Selective oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran over a Cu-acetonitrile complex

Article abstract of DOI:10.1039/c9nj00465c

Herein, Cu(NO₃)₂/acetonitrile was proven to be an efficient oxidation system in the oxidation of 5-HMF to DFF in an aqueous solution. After optimization of the reaction conditions, the DFF yield of 97.0% was obtained under the conditions of 1.5 mmol 5-HMF, 1.5 mmol Cu(NO₃)₂, 0.48 ml acetonitrile, 10 ml H₂O, 80 °C, and 18 h. The oxidation system could be recycled at least three times by adding HNO₃, which avoided the risk of production of Cu-containing waste. The formation of Cu(CH₃CN)₄⁺ in the aqueous solution played an important role in enabling the oxidation of 5-HMF via the redox cycle of Cu²⁺/Cu⁺. To further improve the reaction efficiency, a reactive-extraction process was developed by introducing toluene as the organic phase due to its excellent extraction capacity for DFF. As a result, the reaction time was shortened to 8 h, and the highest DFF yield of 98.9% was achieved.

Full text of DOI:10.1039/c9nj00465c

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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DOI: 10.1039/C9NJ00465C
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Selective oxidation of 5-hydroxymethy furfural into 2,5-formalde
furan over Cu-acetonitrile complex
Zuojun Wei a, Shuwen Xiao a, Mengting Chen b, Meng Lu b, Yingxin Liu*b
Received 00th January 20xx,
Accepted 00th January 20xx
Cu(NO3)2/acetonitrile was proved to be an efficient oxidation system in the oxidation of 5-HMF to DFF in aqueous solution.
After optimization of the reaction conditions, a DFF yield of 97.0% was obtained under the condition of 1.5 mmol 5-HMF,
1.5 mmol Cu(NO3)2, 0.48 ml acetonitrile, 10 ml H2O, 80 °C , 18 h. The oxidation system could be recycled at least three times
by adding HNO3, which avoids the risk of producing Cu-containing waste. The formation of Cu(CH3CN)4+ in aqueous solution
plays an important role in enabling the oxidation of 5-HMF by the redox cycle of Cu2+/Cu+. In order to further improve the
reaction efficiency, a reactive-extraction process was developed by introducing toluene as the organic phase for its excellent
extraction capacity of DFF, through which the reaction time was shorten to 8 h and a highest DFF yield of 98.9% was achieved.
DOI: 10.1039/x0xx00000x
www.rsc.org/
an even higher DFF yield of 96% was achieved.22 Ma reported a
bimetallic Au-Ru nanoparticle supported on reduced graphene
oxide (rGO), which exhibited excellent photocatalytic
performance with a DFF yield of 91%.23 Zhu used precipitation
to support Au on MnO2 and reacted in oxygen at 120 °C for 8 h,
achieved a 5-HMF conversion of 82% and a DFF selectivity of
99%.24 Yuan used Cs/MnOx with oxygen and attained a DFF yield
of 94.7%.25 Chatterjee used Rh/C as hydrogen acceptor to obtain
a DFF yield of 58% in scCO2 without any oxidant.26
Introduction
Nowadays the increasing demand in energy demand causes the
fossil fuel as non-renewable resource being inevitably
exhausted.1, 2 Clean fuels and chemicals derived from biomass
would be a promising alternatives. For example, a number of
derivatives could be prepared from the bio-derived 5-
hydroxymethylfurfural (5-HMF), such as 2,5-dimethylfuran, 2,5-
formylfuran (DFF), 5-formyl-2-furancarboxylic acid and 2,5-
furandicarboxylic acid.1, 3-9 Among them DFF is an important
Cu was also reported as an efficient catalyst for this purpose,
For example, Tong used oxygen as oxidant over Cu-MnO2,
12
chemical precursor for antifungal agents,10 nematicides,11,
obtaining
a
higher DFF yield of 82.5%.27 Jia reported
macrocyclic ligand,13 crosslinkers in the production of polyvinyl
alcohol,14 binders in the process of casting sand15 and electronic
optics16. Therefore, the efficient synthesis of DFF is highly
concentrated over the years.
Cu(NO3)2/VOSO4 as a catalyst and acetonitrile as a solvent, after
reacting at 80 °C for 1.5 h in oxygen atmosphere, a selectivity of
5-HMF and a yield of DFF both reached 99%.28 Hansen reported
CuCl/TEMPO catalyst, after reacting in acetonitrile at room
temperature for 24 h, a DFF yield of 99% were obtained.29 In
those works, N-containing compound seems indispensable in
promoting the catalytic performance of Cu.30 In this article, we
have developed a simple, inexpensive, high-efficient catalytic
oxidation system of Cu-acetonitrile, by which an ever-reported
highest DFF yield of 98.9% was achieved. Furthermore, a
reactive-extraction process was founded to effectively improve
the reaction efficiency and product separation.
Generally, DFF can be prepared by catalytic oxidation of 5-
HMF. In the early time, researchers used tetraacetate,
permanganate, or dichromate as oxidants.17-19 Fox example,
Amarasekara used Mn (III)-Salen as catalyst, sodium
hypochlorite as oxidant, and finally got a DFF yield of 89%.20
Actually, this kind of equivalent oxidants releases toxic wastes
after oxidation, resulting in an environmental burden.
Therefore, researchers have proposed t-butyl hydroperoxide,
hydrogen peroxide or oxygen as relatively greener oxidants. For
example, Ning used 30% t-butyl hydroperoxide as oxidant over
CoxOy-N@TiO2 catalyst, and finally obtain a 5-HMF conversion
of 91% and a DFF yield of 36%.21 Nie used oxygen over Ru/C, and
Materials and methods
Chemicals
5-HMF was purchased from Tengzhou Wutong Fragrances Co.
Ltd. Other chemicals were purchased from Sinopharm Chemical
Reagent China Co. Ltd and directly used without further
purification.
a.Key Laboratory of Biomass Chemical Engineering of the Ministry of Education,
College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda
Road, Xihu District, Hangzhou 310027, P.R. China
b.Research and Development Base of Catalytic Hydrogenation, College of
Pharmaceutical Science, Zhejiang University of Technology, 18 Chaowang Road,
Xiacheng District, Hangzhou 310014, P.R. China
Email: yxliu@zju.edu.cn
Reaction steps
†
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
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Selective oxidation in single phase. The selective oxidation of yield of 66.5% was obtained (Entry 2). Further increasing the
5-HMF was carried out in a 25-ml stainless steel reactor amount of Cu(NO3)2 to 1.5 mmol, a higher DFF yield of 83.1%
equipped with a magnetic stirrer. Typically, 5-HMF (1.5 mmol), was achieved (Entry 3). It was reported that Cu(NO3)2/VOSO4
Cu(NO3)2·3H2O (1.5 mmol), distilled water (10 ml) and an was an efficient catalyst in oxidation of 5-HMF to DFF.28, 31
appropriate amount of acetonitrile were put into the reactor. However, this result seems not well repeated in our case as the
After fully dissolution of the raw materials, the reactor was combination of Cu(NO3)2 and VOSO4 only obtain similar DFF
View Article Online
DOI: 10.1039/C9NJ00465C
heated to 80 °C and maintained for 18 h at 1200 rpm.
yield as adding single Cu(NO3)2 (Entry 4 vs. 2), and single
Reactive-extraction process in dual phase. 5-HMF (1.5 addition of VOSO4 has very poor effect on the reaction, as the
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mmol), Cu(NO3)2·3H2O (1.5 mmol), acetonitrile (2.5 ml), distilled yield of DFF was only 3.8% (Entry 5).
water (5 ml) and organic solvent (5 ml) were put into a 25-ml
stainless steel reactor equipped with a magnetic stirrer. After oxidation, we respectively added CuCl2 and KNO3 into the
sealed, the reactor was heated to 80 °C and maintained for 8 h reaction solution, to find that only very low DFF yields were
-
In order to verify whether Cu2+ or NO3 took effect on the
at 1200 rpm.
obtained. While CuCl2 and KNO3 were added simultaneously, a
satisfactory DFF yield was then achieved. (Entry 8). This proves
that Cu2+ and NO3 synergistically affect the oxidation of 5-HMF
to DFF.
Separation and analysis of the products
Table 2 shows the effect of O2 pressure on the oxidation of 5-
HMF to DMF. During the O2 pressure increased from 0.1 MPa to
1.5 MPa, the conversion of 5-HMF hardly changed (Entry 2-5),
indicating that the pressure of O2 is not a decisive factor in the
reaction. In addition, we are surprised that the conversion of 5-
HMF almost unchanged while the reaction was carried out
under 0.5 MPa N2 and 0.5 MPa O2, respectively (Entry 1, 3),
indicating that O2 did not play any role in the reaction. This
proves that Cu(NO3)2 should at least act as an oxidant for the
reaction.
As reported, Cu+ would coordinate with N-containing
compounds due to the Lewis basicity of the latter.32 The effects
of pyridine, pyrrole, dimethylamine and 1-methyl-2-pyrrolidone
N-containing compounds were further evaluated (Figure 1a).
Results shows that none of the selected N-containing
compound is superior to acetonitrile. We therefore choose
acetonitrile as the coordinating agent for oxidation of 5-HMF to
DFF.
GC-MS. 5-HMF conversion and DFF yield was detected by GC
analysis, which was conducted on Agilent 7890A using an HP-5
capillary column (30.0 m × 320 μm × 0.25 μm) with a carrier gas
of N2 and FID detection. The inlet temperature was 250 °C; the
initial temperature was 100 °C and then was raised to 250 °C at
5 °C·min-1 for 10 min, the detector temperature was 260 °C; split
ratio was 10:1, injection volume was 0.4 μl. The product was
quantified by GC-MS with dodecane as an internal standard.
NMR. After reaction, the reaction mixture was cooled down
and shifted to a 100 ml flask. After removing acetonitrile with a
rotary evaporator, the residue was extracted and crystallized by
dichloromethane to obtain the pure DFF crystal for NMR
1
analysis. The H and 13C NMR were measured by Varian PLUS-
400 nuclear magnetic resonance apparatus. The hydrogen
spectra were scanned at 400 MHz and 29 °C, while the carbon
spectra were scanned at 100.6 MHz and 30 °C.
Results and discussion
Establishment of the oxidation system for oxidation of 5-HMF
It can be seen that acetonitrile acts as more than a solvent in
oxidation of 5-HMF to DFF, since Cu+ coordinated with
acetonitrile as a catalyst system during the reaction,33 we
therefore further screened the solvent. As shown in Figure 1 b,
among the evaluated solvent, N,N-dimethylformamide
achieved the highest yield of DFF, and H2O ranked second. Since
Firstly, we carried out the oxidation of 5-HMF with oxygen as
oxidant, Cu(NO3)2 as co-oxidant and acetonitrile as solvent. As
seen in Table 1, the reaction did not proceed in absence of
Cu(NO3)2 (Entry 1). After 1.0 mmol Cu(NO3)2 was added, a DFF
Table 1 The effect of oxidant on the 5-HMF conversion and DFF yield
Pro-oxidant (mmol)
Entry
Conversion (%)
Yield (%)
Cu(NO3)2
--
VOSO4
--
CuCl2
--
KNO3
--
1
2
3
4
5
6
7
8
0.89
68.3
86.7
68.9
3.9
0.9
1.0
1.5
1.0
--
--
--
--
--
--
--
66.5
83.1
68.4
3.8
1.0
1.0
--
--
--
--
--
--
1.5
--
--
26.8
19.7
92.5
25.9
19.1
88.7
--
--
3.0
3.0
--
--
1.5
Reaction conditions: 1.5 mmol 5-HMF, 15 ml CH3CN, 80 °C , 0.5 MPa O2, 24 h, agitation speed 1200 rpm.
Table 2 The effect of the O2 pressure on the 5-HMF conversion and DFF yield
2 | J. Name., 2012, 00, 1-3
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O2; and (3) Cu(CH3CN)4+ is oxidized to Cu2+ to realizVeietwhAerticcylecOlenlionef
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O2 pressure
(MPa)
0.5 (N2)
0.1 (Air)
0.5
Entry
Conversion (%)
Yield (%)
2+
+
DOI: 10.1039/C9NJ00465C
Cu /Cu (CH3CN)4 redox pair, as shown in Scheme 1.
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3
4
5
89.9
91.5
86.7
90.6
90.1
88.3
90.1
83.1
89.0
88.4
Optimization of the reaction conditions
1.0
1.5
The reaction conditions were optimized in order to obtain
higher DFF yield. The effect of acetonitrile amount on the yield
of DFF is shown in Figure 2a. It can be seen that the yield of DFF
was only 24.0% when the amount of acetonitrile was 0.16 ml.
While the amount of acetonitrile increased, the yield of DFF
increased gradually, and reached 98.2% when the amount of
acetonitrile was 0.48 ml. Further increasing the amount of
acetonitrile decreased the yield of DFF. We therefore consider
that the optimum amount of acetonitrile is 0.48 ml in the
present reaction conditions. As reported, Cu+ would solvate by
acetonitrile molecule instead of water in acetonitrile aqueous
solution. Excess acetonitrile may encapsulate Cu+ and prevent
the contact between 5-HMF and the Cu+ catalyst, leading to a
lower yield of DFF in the end.37, 40
The Cu(NO3)2 amount was optimized in Figure 2b. While the
amount of Cu(NO3)2 increased, the yield of DFF also increased
gradually and reached 95.6% at 1.5 mmol of Cu(NO3)2. Further
increase the amount of Cu(NO3)2 to 2.0 mmol has little effect on
the yield of DFF. We therefore take 1.5 mmol as the optimum
amount of Cu(NO3)2 in the present reaction conditions.
The reaction temperature and time were optimized in Figure
2c and Figure 2d, respectively. With the increase of
temperature the yield of DFF showed a significant ascending
trend and reached 95.8% at 80 °C. Further increasing
temperature to 100 °C, led to a lower DFF yield of 87.1%, which
could be attributed to the thermal instability of DFF. The DFF
yield gradually increased with reaction time and reached 97.0%
at 18 h. After extending the reaction time to 20 h, there was no
significant change in the DFF yield. Therefore, the optimum
reaction time was set at 18 h.
9
Reaction conditions:1.5 mmol 5-HMF, 2.0 mmol Cu(NO3)2, 15 ml CH3CN, 80 °C , 24
h, agitation speed 1200 rpm.
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N,N-dimethylformamide is odorous and toxic, has higher boiling
point, which is not environmentally benign, we finally chose
H2O as the solvent for the oxidation reaction.
Possible reaction mechanism
According to our experimental data, Cu2+, NO3 and acetonitrile
played important roles in catalytic oxidation of 5-HMF into DFF.
The role of acetonitrile should be ascribed that it stabilizes Cu+
through coordinate bond.
As we know, Cu+ is not stable in aqueous solution due to its
spontaneously disproportionation into Cu2+ and Cu.32,
However, it could be stabilized by coordinating with some N-
containing ligands,33, 35-37 such as phenylnitrile, pyridine and
acetonitrile, through which the oxidizing ability of Cu2+/Cu+ will
be verified. For example, the standard electrode potential of
Cu2+/Cu+ is +0.158 V.
34
After coordinating with acetonitrile, Cu2+/Cu+[CH3CN]4
increases its electrode potential to 0.650 V, and is ready to
oxidize 2,6-dimethylphenol into poly (1,4-phenylene ether).33, 38,
39 Hansen studied the catalytic oxidation of 5-HMF to DFF with
CuCl/TEMPO and found that the yield of DFF significant
increased when acetonitrile instead of other solvents, such as
acetone, MIBK, methanol or water, was used.29 They also
concluded that the redox cycle of Cu2+/Cu(CH3CN)4 improved
+
the oxidizing ability of Cu2+ to transform 5-HMF into DFF.
Similar to the aforementioned reports, the plausible
mechanism of the oxidation of 5-HMF to DFF in our case may
be: (1) 5-HMF is oxidized to DFF by Cu2+/Cu+(CH3CN)4 redox pair;
In summary, we have established a catalyst system with
water as solvent and Cu(NO3)2/acetonitrile as oxidant. When 5-
HMF is 1.5 mmol, Cu(NO3)2 is 1.5 mmol, acetonitrile is 0.48 ml,
H2O is 10 ml, a DFF yield of 97% was achieved at 80 °C and 18 h.
-
(2) In a weak acidic solution, NO3 is decomposed into NOx and
Fig. 1 (a) The effect of different nitrogen ligand on the 5-HMF conversion and DFF yield; (b) The effects of different solvent on the 5-HMF conversion and DFF yield
Reaction conditions (a) 1.5 mmol 5-HMF, 1.5 mmol Cu(NO3)2, 15 mmol nitrogen ligand, 80 °C , atmospheric pressure, 24 h, agitation speed 1200 rpm; (b) 1.5 mmol 5-
：
HMF, 1.5 mmol Cu(NO3)2, 1 ml CH3CN, 10 ml solvent, 80 °C , atmospheric pressure, 24 h. agitation speed 1200 rpm.
This journal is © The Royal Society of Chemistry 20xx
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Cu-containing waste water is avoided.
View Article Online
DOI: 10.1039/C9NJ00465C
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Establishment of the reactive-extraction process for oxidation of
5-HMF
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As aforementioned, the optimal reaction time for the oxidation
of 5-HMF to DFF with Cu(NO3)2/acetonitrile was 18 h. In order
to shorten the reaction time, we have developed a reactive-
extraction process, by which toluene was introduced as a
second organic phase, and the formed DFF would be extracted
into the toluene phase during the reaction due to its more
hydrophobic property than 5-HMF. It can be seen from Table 3
that in the presence of single water and single toluene, the
yields of DFF are 30.2% (Entry 1) and 18.6% (Entry 2),
respectively. When the mixture of 5 ml of water and 5 ml of
toluene was used, the yield of DFF increased to 40.3% (Entry 3),
indicating that the addition of toluene in the aqueous phase
accelerates the oxidation of 5-HMF to DFF. Bearing in mind that
acetonitrile may be extracted into the organic phase which
affects the oxidation reaction in the aqueous phase, we further
increased the amount of acetonitrile ranging from 0.48 ml to 3.5
ml. Results showed that the yield of DFF increased with the
amount of acetonitrile and reach plateau of 98.9% at 2.5 ml.
Other four hydrophobic solvents, i.e., benzene, CH2Cl2, methyl
isobutyl ketone (MIBK) and n-butanol were also evaluated as
the extractants, showing that neither of the solvent was
superior to toluene in improving the yield of DFF at the same
amount of 0.48 ml acetonitrile (Entries 7-10). The amount of
Cu(NO3)2 and toluene were further optimized as 1.5 mmol and
5 ml (Entry 5), respectively.
Scheme 1 The reaction mechanism of the oxidation of 5-HMF to synthesize DFF
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by Cu(NO3)2/acetonitrile.
The reaction was further scaled up by adding 5-HMF of 15 mmol.
After reaction, acetonitrile is evaporated, pure DFF (1.58 g of
pale yellow crystal, recovery 85%) was achieved by re-
crystallization with dichloromethane and confirmed by the
followed MS and NMR analysis (Figure s1 and s2, ESI†).
Recyclability of the catalyst system
After each recycle reaction. It can be seen from Figure 3 that the
DFF yield showed a significant descending trend without the
addition of concentrated HNO3. In comparison, the yield of DFF
maintained for at least three cycles if extra HNO3 was added,
indicating that Cu(NO3)2 can be reused and the risk of producing
Conclusion
An catalytic oxidation system was developed using
Cu(NO3)2/acetonitrile as catalyst/oxidant to oxidize 5-HMF to
form DFF. The reaction achieved a high DFF yield of 98.9% under
the condition of 1.5 mmol 5-HMF, 1.5 mmol Cu(NO3)2, 0.48 ml
acetonitrile, 10 ml H2O, 80 °C, and 18 h. The oxidation system
could be recycled at least three times by simply adding HNO3 so
that the risk of producing Cu-containing wastes was avoided.
The plausible oxidation mechanism could be described as: 5-
HMF is oxidized to DFF by Cu2+/Cu(CH3CN)4+ redox pair; then
Fig. 2 (a) The effect of acetonitrile consumption on the 5-HMF conversion and DFF
yield; (b) The effect of Cu(NO3)2 consumption on the 5-HMF conversion and DFF
yield; (c) The effect of temperature on the 5-HMF conversion and DFF yield; (d)
The effect of reaction time on the 5-HMF conversion and DFF yield.
Reaction conditions: (a) 1.5 mmol 5-HMF, 1.5 mmol Cu(NO3)2, 10 ml H2O, 80 °C ,
atmospheric pressure, 24 h, agitation speed 1200 rpm; (b) 1.5 mmol 5-HMF, 0.48
ml CH3CN, 10 ml H2O, 80 °C , atmospheric pressure, 24 h, agitation speed 1200 rpm;
(c) 1.5 mmol 5-HMF, 1.5 mmol Cu(NO3)2, 0.48 ml CH3CN, 10 ml H2O, atmospheric
pressure, 24 h, agitation speed 1200 rpm; (d) 1.5 mmol 5-HMF, 1.5 mmol Cu(NO3)2,
0.48 ml CH3CN, 10 ml H2O, atmospheric pressure, 80 °C , agitation speed 1200 rpm.
Fig. 3 Recyclability of Cu(NO3)2 on the oxidation of 5-HMF to DFF.
Reaction conditions
：1.5 mmol 5-HMF, 1.5mmol Cu(NO3)2, 0.48 ml acetonitrile,
10 ml H2O, 80 °C, atmospheric pressure, 18 h, agitation speed 1200 rpm.
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Table 3 The effect of organic solvent and acetonitrile consumption on the 5-HMF conversion and DFF yield
Organic Solvent
View Article Online
Cu(NO3)2
mmol
Entry
H2O (ml)
CH3CN (ml)
Conversion (%)
Yield (%)
DOI: 10.1039/C9NJ00465C
Extractant
toluene
toluene
toluene
toluene
toluene
toluene
benzene
CH2Cl2
Volume (ml)
1.5
1.5
1
2
3
4
5
6
7
8
9
10
--
5
5
5
5
5
5
5
5
5
5
5
5
5
5
--
10
5
0.48
0.48
0.48
1.5
30.5
18.9
40.5
69.2
99.9
83.8
4.6
30.2
18.6
40.3
67.2
98.9
83.0
4.1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
8
9
5
5
2.5
10
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5
3.5
5
0.48
0.48
0.48
0.48
2.5
5
5.3
4.9
MIBK
5
5.6
5.3
6.1
10
11
butanol
toluene
toluene
toluene
toluene
toluene
toluene
5
6.3
2
79.8
90.7
86.9
14.4
37.1
98.7
79.0
89.8
86.0
14.1
36.6
98.6
12
13
14
15
16
7
2.5
10
5
2.5
1.5
0.5
2.5
1.0
2.0
5
2.5
5
2.5
Reaction conditions: 1.5 mmol 5-HMF, 80 °C, 8 h, atmosphere pressure, agitation speed 1200 rpm.
+
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Cu(CH3CN)4 is oxidized by NOx, which is derived from the
decomposition of NO3- in a weak acidic solution, to Cu2+ to realize
the cycle of Cu2+/Cu+(CH3CN)4 redox pair.
Furthermore, toluene was introduced as a second organic
phase to establish a reactive-extraction process for oxidation of
5-HMF, by which the reaction time was shortened. The highest
DFF yield of 98.9% was achieved with 1.5 mmol of Cu(NO3)2, 2.5
ml of acetonitrile, 5 ml of H2O, 5 ml of toluene at 80 °C , 8 h.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (21878269 and 21476211) and the Natural
Science Foundation of Zhejiang province (LY16B060004 and
LY18B060016).
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