Full Papers
doi.org/10.1002/cplu.202100199
ChemPlusChem
targeting the conversion of 5-HMF to DFF. Our group had
developed series of POM solid catalysts including
Ca ðPO Þ ðOHÞ ½ðNH Þ H PV Mo O �0:42,
1
0
4
6
1:58
4
5
6
8
4
40
a
H PMo V O /SiO mesoporous nanofibers, H PMo V O /CeO
Ca ðPO Þ ðOHÞ ½ðNH Þ H PV Mo O �0:54,
5
10
2
40
2
5
10
2
40
2
10
4
6
1:46
4
5
6
8
4
40
nanofibers,
H PMo V O /chitosan
nanofibers,
and
5
10
2
40
Ca ðPO Þ ðOHÞ ½ðNH Þ H PV Mo O �0:65,
H PMo V O /LDH (LDH represented MgAl-layered double
10
4
6
1:35
4
5
6
8
4
40
5
10
2
40
10,47–49]
[
hydroxide).
Our results showed that the conversion and
selectivity of converting 5-HMF to DFF depended on the
amount of H PMo V O , the basic sites of support, and the
corresponding the POM loading amounts of 15, 26, 31, and
35%, respectively. From the elementary results, it can be
concluded that POM anions exchanged the hydroxide groups in
HAP to achieve its loading.
5
10
2
40
Brønsted acidic sites of the hybrids. Compared to neutral or
acidic supports, basic supports might provide weak basic sites
[
56]
favoring for oxidation of -OH group in 5-HMF. Therefore,
fabrication of multifunctional catalysts through the combina-
tion of POMs and support is more desirable for the conversion
of 5-HMF to DFF. Compared to other solid bases, HAP could
not only provide benefits similar to other supports, but also a
rigid platform to protect the POMs from being attacked.
The FT-IR spectra of as-prepared hybrids were given in
Figure S1, which all presented the four characteristic peaks at
À 1
1028, 960, 874, and 781 cm belonging to νP-Oa, νMo-Od, νM-Oc-M
,
and νM-Ob-M, respectively. It indicated that POMs kept intact as
[
60]
incorporating into matrix of HAP. Compared to PMo V (1051,
4
8
À 1
933, 864, and 752 cm ), the peaks shifted to determine the
interaction between polyanions and HAP through anion-cation
interaction. The X-ray diffraction patterns (XRD) further deter-
mined the structure of PMo V /HAP hybrids (Figure S2). After
Herein,
PMo V /HAP (n)) with various doping amounst (n) of PMo V
4 8
a series of (NH ) H PV Mo O /hydroxyapatites
4 5 6 8 4 40
(
4
8
was synthesized. The molar ratio between the Brønsted acidic
and basic sites could be tailored to control the 5-HMF
conversion to DFF. And the relationship between PMo V /HAP
4
8
POM being loaded on HAP, two kinds of XRD belonging to
hydroxyapatite phase and PMo V phase had been found at
4
8
4 8
and their activity in 5-HMF conversion had also been clarified.
Most importantly, the stability and reusability of PMo V /HAP
was further studied. Meanwhile, the aerobic oxidation of 5-HMF
was achieved under atmospheric pressure, which could fulfill
higher safety and economical needs. There was few reports on
2θ=8.3°, 18.1°, 28.2°, and 33.7°, and 2θ=8.1°, 18.0°, 28.1°, and
33.6° corresponding to HAP and PMo V , respectively. This
indicated that PMo V cluster had been successfully bonded in
4 8
4
8
4 8
À
HAP through exchanging of some OH groups. It can be seen
3
1
from the P MAS NMR that one peak was found at δ=
À 3.00 ppm, which was corresponding to the phosphorus atom
in PMo V /HAP (31) (Figure S3). Compared to PMo V
4 8
5
-HMF oxidation upon oxygen under atmospheric pressure or
[22–26]
upon air so far.
Thus, a new catalytic system is required to
4
8
be developed to achieve 5-HMF oxidation to DFF under
atmospheric pressure with high efficiency. Additionally, the
one-pot conversion of monosaccharides (such as fructose and
glucose) into DFF is of greater value and lower cost in
(À 4.26 ppm), shift occurred for PMo V /HAP (31) due to the
4
8
anion-cation interaction between PMo V and HAP. In the DR-
4
8
UV-Vis spectra (Figure S4), PMo V /HAP exhibited the character-
4
8
istic peak of Keggin anion at 213 nm belonging to oxygen-to-
molybdenum charge-transfer. Therefore, combination of ele-
[45,57,58]
comparison to production of DFF from 5-HMF.
About 8~
3
1
5
1.2% yield of DFF was achieved directly from glucose in the
mentary analysis, FT-IR, XRD, DR-UV-Vis, and P MAS NMR
confirmed the formation of PMo V /HAP through the following
presence of multi-site catalysts under oxygen pressure. By
contrast, it is difficult to achieve a higher yield of DFF
production from monosaccharides under atmospheric pressure,
even thought this condition meets the safety and economical
4
8
equation:
Ca ðPO Þ ðOHÞ
ðNH Þ H PV Mo O !
10
4
6
2þn
4
5
6
8
4
40
[45,57,59]
requirements
Using PMo V /HAP as a catalyst, the multi-
Ca ðPO Þ ðOHÞ ½ðNH Þ H PV Mo O �
4
8
10
4
6
2-
n
4
5
6
8
4
40 2=3n
functional sites could be more suitable for directly producing
DFF from glucose. Therefore, the direct conversion of glucose
into DFF was studied upon PMo V /HAP under atmospheric
Due to the anion-exchanging of hydroxyl group by PMo4V8
À
anion, PMo V molecules were located on the position of OH
4
8
4 8
1
1À
oxygen condition with much more cost-saving.
anion. This indicated that PMo V
anion replaced some
anion was linked by HAP
4
1À
8
1
hydroxy groups in HAP. PMo4V8
through some cation-anion interaction with Ca cation. There-
2
+
À
+
Results and Discussion
fore, there was no interaction between OH in HAP and H in
PMo V . This might provide isolated acid-base catalytic sites as
4
8
Characterization of PMo V /HAP (n)
comparison with other acid-base catalysts by supporting POMs
4
8
[61,62]
through NH linkage.
2
PMo V /HAP (n) hybrids were characterized by elementary
The acidic and basic contents were given in Table S2, which
were 0.26, 0.34, 0.39, and 0.43 mmol/g for acidity and 0.31, 0.27,
0.23, and 0.19 mmol/g for basic sites corresponding to n=15,
26, 31, and 35, respectively. The acidity for PMo V /HAP (n) was
4
8
31
analysis, FT-IR, XRD, P MAS NMR, and DR-UV-Vis. The loading
amounts of PMo V on HAP were about 15, 26, 31, and 35%,
4
8
respectively (Table S2). From the elementary analysis, the
PMo V /HAP hybrids gave the formula as
4
8
mostly relevant to its loading amount in HAP, which deter-
4
8
À
+
mined no direct interaction between OH in HAP and H in
PMo V . These results indicated that PMo V /HAP hybrids
Ca ðPO Þ ðOHÞ ½ðNH Þ H PV Mo O �0:21,
10
4
6
1:79
4
5
6
8
4
40
4
8
4 8
demonstrated a characteristic of having isolated Brønsted acidic
ChemPlusChem 2021, 86, 997–1005
998
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