Synergistic effect of co-reactant promotes one-step oxidation of cyclohexane into adipic acid catalyzed by manganese porphyrins

Article abstract of DOI:10.1139/cjc-2014-0515

The synergistic effect of cyclohexane and cyclohexanone promoted synthesis of adipic acid catalyzed by [Mn^IIIT(p-Cl)PP]Cl with cyclohexane and cyclohexanone as co-reactants. The results showed that the conversions of cyclohexane and cyclohexanone were significantly enhanced because of the cyclohexanone synergistic effect, and the higher selectivity to adipic acid was obtained with dioxygen as an oxidant. The studies indicated that the co-oxidation of cyclohexane and cyclohexanone was influenced by the initial molar ratio of cyclohexanone and cyclohexane, catalyst structure, catalyst concentrations, and reaction conditions. The preliminary mechanism of the co-oxidation reaction of cyclohexane and cyclohexanone using [Mn^IIIT(p-Cl)PP]Cl as the catalyst was proposed.

Full text of DOI:10.1139/cjc-2014-0515

Page 1 of 20
Synergistic effect of co-reactant promotes one-step oxidation of cyclohexane
into adipic acid catalyzed by manganese porphyrins
1
Hui Li , Yuanbin She
2, 1, *
3,*
, Haiyan Fu , Meijuan Cao , Jing Wang and Tao Wang
1
4
1
1
Institute of Green Chemistry & Fine Chemical, College of Environmental & Energy Engineering, Beijing
University of Technology,Beijing100124,PR China
2
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, PR
China
3
School of Pharmaceutical Sciences, South-Central University for Nationalities, Wuhan, Hubei 430074,
PR China
4
Huizhou Research Institute of Sun Yat-sen University, Huizhou, Guangdong 516081, PR China
*
Corresponding authors:
Prof. Dr.Yuanbin She. Tel/Fax: +86 – 0571 –88320533. E-mail: sheyb@zjut.edu.cn.
Dr. Haiyan Fu, Associate Professor. Tel/Fax: +86 – 27 –67843352. E-mail: fuhaiyan@mail.scuec.edu.cn.
1

Page 2 of 20
Abstract: The synergistic effect of cyclohexane and cyclohexanone promoted synthesis of adipic acid
III
catalyzed by [Mn T(p-Cl)PP]Cl with cyclohexane and cyclohexanone as co-reactants. The results showed
that the conversions of cyclohexane and cyclohexanone were significantly enhanced because of the
cyclohexanone synergistic effect, and the higher selectivity to adipic acid was obtained with dioxygen as an
oxidant. The studies indicated that the co-oxidation of cyclohexane and cyclohexanone was influenced by
the initial molar ratio of cyclohexanone and cyclohexane, catalyst structure, catalyst concentrations and
reaction conditions. The preliminary mechanism of the co-oxidation reaction of cyclohexane and
III
cyclohexanone using [Mn T(p-Cl)PP]Cl as the catalyst was proposed.
Key words: Synergistic Effect, Synthesis, Adipic Acid, Cyclohexane and Cyclohexanone,
III
Mn T(p-Cl)PP]Cl
[
1
Introduction
The adipic acid (AA) has proven to be an important synthon for a variety of useful compounds such as
1
, 2
polyamides (Nylon-6,6) and urethane. Currently, the industrial processes rely on two steps: (1) catalytic
oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone (KA oil) by air using cobalt salts
as catalysts, (2) oxidation of KA oil to AA with excessive nitric acid. However, this method has several
drawbacks such as very low conversions (4% to 8%) in the cobalt-catalyzed oxidation of cyclohexane and
the formation of a large amount of the greenhouse gas nitrous oxide (N O) which needs to be recovered and
2
2
,
2 2 2
reused or be converted to Nitrogen (N ) and oxygen (O ) through N O abatement units in industrial plants.
3
Therefore, the development of a green route for the synthesis of AA is highly desirable.
The one-step oxidation of cyclohexane into AA using environmentally friendly air, or molecular oxygen
4
as an oxidant is a green strategy considering the economic and environmental perspectives. Many reports
-9
have been documented for the one-step oxidation of cyclohexane into AA, in which various catalysts were
2

Page 3 of 20
4
applied such as some metal species with isoamyl nitrite in homogeneous system, Nano-Gold and iron
5
6
substituted microporous aluminium phosphates in heterogeneous system. Metalloporphyrins are known to
10-16
2
efficiently activate O and show good performance in the catalyzed oxidation of hydrocarbon,
17, 18
especially for aerobic oxidation of cyclohexane to AA.
The direct aerobic oxidation of cyclohexane to
III
AA catalyzed by Fe T(o-Cl)PP was firstly reported by our group, using molecular oxygen as an oxidant
17
18
without any solvents and promoters. Later, Guo and co-workers reported aerobic oxidation of
III III
cyclohexane to AA using cobalt isooctanoate, Co T(p-Cl)PP or [Fe T(p-Cl)PP] O as catalysts.
2
III
2
Fe T(p-Cl)PP] O gave the best results in selectivity of KA oil and AA. Although some progresses have
[
been made in one-step oxidation of cyclohexane into AA with metalloporphyrins as catalysts, there still
remains great challenge in the field. The selectivity of AA remains unsatisfactory and high temperature is
necessary to initiate a free-radical process in the system with metalloporphyrins as catalysts.
19-21
Weng and coworkers
reported that cyclohexanone as a co-reactant could reduce the induction time of
cyclohexane oxidation and increase the reaction rate of the liquid-phase oxidation of cyclohexane to AA
catalyzed by Co-type weak acid resin (containing 21.25 wt% cobalt ion), CoAPO-5 or supported cerium
22
oxide. Meanwhile, Hermans et al. certified that cyclohexanone could enhance the rate of oxidation of
cyclohexane to AA using complementary experimental and theoretical techniques. In addition,
cyclohexanone is one of the intermediates of cyclohexane oxidation. Therefore, cyclohexanone as a
co-reactant for oxidation of cyclohexane into AA is an attractive strategy. To the best of our knowledge, no
example has been reported for cyclohexanone as co-reactant in oxidation of cyclohexane into AA catalyzed
by metalloporphyrins. Herein, we report an aerobic co-oxidation system of cyclohexane and cyclohexanone
III
in the presence of 5,10,15,20-tetra(4-chlorophenyl)porphyrin manganese(III) chloride ([Mn T(p-Cl)PP]Cl).
The influence of reaction conditions on the synergistic effect as well as the preliminary mechanism of this
3

Page 4 of 20
co-oxidation system was discussed.
2
2
Experimental sections
.1 Instruments and reagents
The starting materials purchased from commercial sources are of analytical grade and used without
further treatment unless indicated. The metalloporphyrins catalysts were prepared according to previously
23-25
published procedures
, and their structures were characterized by UV-Vis, IR, and elementary analysis.
2
.2 General procedures for co-oxidation
The co-oxidation of cyclohexane and cyclohexanone with oxygen as an oxidant catalyzed by
metalloporphyrins was conducted as follows: cyclohexane, cyclohexanone, and metalloporphyrins were
charged into a 50 mL Parr autoclave equipped with a mechanical stirrer and a temperature-controlling
device. After the reactor was flushed with O several times, the mixture was heated to 120 °C. The reactor
2
2
was then charged with O , and the reaction system pressure was maintained at 1.4 MPa. The mixture was
stirred at 120 °C for 5 h. The reactor was cooled to room temperature and the reaction mixture was dissolve
with acetone after reaction. The remaining reactants were analyzed by GC (Agilent HP-5MS) with
methylbenzene as an internal standard. The oxidized products were monitored by GC (Agilent HP-5MS)
and GC-MS after these products were derived by silylation derivatives (BSTFA: TMCS = 99: 1);
2
3
3
-ethylhexanoic acid was used as an internal standard.
Results and discussion
III
.1 Aerobic co-oxidation of cyclohexane and cyclohexanone over [Mn T(p-Cl)PP]Cl
III
Scheme 1 Co-oxidation of cyclohexane and cyclohexanone with dioxygen catalyzed by [Mn T(p-Cl)PP]Cl
4

Page 5 of 20
2
First of all, the aerobic co-oxidation of cyclohexane (1) and cyclohexanone (2) by O in the presence of
III
Mn T(p-Cl)PP]Cl without any solvents has been investigated. The results are summarized in Table 1. The
[
main oxidation products (Scheme1) were AA (3), glutaric acid (GA, 4) and cyclohexanol (5).
III
Table 1 Aerobic co-oxidation of cyclohexane and cyclohexanone catalyzed by [Mn T(p-Cl)PP]Cl
a
Entry 1
2
Initial molar
percentage of 2
0
Conv. (%)
1
Selec. (%)
3
b
(
mmol) (mmol)
2
4
5
Others
1
2
3
4
5
6
7
8
9
1
189
0
189
2
1.6
—
18.8
55.7
48.1
50.1
61.0
65.1
67.5
68.1
68.3
67.9
3.6
32.8
—
5.9
0
100%
1%
—
14.7
0.8
12.1
4.8
2
32.2
13.9
15.4
19.4
18.7
16.0
16.3
16.1
16.4
187
2.1
33.2
32.4
14.3
11.5
10.8
9.6
170
19
10%
5.1
1.9
140.7
116.1
104.9
92.5
84.8
77.1
48.3
26%
10.4
11.9
13.7
18.9
19.0
19.3
16.2
19.6
22.5
26.9
27.1
27.2
5.3
4.8
5.7
6.0
6.4
6.8
72.9
84.1
38%
45%
96.5
51%
104.2
111.9
55%
9.2
0
59%
8.9
a
b
III
-4
2
Reaction conditions: [Mn T(p-Cl)PP]Cl (5×10 mol%), 120 ºC, O Pressure 1.4 MPa for 5 h.
Others include cyclohexylhydroperoxide, 2-hydroxycyclohexanone, cyclohexe-2-one and succinic acid, etc.
Table 1 shows that the oxidation of cyclohexane without cyclohexanone almost did not occur (entry 1).
III
The conversion of cyclohexanone was 14.7% when [Mn T(p-Cl)PP]Cl was used as the catalyst and AA
19, 20
was the main product (entry 2). According to previous reports
, cyclohexanone as a co-reactant could
reduce the induction time of cyclohexane oxidation and increase the reaction rate. Consequently, we
5

Page 6 of 20
decided to introduce cyclohexanone as a co-reactant for one-step oxidation of cyclohexane into AA. To our
delight, as expected, when an initial molar ratio of 48.3/140.7 of cyclohexanone and cyclohexane was used
as a reactant, the conversion of cyclohexane reached 10.4%， which is a remarkable enhancement
compared with cyclohexane alone as a reactant (entry 1). In addition, the conversion of cyclohexanone has
also been raised in the catalytic co-oxidation system. As the initial molar percentage of cyclohexanone
increased from 1% to 51%, the conversion of cyclohexane and cyclohexanone increased (entries 3–8).
Hence, cyclohexane conversion increased gradually as the molar percentage of cyclohexanone in the
mixture increased. These results indicated that an increase in the molar percentage of cyclohexanone could
effectively increase the oxidation rate of cyclohexane. Cyclohexanone could generate free radicals more
easily than cyclohexane which could be attributed to the smaller bond dissociation energies (BDEs) for
22
α-C-H bonds about 9 kcal/mol than C-H bonds in cyclohexane. Therefore, the addition of cyclohexanone
could increase the free-radical concentration. However, the increase of the molar percentage of cyclohexane
in the mixture consumed more free radicals that generated from cyclohexanone and reduced the oxidation
rate of cyclohexanone. When the initial molar percentage of cyclohexanone was 26%, the selectivity of AA
had already reached 61%. However, the selectivity of AA had almost no increase when the initial molar
percentage of cyclohexanone was continued to increase. It is easy to find that this co-oxidation system
could promote the selective formation of AA and restrict the generation of GA and other undesired
products.
The initial molar percentage of cyclohexanone significantly affected the induction time. Fig. 1 shows
that the induction time decreased gradually as the molar percentage of cyclohexanone increased. Induction
time reached approximately 5 min when the single cyclohexanone was the reactant.
6

Page 7 of 20
a
Fig.1 Effects of the initial molar percentage of cyclohexanone on induction time
a
The induction time is the time interval which is from initial reaction to conversion of cyclohexane. The reactants are
monitored by GC through about every 1 minute sampling.
3
.2 Effect of different catalyst and catalyst amounts on the co-oxidation of cyclohexane and
cyclohexanone
The promising result encouraged us to do further optimization on the catalyst. The effect of different
metalloporphyrins on the aerobic co-oxidation of cyclohexane and cyclohexanone was explored. The
results were listed in Table 2. Firstly, the metalloporphyrins with different central metal ions were screened.
It clearly suggested that the nature of central metal ions had a great effect on the catalytic activity of
metalloporphyrins. Compared with cobalt and iron porphyrin with the same ligand T(p-Cl)PP, manganese
porphyrin was the most effective catalyst in the co-oxidation of cyclohexane and cyclohexanone to AA;
Not only high catalytic activity but also good selectivity for AA was obtained for the converts of
cyclohexane and cyclohexanone (entries 1-3). Compared to manganese porphyrin, cobalt porphyrin and
iron porphyrin were slightly less efficient for the co-oxidation of cyclohexane and cyclohexanone. The
reason might be that the reaction pathway has changed and the activation energy of reaction has reduced in
the co-oxidation system. The catalytic activity of the different metalloporphyrins might depend on the
7

Page 8 of 20
stability of different valences of central metal atoms which were probably influenced by their electric
26
potential of the oxidation-reduction pair. The catalytic activity of manganese porphyrins in the
co-oxidation of cyclohexane and cyclohexanone was also related to the peripheral substituent in the
2
porphyrin macro ring. The manganese porphyrin with stronger electron-withdrawing group p-NO gave
lower conversions of cyclohexane and cyclohexanone than group p-Cl (entry 4). The special asymmetry
3 2
A B-type metalloporphyrin with electron-withdrawing groups p-Cl and p-NO did not show higher
catalytic activity than A4 type metalloporphyrins (entry 5). It is not difficult to find that using
III
Mn T(p-Cl)PP]Cl as catalyst gave the good result.
[
a
Table 2 Effect of different structural metalloporphyrins on the co-oxidation of cyclohexane and cyclohexanone
Conv. (%)
1
Select. (%)
3
Entry
Catalyst
2
4
5
Others
19.4
11.8
III
[Mn T(
1
2
3
4
5
p
-Cl)PP]Cl
-Cl)PP]Cl
-Cl)PP]Cl
10.4
11.2
8.8
16.2
15.8
14.3
14.9
15.2
61.0
5.3
14.3
11.4
19.0
13.7
14.7
III
[Co T(
p
56.6
20.2
8.8
III
[Fe T(
p
57.8
14.4
15.5
14.0
III
[Mn T(
p-NO
2
)PP]Cl
9.5
61.6
9.2
III
[Mn (
p-Cl)
3
P(
p-NO )P]Cl
2
9.1
60.1
11.3
a
-4
Reaction conditions: Catalyst (5 × 10 mol%), Cyclohexane (140.7 mmol), Cyclohexanone (48.3 mmol),120 ºC, O
2
Pressure 1.4 MPa for 5 h.
It is well known that the concentration of catalyst has also influenced the co-oxidation of cyclohexane
and cyclohexanone (Fig. 2). The conversions of cyclohexane and cyclohexanone were 3.4% and 6.1%
respectively in the absence of metalloporphyrins. To our surprise, the conversions of cyclohexane and
cyclohexanone were both considerably enhanced and the increased selectivity for AA was also obtained
8

Page 9 of 20
-4
when the catalyst amount was only 1× 10 mol%. Along with the increase of catalyst concentration, the
conversions of cyclohexane and cyclohexanone and the selectivity of AA were also raised. The conversions
-4
III
of cyclohexane and cyclohexanone reached the highest with 5 × 10 mol% [Mn T(p-Cl)PP]Cl. However,
the further increase of catalyst amount could decrease the conversions of cyclohexane and cyclohexanone.
This decreased catalytic efficiency of metalloporphyrins might result from the aggregation of
2
7
metalloporphyrins through π–π interaction.
Fig. 2 Effect of catalyst amounts on the co-oxidation of cyclohexane and cyclohexanone.
2
Reaction conditions: Cyclohexane (140.7 mmol), Cyclohexanone (48.3 mmol), 120 ºC, O Pressure 1.4 MPa for 5 h.
3
3
.3 The effect of different reaction conditions on the co-oxidation of cyclohexane and cyclohexanone
.3.1 Effect of reaction time
Fig. 3 shows the conversions of cyclohexane and cyclohexanone and the yield of different oxidized
product distribution with the reaction time for the aerobic co-oxidation of cyclohexane and cyclohexanone
III
over [Mn T(p-Cl)PP]Cl. The conversions of cyclohexane and cyclohexanone and the yields of oxidized
products both increased slowly within the first 1 h. Actually, there existed the induction time of about 17
min in the initiation period, which was required for the free-radical concentration to reach a minimum
2
critical value for initiation. During this period, reaction pressure kept constant and almost no products
8
were generated. After 1 h, the conversions of both cyclohexane and cyclohexanone and the yield of AA
9

Page 10 of 20
sharply increased until 5 h. The yield of cyclohexanol increased slowly in the initial 2 h, then began to
reduce slightly. This phenomenon indicated that cyclohexanol could be converted to other products in the
catalytic co-oxidation system. To verify this assumption, the aerobic oxidation of cyclohexanol in the
absence and presence of cyclohexanone over metalloporphyrins under the same reaction condition was
investigated. When only cyclohexanol was used as substrate, trace amount of cyclohexanone was detected.
And when cyclohexanol and cyclohexanone were simultaneously applied as substrate in the oxidation
system, a small amount of cyclohexanol was converted. However, the oxidation of cyclohexanone was
inhibited deeply. Therefore, there might be existed a reaction pathway, in which a little amount of
cyclohexanol as an intermediate was converted to other products. Meanwhile, a small amount of
cyclohexanol generated from cyclohexane was an inhibitor to the co-oxidation of cyclohexanone and
29
cyclohexane in a solvent-free system, which was consistent with the previous report.
Fig. 3 Effect of reaction time on the conversions and production distribution.
III
Reaction conditions: [Mn T(p-Cl)PP]Cl(5 × 10 mol%), Cyclohexane(140.7 mmol), Cyclohexanone (48.3 mmol), 120 ºC,
-4
2
O (1.4 MPa)
1
0

Page 11 of 20
3
.3.2 Effect of reaction temperature
Fig. 4 Effect of reaction temperature on the conversions and production distribution.
III
Reaction conditions: [Mn T(
-4
-Cl)PP]Cl (5 × 10 mol%), Cyclohexane(140.7 mmol), Cyclohexanone (48.3 mmol), O
p
2
Pressure 1.4 MPa for 5 h.
The effect of reaction temperature on the co-oxidation of cyclohexane and cyclohexanone was
investigated and the results are shown in Fig. 4. At 100 °C, only 0.6 % cyclohexane and 0.8 %
cyclohexanone were converted. The conversions of cyclohexane and cyclohexanone were significantly
enhanced when the temperature was increased from 100 °C to 120 °C. This finding indicated that high
temperatures favoured the conversion of cyclohexane and cyclohexanone. When the temperature exceeded
120 °C, the conversions of cyclohexane and cyclohexanone increased slightly. This phenomenon might be
30
attributed to the oxidative destruction of catalysts at the higher temperatures. Evident increase in the
selectivity of AA was observed when the reaction temperature was increased from 100 °C to 120 °C. By
contrast, the selectivity of AA decreased when temperature outstripped 120 °C. The reason might be the
increasing selectivity toward undesired products.
3
.3.3 Effect of reaction pressure
Fig.5 shows the effect of reaction pressure on the co-oxidation of cyclohexane and cyclohexanone. The
conversions of cyclohexane and cyclohexanone increased significantly with the rising pressure until 1.4
11

Page 12 of 20
MPa, no remarkable increase was observed beyond 1.4 MPa. Similar to high temperature, the high reaction
pressure also did not promote the selectivity of AA. When the pressure exceeded 1.4 MPa, more undesired
products were produced. The reason might be that acid was oxidized further to form other byproducts under
higher oxygen partial pressure. Therefore, the reaction pressure controlled at 1.4 MPa was optimized for
the co-oxidation of cyclohexane and cyclohexanone.
Fig. 5 Effect of reaction pressure on the conversions and production distribution.
III
Reaction conditions: [Mn T(p-Cl)PP]Cl (5 × 10 mol%), Cyclohexane(140.7 mmol), Cyclohexanone (48.3 mmol), 120 ºC,
-4
5
h
4
. Plausible mechanism for the co-oxidation of cyclohexane and cyclohexanone over
metalloporphyrins
The reaction mechanism of aerobic oxidation of cyclohexane over metalloporphyrins is still not well
26, 31, 32
established and several pathways have been investigated.
Here, the mechanism of co-oxidation of
III
cyclohexane and cyclohexanone to AA using [Mn T(p-Cl)PP]Cl as catalyst was discussed.
The induction period was observed for the aerobic co-oxidation of cyclohexane and cyclohexanone over
III
Mn T(p-Cl)PP]Cl which was consistent with reactions proceeding via a free-radical mechanism. To verify
[
the free-radical mechanism of the catalytic system, 2,6-di-tertbutyl-p-cresol as a free-radical inhibitor was
added to the oxidation system. As expected, the reaction could be subsequently quenched.
1
2

Page 13 of 20
2
As the mimics of the cytochrome P-450, iron porphyrins are known that could activate O to produce a
IV
high valent metal-oxo species (Fe =O) that then oxidizes substrates through a homolytical cleavage of
3
3-35
C-H bond.
The formation mechanism of this metal-oxo species is generally considered via the
formation of a di-iron peroxide intermediates of the type (Por)Fe(III)-O
2
-Fe(III)(Por) and then the O-O
3
bond cleavage thermally, which have been demonstrated by both experiment evidence
6
37-42
and theoretical
43
calculation. Synthetic manganese porphyrins have also been extensively used as P-450 models in
oxidation reactions. Similarly, manganese porphyrins can also activate molecular oxygen and generate
34, 44-46
reactive (Por)Mn(V)=O complex (eq. [1]).
In addition, manganese(V)-oxo porphyrin complexes have
4
5, 47
48
and stopped-flow spectrophotometry . Therefore,
been characterized by X-ray absorption spectroscopy
V
(T(p-Cl)PP)Mn =O] can be considered as active species in the catalytic co-oxidation system of
+
[
cyclohexane and cyclohexanone. In our reaction system, only trace products were detected when
III
cyclohexane was oxidized alone with O by [Mn T(p-Cl)PP]Cl at 120 ºC for 5h. Therefore, the active
2
V
+
species [(T(p-Cl)PP)Mn =O] may initially capture weaker α-H from cyclohexanone to form free radicals
R•-αH=O not C-H bonds in cyclohexane (eq.[2]). Then these free radicals generated from cyclohexanone
rapidly initiated cyclohexyl radicals (R•) from cyclohexane by hydrogen-atom transfer (HAT) (eq.[3])
32
Holger et al reported that AA came from further oxidation of cyclohexanol. The mechanism was as
following. Firstly, the radicals recombine between the initiating cyclohexyl radical (R•) and the OH group
on iron porphyrins complexes to provide cyclohexanol. Then the hydroxylation of cyclohexanol produces
cyclohexane-1,2-diol which goes through C-C bond cleavage to produce AA. To elucidate the role of
cyclohexanol, it was used as reactant under our reaction condition. The result showed that cyclohexanol
was hardly oxidized to cyclohexane-1,2-diol or cyclohexanone by dioxygen as oxidant in our catalyzed
49
50
system, which was different from the results using hydrogen peroxide or t-butyl hydroperoxide as an
1
3

Page 14 of 20
oxidant. Cyclohexanol was even an inhibitor for cyclohexane oxidation in the solvent-free catalyzed system
29
studied here. Therefore, cyclohexanone, AA and other products were afforded through a Haber-Weiss
31, 51
cycle and β C-C cleavage but not the further oxidation of cyclohexanol.
Based on the experimental phenomena, a plausible mechanism was proposed for the co-oxidation of
cyclohexane and cyclohexanone in the presence of Mn porphyrins. In the catalytic co-oxidation system of
cyclohexane and cyclohexanone, high-valence porphyrin intermediates firstly captured α-H from the
•
easily oxidized cyclohexanone to form the free radical R -αH=O (eq.[2]), which is different from oxidation
of cyclohexane alone. The free radical generated from cyclohexanone could rapidly abstract H from
•
cyclohexane to form cyclohexyl radical (R ) (eq.[3]), which advanced the formation rate of the cyclohexyl
•
•
2
radical. The R -αH=O and R trapped O to generated peroxide free radicals which could initiate crossly
more free radicals from cyclohexane and cyclohexanone (eqs.[4]-[9]). The rapidly increase of total
free-radical concentration promoted the propagation of free-radical reactions of both cyclohexane and
52
cyclohexanone. Similar to the thermal autoxidation of cyclohexane, the oxidation of cyclohexane and
α
cyclohexanone in the presence of metalloporphyrin catalysts produced ROOH and R=O(OOH) as the
α
respective initial intermediates. The formed ROOH and R=O(OOH) could be immediately decomposed
51, 53
to ketone and alcohol by Mn porphyrins similarly to Fe porphyrins (eqs.[10]-[14]).
Meanwhile, this
•
reaction also produced RO and R=O(O) . In addition, solid complementary experimental and theoretical
•
α
evidence supports that ketone could also enhance ROOH dissociation by concerted H abstraction to form
•
RO and R -αH=O (eqs. [15] and [16]). RO and R=O(O)
•
22
•
•
α
could react with the substrates (cyclohexane
•
and cyclohexanone) (eqs. [17] and [18]) and ring-open to generate O=CH(CH
2
)
4
CH
2
and
•
54-56
O=CH(CH
)
2 4
C (=O) via β C-C cleavage (eqs. [19] and [20]).
These radicals were further oxidized to
produce AA and other decarboxylated byproducts.
1
4

Page 15 of 20
[
1]
[
[
2]
3]
[
[
[
[
[
[
4]
5]
6]
7]
8]
9]
[
10]
11]
[
[
12]
13]
[
[
14]
15]
[
[16]
[17]
[18]
[19]
[20]
1
5

Page 16 of 20
4
. Conclusions
This work demonstrated the aerobic co-oxidation of cyclohexane and cyclohexanone to AA over
III
Mn T(p-Cl)PP]Cl in the absence of any additives and solvents. The oxidation of cyclohexane and
[
cyclohexanone was enhanced when cyclohexanone was used as a co-reactant. Not only the conversions of
cyclohexane and cyclohexanone were significantly enhanced, but also the higher selectivity to AA was
obtained. Cyclohexanone reduced significantly the induction time of the co-oxidation of cyclohexane and
cyclohexanone to AA. The effect of reaction pressure, reaction temperature, and initial molar ratio of
cyclohexanone and cyclohexane on the co-oxidation system was investigated. Cyclohexanone was
considered as an initiator that rapidly generated free radicals in the presence of metalloporphyrins, which
enhanced cyclohexane oxidation. Further studies on the mechanism are currently underway and will be
reported in due course.
Acknowledgments
This work was supported by the State Key Program, General and Youth Projects of National Natural
Science of China (Grant No. 21036009, 21476270, 21276006, 21076004, 21205145) and the Open Funds
of State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology of Zhejiang University
of Technology (No. GCTKF2014003, GCTKF2014007).
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