Conversion of carbohydrates and lignocellulosic biomass into 5-hydroxymethylfurfural using AlCl3·6H2O catalyst in a biphasic solvent system

Article abstract of DOI:10.1039/c1gc15972k

AlCl₃·6H₂O in a biphasic medium of water/tetrahydrofuran (THF) is effective for the synthesis of 5-hydroxymethylfurfural (HMF) from glucose-based carbohydrates. For glucose, an HMF yield of 61% was achieved in 10 min at 160 °C under microwave heating. The reaction time profile revealed the intermediacy of fructose en route to HMF with a dehydration rate constant that is approximately 4 times that of glucose isomerization to fructose. Addition of NaCl did not increase HMF yields significantly but it diminished lactic acid formation. Disaccharides (maltose and cellobiose) and starch gave good yields of HMF. However, cellulose required a higher temperature (180 °C) and longer reaction time (30 min) to give a modest yield of 37%. Several lignocellulosic biomass variants (corn stover, pine wood, grass, and poplar) were investigated with the AlCl₃· 6H₂O biphasic system. The yields of HMF were modest (20-35%) but high concurrent yields of furfural were observed (51-66%). The described AlCl ₃·6H₂O-NaCl-H₂O/THF biphasic medium has potential because it is economic, nontoxic, and it exhibits fast kinetics (10 min) under microwave heating.

Full text of DOI:10.1039/c1gc15972k

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PAPER
Conversion of carbohydrates and lignocellulosic biomass into
5-hydroxymethylfurfural using AlCl3·6H2O catalyst in a biphasic solvent
system
Yu Yang,a,b Chang-wei Hu*a and Mahdi M. Abu-Omar*b
Received 7th August 2011, Accepted 1st November 2011
DOI: 10.1039/c1gc15972k
AlCl3·6H2O in a biphasic medium of water/tetrahydrofuran (THF) is effective for the synthesis of
5-hydroxymethylfurfural (HMF) from glucose-based carbohydrates. For glucose, an HMF yield of
61% was achieved in 10 min at 160 ◦C under microwave heating. The reaction time profile revealed
the intermediacy of fructose en route to HMF with a dehydration rate constant that is
approximately 4 times that of glucose isomerization to fructose. Addition of NaCl did not increase
HMF yields significantly but it diminished lactic acid formation. Disaccharides (maltose and
cellobiose) and starch gave good yields of HMF. However, cellulose required a higher temperature
(180 ◦C) and longer reaction time (30 min) to give a modest yield of 37%. Several lignocellulosic
biomass variants (corn stover, pine wood, grass, and poplar) were investigated with the
AlCl3·6H2O biphasic system. The yields of HMF were modest (20–35%) but high concurrent
yields of furfural were observed (51–66%). The described AlCl3·6H2O–NaCl–H2O/THF biphasic
medium has potential because it is economic, nontoxic, and it exhibits fast kinetics (10 min) under
microwave heating.
Glucose is the most abundant and cheapest hexose, and
therefore, it is the preferred source for the production of HMF.
1. Introduction
5-Hydroxymethylfurfural (HMF) can be obtained from carbo-
Glucose conversion to HMF proceeds via isomerization of
hydrates and lignocellulosic biomass. It has been hailed as the
glucose to fructose, followed by dehydration of fructose in
central biorenewable chemical from which liquid fuel and other
the presence of a biological catalyst/enzyme,4 base,5 or metal
important chemicals can be produced. HMF can serve as a
chloride.6 Among these catalysts, metal chlorides are favored
precursor for levulinic acid (LA), 2,5-furandicarboxylic acid,
2,5-diformylfuran, dimethylfuran, and dihydroxymethylfuran,
because they catalyze effectively isomerization and dehydration.
Zhao et al. have shown that using CrCl2 in ionic liquid leads to
to name a few.1 Dumesic and coworkers have shown that HMF
HMF yields of 67% at 100 ◦C.6a Hu and co-workers reported a
can be used for the production of liquid alkanes.2 It has also
catalytic conversion of glucose using SnCl4, in which HMF was
been shown that dimethylfuran, produced from hydrogenation
formed in 64% yield.6b These catalyst systems, however, have
of HMF, can directly serve as a high heating value fuel.3
drawbacks, as Cr and Sn are toxic, while ionic liquids are expen-
Hence, the demand for HMF will continue to grow as supply
sive and their application on industrial scale remains prohibitive.
of petroleum feedstock diminishes and the price of petroleum
continues to rise. Excellent yields of HMF can be achieved
AlCl3·6H2O is abundant and cheap; it has also been shown
to catalyze the isomerization of glucose to fructose in aqueous
from the dehydration of fructose. However, obtaining HMF
solution.7 However, in aqueous solution the reaction leads to
from glucose is more difficult because it requires a bifunctional
further degradation of HMF to LA and humins.8 In order to
catalyst that would affect the isomerization of glucose to fructose
and subsequent dehydration of fructose.
remove HMF from the reactant sugar and acidic catalyst as it is
being produced, water immiscible organic solvent can be added
to continuously extract HMF from the aqueous phase.9 Among
aKey Laboratory of Green Chemistry and Technology, Ministry of
Education, College of Chemistry, Sichuan University, Chengdu, Sichuan,
610064, P. R. China. E-mail: gchem@scu.edu.cn
different extracting solvents, tetrahydrofuran (THF) is attractive
because of its high partitioning coefficient and low boiling point.
Moreover, THF is an ideal solvent for further upgrading of
HMF with dihydrogen because THF is a saturated hydrocarbon
and it does not contain functional groups that would be
hydrogenated. Inorganic salts have been used as additives to
bDepartment of Chemistry and the Center for Direct Catalytic
Conversion of Biomass to Biofuels (C3Bio), Purdue University, 560
Oval Drive, West Lafayette, Indiana, 47907, U. S. A.
E-mail: mabuomar@purdue.edu
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increase the partitioning coefficient between THF and water.3,10
NaCl is among the most effective and cheapest salts and has been
shown by Roma´n-Leshkov and Dumesic to significantly improve
partitioning of HMF into the organic phase.10 In this report,
we describe a one-pot synthesis of HMF from carbohydrate
and lignocellulosic biomass using AlCl3·6H2O as a catalyst in
an H2O/THF biphasic medium. The effect of NaCl was also
investigated.
as the mobile phase at a flow rate of 0.6 mL min-1, and the
column temperature was maintained at 338 K. The concentra-
tions of HMF, LA and lactic acid in the organic phase were
determined by gas chromatography (Agilent 6890) equipped
with DB-5 column and flame ionization detector (FID). All
concentrations of carbohydrates in the aqueous phase and
organic products in the aqueous phase and organic phase were
determined by comparison to standards calibration curves. Con-
version of glucose and yield of products are defined as follows:
Glucose conversion = moles of glucose reacted/moles of
starting glucose
2. Experimental
Glucose, maltose, cellobiose, starch, cellulose, AlCl3·6H2O,
NaCl, HCl (37 wt%), THF, HMF, LA, and lactic acid were
all analytical grade, purchased from Sigma-Aldrich, and used
as received. Corn stover was obtained from Purdue University,
Department of Agriculture. Dr Keith Johnson (Purdue Uni-
versity) supplied the switch grass, Mr. Jerry Warner (Defence
LifeSciences, LLC) provided the pine wood, and the USDA
Forest Product Lab (CAFI Consortium) supplied the poplar.
The approximate composition of lignocellulosic biomass was
based on published values.11 For example, corn stover is 27% (by
weight) pentose (xylose and arabinose), 35% hexose (glucose),
11% lignin, and 27% other (ash, protein, etc.).11a
All reactions were performed under microwave heating in
the Discover TM System (CEM Corporation) using a 10 mL
batch reaction vessel. Reaction solutions were mixed using a
magnetic stir bar. The reactor temperature was measured by a
fiber optic sensor. In a typical experiment for the transformation
of glucose in single phase, a 10 mL reaction tube was charged
with glucose (0.25 mmol), AlCl3·6H2O (0.1 mmol), NaCl (0.35 g)
and Millipore water (1 mL); the reaction was heated to 160 ◦C for
10 min. In a typical experiment for the transformation of glucose
and other carbohydrates in biphasic system, a 10 mL reaction
tube was charged with glucose or other carbohydrates (0.25
mmol based on monosaccharide units), AlCl3·6H2O (0.1 mmol),
NaCl (0.35 g), Millipore water (1 mL) and THF (3 mL); the
reaction was heated to 160 ◦C for 10 min. In a typical experiment
for the transformation of biomass in biphasic system, a 10 mL
reaction tube was charged with biomass (0.05 g), AlCl3·6H2O
(0.1 mmol), NaCl (0.35 g), Millipore water (1 mL) and THF
(3 mL); the reaction was heated to 180 ◦C for 30 min. The
reaction was stopped by nitrogen flow cooling. Samples were
filtered with a 0.2 mm syringe filter prior to analysis.
Fructose yield = moles of fructose produced/moles of starting
glucose
HMF yield = moles of HMF produced/moles of starting
glucose
HMF recovery = moles of HMF recovered/moles of starting
HMF
LA yield = moles of LA produced/moles of starting glucose
or HMF
Lactic acid yield = moles of lactic acid produced/2*moles of
starting glucose
For other carbohydrates and lignocellulosic biomass feed-
stock, product yields are defined as follows:
HMF yield = moles of HMF produced/moles of starting
hexoses
Furfural yield = moles of furfural produced/moles of starting
pentoses
The pH value of the H2O–AlCl3·6H2O (3.03) and H2O–
AlCl3·6H2O–NaCl (2.22) solutions (at 25 ◦C) was measured on
an Accumet AB15/15+ pH meter ( 0.01 pH units) calibrated
with standard buffer solutions.
3. Results and discussion
Glucose conversion in pure water in the presence of AlCl3·6H2O
affords a low HMF yield despite the high conversion of glucose
(Table 1). The total combination of HMF, LA, and lactic acid
accounts for only half of the starting glucose. The main reason
for this poor selectivity is that HMF in the presence of an acidic
catalyst is not stable in water due to formation of insoluble
humins from the reaction of HMF with glucose.12 This is
supported by the poor recovery of HMF when used as the staring
material under the same conditions (10 min microwave heating
at 160 ◦C), last column of entry 1 in Table 1. The use of THF in
a biphasic system with water increased HMF yield significantly
to 52% (entry 2, Table 1). Meanwhile, only trace amounts of LA
The aqueous phase was analyzed by HPLC using a Waters
1525 pump, an aminex column HPX-87 column (Agilent), and
Waters 2412 Refractive Index detector. H2SO4 (5 mM) was used
Table 1 Glucose conversion with AlCl3·6H2O in different mediaa
Conv.
(%)
Fructose
yield (%)
HMF
yield (%)
Lactic acid
yield (%)
LA yield
(%)
HMF recoveryc
(LA yieldd) (%)
Entry
Solvent
Catalyst
1
2
3
4
5
Single phase (H2O)
Biphasic (H2O/THF)
Single phase (H2O–NaCl)
Biphasic (H2O–NaCl/THF)
Biphasic (H2O–NaCl/THF)
AlCl3·6H2O
AlCl3·6H2O
AlCl3·6H2O
AlCl3·6H2O
HClb
98
99
98
99
30
3
3
0
0
0
22
52
17
61
12
17
13
—
—
—
10
Trace
29
1
52 (22)
94 (1)
19 (58)
97 (2)
—
2
a Reaction conditions: glucose = 0.25 mmol, AlCl3·6H2O = 0.1 mmol, NaCl = 0.35g, H2O = 1 mL, THF = 3 mL, reaction temperature = 160 ◦C,
reaction time = 10 min. b pH = 1 in HCl (0.1 mmol) c Reaction conditions: HMF = 0.25 mmol, AlCl3·6H2O = 0.1 mmol, NaCl = 0.35 g, H2O = 1 mL,
THF = 3 mL, reaction temperature = 160 ◦C, reaction time = 10 min. d Based on moles of starting HMF.
510 | Green Chem., 2012, 14, 509–513
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was formed, which also supports the notion that THF inhibited
the rehydration of HMF to LA. Furthermore, in this H2O/THF
biphasic medium, the HMF recovery was high (94%) when HMF
was employed as the reactant. The introduction of THF also de-
creased the yield of lactic acid. All of these improvements could
be attributed to the continuous separation of HMF as it is being
produced from the aqueous phase to the organic (THF) phase.
To increase the partitioning coefficient further, NaCl was used
as an additive to the aqueous phase. The effect of NaCl additive
on the conversion of glucose in water was tested first (entry
3, Table 1). In this instance, the yield of HMF decreased and
the yield of LA increased (29%), nearly three times the yield
obtained without NaCl additive. It is also worth noting that in
the presence of NaCl, HMF recovery was only 19% with high LA
yield of 58%. In summary, in single phase aqueous medium NaCl
improved the rehydration of HMF to LA. This improvement
might very well be attributed to the decrease in pH upon
introduction of NaCl (pH = 2.22 versus pH = 3.03). A similar
phenomenon was observed in an aqueous solution of B(OH)3.
Addition of NaCl resulted in an increase of acidity.13 The rate
of HMF hydration to LA is known to be acid dependent.14
Of course, an increase in acidity also facilitates dehydration of
fructose to HMF.14 A notable feature of the NaCl additive is
the absence of lactic acid from the product mixture (entries 1
versus 3). Although HMF was more stable without NaCl in the
single phase aqueous system, the effect of NaCl in the biphasic
medium H2O/THF was worth investigating because of the noted
reduction in lactic acid formation. Even though addition of
NaCl to the biphasic reaction system gave modest improvement
in HMF yield (61 versus 52%), it had marked influence on
selectivity by reducing lactic acid to a trace amount (entries
2 and 4). In any event, the yield of HMF with and without
NaCl using AlCl3·6H2O is superior to employing a strong protic
acid such as HCl (entry 5). Although the acidity of the aqueous
phase is stronger when HCl is used (pH = 1 versus pH = 2.22), the
conversion of glucose is only 30% and the HMF yield only 12%.
Although the amount of glucose relative to AlCl3·6H2O is
2.5 : 1, the reaction proceeds at lower catalyst loadings (Table
2). As the catalyst amount is decreased to 0.025 mmol, glucose
conversion and HMF yield are reduced over the reaction time
of 10 min. A longer reaction time of 30 min with 0.025 mmol
catalyst loading (10 mol%) is required to reach high conversion
and HMF yield (entry 5, Table 2).
Fig. 1 Conversion of glucose as a function of time using AlCl3·6H2O in
H2O/THF biphasic medium. Reaction conditions: glucose = 0.25 mmol,
AlCl3·6H2O = 0.1 mmol, NaCl = 0.35 g in H2O/THF (1 : 3 mL) at 160 ◦C
under microwave heating.
stage of the reaction fructose formation is observed reaching
a maximum at ca. 1 min. It declines to zero as the reaction
reaches completion in 5 min. This behavior supports fructose
as an intermediate for HMF production from glucose in this
system. For a kinetic model of A → I → P (where A is
reactant, I intermediate, and P product) with rate constants k1
and k2, the following two equations describe the ratio of the rate
constants based on the maximum concentration of I and the
/(k -k )
2
time that maximum is reached: [I]max = [A]0(k2/k1)(k
and
2
1
tmax = ln(k2/k1)/k2–k1. The amount of fructose built up relative
to glucose is consistent with a rate of formation of fructose that
is approximately 4 times slower than its subsequent dehydration
to HMF. In other words, with A = glucose, I = fructose, and
P = HMF, k2/k1 ~4. Longer reaction times (> 10 min) lead to
further degradation of HMF to LA and insoluble humins.
We tested AlCl3·6H2O in biphasic H2O/THF with NaCl addi-
tive for the conversion of disaccharides (maltose and cellobiose)
and polysaccharides (starch and cellulose), Fig. 2. HMF yields
Given that the H2O–NaCl/THF biphasic system with
AlCl3·6H2O resulted in the best yield and selectivity for HMF,
we obtained a time profile of the reaction (Fig. 1). In the initial
Table 2 Glucose conversion with AlCl3·6H2O as the catalysta
Catalyst AlCl3·
Glucose
conversion (%)
Fructose
yield (%)
HMF
yield (%)
Entry
6H2O (mmol)
1
2
3
4
5
0.15
0.10
0.050
0.025
0.025b
100
99
88
70
98
0
0
10
15
0
60
61
54
39
65
Fig. 2 HMF yields from various carbohydrates in biphasic H2O–
NaCl/THF system. Reaction conditions: carbohydrate = 0.25 mmol
based on monosaccharide units, AlCl3·6H2O = 0.1 mmol, NaCl = 0.35 g,
H2O = 1 mL, THF = 3 mL, temperature = 160 ◦C, reaction time = 10 min.
aReaction temperature = 180 ◦C, reaction time = 30 min.
a Reaction conditions: glucose = 0.25 mmol, NaCl = 0.35 g, H2O =
1.0 mL, THF = 3.0 mL, reaction temperature = 160 ◦C, reaction time =
10 min. b Reaction time = 30 min.
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Table 3 Comparison of AlCl3·6H2O versus HCl as the catalyst for cellobiose and poplar conversiona
Entry
Starting material
Catalyst
Glucose yield (%)
HMF yield (%)
Xylose yield (%)
Furfural yield (%)
1
2
3
4
Cellobiose
Cellobiose
Poplarc
AlCl3·6H2O
HCl
1
62
0
58
10
26
21
nab
na
0
na
na
51
29
AlCl3·6H2O
Poplarc
HCl
18
0
a Reaction conditions: cellobiose = 0.25 mmol based on glucose units, AlCl3·6H2O or HCl = 0.10 mmol, NaCl = 0.35 g, H2O = 1.0 mL, THF = 3.0 mL,
reaction temperature = 160 ◦C, reaction time = 10 min. b na = not applicable. c Reaction conditions: poplar = 0.05 g, AlCl3·6H2O or HCl = 0.10 mmol,
NaCl = 0.35 g, H2O = 1.0 mL, THF = 3.0 mL, reaction temperature = 180 ◦C, reaction time = 30 min.
from disaccharides were slightly lower than those obtained
from glucose. Therefore, our biphasic system under microwave
heating is effective in hydrolyzing the glycosidic linkage of
disaccharides.15 Conversion of starch to HMF was also quite
efficient and selective (50% yield of HMF). However, cellulose
conversion was quite poor, affording 7% HMF yield under
the same conditions. The majority of the cellulose remained
unconverted as a solid suspension in the aqueous phase. We
reasoned that this poor reactivity with cellulose is most likely
due to ineffective depolymerization and decrystallization with
AlCl3·6H2O at 160 ◦C.16 Indeed a higher temperature was needed
for cellulose conversion. When the reaction was run at 180 ◦C for
30 min, HMF yields increased to 37%. These results demonstrate
that AlCl3·6H2O in biphasic H2O–NaCl/THF system can be
used for conversion of complex carbohydrates to HMF with
reasonable yields.
Our next step was to apply the AlCl3·6H2O biphasic system to
raw biomass variants. HMF yields from corn stover, pine wood,
grass and poplar were 19%, 35%, 23% and 26%, respectively (Fig.
3). These values from raw biomass are lower than those obtained
from pure cellulose. Biomass is more complex than crystalline
cellulose and it contains protein and ash,11 which could affect
the conversion of cellulose in the biomass to HMF. Interestingly,
high furfural yields can be obtained from the raw biomass in our
system (Fig. 3), ranging from 51–66% depending on the source of
lignocellulosic biomass. Raines and Binder reported conversion
of lignocellulosic biomass into furans using CrCl3 and HCl as
co-catalysts.17 Moderate HMF yields can be obtained but with
a low furfural yield (37% furfural yield was based on xylan
analysis of 22.8% for untreated stover). Oktay used HCl for the
conversion of straw biomass into furans.18 Although the furfural
yields were reasonable (46–72% based on different biomass
samples), the HMF yields were very low (< 3%). We have also
compared herein AlCl3·6H2O with HCl to ascertain that the
role of AlCl3·6H2O goes beyond just generating a Brønsted acid
(H3O+). The results for cellobiose and poplar are presented in
Table 3. HCl is clearly less effective at converting cellobiose.
Even though with poplar, HCl gave comparable HMF yield, it
was less effective in converting the resulting glucose and in its
yield of furfural. It should also be noted that HCl was used at
an amount equal to AlCl3·6H2O (0.10 mmol), which results in
a more acidic solution than that of AlCl3·6H2O. In conclusion,
compared to previous reports, our method provides moderate
HMF yields along with quite high furfural yields from varying
biomass sources in a one-pot reaction. Hence, the approach we
describe hereinwithAlCl3·6H2Oand biphasic medium should be
valuable for the conversion biomass into biofuels and platform
chemicals.
4. Conclusions
The introduction of THF to the AlCl3·6H2O aqueous reaction
system increases the yield and selectivity for HMF from glucose.
The addition of NaCl increases the partitioning coefficient of
HMF into a biphasic system and retards the route to lactic
acid, enhancing the yield and selectivity for HMF. Satisfactory
results were obtained with glucose-based disaccharides (maltose
and cellobiose), polysaccharides (starch and cellulose), and raw
biomass. While the yields of HMF from variant lignocellulosic
biomass sources (corn stover, pine wood, poplar, and grass)
were modest, 20–35% based on hexose content, the reaction
produced high yields of furfural (51–66%) concurrently in one-
pot reactions. The described aluminum system has excellent
potential for application because it is cheap and nontoxic.
Fig. 3 Furfural and HMF yields from various lignocellulosic biomass
in biphasic H2O–NaCl/THF system using AlCl3·6H2O under microwave
heating. Reaction conditions: Biomass = 0.05 g, AlCl3·6H2O = 0.1 mmol,
NaCl = 0.35 g, H2O = 1 mL, THF = 3 mL, temp = 180 ◦C, reaction
time = 30 min. Yields of HMF from corn stover, pine wood, grass and
poplar were based on a hexose content of 35%, 26%, 35% and 45% by
weight and yields of furfural were based on a pentose content of 27%,
22%, 23 and 29% by weight, respectively.
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (973 program, No. 2007CB210203),
NNSFC 21072136, PCSIRT (No. IRT0846), the Center for
Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), an
512 | Green Chem., 2012, 14, 509–513
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Energy Frontier Research Center funded by the U. S. Depart-
ment of Energy, Office of Science, and Office of Basic Energy
Sciences under Award Number DE-SC0000997. C3Bio provided
funding for laboratory space, instrumentation, and chemical
supplies at Purdue University. We are also thankful for support
from the China Scholarship Council (Grant 2010624099).
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