Glucose dehydration to 5-hydroxymethylfurfural in ionic liquid over Cr3+-modified ion exchange resin

Article abstract of DOI:10.1039/c4ra09131k

In this work, D001-cc cation-exchange resin was modified by CrCl₃ using an ion exchange method to obtain a Cr³⁺-D001-cc catalyst. FT-IR, UV-vis, SEM, EDX, ICP-OES and Py-IR were used to characterize the catalyst. Subsequently, the effects of the modified metal ion and different cation-exchange resin on the direct conversion of glucose to 5-hydroxymethylfurfural in [Bmim]Cl were examined. The results showed that Cr³⁺-D001-cc resin exhibited excellent catalytic performance. Moreover, with Cr³⁺-D001-cc resin as a catalyst, the influence of the reaction conditions on the dehydration of glucose was investigated in detail. 61.3% yield of 5-hydroxymethylfurfural was achieved in the [Bmim]Cl ionic liquid at 110?°C for 30 min with 0.1 g catalyst. Finally, a recycling experiment was performed. The result indicated that the HMF yield in the Cr³⁺-D001-cc resin and [Bmim]Cl system significantly decreased after six recycles.

Full text of DOI:10.1039/c4ra09131k

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Glucose dehydration to 5-hydroxymethylfurfural in
ionic liquid over Cr3+-modified ion exchange resin
Cite this: RSC Adv., 2015, 5, 9290
*
*
Hui Liu, Hongwei Wang, Yuan Li, Wei Yang, Changhua Song, Huaming Li,
Wenshuai Zhu and Wei Jiang
In this work, D001-cc cation-exchange resin was modified by CrCl3 using an ion exchange method to
obtain a Cr3+-D001-cc catalyst. FT-IR, UV-vis, SEM, EDX, ICP-OES and Py-IR were used to characterize
the catalyst. Subsequently, the effects of the modified metal ion and different cation-exchange resin on
the direct conversion of glucose to 5-hydroxymethylfurfural in [Bmim]Cl were examined. The results
showed that Cr3+-D001-cc resin exhibited excellent catalytic performance. Moreover, with Cr3+-D001-
cc resin as a catalyst, the influence of the reaction conditions on the dehydration of glucose was
investigated in detail. 61.3% yield of 5-hydroxymethylfurfural was achieved in the [Bmim]Cl ionic liquid at
110 ꢀC for 30 min with 0.1 g catalyst. Finally, a recycling experiment was performed. The result indicated
that the HMF yield in the Cr3+-D001-cc resin and [Bmim]Cl system significantly decreased after six recycles.
Received 23rd August 2014
Accepted 9th December 2014
DOI: 10.1039/c4ra09131k
www.rsc.org/advances
are applied to the reaction.8,12,14,15 Among all these solvents,
ionic liquids are promising solvents because of their unusual
properties, such as high thermal and chemical stability, negli-
gible vapor pressure, non ammability, and adjustable solvent
power for organic substances.16,17
1. Introduction
Nowadays, the demand for energy is globally increasing.1
However, fossil-fuels, as a major supply of energy, cannot meet
the ever-increasing need because of the decreasing fossil-fuel
reserves and the environmental pollution brought about by
the utilization of fossil-fuel.2 Therefore, biomass, as an
abundant, renewable, non-polluting and widespread novel
energy resource, has attracted considerable attention in recent
years.3–6 5-Hydroxymethylfurfural (HMF) is just an important
intermediate in the utilization of biomass because it is a
signicant platform compound widely applied in the ne
chemical eld.7,8
Both fructose and glucose are the normal mono-
saccharides used to prepare HMF, but compared with
glucose, fructose can be converted more easily to HMF.9,10
However, the resource of fructose is not abundant in nature,
which leads to high cost during the preparation of HMF.
Therefore, glucose is more suitable than fructose for the mass
industrial production of HMF. In recent years, the dehydra-
tion of glucose to prepare HMF has been investigated by many
researchers.11–13 Despite these efforts, a high yield of HMF
from glucose is difficult to achieve. Therefore, the search for
catalytic systems is of great importance for the dehydration of
glucose to HMF.
Another signicant factor for the dehydration of glucose is
the implementation of catalysts. Various catalysts have been
used to catalyze the dehydration of glucose to HMF.18,19 From
the results of these studies, metal ions, especially Cr3+ and Cr2+,
exhibit excellent catalytic performance. In 2007, chromium
dichloride (CrCl2) and chromium trichloride (CrCl3) were rst
used to catalyze the conversion of glucose to HMF in the pres-
ence of 1-ethyl-3-methylimidazolium chloride([Emim]Cl).20 The
HMF yields of 70% and 45%, respectively, were obtained at 100
ꢀC in 3 h. Subsequently, catalysts with chromium as the catalytic
core were designed to catalyze the conversion of glucose to
HMF. Hu et al. and Yong et al. used chromium(III) chloride
(CrCl3$6H2O) in tetraethylammonium chloride (TEAC) ionic
liquid and N-heterocyclic carbene/CrCl2(CrCl3) to catalyze the
dehydration of glucose to HMF, respectively. The results
showed that chromium ions had excellent catalytic performance
for the dehydration of glucose to HMF.21,22 Moreover, Zhang
et al. studied the dehydration of glucose and fructose catalyzed
by CrII and CrIII chlorides by a combined experiment and
computational study. The results indicated that the ability of
chromium to selectively dehydrate glucose to HMF in [Emim]Cl
did not depend on the oxidation state of chromium.23 Subse-
quently, Jia et al. reported that adjusting the solvent system was
the most effective way of changing the product distribution with
CrCl3$6H2O as a catalyst for the conversion of glucose to HMF. A
controlled amount of water in the non-aqueous system was
favorable for driving the thermodynamic equilibrium for a high
Solvent plays a key role in the catalytic system for the dehy-
dration of glucose to HMF. At present, many solvents, including
water, organic compounds, biphasic solvents and ionic liquids,
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang,
212013, P. R. China. E-mail: lihm@ujs.edu.cn; lh7544@ujs.edu.cn; Fax: +86-511-
88791708; Tel: +86-511-88791800
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HMF yield and desired product selectivity.24,25 However, the
toxicity and pollution of chromium in the environment are well-
known. Therefore, avoiding the drawbacks of chromium while
achieving the appropriate catalytic activity is essential. As seen
in the abovementioned studies, chromium cannot be separated
from the reaction system because of its solubility in the solvent,
leading to environmental damage. It is believed that utilizing an
insoluble solid catalyst with Cr should be a better choice to
decrease the damaging effects.
From our previous work,9 D001-cc cation-exchange resin
has excellent catalytic performance for the dehydration of
fructose to HMF and the HMF yield reaches 93.0% at 75 ꢀC for
20 min. However, a HMF yield of 4.6% could be attained when
glucose was used as the substrate. This might be because that
D001-cc resin contains rich Brønsted acid sites, which favors
the dehydration of fructose, but lacks Lewis acid sites. It was
reported that in the dehydration process of glucose to HMF,
glucose was isomerized rst to fructose, which was then
transformed to HMF by dehydration.26 During the process, the
Lewis acid plays an important role. Different metal ions with
Lewis acidity are exchanged easily into the resin by a simple
ion-exchange method according to the characteristics of the
cation-exchange resin. Moreover, homogeneous catalyst
loaded on the resin can still maintain the catalytic activity,27
which suggests that introducing the metal ion into the cation-
exchange resin can maintain their respective catalytic char-
acteristics. At present, the metal-modied resin is applied to
different organic reaction such as the Suzuki–Miyaura reaction
and the esterication reaction.28–30 However, its catalytic
performance for the dehydration of glucose to HMF has never
been investigated.
2.2 Experimental methods
2.2.1 The modication of cation-exchange resin. To
remove the impurities from D001-cc cation-exchange resin, the
cation-exchange resin was rst washed with deionized water
and subsequently washed with an alcohol solution. Aer these
steps, the resin was dried at 50 ꢀC for 12 h, dipped into a
solution of hydrochloric acid and CrCl3 with stirring at room
temperature for 4 h and then le to stand for 48 h. Finally, the
resin was ltered, washed with deionized water to pH ¼ 6–7 and
ꢀ
dried at 50 C for 12 h. Thus, the D001-cc resin modied with
Cr3+ was obtained, which was dened as Cr3+-D001-cc. The
other metal modied D001-cc resins were obtain under the
same conditions, except for changing CrCl3 to other metal
halides including CrCl2, CuCl2, MoCl5, FeCl3, MgCl2 and AlCl3.
According to the same method, acidied D001-cc resin was
obtained by changing the mixture solution of hydrochloric acid
and CrCl3 to only hydrochloric acid solution.
2.2.2 Catalyst characterization. The content of Na and Cr in
the catalysts was analyzed by an inductively coupled plasma
technique (ICP-OES) using a VISTA-MPX instrument. The FT-IR
spectrum of the catalysts was measured in transmission mode
using a Nicolet Nexus 470 spectrometer. The UV-Vis diffuse
reectance spectrum of the catalysts was recorded on a Shi-
madzu UV-2450. The analytical wavelength ranged from 800 nm
to 190 nm. Before the measurement, the catalyst was ground
ꢀ
into a ne powder and then dried at 50 C for 12 h. The scan-
ning electron microscopy (SEM) and energy dispersive spec-
troscopy (EDS) were measured by FESEM, Model JEOL JSM-
7001F. SEM was employed to analyze the microstructure and
surface topography of the catalyst. EDS was applied for the
quantitative elemental analysis of the catalyst. The type of acidic
sites of the catalysts was determined by infrared spectroscopy of
pyridine adsorption (Py-IR) on a Frontier FT-IR instrument. The
Therefore, according to the above discussion, a series of
cation-exchange resins modied by metal ion were prepared in
this work. First, the effects of different modied metal ions and
cation-exchange resins on the reaction were discussed to choose
a suitable modied metal ion and support. Subsequently, the
effects of the reaction temperature, solvent, catalyst dosages
and different initial fructose loadings on the dehydration of
glucose and the reusability of the resin/ionic liquid system were
also investigated with the optimal resin as a catalyst in a
favorable ionic liquid.
ꢀ
samples were treated at 350 C for 2 h in a vacuum, and then
cooled to room temperature. Subsequently, puried pyridine
vapor was adsorbed at room temperature for 30 min, followed
ꢀ
by desorption at 150 C.
2.2.3 Typical procedure. Glucose was added to the ionic
liquid in a reaction bulb and the mixture was then heated to a
single phase at the reaction temperature. Next, a certain
amount of catalyst was added to the above mixture. Aer a
suitable time, the reaction mixture was cooled rapidly, poured
into a volumetric ask and diluted with pure water. The diluted
solution was ltered through a syringe lter prior to HPLC
analysis.
2. Experimental
2.2.4 Analysis. The sample was analyzed by high perfor-
mance liquid chromatography (HPLC) equipped with an Agilent
TC-C (18) column, a Varian ProStar325 UV-Vis detector and two
Varian ProStar210 pumps. The measured parameters were
consistent with the literature.9 The amount of HMF was calcu-
lated using an external standard method.
2.1 Materials
HMF (99% purity) was purchased from Wutong Spice Co., Ltd.
Fructose (B. R. grade), glucose (A. R. grade), hydrochloric acid
(HCl, A. R. grade), ethanol (A. R. grade) and methanol (CH3OH,
L. C. grade) were purchased from Sinopharm Chemical Reagent
Co., Ltd. 1-Butyl-3-methylimidazolium chloride([Bmim]Cl) was
purchased from Shanghai Cheng Jie Chemical Co., Ltd. All the
chemicals were used without further purication. All Na-type
cation-exchange resins were purchased from Nankai University
Chemical Factory, including D001-cc, D072, D152, NKC-9, 001
ꢁ 1 and 001 ꢁ 7.
For fructose and glucose, the yield of HMF was determined
using the following formula.
nHMF
mHMF ꢁ Mmonosaccharide
MHMF ꢁ mmonosaccharide
Yield ¼
ꢁ 100% ¼
ꢁ 100%
nmonosaccharide
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For disaccharides and polysaccharides, HMF yield was dened
as follows:
nHMF
mHMF ꢁ Mhexose-units
Yield ¼
ꢁ 100% ¼
ꢁ 100%
nhexose-units
MHMF ꢁ mdi=poly-saccharides
where nHMF, nmonosaccharides and nhexose-units are the moles of
HMF, the initial moles of monosaccharides and hexose-units in
di/poly-saccharides, respectively. MHMF, Mmonosaccharides and
Mhexose-units are the molar mass of HMF, monosaccharides and
hexose units in the di/poly-saccharides, respectively. mHMF is
the mass of HMF calculated by HPLC; mmonosaccharides and
mdi/poly-saccharides are the initial mass of monosaccharides and
di-/poly-saccharides.
3. Results and discussion
3.1 Characterization of catalyst
Fig. 1 UV-vis diffuse reflectance of D001-cc and Cr3+-D001-cc.
The content of Na and Cr in D001-cc resin before and aer
modication is listed in Table 1. It can be found in Table 1 that
Na+ in D001-cc resin was not completely exchanged by H+ or by
Cr3+ whether by acidication or modied by CrCl3 through the
ion-exchanging method. However, a large amount of Na+ could
be exchanged when D001-cc resin was immersed in the mixture
solution of CrCl3 and HCl in comparison with the content of
Na+ in H+-D001-cc resin and Cr3+-D001-cc resin.
1450 cmꢂ1 were observed in the spectra of D001-cc and Cr3+
-
D001-cc, which was the principal feature of aromatic
compounds.32 This also indicates that the modication process
by metal ions has no inuence on the framework of D001-cc
resin. This result is in agreement with that of UV-vis diffuse
reectance characterization. In addition, there were two bands
at 1182 cmꢂ1 and 1039 cmꢂ1 in the FT-IR spectrum of D001-cc,
which represented the asymmetric and symmetric stretching
vibration of S]O, respectively.33,34 However, the band at 1182
cmꢂ1 in the FT-IR spectrum of Cr3+-D001-cc was split into two
absorption peaks at 1160 cmꢂ1 and 1224 cmꢂ1, respectively.
Moreover, the intensity of the peak at 1039 cmꢂ1 in Cr3+-D001-cc
resin was reduced and a new band at 900 cmꢂ1 was found aer
the modication of Cr3+. These results indicate that the modi-
cation of D001-cc by metal ions increase the asymmetric
stretching vibration of the S]O in resin.30 This might be
because of the fact that S]O has an interaction with Cr3+, which
leads to an increase in the nonequivalence of S]O.
The UV-vis diffuse reectance spectrum of samples is illus-
trated in Fig. 1. Two absorption peaks at about 236 nm and 270
nm were found in the spectrum of D001-cc resin and Cr3+-D001-
cc, which were represented by the characteristic peak of
benzene.28 Nonetheless, the absorption peaks in Cr3+-D001-cc
were slightly blue-shied and the intensity of the absorption
peaks decreased relative to those in the D001-cc resin. The
result indicates that the introduction of Cr3+ does not inuence
the framework of the resin but changes the distribution of the
electron cloud in the conjugated system. Moreover, no new
absorption peak at 400 nm was observed in Cr3+-D001-cc,
showing that the coordination effect does not exist between Cr3+
and the benzene ring in the D001-cc resin.29 Compared with the
resin D001-cc, two distinct absorption peaks at 435 nm and 625
nm in the spectrum of Cr3+-D001-cc were found, which were
produced by the d–d transitions of Cr3+. This indicates that Cr3+
is in the form of octahedron coordination in the Cr3+-D001-cc.31
Therefore, when Cr3+ is introduced into D001-cc resin by the
ion-exchange method, Cr3+ might interact with the oxygen atom
of S]O in the D001-cc resin, leading to the blue-shi of the
absorption peak of benzene and a decrease in peak intensity.
The FT-IR spectra of D001-cc and Cr3+-D001-cc are shown in
Fig. 2. Three characteristic bands at 1600 cmꢂ1, 1500 cmꢂ1 and
The SEM images and EDS spectra of D001-cc and Cr3+-D001-
cc resin are shown in Fig. 3. Fig. 3a1 and a2 exhibit SEM images
of D001-cc resin; Fig. 3b1 and b2 show the SEM images of
Table 1 Content (mg gꢂ1) of Na and Cr in the catalysts measured by
ICP-OES
Na (mg gꢂ1
)
Cr (mg gꢂ1
)
D001-cc (unmodied)
H+-D001-cc (acidied resin)
Cr3+-D001-cc
68
40
18
—
—
34
Fig. 2 FT-IR spectra of D001-cc resin and CrCl3-modified resin.
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Fig. 3 SEM images and EDS spectra of D001-cc and Cr3+-D001-cc. (a1 and a2) the SEM images of D001-cc, (b1 and b2) the SEM images of Cr3+
-
D001-cc. (a3) EDS spectra of D001-cc, (b3) EDS spectra of Cr3+-D001-cc.
Cr3+-D001-cc resin. It can be observed from the images that the
surface of the unmodied D001-cc resin was quite smooth and
abundant pore channels were found on it. In contrast, the
surface of D001-cc resin became rough but still had consider-
able pore canals aer the modication with Cr3+. These results
suggest that the introduction of Cr3+ has little inuence on the
structure of D001-cc resin. Fig. 3a3 and b3 present the EDS
spectra of D001-cc and Cr3+-D001-cc resin, respectively. It can be
noted that Cr3+ only exists in Cr3+-D001-cc resin, which indi-
cates that Cr3+ can be actually loaded on the resin by a simple
ion-exchange method. However, it is worth noting that Na+ can
be found in Cr3+-D001-cc resin, indicating that Na+ is not
completely exchanged with Cr3+ in the impregnating process.
The result is in agreement with that shown in Table 1.
Fig. 4 shows the Py-IR spectra of D001-cc resin, H+-D001-cc
resin and Cr3+-D001-cc resin. Generally, the vibration bands in
the 1400–1650 cmꢂ1 regions are applied to distinguish the
Brønsted and Lewis acid sites. The bands at 1449 cmꢂ1 and
1537 cmꢂ1 belong to Lewis acidic sites and Brønsted acidic
sites, respectively. The band at 1492 cmꢂ1 corresponds to
Brønsted and Lewis acidic sites (L + B).35–37 As shown in Fig. 4,
Fig. 4 The Py-IR spectra of different samples.
only Lewis acidic sites were found in the D001-cc resin and Cr3+
D001-cc resin, but the band intensity for adsorbed pyridine on
the Lewis acid sites increased and a slight blue-shi was
observed in Cr3+-D001-cc resin compared with the D001-cc
-
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Table 2 Influence of different ions in D001-cc resin on the conversion
resin, indicating that the acidity of D001-cc resin increases aer
modication with Cr3+. In contrast, both Brønsted acid and
Lewis acid were found in H+-D001-cc resin. As listed in Table 1, a
part of Na+ in D001-cc resin is exchanged by the H+ of a
hydrochloric acid solution in the acidied process, which
results in the appearance of Brønsted acidic sites.
According to the above spectra, a diagrammatic sketch of the
preparation of Cr3+-D001-cc resin is proposed, which is shown
in Fig. 5. When D001-cc resin is immersed in the mixture
solution of HCl and CrCl3, most of the Na+ (the exchangeable
cation in resin) are exchanged with Cr3+ through an interaction
between Cr3+ and SO3ꢂ (the xed group in resin). The replaced
Na+ enters the immersion solution.
of glucose to HMFa
Entry
Ion
HMF yield/%
1
2
3
4
5
6
7
8
9
Na+
1.2
9.7
61.3
60.7
15.3
6.2
14.4
20.2
5.4
H+
Cr3+
Cr2+
Cu2+
Mg2+
Fe3+
Al3+
Mo5+
a
Conditions: glucose 0.1 g, [Bmim]Cl 1.5 g, 110 ꢀC, 30 min.
3.2 Inuence of the different ions on the catalytic activity
Zhao.20 It was found that chromic salts can promote the isom-
erization of glucose to fructose, whereas other metal salts can
only improve the mutarotation of a-glucopyranose anomer to
b-glucopyranose anomer. Therefore, Cr2+-D001-cc resin and
Cr3+-D001-cc resin exhibit better catalytic performance than the
other resins. Considering the cost of CrCl2 and CrCl3, CrCl3 was
selected as the modied metal halide to perform the following
experiments.
Table 2 shows the change of catalytic performance of D001-cc
resin before and aer modication. As shown in the table,
unmodied D001-cc resin (entry 1) had little catalytic activity
(HMF yield of 1.2%) for the dehydration of glucose at 110 ꢀC for
30 min. Aer being acidied with HCl, Na+ was exchanged to H+
(entry 2), but the catalytic ability was not improved signicantly
under the same conditions (HMF yield of 9.7%), indicating that
H+-D001-cc resin also had poor catalytic performance for the
dehydration of glucose. Nevertheless, H+-D001-cc resin exhibi-
ted excellent performance for the conversion of fructose to HMF
3.3 Effect of different resin supports on the catalytic activity
The inuence of six different resin supports modied with Cr3+
on the dehydration of glucose to HMF was checked at 110 ꢀC for
30 min, including macroporous strong-acidic cation-exchange
resins (D072, D001-cc and NKC-9), macroporous weak-acidic
cation-exchange resin (D152), and gel strong-acidic cation-
exchange resins (001 ꢁ 1 and 001 ꢁ 7). The result is displayed
in Table 3. When macroporous strong-acidic cation-exchange
ꢀ
in a previous study and the HMF yield reached 93.0% at 75 C
for 20 min.9 Fig. 4 shows that H+-D001-cc resin contains rich
Brønsted acidic sites, favoring the dehydration of fructose.
However, when free Na+ in D001-cc resin was exchanged with
Cr3+, the HMF yield increased to 61.3% (entry 3). This proves
that the introduction of Cr3+ is advantageous in improving the
catalytic performance of the D001-cc resin, which is due to the
introduction of strong Lewis acidic sites aer modication with
Cr3+ (Fig. 4).
Next, the effects of different metal ions (including Cu2+
,
Table 3 Influence of different resin supports on the conversion of
glucose to HMFa
Mo5+, Fe3+, Mg2+, Al3+, Cr2+ and Cr3+) on the dehydꢀration of
glucose to HMF was investigated in [Bmim]Cl at 110 C for 30
min and the result is also listed in Table 2. Aer the modica-
tion with different metal ions, D001-cc resins show higher
catalytic activity than the acidic D001-cc resin, except for the
Mg2+-D001-cc resin. This suggests that the introduction of a
metal ion is in favor of the dehydration of glucose. A more than
60% yield of HMF was achieved in the presence of D001-cc
modied with Cr2+ and Cr3+, whereas less than 20% yield of
HMF was obtained with other ions modied D001-cc resin as a
catalyst. This result is in agreement with that reported by
Entry
Resin
HMF yield/%
1
2
3
4
5
6
001 ꢁ 1
001 ꢁ 7
NKC-9
D072
D001-cc
D152
4.8
5.0
48.8
50.3
61.3
6.9
a
Conditions: glucose 0.1 g, [Bmim]Cl 1.5 g, catalyst 0.1 g, 110 ꢀC,
30 min.
Fig. 5 Diagrammatic sketch of the preparation of Cr3+-D001-cc resin.
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Table 4 Influence of different catalyst dosages on the conversion of
resins (NKC-9, D072 and D001-cc) were applied as the catalyst
support, the catalysts showed good catalytic performance and
48.8%, 50.3% and 61.3% yield of HMF were achieved, respec-
tively. However, when the other types of resins were selected as
the support, not more than 10% yield of HMF yields was
reached. This suggests that macroporous strong-acidic cation-
exchange resin exhibits better catalytic activity than the other
types of resins due to the larger surface area, the stronger acidity
and large number of macrospores. This result is consistent with
our previous work.9 From Table 3, D001-cc resin was selected as
the support for subsequent experiments.
glucose to HMFa
Entry
The dosage of catalyst/g
HMF yield/%
1
2
3
4
5
0
0.7
52.9
61.3
65.1
66.4
0.05
0.1
0.15
0.2
a
Conditions: glucose 0.1 g, [Bmim]Cl 1.5 g, 110 ꢀC, 30 min.
increased and then increased smoothly with increasing catalyst
dosage. Only a yield of 0.7% HMF was obtained without the
catalyst. When 0.05 g of Cr3+-D001-cc resin was added, a 52.9%
yield of HMF was achieved. This result suggests that Cr3+-D001-
cc resin can effectively catalyze the dehydration of glucose to
HMF. Increasing the catalyst dosage from 0.05 g to 2.0 g
resulted in an increase in the yield of HMF from 52.9% to
66.4%, but the rate of the increase lowered considerably. This
means that the yield of HMF will become constant. In previous
work, the addition of more than 0.1 g of D001-cc resin led to a
decrease in the HMF yield for the dehydration of fructose.9
However, in this work, the yield of HMF constantly increased
with increasing catalyst dosage. This suggests that Cr3+ can
improve the stability of HMF by suppressing the side reactions
to increase the HMF yield.20
3.4 Effect of different temperature on the dehydration of
glucose
The inuences of different reaction temperatures on the dehy-
dration of glucose were studied and the result is shown in Fig. 6.
At low temperatures (80 ꢀC and 90 ꢀC), the HMF yield increased
as the reaction proceeded. Moreover, the higher the tempera-
ture was, the faster was the reaction rate. When the reaction
temperature was increased to 100 ꢀC and 110 ꢀC, the initial
HMF yield showed obvious improvement, showing that a high
temperature is benecial for the dehydration of glucose to
HMF. Unlike the reaction at low temperatures, the HMF yield at
high temperatures subsequently decreased smoothly with
increasing time. The time needed to reach the peak yield was
lesser at 110 ꢀC than at 100 ꢀC. This might be because of the fact
that high temperature accelerates not only the dehydration of
glucose to HMF but also the side reactions during the process.
HMF can be decomposed to levulinic acid and formic acid, and
3.6 Inuence of different initial dosage of glucose
HMF also can polymerize with HMF, glucose or intermediate The effect of different initial dosages of glucose on the reaction
products,38 leading to a decrease in HMF yield. Therefore, was investigated. According to Table 5, the HMF yield slowly
110 ꢀC and 30 min were selected as the optimum condition for decreased with increasing initial glucose dosage. When the initial
the dehydration of glucose to HMF.
dosage of glucose was 0.05 g, Cr3+-D001-cc showed a high cata-
lytic activity, resulting in a 70.9% yield of HMF. This suggests that
the excess catalyst dosage relative to glucose can supply more
catalytic sites to accelerate the dehydration of glucose to HMF.
This result is in agreement with that shown in Table 4. However,
the catalyst cannot provide enough catalytic sites for the dehy-
dration of glucose with an increase in the initial dosage of
glucose, which leads to a decrease in the yield of HMF.
3.5 Effect of catalyst dosage on the dehydration of glucose
The effects of the catalyst dosage on the dehydration of glucose
to HMF are listed in Table 4. The yield of HMF rst rapidly
3.7 Conversion of di/poly-saccharides in catalytic system
To study the substrate scope of this catalytic system, ve various
sugars were investigated, including monosaccharide (glucose
Table 5 Influence of different initial dosages of glucose on the yield of
HMFa
Entry
Initial dosage of glucose/g
HMF yield/%
1
2
3
4
5
0.05
0.1
0.2
0.3
0.4
70.9
61.3
51.2
41.9
36.0
Fig. 6 Influence of different temperatures on the conversion of
glucose to HMF. Conditions: glucose 0.1 g, [Bmim]Cl 1.5 g, catalyst
0.1 g.
a
Conditions: [Bmim]Cl 1.5 g, Cr3+-D001-cc 0.1 g, 110 ꢀC, 30 min.
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and fructose), disaccharide (maltose) and polysaccharide
(cellulose and starch). As shown in Table 6, with the increasing
degree of polymerization, the catalytic activity of this catalytic
system gradually decreased. When glucose and fructose were
selected as substrates, a 61.3% and 68.9% yield of HMF was
achieved, respectively. For maltose, only a 22.1% yield was
obtained. Little catalytic activity of Cr3+-D001-cc resin was
observed with polysaccharides as a substrate. This indicates
that this catalytic system cannot quickly catalyze complex
hydrolysis during a short time. According to previous studies,
the conversion of polysaccharide to HMF was quite complicated
and H2O and high temperature were benecial to the reac-
tion.39,40 Therefore, the poor catalytic performance of Cr3+-D001-
cc/[Bmim]Cl system for polysaccharide may result from the
waterless environment and lower temperature employed in the
experiment. Compared with H-type D001-cc,9 the HMF yield for
the dehydration of fructose decreased from 93.0% to 68.9%.
This might be because of the fact that the resin with the Lewis
acidity does not benet the dehydration of fructose or an
increase in reaction temperature leads to some side reactions,
which produces a soluble substance and insoluble polymers.41
Fig. 7 Recycle of the catalytic system. Conditions: glucose 0.1 g,
[Bmim]Cl 1.5 g, Cr3+-D001-cc 0.1 g, 110 ꢀC, 30 min.
the previous runs, as reported by Wang et al.42 However, the
difference in the HMF yields between the rst and second tests
in this work is larger than the result shown in the literature. It
was reported that CrCl4ꢂ could be formed between free Cr3+ and
[Bmim]Cl, which was advantageous to the conversion of
glucose.20,43 Therefore, the reaction solution aer the rst test
was analyzed by ICP-OES. The results showed that Cr existed in
the solution, which indicates that the large increase in HMF
yield in the seconꢂd and third test may be the result of the
3.8 Catalytic system recyclability
As an important aspect of the Cr3+-D001-cc/[Bmim]Cl catalytic
system, the recyclability was tested with a set of experiments.
The dosage of Cr3+-D001-cc, [Bmim]Cl and glucose were 0.1 g,
1.5 g and 0.1 g, respectively. The reaction temperature and time
were 110 ꢀC and 30 min, respectively. The operating details were
consistent with the previous work9 and the result is shown in
Fig. 7. A relatively stable yield of HMF was observed in the rst
four experiments, but the catalytic activity of the Cr3+-D001-cc/
[Bmim]Cl system showed an obvious decrease aer the sixth
run. This indicates that the Cr3+-D001-cc/[Bmim]Cl system can
be reused, but the recycling time is limited. This may be caused
by two aspects. On one hand, the maximum service temperature
of D001-cc resin is between 100 ꢀC and 120 ꢀC. Long-term use at
that temperature results in the damage of resin easily. The
fragments, probably resulting from the Cr3+-D001-cc resin, were
observed aer the h and sixth recycling experiment. On the
other hand, the polymer produced in the reaction may be
adsorbed on the resin, thus poisoning the catalytic activity of
the resin.
formation of CrCl4
.
4. Conclusion
Cr3+ modied D001-cc catalyst could be easily prepared via an
eco-friendly ion-exchanged method. The modied process had
no inuence on the structure of the resin. On the other hand,
Cr3+ could be easily exchanged to the framework of D001-cc
resin. Observed from the results of dehydration of glucose,
chrome ion and macropore strong-acidic cation-exchange resin
could improve the catalytic activity. A 61.3% yield of HMF was
obtained with Cr3+-D001-cc resin as the catalyst and [Bmim]Cl
ꢀ
as the solvent at 110 C for 30 min. Aer six recycling tests, an
obvious decrease in HMF yield was observed for the dehydra-
tion of glucose in the above system.
Acknowledgements
It is interesting that the HMF yields in the two following
recycling tests are even higher than that in the 1st run. This was
possibly due to the residual HMF and unreacted feed stock in
The authors thank the nancial support for this study by the
National Natural Science Foundation of China (21106057 and
21376111) and Advanced Talents of Jiangsu University
(10JDG143).
Table 6 Influence of different substrates on the yield of HMFa
Entry
Substrate
HMF yield/%
References
1
2
3
4
5
Glucose
Fructose
Maltose
Cellulose
Starch
61.3
68.9
22.1
2.4
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