Aerobic oxidation of biomass derived 5-hydroxymethylfurfural into 5-hydroxymethyl-2-furancarboxylic acid catalyzed by a montmorillonite K-10 clay immobilized molybdenum acetylacetonate complex

Article abstract of DOI:10.1039/c4gc00062e

In this study, we have successfully prepared a heterogeneous catalyst (K-10 clay-Mo) by the immobilization of bis(acetylacetonato) dioxo-molybdenum(vi) [MoO₂(acac)₂] on montmorillonite K-10 clay. The structure of the resultant catalyst was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electronic microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy. The catalytic activity of K-10 clay-Mo was tested in the aerobic oxidation of 5-hydroxymethylfurfural (HMF). Although a molecule of HMF contains one hydroxyl group and one aldehyde group, to our surprise, the catalyst showed high catalytic activity for the oxidation of the aldehyde group of HMF into 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). A variety of important reaction parameters such as the reaction temperature, catalyst amount, and solvent were explored. HMFCA could be obtained in a high yield of 86.9% with a HMF conversion of 100% after a short reaction time of 3 h in toluene. More importantly, the catalyst K-10 clay-Mo could be reused several times without a significant loss of its catalytic activity. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00062e

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Aerobic oxidation of biomass derived
5-hydroxymethylfurfural into 5-hydroxymethyl-
2-furancarboxylic acid catalyzed by a
montmorillonite K-10 clay immobilized
molybdenum acetylacetonate complex
Cite this: DOI: 10.1039/c4gc00062e
Zehui Zhang,* Bing Liu, Kangle Lv, Jie Sun and Kejan Deng*
In this study, we have successfully prepared a heterogeneous catalyst (K-10 clay-Mo) by the immobiliz-
ation of bis(acetylacetonato) dioxo-molybdenum(VI) [MoO2(acac)2] on montmorillonite K-10 clay. The
structure of the resultant catalyst was characterized by X-ray diffraction (XRD), X-ray photoelectron spec-
troscopy (XPS), transmission electronic microscopy (TEM), and Fourier transform infrared (FTIR)
spectroscopy. The catalytic activity of K-10 clay-Mo was tested in the aerobic oxidation of 5-hydroxy-
methylfurfural (HMF). Although a molecule of HMF contains one hydroxyl group and one aldehyde group,
to our surprise, the catalyst showed high catalytic activity for the oxidation of the aldehyde group of HMF
into 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). A variety of important reaction parameters such
as the reaction temperature, catalyst amount, and solvent were explored. HMFCA could be obtained in a
high yield of 86.9% with a HMF conversion of 100% after a short reaction time of 3 h in toluene. More
importantly, the catalyst K-10 clay-Mo could be reused several times without a significant loss of its
catalytic activity.
Received 13th January 2014,
Accepted 11th February 2014
DOI: 10.1039/c4gc00062e
www.rsc.org/greenchem
and it can be used as a sustainable precursor for the prepa-
ration of a broad range of chemicals and liquid transportation
Introduction
At present, we are entering an epoch of diminishing avail- fuels.6 In this context, the synthesis of HMF has been exten-
ability of petrochemical resources, which are mainly used to sively studied in various catalytic systems from edible
produce chemical products and energy for our society.1 In biomass.7 The oxidation of HMF can generate several kinds of
order to reduce the dependence on fossil fuels, it is worthwhile furan compounds such as 2,5-furandicarboxylic acid (FDCA)
to develop new routes to effectively convert renewable and 2,5-diformylfuran (DFF). FDCA and DFF have been
resources into fuels and chemicals.2 Unlike other renewable regarded as a promising monomer for the synthesis of furan-
resources, biomass is the only widely available carbon source containing polymers and fine chemicals.8 In recent years,
apart from fossil resources. Therefore, only biomass resources there is an overwhelming research on the synthesis of FDCA
can contribute to the promising development of effective and DFF from the oxidation of HMF.9 The oxidation of the
methods for the production of fuels and chemicals from aldehyde group in HMF can produce another important oxi-
renewable resources.3 In recent years, an enormous effort has dation product HMFCA. HMFCA serves not only as a monomer
been devoted to the development of effective methods for the in the synthesis of various polyesters,10 but also shows anti-
production of fuels and chemicals from biomass.4
tumor activity11 and is a promising building block of an inter-
One of the most attractive directions in biorefinery is leukin inhibitor.12 Unlike the research interest towards the
devoted to the production of furan derivatives, such as furfural synthesis of FDCA and DFF, the synthesis of HMFCA from the
and 5-hydroxymethylfurfural (HMF).5 HMF, which is the oxidation of HMF has remained limited. The reason for this
acidic-catalyzed dehydration product of C6 based carbo- might be due to the selective oxidation of the aldehyde group
hydrates, is widely considered as a versatile platform chemical, into a carboxylic group, leaving the hydroxyl group intact. As
shown in Scheme 1, the oxidation of HMF can generate
various oxidation products including 5-formyl-2-furan-
Department of Chemistry, Key Laboratory of Catalysis and Material Sciences of the
carboxylic acid (FFCA), HMFCA, DFF and FDCA. There are few
State Ethnic Affairs Commission & Ministry of Education, South-Central University
reports on the synthesis of HMF into HMFCA. Mitsukura et al.,
for Nationalities, MinYuan Road 182, Wuhan, P.R. China.
developed a method for the synthesis of HMFCA by the
E-mail: zehuizh@mail.ustc.edu.cn, dengkj@scuec.edu.cn
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by Beijing Chemicals Co. Ltd (Beijing, China). 5-Hydroxy-
methyl-2-furancarboxylic acid (98%) was purchased from J & K
Co. Ltd (Beijing, China). Acetonitrile (HPLC grade) was pur-
chased from Tedia Co. (Fairfield, USA). Montmorillonite K-10
clay was purchased from Ziyi Chemicals Co. Ltd (Shanghai,
China), and the chemical composition (average value) was as
follows: SiO2 (73.0%), Al2O3 (14.0%), Fe2O3 (2.7%), CaO
(0.2%), MgO (1.1%), Na2O (0.6%), K2O (1.9%). All the solvents
were purchased from Sinopharm Chemical Reagent Co., Ltd
(Shanghai, China), and were freshly distilled before use.
Scheme 1 Schematic illustration of the oxidation products during the
oxidation process of HMF.
Catalyst preparation
The catalyst was prepared by the immobilization of the
MoO2(acac)2 complex on the K-10 clay (abbreviated as K-10
clay-Mo). Typically, prior to use, the K-10 clay was dried at
110 °C for 3 h to remove adsorbed water. MoO2(acac)2
(0.6325 g) was firstly dissolved in 50 mL of dry toluene with
magnetic stirring. Then the K-10 clay (1.0 g) was added and
the mixture continuously stirred at room temperature for 24 h.
After reaction, the K-10 clay-Mo catalyst was obtained by fil-
tration and thoroughly washed with hot ethanol several times
to remove any unreacted metal complex. Finally, the K-10 clay-
Mo catalyst was dried at 110 °C for 5 h.
biocatalytic oxidation method using whole cells LF14 as the
catalyst.13 Recently, the classic Cannizzaro reaction has also
been used for the synthesis of HMFCA.14 The Cannizzaro reac-
tion produced a low HMFCA yield, and a high concentration of
alkaline waste. Recently, supported noble metals have been
designed for the synthesis of HMFCA from the aerobic oxi-
dation of HMF.15 Some of them require a high oxygen pressure
or NaOH. For example, Davis et al.,15a have prepared supported
gold for the oxidation of HMF , achieving a HMFCA yield of up
to 90% at 690 kPa O2 in 0.3 M NaOH. Therefore, the develop-
ment of synthetic strategies for the effective synthesis
of HMFCA from HMF is a challenging and promising area in
biorefinery.
Catalyst characterization
Transmission electron microscope (TEM) images were
obtained using an FEI Tecnai G2-20 instrument. The sample
powder was firstly dispersed in ethanol and dropped onto a
copper grid for observation. FT-IR measurements were
recorded on a Nicolet NEXUS-6700 FTIR spectrometer with a
spectral resolution of 4 cm−1 in the wave number range of
500–4000 cm−1. The X-ray powder diffraction (XRD) patterns of
the samples were determined with a Bruker advanced D8
powder diffractometer (Cu Kα). All XRD patterns were collected
Montmorillonite K-10 clay is a type of acidic stratified sili-
cate mineral with a three-layer structure,16 which is commer-
cially available with a low cost. These interlayer cations of the
K-10 clay can undergo exchange with various cations from
external solutions. Therefore, the K-10 clay has been con-
sidered as a promising support for the immobilization of
homogeneous complexes to construct various heterogeneous
catalysts.17 At present, dioxomolybdenum(VI) complexes are
reported to show high catalytic activity for alkene epoxidation
using environmentally benign oxidants such as hydrogen
peroxide and alkyl hydroperoxides.18,19 However, the studies
on the oxidation of alcohols with dioxomolybdenum(VI)
complexes as catalysts are still very limited.20 In 2012, Gao
et al., prepared a new heterogeneous dioxo-molybdenum(VI)
complex, by the reaction of MoO2(acac)2 with aldehyde group-
modified crosslinked polystyrene (CPS) microspheres, and
found that this catalyst showed the ability to activate molecular
oxygen for the oxidation of benzyl alcohols to benzaldehyde.20b
Herein, inspired by the high cation-exchange capacity of
K-10 clay and the ability of dioxomolybdenum(VI) complexes in
the activation of molecular oxygen, a heterogeneous catalyst
was prepared by the immobilization of MoO2(acac)2 on K-10
clay. The prepared catalyst showed high catalytic activity for
the oxidation of HMF into HMFCA with molecular oxygen.
in the 2θ range of 10–80° with a scanning rate of 0.016° s−1
.
X-Ray photoelectron spectroscopy (XPS) was conducted on a
Thermo VG scientific ESCA MultiLab-2000 spectrometer with a
monochromatized Al Kα source (1486.6 eV) at a constant analy-
zer pass energy of 25 eV. The binding energy was estimated to
be accurate within 0.2 eV. All binding energies (BEs) were cor-
rected with referencing to the C1s (284.6 eV) peak of the con-
tamination carbon as an internal standard.
Procedures for the determination of Mo content by ICP-AES
The molybdenum content in the K-10 clay-Mo catalyst was
quantitatively determined by an inductively coupled atomic
emission spectrometer (ICP-AES) on an IRIS Intrepid II XSP
instrument (Thermo Electron Corporation). The acid digestion
was composed of concentrated HNO3 (7.5 mL), 48 wt% HF
(1.5 mL) and 2.5 mL concentrated HCl (2.5 mL). 0.5 g of the
sample was digested in the above acid digestion solution on a
flat electric furnace. After digestion, the sample was made up
to 25 mL with deionized water and centrifuged at 3000 rpm.
The supernatant liquid was taken for ICP-AES analysis. The
Experimental section
Materials and method
MoO2(acac)2 was purchased from Aladdin Chemicals Co. Ltd ICP-AES operation conditions were as follows: incident power:
(Shanghai, China). 5-Hydroxymethylfurfural (98%) was supplied 1.1 kW, carrier gas flow rate: 0.8 L min−1, auxiliary gas flow
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rate: 0.4 L min−1, coolant gas flow rate: 16 L min−1, obser-
vation height: 10 mm.
General procedure for the aerobic oxidation of HMF to
HMFCA
The aerobic oxidation of HMF was carried out in a 25 mL
round bottom flask, which was equipped with a reflux conden-
ser and capped with a balloon. Typically, HMF (1 mmol,
126 mg) was firstly dissolved into toluene (8 mL) with stirring.
The K-10 clay-Mo catalyst was then added to the reaction
mixture, and the vessel flushed with oxygen at a rate of 20 mL
min−1 from the bottom of the reactor, and the reaction was
carried out at 110 °C at a constant stirring rate of 600 revolu-
tions per minute (rpm) to exclude the effect of the stirring rate
on the reaction. Time zero was recorded when oxygen was
flushed into the reaction mixture. After reaction, the K-10 clay-
Mo catalyst was removed from the reaction mixture and the
products were analyzed by HPLC.
Fig. 1 XRD patterns of the samples. (a) K-10 clay; (b) K-10 clay-Mo;
(c) the mechanical mixing of the two solids [MoO2(acac)2 and K-10 clay].
broad 2θ peaks located at about 20, 35, 54 and 62°, which are
assigned to the diffractions of the (110), (105), (210) and (300)
reflections, respectively.21 In addition, a sharp peak at 2θ =
26.5 belongs to quartz.22 On comparison of the XRD pattern of
the K-10 clay with that of the K-10 clay-Mo, there is no differ-
ence between them, which indicated that the clay structure is
retained after the immobilization of MoO2(acac)2. This situ-
ation is consistent with the previous report in the literature for
Mn(III) salen complexes immobilized between the clay layers of
K-10 clay.23 In addition, it was reported that MoO2(acac)2 has
its own characteristic XRD peaks with JCPDS card (00-022-
1833). Therefore, we have mechanically mixed MoO2(acac)2
with K 10 clay and achieved a Mo content of up to 0.98%, and
analyzed the XRD patterns. There was also no XRD peaks
for MoO2(acac)2. Thus, the main reason for the disappearance
of MoO2(acac)2 in the K-10 clay-Mo catalyst was due to the low
content of the Mo.
Identification and quantification of the products
1H NMR and 13C NMR spectra were conducted on a Bruker
DRX-400 spectrometer at 298 K. An Agilent 6890 network GC/
5973 MS equipped with a HP-5 silica capillary column, size
30 m × 0.25 mm was used to determine molecular weights of
the final products.
Quantification of HMF and HFMCA were performed on a
HPLC device using an external standard calibration curve
method. Samples were separated by a reversed-phase C18
column (200 × 4.6 mm) at a wavelength of 280 nm. Acetonitrile
and 0.1 wt% acetic acid aqueous solution (v : v = 30 : 70) were
used as the mobile phase with a flow rate of 1.0 mL min−1
.
The column temperature was maintained at 30 °C. Standard
curves for HMF and HFMCA were then constructed so that the
concentration of HMF and HFMCA in the samples could be
determined.
The valence state of the Mo element in the K-10 clay-Mo
catalyst was characterized by XPS technology. The measure-
ment was carried out with reference to the C 1s binding energy
(284.6 eV) as an internal standard. The survey scan of the Mo
3d region is shown in Fig. 2. It shows Mo 3d lines at 232.4 eV
To calculate the conversion of HMF eqn (1) was used.
HMF conversion ¼ moles of converted HMF=
ð1Þ
moles of starting HMF ꢀ 100%
One molecule of HMF gave rise to one molecule HMFCA,
thus the yield of HMFCA can be calculated using eqn (2).
HMFCA yield ¼ moles of HMFCA=moles of starting HMF
ꢀ 100%
ð2Þ
Results and discussion
Characterization of K-10 clay-Mo
Firstly, the ICP/AES determination showed that the weight per-
centage of the Mo element in the K-10 clay-Mo catalyst was
0.98 wt%. The XRD patterns of the K-10 clay support and the
K-10 clay-Mo catalyst are shown in Fig. 1. The K-10 clay shows
Fig. 2 XPS spectra of the samples (a) the mechanical mixing of the two
a crystalline structure. It is not difficult to distinguish several solids [MoO2(acac)2 and K-10 clay]; (b) the catalyst K-10 clay-Mo.
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Fig. 4 TEM images of the samples. (A) K-10 clay; (B) K-10 clay-Mo.
Fig. 3 FT-IR spectra of spectra of the samples. (a) The mechanical
mixing of the two solids [MoO2(acac)2 and K-10 clay]; (b) K-10 clay; (c)
K-10 clay-Mo.
(3d5/2) and 235.5 eV (3d3/2), which is characteristic of molyb-
denum metal in the +6 oxidation state.24 In order to obtain
deeper insights into the structure of the catalyst, the mechan-
ical mixing of the two solids [MoO2(acac)2 and K-10 clay] was
also analyzed by the XPS technology. As shown in Fig. 2, it was
found that the peaks of Mo 3d5/2 and 3d3/2 were shifted to a
higher binding energy region, which corresponded to 233.0 eV
and 236.1 eV, respectively. The values were derived from the
free MoO2(acac)2. The slight difference of the XPS results
between K-10 clay-Mo and the mixture of the two solids
[MoO2(acac)2 and K-10 clay] indicated that MoO2(acac)2 was
chemically bond on the support K-10 clay.
Fig. 5 The energy-dispersive analysis of X-ray (EDAX) of the samples.
Before the characterization of FT-IR, the samples were
heated at 120 °C in vacuum oven overnight to release any
physisorbed H2O, thus it can exclude the effect of water on the
FT-IR spectra. The strong and wide bands at 3400 cm−1 were
present in the FT-IR spectra of all the samples (Fig. 3), and
it should be attributed to the structural-OH stretching
vibration25 and the band at 1630 cm−1 is related to OH defor-
mation.26 In the low energy region, an intense and broad band
over the range of 1000–1200 cm−1 is observed in both the
spectra of the K-10 clay and K-10 clay-Mo catalyst, which is
attributed to the Si–O stretching vibrations of the tetrahedral
layer. After immobilization of MoO2(acac)2 on the K-10 clay
support two bands appeared at 905 and 950 cm−1, although
the two bands were not distinct. The two bands can be attribu-
ted to the terminal MovO stretching modes.18 In addition, a
band at 1537 cm−1 appeared for the K-10 clay-Mo catalyst,
which corresponds to the vibration of the CvO group of
acac.19 In order gain further insight into the structure of the
catalyst, the FT-IR spectrum of the mechanical mixing of the
two solids [MoO2(acac)2 and K-10 clay] with a Mo content up
to 0.98% was obtained, and it was found that the FT-IR spec-
trum of the K-10 clay-Mo catalyst was almost the same as that
of the mechanical mixing of two solids [MoO2(acac)2 and K-10
clay]. However, there was a shift of the CvO stretching
vibration. The band was at 1532 cm−1 in the mechanical
mixture of the two solids, which was derived from the free
MoO2(acac)2, whereas it was 1537 cm−1 for the K-10 clay-Mo
(a) K-10 clay-Mo; (b) K-10 clay.
catalyst. There results once again verified that MoO2(acac)2 was
supported on the K-10 clay support through chemical bonds.
TEM images of the K-10 clay and K-10 clay-Mo catalyst are
shown in Fig. 4. It reveals that the K-10 clay support presents a
multilayer structure. After the immobilization of MoO2(acac)2
on the K-10 clay support, it is clearly observed that the multi-
layer structure is maintained in the K-10 clay-Mo catalyst, but
the TEM image of K-10 clay-Mo catalyst shows much darker
regions. Furthermore, the K-10 clay and K-10 clay-Mo catalyst
were further characterized by energy-dispersive analysis of
X-ray (EDAX). As shown in Fig. 5, Mo was only presented in the
K-10 clay-Mo catalyst, indicating that MoO2(acac)2 was success-
fully immobilized on the K-10 clay support. In addition, we
also found that the content of Mo in the dark region (1.86%)
was higher than that in the other parts (0.51%). These results
indicated that the distribution of Mo varied over the K-10 clay
material. Our results were consistent with those reported by
Farias et al.27
The aerobic oxidation of HMF into HMFCA under various
conditions
Prior to testing the catalytic activity of the K-10 clay-Mo catalyst
on the aerobic oxidation of HMF, the stability of HMF was
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Table 1 Conversion of HMF into HMFCA under various conditionsa
Table 2 The results of the aerobic oxidation of HMF into HMFCA in
different solventsa
Catalyst
amount (mg) (h)
Time HMF
HMFCA
yield (%)
Entry Catalyst
con. (%)
HMF
HMFCA
yield (%)
HMFCA
Sel.c (%)
Entry
Solvent
Con.b (%)
1b
2
3
4
5
—
—
—
—
150
59
2
2
2
2
2
0.3 0.1
0.7 0.2
42.5
0
0
35.3
23.4
0
0
1
2
3
4
5
6
7
8d
9
DMSO
DMF
p-Chlorotoluene
Toluene
Trifluorotoluene
Acetonitrile
MIBK
7.9 0.4
7.1 0.3
90.3 0.7
92.5 0.5
89.2 0.6
7.0 0.4
31.0 0.7
96.6 0.9
65.9 0.7
7.2 0.5
6.8 0.4
80.9 0.5
81.9 0.4
82.6 0.7
6.2 0.4
26.2 0.6
80.6 0.8
3.0 0.5
91.4
91.2
89.6
88.5
92.6
88.6
84.5
83.4
4.5
K-10 clay
MoO2(acac)2
K-10 clay-Mo 150
26.7
92.5 0.5 81.9 0.4
a Reaction conditions: HMF (70 mg) and a set amount of catalyst were
added to toluene (7 mL), and the reaction was carried out at 110 °C
with the flushing of oxygen at a speed of 20 mL min−1
.
b The reaction
MIBK
Ethanol
was carried out without catalyst and oxygen.
a Reaction conditions: HMF (70 mg) and K-10 clay-Mo (150 mg) were
added to organic solvent (7 mL), and the reaction was carried out at
studied. Although it has always been reported that HMF is not
stable under acidic or alkaline conditions,28 it was observed
110 °C for 2 h with the flushing of oxygen at a speed of 20 mL min−1
.
b Con. is the abbreviation of conversion. c The selectivity (Sel.) was
that HMF was stable in toluene with a recovery of 99.7% after calculated by the use of the average values of HMF conversion and
HMFCA yield. d The reaction time was 8 h.
2 h at 110 °C (Table 1, entry 1). In addition, HMF also showed
high stability even with the flushing of oxygen (Table 1, entry
2). Control experiments were also carried out using the K-10
clay support and MoO2(acac)2 as the catalysts, respectively. A
ditions. As shown in Table 2, the solvent showed a remarkable
HMFCA yield of 35.3% with a HMF conversion of 42.5% were
carried out in a variety of organic solvents under idenitcal con-
effect on the HMF conversion and HMFCA yield. Low HMF
obtained after 2 h in the presence of the K-10 clay (Table 1,
conversions were obtained when the reactions were carried out
entry 3). Further prolonging the reaction time to 12 h meant
in DMSO and DMF with strong polarities and high boiling
that the HMF conversion could reach 97.8% in the presence of
points (Table 2, entries 1 & 2). Among the various solvents, aro-
K-10 clay, with a HMFCA yield of 72.1% after 12 h. These
matic solvents were found to be the best for the transform-
results indicated that the K-10 clay had the ability to catalyze
ation. Similar HMF conversions around 90% and HFMCA
the aerobic oxidation of HMF. Our results were consistent with
yields around 81% were achieved after 2 h when the reactions
the results reported by Dintzner et al., in which they reported
were carried out in p-chlorotoluene, toluene and trifluoroto-
luene (Table 2, entries 3–5). Both the HMF conversion and
HMFCA yield were low when the reaction was performed with
that montmorillonite KSF clay could catalyze the air oxidation
of aliphatic aldehydes to the corresponding carboxylic acids.29
As the composition of montmorillonite K-10 clay is complex,
acetonitrile as the solvent (Table 2, entry 6). When the reaction
including SiO2 (73.0%), Al2O3 (14.0%), Fe2O3 (2.7%), CaO
was performed in methyl isobutyl ketone (MIBK), HMFCA was
(0.2%), MgO (1.1%), Na2O (0.6%), K2O (1.9%), some metal
obtained in a yield of 26.2% with a HMF conversion of 31.0%
elements or metal oxides should be responsible for the cataly-
after 2 h at 110 °C (Table 2, entry 7). A high HMFCA yield of
80.6% could be obtained in MIBK after a long reaction time of
tic ability in the oxidation reactions. For example, Zhu et al.30
reported that Fe2O3 supported on carbon nitride could activate
8 h (Table 2, entry 8). The good results demonstrated the possi-
molecular oxygen.
bility of synthesizing HMFCA from fructose through two steps.
Under the same reaction conditions, a HMF conversion of
The first step involves the biphasic system (MIBK-water) for
26.7% and HFMCA yield of 23.4% were obtained after 2 h
the production of HMF from fructose, in which the de-
using MoO2(acac)2 as the catalyst (Table 1, entry 4). By further
hydration of fructose into HMF was performed in the water
increasing the reaction time to 12 h the HMF conversion
phase, and the product was extracted into the organic MIBK
reached 57.8% and the yield of HMFCA was 38.5%. However,
phase. Then the HMF in MIBK can be oxidized into HMFCA in
the K-10 clay-Mo catalyst showed a higher catalytic activity.
the second step. A HMF conversion of 65.9% was obtained in
A high HMFCA yield of 81.9% was obtained with a HMF con-
ethanol, but HMFCA was only obtained in a low yield of 3.0%
version of 92.5% after 2 h when the reaction was conducted at
(Table 2, entry 9). 5-Ethoxymethylfurfural (EMF) was detected
110 °C in the presence of the K-10 clay-Mo catalyst (Table 1,
as the main product with a yield of 61.5%, which was pro-
entry 5). On assessment of the results given in Table 1 (entries
duced by the acid-catalyzed etherification of the hydroxyl
group in HMF with ethanol. The reason for this is that the
3–5), the K-10 clay and Mo demonstrated an ability for the
aerobic oxidation of HMF into HMFCA, however, they also
demonstrated a synergetic effect on the promotion of the
aerobic oxidation of HMF into HMFCA.
K-10 clay is an acidic material, which is consistent with our
previous results on the synthesis of EMF.31
Effect of reaction temperature and catalyst amount on the
Effect of solvent on the aerobic oxidation of HMF into HMFCA aerobic oxidation of HMF
To study of the solvent effect on the catalytic activity of the With the aim of obtaining the optimal reaction conditions for
K-10 clay-Mo catalyst, the aerobic oxidation of HMF was the aerobic oxidation of HMF into HMFCA, the effect of the
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Table 3 The effect of reaction temperature and catalyst amount on the
aerobic oxidation of HMFa
Table 4 Catalytic oxidation of HMF using various oxidantsa
HMF
Con. (%)
HMFCA
yield (%)
Entry
Oxidant
T (°C)
Time (h)
Catalyst
amount (mg)
T
(°C)
Time
(h)
HMF
Con. (%)
HMFCA
yield (%)
Entry
1b
2b
3c
4d
5d
6e
7e
t-BuOOH
H2O2
O2
O2
O2
Air
Air
80
80
110
110
110
110
110
2
2
3
2
4
2
8
100
5.2 0.6
100
75.2 0.4
100
64.6 0.6
100
0
0
4.7 0.5
86.9 0.8
67.4 0.6
86.1 0.5
58.5 0.4
83.6 0.7
1
2
3
4
5
6
7
8
9
10
150
150
150
150
150
100
50
25
12.5
12.5
110
110
110
80
2
3
12
3
3
2
2
2
2
16
92.5 0.5
81.9 0.4
86.9 0.8
85.4 0.6
22.5 0.8
13.4 0.7
71.9 0.7
57.2 0.6
26.8 0.6
15.3 0.7
84.7 0.8
0
100
100
0
0
0
23.5 0.6
14.3 0.5
81.5 0.9
64.7 0.5
30.5 0.4
17.4 0.7
98.7 0.3
50
0
110
110
110
110
110
a Reaction conditions: HMF (126 mg, 1 mmol), K-10 clay-Mo (150 mg),
and toluene (7 mL). b 7 mmol t-BuOOH or H2O2 were used. c With
flushing of oxygen at a rate of 20 mL min−1
.
d An O2 balloon was
employed. e In an air atmosphere.
a Reaction conditions: HMF (70 mg) and a set amount of catalyst were
added to toluene (7 mL), and the reaction was carried out at different
reaction temperatures with the flushing of oxygen at a speed of 20 mL
various oxidants. As shown in Table 4, the oxidant species
showed a remarkable effect both on HMF conversion and
HMFCA yield. When t-BuOOH was used as the oxidant, the
HMF conversion reached 100% (Table 4, entry 1). However, no
furan compound such as HMFCA, DFF, and FDCA were
detected by HPLC (Table 4, entry 1). Based on the above data,
we can conclude that the furan ring of HMF would be
destroyed during the oxidation process by the strong oxidant
t-BuOOH. In contrast to t-BuOOH, the oxidation reaction
occurred with difficulty using H2O2 as the oxidant (Table 4,
entry 2). Molecular oxygen was found to be the best oxidant in
the aerobic oxidation of HMF into HMFCA. A high HMFCA
yield of 86.9% and HMF conversion of 100% were obtained
after 3 h with the flushing of oxygen at a rate of 20 mL min−1
(Table 4, entry 3). These good results encouraged us to carry
out the oxidation of HMF using an oxygen balloon instead of
with the flushing of oxygen, and the K-10 clay-Mo catalyst also
showed high catalytic activities (Table 4, entries 4 & 5). It only
required 4 h to obtain a high HMFCA yield of 86.1% and HMF
conversion of 100% (Table 4, entry 5). The oxidation reaction
was further conducted in an air atmosphere, and it still
showed excellent results (Table 4, entries 6 & 7). However, a
long reaction time of 8 h was required to obtain a HMF conver-
sion of 100% and HMFCA yield of 83.6% (Table 4, entry 7),
which is due to the low concentration of molecular oxygen in
the solution. The oxidation reaction in the air makes the deve-
loped method much more convenient and economical.
min−1
.
main reaction parameters such as reaction temperature and
catalyst loading were studied. It was noted that the effect of
the reaction temperature was much more obvious than that of
the catalyst loading. The higher the reaction temperature, the
higher the HMFCA yield. It only took 3 h to obtain complete
HMF conversion and the highest HMFCA yield of 86.9%, with
the turnover number (TON) calculated to be 31.5 (Table 3,
entry 2). Interestingly, it was found that HMFCA was stable at
110 °C without further oxidation to other products (Table 3,
entry 3). The HMF conversion and HMFCA yield decreased
sharply when the reaction temperature decreased from 110 °C
to 80 °C and 50 °C. The observed HMF conversions were found
to be 23.5% and 14.3% after the 3 h at the reaction tempera-
ture of 80 °C and 50 °C, respectively, which gave the corres-
ponding DFF yields of 23.5% and 14.3%, respectively (Table 3,
entries 4 and 5).
The effect of the catalyst amount on the aerobic oxidation
of HMF into HMFCA was also studied. As shown in Table 3,
the HMF conversion and HMFCA yield decreased with the
decrease of the catalyst amount (Table 3, entries 1, 6–9). The
increased HMF conversion and HMFCA yield with the increase
of catalyst loading at an early reaction stage can be attributed
to an increase in the availability and number of catalytically
active sites for the aerobic oxidation of HMF. It was noted that
a high HMF conversion of 98.7% and HMFCA yield of 84.7%
were also obtained after 16 h, when 12.5 mg of the K-10 clay-
Mo catalyst was used, with a low mole ratio of Mo to HMF
being 0.24%. In addition, we have tried to increase the content
of the substrate HMF, and found that the maximum HMF con-
centration in toluene was 23 mg mL−1 at the reaction tempera-
ture of 110 °C. Therefore, we have carried out the aerobic
oxidation of HMF with 160 mg in 7 mL with 150 mg of catalyst
at 110 °C, and we found that the HMF conversion was 93.7%
with a HMFCA yield of 79.1% after 12 h.
Time course of the aerobic oxidation of HMF into DFF
In order to obtain deeper insights into the catalytic aerobic
oxidation of HMF into HMFCA over K-10 clay-Mo, the content
of HMF and the oxidation products at different reaction time
points were recorded. The main product of the aerobic oxi-
dation of HMF was HMFCA, and DFF was detected as the only
oxidation product during the aerobic oxidation of HMF into
HMFCA over K-10 clay-Mo. As shown in Fig. 6, the content of
HMF decreased gradually during the reaction process, and the
content of the main oxidation product HMFCA increased
gradually with the increase of the reaction time. HMF was
Oxidation of HMF into HMFCA using various oxidants
In order to explore the scope of the catalytic system for the oxi- completely converted after 3 h, and the highest HMFCA yield
dation of HMF, the oxidation of HMF was carried out using of 86.9% was obtained. In addition, the yield of the byproduct
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Fig. 6 Time course of HMF recovery and product distribution during
the process of the aerobic oxidation of HMF. Reaction conditions: HMF
(70 mg) and K-10 clay-Mo (150 mg) were added to toluene (7 mL), and
the reaction was carried out at 110 °C for 3 h with the flushing of dioxy-
Fig. 7 The recycling experiments of the K-10 clay-Mo catalyst on the
aerobic oxidation of HMF into HMFCA. Reaction conditions: HMF
(70 mg) and K-10 clay-Mo (150 mg) were added into toluene (7 mL), and
the reaction was carried out at 110 °C for 3 h with the flushing of dioxy-
gen at a speed of 20 mL min−1
.
gen at a rate of 20 mL min−1
.
DFF also increased gradually during the reaction course, but
the increase rate was slow. The DFF yield only reached 6.7%
after 3 h. The results indicated that the oxidation of aldehyde
into acid is much easier than the oxidation of hydroxyl into
aldehyde when the aerobic oxidation of HMF was catalyzed by
K-10 clay-Mo. Thus, a high HMFCA yield and selectivity were
obtained in our reaction system. In addition, after 3 h, a
HMFCA yield of 86.9%, DFF yield of 6.7%, and FDCA yield of
1.8% (data not given) were obtained. The total of the deter-
mined furan compounds reached up to 95.4%, which was
close to the HMF conversion. The results indicated that the
furan ring was not destroyed when O2 was compared with
t-BuOOH as the oxidant as discussed above.
Fig. 8 The results of the leaching experiment for the aerobic oxidation
of HMF into HMFCA catalyzed by K-10 clay-Mo by hot filtration after 1 h.
Reaction conditions: HMF (70 mg) and K-10 clay-Mo (150 mg) were
added to toluene (7 mL), and the reaction was carried out at 110 °C for
Catalyst recycling experiments and product identification
The reusability of a heterogeneous catalyst is one of the most
important benefits and makes it useful for commercial appli-
cations. The recovery and reusability of the K-10 clay-Mo cata-
lyst was studied, and the aerobic oxidation of HMF was used
as a model reaction. At the end of the reaction, the catalyst was
separated by simple filtration. Then it was washed with
ethanol and dried in vacuum at 100 °C over night. The reused
catalyst was used for the second cycle under the same reaction
conditions. These processes were repeated for five times to test
the stability of the catalyst. As shown in Fig. 7, the yield of
HMFCA remained almost stable for cycle during the recycling
experiments (86.9% in first cycle versus 83.9% in sixth cycle).
These results indicated that the K-10 clay-Mo catalyst was
stable during the reaction without the leaching of Mo into the
reaction system. The resulting catalyst could be reused without
any significant loss of catalytic activity.
3 h with the flushing of dioxygen at a speed of 20 mL min−1
.
was subjected to stirring at 110 °C for another 2 h under other-
wise the same conditions. As shown in Fig. 7, the HMFCA
yield was maintained at a stable level of 57.2% from 1 h to 3 h.
These results also indicated that there was no leaching of Mo
during the reaction, which once again indicated that the cata-
lyst was stable during the reaction process. Our results were
consistent with the results reported by Farias et al., in which
they used the same catalyst for the epoxidation of vegetable
oils using t-BuOOH as the oxidant in toluene.27 The results
indicated that MoO2(acac)2 chemically bonded with the K-10
clay support. In addition, the reaction solution was also ana-
lyzed by ICP-AES, and there was no Mo detected in the reaction
solution.
In order to further verify the stability of the K-10 clay-Mo
catalyst during the reaction process, the leaching experiment
was also carried out. As shown in Fig. 8, the aerobic oxidation
of HMF was firstly carried out at 110 °C for 1 h in toluene with
the oxygen flow rate at 20 mL min−1, then the catalyst was fil-
tered off. The reaction solution in the absence of the catalyst
Developing a facile and economical method to obtain puri-
fied product is also a very crucial goal in terms of green chem-
istry. When the reaction mixture in toluene was cooled to
room temperature, HMFCA was precipitated out from the
solvent in the form of needle-like crystals with a yellow color.
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However, limited HMFCA and by-product DFF remained in the
solvent. The toluene solvent could be readily purified by evap-
oration under reduced pressure due to the large boiling points
between toluene and the furan compounds. Thus, the solvent
can be reused. The precipitated HMFCA was then easily puri-
fied by washing several times with ethyl acetate. The resulting
HMFCA was characterized by 1H NMR and 13C NMR. The NMR
data were recorded as follows: 1H NMR (400 MHz, CDCl3)
δ (ppm): 10.1 (s, 1H, –COOH), 7.23–7.22 (d, 1H, J = 4.0 Hz,
furan ring), 6.59–6.58 (d, 1H, J = 4.0 Hz, furan ring), 4.65
(s, 2H, –O–CH2-furan ring), 1.60 (s, 1H, –OH). 13C NMR
(100 MHz, CDCl3) δ (ppm): 168.5, 157.2, 152.8, 121.8, 111.8,
64.6.
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Conclusion
In this study, a molybdenum(VI) complex was successfully
immobilized on K-10 clay by the cation exchange method. The
heterogeneous K-10 clay-Mo catalyst showed high catalyst
activity and selectivity in the oxidation of HMF into HMFCA
with molecular oxygen. Under optimal conditions, a HMF con-
version of 100% and HMFCA yield of 86.9% were obtained
after 3 h with the flushing of oxygen at 110 °C. The activity of
the K-10 clay-Mo catalyst is robust, as the oxidation reaction
could occur smoothly in air. More importantly, the catalyst
was stable during the reaction, and could be reused several
times without loss of its activity. These methods represent an
interesting and highly atom-efficient alternative for the oxi-
dation of aldehyde groups into carboxylic groups, as oxygen is
cheap, readily available, and an oxidant that produces water as
the only by-product.
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Acknowledgements
The project was supported by National Natural Science Foun-
dation of China (No. 21203252, 21373275), and the Chenguang
Youth Science and Technology Project of Wuhan City (No.
2014070404010212).
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