Direct synthesis of 2,5-diformylfuran from carbohydrates via carbonizing polyoxometalate based mesoporous poly(ionic liquid)

Article abstract of DOI:10.1016/j.cattod.2018.07.042

A promising atom-effective heterogeneous catalyst derived from the partial carbonization of polyoxometalate based mesoporous poly(ionic liquid) was constructed for the direct (one-pot and one-step) conversion of carbohydrates into 2,5-diformylfuran (DFF). The carbon precursor was synthesized through the free radical copolymerization of carboxylic acid functional ionic liquid and divinyl benzene, followed by the ion-exchange with heteropolyacid H₅PMo₁₀V₂O₄₀ to introduce heteropolyanions. Partial carbonization dramatically enhanced the acid and oxidation properties, rendering the greatly strengthened activity in both the degradation of fructose into 5-hydroxymethylfurfural (HMF) and oxidation of HMF into DFF. As a result, the catalyst exhibited remarkable performance in the direct synthesis of DFF from fructose in a one-pot and one-step reaction, offering a high yield of 87.3% and a maximum turnover number (TON) of 77.7. The catalyst was facilely recovered and reused without apparent deactivation. The one-pot and one-step conversion of glucose into DFF reached the highest yield of 51.2% so far. Other carbohydrates such as inulin and sucrose were also effectively transformed into DFF, displaying good substrate compatibility.

Full text of DOI:10.1016/j.cattod.2018.07.042

Catalysis Today xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Direct synthesis of 2,5-diformylfuran from carbohydrates via carbonizing
polyoxometalate based mesoporous poly(ionic liquid)
⁎
⁎
Qian Wang, Wei Hou, Tongsuo Meng, Qian Hou, Yu Zhou , Jun Wang
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University (former Nanjing University of Technology),
Nanjing, 210009, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A promising atom-effective heterogeneous catalyst derived from the partial carbonization of polyoxometalate
based mesoporous poly(ionic liquid) was constructed for the direct (one-pot and one-step) conversion of car-
bohydrates into 2,5-diformylfuran (DFF). The carbon precursor was synthesized through the free radical co-
polymerization of carboxylic acid functional ionic liquid and divinyl benzene, followed by the ion-exchange with
Biomass conversion
Carbohydrates
Mesoporous
Poly(ionic liquid)s
Heteropolyacids
5 2
heteropolyacid H PMo10V O40 to introduce heteropolyanions. Partial carbonization dramatically enhanced the
acid and oxidation properties, rendering the greatly strengthened activity in both the degradation of fructose
into 5-hydroxymethylfurfural (HMF) and oxidation of HMF into DFF. As a result, the catalyst exhibited re-
markable performance in the direct synthesis of DFF from fructose in a one-pot and one-step reaction, offering a
high yield of 87.3% and a maximum turnover number (TON) of 77.7. The catalyst was facilely recovered and
reused without apparent deactivation. The one-pot and one-step conversion of glucose into DFF reached the
highest yield of 51.2% so far. Other carbohydrates such as inulin and sucrose were also effectively transformed
into DFF, displaying good substrate compatibility.
2
5
,5-diformylfuran
-hydroxymethylfurfural
1. Introduction
simple way to allow multi-functions. For example, the combination of
an acidic and an oxidative catalyst was used in the conversion of
Developing green renewable resources has become growing sig-
fructose into DFF. Alternatively, catalysts with both acid and oxidative
sites were built to fulfill this requirement [18]. However, the activity of
these catalysts in tandem reaction was normally inferior to the one in
the oxidation of HMF into DFF [19–22]. In many cases, two-step re-
action was used to alleviate the interference of the acid and oxidative
sites, though the one-step is more favorable due to the facile operation
[23–26]. Moreover, excessive catalyst or long reaction time was usually
involved in these conversions, causing unsatisfied atom-efficiency with
low turnover number (TON) and turnover frequency (TOF) [27–30].
Particularly, these drawbacks are aggravated in the conversion of the
carbohydrates with more complicated structures (e.g. glucose), which
combines several steps such as isomerization, dehydration, and selec-
tive oxidation [21–32]. Construction of effective catalyst is still on the
way for the synthesis of DFF from carbohydrates, especially in a one-pot
and one-step reaction.
nificantly to alleviate the rapid depletion of fossil resources [1,2].
Biomass is one of the most important ones and exhibits to be the unique
carbon-containing sustainable one [3–8]. Great efforts have been pro-
posed for the transformation of these bio-resources into high value-
added chemicals and biofuels [6–8]. However, establishment of effec-
tive catalytic system is still challengeable. This is exemplified by the
direct synthesis of 2,5-diformylfuran (DFF) from carbohydrates. In this
context, design of highly effective heterogeneous catalyst is extremely
urgent.
DFF is among the most valuable biomass derived platform mole-
cules and has many potential applications such as the monomer of furan
polymers, pharmaceutical intermediates and antibacterial agent [9,10].
Direct synthesis of DFF from carbohydrates is more attractive than that
from 5-Hydroxymethylfurfural (HMF) [11,12]. The advantages include
the abundant available raw materials and simplified synthetic route
In this work, we reported a fascinating strategy to design high-
performance, single component heterogeneous catalyst for the one-pot
and one-step conversion of various carbohydrates into DFF. Catalyst
preparation relied on the partial carbonization of polyoxometalate
(POM) based mesoporous poly(ionic liquid)s (MPILs). POMs are
[
13,14]. This straightforward synthesis is a tandem reaction involving
multi-steps. Therefore, an effective conversion requires the catalyst
possessing multi-functional active sites to synergistically catalyze the
tandem reaction [15–17]. The employment of two or more catalysts is a
⁎
Corresponding authors.
E-mail addresses: njutzhouyu@njtech.edu.cn (Y. Zhou), junwang@njtech.edu.cn (J. Wang).
https://doi.org/10.1016/j.cattod.2018.07.042
Received 18 December 2017; Received in revised form 1 March 2018; Accepted 14 July 2018
0920-5861/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Wang, Q., Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.07.042

Q. Wang et al.
Catalysis Today xxx (xxxx) xxx–xxx
internal multi-functional catalysts with acid/base and redox properties
2.3. Materials synthesis
[
33,34]. Normally, they are dissolvable in polar solvents and hetero-
genization is achieved through immobilization or solidification [35,36].
MPILs characteristics of ionic liquid (IL) moieties in the polymeric
framework are attractive catalysts and catalyst support, due to the ex-
istence of readily available multi-functional groups and IL-derived ion-
exchange properties [37–40]. The combination of POM and MPIL de-
livers a multitude of heterogeneous catalysts with facilely adjustable
multi-functions [41]. MPILs have been used as favorable precursors
towards high-performance carbon materials in many oxidation reac-
tions [42,43]. However, to the best of our knowledge, there are no
examples of the studies related to the carbonization of POM-based
MPIL. Herein, we illustrated that partial carbonization enhanced the
acidity and oxidative properties of carboxylic acid functional MPIL with
Carboxylic acid functional IL [1-vinyl-3-propane carboxylation
imidazolium]Br (VPI-Br) was solvothermally synthesized [39]. Ele-
mental analysis calcd: C, 38.86 wt%; N, 11.24 wt%; H, 4.42 wt%; found:
C, 38.81 wt%; N, 11.24 wt%; H, 4.36 wt%. 1H NMR (300 MHz, D
O,
2 2
TMS) (ppm) = 3.01 (t, 2H, -CH ), 4.72 (t, 2H,-CH ), 5.45 (m, 1H, -CH),
2
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).
MPIL P(DVPI-Br) was synthesized through free radical copolymer-
ization of divinyl benzene (DVB) and VPI-Br by using azobisisobutyr-
onitrile (AIBN) as the initiator. Typically, DVB (0.716 g, 5.5 mmol),
VPI-Br (1.236 g, 5 mmol) and AIBN (0.09 g, 0.55 mmol) were dissolved
in a mixture solution of ethanol (5 mL), H
2
O (20 mL) and ethyl acetate
5−
PMo10
V
2
O
40
(PMoV
2
) anions. The target catalyst was highly active in
(25 mL) under a N atmosphere. The resulting mixture was stirred at
2
the direct synthesis of DFF from various carbohydrates (fructose, glu-
cose, inulin and sucrose). High yield and TON were achieved in a one-
pot and one-step reaction, rendering a highly atom-efficient hetero-
geneous catalyst for this tandem transformation.
353 K for 24 h. The white precipitate was isolated by filtration, washed
with ethanol and ultimately dried at 373 K for 24 h (86% yield).
Elemental analysis: C, 62.7 wt.%, N, 6.7 wt.%, H, 5.6 wt.%.
POM-based MPIL was synthesized through the reaction of P(DVPI-
5 2 5 2
Br) with H PMo10V O40 (H PMoV ), in which the POM anions were
loaded through an ion-exchange process and immobilized via ionic-
bonding interaction. In a typical synthesis, P(DVPI-Br) and an aqueous
2. Experimental section
solution of H
mass ratio for P(DVPI-Br) and H
5
PMoV
2
(1/x g of H
5
PMoV
2
in 20 mL H
2
O; 1/x presents the
2.1. Reagents and materials
5
PMoV
2
) was stirred at room tem-
perature for 24 h. After that, the faint yellow precipitate was collected
by filtration, washed with water and dried at 75 °C in a vacuum drying
All of the following chemicals were commercially available and
used without further treatment: 1-vinylimidazole (Aladdin Industrial
Inc., 99%), 3-bromopropionic acid (Sinopharm Chemical Reagent Co.,
Ltd., 98%), diethenylbenzene (Aladdin Industrial Inc., 80%), phos-
phoric acid (Sigma-Aldrich, ≥85%), vanadium pentoxide (Alfa
Aesar, > 98%), HMF (Shanghai shaoyuan company, > 98%) and car-
bohydrates (glucose, fructose, sucrose and inulin, Aladdin Industrial
Inc., > 99%).
oven for 12 h. The resulted samples were termed PMoV
2
@P-x. PMoV
atmosphere
10 °C min ), affording the partially carbonized samples PMoV @CP-
x-y (y presents the carbonation temperature).
2
@
P-x were carbonized at preset temperature for 1 h in N
2
−
1
(
2
2.4. Catalytic tests
Syntheses of DFF from HMF, HMF from fructose and DFF from
2
.2. Catalyst characterization
carbohydrates (fructose, glucose, sucrose and inulin) were carried out
in a 25 mL glass tube by using the target gas (balloon) and catalyst. In a
typical run for the conversion of glucose into DFF, glucose (1 mmol,
180 mg), catalyst (90 mg) and dimethyl sulfoxide (DMSO, 4 mL) were
1H and 13C NMR spectra were acquired on a Bruker DPX 500
2
spectrometer by using D O as the solvent and TMS (tetramethylsilane)
1
3
as the internal reference. Solid state C NMR spectra were originally
recorded in a Bruker AVANCE-III spectrometer. Attenuated total re-
flection-Fourier transform infrared spectra (ATR-FTIR) were recorded
on an Agilent Cary 660 instrument ranging from 4000 to 800 cm . X-
ray diffraction analysis (XRD) patterns were collected on a Smart Lab
diffractmeter from Rigaku equipped with a 9 kW rotating anode Cu
source (45 kV, 200 mA, 5–80°, 0.2° s ). In situ XRD was collected with
2
heating in the range of 25–600 °C (10 °C min ) under nitrogen (N )
atmosphere. Elemental analyses (EA) were performed with a CHN
elemental analyzer Vario EL cube. Nitrogen sorption experiments were
carried out at 77 K on a BELSORP-MAX analyser. The samples were
degassed at 80 °C for 3 h before analysis. Scanning electron microscopic
2
stirred in the glass tube equipped with an O balloon. After the reaction
at 135 °C for 3 h, isopropanol (0.05 g) was added as the internal stan-
dard. The products were analyzed by gas chromatograph (GC, Agilent
7890B) equipped with a flame ionization detector and a capillary
column (HP-5, 30m × 0.25 mm × 0.32 μm). Reusability was in-
vestigated by in a three-run recycling test. After each run, the catalyst
was separated by centrifugation, washed with water, dried at 100 °C for
12 h, and then calcined at 400 °C for 1 h before recharging into the next
run.
−
1
−
1
−
1
3. Results and discussion
(
SEM) images were viewed on a Hitachi S-4800 field-emission scanning
3.1. Catalysts preparation and characterizations
electron microscope. Transmission electron microscopy (TEM) analysis
was carried out on a JEM-2100 (JEOL) electron microscope (200 kV). X-
ray photoelectron spectra (XPS) was conducted on a PHI 5000 Versa
Probe X-ray photoelectron spectrometer equipped with Al Karadiation
Scheme 1 shows the preparation procedure of the target catalysts
PMoV
copolymerization of IL monomer (VPI-Br) and DVB, 2) immobilization
of PMoV anions through ion-exchange, and 3) carbonization under N
atmosphere. The loading amount of PMoV was facilely adjusted by
varying the initial mass ratio of P(DVPI-Br) to H PMoV , and the car-
bonization degree depended on the temperature. Herein, we took the
typical sample PMoV @CP-5.5-400 as an example. This catalyst was
prepared with the initial P(DVPI-Br)/PMoV mass ratio of 5.5. A
moderate carbonization temperature of 400 °C endowed the partial
carbonization and satisfactory preservation of PMoV anions. Full
characterizations were conducted to reveal the structure of
2
@CP-x-y. The synthesis involves three steps of 1) free radical
(
1486.6 eV). Thermogravimetric (TG) analysis in dry N
2
atmosphere
2
2
proceeded on an STA409 instrument with the heating rate of 10 °C
2
−
1
min . Inductively coupled plasma mass spectrometry (ICP) was
measured at OPTMA 20,000 V. Temperature-programmed desorption
5
2
(
TPD) was analyzed by using Catalyst Analyzer BELCAT-B. Samples
were pretreated at 300 °C for 60 min, and then cooled to 100 °C under
helium (He) gas. NH adsorption was carried out at 100 °C for 60 min
under a NH /He (5% NH and 95% He) gas. After samples were purged
under He gas for 40 min, the temperature was heated to 300 °C (10 °C
2
2
3
3
3
2
−
1
min ). The desorbed gas was determined by a thermal conductivity
detector (TCD).
PMoV
PMoV
2
@CP-5.5-400 plus the two precursors of P(DVPI-Br) and
@P-5.5. Elemental analysis (Table S1) indicated that the N
2
2

Q. Wang et al.
Catalysis Today xxx (xxxx) xxx–xxx
2
Scheme 1. Preparation of PMoV @CP-5.5-400.
Table 1
Textural properties.
of the micropores by PMoV
2
anions, accompanying with the formation
of certain mesopores. PMoV @CP-5.5-400 owned the surface area of
2
2
−1
3 −1
1
64 m g
dicates that the calcination caused the collapse of partial mesopores.
Fig. 3A depicts the XRD patterns of P(DVPI-Br), PMoV @P-5.5 and
PMoV @CP-5.5-400. P(DVPI-Br) demonstrated a wide peak around 2θ
values of 20-30°, indicating that it has an amorphous structure. PMoV
and pore volume of 0.144 cm g . This phenomenon in-
Samples
IL content
(
PMoV
(wt%)
2
b
S
BET
2
Vp
Dp
mmol g 1)a
−
(m g−1)c
(cm g−1)d
3
(nm)e
2
VPI-Br
P(DVPI-Br)
4.05
2.39
2.03
–
–
0
14.3
20.0
–
–
–
2
272
179
164
0.20
0.22
0.14
3.7
4.5
3.5
2
PMoV
PMoV
2
@P-5.5
@CP-5.5-400
presented the typical diffraction peaks for the Keggin type crystal
structure of POM anions at 2θ values of 8–11°, 18–22°, 24–30°, and
2
a
IL content in MPILs calculated via elemental analysis.
Total PMoV loading.
2
BET surface area.
Total pore volume.
Average pore diameter.
33–40° (Fig. 3B) [46]. These characteristic peaks were not observed in
b
c
the XRD pattern of PMoV
are highly dispersed on the polymeric skeleton. PMoV
showed the similar XRD pattern to PMoV @P-5.5, excluding the ag-
gregation of these PMoV anions during the calcination process. In situ
XRD patterns of H PMoV under N atmosphere were collected from 25
to 600 °C (Fig. 3B). The result showed that the Keggin type crystal
structure of H PMoV was kept until a high temperature of 450 °C,
validating that the PMoV anions remained in PMoV @CP-5.5-400. As
2
@P-5.5, implying that these PMoV
2
anions
2
@CP-5.5-400
d
e
2
2
5
2
2
content of P(DVPI-Br) was 6.7%. The monomer DVB has no N element;
therefore these N atoms come from the IL moieties in the ionic frame-
work. The corresponding IL content of P(DVPI-Br) was 2.39 mmol g
−
1
5
2
.
−
1
2
2
PMoV
the loading of POM anions. The N content of PMoV
.1%, lower than the one of PMoV @P-5.5 due to the decomposition of
IL moieties. ICP analysis indicated that the loading amount of PMoV
anions of PMoV @P-5.5 and PMoV @CP-5.5-400 was 14.3% and 20%,
respectively (Table 1).
Fig. 1 displays the SEM images of P(DVPI-Br), PMoV
PMoV @CP-5.5-400. The primary particles of P(DVPI-Br) were small
2
@P-5.5 exhibited the lower IL content of 2.03 mmol g
due to
the temperature increased to 500 °C, the peaks at 9° became weaken,
along with the observation of a new peak at 12°. Additional new peaks
at 23°, 24°, 27° and 33° were more and more apparent with the in-
creasing of the temperature. The emergence of these signals indicates
the formation of MoOx and VOx phase [47,48].
2
@CP-5.5-400 was
4
2
2
2
2
Fig. 4A illustrates the FT-IR spectra of these samples. The spectrum
2
@P-5.5 and
of PMoV
2
showed four main characteristic peaks in the range of
2
−
1
−1
800–2000 cm , in which the peaks at 786, 872, 947 and 1052 cm
spheres with the diameter of about hundreds of nanometers (Fig. 1A).
The spherical particles were loosely packed to form irregular secondary
particles in micrometers. PMoV @P-5.5 and PMoV @CP-5.5-400 de-
2 2
monstrated similar morphology to P(DVPI-Br) (Fig. 1B and C), sug-
gesting the preservation of the morphology during the ion-exchange
and calcination process. This phenomenon also implies a partial car-
stand for P-Oa (central oxygen), M-Ob-M (corner-sharing oxygen; M:
metal in heteropolyacids framework), M-Oc-M (edge-sharing oxygen),
and M-Od (terminal oxygen), respectively [49,50]. P(DVPI-Br) has the
−1
same peaks as reported [39]. The peak at 1720 cm is attributable to
the C]O stretching vibration in the carboxyl group. The peaks at 1157,
−
1
1
448 and 1576 cm for the CeN and C]N stretching vibration reflect
the existence of imidazole ring [51]. Both of the signals for the PMoV
anions and polymeric framework were observable in the spectrum of
PMoV @P-5.5, revealing the immobilization of the PMoV anions on
the MPIL skeleton. The characteristic peaks for the PMoV anions were
still observed in the spectrum of PMoV @CP-5.5-400, indicating that
these PMoV anions remained after calcination, in accordance with the
in situ XRD patterns. However, PMoV @CP-5.5-400 demonstrated
bonization of PMoV
mapping images demonstrated a uniformly distribution of PMoV
over PMoV @CP-5.5-400 (Fig. 1D). TEM image of PMoV @CP-5.5-400
active sites and
2
@P-5.5 towards PMoV
2
@CP-5.5-400. Elemental
2
2
sites
2
2
2
2
additionally reflected a highly dispersion of the PMoV
2
2
the existence of abundant mesopores (Fig. 1E and F).
Qualitative analysis of the porosity was investigated by the nitrogen
sorption experiments (Fig. 2). The nitrogen sorption isotherm of P
2
2
2
(
DVPI-Br) was type IV with a distinctive H
plying the presence of micropores and mesopores [44,45]. Similar ni-
trogen sorption isotherms were observed over PMoV @P-5.5 and
PMoV @CP-5.5-400, suggesting that these pores were preserved in
2
type hysteresis loop, im-
much weakened IR peaks assignable to the IL moieties, suggesting that
these IL moieties were partially decomposed during the calcination.
2
Solid state 13C NMR spectra of PMoV
2
@P-5.5 and PMoV
were collected to further illustrate the variation of the polymeric fra-
mework during the calcination (Fig. 4B). The spectrum of PMoV @P-
.5 presented typical signals attributable to the MPIL network. The two
peaks at 136 and 173 ppm came from the C3 and C8 in the IL moieties
of PMoV @P-5.5 [39]. Both of them disappeared in the spectrum of
2
@CP-5.5-400
2
these two samples. Table 1 summarizes the corresponding textural
properties. Brunauer–Emmett–Teller (BET) surface area and pore vo-
2
5
2
−1
3 −1
lume of P(DVPI-Br) was 272 m g
PMoV @P-5.5 has a slightly lower surface area of 179 m g but larger
pore volume of 0.22 cm g . This variation comes from the occupation
and 0.195 cm g , respectively.
2
−1
2
3
−1
2
3

Q. Wang et al.
Catalysis Today xxx (xxxx) xxx–xxx
2 2 2
Fig. 1. SEM images of (A) P(DVPI-Br), (B) PMoV @P-5.5 and (C) PMoV @CP-5.5-400; (D) elemental (Br, Mo and V) mapping images of PMoV @CP-5.5-400; (E, F)
TEM images of PMoV @CP-5.5-400.
2
PMoV
moieties. The peak at 226 ppm emerged in the spectrum of PMoV
.5 was not observed in previous POM-free MPIL with the same organic
framework. Thus, this signal is tentatively assigned to the C3 of the
2
@CP-5.5-400, additionally reflecting the decomposition of IL
2
PMoV tethered IL moieties, and the peak shift is attributable to the
strong cation-anion interaction. This phenomenon is also suggestive of
the partial decomposition of the IL moieties. In other words, the IL
2
@P-
5
2 2
moieties interacting with PMoV were preserved in PMoV @CP-5.5-
Fig. 2. (A) N
2 2 2
sorption isotherms and (B) pore size distribution curves of P(DVPI-Br), PMoV @P-5.5 and PMoV @CP-5.5-400. The adsorption isotherms for samples
3
−1
PMoV @P-5.5 and P(DVPI-Br) are shifted by 27 and 63 cm g
2
.
4

Q. Wang et al.
Catalysis Today xxx (xxxx) xxx–xxx
2 2 5 2 2
Fig. 3. XRD patterns of (A) P(DVPI-Br), PMoV @P-5.5 and PMoV @CP-5.5-400; (B) In situ XRD patterns of H PMoV under N atmosphere.
4
00. Besides, signals derived from the C1, C2 C11 and C12 were still
PMoV
same temperature (176 °C) as the one in the curve of P(DVPI-Br). Si-
milar weight loss variation was found for PMoV @CP-5.5-400, though
with smaller weight loss in each stair than that for P(DVPI-Br). This
phenomenon indicates that the IL moieties and DVB medicated fra-
2
@CP-5.5-400, an apparent weight loss was observed from the
2
observable in the spectrum of PMoV @CP-5.5-400, evidencing that the
remaining of DVB mediated polymeric framework [52].
Fig. 4C shows the TG curves of P(DVPI-Br) and PMoV
2
2
@CP-5.5-400.
The weight loss below 150 °C comes from desorption of physical ad-
sorbed water. P(DVPI-Br) started to decompose from 176 °C. The weight
loss from 176 to 306 °C is assigned to the partial decomposition of IL
moieties as the starting decomposition temperature of the DVB frame-
work was normally higher than 300 °C [53]. The substantial weight loss
from 306 to 430 °C includes the decomposition of DVB framework and
IL moieties. The deep decomposition of the polymeric framework
happened at high temperature (> 430 °C) and converted most of the
organic compounds into N-doped carbon. In the TG curve of
mework were partially decomposed in the preparation of PMoV
2
@CP-
5.5-400. ESR spectra of PMoV @P-5.5 and PMoV @CP-5.5-400 showed
2
2
a clear enhancement of the signal for the formation of partly reduced V
species (Fig. 4D) [54]. This result indicates that the carbonization
process affects the electronic state of the PMoV anions, causing the
2
formation of partially reduced V(IV) species that were normally active
in many oxidation reactions [55].
2 2
XPS analyses of PMoV @P-5.5 and PMoV @CP-5.5-400 were
2 2 5 2 2 2
@P-5.5, PMoV @CP-5.5-400 and H PMoV @P-5.5 and PMoV @CP-5.5-400; (C)
; (B) 13C MAS NMR spectra of PMoV
Fig. 4. (A) FT-IR spectra of P(DVPI-Br), PMoV
TG curves of PMoV @CP-5.5-400 and P(DVPI-Br); (D) ESR spectra of PMoV
2
2 2
@P-5.5 and PMoV @CP-5.5-400.
5

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Catalysis Today xxx (xxxx) xxx–xxx
Fig. 5. XPS analysis for (A) survey scan; (B) N 1s; (C) C 1s and (D) Mo 3d spectra of PMoV
2
@P-5.5 and PMoV
2
@CP-5.5-400.
performed to represent the electronic states of C, N and Mo (Fig. 5A).
The signals of the V and P species were negligible due to the low
loading amount. Two peaks at 398.3 and 401.3 eV were detected in the
2
N 1 s XPS spectrum of PMoV @P-5.5. They were related to the pyridinic
+
N (eN]) and pyrrolic N (eN ]) in the imidazolinium rings (Fig. 5B)
56]. PMoV @CP-5.5-400 also demonstrated two N 1 s XPS signals at
99.0 and 401.9 eV for the pyridinic and pyrrolic N [57], suggesting
[
2
3
that part of the IL moieties remained in this sample. The variation of the
+
peak location and intensity indicates that the eN ] species were
preferentially decomposed relative to the eN]ones. The C 1 s XPS
spectrum of PMoV
around 284.8, 286.4, and 288.7 eV, corresponding to CeC, CeO, and
C]O/C]N, respectively [58]. The CeO signal of PMoV @CP-5.5-400
slightly shifted to lower binding energy, due to the formation of carbon.
PMoV @P-5.5 represented the typical Mo 3d3/2 (235.4 eV) and 3d5/2
232.2 eV) peaks for the Mo
Mo XPS signal of PMoV @CP-5.5-400 shifted to higher binding energy
2
@P-5.5 (Fig. 5C) was fitted with three peaks at
2
3 2 2
Fig. 6. NH -TPD curves of PMoV @P-5.5 and PMoV @CP-5.5-400.
2
6
+
(
species in PMoV
2
anions (Fig. 5D). The
The result indicates that carbonization at high temperature of 500 and
600 °C further increased the amount of weak acid sites, attributable to
2
of 235.7 and 232.5 eV [57,59].
x x
the formation of MoO and VO . This phenomenon also suggests that
Heteropolyacids are traditional strong acids and their acidity is
normally measured by Hamilton indicator. However, this method is
only suitable for the white samples or the ones with only pale color. The
partial carbonized samples are dark, and thus their acidity was char-
majority of the strong acid sites were damaged due to the decomposi-
tion of POM anions at high temperature. Thus, partial carbonization at
moderate temperature well maintains the strong acid sites and
strengthens the weak acid sites.
3
acterized by NH -TPD analysis. Desorption was stopped at 300 °C by
considering that the organic component will be decomposed at high
temperature (in such case, the desorption peak does not represent the
3.2. Catalytic activities
NH
3
desorption). The desorption temperature of strong acid sites are
-TPD profiles in this work only
normally higher than 400 °C, and NH
reflect the variation of weak acid sites. As shown in Fig. 6, higher
3
PMoV
2
@CP-5.5-400 was employed as a heterogeneous catalyst in
@P-5.5
the synthesis of DFF from carbohydrates. P(DVPI-Br) and PMoV
2
desorption temperature and larger peak area were observed over
were parallel tested to gain insight into the structure-performance re-
lationship. Their catalytic performance was studied in the oxidation of
HMF into DFF, conversion of the typical carbohydrate fructose into
HMF, and straightforward synthesis of DFF from various carbohydrates
of fructose, glucose, inulin and sucrose.
PMoV
clearly illustrates that partial carbonization enhanced the acid strength
and amount of weak acid sites. For comparison, NH -TPD profiles of
PMoV @CP-5.5-500 and PMoV @CP-5.5-600 were collected (Fig. S2).
2 2
@CP-5.5-400 relative to PMoV @P-5.5. This phenomenon
3
2
2
The conversion of carbohydrates into DFF usually involves the
6

Q. Wang et al.
Catalysis Today xxx (xxxx) xxx–xxx
Table 2
Oxidation of HMF into DFF.
Table 3
One-pot and one-step conversion of fructose into DFF.
a
a
Entry
Catalyst
Catalyst dosage (g)
Y
DFF (%)b
TONc
Entry Catalyst
Catalyst dosage (g)
Y
HMF (%)b
Y
DFF (%)c TONd
1
2
3
4
None
P(DVPI-Br)
PMoV
PMoV
–
0.4
–
–
1
2
3
4
None
P(DVPI-Br)
PMoV
PMoV
–
9.1
4.2
9.6
20.0
71.6
–
–
30.3
77.7
0.02
0.02
0.02
14.0
44.7
51.2
0.04
0.04
0.04
60.2
50.5
0.6
2
@P-5.5
@CP-5.5-400
135.7
111.2
2
@P-5.5
2
2
@CP-5.5-
4
00
a
Reaction conditions: HMF 1 mmol, DMSO 4 mL, O
The yield of DFF.
TON = mol DFF/mol vanadium.
2
balloon, 120 °C, 3 h.
b
c
a
b
c
Reaction conditions: fructose 1 mmol, DMSO 4 mL, 130 °C, 3 h.
The yield of HMF.
The yield of DFF.
d
TON = mol DFF/mol vanadium.
dehydration of carbohydrates into HMF and the successive oxidation of
HMF into DFF [16]. The oxidation step is crucial for this reaction.
Hence, we started the catalysis evaluation from the synthesis of DFF
from HMF. As shown in Table 2, the DFF yield was below 1% in the
absence of any catalyst. Owing to the lack of oxidative sites, P(DVPI-Br)
offered a low DFF yield of 14.0%. The DFF yield increased to 44.7% by
DFF with the yield of 71.6% under the same reaction conditions. The
corresponding TON was as high as 77.7. Further altering of the reaction
time and catalyst dosage leaded to a high DFF yield of 87.3% (TON:
4
7.4) (Fig. 8). Such activity in the one-pot and one-step reaction greatly
exceeds previous heterogeneous catalysts under the identical reaction
conditions (Table S3, yields: 39–69.3%; TONs: 3.4–12.3) [23,26,29].
Also, its performance is even superior to or comparable to the ones of
the two-step reactions [32]. It is noted that the activity (yield and TON)
of this catalyst in the conversion of fructose into DFF is close to the one
in the oxidation of HMF into DFF. This interesting finding highlights
that the acidic and oxidative sites in this catalyst were able to smoothly
and independently catalyze each step (dehydration and oxidation), and
thus synergistically catalyzed the conversion of fructose into DFF in a
one-pot and one-step reaction. This is scarcely achieved before.
The kinetic curve indicates the accumulation of HMF in the initial
stage of the reaction, while DFF was gradually produced along with the
using PMoV
2 2
@P-5.5. PMoV @CP-5.5-400 exhibited higher DFF yield of
51.2%, suggesting that partial carbonization strengthens the oxidation
ability, which comes from the formation of more active V(IV) species.
Actually, the corresponding activity depended on the loading amount of
PMoV
vestigated the activity of several control catalysts with different PMoV
content and carbonization degree (Table S2), including PMoV @CP-x-
00 (x = 1.8, 2.7, 11) and PMoV @CP-5.5-y (y = 500, 600). Yields
over PMoV @CP-1.8-400, PMoV @CP-2.7-400 and PMoV @CP-11-400
were 32%, 42.3% and 21.9%, respectively. PMoV @CP-5.5-500 and
PMoV @CP-5.5-600 afforded the DFF yields of 33% and 19%. All of
them are less active than PMoV @CP-5.5-400, verifying that PMoV
2
anions and the carbonization degree. For example, we in-
2
2
4
2
2
2
2
2
2
2
2
decrease of HMF. This proves that PMoV
version of fructose into DFF underwent the formation of HMF (Fig. 8).
The investigation of the PMoV loading amount suggests that
PMoV @CP-5.5-400 was more active than PMoV @CP-x-400 (x = 1.8,
.7, 11) (Table S4), in agreement with their performance in the oxi-
dation of HMF into DFF (Table S2). PMoV @CP-5.5-400 was only
slightly more active than PMoV @P-5.5 in the oxidation step (yield:
1.2% vs 44.7%), but the former presented much higher yield than the
2
@CP-5.5-400 catalyzed con-
content and carbonization degree played a vital role. The influence of
the reaction temperature and catalyst dosage was surveyed in the
PMoV @CP-5.5-400 catalyzed oxidation of HMF into DFF (Fig. 7).
2
Highest DFF yield of 90.4% (TON: 98.1) was obtained by using 0.04 g
catalyst at 130 °C. Maximum DFF TON of 113 was reached by slightly
reducing the catalyst dosage to 0.03 g. Compared with previous work
2
2
2
2
2
(
Table S3), the activity of PMoV
least comparable to the best ones. Particularly, TON value over
PMoV @CP-5.5-400 was among the highest ones under similar reaction
2
@CP-5.5-400 was superior to or at
2
5
latter in the conversion of fructose into DFF (yield: 71.6% vs 20.0%). In
order to gain insightful understanding, we conducted the synthesis of
2
conditions [60,61].
Table 3 demonstrates the performance of P(DVPI-Br), PMoV @P-5.5
2
2
and PMoV @CP-5.5-400 in the one-pot and one-step synthesis of DFF
from the typical carbohydrate fructose. Catalyst-free reaction rendered
a low DFF yield of 4.2%. P(DVPI-Br) exhibited improved activity in the
conversion of fructose into HMF, but was inactive in the oxidation step.
Thus, P(DVPI-Br) catalyzed reaction afforded a high HMF yield of
HMF from fructose by using PMoV
under N atmosphere. Fructose was rapidly converted into HMF when
catalyzed by PMoV @CP-5.5-400, affording the yield of 59.6% within
.5 h. A high yield of 88.2% was achieved at 1 h. It is intensely higher
than that over PMoV @P-5.5 (Fig. 9, yield: 75.2%). This phenomenon
clearly reveals that partial carbonization of PMoV @P-5.5 strengthens
2 2
@CP-5.5-400 and PMoV @P-5.5
2
2
0
2
2
both the acidity and oxidative ability that drives the efficient synergistic
catalysis in this tandem reaction. After reaction, the catalyst
6
0.2% with a low DFF yield of 9.6%. PMoV
enhanced activity, offering the DFF yield of 20.0%. Unexpectedly,
PMoV @CP-5.5-400 resulted in an effective conversion of fructose into
2
@P-5.5 exhibited slightly
PMoV
2
@CP-5.5-400 was facilely recovered and reused without sig-
content in the spent catalyst was
2
nificant deactivation (Fig. S3). PMoV
2
2
Fig. 7. Influence of reaction temperature and catalyst dosage on the PMoV @CP-5.5-400 catalyzed conversion of HMF to DFF. Reaction conditions: HMF 1 mmol,
catalyst 0.03 g, DMSO 4 mL, 3 h, 130 °C, 3 h O
2
balloon. For each figure there is a specific parameter changed.
7

Q. Wang et al.
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 8. Influence of (A) reaction time and (B) catalyst amount on the PMoV
2
@CP-5.5-400 catalyzed one-pot and one-step conversion of fructose into DFF. Reaction
2
conditions: fructose 1 mmol, DMSO 4 mL, O balloon; (A) 120 °C, catalyst 0.08 g; (B) 110 °C, 4 h. For each figure there is a specific parameter changed.
Fig. 10. Yields as a function of reaction time in the PMoV
lyzed conversion of glucose into DFF. Reaction conditions: glucose 1 mmol,
DMSO 4 mL, 135 °C, catalyst 0.09 g, O balloon.
2
@CP-5.5-400 cata-
Fig. 9. Yields as a function of reaction time in the PMoV
lyzed conversion of fructose into HMF. Reaction conditions: fructose 1 mmol,
DMSO 4 mL, 120 °C, catalyst 0.08 g, N balloon.
2
@CP-5.5-400 cata-
2
2
aforementioned, the loading amount of PMoV
the performance. Finely manipulation of the PMoV
additionally increased activity by using PMoV @CP-6-400 (yield:
1.2%). The resulting activity is among the highest ones even compared
2
significantly affected
1
8.1% that is close to the one in the fresh catalyst (Table 1).
content led to
The scope of PMoV @CP-5.5-400 was explored in the transforma-
2
2
tion of other carbohydrates into DFF. The conversion of glucose into
DFF involves three steps (Table 4), and thus is much more difficult than
2
5
with those of two-step reactions (Table S3) [18,32]. Also much high
TON (29.6) was reached compared with previous results (Table S3,
TONs: 1.6–15). Further, inulin and sucrose were effectively converted
into DFF in this straightforward transformation process, rendering the
DFF yields of 61.6% and 20.4% (Table 4), respectively. All of these
emphasize the good substrate compatibility of this catalyst.
2
the transformation of fructose. Exhilaratingly, PMoV @CP-5.5-400 af-
forded a high DFF yield of 46.3% in the one-pot and one-step synthesis
of DFF from glucose. This activity was about 2.6-fold of the one over
PMoV @P-5.5 (17.6%), reflecting an apparent enhanced activity de-
2
rived from the partial carbonization. Kinetic analysis also demonstrated
the conversion of glucose into DFF via HMF (Fig. 10). As
The above catalysis evaluation indicates that partial carbonization
of POM-based MPIL provided highly active heterogeneous catalyst in
the direct synthesis of DFF from carbohydrates. Partial carbonization is
crucial for such good performance. Structure analysis by ESR (Fig. 4D)
and XPS (Fig. 5) spectra demonstrated that partial carbonization caused
the formation of more positive Mo species and more negative V species,
suggesting the electron transfer from Mo to V. Normally, the V species
are the oxidative sites in an oxidation reaction and partial reduced V
Table 4
a
One-pot and one-step conversion of different carbohydrates into DFF.
Entry Substrate Catalyst
PMoV
2
(wt
Y
HMF (%)c
Y
DFF (%)d TONe
b
%
)
1
2
glucose
glucose
PMoV
PMoV
2
@P-5.5
@CP-5-
14.3
21.9
3.4
1.9
17.6
40.7
11.9
17.9
2
4
00
PMoV
00
PMoV
00
PMoV
00
PMoV
00
3
species responded higher activity in many cases [54,62]. NH -TPD test
3
4
5
6
glucose
glucose
inulin
2
2
2
2
@CP-5.5- 20
1.2
0.4
0.6
0.2
46.3
51.2
61.6
20.4
22.3
29.6
29.7
9.8
implied that the weak acid sites were strengthened and strong acid sites
were well preserved after partial carbonization (Figs. 6 and S2). Direct
synthesis of DFF from carbohydrates is a tandem reaction involving an
acid-catalyzed conversion of carbohydrate into HMF and successive
oxidation of HMF into DFF. The enhanced oxidative ability promoted
the activity in the oxidation step (Table 2), while the variation of the
acidity increased the activity in the acid-hydrolysis step (Fig. 9). Owing
to these two enhancements, the obtained catalyst after partial carbo-
nization represented high activity in this tandem reaction. High car-
bonization temperature caused low surface area (Fig. S4 and Table S5)
4
@CP-6-
16.7
4
@CP-5.5- 20
@CP-5.5- 20
4
sucrose
4
a
Reaction conditions: carbohydrates 1 mmol, DMSO 4 mL, catalyst 0.09 g,
1
35 °C, 5 h.
b
2
Total PMoV loading.
c
d
e
The yield of HMF.
and the formation of MoO
x x
and VO (Fig. 3B). By contrast, carboniza-
The yield of DFF.
tion at moderate temperature tended to preserve the high surface area
TON = mol DFF/mol vanadium.
8

Q. Wang et al.
Catalysis Today xxx (xxxx) xxx–xxx
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The authors greatly thank the National Natural Science Foundation
of China (no. U1662107, 21476109, 21136005 and 21303084), Jiangsu
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