Solvent-free aerobic oxidation of toluene over metalloporphyrin/NHPI/CTAB: Synergy and mechanism

Article abstract of DOI:10.1007/s10562-013-1104-5

The solvent-free catalytic oxidation of toluene using molecular oxygen over metalloporphyrin/N-hydroxyphthalimide (NHPI)/cetyl trimethyl ammonium bromide (CTAB) has been developed. The results showed that the type of metalloporphyrin used, catalyst loadin

Full text of DOI:10.1007/s10562-013-1104-5

Catal Lett (2014) 144:333–339
DOI 10.1007/s10562-013-1104-5
Solvent-Free Aerobic Oxidation of Toluene
over Metalloporphyrin/NHPI/CTAB: Synergy and Mechanism
•
Wei Deng Yan-ping Wan Hui Jiang
•
•
•
•
•
Wei-Ping Luo Ze Tan Qing Jiang
Can-Cheng Guo
Received: 18 July 2013 / Accepted: 8 September 2013 / Published online: 12 October 2013
Ó Springer Science+Business Media New York 2013
Abstract The solvent-free catalytic oxidation of toluene
using molecular oxygen over metalloporphyrin/N-hydrox-
yphthalimide (NHPI)/cetyl trimethyl ammonium bromide
(CTAB) has been developed. The results showed that the
type of metalloporphyrin used, catalyst loading, reaction
temperature, and the flow rate of oxygen all influenced the
conversion of toluene and the selectivities of the products.
Furthermore, a co-catalysis was observed between metal-
loporphyrin, NHPI, and CTAB. The toluene conversion
was significantly improved from 10.74 to 40.95 mol% after
adding CTAB to the reaction mixture. Based upon exper-
imental observations, a possible mechanism was also pro-
posed for the present oxidation.
by the chlorination of toluene followed by hydrolysis,
which have obvious drawbacks such as the danger asso-
ciated with the use of Cl2 [4]. Recently, great effort has
been made to develop environmental friendly strategies for
the aerobic oxidation of toluene. For example, several
catalyst systems including metalloporphyrins [5, 6], Au–Pd
nanoparticles [7], Cu–Mn oxides [8], and g-C3N4 nano-
composite [9] have been developed as effective catalysts
for the solvent-free aerobic oxidation of toluene. However,
most of these reactions suffered from harsh conditions such
as high temperature and pressure. More recently, N-hy-
droxyphthalimide (NHPI) has been reported as an effective
mediator for the oxidation of toluene in the presence of
cobalt salt [10], manganese dioxide [11], under mild con-
ditions. However, these processes also have some short-
comings including high catalyst loading, low toluene
concentration, and using acetic acid as solvent, thus lim-
iting their application in industry.
Keywords Toluene oxidation Á Co-catalysis Á
Metalloporphyrin Á NHPI Á CTAB
1 Introduction
Metalloporphyrins,which are widely recognized as
environmentally benign catalysts, have been shown to be
able to catalyze the oxidation of hydrocarbons under mild
conditions [12–15]. Our group have shown that simple
metalloporphyrin catalyzed toluene oxidation using diox-
ygen as oxidant with the selectivity of BA and BAL upto
60 % at the conversion of toluene upto 8.9 % [5]. Aiming
to further improve the catalytic efficiency of metallopor-
phyrin catalysts, we tried to use other additives to co-cat-
alyze the aerobic oxidation of hydrocarbons and we found
that NHPI was an effective co-catalyst [10, 11]. We
recently reported a (T(p-Cl)PP)MnF/NHPI catalyzed the
aerobic oxidation of toluene toward BA and BAL at
atmospheric pressure in acetic acid [16]. However, under
solvent free conditions which are preferred in industrial
processes, we found that the catalytic activity of (T(p-
Cl)PP)MnF/NHPI for the oxidation of toluene was much
Aerobic oxidation of toluene is a very important industrial
process because its main products, benzyl alcohol (BAL)
and benzaldehyde (BA), as well as benzoic acid (BAC), are
widely used to synthesize a large variety of fine chemicals
such as pharmaceuticals, foodstuff, dyes, perfume, and
resins [1]. Currently, BAC is mainly obtained from the
aerobic oxidation of toluene with cobalt carboxylate as
catalyst and bromides as initiators in the presence of acetic
acid as solvent [2, 3], while BA and BAL are synthesized
W. Deng Á Y. Wan Á H. Jiang Á W.-P. Luo Á Z. Tan Á Q. Jiang Á
C.-C. Guo (&)
College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, People’s Republic of China
e-mail: ccguo@hnu.edu.cn
123

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W. Deng et al.
2.2 Toluene Oxidation over Metalloporphyrins/NHPI/
lower. As a continuation of our research involving metal-
loporphyrin-catalyzed oxidations with dioxygen, we now
report the aerobic oxidation of toluene over metallopor-
phyrins/NHPI under solvent-free conditions can be signif-
icantly improved after adding CTAB as the co-catalyst.
The effects of various reaction parameters such as catalyst
components, catalyst concentration, reaction temperature
and the flow rate of oxygen on the conversion of the tol-
uene aerobic oxidation and selectivities of products were
also investigated. The results obtained with the catalyst
system of metalloporphyrin/NHPI/CTAB were not only
much better than those results with metalloporphyrin/
NHPI, metalloporphyrin/CTAB or NHPI/CTAB but also
better than the three results combined, thus clearly showing
the effect of co-catalysis. A possible mechanism for the
synergy of metalloporphyrin/NHPI/CTAB in the toluene
oxidation was also proposed.
CTAB
The catalytic oxidation of toluene with dioxygen at atmo-
spheric pressure under the temperature range from 60 to
100 °C was performed according to the following typical
procedure: 10 g of toluene and precisely weighted catalysts
were added into a 50 mL three-neck flask equipped with a
reflux condenser and a magnetic stirrer. Then while being
stirred, the flask was heated to reaction temperature. Upon
reaching the reaction temperature, oxygen was continuously
introduced into the flask through a flowmeter. The mixture of
toluene oxidation was sampled, characterized by GC–MS
and LC–MS in comparison with the authentic sample,
respectively, and quantified with GC using bromobenzene as
the internal standard [16]. The contents of benzyl hydrogen
peroxide (BHPO) were determined by iodometric method
and analyzed with GC according to the literature [18].
2 Experimental
3 Results and Discussion
2.1 Instruments and Materials
3.1 Toluene Oxidation over Metalloporphyrins/NHPI/
CTAB
GC analysis was performed on a Shimadzu GC-2010
equipped with a 0.5 mm i.d. 25 m PEG 20 M capillary
column and a flame ionization detector. MS spectra were
measured on a Class-5000 GC–MS spectrometer and an
Agilent 1100 LC–MS. UV–Vis absorption spectra were
recorded on a PEL-17 spectrometer. Toluene was purified
and analyzed by GC to ensure the absence of impurities
before use. Pyrrole was redistilled before use. Other
reagents were all analytical grade and used as received.
Porphyrin and their metal complexes were synthesized and
purified according to documented procedures [17, 18].
The oxidation of toluene catalyzed by Metalloporphyrins/
NHPI/CTAB with molecular oxygen was investigated. The
products of toluene aerobic oxidation were identified by
GC–MS and LC–MS, and compared with authentic sam-
ples. The oxidation products are mainly BA, BAL and
BAC, as shown in Scheme 1. The results of toluene oxi-
dation with dioxygen catalyzed by T(p-Cl)PPMnCl/CTAB,
NHPI/CTAB or T(p-Cl)PPMnCl/NHPI/CTAB are shown
in Table 1. When T(p-Cl)PPMnCl/NHPI (20 mL acetic
Scheme 1 Metalloporphyrins/
NHPI/CTAB catalyzed toluene
oxidation
Table 1 Oxidation of toluene catalyzed by T(p-Cl)PPMnCl/NHPI/CTAB
Entries
Catalysts
Solvent
Yield (mol%)
BA
Conv. (mol%)
BAL
BAC
1
2
3
4
5
T(p-Cl)PPMnCl
T(p-Cl)PPMnCl
T(p-Cl)PPMnCl
T(p-Cl)PPMnCl
–
NHPI
NHPI
NHPI
–
CTAB
CTAB
–
–
10.22
7.02
6.14
0.31
2.57
1.44
2.18
1.12
0.11
0.82
28.93
20.68
3.31
0
40.95
30.15
10.74
0.44
AcOHa
AcOHa
CTAB
CTAB
–
–
NHPI
3.30
6.79
Reaction conditions: toluene 10 g, NHPI 2.0 mol%, T(p-Cl)PPMnCl 20 ppm, CTAB 1 mmol, 100 °C, O2 0.05 L/min, reaction time 10 h
a
AcHO 20 mL added
123

Solvent-Free Aerobic Oxidation of Toluene
335
acid is used as solvent) was employed as the catalyst, the
toluene conversion is 10.74 % and the yields of BA, BAL,
and BAC are 6.14, 1.12, and 3.31 %, respectively (Table 1,
entry 3). On the other hand, when NHPI/CTAB was used as
the catalyst, the toluene conversion is 6.79 % and the
yields of BA, BAL, and BAC are 2.57, 0.82, and 3.30 %,
respectively (Table 1, entry 5). Interestingly, the toluene
conversion was significantly increased to 30.15 % if the
oxidation was carried out using T(p-Cl)PPMnCl in com-
bination with NHPI and CTAB as the catalyst in acetic acid
(Table 1, entry 2). Even more surprising is that the toluene
conversion can be elevated to 40.95 % and the yields of
BA, BAL, and BAC were 10.22, 1.44, and 28.93 %
(Table 1, entry 1), respectively, when the oxidation was
carried out in solvent free condition. The toluene conver-
sion over T(p-Cl)PPMnCl/NHPI/CTAB are much higher
than those over T(p-Cl)PPMnCl/NHPI and NHPI/CTAB,
and even better than the two results combined. This marked
improvement of catalytic efficiency suggested co-catalysis
for the T(p-Cl)PPMnCl/NHPI/CTAB catalyst system in the
aerobic oxidation of toluene.
the solvent-free aerobic oxidation of toluene in the pre-
sence of NHPI and CTAB.
3.3 Effect of Catalyst Concentration on the Toluene
Oxidation
3.3.1 Effect of T(p-Cl)PPMnCl Concentration on Toluene
Oxidation
The catalyst concentration is a very important factor
influencing the conversion and product yields in metallo-
porphyrin-catalyzed alkane oxidations [15, 16]. In order to
understand the effect of catalyst on toluene oxidation, we
examined the effect of T(p-Cl)PPMnCl concentration on
the toluene conversion and the selectivities of products in
the presence of 2.0 mol% NHPI and 1.0 mmol CTAB at
90 °C for 8 h. The results are shown in Table 3. When T(p-
Cl)PPMnCl was absent, the conversion of toluene is
5.31 %, the yields of BA, BAL, and BAC were 2.48, 0.68,
2.10 % respectively, and the total selectivity to BA and
BAL reached 59.6 %. This suggested that NHPI and CTAB
could catalyze the toluene aerobic oxidation even without
T(p-Cl)PPMnCl. The toluene conversion increased with
the increase of T(p-Cl)PPMnCl concentration. However
the toluene conversion decreased with further increase of
T(p-Cl)PPMnCl concentration. This observation is consis-
tent with our earlier results that the oxidation reaction can
be inhibited at high concentration of metalloporphyrin [5,
14, 19, 20]. A possible reason could be that high concen-
tration of metalloporphyrins can actually increase the self-
oxidation rate of metalloporphyrin catalyst, which leads to
metalloporphyrin decomposition, thus reducing effective
catalyst concentration [21].
3.2 Effect of the Catalyst Composition on the Toluene
Oxidation
It has been reported that the catalytic activity of metallo-
porphyrin could be greatly affected by the type of metal
ions of the metalloporphyrin catalysts in the aerobic oxi-
dation of toluene [16] or p-xylene [19]. As a result, we
investigated the influence of different metalloporphyrins on
the aerobic toluene oxidation using NHPI and CTAB as co-
catalysts and the results are summarized in Table 2. Four
types of metalloporphyrins, i.e., (T(p-Cl)PPMnCl, T(p-
Cl)PPCo, T(p-Cl)PPFeCl, T(p-Cl)PPCu) combined with
NHPI and CTAB, are chosen as catalysts, and the con-
versions of toluene are 8.32, 7.87, 7.17, and 6.49 %
respectively. The highest toluene conversion was obtained
using T(p-Cl)PPMnCl/NHPI/CTAB as the catalyst. So T(p-
Cl)PPMnCl was chosen as the standard catalyst to examine
3.3.2 Effect of NHPI Concentration on Toluene Oxidation
The aerobic oxidation of toluene catalyzed by NHPI with
Co(OAc)2 as co-catalyst using acetic acid as solvent has
been reported in literature [10]. In this reaction process, a
Table 3 Effect of T(p-Cl)PPMnCl concentration on toluene
oxidation
Table 2 Oxidation of toluene over different metallporphyrins/NHPI/
CTAB
T(p-Cl)PPMnCl (ppm)
Yield (mol%)
Conv. (mol%)
Metallporphyrins
Select. (mol%)
Conv. (mol%)
BA
BAL
BAC
BA
BAL
BAC
0
10
20
30
40
2.48
3.51
2.52
2.42
2.12
0.68
0.92
0.92
0.81
0.72
2.10
2.84
8.68
6.94
6.64
5.31
7.37
T(p-Cl)PPMnCl
T(p-Cl)PPCo
T(p-Cl)PPFeCl
T(p-Cl)PPCu
28.61
28.33
34.45
31.12
4.93
5.59
6.69
4.93
65.14
66.58
57.04
61.94
8.32
7.87
7.17
6.49
12.12
10.25
9.77
Reaction conditions: toluene 10 g, NHPI 2.0 mol%, 100 °C, metall-
porphyrins 20 ppm, CTAB 1 mmol, O2 0.05 L/min, reaction time 6 h
Reaction conditions: toluene 10 g, NHPI 2.0 mol%, CTAB 1 mmol,
90 °C, O2 0.05 L/min, reaction time 8 h
123

336
W. Deng et al.
with the reaction time. However, when NHPI concentration
was 0.5 mol%, the toluene conversion and the selectivities
to BA and BAL changed slowly with reaction time, while
the toluene conversion and the selectivities to BA and BAL
changed drastically with reaction time when NHPI con-
centration was 2.0 mol%. In addition, using 0.5 mol%
NHPI, the maximum conversion of toluene was up to
8.48 mol%, and the total selectivity of BA and BAL was
58.40 % whereas with 2.0 mol% NHPI, the maximum
conversion of toluene reached 33.11 mol%, and the total
selectivity of BA and BAL was 30.4 %. However, when
the concentration of NHPI was increased further to
2.5 mol%, the conversion of toluene was almost unchan-
ged. This behavior might be due to the low solubility of
NHPI in toluene.
Table 4 Effect of NHPI dosages on toluene oxidation
NHPI
(mol%)
Yield (mol%)
BA BAL
0.31
Select.
(mol%)
BA and BAL
Conv.
(mol%)
BAC
0
0.11
0.67
1.61
2.15
1.44
1.30
0
95.45
61.02
49.73
38.61
28.47
27.12
0.44
6.44
0.5
1.0
1.5
2.0
2.5
3.26
5.76
2.34
7.21
15.12
28.93
29.31
14.82
24.84
40.95
40.45
7.44
10.22
9.68
Reaction conditions: toluene 10 g, T(p-Cl)PPMnCl 20 ppm, CTAB
1 mmol, 100 °C, O2 0.05 L/min, reaction time 10 h
yield of 81 % of BAC was obtained in the presence of
10 mol% NHPI. In our study, by fixing the T(p-Cl)PPMnCl
concentration and the amount of CTAB at 20 ppm, 1 mmol
respectively, the influence of NHPI concentration on the
toluene conversion and product yields was investigated
(Table 4). From Table 4, it was observed that the conver-
sion of toluene was only 0.44 % in the absence of NHPI,
which showed that NHPI is essential for the toluene aer-
obic oxidation to proceed efficiently. It was found that the
conversion of toluene and the yields of BA and BAL
increased significantly with the concentration of NHPI
increasing. When the concentration of NHPI reached
2.0 mol%, the conversion of toluene was 40.95 %, and the
yields of BA, BAL, and BAC were 10.22, 1.44, 28.93 %,
respectively. Moreover, the change of the toluene conver-
sion and the selectivities to BA and BAL with reaction time
in the reaction catalyzed by T(p-Cl)PPMnCl/NHPI/CTAB
in the presence of different NHPI concentration are shown
in Fig. 1. It was found that the conversion of toluene
increased and the selectivity to BA and BAL decreased
3.4 Effect of other Reaction Parameters on the Toluene
Oxidation
3.4.1 Effect of Reaction Temperatures on the Toluene
Oxidation
Reaction temperature is a very important factor in the
aerobic hydrocarbon oxidation reactions catalyzed by
metalloporphyrin. Increasing reaction temperature can
shorten the induction period and accelerate the reaction rate
[16, 22]. In this paper, the influence of reaction temperature
on the toluene aerobic oxidation catalyzed by T(p-
Cl)PPMnCl/NHPI/CTAB in the temperature range of
Table 5 Effect of temperature on toluene oxidation
Temp. (oC)
Yield (mol%)
Conv. (mol%)
BA
BAL
BAC
60
70
0
0
0
0
2.12
3.59
3.62
3.77
0.41
0.89
0.95
1.14
1.30
2.93
3.11
3.52
3.85
7.48
7.71
8.48
80
90
100
Reaction conditions: toluene 10 g, NHPI 0.5 mol%, T(p-Cl)PPMnCl
20 ppm, CTAB 1 mmol, O2 0.05 L/min, reaction time 8 h
Table 6 Effect of O2 flows on toluene oxidation
O2 flows (L/min)
Yield (mol%)
Conv. (mol%)
BA
BAL
BAC
0.03
0.05
0.07
2.71
3.62
2.96
0.48
0.95
0.89
2.30
3.11
2.97
5.51
7.37
6.83
Fig. 1 Changes of toluene conversion and selectivities to BA and
BAL with time at different concentration of NHPI. Reaction
conditions: toluene 10 g, T(p-Cl)PPMnCl 20 ppm, CTAB 1 mmol,
100 °C, O2 0.05 L/min
Reaction conditions: toluene 10 g, NHPI 0.5 mol%, 90 °C, T(p-
Cl)PPMnCl 20 ppm, reaction time 8 h
123

Solvent-Free Aerobic Oxidation of Toluene
337
3.4.2 Effect of O2 Flow on the Toluene Oxidation
60–100 °C was investigated. As illustrated in Table 5, we
found that the conversion of toluene was indeed influenced
significantly by the reaction temperature. The rate of tol-
uene oxidation was so slow and no products were detected
when the reaction was conducted at 60 °C for 8 h in the
presence of 10 ppm of T(p-Cl)PPMnCl, 0.5 mol% of
NHPI, and 1 mmol of CTAB. With the increase of reaction
temperature, the conversion of toluene and the yield of
products increased rapidly. However, though the yields of
BA and BAL increased at higher temperature, the selec-
tivity of them decreased somewhat. This could be due to
the fact that BA and BAL could be further oxidized to BAC
at higher temperature.
The influence of the O2 flow on the toluene oxidation was
also investigated and the results are shown in Table 6. With
the increase of O2 flow from 0.03 L/min to 0.05 L/min in
the reaction system, the toluene conversion increased from
5.51 to 7.37 %. When the O2 flow was further increased to
0.07 L/min, the toluene conversion and the yields of pro-
ducts remained almost unchanged. One possible explana-
tion is that when the O2 flow was too high, the rate of
toluene oxidation with O2 is slower than that of O2 mass
transfer in toluene, thus making the excess amount of O2
useless [5].
Fig. 2 Concentration of benzyl
peroxide and conversion of
toluene versus reaction time in
the oxidation of toluene.
Reaction conditions: (a) toluene
10 g, NHPI 2.0 %, T(p-
Cl)PPMnCl 20 ppm, CTAB 1
mmol, 100 °C, O2 0.05 L/min;
(b) HOAc 20 ml added; (c)
without CTAB
Scheme 2 Preliminary
mechanism of toluene oxidation
over T(p-Cl)PPMnCl/NHPI/
CTAB
123

338
W. Deng et al.
3.5 Preliminary Mechanism of Co-catalysis for T(p-
Cl)PPMnCl/NHPI/CTAB in the Aerobic Oxidation
of Toluene
[29]. So for the aerobic oxidation of toluene over T(p-
Cl)PPMnCl/NHPI/CTAB, the toluene oxidation process
might be as follows (Scheme 2): BHPO is first generated
by the catalysis of T(p-Cl)PPMnCl and NHPI, and then it
can be decomposed immediately by CTAB or T(p-
Cl)PPMn(III)Cl. And T(p-Cl)PPMn(III)Cl was oxidized to
high valent T(p-Cl)PPMn(IV)(OH)Cl [14]. The more
active T(p-Cl)PPMn(IV)(OH)Cl then can react with tolu-
ene to produce a benzyl radical.
The aerobic oxidation of hydrocarbons over metallopor-
phyrins is a biomimetic model of C–H activation by
cytochrome P450 monoxygenase [1, 12, 23]. The mecha-
nisms of hydrocarbon oxidations with dioxygen catalyzed
by metalloporphyrins have been extensively investigated
[24, 25]. All previous literatures have suggested that the
aerobic oxidations of hydrocarbons catalyzed by metallo-
porphyrins involve free radical reactions [5, 16]. For the
aerobic oxidation of toluene over T(p-Cl)PPMnCl/NHPI/
CTAB, when 2,6-di-tertbutyl-p-cresol, a free radical
inhibitor, was added into toluene oxidation system, the
reaction was substantially quenched. The result suggested
that the toluene aerobic oxidation catalyzed by T(p-
Cl)PPMnCl/NHPI/CTAB indeed proceeded via a radical
process.
4 Conclusions
The synergy of metalloporphyrin/NHPI/CTAB catalyst in
the aerobic liquid phase oxidation of toluene has been
reported. The toluene conversion with metalloporphyrin,
NHPI and CTAB was higher than those when each catalyst
component was used alone or two of them combined toge-
ther. Experimental results indicated that T(p-Cl)PPMnCl
was the best metalloporphyrin catalyst, affording higher
toluene conversion than other metalloporphyrin catalyst.
The concentration of catalyst, O2 flow, reaction temperature,
and reaction time influenced the toluene aerobic oxidation
significantly. Best results were obtained at 100 °C with
0.05 L/min O2 flow, 20 ppm T(p-Cl)PPMnCl, 2.0 mol%
NHPI, and 1 mmol CTAB as catalyst, the toluene conversion
can be as high as 40.95 %.And we believe that the
improvement after adding CTAB might be due to the fact
CTAB could efficiently decompose BHPOs which are gen-
erated from the toluene oxidation. Further studies of this
catalytic oxidation reaction are underway in our laboratory.
Hydroperoxides can be produced as intermediates in the
hydrocarbon aerobic oxidation catalyzed by metallopor-
phyrins [18] under mild conditions, and they play an
important role in the initiation of the aerobic oxidation of
hydrocarbon [19]. In metalloporphyrin catalyzed aerobic
oxidation of toluene, BHPO could come from the reaction
of molecular oxygen with toluene catalyzed by metallo-
porphyrin [26–28]. NHPI can be an excellent radical cat-
alyst promoter for the aerobic oxidation of toluene through
an active intermediate PINO to produce benzyl radical [10,
16], which was then trapped by O2 to generate BHPO. The
change of BHPO concentration and the conversion of tol-
uene with the reaction time over T(p-Cl)PPMnCl/NHPI
and T(p-Cl)PPMnCl/NHPI/CTAB were investigated and
the results were shown in Fig. 2. When the reaction was
carried out at 100 °C and 0.05 L/min flow of oxygen with
T(p-Cl)PPMnCl/NHPI as the catalyst, the BHPO concen-
tration increased rapidly, while the conversion of toluene
increased slowly. The toluene conversion was about 8 %
when the reaction time was 8 h. However, it is interesting
to find that when 1 mmol of CTAB was added to the
reaction system under the same conditions, the BHPO
concentration was kept at below 0.05 % during the oxi-
dation of toluene and the toluene conversion increased
rapidly with the reaction time. When the reaction time was
8 h, the conversion of toluene was 33 %. One possible
reason could be that the highly volume of R4N cation of
CTAB could be in favor of interacting with the BHPO,
facilitating its decomposition [29, 30]. When 20 mL acetic
acid was added to the reaction system under the same
conditions, the concentration of BHPO was lower than that
when acetic acid was absent. It could be due to that the
coordinating solvent effect of acetic acid reduced the cat-
alytic activity of CTAB to decompose benzyl peroxide
Acknowledgments We thank the financial supports of National
Natural Science Foundation of China (Grants CN21372068).
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