Remarkable effect of simple aliphatic alcohols on the controlled aerobic oxidation of toluene catalyzed by (T(p-Cl)PP)MnF/NHPI

Article abstract of DOI:10.1016/j.molcata.2013.02.012

The remarkable effect of simple aliphatic alcohols on the controlled oxidation of toluene with molecular oxygen[5,10,15,20-tetrakis(p- chlorophenyl)porphinato]manganese fluoride (T(p-Cl)PPMnF) and N-hydroxyphthalimide (NHPI) was reported. The influences o

Full text of DOI:10.1016/j.molcata.2013.02.012

Journal of Molecular Catalysis A: Chemical 372 (2013) 84–89
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
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Remarkable effect of simple aliphatic alcohols on the controlled aerobic
oxidation of toluene catalyzed by (T(p-Cl)PP)MnF/NHPI
∗
Wei Deng, Wei-Ping Luo, Ze Tan, Qiang Liu, Zhao-Min Liu, Can-Cheng Guo
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The remarkable effect of simple aliphatic alcohols on the controlled oxidation of toluene with molecu-
lar oxygen over [5,10,15,20-tetrakis(p-chlorophenyl)porphinato]manganese fluoride (T(p-Cl)PPMnF) and
N-hydroxyphthalimide (NHPI) was reported. The influences of reaction conditions such as the concen-
tration and structure of alcohols, temperature and catalyst concentration on the promotion effect were
also studied. UV–vis spectra were used to investigate the possible structure of reaction intermediate
derived from the catalyst (T(p-Cl)PP)MnF in the toluene oxidation systems in the presence of methanol.
The results showed that the added methanol interacted with catalyst (T(p-Cl)PP)MnF to form the coor-
Received 12 December 2012
Received in revised form 7 February 2013
Accepted 17 February 2013
Available online xxx
Keywords:
Toluene
Metalloporphyrin
Dioxygen
Benzaldehyde
Catalytic oxidation
III
+
dination compound [(T(p-Cl)PP)Mn (ROH)2] , which subsequently was transformed into a high valent
IV
+•
manganese-oxo radical cation intermediate [(T(p-Cl)PP)Mn (O)(CH3OH)] in the oxidation systems. We
IV
+•
deduce that the formation of [(T(p-Cl)PP)Mn (O)(CH3OH)] and its higher reactivity toward toluene may
be responsible for the better results observed. On the basis of the results obtained, a possible mechanism
of the controlled oxidation of toluene with dioxygen over (T(p-Cl)PP)MnF/NHPI in the presence of the
aliphatic alcohols was proposed.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
selectivity of benzaldehyde and benzyl alcohol [5–12] since benz-
aldehyde is easily overoxidized in the oxidation system to benzoic
acid (BAC). So far, most of the selectivities of benzaldehyde and
benzyl alcohol reported are lower than 60%. Consequently, the con-
trolled aerobic oxidation of toluene to benzaldehyde and benzyl
alcohol in good yield and with excellent selectivity remains as a
challenge [13,14].
The controlled oxidation of toluene to benzaldehyde (BA), ben-
zyl alcohol (BAL) is an important transformation in chemical
industry because benzaldehyde and benzyl alcohol are widely used
to synthesize a large variety of diverse chemicals such as pharma-
ceuticals, foodstuff, dyes, perfume, and resins [1,2]. Currently,
benzaldehyde and benzyl alcohol are mainly produced via chlorina-
tion of toluene followed by hydrolysis [3,4], which generates large
quantities of toxic waste, thus causing serious pollution problems.
More importantly, the products produced in the above process are
prohibited to be used in the pharmaceutical and foodstuff fields
because they contain some of the chlorinated impurities. As a result
many researchers have been striving to develop new routes for
the production of benzaldehyde and benzyl alcohol through the
controlled oxidation of toluene using molecular oxygen. Until now
several catalyst systems including g-C N nanocomposite [5,6],
Metalloporphyrins, which are widely recognized as environ-
mentally benign catalysts, have attracted extensive interest for
their excellent catalytic performances in the controlled oxy-
genation of hydrocarbons with molecular oxygen under mild
conditions [10,15–20]. Our group has investigated the selective
aerobic oxidation of toluene using simple metalloporphyrin as the
catalyst and selectivity of benzaldehyde and benzyl alcohol up to
60% with the conversion of toluene up to 8.9% was achieved [10].
Though the results are comparable to those previously reported,
they are far from practical and there is still much room left to be
improved. Since N-hydroxyphthalimide (NHPI) has been reported
as an effective cocatalyst for the metalloporphyrin catalyzed aer-
obic oxidation of hydrocarbons in mild conditions [9,21–24], we
wonder whether adding some NHPI in our reaction mixture could
improve our reaction. Furthermore, researchers have reported
that some protic solvents such as aliphatic alcohols, acting as
ligands, can markedly affect the catalytic activity of metallopor-
phyrin in the oxidation of hydrocarbons or epoxidation of olefins
with peroxides [20,25–27]. So we decided to probe the effect of
adding some alcohols in the T(p-Cl)PPMnF/NHPI catalyzed aerobic
3
4
copper-based oxides [7], Mn(salen) complex [8], and NHPI [9]
have been reported as effective catalyst for the selective oxida-
tion of toluene with different oxidants. However, one of the major
problems associated with the existing technologies is that the con-
version of toluene has to be kept below 10% to obtain relative high
∗ Corresponding author. Tel.: +86 731 88821314; fax: +86 731 88821667.
E-mail address: ccguo@hnu.edu.cn (C.-C. Guo).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.02.012

W. Deng et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 84–89
85
Scheme 1. (T(p-Cl)PP)MnF/NHPI catalyzed toluene oxidation.
GC using bromobenzene as the internal standard [10]. The contents
of hydroperoxides were determined by iodometric method.
3. Results and discussion
3.1. The promotional effect of simple aliphatic alcohols on the
toluene oxidation
The oxidation of toluene catalyzed by (T(p-Cl)PP)MnX/NHPI
with molecular oxygen and the influence of simple aliphatic alco-
hols on the oxidation were investigated. Analysis showed that the
same oxidation products were obtained in all reactions whether
alcohol was added or not (Scheme 1). Out of the four different met-
alloporphyrin catalysts tested (T(p-Cl)PP)MnX (X = F, Cl, Br, OH)),
Fig. 1. T(p-Cl)PPMnX.
oxidation of toluene system. We were pleased to discover that the
catalytic activity of (T(p-Cl)PP)MnF could be remarkably enhanced
by adding some aliphatic alcohol into the reaction mixture in the
aerobic controlled oxidation of toluene. Both the toluene conver-
sion and the selectivity of benzaldehyde and benzyl alcohol could
be significantly improved. UV–vis spectra of the toluene reaction
(T(p-Cl)PP)MnF was found to give slightly better results (Table 1). As
a result, T(p-Cl)PP)MnF was chosen as the standard catalyst to study
the controlled oxidation of toluene. In a typical experiment without
the addition of aliphatic alcohol, the toluene conversions is 12.8%,
the selectivity of benzaldehyde was 39.6%, and the selectivity of
◦
benzyl alcohol was 4.7% at 100 C when the reaction time was 10 h
mixture showed that (T(p-Cl)PP)MnF can be coordinated axially
with aliphatic alcohol to form [(T(p-Cl)PP)Mn(ROH) ] , which is
(Table 2, entry 4). However, when some amount of aliphatic alco-
+
2
hol such as methanol or ethanol was added, the toluene conversion
was increased to 38.2%, 32.1%, while the selectivities of benzalde-
hyde and benzyl alcohol reached 37.5%, 6.3%, respectively (Table 2,
entries 6 and 8). These results are summarized in Table 2.
Interestingly, at 8 h when the conversion of toluene was around
14–15%, the selectivity of benzaldehyde was as high as 62.4–64.8%
transformed in the reaction mixture to high-valent manganese-oxo
IV
+•
redical cations [(T(p-Cl)PP)Mn (O)(CH OH)] , which we believe is
3
the active species for oxidation of hydrocarbon [26]. We reasoned
that the addition of aliphatic alcohol can facilitate the formation
IV
+•
of [(T(p-Cl)PP)Mn (O)(CH OH)] intermediates during the reac-
3
tion process. According to these results, a possible mechanism of
the aerobic oxidation of toluene over the manganese porphyrin and
NHPI was proposed.
(
(
Table 2, entries 5 and 7). When the catalyst was switched from
T(p-Cl)PP)MnF/NHPI to Mn(OAc) /NHPI, the promotional effect
2
observed with methanol or ethanol was not observed (Table 2,
entries 9 and 10).
2
. Experimental
3
.2. The effect of methanol on (T(p-Cl)PP)MnF in the toluene
2.1. Instruments and materials
oxidation
GC analysis was performed on a Shimadzu GC-2010 equipped
The UV–vis spectral changes accompanying the toluene oxi-
dation reaction could be attributed to the change of the
with a 0.5 mm i.d. 25 m PEG 20M capillary column and a flame ion-
ization detector. MS spectra were measured on a Class-5000 GC–MS
spectrometer and an Agilent 1100 LC–MS. UV–vis absorption spec-
tra and changes in absorbance with time were recorded on a PE L-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.
T(p-Cl)PPMnX (X = F, Cl, Br, OH) (structure as Fig. 1) were synthe-
sized and purified according to documented procedures [28,29].
(
(
T(p-Cl)PP)MnF catalyst. So in order to investigate the change of
T(p-Cl)PP)MnF catalyst, the UV–vis spectra of the toluene reac-
tion mixture have been measured and analyzed. Fig. 2 shows the
spectral changes of toluene oxidation over (T(p-Cl)PP)MnF/NHPI in
1
0 h with or without the addition of some CH OH. When 0.08 mol
3
methanol was added, as can be seen from the part A of Fig. 2,
only one characteristic peak (468 nm) could be observed at the
beginning of the toluene oxidation (shown as the bold solid line in
Fig. 2). According to the reported literature [30], this absorbance
2.2. Toluene oxidation over (T(p-Cl)PP)MnX/NHPI
Table 1
The catalytic oxidation of toluene with molecular oxygen at
The catalytic efficiency of (T(p-Cl)PP)MnX on the toluene oxidation.
atmospheric pressure was performed according to the follow-
ing typical procedure: desired amount of reactants and precisely
weighted catalysts were added into a 100 ml three-neck flask
equipped with a reflux condenser and a magnetic stirrer. Then
the flask was heated to reaction temperature while the solution
was agitated by a magnetic stirrer. Upon reaching the reaction
temperature, oxygen was continuously introduced into the flask
through a flowmeter. The mixture of toluene oxidation was sam-
pled at regular intervals, identified by GC–MS, and quantified with
(
T(p-Cl)PP)MnX
Conv. (mol %)
Selectivity (mol%)
BA
BAL
BAC
(
(
T(p-Cl)PP)MnF
T(p-Cl)PP)MnCl
38.2
35.7
29.8
37.5
37.5
38.4
42.9
37.1
6.3
5.8
7.2
6.5
55.3
54.4
47.2
55.1
(T(p-Cl)PP)MnBr
T(p-Cl)PP)MnOH
(
Reaction conditions: toluene 0.13 mol, CH OH 78.1 mmol, (T(p-Cl)PP)MnX 20 ppm,
NHPI 5 mol%, HOAc 20 ml, O2 0.05 L/min, temperature 100 C.
3
◦

8
6
W. Deng et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 84–89
Table 2
Selective oxidation of toluene with molecular oxygen.
Entry
Catalysts
Additives
Time (h)
Con. (mol%)
Selectivity (mol%)
BA
BAL
BAC
1
2
3
4
5
6
7
8
9
–
–
–
–
–
–
10
7
10
10
8
10
8
10
10
10
10
Traces
Traces
Traces
12.8
15.6
38.2
14.5
32.1
5.8
–
–
–
–
–
–
–
–
–
(T(p-Cl)PP)MnF
(T(p-Cl)PP)MnF
(T(p-Cl)PP)MnF
(T(p-Cl)PP)MnF
(T(p-Cl)PP)MnF
(T(p-Cl)PP)MnF
(T(p-Cl)PP)MnF
MeOH
–
NHPI
NHPI
NHPI
NHPI
NHPI
NHPI
NHPI
NHPI
39.6
64.8
37.5
62.4
42.4
17.4
14.2
38.5
4.7
9.1
6.3
10.3
7.1
4.3
3.1
15.4
54.3
24.8
55.3
25.1
49.2
74.2
79.4
0.0
MeOH
MeOH
EtOH
EtOH
MeOH
–
a
Mn(OAc)2
1
1
0
1
Mn(OAc)2a
–
6.3
0.13
–
◦
Reaction conditions: toluene 0.13 mol, CH3OH 78.1 mmol, (T(p-Cl)PP)MnF 20 ppm, NHPI 5 mol%, HOAc 20 ml, O2 0.05 L/min, temperature 100 C.
a
Mn(OAc)2 20 ppm.
produced, while the latter produces FeIV(OH)P directly. Some
researchers [20,34,35] also found that aliphatic alcohols could mod-
ify the electronic properties of metalloporphyrin and facilitate the
heterolytic cleavage of the oxygen–oxygen bond of hydrogen per-
oxide. So in our case, we deduced that the absorbance at 421 nm
peak at 468 nm could be ascribed to the Soret absorbance of
+
[
(T(p-Cl)PP)Mn(CH OH) ] , which came from the (T(p-Cl)PP)MnF
3
2
coordinated with CH OH in the reaction solution. In order to fur-
3
ther confirm the structure of methanol complexed (T(p-Cl)PP)MnF,
a dichloromethane solution of (T(p-Cl)PP)MnF was added methanol
until the concentration of methanol in dichloromethane reached
IV
+•
could be assigned to [(T(p-Cl)PP)Mn (O)(CH OH)] , which could
3
III
+
3
–10 M. Fig. 3 shows the UV–vis spectral changes observed in this
be generated in the reaction of [(T(p-Cl)PP)Mn (ROH) ] with ben-
2
experiment. With the increasing amount of methanol added, the
spectrum shows a decrease in the Soret absorbance (ꢀmax = 458 nm)
for (T(p-Cl)PP)MnF with concomitant increase of absorbance at
zyl hydrogen peroxides. This assignment of the absorbance peak
IV
at 421 nm is consistent with the reported Mn (O)P complexes in
IV
+•
known literatures [31–33]. [(T(p-Cl)PP)Mn (O)(CH OH)] should
3
4
68 nm, the peak of which was also observed in the toluene oxi-
have higher reactivity toward toluene oxidation than [(T(p-
III
+
dation at the same position, which was also consistent with that
of [(T(p-Cl)PP)Mn(CH OH) ] reported in literature [30]. As the
Cl)PP)Mn (ROH) ] , and this could explain why higher conversion
2
+
3
2
of toluene and selectivity toward benzaldehyde and benzyl alcohol
were achieved in the presence of alcohol. It should be pointed out
that similar results were also reported by Groves in the study of the
interaction between cis-␤-methylstyrene and oxomanganese(IV)
porphyrin [31]. In order to show the differences, the UV–vis spectra
of toluene oxidation mixture without methanol have been mea-
sured as well and the results were shown as the part B of Fig. 2.
Only one absorbance peak at 470 nm could be observed which
could be assigned to the coordination compound (T(p-Cl)PP)MnF
complexed with acetic acid. During the reaction, the absorbance at
470 nm remained unchanged. The results implied that it was more
difficult for ((T(p-Cl)PP)MnF to form the active intermediate [(T(p-
toluene oxidation began to take place, two new absorbance peaks
at 421 and 556 nm appeared, and as the reaction progressed,
the intensity of the Soret band at 468 nm decreased significantly
accompanied with substantial increase of absorbance at 421 nm.
For their assignment, we took note of similar metalloporphyrin
complexes in early reports [26,31–33]. It was reported that hydro-
gen peroxides could coordinate axially with iron porphyrin [26],
then the oxygen–oxygen bond of the hydrogen peroxide molecule
coordinated with iron porphyrin could undergo either heterolytic
IV
+•
,
or homolytic cleavage. The former process will lead to Fe (O)P
an active catalyst for olefin epoxidation with FeIV(OH)P species
IV
+•
Cl)PP)Mn (O)(CH OH)] in the oxidation of toluene in acetic acid
3
without the addition of alcohol.
Fig. 2. UV–vis absorption spectral changes observed during toluene oxidation. Reac-
tion conditions: toluene 0.13 mol, (T(p-Cl)PP)MnF 20 ppm, NHPI 5 mol%, HOAc 40 ml,
O2 0.05 L/min, temperature 100 C, reaction time 10 h; A: CH3OH 78.1 mmol, B:
◦
3 2 2
Fig. 3. UV–vis spectral changes of (T(p-Cl)PP)MnF with CH OH in CH Cl . Reaction
conditions: CH3OH: 3.00–10.00 M. (T(p-Cl)PP)MnF: 1.2 × 10 M.
−
5
without alcohols.

W. Deng et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 84–89
87
Table 3
Effect of the concentration of methanol on the toluene oxidation.
Table 5
Effect of the concentration of (T(p-Cl)PP)MnF on the toluene oxidation.
CH3OH (mmol)
Con. (mol%)
Selectivity (mol%)
(T(p-Cl)PP)MnF
ppm)
Conversion
(mol%)
TON (mol/mol)
Yield (mol%)
(
BA
BAL
BAC
BA
0.05
11.5
14.3
BAL
BAC
0.00
19.8
21.1
15.6
31.2
46.9
78.1
13.2
15.4
24.5
38.2
63.9
58.7
50.4
37.5
5.8
6.2
5.6
6.3
29.2
33.1
42.5
55.3
0
10
20
0.13
34.0
38.2
–
155,135
174,333
0.02
2.1
2.4
Reaction conditions: NHPI 5 mol%, CH3OH 78.1 mmol, HOAc 20 ml, O2 0.05 L/min,
◦
Reaction conditions: toluene 0.13 mol, NHPI 5 mol%, (T(p-Cl)PP)MnF 20 ppm, HOAc
0 ml, O2 0.05 L/min, temperature 100 C, reaction time 10 h.
temperature 100 C, reaction time 10 h.
◦
2
of toluene oxidation are obtained. In contrast, when some amount
of isopropanol or tert-butyl alcohol with larger steric hindrance
was added in the reaction solution, the oxidation of toluene was
slowed down considerably. It could be attributed to the larger steric
hindrance of branched-chain alcohols prohibiting molecular oxy-
gen from getting near the catalyst after the catalyst (T(p-Cl)PP)MnF
was coordinated with the added alcohol, consequently the ability of
3
.3. The effect of other reaction parameters on the toluene
oxidation
3.3.1. The effect of the amount of methanol
In order to further study the influence of alcohol on the toluene
oxidation catalyzed by (T(p-Cl)PP)MnF/NHPI, the changes of the
toluene conversion and the selectivity of benzaldehyde and ben-
zyl alcohol were investigated by varying the amount of methanol
added into the reaction solution. The results are listed in Table 3.
When the amount of methanol added was 15.6 mmol, the conver-
sion of toluene and the selectivities of benzaldehyde and benzyl
alcohol are 13.2%, 63.9%, 5.8% respectively. When the amount of
methanol added was further increased, the conversion and the
yield of benzaldehyde and benzyl alcohol increased rapidly. When
(T(p-Cl)PP)MnF activating dioxygen was decreased and the rate of
toluene oxidation was slowed down. Similar phenomenon was also
observed in the investigation of the aerobic oxidation of p-xylene
influenced with p-toluic alcohol by our research group [36].
3.3.3. The effect of the concentration of (T(p-Cl)PP)MnF
The catalyst concentration is a key factor influencing the conver-
sion and product selectivity in metalloporphyrin-catalyzed alkane
oxidations [10,17]. In order to understand the effect of the concen-
tration of (T(p-Cl)PP)MnF on the toluene oxidation, we investigated
the changes of the toluene conversion and the selectivity of
benzaldehyde and benzyl alcohol with different (T(p-Cl)PP)MnF
concentrations, and the results are shown in Table 5. With no cat-
alyst, the toluene conversion was about 0.13%. When 10 or 20 ppm
of (T(p-Cl)PP)MnF was used in the reaction, the toluene conver-
sion was increased to above 34% while the yield of benzaldehyde
was between 11 and 14%. In contrast the yield of benzyl alcohol was
only around 2%. It should be noted that the molar turnover numbers
were more than 150,000 in these reactions. These results illustrate
that the toluene conversion and benzaldehyde/benzyl alcohol yield
increased significantly when a small amount of (T(p-Cl)PP)MnF was
added, and the metalloporphyrin catalyst is a highly efficient cata-
lyst for the selective oxidation of toluene.
7
8.1 mmol of methanol was added, the conversion went up to 38.2%
and the selectivity of benzaldehyde dropped to 37.5%, in other
words, the total yield of benzaldehyde and benzyl alcohol was
1
6.2%. This is understandable since at higher conversion, more alde-
hyde and alcohol were converted to benzoic acids, thus lowering
the selectivities. However, the yield increase can be rationalized
by better coordination of methanol with (T(p-Cl)PP)MnF at higher
concentrations. When the methanol concentration was lower, only
some of the (T(p-Cl)PP)MnF catalysts were coordinated axially
with methanol. When the methanol concentration was higher,
most of the (T(p-Cl)PP)MnF catalysts were converted to the sta-
+
IV
+•
ble [(T(p-Cl)PP)Mn(CH OH) ] or [(T(p-Cl)PP)Mn (O)(CH OH)]
3
2
3
in the process of toluene oxidation. Some literatures [30,34] have
also reported that the increase of alcohol concentration could
change the reductive potentials of metalloporphyrins, thus influ-
encing their catalytic ability in chemical reactions.
3
.3.4. The effect of reaction time on the toluene oxidation
The changes of toluene conversion and the selectivity of benz-
3
.3.2. The effect of aliphatic alcohol structure
In order to gain further insight into the effect of aliphatic alco-
aldehyde and benzyl alcohol of the toluene oxidation with reaction
time are shown in Fig. 4. The plot shows that the conversion of
toluene increased and the selectivity to benzaldehyde and benzyl
alcohol decreased with reaction time. When the reaction time was
hol on the catalytic ability of manganese porphyrin, the changes of
toluene conversion and yields of benzaldehyde and benzyl alcohol
were investigated by adding different aliphatic alcohols. The results
are shown in Table 4. The influences of different aliphatic alcohols
on the toluene oxidation over (T(p-Cl)PP)MnF/NHPI are significant
different. Straight-chain alcohols, such as methanol, ethanol, and
propanol which have smaller steric hindrance, can promote the
catalytic ability of manganese porphyrin and higher conversions
1
0 h, the maximum conversion of toluene was up to 12.8%, and
the selectivities of benzaldehyde and benzyl alcohol was 37.5% and
.3%, respectively, in the absence of any alcohol. When methanol or
6
ethanol was added into the reaction solution, the maximum con-
version of toluene was increased significantly to 38.2% or 32.1%
respectively, and the selectivities of benzaldehyde and benzyl alco-
hol were maintained between 37.5 and 42.4% and 6.3 and 7.1%,
respectively.
Table 4
Effect of different aliphatic alcohols on the toluene oxidation.
Alcohols
Conversion (mol%)
Selectivity (mol%)
3
.3.5. The effect of reaction temperature on the toluene oxidation
Normally temperature is very important on the metallopor-
BA
BAL
BAC
–
12.8
38.2
32.1
17.5
8.4
39.6
37.5
42.4
56.2
75.0
69.0
4.7
6.3
7.1
7.8
9.4
8.3
54.3
55.3
49.2
34.5
14.1
20.8
phyrin catalyzed reactions. Increasing reaction temperature can
shorten the induction period and accelerate the reaction rate of
metalloporphyrin catalyzed reactions [37,18]. In this paper, the
influence of reaction temperature on the toluene oxidation cat-
alyzed by (T(p-Cl)PP)MnF/NHPI with methanol at the temperature
CH3OH
CH3CH2OH
CH3CH2CH2OH
(
(
CH3)2CHOH
CH3)3COH
7.5
◦
Reaction conditions: toluene 0.13 mol, NHPI 5 mol%, (T(p-Cl)PP)MnF 20 ppm, alcohol
8.1 mmol, HOAc 20 ml, O2 0.05 L/min, temperature 100 C, reaction time 10 h.
range of 70–100 C was investigated. As illustrated in Table 6,
the conversion of toluene was influenced significantly by the
◦
7

8
8
W. Deng et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 84–89
through a homolytic cleavage to produce [(T(p-Cl)PP)MnIV(OH)]+
26,35], which is less active for the toluene oxidation, the other
is through a heterolytic cleavage to produce a two-electron oxi-
[
IV
+•
dized [(T(p-Cl)PP)Mn (O)(CH OH)] and benzyl alcohol [15,35].
3
And the heterolytic cleavage could be accelerated by adding some
simple aliphatic alcohol in the reaction solution [34]. In other
words, the added alcohol not only can modify the electronic
properties of metalloporphyrin catalyst but also can facilitate
IV
+•
the formation of [(T(p-Cl)PP)Mn (O)(CH OH)] . The more active
[
3
IV
+•
(T(p-Cl)PP)Mn (O)(CH OH)] then can react with toluene to
3
produce a benzyl radical. This hypothesis is corroborated by the
observation that higher selectivity of benzyl alcohol vs benzal-
dehyde was achieved when some methanol was added into the
reaction solution. The whole reaction process might be as follows:
+
[
(T(p-Cl)PP)MnIII]
−→
RCH + O2
RCH OOH
(1)
(2)
3
2
NHPI
Fig. 4. Changes of toluene conversion and the selectivity of BA and BAL with the
reaction time. Reaction conditions: toluene 0.13 mol, NHPI 5 mol%, (T(p-Cl)PP)MnF
III +
1
III
1
+
[
(T(p-Cl)PP)Mn ] + R OH → [(T(p-Cl)PP)Mn (R OH) ]
2
2
0 ppm, alcohol 78.1 mmol, HOAc 20 ml, O2 0.05 L/min, reaction time 10 h.
+
III
1
[
(T(p-Cl)PP)Mn (R OH )] + RCH OOH
2
2
reaction temperature. The rate of toluene oxidation was so slow
◦
1
at 70 C that there was very little product when the reaction was
R OH
IV
1
+
•
−
→ [(T(p-Cl)PP)Mn (O)(R OH)] + RCH OH
(3)
(4)
(5)
(6)
2
run for 10 h. With the increase of reaction temperature, the conver-
sion of toluene and the yield of benzaldehyde and benzyl alcohol
increased rapidly, although the selectivity of benzaldehyde and
benzyl alcohol decreased somewhat. That could be due to the fact
that benzaldehyde and benzyl alcohol could be oxidized easily at
higher temperature.
III +
[
(T(p-Cl)PP)Mn ] + RCH OOH → [(T(p-Cl)PP)MnIV(OH)]+
2
+ RCH O•
2
[(T(p-Cl)PP)MnIV(O)(R OH)] + RCH3
1
+•
3
.4. The preliminary mechanism for the (T(p-Cl)PP)MnF/NHPI
catalyzed-alcohol promoted selective oxidation of toluene with
dioxygen
→
[(T(p-Cl)PP)MnIV(OH)(R OH)] + RCH2•
1
+
The mechanisms of hydrocarbon oxidations with molecular
oxygen catalyzed by metalloporphyrins [24,33,37,18,39–42], or
metalloporphyrins with co-catalysts [20,21] have been exten-
sively investigated. All previous literatures have suggested that
the aerobic oxidations of hydrocarbons catalyzed by metallopor-
phyrins involve a free radical reaction, which is typically initiated
from alkyl hydroperoxides [10,21]. NHPI can be an excellent rad-
ical catalyst promoter for the aerobic oxidation of hydrocarbons
through an active intermediate PINO to produce alkyl hydroper-
oxides [9,22]. For the aerobic controlled oxidation of toluene over
+
[
(T(p-Cl)PP)MnIV(OH)] + RCH OOH
2
III +
•
→
[(T(p-Cl)PP)Mn ] + RCH OO
2
4. Conclusions
In summary, we discovered that suitable aliphatic alcohols can
exert remarkable beneficial effect on the aerobic controlled oxida-
tion of toluene over (T(p-Cl)PP)MnF/NHPI and good conversion and
product selectivity toward benzaldehyde and benzyl alcohol were
achieved. The UV–vis spectral analysis of the toluene oxidation
mixture showed that an active intermediate species [(T(p-
(
T(p-Cl)PP)MnF/NHPI, benzyl hydrogen peroxide was found to be
produced in the reaction mixture, and its yield was 6.2 mol% when
the reaction time was 5 h. We propose that benzyl hydrogen per-
oxide comes from the reaction of molecular oxygen with toluene
catalyzed by (T(p-Cl)PP)MnF/NHPI [15,21,26,35]. Then the ben-
zyl hydrogen peroxide can coordinate axially to (T(p-Cl)PP)MnF
and the oxygen–oxygen bond of the coordinated benzyl hydro-
gen peroxide can undergo cleavage to produce a high-valent
manganese-oxo porphyrin species through two pathways. One is
IV
+•
Cl)PP)Mn (O)(CH OH)] could be generated in the oxidation. We
3
IV
+•
deduced that the formation of [(T(p-Cl)PP)Mn (O)(CH OH)] is
3
related to the better results observed. Current efforts are under-
way to elucidate the reaction mechanism further and the results
will be reported in due course.
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