Highly Efficient Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid with Heteropoly Acids and Ionic Liquids

Article abstract of DOI:10.1002/cssc.201900651

2,5-Furandicarboxylic acid (FDCA) is regarded as an important bioderived substitute for petrochemically derived terephthalic acid (PTA), which is widely applied in the polymer industry. This work delineates the base-free oxidation of 5-hydroxymethylfurfural (HMF) to FDCA in an ionic liquid/heteropoly acid (IL–HPA) catalytic system. HPAs displayed high activity for selective oxidation; their active center (Mo/V) was activated by O₂ and transformed from oxygen single and double bonds to epoxy groups, resulting in an FDCA yield of 89 % for HPMV₆ (HPM=H₃PMo₁₂O₄₀) in the presence of [Bmim]Cl (1-butyl-3-methylimidazolium chloride) under optimized reaction conditions. The high solubility of imidazole ILs for FDCA improved the affinity of HMF and the active centers of the catalyst and protected the furan ring from oxidative cleavage. Furthermore, multiple hydrogen bonds simultaneously formed between the electronegative anions and hydroxy protons of HMF, as well as the hydrogen atoms of the imidazole rings and hydroxy groups, promoting the transformation to aldehyde groups. Various starting materials were studied, and a moderate FDCA yield was obtained from glucose. This work provides an interesting IL–HPA catalytic system for the base-free synthesis of FDCA from accessible monosaccharides and illustrates the great potential of FDCA production from renewable carbohydrates.

Full text of DOI:10.1002/cssc.201900651

Accepted Article
Title: Highly Efficient Oxidation of 5-Hydroxymethylfurfural to 2,5-
Furandicarboxylic Acid with Heteropoly Acids
Authors: Ruru Chen, Jiayu Xin, Dongxia Yan, Huixian Dong, Xingmei
Lu, and Suojiang Zhang
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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published online in Early View as soon as possible and may be different
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201900651
Link to VoR: http://dx.doi.org/10.1002/cssc.201900651
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ChemSusChem
10.1002/cssc.201900651
FULL PAPER
Highly Efficient Oxidation of 5-Hydroxymethylfurfural to 2,5-
Furandicarboxylic Acid with Heteropoly Acids
Ruru Chen,[a,b] Jiayu Xin, *[a,c] Dongxia Yan, Huixian Dong, Xingmei Lu, Suojiang Zhang*[a,c]
[a]
[a]
[a,c]
Abstract: 5-furandicarboxylic acid (FDCA) is regarded as an
packaging, fibers and clothing.[1a, 4] Compared with petroleum
derived-PET, polyethylene-2,5-furandicarboxylate (PEF) from
polymerization of FDCA is degradable,[ and possesses a better
gas barrier and mechanical strength.[2c, 6]
important bio-derived substitution of petrochemically terephthalic acid
5]
(PTA) applied in polymer industry. This work delineates the base-free
oxidation of 5-hydroxymethylfurfural (HMF) to FDCA in a novel ionic
liquids-heteropoly acids (ILs-HPAs) catalytic system. HPAs displayed
high activity for the selective oxidation, the active center (Mo /V) was
As illustrated in Figure 1, the oxidation of HMF to FDCA is
performed via procedures of either 5-hydroxymethyl-furan-2-
carboxylic acid (HMFA) or furan-2,5-dicarbaldehyde (DFF), and
further oxidative 5-formyl-2-furancarboxylic acid (FFCA) as
intermediates.[
activated by O
bonds to epoxy groups, and the yield of FDCA reached 89% using
HPMV in presence of [Bmim]Cl under optimized reaction conditions.
2
, and transformed from single and double oxygen
7]
6
High solubility of imidazole ionic liquids to FDCA improved the affinity
of HMF and the active centers of catalysts, and prevented the furan
ring from oxidative cleavage. Furthermore, multiple hydrogen bonds
simultaneously formed between electronegative anions and hydroxyl
protons of HMF, 2-H of imidazole rings and hydroxyl groups,
promoting the transformation to aldehyde groups. Various starting
materials were studied, a moderate FDCA yield of was obtained from
glucose. This work provided a novel ILs-HPAs catalytic system for the
base-free synthesis of FDCA directly from more accessible
monosaccharides and illustrated a great potential of FDCA production
from renewable carbohydrates.
O
O
HO
OH
O
O
O
O
O
O
O
O
HMFA
O
O
OH
HO
H
HO
OH
HMF
FFCA
FDCA
O
DFF
Figure 1. Possible oxidative routes for the synthesis of FDCA from HMF.
Currently, synthesis of FDCA is extensively achieved by applying
supported noble metal catalysts in basic solvents with O as
2
oxidant.[
2b, 8]
In the earliest, Pt/C was found to catalyze the
selective oxidation of HMF with a FDCA yield of 69%.[ Zhang
and co-workers developed a series of magnetic material-
supported noble metal catalysts, which provided high FDCA
yields with addition of base and could be easily separated by a
9]
Introduction
Considering the contradiction between increasing demand of bulk
chemicals and the depletion of nonrenewable petroleum
resources, conversion of biomass to chemicals is more
attractive.[1] Among plenty of biomass-derived chemicals,
magnet and efficiently reused.[ Recently, supported Au-Pd alloy
catalysts displayed excellent catalytic activity.[8a,
10]
11]
Further
employing hydrophilic mesoporous poly(ionic liquid) (MPIL) as the
support, it was feasible to control the surface wettability of Au-
Pd/MPIL by varying the cross-linkers with different hydrophilic or
selective oxidation of 5-hydroxymethylfurfural (HMF) could lead to
2,5-furandicarboxylic acid (FDCA), listed as one of the twelve top
hydrophobic properties.[11] Dinesh et al. reported MnCo
O
4
spinel
2
value-added chemicals obtained from biomass by the U.S.
Department of Energy.[2] FDCA is regarded as a promising
alternative to petroleum-derived terephthalic acid (TPA).[3] TPA is
a monomer in the production of polyethylene terephthalate (PET),
which are widely applied in numerous fields including bottle
supported Ru catalyst and provided a high FDCA yield in 10 h.[
Noble metal catalysts showed great performance for FDCA
production, while the major drawback is high costs of metal,
therefore the larger-scale production is limited.[13] A series of no-
noble metal catalysts were employed to the HMF oxidation.[5a, 14]
12]
Zhang et al. first prepared a photocatalyst CoPz/g-C
Na solution, and led to excellent yield of FDCA. Mn-Co
bimetallic oxides with different Mn/Co molar ratio were prepared,
and nanoscale center-hollow hexagon MnCo spinel presented
higher activity, with a FDCA yield of 70.9%.[ Eri Hayashi and co-
workers successfully synthesized MnO with different crystal
structure, and demonstrated that high-surface-area β-MnO
3 4
N , and used
[15]
2
4 7
B O
[
a]
R. Chen, Dr. J. Xin, D. Yan, H. Dong, Dr. X. Lu, Prof. S. Zhang
Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key
Laboratory of Green Process and Engineering, Institute of Process
Engineering
Chinese Academy of Sciences, Beijing 100190, P. R. China
E-mail: jyxin@ipe.ac.cn
2
5a]
O
4
2
2
sjzhang@ipe.ac.cn
presented suitable catalytic activity for the selective oxidation of
HMF to FDCA.[16] The FDCA production with no-noble metal
catalysts is more cost-effective compared with noble metal
catalysts.
[
[
b]
c]
R. Chen
Sino Danish College
University of Chinese Academy of Sciences, Beijing 101408, China
Dr. J. Xin, Dr. X. Lu, Prof. S. Zhang
College of Chemistry and Chemical Engineering
University of Chinese Academy of Sciences, Beijing 100049, China
Using either noble or no-noble metal catalysts, excessive base or
high pH is generally used to produce FDCA salts, preventing
Supporting information for this article is given via a link at the end of
the document.
[17]
FDCA agglomeration and deactivation of catalysts. However,
FDCA salt neutralization to pure FDCA requires subsequent
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ChemSusChem
10.1002/cssc.201900651
FULL PAPER
catalytic hydrolysis with strong acids and generates a large
amount of waste water, increasing the operating cost and time
with additional by-product formation.[18] Furthermore, strong
oxidants were used to activate the catalysts in some reaction
systems, such as or t-BuOOH with acetonitrile.[19] Thus, from the
view of green and sustainability, the conversion of HMF to FDCA
using no-noble metal catalysts in base-free solvents is expected.
Ionic liquids (ILs) with their multifunctionalities and acid-base
properties are in continuous development. Notably, imidazolium-
based ILs show preferable solubility to furan compounds including
HMF and FDCA compared with conventional aqueous solutions
or organic solvents.[20] The oxidation of HMF in imidazole ILs
would eliminate the use of additional base, mineral acids, and
complex downstream process, which is economic feasible and
environmentally friendly. Recently, we demonstrated a base-free
conversion of HMF to FDCA in ionic liquids using Fe-Zr-O
catalysts, and a yield of 60% was obtained.[4b, 20b] Particularly, ILs
are also favored for the synthesis of HMF by acid-catalyzed
dehydration of sugars or cellulose.[21] The merger of the reaction
chemistry of direct conversions of sugars to FDCA with the
solvents having catalytic behavior will extend their utilization in
one-pot synthesis. On the other hand, with strong Brønsted
acidity,[ Keggin-type heteropoly acids (HPAs) are extensively
investigated in biomass conversion, especially hydrolysis of
cellulose.[23] The combination of Brønsted acidity and redox ability
makes HPAs promising for the selective synthesis of FDCA from
HMF and accessible monosaccharides.
using MoO
3
as the catalyst for the reaction and approximately
3
69% of FDCA was obtained. Since MoO readily forms molybdic
acid in water, thereafter exhibiting same valence state (VI) and
[24]
Mo-O bonds as HPM, it was agreed that MoO
oxidative ability to HPM to some degree. On the other hand, HPM
is more tunable and multifunctional compared to MoO as a
catalyst, such as adjusting Brønsted acidity, or selectivity to
products by varying addenda atoms. By contrast, basic Na MoO
performed badly, with a low FDCA yield of 24%. The poor activity
was presumably caused by the basic reaction conditions due to
2 4
Na MoO , which ultimately led to base-induced polymerization of
HMF thereby forming humins. It is revealed, consequently, that
acid-base properties of catalysts greatly influence the
transformation. In addition, Si-based heteropoly acids are
thermally unstable compared with P-based heteropoly acids
according to melting points and presented mostly in liquid state.
Thus, HSM showed relatively poor catalytic performance. In
summary, HPM was the most effective catalyst among heteropoly
acids, and it was Mo that mainly catalyzed the oxidation.
It was reported that, at high temperature, the conversion of HMF
was not stable and degraded into humins significantly.[4b,
Therefore, HMF was highly converted in [Bmim]Cl in the absence
of catalysts, and some insoluble humins caused by HMF
polymerization or cross-linked polymerization were observed after
reaction. Generally, the difference between high conversion and
relative low yield of furanic compounds was a result of the side
reactions due to different catalysts. Ring-opening products
3
showed similar
3
2
4
10b]
22]
Herein, a novel base-free ionic liquids-heteropoly acids (ILs-
HPAs) system was developed in oxidation of HMF to FDCA via
the formation of DFF and FFCA intermediates. By screening
catalysts and solvents, it was indicated that Keggin
2
glycolic acid, and inorganic CO were identified by GC-MS, and
furfural identified by ESI (Figure S4).
phosphormolybdic acid H
3
PMo12
O40 (HPM) was an outstanding
Table 1. Selective oxidation of HMF to FDCA using different catalysts.
catalyst, and [Bmim]Cl served as excellent solvent and co-
catalyst. This novel catalytic system provided efficient FDCA
production with a high FDCA yield under mild conditions, while
no-noble metal catalysts normally required more than 12 h or
even 24 h to achieve high FDCA yield. Considering the
multifunctionality of HPAs, the utilization of cellulose, glucose,
and fructose for the one-pot production of FDCA was investigated,
and a moderate FDCA was obtained. To the best of our
knowledge, this is the first report for the use of heteropoly acids-
ionic liquids to the oxidation of HMF to FDCA.
Entry
Catalysts
HMF Conv.
mol %]
Yield [mol %]
FDCA
[
FFCA
DFF
HPW
99.8±0.3
18.5±2.7
5.3±1.2
0.1±0.0
1
2
3
4
5
6
7
HPM
98.1±1.0
99.7±0.2
99.8±0.1
99.9±0.1
99.5±0.2
99.2±0.5
83.3±3.1
8.6±1.5
1.5±0.3
21.5±1.7
15.3±0.9
1.3±0.5
7.0±1.1
2.4±0.3
1.2±0.1
0.2±0.0
0.8±0.1
0.1±0.0
5.0±0.2
0.7±0.0
HSW[b]
HSM
38.4±3.0
69.5±2.4
23.8±1.3
18.7±0.9
MoO
Na MoO
Blank[a,b]
3
2
4
Results and Discussion
Reaction conditions: 0.1 mmol HMF, 1 g [Bmim]Cl, 0.05 g catalysts, 140 °C,
MPa O , 6 h. [a] No catalysts were used. [b] HMFA was the intermediate
1
2
rather than DFF, and the molar content of HMFA was 0.2%.
Catalyst Preparation and Characterization
Four types of Keggin HPAs, including HPM, HPW, HSM, HSW
were tested for the reaction. As summarized in Table 1, all tested
HPAs catalysts provided nearly full conversion of HMF under the
reaction conditions, among which HPM showed high selectivity
towards FDCA with a yield of 83%. The two heteropoly tungstic
acids (HPW and HSW), however, showed poor catalytic
behaviors. FDCA and furanic intermediates were synthesized in
total yields of less than 30%, although 99% conversion of HMF
were achieved. Therefore, it was determined that Mo mainly
favored the selective oxidation, which was then confirmed by
In order to improve the catalyst performance, enhancing the
selectivity of FDCA, the catalyst HPM was modified by varying
addenda atoms. As reported in the conversion of cellulose to
formic acid, introduction of vanadium into HPM could promote the
selectivity, and then it would mainly be V that favored the
oxidation.[25] In addition, on the basis that HPMV
with over 3
n
26]
vanadium atoms are especially easy to re-oxidise,[ the oxidation
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ChemSusChem
10.1002/cssc.201900651
FULL PAPER
rate was expected to be significantly improved. HPMV
the reaction by change of V(V) to V(IV) mostly. HPMV
n
catalyzed
was
peak at 8.6° vanished, reflecting a shrinking unit cell due to the
replacement of Mo atoms with smaller V atoms. It was indicated
that V was present in the primary structure of Keggin, and the
symmetry of anion structure decreased by the addition of V.
n
successfully synthesized and confirmed by FTIR, XRD, ICP, ESR
and XPS analysis. Table S3 shows the list of prepared HPAs
together with the results for the ICP of dried compounds, and it
was indicated that all HPAs were obtained in good purity and
desired metal component. The ESR spectra (Figure S3) of
HPMV
n
catalysts shows the characteristic signal for V5+ species
with hyperfine structure, according to Lingaiah et al.[27] The
intensity of spectra increased with V atoms increase in HPAs,
During the catalyst preparation, insoluble V
2 5
O was removed by
filtration. No characteristic peaks about crystal structure of V
2
O
5
was shown in the XRD patterns, thus all vanadic species were
presented in the form of heteropoly anions.
n
confirming that more V(IV) exists in HPMV with a larger value of
n, also proved by XPS.[28] Afterwards, the catalyst ability was
tested for the oxidation, and HMF conversion, furanic yields were
a
b
determined. The FTIR spectra of HPMV
Four main vibration bands emerged around 1064, 961, 869, and
85 cm-1 were assigned to the P-O, M=O
(terminal oxygen), M-
-M (corner-sharing oxygen), and M-O -M (edge-sharing
oxygen), respectively, which are the characteristic of HPAs with a
n
are depicted in Figure 2.
c
7
O
d
b
c
d
[
26, 28-29]
Keggin structure (M represents Mo or V).
Typical V
2
O
5
e
f
-1
bands at 1021, 829 and 617 cm , related to V=O, asymmetric and
symmetric vibration of V-O-V,[30] were not presented
independently in the spectra but showed a combining influence
on the four vibration bands of Keggin structure. With the increase
of introduced vanadium atoms, the characteristic bands P-O and
g
10
20
30
40
50
60
70
80
M=O
d
became broader and shifted to lower wavenumbers and M-
2 Theta (degree)
O
c
-M shifted to higher wavenumbers, further indicating the
incorporation of V in the primary structure and the destruction of
Keggin structure cetro-symmetry.[31]
Figure 3. XRD spectra of HPMV
HPMV , f: HPMV , g: HPMV
n
, a: HPM, b: HPMV
1
, c: HPMV
2
3
, d: HPMV , e:
4
5
6
.
P-O
M=Od
a
M-O -M
M-O -M
b
c
The valence change of catalysts plays a key role in the oxidation
pathway. Reduced Mo and V promote electron and oxygen
transfer between reactants and catalysts. To gain more insight
into the reaction mechanism, XPS analysis of catalysts was
performed. The XPS spectra of synthesized samples were shown
b
c
d
e
f
in Figure 4 below. As shown in Figure 4A, there was only binding
energy of Mo6+ presented at 233.4 eV, and almost no Mo5+ was
g
found in V-free HPM, revealing a stable Keggin structure. By
contrast, in V-substituted HPMV
n
, the binding energy of Mo 3d5/2
consisted of two peaks corresponding to Mo6 and Mo5+ species
at 233.4 and 232.1 eV, respectively, suggesting the presence of
partially reduced catalysts during preparation with little content of
+
1200
1100
1000
900
800
700
600
-
1
5+ [31-32]
5+
Mo .
In addition the relative content of reduced Mo grew
Wavenumber (cm )
with increasing V atoms, attributing to the removal of the bridging
oxygen in the orthorhombic distortion with defect Keggin unit.[
Spectrum of V 2p3/2 in HPMV was depicted in Figure 4B, two
broad peaks of V were observed, 509.6 and 512.7 eV, which
could be deconvoluted to V4+ and V , respectively. The existence
of both V4+ and V5+ indicated the exchange of electrons between
V and Keggin structure, and further suggested the formation of
31]
Figure 2. FT-IR spectra of HPMV
HPMV , f: HPMV , g: HPMV
n
, a: HPM, b: HPMV
1
, c: HPMV
2 3
, d: HPMV , e:
5+
4
5
6
.
2
+
The XRD patterns of synthesized HPAs are shown in Figure 3. A
series of peaks at 8~11°, 18~20°, 25~30°, denoted the triclinic
crystal phase of Keggin structural HPM.[28] The XRD pattern
containing V species exhibited some broader and weaker peaks
compared with HPM. With the increase of the introduced
vanadium atoms, the peaks at 10.4°, 20.1° was broaden. The
monomeric pervanadyl (VO ) cation, which was favorable for the
6
+
5+
selectivity to FDCA. The Mo /Mo molar ratio in HPMV
1
5+
was
4+
obtained according to the relative peak area, as well as V /V .
5
+
6+
In HPM catalyst, there was almost no Mo , indicating that Mo
was stable in Keggin unit and hard to be reduced during the
preparation. On the contrary, in HPMV
, the content of Mo5+ was
1
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ChemSusChem
10.1002/cssc.201900651
FULL PAPER
increased because the removal of bridging oxygen, suggesting
the formation of partially reduced catalysts during synthesis.
higher degree of V substitution, the yield of FDCA rose as well.
The same tendency was observed for the total yield. With V
increasing, the dissociation of the parent HPMV
n
led to VO2+
cation and complex thereof. The leaching VO2+ has a higher redox
activity,[26] therefore favorable for the selective oxidation to FDCA.
On the other hand, higher V-substituted HPAs possesses
improved pH or reduced acidity, leading to lower selectivity to
FDCA. Considering two aspects, for n<4 decreased acidity mainly
2
+
influenced the activity of HPAs, whereas free pervanadyl (VO )
increased oxidative selectivity for n>4. However, it was indicated
that HPAs were less stable and irreversibly catalytic with even
higher V content. Overall, HPMV
6
provided highest FDCA yield of
89% among series of V-substituted HPAs.
Oxidation of HMF to FDCA in Various Solvents
The application of ionic liquid [Bmim]Cl in the oxidation of HMF to
FDCA was reported to increase the solubility of FDCA,
furthermore create base-free system.[20b] The influence of ILs on
the reaction was discussed, by comparing with various solvents
1
Figure 4. XPS spectra of A: Mo3d5/2, B: V2p3/2 of HPAs. (a: HPM, b: HPMV , c:
2 2 2
including H O, H O , DMSO, toluene, Choline chloride (ChCl),
HPMV
2
, d: HPMV
3
, e: HPMV
4
, f: HPMV
5
, g: HPMV
6
)
and 1-Butylpyridinum chloride ([BPy]Cl), shown in Table 3. The
oxidation was substantially determined by the solubility of FDCA,
2 2 2
and H O, H O , DMSO, toluene as well as ChCl revealed poor
The synthesized HPMV
n
was employed to the selective oxidation
solubility compared with ILs. Consequently, though HMF was
totally converted, little furanic compounds were synthesized due
to insolubility of FDCA, and humins from polymerization of HMF
were the main byproducts. Reversely, selectivity to FDCA was
highly enhanced using ionic liquids solvents, attributing to the
excellent solubility of FDCA. In this way, FDCA was less adsorbed
on the surface of catalysts, thereby ensuring the activation of
catalysts. In addition, base-free condition avoided base-induced
polymerization of HMF,[7a] and eliminated further acidification of
FDCA salts, making the reaction more environmentally friendly
and cost-efficient.
of HMF to FDCA and the results are demonstrated in Table 2. As
stated previously, full HMF conversion was easily achieved
regardless of the number of V atoms. To state the synergetic
effect of V in Keggin structure, the catalytic performance of V
was conducted as well. V showed poor selectivity to FDCA,
with only 0.5 yield, even lower than that in blank reaction. In
reaction system, V catalyzed the conversion of HMF to other
2 5
O
2
O
5
2
O
5
inorganic byproducts, but restrained the selective oxidation to
FDCA. V promoted effective selectivity to FDCA in Keggin HPAs,
and show synergetic effect with Mo. With the increase of V atoms
in Keggin structure, the selectivity to different furanics varied
significantly.
Table 3. The selective oxidation of HMF to FDCA in different solvents.
Table 2. The effect of V species in HPAs on the oxidation of HMF to FDCA.
Entry
Solvents
HMF Conv.
mol %]
Yield [mol %]
FDCA
Entry
Catalysts
HMF Conv.
mol %]
Yield [mol %]
FDCA
[
FFCA
DFF
1
2
3
4
5
6
7
H
H
2
O
O
99.9±0.1
97.3±0.9
100±0
0
0
0
[
FFCA
DFF
0
0.6±0.1
8.4+0.7
0.3
0.4±0.0[a]
2
2
1
2
3
4
5
6
7
V
2
O
5
96.7±0.2
99.4±0.4
99.7±0.2
99.8±0.1
99.8±0.1
99.4±0.1
99.4±0.3
0.5±0.1
0
0
DMSO
Toluene
ChCl
0.1
0
0.6[a]
HPMV
HPMV
HPMV
HPMV
HPMV
HPMV
1
2
3
4
5
6
70.6±2.7
68.8±3.8
45.0±2.1
23.8±4.5
44.6±2.7
88.8±2.5
1.3±0.5
3.9±1.0
4.1±0.8
4.3±0.5
2.9±0.3
0.7±0.0
0.9±0.2
0.5±0.1
0.3±0.0
0
99.6
0.2
100±0.0
100±0.0
100±0.0
3.7±0.4
0.7±0.1
88.8±3.9
0.6±0.0
9.6±0.5
1.1±0.1
0.2±0.0
27.5±1.3
0
[BPy]Cl
[Bmim]Cl
0.8±0.1
1.1±0.1
Reaction conditions: 0.1 mmol HMF, 1 g solvents, 0.05 g catalysts, 140 °C,
MPa O , 6 h. [a] HMFA was the intermediate rather than DFF.
1
2
Reaction conditions: 0.1 mmol HMF, 1 g [Bmim]Cl, 0.05 g catalysts, 140 °C,
MPa O , 6 h.
1
2
The oxidation of HMF in [Bmim]Cl afforded full HMF conversion
and highest FDCA yield among ionic liquids. After reaction in
The yield of FDCA firstly declined with vanadium content
increasing, from 83% (HPM) to 23.8% (HPMV ), then with a
[
BPy]Cl under identical conditions, approximately 38%
4
intermediates including DFF and FFCA were obtained, and DFF
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ChemSusChem
10.1002/cssc.201900651
FULL PAPER
was the main product, exhibiting slower transformation and lower
selectivity compared with that in [Bmim]Cl. The lower activity of
HPM in [BPy]Cl might be ascribed to the presence of the basic
nitrogen atoms on the pyridinium ring.[33] This ultimately made the
protons related to nitrogen atoms difficult to dissociate, and then
reduction in the acidity of the solvents affected the selective
oxidation.
Table 4. The effect of ILs with different anions and cations.
Entry
Solvents
HMF Conv.
mol %]
Yield [mol %]
[
FDCA
0
FFCA
DFF
[Bmim]BF
4
100±0.0
2.3±0.5
0.3±0.0
1
2
3
4
5
6
7
8
9
[Bmim]PF
[Bmim]Br
[Bmim]Cl
[Emim]Cl
[Amim]Cl
[Hmim]Cl
[Omim]Cl
6
99.8±0.1
100±0
0
0.2±0.0
5.0±1.3
1.1±0.1
2.2±1.0
1.8±0.3
0
3.1±1.1
70.7±2.2
0
It is worth noting that [Bmim]Cl could also provide 18.7% FDCA
without catalysts (Table 1, Entry 7), revealing that ionic liquid
14.7±5.1
88.8±3.9
86.3±2.8
79.1±5.9
70.9±3.0
48.0±3.5
7.6±2.1
99.5±0.3
98.9±0.7
98.8±0.5
95.1±2.7
96.7±3.1
100±0
[
Bmim]Cl possesses moderate catalytic ability to the oxidation of
1.7±0.2
6.1±0.7
2.1±0.8
1.4±0.3
0.1±0.0
HMF. However, the reaction pathway differs from HPAs, via
intermediate HMFA. As a catalyst, [Bmim]Cl is competing with
HPAs, however less effective and affording a lower yield. In the
presence of HPAs, HMFA from catalytic [Bmim]Cl was negligible,
and [Bmim]Cl rather acted as a co-catalyst by strong hydrogen
bonds with furanics, and electron transfer between heteropoly
anions and reactants. Hence, it was concluded that [Bmim]Cl
acted not only as a solvent but a co-catalyst in the oxidation of
HMF to FDCA
15.1±1.9
4.8±1.6
[C14mim]Cl
Reaction conditions: 0.1 mmol HMF, 1 g solvents, 0.05 g catalysts, 140 °C,
MPa O , 6 h
1
2
Optimization of the Oxidation of HMF
Figure 5a shows the dependence of HMF conversion and yields
of FDCA and intermediates on the reaction time. It is clear that
HMF converted rapidly in the beginning and the conversion
reached 100% within 1 h, revealing that the transformation of
HMF was readily complete in a short time. On the other hand,
products were greatly influenced by the reaction time. The FDCA
yield increased with reaction time, where the increasing rate in the
first two hours was comparatively faster than that at later stages,
showing a yield of 88.8% after 6 h. Meanwhile, as intermediates,
the yield of FFCA and DFF was the highest in the beginning and
decreased with time. Furthermore, the decrease of DFF was
faster than that of FFCA, and the main reason was that (1) FFCA
was the downstream product of continuous DFF oxidation, and it
was accumulated and consumed simultaneously; (2) the
oxidation rate of DFF was faster than that of FFCA. As presented,
the highest FDCA yield was achieved when FFCA totally
transformed. Since the yield of DFF was already reduced to 0
within 2 h, the increase of FDCA yield was mainly determined by
the transformation of FFCA, indicating that the oxidation of FFCA
to FDCA was the rate-determining step. Prolonging the reaction
time to 7 h, a slight decrease of the FDCA yield was observed,
and only 69% FDCA was obtained after reaction for 24 h. The
reduction of FDCA yield presumably was due to the deep
Since imidazole ionic liquids displayed high dissolving capacity to
FDCA,[
20b]
and concerted promoting effect on the oxidation,
various imidazole ILs with different cations and anions were
employed as the solvents for the reaction. The results are
summarized in Table 4. On the basis of the same cation [Bmim] ,
+
it is observed that different anions substantially influenced the
-
oxidation. Among tested anions, Cl led to highest FDCA yield,
4 6 2
while BF displayed poorer catalytic ability and
-, PF -, NTf -
scarcely any product was obtained. Br- mainly produced
intermediates DFF and FFCA under the same conditions, with a
total furanic yield of 24% and 90%, respectively. The influence of
anions is discussed from two aspects. Firstly, electronegativity of
anions determines the strength of hydrogen bond between anions
and HMF molecule. Higher electronegative anions help the
-
-
deprotonation and promote the oxidation of -OH. Unlike Cl or Br ,
-
BF
4
-, PF
6
-, NTf
2
anions cannot form hydrogen bonds with HMF,
because hydroxyl group is in shielding cone of the aromatic
-
-
-
4 6 2
imidazolium ring in ILs with BF , PF , NTf anions (see Figure
S5). Anions of ionic liquids dramatically influenced the reaction,
and ionic liquids with halide ions provided preferable yields.
By varying the alkyl substituent on cation of ILs, it was revealed
that ILs with long alkyl substituent on the cation led to lower
+
+
+
+
selectivity. [Bmim] , [Amim] , [Emim] , and [Hmim] showed better
performance on the catalytic reaction, whereas lower results were
2
oxidation and degradation to inorganic compounds such as CO .
The reaction temperature was another important factor
influencing the conversion of HMF and product selectivity.
Experiments were conducted at different temperatures between
+
+
obtained in [Omim] and [C14mim] , showing 48% and 7.6% yield
of FDCA, respectively. Presumably due to the unfavorable steric
effect of long alkyl chain, the catalytic activity of HPA was
restrained. In addition, as electron-donating group, longer alkyl
chain diminished the activity of acidic proton on the imidazolium
cation. On the other hand, aggregation of ILs is easier with longer
alkyl chains, and the well-defined aggregates hinder the reaction
as well.
80 and 160 °C under otherwise similar reaction conditions. As
shown in Figure 5b, the conversion of HMF increased from 90%
to 100% as the temperature changed from 80 to 100 °C and kept
stable with a further temperature increase. Thus, HMF was easy
to be converted at low temperature, and 100 °C was sufficient for
complete conversion of HMF. On the other hand, selectivity of
FDCA and intermediates varied with the temperature. At
temperatures lower than 100 °C, the main products were
intermediates DFF and FFCA, whereas little FDCA was obtained.
As the temperature increased, the yield of DFF was reduced
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ChemSusChem
10.1002/cssc.201900651
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sharply and it was totally converted at 120 °C, meanwhile,
moderate FDCA and FFCA yields of 50% was achieved. The
optimal temperature for the oxidation of HMF to FDCA was
presented as 140 °C, and barely no intermediates and substrate
remained. It was demonstrated that relatively low temperature
was advantageous to the oxidation of hydroxyl to aldehyde, while
the transformation of aldehyde to carboxyl required higher
temperature and more energy. Further rising temperature
resulted in a decrease of FDCA yield, presumably because of (1)
the decomposition of ionic liquids, which was confirmed by HPLC,
The FDCA yield firstly increased with the catalyst amount from
69%, and best result was obtained with 0.05 g catalysts, which
was attributed to the increase in the availability and number of
catalyst active sites. Further increase in catalyst/HMF ratio led to
a decrease of the FDCA yield. Thus, 0.05 g catalysts under the
reaction condition provided sufficient active sites for the oxidation,
and the decrease may be a result of deep oxidation and
decomposition of products on excess active sites.
Catalyst was successively reused three times under optimal
conditions for the oxidation of HMF to FDCA. The FDCA yields
were significantly reduced from 88.8% in the first run to 75.1% in
the third run. It was mainly because the instability of catalyst
during the recovery, Mo6+ is leaching from catalyst. Specific
results of recycling are shown in Table S6.
(
2) degradation of HMF, and (3) polymerization and resinification
of HMF to humins.
1
00
a
100
80
60
40
20
0
b
8
6
4
2
0
0
0
0
0
Reaction Mechanism
FDCA yield
FFCA yield
HMF conversion
DFF yield
FDCA yield
FFCA yield
HMF conversion
DFF yield
To gain insight into the reaction and improve the selectivity of
FDCA, an integrated reaction mechanism is proposed, as
illustrated in Figure 6. As for HPM, Mo(VI) acted as the active sites
where the reaction takes place, and then the oxygen transfer is
1
2
3
4
5
6
7
80
100
120
140
160
Time (h)
Temperature (°C)
completed by oxygen atmosphere via a redox cycle. For HPMV
n
c
d
1
00
100
80
60
40
20
0
(n=1~6), it was rather V(V) that principally promoted the oxidation.
For the sake of clarity, it is assumed that at least one of the three
protons in HPA remains associated with the anion.
8
6
4
2
0
0
0
0
0
FDCA yield
FFCA yield
HMF conversion
DFF yield
FDCA yield
FFCA yield
HMF conversion
DFF yield
0
.5
1.0
1.5
2.0
2.5
1
2
3
4
5
6
7
8
9
Pressure (MPa)
catalyst amount /g
a
f
Figure 5. Dependence of HMF conversion, yields of FDCA and intermediates
on reaction conditions. (a) reaction time; (b) reaction temperature; (c) oxygen
pressure; (d) catalyst amount (g/g ILs). Default reaction conditions: 0.1 mmol
b
2
HMF, 1 g [Bmim]Cl, 0.05 g catalysts, 140 °C, 1 MPa O , 6 h.
c
e
The pressure in the reaction system determines the oxygen
availability in solvents and oxygen mass transfer from the gas
phase to the liquid phase, and further influences the re-oxidation
of catalysts. The effect of oxygen pressure on the oxidation
reaction was investigated and shown in Figure 5c. Contrary to the
results from changing temperature and time, pressure variation
did not exert obvious influence on the reaction. Intermediates
FFCA and DFF could be effectively obtained under low pressure
d
Figure 6. Proposed reaction mechanism of the oxidation of HMF to FDCA.
(
M=Mo, V)
(
0.5 MPa). The yield of FDCA achieved maximum value (88.8%)
Firstly, hydroxyl group of HMF interacted with ILs and multiple
hydrogen bonds were formed with the [Bmim]+ cations and Cl-
under 1 MPa, afterwards slightly reduction with the pressure
increasing. Moderate pressure favored the dissolving of oxygen
into solvents and further promoted the oxidation on the active
sites of the catalyst. However, excessive pressure resulted in
deep oxidation and thereby decomposition of products.
The aerobic oxidation of HMF to FDCA was conducted with
different catalyst amount (g/g ILs). As shown in Figure 5d, the
6
conversion of HMF was 99% with 0.01 g HPMV addition and kept
anions, leading to HMF-ILs complex a (proton in [Bmim]+ with O,
-
and Cl with H). In the presence of O
2
, β-hydride elimination
subsequently occurred, affording the corresponding aldehyde b
DFF) accompanied with water. Secondly, with HPAs as catalysts,
(
electron transfer occurred and formed a protonated aldehyde c,
then the aldehyde group underwent reversible hydration to a
hydrate intermediate
d
(geminal diol) and subsequent
stable with the variation of catalyst amount. On the other hand,
the yield of FDCA was relatively affected by the catalyst amount.
dehydrogenation to produce the corresponding carboxylic acid e
+
with eliminating H . Consequently, intermediate FFCA was
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ChemSusChem
10.1002/cssc.201900651
FULL PAPER
generated by the epoxy groups and Mo=O was reduced, which
Substrate Exploration
would be further re-oxidized by O
intensive UV spectrum peak of Mo=O in HPM is located at λ = 210
nm, and the bonds of Mo and O were single and double bonds.[34]
2
. As shown in Figure 7, the
Since the source of HMF is restrained and results in a higher cost,
the synthesis of FDCA from accessible and inexpensive biomass
or carbohydrates is further studied. It was expected to apply HPAs
to the direct synthesis of FDCA from hexoses or cellulose, and
the results are summarized in Table 5.
2
When HPM was exposed to high O pressure, one new UV
spectrum peak appeared around λ = 312 nm, indicating the epoxy
groups were formed. The epoxy groups played a crucial role in
further oxidation of -CHO to -COOH and DFF advanced oxidation
to FFCA. The strong π–d interaction between the imidazole and
Mo center resulted in a UV spectrum peak tailing of epoxy
groups,[
34]
which was responsible for the higher activities of
heteropoly acids in imidazole ionic liquids. Finally, the other
aldehyde group was continuously catalyzed via the same
oxidation route and finally provided FDCA. Here, the molybdic
species exhibited both Brønsted acidity and redox activity during
the oxidation.
O
Mo
O
O
Mo
HPM-O
HPM
2
2
00
300
400
500
λ / nm
Figure 8. NMR spectra of [Bmim]Cl and HMF, a. 1H NMR spectra of [Bmim]Cl,
HMF and mixture of [Bmim]Cl and HMF; b. 13C NMR spectra of [Bmim]Cl, HMF
and mixture of [Bmim]Cl and HMF.
2
Figure 7. UV spectra of Phosphomolybdic acid (HPM) within and without O .
The HMF-ILs complex is confirmed by NMR spectra. By
comparing the H NMR spectra of pure [Bmim]Cl, HMF and the
Table 5. The synthesis of FDCA from different substrates and systems.
1
mixture (Figure 8a), it is clear that many characteristic resonance
signals of HMF and [Bmim]Cl changed after mixing, which verifies
the special interaction between HMF and N-heterocyclic carbenes
Entry
substrates
Base
Process
FDCA
yield
Time
[h]
ref
[
mol %]
(
NHCs) originated from [Bmim]Cl.[35] The resonance band of the
hydroxyl group of HMF shifted towards the downfield region from
.56 ppm to 5.88 ppm after [Bmim]Cl was mixed with HMF, due
Fructose[a]
Glucose[a]
Fructose
-
-
One pot
One pot
30.7
48.6
9
9
-
-
1
2
5
to the hydrogen bond formation between Cl- anions and hydroxyl
protons. Meanwhile, the band attributed to the active proton 2-H
of imidazole ring (5) shifts from 9.54 ppm to 9.44 ppm, indicating
the interaction and coordination between hydroxyl groups and the
Triphasic
system
[
36]
3
4
Na
2
CO
3
78
25
30
70
Biphasic
system
[37]
Fructose
-
2
-H. It is worth noting that chemical shifts of -CHO assigned to
HMF is negligible, therefore, hydroxyl group is transformed in
advance and HMF is easily oxidized to DFF rather than HMFA. In
contrast, the 13C NMR of [Bmim]Cl and HMF (Figure 8b) showed
no obvious chemical shifts after mixing, suggesting the integration
of imidazole ring and furan ring.
[
1
a] Reaction conditions: 0.1 mmol substrates, 1 g [Bmim]Cl, 0.05 g HPMV
40 °C, 1 MPa O
6
,
2
.
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ChemSusChem
10.1002/cssc.201900651
FULL PAPER
Table 6. Comparison of different catalytic system for the oxidation of HMF to FDCA.
Yield
Ref
Entry
Catalyst
Oxidant
Additives
Temp. (℃)
Time (h)
Conv. (%)
(%)
[
[
[
[
[
[
[
[
9]
1
2
3
4
5
6
7
8
9
Pt/C
40 bar air
2 equiv Na
2
CO
3
100
110
25
6
99
69
8c]
15]
13]
10a]
38]
5a]
4b]
Pt/C-O-Mg
1 MPa O
1 bar air
2 MPa O
TBHP
2
-
12
14
12
12
5
100
99
97
CoPz/g-C
MoO -CeO
Fe -CoO
MnFe
MnCo
3
N
4
2
Na B
O
4 7
96
x
2
2
2 equiv KHCO
3
110
80
99
91
3
O
4
x
DMSO
97
68
2
O
4
TBHP
-
100
100
160
140
100
99.5
99.9
100
85
2
O
4
1 MPa O
2 MPa O
1 MPa O
2
2
2
3 equiv KHCO
[Bmim]Cl
[Bmim]Cl
3
24
24
6
70.9
60.6
89
Fe-Zr-O
HPAs
-
FDCA was obtained with a yield of 6% from cellulose, 30.7% from
fructose, and 48.6% from glucose. FDCA obtained from cellulose
is a combined result of hydrolysis of cellulose to hexose,
dehydration of hexose to HMF, and eventual oxidation of HMF.
Catalysts employed are required to be multi-functional for all the
procedures. Obviously, lower FDCA yield was accompanied with
a longer reaction pathway, as more byproducts were presented
on the active sites of catalysts, and side reactions occurred,
thereby restraining the selective oxidation. During the glucose
dehydration to HMF, it was proposed that 1,2-enediol was an
intermediate stabilized by heteropoly anions, and no isomerized
fructose was formed by HPAs in ionic liquid system. Brønsted
acids induce the transformation of glucose to intermediate 1,2-
enediol and then oxidation of HMF to FDCA, no fructose
presented as intermediate.[33] Hence, the dehydration of glucose
and fructose to HMF underwent different reaction paths, and there
were wilder side reactions from fructose.
great capital differences mainly including catalysts, oxidants and
base additives. It is regarded more economically and
environmentally feasible to employ ILs-HPAs reaction system to
the synthesis of FDCA.
Conclusions
In summary, a novel base-free IL-HPA catalytic system was
developed for the selective oxidation of HMF to FDCA. Strong
hydrogen bonds were formed between halide anions, 2-H of
imidazole ring and HMF, promoting the catalytic oxidation.
[
Bmim]Cl served as excellent solvent and co-catalyst, and by
introducing V atoms in HPAs, it was found that HPMV was as an
6
outstanding catalyst. The active M (Mo /V) center transformed
from single or double oxygen bonds to epoxy groups in oxygen
atmosphere, that promote electron and oxygen transfer between
furanics and Keggin anions. Over 99% HMF conversion and 89%
FDCA yield were achieved at 140 °C in 6 h. Meanwhile, ILs-HPAs
exhibited moderate activity in the direct conversion of hexoses to
FDCA, and provided 48% FDCA yield from glucose. Overall, this
study provided convenient access to FDCA from biomass-derived
feedstocks, and avoided the high cost of noble metal catalysts,
the use of corrosive inorganic base and acid and the complex
industrial procedure. Therefore, it can be regarded more
economically and environmentally feasible to employ ILs-HPAs
reaction system to the synthesis of FDCA.
Compared with previous synthesis of FDCA from fructose (Table
5
, Entry 3,4) using triphasic or biphasic reaction system as well
as noble metal catalysts,[36-37] the conversion of sugars to FDCA
in ILs-HPAs system could be performed via one pot and one step,
simplifying the reactor and reducing the cost. The reaction time
was significantly reduced to less than 10 h, which greatly
improved the production efficiency. Glucose is more accessible
and earth-abundant than fructose, and it was first reported as
substrate in ILs-HPAs, and provided a moderate FDCA yield,
which is quite prior to other reported reaction system.
The results of FDCA synthesis from HMF in different reaction
system were listed in Table 6 to illustrate the advantages of the
novel catalyst system. Compared with other noble metal catalysts
or no-noble metal catalysts, this novel catalyst system eliminated
the use of expensive noble metal, additive base, inorganic acids
for FDCA salt neutralization and complex industrial procedure.
Moreover, high FDCA yield was achieved in relative shorter time,
while 12 h to 24 h were required for other catalytic system. A
rough capital of process was also listed in Table S8, and there are
Experimental Section
Catalysts preparation and characterization
Typical H
method as follows. 11 mmol MoO
into a 250 ml round flask with 100 ml water, subsequently mixed
well at 80 °C with vigorous stirring. Afterwards, 1 mmol H PO
4
PMo11
V
1
O40 (HPMV
1
) was fabricated using a typical
3
and 1 mmol V was added
2 5
O
3
4
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ChemSusChem
10.1002/cssc.201900651
FULL PAPER
(
85%) was added dropwise into the system, which was refluxed
Keywords: 5-Hydroxymethylfurfural • 2,5-Furandicarboxylic
at 110 °C with vigorous stirring for 5 h. During the heating, a color
change was observed from turbid to red and transparent.
Eventually, the solution was evaporated at 80 °C, obtaining a
Acid • Heteropoly Acids • Ionic Liquids • Biomass
Conflict of interest
bright orange solid. Other heteropoly acids HPMV
synthesized via similar procedure, only with different quantity. The
commercial H 40 (HPM), H 40 (HPW), H SiMo12
HSM), H 40 (HSW) were directly used as received.
HVOPMo12 40 was prepared as follows. Required V was first
dissolved in oxalic acid solution, and the new solution was
obtained and turned to blue after stirring 2 h at 80 °C. H
was added afterwards, which was continuously stirred to solid.
HVOPMo12 40 was finally obtained. /H
prepared by following the procedure. Required amount of
40 and V were added into 30 ml deionized water,
followed by stirring for 2 h. The mixture was then heated with
stirring at 80 °C , V /H 40 was finally obtained.
n
(n=2~6) were
The authors declare no conflict of interest
3
PMo12
O
3
PW12
O
4
O
40
(
4
SiW12
O
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thermo-controller. In a typical procedure, 0.1 mmol HMF, 0.01 g
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for a certain time, reactor was quickly cooled to room temperature
and carefully depressurized. The major products were identified
by high performance liquid chromatography (HPLC) of Shimazu
LC-20A system using a method below. Compounds including
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Entry for the Table of Contents
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Ruru Chen, Jiayu Xin,* Dongxia Yan,
Huixian Dong, Xingmei Lu, Suojiang
Zhang*
Page No. – Page No.
Highly Efficient Oxidation of 5-
Hydroxymethylfurfural to 2,5-
Furandicarboxylic Acid with
Heteropoly Acids
Base-free synthesis of FDCA from HMF and glucose in the novel ionic liquids-
heteropoly acids (ILs-HPAs) reaction system.
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