A zinc sulfide-supported iron tetrakis (4-carboxyl phenyl) porphyrin catalyst for aerobic oxidation of cyclohexane

Article abstract of DOI:10.1039/c4ra15991h

Zinc sulfide-supported iron tetrakis (4-carboxyl phenyl) porphyrin (Fe TCPP/ZnS) was prepared and used for aerobic cyclohexane oxidation. X-ray diffraction, ultraviolet-visible spectroscopy and Fourier-transform infrared spectroscopy were carried out. The effects of oxygen pressure, reaction temperature, amount of iron tetrakis (4-carboxyl phenyl) porphyrin (Fe TCPP) and reaction time on the Fe TCPP/ZnS-catalyzed cyclohexane oxidation process were investigated. Fe TCPP/ZnS exhibited excellent activity for aerobic cyclohexane oxidation. Under optimal reaction conditions, the turnover number, cyclohexane conversion, cyclohexanone and cyclohexanol yields were 8.6 × 10⁵, 64.9% and 24.4%, respectively. The stability of Fe TCPP was improved after immobilization on zinc sulfide (ZnS), and the catalyst maintained nearly original levels of activity after several reaction cycles.

Full text of DOI:10.1039/c4ra15991h

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A zinc sulfide-supported iron tetrakis (4-carboxyl
phenyl) porphyrin catalyst for aerobic oxidation of
cyclohexane
Cite this: RSC Adv., 2015, 5, 24788
*
*
Yue-xiu Jiang, Tong-ming Su, Zu-zeng Qin and Guan Huang
Zinc sulfide-supported iron tetrakis (4-carboxyl phenyl) porphyrin (Fe TCPP/ZnS) was prepared and used for
aerobic cyclohexane oxidation. X-ray diffraction, ultraviolet-visible spectroscopy and Fourier-transform
infrared spectroscopy were carried out. The effects of oxygen pressure, reaction temperature, amount of
iron tetrakis (4-carboxyl phenyl) porphyrin (Fe TCPP) and reaction time on the Fe TCPP/ZnS-catalyzed
cyclohexane oxidation process were investigated. Fe TCPP/ZnS exhibited excellent activity for aerobic
cyclohexane oxidation. Under optimal reaction conditions, the turnover number, cyclohexane
conversion, cyclohexanone and cyclohexanol yields were 8.6 Â 105, 64.9% and 24.4%, respectively. The
stability of Fe TCPP was improved after immobilization on zinc sulfide (ZnS), and the catalyst maintained
nearly original levels of activity after several reaction cycles.
Received 8th December 2014
Accepted 2nd March 2015
DOI: 10.1039/c4ra15991h
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stability of porphyrins toward the oxidant.9 To improve the
catalytic performance of the metalloporphyrins, immobilization
of metalloporphyrins
boehmite,9,12 halloysite,13 zinc hydroxide nitrate,14 ferroferric
oxide,15 polymeric microspheres,16,17 micro and mesoporous
supports,18 porous metal–organic frameworks,19 and some
other supports20–23 has been investigated.
Introduction
on chitosan,10 organosilicon,11
Selective cyclohexane oxidation is of great signicance for
chemical industries, because the products of the reaction,
cyclohexanone and cyclohexanol, are important intermediates
in the manufacture of nylons 6 and 66.1 The commercial process
for cyclohexane oxidation is carried out at 413–443 K and 1.0–
2.0 MPa and affords only 4% cyclohexane conversion.2
Considering this low conversion and the fact that the process
promotes severe issues, such as corrosion and pollution,
developing highly effective and environment-friendly hetero-
geneous catalysts for cyclohexane oxidation remains an urgent
necessity. In catalytic oxidation of cyclohexane by FeP(PF6)4,
FePCl4, FeP-NaYimp, and Fe-NaY, the cyclohexanol yields reach
30%, 15%, 25%, and 38%, respectively.3 However, the activity
and stability of the catalyst should be further improved.
Metalloporphyrins, as model catalysts of cytochrome P-450,
have recently been recognized as a valuable biomimetic cata-
lysts for alkane oxidation and many other oxidation reactions
with high selectivity and efficiency.4–7 However, metal-
loporphyrins are unstable, difficult to reuse, and degrade easily
in the presence of inert substrates,8 which limits their practical
application in industrial processes. These disadvantages can be
overcome by immobilization on supports. Many supports can
provide a suitable micro-environment for various reactions and
site isolation of metalloporphyrins, reduce the formation of
catalytically inactive m-oxo porphyrin dimers, and increase the
Carbon nanotube-supported iron porphyrins24 and
boehmite-supported iron tetraphenylporphyrins12 are conven-
tionally prepared aer iron porphyrins immobilization on
carbon nanotubes or boehmite to improve the performance of
the porphyrins. And from the previous study, we found that the
stability and catalytic activity of the metalloporphyrin can be
enhanced when used chitosan or boehmite as the
support.8,9,12,25,26 However, cyclohexane conversion, cyclohexa-
none and cyclohexanol yields remain very low. As such, nding
new supports which can signicantly improve the cyclohexa-
none and cyclohexanol yields catalyzed by iron porphyrins is
necessary. Zinc sulde (ZnS) materials are promising supports
because they are chemically more stable and technologically
more advanced than other chalcogenides. These materials have
been used as supports to promote electron transfer reactions at
lower potentials and catalyze electrochemical oxidation of
enzymes to thiocholine.27 However, the use of ZnS as metal-
loporphyrins supports has not been reported.
In this study, zinc sulde-supported iron tetrakis (4-carboxyl
phenyl) porphyrin (Fe TCPP/ZnS) was initially prepared and
used for the aerobic oxidation of cyclohexane to cyclohexanone
and cyclohexanol. Fe TCPP/ZnS was characterized by X-ray
diffraction (XRD), ultraviolet-visible spectroscopy (UV-vis),
Fourier-transform infrared (FT-IR) spectroscopy, and X-ray
photoelectron spectrometer (XPS). The effects of oxygen
Guangxi Key Laboratory of Petrochemical Resource Processing and Process
Intensication Technology, School of Chemistry and Chemical Engineering, Guangxi
University, Nanning 530004, PR China. E-mail: qinzuzeng@gmail.com; huangg66@
126.com; Fax: +86 771 3233718; Tel: +86 771 3233718
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ꢀ
pressure, reaction temperature, amount of Fe TCPP and reac- The mixture was stirred at 300 rpm and heated to 145–175 C.
tion time on the Fe TCPP/ZnS-catalyzed cyclohexane oxidation O2 was continuously pumped into the reaction system and the
process, as well as the reuse of catalysts for cyclohexane oxida- pressure was kept at 0.6–1.1 MPa. O2 ow was measured with a
tion were investigated. This study developed an efficient catalyst rotameter, and the tail gas was determined with a digital oxygen
consisting of iron porphyrin immobilized on ZnS. The catalysts detector. Gas chromatography was used to analyze reaction
obtained catalysts displayed excellent cyclohexanone and mixture samples using an internal standard method with
cyclohexanol yields during cyclohexane oxidation.
chlorobenzene as the standard substance. To study the subse-
quent reuse of Fe TCPP/ZnS, the catalysts were separated from
the reaction mixture immediately aer use, dried naturally at
room temperature, and used in the next catalytic cyclohexane
oxidation reaction.
Experimental section
Preparation of TCPP catalysts
A total of 2.22 g (0.0148 mol) of 4-carboxybenzaldehyde and 70
mL of propionic acid were added to a 250 mL three-necked,
round-bottomed ask with stirring at a speed of 100 rpm,
Upon complete dissolution of 4-carboxybenzaldehyde, the
solution was heated to reux the propionic acid. Then 1.02 mL
(0.0148 mol) of pyrrole was dropped slowly into the solution
using a funnel. Aer reuxing in the dark for 1 h, the solution
was cooled to room temperature and ltered. The ltered cake
was vacuum dried aer washing with deionized water and TCPP
was obtained.
Characterization of Fe TCPP/ZnS catalysts
XRD patterns of ZnS and Fe TCPP/ZnS were obtained using a D/
MAX-2600/PC X-ray powder diffraction instrument (Rigaku
Corporation), equipped with a Cu Ka radiation source. UV-vis
spectra of supported and unsupported catalysts in dimethyl
formamide were recorded using a UV-2102 PCS spectrometer
(Shanghai Nylon Co., Ltd.). FT-IR spectra of ZnS and Fe TCPP/
ZnS were recorded on a Nexus 470 FT-IR spectrophoto-meter
(Nicolet Instrument Corp.) from 400 cmÀ1 to 4000 cmÀ1; KBr
was used as the background. The oxidation state and surface
composition were analyzed using an X-ray photoelectron spec-
trometer (XPS) (Kratos Ultra Axis DLD), equipped with an Al Ka
radiation source, at 150 W with a pass energy of 40 eV.
Preparation of Fe TCPP catalysts
TCPP (0.5 g) and N,N-dimethyl formamide (80 mL) were added
to a 250 mL three-necked, round-bottomed ask with stirring at
a speed of 100 rpm. Upon complete dissolution of TCPP, the
solution was heated to reux and 1.5 g of FeCl2$4H2O was added
to the mixture. Reuxing was continued for 1 h, and the reac-
tion process was tracked using a thin layer chromatography
plate. Aer completion of the reaction, the temperature of the
reaction solution was cooled to below 100 ꢀC. Hydrochloric acid
(6.0 mol LÀ1) was dropped slowly into the solution until
complete dissolution of the red solid. The solution was cooled
to room temperature and 100 mL of deionized water was added
to it. The solution was then allowed to stand for one night and
ltered. The ltered cake was vacuum dried aer washing with
deionized water and Fe TCPP was obtained.
Results and discussion
Characterization of Fe TCPP/ZnS material
When Fe TCPP was anchored on white ZnS, a light green solid
was obtained. This indicates that the presence of Fe TCPP on
the ZnS. On the XRD patterns of ZnS and Fe TCPP/ZnS (Fig. 1),
characteristic diffraction peaks of ZnS were found at 28.56ꢀ,
47.50ꢀ, and 56.37ꢀ; these peaks indicate that the support has a
cubic lattice structure (JCPDS 05-0566).28 The diffraction peaks
of Fe TCPP/ZnS were similar to those of ZnS because of the low
content of immobilized Fe TCPP in the former. This result
indicates that Fe TCPP immobilization does not change the
crystal structure of ZnS, which favors Fe TCPP immobilization
on the ZnS surface.
Preparation of Fe TCPP/ZnS catalysts
A total of 200 mL of 1.3 mol LÀ1 Na2S solution was slowly added
to 200 mL of 1.3 mol LÀ1 ZnSO4 solution with stirring to yield a
white precipitate. The precipitate was ltered and washed with
2À
distilled water until SO4 ion were no longer detected in the
washings; here, 0.05 mol LÀ1 BaCl2 solution was used as a
detector. The precipitate was added then to ethanol (150 mL) in
a three-necked ask, and the mixture was stirred at high speed
for 0.5 h. A solution of Fe TCPP (25 mg) in absolute ethanol (20
mL) was slowly added to the suspension, and the mixture was
heated at 65 ꢀC with rapid stirring for 5 h. The suspension was
ltered, and the solid was washed with distilled water. The
ꢀ
ltered cake was dried at 0.09 MPa and 160 C for 5 h and Fe
TCPP/ZnS was obtained.
Cyclohexane oxidation catalyzed by Fe TCPP/ZnS with O2
A 250 mL autoclave reactor was charged with a measured
amount of Fe TCPP or Fe TCPP/ZnS and cyclohexane (200 mL). Fig. 1 XRD patterns of ZnS (a) and Fe TCPP/ZnS (b).
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Fig. 2 shows the UV-vis spectra of Fe TCPP and Fe TCPP/ZnS.
The Soret peak of Fe TCPP at room temperature was observed at
418 nm. Aer Fe TCPP immobilization on ZnS, the Soret peak
violet shied to 415 nm and a new peak was observed at 460 nm,
which may be explained by interactions between the metal-
loporphyrin complex and the surface of the solid support. This
nding indicates that Fe TCPP is immobilized on ZnS through
coordination bonds, although no change in the macrocyclic
complex structure of the support is observed; the saddle-shaped
Fe TCPP may be placed on the ZnS surface in a single layer
during immobilization,29 resulting in the violet shi of the Soret
peak.
Fig. 3 FT-IR spectra of ZnS (a) and Fe TCPP/ZnS (b).
The FT-IR spectrum of ZnS is similar to that of Fe TCPP/ZnS
(Fig. 3). Absorption peaks between 3410 and 3465 cmÀ1 are due
to OH groups on the ZnS surface.30 In the FT-IR spectra of Fe
TCPP/ZnS, peaks observed at 1107, 619, and 473 cmÀ1 aÀre1
assigned to Zn–S vibrations.30,31 The peak observed at 1640 cm
in the spectrum of ZnS shied to 1624 cmÀ1 upon Fe TCPP
immobilization on ZnS. The peak at 1001 cmÀ1 disappeared and
7.7% to 38.1% and 11.3%, respectively, which indicated that the
chemical state of C might be affected by immobilizing on ZnS.
In the O 1s spectrum, the binding energy of O 1s in the Fe
TCPP was 531.9 eV and 533.5 eV, respectively, which assigned to
the carboxylic O (C]O) and hydroxyl O (C–OH).35 In the Fe
TCPP/ZnS, the binding energy of O 1s in hydroxyl (533.5 eV) and
in carboxylic (531.9 eV) was slightly shied to 533.3 eV and
531.7 eV, respectively. And the percentage of hydroxyl O was
decreased from 52.2% to 9.1% as well as that of the carboxylic O
was increased from 47.8% to 90.9%. however, aer Fe TCPP
immobilizing on ZnS. Meanwhile, to compare from the oxygen
amounts of the two samples, which usually are represented by
the XPS tting peaks area,36 the amount of carbonyl O is
increased greatly and almost about double in the peak area to
the one before Fe TCPP immobilizing on ZnS; and on the
contrary, the amount of hydroxyl O is greatly reduced, and
which is only 17% of the one before Fe TCPP immobilizing on
ZnS. In general, the H readily ionized from C–OH to form free
radicals, which lead to the change of the external environment
of O in the C–OH. as show in Table 1, the amount of C–OH is
decreased, which means free radicals are easily formed and
reacted aer Fe TCPP immobilized on ZnS.
two new absorption peaks were observed at 931 and 1193 cmÀ1
.
This result indicates that Fe TCPP is immobilized on ZnS by
coordination bonds, i.e., Zn–S–Fe, which strengthens the
stretching vibration frequency of the S–Fe bond, is formed. We
therefore suggest that Fe TCPP is immobilized on ZnS such that
sulfur atoms use one of the electron pairs of sulfur ion to
coordinate with an iron ion, thereby forming an Fe TCPP/ZnS
complex.
XPS analysis of Fe TCPP and Fe TCPP/ZnS
X-ray photoelectron spectroscopy (XPS) was applied to study the
composition of elements on or near the surface of the Fe TCPP
and Fe TCPP/ZnS (Fig. 4 and Table 1). From Fig. 4a, at least 3
types of C species are tted with bands at 284.8 eV (90.1%),
286.8 eV (2.2%), and 288.8 eV (7.7%), are found on Fe TCPP,
which attribute to the existing of C species in C–H, C–OH, and
C]C, respectively.32–34 In C 1s spectrum of Fe TCPP/ZnS, the
binding energy (284.8 eV) of C 1s of C–H was the same as that
(284.8 eV) of Fe TCPP. The binding energy of C 1s in the C–OH,
and C]C was blue shi by 1.2 eV and 0.2 eV, and the
percentage of C in C–OH and C]C increased from 2.2% and
Fig. 2 UV-vis spectra at room temperature. Dimethyl formamide
solution of Fe TCPP (a), suspension of Fe TCPP/ZnS in dimethyl Fig. 4 XPS spectra of C 1s, O 1s, N 1s, and Fe 2p for Fe TCPP (a) and Fe
formamide (b).
TCPP/ZnS (b).
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Table 1 Binding energy of core electrons of Fe TCPP and Fe TCPP/ZnS catalysts
Fe TCPP
Fe TCPP/ZnS
Percent of valence
Existential form
of elements
Percent of valence
statea/%
XPS spectra
C 1s
BE/eV
Peak area/unit
BE/eV
state/%
Peak area/unit
C–H
284.8
286.8
288.8
531.9
533.5
398.7
401.4
—
90.1
2.2
7.7
47.8
52.2
88.1
11.9
67 148
1623
5731
18 448
20 117
8178
284.8
285.6
288.6
531.7
533.3
50.6
38.2
11.3
90.9
9.1
11 619
8761
2596
35 845
3576
C–OH
C]C
C]O
C–OH
N–Fe
N–C
O 1s
N 1s
S 2p
1105
400.1
161.6
162.9
100
66.9
33.1
2887
25 716
12 701
S–Fe
S–Fe
—
a
Surface concentrations of different C, O, N, and S states are in parentheses.
The binding energy located at 398.7 eV and 401.4 eV in the N at higher temperatures may be attributed to product oxida-
1s spectrum of Fe TCPP are ascribed to the nitrogen in the Fe–N tion.46 In the present experiment, the optimal reaction
ꢀ
when Fe TCPP was formed and the pyrrolic nitrogen in the temperature was 160 C.
porphyrin core, respectively.37–39 The results of N 1s spectrum of
To determine whether or not higher pressures are benecial
Fe TCPP indicate that there are at least two forms of nitrogen to product yields, the effect of increasing pressure from 0.6 MPa
atoms in the Fe TCPP catalyst. The percentage of Fe–N is 88.1%, to 1.0 MPa was investigated at 160 ꢀC using Fe TCPP/ZnS as the
and the percentage of pyrrolic nitrogen is only 11.9% (Table 1). catalyst; relevant results are shown in Fig. 5B. Cyclohexanone
However, in the N 1s spectrum of the Fe TCPP/ZnS, only one and cyclohexanol yields increased with increasing pressure and
characteristic peak located at 400.1 eV was observed and then decreased aer a certain pressure was reached under the
matches the contributions from the pyrrolic nitrogen in the reaction conditions. Cyclohexanone and cyclohexanol yields
porphyrin core, indicating the four equivalent nitrogen atoms were the highest (24.4%) when the pressure was 0.8 MPa.
in the complex.40,41
Higher oxygen pressures indicate higher initial dissolved
For S 2p XPS spectrum in the Fe TCPP/ZnS, the characteristic oxygen concentrations in the liquid phase. Aer a certain
peaks of S 2p3/2 and S 2p1/2 are located at 161.6 eV and 162.9 eV,
which are 161.9 eV and 163.2 eV in a pure ZnS.42 Aer immo-
bilization on ZnS, the binding energy of S 2p3/2 and S 2p1/2 blue
shied by 0.3 eV, respectively, which may be caused by the
electronic interaction between the Fe ion and the ZnS substrate,
indicating that coordination bonds (S–Fe) may be formed,43–45
which reduced the redox potential of the Fe ion in Fe TCPP/ZnS.
This results is helpful for oxygen activation by the metal-
porphyrin and improves the catalytic activity of the
metalporphyrin.
Effect of reaction conditions on Fe TCPP/ZnS-catalyzed
cyclohexane oxidation
Fig. 5A shows the effect of reaction temperature on the cyclo-
hexane oxidation reaction. When the temperature was below
160 ꢀC, the cyclohexanone and cyclohexanol yields and turnover
number (TON) increased quickly with increasing temperature.
Cyclohexanone and cyclohexanol yields increased from 10.1%
Fig. 5 Effects of reaction temperatures (A), reaction pressures (B),
amount of Fe TCPP (C) and reaction time (D) on the cyclohexane
oxidation catalyzed by Fe TCPP/ZnS. Reaction conditions: (A) cyclo-
5
ꢀ
ꢀ
(145 C) to 24.4% (160 C) and TON increased from 2.1 Â 10
(145 ꢀC) to 8.6 Â 105 (160 ꢀC). However, at temperatures was
higher than 160 ꢀC, the TON, cyclohexane conversion, selec- hexane, 200 mL; oxygen pressure, 0.8 MPa; Fe TCPP, 1.0 mg; reaction
time, 4 h, tail gas flow, 0.04 m3
h
À1. (B) Cyclohexane, 200 mL;
tivity, and cyclohexanone and cyclohexanol yields decreased
from 8.6 Â 105, 64.9%, 37.6%, and 24.4% (160 ꢀC) to 3.7 Â 105,
54.9%, 25.1%, and 13.8% (165 ꢀC), respectively. The decrease in
cyclohexane conversion could be due to the low stability of the
temperature, 160 ꢀC; Fe TCPP, 1.0 mg; reaction time 4 h, tail gas flow,
0.04 m3 hÀ1. (C) Cyclohexane, 200 mL; temperature, 160 ꢀC; oxygen
pressure, 0.8 MPa; reaction time, 4 h, tail gas flow, 0.04 m3 À1. (D)
h
Cyclohexane, 200 mL; oxygen pressure, 0.8 MPa; temperature, 160
catalyst at higher temperatures, and the reduction in selectivity ꢀC; Fe TCPP, 1.0 mg, tail gas flow, 0.04 m3 hÀ1
.
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oxygen concentration is reached at 160 ꢀC, further increases in
oxygen pressure cannot markedly accelerate the reaction rate
and cyclohexane conversion reaches nearly maximum values.
Higher oxygen pressures in the reaction system can promote the
formation of by-products, which are responsible for the low
selectivities of cyclohexanone and cyclohexanol.10 Catalysts can
be destroyed by high oxygen concentrations,26 thereby
decreasing cyclohexane conversion. Based on the results,
oxygen pressure is an important parameter inuencing cyclo-
hexane oxidation and 0.8 MPa is the optimal pressure
promoting both activity and selectivity.
Fig. 5C shows that the optimal Fe TCPP value. As the Fe TCPP
amount increased to 1.0 mg, maximum cyclohexanone and
cyclohexanol yields were observed. Cyclohexanone and cyclo-
hexanol yields subsequently decreased as the Fe TCPP amount
continued to increase. Excessive addition of Fe TCPP may
improve interactions between activated intermediates and
suppress the formation of cyclohexanone and cyclohexanol.47
The results indicate that the polymer microenvironment of
metalloporphyrins catalysts is an important factor inuencing
product yields. This microenvironment not only facilitates
catalytic action but also effectively protects metalloporphyrins
from dimer formation and destruction, thereby resulting in
high catalytic activity.16
Fig. 6 Comparison of cyclohexane oxidation catalyzed by Fe TCPP/
ZnS and Fe TCPP. Reaction conditions: cyclohexane, 200 mL; oxygen
pressure, 0.8 MPa; temperature, 160 ꢀC; Fe TCPP, 1.0 mg; reaction
time, 4 h.
oxidation. When the chitosan-supported TPPFeCl,26 CoII TPP/
BM,25 CoII TCPP/AlOOH,9 Mn(III)TPP–Au/SiO2,48 DP-IX-DME-
Co(II),49 and CTSPFe50 Fe TNPP/BM,12 and Fe T(p-MeO)PP/BM8
was used as the catalyst, the cyclohexanone and cyclohexanol
yields were 9.3%, 8.5%, 14.3%, 4.8%, 15.7%, 6.4%, 5.3%, and
4.7%, respectively. When ZnO was used as the support for Fe
(TPFPP), the stability of the Fe (TPFPP) was improved, aer
reused for 11 times, the cyclohexanone and cyclohexanol yields
just decreased from 22.5% to 18.1%.43
In the present study, the cyclohexanone and cyclohexanol
yields reached 24.4% on Fe TCPP/ZnS, which was much
higher than that of the previous studies. The reason is
probably that the Fe TCPP/ZnS has a better performance for
activating molecular oxygen than the Fe TCPP. And according
to the results of UV-vis spectra and FT-IR spectra, the sulphur
atom of ZnS may use one of the electron pairs to coordinate
with a iron ion and cause the Fe TCPP immobilize on ZnS by
chemical coordination, which may improve the activity of Fe
TCPP. However, in the catalytic reaction process, Fe TCPP is
superior to Fe TCPP/ZnS in promoting cyclohexanone and
cyclohexanol selectivity. This result may be due to improve-
ments in the catalytic activity of Fe TCPP when immobilized
on ZnS, which increases the number of activated oxygen
molecules. An abundance of activated oxygen molecules
allows continuous oxidation of cyclohexanone and cyclo-
hexanol to other by-products, such as acids and esters. In
conclusion, ZnS is an excellent support that provides vital
assistance for metalloporphyrins catalyzing cyclohexane
oxidation.
Fig. 5D illustrates changes in the selectivity and yield of
cyclohexanone and cyclohexanol, cyclohexane conversion,
ꢀ
and TON under an optimal temperature of 160 C and pres-
sure of 0.8 MPa catalyzed by Fe TCPP/ZnS. In generally, yields
increased with increasing reaction time. From 0 h to 0.5 h,
cyclohexanone and cyclohexanol yields slightly increased.
This phenomenon may be attributed to the fact that the
supported catalyst undergoes induction and activation in the
present reaction conditions and that cyclohexane is oxidized
to only very small amounts of main products.10 Cyclohexa-
none and cyclohexanol yields sharply increased aer 4 h of
reaction and then showed maxima (24.4%). Cyclohexanone
and cyclohexanol selectivity decreased with increasing reac-
tion time, possibly because these products are more readily
oxidizable than cyclohexane. Besides the limited conversion
observed, other oxidation products, such as adipic acid and
ester, were observed. Fe TCPP/ZnS catalyzed cyclohexane
oxidation under optimal conditions to give cyclohexanone
and cyclohexanol with maximum yields (24.4%).
Comparison of cyclohexane oxidation catalyzed by Fe TCPP/
ZnS and Fe TCPP
Cyclohexane oxidation catalyzed by reused Fe TCPP/ZnS
To investigate the effect of ZnS on the cyclohexane oxidation
reaction catalyzed by metalloporphyrins, the catalytic activities When Fe TCPP/ZnS is reused four times, cyclohexane conver-
of Fe TCPP/ZnS and Fe TCPP were studied under the same sion, cyclohexanone and cyclohexanol yields and TON remain
conditions; relevant results are shown in Fig. 6.
higher than those obtained from Fe TCPP catalysis (Table 2).
Fe TCPP/ZnS exhibited more notable activities than Fe TCPP The Fe TCPP/ZnS results are attributed to the catalytic activity
during cyclohexane conversion. When Fe TCPP/ZnS and Fe promotion of Fe TCPP by immobilization on ZnS, ZnS offers
TCPP were used as catalysts aer 4 h of reaction, cyclohexane protection from destructive oxidation by oxygen,51 prevents Fe
conversion rates of 64.9% and 35.6%, respectively, were TCPP agglomeration, and improves the mechanical strength of
observed. These conversion rates indicate that interactions Fe TCPP, thereby rendering the Fe TCPP catalyst more stable,
between ZnS and Fe TCPP enhance their catalytic activity during active, easily recoverable and reusable.
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Table 2 Cyclohexane oxidation catalyzed by reused Fe TCPP/ZnSa
ROOH + [O]FeIVTCPP/ZnS]+c / ROc + OHÀ
(3)
(4)
(5)
(6)
Catalysts
Run
Conversion/% Selectivity/% TON/(Â 105) Yield%
ROOH + [O]FeIVTCPP/ZnS]+c / ROOc + H+
ROc + ROOH / ROH + ROOc
ROc + RH / ROH + Rc
Fe TCPP/ZnS 1
64.9
49.5
40.9
41.6
37.6
42.5
46.8
41.2
42.0
47.9
8.6
6.8
5.8
5.8
6.8
5.1
24.4
21.1
19.2
17.1
20.5
17.1
2
3
4
Average 49.2
1 35.6
Fe TCPP
The radical chain stops when two cyclohexyl radicals collide,
and radical coupling occurs to form R–R groups; two alkyl
peroxy radicals (ROOc) may also react to form ketone (R]O) and
oxygen. The catalytic mechanisms of supported and unsup-
ported metalporphyrins are similar, but the redox potential of
the Fe ion in Fe TCPP is fairly low because of the Zn–S–Fe bond
formed between the Fe TCPP and ZnS, which was supported by
the XPS results. The coordination bonds (S–Fe) promotes the
catalysis of the Fe TCPP for the cyclohexane oxidation, which
a
Reaction conditions: cyclohexane, 200 mL; oxygen pressure, 0.8 MPa;
temperature, 160 ꢀC; Fe TCPP, 1.0 mg; reaction time, 4 h, tail gas ow,
0.04 m3 hÀ1
.
Suggested mechanism for the cyclohexane oxidation catalyzed
by Fe TCPP/ZnS
The catalytic oxidation reaction mechanism of metalporphyrin was similar to the results of our previous studies.43 Therefore,
has recently been reported; metalporphyrin intermediates the cyclohexane conversion, cyclohexanone and cyclohexanol
possess an important function in the oxidation reaction. Given yields can be improved by this process.
current limitations in science and technology, many interme-
diates cannot be detected, so the reaction mechanism pre-
sented here is hypothetical.35 Lyons36 proposed a hypothetical
catalytic oxidation hypothetical mechanism of iron porphyrins,
which is similar to that of the P-450 enzyme. The reaction
mechanism of cyclohexane oxidation catalyzed by Fe TCPP/ZnS
is similar to that of hydrocarbon oxidation catalyzed by metal-
porphyrins, i.e., a radical reaction mechanism. The suggested
mechanism is shown in Fig. 7.
Conclusions
ZnS promoted the catalytic activity of Fe TCPP for the aerobic
oxidation of cyclohexane by forming coordination bonds with
Fe TCPP. Under optimal conditions, the TON, cyclohexane
conversion and cyclohexanone and cyclohexanol yields were 8.6
 105, 64.9%, and 24.4%, respectively. Fe TCPP/ZnS could
continuously be reused four times in the aerobic oxidation of
cyclohexane to cyclohexanol and cyclohexanone with little
change of catalytic performance. The average cyclohexane
conversion and cyclohexanone and cyclohexanol yields were
49.2% and 20.5%, respectively. ZnS signicantly improved the
catalytic activity of Fe TCPP for the aerobic oxidation of cyclo-
hexane and is thus an excellent support material. Fe TCPP/ZnS
catalyst, which possessed much higher catalytic activity than Fe
TCPP, was conveniently prepared and was easily recovered by
simple isolation from the reaction mixture, and it was a
promising catalyst in industrial applications.
The suggested mechanism of cyclohexane oxidation catalyzed
by Fe TCPP/ZnS is as follows: FeIII TCPP Cl/ZnS loses axial
chlorine atoms and FeIII TCPP Cl/ZnS is reduced to FeII TCPP/
ZnS, which reacts with O2 to form O2FeIIITCPP/ZnS. However,
the peroxy bond of O2FeIIITCPP/ZnS is easily fractured, and
[O]FeIVTCPP/ZnS]+c is generated. [O]FeIVTCPP/ZnS]+c inter-
mediates show high activity for capturing hydrogen atoms of
cyclohexane (RH) and form cyclohexyl radicals (Rc), which causes
the radical chain. Aer initiation of the radical chain, six radical
chain transfer processes may occur according to eqn (1) to (6).
Rc + O2 / ROOc
(1)
(2)
Acknowledgements
ROOc + RH / ROOH + Rc
This work was supported by the National Natural Science
Foundation of China (no.: 51063001 and 51363001), the
Guangxi Scientic and Technological Project (12118008-12-3),
the Guangxi Natural Science Foundation (2012GXNSFAA053016
and 2014GXNSFDA118009) and the Open Project of Guangxi
Key Laboratory of Petrochemical Resource Processing and
Process Intensication Technology (2012K06).
Notes and references
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Fig. 7 Suggested mechanism of cyclohexane oxidation catalyzed by
Fe TCPP/ZnS.
This journal is © The Royal Society of Chemistry 2015
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