One-pot synthesis of sulfonated graphene oxide for efficient conversion of fructose into HMF

Article abstract of DOI:10.1039/c6ra23420h

Sulfonated graphene oxide (SGO) is a promising solid acid catalyst for many acid-catalyzed reactions, but its synthesis typically requires complicated post-functionalization. Herein, a one-pot method was developed to directly synthesize SGO. Characterizations showed that the SGO has abundant acid sites, including SO₃H (1.62 mmol g^-1), OH and COOH, and a two-dimensional (2D) structure. An efficient reaction system using SGO as a solid acid catalyst was developed for the dehydration of fructose into 5-hydroxymethylfurfural (HMF). A high yield of HMF up to 94% was obtained with a low loading of SGO. Moreover, the SGO is also capable of converting high-concentration fructose (20 wt%) with an acceptable yield of 67%. The high catalytic activity was attributed to the abundant active sites and the 2D structure which enhances the accessibility of the active sites.

Full text of DOI:10.1039/c6ra23420h

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One-pot Synthesis of Sulfonated Graphene Oxide for Efficient
Conversion of Fructose into HMF
Qidong Hou, Weizun Li, Meiting Ju*, Le Liu, Yu Chen, Qian Yang
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Tianjin Engineering Research Center of Biomass Solid Waste Resources Technology,
College of Environmental Science and Engineering, Nankai University, Tianjin
300071, PR China
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Abstract: Sulfonated graphene oxide (SGO) is a promising solid acid catalyst for
many acidꢀcatalyzed reactions, but its synthesis typically requires complicated
postꢀfunctionalization. Herein, a oneꢀpot method was developed to directly synthesize
SGO. Characterizations showed that the SGO has abundant acid sites, including SO3H
(1.62 mmol gꢀ1), OH and COOH, and twoꢀdimension (2D) structure. An efficient
reaction system using SGO as solid acid catalyst was developed for the dehydration of
fructose into 5ꢀhydroxymethylfurfural (HMF). A high yield of HMF up to 94 % was
obtained with a low loading of SGO. Moreover, the SGO is also capable to convert
high concentration fructose (20 wt%) with an acceptable yield of 67%. The high
catalytic activity was attributed to the abundant active sites and the 2D structure
which enhances the accessibility of the active sites.
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Keywords：5ꢀhydroxymethylfurfural; fructose; glucose; sulfonated graphene oxide;
ionic liquid
1. Introduction
*
Corresponding author. Email:jumeit@nankai.edu.cn; Tel: +86 13820988813；
Address: College of Environmental Science and Engineering, Nankai University, 94 Weijin Street,
Tianjin, 300071, China.
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The fossil fuel reserves exhaustion and environmental issue are aggravating in
recent years, which serious threating the sustainable development 1, 2. Therefore, the
development of sustainable industry based on renewable feedstock is extremely
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important . Lignocellulosic biomass is a promising alternative feedstock because of
its abundance, environmental benignity and renewability. Therefore, biorefinery
technologies have been greatly developed to upgrade biomass into valueꢀadded
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chemicals, materials and fuels . In biorefinery, the transformations of biomass into
platform chemical 5ꢀhydroxymethylfurfural (HMF), a triple dehydration product of
hexoses, is a key reaction1. As a versatile intermediate, HMF can be converted to
various valueꢀadded chemicals, including levulinic acid (LA), γꢀvalerolactone (GVL)
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and furanꢀ2,5ꢀdicarboxylic acid (FDCA) . The conversion of cellulosic biomass into
HMF involves a series of complicated reactions, including the hydrolysis of cellulose
into glucose, isomerization of glucose to fructose, and the dehydration of fructose to
HMF 6, 7. The deconstruction of lignocellulose into sugars are catalyzed by acids or
enzymes in the presence of water 8. However, the presence of large amounts of water
is harmful to the dehydration process. The direct conversion of lignocellulose and
cellulose usually leads to relative yield and selectivity of HMF 9, 10. Therefore,
rational design of reaction system for each step is very important for the establishment
of commercially viable HMF production process 8.
Many homogenous catalysts, such as H2SO4, HCl, H3PO4 and transitionꢀmetal
ions, could convert fructose into HMF 11. However, these homogeneous catalysts
suffer from several drawbacks, including environmental pollutions and separation
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problems. To solve the disadvantages of homogenous catalysts, some solid acids,
including Amberlyst®ꢀ15, polyꢀbenzylic ammonium chloride resin, sulfonic
acidꢀfunctionalized metalꢀorganic frameworks (MOFꢀSO3H), phosphotungstic acid
encapsulated in metal organic framework and titanium hydrogenphosphate were
used for the dehydration fructose to HMF 6, 12ꢀ15. Moreover, carbonꢀbased solid acids
bearing SO3H, COOH, and phenolic OH groups prepared by sulfonation of
carbonized biomass were reported to be powerful solid acid catalysts for biomass
conversion16ꢀ20. However, high loading of solid catalyst is typically required due to
the limited accessible active sites in these solid catalysts, as will obstacle the practical
application of these catalysts on a large scale 21. Therefore, it’s very necessary to
increase the accessibility of active sites in solid acid catalysts to improve their
catalytic performance.
Compared with conventional solid acid catalysts, graphene oxide (GO), which is
prepared by chemical oxidation of natural graphite followed by exfoliation, seems to
be more fascinating due to its abundant functional groups, including COOH and OH,
as well as its unique twoꢀdimensional (2D) structure 21. Several reports have shown
that GO and its derivatives, sulfonated graphene oxide (SGO), are highly active
catalyst for many acidꢀcatalyzed reactions, such as hydrolysis of cellulose, hydrolysis
of glycitein and hydration of alkynes 21, 22. However, the preparation of SGO requires
complicate and harsh processes: chemical oxidation of natural graphite, reduction of
GO by reductant and sulfonation of reduced GO by concentrated H2SO4, fuming
sulphuric acid or chlorosulfonic acid 21, 23
.
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In this work, we develop a oneꢀpot method to synthesize SGO for the first time.
The prepared SGO was characterized by elemental analysis, FTꢀIR, Raman, XRD,
XPS, TGA and TEM. To evaluate its catalytic performance, the prepared SGO was
used to catalyze the dehydration of fructose and glucose into HMF. The influence of
reaction temperature, time, catalyst loading and reaction medium were investigated to
optimize the reaction conditions.
2. Experimental
2.1. Materials and reagents
Fructose (99%), glucose (99%), 5ꢀHydroxymethylfurfural (99%) and graphite
powder (99%) were purchased from Tianjin Heowns Biochem LLC (Tinjin, China).
The graphite powder was ballꢀmilled and then sifted with a 100 mesh sieve. The sifted
graphite powder (<150 ꢁm) was used for the preparation of graphene oxide. Ionic
liquids, 1ꢀallylꢀ3ꢀmethylimidazolium chloride (AMIMCl) was purchased from
Shanghai Chengjie Chemical Reagent Co. Ltd., China. All the other chemical reagents
were purchased from commercial sources in China and used as received.
2.2. Catalyst preparation
GO and SGOs were prepared using different approaches, as described in Scheme
1. GO was prepared by Modified Hummers method according the literature 24. Briefly,
graphite powder (3.0 g) and NaNO3 (1.5 g) was added to concentrated H2SO4 (69 mL)
in an ice bath. Keeping the reaction temperature below 20 °C, KMnO4 (9.0 g) was
added slowly in portions to the solution. Then the system was warmed to 35 °C with
stirring and kept for 30 min. After water (138 mL) was added slowly, the system was
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selfꢀheated to 98 °C and the reaction temperature was maintained at 98 °C for 40 min.
Afterward, the reaction mixture was cooled, diluted and further treated with additional
30% H2O2 (3 mL). After air cooling, the mixture was centrifuged (10000 rpm for 30
min), and the supernatant was decanted away. The remaining solid material was then
washed in succession with water, 30% HCl and ethanol to remove remnant salt. The
obtained solid was dewatered by freeze drying to obtain the graphite oxide powder.
SGOꢀ1 and SGOꢀ2 was prepared by Tours method according to previous report25.
For SGOꢀ1, graphite powder (3.0 g) was added to a 9:1 mixture of concentrated
H2SO4/H3PO4 (360:40 mL) in an ice bath. Keeping the reaction temperature below
20 °C, KMnO4 (9.0 g) was added slowly in portions to the solution. Afterward, the
reaction mixture was then heated to 50 °C and stirred for 12 h. After the reaction, the
mixture was purified as described for GO. SGOꢀ2 was prepared according the same
process by changing the amount of KMnO4 from 9.0 g to 18.0 g.
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SGOꢀ3 was prepared according to modified Tours method. Briefly, graphite
powder (3.0 g) and NaNO3 (1.5 g) was added to a 9:1 mixture of concentrated
H2SO4/H3PO4 (360:40 mL) in an ice bath. Keeping the reaction temperature below
20 °C, KMnO4 (18.0 g) was added slowly in portions to the solution. Afterward, the
reaction mixture was then heated to 50 °C and stirred for 12 h. After the reaction, the
mixture was purified as described for GO.
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Modified Hummers
GO
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KMnO4(9 g), NaNO3(1.5 g) H2SO4 (69 mL)
Graphite powder
(3 g)
Tours
SGO-1
KMnO4(9 g), H2SO4 (360 mL), H3PO4 (40 mL)
Tours
SGO-2
SGO-3
KMnO4(18 g), H2SO4 (360 mL), H3PO4 (40 mL)
Modified Tours
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KMnO4(18 g), NaNO3(1.5 g), H2SO4 (360 mL), H3PO4 (40
Scheme 1 Preparation of GO and SGOs by different approaches.
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2.3. Catalyst characterization
The elemental composition of the samples was determined by elemental analysis
by using an elemental analyzer (Euro EA3000, America). The acidity distributions
were measured by the Boehm titration method 17. Xꢀray photoelectron spectra were
performed using a ESCALAB 250Xi Xꢀray Photoelectron Spectrometer (Thermo
Fisher Scientific, America). A PHI ACCESS ESCAꢀV6.0 F software package was
used for data analysis. A Shirleyꢀtype background was subtracted from the signals. All
recorded spectra were fitted using GaussianꢀLorentzian curves to more accurately
determine the binding energy of the different element core levels. The powder Xꢀray
powder diffraction (XRD) patterns were recorded on a X'pert MFD diffractometer
using CuꢀKα radiation (λ=1.5406 Å) generated at a voltage of 40 kV and a current of
40 mA. Scans were obtained from 2θ = 5 to 70° at a scanning rate of 5° min−1. The
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Fourier transform infrared (FTIR) spectra were recorded using a FTS 6000 FTIR
spectrometer (Bioꢀrad, USA). The test samples were prepared using KBrꢀdisk method.
The spectra were measured from 4000 to 400 cm−1 at a resolution of 2 cm−1 in the
transmission mode. The Raman spectra were recorded with a DXR Raman system
(Thermo Fisher Scientific, America) with laser frequency of 514 nm as an excitation
source. The surface morphology of all the samples was analyzed by a Tecnai G2 F20
SꢀTWIN high resolution transmission electron microscope (FEI, America).
2.4. Catalytic tests
In a typical procedure, 10 mg catalyst was added into 2 g of DMSO in a 10 ml
quartz tube reactor. The catalyst was sufficiently dispersed into DMSO by
sonication at 400W for 30 min, and then 100 mg of fructose was added to the reactor.
The reactor was sealed, and then heated to 120 °C in an oil bath and maintained for 6h.
After the desired reaction time elapsed, the reactor was cooled to room temperature
with cold water bath within 1 min, and water (1.0 g) was added to the reactor. The
sample was diluted with water and then filtered with
a
0.45 ꢁm
polytetrafluoroethylene filter membrane to remove the insoluble solid. The
concentration of HMF was measured with a UV3100PC spectrophotometer (Shanghai
Meipuda instrument Co., LTD, China) at 284 nm by using the standard curve method.
The concentration of glucose and fructose was measured with a highꢀperformance
liquid chromatography (HPLC) systems equipped with an evaporative light scattering
detector (SofTA, ELSD Model 300s) and a Xtimate ®SugarꢀCa analytical column
(7.8×300mm, 5µm). The column oven temperature was set as 80 °C, and the mobile
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phase was ultrapure water at a flow rate of 0.50 mL/min. The sugar conversion, HMF
yield and selectivity were calculated according to the following equations:
(moles of starting sugar − moles of remaining sugar)
ꢀ
ꢁ
Sugar conversion mol% =
×100%
moles of starting sugar
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(1)
moles of produced HMF
ꢀ
ꢁ
HMF yield mol% =
×100%
moles of starting sugar
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(2)
×100%
moles of produced HMF
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HMF selectivity mol% =
(moles of starting sugar − moles of remaining sugar)
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(3)
All results were replicated at least three times, and reproducibility of sugar conversion,
HMF yield and HMF selectivity were within 3% standard deviation.
3. Results and discussion
3.1. Catalyst structure and properties
3.1.1. Composition and acidity distributions
After drying, all the samples were subjected to elemental analysis. As shown
in Table 1, the C content in GO, SGOꢀ1, SGOꢀ2 and SGOꢀ3 are 53.51, 50.31, 45.45,
and 44.52 wt%, respectively, revealing that the overall order of oxidation is SGOꢀ3 >
SGOꢀ2 > SGOꢀ1 > GO. The S content in GO, SGOꢀ1, SGOꢀ2, and SGOꢀ3 are 1.43,
4.21, 2.54 and 5.19 wt%, respectively. Since all the sulfur atoms are present in the
form of SO3H groups21, 26, as is demonstrated by Xꢀray photoelectron spectroscopy in
section 3.1.3, the corresponding concentrations of SO3H were calculated to be 0.45,
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1.31, 0.80 and 1.62 mmol gꢀ1. The acidity distributions were analyzed by the Boehm
method and are listed in Table 2, indicating those materials contain abundant acid
sites, including carboxylic groups, phenolic groups, lactonic groups and sulfonic
group.
Table 1
Elemental analysis of the GO and SGO samples.
Sample
C
H
O
N
S
Concentration of ꢀSO3H
[wt%] [wt%] [wt%] [wt%]
[wt%]
1.43
4.21
2.54
5.19
[mmol gꢀ1]
0.45
GO
53.51
2.01
2.01
2.29
2.20
42.97
43.23
48.73
47.98
0.10
0.27
0.03
0.14
SGOꢀ1 50.31
SGOꢀ2 45.45
SGOꢀ3 44.52
1.31
0.80
1.62
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Table2
Acidity distributions of the GO and SGO samples.
Sample Sulfonic groups Carboxylic groups Phenolic groups Lactonic groups Total acid sites
(mmol/g)a
(mmol/g)b
(mmol/g)c
(mmol/g)d
f
[mmol gꢀ1]e
GO
0.45
1.31
0.80
1.62
0.55
0.48
0.63
0.50
0.68
0.54
0.75
0.59
0.23
0.17
0.24
0.14
1.91
2.50
2.42
2.85
SGOꢀ1
SGOꢀ2
SGOꢀ3
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a Calculated from S content by elemental analysis.
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Obtained by subtracting concentration of sulfonic groups from titration results with
NaHCO3.
c
Obtained by subtracting titration results using NaHCO3 from calculated values of
Na2CO3.
d
Obtained by subtracting titration results using Na2CO3 from calculated values of
NaOH.
e Obtained by titration with NaOH.
3.1.2. FTIR spectroscopy
As shown in Figure. 1, all of the samples show a band between 1732 and 1739
cmꢀ1 attributed to the C=O stretch vibration, which is associated with carboxylic acids
and carbonyl moieties 23. The bands at 1574 and 1443 cmꢀ1 are assigned to the
skeleton vibration of the C=C bonds of unoxidized graphitic domains 27. The FIIR
spectra of SGOꢀ1, SGOꢀ2 and SGOꢀ3 show significant bands at 1393, 1057 and 857
cmꢀ1, which were identified as O=S=O stretching vibration 23, ꢀSO3ꢀsymmetrical
stretching vibration, and SꢀOH stretching vibration, respectively. These bands are the
characteristic absorption of SO3H, confirming that SO3H is grafted on the graphene
surface during the oxidation process.
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Figure. 1 FTIR spectra of GO and SGOs prepared by different methods.
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3.1.3. X-ray photoelectron spectroscopy
The wideꢀrange scanning XPS spectra (Figure. 2a) show the absorption peaks of
C with a binding energy of 286 and 978 eV, the absorption peaks of O with a binding
energy of 533 eV, and the absorption peak of S with a binding energy of 168 eV,
confirming that the existence of C, O, and S element in all the samples 21. The
highꢀresolution C 1s XPS spectra (Figure. 2b) show peaks corresponding to CꢀC,
CꢀO, C=O and COOH, confirming the presence of these oxygenꢀcontaining groups 21,
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To investigate the existing state of sulfur, the highꢀresolution S 2p spectra were
recorded. As shown in Figure. 2c, there is a single Gaussian distribution peak at 169.0
eV corresponding to SO3H in all the samples 26, while the peak at 163.9 eV
corresponding to sulphide is absent 24. However, whether the peak around 168 eV
should be assigned to CꢀS bond energy site or to OꢀS bond energy site is contradictory
in different literatures. Zhao et.al attributed the peak at 168.2 eV in GO to sulphate,
24
that is OꢀS bond energy site between SO3H and O . Wei et.al prepared GO using a
similar method, but they assigned the peak at 167.8 to CꢀS bond energy site between
SO3H and C 21. Although it’s difficult to distinguish CꢀS and OꢀS bond, the
highꢀresolution S 2p spectra demonstrated that all the S was grafted on the graphene
nanosheets in the form of SO3H. It is notable that the concentration of SO3H in
SGOꢀ1 and SGOꢀ3 is comparable with that of previously reported SGO prepared by
postꢀmodification method 21, 23. The total acidity sites determined by acidꢀbase
titration are listed in Table 2, also supporting the relative high acid strength of SGOꢀ1
and SGOꢀ3. The postꢀmodification method usually involves complicated and harsh
processes, including partial reduction of GO with hydrazine and sulfonation of
reduced GO with sulfanilic acid, chlorosulfonic acid, concentrated sulfuric acid or
fuming sulphuric acid 21, 23, 28, 29. In the present work, the SGO bearing abundant
SO3H functional groups can be obtained directly though oneꢀpot reaction. Compared
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with previous method, our method is very convenient and effective to prepare SGO.
(a)
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(b)
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(c)
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Figure. 2 Wideꢀrange scanning XPS spectra (a), highꢀresolution C 1s XPS spectra (b),
and highꢀresolution S 2p XPS spectra (c) of GO and SGOs prepared by different
methods.
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3.1.4. X-ray powder diffraction
The Xꢀray powder diffraction (XRD) spectrum was used to characterize the
crystalline nature of the GO and SGOs (Figure. 3). For all samples, the diffraction
peak at 2θ= 26.4° that characterizes the 0.335 nm interlayer distance of pristine
graphite27 has disappeared completely, indicating that almost no starting material
(graphite flakes) are present in these samples. The interlayer spacings are 0.82, 0.87,
0.88 and 0.93 nm for GO, SGOꢀ1, SGOꢀ2 and SGOꢀ3, respectively. Usually the
degree of oxidation of GO is considered to be proportional to the interlayer spacing,
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since the interlayer spacing increases as the polar oxygenꢀcontaining groups increase
during the oxidation process 25. The degree of oxidation revealed by XRD spectra is
consistent with elemental analysis, demonstrating that SGO with higher degree of
oxidation could be obtained through Tour’s method and modified Tour’s method.
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Figure. 3 XRD spectra of GO and SGOs prepared by different methods.
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3.1.5. Raman spectroscopy
All these solid materials were characterized with Raman spectroscopy, a
nonꢀdestructive technique which is widely used to analyze the structure of carbon
material.
As shown in Figure. 4, both D peaks at around 1590 cmꢀ1 and G peaks at around 1350
cmꢀ1 were identified in all samples 25, suggesting that all four materials were grossly
similar. The ratio between the area of the D and G bands (ID/ IG) is generally used a
measure for the oxidation level and the size of sp2 ring clusters in a sp3/sp2 hybrid
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network of carbon atoms 27. The ID/ IG ratio is 1.35, 1.46, 1.56 and 1.58 for the GO,
SGOꢀ1, SGOꢀ2 and SGOꢀ3, respectively. The increased ID/ IG ratio is consistent with
the increased structural disorder originating from the modification of the carbon
surface by the introduction of sulfur groups and oxygenꢀcontaining groups.
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Figure. 4 Raman spectra of GO and SGOs prepared by different methods.
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3.1.6. Transmission electron microscopy
The morphology of the asꢀsynthesized GO and SGOs was investigated using
highꢀresolution transmission electron microscope (HRꢀTEM). The TEM images
(Figure. 5) show that all samples have irregular edges and transparent lamella,
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indicating that they are composed of large flakes of GO with a few layers thick.
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Figure. 5 Highꢀresolution transmission electron microscope images of GO and SGOs
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prepared by different methods.
3.2. Conversion of fructose into HMF
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3.2.1. Conversion of fructose into HMF with different catalysts
To evaluate the performance of the prepared GO and SGOs as a heterogeneous
solid acid catalyst, the conversion of fructose into HMF was conducted in the medium
of DMSO. DMSO could not only dissolve fructose, but also promote the dispersion of
catalyst after ultrasonic exfoliation. Figure. 6 shows the catalytic performance of
different catalysts for the dehydration of fructose to HMF in DMSO at 120 °C for 60
min. Among the investigated catalysts, SGOꢀ3 shows the best catalytic performance
during the reaction process and reaches a maximum value of fructose conversion of
89% with high HMF yield (85%). SGOꢀ1 can also afford a good HMF yield (81%),
while the fructose conversion and HMF yield for GO and SGOꢀ2 are relative low. The
catalytic activity is influenced not only by the density of active sites including
strongly acidic SO3H groups and the weakly acidic OH, COOH groups, but also by
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the accessibility of active sites 24. Compared with other samples, SGOꢀ3 has the
highest degree of oxidation with the highest density of SO3H group and largest
interlayer spacing, thus leading to the superior catalytic performance. Apart from
fructose conversion and HMF yield, HMF selectivity for different catalysts was also
evaluated. All the catalysts could convert fructose into HMF with an excellent
selectivity (around 95%), suggesting rare byꢀproduct was formed under the
investigated conditions.
100
90
80
Fructose conversion
70
5-HMF yield
60
5-HMF selectivity
50
40
GO
SGO-1 SGO-2 SGO-3
293
294
295
296
Figure. 6. The comparison of catalytic performance of GO and SGOs prepared by
different approaches. Reaction conditions: fructose (100 mg), catalyst (10 mg),
DMSO (2 mL), 120 °C, 1h.
297
298
299
300
301
302
303
3.2.2. Influence of temperature and catalyst loading on HMF yield
In the subsequent experiment, the influence of reaction temperature and time on
fructose dehydration to HMF was studied. Generally, high temperature could not only
promote the dehydration of fructose, but also accelerate the sideꢀreactions, including
polymerization of HMF with sugars to form humins, and rehydration of HMF to form
levulinic acid and formic acid 11. As shown in Figure. 7a, at 80 °C the yield of HMF
increased slowly to 59 % in 240 min. When the temperature increased to 120 °C, the
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306
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yield of HMF increased to 85% only in 60 min. When the temperature was further
evaluated to 140 °C, the yield of HMF increased quickly to 84% at the initial 60 min,
but the prolonged reaction time will lead to obvious decrease of HMF yield, possibly
as a consequence of the further reaction of produced HMF.
Afterwards, we tested the effect of catalyst loading on the conversion of fructose
at 120 °C. As shown in Figure. 7b, in the absence of catalyst the HMF yield was less
than 25% during the reaction time of 120 min. When the catalyst loading was
increased from 10 mg to 20 mg, there was a slight decrease of the HMF yield, which
may result from the excess acidꢀactive sites that catalyze not only the dehydration of
fructose to HMF but also the degradation of the formed HMF into other products,
314
315
such as humins 17.
(a)
100
90
80
70
80 ℃
60
100 ℃
50
120 ℃
40
140 ℃
30
20
0
60
120
180
240
Time (min)
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317
(b)
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100
90
80
70
60
50
40
30
20
10
0
20 mg
10 mg
0 mg
0
50
100
150
Time （min）
318
319
320
321
Figure. 7 Effect of temperature (a) on the yield of HMF using 10 mg SGOꢀ3 and
effect of catalyst loading (b) on the yield of HMF at 120 °C. Reaction conditions:
fructose (100 mg), DMSO (2 mL).
322
323
324
325
326
327
328
329
330
331
332
333
334
3.2.3. Influence of solvent on HMF yield
It has been shown that DMSO is very effective reaction medium for fructose
dehydration, as is consistent with previous reports 11, 30. In order to search for
alternative
reaction
medium,
several
different
solvents
including
1,3ꢀdimethylꢀ2ꢀimidazolidinone (DMI), γꢀvalerolactone (GVL) and ionic liquid
1ꢀallylꢀ3ꢀmethylꢀmethylimidazolium chloride (AMIMCl) were tested as reaction
medium for the production of HMF. DMI and GVL can’t give HMF in high yield in
this reaction conditions. When ionic liquid AMIMCl was used as reaction medium,
the catalyst can’t be exfoliated by ionic liquid AMIMCl after 2h ultrasonic treatment
31
(Figure. 8a) , as is in accord with the previous report . Although SGO can’t form a
stable dispersion in AMIMCl, we observed relative high HMF yield of 81% from
AMIMCl using SGO as catalyst (Figure. 8b). It has been reported that HMF yield up
to 72% with a fructose conversion approaching 100% was obtained from ionic liquid
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336
337
338
339
340
[EMIM]Cl in the absence of catalyst 32. In order to affirm that the excellent HMF
yield was not just the consequence of ionic liquid, we did a control experiment
without catalyst. As shown in Figure. 8c, HMF yield was less than 53% during the
whole reaction process in the absence of catalyst. When a mixture of AMIMCl and
DMSO (w:w 1:1), which could not only dissolve high concentration of fructose but
also disperse SGO, was used as reaction medium, an superior HMF yield up to 94%
341
342
with a fructose conversion approaching 100% was obtained.
(a)
343
344
(b)
AMIMCl
100
AMIMCl-
DMSO(1:1)
80
60
40
20
0
DMSO
DMI
GVL
0
60
120
180
Time (min)
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346
(c)
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100
80
60
40
20
0
AMIMCl-
DMSO(1:1)
DMSO
AMIMCl
0
60
120
180
Time (min)
347
348
Figure. 8 Photographic images of SGOꢀ3 dispersed in different solvents (a). Effect of
solvents on the yield of HMF using SGOꢀ3 (10 mg) as catalyst (b) and without
catalyst (c). Reaction conditions: fructose (100 mg), solvent (2 mL), 120 °C.
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
3.2.4. Influence of initial fructose concertation on HMF yield
One major disadvantage of solid acid catalysts is that they can’t process high
concentration feed, as serious restrict their practical applications. As mentioned above,
the unique 2D structure of SGO may enhance the accessibility of the active sites. In
the subsequent experiment, we increased initial fructose concentrations to investigate
their influence on HMF yield. As shown in the Table 3, along with the increase of the
fructose amount from 5 to 10%, the HMF yield decreased from 95 to 88%. Even if the
initial concentration of fructose increases to 20 wt%, the HMF yield remain
acceptable (67 %). It should be noted that the loading of catalyst in the present work
is much lower than that of other solid acid, while the HMF yield here is comparable to
or better than the reported values. Although the reaction conditions, including
temperature, time and solvent aren’t completely same, this comparison demonstrates
that SGOꢀ3 prepared in this work has superior catalytic performance. Since both the
type and density of acid sites in SGOꢀ3 are similar with other carbonaceous solid
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366
acids, the superior catalytic performance of SGOꢀ3 is mainly owing to synergy of the
abundant acid sites and its easily accessible 2D structure.
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368
369
Table 3
Comparison of the catalytic performance of the SGOꢀ3 with other reported solid acid
catalysts
Entry
Catalyst
Catalyst loading
Fructose loading
HMF yield
Refs.
(wt%)
0.5
0.5
0.5
0.5
6
(wt%)
5
(%)
94
1
2
SGOꢀ3
SGOꢀ3
This work
10
15
20
10
20
5
88
This work
3
SGOꢀ3
75
This work
4
SGOꢀ3
67
This work
5
MILꢀ101(Cr)ꢀSO3H
CꢀSO3H
76
5
6a
7a
8b
5
71
13
13
24
CꢀSO3H
1
<50
< 50
βꢀcyclodextrinꢀSO3H
0.5
1
a
370
371
372
373
CꢀSO3H is carbonaceous solid material prepared by incomplete hydrothermal carbonization of
13
cellulose followed by both chemical activation with KOH and sulfonation with H2SO4
.
b
βꢀcyclodextrinꢀSO3H is carbonaceous solid acid catalyst prepared by one step hydrothermal
carbonization of βꢀcyclodextrin and pꢀTSA 24
.
374
375
376
377
378
379
3.2.5. Recycling of catalyst
To study the recyclability and stability of catalyst, recycling experiment was
performed for five cycles. In this experiment, fructose dehydration was catalyzed by
SGOꢀ3 in DMSO at 120 °C for 60 min. After five recycles, the conversion of fructose
and HMF yield are 85 % and 81%, respectively (Figure. 9). This result indicates that
the catalyst can be reused with slightly loss of catalytic activity.
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100
80
60
40
20
0
Fructose
conversion
HMF yield
1
2
3
4
5
Recyling number
380
381
382
Figure. 9 Catalyst recycling. Reaction conditions: fructose (100 mg), SGOꢀ3 (10 mg),
DMSO (2 mL), 120 °C, 60 min.
383
384
385
386
387
388
389
390
391
392
3.3. Conversion of glucose into HMF
Besides fructose, the reaction system was further extended to produce HMF using
glucose as feedstock. At the optimized reaction conditions for fructose, both the
conversion of glucose and HMF yield were remarkably inferior. We optimized the
reaction conditions by increasing the reaction temperature and reaction time. However,
even at 140 °C for 6h, the yield of HMF is around 30% with a glucose conversion less
than 60%. As commonly considered, isomerization of glucose to fructose is a
necessary step in the transformation of glucose to HMF. The low yield of HMF
suggest that the catalyst is not capable to catalyze the isomerization of glucose to
fructose.
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70
60
50
40
30
20
10
0
120 ℃ HMF yield
140℃ HMF yield
120℃ glucose
conversion
140℃ glucose
conversion
0
100
200
300
400
500
Time (min)
393
394
Figure. 10 Influence of reaction temperature and time on the dehydration reaction of
395
396
glucose. Reaction conditions : SGOꢀ3 (10 mg), glucose (100 mg), AMIMClꢀDMSO
(w: w 1
：1，2 mL), 120 °C.
397
398
399
400
401
402
403
404
4. Conclusion
GO and SGOs were synthesized using different approaches, and the
characterization indicated that SGO bearing abundant SO3H can be obtained through
oneꢀpot reaction according to modified Tours method. Using the SGO as solid acid
catalyst, we developed an efficient reaction system for the dehydration of fructose into
HMF. A high yield of HMF up to 94 % was obtained with a low loading of catalyst.
The excellent catalytic activity of SGO is owing to the abundant active sites and its
2D structure. In addition, the catalyst exhibits good reusability.
405
Acknowledgements
406
407
408
This work was supported by the International Joint Research Projects in the
Science & Technology Pillar Program of Tianjin, China (13RCGFSF14300), Research
Projects in the Science & Technology Pillar Program of Tianjin, China
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(14TXGCCX00012), Research Projects in the Science & Technology Program of
Jinnan District Tianjin, China (2015JNKW0005) and Research Projects in the Science
& Technology Pillar Program of Tianjin, China (15JCTPJC63300).
412
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Sulfonated graphene oxide prepared through one-pot method show excellent
catalytic performance for the dehydration of fructose into 5-hydroxymethylfurfural.