Aerobic oxidation of ethylbenzene co-catalyzed by n-hydroxyphthalimide and oxobis(8-Quinolinolato) vanadium (IV) complexes

Article abstract of DOI:10.1016/S1872-2067(10)60255-0

The mediation effect of vanadium compounds on the N-hydroxyphthalimide (NHPI)-catalyzed aerobic oxidation of ethylbenzene (EB) was investigated at 90 °C in benzonitrile. Among the vanadium mediators examined, a series of oxobis(8-quinolinolato) vanadium (

Full text of DOI:10.1016/S1872-2067(10)60255-0

CHINESE JOURNAL OF CATALYSIS
Volume 32, Issue 8, 2011
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2011, 32: 1342–1348.
RESEARCH PAPER
Aerobic Oxidation of Ethylbenzene Co-catalyzed by
N-Hydroxyphthalimide and Oxobis(8-Quinolinolato)
Vanadium (IV) Complexes
QIN Jinwei, FU Zaihui*, LIU Yachun, HE Xiangling, ZHANG Dexi, WU Wenfeng, WANG Yanlong, GONG
Xinglang, DENG Xiaolin, WU Haitao, ZOU Yanhong, YU Ningya, YIN Dulin
Key Laboratory of Resoruce Fine-Processing and advanced materials of Hunan Province and Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research Ministry of Education of China, College of Chemistry and Chemical Engineering, Hunan
Normal University, Changsha 410081, Hunan, China
Abstract: The mediation effect of vanadium compounds on the N-hydroxyphthalimide (NHPI)-catalyzed aerobic oxidation of ethylbenzene
(EB) was investigated at 90 °C in benzonitrile. Among the vanadium mediators examined, a series of oxobis(8-quinolinolato) vanadium
(IV) complexes (VIVOQ2), which were prepared by the coordination of 8-hydroxyquinoline or its derivatives with oxo-
bis(2,4-pentanedionate) vanadium (IV) (VIV O(acac)2) showed a better mediation effect than VIVO(acac)2, NH4VO3, and V2O5 and they gave
about 60%–69% EB conversion and 97% of acetophenone (AcPO) selectivity under optimum reaction conditions. This is due to the dual
catalysis effects of these V mediators on the transformation of NHPI to the phthalimide-N-oxyl (PINO) radical and the decomposition of
1-phenylethyl hydroperoxide to AcPO, as supported by UV-Vis spectral characterization.
Key words: n-hydroxyphthalimide; oxobis(8-quinolinolato) vanadium (IV); aerobic oxidation; ethylbenzene; acetophenone
Acetophenone (AcPO) is an important intermediate and has
been widely used for the synthesis of compounds such as per-
fumes, pharmaceuticals, resins, alcohols, and esters. Therefore,
there has been considerable interest in the development of
highly efficient processes for the production of AcPO from
ethylbenzene (EB) [1]. For the oxidation of EB, t-butyl hy-
droperoxide (TBHP) [2,3] and H2O2 [4–6] are usually used as
the oxidant because they provide a good yield of acetophenone
under relatively mild conditions but the costly oxidants make
these processes lack competitive capacity. From an economic
and environmental viewpoint, air and molecular oxygen are
more attractive than TBHP or H2O2 and they are considered to
be the ultimate oxidants [7]. Many efficient catalysts have been
developed for the catalytic partial oxidation of EB with mo-
lecular oxygen [8] but these processes generally need to em-
ploy harsh operating conditions because the triplet nature of
molecular oxygen hampers the reaction with EB in its singlet
state, which rarely leads to the chemistry being selective [9].
N-hydroxyphthalimide (NHPI) is usually used to improve
the aerobic oxidation of hydrocarbons combined with media-
tors [10] such as metal compounds [11–20], hemin [21], an-
thraquinones [22], o-phenanthroline [23], alkaline-earth chlo-
rides [24], oxime [25], azodiisobutyronitrile (AIBN) [26],
Ce(IV) [27], I2/HNO3 [28], NO2 [29], and quaternary ammo-
nium salts [30]. Among these catalyst systems, NHPI com-
bined with transition metal ions (e.g. cobalt and manganese) is
well-known as the Ishii catalytic system, which has excellent
catalytic activity for the aerobic oxidation of different kinds of
organic substrates [8,11–20]. For example, Ishii et al. [31] and
Sheldon et al. [32] reported that NHPI and Co(acac)2 in an
acetic acid medium could efficiently catalyze the aerobic oxi-
dation of EB to the corresponding AcPO in good yield but the
use of a corrosive acetic acid solvent hampers its industrial
application. Vanadium and its derivatives are well known effi-
cient catalysts for the oxidation of various organic substrates
like alkenes, alcohols, aromatic, and aliphatic hydrocarbons
Received 28 February 2011. Accepted 13 May 2011.
*Corresponding author. Tel: +86-731-88872576; Fax: +86-731-88872531; E-mail: fzhhnu@tom.com
This work was supported by the National Natural Science Foundation of China (20873040, 20573035) and the Natural Science Foundation of Hunan Province
(10JJ2007).
Copyright © 2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60255-0

QIN Jinwei et al. / Chinese Journal of Catalysis, 2011, 32: 1342–1348
halides [33] but few reports exist about the use of vana-
filtrate was analyzed on an Agilent 6890N gas chromatograph
(GC) with a HP-5 quartz capillary column (30 m u 0.32 mm u
0.25 ȝm) and flame ionization detector (FID) using ultra-pure
nitrogen as a carrier gas with anisole as an internal standard.
Both the injector and the detector were at 250 °C and the
column temperature was 120 °C. The products AcPO and
1-phenylethanol (PEA) were satisfactorily identified by com-
paring the GC-MS spectra with those of authentic samples. The
GC-MS contained a quartz capillary column HP-5 (30 m u 0.32
mm u 0.25 ȝm) and the column temperature was initially 100
°C and held there for 2 min. It was then increased at a rate of 4
°C/min to 250 °C and held there for 1 min. The carrier gas was
ultra-pure nitrogen and it was added at a constant flow of 1
ml/min. The injector, interface and ion-source were kept at 250,
250, and 200 °C, respectively. Electron impact mass spectra
were obtained at a 70 eV standard electronic impact. The se-
lectivity for alkyl hydroperoxide (PEHP) in the products was
measured based on the change of selectivity for AcPO before
and after the filtrate was treated with triphenylphosphine for 1
h [37]. A PhCN (18 ml) solution of EB (1272 mg, 12 mmol),
NHPI (1.2 mmol), and VIVOQ2 (0.006 mmol) were placed in a
three-necked flask equipped with a water-cooled condenser.
The mixture was stirred at 90 °C for 12 h and then extracted
with Et2O. The organic layer was dried over MgSO4. The
products were separated from the solvent under reduced pres-
sure and purified by column chromatography on silica gel
(n-hexane:AcOEt = 20:1) to give the corresponding AcPO
(748.9 mg, 52 mol%).
dium-containing compounds as mediators for NHPI-based
aerobic oxidation [34]. Herein, we report initial results that we
obtained upon using a small amount of oxobis(8-quinolinolato)
vanadium (IV) (VIVOQ2) to significantly accelerate the
NHPI-catalyzed aerobic oxidation of EB to AcPO in benzoni-
trile (PhCN) under 101.325 kPa O2.
1 Experimental
1.1 Preparation of VIVOQ2 complexes
The general preparation procedure for the VIVOQ2 com-
plexes is as follows. Oxobis(2,4-pentanedionate) vanadium
(IV) (VIVO(acac)2) was prepared as reported previously [35].
First, to
a stirred mixture of CH2Cl2 (30 ml) and
8-hydroxyquinoline or its derivatives (10 mmol), a 5 ml of
CH2Cl2 solution containing 5 mmol of VIVO(acac)2 was added
dropwise within 15 min and the reaction mixture was refluxed
for 2 h. The resulting precipitate was filtered and washed with
CH2Cl2 at least three times. After air drying, a deep-black solid
(yield: 85%–95%) was obtained and denoted 1a to 1d (see
Scheme 1). The vanadium content of 1a as measured by a
UV-3310 spectrophotometer using 5-Br-PADAP and H2O2 as
chromogenic agents [36] was 14.08 wt% and this was in good
agreement with the calculated value (14.36 wt%) from the
formula of 1a.
1.2 UV-Vis spectral measurements
2 Results and discussion
The solid and liquid UV-Vis spectral measurements of the
VIVOQ2 complexes were recorded from 200 to 700 nm on a
UV-3310 spectrophotometer (Hitachi).
2.1 Characterization of the VIVOQ2 complexes
Diffusion reflection UV-Vis spectra of the solid VIVOQ2
complexes (1a–1d) are very similar to each other (Fig. 1). They
consist of two bands in the 230–350 nm region, which may be
assigned to SoS* transitions in the ligands. The complexes
1.3 Oxidation of EB
Catalytic oxidation was performed in a 100 ml of
three-necked flask equipped with a water-cooled condenser.
After the desired amount of substrate, the catalysts and PhCN
were added into the reactor and the air in the reactor was re-
placed over three cycles with dioxygen (O2). The reaction
mixture was heated to an appropriate temperature under a pure
O2 atmosphere under magnetic stirring. After the desired reac-
tion time had elapsed the reaction mixture was naturally cooled
to room temperature and filtered to remove the catalyst. The
251
1a
320
394
650
1d
1c
1b
285
335
338
448
433
693
700
283
1: VIVOQ2
262
300
O
O
O
V
N
1a: R1 = H, R2 = H
1b: R1 = Cl, R2 = Cl
1c: R1 = Br, R2 = Br
1d: R1 = Cl, R2 = I
329
N
407
642
600
R1
=
200
400
Wavelength (nm)
Diffusion reflectance UV-Vis spectra of solid VIVOQ2
500
700
800
O
N
R2
N
O
Fig. 1.
Scheme 1. Structures of the VIVOQ2 complexes.
complexes.

QIN Jinwei et al. / Chinese Journal of Catalysis, 2011, 32: 1342–1348
also exhibit a broad band at 370–470 nm probably as a result of
EB conversion and 90% selectivity for AcPO (entry 5). Entries
6–10 show that all the organic V mediators gave a better pro-
motion effect than both the inorganic mediators and the
VIVOQ2 complexes were superior to VIVO(acac)2 in improving
the conversion and selectivity (entries 6–9 vs entry 10). Among
the VIVOQ2 complexes, 1a was found to be the most efficient
and about 69% EB conversion and 97% selectivity for AcPO
was obtained when using 10 mol% NHPI-coupled with 0.05
mol% 1a (entry 6). The three halogenated VIVOQ2 complexes
gave about 60%–64% conversion and 92%–97% selectivity for
the NHPI-catalyzed EB oxidation (entries 7–9). These
NHPI-coupled V complex systems generally give higher ca-
talysis efficiencies than the NHPI-coupled Co(OAc)2 system
(entry11) under the same reaction conditions. This is due to
Co(OAc)2 having poor solubility in PhCN.
ligand to metal V(IV) charge transfer (LMCT), as reported in
literature[38]. In addition, a weak and broad band in the 500–700
nm region may be attributed to a spin-allowed d-d transition of
the central metal V(IV) ions [38]. The halogenated VIVOQ2
samples commonly result in a red shift of the LMCT band and
this red shift effect gradually strengthens for the brominated
and iodonated VIVOQ2 samples, which is most likely due to an
increase in the electron-donating effect of the halogen sub-
stituents on the ligand’s aryl ring.ꢀ
2.2 Oxidation of EB
Table 1 lists data for the oxidation of EB with O2 upon
co-catalysis by NHPI and various organic and inorganic V
compounds at 90 °C in PhCN. Entries 1 and 2 show that the
oxidation reaction hardly occurs without a catalyst or in the
presence of the VIVOQ2 complex 1a. When 10 mol% NHPI was
used as the catalyst (see entry 3) about 18% EB was oxidized to
AcPO (selectivity, 46%), PEA (selectivity, 23%), and PEHP
(selectivity, 31%) showing that NHPI plays an important role
in this reaction. When NHPI was used with various V com-
pounds to co-catalyze the reaction, EB conversion and AcPO
selectivity improved significantly while PEHP selectivity de-
creased drastically (entries 4–10) and this is consistent with the
results reported by Recupero et al. [10]. Among the V media-
tors examined the mediation effect of V2O5 was found to be the
lowest most likely because of the poor solubility of V2O5 in the
reaction medium (entry 4). Another inorganic mediator
NH4VO3 promoted the reaction well and afforded about 46%
2.3 Effect of temperature
The effect of reaction temperature on the aerobic oxidation
of EB was determined using 10 mol% NHPI and 0.05 mol% 1a
as co-catalysts and the results are shown in Fig. 2. As expected,
increasing the temperature from 60 to 90 °C resulted in a per-
sistent increase in EB conversion and AcPO selectivity because
of the accelerated effect of temperature on NHPI and 1a
co-catalyzed the oxidation of EB to AcPO. However, an at-
tempt to further increase the temperature (100 °C) resulted in a
slight decrease in EB conversion and AcPO selectivity. This
was likely due to the partial degradation of NHPI in protic
solvents at high temperature as reported by Saha et al. [39] after
they studied the oxidation of p-xylene.
Table 1 Aerobic oxidation of EB catalyzed by the vanadium com-
2.4 Effect of reaction time
plexes/NHPI system
O
OH
OOH
The effect of reaction time on the aerobic oxidation of EB
was investigated using 10 mol% NHPI and 0.05 mol% 1a as
co-catalysts. As shown in Fig. 3, EB conversion continuously
increased with an increase in the reaction time from 2 to 8 h
and, thereafter, it was mostly invariable. EB conversion in-
creased sharply during the early to middle stages (2–8 h),
V complexes/NHPI
O2, PhCN
+
+
AcPO
PEA
PEHP
Conversiona
Selectivitya (%)
Entry
Catalyst
(%)
trace
trace
18
AcPO
—
PEA
—
—
23
10
8
PEHP
—
—
31
12
2
1
2
blank
1a
—
100
90
3
NHPI
46
4
V2O5/NHPI
NH4VO3/NHPI
1a/NHPI
26
78
5
46
90
80
6
69
97
3
0
Conversion
7
1b/NHPI
1c/NHPI
64
97
3
0
70
Selectivity for AcPO
8
62
95
5
0
60
50
40
9
1d/NHPI
VO(acac)2/NHPI
60
96
4
0
10
54
92
5
3
11b Co(OAc)2/NHPI
20
67
20
13
Reaction conditions: PhCN 6 ml, EB 4 mmol, NHPI 0.4 mmol, catalyst
a
60
70
Temperature (oC)
Fig. 2. Effect of reaction temperature on the aerobic oxidation of EB.
80
90
100
0.002 mmol, O2 101.325 kPa, temperature 90 °C, time 12 h. The data
were obtained by GC analysis using anisole as an internal standard.
b0.002 mmol Co(OAc)2.

QIN Jinwei et al. / Chinese Journal of Catalysis, 2011, 32: 1342–1348
Table 2 Effect of NHPI concentration on the aerobic oxidation of EB
100
90
80
70
60
50
40
30
20
10
NHPI
(mol%)
5.0
VIVOQ2
(mol%)
0.05
0.05
0.05
0.05
0.04
0.1
Conversion
Selectivitya (%)
Entry
(%)
47
65
69
63
61
68
64
56
AcPO PEA PEHP
1
2
3
4
5
6
7
8
93
96
97
97
97
97
97
95
7
4
3
3
3
3
3
5
0
0
0
0
0
0
0
0
7.5
10.0
12.5
12.5
12.5
10.0
7.5
Conversion
0.1
Selectivity for AcPO
0.1
Reaction conditions: PhCN 6 ml, EB 4 mmol, O2 101.325 kPa, tempera-
0
2
4
6
8
10 12 14 16 18
a
Reaction time (h)
ture 90 °C, time 12 h. The data were obtained by GC analysis using
anisole as an internal standard.
Fig. 3. Effect of reaction time on the aerobic oxidation of EB.
increased to 12.5 mol% the conversion was reduced to 63%
(entry 4). Interestingly, at a high NHPI concentration (12.5
mol%) the conversion was improved by increasing the con-
centration of the V mediator 1a (entries 4–6) but the opposite
was found when using a low concentration of NHPI (entries
7–8). The existence of an optimal NHPI/VIVOQ2 molar ratio
(about 125–200) implies that a synergistic catalysis effect
exists between NHPI and VIVOQ2.
which matches the expected chain propagation process during
radical oxidation. AcPO selectivity increased slowly with a
reaction time up to 12 h after which it remained almost un-
changed.
2.5 Effect of VIVOQ2 concentration
The effect of VIVOQ2 concentration was investigated using
1a as an example at a NHPI concentration of 10 mol% and a
reaction temperature of 90 °C. As shown in Fig. 4, EB con-
version and AcPO selectivity increased drastically with an
increase in the concentration of 1a from 0 to 0.05 mol%. After
that the conversion decreased slightly and the selectivity for
AcPO remained almost constant with a further increase in
VIVOQ2 concentration.
2.7 Reaction mechanism
The NHPI/VIVOQ2 co-catalyzed EB oxidation process was
further monitored by UV-Vis spectroscopy. Figure 5 shows a
stepwise overlay of the UV-Vis spectra for NHPI and the
VIVOQ2 complex 1a in acetonitrile at room temperature. The
UV-Vis spectrum of NHPI has an absorption band with a Ȝmax
0.8
2.6 Effect of NHPI concentration
328
(5)
(4)
(3)
0.06
0.04
0.02
0.00
(9)
(8)
(7)
Table 2 lists the effect of NHPI concentration on the reac-
tion. By maintaining a 1a concentration of 0.05 mol%, an
increase in NHPI concentration from 5 to 10 mol% resulted in a
continuous increase in EB conversion from 47% to 69% and an
increase in selectivity for AcPO from 93% to 97%, respectively
(Table 2, entries 1–3). However, when the concentration was
0.6
0.4
0.2
0.0
(6)
328
(1,2)
325
350
375
Wavelength (nm)
100
90
80
70
60
50
40
300
400
500
Wavelength (nm)
600
700
Fig. 5. Liquid UV-Vis spectra of the mixture of NHPI and VIVOQ2 (1a)
in acetonitrile treated with O2 at 80 °C. (1) A solution of NHPI in acetoni-
trile (2.0×10í4 mol/L, 8 ml); (2) After the addition of a solution of
VIVOQ2 in acetonitrile (1 mol/L, 0.2 ȝl); (3) After treatment with O2 at 80
°C for 20 min; (4) For 40 min; (5) For 60 min. Inset is the liquid UV-Vis
spectra of NHPI in acetonitrile after treatment with O2 at 80 °C. (6) A
solution of NHPI in acetonitrile (2.0×10í4 mol/L, 8 ml); (7) After treat-
ment with O2 for 20 min; (8) For 40 min; (9) For 60 min.
30
20
10
Conversion
Selectivity for AcPO
0.0
0.1
0.2
0.3
0.4
0.5
VIVOQ2 concentration (mol%)
Fig. 4. Effect of VIVOQ2 concentration on the aerobic oxidation of EB.

QIN Jinwei et al. / Chinese Journal of Catalysis, 2011, 32: 1342–1348
of ~295 nm, which is due to a ʌĺʌ* transition (see curve (1)).
V(Č)
ROOH
V(č)
R=O
When a trace amount of 1a in acetonitrile (1 mol/L, 2 Pl) was
added to solution (1) its spectrum did not show any change (see
curve (2)). When solution (2) was treated for 20 min in the
presence of O2 at 80 °C, a new shoulder peak appeared at a Ȝmax
of ~328 nm (see curve (3)) and it became more obvious when
the treatment time was increased to 40 or 60 min (see curves (4)
and (5)). This shoulder peak was also observed by Figiel et al.
[34] in the UV-Vis analysis of NHPI solutions in the presence
of [n-Bu4N] VO3 and they attributed it to the formation of
PINO· radical species by ESI-MS characterization. Moreover,
Yoshino et al. [13] and Sheldon et al. [32] also proved the
existence of PINO· by EPR after the exposure of NHPI to
dioxygen in benzonitrile at 80 °C over 45 min. The inset in Fig.
5 shows that the cleavage of NHPI to PINO· in the absence of
1a, as reported in Refs. [13,32] could also occur in acetonitrile
in the presence of O2 at 80 °C (see curves (6)–(9)) but its
cleavage rate was slower than that in the presence of 1a (curves
(6)–(9) vs (3)–(5)). This accounts for 1a promoting this
cleavage process.
ꢀ
ꢀ
R H
V(Č)
V(č)
PINO
NHPI
R OH
O
PINO
2
ROO
R
NHPI
O
2
Scheme 2. Catalytic cycle of the VIVOQ2/NHPI system for EB oxida-
tion.
formation reaction continuously proceeded when the treatment
time was increased from 30 to 120 min, as shown by the spec-
tral curves (3)–(5) in Fig. 6. Interestingly, when solution (5)
was treated with a solution of NHPI in acetonitrile (0.16 mol/L,
1 ȝl) for 1 h at 80 °C, a comeback phenomenon of the oxova-
nadium (IV) band was observed (see curve (6) in the inset), and
this comeback trend became more obvious with an increase in
the treatment time (see curves (7) and (8)). This spectral
change supports the assertion that the oxoperoxovanadium (V)
species participates in the cleavage process of NHPI to PINO
by reduction to the oxovanadium (IV) species.
Figure 6 shows the spectral change of complex 1a in ace-
tonitrile before and after treatment with O2 at 80 °C. A strong
adsorption band with a Ȝmax of ~378 nm and a very weak band
with a Ȝmax of ~302 nm were observed in the UV-Vis spectrum
of fresh 1a (see (1)) and this is attributed to oxovanadium (IV)
and oxoperoxovanadium (V) species, respectively [38,40].
After 1a was treated for 30 min with O2 at 80 °C, the oxova-
nadium (IV) band decreased obviously with a concomitant
increase in the oxoperoxovanadium (V) band (see curve (2))
indicating that the present reaction conditions can lead to the
transformation of oxovanadium (IV) to oxoperoxovanadium
(V), as reported by Maurya et al. [38,40] upon studying the
interaction between V(IV) and H2O2. Moreover, this trans-
Based on the results given here as well as those in previous
studies [10–30], it is apparent that the above experimental data
are better explained according to the reaction pathway given
in Scheme 2. First, NHPI may be cleaved to form a PINO· free
radical under an O2 atmosphere and with heating. This cleav-
age process can be accelerated significantly by the transfer of
its hydrogen atom to the V(V) species that is generated in situ
from the reaction of VIVOQ2 with O2, as supported by the above
UV-Vis spectral characterization. The next step involves the
abstraction of a secondary H atom from EB by PINO· and the
resulting hydrocarbon radical is trapped by dioxygen through a
well-known reaction pathway [41]. Another effect of the VI-
VOQ2 mediator, as reported in many papers, is the ability to
catalyze the decomposition of the in situ produced PEHP,
which can lead to a significant increase in AcPO selectivity.
0.5
378
(8)
(7)
(6)
0.20
(1)
(2)
0.4
0.3
0.2
0.1
0.0
378
302
(5)
0.15
0.10
(3)
(4)
3 Conclusions
330 355 380 405 430
Wavelength (nm)
(5)
An oxobis(8-quinolinolato) vanadium (IV) complex was
developed to efficiently promote the aerobic oxidation of
ethylbenzene to acetophenone upon catalysis by NHPI. This
catalytic system has some distinct characteristics: (a) noncor-
rosive benzonitrile can be used as a medium; (b) small amounts
of VIVOQ2 catalyst (0.05 mol %) are required; and (c) excellent
AcPO selectivity (97%) is obtained. Improving this catalytic
system by reducing the required amount of NHPI and by en-
hancing its catalytic efficiency will provide a promising strat-
egy for the industrial synthesis of acetophenone from ethyl-
benzene.
300
400
500
Wavelength (nm)
600
700
Fig. 6. Liquid UV-Vis spectra of the mixture of VIVOQ2 (1a) in ace-
tonitrile treated with O2 at 80 °C. (1) A solution of VIVOQ2 in acetonitrile
(1×10í6 mol/L, 8 ml); (2) After treatment with O2 at 80 °C for 30 min; (3)
For 60 min; (4) For 90 min; (5) For 120 min. The inset is UV-Vis spectra
obtained after adding a solution of NHPI in acetonitrile (0.16 mol/L, 1 ȝl
to solution (5). (6) 60 min later; (7) 120 min later; (8) 180 min later.

QIN Jinwei et al. / Chinese Journal of Catalysis, 2011, 32: 1342–1348
3687
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