Solvent-free selective oxidation of C-H bonds of toluene and substituted toluene to aldehydes by vanadium-substituted polyoxometalate catalyst

Article abstract of DOI:10.1016/j.molcata.2012.11.022

A novel homogeneous vanadium-substituted polyoxometalate catalyst of [n-Bu₄N]₃H₃[PW₉V₃O ₄₀] showed good catalytic activity for the activation of CH bonds of toluene and substituted toluene with TBHP (70% aq) under solvent-free conditions. Under the optimal conditions, a 65% yield of benzaldehyde and a TON of 5221 were obtained over this catalyst at 343 K. This catalyst also showed good conversion and selectivity for the oxidation of a series of substituted toluene. The vanadium-substituted Keggin-POM was not decomposed to some species upon the treatment of 530-fold of TBHP by IR characterization. A free radical mechanism was proposed based on the experimental results and the literature supports.

Full text of DOI:10.1016/j.molcata.2012.11.022

Journal of Molecular Catalysis A: Chemical 368–369 (2013) 152–158
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Solvent-free selective oxidation of C H bonds of toluene and substituted toluene
to aldehydes by vanadium-substituted polyoxometalate catalyst
∗
∗
Baochun Ma , Zhenxin Zhang, Wenfeng Song, Xiaoling Xue, Yongze Yu, Zhensheng Zhao, Yong Ding
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 June 2012
Received in revised form 31 October 2012
Accepted 28 November 2012
Available online xxx
4 3 3 9 3
A novel homogeneous vanadium-substituted polyoxometalate catalyst of [n-Bu N] H [PW V O40]
showed good catalytic activity for the activation of C H bonds of toluene and substituted toluene with
TBHP (70% aq) under solvent-free conditions. Under the optimal conditions, a 65% yield of benzaldehyde
and a TON of 5221 were obtained over this catalyst at 343 K. This catalyst also showed good conversion
and selectivity for the oxidation of a series of substituted toluene. The vanadium-substituted Keggin-POM
was not decomposed to some species upon the treatment of 530-fold of TBHP by IR characterization. A
free radical mechanism was proposed based on the experimental results and the literature supports.
Keywords:
Polyoxometalates
Toluene
©
2012 Elsevier B.V. All rights reserved.
Benzylic C H bonds
Benzaldehyde
Oxidation
1
. Introduction
supported metal complexes [8] and metal containing molecular
sieves [9] as heterogeneous catalyst. Although many procedures
for the catalytic oxidation toluene and substituted toluene in liq-
uid have been developed, most of them suffered the low yield of
benzaldehyde or the low selectivity to benzaldehyde or the dif-
ficulty for the recovery of the catalyst. Therefore, developing new
pathway to oxidize the toluene and substituted toluene under mild
condition and improving the yield of aldehyde are of great demand
and importance.
Polyoxometalates (POMs) are molecular metal-oxygen clusters
with discrete structures and they have led to many applications in
various areas. The application of POMs to catalysis has attracted
much attention, and the versatility and accessibility of POMs have
been used to various catalytic oxidation process, such as olefin
epoxidation [10], alcohol oxidation [11] and pyridine oxidation
[12]. However, only few research concerns on oxidation of toluene
and substituted toluene using POMs as catalysts under mild condi-
tion [2,13] and the catalytic activity of the catalysts is low and the
selectivity to aldehyde is not satisfied, either. Recently, Mizuno and
co-workers reported a divanadium-substituted phosphotungstate
that catalyses the stereo- and regioselective hydroxylation of
alkanes with hydrogen peroxide as the sole oxidant [14]. Although
Direct functionalization of hydrocarbons by catalytic oxidation
of C H bonds to form oxygenated products under mild conditions
is a major challenge in view of industrial aspects [1]. Toluene and
substituted toluene, as a kind of the hydrocarbons, are used as the
most important precursors of various fine chemicals. For toluene,
the oxygenated products mainly include benzyl alcohol, benzalde-
hyde and benzoic acids. Among these products, benzaldehyde is the
most desirable products. However, benzaldehyde is easily trans-
formed to corresponding carboxylic acid due to over oxidation.
By traditional method, benzaldehyde can be yielded by side-chain
chlorination of toluene followed by saponification of the result-
ing dichloromethyl group to form the aldehyde [2]. However, the
product generated by this method contains the chlorinated by-
products, which do not meet food and drug grade specifications.
Another method of oxidizing toluene is vapor phase oxidation with
oxygen but the reaction is carried out at high pressure and high
temperature and seems too harsh for improving the selectivity of
benzaldehyde [3].
Until now, catalytic oxidation systems in liquid phase for
toluene and substituted toluene were limited to a few examples
such as iridium (III) chloride [4], metal complexes [5], 10-methyl-9-
phenylacridinium ion [6], xanthone [7] as homogeneous catalysts;
the catalyst showed high activity for catalytic oxidation of C
bonds of cyclic akkanes, other types of C H bonds such as benzylic
H bonds are not investigated in their publication.
Here, we reported tri-vanadium-substituted phospho-
tungstate, [n-Bu N] H [PW V O40] (I), as efficient homogeneous
H
C
a
4
3
3
9
3
∗ Corresponding authors. Tel.: +86 931 8619200; fax: +86 931 8912582.
E-mail addresses: mabaochun@lzu.edu.cn (B. Ma), dingyong1@lzu.edu.cn,
dingyong1973@163.com (Y. Ding).
catalyst for direct activate the C H bonds of toluene and substi-
tuted toluene with tert-butyl hydroperoxide (TBHP) as an oxidant
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.11.022

B. Ma et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 152–158
153
under solvent-free conditions. For the first time, the catalyst was
used to catalyze oxidation of toluene and substituted toluene.
These compounds could be converted to corresponding alde-
hyde in high yields, and thousands of catalytic turnovers were
obtained. A 65% yield of benzaldehyde and a TON of 5221 were
obtained over this catalyst, which was the highest value among
those reported for TBHP-based catalytic system so far [4–9]. These
vanadium-substituted polyoxometalates catalysts were character-
800, 597, 515, 449. Calcd for [n-Bu N] H [PW V O40]: C, 17.96; H,
4 3 3 9 3
3.46; N, 1.31. Found: C, 17.95; H, 3.71; N, 1.28.
2
.3.3. [n-Bu N] H[PW10V2O40] [14]
4
4
Cs5[PV W10O40] was synthesized according to published lit-
2
erature procedures and characterized by infrared spectroscopy
20]. The tetra-n-butylammonium salt of the monoprotonated
deritived [n-Bu N] H[PW V O40] was prepared using a cation-
[
4
4
10
2
3
1
ized by FT-IR, P NMR, TGA and elemental analysis.
exchange reaction. NaVO3 (0.4 mmol) was dissolved in 40 mL hot
water. On cooling, the pH of the solution was adjusted to 2.0
with 3 M HCl. Cs5[PV2W10O40] (1.27 g, 0.37 mmol) was dissolved
in the solution and the insoluble materials removed by filtration.
Tetra-n-butylammonium bromide (0.6 g, 1.8 mmol) was added
with vigorous stirring. The precipitate was collected by filtration,
washed 3 times with 130 mL water, and dried under high vacuum.
2
. Experimental
2.1. Materials
All materials used were of analytical reagent grade and used
without further purification unless otherwise stated.
3
1
−1
P NMR (DMSO-d6): −14.30 ppm. IR spectrum (KBr, cm ): 1096,
1
062, 1040, 1000, 953, 871, 805, 751, 534, 488.
2.2. Characterization techniques
2
.3.4. [n-Bu N] [PW11VO40]
4 4
Infrared spectra were recorded on a Nicolet FT-IR-360 FT-IR
K4[PW11VO40] was synthesized according to published lit-
spectrometer. The catalysts were measured using 2–4% (w/w)
KBr pellets prepared by manual grinding. 31P NMR spectra were
recorded on a Varian Unity-300 MHz NMR spectrometer, using
DMSO-d6 as the solvent; 31P MAS NMR spectra were recorded
erature procedure [16] and characterized by IR spectroscopy.
The tetra-n-butylammonium salt of the [n-Bu4N]4 [PW11VO40]
was prepared as follows: The yellow solid (1 g, 0.34 mmol) of
K4[PW11VO40] was dissolved in 20 mL of distilled water under stir-
ring at room temperature. To this solution tetra-n-butylammonium
bromide (0.55 g, 1.7 mmol) was added. The mixture was stirred
vigorously and acidified by one drop of HCl (conc.). Acidification
initiated precipitation of quaternary ammonium salt of POM. The
precipitate was filtered off, washed 3 times with water, and dried
31
at 9.4 T on a Bruker Avance-400 wide bore spectrometer. The
P
MAS NMR spectra of solid catalyst with high-power proton decou-
pling were performed at 161.9 MHz with BB MAS probe head using
4
mm ZrO rotors and 3.8 ␮s pulse and 2 s repetition time and 4096
2
scans, with samples spun at 10 kHz and referenced to 85% H PO .
3
4
31
Chemical elemental analysis of the catalysts was done on an ICP
atomic emission spectrometer (IRIS ER/S), and C, H, and N contents
were measured on a German Elementar Vario EL spectrometer. TGA
was recorded on Perkin Elmer PE7. GC analyses were performed
on Shimadzu GC-9AM with a flame ionization detector equipped
with SE-54 capillary (internal diameter = 0.25 mm, length = 30 m).
GC–MS were recorded on Finnigan Trace DSQ (Thermo Electron
Corporation) at an ionization voltage of 70 eV equipped with a
DB-5 capillary column (internal diameter = 0.25 mm, film thick-
ness = 0.25 lm, length = 30 m). 1H NMR and C NMR spectra were
recorded on a Bruker AM-400 and Varian mercury 300 MHz spec-
trometer with TMS as an internal standard and CDCl3 as solvent.
under high vacuum. P NMR (DMSO-d6): −14.18 ppm. IR spectrum
−
1
(KBr, cm ): 1099, 1077, 983, 883, 790, 596, 521.
K6H[SiW9V3O40]·3H2O was synthesized according to published
literature procedure [16] and characterized by IR spectroscopy. IR
−
1
spectrum (KBr, cm ): 1008, 958, 914, 798.
2.3.5. [n-Bu N] H[SiW V O40]
4
6
9 3
K H[SiW V O40] (0.5 g, 0.18 mmol) was dissolved in 50 mL of
6
9
3
distilled water under stirring at room temperature. To this solution
tetra-n-butylammonium bromide (1.4 g, 4.3 mmol) was added. The
mixture was stirred vigorously and acidified by one drop of HCl
(conc.). Acidification initiated precipitation of quaternary ammo-
nium salt of POM. The precipitate was filtered off, washed 3 times
13
−
1
with water, and dried under high vacuum. IR spectrum (KBr, cm ):
005, 956, 914, 796.
2.3. Synthesis of polyoxometalates
1
2
.3.1. Na H[ꢀPW O34][15a]
8
9
Firstly, Na WO ·2H O (30.0 g, 91 mmol) was dissolved in 37 mL
2.3.6. Cs [PW V O40] [16]
6 9 3
2
4
2
of distilled water. After adding 85% H PO (0.75 mL, 12 mmol),
NaVO3 (0.61 g, 5 mmol) was added to 20 mL (1.0 M) of
sodium acetate/acetic acid buffered at pH = 4.8. Then, 4 g of
Na H[ꢀPW O34] (1.66 mmol) was added to the above solution and
3
4
5
.5 mL of glacial acetic acid was added to the solution with stirring.
After a few seconds the solution became cloudy and about 1 min
later a heavy white precipitate had formed. The obtained solid was
first dried under aspiration for 24 h, and then the aspirated solid
was left in air for 24 h. After that, the resulting solid was dried at
8
9
◦
stirred at 25 C for 48 h. To This solution, a saturated solution of CsCl
was added, until no further precipitation occurred. The product was
◦
filtered and washed with 2 × 20 mL of 25 C water. Then the solid
◦
−1
1
9
40 C for about 1.5 h. IR spectrum (KBr, cm ): 1171, 1057, 994,
47, 896, 818, 742, 653, 624, 588, 507, 470, 417.
was dried in air to give 4.6 g of tan-orange powder. IR spectrum
−
1
31
(KBr, cm ): 1085, 1055, 962, 876, 788, 597, 509, 481. P MAS
NMR: −13.3 ppm.
2
.3.2. [n-Bu N] H [PW V O40]
4 3 3 9 3
NaVO3 (1.3 mmol) was dissolved in 13 mL of hot water. On
cooling, the pH of the solution was adjusted to 2.0 with 3 M
HCl. Cs [PW V O40] (0.4 g, 0.12 mmol) was dissolved in the solu-
2.4. Catalytic reaction
A acetonitrile solution of [n-Bu N] H [PW V O40] (80 mg of [n-
6
9
3
4
3
3
9
3
tion and the insoluble materials were removed by filtration.
Tetra-n-butylammonium bromide (0.2 g, 0.61 mmol) was added
with vigorous stirring. The precipitate was collected by filtration,
Bu N] H [PW V O40] in 100 mL acetonitrile) was firstly prepared.
4 3 3 9 3
The catalytic reaction was performed in a round-bottomed flask
equipped with an oil bath, a magnetic stirring bar and a reflux con-
denser. The oxidation was carried out as follows: catalyst (0.02 mM)
in 0.25 mL acetonitrile, toluene (8.3 M, 2.7 mL), TBHP (70%, 167 mM)
were charged in the reaction vessel. The reaction was carried out
31
washed 3 times with water, and dried under high vacuum. P NMR
−
1
(
2
DMSO-d ): −13.7 ppm. IR spectrum (KBr, cm ): 3497, 2961, 2936,
6
873, 2735, 1633, 1482, 1381, 1152, 1106, 1087, 1054, 957, 884,

1
54
B. Ma et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 152–158
3
.2. Catalytic performance of [n-Bu N] H [PW V O40]
4 3 3 9 3
[
n-Bu N] H [PW V O40] as a new homogeneous catalyst was
4 3 3 9 3
applied to the oxidation of benzylic C H bonds of toluene and its
derivatives with TBHP. Several experiments were carried out to
test the catalytic activity of I. In reactions performed by the proce-
dure as described in the experimental section, oxidation of toluene
and its derivatives to corresponding aldehydes was quite effective.
Stoichiometry of the reaction in case of benzaldehyde and benzyl
alcohol could be given by Eqs. (a) and (b). Selectivity, Yield and
TON were determined by gas chromatography using an internal
standard technique and were based on TBHP.
catalyst
oxidant
O
+
2 TBHP
+
2 t-BuOH
(a)
catalyst
oxidant
OH
+
TBHP
+
t-BuOH
(b)
Reaction conditions of oxidation of benzylic C H bonds were
studied and optimized using toluene as a model substrate. Sev-
eral kinds of POMS were applied to the oxidation of toluene with
TBHP. The results (Table 1) showed the catalyst of I behaved as
the most effective catalyst for the oxidation of toluene. A 65%
yield of benzaldehyde was achieved and TON was 5221 (entry 1).
It can be observed that vanadium existing in the polyoxometala-
tes had effect on the catalytic performance. The more vanadium
existed in polyoxometalate, the catalyst performed more effec-
tive (entries 1–3). Concerning the same numbers of vanadium
in the polyoxometalate, the one whose heteroatom is silicon
showed a poorer activity than that of POM whose heteroatom is
phosphorus (entry 4). Some other vanadium-substituted polyoxo-
Fig. 1. FT-IR spectra of (a) fresh catalyst of [n-Bu4N]4H3[PW9V3O40], (b) fresh
catalyst of Cs6[PW9V3O40], (c) used catalyst of Cs6[PW9V3O40] and (d) [n-
Bu4N]4H2[PW9V3O40] treated with TBHP for 24 h.
at 343 K was monitored by TLC. The reaction solution was periodi-
cally sampled by syringe and analyzed by a Shimadzu GC-9AM with
a flame ionization detector equipped with SE-54 capillary (internal
diameter = 0.25 mm, length = 30 m). Assignments of products were
made by comparison with the retention time of authentic reference
compounds. The products were identified by GC in combination
with mass spectrometry and NMR spectra with those of authentic
samples.
metalates with the cation of K , H and Cs+ were also investigated
+
+
(
entries 5–9), such as K [PW V O ]·4H O, K H[SiW V O ]·3H O,
6
9
3
40
2
6
9
3
40
2
H5PMo10V O40, K5[PW10V O ] and Cs [PW V O ]. However,
2
2
40
6
9
3
40
these polyoxometalates showed lower activity (yield = 9%, 19%,
1
2%, 13%, 10%, respectively) compared with the corresponding
+
POMs whose cations were quaternary ammonium ([n-Bu N] ). The
4
3
. Results and discussion
solubility of POMs containing quaternary ammonium as cations is
far better than those POMs containing K , so the catalytic activity
+
3.1. Catalyst characterization
of former is better than the latter’s.
Using the lacunary without vanadium-substituted or saturated
polyoxometalates, Na8HPW9O34, H4SiW12O40 and H3PW12O40, the
yields were poor (entries 10–12). For simple salt containing vana-
dium such as NaVO3, the catalytic activity was bad and only 7% of
yield was obtained (entry 13). A simple mixture of Na8HPW9O34
and NaVO3 showed low catalytic activity too (entry 14). There was
no benzaldehyde was detected when no catalyst was added to the
reaction (entry 15). Based on the catalytic results, I was the best
catalyst for the oxidation of toluene under the reaction conditions.
Some oxidants were studied for the oxidation of toluene cat-
alyzed by I. Different types of peroxides, such as TBHP, di-tert-butyl
peroxide (DTBP), hydrogen peroxide, were used to this reaction.
However, only TBHP can oxidize toluene to benzaldehyde with high
selectivity and yield (entry 1). Oxygen (1 atm) does not work at
all (entry 18), which autooxidation reaction was ruled out. Thus,
the best oxidant for oxidation of toluene was TBHP. No any prod-
ucts were detected when only catalyst I was added to the toluene
without oxidant (entry 19).
Fig. 1 shows the FT-IR spectra of the obtained POMs cata-
6−
lysts. The characteristic bands of [PW V O40] are at 1087, 1054,
9
3
−
1
9
57, 884, 800, 597, 515, 449 cm
[15], in which are similar
to the previously reported data for the same POM anion. The
6−
FT-IR spectrum indicated that the POM anion of [PW V O40]
9
3
was retained after forming the tetrabutylammonium salt and
cesium salt. The spectrum of 31P NMR gave the detailed struc-
ture information of the synthesized I. Only one signal was found
at −13.75 ppm (Fig. 2a), which demonstrated that the pure sin-
6−
gle structure of [PW V O ]
40
was obtained after formation of
9
3
[
n-Bu N] H [PW V O40].
4 3 3 9 3
The effect of reaction temperature on the activity of oxidation
of toluene was investigated. Reactions were carried out at various
temperatures, 283, 303, 333, 358 and 373 K (Fig. 3). With increasing
temperature, the activity of the reaction was gradually increased.
When the reaction was carried out at 283 K and 303 K, although
the selectivity to benzaldehyde was high (303 K, 99%), the activ-
ity of the catalyst was poor (303 K, yield = 4%; 298 K, yield = 0).
Fig. 2. (a) 31P NMR spectrum of [n-Bu4N]3H3[PW9V3O40] in DMSO-d6; (b) 31P MAS
NMR spectrum of Cs6[PV3W9O40].

B. Ma et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 152–158
155
Table 1
Oxidation of toluene with various oxidants catalyzed by different POMs catalysts.
a
Entry
Catalyst
Oxidant
Selectivity to benzaldehyde (%)
Yield of benzaldehyde (%)
TON
1
2
3
4
5
6
7
8
9
[n-Bu4N]3H3[PW9V3O40]
[n-Bu4N]4H[PW10V2O40]
[n-Bu4N]4[PW11VO40]
[n-Bu4N]6H[SiW9V3O40]
K6[PW9V3O40]·4H2O
K6H[SiW9V3O40]·3H2O
H5PMo10V2O40
K5[PW10V2O40]
Cs6[PW9V3O40]
Na8HPW9O34
H4SiW12O40
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
DTBP
O2
83
86
95
92
95
97
95
95
99
99
99
95
98
99
0
65
54
25
36
9
19
12
13
10
4
1
8
7
7
0
5
0
0
5221
4337
1992
2894
766
1565
996
1073
796
319
79
613
553
558
0
399
0
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
H3PW12O40
NaVO3
b
Na8HPW9O34 + NaVO3
No
[n-Bu4N]3H3[PW9V3O40]
[n-Bu4N]3H3[PW9V3O40]
[n-Bu4N]3H3[PW9V3O40]
[n-Bu4N]3H3[PW9V3O40]
99
0
0
0
0
No
0
0
a
Reaction condition: catalyst (0.06 ␮mol) in 0.25 mL acetonitrile, toluene (8.3 M, 2.7 mL), TBHP (70%, 167 mM). Reaction time 6 h, reaction temperature 343 K. By-product
was benzyl alcohol.
b
Na8HPW9O34 0.062 ␮mol, NaVO3 0.19 ␮mol. Selectivity (%) = [aldehyde (mol) × 2]/[aldehyde (mol) × 2 + alcohol (mol)]. TON = [aldehyde(mol)]/catalyst(mol).
Increasing the reaction temperature result in the oxidation of
toluene faster, while the selectivity of aldehyde decreased and the
selectivity to benzyl alcohol increased (Fig. 3c). When the reaction
was carried out at 343 K, the reaction rate markedly increased. A
The amount of the catalyst had effect on the oxidation of toluene.
Fig. 4 shows the catalytic performance of toluene oxidation under
the condition of different catalyst loadings. With increasing of the
amount of I, the conversion of toluene increased while the selec-
tivity of aldehyde decreased. When the catalyst amount increased
to 1.2 mol‰, the improvement of the conversion and selectiv-
ity was quite limited. Although using more catalyst led to higher
conversion, but it made the TON smaller and the catalyst thrift-
less. Therefore, a 0.12 mol‰ of catalyst loading afforded satisfied
6
5% yield of benzaldehyde was achieved after 6 h at 343 K. Although
further increasing the temperature was beneficial for the improve-
ment of the conversion, its magnitude was quite limited and the
selectivity of benzaldehyde dropped. Therefore, 343 K was suitable
for oxidation of toluene using I as catalyst.
Table 2
Oxidation of various toluene derivatives catalyzed by I.a
Entry
1
Substrate
Product
Time (h)
6
Conversion (%)
76
Selectivity (%)
83
TON
O
5221
O
2
3
4
4
94
99
92
86
6974
6866
O
O
4
4
88
93
6600
O
5
6
4
4
90
92
92
73
6677
5416
O
7
8
4
6
99
58
96
89
7665
4163
O
Cl
Cl
9
6
25
92
1854
a
Reaction condition: substrate 25 mmol, TBHP 0.5 mmol, acetonitrile 0.25 mL, catalyst I 0.062 ␮mol, reaction temperature 343 K. By-product was the corresponding alcohol.
Other conditions are same as Table 1.

1
56
B. Ma et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 152–158
(
a) 100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
Time (h)
(
b) 100
9
9
8
8
7
7
6
6
5
0
5
0
5
0
5
0
Fig. 4. Oxidation of toluene with TBHP catalyzed by I with different catalyst load-
ings. (ꢀ) Selectivity of benzaldehyde, (ꢂ) Conversion, (ꢁ) TON. Reaction condition:
toluene (8.3 M, 2.7 mL), TBHP (70%, 167 mM). Acetonitrile 0.25 mL, reaction temper-
ature 343 K, reaction time 6 h.
of filtrate and the remaining sample were continued and were mon-
itored by GC. Within the next 2.5 h reaction time, both the filtrate
and the remaining sample afforded further conversion of toluene,
which suggested there existed active species in the filtrate (Fig. 5).
Compared with the filtrate, a 5% higher conversion of toluene was
found in the remaining sample, indicating that the insoluble part
of the catalyst could also catalyze the oxidation of toluene.
0
1
2
3
4
5
6
7
3.3. Oxidation of substituted toluene
Time (h)
4
3
3
2
2
1
1
0
(
c)
What happens for the alkyl benzene with electron-donating
group and the one with electron-withdrawing group in this cat-
alytic oxidation system?
5
0
5
0
5
0
5
0
The catalyst of I was applied to oxidation of other types of
alkyl benzene with TBHP at 343 K. The results in Table 2 show
that I effectively catalyze the oxidation of various alkyl benzene
with electron-donating groups, good yield (∼80%) and selectiv-
ity (∼90%) to the corresponding aldehydes were obtained (entries
1
–5). Thousands of TONs were got under the reaction conditions. It
is found that the ring-substituent group could affect activity of the
reaction. For the toluene derivatives which had electron donating
1
00
90
80
0
1
2
3
4
5
6
7
Time (h)
Fig. 3. Oxidation of toluene with TBHP catalyzed by I at different temperatures.
a) Conversion, conversion (%) = [aldehyde (mol) × 2 + alcohol (mol)]/TBHP (mol), (b)
selectivity of benzaldehyde, (c) selectivity of benzyl alcohol, (ꢀ) 363 K, (ꢁ) 343 K,
ꢂ) 328 K, (×) 303 K, (᭹) 283 K. Reaction condition: catalyst (0.02 mM) in 0.25 mL
acetonitrile, toluene (8.3 M, 2.7 mL), TBHP (70%, 167 mM).
(
7
6
5
0
0
0
(
conversion and selectivity and
.12 mol‰ of catalyst was used for later experiment.
The solubility of I in the present reaction solution (toluene as
both reactant and solvent) was not high. When a high loading
1.2 mol‰) of catalyst was added to the reaction, some of the cata-
lyst became insoluble. In order to confirm the catalytic system was
homogeneous, hot separation of catalyst experiment with high cat-
alyst loading (12 mol‰) was carried out as follows: A half of the
reaction mixture was filtrated to separate the insoluble catalyst of
I after 1.5 h. The filtrate of the catalyst was quickly transferred into
a fresh flask that had been thermally stabilized in the same oil bath
and was heated at 343 K. Meanwhile, the remaining sample was
left to be heated at the same temperature. The oxidation reaction
a
high TON. Consequently,
40
0
3
0
20
Removal of insoluble catalyst
(
1
0
0
0
1
2
3
4
Time (h)
Fig. 5. Evidence of homogeneous catalysis of I in the oxidation of toluene. (ꢂ) Con-
tinuous reaction; (ꢀ) insoluble catalyst was removed by hot filtration after 1.5 h
reaction. Reaction condition: catalyst (0.02 mM) in 0.25 mL acetonitrile, toluene
(8.3 M, 2.7 mL), TBHP (70%, 167 mM), reaction temperature 343 K.

B. Ma et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 152–158
100
157
group, the reaction rate was faster than that of toluene, and the
reaction was almost completed in 4 h. Another toluene derivative
of p-methoxy-toluene containing the electron donating group of
methoxy also showed the similar high reaction rate and conversion
except for a relative lower selectivity to 4-methoxybenzaldehyde
9
0
80
7
0
Addition of BHT
(
73%, entry 6). The reason should be ascribed to the high reac-
tivity of electron donating group of methoxy. Thus, the substrate
were oxidized to some other products (alcohol and carboxylic acid)
besides 4-methoxybenzaldehyde. For ethyl benzene, the methyl
group could not be oxygenated; only benzylic C H bonds were oxi-
dized to the carbonyl compound (entry 7) with high yield (95%) and
selectivity (96%) to acetophenone.
60
5
0
40
3
0
For those toluene derivatives containing electron-withdrawing
group, the activity was less than that of the ones having electron-
donating groups. The electron withdrawing group made the
benzene ring electron deficient and inactivate. So, it was harder
for them to be oxidized. p-Chlorotoluene gave a 89% selectivity
to 4-chlorobenzaldehyde and a 52% yield (entry 8). The toluene
derivative of o-nitro-toluene which has a more stronger electron
withdrawing group of nitro-group gave a poor reactivity; a 92%
selectivity to 2-nitrobenzaldehyde and a 23% yield were obtained
after 6 h (entry 9).
20
1
0
0
0
1
2
3
4
Time (h)
Fig. 6. Evidence of free-radical reaction mechanism for the oxidation of toluene.
Reaction condition: catalyst (0.02 mM) in 0.25 mL acetonitrile, toluene (8.3 M,
2.7 mL), TBHP (70%, 167 mM), 2,6-bis(1,1-dimethylethyl)-4-methylphenol (BHT)
23 ␮mol, reaction temperature 343 K. (ꢀ) Conversion of toluene with addition of
BHT after 2 h, (ꢁ) selectivity of benzaldehyde with addition of BHT after 2 h, (ꢂ)
conversion of toluene with addition of BHT at the beginning.
3.4. Mechanism for the toluene oxidation
The catalysts of the traditional catalytic systems based on
phosphorus-contained POMs with peroxide are reported to be
decomposed to some phosphatooxotungstate peroxo fragments
At the beginning of the oxidation, the addition of radical scav-
enger 2,6-di-tert-buty-4-methylphenol (BHT, 370 equivalent with
respect to catalyst of I) completely stopped the oxidation, no any
products were detected. When the radical scavenger was added to
the reaction system after 2 h, the conversion of toluene just reached
46%. When the reaction time was prolonged, the conversion and
the selectivity kept constant without any change, which differed to
the toluene oxidation catalyzed by I without the addition of radi-
cal scavenger. For the normal oxidation, the conversion of toluene
increased with reaction time continued. All these data showed the
oxidation was underwent a free-radical mechanism (see Fig. 6).
Based on our experiments and previous researches of toluene
oxidation with TBHP [2,8c,19], a proposed reaction mechanism
was shown in Scheme 1, in which suggested that two kinds
of radicals existed for the present catalytic system. To begin
with, the polyoxometalate I (a) reacted with TBHP to form [n-
[
17] or some vanadatooxotungstate peroxo species [12c,14,18],
which actually catalyzed the oxidation of organic compounds.
However, this is not the case for [n-Bu N] H [PW V O40] based
4
3
3
9
3
on the characterization and catalytic experiments. This vanadium-
substituted POM was treated with 530-fold of TBHP for 24 h
Fig. 1d), which showed the same IR bands as that of the fresh
(
POM. The characteristic bands in FT-IR of phosphatooxotungstate
and vanadatooxotungstate species were not found, indicating that
phosphatooxotungstate or vanadatooxotungstate fragments did
not generated by treating [n-Bu N] H [PW V O40] with TBHP.
4
3
3
9
3
Therefore, there should exist other pathway for the oxidation of
toluene mediated by I with TBHP.
In order to clarify the reaction mechanism of this catalytic oxi-
dation process, we did the following experiments:
Bu N] H [PV W O40(OH)] (b) and the first type of radical of
4
3
3
3
9
CH3
t-BuOH
t-BuOO
CH2
CH2
CH2OH
b
a
t-BuO
V
t-BuOOH
OH
H
O
V
O
Ot-Bu
H2O
[
TBA] [H PV W O40(OOt-Bu)]
3
3
3
9
c
CH2OOt-Bu
CHO
t-BuOOH
t-BuOH
[
TBA] [H PV W O40(OH)] [TBA]3[H3PV3W9O40]
3
3
3
b
9
a
t-BuO
t-BuOOH
Scheme 1. Proposed catalytic mechanism for the oxidation of toluene with TBHP catalyzed by I.

1
58
B. Ma et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 152–158
•
•
t-BuO . Then the t-BuO reacted with TBHP to generate another
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•
radical of t-BuOO . Benzyl radical was produced by the reac-
tion between t-BuOO and toluene. The benzyl radical then
•
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4
3
3
3
9
[
peroxo species of c was vanadium based peroxide species [14],
so vanadium content had positive effect on the reaction activ-
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(
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4
3
3
3
9
same time, catalyst was regenerated to the original form of [n-
Bu N] H [PW V O40] (I or a). Finally, the benzyl cation reacts with
hydroxyl anion to yield benzyl alcohol.
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[
(
According to the proposed mechanism, two experimental
phenomena could be explained. First, although benzaldehyde was
the oxygenated product of benzyl alcohol, the mechanism indi-
cated the pathway of forming benzaldehyde and benzyl alcohol
was independent. Toluene was not initially oxidized to form ben-
zyl alcohol, which then generated benzaldehyde. The experiment
showed the same result. The temperature–time curve (Fig. 3b and
c) showed that at the beginning of the reaction no benzyl alco-
hol was produced and benzaldehyde dominated the main product.
With the reaction time continuing, the by-product of benzyl alcohol
increased immediately in 1 h. Then, during the whole 7 h reaction
duration, the selectivity between benzaldehyde (83%) and benzyl
alcohol (16%) nearly kept constant. Second, there existed some
intermediate species such as benzyl radical and benzyl cation. The
existence of these species answered for the product distribution of
ethyl benzene oxidation (Table 2, entry 7), in which only benzylic
(
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[
[
[
5
C
H bond could be oxidized. It is well known that benzylic radical
3
(
or cation was more stable than methylic radical or cation.
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4
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(
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In summary, we developed an effective catalytic sys-
1486–1489.
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4
3
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(
6
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mild conditions (343 K, 4–6 h) with high selectivity and yield. Only
[
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[
0.12 mol‰ of catalyst was needed for the reaction and up to 7665
(
b) P.J. Domaille, G. Watunya, Inorg. Chem. 25 (1986) 1239–1242;
(
ethyl benzene) of TONs were achieved. Based on our experiments
(
c) K. Nomiya, H. Yanagibayashi, C. Nozaki, K. Kondoh, E. Hiramatsu, Y. Shimizu,
and previous researches of toluene oxidation, a reaction mecha-
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present catalytic system.
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Acknowledgments
(
2
(
This work was financially supported by the National Natural Sci-
ence Foundation of China (Grant Nos. 21172098 and 21173105), the
Program for Changjiang Scholars and Innovative Research Team in
University (IRT1138) and National fundamental scientific talented
person training fund-the ability to improve project (J1103307).
(
(
3
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