Y. Fu et al.
ture of oxidation was very high and alkali bases were added to
prevent decomposition. Phosphovanadomolybdates with
Strikingly, the glucose oxidation process could be catalyzed
by H PV Mo O at a lower temperature (373 K) with a higher
5
2
10 40
(
3+x)À
a Keggin structure, [PV Mo O ]
(particularly for x=2),
selectivity of FA and a maximal yield of 36% (Table 1, entry 8).
When compared with the oxidation results for catalysis by
H PMo O , we found that more FA was generated when two
x
12Àx 40
perform well in liquid-phase aerobic catalytic oxidation reac-
5
À
tions and [PV Mo O ] can be re-oxidized by oxygen after
2
10 40
3
12 40
[
20]
oxidation of the substrate.
In addition, a Keggin-structure
molybdenum atoms in the Keggin-structure heteropolyacid
were substituted by vanadium atoms. This result indicated that
the vanadium atom played an important role in glucose oxida-
tion. Other vanadium catalysts, such as V O , NaVO , and some
heteropolyacid has been used as a homogeneous catalyst in
biomass conversion, particularly in cellulose conversion.
H PW O combined with Ru/C can efficiently catalyze the
3
12 40
2
5
3
[
21]
conversion of cellulose into sugar alcohol. The results for the
acid hydrolysis of cellulose catalyzed by a Keggin-structure het-
eropolyacid were better than those obtained for common min-
homogeneous vanadium oxides, gave rise to a much lower
conversion rate of glucose and yield of FA (Table 1, entries 9–
18). Complex products with a low selectivity of FA were detect-
ed when homogeneous vanadium oxides were used at 423 K.
Thus, we used H PV Mo O as the catalyst for the glucose oxi-
[
22]
eral acids. In the study of glucose oxidation catalyzed by
H PV Mo O , we found good yields of FA when using air
5
2
10 40
5
2
10 40
as the oxidant. Efficient conversion of cellulose into FA also
occurred.
dation and explored the best reaction conditions.
Effect of reaction temperature
First, we investigated the effect of reaction temperature
(
Figure 1) on the formation of FA by glucose oxidation. Experi-
Results and Discussion
ments were carried out at 353, 373, 398, and 423 K. Higher
conversion rates were observed at 398 and 423 K. At these
temperatures, complete conversion of glucose was detected in
less than 1 h and the maximum yield of FA was about 30%.
Under these conditions, the selectivity of FA was low.
Catalyst screening
Initially, weakly oxidative heteropolyacids and the mineral acid
H SO were selected for glucose oxidation (Table 1, entries 1–
2
4
6). No FA was detected when H PW O and H PMo O were
3 12 40 3 12 40
When the temperature was 373 K, complete conversion of
glucose and a yield of 47% of FA were obtained in 3 h. Howev-
er, the yield of FA decreased when the reaction time was ex-
tended, indicating that decomposition of FA occurred.
When the temperature was reduced to 353 K, a slower con-
version rate of glucose was observed and the highest yield of
HCOOH was 30% in 12 h. Thus, we selected 373 K as the opti-
mized reaction temperature.
used (Table 1, entries 1–4). When H SO was employed, no oxi-
2
4
dation occurred at 373 K (Table 1, entry 6); however, when the
temperature increased to 423 K, acid hydrolysis of glucose to
levulinic acid and FA occurred, but gave only 9% yield of FA
(Table 1, entry 5). These results demonstrate that the yield of
FA has no relationship with the acidity of the catalyst.
[
a]
Table 1. Results for glucose oxidation into FA with different catalysts.
[
b]
Effect of substrate concentration
Entry
Catalyst
t
T
[K]
Conversion
[%]
Yield
[h]
[mol%]
We prepared glucose solutions with different concentrations
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
3
3
3
3
2
2
5
5
PMo12
PMo12
O
O
40
40
3
2
3
2
3
2
3
2
3
2
3
2
3
2
3
2
3
2
423
373
423
373
423
373
423
373
423
373
423
373
423
373
423
373
423
373
100
0
50
0
80
0
100
90
100
33
90
25
99
52
99
65
99
55
0
0
5
0
9
(
1, 2.5, and 5%) for the oxidation reaction and the results are
shown in Figure 2. When the substrate concentration in-
creased, the yield of FA and the conversion rate of glucose
clearly decreased. Reaction with a 1 wt% aqueous solution of
glucose gave a 55% yield of FA with complete decomposition
of glucose in 3 h. However, only a 36% yield of FA was detect-
ed when the concentration of the solution was 5 wt%. In addi-
tion, FA yields decreased in all cases as the reaction time was
extended. These results showed that a low concentration was
beneficial to the oxidation reaction.
PW12
PW12
O
O
40
40
SO
SO
4
4
0
PV
PV
2
Mo10
Mo10
O
O
40
40
29
36
30
12
27
10
5
24
6
27
7
2
V
V
2
O
2
O
5
5
10
1
1
NaVO
NaVO
3
3
1
1
1
1
1
1
1
2
3
4
5
6
7
8
VOHPO
VOHPO
4
4
VOPO
VOPO
4
Effect of catalyst loading
4
VOSO
VOSO
4
4
We carried out the reaction with different catalyst loadings,
and the results are shown in Figure 3. It was found that the
catalyst concentration had a great impact on the conversion
rate of glucose, but little effect on the yield of FA. When
2.5 mol% catalyst was added, complete conversion of glucose
required a longer reaction time of about 12 h, whereas when
the catalyst concentration was increased to 10%, glucose was
25
[a] Reaction procedure: glucose (2.5 g) was added to H
2
O (50 mL), then
mixed with catalyst, which contained 10 mol% vanadium. The concentra-
tion of H was equivalent to H PV Mo10O40 when H SO was used. The
5 2 2 4
pressures of oxygen were 1 and 2 MPa when the reaction temperatures
were 423 and 373 K, respectively. [b] The yield of FA was calculated based
on 1 mol glucose producing 6 mol FA.
+
&2
&
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