S.J. Liu et al. / Chinese Chemical Letters 22 (2011) 221–224
223
Table 3
a
The effect of temperature on the selective reduction of NT .
b
Conv. (NT) (%)
b
Selec. (M-PHA) (%)
b
Yield (M-PHA) (%)
b
Yield (M-AN) (%)
Entry
T (8C)
1
2
3
4
5
6
7
0
34
47
59
36
23
16
15
88
94
92
92
91
88
87
30
44
54
33
21
14
13
1
2
3
2
1
1
1
15
25
40
50
60
70
a
Reaction conditions: NT 10 mmol, Zn 30 mmol, H
Determined by HPLC.
2
O 5 mL + ethanol 15 mL, PCO2 0.1 MPa, and 12 h.
b
Indeed, the addition of ethanol increased the reaction selectivity to M-PHA. Without ethanol in the reaction system,
the conversion of NT was 77% and the selectivity of M-PHA was 65% (Table 2, entry 1). When the mixture of 10 mL
H O and 10 mL ethanol was employed, the selectivity toward M-PHA increased to 80%, but the conversion of NT
2
decreased to 61%. Further increasing the amount of ethanol (H O/ethanol = 5 mL/15 mL) resulted in a negative effect
2
on the conversion of NT and a positive effect on the selectivity to M-PHA. In the condition, prolonging the reaction
time to 12 h, a 59% conversion of NT and a 92% selectivity of M-PHA were realized. These results demonstrated that
with the increase of the ethanol portion, the selectivity of M-PHAwas improved, meanwhile, the reaction rate became
low. Ethanol added into the system favors the desorption of M-PHA from the Zn surface, so the selectivity of M-PHA
is elevated. On the other hand, the dielectric constant of the solution decreases with the addition of ethanol. As it is
known, the dielectric constants of H O and ethanol are 78.3 and 24.5, respectively. The lower the dielectric constant is,
2
the worse it is for the acidic proton to dissociate from the acid [9]. So the acid strength drops with increasing the
amount of ethanol, which leads to the low reaction rate. To balance the conversion and the selectivity of the reaction,
the following experiments are carried out with the mixture solvent of 5 mL H O and 15 mL ethanol.
2
The effect of different reaction parameters on the selective reduction of NT in Zn/H O/CO system with the
2
2
addition of ethanol was systematically examined. Firstly the reaction results at different temperature were shown in
Table 3.
Apparently, in the temperature range from 0 8C to 70 8C, the conversion of NTand the yield of M-PHA reached to a
maximum at 25 8C. The trend is similar to that obtained without ethanol, but over the range of this reaction
temperature, the selectivity of M-PHA is high and the yield of M-AN, the by-product for the full reduction of NT, is
low.
The reaction results with different amount of Zn used and reaction time were illustrated in Fig. 2. With prolonging
the reaction time the conversion of NT increased, but NT was not converted completely during 24 h as the molar ratio
of Zn to NT was equal to 2 and 3. With further increase of the molar ratio of Zn to NT (Zn/NT = 4 and 5), the yield of
M-PHA was as high as 89% (14 h and 10 h, respectively) and the selectivity of M-PHA was almost constant. Then
further prolonging the reaction time led to an obvious increase in the amount of M-AN, so the yield of M-PHA
decreased.
[()TD$FIG]
1
00
c
100
80
60
40
20
0
d
b
a
8
6
4
2
0
0
0
0
0
d'
c'
b'
a'
0
4
8
12 16 20 24 28
t/ h
0
4
8
12 16 20 24 28
t/ h
2
Fig. 2. The effect of reaction time and amount of Zn on the reduction. Reaction conditions: NT 10 mmol, H O 5 mL + ethanol 15 mL, 25 8C, and
0
0
0
0
PCO2 0.1 MPa. a, a (Zn 20 mmol), b, b (Zn 30 mmol), c, c (Zn 40 mmol), and d, d (Zn 50 mmol).