Conjugated copper(II) porphyrin polymer and N-hydroxyphthalimide as effective catalysts for selective oxidation of cyclohexylbenzene

Article abstract of DOI:10.1016/j.catcom.2017.02.011

A nanomaterial catalyst of azo-bridged Cu(II) porphyrin polymer (Cu^II-APP) was synthesized and characterized by scanning electron microscopy, transmission electron microscopy and N₂adsorption measurement. Cu^II-APP had a nanoporous structure, with the particle size of about 30?nm. Owing to the special structure, Cu^II-APP acted as an efficient heterogeneous catalyst for selective oxidation of cyclohexylbenzene into cyclohexylbenzene-1-hydroperoxide. When N-hydroxyphthalimide was used as co-catalyst, this binary catalyst system showed an obvious synergic effect. After being recovered and reused, Cu^II-APP and NHPI still had high catalytic activities.

Full text of DOI:10.1016/j.catcom.2017.02.011

Accepted Manuscript
Conjugated
copper(II)
porphyrin
polymer
and
N-
hydroxyphthalimide as effective catalysts for selective oxidation
of cyclohexylbenzene
Zhiwei Tan, Junhui Zhu, Weijun Yang
PII:
DOI:
Reference:
S1566-7367(17)30062-6
doi: 10.1016/j.catcom.2017.02.011
CATCOM 4939
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 November 2016
13 February 2017
14 February 2017
Please cite this article as: Zhiwei Tan, Junhui Zhu, Weijun Yang , Conjugated copper(II)
porphyrin polymer and N-hydroxyphthalimide as effective catalysts for selective oxidation
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affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/
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ACCEPTED MANUSCRIPT
Conjugated copper(II) porphyrin polymer and
N-hydroxyphthalimide as effective catalysts for selective oxidation of
cyclohexylbenzene
Zhiwei Tan, Junhui Zhu, Weijun Yang[a]*
[a] Weijun Yang
College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082,
Hunan, China.
Tel: +86-731-88821449, Fax: +86-731-88713642, E-Mail: wjyang@hnu.edu.cn
College of Chemistry and Chemical Engineering, Hunan University, Changsha
410082, Hunan, China
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Abstract
A nanomaterial catalyst of azo-bridged Cu(II) porphyrin polymer (CuII-APP) was
synthesized and characterized by scanning electron microscopy, transmission electron
microscopy and N2 adsorption measurement. CuII-APP had a nanoporous structure,
with the particle size of about 30 nm. Owing to the special structure, CuII-APP acted
as an efficient heterogeneous catalyst for selective oxidation of cyclohexylbenzene
into cyclohexylbenzene-1-hydroperoxide. When N-hydroxyphthalimide was used as
co-catalyst, this binary catalyst system showed an obvious synergic effect. After being
recovered and reused, CuII-APP and NHPI still had high catalytic activities.
Keywords: catalytic oxidation; N-hydroxyphthalimide; conjugated metalloporphyrin polymer;
cyclohexylbenzene-1-hydroperoxide; cyclohexylbenzene
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1. Introduction
Conventionally, phenol is industrially produced by the oxidation of cumene into
cumene hydroperoxide, and then decomposition by acid catalyst [1]. The cumene
oxidation method, however, generates a large amount of acetone as undesirable
by-product. Hence, developing a by-product-free method for phenol production is of
great economic significance. Until now, phenol has been successfully synthesized by
cyclohexylbenzene (CHB) oxidation, benzene oxidation and cyclohexane oxidation
[2-5], among which liquid-phase aerobic oxidation of CHB is a new green synthetic
process, with 100% atom utilization. At present, CHB is oxidized into
cyclohexylbenzene-1-hydroperoxide
(CHBHP)
mainly
by
using
N-hydroxyphthalimide (NHPI) as catalyst, then giving phenol and cyclohexanone
after decomposition by acid catalyst [5-7].
NHPI has been widely used as a radical catalyst promoter for the aerobic
oxidation of organic compounds under mild reaction conditions [8, 9]. This
homogeneous catalyst can be recovered easily by filtration and adsorption [10].
Co-catalyst or initiator is required for NHPI to generate the active intermediate,
phthalimide-N-oxyl (PINO) radical, in the catalytic reaction. PINO radical can
effectively extract a hydrogen atom from substrates to transforms back into NHPI.
Recently, metalloporphyrin and NHPI have been reported to catalyze the oxidation of
toluene and ethylbenzene [11, 12]. Up to now, metalloporphyrin and NHPI have never
been employed as co-catalysts for the oxidation of CHB.
Metalloporphyrins, as biomimetic catalysts with high catalytic activities to C-H
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bond and C=C bond, have been widely used in the selective oxidation of alkanes,
alkenes and alcohols [13, 14]. However, metalloporphyrins are prone to oxidation and
degradation during reactions. In recent years, in order to solve these drawbacks,
several kinds of heterogeneous metalloporphyrin catalysts have been developed
[15-18], of which conjugated metalloporphyrin polymers have attractive physical and
chemical properties. Conjugated metalloporphyrin polymer can efficiently catalyze
the oxidation reaction of sulfides, olefins, alcohols and toluene [16, 19-21].
In this study, we synthesized a new azo-bridged conjugated copper(II) porphyrin
polymer (CuII-APP). CuII-APP showed high catalytic activity and selectivity for the
aerobic oxidation of C-H bond on the tertiary carbon of CHB. Furthermore, NHPI
was used as co-catalyst to enhance the catalytic performance of CuII-APP and to
moderate the reaction conditions. After reaction, CuII-APP remained stable as well as
easily recoverable and reusable.
2. Experimental Section
2.1 Synthesis of CuII-APP
Cu(II) meso-tetra(4-nitrophenyl)porphyrin (Cu-TNPP, 0.25 mmol), Cu(II)
meso-tetra(4-aminophenyl)porphyrin (Cu-TAPP, 0.25 mmol) and KOH powders (5
mmol) were added to a three-necked flask equipped with a condenser tube and a
nitrogen balloon, then degassed in vacuum and added with DMF (40 ml) under
nitrogen atmosphere. The mixture was heated to 150 °C and reacted for 24 h with
vigorous stirring. After reaction, the precipitate was collected by filtration and
washing with hot deionized water. Then the solid product was purified by extraction
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with DMF several times until the solution was colorless, giving CuII-APP (Fig. 1) as
purple powders in 49.7% yield.
2.2 Typical Procedure for Aerobic Oxidation of CHB
A mixture of CHB, solvent and catalyst was added in a three-necked flask equipped
with a condenser tube, a thermometer, an oxygen inlet pipe and a magnetic stirrer.
After heating to the desired temperature in an oil bath under N2 atmosphere,
molecular oxygen was bubbled into the three-necked flask at a certain flow rate, and
the mixture was regularly sampled and analyzed during reaction (Fig. 2).
3. Results and Discussion
3.1 Characterizations of CuII-APP
The structure of CuII-APP was analyzed by Fourier transform infrared spectroscopy
(FT-IR) (Fig. S3). The IR spectra of CuII-APP, Cu-TNPP and Cu-TAPP are highly
similar, indicating that CuII-APP contained a high content of copper porphyrin in the
polymer framework. The characteristic peak of Cu-N vibration band at 998.8 cm−1 in
the spectrum of CuII-APP is the same as those of Cu-TNPP and Cu-TAPP. In contrast
to Cu-TNPP and Cu-TAPP, the characteristic peak of CuII-APP at 1452 cm−1
originates from the azo bonds by the coupling reaction of nitro groups and amidogen
groups.
The UV–visible absorption spectrum of CuII-APP obviously red shifts compared to
those of Cu-TNPP and Cu-TAPP (Fig. S4). The Soret bands of Cu-TNPP (418 cm-1)
and Cu-TAPP (428 cm-1) shift to 447 cm-1, and the Q-bands of Cu-TNPP (538 cm-1)
and Cu-TAPP (544 cm-1 and 590 cm-1) shift to 552 cm-1 and 597 cm-1. The shifts can
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be explained by the polymerization of copper porphyrins into a Π-Π conjugated
planar structure [22], indicating the formation of azo-bridged copper porphyrin
polymer.
The morphology of the catalyst was investigated by field emission scanning
electron microscopy (FE-SEM) and high-resolution transmission electron microscopy
(HR-TEM) (Fig. S5). FE-SEM images show that CuII-APP has a porous structure
composed by circular particles, with rough surface (Fig. S5 a). The magnified
FE-SEM image displays that the diameter of circular particles is about 20–40 nm (Fig.
S5 b). HR-TEM image further proves that the circular particle is about 30 nm (Fig. S5
c). The magnified HR-TEM image displays a large number of nanopores on the
surface of CuII-APP (Fig. S5 d). N2 adsorption measurement was used to investigate
the specific surface area and pore size distribution of CuII-APP (Fig. S6). According to
the multi-point BET method, the surface area of CuII-APP is as high as 217 m2g-1 and
the total pore volume is 0.1841 cm3g-1. The nitrogen adsorption and desorption
isotherm (Fig. S6 a) displays a H4 hysteresis loop, suggesting the existence of
micropores and mesopores in CuII-APP. Besides, the main pore diameters are about
1.2 and 1.9 nm (Fig. S6 b).
TG analysis was used to study the thermal stability of CuII-APP (Fig. S7). CuII-APP
remained stable and hardly underwent decomposition at below 153 °C. The TG curve
begins to decline at 153 °C due to the decomposition of azo bonds, and porphyrin ring
began to decompose at 349 °C. At the end of the test, the sample still had 60.5%
residual.
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3.2 Catalytic Applications in Oxidation of CHB
3.2.1 Catalytic activities of CuII-APP at different temperatures
Initially, in order to study the catalytic activity of CuII-APP, the reaction was carried
out without any solvent or co-catalyst (Table 1). The products were qualitatively
analyzed by GC-MS (Fig.S10-17). Qualitative analysis results indicated that the main
catalytic product was CHBHP and the by-products included 1-phenylcyclohexanol,
cyclohexylbenzene-2-hydroperoxide,
cyclohexylbenzene-4-hydroperoxide,
1-phenylcyclohexene, 1-phenyl-1-hexanone and 6-hydroxy-1-phenyl-1-hexanone.
CHB was prone to autoxidation at high temperatures, especially excess 120 °C.
Compared with the autoxidation process at 120 °C, the presence of CuII-APP greatly
accelerated the oxidation reaction and maintaining the high CHBHP selectivity. The
conversion of CHB increased significantly with rising reaction temperature. CuII-APP
had high catalytic activity with 37.59% CHB conversion at 130 °C for 6 h, but the
selectivity of CHBHP decreased. Moreover, temperature seldom affected the
selectivity of CHBHP at below 120 °C. High temperature led to the decomposition of
CHBHP and the formation of by-products. Therefore, the reaction temperature should
not exceed 130°C.
3.2.2 Comparison of catalytic activities of CuII-APP, Cu-TAPP and Cu-TNPP
The oxidation of CHB catalyzed by CuII-APP and its monomers was studied to
investigate the catalytic activity of CuII-APP, using the same amount of Cu2+ under
similar reaction conditions (Table 1). When CuII-APP and its monomers were used,
the selectivities of CHBHP were almost similar, but the conversion of CHB catalyzed
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by CuII-APP was much higher than those by Cu-TNPP and Cu-TAPP. This
heterogeneous catalyst had excellent catalytic activity for CHB oxidation. The higher
catalytic activity of CuII-APP can mainly be attributed to the following reasons. First,
the porous structure of CuII-APP facilitated the interactions between O2, CHB and
Cu2+ active centers. Second, the particle size of CuII-APP (about 30 nm) was much
smaller than those of other conjugated metalloporphyrin polymer catalysts (about
100-200 nm) reported before. Small particles promoted the substrates to enter
CuII-APP, thereby fully utilizing the catalytic active center. Last but not least, the
conjugated structure of CuII-APP allowed electron transfer in the catalyst to increase
the catalytic activity during reaction.
3.2.3 Effect of NHPI as co-catalyst
The effect of NHPI as co-catalyst was studied to increase the conversion and
selectivity, and to moderate the reaction conditions (Table 2). NHPI can be thermally
initiated at high temperature without an initiator. The presence of CuII-APP further
increased the reaction conversion rate with the same selectivity of CHBHP at 110 °C.
It effectively initiated NHPI and catalyzed the substrate during reaction. When the
reaction was carried out at the saturated concentration of NHPI (0.144 mol%) at
110 °C, the conversion of CHB was 23.99%, which was significantly higher than that
without NHPI at 110 °C for 6 h. When 0.5 mol% NHPI was added, the conversion of
CHB and the selectivity CHBHP were markedly augmented. After reaction, NHPI
was almost completely dissolved under solvent-free condition. The further dissolution
of NHPI was mainly attributed to the following reasons. Firstly, NHPI was unstable
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under aerobic oxidation conditions at high temperature (T >100°C) and prone to
partly decomposed during the reaction [23]. Secondly, during the reaction, the polarity
of the reaction mixture gradually increases due to the formation of polar products. The
increased polarity of the reaction mixture can increase the solubility of NHPI and
cause its further dissolution. The excess NHPI can maintain its saturated
concentration during the reaction, thus increasing the CHB conversion.
3.2.4 Oxidation of CHB in the presence of solvent
In order to prevent the decomposition of CHBHP and NHPI, we added
chlorobenzene as solvent to dilute the concentration of product and conducted the
reaction at a lower temperature (Table 3). The conversion rate remarkably increased
when the solvent was used at 90 °C. NHPI initiated the oxidation reaction of CHB in
the presence of chlorobenzene as solvent at low temperature. The presence of
CuII-APP efficiently induced NHPI to participate in the catalytic reaction cycle. The
conversion of CHB was up to 44.81% after 8 h. In this catalytic environment, the
selectivity of co-catalysts to the tertiary carbon atom of CHB was greatly increased.
However, the yield of 6-hydroxy-1-phenyl-1-hexanone also increased due to side
reaction. Compared to copper chloride, CuII-APP had higher selectivity of CHBHP
because copper porphyrin had no catalytic activity against the decomposition of alkyl
hydroperoxides [24]. After reaction, CuII-APP and part of NHPI were presented as
solid precipitates in the cold reaction mixture when chlorobenzene was used as
solvent.
3.2.5 Effect of Catalyst Amount
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The effect of CuII-APP amount was evaluated (Table 3). Without CuII-APP, the
conversion of CHB was only 12.13%. When 0.0125 mol% CuII-APP was added in the
reaction system, the conversion increased 3-fold after 8 h. The conversion rate rose
with increasing catalyst amount. However, the selectivity of product decreased with
increasing amount of CuII-APP. Therefore, 0.025 mol% CuII-APP and 1 mol% NHPI
were optimum for this reaction.
3.2.6 Recyclability Tests
To evaluate the stability of CuII-APP in CHB oxidation, we repeatedly applied this
heterogeneous catalyst. CuII-APP and NHPI were recovered by filtration after the
reaction mixture was cooled to 0 °C, and then they were washed with acetone several
times to separate NHPI. The filter residue was dried in vacuum to give CuII-APP, with
the recovery rate of over 95% each time. Then NHPI was recovered by removing
solvent from the acetone filtrate using a rotary evaporator. NHPI partly decomposed
and dissolved during reaction, and the recovery rate of NHPI was about 60% when
chlorobenzene was used as solvent. The recyclability test was carried out five times
under the same reaction conditions (Fig. S8). After 5 cycles, CuII-APP still had high
catalytic activity and selectivity. As suggested by the unchanged FT-IR spectrum of
CuII-APP (Fig. S9) after oxidation reaction, it remained stable.
3.3 Plausible Reaction Process
Based on the results above and the plausible mechanism for aerobic oxidation
catalyzed by metalloporphyrin and NHPI reported previously [12, 25-29], we
proposed a plausible reaction process for the CHB oxidation catalyzed by CuII-TAPP
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and NHPI (Fig. 3). CuII-TAPP is converted into high-valent oxo-Cu intermediate
[TPP-CuIV=O] by activating oxygen molecule, and [TPP-CuIV=O] captures a
hydrogen atom from NHPI to form [TPP-CuIII-OH] and a PINO radical [12, 26, 27].
The hydrogen atom from H-O bond of NHPI can be transferred to the [TPP-CuIII-OH]
intermediate, forming a PINO radical to participate in the free radical reaction.
[TPP-CuII-OH2] is converted back into CuII-APP by desorbing H2O ligand [30, 31].
PINO transforms back into NHPI by extracting a tertiary hydrogen from CHB [12].
CHB radical is further oxidized into CHBHP. NHPI can efficiently react with
CHB-peroxy radicals and the copper porphyrin intermediate. In the reaction system,
NHPI can accelerate the radical oxidation of CHB.
4. Conclusions
In summary, we synthesized a conjugated Cu(II) porphyrin polymer bridged by azo
groups, as an effective heterogeneous catalyst for selective oxidation. This
nanomaterial catalyst exhibited good catalytic performance for the oxidation of CHB
into CHBHP, and had a much higher catalytic activity than those of Cu-TNPP and
Cu-TAPP. When NHPI was used as co-catalyst, the catalytic performance of CuII-APP
was remarkably enhanced, and the reaction was also allowed under mild conditions.
Under optimum conditions, the CHB conversion and the CHBHP selectivity were as
high as 43.26% and 89.62%, respectively. In addition, CuII-APP could be easily
recovered and reused, with high stability and catalytic activity in at least several
catalytic cycles.
Acknowledgements
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The authors thanks for the financial support from the National Nature Science
Foundation of China (Grant No. 21576074).
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Figure Caption
Fig. 1. Schematic representation of the synthesis of CuII-APP.
Fig. 2. Schematic representation of the oxidation of CHB.
Fig. 3. Schematic representation of plausible reaction process of CHB oxidation catalyzed by
CuII-APP and NHPI.
Table Caption
Table 1. Aerobic Oxidation of CHB Catalyzed by CuII-APP, Cu-TAPP and Cu-TNPP
Table 2. Aerobic oxidation of CHB catalyzed by CuII-APP and NHPI
Table 3. Aerobic oxidation of CHB in the presence of solvent.
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Figure 1
Figure 2
Figure 3
17

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Table 1. Aerobic Oxidation of CHB Catalyzed by CuII-APP, Cu-TAPP and Cu-TNPP
CHB
Selectivity * [%]
Entry Catalyst
T [oC]
Time [h] Conversion
[%]
CHBHP
A
B
C
D
E
F
CuII-APP
CuII-APP
CuII-APP
110
120
130
110
120
130
120
120
8
8
6
8
8
6
8
8
7.98
24.34
37.59
1.21
84.10
81.03
72.69
87.10
83.45
75.48
85.84
83.06
3.02 0.65 4.61 0.62 0.94 6.05
4.70 0.39 4.32 0.32 0.76 8.46
7.04 0.55 5.78 0.56 1.48 12.88
1.97 0.42 3.85 0.55 0.62 5.49
1.50 0.82 4.12 0.47 0.80 8.83
4.19 0.82 5.08 0.67 1.57 12.17
1.96 0.47 4.16 0.12 0.88 6.54
2.05 0.56 5.14 0.21 0.70 8.28
1
2
3
–
4
5
6
7
8
–
6.23
–
24.93
10.10
12.38
Cu-TAPP
Cu-TNPP
Conditions: CHB (0.3 mol), catalysis (2.5×10-3 mol% Cu2+), 1 atm of O2 (100 ml / min).
* 1-Phenylcyclohexanol (A), 2-Phenylcyclohexanol (B, after reduction), 4-Phenylcyclohexanol (C, after reduction),
1-Phenylcyclohexene (D), 1-Phenyl-1-hexanone (E), 6-Hydroxy-1-phenyl-1-hexanone (F).
Table 2. Aerobic oxidation of CHB catalyzed by CuII-APP and NHPI
CHB
Conversion
[%]
Selectivity * [%]
Temperature Time
Entry
[oC]
[h]
CHBHP
A
B
0
C
D
0
E
F
0
1[a]
2[a]
3[a]
4[a]
80
90
8
8
8
6
2.26
96.94
95.38
93.26
91.40
0.52
2.20
0.33
9.64
0.97 0.19 1.72 0.56 0.29 0.89
1.64 0.25 2.85 0.64 0.32 1.03
2.34 0.16 2.89 0.54 0.41 2.25
100
110
20.46
35.72
5[b]
110
6
23.99
92.75
1.71
0.54 2.11 0.23 0.64 2.02
6[c]
7[c]
110
120
6
4
25.22
29.56
92.41
87.74
2.04 0.60 2.24 0.23 0.34 2.11
2.73 0.36 3.63 0.37 0.82 4.34
Conditions: CHB (0.3 mol), 1 atm of O2 (100 ml/min). [a] CuII-APP (2.5×10-3 mol% Cu2+), NHPI
(0.5 mol%). [b] CuII-APP (2.5×10-3 mol% Cu2+), NHPI (0.14 mol%). [c] NHPI (0.5 mol%).
* 1-Phenylcyclohexanol (A), 2-Phenylcyclohexanol (B, after reduction), 4-Phenylcyclohexanol (C,
after
reduction),
1-Phenylcyclohexene
(D),
1-Phenyl-1-hexanone
(E),
6-Hydroxy-1-phenyl-1-hexanone (F).
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ACCEPTED MANUSCRIPT
Table 3. Aerobic oxidation of CHB in the presence of solvent.
CHB
Selectivity* [%]
T
[oC]
Entry
Catalyst
Conversion
[%]
CHBHP
A
B
C
D
E
F
CuII-APP +NHPI
CuII-APP +NHPI
CuII-APP +NHPI
CuII-APP +NHPI
CuII-APP +NHPI
CuCl2+NHPI
NHPI
70
80
90
90
90
90
70
80
90
14.49
28.35
44.81
43.26
39.02
36.54
5.72
96.37
93.21
84.35
89.62
92.56
69.58
99.1
0.8 0.07 1.03 0.13 0.33 1.23
1.16 0.09 1.38 0.21 0.43 3.51
5.98 0.10 1.43 0.20 0.65 7.27
1.40 0.06 1.41 0.17 0.60 6.71
1.06 0.09 1.36 0.15 0.36 4.39
5.64 0.18 3.10 0.79 2.30 18.39
1[a]
2[a]
3[a]
4[b]
5[c]
6[d]
7[e]
8[e]
9[e]
0.25
0.23
0
0
0.65
0.89
0
0
0
0
0
0
NHPI
8.44
98.77
0.11
NHPI
12.13
98.29
0.21 0.10 0.92 0.16 0.32
Conditions: CHB (0.03 mol), chlorobenzene 0.3 mol, 1 atm of O2 (50 ml/min), 8 h reaction time. [a] CuII-APP
(0.05 mol% Cu2+), NHPI (1 mol%). [b] CuII-APP 0.025 mol%, NHPI (1 mol%). [c] CuII-APP 0.0125 mol%, NHPI
(1 mol%). [d] CuCl2 0.025 mol%, NHPI (1 mol%). [e] NHPI 1 mol%
* 1-Phenylcyclohexanol (A), 2-Phenylcyclohexanol (B, after reduction), 4-Phenylcyclohexanol (C, after reduction),
1-Phenylcyclohexene (D), 1-Phenyl-1-hexanone (E), 6-Hydroxy-1-phenyl-1-hexanone (F).
Graphical abstract
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ACCEPTED MANUSCRIPT
Highlights
• A kind of metalloprophyrin conjugated polymer (CuII-APP) was synthesized.
• CuII-APP was a nano-material catalyst with porous structure.
• CuII-APP exhibited high catalytic activity to cyclohexylbenzene.
• N-hydroxyphthalimide exhibited excellent co-catalytic effect with CuII-APP.
• CuII-APP still maintained high catalytic activity after 5 catalytic cycle.
20