Catalytic oxidation of cyclohexane with dioxygen over boehmite supported trans-A2B2 type metalloporphyrins catalyst

Article abstract of DOI:10.1016/j.molcata.2014.02.002

Metalloporphyrin-catalysed direct functionalization of saturated CH bonds is one among the topical and challenging areas within the chemical industry. In this work, three types of unsupported and boehmite supported trans-A ₂B₂-metalloporphyrin chloride complexes (Co-D(p-Cl)PPCl, Fe-D(p-Cl)PPCl, Mn-D(p-Cl)PPCl) were prepared and characterized. It has been found that these trans-A₂B₂-metalloporphyrins have high catalytic activity in cyclohexane oxidation under dioxygen. The boehmite supported metalloporphyrins show better catalytic performance and are more stable than unsupported metalloporphyrins. Among these supported metalloporphyrin catalysts, Co-D(p-Cl)PPCl/BM was better than Fe-D(p-Cl)PPCl/BM and Mn-D(p-Cl)PPCl/BM, the cyclohexane conversion and selectivity to KA oil are 6.83% and 83.30%, respectively, and the turnover number is 1.05 × 10 ⁷. The Co-D(p-Cl)PPCl/BM catalyst can be facilely recovered and was recycled up to six times without significant decrease in catalytic performance.

Full text of DOI:10.1016/j.molcata.2014.02.002

Journal of Molecular Catalysis A: Chemical 386 (2014) 95–100
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic oxidation of cyclohexane with dioxygen over boehmite
supported trans-A2B2 type metalloporphyrins catalyst
Yujia Xie, Fengyong Zhang, Pingle Liu∗, Fang Hao, He’an Luo
College of Chemical Engineering, Xiangtan University, Xiangtan 411105, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Metalloporphyrin-catalysed direct functionalization of saturated C H bonds is one among the topical and
challenging areas within the chemical industry. In this work, three types of unsupported and boehmite
supported trans-A2B2-metalloporphyrin chloride complexes (Co-D(p-Cl)PPCl, Fe-D(p-Cl)PPCl, Mn-D(p-
Cl)PPCl) were prepared and characterized. It has been found that these trans-A2B2-metalloporphyrins
have high catalytic activity in cyclohexane oxidation under dioxygen. The boehmite supported metallo-
porphyrins show better catalytic performance and are more stable than unsupported metalloporphyrins.
Among these supported metalloporphyrin catalysts, Co-D(p-Cl)PPCl/BM was better than Fe-D(p-
Cl)PPCl/BM and Mn-D(p-Cl)PPCl/BM, the cyclohexane conversion and selectivity to KA oil are 6.83%
and 83.30%, respectively, and the turnover number is 1.05 × 107. The Co-D(p-Cl)PPCl/BM catalyst can be
facilely recovered and was recycled up to six times without significant decrease in catalytic performance.
Received 31 October 2013
Received in revised form 20 January 2014
Accepted 1 February 2014
Available online 8 February 2014
Keywords:
Trans-A2B2 types metalloporphyrins
Boehmite
Cyclohexane oxidation
Dioxygen
Supported catalyst
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
from the homogeneous media at the end of the first reaction
[11]. Immobilization of the metalloporphyrins on inert and stable
Although many types of metalloporphyrins (MPs) have been
largely studied for their catalytic performance in oxidation reac-
tions, the application of metalloporphyrins in synthetic chemistry
and chemical industry is not popular because of its economic prob-
lem. Therefore, the selective oxidation of hydrocarbons and other
organic compounds is still a challenging and promising subject.
Cyclohexane oxidation with air or dioxygen in the absence of addi-
tives and solvents is one of the most important industrial process to
produce KA oil (cyclohexanol/cyclohexanone) or adipic acid, which
are key intermediates used principally for the production of capro-
lactam, nylon-6,6 and nylon-6 [1,2].
Like cytochrome P-450, metalloporphyrins have been proven
to be efficient oxidation catalysts, which are known to activate
O2 and perform hydrocarbon oxidation into the corresponding
oxygenic compounds under mild condition [3–6]. Homogeneous
systems based on metalloporphyrins have achieved good activities
and high selectivities in catalytic oxidation of hydrocarbons. How-
ever, the metalloporphyrins are inclined to aggregation through
␲–␲ interactions or decompose during the catalytic oxidation
process [7–10]. Furthermore, it is hard to recover and recycle
inorganic supports is one of the most effective ways to overcome
the above drawbacks. In addition, much efforts have been devoted
to develop heterogeneous metalloporphyrin catalysts on various
supports [12–16], which referred that the turnover number (TON)
is 18.92, 1.54 × 105 and 7.91 × 104.
Boehmite (BM) has attracted much attention because of their
unique structure, high surface energy and easy preparation
[17–19]. Some of them referred that the turnover number (TON)
is 1.14 × 105 and 2.3 × 105. Boehmite, a principal oxo-hydroxide of
aluminum, which is also the topological precursor for synthesizing
␥-Al2O3, is widely used as adsorbent, catalyst, catalyst support, and
composite material. Boehmite can be easily prepared in nanopar-
ticle form of high surface energy and good thermal stability. It also
possesses an oxygen atom acting as an electron pair donor, thus
providing an opportunity of coordination to the metal ion of the
metalloporphyrin. Furthermore, metalloporphyrin can be firmly
adsorbed onto its surface. Boehmite supported A4-type metallo-
porphyrin (Mn TPP) has been utilized in cyclohexane oxidation by
Huang [20], the single-pass yield of KA oil is around 5% and the
turnover number (TON) is 1.49 × 105.
In this paper, several trans-A2B2 types metalloporphyrin chlo-
ride complexes (Co-D(p-Cl)PPCl, Fe-D(p-Cl)PPCl, Mn-D(p-Cl)PPCl)
were synthesized (Scheme 1) and immobilized on boehmite. The
relationship between the central metal ion of trans-A2B2 type
∗
Corresponding author. Tel.: +86 73158293545; fax: +86 73158298267.
E-mail address: liupingle@xtu.edu.cn (P. Liu).
1381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2014.02.002

96
Y. Xie et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 95–100
Scheme 1. Synthesis of trans-A2B2-metalloporphyrins.
metalloporphyrins and their catalytic performance in cyclohexane
oxidation was studied.
light-brown suspension was filtrated and washed with distilled
water, and the cake was vacuum-dried at 170 ◦C for 10 h.
2.4. Catalysts characterization
2. Experimental
2.4.1. UV–vis
2.1. Reagents
UV–vis spectra was obtained by UV-2550 spectrophotometry
with a scan range of 300–800 nm for trans-A2B2-metalloporphyrin
chloride complexes using 1 cm quartz cuvette. MsDClPPCl/BM
was characterized by using barium sulphate as reference, a small
amount of BaSO4 was pressed into thin pellets, and some samples
were put on the pellets and pressed again, then it was placed in the
sample holder to assay their spectra.
Aluminum nitrate nonahydrate (98%), NH3 aqueous (NH3 27%)
and absolute ethanol were all obtained from Aladdin Reagent. No
impurities were found in the cyclohexane by GC analysis before
use.
2.2. Trans-A2B2-metalloporphyrin chloride complexes
(MsDClPPCl)
2.4.2. FTIR
Fourier transform infrared (FT-IR) spectra were recorded on a
Nicolet 380 spectrometer. The spectra of the samples were acquired
in the wave number range of 400–4000 cm−1. The samples were
ground to fine powders, mixed with KBr, and then were pressed into
thin pellets and placed in the sample holder of the spectrometer to
assay their spectra.
All reagents and solvents used for the synthesis of porphyrin
were of analytical grade and were obtained commercially. MsDClP-
PCl (CoDClPPCl, FeDClPPCl, MnDClPPCl) was prepared by referring
to the following procedures [21,22]. D-(p-Cl)PP (0.25 mol) was
dissolved in DMF (50 mL). The solution was heated to reflux
with stirring. Then, cobalt acetate, ferrous chloride or manganous
acetate (1.50 mmol) was added in three portions within 30 min.
When the thin-layer chromatography (silica) indicated no free base
porphyrin, the solution was cooled to 70 ◦C, and 40 mL of 6 M HCl
was added and stirred for 4 h. The solution was cooled and the
appeared solid was filtrated and washed with 3 M HCl until the
filtrate no longer appeared red. The obtained solid was vacuum-
dried, and gave 89–98% yield of MsDClPPCl (CoDClPPCl, FeDClPPCl,
MnDClPPCl).
2.4.3. Thermal analysis
TG/DTG curves were carried out on a TGA Q50 using air as purge
gas (80 mL min−1). The temperature is between 30 and 800 ◦C with
a heating rate of 10 ◦C min−1
.
2.4.4. X-ray diffraction analysis
X-ray diffraction (XRD) patterns were collected on a Japan
Rigaku D/Max 2550 VB+ 18 kW X-ray diffractometer under the con-
ditions of 40 kV, 30 mA, Cu K␣ radiation, with a scanning rate of
2◦/min in the range of 2ꢀ = 20–80◦.
2.3. Preparation of trans-A2B2-metalloporphyrin chloride/BM
catalyst
2.4.5. Inductively coupled plasma (ICP)
The trans-A2B2-metalloporphyrin chloride/BM (MsDClP-
PCl/BM) catalysts were prepared by referring to the following
procedures [20]. Aluminum nitrate nonahydrate (0.25 mol) was
dissolved in distilled water (500 mL) for 30 min under vigorous
stirring at room temperature, and 1.45 mol of NH3 aqueous was
slowly added into the solution. After the white aluminum hydrox-
ide precipitate was formed for 1 h, the mixture was filtrated and
washed with distilled water until no NH3 aqueous or NH4Cl was
detected. Then, the precipitate was added to 250 mL of ethanol
in a three-necked flask with high stirring for 1 h. Subsequently,
0.015 mmol of MsDClPPCl ethanol solution were slowly added
into the above suspension and stirred for another 40 min, and
the mixture was heated to 50 ◦C with rapid stirring for 6 h. The
The amount of MsDClPPCl/BM (Co, Fe and Mn) per gram of
boehmite was determined by Inductively Coupled Plasma Atomic
Emission Spectometer (IRIS Intrepid II XSP ICP-AES) (Thermo Elec-
tron Co.) under a microwave pressure digestion (MDS 200; CEM)
with hydrofluoric and aquaregia. The samples were digested by a
traditional acid method (HF, HNO3, HClO4 and HCl), diluted ade-
quately and analyzed for Co, Fe and Mn.
2.5. Procedures for the catalytic test
Cyclohexane oxidation was carried out in a 50 mL autoclave
reactor with a magnetic stirrer in the absence of solvent. Typically,

Y. Xie et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 95–100
97
FTIR spectra were obtained in order to observe the interactions
between OH groups on the surface of boehmite and metallopor-
phyrin complexes. However, the main characteristic peak of the
immobilized metalloporphyrin complexes could not be observed
due to the very low amount of metalloporphyrins on the surface of
boehmite.
Thermal analysis was used to verify the stability of the sup-
ported catalysts designed for oxidation reactions. For all samples,
there is a rapid weight loss between 60 and 140 ◦C. The weight loss
of the supported catalysts reduced during 140–240 ◦C indicating
that the weight loss of MsDClPPCl/BM was too small to change the
structure of the complexes. This can also be seen from the corre-
sponding DTG curves. Thus, the structure of the supported catalysts
will not lead to decomposition since the cyclohexane oxidation was
carried out at 150–160 ◦C.
The XRD patterns of boehmite, CoDClPPCl/BM, FeDClPPCl/BM
and MnDClPPCl/BM exhibit characteristic broad peaks of the
boehmite phase. The easy formation of nanoparticles enhances the
dispersion and adsorption of the metalloporphyrin. And the typ-
ical peaks pattern at 2ꢀ = 28, 38, 49 and 65◦ for all the samples
of boehmite and boehmite supported metalloporphyrins were in
agreement with the literature [24].
The amount of metalloporphyrin on the support was deter-
mined by Inductively Coupled Plasma Atomic Emission Spectome-
ter (IRIS Intrepid II XSP ICP-AES). The results show that the amount
of CoDClPPCl, FeDClPPCl and MnDClPPCl was 0.276, 0.284 and
0.056 mg g−1 of boehmite respectively.
Fig. 1. UV–vis spectra of BM and BM supported metalloporphyrin.
catalysts and cyclohexane (15.6 g) were added into the autoclave
reactor. And the reactor was sealed and heated to the setting tem-
perature. Then it was pressurized to the setting pressure with the
molecular oxygen under stirring. After the reaction, the reactor was
cooled to the ambient temperature. The mixture was dissolved in
ethanol and the catalysts were removed by filtration. And the cat-
alysts were washed in alcohol and dried, and then recycled in the
next reaction. The samples of the reaction mixture were identi-
fied by GC–MS and LC–MS. The acid in the product can mainly
be attributed to the succinic acid, glutaric acid and adipic acid.
The cyclohexanol and cyclohexanone in the product were analyzed
by gas chromatography with the internal standard method using
chlorobenzene as the internal standard. The total acid in the prod-
uct was analyzed by the chemical titration method. The total ester
in the product was analyzed by the chemical titration method with
a solution of hydrochloric acid.
3.2. Catalytic performance
The results of catalytic performance of cobalt, iron and man-
ganese metalloporphyrins are described in Table 2. Under the same
reaction conditions (entries 1–3 and 5–7), Co-D(p-Cl)PPCl presents
the highest selectivity to KA oil, and Mn-D(p-Cl)PPCl exhibits the
highest cyclohexane conversion. Comparing the consequences of
entry 4 and 5, we could find that the cyclohexane conversion
decreases in a certain degree while the selectivity to KA oil increases
by 21.61%. During the cyclohexane oxidation process, we observe
that the induction period varies with the different metallopor-
phyrin catalysts, Co-D(p-Cl)PPCl may activate oxygen molecules
more quickly than the other two metalloporphyrin catalysts. The
reason may be that different central metal ions changed the original
electric potential of the MsDClPPCl.
Table 3 shows the results of cyclohexane oxidation catalyzed
by MsDClPPCl/BM. It was found that boehmite supported met-
alloporphyrin catalysts show better catalytic performance than
unsupported catalysts. When the reaction was catalyzed by
boehmite supported metalloporphyrins under 1 MPa in the absence
of solvent, the selectivity to KA oil increases 12–17% at almost
the same cyclohexane conversion. Similarly, compared with the
results of supported and unsupported catalysts, the selectivity to
KA oil increases 6–10% at the cyclohexane conversion of around
15% under 2 MPa in the absence of solvent. And the TON is more
than one order of magnitude higher than that of unsupported
catalyst. Among these boehmite supported metalloporphyrin cat-
alysts, Co-D(p-Cl)PPCl/BM presents the best catalytic performance
in cyclohexane oxidation. This is in accordance with the known dif-
ferences in chemistry of the concerned metalloporphyrins [25,26].
The cobalt porphyrins are active in one-electron redox processes,
such as the activation of dioxygen, while iron and manganese por-
phyrins are more likely to activate single oxygen donors, such as
PhIO, H2O2, t-BuOOH, or NaOCl in a two-electron redox reaction.
It can be seen from Tables 2 and 3 that MsDClPPCl/BM presents
better catalytic performance in cyclohexane oxidation. At the same
time, MsDClPPCl/BM catalysts show a longer induction period
than that of MsDClPPCl. In metalloporphyrins/O2 system [27,28],
3. Results and discussion
3.1. Characterization of the catalysts
UV–vis and FTIR spectroscopies were used to confirm the cobalt,
iron and manganese MsDClPPCl. The results of UV–vis absorp-
tion measurements of MsDClPPCl are shown in Table 1. It is well
known that porphyrin has one soret band and four Q band absorp-
tion peaks in the ultraviolet visible region. The number of Q band
usually reduces to one or two owing to the increase of sym-
metry of the molecular structure when metalloporphyrins form.
The results of UV–vis absorption in Table 1 show that MsDClPPCl
forms. Furthermore, IR spectra of these metallic complexes shows
some characteristic bands; the strong absorption band appeared
at 1000 cm−1, an increased intensity of the band at 1350 cm−1
corresponding to C N stretching, and the absence of the band at
3300 cm−1 indicates that the metalloporphyrins form, that is, N
,
H
stretching of the intramolecular hydrogen bonds usually presents
in the free base porphyrin. These contributions directly argue for
the formation of the corresponding metallic complexes [23].
CoDClPPCl/BM shows bright red in color. FeDClPPCl/BM and
MnDClPPCl/BM show light green in color, which indicates that
metalloporphyrins are successfully supported on boehmite.
UV–vis spectra of the boehmite supported metalloporphyrins
are shown in Fig. 1. CoDClPPCl/BM, FeDClPPCl/BM and MnD-
ClPPCl/BM show the Soret peak at 432 nm, 417 nm and 480 nm
respectively. Besides, FeDClPPCl/BM and MnDClPPCl/BM present
aborption peaks at 574 nm and 614 nm. The UV–vis spectra confirm
that these supported materials contain metalloporphyrin species.

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Y. Xie et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 95–100
Table 1
Results of UV–vis absorption measurements of MsDClPPCl.a
Metalloporphyrin
Soret band (nm), ε(L mol−1 cm−1
)
Q band (nm), ε (L mol−1 cm−1
)
CoDClPPCl
FeDClPPCl
MnDClPPCl
DClPP
428(2.24 × 105)
417(1.69 × 105)
480(1.39 × 105)
419(2.97 × 105)
544(0.24 × 105)
509(0.20 × 105)
584(0.13 × 105)
515(1.39 × 104)
619(0.13 × 105)
551(0.60 × 104)
591(0.44 × 104)
647(0.32 × 104)
a
The absorption spectra were measured in chloroform solution.
Table 2
Results of cyclohexane oxidation catalyzed by MsDClPPCl.
Entry
Catalysts
Conversion (%)
Selectivity (%)
K–A
K/A (mol ratio)
TON (×106)d
Yield of KA oil (%)
1a
2a
3a
4b
5c
6c
7c
Co-D(p-Cl)PPCl
Fe-D(p-Cl)PPCl
Mn-D(p-Cl)PPCl
Co-D(p-Cl)PPCl
Co-D(p-Cl)PPCl
Fe-D(p-Cl)PPCl
Mn-D(p-Cl)PPCl
6.95
7.71
8.41
23.44
15.14
15.13
16.31
68.77
68.22
65.40
40.15
61.76
58.83
55.91
0.49
0.74
0.65
1.12
0.76
0.92
0.86
0.55
0.61
0.67
0.93
1.21
1.20
1.29
4.78
5.26
5.50
9.41
9.35
8.90
9.12
Reaction conditions: cyclohexane (20 mL), catalysts (1.8 mg), 155 ◦C for 2 h, oxygen pressure (1 MPa).
a
b
c
Reaction conditions: cyclohexane (10 mL), acetonitrile (10 mL), catalysts (1.8 mg), 155 ◦C for 2 h, oxygen pressure (2 MPa).
Reaction conditions: cyclohexane (20 mL), catalysts (1.8 mg), 155 ◦C for 2 h, oxygen pressure (2 MPa).
The turnover number (TON) is the value of 2 h reaction time, calculated by mol product (ketone + alcohol + acid + ester)/mol trans-A2B2-metalloporphyrin.
d
Table 3
Results of cyclohexane oxidation catalyzed by MsDClPPCl/BM.
Entry
Catalysts
Conversion (%)
Selectivity (%)
K–A
K/A (mol ratio)
TON (×107)c
Yield of KA oil (%)
1a
2a
3a
4b
5b
6b
CoDClPPCl/BM
FeDClPPCl/BM
MnDClPPCl/BM
CoDClPPCl/BM
FeDClPPCl/BM
MnDClPPCl/BM
6.17
7.05
6.20
14.02
14.77
14.97
84.27
80.43
82.26
68.12
63.98
64.80
0.38
0.37
0.38
0.59
0.55
0.57
0.64
0.69
3.16
1.45
1.45
7.64
5.20
5.67
5.10
9.55
9.45
9.70
a
Reaction conditions: cyclohexane (20 mL), MsDClPPCl/BM (CoDClPPCl 0.138 mg, FeDClPPCl 0.142 mg, MnDClPPCl 0.028 mg), 155 ◦C for 2 h, oxygen pressure (1 MPa).
Reaction conditions: cyclohexane (20 mL), MsDClPPCl/BM (CoDClPPCl 0.138 mg, FeDClPPCl 0.142 mg, MnDClPPCl 0.028 mg), 155 ◦C for 2 h, oxygen pressure (2 MPa).
The turnover number (TON) is the value of 2 h reaction time, calculated by mol product (ketone + alcohol + acid + ester)/mol MsDClPPCl.
b
c
Scheme 2. Proposed mechanisms of the supported catalyst.

Y. Xie et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 95–100
99
Table 4
Results of cyclohexane oxidation catalyzed by CoDClPPCl/BM.
Entry
CoDClPPCl catalysts (mg)
Conversion (%)
Selectivity (%)
K–A
K/A (mol ratio)
TON (×107)d
Yield of KA oil (%)
1a
2a
0.22
0.14
0.12
0.10
0.44
0.22
0.14
0.04
0.10
5.73
6.17
6.26
83.77
84.27
82.10
71.19
68.58
67.22
68.12
61.18
57.60
0.35
0.38
0.37
0.38
0.61
0.62
0.59
0.70
0.72
0.38
0.64
0.76
1.05
0.48
0.97
1.46
5.33
21.53
4.80
5.20
5.14
5.19
10.00
9.86
9.55
9.99
8.64
3a
4a,c
5b
7.29
14.58
14.67
14.02
16.33
15.00
6b
7b
8b
9b,c
a
Reaction conditions: cyclohexane (20 mL), CoDClPPCl/BM catalysts, 155 ◦C for 2 h, oxygen pressure (1 MPa).
Reaction conditions: cyclohexane (20 mL), CoDClPPCl/BM catalysts, 155 ◦C for 2 h, oxygen pressure (2 MPa).
Reaction conditions: unsupported catalysts.
b
c
d
The turnover number (TON) is the value of 2 h reaction time, calculated by mol product (ketone + alcohol + acid + ester)/mol CoDClPPCl.
Table 5
Recycle of CoDClPPCl/BM catalyst.
Catalysts
Entry
Conversion (%)
Selectivity (%)
K–A
K/A (mol ratio)
TON (×107)b
Yield of KA oil (%)
1a
2
3
4
5
6.17
6.52
6.93
6.87
7.54
6.94
84.27
83.27
82.25
83.26
82.76
84.01
0.38
0.46
0.50
0.49
0.45
0.45
0.64
0.77
0.94
1.19
1.39
1.38
5.20
5.43
5.70
5.72
6.24
5.83
CoDClPPCl/BM
6
Average
6.83
83.30
0.46
1.05
5.69
a
Reaction conditions: cyclohexane (20 mL), CoDClPPCl/BM catalysts (CoDClPPCl 0.138 mg), 155 ◦C for 2 h, oxygen pressure (1 MPa).
The turnover number (TON) is the value of 2 h reaction time, calculated by mol product (ketone + alcohol + acid + ester)/mol CoDClPPCl.
b
Ms(III)DClPPCl loses a chlorine radical to form Ms(II)DClPP•, which
3.3. Effect of the catalyst amount
combines with an oxygen molecule to form an activated radical
species Ms(III)OODClPP•, and then form the high-valent metal-oxo
active species. And the intermediate Ms(IV) = ODClPP• can easily
capture a hydrogen atom from cyclohexane to afford an alkyl rad-
ical and then form the products [29]. The chlorine radical can also
easily capture a hydrogen atom from cyclohexane to afford an
alkyl radical, which combines with an oxygen molecule to form the
activated peroxoradicals species (ROO•), then it may form organic
acids. The corresponding active species for the supported cata-
lyst [20] were mainly Ms(IV) = ODClPP•/BM (Scheme 2). The longer
induction period of Ms(III)DClPPCl/BM may be that the formation of
Ms(IV) = ODClPP• is faster than Ms(IV) = ODClPP•/BM. The existence
of BM hindered the supported catalysts and dioxygen to form the
active species Ms(IV) = ODClPP•/BM, which can capture a hydrogen
atom from cyclohexane to afford an alkyl radical and then form
the products. The chlorine radical and Ms(IV) = ODClPP•/BM can
also easily capture a hydrogen atom from cyclohexane to afford
an alkyl radical, and then it combines with an oxygen molecule to
form the activated peroxoradicals species (ROO•) which may form
organic acids. That is, the steric hindrance of BM decreases the oxy-
gen activation ability of the supported metalloporphyrin catalysts.
This steric hindrance leads to increase the selectivity to KA oil and
prevents from further oxidation of KA oil to byproducts. It may be
the reason why the supported catalyst shows better catalytic per-
formance than the unsupported catalyst, which is consistent with
the results of Tables 2 and 3.
The effect of the amount of CoDClPPCl/BM catalyst was inves-
tigated and the results are shown in Table 4. The cyclohexane
conversion and selectivity to KA oil change very little when the
amount of CoDClPPCl/BM catalyst increases from 0.12 to 0.22 mg
under 1 MPa or increase from 0.14 to 0.44 mg under 2 MPa. How-
ever, the selectivity to KA oil decreases significantly when the
catalyst amount declines from 0.14 to 0.04 mg under 2 MPa. Sim-
ilarly, the boehmite supported Co-DClPPCl catalyst show better
catalytic performance than the unsupported catalyst, the selectivity
to KA oil increases by more than 11%.
3.4. Recycle of the CoDClPPCl/BM catalyst
Recycling tests with repeated use of CoDClPPCl/BM catalyst in
six consecutive reactions were carried out under the typical reac-
tion conditions, the recycled catalysts were washed with alcohol,
dried and then used for the next reaction. The results are shown
in Table 5. It can be seen from Table 5 that the change of catalytic
activity and selectivity could be ignored. However, the cyclohexane
oxidation catalyzed by the unsupported catalysts gives lower selec-
tivity to KA oil (65.40–68.77%) and lower TON (0.55–0.67 × 106).
Furthermore, unsupported catalysts are difficult to recover and
reused. Thus, the advantages of the bohmite supported CoDClPPCl
catalyst were obvious.
In general, the electron-withdrawing metalloporphyrins are
very strong for activating O2 because the electron deficiency of the
central metal ion is willing to accept a pair of electrons donated by
O2. The potential order of metal ions is different. The synergistic
effect on the potential and the formation of active species could
give rise to the long induction period and different catalytic perfor-
mance. This may be the reason why cobalt metalloporphyrin shows
better results than that of iron and manganese metalloporphyrin.
4. Conclusion
Unsupported and boehmite supported metalloporphyrin cata-
lysts were prepared and characterized. The prepared catalysts were
used to mimic cytochrome P-450 in cyclohexane oxidation process.
In general, boehmite supported metalloporphyrin catalysts show
better catalytic performance than unsupported metalloporphyrins,
the selectivity to KA oil is improved 12–17% under the cyclohexane

100
Y. Xie et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 95–100
conversion of 6–8%. And CoDClPPCl/BM presents the best results, it
gives the selectivity to KA oil of 83.30% at the cyclohexane conver-
sion of 6.83% and TON reaches 1.05 × 107. This complex may prove
to be an active and reusable catalyst for industrial cyclohexane
oxidation process.
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This work was supported by the NSFC (21276218), program
for New Century Excellent Talents in University (NCET-10-0168),
SRFDP (20124301110007) and Scientific Research Fund of Hunan
Provincial Education Department (13k043, CX2012B253).
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