Acetylacetone as an oxygen activator to improve efficiency for aerobic oxidation of toluene and its derivatives by using cobalt: Meso-tetraphenylporphyrin

Article abstract of DOI:10.1039/d0nj01575j

The activation of dioxygen is the major challenge in the catalytic oxygenation of hydrocarbons under mild conditions. In this study, the catalytic efficiency for the aerobic oxidation of toluene and its derivatives by using cobalt meso-tetraphenylporphyrin was enhanced by using acetylacetone as the oxygen activator. The influences of reaction conditions such as solvent, different metalloporphyrin catalysts, temperature, pressure and acetylacetone loading were studied, as well as the kinetics of the oxidation. Various toluene derivatives could also be oxidized to the corresponding products in satisfactory yields with this catalytic system. Based on the characterization of in situ electron paramagnetic resonance (EPR) and in situ UV-vis spectroscopy, the catalytic mechanism was also proposed, in which acetylacetone served as the key initiator of the free radical in activating dioxygen. This journal is

Full text of DOI:10.1039/d0nj01575j

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Acetylacetone as oxygen activator to improve efficiency for the
aerobic oxidation of toluene and its derivatives by cobalt meso-
tetraphenylporphyrin
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
b
b,c
Xian-Tai Zhou , Hong-Yu Chen , Qi Han , Meng Lv , Hong-Bing Ji*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The activation of dioxygen is the major challege in the catalytic oxygenations of hydrocarbon undre mild conditions. In this
study, catalytic efficiency for the aerobic oxidation of toluene and its derivatives by cobalt meso-tetraphenylporphyrin was
enhanced by using acetylacetone as oxygen activator. The influences of reaction conditions such as solvent, different
metalloporphyrins catalyst, temperature, pressure and acetylacetone loading were studied, as well as the kinetics of the
oxidation. Various toluene’s derivates could also be oxidized to corresponding products in satisfactory yields with this
catalytic system. Based on the characterization of in situ EPR (Electron Paramagnetic Resonance) and in situ UV–vis
spectroscopy, the catalytic mechanism was also proposed, in which acetylacetone served as the key inititor of free radical
in activating dioxygen.
polymerization have been developed as heterogeneous catalyst used
3
1-35
Introduction
in the oxygenation of hydrocarbons.
Recently, one kind
conjugated nanoporpous alkynyl magnanese porphyrin framework
was used in the aerobic oxidation of toluene. The heterogeneous
catalyt showed excellent activity with a conversion of toluene up to
Selective catalytic oxygenations of hydrocarbon with dioxygen as
1
-7
the terminal oxidant play a key role in the chemcial industry. As a
model of aromatic molecule, the oxidation of toluene to
benzaldehyde, benzyl alcohol and benzoic acid is gathering much
attention, as all products are key intermediates for the synthesis of
pharmaceuticals, insecticides and perfumes etc.
reports on gas phase oxidation of toluene using molecular oxygen as
oxidant, the reaction involve harsh conditions and are poorly
3
6
10.2%. Manganese tetraphenylporphyrin supported on chitosan
used as catalyst for toluene oxidation yielded a 5.9% conversion at
o
37
195 C and 0.6MPa.
8
-11
There are some
The activation of dioxygen under mild conditions is still the
major challenges faced in metalloporphyrins-catalyzed oxidation of
C-H bonds. Recently, our group reported the biomimetic aerobic
epoxidation of olefins, toluene oxidation with isopropylbenzene and
1
2-15
selective to specific products.
phase oxidation must be limited to avoid over-oxidation of reactant
into CO at high reaction temperature. As compared with gas-solid
Meanwhile, the process of vapor-
3
8-39
cyclohexene as NAD(P)H-like compounds, respectively.
We
2
initiated continously our study by examining the compound could act
as the inititor for activation of molecular oxygen in the oxidation of
hydrocarbon. As one kind of carbonyl compounds, acetylacetone
with two carbonyl groups, dehydrogenation of second carbon could
occur easily to form free radical. Therefore, we supposed
acetylacetone has a potential as excellent inititor in biomimetic
aerobic oxidation of non-activated C–H bonds under mild conditions.
As we expected, in this work, toluene and its derivatives were
smmothly converted to corresponding products in cobalt porphyrin
catalyzed system with acetylacetone. Based on the results obtained
from experiments as well as in situ electron paramagnetic resonance
(EPR) and UV–vis spectroscopy, the mechanistic understanding the
role of acetylacetone towards in the activation of molecular oxygen
has been presented.
system, the liquid-phase oxidation of toluene has some advantages
such as high selectivity, mild reaction conditions.16-18 Many efforts
have been dedicated to the liquid phase oxidation of toluene using a
variety of catalysts such as iodine modified mesoporous Mg-Al mixed
1
9
20
oxides, modified HZSM-5 zeolites, hydrophilic ionic liquids with
2
1
22
23
x
cobalt naphthenate, MnO /SBA, iron-vanadium oxide etc.
In last decade, the aerobic oxidation of hydrocarbons catalyzed
by metalloporphyrins under mild conditions has attracted much
attention due to the efficiency and friendly conditions.
porphyrins presented high efficiency and selectivity for the aerobic
oxidation of toluene in homogeneous phase, in which the conversion
2
4-29
Cobalt
3
0
of toluene reached up to 8.9%. To achieve the reuse of catalyst,
metalloporphyrins immobilized on solid supports, metalloporphyrin
a. School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai,
519082, PR China.
Fine Chemical Industry Research Institute, School of Chemistry, Sun Yat-sen
University, Guangzhou 510275, PR China
School of Chemical Engineering, Guangdong University of Petrochemical
Technology, Maoming 525000, P. R. China
Results and discussion
Using toluene as model substrate, the reaction conditions were
optimized with metalloporphyrins as catalyst. The main products in
b.
c.
This journal is © The Royal Society of Chemistry 20xx
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the oxidation of toluene are benzyl alcohol (BAL), benzaldehyde (BA) out with either CoTPP or acetylacetone alone (entrVieiesw2ArtaicnledO3nlinine
and benzoic acid (BAC) (Table 1). In control experiments without the Table 1). This results also indicate that DaOceI:t1y0la.1c0e3t9o/nDe0NpJl0a1y5s75aJ
catalyst and acetylacetone, no obvious conversion was observed significant role in the oxygenation of C–H bond of toluene.
(entry 1 in Table 1). On the other hand, the reaction could be carried
Table 1. The oxidation of toluene catalyzed by metalloporphyrins with acetylacetonea
Amount
10 mmol)
Select. (%)
BAL
Entry
Catalyst
None
CoTPP
None
Solvent
Conv. (%)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
-3
BA
BAC
b
1
2
0
1.5
0
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMSO
HAc
-
-
-
-
b
6.2
7.6
11.9
10.2
14.6
10.1
10.5
5.2
9.7
14.4
7.8
4.1
3.0
2.7
o
35.3
1.8
12.3
5.9
10.2
13.2
8.5
11.4
10.4
10.5
10.9
12.8
6.6
52.4
92.3
58.8
54.5
61.4
52.6
54.9
44.9
52.2
48.3
36.8
48.3
38.3
71.0
3
4
5
6
7
8
9
MnTPPCl
FeTPPCl
CoTPP
ZnTPP
CuTPP
CoTPP
CoTPP
CoTPP
CoTPP
CoTPP
CoTPP
CoTPP
1.5
1.5
1.5
1.5
1.5
0.2
1.0
2.0
1.5
1.5
1.5
1.5
31.1
32.3
30.1
40.0
34.8
44.6
36.9
38.9
56.6
50.3
57.5
4.8
1
1
1
1
1
1
0
1
2
3
4
5
1.4
4.2
24.2
DCE
DMF
a
b
Toluene (10 mmol), acetylacetone (10 mmol), solvent (10 mL), O
In the absence of acetylacetone.
2
(1.0 MPa), 165 C, 4 h.
Table 2. Effect of initiator on toluene oxidation catalyzed by cobalt porphyrina
Select./%
BAL
Entry
1
Initiator
Conv./%
1.2
BA
BAC
O
O
40.3
30.8
10.5
8.5
28.9
57.2
61.4
EtO
OEt
O
O
2
3
4.6
32.3
30.1
OEt
O
O
O
O
14.6
4
5
6
11.2
20.6
21.6
28.7
37.5
12.3
9.4
59.0
53.4
66.7
O
O
O
O
O
25.5
7.8
OEt
a
-3
o
Toluene (10 mmol), CoTPP (1.5×10 mmol), initiator (10 mmol), MeCN (10 mL), 165 C, O
2
(1.0 MPa), 4 h.
The catalytic activity of metalloporphyrins for the aerobic metalloporphyrins on the aerobic oxidation of toluene was first
oxidation of toluene has been reported to be significantly affected by investigated. Catalytic performances of metalloporphyrins with
their chelated metal ions. Accordingly, the effect of different different metal ions combined with acetylacetone were examined.
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Comparing the catalytic activities of different metalloporphyrins conversion increased from 3.3% to 14.6%. However,Vnieow AsritgicnleifOicnalinnet
catalysts, it was found that cobalt porphyrin was the most effective increase was obtained with the continuaDl OinI:c1r0e.a10s3e9/oDf0NreJ0a1c5t7io5nJ
since the conversion of toluene reached up to 14.6% (entries 4~8). temperature. With increasing temperature, selectivity for benzoic
The catalytic activity of different catalysts is probably influenced by acid also increased all the time.
their electric potential and the stability of different valences of metal
4
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atom.
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100
Subsequently, the catalyst amount was examined for the aerobic
oxidation of toluene. Lower catalysts usage presented less effficiency
under the same reaction conditions (entries 9~10). But no significant
difference in effficiency was observed when the amount of catalyst
was increased (entry 11). The oxidation reactions were also studied
in other common organic solvents, such as DMSO, HAc, DCE and
DMF. However, the conversions were much lower than those in
MeCN (entries 12~15). Due to the solvent effect, more stable
acetylacetone radical could be obtained in MeCN solvent, thus
exhibited better activity for toluene oxidation.40 Meanwhile, the
selectivity of product also varied dramatically with different solvents.
For example, in DCE the selectivity of BA was up to 57.5 % (entry 14)
8
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BAC
60
1
0
5
0
4
2
0
0
0
BA
BAL
0
1
2
3
4
n
/n
ace sub
,
which was much higher than those in DMF (entry 15). It could be
Fig. 1 Effect of acetylacetone amount on the toluene oxidation.
ascribed to the quite different solubility of benzaldehyde and benzyl
alcohol in various solvents.
As mentioned above, acetylacetone plays key role in the
activation of molecular oxygen. According to our previous works, a
suitable initiator for free radical chain should satisfy the
-3
Toluene (10 mmol), CoTPP (1.5×10 mmol), MeCN (10 mL), O
2
(1.0
o
MPa), 165 C, 4 h.
20
100
80
3
9
requirements of facile dehydrogenation. As to acetylacetone, the
free radical is generated through the dehydeogenation of secondary
carbon. Therefore, the electronic effect and steric hindrance of
substrate groups in the structure of acetylacetone is important for
the generation rate of radical. The oxidations of toluene in presence
of acetylacetone and its derivatives were examined (Table 2).
As the two methyl groups of acetylacetone were replaced by
ethoxy repectively, the efficiency of the oxidation decreased sharply,
toluene was converted in only 1.2% conversion (entry 1, Table 2).
Compared with two ethoxy groups replacement, one ethoxy group
gave the sighly higher conversion of toluene was obtained (entry 2,
Table 2). Hence, There are both steric hindrance and electronic
effects for acetacetone that -positioin of carbonyl were substituted
by groups. When the position of secondary carbon of acetacetone
was replaced by other substituents, the effiency of toluene oxidation
has been greatly improved, whether it is the electron withdrawing
groups or electron-donating groups (entries 4~6). It indicates that
the electron effect of the initiator is more closely related to the
efficiency than that of steric hindrance.
From the viewpoints of the commercial feasibility and
efficiency, acetylacetone is the preferred initiator for the toluene
aerobic oxidation catalyzed by CoTPP. The effects of initiator amount
on toluene oxidation were investigated. As shown in Fig. 1, the
reaction rate increased with the increasing acetylacetone amount.
When the molar ratio between initiator and toluene was 1:1, 14.6%
conversion was obtained. However, as continual increasing in the
acetylacetone amount, no significant increased efficiency and
selectivity was obtained.
1
1
5
0
5
BAC
6
4
2
0
0
0
BA
BAL
0
1
0
30
140
150
160o
170
180
Temperature ( C)
Fig. 2 Effect of temperature on the aerobic toluene oxidation.
Toluene (10 mmol), CoTPP (1.5×10 mmol), acetylacetone (10
-3
2
mmol), MeCN (10 mL), O (1.0 MPa), 4 h.
20
100
80
60
40
20
0
1
5
BAC
BA
10
5
BAL
0
0
.4
0.6
0.8
1.0
1.2
1.4
1.6
Pressure (MPa)
Fig. 2 shows the effect of reaction temperature on the aerobic
oxidation of toluene in the presence of acetylacetone. As can be
clearly observed in Fig. 2, toluene conversion increased with the rised
temperature. Furthermore, with increasing reaction temperature
from 135°C to 165°C, the reaction rate significantly increased, where
Fig. 3 Effect of pressure on the aerobic toluene oxidation. Toluene
(10 mmol), CoTPP (1.5×10 mmol), acetylacetone (10 mmol), MeCN
-3
o
(10 mL), 165 C, 4 h.
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For establishing the optimum pressure for the aerobic oxidation
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of toluene, the effect of reaction pressure on the reaction was also
investigated, and Fig. 3 summarizes the results obtained. Toluene
conversion increased to the maximum at a molecular oxygen
pressure of 1.0 MPa, whereas conversion slightly declined with
increasing pressure of greater than 1.0 MPa. Selectivity for benzonic
acid was marginally affected by reaction pressure. Hence, the
optimal reaction pressure for the aerobic oxidation of toluene is 1.0
MPa.
With the optimized conditions in hand, the scope of different
toluene derivatives for the aerobic oxidation catalyzed by CoTPP with
acetylacetone as initiator was investigated as shown in Table 3.
A series of toluene derivatives with electron-donating and -
withdrawing groups on the phenyl ring were subjected to the
reaction, affording desired products (Table 3). This result is indicative
of good tolerance. para-Methoxytoluene (entry 1) exhibited better
conversion compared to the other derivatives bearing electro-
withdrawing groups (entry 1 vs entries 3~6). Moreover, in this
biomimetic catalytic system, it is interesting to find that aldehyde is
the main product for the substrates bearing electro-withdrawing
groups (entries 3~6).
The profile of cobalt porphyrin mediated aerobic oxidation of
toluene in presence of acetylacetone as initiator is shown in Fig. 4.
The conversion of toluene, as well as the yield of oxidized products,
slowly increased within the first 2 h. Then, the reaction rate rapidly
increased. An induction period was apparently observed for the
aerobic oxidation of toluene, which was consistent with reactions
involving radicals. For verifying the free-radical mechanism, 2,6-di-
tert-butylphenol (1 mmol) serving as the free-radical inhibitor, was Fig. 5 (a) Plot of the pseudo first-order rate constant (k) under
used. After the addition of this inhibitor, the oxidation was different reaction temperatures. (b) Arrhenius plot of rate constant
subsequently quenched. Therefore, the oxidation catalyzed by cobalt for the oxidation of toluene catalyzed cobalt porphyrins.
porphyrin should be involve radical species, which was generated
20
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DOI: 10.1039/D0NJ01575J
100
8
6
4
2
0
0
0
0
0
BAC
BA
1
0
5
0
0
1
2
3
4
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0
1
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0
1
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0
1
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
BAL
0
60 120 180 240 300 360
Time (min)
Fig. 4 Reaction profile for the oxidation of toluene in the presence of
-3
acetylacetone. Toluene (10 mmol), CoTPP (1.5×10 mmol),
o
acetylacetone (10 mmol), MeCN (10 mL), O
2
(1.0 MPa), 165 C.
from acetylacetone. In addtion, the selectivity towards bezonic acid
is always increasing, accompanied by the declined selectivitiy of spectrum every 10 min. As shown in Fig. 6, as the reaction
benzyl alcohol.
proceeded, the Soret band gradually decreased at 412nm,
To explore the kinetic behavior over the aerobic oxidation of accompanied with the increase of the peak at 430 nm. Such changes
tulene catalyzed by cobalt porphyrin in the presence of in the in situ UV–vis spectra have been reported to be attributed to
acetylacetone, factors influencing the rate of oxidation have been the generation of the high-valent cobalt porphyrins during oxidation.
studied. The reaction rate of the catalytic oxidation was determined
through the consumption of toluene analyzed by GC-MS and GC. The
first-order rate constant was proportional to the substrate
concentration at different temperature. The obtained rate constants
were presented in Fig. 5.
The spectrophotometer was programmed for acquiring UV–vis
4
3-44
The present oxidation study was carried out in the range (428-
4
43K). The rate constants apparently increased with the rising
reaction temperature (Fig. 5a). The plot of lnk vs 1/T was a straight
line. From Arrhenius plot, the values of energy of activation (E ) were
a
calculated. The calculated activation energy from the slope was 97.0
kJ/mol.
As has been previously reported, the mechanism of
metalloporphyrin-catalyzed oxidation involves the generation of
high-valence species. For confirming the formation of a high-valent
cobalt porphyrin intermediate, in situ UV–vis spectra were recorded
on
a
spectrophotometer connected to an AvaSpec-2048
spectrometer under the given reaction conditions. Figure 6 shows Fig. 6 In-situ UV-Vis spectra of aerobic oxidation of toluene. Toluene
the in situ UV–vis spectra of cobalt porphyrins during the reaction.
-
3
(5 mmol), CoTPP (1.0×10 mmol), acetylacetone (8 mmol), MeCN (10
o
mL), O
2
(1.2 MPa) at 165 C, and monitored at interval 10 min.
4
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Table 3. Aerobic oxidation of toluene and its derivatives with CoTPP and acetylacetonea
CHO
COOH
CoTPP, acetylacetone,
o
O , MeCN, 4h, 165 C
2
R
R
R
1
2
3
Entry
Substrate
Main product
Conv. (%) b
Select. (%) b
1
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4
5
6
R = MeO (1a)
R = H (1b)
R = F (1c)
R = Cl (1d)
R = Br (1e)
3a
3b
2c
2d
2e
2f
18.6
14.6
12.7
13.5
7.8
65.6
61.4
81.9
85.9
80.8
96.9
R = NO
2
(1f)
6.4
a
-3
o
2
Substrate (10 mmol), acetylacetone (10 mmol), CoTPP (1.5×10 mmol), MeCN (10 mL), O (1.0 MPa), 165 C, 4 h.
For obtaining further insights into the role of acetylacetone, in
situ EPR spectra for the oxidation of acetylacetone were recorded
during the reaction. Aerobic catalytic oxidation in CH CN was
performed using a Wilmad WG-810-A quartz-flat cell using a mixture
of the CH CN solution of acetylacetone and DMPO (free-radical spin-
3
3
trapping agent). The cell was sealed after filling dioxygen. EPR
analyses were performed at a microwave frequency of 9.05 GHz at
23 K. Stable DMPO radical adducts were confirmed via the
4
comparison of hyperfine splitting constants.
Fig. 7 shows the results obtained from the in situ EPR of the
oxidation of acetylacetone at different reaction temperature. As has
been reported previously, in the cyclohexene-mediated oxidation of
toluene, the α-hydrogen atom of cyclohexene was easily abstracted
under high temperature, affording an allylic radical.40 As shown in
Fig. 7, the only stable DMPO radical adduct was obtained when
acetylacetone was immersed in dioxygen. The six lines shown in
Fig. 7 In-situ EPR spectra of aerobic acetylacetone oxidation at
different temperatures. a) In the absence of cobalt porphyrins. b) In
the presence of cobalt porphyrins.
Based on the above mentioned discussions, a plausible
mechanism was proposed for the aerobic oxidation of toluene in the
presence of cobalt porphyrins and acetylacetone as the radical
initiator, as shown in Fig. 8.
Figure 7 are a typical signal of acetylacetone radical (a
H() = 22.62 G). With the increase of reaction temperature, the
N
= 14.77 G,
a
The reaction was proposed to be initiated from the CoTPP-
promoted dehydrogenation of acetylacetone, companied the
formation of cobalt porphyrins peroxide. Acetylacetone radical then
reacted with dioxygen to give an acetylacetone peroxyl radical. And
high-valent species was generated by the reaction between
acetylacetone peroxyl radical and cobalt porphyrins. Benzyl radicals
was generated from the dehydrogenation of toluene by this high-
valent species. The redox cycle of catalyst was achieved by the
reaction of the cobalt porphyrins peroxide with benzyl radicals,
followed to form corresponding oxidized product.
intensity of characteristic peak is also increasing. The signal of
acetylacetone radical was not observed clearly at room temperature
in absence of cobalt porphyrins catalyst (Fig. 7a). However, the
generation rate of acetylacetone radical was enhanced obviously in
the presence of cobalt porphyrins catalyst (Fig. 7b).
Peroxyl radical roles as active oxy radical in the catalytic
oxidation. In the catalyst control experiment, 7.6% conversion was
obtained in the presence of acetylacetone alone (entry 3 in Table 1).
Therfore, it indicates that acetylacetone peroxyl radical acts as the
active species, could initiate the generation of the benzyl radical. And
in presence of cobalt porphyrins, high-valent cobalt porphyrins
species could be generated by the combination between CoTPP and
acetylacetone peroxyl radical.
This journal is © The Royal Society of Chemistry 20xx
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OO
This work was financially supported by the National Key Research
O2
View Article Online
PorCo(II)
PorCo(III)
DOI: 10.1039/D0NJ01575J
and Development Program of China (2016YFA0602900), the National
O
O
O
O
Natural Science Foundation of China (No. 21938001 and 21878344),
the National Natural Science Foundation of China-SINOPEC Joint
Fund (No. U1663220), Guangdong Provincial Key R&D Programme
(2019B110206002), the Local Innovative and Research Teams Project
of Guangdong Pearl River Talents Program (2017BT01C102),
Research and Innovation Team Construction Project of Guangdong
University of Petrochemical Technology
O2
OOH
PorCo(III)
O
O
CH2OH
CH2
OO
PorCo(II)
PorCo(II)
O
O
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Por(III)Co OO
Notes and references
Fig. 8 Plausible mechanism for CoTPP-catalyzed oxidation of toluene
in presence of acetylacetone.
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View Article Online
DOI: 10.1039/D0NJ01575J
Graphical Abstract
Acetylacetone as oxygen activator to improve efficiency for the
aerobic oxidation of toluene and its derivatives by cobalt meso-
tetraphenylprophyrin
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a
b
a
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b,c*
Xian-Tai Zhou , Hong-Yu Chen , Qi Han , Meng Lv , Hong-Bing Ji
aSchool of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai,
519082, PR China
bFine Chemical Industry Research Institute, School of Chemistry, Sun Yat-sen University,
Guangzhou 510275, PR China
cSchool of Chemical Engineering, Guangdong University of Petrochemical Technology,
Maoming 525000, P. R. China
*Corresponding author. Tel.: +86-20-84113653; fax: +86-20-84113654.
E-mail: jihb@mail.sysu.edu.cn (H. B. Ji)
An efficient system comprising acetylacetone and cobalt tetraphenylporphyrin was
developed for the aerobic oxidation of toluene and its derivatives, in which acetylacetone
served as the key inititor of free radical in activating dioxygen.
1