Fabrication of Trifunctional Polyoxometalate-Decorated Chitosan Nanofibers for Selective Production of 2,5-Diformylfuran

Article abstract of DOI:10.1002/cssc.201901384

Trifunctional catalysts based on polyoxometalate (POM) decorating chitosan nanofibers (H₅PMo₁₀V₂O₄₀/chitosan nanofibers, abbreviated as HPMoV/CS-f), synthesized by using the electrospinning method, realized highly efficient oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF). Decorating chitosan nanofibers with POMs generated enhanced catalytic activity by merging their unique individual properties of redox ability, Br?nsted acidity, basicity, and nanofiber structure with higher surface area. As a result, HPMoV/CS-f(25) (with 25 representing the POM amount) was found to be the most active catalyst in the aerobic oxidation of HMF, resulting in 94.1 % DFF yield at 96.2 % conversion in DMSO at 120 °C for 6 h, whereas 56.2 % DFF yield at 95.0 % conversion was obtained in water at 140 °C for 8 h. Importantly, DFF could be produced in one pot in one step to give 61.9 and 31.4 % yield, respectively, directly from fructose and glucose under the reaction conditions of 140 °C, 6 h in DMSO, which was owing to the suitable balance of Br?nsted acidity and basicity of the trifunctional HPMoV/CS-f(25). Moreover, HPMoV/CS-f showed good stability and ability to be reused at least ten times without leaching of the POMs from the chitosan nanofibers.

Full text of DOI:10.1002/cssc.201901384

Accepted Article
Title: Fabrication of trifunctional polyoxometalate - decorating chitosan
nanofibers in highly selective production of DFF from 5-HMF,
fructose and glucose
Authors: Yiming Li, Peili Li, Ping Cao, Ying Li, Xiaohong Wang, and
Shengtian Wang
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content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201901384
Link to VoR: http://dx.doi.org/10.1002/cssc.201901384
A Journal of
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10.1002/cssc.201901384
ChemSusChem
FULL PAPER
Fabrication of trifunctional polyoxometalate - decorating chitosan
nanofibers in highly selective production of DFF from 5-HMF,
fructose and glucose
Yiming Li, Peili Li, Ping Cao, Ying Li, Xiaohong Wang * and Shengtian Wang
Abstract: Trifunctional polyoxometalate (POM) decorating chitosan
nanofibers (H5PMo10V2O40/chitosan nanofibers, abbreviated as
HPMoV/CS-f) had been synthesized using electrospinning method,
which realized highly efficient oxidation of 5-hydroxymethylfurfural (5-
HMF) to 2,5-diformylfuran (DFF). Decorating chitosan nanofibers by
POMs generated enhanced catalytic activity by emerging their unique
individual properties of redox ability, Brønsted acidity, basic property,
and nanofiber structure with higher surface area. As results,
HPMoV/CS-f (25) (25, represented the POM amount) was found to be
most active in aerobic oxidation of 5-HMF to give 94.1 % yield of DFF
at 96.2 % conversion in dimethyl sulfoxide (DMSO) at 120 °C for 6 h,
while 56.2 % yield of DFF at 95.0 % conversion was obtained in water
system at 140 °C for 8 h. Most importance was that DFF could be
produced in one-pot one-step to give 61.9 % and 31.4 % yields directly
from fructose and glucose under the reaction conditions as 140 °C , 6
h in DMSO, which was due to the suitable balancing of Brønsted
acidity and basicity of the trifunctional HPMoV/CS-f (25). Moreover,
HPMoV/CS-f showed good stability and duration for being reused at
least ten times without leaching of POMs from chitosan nanofibers.
5-hydroxymethylfurfural (HMF) has drawn increasing attention as a
member of bio-platform chemicals, [5] which could be synthesized by
acid-catalyzed dehydration of carbohydrates including mono- or poly-
[6]
saccharides.
chemicals
HMF could be converted into a series of bulk
and
intermediates
through
various
chemical
[7]
transformations including maleic anhydride (MA),
furandicarboxylic acid (FDCA) and 2,5-diformylfuran (DFF), [9] 2,5-
2,5-
[8]
[10]
bishydroxymethyl furan (BHMF),
2,5-di-hydroxy-methyl-
[11]
[12]
tetrahydrofuran (THFDM),
2,5-dimethylfuran (DMF),
and 3-
hydroxymethylcyclopentanone
(HCPN)
and
3-
hydroxymethylcyclopentanol (HCPL) (Scheme S1). [13] Among these,
selective oxidation of HMF was one of the most essential
transformations in biorefinery to produce DFF, FDCA or MA. DFF is a
versatile compound that can be used as a precursor in the synthesis
[14]
[16]
of functional polymers,
pharmaceuticals, [15] antifungal agents,
furan-urea resins, [17] heterocyclic ligands,
and other value added
[18]
products. However, several kinds of furan compounds like FDCA, 5-
formyl-2-furancarboxylic acid (FFCA), 5-hydroxymethyl-2-
furancarboxylic acid (HMFCA), companied with DFF can possibly be
formed during the HMF oxidation. Highly selective oxidation of HMF
is important for DFF production. By now, both homogeneous and
heterogeneous metal catalysts have been explored for the oxidation
Introduction
[19-21]
of HMF to DFF using various oxidants.
From the “Green
Chemistry” concept, aerobic oxidation of HMF to produce DFF is a
challenge with a promising application. Various solid catalysts had
been designed to achieve HMF convert to DFF under aerobic
oxidative conditions (Table S1). Among all, materials containing
vanadium had been paid more attentions, which showed the
conversion ranging from 84 to 100 % and DFF yields of 82 ~ 99.9 %
Chitosan is a kind of natural polymers with potentials for biologicals,
industrial treatment of waste water, and catalyst supports due to its
[1]
high adsorbing or chelating ability.
For various applications,
chitosan has been decorated with some functional groups or noble
[2]
metals.
Polyoxometalates (POMs) functionalizing chitosan had
been synthesized by doping POM molecules into chitosan through
anion-cation interaction between protonated amino groups in chitosan
[22-25]
at 120 °C to 140 °C for about 3 ~ 11 h in DMSO.
Polyoxometalates (POMs) containing vanadium also showed
potentials for oxidation of HMF to give a series of products including
DFF [9] and MA [7]: Cs3HPMo11VO40 gave 99 % conversion and 99 %
yield of DFF at 110 °C for 6 h in DMSO under N2 and O2 in one-pot
two-step process; H5PMo10V2O40 presented 100 % conversion and
64 % yield of MA at 90 °C for 8 h in acetonitrile under 10 atm of O2. It
can be concluded that the existence of Lewis base could favor for
activating the hydroxyl group hence promoting the formation of DFF.
[3]
and polyanions. Such POM-chitosan hybrids exhibited larger size
more than 0.1 μm and surface area lower than 0.4 cm3/g.
Electrospinning has been recognized as an efficient technique for
fabrication chitosan nanofibers ranging from 5 to 500 nm with amazing
characteristics of very large surface area-to-volume ratio. [4] Therefore,
fabrication of POM-functionalizing-chitosan hybrids with good
physical-chemical characteristics is desirable, especially for catalysis.
[26]
Therefore, the combination of Lewis base and POMs to fabricate
multifunctional hybrids might open a new way for production of DFF
from 5-HMF or even from monosaccharides.
Y. Li, P. Li, P. Cao, Y. Li, Prof. X. Wang, S. Wang
Key Lab of Polyoxometalate Science of Ministry of Education,
Faculty of Chemistry
Northeast Normal University
Changchun 130024, P. R. China
HMF is a dehydrated product from fructose and also could be
obtained from glucose through isomerization to fructose then
dehydration in presence of basic or Lewis acidic catalysts (Scheme
1). [27] Compared to HMF as feedstocks, direct production of DFF from
saccharides is more economics with well-tolerance to feedstocks and
energy-release. By now, direct synthesis of DFF from fructose had
already been achieved using POMs as catalysts. Liu’s group reported
that 58 % yield of DFF was obtained directly from fructose through
H3PMo12O40 with 100 % conversion under reaction conditions as
E-mail: wangxh665@nenu.edu.cn
Supporting information for this article is given via a link at the end of
the document
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901384
ChemSusChem
FULL PAPER
[28]
160 °C, 6 h in DMSO.
The catalytic activity was attributed to the
peaks in the range of 1100-700 cm-1, which are attributed to the
Keggin unit of υas(P-Oa), υas(Mo-Od), υas(Mo-Ob-Mo), and υas(Mo-Oc-
Mo), respectively. Meanwhile, υas(P-Oa), υas(Mo-Od), υas(Mo-Ob-Mo),
and υas(Mo-Oc-Mo) were observed with some blue shifts after
formation of HPMoV/CS materials. Such shifts were due to the strong
co-existence of Brønsted acid and redox centers. Then
Cs3HPMo11VO40 was adopted in one step dehydration-oxidation of
fructose to DFF with 60 % yield at > 99 % conversion under reaction
[9]
conditions as 120 °C, 8 h in DMSO. And Ghezali et al conducted
+
[33]
the production of DFF directly from fructose and inulin over
H(3+n)PMo(12-n)VnO40 (n = 0 ~ 2) in choline chloride/DMSO solvent,
interaction between POM anion and -NH3 in chitosan nanofiber.
The typical vibrational peak of -NH2 in chitosan nanofiber was
+
which H4PMo11V1O40 was found to be most active to give 84 % yield
observed to shift from 1578 cm-1 to 1520 cm-1 belonging to -NH3 , [34]
[29]
+
of DFF at 120 °C for 360 min.
Recently, Lee’s group achieved
further determined the interaction between PMo10V2O405- and -NH3 .
H3PMo12O40 encapsulated on MIL-101 (PMA-MIL-101) to catalyze
The XRD of HPMoV/CS-f (n) was used to study the dispersion of
the Keggin POM unit throughout the nanofiber hybrids (Fig. S2). A
broad peak centered at 2θ of 19.7° was observed for all nanofibers,
which was assigned to the chitosan. Meanwhile, there were no
diffraction peaks corresponding to the parent H5PMo10V2O40 in all
HPMoV/CS-f except HPMoV/CS-f (35). This indicated that the POM
molecules were homogeneously dispersed in the mattric of chitosan
as the loading amount lower than 30 %. Increasing the loading
amount to 35 %, the diffraction peaks at 2θ of 21.2°, 26.7° and 29.5°
were observed belonging to H5PMo10V2O40, which showed that POM
molecules unevenly dispersed or aggregated across the nanofibers in
higher loadings. The DR-UV-vis (Fig. S3) gave the characteristic peak
at 210 nm corresponding to Keggin structure. Meanwhile, the
absorption intensity of the peak increased gradually as increasing the
loading amount of HPMoV on chitosan nanofiber.
fructose convert into DFF in DMSO with 75.1 % yield of DFF at 81 %
[30]
fructose conversion at 110 °C for 1 h.
Such PMA-MIL-101
overcame the drawbacks that one-pot conversion of fructose to DFF
might suffer from low yield of DFF due to the co-existence of oxidative
catalysts favoring for humin or the side-products formation. [31, 32] Also
PMA-MIL-101 showed good reusability without separation procedures.
Nevertheless, more efficient and recyclable catalysts containing multi-
functionalized active sites are more desirable in one-pot one-step
production of DFF from fructose even glucose.
Scheme 1 Reaction pathways for production of 5-HMF then to DFF from
glucose in presence of multi-functional catalysts
The 31P MAS NMR for HPMoV/CS-f (25) gave two peaks at -4.54
and -2.19 ppm (Fig. S4a), which one was attributed to the existence
Herein, we designed to synthesize chitosan nanofibers being
decorated by H5PMo10V2O40 (HPMoV) to check the physical-chemical
properties on production DFF from HMF, and even from fructose or
glucose. Firstly, chitosan nanofibers acted as a support to load POMs
forming heterogeneous catalyst with large surface area-to-volume
ratio. Secondly, such HPMoV/chitosan nanofiber hybrids could adjust
Lewis basicity, Brønsted acidity and redox potentials through simply
changing its components, which resulted in controllable triple sites
favoring for HMF to DFF, even for fructose or glucose. This might
provide some information for targeting high conversion of HMF and
selectivity to DFF in the absence of bases in the reaction system, but
also give some advice on designation of triple-functionalized POMs
catalysts in other organic synthesis. Thirdly, DFF could be produced
from glucose via base-catalyzed isomerization, acid-catalyzed
dehydration, and successive selective oxidation by one-pot one-step
reaction through HPMoV/chitosan nanofiber and O2.
5-
of PV2Mo10O40 anion (-4.55 ppm) and one was assigned to the
+
interaction between PV2Mo10O405- anion and -NH3 group in chitosan
nanofibers. [35] These results indicated the structural integrity of POM
anion in hybrids, and also permitted their less leaching from chitosan
support. The SEM images and EDX of HPMoV/CS-f (n) were given in
Fig. 1, showing that all HPMoV/CS-f (n) were consisted of uniform
fibers with diameters of 180-200 nm in good dispersity. And the EDX
results for HPMoV/CS-f were similar to those of elementary analysis
with HPMoV loading amounts as 5, 12, 25, 30 and 35 wt%,
respectively.
The XPS analysis mainly reflects the composition and chemical
elementary state of the surface and the inferior surface of sample. [36]
The binding energy of HPMoV/CS-f (25) was given in Fig. 2a. It can
be found the peaks of the O1s, N1s, C1s, V2p, P2p and Mo3d at 532.4
eV, 399.1 eV, 284.6 eV, 516.3 eV, 133.4 eV and 232.4 eV,
respectively. This can prove that element O exists as O2-, N as N3-, C
as C4+, P as P5+, V as V5+, and Mo as Mo6-. The XPS for N1s (Fig. 2b)
presented the peak of -NH2 at 399.0 eV and protonated -NH3+ at 401.5
eV in HPMoV/CS-f (25). This could be explained as that the proton
from H5PMo10V2O40 reacted with some of -NH2 in chitosan through
Results and Discussion
Characterization of HPMoV/CS-f
+
PMo10V2O40···H···NH2 to form protonated -NH3 . Furthermore, some
The elemental analyses of HPMoV/CS-f (n) with different loading
amount of H5PMo10V2O40 were given in Table S2. 0These results gave
the molar ratio of P: Mo: V = 1: 10: 2, implying that H5PMo10V2O40 kept
Keggin structure during the preparation. And the loading amounts of
HPMoV on chitosan nanofiber were 5 wt% for HPMoV/CS-f (5), 12 wt%
for HPMoV/CS-f (12), 25 wt% for HPMoV/CS-f (25), 30 wt% for
HPMoV/CS-f (30), and 35 wt% for HPMoV/CS-f (35), respectively.
of amino groups did not react with heteropolyacids to retain some of
basicity.
Furthermore, the stability of HPMoV/CS-f (n) was studied by
thermogravimetric analysis (TGA), which gave the decomposition
temperatures at 100 and 200 ~ 250 °C with the mass losses as around
8 % and 27 %, respectively (Fig. S5). The first mass loss was
contributed to the loss of crystal water, while the second one was
assigned to the dehydration of hydroxyl group in chitosan[37] similar to
chitosan. It was indicated that the hybrids were stable within 500 °C.
5-
The structural integrity of the Keggin anion PMo10V2O40 was
confirmed by IR spectroscopy (Fig. S1). The IR spectra of
H5PMo10V2O40 (1051, 964, 864, and 788 cm-1) and HPMoV/CS-f (n)
(1044, 934, 859, and 782 cm-1) gave almost the same characteristic
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10.1002/cssc.201901384
ChemSusChem
FULL PAPER
f (25) (0.21 mmol g-1) > HPMoV/CS-f (30) (0.16 mmol g-1) >
HPMoV/CS-f (35) (0.11 mmol g-1).
Fig. 3 CO2-TPD spectra of chitosan nanofiber, HPMoV/CS-f (5), HPMoV/CS-f
(12), HPMoV/CS-f (25), HPMoV/CS-f (30) and HPMoV/CS-f (35)
The catalytic activity for HPMoV/CS-f (n) in HMF oxidation
Scanning the activity in HMF oxidation
In order to evaluate oxidative activity of POMs/CS nanofibers, we
tested in the conversion of HMF under reaction conditions as 100 mg
of 5-HMF and 60 mg of catalyst at 120 °C , 0.8 MPa for 6 h in 4 mL
DMSO. As shown in Table 1, various catalysts including chitosan,
chitosan nanfiber, HPMoV/CS (25) without nanofiber morphology,
HPMoV/CS-f (5), HPMoV/CS-f (12), HPMoV/CS-f (25), HPMoV/CS-f
(35) and H5PMo10V2O40 had been evaluated in aerobic oxidation of
HMF. It can be seen that without any catalysts, HMF was only oxidized
with 22.0 % conversion and 48.0 % selectivity to DFF, showing that
the oxidative ability of oxygen was not strong enough to confirm the
oxidation of HMF under our reaction conditions. Adding chitosan, the
conversion of HMF was only increased to 23.3 %, but DFF selectivity
increased to 77.3 %. Chitosan is a kind of basic biopolymer with a lot
of amino groups, which could interact with -OH group of HMF through
Fig. 1 SEM images (a) and EDX patterns (b) of HPMoV/CS-f (n)
Fig. 2 (a) XPS spectra of the chitosan nanofiber and HPMoV/CS-f (25). (b) N
1s XPS of the chitosan nanofiber and HPMoV/CS-f (25).
hydrogen bond as chitosan-NH2···
of the hydroxyl was favored,
. Hence the activation
and the generation of DFF was
[38]
As chitosan is a kind of basic biomacromolecules containing a large
number of -NH2 groups. Here, the purpose that we selected chitosan
as a support for loading of POMs was to fabricate trifunctional hybrids
combined POMs with Brønsted acidity and redox property, and
chitosan nanofibers with basicity. It can be seen that all samples
presented a similar CO2 desorption temperature centered around
280-330 °C, which were assigned to the contribution of chitosan (Fig.
3) since HPMoV/CS-f (n) and chitosan nanofiber were stable within
500 °C. Meanwhile, increasing of loading amount of H5PMo10V2O40,
the desorption temperature showed a decreasing trend from 317.3 °C
(pure chitosan) to 291.8 °C (HPMoV/CS-f (25). This indicated that
basicity of the hybrids decreased depending on the increasing of POM
loading amount, which also determined that H5PMo10V2O40 molecule
homogeneously dispersed in chitosan matrix through interaction with
amino groups hence decreasing the amount of -NH2. The CO2
desorption temperature of HPMoV/CS-f (35) at 291.5 °C was lower
than that of HPMoV/CS-f (25) at 291.8 °C, showing that there were
some aggregation of POM molecules as increasing of its loading
amount. Nevertheless, POM/CS-f (n) hybrids presented strong
basicity in order of chitosan nanofibers (0.62 mmol g-1) > HPMoV/CS-
f (5) (0.46 mmol g-1) > HPMoV/CS-f (12) (0.37 mmol g-1) > HPMoV/CS-
improved. When chitosan was electrospinned to form nanofibers, the
activity of chitosan nanofiber was improved to 27.5 % of conversion
but 85.5 % of selectivity. Although the surface area of chitosan
nanofiber was larger than that without nanofiber structure, the
catalytic activity in aerobic oxidation of HMF was /not increased
significantly. This indicated that chitosan was not active in oxidative
reaction. The selectivity to DFF was increased from 73.3 to 85.5 %,
which was due to the increase of surface basicity in chitosan
nanofibers. HPMoV presented strong redox capacity and strong
[39]
Brønsted acidity,
which gave 58.9 % selectivity at 98.2 %
conversion. The relationship between their activity of HPMoV/CS-f (n)
and the HPMoV loading amount on chitosan nanofiber was also
studied: (1) All HPMoV/CS-f hybrids showed higher activity than
HPMoV and chitosan nanofibers. For same loading amount of HPMoV,
HPMoV/CS nanofibers showed higher activity than HPMoV/CS non-
nanofibers; (2) The conversion of HMF followed the order of
HPMoV/CS-f (5)
< HPMoV/CS-f (12) < HPMoV/CS-f (25) <
HPMoV/CS-f (30) < HPMoV/CS-f (35), which was similar to the
increasing of HPMoV loading amount. The HMF conversion was
62.7 % for HPMoV/CS-f (5), while increased to ~100 % for
HPMoV/CS-f (35). The influence of loading amount of HPMoV on
HMF conversion was also observed in Fig. 4a of conversion verse
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10.1002/cssc.201901384
ChemSusChem
FULL PAPER
significant differences at 1666 cm-1 (Fig. S6a) assigned to amide
vibrations representing the C=O groups along HMF involved in the
intrasheet hydrogen bonds with the NH2 group along the chitosan. [40]
reaction time, while the maximum conversion upon HPMoV/CS-f was
achieved at different time depending on the HPMoV loading amount.
It was attributed to the increasing of redox sites and also the
increasing in Brønsted acidity, which was similar to Chen’s report as
stronger Brønsted acidity of H5PMo10V2O40 influencing the equilibrium
Table 1 Comparison of catalysts used in HMF oxidation and product distribution.
[9]
shift for HMF oxidation;
(3) The selectivity to DFF increased as
HMF
conver
-sion
Yield (%)
Basic
content
(mmol·g-1)
enhancement of HPMoV loading amount from 5 % to 25 %, then
decreased as further increasing HPMoV amount to 35 %. And the
maximum value was obtained as 91.8 % as using HPMoV/CS-f (25)
with TOF as 14.73 h-1 (TOF = converted HMF (mmol)/actual usage of
HPMoV in chitosan (mmol) × h). Based on the previous reports, the
coexistence of basic center and redox center might affect the
oxidation of HMF and the distribution of the products [38]: there are two
main competitive reactions in HMF oxidation as alcohol oxidation to
DFF in the presence of slightly basic or neutral medium, and the
aldehyde oxidation to HMFCA in strong basic media. Then DFF and
HMFCA are further oxidized to FFCA under a strong basic condition.
Catalyst
DFF
HMFCA
FDCA
(%)
22.4
23.3
None
Chitosan
Chitosan
nanofiber
HPMoV/CS
(25) without
nanofiber
HPMoV/CS-f
(5)
HPMoV/CS-f
(12)
HPMoV/CS-f
(25)
-
10.6
18.0
-
-
0.69
0.4
0.2
0.62
27.5
23.5
0.1
1.3
0.5
1.3
0.23
76.6
62.9
0.46
0.37
0.21
0.16
62.7
88.4
96.2
98.2
47.8
73.5
94.1
84.4
2.0
1.7
1.3
6.7
0.3
0.5
0.4
1.4
[38]
Therefore, the properties for POM/chitosan nanofibers as
distribution of Brønsted acidity or Lewis basic sites and redox
potentials might influence HMF conversion and also product
selectivity as well. In presence of HPMoV/CS-f, DFF yields increased
firstly then decreased as reaction time changing from 0 to 6 h then to
8 h (Fig. 4b). For HPMoV/CS-f (n), the highest yields of DFF varied
according to the difference of POM loading amounts as HPMoV/CS-f
(5) (Brønsted acidity/Lewis basicity = 1.5: 100) ＜ HPMoV/CS-f (12)
HPMoV/CS-f
(30)
HPMoV/CS-f
(35)
H5PMo10V2O40
0.11
-
100.0
98.2
81.4
58.9
2.6
4.3
1.7
4.2
Reaction conditions: HMF (100 mmg), catalyst usage (60 mg), DMSO (4 mL),
120 °C, 6 h, O2 (0.8 MPa)
(B/L = 2.7: 100) ＜ HPMoV/CS-f (35) (B/L = 21.8: 100) ＜ HPMoV/CS-
f (30) (B/L = 16.8: 100) ＜ HPMoV/CS-f (25) (B/L = 8.9: 100). It can
be seen that the oxidation of formyl group of HMF to HMFCA was
observed around all reaction procedure with the yield lower than 10 %
to 5 %, which indicated that the existence of Brønsted acid and Lewis
basicity of hybrid materials did not favor for oxidation of formyl group
of HMF (Fig. 4c). Meanwhile, the by-product of FDCA was observed
at the end of the reaction, showing not stronger enough oxidative
ability of O2 to oxidize DFF and HMFCA to FDCA through HPMoV/CS-
f (n) catalysts (Fig. 4d). As results, HPMoV/CS-f (25) with B/L = 8.9:
100 gave the highest yield as 94.1 % of DFF with 1.3 % HMFCA and
0.4 % FDCA among all chitosan-POM nanofibers. For HPMoV/CS-f
(25), the total yield of DFF, FDCA and HMFCA after 6 h was close to
the conversion of HMF being tested by TOC (total organic carbon).
The TOC still kept balancing at 6 h with only decreasing of 2.6 %. This
indicated that no other by-products being produced under our reaction
conditions.
The higher efficiency of HPMoV/CS-f (n) was also contributed to the
interaction between -OH from HMF and -NH2 from chitosan, which
could concentrate 5-HMF around the active sites of HPMoV. The
adsorption capacity of the catalysts on HMF also depended on the
composition and structure of hybrids as: HPMoV/CS-f (5) (11 mmol/g)
> HPMoV/CS-f (12) (6.4 mmol/g) > HPMoV/CS-f (25) (3.5 mmol/g) >
HPMoV/CS-f (30) (2.7 mmol/g) > HPMoV/CS-f (35) (2.1 mmol/g) >
HPMoV/CS (25) (1.7 mmol/g). It can be seen that increasing POM
amount gave rise to decreasing of adsorption of HMF, indicating that
chitosan mattric played a main role on adsorption effect. Meanwhile,
HPMoV/CS-f (25) with nanofiber structure presented almost two times
higher than that of HPMoV/CS-f (25) without nanofiber structure. This
was attributed to the enhancing surface area and increasing surface
basic sites of chitosan nanofibers. The essential adsorption of HMF
by HPMoV/CS-f (n) was further determined by IR spectrum of
HPMoV/CS-f (25) after absorbing HMF (Fig. S6). Compared with fresh
HPMoV/CS-f (25), the IR spectrum of adsorbing HMF displayed
Fig. 4 Catalytic performance for various catalysts in oxidation of 5-HMF by O2.
Reaction conditions: 100 mg of 5-HMF, 60 mg of catalyst, and 4 mL solvent at
120 ℃ for 6 h.
The influence of catalyst dosage, temperature and usage of HMF
oxidation catalyzed by HPMoV/CS-f (25) was optimized (Fig. S7).
Varying amounts of HPMoV/CS-f (25) from 40 to 80 mg were used for
the DFF production at 120 °C for 6 h. As shown in Fig. S7a, the
conversion increased from 83.1 % to 100 % as increasing the amount
of HPMoV/CS-f (25) from 40 mg to 80 mg, respectively. It is clear that
the catalyst dosage had a significant effect on the oxidation of HMF.
The yield of DFF increased when the catalyst usage was increased
from 40 mg to 60 mg, and then sharply reduced as further increasing
the usage of catalyst to 80 mg. This was contributed to the subsequent
oxidation of DFF to FDCA. These results showed that the usage of
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10.1002/cssc.201901384
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catalyst as 60 mg was sufficient in the oxidation reaction. Fig. S7b
showed the influence of temperature on oxidation of HMF in presence
of HPMoV/CS-f (25). The conversion increased with the increase of
the temperature from 90 to 130 °C. It can be seen that increasing to
120 °C, the yield of DFF reached the maximum. Further enhancement
of temperature led to decreasing DFF yield due to subsequent
oxidation occurred. The effect of HMF usage of was studied at 80, 90,
100, 110 and 120 mg under similar conditions. Conversion of HMF
was found to be decreased with increasing HMF usage (Fig. S7c). It
can be seen that increasing to 100 mg, the yield of DFF reached the
maximum. All further experiments were carried out by keeping 100
mg of HMF.
the reaction conditions as catalyst (60 mg), fructose (100 mg), DMSO
o
(4 mL), 140 C and 6 h with 0.8 MPa of O2 (Fig. 5a). It can be seen
that hybrids with higher amount of Brønsted acidity gave the highest
conversion of fructose, while HPMoV/CS-f (35) showed highest
conversion efficiency. The yields of DFF depended on the acid-base
distribution of the catalysts. Chitosan nanofiber was a kind of solid
Lewis base, which could not catalyze fructose dehydration, hence no
DFF or HMF generated. H5PMo10V2O40 is a kind of Brønsted acid-
redox POMs, which gave 40.3 and 9.2 % yields of DFF and HMF at
71.8 % conversion of fructose. As expected, the DFF yield increased
significantly as increasing loading amount of HPMoV on chitosan
nanofibers. For all HPMoV/CS-f (n), the yields of 5-HMF were lower
than 16.4 %, showing that the oxidation of 5-HMF was faster than
dehydration of fructose due to the stronger redox ability of HPMoV.
The influence of loading amounts of HPMoV on DFF yield and
fructose conversion was similar to those in oxidation of 5-HMF, while
the HPMoV loading amount of 25 % was suitable for transformation.
Upon HPMoV/CS-f (25), the maximum DFF yield of 61.9 % was
achieved with 14.1 % yield of HMF at 83.1 % conversion of fructose.
Due to the existence of basicity of HPMoV/CS-f (n), it might be more
available for production of DFF directly from glucose. As expected,
yield of DFF was achieved as 31.4 % upon HPMoV/CS-f (25) at
almost 82.3 % conversion of glucose (Fig. 5b) under reaction
conditions similar to fructose. By now, direct production of DFF from
glucose had been achieved by using a base-acid-redox combination
Besides the above conditions, solvents also played a principal role
in the conversion of HMF and selectivity of DFF. Here, six kinds of
solvents were used including organic solvents of DMF (N, N ‐
dimethylformamide), C2H5OH, Toulene, MIBK (methyl isobutyl
ketone), DMSO and H2O (Fig. S8). It also can be seen that the
selectivity of DFF depended on the polarity of solvents using
HPMoV/CS-f (25) catalyst, which strong polarity and high boiling point
for solvent were beneficial for HMF conversion. In these organic
solvents, where DMSO emerged as the best solvent, which a good
HMF conversion of 96.2 % and 94.1 % yield of DFF were obtained
after 6 h. And the HMF conversion (51.4 %) and 20.1 % yield of DFF
were obtained in water at 120 ℃ for 6 h. 95.0 % conversion of HMF
and 56.2 % yield of DFF were obtained in water as temperature was
increased to 140 °C and time was prolonged to 8 h. By now, the best
efficiency of 98 % selectivity to DFF at 97.8 % HMF conversion was
of HT (Mg-Al hydrotalcite), Amberlyst-15 and Ru/HT as
a
heterogeneous catalyst giving 98 % conversion and 25 % yield. [43] It
can be seen that there were three main products being generated as
fructose, 5-HMF and DFF in presence of HPMoV/CS-f (n), indicating
that the co-existence of Brønsted acidity, Lewis basicity and redox
potential synergistically influenced the glucose oxidation (Fig. 5b).
[38]
obtained upon MgO·CeO2 in water system under 100 °C for 15 h.
By now, aerobic oxidation of 5-HMF to produce DFF in water has not
been concerned using POMs as catalysts before. And the study on
this field would be done further. Fig. S8 also presented the influence
of the O2 pressure on the conversion and selectivity in the oxidation
of HMF. By increasing the O2 pressure from 0.4 to 1.2 MPa, the
conversion increased from 88.7 to 98.2 %, indicating higher oxygen
pressure increased the HMF conversion. However, the yields of DFF
at 0.4 MPa, 0.8 MPa and 1.2 MPa changed as 82.3 %, 94.1 % and
72.8 %, respectively. Higher oxygen amount could increase the HMF
conversion but decrease the DFF yield due to the occurrence of over-
oxidation. Therefore, the best considerable oxygen pressure was 0.8
MPa for oxidation of HMF to DFF.
The catalytic activity of HPMoV/CS-f in oxidation of
monosaccharides
Compared to production of DFF from HMF, the direct one-pot
conversion of monosaccharides (fructose and glucose) into DFF with
[22, 41, 42]
high yield is desirable and still a great challenge,
which
involves the selective dehydration of carbohydrates into HMF over
acid sites and the consequent oxidation of HMF to DFF over redox
[22]
sites.
However, production of DFF was often obtained in one-pot
and two-step containing acidification and oxidation reaction facing one
major drawbacks as the formation of humins and other undesired by-
[40, 43]
products due to the co-existence of acid and redox sites.
Moreover, the existence of acid sites might cause the degradation of
formed HMF to levulinic acid or lactic, et al. Therefore, designation of
new multifunctional catalysts with suitable distribution of acid sites and
redox sites is still desirable. On this concept, HPMoV/CS-f (n) might
be a good candidate for production of DFF through monosaccharides.
Initially, their activity was evaluated in conversion of fructose under
Fig. 5 One-pot, one-step synthesis of DFF from fructose (a) or glucose (b).
Reaction conditions: catalyst (60 mg), fructose or glucose (100 mg), DMSO (4
mL), 0.8 MPa O2, 140 oC, 6 h.
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10.1002/cssc.201901384
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The time course for one-pot one-step conversion of glucose into Conclusions
DFF was also investigated in presence of HPMoV/CS-f (25) (Fig. S9).
It was found that fructose and HMF were generated firstly then to DFF,
while fructose and HMF were the main products at the initial stage of
4 h with the yields of 29.8 and 18.4 %, while the yield of DFF was
obtained as 16.3 %. The yields of fructose and HMF then decreased
as prolonging reaction time, while yield of DFF increased to 31.4 % at
6 h. The TOC still kept balancing at 6 h, determining no deep oxidation
to CO2 occurrence. The products verse reaction time determined that
glucose underwent isomerization to fructose by base sites,
dehydration to HMF upon Brønsted acidic sites, and final oxidation to
DFF by redox sites. The optimization of the reaction conditions in
production of DFF form fructose and glucose were undergoing.
Catalyst stability and reusability
A series of trifunctional POM decorated chitosan nanofibers
HPMoV/CS-f (n) had been prepared through electrosinning and
characterized by IR, 31P MAS NMR, XRD, XPS, SEM spectroscopy
and CO2-TPD. Among all hybrids, HPMoV/CS-f (25) showed highest
efficiency in aerobic oxidation of HMF with 96.2 % conversion and
94.1 % yield of DFF in DMSO, which was contributed to its unique
properties of co-existence of Lewis basicity, Brønsted acidity, redox
potentials, nanofiber morphology, suitable surface acid-base sites,
and rational molar ratio for Brønsted acidity to Lewis basicity as 8.9:
100. Direct transformation of fructose and glucose was achieved in
one-pot one-step process, which presented 61.9 % yield of DFF at
83.1 % fructose conversion and 31.4 % yield at 82.3 % glucose
conversion. HPMoV/CS-f was easily separated through filtration and
was reused for at least ten times without obvious loss of its catalytic
activity. Furthermore, HPMoV/CS-f (25) showed potentials in
converting 5-HMF to DFF in water system with 56.2 % yield at 95.0 %
conversion.
Recovery and stability are important properties for practical
heterogeneous catalysts. In each run, the catalyst was separated from
the reaction mixture by centrifugation then washed with DMSO and
ethanol. The obtained solid was dried at 40 °C overnight for reuse.
The HPMoV/CS-f (25) catalyst could be reused for ten runs without
significant loss of the activity under identical reaction conditions. DFF
yield was maintained over 91 % and HMF conversion was kept >
94.2 % after ten runs, which indicated that the HPMoV/CS-f (25)
catalyst was stable and little leaching of HPMoV from chitosan support.
The above filtrate was checked by UV-vis spectroscopy to determine
the leaching of HPMoV/CS-f (25) into reaction mixture (Fig. S10).
Leaching experiments was carried out to check the loss of HPMoV
from support during the reaction (Fig. S11). There was little increasing
in HMF conversion and DFF yield for 5, 6, 7 and 8 h as the catalyst
was filtered out at 4 h compared to the common experiment. It can be
proved that HPMoV did not leaching from the support during the
reaction showing higher stability. The stability of the catalyst was
performed with the reaction time in the range of 0.5-6 h for ten cycles.
The final leaching amount of HPMoV/CS-f (25) was about 1.7 % of
the initial amount (Fig. 6) due to the personal operation. The used
HPMoV/CS-f (25) catalyst was characterized by DR-UV-vis (Fig. S12),
31P MAS NMR, FT-IR and XPS (Fig. S4b, Fig. S6 and Fig. S13), which
showed no change compared to the fresh one. The SEM image (Fig.
S14a) and EDAX (Fig. S14b) of reused HPMoV/CS-f (25) also
showed no change in morphology and element compositions. The
above results determined that HPMoV/CS-f (25) presented high
stability and long duration.
Experimental Section
The synthesis and characterization of the hybrids were given in
Supporting Information. And the oxidative experiment was also in
Supporting Information.
Acknowledgements
This work was supported by National Natural Science Foundation
of China (Grants 51578119), the Changchun Science and Technology
Bureau (17DY025), and the major projects of Jilin Provincial Science
and Technology Department (20180414069GH).
Keywords: polyoxometalate • electrospinning nanofibers • 5-HMF •
monosaccharide • DFF.
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