Functionalized expanded corn starch-anchored Cu(I): An efficient and recyclable catalyst for oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran

Article abstract of DOI:10.1002/aoc.5411

The development of new strategies for the synthesis of biomass-based non-precious metal heterogeneous catalysts has recently received great interest from chemists because of the advantages of these catalytic systems being sustainable, low cost and green. An expanded corn starch-supported CuBr catalyst (ECS-SB-CuBr) has been successfully prepared and well characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis, powder X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy/energy-dispersive X-ray spectroscopy. Further, ECS-SB-CuBr was used as a heterogeneous catalyst for the selective oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-diformylfuran (DFF) and a full HMF conversion is obtained with 98% DFF yield in acetonitrile under ambient pressure of dioxygen at 50°C. The catalyst also showed good reusability, could be easily recovered through filtration and washing and was reused in at least six consecutive runs with virtually no loss of catalytic performance.

Full text of DOI:10.1002/aoc.5411

Received: 23 July 2019
DOI: 10.1002/aoc.5411
Revised: 9 November 2019
Accepted: 11 November 2019
C O M M U N I C A T I O N
Functionalized expanded corn starch-anchored Cu(I): An
efficient and recyclable catalyst for oxidation of
5
-hydroxymethylfurfural to 2,5-diformylfuran
1
,2
2
2
2
2
Mei Hong
| Jie Min | Shuangyan Wu | Jiatong Li | Jing Wang
|
2
2
2
1,2
Lifen Wei | Zhi Ling | Kun Li | Shifa Wang
1
Jiangsu Co-Innovation Center of
Abstract
Efficient Processing and Utilization of
Forest Resources, Nanjing Forestry
University, Nanjing, 210037, China
The development of new strategies for the synthesis of biomass-based non-
precious metal heterogeneous catalysts has recently received great interest
from chemists because of the advantages of these catalytic systems being sus-
tainable, low cost and green. An expanded corn starch-supported CuBr catalyst
2
College of Chemical Engineering,
Nanjing Forestry University, Nanjing,
210037, China
(
ECS-SB-CuBr) has been successfully prepared and well characterized using
Correspondence
Fourier transform infrared spectroscopy, thermogravimetric analysis, powder
X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron
microscopy/energy-dispersive X-ray spectroscopy. Further, ECS-SB-CuBr was
Mei Hong. College of Chemical
Engineering, Nanjing Forestry University,
Nanjing 210037, People's Republic of
China.
used as
a
heterogeneous catalyst for the selective oxidation of
Email: hongmei-613@njfu.edu.cn
5
-hydroxymethylfurfural (HMF) into 2,5-diformylfuran (DFF) and a full HMF
Funding information
conversion is obtained with 98% DFF yield in acetonitrile under ambient pres-
the Open Foundation of Jiangsu Key
Laboratory of Biomass Energy and
Materials, Grant/Award Number:
JSBEM201915; This work was also
financially supported by the Priority
Academic Program Development of
Jiangsu Higher Education Institutions
ꢀ
sure of dioxygen at 50 C. The catalyst also showed good reusability, could be
easily recovered through filtration and washing and was reused in at least six
consecutive runs with virtually no loss of catalytic performance.
K E Y W O R D S
(
PAPD)
2,5-diformylfuran, 5-hydroxymethylfurfural, copper, expanded corn starch, heterogeneous
catalyst
1
| INTRODUCTION
Thus, for DFF production, highly selective oxidation of
HMF is important.
[5]
[6]
2
,5-Diformylfuran (DFF) is a versatile compound that
In several cases late transition metals like Pd, Pt,
[
7–9]
[10]
[11,12]
has various applications, including as a polymeric mono-
mer and a promising cross-linking agent for battery sepa-
ration, in pharmaceuticals, antifungal agents, furan–urea
resins, heterocyclic ligands and other value added
Ru,
Ag
and Au
have been used as efficient
catalysts for selective oxidation of HMF to DFF. From the
practical and environmental points of view, there is a
strong demand for exploring efficient catalytic systems
using inexpensive and nontoxic first-row transition metal
(Cu, Fe, etc.)-based catalysts and nontoxic air or oxygen
as the terminal oxidant. To this end, many homogeneous
[1,2]
products,
which is synthesized predominantly via the
oxidation of 5-hydroxymethylfurfural (HMF). However,
this transformation can be accompanied by the formation
of several other kinds of furan compounds like
copper catalysts have been applied in the presence of O
2
[
13,14]
2
,5-furandicarboxylic acid, 5-formyl-2-furancarboxylic
and offered high DFF yield.
Ma et al. reported a cat-
[
3,4]
acid and 5-hydroxymethyl-2-furancarboxylic acid.
alyst system consisting of Cu (NO ) and VOSO (1:1),
3
2
4
Appl Organometal Chem. 2019;e5411.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 8
https://doi.org/10.1002/aoc.5411

2
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HONG ET AL.
which showed excellent activity for the conversion of
identified that Schiff base-functionalized expanded corn
starch-supported CuBr (ECS-SB-CuBr) in the presence of
TEMPO as a co-catalyst was an efficient catalyst for pro-
moting the selective aerobic oxidation of HMF into DFF
ꢀ
HMF (99%) to yield DFF (99%) in acetonitrile at 80 C,
[15]
1
0 bar O , after 90 min.
Kompanets et al. reported Cu
2
(
NO ) as an efficient catalyst for the production of DFF
3
2
ꢀ
from HMF in the presence of N-hydroxyphthalimide as a
under ambient pressure of dioxygen at 50 C in acetoni-
ꢀ
co-catalyst, yielding 71% DFF in acetonitrile, at 50 C,
trile (Scheme 1). In addition, the catalyst is stable to air
and moisture and can be easily recovered and reused. To
the best of our knowledge, this is the first example of cat-
alytic aerobic oxidation of HMF to DFF catalyzed by
biomass-supported CuBr using a low-cost and more sus-
tainable method.
[
16]
1
1
bar O2.
Tong et al. showed that CuI assisted by
-hydroxybenzotriazole afforded 93% conversion of HMF,
with 99% selectivity towards DFF, in dimethylsulfoxide at
ꢀ
[16]
1
30 C, 3 bar O , after 10 h reaction time.
Riisager and
2
co-workers also developed homogeneous Cu-based cata-
lytic systems bearing various organic ligands in combina-
tion with 2,2,6,6-tetramethylpiperidinooxy (TEMPO) as a
highly efficient method for the aerobic oxidation of HMF
2 | RESULTS AND DISCUSSION
[17]
to DFF using O or air as the terminal oxidant.
How-
2
ever, these methods generally suffer from some draw-
2.1 | Preparation and characterization of
ECS-SB-CuBr catalyst
backs, e.g. high pressure of O , low product yield and the
2
use of high-boiling-point solvents. Also, the synthesis of
DFF proceeds in the presence of copper salts as homoge-
neous catalysts, which makes the separation and recovery
of the copper catalysts very difficult. In addition, it is
probable that the obtained DFF can be contaminated by
copper metal in a homogeneous catalytic system. There-
fore, separation from the products and recycling of the
copper catalysts remains a key issue. In contrast, hetero-
geneous catalytic systems are beneficial for industrial
applications because of the ease of handling and purifica-
tion of products.
As shown in Scheme 2, the general design of the ECS-SB-
CuBr catalyst was first to modify expanded corn starch
(ECS) using 3-aminopropyltriethoxysilane to obtain an
amine-functionalized surface (ECS-NH ). Then, the for-
2
mation of Schiff base-modified ECS (ECS-SB) was
achieved through the Schiff base condensation between
ECS-NH with 2-pyridinecarboxaldehyde. ECS-SB, which
2
possessed bidentate nitrogen-binding sites, reacted with
copper cations, and ultimately yielded the copper-
incorporated heterogeneous catalyst (ECS-SB-CuBr).
ECS-SB-CuBr was characterized using various
methods, such as Fourier transform infrared (FT-IR)
spectroscopy, thermogravimetric analysis (TGA), powder
X-ray diffraction (XRD) and scanning electron micro-
scopy/energy-dispersive X-ray spectroscopy (SEM/EDX).
At first, ECS, ECS-SB and ECS-SB-CuBr were character-
ized using FT-IR spectroscopy (Figure 1). FT-IR spectros-
copy provided useful information on the reaction of the
NH2 group with 2-pyridinecarboxaldehyde. Compared
with ECS, the spectrum of ECS-SB contains two weak
In recent years, low-cost and green synthesis of
supported catalysts from biomass has attracted growing
[18,19]
interest.
In continuation of our efforts to develop
green, highly efficient and practical chemical methods as
well as a focus on the application of biomass-based het-
[20]
erogeneous catalysts in organic reactions,
we
−
1
bands at 1568 and 1589 cm , associated with pyridyl
[
21]
SCHEME 1 Oxidation of HMF to DFF
ring stretching vibrations.
In addition, the bands at
SCHEME 2 Synthesis of ECS-SB-CuBr

HONG ET AL.
3 of 8
chemical treatments, the catalyst still had high thermal
stability (Figure 2b–d).
Surface morphologies of the products at each chemi-
cal modification step are presented in Figure 3. As a
result of chemical treatment, each sample exhibited dif-
ferent morphological features, showing that modification
of the support materials was achieved. Cu element was
detected on the whole ECS-SB-CuBr surface (scanning
area in Figure 3d) using EDX. Figure 3f shows that the
distribution of Cu (red) element is uniform in the ECS-
SB-CuBr catalyst.
The XRD pattern of ECS-SB-CuBr is shown in
ꢀ
Figure 4. The broad peak at 10–30 can be attributed to
starch due to its amorphous nature. Three additional
FIGURE 1 FT-IR spectra of (a) ECS, (b) ECS-SB, (c) as-
prepared ECS-SB-CuBr and (d) recovered ECS-SB-CuBr after six
consecutive trials
peaks are clearly visible in the diffraction pattern of the
ꢀ
ꢀ
ꢀ
and 53.7
ECS-SB-CuBr at 2θ
=
27.1 , 45.0
corresponding to the (111), (220) and (311) lattice planes
77 cm− assigned to the Cu—N bond
1
[22]
stretching
of cubic CuBr,
[23]
implying the presence of CuBr.
4
vibration confirmed the interaction between nitrogen
atoms in the Schiff base and copper cations, suggesting a
Cu—N coordination in the as-synthesized ECS-SB-CuBr.
The thermal stability of catalysts is essential for
retaining their activity in longer reaction times, and ther-
mally stable catalysts are especially important for indus-
trial applications requiring high-temperature treatment.
The thermal stability of the ECS-SB-CuBr catalyst in each
step of production was investigated using TGA at a
Furthermore, the electronic state of the Cu element
in the catalyst was analyzed using X-ray photoelectron
spectroscopy (XPS). Figure 5 shows the binding energies
of Cu 2p for the as-prepared ECS-SB-CuBr catalyst. The
+
distinct peaks at 932 and 952 eV are ascribed to Cu 2p
3
/2
+
[24]
and Cu 2p1/2
,
respectively. The peaks at 934 and
[
25]
954 eV confirm the presence of Cu(II) species,
proba-
bly arising from oxidation of Cu during catalyst prepara-
+
[
26,27]
tion.
The XPS atomic percentages (N, 2.3%; Cu,
ꢀ
−1
heating rate of 10 C min in nitrogen over a range of
1.1%) show that the atomic ratio of nitrogen to copper is
about 2 in ECS-SB-CuBr, proving that one copper atom
coordinates with two nitrogen atoms in the ECS-SB-CuBr
catalyst.
ꢀ
3
0–800 C. Materials that are thermally resistant make
them ideal support materials for catalyst systems. ECS
exhibited marked thermal resistance as depicted in
ꢀ
Figure 2a (thermal degradation temperature of 259 C),
demonstrating the suitability of ECS as a catalyst support.
Also, thermograms corresponding to each chemical mod-
ification are presented in Figure 2. As seen, despite the
2.2 | Selective aerobic oxidation of HMF
to DFF
We continued our investigations by using the Cu-based
catalyst for selective aerobic oxidation of HMF to DFF
[17]
under ambient pressure. Based on the literature,
in
our preliminary experiment, a mixture of 1 mmol of
HMF, 10 mol% of TEMPO and 10 mol% of CuCl in
ꢀ
acetonitrile was stirred at 50 C under ambient pressure
of dioxygen for 24 h to afford DFF in 36% yield (Table 1,
entry 1). To examine the activity of other copper(I)
sources, Cu O, CuBr and CuI were used as catalysts.
2
CuBr showed the best yield of DFF (98%) (Table 1,
entries 2–4). Copper(II) or other metal catalysts such as
NiBr showed no catalytic activity. After the catalyst, var-
2
ious oxidants (air, tert-butyl hydroperoxide (TBHP) and
H O ) were evaluated, and from the results oxygen gas
2
2
appeared to be best along with CuBr (Table 1, entries
7–9). Various solvents including HAc, EtOH, water,
dichloroethane (DCE) and EtOAc and solvent-free
FIGURE 2 TGA curves of (a) ECS, (b) ECS-NH
d) as-prepared ECS-SB-CuBr and (e) recovered ECS-SB-CuBr after
six consecutive trials
2
, (c) ECS-SB,
(

4
of 8
HONG ET AL.
FIGURE 3 SEM images of (a) ECS,
(
b) ECS-NH
prepared ECS-SB-CuBr and
e) recovered ECS-SB-CuBr after six
2
, (c) ECS-SB, (d) as-
(
consecutive trials. EDX mapping of Cu
of (f) ECS-SB-CuBr and (g) recovered
ECS-SB-CuBr after six consecutive trials
conditions were further examined; acetonitrile was still
the optimum solvent. In order to allow recycling of the
copper salts, the supported ECS-SB-CuBr catalyst was
employed as heterogeneous catalyst. To our delight, a
complete conversion of HMF with excellent selectivity
The performance of the catalytic system was investi-
gated for a series of different conditions, varying the cata-
lyst and TEMPO dosage, reaction time and temperature
(Table 2). As can be seen, the addition of ECS-NH -CuBr,
2
CuBr or TEMPO alone did not show any conversion
(
98%) to DFF was achieved using 100 mg of ECS-SB-CuBr
(Table 2, entries 1–3). The blank reaction was also carried
ꢀ
with 10 mol% TEMPO after 24 h of reaction at 50 C in
acetonitrile (Table 1, entry 16).
out with only ECS, ECS-NH and ECS-SB supports as cat-
2
alysts and HPLC analysis indicated that there were no
FIGURE 4 XRD patterns of (a) as-prepared ECS-SB-CuBr and
FIGURE 5 Cu 2p XPS spectra of (a) as-prepared ECS-SB-CuBr
(
b) recovered ECS-SB-CuBr after six consecutive trials
and (b) recovered ECS-SB-CuBr after six consecutive trials

HONG ET AL.
5 of 8
a
TABLE 1 Optimization of aerobic oxidation of HMF using Cu-based catalysts
TEMPO
(mol%)
Conversion of HMF
Solvent (%)
Yield of DFF
(%)
Selectivity
(%)
b
Entry Catalyst
Oxidant
1
2
3
4
5
6
7
8
CuCl
CuBr
10
10
10
10
10
10
10
—
O
O
O
O
O
O
2
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
CN
CN
CN
CN
CN
CN
CN
CN
36
100
22
36
98
100
98
91
96
—
—
93
85
2
2
2
2
2
Cu
2
O
20
CuI
25
24
CuBr
2
Trace
Trace
15
Trace
—
NiBr
2
CuBr
CuBr
Air
14
TBHP (1.5
equiv.)
42
36
9
CuBr
—
H
2
O
2
(1.5
equiv.)
CH
3
CN
NR
—
—
10
11
12
13
14
15
16
CuBr
CuBr
10
10
10
10
10
10
10
O
2
O
2
O
2
O
2
O
2
O
2
O
2
AcOH
EtOH
9
3
33
20
—
94
91
92
98
20
4
CuBr
H
2
O
NR
31
—
29
30
77
98
CuBr
DCE
EtOAc
—
CuBr
33
CuBr
84
c
ECS-SB-CuBr
CH
3
CN
100
a
ꢀ
Reaction conditions: HMF (1 mmol), catalyst (10 mol%), solvent (5 ml), O (1 atm), 50 C, 24 h.
2
b
NR, no reaction.
c
ECS-SB-CuBr (100 mg).
a
TABLE 2 Effects of reaction parameters on aerobic oxidation of HMF catalyzed by ECS-SB-CuBr
Catalyst loading
(mg)
TEMPO
(mol%)
T
( C)
Time
(h)
Conversion of
HMF (%)
Yield of
DFF (%)
Selectivity
(%)
ꢀ
b
Entry Catalyst
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
—
—
—
10
10
—
10
10
10
10
10
10
6
50
50
50
50
50
50
50
50
50
50
50
40
80
50
50
24
24
24
24
24
24
24
24
24
24
24
24
24
18
20
NR
NR
NR
NR
NR
NR
70
—
—
c
CuBr
ECS-SB-CuBr 100
—
—
—
—
69
87
89
64
81
46
69
35
94
—
—
—
—
98
98
99
96
97
97
96
100
99
ECS
100
100
100
ECS-NH
2
ECS-SB
ECS-SB-CuBr 60
ECS-SB-CuBr 80
ECS-SB-CuBr 120
ECS-SB-CuBr 100
ECS-SB-CuBr 100
ECS-SB-CuBr 100
ECS-SB-CuBr 100
ECS-SB-CuBr 100
ECS-SB-CuBr 100
89
90
0
1
2
3
4
5
67
8
83
10
10
10
10
47
72
35
95
a
Reaction conditions: HMF (1 mmol), CH
3
CN (5 ml), O
2
(1 atm).
b
NR, no reaction.
c
CuBr (10 mol%).

6
of 8
HONG ET AL.
products (Table 2, entries 4–6). Therefore, CuBr was the
main catalytic center in the oxidation of HMF. As evident
from Table 2, the HMF conversion and DFF yield
increased with an increase of the catalyst amount, which
can be attributed to an increase in the availability and
number of catalytically active sites for the aerobic oxida-
tion of HMF. Furthermore, an increase in the amount of
ECS-SB-CuBr to 120 mg caused a decrease in the conver-
sion of this reaction to 90% (Table 2, entry 9), which can
be related to the fact that excess heterogeneous catalyst
may cause an issue with reactant interaction with active
sites and the mass transfer limitation leads to a decrease
in DFF yield. The conversion of HMF is elevated to 67%
when 6 mol% TEMPO was added (Table 2, entry 10). A
further increase in the amount to 8 mol% led to HMF
conversion of 83%. Decreasing the reaction temperature
FIGURE 6 Recycling activity of ECS-SB-CuBr in the
oxidation of HMF to DFF
ꢀ
to 40 C slowed the reaction, affording a 46% yield of DFF
(Figure 4a,b), which thus indicated that the present cata-
lyst system was thermally stable. XPS analysis was fur-
ther carried out to determine the composition and the
surface electronic state of ECS-SB-CuBr after six recycles
(Figure 5b). A similar Cu 2p band profile can be observed
after ECS-SB-CuBr had been used for six catalytic cycles,
which confirms the strong stability of the copper complex
during the catalytic process. All these results suggested
that there was no significant difference between the fresh
and the reused ECS-SB-CuBr catalysts.
(
8
Table 2, entry 12). When the reaction was conducted at
0 C, the yield of DFF was only 69%, which was presum-
ꢀ
ably due to the mass transfer resistance of oxygen from
oxygen gas on the liquid surface down to liquid solution.
ꢀ
Subsequently, the reaction was investigated at 50 C for
different periods of time (Table 2, entries 14 and 15). A
9
4% yield of DFF was obtained even when the reaction
time was reduced to 20 h. The data show that prolonging
the reaction time is favorable for an increase of both
HMF conversion and DFF yield. This implies that
increasing the reaction time is one way to obtain high
conversion and yield for low-activity catalysts.
3 | CONCLUSIONS
We have successfully developed a new, low-cost and
biomass-based heterogeneous catalyst, ECS-SB-CuBr, for
the first time. ECS-SB-CuBr was applied as an active cata-
lyst in oxidation of HMF to DFF. In acetonitrile, a 98%
yield of DFF was obtained after 24 h with ambient pres-
2
.3 | Reusability of catalyst
In a final set of experiments, we further assessed the sta-
bility and reusability of the ECS-SB-CuBr catalyst in the
oxidation reaction, because it is crucial to confirm that
the highly active catalyst is recyclable. As shown in
Figure 6, there was no discernible loss in activity or selec-
tivity. The results were in accordance with inductively
coupled plasma experiments because only trace amounts
of copper (90 ppb) were detected in the resultant solution
collected by hot filtration after 1 h.
Subsequently, the FT-IR spectrum of the reused ECS-
SB-CuBr catalyst was obtained, as shown in Figure 1d,
which revealed that the typical absorption peak of the
Cu—N bond was still present, and also the TGA curve of
the reused catalyst was similar to that of the fresh one
ꢀ
sure of dioxygen at 50 C. Furthermore, the introduced
catalyst was stable and could be reused in more than six
successive runs without loss of catalytic activity. To the
best of our knowledge, functionalized materials
supporting CuBr have not been explored as catalysts in
the oxidation of HMF to DFF and hence we believe that
the results of this study would make an important contri-
bution to the existing methods for DFF synthesis.
4 | EXPERIMENTAL
(Figure 2d,e). The surface morphology and EDX mapping
4.1 | Materials and instrumentation
image of Cu (red) element of the reused catalyst are
shown in Figure 3e,g, which indicated that the recycling
of the catalyst did not cause any significant change in the
morphology. In addition, the XRD pattern of the reused
catalyst was also similar to that of the fresh one
All chemicals were purchased from Energy Chemical
Company (Shanghai, China). High-amylose corn starch
was purchased from Guomin Starch Chemistry
(Shanghai, China) Co. Ltd.

HONG ET AL.
7 of 8
FT-IR spectra were recorded with a Nicolet 380 FT-IR
completion of the reaction, the mixture was cooled to
room temperature and filtered, and the remaining solid
was washed with acetonitrile to separate the catalyst.
Then, the recovered catalyst was dried and used in sev-
eral runs in the same manner as described for the first
run. Samples were collected and analyzed via HPLC.
instrument. TGA and differential thermal analyses were
carried out with a TG 209F1 thermal analyzer under
ꢀ
−1
nitrogen atmosphere at a heating rate of 10 C min .
SEM/EDX was conducted using a Quanta 200. Powder
XRD patterns of ECS-SB-CuBr were recorded with a Rig-
aku D/Max 2400 X-ray diffractometer using Cu Kα
(
λ = 0.15406 nm) radiation. XPS spectra were recorded
with a Kratos Axis Ultra DLD spectrometer, and the
4.4 | Product analysis
1
binding energy was calibrated using the C 1s peak. H
13
NMR and C NMR spectra were recorded with Bruker
DRX-600 instruments.
HMF and DFF were analyzed with HPLC (Agilent 1200)
using a reversed-phase C18 column (250 × 4.6 mm) at
ꢀ
2
5 C with a detection wavelength of 280 nm. The mobile
phase was acetonitrile and 0.1 wt% acetic acid aqueous
−
1
4
.2 | Synthesis of catalyst
solution (65:35 v/v) at 0.5 ml min . The HMF conver-
sion and DFF yield were expressed as mol% in terms of
the total amount of HMF. The amounts of HMF and DFF
in the samples were calculated by interpolation from cali-
bration curves. Calibration curves for the observed prod-
ucts were constructed by injecting known concentrations
of commercial reference products.
4
.2.1 | Synthesis of ECS-supported imine
ligand (ECS-SB)
ECS and ECS-NH were synthesized using our previously
2
[28]
reported procedure.
fied method reported by Xiao and co-workers.
-Pyridinecarboxaldehyde (1.3338 g, 12 mmol) was added
to a mixture of 3-aminopropyl-functionalized ECS
2.4445 g) in super-dry ethanol (60 ml) in a 100 ml
ECS-SB was prepared via a modi-
[
22]
2
4.5 | Product isolation
(
round-bottomed flask. The reaction mixture was stirred
under reflux for 24 h, and then filtered. The resulting
solid was washed with toluene and then further washed
with ethanol to remove any unreacted aldehyde. It was
dried in a freeze-dryer overnight to furnish the
corresponding ECS-SB material.
DFF was purified by flash column chromatography on
silica gel (eluent was petroleum ether–EtOAc, 7:3), where
a colorless band was collected and the solvent removed
1
in vacuo resulting in a white crystalline compound. H
NMR (600 MHz, CDCl , δ, ppm): 7.35 (s, 2H), 9.80 (s,
3
13
2H); C NMR (150 MHz, CDCl , δ, ppm): 119.35 (2C),
3
154.19 (2C), 179.20 (2C).
4
.2.2 | Synthesis of ECS-SB-CuBr
ACKNOWLEDGMENTS
This work was financially supported by the Open Foun-
dation of Jiangsu Key Laboratory of Biomass Energy and
Materials (JSBEM201915). This work was also financially
supported by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions (PAPD).
ECS-SB (2 g) was suspended in absolute ethanol (20 ml).
Then, copper bromide (1.92 g, 13 mmol) in absolute etha-
nol was added and the mixture was kept under refluxing
condition for 24 h. The reaction mixture was cooled to
room temperature. Finally, the resulting material, ECS-
SB-CuBr, was filtered out, washed thoroughly with etha-
nol and acetonitrile and freeze-dried.
ORCID
Mei Hong https://orcid.org/0000-0002-6098-0368
REFERENCES
4
.3 | General procedure for oxidation of
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