratio became higher. After 30 min since
the reaction started, the yields of FF in the solvent with
different GVL-water ratios were 26.86% (ratio 1:1), 41.21%
Figure 1 shows the decomposition and conversion results of
xylose catalyzed by Lewis acid at 140°C in solvents with
different GVL-water ratios. When the reaction occurred in
pure-water solvent (ratio 0:1), the FF yields were less than 30%,
and the corresponding time-dependent curve was not drawn
in Figure 1. The conversion rate of xylose reached 99.80% in
pure water after 240 min since the reaction started, whereas
the FF yield was only 29.05%. This might be due to the
(ratio 2:1), 61.64% (ratio 4:1) and 60.86% (ratio 19:1),
indicating that an adequate amount of GVL favored the
formation of FF. The reason might be that GVL inhibited the
occurrence of secondary reactions involving FF. Gürbüz et al.
studied the conversion of xylose in a similar solvent system
(
GVL:water 9:1) and also found that the FF yield achieved is
1
5
significantly lower in pure water than in the mixed solvent.
This was because the presence of water could promote several
FF degradation reactions, which mainly include fragmentation,
32
resinification, and condensation. In fragmentation reactions,
FF became several smaller molecules such as formic acid,
formaldehyde and acetaldehyde. In resinification reactions,
two FF molecules reacted with each other, while in
condensation reactions FF coupled with pentose molecules or
intermediates. These disadvantageous reactions were proved
1
5
to be hindered in GVL by Gürbüz et al. On the other hand,
water is a polar protic solvent with very high polarity so that
the formed FF could easily degrade to produce byproducts in
pure-water solvent, but GVL served as a polar aprotic solvent
which inhibited these negative reactions and raised the
3
1
activation energy for FF degradation. The apparent activation
energy for FF degradation was calculated to increase from 85
-1
-1
kJ mol to 105 kJ mol when GVL was used as the solvent
3
1
instead of water.
When the reaction continued for longer time, the
formation of FF presented different trends at different GVL-
water ratios. At the ratio 19:1 the FF yield continuously
decreased with the time going by. In other solvent
composition with the existence of GVL, the FF yield rose first
and then showed a gradual descent, and it reached the
maximum value 75.43% at ratio 4:1 after the reaction lasted
for 120 min. This declining tendency was probably caused by
low water content in the reaction system when the GVL
concentration was relatively high, which resulted in the
Figure 1 - Effect of solvent composition on xylose conversion and FF yield (140 °C, 0.1 M
IL, 30 mg/mL xylose)
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hindered dehydration process from xylulose (an intermediate)
Journal Name
3
3
to FF. Besides, De et al. came up with the catalytic
mechanism of AlCl as a Lewis acid that it would first be
hydrolyzed to become a metal complex ion [Al(OH)(H
3
2
5
]
+
2
O)
+
and H , which could behave as Lewis acid and Brønsted acid,
3
4
respectively. If the water concentration is considerably low, it
will be hard for AlCl to hydrolyze. In addition, the amount of
water formed from xylose dehydration is much less than the
3
1
5
original water that exists in the solution, and therefore the FF
yield at a GVL-water ratio of 19:1 was lower than at other
ratios.
Opposite results were found in our previous research at
the ratio of 19:1, where we used Brønsted acidic ionic liquids
2
6
as catalysts to convert xylose into FF. In the previous
research, the FF yield at ratio 19:1 was higher than all the
other solvent compositions, and it did not begin to decrease
until 180 min. However, GVL presented more powerful
inhibition effects on the xylose dehydration at ratio 19:1 in this
research where we used Lewis acidic catalyst instead of
Brønsted acidic one, which could be explained by the different
catalytic mechanisms that the two kinds of catalyst possessed.
Choudhary et al. proposed that there were two reaction
pathways for xylose conversion when catalyzed by Lewis acid
3
3
or Brønsted acid. One was direct dehydration of xylose to
lose three molecules of water with Brønsted acid as catalyst
and then FF was produced. The other included two steps:
xylose firstly became xylulose via isomerization catalyzed by
Lewis acid, and subsequently xylulose lost three molecules of
water to form FF with the presence of Brønsted acid. In this Figure 2 - Effect of solvent composition on arabinose conversion and FF yield (140 °C,
research, the Lewis acidic catalyst containing metallic chloride 0.1 M IL, 30 mg/mL arabinose)
+
was adopted so that the necessary Brønsted acid (H ) could
and 30.25% (ratio 19:1). The highest FF yield of 58.23% was
3
+
only be supplied from the hydrolyzation of the metal ion (Al ).
Hence the reduced water content caused by a large sum of
GVL addition was adverse to the xylose dehydration process.
As shown in Figure 2, similar to the reaction rule of xylose,
arabinose was rapidly converted in pure-water solvent in a
short period of time at the same temperature. The conversion
rate of arabinose reached 69.94% after 30 min, and as the
reaction continued the conversion rate further increased to a
maximum value of 98.00%. The yield of FF from arabinose first
appeared as an ascent and then decreased as the reaction
time prolonged, and a maximum FF yield of 33.12% was
obtained when reaction lasted for 120min. The conversion rate
of arabinose in a short time space from the beginning rose
dramatically with the increase of GVL ratio. Especially, when
the GVL-water ratio was 19:1, the arabinose conversion
jumped up rapidly to over 95% within 30 min and maintained
at around 98% after 60 min. This trend indicated that the
existence of GVL might also decrease the activation energy
related to the arabinose conversion as well as increase the
reaction rate.
achieved at ratio 4:1 when arabinose reacted for 120 min. In-
depth analysis of the FF yields in solvents with different
compositions suggested that large amounts of FF underwent
secondary reactions to form unwanted chemicals, because at
each ratio FF yield showed a decline when the reaction time
was relatively long. For instance, at ratio 19:1 the FF yield
dropped all the time from the very beginning of the reaction,
and it reached as low as 24.08% after 240 min. Wijittra et al.
illustrated in their research that when producing FF from
pentoses, FF would decomposed constantly throughout the
35
reaction process so that the overall yield was lessened. When
compared with Brønsted acidic ionic liquid, the yield of FF from
arabinose catalyzed by Lewis acidic ionic liquid remarkably
26
increased from 24.46% to 58.23%. Arabinose was likely to
undergo two different reaction pathways when catalyzed by
Lewis acid or Brønsted acid, which might be similar to the
33
reaction pathways of xylose proposed by Choudhary et al. In
this way, a reasonable conclusion could be reached: the direct
dehydration of arabinose catalyzed by Brønsted acid needed
pretty high activation energy and reaction temperature, while
the isomerization catalyzed by Lewis acid and subsequent
dehydration to form FF required much lower activation energy.
Hence the reaction process was dramatically promoted when
using Lewis acidic catalyst.
Moreover, the addition of GVL into the reaction system
significantly accelerated the formation of FF, and within a
certain range of GVL-water ratio FF yield would increase as the
ratio became higher. After 30 min since the reaction started,
the yields of FF in the solvent with different GVL-water ratios
were 30.79% (ratio 1:1), 42.24% (ratio 2:1), 43.05% (ratio 4:1)
4
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For both xylose and arabinose, their tautomeric deemed that the steric positioning of the hydroxyls in the
compositions were found to be altered by GVL, and the reactive forms of aldopentoses was an important controlling
conformers in the solvent of GVL-water (4:1) were identified to factor of FF formation rate. Furthermore, they compared the
be more reactive than in pure water, as shown in Table 1. reactivity of different aldopentoses by means of reaction
There has been much research about anomeric equilibria of kinetic analysis, and the results revealed that arabinose had
xylose and arabinose in aqueous solution, and all the results the lowest reactivity as well as the reaction rate constant
showed that the cyclic form (pyranose and furanose) being only half of that of xylose.
dominated in the conformers of these two pentoses for over
3
6-39
Effect of Reaction Temperature
99%.
Moreover, no peaks were found near the possible
chemical shifts for the acyclic forms in the NMR spectrum. Figure 3 depicts the influence of reaction temperature on
Therefore, only the four cyclic kinds of conformer were taken xylose conversion and FF formation. Only a little xylose
into account in this study. Angyal calculated the converted at 120 °C after reaction proceeded for 30 min,
conformational free energy for D-aldopyranoses and indicated whereas the conversion rate jumped up to 93.86% after xylose
that stable conformers had lower free energy than active reacted for 240 min. The corresponding FF yield after 240 min
36
ones. Xylose existed in water mainly as pyranose, in which arrived at a highest value of 63.55%. When the reaction
the α form and the β form accounted for 36.48% and 61.95%, temperature further increased, the conversion rate of xylose
3
respectively. The β form was more stable than the α form displayed a significant ascent to over 90% within a short time.
8
1
3
according to their free energies. However, based on the C- The highest yields of FF at increasing temperatures were
NMR analysis results in this study, the composition was 69.05% (130 °C, 180 min), 75.43% (140 °C, 120min) and
reversed in GVL-water solution, where the β form decreased 71.14% (150 °C, 60min). All of these results suggested that
to 30.85% while the α form increased to 67.24%. Hence it reaction temperature had considerable effects on the
could be concluded that GVL favored the reactive forms of conversion of xylose, and high temperature could shorten the
xylose, and therefore the FF yield was enhanced. As for time that FF yield needed to reach the maximum value, which
arabinose, the solvent effect on its conformation was similar was consistent with what Choudhary et al. found in their
3
3
to xylose. The content of α-arabinopyranose and β- research. Although high reaction temperature could cut
arabinopyranose in GVL-water solvent was opposite to that in down the time within which xylose converted to FF, the
aqueous solution. The latter was more reactive than the
former and its proportion increased from 32.23% to 38.19%.
Additionally, it was obvious that the amount of
arabinofuranoses also increased in GVL-water solvent. In
general, the pyranose form was less active than the furanose
0, 41
4
form.
In this way, the reactivity of arabinose was further
improved in GVL-water solvent.
Through comparison between the conversion of the two
pentoses in the same reaction system, it could be found that
the yield of FF from xylose had a higher maximum value of
7
5.43% than that of 58.23% from arabinose, which was in
4
2
accordance with the results observed by Garrett et al. They
Table 1 Percentages of cyclic forms of D-xylose and D-arabinose and
their conformational free energies
b
Percent in solution
in GVL-water in water
Free energy
Compound
a
(kcal/mol)
D-xylose
α-xylopyranose
β-xylopyranose
α-xylofuranose
β-xylofuranose
D-arabinose
67.24 (±0.23) 36.48 (±0.33)
30.85 (±0.19) 61.95 (±0.39)
00.97 (±0.02) 00.86 (±0.06)
00.94 (±0.10) 00.69 (±0.07)
1.9
1.6
α-arabinopyranose
β-arabinopyranose
α-arabinofuranose
β-arabinofuranose
a
43.33 (±0.13) 58.68 (±0.15)
38.19 (±0.21) 32.23 (±0.15)
12.13 (±0.06) 05.60 (±0.00)
06.35 (±0.11) 03.55 (±0.06)
1.95
2.2
Figure 3 - Effect of reaction temperature on xylose conversion and FF yield (GVL-water
4:1), 0.1 M IL, 30 mg/mL arabinose)
Data from ref. 39. The acyclic conformers accounted for less than 0.20% and
b
(
were not listed. Data from ref. 36.
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secondary reactions involving FF were also favored leading to high reaction temperature not only accelerated the reaction
decreased FF yield. Besides, when the reaction time was pretty process but also promoted the negative consumption of FF.
long at high temperature, some insoluble humin was formed in
By comparison of the conversion rules between xylose and
the solution and on the inner surface of the glass reactor, arabinose at different temperatures, it was obvious that
which mainly came from the condensation reactions between increased temperature greatly enhanced the conversion of
4
6
xylose, active intermediates and FF and the further xylose, which corresponded to the results from Danon et al.
4
3-45
aromatization of polymers.
In spite of this, it was observed They demonstrated that the activation energy required for
from the experiment that the amount of humin was more in xylose dehydration was higher than that of arabinose,
pure-water solvent than in GVL-water solvent, and therefore it resulting in the more dependence of xylose on reaction
could be inferred that the existence of GVL was able to impede temperature than that of arabinose. The variation rules of FF
the formation of insoluble humin to a certain degree.
yield with different temperatures for the two pentoses are
As for arabinose, the effect of reaction temperature on its similar, and the yield of FF from arabinose is generally lower
conversion and FF yield was described in Figure 4. At 120 °C than that of xylose, which was due to its low reactivity
the conversion rate of arabinose rose dramatically as the discussed above.
reaction time went by, from 48.53% after 30 min to 98.52%
Effect of Catalyst Dosage
after 240 min. In contrast, when the temperature went up to
130 °C, 140 °C and 150 °C, the conversion rates of arabinose
Figure 5 showed the effect of catalyst dosage on pentose
reached high values at the very beginning, and as the reaction conversion and FF yield at 140 °C after the pentoses reacted
continued they just showed a slight increase for less than 15%. for 120 min. It has been reported that ionic liquids might
4
7-50
At each temperature, the yield of FF first presented a growth decompose at elevated temperatures,
and their
and then a decline as the reaction proceeded because of the decomposition kinetics was commonly measured by
5
1, 52
occurrence of secondary reactions involving FF. At 140 °C the thermogravimetric analysis.
highest yield of FF was 58.23% when the reaction lasted for analysis for the ionic liquid used in this research showed no
20 min, while at 150 °C the FF yield stopped going up after 60 evident decomposition at the temperatures below 150 °C (see
However, thermogravimetric
1
min and appeared a decreasing tendency afterwards. This ESI), which guaranteed its steady performance. When no
phenomenon was in accordance with xylose conversion that catalyst was added, the conversion rates of pentose were
Figure 4 - Effect of reaction temperature on arabinose conversion and FF yield (GVL-
water (4:1), 0.1 M IL, 30 mg/mL arabinose)
Figure 5 - Effect of catalyst dosage on pentose conversion and FF yield (GVL-water
(4:1), 140 °C, 120 min, 30 mg/mL arabinose)
6
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lower than 10% (corresponding bars were not drawn in Figure From a practical point of view, high initial concentration of
), and FF could not be detected by HPLC. Although pentose would be beneficial for the production of FF as well as
5
dehydration reactions of monosaccharides were reported to the economy. In this way, different pentose dosages were
proceed without acids addition, they usually required applied in the experiment, and the corresponding degradation
5
3
sufficiently high reaction temperatures above 200 °C, which characteristics of the two pentoses were investigated. The
was much higher than the temperatures adopted in this study. results are reflected in Figure 6. Through comparison and
Low temperature was not able to favor the self-ionization of analysis, it could be clearly observed that the initial dosage of
water, and therefore the dehydration reactions could hardly pentose had little influence on the conversion rate, whereas
5
4, 55
be catalyzed to generate FF.
After adding a little catalyst the FF yield was noticeably affected. When the xylose dosage
(0.02 M), the conversion rates of xylose and arabinose both and arabinose dosage rose from 0.2 M to 2 M, the
reached higher than 95%, and the corresponding FF yields corresponding conversion rates were all higher than 90%,
were 70.73% and 57.09%. As the catalyst dosage further while the yields of FF decreased from 79.76% and 58.70% to
increased, the conversion rates showed a light ascent, and the 47.98% and 36.58%, respectively, which indicated that high
corresponding FF yield first rose and then fell down. Hence it pentose concentration hindered the formation of FF. This
could be deduced that catalyst dosage had little influence on negative effect corresponded to what Danon et al. found, and
the conversion of both the two pentoses. In fact, after 60 min they concluded that the degradation products from pentose
since the reaction occurred the conversion rates were already throughout the reaction process would facilitate the
4
6
over 90%. The highest FF yield of 79.76% from xylose was degradation of FF. in order to overcome these side reactions,
obtained when adding 0.04 M catalyst. Yang et al. carried out simultaneous extraction was recommended in FF production
6
0
xylose conversion to produce FF in biphasic solvent reaction at high initial substrate load.
system of water-THF with AlCl ·6H O as Lewis acidic catalyst
and achieved 80% yield of FF. Serrano-Ruiz selected Brønsted
acidic ionic liquid as catalyst and converted xylose into FF in Considering the cost and the possibility of practical application,
3
2
2
4
Conversion of Xylan and Raw Biomass into FF
5
6
aqueous solvent with a maximum FF yield being 74%. The a reaction system developed for the production of FF is
highest yield of FF from arabinose (58. 70%) was reached when supposed to be assessed with real raw biomass. Bernal et al.
the catalyst dosage was 0.04 M, which was much higher than
in our previous research (24.46%) with Brønsted acidic ionic
2
liquid as catalyst. Gallo et al. used H
6
2 4
SO as a Brønsted acidic
catalyst and converted arabinose into FF in GVL-water solvent
5
7
with a highest FF yield of 44%. The above results indicated
that Lewis acidic ionic liquid combined with GVL-water solvent
system exhibited promising opportunities for large-scale
economic production of FF from the catalytic conversion of
pentose.
As for the decreasing trend of FF yield along with
increasing catalyst dosage, high concentration of acidic proton
produced from the hydrolyzation of Lewis acidic catalyst might
be the cause. The acidic protons could promote side reactions
that led to tremendous consumption of FF after the pentose
1
8
reacted for a long time, such as Diels−Alder reacꢀon. The
earliest research on the degradation behavior of FF is from
3
2
Dunlop et al. These series of degradation reactions could
include the destruction of FF structure in aqueous solution
with the presence of acid, the formation of resins and the
formation of formic acid from the hydrolytic fission of the
3
2
aldehyde group. Diels−Alder reacꢀon might also occur since
the oxygen atom of the aldehyde group on FF forms one
hydrogen bond with the surrounding water, which significantly
5
8
increased the dipole moment of the FF molecule. Hydrogen
+
3
bonds with the protonated water (H O ) created by the
hydrolyzation of Lewis acidic catalyst further increase the
dipole moment of FF. Thus the electron density of the
aldehyde group on FF jumped up while that of furan ring fell
down, leading to accelerated Diels−Alder reacꢀon between
5
9
two furan molecules.
Figure 6 - Effect of pentose dosage on pentose conversion and FF yield (GVL-water
(4:1), 140 °C, 120 min, IL 0.04 M)
Effect of Initial Pentose Dosage
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Journal Name
developed a two-step approach to convert corn stover respectively. Consequently, it was easier for CS to be
hemicellulose into FF in aqueous solvent with niobium converted to FF. Although the hemicellulose contents of ES
phosphate as acidic catalyst, and up to 23 mol% FF yield was and TC were similar, their hemicellulose compositions differed
obtained at 170 °C with respect to the pentosan units in the greatly from each other. The backbone of hardwood (ES)
6
1
starting raw biomass. Bruce et al. utilized the solvent of GVL- hemicellulose is mainly composed of xylose units connected
water (9:1), which was similar to the solvent in this research, through β-1,4-glucosidic bonds, whereas the backbone of
and produced FF from switchgrass with commercial SAPO-34 softwood (TC) hemicellulose consists of mannose and glucose
4
zeolite as catalyst. The best FF yield of 31% was obtained at that are much harder to be converted to FF.
6
90 °C after switchgrass was reacted for 8 h. In this study,
2
1
Catalytic Mechanism of Lewis Acidic ionic liquid
xylan, Eucalyptus saligna (ES), Tsuga chinensis (TC) and corn
stalk (CS) were chosen as raw material for FF production. Xylan For the whole catalytic reaction process of monosaccharides,
accounts for most of the hemicellulose polysaccharides in we proposed a possible reaction pathway along which xylose
6
herbaceous and hardwoods biomass. ES and TC represent and arabinose were converted into FF with the catalysis of
hardwood and softwood, respectively. CS is
agricultural straw and is widely available in China. Experiments proportions of [bmim]Cl and AlCl
with these four samples were carried out in optimized process, the Lewis acidic ionic liquid [bmim]Cl-AlCl
conditions, in which the best FF yields were achieved from in the formation of more than one anion species, such as
a
typical Lewis acid, as shown in Figure 8. According to the relative
during the preparation
will result
3
3
-
-
-
xylose and arabinose, and the respective FF yields are shown in [AlCl
Figure 7. The FF yield from xylan rose rapidly after the reaction acidity. The [bmim]Cl-AlCl
lasted for 30 min, and reached the maximum value (69.66%) research consisted of 0.5 mole fraction of AlCl
when xylan was reacted for 120 min. This yield was not as high fraction of [bmim]Cl, and for this composition [AlCl
4
] , [Al
2
Cl
7
]
and [Al
3
Cl10] , all of which appear Lewis
catalyst that was adopted in this
and 0.5 mole
6
3
3
3
-
4
]
6
4
as that from pure xylose, which could be attributed to dominated among the anion species. On the basis of the
incomplete hydrolyzation and the interference from other isomerization process put forward by Roman-Leshkov et al., in
2
components in xylan. Afterwards its FF yield reflected a which glucose was converted into fructose with Lewis acid as
4
-
decreasing tendency. As for raw biomass materials, all the catalyst, we inferred that [AlCl
4
] anion affected the normal
6
5
three species had much lower FF yield than xylan owing to hydride shift in the xylose-xylulose isomerization process.
their complicated structure and composition that made it Binder et al. made use of isotopic labeling and demonstrated
difficult for the polysaccharides to hydrolyze. A relatively fast that this isomerization reaction was mainly caused by 1,2-
6
6
conversion process occurred within 120 min, and after that the hydride shift. The isomerization reaction started with the
FF yields appeared to stabilize. Actually, their FF yield was still opening of the sugar ring. Then a xylose-aluminum complex
increasing slowly after 240 min since the reaction started, was formed from xylose and ionic liquid with the two oxygen
3
+
indicating more and more monosaccharides being released atoms O1 and O2 chelated with the metal atom, and then Al
from polymers to participate in subsequent dehydration cation behaved as Lewis acid catalyst to get a hydrogen atom
reactions. The highest FF yields of ES, TC and CS at 240 min to transfer from C2 carbon to C1 carbon, leading to the
were 34.82%, 23.68% and 47.96%, respectively. There were formation of pronated water and intermediate 1. Proton back-
also some differences among the conversion behavior of these donation from the water molecule to the C1 carbon atom of
three kinds of raw biomass. Generally, the order of FF yield the carbohydrate molecule resulted in the formation of
was CS > ES > TC. According to the compositional analysis, CS xylulose. Choudhary et al. studied the isomerization process of
had the highest hemicellulose content of 31.79%, whereas the xylose catalyzed by Sn-beta, a zeolite catalyst with Lewis
contents in ES and TC were only 15.24% and 13.83%, acidity, and the results was similar to the xylose isomerization
6
7
pathway depicted in Figure 7. Once xylulose was formed, it
would rapidly change into intermediate (1,2-enediol) via
tautomerism with the presence of H , and then would
undergo consecutive dehydration reactions to form FF.
2
+
2
1
0
The conversion process of arabinose was similar to that of
xylose: arabinose first isomerized to be ribulose, whose
tautomer
3 dehydrated continuously to form FF with the
+
presence of H . This process was in accordance with the
mechanism that Lü et al. proposed, in which arabinose was
6
8
converted into FF in aqueous solvent system. Thus [bmim]Cl-
AlCl is a promising Lewis acidic ionic liquid catalyst for the
efficient conversion of xylose and arabinose to FF.
3
Figure 7 – FF yield from xylan, ES, TC and CS based on pentose analysis (5 mL solvent
GVL-water (4:1)), 140 °C, IL 0.04 M, 30 mg/mL xylan or raw biomass)
(
8
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Figure 8 - Proposed mechanism of the conversion of xylose and arabinose to FF catalyzed by IL
Conclusion
Acknowledgements
In this research, an efficient and green reaction system for the The authors are grateful for the financial support from the
production of FF from pentose was developed. GVL-water National Natural Science Foundation of China (51621005 and
solvent system was used and Lewis acidic ionic liquid was 51476142) and the National Science and Technology
selected as catalyst. Maximum FF yield 79.76% (from xylose) Supporting Plan Through Contract (2015BAD15B06).
was achieved at 140 °C. It was found that Lewis acidic ionic
liquid could efficiently promote 1,2-hydride shift in the
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