Synthesis of porous polymeric metalloporphyrins for highly efficient oxidation of cyclohexane in heterogeneous systems

Article abstract of DOI:10.1039/c7nj00564d

Herein, porphyrin-conjugated organic polymers, Mn(iii)P-CMP and Fe(iii)P-CMP, were synthesized by Suzuki coupling reaction with manganese/iron tetraphenylporphyrin (T(p-Br)PPMn/FeCl) and 1,4-phenyldiboronic acid as building blocks. The structures of the p

Full text of DOI:10.1039/c7nj00564d

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Synthesis of porous polymeric metalloporphyrins
for highly efficient oxidation of cyclohexane in
heterogeneous systems†
Cite this:DOI: 10.1039/c7nj00564d
Yongjin Li, Chuanrong Liu and Weijun Yang
*
Herein, porphyrin-conjugated organic polymers, Mn(III)P-CMP and Fe(III)P-CMP, were synthesized
by Suzuki coupling reaction with manganese/iron tetraphenylporphyrin (T(p-Br)PPMn/FeCl) and
1,4-phenyldiboronic acid as building blocks. The structures of the pore and surface were characterized
by nitrogen adsorption and desorption isotherm, FE-SEM, and HR-TEM. The electrochemical behavior of
Mn(III)P-CMP was tested by cyclic voltammetry. For the catalytic oxidation of higher inert C–H bond in
cyclohexane, Mn(III)P-CMP and Fe(III)P-CMP with hyper-conjugated and microporous structures
functioned better than the monomer metalloporphyrin. With MnP-CMP as the preferred catalyst, after
3.5 h of reaction, the yield of cyclohexanol and cyclohexanone was 21.6%, and the conversion of
cyclohexane reached 29.9% at the Mn3+/cyclohexane molar ratio of 1/168 000, 150 1C, and 0.8 MPa.
Mn(III)P-CMP and Fe(III)P-CMP were structurally stable and insoluble in common solvents and hardly
degraded below 300 1C; hence, they had excellent reusability.
Received 16th February 2017,
Accepted 26th June 2017
DOI: 10.1039/c7nj00564d
rsc.li/njc
The commonly used method to solve this problem is the
immobilization of monomer metalloporphyrin on a solid support
1. Introduction
Owing to its industrial importance, the oxidation of cyclohexane surface that converts metalloporphyrin catalytic system to hetero-
to corresponding products has been investigated for decades, geneous catalysis.16–18 On the one hand, it improved the stability
and some meaningful research results have been achieved.1–4 of metalloporphyrin via the adsorption properties of porphyrin on
Cytochrome P450 can efficiently activate oxygen molecules for different solid support surfaces. On the other hand, it also improved
the highly selective catalytic oxidation of the C–H bond to form the catalytic efficiency of metalloporphyrin due to the intrinsic
corresponding products under mild conditions.5,6 Metalloporphyrin, characteristics of solid support. However, the bond between
which has a structure similar to that of the active center of porphyrin and solid support surface is not stable. In addition, the
cytochrome P450 enzyme, also functions similarly and is green, activity of the supported metalloporphyrin rapidly declines when it
efficient, environmentally friendly, and reactive under mild is reused.19 This problem may be solved by aggregating metallo-
conditions.7–10 Tetraphenylmetalloporphyrin has been used as porphyrins into special insoluble macromolecular polymers.20–24
a biomimetic catalyst for the catalytic oxidation of cyclohexane, Side chains outside the metalloporphyrin macrocycle are bridged
which elevates the conversion rate of cyclohexane from 4% to by the groups with conjugated effect to form a hyper-conjugated
10% with the reaction time reduced to half as compared to that micro–mesoporous polymer (CMP).25–27 Recently, researchers have
under the original, non-catalytic conditions with cyclohexanol endeavoured to prepare CMPs with new structures.28–30 Jiang et al.
or cyclohexanone as additives. Moreover, the total selectivity of reported a Fe-based micro–mesoporous-conjugated porphyrin
cyclohexanol and cyclohexanone is increased from 78% to 85% polymer that catalytically oxidized organic sulfur and olefin double
or higher.11–13 However, metalloporphyrin can be easily bond compounds.31,32 CMPs in which porphyrin monomers were
degraded by the high concentration of peroxide produced in connected with stable covalent bonds were superior to the supported
the reaction system; this causes the loss of oxidation activity and porphyrin. The hyper-conjugated effect enhanced the stability of
recyclability as well as considerable high cost in industrial porphyrin, and the CMPs with micro–mesoporous and large surface
application.14,15
area allowed more exposure of porphyrin active reaction sites on the
surface of polymer, which clearly improved the catalytic efficiency. A
series of phenylimine-bridged and alkynyl-bridged manganese
porphyrin polymers was synthesized and applied to aerobic
oxidation of alcohols and toluene in our previous works, which
Department of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, Hunan, China. E-mail: wjyang@hnu.edu.cn;
Fax: +86 731 88713642; Tel: +86 731 88821449
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj00564d showed good catalytic activity.33,34
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Herein, the conjugated porous metalloporphyrin polymers
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FeP-CMP and MnP-CMP coupled with phenylene were synthe-
sized using metalloporphyrin as the monomer and applied in
the highly efficient catalytic oxidation of higher inert C–H bond
in cyclohexane. Due to their high stability, FeP-CMP and MnP-
CMP resisted the damages caused by high-concentration of
cyclohexane peroxide and high temperature, such that the
catalytic activities were retained, and the catalysts could be
reused. Due to the synergistic effect of hyper-conjugation and
numerous pores, the catalytic activities were found to be higher
than those of monomer metalloporphyrin, and the selectivity of
the target products alcohol and ketone augmented as well.
Accordingly, CMPs are potentially eligible heterogeneous catalysts
for cyclohexane oxidation.
Fig. 1 Schematic of the MnP-CMP synthesis.
Mn(III)P-CMP was synthesized via the following steps: a
mixture of T(p-Br)PPMnCl (0.20 mmol), K2CO3 (1.60 mmol),
bis(tricyclohexylphosphine)palladium(0) (20 mmol), 1,4-phenyl-
diboronic acid (0.80 mmol), 1,4-dioxane (16.0 mL), and aqueous
solution (4.0 mL) was added to a reaction flask, degassed by
three freeze–pump–thaw cycles, purged with argon, and stirred
at 125 1C for 24 h. TLC method was used to detect excess 1,4-
phenyldiboronic acid. Finally, the mixture was allowed to cool at
room temperature and poured into plenty of water. The pre-
cipitate was obtained by filtration, thoroughly washed with THF,
methanol and trichloromethane, and then dried in vacuum. The
solid was rigorously washed by Soxhlet extraction for 24 h with
THF, methanol, and trichloromethane until the solvent in the
Soxhlet extractor did not exhibit porphyrin peaks in UV-vis
detection and then dried in vacuum to obtain Mn(III)P-CMP as
dark green solid particles in 91.12% yield, Mn content: 4.98%.
The synthesis method of Fe(III)P-CMP was the same as that of
Mn(III)P-CMP, giving atroviren solid particles in 90.36% yield, Fe
content: 4.22%.
2. Experimental
2.1. Reaction reagents and instruments
Bis(tricyclohexylphosphine)palladium(0), 1, 4-dioxane, 1,4-phenyl-
diboronic acid, and other reagents were analytical grade and
purchased from J&K Chemicals, and 1,4-dioxane was distilled over
benzophenone ketyl under argon before use. T(P-Br)PPMnCl and
T(p-Br)PPFeCl were synthesized by our group according to the
documented procedures and characterized by IR, UV-vis, 1H-NMR,
and MS spectroscopic methods (ESI†).35
The specific surface areas and pore size distribution were
detected by Beckman Coulter, Inc. SA 3100 accelerated surface
and porosity analyzer with nitrogen as the probing gas at 77 K.
The specific surface areas were calculated by the Brunauer–
Emmett–Teller (BET) method, and the NLDFT theory was utilized
to estimate the pore size, pore volume, and pore distribution. Field
emission scanning electron microscopy (FE-SEM) images were
obtained using a JEOL model JSM-5600Lv microscope. Before
measurements, gold was sprayed on the surface of the samples.
High resolution transmission electron microscopy (HR-TEM)
images were obtained using a JEOL model JEM-3010 microscope.
FT-IR spectra were obtained using a Fourier transform infrared
spectrometer (IRrestige-21, Shimadzu), and the sample was
completely dried and diluted by potassium bromide. UV-vis
absorption spectra were obtained using a Cary 100 UV-Vis
spectrophotometer from Agilent technologies, and the diffuse
reflectance spectra were obtained via the spectrophotometer
equipped with integration sphere, and the sample was diluted
with BaSO4. Elemental analyses of C, H, and N were carried out
using a vario ELIII model from German Elementar corporation.
Elemental analyses of Mn/Fe were carried out by atomic absorption
spectrometry using a Perkin Elmer Aanalyst 700/800. Elemental
analyses of Br and Cl were carried out by combustion.
2.3. Cyclic voltammetry of Mn(III)P-CMP
Cyclic voltammetry was performed using a CHI66OB electro-
chemical workstation with a three-electrode cell system in
which a glassy carbon electrode after loading MnP-CMP, a
calomel electrode, and a Pt wire were used as the working,
reference, and counter electrodes, respectively. For the preparation
of the working electrode, 6.0 mg of Mn(III)P-CMP sample was
dispersed in 0.5 mL of solvent mixture containing 11 mL of Nafion
(5 wt%) and 0.49 mL of ethanol by sonication for more than 1 h to
obtain a stable suspension. Subsequently, 10 mL portion of the
sample solution was slightly dropped on the surface of the
pre-polished glassy carbon electrode. The electrodes were dried
overnight at room temperature for measurement. The electro-
chemical experiments were conducted in 0.1 mol LÀ1 (CH3CH2)4NBr
CH2Cl2 solution and cyclically scanned at a scan rate of 50 mV sÀ1 at
room temperature after Ar gas was purged for 10 min.
2.2. Synthesis of Fe and Mn porphyrin polymers
2.4. Catalytic oxidation of cyclohexane
To connect porphyrin monomers to form polymers, we chose
T(p-Br)PPMnCl and 1,4-phenyldiboronic acid with two boronates Cyclohexane oxidation with air was catalyzed by Mn(III)P-CMP.
as building blocks in the presence of bis(tricyclohexylphosphine)- Cyclohexane 200 mL (1.85 mol) and Mn3+/cyclohexane =
palladium(0) as the catalyst under alkaline condition to synthesize 1/168 000 (molar ratio) of Mn(III)P-CMP were added to a 250 mL
Mn(III)P-CMP by the Suzuki cross-coupling reaction (Fig. 1).
autoclave reactor. The stirring speed was set at 400 rpm, with the
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needle valve and rotameter switch controlling and metering the
At 77 K, Mn(III)P-CMP displayed a typical type-II and S-type
gas flow rate, respectively. The mixture was rapidly sampled and sorption isotherm curve along with a strong adsorption at low
analyzed during the reaction. Reaction products were identified pressures (P/P0 o 0.1) (Fig. 2a and c), suggesting the coexistence
by GC-MS (Shimadzu QP-5000 MS). The samples were analyzed of micro- and mesopores in the framework. Fig. 2b and d also
by GC using chlorobenzene as an internal standard. Chromato- show that there are a large number of micro- and mesopores in
graphic analyses were conducted using a Shimadzu GC-17A series the framework. The main pore diameters were about 1.12 nm
gas chromatograph (helium as the carrier gas) equipped with an and 2.58 nm, and the contributions to the pore volume were
FID detector and a PEG-20000 (25 m  0.25 mm) capillary column. 32% and 33% respectively. As evidenced by the gentle curve
The contents of adipic acid, ester, and hyperoxide were measured between 1.12 nm and 2.58 nm, the pore size distribution was
by chemical titration according to a documented procedure.36
relatively uniform and the contribution to the pore volume was
12% (Fig. 2b). Except for the pores with the diameter of 1.06 nm,
the pore sizes were unevenly distributed (Fig. 2d). The contribution
of 1.06 nm-wide pores to the volume was 26%, whereas that of the
total micropores (micropore volume: 0.196 cm3 gÀ1) was 36%. On
the one hand, the pore structures of Mn(III)P-CMP and Fe(III)P-CMP
also verified porphyrin polymerization. On the other hand, the
pore structures enhanced the surface area of the polymers that
also enhanced the porphyrin reactive sites on the surface of
polymers during the reaction.
3. Results and discussion
3.1. Mn/FeP-CMP characterization
Mn(III)P-CMP and Fe(III)P-CMP conjugated metalloporphyrin
polymers had large surface areas and porous structures. The
BET surface areas of Mn(III)P-CMP and Fe(III)P-CMP were as
high as 325 m2 gÀ1 and 296 m2 À1, respectively (Table 1).
g
Moreover, the BET surface area of the monomer metalloporphyrin
powders was much lower than that of CMPs and almost without
pores. Therefore, Mn(III)P-CMP is an excellent micro- and
mesoporous polymer.
Monomer T(p-Br)PPMnCl had significantly less amount of
nitrogen adsorption at the same pressure and did not show
strong adsorption at low pressures (P/P0 o 0.1) as compared to
Mn(III)P-CMP (Fig. 2e). The BET surface area was only 16.72 m2 gÀ1
,
which mostly belonged to external specific surface area and pore
accumulation. Thus, it was a typical non-porous material.
FE-SEM images (Fig. 3a and b) indicated that Mn(III)P-CMP
comprised about 0.2–5.0 mm monoliths originating from the
overlapping of circular flakes that were about 200–500 nm in
diameter. Transition from a molecular level framework to
nanoscale layers and further to microscale monoliths attracted
significant attention because it was ubiquitous in biological
catalytic systems. The columnar structure was accumulated by
nanoscale layers of circular porphyrin flakes among which
Table 1 Specific surface areas and pore volumes of Mn(III)P-CMP, Fe(III)P-
CMP, and T(p-Br)PPMnCl
SBET
Smicro
Vtotal
Vmicro
(m2 gÀ1
)
(m2 gÀ1
)
(cm3 gÀ1
)
(cm3 gÀ1
)
Mn(III)P-CMP
Fe(III)P-CMP
T(p-Br)PPMnCl
325
296
16
143
115
2.048
0.512
0.545
0.225
0.196
—
2.275 Â 10À4
SBET: total BET; Smicro: micropore BET; Vtotal: total pore volume; and
Vmicro: micropore volume.
Fig. 2 Nitrogen adsorption (K) and desorption (J) isotherm profiles of MnP-CMP (a), FeP-CMP (c), and T(p-Br)PPMnCl (e) at 77 K; pore size distribution
(J) and cumulative pore volume (K) of MnP-CMP (b) and FeP-CMP (d).
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wide-range flow of electrons and delocalization. As a result,
Mn3+ ions in the middle of porphyrin rings are further activated
and easily form active valence ions Mn2+, thus promoting the
catalytic reaction.
The phenylene bridges in four directions of porphyrin rings
enable Mn(III)P-CMP to be a highly developed micro- and
mesoporous and hyperconjugated metalloporphyrin polymer;
hence, the catalytic oxidation activity of Mn(III)P-CMP towards
C–H bond is significantly augmented. Furthermore, the XRD
profile confirms the amorphous character of Mn(III)P-CMP
(ESI,† Fig. S8). Owing to the covalent attachment and cross-
linked structure, Mn(III)P-CMP is stable and insoluble in water,
dichloromethane, THF, acetone, methanol, DMSO, and some
other organic solvents. The thermal property of Mn(III)P-CMP
was characterized by thermogravimetric analysis (ESI,† Fig. S9).
Mn(III)P-CMP is stable under 300 1C, indicating that Mn(III)P-CMP is
highly thermally stable. Accordingly, Mn(III)P-CMP should have
excellent catalytic oxidation performance and favorable reusability.
Fig. 3 FE-SEM images of MnP-CMP (a) and FeP-CMP (b); HR-TEM images
of MnP-CMP (c) and FeP-CMP (d).
3.2. Catalysis of air oxidation of cyclohexane by Mn(III)P-CMP
there was network interpenetration. Their surfaces were quite
rough and HR-TEM images showed that there were nanopores To research the catalytic activity and product formation rules of
on the surface of Mn(III)/Fe(III)P-CMP (Fig. 3c and d), consistent conjugated metalloporphyrin polymers during the air oxidation
with the pore size analysis results. The pore structure and the of cyclohexane, cyclohexane was oxidized with air at 150 1C and
roughness of the surface implied more reactive site exposure to 0.8 MPa with Mn(III)P-CMP as the catalyst, and the mixture of
the substance, which would promote the catalytic reaction.
reaction products was sampled every 30 min. GC-MS analysis
As shown in Fig. S5 (ESI†), the IR spectrum of T(p-Br)PPMnCl results showed that the main products were cyclohexanol and
was different from that of Mn(III)P-CMP, which indicates that cyclohexanone, and the by-products were mainly adipic acid,
the reaction occurred on the other side. The characteristic peak dicyclohexyl adipate, and cyclohexyl hydroperoxide. The contents
at 475 cmÀ1 in the spectrum of T(p-Br)PPMnCl is assigned to of cyclohexanone and cyclohexanol analyzed by GC and the
C–Br, which attenuates and even disappears in Mn(III)P-CMP contents of adipic acid, ester, and cyclohexyl hydroperoxide
due to consumption of C–Br during the coupling reaction. measured by chemical titration are shown in Fig. 4. The blank
Moreover, Mn(III)P-CMP exhibits a characteristic Mn–N vibration tests showed that cyclohexane could not be oxidized at the same
band at 1009.1 cmÀ1, which is close to that of T(p-Br)PPMnCl temperature and pressure in the absence of Mn(III)P-CMP and
(1010.9 cmÀ1). The bands at 3035 cmÀ1 correspond to the C–H other additives, which indicated that Mn(III)P-CMP acted as a
stretches in phenyls, and those at 1516 cmÀ1 and 1608 cmÀ1 catalyst during cyclohexane oxidation with air. The literature
correspond to CQC stretches in phenyls. Thus, T(p-Br)PPMnCl also had a similar blank experimental report.12
had already been introduced into the Mn(III)P-CMP porous
structure in which manganese ion was still stably coordinated.
Changes in the product distribution of cyclohexane oxidation
with time are shown in Fig. 4. The catalysis of Mn(III)P-CMP for
The UV-vis spectrum of Mn(III)P-CMP solid and that of cyclohexane oxidation with air did not have a significant induction
T(p-Br)PPMnCl in CHCl3 solution were also obtained and period (Fig. 4a). To explore the reaction mechanism, a radical
compared (ESI,† Fig. S6). In the curve, a strong absorption scavenger, hydroquinone, was added to the reaction system in
band at 478 nm is the Soret band of T(p-Br)PPMnCl and those the initial stage. Since the oxidation reaction hardly occurred, it
at 581 and 618 nm are the Q-bands. Mn(III)P-CMP also shows was a radical type reaction. Moreover, there was a certain
the same typical UV-vis spectrum as T(p-Br)PPMnCl: there are amount of cyclohexyl hydroperoxide in the products. Hence,
Soret bands at 462 and 502 nm and Q-bands at 583 and 622 nm. cyclohexyl radical, which was generated via the decomposition
Compared with those in T(p-Br)PPMnCl monomer, the Soret of cyclohexyl hydroperoxide, was the major active radical in the
bands in MnP-CMP divide as 462 and 496 nm, belonging to reaction system. In addition, the mechanism via which air
blue- and red-shifted Soret bands of porphyrin, because the oxidation of cyclohexane was catalyzed by Mn(III)P-CMP was
conjugate bridge of phenylene increases the molecular connectivity similar to that by monomer metalloporphyrin.39
and the electronic coupling in Mn(III)P-CMP, which causes the Soret
band to clearly split in Mn(III)P-CMP.37,38
The yields of cyclohexanol and cyclohexanone rapidly and
linearly increased in the first 3 h. The cyclohexyl hydroperoxide
The first reduction potential of Mn(III)P-CMP is significantly content reached a maximum (2.0%) at 1.5 h and began to
reduced as compared to that of T(p-Br)PPMnCl (shifts from gradually decline but remained relatively high at 3 h. As the
À0.28 V to À0.12 V) (ESI,† Fig. S7), indicating that phenylene time further increased, the yields of cyclohexanol and cyclohexanone
has been introduced into the polymers. As a conjugated bridge reached highest (12.8% and 11.5%) at 4 h and 3.5 h, respectively,
between porphyrin monomers, phenylene is advantageous to and plummeted soon afterwards. In the beginning of the reaction,
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Fig. 4 Cyclohexane conversion and reaction product distribution in the cyclohexane oxidation catalyzed by MnP-CMP; Mn3+/cyclohexane = 1/168 000
(molar ratio), Vair = 1000 mL minÀ1, P = 0.8 MPa, T = 150 1C, and cyclohexane: 200 mL (1.85 mol).
the cyclohexane conversion rate increased faster (Fig. 4b) and then selectivity ranged from 70.3% to 85%, and the maximum yield of
slowed down, reaching 29.9% at 3.5 h, whereas the yields of alcohol and ketone was found to be 21.6% at 150 1C. When the
cyclohexanol and cyclohexanone were maintained at maximum temperature was 160 1C, the total contents of alcohol and
(21.6%). However, at 3 h, white or light yellow solid acid ketone peaked at 1.5 h. At the same time, the conversion rate
precipitates were produced, and the content and selectivity of of cyclohexane was 26.3%, but the selectivity of oxidation
alcohol and ketone were lower at the same time, indicating deep reaction was hardly controllable and the total selectivity of
oxidation of cyclohexane. Cyclohexane was oxidized with air alcohol and ketone was only 66.4%. Moreover, the by-products
over Fe(III)P-CMP under the same reaction conditions. The yield significantly increased and the reaction system had already
of cyclohexanol and cyclohexanone was 19.21%, and that of acid generated plenty of acid and ester. As the reaction proceeded, the
was 1.93%. The conversion of cyclohexane reached 27.51%.
selectivity of alcohol and ketone sharply dropped to 61.3% at 2 h.
3.3. Effect of reaction temperature on cyclohexane oxidation
3.4. Comparison of the catalytic activities of the monomer
metalloporphyrin and conjugated polymer porphyrin
Cyclohexane oxidation with air catalyzed by Mn(III)P-CMP was
highly sensitive to reaction temperature. The reaction could not The aerobic oxidation of cyclohexane catalyzed by Mn(III)P-CMP
take place until the temperature was above 120 1C. The effect of and Fe(III)P-CMP had higher selectivity and conversion rates
reaction temperature on the product selectivity and cyclohexane than of that catalyzed by T(p-Br)PPMnCl and T(p-Br)PPFeCl,
conversion rate was evaluated at 135 1C, 140 1C, 145 1C, 150 1C, and the results are listed in Table 3 and Fig. 6. The catalysis of
155 1C and 160 1C (Table 2 and Fig. 5).
cyclohexane oxidation over T(p-Br)PPFeCl with air had a about
Fig. 5 indicated that the conversion rate of cyclohexane 0.5 h significant induction period.
obviously increased as the reaction temperature increased.
In the beginning of the reaction, the conversion rate of
Moreover, it also showed that the reaction temperature influenced cyclohexane linearly increased. Compared with those obtained
the yields of alcohol and ketone; the higher the temperature, the by Mn(III)P-CMP and Fe(III)P-CMP, the conversion rates of
low the selectivity of cyclohexanol and cyclohexanone. The total cyclohexane catalyzed by T(p-Br)PPMnCl and T(p-Br)PPFeCl
content of alcohol and ketone reached maximum at 6 h and 135 1C were significantly slower later in the reaction (after 3 h); thus,
(Table 2); however, it did not exceed 10.0%. At 140 1C, 145 1C, the catalyst activity had deteriorated by this time. Their catalytic
150 1C, and 155 1C, the total contents of alcohol and ketone activities followed a descending order of Mn(III)P-CMP 4
reached maxima at 4.5 h, 4 h, 3.5 h, and 3 h, respectively; the total Fe(III)P-CMP 4 T(p-Br)PPMnCl 4 T(p-Br)PPFeCl. Alcohol and
Table 2 Effect of reaction temperature on cyclohexane oxidationa
Reaction temperature
135 1C
140 1C
145 1C
150 1C
155 1C
160 1C
Reaction timeb (h)
Cyclohexane conversion (%)
Yields of alcohol and ketone (%)
Catalyst mole TONc
6.0
4.5
4.0
3.5
3.0
1.5
12.5
10.6
21 000
19.4
14.5
32 592
21.4
16.0
35 952
29.9
21.6
50 232
29.3
18.8
49 224
26.3
17.5
44 184
a
Catalyst: Mn(III)P-CMP, Mn3+/cyclohexane = 1/168 000 (molar ratio), Vair = 1000 mL minÀ1, P = 0.8 MPa, and cyclohexane: 200 mL (1.85 mol).
Time of maximum (alcohol + ketone) yields. TON was calculated using the following equation: TON = (consumed cyclohexane)/(amount of Mn).
b
c
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Fig. 5 Selectivity of alcohol and ketone and cyclohexane conversion in the cyclohexane oxidation catalyzed by MnP-CMP at different temperatures;
catalyst: MnP-CMP, Mn3+/cyclohexane = 1/168 000 (molar ratio), Vair = 1000 mL minÀ1, P = 0.8 MPa, and cyclohexane: 200 mL (1.85 mol).
ketone were the main products during the porphyrin-catalyzed with the increasing aperture of polymerized porphyrin. Fe(III)P-
oxidation of cyclohexane, and there was also a small amount of CMP, whose pore size distribution was mainly concentrated at
adipic acid with high economic value. We also compared the 1.06 nm, had slightly lower acid selectivity than Mn(III)P-CMP,
conversion rates of cyclohexane as well as the selectivity of whose pores were distributed at 1.12 nm and 2.58 nm. Hence,
alcohol, ketone, and adipic acid at the highest yield of alcohol this polymerized porphyrin can improve the selectivity of the
and ketone (Table 3).
main product in the cyclohexane oxidation reaction owing to its
Although the yields of alcohol and ketone reached highest at unique porous structure and shape-selective effect and make the
3 h with T(p-Br)PPFeCl as the catalyst, they were only 9.4%, selectivity of alcohol and ketone less dependent on the conversion
which were significantly lower than those of other catalysts. rate of cyclohexane, simultaneously maintaining high cyclohexane
When porphyrin polymer was used as the catalyst, the conversion conversion rate as well as selectivity of alcohol and ketone.
rate of cyclohexane and the selectivities of alcohol and ketone
3.5. Reusability of metalloporphyrin polymers
were elevated by 10% as compared to those obtained by employing
monomer metalloporphyrin. The selectivities of alcohol and
ketone followed a decreasing order of Mn(III)P-CMP 4 Fe(III)P-
CMP 4 T(p-Br)PPMnCl 4 T(p-Br)PPFeCl, i.e. porphyrin polymer
had significantly higher selectivity than monomer metalloporphyrin.
Additionally, the selectivity of porphyrin polymer for adipic acid was
obviously lower than that of monomer metalloporphyrin. However,
under the same conditions, the conversion rate of cyclohexane
catalyzed by porphyrin polymer was higher than that obtained
by monomer metalloporphyrin; hence, the yield of adipic acid
remained higher.
The developed porous structure and large specific surface
area in porphyrin polymer enlarged the contact area between
substrate, oxygen, and catalyst; this provided more micro-
reaction sites and improved the efficiency of the catalytic
reaction. The content of acid in the product slightly increased
Monomer metalloporphyrin is prone to degradation and inactivation
in a homogeneous catalytic oxidation system. At the end of the
reaction with Mn(III)P-CMP or Fe(III)P-CMP as the catalyst, the
mixture was centrifuged and the supernatant that was detected
by UV-vis spectroscopy did not contain metalloporphyrin. The
metal leaking experiment was also performed. The filtrate
obtained after fifth reaction cycle was tested by atomic absorption
spectrometry using a Perkin Elmer analyst 700/800. The
concentration of Mn in the filtrate was 1.14 Â 10À5 mg mLÀ1
and it was about 0.4% of the amount in the original solution
(3.0 Â 10À3 mg mLÀ1) (Mn3+/cyclohexane = 1/168 000). Moreover,
87% of Mn(III)P-CMP catalyst was recovered by centrifugation,
filtration, and thorough washing with THF, methanol, and
trichloromethane in the fifth recovery experiment. Thus, it can
be seen that the loss mostly occurred in the recovery process
Table 3 Influence of different catalysts on the cyclohexane oxidation reactiona
Reaction temperature
T(p-Br)PPFeCl
T(p-Br)PPMnCl
Fe(III)P-CMP
Mn(III)P-CMP
Reaction timeb (h)
3.0
3.5
4.0
3.5
Cyclohexane conversion (%)
Selectivity of alcohol and ketone (%)
Selectivity of adipic acid (%)
Yields of alcohol and ketone (%)
Yields of adipic acid (%)
Catalyst mole TONc
15.6
60.2
17.0
9.4
2.6
20 267
19.2
62.3
18.8
11.9
3.6
27.5
69.8
7.0
19.2
1.9
29.9
72.3
8.9
21.6
2.7
24 895
46 200
50 232
a
b
Catalyst: M3+/cyclohexane = 1/168 000 (molar ratio), Vair = 1000 mL minÀ1, T = 150 1C, P = 0.8 MPa, cyclohexane: 200 mL (1.85 mol). Time of
c
maximum (alcohol + ketone) yields. TON was calculated using the following equation: TON = (consumed cyclohexane)/(amount of Mn/Fe).
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MnP-CMP at the Mn3+/cyclohexane molar ratio of 1/168 000
and air flow rate of 1000 mL minÀ1. The total yield of cyclohexanol
and cyclohexanone was 21.6%, and the conversion of cyclohexane
reached 29.9%. In addition, polymer porphyrin had a recovery rate
of over 87%. After being recycled 5 times, the conversion rates of
cyclohexane exceeded 25% and 23% when Mn(III)P-CMP and
Fe(III)P-CMP were used, respectively. By realizing heterogeneous
catalysis, metalloporphyrin-conjugated polymers can replace the
corresponding metalloporphyrin for industrial catalytic oxidation
and thus have significant application prospects.
Acknowledgements
The financial support of the National Natural Science Foundation
of China (Grant No. 21576074) is gratefully acknowledged. We are
also grateful for the financial support of Hunan University.
Fig. 6 Cyclohexane conversion in the cyclohexane oxidation catalyzed
by MnP-CMP, FeP-CMP, T(p-Br)PPMnCl, and T(p-Br)PPFeCl; M3+/cyclo-
hexane = 1/168 000 (molar ratio), Vair = 1000 mL minÀ1, T = 150 1C,
P = 0.8 MPa, and cyclohexane: 200 mL (1.85 mol).
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