Nanobelt α-CuV2O6 with hydrophilic mesoporous poly(ionic liquid): A binary catalyst for synthesis of 2,5-diformylfuran from fructose

Article abstract of DOI:10.1039/c6cy02561g

Direct transformation of renewable biomass-derived carbohydrates into value-added chemicals usually involves a tandem reaction, and therefore demands catalysts with different versatile isolated active sites that can independently take action in each step. In this work, the task-specific binary catalyst nanobelt α-CuV₂O₆ with the mesoporous poly(ionic liquid) (MPIL) derived solid acid P(EVPI-Br) was constructed for the direct synthesis of 2,5-diformylfuran (DFF) from fructose. α-CuV₂O₆ gave a >99.9% DFF yield in the aerobic oxidation of 5-hydroxymethyl furfural (HMF) under atmospheric O₂ (130 °C, 3 h). The binary catalyst α-CuV₂O₆ with P(EVPI-Br) afforded a 63.1% DFF yield in the one-pot and one-step conversion of fructose into DFF (O₂, 135 °C, 3.5 h). Stepwise addition of the two catalyst units further increased the DFF yield to 76.1%. The hydrophilic surface of P(EVPI-Br) enabled the preferential adsorption of fructose on the acid sites whereas α-CuV₂O₆ showed strong adsorption of HMF but weak affinity to fructose. Such surface wettability-controlled adsorption features inhibited the oxidation of fructose and facilitated the transfer of HMF to the oxidative sites on α-CuV₂O₆ and its timely conversion into the final product, thanks to the high activity of α-CuV₂O₆. Moreover, facile catalyst recovery and good reusability were observed in the synthesis of DFF from both HMF and fructose. This work highlights the potential of manipulating the surface wettability of a binary catalyst towards an efficient multifunctional catalysis system for biomass-related tandem reactions.

Full text of DOI:10.1039/c6cy02561g

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Nanobelt α-CuV2O6 with hydrophilic mesoporous
poly(ionic liquid): a binary catalyst for synthesis of
2,5-diformylfuran from fructose†
Cite this: DOI: 10.1039/c6cy02561g
*
*
Wei Hou, Qian Wang, Zengjing Guo, Jing Li, Yu Zhou and Jun Wang
Direct transformation of renewable biomass-derived carbohydrates into value-added chemicals usually in-
volves a tandem reaction, and therefore demands catalysts with different versatile isolated active sites that
can independently take action in each step. In this work, the task-specific binary catalyst nanobelt
α-CuV2O6 with the mesoporous polyIJionic liquid) (MPIL) derived solid acid P(EVPI-Br) was constructed for
the direct synthesis of 2,5-diformylfuran (DFF) from fructose. α-CuV2O6 gave a >99.9% DFF yield in the
aerobic oxidation of 5-hydroxymethyl furfural (HMF) under atmospheric O2 (130 °C, 3 h). The binary
catalyst α-CuV2O6 with P(EVPI-Br) afforded a 63.1% DFF yield in the one-pot and one-step conversion of
fructose into DFF (O2, 135 °C, 3.5 h). Stepwise addition of the two catalyst units further increased the DFF
yield to 76.1%. The hydrophilic surface of P(EVPI-Br) enabled the preferential adsorption of fructose on the
acid sites whereas α-CuV2O6 showed strong adsorption of HMF but weak affinity to fructose. Such surface
wettability-controlled adsorption features inhibited the oxidation of fructose and facilitated the transfer of
HMF to the oxidative sites on α-CuV2O6 and its timely conversion into the final product, thanks to the high
activity of α-CuV2O6. Moreover, facile catalyst recovery and good reusability were observed in the synthesis
of DFF from both HMF and fructose. This work highlights the potential of manipulating the surface wetta-
bility of a binary catalyst towards an efficient multifunctional catalysis system for biomass-related tandem
reactions.
Received 7th December 2016,
Accepted 18th January 2017
DOI: 10.1039/c6cy02561g
rsc.li/catalysis
based biopolymers, pharmaceuticals, antifungal agents and
ligands.5,8 Although DFF can be obtained from the well
Introduction
Among the most abundant renewable resources, biomass car-
bohydrates are promising alternative feedstocks for the pro-
duction of fuels and chemicals. Currently, growing interest is
devoted to the transformation of biomass to value-added
chemicals that usually rely on the petroleum industry.1–9
Chemocatalysis is attractive as many chemicals can be pro-
duced from biomass through task-specifically designing effi-
cient catalytic processes.6 In this context, one significant task
is to explore high performance catalysts for the synthesis of
downstream platform compounds.7–9
known platform compound 5-hydroxymethylfurfural (HMF),10
direct synthesis of DFF from carbohydrates such as fructose
has received widespread attention because of the easy avail-
ability of the raw materials. Such consolidation of a multi-
step into a one-pot process accommodates the demand of
sustainable development due to the low energy requirement
and decreased waste release. Direct transformation of carbo-
hydrates into DFF involves the selective dehydration of carbo-
hydrates into HMF over acid sites and the successive oxida-
tion of HMF to DFF over redox sites.11 Several catalytic
systems using dioxygen (O2) have been developed and be-
come efficient in the one-pot and two-step approach.12–16
However, the one-pot and one-step strategy faces one major
bottleneck, i.e., the oxidation of carbohydrates to humins or
other undesired by-products due to the co-existence of acid
and redox sites.11,17 Moreover, the efficient transformation of
carbohydrates into DFF requires the rapid oxidation of HMF
to DFF so as to minimize the most possibly fast degradation
of the formed HMF. Owing to such difficulty, the more desir-
able one-pot and one-step process is still seldom reported.
2,5-Diformylfuran (DFF) is an important platform chemical
with wide potential applications as a precursor of furan-
State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Chemical Engineering, Nanjing Tech University (former Nanjing University of
Technology), Nanjing, 210009, PR China. E-mail: njutzhouyu@njtech.edu.cn,
junwang@njtech.edu.cn; Fax: +86 25 83172261; Tel: +86 25 83172264
† Electronic supplementary information (ESI) available: Fig. S1–S11, Tables S1–
S4 and Scheme S1. See DOI: 10.1039/c6cy02561g
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The oxidation of HMF to DFF has been extensively investi-
lytic performances of the binary catalyst are discussed in
depth by studying the influence of the surface wettability of
MPIL and the morphology of α-CuV2O6, as well as their ad-
sorption behaviors toward fructose and HMF.
gated over both homogeneous and heterogeneous catalysts,
including noble metal nanoparticles (e.g., Ru, Pd, Au and Pt),
transition metal oxides and carbons.18–27 Transition metal-
based materials are preferable due to their low costs and rela-
tively favorable activity. Many reports demonstrated that the Experiment
combination of V5+ and Cu2+ afforded excellent catalytic per-
Materials and methods
formance in the aerobic oxidation of HMF to DFF.23–27 For
All the chemicals (solvents and reagents) were of analytical
example, VOSO4/CuIJNO3)2 gave a 99% yield in the homoge-
neous aerobic oxidation of HMF.24 V2O5@Cu-MOR zeolite
gave a high DFF yield of 91.5% with good reusability in the
heterogeneous conversion of HMF into DFF.28 Cu/V-based
catalysts are also active in many typical chemical conversions
such as reduction,29 oxidation,30,31 photocatalysis,32,33
electrocatalysis,34 and so on. As a family of typical Cu/V-based
materials, copper vanadium oxides (CVOs) including CuV2O6,
Cu2.33V4O11 and Cu1.1V4O11 have been investigated as battery
materials for a long time because of their electrochemical
properties.35 Some CVOs were also utilized in solar thermo-
grade and used as received. X-ray diffraction (XRD) patterns
were collected using a Smart Lab diffractometer (Rigaku Cor-
poration) equipped with a 9 kW Cu rotating-anode source at
40 kV and 200 mA (10–70°, 0.2° s−1). The morphology was in-
vestigated using a field emission scanning electron micro-
scope (FESEM; Hitachi S-4800, accelerated voltage: 5 kV).
Transmission electron microscopy (TEM) images were
obtained by using a JEOL JEM-2100F 200 kV field-emission
transmission electron microscope. N2 sorption isotherms
were measured at 77 K using a BELSORP-MINI analyzer. Be-
fore measurement, the samples were degassed for 3 h at 100
°C. The surface chemical composition was determined by
X-ray photoelectron spectroscopy (XPS) using a PHI 5000
Versa Probe X-ray photoelectron spectrometer equipped with
Al Kα radiation (1486.6 eV). Fourier transform infrared (FTIR)
spectra were recorded on an Agilent Cary 660 FT-IR instru-
ment (KBr disks) ranging from 4000 to 400 cm−1.
Temperature-programmed reduction (H2-TPR) was carried
out using a glass flow system in flowing 10% H2/Ar (30 cm3
min−1) with a heating rate of 10 K min−1. The signal was
detected by using a Gow-Mac thermal conductivity detector
(TCD). Elemental analyses were performed using a Vario EL
cube CHN elemental analyzer. Thermogravimetric (TG)
curves were obtained using an STA409 instrument in dry air
atmosphere at a heating rate of 10 °C min−1. Solid state 13C
NMR spectra were obtained using a Bruker AVANCE-III
spectrometer in a magnetic field strength of 9.4 T corre-
sponding to the Larmor frequency of 100 MHz for 13C nuclei
with a CP/MAS unit. 1H NMR spectra were acquired using a
Bruker DPX 500 spectrometer by using D2O as the solvent
and TMS (tetramethylsilane) as the internal reference. Induc-
tively coupled plasma mass spectrometry (ICP) was carried
out using an OPTIMA 20000 V.
chemical water splitting,36 oxidation of toluene37 and SO3
−
decomposition.38 Nonetheless, application of CVOs as cata-
lysts is still limited, and even rare in liquid phase catalysis.
On the other hand, mesoporous polyIJionic liquid)s (MPILs)
containing ionic liquid units in the framework are promising
multifunctional polymers and have demonstrated various ad-
vantages in the catalysis field.39 Their structure, chemical
composition and porosity can be facilely adjusted through
designing specific monomers at the molecular level.40 Fur-
ther, their surface wettability can be facilely adjusted by
choosing the required cross-linkers, and this property is of
great significance in liquid-phase processes. Several
polyIJionic liquid)s have been used as heterogeneous catalysts
in biomass conversions and other reactions,41–44 but none of
them in conjunction with a redox catalyst is used in the di-
rect synthesis of DFF from carbohydrates.
In this work, we aim to develop a binary catalyst com-
posed of an acid and a redox unit for the direct synthesis of
DFF from fructose. The α-CuV2O6 nanobelt (denoted as
α-CuV2O6) is synthesized by a hydrothermal route and used
as the redox catalyst for the aerobic oxidation of HMF to DFF
under atmospheric O2. The MPIL-derived solid acid P(EVPI-
Br) with a hydrophilic surface is prepared through the free
radical copolymerization of the IL monomer [1-vinyl-3-
propane carboxylation imidazolium]Br and the hydrophilic
cross-linker ethyleneglycol dimethacrylate (EGDMA). The use
of α-CuV2O6 and PIJEVPI-Br), individually or as a binary cata-
lyst, in catalysis is systematically assessed and investigated
for the synthesis of DFF both from HMF and directly from
fructose, as well as the synthesis of HMF from fructose. The
results reveal that α-CuV2O6 can quantitatively convert HMF
to DFF (yield > 99.9%) within a short reaction time, and hy-
drophilic P(EVPI-Br) is active in the dehydration of fructose
to HMF. The binary catalyst α-CuV2O6 with P(EVPI-Br) proved
to be very efficient in the one-pot and one-step conversion of
fructose to DFF. Moreover, the catalysts can be facilely sepa-
rated by centrifugation and show stable reusability. The cata-
Catalyst preparation
Nanobelt-like α-CuV2O6 was synthesized by a hydrothermal
route (Scheme 1). Typically, 2 mmol NH4VO3 was dissolved in
20 mL water and stirred at 80 °C for 30 min, followed by the
addition of CuIJNO3)2 solution (1 mmol CuIJNO3)2 in 20 mL
water), immediately giving a yellow precipitate. The mixture
was stirred at 80 °C for 30 min with the suspension color
changing from yellow to orange. After that, the mixture was
transferred to a 50 mL Teflon-lined stainless steel autoclave
and maintained at 210 °C for 18 h. The faint yellow solid was
collected by centrifugation, washed with water and ethanol,
and dried at 80 °C for 10 h to give the final product. Using a
similar procedure, the counterpart samples CuxVy were
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was vacuumed, sheathed with an O2 balloon, and then
heated to the required temperature. The products were ana-
lyzed by using a gas chromatograph (GC, Agilent 7890B)
equipped with a flame ionization detector and a capillary col-
umn (HP-5, 30 m × 0.25 mm × 0.25 μm). The conversion of
HMF and selectivity to DFF was measured by using methyl
isobutyl alcohol as the internal reference.
The synthesis of HMF or DFF from fructose was carried
out using the same procedure but by replacing HMF with
fructose. The products were analyzed by GC analysis while
the conversion of fructose was monitored by liquid
chromatography-tandem mass spectrometry (LC-MS/MS). An
Agilent 1260 LC system equipped with an EC-C18 column
and equilibrated with 90% solvent A (water) and 10% solvent
B (methanol) was used for the chromatographic separation. A
G6420A mass spectrometer was used for the MS/MS analysis
and operated in positive ESI mode. 2 μL of the sample was
injected for each run. The mass spectral data of the precursor
and product ions were collected in multiple reaction-
monitoring (MRM) modes. Peak area determinations for all
samples, blanks, and standards were processed using the
Analyst software version B.06.00.
Scheme
1 Synthesis of α-CuV2O6 and MPIL P(EVPI-Br) for the
conversion of HMF and fructose into DFF.
prepared by varying the initial molar ratio of CuIJNO3)2 and
NH4VO3 (x : y). Bulk α-CuV2O6 (denoted as α-CuV2O6-B) was
prepared through a solid state grinding method.45 Briefly,
CuO and V2O5 with equal molar ratios were ground using an
agate mortar for 30 min, and then calcined at 620 °C for
48.5 h.
The IL monomer [1-vinyl-3-propane carboxylation
imidazolium]Br (VPI-Br) was synthesized as follows.46
1-Vinylimidazole (1.811 g, 0.02 mol) and 3-bromopropionic
acid (3.06 g, 0.02 mol) were dissolved in 20 mL ethanol, and
then stirred at 75 °C for 24 h. After that, a white solid formed
by adding acetonitrile was separated by rotary evaporation,
washed with acetonitrile and vacuum dried at 50 °C for 24 h
to give the final product VPI-Br (90% yield). Elemental analy-
sis calcd: N, 11.24 wt%; C, 38.86 wt%; H, 4.42 wt%; found: N,
11.24 wt%; C, 38.81 wt%; H, 4.36 wt%. 1H NMR (300 MHz,
D2O, TMS) (ppm) = 3.01 (t, 2H, –CH2), 4.72 (t, 2H, –CH2),
5.45 (m, 1H, –CH), 5.81 (m, 1H, –CH), 7.17 (m, 1H, –CH),
7.63 (s, 1H, –CH), 7.79 (s, 1H, –CH), 9.12 (s, 1H, –CH)
(Fig. S1†).
The mesoporous polyIJionic liquid) (MPIL) sample P(EVPI-
Br) was prepared through free radical copolymerization of
the IL monomer VPI-Br and ethyleneglycol dimethacrylate
(EGDMA) by using azobisisobutyronitrile (AIBN) as the initia-
tor (Scheme 1). VPI-Br (1.235 g, 5 mmol), ED (1.09 g, 5.5
mmol) and AIBN (0.09 g, 0.55 mmol) were dissolved in a mix-
ture solution containing ethyl acetate (25 mL), ethanol (5
mL) and water (5 mL), and then the solution was stirred at
80 °C for 24 h. A white solid was collected by filtration,
washed with water and ethanol, and then dried in air. Simi-
larly, the MPIL sample P(DVPI-Br) was prepared through the
copolymerization of VPI-Br (1.235 g, 5 mmol) and divinyl-
benzene (DVB, 0.716 g, 5.5 mmol) (Scheme S1†). Sulfonic
acid-modified SBA-15 (SBA-15-SO3H) was prepared according
to the literature.47
The catalyst reusability was investigated by conducting a
five-run recycling test. After reaction, the catalyst was sepa-
rated by centrifugation, washed and dried at 100 °C for 6 h,
and then reused in the next run.
Adsorption of reactants and intermediates
The adsorption of fructose and HMF were studied by means
of a reaction tube. Typically, P(EVPI-Br) (0.09 g), adsorbed
species (1 mmol) and DMSO (4 mL) were stirred in the reac-
tor at room temperature for a quarantine period. After that,
the solid was removed by filtration and the filtrate was ana-
lyzed by LC-MS/MS. The adsorption amounts were deter-
mined from the variation in adsorbate concentration.
Results and discussion
Synthesis and characterization of α-CuV2O6
Scheme 1 shows the synthetic procedure for α-CuV2O6. The
SEM image (Fig. 1A) indicates the nanobelt-like morphology
with a width of 50–200 nm, which is further illustrated by the
TEM image (Fig. 1C). The representative high resolution TEM
image along the [110] crystal planes (Fig. 1D) displays the
clear lattice fringe perpendicular to the longitudinal direction
of the nanobelt with an interplanar distance of 0.33 nm.35
The XRD pattern (Fig. 2A) demonstrates the well crystallized
phase of α-CuV2O6 (JCPDS no.30-0513). The crystal structure
of α-CuV2O6 consists of VO6 octahedral double layers. The
Cu2+ ions in the interlayer space coordinate with six oxygen
atoms to form CuO6 octahedral chains and Cu–O–V units
(Scheme 1). ICP analysis shows that the molar ratio of Cu to
V is 1 : 2 (Table S1†), which is in good accordance with the
formula of α-CuV2O6.
Catalytic test
The synthesis of DFF from HMF was carried out in a 25 mL
glass tube capped with a balloon filled with the required gas.
Typically, HMF (1 mmol, 126 mg) was dissolved in DMSO
(4 mL), followed by the addition of the catalyst. The reactor
The above α-CuV2O6 sample is obtained with a Cu/V ratio
of 1/2 with CuIJNO3)2 and NH4VO3 as precursors. The
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ratio of CuIJNO3)2 and NH4VO3 is crucial for the synthesis of
α-CuV2O6.
Additional characterization studies on α-CuV2O6 are
performed. The TG curve (Fig. 2B) reveals excellent thermal
stability. The mass loss at low temperatures comes from wa-
ter desorption. The N2 sorption isotherm is of type IV
(Fig. 2C), suggesting its mesoporous structure. The surface
area is 43 m2 g−1 with a pore volume of 0.09 cm3 g−1. The XPS
survey spectrum reflects the existence of Cu and V species,
based on the V 2p and Cu 2p XPS spectra in Fig. 2D and E.
The two peaks for V2p 3/2 (517.3 eV) and V2p 1/2 (524.7 eV)
correspond to V5+ species. The signals at 934.7 and 954.5 eV
are ascribed to Cu2+ 2p 3/2 and 2p 1/2, respectively. The
strong satellite peak at around 942 eV is a typical characteris-
tic of Cu2+ species.35 Therefore, Cu and V in α-CuV2O6 exist
in the form of Cu2+ and V5+. Fig. 2F shows the H2-TPR pro-
files of α-CuV2O6 plus the three controls CuO, V2O5 and bulk
α-CuV2O6 (α-CuV2O6-B). α-CuV2O6-B possesses the same crys-
talline phase as α-CuV2O6 (Fig. S4†) but exhibits a different
morphology of aggregated irregular particles at the μm level
(Fig. S5†). CuO exhibits two broad peaks at low reduction
temperatures centered at 300 and ∼380 °C, attributable to
the two step reduction of the Cu2+ species, i.e. the reduction
of Cu2+ to Cu+ at low temperatures and then to Cu0 at high
temperatures. V2O5 displays reduction peaks at high tempera-
Fig. 1 SEM images of (A) α-CuV2O6 and (B) PIJEVPI-Br); (C and D) TEM
images (inset: the lattice fringe) of α-CuV2O6.
tures of ∼675 °C for V2O5 → V6O13, ∼705 °C for V6O13
→
V2O4 and ∼780 °C for V2O4 → V2O3.48 α-CuV2O6 shows a
higher reduction temperature than CuO and a lower one
compared to V2O5, implying that the presence of Cu species
promotes the reduction of V species due to the formation of
a Cu–O–V structure.49 α-CuV2O6-B demonstrates a higher re-
duction temperature than α-CuV2O6, suggesting that the
α-CuV2O6 nanobelt is more easily reduced. The reduction be-
comes easier because of the dramatic increase of exposed lat-
tice oxygen when the particle size of nanocrystals decreases.50
This is one reason why α-CuV2O6 with the small-sized nano-
belt morphology is more easily reduced. Further, in a multi-
metal catalyst, the electronegativity of the metals significantly
affects the reduction properties through the M1–O–M2 link.51
Thus in this work, α-CuV2O6 shows a lower reduction temper-
ature than V2O5 because the presence of Cu species promotes
the reduction of V species by forming a Cu–O–V structure.
The electrons can be captured by Cu2+ through the Cu–O–V
link due to the higher electronegativity of Cu (1.9) than that
of V (1.63). Consequently, the reduction of V–O in α-CuV2O6
is easier than that in pure V2O5.
Fig. 2 (A) XRD pattern, (B) TG curve, (C) N2 sorption isotherm (inset:
pore size distribution curve), and (D) V 2p and (E) Cu 2p XPS spectra of
α-CuV2O6; (F) H2-TPR profiles of α-CuV2O6 and controls.
influence of different feeding ratios (Cu/V = 2/1, 1/1, and 1/4)
is investigated, respectively yielding the samples Cu2V1,
Cu1V1 and Cu1V4. The XRD patterns indicate that these sam-
ples have different crystalline phases (Fig. S2†). Besides the
α-CuV2O6 phase, there exist other phases of volborthite such
as Cu3IJOH)2V2O7·2H2O, Cu2.33V4O11 and Cu0.4V2O5 in Cu2V1,
Cu1V1 and Cu1V4, respectively (Table S1†). Correspondingly,
they demonstrate a mixed morphology of spheres and nano-
belts (Fig. S3†). These results suggest that a suitable feeding
Synthesis and characterization of MPIL P(EVPI-Br)
The MPIL sample P(EVPI-Br) is synthesized through a copoly-
merization process (Scheme 1). The SEM image reveals that
P(EVPI-Br) is composed of aggregates of nanoparticles
(Fig. 1B). Fig. 3A depicts the solid state 13C NMR spectrum.
The chemical shifts at 24 and 28 ppm correspond to the
methyl unit that is connected with the carbon atoms in the
alkyl chain. The broad overlapping peak at 31 ppm is
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The type IV nitrogen sorption isotherm and the correspond-
ing pore size distribution curve of P(EVPI-Br) in Fig. 3D reveal
the mesoporous structure. P(EVPI-Br) has a BET surface area
of 74 m2 g−1, total pore volume of 0.09 cm3 g−1 and average
pore size of 3.8 nm (Table 1). The contact angle is 0° for
PIJEVPI-Br), suggesting its super hydrophilicity (Fig. S6† and
Table 1). For comparison, hydrophobic MPIL P(DVPI-Br) is
prepared by using the hydrophobic cross-linker DVB and the
same IL monomer. Its structure is confirmed by the 13C NMR
spectrum (Fig. S7†), elemental analysis (Table S2†) and the
nitrogen sorption experiment (Fig. S8† and Table 1). P(DVPI-
Br) possesses a similar IL content and surface area to P(EVPI-
Br) but exhibits a high contact angle of 120° (Fig. S6† and
Table 1), reflecting its strong hydrophobicity.
Fig. 3 (A) 13C MAS NMR spectrum, (B) FTIR spectrum, (C) TG curve
and (D) N2 sorption isotherm (inset: pore size distribution curve) of
PIJEVPI-Br).
Aerobic oxidation of HMF to DFF
The nanobelt α-CuV2O6 is used as the heterogeneous catalyst
in the aerobic oxidation of HMF to DFF under atmospheric
O2 (Table 2). Auto-oxidation of HMF in the absence of a cata-
lyst only gives a low DFF yield of 6.0% and a selectivity of
30.0% (entry 1). V2O5 can catalyze the reaction, giving a DFF
yield of 65.8%. Cu–V bimetallic catalysts improve the activity
(entries 3–8). The DFF yield and selectivity firstly increase
and then decrease with elevated Cu content in the catalyst.
The highest yield of >99.9% (selectivity >99.9%) is attained
with α-CuV2O6 (entry 5). CuO shows a low DFF yield of 36.6%
and an inferior selectivity of 45.3% (entry 9). The mixture of
CuO and V2O5 with the same Cu and V content as α-CuV2O6
gives a DFF yield of 78.2% (entry 10), which is higher com-
pared to CuO or V2O5, but still inferior to α-CuV2O6.
Such a phenomenon suggests that the remarkable activity
of α-CuV2O6 comes from the special bimetallic synergistic
effect in the Cu–O–V unit rather than the simple combined
activities of CuO and V2O5. α-CuV2O6-B exhibits much
lower activity (yield: 41.2%, selectivity: 73.2%, entry 11)
than nanobelt α-CuV2O6, implying that a bulky particle plays
a detrimental role. Compared with α-CuV2O6-B, α-CuV2O6
which has a smaller particle size, exhibits good dispersion in
the reaction solution (Fig. S9†), benefiting from the better ac-
cessibility of the active sites. Further, the lower reduction
attributable to C12 and C9. The peak at 44 ppm is indexed to
the C7 and C8 atoms. The peaks at 120, 128 and 138 ppm are
ascribed to the C13, C4 and C3 atoms in the imidazole ring.
The signal at 145 ppm comes from the C6 atom connected to
the imidazole ring. The broad overlapping peak at 176 ppm
is indexed to the two carbon atoms attached to the oxygen. A
very weak and overlapped signal at 174 ppm is observable,
implying the presence of a carboxyl carbon atom (C2) in the
framework.52 All of these signals confirm the polymeric struc-
ture. In the FTIR spectrum (Fig. 3B), the bands at 1721 and
1167 cm−1 are ascribed to the CO and C–O stretching vibra-
tions in the ester group.53 The band at 1643 cm−1 evidences
the existence of the imidazolium moiety.54 The IL content of
P(EVPI-Br) is 2.63 mmol g−1, calculated from the elemental
analysis (Table S2†). The TG curve shows that P(EVPI-Br)
starts to decompose at a relatively high temperature of >210
°C, implying a good thermal stability (Fig. 3C). The weight
loss at temperatures ranging from 210 to 325 °C indicates the
decomposition of the imidazolium unit. The successive deg-
radation of the polymeric network happens at 325–450 °C.
Table 1 Textural properties and catalytic performance in the conversion of fructose into HMFa
Contact angle (°) Acid siteb (mmol g−1
)
SBETc (m2 g−1
)
Vpd (cm3 g−1
)
Catalyst dosage (g) YHMF (%) YDFFf (%)
e
Entry Catalyst
1
2
3
4
5
None
P(EVPI-Br)
P(DVPI-Br)
Amberlyst-15 70
SBA-15-SO3H 20
—
0
120
—
—
74
126
45
690
—
—
14.3
3.4
2.63
2.61
4.3
0.13
0.24
0.31
0.99
0.08
0.08
0.045
0.082
83.2/71.2g
93.1/83.1g
86.1
1.2/10.3g
1.6/8.9g
1.7
2.7
76.7
1.2
a
b
Reaction conditions: fructose (1 mmol), catalyst (0.21 mmol H+), solvent (DMSO, 3 mL), 130 °C, 1 h, N2 balloon volume. The number of
c
d
e
acid sites is calculated based on the S or N content that is measured by elemental analysis. BET surface area. Total pore volume. GC yield
of HMF. f GC yield of DFF. g Oxygen balloon.
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Table 2 Aerobic oxidation of HMF to DFFa
Entry
Catalyst
Catalyst dosage (g)
YDFF (%)b
Sel. (%)c
1
None
—
6
30
2
3
4
V2O5
Cu2V1
Cu1V1
α-CuV2O6
α-CuV2O6
α-CuV2O6
Cu1V4
CuO
CuO + V2O5
α-CuV2O6-B
0.028
0.04
0.04
0.04
0.04
0.04
0.04
0.012
0.012 + 0.028
0.04
65.8
82.6
89.1
>99.9
>99.9
16.3
70.5
36.6
78.2
41.2
68.3
90.2
92.2
>99.9
>99.9
83.1
76.7
45.3
78.3
73.2
5
6d
7e
8
9
10
11
a
b
c
Reaction conditions: HMF (1 mmol), solvent (DMSO, 4 mL), O2 balloon (1 bar), 130 °C, 3 h. GC yield of DFF. Selectivity to DFF = mmol
[DFF]/mmol [initial HMF] × 100%. d In the presence of butylated hydroxytoluene (BHT, 0.1 mmol). e N2 balloon.
temperature of α-CuV2O6 than that of α-CuV2O6-B suggests
more oxygen mobility, which also facilitates the activity.
Fig. 4 demonstrates the activity of α-CuV2O6 under differ-
ent reaction conditions. Various solvents are used in the reac-
tion (Fig. 4A). DMSO gives the best performance while the
others present inferior activities. This can be attributed to its
high boiling point and high polarity. Earlier research
suggested that the oxidation of HMF with O2 in polar sol-
vents was better than that in non-polar ones due to the im-
proved dissolution of O2 in the polar solvent.21 Moreover,
DMSO is known to promote dehydration of fructose and sta-
bilize HMF, and thus one-pot conversion of fructose to DFF
was developed by using DMSO.55 The influence of tempera-
ture is investigated from 100 to 140 °C by using DMSO
(Fig. 4B), indicating that the medium temperature of 130 °C
affords the highest yield. The yield as a function of reaction
time is of a “volcano” type (Fig. 4C) and the highest yield is
obtained at 3 h. Fig. 4D shows the influence of the catalyst
amount. A low catalyst amount gives low conversion and
selectivity, and the highest activity is attained by using 0.04 g
catalyst.
Reaction under a N2 atmosphere is carried out to under-
stand the catalytic behavior of α-CuV2O6. DFF is produced
with a yield of 16.3% and a selectivity of 83.1% (Table 2,
entry 7), suggesting that the lattice O species take part in the
reaction. The XPS analyses (Fig. S10†) indicate that the recov-
ered sample after the reaction under an O2 atmosphere
returns back to its initial highest valence state (Cu2+ and
V5+). By contrast, in the XPS spectra of the sample isolated
from the N2-mediated reaction, Cu+ and V4+ species are ob-
servable, accompanied by the decreased intensity of the sig-
nal at 530.7 eV for the O species in Cu–O–V units.46 Such XPS
comparison suggests that the lattice O in the Cu–O–V units is
involved in the catalysis process, causing the reduction of
Cu2+–O–V5+ to Cu+–□–V4+ (□ represents an oxygen vacancy).49
Besides, no deactivation is observed in the presence of butyl-
ated hydroxytoluene (BHT, 20 wt%), a free radical quencher
(Table 1, entry 6), thus excluding the possible free radical in-
termediates in the reaction.
Based on these phenomena and previous corresponding
studies,49,56,57 a potential Mars–van Krevelen type mechanism
as shown in Scheme 2 is proposed for the α-CuV2O6 catalyzed
aerobic oxidation of HMF to DFF. Firstly, HMF is adsorbed
on the surface of α-CuV2O6 to form an adsorbed alcoholate.
Lattice O in Cu–O–V units inserts into the activated HMF, re-
ducing V5+ to V4+ and Cu2+ to Cu+. Then the nucleophilic re-
action between –OH and C+ generates DFF and water. The re-
oxidation of Cu+–□–V4+ with O2 regenerates the Cu2+–O–V5+
species. The Cu–O–V units promote the mobility of the O spe-
cies near V. The co-existence of the Cu2+ species is able to im-
prove the adsorption of HMF to form CuIJII)–HMF intermedi-
ates, which plays a dominant role in the conversion of
Fig. 4 Influence of (A) solvents, (B) reaction temperature, (C) time and
(D) catalyst amount on the α-CuV2O6 mediated oxidation of HMF to
DFF. Reaction conditions: HMF (1 mmol), α-CuV2O6 (0.04 g), solvent (4
mL), O2 balloon (1 bar), 130 °C, 3 h. The reaction temperature and time
are 90 °C and 6 h, respectively, for (A). The solvent is DMSO for (B), (C)
and (D).
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P(EVPI-Br) with α-CuV2O6 affords DFF from fructose with a
high yield of 63.1% in this one-pot and one-step conversion
process (Table 3, entry 4).
In a heterogeneous catalytic process, the surface wettabil-
ity of the solid catalyst plays an important role in the accessi-
bility of reactants and successive desorption of products.58,59
For comparison, a parallel test is performed on three other
solid acids SBA-15-SO3H, Amberlyst-15 and hydrophobic MPIL
P(DVPI-Br) with contact angles of 20, 70 and 120°, respec-
tively (Fig. S6† and Table 1). Other structural information on
SBA-15-SO3H and Amberlyst-15 is presented in Fig. S8,†
Table 1 and S2.† The DFF yield continuously decreases as the
hydrophobicity of the solid acid increases (Table 3), although
all of them can efficiently catalyze the first step (i.e., fructose
into HMF, Table 1). In particular, compared with hydrophilic
PIJEVPI-Br), hydrophobic P(DVPI-Br) exhibits high activity in
the conversion of fructose to HMF, but shows a much lower
DFF yield of 33.5% in the one-pot and one-step conversion of
fructose to DFF. These two MPIL-based solid acids exhibit
the same type and density of acidic sites plus a similar sur-
face area, thus their different activities can be ascribed to the
discrepancy in their surface wettability. Such comparison
clearly reveals the improvement of the hydrophilic surface for
this tandem reaction. Further, the special IL unit in P(EVPI-
Br) enables the co-existence of H+ and Br−, which was proved
to be a promoting parameter for the formation of DFF from
fructose.55 This also accounts for the superior performance
of P(EVPI-Br) to SBA-15-SO3H and Amberlyst-15.
Fig. 5 shows the performance of the binary catalyst
PIJEVPI-Br)–α-CuV2O6 in the one-pot and one-step conversion
of fructose into DFF under different conditions. The results
indicate that the dosage of the two catalyst components af-
fects the reaction, demonstrating “volcano”-type profiles. This
can be due to the fact that the rate of creating the HMF inter-
mediate should match the rate of the successive oxidation
step in a tandem reaction process. The temperature signifi-
cantly affects the rates of both reactions, thus presenting a
sharp “volcano”-type variation tendency. The profiles for the
yields of HMF and DFF as a function of reaction time clearly
Scheme
2 Possible reaction route for the α-CuV2O6 catalyzed
aerobic oxidation of HMF to DFF with O2.
HMF.57 Further, the Cu2+ species also prohibit the oxidative
cleavage of the C–C bond,25 thus favoring a high DFF yield.
Also, the nanobelt morphology promotes the mass transfer
and the mobility of the O species. All of these result in a high
DFF yield over α-CuV2O6.
One-pot and one-step conversion of fructose into DFF
Direct synthesis of DFF from fructose is performed by using
the binary catalyst P(EVPI-Br) with α-CuV2O6, in which the
former is used as a solid acid for the dehydration of fructose
to HMF while the latter catalyzes the in situ conversion of
HMF to DFF. P(EVPI-Br) is a hydrophilic acidic MPIL material
tethered with –COOH groups that can efficiently produce
HMF from fructose under both N2 and O2 atmospheres
(Table 1). The combination of P(EVPI-Br) and α-CuV2O6 is
applied in the one-pot and one-step conversion of fructose
into DFF (Table 3). In the absence of a catalyst, fructose can
be converted but the DFF yield is low (entry 1). Further, a still
low DFF yield is observed by using either α-CuV2O6 or
P(EVPI-Br) (entries 2 and 3). Notably, the binary catalyst
Table 3 Synthesis of DFF from fructosea
Catalyst dosage (g)
b
c
d
CFRU
YHMF
YDFF
Acid catalyst
α-CuV2O6
Acid catalyst
(%)
(%)
(%)
None
None
—
0.05
—
0.05
0.05
0.05
0.05
—
—
86
91
12.3
4.6
37.2
0.8
3.7
2.3
5.6
9.3
P(EVPI-Br)
P(EVPI-Br)
P(DVPI-Br)
Amberlyst-15
SBA-15-SO3H
0.09
0.09
0.09
0.036
0.054
100
100
100
100
100
15.2
63.1
33.5
36.4
46.9
3.6
a
b
Reaction conditions: fructose (1 mmol), solvent (DMSO, 3 mL), O2 balloon (1 bar), 135 °C, 3.5 h. Conversion of fructose determined by LC-
MS/MS analysis. c GC yield of HMF. d GC yield of DFF.
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version of fructose and simultaneous formation of HMF with
a yield of 83.2% at 1 h over P(EVPI-Br) under atmospheric N2
conditions. The second step is the further addition of
α-CuV2O6 without catalyst separation under atmospheric O2
conditions, affording DFF from the aerobic oxidation of
HMF. The overall yield of DFF is 76.1% at 4.5 h based on
fructose.
Catalyst recycling
The catalytic reusability of α-CuV2O6 and PIJEVPI-Br)–α-
CuV2O6 is investigated in the aerobic oxidation of HMF to
DFF and the direct synthesis of DFF from fructose in a one-
pot and one-step process, respectively (Fig. 7). In a five-run
recycling test, a decline of the activity is rarely observed in
the α-CuV2O6 catalyzed conversion of HMF to DFF, reflecting
its stable reusability. The XRD pattern and SEM image of the
recovered α-CuV2O6 after the 5th run (Fig. S11†) are almost
the same as those of the fresh one. Neither Cu nor V in the
reacted solution is detectable by ICP. Furthermore, a three-
run recycling test with low conversion and yield is carried out
by reducing the reaction time to 1 h (Fig. S12†). No apparent
decrease in conversions and yields is observed, confirming
the excellent reusability of the catalyst. All of these support
the good stability of α-CuV2O6. The recycling of PIJEVPI-Br)–α-
CuV2O6 in the tandem reaction is performed at 135 °C by
using 50 mg of α-CuV2O6 and 90 mg of PIJEVPI-Br). After the
reaction, the binary catalyst was separated by centrifugation,
washed with water and ethyl alcohol, dried at 100 °C, and
then reused in the next run. A stable DFF yield is obtained in
the five-run recycling test, reflecting the good reusability of
the binary catalyst.
Fig. 5 Catalytic behavior of PIJEVPI-Br)–α-CuV2O6 in the one-pot and
one-step conversion of fructose into DFF. Reaction conditions: fruc-
tose (1 mmol), α-CuV2O6 (0.04 g), P(EVPI-Br) (0.09 g), O2 balloon (1
bar), solvent (DMSO, 3 mL), 130 °C, 3 h. (A) The amount of P(EVPI-Br)
is varied by using 0.04 g α-CuV2O6; (B) the amount of α-CuV2O6 is
varied by using 0.09 g PIJEVPI-Br); (C) the reaction temperature is var-
ied with a reaction time of 3 h; (D) the reaction time is varied at 135 °C
(dotted line: the hot filtration result). The amount of α-CuV2O6 is 0.05
g for (C) and (D).
reflect the tandem reaction feature involving the formation of
HMF and its oxidation to DFF.
In a separate test, the reaction is stopped at 2.5 h by im-
mediate hot filtration to remove the binary catalyst. The fil-
trate solution is further put into the reactor to continue the
reaction under the initial conditions. The dotted line in
Fig. 5D shows that the yield no longer increases with time,
confirming the heterogeneous nature of the catalyst.
Insight into the reaction
The binary catalysts P(EVPI-Br) and α-CuV2O6 are further
investigated in the one-pot and two-step synthesis of DFF
from fructose. Fig. 6 shows the plots of HMF and DFF yields
as a function of reaction time. The first step is the whole con-
In a heterogeneous tandem reaction, by using a binary cata-
lyst, adsorption of reactants and intermediates over different
catalyst components will play a vital role, because adsorption
reflects the accessibility and diffusion of the reactants to the
active sites, which is a prerequisite for the smooth progress
of the whole reaction.55,60 The adsorption of fructose and
HMF is investigated to gain deep understanding of the
Fig. 7 Recyclability of (A) α-CuV2O6 in the oxidation of HMF and (B)
PIJEVPI-Br)–α-CuV2O6 in the one-pot and one-step conversion of fruc-
tose into DFF. Reaction conditions for A: HMF (1 mmol), α-CuV2O6
(0.04 g), solvent (DMSO, 4 mL), O2 balloon (1 bar), 130 °C, 3 h. Reac-
tion conditions for B: fructose (1 mmol), α-CuV2O6 (0.05 g), P(EVPI-Br)
(0.09 g), O2 balloon (1 bar), solvent (DMSO, 3 mL), 135 °C, 3.5 h.
Fig. 6 One-pot and two-step conversion of fructose into DFF. Reac-
tion conditions: fructose (1 mmol), P(EVPI-Br) (0.08 g), α-CuV2O6 (0.04
g), solvent (DMSO, 3 mL), 135 °C. The 1st step was under a N2 balloon
(1 bar) and the 2nd step was under an O2 balloon (1 bar) with the addi-
tion of α-CuV2O6.
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behavior of PIJEVPI-Br)–α-CuV2O6 in the one-pot and one-step
conversion of fructose into DFF. We firstly conducted the ad-
tability. The surface wettability of solid acids greatly affects
the adsorption of fructose and a hydrophilic surface favors
high adsorption. This can be attributed to the fact that fruc-
tose contains abundant hydroxyl groups, and thus is prefer-
entially adsorbed on the hydrophilic surface.5,61 It should be
noticed that though Amberlyst-15 is more hydrophilic than
PIJDVPI-Br), it exhibits weaker adsorption of fructose due to
its low surface area. During the adsorption of HMF, SBA-15-
SO3H which has the largest surface area gives the highest ad-
sorption amount, while Amberlyst-15 which has a low surface
area exhibits weak adsorption. In general, a large surface area
and pore volume favor a high adsorption amount. As a result,
adsorption depends on both the surface wettability and sur-
face area. The hydrophilic sample with a large surface area
usually exhibits high adsorption for fructose. Here, in order
to gain insight into the influence of surface wettability, fur-
ther comparison is made between the two MPILs P(EVPI-Br)
and PIJDVPI-Br), which possess similar chemical compositions
and surface areas but dramatically different surface wettabil-
ities. Hydrophilic P(EVPI-Br) exhibits stronger adsorption for
fructose but weaker adsorption for HMF than hydrophobic
PIJDVPI-Br). This result apparently shows that the preferential
adsorption of fructose on P(EVPI-Br) is closely related to the
surface wettability. The hydrophilic surface of P(EVPI-Br) is
thus responsible for the special adsorption behavior of the
PIJEVPI-Br)–α-CuV2O6 catalysis system and its catalytic
performance.
The robust activity of α-CuV2O6 in the oxidation of HMF
to DFF also accounts for the superior performance of the bi-
nary catalyst in the synthesis of DFF from fructose. Table S3†
summarizes the typical and most efficient heterogeneous cat-
alysts in the aerobic oxidation of HMF to DFF under atmo-
spheric pressure. By comparison, α-CuV2O6 exhibits compara-
ble activity under similar or even milder conditions.11,14,15,30
Noticeably, a quantitative yield of DFF is achieved under am-
bient O2 pressure within a short reaction time of 3 h, which
is the shortest time under identical conditions so far.19–23
Further, a lower catalyst dosage of α-CuV2O6 is required com-
pared with the previous efficient catalyst K-OMS-2.21 Such
rapid and selective oxidation of HMF to DFF enables the
timely conversion of the formed HMF into DFF in a one-pot
and one-step process, without much further degradation of
HMF. Consequently, a high yield can be obtained at 130–135
°C for 3–3.5 h; the application of this medium temperature
also prevents the potential decomposition of DMSO into
Me2SO2 and Me2S at high temperatures above 150 °C.11 Com-
pared with previous works (Table S4†), our binary catalyst
can attain comparable activity with stable reusability under
mild conditions.
sorption study by using
a single component solution
containing fructose or HMF (Fig. 8A and B). The acid cata-
lysts show different adsorption amounts of fructose in the or-
der P(EVPI-Br) > SBA-15-SO3H > P(DVPI-Br) > Amberlyst-15,
and HMF in the order SBA-15-SO3H > P(DVPI-Br) > P(EVPI-
Br) > Amberlyst-15. It is clear that P(EVPI-Br) can preferen-
tially adsorb fructose but presents low affinity towards HMF.
Such favorable adsorption enables the facile access of fruc-
tose towards the acidic sites and then its further conversion
to HMF. Moreover, α-CuV2O6 exhibits low adsorption of fruc-
tose but preferential adsorption of the HMF intermediate
(Fig. 8B). This adsorption behavior will inhibit the access of
fructose towards the oxidative sites, and thus weaken the oxi-
dation of fructose. The preferential adsorption of HMF over
α-CuV2O6 will improve the access of the formed HMF towards
the oxidation sites and its subsequent transformation to DFF
in this tandem reaction. Briefly, the above advantageous ad-
sorption behaviors over the binary catalyst P(EVPI-Br) with
α-CuV2O6 effectively weaken the interference of the two reac-
tions involved in one-pot, thus giving a high DFF yield.
During the reaction, both fructose and HMF exist, and
thus we further carry out a competitive adsorption test by
using a two-component solution containing both of them. A
similar adsorption behavior is observed to the situation using
a single component solution, albeit a slightly lower adsorp-
tion amount is observed over each catalyst unit
(Fig. 8C and D). This phenomenon suggests that preferential
adsorption still happens in the real reaction system, which
could have promoted the performance of the binary catalyst.
The adsorption behavior depends on various parameters
such as surface area, chemical composition and surface wet-
Conclusions
Fig.
8 Adsorption of (A) fructose and (B) HMF in the single-
component solution; the adsorption test was performed by placing
0.09 g adsorbent in 4 mL DMSO containing 1 mmol adsorbate. Adsorp-
tion of (C) fructose and (D) HMF in a two-component solution; the ad-
sorption test was performed by placing 0.09 g adsorbent in 4 mL
DMSO containing 1 mmol fructose and HMF.
In summary, one-pot hydrothermally synthesized nanobelt-
like α-CuV2O6 acts as a robust heterogeneous catalyst in the
aerobic oxidation of HMF to DFF under ambient O2 pressure,
affording a 99.9% yield at 130 °C and 3 h. The unique Cu–O–
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V units in α-CuV2O6 account for the remarkable activity. The
combination of α-CuV2O6 with the hydrophilic MPIL-derived
solid acid P(EVPI-Br) serves as an efficient binary catalyst in
the direct synthesis of DFF from fructose in a one-pot and
one step process, providing a DFF yield of 63.1% at 135 °C
and 3.5 h. Two advantages are responsible for such efficient
catalysis: 1) the enhanced adsorption of fructose on hydro-
philic P(EVPI-Br) and 2) the preferential adsorption and
timely and efficient oxidation of HMF on α-CuV2O6. After re-
action, the catalyst can be facilely recovered and reused,
exhibiting excellent reusability in synthesizing DFF directly
from fructose.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. U1662107, 21476109, 21136005
and 21303084), the Jiangsu Provincial Science Foundation for
Youths (No. BK20130921 and BK20160976), the Specialized
Research Fund for the Doctoral Program of Higher Education
(No. 20133221120002), the China Postdoctoral Science Foun-
dation (2016 M590447), and the Project of Priority Academic
Program Development of Jiangsu Higher Education Institu-
tions (PAPD).
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