Iron oxide encapsulated by ruthenium hydroxyapatite as heterogeneous catalyst for the synthesis of 2,5-diformylfuran

Article abstract of DOI:10.1002/cssc.201402402

Magnetic γ-Fe₂O₃ nanocrystallites encapsulated by hydroxyapatite (HAP), HAP@γ-Fe₂O₃, were prepared followed by cation exchange of Ca²⁺ on the external HAP surface with Ru³⁺ to give the γ-Fe₂O₃@HAP-Ru catalyst. The structure of the as-prepared catalyst was characterized, and its catalytic activity was studied in the aerobic oxidation of 5-hydroxymethylfurfural (HMF). γ-Fe₂O₃@HAP-Ru showed a high catalytic activity for the aerobic oxidation of HMF into 2,5-diformylfuran (DFF). A high DFF yield of 89.1% with an HMF conversion of 100% was obtained after 4 h at 90°C. Importantly, the synthesis of DFF from fructose was realized by two consecutive steps. The dehydration of fructose in the presence of a magnetic acid catalyst (Fe₃O₄@SiO₂-SO₃H) produced HMF in a yield of 90.1 %. Then the Fe₃O₄@SiO₂-SO₃H catalyst was removed from the reaction solution with a permanent magnet, and HMF in the resulting solution was further oxidized to DFF with a yield of 79.1% based on fructose. The synthesis of DFF from fructose by two steps avoids the tedious separation of the intermediate HMF, which saves time and energy.

Full text of DOI:10.1002/cssc.201402402

CHEMSUSCHEM
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DOI: 10.1002/cssc.201402402
Iron Oxide Encapsulated by Ruthenium Hydroxyapatite as
Heterogeneous Catalyst for the Synthesis of 2,5-
Diformylfuran
[
a]
Zehui Zhang,* Ziliang Yuan, Dingguo Tang, Yongshen Ren, Kangle Lv, and Bing Liu
Magnetic g-Fe O nanocrystallites encapsulated by hydroxyapa-
tose was realized by two consecutive steps. The dehydration
of fructose in the presence of a magnetic acid catalyst
(Fe O @SiO ꢀSO H) produced HMF in a yield of 90.1%. Then
2
3
tite (HAP), HAP@g-Fe O , were prepared followed by cation ex-
2
3
2
+
3+
change of Ca on the external HAP surface with Ru to give
3
4
2
3
the g-Fe O @HAP-Ru catalyst. The structure of the as-prepared
the Fe O @SiO ꢀSO H catalyst was removed from the reaction
2
3
3
4
2
3
catalyst was characterized, and its catalytic activity was studied
in the aerobic oxidation of 5-hydroxymethylfurfural (HMF).
g-Fe O @HAP-Ru showed a high catalytic activity for the aero-
solution with a permanent magnet, and HMF in the resulting
solution was further oxidized to DFF with a yield of 79.1%
based on fructose. The synthesis of DFF from fructose by two
steps avoids the tedious separation of the intermediate HMF,
which saves time and energy.
2
3
bic oxidation of HMF into 2,5-diformylfuran (DFF). A high DFF
yield of 89.1% with an HMF conversion of 100% was obtained
after 4 h at 908C. Importantly, the synthesis of DFF from fruc-
Introduction
The growing concern over the depletion of fossil fuel has re-
sulted in a growing interest in the production of energy and
Scheme 1). FDCA is a highly promising bio-based building
block for resins and polymers, thus it can be used as a promis-
ing replacement for terephthalic acid (TA), a petroleum-based
[
1–3]
chemicals from renewable resources.
Although renewable
energy can be produced from renewable sources such as
wind, solar, and geothermal, none of these renewable sources
can be used to produce organic chemicals. Biomass is the only
carbon-containing renewable resource and it is attractive as
[
4–6]
a feedstock for energy and the chemical industry.
In this
context, much effort has been devoted to the transformation
of abundant biomass resources into chemicals and liquid fuels.
5-Hydroxymethylfurfural (HMF), the acid-catalyzed dehydra-
tion product of C -based carbohydrates, has two functionalities
6
attached to a furan ring. It can be used as a versatile precursor
for the synthesis of transportation fuels, pharmaceuticals, and
[
7,8]
other petroleum-derived chemicals.
Therefore, there has
Scheme 1. Oxidation products of HMF.
been an overwhelming interest in the synthesis of HMF from
monosaccharides, polysaccharides, cellulose, and lignocellulo-
[
9–13]
[14,15]
ses.
monomer in the plastics industry.
DFF is another important
Recently, the oxidation of HMF has received increasing at-
tention. The oxidation of HMF can generate several kinds of
oxidation products such as 2,5-diformylfuran (DFF), 5-hydroxy-
methyl-2-furancarboxylic acid (HMFCA), 5-formyl-2-furancar-
boxylic acid (FFCA), and 2,5-furandicarboxylic acid (FDCA;
oxidation product of HMF that is useful in many fields, such as
the synthesis of Schiff bases, pharmaceuticals, antifungal
[16–18]
agents, and organic conductors.
Early reports on the oxidation of HMF into DFF focused
mainly on classic oxidation methods, which had some draw-
backs such as their high cost and the production of large
[
19,20]
amounts of waste.
mentally friendly methods for the oxidation of HMF into DFF
with O is required. Some homogeneous catalysts have been
Therefore, the development of environ-
[a] Prof. Z. H. Zhang, Z. L. Yuan, Dr. D. G. Tang, Dr. Y. S. Ren, Dr. K. L. Lv,
Dr. B. Liu
Key Laboratory of Catalysis and Material Sciences
State Ethnic Affairs Commission & Ministry of Education
South-Central University for Nationalities
MinYuan Road 182, Wuhan (PR China)
Fax: (+86)27-67842752
2
[21–23]
used for the aerobic oxidation of HMF into DFF,
but the
major drawback of homogeneous catalysts is the difficulty of
their recovery. Therefore, the aerobic oxidation of HMF with
heterogeneous catalysts is an intriguing prospect in biorefiner-
ies. Recently, some heterogeneous catalytic systems have been
E-mail: zehuizh@mail.ustc.edu.cn
ꢀ
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[
24–26]
reported for the aerobic oxidation of HMF into DFF.
was determined to be 2.0 wt% by inductively coupled plasma
atomic emission spectroscopy (ICP-AES).
Although heterogeneous catalysts can be recycled, their tedi-
ous recovery by filtration or centrifugation and the inevitable
loss of solid catalysts during the separation process still limit
their application.
The Ru dispersion was calculated from H temperature-pro-
2
grammed desorption (H -TPD) measured by using a Zeton Al-
2
tomare AMI-200 unit. The g-Fe O @HAP-Ru (50 mg) catalyst
2
3
Recently, magnetic materials have opened up a new avenue
to introduce efficient systems to facilitate catalyst recovery in
was reduced at 3508C for 12 h using a flow of high-purity H2
and then cooled to 1008C under a H stream. The sample was
2
[
27]
different liquid-phase reactions. Iron oxides are usually used
as magnetic materials but they tend to aggregate. It has been
demonstrated that the encapsulation of iron oxide nanoparti-
cles prevents their aggregation and improves their stability. Re-
held at 1008C for 1 h under flowing Ar to remove weakly
bound physisorbed species prior to increasing the temperature
slowly to 3508C. At this temperature, the catalyst was held
under flowing Ar to desorb the remaining chemisorbed hydro-
gen, and the thermal conductivity detector (TCD) recorded the
cently, g-Fe O3 encapsulated by hydroxyapatite (HAP), g-
2
Fe O @HAP, as a magnetic support has been used to construct
signal until it returned to the baseline. The chemisorbed H
2
3
2
[
28,29]
ꢀ1
various catalysts for many chemical reactions.
For example,
uptake at 1008C was determined to be 82.7 mmol g catalyst.
H2
[
29]
[31]
Sheykhan et al. prepared n-propyl sulfamic acid covalently
supported on hydroxyapatite-encapsulated magnetic nanopar-
ticles for the Friedlꢁnder reaction. One important characteristic
According to the chemisorption stoichiometry H/Ru=1:1,
the Ru dispersion was determined to be 83.5%, which indicat-
2
+
ed that the Ru was exchanged well with the surface Ca
.
2
+
of HAP is that Ca in its framework can be exchanged with
other metal cations, which allows the introduction of active
sites. It was reported that Ru-exchanged hydroxyapatite (HAP-
Ru) showed a high catalytic activity in the oxidation of alcohols
The procedure for the synthesis of Fe O @SiO ꢀSO H is
3
4
2
3
shown in Scheme 2B. The core–shell silica-coated Fe O nano-
3
4
particles (Fe O @SiO ) were prepared according to our previous
3
4
2
[27]
work. The surface of Fe O @SiO was then grafted with g-
3
4
2
[
30]
with O2. Herein, HAP-Ru-encapsulated magnetic g-Fe O (g-
mercaptopropyl groups. Finally, the thiol groups in
2
3
Fe O @HAP-Ru) was prepared and used for the oxidation of
Fe O @SiO ꢀSH were oxidized to sulfonic acid groups by H O
2
3
3
4
2
2
2
HMF into DFF with O . Importantly, a magnetic acid catalyst
to obtain the Fe O @SiO ꢀSO H acid catalyst.
2
3
4
2
3
+
functionalized with sulfonic acid (Fe O @SiO ꢀSO H) was also
The amount of H in Fe O @SiO ꢀSO H was determined by
3
4
2
3
3
4
2
3
prepared and used to catalyze the dehydration of fructose into
HMF. The resulting reaction solution was used directly without
separation for the synthesis of DFF by the in situ oxidation of
HMF without the need to purify the HMF.
an acid–base titration method according to Equation (1). The li-
+
berated H O was titrated by standard NaOH. The amount of
3
+
H
in the Fe O @SiO ꢀSO H catalyst was determined to be
3
4
2
3
ꢀ1
0.9 mmolg .
ꢀ
1
þ
Fe O @SiO ꢀSO H þ H O ! Fe O @SiO ꢀSO þ H O
ð1Þ
3
4
2
3
2
3
4
2
3
3
Results and Discussion
Fe O @SiO ꢀSO H and g-Fe O @HAP-Ru were characterized
3
4
2
3
2
3
by XRD, and the results are shown in Figure 1. Characteristic
Catalyst preparation and characterization
diffraction peaks for the Fe O @SiO ꢀSO H sample were ob-
3
4
2
3
The procedures for the synthesis of the g-Fe O @HAP-Ru cata-
served clearly at 2q=30.2, 35.4, 43.3, 53.7, 57.3, and 62.98 (Fig-
ure 1A), which are attributed to the (220), (311), (400), (422),
(511), and (440) Bragg reflections of the face-centered cubic
2
3
2
+
lyst are shown in Scheme 2A. The coprecipitation of Fe and
3
+
Fe ions in alkaline solution under a N atmosphere produced
2
Fe O nanoparticles. The Fe O nanoparticles were then coated
lattice of Fe O4 nanoparticles, respectively (JCPDS No. 19–
3
4
3
4
3
2
+
3ꢀ
with HAP, which was formed from Ca and PO4 at pH 11.
The resultant material was calcined at 3008C for 3 h to give
the g-Fe O @HAP support. Finally, g-Fe O @HAP was stirred in
0629). In the XRD pattern of g-Fe O @HAP-Ru, peaks at 2q=
2
3
35.8, 43.7, 53.7, 57.3, and 63.08 were assigned to the reflections
[28]
of g-Fe O .
Furthermore, several well-resolved diffraction
2
3
2
3
2
3
an aqueous solution of RuCl at room temperature to produce
peaks at 2q=25.9, 31.8, 32.9, 34.1, 39.8, 46.7, and 49.58 were
observed clearly in the pattern of g-Fe O @HAP-Ru, which
3
the g-Fe O @HAP-Ru catalyst. During the cation-exchange pro-
2
3
2
3
[28]
cess, the black solution became clear, and the content of Ru
were characteristic of HAP. The XRD results indicated that g-
Fe O nanoparticles were encapsulated successfully
2
3
with HAP.
FTIR spectra of Fe O @SiO ꢀSO H and g-
3
4
2
3
Fe O @HAP-Ru are shown in Figure 2. The spectrum
2
3
of Fe O @SiO ꢀSO H was similar to that seen in the
3
4
2
3
[
32]
literature for silica-supported alkyl sulfonic acids.
A
ꢀ1
broad peak at n˜ =3000–3700 cm centered around
ꢀ
1
ꢀ1
n˜ =3400 and a sharper peak at n˜ =1640 cm were
observed, which are assigned to the OꢀH stretching
vibration of surface hydroxyl groups and physisor-
[32]
bed water, respectively. An intense peak around
ꢀ
1
Scheme 2. Preparation of (a) g-Fe
2
O
3
@HAP-Ru and (b) Fe
3
O
4
@SiO
2
ꢀSO
3
H.
n˜ =1040 cm was attributed to the asymmetric Siꢀ
ꢀ
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Figure 1. XRD patterns of (a) Fe
3
O
4
@SiO
2
ꢀSO
3 2 3
H and (b) g-Fe O @HAP-Ru.
2 3
Figure 3. TEM image of g-Fe O @HAP-Ru.
TEM images of the as-prepared g-Fe O @HAP-Ru catalyst are
2
3
shown in Figure 3. The as-synthesized g-Fe O @HAP-Ru has
2
3
a spindle-shaped morphology. In addition, slight aggregation
was observed in the TEM images. g-Fe O3 is visible in the
2
center of the g-Fe O @HAP-Ru (dark spots). The results indicate
2
3
that g-Fe O was encapsulated by HAP. The encapsulation of g-
2
3
Fe O by HAP was verified by X-ray photoelectron spectrosco-
2
3
py (XPS), in which no XPS peaks for Fe were observed. The
XPS results also indicated that g-Fe O was encapsulated by
2
3
HAP.
The surface composition of g-Fe O @HAP-Ru was also stud-
Figure 2. FTIR spectra of (a) Fe
3
O
4
@SiO
2
ꢀSO
3 2 3
H and (b) g-Fe O @HAP-Ru.
2
3
ied by XPS. The survey scan XPS spectrum of g-Fe O @HAP-Ru
2
3
ꢀ
1
OꢀSi stretching vibration, and a weak peak at n˜ =800 cm
(Figure 4a) shows peaks that correspond to the HAP frame-
work elements (Ca, P, O) and Ru. It was reported the broaden-
ing of the Fe2p1/2 peak (binding energy, BEꢁ711 eV) and
was caused by the symmetric SiꢀOꢀSi stretching vibration. The
presence of surface organic functional groups in the
Fe O @SiO ꢀSO H catalyst was proved clearly by FTIR spectros-
Fe2p peak (BEꢁ724 eV) at high energy is characteristic of
3
4
2
3
3/2
2
+
[39]
copy. The CꢀH stretch and CꢀH bending vibrations are visible
Fe in Fe O . There were no peaks at BEꢁ711 and 724 eV.
2
3
ꢀ
1
at n˜ =2920 and 1420 cm , respectively. The FTIR absorption of
the O=S=O asymmetric and symmetric stretching modes was
These results also indicated that g-Fe O was successfully en-
2 3
capsulated by HAP.
ꢀ
1
at n˜ =1240 and 1123 cm , respectively, and that of the SꢀO
stretching mode appeared at n˜ =647 cm . In addition,
To gain an insight into the valence state of the Ru species in
ꢀ
1 [33]
the g-Fe O @HAP-Ru framework, high-resolution XPS of Ru3p
2
3
ꢀ
1
a peak at n˜ =587 cm was also observed, which was assigned
to FeꢀO vibrations of the Fe O core in the Fe O @SiO ꢀSO H
and Ru3d were measured (Figure 4b and c, respectively). The
C1s peak was taken as the reference at BE=284.9 eV. The
Ru3d3/2 signal was overlapped completely with the C1s signal,
3
4
3
4
2
3
[
34]
catalyst.
The FTIR spectrum of g-Fe O @HAP-Ru is shown in Fig-
but the Ru3d peak was discerned at BE=281.9 eV. Metallic
2
3
5/2
0
ure 2B. The characteristic n(OH) vibrations of lattice hydroxyl
Ru invariably has binding energies at 279.8ꢂ0.2 eV, for RuO
2
ꢀ
1
groups at n˜ =3570 and 640 cm were observed, which were
peaks at BE=281 and 282.1 eV are found, and RuCl has a char-
3
[
35]
ꢀ1
[40]
produced by HAP. A broad peak centered around n˜ =3400
acteristic peak at BE=281.8 eV. The peak at BE= 281.9 eV
ꢀ
1
and a peak at n˜ =1640 cm
are attributed to adsorbed
observed for Ru3d5/2 indicated clearly that the Ru in g-
[
35]
3+
4+
water. The asymmetric and symmetric stretching modes of
Fe O @HAP-Ru is in an oxidized state, probably Ru or Ru .
2 3
3
ꢀ
3+
4+
PO4 groups were observed at n˜ =1100, 1045, 960, 600, and
To differentiate between Ru and Ru , the Ru3p3/2 spectrum
ꢀ
1 [35]
5
65 cm . The characteristic stretching vibration of the FeꢀO
of g-Fe O @HAP-Ru was also collected. The Ru3p peak was
2
3
3/2
ꢀ
1
bond at n˜ =598 cm overlapped with one of the stretching vi-
observed at BE=463.2 eV (Figure 4c), which is similar to that
3
4
ꢀ [36]
ꢀ1
III
brations of PO
.
The band at n˜ =875 cm indicated that
of Ru-HAP prepared conventionally that involves Ru species.
2
ꢀ
[37]
IV
HPO4 was present in the catalyst as an impurity. In addi-
tion, carbonate impurities in the catalyst were identified by
In addition, it was reported that the Ru3p3/2 peak with Ru
was up to BE=465.2 eV, which was higher than the value of
ꢀ
1
III
[41]
a set of signals in the n˜ =1600–1300 cm region, which origi-
nate from CO adsorption from air during sample handling.
BE=463.2 eV for Ru species. The valence state of Ru in g-
[
38]
Fe O @HAP-Ru was consistent with the results reported by
2
2
3
ꢀ
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2 3
Figure 5. Hysteresis loops for the g-Fe O @HAP-Ru catalyst at 300 K.
Effect of the solvents on the aerobic oxidation of HMF
To find a suitable solvent for the aerobic oxidation of HMF,
some common organic solvents and water were tested
(
Table 1). HMF conversion and DFF selectivity were affected
greatly by the solvent. Although both DMSO and DMF are
strongly polar solvents with high boiling points, a higher HMF
conversion was obtained in DMSO than in DMF (Table 1, en-
tries 1 and 2). The g-Fe O @HAP-Ru catalyst showed high cata-
2
3
lytic activity in aromatic solvents such as toluene, trifluoroto-
luene, and 4-chlorotoluene (Table 1, entries 3–5). The best
result was obtained if the oxidation of HMF was performed in
4-chlorotoluene (Table 1, entry 5). An HMF conversion of 100%
and a DFF yield of 81.4% were obtained in 4-chlorotoluene
after 2 h (Table 1, entry 5). A moderate HMF conversion of
47.5% was obtained in methyl isobutyl ketone (MIBK), but it
gave the highest selectivity to DFF in 85.0% (Table 1, entry 6).
As MIBK is usually used as an organic extractant in biphasic
systems (MIBK/water) for the conversion of fructose into HMF,
our method shows the potential for the synthesis of DFF from
fructose by two consecutive steps, which comprise the produc-
tion of HMF from fructose in a biphasic system (MIBK/water)
and the subsequent oxidation of HMF in MIBK. Poor results
were obtained if reactions were performed in low-boiling-
2 3
Figure 4. XPS spectra of the samples: (a) Survey scan ofg-Fe O @HAP-Ru,
(b) Ru3d region, and (c) Ru3p region.
[
42]
Wuyts et al.
They confirmed that the oxidation
III
state of Ru in Ru-HAP was Ru , in which the method
for the preparation of Ru-HAP was similar to ours by
2 3
Table 1. Results of the aerobic oxidation of HMF into DFF catalyzed by g-Fe O @HAP-
Ru in various solvents.
stirring the HAP in RuCl solution.
3
[
a]
It is important that the prepared catalyst should
possess sufficient paramagnetic properties for practi-
cal applications. Magnetic hysteresis was measured
Entry Solvent
e/e
0
(258C),
t
HMF conversion DFF yield DFF selectivity
Ref. [43]
[h] ]%]
[%]
[%]
for the g-Fe O @HAP-Ru catalyst in an applied mag-
2
3
1
2
3
4
5
6
7
8
9
DMF
DMSO
toluene
benzotrifluoride
4-chlorotoluene
MIBK
acetonitrile
ethanol
37.0
46.7
2.4
–
–
–
37.5
24.5
80.1
12
12
4
2
2
12
12
12
12
36.6
82.4
98.1
97.6
100
47.5
17.5
23.7
17.7
27.9
76.2
71.5
69.4
82.3
81.4
85.0
57.7
66.6
60.5
netic field at 300 K with the field sweeping from
4000 to +4000 Oe. The isothermal magnetization
curve of g-Fe O @HAP-Ru at 300 K displayed a rapid
58.9
68.3
80.3
81.4
40.4
10.1
15.8
10.7
ꢀ
2
3
increase with the increasing applied magnetic field
because of superparamagnetic relaxation (Figure 5).
ꢀ
1
The saturation magnetization reached 6.1 emug .
The magnetization is sufficient for magnetic separa-
tion by a permanent magnet.
water
[
(
2 3
a] Reaction conditions: g-Fe O @HAP-Ru (150 mg), HMF (100 mg), 1108C, solvent
7 mL), O (20 mLmin ).
2
ꢀ
1
ꢀ
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point solvents such as acetonitrile and ethanol
2 3
Table 2. Results of the aerobic oxidation of HMF into DFF catalyzed by g-Fe O @HAP-
Ru at different reaction temperatures.
(Table 1, entries 7 and 8). A poor catalytic per-
[a]
formance was also observed if the oxidation of HMF
was conducted in water (Table 1, entry 9).
Entry
T
[
t
[h]
HMF conversion
[%]
DFF yield
[%]
DFF selectivity
[%]
FDCA yield
[%]
o
C]
Generally, the reaction solvent showed a remark-
able effect on chemical reactions. The nature of
these solvent effects is complicated because of their
dependence on the catalyst properties and reaction
1
2
3
4
5
6
7
110
90
70
70
70
50
25
2
4
4
100
100
43.2
97.5
100
41.5
11.4
81.4
89.1
38.3
84.8
83.4
36.6
10.7
81.4
89.1
88.6
86.9
83.4
88.1
93.8
13.9
9.3
2.2
6.8
9.7
3.7
0.4
12
14
12
12
[
44]
conditions and their origin remains unclear. In ad-
dition, the properties of the solvent, such as the po-
larity, dielectric constant, steric hindrance, and acid–
base properties, also have a great effect on the
chemical reaction. The aerobic oxidation of HMF
over g-Fe O @HAP-Ru did not seem to follow any
2 3
[a] Reaction conditions: g-Fe O @HAP-Ru (150 mg), HMF (100 mg), 4-chlorotoluene
ꢀ
1
(
2
7 mL), O (20 mLmin ).
2
3
trend in terms of solvent dielectric constant (a rela-
tive measure of the solvent polarity; Table 1), which suggested
ture and the reaction time. With the same HMF conversion of
100%, the lowest selectivity of DFF (81.4%) was obtained in
2 h at 1108C (Table 2, entry 1), which indicates that a high reac-
tion temperature benefited the subsequent oxidation of DFF
into FDCA. The highest selectivity of DFF was obtained in
89.1% at 908C after 4 h (Table 2, entry 2). If the reaction tem-
perature was further decreased to 708C, a long reaction time
of 14 h was needed to achieve full HMF conversion, but the se-
lectivity was moderate (Table 2, entry 5).
that the effect of the solvent on the g-Fe O @HAP-Ru-catalyzed
2
3
oxidation of HMF was more than a polar effect. Interestingly,
solvents that contain heteroatoms (N, O) led to lower HMF
conversions and DFF yields than aromatic solvents. One possi-
ble reason is the stronger interactions between the solvent
(the heteroatoms N, O as the electron donor) and the catalytic
center (Ru as the electron acceptor). This phenomenon was
also observed by Lignier et al. who used Au/TiO for the oxida-
2
[
45]
tion of alcohols. They found that solvents that did not con-
tain heteroatoms (N, O, S) resulted in significant product
yields. In addition, our results also were consistent with previ-
ous results, in which HAP-supported Ru catalysts showed
a high catalytic activity in aromatic solvents for the oxidation
Effect of the catalyst amount on the aerobic oxidation of
HMF
The results of the aerobic oxidation of HMF into DFF with dif-
[
46]
of alcohols.
ferent amounts of g-Fe O @HAP-Ru are shown in Table 3. The
2
3
HMF conversion and DFF yield increased with the increase of
the catalyst amount at the same reaction time point. For exam-
ple, HMF conversions reached 27.5, 51.9, and 100% after 4 h
with the catalyst amounts of 25, 50, and 100 mg, respectively,
and the corresponding DFF yields were 24.3, 45.9, and 83.4%,
respectively (Table 3, entries 1–3). The increased HMF conver-
sion with an increase of the catalyst amount in the same reac-
tion time period is attributed to an increase in the availability
and number of catalytically active sites. However, the selectivi-
ty of DFF was not affected notably by the catalyst amount
Effect of reaction temperature on the aerobic oxidation of
HMF
The effect of the reaction temperature on the aerobic oxida-
tion of HMF into DFF was studied over g-Fe O @HAP-Ru in 4-
2
3
chlorotoluene (Table 2). Higher reaction temperatures led to
higher HMF conversions. Only 2 h was needed to obtain an
HMF conversion of 100% with a DFF yield of 81.4% at 1108C.
If the reaction temperature decreased to 908C, a longer reac-
tion time of 4 h was required to obtain a 100% HMF
conversion with a DFF yield of 89.1% (Table 2, en-
Table 3. Results of the aerobic oxidation of HMF into DFF with different amounts of
g-Fe O @HAP-Ru.
2 3
tries 1 and 2). FDCA was the main byproduct, which
was formed by the further oxidation of DFF. Interest-
ingly, the oxidation reaction could proceed smoothly
at a low temperature of 708C, but a long reaction
time was required to obtain a high HMF conversion
[
a]
Entry Catalyst amount
[mg]
t
HMF conversion DFF yield DFF selectivity TON
[h] [%]
[%]
[%]
1
2
3
4
5
6
7
8
9
25
50
100
150
4
4
4
2
4
4
4
8
12
27.5
51.9
100
100
2.7
3.1
74.2
100.0
82.7
24.3
45.9
83.4
81.4
0
87.4
88.6
83.4
81.4
0
44.1
41.6
40.1
26.7
–
(Table 2, entries 4 and 5). An HMF conversion of
1
1
00% and a DFF yield of 83.4% were obtained after
4 h at 708C (Table 2, entry 5). A moderate HMF con-
without catalyst
version of 41.5% and a DFF yield of 36.6% were ob-
tained after 12 h at 508C (Table 2, entry 6), whereas
the oxidation of HMF hardly occurred at room tem-
perature (Table 2, entry 7). In addition, as the reaction
comprised the oxidation of HMF into DFF and the
subsequent oxidation of DFF into FDCA, the selectivi-
ty of DFF depended on both the reaction tempera-
[b]
150
100
100
100
0
0
–
[
[
[
c]
c]
d]
65.1
84.9
65.6
87.7
84.9
79.3
29.7
40.1
33.2
[
a] Reaction conditions: A set amount of g-Fe
2
O
3
@HAP-Ru, HMF (100 mg), 4-chloroto-
ꢀ1
luene (7 mL), 1108C, O
2
(20 mLmin ). [b] 150 mg of g-Fe @HAP was used. [c] Under
2 3
O
2
O balloon. [d] In air.
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(
Table 3, entries 1–3). With a further increase of the catalyst
low yields. The FDCA and HMFCA yields were 6.8 and 6.4%, re-
spectively, after 12 h. The total yield of DFF, FDCA, and HMFCA
after 12 h was close to the conversion of HMF, which indicated
that no other byproducts were formed in the aerobic oxidation
of HMF.
amount to 150 mg, a short reaction time of 2 h was enough to
obtain an HMF conversion of 100% and a DFF yield of 81.4%
(Table 3, entry 4).
The turnover number (TON) of the oxidation of HMF over g-
Fe O @HAP-Ru was calculated (Table 3). In contrast to the
Interestingly, it was found that HMFCA was formed mostly
at the beginning of the experiment, whereas the yields of both
FDCA and DFF increased linearly. The oxidation of the hydroxyl
group in HMF produces DFF (this is the main reaction in our
case), and the oxidation of the aldehyde group in HMF produ-
ces HMFCA (one of the minor byproducts in our case;
Scheme 1). Both DFF and HMFCA can be further converted
into FDCA. FDCA was also detected as one of the minor by-
products in our case. Therefore, according to the reaction
routes of HMF, it is not difficult to understand why HMFCA was
formed mostly at the beginning of the experiment and why
the yields of both FDCA and DFF increased linearly. DFF is the
major product, the yield of which increased with the increase
of the reaction time. FDCA was the final oxidation product, the
yield of which also increased during the reaction process as it
can be produced from the oxidation of DFF or HMFCA. The
yield of HMFCA fluctuated at a level of 6% after the beginning
of the reaction. The reasons might be that HMFCA was not
formed mainly from HMF and that the HMFCA formed during
the reaction was further converted into FDCA.
2
3
trend of the HMF conversion and DFF yield, the TON decreased
with the increase of the catalyst amount. Unlike chemical reac-
tions with homogeneous catalysts, liquid-phase reactions with
heterogeneous catalysts are limited by mass transfer in the re-
action system. The substrate (HMF) in the liquid solution is re-
quired to reach the active sites (Ru). With the same HMF con-
centration, a larger amount of the catalyst resulted in the
slower mass transfer of the substrate from the liquid solution
to the active sites. Therefore, the TON decreased with the in-
crease of the catalyst amount under the otherwise same condi-
tions, which means that the reaction results are a convolution
of kinetics and mass transfer. Control experiments were per-
formed without catalyst or in the presence of g-Fe O @HAP,
2
3
and no DFF was determined in both cases (Table 3, entries 5
and 6). These results indicated that the Ru species were the
active sites for the aerobic oxidation of HMF into DFF.
The high catalytic activity of g-Fe O @HAP-Ru inspired us to
2
3
perform the oxidation of HMF under less rigorous conditions.
Therefore, reactions were performed under an O balloon or in
2
air (Table 3, entries 7–9). An HMF conversion of 100% and
a DFF yield of 84.9% were obtained after 8 h under an O bal-
2
Synthesis of DFF from fructose through two consecutive
steps
loon (Table 3, entry 8). Good results were also achieved if the
reaction was conducted in air (Table 3, entry 9). The high cata-
lytic activity of g-Fe O @HAP-Ru under an O balloon or in air
As the aerobic oxidation of HMF over g-Fe O @HAP-Ru gave
2
3
2
2
3
makes this method much more attractive in practical applica-
tions from an economical viewpoint because of the mild oper-
ation conditions.
an excellent DFF yield, we tried to obtain DFF directly from
fructose in the presence of the binary catalysts, namely,
Fe O @SiO ꢀSO H and g-Fe O @HAP-Ru, in which Fe O @SiO ꢀ
3
4
2
3
2
3
3
4
2
SO H was used to catalyze the dehydration of fructose into
3
HMF and g-Fe O @HAP-Ru was used for the in situ oxidation of
2
3
Time course of the product distribution
HMF into DFF. 4-Chlorotoluene was the best solvent for the
aerobic oxidation of HMF into DFF, and DMSO was a superior
To gain more insight into the oxidation of HMF into DFF, the
time course of HMF recovery and product yields were recorded
[47]
organic solvent for the dehydration of fructose into HMF.
(
Figure 6). The content of HMF decreased gradually during the
reaction process, and the DFF yield increased gradually from
1.4% after 2 h to 84.8% after 12 h. Furthermore, FDCA and
Therefore, a mixture of DMSO and 4-chlorotoluene (1:4 v/v)
was used as the solvent for the synthesis of DFF from fructose
through two consecutive steps.
3
HMFCA were determined as the byproducts of the reaction in
Initially, the one-step conversion of fructose into DFF was
performed in the presence of Fe O @SiO ꢀSO H and g-
3
4
2
3
Fe O @HAP-Ru under O . However, the one-step reaction re-
2
3
2
sulted in very low yields of HMF (1.4%) and DFF (3.5%) at
10 8C after 4 h. Our results were consistent with those report-
ed recently on the synthesis of DFF from fructose in one step
1
[48]
using two binary catalysts. The low DFF yield from fructose
in a one-step reaction was mainly because of the possible oxi-
dation of fructose, which results in undesired byproducts. For
[49]
example, Heinen et al. reported that the oxidation of fruc-
tose with O on Pt/C catalysts formed two major products, 2-
2
keto-d-gluconic acid and d-threo-hexo-2,5-diulose (5-ketofruc-
tose). Therefore, to realize the synthesis of DFF from fructose,
two consecutive steps were applied for the conversion of fruc-
tose into DFF (Scheme 3). Firstly, the dehydration of fructose
was performed in the presence of Fe O @SiO ꢀSO H under an
Figure 6. Time course of the product distribution. Reaction conditions: g-
Fe
2 3 2
O @HAP-Ru (150 mg), HMF (100 mg), 4-chlorotoluene (7 mL), O
ꢀ
1
(20 mLmin ), 708C.
3
4
2
3
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patite (g-Fe O @HAP-Ru). A high DFF yield of 89.1%
2
3
and an HMF conversion of 100% were obtained
under the optimal reaction conditions. Importantly,
the synthesis of DFF from fructose was achieved by
acid-catalyzed dehydration and subsequent aerobic
oxidation in a two-step reaction. Two consecutive
steps gave DFF in a 79.1% yield. Both the magnetic
acid catalyst functionalized with sulfonic acid (Fe O @SiO ꢀ
Scheme 3. Consecutive conversion of fructose into DFF.
air atmosphere. The HMF yield increased gradually, and the
maximum HMF yield of 90.1% was obtained after 2.5 h at
3
4
2
SO H) and g-Fe O @HAP-Ru could be separated easily from the
3
2
3
1
10 8C (Figure 4). The reaction solution was separated readily
reaction solution with a permanent magnet and reused several
times without significant loss of their catalytic activity. This
method shows potential in the conversion of abundant carbo-
hydrates into valuable bulk chemicals.
from Fe O @SiO ꢀSO H with a permanent magnet, and g-
3
4
2
3
Fe O @HAP-Ru was added into the resultant reaction solution.
2
3
The oxidation reaction was performed under O . The HMF con-
2
tent decreased gradually, and the DFF yield increased gradually
(Figure 7). After 24 h, a high DFF yield of 79.1% was obtained
from fructose.
Experimental Section
Materials
FeSO ·7H O (99.5%), FeCl ·6H O (99.5%), tetraethoxysilane (TEOS,
4
2
3
2
9
9.5%), Ca(NO ) ·4H O, and (NH ) HPO were purchased from Sino-
3 2 2 4 2 4
pharm Chemical Reagent Co., Ltd. (Shanghai, China). RuCl ·xH O
3
2
(
(
38.0–42.0% Ru basis) and g-mercaptopropyltrimethoxysilane
MPTMS) were purchased from Aladdin Chemicals Co. Ltd. (Beijing,
China). Fructose was purchased from Sanland-Chem International
Inc. (Xiamen, China). HMF (98%) was purchased from Beijing
Chemicals Co. Ltd. (Beijing, China). DFF was purchased from the
Figure 7. Conversion of fructose into DFF by two consecutive steps. Reac-
tion conditions: First step: Fructose (143 mg, 0.8 mmol) was dissolved in
DMSO (1 mL) and 4-chlorotoluene (4 mL), Fe
added to the mixture, and the reaction was performed at 1108C. Second
step: g-Fe @HAP-Ru (150 mg) was added to the reaction solution, and the
3
O
4
@SiO
2
ꢀSO
3
H (150 mg) was
2
O
3
ꢀ1
2
oxidation reaction was performed with an O flow rate of 20 mLmin .
Catalyst recycling experiments
The recycling of the g-Fe O @HAP-Ru catalyst was studied. The
2
3
aerobic oxidation of HMF was used as a model reaction. The
Figure 8. Separation of the catalyst: (a) After reaction and b) separation
reaction was performed at 908C with 150 mg of g-Fe O @HAP-
using a permanent magnet.
2
3
Ru. After the reaction, the g-Fe O @HAP-Ru catalyst was sepa-
2
3
rated easily from the reaction mixture with a permanent
magnet (Figure 8). The catalyst was washed with water and
ethanol and dried at 808C overnight in a vacuum oven. The
spent catalyst was used for the second cycle, and the reaction
was performed under the same conditions. Other cycles were
also performed as described above. The DFF yields remained
stable (Figure 9), which indicated that there was no significant
loss of the catalytic activity.
Conclusions
We have developed an efficient and environmentally benign
method for the aerobic oxidation of 5-hydromethylfurfural
(HMF) to 2,5-diformylfuran (DFF) over a magnetic catalyst com-
posed of g-Fe O encapsulated with Ru-exchanged hydroxya-
2 3
Figure 9. Recycling of g-Fe O @HAP-Ru.
2
3
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J&K Chemical Co. Ltd. (Beijing, China). Acetonitrile (HPLC grade)
was purchased from Tedia Co. (Fairfield, USA). All other reagents
were purchased from local supplies (Wuhan, China). All the sol-
vents were freshly distilled before use.
(1486.6 eV) at a constant analyzer pass energy of 25 eV. The BE was
estimated to be accurate within 0.2 eV. All BEs were corrected with
reference to the C1s peak (BE=284.9 eV) of carbon contaminants
as an internal standard.
Magnetization measurements were performed by using a physical
property measurement system (PPMS-9T) with VSM option from
Quantum Design. Applied magnetic fields H between ꢀ30 and
30 kOe at 300 K were used in the experiments.
Preparation of g-Fe O @HAP-Ru
2
3
Fe O nanoparticles were prepared and characterized as described
3
4
2
+
3+
in our previous work by the coprecipitation of Fe and Fe in an
[27]
alkaline solution under N2.
g-Fe O @HAP was prepared in situ
2 3
without the separation of Fe O nanoparticles. After the formation
Aerobic oxidation of HMF under atmospheric pressure
3
4
of Fe O nanoparticles, a solution (100 mL) of Ca(NO ) ·4H O
3
4
3 2
2
In a typical run, HMF (1 mmol, 126 mg) was dissolved in 4-chloro-
(
7.95 g, 33.7 mmol) and (NH ) HPO (2.64 g, 20 mmol) adjusted to
4 2 4
toluene (7 mL) with magnetic stirring. g-Fe O @HAP-Ru was added
2
3
pH 11 were added dropwise to the obtained Fe O nanoparticles
3
4
into the reaction mixture, pure O was flushed at a rate of
2
over 30 min with mechanical stirring. The resulting mixture was
heated at 908C for 2 h. Then the mixture was cooled to RT and
aged overnight. The dark brown precipitate was separated with
a permanent magnet, washed repeatedly with deionized water
until a neutral solution was obtained, and dried at 608C overnight
under vacuum. The as-synthesized sample was calcined at 3008C
for 3 h, and a reddish-brown powder (g-Fe O @HAP) was obtained.
ꢀ
1
2
0 mLmin from the bottom of the reactor, and the reaction was
performed at 1108C. The mechanical stirrer was set at a constant
rate of 600 rpm. The point at which O was flushed into the reac-
2
tion mixture was taken to be t=0. After reaction, the Fe O @HAP-
2
3
Ru catalyst was separated from the reaction mixture with a perma-
nent magnet, and the products were analyzed by HPLC.
2
3
2
+
Details of the use of an O
action system was expelled by a flush of pure O . The top of the
reactor equipped with a condenser was sealed with an O balloon.
2
balloon are as follows: The air in the re-
2
The cation exchange of Ca in g-Fe O @HAP was performed by
2
3
2
the reaction of g-Fe O @HAP (1.0 g) with an aqueous RuCl ·xH O
2
3
3
2
ꢀ
3
solution (40 mL, 5.0ꢂ10 m) at RT for 24 h. The catalyst was sepa-
rated with a permanent magnet, washed repeatedly with deion-
ized water until no Ru was detected, and dried under vacuum
overnight. The obtained catalyst was denoted as g-Fe O @HAP-Ru,
The volume of O in the balloon was approximately 800 mL, which
2
was far larger than the stoichiometric amount of O required for
the oxidation of HMF into DFF.
2
2
3
and the content of Ru was determined to be 2 wt%.
The procedure of the oxidation of HMF in the air was almost the
same with a flush of O and the reaction system was exposed to
2
the air.
Preparation of sulfonic acid groups
Silica-coated Fe O nanoparticles (Fe O @SiO ) were prepared and
3
4
3
4
2
Synthesis of DFF from fructose by two consecutive reactions
[27]
characterized as described in our previous work. Supported sul-
[3]
fonic acid was prepared by the oxidation of surface thiol groups.
Firstly, the dehydration of fructose was performed. Briefly, fructose
(0.8 mmol, 143 mg) was dissolved in 4-chlorotoluene (4 mL) and
DMSO (1 mL) at 1108C. Then Fe O @SiO ꢀSO H (150 mg) was
To a solution of MPTMS (1 g) in ethanol (10 mL) and water (10 mL)
was added Fe O @SiO (250 mg). The mixture was sonicated for
3
4
2
3
4
2
3
1
5 min and stirred under reflux overnight. The Fe O @SiO -support-
added into the reaction solution to promote the dehydration of
fructose into HMF at 1108C for 2.5 h. After the formation of HMF,
Fe O @SiO ꢀSO H was separated from the reaction solution with
3
4
2
ed thiol (Fe O @SiO ꢀSH) was recovered magnetically and washed
3
4
2
three times with water (20 mL). Fe O @SiO ꢀSH was oxidized with
3
4
2
3
4
2
3
H O (10 mL, 30%) in a mixed solution of water (10 mL) and etha-
a permanent magnet, and HMF in the resultant reaction solution
2
2
nol (10 mL) at RT overnight. The product was recovered magneti-
cally, washed three times with water (20 mL), and reacidified with
H SO (10 mL, 0.1m). Fe O @SiO modified with sulfonic acid
was further oxidized to DFF with O catalyzed by g-Fe O @HAP-Ru.
The procedure of the oxidation reaction was as described above.
2
2
3
2
4
3
4
2
(
Fe O @SiO ꢀSO H) was washed three times with water and dried
3
4
2
3
under vacuum at RT overnight.
Analytical methods
HMF and DFF were analyzed by using a VARIAN ProStar 210 HPLC
system. Samples were separated by a reversed-phase C18 column
Catalyst characterization
(200ꢂ4.6 mm) with a detection wavelength of 280 nm. The mobile
All XRD patterns were collected in the 2q range of 10–808 with
phase was acetonitrile and 0.1 wt% acetic acid aqueous solution
(30:70 v/v) at 1.0 mLmin . The column oven temperature was
ꢀ1
ꢀ1
a scanning rate of 0.0168s
.
2ꢀ1
kept at 258C. The contents of HMF and DFF in samples were calcu-
lated by the external standard calibration curve method, for which
the calibration curves were constructed based on the pure com-
pounds.
TEM images were obtained by using an FEI Tecnai G -20 instru-
ment. The sample powder was dispersed in ethanol and dropped
onto a copper grid for observation.
FTIR spectra were recorded by using a Nicolet NEXUS-6700 FTIR
ꢀ1
spectrometer with a spectral resolution of 4 cm in the wave
ꢀ1
number range of 500–4000 cm . Powder XRD patterns of samples Acknowledgements
were determined by using a Bruker advance D8 powder diffrac-
tometer (CuKa).
This project was supported by the National Natural Science Foun-
XPS was conducted by using a Thermo VG scientific ESCA Multi-
Lab-2000 spectrometer with a monochromatized AlKa source
dation of China (Nos. 21203252, 21373275), the Natural Science
Foundation of Hubei Province of China (No. 2012FFB07405), and
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25] I. Sꢄdaba, Y. Y. Gorbanev, S. Kegnæs, S. S. R. Putluru, R. W. Berg, A. Riisag-
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Published online on && &&, 0000
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
9
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ÞÞ
These are not the final page numbers!

FULL PAPERS
Z. H. Zhang,* Z. L. Yuan, D. G. Tang,
Y. S. Ren, K. L. Lv, B. Liu
&& – &&
Iron Oxide Encapsulated by
Ruthenium Hydroxyapatite as
Heterogeneous Catalyst for the
Synthesis of 2,5-Diformylfuran
Magnetic attraction: We have demon-
strated an efficient and environmentally
benign magnetic catalyst for the aerobic
oxidation of 5-hydroxymethylfurfural
(HMF) to 2,5-diformylfuran (DFF). A high
DFF yield of 89.1% and an HMF conver-
sion of 100% were obtained after 4 h at
908C.
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&10
&
ÞÞ
These are not the final page numbers!