Silica coated magnetic Fe3O4 nanoparticles supported phosphotungstic acid: A novel environmentally friendly catalyst for the synthesis of 5-ethoxymethylfurfural from 5-hydroxymethylfurfural and fructose

Article abstract of DOI:10.1039/c3cy00223c

In this study, a magnetically-recoverable catalyst (Fe₃O ₄@SiO₂-HPW) was prepared by the application of phosphotungstic acid (HPW) supported on silica-coated Fe₃O ₄ nanoparticles. The prepared samples were characterized by XRD, TEM, FT-IR, and N₂-adsorption-desorption isotherms. The content of W in Fe₃O₄@SiO₂-HPW was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and its surface acidity was determined by a potentiometric titration with n-butylamine. Fe ₃O₄@SiO₂-HPW showed an excellent catalytic activity for the synthesis of EMF from HMF and fructose. Under optimal reaction conditions, EMF was obtained in a high yield of 83.6% by the etherification of 5-hydroxymethylfurfural. EMF could also be synthesized directly from fructose in a yield of 54.8% via a one-pot reaction strategy. After reaction, the catalyst Fe₃O₄@SiO₂-HPW could be easily separated from the reaction mixture with an external magnetic field, and it could be reused at least five times without any loss of its catalytic activity.

Full text of DOI:10.1039/c3cy00223c

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Cite this: DOI: 10.1039/c0xx00000x
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Silica coated magnetic Fe3O4 nanoparticles supported phosphotungstic
acid: a novel environment-friendly catalyst for the synthesis of 5-
ethoxymethylfurfural from 5-hydroxymethylfurfural and fructose
Shuguo Wang, Zehui Zhang*, Bing Liu and Jinlin Li
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
In this study, a magneticallyꢀrecoverable catalyst (Fe3O4@SiO2ꢀHPW) was prepared by the application of
phosphotungstic acid (HPW) supported on silicaꢀcoated Fe3O4 nanoparticles. The prepared samples were
characterized by XRD, TEM, FTꢀIR, and N2ꢀadsorptionꢀdesorption isotherm. The content of W in
10 Fe3O4@SiO2ꢀHPW was measured by inductively coupled plasma atomic emission spectroscopy (ICPꢀ
AES) and its surface acidity was determined by a potentiometric titration with nꢀbutyl amine.
Fe3O4@SiO2ꢀHPW showed an excellent catalytic activity for the synthesis of EMF from HMF and
fructose. Under an optimal reaction conditions, EMF was obtained in a high yield of 83.6% by the
etherification of 5ꢀhydroxymethylfurfural. EMF could also be synthesized directly from fructose in a
15 yield of 54.8% via a oneꢀpot reaction strategy. After reaction, the catalyst Fe3O4@SiO2ꢀHPW could be
easily separated from the reaction mixture with an external magnetic field, and it could be reused at least
five times without any loss of its catalytic activity.
30 and 2,5ꢀhydroxymethylfuran8) and valuable intermediates for fine
1. Introduction
chemistry.9 Therefore, extensive studies have been carried out for
the transformation of carbohydrates and cellulosic biomass into
HMF. 10ꢀ15
With diminishing fossil fuel, much effort has been devoted to the
20 production of fuels and chemicals from renewable sources.1ꢀ3
Abundant, renewable biomass resource is a promising alternative
to petroleum for production of chemicals and liquid fuels.4ꢀ5 In
recent years, various chemicals have been successfully
synthesized from biomass. Among the many biomassꢀderived
25 chemicals, 5ꢀhydroxymethylfurfural (HMF) is considered an
excellent platform molecule. HMF serves as a platform chemical
for the production of a wide range of chemicals and biofuels,
which can be converted to energy products (2,5ꢀdimethylfuran),6
monomers for highꢀvalue polymers (2,5ꢀfurandicarboxylic acid7
5ꢀEthoxymethylfurfural (EMF), the etherification product of
35 HMF, has been proposed as a potential liquid biofuel for the
future.16 EMF is presented in liquid with a high boiling point of
235 oC. It has a high energy density of 8.7 kWh/L, which is close
to the standard gasoline (8.8 kWh/L).17 EMF can be directly
synthesized from HMF or 5ꢀchloromethylfurfural (CMF) with
40 high yields.18ꢀ20 However, the largeꢀscale synthesis of EMF from
pure HMF or CMF is limited due to the high price of HMF and
CMF. It is much more attractive to synthesize EMF from
inexpensive and renewable fructose via a oneꢀpot reaction
strategy. As shown in scheme 1, the oneꢀpot reaction integrates
45 the dehydration of fructose into HMF and the followed
etherification of HMF into EMF, which saves time and energy. In
recent years, some methods were reported on the synthesis of
EMF from fructose. Yang et al. reported that an EMF yield of
65% could be obtained from fructose in the presence of
50 heteropolyacid H3PW12O40 under microwave heating.21 Kraus et
al. reported that EMF could be achieved with an approximate
yield of 55% from fructose catalyzed by acidic ionic liquids.22
However, the procedures for the catalyst recycling of the above
mentioned methods were complex, as H3PW12O40 and acidic
55 ionic liquids were dissolved in the solvents. Balakrishnan et al.
Dr. S.G. Wang, Dr. Z. H. Zhang, Dr. B. Liu, Prof. J. L. Li, Department
of Chemistry, key Laboratory of Catalysis and Material Sciences of
the State Ethnic Affairs Commission & Ministry of Education.
South-Central University for Nationalities
MinYuan Road 708, Wuhan, R.P. China
Fax: (+)86-27-67842572
E-mail: zehuizh@mail.ustc.edu.cn
This journal is © The Royal Society of Chemistry [year]
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50 2.2.1 Synthesis of Fe3O4
prepared a heterogeneous catalyst by the graft sulfuric acid on the
surface of silica, and it showed high catalytic activity on the
synthesis of EMF from fructose with a yield of 70%.23 Although
the heterogeneous catalyst demonstrated superior advantages in
terms of catalyst recycling in comparison with homogeneous
catalysts, the tedious recovery procedure via filtration or
centrifugation and the inevitable loss of solid catalysts in the
separation process still limited their application.
Fe3O4 was synthesized according to the known procedure by coꢀ
precipitation method.27 FeCl3.6H2O (2.703 g, 10 mmol) and
FeSO4.7H2O (1.390 g, 5 mmol) were dissolved in 50 mL of
5
o
deionized water under vigorous mechanical stirring at 80 C for
55 10 min under a nitrogen atmosphere. 8 mL of NH3.H2O was
added dropwise into the mixture solution for 5 min. Then 1 mL of
1M citric acid was added. After continuous stirring for 2 h, the
magnetite precipitates were collected by a permanent magnet, and
washed to pH = 7 using deionized water. Finally, the black
60 precipitates were washed twice by ethanol and dried in vacuum at
70 oC overnight to obtain Fe3O4 nanoparticles.
CH2OH
EtOH
H+
O
O
CH2OH
C
H+
OH
O
O
O
O
OH
OH
OH
Fructose
EMF
HMF
10
Scheme 1. Synthesis of EMF from fructose in ethanol.
2.2.2 Synthesis of Fe3O4@SiO2
Firstly, Fe3O4 nanoparticles (2.0 g) was dispersed in a mixture
solvent of ethanol (70 mL) and H2O (10 mL) by sonication for 15
65 min. Then NH3.H2O (5 mL) and TEOS (5.0 mL) were added
successively. The mixture was stirred for 24 h under vigorous
mechanical stirring. Then Fe3O4@SiO2 precipitate was collected
by a permanent magnet, and rinsed repeatedly with deionized
water until the filter was neutral. Then Fe3O4@SiO2 was washed
70 with ethanol three times and dried in a vacuum.
Recently, magnetic nanoparticles (MNPs) have appeared as a
new kind of catalyst support due to their good stability, and facile
separation by magnetic forces.24 The unique magnetic separation
property makes MNPs much more effective than conventional
15 filtration or centrifugation as it prevents loss of the catalyst.
However, MNPs are readily aggregated due to the self
interactions. In order to prevent the aggregation, the surface of
MNPs was usually modified with silica layer, together providing
a chemically inert surface for the chemical modification. The
20 Kegginꢀtype heteropoly acids such as H3PW12O40 (HPW) have
been extensively applied to catalyze many chemical reactions due
to the high Brønsted acidity.25, 26 Herein, in this study, driven by
the excellent catalytic activity of H3PW12O40 (HPW) and the
unique magnetic separation property of Fe3O4, a magneticallyꢀ
25 recoverable catalyst (Fe3O4@SiO2ꢀHPW) was prepared by the
application of phosphotungstic acid (HPW) supported on silicaꢀ
coated Fe3O4 nanoparticles (Fe3O4@SiO2). Fe3O4@SiO2ꢀHPW
was used to catalyze the synthesis of EMF from HMF and
fructose with ethanol as the solvent. This new catalytic system
30 possesses both high separation efficiency and a relatively high
catalytic activity for the synthesis of a potential liquid fuel EMF
from renewable resource.
2.2.3 Synthesis of Fe3O4@SiO2-HPW
Fe3O4@SiO2ꢀHPW was prepared by a similar method which was
used to support HPW on the surface of silica.28 Fe3O4@SiO2 (2.0
g) was firstly dispersed in 30 mL of deionized water by
75 sonication for 30 min. Then an aqueous solution of HPW (0.8 g
in 5mL of water) was added. After stirring for 24 h, the catalyst
was collected by a permanent magnet, and washed three times
with deionized water (20 mL ×3). Finally, the catalyst was dried
at 100 oC for 12 h.
80 2.3 Catalyst characterization
Transmission electron microscope (TEM) images were obtained
using an FEI Tecnai G2ꢀ20 instrument. The sample powder were
firstly dispersed in ethanol and dropped onto copper grids for
observation. FTꢀIR measurements were recorded on a Nicolet
85 NEXUSꢀ6700 FTIR spectrometer with a spectral resolution of 4
cmꢀ1 in the wave number range of 500ꢀ4000 cmꢀ1. Xꢀray powder
diffraction patterns of samples were determined with a Bruker
advanced D8 powder diffractometer (Cu Kα). The scan ranges
were 10–80◦ with 0.016◦ steps, respectively. The content of W
90 was determined by ICP/AES on an IRIS Intrepid II XSP
instrument (Thermo Electron Corporation).
N2 absorptionꢀdesorption experiment was carried out at −193
ºC using a Quantachrome Autosorbꢀ1ꢀCꢀMS instrument. The
sample was subjected to be outgassed for 6 h at 200 ºC before the
95 experiment. The surface area was obtained by using the BET
method. The pore size was evaluated from the desorption
branches of the isotherms by using the Barrett–JoynerꢀHalenda
(BJH) method.
2. Experimental Section
2.1 Materials and Methods
35 Kegginꢀtype 12ꢀtungstophosphoric acid was purchased from
Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Aqueous
NH3 (28 wt.%), ethanol (99.5%), FeSO4.7H2O (99.5 %),
FeCl3.6H2O (99.5 %), tetraethoxysilane (TEOS, 99.5%) and ethyl
levulinate (99.0%) were purchased from Sinopharm Chemical
40 Reagent Co., Ltd. (Shanghai, China), and ethanol (99.5%) was
freshly distilled before use. 5ꢀHydroxymethylfurfural (98%) was
supplied by Beijing Chemicals Co. Ltd. (Beijing, China). 5ꢀ
Ethoxymethylfurfura (98%) was purchased from Hangzhou
Imaginechem Co., Ltd. (Zhejiang, China). Fructose was
45 purchased from SanlandꢀChem International Inc. (Xiamen,
China). Acetonitrile (HPLC grade) was purchased from Tania Co.
Ltd. (Fairfield, USA). All other reagents were provided by local
supplies (Wuhan, China) without further purification.
2.4 Titration of the catalyst acidity
100 The acidity of Fe3O4@SiO2ꢀHPW was probed by potentiometric
titration.29 Fe3O4@SiO2ꢀHPW (0.05 g) was suspended in
acetonitrile, and stirred for 3 h. Then the suspension was titrated
2.2 Preparation of the catalyst
2
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with 0.05 M nꢀbutylamine in acetonitrile at 0.05 ml/min. The
3.1 Preparation and characterization of the catalyst
electrode potential variation was measured with an Orion 420
digital A model by using a doubleꢀjunction electrode.
The magnetic nanoparticles (Fe3O4@SiO2) supported HPW
55 catalyst (Fe3O4@SiO2ꢀHPW) was prepared according to the
2.5 Typical procedure for the synthesis of EMF
2.5.1 Synthesis of EMF from HMF
following procedure, as shown in Scheme 2.
Fe3O4
5
Fe (II )
TEOS
NH4OH
Fe3O4@SiO2 MNP
Fe3O4
+
NH4OH
In a typical experiment for the synthesis of EMF from HMF,
HMF (126 mg, 1 mmol) , Fe3O4@SiO2ꢀHPW (150 mg) and 5 mL
of ethanol were charged in a 15ꢀmL flask coupled with a reflux
condenser under a nitrogen atmosphere, and the condenser was
Fe (III)
HPW
Keggin Unit
10 sealed with a balloon charged with nitrogen to avoid the release
of ethanol during the reaction. The reactor was placed into a
preheated oil bath kept at 100 C and stirred magnetically at 600
rpm for a given reaction time. Time zero was taken when the
reactor was immersed in the oilꢀbath heating device. Small
15 aliquots were withdrawn from the reaction mixture at a given
reaction time, and diluted with deionized water to a certain
concentration for HPLC detective range. 30
Fe3O4@SiO2-HPW
o
Scheme 2. Schematic illustration for the synthesis of
Fe3O4@SiO2ꢀHPW.
60
The magnetic nanoparticles (MNP) Fe3O4 was easily prepared
via coꢀprecipitation method. Under a nitrogen atmosphere, an
aqueous mixture of ferric and ferrous salts precipitated as an
aqueous dispersion of Fe3O4 MNP via the addition of strong
alkaline solutions of 28 wt. % NH3.H2O. Considering the
2.5.2 Synthesis of EMF from fructose
The reaction device was the same as the above for the synthesis
20 of EMF from HMF. Firstly, fructose (180 mg, 1 mmol) was
added into 5 mL of ethanol under stirring until a clear solution
was formed. Then Fe3O4@SiO2ꢀHPW (150 mg) was added into
the fructose solution. Time zero was taken when the catalyst was
added into the reaction mixture. Otherwise steps were carried out
25 in a similar way as the above description.
65 aggregation tendency of the naked Fe3O4 MNP and acid
corrosion problem, coating the magnetic nanoparticles with silica
should be a good solution. Furthermore, the outer shell of silica
also provides suitable sites (SiꢀOH groups) for surface
functionalization with the precursor HPW. Fe3O4@SiO2 MNP
70 was obtained by the treatment of Fe3O4 with tetraethyl
orthosilicate (TEOS) in an alkaline ethanol–water solution under
strong alkaline conditions. Finally, HPW was immobilized on the
support Fe3O4@SiO2 to construct a magneticallyꢀrecoverable
catalyst (Fe3O4@SiO2ꢀHPW). The XRD diffraction patterns of
75 Fe3O4@SiO2 and Fe3O4@SiO2ꢀHPW are shown in Fig. 1. The
diffraction peaks in Fe3O4@SiO2 and Fe3O4@SiO2ꢀHPW can be
assigned to the planes of inverse cubic spinel structured Fe3O4,
which match well with the standard Fe3O4 sample (JCPDS file
No. 19ꢀ0629).31 In addition, it should be pointed out that no
80 separate crystal phase characteristic of bulk HPW existed in
Fe3O4@SiO2ꢀHPW, which confirms the high dispersion of HPW
on the support Fe3O4@SiO2. 32
2.6 Determination of the products
Quantitative analyses of HMF and EMF were conducted on a
ProStar 210 HPLC system using external standard calibration
curve method. Samples were separated using a reversedꢀphase
30 C18 column (200×4.6 mm) at 25 °C with a detection wavelength
of 280 nm. Acetonitrile and 0.1 wt.% acetic acid aqueous solution
(V:V = 15:85) were used as the mobile phase at a flow rate of 1.0
mL/min, and the column temperature was maintained at 303 K.
The content of HMF and EMF in samples were obtained directly
35 by interpolation from calibration curves, with a coefficient of
0.999.
The byꢀproducts such as ethyl levulinate was analyzed by gas
chromatography (GC) on a 7890F instrument with a crosslinked
capillary FFAP column (30 m × 0.32 mm × 0.4 mm) equipped
40 with a flame ionization detector. Operating conditions were as
follows: The flow rate of the N2 carrier gas was 40 mL/min, the
Fe3O4@SiO2-HPW
Fe3O4@SiO2
o
injection port temperature was 250 C, the oven temperature was
190 oC, and the detector temperature was 280 oC. The peak of
ethyl levulinate was identified by comparison of standard
45 compound and quantified based on the internal standard method.
2.7 Recycling of catalyst
10
20
30
40
50
60
70
80
After reaction, the catalyst Fe3O4@SiO2ꢀHPW was seperated
from the reaction mixture by a permanent magnet. Then the
catalyst was washed three times with 10 mL of water, three times
2
θ
o
50 with 10 mL of ethanol, and dried at 100 C in a vacuum oven
over light.
Fig. 1 XRD patterns of Fe3O4@SiO2 and Fe3O4@SiO2ꢀHPW.
Catal.Sci.Technol., [2013], [vol], 00–00 |
3. Results and Discussion
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Transmission electron micrograph image of Fe3O4@SiO2 NP is 45 between 1100 and 500 cm−1: oxygen atoms connected to three W
shown in Fig. 2. A typically core–shell structure of Fe3O4@SiO2
NP was present. The dark core of the Fe3O4 and the grey silica
shell were clearly observable. The typical silica shell thickness
was assessed to be around 10 nm. However, some
atoms and P heteroatom (Oa), bridging oxygen atoms (Ob and Oc),
and terminal oxygen atoms (Od), which are responsible for the
fingerprint bands of the Keggin ion. In the case of the spectra of
Fe3O4@SiO2ꢀHPW, four kinds of oxygen atoms were stated as
5
aggregations/coalescences of individual Fe3O4@SiO2 appeared, 50 follows: PꢀOa bands (1088 cmꢀ1), bridge WꢀOb/c bands (893 and
and the aggregations might be performed during the coating
process. The aggregations thus resulted in the form of bigger
structures with a nonꢀspherical morphology. We also tried to get
803 cmꢀ1), WꢀOd bands (986 cmꢀ1), and the bending vibration of
PꢀO (592 cmꢀ1), respectively. It should be pointed out that the
band of PꢀOa band was masked by the Si–O–Si absorption band
in the FTꢀIR spectra of Fe3O4@SiO2ꢀHPW. The FTꢀIR spectra
10 a wide range of TEM containing three layers for Fe3O4@SiO2ꢀ
HPW, from which we would clearly seen a layer of HPW on the 55 indicated that Keggin structure of HPW was maintained in
out surface of Fe3O4@SiO2, but in vain. One important reason
might be that the loading of HPW was low to give a distinct
image in a wide rage. As detected by element analysis, the weight
15 percentage of HPW was 26.8% in Fe3O4@SiO2ꢀHPW. In fact,
Fe3O4@SiO2ꢀHPW. The content of W in the sample was
determined to be 20.5%, indicated that a loading at ca. 26.8 wt%
HPW was obtained.
Rafiee
et
al.
prepared
silica
spheres supported HPW (SiꢀHPW), and SiꢀHPW was subjected to
the TEM analysis.33 Although the weight percent of HPW was
reached 40 wt%, it could not observe a typical layer of HPW.
Fe3O4
Fe3O4@SiO2
Fe3O4@SiO2-HPW
4000 3500 3000 2500 2000 1500 1000
-1
500
Wavenumber
(
cm
)
60 Fig. 3 IR spectra of (a) Fe3O4, (b) Fe3O4@SiO2 and (c)
20
Fe3O4@SiO2ꢀHPW.
Fig. 2 Transmission electron microscope (TEM) image of
Fe3O4@SiO2
Table 1. Textural properties of Fe3O4@SiO2ꢀHPW and
Fe3O4@SiO2
Sample
Surface area
(m2/g)
Pore volume Pore size
In order to further confirm the silica layer of Fe3O4@SiO2 and
the Keggin structure of HPW in Fe3O4@SiO2ꢀHPW, FTꢀIR was
(cc/g)
(nm)
Fe3O4@SiO2
44.5
27.6
0.101
1.98
25 employed to give detailed structure of Fe3O4, Fe3O4@SiO2 and
Fe3O4@SiO2ꢀHPW. A band was presented around 1630 cmꢀ1 for
the three samples, which was attributed to adsorbed water.34 The
characteristic absorption band at 583 cmꢀ1 was observed for
Fe3O4 (curve a), which was due to the Fe–O vibrations.35 In
30 addition, a wide broad peak was also observed around 3400 cmꢀ1,
which indicated that hydroxyl groups were presented on the
surface of Fe3O4 MNP. In the case of Fe3O4@SiO2 NP (curve
2), a band appeared at 1088 cmꢀ1 was attributed to the Si–O–Si
antisymmetric stretching vibrations, and the band at 800 cm−1
35 was also an evidence of the symmetric stretching vibration of the
rocking mode of SiꢀOꢀSi. The presence of SiꢀOH symmetric
stretching vibration was also confirmed by the absorption band at
950 cm−1. The O–H stretching band around 3423 cm−1 were both
presented in Fe3O4@SiO2 and Fe3O4. Considering the FTꢀIR
40 spectra of Fe3O4 and Fe3O4@SiO2, it was clearly concluded that
Fe3O4 was successfully coated with silica layer. It is well known
that Keggin ion structure is consisted of a PO4 tetrahedron, which
is surrounded by four W3O9 groups formed by edgesharing
octahedral.36 There are four different types of oxygen atoms
Fe3O4@SiO2ꢀHPW
0.064
1.85
In order to study the surface area and porous nature of the
65 materials, the N2 adsorptionꢀdesorption isotherms of Fe3O4@SiO2
and Fe3O4@SiO2ꢀHPW were conducted. A typical isothermal of
the Fe3O4@SiO2 and Fe3O4@SiO2ꢀHPW samples are shown in
Fig. 4. Isotherms of Fe3O4@SiO2 and Fe3O4@SiO2ꢀHPW showed
a type IV isotherm, and the samples were microporous material.
70 With the loading of HPW, the hysteresis loop became shorter,
corresponding to a reduction of pore volume. Table 1 shows
values of BET surface area, pore size, and pore volume for
Fe3O4@SiO2 and Fe3O4@SiO2ꢀHPW. It was noted that
Fe3O4@SiO2 had much higher surface area and pore volume than
75 Fe3O4@SiO2ꢀHPW. It seems logical as the loading of HPW was
dispersed and deposited on the surface of Fe3O4@SiO2,
decreasing the pore diameter and thus diminishing the surface
area.
4
| Catal.Sci. Technol., [2013], [vol], 00–00
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OH (silanol groups) on the surface of Fe3O4@SiO2 are always
25 weakly acidic in nature. The immobilization of HPW on the
surface of Fe3O4@SiO2 exhibited a very strong acidity with an Ei
value of 584 mV. The strong acidity of Fe3O4@SiO2ꢀHPW was
due to the replacement of the low acidity of silanol groups with
stronger acidic sites of HPW, which caused an increase in the
30 number and strength of the acid sites of Fe3O4@SiO2ꢀHPW.
110
100
90
80
70
60
50
40
30
20
10
0
Fe O @SiO
3
4
2
Fe O @SiO -HPW
3
4
2
3.3 Synthesis of EMF from HMF in the presence of various
catalysts
0.0
0.2
0.4
Partial pressure
0.6
P/P0
0.8
1.0
The catalytic activity of Fe3O4@SiO2ꢀHPW was evaluated by the
synthesis of EMF from the etherification of HMF. In order to
35 evaluate the advantages of Fe3O4@SiO2ꢀHPW, various catalysts
were employed to catalyze the etherification of HMF by ethanol
(Table 2). No EMF was detected after reaction, when the reaction
was carried out in the absence of catalyst (Table 2, Entry 1). In
the meantime, negligible EMF yield of 0.2% was also noted in
40 the presence of Fe3O4@SiO2 (Table 2, Entry 2). However, the
yield of EMF was largely increased to 76.9% with the HMF
conversion of 89.5% when Fe3O4@SiO2ꢀHPW was used (Table 2,
Entry 3). These results were in line with the results obtained by
potentiometric titration. Taking the above results into
45 consideration, HPW was the active catalytic species in
Fe3O4@SiO2ꢀHPW for the synthesis of EMF from the
etherification of HMF. HPW was also employed to catalyze the
etherification of HMF. In order to give a reasonable comparison
between HPW and Fe3O4@SiO2ꢀHPW, the amount of HPW was
50 the same as the content of HPW in Fe3O4@SiO2ꢀHPW. HMF
conversion and EMF yield with HPW as catalyst were close to
those with Fe3O4@SiO2ꢀHPW as catalyst (Table 2, Entry 2 vs
Entry 4). Unlike HPW, Fe3O4@SiO2ꢀHPW was insoluble in
ethanol, but the catalytic activity was comparable with the
55 homogeneous HPW. This was presumed to be due to the
adsorption of reactants on the surface of the catalyst, increasing
the local concentration of reactants around the active sites of the
Fe3O4@SiO2ꢀHPW and promoted the reaction effectively. More
importantly, a distinct disadvantage of the use of HPW was that
60 the procedures for the catalyst recycling was complex, as HPW
was dissolved in ethanol. In addition, two kinds of the
representative solid catalysts were also studied for the synthesis
of EMF. As expected, HMF conversion and EMF yield were low
when the low acidity of HYꢀzeolite was used (Table 2, Entry 5).
65 The sulfonated NKCꢀ9 resin with a strong acidity could also
effectively catalyze the etherification of HMF with the
conversion of 93.3% and EMF was obtained in a yield of 78.8%.
However, the structure of NKGꢀ9 resin was destroyed seriously.
The globose catalyst was pulverized after reaction, therefore
70 catalyst recycling of NKGꢀ9 resin was difficult. Xu et al. also
reported the synthesis of EMF from the etherification of HMF
catalyzed by MCMꢀ41ꢀsupported H4SiW12O40 catalyst, and an
EMF yield of 77.3% was obtained, which was similar with our
result (Table 2, Entry 7 vs Entry 3).20 Meanwhile, the recycling
75 experiments were carried out. Whereas, the recycling procedures
for MCMꢀ41ꢀsupported H4SiW12O40were much more complex as
compared with our magnetic Fe3O4@SiO2ꢀHPW catalyst.
Considering all the above discussed results, Fe3O4@SiO2ꢀHPW
not only had an appropriate acidity to promote the etherification
(
)
Fig. 4 Adsorption isotherm of Fe3O4@SiO2 and Fe3O4@SiO2ꢀ
HPW.
3.2 Surface acidity measurements
600
Fe3O4@SiO2-HPW
Fe3O4@SiO2
500
400
300
200
100
0
-100
-200
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
mmol amine / g catalyst
5
Fig. 5 Potentiometric titration of Fe3O4@SiO2 and Fe3O4@SiO2ꢀ
HPW.
The acidity of Fe3O4@SiO2ꢀHPW was determined by
a
potentiometric titration method. This method is based on the
10 measured potential difference, is mainly determined by the acidic
environment around the electrode membrane. nꢀButylamine is
used as an organic base for the potentiometric titration of acid,
which has a basic dissociation constant of approximately 10ꢀ6.
Using this technique, it was able to evaluate the strength of the
15 acidity of Fe3O4@SiO2ꢀHPW. The potentiometric titration curves
of Fe3O4@SiO2 and Fe3O4@SiO2ꢀHPW are shown in Fig. 5.
According to the previous references,37 the initial electrode
potential (Ei) indicated the maximum acid strength of the acid
sites. The acid strength can be quantified according to the
20 following scales: Ei > 100 mV (very strong sites), 0 < Ei < 100
mV (strong sites), −100 < Ei < 0 mV (weak sites) and Ei < −100
mV (very weak sites). As shown in Fig. 5, Fe3O4@SiO2 showed a
weak acid sites with an Ei value of ꢀ60 mV. The presence of Siꢀ
This journal is © The Royal Society of Chemistry [year]
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Catalysis Science & Technology
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of HMF into EMF, but also was convenient in the catalyst
recycling.
90
80
70
60
50
40
30
20
10
0
Table 2. Synthesis of EMF from HMF catalyzed by various
catalysts.a
HMF
EMF
(%)
Catalyst
amount
(mg)
Conversion Yield
Entry
Catalyst
(%)
0.3
150 mg
100 mg
50 mg
1
2
ꢀ
ꢀ
0
Fe3O4@SiO2
100
100
27
1.7
0.2
76.9
80.3
6.5
0
2
4
6
8
10
12
Fe3O4@SiO2ꢀ
HPW
Time (h)
3
89.5
91.0
7.0
100
90
80
70
60
50
40
30
20
10
0
4
H3PW12O40
HYꢀZeolite
NKCꢀ9 resin
5b
6
100
100
ꢀ
50 mg
100 mg
150 mg
93.3
92.0
78.8
77.3
MCMꢀ41ꢀ
H4SiW12O40
7b
a Reaction conditions: a mixture of HMF (126 mg, 1 mmol) and
catalyst were added into 5 ml of ethanol, and the reaction was
carried out at100 oC for 11 h.
0
2
4
6
8
10
12
Time(h)
b Ref. 18. The reaction conditions: HMF (3 mmol), acid catalyst
(0.051 mmol H+), anhydrous ethanol (1.80 mL), oil bath at 90
oC for 2 h.
Fig. 6 The effect of catalyst loading on etherification of HMF
35 catalyzed by Fe3O4@SiO2ꢀHPW in ethanol. Reaction conditions:
HMF (126 mg, 1 mmol) and a set amount of Fe3O4@SiO2ꢀHPW
were added into 5 mL of ethanol, and the reaction was carried out
at 100 oC.
5
3.4 Effect of Fe3O4@SiO2-HPW loading on the Synthesis of
EMF from HMF
With the aim of optimizing the catalytic system, the effect of
catalyst loading on the synthesis of EMF by the etherification of
HMF was also studied. As shown in Fig. 6, HMF conversion and
3.5 Synthesis of EMF from fructose
40 As the dehydration of fructose into HMF and the etherification of
HMF into EMF are both acidicꢀcatalyzed reactions, the
integration of the two steps in one pot reaction is logically
possible, which saves time and energy. Therefore, synthesis of
EMF from fructose through oneꢀpot reaction was carried out. As
45 shown in Fig. 7, the reaction temperature had a remarkable effect
10 EMF yield were increased with the increase of the catalyst
loading at the same reaction time point. Taking the reaction time
of 1 h as an example, HMF conversions were reached in 10.1%,
28.6% and 38.6% with the catalyst loading at 50 mg, 100 mg, and
150 mg, respectively, and the corresponding EMF yields were
15 8.9%, 20.1% and 31.2%, respectively. An increase of HMF
conversion and EMF yield with an increase of the catalyst
loading could be attributed to an increase in the availability and
number of catalytically active sites. When the reaction was
stopped at 11 h, the maximum EMF yields were obtained in
20 71.7%, 76.9% and 83.6% with the amount of Fe3O4@SiO2ꢀHPW
at 50 mg, 100 mg and 150 mg, respectively, corresponding to the
conversion of HMF in 84.4%, 89.5% and 97.9%. Therefore 150
mg was selected as an appropriate catalyst loading in order to get
a high EMF yield in the following experiments. It seems that the
25 catalyst amount was a little high in order to get a high EMF yield.
According to the elemental analysis, the weight percent of the
supported HPW was 26.8%, thus 150 mg of Fe3O4@SiO2ꢀHPW
was equaled to 40 mg of HPW. Although the catalyst
Fe3O4@SiO2ꢀHPW could be readily recycled by a permanent
30 magnet, it was necessary to develope a novel catalyst with a high
catalyst activity to reduce the catalyst amount, which is potential
in practical application for the synthesis of liquid fuel.
o
on the yield of EMF. When the reaction was conducted at 70 C,
both HMF yield and EMF yield were rather low during the
reaction process. EMF yield was slowly increased from 0.2% for
2 h to 5.2% for 24 h, and HMF yield was also kept at a low level.
50 At the end of reaction, the conversion of fructose was detected to
be 9.7%. The results indicated that the first step of the
o
dehydration of fructose into HMF was difficult at 70 C, but it
was difficult to assess the followed etherification of HMF into
o
EMF at 70 C. Therefore, the etherification of HMF into EMF
o
55 was also studied at 70 C, as shown in Fig. 8. EMF yield was
reached in 47.0% with HMF conversion of 53.6% for 11 h,
indicating that the etherification reaction was relatively easier
than the dehydration reaction.
EMF yield was largely increased from 2.8% for 2 h to 54.8%
60 for 24 h when the oneꢀpot reaction was carried out 100 oC. HMF
yield was initially increased from 5.7% for 2 h to 10.8% for 8 h,
and then it was kept at a stable level around 11%. Compared the
o
o
results at 100 C with those at 70 C, the dehydration of fructose
into HMF was promoted at a high reaction temperature. In
65 addition, as HMF yield was almost stable from 8 h to 24 h, it was
6
| Catal.Sci. Technol., [2013], [vol], 00–00
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o
concluded that the dehydration rate of fructose into HMF was
close to the etherification rate of HMF. When the reaction
temperature was further increased to 120 C, EMF yield at the
98.9% for 11 h at 100 C and 120 oC, respectively, and the
o
maximum EMF yields were obtained in 64.0% for 9 h at 120 C
and 83.6% for 11 h at 100 oC. Thus, the side reactions were much
o
early stage was higher than that at 100 oC. Whereas, it was lower 35 more serious at 120 C than those at 100 C. After reaction, the
o
o
5
than that at 100 oC at the late stage. The maximum EMF yield at
120 oC was 41.9% for 24 h, which was lower than that at 100 oC.
Fructose conversions were detected to be 94.7% and 100% for
catalyst was removed from the reaction mixture by a permanent
magnet, and some insoluble black humins were observed, which
was formed by the polymerization and crossꢀpolymerization of
HMF.39 On the other hand, ethyl levulinate as one of the major
o
o
24 h at 100 C and 120 C, respectively. In our experiment, the
reaction mixture was analyzed by gas chromatography at the end 40 byꢀproducts was beneficially promoted at a high reaction
10 of the reaction time of 24 h, and the yields of ethyl levulinate
temperature, which was caused by the alcoholysis of HMF. It was
reported that ethyl levulinate could be reached in a high yield of
o
o
were determined to be 31.7% at 120 C and 16.8% at 100 C,
respectively. These results indicated that the yield of ethyl
levulinate was increased with the increase of reaction temperature.
o
62% when HMF was treated in ethanol at 120 C for 30 h in the
presence of acid catalyst.23 At the end of the reaction time of 11 h,
o
In general, the selectivity of EMF from fructose was much lower 45 ethyl levulinate yields were determined to be 26.3% at 120 C
15 than that from HMF, which could be attributed to the following
and 5.8% at 100 oC, respectively. These results verified the major
byꢀproduct for the synthesis of EMF from HMF was caused by
the alcoholysis of HMF. It also indicated that ethyl levulinate
yield was increased with the increase of reaction temperature.
50 Other byꢀproducts such as humins and the selfꢀetherification
product [5,5'(oxyꢀbis(methylene))bisꢀ2ꢀfurf ural] of two
molecules of HMF was also quantified and the total carbon
balance was calculated (ESI, Table 1)
two reasons. On the one hand, dehydration of fructose into HMF
was not effective in ethanol with Fe3O4@SiO2ꢀHPW as catalyst.
On the other hand, some side reactions were presented such as the
rehydration of HMF into levulinic acid.38
55
o
120
100
C
50
45
40
35
30
25
20
15
10
5
o
C
o
70
C
90
o
120
C
80
70
60
50
40
30
20
10
0
o
70
C
o
100
C
0
0
5
10
15
20
25
Time (h)
0
2
4
6
8
10
12
14
12
10
8
Time (h)
100
90
80
70
60
50
40
30
20
10
0
o
70
C
o
120
100
C
o
100
120
C
C
o
C
o
o
70
C
6
4
2
0
0
2
4
6
8
10
12
0
2
4
6
8
10 12 14 16 18 20 22 24 26
Time (h)
Time (h)
20
55 Fig. 8 Sytheis of EME from HMF catalyzed by Fe3O4@SiO2ꢀ
HPW at different reaction temperatures. Reaction conditions:
HMF (126 mg, 1 mmol) and Fe3O4@SiO2ꢀHPW (150 mg) were
added into 5 mL of ethanol, then the reaction was carried out at
desired temperature.
Fig. 7 Sytheis of EME from fructose catalyzed by Fe3O4@SiO2ꢀ
HPW at different reaction temperatures. Reaction conditions:
Fructose (180 mg, 1 mmol) and Fe3O4@SiO2ꢀHPW (150 mg)
were added into 5 mL of ethanol, then the reaction was carried
25 out at desired temperature.
60 3.6 Catalyst recycling experiments
In order to give some insights into the difference between the
synthesis of EMF at 100 oC and that at 120 oC, synthesis of EMF
from HMF was also carried out at 100 oC and 120 oC,
respectively. As shown in Fig. 8, HMF conversion was higher at
30 120 oC than that at 100 oC, but EMF yield at 120 oC was lower
than that at 100 oC. HMF conversions were reached in 97.9% and
The reuse of a catalyst is highly preferable in terms of green
chemistry. Thus, the recycling experiments of Fe3O4@SiO2ꢀHPW
were carried out by using the synthesis of EMF from HMF as a
model reaction. After reaction, the reaction mixture was muddy
65 (Fig. 9a). The catalyst Fe3O4@SiO2ꢀHPW could be easily
This journal is © The Royal Society of Chemistry [year]
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collected with the aid of a permanent magnet (Fig. 9b). Then
liquid solution was removed from the mixture to leave the
catalyst Fe3O4@SiO2ꢀHPW. Finally, the catalyst was washed by
ethanol to remove the adsorbed products, dried under vacuum and
reused in a subsequent reaction. Other catalytic cycles were
repeated as described above. As shown in Fig. 10, in a test of six
cycles, the catalyst could be reused without any significant loss of
catalytic activity (83.2% in first cycle versus 78.8% in sixth
cycle). In order to further verify the stability of Fe3O4@SiO2ꢀ
10 HPW during the reaction process, a control experiment was also
carried out. Synthesis of EMF from HMF was firstly carried out
at 100 oC in the presenc of Fe3O4@SiO2ꢀHPW for 3 h, then
Fe3O4@SiO2ꢀHPW was removed from the reaction reaction
mixture. The liquid mixture was continued to be stirred at 100 oC
15 without Fe3O4@SiO2ꢀHPW. As shown in Fig. 11, EMF yield was
stable after the removal of Fe3O4@SiO2ꢀHPW. These results once
again verified Fe3O4@SiO2ꢀHPW was stable under our reaction
conditions. The liquid mixture was subjetced to the elemental
analysis by ICPꢀAES technology, and no W was detected, which
20 also indicated that the supported HPW was not released into the
reaction mixture from Fe3O4@SiO2ꢀHPW. We have also
characterized the catalyst after reused for 6 times by FIꢀIR (ESI,
Fig. S1) and XRD (ESI, Fig. S2). Compared the reused catalyst
with the fresh catalyst, no distinct difference was observed.
80
60
40
20
0
5
1
2
3
4
5
6
Recycle
Fig. 10 Recycle experiments of the catalyst Fe3O4@SiO2ꢀHPW.
Reaction conditions: HMF (126 mg, 1 mmol) and Fe3O4@SiO2ꢀ
HPW (150 mg) were added into 5 mL of ethanol, then the
50 reaction was carried out at 100 oC.
90
80
70
Catalystfiltrationafter3h
Withoutcatalystfiltration
25
In addition, the recycling experiments of the catalyst for the
synthesis of EMF from fructose was also carried out, and EMF
yields were kept stable around 54% for six runs (ESI, Fig S3).
These results also indicated that the Fe3O4@SiO2ꢀHPW was
stable. Incidentally, some silica supported HPW catalysts also
60
50
40
30
30 showed high stability for the liquid organic reactions.40 Whereas,
it was also reported that the activity of some silica supported
HPW catalysts was decreased during the recycling expriments.
Liu et al., reported the toluene alklylation with 1ꢀoctene catalyzed
by MCMꢀ41 supported heteropoly acids, and found the catalytic
35 acitivity was lost during the recycling expriments.41 They
considered that the catalyst after reaction could not be fully
recovered by filtration, as the particle size was very small,
leading to a loss of catalytic activity. Compared with the filtration
method, the method for the recovery of the catalyt by a
40 permanent magnet obviously avoids the catalyst loss during the
recycling expriments (as shown in Fig. 9b).
0
2
4
6
8
10
12
Time(h)
Fig. 11 The results of the control expriments after hot filtration
for the synthesis of EMF from the etherification of HMF.
Reaction conditions: HMF (126 mg, 1 mmol) and Fe3O4@SiO2ꢀ
55 HPW (150 mg) were added into 5 mL of ethanol, then the
reaction was carried out 100 oC.
4. Conclusion
In conclusion, we have developed a novel magnetic Fe3O4@SiO2
MNP supported heteropolyacid H3PW12O40 catalyst for the
60 synthesis of EMF from HMF and fructose. EMF was obtained
with a high yield of 83.6% by the etherification of HMF, and a
moderate EMF yield of 54.8% was also obtained from fructose
through oneꢀpot reaction. The special properties of HPW
provided sufficient acidic sites for the excellent catalytic acitivity
65 of Fe3O4@SiO2ꢀHPW. Moreover, Fe3O4@SiO2ꢀHPW could be
readily removed from the reaction mixture by a permanent
magnet, and it could be reused for several times without loss of
its catalytic activity. In addition to the application for the
synthesis of EMF, the environmentally benign catalyst
70 Fe3O4@SiO2ꢀHPW would have a promising potential for other
acidꢀcatalyzed chemical reactions.
Fig. 9 Separation of the Fe3O4@SiO2ꢀHPW catalyst from the
45 reaction mixture using an external magnet.
Acknowledgements
8
| Catal.Sci. Technol., [2013], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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35 C. He, T. Sasaki, Y. Shimizu and N. Koshizaki, Appl. Surf. Sci.,
2008, 254, 2196.
70 36 D. S. Pito, I. Matos, I. M. Fonseca, A. M. Ramos, J. Vital and J. E.
Castanheiro, Appl. Catal. A, 2010, 373, 140.
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The Project was supported by National Natural Science
Foundation of China (No. 21203252, 21206200), and the Special
Fund for Basic Scientific Research of Central Colleges, Southꢀ
Central University for Nationalities (CZY10006 ).
38 R. Weingarten, W. C. Conner and G. W. Huber, Energy Environ.
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