One-pot conversion of carbohydrates into 5-ethoxymethylfurfural and ethyl d-glucopyranoside in ethanol catalyzed by a silica supported sulfonic acid catalyst

Article abstract of DOI:10.1039/c3ra41043a

In this study, prepared silica supported sulfonic acid was used as a heterogeneous catalyst for the production of 5-ethoxymethylfurfural (EMF) from fructose based carbohydrates and the synthesis of ethyl d-glucopyranoside from glucose based carbohydrates in ethanol. EMF was obtained in a high yield of 83.8% from 5-hydroxymethylfurfural (HMF) after 10 h, and a 63.1% yield was obtained from fructose at 100°C for 24 h. Temperature experiments demonstrated that a higher reaction temperature (in the case of 120°C) resulted in side reactions such as polymerization of HMF and alcoholysis of HMF into ethyl levulinates. When di- and poly-saccharides (sucrose and inulin) were used, the fructose moieties in sucrose and inulin were also successfully converted into EMF. However, the silica supported sulfonic acid catalyst was inert for the production of EMF from aldose based carbohydrates such as glucose and cellobiose. Ethyl d-glucopyranoside was formed in a high yield of 91.7% from glucose. More importantly, the catalyst could be reused several times without losing its catalytic activity with an average EMF yield of around 60% from a one-pot reaction of fructose. This work provides a good outlook for the conversion of carbohydrates into fine chemicals and biofuel additive. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41043a

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One-pot conversion of carbohydrates into
5-ethoxymethylfurfural and ethyl D-glucopyranoside in
ethanol catalyzed by a silica supported sulfonic acid
catalyst
Cite this: RSC Advances, 2013, 3,
12313
Bing Liu and Zehui Zhang*
In this study, prepared silica supported sulfonic acid was used as a heterogeneous catalyst for the
production of 5-ethoxymethylfurfural (EMF) from fructose based carbohydrates and the synthesis of ethyl
D-glucopyranoside from glucose based carbohydrates in ethanol. EMF was obtained in a high yield of
83.8% from 5-hydroxymethylfurfural (HMF) after 10 h, and a 63.1% yield was obtained from fructose at
100 uC for 24 h. Temperature experiments demonstrated that a higher reaction temperature (in the case of
120 uC) resulted in side reactions such as polymerization of HMF and alcoholysis of HMF into ethyl
levulinates. When di- and poly-saccharides (sucrose and inulin) were used, the fructose moieties in sucrose
and inulin were also successfully converted into EMF. However, the silica supported sulfonic acid catalyst
was inert for the production of EMF from aldose based carbohydrates such as glucose and cellobiose. Ethyl
D-glucopyranoside was formed in a high yield of 91.7% from glucose. More importantly, the catalyst could
be reused several times without losing its catalytic activity with an average EMF yield of around 60% from
a one-pot reaction of fructose. This work provides a good outlook for the conversion of carbohydrates into
fine chemicals and biofuel additive.
Received 2nd March 2013,
Accepted 29th April 2013
DOI: 10.1039/c3ra41043a
www.rsc.org/advances
drop-in biofuel candidates such as 5-ethoxymethylfurfural
(EMF) and 2,5-dimethylfuran (DMF) can also be obtained by
Introduction
the treatment of HMF.10–12 In the past decades, considerable
work has been devoted to the production of HMF in various
systems including water, organic solvents, ionic liquids (ILs)
and using a biphasic system. Moderate to high HMF yields
were obtained from fructose and glucose, even directly from
cellulose and lignocelluloses.13–16
With the development of the global economy, fossil resources
are expected to shrink significantly in the next few decades.
Therefore, in the near future, the world will need to gradually
develop renewable resources to replace the use of fossil
resources for energy consumption and platform chemicals.1–3
Renewable biomass with an estimated annual production of
1.0 6 1011 tons per year is considered to be a promising
alternative to fossil resources to supply sustainable raw
material for the production of biofuels and chemicals.4–6 In
the past decades, much effect has been devoted to the
development of efficient catalytic routes for the production
of biofuels and bio-chemicals from biomass resources.7
Among the bio-based chemicals, 5-hydroxymethylfurfural
(HMF) is recognized as one of the top building-block
chemicals according to the ‘‘Top Value Added Chemicals’’
proposed by the US Department of Energy.8 It can be used as a
valuable intermediate for the production of fine chemicals,
pharmaceuticals and furan-based polymers.9 In addition,
Recently, EMF derived from HMF is considered to be one of
the promising next-generation biofuels. It has a high energy
density of 8.7 kWh L21, which is comparable with those of
standard gasoline (8.8 kWh L21) and diesel fuel (9.7 kWh L21),
and significantly higher than the widely familiar bio-ethanol
(6.1 kWh L21).17 Surprisingly, the synthesis of EMF from
carbohydrates has remained largely unexplored, despite its
good prospect as a biofuel. One of the quantitative conversion
methods for the synthesis of EMF was via the treatment of
5-chloromethylfurfural (CMF) with ethanol.18 The intermedi-
ate CMF was produced by the treatment of carbohydrates in a
solution of lithium chloride (5 wt.%) in concentrated hydro-
chloric acid, and it was extracted by 1,2-dichloroethane.
Alternatively, the synthesis of EMF has also been reported by
the direct etherification of HMF with ethanol in the presence
of acid catalysts.19,20 Although a high EMF yield was achieved
from HMF or CMF, it was desirable to carry out the research
Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs
Commission & Ministry of Education, College of Chemistry and Material Science,
South-Central University for Nationalities, Wuhan, 430074, China.
E-mail: zhangzehui@mail.scuec.edu.cn; zehuizh@mail.ustc.edu.cn;
Fax: +8627-67842752; Tel: +8627-67842752
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for the synthesis of EMF directly from carbohydrates without
the separation of the intermediate such as HMF, which saves
time and energy. Fructose is regarded as a preferable feedstock
for the production of HMF with a high conversion and a high
selectively. Recently, some methods were developed for the
synthesis of EMF from fructose, which combines the dehydra-
tion of fructose into HMF followed by the etherification of
HMF to EMF. Recently, 5-alkoxymethylfurfural ethers have
been synthesized directly from fructose using ionic liquids
(imidazolium propanesulfonic acids) as catalysts with an
approximate yield of 55%.21 Yang et al. reported that EMF
could be synthesized from fructose in a yield of 65% catalyzed
by the heteropolyacid H3PW12O40 under microwave heating.22
However, H3PW12O40 was soluble in ethanol and so recycling
of the catalyst was difficult. Although the authors carried out
the recycling experiments, the procedure seemed tedious. The
catalyst was recovered by the distillation of solvents and
products under vacuum. In our group, we have also developed
a new method for the synthesis of fructose into EMF in a high
side. Although ethyl D-glucopyranoside is not a liquid fuel, it
been widely used as a biodegradable surfactant and a chemical
intermediate.28 This method generates a sustainable strategy
for the conversion of biomass into biofuels and chemicals.
Experimental section
Materials and methods
3-Mercaptopropyltrimethoxysilane (99.8%) was purchased
from Aladdin Chemical Reagent Co., (Shanghai, China). 35
wt.% H2O2 was purchased from Tianjin Damao Chemical
Reagent Factory (Tianjin, China). Ethanol (99.5%), n-hexade-
cylamine and tetraethoxysilane (TEOS, 99.5%) were purchased
from Sinopharm Chemical Reagent Co., Ltd. (Shanghai
China), and ethanol (99.5%) was freshly distilled before use.
5-Hydroxymethylfurfural (98%) was supplied by Beijing
Chemicals Co. Ltd. (Beijing, China). 5-Ethoxymethylfurfura
(98%) was purchased from Hangzhou Imaginechem Co., Ltd.
(Zhejiang, China). Ethyl D-glucopyranoside was purchased
from Feiyang Biotechnology Co. Ltd. (Shangdong, China).
Sucrose, inulin and cellobiose were purchased from J & K Co.
Ltd. (Beijing, China). Fructose was puchased from Sanland-
Chem International Inc. (Xiamen, China). Glucose was
purchased from ABCR GmbH & Co. (Karlsruhe, Germany).
Acetonitrile (HPLC grade) was purchased from Tedia Co.
(Fairfield, USA). All other reagents were provided by local
supplies (Wuhan, China) without further purification.
yield catalyzed by an organic–inorganic hybrid solid catalyst,
23
[MIMBS]3PW12O40
.
Due to the high hygroscopicity of this
material, caution is needed with regards to the storage and
utilization of [MIMBS]3PW12O40. Therefore, there is still a high
demand to develop new green chemistry catalytic routes for
the synthesis of EMF from carbohydrates.
Grafting soluble acids on the surface of an inorganic
insoluble support is a strategy to transform high-performance
soluble acids into heterogeneous solid catalysts, which have
advantages of a strong acid site, large surface area, and they
are easily separated and recycled.24,25 Over the last decades,
sulfonic acid (–SO3H) was usually chosen as a functional
group, because it is not only known for its efficient proton acid
catalytic ability, but it can also be easily incorporated into
many different materials.26,27
Herein, in continuation with our investigation on the
development of a new catalytic system and the expansion in
the utilization of carbohydrate substrates, propyl sulfonic
groups were immobilized on mesoporous silica to construct
silica supported sulfonic acid as a heterogeneous catalyst, and
its catalytic activity was studied for the synthesis of EMF from
carbohydrates (Scheme 1). This catalyst was found to be
effective for the conversion of fructose unit based carbohy-
drates into HMF and it also demonstrated a good activity for
the direct etherification of glucose into ethyl D-glucopyrano-
Preparation of the catalyst
Silica sulfate was prepared according to a known proce-
dure.29,30 Typically, 3.2 g (0.013mol) of n-hexadecylamine was
dissolved in 100 mL mixed solution of ethanol and water (the
volume ratio of ethanol to water: 21 : 27) at room temperature.
Then 8.3 g of tetraethoxysilane (TEOS, 0.039 mol) and 1.97 g of
3-mercaptopropyltrimethoxysilane (MPTMS, 0.01 mol) were
added to the above mixture in sequence. The resulting
solution was stirred for 24 h at room temperature and the
slurry was filtered to obtain a white solid. n-Hexadecylamine
was removed through soxhlet extraction by ethanol at reflux
temperature for 24 h to give rise to propyl thiol modified
mesoporous silica. Then, thiol groups were oxidized by 35%
aqueous H2O2 (2 g g21 of solid) at room temperature for 24 h
in a methanol/water mixture. After reaction, the white solid
was washed with ethanol and water, and then the resulting
solid was stirred for 4 h in 0.1 M H2SO4 (1 g of solid per 100 mL
of solution). Finally, the prepared silica supported propyl
sulfonic acid (Silica-SO3H) catalyst was filtered and washed
extensively with deionized water until the filtrate was neutral.
The recovered Silica-SO3H was dried in an oven at 100 uC in
vacuum over night.
Catalyst characterization
Fourier transform infrared (FT-IR) measurements were
recorded on a Nicolet NEXUS-6700 FTIR spectrometer with a
spectral resolution of 4 cm21 in the wave number range of
500–4000 cm21. N2 absorption–desorption experiments were
carried out at 2193 uC using a Quantachrome Autosorb-1-C-
Scheme 1 Illustration for the synthesis of EMF and ethyl D-glucopyranoside
from carbohydrates in ethanol.
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MS instrument. Silica-SO3H was subjected to outgassing for 6
h at 200 uC before the experiment. The surface area was
obtained by using the BET method. The pore size distribution
was evaluated from the desorption branches of the isotherms
by using the Barrett–Joyner–Halenda (BJH) method. The
content of sulfur and carbon was conducted on a 2400 Series
II CHNS/O Elemental Analyzer (Perkin Elmer).
Titration of solid
0.1 g of the solid catalyst was added to 10 mL of aqueous KCl
(0.1 M) and stirred for 30 min. The resulting solution was
titrated by a standard solution of KOH (0.02 M) and the pH
evolution was determined by a Metrohm pH meter.
Fig. 1 FTIR spectrum of the Silica-SO3H catalyst.
Synthesis of EMF from the etherification of HMF
Firstly, HMF (126 mg, 1 mmol), ethanol (5 mL) and Silica-SO3H
catalyst (200 mg) were added into a 10 mL round bottom flask with
a condenser, which was sealed with a balloon. Then, the reaction
mixture was carried out at a set temperature for the desired
reaction time under an atmospheric pressure with a magnetic
stirrer. Samples were withdrawn, diluted with water, centrifuged at
12 000 rpm for 5 min, and analyzed by an HPLC system.31
groups. The band near 1088 cm21 was assigned to the
asymmetric stretching modes of Si–O–Si bond, indicating the
structure of support silica.32 Three bands appeared at 1200–
1250, 1010–1100 and 685 cm21, which were attributed to the
OLSLO asymmetric and symmetric stretching vibration and S–
O
stretching vibration of the sulfonic groups (–SO3H),
respectively.32 The band presented at 805 cm21 was assigned
to the C–S stretching vibration. In addition, the bands at 2927
cm21 and 612 cm21 were attributed to the stretching vibration
and the bending vibration of the C–H bond in the propyl
sulfonic acid group, respectively. These results clearly indi-
cated that the propyl sulfonic acid group was successfully
grafted onto the surface of the silica support.
The textural properties of the catalysts were obtained from
nitrogen adsorption experiment. The surface area of the
prepared Silica-SO3H was determined to be 1102 m2 g21. The
pore diameter was calculated to be 2.87 nm by the BJH
method. The H+ exchange capacity was 0.20 mmol g21, which
was determined by the potentiometric titration. The sulfur
content in the Silica-SO3H catalyst was determined to be 0.66
wt.% and the carbon content was determined to be 0.73 wt.%
by elemental analysis. Thus the calculated mole ratio of
H : S : C is equal to 1 : 1 : 2, which is consistent with the
structure of propylsulfonic acid.
Synthesis of EMF from fructose or ethyl D-glucopyranoside
through a one-pot reaction
D-Fructose (180 mg, 1 mmol) or glucose (180 mg, 1 mmol) was
firstly dissolved in ethanol (5 mL) at a set temperature. Then
the Silica-SO3H catalyst (200 mg) was added into the reaction
system, and the reaction was carried out at a set temperature.
Otherwise the procedure followed in a manner similar to that
discussed above.
Determination of the products
HPLC analysis of HMF and EMF were done on VARIAN ProStar
210 HPLC system coupled with a UV detector. Samples were
separated by a reversed-phase C18 column (200 6 4.6 mm).
The column temperature was maintained at 25 uC. The
optimized mobile phase consisted of acetonitrile and 0.1
wt.% acetic acid aqueous solution with the volume ratio at
15 : 85. The flow rate was set at 1.0 mL min21. The detection
wavelength was 280 nm. The content of HMF and EMF in
samples were obtained directly by interpolation from calibra-
tion curves, with a coefficient of 0.999.
Effect of reaction temperature on the conversion of HMF into
EMF
Ethyl D-glucopyranoside was separated with an aminex HPX-
87 column (Agilent) and Waters 2412 Refractive Index detector.
As HMF is the key intermediate for the one-pot conversion of
fructose into EMF, the success of the preparation of EMF from
HMF is crucial to being able to use the one-pot reaction.
Therefore, the etherification of HMF into EMF was initially
carried out in ethanol. Firstly, the etherification of HMF by
ethanol was conducted at 80 uC, 100 uC and 120 uC in order to
study the effect of the reaction temperature, and the results are
shown in Fig. 2, from which, it was noted that reaction
temperature had a remarkable effect on the etherification of
HMF. A general rule of the effect of reaction temperature can
be concluded as follows: the higher the reaction temperature,
the higher the etherification rate. When the reaction tempera-
ture was set to 80 uC, HMF conversion and EMF yield reached
73.9% and 64.6% after 12 h. However, it only took 10 h to
obtain a maximum EMF yield of 83.8% with HMF conversion
Recycling of the catalyst
At the end of the reaction, the Silica-SO3H catalyst was
recovered by filtration. The recovered catalyst was washed
three times with 10 mL of water, three times with 10 mL of
ethanol, three times with 10 mL of diethyl ether, and then it
was dried at 80 uC over night in a vacuum oven.
Results and discussion
Catalyst characterization
The IR spectra of the Silica-SO3H catalyst is shown in Fig. 1.
The absorption at 3447 cm21 was attributed to the hydroxyl
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Fig. 3 The effect of catalyst loading on the etherification of HMF by ethanol.
Reaction conditions: HMF (1 mmol) and a set amount of Silica-SO3H were added
into 5 mL ethanol and then the reaction was carried out at 100 uC.
only 5.1% at 1 h with 100 mg Silica-SO3H. Whereas, yields of
15.7%, 21.9% and 35.6% were reached with the amount of
Silica-SO3H being 150, 200 and 250 mg, respectively. A higher
etherification rate of HMF with an increase in catalyst loading
can be attributed to an increase in the availability and number
of catalytically active sites. The maximum yields of EMF relied
on the catalyst loading. A maximum EMF yield was obtained of
77.8% after 8 h with 250 mg Silica-SO3H, and that increased to
83.8% yield with 200 mg Silica-SO3H after 10 h. The reason
might be that more catalyst loading accelerated the formation
of EMF from HMF while it also favored the alcoholysis of HMF
into alkyl levulinates, which offset the increase of EMF yield.
In our experiment, the reaction mixture was analyzed by gas
chromatography at the end of the reaction after 12 h, and ethyl
levulinate yields were determined to be 23.4%, 14.8%, 10.5%
and 7.9%, corresponding to the catalyst loading of 250 mg, 200
mg, 150 mg and 100 mg, respectively. These results verified the
major by-product for the synthesis of EMF from HMF was
caused by the alcoholysis of HMF. It was also noted that the
maximum yield of EMF could not remain stable, when the
reaction time was prolonged. The maximum EMF yield of
77.8% at 8 h slowly decreased to 69.7% after 12 h at 100 uC
with a catalyst loading of 250 mg. These results indicate that
the EMF product was also unstable during the reaction
process. Yang et al.37 have subjected EMF to heating at 160
uC for 1 h, and found that EMF was only recovered in an 82%
yield, accompanying the formation of HMF and alkyl
levulinates. Therefore, the appropriate amount of Silica-SO3H
was chosen as 200 mg in subsequent experiments if not
otherwise indicated.
Fig. 2 Effect of reaction temperature on the etherification of HMF in ethanol.
Reaction conditions: HMF (126 mg, 1 mmol) and Silica-SO3H (200 mg) were
added into 5 mL ethanol and then the reaction was carried out at a set
temperature.
at 96.5%, when the reaction temperature was increased from
80 uC to 100 uC. Increasing the reaction temperature further to
120 uC, a 98.8% conversion of HMF after 12 h was the highest
of the three different temperatures. However at 120 uC the
EMF yield was only 60.8% after 12 h, which was the lowest for
the three different reaction temperatures. Comparing the
results of HMF conversion and EMF yield, the selectivity of
EMF decreased with the increasing reaction temperature,
which was due to higher temperatures causing side reactions
that formed undesired byproducts. On the other hand, it was
reported that HMF itself was not stable at high temperatures.
In our experiments, some black, insoluble sediment by-
product termed humins were observed when the reaction
time was 4 h at 120 uC, which attenuated the EMF yield. Other
research groups also observed the formation of humins, which
was formed through the polymerization and cross-polymeriza-
tion of HMF.33–35 On the other hand, side products such as
alkyl levulinates were favored to be formed at high tempera-
ture from the alcoholysis of HMF. Balakrishnan et al.36
reported that ethyl levulinate was obtained in a 62% yield
with 5 mol% H2SO4 at 120 uC for 30 h. Taking into account the
effect of reaction temperature, 100 uC was chosen as the best
reaction temperature for etherification of HMF into EMF.
One-pot dehydrative etherification of fructose into EMF
Although EMF was successful synthesized from HMF by the
etherification with Silica-SO3H as a catalyst, it would be
desirable to synthesize EMF directly from fructose through a
one-pot reaction, due to the dehydration of fructose into HMF
and the etherification of HMF into EMF both requiring the
acid catalyst. Therefore, synthesis of EMF from one-pot
dehydrative etherification of fructose was carried out in the
presence of Silica-SO3H at 80 uC, 100 uC and 120 uC,
respectively, and the yields of the intermediate HMF and the
product EMF are shown in Fig. 4. Seeing the curves of HMF
Effect of catalyst loading on the etherification of HMF into
EMF
Secondly, the influence of catalyst dosage on the etherification
of HMF into EMF was investigated and the results are shown
in Fig. 3. From Fig. 3, it is noted that increasing the catalyst
loading led to an increase of EMF yield. Taking the early
reaction stage at 1 h as an example, the yield of EMF yield was
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increased as the reaction time increased. The highest EMF
yields reached were 55.9%, 63.1% and 53.1% at 80 uC, 100 uC
and 120 uC, respectively. No fructose was detected at 24 h for
the three different temperatures. The lowest EMF yield at 120
uC was also caused by the side reactions as described above.
After the complete reaction at 100 uC and 120 uC, some
insoluble humins were also observed, when the reaction
mixture was diluted with water.
Conversion of various substrates in ethanol catalyzed by Silica-
SO3H
Various carbohydrates were used as substrates for the
production of EMF in order to explore the scope of our
catalytic system, and the results are shown in Table 1. As
described above, an EMF yield of 63.1% was obtained from
fructose after 24 h at 100 uC, with the HMF yield remaining at
11.2%. Then fructose based di- and polysaccharides (sucrose
and inulin) were subjected to heating at 100 uC under the same
reaction conditions as described for fructose. A high yield of
EMF was obtained of 60.7% from inulin after 24 h, which was
comparable with that from fructose. These results indicated
that Silica-SO3H could also effectively cleave the glycosidic
bonds as well as showing catalytic activity for the dehydration
of fructose and the etherification of HMF. Unfortunately, EMF
yield only reached 34.9% after 24 h using sucrose, which was
almost half of that from pure fructose. No sucrose was
detected after the reaction. More importantly, the monosac-
charides, fructose and glucose, also showed negligible detec-
tion. Interestingly, ethyl D-glucopyranoside (EGL) was also
detected by HPLC and it was obtained in a yield of 45.0% using
sucrose. Thus, it indicated that EGL was formed from the
glucose part of sucrose. Furthermore, a high EGL yield was
obtained of 91.7% after 12 h, when pure glucose was used as
the substrate. Interestingly, an EGL yield of 85.9% after 12 h
was also obtained when cellobiose was used. These results
indicated that glycosylation of the hydroxyl group at C1 in
glucose was readily promoted under the acidic conditions.
However, the isomerization of glucose into fructose was much
more difficult under the acidic conditions, which was
generally accepted to be a key step for the effective synthesis
of EMF in ethanol.31 Unlike EMF, the product EGL is not a
liquid fuel, but it has wide applications such as a biodegrad-
Fig. 4 Effect of reaction temperature on the one-pot dehydrative etherification
of fructose in ethanol. Reaction conditions: fructose (180 mg, 1 mmol) and
Silica-SO3H (200 mg) were added into 5 mL of ethanol and then the reaction
was carried out at a set temperature.
yields at different reaction temperatures, it was noted that
HMF yields after 1 h decreased with the decrease of reaction
temperature. The results indicated that a high temperature
was beneficial to promote the dehydration of fructose into
HMF, which was similar to the etherification of HMF into EMF
as described above. Seeing the trends of HMF yields at
different reaction temperatures, it was also found that the
HMF yield at 80 uC firstly increased from 9.7% after 1 h to
26.7% after 8 h, and then kept stable around 27%. It indicated
that the dehydration rate of fructose and the etherification rate
of HMF were comparable at 80 uC. However, HMF yields
decreased gradually at 100 uC and 120 uC, indicating that the
etherification of HMF into EMF was faster than the dehydra-
tion of fructose into HMF. The yields of EMF gradually
able surfactant,
intermediate.38
a cosmetic humectant and a chemical
Table 1 The results of products yields using various carbohydrates in ethanol catalyzed by HSM-SO3Ha
Entry
Substrate
Reaction time (h)
EGL yield (%)
HMF yield (%)
EMF yield (%)
1
2
3
4
Fructose
Sucrose
Inulin
Glucose
Cellobiose
Fructose
Glucose
24
24
24
12
12
24
12
0
45.0
0
91.7
85.9
ND
ND
11.2
5.9
6.5
0
0
ND
ND
63.1
34.9
60.7
0
0
ND
Nd
5
6b
7b
a
Reaction conditions: Substrates (180 mg) and 200 mg Silica-SO3H was added into 5 mL ethanol and the reaction was carried out at 100 uC.
Other conditions were the same as above, unless the reaction was carried out without the catalyst. ND means not detected.
b
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Fig. 5 Recycling experiments of the Silica-SO3H catalyst.
application in large scale synthesis and from the industrial
point of view.
Scheme 2 Possible reaction pathway of the formation of EMF and EGL from
hexoses in ethanol catalyzed by Silica-SO3H.
According to previous results, the possible reaction pathway
for the conversion of hexoses into ethyl D-glucopyranoside and
EMF can be described as shown in Scheme 2. For ketoses such
as glucose, it only catalyzed the glycosylation of glucose with
ethanol to form ethyl D-glucopyranoside, which was similar to
previous results reported that used Brønsted acid catalysts in
methanol and butanol.39,40 For the aldose, in the case of
fructose, the release of one molecular water from D-fructopyr-
anose under acidic conditions resulted in the formation of a
fructofuranosyl oxocarbenium ion. The fructofuranosyl oxo-
carbenium ion could then convert into an enol furan
intermediate by the release of H+, which was subsequently
dehydrated to molecular water to produce the intermediate
HMF. Then, the hydroxyl group was activated by the acid
catalyst, and substituted by ethanol to give rise to the final
product, EMF. On the other hand, some byproducts such as
humins and ethyl levulinates were also formed under acidic
conditions.
Conclusions
In conclusion, synthesis of EMF from fructose based carbohy-
drates was efficiently developed through a one-pot reaction.
The easily prepared silica supported sulfuric acid Silica-SO3H
catalyst showed a high catalytic activity for the etherification of
HMF in ethanol with a high EMF yield of 83.8% at 100 uC. EMF
was also successfully synthesized directly from fructose and
inulin, with yields of 63.1% and 60.7%, respectively. However,
Silica-SO3H was inert for the synthesis of EMF from aldose
based carbohydrates such as glucose. It can effectively catalyze
the glycosylation of glucose into ethyl D-glucopyranoside with
a
high yield of 91.7%. Finally, reusability experiments
demonstrated that Silica-SO3H could be reused several times
without losing its catalytic activity. This catalytic system
provides a new catalytic process for the conversion of
abundant renewable carbohydrates to high-heating value
liquid biofuels and bio-based chemicals.
Catalyst reuse experiments
Finally, the reusability and stability of the Silica-SO3H catalyst
was evaluated. The reuse experiment was carried out by a
model reaction of the one-pot dehydrative etherification of
fructose in ethanol at 100 uC, and the detailed procedures are
described in the experimental section. After the completion of
the reaction after 24 h with the use of fresh catalyst, the
reaction mixture was filtered, and then washed with distilled
water, ethanol and diethyl ether successively. The reused
catalyst was placed under vacuum to remove the solvents at 80
uC over night. Then the recovered catalyst was added into a
fresh solution of the reaction mixture and the second cycle was
carried out under the same conditions. Fig. 5 shows the results
from recycling Silica-SO3H for six consecutive runs. It was
found that the yields of EMF were kept almost stable. The EMF
yield of the first run was 63.1%, and was 57.5% for the sixth
time. Therefore, the Silica-SO3H catalyst can be reused without
losing its catalytic activity, which is favorable for the potential
Acknowledgements
The project was supported by the National Natural Science
Foundation of China (No. 21203252 and 21206200).
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