Polyaniline-grafted VO(acac)2: An effective catalyst for the synthesis of 2,5-diformylfuran from 5-hydroxymethylfurfural and fructose

Article abstract of DOI:10.1002/cctc.201500119

Herein, polyaniline-grafted vanadyl acetylacetonate [VO(acac)₂] was prepared and used as an effective catalyst for the oxidation of 5-hydroxymethylfurfural (HMF) under atmospheric oxygen pressure. A high HMF conversion of 99.2 % was obtained after 12 h at 110 C, and 2,5-diformylfuran (DFF) was obtained in 86.2 % yield. More importantly, the one-pot conversion of fructose into DFF was also realized with two binary catalysts: Amberlyst-15 and polyaniline-VO(acac)₂. Amberlyst-15 was used for the acid-catalyzed dehydration of fructose into HMF, followed by the in situ oxidation of HMF catalyzed by polyaniline-VO(acac)₂. DFF could be obtained in a yield of 42.1 % from fructose by using the one-pot reaction, whereas two consecutive steps gave a higher DFF yield of 71.1 %. Owing to the facile catalyst preparation and low cost, this method showed a promising potential for the synthesis of DFF from abundant carbohydrates.

Full text of DOI:10.1002/cctc.201500119

DOI: 10.1002/cctc.201500119
Full Papers
Polyaniline-Grafted VO(acac)2: An Effective Catalyst
for the Synthesis of 2,5-Diformylfuran from
5-Hydroxymethylfurfural and Fructose**
Fenghua Xu and Zehui Zhang*[a]
Herein, polyaniline-grafted vanadyl acetylacetonate [VO(acac)2]
was prepared and used as an effective catalyst for the oxida-
tion of 5-hydroxymethylfurfural (HMF) under atmospheric
oxygen pressure. A high HMF conversion of 99.2% was ob-
tained after 12 h at 1108C, and 2,5-diformylfuran (DFF) was ob-
tained in 86.2% yield. More importantly, the one-pot conver-
sion of fructose into DFF was also realized with two binary cat-
alysts: Amberlyst-15 and polyaniline–VO(acac)2. Amberlyst-15
was used for the acid-catalyzed dehydration of fructose into
HMF, followed by the in situ oxidation of HMF catalyzed by
polyaniline–VO(acac)2. DFF could be obtained in a yield of
42.1% from fructose by using the one-pot reaction, whereas
two consecutive steps gave a higher DFF yield of 71.1%.
Owing to the facile catalyst preparation and low cost, this
method showed a promising potential for the synthesis of DFF
from abundant carbohydrates.
Introduction
Nowadays, much attention is paid to the search of renewable
resources to supply chemicals and fuels owing to the decrease
in fossil resources.[1] Biomass is a promising renewable feed-
stock that can serve as a substitute of fossil resources because
it is inexpensive and abundant in the earth.[2–5] In addition, bio-
mass maintains the carbon balance in the earth, as biomass is
synthesized with carbon dioxide and water using sunlight as
the energy source. In this context, much effort has been devot-
ed to the conversion of biomass into useful chemicals and val-
uable fuels.[6,7]
the oxidation products of HMF, has also been found to be
useful in many fields. It can be used as a versatile precursor for
the synthesis of furanic polymers, pharmaceuticals, antifungal
agents, nematocides, fluorescent materials, and porous organic
frameworks.[19–23] DFF is generally obtained by the selective oxi-
dation of the hydroxyl group in HMF. However, the existing
furan and aldehyde functionalities in HMF make it susceptible
to undergo many side reactions, such as overoxidation to
FDCA and cross-polymerization to produce unwanted byprod-
ucts.
Among various biomass-based chemicals, 5-hydroxymethyl-
furfural (HMF) is one of the most promising platform chemicals
in the biorefinery area. Therefore, the synthesis of HMF from
various carbohydrates with various homogeneous and hetero-
geneous catalysts has been extensively studied.[8–10] Nowadays,
there is a growing interest in the transformation of HMF into
valuable chemicals and liquid fuels.[11–14] The catalytic oxidation
of HMF can generate several important furan compounds,
such as 2,5-diformylfuran (DFF), 5-hydroxymethyl-2-furancar-
boxylic acid, and 2,5-furandicarboxylic acid (FDCA).[15–17] FDCA
has been identified as one of the top 12 valuable chemicals
from biomass by the U.S. Department of Energy. It has a struc-
ture similar to that of petroleum-derived terephthalic acid;
thus, it can serve as a substitute for terephthalic acid in manu-
facturing poly(ethylene terephthalate) plastics.[18] DFF, one of
In early reports, the oxidation of HMF to DFF was performed
with conventional oxidants such as NaOCl,[24] BaMnO4,[25] and
C5H5NH[CrO3Cl].[26] These methods not only required stoichio-
metric oxidants but also released serious toxic wastes. There-
fore, the use of molecular oxygen or air and heterogeneous
catalysts is highly favored from both sustainable chemistry and
economical point of views. To date, several metal-based cata-
lytic systems such as Pt–Bi/C,[27] hydrotalcite supported Ru
nanoparticles (Ru/HT),[28] and KMn8O16·nH2O[29] have been re-
ported for the aerobic oxidation of HMF with varied catalytic
performances. Among the heterogeneous catalysts, vanadium-
based catalysts have attracted much attention for the aerobic
oxidation of HMF to DFF because the cost of vanadium-based
catalysts is low as compared with other metal catalysts such as
ruthenium and platinum. Carlini et al. reported that a high
HMF conversion of 98% was obtained with the vanadyl phos-
phate catalyst in methyl isobutyl ketone (MIBK) under an
oxygen pressure of 0.1 MPa, but the selectivity toward DFF
was only 50%.[30] Later, Riisager et al. reported that the use of
the zeolite-supported vanadia catalyst afforded 50% HMF con-
version with approximately 45% DFF yield; however, the cata-
lyst lost its catalytic activity significantly owing to leaching of
active metal species.[31]
[a] F. Xu, Prof. Z. Zhang
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 182, Wuhan (P.R. China)
Fax: (+86)27-67842752
E-mail: zehuizh@mail.ustc.edu.cn
[**] acac=acetylacetonate.
ChemCatChem 0000, 00, 0 – 0
1
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
However, the large-scale synthesis of DFF from HMF is limit-
ed owing to the high cost of HMF. Because HMF can be readily
generated from the dehydration of fructose in the presence of
an acid catalyst, the synthesis of DFF from fructose is desirable
through an integrated chemical process in which multistep re-
actions are consolidated into a one-pot reaction. Therefore, the
one-pot conversion of fructose into DFF with a series of com-
bined catalytic systems has received great attention.[28,29] For
example, Takagaki et al.[28] demonstrated a one-pot conversion
of fructose into DFF via two consecutive steps with use of Am-
berlyst-15 and Ru/HT catalysts in DMF. First, HMF was produced
from the dehydration of fructose in the presence of Amberlyst-
15, and then HMF was oxidized in situ to DFF over the Ru/HT
catalyst. DFF was obtained from fructose in a yield of 49%.
Chen et al. demonstrated the dehydration of fructose into HMF
catalyzed by protonated graphitic carbon nitride [g-C3N4(H+)]
as a solid acid as well as the aerobic oxidation of HMF to DFF
promoted by vanadium-doped g-C3N4, which gave DFF in ap-
proximately 63% yield.[32]
line hydrochloride was then achieved with aqueous ammonia
(3 wt%). Polyaniline can be used as an effective support for
the immobilization of a transition metal complex. On the one
hand, it is insoluble in common organic solvents and water. On
the other hand, there are many nitrogen atoms in polyaniline
that have lone pair electrons to coordinate with the free orbit
of the transition metal complex. Thus, the polyaniline-support-
ed transition metal complex can be used as an effective and
stable heterogeneous catalyst. The weight percentage of vana-
dium was determined to be 6.3% from inductively coupled
plasma atomic emission spectroscopy analysis.
The nitrogen adsorption–desorption isotherms of the poly-
aniline–VO(acac)2 catalyst are shown in Figure 1. The isotherm
In recent years, polymer-supported metal complexes have
gained much attention in organic synthesis because they offer
high economic prospects in developing reusable catalysts.[33] In
polymer-supported catalysts, the fixed position of the metal
ion on the polymer matrix may contribute to greater isolation
of catalytically active sites and can thus increase the catalytic
activity. Polyaniline has recently received a great interest in cat-
alytic applications owing to its ease of synthesis, performance
stability, and low synthesis cost. It can be synthesized easily by
the oxidation of the monomer in the liquid phase. Recently,
polyaniline-supported metal catalysts have been used as heter-
ogeneous catalysts for some chemical reactions.[34,35] Herein,
these findings have motivated us to explore the use of polyan-
iline to immobilize vanadium complex and use it as a new het-
erogeneous catalyst for the oxidation of HMF with molecular
oxygen. More importantly, much more attention has been de-
voted to the one-pot conversion of fructose into DFF in two
ways.
Figure 1. Nitrogen adsorption–desorption isotherms of the catalyst.
is a typical type IV isotherm with H3-type hysteresis loop. The
BET surface area of the catalyst was determined to be
31.5 m2 gꢀ1, and its pore volume was 0.21 cm3 gꢀ1. However,
the BET surface area and pore volume of the parent polyaniline
were calculated to be 41.9 m2 gꢀ1 and 0.36 cm3 gꢀ1, respective-
ly. This decrease in surface area and pore volume of the cata-
lyst was due to VO(acac)2 in the porous framework of the
parent material. The BET results indicated that VO(acac)2 was
grafted onto the surface and pores of polyaniline.
Results and Discussion
Catalyst preparation and characterization
The FTIR spectra of polyaniline and polyaniline–VO(acac)2
are shown in Figure 2. With regard to the structure of polyani-
line, the characteristic peaks at 1583 and 1490 cmꢀ1 were as-
signed to the C=C stretching mode of quinoid and benzenoid
rings.[36] Importantly, the band at 1301 cmꢀ1 was assigned to
the CꢀN stretching vibration of secondary aromatic amine in
the alternative unit of quinoid–benzenoid sequences of poly-
aniline. Another strong peak at 1149 cmꢀ1 was assigned to the
in-plane bending vibration of the CꢀH bond, and the peak at
825 cmꢀ1 was attibuted to the out-plane bending of the CꢀH
bond.[37]
The synthesis of polyaniline–vanadyl acetylacetonate
[VO(acac)2] catalyst was performed in two steps (Scheme 1).
First, the polymerization of aniline under acidic conditions in
the presence of the oxidant reagent (NH4)2S2O8 produced the
polyaniline hydrochloride salt. The deprotonation of polyani-
With regard to the characteristic peaks of VO(acac)2, the
peak at approximately 1575 cmꢀ1 was assigned to the strech-
ing vibration v9 of the C=O bond.[38] The peak at approximately
930 cmꢀ1 was assigned to the streching vibration v3 of the C=
O bond,[38] and the peak at 640 cmꢀ1 was attibuted to the
Scheme 1. Synthesis of polyaniline–VO(acac)2 catalyst.
&
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
2
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
Figure 2. FTIR spectra of a) polyaniline and b) the polyaniline–VO(acac)2
catalyst.
stretching vibration of the VꢀO bond.[38,39] On comparing the
FTIR spectra of the catalyst and the support, only two distinct
peaks at approximately 930 and 640 cmꢀ1 were observed in
the catalyst. These results indicated that VO(acac)2 was immo-
bilized on the polyaniline support. As shown in Figure 2b, the
characteristic peaks of the catalyst in the region from 1100 to
1600 cmꢀ1 were similar to those of the parent polyaniline.
The results of X-ray photoelectron spectroscopy (XPS) of
polyaniline–VO(acac)2 are presented in Figure 3a. The strong
signal of carbon with the C1s binding energy of approximately
285 eV was observed, and its atomic percentage was approxi-
mately 76.1%, which included adventitious carbon and the
sample carbon. A peak at 399 eV was an evidence of the pres-
ence of nitrogen, which was assigned to the binding energy of
N1s. Two peaks with the binding energy of approximately 531
and 516 eV were attributed to O1s and V2p, respectively,
which confirmed the presence of oxygen and vanadium in the
catalyst. The atomic percentage of vanadium and oxygen was
determined to be 1.54 and 8.05%, respectively, from XPS anal-
ysis, which was close to the atomic molar ratio of vanadium
and oxygen in VO(acac)2. These results suggested that
VO(acac)2 should be immobilized through the coordination of
the nitrogen atom in polyaniline with vanadium in VO(acac)2.
In addition, the atomic percentage of nitrogen was 8.81%, and
it was much higher than that of vanadium, which indicated
that only part of nitrogen in polyaniline was used to immobi-
lize VO(acac)2. In addition, the survey spectrum showed the
presence of a small emission peak at approximately 270 eV
and it was assigned to the binding energy of Cl2s. This result
indicated that chlorine ions still remained on the catalyst sur-
face.
Figure 3. XPS spectra of a) the polyaniline–VO(acac)2 complex, b) O1s and
V2p, and c) O1s.
was larger than 518 eV. However, the binding energy of vana-
dium was 517. 3 eV when (VO)2P2O7 was treated in nitrogen at-
mosphere. In the high-resolution XPS spectrum of vanadium,
there was no peak for vanadium(V). Thus, we can conclude
that VO(acac)2 in the catalyst was stable. The high-resolution
XPS spectrum of vanadium should present only one peak for
vanadium(IV) with the binding energy of 170.0 eV. Cornaglia
et al. reported that the difference in the binding energy of O1s
and V2p3/2 in VOHPO4 [vanadium(IV)] was 14.7 eV, whereas it
was 12.9 eV in V2O5 [vanadium(V)]. As shown in Figure 3b, the
difference in the binding energy of VꢀO1s (531.7 eV) and
V2p3/2 (517.0 eV) in our prepared catalyst was 14.7 eV. Thus,
this result again confirmed that the oxidation state of vanadi-
um in our prepared catalyst was +4, which was in accordance
The high-resolution XPS spectra of O1s and V2p are shown
in Figure 3c. The high-resolution XPS spectrum of O1s has
been deconvoluted into three peaks. The binding energy of
the three deconvoluted peaks was 530.2, 531.7, and 533.2 eV,
and these peaks were assigned to C=O, VꢀO, and CꢀO bonds,
respectively. The binding energy of vanadium(V) is usually
larger than 518 eV. For example, Cornaglia et al.[40] treated
(VO)2P2O7 in oxygen atmosphere at different temperatures and
found that the binding energy of vanadium in all the samples
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
3
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
with the results reported by Cornaglia et al. Our results were
consistent with the previous results reported by Pereira
et al.,[39] which indicated that the valence state of vanadium(IV)
in VO(acac)2 did not change after immobilization.
ments were initially performed in various solvents to study the
effect of the reaction solvent on the oxidation of HMF, as dif-
ferent solvents show different properties such as polarity, die-
lectric constant, steric hindrance, and acid–base property,
which would affect chemical reactions.[43] The aerobic oxidation
of HMF was greatly affected by the reaction solvent (Table 1).
In addition, the binding energy of V2p3/2 was found to be
517.0 eV, though it was reported that the binding energy of
the V2p3/2 peak of free VO(acac)2 was 516.8 eV.[41] These results
suggested that VO(acac)2 should be chemically bonded to the
support through the coordination of lone pair electrons of ni-
trogen with the free orbit of vanadium, as shown in Scheme 1.
Jarrais et al. also found that the binding energy of vanadium(V)
in free VO(acac)2 was 516.8 eV, which shifted to 517.0 eV after
VO(acac)2 was bonded to the surface of the amine-functional-
ized active carbon.[41]
Table 1. Results of the oxidation of HMF in various solvents.[a]
Entry
Solvent
HMF conversion
[%]
DFF yield
[%]
FDCA yield
[%]
1
2
3
4
5
6
ethanol
acetonitrile
DMSO
MIBK
toluene
16.1
17.2
43.6
91.7
94.1
99.2
8.7
14.1
31.1
70.1
62.1
86.2
4.1
2.0
7.6
17.6
29.6
11.7
Thermogravimetric analysis (TGA) was performed to detect
the difference between polyaniline and the polyaniline–vanadi-
um complex, and the results are presented in Figure 4. Ther-
4-chlorotoluene
[a] Reaction conditions: polyaniline–VO(acac)2 (80 mg), HMF (100 mg), sol-
vent (8 mL), oxygen flow rate=30 mLminꢀ1, T=110 8C.
Generally speaking, the strong polar solvents were not benefi-
cial for the aerobic oxidation of HMF over the polyaniline–
VO(acac)2 catalyst (Table 1, entries 1–3 vs entries 4–6). Low
HMF conversions were obtained in ethanol and acetonitrile
with low boiling point and strong polarity (Table 1, entries 1
and 2). A moderate HMF conversion of 43.6% was obtained in
DMSO with strong polarity (Table 1, entry 1). High HMF conver-
sions were obtained in MIBK, toluene, and 4-chlorotoluene
with relatively low polarity (Table 1, entries 4–6). However, the
product distribution was quite different in each solvent. In all
cases, DFF was determined to be the major oxidation product
during the formation of FDCA. The ratio of DFF to FDCA
strongly depended on the specific solvent. Although a high
HMF conversion of approximately 90% was obtained in MIBK,
toluene, and 4-chlorotoluene (Table 1, entries 4–6), the selectiv-
ity toward DFF in MIBK and toluene was much lower than that
in 4-chlorotoluene (Table 1, entries 4 and 5 vs entry 6). With
regard to both HMF conversion and DFF selectivity, 4-chloroto-
luene was found to be the best solvent, which afforded a high
HMF conversion of 99.2% and a DFF selectivity of 86.2%.
Figure 4. TGA of a) polyaniline and b) the polyaniline–VO(acac)2 complex.
mal analysis was performed in nitrogen atmosphere at a heat-
ing rate of 108Cminꢀ1 in the temperature range of 40–7008C.
The small weight loss was almost the same between the sup-
port and the catalyst before 1508C in nitrogen atmosphere,
which was due to the loss of physically absorbed water. In the
temperature range of 150–4508C, the weight loss of the cata-
lyst was much higher than that of the support, which was due
to the decomposition of the acetylacetonate complex in the
catalyst. Baltes et al. reported that the thermal decomposition
of alumina–VO(acac)2 underwent three steps in the tempera-
ture range of 150–3258C.[42] However, polyaniline started to de-
compose at 3008C and decomposed quickly at approximately
6008C. As shown in Figure 4, the weight loss of polyaniline
was slightly greater than that of the polyaniline–vanadium
complex, which was due to the decomposition products of va-
nadium species, mainly vanadium oxide, present in the
sample.
Effect of the reaction temperature on the oxidation of HMF
As discussed in the above section, the oxidation of HMF in 4-
chlorotoluene at the reaction temperature of 1108C gave
a DFF yield of 86.2% and an FDCA yield of 11.7%. To increase
the selectivity toward DFF and to inhibit the overoxidation of
DFF to FDCA, oxidation reactions were performed at low reac-
tion temperatures of 70 and 908C, and the results are present-
ed in Figure 5. It can be seen that the reaction temperature
significantly affected HMF conversion and DFF selectivity. HMF
conversion decreased gradually with the decrease in the reac-
tion temperature from 110 to 708C. Almost quantitative HMF
conversion (99.2%) was observed at 1108C, whereas it sharply
decreased to 55.2% at 908C. With a decrease in the reaction
temperature to 708C, the oxidation of HMF proceeded slug-
Aerobic oxidation of HMF in various solvents
The catalytic activity of the as-prepared polyaniline–VO(acac)2
catalyst was evaluated by the aerobic oxidation of HMF. Experi-
&
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
4
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
Figure 5. Results of HMF oxidation at different reaction temperatures. Reac-
tion conditions: polyaniline–VO(acac)2 (80 mg), HMF (100 mg), 4-chloroto-
luene (8 mL), oxygen flow rate=30 mLminꢀ1, t=12 h.
Figure 6. Results of the one-pot conversion of fructose catalyzed by two
binary catalysts. Reaction conditions: fructose (145 mg) was first dissolved in
a mixed solvent of DMSO and 4-chlorotoluene. Then, Amberlyst-15 (100 mg)
and polyaniline–VO(acac)2 (80 mg) were added to the fructose solution. The
reaction was performed at 1108C with an oxygen flow rate of 30 mLminꢀ1
.
gishly and afforded a low HMF conversion of only 25.7%. DFF
and FDCA were also detected as the oxidation products at the
three reaction temperatures. DFF selectivity (also the ratio of
DFF to FDCA) increased with the decrease in reaction tempera-
ture. DFF selectivity was calculated to be 86.8% at 1108C, and
it was 90.7 and 92.2% at 70 and 508C, respectively.
starting from fructose. In addition, a low FDCA yield of 6.1%
was observed.
Synthesis of DFF from fructose by two consecutive steps
To further increase DFF yield from fructose, two consecutive
steps were applied for the synthesis of DFF from fructose to
reduce side reactions (Scheme 2). First, HMF was produced by
Synthesis of DFF and FDCA from fructose by using the one-
pot method
Although DFF could be produced from the oxidation of HMF
with polyaniline–VO(acac)2 as the catalyst, this method is limit-
ed to the large-scale synthesis of DFF owing to the high cost
of HMF. Therefore, researchers tried to produce DFF directly
from renewable carbohydrates such as fructose. To our knowl-
edge, there are few examples of the one-pot conversion of
fructose into DFF. To realize the one-pot conversion of fructose
into DFF, we should solve the problem of solubility of fructose.
DMSO was reported to have the strong ability to dissolve fruc-
tose and thus was the excellent reaction medium for the dehy-
dration of fructose into HMF. As described above, the polyani-
line–VO(acac)2 catalyst showed high catalytic activity for the
oxidation of HMF in 4-chlorotoluene and moderate activity in
DMSO. Therefore, a mixed solvent of DMSO and 4-chloroto-
luene with the volume ratio of 1:4 was used as the reaction
solvent. The synthesis of DFF from fructose was performed by
using the one-pot reaction catalyzed by two binary catalysts,
Amberlyst-15 and polyaniline–VO(acac)2, simultaneously, in
which Amberlyst-15 was used as the acid catalyst for the dehy-
dration of fructose into HMF and polyaniline–VO(acac)2 was
used for the in situ oxidation of HMF to DFF. The results of the
one-pot conversion of fructose into DFF are presented in
Figure 6. It can be seen that HMF yield first increased and then
decreased during the reaction. The initial increase in HMF yield
was due to the high concentration of fructose in an early reac-
tion stage, and the decrease in HMF yield was due to the flow-
ing oxidation of HMF to DFF and FDCA. The highest HMF yield
of 37.1% was obtained after 2 h. The DFF yield continuously
increased during the reaction process. The highest DFF yield
was observed to be 42.1% after 12 h in a one-pot reaction
Scheme 2. Synthesis of DFF from fructose by two consecutive steps.
the dehydration of fructose catalyzed by Amberlyst-15.
Second, the Amberlyst-15 catalyst was removed from the reac-
tion solution, and then the polyaniline–VO(acac)2 catalyst was
added to the resultant reaction solution to perform the follow-
ing oxidation of HMF to DFF. In the first step, HMF was pro-
duced by the dehydration of fructose in the presence of Am-
berlyst-15 (Figure 7). This process was fast and effective, which
gave a high HMF yield of 91.8% after 1.5 h at 1108C. Then,
Amberlyst-15 was removed from the reaction mixture and
polyaniline–VO(acac)2 was added to the resultant reaction solu-
tion to promote the oxidation of HMF. In the second step, the
HMF content decreased and DFF yield increased gradually.
After the reaction time of 12 h in the second step, DFF was ob-
tained from fructose in a yield of 71.1%. In addition, FDCA was
produced in a small amount of 9.3%. DFF yield obtained by
using the two consecutive method was much higher than that
obtained by using the one-pot reaction. One of the main rea-
sons for the lower DFF yield from fructose via a one-pot reac-
tion was the possible oxidation of fructose, which resulted in
other undesired byproducts.[44] For example, the oxidation of
fructose over the Pt/C catalyst generated two oxidation prod-
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
5
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
Figure 7. Results of the synthesis of DFF from fructose via two consecutive
steps. Reaction conditions: fructose (145 mg) was first dissolved in a mixed
solvent of DMSO and 4-chlorotoluene. Then, the dehydration of fructose
was performed at 1108C for 2 h. Finally, HMF oxidation in the reaction solu-
tion was performed with polyaniline–VO(acac)2 (80 mg) at 1108C with an
Figure 8. Results of recycling experiments of the polyaniline–VO(acac)2 cata-
lyst.
oxygen flow rate of 30 mLminꢀ1
.
of HMF, the method developed in our study was compared
with other methods for the oxidation of HMF to DFF over va-
nadium-based catalysts, and the results are summarized in
Table 2. To our knowledge, the use of homogeneous VOSO4
with Cu(NO3)2 showed the best results for the oxidation of
HMF to DFF, but the catalyst was difficult to recycle (Table 2,
entry 1). Active carbon-supported V2O5 (AC/V2O5) and H-beta-
supported V2O5 (H-beta/V2O5) could catalyze the oxidation of
HMF to DFF with a relative selectivity; however, these catalytic
systems were used under atmospheric oxygen pressure
(Table 2, entries 2 and 3). g-C3N4–V2O5·VO2 showed much
higher catalytic activity than AC/V2O5 and H-beta/V2O5, as it
could catalyze the oxidation of HMF under atmospheric
oxygen pressure. The reason was that VO2 in the catalyst accel-
erated electron transfer between vanadium(V) and vanadiu-
m(IV), which was generally believed to be the redox cycle
during the oxidation of HMF to DFF (Table 2, entry 4). Vanadi-
um phosphate oxide-based heterogeneous catalysts also
showed the catalytic activity for the oxidation of HMF to DFF
(Table 2, entry 1). Although a high HMF conversion of 98%
could be obtained in the presence of VOPO4·2H2O, the selec-
tivity toward DFF was only 50% (Table 2, entry 5). The intro-
duction of alkyl chain C14 into vanadium phosphate oxide
(C14VOPO4) increased the DFF selectivity. However, the activity
of the catalyst decreased significantly and HMF conversion de-
creased from 91% in the first run to 62% in the third run
ucts 2-keto-d-gluconic acid and d-threo-hexo-2,5-diulose (5-ke-
tofructose).[44]
Catalyst recycling experiments
The recycling experiment of the polyaniline–VO(acac)2 catalyst
was studied. The aerobic oxidation of HMF in 4-chlorotoluene
at 1108C was used as the model reaction. Polyaniline–
VO(acac)2 was insoluble in the reaction solvent and hence can
be easily separated by centrifugation to avoid the weight loss
of the catalyst. Then, the spent catalyst was washed several
times with water and ethanol to remove any residue that re-
mained in the catalyst. Finally, the spent catalyst was dried in
a vacuum oven at 608C. The spent catalyst was used in the
next run under the same reaction conditions used for the first
run. The results of recycling experiments of polyaniline–
VO(acac)2 are presented in Figure 8. Notably, HMF conversion
and DFF yield showed a small decrease from the first run to
the sixth run. HMF conversion decreased from 99.2% in the
first run to 85.3% in the sixth run, and DFF yield decreased
from 86.2% in the first run to 77.2% in the sixth run. The
weight percentage of vanadium in the spent catalyst after the
sixth run was determined to be 5.6% from inductively coupled
plasma atomic emission spectroscopy analysis, and it was
6.3% in the fresh catalyst. There-
fore, the small decrease in the
Table 2. Comparison of our method with other reported methods.
catalytic activity was due to
minor leaching of vanadium in
the catalyst.
Entry
Catalyst
Oxygen pressure
[bar][a]
HMF conversion
[%]
DFF yield
[%]
Reference
1
2
3
4
5
6
7
Cu(NO3)2/VOSO4
AC/V2O5
H-beta/V2O5
g-C3N4–V2O5·VO2
VOPO4·2H2O
C14VOPO4
1
99
95.2
84
99
98
99
91.8
82
82
49
[45]
[46]
[31]
[32]
[30]
[46]
this work
2.8 (O2)
10 (O2)
1
1
1
1
Comparison of our method for
the synthesis of DFF from HMF
with other reported methods
99
99.2
83
86.2
To evaluate the advantages of
our catalytic system for the syn-
thesis of DFF from the oxidation
polyaniline-VO(acac)2
[a] 1 bar=100 kPa
&
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
6
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
The deprotonation of polyaniline hydrochloride was achieved with
aqueous ammonia (3 wt%). The deprotonated polymer was
washed with water, methanol, and diethyl ether and dried in
a vacuum oven for 24 h.
(Table 2, entry 6). Compared with the above-mentioned
method using the vanadium catalyst, our method did not re-
quire to perform the reaction under high atmospheric oxygen
pressure and could gave high HMF conversion and DFF selec-
tivity (Table 2, entry 7).
Synthesis of polyaniline-supported vanadium(VI) complex
Conclusions
First, VO(acac)2 (0.5 g) was dissolved in dichloromethane (50 mL)
and stirred with a magnetic stirrer. Then, polyaniline (1.0 g) was
added and continuously stirred at RT for 48 h. After the completion
of the reaction, the resultant precipitate was obtained by filtration
and washed several times with hot ethanol to remove any unreact-
ed metal complex. Finally, the catalyst was dried under vacuum at
608C for 12 h, which was labeled as polyaniline–VO(acac)2.
Herein, the immobilization of vanadyl acetylacetonate
[VO(acac)2] on polyaniline was performed and well character-
ized by using several technologies. The polyaniline–VO(acac)2
catalyst showed high activity for the aerobic oxidation of 5-hy-
droxymethylfurfural (HMF) to 2,5-diformylfuran (DFF). The oxi-
dation of HMF proceeded smoothly under atmospheric oxygen
pressure. A high HMF conversion of 99.2% was obtained after
12 h at 1108C, and DFF and 2,5-furandicarboxylic acid were ob-
tained in a yield of 86.2 and 11.7%, respectively. More impor-
tantly, the synthesis of DFF directly from fructose was also
studied by using two methods using the two binary catalysts:
Amberlyst-15 and polyaniline–VO(acac)2. Amberlyst-15 was
used for the acid-catalyzed dehydration of fructose into HMF,
and polyaniline–VO(acac)2 was used for the oxidation of HMF.
The one-pot method produced DFF in a yield of 42.1%, where-
as the two consecutive method gave a higher DFF yield of
71.1%. In addition, the polyaniline–VO(acac)2 catalyst could be
reused despite the slight loss of its catalytic activity. Owing to
the low cost of the catalyst, this method showed a promising
potential for the conversion of abundant carbohydrates into
valuable bulk chemicals.
Catalyst characterization
The TEM images were obtained with an FEI Tecnai G2 F20 instru-
ment. The sample powder was dispersed in ethanol and dropped
onto copper grids for observation. The XRD patterns of the sam-
ples were collected with a Bruker D8 ADVANCE X-ray diffractome-
ter using CuKa radiation. All XRD patterns were collected in the 2q
range of 10–808 (scanning rate: 0.0168sꢀ1). The XPS spectra were
recorded on a Thermo VG Scientific ESCA MultiLab 2000 spectrom-
eter with a monochromatized AlKa X-ray source (1486.6 eV) at
a constant analyzer pass energy of 25 eV. The binding energy was
estimated to be accurate within 0.2 eV. All binding energies were
corrected referencing to the C1s (284.6 eV) peak of the contami-
nated carbon used as an internal standard. The FTIR spectra were
recorded on a Nicolet Nexus 6700 FTIR spectrometer with a spectral
resolution of 4 cmꢀ1 in the wavenumber range of 500–4000 cmꢀ1
.
TGA was performed with a Netzsch TG209 instrument at a heating
rate of 10 Kminꢀ1 and a nitrogen flow of 20 mLminꢀ1. Nitrogen ad-
sorption–desorption isotherms were measured with an Autosorb-
1 (Quantachrome, USA) at 77 K. Before the measurement, all the
samples were degassed in a vacuum line at 2008C for 6 h. The BET
surface area was determined by a multipoint BET method using
the adsorption data in the P/P0 range of 0.05–0.3. The mesoporous
pore size distributions were derived from desorption branches of
isotherms by using the BJH model.
Experimental Section
Materials
Aniline and ammonium persulfate were purchased from Sino-
pharm Chemical Reagent Co., Ltd. (Shanghai, China). VO(acac)2 and
Amberlyst-15 were purchased from Aladdin Chemicals Co. Ltd.
(Beijing, China). HMF (98%) was purchased from Beijing Chemicals
Co. Ltd. (Beijing, China). DFF and FDCA were purchased from the
J&K Chemical Co. Ltd. (Beijing, China). All solvents were purchased
from Sinopharm Chemical Reagent Co., Ltd.. Aniline was purified
by distillation before use, and all other chemicals were of analytical
grade and used without further purification. Acetonitrile (HPLC
grade) was purchased from Tedia Co. (Fairfield, USA).
General procedure for the aerobic oxidation of HMF
In a typical run, HMF (0.8 mmol, 100 mg) was first dissolved in 4-
chlorotoluene (8 mL) and stirred with a magnetic stirrer. Then,
polyaniline–VO(acac)2 (80 mg) was added to the reaction mixture
and oxygen was flushed (flow rate: 20 mLminꢀ1) from the bottom
of the reactor under atmospheric pressure, and the reaction was
performed at 1108C at a constant stirring rate of 600 rpm. Time
zero was recorded when oxygen was flushed into the reaction mix-
ture. After the completion of the reaction, the catalyst was re-
moved from the reaction mixture and the reaction mixture was an-
alyzed by using HPLC.
Synthesis of polyaniline
Polyaniline was synthesized as described in a previous report.[36] In
a typical synthesis, freshly distilled aniline (5 mL, 55 mmol) was first
dissolved in HCl (63 mL, 1.5m) in a 250 mL round flask. The flask
was placed in an ice-salt bath, and the temperature was main-
tained at 08C. Then, a solution of ammonium persulfate (55 mmol)
in HCl (63 mL, 1.5m) was added slowly for 1 h to the above solu-
tion at 08C to keep the reaction temperature not higher than 58C,
as aniline polymerization is strongly exothermic. After the com-
plete addition, the mixture was stirred for an additional 4 h. The re-
sultant precipitate was separated by filtration and washed consec-
utively with water (3ꢁ30 mL), methanol (2ꢁ25 mL), and diethyl
ether (2ꢁ15 mL) to remove the oligomers and any possible by-
products. The polymer was then dried in a vacuum oven for 24 h.
Quantification of the products
The amounts of furan compounds in the reaction solution were an-
alyzed with a ProStar 210 HPLC system using the external standard
calibration curve method. Furan compounds could be separated
with a reversed-phase C18 column (200ꢁ4.6 mm) at a wavelength
of 280 nm. Acetonitrile and acetic acid aqueous solution (0.1 wt%;
30:70 v/v) were used as a mobile phase, and the flow rate was
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
7
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
1.0 mLminꢀ1. The column temperature was maintained at 308C.
The amounts of furan compounds in the samples were obtained
directly by interpolation from calibration curves. The retention
times of HMF, DFF, and FDCA were 3.0, 3.5, and 2.8 min, respective-
ly.
[15] Z. H. Zhang, B. Liu, K. L. LV, J. Sun, K. J. Deng, Green Chem. 2014, 16,
2762–2770.
[16] B. Liu, Y. S. Ren, Z. H. Zhang, Green Chem. 2015, 171, 1610–1617.
[17] G. S. Yi, S. P. Teong, X. K. Li, Y. G. Zhang, ChemSusChem 2014, 7, 2131–
2135.
[18] B. S. Wu, Y. T. Xu, Z. Y. Bu, L. B. Wu, B. G. Li, P. Dubois, Polymer 2014, 55,
3648–3655.
The conversion of HMF was calculated by using Equation (1).
[19] M. Baumgarten, N. Tyutyulkov, Chem. Eur. J. 1998, 4, 987–989.
[20] K. T. Hopkins, W. D. Wilson, B. C. Bendan, D. R. McCurdy, J. E. Hall, R. R.
Tidwell, A. Kumar, M. Bajic, D. W. Boykin, J. Med. Chem. 1998, 41, 3872–
3878.
moles of converted HMF
moles of starting HMF
ð1Þ
HMF conversion ð%Þ ¼ ð
Þ ꢁ 100
[21] M. D. Poeta, W. A. Schell, C. C. Dykstra, S. Jones, R. R. Tidwell, A. Czarny,
M. Bajic, M. Bajic, A. Kumar, D. Boykin, J. R. Perfect, Antimicrob. Agents
Chemother. 1998, 42, 2495–2502.
[22] A. Gandini, M. N. Belgacem, Prog. Polym. Sci. 1997, 22, 1203–1379.
[23] J. Ma, Z. Du, J. Xu, Q. Chu, Y. Pang, ChemSusChem 2011, 4, 51–54.
[24] A. S. Amarasekara, D. Green, E. McMillan, Catal. Commun. 2008, 9, 286–
288.
[25] F. W. Lichtenthaler, Acc. Chem. Res. 2002, 35, 728–737.
[26] L. Cottier, G. Descotes, E. J. Viollet, J. Lewkowski, R. SkowroÇski, J. Heter-
ocycl. Chem. 1995, 32, 927–930.
[27] A. Giroir-Fendler, P. Denton, A. Boreave, H. Praliaud, M. Primet, Top.
Catal. 2001, 16–17, 237–241.
One molecule of HMF produced one molecule of DFF or FDCA;
thus, DFF or FDCA yield was calculated by using Equation (2) or
Equation (3), respectively.
moles of DFF
ð2Þ
ð3Þ
DFF yield ð%Þ ¼ ð
Þ ꢁ 100
Þ ꢁ 100
moles of starting HMF
moles of FDCA
moles of starting HMF
FDCA yield ð%Þ ¼ ð
[28] A. Takagaki, M. Takahashi, S. Nishimura, K. Ebitani, ACS Catal. 2011, 1,
1562–1565.
[29] Z. Z. Yang, J. Deng, T. Pan, Q. X. Guo, Y. Fu, Green Chem. 2012, 14,
2986–2989.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (grant nos 21203252, 21373275, and 21206200),
the Natural Science Foundation of Hubei Province, China (grant
no. 2012FFB07405), and the Chenguang Youth Science and Tech-
nology Project of Wuhan City (grant no. 2014070404010212).
[30] C. Carlini, P. Patrono, A. M. R. Galletti, G. Sbrana, V. Zima, Appl. Catal. A
2005, 289, 197–204.
[31] I. Sꢂdaba, Y. Y. Gorbanev, S. Kegnæs, S. S. R. Putluru, R. W. Berg, A. Riisag-
er, ChemCatChem 2013, 5, 284–293.
[32] J. Z. Chen, Y. Y. Guo, J. Y. Guo, L. S. Song, L. M. Chen, ChemCatChem
2014, 6, 3174–3181.
[33] K. C. Basavaraju, S. Sharma, A. K. Singh, D. J. Im, D. P. Kim, ChemSusChem
2014, 7, 1864–1869.
[34] U. Mandi, M. Pramanik, A. S. Roy, N. Salam, A. Bhaumik, S. M. Isla, RSC
Adv. 2014, 4, 15431–155440.
Keywords: fructose · 5-hydroxymethylfurfural · polyaniline ·
oxidation · supported catalysts
[35] H. A. Patel, A. L. Patel, A. V. Bedekar, Appl. Organomet. Chem. 2015, 29,
1–6.
[36] M. Nandi, R. Gangopadhyay, A. Bhaumik, Microporous Mesoporous
Mater. 2008, 109, 239–247.
[1] G. W. Huber, S. Iborrra, A. Corma, Chem. Rev. 2006, 106, 4044–4098.
[2] R. Ciriminna, P. D. Cara, J. A. Lopez-Sanchez, M. Pagliaro, ChemCatChem
2014, 6, 3053–3059.
[3] S. Sreekumar, Z. C. Baer, E. Gross, S. Padmanaban, K. Goulas, S. Gunbas,
S. Alayoglu, H. W. Blanch, D. S. Clark, F. D. Toste, ChemSusChem 2014, 7,
2445–2448.
[4] K. Anastasakis, A. B. Ross, Fuel 2015, 139, 546–553.
[5] V. V. Ordomsky, J. Van der Schaaf, J. C. Schouten, T. A. Nijhuis, J. Catal.
2012, 287, 68–75.
[37] S. Trakhtenberg, Y. Hangun-Balkir, J. C. Warner, F. F. Bruno, J. Kumar, R.
Nagarjan, L. Samuelsonson, J. Am. Chem. Soc. 2005, 127, 9100–9104.
[38] K. Nakamoto, P. J. McCarthy, A. Ruby, A. E. Martell, J. Am. Chem. Soc.
1961, 83, 1066–1069.
[39] C. Pereira, J. F. Silva, A. M. Pereira, J. P. Araffljo, G. Blanco, J. M. Pintadoc,
C. Freire, Catal. Sci. Technol. 2011, 1, 784–793.
[6] T. J. Morgan, R. Kandiyoti, Chem. Rev. 2014, 114, 1547–1607.
[7] Y. Xiong, Z. H. Zhang, X. Wang, B. Liu, J. T. Lin, Chem. Eng. J. 2014, 235,
349–355.
[8] S. H. Xiao, B. Liu, Y. M. Wang, Z. F. Fang, Z. H. Zhang, Bioresour. Technol.
2014, 151, 361–366.
[40] L. M. Cornaglia, E. A. Lombardo, Appl. Catal. A 1995, 127, 125–138.
[41] B. Jarrais, A. R. Silva, C. Freire, Eur. J. Inorg. Chem. 2005, 4582–4589.
[42] M. Baltes, P. V. Der Voort, B. M. Weckhuysen, R. R. Rao, G. Catana, R. A.
Schoonheydt, E. F. Vansant, Phys. Chem. Chem. Phys. 2000, 2, 2673–
2680.
[9] J. B. Binder, R. T. Raines, J. Am. Chem. Soc. 2009, 131, 1979–1985.
[10] T. F. Wang, M. W. Nolte, B. H. Shanks, Green Chem. 2014, 16, 548–572.
[11] R. J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J.
Heeres, J. G. de Vries, Chem. Rev. 2013, 113, 1499–1597.
[12] Z. H. Zhang, J. D. Zhen, B. Liu, K. L. Lv, K. J. Deng, Green Chem. 2015, 17,
1308–1317.
[13] V. Choudhary, S. H. Mushrif, C. Ho, A. Anderko, V. Nikolakis, N. S. Marin-
kovic, A. I. Frenkel, S. I. Sandler, D. G. Vlachos, J. Am. Chem. Soc. 2013,
135, 3997–4006.
[43] R. R. Sever, T. W. Root, J. Phys. Chem. B 2003, 107, 4080–4089.
[44] A. W. Heinen, J. A. Peters, H. van Bekkum, Carbohydr. Res. 1997, 304,
155–164.
[45] C. A. Antonyraj, B. Kim, Y. Kim, S. Shin, K.-Y. Lee, I. Kim, J. K. Cho, Catal.
Commun. 2014, 57, 64–68.
[46] F. L. Grasset, B. Katryniok, S. Paul, V. Nardello-Rataj, M. Pera-Titus, J.-M.
Clacens, F. De Campo, F. Dumeignil, RSC Adv. 2013, 3, 9942–9948.
Received: February 8, 2015
[14] S. G. Wang, Z. H. Zhang, B. Liu, J. L. Li, Catal. Sci. Technol. 2013, 3, 2104–
2112.
Published online on && &&, 0000
&
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
8
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
F. Xu, Z. Zhang*
&& – &&
Polyaniline-Grafted VO(acac)2: An
Effective Catalyst
for the Synthesis of 2,5-Diformylfuran
from
5-Hydroxymethylfurfural and Fructose
Take baby steps: Polyaniline-grafted va-
nadyl acetylacetonate shows high cata-
lytic activity for the oxidation of 5-hy-
droxymethylfurfural (HMF) under atmos-
pheric oxygen pressure. A high HMF
conversion of 99% and a 2,5-diformyl-
furan (DFF) yield of 86% are obtained
after 12 h. More importantly, the synthe-
sis of DFF directly from fructose is also
realized with two binary catalysts:
Amberlyst-15 and polyaniline–vanadyl
acetylacetonate.
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
9
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ