Q. Zhao et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 79–82
81
withdrawing ability. The enhanced >NO H strength of the catalysts
O
N
O
HO
O
COOH
+
make the corresponding N-oxyl radical have stronger H-abstraction
ability [41,42]. Therefore, the synergy between NHQI and Car-NHPI
might be attributed to the acid-base neutralization and the radical
exchange between electronegative Car-PINO radical and proto-
nated NHQI. The verified experiments were designed to detect the
roles of above interactions in the oxidation.
OH
OH
O
N OH
N
+
O
O
neutralization
pyridinium
salts
O
O
HOO
3.1.1. The effect of acid-base neutralization on the oxidation
N
N OH
To confirm whether the acid-base neutralization in the Car-
N
H
COO
heat
NHPI/NHQI catalytic system was responsible for the good catalytic
performance in the oxidation, 10 mol% benzoic acid or 10 mol%
trichloroacetic acid was added to this catalytic system to replace
Car-NHPI. The addition of benzoic acid or trichloroacetic acid
slightly lowered the conversion of ethylbenzene, but the selec-
tivity of AcPO increased from 40% to 49% or 53%, respectively
O
O
N O
O
N
O
NO H
OO
N
OH
(Table 1, entries 3, 11, and 12). This indicated that the neutralization
N
H
COO
O
between alkaline NHQI and the acid had little effect or even nega-
tive effect on the conversion of ethylbenzene. It can be explained
by the difficult production of the related protonated N-oxyl rad-
icals due to the higher BDE value of its >NO H bond. Meanwhile,
the acid-base neutralization played a positive role in enhancing the
selectivity of AcPO, which increased from 46 % to 67 % by adding
O
radical
exchange
O2
O
N
O
H
COO
N
OH
N
O
1
0 mol% pyridine to 10 mol% Car-NHPI catalytic system (Table 1,
entries 4 and 13). The explanation may be that some pyridinium
salts produced from acid-base neutralization in our system can
decompose PHEP to AcPO, thereby improving the AcPO selectivity,
which is also supported by other literatures [32,43,44].
O
O
Fig. 2. A plausible reaction path for the oxidation of ethylbenzene with molecular
oxygen.
3
.1.2. The effect of radical exchange on the oxidation
To investigate only the function of interaction between N-
>
NO H BDE, which could abstract a hydrogen atom from substrate
to form alkyl radical more easily [27,28,41]. The following process
was that the alkyl radicals were captured by molecular oxygen
to generate peroxy radical, which was converted to hydroperox-
ide through its H-abstraction from >NO H bond. Ultimately, the
pyridinium salts helped decompose hydroperoxide into ketone
products.
oxyl radical and N-hydroxyl, the effect of pyridinium salt on
the oxidation must be excluded. Therefore, the NHPI/NHQI and
NHPI/Car-NHPI catalytic systems were used to the oxidation of
ethylbenzene, respectively. The conversion of ethylbenzene was
5
0% using 10 mol% catalyst amount of NHPI/NHQI catalytic sys-
tem, which was higher than that using NHQI or NHPI alone,
and the selectivity of AcPO was, however, almost equal to that
with NHQI (Table 1, entries 2, 3, and 9). The interpretation was
that in NHPI/NHQI catalytic system, NHPI with relatively lower
3
.2. Influence of reaction conditions on ethylbenzene oxidation
catalyzed by Car-NHPI/NHQI catalytic system
>
NO H BDE could produce PINO more easily, which facilely trans-
Due to its good catalytic performance, the Car-NHPI/NHQI cat-
alytic system was chosen for further oxidation studies.
formed NHQI to the corresponding N-oxyl radical through radical
exchange. The yielded radical was more active than PINO, conse-
quently improving the conversion of ethylbenzene. This process
was also confirmed by the similarity of products selectivity in
NHPI/NHQI catalytic system to that in NHQI. Correspondingly, the
conversion of ethylbenzene by NHPI/Car-NHPI catalytic system was
higher than that by Car-NHPI with the same catalytic amount, and
their selectivity was quite similar (Table 1, entries 2, 4, and 10).
These results indicated that the radical exchange between PINO and
NHPI analogues could increase the conversion of ethylbenzene, but
had no obvious effect on the selectivity of AcPO.
3.2.1. Solvent effect
Due to the poor solubility of NHPI in nonpolar organic sol-
vents, the NHPI-catalyzed oxidations were often conducted in polar
solvents, such as acetic acid or acetonitrile [45]. In this section,
Car-NHPI/NHQI catalytic system was applied in different common
Table 2
a
Oxidation of ethylbenzene under different reaction conditions.
Entry Solvent
Temp.
( C)
Conv.
(%)
Select. (%)
AcPO
◦
3
.1.3. Suggested catalytic mechanism of oxidation
The above results revealed that the combined radical exchange
PEA
PEHP
1
2
3
4
5
6
7
8
9
CH3CN
PhCF3
CH3COOH
PhCN
PhCN
PhCN
PhCN
PhCN
PhCN
PhCN
80
17
15
52
12
23
48
53
70
73
72
trace
trace
26
35
41
45
54
66
67
15
15
21
trace
7
20
22
10
16
9
83
78
42
63
48
32
27
14
3
and acid-base neutralization largely improved the performance of
Car-NHPI/NHQI catalytic system. A plausible catalytic mechanism
of Car-NHPI/NHQI in the oxidation of ethylbenzene by molecular
oxygen was proposed in Fig. 2.
Firstly, the pyridinium salts involving deprotonated Car-NHPI
and protonated NHQI were formed by acid-base neutralization
between Car-NHPI and NHQI, then the deprotonated Car-NHPI was
converted to deprotonated Car-PINO by heating due to its low
100
110
80
90
100
110
120
120
130
b
10
65
8
>
NO H BDE. The next step contained the radical exchange between
a
Reaction conditions: 10 mmol ethylbenzene, 25 ml solvent, 1 mmol NHQI,
Car-PINO and protonated NHQI, and the resulting N-oxyl radical of
protonated NHQI had a better H-abstraction ability due to its larger
1 mmol Car-NHPI, 1 atm O , 15 h.
2
b
Reaction time was prolonged to 24 h.