Aerobic oxidation of biomass-derived 5-hydroxymethylfurfural to 2,5-diformylfuran with cesium-doped manganese dioxide

Article abstract of DOI:10.1039/c8cy01246f

The selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) has attracted much attention in recent years. Cesium-doped manganese dioxide (Cs/MnO_x) was prepared by a soft template method, and was found to be active for the selective oxidation of HMF to DFF with molecular oxygen. Various reaction conditions have been investigated, and DFF was attained with a high yield of 94.7% at a HMF conversion of 98.4% at 100 °C and 10 bar O₂ in N,N-dimethylformamide. Cs/MnO_x demonstrated much higher catalytic activity than un-doped MnO_x, suggesting that the introduction of Cs into MnO_x plays a crucial role in the oxidation of HMF to DFF. Kinetic studies demonstrated that the oxidation of HMF to DFF depends on both the HMF and oxygen concentration, with the reaction order being 0.44 and 0.53, respectively. Compared with other heterogeneous non-noble metal catalysts, Cs/MnO_x demonstrated a much lower activation energy for the oxidation of HMF to DFF. The catalyst can be reused without much loss of catalytic activity, suggesting that the catalyst is highly stable. Furthermore, the direct production of HMF from fructose was also successfully realized by a one-pot reaction strategy via two consecutive steps.

Full text of DOI:10.1039/c8cy01246f

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Aerobic Oxidation of Biomass-Derived 5-Hydroxymethylfurfural to
2,5-Diformylfuran with Cesium-Doped Manganese Dioxide
Received 00th January 20xx,
Accepted 00th January 20xx
Ziliang Yuan, Bing Liu*, Peng Zhou, Zehui Zhang*, Quan Chi
DOI: 10.1039/x0xx00000x
The selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) has attracted much attention in
recent years. A cesium-doped manganese dioxide (Cs/MnOx) was prepared by the soft template method, and was found to
be active for the selective oxidation of HMF into DFF with molecular oxygen. Various reaction conditions have been
www.rsc.org/
o
investigated, and DFF was attained in a high yield of 94.7% at HMF conversion of 98.4% at 100 C and 10 bar O2 in N,N-
dimethylformamide. Cs/MnOx demonstrates much higher catalytic activity than the un-doped MnOx, suggesting that the
introduction of Cs into MnOx played a crucial role in oxidation of HMF into DFF. Kinetic studies demonstrated that the
oxidation of HMF into DFF depended both on HMF and oxygen concentration with the reaction order being 0.44 and 0.53,
respectively. Compared with other heterogeneous non- noble metal catalysts, Cs/MnOx demonstrates much lower
activation energy of the oxidation of HMF into DFF. The catalyst can be reused without much loss of catalytic activity,
suggesting that the catalyst was highly stable. Furthermore, the direct production of HMF from fructose was also
successfully
realized
by
the
one-pot
reaction
strategy
via
two
consecutive
steps.
substitute of terephthalic acid as a polymer monomer for the
synthesis of polyethylene furanoate (PEF).8 Similar as FDCA, DFF has
also been serves as a useful precursor in the synthesis of functional
polymers, pharmaceuticals, antifungal agents, furan−urea resins,
and heterocyclic ligands.9 The history for the oxidation of HMF
could be traced back to nineteen century. In early days, oxidation of
HMF to DFF has been performed by use of stoichiometric oxidants,
including NaOCl, BaMnO4 or pyridinium chlorochromate.10-12
Obviously, these methods are unsatisfactory, because of the high
cost of the oxidant as well as the release of high toxic wastes.
Therefore, it is attractive for the synthesis of DFF via the catalytic
oxidation process especially with molecular oxygen as the oxidant.
Both homogeneous and heterogeneous catalysts have been studied
for the oxidation of HMF into DFF. Although some homogeneous
catalytic systems were very effective for the oxidation of HMF into
DFF with nearly quantitatively yields,9 these methods rely on
homogeneous catalyst systems that are not environmentally
friendly and recyclable. Accordingly, significant efforts have been
devoted to develop heterogeneous catalysts for the oxidation of
HMF into DFF.13-15
Introduction
In the current, there has been a growing increase in the
requirement of energy and chemicals for the world, but the non-
renewable resources have been depleted continuously. Therefore,
the search of renewable resources for the production of chemicals
as well as fuels has attracted great attention by the government as
well as the chemists.1-3 Biomass including lignocellulose and
triglycerides, as one of the most abundant renewable resources,
has received significant attention as a substitute for non-renewable
fossil resources such as crude oil, coal, and natural gas, from which
various high value-added products such as biofuels, commodity
chemicals, and bioplastics can be manufactured.4,5 Many routes
have been exploited for the transformation of biomass into
chemicals and liquid fuels. Among them, one important route
towards an effective exploitation of biomass is the catalytic
conversion of biomass into 5-hydroxymethylfurfural (HMF) and the
subsequent conversion of HMF into other valuable chemicals and
liquid fuels. 6,7
Catalytic oxidation of HMF can generate several important
furan chemicals, such as 2,5-diformylfuran (DFF), 5-hydroxymethyl-
2-furancarboxylic acid (HMFCA), and 2,5-furandicarboxylic acid
(FDCA). However, the selective oxidation of HMF into the specific
furan compound is challenging. In recent years, selective oxidation
of HMF into DFF and FDCA has been studied extensively. FDCA has a
similar structure with terephthalic acid, thus FDCA can sever as a
Generally speaking, the heterogeneous catalysts that studied
for the oxidation of HMF into FDCA are mainly based on vanadium,
ruthenium and manganese catalysts. Supported Ru nanoparticles
such as Ru/C and Ru/Al2O3 demonstrated relative higher activity for
the oxidation of HMF into DFF.16-18 For example, Liu and co-workers
reported that Ru/C catalyst can promote the oxidation of HMF into
DFF with a yield of 96% under optimized reaction conditions.16
However, some supported ruthenium catalysts are not stable,
suffering the leaching and the growth of metal nanoparticles.18 In
addition, ruthenium based catalysts are high cost, which also limits
its application in the large-scale synthesis of DFF with HMF.
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Specific surface areas of the supports or the catalysts were
measured by N2 physisorption by using a Micromeritics Tristar II
3020. Transmission electron microscope (TEM) images were
obtained using an FEI Tecnai G2-20 instrument. The sample powder
were firstly dispersed in ethanol and dropped onto copper grids for
observation. X-ray powder diffraction (XRD) patterns of samples
Vanadium oxides and salts were also used for the oxidation of HMF
into DFF.19-22 Compared with ruthenium catalysts, vanadium based
catalysts are much more cheap. However, they showed lower
activity, lower selectivity and lower stability. For example, Riisager
and co-workers reported that over 60% of the total catalyst activity
is a result of the catalytic species dissolved from the V2O5/H-ZSM-5
and V2O5/H-mordenite catalysts.23,24 Compared with the ruthenium
and vanadium catalysts, manganese oxide-based catalysts have
proved to be effective and stable for the oxidation of HMF into DFF,
especially OMS-2 based catalysts. Despite these promising results,
these current methods still demonstrate some drawbacks such as
the use of high catalyst loading to keep a high conversion.
Therefore, there still exists the room to develop new
heterogeneous manganese catalysts for the effective and selective
oxidation of HMF into DFF with molecular oxygen.
were determined with
diffractometer (Cu Kα). All XRD patterns were collected in the 2θ
range of 10◦–80◦ with scanning rate of 0.016 ◦/s. X-ray
photoelectron spectroscopy (XPS) was conducted on a Thermo VG
scientific ESCA MultiLab-2000 spectrometer with
a
Bruker advanced D8 powder
a
a
monochromatized Al Kα source (1486.6 eV) at constant analyzer
pass energy of 25 eV. The binding energy was estimated to be
accurate within 0.2 eV. All binding energies (BEs) were corrected
referencing to the C1s (284.6 eV) peak of the contamination carbon
as an internal standard.
General procedure of the oxidation of HMF
It was reported that the addition of trace amount of other
elements into the manganese oxide structure could improve its
catalytic activity.23,25 For example, Suib and co-workers reported
that the incorporation of a trace amount of Cs in the manganese
oxide resulted in superior activity enhancement in the aerobic
oxidation of amines to imines and oxidative cross-condensation of
two different amines, due to the change of the structure of the
catalyst such as the increase of basicity.23 Considering the fact that
the introduction of Cs in the manganese oxide greatly increased the
catalytic activity towards the oxidation of amines, we would like to
perform the oxidation of HMF into DFF by the use of Cs-doped
manganese oxide for this transformation without the use of
additive.
The oxidation reactions were conducted in a 40 mL autoclave.
Typically, HMF (63.0 mg, 0.5 mmol), Cs/MnOx (20 mg) and the
solvent (10 mL) were added into the autoclave. Then the autoclave
was purged five times with O2 to completely remove the air. After
being sealed, the autoclave was charged with 10 bar O2 at room
o
temperature, then it was heated from room temperature to 100 C
and then the reaction was performed at 100 oC for 12 h with a
mechanic stirring at 1000 RPM (Revolutions per minute).
Afterwards, the autoclave was cooled to room temperature,
depressurized. The chemicals in the reaction solution were
subjected to HPLC analysis.
One-pot transformation of fructose into DFF
Synthesis of DFF from fructose was performed by one-pot, two step
strategy. Briefly, fructose (90 mg, 0.5 mmol), the Amberlyst-15
catalyst (20 mg) and Dimethyl sulfoxide (DMSO, 10 mL) were added
into the autoclave, and the dehydration of fructose into HMF was
performed at 110 oC for 5 h in the air. Then the acid catalyst was
removed via centrifugation. The followed step of the oxidation of
HMF was performed by addition of the Cs/MnOx catalyst. The
oxidation of HMF was conducted at 110 oC for 10 h under 10 bar
oxygen pressure. Other steps were the same as described above.
Experimental Section
Materials
Pluoronic P123, (PEO20PPO70PEO20, molar mass 5800 g·mol−1) was
purchased from Sigma-Aldrich. HMF (98%) was purchased by Beijing
Chemicals Co. Ltd. (Beijing, China). DFF was purchased from the J&K
Chemical Co. Ltd., (Beijing, China). Manganese acetate tetrahydrate
(Mn(OAc)2·4H2O) was purchased from Aladdin Chemicals Co. Ltd.
(Beijing, China). All other chemical and solvents were purchased
from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Acetonitrile (HPLC grade) was purchased from Tedia Co. (Fairfield,
USA).
Product analysis
Furan compounds were analyzed by a UltiMate 3000 HPLC system
equipped with a UV detector. Before analysis, all samples were
filtered through a syringe filter (VWR, 0.22 μm PTFE). The furan
products were separated by a reversed-phase C18 column (250 ×
4.6 mm) and detected by with a UV detector at a wavelength of 266
nm. Acetonitrile and aqueous solution at the volume ratio of 25:75
Preparation of Cs-doped MnOx
In a typical synthesis, Mn(OAc)2·4H2O (4.90 g, 20 mmol) and 1-
butanol (13 mL) were added into a 100 mL round-bottomed flask
with a magnetic stirring. To this solution, Pluoronic P123 (1.97 g,
0.34 mmol) and 65 wt.% HNO3 (1.44 mL, 0.032 mmol) were added
and the mixture was continuously to be stirred at room
temperature until a solution was observed. Then, Cs2CO3 (32.6 mg,
0.1 mmol) was added to make the Mn/Cs molar ratio of 100/1. The
resulting clear solution was then maintained to be stirless at 120 °C
for 3 h under air. Then, the mixture was subjected to be filtrated,
and the black powder was washed with excess water and ethanol,
and dried in a vacuum oven overnight. The dried black powders
were further heated at 150 °C for 12 h and cooled to room
temperature under ambient conditions, followed by a second
heating step of 250 °C for 3 h. Mesoporous MnOx was prepared in
the same way without the addition of Cs2CO3.
o
was used as the mobile phase at 30 C with the flow rate was set at
0.8 mL/min. The calibration was controlled for drift by applying a
standard solution of reference compounds of known concentration
prior to every HPLC run.
HMF conversion and DFF yield are defined as follows:
HMF conversion = moles of HMF/moles of starting HMF×100%
DFF yield = moles of DFF/moles of starting HMF ×100%
Results and Discussion
Structural characterization of Meso Cs/MnOx
The as-prepared catalysts were firstly characterized by the XRD
technology. It was observed that the XRD patterns of MnOx and
Cs/MnOx demonstrated a huge difference. As displayed in Fig. 1a,
Catalyst characterization
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many sharp reflections appear in the XRD pattern of MnOx distribution. Cs/MnOx has a narrow size distribution centering at 36
indicating the high crystallinity of the product. All the reflections in nm, while MnOx had a large size distribution. The Brunauer–
the MnOx sample can be identified as cubic-phase Mn2O3 (JCPDS Emmett–Teller (BET) surface area, and pore volumes of MnOx were
Card, No 41-1442).26 The characteristic peaks at 2θ = 23.0°, 33.1°, determined to be 41.4 m2·g−1 and 0.11 m3·g−1. However, the surface
38.3°, 45.0°, 49.3°, 55.2° and 66.1° are diffraction planes of (211), area and the pore volume of Cs/MnOx greatly increased to 166.6
(222), (400), (332), (431), (440) and (622) respectively. However, m2·g−1 and 0.67 m3·g−1, respectively. These results indicated that the
the XRD peaks of the Cs/MnOx are too weak, which indicated the dope of Cs in MnOx not only destroy the crystalline nature of the
poorly crystalline nature of the material. XRD pattern (Fig. 1b) materials and change the valence state of Mn, but also it can
indicates that the product with poor crystallinity can be identified enhance the surface area and the pore volume of the catalyst.
as orthorhombic-phase g-MnO2 (JCPDS Card, No 44-0142) as well as
cubic-phase Mn2O3 (JCPDS Card, No 41-1442).26 These results
indicated that the introduction of Cs+ during the preparation of
MnOx caused the decrease of the crystalline nature of the
materials, but an increase of the valence state of the manganese.
(a)
80
60
40
20
0
0.12
0.08
0.04
0.00
(a)
Mn2O3(PDF#41ꢀ1442)
0
20
40
60
80
100
120
Pore Dimeter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
(b)
500
400
300
200
100
0
10
20
30
40
50
60
70
80
2
θ / degree
1.6
1.2
0.8
0.4
0.0
(b)
gꢀMnO2(PDF#44ꢀ0141)
Mn2O3(PDF#41ꢀ1442)
0
20
40
60
80
100
120
Pore Dimeter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Fig. 2 Nitrogen adsorption–desorption isotherm and pore size distribution (inset)
of MnOx (a) and Cs/MnOx (b).
10
20
30
40
50
60
70
80
The enhanced surface area and the pore volume of Cs/MnOx
were also reflected by the SEM images. Fig. 3 shows the SEM
images of MnOx and Cs/MnOx. SEM image of MnOx (Fig. 3a)
revealed the random packing of nano-oxide particles with tight
structure. Seeing from the TEM image of Cs/MnOx (Fig. 3b), it was
observed that the size of nano-oxide particles became smaller,
while much more pores were present. The SEM results were
consistent with the BET results that the Cs/MnOx catalyst had a
larger surface area and larger pore volume.
2
θ
/ degree
Fig. 1 X-ray diffraction patterns of MnOx (a) and Cs/MnOx (b).
The textural properties of the prepared samples were
characterized by N2 adsorption–desorption measurements. The
isotherms and the Barrett–Joyner–Halenda (BJH) pore size
distributions of the samples are shown in Fig. 2. N2 adsorption–
desorption isotherm curves of MnOx and Cs/MnOx both show a type
IV adsorption isotherm with a H4 hysteresis loop, which indicated
MnOx and Cs/MnOx both processed mesoporous structures. In
addition, MnOx and Cs/MnOx showed a huge difference in the size
The surface valence state and concentration of manganese in
MnOx and Cs/MnOx were investigated by X-ray photoelectron
spectroscopy (XPS) analysis. As shown in Fig. 4, the Mn 2p XPS
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spectra consist of two peaks, which were assigned as Mn 2p3/2 and indicating the presence of multiple oxygen species. As shown in Fig.
Mn 2p1/2 core levels. The binding energies of 641.2 and 642.1 eV 5a, three types of surface oxygen species can be identified from
can be attributed to the Mn3+ and Mn4+ oxidation state, their binding energies. The peaks at low binding energies (529.3–
respectively. Besides the two peaks, the third peak with the binding 529.4 eV), medium binding energies (530.4–530.5 eV), and high
energy at 643.8 eV, which is attributed to incomplete binding
energies (531.7–532.0 eV) are ascribed to the lattice
decomposition of Mn nitrate owing to relatively low calcination oxygen atoms, oxygen vacancies or surface adsorbed O (OH) groups
temperature.27 In according with the XRD results, the main , and adsorbed water, respectively.28 Interestingly, it was noted that
oxidation state for MnOx is +III, while the main oxidation state of the Cs/MnOx catalyst have a higher peak area of the oxygen
Cs/MnOx is +IV as determined by XPS technology.
vacancies or surface adsorbed O (OH) groups, which was believed
to be beneficial for the oxidation reaction.
(a)
O1s
63%
19%
532
18%
536
534
530
528
Binding energy (eV)
(b)
O1s
60%
16%
532
24%
Fig. 3 SEM images of theMnOx (a) and Cs/MnOx (b).
2p3/2
Mn 2p
536
534
530
528
526
2p1/2
Binding energy (eV)
642.1 eV
Mn4+
643.8 eV
CsꢀMn
Fig. 5 The high resolution XPS spectra of the O 1s in the MnOx (a) and Cs/MnOx
(b) catalyst
.
Aerobic oxidation of HMF over Cs/MnOx in different solvents
Mn
641.2 eV
Mn3+
The catalytic activity of the as-prepared Cs/MnOx catalyst was
evaluated by the oxidation of HMF. Initially, the reaction was
performed at the temperature of 100 oC and 10 bar oxygen
pressure in different solvents, as different solvents have different
properties such as polarity and acid-base properties, which would
influence the catalytic activity of the Cs/MnOx catalyst. As shown in
Table 1, the catalytic activity of Cs/MnOx was observed to be greatly
affected by the reaction solvents. Reactions in strong polar solvents
such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide
(DMSO) produced much higher HMF conversion than other solvents
(Table 1, Entries 1, 2 vs 3~7). Other researchers also found that
DMF was a good solvent for the oxidation of HMF over manganese
660
655
650
645
640
635
Binding energy (eV)
Fig. 4 The high resolution XPS spectra of the Mn 2P in the Cs/MnOx and MnOx
catalyst.
O1s XPS spectra were used to probe the surface oxygen
species of MnOx and Cs/MnOx (Fig. 5). Both of them showed a
broad peak of binding energy in the range of
∼526 to 536 eV,
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based catalysts.29 The possible reason should be that DMF with a catalytic performance of the Cs/MnOx catalyst can be attributed to
strong polarity can well dissolve HMF, and also the Cs/MnOx the following reasons.
catalyst has a good dispersion in DMF. Besides DMF and DMSO, the
First, the crystalline component of the Cs/MnOx catalyst
oxidation of HMF in the solvents with strong polarity also produced
characterized by XRD (Fig. 1b) is MnO2 and Mn2O3, while the
relative higher HMF conversions than the reactions in the solvents
crystalline component is Mn2O3 for the MnOx catalyst. MnO2 should
with weak polarity (Table 1, Entries 3~5 vs 6~7), which also
demonstrate stronger oxidation ability than Mn2O3 in the MnOx
confirmed the above proposed reasons. However, H2O with the
catalyst. XPS results also revealed that the valence state of Mn in
strongest polarity among all of the tested solvents was proved to be
the Cs/MnOx catalyst is +IV with a part of Mn in +III, while that was
inferior to organic solvents, which gave rise to the lowest HMF
in reverse for the valence state of Mn in MnOx. Second, much more
conversion (Table 1, Entry 8). The poor catalytic performance of the
defect and oxygen vacancies were also observed in the Cs/MnOx
Cs/MnOx catalyst in water was possibly due to the fact that water
catalyst. The defects and oxygen vacancies in the catalysts were
can stabilize HMF via the formation of hydrogen bond. As far as the
also beneficial for the oxidation reaction with respect to the
oxidation products, DFF was produced with a high selectivity in all
enhancement of the absorption and activation of the substrates as
solvents. Besides DFF, a trace amount of the other oxidation
well as the oxygen molecules. Thirdly, BET and SEM results revealed
product 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) was also
that much larger surface area and much more abundant pores were
detected by HPLC, which indicates that DFF is stable in DMF in the
present in the Cs/MnOx catalyst, which promoted the oxidation of
presence of the Cs/MnOx catalyst under the reaction conditions
HMF. Much more substrates would absorb on the active sites of the
employed in this work. The results listed in Table 1 reveals that
catalysts, which have larger surface area and much more abundant
DMF was the best solvent for the oxidation of HMF into DFF, which
pores. In addition, the mass transfer of the substrates would
produced both the highest conversion and the highest selectivity.
become much more free in the Cs/MnOx catalyst with larger surface
Table 1. The results of HMF oxidation in different solvents. a
area and more abundant pores. Fourthly, it was reported that the
doped Cs in the Cs/MnOx catalyst also enhanced the basicity of the
Con.
(%)
Sel.DFF Sel.HMFCA
catalyst.25 The basic sites of the Cs/MnOx can be helpful for the
cleavage of C-H bond, which also promote the oxidation of HMF
into DFF. In addition, we have also used the commercial MnO2 as
the catalyst for the oxidation of HMF, which produced a very low
conversion (Table 1, Entry 11). These results further confirmed that
addition of Cs as well as the presence of Mn3+ were very important
for the oxidation of HMF into DFF.
Entry
Catalyst
Solvent
(%)
(%)
1
2
3
4
5
6
7
8
9
DMF
DMSO
76.8
71.6
42.3
38.1
27.1
11.3
10.9
8.3
98.1
1.9
Cs/MnOx
Cs/MnOx
Cs/MnOx
Cs/MnOx
Cs/MnOx
Cs/MnOx
Cs/MnOx
Cs/MnOx
-
89.8
98.2
98.2
97.1
96.9
95.3
76.2
-
10.2
1.8
1.8
2.9
3.0
4.6
23.8
-
iso-Propanol
Acetonitrile
THF
Giving the above experiments results and the previous work on
the oxidation of HMF with manganese catalysts,
a plausible
mechanism was proposed for the oxidation of HMF by the Cs/MnOx
catalyst (Scheme 1). The oxidation of HMF into DFF is proposed to
proceed via the Mn4+/Mn3+ redox cycles including the HMF
oxidation with the lattice oxygen atoms on OMS-2 and the
Toluene
Ethyl acetate
H2O
30
reduction of Mn4+ to Mn3+ (Step 1) Then the reoxidation of Mn3+
to Mn4+ as well as the replenishment of the consumed lattice
oxygen atoms (i.e., oxygen vacancies) occurred by molecular oxygen
(Step 2) to finish the catalytic cycle.
DMF
0
10b
11
a
MnOx
DMF
10.2
2.4
85.3
89.8
14.3
0
MnO2
DMF
Reaction conditions: HMF (63 mg, 0.5 mmol), catalyst
(Cs/MnOx, 20 mg), solvent (10 mL), O2 (10 bar), 100 °C and 4 h. b
catalyst (MnOx, 20 mg), 10 h. c MnO2 (20 mg), 10 h.
Some control experiments were also performed in order to
give some insights into the oxidation of HMF over the Cs/MnOx
catalyst. The reaction was performed in the absence of the catalyst,
affording no HMF conversion under the same conditions (Table 1,
Entry 9). For comparison, the reaction was also performed by the
use of MnOx as the catalyst (Table 1, Entry 10), which was prepared
by the same method for the Cs/MnOx catalyst without the addition
of Cs2CO3. The conversion of HMF was attained in a low value of
10.2% after 10 h under 10 bar oxygen pressure at 100 oC, while that
was high up to 76.8% under the same conditions by the use of
Cs/MnOx (Table 1, Entries 1 vs 10). Interestingly, the selectivity of
DFF from HMF oxidation catalyzed by Cs/MnOx was also higher than
that with MnOx (Table 1, Entries 1 vs 10). These results suggested
that the introduction of Cs into MnOx not only improve the catalytic
activity, but also enhance the selectivity of DFF. The superior
Scheme 1. Proposed reaction mechanism of HMF oxidation over the Cs/MnOx
catalyst.
Aerobic oxidation of HMF over Cs/MnOx at different temperatures
With DMF as the best solvent, we would like to perform other
reactions by the optimization of the reaction conditions with the
aim to further improve DFF yield. Fig. 6 lists the results of HMF
oxidation at different reaction temperature under 10 bar oxygen
pressure in DMF. It was noted that the reaction temperature
showed a great effect on the catalytic performance of Cs/MnOx
towards the oxidation of HMF. Generally, HMF conversion greatly
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increased with an increase of the reaction temperature and then
the increasing trend became slow. For example, a low HMF
conversion of 17.7% was attained at the low reaction temperature
of 40 oC, and it greatly increased to 41.2% at 60 oC. To the best of
our knowledge, there were few reports on the oxidation of HMF
into DFF under such low reaction temperatures. The high catalytic
activity of the Cs/MnOx catalyst at low reaction temperatures
demonstrates this method is promising for the practical application
in the synthesis of DFF from HMF, as it is energy-saving for
reactions at low temperatures. Further increasing the reaction
temperature to 80 and 100 oC, HMF conversion still greatly
increased to 65.2% and 92.0%, respectively. These results indicated
that the oxidation of HMF was sensitive to the reaction
temperature. Nearly quantitative HMF conversion was achieved
after 10 h at 110 oC under 10 bar oxygen pressure.
120
100
80
60
40
20
0
Con. (%)
Sel. of DFF (%)
Sel. of HMFCA (%)
5
10
20
30
40
As far as the reaction selectivity of the products, the reaction
temperature almost showed no influence on DFF selectivity. DFF
selectivity was maintained in the range from 95.4%~97.9% at
different temperatures. Besides the oxidation product of DFF,
HMFCA was also determined with a low selectivity. The total
selectivity of DFF and HMFCA was up to 100% without no other side
reactions. In fact, some side reactions such as the condensation and
polymerization of HMF into humins were often encountered
especially in the presence of an acid or base catalyst.31 To our
pleasure, there were no such side reactions in our catalytic system.
Catalyst amount (mg)
Fig. 7 The results of HMF oxidation into DFF with different amount of Cs/MnOx
catalyst. Reaction conditions: HMF (63 mg, 0.5 mmol), DMF (10 mL), O2 (10 bar),
100 °C and 4 h.
As stated above, the oxidation of HMF into DFF in the solvent of
DMF with molecular oxygen as the oxidant was promoted by the
Cs/MnOx catalyst. Therefore, the effect of the catalyst loading on
the oxidation of HMF was studied and these results are depicted in
Fig. 7. HMF conversion gradually increased with an increase of the
catalyst loading and then remained stable. For example, HMF
conversion was achieved in 35.5% after 4 h by the use of 5 mg of
the Cs/MnOx catalyst and it greatly increased to 54.8% under the
same reaction conditions by the use of 10 mg of the Cs/MnOx
catalyst. The increase of HMF conversion with an increase amount
of the Cs/MnOx catalyst was due to the fact that many more
catalytic sites were accessed to HMF with higher catalyst loading.
Further increasing the catalyst loading from 10 mg to 40 mg, HMF
conversion contentiously increased with the increase of the catalyst
loading. A high HMF conversion of 98.8% was attained after 4 h at
100 oC with 40 mg of the Cs/MnOx catalyst. As far as the selectivity
of DFF, it remained stable around 96.0~98.3% with all different
catalyst loading, suggesting that the selectivity of the products were
not influenced by the catalyst loading.
120
Con.(%)
Sel. of DFF (%)
Sel. of HMFCA (%)
100
80
60
40
20
0
40
60
80
110
Temperature (°1C00)
Time course of the oxidation of HMF into DFF over the Cs/MnOx
catalyst
Fig. 6 The effect of reaction temperature on the oxidation of HMF. Reaction
conditions: HMF (63 mg, 0.5 mmol), catalyst (Cs/MnOx, 20 mg), solvent (DMF, 10
mL), O2 (10 bar) and 10 h.
To get more insights into the oxidation of HMF into DFF, the time
course of HMF oxidation into DFF over the Cs/MnOx catalyst was
also recorded. Fig. 8 shows the distribution of products at different
reaction time points. The molar percentage of DFF was observed to
gradually increase during the reaction process. On the meanwhile,
the molar percentage of HMF was noted to decrease during the
reaction process. After the reaction time of 12 h, a HMF conversion
of 98.4% and a DFF yield of 94.7% were attained at 100 oC and 10
bar oxygen pressure. HMFCA as the side product was also detected
with a little amount during the reaction process. These results
indicated that the transformation of HMF into DFF was a direct way
without the formation of the intermediate.
Aerobic oxidation of HMF with different amount of Cs/MnOx
6 | J. Name., 2012, 00, 1-3
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100
80
60
40
20
0
100
80
60
40
20
DFF
1.6
1.2
HMF
DFF
HMFCA
0.8
0.4
HMFCA
8.0
4.0
12.0
16.0
0
0
2
4
6
8
10
12
O2 Pressure (bar)
Time (h)
Fig. 10 Dependence of activities and selectivities to DFF and HMFCA on O2
pressures for aerobic oxidation of HMF over the Cs/MnOx catalyst. Reaction
Fig. 8 The time course of HMF oxidation into DFF over the Cs/MnOx catalyst.
Reaction conditions: HMF (63 mg, 0.5 mmol), catalyst (Cs/MnOx, 20 mg), solvent
(DMF, 10 mL), O2 (10 bar) and 100 °C.
conditions: HMF concentration (100 mmol/L), 1~16 bar O2, 100 °C, Cs/MnOx (20
mg), DMF (10 mL).
Kinetic and mechanism studies for the oxidation of HMF
The oxidation of HMF was also studied at different oxygen
pressures to investigate its influence on the catalytic performance
of the Cs/MnOx catalyst. The oxidation of HMF was performed in
the air at 100 oC with a high HMF concentration of 100 mmol/L.
Similarly, the product selectivity was not sensitive to the O2
pressures. As shown in Fig. 10, the selectivity of DFF and the
selectivity of HMFCA remained 98.5~99.1% and 0.9~1.4% at similar
HMF conversions (~30%), respectively, when O2 pressures increased
from 1 to 16 bar. However, the effect of O2 pressures on the
activities strongly depends on the HMF concentration. The activities
of the Cs/MnOx catalyst increased almost linearly from 0.68 to 1.38
mmolHMF/(mg·h) with increasing O2 pressures from 1 to 8 bar, and
then a slow trend was observed. The reaction order was estimated
to be 0.53 with respect to HMF concentrations from the linear
range, similar to the order in the HMF concentrations, indicating
the dissociative chemisorption of O2 and HMF on the active sites of
the Cs/MnOx catalyst, as observed on Ru/C.16 This is due to the
increase of oxygen concentration in the reaction solution. The
enhancement of the catalyst activity of the Cs/MnOx catalyst with
an increase of oxygen pressure was due to increase of oxygen
concentration in the reaction solution. The higher concentration of
oxygen in the reaction solution resulted in much more surface
coverage of oxygen species. Fu and coworkers29 also found that the
change of oxidant from air to oxygen increased the HMF conversion
dramatically from 39% to 99% on OMS-2.
In order to give the insight into the oxidation of HMF into DFF over
the Cs/MnOx catalyst, the effect of HMF concentration, the effects
of oxygen pressure as well as the activation energy were also
studied. Fig. 9 shows the effect of HMF concentrations on the
activity and selectivity for HMF oxidation over the Cs/MnOx catalyst
at 100 °C and 10 bar O2 pressure. DFF selectivity remained nearly
stable (97.8~99.2%) independent with the HMF concentration, and
also the HMFCA selectivity was almost unchanged (0.5~2%). At
similar HMF conversions (~30%), the activities increased almost
linearly from 0.79 to 1.78 mmolHMF/(mg·h) with increasing HMF
concentrations from
5 to 30 mmol/L, which then increased
gradually to reach a constant value of ~1.8 mmolHMF/(mg·h) at
above 60 mmol/L. Such effect indicates the saturated adsorption of
HMF on the active surfaces of Cs/MnOx with the HMF concentration
of 30 mmol/L for 20 mg of the Cs/MnOx catalyst. Similar
phenomenon were also observed over other catalytic systems for
HMF oxidation with Ru/C and OMS-2 catalysts.16,31 The reaction
order was calculated to be 0.44 with respect to HMF concentrations
from the linear range.
100
2.0
1.6
1.2
0.8
DFF
80
60
40
20
0
HMFCA
0
20 40 60 80 100 120 140 160
HMF Concentration, mmol/L
Fig. 9 Dependence of activities and selectivities to DFF and HMFCA on HMF
concentrations for aerobic oxidation of HMF on Cs/MnOx. Reaction conditions:
100 °C, O2 (10 bar), HMF (5~150 mmol/L), Cs/MnOx (2~20 mg) and DMF (10 mL).
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Fig. 11 Dependence of activities and selectivities to DFF and HMFCA on reaction
temperatures for aerobic oxidation of HMF over the Cs/MnOx catalyst. Reaction
conditions: HMF concentration (100 mol/L), O2 (10 bar), 353–393 K, Cs/MnOx (20
mg), DMF (10 mL).
100
80
60
40
20
0
4
DFF
ꢀ3.6
ꢀ4.0
ꢀ4.4
Fig. 11 shows the effect of the reaction temperature on the
oxidation of HMF over the Cs/MnOx catalyst at similar HMF
conversions (~30%). It was noted that the catalytic activity
increased dramatically from 0.52 to 4.17 mmolHMF/(mg·h) with
increasing the temperature from 353 to 393 K and 10 bar oxygen
pressure. The apparent activation energy (Ea) of HMF oxidation
over the Cs/MnOx catalyst was estimated to be 58.0 kJ/mol.
Compared with other reported methods for the oxidation of HMF
into DFF over non-noble metal catalysts (Table 2), the Cs/MnOx
catalyst has a much more activation energies than the VOx/TiO2
catalyst (67–77 kJ/mol)30,32 and OMS-2 catalyst (81.0 kJ/mol).29
These results show that OMS-2 is advantageous for the aerobic
oxidation of HMF, which can promote the oxidation of HMF into
DFF at low temperatures (Fig. 6).
3
2
1
0
Ea=58.0 kJ/mol
2.70 2.75 2.80 2.85
1000/T (1/K)
HMFCA
370
350
360
380
390
Temperature (K)
Table 2. Comparison of the oxidation of HMF over the Cs/MnOx catalyst with other catalysts.
Temperature
Con.
(%)
Sel. of DFF
(%)
Ea
(kJ·mol-1)
Entry
Catalyst
Solvent
References
(℃
)
1
2
3
Cs/MnOx
VOx/TiO2
OMS-2
DMF
DMSO
DMF
110
125
110
99.8
99
96.7
71
58.0
67–77
81.0
This work
Ref. 30,32
Ref. 29
100
97.2
One-pot conversion of fructose into DFF
fructose was high up to 99% (Table 3, Entry 2). The low yield of DFF
from fructose in the one-step reaction may be due to the formation
of humins or the undesired oxidation of fructose by Cs/MnOx in the
presence of molecular oxygen. In fact, a control experiment was
also performed by the only use of the Cs/MnOx catalyst (Table 3,
Entry 3). In fact, a high fructose conversion of 99% was also
attained, but there were no products of HMF and DFF. The possible
reaction in the presence of the Cs/MnOx catalyst was the oxidation
of fructose.29 To avoid the side reactions, a “one-pot, two-step
strategy” was adopted for the synthesis of DFF from HMF. The first
step of the dehydration of fructose produced HMF in a yield of 90%
at 110°C after 5 h in DMSO, and then the oxidation of HMF was
further conducted at 110 oC after 10 h under 10 bar oxygen
pressure. To our pleasure, DFF was produced in an overall yield of
82% by this method (Table 3, Entry 4), which further confirms that
the two-step strategy can avoid the oxidation of fructose by the
Cs/MnOx catalyst.
Table 3. One-pot synthesis of DFF from fructose using Amblyerst-15 and
Cs/MnOx.
Oxidation Con. HMF
catalyst
DFF
Yieid (%)
Entry
Acid catalyst
(%)
(%)
1a
2b
3c
4d
Amberlyst-15
Amberlyst-15
-
-
>99
90
<1
<5
0
Cs/MnOx
Cs/MnOx
Cs/MnOx
>99
>99
>99
10
0
Amberlyst-15
<1
82
a Fructose (0.5 mmol), Amberlyst-15 (0.02 g), DMSO (10 mL), Air,
b
110 °C and 5 h. One step reaction. Reaction conditions:
Amberlyst-15 (0.02 g) and Cs/MnOx (0.02 g) in 110 °C, O2 (10
bar) for 15 h. c Cs/MnOx (0.02 g), DMSO (10 mL), O2 (10 bar), 110
Catalyst recycling experiments
d
°C and 5 h.
Two-step reaction catalyst separation. The first
The stability and recyclability of the Cs/MnOx catalyst were finally
studied, and the oxidation of HMF was used as the model reaction,
which was performed at 373 K and 10 bar O2. After the first run, the
Cs/MnOx catalyst was collected via centrifugation and washed with
large amount of water and ethanol. Then the spent Cs/MnOx
catalyst was dried and used for the second run. As shown in Fig. 12,
HMF conversion remained high during the recycling runs. The ICP-
AES analysis of the filtrate after each reaction cycle revealed that
the leaching of the manganese species into the reaction solution
was below the limit (<0.1%). However, a slight decrease of the HMF
conversion was still observed. The possible reason should be caused
by the deposition of trace amount of humins on the surface of the
Cs/MnOx catalyst,31 which blocked the active sites of the catalyst.
step is the same as Entry 1, The second step: Cs/MnOx (20 mg)
was added, and the reaction was performed at 110 oC under O2
(10 bar) pressure for 10 h.
The excellent catalytic performance of the Cs/MnOx towards the
oxidation of HMF into DFF inspired us to carry out the synthesis of
DFF directly from the readily available and cheap fructose via one-
pot reaction strategy by the use of using of the commercial
Amberlyst-15 as the acid catalyst and Cs/MnOx. As shown in Table
3, HMF can be produced HMF from fructose in a yield of 90% at
110°C after 5 h in DMSO (Table 3, Entry 1). Then we started to
perform the one-pot transformation of fructose into DFF by the
simultaneous use of Amberlyst-15 and Cs/MnOx. However, a poor
yield of DFF was obtained in this case, but the conversion of
8 | J. Name., 2012, 00, 1-3
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2
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a mesoporous Cs-promoted manganese oxide
(Cs/MnOx) was successfully prepared, and it can be used as an
efficient, selective, and reusable, and therefore environmentally
benign catalyst for the aerobic oxidation of HMF into DFF. The
Cs/MnOx catalyst was able to activate molecular oxygen for the
oxidation of HMF into DFF, affording a high HMF conversion of
o
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larger surface area and more abundant pores as compared with the
MnOx catalyst. Kinetic studies showed that the Cs/MnOx catalyst
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O2 concentrations. It was also calculated that the Cs/MnOx catalyst
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compared with the other kinds of heterogeneous non-noble metal
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experiments without the decrease of the catalytic activity and
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from fructose via acid-catalyzed dehydration and successive aerobic
oxidation in a one-pot reaction by two steps, which afforded DFF in
an overall yield of 82%.
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Conflicts of interest
There are no conflicts to declare.
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This work was financially supported by the National Natural Science
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