Remarkable effect of PEG-1000-based dicationic ionic liquid for N-Hydroxyphthalimide-catalyzed aerobic selective oxidation of alkylaromatics

Article abstract of DOI:10.5562/cca2051

PEG 1000-based functional dicationic acidic ionic liquid (PEG ₁₀₀₀-DAIL) was used for the first time as the reaction solvent for the N-Hydroxyphthalimide (NHPI)-cobalt acetate(Co(OAc)₂) catalyzed aerobic oxidations of alkylaromatics to the corresponding acids. It enhanced the efficient catalytic ability of NHPI: 99.9 % conversion of toluene with 99.5 % selectivity for benzoic acid could be obtained at 80 °C in 10 h and ethylbenzene was selectively oxidized to benzoic acid. Several alkylaromatics were efficiently oxidized to their corresponding acids under mild conditions. For substituted toluene, the conversions of substrates and the selectivity of products was affected by the position and kind of substituted groups, respectively. Both the catalyst and PEG₁₀₀₀-DAIL could be reused at least eight times without significantly decreasing the catalytic activity.

Full text of DOI:10.5562/cca2051

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CROATICA CHEMICA ACTA
CCACAA, ISSN 0011-1643, e-ISSN 1334-417X
Croat. Chem. Acta 85 (3) (2012) 277–282.
http://dx.doi.org/10.5562/cca2051
Original Scientific Article
Remarkable Effect of PEG-1000-based Dicationic Ionic Liquid for
N-hydroxyphthalimide-catalyzed Aerobic Selective
Oxidation of Alkylaromatics
*
Tingting Lu, Ming Lu, Wang Yu, and Zhongjie Liu
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China
RECEIVED JANUARY 16, 2012; REVISED APRIL 12, 2012; ACCEPTED MAY 25, 2012
Abstract. PEG 1000–based functional dicationic acidic ionic liquid (PEG1000 –DAIL) was used for the first
time as the reaction solvent for the N-Hydroxyphthalimide (NHPI)-cobalt acetate(Co(OAc)2) catalyzed
aerobic oxidations of alkylaromatics to the corresponding acids. It enhanced the efficient catalytic ability
of NHPI: 99.9 % conversion of toluene with 99.5 % selectivity for benzoic acid could be obtained at
80 °C in 10 h and ethylbenzene was selectively oxidized to benzoic acid. Several alkylaromatics were
efficiently oxidized to their corresponding acids under mild conditions. For substituted toluene, the con-
versions of substrates and the selectivity of products was affected by the position and kind of substituted
groups, respectively. Both the catalyst and PEG1000 –DAIL could be reused at least eight times without
significantly decreasing the catalytic activity.(doi: 10.5562/cca2051)
Keywords: aerobic oxidation, alkylaromatics, NHPI, PEG1000 –DAIL, ionic liquid
INTRODUCTION
ple of an aerobic oxidation with NHPI catalysts in ionic
liquid.10 Liu et al. reported oxidation of alkylbenzenes
(such as ethylbenzene and n-propylbenzene) using
NHPI with Co(II), Mn(II) or Ni(II) in various ILs (in-
cluding [Hex-mim]BF4, [Bmim]PF6, [Bmim]BF4 and
[Omim]BF4) with molecular oxygen.11 Later, Yavari
reported a mild oxidation reaction of xylenes to
phthalic acids with N-hydroxyphthalimide and HNO3 in
bmim[OMS].12 However, these ILs are inevitably asso-
ciated with one or more disadvantages, such as low
recovery ratio, high cost or difficulty of synthesis and
the low selectivity. To improve catalyst recovery and
the selectivity, some novel ionic liquid like PEG based
ionic liquid have been considered due to their some
advantages, such as high conversions and selectivity,
stability at high temperatures, and reusability in the
reaction.13
We now report an efficient and convenient pro-
cedure for the oxidation of aromatic hydrocarbons
catalyzed by NHPI in PEG 1000-based dicationic
ionic liquid (PEG1000 –DAIL)14 (Scheme 1). Oxidation
was surveyed using a host of aromatic substrates with
similar reactivity and high selectivity of acid, wherein
both the catalyst and PEG1000 –DAIL can be success-
fully recovered and reused. The easily recovered ionic
liquid opened up the possibility of a more economic
process.
The catalytic oxidation of alkylaromatics with molecu-
lar oxygen is of major industrial importance, and im-
proving its efficiency and selectivity for the value-
added products remains a challenge.1 The success of
these oxygenations always depends largely on the
development of catalysts to promote productivity, the
rate of reaction, as well as the selectivity of products.2
In recent years, due to its highly catalytic efficiencies
even under mild conditions for the aerobic oxidation of
various organic compounds in the presence of some
metallic compounds or non-metallic compounds, N-
hydroxyphthalimide (NHPI) is attracting continuous
attention.3−6 However, the progress of using NHPI
suffers from difficulties encountered in the reuse of
catalyst for new reactions7 and employed corrosive
acetic acid.8
Ionic liquids (ILs) are a special class of molten
salts composed of organic cations and inorganic or
organic anions. They have many excellent advantages
such as negligible volatility, excellent thermal stability,
remarkable solubility, and a variety of available struc-
tures.9 It should be the most appropriate solvent to
replace acetic acid for the NHPI catalyzed aerobic
oxidation of hydrocarbons and a few examples have
been reported. In 2005, Wang reported the first exam-
* Author to whom correspondence should be addressed. (E-mail: tt252525@gmail.com)

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278
T. Lu et al., N-hydroxyphthalimide-catalyzed Aerobic Selective Oxidation of Alkylaromatics in PEG1000–DAIL
Scheme 2. Aerobic oxidation of Toluene by NHPI in
PEG1000 −DAIL.
The PEG1000 –DAIL was prepared by the proce-
dure given in the literature (Scheme 1).14
Scheme 1. The preparation of the PEG-DAIL.
The substrate (0.05 mol), PEG1000 –DAIL (2.8 ×
10−4 mol), NHPI (5 × 10−3 mol, 10 mol %), and Co(OAc)2
(2.5 ×10−4 mol, 0.5 mol %) were placed in a three-necked
flasks. The flask was purged with a stream of O2 at a flow
rate of 20 mL min−1. The reaction mixture was stirred at a
specific temperature for a specific time (Table 2), and the
reaction progress was monitored by HPLC. After comple-
tion of the reaction, the mixture was cooled to room tem-
perature and extracted with 10ml ether for three times.
After concentration of the ether solution, the products
EXPERIMENTAL SECTION
All starting materials were purchased from commercial
sources and used without further treatment. H NMR
1
(500 MHz) and 13C NMR (125 MHz) were recorded on
a Bruker 500 spectrometer using D2O as the solvent
with tetramethylsilane (TMS) as an internal standard. IR
spectra were recorded on a Bruker Vector 22 infrared
spectrometer, as KBr pellets with absorption in cm−1.
High performance liquid chromatography (HPLC) ex-
periments were performed on a liquid chromatograph
(Dionex Softron GmbH, America), consisting of a pump
(P680) and UV/Vis light detector system (170U). The
experiments were performed on a Diacovery C18 col-
umn; ø 4.6 × 250 mm. The conversions of the substrates
and the selectivities of products were estimated from the
peak areas based on the internal standard technique. The
products were determined in some cases by comparison
of their HPLC with those of authentic samples.
were taken for HPLC measurement and the PEG1000
DAIL was reused without any treatment.
–
RESULTS AND DISCUSSION
For the initial study, toluene was selected as the model
substrates (Scheme 2) to optimize the reaction condi-
tions (Table 1).
It is indicated that the oxidation of toluene with O2
(1 atm) catalyzed by NHPI (10 mol %) combined with
Table 1. Oxidation of toluene with O2 by NHPI under various conditions(a)
Run
NHPI
Additive (mol %)
Solvent(b)
Conversion / %
Products selectivity / %
(mol %)
BOL
0.3
7.2
BAl
n.d. (c)
1.7
20.0
n.d.
BAC
1
2
3
4
5
6
10
5
10
0
10
10
Co(OAc)2 (0.5)
Co(OAc)2 (0.5)
PEG1000-DAIL
PEG1000-DAIL
PEG1000-DAIL
PEG1000-DAIL
PEG1000-DAIL
PEG1000-DAIL
99.9
58.9
48.7
n.d.
90.1
92.0
99.5
91.1
78.1
n.d.
74.0
41.7
1.9
n.d.
26.0
58.3
Co(acac)2 (0.5)
Mn(OAc)2 (0.5)
Co(OAc)2 (0.5) +
Mn(OAc)2 (0.05)
Dimethylglyoxime (10)
anthraquinone (1.25)
ABIN (3)
n.d.
n.d.
7
10
PEG1000-DAIL
100
2.3
n.d.
97.7
8
9
10
10
10
10
10
10
10
10
10
10
PEG1000-DAIL
PEG1000-DAIL
PEG1000-DAIL
PEG1000-DAIL
HOAc
CH3CN
CH2Cl2
[Hex-mim]BF4
[Bmim]PF6
PEG 1000-DIL
89.8
87.9
37.2
14.0
84.2
52.1
61.4
31.2
16.7
98.5
94.6
n.d.
0.6
69.3
n.d.
n.d.
3.6
12.5
n.d.
7.1
n.d.
29.9
30.0
30.7
2.7
45.0
41.2
47.2
8.8
5.00
70.1
69.4
n.d.
96.1
52.9
55.0
39.9
90.1
6.2
10
11(d)
12(e)
13
14
15
16
17
nitric acid (5)
Co(OAc)2 (0.5)
Co(OAc)2 (0.5)
Co(OAc)2 (0.5)
Co(OAc)2 (0.5)
Co(OAc)2 (0.5)
Co(OAc)2 (0.5)
86.7
(a) Toluene (0.05 mol) was allowed to react with O2 (1atm) in the presence of NHPI and additive in solvent at 80 °C for 10 h.
(b) PEG1000 –DAIL was 2.8 × 10-4 mol and other solvent was 10 ml.
(c) Not detected.
(d) The concentration of nitric acid was 67 %.
(e) The reaction temperature is 25 °C and the time is 20 h.
Croat. Chem. Acta 85 (2012) 277.

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T. Lu et al., N-hydroxyphthalimide-catalyzed Aerobic Selective Oxidation of Alkylaromatics in PEG1000–DAIL
Table 2. Aerobic Oxidation of Various Substrates Catalyzed by NHPI in PEG1000 −DAIL(a)
279
Run
1
Substrate
Times
Conversion / %
Main Products and Selectivity / %
8
8
94.5
99.9
94.3
94.1
5.3
2
5.6
3
4
11
11
93.3
91.2
90.7
89.3
9.1
5.8
5
6
11
12
84.1
83.5
89.0
75.3
6.0
20.1
7
12
98.2
95.7
3.1
8
9
12
8
80.8
93.8
96.9
96.9
2.3
1.2
10
8
90.9
96.0
3.9
11
12
13
6
75.6
86.3
90.8
94.1
84.0
82.4
4.0
11
11
12.0
15.6
14
15
16
17
11
13
13
13
88.8
90.9
90.1
90.3
80.3
81.4
15.8
18.4
24.0
23.8
COOH
CHO
75.4
73.0
Cl
Cl
18
19
13
24
75.3
0
72.0
23.5
(a)
Reaction conditions: 0.05 mol substrate, 10 mol % NHPI, 0.5 mol % Co(OAc)2, 2.8 × 10−4 mol PEG1000 −DAIL, 80 °C,
1 atm O2. Coversions and selectivities were determined by HPLC.
Croat. Chem. Acta 85 (2012) 277.

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280
T. Lu et al., N-hydroxyphthalimide-catalyzed Aerobic Selective Oxidation of Alkylaromatics in PEG1000–DAIL
Figure 1. Time-dependence curve for the aerobic oxidation of
toluene(a)
(a) Reaction condition: 0.5 mol toluene, 0.5 mol % Co(OAc)2,
10 mol % NHPI, 2.8 × 10−4 mol PEG1000 −DAIL, 80 °C.
Figure 2. Temperature-dependence curve for the aerobic
oxidation of toluene(a)
(a)
Reaction condition:0.5 mol toluene, 0.5 mol% Co(OAc)2,
Co(OAc)2 (0.5 mol %) gave benzyl alcohol (BOL),
benzoic acid (BAC) in 0.3 % and 99.5 % selectivity,
respectively, at 99.9 % conversion (Table 1, Run 1).
This shows that toluene is completely oxidized with O2
by NHPI to give selectively BAC. When the amount of
NHPI was halved, the conversion was sharply decreased
and a considerable amount of BOL and benzaldehyde
(BAl) was formed (Run 2). Removing Co(OAc)2 from
the oxidation system resulted in lower conversion with
the concomitant gradual increase of BAl and the de-
crease of BAC in the selectivities (Run 3). However, no
oxidation was induced when PEG1000 –DAIL was used
alone (Run 4). Substitution of Co(OAc)2 by Co(acac)2
(Ref. 15) or MnO2 (Ref. 3) led to a slightly lower con-
version with comparable selectivity toward the for-
mation of BAC (Run 5 and 6). When both Co(OAc)2 and
Mn(OAc)2 were used,15 the conversion increased but the
selectivity for BAC was slightly decreased (Run 7).
In the previous papers, it was shown that some
10 mol% NHPI, 2.8 × 10−4 mol PEG1000 −DAIL, 10 h.
most appropriate solvent (Run 12), the employment of
PEG1000 –DAIL led to a higher conversion and selectivi-
ty of BAC and lower selectivity of side product BAl
(Run 1). It was indicated PEG1000 –DAIL increased the
efficient catalytic ability of NHPI. In presence of ace-
tonitrile and dichloromethane, the conversions and
selectivities of BAC were relatively low (Run 13 and
14). Traditional ionic liquids [Hex-mim]BF4 and
11
[Bmim]PF6 were also tested, however, poor conver-
sions were observed (Run 15 and 16). The reaction in
PEG1000-DIL (PEG 1000-based dicationic ionic liquid)
resulted in slightly lower conversion and selectivity due
to its lack of acidity (Run 17).
In order to obtain information on the reaction
course of the present reaction, the oxidation of toluene
with O2 (1 atm) under the influence of NHPI and
Co(OAc)2 was monitored as a function of time and
temperature (Figure 1 and 2).
non-metallic compounds such as azobisisobutyronitrile
(ABIN)16 or HNO3 also can abstract the hydrogen
12
atom from the hydroxyimide moiety of the NHPI to
generate phthalimide N-oxyl (PINO) which abstracts the
hydrogen atom from the methyl moiety of toluene.
Thus, the oxidation of toluene was examined by non-
metallic radical initiator. Unsatisfactorily, when
dimethylglyoxime6 and anthraquinone17,18 were em-
ployed, the conversion with selectivity evidently de-
clined (Run 8 and 9). And in the presence of the ABIN
(Run 10), the reaction proceeded in much low conver-
sion to result in lots of undesired products (BAl). To our
disappointment, the conversion reached only 14.0 %
and no benzoic acid appeared when HNO3 was used
instead (Run 11).
The changes of product distribution of toluene oxi-
dation with time are shown in Figure 1. As illustrated,
the conversion of toluene increased with prolonging the
reaction time and the selectivity of BOL increased rapid-
ly in the initial 2.0 h. In the same period of time, the
selectivity of BAC increased slowly. With the further
increase of time, the selectivity of BAC continuously in-
creased, but the selectivity of BOL decreased due to the
continuous oxidation of BOL to BAC. However, after 10
h the conversion was changed slowly and became stag-
nated near 99.9 %. As a result, in a 10 h reaction course,
99.9 % toluene was oxidized with 99.5 % selectivity of
BAC. The change of the selectivity of BAl was similar
to BOL except the concentrate was always low.
Subsequently, several solvents were evaluated. In
contrast to the traditional NHPI/Co(OAc)2 catalyzed
aerobic oxidation of toluene where acetic acid8 was the
The effect of temperature on oxidation of toluene
was investigated and shown in Figure 2. Since the solu-
Croat. Chem. Acta 85 (2012) 277.