Synthesis of a porphyrinic polymer for highly efficient oxidation of arylalkanes in water

Article abstract of DOI:10.1016/j.catcom.2015.03.031

A highly stable covalent-porphyrinic framework Mn-CPF-1 was synthesized by reaction of cyanuric chloride and tetraphenylamine porphyrin (TAPP) and subsequent metallation. Mn-CPF-1 exhibits remarkable catalytic activity and stability on catalytic oxidation of arylalkanes in water under mild conditions, which is much superior to the homogeneous analog Mn-tetraphenylporphyrin (Mn-TPP) under identical conditions.

Full text of DOI:10.1016/j.catcom.2015.03.031

Catalysis Communications 66 (2015) 116–120
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Synthesis of a porphyrinic polymer for highly efficient oxidation of
arylalkanes in water
⁎
Chao Zou, Min Zhao, Chuan-De Wu
Center for Chemistry of High-Performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly stable covalent-porphyrinic framework Mn-CPF-1 was synthesized by reaction of cyanuric chloride and
tetraphenylamine porphyrin (TAPP) and subsequent metallation. Mn-CPF-1 exhibits remarkable catalytic activ-
ity and stability on catalytic oxidation of arylalkanes in water under mild conditions, which is much superior to
the homogeneous analog Mn-tetraphenylporphyrin (Mn-TPP) under identical conditions.
© 2015 Elsevier B.V. All rights reserved.
Received 10 February 2015
Received in revised form 20 March 2015
Accepted 21 March 2015
Available online 26 March 2015
Keywords:
Biomimetic
Heterogeneous catalysis
Metal-organic framework
Metalloporphyrin
Porous organic polymer
1. Introduction
arylalkanes in water, which is significantly superior to the correspond-
ing homogeneous counterpart Mn-tetraphenylporphyrin (Mn-TPP).
Since monooxygenases were discovered as the highly efficient
metalloenzymes on oxidation reaction, extensive endeavors have
been focused on the development of synthetic metalloporphyrins to
mimic the catalytic properties of cytochrome P450 [1,2]. However, the
homogeneous artificial metalloporphyrins heavily suffered from cata-
lytic deactivation by suicidal self-oxidation [3]. To improve the stability
of metalloporphyrins, great effort has been focused on immobilization
of the metalloporphyrin moieties onto solid supports to develop their
heterogeneous counterparts [4]. To tune the pore structures and modify
the functional groups of porous metal-organic frameworks (MOFs),
these porous materials have been extensively explored as the very
promising heterogeneous catalysts [5,6]. In fact, immobilization of the
metalloporphyrin moieties onto the pore surfaces of MOFs has generat-
ed many unique and highly active heterogeneous catalysts [7–10].
Porous organic polymers are an emerging class of porous materials
that have control over structural parameters, including compositions,
porosities and functionalities [11–19]. Compared with MOF materials,
porous organic polymers are robust and stable in that they are connect-
ed by strong covalent bonds. To synthesize stable and porous organic
materials with accessible active sites for highly efficient heterogeneous
catalysis, we have reacted tetraphenylamine porphyrin (TAPP) with
cyanuric chloride, which resulted in a porous covalent porphyrinic
framework (CPF) CPF-1. The manganese(III) metalated solid Mn-CPF-
1 exhibits remarkable activity and stability on catalytic oxidation of
2. Experimental
2.1. Synthesis of CPF-1
Cyanuric chloride (360 mg, 2 mmol) in THF (40 mL) was dropwisely
added to a mixture of TAPP (500 mg, 0.75 mmol) and DIPEA (1 mL, ex-
cess) in THF (40 mL) under stirring in an ice-bath for 8 h. The mixture
further reacted at room temperature for 16 h, and refluxed at 90 °C for
48 h under stirring. Brown solid was collected by filtration and washed
with THF, CHCl3, EtOH and H2O. Yield: 795 mg. Anal. Calcd. for
C
156H144N48O33 (%): C, 58.20; H, 4.51; N, 20.88. Found (%): C, 55.26; H,
4.74; N, 20.89. IR (KBr pellet): ν/cm−1 = 1668(w), 1603(s), 1540(s),
1470(m), 1390(m), 1349(m), 1308(w), 1278(w), 1220(m), 1178(m),
1018(m), 981(s), 964(s), 942(w), 846(m), 795(s), 721(m), 644(w),
595(w), 558(w), 528(w).
2.2. Synthesis of Mn-CPF-1 by post-metalation
A mixture of CPF-1 (50 mg) and MnCl2·4H2O (100 mg) in NMP
(10 mL) was heated at 100 °C for 12 h. The reaction mixture was cooled
down to room temperature, and poured into MeOH (100 mL). The solid
was collected by filtration, and sufficiently washed with diluted HCl aque-
ous solution (0.1 M) and methanol to remove the surface adsorbed metal
ions, and dried under vacuum at room temperature for 12 h. Anal. Calcd.
for C156H138N48O33Mn3Cl3 (%): C, 53.78; H, 3.99; N, 19.30; Mn, 4.73. Found
⁎
Corresponding author.
E-mail address: cdwu@zju.edu.cn (C.-D. Wu).
(%): C, 53.23; H, 4.10; N, 17.98; Mn, 3.54. IR (KBr pellet): ν/cm−1
=
http://dx.doi.org/10.1016/j.catcom.2015.03.031
1566-7367/© 2015 Elsevier B.V. All rights reserved.

C. Zou et al. / Catalysis Communications 66 (2015) 116–120
117
Scheme 1. Schematic representation of the synthesis procedure for CPF-1.
1735(w), 1654(m), 1597(m), 1560(m), 1466(m), 1383(w), 1333(w),
1223(m), 1178(m), 1006(s), 981(s), 932(w), 850(m), 796(s), 713(m),
660(w), 561(w), 531(w), 472(w), 437(w).
was determined by GC–MS, and compared with the authentic sam-
ples analyzed under the same conditions, while the yield was ob-
tained by GC analysis in the presence of an internal standard of
naphthalene.
2.3. A typical procedure for the oxidation of ethylbenzene
2.4. EPR spin-trapping experiment
A mixture of Mn-CPF-1 (0.01 mmol based on Mn), ethylbenzene
(0.1 mmol) and TBHP (0.15 mmol) in water (5 mL) was stirred at
room temperature for 12 h. The mixture was centrifuged, and the
solid was washed with CH3CN for several times. The recovered
solid catalyst was used in the successive run. The supernatant
was extracted with hexane (2 mL). The identity of the product
A mixture of catalyst Mn-CPF-1 (0.02 mmol), ethylbenzene
(2.0 mmol) and TBHP (3.0 mmol) in water (5 mL) was stirred at room
temperature for 3 h, which was centrifuged. The supernatant (0.5 mL)
was added to 0.5 mL of DMPO aqueous solution (1.6 M). The liquid
phase was analyzed on a Bruker ESRA-300 at room temperature.
Fig. 1. SEM (a, b) and TEM (c, d) images for CPF-1 (left column) and Mn-CPF-1 (right column).

118
C. Zou et al. / Catalysis Communications 66 (2015) 116–120
Table 1
binding energies for Mn 2p3/2 and 2p1/2 are 641.2 and 653.4 eV, respec-
Selective oxidation of arylalkanes.a
tively, which are the characteristic binding energies for the trivalent Mn
ions [21]. Compared with the typical binding energy of free MnIII 2p3/2 at
642.6 eV, the binding energy at 641.2 eV is negatively shifted. This result
indicates that MnIII ions should strongly coordinate with the porphyrin
pyrroles in Mn-CPF-1 because electron donation from pyrroles to MnIII
ions can increase the electron density around MnIII ions [22]. The
retained uniform morphology of Mn-CPF-1 was proved by SEM and
TEM images (Fig. 1, right column). The solid samples of CPF-1 and
Mn-CPF-1 were activated by heating at 90 °C for 24 h to study the gas
sorption properties. As shown in Fig. S6, the activated CPF-1 takes up
57.3 cm3/g CO2 with a surface area of 621 m2/g, and Mn-CPF-1 adsorbs
42.8 cm3/g CO2 with a surface area of 433 m2/g.
Entry
1
Substrate
Product
Yield%b
N99
2
3
91
73
To prove that Mn-CPF-1 is an efficient heterogeneous catalyst, we
have conducted the arylalkane oxidation experiments. Because H2O2
very quickly decomposed by Mn-CPF-1 when it was added to the reac-
tion mixture, the arylalkane substrates cannot be effectively oxidized.
Therefore, the ethylbenzene oxidation by Mn-CPF-1 was performed in
H2O at room temperature using tertbutylhydroperoxide (TBHP) as
oxidant (Table 1). GC–MS analysis showed that only one product of
acetophenone was generated with an excellent yield of N99% (Table 1,
entry 1). Considering that the axial ligands on porphyrin metal sites
should play important roles for the activation of peroxides, we have
therefore examined the additive effects of imidazole and pyridine.
After imidazole and pyridine were added to the reaction mixtures, the
acetophenone yields decreased to 14.3 and 12.1%, respectively. These ex-
periments indicate that the catalytic efficiency of Mn-CPF-1 is significant-
ly affected by imidazole and pyridine, because these small bases can block
the porphyrin metal sites that are inaccessible to peroxides and substrate
molecules. To prove that the porphyrin metal sites in the pores of Mn-
CPF-1 are accessible to ethylbenzene and acetophenone molecules trans-
ferred through the open channels, we have studied the sorption property
of Mn-CPF-1 by UV–Vis absorption spectroscopy. The UV–Vis spectra
clearly indicate that the pores of Mn-CPF-1 are accessible to the ethylben-
zene and acetophenone molecules, and the uptakes are of 5.93 and 0.40
molecules per porphyrin unit of Mn-CPF-1, respectively. These results
suggest that the porphyrin MnIII sites in the channel walls of Mn-CPF-1
are accessible to ethylbenzene molecules, thus achieving much higher
catalytic efficiency on oxidation of ethylbenzene. Moreover, the selective-
ly preferred sorption of the substrate molecules over the product mole-
cules by Mn-CPF-1 can significantly improve the oxidation efficiency by
enrichment of ethylbenzene molecules and repellence of acetophenone
molecules in the pores of Mn-CPF-1.
4
5
57
4.3
6
76c
7
8
9
N99d
52e
91c
a
Mn-CPF-1 (0.01 mmol based on metalloporphyrin unit), substrate (0.1 mmol) and t-
BuOOH (0.15 mmol) in H2O (5 mL) were stirred at room temperature for 12 h.
b
Based on GC-MS analysis.
Catalyzed by Mn-TPP.
The fifteenth cycle.
c
d
e
After reaction proceeded for 2 h, the solid catalyst was removed by centrifugation.
3. Results and discussion
Considering that cyanuric chloride is an excellent regent for the syn-
thesis of porous polymeric materials [20], we have reacted cyanuric
chloride with tetraphenylamine porphyrin (TAPP) in a simple one-pot
procedure under mild conditions (Scheme 1). As shown in the UV–Vis
spectra (Fig. S1), when cyanuric chloride in THF was dropwisely
added to a mixture of TAPP and N,N-diisopropylethylamine (DIPEA) in
THF, the maximum absorption peak of the reaction solution shifted
from 432 to 424 nm, and further decreased gradually. As shown in the
X-ray photoelectron spectra (XPS) of the collected solid samples, the
typical band peak at 199.9 eV for Cl 2p gradually decreased and finally
disappeared after 72 h (Fig. S2). 13C solid-state NMR (CP/MAS) spectros-
copy clearly indicates that CPF-1 contains triazine and porphyrin moie-
ties (Fig. S3). The FT-IR spectrum of CPF-1 shows that the N-H stretch
band around 3351 cm−1 almost disappeared in comparison with that
of TAPP, which suggests the formation of tertiary amines (Fig. S4). The
uniform morphology of CPF-1 was confirmed by SEM and TEM images
(Fig. 1, left column).
Mn-CPF-1 can also catalytically oxidize various arylalkanes under the
identical conditions (Table 1, entries 2–5). When the size of substrate
CPF-1 is very stable in common organic solvents, boiling water, and
even concentrated NH3 and HCl aqueous solutions. Therefore, the post-
metalation of CPF-1 was simply realized by heating a mixture of CPF-1
and MnCl2 in N-methyl-2-pyrrolidone (NMP) at 100 °C for 12 h. Atomic
absorption spectroscopy (AAS) reveals that Mn-CPF-1 contains 3.54%
Mn ions, which are slightly below the capacity of theoretical metalation
(4.73%). As shown in the XPS spectrum of Mn-CPF-1 (Fig. S5), the
Fig. 2. Yield versus catalytic cycle for the oxidation of ethylbenzene by Mn-CPF-1 (black)
and Mn-TPP (red).

C. Zou et al. / Catalysis Communications 66 (2015) 116–120
119
the catalytic efficiency. The yield of acetophenone is of 91% catalyzed by
Mn-TPP under the identical conditions (Table 1, entry 9). However, signif-
icantly different to the catalytic property of Mn-CPF-1, Mn-TPP loses its
catalytic activity after three cycles (Fig. 2). These results indicate that
the catalytic activity and stability of heterogeneous Mn-CPF-1 are appar-
ently superior to those of the homogeneous Mn-TPP.
It has been speculated that oxidation of ethylbenzene by
metalloporphyrin catalysts involves the formation of reactive
oxometalloporphyrin intermediates and hydrogen abstraction from
benzylic carbon atom [23]. We have used the electron paramagnetic
resonance (EPR) spectroscopy, employing the spin-trapping technique
to monitor the radical intermediates. When the diamagnetic spin trap
molecule, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), was added to
the catalytic reaction solution, EPR spectrum of the supernatant exhibits
a clear four-line hyperfine pattern of 1:2:2:1 intensity ratios with the
hyperfine coupling constants of aN = 14.71 G, aβ = 15.09 G and
H
g = 2.011 (Fig. 4). The signal is characteristic for the known DMPO–
•OC(CH3)3 radical adduct [24]. The free-radical pathway is further con-
firmed by drastically decreasing the catalytic efficiency of Mn-CPF-1
when free radical scavengers of hydroquinone and iodine were added
to the reaction mixtures, in which the acetophenone yields are of 5.3
and 5.8%, respectively. According to the free-radical pathway, the ho-
mogeneous oxometalloporphyrin intermediates easily form the catalyt-
ically inactive μ-oxo metalloporphyrin dimers, which cannot survive for
long term successive usage. However, incorporation of the catalytically
active metalloporphyrin species into porous Mn-CPF-1 can block the
formation of catalytically inactive di-metalloporphyrin species, which
can significantly sustain their high catalytic efficiency.
Fig. 3. The oxidation of ethylbenzene catalyzed by Mn-CPF-1 catalyst (blue); and after 2 h
reaction, the solid catalyst was removed by centrifugation (red).
increases, the conversion of substrate gradually decreases. In the 4-
ethylbiphenyl oxidation experiment, the yield of the oxidation product
1-(biphenyl-4-yl)ethanone is of only 4.3% (entry 5), whereas the homo-
geneous molecular catalyst Mn-tetraphenylporphyrin (Mn-TPP) can
smoothly oxidize 4-ethylbiphenyl into 1-(biphenyl-4-yl)ethanone
(entry 6). These results might be ascribed to the negligible adsorption of
4-ethylbiphenyl substrate in water by Mn-CPF-1 as confirmed by UV–
Vis spectroscopy, even though the size of 4-ethylbiphenyl is not very
large (~4.3 × 4.3 × 11.5 Å3).
4. Conclusions
Catalyst Mn-CPF-1 can be easily recovered by centrifugation to re-
move the supernatant, and reused for 15 cycles without deterring the
high catalytic activity (Table 1, entry 7; Fig. 2). After ethylbenzene and ad-
ditional TBHP were added to the filtrate, the mixture was stirred at room
temperature for another 12 h. GC analysis indicates that the mixture is
unreactive. Moreover, after the reaction proceeded for 2 h, the solid cata-
lyst was removed by centrifugation. The interrupted reaction did not pro-
ceed anymore under the same conditions, even though additional TBHP
was added to the solution (entry 8; Fig. 3). These results demonstrate
that the catalyst system is heterogeneous in nature. Because the amino
groups on Mn-TAPP or its dialkylamino derivatives are highly reactive
with porphyrin metal centers, these metalloporphyrins are insoluble in
water. Therefore, to facilitate the homogeneous catalysis for comparison,
we have used Mn-TPP as the reference homogeneous catalyst, even
though the electronic nature of different metalloporphyrins might affect
In conclusion, to make use of porphyrin TAPP and cyanuric chloride,
we have successfully constructed a porous porphyrinic material CPF-1.
The manganese(III) ion metalated catalyst Mn-CPF-1 exhibits remark-
able efficiency and selectivity on the oxidation of ethylbenzene, in
which N99% yield has been realized for the transformation of ethylben-
zene to acetophenone. Mn-CPF-1 is much superior to the molecular Mn-
TPP counterpart in terms of catalytic activity and stability. We expect
that this work will facilitate much more extensive research on porous
porphyrinic materials for heterogeneous catalysis, and thus leads to
the discovery of some industrially useful materials in the near future.
Acknowledgments
We are grateful for the financial support of the National Natural Sci-
ence Foundation of China (Grant Nos. 21073158 and 21373180), and
the research fund of Key Laboratory for Advanced Technology in Envi-
ronmental Protection of Jiangsu Province (Grant No. AE201301).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.03.031.
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