Enhanced catalytic performance of porphyrin cobalt(II) in the solvent-free oxidation of cycloalkanes (C5~C8) with molecular oxygen promoted by porphyrin zinc(II)

Article abstract of DOI:10.1016/j.catcom.2019.105809

Dual-metalloporphyrins catalytic system based on T(p-Cl)PPCo and T(p-Cl)PPZn was presented to enhance the oxidation of cycloalkanes, especially for cyclohexane, the selectivity towards KA oil increasing from 90.7% to nearly 100.0%, meanwhile the conversion increasing from 3.42% to 4.29%. Enhancement on conversion and selectivity was realized simultaneously. In the dual-metalloporphyrins system, T(p-Cl)PPCo served the role to activate molecular oxygen and promote the decomposition of cyclohexyl hydroperoxide, and T(p-Cl)PPZn catalyzed the decomposition of cyclohexyl hydroperoxide to avoid unselective thermal decomposition. This protocol is also very applicable to other cycloalkanes and will provide a applicable strategy to enhance the oxidation of alkanes.

Full text of DOI:10.1016/j.catcom.2019.105809

CatalysisCommunications132(2019)105809
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Short communication
Enhanced catalytic performance of porphyrin cobalt(II) in the solvent-free
oxidation of cycloalkanes (C5~C8) with molecular oxygen promoted by
porphyrin zinc(II)
Hai-Min Shen, Long Zhang, Jin-Hui Deng, Jing Sun, Yuan-Bin She⁎
College of Chemical Engineering, State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014,
China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cycloalkane
Oxidation
Metalloporphyrin
Oxygen
Catalysis
Dual-metalloporphyrins catalytic system based on T(p-Cl)PPCo and T(p-Cl)PPZn was presented to enhance the
oxidation of cycloalkanes, especially for cyclohexane, the selectivity towards KA oil increasing from 90.7% to
nearly 100.0%, meanwhile the conversion increasing from 3.42% to 4.29%. Enhancement on conversion and
selectivity was realized simultaneously. In the dual-metalloporphyrins system, T(p-Cl)PPCo served the role to
activate molecular oxygen and promote the decomposition of cyclohexyl hydroperoxide, and T(p-Cl)PPZn cat-
alyzed the decomposition of cyclohexyl hydroperoxide to avoid unselective thermal decomposition. This pro-
tocol is also very applicable to other cycloalkanes and will provide a applicable strategy to enhance the oxidation
of alkanes.
1. Introduction
compared with other oxidants, the activation of molecular oxygen in
the oxidation of cycloalkanes is very difficult. Thus the industrial oxi-
The selective oxidation of alkanes to corresponding alcohols, alde-
hydes, ketones and carboxylic acids is an extremely important and
challenging transformation in both chemical industry and academic
research, in which the abundant and inexpensive alkanes in petroleum
and natural gas are converted to high-value chemical intermediates and
fine chemicals [1,2]. Among these significant reactions, the oxidation of
cycloalkanes to cycloalkanols and cycloalkanones has gained a con-
tinuous and considerable interest, especially for the cyclohexane [3,4],
since the obtained cyclohexanol and cyclohexanone (KA oil) can be
further converted to caprolactam and hexanedioic acid which are the
irreplaceable intermediates in the production of various polymers such
as nylon-6 and nylon-66 [5,6]. And the oxygenated products from other
cycloalkanes, such as cyclopentane, cycloheptane and cyclooctane, not
only are important precursors of a series of macromolecule polymers
[7], but also are building blocks in several valuable chemicals [8]. In
order to realize these essential transformations, several oxidants have
been explored, including iodosylbenzene [9], m-chloroperoxybenzoic
acid [10], tert-butyl hydroperoxide [11], hydrogen peroxide [12], mo-
lecular oxygen [13], and so on. It is obvious that molecular oxygen is
the most promising one according to the requirements of green chem-
istry, which possesses low cost, readily availability and environmental
harmlessness with innocuous water as the only by-product. But
dation of cyclohexane is conducted at 150–170 °C and 1.0–2.0 MPa
oxygen pressure employing homogeneous cobalt salt as catalyst with
conversion of 3%–8% for cyclohexane and selectivity of 80%–85% to-
wards KA oil [14,15], which suffers from the low selectivity seriously.
To some extent, the low selectivity in oxidation of cycloalkanes can be
attributed to the high reaction temperature, because the autoxidation of
cycloalkanes and thermal decomposition of cycloalkyl hydroperoxides
which are the main intermediate products in the oxidation of cy-
cloalkanes [16], would occur at high temperature. Both of them process
through unselective free radical chain reaction pathway, and give rise
to a lot of undesired oxydates, including hexanedioic acid which is very
difficult to separate from the oxidation products. Despite the fact that
most of the KA oil is employed as intermediate to produce hexanedioic
acid, the formation of hexanedioic acid in the production of KA oil is a
troublesome issue. Hence, it is highly urgent to develop the selective
oxidation of cycloalkanes to cycloalkanols and cycloalkanones under
mild conditions.
To smooth the oxidation of cycloalkanes to their corresponding al-
cohols and ketones using molecular oxygen, various catalytic systems
have been explored, including transition metal complex catalysis [17],
metal nanoparticle catalysis [18], molecular sieve catalysis [19], pho-
tocatalysis [20], biocatalysis [21], and so on. Synthetic
⁎ Corresponding author.
E-mail address: haimshen@zjut.edu.cn (Y.-B. She).
https://doi.org/10.1016/j.catcom.2019.105809
Received 3 July 2019; Received in revised form 30 August 2019; Accepted 1 September 2019
Availableonline02September2019
1566-7367/©2019ElsevierB.V.Allrightsreserved.

H.-M. Shen, et al.
CatalysisCommunications132(2019)105809
metalloporphyrins which are the chemical models of cytochrome P-450
enzymes, are a type of transition metal complexes and have been widely
applied in the oxidation of cycloalkanes as biomimetic catalysts for
their high efficiency in activation of molecular oxygen and satisfying
selectivity in oxidation of CeH bond [9,13,17,22,23]. A number of
satisfying results were achieved. For example, Huang and co-workers
reported tetrakis(4-carboxylphenyl)porphyrin iron(III) chloride, tet-
rakis(pentafluorophenyl) porphyrin iron(III) chloride, tetrakis(4-sulfo-
natophenyl)porphyrin iron(III) chloride and tetrakis(4-carboxylphenyl)
porphyrin iron(II), cobalt(II), manganese(II), which were immobilized
onto zinc oxide(ZnO), zinc sulfide(ZnS) and chitosan respectively to
boost their stability and catalytic performance, were employed as cat-
alysts in the oxidation of cyclohexane at the temperature range of
150–165 °C, and the most satisfying selectivity towards KA oil was
56.5% with the conversion of 39.8% [17,22,24]. Liu and co-workers
reported 5,15-di(4-chlorophenyl)-10,20 -diphenylporphyrin cobalt(III)
chloride immobilized onto zinc oxide(ZnO) catalyzed oxidation of cy-
clohexane with molecular oxygen at 155 °C, and the selectivity towards
KA oil reached up to 72.1% with the conversion of 13.1% [25]. Yang
and co-workers reported the synthesis of porous polymeric metallo-
porphyrins using tetrakis(4-bromophenyl)porphyrin iron(III) chloride,
tetrakis(4-bromophenyl) porphyrin manganese(III) chloride and 1, 4-
phenyldiboronic acid as building blocks, which were applied in the
oxidation of cyclohexane at 150 °C, and the selectivity was up to 72.2%
with the conversion of 29.9% [26]. Pamin and co-workers investigated
the performance of three generations of porphyrin cobalt(II) in the
oxidation of cycloalkanes with molecular oxygen systematically, and
found that the generation II porphyrin cobalt(II) showed higher activity
[13]. Evidently, the oxidation of cyclohexane with molecular oxygen
was promoted by metalloporphyrins, especially for the conversion and
yield, but still suffered from the high temperature and low selectivity,
which would lead to a lot of undesired by-products and pollute the
environment. Thus, how to increase the selectivity and lower the re-
action temperature is still an enormous challenge lied in the progress of
cycloalkanes oxidation.
In our attempt to increase the selectivity in oxidation of cycloalk-
anes catalyzed by metalloporphyrins with molecular oxygen [27–29],
the formation of carboxylic acid and other undesired by-products from
unselective free radical reaction occurred at higher temperature, be-
came the primary issue to consider. To solve this problem, an idea to
conduct the oxidation of cycloalkanes at lower-temperature and de-
compose the cycloalkyl hydroperoxides through catalytic pathway
emerged in our minds. After systematic investigation on the autoxida-
tion temperature of cycloalkanes(C5-C8) [30], the catalytic oxidation of
cycloalkanes was conducted at the temperature not higher than their
autoxidation temperature in our following work. Meanwhile, in order
to realize the catalytic decomposition of cycloalkyl hydroperoxides,
dual-metalloporphyrins system was introduced to the catalytic oxida-
tion of cycloalkanes, in which one metalloporphyrin mainly served the
decomposition of cycloalkyl hydroperoxides. Encouraging outcomes
were obtained, especially for the selectivity towards KA oil. The se-
lectivity towards KA oil was increased from 90.7% to nearly 100.0%,
meanwhile the conversion of cyclohexane was also increased from
3.42% to 4.29%. Thus, in this work we reported the enhanced perfor-
mance of porphyrin cobalt(II) in the solvent-free oxidation of cy-
cloalkanes (C5~C8) with molecular oxygen promoted by porphyrin
zinc(II), which not only reveals a strategy to enhance the conversion
and selectivity in the oxidation of cycloalkanes, but also act as an im-
portant reference in the selective oxidation of other alkanes. To the best
of our knowledge, our work is a very scarce example in the oxidation of
alkanes, in which the conversion and selectivity is enhanced simulta-
neously through the delicate cooperation of two different metallopor-
phyrins.
2. Experimental
2.1. Materials and reagents
Cyclohexane in 99% purity was purchased from Hangzhou
Shuanglin Chemical Reagent Co. Ltd., China. Cyclopentane and cyclo-
heptane in 98% purity were purchased from TCI Co. Ltd. Cyclooctane in
99% purity was purchased from Alfa Aesar Co. Ltd. All of the above
cycloalkanes were dried over sodium and redistilled before use. All the
other common reactants and reagents were analytical grade, and all of
them were used as received without further purification unless other-
wise noted.
2.2. Catalytic oxidation of cycloalkanes
The catalytic oxidation of cycloalkanes employing metallopor-
phyrins as catalysts and molecular oxygen as oxidant was conducted in
a 100 mL autoclave reactor equipped with an inner Teflon liner and a
magnetic stirrer. In
a
typical procedure, metalloporphyrin
(1.2 × 10−3%, mol/mol) was suspended in cycloalkane (200 mmol) in
the 100 mL autoclave reactor, and the reactor was heated to the desired
reaction temperature after sealing. When the reaction temperature
reached, oxygen was fed into the autoclave to obtain a desired pressure.
The resultant reaction mixture was stirred at the desired temperature
and desired pressure for 8.0 h, then cooled to room temperature.
Triphenylphosphine (25 mmol, 6.5573 g) was added into the reaction
mixture to reduce the cycloalkyl hydroperoxide to corresponding cy-
cloalkanol quantitatively. After stirring for 30 min, the reaction mixture
was dissolved in acetone and transferred into a 100 mL volumetric flask
totally. Then the GC and HPLC analyses were performed to determine
the yields of the oxidized products. The oxidized products were de-
termined by comparison with the authentic samples. The yields of cy-
cloalkanol and cycloalkanone were determined by GC employing to-
luene as internal standard, and the yield of cycloalkyl hydroperoxide
was determined through the amount of O=PPh3 obtained from re-
duction of cycloalkyl hydroperoxide with PPh3 using GC. GC analyses
were performed on a Thermo Scientific Trace 1300 instrument using a
TG-5MS column (30 m × 0.32 mm × 0.25 μm). The yields of aliphatic
diacids were determined by HPLC with benzoic acid as internal stan-
dard. HPLC analyses were performed on a Thermo Scientific Ultimate
3000 chromatography equipped with a Ultimate 3000 Photodiode
Array
Detector
using
a
Amethyst
C18eH
column
(250 mm × 4.6 mm × 0.25 μm).
3. Results and discussion
In this work, the catalytic oxidation of cycloalkanes was carried out
in a 100-mL stainless steel autoclave reactor equipped with an Teflon
liner and a magnetic stirrer under solvent-free condition with me-
talloporphyrins as catalysts and molecular oxygen as oxidant as shown
in Scheme 1. The representative metalloporphyrins employed in this
work were tetrakis(2-chlorophenyl)porphyrin cobalt(II) (T(o-Cl) PPCo),
tetrakis(3-chlorophenyl)porphyrin cobalt (II) (T(m-Cl)PPCo), tetrakis(4
-chlorophenyl)porphyrin cobalt(II) (T(p-Cl)PPCo), tetrakis(4-chlor-
ophenyl) porphyrin iron(II) (T(p-Cl)PPFe), tetrakis(4-chlorophenyl)
porphyrin manganese(II) (T(p-Cl)PPMn), tetrakis(4-chlorophenyl)por-
phyrin zinc(II) (T(p-Cl)PPZn), which were synthesized through the
Scheme 1. Selective oxidation of cycloalkanes (C5-C8) with molecular oxygen
catalyzed by metalloporphyrins
2

H.-M. Shen, et al.
CatalysisCommunications132(2019)105809
Table 1
Catalytic oxidation of cyclohexane with O2 catalyzed by various metalloporphyrins.a
Entry
Catalyst
Conversionb (%)
Selectivityb (%)
Alcohol
Ketone
Hydroperoxide
Total
1
2
3
4
5
6
7
8d
∕
N. D.
3.88
3.72
3.43
3.54
1.08
N. D.
3.48
N. D.
39.9
45.2
42.3
43.8
19.4
N. D.
41.7
N. D.
44.3
41.1
43.4
29.4
31.5
N. D.
41.1
N. D.
4.4
4.0
N. D.c
88.6
90.3
90.7
96.6
100.0
N. D.
94.6
T(o-Cl)PPCo
T(m-Cl)PPCo
T(p-Cl)PPCo
T(p-Cl)PPMn
T(p-Cl)PPFe
T(p-Cl)PPZn
T(o-Cl)PPCo
T(p-Cl)PPZn
T(m-Cl)PPCo
T(p-Cl)PPZn
T(p-Cl)PPCo
T(p-Cl)PPZn
T(p-Cl)PPMn
T(p-Cl)PPZn
T(p-Cl)PPFe
T(p-Cl)PPZn
5.0
23.4
49.1
N. D.
11.8
9
3.01
4.18
1.25
1.09
44.2
42.6
28.8
6.4
45.2
33.0
20.8
24.8
5.3
94.7
96.4
96.0
91.8
10
11
12
20.8
46.4
60.6
a
Reaction condition: cyclohexane (200 mmol, 16.83 g), metalloporphyrin (1.2 × 10−3%, mol/mol), O2 (1.40 MPa), 120 °C, 8.0 h.
The conversion and selectivity of cyclohexanol and cyclohexanone were determined by GC after treatment with PPh3 employing toluene as internal standard,
b
and the selectivity of cyclohexyl hydroperoxide was determined through the amount of O=PPh3 obtained from reduction of cyclohexyl hydroperoxide with PPh3.
The products were determined by comparison with the authentic samples.
c
No obvious reaction was detected.
d
The molar ratio of two metalloporphyrins was 1:1, and the total concentration of metalloporphyrins was kept at 1.2 × 10−3%, mol/mol.
condensation of freshly distilled pyrrole with corresponding chlor-
obenzaldehyde in propionic acid followed by the metallation in DMF
[31]. All the metalloporphyrins were purified by silica column chro-
matography and characterized through 1H NMR, 13C NMR and ESI-MS.
With metalloporphyrins in hand, the catalytic oxidation of cyclohexane
was selected as probe reaction to test and verify our ideas at the reac-
tion temperature of 120 °C, at which the autoxidation would not
happen. And the major products were KA oil, cyclohexyl hydroperoxide
and a little of carboxylic acids which were analyzed by GC and HPLC to
determine the conversion and selectivity. Because a small amount of
cyclohexyl hydroperoxide could be converted to KA oil smoothly after
reaction, the selectivity towards cyclohexyl hydroperoxide was con-
sidered as a part of KA oil. As shown in Table 1, no oxidation occurred
in the absence of metalloporphyrins (Entry 1). When the metallopor-
phyrins were introduced to the reaction, the conversion of cyclohexane
reached up to 3.43%–3.88% for porphyrin cobalt and porphyrin man-
ganese with an acceptable selectivity of 88.7%–96.6% towards KA oil.
But there are still some hexanedioic acid and undesired by-products
formed, especially for the porphyrin cobalt. As for T(p-Cl)PPFe, a lower
catalytic performance was observed, and T(p-Cl)PPZn did not exhibit
any catalytic activity in the oxidation of cyclohexane at all. However,
when dual-metalloporphyrins systems were applied to the oxidation of
cyclohexane, the catalytic performance of T(p-Cl)PPCo was enhanced
considerably by T(p-Cl)PPZn. The conversion of cyclohexane increased
from 3.43% to 4.18% meanwhile the selectivity towards KA oil in-
creased from 90.7% to 96.4% (Entry 3, Entry 10). The increase in the
conversion did not consume the selectivity. Increase in both of the
conversion and selectivity were realized as a result of the promotion of
porphyrin znic. Thus, T(p-Cl)PPCo employed as catalyst combined with
T(p-Cl)PPZn as promoter was considered as the most successful dual-
metalloporphyrins system in this work. For other dual-metallopor-
phyrins systems, the unsatisfying catalytic performance might be as-
cribed to the undesired interactions between two metalloporphyrins
such as the formation of dimmers and the intermolecular redox [32,33].
With the optimized dual-metalloporphyrins system established, the
molar composition of T(p-Cl)PPCo and T(p-Cl)PPZn was investigated
Table 2
Effect of molar ratios between T(p-Cl)PPCo and T(p-Cl)PPZn on the catalytic
oxidation of cyclohexanea.
Entry Molar
ratiob
Conversionc (%) Selectivityc (%)
Alcohol Ketone Hydroperoxide Total
1
2
3
4
5
1.0:1.0
1.0:1.5
1.0:2.0
1.0:2.5
1.0:3.0
4.18
4.28
4.29
3.90
3.55
42.6
52.6
49.7
55.6
45.6
33.0
37.4
31.7
33.4
29.9
20.8
8.6
18.6
11.0
24.5
96.4
98.6
100.0d
100.0
100.0
a
Reaction condition: cyclohexane (200 mmol, 16.83 g), T(p-Cl)PPCo
(6.0 × 10−4%, mol/mol), T(p-Cl)PPZn as shown in table, O2 (1.40 MPa),
120 °C, 8.0 h.
b
m[T(p-Cl)PPCo]: m[T(p-Cl)PPZn]
Same with Table 1.
No obvious by-product was detected.
.
c
d
Table 3
Decomposition of tert-butyl hydroperoxide in the presence or absence of me-
talloporphyrins.a
Entry Metalloporphyrin Decomposition ratiob
(%)
Yieldc (%) Selectivityc (%)
1
2
3
4
/
5.2
N. D.
30.0
39.3
16.9
N. D.
81.3
66.6
77.9
T(p-Cl)PPZn
T(p-Cl)PPCo
T(p-Cl)PPCo
T(p-Cl)PPZn
36.9
59.0
21.7
a
Reaction condition: t-BuOOH (5.0 mmol, 70% aqueous solution, 0.6437 g)
in cyclohexane (20 mmol, 1.6832 g), metalloporphyrin (2.5 × 10−3 mmol),
80 °C, 4.0 h.
b
Determined by GC employing toluene as internal standard through the
amount of O=PPh3 obtained from reduction of the residual t-BuOOH with
PPh3. The products were determined by comparison with the authentic sam-
ples.
c
Yield and selectivity of tert-butanol, determined by GC employing toluene
deeply, in which the loading of T(p-Cl)PPCo was set as 6.0 × 10−4
%
as internal standard.
(mol/mol), and the molar ratios between T(p-Cl)PPCo and T(p-Cl)PPZn
varied from 1: 1.0 to 1: 3.0. It was illustrated in Table 2 that, when the
3

H.-M. Shen, et al.
CatalysisCommunications132(2019)105809
Table 4
Oxidation of cycloalkanes (C5-C8) catalyzed by dual-metalloporphyrins system based on T(p-Cl)PPCo and T(p-Cl)PPZn.a
Entry
Cycloalkane
Conversionb (%)
Selectivity (%)b
Alcohol
Ketone
35.3
Hydroperoxide
7.5
Total
54.2
1c
2c
1.84
4.29
11.4
49.7
31.7
18.2
18.6
81.8
100.0f
100.0
3d
4e
2.20
2.29
N. D.g
N. D.
16.2
83.8
100.0
a
Reaction condition: cycloalkane (200 mmol), T(p-Cl)PPCo (6.0 × 10−4%, mol/mol), T(p-Cl)PPZn (1.2 × 10−3%, mol/mol), O2 (1.40 MPa), 8.0 h.
Same with Table 1.
120 °C.
110 °C.
b
c
d
e
f
100 °C.
No obvious by-product was detected.
No obvious cycloalkanol was detected.
g
Table 5
Catalytic performance of some metalloporphyrins in the oxidation of cyclohexane with molecular oxygen under solvent-free condition.
Entry
Catalyst
Temperature (°C)
Conversion (%)
Selectivitya (%)
Ref.
1
2
3
4
5
6
7
8
T(penta-F)PPCo immobilized onto chitosan
T(p-COOH)PPCo immobilized onto ZnS
T(p-COOH)PPMn immobilized onto ZnS
T(p-COOH)PPFe immobilized onto ZnS
T(p-COOH)PPMnCl immobilized onto chitosan
T(p-COOH)PPFeCl immobilized onto chitosan
T(p-SO3H)PPFeCl immobilized onto chitosan
T(p-COOH)PPMn
165
165
160
160
155
155
155
150
150
150
150
150
150
135
120
120
38.5
67.8
46.1
51.8
14.7
17.3
51.4
9.76
7.98
23.4
39.8
29.9
27.5
12.5
3.43
4.29
62.8
34.2
46.1
42.3
76.9
75.3
55.8
74.7
91.0
64.0
56.5
72.3
69.8
84.8
90.7
100.0
[34]
[17]
[17]
[17]
[35]
[35]
[24]
[23]
[23]
[22]
[22]
[26]
[26]
[26]
This work
This work
9
T(p-COOH)PPMn immobilized onto ZnO
T(penta-F)PPFeCl
10
11
12
13
14
15
16
T(penta-F)PPFeCl immobilized onto ZnO
Porous polymeric T(p-Br)PPMnCl
Porous polymeric T(p-Br)PPFeCl
Porous polymeric T(p-Br)PPMnCl
T(p-Cl)PPCo
T(p-Cl)PPCo & T(p-Cl)PPZn
a
Selectivity towards KA oil.
molar ratio of T(p-Cl)PPCo to T(p-Cl)PPZn was changed from 1.0:1.0 to
1.0:1.5, the selectivity towards KA oil increased from 96.4% to 98.6%.
Further increasing the ratio of T(p-Cl)PPZn to 1.0: 2.0, no obvious by-
product was detected with satisfying conversion of 4.29%. Thus, with
the promotion of T(p-Cl)PPZn, it was achieved to enhanced the con-
version and selectivity in the oxidation of cyclohexane catalyzed by T(p-
Cl)PPCo. With the acceptable conversion of 4.29%, nearly 100% se-
lectivity towards KA oil was obtained, which would simplify the se-
paration process and reduce the production costs remarkably in the
industrial oxidation of cyclohexane. As mentioned above, the exciting
conversion and selectivity in our work was mainly attributed the low-
temperature oxidation and the catalytic decomposition of cyclohexyl
hydroperoxide promoted by T(p-Cl)PPZn. At lower reaction tempera-
ture, the unselective autoxidation of cyclohexane, cyclohexanol and
cyclohexanone, and the unselective thermal decomposition of cyclo-
hexyl hydroperoxide were inhibited. On the other hand, the cyclohexyl
hydroperoxide was decomposed to cyclohexanol catalyzed by T(p-Cl)
PPZn. Both of them reduced the formation of undesired by-products and
improved the selectivity in the oxidation cyclohexane. The catalytic
performance of T(p-Cl)PPZn in the decomposition of cyclohexyl hy-
droperoxide was verified in our work employing tert-butyl hydroper-
oxide as representative. It was demonstrated in Table 3 that when tert-
butyl hydroperoxide was stirred in cyclohexane at 80 °C for 4.0 h, the
decomposition ratio was just 5.2%, but when T(p-Cl)PPCo or T(p-Cl)
PPZn was added, the decomposition reached up to 59.0% and 36.9%
respectively with tert-butanol as major product. Both of T(p-Cl)PPCo
and T(p-Cl)PPZn can catalyze the decomposition of tert-butyl hydro-
peroxide and T(p-Cl)PPZn possesses better selectivity towards tert-bu-
tanol. With the results obtained in Table 1 that T(p-Cl)PPCo was an
effective catalyst in the oxidation of cyclohexane with molecular
oxygen and T(p-Cl)PPZn did not exhibit any catalytic activity, and the
catalytic mechanism in the oxidation of cyclohexane with molecular
oxygen catalyzed by metalloporphyrins that molecular oxygen was
activated by metalloporphyrins [17,24,25], it could be concluded that
in this dual-metalloporphyrins catalytic system, T(p-Cl)PPCo not only
served the role to activate molecular oxygen, but also possessed the
function to promote the decomposition of cyclohexyl hydroperoxide,
and T(p-Cl)PPZn served the decomposition of cyclohexyl hydroperoxide
exclusively. Their delicate cooperation made the high selective oxida-
tion of cyclohexane realized. As for the optimized molar ratio of T(p-Cl)
PPCo to T(p-Cl)PPZn was 1.0: 2.0, it should be attributed to the high
efficiency of T(p-Cl)PPCo in activation of molecular oxygen and for-
mation of cyclohexyl hydroperoxide, and the lower transformation rate
of cyclohexyl hydroperoxide to cyclohexanol and cyclohexanone. More
4

H.-M. Shen, et al.
CatalysisCommunications132(2019)105809
catalytic active centers were needed to enhanced the transformation of
cyclohexyl hydroperoxide to cyclohexanol and cyclohexanone. What
should be mentioned additionally is that cyclohexanol and cyclohex-
anone at negligible levels compared with tert-butanol were also de-
tected in the decomposition of tert-butyl hydroperoxide in cyclohexane.
Based on the results obtained from the selective oxidation of cy-
clohexane catalyzed by the dual-metalloporphyrins system, the sub-
strates were extended from cyclohexane to cyclopentane, cycloheptane
and cyclooctane. All the catalytic transformations were conducted at
the temperature no autoxidation would occur. As shown in Table 4,
nearly 100% selectivity towards alcohol, ketone and hydroperoxide was
achieved for the cycloalkanes involved, except for the cyclopentane.
The lower selectivity in cyclopentane oxidation was mainly attributed
the higher strain in cyclopentane molecule. And the major product in
the oxidation of cyclopentane is pentanedioic acid illustrated in Tables
S1 and S2. For the satisfying substrate tolerance, the dual-metallopor-
phyrins catalytic system presented in this work has enormous potential
application in the selective oxidation of cycloalkanes, and will serve as
a applicable strategy in the enhancement of various alkanes oxidation.
At last, the catalytic oxidation of cyclohexane in this work was
compared with some reported cyclohexane oxidation systems em-
ploying metalloporphyrins as catalysts and molecular oxygen as oxidant
under solvent-free condition, as shown in Table 5. It is clear that, in the
term of conversion, the dual-metalloporphyrins catalytic system in this
work is not so competitive, but in the aspects of reaction temperature
and selectivity towards KA oil, our protocol in the oxidation of cyclo-
hexane is very promising without any obvious by-product produced,
which will not only simplify the separation process remarkably in the
industrial oxidation of cyclohexane, but also cut down the chemical
waste discharged to the environment. Although the conversion in this
work is comparable to the industrial level, the attempt to further im-
prove the conversion of cyclohexane with high selectivity is still in
process in our group at present.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2019.105809.
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In conclusion, a strategy to enhance the selectivity in the catalytic
oxidation of cycloalkanes was presented in this work, especially for the
cyclohexane, the selectivity towards KA oil was increased from 90.7%
to nearly 100.0%, meanwhile the conversion of cyclohexane was in-
creased from 3.42% to 4.29%. The increase in the conversion and se-
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oxidation of cyclohexane were mainly attributed to the low tempera-
ture reaction and the delicate cooperation between T(p-Cl)PPCo and
T(p-Cl)PPZn in the dual-metalloporphyrins catalytic system, in which
T(p-Cl)PPCo served the role to activate molecular oxygen and promote
the decomposition of cyclohexyl hydroperoxide, and T(p-Cl)PPZn could
catalyze the decomposition of cyclohexyl hydroperoxide selectively.
For the low temperature reaction and the delicate cooperation of T(p-
Cl)PPCo and T(p-Cl)PPZn, the unselective autoxidation of cyclohexane
and the unselective thermal decomposition of cyclohexyl hydroper-
oxide were inhibited, resulting in the higher selectivity. This protocol to
enhance the oxidation of cyclohexane is also very applicable to other
cycloalkanes, such as cyclopentane, cycloheptane and cyclooctane. To
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selectivity is still in process in our group at present.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21878275, 21476270, 21776259,
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