Promoted role of Cu(NO3)2 on aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran over VOSO4

Article abstract of DOI:10.1016/j.apcata.2014.05.031

The promoted effect of Cu(NO₃)₂ on aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) catalyzed by VOSO₄ in acetonitrile was intensively investigated. It was revealed that Cu(NO₃)₂ facilitated the activation of VOSO ₄ to generate active V⁵⁺ species via the generation of NO_x gas. The high DFF selectivity is ascribed to Cu²⁺ cation which can effectively prohibit oxidative CC bond cleavage reaction of HMF and prevent radical reaction of DFF to humins. In addition, the polarity of solvent plays a great role on high selectivity of DFF.

Full text of DOI:10.1016/j.apcata.2014.05.031

Accepted Manuscript
Title: Promoted role of Cu(NO3)2 on aerobic oxidation of
5-hydroxymethylfurfural to 2,5-diformylfuran over VOSO4
Author: Xiuquan Jia Jiping Ma Min Wang Zhongtian Du
Fang Lu Feng Wang Jie Xu
PII:
S0926-860X(14)00365-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.05.031
APCATA 14851
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Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
10-3-2014
12-5-2014
26-5-2014
Please cite this article as: X. Jia, J. Ma, M. Wang, Z. Du, F. Lu,
F. Wang, J. Xu, Promoted role of Cu(NO3)2 on aerobic oxidation of 5-
hydroxymethylfurfural to 2,5-diformylfuran over VOSO4, Applied Catalysis A, General
(2014), http://dx.doi.org/10.1016/j.apcata.2014.05.031
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Promoted role of Cu(NO3)2 on aerobic oxidation of 5-hydroxymethylfurfural to
2,5-diformylfuran over VOSO4
Xiuquan Jia,‡a,b Jiping Ma,‡a Min Wang,a Zhongtian Du,a Fang Lu,a Feng Wanga and
Jie Xua*
a Dalian National Laboratory for Clean Energy, State Key Laboratory of Catalysis,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.
R. China.
b University of Chinese Academy of Sciences, Beijing 100049, P. R. China
* Corresponding author: Prof. Jie Xu; E-mail Address: xujie@dicp.ac.cn; Fax: +86
411 84379245
‡ X. Q. Jia and J. P. Ma contributed equally to this work.
Highlights
ꢀ The essential role of Cu(NO3)2 how to activate VOSO4 is investigated.
ꢀ The role of Cu2+ cation on high 2,5-diformyfuran selectivity is discussed.
ꢀ Solvent polarity depended 2,5-diformyfuran selectivity is observed.
ꢀ Kinetic studies have been done to evaluate the possible reaction mechanism.
Abstract
3
Page 1 of 24

The promoted effect of Cu(NO3)2 on aerobic oxidation of 5-hydroxymethylfurfural
(HMF) to 2,5-diformylfuran (DFF) catalyzed by VOSO4 in acetonitrile was
intensively investigated. It was revealed that Cu(NO3)2 facilitated the activation of
VOSO4 to generate active V5+ species via the generation of NOx gas. The high DFF
selectivity is ascribed to Cu2+ cation which can effectively prohibit oxidative C-C
bond cleavage reaction of HMF and prevent radical reaction of DFF to humins. In
addition, the polarity of solvent plays a great role on high selectivity of DFF.
Keywords: Catalysis; Copper nitrate; 2,5-Diformylfuran; Oxidation; Sustainable
chemistry
1. Introduction
Catalytic conversion of renewable biomass resources to value-added chemicals is
significant for sustainable chemistry [1]. 5-Hydroxylmethylfurfural (HMF), a versatile
biomass-derived platform chemical, can be obtained by dehydration of carbohydrates
in the presence of acid catalysts [2]. A growing interest has been devoted to catalytic
transformation of HMF to valuable chemicals in the past few years [3].
2,5-Diformylfuran (DFF), via selective oxidation of HMF, indeed finds applications
as intermediate for pharmaceuticals, macrocyclic ligands and others [4]. Moreover,
DFF gained interests as a monomer for synthesis of functional materials [5]. For
example, a fluorescent material, furan-based imine-linked porous organic frameworks
and a 2,5-diformylfuran-urea resin have been synthesized using DFF as the key
monomer.
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However, owing to the presence of both a furan ring and an aldehyde group in the
molecule, the oxidation of HMF to DFF often involves many side reactions, for
example,
overoxidation
to
5-hydroxymethyl-2-furancarboxalic
acid
or
2-formyl-5-furancarboxylic acid or 2,5-furandicarboxalic acid, oxidative C-C bond
cleavage, decarbonylation, and cross-polymerization. Therefore, it is challenging to
convert HMF into DFF with high selectivity via catalytic oxidation with molecular
oxygen as oxidant. Recently, great effort has been made in selective aerobic oxidation
of HMF to DFF and a number of catalytic systems have been developed. The catalytic
systems include Co/Mn/Zr/Br [6], Ru-based catalysts [7], TEMPO/Cu(I) and
Fe3O4@SiO2–TEMPO catalysts [8], and Mn-based catalysts [9]. It was reported that
vanadium-based catalysts were active in the oxidation of alcohols [10] and have been
explored for the oxidation of HMF to DFF [11]. For example, vanadyl phosphates
have been investigated by Carlini et al. and an HMF yield of 84% with DFF
o
selectivity of 97% was achieved in dimethyl sulfoxide (DMSO) at 150 C [11h].
Immobilized vanadyl-pyridine complexes reported by Corma and co-workers gave
>99% selectivity of DFF at HMF conversion of 82% in triflurotoluene at 130 oC [11g].
A reaction selectivity towards DFF of >99% at 84% HMF conversion was obtained
with V2O5/H-beta catalyst in DMSO at 125 oC [11a].
Recently, our group reported a highly efficient catalyst system consisting of
Cu(NO3)2/VOSO4, and 99% DFF selectivity at 99% HMF conversion was obtained in
o
acetonitrile at 80 C [5a].We found that VOSO4 alone was nearly inactive in the
oxidation of HMF to DFF. Interestingly, with the addition of Cu(NO3)2, conversion of
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HMF and selectivity for DFF were both remarkably enhanced with nearly quantitative
conversion of HMF to DFF under mild conditions. The promoting effect of Cu(NO3)2
is still unclear. Herein, the essential role of the Cu(NO3)2 for high activity and
selectivity in the oxidation of HMF to DFF catalyzed by VOSO4 in acetonitrile was
investigated in detail. Moreover, the polarity of solvent plays a great role on
selectivity of DFF. Kinetic studies are also carried out to establish a detailed reaction
mechanism.
2. Experimental
2.1. Materials
HMF was purchased from Wutong Aroma Chemicals Co. Ltd. Cu(NO3)2·3H2O,
Ce(NO3)3·6H2O, Ni(NO3)2·6H2O, Mn(NO3)2·6H2O, Co(NO3)2·6H2O, NaNO3,
Fe(NO3)3·9H2O, toluene, dichloromethane, n-butanol, iso-propyl alcohol (IPA),
tetrahydrofuran (THF), ethanol, methanol, acetonitrile, DMSO and N,N-dimethyl
formamide (DMF) were from Tianjin Kermel Chemical Reagent Development Center,
China. Acrylonitrile was from Tianjin Guangfu Fine Chemical Research Institute.
Benzonitrile (Chemical Purity), tert-butyl hydroperoxide and hydroquinone were
from Sinopharm Chemical Reagent Co. Ltd. VOSO4·xH2O was from Shanghai
Lvyuan
Fine
Chemical
Co.
Ltd.
4-(2-pyridylazo)resorcinol
(PAR),
iodo-nitro-tetrazolium chloride (INT), 1,2,-diaminocyclohexane-N,N,N’,N’-tetraacetic
acid (CDTA), sodium acetate, 2,6-di-tert-butyl-p-cresol and azodiisobutyronitrile
were obtained from Aladdin Chemistry Co. Ltd. Acetonitrile was HPLC grade. A trace
amount of water in acetonitrile was removed by using 4A molecular sieve.
6
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VOSO4·xH2O (USP28) was dried at 120 oC overnight. Other reagents were analytic
grade unless otherwise specified and used as received.
2.2. Reaction system and analysis method
The aerobic oxidation of HMF was carried out in a 50-mL stainless steel
autoclave equipped with a magnetic stirrer, a pressure gauge and automatic
temperature control apparatus. Typically, VOSO4 (0.2 mmol, 34.3 mg),
Cu(NO3)2·3H2O (0.2 mmol, 48.3 mg) and HMF (10 mmol, 1.26 g) were put into the
autoclave with Teflon lining, followed by 5 mL acetonitrile. After the autoclave was
o
closed, oxygen was added (0.1 MPa). The autoclave was then heated to 80 C within
ca. 20 min. The reaction temperature was maintained at 80 oC for 1.5 h. Oxygen was
recharged if consumed during the oxidation. The autoclave was cooled to room
temperature and depressurized carefully. A sample of the reaction mixture was taken
for GC analysis, which were conducted on Agilent GC 7890D equipped with
19095J-323 capillary column (30 m × 530 μm × 1.5 μm) and a flame ionization
detector. The quantitative results were based on the internal standard method using
mesitylene as internal standard. The results reported as conversion and selectivity are
expressed in mol% based on the total HMF intake.
2.3. Detection of NOx by MS
Mass spectra were performed on a mass analyzer (Thermal Star, GSD 320,
Germany) with argon flow of 50 mL·min-1.
Sample: To a 20 mL sealed sample vial equipped with a magnetic stirring, 1
mmol VOSO4 and 1 mmol Cu(NO3)2 were added. The sealed sample vial was kept at
7
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ambient temperature with stirring. The reaction was stopped after 1.5 h, and the mass
spectra of the afforded NOx were recorded.
2.4. Detection of V5+ species by UV-vis
The UV-vis spectrum was recorded on a SHIMADZU UV-2550 spectrometer
with quartz cells at 298 K. The reference cell was filled with the same concentration
of CDTA and pH 5.6 buffer solutions. Wavelength range: 190-900 nm.
Sample: for reaction mixture: 0.2 mmol VOSO4, 5 mL acetonitrile charged with
NOx generated from the mixture of VOSO4 and Cu(NO3)2, 80 oC, 0.1 MPa O2, 1.5 h.
Then cooled to room temperature and diluted to 25 mL with acetonitrile.
Coordination process: 1 mL diluted mixture, 2 mL 0.125 M aqueous solution of
CDTA, 2 mL pH 5.6 buffer of acetate, 1 mL 2 × 10-3 M aqueous solution of PAR, 0.5
mL 3 × 10-3 M acetonitrile solution of INT, then dilute to 25 mL with acetonitrile.
After developing color 15 min, UV-vis spectrum was recorded.
3. Results and Discussion
3.1. The role of Cu(NO3)2
Fig. 1. The effect of molar ratio of Cu(NO3)2 on the HMF conversion and DFF
selectivity. Reaction conditions: 10 mmol HMF, 0.4 mmol catalysts, 5 mL solvent, 80
8
Page 6 of 24

oC, 0.1 MPa O2, 1.5 h.
As shown in Fig. 1, the molar ratio of Cu(NO3)2 to Cu(NO3)2 and VOSO4
apparently influenced the reactivity and selectivity. With increase of the molar ratio of
Cu(NO3)2, the conversion of HMF firstly increased and then decreased after the molar
ratio of Cu(NO3)2 was up to 50.0%. Whereas, the selectivity display another behaves.
High selectivity could maintain when Cu(NO3)2 was the major composition. The
selectivity gradually decreased with further increasing the amount of VOSO4. These
results revealed that Cu(NO3)2 could effectively prohibit the side reactions and thus
obtaining high DFF selectivity. The optimization of molar ratio of Cu(NO3)2 to
VOSO4 is 1, and at this point the conversion of HMF and selectivity for DFF were up
to more than 99%. Lower than 10% HMF conversion was observed when solely
VOSO4 or Cu(NO3)2 was used. This suggested that neither VOSO4 nor Cu(NO3)2 was
the active species.
Fig. 2. Interaction between Cu(NO3)2 and VOSO4 characterized by in situ FT-IR.
Reaction conditions: 0.2 mmol Cu(NO3)2, 0.2 mmol VOSO4, 5 mL CH3CN, room
temperature, 0.1 MPa O2, 10 h. Red dotted line is standard VOSO4, green dotted line
is standard Cu(NO3)2.
In our previous paper, it was demonstrated that V5+ species were the active
9
Page 7 of 24

species which could directly oxidize HMF to DFF. However, in the catalytic system
of Cu(NO3)2/VOSO4, the initial vanadium species are V4+. This indicates that the V5+
species were firstly formed mediated by Cu(NO3)2. In order to make sure how
Cu(NO3)2 facilitates the V5+/V4+ redox cycle, we investigated the interaction between
Cu(NO3)2 and VOSO4 using in situ Fourier transform infrared (FT-IR) (Fig. 2). When
Cu(NO3)2 and VOSO4 were mixed together in CH3CN, the colour of the solution
changed from blue to brown. Simultaneously, the decrease of the intensity of the
characteristic peaks of Cu(NO3)2 at 1493, 1309 and 1288 cm-1 was clearly observed.
This suggested that Cu(NO3)2 may gradually decompose and NOx gas occur even at
room temperature. In order to confirm this hypothesis, we then mixed VOSO4 and
Cu(NO3)2 under solid state. Again, we found that brown gas generated when
equivalent molar of VOSO4 was mixed with Cu(NO3)2 at room temperature under air
atmosphere (Fig. S1a, supporting information). Whereas, Cu(NO3)2 is stable when it
is stirred under the same conditions (Fig. S1b, supporting information).
Subsequently, the brown gas was characterized by mass spectrometry (MS). The
peaks with m/z of 30, 44 and 46 were detected by MS (Fig. S2, supporting
information). The peak intensity ratio of 30 to 46 is much higher than that of pure
NO2 (Fig. S3, supporting information). Also, the intensity of 44 of the brown gas is
much higher than that of air (Fig. S4, supporting information). It is probably that NO
and N2O could not be excluded from the components of the in situ formed NOx gas.
However, it was believed that NO can quickly convert to NO2 under air atmosphere
even at room temperature. It seems that it is enigmatical that NO existed in the brown
10
Page 8 of 24

gas generated from Cu(NO3)2 and VOSO4 under air atmosphere. Hence, the brown gas
was further characterized by FT-IR (Fig. S5, supporting information). Four Typical
bands of NO2 centred at 2918, 2812, 1628 and 1601 cm-1 were detected.[12] Another
absorption band at 1806 cm-1 associated with nitroso stretch is approximate to that of
N2O3 which could decompose into NO and NO2.[13] N2O3 with m/z of 76 was not
detected by mass spectroscopy could arise from that N2O3 is unstable under MS
detection conditions and may degrade to NO and NO2 during the analysis process.
Fig. 3. Detection of V5+ species using UV-vis spectroscopy.
It is well-known that NOx is oxidative gas [14]. The V5+ species may be formed
from the oxidation of VOSO4 by NOx. In previous work, we have shown that the
UV-vis spectroscopy could detect the V5+ species after chelation [5a]. In order to
further confirm whether this brown gas derived from the interaction of Cu(NO3)2 and
VOSO4 could oxidize VOSO4 to V5+ species, we directed the brown gas into the
acetonitrile solution of VOSO4 and treated the mixture under the reaction conditions,
followed by pretreating with chelating agents of PAR and INT in the presence of
masking agent CDTA to detect by UV-vis spectroscopy (Fig. 3). The peak at ca. 385
nm is assigned to a π-π* electronic transition of PAR. A new absorption band
11
Page 9 of 24

appeared at ca. 552 nm which is characteristic of complex of V5+ with PAR and INT
[15]. The above results suggest that the brown gas derived from the interaction of
Cu(NO3)2 and VOSO4 can oxidize VOSO4 to V5+ species, thus realizing the V5+/V4+
redox cycle. To verify the obtained V5+ species can oxidize HMF to DFF, control
experiment was performed under the same reaction conditions. A 97% conversion of
HMF with 92% selectivity for DFF was obtained in the presence of CuSO4, which is
comparable to that of Cu(NO3)2/VOSO4 catalytic system.
Table 1 Oxidation of HMF catalyzed by VOSO4.a
Entry Metal nitrate Conversion (%) Selectivity b (%)
1
2
3
4
5
6
7
8
-
24
99
11
13
20
22
30
34
12
99
27
33
58
75
11
51
Cu(NO3)2
Ce(NO3)3
Ni(NO3)2
Mn(NO3)2
Co(NO3)2
NaNO3
Fe(NO3)3
a Reaction conditions: 0.2 mmol VOSO4, 0.2 mmol metal nitrate, 10 mmol HMF,
5 mL CH3CN, 80 oC, 0.1 MPa O2, 1.5 h. Conversion and selectivity were determined
by GC using mesitylene as internal standard. For Mn(NO3)2, 50% aqueous solution
was used. b Selectivity for DFF.
As have been demonstrated above, Cu(NO3)2 played a vital role in catalytic
12
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performance. The addition of Cu(NO3)2 not only increased the activity but also the
DFF selectivity (Table 1, entry 2). When other metal nitrate such as Ce(NO3)3,
Ni(NO3)2, Mn(NO3)2, Co(NO3)2, NaNO3, or Fe(NO3)3 was employed instead of
Cu(NO3)2, they show lower activity and selectivity under the same reaction conditions.
HMF conversions were only 11-34% with 11-75% selectivity for DFF (Table 1,
entries 3-8). It has been demonstrated that VOSO4 can promote Cu(NO3)2 degradation
to NOx which oxidized V4+ species to V5+ active species. The UV-vis results implied
that other tested metal nitrate could not degrade to NOx in the presence of VOSO4
under the same conditions (Fig. S6, supporting information). These should account for
lower activity for other tested metal nitrate than that of Cu(NO3)2.
Fig. 4. The dependence of HMF conversion and DFF selectivity on O2 partial pressure.
Reaction conditions: 10 mmol HMF, 0.2 mmol VOSO4, 0.2 mmol Cu(NO3)2, 5 mL
CH3CN, 80 oC, 50 min.
The effect of O2 pressure on the HMF conversion and DFF selectivity was further
examined (Fig. 4). When the O2 pressure increased from 0.1 to 2.0 MPa, the HMF
conversions were about 60% after 50 min, indicating that the reaction was not
controlled by an oxygen mass-transfer process. Furthermore, O2 pressure showed little
effect on the DFF selectivity. Over 94% selectivity of DFF were obtained in the range
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of O2 pressure tested. The phenomena are very different from our previous study that
vanadium catalyzed oxidative cleavage of HMF to maleic anhydride (MA) is very
sensitive to partial oxygen pressure. The yield of MA increased with increasing partial
oxygen pressure [16]. In the absence of Cu(NO3)2, only 7% DFF selectivity at 94%
HMF conversion was obtained under 1.0 MPa O2 pressure. In order to easily examine
the by-products via gas chromatography-mass spectrometry (GC-MS), the obtained
reaction mixture was followed treatment with methanol catalyzed by BF3·(Et)2O. The
main by-products are dimethyl fumarate, monomethyl maleate, dimethyl maleate and
maleic anhydride. Other by-products, such as methyl glycolate, dimethyl oxalate,
dimethyl malonate, methyl levulinate and dimethyl succinate were also examined (Fig.
S7, supporting information). These results indicated that Cu2+ cation can effectively
prohibit oxidative C-C bond cleavage of HMF.
Table 2 Influences of catalyst systems and additives on DFF stability. a
Entry
Catalyst
Copper salts
Additive
Recovery (%)
1
2
3b
4
5
6
7
-
-
-
99
74
35
99
98
97
69
VOSO4
VOSO4
-
-
-
-
-
Cu(NO3)2
Cu(NO3)2
CuSO4
-
-
VOSO4
VOSO4
VOSO4
-
-
ADBNc
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8
VOSO4
VOSO4
VOSO4
-
-
-
TBHPd
BQe
67
98
99
9
10
DTBPf
a Reaction conditions: 0.1 mmol VOSO4, 0.1 mmol copper salt, 0.25 mmol additive, 5
o
mmol DFF, 5 mL CH3CN, 80 C, 0.1 MPa O2, 3 h. Conversion and selectivity was
determined by GC using mesitylene as the internal standard. b 1.0 MPa O2 was used.
c
d
e
ADBN: Azodiisobutyronitrile.
TBHP: Tert-butyl hydroperoxide.
BQ:
Benzoquinone. f DTBP: 2,6-Di-tert-butyl-p-cresol.
It was reported that Cu2+ cation is an excellent carbon radical trapper frequently
used in oxidation reactions [17]. We then investigated the stabilization of DFF by
Cu2+ cation. DFF can keep stable in the absence of catalysts. 99% DFF recovery was
obtained (Table 2, entry 1). Only 74% DFF recovery was achieved in the presence of
VOSO4 (Table 2, entry 2). Under more severe conditions, DFF is more likely to
convert to by-products (Table 2, entry 3). In contrast, nearly quantitative recovery of
DFF were obtained in the presence of cupric salts whether with or without VOSO4
(Table 2, entries 4-6).
Fig. 5. MALDI TOF mass spectrum of humins derived from DFF (Table 2, entry 3).
15
Page 13 of 24

It is well known that furan derivatives, such as furfuryl alcohol and furfural are
likely to cross-link by radical reaction to form soluble or insoluble humins in the
presence of metal salts even under the ambient conditions. The HMF oxidation
product of DFF, bearing reactive furan ring and aldehyde group, may be unstable in
the presence of catalysts owing to the radical reactions. The recovery of DFF further
decreased when free radical initiator such as azodiisobutyronitrile or tert-butyl
hydroperoxide was added (Table 2, entries 7 and 8). While, nearly quantitative
recovery of DFF was obtained in the presence of radical inhibitor such as
benzoquinone or 2,6-di-tert-butyl-p-cresol (Table 2, entries 9 and 10). MALDI TOF
mass spectrum of the products derived from DFF demonstrates that humins was
formed via radical reaction (Fig. 5). Further comparison was examined between DFF
and the humins derived from DFF by FT-IR spectroscopy (Fig. S8, supporting
information), it shows an 8 cm-1 red shift of the C=O stretching vibrational absorption
in the humins, and the absorption of furan rings was unconspicuous, implying that
polymerization between furan rings occurred. These results confirmed that free
radical reaction was the main side reaction and Cu2+ cation can inhibit the free radical
reaction, thus achieving high DFF selectivity. These phenomena were also observed
by Yin et al. in the aerobic oxidation of furfural to maleic acid [18].
16
Page 14 of 24

Scheme 1 Promoted role of Cu(NO3)2 on aerobic oxidation of HMF to DFF catalyzed
by VOSO4.
Based on the above results, the role of Cu(NO3)2 is more clear. Firstly, with the
assistance of VOSO4, Cu(NO3)2 was degraded to NOx, which subsequently oxidized
VOSO4 to active V5+ species to further oxidize HMF to DFF. Secondly, Cu2+ cation
can effectively prohibit oxidative C-C bond cleavage of HMF and prohibit radical
reaction to keep high DFF selectivity under the reaction conditions. Promoted effect
of Cu(NO3)2 on aerobic oxidation of HMF to DFF catalyzed by VOSO4 was
summarized in Scheme 1.
3.2. Solvent effect
Fig. 6. The dependence of HMF conversion and DFF selectivity on solvent. Reaction
conditions: 10 mmol HMF, 0.2 mmol VOSO4, 0.2 mmol Cu(NO3)2, 5 mL solvent, 80
oC, 0.1 MPa O2, 1.5 h.
As shown in Fig. 6, the solvent has significant influence on the reactivity of HMF
and the selectivity of DFF. Acetonitrile shows the best performance among the
solvents tested with different polarity (described quantitatively by ET(30) [19] with
unit of kcar mol-1), including toluene (33.9), THF (37.4), dichloromethane (40.7),
benzonitrile (41.5), DMF (43.2), DMSO (45.1), acetonitrile (45.6), acrylonitrile (46.7),
17
Page 15 of 24

IPA (48.4), n-butanol (49.7), ethanol (51.9), methanol (55.4) and water (63.1). Over
99% HMF conversion and DFF selectivity are achieved in acetonitrile.
The dependence of DFF selectivity on the solvent polarity is also studied (Fig. 7).
It seems that solvents with medium polarity (ET(30) is close to 46 kcar mol-1) could
keep high selectivity of DFF. Poor selectivity is observed in solvent with too low or
strong polarity. This suggested that DFF selectivity was very closely related to the
solvent polarity. In order to further verify the hypothesis, mixed solvents of
acetonitrile/toluene (v/v = 4) and acetonitrile/acrylonitrile (v/v = 4) with polarity of
43.3 and 45.8 were tested, and moderate DFF selectivity of 86% and 88% were
obtained, respectively.
Fig. 7. The dependence of DFF selectivity on the solvent polarity (the blue star and
diamond stand for mixed solvent of acetonitrile/toluene and acetonitrile/acrylonitrile
with volume ratio of 4, respectively).
3.3. Kinetic study
The dependence of the HMF concentration with reaction time at different reaction
temperature was studied (Fig. 8a). A linear relationship exists between ln(C0/Ct) and
reaction time (s), indicating that the reaction is a pseudo-first-order reaction with
18
Page 16 of 24

respect to HMF. This is different from the heterogeneous Ru/C catalyzed oxidation of
HMF to DFF reported by Liu’s group [7a]. The apparent activation energy, Ea, is
o
determined from the Arrhenius plot in the temperature range of 40-80 C (Fig. 8b).
The least-square fit analysis yields Ea value of 66 kJ mol-1.
Fig. 8. (a) Relationship of In(C0/Ct) and reaction time t of HMF oxidation to DFF. (b)
Arrhenius plot for the oxidation of HMF. Reaction conditions: 20 mmol HMF, 0.4
mmol VOSO4, 0.4 mmol Cu(NO3)2, 10 mL CH3CN, 0.1 MPa O2.
4. Conclusion
In summary, the promoted role of Cu(NO3)2 on aerobic oxidation of HMF to DFF
catalyzed by VOSO4 in acetonitrile was extensively investigated. How Cu(NO3)2
activate VOSO4 to generate V5+ species can be clear. It was revealed that Cu(NO3)2
can facilitate the V5+/V4+ redox cycle via the generation of NOx gas, thus achieving
high activity. Moreover, the Cu2+ cation not only can effectively prohibit oxidative
C-C cleavage reaction of HMF but also can prohibit radical reaction of DFF to
humins, thus obtaining high selectivity. In addition, the polarity of solvent plays a
great role on high selectivity of DFF. Kinetic study shows that it is a
pseudo-first-order reaction with respect to HMF, and apparent activated energy of 66
19
Page 17 of 24

kJ mol-1.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21233008, 21303183 and 21203180).
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Figure, Scheme and Table Captions:
Fig. 1. The effect of molar ratio of Cu(NO3)2 on the HMF conversion and DFF
selectivity.
Fig. 2. Interaction between Cu(NO3)2 and VOSO4 characterized by in situ FT-IR.
Fig. 3. Detection of V5+ species using UV-vis spectrum.
Fig. 4. The dependence of HMF conversion and DFF selectivity on O2 partial
pressure.
Fig. 5. MALDI TOF mass spectrum of humins derived from DFF.
Fig. 6. The dependence of HMF conversion and DFF selectivity on solvent.
Fig. 7. The dependence of DFF selectivity on the solvent polarity.
Fig. 8. (a) Relationship of In(C0/Ct) and reaction time t of HMF oxidation to DFF.
(b) Arrhenius plot for the oxidation of HMF.
Scheme 1 Promoted role of Cu(NO3)2 on aerobic oxidation of HMF to DFF catalyzed
by VOSO4.
Table 1 Oxidation of HMF catalyzed by VOSO4.
Table 2 Influences of catalyst systems and additives on DFF stability.
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Graphical abstract
Promoted role of Cu(NO3)2 on aerobic oxidation of 5-hydroxymethylfurfural to
2,5-diformylfuran over VOSO4
Xiuquan Jia,‡a,b Jiping Ma,‡a Min Wang,a Zhongtian Du, a Fang Lu,a Feng Wanga and
Jie Xua*
26
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