Chromium halides mediated production of hydroxymethylfurfural from starch-rich acorn biomass in an acidic ionic liquid

Article abstract of DOI:10.1016/j.carres.2010.11.009

Chromium halides were introduced for the sustainable production of hydroxymethylfurfural (HMF) from raw acorn biomass using an acidic ionic liquid. The free sugars (glucose and maltose) released by the acidic hydrolysis of the biomass were confirmed by the FT-IR absorption bands around 995-1014 cm ^-1 and HPLC. FESEM analysis showed that the acorn biomass contains various sizes of starch granules and their structures were severely changed by the acidic hydrolysis. An optimal concentration of HCl for the HMF yields was 0.3 M. The highest HMF yield (58.7 + 1.3 dwt %) was achieved in the reaction mixture of 40% [OMIM]Cl + 10% ethyl acetate + 50% 0.3 M HCl extract containing a mix of CrBr₃/CrF₃. The combined addition of two halide catalysts was more effective in the synthesis of HMF (1.2-fold higher on average) than their single addition. The best productivity of HMF was found at 15% concentration of the biomass and at 50%, its relative productivity declined down to ca. 0.4-fold.

Full text of DOI:10.1016/j.carres.2010.11.009

Carbohydrate Research 346 (2011) 177–182
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Chromium halides mediated production of hydroxymethylfurfural
from starch-rich acorn biomass in an acidic ionic liquid
Jin-Woo Lee a, Myoung-Gyu Ha b, Young-Byung Yi a, Chung-Han Chung a,
⇑
a Department of Biotechnology, Dong-A University, 840, Ha-Dan-Dong, Sa-Ha-Gu, Busan 604-714, South Korea
b High-Technology Components & Materials Research Center, Korea Basic Science Institute, Busan 618-230, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Chromium halides were introduced for the sustainable production of hydroxymethylfurfural (HMF) from
raw acorn biomass using an acidic ionic liquid. The free sugars (glucose and maltose) released by the
acidic hydrolysis of the biomass were confirmed by the FT-IR absorption bands around 995–1014 cmÀ1
and HPLC. FESEM analysis showed that the acorn biomass contains various sizes of starch granules
and their structures were severely changed by the acidic hydrolysis. An optimal concentration of HCl
for the HMF yields was 0.3 M. The highest HMF yield (58.7 + 1.3 dwt %) was achieved in the reaction mix-
ture of 40% [OMIM]Cl + 10% ethyl acetate + 50% 0.3 M HCl extract containing a mix of CrBr3/CrF3. The
combined addition of two halide catalysts was more effective in the synthesis of HMF (1.2-fold higher
on average) than their single addition. The best productivity of HMF was found at 15% concentration
of the biomass and at 50%, its relative productivity declined down to ca. 0.4-fold.
Received 31 August 2010
Accepted 10 November 2010
Available online 13 November 2010
Keywords:
Acorn starch
Chromium halides
Hydroxymethylfurfural
Acidic ionic liquid
FT-IR
Ó 2010 Elsevier Ltd. All rights reserved.
Raw biomass
1. Introduction
simple sugars (mainly glucose) and transformed into HMF by sim-
ple processes.12–14 In particular, the direct use of raw biomaterials
In current, the sustainability issue in the chemical industry has
been considerably spread because of its importance on the protec-
tion of environment and the utilization of energy against the
depletion of fossil fuel. Now, the most attractive resources for
chemical sustainability can be derived from plant kingdom be-
cause plants are the reservoir of carbon sources that can be used
as platform intermediates for the production of diverse carbon-de-
rived compounds.1–4 For example, hydroxymethylfurfural (HMF),
which is the platform chemical for the syntheses of various carbon
compounds including liquid biofuels, such as dimethylfuran
(DMF), liquid alkanes, thermo-resistant polymers and complex
macrocycles, and some pharmaceutical precursors, can be synthe-
sized from biomass-derived carhohydrates.5–7 Recent studies
showed that HMF can be produced from various carbohydrate
sources, such as fructose, glucose, sucrose, starch, inulin, cellulose,
lignocellulose, and other simple sugars.5,8–13 Among these, cellu-
losic and lignocellulolic biomass resources are more abundant than
other ones. However, more improved biorefinery technologies are
required for the synthesis of HMF from them because not only its
yields are low but also its synthetic process is not easy when raw
cellulosic/lignocellulosic biomass materials are directly used. In
contrast, starch, which is the most abundant carbohydrate con-
served in many plants, can be more easily depolymerized into
containing high amounts of starch eliminates its purification step
and thus can be contributive to reduction of energy and CO2 emis-
sion in synthesizing HMF. In this context, raw acorn is a good bio-
material for the sustainable production of HMF because it is
obtainable from a wild perennial tree which can grow in the barren
soils, meaning that there is no need for its systematic farming, and
contains high amounts of starch (ca. 70–72% by dry weight).
HMF is synthesized from carbohydrates via thermal dehydra-
tion reaction in the presence of acid and metal catalysts.5 A recent
work demonstrated that HMF could be produced from the purified
acorn starch in the acidic ionic liquid (1-octyl-3-methylimidazo-
lium chloride, [OMIM]Cl).13 Because [OMIM]Cl, a disubstituted
imidazolium-based ionic liquid, has high miscibility with water,
less viscosity, high thermal stability, low vapor presure, low
melting points, and other physico-chemical properties favorable
to chemical reactions, it can be used as both solvent and catalyst
for the synthesis of HMF.12,13,15
Chromium halides have been primarily used as a cocatalyst to
promote the chemical oxidation reaction of various alkyl com-
pounds.16 Some recent reports demonstrated that chromium-based
catalysts and other metal halides play a key role in the synthetic
reactions of some organic compounds including HMF.8,12,13,17–19
They provided evidence that the yields of HMF was highly im-
proved by the addition of lithium halides and chromium chloride
to its synthetic processes using sugar polymers, such as cellulose,
lignocellulose, and starch.8,13,19 In our work, we examined the cat-
alytic activity of 4 different chromium halides on the yield of HMF
⇑
Corresponding author. Tel.: +82 51 200 7583; fax: +82 51 200 6536.
E-mail address: chchung@donga.ac.kr (C.-H. Chung).
0008-6215/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2010.11.009

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J.-W. Lee et al. / Carbohydrate Research 346 (2011) 177–182
in an acidic ionic liquid using raw acorn biomass and the structural
changes of its chemical components were characterized by FT-IR
spectroscopy in order to provide basic information about the sus-
tainable production of HMF.
ethanol. The FT-IR analysis was performed with microscopic FT-
IR/Raman spectroscopy equipped with a diamond crystal ATR
(attenuated total reflectance) accessory (Vertex 80 V, Bruker).
About 2 mg of the sample was placed onto the ATR crystal. The
FT-IR spectra were recorded on Bruker spectrometer (Germany)
in the spectral range of 4000–550 cmÀ1 with a resolution of
4 cmÀ1. Between determinations, the crystal was carefully cleaned
with ethyl alcohol and memory effects were avoided by inconsec-
utively recording the replica spectra.
2. Experimental
2.1. Reaction procedure
The acorn samples were dehulled, sliced into small pieces, and
dried. The dried pieces were then pulverized into powder (Fig. 1)
using a sample mill (Cyclotech 1093). Four chromium halides,
CrCl2 (97%, anhydrous), CrCl3 (99.5%, 6H2O), CrBr3 (crystalline,
6H2O), and CrF3 (98%, hydrate), were purchased from a commercial
supplier (Alfa Aesar, Mass., USA). [OMIM]Cl was obtained from
Merck (Germany) and other chemicals were also purchased from
commercial suppliers.
In the first step, the extracts of the acorn powder were prepared
using 10% (w/v) powder suspended in four different HCl concentra-
tions (0, 0.3, 0.5, and 1 M) by heating at 80 °C for 2 h with stirrings.
After the suspensions were centrifuged at 13,500g for 60 min, the
supernatants (extracts) were collected and stored for further uses
(Fig. 1).
The effects of HCl and the reaction time on the yields of HMF
were investigated in a reaction mixture containing the reaction
solvent (4 g [OMIM]Cl + 1 mL ethyl acetate) mixed with 5 mL of
the extract prepared in different concentrations of HCl as described
above at 120 °C for the specified reaction time (30, 60, 90, or
120 min) using a heating mantle. For the reaction with halide cat-
alysts, the reactant containing [OMIM]Cl and the catalyst was pre-
heated at 120 °C for 15 min before the biomass extract and ethyl
acetate were added. For the effect of single addition of the cata-
lysts, 0.2 g of each catalyst was added to the reaction mixture.
For the combined addition of two catalysts, 0.1 g each of two cat-
alysts (total 0.2 g) was mixed with the reaction mixture. The reac-
tions were conducted for 60 min or 90 min.
2.3. FESEM analysis
For field emission scanning electron microscopy (FESEM), the
samples were vacuum-dried and then they were coated with plat-
inum (Pt) (20 nm thick) using a vacuum ion sputter (108auto, Cres-
sington, UK). Each Pt-coated specimen was then observed with a
field emission scanning electron microscope (JEM-6700F, JEOL, Ja-
pan) at the probe energy level of 5.0 KV and its image was photo-
graphed at magnifications of 1000Â or 3000Â.
2.4. HMF and sugar analysis by HPLC
HMF and sugars were analyzed with a high performance liquid
chromatograpy (HPLC, Waters). An absorbance detector (Waters)
and a refractometer (Waters) were used for their determination.
HMF was quantified with a Waters XBridge C18 reversed-phase col-
umn (4.6 mm  150 mm, 5
lm) using a gradient mobile phase at a
flow rate of 0.7 mL/min (UV at 320 nm). The gradient condition
was as follows: 100% (v/v) water phase for 2 min, transition and
gradient phase in the ratio of 80% (v/v) water and 20% (v/v) MeOH
for 8 min, and 100% (v/v) water for 20 min including transition
time. For quanitification of sugars, a YMC-Pack Polyamine II col-
umn (4.6 mm  250 mm, S-5
lm, 12 nm) was used with a mobile
phase (75:25, acetonitrile/water) at a flow rate of 1 mL/min.
3. Results and discussion
For the test of HMF productivity, nine different concentrations
(10, 15, 20, 25, 30, 35, 40, 45, and 50%) of the acorn biomass were
prepared in 0.3 M HCl and hydrolyzed at 80 °C for 2 h. The reaction
for the production of HMF was performed at 120 °C for 90 min in
the same reaction mixture with a mix of two catalysts (each
0.1 g; CrCl3/CrF3 because of much cheaper than other ones) as de-
scribed above. The relative productivity of HMF was determined
using the HMF yield obtained from 10% of the acorn biomass as a
control (a value = 1).
3.1. Characterizations of raw acorn biomass and its chemical
components by FT-IR and FESEM analyses
Acorn, which contains high amount of starch (70–72% by dry
weight), can be an excellent raw biomaterial for high yield of
HMF because starch is a good feed material in the synthesis of
HMF.13 Starch hydrolysis to simple sugars is an important step
for high yield of HMF (Fig. 1). Thus, the first goal of this study
was to establish the reaction condition allowing efficient hydroly-
sis of raw acorn to carbohydrates which can be converted into
HMF. Its efficiency was monitored by FT-IR20,21 and FESEM analy-
sis.22 Figure 2 shows FT-IR spectra of the acorn biomass and its
component carbohydrates. In the raw acorn biomass, some major
absorbance peaks, which typically appear for C–OH and C–O–C
stretching modes of carbohydrates23,24 were identified between
900 and 1200 cmÀ1 region (BP in Fig. 2A). These infrared spectra
All measurements were estimated with the mean values of at
least four independent reactions and expressed with standard
deviations.
2.2. FT-IR analysis
Fourier transform infrared (FT-IR) method was used to measure
the sugar samples prepared by precipitating the extracts with
Figure 1. Synthetic process of hydroxymethylfurfural (HMF) from raw acorn biomass. The acorn samples harvested were peeled, sliced, dried, and pulverized into powder
using a sample mill. Then, the sample powder was suspended in the given HCl solution and hydrolyzed at 80 °C for 2 h. The hydrolyzed suspension was centrifuged at 13,500g
for 60 min and the supernatant was collected and used as the biomass extract for the synthesis of HMF. Soln; solution and IL; ionic liquid.

J.-W. Lee et al. / Carbohydrate Research 346 (2011) 177–182
179
were compared with those from the native acorn starch prepara-
tions (SP in Fig. 2A). Their band pattern was very similar with each
other, confirming that the main component of acorn biomass
would be starch because the infrared absorption peaks around
997 and 1149 cmÀ1 were detected in both samples (Fig. 2A), which
(1149 cmÀ1) contributing to pyranose sugars20 was present and
the strong peak around 995 cmÀ1 was found (Fig. 2B), which ap-
pears for glucose28 as shown in the absorption band of glucose
standard (Fig. 2B–G). HPLC analysis supported the above spectral
characterization from FT-IR determination by providing evidence
that the free glucose and maltose in the hydrolysates from the
raw acorn biomass were present after acid hydrolysis (Fig. 3).
Figure 4 reveals the structural changes of the raw acorn biomass
by acid hydrolysis. The FESEM observation before acid hydrolysis
displays the native image of raw acorn biomass sample, showing
a variety of different shapes (some are spherical or ellipsoidal)
would be attributed to the
a-1,4/1,6-glycosidic linkages (a-C1–O–
C4/ -C1–O–C6 vibration modes) present in starch molecules and
a
the C–C stretching appearing in pyranose sugars.20,24–27 However,
when an acid (HCl) was applied, a small change in the FT-IR spectra
was observed around the peak region of 995–1014 cmÀ1 in the acid
hydrolysates (Fig. 2B), which was not found in the raw acorn bio-
mass (Fig. 2A). This shift of the absorption band after acid hydroly-
sis may indicate that some changes in the chemical structure of the
acorn starch occurred, implying its depolymerization into glucose
units or other small pyranose sugars such as maltose. The reason
for this explanation is based on the fact that the peak region
and sizes that ranged from approximately 3 lm to 8 lm. This im-
age also shows the presence of some debris and the substantial
amounts of starch granules in the biomass sample (Fig. 4A). In con-
trast, the FESEM observation after acid hydrolysis (in 0.3 M HCl
solution) reveals completely disrupted surfaces of the starch gran-
ules, which are typically found by acid hydrolysis (Fig. 4B). This
disrupted image may explain that the starch and other carbohy-
drates contained in the starch granules would have been released
into its solution medium.29,30
3.2. Effects of HCl concentration and reaction time on the yields
of HMF from raw acorn biomass
It is known that acid catalysts play an important role in the syn-
thesis of HMF.5 Some recent publications have demonstrated that
HCl and H2SO4 are effective acid catalysts for its synthesis from
carbohydrates, such as fructose, glucose, sucrose, and starch in
the presence of an ionic liquid.12,13,31 In this experiment, the effects
of HCl concentration and reaction time on the yields of HMF were
surveyed to select an optimal concentration of HCl and reaction
time in synthesizing HMF from raw acorn biomass. As seen in
Figure 5, the absence of HCl (at 0 M HCl) showed negligible yields
of HMF, whereas the presence of HCl in the reaction mixture
positively affected the synthesis of HMF. Its highest yield
(28.4 + 1.6 dwt %; % of dry weight) was achieved in the reaction
mixture with 0.3 M HCl by undergoing its reaction for 90 min, indi-
cating that 0.3 M HCl would be the most effective for the synthesis
of HMF from raw acorn biomass. However, above 0.3 M HCl, its
yields began to decline (Fig. 5). The negative effect at higher HCl
concentrations (0.5 and 1 M HCl) on the yield of HMF may be
due to the overactivity of H+ caused probably by high H+ density
derived from its high concentrations.32 This explanation was sup-
ported by other works that HCl concentrations above a critical level
Figure 2. FT-IR spectral profiles of raw acorn biomass and its hydrolysis products.
(A): FT-IR band patterns of raw acorn biomass and its starch: SP; starch powder, BP;
biomass powder. (B): FT-IR band patterns of the hydrolysis products of the raw
acorn biomass: A; the hydrolysate of acorn starch in 0.3 M HCl, B; the hydrolysate of
acorn biomass in 0.3 M HCl, C; the hydrolysate of acorn biomass in 0.5 M HCl, D; the
hydrolysate of acorn biomass in 1 M HCl, and G; glucose standard.
Figure 3. HPLC analysis of the hydrolysates of the acorn biomass before and after
acid hydrolysis.

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J.-W. Lee et al. / Carbohydrate Research 346 (2011) 177–182
(Fig. 5). The above results illustrate that the interaction between
HCl concentration and the reaction time could be a crtical factor
for the high yield of HMF from the acorn biomass. In particular,
although HCl functions as an important factor in synthesizing
HMF, there has been no direct evidence on its reaction mechanism.
Nonetheless, one expnation may be possible: As a Brønsted acid
catalyst, the proton from HCl would act as a mediator for the
hydrolysis of starch present in the acorn biomass into simple sug-
ars that are convertible into HMF by attacking the glycosidic oxy-
gen in the starch, resulting in protonated glycosides. These
protonated forms of the glycosides are rapidly liberated into free
sugars (mainly glucose units in this case). The interaction between
H+ activity (from HCl) and the dual properties (cation and anion) of
[OMIM]Cl present in the reaction medium would help induce the
dehydration reaction of the free sugars into HMF by forming the
key intermediates such as 1,2-enediol via their isomerization reac-
tion.5,33,35,36 However, more studies are required for further de-
tailed explanation.
3.3. Effects of chromium halides on the yields of HMF
It is known that chromium halides, such as chromium bromide
and chloride promote the reaction rate in organic synthesis by cat-
alyzing its oxidation reactions.16 A recent study first introduced
CrCl2 for high yields of HMF from the purified carbohydrates.18 In
our work, four chromium halides (CrCl2, CrCl3, CrBr3, and CrF3)
were employed to scrutinize their effects on the yields of HMF that
are produced directly from raw acorn biomass. Figure 6 displays
that the addition of halide catalysts contributed to an increase in
the HMF yields on the whole. Moreover, the combined addition
of two halide catalysts enhanced the HMF yields up to ca. twofold
compared to that with no catalyst. The highest yield of HMF
(58.7 + 1.3 dwt %) was observed in the reaction mixture with the
combined catalysts of CrBr3/CrF3 by undergoing its reaction for
90 min (Fig. 6). Among the reactions with single addition of the ha-
lide catalysts, CrCl2 was most effective for the HMF yield
(46.7 dwt % at 30 min and 52.8 dwt % at 90 min reaction time),
whereas CrF3 was lowest in its yield (4.1 dwt % at 30 min and
36.7 dwt % at 90 min reaction time). The positive effect of the ha-
lide catalysts on the HMF yield may be involved with the interac-
tive coordination mechanism of the halide catalysts with the free
carbohydrates (mainly glucose) released from the acorn biomass
by acid hydrolysis in the presence of [OMIM]Cl ionic liquid.18 This
mechanism is based on the fact that, in the presence of [OMIM]Cl,
the Cr metal with halides plays a critical role by coordinating with
Figure 4. FESEM observation of the acorn biomass before and after acid hydrolysis.
(A): FESEM image of the raw acorn biomass and (B): FESEM image of the raw acorn
biomass after its hydrolysis in 0.3 M HCl.
decreased the yields of HMF.13,33,34 Another finding was that a ra-
pid increase in the yields of HMF occurred after 30 min reaction
time in all reactions except no HCl, and after 60 min (at 0.5 M
and 1 M HCl) or 90 min (at 0.3 M HCl), its yields began to decrease
Figure 5. Effect of HCl concentration and reaction time on the yields of hydroxymethylfurfural (HMF) synthesized from the raw acorn biomass.

J.-W. Lee et al. / Carbohydrate Research 346 (2011) 177–182
181
Figure 6. Effect of chromium halide catalysts and reaction time on the yields of hydroxymethylfurfural (HMF) synthesized from the raw acorn biomass.
the hydroxyl proton located at the anomeric carbon of the free glu-
cose. This may lead to isomerization of the glucose to fructofura-
nose via glucose mutarotation by forming an enolate in the
chromium halide–sugar complex, the key intermediate in their
pathways. Through their consecutive reactions, the intermediate
products are rapidly dehydrated into HMF.18,37
single addition because its yields in the former were higher (ca.
1.2-fold on average) than in the latter. However, contrary to our
expectations, the highest promotion effect on the HMF yield was
achieved in the combined addition of CrBr3/CrF3 and the lowest
was found in CrCl3/CrBr3 (Fig. 6). These data suggest that their pro-
motion efficiency may be dependent on the synergistic coordina-
Another interesting finding was that the effect of the halide cat-
alysts on the HMF yield was different with each other (Fig. 6), sug-
gesting that their catalytic activity varied with the type of halide.
The order of the promotion effect on their catalytic activity was
CrCl2 > CrCl3 > CrBr3 > CrF3. Moreover, the promotion effect was
higher in chloro-halide groups (CrCl2 and CrCl3) than in other ha-
lide ones (CrBr3 and CrF3) (Fig. 6). This difference may result from
their different coordination ability of halo-groups with chromium
center.17 That is, the reason for higher catalytic activity of chloro-
groups would be due to their stronger coordination ability with
chromium center than that of other two halo-groups (bromo-
and fluoro-group).
tion ability of the two catalysts (as a ligand) used with
chromium metal under the given reaction condition.8,17,19 In addi-
tion, the reaction time should be considered for high HMF yield
from the raw acorn biomass because after 90 min reaction time,
insoluble solid products began to occur (data not shown). Our data
indicated that the optimal reaction time would be between 60 and
90 min (Fig. 6) and lengthening the reaction time resulted in the
formation of some insoluble solid products (data not shown).
3.4. Effects of biomass concentration on the productivity of
HMF
Figure 6 also provides data that the combined addition of two
halide catalysts to one reaction was more effective in the synthesis
of HMF from the raw acorn biomass compared with that of their
The productivity of HMF is an element of economical impor-
tance for its industrial uses because it could be a factor in
determining the production cost of HMF. Its productivity was
Figure 7. Effect of the concentration of the raw acorn biomass on the productivity of hydroxymethylfurfural (HMF).

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J.-W. Lee et al. / Carbohydrate Research 346 (2011) 177–182
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As shown in Figure 7, the highest productivity was identified with
a value of ca. 1.2-fold greater at 15% concentration of the biomass
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We provide a new route to the sustainable production of HMF
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can grow in the wild soils and is a non-food plant. FT-IR spectros-
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combined addition of two halide catalysts was more effective for
the synthesis of HMF than their single addition. The highest yield
of HMF occurred in the reaction mixture containing a mix of
CrBr3/CrF3. The best productivity was achieved at 15% concentra-
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This research was supported by a fund from Dong-A University.
The authors greatly acknowledge the financial support.
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