Graphene oxide as a facile acid catalyst for the one-pot conversion of carbohydrates into 5-ethoxymethylfurfural

Article abstract of DOI:10.1039/c3gc41109e

Graphene oxide obtained by the Hummers method was discovered to be an efficient and recyclable acid catalyst for the conversion of fructose-based biopolymers into 5-ethoxymethylfurfural (EMF). EMF yields of 92%, 71%, 34% and 66% were achieved when 5-hydroxymethylfurfural (HMF), fructose, sucrose and inulin were used as starting materials, respectively.

Full text of DOI:10.1039/c3gc41109e

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Graphene oxide was used as a facile carbon catalyst for converting renewable carbon source into
a potential biofuel 5-Ethoxymethylfurfural.

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Graphene oxide as a facile acid catalyst for the one-pot conversion of
carbohydrates into 5-ethoxymethylfurfural
a,b
a
a
a,c
a
d
Hongliang Wang, Tiansheng Deng, Yingxiong Wang, Xiaojing Cui, Yongqin Qi, Xindong Mu and
a
a,c
Xianglin Hou,* Yulei Zhu*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Graphene oxide obtained by Hummers method was
discovered to be an efficient and recyclable acid catalyst for
the conversion of fructose-based biopolymers into
5-ethoxymethylfurfural (EMF). EMF yield of 92%, 71%,
material, while it is still troublesome to achieve high yields of
EMF directly from fructose or other cheaper biopolymers by a
one-pot process.
50
Carbon nanomaterials hold promise in the development of
sustainable catalysts for green chemistry with low emission and
1
0
3
4% and 66% were achieved when HMF, fructose, sucrose
2
0, 21
high selectivity.
Graphene-based materials in the growing
and inulin were used as start materials, respectively.
field of “carbocatalysis” have become importance in recent years
due to their unique chemical properties, high mechanical
resistance, and propitious to the charge generation and
transportation in catalysis processes. Graphene oxide (GO) has
functioned primarily as a precursor in the large-scale production
of graphene. One of the most widely applied ways for the
preparation of GO is the Hummers method, which is based on the
The diminishing fossil reserves and environmental problems
necessitate the development of processes to use sustainable, green,
and environmentally benign resources for fuels and bulk
55
2
1
1
2
2
3
3
4
4
5
0
5
0
5
0
5
1
-3
chemicals production. As a widely available and sustainable
carbon source, biomass has the potential for the replacement of
4
current non-renewable fossil resources. It is predicted by the
roadmap for Biomass Technologies in the U.S. that 18% of all 60 exhaustive oxidation of graphite in strong acid conditions (in
5
concentrated sulfuric acid) by using permanganate and hydrogen
manufactured chemicals will originate from biomass by 2020.
The boom in the chemical industry based on biomass demands
the development of green and economical technologies for the
efficient conversion of biomass into important platform
2
2
peroxide.
alcohols, epoxides, and carboxylates,) as well as a small quantity
of sulfate groups were introduced into the graphene plane during
A variety of oxygen-containing functionalities
(
molecules, as well as the ways for converting these platform 65 the synthesis of GO in the harsh conditions. These extrinsic
6
, 7
molecules into various base chemicals.
-Hydroxymethylfurfural (HMF),
functional groups make GO an excellent catalyst for various
synthetic transformations. To date, GO has been tested to be a
powerful catalyst in the oxidation of benzylic alcohols,
5
produced
from
carbohydrates such as fructose, glucose, sucrose, inulin and even
cellulose, has been envisaged as one of the top building block
2
3-25
unsaturated hydrocarbons, sulfides and thiols.
In addition, GO
8
-11
chemicals in biorefinery processes.
It can be transformed into 70 obtained by the conventional Hummers method is reported to be a
a number of important compounds with a high potential in fuel or
polymer applications, including furan derivatives such as
highly efficient, recyclable acid catalyst for the ring opening of
epoxide.
2
6
2
2
,5-furandicarboxylic
,5-bishydroxymethylfuran, 2,5-dimethylfuran, and nonfuranic
acid,
2,5-diformylfuran,
compounds such as levulinic acid, 1,6-hexanediol, adipic acid,
4
, 12-14
caprolactam, and caprolactone.
EMF), synthesized from etherification of HMF with ethanol, has
attracted much attention as a potential biofuel alternative
5-Ethoxymethylfurfural
(
1
5-18
-1
recently.
2
The energy density of EMF is 30.3 MJ L , which is
9% greater than that of ethanol (23.5 MJ L ) and very close to
that of diesel (33.6 MJ L ). EMF has generated positive results
-
1
75
Scheme 1 The conversion of carbohydrates to EMF catalyzed
by GO.
-
1 19
with a significant reduction of soot and SO emissions in engine
X
1
9
Intrigued by the previous studies, we were interested to
examine if GO, a readily available and inexpensive material, can
be used as an efficient carbon catalyst in the transformation of
renewable carbon source into useful chemicals. GO obtained
directly by the Hummers method without any further treatment
was used since it not only could be conveniently prepared but
tests. Heteropolyacids, H-form zeolites, mesoporous silica, ion
exchange resins, and hybrid solid catalyst of
MIMBS] PW O have been used for the synthesis of EMF
a
80
[
3
12 40
1
5-18
from both HMF and fructose in ethanol.
Acceptable yields of
EMF have been obtained when HMF was used as the start
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also contains lots of recoverable Brønsted acid sites. Typically,
HMF conversion of 96% with a catalyst loading of 20 mg,
graphite powder (8000 meshes, 5 g), suspended in sulfuric acid 60 indicating GO an excellent acid catalyst for this reaction.
(
115 mL, 98 wt %), was oxidized by NaNO (2.5 g), KMnO (15
Furthermore, the effect of GO loading on the catalytic
performances was explored. The yield of EMF was decreased to
83% at a low HMF conversion of 85% when the GO loading was
reduced to 10 mg. An increase in the catalyst loading from 20 mg
3
4
g) and H O (50 mL, 30wt %) in sequence. After that, it was
washed by HCl (1 L, 1 mol·L ) and a large amount distilled
water. Then the graphite oxide suspension was ultrasonicated to
2
2
-
1
5
0
5
0
5
0
5
0
5
0
5
form the graphene oxide. Finally the GO suspension was dried to 65 to 30 mg resulted in a slight improvement in HMF conversion,
get the GO powder. The detailed procedure of GO preparation
could be seen in ESI† The performances of GO as an acid
catalyst was tested for the conversion of carbohydrates into EMF,
including the hydrolysis of biopolymers into fructose,
however caused a decrease in EMF yield. Increase of the GO
loading to 40 mg resulted in a further decrease of the EMF yield.
The promotion of side-reactions including the alcoholysis of
HMF to LAE and the formation of insoluble products was
1
1
2
2
3
3
4
4
5
5
dehydration of fructose into HMF and etherification of HMF into 70 observed during the increase of GO loading. Thus, a low catalyst
EMF (Scheme 1). It is known that all the aforementioned
reactions (hydrolysis, dehydration and etherification) are readily
promoted by acid catalysts.
loading resulted in a low HMF conversion that led to a low EMF
yield; a high catalyst loading resulted in a high HMF conversion
but may also induce side reactions that led to a low EMF yield.
Therefore, the amount of catalyst should be properly added, and
In an optimized experiment, 0.5 mmol (63 mg) HMF was
added into a 5 mL micro autoclave and dissolved with 1 mL 75 20 mg GO was proved to be the best for the present reaction
system.
Table 1 Etherification of HMF with ethanol into EMF over
ethanol, and then heated to 100 °C in the presence of 20 mg GO
for 12 h. After the reaction, GO was removed by filtration. HMF
was found to be transformed into EMF with a yield of 92% and a
conversion of 96%, as determined by NMR and LC/MS. During
the reaction, some active sites on the GO surface may dissolve
into the solution, and these leached species may act as new active
sites. To rule out this possibility, the reaction was first carried out
with GO under the same conditions for 6 h, and then GO was
filtered from the reaction mixture. An EMF yield of 74% was
achieved. The solution in the absence of GO was continued to stir
at 100 °C for another 6 h, and no further HMF conversion and
product formation were observed. These results indicate that the
catalysis indeed occurs on the surface of GO and GO is a truly
heterogeneous catalyst. Furthermore, when GO was replaced by
the natural flake graphite (used as the starting material to prepare
GO) or the reduced graphene oxide (R-GO, the
oxygen-containing functionalities and sulfate groups were
removed by thermal treatment at 400 °C ) under the same reaction
conditions, no reactions were observed. This result reveals that
extrinsic functional groups existed in GO play a vital role in the
catalytic etherification of HMF with ethanol.
a
GO and other catalysts.
Entry
Catalyst
Loading HMF conversion EMF yield
(%)
(%)
1
GO
20 mg
20 mg
96
—
92
—
—
54
61
82
59
83
89
85
b
2
graphite
R-GO
H SO
c
3
20 mg
—
4
5
6
7
8
9
3 mol%
3 mol%
5 mol%
100
100
100
99
2
4
p-TSA
H PW O
3
12 40
Amberlyst-15 10 mg
GO
GO
GO
10 mg
30 mg
40 mg
85
To evaluate the catalytic performance of GO, various other
common acid catalysts were used in the identical reaction
conditions and the results are summarized in Table 1.
Homogeneous acids such as H SO , p-toluenesulfonic acid
99
2
4
1
0
99
(
p-TSA) and H PW O were tested in the reaction, and yields of
3 12 40
EMF were 54%, 61% and 82% with complete conversion of
HMF, respectively. These results are consistent with previous
a
Reaction conditions: 0.5 mmol HMF with specific catalyst was
b
mixed in 1 mL ethanol and reacted at 100 °C for 12 h. Natural
flake graphite was purchased from Sinopharm Chemical Reagent
Co. Ltd. China and used without further purification. Reduced
graphene oxide was prepared by thermal treatment at 400 °C .
1
5, 17
studies.
The main by-product was detected to be ethyl
c
levulinate (LAE). Moderate yields of EMF with high conversions
of HMF were achieved by using H SO and p-TSA probably due
2
4
to the strong acidity of the two catalysts, which could give rise to
ring-opening alcoholysis of HMF and thus leads to the formation
of ethyl levulinate. A relatively high yield of EMF was obtained
when H PW O was used as the catalyst. However, it was
8
0
The effects of reaction temperature and reaction time on the
etherification of HMF were also studied (Fig. 1). At a low
temperature of 80 °C , the yield of EMF was only 68% for 12 h.
Even if the reaction time was prolonged to 20 h, the EMF yield
3
12 40
dissolved in the reaction mixture, making it difficult in separation
and recycle. Amberlyst-15, a widely used heterogeneous acid
catalyst, was also subjected into this reaction, and it was found
that the yield of EMF as well as the conversion of HMF was
similar to that use of H SO . As listed in table 1, among all the
85
slightly increased to 80%. While, it was significantly improved to
9
1
2% within 12 h by just increasing the reaction temperature to
00 °C , indicating the high temperature enhanced the rate of
2
4
HMF conversion. When the reaction was further carried out at
20 °C , EMF yields were obviously higher than those at 100 °C
screened catalysts, GO exhibited the highest yield up to 92% at a
1
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in the initial 8 h, and the maximum yield of 85% was obtained
within 7 h. In order to better understand the effect of temperature
and 18%, respectively, when fructose instead of HMF was used
as the start material in the reaction under conditions similar to
on the etherification of HMF to EMF, we have constructed the 35 those used in the etherification reaction (Table 2, Entry 1). The
kinetics profiles at different temperatures and calculated the
activation energy. The detailed calculation method could been
seen in ESI† The value of k (rate constant for HMF conversion)
increased from 1.07 h to 0.219 h and 0.418 h when the
temperature increased from 80 °C to 100 °C and 120 °C , 40 the reaction at 100 °C may root in two aspects. The one is that
total yield of furan products was much low (27%) as compare
with the high conversion of fructose (95%). LAE was detected,
but its yield was low, too. Moreover, a small amount of brownish
black humins was observed. The low selectivity towards furans in
5
0
5
-
1
-1
-1
respectively. Based on the rate constants we obtained, the
activation energy (Ea) was calculated by the Arrhenius equation,
and the apparent activation energy of the catalytic system was to
there may be existence of abundant of partially dehydrated
intermediates produced from the process of fructose dehydration
to HMF at a low reaction temperatures (<130 °C), and the other is
that side reactions such as polymerization of these intermediates
1
-
1
be about 39.93 kJ mol .Thus, it could be confirmed that
2
7
temperature has a positive effect on promoting this reaction, the 45 and alcoholysis of HMF have occurred in ethanol. In our
1
6
higher the reaction temperature, the higher the etherification rate.
However, HMF is unstable at high temperatures since it can be
readily transformed into LAE and humins by alcoholysis and
polymerization. This leads to a low yield of the EMF as shown in
Fig. 1.
previous studies, it was found that adding a certain volume of
DMSO (dimethylsulfoxide) into ethanol could efficiently
improve the yields of furan products without causing much
trouble in the product separation. Thus, 0.3 mL DMSO mixed
50 with 0.7 mL ethanol was used as the reaction solution. The yield
of HMF was significantly enhanced when DMSO was introduced
1
(Table 2, Entry 2), indicating that DMSO plays a positive role in
the promotion of dehydrated intermediates into HMF.
55
Table 2 Production of EMF from various carbohydrates over
GO under different conditions.
a
Entry Substrate Tempe. Cat.
Substrate
HMF EMF
yield yield
(
°C ) Loading conversion
(mg)
(%)
(%)
(%)
b
1
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Sucrose
Inulin
100
20
20
20
30
30
40
30
30
30
95
98
9
31
14
9
18
23
64
71
62
67
34
66
3
2
100
130
130
150
130
130
130
130
2
0
3
4
5
6
7
8
9
100
100
100
100
100
100
99
3
6
4
7
Glucose
1
a
Reaction conditions: 0.5 mmol carbohydrate based on
monosaccharide with specific amount of GO mixed in 0.7 mL
ethanol and 0.3 mL DMSO reacted for 24 h. 0.5 mmol fructose
with GO mixed in 1 mL ethanol and reacted at 100 °C for 24 h.
Fig. 1 Effects of reaction temperature and reaction time on the
conversion of HMF to EMF. Reaction conditions: 0.5 mmol
HMF, 20 mg GO, 1 mL ethanol.
b
2
5
HMF can be obtained from dehydration of carbohydrates,
Subsequently, effects of reaction temperature and catalyst
especially readily from fructose, by using acid catalysts which are 60 loading on the performance of GO were investigated in
similar to those used in the aforementioned etherification reaction.
Thus one-pot synthesis of EMF from low-cost carbohydrates was
investigated in this work and the results are listed in Table 2.
Initially, fructose was used as the substrate in the reaction. The
DMSO-ethanol solution. The total yield of furan products was
increased from 54% to 78%, when the temperature increased
from 100 °C to 130 °C . These results reveal that the dehydration
of fructose requires higher temperature than that for etherification
3
0
main products detected were HMF and EMF with yields of 9% 65 of HMF. The yield of EMF was further improved when the GO
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loading was increased to 30 mg. However, further increasing the
mixture by filtration, and washed by 95% ethanol and acetone for
three times, respectively. Then the washed GO was dried in an
oven at 60 °C for 12 h. It was found the activity of the catalyst for
the conversion of fructose to EMF had a little drop but not
reaction temperature or GO loading resulted in a drop in the furan
products yield, since side reactions could be promoted by high
temperature or overmuch active acid sites. Moreover, the
5
0
5
conversion of fructose and the formation of products as a function 50 obviously after four consecutive reuses (Fig. S1, ESI†), and this
of reaction time were carefully studied. As depicted in Fig. 2,
complete fructose conversion is achieved within 2 h. In the initial
stage of the reaction HMF is observed with a maximum yield of
might be due to the fact that GO may undergo partial thermal
reduction (some oxygen-containing functionalities were lost
under thermal treatment) with long time exposure of high
2
9
5
4% at about 6 h, and then it decreases gradually. Meanwhile, the
temperature. The partial thermal reduction could lead to a minor
1
yield towards EMF gradually increases to a maximum of 71% at 55 decrease in the acid strength of GO, and this was supported by
about 24 h. This behaviour supports that the conversion of
fructose in ethanol–DMSO system is a typical consecutive
reaction, and HMF is an intermediate in EMF production from
fructose.
results of the Infrared Fourier Transform spectroscopy (FT-IR)
(Fig. S3, ESI†) and the ammonia adsorption Diffuse Reflectance
Infrared Fourier Transform spectroscopy (DRIFT) (Fig. S4,
ESI†). While, it is not troublesome to hand that partial thermal
60 reduced GO in the Hummers method conditions to recover the
fully catalytic activity of the catalyst. Therefore, GO can be
reused as a green and efficient acid catalyst in practical usage.
1
Conclusions
In conclusion, we discovered that GO synthesized from
6
7
7
8
8
5
0
5
0
5
Hummers oxidation of graphite can be used as a facile acid
catalyst for the one-step conversion of fructose-based
carbohydrates into a high-heating value liquid biofuels. GO
showed an excellent catalytic activity for the etherification of
HMF in ethanol with a high EMF yield of 92%. GO also
performed well in the one-pot conversion of fructose, sucrose and
inulin in DMSO-ethanol solvent system into EMF with yields of
7
1%, 34% and 66%, respectively. The reaction temperature and
catalyst loading had a notable effect on EMF yield. Compare with
the etherification reaction, the hydrolysis and dehydration
reactions need higher reaction temperature and catalyst loading.
Finally, GO can be reused for several times without much loss in
catalytic activity. Given the fact that the production of GO is now
being scaled up industrially, it may open up a new way to apply
GO in the catalytic conversion of biomass-based compounds into
value-added chemicals and open a new route in green and
sustainable industry.
Fig. 2 Fructose conversion and products distribution as a function
of time over GO catalysis. Reaction conditions: 0.5 mmol
fructose, 30 mg GO, 0.3 mL DMSO, 0.7 mL ethanol, T= 130 °C .
2
2
3
3
4
4
0
5
0
5
0
5
Next, fructose based disaccharide (sucrose) and polysaccharide
(
inulin) were tested as the start materials over GO catalysis.
Sucrose is the most abundant and cheapest disaccharide, and one
molecule of sucrose contains one molecule of glucose and one
molecule of fructose. No sucrose was detected after the reaction;
a 4% yield of HMF and a 34 % yield of EMF were obtained.
When glucose was subjected to this reaction, the conversion was
high (99%) however the total furan products yield was low (4%).
The major product was found to be ethyl glucoside. These results
suggest that only fructose in sucrose can be converted into HMF
and EMF, and GO cannot be used as the catalyst for the
isomerization of glucose to fructose. Much higher EMF yield
This work was financially supported by the National Key
Basic Research Program of China (973 Program) (No:
2
012CB215305), Science Foundation of Shanxi (2013011010-6),
Science Foundation for Youths of Shanxi (2011021011-2,
012021009-2) and the Open Foundation of Key Laboratory of
2
Biofuels, Chinese Academy of Sciences (CASKLB201307).
Notes and references
(66%) was obtained, when inulin was used as the substrate for the
a
Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan,
reaction. This result may be due to the fact that one molecule of
9
9
0
5
030001, P.R. China Fax: (+86) 351-4041153, E-mail: houxl@sxicc.ac.cn,
zhuyulei@sxicc.ac.cn.
inulin contains one molecule of glucose and 1–59 molecules of
2
8
fructose. All the results indicate GO can not only catalyze
reactions of etherification and dehydration, but also can catalyze
the reaction of hydrolysis. Thus, GO can be used as a facile and
efficient acid catalyst for the one step conversion of abundant and
cheaper fructose based carbohydrates into a promising biofuel of
EMF.
b
University of Chinese Academy of Sciences, Beijing, 100049, P.R.
China.
c
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,
Chinese Academy of Sciences, Taiyuan, 030001, P.R. China.
d
Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and
Bioprocess Technology, Chinese Academy of Sciences, Qingdao,
Shandong, 266101,P.R. China.
Finally, the reusability and stability of GO was investigated for
the one-step conversion of fructose into EMF in DMSO-ethanol 100 † Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
solvent system under identical conditions as described in Fig. 2.
After reactions, the solid GO was separated from the reaction
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Page 6 of 6
‡
Footnotes should appear here. These might include comments relevant
2
4. D. R. D. D. R. Dreyer, H. P. Jia, A. D. Todd, J. Geng and C. W.
Bielawski, Organic & biomolecular chemistry, 2011, 9,
7292-7295.
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
5
6
6
5
0
5
2
2
5. H. P. Jia, D. R. Dreyer and C. W. Bielawski, Tetrahedron, 2011, 67,
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